Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
TITLE
"TOTALLY SUBMERGED PRESSURIZED PULP WASHER
AND METHOD OF OPERATION THEREOF"
Field of the Invention:
The present invention relates generally to pulp washing technology. More
specifically, the present invention is a new and improved liquid filled
pressurized pulp
washer and methods of operation thereof.
BACKGROUND OF THE IN1~'ENTION
An example of a currently-available liquid filled pressurized pulp washer is
shown
and described in U.S. Patent No. 4,827,741 to Luthi. In such liquid filled
pressurized
washers, free air is precluded from the washer vat and foaming of the wash
liquor in the
vat and other undesirable chemical reactions are avoided. One primary
disadvantage to
the currently available liquid filled pressurized washers like the one
disclosed in the '741
patent is their susceptibility to a phenomenon known as "mat collapse".
Specifically, in pressurized pulp washers, a pulp mat is formed from a pulp
slurry by
;gassing the slurry between a rotating permeable drum and a containing baffle.
The
pressure on the outside of the drum is higher than the pressure on the inside
of the drum
and filtrate flows from the slurry into the drum to form the mat on the
outside of the drum.
In actuality, the mat is a liquid filled fibrous lattice structure that is
permeable.
Maintaining the permeability of the mat structure is essential because the mat
must
be washed after formation so as to remove as much black liquor from the pulp
as possible
during the washing process. If the washer is operated at too high of a
pressure differential
across the mat, the mat can structurally collapse, compress, and become
substantially
impermeable. Collapsing and compressing of the mat substantially precludes the
flow of
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wash liquid through the collapsed interstices of the fibrous mat and through
the densified
impermeable mat skin of collapsed fiber that is created on the surface of the
drum when
a mat collapses. As a result, the vat fills with incoming thickened slurry and
the washing
operation becomes inoperable.
In an optimum operation) the flow of wash liquid through the interstices of
the fibrous
mat approaches a maximum flowrate developing an optimum pressure resistance
and at
a maximum pressure differential ("maximum ep"). At the point where incipient
mat collapse
occurs, the pressure differential across the mat has exceeded the maximum op
and has
reached a critical pressure differential ("critical op") beyond which plugging
occurs.
Because the typical mat operating pressure differential and the critical
pressure
differential are relatively small relative to the washer operating pressure,
variations and/or
instantaneous increases in either the wash liquid pressure andlor surges of
wash liquid
flow andlor surges of pulp slurry feed pressure andlor surges of pulp slurry
feed flow andlor
changes in mat drainage characteristics can increase the pressure on the mat
and hence
the pressure differential across the mat so that it exceeds the critical np
which often results
in the mat collapse and the sealing of the washer drum surface due to the
impermeability
of the collapsed mat andlor the aforenoted compacted mat skin thereby making
the washer
inoperable.
Mat collapse is all too frequent an occurrence due to fluctuating resistance
and the
small pressure drop across the mat and due to the small difference between a
satisfactory
mat operating pressure differential and the critical pressure differential.
Specifically, typical
mat operating pressure differentials will be in the range from 1 to 5 inches
of water (2.5 to
12.5 cm) while the operating pressure of the vat will be in the range of 3 psi
or higher (210
cm water or higher).
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The problem is compounded by liquid incompressibility and the way in which the
washing liquid and pulp slurry are injected into the vat. Specifically,
centrifugal pumps are
used to inject both the washing liquid and pulp slurry. These pumps are known
to generate
pressure pulsations and attendant surges which can have a significant
influence on the
pressure drop across the mat due to the incompressibility of liquid.
Accordingly, a surge
from either the pump that supplies the wash liquid or pulp slurry to the vat
can cause the
pressure drop across the mat to reach or exceed the critical ep resulting in
mat collapse
and subsequent washer plugging.
Further, mat collapse occurs due to the inability of pressure and flow
controls and
sensors to adequately react to minuscule changes in the mat drainage
resistance. Hence)
currently available equipment is unable to consistently maintain the pressure
drop across
the mat in a liquid filled pressurized washer below the critical ep at all
times.
Accordingly, there is a need for an improved liquid filled pressurized pulp
washing
system and method which avoids mat collapse by providing increased control of
the
operating parameters that can contribute to mat collapse and which further
increases
throughput while avoiding mat collapse.
SUMMARY OF THE INVENTION
The present invention satisfies the aforenoted need by providing a method of
forming and washing a pulp mat that comprises the steps of introducing a pulp
slurry into
a pulp washer at a first inlet, the pulp washer including a rotating permeable
drum and
forming a pulp mat on the drum. Wash liquid is supplied and flows freely from
a wash liquid
reservoir which is partially filled with wash liquid. The gaslwash liquid
interface level in the
primary wash liquid reservoir is hydraulically disposed above the drum or the
top of the vat
so that the drum is submerged and the vat remains filled with washing liquid
during
operation. Gas from a filtrate reservoir may be used to pressurize the primary
wash liquid
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reservoir and to create an additional positive differential pressure between
the primary
wash liquid reservoir and the pulp washer.
The method includes the step of introducing wash liquid under pressure from
the
primary wash liquid reservoir into the pulp washer at a second inlet disposed
downstream
of the first inlet. The second inlet provides communication between the pulp
washer vat
and the primary wash liquid reservoir. The method further includes the step of
measuring
the level of wash liquid in primary wash liquid reservoir and adjusting the
rotational speed
of the drum in response to the level of wash liquid. Specifically, at a
constant preset wash
liquid flow and tonnage, if the wash liquid level drops below a predetermined
optimal level
for the current speed of the drum, the speed of the drum is reduced to
optimize mat
drainage and increase washing efficiency. On the other hand, if the level of
wash liquid
rises above a predetermined optimal level for the speed of the drum, the
current speed of
the drum is increased to avoid mat collapse and optimize washing efficiency.
In an embodiment, the primary wash liquid reservoir is integrally connected to
the
pulp washer.
In an embodiment, the mat is removed from the pulp washer without exposing the
mat to gas during the formation and washing processes.
In an embodiment, the method further comprises the step of pumping wash liquid
into the primary wash liquid reservoir from a make up wash liquid reservoir to
replenish the
wash liquid supply in the primary wash liquid reservoir.
In an embodiment, the filtrate is drained from the drum into a filtrate
reservoir. The
filtrate reservoir further includes a space for accommodating gas. The space
in the filtrate
reservoir that accommodates the gas is in communication with the primary wash
liquid
reservoir and the method further includes the step of pumping gas from the
filtrate reservoir
to the primary wash liquid reservoir.
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In an embodiment, the step of pumping filtrate gas into the primary wash
liquid
reservoir further comprises the substeps of controlling the filtrate gas
pressure in the
primary wash liquid reservoir by monitoring the filtrate gas pressure in the
primary wash
liquid reservoir, comparing the measured filtrate gas pressure in the primary
wash liquid
reservoir with a predetermined value, and, in the event the measured pressure
is greater
than the predetermined value, recirculating filtrate gas back to the filtrate
reservoir or, in
the event the measured pressure is less than the predetermined value, pumping
additional
filtrate gas into the primary wash liquid reservoir.
In an embodiment, the step of pumping filtrate gas from the filtrate reservoir
to the
primary wash liquid reservoir further comprises the sub-steps of controlling
the filtrate gas
pressure in the primary wash liquid reservoir by measuring the filtrate gas
pressure in the
primary wash liquid reservoir, comparing the measured filtrate gas pressure
value in the
primary wash liquid reservoir with a predetermined value, and, in the event
the measured
pressure is greater than the predetermined value, recirculating the filtrate
gas to a gas
handling system or, in the event the measured pressure is less than the
predetermined
value, pumping additional gas into the primary wash liquid reservoir.
In an embodiment, the present invention provides an improved pulp washer that
includes a primary wash liquid reservoir partially filled with wash liquid.
The surface of the
wash liquid is disposed above the drum and the vat to provide a hydrostatic
head between
the primary wash liquid reservoir and the vat and to insure that the vat
remains filled and
the mat submerged during operation. The pulp washer of the present invention
includes
a wash liquid level control for measuring the surface level of the wash liquid
in the primary
wash liquid reservoir and comparing that surface level to a predetermined
surface level for
the current operating conditions of the washer and rotational velocity of the
rotating drum.
If the surface level of the wash liquid is higher than a predetermined optimum
level, the
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controller increases the speed of rotation of the drum to reduce mat basis
weight, increase
wash liquid use, decrease mat pressure resistance, avoid mat collapse and
optimize
washing efficiency. If the surface falls below a predetermined optimum level,
the controller
decreases the rotational velocity of the drum to optimize mat drainage and
increase
washing efficiency.
In an embodiment, pressurized gas is used to pressurize the primary wash
liquid
reservoir and provide an additional positive differential pressure between the
primary wash
liquid reservoir and the vat.
In an embodiment, a pump is also provided for pumping filtrate gas from the
filtrate
reservoir to the primary wash liquid reservoir.
In an embodiment, the primary wash liquid reservoir further comprises a wash
liquid
level control system that includes at least one sensor for sensing the level
of wash liquid
in the primary wash liquid reservoir. The sensor is in communication with a
controller that
controls the rotational velocity of the drum.
In an embodiment, the wash liquid control system also includes a pressure
sensor
for sensing the filtrate gas pressure in the primary wash liquid reservoir.
The primary wash
liquid reservoir further includes an outlet for releasing excess filtrate gas
from the primary
wash liquid reservoir. The outlet includes an adjustable valve which is
connected to an
actuator that is in communication with a controller. In the event the filtrate
gas pressure
in the primary wash liquid reservoir becomes too high, the controller sends a
signal to the
actuator to open the valve at the outlet thereby enabling pressurized filtrate
gas to be
transmitted from the primary wash liquid reservoir back to the filtrate
reservoir or other
suitable gas handling system.
In an embodiment, an improved pulp washer is provided which includes multiple
counter current washing zones. The filtrate reservoir is divided into two
parts -- a primary
CA 02270382 1999-04-28
filtrate side and a secondary filtrate side. The primary filtrate side
receives pressate (or
fluid that is initially pressed out of the pulp slurry) from the forming zone
of the drum and
twice-used wash liquid from a preliminary washing zone of the drum. The
secondary filtrate
side receives once-used wash liquid from a primary washing zone of the drum.
The wash
liquids reservoir is also divided into two parts -- a preliminary wash liquid
side and a fresh
wash liquid side. The preliminary wash liquid side receives once-used wash
liquid from the
secondary filtrate side of the filtrate reservoir. The fresh wash liquid side
of the wash
liquids reservoir receives fresh wash liquid from a fresh wash liquid make up
source. The
speed cf the drum i~ controlled in response to changes in the level of once-
used wash
liquid in the preliminary wash liquid side of the wash liquids reservoir or,
alternatively, by
the level of fresh wash liquid in the fresh wash liquid side of the wash
liquids reservoir.
It is therefore an advantage of the present invention to provide a method of
washing
pulp using a rotating permeable drum whereby the speed of the drum is
controlled based
upon the permeability of the pulp mat.
Another advantage of the present invention is that the speed of rotation of
the
permeable drum, and therefore the speed and efficiency of the washing process,
is
controlled by the level of wash liquid in the wash liquid reservoir.
Yet another advantage of the present invention is that the wash liquid is
introduced
into the vat under a hydrostatic head and is not pumped from the wash liquid
reservoir to
the vat. Accordingly, pulsations or flow disturbances caused by a pump are not
transmitted
to the vat by the washing liquid.
Another advantage of the present invention that the formed and washed pulp mat
is not exposed to pressurized gas prior to removal from the pulp washer.
Another advantage of the present invention is that the speed of the formation
and
washing process is controlled by variations in the wash liquid level in the
wash liquid
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reSerV01 r.
Other objects and advantages of the present invention' will become apparent
upon
reading the following detailed description and appended claims, and upon
reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of this invention, reference should now be
made
to the embodiments illustrated in greater detail in the accompanying drawings
and
described below by way of an example of the invention.
In the drawings:
Figure 1 is a schematic illustration of a prior art pressure pulp washing
system;
Figure 2 is a schematic illustration of another prior art pulp washing system;
Figure 3 is an illustration of a pressure pump through pulp washing system
made
in accordance with the present invention;
Figure 4 is a schematic illustration of another pulp washing system made in
accordance with the present invention;
Figure 5A is an enlarged sectional view of the outlet to the vat of the pulp
washing
system of the present invention particularly illustrating a means for
dislodging the mat from
the rotating drum;
Figure 5B is an enlarged sectional view of the means for sealing the vat from
the
discharge chamber and the expansion of the mat from the drum following its
release from
a compacting means shown in Figure 5A;
Figure 6 is a schematic illustration of yet another pulp washing system made
in
accordance with the present invention; and
Figure 7 is a schematic illustration of yet another pulp washing system made
in
accordance with the present invention.
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It should be understood that the drawings are not necessarily to scale and
that the
embodiments are sometimes illustrated by graphic symbols, phantom lines)
diagrammatic
representations and fragmentary views. In certain instances, details which are
not
necessary for an understanding of the present invention or which render other
details
difficult to perceive may have been omitted. It should be understood, of
course, that the
invention is not necessarily limited to the particular embodiments illustrated
herein.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
To best understand the method and apparatus of the present invention, it is
helpful
to consider the prior art pulp washing systems of the prior art as shown in
Figures 1 and
2. Turning first to Figure 1, a washer 10 includes a vat 11 that houses a
rotating permeable
drum 12. The rotating permeable drum 12 rotates at a speed that may controlled
by a
controller such as that as shown at 13. The drum 12 also rotates within a
number of
baffles, two of which are shown at 14 and 15.
In operation, a pulp slurry from a pulp slurry reservoir) as shown
schematically at 16,
is pumped or injected into the inlet 17 through a conduit 18. In the
embodiment shown in
Figure 1, the slurry reservoir 16 is pressurized and the flow of slurry
through the conduit
18 is controlled through an automated control valve 19.
After a slurry is injected through an inlet 17, the slung is drawn towards the
drum 12
because the pressure at the outside of the drum 12 is greater than the
pressure inside the
drum 12. Thus, the slurry is drawn towards the drum 12 and a mat (not shown)
is formed
against the drum 12 in a section of the drum 12 shown at 21 is referred to as
a forming
section 21. As the mat is formed, liquid which will hereinafter be referred to
as pressate
is drained off the mat through the perforate surface of drum 12 and is
discharged out of the
drum 12 through a filtrate outlet 21a.
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Pressate is the liquid that is squeezed out of the mat in the forming zone 21.
I n the
system 10 shown in Figure 1, the pressate consists mainly of black liquor. The
term filtrate
is used for wash liquid that is removed from the mat or a combination of wash
liquid
removed from the mat and pressate downstream of the forming zone 21 in an area
such
as a washing zone 22. For two stage processes) the term filtrate is used for
the
combination of once and twice used wash liquid that is removed from the mat or
once and
twice used wash liquid in combination with pressate. Because the pressate and
the used
wash liquid are stored in a common reservoir after removal from the vat, such
reservoirs
will be referred to as filtrate reservoirs (see reference numeral 56 in Figure
2; 115 in
Figures 3, 4 and 6; and 170 in Figure 7).
After formation of the mat in the forming section 21, the drum 12 rotates
towards the
washing zone 22. In the washing zone 22, the mat is washed with wash liquid
injected into
the vat 11 through a wash inlet 23 and conduit 24. The baffle 14 compacts the
mat during
the initial washing process. Flow of wash liquid through the mat is controlled
by an
automated valve control system 25. The wash liquid is pumped to the inlet 23
from a wash
liquid reservoir 26 by a pump 27. Gas pressure venting is provided to the top
portion of the
wash liquid reservoir 26 through a gas line shown at 31. In the event the gas
pressure
exceeds an acceptable value, air enters into reservoir 26 through vacuum
pressure relief
valve shown at 29 or out of reservoir 26 through conduit 34 to a gaslvapor
recovery system
shown schematically at 31.
I n the embodiment illustrated, a control system 32 includes a sensor 33 for
monitoring the gas pressure inside the reservoir 26. In the event the gas
pressure reaches
an unacceptable level, the controller 32 opens a valve 34a to release gas
through conduit
34 to the gas/vapor recovery system 31. Wash liquid is supplied to the wash
liquid
reservoir 26 from a make up wash liquid reservoir shown schematically at 35. A
level
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controller 36 is employed which includes a sensor 37 to indicate when the wash
liquid level
in the wash liquid reservoir tank 26 has reached an acceptable level. When
such an
acceptable level is reached, the controller 36 closes a valve 38.
Because wash liquid is pumped directly into the vat 11 by the pump 27, the
system
shown in Figure 1 is susceptible to mat collapse because all pumps, including
the pump
shown at 27, deliver small surges, pulsations and variations in the wash
liquid flow. These
small surges and variations create instantaneous pressure changes on the mat
and in the
vat wash liquid and are caused in part by the incompressibility of the wash
liquid being
supplied to the vat 11.
As a result) if the washer 10 is being operated anywhere close to the critical
Op, mat
collapse can occur. Still further, the controller 13 shown in Figure 1 may be
used to adjust
the angular velocity of the drum 12 by measuring vat pressure or the flow of
filtrate through
the outlet 21 a or the flow of wash liquid through the conduit 18. The
controller 13 has no
way to measure or gauge the changes in permeability of the mat formed on the
drum 12.
Hence, the controller 13 has no direct way to compensate for minuscule changes
in the
mat drainage characteristics and, as discussed above, minuscule changes in the
system
can mean the difference between the system operating below the critical Op or
going above
the critical ~p and plugging the washer.
The same problems are inherent in the prior art design shown in Figure 2.
Specifically, Figure 2 illustrates the use of a dual "series countercurrent"
washer system
employing washers 40 and 41. The washer 40 includes a vat 42 into which a
slurry is
supplied through the inlet 43 by way of a conduit 44 and slurry reservoir or
supply shown
schematically at 45. The flow of slurry through the conduit 44 is controlled
by an automatic
valve system shown at 46. The washer 40 operates in a manner similar to the
washer 10
shown in Figure 1. The wash liquid used in the washer 40 is provided from a
primary
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filtrate reservoir 47 which collects pressate and tha once used wash liquid
through an outlet
48 of a vat 49 of the washer 41. Similar to the washer 10 shown in Figure 1,
the primary
filtrate is pumped to the vat 42 by way of a pimp 51. While the flow of
primary filtrate
through a conduit 52 and into a wash liquid inlet 53 is controlled by an
automatic valve 54,
pulsations andlor surges caused by the centrifugal pump 51 will be directly
communicated
to the vat 42 by way of the liquid flowing through the conduit 52.
Pressate and once used primary filtrate exit the washer 40 by way of an outlet
55
and enter the secondary filtrate reservoir 56. A pump 57 is used to recycle
malodorous gas
from reservoir 56 to the top of the reservoir 47. Vacuum relief valves are
shown at 58, 59.
The level control system 61 is used to control the level of liquid in the
reservoir 56. A
controller 62 may be used to control the angular velocity of the drum (not
shown) of the
washer 40. After the pulp mat is dislodged from the drum, it exits the washer
40 through
the pulp outlet 63. The pressure in and the rate of flow out of outlet 63 is
controlled by the
pressure control system 65. Pulp is then discharged into a reservoir 66 where
it is diluted
with liquid pumped from the primary filtrate reservoir 47 by the pump 67. The
level of slurry
in the reservoir 66 is controlled by the level control system 69, and the
slurry pump out
consistency is controlled by consistency control system 68.
The slurry is then pumped from the reservoir 66 to a slung inlet 72 of the
washer 41
by the pump 71. The flow of slurry through a conduit 73 may be measured by the
flow
meter system 74. Fresh wash liquid is provided to the washer 41 through an
inlet 75 and
from a fresh wash liquid reservoir 76. The fresh wash liquid is pumped from
the reservoir
76 by a pump 77. The flow through a conduit 78 is controlled by an automatic
valve 79.
Control systems 81, 82 operate in a manner similar to that discussed above
with respect
to control systems 32) 36 of Figure 1 respectively. As discussed above with
the centrifugal
pump 51, the pump 77 is in direct communication with the vat 49 and therefore
any
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pulsations andlor surges created by the pump 77 will be directly communicated
to the vat
49 by way of the wash liquid proceeding through the conduit 78. As a result,
pulsations
generated by the pump 77, or the pump 51 for that matter, can greatly
contribute to the
causation of mat collapse and the plugging of the washers 41 ( 40
respectively. Further,
the same is true with respect to pump 71 which, as discussed above, pumps
slurry from
the reservoir 66 to the inlet 72 of the washer 41. Like the pump 77, the pump
71 can cause
pulsations andlor surges thereby contributing to mat collapse and the plugging
of the
washer 41.
To overcome the deficiencies discussed above with respect to Figures 1 and 2
and
to more accurately control the dynamics inside the washer vat, the system 100
shown in
Figure 3 ~Nas developed. A washer 101 includes a vat 102 which includes a
slurry inlet 103
that is in communication with a slurry supply means shown only schematically
at 104. The
flow of slurry through the conduit 105 is controlled by a flow control system
106. After
formation of a mat on a drum 107, and compaction of the mat on the drum 107 by
a baffle
108, the mat is continuously flushed with wash liquid contained in a primary
wash liquid
reservoir 112. In the embodiment shown in Figure 3, the primary wash liquid
reservoir 112
is connected directly to the vat 102. The primary wash liquid reservoir 112 is
only partially
full and includes a space 113 for accommodating gas pumped into the wash
liquid reservoir
112 by a pump 114 from a filtrate reservoir 115. The filtrate reservoir 115
contains
pressate and used wash liquid transmitted from the drum 107 by way of the
outlet conduit
106a. The gas pressure provided in the space 113 pressurizes the primary wash
liquid
reservoir 112 and, in addition to the hydrostatic head, it eliminates the need
for any pump
disposed between the feed wash liquid reservoir 112 and the washer vat 102.
Fresh wash liquid is supplied to the primary wash liquid reservoir 112 by a
make-up
secondary wash liquid reservoir 116. Specifically, a pump 117 is provided to
pump wash
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liquid through the inlet 118. The hydrostatic head between the wash liquid in
the feed
reservoir 112 and the washer vat 102 is indicated at "Y". Cont~ of of make up
wash liquid
from the pump 117 through the conduit 119 is provided by a flow control system
121.
Similar to the reservoir 26 shown in Figure 1, the make-up reservoir 116 is
equipped with
a pressure control system 122 for releasing gas pressure which, in tum is
supplied through
a gas conduit 123 from an outlet 123a of the primary wash liquid reservoir
112. A pressure
control system 125 releases gas pressure from the feed reservoir 112 through
the conduit
123 as needed. Fresh wash liquid is supplied to the make up reservoir 116 by a
wash
liquid supply shown only schematically at 126. The flow of fresh make-up wash
liquid to
the make up reservoir 116 is controlled by a level control system 127. The
mixture of
pressate and used wash liquid is discharged from the filtrate reservoir 115 on
an as needed
basis by way of the pump 128 and level control system 129.
The washing control system shown schematically at 124 maintains the level of
wash
liquid 111 in the reservoir 112 as follows. First, sensors 132, 133 monitor
the level 131 of
liquid 111 in the reservoir 112. An increase in the liquid height "Y" would
result from a
decrease in permeability of the mat formed on the drum 107. In order to avoid
mat
collapse; the drum speed must be increased. Accordingly, if the liquid height
"Y" increases
to a height above a predetermined preferred height for a particular angular
velocity of the
drum 107, the controller 124 will increase the angular velocity of the drum
107 in order to
optimize washing efficiency, avoid mat skinning and avoid mat collapse and
washer
plugging. In contrast, in the event the liquid height "Y" drops below a
predetermined
preferred height for a particular angular velocity of the drum 107, the
reduction in height "Y"
would be due to an increase in the permeability of the mat. As a result, the
drum speed
is too fast and the controller will accordingly reduce the angular velocity of
the drum 107
in order to optimize mat drainage and increase washing efficiency.
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Thus, the angular velocity of the drum 107 is controlled by the level
controller 124
in response to permeability changes of the mat formed on the drum 107. These
changes
in permeability result in and are measured directly by the changes in liquid
level height Y
in the wash liquid reservoir 112. Thus, the control system or controller shown
at 124, 125
controls the angular velocity of the drum 107 as well as the pressure in the
space 113.
The gas pressure in the space 113 of the reservoir 112 is also monitored by a
sensor 133. In the event the pressure becomes too high, the pressure control
system 125
which releases gas through the conduit 123 to the make up wash liquid
reservoir 116.
Further, in order to provide for gravity filtrate flow and eliminate the need
for a pump in the
outlet line 106, a hydrostatic head represented by "X" is needed between the
drum 107 and
liquid 134 contained in the filtrate reservoir 115. This head "X" is
maintained by the level
control system 129.
A second embodiment is illustrated in Figure 4 and, due to the similarities
between
the embodiments shown in Figures 3 and 4, like reference numerals will be used
to
describe like or similar parts. Again, a level controller 124 is used to
maintain the
hydrostatic head "Y" between free surface 131 of the wash liquid 111 in the
primary feed
wash liquid reservoir 112 and the top of the vat 102 in order to eliminate the
need for a
pump in a conduit 134 between the primary wash liquid reservoir 112 and the
vat 102. The
level controller 124 monitors the height "Y" of the wash liquid 111 and, in
the case of an
increase in the height Y of the liquid due to a decrease in permeability of
the mat formed
on the drum 107, the controller 124 increases the angular velocity of the drum
107.
Further, in the case of a drop or lowering of the height "Y" of the liquid 111
in the primary
wash liquid reservoir 112, the controller 124 will decrease the angular
velocity of the
rotating drum 107. Thus, by monitoring the height Y of the wash liquid 111,
the controller
124 controls the angular velocity of the drum 107 based upon changes in
permeability of
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the mat during rotation of the drum 107 through the formation and washing
zones.
The primary difference between the system 100 shown in Figure 3 and the system
100a shown in Figure 4 is that the gas pressure in the primary feed wash
liquid reservoir
112 of Figure 4 may have to be greater than the gas pressure in the reservoir
112 of Figure
3 in order to compensate for added friction loss in the conduit 134 of Figure
4 and any
change in the elevation of the liquid surface 131 of the reservoir 112 of
Figure 4 relative
to the top of the vat 102 of Figure 4.
Figures 5A and 5B illustrate the pulp outlet shown at 135 in Figure 6 where
the wash
liquid feed 134 is disposed at the top of the vat 102 and adjacent to the pulp
outlet 135.
Specifically, a downstream end 136 of the baffle 109 is slidably attached to a
vat section
137 by way of the sandwiching of a lip 138 of the baffle 109 between a seal
139 and a
ledge 140. The baffle 109 can slide in the direction of the arrows 141 and 142
as shown
in Figure 5B. A doctor blade is shown at 143 which peels the pulp mat 144 off
of the drum
107. Continuous rotation of the drum 107 in the direction of the arrow 145
causes the pulp
to exit through the outlet 135 as shown in Figure 5A.
A third system 100c is illustrated in Figure 6. Again, like reference numerals
are
used to define like or similar parts. A hydrostatic head "Y" is maintained
between the wash
liquid 111 and the wash liquid reservoir 112 and the top of the vat 102. The
controller 124
controls the speed of the drum 107 in response to the changes in the height or
hydrostatic
head Y. Again, no pump is disposed between the primary wash liquid reservoir
112 and
the vat 102. Instead, the conduit 134 directly connects the reservoir 112 to
the vat 102 and
includes a check valve 151.
Finally, turning to Figure 7, a system 160 which includes multiple counter
current
wash zones is illustrated. Specifically, a vat 161 houses a permeable drum 162
and baffles
163) 164, 165. A pulp inlet is shown at 166. As the drum 162 is rotated in the
direction of
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the arrow 167, pressate is extracted from the slurry as the mat is formed on
the drum 162
at a forming zone shown generally at 162a. The pressate exits the permeable
drum 162
through a conduit 168 and enters a primary filtrate side 169 of a filtrate
liquids reservoir
170. In contrast, downstream in a fresh liquid or primary wash zone 171, fresh
wash liquid
is passed through the mat and exits the permeable drum 162 through an outlet
conduit 172
which is in communication with a secondary filtrate side 173 of the filtrate
liquids reservoir
170. The secondary filtrate is recirculated to a preliminary wash liquid side
173' of a
washing liquids reservoir 175 by way of a pump 176 and conduit 177. The wash
liquid
supplied to the vat 161 from the preliminary side 173' of the washing liquids
reservoir 175
is recycled filtrate wash liquid and therefore is supplied through the conduit
178 and after
passing through the mat combines with the pressate in a preliminary washing
zone shown
at 179 before it is routed to the primary filtrate side 169 of the reservoir
170.
It will be noted that the forming zone 162a is the area that includes the
periphery of
the drum 162 where the mat is initially formed as well as the area inside the
drum where
the pressate is collected. Similarly, washing zones such as 179 and 171, as
discussed
below, include the outer periphery of the drum 162 where the mat is washed as
well as the
areas inside the drum where used wash liquid is collected.
I n contrast, the washing zone 171 is a final or primary washing zone prior to
the
discharge of the washed pulp through the outlet 181. Hence, the wash liquid
provided to
the final washing zone 171 is provided from a clean fresh wash liquid side 182
of the
reservoir 175 through the conduit 183. Fresh wash liquid make up is provided
to the fresh
wash liquid side 182 through a conduit 184 which is in communication with a
fresh wash
liquid make up supply shown schematically at 185. The supply of fresh wash
liquid make
up to the fresh wash liquid side 182 of washing liquids reservoir 175 is
controlled by an
automatic valve 186. The washing liquids reservoir 175 is pressurized by gas
supplied
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from the filtrate liquids reservoir 170 through a conduit 187.
The pressure in the washing liquids reservoir 175 is controlled by a pressure
control
system 188. Controller 189 monitors the level of secondary filtrate in the
secondary filtrate
wash liquid side 173' of the washing liquids reservoir 175. In the event the
level in the
secondary filtrate side 173' rises due to a decrease in permeability of the
formed mat, the
controller 189 will increase the angular velocity of the drum 162. In
contrast, if the level in
the secondary filtrate side 173' drops due to an increase in permeability of
the formed mat,
the controller 189 will reduce the angular velocity of the drum 162. The
controller 189 may
also monitor the fresh wash liquid side 182 and adjust the angular velocity of
the drum 162
in the same manner depending upon a rise or drop in the level of fresh wash
liquid in the
fresh side 182 of the washing liquids reservoir 175.
From the above description, it is apparent that the objects and advantages of
the
present invention have been achieved. While only certain embodiments have been
described above, alternative embodiments and various modifications will be
apparent from
the above description to those skilled in the art. These and other
alternatives are
considered equivalents and within the spirit and scope of the present
invention.
