Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTINUOUS DIGESTER
a. Background of the Invention
The present invention relates generally to
the art of wood pulp digesters, and more
particularly to a continuous digester.
b. Background Art
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 F). The
mixture 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 chips 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.
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
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.
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 -
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:
i. an initial inlet means to initially
introduce processing liquid into the
pressure vessel at an 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
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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;
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;
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
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.
said liquid inlet means and said
liquid outlet means.
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.
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.
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.
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.
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
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recovered alcohol back to liquid flow means to be
recirculated into the liquid flow means.
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.
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.
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
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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.
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. Screenmeans are
located at longitudinally spaced locations_at the
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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.
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.
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.
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
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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.
In preferred form, the pressure vessel is
aligned so its major alignment component is
horizontal.
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.
Other details of the present invention will
become apparent from the following detailed
description.
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~
Brief Description of the Drawings
Figure 1 is a schematic drawing illustrating
the main components and system of a prior art
digesting system;
Figure 2 is a somewhat schematic side
elevational view illustrating the digesting system
of the present invention;
Figure 3 is a transverse sectional view
illustrating a typical cross section of the
digester of the present invention;
Figure 4 is a transverse sectional view
showing a liquid inflow module used in the
digester of the present invention;
Figure 5 is a sectional view taken along line
5-5 of Figure 4;
Figure 6 is a transverse sectional view,
similar to Figure 4, showing a liquid outflow
module of the present invention;
Figure 7 is an isometric view of one of the
hydraulic actuators used in the liquid outflow and
liquid inflow modules of the present invention;
Figure 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.
Figure 9 is a transverse sectional view of
the pulp diluting module of the digester of the
present invention;
Figure 10 is a transverse sectional view
showing the liquid outlet unit for the wood chip
inflow section of the present invention;
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Figures 11 through 14 are semi-schematic
views, similar to Figure 2, showing in an enlarged scale four sections of the
digesting system
illustrated in Figure 2;
Figure 15 is a view similar to Figure 4,
showing a cross sectional configuration of a
modified form of a liquid inflow module;
Figure 16 is a view similar to Figure 6,
showing a modified version of the outflow module;
Figure 17 is a view similar to Figure 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;
Figure 18 is a view similar to Figure 17
showing an outlet module utilizing the second
embodiment of the present invention;
Figures 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 Figure 19, and continuing
all the way to the opposite end of the digester in
Figure 29;
Figure 30 is a schematic side elevational
view of a third embodiment of the present
invention;
Figures 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;
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Figure 32 is a sectional view of an inlet
port section of one of the cross flow rings of the
present invention;
Figure 33 is a longitudinal sectional view
showing flow patterns in Displacement Wash Zone A
Figure 34 is a view similar to Figure 30, but
showing additionally the various cross flow rings
and their angular orientation;
Figure 35 is a view similar to Figure 30, but
drawn to an enlarged scale, and only showing the
left half of the digesting system;
Figure 36 is a view similar to Figure 30, but
drawn to an enlarged scale, and showing the right
half of the digesting system of Figure 30;
Figures 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 Figure 37, the
middle portion being shown in 38, and the upstream
portion being shown in Figure 39;
Figure 40 is a view similar to Figure 32,
showing a modified form of a cross flow ring;
Figure 41 is a view similar to 40, but
showing a yet further modified version of the
cross flow ring;
Figure 42 is a table showing various values,
namely temperature, alcohol contents, and
dissolved solid contents, at various locations in
, the digester;
Figure 43 is a semi-schematic view showing a
modification of the cooking zone 1 shown in Figure
30;
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Figure 44 is a schematic view illustrating
the evaporation and recovery system of the present
invention;
Figure 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.
.
,
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. Description of the Preferred Emboliments
l. Typical Prior_Art Digesting Process
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 Figure 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.
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 B. As seen in Figure 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
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cooking liquor can be controlled by a magnetic
flow meter.
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.
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.
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 C
(to 135 ) are controlled in the diffusion zone by
an auxiliary wash liquor circulation and heater
system.
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
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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.
A. FIRST MBODIMEATT
2. The Overall Structure and Operation of a
First Embodiment of the Digester
In this section there will be a brief
description of the overall construction and
operation ofthe 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.
To describe the first embodiment of the
present invention, reference is first made to
Figures 2 and 3. Figure 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.
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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.
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.
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
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place adjacent to the wood chip inlet end 104 of
ti the digester 102.
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.
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.
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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.
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.
3. Typical Cross Sectional ConfiQuration of
the Digester 102 --
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
Figure 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 C or higher.
This vessel comprises a cylindrical containing
wall ill which extends the entire length of the
digester 102, and it is enclosed at the ends.
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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.
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).
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 Figure 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.
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
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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.
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 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.
As indicated above, what is shown in Figure 3
is a typical cross section of the digester 102.
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These sections can be made in modules or units of,
for example, four feet long, or as longer
sections. The module shown in Figure 3 is given
the general numerical designation 146.
4. Fluid Inlet Unit of the Digester 102
Reference is made to Figure 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 Figure 4 is
substantially the same as shown in Figure 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.
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 screen152, 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.
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
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reciprocating manner through 1800 of rotation.
Thus, the actuator 164 will rotate the screen
assembly 158 180 in one direction, then 180 in
the opposite direction.
There is provided a longitudinally aligned
wiper blade 168 (Figure 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 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.
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.
There are also partial bracing plates 178,
but the inner edge 180 of these plates 178 is 30 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
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assembly 158 and the adjacent wall portion 186 of
the vessel 110.
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.
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
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 thedigester, faster at the forward
end and slower at the rear end of the digester).
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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.
5. Fluid Outlet Unit of the Digester 102
Reference is made to Figure 6, which shows a
module 190 which is constructed substantially
identically to the module 148 shown in Figure 4,
except that the module 190 is inverted 180
relative to the module 148 of Figure 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
Figure 4 will be given like numerical designations
with an "a" suffix distinguishing those of the
module 190 shown in Figure 6.
Thus, there is a screen assembly 158a
comprising the screen 152a, the support arms 154a,
and the hub 156a. There is a rotary actuator Z64a
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.
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 30 exists within the
digesting chamber 118.
Also, since the flow from the digesting
chamber 118 is outwardly through the screen 152a,
there may be some tendency for the wood chips
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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.
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.
To describe the rotary actuators 164 and
164a, reference is made to Figure 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
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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 paths of revolution.
As described above, this causes the 1800
alternating rotation of the two screen assemblies
152 and 152a.
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.
6. Flow Patterns in Displacement Zones
Reference is now made to Figure 8 which
illustrates a typical displacement flow pattern
between an inlet module 148 and an outlet
module 190.
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 12.5a, which means
that there are two parts liquid within the fibers
and five parts liquid surrounding the fibers.
In the right hand part of Figure 8, the flow
of the liquor moving into the displacement zone is
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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.
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.
With further reference to Figure 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.
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.
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.
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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.
7. Pulp Diluting Unit of the Digester
Reference is made to Figure 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
bl' suffix distinguishing those of the modular
unit 220.
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 12 1/2a consistency in the
digester 102) to about 2 s to 496 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
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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.
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 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.
8. Liquid Outlet Unit For The Wood
Chip Inflow
Reference is made to Figure 10 which shows a
module 230 which in terms of structure is
substantially identical to the module 220 of
Figure 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.
When the wood chips are introduced into the
rear end 104 of the digester 102, they are first
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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.
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.
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.
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.
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9. Overall Operati.on of the Digesting
System 100
Reference is now made back to Figure 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.
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
1.s 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.
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 s. 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).
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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.
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 Figure 2 and also with
reference to Figures 11 through 14 which show
respective portions of the digester system 100 of
Figure 2, but drawn to an enlarged scale.
10. OT)eration at the Dilution Zone and the
Hot Wash Zone
Reference is first made to Figure 2 and
Figure 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
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33
of filtrate from the pulp during the final
displacement washing in the washer 108.
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.
In the schematic drawing of Figure 2 and in
Figure 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 Figure 9.
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 118 by means of
the module 220 and then flows with the pulp fiber
through the outlet nozzle 244 (blow nozzle) into
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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.
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.
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
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downwardly to flow outwardly through the screen
150a of the module 190-2.
The action of these modules 148-1 and 2 and
190-1 and 2 have thus accomplished a two stage hot
5 wash, and this is accordingly designated in Figure
2 as the "hot wash" zone of the digester 102.
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
10 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.
15 11. Operation of the Diffusion Wash Zone,
High Heat Alcohol Wash Zone, Cooking Zone 3
and Displacement - Zone A
At a location between the pump 252 and the
module 148-3, there is shown schematically at 254
20 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
25 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.
There are at that high heat alcohol wash zone
30 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
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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".
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 Figure 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.
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-
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.
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.
12. Operation at Cooking Zone 2,
Displacement Zone B, Cooking Zone 1
and Displacement Zone C
Reference is made to Figures 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
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38
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".
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 module 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 267 which optionally can be
used to add ethanol, through a heat exchanger 268
to the unit 148-9, and after passing through the
units 190-9, 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.
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13. Operation of the Impregnation Zone.
Initial Heating Zone, Wood Chip and
Liquid Recirculation and Black Liquor
Removal
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.
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 Figure 10) to
serve as make-up liquor for the liquid which
carries the wood chips into the digester-
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
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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.
5 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
10 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.
14. Possible Modifications
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 Figures 15 and 16. The components
shown in Figure 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 Figure
15.
In Figure 15, instead of providing only one
flow inlet assembly 150d as shown in Figure 4,
there are provided two such flow inlet assemblies
150d and likewise two inlet conduits 182d.
The other modification as shown in Figure 16
shows the liquid flow outlet module. Components
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of this modification in Figure 16 which are
similar to prior components will be given like
numerical designations, with an "e" suffix
distinguishing those shown in Figure 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.
It is believed that the mode of operation of
these modules Figures 15 and 16 are readily
apparent from the prior description. The flow
from the assemblies 150d in Figure 15 is at a
downward and lateral slant in a downstream
direction.
B. SECOND EMBODIMENT
15. Introduction to the Second Embodiment
This second embodiment of the_present
invention is shown in Figures 17 through 29, which
can be described as follows.
In the second embodiment there is a further
modified version of an inlet module as shown in
Figure 17, and an outlet module shown in Figure
18.
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 Figure 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.
In Figure 18, there is shown a fluid outlet
unit or module 190g having an outlet assembly 150g
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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
Figures 17 and 18 is incorporated in the second
embodiment which is described later herein.
Figures 19 through 29 are semi-schematic
views, similar to Figures 11 through 14, showing
11 different sections of a second embodiment of
the digester of the present invention.
The basic operation of this second embodiment
is in many respects substantially similar to the
operation of the first embodiment as shown in
Figures 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.
In Figures 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
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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 Figures 19 - 29.
16. Descrigtion of the Second Embodiment
In Figure 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 Figure 11,
indicated at 242 are not present in this second
embodiment.
In the right half of Figure 19, in the entire
showing of Figure 20 and in the left part of
Figure 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 Figures 17 and
18. Thus, there are eight washing stages. it
will be noted from the processing information on
Figures 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.
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It was indicated earlier herein that the
modified arrangement of the flow inlet and flow
outlet modules as shown in Figures 17 and 18 is
utilized in the second embodiment. The
arrangement shown in Figures 17 and 18 is utilized
in an alternating pattern with the inflow and
outflow modules shown in Figures 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.
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
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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.
In the right hand portion of Figure 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 85o 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 85a alcohol is added, together with
the flow from location A and a comparable amount
of the filtrate is discharged at the location "B11
to be recirculated into the digester at a further
upstream location.
In Figure 22, there is shown cooking zone 3,
In this cooking zone, the cooking liquor around
and inside the pulp enters the cooking zone with
10o dry solids (see the left part of Figure 23),
and exits from cooking zone 3 at 12% dry solids
(see the left part of Figure 21) -
In the left part of Figure 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.
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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".
In the right hand part of Figure 23, there is
shown the downstream portion of cooking zone 2,
and the rest of cooking zone 2 is shown in Figure
24. It will be noted that at the entry portion of
cooking zone 2 (see the left hand part of-Figure
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 Figure 23).
In the left hand side of Figure 25, there is
a displacement wash zone at the end of the cooking
zone 1.
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".
In the right hand part of Figure 25 and in
Figure 26, there is shown cooking zone 1. it will
be noted that the pulp slurry entering cooking
zone 1(see the left hand of Figure 27) is at 11%-
dry solids, and at the downstream end of cooking
zone 1 the dissolved solids is at 14%.
In Figures 27 and 28, and also in the left
part of Figure 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
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outlet modules recirculating the filtrate in an
upstream direction. As can be seen in the middle
of Figure 27, the outflow from locations "C" and
"D" further downstream are added to the flow of
S filtrate. It can be seen from observing the
operating values shown in Figures 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 Figure 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.
with reference to Figure 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.
In the right part of Figure 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.
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
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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.
C. THIRD EMBODIMENT OF THE INVENTION
17- Introduction To The Third Embodiment
The third embodiment is shown in Figures 30
through 41. In this section, there will first be
an overall description of the system of the third
embodiment with reference to Figure 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:
a. the cross flow rings and their mode of
operation;
b. the wood chip feed assembly, and filtrate
recirculation zone;
c. the impregnation zone and its mode of-
operation.
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.
To give an overview of this third embodiment,
reference is made first to Figure 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
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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.
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. patent 5,482,594 entitled "LIQUID REMOVAL
APPARATUS AND METHOD FOR WOOD PULP , issued on
January 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 20o to 300),
=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.
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
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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.
In Figure 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 Figure 43.
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.
The digester 402 comprises, in terms of
function, seven sections, which will be identified
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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:
a. dilution zone;
b. displacement wash zone A
c. cooking zone 2
d. displacement wash zone B
e. cooking zone 1
f. impregnation zone
g. wood chip filtrate recirculation zone
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.
18. Cross Flow Ring System
This cross flow ring system will be described
with reference to Figure 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
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laterally across the digesting cha.mber 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.
The cross flow function in the present
invention is accomplished by sets or sections of
cross flow rings 430 (Figure 32) positioned around
the outside of the cylindrical sidewall 411 of the
digester pressure vessel 410- Reference is first
made to Figures 31 and 32. It can be seen in
Figure 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.
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 Figures 31A and
31B, there is an inflow chamber section 440a which
has an arcuate length of about 900, and an outflow
chamber section 440b which has an arcuate length
of about 180 . At the inflow chamber 440a, the 30 sidewall 411 is formed with
several (four as shown
herein) slots 442 which slant relative to the
longitudinal center axis 444 (see Figure 33) of
the digester 402 at an angle of about 45 , and
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which extend circumferentially in a 900 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 450
relative to the longitudinal axis 444 (see Figure
33). However, the arc length of the slots 442b is
180 . This 45 slant angle could be varied, for
example, between 20 to 80 -
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.
As can be seen in Figures 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 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
Figure 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 Figure 31B, the
inflow passageway 440a has been rotated to the
left 90 , with the opposite outflow passageway
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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.
At this point, a clarification should be made
with reference to the flow lines shown in Figures
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.
To follow this line of thought further,
reference is now made to Figure 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
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indicated at 430-1 (out). The remaining flow
lines 454 proceed in a downstream direction toward
the dilution zone.
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 430-
2 (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 washstation 552) in
displacement wash zone "A". (This can be seen by
examining Figure 37).
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
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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.
It should be pointed out that Figure 33 is
not drawn with total accuracy, since the inlet
ports 430-1 through 4 are shown in Figure 33 as
having the same angular orientation. Actually,
(as illustrated in Figures 31A and 31B), the
angular position of the rings 430 is in an -
alternating pattern so that if one inlet port 446a 30 is at a lower right hand
position (as shown in
Figure 31A) the next adjacent inlet port 446a is (as shown in Figure 31B) at
the lower left hand
position. Therefore, with reference to Figure
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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 900 from the diametrically opposed
position.
19. Wood Chip Feed Assembly and Filtrate
Recirculating Zone
The wood chip feed assembly 422 can best be
explained with reference to Figure 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
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460 to the evaporation and recovery system 418.
(See Figure 36).
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.
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.
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.
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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.
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 product (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
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makeup liquid is supplied from the upstream end of
the impregnation zone. Thus, there is shown in
Figure 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.
20. The Imgregnation Zone
In the impregnationzone (see Figure 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.
The filtrate from the alcohol displacement
wash station of displacement wash zone "B" (See
Figure 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
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w
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.
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.
The heat exchanger 512 brings the temperature
of the filtrate up to 205 C so that the
temperature of the filtrate flowing downstream
through the cooking zone 1 is at a sufficiently
high temperature, for example 195 C. The
remaining two heat exchangers 514 and 516 are used
to adjust the filtrate temperature for the two
impregnation sections 502 and 504.
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
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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.
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.
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 30 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
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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.
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
Figure 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.
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.
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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.
Delignification is accomplished mainly in the
cooking zones 1 an 2. Thus, the liquor that is
directed from the end of cooking zone 1 and is
directed through the line to the evaporation and
recovery system 418 provides a high percentage of
the lignin that is extracted from the wood chips.
21. Overall Operation of the Third
Embodiment
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
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of the digester where there is the dilution zone
and the proceeding through the following zones in
sequence to and including cooking zone 1. As this
is done, reference will be made to the other
5 components related to the operation in those
zones.
Reference is first made to Figure 30. As
previously described in Section 17, a pulp slurry
is discharged through the blow valve 416 and
10 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
15 filtrate is directed into the dilatation 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
20 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
25 blow tank to dilute the pulp slurry flowing into
the washer 408.
As indicated previously, the washer 408 is
desirably the same as (or quite similar to) the
washer described in U.S. Patent 5,482,594. In the
30 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
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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.
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
glow tank 407 to evaporation and recovery system
418.
With reference to Figure 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
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washer 408 travels through a line 532 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.
The components in the five zones beginning
from the dilution zone through to cooking zone 1
will now be described under appropriate headings.
a. The Dilution Zone
With reference to Figure 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 Figure 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
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as, or similar to, the agitator as shown in Figure
39 in the wood chip feed assembly. 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 Figure 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.
b. Displacement Wash Zone A
Reference is made to Figure 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
Figure 37, only the first two cross flow rings
have been given the designation of 430 (to keep
Figure 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 30 another, and two of
these pumps have been
designated at 554 in Figure 37 (again to keep
Figure 37 from becoming too cluttered), and the
other pumps have not been given numerical
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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.
With reference to Figure 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 Figure 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
Figures 31a and 31b, this will not be described
further in this portion of the text.
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 C. This is to maintain
the blowout temperature of the pulp/filtrate
mixture that passes out the blow valve at about
74 C.
Also, as indicated previously, the outflow
from the fourth cross flow ring (designated 430-4
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in Figure 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.
Also, it can be seen in Figure 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.
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 C.
The tank 420 derives its alcohol from the
evaporation and recovery plant, as does the
aforementioned tank 421.
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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 548, as
illustrate in Figure 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).
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 Figure 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 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 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 C,
with an alcohol content of 15% and dry solids of
0.06. The discharge of pulp from the washier 408
is at 50 C, with an alcohol content of 0.2%, and
a dissolved solid content of 0.1%. With the
assumed values given previously in this text, this
pulp would be at the 20% consistency.
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However, it is within the capability of the
washer 408, as described in the U.S. Patent
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).
Reference is now made back to the table in
Figure 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 C, has an alcohol
content of 57%-, and a dry solids content of 4.50.
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 C), a much lower alcohol content
(15!k), and a much smaller dissolved solid content
(0.06 s). Thus, it can be seen that the net
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filtrate/liquor flow that is recirculated in an
upstream direction increases in dry solids content
substantially (0.06%- to 4.50) and also increases
in alcohol content (15% to 570). Further, there
is a substantial rise in temperature in an
upstream counterflow direction (68 C to 188 C).
The plurality of wash sections provides fast
cooling of the liquor to cause the digesting
process to substantially cease.
With further reference to lines 3 through 8
of Figure 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 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 5.99. to 0.070, and the alcohol
content has dropped from 59o 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
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evaporation and recovery plant 418. Further,
there is a net upstream flow of the digesting
alcohol.
c. Cooking Zone 2
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 C. (As can be seen in Figure 30, there are a
couple of heat exchangers 570 which maintain this
cooking temperatures).
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 (0.30).
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.
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d. Displacement Wash Zone B
Reference is made to Figure 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 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 Figure 36).
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 C. The outflow from
the third filtrate displacement wash section 574
is from its furthest upstream ring of that section
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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 Figure 36).
The outflow from the most upstream cross flow
ring of the second filtrate displacement wash zone
552 (see Figure 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_1g.
e. Cooking Zone 1
Reference is made to Figures 31 and 36- The
flow of the pulp/filtrate leaving the impregnation
zone has a dry solids content of about 3.3s, and
this dry solids content increases to about 10_40
as the flow is exiting from the cooking zone 1.
The temperature in cooking zone 1 is maintained at
about 195 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
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flow through cooking zone 1 would be between 30 to
60 minutes.
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 Figure 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 9o 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 Figure
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 Figure 42 where the
percentage of dry solids is given for the
discharge of the washer 408.
f. Other Modifications and Further Comments
It is apparent that various modifications
could be made in the present invention without
departing from the basic teachings thereof. For
example, Figure 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
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78
is shown in Figure 32 will be given like numerical
designations, with a prime (') designation
distinguishing those of the arrangement in Figure
40. As shown in Figure 40, the ring 430' has
simply a circumferential curved plate 432' without
the flanges 434 as shown in Figure 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 Figure 32, and
an opening is drilled in the surrounding wall 432'
to receive the fitting 445a'.
Figure 41 shows substantially the same
arrangement as shown in Figure 40, but shows a
further modification of the ring 430. The
components of this further modification to Figure
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 Figure 41.
The ring 430" has the chamber 440"??? cut out
of the wall 411", as in Figure 40. However,
instead of having the slots 440, there are a
plurality of slanted bore holes 5698 formed in the
side wall 411'. Thus, the flow is through these
bore holes 598 on one side of the ring 598", and
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out through similar bore holes 598 in the opposite
side of the ring 598".
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 the 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.
22. The Evaporation and Recovery System
As a further modification, as shown in Figure
43, the cooking zone 1 has been divided into
cooking zone lA and cooking zone 1B. The liquor
is extracted at the end of each of cooking zones
lA 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.
Figure 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
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421. The system 418 removes substantially all of
the alcohol, and a substantial amount of the water
from the pulping liquo_rs directed into the system
418.
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-li and E-2i.
These two evaporators E-li 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.
In Figure 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.
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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-li. The liquid is
recirculated by the pump P-1 upwardly through a
flow line and back into the tank of E-li, and into
the upper end of a heat exchanger 712. Also, a
portion from the flow from the pump P-i is
conveyed by the pump P-2 into a first separator S-
1. This separator S-i can be one of a number of
different types of separators. The portion of the
liquor which is extracted in the separator S-i 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.
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-lc and
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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-lc and E-2c in substantially the same
manner as described previously with respect to the
evaporators of the first section, namely E-li 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.
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 by-
products, 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 by-
product 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
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suspension. This better enables the lignin to be
spun off by centrifugal force to free most of the
extraneous materials (oils, etc.).
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.
While the evaporating system shown in Figure
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.
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]] 726 flows in a
recirculating pattern through all three of the
heat exchangers 736 in series, which recirculating
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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 S. 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.
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
evaporating 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-li and E-lc,
The vapor collected in the evaporators E-li
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 begiizs at the right hand part of -Figure
44 and extends all the way to the left hand side =
of Figure 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
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"A" therein, and picks up again at the left hand
side of the figure 750 where there is another
circle with the designation "A therein-
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-lc.
To review the overall operation of the
recovery system of Figure 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-li and E-2i, and from the
second evaporator section (evaporators E-lc 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.
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The condensate from the heat exchangers in
the seven evaporating units can be treated in a
conventional manner.
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. =
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.
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-li 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.
23. Final Comments -
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
Figure 39), the wood chip/pulp/filtrate flow
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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 448.
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.
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.
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As indicated above, there are various
modifications which can be made to the present
invention without departing from the basic
teachings thereof.
