Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INV NTI~N
Field of the Invention
The present invention is in ~he fi~ld of chemical
cooking of wood chips to produce pulp suitable for manufacture
of paper and involves sequential use of various cooking
liquors in the same digester for predetermined cooking times,
to increase the efficiency of the process and to conserve
energy.
Descrl tion of the Prior Art
There are numerous types of processe~ ~or batch
digestion of wood chips in the manufacture of paper. The
digestion usually takes place in a digester specifically
built for that purpose, the digester being filled with the
wood chips which are usually compacted therein. ~ot
solutions of sodium hydroxide alone or in admixture with
sodium sulfide are then charged into the digester. The
temperature of the digestar can be controlled through
the introd~ction of ste~m and ater maintaining the chips
in contact with the cooking li~uor for a predetermined period
of time, a blow valve in ~he digester can ~e opened to
dump the contents into a blow tank.
There is a substantial amount of heat loss in
carrying out the batch digestion process and while many
systems have been suggested or minimizing this heat loss,
none has been particularly efective. Some paper manu-
facturers have gone to continuous digesting processes in order
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to Improve the efficiency of the cooking operation, but the
equipment costs for a continuous digesting system are very
high.
More recently, an improved type of batch process
has been designed for overcoming these difficulties. In
this new process, the wood chips are cooked in ~he digester
and the hot black liquor which results i5 removed by
displacement with a filtrate from the washing ~ection.
This filtrate is added to the bottom of the digester and
pushes up the hot spent liquor through the chip column
wi~hout a subskantial in~erminglin~ o the two liquld~.
The displaced hot black liquor is then dixected into a
pressurized acc~mulator. The digester is then emptied by
adding steam to the top of the digester which forces the
pulp out through a blow valve into a blow tank. After the
pulp has been blo~n from the diyester, it is uni~ormly filled
with chips.
Hot black li~uor from the accumulator i5 pumped
into the bottom of the digester where it heats the chips.
In this stage,,an excess of black liquor i5 employed~ more
than the capacity of the digester so that excess black
liquor is discharged from the top of the aigester and is
transferrPd to a we~k black liquor tank. Fresh white liquor
is then used to displace the black liquor from the bottom
of the digester and ~he resulting spent liquor is passed
to a weak black liquor storage space. The contents of the
digester are then heated with steam to the desired cooking
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temperatures and held there for the required cookina times.
When the contents of the digester have reached the cooking
temperature, the steam introduction stops. After cooking,
the hot liquor is removed as in the originally described
step, and the cyle starts over again. This type of process
is described in Canadian Patent No. 1,179,807, dated
December 27, 1984, and assigned to the assignee of the
present application.
SUMMARY OF THE INVENTION
The present invention provides a multi-stage wood
chip cooking process utilizing a single digester wherein the
wood chips are introduced into the digester and soaked in
a warm black liquor to remove most oE the air from the
digester and the ch:Lps. ~fter a 9uitable soalc.Lng per.Lod,
the warm blaclc llquor is displaced Erom the digester with
a mixture of a first hot black liquor and hot white liquor
having a relatively high proportionate amount of white liquor.
The temperature of the digester contents is raised to a
cooking temperature usually by circulating the contents
through a heat exchanger through which steam is added~
AEter a suitable cooking period which will be the longest
of the multi-stage process, the liquor is displaced from
the digester with a mixture of a second hot black liquor
and hot white liquor. This mixture has a proportionate
amount of hot white liquor lower than in the first liquor.
The temperature of the digester is again raised to a cooking
temperature and after the chips have attained a predetermined
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degree of cooking, the liquor is displaced in the digester
with a liquid filtra~e derived from pulp washing. Finally,
the contents of the digester are emptiled by applying gas
pressure to the interior of the digester.
Stated more generally, the present invention
involves a multi-stage wood chip cooking process in which
the chips are sequentially cooked i~ a digester in a series
of cooks Cl, C2, C3 ... Cn. The cook Cl is carried out
with a liquid I.l having a relatively high proportionate amount
of white liquox and for a r~latively long cooking time T1.
Cook C2 i9 carried out with a liquor L2 havlng a proportionate
amount of white liquor less than Ll and for a time shorter
than Tl. Succeeding cooks are carried out khrough cook Cn at
successively lower proportionate amounts of white liquor
and successively shorter timesO As few as two stages can be
used, but three are preferred. More than thxee can be used
where necessary or desirable.
In the case of a three-stage process for pulping
softwood chips using the kraft process, the following
conditions may apply. The tokal white li~uox may typically
constitute about 25% of the liquid capacity of the digester
minus the volume of chips in the digester. The first cooking
is carried out with an amount of white liquor comprising/
5Q to 75% of the total white liquor for a period of 25 to 40
minutes~ the second cooking is carried out with an amount of
white liquor comprising 10 to 30~ of the total for a period of
10 to 20 minutes, and the third cooking is carri~d out with
an amount of white li~uvr comprising 5 to 20% of the total
for a period of 5 ko 15 minutes.
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BRIEF DESCRIPTION OF THE DRAWING
The single Figure of the drawing illustrates
schematically an ins~allation for carrying out ~he multi-
stage wood chip cooking process of ~h~ present invention.
DESCRIPTION OF THE P~ EFERRED EMBODIMENTS
_ _
In the attached Figure of the drawing, reference
numeral 10 has been applied generally to a digester of the
conventional type including a removable lid 11. The
contents o~ the dige~ter can be heated to a cooking
temperature by pumping them through a pump 12 and a valve 13
through a heat exchanger 14 having a steam inlet line 15
and a steam condensate outlet line 16.
A warm liquor accumulator 17 stores black liquor
at a relatively low temperature. This warm black li~uor,
at a temperature substantially below that required for
cooking,is initially pumped b~ means of a pump 18 through
a line 19 controlled by a valve 20 into the base of the
digester 10 through an inlet valve 21.
The system al50 includes a first hot liquor
accumulator 22 which contains the liquor for the first stage
cook. This black liquor at a relatively high temperature is
pumped out through a pump 23 and proceeds through a valve 24,
through the valve 21 and into the base of the digester.
A hot white liquor accumulator 25 serves as a s~orage
vessel for the fresh hot white liquor which is pumped out
of the accumulator 25 by means of a pump 26 through a
flow regulator 27 and is thereupon combined with the
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discharge of the pump 23 from the first hot black liquox
accumulator 22. The flow regulator 27 can be used to set
the relative proportions between hot white liquor and
hot black liquor in the first cooking ~step.
The ho~ white liquor is prahleated after it is
introduced through an inlet line 28 through a heat exchanger
29 before entering the hot white liquor accumulator 25.
The heat exchange is accomplished by withdrawing a portion
of the hot black liquor from the accumulator 22 through a
pump 30 and a line 31. This hot black liquor is al~o used
as the heat exchange li~uid ~or a second heat ~x~hanger 32
which will be described subsequently. The hot black liquor
passing through the two heat exchangers 29 and 32 is removed
as a warm but not hot liquid through a line 33 whereupon
it is delivered to the warm liquor accumulator 17.
Periodically, the warm black li~uor from the accumulator 17
is discharged ~hrough a line 66 and passed to a black liquor
evaporator.
A second hot liquor accumulator 34 is used to
store the black liquor for the second cook. A pump 35
delivers a stream of the hot black liquor which is at a
lower temperature than the hot black liquor in the accumulator
22 into combination with hot white liquor from the accumulator
25. The hot white liquor is delivered by means of a pump 36
through a flow regulator 37 where it is combine~ with the
discharge of the pump 35, the combined discharge then
passing through a valve 38 and into the ~ase of the digester
through the valve ~1. The relative proportion of hot white
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l iquor in this cook will be less than the proportion used
in the first cook~ a~d the cooking templ3rature will be
less. Steam is optionally introduced into the second hot
liquor accumulator 34 by means of a steam line 39.
A third hot liquor ~ccumulator 40 containing
hot black liquor for the third cook is provided with a
pun~p 41 for the discharge of its contents. A pump 42
associated with the hot white liquor accumulator ~5
delivers a metered amount o~ hot white liquor through
a line 43 and a ~low regulator 4~ into admlxture wikh the
hot black liquor being pumped through the pump ~1. The
combined stream passes through a valve 45, and through the
valve 21 into the base of the digester 10. The combined
stream for the third cook has a lesser concentration of
white li~uor than the previous cooking liquors and is at
a lower temperature.
A pulp washer filtrate recovered from another
portion of the papermaking plant (not shown) is introduced
into an accumulator 46 through a line 47. A pump 48 is
provided to deliver the filtrate through a valve 49 and
into the base of the digester through the valve 21.
A portion of the filtrate in the accumulator 46 may be
pumped by means of pump 50 through a line 51 and then through
the heat exchanger 32 where it is passed in heat exchange
relationship with the hot black liquor from the first hot
liquor accumulator 22. This prehea~ed filtrate is then
directed to a hot filtrate accumula~or 52. A pump 53
delivers the heated filtrate through the valve 49 and into
the digester 10.
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Finally, there is provided a discharge valve 54
for emptying the con~ents of the digester 10. For this
purpose, air or other fluid is introduced through an inlet
line 55 at the completion of the cook, whereupon ~alve 54
is opened and the contents of the digester are transfexred
to a blow tank or another receptacle through a discharge
line 56.
The various cooking liquors are then returned to
the accumulators upon completion o the individuAl stages
o~ the cook. ~ llne S7 and valve 58 are used to r~turn
a weak cooking liquor to the warm liquor accumulator 17.
A line 59 and a valve 60 are used to deliver hot black
liquor to the first hot liguor accumulator 22. Similarly,
a line 61 and a valve 6~ return li~uor from the digester 10
to the second hot liquor accumulator 34. Material is
recycled to the third hot liquor accumulator 40 by means
of a line 63 and a valve 64.
The process of the present invention can be used
with a modified kraft or soda pulping process. The multi-
stage system preferably consists of three stages as shown
in the drawing but may employ two stages or more than three.
The multi-stage cook removes the non-cellulosic material
from the wood chips in a manner so as to improve the pulp
~ield, improve the pulp quality as measured by the aver ge
molecule size, and improve the pulp brightness. Additionally,
this new process permits pulps produced for further pr~cessing
by bleachi~g to be more completely delignified and thus
require a milder bleaching treatment using reduced quantities
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of bleach chemicals. A further advantage of this invention
is ~hat most of the pulp washing to remove spent cooking
chemlcal and dissolved organic matter is done in the
digester.
~ he conditions of time, temperature and active
cooking chemical concentration can, within reasonable limits,
be adjusted between the stages of cooking so as to optimize
the desired pulp properties from the wood chips being used.
This feature provides greater ~lexibility in the pu:Lping
operation.
rrh~ followlng desaription is given to ~how the
o~erall process sequence.
The empty digester 10 is filled by removal o~ the
lid 11 with wood chips. These chips may be compacted in
order to increase the quantity of chips charged, and to
provide a more uniform chip density. It i5 preferable that
overly thick chips (more than 6 millimeters) be removed
~rom the chips supply.
With the digester 10 closed, warm blac]c liquor
from the accumulator 17 is pumped by means of pump 18
through the line 19 to ~he valve 21 into the bot~om of the
digester which is substantially filled wi~h chips. The
digester is completely filled with this liquor and some
excess is supplied. The excess leaves the digester by means
of an extraction screen (not shown) located in the top dome
of the digester 10. The excess liquor is returned to the
warm liquor accumulator 17 through the line 57. This
initial soaki~g with the warm liquor at a temperature
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considera~ly below cooking temperature serves to remove
most of ~he air from the digester and the chips, warms the
chips, and neutralizes some of the organic acids associated
with the wood chips. The excess weak black liquor generated
in the pulping and washing system is periodically discharged
to the black liquor evaporators through the discharge line 66.
Hot black liquor from the first hot black liquor
accumulator 22 and hot white liquor from the hot white liquor
accumulator 25 are pumped together by means of pumps 23 and
26, respectively, through the valve 24 into the bottom o~
the dige3ter which i~ n4~ ~illed with warm black llquox.
The di~placed warm liquor leaves the dige~ter via the
extraction screen in the top dome of the digester and is
returned to the warm black liquor accumulator 17 through
the line 57.
Liquor from the first hot black liquor accumulatvr 22
is not only used for filling the digester 10 but is also
used to preheat the fresh white liquor in the heat exchanger 29
and also to preheat the ~irst portion o~ the washer filtrate
in the heat exchanger 32. The black liquor leavin~ the two
heak exchangers goes to the warm liquor accumulator 17 by
means of the line 33.
The hot wh~te liquor entering through the line 28
is also heated by the heat exchanger 29 be~o~e it arrives
at the hot white liquor accumulator 25.
The temperature of the contents of the digester 10
filled with the mixture of hot black liqu4r rom the first
hot liquor accumulator 22 and the hot white li~u~r accumulator
25 iB raised to the desired cooking ~emperature by circulating
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the contents of ~he digester through the valve 13 and
heat exchanger 14 under the action of the pump 12. Forced
circulation of liquor in the digester is preferred to
insure uniform distribution of temperature and chemicals
throughout ~he digester lO. In the case of a kraft process
cooking for the three-stage sequence, the first cooking
can take place with a liquor containing 50 to 75% o:E the
total white liquor used and a cooking time of 25 to 40 minutes.
No cooking operation in thi~ multi-stage process requires as
much as a 60 minute cook.
~ t the conclu~ion oE the desired ~ir~t ~tag~ cooking
time, hot black liquor frQm the second hot liquor accumulator
34 and hot white liquor from the accumulator 25 are pumped
together through pumps 35 and 36, respectively, into the
bottom of the digester lO. Proportioning of the relative
amounts is accomplished by the flow controller 37. This
second cooking liquor contains a lower proportionate amount
of white liquor and is used for a lesser cooking time than
the first cook. Typically, the second cooking is carried
out with a white liquor constituting lO ~o 30% o~ the total
for a period of lO to 20 minutes.
The digester is brought up to cooking temperature
by circulating ~he liquor through the heat exchanger 14.
Alternatively, steam can be added to ~he second
hot liquor accumulator 34 through the steam line 39.
A~ the expiration of the desired sPcond stage
cooking time, hot black liquor from the third hot black
liquor accumulator 40 and hot white liquor through the
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pump 42 and flow regulator 44 are combined and pumped together
through valve 45 into the base of the digester 10. The
displaced black liquor leaves the digester 10 and passes
through line 61 and valve 62 into the second hot black
liquor accumulator 34.
The temperature of the digester contents filled
with the hot black li~uor and hot white liquor is then
raised to the desired cooking temperature by circulating
through the heat exchanger 14. Alternatively, the heat
exchanger can be eliminated and the steam can be injected
direatl~ into a circulating llne whereby the contents o
the digester are withdrawn from the top and pumped into the
bottom by means of the pump 12. As another alternative,
steam can be added to the third black liquor accumulator 40.
Typical conditions for the third cooking cycle
include a white liquor fraction of 5 to 20% of the total
and a cooking time of 5 to 15 minutes.
At the expiration of the desired third stage cooking
time, filtrate from the pulp washing operation is pumped
into the bottom of the digester. The displaced black
liquor leaves the digester from the top and goes to the
third hot black liquor accumulator 40 through the line 63
and valve 64.
The first portion of the filtrate is pumped from
the accumulator 46 through a pump 50 into hea~ exchange
relationship with the hot black liquor circulating through
the heat exchanger 32. Preheating the first portion of the
filtrate reduces the total steam required in the pulping
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system, permits the wash water added to the liquox system
to be efficiently used by counter-current flow and maintains
a low concentration of black liquor in the final cooking
stages.
After the hot black liquor has been displaced from
the third stage cook with the washer filtrate, compressed air
is introduced through a line 55 into the top of the digester
and the contents of the digester, pulp and washer filtrate
are forced out of the bottom of the digester through a
valve 54 into a suitable storage chest or blow tank by means
of the line 56. The discharge of the chips from the
digester by means of a curtain o air is more ully described
ln Canadian patent appl:Lea-tion Serial No. 42~,978.
Typical c~ookincJ conclltions eor a three-sta~e
process pulping soEtwood ehips using the kraft process are
given in the following table:
Final
%Total Total Maximum Final Black
White Cooking Cooking Pulp Liquor
Stage Liquor Time(min.) Temp. ~F Kappa Nr. Solids,%
First 69 50 340 100 25
Second 22 30 338 40 12
Third 9 25 335 25 4
The present invention permits the use of relatively
short total cycle times and thus improves pulp production
rates. Typical times for the various functions in a three-
stage process according to the present invention are:
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Function TimP in Minutes
Chip Filling 15
Warm Liquor Fill 20
First Hot Liguor Fill 15
First Stage Cook 35
; Second Bot Liquor Fill 15
Second Stage Cook 15
Third Hot Liquor Fill 15
Third Stage Cook lO
Hot Li~uor Displacement 20
Blowing 15
Spare 5
Total Digester Cycle Time 180
While the drawings illustrate a three-stage process
and this is the preferred embodiment, the invention is more
general than that. Basically, the invention involves a
multi-stage wood chip cooking process in which the chips are
sequentially cooked in a digestex in a series of cooks Cl,
C2, C3 ... Cn. The cook Cl is carried out with a liquor Ll
having a relatively high proportionate amount of white liquor
and for a relatively long cooking time T1. Cook C2 is carried
. out with a liquor L2 having a proportionate amount of white
liquor less than Ll and for a time shorter than Tl. Succeeding
cooks through cook Cn are carried out in successively lower
proportionate amounts of white liquor and successfully shorter
times.
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The total auantity of white li~uor used in a digester
is determined by (1) the degree of pulping or extent of deligni-
fication desired, ~2) the guan~ity of wood chips chargsd on an
oven ~ry basis, and (3) the concentration of the active cooking
chemicals, sodium hydroxide and sodi~m sulphide, in the whi~e
liquor. For example, it is found ~hat an active cooking chemical
application, expressed as sodium oxide, of 15% on oven dry wood
is required to achieve a properly delignified pulp. In a 6,000
cublc foot digester containing 60,000 pounds of bone dry wood
there is a need or lS percent of 60,000 or 9,000 poun~s o~ acti~e
alkall. The volume o white liquor, found by te~t to contain
6.0 pounds of active alkali per cubic foot, required for the
charge is then calculated to be 1,500 cubic feet.
In the above example, the entire quantity of white
liquor is added to the charge in the initial filling operation
in a conventional batch pulping system. With the new multi-
stage process, assuming the same total white liquor usage, the
application in a three-stage system could be as follows:
~hite Liquor Percent of
~ arye ~ 1 Ch~
F ~ t 1,000 cu. ft. 66.7%
Second 300 cu. ft. 20.0%
Third 200 cu. ft. 13.3%
Tbta~1,500 cu. ft. 100.0%
It will be evident tha~ various modi~ications can
be made to the described embodiments withcut departing from
the scope of the present invention.
