Note: Descriptions are shown in the official language in which they were submitted.
1
I1'~R.EGNATION KITH BLACK L~'UOR PRIOR TO
4~"HITL' LI UOR INTRODUCTION
BACKGROUND AND SUhiI~IARY OF TI3E INVENTION
For several decades, continuous cooking of
- comminuted cellulosic fibrous material (e. g. wood
chips) to produce pulp, particularly kraft pulp, has
been an extremely efficient system for producing the
pulp. Whenever continuous cooking is practiced,
however, care must be taken to ensure uniformity of
the end product. An important part of ensuring
uniform quality of the end product is ensuring
proper impregnation of the material with coo_.king
liquor (white liquor). It has long been known that
freshly cut wood, with high moisture content, will
impregnate quickly and uniformly, whereas stored
wood, with low moisture and high air content,
requires efficient presteaming and favorable
impregnation conditions in order to be uniformly
impregnated. Despite the fact that this has been
known for many years, this fact has not normally
been effectively utilized to ensure uniform
impregr~tion of cellulosic material, such as wood
chips.
As energy conservation becomes more important,
it also becomes important to minimize energy use
during cooking. Typically, a substantial amount of
energy in the form of high pressure steam is
consumed in a continuous pulping facility by heating
the white liquor to an appropriate cooking
temperature (e. g. approximately 160-180°C).
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2
According to the present invention, a method of
continuously treating comminuted cellulosic fibrous
material to produce paper pulp, particularly kraft
pulp, is provided which has enhanced uniformity and
minimized energy requirements. According to the
present invention, impregnation by rapid diffusion
:i.s accomplished so that -the uniformity of treatment
is enhanced, and preferably the cooking liquor is
passed in heat exchange relationship with black
liquor withdrawn from a digester so as to preheat
the cooking liquor. This minimizes the consumption
of high pressure steam in a conventional indirect
heater.
According to one aspect of the present
invention, a method of effecting continuous kraft
cooking of comminuted cellulosic fibrous material is
provided. The method comprises the steps of
continuously: (a) Effecting rapid displacement
heating of comminuted cellulosic fibrous material
moving in a first flow path by introducing black
liquor, substantially at cooking temperature, into
the material moving in the first f low path. Then -
(b) effecting rapid diffusion impregnation o~ the
material moving in a second flow path, after the
first flow path, by introducing white liquor into
the material moving in the second flow path. And
then (c) effecting continuous kraft cooking of the
material. Step (b) is preferably practiced by
introducing the white Liquor substantially at
cooking temperature into the second flow path, after
it has been passed into heat exchange relationship
with the black liquor substantially at cooking
temperature.
3
According to another aspect of the present
invention, a method of continuously treating
comminuted cellulosic fibrous material to produce
kraf_t pulp comprises the steps of continuously: (a)
Slurrying comminuted cellulosic fibrous material
with lic~.iid to produce a slurry. (b) Feeding the
slurry into a soaking and heating zone. (c) Heating
and soaking the s7.urry with heating and soaking
liquid in the heating and soaking zone. (d) Feeding
the slurry from the heating and soaking zone into an
impregnation and cooking zone. (e) Treating the
slurry with kraft cooking liquor at cooking
temperature in the impregnation and cooking zone to
effect digestion of the material in the slurry, to
produce kraft pulp. (f) ~lithdrawing black liquor
from the impregnation and cooking zone, at cooking
temperature.. And, (g) feeding at least a portion of
the black liquor withdrawn in step (f) to the
soaking and heating zone to provide at least a part
of the soaking and heating liquid in step (c).
There is preferably also the further step (h) of
preheating the cooking liquor before feeding it into -
the impregnation and cooking zone by passing it into
heat exchange relationship with a first portion of
the black liquor withdrawn in step (f), while a
second portion of the black liquor withdrawn in step
(f) is used in step (g).
Step (b) may be practiced in part by
withdrawing a portion of the liquid from the slurry
at a withdrawal area above the heating and soaking
zone, pressurizing the withdrawn liquid, and
introducing the pressurized withdrawn liquid into
the slurry, above the withdrawal area, with a
4
primarily downward vector. Steps (c) and (g) are
preferably practiced by withdrawing a portion of_ the
liquid from the slurry in the heating and soaking
zone at a withdrawal area, recirculating the
withdrawn liquid back into the heating and soaking
zone above the withdrawal area, and adding black
liquor from step (f) to the recirculating withdrawn
liquid. Step (a) is preferably practiced by
slurrying the material wi'h black liquor. Steps
(b)-(f) are preferably practiced in a single
vertical vessel, the material moving downwardly in
the vessel during treatment.
According to a still further aspect of the
present invention, a method of treating a slurry of
comminuted cellulosic fibrous material in an upright
vessel having a top and bottom, and at least first,
second, and third sets of screens, vertically spaced
from each other with the first set of screens the
topmost screens, and the third set the bottom most,
is provided. This method comprises the steps of:
(a) Introducing the slurry into the top of the
vessel, to flow downwardly therein. (b) Withdrawing -
a part of the liquid from the slurry with the first
set of screens, and recirculating the withdrawn
liquid back into the vessel above the first set of
scraens. (c) Withdrawing a part of the liquid from
the slurry with the second set of screens, and
recirculating the withdrawn liquid back into the
vessel above the second set of screens. (d)
Withdrawing a part of the liquid from the slurry
with the third set of screens, and passing at least
a portion of the withdrawn liquid from the third set
of screens into the withdrawn liquid from the first
CA 02050818 2001-10-O1
23402-147
set of screens, to be introduced into the vessel with the
recirculated liquid from step (b). And, (e) discharging
pulp from the bottom of the vessel, below the third set of
screens. A fourth set of screens is also preferably
5 utilized, above the first set. Liquid is withdrawn through
the fourth set of screens, pressurized, and then introduced
above the fourth set. of screens with a substantially
downward velocity vector so as to assist in movement of the
material.
According to one aspect of the present invention,
there is provided a method of continuously treating
comminuted cellulosic fibrous material to produce kraft
pulp, comprising the steps of continuously: (a) slurrying
comminuted cellulosic fibrous material with liquid in a
high-pressure feeder to produce a slurry; (b) feeding the
slurry into a soakirig and heating zone; (c) heating and
soaking the slurry with heating and soaking liquid in the
heating and soaking zone; (d) feeding the slurry from the
heating and soaking zone into an impregnation and cooking
zone; (e) treating the slurry with kraft cooking liquor at
cooking temperature in the impregnation and cooking zone to
effect digestion of the material in the slurry, to produce
kraft pulp; (f) withdrawing black liquor from the
impregnation and cooking zone, at cooking temperature; and
(g) feeding at least a portion of the black liquor at
cooling temperature withdrawn in step (f) to the soaking and
heating zone to provide at least a part of the soaking and
heating liquid in step (c) to effect rapid displacement
heating of the wood chips, and then effecting rapid
diffusion impregnation with white hot liquor.
According to another aspect of the present
invention, there is provided a method of effecting
continuous kraft cooking of comminuted cellulosic fibrous
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5a
material, comprising the steps of continuously: (a)
effecting rapid displacement heating of comminuted
cellulosic fibrous material moving in a first flow path by
introducing black liquor, substantially at cooking
temperature, into the material moving in the first flow
path; then (b) effecting rapid diffusion impregnation of the
material moving in a second flow path, after the first flow
path, by introducing white liquor into the material moving
in the second flow path; and then (c) effecting continuous
kraft cooking of the material.
According to still another aspect of the present
invention, there is provided a method of treating a slurry
of comminuted cellulosic fibrous material in an upright
vessel having a top and a bottom, and at least first, second
and third sets of screens, vertically spaced from each other
with the first set of screens the topmost screens, and the
third set the bottommost, comprising the steps of: (a)
introducing the slurry into the top of the vessel, to flow
downwardly therein; (b) withdrawing a part of the liquid
from the slurry with the first set of screens, and
recirculating the withdrawn liquid back into the vessel
above the first set of screens; (c) withdrawing a part of
the liquid from the slurry with the second set of screens,
and recirculating the withdrawn liquid back into the vessel
above the second set of screens; (d) withdrawing a part of
the liquid from the slurry with the third set of screens,
and passing at least a portion of the withdrawn liquid from
the third set of screens into the withdrawn liquid from the
first set of screens, to be introduced into the vessel with
the recirculated liquid from step (b); and (e) discharging
pulp from the bottom of the vessel, below the third set of
screens.
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5b
It is the primary object of the present invention
to provide for the continuous cooking of pulp with
substantial energy economy and simultaneously improved
uniformity of the end product. This and other objects of
the invention will become clear from an inspection of the
detailed description of the invention, and from the appended
claims.
BRIEF DESCRIPTION OF THE DRAWING
FIGURE 1 is a schematic view of an exemplary
apparatus for practice of the method according to the
present invention.
23402-147
CA 02050818 2001-10-O1
6
DETAILED DESCRIPTION OF THE DRAWING
Many of the components of the apparatus
illustrated in FIGURE 1 are the same as, or
comparable to, parts of the apparatus of U.S. patent
3,802,956.
That patent shows
apparatus for effecting continuous pulping,
particularly kraft pulping, of comminuted cellulosic
fibrous material, with a view toward proper
impregnation of the material.
Apparatus of the system of FIGURE 1 that is
generally comparable to that in said patent
3,802,956 includes a chips bin 11 (which may be a
steaming chips bin), communicating with a low
pressure valve 13 with a conventional steaming
vessel 14. Low pressure steam, e.g. one atmosphere
over pressure, is supplied to the vessel 14 through
a conduit 15, ar_d air is driven off through conduit
17. The chips ultimately pass through chute 18 into
the conventional high pressure feeder 19,
circulating liquid being forced by pump 20 to -
entrain chips -- or like cellulosic fibrous material
-- droning from chute 18 into the transfer device
19 in the circulating liquid.
According to the invention, the chips from high
pressure feeder 19 pass in conduit 21 to the top of
a substantially vertical digester 22. The slurry of
chips in liquid passes into apparatus at the top of
the digester 22 for separating a portion of the
liquid, the apparatus preferably comprising a
strainer girdle 25 with a feed screw 27 therein, and
the withdrawn liquid passing in conduit 23 back to
7
the pump 20. A screen 28 is provided adjacent to
the top of the digester 22, but below the girdle 25
and screw 27, connected to pump 26, which in turn is
connected to nozzles 24. A portion of the liquid
withdrawn from the slurry through screen 28 is
pressurized by pump 26 and then forced through the
nozzles 24 back into the vessel, above the screen
28, with a substantially downward velocity vector.
This system is primarily for producing a downward
thrust on the material for operational purposes, so
that it flows concurrently with liquid initially
down into the digester 22. A portion of the liquid
withdrawn by screen 28 is passed via conduit 30 to
the second effect 90 of a flash tank system, as
hereinafter described.
The chips are flushed into the high pressure
feeder 19 through chute 18 via liquid flowing in
loop 37 under the influence of pump 35. Excess
liquid passes into level tank 39 and is returned
therefrom through conduit 41 and pump 29 to the high
pressure side. Additionally, make-up liquor --
which may be b~.ack liquor from source 33, white
liquor from source 31, or a combination of both (as
meteredmby valve 48), and valves 10, 12, may be
provided. This liquid passes to the top exterior of
the strainer girdle 2S -- i.e. the high pressure
loop -- via conduit S0.
Below the screen 28 adjacent the top of the
vessel 22 is a soaking and heating zone. A screen
40, connected to a pump 42, is provided in the
soaking and heating zone. The pump 42 withdraws a
portion of the liquid from the slurry moving
downwardly in the vessel 22, and recirculates it in
8
loop 43 to the inlet tube 46 above the screen 40.
Liquid introduced thereby tends to flow
countercurrently to the chips moving downwardly in
the vessel 22, ultimately being withdrawn by screen
28. A heater, such as an indirect heater (heating
liquid with steam), may be provided in the
circulation loop 43, but is not necessary according
to the invention.
Below the screen 40, an impregnation and
cooking zone is provided. Within the impregnation
and cooking zone are one or more screens 63, 87, and
92. A portion of the liquid in the slurry moving
downwardly at the screen 63 is withdrawn therefrom
via pump 65, and passed through conventional
indirect steam heater (heat exchanger) 67 to be
heated, the withdrawn liquid being recirculated and
introduced at central return conduit 69 into the
chips column above the screen 63. According to the
present invention, kraft cooking liquor (wh.ite
liquor) from source 31 may be pumped by pump 36 to
this recirculation loop, e.g. added to the pump 65
to be introduced with the recirculated liquid.
The screens 87 and 92 withdraw black liquor
from tl~ digester 22. A portion of the black liquor
2.5 withdrawn by screen 92 under the influence of pump
93 may be recirculated into the digester 22 and
introduced at central conduit 95,~while another
portion passes through valve 32 ultimately to the
second stage flash tank 90. The conduit 91 from the
bottom of the flash tank 90 may be used to supply
black liquor to the source 33. Also, according to
the present invention, black liquor withdrawn
through screen 87 passes into conduit 110, which is
connected to the circulation loop 43 including the
pump 42, so that it is introduced at the return
conduit 46 unto the vessel. The black liquor in
conduit 110 is substantially at cooking temperature
(e. g. about 160-180°C).
Another portion of the liquor withdrawn with
screens 87 -- which also is at substantially cooking
temperature --- is used to preheat white liquor
introduced from source 31 into pump 65. This may
preferably be accomplished by passing the conduit
112 and the conduit 38 into a heat exchanger 114,
whereby the black hot liquor in conduit 112 comes
into heat exchange relationship with the cool white
liquor in conduit 38, preheating it before it is
introduced into pump 65. Typically, the heat
exchanger 114 would preheat the white liquor from
about 70°C to about 160°C. Little additional energy
input is required from the heater 67. After losing
a substantial portion of its heat value in heat
exchanger 114, the remaining black liquor in conduit
112 is passed to a first stage flash tank 89.
The arrangement described above, in addition to -
obviously minimizing energy requirements, also
effect ~ very uniform impregnation. The black liquor
introduced from conduit 110 soaks and heats the
chips, effecting rapid displacement heating
thereof. This rapid displacement heating makes the
chips much more amenable to impregnation with white
liquor. 'that means when the chips pass downwardly
into the impregnation and cooking zone, and are
confronted with hot white liquor, rapid diffusion
impregnation of the chips with white liquor occurs.
The chips are then cooked at a temperature of about
10
160-180°C, and then passed downwardly into the lower
part of the digester 2?. where they are preferably
washed.
Near the bottom of the vessel 22 a screen 79 is
o connected to a pump 81, heater 83, and central
supply conduit 85. Washing liquid is introduced
into the conduit 71 to flow countercurrently (that
is upwardly) with respect to the downward .flowing
chips, with a portion of the liquid being withdrawn
through screen 79, heated, and recirculated back
into conduit 85. The washed, kraft pulp, may then
be discharged from the bottom of the vessel
utilizing a conventional rotary scraper 99, and
a
throttling device 101 if desired.
While the method has been described with
respect to a single upright vessel 22, the method
may be practiced otherwise (e.g. with more than
one
vessel), although the single vessel arrangement
of
FIGURF 1 is greatly preferred.
According to one aspect of the method of the
present invention, a slurry of comminuted cellulosic
fibrous material (e. g. wood chips) in liquid (e.
g..
black liquor combined with steam which condensed
on
the chi's from steaming vessel 14, and any natural
liquid in the chips) is fed to the top of tha
digester 22, and under the influence of the
downwardly directed flow of liquid from nozzles
24
moves downwardly in the vessel 22. At a first set
of screens 40 part of the liquid in the slurry
may
be removed, and introduced back into the vessel,
with black liquor withdrawn through screen 87 by
conduit 110 being introduced into the recirculation
loop 43. Thus in the vicinity of the screen 40,
and
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11
above, tie chips are soaked and heated, the blac)c
liquor in conduit 110 being substantially at cooking
temperature. This rapid displacement heating of the
chips makes them susceptible to subsequent
impregnation during the continuous processing
thereof.
As the chips move downwardly further in the
vessel 22 they encounter the second set of screens
63. A portior_ of the liquid is withdrawn
therethrough, heated by heater 67, and reintroduced
by central conduit 69. White liquor from source 31,
pumped by pump 36, in conduit 38 is added to the
pump 65 in this recirculation loop. Preferably the
white liquor is preheated by passing it through the
heat exchanger 114, in heat exchange relationship
with black liquor in conduit 112. Since the chips
are susceptible to impregnation due to their soaking
and heating by rapid displacement heating, rapid
diffusion impregnation of the chips with white
liquor occurs adjacent the screen 53 and
thereabove. Below the screen 63, kraft cooking of
the chips takes place. The chips are ultimately ' -
washed at or above screen 79, and are subsequently
discharged from the bottom of the vessel.
It will thus be seen that according to the
present invention a method of effecting substantial
energy economy while simultaneously improving
uniformity of the pulp produced in a continuous
kraft pulping process, is provided. ' While the
invention has been herein shown and described in
what is presently conceived to be the most practical
and preferred embodiment thereof, it will be
apparent to those of ordinary skill in the art that
12
many modifications may be made thereof within the
scope of the invention, which scope is to be
accorded the broadest interpretation of the appended
claims so as to encompass all equivalent methods and
procedures.
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