METHOD AND APPARATUS FOR PUMPING SAWDUST FEED IN A SAWDUST PULPING SYSTEM

Abstract

A method of producing chemical cellulose pulp from sawdust utilizing a static down-flow retention vessel comprising the steps of continuously pressurizing a flow of sawdust using a progressive cavity pump to produce a pressurized sawdust feed for a treatment vessel, passing the pressurized sawdust feed at super-atmospheric pressure downwardly in the treatment vessel, adding treatment white black liquor to the treatment vessel, and treating the pressurized sawdust feed in the treatment vessel with treatment white black liquor to form a treated sawdust, and discharging treated sawdust from the treatment vessel.

Claims

1. A method of producing chemical cellulose pulp from sawdust utilizing a static down-flow retention vessel, comprising the steps of continuously: using a progressive cavity pump to pressurize a flow of sawdust to produce a pressurized sawdust feed for a treatment vessel; passing the pressurized sawdust feed at super-atmospheric pressure downwardly in the treatment vessel; adding treatment white black liquor; treating the pressurized sawdust feed in the treatment vessel with treatment white black liquor to form a treated sawdust; and discharging treated sawdust from the treatment vessel.

2. The method of claim 1, wherein the flow of sawdust is maintained at between about 5% and about 20% consistency as said flow of sawdust moves from the progressive cavity pump to the treatment vessel.

3. The method of claim 1, wherein the flow of sawdust is maintained at between about 20% and about 35% consistency as said flow of sawdust moves from the progressive cavity pump to the treatment vessel.

4. The method of claim 1, wherein the flow of sawdust is maintained at between about 5% and about 35% consistency as said flow of sawdust moves from the progressive cavity pump to the treatment vessel.

5. The method of claim 1, wherein the treatment white black liquor is added to a sawdust chute.

6. The method of claim 1, wherein the progressive cavity pump has an outlet.

7. The method of claim 6, wherein the treatment white black liquor is added to the outlet of the progressive cavity pump.

8. The method of claim 1, wherein the treatment white black liquor is added directly to the treatment vessel.

9. The method of claim 8, wherein treatment white black liquor is additionally added to a sawdust chute.

10. The method of claim 6, wherein treatment white black liquor is additionally added to a sawdust chute.

11. The method of claim 6, wherein treatment white black liquor is additionally added directly to the treatment vessel.

12. The method of claim 7, wherein treatment white black liquor is additionally added directly to the treatment vessel.

13. The method of claim 7, wherein treatment white black liquor is additionally added to a sawdust chute.

14. The method of claim 1, wherein no purge steam is added to the progressive cavity pump.

15. The method of claim 1, wherein purge steam is added to the treatment vessel through a digester purge steam upper.

16. The method of claim 1, wherein purge steam is added to the treatment vessel through a digester purge steam lower.

17. A system for producing chemical pulp from sawdust, comprising: a receiving vessel to receive a flow of sawdust; a feed screw for transporting the flow of sawdust from the receiving vessel to a progressive cavity pump, wherein said progressive cavity pump pressurizes and transports the flow of sawdust to a treatment vessel; treatment white black liquor fed to the treatment vessel; and treated sawdust discharged from the treatment vessel.

18. The system of claim 17, wherein the flow of sawdust is maintained at between about 20% and about 35% consistency as said flow of sawdust moves from the progressive cavity pump to the treatment vessel.

19. The system of claim 17, wherein the flow of sawdust is maintained at between about 5% and about 35% consistency as said flow of sawdust moves from the progressive cavity pump to the treatment vessel.

20. The system of claim 17. wherein treatment white black liquor is added to one or more of: a sawdust chute, an outlet of the progressive cavity pump, or the treatment vessel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing will be apparent from the following more particular description of exemplary embodiments of the disclosure, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, with emphasis instead being placed upon illustrating the disclosed embodiments.

[0016] FIG. 1 is a schematic of a conventional sawdust pulping system using a rotary valve.

[0017] FIG. 2 is a side schematic view of a conventional sawdust pulping system using dilution and a slurry pump.

[0018] FIG. 3 is a schematic view of an exemplary embodiment of a system according to the present disclosure using a progressive cavity pump.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The following detailed description of the preferred embodiment is presented only for illustrative and descriptive purposes and is not intended to be exhaustive or to limit the scope and spirit of the invention. The embodiments were selected and described to best explain the principles of the invention and its practical application. One of ordinary skill in the art will recognize that many variations can be made to the invention disclosed in this specification without departing from the scope and spirit of the invention.

[0020] The present disclosure describes a feed system for a sawdust pulping process where a progressive cavity pump is used in place of a rotary feeder to pressurize and transport sawdust feed material from a receiving vessel to a treatment vessel.

[0021] FIG. 1 is a schematic view of a conventional sawdust pulping system 100 with a rotary valve 105 configured to feed sawdust 101 into the sawdust chute 122, 127. Sawdust 101 is fed to a feed conveyor 102. Recycled digester rejects 103 from other digesters in other pulping systems may also be fed to feed conveyor 102. The teed conveyor 102 transports sawdust 101 and recycled digester rejects 103 to a receiving vessel 104. The receiving vessel 104 may receive exhaust steam 126. Exhaust steam 126 is purge steam 112 which has been distributed to the rotary valve 105 to aide in purging solid material caught in the pockets of a rotary valve 105. Sawdust 101 from the receiving vessel 104 is discharged to a feed screw 124 located at the discharge end of the receiving vessel 104. Sawdust 101 in the feed screw 124 moves from the receiving vessel 104 into an upper section of sawdust chute 127, connecting the discharge end of the feed screw 124 and the inlet pockets of the rotary valve 105.

[0022] The rotary valve 105 is comprised of multiple pockets to receive sawdust 101 feed material from the upper section of the sawdust chute 127 at atmospheric or near atmospheric pressure and pressurize the sawdust 101 while within the rotary valve 105 to the operating pressure of the treatment vessel 106. The operating pressure of the treatment vessel 106 may be between 2 bar and 15 bar absolute. Purge steam 112 may be distributed through purge steam distributor 114 to rotary valve 105 as chute purge steam 112. From rotary valve 105, pressurized sawdust 101 is transported via sawdust chute lower section 122 where treatment white black liquor 119 is added as the pressurized sawdust 101 enters the treatment vessel 106.

[0023] Black liquor 110 (source of black liquor 110 may be elsewhere in the mill) may be pumped via black liquor pump 111 to a stream of white liquor 108 associated with white liquor pump 109. White liquor 108 and black liquor 110 may be combined to give a combined white black liquor 113. Combined white black liquor 113 is heated in a heat exchanger 107 using purge steam 112 from purge steam distributor 114. It may be desirable to bypass heat exchanger 107 with at least a portion of the combined white black liquor 113 as unheated white black liquor 117. Heated white black liquor 118 may be mixed with at least a portion of unheated combined white black liquor 117 to form treatment white black liquor 119. Treatment white black liquor 119 is used in the treatment vessel 106 to chemically treat the pressurized sawdust 101 entering treatment vessel 106.

[0024] Purge steam 112 from purge steam distributor 114 is typically provided to treatment vessel 106 as digester purge steam upper 115 and digester purge steam lower 116 to aide in the movement of sawdust 101 through the treatment vessel 106. It is possible for only one of the digester purge steam upper 115 or digester purge steam lower 116 to be used.

[0025] Sawdust 101 from treatment vessel 106 is discharged into surge tube 121. From surge tube 121, treated sawdust 123 may be sent for further processing, including depressurization, cooling, washing, bleaching, etc. Volatile gases are removed from treatment vessel 106 via DNCG release 120.

[0026] FIG. 2 shows a schematic diagram of a conventional sawdust pulping system 200 with dilution and a slurry pump 10 for pulping finely divided comminuted cellulose material referred to as sawdust 101 herein. The sawdust 101 is fed continuously by feed conveyor 11 into a receiving vessel 12 where pretreatment may take place. Pretreatment may consist of steaming or treatment with black liquor 110 or some other strength or yield enhancing chemical, for example polysulfide or anthraquinone and their derivatives. Treatment and retention in receiving vessel 12 may be from 5 minutes to 60 minutes, but is preferably between 5 minutes and 20 minutes. The receiving vessel 12 may operate at atmospheric or super-atmospheric pressures.

[0027] The receiving vessel 12 may exhibit single-convergence and side relief as disclosed in U.S. Pat. No. 5,500,083 and U.S. Pat. No. 5,628,873. Receiving vessel 12 discharges into a conveyor 13 which includes a conventional conveying screw as shown in FIG. 1, or any other conventional means of conveying the pretreated sawdust may be provided. The conveyor 13 typically comprises a screw 13 driven by a drive device such as an electric motor 13, for example a variable speed electric motor. If the conveyor 13 is pressurized, some form of pressure-isolation device can be used between the receiving vessel 12 and the conveyor 13. For example, a star-type feeder, such as rotary valve 14, may be used. The conveyor 13 is a first mixer for mixing steam and cooking liquor with the sawdust 101.

[0028] Cooking liquor, for example Kraft white liquor, is added to the conveyor 13 in white liquor line 43 to begin the impregnation of the material with cooking chemicals. Steam may be, but is not necessarily, added to the conveyor 13 via steam line 15 to begin the heating or continue the heating of the material begun in the vessel 12 and to remove unwanted air from the material. The conveyor 13 may also include a vent 16 for releasing non-condensable gases (NCG) to a conventional NCG collection system. A slurry having a consistency of about 25% or more and a temperature of between about 125 F. to 175 F. may be discharged from conveyor 13.

[0029] The conveyor 13 discharges to a teed chute 17 in which the sawdust 101 slurry is diluted to a consistency of between about 5% to about 15%. The temperature of the sawdust 101 slurry in the feed chute 17 may be between about 150 F. to about 250 F. The feed chute 17 feeds a conventional slurry pump 18. The slurry pump 18 pressurizes and transfers the sawdust 101 slurry to a conventional dewatering conveyor 19 via slurry conduit 20. The slurry may be diluted to lower the consistency of about 5% to about 10% in the slurry conduit 20, e.g. by dilution liquid (e.g. recirculated liquor, filtrate, or hot water), added via dilution liquor conduit 21. The dewatering conveyor 19 may be a conventional separator such as a top separator or an inverted top separator or another suitable conveyor.

[0030] The liquor removed from this dewatering conveyor 19, via hot liquor line 22 is typically at about 250 F. to about 300 F., may be used as the source of dilution in the dilution liquor conduit 21, after being pressurized in pump 23 and heated in heat exchanger 26. All or part of hot liquor in hot liquor line 22 may be flashed to produce a source of steam using conventional flash tank 24. For example, the pressure of the hot liquor in hot liquor line 22 may be decreased under controlled conditions, i.e. flashed, in flash tank 24 to produce a source of contaminated steam 25 and hot flashed liquor 25. The contaminated steam 25 may be used as the source of steam introduced to the conveyor 13 or receiving vessel 12. This contaminated steam 25 may be supplemented by clean steam as needed. The hot flashed liquor 25 from flash tank 24 may be used as the source of dilution liquid in teed chute 17, or elsewhere.

[0031] The dewatering conveyor 19 increases the consistency of the sawdust 101 slurry to between about 20% to about 40% and discharges the sawdust 101 slurry to a conventional steam mixer 27. The steam mixer 27 may be any conventional device (e.g. having an internal conveying screw) for introducing steam to the slurry and heating the slurry to cooking temperature, typically about 250 F. to about 350 F. (from 2 bar to 10 bar), while the slurry's consistency is being diluted by the steam addition to between about 15% to about 35%.

[0032] The sawdust 101 slurry discharged from the steam mixer 27 proceeds to a retention vessel/digester 28 in which the cooking reaction is allowed to proceed. The retention time in the retention vessel/digester 28 may range from about 30 minutes to about 6 hours. It should be noted that retention vessel/digester 28 is static, that is, retention vessel/digester 28 does not include any real cooking circulations or associated screens, because cooking circulations would be difficult to operate for such a finely comminuted material as sawdust 101. The retention vessel/digester 28 need not include an agitator at a retention vessel/digester discharge 29 but preferably includes as the discharge 29 a non-mechanical means, such as a single-convergence outlet with side relief as illustrated schematically in FIG. 2 or liquid discharge jets or nozzles.

[0033] The material is discharged through discharge 29 from retention vessel/digester 28, typically at between about 5% and about 20% consistency, and is transferred, while still at cooking temperatures and pressures (and without destructive reduction of pressure), via transfer conduit 30 to a second treatment vessel 31. In the second treatment vessel 31 the cooked, hot, pressurized material is cooled by means of filtrate from filtrate line 32. The heat of the treated material entering second treatment vessel 31 is removed via liquid extraction line 33 and used, for example, as a source of heat for heat exchanger 26. The hot liquor in liquid extraction line 33 is cooled somewhat in heat exchanger 26 and may then be sent to a conventional chemical recovery system, for example, to one or more flash tanks, to evaporators, to a recovery boiler, etc. The liquor in liquid extraction line 33 may also be used to treat material in receiving vessel 12, conveyor 13 or feed chute 17.

[0034] The second treatment vessel 31 may be a pressure diffuser where the cooked sawdust 101 is typically cooled by diffusing the cooler liquid from filtrate line 32, typically brownstock washer filtrate, through a pulp bed of cooked sawdust 101 (pulp) formed in the second treatment vessel 31. The pulp is cooled to below cooking temperature (e.g. below about 250 F.) in the second treatment vessel 31. The hot cooking liquor is displaced by the cooler liquid in this process and the hot displaced liquor is extracted as is conventional from the bottom of the pressure diffuser (in liquid extraction line 33). The cooled pulp is discharged from the top 34 of the second treatment vessel 31 and passed by cooled material discharge conduit 35 to a high density brown stock storage vessel 36 or the like. The pulp stored in the high density brown stock storage vessel 36 may be further treated by, for example, washing or bleaching, and sent to a paper, board, or pulp machine.

[0035] FIG. 3 is a schematic view of an embodiment of a system according to the present disclosure using a progressive cavity pump. A sawdust pulping system with a progressive cavity pump 300 has been conceived. Sawdust 101 is fed to a receiving vessel 104. Sawdust 101 is discharged from receiving vessel 104 into feed screw 124. Connected to feed screw 124 is a thick slurry pump, specifically a progressive cavity pump 225. The progressive cavity pump 225 has a stator piece 226 attached to the housing 228 and a rotor 227 within the housing 228. An inlet 229 is operatively connected to feed screw 124 and an outlet 230 is operatively connected to the treatment vessel 106. In some instances, the outlet 230 may be connected to a sawdust chute 122A or it may be connected directly to treatment vessel 106. If sawdust chute 122A exists, it may be a pipe and may be short in length, such as 1 foot (ft.) to 2 ft. in length. If the sawdust chute 122A is used, treatment white black liquor 119 may be added to sawdust chute 122A. It is also possible to add treatment white black liquor 119 to the outlet 230 of the progressive cavity pump 225. If desired, it is also possible to add treatment white black liquor 119 directly to treatment vessel 106. If desired, any combination of locations for treatment white black liquor 119, 119, 119 may be used.

[0036] When using progressive cavity pump 225, it is not necessary to provide purge steam 112 to the progressive cavity pump, but it may be desirable to add purge steam 112 to treatment vessel 106 through one or both digester purge steam upper 115 and digester purge steam lower 116. Any volatile gases produced in treatment vessel 106 are removed via DNCG release 120. As part of the system purge, steam 112 may first flow to a steam distributor 114 and then to treatment vessel 106.

[0037] While the invention has been particularly shown and described with references to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.