U.S. patent application number 09/981837 was filed with the patent office on 2002-03-07 for method and apparatus for pulping sawdust.
This patent application is currently assigned to ANDRITZ-AHLSTROM INC.. Invention is credited to Chamblee, J. Wayne, Chasse, R. Fred, Marois, Marco, Miele, Jay J., Prough, J. Robert, Weston, John D..
Application Number | 20020026990 09/981837 |
Document ID | / |
Family ID | 24074673 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020026990 |
Kind Code |
A1 |
Miele, Jay J. ; et
al. |
March 7, 2002 |
Method and apparatus for pulping sawdust
Abstract
Chemical cellulose pulp is made from sawdust utilizing a static
down-flow retention vessel. By adding steam and cooking liquor to a
flow of sawdust a heated slurry, at a cooking temperature of about
250-350.degree. F., is produced. The heated slurry is, at
superatmospheric pressure, moved downwardly in the static down-flow
retention vessel while cooking temperature is maintained, for a
time period of about 0.5-6 (preferably 1 to 3) hours, the slurry
having a consistency of about 5-30%. At superatmospheric pressure,
without significant reduction in pressure from the retention
vessel, the slurry is cooled to well below cooking temperature by
diffusing cooling liquid through it, as in a conventional pressure
diffuser. The discharge from the retention vessel is preferably
substantially solely gravity action (e.g. using a discharge with
single convergence and side relief). Various mixing, diluting,
thickening, steaming, and pumping devices are utilized in the
system from initial steaming of the sawdust to passage into the top
of the retention vessel.
Inventors: |
Miele, Jay J.; (Queensbury,
NY) ; Marois, Marco; (Queensbury, NY) ;
Chasse, R. Fred; (Queensbury, NY) ; Chamblee, J.
Wayne; (Queensbury, NY) ; Weston, John D.;
(Queensbury, NY) ; Prough, J. Robert; (Queensbury,
NY) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
1100 North Glebe Road, 8th Floor
Arlington
VA
22201-4714
US
|
Assignee: |
ANDRITZ-AHLSTROM INC.
|
Family ID: |
24074673 |
Appl. No.: |
09/981837 |
Filed: |
October 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09981837 |
Oct 19, 2001 |
|
|
|
08520941 |
Aug 31, 1995 |
|
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|
Current U.S.
Class: |
162/19 ; 162/246;
162/40; 162/52; 162/60 |
Current CPC
Class: |
D21C 3/24 20130101; D21C
1/02 20130101; D21C 3/266 20130101 |
Class at
Publication: |
162/19 ; 162/60;
162/52; 162/40; 162/246 |
International
Class: |
D21C 003/26; D21C
007/06; D21C 007/08; D21C 009/02; D21C 001/02 |
Claims
What is claimed is:
1. A method of producing chemical cellulose pulp from sawdust
utilizing a static down-flow retention vessel, comprising the steps
of continuously: (a) adding steam and cooking liquor to a flow of
sawdust to produce a heated slurry of sawdust and cooking liquor at
a consistency of between about 15-35%, and a cooking temperature of
between about 250-350 degrees F.; (b) passing the heated slurry
from step (a) at superatmospheric pressure downwardly in the static
down-flow retention vessel, and retaining the slurry in the
retention vessel at cooking temperature between about 0.5-6 hours,
and then discharging it at a consistency of between about 5-20%
from the retention vessel; and (c) at superatmospheric pressure,
without significant reduction in pressure from the retention
vessel, cooling the slurry discharged from the retention vessel by
diffusing cooling liquid therethrough so that the temperature of
the slurry drops below cooking temperature, and cooking thereof is
terminated.
2. A method as recited in claim 1 wherein step (b) is practiced to
discharge the slurry from the retention vessel without mechanically
acting on the slurry.
3. A method as recited in claim 1 wherein step (b) is practiced to
discharge the slurry from the retention vessel substantially by
gravity action alone.
4. A method as recited in claim 2 wherein step (a) is practiced by
initially forming a slurry at a first consistency greater than
about 20%, and then successively: diluting and heating the slurry
so that it has a readily pumpable second consistency of less than
20%; rethickening the slurry to a consistency of greater than about
20%; and then heating the slurry.
5. A method as recited in claim 1 wherein steps (a)-(c) are
practiced to produce a chemical cellulose pulp having a Kappa
number of between about 10-30, with a yield of between about
38-45%.
6. A method as recited in claim 2 wherein steps (a)-(c) are
practiced to produce a chemical cellulose pulp having a Kappa
number of less than about 24 with a yield of about 39-42%.
7. A method as recited in claim 2 comprising the further step of
pre-steaming the sawdust prior to step (a) in a steaming vessel,
and discharging the pre-steamed sawdust from the steaming vessel
substantially by gravity action alone.
8. A method as recited in claim 2 wherein step (c) is practiced by
upflowing the suspension through a pressure diffuser at a
consistency of about 5-20%.
9. A method as recited in claim 3 wherein step (a) is practiced by:
diluting the slurry so that it has a diluted consistency of about
10% or less, and pumping the diluted consistency slurry to an
elevated level near the top of or above the retention vessel;
thickening the slurry at the elevated level to a consistency of
about 20-40%; and steaming the thickened elevated slurry to
increase the temperature thereof.
10. A method as recited in claim 2 wherein step (a) is practiced to
heat the slurry to a cooking temperature of between about 300-330
degrees F., and step (b) is practiced by maintaining the cooking
temperature in the retention vessel about 1-3 hours.
11. A method as recited in claim 2 comprising the further steps of
washing and bleaching the pulp from step (c).
12. A system for producing chemical pulp from sawdust, comprising:
a static down-flow superatmospheric pressure retention vessel
having a top for receipt of a sawdust slurry, and a bottom for
discharge of chemical pulp; a first mixer for mixing steam and
cooking liquor with sawdust to form an initial slurry; subsequent
means for diluting, raising the temperature to cooking temperature,
and pressurizing the initial slurry to provide a slurry suitable
for cooking, and elevating the slurry to the top of said retention
vessel to feed slurry into the top of the retention vessel; a
non-mechanical discharge from the bottom of said retention vessel;
and a superatmospheric pressure vessel connected to said
non-mechanical discharge for displacing cooking liquid from the
pulp after the pulp is discharged from the bottom of said retention
vessel to lower the temperature thereof below cooking
temperature.
13. A system as recited in claim 12 wherein said subsequent means
comprises a thickener substantially at or above the top of said
retention vessel, and connected to a steam mixer, said steam mixer
connected to said top of said retention vessel and above said
retention vessel.
14. A system as recited in claim 13 wherein said non-mechanical
discharge comprises a discharge with single-convergence and side
relief.
15. A system as recited in claim 14 wherein said first mixer
comprises a screw conveyor mixer.
16. A system as recited in claim 15 wherein said subsequent means
comprises: a discharge chute having a top portion connected to said
screw conveyor mixer, and a bottom portion; dilution liquid
addition means to said discharge chute; a pump adjacent said
discharge chute bottom portion and a conduit extending from said
pump to said thickener; and dilution liquid addition means
connected to said conduit from said pump.
17. A system as recited in claim 12 wherein said superatmospheric
pressure vessel comprises a pressure diffuser.
18. A system as recited in claim 12 wherein said subsequent means
comprises: a discharge chute having a top portion connected to said
first mixer, and a bottom portion; dilution liquid addition means
to said discharge chute; a pump adjacent said discharge chute
bottom portion and a conduit extending from said pump; dilution
liquid addition means to said conduit from said pump; a thickener
substantially at or above the top of said retention vessel and
connected to said conduit from said pump; and a steam mixer
connected to said thickener and the top of said retention
vessel.
19. A system as recited in claim 18 further comprising a second
conduit from said thickener connected to said dilution liquid
addition means to said conduit from said pump, and a heat exchanger
for heating liquid in said second conduit disposed between said
thickener and said dilution liquid addition means.
20. A system as recited in claim 19 further comprising a flash tank
connected to said second conduit and including a flash steam outlet
and a liquid outlet, said flash steam outlet connected to said
dilution liquid addition means to said discharge chute, and said
flash steam outlet connected to said first mixer.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
[0001] Many forms of naturally occurring cellulose are used to
produce chemical pulps for the production of paper. The form used
depends upon the availability of the material and the capability of
the pulping equipment. One of the most common forms is the wood
chip, either made from hardwoods or softwoods, but any other form
of comminuted cellulose material may be used including grasses or
agricultural waste, for example, bagasse and cornstalks.
[0002] An additional source of cellulose is the waste from saw
mills, namely sawdust. Especially in lumber producing regions there
is a plentiful supply of sawdust that can be pulped to produce wood
pulp. The pulping of sawdust has both advantages and disadvantages.
One advantage for using sawdust as a source of cellulose is that
the smaller sawdust particles are relatively easy to impregnate
with cooking liquor. For this reason the pretreatment systems for
chemical pulping of sawdust are less complex than those used to
impregnate wood chips, which are generally more difficult to
impregnate than sawdust.
[0003] One disadvantage of chemical pulping sawdust is that sawdust
can be resistant to the flow of cooking liquors. The finely
dividing material tends to form a compact matrix when exposed to a
liquid flow and limit flow through the material, if not prevent it
altogether. For example, since batch digesters are highly dependent
upon the capability of providing a cooking liquor circulation
through the medium being pulped, it is difficult--if not
impossible--to pulp sawdust in a conventional batch digester. Also,
conventional continuous digesters, such as Kamyr.RTM. continuous
digesters, also have difficulty handling sawdust without
incorporating some form of special rotating liquid distribution
device.
[0004] One common method used to continuously pulp sawdust is by
using a drag-chain type digesters, for example, an M&D-type
digester as shown in FIG. 138 of Volume 5 of TAPPI's Pulp and Paper
Manufacture (1989), Grace, ed. These type of digesters consist of
an inclined vessel through which sawdust is conveyed through the
cooking liquor by means of a conveyor mechanism. However, this
conveyor mechanism and its related hardware requires continuous
maintenance that makes this type of system unsatisfactory in modern
pulp mills.
[0005] Another mechanical disadvantage of the M&D-type digester
for treating sawdust, and the like, is the rotary feed valve used.
A typical device is shown in FIG. 139 of Grace. This rotary valve
is a typical star-type feeder that inherently experiences an
unbalanced pressure load due to the large pressure difference
between the inlet and outlet of the valve. This load imbalance
typically causes bearing wear requiring repeated maintenance.
[0006] In addition to the mechanical disadvantages, these
M&D-type systems also have process disadvantages that make
these systems less efficient than desired. One characteristic of
the M&D-type process is the relatively short retention times.
Two aspects of this type of digester limit the retention time: (a)
steam heaving and (2) mechanical conveyance. Since the
impermeability of sawdust prevents the sawdust from being heated by
liquor displacement, the sawdust is heated by direct exposure to
steam. The steam or vapor space required to expose the material to
steam consumes some of the space that could be used for cooking
retention time and hence limits the retention time.
[0007] The mechanical conveyor used in an M&D-type digester,
referred to as a "drag conveyor", also limits the retention time
because of the physical limitations of the size of the conveyor. It
is simply too costly to manufacture a larger mechanical conveyor to
achieve longer retention times.
[0008] As a result, the retention times provided by such a digester
are limited to less than 1 hour, typically less than 30 minutes.
Typically, additional cooking retention time is obtained when
treating sawdust by following the M&D-type digester by ne or
more retention vessels, or by "piggy-backing" two or more inclined
digesters.
[0009] These characteristic short retention times also affect the
cooking temperatures that are used. In order to obtain the proper
degree of cooking, for example, to achieve a desired H factor, a
relatively higher temperature must be used because of the shorter
retention time. For example, if a typical cook requiring 2 hours
retention time is limited to only 1/2 hour in an M&D-type
digester, the cooking temperature must be increased from
approximately 325.degree. F. to 360.degree. F. to achieve a
comparable cook. this increase in cooking temperature increases the
amount of high-pressure steam needed to maintain the higher cooking
temperature. Therefore, the M&D-type digester is not as energy
efficient as a digester capable of longer retention times.
[0010] These higher temperatures also consume more cooking
chemicals and can potentially increase fiber damage. The rate of
reaction of cooking chemicals with cellulose is highly dependent
upon the prevailing temperature. The higher the temperature the
faster and more aggressive the reaction. For kraft systems of the
M&D type, the higher cooking temperatures, required for the
shorter cooking times, result in higher reaction rates. This
typically can cause increased chemical consumption and increased
cellulose degradation.
[0011] The disadvantages of the M&D-type digester for cooking
sawdust, and the like, are also seen in the "Pandia"-type digester
shown in FIGS. 141 and 143 of Grace.
[0012] Another conventional continuous sawdust pulping system,
shown in Grace, FIG. 133, and Smook, Handbook for Pulp and Paper
Technologists, 1982, page 86 (FIGS. 8-17), comprises a cylindrical
vessel fed by two horizontal screw conveyors and a pocket feeder,
for example, a Kamyr.RTM. asthma feeder. This type of vessel is a
steam-phase type in which a liquid level is maintained below the
top of the vessel and steam is added to the space above the liquid
level. The sawdust fed to this vessel by the pocket feeder is
heated to cooking temperature by the added steam. This steam
heating avoids the impractical practice of circulating heated
liquor to heat to cooking temperature.
[0013] As described by Grace, the pulp in this type of digester is
cooled by introducing wash filtrate to the bottom of the digester
and extracting it by means of a centrally-located rotating
cylindrical screen. (See U.S. Pat. No. 3,475,271 of Laakso.)
However, due to the impermeability of finely divided material like
sawdust, this method of extraction has been shown to be
unstable.
[0014] This "asthma-feeder" style sawdust cooking system also has
the disadvantage that the feed system is located above the digester
vessel.
[0015] This is because the asthma feeder is limited to transporting
the sawdust a short distance. This limits the size and flexibility
of such installations.
[0016] Another sawdust pulping system is shown in Canadian patent
1,242,055. This patent discloses the use of a conventional slurry
pump to feed a slurry of sawdust and cooking liquor to a
cylindrical digester. This transfer of medium consistency slurry by
means of a pump prior to cooking is not energy efficient.
Typically, such pumps are limited to medium consistency slurries of
between 8 and 16% consistency. In heating such a slurry to cooking
temperature the excess liquid volume must also be heated to cooking
temperature. For example, a 12% slurry contains 7.33 lbs. of liquid
per lb. of fiber. In contrast, a 30% slurry contains 2.33 lbs. of
liquid per lb. of fiber, or less than a third of the liquid per lb.
of fiber. The lower consistency slurry requires additional energy
to heat this excess liquid to cooking temperature.
[0017] Furthermore, no effort is made to minimize the mechanical
action on the pulp or to recover heat from the cooked pulp slurry.
Excessive mechanical action on sawdust slurries can be damaging to
fiber properties, and is otherwise undesirable.
[0018] The present invention avoids these limitations of prior art
continuous cooking systems for sawdust, and other finely divided
comminuted fibrous material by first eliminating the need for high
pressure mechanical feeders and conveyors; second, by discharging
hot, pressurized cooked sawdust without cooling and without the aid
of a rotating discharge device; and third by recovering the heat of
the cooking reaction in an efficient economical manner.
[0019] The invention addresses the problems inherent in treating
sawdust, or other finely divided source of cellulose material
(which is within the scope of the term "sawdust" as used in the
present specification and claims, e.g. initial cellulose particles
which flow more like a powder than they flow like conventional wood
chips), and provides for more efficient pulping, requiring less
maintenance. The invention is practiced utilizing a static
retention vessel. A "static" vessel is one without any significant
internal circulation, which internal circulation typically include
(in conventional continuous digesters for example) screens,
conduits, pumps, heaters, and the like. While steam or heated
liquid may be added to the pulp in the retention vessel, to ensure
that it is retained at cooking temperature (although that is not
normally necessary), there is no attempt to draw liquid uniformly
through the vessel as in conventional batch and continuous
digesters.
[0020] According to one aspect of the present invention a method of
producing cellulose pulp from sawdust utilizing a static down-flow
retention vessel is provided. The method comprises the steps of
continuously: (a) Adding steam and cooking liquor to a flow of
sawdust to produce a heated slurry of sawdust and cooking liquor at
a consistency of between about 10-35%, preferably 20-30%, and a
cooking temperature of between about 250-350.degree. F. (b) Passing
the heated slurry from step (a) at superatmospheric pressure
downwardly in the static down-flow retention vessel, and retaining
the slurry in the retention vessel at cooking temperature between
about 0.5-6 hours, and then discharging it at a consistency of
between about 5-20% from the retention vessel. And, (c) at
superatmospheric pressure, without significant (i.e. destructive to
the fiber) reduction in pressure from the retention vessel, cooling
the slurry discharged from the retention vessel by diffusing
cooling liquid therethrough so that the temperature of the slurry
drops below cooking temperature, and cooking thereof is
terminated.
[0021] Step (b) is preferably practiced to discharge the slurry
from the retention vessel without mechanically acting on the slurry
(that is no mechanical agitator, pump, or like structure being
provided). In fact it is desirable to discharge the slurry from the
retention vessel substantially by gravity action alone (as by using
a discharge having single convergence and side relief).
[0022] Step (a) may be practiced by initially forming a slurry at a
first consistency greater than about 20%, and then successively:
diluting and heating the slurry so that it has a readily pumpable
second consistency of less than 20%; rethickening the slurry to a
consistency of greater than about 20%; and then diluting and
heating the slurry. Steps (a) through (c) are typically practiced
to produce a chemical cellulose pulp having a Kappa No. of between
about 10-30 (e.g. less than 24) with a yield of about 38-45% (e.g.
about 39-42%).
[0023] There may also be the further step of pre-steaming the
sawdust prior to step (a) in a steaming vessel and discharging the
presteamed sawdust from the steaming vessel substantially by
gravity action alone. There are also typically the further steps of
washing and bleaching the pulp from step (c) depending upon the
final product to be produced. Step (c) is also typically practiced
by upflowing the suspension through a pressure diffuser at a
consistency of about 5-20%. Step (a) is typically practiced to heat
the slurry to a cooking temperature of between about
300-330.degree. F., and step (b) is practiced by maintaining the
cooking temperature in the retention vessel about 1-3 hours.
[0024] Step (a) may be practiced by: diluting the slurry so that it
has a diluted consistency of about 20% (e.g. about 10%) or less,
and pumping the diluted consistency slurry to an elevated level
near the top of or above the retention vessel; thickening the
slurry at the elevated level to a consistency of about 20-40%; and
steaming the thickened elevated slurry to increase the temperature
thereof while diluting it to a consistency of about 5-20%.
[0025] According to another aspect of the present invention a
system for (continuously) producing chemical pulp from sawdust is
provided. The system preferably comprises the following components:
A static down-flow superatmospheric pressure retention vessel
having a top for receipt of a sawdust slurry, and a bottom for
discharge of chemical pulp. A first mixer for mixing steam and
cooking liquor with sawdust to form an initial slurry. Subsequent
means for diluting, raising the temperature to cooking temperature,
and pressurizing the initial slurry to provide a slurry suitable
for cooking, and elevating the slurry to the top of the retention
vessel to feed slurry into the top of the retention vessel. A
non-mechanical discharge from the bottom of the retention vessel.
And, a superatmospheric pressure vessel connected to the
non-mechanical discharge for diffusing cooling liquid into pulp
after the pulp is discharged from the bottom of the retention
vessel to lower the temperature thereof below cooking
temperature.
[0026] The subsequent means may comprise a thickener substantially
at or above the top of the retention vessel, and connected to a
steam mixer, the steam mixer connected to the top of the retention
vessel and above it. The first mixer may comprise a screw conveyor
mixer. The non-mechanical discharge may comprise a discharge with
single-convergence and side relief. The subsequent means may
comprise: a discharge chute having a top portion connected to the
screw conveyor mixer, and a bottom portion; dilution liquid
addition means to the discharge chute; a pump adjacent the
discharge chute bottom portion and a conduit extending from the
pump to the thickener; and/or dilution liquid addition means
connected to the conduit from the pump. The superatmospheric
pressure vessel preferably comprises a pressure diffuser.
[0027] The system may further comprise a second conduit from the
thickener connected to the dilution liquid addition means to the
conduit from the pump, and a heat exchanger for heating liquid in
the second conduit disposed between the thickener and the dilution
liquid addition means. A flash tank may be connected to the second
conduit and includes a flash steam outlet and a liquid outlet, the
flash steam outlet connected to the dilution liquid addition means
to the discharge chute, and the flash steam outlet connected to the
first mixer. Though the invention is disclosed for use with
sawdust, one skilled in the art would recognize that for various
aspects of the invention any other form of comminuted fibrous
material may be used, for example, wood chips, agricultural waste
or grass.
[0028] It is the primary object of the present invention to simply
and effectively produce a relatively low Kappa No. chemical pulp,
with relatively high yield, from sawdust. This and other objects of
the invention will become clear from an inspection of the detailed
description of the invention, and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic side view of a typical embodiment of a
system according to the present invention;
[0030] FIG. 2 is a side schematic view of a typical heat exchanger
used with the system of FIG. 1; and
[0031] FIG. 3 is a side schematic view of a further typical
embodiment of a system according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 shows a schematic diagram of a typical system 10 for
pulping finely divided comminuted cellulose material referred to as
"sawdust" herein. The sawdust is fed continuously by conveyor 11
into a pretreatment vessel 12. Pretreatment my consist of steaming
or treatment with black liquor or some other strength or yield
enhancing chemical, for example polysulfide or anthraquinone and
their derivatives. Treatment and retention in vessel 12 may be from
5 to 60 minutes, but is preferably between 5 and 20 minutes. The
vessel 12 may operate at atmospheric or super-atmospheric
pressures.
[0033] The treatment vessel, 12, may exhibit single-convergence and
side relief as disclosed in pending U.S. patent applications Ser.
Nos. 08/189,546 filed on Feb. 1, 1994 (10-926) and 08/366,581 filed
on Dec. 30, 1994 (10-1020). Such a retention vessel is sold under
the trademark "Diamondback" by Ahistrom Kamyr Inc. of Glens Falls,
N.Y.
[0034] The vessel 12 discharges into a conveyor 13 which includes a
conventional conveying screw as shown in FIG. 1, or any other
conventional means of conveying the pretreated sawdust may be
provided. The conveyor 13 typically comprises a screw 13" driven by
a drive device such as an electric motor 13"", for example a
variable speed electric motor. If the conveyor 13 is pressurized,
some form of pressure-isolation device can be used between the
vessel 12 and the conveyor 13. For example, a star-type feeder,
such as an Ahistrom Kamyr low pressure feeder 14 may be used. The
conveyor 13 is a first mixer for mixing steam and cooking liquor
with the sawdust.
[0035] Cooking liquor, for example kraft white liquor, is added to
the conveyor 13 in line 43 to begin the impregnation of the
material with cooking chemicals. Steam is also preferably added to
the conveyor 13 via line 15, to begin the heating, or continue the
heating begun in the vessel 12, of the material and to remove
unwanted air form the material. The conveyor 13 may also include a
vent 16 for releasing non-condensable gases (NCG) to a conventional
NCG collection system. A slurry having a consistency of about 25%
or more and a temperature of between about 125.degree.
F.-175.degree. F. is discharged from conveyor 13.
[0036] The conveyor 13 discharges to a feed chute 17 in which the
sawdust slurry is diluted to a consistency of between about 5 to
20%, preferably about 10 to 15%. The temperature of the slurry in
the chute 17 may be between about 150 to 250.degree. F., typically
about 160 to 200.degree. F. The chute 17 feeds a conventional
slurry pump 18. The pump 18 pressurizes and transfers the heated
material and cooking liquor slurry to a conventional dewatering
conveyor 19 via conduit 20. The slurry may be diluted to lower the
consistency thereof by at least 2%, and preferably between about
5-10%, in the conduit 20, e.g. by dilution liquid (e.g.
recirculated liquor, filtrate or hot water), added via conduit 21,
to a consistency of between about 3 and 15%, typically about 5 to
10%. The dewatering conveyor 19 may be a conventional separator
such as a "top separator" or an "inverted top separator" as sold by
Ahlstrom Kamyr. This conveyor 19 may alternatively be a "Stocker"
as sold by A. Ahistrom Corp. of Helsinki, Finland.
[0037] The liquor removed from this dewatering conveyor 19, via
line 22, which is typically at about 250 to 300.degree. F., may be
used as the source of dilution in the conduit at 21, after being
pressurized in pump 23 and heated in heat exchanger 26, and/or all
or part of it may be flashed to produce a source of steam using
conventional flash tank 24. For example, the pressure of the hot
liquor 22 may be decreased under controlled conditions, i.e.,
flashed, in flash tank 24 to produce a source of contaminated steam
in line 25. The steam in line 25 may be used as the source of steam
introduced to the conveyor 13 or vessel 12. This contaminated steam
may be supplemented by clean steam as needed. The hot flashed
liquor from tank 24, in line 25", may be used as the source of
dilution liquid in chute 17, or elsewhere.
[0038] The dewatering conveyor 19 increases the consistency of the
slurry to between about 20-40% and discharges the slurry to a
conventional steam mixer 27. The steam mixer 27 may be any
conventional device (e.g. having an internal conveying screw) for
introducing steam to the slurry and heating the slurry to cooking
temperature, typically about 250 to 350.degree. F., preferably
about 300 to 330.degree. F., while its consistency is being diluted
by the steam addition to between about 15-35%, preferably 10-20%.
The structures 17, 18, 20, 19, 27, 22, 24, etc. between first mixer
13 and the discharge from steam mixer 27 are one exemplary
embodiment of subsequent means for diluting, raising the
temperature to cooking temperature, and pressurizing the slurry
from conveyor mixer 13 before cooking. A wide variety of other
conventional pressurizing, temperature raising, and dilution and
thickening devices may be provided.
[0039] The steam-heated slurry is discharged from the mixer 27 to a
retention vessel/digester 28 in which the cooking reaction is
allowed to proceed. The retention time in vessel 28 may range from
about 30 minutes to about 6 hours but is typically about 1 to 3
hours, preferably 1 to 11/2 hours. Note that vessel 28 is static,
that is it does not include any real cooking circulations, and
associated screens, because cooking circulations would be difficult
to operate for such a finely comminuted material as sawdust. The
vessel 28 need not include an agitator at its discharge 29 but
preferably includes as the discharge 29 a non-mechanical means,
such as a single-convergence outlet with side relief as illustrated
schematically in FIG. 1, and as discussed previously for vessel 12,
and/or liquid discharge jets or nozzles.
[0040] The material discharged through discharge 29 from vessel 28,
typically at between about 5 and 20% consistency, is transferred,
while still at cooking temperatures and pressures (and without
destructive reduction of pressure), via conduit 30 to a second
treatment vessel 31. In treatment vessel 31 the cooked, hot,
pressurized material is cooled by means of filtrate from line 32.
The heat of the treated material entering vessel 31 is removed via
liquid extraction line 33 and used, for example, as a source of
heat for heat exchanger 26. The hot liquor in line 33 is cooled
somewhat in heat exchanger 26 and may then be sent to a
conventional chemical recovery system, for example, to one or more
flash tanks, to evaporators, a recovery boiler, etc. The liquor in
line 33 may also be used to treat material in vessels 12, 13 or
17.
[0041] The vessel 31 is preferably an MC.RTM. Pressure Diffuser as
sold by Ahistrom Kamyr. The cooked material is typically cooled by
diffusing the cooler liquid from line 32, typically brownstock
washer filtrate, through the pulp bed. The pulp is cooled to below
cooking temperature (e.g. below about 250.degree. F.) in vessel 31.
The hot cooking liquor is displaced by the cooler liquid in this
process and the hot displaced liquor is extracted as is
conventional from the bottom of the pressure diffuser (in line 33).
The cooled material is discharged from the top 34 of the vessel 31
and passed by conduit 35 to a high density brown stock storage
vessel 36 or the like. The material stored in vessel 36 may be
further treated by, for example, washing or bleaching, and sent to
a paper, board or pulp machine.
[0042] FIG. 2 illustrates the typical temperatures around the
conventional, non-contact heat exchanger 26. Hot extract in line 33
from the cooling vessel [e.g. a pressure diffuser] 31 is typically
between about 250-350.degree. F., preferably between about 300
-325.degree. F., and is cooled at least about 25.degree. F. in heat
exchanger 26 to between about 200-300.degree. F., preferably about
275 to 300.degree. F. The liquor from the dewatering conveyor 22
and pump 23 [i.e. 38 in FIG. 2] is normally between about
200-300.degree. F., typically about 260 to 280.degree. F. The
liquid in line 38 is heated at least about 25.degree. F. to about
270 to 325.degree. F., typically about 290 to 310.degree. F., in
heat exchanger 26 before entering conduits 21 then 20. The material
slurry in conduit 20 is typically heated by the addition of liquid
from line 21 from between about 150-250.degree. F., typically
between about 160-200.degree. F., by at least about 50.degree. F.,
e.g. to between about 200 to 300.degree. F., typically to between
about 270 to 290.degree. F.
[0043] The now cooler, but still hot (e.g. about 290.degree. F.)
liquid from line 33 is discharged from heat exchanger 26 into line
40. It may then be used for heat recovery elsewhere before being
passed to recovery in line 41, e.g. by preheating white liquor in
heat exchanger 42, pre-heated white liquor (e.g. for addition to
line 15) being discharged in line 43 from preheater 42.
[0044] Using the process and apparatus described, for example, a
primarily or completely softwood sawdust can be pulped to a Kappa
number between about 10-30, typically about 20-24 (e.g. about 22).
The pulp yield will typically range from 38 to 45%, typically about
39-42% (e.g. about 40%).
[0045] FIG. 3 illustrates an additional embodiment of a system for
pulping sawdust, or similar finely divided comminuted cellulose
material. The system 50 includes pretreatment vessel 51, typically
including an outlet having single convergence and side relief (e.g.
a "Diamondback".TM. chip bin), a slurry pump 52; a heat exchanger
53; a continuous digester 54; and a pressurized washer, typically a
pressure diffuser 55.
[0046] The distinct feature of the FIG. 3 embodiment compared to
the FIG. 1 embodiment is the heat exchanger 53. Instead of using
the heat recovered from the washer 55 to indirectly heat dilution
liquor which is used to heat the slurry before cooking, the hot
liquor extracted from the washer 55 is used directly in an indirect
heat exchanger 53 to heat the cellulose slurry prior to cooking in
digester 54. By doing so, the need for the dewatering conveyor 19
and steam mixer 27 of FIG. 1 is eliminated.
[0047] The heat exchanger 53 may be of the lamellar type with
alternating vertical or horizontal lamellar heating elements
through which the slurry passes, or of a wide variety of
conventional designs used in the pulp and paper art.
[0048] In one typical application of the system shown in FIG. 3,
cellulose material, e.g. sawdust or wood chips, water and cooking
liquor, typically kraft white liquor, are added to the pretreatment
vessel 51. Since the incoming cellulose material is typically
composed of about 50% cellulose and 50% water, the cellulose is
added at a rate of 2 tons of cellulose, or wood fiber, per ton of
pulp produced (t/tp); and water is introduced at a rate of 2 t/tp.
Additional liquid is typically added as steam or cooking liquor at
a rate of 4 t/tp; this liquid typically contains approximately 0.6
t/tp dissolved solid material.
[0049] After combining the cellulose and liquid in vessel 51 to
create a slurry of material it is pumped by slurry pump 52 at a
consistency of between about 20 and 30%, typically about 27%. This
slurry now typically contains 5.4 t/tp liquid, 2.0 t/tp cellulose,
and 0.6 t/tp dissolved solids. In passing through heat exchanger
53, the slurry temperature is typically raised from approximately
200.degree. F. to a cooking temperature of about 325.degree. F.
before passing to the digester 54. The slurry temperature may have
to be augmented by an additional heating device, for example, a
steam mixer as in FIG. 1, to obtain cooking temperature should the
temperature increase in the heat exchanger 53 not be
sufficient.
[0050] The cooked material is discharged from the digester 54,
again typically without the aid of any mechanical discharge device,
at a consistency of between about 10 and 20%, typically about
15.6%. Due to the pulping process, again assuming a 50% yield, the
pulp slurry contains approximately 5.4 t/tp liquid, 1.0 t/tp
cellulose fiber, and 1.6 t/t5p dissolved solids.
[0051] The hot, solids-containing pump is then passed, while still
at digester temperature, e.g., 300-350.degree. F., and digester
pressure, e.g., 140-180 psi, to a pressurized washer 55. The
washer, which is typically an MC.RTM. Pressure Diffuser as sold by
Ahistrom Kamyr of Glens Falls, N.Y., is used to diffusion wash,
dilute, and displace the hot cooking liquor. The wash water is
typically applied at a rate of 9.4 t/tp (e.g., for a typical
dilution factor of 2.0) to produce a cleaner, cooler pulp at
between about 8 and 16% consistency, typically about 12%. The fiber
slurry now contains approximately 1 t/p (by definition) and 7.4
t/tp liquid.
[0052] The hot extraction liquor removed from the washer 55, that
is, the black liquor at between about 300 and 350.degree. F.,
typically 325.degree. F., is used as the heat source in heat
exchanger 53. This black liquor typically contains approximately
7.4 t/tp liquid and 1.6 t/tp dissolved solid material,
corresponding to a black liquor solids concentration of between
about 15 and 20% dry solids.
[0053] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *