Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a novel
process and means for controlling variables in a con-
tinuous process for digestion of cellulose such as
described in U.S. Patents No. 3,041,232 and 3,200,032.
More specifically, the present invention contemplates
a method and apparatus for determining the degree of
delignification of the cellulose material in the di-
gester by measuring the increase in temperature
occurring in the cooking zone of the digester. The
invention is based on the finding that the magnitude
of the temperature rise in the cooking zone, due to
the exothermic digestion reaction taking place therein,
is a reliable and accurate way of measuring the rate
of reaction or degree of delignification in the di-
gester.
The present invention is particularly advan-
tageous for use with a digester system of continuously
monitoring and controlling a continuous process for
digestion of cellulose utilizing means programmed to
determine (l) the mass of wood fed to the digester,
(2) the mass of pulp solids being withdrawn from the
digester, (3) the desired amount of chemicals to be
fed to the digester and (4) the desired temperature
of the reaction zone of the digester.
The total mass of pulp slurry flowing from
the digester is continuously monitored by a gamma
absorption device and by a flow measurement. Addi-
tionally, the total mass of liquid in the digester
effluent is continuously measured by a refractometer.
The gamma absorption device determines the density of
the pulp slurry by measuring the amount of gamma
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particles absorbed by the slurry. The re~ractometer
determines the density of the liquid portion of the
slurry along with the total mass measurement by
measuring the refractive index of the liquor. These
measurements may be continuously fed to a computer
which, in turn, is programmed to adjust the amount of
wood chips, water and digesting liquor fed to the
digester as necessary to maintain the desired solids
content in the digester product.
The combination of gamma absorption device,
refractometer and flow measurement device nicely
serves to differentiate the composition of the slurry
or effluent from the digester, and thus is extremely
beneficial towards automatic control of the digester
operation. Quite unexpectedly, the gamma absorption
device and refractometer give reliable and accurate
measurements of the density of a pulp slurry and the
liquid portion of the slurry, respectively, even
though the densities of the solid and liquid involved
are not greatly different and the solids are not sep-
arated from the liquid in the slurry prior to measure-
ment with the refractometer~
Also, a gamma absorption device may be used
to accurately monitor the amount of wood chips fed to
the digester. One of the principal sources of process
variation in the digester is the non-uniformity of
wood chip loading. The chip meters used to feed the
chip to the digester are usually volume type meters
wherein a controlled volume of chips is obtained.
The accurate measurement of chip volume allows the
control of residence time or cooking time in the
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digester (that is, if the effluent from the digester
is controlled effectively). However, due to non-uni-
formity of the chip composition, such as size distri-
bution of chip particles and porosity of the chip,
the volumetric control does not produce a uniform
mass of chips entering the digester. The modifica-
tion proposed herein permits continuous measurement
of the mass of the wood which is fed to the digester
and thus offers a closer control on the digestion
than hitherto possible. Briefly stated, the measure-
ment of wood chips fed to the digester is correlated
with the amount of digesting liquor fed to the di-
gester so that the amount of liquor is controlled
and adjusted as necessary to accommodate for any
variation in the amount of wood chips fed to the di-
gester.
The accurate monitoring of wood chips fed to
the digester is accomplished by measuring the differ-
ence in density of the liquor flow feeding the chip
to the digester before and after the chips have been
introduced therein. As described in U.S. Patent
3,041,232, the chip is characteristically charged
first into a feed conduit and carried by liquor which
is circulating therein into the digester. The di-
gestor liquor is then separated from the fiber ma-
terial and the liquor returned to the circulation
conduit. The density of the liquor is measured by
a gamma absorption device just prior to when the chips
are introduced therein and again just after the chips
have been introduced. The resulting density measure-
ments may then be fed, along with a total flow
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measurement, to a computer which is programmed to
modify the amount of fresh digester liquor fed to~
the system based on the amount of wood chips deter-
mined by the density and flow measurements. Here
again, the excellent control of wood and fresh di-
gester liquor resulting from the use of a means in-
cluding a gamma adsorption device is quite unexpected,
as it could not have been predicted that there could
be a significant and meaningful density variation
between the digester liquor without chips and the
digester liquor containing the wood chips to give an
indication of amount of wood chips supplied in unit
time.
One may combine the use of the gamma ab-
sorption device on both the wood feed to the digester
and the pulp effluent from the digester in combina-
tion with flow measurements in each line and also a
refractometer on the effluent from the digester. All
these measurements may be fed to a computer which, in
turn, provides for the correct amount of fresh digester
liquor to be fed to the digester, the correct temper-
ature to be maintained in the digester and correct
flow rate of effluent from the digester.
The correct temperature and time for proper
delignification has been determined in the prior art
by empirical relationships. In the conventional prior
art continuous digester, following the cellulose ma-
terial feed to the digester and the impregnation zone
wherein the cellulose material is impregnated with
the digesting liquor, there is at least one and possibly
two heating zones which bring the cellulose and liquor
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up to the cooking or delignification temperature.
Following the heating zone or zones, the cellulose
and liquor remain at the cooking or delignification
temperature for a time period required to complete
the desired delignification. As mentioned above,
the practice has been to determine the prior temper-
ature and time from empirical relationships. The
correctness of the time and temperature at which the
cellulose and liquor are allowed to cook is deter-
mined by testing the product produced. This pro-
cedure has several disadvantages. The temperature is
usually controlled at a predetermined set point and
no adjustment is made for change in the properties of
the cellulose feed material. The procedure for deter-
mining the correctness of the cooking time and temper-
ature involves a rather lengthy delay and, therefore,
the correct time and temperature and subsequently the
proper degree of delignification for the particular
cellulose being cooked is never really accomplished.
It has now been found that the degree of de-
lignification can be determined continuously and con-
temporaneously with the cooking of the cellulose ma-
terial. The cooking or delignification reaction is
exothermic and thus there is a temperature rise in
the cooking zone of the digester. The magnitude of
this temperature rise in the cooking zone has been
found to be proportioned to the rate of delignifica-
tion at any given production rate and at steady con-
dition, this measurement of the temperature rise
gives a reliable instantaneous and continuous indica-
tion of the degree of delignification being accom-
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plished in the digester.
In addition, it has also been found that themeasurement of the temperature rise in the cooking
zone of the digester can be effectively combined in
a computer control system. Computer control systems
for continuous digesters are well known in the art,
such as the computer control system at GUlf States
Paper Corporation at Demopolis, Alabama, and such
computer control systems are readily adaptable to
computing a wide variety of outputs from given inputs
to control further process functions. Computer con- ;
trol per se does not form any part of the present in-
vention. In such a scheme, the temperature rise of
the cellulose and liquor in the cooking zone of the
digester is fed to the computer along with measure-
ments of the digester liquor fed, wash water flow to
the digester makeup water or black liquor feed to the
digester, effluent flow from the digester, the density
of liquor portion of the effluent flow from the di-
gester, and the density of the effluent flow from the
digester. In addition, the computer will be fed the
amount of steam used to heat the cellulose material
and liquor in the heating zone of digester and the
temperature increase obtained.
The computer is programmed to determine the
apparent heat capacity of the cellulose and liquor
slurry in the digester, the computer will also be pro-
grammed so as to control the cellulose feed rate, di-
gester liquor feed rate, makeup water or black liquor
feed rate and the effluent withdrawn from the digester.
The control of the feed and withdrawal of materials
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from the digester will insure that a constant preset
mass of material will be moving through the reactor.
The computer is programmed to determine the exothermic
heat produced. The amount of exothermic heat produced
is proportional to the degree of delignification and
thus the degree of delignification can be determined.
The invention is more fully explained with re- -
ference to the accompanying drawing which diagrammati-
cally illustrates a preferred embodiment of the digest-
ing plant and process according to the invention.
Referring to the illustrated embodiment shown
in the drawing, the numeral 11 designates an upright
cylindrical digester of essentially uniform cross-
sectional area, and a length equal to about ten times
the diameter. At its upper end, the digester is pro-
vided with appropriate charging or inlet means 13 or a
type well known in the art, for example, as shown in
United States Patent ~o. 2,459,180, utilizing a screw
conveyor in the charging device which is shown in more
detail in Patent No. 2,459,180. A slurry of commi-
nuted cellulosic material, such as wood chips sus-
pended in digester liquor, is charged by the means 13
into the digester continuously. The thus charged
cellulosic material moves as a compact column verti-
cally downward through the digester and is dischargedas a slurry of digested pulp suspended in water into
conduit 15 by means of a suitable discharging device,
such as that shown in United States Patent ~o.
2,938,824. During its passage through the digester,
the cellulosic material is su~jected to various
treatment areas which are indicated as the separate
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zones a, b, and c, respectively.
In the ~irst comparatively short zone a, im-
pregnation of the wood chips or cellulosic fiber ma-
terial takes place between the digesting liquor and
the chips. The digesting liquor preferably consists
of sulphate lye, e.g., sodium or calcium sulphate lye.
A sieve 17 is located at the upper end of the
digester which separates the wood chips from at least
part of the digester liquor and the separated digester
liquor is removed in conduit 19. The withdrawn di-
gester liquor flows via conduit 19 to the intake of
pump 21. A gamma absorption device 23 is situated
on the conduit l9 between the digester 11 and pump 21.
This device measures the amount of gamma absorption
of the digester liquor and sends this information to
a computer, which is not shown in the drawing. The
computer determines the density of the digester
liquor flowing in conduit 19 from the measurement of
its gamma absorption. Fresh digester liquor and
make-up black liquor are fed to the digester liquor
in conduit l9 between the gamma absorption device and
the intake of pump 21. The flow of fresh digester
liquor and make-up black liquor are controlled by the
computer as more fully disclosed hereinafter.
The digester liquor is then cycled by pump 21
through a wood chip feeding device 24. Such a device
is described in U.S. Patent 3,041,232. Wood chips are
fed to the circulating digester liquor by feeding
device 24, and then the slurry of chips and liquor is
fed by conduit 25 to the feeding device 13 on the top
of digester 11. A gamma absorption device 27, a flow
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meter 29 and a temperature sensing device 31 are
located in conduit 25 between the chip feeder 24 and
the digester ll. The gamma absorption device mea-
sures the gamma ray absorption of the wood chip,
liquor slurry and sends such measurement to a com-
puter. The flow measurement and the temperature
measurement are also fed to the computer. The computer
is programmed to determine the density of the liquor
flowing in conduit 19 and the density of the slurry
of wood chips and liquor flowing in conduit 25. The
computer is further programmed to determine from the
difference in densities of the liquor and slurry, the
mass of wood chips being fed to the digester.
In the drawing, the wood chip feeding device
24 has combined therein a steaming chamber 26. The
condensate fed to the chamber 26 is measured and this
measurement introduced into the computer to be used
in the mass balance of the digester.
The wood chip liquor slurry is fed to the top
of the digester ll and progresses slowly down the
column under the action of gravity. As explained
above, a portion of the li~uor is separated from the
wood chips by sieve 17 and withdrawn in conduit 19
to be recycled back to the top of the digester 11
after having added thereto necessary chemicals and
wood chips.
The accuxate monitoring of wood chips ~ed to
~he digester is accomplished by measuring the differ-
ence in density of the li~uor flow feeding the chip
to the digester before and a~ter the chips have been
introduced therein. The density of the liquor is
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measured by a gamma absorption device 27 ~ust prior
to when the chips are introduced into 13 and again by
23 just after the chips have been introduced. The
resulting density measurements may then be fed, along
with a total flow measurement from 29, to a computer
which is programmed to modify the amount of fresh di-
gester liquor fed to the system based on the amount of
wood chips determined by the density and flow measure-
ments. Here again, the excellent control of wood and
fresh digester liquor resulting from the use of a means
including a gamma adsorption device is quite unexpected,
as it could not have been predicted that there could be
a significant and meaningful density variation between
the digester liquor without chips (in 19) and the di-
gester liquor containing the wood chips (in 25) togive an indication of amount of wood chips supplied in
unit time.
As the wood chips and liquor progress down the
length of the digester 11, through section b, deligni-
fication or cooking of the cellulosic material takesplace. At the top of section b a side stream of cellu-
losic material and liquor is withdrawn in conduit 33
and pumped by pump 35 through heater 37 and conduit 39
back to the digester. The cellulosic material and
liquor are heated to the delignification or cooking
temperature by the heat added in heater 37. The
temperature of the slurry withdrawn in conduit 33 is
measured by means 36 and the temperature of the slurry
in conduit 39 is measured by means 38, along with the
flow rate of material in conduit 39 being measured by
flow meter 40. The temperature and flow measurements
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12
together with the flow of steam to the heater are
input to the computer which is programmed to determine
the heat capacity of the wood chip liquor slurry.
Alternati~ely, the heat capacity of the wood chip
liquor slurry can be determined by measuring the
temperature of the material being introduced to the
digester in conduit 25, the temperature of the ma-
terial withdrawn in conduit 33 and the heat added by
the steam in heater 37. The quantity of material
being heated between the top of the digester 11 and
the withdrawn conduit 33 can be obtained by a mass
balance around section a of the digester 11. The
computer can easily be programmed to accomplish this
mass balance and determine the heat capacity of the
material in the digester.
At the lower end of section b, a sidestream
of material can be recycled as shown through conduit
41, pump 43 and conduit 45 back to the digester 11 and
the temperature of this side stream measured by means
42. Alternatively, a thermowell, thermocouple or
other means (shown in dotted line at 42') for deter-
mining the temperature of the material at the lower
end of section b could be utilized in place of the
circulating side stream. The temperature of the ma-
terial at the lower end of section b is input to thecomputer which is programmed to determine the heat of
the exothermic delignification reaction.
In section c of digester 11, the delignified
cellulosic material is washed by wash water which is
introduced at the bottom of the digester 11 through
conduit 49. The flow of water into the digester is
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controlled by the computer. In section c, the cellu-
losic material moves counter-currently with respect
to the wash water. The wash water and the digesting
liquors are withdrawn from the digester 11 through
conduit 47.
A slurry of cellulosic material and water is
withdrawn from the bottom of digester 11 in conduit
15 and sent to storage or for further processing. A
gamma absorption device, a pressure reducer, a re-
fractometer and a flow meter are installed in conduit15 and as more fully explained above, the measurements
from these devices are input to a computer which is
programmed to determine the mass of cellulose being
withdrawn and the amount of water being withdrawn from -
the bottom of the digester.
In operation, the digesting process is con-
trolled by the amount of delignification desired in
the cooking or delignification section of the digester.
The computer performs a mass balancing of materials
fed to and withdrawn from the digester. Specifically,
the computer measures the mass of cellulose material
being fed to the digester. The correct flow of di-
gester chemicals is then determined and controlled by
the computer. Withdrawal of digested pulp is con-
trolled by the computer by balancing the mass of cel-
lulosic material being fed to the reactor with that
being withdrawn. The temperature rise of the reacting
materials as they move through the coo~ing or deligni-
fication zone is measured and the computer determines
the holdup time in the delignification section or
cooking zone from the mass balance on materials fed
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to and from the digester. The computer then cal-
culates the exothermic heat produced by the reaction.
As explained heretofore, the amount of delignifica-
tion is proportional to the exothenmic heat produced
by the delignification reactionO Having determined
the amount of delignification occurring in the di-
gester, the computer compares such with the set point
or standard and adjusts the amount of cellulosic ma-
terial being fed to the digester accordingly.
If the amount of delignification is less than
that desired, the computer would reduce the amount of
cellulosic material being fed to the digester and thus
the holdup time in the cooking zone or delignification
zone would be increased, thus increasing the amount of
delignification taking place per unit of mass of cel-
lulosic material. In a similar manner, if the amount
of delignification is in excess of that desired, the
computer would increase the mass o~ cellulosic ma-
terial being fed to the digester. The hold up time
in the cooking zone would thus he reduced, and the
delignification per unit of mass of cellulose material
would also decrease.
It is to be understood that the present in-
vention is not limited to the digesting chemicals or
particular apparatus described herein, but can be
applied equally well with other digesting chemicals
and apparatus amenable to continuous operation in a
single digester or vessel. Although only a single
stage digesting process has been described herein,
~he present invention is understood not to be limited
to such~ The present invention is amenable to two
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stage continuous operations such as described in
U.S. Patent 3,200,032 as well. To permit such use
of the present invention in a two stage digesting
process, such as described in the above-mentioned
patent 3,200,032, it is only necessary to determine
the increase in temperature in each digesting stage.
It is also to be understood from the above
description of the present process that many changes
may be made without departing from the inventive
scope thereof as defined in the following claims.
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