Patent Application
20170175328
Many forms of naturally occurring cellulose are used to produce chemical pulps for the production of paper. One source of cellulose is the waste from saw mills, namely sawdust. The pulping of sawdust has both advantages and disadvantages. One advantage for using sawdust as a source of cellulose is that smaller sawdust particles are relatively easy to impregnate with cooking liquor. For this reason the pretreatment systems for the chemical pulping of sawdust are less complex than those used to impregnate wood chips, which are generally more difficult to impregnate than sawdust.
One disadvantage of the chemical pulping sawdust is that sawdust can be resistant to the flow of cooking liquors. The finely divided sawdust material tends to form a compact matrix when exposed to liquid flow. Exposure to this compact matrix can limit liquid's flow through the sawdust material, if not prevent liquid flow altogether. For example, because batch digesters are highly dependent upon the capability of providing a cooking liquor circulation through the medium being pulped sawdust), it is difficult, if not impossible, to pulp sawdust in a conventional batch digester. Also, conventional continuous digesters have difficulty handling sawdust without incorporating some form of special liquid distribution device. Conventional continuous sawdust pulping systems are known in the industry and are described in U.S. Pat. No. 3,684,651, EP 0157279, and U.S. Pat. No. 6,379,504 (all of which are included by reference in this document).
EP '279 introduces the use of a fluidizing, high-speed, degassing, centrifugal pump (such as an MC® brand pump manufactured by Kamyr AB as disclosed in U.S. Pat. Nos. 4,435,193 and 4,410,337) to pump a slurry of sawdust to a treatment step where the sawdust may be subjected to chemical treatment to produce pulp. A fluidizing, high-speed, degassing, centrifugal pump is required because the fine particles of the sawdust feed material tend to act as a solid when in a slurry and do not filter well. The fluidizing, high-speed, degassing, centrifugal pump has a fluidizer at the inlet of the pump. The fluidizer may have spokes through which the sawdust slurry is fed. As the sawdust slurry passes through the spokes, the spokes break apart the clumps of sawdust and thereby allow the solid-like slurry of sawdust feed material to be broken from a solid form into a fluidized suspension where there is no phase separation (e.g. no separation of the solids from the liquids). The fluidized suspension operates more like a Newtonian fluid than the non-fluidized suspension. Gases retained in the slurry of sawdust feed material must be removed from the sawdust feed material suspension. The removal of gases can also be accomplished by the fluidizing, high-speed, degassing, centrifugal pump. The disadvantage of the fluidizing, high-speed, degassing, centrifugal pump is the fluidizing requirement, because the fluidizer is prone to clogging. Replacing a clogged fluidizer requires the operators to deactivate the fluidizing, high-speed, degassing, centrifugal pump, which results in loss of production. Great care should generally be taken, however, to achieve sufficient fluidization in order to form a pumpable suspension of sawdust material.
Another method for processing sawdust using a conventional continuous digester is given in U.S. Pat. No. 6,379,504. The '504 patent utilizes a static retention vessel. A “static” retention vessel is a retention vessel without any significant internal circulation. Internal circulation typically includes (in conventional continuous digesters, for example) screens, conduits, pumps, heaters, and the like. While steam or heated liquid may be added to the pulp in the retention vessel, to ensure that the pulp is retained at cooking temperature (although that is not normally necessary), static vessels do not attempt to draw liquid uniformly through the vessel. In this manner, static retention vessels differ from conventional batch and continuous digesters. This conventional method involving a static retention vessel also includes a slurry pump. The slurry pump is located after a chute where the sawdust feed stock is diluted from an initial solids consistency of 20% to 35% at the inlet of the chute to a consistency of 10% to 15% (typically referred to as a “medium consistency slurry”) at the outlet of the chute where an inlet to the slurry pump is positioned. Transfer of medium consistency slurry by means of a pump prior to cooking is not energy efficient. Typically, such pumps are limited to medium consistency slurries of between 8% and 16% consistency. In heating such a slurry to cooking temperature, the excess liquid volume must also be heated to cooking temperature. For example, a 12% slurry contains 7.33 lbs. of liquid per pound of fiber. In contrast, a 30% slurry contains 2.33 lbs. of liquid per pound of fiber, or less than a third of the liquid per pound of fiber. The lower consistency slurry requires additional energy to heat excess liquid to cooking temperature.
The fluidizing, high-speed, degassing, centrifugal pump of EP '279 and the slurry pump of the '504 patent both have the disadvantages of being part of a multipart, complicated, energy inefficient feed system requiring dilution liquid be added to be within a pumpable consistency. Additionally, excessive mechanical action on sawdust slurries can be damaging to fiber properties, and is otherwise undesirable.
U.S. Pat. No. 3,684,651 describes a pulping system for sawdust where washed and dewatered sawdust raw material is subjected to chemical treatment in a vapor phase. The method of the '651 patent involves a step where washing and impregnation of the sawdust feed material is combined and the feed sawdust material is in a finely divided state. This method provides a simple feed system, but nevertheless a feed system requiring multiple screw conveyors and a rotary feeder prior to the chemical treatment vessel.
Another common method used to continuously pulp sawdust is by using drag-chain type digesters, for example, an M&D-type digester as shown in FIG. 138 of Volume 5 of TAPPI's Pulp and Paper Manufacture (1989), Grace, ed. These types of digesters have an inclined vessel through which sawdust is moved through the cooking liquor by means of a conveyor mechanism. The method of a drag-chain type digester is simplified compared to other sawdust pulping methods, especially the method of U.S. Pat. No. 3,684,651. The drag-chain type digester, however, does require a conveyor mechanism and a rotary valve. Other related hardware requires continuous maintenance that makes a drag-chain type digester system, especially the rotary valve, less than ideal in most pulp mills. The rotary valve is a typical star-type feeder that inherently experiences an unbalanced pressure load due to the large pressure difference between the inlet and outlet of the valve. This load imbalance typically causes significant wear and requires regular rebuilding of the valve.
The present disclosure avoids the limitations presented by prior art continuous cooking systems for sawdust, and other finely divided comminuted fibrous material, by first eliminating the need for a rotary valve as a sawdust feeder to the treatment vessel. Second, the present disclosure eliminates the need for diluting the sawdust feed material to medium consistency prior to pumping the sawdust feed material to a treatment vessel.
This disclosure addresses the problems inherent in treating sawdust, or other finely divided sources of cellulose material (which is within the scope of the term “sawdust” as used in the present specification and claims, e.g. initial cellulose particles that flow more like a powder than like conventional wood chips), and provides for more efficient pulping, and thereby allows for less maintenance. The invention is practiced using a progressive cavity pump attached to the feed screw at the discharge of the receiving vessel. The progressive cavity pump has an inlet and an outlet. The inlet is operatively connected to the receiving vessel and the outlet is operatively connected to the treatment vessel.
It is known to use pumps, specifically a fluidizing, high pressure, degassing, centrifugal pump or a medium slurry pump in sawdust feeding sections of a sawdust pulping system. It has, however, previously been thought that a thick stock pump such as a progressive cavity pump could not be used in such a position.
A progressive cavity pump is a rotary positive displacement pump utilizing a single helical rotor within a stator within a cylindrical casing or housing. The stator has a double helical shape and is mounted or otherwise connected to the casing or housing. As the rotor moves material (in this case sawdust) in a helical motion through the stator area within the casing or housing, the sawdust is pressed and the pressure of the sawdust is increased by the motion of the progressive cavity pump. Sawdust generally exits the progressive cavity pump at a pressure higher than the entering sawdust. The sawdust also exits the progressive cavity pump without the addition of steam as pressurizing medium or liquid to form a slurry.
Until recent developments to progressive cavity pumps, progressive cavity pumps were small in size, which limited a progressive cavity pump's throughput capacity and thus encouraged the use of multiple pumps operating in series to handle the flow of material in a conventional sawdust pulping system. Progressive cavity pumps also have pressure limitations. Additionally, the costs associated with purchasing and maintaining multiple progressive cavity pumps were considered to be unreasonable.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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%.
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
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.
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.
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.
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.
CROSS-RELATED APPLICATION This application is a non-provisional application claiming priority to U.S. Provisional Patent Application No. 62/270,494 filed Dec. 21, 2015, the entirety of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62270494 | Dec 2015 | US |
