U.S. patent application number 10/418481 was filed with the patent office on 2003-11-06 for continuous digester.
Invention is credited to Salminen, Reijo K..
Application Number | 20030205343 10/418481 |
Document ID | / |
Family ID | 26668210 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030205343 |
Kind Code |
A1 |
Salminen, Reijo K. |
November 6, 2003 |
Continuous digester
Abstract
A continuous pulp digester having an elongate horizontally
aligned pressure vessel through which the wood chips and digesting
fluid flow in a forward direction. In a first and second
embodiments within the pressure vessel there is an inner container
defining an elongate chamber or passageway having a square cross
sectional configuration. In a third embodiment the digesting
chamber is cylindrical. At locations along the digester, there are
several pair of liquid flow inlets and liquid flow outlets which
enable filtrate from a pulp washer and a digesting agent to be
moved into and across the digesting chamber to flow out the flow
outlets. These are recirculated in a net upstream flow pattern
toward the outlet end of the digester, and discharged as black
liquor at different locations in the digester.
Inventors: |
Salminen, Reijo K.;
(Bellingham, WA) |
Correspondence
Address: |
HUGHES LAW FIRM, PLLC
Suite 201
2801 Meridian St.
Bellingham
WA
98225
US
|
Family ID: |
26668210 |
Appl. No.: |
10/418481 |
Filed: |
April 18, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10418481 |
Apr 18, 2003 |
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10084289 |
Feb 25, 2002 |
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10084289 |
Feb 25, 2002 |
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09882789 |
Jun 14, 2001 |
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09882789 |
Jun 14, 2001 |
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09141466 |
Aug 27, 1998 |
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09141466 |
Aug 27, 1998 |
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08939595 |
Sep 29, 1997 |
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08939595 |
Sep 29, 1997 |
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08672458 |
Jun 28, 1996 |
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5680995 |
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60004474 |
Sep 28, 1995 |
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60000830 |
Jun 29, 1995 |
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Current U.S.
Class: |
162/40 ; 162/239;
162/246; 162/249; 162/250; 162/251; 162/52; 162/60; 162/77 |
Current CPC
Class: |
D21C 7/00 20130101; D21C
7/06 20130101; D21C 7/08 20130101; D21C 3/24 20130101 |
Class at
Publication: |
162/40 ; 162/60;
162/77; 162/239; 162/246; 162/251; 162/249; 162/250; 162/52 |
International
Class: |
D21C 003/20; D21C
009/02; D21C 007/06; D21C 007/08; D21C 007/14; D21C 007/10 |
Claims
What is claimed:
1. A continuous digester system comprising: a. a pressure vessel
having a lengthwise axis, a rear upstream inlet end having a wood
chip intake means, and a front outlet end having a pulp outlet
means, said vessel having an elongate processing chamber through
which wood chips travel forwardly in the presence of a digesting
agent while being transformed into pulp, with the pulp being
discharged from the pulp outlet means at the front outlet end of
the vessel; b. liquid flow means to circulate processing liquid
through said digester to carry dissolved solids with said
processing liquid, said flow means comprising; i. initial inlet
means to initially introduce processing liquid into the pressure
vessel at an initial inlet downstream location; ii. a plurality of
processing liquid inlet means at inlet locations along the
lengthwise axis of the pressure vessel to introduce processing
liquid into the processing chamber; iii. a plurality of processing
liquid outlet means at outlet locations along the lengthwise axis
of the pressure vessel to extract processing liquid from said
processing chamber, said outlet locations being spaced laterally
from said inlet locations, so that flow of said processing liquid
from each of said inlet means to related outlet means has a lateral
flow component through said processing chamber; iv. recirculating
means comprising a plurality of interconnecting line means, at
least some of said interconnecting line means connecting at least
some of the outlet means with related inlet means at further
upstream locations to direct processing liquid from said at least
some of said liquid outlet means through related interconnecting
line means to further upstream locations to flow through the
related liquid inlet means into the processing chamber and
laterally in the processing chamber to other outlet means to again
be recirculated through related interconnecting line means to other
inlet means; v. liquor outlet means to discharge liquor, said
liquor outlet means being upstream of the initial downstream
location and upstream of at least some of said liquid inlet means
and said liquid outlet means; c. said digester system being
characterized in that the processing liquid moving in a
recirculating pattern through the processing chamber and through
said recirculating means carries dry solid content extracted from
the wood chips during processing in the processing chamber in a net
upstream flow pattern to be discharged from the processing chamber
at said liquor outlet means.
2. The system as recited in claim 1, wherein there is a washer to
receive pulp from the digester and to dewater and wash the pulp, a
substantial portion of filtrate from the washer being directed into
the initial inlet means as said processing liquid to move through
said recirculating means in said net upstream direction.
3. The system as recited in claim 2, wherein a digesting agent is
introduced into said liquid flow means to flow through said
recirculating means and through said processing-in a net upstream
direction to extract dry solids content from said wood chips being
processed and carry said dry solids content in a net-upstream
direction.
4. The system as recited in claim 3, said system further comprising
an evaporation and recovery means to receive liquor discharged from
said pressure vessel at a plurality of discharge locations at
different operating locations in said pressure vessel so as to
extract liquor having different characteristics from different
extraction locations.
5. The system as recited in claim 4, wherein said digesting agent
is alcohol, with said evaporation and recovery means extracting
said alcohol from said black liquor and recirculating said recovery
alcohol back to said liquid flow means to be recirculated into said
liquid flow means.
6. The system as recited in claim 1, wherein said system comprises
an impregnation zone located in said pressure vessel at an upstream
location, at least one cooking zone located downstream of said
impregnation zone, and at least one wash displacement zone located
downstream of said cooking zone, at least some of said liquid inlet
means and said liquid outlet means being located at said
displacement wash zone to receive said processing liquid and
recirculate said processing liquid sequentially through related
pairs of said liquid inlet means and said liquid outlet means, said
flow means further comprising means to move the processing liquid
from the wash displacement zone to an upstream location to be
directed into said cooking zone, to flow in a downstream direction
in the processing chamber toward said displacement wash zone.
7. The system as recited in claim 6, wherein at least some
processing liquid from said displacement wash zone is recirculated
ultimately to said impregnation zone to flow downstream in said
vessel through said impregnation zone and into said cooking
zone.
8. The system as recited in claim 7, wherein liquor is extracted
from the impregnation zone and directed to said evaporation and
recovery means for processing.
9. The system as recited in claim 8, wherein liquor is extracted
from said cooking zone and directed to said evaporation and
recovery means for processing.
10. The system as recited in claim 1, wherein said liquid flow
means comprises at least one displacement wash zone, having a
downstream end and an upstream end, with a plurality of said liquid
inlet means being positioned at longitudinally spaced inlet
locations along a length of said displacement wash zone and a
plurality of said liquid outlet means positioned at spaced
locations along a length of said displacement wash zone, said
pluralities of liquid outlet and liquid inlet means being arranged
so that there are a related first downstream and a second upstream
liquid inlet means being arranged to a related first downstream and
second upstream outlet means in a manner that at least a portion of
processing liquid from said first downstream inlet means flows
through said processing chamber to pass into said first downstream
outlet means, with at least a portion of flow into said first
downstream inlet means being recirculating through said
recirculating means to said second upstream inlet means, with at
least a portion of the flow from said second liquid inlet means
flowing across said processing chamber to said second upstream
inlet means, with at least a portion of the flow from said second
upstream inlet means being recirculated by said recirculating means
in an upstream direction, thus accomplishing said net upstream flow
of processing liquid.
11. The system as recited in claim 1, wherein said pressure vessel
has a generally cylindrical cross sectional configuration
transverse to its lengthwise axis, and said digester system
comprises an inner-containing means positioned within said pressure
vessel, with said inner-containing means defining the elongate
processing chamber, said inner-container means comprises at least
in part planar wall surfaces.
12. The system as recited in claim 11, wherein there are inlet
screen means and outlet screen means located at longitudinally
spaced locations at said planar wall surfaces, at least some of
said liquid inlet means passing liquid into said processing
chambers through related screen means, and at least some of said
liquid outlet means discharging processing liquid through related
screen means, at least some of said screen means having propeller
blade means which move across related screen means to prevent
obstruction of flow through said screen means.
13. The system as recited in claim 1, wherein said pressure vessel
comprises a generally cylindrical sidewall, which defines the
processing chamber as a generally cylindrical processing chamber,
at least one of said liquid inlet means and liquid outlet means
comprises liquid passageway means formed in said cylindrical
sidewall, said liquid passageway means having flow axes, said flow
axes being slanted in a radially inward and forward direction.
14. The system as recited in claim 13, wherein said liquid flow
means comprises a plurality of circumferential ring assemblies
positioned at longitudinally spaced locations along said sidewall,
each of said ring assemblies defining a flow chamber to communicate
with related passageway means extending through said wall
member.
15. The system as recited in claim 1, wherein said plurality of
liquid inlet means and said plurality of liquid outlet means are
arranged in alignment pairs, having an alignment flow path between
the liquid inlet means and the liquid outlet means of a related
pair, at least some of said pairs of liquid inlet and liquid outlet
means being arranged in an alternating pattern, whereby cross flow
of processing liquid between adjacent alternating pairs have
different flow directions through said processing chamber.
16. The system as recited in claim 1, wherein said elongate
processing chamber is defined by a longitudinally extending chamber
wall means, said liquid inlet means and said liquid outlet means
being positioned at said chamber wall means in a manner that said
liquid-inlet means causes processing liquid to flow through said
chamber wall means into said processing chamber, and said liquid
outlet means extracts processing liquid from said processing
chamber through said chamber wall means, said liquid inlet means
and said liquid outlet means being arranged in related alignment
pairs where at least some of the liquid from the liquid inlet means
flows in a flow path substantially across said processing chamber
to its related liquid outlet means.
17. The method as recited in claim 1, wherein there is an
evaporation and recovery means to receive liquor discharged from
said pressure vessel, said evaporation and recovery means
comprising at least first and second heat exchange means to cause
evaporation of liquid from said liquor, and first and second
separator means, said first evaporator means being arranged to
initially receive liquor from said pressure vessel and to discharge
liquor from said first heat exchange means, means to direct liquor
from-said first heat exchange means to said first separator means
said first separator means to separate a portion of the liquor from
to the first evaporator means, means to direct remaining liquor
from said first separator means to said second heat exchange means
where said remaining liquor is subjected to a further heat exchange
process, means to direct liquor from said second evaporator means
to said second separator means to extract a portion of the liquor
from said second heat exchanger means.
18. The system as recited in claim 1, wherein there are: a. at
least one impregnation zone at an upstream location in said
pressure vessel; b. first and second cooking zones, with said first
cooking zone being positioned downstream of said impregnation zone,
and said second cooking zone being located downstream of said first
cooking zone; c. first and second displacement wash zones, with
said first displacement wash zone being positioned downstream of
said second cooking zone, and said second displacement wash zone
being positioned between said first and second cooking zones; d.
each of said displacement wash zones having a downstream end and an
upstream end, with a plurality of said liquid inlet means being
positioned at longitudinally spaced inlet locations along a length
of said displacement wash zone and a plurality of said liquid
outlet means positioned at spaced locations along a length of said
displacement wash zone, said pluralities of liquid outlet and
liquid inlet means being arranged so that there are a related first
downstream and a second upstream liquid inlet means being arranged
to a related first downstream and second upstream outlet means in a
manner that at least a portion of processing liquid from said first
downstream inlet means flows through said processing chamber to
pass into said first downstream outlet means, with at least a
portion of flow into said first downstream inlet means being
recirculated through said recirculating means to said second
upstream inlet means, with at least a portion of the flow from said
second liquid inlet means flowing across said processing chamber to
said second upstream inlet means, with at least a portion of the
flow from said second upstream inlet means being recirculated by
said recirculating means in an upstream direction, thus
accomplishing net upstream flow of processing liquid in said
displacement wash zone; e. said recirculating means interconnecting
said first and second wash zones with said first and second cooking
zones and said impregnation zone in a manner that within said
processing chamber, there is a substantially continuous flow of
pulp and processing liquid in a downstream direction from the inlet
end to the outlet end, and a substantially continuous flow of
processing liquid from said displacement wash zones through said
recirculating means to upstream locations into said first and
second cooking zones and into said impregnation zone, whereby
dissolved solids are carried through said recirculating means in a
net upstream direction, while wood chips being processed into pulp
and the processing liquid in the digester travel in downstream
direction.
19. The system as recited in claim 1, wherein said pressure vessel
is aligned so that its major alignment component is horizontal.
20. A method of digesting wood chips, said method comprising: a.
providing a pressure vessel having a lengthwise axis, a rear
upstream inlet end having a wood chip intake means, and a front
outlet end having a pulp outlet means, said vessel having an
elongate processing chamber; b. feeding wood chips through said
wood chip intake means into said processing chamber and causing
said wood chips to travel forwardly in said processing chamber in
the presence of a digesting agent while being transformed into
pulp, and discharging the pulp from the pulp outlet means at the
front outlet end of the vessel; c. circulating processing liquid
through said digester to carry dissolved solids with said
processing liquid by: i. initially introducing processing liquid
into the pressure vessel at an initial inlet downstream location;
ii. directing processing liquid through a plurality of processing
liquid inlet means at inlet locations along the lengthwise axis of
the pressure vessel into the processing chamber; iii. directing
processing liquid from said processing chamber through a plurality
of processing liquid outlet means at outlet locations along the
lengthwise axis of the pressure vessel to extract processing liquid
from said processing chamber, with said outlet locations being
spaced laterally from said inlet locations, so that flow of said
processing liquid from each of said inlet means to related outlet
means has a lateral flow component through said processing chamber;
iv. recirculating said processing liquid through a plurality of
interconnecting line means, with at least some of said
interconnecting line means connecting at least some of the outlet
means with related inlet means at further upstream locations, by
directing processing liquid from said at least some of said liquid
outlet means through related interconnecting line means to further
upstream locations to flow through the related liquid inlet means
into the processing chamber and laterally in the processing chamber
to other outlet means to again be recirculated through related
interconnecting line means to other inlet means; v. discharging
liquor through liquor outlet means at at least one location
upstream of the initial downstream location and upstream of at
least some of said liquid inlet means and said liquid outlet means;
d. said method being characterized in that the processing liquid
moving in a recirculating pattern through the processing chamber
and through said recirculating means carries dry solid content
extracted from the wood chips during processing in the processing
chamber in a net upstream flow pattern to be discharged from the
processing chamber at said liquor outlet means.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority benefit of U.S. Provisional
Patent Application No. 60/000,830 filed Jun. 29, 1995, Provisional
Patent Application No. 60/004,474 filed Sep. 28, 1995, U.S. patent
application Ser. No. 08/672,458, filed Jun. 28, 1996, and U.S.
patent application Ser. No. 10/084,289 filed Feb. 25, 2002.
a. BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to the art of wood
pulp digesters, and more particularly to a continuous digester.
b. BACKGROUND ART
[0003] In a typical continuous pulp digester, the wood chips and
the white liquor are fed into the upper end of a vertically aligned
digester, with the interior of the digester defining a cylindrical
digesting chamber maintained at a relatively high pressure (e.g.
200 PSI) and high temperature (e.g. approximately 380.degree. F.).
The ixture of chips and white liquor moves slowly and downwardly
through the digester so that the total dwell time within the
digester is generally between about two to four hours. During the
period that the wood ships are in the digester, the white liquor
reacts with the material in the wood chips to break down certain
organic compounds in the wood chips so as to "delignify" the
pulp.
[0004] At several locations along the length of the digester,
portions of the liquid are extracted, either to be recirculated
back into the digester, sent to an evaporator, or possibly to be
processed elsewhere in the system. To retain the wood chips that
are being processed in the digester, the liquid is extracted
through sets of screens which are generally placed in sets at
vertical locations circumferentially around the digester.
SUMMARY OF THE INVENTION
[0005] The continuous digester system of the present invention
comprises a pressure vessel having a lengthwise axis, a rear
upstream inlet end having a wood chip intake means, and a front
outlet means having a pulp outlet means. The vessel has an elongate
processing chamber through which wood chips travel forwardly in the
presence of a digesting agent while being transformed into pulp,
with the pulp being discharged from the pulp outlet means at the
front outlet end of the vessel. There is a liquid flow means to
circulate the processing liquid through said digester system to
carry dissolved solids with said processing liquid, said flow means
comprising the following:
[0006] i. an initial inlet means to initially introduce processing
liquid into the pressure vessel at an an initial inlet downstream
location;
[0007] ii. a plurality of processing liquid inlet means at inlet
locations along the lengthwise axis of the pressure vessel to
introduce processing liquid into the processing chamber;
[0008] iii. a plurality of processing liquid outlet means at outlet
locations along the lengthwise axis of the pressure vessel to
extract processing liquid from said processing chamber, said outlet
locations being spaced laterally from said inlet location so that
flow of said processing liquid from each of said inlet means to
related outlet means has a lateral flow component through said
processing chamber;
[0009] iv. recirculating means comprising a plurality of
interconnecting line means, at least some of said interconnecting
line means connecting at least some of the outlet means with
related inlet means at further upstream locations to direct
processing liquid from said at least some of said liquid outlet
means through related interconnecting line means to further
upstream locations to flow through the related inlet means into the
processing chamber and laterally in the processing chamber to other
outlet means to again be recirculated through related
interconnecting line means to other inlet means;
[0010] v. liquor outlet means to discharge liquor for further
processing, said liquor outlet means being upstream of the initial
downstream location and upstream of at least some of said liquid
inlet means and said liquid outlet means.
[0011] The digesting system is characterized in that the processing
liquid moving in a recirculating pattern through the processing
chamber and through the recirculating means carries dry solid
content extracted from the wood chips during processing in the
processing chamber in a net upstream flow pattern to be discharged
from the processing chamber at said liquor outlet means.
[0012] In the preferred system, there is a washer to receive pulp
from the digester and to dewater and wash the pulp. A substantial
portion of filtrate from the washer is directed into the initial
inlet means to move through the recirculating means into said net
upstream direction.
[0013] Also, in the preferred form, the digesting agent is
introduced into the liquid flow means to flow through the
recirculating means in a net upstream direction to extract dry
solids content from the wood chips being processed and carry these
in the net upstream direction.
[0014] In the preferred form, the system further comprises an
evaporation and recovery means to receive liquid discharged from
the pressure vessel at a plurality of discharge locations at
different operating locations in said pressure vessel so as to
extract liquor having different characteristics from different
extraction locations.
[0015] In the preferred embodiments of the present invention, the
digesting agent is alcohol. The evaporation and recovery means
extracts the alcohol from the liquor and recirculates the recovered
alcohol back to liquid flow means to be recirculated into the
liquid flow means.
[0016] Also, in the preferred form the system comprises an
impregnation zone located in the pressure vessel at a more upstream
location. There is at least one cooking zone located downstream of
the impregnation zone, and at least one wash displacement zone
located downstream of the cooking zone. At least some of the liquid
inlet means and liquid outlet means are located at said
displacement wash zone to receive the processing liquid and
recirculate the processing liquid sequentially through related
pairs of the liquid inlet means and the liquid outlet means. The
flow means further comprises means to move the processing liquid
from the wash displacement zone to-an upstream location to be
directed into the cooking zone to flow in a downstream direction in
the processing chamber toward the displacement wash zone.
[0017] Desirably, processing liquid from the dislacement wash zone
is recirculated to the impregnation zone to flow downstream in said
vessel through said impregnation zone and into said cooking zone.
Also, liquor is extracted from the impregnation zone and directed
to the evaporation and recovery means for processing. In this
arrangement, liquor also is extracted from the cooking zone and
directed to the evaporation and recovery means for processing.
[0018] In the system of the present invention, the net upstream
flow created by the liquid flow means comprises at least one
displacement zone having a downstream end and an upstream end, with
a plurality of a liquid outlet means being positioned at
longitudinally spaced locations along a length of the displacement
wash zone and a plurality of the outlet means positioned at spaced
locations along the length of the displacement zone. The
pluralities of liquid outlet and liquid inlet means are arranged so
that there are related first downstream and second upstream liquid
inlet means being arranged relative to related first downstream and
second upstream outlet means in a manner that at least a portion of
flow from the first downstream inlet means flows through the
processing chamber to pass into the first downstream outlet means.
Then at least a portion of the flow into the first downstream inlet
means is recirculated to the second upstream inlet means, with at
least a portion of the flow from the second liquid inlet means
flowing across the processing chamber to the second upstream inlet
means. At least a portion of the flow from the second upstream
inlet means is recirculated by the recirculating means in an
upstream direction. In this manner, the net upstream flow of
processing liquid is accomplished.
[0019] The pressure vessel has a generally cylindrical cross
sectional configuration transverse to its lengthwise axis. In two
embodiments, there is inside the pressure vessel an
inner-containing means, defining the elongate processing chamber,
and comprising at least in part planar wall surfaces. Screen means
are located at longitudinally spaced locations at the planar wall
surfaces, so that at least some of the liquid inlet means passes
liquid into the processing chamber through the screen means, and at
least some of the liquid outlet means discharges processing liquid
through related screen means. Desirably, there is propeller blade
means which move across related screen means to prevent obstruction
of flow through the screen means.
[0020] In another configuration, the cylindrical sidewall itself
defines the processing chamber. At least one of the liquid inlet
means and outlet means comprises liquid passageway means formed in
the cylindrical sidewall, with the passageway means having flow
axes which are slanted in a radially inward and forward direction.
This flow means desirably comprises a plurality of circumferential
ring assemblies positioned at longitudinally spaced locations along
the sidewall. Each ring assembly defines a flow chamber to
communicate with related passageway means extending through the
wall member.
[0021] Also, in a preferred form, adjacent pairs of aligned liquid
inlet means and liquid outlet means are arranged in an angularly
alternating relationship, so that a cross flow of processing liquid
between such adjacent alternating pairs have different flow
directions through the processing chamber.
[0022] In a preferred form, the evaporation and recovery means
comprises at least first and second heat exchange means and first
and second separator means. The first evaporator means initially
receives the liquor from the pressure vessel and after evaporation
discharges liquor which is then directed to the first separator
means, where a portion of the liquor is separated. The remaining
liquor is directed to the second heat exchange. Then the liquor
from the second heat exchange means is directed to the second
separator means to extract another portion of the liquid from the
second heat exchange means.
[0023] In preferred form, the pressure vessel is aligned so its
major alignment component is horizontal.
[0024] In the method of the present invention, a pressure vessel
and flow system is provided as described above. The flow of the
wood chips is in a downstream direction, while there is a
recirculation of the processing liquid in an upstream direction, as
described above.
[0025] Other details of the present invention will become apparent
from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic drawing illustrating the main
components and system of a prior art digesting system;
[0027] FIG. 2 is a somewhat schematic side elevational view
illustrating the digesting system of the present invention;
[0028] FIG. 3 is a transverse sectional view illustrating a typical
cross section of the digester of the present invention;
[0029] FIG. 4 is a transverse sectional view showing a liquid
inflow module used in the digester of the present invention;
[0030] FIG. 5 is a sectional view taken along line 5-5 of FIG.
4;
[0031] FIG. 6 is a transverse sectional view, similar to FIG. 4,
showing a liquid outflow module of the present invention;
[0032] FIG. 7 is an isometric view of one of the hydraulic
actuators used in the liquid outflow and liquid inflow modules of
the present invention;
[0033] FIG. 8 is a longitudinal sectional view of a portion of the
digester illustrating the flow pattern from an inlet module to an
outlet module of the digester.
[0034] FIG. 9 is a transverse sectional view of the pulp diluting
module of the digester of the present invention;
[0035] FIG. 10 is a transverse sectional view showing the liquid
outlet unit for the wood chip inflow section of the present
invention;
[0036] FIGS. 11 through 14 are semi-schematic views, similar to
FIG. 2, showing in an enlarged scale four sections of the digesting
system illustrated in FIG. 2;
[0037] FIG. 15 is a view similar to FIG. 4, showing a cross
sectional configuration of a modified form of a liquid inflow
module;
[0038] FIG. 16 is a view similar to FIG. 6, showing a modified
version of the outflow module;
[0039] FIG. 17 is a view similar to FIG. 15 showing a cross
sectional configuration of a further modified form of a liquid
inflow module, incorporated in a second embodiment of the present
invention;
[0040] FIG. 18 is a view similar to FIG. 17 showing an outlet
module utilizing the second embodiment of the present
invention;
[0041] FIGS. 19 through 29 are schematic side elevational views
showing the entire second embodiment, with various sections of the
digester being shown in sequence, beginning from a front downstream
location in FIG. 19, and continuing all the way to the opposite end
of the digester in FIG. 29;
[0042] FIG. 30 is a schematic side elevational view of a third
embodiment of the present invention;
[0043] FIGS. 31A and 31B are cross-sectional views of two of the
cross flow rings of the present invention, these Figures
illustrating the different angular orientation of adjacent cross
flow rings;
[0044] FIG. 32 is a sectional view of an inlet port section of one
of the cross flow rings of the present invention;
[0045] FIG. 33 is a longitudinal sectional view showing flow
patterns in Displacement Wash Zone A FIG. 34 is a view similar to
FIG. 30, but showing additionally the various cross flow rings and
their angular orientation;
[0046] FIG. 35 is a view similar to FIG. 30, but drawn to an
enlarged scale, and only showing the left half of the digesting
system;
[0047] FIG. 36 is a view similar to FIG. 30, but drawn to an
enlarged scale, and showing the right half of the digesting system
of FIG. 30;
[0048] FIGS. 37 through 39 are three views drawn to a further
enlarged scale, showing three different portions of the digester,
with the downstream portion being shown in FIG. 37, the middle
portion being shown in 38, and the upstream portion being shown in
FIG. 39;
[0049] FIG. 40 is a view similar to FIG. 32, showing a modified
form of a cross flow ring;
[0050] FIG. 41 is a view similar to 40, but showing a yet further
modified version of the cross flow ring;
[0051] FIG. 42 is a table showing various values, namely
temperature, alcohol contents, and dissolved solid contents, at
various locations in the digester;
[0052] FIG. 43 is a semi-schematic view showing a modification of
the cooking zone 1 shown in FIG. 30;
[0053] FIG. 44 is a schematic view illustrating the evaporation and
recovery system of the present invention;
[0054] FIG. 45 is a graph illustrating the content of a typical
wood cell, showing at the left hand side the outer surface of the
wood cell, and at the right hand side the center of the wood
cell.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0055] 1. Typical Prior Art Digesting Process
[0056] It is believed that a clearer understanding of the present
invention will be obtained by first reviewing the digesting portion
of a typical pulp mill for which the present invention is
particularly adapted. With reference to FIG. 1, the wood chips are
first subjected to magnetic separation of tramp iron and screening
at location 1, and then directed into a surge bin of a hopper
indicated at 2. From the hopper, the chips flow into a chip meter 3
which controls the rate of flow of the chips which then pass into a
low pressure feeder 4.
[0057] The feeder 4 directs the chips into a steaming vessel 5 that
is kept at between 15 to 20 PSI where the chips are pre-steamed.
The chips are then directed from the steaming vessel 5 into the
chip chute 6, from which the chips move to a high pressure feeder
7. The chips are flushed into the feeder by means of a chip chute
circulating pump 8. As seen in FIG. 1, the flow from the pump 8
into the chip chute 6 and to the feeder 7 is in a counterclockwise
direction. Liquor level of the chip chute 6 is controlled by the
level tank 9. The wood chips mixed with a certain amount of liquor
are then moved from the feeder 7 through a line 11 into a top
strainer 12 to the top of the digester 14. A high pressure pump 10
introduces the cooking liquor to the digester, as well as the
excess liquor from the chip chute level tank 9. The volume of the
cooking liquor can be controlled by a magnetic flow meter.
[0058] In general, the digester pressure is controlled so as to be
at about 200 PSI. The chips and the cooking liquor gradually move
downwardly in the digester, first passing into an upper
impregnation Zone I and then to the heating Zone II.
[0059] The temperature is raised in two steps by two cooking
circulating systems, which comprise extraction strainers, pumps and
central circulating chambers. Three heaters 13 are shown. After
heating, the chips and liquor pass downwardly through the cooking
zone III of the digester. As the chips then pass into the lower
washing zone IV of the digester, extracted wash liquor is
circulated through the chips to provide a quench of the cooking
reaction. The chips continue to pass downwardly in the washing zone
IV, then to be discharged. The entire sequence is arranged so that
the duration of the digesting process is about one and one half to
four hours.
[0060] Wash liquor from a subsequent filtrate tank or fresh hot
water is pumped into the bottom of the digester and flows inwardly
countercurrently to the chip flow. Elevated temperatures of
125.degree. C. (to 135.degree.) are controlled in the diffusion
zone by an auxiliary wash liquor circulation and heater system.
[0061] At various locations in the digester, the liquor is
recirculated to an upper location. A portion of the liquor that is
extracted between zone III and zone IV is directed to a flash tank
17, and thence to flash heat evaporators. The pulp that is
extracted from the bottom of the digester is directed to a blow
unit 16 which has a pressure reducing function, and then further
directed to a brown stock washer 19 and/or to some other location
for further processing as indicated schematically at 20.
[0062] A. First Embodiment
[0063] 2. The Overall Structure and Operation of a First Embodiment
of the Digester
[0064] In this section there will be a brief description of the
overall construction and operation of the digester and the
digesting system of the first embodiment of the present invention.
Then in the following sections there will be descriptions of the
main structural components which are particularly adapted for use
in this first embodiment, and also a more detailed description of
the overall operation of the digesting system and other aspects and
variations of the same.
[0065] To describe the first embodiment of the present invention,
reference is first made to FIGS. 2 and 3. FIG. 2 is a schematic
side elevational view of the digesting system 100 of the present
invention. This digesting system 100 comprises an elongate,
horizontally aligned digester 102, having a wood chip inlet end 104
and a pulp outlet end 106. At the pulp outlet end 106 there is a
washer 108 which receives the pulp slurry from the digester 102 to
dewater and wash the pulp and discharge it for further processing.
In addition, the washer 108 cooperates with the digesting system
100 to recirculate filtrate from the washer back into the digester
102 near the exit end 106 thereof.
[0066] The wood chips are introduced into the inlet end 104 through
an inlet of the digester 102 by conventional means and are mixed
with the liquor in the digester. Over a period of several hours
(e.g. usually two to four hours), the wood chips move continuously
down the length of the digester 102 and proceed through various
processing zones. When the wood chips reach the exit end 106, these
have been substantially delignified, and the pulp is diluted with
filtrate from the washer 108 and then passed into the washer
108.
[0067] In the following description, the term "wash water" means
the fresh water which is introduced into the washer 108. The term
"filtrate" shall refer to the liquid which is removed from the pulp
in the washer 108 during the dewatering operation (which will be
called the "dewatering filtrate"), and the effluent which is
discharged from the washer to be utilized at another location of
the digester (this being called the "washer discharge filtrate".
The term "liquor" or "liquors" shall refer to all of the liquid
which is in the digester and has as one of its ingredients the
digesting ingredient (which in this preferred embodiment is ethyl
alcohol). Finally, the term "black liquor" shall refer to the
liquor which is discharged from the digester for further
processing. This discharge of black liquor takes place adjacent to
the wood chip inlet end 104 of the digester 102.
[0068] The term "forward" shall denote a direction extending from
the wood chip intake end 104 to the pulp discharge end 106, so that
the rear end will be at 104 and the forward-end will be the end at
106. The term "downstream" shall denote the direction of flow of
the wood chips which are being processed in the digester, this
direction being from the end 104 to the end 106, and the term
"upstream" shall denote the opposite direction. The term "inner"
shall denote proximity to a longitudinal center axis of the
digester 102, and the term "outer" or "outward" shall denote a
direction away from the longitudinal center axis of the digester
102 and/or a location more distant from the longitudinal center
axis or line of the digester 102.
[0069] The filtrate from the washer in addition to being
recirculated to dilute the pulp that exits from the digester end at
106, provides part of the liquid to form the liquor which is used
in the digesting process within the digester 102. This is
accomplished in a manner that the filtrate from the washer 108
enters the digester vessel 102 adjacent to the downstream end, and
then is recirculated through the digester in a manner that the "net
flow path" is in an upstream direction. This will be described in
more detail later herein, but the following will give a brief
summary of how this is accomplished.
[0070] In the preferred embodiment, the digesting ingredient is
ethyl alcohol. As will be discussed more fully hereinafter, the
present invention particularly adapts itself for the effective use
of ethyl alcohol and solves problems which have been experienced in
the prior art where ethyl alcohol is used as the digesting
ingredient. However, within the broader scope of the present
invention, it is to be understood that other digesting ingredients
could be used and derive a good portion of the benefits of the
present invention.
[0071] There are five main components in the digester 102 which are
combined with other components of the system. As indicated above,
there will now be descriptions of each of these five components in
more detail.
[0072] 3. Typical Cross Sectional Configuration of the Digester
102
[0073] The term digester 102 shall refer not only to the high
pressure vessel which is the containing structure, but also to
those components within the containing structure. Thus, with
reference to FIG. 3, this digester 102 comprises a high pressure
container 110 which has a cylindrical cross sectional
configuration. This vessel 110 is typically made of a high strength
steel capable of withstanding pressures up to as high as 500 PSI,
and temperatures as high as 200.degree. C. or higher. This vessel
comprises a cylindrical containing wall 111 which extends the
entire length of the digester 102, and it is enclosed at the
ends.
[0074] The rear end of the digester vessel 110 is closed by a
substantially hemispherical rear wall 112, and the front end is
closed by a substantially hemispherical forward wall 114.
[0075] Positioned within the pressure vessel 110 is an inner
container 116 which has a substantially square cross sectional
configuration and which extends substantially the entire length of
the digester 102 (See FIG. 3).
[0076] This inner container 116 defines an elongate chamber or
passageway 118 (also having a square cross sectional configuration)
which is the digesting area. This area 118 contains what are
initially the wood chips and the digesting liquid (i.e. the
liquor). The inner container 116 has an upper wall 120, a lower
wall 122 and right and left sidewalls 124 and 126, respectively.
These walls are joined to one another at corner locations which are
designated (beginning at the upper right hand corner of FIG. 3 and
proceeding counterclockwise) 128, 130, 132 and 134. These corners
128 through 134 join directly with the inside surface 136 of the
vessel wall. Alternatively, these corners 128 through 134 could be
spaced inwardly from the vessel inner surface of the wall 111 136
and yet joined to the vessel 110 so that the areas surrounding the
inner container 116 can communicate with one another.
[0077] Positioned between the inner surface 136 of the vessel 110
and the inner container 116, there is a plurality of reinforcing
plates 138 welded or otherwise joined both to the outer pressure
vessel 110 and the inner container 116. These plates 138 are
positioned around all four walls of the inner container 116 at
longitudinally spaced intervals along the length of the digester
102. These plates are provided with openings 140 so that a pressure
equalizing fluid (either gaseous or liquid) can communicate between
the areas 142 outside of the square container 116 between adjacent
pairs of plates 138.
[0078] As indicated previously, the digesting process takes place
at pressures as high as 200 to 500 PSI and temperatures as high as
150 to 200.degree. C. The pressure vessel 110 is designed to
withstand these high pressures and also to provide thermal
insulation. Accordingly, the temperature and pressure levels within
the inner chamber 118 should be substantially the same as the
pressure and temperature of the areas 142 between the inner
container 116 and the pressure vessel 110. This is accomplished by
filling the areas 142 (which areas 142 have in cross section the
shape of a segment of a circle) with a liquid or gaseous medium
which would be kept at the same pressure as exist with the chamber
118 of the inner container 116. For this purpose, there are shown
nozzles 144 which communicate with a fluid such as steam, or
possibly a suitable liquid (e.g. condensate) to fill these areas
142 surrounding the inner container 116. These nozzles 144 will be
provided for all space or areas of the vessel 110 which surround or
are adjacent to the inner container 116.
[0079] As indicated above, what is shown in FIG. 3 is a typical
cross section of the digester 102. These sections can be made in
modules or units of, for example, four feet long, or as longer
sections. The module shown in FIG. 3 is given the general numerical
designation 146.
[0080] 4. Fluid Inlet Unit of the Digester 102
[0081] Reference is made to FIG. 4, where there is shown a fluid
inlet module 148 of the digester 102. It can be seen that the cross
sectional configuration of this section in FIG. 4 is substantially
the same as shown in FIG. 3, in that there is the surrounding
pressure vessel 110, the bracing or reinforcing plates 138, and the
inner container 116. However, the top wall 120 of the inner
container 116 is omitted. In its place, there is provided a fluid
inlet assembly generally designated 150.
[0082] In place of the top wall 120, there is provided an inlet
screen 152 which is positioned in a plane extending between the two
top corner lines 128 and 130. This screen 152 has a circular
configuration and is mounted to a plurality of radially extending
bracing arms 154 which in turn connect to a central hub 156. The
screen assembly 158 (made up of the screen 152, the bracing arms
154 and the hub 156, is rotatably mounted within a surrounding
plate 160 which has an outer square perimeter and a circular cutout
to receive the screen 152 and the bracing arms 154.
[0083] The hub 156 is connected (e.g. by the nut and drive shaft
connection 162) to a hydraulic actuator 164. This hydraulic
actuator 164 has an output shaft 166 which is caused to rotate in a
reciprocating manner through 180.degree. of rotation. Thus, the
actuator 164 will rotate the screen assembly 158 180.degree. in one
direction, then 180.degree. in the opposite direction.
[0084] There is provided a longitudinally aligned wiper blade 168
(FIG. 5) that extends diametrically across the entire screen 152 in
a forward to rear direction. This wiper blade 168 remains
stationary, and one means of accomplishing this is, as shown
herein, to connect opposite ends 170 of the blade 168 to the plate
structure 160 that surrounds the screen assembly 158. As the screen
assembly 158 rotates through its 180.degree. paths of travel, all
portions of the screen 152 pass by the adjacent edge sections 171
of the blade 168 to maintain the screen 152 free of any matter that
might clog the screen 152. This could be, for example, wood chips
or pulp fiber bundles.
[0085] The screen assembly 158 and the rotary actuator 164 are
mounted to a flat circular mounting plate 172 that is in turn
mounted to a cylindrical plate 174 that is in turn welded or
otherwise joined at 176 to an opening in the vessel wall 110. This
mounting plate 172 and the cylindrical plate 174 are of steel
construction and of sufficient strength to withstand the pressures
within the vessel 110.
[0086] There are also partial bracing plates 178, but the inner
edge 180 of these plates 178 is spaced a short distance outwardly
of the arms 154. There is a fluid inlet nozzle 182 which leads into
the area or space 184 which is between the screen assembly 158 and
the adjacent wall portion 186 of the vessel 110.
[0087] The plate 160 which surrounds the screen assembly 158 has a
square configuration around its perimeter. At the forward and rear
edges of the plate 160, there are forward and rear isolating plates
188 which form an isolated chamber which is defined at the inner
location by the screen 152 and the surrounding plate 160, on the
outside by the adjacent portion 186 of the vessel 110, and at the
forward and rear ends by the forward and rear plates 188.
[0088] In operation, the effluent which is to be directed into the
vessel 110 is directed through the inlet nozzle 182 at a pressure
slightly higher than the fluid within the chamber 118. This fluid
entering through the nozzle 182 distributes itself throughout the
area or volume 184 behind the screen assembly 158 and thus passes
substantially uniformly through the screen 152 into the chamber
118. As this happens, the screen assembly 158 is rotated by the
actuator 164 through the 180.degree. arcuate path of travel to wipe
the screen 152 free of any material which might clog the screen
152. This reciprocating rotation of the screen assembly 158 may not
need to be done continuously, but could be done intermittently to
keep the screen 152 open. (e.g. every two to fifteen minutes or
longer). The screen is rotated slowly (e.g. one to five revolutions
per minute depending on the position in the digester, faster at the
forward end and slower at the rear end of the digester).
[0089] As indicated above, this modular fluid inlet unit 148 is
used at different locations along the length of the digester 102 to
direct fluid into the digester 102. The various functions performed
by this unit 148 will be discussed later herein.
[0090] 5. Fluid Outlet Unit of the Digester 102
[0091] Reference is made to FIG. 6, which shows a module 190 which
is constructed substantially identically to the module 148 shown in
FIG. 4, except that the module 190 is inverted 1800 relative to the
module 148 of FIG. 4. With this similarity of structure, for
convenience components of this module 190 which correspond to
similar components of the module or unit 148 of FIG. 4 will be
given like numerical designations with an "a" suffix distinguishing
those of the module 190 shown in FIG. 6.
[0092] Thus, there is a screen assembly 158a comprising the screen
152a, the support arms 154a, and the hub 156a. There is a rotary
actuator 164a along with its output shaft 166a, and also the wiper
arm 168a. Other components will simply be given numerical
designations with the "a" suffix without further verbal
description.
[0093] The main difference in this unit 190 is that the nozzle
182a, instead of being an inlet nozzle is an outlet nozzle. Thus,
the nozzle 182a is operated at a pressure slightly below that which
exists within the digesting chamber 118.
[0094] Also, since the flow from the digesting chamber 118 is
outwardly through the screen 152a, there may be some tendency for
the wood chips which are being processed into pulp to tend to
accumulate on the screen 152a. The rotation of the screen assembly
158a relative to the blade 168a alleviates this problem.
[0095] As will be discussed later herein, most all of the inlet
units 148 are positioned so that each is adjacent to, and
immediately upstream of, a related outlet unit 190. The pressure
differential from the chamber 184 of the inlet unit 148 to the
chamber 118 and thence to the outlet chamber 184a is such that it
causes a fluid flow into the chamber 118 (through the screen 152)
and outwardly from the chamber 118 (through the screen 152a). As
the wood chips/pulp and liquor move forwardly in the chamber 118,
the fluid flows outwardly through the screen 152 displaces the
liquor in the chamber 118 downwardly so that as the displaced fluid
moves downwardly through the chamber 118 it is also moving
forwardly. As this liquor reaches the lower wall of the inner
container 116, it is then adjacent to the outlet screen 152a so
that this displaced liquid then flows out the lower conduit 182a.
This operation is accomplished at various locations along the
length of the digester 102 and will be described more completely in
the next section.
[0096] To describe the rotary actuators 164 and 164a, reference is
made to FIG. 7. This actuator 164 or 164a is, or may be of more or
less conventional design. There are upper and lower reciprocating
pistons 192, each of which have gear teeth 194 which engage with
gear teeth 196 connected to the shaft 166. Hydraulic fluid is
directed alternatively into the chambers 198 at opposite ends of
each piston 194. The fluid is directed into, and discharged from,
the chambers 198 in a manner to reciprocate the pistons oppositely
to one another and thus move the shaft 166 through alternating
180.degree. paths of revolution. As described above, this causes
the 180.degree. alternating rotation of the two screen assemblies
152 and 152a.
[0097] In the area of the rack and pinion gear teeth 194 and 196,
there is maintained an oil pressure moderately higher than the
pressure in the digesting chamber 118. Seals are provided around
the shaft 166 to prevent the oil in this area from flowing into the
interior of the digester 102. Also, a seal is provided between the
housing of the actuator 164 and the plate 172.
[0098] 6. Flow Patterns in Displacement Zones
[0099] Reference is now made to FIG. 8 which illustrates a typical
displacement flow pattern between an inlet module 148 and an outlet
module 190.
[0100] As the wood chips are introduced into the rear end 104 of
the digester 102 and progress through the digester 102, they lose
their character as wood chips and become delignified pulp fibers.
The consistency of the pulp in the digester is typically about
12.5%, which means that there are two parts liquid within the
fibers and five parts liquid surrounding the fibers.
[0101] In the right hand part of FIG. 8, the flow of the liquor
moving into the displacement zone is indicated by the flow lines
202. The velocity of the liquor further upstream is about one and
one half feet per minute. The average velocity of the liquor which
flows from the screen 152a of the unit 190 is about three feet per
minute. The pulp fibers tend to move through the digesting chamber
or passageway 118 more as a plug, moving at the one and one half
foot per minute rate of travel.
[0102] The diagonal cross flow between the units 148 and 190 does
not have any significant tendency to compress this plug of pulp
fibers, but the flow passes through the spaces surrounding the pulp
fibers.
[0103] With further reference to FIG. 8, the initial flow of liquor
from the screen 152 is indicated by the lines at 204. This flow
forms an interface at 206 with the flow 202. It can be seen that
that interface plane 206 extends longitudinally in a downstream
direction at a downward slant. The result is that the interface
plane 206 extends to about a mid-location at 208 of the screen
152a. The flow of the liquor 202 displaced through the screen 152a
is illustrated by the lines at 210.
[0104] It is to be understood that the interface plane 208 is not a
clearly defined plane and the adjacent liquors tend to combine to
some extent in a mixing zone along the plane 208.
[0105] A portion 212 of the flow of liquor from the screen 152
follows a flow path to the screen 152a at 214. Another portion of
the liquor from the screen 152 follows more of a diverging
downstream flow, indicated by the lines 216.
[0106] From the above description, it is apparent that a high
percentage of the liquid flow at 202 is displaced into the screen
152a for recirculation in an upstream direction. Also, a
significant percentage of the flow from the screen 152 is directed
in a downstream direction. However, this flow at most locations is
subjected to a further downstream displacing action to be
recirculated back up to an upstream location. The advantage of this
will become more apparent in reviewing the later description of the
overall operation of the system 100 of the present invention.
[0107] 7. Pulp Diluting Unit of the Digester
[0108] Reference is made to FIG. 9 which is a cross sectional view
if the diluting unit or module 220. This module 220 is located near
the outlet 106 of the digester 102. Components of this diluting
module 220 which are the same as, or similar, components of the
prior modular units will be given like numerical designations, with
a "b" suffix distinguishing those of the modular unit 220.
[0109] The function of this diluting unit 220 is to deliver a large
portion of the filtrate from the washer 108 into the front end of
the chamber 118 to bring the consistency of the wood pulp/liquor
mix (which is at about 121/2% consistency in the digester 102) to
about 2% to 4% consistency. It can be seen that the four walls of
the inner container 116 are removed and replaced by four inlet flow
assemblies 150b, each with its screen assembly 158b. Each screen
assembly 158b has, as in the prior modular units 148 and 190 the
hydraulic actuator 164b (or hydraulic motor) and the associated
mounting plates 172b and 174b. All of the nozzles 182b are inlet
nozzles so that there is a net inflow of filtrate from the washer
108 into the chamber 116 from all four sides.
[0110] There is connected to each rotary actuator shaft 166b a
related mixing arm 222 which has a radially outward right angle
elbow section 224 positioned laterally of the axis of the shaft
166. This elbow section 224 connects to a forearm section 226 that
terminates at a middle location 228. The hydraulic actuators 164b
could be arranged to rotate through 360.degree. paths of travel, in
an alternating pattern. Or each actuator 164b could be a
continuously rotating motor, rotating only in one direction. These
rotating arms 222 mix the incoming filtrate with the pulp in the
dilution zone.
[0111] 8. Liquid Outlet Unit for the Wood Chip Inflow
[0112] Reference is made to FIG. 10 which shows a module 230 which
in terms of structure is substantially identical to the module 220
of FIG. 9. Components of this module 230 which are similar to
components described previously herein will be given like numerical
designations, with a "c" suffix distinguishing those components of
this module 230.
[0113] When the wood chips are introduced into the rear end 104 of
the digester 102, they are first mixed with liquid in a prior art
manner so that they flow "readily into the digester 102. Black
liquor is used for this purpose. However, after the wood chips are
introduced, it is desirable to displace this liquor to maintain the
derived liquor to wood ratio in the digester 102.
[0114] Positioned above the module 230 is an accumulator tank 232
which is filled to about its mid-height with this recirculation
liquor, as at 234. The upper half of the chamber defined by the
container 230 is designated 236 and contains a pressurized gas,
typically nitrogen. This communicates through two tubes 238 to the
uppermost fluid assembly 150c which functions either as an inlet
assembly or an outlet assembly, depending upon conditions in the
digesting chamber 116. If the inflow of fluid into the inlet 104 of
the digester 102 is for a period of time greater than the outflow
at the opposite end 106, then this extra fluid is able to pass
upwardly through the conduits 238 into the accumulator tank 232. If
the opposite situation occurs, then fluid will flow from the tank
232 into the chamber 118.
[0115] In addition of the function as an accumulator, the module
230 acts also as a separator of air and gases that are coming in
with the wood chips. These gases are vented from the area 236
periodically.
[0116] The other three assemblies 150c are all fluid outlet
assemblies. These function to carry away the excess liquor which
accompanies the wood chips that are being introduced into the
digester 102.
[0117] 9. Overall Operation of the Digesting System 100
[0118] Reference is now made back to FIG. 2 which shows somewhat
schematically the entire digesting system. It is believed that the
more detailed description of the overall operation of the preferred
embodiment will be better understood by first providing some
introductory comments, some of which have been made earlier
herein.
[0119] As indicated earlier, the wood chips are introduced at 104
and these move continuously downstream along the length of the
digester 102. As the wood chips pass through various zones, they
are subjected to several processing steps to delignify the wood
chips, and to cause these to become pulp fibers. The pulp with the
liquor carrying the pulp to the front discharge end 106 is first
diluted and cooled with the filtrate from the washer 108 and then
discharged into the washer for dewatering and washing.
[0120] As described previously, the filtrate from the washer 108
serves to dilute the pulp near the discharge end 106 so that it can
be discharged at a consistency of 2 to 4%. Further, the filtrate
from the washer 108 is delivered into the digester 102 near the
outlet 106 end first to accomplish displacement washing of the pulp
near the outlet end 106, and then to be moved further upstream to
be combined with digesting ingredients (in this preferred
embodiment ethyl alcohol) to provide the proper concentration of
the digesting ingredient(s).
[0121] As indicated previously, the digesting liquid, hereinafter
called the "liquor", is recirculated through the digester in a
fashion so that there is a net upstream movement of the liquor from
the front discharge end 106 toward the rear inlet end 104, this
being accomplished by extracting the liquor from the digester 102
at downstream locations and moving it upstream to be reinjected
into the digester 102. As the liquor is moved further upstream, the
liquor acquires a higher concentration of the lignin and other
organic matter extracted form the wood chips, and thus, in the
terminology of the pulp industry, becomes higher in dry solids
(D.S.) content as it is recirculated in a continuous upstream
fashion. The liquor is eventually discharged as black liquor at an
upstream location indicated at 240.
[0122] With the foregoing being given as introductory comments,
there will now be a more detailed description of this process. This
will be done with reference to FIG. 2 and also with reference to
FIGS. 11 through 14 which show respective portions of the digester
system 100 of FIG. 2, but drawn to an enlarged scale.
[0123] 10. Operation at the Dilution Zone and the Hot Wash Zone
[0124] Reference is first made to FIG. 2 and FIG. 11. All of the
filtrate from the washer 108 is transferred to, or recirculated in,
the digester 102. This filtrate is the liquid which is removed from
the pulp during the dewatering process, and also the liquid which
is the outflow of filtrate from the pulp during the final
displacement washing in the washer 108.
[0125] Typically, the consistency of the pulp being processed in
the digester is at about 12-13%. Thus, there are seven parts of
liquor to one part wood fibre. To cause the proper discharge of the
pulp into the washer 108, it is generally desirable to dilute the
pulp to about 2% consistency. This is accomplished in the present
invention by directing the major portion of the filtrate from the
washer 108 by pumps 241 through a pair of heat exchangers 242 (to
extract heat from the filtrate) into the filtrate inlet module 220.
As the filtrate enters through the inlet nozzles 182b, it flows
through the screens 152b into the chamber 118. The mixing arms 222
rotate slowly so that these mix the wood pulp with the filtrate.
Then the filtrate is continuously discharged through a digester
blow nozzle which is indicated schematically at 244.
[0126] In the schematic drawing of FIG. 2 and in FIG. 11, the
module 220 is shown somewhat schematically outside of the pressure
vessel 110. This is done merely for purposes of illustration, and
it is to be understood that this module 220 is positioned within
the pressure vessel 110 as illustrated in FIG. 9.
[0127] From the above description, it is apparent that the major
part of the filtrate is simply recirculated from the washer 108
through the heat exchangers 242 into the chamber 116 by means of
the module 220 and then flows with the pulp fiber through the
outlet nozzle 244 (blow nozzle) into the washer 108. In the washer
108, the diluted wood pulp is first dewatered, then washed through
several cycles, and discharged. Fresh wash water is directed into
the washer 108 through the inlet 246.
[0128] A substantially smaller portion of the filtrate is directed
by a pump 247 first through a heat exchanger 248 which adds heat to
the filtrate, and then is directed into the furthest downstream
fluid inlet module 148-1 so that the filtrate flows downwardly
through the flow inlet assembly 150 at the top of the module 148-1.
The filtrate flowing from the assembly 150 moves downwardly, and at
the same time, due to the forward flow of the liquor carrying the
pulp in a downstream direction, the net flow of the filtrate is in
a slanted downward and forward direction. The effect of this is, as
described above, that the liquor presently in the digester 102
directly below the inlet assembly 150 is displaced downwardly and
outwardly through the outlet assembly 150a of the unit 190-1. This
initial displacement of the liquor accomplished by the combined
action of the modules 148-1 and 190-1 is one stage of a final hot
displacement wash accomplished by the filtrate derived directly
from the washer 108.
[0129] Then the liquor which flows into the module 190-1 is moved
by the pump 250 into a further upstream module 148-2 which moves
the liquor displaced by the module 148-1 into the liquor stream to
displace the liquor immediately below the inlet assembly 150 of the
module 148-2 downwardly to flow outwardly through the screen 150a
of the module 190-2.
[0130] The action of these modules 148-1 and 2 and 190-1 and 2 have
thus accomplished a two stage hot wash, and this is accordingly
designated in FIG. 2 as the "hot wash" zone of the digester
102.
[0131] At this point, it should be noted that the wash has been
accomplished by the filtrate which enters at 148-1. The wash liquid
that remains in the pulp moves downstream to the area of the
dilution module 220. The liquor collected in the module 190-2 is
moved by means of a pump 252 upstream toward a third inflow module
148-3.
[0132] 11. Operation of the Diffusion Wash Zone, High Heat Alcohol
Wash Zone, Cooking Zone 3 and Displacement-Zone A
[0133] At a location between the pump 252 and the module 148-3,
there is shown schematically at 254 an inlet nozzle where ethyl
alcohol is added in a sufficient quantity to the liquor from the
module 190-2 to cause the liquor flowing into the digester 102 to
be sixty percent ethyl alcohol by weight and forty percent water by
weight. The liquor then flows through a heat exchanger 256 to raise
the temperature, and then is delivered to what is designated as the
"high heat alcohol wash" zone.
[0134] There are at that high heat alcohol wash zone two flow inlet
modules 148-3 and 148-4 and two flow outlet modules 190-3 and
190-4. The liquid displacement operation proceeds in this high heat
alcohol wash zone in the same manner as described previously
relative to the further downstream hot wash zone. The net effect is
that the alcohol liquor mixture displaces most of the liquor which
is flowing into the high heat alcohol wash zone and delivers it
through a pump 256 in an upstream direction. Also the rate of flow
of the liquor into and through the inlet assembly 150 of the module
148-3 is sufficiently high so that a substantial amount of this
liquor ends up flowing out of the module 190-4 and thence is pumped
at 257 in an upstream direction. At a location upstream of the pump
257 there is an injection nozzle 258 by which an alcohol water
combined liquor can be delivered into the line as needed to adjust
the pH if this is required. The liquid then flows through the heat
exchanger 260 and is delivered to a further upstream location which
is designated "displacement zone A".
[0135] These modules 148-3 and 4 and 190-3 and 4 which comprise the
high heat alcohol wash zone are spaced a distance upstream from the
hot wash zone so that there is between these two zones an
interconnecting section of the digester which has the typical cross
section, as shown in FIG. 3. The rate of travel of the liquor with
the pulp is such that it takes about twenty minutes for the
liquor/pulp mixture to travel from the high heat alcohol wash zone
to the hot wash zone. This twenty minute period of travel is
through what is termed the "diffusion wash" zone where the lignin
and other material in the pulp diffuses outwardly into the
liquor.
[0136] As indicated above, the liquor that flows through the heat
exchanger 260 is then delivered to that section of the digester
which is designated as displacement zone "A". At this zone, there
are two liquor inlet modules 148-5 and 6, and two liquid outlet
modules 190-5 and 6. The liquor entering the module 148-5
discharges the liquor through the fluid flow assembly 150 to
displace the liquor in the chamber 118 outwardly through the module
190-5 from which the liquor is delivered by the pump 261 to the
module 148-6 which in turn delivers the liquor through the chamber
118 to displace the liquor in the chamber 118 to flow into the unit
190-6, where the pump 262 moves the liquor upstream.
[0137] Between the displacement zone A and the high heat alcohol
wash zone, there is a section of the conduit which is designated
"cooking zone 3". At this cooking zone 3, the digester section is
of sufficient length so that the dwell time of the liquor pulp
mixture in traveling from the displacement zone "A" to the high
heat wash zone is between about thirty to forty minutes.
[0138] 12. Operation at Cooking Zone 2, Displacement Zone B,
Cooking Zone 1 and Displacement Zone C
[0139] Reference is made to FIGS. 2 and 13. The pump 262 delivers
the liquor by an inlet 264 where ethanol can be added as required,
and this liquor is passed through a heat exchanger 265. At a
location further upstream from the displacement zone "A", there is
displacement zone "B". This displacement zone B comprises the
modules 148-7 and 8 and 190-7 and 8. These function in
substantially the same way as the modules 148-5 and 6 and 190-5 and
6 in the displacement zone "A" so the operation of these will not
be repeated herein. The liquor and pulp mixture that flows from the
displacement zone "B" to the displacement zone "A" travels through
a section of the digester 102 which has a length such that the
dwell time is about thirty to forty minutes. This section of the
digester is designated "cooking zone 2".
[0140] Further upstream from the displacement zone "B", there is
what is called a displacement zone "C". This comprises the modular
units 148-9 and 10 and 190-9 and 10. The mode of operation of these
modular units is substantially the same as those in the
displacement zone "A" and displacement zone "B", so that operation
will not be described further herein. The liquor from the
unit-190-8 is delivered by a pump 266 by an ethanol inlet nozzle
which optionally can be used to add ethanol, through a heat
exchanger 268 to the unit 148-9, and after passing through the
units 160-9, 148-10, and 190-10, the liquor from 190-10 is
delivered further upstream by a pump 269. The section of the
digester between the displacement zone "B" and the displacement
zone "C" comprises the cooking zone 1, and this section of the
digester is sufficiently long so that the dwell time of the liquor
and pulp mixture from the displacement zone "C" to displacement
zone "B" is approximately thirty to forty minutes.
[0141] 13. Operation of the Impregnation Zone, Initial Heating
Zone, Wood Chip and Liquid Recirculation and Black Liquor
Removal
[0142] Immediately upstream of the displacement zone C there is the
impregnation zone. The section of the digester defining this
impregnation zone is sufficiently long so that the dwell time of
the wood chip liquor mixture is about ten to twenty minutes.
[0143] The liquor from the module 190-10 is pumped upstream through
a heat exchanger 270 to add heat and this is delivered into the
initial heating zone. Also there is a nozzle 271 for optional
addition of ethanol as needed. At this initial heating zone, there
are the modules 148-11 and 12 and 190-11 and 12. The displacement
operation takes place as described with regard to the previous
displacement zones. Most of the liquor passing through the initial
heating zone moves through the modules 148-11, 190-11, 148-12, and
190-12 and is discharged at 240 as black liquor, for further
processing. A smaller portion of the black liquor flows upstream to
the location of the liquid outlet unit 230 (shown in FIG. 10) to
serve as make-up liquor for the liquid which carries the wood chips
into the digester.
[0144] The wood chips are, as indicated earlier, mixed with a
carrying liquid (i.e. black liquor) and introduced into the wood
chip inlet 105 in a conventional manner. Most of this black liquor
that carries the wood chips is discharged through the module 230 to
be recycled to the wood chip feeder unit again carry additional
wood chips into the inlet 105. Since the manner in which this is a
done is already known in the prior art, this will not be described
further herein.
[0145] The black liquor discharged at 240 is further processed for
alcohol recovery and evaporation to recover the ethyl alcohol for
reuse and also to provide the by-product or by-products from the
black liquor More particularly the black liquor can have the liquid
content reduced (e.g. by evaporation) and then be spray dried or
otherwise dried to produce a by-product in a powder form which has
desirable properties as an animal feed supplement or other
uses.
[0146] 14. Possible Modifications
[0147] It is to be recognized, of course, that within the scope of
the present invention, various modifications could be made in the
present invention without departing from the basis teachings
thereof. One such modification is illustrated in FIGS. 15 and 16.
The components shown in FIG. 15 which correspond to components
described previously herein will be given like numerical
designations, with a "d" suffix distinguishing those of the module
shown in FIG. 15.
[0148] In FIG. 15, instead of providing only one flow inlet
assembly 150 as shown in FIG. 4, there are provided two such flow
inlet assemblies 150d and likewise two inlet conduits 182d.
[0149] The other modification as shown in FIG. 16 shows the liquid
flow outlet module. Components of this modification in FIG. 16
which are similar to prior components will be given like numerical
designations, with an "e" suffix distinguishing those shown in FIG.
16. Thus, it can be seen that there are two liquid outflow
assemblies 150e, and also outlet nozzles 182e for each of these
assemblies 150e.
[0150] It is believed that the mode of operation of these modules
FIGS. 15 and 16 are readily apparent from the prior description.
The flow from the assemblies 150d in FIG. 15 is at a downward and
lateral slant in a downstream direction.
[0151] B. Second Embodiment
[0152] 15. Introduction to the Second Embodiment
[0153] This second embodiment of the present invention is shown in
FIGS. 17 through 29, which can be described as follows.
[0154] In the second embodiment there is a further modified version
of an inlet module as shown in FIG. 17, and an outlet module shown
in FIG. 18.
[0155] Components of this modified version of the inlet module will
be given numerical designations similar to the prior embodiments,
with an "f" suffix distinguishing those components of the modified
version of FIG. 17. There is shown an inlet module 148f, and this
has a single flow inlet assembly 150f located at one side of the
module 148f.
[0156] In FIG. 18, there is shown a fluid outlet unit or module
190g having an outlet assembly 150g positioned at the side of the
digester opposite to the side at which the flow inlet module 148f
is located. The outlet module 190g is located just downstream of
the inlet module 148f so that the flow of the filtrate from the
inlet assembly 150f to the outlet assembly 150g is laterally across
the chamber and also downstream toward the flow outlet assembly
150g. As will be disclosed later herein, this particular
modification shown in FIGS. 17 and 18 is incorporated in the second
embodiment which is described later herein.
[0157] FIGS. 19 through 29 are semi-schematic views, similar to
FIGS. 11 through 14, showing 11 different sections of a second
embodiment of the digester of the present invention.
[0158] The basic operation of this second embodiment is in many
respects substantially similar to the operation of the first
embodiment as shown in FIGS. 11 through 14. Therefore, the overall
structure and operation of this second embodiment will not be
discussed in detail in this description of the second embodiment,
but will be described only generally. Those components or sections
of the digester which are somewhat different in structure and/or
function from components of the first embodiment will be
indicated.
[0159] In FIGS. 19 through 29, there is presented various technical
information concerning the overall operation. More specifically,
flow rates are indicated in liters per minute. Temperatures at
various locations are indicated. The alcohol content is indicated
at various locations. The percent of dissolved solids is also
indicated. It is believed that this information, in conjunction
with the prior description of the operation of the various
components quite adequately discloses the operation of this second
embodiment. In the following paragraphs, there will be discussion
of the second embodiment with reference to the individual drawings
of FIGS. 19-29.
[0160] 16. Description of the Second Embodiment
[0161] In FIG. 19 in the left hand side of the drawing, there is
designated in the lower part the "dilution zone". This functions in
substantially the same way as described previously herein with
reference to Section 10. One difference is noted, in that the heat
exchangers shown in FIG. 11, indicated at 242 are not present in
this second embodiment.
[0162] In the right half of FIG. 19, in the entire showing of FIG.
20 and in the left part of FIG. 21, there is shown the displacement
wash zone. In this displacement wash zone, there are eight pair of
modules, each pair comprising a flow inlet module and a flow outlet
module, such as indicated at 148 and 190 in the description of the
prior embodiment, or as shown in FIGS. 17 and 18. Thus, there are
eight washing stages. It will be noted from the processing
information on FIGS. 19, 20 and 21 that the temperature of the
filtrate increases in an upstream direction. The percentage of dry
solids in the pulp slurry decreases in a downstream direction. The
alcohol content also decreases in a downstream direction.
[0163] It was indicated earlier herein that the modified
arrangement of the flow inlet and flow outlet modules as shown in
FIGS. 17 and 18 is utilized in the second embodiment. The
arrangement shown in FIGS. 17 and 18 is utilized in an alternating
pattern with the inflow and outflow modules shown in FIGS. 4 and 6,
respectively. Thus, the pulp slurry will be subjected to a
displacement wash operation in which at one location the
displacement flow would be downstream and vertically and then at a
subsequent location the displacement flow would travel laterally
and downstream. This is believed to have certain benefits. For
example, if the chips are oriented in one way where the flow may be
obstructed or ineffective in the vertical direction, the flow in
the lateral direction may accomplish better liquid contact with the
chips. Also, the opposite may be true (i.e. the vertical flow
accomplishing better liquid contact at some location of wood chips
than the horizontal flow. This alternating pattern of vertically
and laterally oriented pairs of flow inlet and flow outlet modules
is repeated in all parts of the digester.
[0164] Also, it is possible that the vertical flow can be
accomplished at different locations and in somewhat different
manner in that at one location the flow could be at a downward
slant, and in another location the flow from the inlet flow
assembly to the outlet flow assembly could be at an upward slant.
Further, with regard to the laterally disposed pairs of flow inlet
and flow outlet assemblies, these could be arranged so that the
flow would first be laterally across the digester in one direction,
an din a subsequent pair flow inlet and flow outlet assembly, the
lateral cross flow would be in the opposite direction.
[0165] In the right hand portion of FIG. 21 there is shown the high
heat alcohol wash zone. It can be seen that downstream of the high
heat alcohol wash zone a quantity of liquid having 85% alcohol
content is being added to the flow of filtrate, and a corresponding
quantity of the filtrate already in the flow line is discharged at
location "A" and directed further upstream to be entered back into
the digester. Also, at the upstream part of the high heat alcohol
wash, and additional quantity of 85% alcohol is added, together
with the flow from location A and a comparable amount of the
filtrate is discharged at the location "B" to be recirculated into
the digester at a further upstream location.
[0166] In FIG. 22, there is shown cooking zone 3, In this cooking
zone, the cooking liquor around and inside the pulp enters the
cooking zone with 10% dry solids (see the left part of FIG. 23),
and exits from cooking zone 3 at 12% dry solids (see the left part
of FIG. 21).
[0167] In the left part of FIG. 23, there is the "displacement wash
at the end of the cooking zone 2". This comprises two pair of flow
inlet and flow outlet modules. It will be noted that there is a
heat exchanger leading into the first flow inlet module to add heat
to the flow of filtrate. It will also be noted that at the upstream
end of the displacement wash for cooking zone 2, there is added
liquid of 56% alcohol content, and there is a corresponding outflow
indicated at location "C".
[0168] In the right hand part of FIG. 23, there is shown the
downstream portion of cooking zone 2, and the rest of cooking zone
2 is shown in FIG. 24. It will be noted that at the entry portion
of cooking zone 2 (see the left hand part of FIG. 25), the dry
solids content of the pulp slurry is 10%, and at the downstream end
of the cooking zone 2, the dry solids content is 13% (see FIG.
23).
[0169] In the left hand side of FIG. 25, there is a displacement
wash zone at the end of the cooking zone 1.
[0170] It will be noted that at the upstream end of the
displacement wash zone of cooking zone 1, there is an inflow of the
filtrate that was discharged further downstream at location "B",
and there is also an inflow of additional liquid at 56% alcohol
content. There is a corresponding outflow at location "D".
[0171] In the right hand part of FIG. 25 and in FIG. 26, there is
shown cooking zone 1. it will be noted that the pulp slurry
entering cooking zone 1 (see the left hand of FIG. 27) is at 11%
dry solids, and at the downstream end of cooking zone 1 the
dissolved solids is at 14%.
[0172] In FIGS. 27 and 28, and also in the left part of FIG. 29,
there is shown the impregnation zone. In this impregnation zone
there are ten pairs of a flow inlet module and a flow outlet
module, with each pair of the flow inlet and flow outlet modules
recirculating the filtrate in an upstream direction. As can be seen
in the middle of FIG. 27, the outflow from locations "C" and "D"
further downstream are added to the flow of filtrate. It can be
seen from observing the operating values shown in FIGS. 27, 28 and
29 that as the filtrate recirculates in an upstream direction in
the digester, the temperature decreases and the alcohol content
decreases in the recirculating filtrate. In FIG. 28, just
downstream of the first four rearmost pair of flow inlet and flow
outlet modules, there is a discharge of the filtrate at location
"E", and this is directed to the alcohol recovery location.
[0173] With reference to FIG. 29, it can be seen that the outflow
from the furthest upstream flow outlet module is discharged at
location "F" and directed to the chip feed-in station to be mixed
with the wood chips that are directed into the inlet of the
digester.
[0174] In the right part of FIG. 29, there is illustrated the
dewatering section. It will be noted that there are two dewatering
modules, and the outflow from these dewatering modules at locations
""G" and "H" is directed to the chip feed-in station. The slurry of
wood chips and filtrate are directed into the inlet end of the
digester through the valve, such as indicated at 105 in the first
embodiment.
[0175] In the description of the first embodiment, it was indicated
that a pressure equalizing fluid (either gaseous or liquid)
surrounding the square container 116 is utilized. It is believed to
be desirable to utilize a pressurized liquid which has
substantially the same composition as the filtrate which is inside
the square container. This would better insure pressure
equalization at different temperature levels.
[0176] C. Third Embodiment of the Invention
[0177] 17. Introduction to the Third Embodiment
[0178] The third embodiment is shown in FIGS. 30 through 41. In
this section, there will first be an overall description of the
system of the third embodiment with reference to FIG. 30. Following
this, there will be three sections of text (Sections 18-20) devoted
to three portions of the system of this third embodiment which
merit more detailed discussion, these being:
[0179] a. the cross flow rings and their mode of operation;
[0180] b. the wood chip feed assembly, and filtrate recirculation
zone;
[0181] c. the impregnation zone and its mode of operation.
[0182] After this, in Section 21 there will be a general
description of the overall operation of the third embodiment, and
in Section 22 there will be a description of the evaporating and
recovery system, followed by a brief summary of Section 23.
[0183] To give an overview of this third embodiment, reference is
made first to FIG. 30. The digester system 400 comprises a digester
402 having a rear chip inlet end 404, and a forward pulp outlet end
406. There is a blow tank 407 which receives the diluted pulp from
the front outlet of the digester 402, and a washer 408 which
receives the pulp from the blow tank 407.
[0184] The filtrate from the washer 408 is in turn directed into
the digester 402 at its outlet end 406, and the manner in which
this is accomplished will be described later herein. While various
pulp washers that already exist in the prior art could be used in
the present invention, particular advantages can be obtained by the
washer 408 being the same as, or similar to, the washer described
in U.S. Pat. No. 5,482,594 entitled "LIQUID REMOVAL APPARATUS AND
METHOD FOR WOOD PULP", issued on Jan. 9, 1996, the inventor being
the same as the inventor in the present patent application. One of
the reasons for this is that this particular washer enables the
pulp that is received from the digester 402 to be washed at several
atmosphere pressure and at a high pulp consistency (7-10 bars and
20% to 30%), respectively). This is accomplished in a quite
effective manner so that the amount of alcohol which is lost (ethyl
alcohol being the preferred digesting agent) and carried out with
the washed pulp is relatively small and the loss due to evaporation
is practically nil, thus enhancing the economy of operation of the
present invention.
[0185] The digester 402 comprises an elongate pressure vessel 410
having a cylindrical sidewall 411, a rear wall 412 into which the
wood chips carrying filtrate is directed, and a front wall 414
through which the diluted pulp is discharged through an appropriate
blow valve 416. One significant difference in this third embodiment
is that it does not have the square cross section inner container
that is present in the first two embodiments. Nor does this third
embodiment have the inflow and outflow modules as described in the
first two embodiments. Instead, their is provided a cross flow ring
system (mentioned very briefly above) which will be described in
more detail later herein in Section 18.
[0186] In FIG. 30 there is shown only schematically an evaporating
and recovery plant 418 to which the filtrate from the digester is
directed. This evaporation and recovery plant 418 recovers the
alcohol from the filtrate and directs this to a pair of alcohol
supply tanks 420 and 421, from which the alcohol is directed back
into the digester 402. Also, the evaporation and recovery plant 418
accomplishes the recovery of the dry solids (i.e. organic material
derived from the wood chips during the digesting process) in a
quite advantageous manner which also will be described in more
detail later herein with regard to the description of the
impregnation zone of the present invention. This will be described
later with reference to FIG. 43.
[0187] At the inlet end of the digester 402, there is the wood chip
feed assembly 422 which comprises a wood chip and filtrate supply
section 424 and a pump section 426 to receive the diluted wood
chips from the supply section 424 and direct these into the inlet
end 404 of the digester 402.
[0188] The digester 402 comprises, in terms of function, seven
sections, which will be identified below in the order in which they
are placed, beginning at the forward end 406 of the digester 402,
and proceeding on to the rear end 404, these being:
[0189] a. dilution zone;
[0190] b. displacement wash zone A
[0191] c. cooking zone 2
[0192] d. displacement wash zone B
[0193] e. cooking zone 1
[0194] f. impregnation zone
[0195] g. wood chip filtrate recirculation zone
[0196] It is believed that the operation of these seven zones would
be in large part understood from a review of the description of the
first embodiment. In the later sections, these will be described in
more detail. In the following three sections, there will be, as
indicated above, a description of three sections of the third
embodiment.
[0197] 18. Cross Flow Ring System
[0198] This cross flow ring system will be described with reference
to FIGS. 31 through 33. By way of introduction, it will be recalled
that in the first two embodiments, the cross flow of the filtrate
and alcohol is accomplished by having at various locations along
the length of the digester an upstream inflow module 148, which is
immediately followed in the downstream direction by an outflow
module 190. Thus, at various locations, there is an inflow of
filtrate (along with alcohol at some locations) which travels
laterally across the digesting chamber and also downstream to the
opposite side where a portion of this flow would be taken out by
the outflow module, with another portion of that flow continuing
downstream in the digester.
[0199] The cross flow function in the present invention is
accomplished by sets or sections of cross flow rings 430 (FIG. 32)
positioned around the outside of the cylindrical sidewall 411 of
the digester pressure vessel 410. Reference is first made to FIGS.
31 and 32. It can be seen in FIG. 32 that the ring 430 has a "U"
shaped cross sectional configuration, and thus comprises an outer
circumferential plate portion 432 which is spaced outwardly from
the digester sidewall 411, and a pair of radially inwardly
extending flanges 434 which are welded at 436 to the outer surface
438 of the digester sidewall 411. The ring 430 defines a
circumferential chamber 440 which has a depth dimension (indicated
at "a") to provide an adequate cross section for filtrate flow.
[0200] Each ring 430 extends entirely around the circumference of
the digester sidewall 411. The chamber 440 is actually divided into
two arcuate sections. As can be seen in both FIGS. 31A and 31B,
there is an inflow chamber section 440a which has an arcuate length
of about 90.degree., and an outflow chamber section 440b which has
an arcuate length of about 180.degree.. At the inflow chamber 440a,
the sidewall 411 is formed with several (four as shown herein)
slots 442 which slant relative to the longitudinal center axis 444
(see FIG. 33) of the digester 402 at an angle of about 45.degree.,
and which extend circumferentially in a 90.degree. arc. Thus, the
plane occupied by each slot 442a has the configuration of a segment
of a conical surface. In like manner, there are four similar slots
442b leading into the chamber or plenum 440b, these also slanting
inwardly toward the center axis 444 in a downstream direction at an
angle of about 45.degree. relative to the longitudinal axis 444
(see FIG. 33). However, the arc length of the slots 442b is
180.degree.. This 45.degree. slant angle could be varied, for
example, between 20.degree. to 80.degree..
[0201] There is an inlet fitting 446a leading through the outer
wall 432 at the location of the inflow chamber 440a, this defining
an inlet passageway into the chamber 440a. In like manner, there is
an outflow fitting 446b leading from the outflow chamber 440b.
[0202] As can be seen in FIGS. 31 and 31B, part of the flow from
the chamber 440a is directed across the main interior chamber 448
of the digester 402 to displace liquid in the chamber 448 through
the outflow passageways 440b. However, angular positions of the
passageways 440a and 440b of each set are offset 90.degree. in an
alternating pattern. Thus, longitudinally adjacent cross flow rings
430 are angularly offset relative to one another by ninety degrees.
It can be seen that in the upper FIG. 31A, the inflow passageway
440a and the outflow passage 440b are directly opposite one another
and their alignment slants upwardly to the left. In the lower
showing at FIG. 31B, the inflow passageway 440a has been rotated to
the left 90.degree., with the opposite outflow passageway 440b
again being positioned diametrically opposite to the passageway
440a, so the alignment slants upwardly to the right. Obviously the
rings 430 could be angularly positioned in the same alignment, and
also the alternating pattern could be modified, as well as the
angular displacement. These changes in flow direction enhance the
effectiveness of the displacement wash, since the cross
displacement wash is performed in alternating direction.
[0203] At this point, a clarification should be made with reference
to the flow lines shown in FIGS. 31A and 31B. There is a constant
flow of the liquid and pulp in the digester pressure vessel 410 in
a forward direction. As the liquid is discharged from the inflow
chamber 440a to pass into the chamber 448, in addition to having a
flow component across the chamber 448, the flow also has a travel
component in a forward direction toward the front end of the
digester 402.
[0204] To follow this line of thought further, reference is now
made to FIG. 33, where there are shown the four cross flow rings
430 which comprise the most forward section of the displacement
wash zone "A". For convenience of explanation, the four rings 430
have been designated 430-1 through 430-4, in an upstream direction.
The excess part of the filtrate from the washer 408 is directed
into the inlet port of the ring 430-1. The flow Of this filtrate is
illustrated by the arrows and the dotted lines. It can be seen that
one of these dotted lines 452 moves across to exit out of the
outlet port indicated at 430-1 (out). The remaining flow lines 454
proceed in a downstream direction toward the dilution zone.
[0205] The outflow from the ring 430-1 is pumped through a
recirculating conduit positioned outside the digester vessel 410 to
the inlet port at 430-2 (in). It can be seen that there is again
cross flow where one of the flow lines 456 travels from the port
430-2 (in) across to the outlet port 4302 (out), while other flow
lines 458 travel across the chamber 448 and somewhat downstream
toward the outlet port at 430-1 (out). This pattern repeats itself
relative to the next rings 430-3 and 430-4. The flow from 430-4
(out) is directed upstream to re-enter into the digester at the
inlet port of the most forward ring 430 in the second filtrate
displacement wash zone (which is referred to in a later portion of
this text as wash station 552) in displacement wash zone "A". (This
can be seen by examining FIG. 37).
[0206] To explain the effect of this flow pattern, let us examine
the flow which goes out the port 430-1 (out) and is directed into
the port 430-2 (in). It can be seen that part of the flow
(indicated by the broken lines 458) passes through the chamber 448
to go to the outlet port at 430-1 (out). However, a portion of this
flow (indicated by the broken line 456) travels straight across to
pass outwardly through the outlet port 430-2 (out). Further, the
flow from the port 430-2 (out) moves through a recirculating
conduit upstream to re-enter at the inlet port 430-3 (in). Thus, it
can be seen that there is an overall circulating pattern where a
greater portion of the flow from one inlet port migrates downstream
to the next adjacent outlet port, and another portion of the flow
coming through the inlet port passing straight across the chamber
448 to exit from the outlet port straight across and then to travel
upstream through a recirculating conduit to next upstream inlet
port. The overall effect of this pattern is that there is a net
movement of the filtrate in an upstream direction in that there is
an increment of downstream travel inside the vessel 410, and then a
slightly greater increment of upstream travel through the
recirculating pattern of the filtrate. Thus, in the digester vessel
410 there is a constant flow of filtrate downstream toward the
outlet end 406. Outside the digester vessel 410 there is a counter
flow upstream in the recirculating conduits, then across and
through the chamber 448, then further upstream through the outside
recirculating conduits, etc. The overall effect of this will be
described more fully later herein.
[0207] It should be pointed out that FIG. 33 is not drawn with
total accuracy, since the inlet ports 430-1 through 4 are shown in
FIG. 33 as having the same angular orientation. Actually, (as
illustrated in FIGS. 31A and 31B), the angular position of the
rings 430 is in an alternating pattern so that if one inlet port
446a is at a lower right hand position (as shown in FIG. 31A) the
next adjacent inlet port 446a is (as shown in FIG. 31B) at the
lower left hand position. Therefore, with reference to FIG. 31A,
the upper part of the cross flow would go diametrically across to
the inlet slots at 440b, while part of the flow would travel
across, but with a downstream component of travel, and off to one
side, since the downstream inlet slot 440b is displaced 90.degree.
from the diametrically opposed position.
[0208] 19. Wood Chip Feed Assembly and Filtrate Recirculating
Zone
[0209] The wood chip feed assembly 422 can best be explained with
reference to FIG. 39. There is a filtrate tank 460 in which is
positioned a feed tube 462, and a measuring auger 464 which takes
wood chips from a wood chip bin 466 and feeds these into the upper
end of the feed tube 462. To move the wood chips downwardly in the
feed tube 462, there is provided a vertical auger 468 that is
rotated by a motor 470. The filtrate 472 in the tank 460 flows
through a plurality of openings extending downwardly along in the
sidewall of the feed tube 462. Thus, the flow of the filtrate 472
is through the sidewall of the feed tube 462 and into the
passageway 474 in the feed tube 462. At the lower end of the feed
tube 462, there are eight rotating agitators 476 which mix the wood
chips into suspension with the filtrate. At the lowermost part of
the feed tube 462 there is a rock and iron sump 478 having a blow
valve 480 by which the rocks and metal particles can be extracted
periodically. At the lower end of the tank 460, there is a clean
out valve 481. Also, there is a vapor line 493 from the filtrate
tank 460 to the evaporation and recovery system 418. (See FIG.
36).
[0210] The out flow from the feed tube 462 is directed through an
outlet 482 to the aforementioned pump section 426 of the wood chip
feed assembly 422. There are four centrifugal pumps 484 which
operate in series in order to raise the wood chip and filtrate
slurry to a sufficiently high pressure to enter the digester at the
inlet valve 486.
[0211] In operation, when the pumps 484 start operating, the wood
chip and filtrate slurry is drawn from the flow passageway 474, so
that the liquid level in the passageway 474 drops. The arrangement
of the feed tube 462 with its vertically spaced openings
automatically adjusts the level of the filtrate within the feed
tube passageway 474, since when the filtrate level in the
passageway 474 drops to a lower level, the higher level of the
filtrate 472 in the tank 460 causes an increased flow through the
sidewall openings of the feed tube 462 into the passageway 474.
[0212] To enable the centrifugal pumps 484 to pump the wood
chip/filtrate slurry, it is necessary that there be approximately
twenty four parts filtrate to one part wood chips by weight. This
ratio is maintained by operating the measuring auger 464 so as to
obtain a desired rate of feeding for the wood chips, and also
sizing and operating the pumps 484 so that the volumetric flow
through the pumps 484 properly matches the feed rate of the wood
chips to obtain this ratio.
[0213] Since a wood chip to liquid ratio of twenty four to one is
substantially greater than the desired wood chip to liquid ratio
which is directed into the impregnation zone of the digester, it
becomes necessary to withdraw most of the filtrate from the wood
chip filtrate slurry entering the digester 402 and return this
filtrate to the tank 460. This is accomplished at the wood chip
filtrate recirculating zone which is immediately downstream of the
inlet valve 486. This is accomplished by two recirculating rings
488. Each of the two recirculating rings 488 has substantially the
same configuration as the cross flow rings 430, except that the
plenum chamber extends entirely around the circumference of the
digester sidewall 411. Also, each ring 488 has four extraction
fittings 490 which withdraw the filtrate from the interior of the
digester vessel 410. This filtrate flows through the lines 492 into
the line 494 and back to the tank 460 through an inlet at 496.
[0214] It is necessary to supply makeup filtrate to the tank 460.
The reason for this is as follows. It takes approximately two tons
of oven dry wood chips to produce (at fifty percent yield) one ton
of pulp. The moisture content of two tons of oven dry wood chips is
usually between about one ton to two tons of water, depending upon
how dry or wet the wood chips are. If the wood chips are rather
dry, they absorb liquid as they pass downwardly through the feed
tube 462 and through the pumps 484. Thus, the extra liquid which is
absorbed into the wood chips needs to be made up. This makeup
liquid is supplied from the upstream end of the impregnation zone.
Thus, there is shown in FIG. 39 several arrows 498 illustrating the
flow of this makeup filtrate. There is a suitable level sensor at
the tank 460 to detect when the makeup liquid is required, and the
flow control valve 496 in line 497 to the recovery plant is
operated in a manner that the flow through the line 494 to the tank
460 is sufficient to maintain the level in the tank 460.
[0215] 20. The Impregnation Zone
[0216] In the impregnation zone (see FIG. 36), there is a
downstream impregnation section 502 and an upstream impregnation
section 504. Further, there are three displacement wash sections,
one displacement wash section 506 being immediately downstream of
the impregnation section 502, a second displacement wash station
508 between the impregnation sections 502 and 504, and the third at
510, immediately upstream of the impregnation section 504. There
are three heat exchangers, 512, 514, and 516, being positioned
immediately downstream of the three displacement wash sections 506,
508 and 510.
[0217] The filtrate from the alcohol displacement wash station of
displacement wash zone "B" (See FIG. 30) is directed through the
line 518 into the heat exchanger 512, and thence through the
displacement washing section 506 to exit through the line 520. The
flow of filtrate through the line 520 is then directed through the
heat exchanger 516 into the most upstream displacement wash section
in the impregnation zone. Then the outlet flow from the upstream
cross flow ring of the displacement wash section 510 travels
through the line 497 to the evaporation and recovery plant 418.
[0218] The outflow of the filtrate from the third displacement wash
section in the displacement wash zone "B" flows through a line 524
to pass through the heat exchanger 514 and thence pass through the
two cross flow rings of the middle displacement wash section 508.
The filtrate discharge from displacement wash section 508 travels
through line 526 to the evaporation and recovery plant 418.
[0219] The heat exchanger 512 brings the temperature of the
filtrate up to 205.degree. C. so that the temperature of the
filtrate flowing downstream through the cooking zone 1 is at a
sufficiently high temperature, for example 195.degree. C. The
remaining two heat exchangers 514 and 516 are used to adjust the
filtrate temperature for the two impregnation sections 502 and
504.
[0220] The withdrawal of the filtrate through the lines 497 and
526, which has a dry solids content of about seven percent, has
certain advantages. In the impregnation zone there is being
extracted from the wood chips a relatively large percentage of
extraneous materials present in the wood chips. Examples of
extraneous materials are volatile acids, volatile oils, resin and
fatty acid fractions, etc. Volatile oils have considerable economic
value because of these being a source of turpentine, pine oil, and
other organic matter. Softwood species, such as pines and cedars,
are particularly rich in this fraction. These are often termed
"essential oils". The major components of resin and fatty acid
fractions include resin acids, fatty acids, turpene, and other
esters and unsaponifables (waxes, sterols, etc.). Sterols include
B-sitrosterol which has been researched for medical purposes and
has proven to be extremely valuable. These are discussed in more
detail in the book, "Second Edition, Volume 1, The Pulping of Wood"
by Ronald G. MacDonald and John N. Franklin, published by
McGraw-Hill Book Company.
[0221] Removal of extraneous materials opens up the pores and
cavities in the wood chips to allow the alcohol digesting liquid to
enter into the wood chips and accomplish the delignification. It is
very valuable to extract these extraneous materials early so that
they are not subjected to possible deterioration in subsequent
cooking zones in the digester and that they are not diluted by
other organic matter dissolved later in the cooking zones 1 and 2.
Also, some of the extraneous materials can also have negative
effects in the delignification process if these remain and are
present in the cooking zones.
[0222] Thus present analysis indicates that by extracting the
filtrate flow from the impregnation zone and treating this
separately certain advantages would be produced in that this
portion of the organic material has a quite different composition
and value than other portions of organic matter which are later
extracted from the wood chips. In the present invention, by
diverting this organic material that is extracted in the
impregnation zone at an early stage and recovering it separately,
with the alcohol being evaporated, the further processing of this
organic material into certain desirable by-products is
substantially enhanced.
[0223] Another benefit derived in this impregnation zone is that
the filtrate that flows through the impregnation section 502 is
recycled back to the upstream end of the impregnation section 504.
By maintaining some of this extracted organic material present in
the impregnation zone, in accordance with present analysis, an
enhanced extraction process in the impregnation zone is
accomplished. Thus, the term "impregnation zone" is somewhat
incomplete, since this zone serves a dual function of both
impregnation and extraction. To comment on this further, reference
is made to FIG. 45 which represents the layers of a single cell of
wood, where the right side of the figure represents the
longitudinal center line of the cell, and the left side represents
the outer surface. It can be seen that the distribution in the cell
is such that more of the lignin is nearer the outer layer, while
the center part is rich in hemicellulose.
[0224] The liquor at the end of the first cooking zone is high in
lignin content and low in extraneous materials, and is therefore
heavier in composition. This is the liquor that is directed through
the line 586 to be evaporated through an evaporating system
separate from the impregnation zone.
[0225] Most of the liquor that is removed immediately after cooking
zone 2 remains in the pulp/liquor flow in the middle of the
displacement wash zone B and is pumped to the end of the
impregnation zone. This liquor is rich in hemicellulose, and some
of this migrates into the impregnation zone discharge liquor, thus
adding hemicellulose to the extraneous materials stream going to
the evaporation and recovery system 418.
[0226] Delignification is accomplished mainly in the cooking zones
1 and 2. Thus, the liquor that is directed from the end of cooking
zone 1 and is directed through the line 480 to the evaporation and
recovery system 418 provides a high percentage of the lignin that
is extracted from the wood chips.
[0227] 21. Overall Operation of the Third Embodiment
[0228] This section is not only to describe the overall operation
of the present invention, but also to provide more specific
information about the other sections (i.e., zone) of the digester
system which have not been described in detail in the prior
sections related to this third embodiment. In sections 19 and 20,
there was presented a more detailed description of the wood chip
feed assembly, and also the seventh and sixth zones of the digester
402, namely the wood chip feed assembly and filtrate recirculation
zone, and the impregnation zone. In the following description, each
of the five other zones will be described in sequence, beginning at
the front end of the digester where there is the dilution zone and
then proceeding through the following zones in sequence to and
including cooking zone 1. As this is done, reference will be made
to the other components related to the operation in those
zones.
[0229] Reference is first made to FIG. 30. As previously described
in Section 17, a pulp slurry is discharged through the blow valve
416 and directed to the blow tank 407. The pulp slurry from the
glow tank 407 is further diluted and directed into the washer 408.
Part of the filtrate from the washer 408 is directed into the
digester 402 at two locations. First, some of the filtrate is
directed into the dilution zone to mix with the pulp and filtrate
that is received from the upstream displacement Wash Zone A, with
this mixture then being discharged through the blow valve 416.
Second, the excess filtrate from the washer 408 is pumped into the
inlet port of ring 430-1 to start the displacement wash in the most
downstream section of the displacement wash zone A. Also, another
part of the filtrate from the washer 408 is directed to the outlet
of the blow tank to dilute the pulp slurry flowing into the washer
408.
[0230] As indicated previously, the washer 408 is desirably the
same as (or quite similar to) the washer described in U.S. Pat. No.
5,482,594, the subject matter of which is hereby being incorporated
by reference into the present patent application. In the normal
mode of the operation of the washer 408, the pulp slurry enters the
washer at about a 2% consistency, which means that there is one
part pulp to forty-nine parts liquid. In this instance, for purpose
of description, it will be assumed that the washer 408 is being
operated so that the pulp entering the washer 408 is at 2%
consistency. Assuming a dilution factor of 1, and the pulp
discharge from the washer 408 at 20% consistency (which means 4
parts water to one part pulp), there is a total wash water quantity
of five parts wash water to one part pulp. When this is combined
with the forty nine parts water for one part pulp (49 plus 5), the
total liquid entering the washer 408 equals 54 parts liquid to 1
part pulp.
[0231] The pressure in the digester is at about thirty bars, and
there is a pressure reduction of about fifteen to twenty bars, when
the pulp passes through the blow valve 416. The blow tank 407
stores the pulp and the liquid so that liquid can be properly fed
into the washer 408. (The washer 408 may not be operating at the
very same time that there is a discharge from the blow valve 416 of
the digester 402, so the blow tank 407 also acts as a buffer.)
There is a line 529 that directs the flushed alcohol/water vapors
from the blow tank 407 to evaporation and recovery system 418.
[0232] With reference to FIG. 35, of the 54 parts of liquid that
enter the washer 408, 33 parts liquid go into the discharge line of
the blow tank at 530 to dilute the pulp flowing from the blow tank
407 from about 6% consistency down to about 2% consistency. Ten
parts of the liquid from the washer 408 travels through a line 432
to pass into dilution inlet ring 534 in the dilution zone to dilute
the pulp in the digester from about 14% to about 6%. Seven parts of
liquid from the washer 408 flow through a line 536 to pass through
a heat exchanger 538 into the downstream end of the displacement
wash zone A. Three pumps 540 in series are provided to pump the
liquid from the washer 408 through the line 532 to the dilution
inlet rings 534, and another three pumps 542 in series are provided
to pump the filtrate from the washer 408 through the heat exchanger
538 and into the displacement wash zone A.
[0233] The components in the five zones beginning from the dilution
zone through to cooking zone 1 will now be described under
appropriate headings.
[0234] a. The Dilution Zone
[0235] With reference to FIG. 37, it can be seen that the dilution
zone comprises two longitudinally spaced identical filtrate inlet
stations, with an inlet ring 534 being at each station. One of the
inlet rings 534 is shown in transverse section in FIG. 37, and it
can be seen that there are four inlet fittings 544 positioned at
90% intervals around the circumference of the inlet ring 534. Also,
there are four rotating agitators 546 which (as the name implies)
serve the function of mixing the effluent flowing inwardly through
the fittings 544 with the pulp and liquid flowing downstream in the
digester 402. These rotating agitators 546 can be the same as, or
similar to, the agitator as shown in FIG. 39 in the wood chip feed
assembly.
[0236] The two inlet rings 534 each have an outer ring structure
which is the same as (or similar to) the ring structure of the
cross flow ring as shown in FIG. 32, where the liquid flows
inwardly through the circumferential member 432 and into the
chamber 440 and further inwardly through the slanted slots 442.
Accordingly, it is believed that a detailed description of these
two inlet ring stations 534 is not required. The function of this
dilution zone is to bring the consistency of the pulp from about 1
part pulp to 6 parts liquid to approximately 1 part pulp to 16
parts liquid, which is the desired consistency at which the pulp
and liquid is discharged from the blow valve 416.
[0237] b. Displacement Wash Zone A
[0238] Reference is made to FIG. 37 where there are shown three
displacement wash stations indicated at 548, 550 and 552. Each of
these stations comprises four cross flow rings 430. In FIG. 37,
only the first two cross flow rings have been given the designation
of 430 (to keep FIG. 37 from becoming too cluttered), it being
understood that the other cross flow rings are also cross flow
rings 430. Also, there are pumps directing the flow from one cross
flow ring 430 to another, and two of these pumps have been
designated at 554 in FIG. 37 (again to keep FIG. 37 from becoming
too cluttered), and the other pumps have not been given numerical
designations. The cross flow rings 430 in the first displacement
wash section were described in detail previously in this text in
Section 18. Accordingly, that description will not be repeated in
this portion of the text.
[0239] With reference to FIG. 37, there is shown in cross section
each of the twelve cross flow rings 430, in displacement wash zone
A, and the cross sectional view of each such ring 430 is positioned
in alignment below its related cross flow ring 430 which is shown
in a side elevational view in the upper part of FIG. 37. It will be
noted that the inlet and outlet fittings 446a and 446b,
respectively, are angularly positioned in an alternating pattern so
that one ring 430 will have the inlet and outlet fittings 446a and
446b aligned in an upward slant to the left, and the next adjacent
ring 430 will have its inlet and outlet fittings 446a and 446b
oriented in a direction slanting upwardly to the right. Since this
was described previously herein relative to FIGS. 31a and 31b, this
will not be described further in this portion of the text.
[0240] Flow from the washer 408 flows through the line 536 into the
heat exchanger 538. The heat exchanger 538 operates in a manner
that the flow from the heat exchanger 538 going into the digester
is at about 68.degree. C. This is to maintain the blowout
temperature of the pulp/filtrate mixture that passes out the blow
valve at about 74.degree. C. Also, as indicated previously, the
outflow from the fourth cross flow ring (designated 430-4 in FIG.
33) flows through a line 556 into the most forward cross flow ring
430 in the further upstream filtrate displacement wash station 552.
It can be seen that the flow from the initial filtrate displacement
wash station 548 "leapfrogs" the middle alcohol wash station 550 to
pass into the further upstream filtrate displacement wash station
552 of displacement wash zone A.
[0241] Also, it can be seen in FIG. 37 that to provide
recirculating flow between adjacent cross flow rings 430, there are
related recirculating conduits 558, each of which connects to the
outlet fitting of its related ring 430 and leads through a pump 554
to the inlet fitting of the adjacent upstream cross flow ring 430.
Further, it will be noted that there are broken lines 560 leading
from many of these recirculating conduits 558, and these represent
conduits leading from these recirculating conduits 558 to collect
any accumulation of gaseous substances and redirect these through a
line 562 back to the evaporation and recovery plant.
[0242] The middle alcohol displacement wash zone 550 receives a
flow of alcohol from the accumulator tank 420 through a pump 563
and a line 564, which directs the alcohol through a heat exchanger
566 and thence through a pump 554 to flow into the most forward
ring 430 of the alcohol displacement wash station 550. The heat
exchanger 566 raises the temperature of the alcohol to about
135.degree. C. The tank 420 derives its alcohol from the
evaporation and recovery plant, as does the aforementioned tank
421.
[0243] The alcohol passing into the most forward ring 430 of the
alcohol wash station 550 recirculates in a cross flow pattern
through the digester in substantially the same manner as the flow
pattern that was described previously herein relative to the cross
flow pattern in the most forward filtrate displacement wash section
538, as illustrate in FIG. 33. The flow from the furthest upstream
cross flow ring 430 of the middle alcohol wash station 550 flows
through a line 568 to pass into the middle filtrate displacement
wash station of the displacement wash zone B (which will be
described later herein).
[0244] It is believed that in order to present a better
understanding of the present invention, it would be helpful to
review at this time the first part of the table presented in FIG.
42. In line 1, the flow of the filtrate flowing into the washer 408
(designated the "RKS-Washer"), which in this case is pure water, is
at 30.degree. C., with zero alcohol content and zero dry solids.
The flow of pulp and the filtrate from the blow tank 407 is at a
temperature 74.degree. C., with an alcohol content of 17.5% alcohol
and 0.07 dry solids. The filtrate which is discharged from the
washer 408 (and is directed back to the digester 402) is at
74.degree. C., with an alcohol content of 15% and dry solids of
0.06. The discharge of pulp from the washer 408 is at 50.degree.
C., with an alcohol content of 0.2%, and a dissolved solid content
of 0.01%. With the assumed values given previously in this text,
this pulp would be at 20% consistency.
[0245] However, it is within the capability of the washer 408, as
described in the U.S. Pat. No. 5,482,594, to obtain even higher
consistencies of the pulp being discharged. Also, it should be
noted that the only net outflow of liquid from the washer 408 is
the liquid which remains in the pulp which is being discharged from
the washer 408. The rest of the filtrate from the washer 408 is
directed back into the digester 402. As indicated previously, in
the overall digester apparatus, there is a net upstream flow of
filtrate. This occurs as follows. there is a substantially constant
volumetric flow downstream from the rear end 404 to the front end
406 in the chamber 448 of the digester 402. However, at the same
time there is a grater flow through the recirculating conduits 558
and in the other lines (i.e., 556, 568 and others) that carry the
filtrate to further upstream locations).
[0246] Reference is now made back to the table in FIG. 42, and
specifically to the values given for the filtrate/liquor flow, at
the reference lines 3 through 8. It can be seen that the filtrate
which flows outwardly from the most upstream ring 430 of the second
filtrate wash zone 552 is at a temperature of 188.degree. C., has
an alcohol content of 57%, and a dry solids content of 4.5%. On the
other hand, the filtrate/liquor flow raveling into the most
downstream ring 430 of the first filtrate wash section 548 has a
much lower temperature (68.degree. C.), a much lower alcohol
content (15%), and a much smaller dissolved solid content (0.06%).
Thus, it can be seen that the net filtrate/liquor flow that is
recirculated in an upstream direction increases in dry solids
content substantially (0.06% to 4.5%) and also increases in alcohol
content (15% to 57%). Further, there is a substantial rise in
temperature in an upstream counterflow direction (68.degree. C. to
188.degree. C.). The plurality of wash sections provides fast
cooling of the liquor to cause the digesting process to
substantially cease.
[0247] With further reference to lines 3 through 8 of FIG. 42, on
the right hand side values are given for the pulp chip flow. This
is the flow which is traveling in the main chamber 448 of the
digester vessel 410 in a forward direction toward the discharge end
406. It can be seen that the filtrate flowing from the cooking zone
2 into the portion of the chamber 448 at the start of the
displacement wash zone A is at a relatively high temperature
(195.degree. C.). It has a dry solids content of 5.9%. By the time
this filtrate in the digester chamber 448 reaches the downstream
end of the displacement wash zone A, the dry solids have dropped
from 5.9% to 0.07%, and the alcohol content has dropped from 59% to
17.5%. Thus, it can be seen that as the pulp/filtrate flow proceeds
downstream in the chamber 448 in the displacement wash zone A, it
is becoming cooler, clearer, and with less alcohol content. On the
other hand, the recirculating counter current flow in an upstream
direction is extracting the dry solids from the filtrate in the
digester chamber 448 in displacement wash zone a, and carrying this
further upstream for eventual discharge into the evaporation and
recovery plant 418. Further, there is a net upstream flow of the
digesting alcohol.
[0248] c. Cooking Zone 2
[0249] The pulp and filtrate mixture which leaves the displacement
wash zone B travels downstream in the chamber 448 through the
cooking zone 2 is maintained at a temperature of approximately
195.degree. C. (As can be seen in FIG. 30, there are a couple of
heat exchangers 570 which maintain this cooking temperatures).
[0250] The filtrate in the chamber 448 leaving the displacement
wash zone B has just traveled through the second alcohol
displacement wash section (to be identified and discussed in the
next section of this text) and thus has a relatively high alcohol
content (59%), and also a rather low level of dry solids
(0.3%).
[0251] As the pulp/liquor flow proceeds in a continuous manner
downstream in the cooking zone 2, the dry solids content in the
liquor increases, so that the pulp/liquor passing from cooking zone
2 and into displacement wash zone A has the dry solids content
increase from 0.3% (at the start of cooking zone 2) to 5.9% (at the
end of cooking zone 2). The time which it would take for a portion
of the pulp/liquor mixture to pass through the cooking zone 2 would
generally be between about 30 to 60 minutes.
[0252] d. Displacement Wash Zone B
[0253] Reference is made to FIG. 38, where it can be seen that in
displacement wash zone B there are three wash sections, namely, a
first alcohol displacement wash section 572, and two additional
filtrate displacement wash sections 574 and 576. Each of these wash
sections 572, 574 and 576 comprises three cross flow rings 430. The
inflow to the alcohol wash section 572 is from the alcohol
accumulator tank 421 through a pump line 577 and 578 which in turn
directs the alcohol through the heat exchanger 580 which raises the
temperature of this alcohol to about 195.degree. C., with the
alcohol then passing into the furthest downstream ring 430 via pump
581. There is generally the same upstream recirculating flow
pattern through the three rings 430 at the alcohol wash section
572, and this flow exits from the third ring 430 to pass through
the line 518 to then flow through the heat exchanger 512 into the
furthest downstream displacement wash section 506 of the
impregnation zone (see FIG. 36).
[0254] The inflow into the most downstream cross flow ring of the
third filtrate displacement wash zone 574 (which is in the middle
of displacement wash zone B) is from the line 568 which receives
the outflow from the furthest upstream ring 430 of the first
alcohol displacement wash section 550. The flow through the line
568 passes through a heat exchanger 582 which raises the
temperature of this filtrate to about 195.degree. C. The outflow
from the third filtrate displacement wash section 574 is from its
furthest upstream ring of that section 574 through the
aforementioned line 524 to pass through the heat exchanger 514 into
the downstream cross flow ring of the middle displacement wash zone
508 of the impregnation zone (see FIG. 36).
[0255] The outflow from the most upstream cross flow ring of the
second filtrate displacement wash zone 552 (see FIG. 37) is through
the line 583 to pass through a heat exchanger 584 and enter into
the furthest downstream ring of the fourth filtrate displacement
wash section 576 (which is the furthest upstream section of the
displacement wash zone B). The flow from the most upstream ring 430
in the section 576 is through the line 586 to the evaporation and
recovery plant 418. It will be noted that the flow into the line
586 is mostly from the cooking zone 1, and that at the downstream
end of the cooking zone 1, there is a dry solids content in the
liquor in the digester chamber 448 of 10.4%. The dry solids passing
outwardly from the most upstream ring 430 of the fourth filtrate
displacement wash section 576 is about 9.1%.
[0256] e. Cooking Zone 1
[0257] Reference is made to FIGS. 31 and 36. The flow of the
pulp/filtrate leaving the impregnation zone has a dry solids
content of about 3.3%, and this dry solids content increases to
about 10.4% as the flow is exiting from the cooking zone 1. The
temperature in cooking zone 1 is maintained at about 195.degree.
C., and three heat exchangers 590 are provided along the length of
cooking zone 1 for this purpose, and the time for the pulp/liquor
to flow through cooking zone 1 would be between 30 to 60
minutes.
[0258] To appreciate some of the benefits derived from the present
invention, it would be helpful to pause at this point and review
some of the values presented in the chart of FIG. 42. An analysis
of the values presented in that chart will indicate that due to the
recirculating upstream flow provided in the present invention, the
dry solids content of the filtrate exiting from the digester 402
through the line 586 into the evaporating and recovery plant 418 is
about 9% and is derived not only from the dry solids extracted from
the pulp in cooking zone 1, but also partially from the dry solids
extracted from the pulp in the cooking zone 2. Also, it becomes
evident that by reviewing the overall chart FIG. 42, the dry solids
that eventually leave the entire system in a path other than
through the eh lines directed back to the evaporation and recovery
plant 418, are only a very, very small fraction of the dry solids
extracted from the pulp. This can be seen by examining reference
line 2 of the chart of FIG. 42 where the percentage of dry solids
is given for the discharge of the washer 408.
[0259] f. Other Modifications and Further Comments
[0260] It is apparent that various modifications could be made in
the present invention without departing from the basic teachings
thereof. For example, FIG. 40, there is shown a modified form of
the cross flow ring 430. Components of this cross flow ring which
are similar to a ring that is shown in FIG. 32 will be given like
numerical designations, with a prime (') designation distinguishing
those of the arrangement in FIG. 40. As shown in FIG. 40, the ring
430' has simply a circumferential curved plate 432' without the
flanges 434 as shown in FIG. 32. To form the chamber of 440', the
outer wall 411' is recessed, and thus the chamber 440' is defined
by the surfaces 592, 594 and 596 that are formed out of the
digester side wall 411' at the time the chamber 440' is formed
(e.g., by simply machining a circumferential recess into the wall`)
These portions of the recess 440' which are not to serve as flow
passageways can simply be filled in with metal solder or other
filler material. The intake fitting 446a' is formed as shown in
FIG. 32, and an opening is drilled in the surrounding wall 432' to
receive the fitting 446a'.
[0261] FIG. 41 shows substantially the same arrangement as shown in
FIG. 40, but shows a further modification of the ring 430. The
components of this further modification to FIG. 41 will be given
numerical designations corresponding to those in 440, except that
there will be a double prime ("" to distinguish those in the
arrangement in FIG. 41.
[0262] The ring 430" has the chamber 440"??? cut out of the wall
411", as in FIG. 40. However, instead of having the slots 440a,
there are a plurality of slanted bore holes 598 formed in the side
wall 411". Thus, the flow is through these bore holes 498 on one
side of the ring 430", and out through similar bore holes 498 in
the opposite side of the ring 430".
[0263] Another possibility is that during the operation of the
digester 402, it could be subjected to vibrations for various
purposes (e.g., to enhance the diffusion of dry solids from the
inside of pulp fibers and/or chips and to dislodge chips that may
have been stuck in inlets and outlets). Also, the digester could be
rotated about its lengthwise axis back and forth for this same
purpose or other purposes.
[0264] 22. The Evaporation and Recovery System
[0265] As a further modification, as shown in FIG. 43, the cooking
zone 1 has been divided into cooking zone 1A and cooking zone 1B.
The liquor is extracted at the end of each of cooking zones 1A and
1B, and these are directed through separate liquor streams into the
evaporation and recovery system 418. Present analysis would
indicate that the composition of the liquor from these two separate
locations of the cooking zone 1A and 1B would differ so that the
separation of by-products would be enhanced.
[0266] FIG. 44 shows the evaporating and recovery system of the
present invention. There are three liquor inputs into the system
418. Two of these are through the lines 526 and 497 from
impregnation zone. The third is through line 586 from the end of
cooking zone 1. The alcohol that is recovered in the system 418 is
discharged to two locations, one to the first alcohol wash tank 420
and the second to the second alcohol wash tank 421. The system 418
removes substantially all of the alcohol, and a substantial amount
of the water from the pulping liquors directed into the system
418.
[0267] The system 418 comprises an alcohol recovery system 702,
which may be conventional and in this instance comprises a
condensate stripper 704, an alcohol distillation column 706, and an
alcohol condenser 708. The system 418 also comprises an evaporating
system which comprises three sets of evaporating units. These
evaporating units (which in the industry are called "vapor bodies")
can be conventional, and each comprise a containing tank, a heat
exchanger, a liquor circulation means, a vapor supply line and a
condensate removal system. There is a first set, comprising first
and second stage evaporator units designated E-1i and E-2i. These
two evaporators E-1i and E-2i receive liquor from the impregnation
zone. There is a second set of evaporator units comprising two
evaporator units E-1C and E-2C which receive liquor from the
downstream end of cooking zone 1. Then there is a third evaporating
section having three stages or units, these being designated,
respectively, E-3, E-4 and E-5.
[0268] In FIG. 43, the flow of the liquid material in the recovery
system is shown in solid lines, while the flow of vapor is shown in
broken lines. First, there will be a description of the manner in
which the liquid flow passes through the system, and then a
description of the flow of the steam added to the system and the
flow of alcohol and water vapor evaporated from the liquor.
[0269] The flow of liquid from the impregnation zone flows through
the inlet lines from 526 and 497 into a blow tank 710, and thence
into the tank of the first evaporator stage E-1i. The liquid is
recirculated by the pump P-1 upwardly through a flow line and back
into the tank of E-1i, and into the upper end of a heat exchanger
712. Also, a portion from the flow from the pump P-1 is conveyed by
the pump P-2 into a first separator S-1. This separator S-1 can be
one of a number of different types of separators. The portion of
the liquor which is extracted in the separator S-1 is indicated by
the arrow 714. The remaining portion of the liquid from the
separator S-1 is directed through the line 716 into the second
stage evaporator E-2i.
[0270] The second stage evaporator unit E-2i has pumps P-3 and P-4
which operate in substantially the same manner as the pumps P-1 and
P-2, with a portion of the liquor being directed to the second
stage separator S-2. The portion extracted from the separator is
discharged through the discharge line 720. The other portion of the
liquor is recirculated upwardly and into the heat exchanger 722 of
the second stage E-2i. The unextracted liquor from the separator
section S-2 is directed through the line 724 and thence into a line
726 leading into the middle evaporator section comprising the three
evaporator stages or units, E-3, E-4 and E-5. Attention is now
directed to the evaporator units in the second section, namely
units E-1c and E-2c. These have pumps P-5, P-6, P-7 and P-8. Also,
there are two by-product separators S-3 and S-4. The flow from the
line 586 at the downstream end of cooking zone 1 enters the blow
tank 728, with the liquid passing through the evaporator units E-1c
and E-2c in substantially the same manner as described previously
with respect to the evaporators of the first section, namely E-1i
and E-2i. The liquor stream that is extracted in the separation
process from the two separators S-3 and S-4 are designated 730 and
732, respectively. The flow from the separator S-3 which is not
extracted in the separation process goes through line 734 into the
line 726 to flow into the center evaporator section.
[0271] Various separating techniques could be used in one or more
of the separators S-1, S-4. For example, a conventional centrifuge
could be utilized, where oils are being separated since the oils
are less dense than the lignin. Conventional filters also could be
used, or systems where an added substance reacts with the desired
byproducts, making these heavier or lighter so that they either
sink to bottom or flow to the top. Or the added substance could
make the desired byproduct stickier, or possibly heavier so that it
could be more easily separated by a centrifuge. Further, in the
evaporation process, the alcohol will evaporate more rapidly than
the water because of its lower boiling point and other
characteristics. Thus, since alcohol is the dissolving agent, when
it evaporates it frees the organic solids from suspension. This
better enables the lignin to be spun off by centrifugal force to
free most of the extraneous materials (oils, etc.).
[0272] The by-product(s) removed by separator S-1 have a rather
different composition than those separated by the separator S-2,
since the liquor which goes into the separator S-2 has practically
all of the alcohol removed therefrom. This is also true with regard
to the separation that takes place at the Separators S-3 and S-4,
with most all of the alcohol being removed from the liquor that
goes to S-4.
[0273] While the evaporating system shown in FIG. 44 has shown only
two liquor streams going into the evaporating process, it is within
the scope of the present invention to have yet more separate liquor
streams. For example, the two streams from the lines 526 and 497
from the impregnation zone could be treated separately. Also, more
than two cooking zones could be provided, and liquor streams could
be taken from locations at each such cooking zone.
[0274] In the center evaporator section, there are three
evaporating units/stages E-3, E-4 and E-5, and each stage comprises
its own heat exchangers and recirculating components as previously
described. The liquor flow in the stages are each handled
separately, but the vapors are mixed to comprise one vapor stream.
The three heat exchangers are each designated 736. The flow of
liquor from the line 326 flows in a recirculating pattern through
all three of the heat exchangers 736 in series, which recirculating
pattern is or may be conventional in the prior art. Accordingly,
this will not be described in detail herein. Specifically the
liquor flow progresses from stage 3 to stage 4 and then to stage 5.
The discharge of liquor from evaporation unit E-5 is through a line
738, into a blow tank 740, with the liquor being discharged through
the line 741 to be delivered to the spray dryer.
[0275] Attention is now directed toward the flow of steam and vapor
in the evaporation and recovery system 418. The steam is directed
into the system 418 through a steam line 742 to the center section
of evaporator units and is directed through three steam lines into
the three heat exchangers 736 in the third, fourth and fifth stages
E-3, E-4 and E-5. The vapors resulting from evaporators in E3, E4
and E5 is then directed through the lines 746, 748 to,
respectively, heat exchangers in the two evporating sections E-2i
and E-2c. Then vapor from the evaporator units E-2i and E-2c of the
second evaporating section. The vapor is then directed through the
two heat exchangers of the evaporating sections E-1i and E-1c,
[0276] The vapor collected in the evaporators E-1i and E-1c into
the line 750, which leads directly into the heat exchanger of
condensate stripper 704. For convenience of illustration, since the
line 750 begins at the right hand part of FIG. 44 and extends all
the way to the left hand side of FIG. 44 where it enters the
condensate stripper 704, the line 750 is not shown extending all
the way across the page. Rather, as shown, the line 750 ends at a
circle with a designation "A" therein, and picks up again at the
left hand side of the FIG. 750 where there is another circle with
the designation "A" therein.
[0277] The vapor discharged from the blow tank 740 travels through
two lines 752 and 754 to be delivered to, respectively, the heat
exchanger 712 of the evaporator E-1i and to the heat exchanger in
the evaporator E-1c.
[0278] To review the overall operation of the recovery system of
FIG. 43, several items should be noted. First, the liquor from the
impregnation zone is treated separately, and portions of this
liquor are extracted at two separating stages S-1 and S-2. This is
to recover some of the liquor components which are removed from the
fibers at an earlier stage in the overall digesting system. Then
the liquor from the downstream end of cooking zone 1 also has
portions thereof separated at relatively early stages in the
evaporation process, namely separation stage S-4 and S-3. The
reason for this is that this earlier extracted liquor has a
somewhat different character than the liquor going through the
entire evaporation process. The liquor that travels from the first
evaporator section, E-1i and E-2i, and from the second evaporator
section (evaporators E-1c and E-2c) is delivered into the central
evaporating section (evaporating stages E-3, E-4 and E-5) where it
goes through a further evaporation process, and as indicated
previously, is discharged at 741 to be delivered to the spray
dryer.
[0279] The condensate from the heat exchangers in the seven
evaporating units can be treated in a conventional manner.
[0280] Any condensate which has such a low percentage of alcohol
content so that further alcohol recovery would be uneconomical
would be discharged from the recovery system. The condensate that
has a sufficiently high percentage of alcohol therein for
economical recovery is directed to the alcohol recovery
section.
[0281] The non-condensable gases which enter the recovery system
through the line 562 can be treated in a conventional manner in the
recovery system. Accordingly, these will not be discussed further
herein.
[0282] It is to be understood that within the scope of the present
invention, there could be additional liquor streams from other
portions of the digester entering into the recovery system, and
these could be treated in separate evaporator sections, so that
there would be additional sets of evaporators, such as indicated in
the first section at E-1i and E-2i, and also in the second
evaporator section (evaporators E-1c and E-2c). Likewise, there
could be additional by-product separator sections such as those
shown at S-1 through S-4.
[0283] 23. Final Comments
[0284] To review briefly some of the desirable features of the
present invention, it will be noted that except for the flow area
at 498 (see FIG. 39), the wood chip/pulp/filtrate flow within the
digester 402 is a substantially continuous downstream flow through
the length of the digesting chamber 448. Thus, the wood chips
initially introduced into the digester are in the process of
becoming pulp as they flow through the digester along with the
liquid in the digester chamber 448. The counter current flow (i.e.,
recirculating flow) is accomplished in a manner so that there is a
cross flow transverse to the main forward flow through the digester
chamber 443. Yet, there is overall a net upstream flow so that all
of the filtrate from the washer 408 (except for the liquid that is
discharged with the washed pulp) is directed into the downstream
end of the digester 402 and is discharged at further upstream
locations. Further, it can be seen that the overall migration of
the dry solids is also in an upstream direction in the digester
402.
[0285] While alcohol is a preferred digesting agent in the present
invention, other digesting agents could be used. For example, the
present invention could be adapted for the Kraft process, sulfite
process, or other, digesting processes. Further within the scope of
the present invention, while the present invention is particularly
adapted for the digesting of wood products, it could be utilized
for other materials such as hemp, linen and other plant material.
Also, while in the preferred form, the digester has its
longitudinal axis horizontally aligned, within the broader scope,
the digester could be positioned vertically, or on a slant to both
the horizontal and vertical.
[0286] As indicated above, there are various modifications which
can be made to the present invention without departing from the
basic teachings thereof.
* * * * *