HEATING OF HYDRAULIC DIGESTERS

Abstract

A method and system for heating a hydraulic digester, such as a single-vessel hydraulic digester, which has a top separator, a level of chips and a liquid phase above the level of chips. The method includes: a. entraining chips in liquor to produce a slurry having a first temperature, b. feeding the slurry of chips to the digester through the top separator, c. supplying direct steam to the liquid phase between the level of chips and the top separator to heat the slurry to a second temperature for the impregnation stage, wherein the steam is fed by using at least one steam injector having a plurality of steam discharge openings; and d. heating the slurry to a third temperature and cooking the chips in a cooking zone, and withdrawing the cooked pulp from the bottom of the digester.

Claims

1. A method of producing chemical pulp in an impregnation stage and a cooking stage, using a hydraulic digester having a top separator, a level of chips and a liquid phase above the level of chips, comprising at least the following steps: a. entraining chips in liquor to produce a slurry having a first temperature, b. feeding the slurry of chips to the digester through the top separator, c. supplying direct steam to a liquid phase between the level of chips and the top separator to heat the slurry to a second temperature for the impregnation stage, wherein the steam is fed by using at least one steam injector having a plurality of steam discharge openings; and d. heating the slurry to a third temperature and cooking the chips in a cooking zone in the hydraulic digester, and withdrawing the cooked pulp from a bottom of the hydraulic digester.

2. The method according to claim 1, wherein the first temperature is below 110° C.

3. The method according to claim 1, wherein the second temperature is 1 to 40° C. higher than the first temperature.

4. The method according to claim 1, wherein the supplying of the direct steam includes feeding the direct steam through steam discharge openings having a diameter in a range of 0.1 to 15 mm to produce small bubbles in the liquid phase of the hydraulic digester.

5. A continuous hydraulic digester system for producing chemical pulp from cellulose chips, comprising: a digester vessel having a top, a bottom and a wall, a top separator at the top of the vessel configured to introduce a slurry of chips into the digester vessel; and at least one steam injector having a plurality of steam discharge openings, said at least one steam injector being located below the top separator and is configured to introduce steam to a liquid phase in the digester vessel during the operation of the continuous hydraulic digester.

6. The continuous hydraulic digester system according to claim 5, wherein the at least one steam injector comprises a tube which extends to an interior of the digester vessel and which is connected to a steam source.

7. The continuous hydraulic digester system according to claim 5, wherein the discharge openings are holes each having a diameter in a range of 0.1 to 15 mm.

8. The continuous hydraulic digester system according to claim 5, wherein the at least one steam injection includes at least two spaced apart steam injectors which are each disposed along the circumference of the digester wall.

9. A steam injector disposed in a wall of a hydraulic digester and arranged to introduce steam to the digester, wherein the steam injector comprises a tube extending to an interior of the digester and having a plurality of steam discharge openings.

10. The steam injector according to claim 9, wherein the discharge openings are distributed along the circumference and the length of a wall of the tube and the discharge openings are grouped on the wall in a continuous zone or grouped in separate zones.

11. The steam injector according to claim 9, wherein the discharge openings are circular holes each having a diameter of 0.1 15 millimeters.

12. All The steam injector according to claim 9, wherein the discharge openings are configured as gaps or slots.

13. The steam injector according to claim 9, wherein the at least one steam injector is arranged to introduce steam to a liquid phase in the digester and during the operation of the digester.

14. The method of claim 1 wherein the step of supplying the direct steam includes heating the slurry to the second temperature while the slurry is in the liquid phase of the hydraulic digester.

15. The method of claim 1 wherein the at least one steam injector includes a conduit extending through an outer wall of the hydraulic digester and the plurality of steam discharge openings are in the liquid phase of the hydraulic digester.

16. The continuous hydraulic digester system of claim 15 wherein the wall extends entirely around a perimeter of the digester vessel and extends between the top and bottom of the digester vessel, wherein the top, bottom and wall form an enclosed interior of the digester vessel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIGS. 1 and 2 illustrate the top sections of two conventional continuous digesters. The top of a vapor-phase digester, 10, is shown in FIG. 1; a hydraulic digester, 20, is shown in FIG. 2.

[0018] FIG. 3 is a view like that of FIGS. 1 and 2 of a typical inlet and upper section of a digester according to the present invention,

[0019] FIGS. 4 a and 4b illustrate embodiments of a steam injector, and

[0020] FIG. 5 illustrates locations of steam injectors in a wall of a digester.

[0021] The digesters in FIGS. 1 and 2 typically receive a slurry of comminuted cellulosic fibrous material, typically wood chips, in cooking liquor, such as kraft white liquor. The slurry is typically first treated in a feed system. The vapor-phase digester of FIG. 1 is typically fed a slurry of chips and liquor in conduit 11. The slurry is introduced to the digester using a conventional vertically-oriented screw conveyor 12 known in the art as an “inverted top separator”. The slurry is transported upwardly in the separator 12 and chips and liquor are discharged from the top of the separator 12 as shown by arrows 13. As the slurry is transported upwardly, excess liquor is removed from the slurry using a cylindrical screen 14 and returned to the feed system by way of conduit 15. The chips and liquor 13 discharged from separator 12 fall through a gas-filled zone 16 onto a chip pile 17. In order to continue the steam heating of the chips, the level of the chip pile 17 is maintained above the level of the cooking liquor 18, as seen in FIG. 1. After steam heating, the chips are immersed in cooking liquor, passing below the liquid level shown at 18 in FIG. 1, and the cooking processes continues. In order to improve the distribution of heat across the chip column and chip pile 17, a vapor-phase digester 10 typically also includes a liquor removal screen 19 and circulation 21, for drawing liquor radially outward, removing it and returning it via a centrally-located pipe 24 to the chip column. Circulation 21 typically includes a pump 25 and may include a liquor heater 25′. The liquor removal screen 19 and the associated circulation 21 (including pump 25 and pipe 24) are referred to in the art as the “trim circulation”. Below the trim circulation screen 19, with a more uniform distribution of heat and chemical, the cooking process continues. Excess pressure, for ex-ample, pressure introduced by the gases introduced with the incoming chip slurry, is typically vented using a conventional pressure relief device, shown schematically at 28 in FIG. 1. The temperature in zone 16 is monitored and controlled by adding pressurized steam via conduit 22 from steam source 23.

[0022] Similar to the vapor phase digester 10 of FIG. 1, the conventional hydraulic digester 20 in FIG. 2 receives a slurry of chips and liquor from a feed system via conduit 60. The slurry is introduced to the digester 20 by a conventional “top separator” 61, which is a downwardly directed screw-conveyor. The liquor introduced by separator 61 is shown as a double arrow 62; the chips by single arrow 63. As the slurry is transported downwardly by conveyor 61, excess liquor is removed from the slurry through a cylindrical screen 64 and returned to the feed system (e.g. high pressure feeder) by conduit 65. The chips introduced by the separator 61 produce a level of chips 66. Since digester 20 is hydraulically full, the zone 67 above the chip level 66 is filled with liquid, so that no gaseous zone typically exists.

[0023] In FIG. 2 the chips on the top of pile 66 are typically not heated to full cooking temperature, but are treated in the impregnation zone where the temperature is typically at the same level as the temperature in the feed system. Then the chips must be heated before cooking commences. This is typically done utilizing one or more heated cooking circulation loops 70. Heating may be performed co-currently or counter-currently; the circulation loop 70 shown in FIG. 2 heats the chips counter-currently. The slurry first passes a liquor-removal (withdrawal) screen 71 which removes liquor from the slurry through conduit 78. Liquor removed via conduit 78 may be forwarded to chemical recovery. This liquor removal draws free liquor, shown by a double arrow 76, counter-currently past the downwardly flowing chips, shown by a single arrow 77. The heated liquor 76 is obtained from circulation 70. The liquor is first removed from the slurry via screen 72 via conduit 73 and a pump 79, heated in an indirect steam heater 74 (e.g. to a temperature of 140° C. to 170° C.), and returned to the vicinity of screen 72 by a centrally located return conduit 75. Cooking liquor, for example, kraft white liquor, is typically added to this circulation. After heating to cooking temperature in circulation 70, the slurry can be cooked and otherwise further treated below screen 72.

[0024] The temperature in the impregnation zone is typically 100-120° C. Cooking temperature in the cooking zone is typically between 140° C. and 180° C. So there is a large temperature difference between the impregnation zone temperature at the top of the single-vessel hydraulic digester and the cooking zone. Due to the large temperature difference it can be difficult to heat the chips and liquor evenly by the cooking circulations. If the heating is not even some chips are cooked less than the others and the pulp quality is uneven. This may result in a high amount of uncooked material in the pulp. The larger the temperature difference between the impregnation zone in the top and the cooking zone is the more difficult it is to reach an even heating result.

[0025] This can be solved by the new method presented herein. FIG. 3 illustrates the sys-tem which can be used to realize the new method.

[0026] Similar to FIG. 2, the conventional hydraulic digester 68 in FIG. 3 receives a slurry of chips and liquor from a feed system (not shown) via conduit 60′. The feed system may be unpressurized or slightly pressurized, and the temperature of the slurry is about 110° C. or below. The slurry is introduced to the digester 68 by a conventional “top separator” 61′, which is a downwardly directed screw-conveyor. The liquor introduced by separator 61 is shown as an arrow 62′; the chips by an arrow 63′. As the slurry is transported downwardly by conveyor 61′, excess liquor is removed from the slurry through a cylindrical screen 64′ and returned to the feed system (e.g. high pressure feeder or pumps) by conduit 65. The chips introduced by the separator 61′ produce a level of chips 66′. Since the digester 68 is hydraulically full, the zone 67′, i.e. the liquid phase, above chip level 66′ is filled with liquid, so that no gaseous zone typically exists.

[0027] The digester wall 43 having a continuously curved cross-section is provided with steam injectors 40, which comprise tubes 41 extending to the interior of the digester 68 through the wall. The tubes are connected to a steam source (not shown) for leading steam (arrow 42) to the digester. The length of the tube 41 inside the digester may be 150-2500 millimeters (mm), typically 200-600 mm. The tubes are located above the level of chips 66′ and below the lower edge of the top separator 61′ so that the steam is directed to the liquid phase 67′. The tubes are typically located 0.1-5.0 meters (m) below the top separator 61′ in the vertical direction. When the steam is fed, a temperature increase can be from 1 to 40° C., preferably from 5 to 30° C.

[0028] The tube 41 has a plurality of openings 50 (FIGS. 4a and 4b) for discharging steam to the liquor phase 67′ above the chip level 66′. Typically the openings are circular small holes having a diameter (D) of 0.1-15 millimeters (mm), preferably 1.5-5.0 mm. The holes can be configured, typically, as circular holes, but also as gaps or slots. The term “hole” should therefore not be given any restrictive meaning, but should cover all through openings, slots, etc., regardless of shape.

[0029] The openings 50, typically hundreds of small holes, are distributed along the circumference and the length of the tube wall 52 as a continuous zone 51 or as separate zones. The separate zones may be disposed spaced apart along the length and/or circumference of the tube. The number of the holes 50 depends on the steam flow required for heating the chip suspension, and thus the zone or zones can cover adequate portion(s) of the tube wall. Some portions of the tube wall may be unperforated. For instance, the tube end 53 and/or the portion 54 closest to the digester wall may be unperforated, whereas the portion 55 therebetween is perforated partially or entirely.

[0030] FIG. 5 shows that there may be more than one injector 40 (tubes 41) disposed along the circumference of the digester wall 43 so that the tubes 41 may be equally or unequally spaced apart from each other. The distance between the tubes may depend e.g. on the construction of the top part of the digester.

[0031] As shown in FIG. 5, the steam flow from the steam openings 50 is directed radially (an arrow 57) and/or circumferentially (an arrow 56) in the digester. The steam flow along a circumferential direction may intensify heat transfer in the liquid phase. The direction of the steam flow may be defined by the location of the perforated and un-perforated zones in the tube wall.

[0032] It appears that adding direct steam via steam injectors solves the dominant problem regarding hydraulic digester operation. This problem has been too large a temperature difference between impregnation and cooking zones. All hydraulic digesters would benefit from the steam addition, especially those hydraulic digesters in which the impregnation zone temperature has been only about 100° C.

[0033] Although only some preferred embodiments of the method according to the invention have been described in the above, the invention covers all such modifications and variations that are included in the scope defined in the claims.