Patent Publication Number: US-6339859-B1

Title: Control system for a filtrate splitting device

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the pulp and paper industry. More specifically, the present invention relates to apparatuses for washing pulp and, more specifically, to apparatuses for washing pulp which include a plurality of discharge conduits. The present invention is directed toward devices for regulating the flow between the discharge conduits to compensate for variations in the operation of the apparatus and in the feed slurry. 
     BACKGROUND OF THE INVENTION 
     In currently used methods of making pulp from wood stock, the wood, which may be in the form of wood chips, is heated in a digester. In the digester, the lignin is chemically dissolved and heated to free the cellulose fibers so they can be reformed into paper. The end product of the digestion process is cooked pulp fibers. 
     The cooked pulp fibers are then blown into a tank where the steam flashes off. Black liquor is added to the blow tank to dilute the pulp and form a slurry. The pulp slurry is then further diluted and transferred to a pulp washer. 
     In the pulp washer, a mat or sheet is formed from the slurry and the black liquor is extracted as filtrate. Wash liquor is added to displace the black liquor and is also subsequently extracted. The extracted liquids are recycled to the extent possible and their recycled use depends upon the solids content in the extracted liquid. It is therefore a fairly common practice to “split” the filtrate or extracted liquid into a plurality of discharge conduits, depending upon the solids content in the extracted liquid. For example, in rotating drum filters, it is common to extract the initial black liquor filtrate separately and then split the wash liquor filtrate into “cloudy” or “weak” and “clear” or “strong” streams. Some devices employ a third discharge conduit used to extract a “very clear” stream later in the cycle. In other pulp washers or thickeners, the discharge stream can be split into a number of different discharge streams. 
     Typically, a movable assembly is provided, referred to as a splitter or splitter valve, that segregates the liquor or filtrate flow into two or more flow streams. Typical prior art splitter valves are either fixed or manually adjustable. As a result, they are installed for a specific set of operating conditions and their performance deteriorates as these conditions change. In most pulp mills, the parameters which affect the most efficient position of the splitter valve can vary constantly and therefore the currently available fixed or manually adjustable splitter valves seldom operate at their optimum setting or position. 
     Further, when operating a multi-stage washer, increasing the speed of the operation or the speed of rotation of the drum can result in dirty black liquor being carried forward to the subsequent washing stage. As a result, the efficiency of the washing operation is compromised. One condition contributing to the forward movement of black liquor into a second stage or a washing stage is the inappropriate setting of the splitter valve. If the splitter valve were set to permit a sufficient discharge rate of the black liquor, it would not be carried forward into a subsequent washing stage. 
     Finally, if the splitter valve is not accurately positioned, the concentration of solids in the collected filtrate stream will be adversely affected. Specifically, a “clear” or “weak” stream could result which has an unacceptably high solids concentration. Further, “cloudy” or “strong” streams with unacceptably low solids concentrations could result thereby adversely affecting the system&#39;s efficient use of wash liquor and efficient recycling of the various wash liquor discharge streams. 
     Accordingly, there is a need for an improved system for controlling the position of the splitter valve in pulp washing apparatuses. Improvements in the control of splitter valves in the system will result in faster operation of the apparatuses as well as improved recycling of filtrate streams. 
     SUMMARY OF THE INVENTION 
     In satisfaction of the aforenoted needs, the present invention provides an adjustable splitter valve and control system for a pulp washing apparatus that includes a displacement zone for displacing fluid from a pulp mat formed from a pulp slurry and a plurality of discharge conduits for discharging fluid from the displacement zone. The splitter valve and control system includes a movable valve body for directing fluid from the displacement zone to one or more discharge conduits. The valve body is connected to an actuator and the actuator is in communication with a controller. The controller is in communication with at least one sensor disposed in each discharge conduit. The sensors measure at least one physical property of the fluid that flows through each discharge conduit when the valve body is in a position so as to direct fluid through that conduit. Each sensor sends a signal indicating a measured value for the physical property of the fluid flowing through each discharge conduit to the controller. The controller then compares the measured value to a predetermined value range for the respective discharge conduit. In the event the measured value falls outside of a predetermined value range for the conduit or one or more conduits, the controller sends a signal to the actuator to move the valve body to a more appropriate position. 
     In an embodiment, the sensors measure the conductivity of the fluid flowing through the discharge conduits. 
     In an embodiment, the sensors measure the solids concentration of the fluid flowing through the discharge conduits. 
     In an embodiment, the plurality of discharge conduits includes a first conduit for primarily discharging black liquor from the washing apparatus, the black liquor having a high solids concentration and high conductivity. The discharge conduits also include a last conduit for discharging weak, or substantially clean wash liquor from the washing apparatus. The weak, or very clean wash liquor, has a low solids concentration and a low conductivity. The plurality of discharge conduits also include one or more discharge conduits spaced between the first discharge conduit and the last discharge conduit. These additional discharge conduits discharge fluid having a progressively decreased dissolved solids content and a decreased conductivity as the conduits are spaced closer to the last discharge conduit. In other words, the discharge conduit disposed immediately adjacent to the first discharge conduit discharges fluid having a relatively high dissolved solids content and high conductivity in comparison to the fluid discharged by the discharge conduit disposed immediately adjacent to the last discharge conduit. 
     In an embodiment, the present invention provides a method for controlling the position of a filtrate splitting device in a pulp washer that includes a plurality of discharge conduits as described above, each conduit including a sensor that is in communication with a controller, the controller being in communication with an actuator that translates signals from the controller to a repositioning movement of the splitter valve. The method includes the steps of measuring a physical property of the fluid being discharged from one or more of the discharge conduits, transmitting a signal reflective of the measured value to the controller, comparing the measured value with a predetermined operating range and, in the event the measured value falls out of the predetermined optimal range, sending a signal to the actuator to thereafter reposition the splitter valve. 
     In an embodiment, the sensors measure the conductivity of the fluid being discharged through each conduit and send a signal reflective of the measured conductivity to the controller. The controller then compares the measured conductivity value with a preferred conductivity range for each discharge conduit. If the measured value in the first conduit is less than the preferred range, the controller sends a signal to the actuator to move the splitter valve towards the first discharge conduit so that less of the lower conductivity (low solids concentration) fluid is discharged to the first conduit. In the event the measured conductivity value in the first conduit is higher than the preferred conductivity range for the respective discharge conduit, the controller sends a signal to the actuator to move the splitter valve towards the second discharge conduit to thereby reduce the amount of the low conductivity (low solids concentration) fluid being discharged to the first conduit from the pulp washer and to thereby minimize the rate at which wash liquor is consumed. In both cases, the splitting of high and low solids concentration liquors is optimized. 
     In an embodiment, as the splitter valve moves from the first discharge (black liquor) conduit towards the last (very weak) discharge conduit, the percentage of weak liquor or filtrate that is removed from the pulp washer decreases. As a result, a smaller quantity of wash liquor is utilized in the pulp washer. 
     It is therefore an advantage of the present invention to provide an improved method of controlling the position of a splitter valve for a pulp washer. 
     Another advantage of the present invention is that it reduces the use of clean wash liquor in pulp washers. 
     Another advantage of the present invention is that it more efficiently splits the filtrates and liquors discharged from a pulp washer for a more efficient recycling of these materials. 
     Yet another advantage of the present invention is that it enables pulp washers to be operated at a faster rate. 
     Still another advantage of the present invention is that it enables pulp washers to be operated at higher efficiencies. 
     These and other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present 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: 
     FIG. 1 is a schematic illustration of the adjustable splitter valve and control system therefor as incorporated into the discharge piping of a pulp washing apparatus; 
     FIG. 2 is another schematic illustration of the adjustable splitter valve and control system of the present invention as incorporated into the discharge piping system of a pulp washing apparatus, particularly illustrating the communication between sensors disposed in each discharge conduit and the controller; 
     FIG. 3 is a schematic illustration of another embodiment of the present invention employing two sensors, one on either side of the splitter valve. 
     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 
     Turning first to FIG. 1, a pulp washing apparatus shown schematically at  10  which includes a plurality of discharge pipes shown at  11 - 24 . It will be known that the number of discharge pipes for a pulp washer  10  may vary greatly. Some pulp washers include as little as two discharge pipes; others include as many as the fourteen illustrated in FIGS. 1 and 2 or more. In addition to a pulp slurry inlet (not shown), the pulp washer  10  also includes a first wash liquor feed  25  and a second wash liquor feed  26 . The pulp washer  10  may be any type of pulp washer, including the rotating filter types such as the drum filters or disc filters. The discharge conduit  11 , shown at the left in FIGS. 1 and 2, will be hereinafter referred to as the first discharge conduit  11  because it is the conduit that discharges primarily black liquor or other high solids concentration filtrate that has been displaced from the pulp mat (not shown) in the washer  10 . The liquor filtrate discharged by the first discharge conduit  11  has a high solids content and high conductivity. 
     In contrast, the last discharge conduit  24  is intended to discharge primarily weak or “very weak” liquor or filtrate having a low solids concentration and a low conductivity. The discharge conduits  12 - 23  disposed between the first discharge conduit  11  and the last discharge conduit  24  discharge filtrate having decreasing solids concentrations and decreasing conductivity from left to right in FIGS. 1 and 2. That is, the discharge conduit  12 , which is disposed immediately adjacent to the first discharge conduit  11 , discharges fluid having a higher solids concentration and a higher conductivity than the discharge conduit  23 , which is disposed immediately adjacent to the last discharge conduit  24 . The relationship between the decreasing solids concentrations from left to right in FIG. 1 is illustrated by the line shown at  27 . 
     In order to adjust the split between one, two or more conduits  11 - 24 , a movable splitter valve  28  is provided. The splitter valve  28  can be moved in both the left and right directions as indicated by the arrow  29 . Movement of the splitter valve  28  is actuated by the actuator  31  which is in communication with a controller  32 . A cumulative discharge conduit is shown at  33  which can be used to collect the discharge filtrate. Also, the discharge conduits  11 - 24  may be connected to separate receiving receptacles or fluid handling systems (not shown). 
     Turning to FIG. 2, the controller  32  is in communication with a plurality of sensors for each discharge conduit  11 - 24 , the sensors being shown schematically at  35 - 48 . Each sensor  35 - 48  can measure a physical property of the filtrate or fluid being discharged through the conduit  11 - 24 . For example, each sensor  35 - 48  can measure the conductivity (or resistivity) of the filtrate being discharged. A signal is generated and sent to the controller  32 . The controller  32  preferably has a memory in which acceptable conductivity ranges for the conduits  11 - 24  is stored. By way of an example, if the conductivity measured by the sensor  41  for the discharge conduit  17  is greater than the preferred range stored in the controller  32  for the discharge conduit  17 , the controller will send a signal to the actuator  31  to move the splitter valve  28  to the right which will result in an increase in the percentage of strong filtrate being discharged from the pulp washer  10 . One preferred movement would be to move the splitter valve  28  between the discharge conduits  18  and  19  in this occurrence. 
     Also, by way of an example, if the sensor  42  for the discharge conduit  18  measures a conductivity value that is less than the preferred conductivity range stored in the controller  32  for the discharge conduit  18 , the controller  32  will send a signal to the actuator  31  to move the splitter valve  28  to the left which will result in a decrease in the percentage of strong filtrate being discharged from the pulp washer  10 . Accordingly, if the measured conductivity values are too low for the specific discharge conduits being measured, the valve  28  will be moved to the left to withdraw less of the low conductivity (weak) filtrate from the washer  10 . If the measured conductivity values are high, the splitter valve  28  will be moved to the right resulting in a reduction of the weak filtrate discharge and a more efficient use of the wash liquor. 
     FIG. 3 is an illustration of a simplified and therefore less expensive embodiment of the present invention that employs two sensors  31   a  and  44   a  on conduits  15  and  20  respectively. The embodiment illustrating the use of fourteen sensors shown in FIG. 2 is anticipated to be useful for experimental purposes and, unless the economics of conductivity sensors changes, it is anticipated that the use of less than fourteen sensors would be preferable. Therefore, the use of two sensors, one on either side of the splitter valve  28  is substantially more feasible than the design shown in FIG.  2 . Further, the present invention can be carried out using only one sensor disposed on either side of the splitter valve  28 . Therefore, the present invention can be carried out with as little as one sensor. 
     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 set forth, alternative embodiments and various modifications will be apparent from the above description to those skilled in the art. For example, the number of discharge conduits can vary greatly. Further, in addition to pulp washers, the present invention applies to other washing apparatuses as well and, therefore, black liquor may not be the filtrate that is initially displaced in the washer, but other types of dirty liquor or filtrate may be initially displaced as well. Also, in addition to conductivity, resistivity or fluent clarity can be measured in order to estimate the solids concentration or suspended solids of the discharge liquor. Any measurement that provides an indication of the solids concentration of the discharge liquor can be utilized. These and other alternatives are considered equivalents and within the spirit and scope of the present invention.