CA1179807A

CA1179807A - Process for digesting cellulosic material

Info

Publication number: CA1179807A
Application number: CA000396961A
Authority: CA
Inventors: Bertil Fagerlund
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-03-05
Filing date: 1982-02-24
Publication date: 1984-12-27
Also published as: BR8201145A; JPS57176282A; SE463267B; JPH0244957B2; FI79724B; FI820643L; SE8201334L; FI79724C

Abstract

PROCESS FOR DIGESTING CELLULOSIC MATERIAL

Abstract of the Disclosure Hot spent liquor, produced in a digester as the result of cooking a mass of cellulosic material with cooking liquor, is displaced from the top of the digester by pumping under pressure a displacing liquid into the base of the digester. The hot liquor so displaced is collected in an accumulator, and used to preheat another mass of cellulosic material as a preliminary to cooking of this other mass of material.

Description

~7~

Background and Summary This invention relates to the digestion of cellulosic material such as wood chips in a batch-type process. The invention more particularly concerns a process for digesting cellulosic material featuring an efficient and practical manner of recovering heat used in the process.
In a conventional batch process for digestin~ wood chips, a digester is filled with chips and the digester is then charged with cooking chemical, which in a soda process comprises essentally a solution of sodium hydro~ide, and in a kraf-t process comprises such a solution with the further inclusion of a sulphur compound. The digester is then sealed, and with steam the tempera-ture of the digester is brought up to cooking temperature. At the conclusion of the cook a blow valve in the digester is opened and the contents of the digester discharged into a blow tank.
Much of the heat energy acquired by the contents of the digester during the processing of the pulp exits through the blow tank with exhaust vapors. To recover such energy attempts have been made to pass such vapors through various forms of heat recovery systems. These recovery systems, however, have not been truly efficient. To conserve energy costs, some pulp manufacturers have chosen to install continuous diyestion processes. Such a process ordinarily is characterized by a more efficient utilization of heat than is achieved with a conventional batch process.
However, the cost of the equipment needed in a continuous process is substantially greater than the cost of the equipmen-t xequired in a batch-type process.
The prior patent art of which I am aware illustrates various attempts of others to recover heat in spent liquor produced in a pulping process. An example of such prior art is U.S. Patent 1,697,032 disclosing a sulphite process for cooking pulp. In the process described in this patent, hot liquor recovered in a blow pit is caused to flow down throuyh a digester ~7~ 7 together with wood c~ips, thereby to ohtain greater packing of the chips, and also some heating of the chips. However, with this process, as in the case of conventional batch-type soda and kraft processes described above, in discharging the contents of the digester into the blow pit, considerable energy exits from the blow pit with exhaust vapors.
U.S. Patent 2,195,378 discloses a process for chemically producing pulp, wherein white liguor is heated within an accumu-lator by a heating coil heated by pumping spent liquor through the coil. There are practical problems involved in removing spent liquor from a cooked mixture in a digester using a pump as disclosed. Furthermore, it is difficult to recover heat energy in a practical manner from spent liquor having the temperature that such has when leaving the heating coil described in the patent.
Generally, therefore, it is an object of the invention to provide a process for the batch-type digestion of cellulosic material which makes possible the efficient recovery of heat energy in a practical manner.
With steam being the usual source of energy for operating a digestion process, a corollary of the above is to provide a batch-type digestion process having reduced steam requirements.
A further object of the invention is to provide a method for digesting cellulosic material, wherein heat is recovered from spent liquor residing in the digester after the final cook in a manner promoting efficient recovery even -though relatively low temperature levels are involved.
As contemplated by the invention, spent liquor in the digester after cooking of the cellulosic material is displaced from the digester by admitting a displacing liquid which expels the spent liquor while replacing it in the digester. Pressure in the digester is maintained during this displacement to prevent flashing of the liquor. Spent liquor recovered in this manner is then utilized to supply heat to a subsequent digester charge.
Further contemplated in a specific embodiment of the invention is the collection of liquids displaced from a digester at two temperature levels. Liquid at a lower temperature level may be utilized in the initial preheating of a charge of cellu-losic material within a digester by immersing the cellulosic material in the lower temperature liquid. Liquid at a higher temperature level may be utilized in further heating the cellu-losic material, through displacement of the lower temperatureliquid within the digester with the higher temperature liquid.
Other features and advantages of the invention include a more efficient use of cooking chemical in the digestion pro-cess and reduced use of cooking chemical; higher yields and improved quality of pulp by reason of greater selectivity in the delignification reaction which occurs; the capability of using the digester as an instrumentality performing a washing opera-tion in the process; and control of scaling in heat exchanger equipment. Furthermore, existing facilities for carrying out batch digestion processes are readily converted to perform the method of the invention.
Thus, in one aspect the invention provides in the digesting of celluloslc material, where a digester is filled with a charge comprising cellulosic material and cooking liquor, and cooking occurs in the digester at an elevated temperature and superatmospheric pressure to produce pulp fibers, with con-version of the cooking liquor to hot spent liquor, the method comprising: at the conclusion of the cooking of one digester charge, maintaining the elevated pressure within the digester, pumping displacement liquid into an end of the digester to expel hot spent liquor through an outlet of the digester by displacing the hot spent liquor from the digester, and transferring the heat ~L~7~ 7 of the hot spent liquor so displaced to another digester charge to prepare such for cooking.
In another aspect the invention provides in a ba-tch digestion process wherein a digester is charged with a mass of cellulosic material and liquor, and the charge is then cooked at an elevated temperature and superatmospheric pressure to obtain within the digester a column of delignified pulp extending along the axis of the digester, the method comprising after cooking of the charge and the production of the column of pulp, and while retaining the column of pulp in the digester, pumping under pressure a displacement liquid into an end of the digester whereby the displacement liquid initially replaces by displace-ment hot spent liquor in a region encompassing the cross section of the column of pulp which is spaced from an end of the pulp column, and with continued pumping causing the displacement liquid further progressively to fill the digester and progres-sively replace hot spent liquor in a direction extending from said region toward said end of the pulp column, with such displacement liquid essentially quantitatively replacing by displacement hot spent liquor in a zone enlarging along the length of the pulp column, collecting the hot spent liquor so replaced, and transferring by conduction the sensible heat of the hot spent liquor to another mass of cellulosic material to preheat this other mass and prepare it for digestion.
In yet another aspect the invention provides in the digesting of cellulosic material to produce fibers therefrom, where the material is cooked with a fresh liquor to produce the fibers and spent liquor, the improvement which comprises prior to cooking the material, immersing the material in a digester with a preheating liquid at a relatively low temperature to raise the temperature of the material, replacing the preheating liquid at a relatively low temperature in the diges-ter with - 3a -~ .
e another preheating liquid at a higher temperature whereby the material becomes immersed in the liquid at a higher temperature and the temperature of the material .is further raised, replacing at least a portion of the preheating liquid at higher tempera-ture in the digester with fresh liquor, cooking the material at the temperature required for cooking) and recovering spent liquor produced by the cooking at substantially the temperature at which cooking was performed, said recovering s-tep including pumping displacement liquid into an end of the digester to pro-gressively displace the hot spent liquor quantitatively, thepreheating liquids at a relatively low temperature and at a higher temperature being derived from the spent liquor so re-covered.
These and other objects and advantages of the invention will become more fully apparent as the following description is read in conjunction with the accompanying drawings, wherein:
Figure 1 is a schematic view illustrating, in simpli-fied form, a digester and equipment associated with the digester which may be utilized in carrying out an embodlment of the invention;
Figures 2 through 5 are simplified schematic drawings illustrating principal equipment used in different stages of the digestion process; and Figure 6, which appears on the same sheet as Figure 1, is a graph illustrating the typical relationship existing between the temperature of liquor displaced from a digester and the amount of liquor displaced, expressed as a per s. - 3b -,~, cent of the total liquid-carrying capacity of the digester.

Detailed Description of the Invention As indicated above, it is conventional, in the chemical digestion of pulp, following a batch-type process, to charge a digester with cellulosic material, i.e. wood chips, and then to introduce into the digester a reactive liquor including reactive chemical. In the case of the soda process, the reactive liquor, known as white liquor, is essentially an aqueous solution of sodium hydroxide. In the case of a kraft process, the white liquor includes a sulphur compound. Digestion occurs with the contents of the digester at an elevated temperature and pressure, the temperature within the digester typically being within the range of from 330 to 350F (165 to 177C~.
According to the present invention, at the conclusion of the cooking period, and with the maintaining of pressure in the digester, a displacement liquid is pumped into the bottom of the digester. As this displacement liquid fills up the digester from the bottom, it progressively expels and replaces the hot spent liquor or black liquor produced by the cook. This hot black liquor, it has been observed, leaves the digester at essen-tially the temperature of the diyester at the conclusion of the cooking period, and may be collected in a high temperature, black liquor accumulator.
In a specific embodiment of the invention, it is preferred to employ, as the displacement liquid, the filtrate obtained from washing the pulp or delignified fibers obtained in earlier digestion of chips. Such filtrate has a temperature od3~ 7 elevated from the usual room temperature (by reason of passing through the warm fibers), and in practicing the invention, such filtrate might be expected to have a temperature within the range of from 140 to 165F (60 to 74C).
When the volume of liquid displaced from the digester approaches the liquid-carrying capacity of the digester, i.e., the volume of the digester minus the volume taken up by the solids within the digester, the temperature of the liquid leaving the digester falls off, with such li~uid becoming essentially the filtrate used in the displacement process. This liquid will have a somewhat higher temperature however, than the original filtrate, because of heating by the pulp fibers contained within the digestor.
The spent liquor at relatively high temperature which is displaced from the digester may be collected in a high tempera-ture accumulator, and the lower temperature liquid which follows from the digester may be collected in a low temperature accumulator.
These liquids, in turn, may be utilized in supplying part of the heat required to obtain proper cooking temperature in subsequent batches of cellulosic material. In performing this heating function, any liquor from the high temperature accumulator which is lowered in temperature by exchanging heat may be collected in the low temperature accumulator.
Apparatus which is usable in carrying out the invention is schematically illustrated in Fig. 1. It should be understood that the illustration is schematic in form, with many instrumen talities such as gages, pressure vents, pumps and valves which would characterize an actual installation being eliminated from the drawing for reasons of simplicity.
Referring to Fig. 1 of the drawing, illustrated at 10 is a digester of a type typically used in the chemical digestion of wood chips. Although not specifically illustrated, such a digester is provided with a conventional detachable cover, which 7~8~.

is removed for the purpose of charging the digester with wood chips.
Connecting with the top of the digester, and providing a path for the flow of liquid from the digester to a hlack liquor storage 12, is a valve-controlled conduit including conduit 14 and valve 16. In the making of pulp, spent black liquor recovered in storage 12 is converted, using conventional procedures, into white liquor which provides the active chemical required in the digestion of the wood chips. Details of this procedure are eliminated from this disclosure as they are unnecessary to an understanding of the invention.
Illustrated at 20 is a low temperature accumulator. A
valve-controlled conduit including conduit 22 and valve 24 provides a path for the flow of liquid from the top of the digester into the low temperature accumulator. Liquid contained within the low temperature accumulator may be returned to the digester by actuating pump 26 which removes liquid from the accumulator and pumps such throuyh conduit 28 and valve 30 into the bottom of the digester.
As earlier described, in carrying out the invention, spent liquor in the digester, after the cooking of the chips, is displaced from the digester with a displacement liquid, and preferably such is filtrate recovered from the first washer conventionally relied upon -to wash pulp after such has been removed from the digester. A first washer filtrate storage is indicated at 32 which is supplied with filtrate through conduit 34. Filtrate may be removed from storage 32 and pumped into the base of the digester utilizing pump 36, conduit 38 and valve 40.
Hot spent liquor on being expelled from the digester is collected in a high temperature accumulator shown at 42. The liquor travels to the accumulator through valve 44 and conduit 46. Liquid within the high temperature accumulator may be intro-~7~8Ci7 duced into the base of the digester utilizing pump 48, conduit 50and valve 52 which connect with the bottom of the digester.
According to an embodiment of the invention, a portion of the liquor within the high temperature accumulator may be used preliminarily to heat white liquor containing reactive chemical introduced into the digester for cooking purposes. Thus, indicated at 54 is a heat exchanger. Hot liquor from the high temperature accumulator is pumped into the heat exchanger using pump 56 and conduit 58. Such liquor, after transferring much of its heat to the white liquor travelling through the exchanger, travels through conduit 60 into the low temperature accumulator ~0.
Shown at 62 is a hot white liquor storage receptacle.
White liquor pumped through the heat exchanger and entering the exchanger through conduit 63 travels through conduit 64 into receptacle 62. Hot white liquor is transferred from the receptacle to the base of the digester 10 by pump 66 which pumps the liquor through conduit 68 and valve 70.
If desired, means may be provided for heating the contents of receptacle 62 above -the temperature of the liquor admitted to the receptacle by conduit 64. Such ma~ be done by utilizing various conventional heating systems. As illustrated in the drawing, heating i5 performed utilizing a steam-operated heat exchanger 82 supplied steam through line 83, and by circulating the contents of receptacle 62 through the exchanger utilizing pump 84.
Additional heating means is provided for heating the contents of the digester up to final cooking temperature. Such heating means again may take various forms. In the particular equipment shown heating is performed with a steam-operated heat exchanger 86 suppl:ied æteam through line 87. Liquid from the digester enters the heat exchanger through conduit 88 and pump 90. Liquid leaving the exchanger is directed into the digester ~9~7 adjacent its top and bottom by conduits 92, 94.
The process of the invention will now be described as employed in a batch-type digestion of wood chips. In this dis-cussion, typical temperature and volume relationships will be indicated, although it should be understood that these figures will vary from installation to installation. To initiate this explanation, it will be assumed that the high temperature accumu-lator contains liquid, i.e. hot black liquor, at a temperature of approximately 325~F (163C). The low temperature accumulator contains liquid at approximately 210F (98.9C) How these conditions are obtained will be come apparent at a later part of this dis-cussion.
Initially, the digester is filled with wood chips and such typically may have a temperature which is approximately room temperature, or 60F (15.5).
After closing off the top of the digester, alld referring to Fig. 2 of the drawings, liquid from low temperature accumulator 20 is pumped into the digester completely to fill the digester (and to expel the air from the digester), and to immerse the chips therein with the liquid. Through direct contact of the liquid with the chips, the temperature of the chips is raised.
Assuming by way of example, the use of a digester having an internal volume of 6000 cubic feet (170 cubic meters), when such is filled with chips, roughly 400 cubic feet (11~3 cubic meters) will be taken up by the wood material. This leaves s6no cubic feet (159 cubic meters) as the liquid-carrying capacity of the digester when such is filled with chips.
In filling with liquid from the low temperature accumu-lator, ordinarily slightly more than the liquid-carrying capacity would be introduced to the digester, for instance, 1.5 to 1.7 times this capacity, so that, in effect, there is a flushing of the chips with the low temperature liquid. This treatment with ~ t7 low temperature liquid has the effect of raising the temperature of the contents o~ the digester to approximately 200F (~3C).
Excess low temperature liquid on leaving the digester is directed to black liquor storage 12.
After such treatment with low temperature liquid initially to raise the temperature of the chips in the manner described, and referring to Fig. 3, liquor from high temperature accumulator 42 is pumped into the base of the digester. During such pumping valve adjustments are made whereby the top of the digester is connected to the low temperature accumulator by line 22 as illustrated in Fig. 3. The high temperature liquor intro-duced into the base of the digester displaces -the low temperature liquid previously introduced to the digester, and such low temp-erature liquid returns to the low temperature accumula~or.
Pumping is performed against a back pressure valve provided in line 22 whereby a pressure condition is maintained in the digester preventing flashing of the high temperature liquor.
It has been found that in this and other displacement steps which may be performed in the process, displacement of liquid within the digester by another liquid is substantially quantitative, which is to say that during the introduction of displacement liquor, up until the time that the liquid-carrying capacity of the digester has been reached, liquid leaving the digester is essentially the liquid initially in the digester~
After introducing an amount of displacing liquid which is equal to the liquid carrying capacity of the digester, and with the introduction of more displacing liquid, the liquid leaving the digester becomes essentially -the displacing liquid. This is demonstrated by making temperature determinations of the liquid leaving the digester as will later be discussed.
With the introduction of high temperature liquor in an amount equal to the liquid-carrying capacity of the digester, _g_ pumping may be stopped. The amount oE high temperature liquor pumped also may be effected by the amount present in the high temperature accumulator, and how much may be pumped with a balancing of the system. Through direct contact of the high temperature liquor with the chips in the digester, the temperature of the chips is raised to nearly the temperature of the high temperatur~ liquor, for example, a temperature o~ approximately 290 to 300F (lA3 to 149C?.
Referring to ~ig. 4, at the conclusion oE this step in the process, and with the chips now at their higher temperature level, white liquor from the hot white liquor storage receptacle 62 may be pumped into the base of the digester utilizing pump 66.
With suitable valving adjustments made, hot liquor displaced from the digestor is returned to high temperature accumulator 42 as shown in Fig. 4. In a typical pulp process, from 50% to 70% of the liquid-carrying capacity of the digester is introduced at this time, since this is the extent of the white liquor usually needed to introduce into the digester the reactive chemical needed for digestion. Pumping is performed against a back pressure valve provided in line 46 whereby a pressure condition is maintained in the digester preventing flashing of liquor.
In practice, to save digester time, and to adjust white liquor concentration, the latter part of the hot black liquor fill and the white liquor fill may be done simultaneously.
As may be seen with reference to Fig. 1, hot black liquor from the high temperature accumula-tor may be pumped through heat exchanger 54 preliminarily to heat the white liquor pumped through the exchanger and delivered to storage receptacle 52. If it is desired to raise the temperature of the white liquor within storage receptacle 62 above the temperature produced by passing the liquor through exchanger 54, steam-operated heat exchanger 82 may be actuated, with circulation of white liquor through the exchanger using pump 84.
In the process of the invention, the temperature of the white liquor contained in the storage receptacle 62 might be within the range of 300 to 325F (150 to 163C).
With the digester now filled with the proper amount of white liquor, and the contents of the digester having a temper~ture within the range of 300 to 325F, the temperature of the conten-ts of the digester may be brought up to the temperature of the usual cook, i.e. 330 to 350F (165 to 177C), utilizing steam-operated heat exchanger 86, and with circulating of the contents of the digester through the exchanger employing pump 90.
At the conclusion of the cooking period, the hot black or spent liquor now in the digester is displaced from the diges-ter by pumping into the bottom of the digester stored filtrate from filtrate storage 32, as illustrated in Fig. 5. The liquor initial-ly leaving the digester is directed to the high temperature accumulator with such having, for example, a temperature of approximately 330F (165C). Pressure is maintained within the digester to prevent flashing of liquor.
As earlier discussed, the displacement of liquid within the digester through the pumping of a displacement liguid into the base of -the digester is substantially quantitative, and this is borne out by temperature measurements made of the liquid leaving the digester. Thus, temperature measurements have been made of the liquid leaving the digester when stored filtrate from filtrate storage 32, is pumped into the base of the digester.
Fig. 6 illustrates graphically the results obtained from such measurements. As can be seen with reference to Fig. 6, which char-ts the temperature of the liquid leaving the digester in relation to the vo:Lume of liquid introduced into the digester expressed as percent of the liquid-carrying capacity of the digester, up until the time -that the volume oE the pumped dis-~L~7~ 7 placing liquid equals substantially the liquid-carrying capacity of the digester, the tempera-ture of the liquid expelled remains substantially constant and at a temperature which is only slightly below the cooking temperature employed. Upon the volu~e of the displacing liquid introduced into the digester equaling the liquid-carrying capacity of the digester, the temperature of the liquid displaced drops off rather sharply. In practicing the invention, the filtrate pumped into the digester may conveniently be in an amount equaling approximately 120% of the liquid-carrying capacity of the digester. Liquid disl~laced during pumping into the digester of a volume equaling approximately the liquid-carrying capacity of the digester is directed to the high temper-ature accumulator. Remaining liquid traveling from the digester is directed to the low temperature accumulator, such having an average tempera~ure approximately mid-way between the temperature of the filtrate used and the temperature of the hot liquor initially displaced.
After recovering the high and low temperature liquids as above described, the digested pulp fibers in the digester may be cleared from the digester in any of a number of ways. For instance, and referring to Fig. 1, pressurized st~am may be introduced to the top of the digester to raise the pressure within the digester (insignificant increase in temperature occur-ring), and blow off line 96 opened, whereby the contents of the digester transfers to -the usual blow tank. ~lternatively, ~he contents of the digester may be removed ~rom the digester utilizing a flushing fluid such as filtrate from storage 32 introduced through conduit 98. Whatever approach is used, after -the pulp fibers are removed from the digester such are subsequently subjected to a washing, with the water used in such washing producing the first washer filtrate which is collected in filtrate storage 32. Since such washing liquid contacts warm pulp (and warm water is used for washing), the filtrate in the filtrate storage has a temperature elevated above room temperature.
Liqui~ supplied high temperature accumulator, in the embodimen-t of the invention described" is the spent liquor dis-placed from the digester at the conclusion of the cook. Liquid supplied low temperature accumulator is the hot black liquor which passes through exchanger 5~, ancl filtrate used in the final stage of displacing liquor from the cligester. These sources, and cooling resulting from heat transfer to chips, result in the liquid temperatures present in the accumulators.

Briefly summarizing some of the advantages of the process outlined and of modifications o~ this process, batch delignification may ~e employed with considerably less steam required to perform the heating operation then needed following conventional practice. This is because hot spent cooking liquor extracted from the digester is used as a principal heat source for succeeding digester batches.
Another advantage attained by the invention is a reduction in the white liquor needed to digest a charge of material. Residual active chemical present in spent liquor and in washing filtrate is reintroduced into the system, and this residual chemical is available for reaction with wood acids to neutrali~e these acids and for other initial reactions of the cook, before delignification with new white liquor occurs. As a con-sequence, cooking chemical in white liquor is not needed to perform these initial reactions. A somewhat related feature and advantage is greater selectivity in the delignification which occurs in the digester during the cook. At the time cooking occurs, there is a high concentration of active alkali which produces relatively rapid delignification which may be terminated before destruction of useful cellulose. Special chemicals intro-duced into a cooking li~uor to improve its charac-teristics, also are conserved by reason of this reintroduction into the system.
Yet a further advantage of the invention is the obtaininy of significant washing of the pulp in the digester as a result of displacement of the spent liquor after the cook with washing filtrate liquid. In effect, the digester is used as a containment vessel during the first washing of the pulp produced. In a new installation, this feature enables reduced cost for washing installations. In an existing installation, another washing step is permitted, with the advantages which flow from this.
Using conventional procedures, calcium contained in wood tends to dissolve to produce an increasing calciu~ ion concentration in the liquor during the initial phases o~ a cook when the load on heat exchanging equipment employed to heat the charge is high. With the hydroxyl ion concentration at this time being high, calcium carbonate starts precipitating, producing scaling in strainers and heat exchangers. Following the present invention, calcium ions are formed during contacting of the chips with the black liquor as a preliminary to charging with white liquor and hefore the hydroxyl ion concentration is high. When the white liquor is charged into the digester, there is an im-mediate and drastic increase in hydroxyl ions, and calcium car-bonate precipitates in the chip column. At this time there is no load on strainers and heat exchangers, and precipitation of the calcium carbonate in the strainers and heat exchangers is minimized.
Thus, the invention offers control of scaling which has been a problem ~ith conventional procedures.
A further advantage of the process outlined arises through the use of a hot liquor under pressure to displace another liquid in the digester. Pressure impregnation of the chips with the liquor under pressure occurs.
Finally, it should be noted that a conventional instal lation may be converted quite readily to apparatus utilizable in 7~
practicing the invention. This makes practicing the invention attractive to pulp processors who have been using conventional equipment and wish to lower their energy requirements.
The invention has been described in connection with the use o~ a single digester. Obviously~ operating principles o~ the invention are applicable to digestion processes using one or multiple digesters.
While a particular embodiment o~ the invention has been described, modification and variations are possible without departing ~rom the invention. Included within the invention, therefore, are such modi~ications and variations that would be apparent to one skilled in the art.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a batch digestion process wherein a digester is charged with a mass of cellulosic material and liquor, and the charge is then cooked at an elevated temperature and super-atmospheric pressure to obtain within the digester a column of delignified pulp extending along the axis of the digester, the method comprising after cooking of the charge and the production of the column of pulp, and while retaining the column of pulp in the digester, pumping under pressure a displacement liquid into an end of the digester whereby the displacement liquid initially replaces by displacement hot spent liquor in a region encompass-ing the cross section of the column of pulp which is spaced from an end of the pulp column, and with continued pumping causing the displacement liquid further progressively to fill the digester and progressively replace hot spent liquor in a direc-tion extending from said region toward said end of the pulp column, with such displacement liquid essentially quantitatively replacing by displacement hot spent liquor in a zone enlarging along the length of the pulp column, collecting the hot spent liquor so replaced, and transferring by conduction the sensible heat of the hot spent liquor to another mass of cellulosic material to preheat this other mass and prepare it for digestion.

2. The method of claim l, wherein said other mass of material is contained within a digester, and transfer of heat to said other mass to preheat this other mass is performed by pumping the hot spent liquor into the digester and immersing said material with the hot spent liquor.

3. The method of claim 2, wherein with pumping of the hot spent liquor into the digester containing the other mass of cellulosic material the hot spent liquor displaces another li-quid in the digester having a lower temperature than the temperature of the hot spent liquor.

4. The method of claim 2, wherein after pumping of the hot spent liquor into the digester containing the other mass of cellulosic material, fresh liquor is pumped into the digester containing the other mass of material with such displacing at least a portion of the hot spent liquor, and heat is transferred from the hot spent liquor so displaced to yet another mass of cellulosic material.

5. The method of claim 3, wherein heat is transferred from the displaced liquid at lower temperature to a third mass of cellulosic material contained within a digester by pumping the liquid into the digester containing the third mass of material and immersing the material with the lower temperature liquid.

6. The method of claim 1, wherein after pumping of displacement liquid to produce essentially quantitative replacement by displacement of the hot spent liquor, additional displacement liquid is pumped into the digester to cause replacement of earlier pumped displacement liquid, the hot spent liquor is collected in one region with such at a temperature which is essentially the temperature of the cook, the displacement liquid replaced by the additional pumping of displacement liquid is collected in a region separate from said one region at a temperature which is somewhat below the temperature of the hot spent liquor, and the sensible heat of the replaced displacement liquid is transferred by conduction to a mass of cellulosic material to preheat it for digestion.

7. In the digesting of cellulosic material to produce fibers therefrom, where the material is cooked with a fresh liquor to produce the fibers and spent liquor, the improvement which comprises prior to cooking the material, immersing the material in a digester with a preheating liquid at a relatively low temperature to raise the temperature of the material, replacing the preheating liquid at a relatively low temperature in the digester with another preheating liquid at a higher temperature whereby the material becomes immersed in the liquid at a higher temperature and the temperature of the material is further raised, replacing at least a portion of the preheating liquid at higher temperature in the digester with fresh liquor, cooking the material at the temperature required for cooking, and recovering spent liquor produced by the cooking at substantially the temperature at which cooking was performed, said recovering step including pumping displacement liquid into an end of the digester to progressively displace the hot spent liquor quanti-tatively, the preheating liquids at a relatively low temperature and at a higher temperature being derived from the spent liquor so recovered.

8. The method of claim 7, wherein the preheating liquid at a relatively low temperature is replaced by the preheating liquid at higher temperatures by pumping the liquid at higher tempera-ture into the digester with such displacing the liquid at a rela-tively low temperature, the preheating liquid at higher tempera-tures is replaced with fresh liquor by pumping fresh liquor into the digester with such displacing the liquid at higher tempera-ture, and the spent liquor is recovered by pumping a displacement liquid into the digester with such displacing the spent liquor.

9. The method of claim 8, wherein the displacement liquid is washer filtrate obtained through washing of fibers.

10. In the digesting of cellulosic material, where a digester is filled with a charge comprising cellulosic material and cooking liquor, the cooking then occurs in the digester at an elevated temperature and superatmospheric pressure to produce pulp fibers and conversion of the cooking liquor to hot spent liquor, the method comprising at the conclusion of the cooking of one digester charge and with the maintaining of pressure within the digester, pumping displacement liquid into the base of the digester which expels hot spent liquor from the top of the digester by displacing such from the digester, and transfer-ring the heat of -the hot spent liquor so displaced to another digester charge to prepare such for cooking.

11. The method of claim 10, wherein the heat of the hot spent liquor is transferred to another digester charge by heat-ing the cooking liquor of the charge.

12. The method of claim 10, wherein the heat of the hot spent liquor is transferred to another digester charge through contacting the cellulosic material of the charge with the liquor to directly transfer the heat to the material, and by heating the cooking liquor of the charge before such liquor is intro-duced to the digester.

13. In the digesting of cellulosic material, where a digester is filled with a charge comprising cellulosic material and cooking liquor, and cooking occurs in the digester at an elevated temperature and superatmospheric pressure to produce pulp fibers, with conversion of the cooking liquor to hot spent liquor, the method comprising: at the conclusion of the cooking of one digester charge, maintaining the elevated pressure within the digester, pumping displacement liquid into an end of the digester to expel hot spent liquor through an outlet of the digester by displacing the hot spent liquor from the digester, and transferring the heat of the hot spent liquor so displaced to another disgester charge to prepare such for cooking.