U.S. patent application number 13/257606 was filed with the patent office on 2012-05-24 for method and arrangement for clarifying green liquor.
This patent application is currently assigned to CLEANFLOW AB. Invention is credited to Lennart Borjeson, Lennart Kallen, Christofer Lindgren, Mikael Lindstrom.
Application Number | 20120125849 13/257606 |
Document ID | / |
Family ID | 42828551 |
Filed Date | 2012-05-24 |
United States Patent
Application |
20120125849 |
Kind Code |
A1 |
Lindstrom; Mikael ; et
al. |
May 24, 2012 |
METHOD AND ARRANGEMENT FOR CLARIFYING GREEN LIQUOR
Abstract
Green liquor clarification comprising filtering of a flowing
suspension containing solids, wherein the suspension is brought
into contact with a first filter unit (4), said 5 filter unit (4)
comprising one or several filter elements (12) comprising one or
several filter bodies (3) having filter channels (33) within the
filter bodies (3) with a filtering layer (32), a part of the
suspension is forced to pass through the filtering layer (32) from
a first/inner surface (32A) to a second/outer surface (32B) of the
filtering layer (32) forming a filtrate while the solids
substantially remains in a residual part of the suspension forming
a slurry and where the filtering layer (32) is made of a membrane
material with pores, said pores having a pore size of 0.1-10
micrometer, more preferred 0.1-5 micrometer and most preferred
0.2-1.0 micrometer.
Inventors: |
Lindstrom; Mikael; (Lidingo,
SE) ; Lindgren; Christofer; (Stockholm, SE) ;
Borjeson; Lennart; (Hammaro, SE) ; Kallen;
Lennart; (Forshaga, SE) |
Assignee: |
CLEANFLOW AB
Karlstad
SE
|
Family ID: |
42828551 |
Appl. No.: |
13/257606 |
Filed: |
March 30, 2010 |
PCT Filed: |
March 30, 2010 |
PCT NO: |
PCT/SE2010/050348 |
371 Date: |
October 26, 2011 |
Current U.S.
Class: |
210/650 ;
210/500.21 |
Current CPC
Class: |
B01D 2325/02 20130101;
B01D 71/02 20130101; B01D 61/142 20130101; B01D 69/04 20130101;
B01D 2311/25 20130101; B01D 2317/02 20130101; B01D 61/147 20130101;
D21C 11/0078 20130101 |
Class at
Publication: |
210/650 ;
210/500.21 |
International
Class: |
B01D 61/00 20060101
B01D061/00; B01D 39/00 20060101 B01D039/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 2, 2009 |
SE |
0950213-9 |
Claims
1-38. (canceled)
39. A method for clarifying green liquor comprising: filtering a
flowing suspension containing solids, wherein the suspension is
brought into contact with a first filter unit, said filter unit
comprising one or several filter elements, comprising one or
several filter bodies having filter channels within the filter
bodies with a filtering layer, a part of the suspension is forced
to pass through the filtering layer from a first/inner surface to a
second/outer surface of the filtering layer forming a filtrate
while the solids substantially remains in a residual part of the
suspension forming a slurry wherein the filtering layer comprises a
membrane material having pores, and said pores having a pore size
of 0.1-10 micrometer.
40. The method according to claim 39, wherein the pores have a size
of 0.2-1.0 micrometer.
41. The method according to claim 39, wherein said suspension is
forcibly caused to flow into said filter body.
42. The method according to claim 39, wherein the flowing
suspension has a Reynolds Number between 10,000-45,000.
43. The method according to claim 42, wherein the flowing
suspension has a Reynolds Number between 12,000-17,000.
44. The method according to claim 39, wherein said membrane
material is ceramic.
45. The method according to claim 39, wherein the suspension is
forcibly caused to flow into said filter unit by a pump.
46. The method according to claim 39, wherein said filter unit is
connected to an already existing cleaning unit for green
liquor.
47. The method according to claim 46, wherein said filter unit is
arranged as a first filtering step before final cleaning of the
remaining slurry in the already existing cleaning unit.
48. The method according to claim 46, wherein the filter unit is
arranged to filter already cleaned green liquor.
49. An arrangement for green liquor clarification comprising
filtering of a flowing suspension containing solids, wherein the
suspension is brought into contact with at least one filter unit,
said filter unit comprising one or several filter elements,
comprising one or several filter bodies having filter channels
within the filter bodies with a filtering layer, a part of the
suspension is forced to pass through the filtering layer from a
first/inner surface to a second/outer surface of the filtering
layer forming a filtrate while the solids substantially remains in
a residual part of the suspension forming a slurry wherein the
filtering layer comprises a membrane material having pores, and
said pores having a pore size of 0.1-10 micrometer.
50. The arrangement according to claim 49, wherein said pores have
a pore size of 0.2-1.0 micrometer.
51. The arrangement according to claim 49, wherein said suspension
is forcibly caused to flow into said filter body.
52. The arrangement according to claim 49, wherein the flowing
suspension has a Reynolds Number between 10,000-45,000.
53. The arrangement according to claim 52, wherein the flowing
suspension has a Reynolds Number between 12,000-17,000.
54. The arrangement according to claim 49, wherein said membrane
material is ceramic.
55. The arrangement according to claim 49, wherein the suspension
is forcibly caused to flow into said filter unit by a pump.
56. The arrangement according to claim 49, wherein said filter unit
is connected to an already existing cleaning unit for green
liquor.
57. The arrangement according to claim 56, wherein said filter unit
is arranged as a first filtering step before final cleaning of the
remaining slurry in the already existing cleaning unit.
58. The arrangement according to claim 56, wherein the filter unit
is arranged to filter already cleaned green liquor.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the pulp industry, more
specifically to the chemical recovery of the chemical pulping
process. The invention describes a filtering method and arrangement
for clarifying green liquor.
BACKGROUND INFORMATION
[0002] During the pulping of the chips lignin and other substances
in the chips are dissolved into the pulping liquid.
[0003] In order to recover the pulping chemicals (as well as to
produce energy) the black liquor is concentrated and then sent to a
recovery boiler. The concentrated black liquor is combusted in the
recovery boiler and heat is extracted from the hot flue gases. The
sodium and sulfur compounds are recovered as sodium carbonate and
sodium sulfide. The sodium carbonate and sodium sulfide exit the
recovery boiler in a molten state and are dissolved in a water
solution (weak white liquor) thus forming green liquor.
[0004] The green liquor formed also contains small amounts of solid
material, known as green liquor dregs or green liquor sludge.
Because of the content of dregs in the green liquor, the green
liquor needs to be clarified with respect to these dregs. One way
to clarify the green liquor is by sedimentation another way is by
filtration.
[0005] The chemical recovery is very often a bottleneck in the
production of cellulose pulp and when the mills raise the
production there is a need of raising the capacity of the chemical
recovery process. Building larger apparatus for sedimentation of
the green liquor is, however, a costly and complicated choice since
the diameter the tank becomes large. Generally, clarification by
sedimentation results in regenerated cooking chemicals with a high
content of sludge. It is of great importance to keep the sludge
content a low level in the cooking chemicals, otherwise the sludge
will cause scaling and plugging in the pulping plant and the
evaporation plant, which leads to unwished and unplanned, costly
operation stops.
[0006] A general problem with filtration of green liquor is the low
filtering capacity because of the poor filterability of the green
liquor. The dregs in the green liquor form a dense cake on the
filtering layer and blind the filtering medium thus lowering the
filtering capacity even more. The filter cake must be removed and
thereby causing production stops leading to lowered productivity
and higher operating costs.
[0007] The document U.S. Pat. No. 5,3618,443 relates to
clarification of green liquor by falling-film filtration. The
filtering material is made of textile cloth. The apparatus
described comprises a pressurized vessel in which several filter
elements are mounted in a vertical position, or generally vertical
position, and the liquid to be filtered flows due to the
gravitational force along the filtering layers on the outside of
the filter elements. Due to pressure difference, caused by
pressurized gas, between outer and inner surfaces of the filter
elements, the filtrate penetrates the filter surface from the outer
side to the inner side of the filter surface of the elements and
reaches the filtrate channel surrounded by the filtering
layers.
[0008] A problem with prior art technique described above is the
low filtering efficiency leading to large equipments and with high
investment costs and a relative high energy consumption.
[0009] In order to solve the problems connected with clarification
of green liquor, a filtering technique using a new kind of filter
has been investigated. The filtering technique, hereinafter called
cross-flow filtration, is described in a final thesis at the Royal
Institute of Technology, Stockholm, by Fredrik Brostrom, 2007,
TRITA-CHE-report 2007:66 ISSN 1654-1081.
[0010] The cross flow-filter comprises a long tubular ceramic
membrane element perforated with several channels in its length
direction. The green liquor is transported into the channels and
while flowing through the channels the filtrate passes through the
porous membrane channel walls and flows in a radial direction from
the inside to the outside of the channels. The pore size of the
ceramic membrane used was 45 .mu.m.
[0011] However, in the thesis work the green liquor filtrate flow
decreased rapidly, within 1 day the flow was 1/6 of the initial
flow. This should according to the author be solved by controlling
the inlet flow to the cross-flow filter.
SUMMARY OF THE INVENTION
[0012] It is an object of the present invention to overcome the
drawbacks and disadvantages of the above described filtering
methods for clarification of green liquor.
[0013] Surprisingly, we have found that a filter pore size >0.10
.mu.m but significantly lower than the 45 .mu.m used in the thesis
work, preferably a filter pore size of 0.1-10 .mu.m, more preferred
0.1-5 .mu.m and most preferred 0.2-1.0 .mu.m and controlling the
green liquor filtrate flow prevents the observed capacity
decrease.
[0014] According to one aspect of the invention, since the flowing
suspension is forcibly caused to flow, one important feature of the
invention is the possibility to place the filter elements in any
direction and any inclination to the gravitational force since it
is a source of energy and not gravitational forces that forces the
suspension to flow. If the filtering arrangement is to be mounted
into an existing system and the space is limited the possibility to
place the filter elements in any direction/any inclination is a
great advantage. As shown in FIG. 1 the suspension to be filtered
is forced to flow upstream against the direction of the
gravitational force.
[0015] According to another aspect of the invention Reynold Number
is preferably higher than 10,000, but lower than 45,000, more
preferred 10,000-25,000, and most preferred 12,000-17,000.
[0016] According to yet another aspect of the invention the green
liquor filtrate flow should be less than 50% of the flowing
suspension, preferably less than 40% and most preferred less than
30% but not lower than 5%.
[0017] According to another aspect of the invention, the use of
converging cross-sectional area of the filter channels as an
alternative to cylindrical facilitates constant flow rate or
constant Reynold Number trough the filter channel.
[0018] According to another aspect of the invention, the flow of
filtrate is regulated by controlling the pressure of the green
liquor flowing in to the filter and the Reynolds Number is
regulated by controlling the flow of slurry out from the
filter.
[0019] According to still another aspect of the invention, the
cross-flow filter is connected in series with at least one
additional cross-flow filter in such a way that a partial flow of
the slurry from the first filter unit is led to an inlet of the
additional filter unit for an additional filtration.
[0020] According to yet another aspect of the invention, the
cross-flow filter is connected in parallel with at least one
additional cross-flow filter.
[0021] According to yet another aspect of the invention, the
cross-flow filter is connected to an already existing cleaning unit
for green liquor in order to boost the capacity of the existing
cleaning plant. The filter unit is arranged as a first filtering
step before final cleaning in the already existing cleaning
unit.
[0022] According to still another aspect of the invention, the
cross-flow filter is connected to an already existing cleaning unit
for green liquor in order to increase the degree of purity of the
already cleaned green liquor. The filter unit is arranged to filter
already cleaned green liquor.
[0023] According to yet another aspect of the invention, the cross
flow filter is a replacement for an existing cleaning unit for
green liquor,
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The invention will be described in more detail with
reference to the enclosed figures, in which:
[0025] FIG. 1 schematically shows a preferred embodiment of an
arrangement for cleaning green liquor using cross-flow filtration
for practising the invention;
[0026] FIG. 2 shows an alternative arrangement with two cross-flow
filters connected in series;
[0027] FIG. 3 shows an arrangement where the cross-flow filter is
used as a capacity booster of an existing green liquor cleaning
unit;
[0028] FIG. 4 shows an arrangement where an existing green liquor
cleaning unit is complemented with a cross-flow filter in order to
increase the degree of purification of the already cleaned green
liquor;
[0029] FIG. 5 shows a cross-sectional side view of A) a filter body
with a cylindrical filter channel and B) a filter body with a
converging filter channel;
[0030] FIG. 6 shows a front view of a filter element having several
filter bodies with filter channels as well as the end plate;
and
[0031] FIG. 7 shows a cross-sectional front view of a filter
element filled with supporting structure having several filter
channels
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] The cross-flow filter consists of a filter unit comprising
one or several filter elements. These filter elements are provided
with a plurality of filter bodies, each filter body comprising a
filter channel.
[0033] The filter body also comprises porous filter walls
surrounding the channels. Each filter body comprises a supporting
structure and in conjunction with the filter walls, which may be
integral therewith. In a preferred embodiment the filtering layer
is applied as a coating on the inside of the supporting structure.
The coating may in some preferred installations have filter pores
of a filter pore size of 0.2 to 1.0 micrometer. In other
installations other pore sizes may be more advantageous, but would
normally be within the range 0.1 to 10 micrometer.
[0034] The filter walls in this embodiment may be made of a ceramic
material. The interior diameter of the filter channels provided
within the filter elements is 1-10 mm and the length of the filter
channels is preferably 0.5-3 m, more preferred 0.7-2.2 m and most
preferred 0.8-1.5 m.
[0035] The green liquor is forcibly caused to flow into the filter
channels of the filter unit. The passage of the green liquor
through the filtering layer--the membrane--is forced by a pressure
difference between the in- and outside of the filter walls. The
pressure inside the channels is 0.2-2 bar higher than the pressure
outside the filter walls thereby forcing some of the green liquor
to pass through the inside of the channel walls to the outside of
the walls in a radial direction and perpendicular to the direction
of the channels.
[0036] Since the size of the pores of the membrane is of micrometer
scale the passage of the dregs through the membrane is prevented
and the dregs continue their flow in the remaining suspension
through the channels to the end opposite to the inlet and leaves
the filter body as a slurry.
[0037] Exemplary filtering arrangement according to the invention
illustrated in FIG. 1 comprises filter unit 4 comprising a filter
housing 10, in which one or more filter elements 12 is mounted. The
filter element 12 comprises longitudinal filter bodies, which
preferably are in the form of tubes and having filter channels
within the filter bodies. The filter channels have a first surface
on the inside of the filter walls surrounding the filter channels
and a second surface on the outside of the filter walls surrounding
the filter channels.
[0038] Chemicals to be recovered from the soda boiler is led via
conduit 25 into tank 40 and is dissolved in weak liquor, forming
green liquor. The green liquor is led via conduit 26 to a pump 61
and pumped via conduit 27 into tank 41. From tank 41 via conduit 20
the suspension to be filtered passes pump 62 and is pumped via
conduit 30 to an inlet 13 and further into the filter channels
within the filter element 12. A part of the suspension is forced to
pass the filtering layer from an inside/a first/inner surface to an
outside/a second/outer surface of the filtering layer and forming a
filtrate while the solids substantially remains in a residual part
of the suspension, forming a slurry. The cleaned filtrate is
collected in the filter housing 10 and the filtrate is led from the
filter housing 10 through conduit 21 to a collecting tank 42. The
cleaned filtrate (cleaned green liquor) is then led via conduit 28
to a pump 63 and is pumped via conduit 29 to the white liquor
preparation (not shown).
[0039] The slurry, containing the dregs, passes outlet 14 and a
partial flow of the slurry passes valve 50 and is recirculated via
conduit 24 to tank 41 while another partial flow of the slurry
passes valve 51 and is via conduit 23 led to a mud filter 70 for
dewatering and thereafter via conduit 24 to a landfill 80.
[0040] FIG. 2 is an alternative to the arrangement shown in FIG. 1
and shows two filter units 4, 4' connected in series. A partial
flow of the slurry received from the first filter unit 4 passes
control valve 51 and is led via conduit 23 to a tank 43. In the
tank 43 the incoming slurry from the first filter stage is
collected and mixed with the recirculated slurry from the second
filtering stage. The slurry is then led via conduit 200 to the pump
620 and is pumped via inlet 130 into the second filter unit 4' and
forced to flow through the filter channels. The filtrate is
collected in filter housing 100 and is led from the filter housing
100 via a conduit 210 to the collecting tank 42. The filtrate is
then led via conduit 28 to a pump 63 and is pumped via conduit 29
to the white liquor preparation (not shown).
[0041] The up-concentrated slurry passes outlet 140 and a partial
flow of the slurry passes valve 500 and is recirculated via conduit
240 to tank 43 and is mixed with the slurry received from the first
filtering stage via conduit 23.
[0042] Another partial flow of the up-concentrated slurry passes
valve 510 and is via conduit 230 led to a mud filter 70 for
dewatering and is afterwards led via conduit 24 to landfill 80.
[0043] The first filter unit 4 may for instance be used under
milder conditions, i.e. lower Reynolds Number which means decreased
cavitation within the filter channels leading to decreased wear of
the filter bodies in order to increase its life-time. Because the
slurry concentration is lower this filter will be less prone to
clogging and therefore a filter with a larger pore diameter may, in
some applications, beneficially be used in this position. In
addition, the need to stop the filtration to clean the filter is
decreased leading to higher availability and production. The second
filter unit 4' could then be used at tougher conditions due to the
higher dregs concentration, with higher flow velocities, smaller
pore diameters, and more frequent cleaning, compared to the first
filter unit 4
[0044] Alternatively, both filter units may be used under the same
conditions.
[0045] Another advantage of two filter units in series can be of
space-saving reasons. It may sometimes be easier to fit in two
smaller filter housings than one big into an already existing
chemical recovery unit in the mill. Of course, more than two filter
units can be connected in series in order to either get enough
capacity or to get enough degree of purification or of
space-limited reasons where one or two bigger filter units are
replaced by several smaller filter units which are easier to fit in
into the existing chemical recovery unit.
[0046] It is of course possible to connect one or more filter units
in parallel to the first filter unit, resulting in a "first set of
filter units in parallel", and to recirculate a partial flow of the
dregs slurry from this "first set of filter units in parallel"
while another partial flow of the dregs slurry from the "first set
of filter units in parallel" is led to a filter unit connected in
series with the "first set of filter units in parallel". This
alternative is not shown.
[0047] An alternative of having only one filter unit for further
filtering the dregs slurry from the "first set of filter units in
parallel" is to have two or several filter units connected in
parallel, resulting in a "second set of filter units in parallel".
Connecting a "first set of filter units in parallel" with "second
set of filter units in parallel" result in increased capacity of
the clarification process as well as in a very well purified green
liquor. It may also be easier to fit in several small filters than
one or two big filters into an existing chemical recovery plant, as
mentioned above. This alternative is not shown.
[0048] FIG. 3 shows a capacity booster arrangement. The filter unit
4 is used as a capacity booster of an existing green liquor
cleaning plant and the chemicals to be recovered from the soda
boiler is led via conduit 25 into tank 40 and is dissolved in weak
liquor, forming green liquor. A partial flow of the green liquor is
led via conduit 26 to a pump 61 and pumped via conduit 30 to the
inlet 13 of the filter elements 12 of the filter unit 4. The
cleaned filtrate is led via conduit 21 to the white liquor
preparation while the slurry passes outlet 14 and is via conduit 22
led to further purification in the green liquor clarifier 90.
[0049] Another partial flow of the green liquor is led from the
dissolving tank 40 via conduit 260 to a pump 611 and the green
liquor is pumped via conduit 270 to the green liquor clarifier 90.
The cleared green liquor is led away by conduit 210. The dregs
slurry leaves the green liquor clarifier 90 via conduit 220.
[0050] FIG. 3 schematically shows either a way to decrease the load
of the green liquid clarifier by cross-flow filtration of a partial
flow of the green liquor or a way to increase the capacity of the
chemical recovery unit by introducing a cross-flow filtration step
for taking care of a partial flow of the increasing amounts of
green liquor needed to be cleaned when the clarifier is already
working at its maximum capacity. The combined green liquor flow in
conduits 21 and 220 is led to the white liqour preparation and will
have a substantially reduced content of dregs due to the fact that
the liquor in conduit 21 is virtually free from dregs while the
decreased load on the clarifier 90 will decrease the dregs content
in conduit 220 as well.
[0051] Alternatively, all green liquor from tank 40 may be passed
through conduit 26 to the filter unit 4 to maintain a suitable flow
of liquor through the filter channels before passing the excess
liquor to the clarifier 90 through conduit 22.
[0052] FIG. 4 shows an arrangement for polishing of clarified green
liquor. An existing green liquor cleaning plant is complemented
with a filter unit 4 in order to increase the degree of
purification of the already cleaned green liquor. The clarified
green liquor from the clarifier 90 is led via conduit 210 to a
collecting tank 41 and is by pump 62 forced to flow via conduit 30
to the inlet 13 of the filter elements 12 of the filter unit 4 and
further in to the filter channels where the clarified green liquor
is filtrated for final separation of solids. The cleaned filtrate
is led away by conduit 21 and has a very high degree of
purification.
[0053] The recirculation of the partial flow of the slurry
(corresponding to conduit 24 in FIG. 1, conduits 24 and 240 in FIG.
2 and conduit 22 in FIG. 4) secures that Reynolds Number of the
flow is maintained during the flow along the filter channels.
Otherwise, in the case of no recirculation, the velocity of the
suspension will decrease along the passage of the filter channels,
which will lead to the build-up of a filter cake on the filtering
layers, which in turn will lower the filtering efficiency. The
recirculation also insures that the desired relationship between
the inlet flow and the filtrate flow is maintained at the
predetermined level.
[0054] FIG. 5A shows a cross-sectional side view a filter body 3 in
its length direction. The filter body 3 has a cylindrical shape and
comprises a supporting structure 31 and a filter channel 33. The
inside surface of the supporting structure 31 has a filtering layer
32 covering the whole inside surface of the supporting structure
31. The filtering layer 32 has a first/an inner surface 32A and a
second/outer surface side 32B and is provided with pores in the
range of about 0.2 to 1.0 micrometer.
[0055] The arrow 1 shows the direction of the incoming flowing
suspension. The arrow 2 shows the direction of a portion of the
flowing suspension passing through, firstly, the first/inner
surface of the filter layer 32A of the filtering layer 32, then
through the filtering layer 32 and further through the second/outer
surface 32B of the filtering layer 32 and still further through the
supporting structure 31 to the outside of the supporting structure
31. The portion of the flowing suspension has now been filtrated
thus forming a filtrate consisting of cleaned green liquor.
[0056] FIG. 5B shows a cross-sectional side view of a filter body 3
with a converging filter channel 33. An alternative to cylindrical
filter channels 33 with a circular cross-sectional area would be to
use filter channels 33 having an converging cross-sectional area.
Suspensions flowing in filter channels 33 with a converging
cross-sectional area will maintain its Reynolds Number or flow rate
during the flow within the entire length of the filter channels
despite the fact that the volumetric flow rate within the channels
decreases as a result of the penetration of suspension through the
filter surfaces. The suspension will maintain its velocity or
Reynolds Number and no filter cake will be built up on the filter
surface thanks to the maintained velocity.
[0057] FIG. 6 shows a front view of a filter element 12 having
several filter bodies 3. The filter body 3 comprises a supporting
structure 31 and a filter channel 33. The inside surface of the
supporting structure 31 has a filtering layer 32 covering the whole
inside surface of the supporting structure 31. The filtering layer
32 has a first/inner surface 32A and a second/outer surface
32B.
[0058] FIG. 6 also shows an end plate 34 covering the whole end
area of the filter element 12 except for the filter channels 33.
The end plate 34 is fixed to the end of the filter bodies 3 and has
through bores of the same diameter as the diameter of the filter
channels 33. The flowing suspension to be filtered reaches the end
plate 34 and passes through the through bores of the end plate 34
and flows into the filter channels 33 whereby a portion of the
flowing suspension is to be filtered while another portion of the
flowing suspension continues to flows in the filter channel 33. The
end plate 34 steers the incoming flowing suspension to the filter
channels 33. Another feature of the end plate 34 is that is that it
supports the filter bodies 3 and keeps them fixed in their
positions.
[0059] In FIG. 6 the filter bodies 3 are placed so tightly to each
other that each filter body 3 is in contact with neighbouring
filter bodies 3. The filter bodies 3 may be placed so close to each
other that the wall 31 of a filter channel also is part of the wall
of the neighbouring filter bodies.
[0060] The filter bodies shown in FIG. 6 are cylindrical thus
giving rise to vacant spaces between the filter bodies.
[0061] It is understood that the filter bodies 3 do not have to be
placed so tightly to each other that each filter body 3 is in
contact with the neighbouring filter bodies 3. Thanks to the end
plate 34 which supports and fixes the filter bodies 3 the filter
bodies within the filter element 12 could be sparsely placed thus
not being in contact with each other.
[0062] FIG. 7 shows cross-sectional front view of a filter element
12. The filter element 12 has a cylindrical form and a circular
cross-sectional area. In this embodiment the filter element 12 is
homogeneously filled with supporting structure 31. The supporting
structure 31 is perforated in its length direction with filter
bodies 3. The filter body 3 comprises a filter channel 33. The
inside surface of the filter body 3 is covered its whole surface
with a filtering layer 32 which has a first filtering layer 32A and
a second filtering layer 32B. The flowing suspension to be filtered
flows inside the filter channels 33 and a portion of the flowing
suspension passes through, firstly, the first/inner surface of the
filter layer 32A of the filtering layer 32, then through the
filtering layer 32 and further through the second/outer surface 32B
of the filtering layer 32 and still further through the supporting
structure 31 to the outside of the supporting structure 31 and to
the outside of the filter element 12 to be collected in the filter
housing 10, 100, shown in FIGS. 1 and 2.
[0063] In accordance with an embodiment of the invention the green
liquor filtrate flow should be a certain predetermined fraction of
the total flow of green liquor through the filter channels, at the
same time the total flow of green liquor through the filter has to
be controlled in order to maintain the desired Reynolds Number.
With reference to FIG. 3, the flow and pressure of the filtrate in
conduit 21 is measured together with the pressure in conduit 30 and
the flow in conduit 22. The flow of filtrate is set by mill
conditions that require a certain production of purified green
liquor, the pump 61 is controlled to provide such a pressure in
conduit 30 that this requirement is met. At the same time the flow
in conduit 22 is set with a valve (not shown), so that the desired
Reynolds Number is maintained in the filter bodies. The pressure
difference between conduits 21 and 30 provide information on the
condition of the filter elements and is used to determine when the
unit has to be shut down for cleaning.
[0064] Reynolds Number is used to characterize different flow
regimes, such as laminar or turbulent flow: laminar flow occurs at
low Reynolds numbers, where viscous forces are dominant, and is
characterized by smooth, constant fluid motion, while turbulent
flow occurs at high Reynolds numbers and is dominated by inertial
forces, which tend to produce random eddies, vortices and other
flow fluctuations.
[0065] For flow in a pipe or tube, the Reynolds number is generally
defined as:
Re = .rho. VD .mu. = VD v = QD vA ##EQU00001##
where:
[0066] V is the mean fluid velocity in (SI units: m/s)
[0067] D is the diameter (m)
[0068] .mu. is the dynamic viscosity of the fluid (Pas or
Ns/m.sup.2)
[0069] .nu. is the kinematic viscosity (.nu.=.mu./.rho.)
(m.sup.2/s)
[0070] .rho. is the density of the fluid (kg/m.sup.3)
[0071] Q is the volumetric flow rate (m.sup.3/s)
[0072] A is the channel cross-sectional area (m.sup.2)
As will be understood by those skilled in the present field of art,
numerous changes and modifications may be made to the above
described and other embodiments of the present invention, without
departing from its scope as defined in the appending claims.
[0073] For example, the filtration of the flowing suspension could
of course be arranged to take place in the opposite direction
meaning that the suspension flows on the outside of the filter
elements and that the filtrate passes the filter wall of the
elements from the outside to the inside, meaning that the filtrate
will flow on the inside the filter channels within the filter
elements.
[0074] Instead of a pump 61, 62 or 620 for forcibly causing the
suspension to flow into the filter, another arrangement could be
used, such as one pump to remove filtrate from the filter and
another pump to remove the slurry, or the tank could be pressurised
causing the liquor to flow through the filter or the tank 40 may be
elevated so that gravity provides the necessary pressure
differential. The pump can be hydraulic or electrically driven.
[0075] The filter elements 12, may have other forms such as plate
and frame, capillaries, tubes, lamellas, or discs etc.
[0076] As shown in FIG. 5B, the filter body 3 has a converging form
but the filter body 3 could of course have a cylindrical elongation
while the filter channel could be converging thus having a
converging cross-sectional area.
[0077] The cross-sectional area of the filter bodies and of the
filter channels could of course have other forms such as triangular
or rectangular forms. If using filter bodies with other forms than
cylindrical, the volume of the vacant spaces between the filter
bodies in the filter elements will vary and depend on the form of
the filter bodies.
[0078] Other porous materials than ceramic materials could be used
as the support structure 31, e.g different polymers and
graphite.
[0079] Regarding the filtering layer 32, other porous materials
could be used, e.g polymers.
[0080] It is understood that the supporting structure 31 of the
filter body 3 in another embodiment could have the functions of
both being a supporting structure as well as being the filtering
layer itself. No extra covering with filtering properties on the
inside surface of the supporting structure 31 is then needed.
[0081] FIGS. 3 and 4 show an existing green liquor cleaning plant
complemented with a cross-flow filter. The clarifier 90 in the
existing green liquor cleaning plant is an example of common prior
art technique for clarifying green liquor. It is understood that
already existing green liquor cleaning plants comprising other
prior art technique, such as conventional green liquor filters,
also can be complemented with a cross-flow filter.
[0082] It is also understood that there of course may exist other
solutions within the scope of the invention on how to regulate the
flow of the filtrate and the Reynolds Number.
[0083] The cross-flow filter can be used for purifying other
suspensions within the kraft pulp mills, e.g. white liquor.
[0084] The cross flow filter may be a replacement for an existing
cleaning unit for green liquor.
[0085] Following advantages are achieved with the present invention
when applying it on the filtration of green liquor or similar
suspensions:
[0086] The filtering process is a continuous process with no build
up of a filter cake which are desirable features.
[0087] It is also a very effective process leading to a very high
separation degree of dregs, up to almost 100%.
[0088] The filtrated green liquor is almost free from slurry.
[0089] Under normal operating conditions the characteristic green
color of the green liquor is removed with the dregs thereby
simplifying the identification both of disturbances in the
filtration process and in the recovery furnace.
[0090] The investment costs for the cross-flow filtration equipment
is only a fraction of the investment costs for conventional
cleaning systems.
[0091] The space required is much smaller than the space required
for the sedimentation tanks
[0092] There is no contact with the surrounding air, nor is
pressurized air used in the equipment, thereby minimizing
oxidation/degradation of the valuable sulfide content of the green
liquor.
[0093] The closed system with no contact with surrounding air or
use of vacuum means that the temperature of the green liquor is
maintained at a high level.
[0094] The modular design of the filters facilitates a incremental
capacity increase with minimal investment cost.
[0095] The simple system with few moving parts means less labor is
needed for oversight and maintenance.
[0096] Benefits due to less particles in the green liquor: [0097]
Less lime make-up or decreased content of inerts in the lime at the
same make-up rate. [0098] Less dregs carryover improves
clarification of white liquor and improves mud dewatering and lower
energy consumption. [0099] Efficient removal of non-process
elements for minimum operation cost and low landfill volumes.
* * * * *