Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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 -
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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
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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
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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-
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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
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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
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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,
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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
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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-
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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
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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
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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.
