Abstract:
A method for feeding a cellulosic fibrous material to a high pressure processing vessel including: pressurizing a low pressure flow of the fibrous material in a high pressure transfer device; discharging a high pressure flow of the fibrous material from the transfer device to a conduit in fluid communication with the processing vessel; discharging a low pressure flow of fluid and fibrous material from the transfer device; downstream of the transfer device, pressurizing the discharged low pressure flow; merging the pressurized low pressure discharge flow with the discharged high pressure flow, and transferring the merged flow to the processing vessel.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Application Ser. No. 60/581,382, filed Jun. 22, 2004, the entirety of which is incorporated by reference.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a method and system for feeding comminuted cellulosic fibrous material (“chips”) to a treatment vessel, such as a continuous digester.  
         [0003]     The reaction of pulping chemicals with comminuted cellulosic fibrous material to produce a chemical pulp requires temperatures ranging between 140-180 degree Celsius (C.). Since the aqueous chemicals used to treat the material boil at such temperatures, commercial chemical pulping is typically performed in a pressure-resistant vessel, e.g., a continuous digester, under pressures of at least about 10 bars gauge (approximately 150 psi gauge). To maintain this pressure, especially when performing a continuous pulping process, a high pressure feeder (HPF) increases the pressure of the chip slurry entering the vessel, e.g., a digester, to a pressure level at or above the pressure in the vessel to ensure that the pressure is not lost when introducing material to the pressure vessel.  
         [0004]     The present invention relates to the transfer system for feeding chips to a high pressure continuous digester and/or to other high pressure chip processing systems. High pressure chip processing systems typically include a HPF, such as is shown in U.S. Pat. No. 6,669,410. The HPF receives a low-pressure slurry of comminuted cellulosic fibrous material (“chip slurry”) and outputs a high-pressure chip slurry. The high pressure slurry is suitable for introduction into a continuous digester, chip steaming vessel and other high pressure chip processing systems.  
         [0005]     Typically, high pressure feeders contain a pocketed rotor which acts as a means for transferring a slurry of material from a low pressure to a high pressure while also acting as a valve for preventing loss of pressure. The rotor has a chamber for transferring low pressure slurry to a high pressure stream. An HPF generally has a stationary housing with a low pressure inlet port at its top (12:00 position), low pressure outlet port at its bottom (6:00 position), a high pressure inlet port at a first side (9:00 position) and an high pressure outlet port at an opposite side (3:00 position). A rotor in the feeder housing alternately opens the pair of low pressure inlet and outlet ports and then opens the pair of high pressure inlet and outlet ports. The low pressure ports are not open while the high pressure ports are open and vice versa. When the low pressure inlet and outlet ports are opened, a new volume of chip slurry enters the rotor chamber and some liquid is purged through the outlet. When the rotor opens the high pressure inlet and outlet, high pressure liquid enters and flushes the chip slurry in the rotor chamber through the high pressure outlet into a high pressure conduit.  
         [0006]     The top port (12:00) of the feeder housing of the HPF is the low-pressure inlet port into which a slurry of chips and liquid is introduced to the feeder. The top port has historically been the low pressure chip slurry input. However, due to the pump-feeding which characterizes the LO-LEVEL™ Feed System marketed by Andritz Inc. of Glens Falls, N.Y., the pressurized slurry flow from the slurry pump may be introduced to a low-pressure inlet of the HPF which is oriented wherever necessitated by the installation. The pump-fed slurry can be introduced to a port located physically on top, on either side, on the bottom of the HPF, or even to a port oriented at an oblique angle, that is, at any angle of orientation desired.  
         [0007]     As the low-pressure slurry is introduced to the low-pressure inlet of the HPF, one or more of the through-going pockets of the rotating rotor receive the slurry. The low-pressure outlet of the HPF is located opposite the low-pressure inlet. As the slurry is introduced to the low-pressure inlet and the first end of one of the through-going pockets, the slurry flows into the rotor pocket and toward a second and opposite end of the pocket, in this case, toward the lower end of the pocket, and toward the low-pressure outlet. The low-pressure outlet port of the HPF is typically provided with a screen element, for example, a cast horizontal bar type screen element (see for example the screen element in U.S. Pat. No. 5,443,162). This screen element retains the chips in the slurry within the rotor of the feeder and allows some of the liquid in the slurry to pass out of the second end of the pocket and through the screen. The liquid discharged from the low-pressure outlet has in the past been recirculated back into the chip feed system to a location upstream of the HPF in the flow of the chip slurry. A difficulty with the screen for the low pressure outlet is that some chips pass through the screen. These chips are then unavailable for further processing in the digester.  
         [0008]     The chips that are introduced to the rotor pocket of the HPF, including those chips retained by the screen element, are transported by the rotation of the rotor. After a typical one-quarter revolution of the rotor, the first end of the pocket that was once in communication with the low-pressure inlet is in communication with the HPF high pressure outlet. The high-pressure outlet typically communicates with the inlet of a digester, either a continuous or batch digester, via one or more conduits. At the same time, the rotation of the rotor also places the second end of the through-going pocket, which was just in communication with the low-pressure outlet, in communication with the high-pressure inlet. The high pressure inlet typically receives a flow of high-pressure liquid from a high-pressure hydraulic pump. The pressure of this liquid typically ranges from about 5 to 15 bar gauge, and is typically about 7-10 bar gauge. This high-pressure liquid displaces the slurry of chips and liquid from the through-going pocket and out of the high-pressure outlet and ultimately to the inlet of the digester.  
         [0009]     As the rotor continues to rotate, the second end of the pocket which received the high-pressure fluid then is placed in communication with the low-pressure inlet and receives another supply of slurry from the conduit connected to the low-pressure inlet. Similarly, the first end of the pocket is rotated into communication with the low-pressure outlet of the housing, having the screen element. The process described above then repeats itself such that during one complete revolution of the rotor each through-going pocket receives and discharges two charges of chips and liquid. The rotor typically contains at least two, typically four, through-going pockets such that the rotor is repeatedly receiving slurry from the low-pressure inlet and discharging slurry out the high-pressure outlet. The ends of the these pockets act as both an inlet for slurry and an outlet depending upon the orientation of the rotor.  
         [0010]     A difficulty has arisen in certain HPFs operating at relatively high rotor speeds. The difficulty is that excessive amounts of chips tend to accumulate on and pass through the screen in the HPF housing at the low-pressure outlet (6:00 position). The chips in the low pressure discharge conduit are lost to the chip processing process and can clog the liquor processing equipment receiving the low pressure discharge of the HPF.  
       SUMMARY OF THE INVENTION  
       [0011]     A method is disclosed for feeding a cellulosic fibrous material to a high pressure processing vessel comprising: pressurizing a low pressure flow of the fibrous material in a high pressure transfer device; discharging a high pressure flow of the fibrous material from the transfer device to a conduit in fluid communication with the processing vessel; discharging a low pressure flow of fluid and fibrous material from the transfer device; downstream of the transfer device, pressurizing the discharged low pressure flow; and merging the pressurized low pressure discharge flow with the discharged high pressure flow  
         [0012]     In a second embodiment, a method is disclosed for feeding a cellulosic fibrous material to a high pressure processing vessel comprising: pressurizing a low pressure flow of the fibrous material in a high pressure transfer device; discharging a high pressure flow of the fibrous material from the transfer device to a conduit in fluid communication with the processing vessel; extracting a liquor from the slurry of the fibrous material in the processing vessel; introducing a portion of the extracted liquor into a high pressure inlet of the high pressure transfer device, and introducing a second portion of the extracted liquor into the low pressure flow of fibrous material upstream of the transfer device.  
         [0013]     Also disclosed is an apparatus for feeding a cellulosic fibrous material to a high pressure processing vessel, said apparatus comprising: a high pressure transfer device receiving a low pressure flow of the fibrous material; a first high pressure conduit connected to a high pressure output of the transfer device to receive a high pressure flow of the fibrous material; a low pressure outlet conduit connected to a low pressure outlet of the transfer device; at least one pump to pressurize a flow in the low pressure outlet conduit and introducing the pressurized flow into a second high pressure conduit; a third high pressure conduit receiving the high pressure flow from the first high pressure conduit and the pressurized flow from the second high pressure conduit, wherein said third high pressure conduit transfers high pressure flow to the processing vessel.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic diagram of a system for feeding a slurry of comminuted cellulosic fibrous material to a continuous digester or other high pressure vessel.  
         [0015]      FIG. 2  is a schematic diagram of a system for feeding a slurry of comminuted cellulosic fibrous material to a continuous digester or other high pressure vessel having a second embodiment of a chip feed system.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Though the systems shown and described in  FIGS. 1 and 2  are continuous digester systems, it is understood that the method and system of the present invention can also be used to feed one or more batch digesters, an impregnation vessel connected to a continuous digester, or other high pressure processing system. The continuous digesters may be used for kraft pulping, sulfite pulping, soda pulping or equivalent processes.  
         [0017]      FIG. 1  illustrates an exemplary chip feed system  10  for feeding a slurry of comminuted cellulosic fibrous material, for example, softwood chips, to the top of a continuous digester  11 . The digester  11  typically includes a top separator having a liquor removal screen  12  near the inlet of the digester  13  for removing excess liquor from the slurry and returning it to feed system  10 . The digester  11  may also includes at least one liquor removal screen  14  for removing spent cooking liquor during or after the pulping process.  
         [0018]     The digester  11  also typically includes one or more additional liquor removal screens (not shown) which may be associated with cooking liquor circulation, digester cooking circulation or digester circulation having a liquor removal conduit and a dilution liquor addition conduit. Cooking liquor, for example, kraft white, black, or green liquor, may be added to these circulations. The digester  11  also includes an outlet  15  for discharging the chemical pulp produced which may be passed on to further treatment such as washing or bleaching.  
         [0019]     The chip feed system  10  receives comminuted cellulosic fibrous material  20  introduced into a chip bin  21 . Typically, the material  20  is softwood or hardwood chips but any form of comminuted cellulosic fibrous material, such as sawdust, grasses, straw, bagasse, kenaf, or other forms of agricultural waste or a combination thereof, may be used. Though the term “chips” is used in the following discussion to refer to the comminuted cellulosic fibrous material, it is to be understood that the term is not limited to wood chips but refers to any form of the comminuted cellulosic fibrous materials listed above, or the like.  
         [0020]     The chip bin  21  may be a conventional bin with vibratory discharge or a DIAMONDBACK™ steaming vessel, as described in U.S. Pat. No. 5,500,083 and sold by Andritz Inc. The bin  21  may include an airlock device at its inlet and a means for monitoring and controlling the level of chips in the bin and a vent with an appropriate mechanism for controlling the pressure within the bin. Steam, either fresh or steam produced from the evaporation of waste liquor (i.e., flashed steam), is typically added to bin  21  via one or more conduits  22 .  
         [0021]     The bin  21  typically discharges to a metering device  23 , such as a screw-type metering device. The metering device  23  discharges to a pressure isolation device  24 , such as a low-pressure feeder. The pressure isolation device  24  isolates a pressurized horizontal treatment vessel  25  from the essentially atmospheric pressure that exists in the chip feed system that is upstream of the isolation device  24 .  
         [0022]     The treatment vessel  25  is used to treat the chip material with pressurized steam, for example steam at approximately 10-20 psig. The vessel  25  may include a screw-type conveyor. Clean or flashed steam may be added to the vessel  25  via one or more conduits  28 .  
         [0023]     After treatment in vessel  25 , the chip slurry is transferred to a high-pressure transfer device  27 , such as a High-Pressure Feeder (HPF) sold by Andritz Inc., of Glens Falls, N.Y. Typically, the steamed material is transferred to the feeder  27  by means of a conduit or chute  26 , such as a chip chute. Heated cooking liquor, for example, a combination of spent kraft black liquor and white liquor, is typically added to the chute  26  via a conduit  29  so that a slurry of material and liquor is produced in chute  26 . The pressurized treatment vessel  25  and the pressure isolation device  24  may be replaced by a steaming vessel such as is disclosed in U.S. Pat. No. 5,000,083, and sold by Andritz as a DIAMONDBACK™ steam vessel.  
         [0024]     The High-Pressure Feeder (HPF)  27  includes a rotor housing comprising a low pressure inlet (at a 12:00 position) connected to the chip chute  26 , and a low pressure outlet (opposite to the low pressure inlet and at a 6:00 position) connected to a conduit  30 . The HPF housing also includes a high-pressure inlet (at a 9:00 position) connected to conduit  33 , and a high-pressure outlet (opposite to the high pressure inlet and at a 3:00 position) connected to a conduit  34 .  
         [0025]     The HPF, and particularly its pocketed rotor  35 , may be driven by a variable-speed electric motor and speed reducer (not shown). The low pressure inlet accepts the heated slurry of chips from chute  26  into a pocket of the rotor  35 . A screen  36  in the HPF housing at the low-pressure outlet, retains the chips in the rotor pocket but allows liquor to pass through the rotor to be removed via conduit  30 .  
         [0026]     As the rotor  35  turns the chips that are retained within the rotor pocket are exposed to high pressure liquid entering at the 9:00 high-pressure inlet via conduit  33 . The high-pressure liquor flowing into the rotor pocket flushes the chips out of the feeder, through the 3:00 high pressure outlet and into conduit  34 . The high-pressure chip slurry flows through conduit  34  and to the top of digester  11 .  
         [0027]     Upon reaching the inlet of digester  11  some of the excess liquor used to slurry the chips in conduit  34  is removed from the slurry via screen  12 . The excess liquor removed via screen  12  is returned to the inlet of pump  32  via conduit  33 . The liquor in conduit  33 , to which fresh cooking liquor may be added, is pressurized in pump  32  and passed in conduit  33  for use as high pressure liquid to flush chips through the feeder  27 .  
         [0028]     Low-pressure liquor (and the chips that pass through the screen  36 ) discharged from the low-pressure outlet (6:00 position) from the feeder  27  flow into the conduit  30 . The flow of liquor and chip slurry in conduit  30  is pressurized by pump(s)  37 , which is at least one pump and preferably is a serial arrangement of one to four screw impeller pumps. These pump(s)  37  increase the liquor-chip slurry from conduit  30  so that the slurry in conduit  31  is at the same high pressure as the chip slurry conduit  34 .  
         [0029]     The pressurized chip slurry flow in conduit  31  is combined with the pressurized flow in conduit  34  at a conduit junction  38 . The high pressure flows of slurries in conduits  31  and  34  merge into conduit  39  between the conduit junction  38  and the top separator  12  of the digester vessel  11 . The chip slurry flow in conduit  31  may have a substantially greater ratio of liquor to chips than the liquor to chip ratio in conduit  34 . Accordingly, adding the high liquor content flow from conduit  31  may increase the ratio of liquor to chips in conduit  39 .  
         [0030]     The volume of flow through conduits  30 ,  31  may be relatively small compared to the volume in conduit  34 . By merging the chip slurry from the low pressure discharge (conduits  30 ,  31 ) with the chip slurry from the high-pressure discharge (conduit  34 ) the chips in conduit  30  are directed to the digester. Chips in the low pressure discharge flow (conduit  30 ) that would otherwise clog conventional liquor processing systems are introduced to the digester.  
         [0031]     The pump(s)  37  may each be a screw centrifugal impeller slurry pumps, or other pressurizing and transferring devices, such as a piston-type solids pump or a high-pressure eductor. Preferably, multiple pressurizing and slurrying pumps  37 , e.g., up to four, are used to transfer the slurry through conduits  30 ,  31  and pressurize the low pressure discharge flow. The pumps  37  may be a screw centrifugal impeller pump such as sold by Hidrostal Ltd. of Newbury England and/or Wemco® pumps supplied by Weir Specialty pumps of Salt Lake City, Utah.  
         [0032]     An optional conduit  40  directs a portion of the liquor from conduit  33  (which has liquor from the top separator of the digester) to the chip feed system, such as to chip chute  26 , chip bin  21  or treatment vessel  25 . A valve (not shown) may be used to control the portion of the flow in conduit  33  that is directed to conduit  40 . The valve controlling the flow of conduit  40  is preferably in conduit  29 , but may be anywhere in the loop defined by conduits  33 ,  40 ,  45  and  29 . A sand separator  42  and an in-line drainer  43  may be included and between conduit  40  and conduit  29 . The sand separator  42  may be a cyclone-type separator for removing sand and debris from the liquor. The in-line drainer  43  may be a static screening device which removes excess liquor from conduit  45  and passes it to conduit  46  which may lead to a level tank ( FIG. 2 ). The liquor not removed by the in-line drainer passes through conduit  29  and is reintroduced into the chip feed system.  
         [0033]      FIG. 2  illustrates another chip feed system  110  for feeding chips to a digester. This system  110  uses processes and equipment described in U.S. Pat. Nos. 5,476,572, 5,622,598 and 5,635,025. This equipment and the processes they are used to effect are collectively marketed under the trademark Lo-Level™ by Andritz Inc. The components in  FIG. 2  which are identical to those that appear in  FIG. 1  are identified by the same reference numbers. Those components which are similar or which perform similar functions to those that appear in  FIG. 1  have their reference numbers that appear in  FIG. 1  prefaced by the numeral “1”.  
         [0034]     Similar to the chip feed system  110  of  FIG. 1 , chips  20  are introduced to steaming vessel  121  where they are exposed to steam introduced via conduit  22 . The vessel  121  discharges to metering device  123 , and to a conduit  126 , which is preferably a Chip Tube as sold by Andritz Inc. Cooking liquor is typically introduced to tube  126  via conduit  55 , similar to conduit  29  of  FIG. 1 . Since the vessel  121  is preferably a DIAMONDBACK™ steaming vessel as described in U.S. Pat. No. 5,000,083, no pressure isolation device,  24  in  FIG. 1 , or pressurized steaming vessel  25  in  FIG. 1 , may be needed. As disclosed in U.S. Pat. No. 5,476,572, instead of discharging the slurry of chips and liquor directly to HPF feeder  27 , a high-pressure slurry pump  51  fed by conduit  50  is used to transport the chips to the HPF feeder  27  via conduit  52 .  
         [0035]     The pump  51  is preferably a screw centrifugal impeller pump or other pump of the same type as pumps  37 . The chips that are passed via pump  51  are transported to digester  11  by feeder  27  in a manner similar to what was shown and described with respect to  FIG. 1 .  
         [0036]     As with the embodiment shown in  FIG. 1 , a portion of the liquor extracted from the digester  11  and that flows into conduit  33  may be optionally diverted into conduit  40 . The liquor in conduit  40  passes through a sand separator  42 , conduit  45 , an in-line drainer  43 , and conduit  144  to a liquor level tank  53 .  
         [0037]     The level tank  53  ensures a sufficient supply of liquor to the inlet of the pump  51 , via conduit  54 . This tank  53  may also supply liquor to chip tube  126  via conduit  55 . This level tank  53  also allows the operator to vary the liquor level in the chip feed system such that, if desired, the liquor level may be elevated to the metering device  123  or even to the bin  121 .  
         [0038]     Excess liquor removed by the in-line drain  43  may optionally flow into a second level tank  60  via conduit  61  connected to the in-line drain. Liquor from the tank  60  may flow to the digester  11  via conduit  62 , pump  63  and conduit  64 .  
         [0039]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.