US3303088A

US3303088A - Continuous liquid-phase rapid pulping

Info

Publication number: US3303088A
Application number: US274186A
Authority: US
Inventors: Adolf W Gessner
Original assignee: Lummus Co
Current assignee: CB&I Technology Inc
Priority date: 1963-04-19
Filing date: 1963-04-19
Publication date: 1967-02-07
Anticipated expiration: 1984-02-07
Also published as: SE301412B; NO115644B

Description

Feb. 7, 1967 w ss ER 3,303,088

CONTINUOUS LIQUID-PHASE RAPID FULPING Filed April 19, 1963 file fl "l7 el 3 IMPREG.

ZONE WD) 2a 7 I, J- 71 30 25 COOKING 26 27 A 6? K2 W ZONE WHITE r 7 R LIQUOR A I 3 33 A 32 Q 17 BLACK LIQUOR TO RECOVERY INVENTOR ADOLF W. GESSNER ATTORNEY United States Patent 3,303,088 CONTINUQUS LIQUID-PHASE RAPID PULPING Adolf W. Gessner, Montclair, N.J., assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Apr. 19, 1963, Ser. No. 274,186 Claims. (Cl. 162-19) This invention relates to the delignification of cellulosic materials to prepare cellulosic fibers and, more particularly, to an improved process and apparatus for effecting the continuous and rapid digestion of cellulosic materials.

. Prior continuous processes and apparatus for effecting the delignification of cellulosic material have been directed to the simulation of the processing steps of batchwise operation in a continuous manner. Accordingly, such continuous pulping processes and apparatus have been designed to reproduce the time, temperature, and pressure relationships of the processing steps of batch operation utilizing continuous processing techniques. For instance, using kraft liquors, the batch digester, once having reached a temperature in the range of from 338 F. to 356 F., is maintained at such temperature for a period of from two to three hours to effect delignification. Continuous processes designed for kraft liquor operation retain the cellulosic material being treated at a temperature within such temperature range for the same two to threehour period. Savings in utilities, of course, are nevertheless realized through continuous operation.

A recent process for continuously digesting fibrous material has found commercial acceptance in the industry. In accordance with this process, cellulosic material is introduced by a screw-type feeder into the digester wherein the cellulosic material continuously passes downwardly through the various zones of the digester. In the upper portion of the digester, impregnation of the cellulosic material is effected at an impregnation temperature within the range of from 60 C. to 120 C. The material is thereafter heated to a temperature of 140 C. to 180 C. and higher whereby delignification of the cellulosic material is accomplished in a section below the impregnation section of the digester. Since a requirement of the process is to effect digestion in the liquid phase and at a pressure whereby no vaporization of the cooking liquor is effected, a compression liquid is introduced into the lower portion of the digester to maintain the pressure within the digester above the pressure at which vaporization of the digestion liquor will occur. The digested cellulosic material, together with spent cooking liquor and compression liquor are withdrawn at a temperature of 120 C. from the lower portion of the digester and passed to subsequent units for further treating. In accordance with this process, the cellulosic material is maintained at delignifying temperatures in the digestion zone of the digester for a period of time normally required for the delignification of cellulosic materials in a batch digester, i.e., two to three hours.

No liquid-phase delignification process has been developed which substantially reduces the time period required for effecting the delignification of the cellulosic material while improving or maintaining the quality of the resultant pulp, as well as substantially reducing the size of the digester and related equipment for any desired capacity.

A primary object of my invention is to efiiect the continuous liquid-phase delignification of cellulosic materials at temperatures and pressures normally employed in batch operations in time periods which are substantially less than the time periods heretofore required to effect the delignification of cellulosic material either in batch or continuous operation.

Another object of my invention is to provide a continuous process for rapidly effecting the delignification of cellulosic materials wherein the cellulosic material is rapidly heated to impregnation temperature by partially spent cooking liquor.

A further object of my invention is to provide a com tinuous process for effecting the delignification of cellulosic material by rapidly heating the impregnated cellulosic material to delignifying temperatures.

Still another object of my invention is to provide a continuous process for effecting the delignification of cellulosic material having reduced utility requirements.

A still further object of my invention is to provide a continuous process for effecting the delignification of cellulosic material in processing equipment substantially smaller than equipment heretofore utilized.

Another object of my invention is to provide a continuous process for effecting the delignification of cellulosic material wherein the delignification reaction is rapidly halted in a cooling zone of the apparatus and the delignified material is withdrawn from said zone at a temperature below the boiling point of the cooking liquor at atmospheric pressure whereby cellulosic fiber damage is essentially minimized.

A further object of my invention is to provide a continuous process for effecting the digestion of cellulosic material in a digestion vessel having a cooling zone comprised of at least two sub-sections whereby the liquid flow through each sub-secti0n is concurrent with the flow of the material, but the liquor flow between each sub-section is countercurrent.

Other objects and a fuller understanding of my invention may be had by referring to the following description taken in conjunction with the accompanying drawing in which the figure is a partial schematic flow diagram illustrating a preferred embodiment of my invention for effecting the rapid and continuous digestion of cellulosic material in liquid phase for the preparation of cellulosic fibers of high strength and low bleachability requirements.

In accordance with my invention, cellulosic material, after steaming, is introduced into a feed tank wherein the steamed cellulosic material is submerged in a heated cooking liquor. A slurry of the cellulosic material and liquor is introduced into the upper portion of a digester having an impregnation zone, a digestion zone, and a cooling zone. A major portion of the liquor utilized for carrying the cellulosic material to the upper portion of the digestion zone is withdrawn from such upper portion and returned to the feed tank. The consistency of the slurry introduced into the upper portion of the digestion zone is increased by the withdrawal of such recycle liquor.

The chip-liquor slurry is contacted with partially spent cooking liquor at a temperature of from about 360 F. to about 385 F. to rapidly raise the temperature of the chip-liquor slurry to about 240 F. to about 280 F. The cellulosic material is passed through the impregnation zone for a time period of about 10 to 35 minutes, preferably from about 20 to about 30 minutes, whereby the cellulosic material is substantially impregnated. Thereafter, the chip-liquor slurry is contacted with another stream of cooking liquor heated to a temperature of from about 370 F. to about 400 F. to rapidly raise the temperature of the cellulosic material to a temperature within the range of from about 330 F. to about 385 F. The cellulosic material is passed downwardly through the digestion zone of the digester for a time period of about 10 to 35 minutes, preferably from about 20 to about 25 minutes, to effect delignification of the cellulosic material. The solids consistency of the chip-liquor slurry in both the impregnation zone and the top of the cooking zones is maintained substantially equal and preferably within the range of from about 10% to about 20% by weight. Variations of consistency of not more than 3% are preferred.

The delignified material or cellulosic fibers passing downwardly from the digestion zone of the digester has a solids consistency of about to about by weight, as a result of the solubilization of approximately onehalf of the solids introduced into the digester. A portion of the liquor is withdrawn from the material to increase the solids consistency to about 10% to about 20% by weight. The delignified material is now passed through a cooling zone to reduce the temperature of the delignified material to a temperature below the atmospheric boiling point of the cooking liquor, normally about 212 F. to about 220 F. thereby substantially eliminating fibre damage due to steam flashing during subsequent pressure reduction. The delignified material is subsequently passed to a screen to remove knots and undigested cellulosic material, and thereafter washed to remove substantially all of the spent cooking liquor and then passed to subsequent units for further processing.

The cooling zone of the digester is'comprised of at least two sub-sections. The flow of the delignified material and liquor through such sub-sections is concurrent; however, the sequence of contact between the cooling liquor and material to be cooled in such sub-sections is in effect countercurrent. The cooling liquor is first introduced into the lower cooling sub-section and contacts the delignified material which has been partially cooled in the upper cooling sub-section. The cooling liquor is then withdrawn from the lower cooling sub-section and is introduced into the upper cooling sub-section from which the liquor is withdrawn after partially cooling the material passing into the upper cooling sub-section from the delignification zone. The object of this countercurrent flow arrangement is to cool the delignified material below the atmospheric boiling point of the liquor with a substantially smaller quantity of cooling liquor than would be required with concurrent flow. The object of maintaining concurrent flow within each individual cooling sub-section is to avoid the difficulties associated with countereurrent flow of liquor and delignified material, i.e., excessive fluid pressure drop resulting in the compression of the delignified solids, which may cause even higher fluid pressure drops and eventual plugging of the equipment, or extremely low velocities of the liquor relative to the solids which necessitate uneconomically large vessels for a given throughput of materials.

As will be readily apparent, the combined impregnation-delignification time period in accordance with the present process is significantly less than impregnationdelignification time periods of processes presently practiced, whether batch or continuous.

Now referring to the drawing, wood chips or like cellulosic material prepared in a normal manner and at ambient temperatures are introduced through line 1 into storage bin 2. The wood chips are withdrawn from storage bin 2 through line 3 controlled by rotary valve 4, and passed into a screw conveyor 5 driven by motor 6. The wood chips are preheated as they are passed through the screw conveyor 5 by low pressure steam introduced into screw conveyor 5 through line 7. Steam condensate is withdrawn from the screw conveyor 5 through line 8. The thus preheated and steamed chips are passed from screw conveyor 5 through line 9, controlled by rotary valve 10, and are introduced into feed tank 11. The preheated chips in feed tank 11 are submerged in a liquor formed of cooking liquor and recycled liquor introduced into the feed tank 11 through lines 12 and 13, respectively.

A slurry of wood chips and liquor is withdrawn from the storage tank 11 through line 14 by pump 15 and passed through line 16 into the upper portion of a digester, generally indicated as 17. The consistency of the chipliquor slurry introduced into the digester 17 is from about 1% to 6%, preferab y about 4%, and is determined y 5% to about 25%, preferably 10% to 20%.

4 various factors, such as the size of the chips, temperature and pressure of digestion, the construction of pump 15, etc. Below the point at which the slurry is introduced, a major portion of the liquor is withdrawn through a strainer screen 18, or other suitable devices known in the art, and is passed through line 13 to the feed tank 11, thereby increasing the consistency of the slurry.

Immediately below the screen 18, partially spent cooking liquor at a temperature of about 350 F. to about 385 F. in line 19 is introduced through a plurality of nozzles, generally indicated as 20, into the chip-liquor slurry. The velocity at which the partially spent cooking liquor is introduced into the chip-liquor slurry is sufiicient to effect turbulent mixing of the slurry and partially spent cooking liquor and thereby rapidly heat the chipliquor slurry to a temperature between about 240 F. to about 280 F. Thus, the cellulosic material is heated to impregnation temperatures within from about 10 to about 30 seconds. Excess liquor is withdrawn from a strainer screen 21 through line 22 to maintain the consistency of the slurry passing through the impregnation zone at about The chipliquor slurry is caused to pass through the impregnation zone for a period preferably of from about 20 to about 30 minutes to effect substantial impregnation of the cellulosic material.

At the lower portion of the impregnation zone, hot cooking liquor in line 23 is introduced through a plurality of nozzles, generally indicated as 24, into the chip-liquor slurry passing from the impregnation zone. The cooking liquor is heated to a temperature of from about 360 F. to about 400 F. and is passed through the nozzles 24 at a velocity such that turbulent mixing of the chips and heated cooking liquor effects rapid heating of the chips to a temperature of from 330 F. to 385 F. A strainer screen 25 is positioned below the nozzles 24 and excess liquor is withdrawn through line 26 by pump 27 and is heated in heat exchanger 28 to form the heated cooking liquor introduced into the chip-liquor slurry through line 23. Steam in line 29 is introduced into the heat exchanger 28 to heat the cooking liquor, with steam condensate being withdrawn therefrom through line 30. The consistency of the chip-liquor slurry passing into the digestion zone of the digester 17 is substantially the same as that of the chip-liquor slurry passing through the impregnation zone. The consistency of the slurry in the digestion zone is maintained at about 5% to about 25 preferably from 10% to 20%. The consistencies of the slurries passing through the impregnation and digestion zones may be independently controlled by recirculation of liquor through lines (not shown), but are usually maintained at about the same percentage. In some instances, the consistencies may vary depending on the species of cellulosic material, temperature of impregnation and delignification and utility requirement considerations.

The now heated chip-liquor slurry is caused to pass through the digestion zone of the digester 17 for a time period of about from 15 to about 30 minutes, preferably from 15 to about 30 minutes, preferably from 20 to 25 minutes, to effect solubilization of the lignins in the cellulosic material to produce cellulosic fibers. Partially spent cooking liquor is withdrawn through a strainer screen 31 positioned at the lower portion of the digestion zone. A portion of the liquor withdrawn from the screen 31 is passed through line 32 by pump 33 and constitutes the partially spent cooking liquor introduced into the chip-liquor slurry in line 19 to elfect the rapid heating of the wood chips to the impregnation temperature thereof. The remaining portion of the partially spent cooking liquor withdrawn through screen 31 is passed through line 32a controlled by valve 33a into a flash drum 34. The liquor is flashed in the drum 34 and forms the steam which is introduced into the screw conveyor 5 through line 7 controlled by valve 35.

Below the screen 31, a cooled and partially spent liquor stream in line 36 is introduced into the digester 17 through a plurality of nozzles, generally indicated as 37, to effect a rapid and partial cooling of the now delignified material, as well as a partial washing of the spent cooking liquor from such material. The nozzles 37 function in a manner similar to the

nozzles

20 and 24 in that turbulent mixing is effected to rapidly cool the delignified material. Excess liquor is withdrawn through a strainer screen 38 and passed through line 39 for further processing as hereinafter described.

Below screen 38, a second cooling liquor stream in line 40 is introduced through a plurality of nozzles, generally indicated as 41, into the partially cooled material to effect further cooling of the material, as well as a further washing thereof. The material is thus cooled to a temperature below or just slightly above the boiling point of the liquor at atmospheric pressure. Excess liquor is withdrawn from the lower portion of the cooling zone of the digester 17 through strainer screen 42 and is passed through line 43 by pump 44 and constitutes the first cooling liquor stream in line 36 introduced through the nozzles 37 into material passing through the digester 17.

In the event that the temperature of the delignified material at the exit of the lower cooling sub-section is slightly above the atmospheric boiling point of the liquor, or that the slurry consistency at this point is too high for smooth discharge through the pressure reducing valve 48, further cooling liquor may be introduced through line 45 and a plurality of nozzles, generally indicated as 46, to cool the material to a temperature below the atmospheric boiling point of the liquor and/ or to reduce the slurry consistency to a level at which the flow through the reducing valve 48 is smooth and mechanical fibre damage minimized. Thus, the consistency of the delignified material withdrawn from the digester is normally of from about 3% to about 15 preferably from 5% to The quantity of liquor introduced into the downwardly flowing material in digester 17 through the

nozzles

37 and 41 is substantially equal to the quantity of liquor withdrawn through the

screens

38 and 42.

The now cooled delignified material is withdrawn through line 47, controlled by reducing valve 48. Since the temperature of the delignified material and liquor withdrawn from the digester is below the boiling point of the liquor at atmospheric pressure, there is no flashing of steam as the material and liquor pass through the reducing valve 48 and, consequently, fibre damage is minimized. The material passing through the valve 48 is passed through line 49 and contacted with a wash water stream in line 50. The combined stream is passed through line 51 to a screen 52 wherein the cellulosic fibers are freed of knots and undissolved cellulosic material. The wash water stream in line 50 is introduced into the delignified material to reduce the solids consistency and thereby to facilitate the removal of knots and undissolved cellulosic material. Knots and undissolved cellulosic material are withdrawn from the screen 52 through line 53. The delignified material is withdrawn from the screen 52 through line 54 and is passed to a rotary vacuum washer 55. In the rotary vacuum washer 55, wash water is introduced through line 56 to leach out spent cooking liquor from the delignified material or cellulosic fibres. Washed pulp is withdrawn from rotary vacuum wash 55 through line 57 and is passed to subsequent units (not shown) for further processing.

The wash water from washer 55 is withdrawn through line 58 and is separated into two portions in

lines

59 and 60. The portion in line 59 is passed by pump 61 and introduced through line 50 into the delignified material in line 49. The portion in line 60 is passed by pump 62 and introduced into the digester 17 through line 40 and constitutes the aforementioned second cooling stream. A portion of the Wash water in line 40 may be introduced into the digester through line 45.

Partially spent cooking liquor is withdrawn from flash drum 34 through line 63. The liquor streams in

lines

22 and 63, together with a portion of the black liquor in line 39 are combined and passed through line 64 to be combined with the fresh cooking liquor or white liquor in line 65 for introduction into the storage tank 11 through line 12. The remaining portion of the black liquor in line 39 is passed through line 66 to a black liquor recovery unit (not shown) for treatment of the black liquor to recover the chemical values thereof.

It is apparent from the aforementioned description of the two-stage cooling zone that the liquor and delignified material is passed concurrently through the individual sections of the cooling zone of the digester 17. Countercurrent flow of delignified material and liquor is avoided since at even low relative velocities of delignified material to liquor, high fluid pressure gradients may be formed which lead to excessive compression of the material and possible plugging of the equipment. It is noted, however, that the two sections of the cooling zone are designed to provide for a countercurrent sequence of contact of the cooling liquor with the delignified material while providing for concurrent flow of material and liquor within each cooling subsection. In this manner, the delignified material may be rapidly cooled to a temperature below the boiling point of the cooking liquor prior to being withdrawn from the digester 17 through line 47.

The countercurrent flow of the liquor between the two stages of the cooling zone effects a substantial washing of the delignified material before the material is withdrawn from the digester 17. Consequently, the cellulosic fibres passed to the rotary vacuum washer 55 through line 54 contain a substantially smaller quantity of black liquor solids which permit final washing of the cellulosic fibres in fewer stages compared to pulp prepared in known batch or continuous processes.

As an example of my invention, on an hourly flow basis, 3600 pounds of wood chips (oven-dry basis) and 2400 pounds of moisture are introduced into the chip bin 2 through line 1, and passed via line 3 into rotary screw 5. Steam at 15 p.s.i.g. is introduced into the screw conveyor 5 through line 7. Preheated chips are withdrawn from the screw conveyor 5 and introduced into feed tank 11 at about 250 F. 6000 pounds of alkaline white liquor in line 65 at a temperature of 70 F., together with a combined liquor stream in line 64, of 12,000 pounds and the recycle liquor in line 13 of 48,000 pounds are continuously introduced into the feed tank 11. The temperature of the chip-liquor slurry in the tank 11 is maintained at a temperature of 250 F.

72,000 pounds of chip-liquor slurry at a consistency of 5% are withdrawn through line 14 by pump 15 and introduced into the digester 17 through line 16. 48,000 pounds of the liquor are withdrawn through screen 18 and returned to the feed tank 11 through line 13. 3400 pounds of partially spent cooking liquor in line 19 at a temperature of 360 F. are introduced into the chipliquor slurry through the nozzles 20 below the screen 18 and rapidly raise the temperature of the slurry to 265 F.

3400 pounds of cooking liquor are withdrawn through screen 21 to form chip-liquor slurry having a consistency of 15%. The slurry is caused to pass through the impregnation zone at a flow rate whereby the chips are substantially impregnated after 20 minutes. Above the screen 25, 101,000 pounds of a cooking liquor withdrawn from screen 25 through line 26 and heated in heat exchanger 28 to a temperature of 380 F. are introduced through nozzles 24 into the chip-liquor slurry to rapidly raise the temperature of the cellulosic material to the delignification temperature of 360 F. The slurry is maintained at a consistency of 15% as it passes through the digestion zone of the digester 17. The chips are passed through the zone at a flow rate whereby the chips are substantially delignified after 20 minutes. 12,000

pounds of partially spent cooking liquor are withdrawn from screen 31 at a temperature of 360 F. 3400 pounds of the liquor withdrawn from screen 31 are passed through line 32 by pump 33 and introduced into the chipliquor slurry through nozzles 20. 8600 pounds of the liquor withdrawn from screen 31 are passed through line 32 to flash drum 34 wherein the pressure is reduced to about 35 p.s.i.g. 665 pounds of steam at a temperature of about 280 F. are passed to the screw conveyor through line 7 and valve 35. The liquor withdra'wnfrom flash drum 34 through line 63 at a temperature of about 280 F. is passed through

lines

64 and 12 to the feed tank 11.

15,600 poundsof liquor at a temperature of 209 F. are introduced through the nozzles 37 into the chipliquor slurry and decrease the consistency of the material and reduce the temperature of the material to a temperature of about 267 F. by the time the material reaches the screen 38. 15,600 pounds of liquor at a temperature of 267 F. are withdrawn from screen 38 and passed through line 39. A minor portion of the black liquor is returned via

lines

64 and 12 to feed tank 11 and the remaining portion of the black liquor is withdrawn through line 66 for recovery of cooking chemicals.

The partially cooled delignified material is further contacted immediately below the screen 38 with 15,600 pounds of an additional cooling liquor in line 40 having a temperature of 170 F. The material is now rapidly cooled to a temperature of 209 F. by the time it reaches the screen 42. 15,600 pounds of liquor at a temperature of 209 F. are withdrawn through screen 42 and line 43 and passed by pump 44 through line 36 and are introduced into the material through nozzles 37 located below the screen 31, as described above.

The consistency of the chip-liquor slurry is decreased to by injection of 5000 pounds of liquor through line 45 to permit effective flow of the delignified material and liquor through the reduction valve 48. This cools the material to about 196 F. The delignified material and cooking liquor in line 48 are contacted with a wash liquor in line 50 having a temperature of 170 F., the combined stream being passed to screen 52 to remove knots and undissolved cellulosic material. The delignified material is then passed to rotary vacuum washer unit 55 wherein it is contacted with wash water in line 56 having a temperature of 150 F. 1800 pounds of washed pulp or cellulosic fibres (oven-dry basis) are thereupon withdrawn from the rotary vacuum washer unit 55 at line 57 and are passed to subsequent processing units (not shown).

While a prefered embodiment of my invention has been illustarted and described, variations thereof may be made by one skilled in the art, and, therefore, the invention as disclosed hereinabove is intended to be limited only by the scope of the disclosure and the dependent claims.

I claim:

1. A process for preparing pulp by the continuous delignification of cellulosic material at superatmospheric pressure in a digester having an impregnation zone, a delignification zone and a cooling zone, which comprises:

introducing a preheated slurry of cellulosic material and cooking liquor into the upper portion of said digester;

rapidly heating said slurry to a temperature of from about 240 F. to about 280 F.; effecting impregnation of said cellulosic material in said impregnation zone of said digester;

rapidly heating said impregnated material to a temperature of from about 330 F. to about 385 F.; effecting delignification of said cellulosic material in said delignification zone of said digester;

cooling said delignified material in a cooling zone comprised of at least two sub-stages in said digester; and withdrawing said delignified material from said digestion zone.

2. The process of claim 1 wherein the cellulosic material is maintained in the impregnation zone for about 20 to about 30 minutes.

3. The process of claim 2 wherein the cellulosic material is maintained in the delignification Zone for about 20 to about 25 minutes.

4. The process as defined in claim 3, wherein the consistency of the cellulosic material and cooking liquor passing through said impregnation zone, delignification zone and cooling zone is from about 10% to about 20%.

5. The process of claim 4 wherein partially spent cooking liquor is withdrawn from a lower portion of said delignification zone and is introduced into an upper portion of said impregnation zone to effect the rapid heating of said slurry.

6. A process for preparing pulp by the continuous delignification of cellulosic material at superatmospheric pressure in a digester having an impregnation zone, a delignification zone and .a cooling zone, which comprises:

rapidly heating said slurry to a temperature of from about 240 F. to about 280 F.;

effecting impregnation of said cellulosic material in said impregnation zone of said digester; rapidly heating said impregnated material to a temper ature of from about 330 F. to about 385 F.;

etfecting delignification of said cellulosic material in said delignification zone of said digester;

introducing a first cooling liquor stream into the upper portion of said cooling zone to partially cool said material and to leach out spent cooking liquor;

withdrawing a portion of the liquor stream from a point intermediate of said upper and lower portions of said cooling zone;

introducing a second cooling liquor stream into said delignified material below said intermediate point to further cool said material;

withdrawing a portion of the liquor stream from a lower portion of said cooling zone and passing said liquor stream to the upper portion-of said cooling zone as said first cooling liquor stream;

and withdrawing delignified material from said digester below said cooling zone.

7. The process as defined in claim 6, wherein said cellulosic material is held in said impregnation zone for about 20 to about 30 minutes.

8. The process as defined in claim 7, wherein said cellulosic material is held in said delignification zone for about 20 to about 25 minutes.

'9. The process as defined in claim 8, and additionally comprising contacting said cellulosic material with steam to preheat said material and forming a slurry of about 1% to about 6% with cooking liquor prior to introducing said slurry into said digester.

10. A process as defined in claim 8 wherein partially spent cooking liquor is withdrawn from the lower portion of said delignification zone and is introduced into the upper portion of said impregnation zone to effect the rapid heating of said slurry.

11. A process as defined in claim 8 wherein a portion of the black liquor withdrawn from said intermediate point of said cooling zone is passed to a recovery unit.

12. A process as defined in claim 10 wherein a portion of the liquor withdrawn from the lower portion of said delignification zone is passed to a flash zone and the steam formed therein is used to preheat said cellulosic mate rial.

13. A process as defined in claim 8 wherein the delignified material withdrawn from said digester is washed with water and a portion of said wash water is passed to an intermediate point in said cooling zone of said digester as said second cooling liquor stream.

14. The process of claim 8 wherein the solids consist- 9 10 ency in each of said zones is maintained at a value between withdrawing a portion of the liquid from a lower porabout 10% and about 20%. tion of said cooling zone and passing said portion of 15. In a process for preparing pulp by the continuous liquid withdrawn from the lower portion to the upper delignification of cellulosic material wherein the celluportion of said cooling zone as said first cooling liquid losic material is successively passed through an impregna- 5 stream; and tion zone, a delignification zone, and a cooling zone, an Withdrawing delignified material from said digester beimprovement comprising: law said cooling zone.

introducing a first cooling liquid stream into an upper portion of said cooling zone to partially cool the References Cited y the Examine! delignified cellulosic material and to leach out spent 10 UNITED STATES PATENTS hquor 3,097,987 7/1963 Solman 16217 Withdrawing a portion of the liquid from an intermediate portion of said cooling zone; introducing a second cooling liquid stream below the DONALL H SYLVESTER Primary Examiner intermediate portion of said cooling zone to further 15 cool the material; HOWARD R. CAINE, Examiner.

3,200,032 8/1965 Richter et a1. 162-19

Claims

1. A PROCESS FOR PREPARING PULP BY THE CONTINUOUS DELIGNIFICATION OF CELLULOSIC MATERIAL AT SUPERATMOSPHERIC PRESSURE IN A DIGESTER HAVING AN IMPREGNATION ZONE, A DELIGNIFICATION ZONE AND A COOLING ZONE, WHICH COMPRISES: INTRODUCING A PREHEATED SLURRY OF CELLULOSIC MATERIAL AND COOKING LIQUOR INTO THE UPPER PORTION OF SAID DIGESTER; RAPIDLY HEATING SAID SLURRY TO A TEMPERATURE OF FROM ABOUT 240*F. TO ABOUT 280*F.; EFFECTING IMPREGNATION OF SAID CELLULOSIC MATERIAL IN SAID IMPREGNATION ZONE OF SAID DIGESTER; RAPIDLY HEATING SAID IMPREGNATED MATERIAL TO A TEMPERATURE OF FROM ABOUT 330*F. TO ABOUT 385*F.; EFFECTING DELIGINIFICATION OF SAID CELLULOSIC MATERIAL IN SAID DELIGNIFICATION ZONE OF SAID DIGESTER; COOLING SAID DELIGNIFIED A COOLING ZONE COMPRISED OF AT LEAST TWO SUB-STAGES IN SAID DIGESTER; AND WITHDRAWING SAID DILIGNIFIED MATERIAL FROM SAID DIGESTION ZONE.