Continuous solvent pulping process

Abstract: A continuous solvent pulping process is practiced with oxygen free gas (e.g. nitrogen) purges of all major treatment vessels (23, 24, 85, 103, 105) during the time when the process is arrested or terminated. The wood chips or other cellulosic fibrous material to be pulped is steamed in a first horizontal steaming zone (23) at a pressure of about 10-20 psi, and then in a second horizontal steaming zone (24) at a pressure of about 20-75 psi. Gases, including vaporized solvent (e.g. ethanol or other alcohol) are vented (via 30, 36) from the steaming zones, and solvent is added (at 39) to the steamed material prior to feeding to a high pressure feeder (11) . The high pressure feeder introduces the material into the top of a single digesting vessel (12), liquid and chips being separated at the top of the digester vessel without mechanical means that could cause a spark. Lignin containing liquid is withdrawn from a central portion (14) of the digester and passed through flash tanks (51-53) and ultimately for lignin and alcohol recovery. Washing -- which also may be practiced using solvent pulp from a batch system -- is accomplished by first continuously passing the pulp to a pressure diffuser (85), then to a first multi stage drum displacer washer (103), and then to a second multi stage drum displacer washer (105).

Description: One alternative to the production of paper pulp by conventional kraft and sulfite chemical pulping technologies is solvent pulping to which the present invention relates. Most proposed solvent pulping processes, such as disclosed in US-A-4,764,596 and 4,100,016, use alcohol as a solvent, particularly an ethanol and methanol mixture. The alcohol is introduced with wood chips into a batch digester, and after cooking the material is subjected to three different washes in the batch digester, the first wash with a slightly weakened cooking liquor (containing alcohol), the second wash with a weak cooking liquor, and the third wash with water. One of the proposed advantages of the solvent pulping technique is that lignin may be recovered from the "black liquor" produced from the process (a solution of lignin in a water miscible organic solvent such as a lower aliphatic alcohol). It is necessary, in order to make the system economical to recover as much of the alcohol as possible. Significant markets may also develop for the lignin, which may make solvent pulping economical and advantageous. At the present time, there are no known large scale commercial installations in which solvent pulping is practiced. One of the most significant reasons for this is the inability to recover a substantial enough portion of the alcohol. If one utilizes a batch digester, with washing in the digester, as described above, the alcohol consumption may be such as to make the procedure economically unattractive. There are certain problems associated with proposed solvent pulping systems. One is the potential safety hazard as a result of solvent vapor, oxygen (i.e. an oxidative gas), and a condition -- such as a spark -- capable of producing an explosion, combining. In order to guard against this, when the operation of the batch digester is being arrested or terminated, any portions thereof where vapor can collect are purged with nitrogen, or a like substantially oxygen free gas. It has been recognized for many years that the solvent pulping process could theoretically be improved if it were made continuous, such as the majority of commercial kraft and sulfite pulping systems. However the safety problems described above, plus the need for equipment to maintain sufficient pressures to accommodate solvent pulping (which pressures are much higher than for kraft pulping) made the realization of that ideal difficult to achieve. It was also recognized that the lack of recovery of a substantial portion of the alcohol as a result of washing was a major drawback, but techniques for significantly reducing the alcohol loss were not envisioned. According to the present invention, it is possible to make the solvent pulping process continuous. The method of solvent pulping to accomplish the objectives set forth above is practiced by the steps of continuously: (a) Steaming the material to remove the air therefrom. (b) Mixing the material with solvent pulping liquid to produce a mixture. (c) Feeding the mixture of material and solvent pulping liquid under pressure to the top of the vessel. (d) Separating some liquid from the material at the top of the vessel in a manner positively precluding the generation of electrical or mechanical sparks. (e) Returning the separated liquid from step (d) to step (b). (f) Withdrawing a liquor having a high concentration of dissolved lignin from a central portion of the vessel; and (g) withdrawing produced pulp from the bottom of the vessel. Step (d) is practiced by the steps consisting essentially of providing a plurality of screens at the top of the vessel, withdrawing liquid through at least one screen while liquid is not being withdrawn through at least one other screen, and periodically switching which screens liquid is and is not being withdrawn through. In a preferred embodiment of the present invention, the method comprises, between the top of the vessel and the central portion of the vessel, the further steps of (h) withdrawing a liquid having lignin therein from the vessel (via 58, 59); (i) removing a portion of the withdrawn liquid (via 60) and introducing it (via 50) in a first flash tank (51); (j) replacing the removed liquid with solvent pulping liquid (via 63) to reduce the solids ratio of the liquid in the vessel; (k) heating the withdrawn liquid (at 65); and (l) recirculating the heated liquid into the vessel (via 66). In a further preferred embodiment of the method, the liquid is withdrawn between the top and the central portion from screens; and the recirculated liquid is introduced into the vessel above the screens. In another preferred embodiment of the method, it comprises the further steps of (m) withdrawing (through 76) a liquid having lignin therein; (n) removing a portion of the withdrawn liquid (via 77) and flashing it (at 51, 52, 53); and (o) removing a portion of the withdrawn liquid (via 78), heating the liquid (80) and then reintroducing it into the vessel adjacent the top thereof (via 81). In the practise of step (m), the further step (p) of adding fresh solvent pulping liquid to the second stream prior to heating thereof is comprised. In a further preferred embodiment of the method, a first steaming zone is utilized and a second steaming zone in series with the first zone, wherein step (a) is practised by the substeps: (i) adding steam (26) to the material in the first steaming zone (23) to an introduction plenum (27) along a significant part of the middle portion of the first steaming zone (23) while maintaining the pressure at about 69-138 kPa (10 to 20 psi); (ii) isolating the first steaming zone from the second steaming zone (via 25); (iii) maintaining the pressure in the second steaming zone at about 138-517 kPa (20 to 75 psi); (iv) purging the second steaming zone with steam from a medium pressure steaming zone (35) by introducing steam into the material (at 34) to flow concurrently with the material into and through the second steaming zone (24); and (v) venting gases from the first and second steaming zones (via 30, 36). In a further preferred embodiment the material is moved horizontally within the first and second steaming zones. Preferably, the method comprises further the steps of (vi) recovering ethanol from the gas vented in substep (v). Also preferably step (a) is further practised by the substep of (vii) purging the second steaming zone with a substantially oxygen free gas when the practice of steps (a) to (g) is arrested or terminated. In a further preferred embodiment of the method, the substep (vii) is practised by using nitrogen gas. In another preferred embodiment of the method, step (a) is practised by the substeps of continuously (viii) adding steam to material in the steaming zone while maintaining the pressure at about 69-517 kPa (10-75 psi) (10, Fig. 1A); (ix) venting gases from the steaming zone (via 31); and (x) purging the steaming zone with a substantially oxygen free gas when the practice of steps (a) to (g) is arrested or terminated (via 41). It is the primary object of the present invention to provide a continuous solvent pulping method. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 (i.e. FIGURES 1A - 1C) is a schematic view of an exemplary apparatus for practicing continuous solvent pulping according to the invention. DETAILED DESCRIPTION OF THE DRAWINGS Exemplary apparatus for continuous solvent pulping of comminuted cellulosic fibrous material, such as wood chips, is illustrated schematically in FIGURES 1A - 1C. The major components of the apparatus include a system for steaming the material to remove the air therefrom, a high pressure feeder and associated components -- illustrated generally by reference numeral 11 -- for feeding the slurried chips to the digesting vessel; and the upright continuous digesting vessel shown generally by reference numeral 12. The digester (extractor) 12 has associated therewith non-sparking liquid/material separation means 13 at the top thereof, a central extraction area and system 14 for the withdrawal of lignin containing liquid, and a pulp discharge 15 at the bottom thereof. Also a recirculation system 16 is provided between the central portion system 14 and the top separation system 13. The steaming apparatus (FIG. 1A) is not novel. In a conventional kraft system, a chips bin 20 is provided, connected via a chip meter 21 and low pressure feeder 22 to a horizontal steaming vessel 23. However the horizontal steaming vessel 23 is then typically connected directly to the high pressure feeder 11. Such an arrangement is not satisfactory for solvent pulping, however. According to the invention it is necessary to utilize a second horizontal steaming vessel 24 with a second low pressure feeder 25 isolating the two steaming vessels. The vessel 23 is operated at a much lower pressure than the vessel 24. Typically the Dressure in vessel 23 is about 69-138 kPa (10-20 psi). In the vessel 24 the pressure is typically about 138-517 kPa (20-75 psi), preferably about 310 kPa (45 psi). Steaming may be done in the chips bin 20, as is conventional, and steaming is done in the first steaming vessel 23 by passing low pressure steam from source 26 to an introduction plenum 27 along a significant part of the middle portion of the vessel 23, as is conventional. Chips are introduced into the vessel 23 from the low pressure feeder 22 into chips inlet 28, and pass out of the vessel 23 through chips outlet 29. Gases -- including solvent vapor -- are vented through vertically extending vent pipe 30 which is connected to conduit 31 which ultimately passes to a condenser 32, for removal of the alcohol therefrom. A first vertical conduit 32 is provided between the second low pressure feeder 25 and the chips inlet 33 to the second steaming vessel 24. Steam from medium pressure steam source 35 is introduced into the conduit 32 at introduction port 34 (just below feeder 25) to purge the chips, the steam and chips together entering the vessel 24 through the chips inlet 33. This minimizes the possibility that solvent vapor will pass backwardly through the system. Gases are vented from the vessel 24 by gas vent 36, which is near the chips outlet 37, and extends upwardly from the vessel 24. Extending downwardly from the chips outlet 37 -- generally opposite the vent 36 -- is a second vertical conduit 38, which is connected to the high pressure feeder 11. Within the conduit 38 the chips are slurried with solvent cooking liquor, the solvent -- e.g. a mixture of 90% ethanol and 10% methanol -- is introduced at port 39. When the steaming operation is arrested or terminated, one must be careful that no solvent vapors collect in pockets within any of the vessels. If such collection occurs, a very large safety hazard occurs, since if the vapor-mixes with oxygen -- if the temperature conditions are right, or if there is a spark -- an explosion can occur. In order to preclude this possibility, according to the invention means are provided for introducing a purging gas into the conduit 38 at port 40 to flow countercurrent to the normal flow of chips through the vessel 24, etc. The purging gas is preferably provided through conduit 41 from a source of pure nitrogen 42 or the like. It is to be understood that any substantially oxygen free gas (meaning any gas not having oxygen or any oxidative -- or solvent, such as alcohol -- component) that is economical may be utilized. "Pure" nitrogen (that is a gas containing substantially all nitrogen, although certainly impurities will exist) is best suited from the cost standpoint . The high pressure feeder 11 (FIG. 1A) according to the invention must be specially designed. It must be capable of withstanding pressures much greater than for conventional chemical pulping systems. While it is possible to beef up a conventional Kamyr® high pressure feeder so that it can handle 4826 kPa (700 psi) (rather than the 2068 kPa (300 psi) that is conventional), alternatively a Kamyr® shoe feeder can be utilized, such as disclosed in US-A-4,516,887 and/or 4,430,029. The rest of the components associated with the high pressure feeder 11, such as a low pressure pump 42, high pressure pump 43, sand separator, level tank, etc. (all unnumbered) are conventional, except that they must be capable of withstanding the larger pressures typically encountered in a solvent pulp process. From the high pressure feeder 11 the steamed chips entrained in solvent and water are passed in line 44 to the top 45 of the digester 12 (FIG. 1B). As previously indicated, the top 45 of the digester 12 includes a solids/liquid separator separating apparatus 13, however the apparatus 13 is not conventional in one vessel hydraulic digesting systems. Instead of a screw and perforated cylinder, or the like, as is conventional, the solids/liquid separator 13 comprises a plurality of screens 46, and a switching means 47 for controlling which of the screens 46 has extraction therethrough, and which screens are dormant (i.e. have no extraction therethrough). Typical screen switching systems are shown in US-A-4,547,264, and the references cited therein. The liquid that is withdrawn passes into conduit 48, and then is returned to the high pressure feeder 11. It would not typically be expected that a non-mechanical, spark free liquid/material separation system such as the system 13 could be utilized to effectively accomplish its separating function. However it is possible, according to the invention, because the alcohol cooking liquor has a specific gravity much less than the typical kraft cooking liquor. The alcohol-water mixture which carries the chips in the line 44 typically has a specific gravity of about 0.6-0.8 (depending upon temperature and being very sensitive to the temperature). The same liquid in a kraft system has a specific gravity of about 1.0-1.05. This means that the buoyancy of the chips in the liquid is much less, and therefore the chips will have a tendency to move downwardly in the vessel 12 more quickly. The downward movement of the chips is illustrated by arrow 49 in FIGURE 1B. As previously mentioned, extraction of lignin rich liquid from the digester 12 occurs at the central portion system 14 thereof. The lignin rich liquid is extracted through the screens of the system 14 into line 50, and then passes to a series of flash tanks, e.g. first, second, and third flash tanks 51-53 (FIG. 1C). In each case, a mixture of water and solvent vapor, generally enriched in solvent concentration flashes off of the liquid, and the liquid is concentrated, the concentrated liquid ultimately passing in line 54 to liquor recovery stage 55 where the lignin and alcohol are recovered in a known manner (e.g. see US-A-4,764,596 for one example). The vapor mixture which flashes off from the tanks 51-53 passes into lines 54 through 56, and depending upon its pressure is ultimately used elsewhere within the system, e.g. as process heat in the solvent recovery system. Between the top of the vessel 45 and the central extraction portion 14 a recirculation screen and system means is provided, shown generally by the reference numeral 16. This system includes, for example, screens 57 from which liquid is withdrawn in conduits 58 and 59. At the level of the screens 57, some of the lignin has already dissolved, therefore the liquid in the conduits 58, 59 has lignin therein. In order to maintain the liquid/material ratio at the desired high proportion of liquid, according to the invention a portion of the liquid from the conduits 58, 59 is removed in conduit 60. Conduit 60 includes an isolation valve 61 and a flow control valve 62 therein. The lignin rich liquid in conduit 60 is introduced into the conduit 50 just before first flash tank 51. The rest of the liquid removed in the conduits 58, 59 -- as well as a source of fresh solvent in conduit 63, to reduce the solids ratio of the liquid -- is passed by pump 64 to conventional indirect heater 65, and is ultimately recirculated in line 66 to a portion of the interior of the digester 12 above the screens 57. The line 63 also includes an isolation valve 63' and a flow control valve 63''. In the exemplary embodiment illustrated in FIGURE 1B, a second set of screens 67, with corresponding conduits, heater, and recirculation path (unnumbered -- see FIG. 1C) is also utilized, and an additional heater is provided in case one of the two normally used heaters malfunctions. The chips continue to flow downwardly in the vessel 12 past the central portion 14, as illustrated by arrow 68, however while the solvent flows downwardly in the top portion of the vessel -- as illustrated by arrow 69 -- below the extraction portion 14 the liquid flows countercurrent to the chips, as illustrated by arrow 70. A conventional scraper 71 is provided at the bottom 72 of the vessel, with the pulp extracted in pulp outlet 15 connected to blow line 73. According to the invention, again -- in order to handle the relatively large volume of liquid compared to kraft or sulfite processes -- the extra sets of screens 74, 75 are utilized. A portion of the liquid withdrawn in conduits 76 from the screens 74 passes in line 77 to be flashed in the flash tank 51, while the rest is recirculated in conduit 78, under the influence of pump 79, beipg passed to heater 80 and then ultimately returned via conduit 81 to the top of the digester 12. The purpose of splitting the flows into conduits 77, 78 is to remove some of the solids and replace them with liquid, the fresh liquid containing solvent being added in conduit 82. Conduit 82 -- which supplies fresh liquid both to the conduit 78 and the conduit 63 -- is ultimately connected up to the filtrate stage 83 from the washing system. In the entire solvent pulping process of FIGURES 1A - 1C, it is necessary to maintain the pressure above the vapor pressure of the alcohol-water mixture at all points. With one particular useful mixture of alcohol and water, the pressure would be maintained at about 2930-3102 kPa (425-450 psi) However it is conceivable that the pressure could be as high as 4137 kPa (600 psi), therefore the vessel 12 should be constructed to accommodate such a pressure. Within the digester 12 the temperature is approximately the same as for the batch solvent pulping process. That is typically in the vessel 12 between the screens 74 and 57 the temperature will be about 182-204°C (360-400°F). Both above and below those points the temperature will be less; for example the temperature in pulp discharge 15 is 87°C (190°F). It will thus be seen that according to the present invention it is possible to make a solvent pulping process continuous. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and procedures.

Claims: A method of solvent pulping comminuted cellulosic fibrous material using a vertical digester (12) vessel having a top (13) and a bottom (15), comprising the steps of: (a) steaming the material to remove the air therefrom; (b) mixing the material with solvent pulping liquid (at 38, 39) to produce a mixture; characterized by the steps of continuously (c) feeding (with 11) the mixture of material and solvent pulping liquid under pressure to the top of the vessel; (d) separating (with 46, 47) some liquid from the material at the top of the vessel in a manner positively precluding the generation of electrical or mechanical sparks; (e) returning (via 48) the separated liquid from step (d) to step (b); (f) withdrawing (through 14, 50) a liquid having a high concentration of dissolved lignin from a central portion of the vessel; and (g) withdrawing produced pulp from the bottom (15) of the vessel. The method of claim 1 wherein step (d) is practiced by only the steps of providing a plurality of screens (46) at the top of the vessel, withdrawing liquid through at least one screen while liquid is not being withdrawn through at least one other screen, and periodically switching (through 47) which screens liquid is and is not being withdrawn through. The method of claim 1 comprising, between the top (13) of the vessel and the central portion (14) of the vessel, the further steps of: (h) withdrawing a liquid having lignin therein from the vessel (via 58, 59); (i) removing a portion of the withdrawn liquid (via 60) and introducing it (via 50) in a first flash tank (51); (j) replacing the removed liquid with solvent pulping liquid (via 63) to reduce the solids ratio of the liquid in the vessel; (k) heating the withdrawn liquid (at 65); and (l) recirculating the heated liquid into the vessel (via 66). The method of claim 3 wherein the liquid withdrawn between the top and the central portion is withdrawn (via 58, 59) from screens (57); and wherein the recirculated liquid is introduced into the vessel (via 66) above the screens (57). The method of claim 1 or 3 comprising the further steps of: (m) withdrawing (through 76) a liquid having lignin therein; (n) removing a portion of the withdrawn liquid (via 77) and flashing it (at 51, 52, 53); and (o) removing a portion of the withdrawn liquid (via 78), heating the liquid (80) and then reintroducing it into the vessel adjacent the top thereof (via 81). The method of claim 5 comprising, in the practice of step (m), the further step of (p) adding fresh solvent pulping liquid to the second stream (via 82) prior to heating thereof. The method of claim 1, utilizing a first steaming zone (23) and a second steaming zone (24) in series with the first zone, wherein step (a) is practiced by the sub-steps of: (i) adding steam (26) to the material in the first steaming zone (23) to an introduction plenum (27) along a significant part of the middle portion of the first steaming zone (23) while maintaining the pressure at about 69-138 kPa (10 to 20 psi); (ii) isolating the first steaming zone from the second steaming zone (via 25); (iii) maintaining the pressure in the second steaming zone at about 138-517 kPa (20 to 75 psi); (iv) purging the second steaming zone with steam from a medium pressure steaming zone (35) by introducing steam into the material (at 34) to flow concurrently with the material into and through the second steaming zone (24); and (v) venting gases from the first and second steaming zones (via 30, 36). The method of claim 7 wherein the material is moved horizontally within the first and second steaming zones. The method of claim 7 comprising the further step of (vi) recovering ethanol from the gases vented in sub-step (v). The method of claim 7 wherein step (a) is further practiced by the sub-step of (vii) purging the second steaming zone with a substantially oxygen free gas (via 41) when the practice of steps (a) to (g) is arrested or terminated. The method of claim 10 wherein sub-step (vii) is practiced by using nitrogen gas. The method of claim 1 where step (a) is practiced by the sub-steps of continuously: (viii) adding steam to material in the steaming zone while maintaining the pressure at about 69-517 kPa (10-75 psi)(10, Fig. 1A); (ix) venting gases from the steaming zone (via 31); and (x) purging the steaming zone with a substantially oxygen free gas when the practice of steps (a) to (g) is arrested or terminated (via 41).

Inventors: GREENWOOD BRIAN F, LEBEL DAVID J, PHILLIPS JOSEPH R, GREENWOOD, BRIAN F., LEBEL, DAVID J., PHILLIPS, JOSEPH R.
Applicants: ALCELL TECH INC, ALCELL TECHNOLOGIES INC.
Application year: 1991
Main classification: D21C3
IPC codes: D21C3, D21C7, D21C9