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(11) | EP 0 955 405 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Totally submerged pressurized pulp washer and method of operation thereof |
| (57) A pulp mat is formed on a surface of a permeable rotating drum (107) disposed in
a pressurized liquid filled washing apparatus (101). Pressurized washing liquid (111)
is delivered under the force of gravity or a hydrostatic head from a wash liquid reservoir
(112). The wash liquid reservoir includes both wash liquid and gas (113) supplied
from a filtrate reservoir. The gas/liquid interface in the wash liquid reservoir is
hydraulically disposed above the top of the washer vat (102) to thereby provide a
hydrostatic head between the wash liquid reservoir and the washer vat. The interface
is monitored and used to control the speed of the rotating drum. In the event the
gas/liquid interface rises above a predetermined optimum value for the current operating
speed of the drum, the speed of the drum is increased in order to avoid mat collapse
and optimize washing efficiency. In the event the gas/liquid interface drops below
a predetermined optimum level, the speed of the drum is reduced to increase washing
efficiency. |
Field of the Invention:
[0001] 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 INVENTION
[0002] 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".
[0003] Specifically, in pressurized pulp washers, a pulp mat is formed from a pulp slurry by passing 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.
[0004] 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 wash liquid through the collapsed interstices of the fibrous mat and through the densified impermeable flat 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.
[0005] 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 Δp"). At the point where incipient mat collapse occurs, the pressure differential across the mat has exceeded the maximum Δp and has reached a critical pressure differential ("critical Δp") beyond which plugging occurs.
[0006] 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 and/or surges of wash liquid flow and/or surges of pulp slurry feed pressure and/or surges of pulp slurry feed flow and/or 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 Δp which often results in the mat collapse and the sealing of the washer drum surface due to the impermeability of the collapsed mat and/or the aforenoted compacted mat skin thereby making the washer inoperable.
[0007] 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).
[0008] 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 Δp resulting in mat collapse and subsequent washer plugging.
[0009] 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 Δp at all times.
[0010] 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
[0011] 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 gas/wash 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 reservoir and to create an additional positive differential pressure between the primary wash liquid reservoir and the pulp washer.
[0012] 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.
[0013] In an embodiment, the primary wash liquid reservoir is integrally connected to the pulp washer.
[0014] In an embodiment, the mat is removed from the pulp washer without exposing the mat to gas during the formation and washing processes.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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 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.
[0020] 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.
[0021] In an embodiment, a pump is also provided for pumping filtrate gas from the filtrate reservoir to the primary wash liquid reservoir.
[0022] 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.
[0023] 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.
[0024] 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 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 of the drum is 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, altematively, by the level of fresh wash liquid in the fresh wash liquid side of the wash liquids reservoir.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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 reservoir.
[0030] 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
[0031] 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.
[0032] 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.
[0033] 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
[0034] 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. Tuming 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.
[0035] 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.
[0036] After a slurry is injected through an inlet 17, the slurry 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.
[0037] Pressate is the liquid that is squeezed out of the mat in the forming zone 21. Ir 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).
[0038] 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 gas/vapor recovery system shown schematically at 31.
[0039] In 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 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.
[0040] 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.
[0041] As a result, if the washer 10 is being operated anywhere close to the critical Δp, 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 21a 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 Δp or going above the critical Δp and plugging the washer.
[0042] 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 filtrate reservoir 47 which collects pressate and the 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 pump 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 and/or 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.
[0043] 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.
[0044] The slurry is then pumped from the reservoir 66 to a slurry 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 pulsations and/or 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 and/or surges thereby contributing to mat collapse and the plugging of the washer 41.
[0045] 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 was 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.
[0046] 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 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". Control 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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 the mat during rotation of the drum 107 through the formation and washing zones.
[0051] 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 1 12 of Figure 4 may have to be greater than the gas pressure in the reservoir 1 12 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.
[0052] 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.
[0053] 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.
[0054] 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 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.
[0055] 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.
[0056] In 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 from the filtrate liquids reservoir 170 through a conduit 187.
[0057] 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.
[0058] 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.
introducing a pulp slurry into a pulp washer vat, the vat including a rotating permeable drum,
rotating the drum at an angular velocity,
forming a pulp mat on the rotating drum,
introducing wash liquid from a primary wash liquid reservoir into the vat, the primary wash liquid reservoir having a gas/wash liquid interface, the interface being disposed at a heigh vertically above the rotating drum,
submerging the mat in the wash liquid,
measuring the height of the gas/wash liquid interface,
controlling the angular velocity of the drum in response to the measured height of the gas/wash liquid interface by increasing the angular velocity of the drum as the measured height of the interface increases and decreasing the angular velocity of the drum as the measured height of the interface decreases.
introducing a pulp slurry into a pulp washer vat at a first inlet, the vat including a rotating permeable drum,
rotating the drum at an angular velocity,
forming a pulp mat on the drum,
removing pressate from the mat,
transporting the pressate removed from the mat to a filtrate reservoir, the filtrate reservoir including a space for accommodating gas,
pumping gas from the filtrate reservoir to a primary wash liquid reservoir containing a level of a wash liquid to pressurize the primary wash liquid reservoir with the gas and to create a positive differential pressure between the primary wash liquid reservoir and the vat,
measuring the level of wash liquid in the primary wash liquid reservoir,
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 providing communication between the pulp washer and a primary wash liquid reservoir, and
controlling the angular velocity of the drum and the level of wash liquid in the primary wash liquor reservoir in response to the measured level of wash liquid in the primary wash liquid reservoir by decreasing the angular velocity of the drum and decreasing the flow of filtrate through the mat when the level falls below a predetermined optimum level, and by increasing the angular velocity of the drum and increasing the flow of filtrate through the mat when the level rises above the predetermined optimum level.
controlling the gas pressure in the primary wash liquid reservoir by measuring the gas pressure in the primary wash liquid reservoir and comparing the measured gas pressure value in the primary wash liquid reservoir with a predetermined value, and
in the event the measured value is greater than the predetermined value, removing gas from the primary wash liquid reservoir,
in the event the measured value is less than the predetermined value, pumping additional gas into the primary wash liquid reservoir.
introducing a pulp slurry into a pulp washer vat at a first inlet, the vat including a rotating permeable drum,
rotating the drum at an angular velocity,
forming a pulp mat on the rotating drum,
removing pressate from the mat,
transporting the pressate removed from the mat to a filtrate reservoir, the filtrate reservoir including a space for accommodating gas,
pumping gas from the filtrate reservoir to a primary wash liquid reservoir containing a wash liquid to pressurize the primary wash liquid reservoir with gas from the filtrate reservoir and to create a positive pressure in the pulp washer vat, the primary wash liquid reservoir having a gas/wash liquid interface, the interface being disposed at a height vertically above the vat,
controlling gas pressure in the primary wash liquid reservoir by measuring the gas pressure in the primary wash liquid reservoir,
comparing the measured gas pressure value in the primary wash liquid reservoir with a predetermined value, and
in the event the measured value is greater than the predetermined value, releasing gas from the primary wash liquor reservoir,
in the event the measured value is less than the predetermined value, pumping additional gas into the primary wash liquid reservoir,
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 providing communication between the pulp washer and a primary wash liquid reservoir,
submerging the mat in the wash liquid,
measuring the height of the gas/wash liquid interface, and
controlling the angular velocity of the drum in response to the measured height of the gas/wash liquid interface by increasing the angular velocity of the drum as the measured height of the interface increases and decreasing the angular velocity of the drum as the measured height of the interface decreases.
a wash liquid inlet for introducing a wash liquid into a washing zone disposed downstream of the pulp inlet and upstream of the pulp outlet, the wash liquid inlet being in communication with a primary wash liquid reservoir,
the primary wash liquid reservoir being partially filled with wash liquid, the primary wash liquid reservoir further comprising a wash liquid level control comprising at least one sensor for measuring the level of wash liquid in the primary wash liquid reservoir, the sensor being in communication with a controller, the controller increasing the speed of the drum if the measured level of the wash liquid rises above a predetermined upper limit for the speed at which the drum is rotating and the controller decreasing the speed of the drum if the measured level of wash liquid falls below a predetermined lower limit for the speed at which the drum is rotating.
a wash liquid inlet for introducing a wash liquid into the vat in a washing zone disposed downstream of the pulp inlet and upstream of the pulp outlet, the wash liquid inlet being in communication with a primary wash liquid reservoir,
the primary wash liquid reservoir being in communication with the filtrate reservoir space that accommodates gas, the primary wash liquid reservoir being partially filled with wash liquid and including a space for accommodating pressurized gas for pressurizing the primary wash liquid reservoir and providing a positive differential pressure between the primary wash liquid reservoir and the vat,
the primary wash liquid reservoir further comprises a wash liquid level control comprising at least one sensor for measuring the level of wash liquid in the primary wash liquid reservoir, the sensor being in communication with a controller, the controller increasing the speed of the drum if the measured level of the wash liquid level rises above a predetermined upper limit for the speed at which the drum is rotating and the controller decreasing the speed of the drum if the measured wash liquid level is lowered below a predetermined lower limit for the speed at which the drum is rotating.
a first wash liquid inlet for introducing preliminary wash liquid into a preliminary washing zone disposed downstream of the pulp inlet and upstream of the pulp outlet, the first wash liquid inlet being in communication with a preliminary wash liquid side of a wash liquids reservoir,
a second wash liquid inlet for introducing fresh wash liquid into a primary washing zone disposed downstream the preliminary washing zone and between the pulp outlet and the preliminary washing zone, the second wash liquid inlet being in communication with a fresh wash liquid side of the wash liquids reservoir,
the wash liquids reservoir having a common gas space and further comprising a fresh wash liquid side with a divider disposed between the fresh wash liquid side and the preliminary wash liquid side, the preliminary wash liquid side of the wash liquids reservoir being partially filled with used wash liquid, the wash liquids reservoir further comprising a used wash liquid level control comprising at least one sensor for measuring the level of used wash liquid in the preliminary wash liquid side of the wash liquids reservoir, the sensor being in communication with a controller, the controller increasing the speed of the drum if the measured level of used wash liquid in the preliminary wash liquid side rises above a predetermined upper limit for the speed at which the drum is rotating and the controller decreasing the speed of the drum if the measured used wash liquid level falls below a predetermined lower limit for the speed at which the drum is rotated,
the fresh wash liquid side of the wash liquids reservoir being partially filled with fresh wash liquid, the wash liquids reservoir further comprising a fresh wash liquid level control comprising at least one sensor for measuring the level of fresh wash liquid in the fresh wash liquid side of the wash liquids reservoir, the sensor of the fresh wash liquid level control being in communication with the controller, the controller increasing the speed of the drum if the measured level of fresh wash liquid rises above a predetermined upper limit for the speed at which the drum is rotating and the controller decreasing the speed of the drum if the measured level of wash liquid falls below a predetermined lower limit for the speed at which the drum is rotating.
the primary filtrate side of the filtrate reservoir receiving pressate from inside the drum disposed radially inward from a forming zone disposed outside of the drum and twice-used wash liquid from inside the drum at a point disposed radially inward from the preliminary washing zone, the secondary filtrate side of the filtrate reservoir receiving once-used wash liquid from inside the drum at a point disposed radially inward from the primary washing zone.