METHOD AND APPARATUS TO DECREASE STEAM CONSUMPTION IN CONVENTIONAL BATCH DIGESTERS

Abstract

Method and apparatus for conventional batch digesters to retain thermal energy typically lost in the flashing blow operation by transferring the thermal energy to fresh cooking liquors from hot spent cooking liquor.

Description

This application claims priority from U.S. Provisional Application Ser. No. 60/672,589 filed Apr. 19, 2005

BACKGROUND OF THE INVENTION

This present invention relates to a method and apparatus for saving the energy consumption required for digesting cellulosic material using conventional batch reactors.

In conventional batch pulping operations, the wood material (softwoods, eucalyptus, or hardwoods) or the lignocellulosic crop material (e.g., bagasse, bamboo, kenaf, reeds, and so forth) is heated and reacted with cooking liquor for a given time at a specified temperature. The cooking chemistry may be kraft (sulfate), sulfite or other. After the required heating and reaction at high temperature, the contents of the batch reactor are removed to prepare for the next batch. Conventional batch reactors (digesters) used for producing fibrous cellulose from fiber bearing sources, such as wood, have traditionally been emptied by opening the hot pressurized vessel to an atmospheric pressure tank. This process is referred to as a “blow” and the atmospheric pressure tank is referred to as a “blow tank”.

Recently, new processes have emerged that reuse the spent cooking liquors that have been displaced from previous batch cooks (spent liquor) to save energy and to take advantage of the residual chemicals in the spent liquor. These systems remove the hot liquor from the pressurized digester before the “blow” thereby cooling the contents which precludes the violent flashing that occurs from high temperature liquor flashing into an atmospheric pressure blow tank from a pressurized digester. This requires that stored spent liquor referred to as the “displacing liquor”, is pumped into the digester to replace the liquor inside the digester, referred to as the “displaced liquor”, with the “displaced liquor” exiting the digester. Hence, they are referred to as Displacement Batch Digesters. Therefore, the thermodynamic mechanism utilized to transfer the thermal and chemical energy is direct contact mixing of liquors, cellulosic bearing material, and cellulose fiber at the appropriate times during the cooking cycle. The direct contact mixing and reuse of the hot spent cooking liquor requires a tank farm composed of atmospheric tanks as well as pressure vessels to store “displaced liquor” at temperatures above 100° C. The pressure vessels are referred to as accumulators since they accumulate the displaced return liquors, at temperatures above atmospheric flash points, from the digester for reuse. Since the cooled digester does not have pressure and thermal energy to flash during the blow, the contents or product pulp fiber in the digester, must be pumped out of the digester in what is sometimes referred to as the “pump-out” or “discharge” operation. The displacement digester systems have been commercialized by various companies and are marketed under trade names such as RDH, Superbatch, CBC and DDS.

The displacement batch digester systems require substantial capital investment for the pumps, valves, instruments and vessels required to recover, store and reuse the spent cooking liquors. The benefit in energy savings is also substantial, e.g. up to 2 metric tons of steam per metric ton pulp, but require several years for payback of the initial investment. For this reason, some fiber producing companies do not want to invest in these types of systems.

U.S. Pat. Nos. 4,814,042; 6,719,878; 5,800,674; 4,578,149; 4,601,787; 6,103,057; 5,059,284; 5,080,757; 4,849,052; 6,139,689; 4,764,251; 4,670,098; 4,764,251; H1,681; 4,764,251; 6,306,252; 6,346,166; 6,346,167; 6,350,348; 6,391,628; 6,451,172 and 6,514,380 disclose various aspects for the delignification (cooking) process of displacement batch digesters for providing cellulosic pulp.

Thus there is a need for minimal investment to provide for transfer of thermal energy while still achieving a savings of at least one metric ton of steam or more per metric ton of pulp produced.

BRIEF SUMMARY OF THE INVENTION

The present invention pertains to methods and apparatus for conventional batch digesters to transfer thermal energy between each other or between cooking liquors using indirect contact methods without interfering with the production of cellulose fiber.

Thus in one aspect the present invention is a process for producing pulp using at least two batch digesters comprising; removing hot spent cooking liquor from a first digester, passing the hot spent cooking liquor in heat exchange with a supply of fresh cooking liquor to be supplied to a second digester thereby heating said fresh cooking liquor.

In another aspect, the present invention includes withdrawing the spent cooking liquor from one of prior to or after the spent cooking liquor is passed through a heater used to heat recirculating cooking liquor in a first digester.

In yet another aspect, the present invention includes use of accumulation to store pre-heated fresh cooking liquor from a first digester.

In still another aspect, the present invention includes the process wherein when the conclusion of a cooking cycle in a second digester is completed; hot spent cooking liquor from the first digester is used to preheat fresh cooking liquor supplied to the first digester.

In a further aspect the present invention is a process including the step of withdrawing cooked pulp from the first digester by means of a pump upstream of a blow tank for receiving cooked pulp.

In still a further aspect, the present invention is a system for producing pulp from lignocellulosic material comprising in combination, at least two batch digesters, each digester having means to introduce lignocellulosic material and cooking liquor into each of the digesters, means to re-circulate cooking liquor with intermediate heating as required, and means to withdraw cooked pulp from the digesters, and heat exchange means to use spent cooking liquor from any one digester and the end of a cooking cycle to pre-heat fresh cooking liquor prior to introduction into any other digester at the beginning of a cooking cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a conventional batch digester representing prior art.

FIG. 2 is a schematic representation of a first embodiment of the present invention.

FIG. 3 is a schematic representation of another embodiment of the present invention.

FIG. 4 is a schematic representation of yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Conventional batch digesters typically recirculate the cooking liquor while heating the digester with a steam heating. The steam may be injected directly or applied through a steam heat exchanger. The heating device is typically placed at the discharge of the recirculation pump. The liquor removed from the digester is heated in the heating device and then returned to the top and/or the bottom of the digester.

Referring to FIG. 1 the conventional cooking cycle in digester 3 begins with feed cellulose bearing material entering through feed line 23. Simultaneously, the proper ratio of black liquor in tank 1 and white liquor in tank 2 are fed to mixing line 22 through line 20 and line 21, respectively. The mixture of feed material from line 23 along with the black and white cooking liquor mixture from line 24 fills digester 3. Digester 3 is sealed and recirculation pump 4 is started pulling cool fresh cooking liquor from digester 3 through line 40 and pumping it through line 41 to the digester heating device 5, which can be a direct or indirect steam heater. The heated cooking liquor leaves the heating device 5 through line 42 and is sent to the top of the digester through line 43 and/or to the bottom of the digester through line 44, as necessary. The heating is continued until the contents of the digester 3 reach the desired cooking temperature. The contents of the digester 3 are then held at the cooking temperature the required time. After the contents of digester 3 are held for the required time to complete a cooking cycle they are flashed into blow tank 96 through line 90. Note that Line 90 is a common “blow” line for all digesters in the cooking system. Hence, the system “blow” line and “blow” tank are shared resources for all the digesters. At this time digester 3 is now ready to start a new cooking cycle such as described above.

Therefore, in one aspect the present invention utilizes a heat exchanger that would transfer the heat from a hot digester that has finished the required high temperature reaction to a digester that is in the beginning stage or step of heating newly charged cellulosic material and cooking liquor introduced into the digester. The new heat exchanger is piped with piping manifolds so that all the digesters in a train or group can transfer, in any combination, hot to cold (heating phase). The existing recirculation pumps of individual digesters would be used to pump the liquors through the appropriate side of the new heat exchanger and then returned to the same digester after cooling or heating. The contents of a hot digester would be cooled by transferring the thermal energy to a cooler digester at a temperature that would preclude a hot blow operation to empty the digester. Therefore, a pump would discharge the digester similar to the displacement batch digester systems. In the process no liquor is displaced from the digester, only recirculated, as in the conventional batch cooking prior art and therefore is an improvement of prior art conventional batch cooking. This arrangement requires an even number of digesters and exact timing in the cooking cycles of the digesters. In an upset timing condition when there is a hot digester but no digester ready to be heated which would cool the hot digester, the hot digester can be cooled using an additional heat exchanger and cool water to reduce the temperature of the spent hot cooking liquor. Another option in case of upset timing is to arrange the system for both a conventional hot “blow” since the digester contents would not be cooled, in addition to a new pump discharge.

In another aspect of the present invention the timing difficulties noted above can be eliminated by preheating and storing new cooking liquor in a pressure vessel. The preheated liquor is heated by a digester ready to be cooled before filling the next digester with cellulosic material and cooking liquor. The hot digester that would be cooled would use its recirculation pump as discussed above to pump the liquor through a heat exchanger. The fresh cool cooking liquor would be pumped through the same heat exchanger to absorb the thermal energy and exit the heat exchanger to a pressure vessel for use as warm cooking liquor that is waiting to be used in the next digester undergoing a filling operation. In effect, this would result in preheated cooking liquor. The next digester that is filled with cellulosic material would use this preheated cooking liquor. Since this preheated liquor would be stored in a pressure vessel, this modification would require an additional tank and pump. A water cooled heat exchanger could also be used in the case of an extreme timing upset when the new pressure vessel is full which would preclude the ability to accept newly heated cooking liquor.

In another aspect of the present invention the displacing of the hot spent liquors can be incorporated which is similar to prior art displacement digester systems. The displacing of the hot spent liquors gives benefit in cleaning (washing) the cellulosic fiber while removing the thermal energy. In this arrangement the “displaced liquor” would yield the thermal energy in a heat exchanger to preheat cooking liquor instead of transferring it to recirculating liquor from a cooler digester as discussed above. A water cooler would be used to maintain the temperature of the displaced spent liquor below flash point for further processing in the pulp mill. This method does not reuse the spent cooking liquor for further cooking as in the displacement batch systems since the “displaced liquor” would be directly sent to the evaporator plant for further processing. A commercial prior art system known as “Cold Blow” uses one pressure vessel to store the “displaced” liquor for reuse but differs from this invention because the storage and reuse of the spent liquor for thermal savings is by direct contact of the spent cooking liquor with the subsequent digester starting a new cooking cycle. The disadvantage of this system, the Cold Blow system, is that the thermal transfer cannot be controlled as in a heat exchanger and the spent liquor has a large amount of dissolved high molecular weight by-products from the delignification reactions (cooking). With the process and apparatus of the present invention dissolved by-products are precluded from interfering with subsequent delignification reactions while still allowing for thermal transfer and recycle.

In all aspects of the present invention the hot spent cooking liquor is not reused as in displacement batch digester systems. The use of an additional or new heat exchanger mitigates the loss of thermal energy during a conventional “blow” operation by transferring the thermal energy to another digester that is in the heating phase or to fresh cooking liquor waiting to be used in another digester. The amount of thermal energy retained would decrease the steam consumption one or more metric tons per ton of pulp produced. The following table is a summary of the three configuration aspects described above.

SUMMARY TABLE OF INVENTION CONFIGURATIONS
		Water
	New Heat	Heat	Storage
Embodiment	Exchanger	Exchanger	Vessel	Notes
I	YES	Optional	NO	1. requires even number
				of digesters
				2. requires exact digester
				sequencing timing
II	YES	Optional	YES	1. any number of
				digesters
				2. timing problems
				mitigated
III	YES	Optional	YES	1. same as embodiment
				II
				2. cleans pulp in digester

Referring to FIG. 2, the cooking cycle in digester 3 begins with feed cellulose bearing material entering in through feeder line 23. Simultaneously, the proper ratio of black liquor in tank 1 and white liquor in tank 2 are fed to mixing line 22 through line 20 and line 21, respectively. The mixture of feed material from line 23 along with the black and white cooking liquor mixture from line 24 fills digester 3. Digester 3 is sealed and recirculation pump 4 is started pulling cooking liquor from digester 3 through line 40 and pumping through line 41. This part of the process is the same as the prior art.

In the process of the present invention the timing of one of the other digesters would be such that, e.g. Digester 8, has simultaneously finished the required time at high temperature. The recirculation pump 9 for digester 8 is started to pull hot spent cooking liquor from digester 8 through line 80 and pumping it through line 81. The pumped hot spent cooking liquor would be diverted at point C to line 64. The hot spent cooking liquor travels through a recovery heat exchanger 6 and returns downstream of point C via line 63. This liquor returns to digester 8 via lines 82, 83, and 84 at a lower temperature to cool the contents of digester 8. An alternate process would be to divert the hot spent cooking liquor at point D in line 82 pass it through the heat exchanger 6 and return the cooled spent cooking liquor downstream of point D to digester 8 via lines 83, 84. Any combination of appropriate connections on lines 80, 81, 82, 83, and 84 could be used with this invention.

At the same time digester 8 is finishing its cooking cycle, digester 3, which has been filled, is now ready for heating. Pump 4 starts pulling fresh cool cooking liquor through line 40 and pumping it through line 41 to digester heating device 5. The fresh cool cooking liquor is diverted at point A via line 60 to the recovery heat exchanger 6. The fresh cool cooking liquor from digester 3 heated by the hot spent cooking liquor from digester 8 in the new recovery heat exchanger 6 would be returned to digester 3 heating device 5 through lines 61 and 41. The digester heating device 5 would increase the fresh cooking liquor temperature further by the use of direct or indirect steam contact. The fresh heated cooking liquor returns to digester 3 through lines 42, 43, and 44 as necessary for even heating, from top to bottom of digester 3. Alternatively, the fresh cool cooking liquor of digester 3 could be diverted at point B on line 42 to heat exchanger 6 and returned to digester 3 heater 5 downstream of point B in line 42 to digester 3 via lines 43, and 44. This transfer of heat from the hot spent cooking liquor of digester 8 to the fresh cool cooking liquor of digester 3 continues until the temperatures of the respective liquors in the digester reach the recovery heat exchanger 6 design temperatures.

According to the invention the contents of digester 8 having been cooled by transferring thermal energy to digester 3 would be emptied into the blow tank 96 by diverting the digester 8 contents from line 90 to line 91 to use pump 95 as the energy source. Pump 95 would transfer the contents through line 92 to blow tank 96. Pump 95 is required because the thermal energy of the digester 8 contents have been transferred to digester 3 using the new recovery heat exchanger 6 and is not capable of a flash (blow) discharge as in the prior art. Digester 8 is now ready to start another cooking cycle.

In the event that digester 8 has attained the proper time period at high temperature and is now ready to transfer thermal energy and digester 3 is not ready for receiving the thermal energy, an optional cooling water heat exchanger 7 is used to absorb the thermal energy for use in heating water. In this case, the liquor leaving recovery heat exchanger 6 would be at the same temperature as when it entered the heat exchanger since digester 3's fresh cool cooking liquor is not available to absorb the thermal energy. The hot spent cooking liquor from digester 8 would be diverted via line 62 to the cooling water heat exchanger 7 to heat water in line 66 to be used in downstream devices connected to line 67. The cooled spent cooking liquor would leave the cooling water heat exchanger in line 65 to an appropriate connection on lines 81, 82, 83 or 84. If the optional cooling water heat exchanger 7 is not used, the contents of digester 8 would be flashed into blow tank 96 through line 90 as in the prior art case described above with no thermal savings.

Referring to FIG. 3, digester 8 has finished the required time at high temperature. At the time one of the other digesters (e.g. digester 3) would be ready for start of a cooking cycle, digester 8 has simultaneously finished the required time at high temperature. Digester 8 recirculation pump 9 is started to pull hot spent cooking liquor from digester 8 through line 80 and pump it through line 81 to digester heater 10. The pumped hot spent cooking liquor would be diverted at point C via line 64 to the new recovery heat exchanger 6 and returned downstream of point C traveling via line 63. The cooled liquor returns to digester 8 in lines 82, 83, and 84 at a lower temperature cooling the contents of digester 8. An alternate configuration would be to divert the hot spent cooking liquor at point D on line 82 to recovering heat exchanger 6 and return the cooler hot spent cooking liquor downstream of point D in line 82. Any combination of appropriate connections on lines 80, 81, 82, 83, and 84 could be used with this invention.

In FIG. 3 black liquor from tank 1 and white liquor from tank 2 flow through lines 20 and 21 and are mixed in line 22. The mixture travels in line 24 to a new fresh cool cooking liquor pump 35. Optionally mixing tank 12 may be placed between line 22 and pump 35 to further mix the black and white liquors. Line 69, allows cool fresh cooking liquor to flow through the recovery heat exchanger 6 to be heated by hot spent cooking liquor from digester 8. The fresh cool cooking liquor, heated by the hot spent cooking liquor from digester 8 in the new recovery heat exchanger 6 is referred to as warm cooking liquor which, flows to pressure accumulator 11 through line 67 for storage and subsequent use on demand. Therefore, whenever digester 8 has completed the required time at high temperature, the hot spent cooking liquor is able to transfer thermal energy to the fresh cool cooking liquor. This process eliminates the necessity for digester 8 and digester 3 to operate on precisely the same time regimens.

In the event that digester 8 has attained the proper time period at high temperature and is now ready to transfer thermal energy but there is no fresh cool cooking liquors in supply tanks 1 and 2 or in mixing tank 12, or if there is too much warm cooking liquor in the warm cooking liquor accumulator 11, an optional cooling water heat exchanger 7 could be used to absorb the thermal energy. In this case, the liquor leaving recovery heat exchanger 6 via line 63 would be at the same temperature as when it entered heat exchanger 6. Hot spent cooking liquor would be diverted through line 62 to the cooling water heat exchanger 7 to heat water in line 66 to be used on connected downstream devices on line 67. The cooled spent cooking liquor would leave the cooling water heat exchanger in line 65 for introduction downstream of the diversion points C or D. Any combination of appropriate connections on lines 80, 81, 82, 83, and 84 could be used with this invention. If the optional cooling water heat exchanger 7 is not used, the contents of digester 8 would be flashed into blow tank 96 through line 90 as described above.

The contents of digester 8, having been cooled by transferring thermal energy to the warm cooking liquor using the method and apparatus of the invention, would be emptied by diverting the digester 8 contents from line 90 to line 91 using pump 95 as the energy source. Pump 95 would transfer the contents through line 92 to blow tank 96. Digester 8 is now ready to start another cooking cycle.

The cooking cycle in digester 3 begins at any time with feed cellulose bearing material entering in through feeder line 23. Before, during and or after any amount of feed material has entered digester 3, warm cooking liquor transfer pump 13 starts to transfer warm fresh cooking liquor to digester 3 from the warm cooking liquor accumulator 11. An alternate configuration is to use the digester 3 recirculation pump 4 to transfer warm cooking liquor from accumulator 11. In this case, the warm cooking liquor would be transferred from the warm cooking liquor accumulator 11 through line 25 to the suction side of pump 4. The transfer of warm cooking liquor stops after the required amount has entered the digester. Digester 3 is now ready to continue heating using the prior art recirculation pump 4 and heating device 5 using lines 40, 41, 42, 43, and 44. Due to the transfer of thermal energy to the cool cooking liquor in the recovery heat exchanger 6, the heating requirement of digester 3 heating device 5 is deceased.

Referring to FIG. 4 this embodiment of the invention incorporates displacement digester displacing action with cool washer filtrate to displace the hot spent liquor from a digester that has achieved the required time at high temperature. The excess thermal energy in the “displaced liquor” is transferred by the recovery heat exchanger 6 to fresh cool cooking liquor. In this case, digester 8 has finished the required time at high temperature. Recovery pump 14 is started and washer filtrate liquor, the “displacing liquor”, from the washing area or black liquor from tank 1 is withdrawn and pumped into digester 8 using lines 25 and 26. The hot spent cooking liquor in digester 8 is displaced from the digester through line 27 and flows to the recovery heat exchanger 6.

Black liquor from tank 1 and white liquor from tank 2 flow through lines 20 and 21 are mixed in line 22. The mixture travels in line 24 to a new fresh cool cooking liquor pump 35. If required a mixing tank 12 can be interposed in line 24 up-stream of cooking liquor pump 35. Cool fresh cooking liquor flows through line 69 to the recovery heat exchanger 6 to be heated by hot spent cooking liquor exiting digester 8 via line 27. The fresh cool cooking liquor, heated by the hot spent cooking liquor from digester 8 in the new recovery heat exchanger 6, referred to as warm cooking liquor, flows to pressure accumulator 11 through line 67 for storage and subsequent use on demand. Therefore, whenever digester 8 has completed the required time at high temperature, the hot spent cooking liquor can be used to transfer thermal energy to the fresh cool cooking liquor. This process eliminates the requirement of digester 8 and digester 3 to be on the same cooking cycle times.

In the event that digester 8 has attained the proper time period at high temperature and is now ready to transfer thermal energy but there is no fresh cool cooking liquors in supply tanks 1 and 2 or in mixing tank 12, or if there is too much warm cooking liquor in the warm cooking liquor accumulator 11, an optional cooling water heat exchanger 7 could be used to absorb the thermal energy. In this case, the liquor leaving recovery heat exchanger 6 in line 62 would be at the same temperature as when it entered heat exchanger 6. The hot spent cooking liquor would be diverted through line 62 to the cooling water heat exchanger 7 to heat water in line 66 to be used in downstream devices connected via line 67. The cooled spent cooking liquor would leave the cooling water heat exchanger in line 70. If the optional cooling water heat exchanger 7 is not used, the contents of digester 8 would be flashed into blow tank 96 through line 90 as is described above.

Digester 8 contents now having been cooled by transferring thermal energy to the warm cooking liquor using this invention would be emptied by diverting the digester 8 contents from line 90 to line 91 using pump 95 as the energy source. Pump 95 would transfer the contents through line 92 to blow tank 96. Digester 8 is now ready to start another cooking cycle.

The cooking cycle in digester 3 can begin at any time with feed cellulose bearing material entering in through feeder line 23. Before, during and or after any amount of feed material has entered digester 3, warm cooking liquor transfer pump 13 starts to transfer warm fresh cooking liquor to digester 3 from the warm cooking liquor accumulator 11. An alternate configuration is to use the digester 3 recirculation pump 4 to transfer warm cooking liquor from accumulator 11. In this case, the warm cooking liquor would be transferred from the warm cooking liquor accumulator 11 through line 25 to the suction side of pump 4. The transfer of warm cooking liquor stops after the required amount has entered the digester 3. Digester 3 is now ready to continue heating using the prior art recirculation pump 4 and heating device 5 using lines 40, 41, 42, 43, and 44. Due to the transfer of thermal energy to the cool cooking liquor in the recovery heat exchanger 6, the heating requirements from digester 3 heating device 5 is decreased.

All the processes described above use a new recovery heat exchanger 6 to recover thermal energy from hot spent cooking liquor and transfer the energy to fresh cool cooking liquor either directly to a different digester, as in FIG. 2, or to a storage tank, as in FIGS. 3 and 4 for subsequent use. All the processes allow for use of a backup cooling water heat exchanger 7. All the processes allow for piping connections at the digesters to be before or after the digester heating device. All of the processes use a digester content discharging pump 95 or allow for conventional prior art flash blow in the event of timing or liquor inventory upsets. A two or more digester system is able to use this invention by utilizing piping manifolds at the new recovery heat exchanger 6 inlet and outlet connections to accommodate any combination of hot and cool liquor for heat transfer as needed.

The processes of the present invention are simpler and less costly since they do not require large vessels for direct contact mixing to transfer the thermal and chemical energy for reuse. The processes of the invention use indirect methods of thermal transfer at the appropriate time in the cooking cycle which implies that there would be no chemical recycle. Thus, the processes of the invention are a less costly option for thermal savings only.

Having thus described my invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims which should be read without limitation.

Claims

1. In a process for producing pulp using at least two batch digesters the improvement comprising:

removing hot spent cooking liquor from a first digester;

passing said hot spent cooking liquor in heat exchanger with a supply of fresh cooking liquor to be supplied to a second digester thereby heating said fresh cooking liquor.

2. A process according to claim 1 including the step of withdrawing said spent cooking liquor from one of prior to or after said spent cooking liquor is passed through a heater used to heat recirculating cooking liquor in said first digester.

3. A process according to claim 1 including the step of an accumulation to store pre-heated fresh cooking liquor from said first digester.

4. A process according to claim 1 the step of, at the conclusion of a cooking cycle in said second digester is completed, hot spent cooking liquor from said first digester is used to pre-heat fresh cooking liquor supplied to said first digester.

5. A process according to claim 1 including the step of using spent cooking liquor to heat water for use in one of said pulping process or selected processes or equipment related to said pulping process.

6. A process according to claim 1 including the step of withdrawing said cooked pulp from said first digester by means of a pump upstream of a blow tank for receiving said cooked pulp.

7. A system for producing pulp from lignocellulosic material comprising in combination:

at least two batch digesters each digester having means to introduce lignocellulosic material and cooking liquor into each of digesters, means to re-circulate cooking liquor with intermediate heating as required, and means to withdraw cooked pulp from said digesters; and

heat exchange means to use spent cooking liquor from one digester at the end of a cooking cycle to pre-heat fresh cooking liquor prior to introduction into said other digester at the beginning of a cooking cycle.