Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Procedure and apparatus for controlling the cellulose bleaching
process
The present invention concerns a procedure for controlling the
cellulose bleaching process wherein the degree of bleaching of the
pulp meant to be bleached is optically measured, on the basis of
such optical measurement is formed a control signal corresponding
to the pulp s degree of bleaching, and the dispensing of bleaching
chemicals to the pulp is controlled by the said control signal. The
invention further concerns apparatus for controlling the cellulose
bleaching process comprising a measuring means for the forming of a
control signal corresponding to the degree of bleaching of the pulp
that is going to be bleached, and a control means for controlling
the dispensing of bleac.hing chemicals under control by said control
signal with the aid of dispensing means.
Heretofore, the dispensing of bleaching chemicals in connection
with the bleaching process has usually been controlled on the basis
of measurements of the bleached pulp s residual chemical contents.
In the residual chemicals measurement is determined, at a given
stage in the process, the content of bleaching chemicals thac have
failed to react, for instance by an electro-chemical measuring
method. In the measuring instrument a signal is generated which is
e2uivalent to the concentration of the chemicals, and this signal
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is conducted so that it controls the dispensing of bleaching chem-
icals to the pulp, with the aid of control means, controllers and
dispensing means.
Also known in prior art is the optical controlling method defined
in the lntroductory part, wherein the pulp's degree of bleachin~ is
measured optically and the dispensing of bleaching chemicals to the
pulp is controlled with the aid of a signal formed on the basis of
said optical measurement, with the ald of measuring, control and
dispensing means. It is thus understood that the optical measure-
ment is based on determLnation of the result of bleaching made from
the pulp that is being bleached, while the residual chemicals
measurement is based on determination from the bleached pulp of the
chemicals that have failed to react.
In order to be able to implement in a satisfactory way the control
of chemicals dispensing in the chlorination step, one must be able
to perform the requisite measurements as soon as about 0.5 to 3
min. after the chemical dispensing operations. This implies in
practice that not nearly all of the chemlcal has been used up in
its reaction with lignin. The reaction rate is also decLsively
influenced, individually in each instance, by the reaction temper-
ature, viscocity, efficiency of agitation, etc. However, usually
these factors are not greatly variable in one plant~ whereby their
effect on the result of control is not decisive within one plant.
Therefore, the overwhelmingly most important process variable will
be the unc~ntrolled variation in the kappa number of the pulp
arriving at the bleaching plant, in such manner that the higher the
kappa number of the arriving pulp, the longer is the time required
for a given quantity of chemicals to oxidize and dissolve the
lignin from the pulp.
Taking into account the factors mentioned, that is the kappa number
and the reaction time, it is possible to explain completely why the
control based on residue measurement over-doses, and control based
on optical measurement under-doses, hard and soft pulps. For ex-
ample in the instance of increasing incoming kappa, the slowing
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down of ~he reactLon rate causes the resLdual quantlty at the point
of measurement to increase. This pheno~enon is even further accen-
tuated because owing to the conventional delay of measurement the
point of operation still lies on the steep portion of the reaction
graph. The consequence is that the control endeavours to attain i~s
set-point value by reducing the dose of chemicals, and the result
is therefore subchlorination. The opposite phenomenon occurs in the
event of diminishing kappa at entrance.
In case the control ls based on optical measurement, the situation
is better insofar as the measurement yields direct information as
to the chlorination of the pulp. Secondly, owing to the rapid kappa
descent the point of operation is not on the steep portion of the
graph. Therefore the only remaining factor that has an effect on
the control will usually be the variations of reaction rate due to
fluctuating incoming kappa. However, as can be deduced from the
foregoing, their influence is not equally strong as that in control
based on residue measuring. It follows that the over~ and under-
chlorinations incurred are also clearly less with equal kappa
fluctuations. But it is a fact that fully satisfactory results
cannot be achieved by residue measurement any more than by measur-
ing the degree of oxidation and by chemicals dispensing carried out
on their basis. Overdosage of chemicals implies excessive~consump-
tion of chemicals, while underdosage results in insufficient
bleaching and thereby in inferior quality.
As set forth in the foregoing, it is a fact that today satisfactory
results are not achieved in all and any conditions, owin~ to qual-
ity and other reasons, by residual chemicals measurement nor by
optical measurement.
The object of the present invention is to eliminate the drawbacks
mentioned. It is a particular object of the invention to provide a
procedure, and apparatus, for such controlling of the cellulose
bleaching process that the bleaching will take place without either
under- or overdosage of the bleaching chemicals. It is a further
object of the invention to provide a new procedure, and apparatus,
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for such controlling of the bleaching process that control of the
bleaching process takes place with more scrupulous obse~vation than
before of the kappa number of the pulp that is being bleached.
Regarding the features characteristic of the invention, reference
is made to the claims section.
The invention is based on the principle that optically performed
measurement of the pulp's degree o~ bleaching is selected to be the
basis for control of the chlorinating step in the bleaching pro-
cess. It is possible by simultaneously measuring the residual
chemicals content, either from the same process step where the
determination of degree of bleaching is made, alternatively before
the degree of bleaching determination, or preferably e.g. about 5
min. thereafter, to measure continuously also on line the hard-
ness of the incoming pulp, i.e., its kappa number. Since measure
ment of the optical degree of bleaching is known to cause overdos-
age in the case of hard pulps, this implies at the same time a
powerful increase of the residual chemicals content at the begin-
ning of the reaction step. It is then possible by means of a signal
derived from the residual chemicals determination to correct the
set-point of the optical measurement consistent with the kappa
variations. - -
Heretofore there has been no use of the combined optical determi-
nation of the puLp's degree of bleaching and of the residual chem-
icals measurement in the bleaching in combination to control the
bleaching chemicals supply.
Thanks to the present invention, the dispensing of bleaching chem-
icals can be effected with substantially higher accuracy than
heretofore, taking the kappa variations of the pulp that is being
bleached into account. When using the procedure and/or apparatus of
the invention, overdosage, and on the other hand underdosage as
well, of the chemicals is avoided. Thanks to the invention, the
excess consumption of bleaching chemicals is reduced, while on the
other hand quality fluctuatLons caused by insufficiently bleached
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pulp batches are eliminated.
~he invention is described in detail in the following with the aid
of embodiment examples, referring to the drawings attached,
wherein:
Fig. 1 illustrates the variations in residual chemical level and
;he incomlng kappa, plotted over time, for softwood,
Fig. 2 illustrates the variations in residual chemical level and
the incoming kappa, plotted over time, for birch wood,
.
Fig. 3 illustrates the variations in resid;ual chemical level when
the sald measurement has been applied in correction of the set-
point value derived from the optical degree of bleaching measure-
m?nt in the control of chemicals dispensing,
Fig. 4 presents schematically an apparatus according to the
invention,
Fig. 5 presents schematically another apparatus according to the
inventton, and
Fig. 6 presents schematically a third apparatus according to the
invention.
In experiments carried out in an industrial plant, the changes of
the residual chemicals level was measured with varying kappa
number; in these experiments the control of chemicals dispensing
W2S based on measuring the optical degree of bleaching. ~oth
sensors, i.e., an opttcally operating Cormec' sensor and an
electrochemically operating Polarox sensor, had a delay of about
2 min. from the dispensing of chemicais.
Exa~ples 1 - 3
In the experiments the residual chemicals level of the pulp was
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determined, as a function of tlme, and the incoming kappa similarly
as a functlon of time, for softwood and with bleaching chemicals
dispensing performed on the basls of degree o~ bleaching measure-
ment (Fig. 1), and the same for birch wood (Fig. 2). Furthermore,
in the experiments the variation of residual bleaching chemicals
level was determined in the case in which the measurement in ques-
tion was applied for correction of the chemicals dispensing set-
point value formed on the basis of optical degree of bleaching
measurement (Flg. 3), the measuring and control arrangements being
as Fig. 4 shows. In Fig. 3 the correlation between residual content
and incoming kappa is 0.9, in other words, during joint operation
of both sensors the residual chemicals measurement correlated
exactly with the varLatLon of the kappa number.
In Fig. 4 is depicted an apparatus according to the invention for
controlling the bleachLng process. The apparatus comprises a
bleachLng reactor 7, to which the pipeline 8 supplies pulp to be
bleached. In the pipeline 8 has been mounted an optically operating
measuring means 1 for measuring the degree of bleaching of the pulp
and for forming a control signal corresponding to the degree of
bleaching. The apparatus further includes control means 2 and 3 for
controlling the dispensing of the bleaching chemicals C12 and C10
into the pipelLne 8 with the aid of dispensing means 4 and 5. As
taught by the invention, an electro-chemLcally operating analyser 6
has been provided to measure the residual chemicals in the pipeline
8 with the same -10 min. delay, i.e., in time between O and 10,
preferably about 5-10 min. after the optical measuring means 1. The
analyser 6 forms a correction signal for correction of the bleach-
ing chemicals dispensing in accordance with the residual co~tent
which Lt measured. The correction sLgnal has been conducted to
correct the control signal of the measuring means 1 over modulators
9 and 10, to the control means 2 and 3. The apparatus controls the
supplying of bleaching chemicals on the basis of measurement of the
pulp's optLcal degree of bleachLng, corrected by the residual
chemicals measurement.
The arrangement depicted Ln FLg. 4 is based on so-called cascade
control. The chemicals control means, that is the flow control
circuits 2,3, receive their external set-polnt from a modulator
based on the optical measure~ent 1, that is from a whiteness con-
trol circuit 10. This, in its turn, receives an external set-point
value dependent on the residue measurement 6 and affecting the
overall control. This has in practice been implemented e.g. by
carrying the residue measurement signal from the analyser 6 to a
pure P controller, that is to the modulator 9, of which the output
is directed to become the set-point for the whiteness control
circuit. It is possible to change this relationship, according to
the situation, by altering the proportional band width of the P
controller 9.
The embodiment depicted in Fig. 5 is largely equivalent to that of
Fig. 4. In Fig. 5, the signals derived from the optical measurement
1 and from the residue measurement 6 have been conducted over mod-
ulators 9,10 to a proportion relay 13 to constitute a so-called
control signal, which has been carried to be the set-point value
for the flow controller 2. It is possible in the proportion relay
13 to take into account in such manner as is desired, the influence
of the residue measurement on the optical measurement.
The apparatus depicted in Fig. 6 is based on compensation, or cal-
culation. The fundamental idea is then that the signal ~rom the
optical measurement 1 is corrected by calculation with the aid of
the residue measurement 6, for instance according to the formula:
CONTROL VARIABLE = K x OPTICAL + (l-K) x ~ESID~E
This corresponds in practice to the situation in which the set-
point of the optical control circuit is altered with the aid of the
residue measurement. The calculation is performed e.g. with a cal-
culator unit 9,10 known in itself in the art. It is obviously pos-
sible in the calculator unit to establish programmed controllers
according to analog control and to implement the control on the
sa~e basis.
In Fig. 6, the optical measurement 1 and the resldue measurement 6
are made in the same process step, i.e., with the same delay after
addition of the chemicals from pipeline 11 into pipeline 8.
Example 4
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In a plant run, dispensing oi chemicals was carried out by a pro-
cedure according to the invention, with the arrangement shown in
Fig. 4. The optical measurement was made with so-called "Cormec"
equipment, optically, and the residue measurement was made with a
so-called "Polarox" apparatus, electro-chemically. In this meas-
urement the kappa number oi the incoming pulp 8 and the intermedi-
ate kappa were determined. The reEerence method employed was opti-
cal bleaching chemicals control with the "Cormec" apparatus; it has
previously been found in practice that control based on residue
measurement alone yields a result inferior to that obtained with
optical ~easurement.
Table 1
RESULTS OF CORMEC CONTROL
Mean Max. Min. Standard Deviation
Incoming kappa36.9 46 28 3.20
Intermediate kappa 6.4 7.9 5.1 0.50
RESULTS OF CORMEC + POLAROX CONTROL
Mean Max. Min. Standard Deviation
Incoming kappa33.6 44 28 3.27
Intermediate kappa 5.6 7!9 4.5 0.31
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In the embodiment examples presented above, the residual chem cals
determination was made from the same process step as the forming of
the control signal, or alternatively from the process step follow-
ing after the forming of the control signal. If-desired, ho~ever,
the residual chemicals determination may e~ually be made from the
process step preceding the forming of the control signalJ i.e.,
before the optical measurement. This is particularly to be conte~-
plated when the process involves Long delay periods. Thus, when the
optiral delay is for instance on the order of 10 min., the residual
content of chemicals may be determined, in time, e.g. 0-1 min. or
even several minutes, such as 1-10 min., before the optical meas-
urement.
The invention is not confined to the examples that have been pre-
sented: its applications may vary within the scope of the claims
following below. Then, the procedure in question, and the appara-
tus, is appropriate for controlling the dispensing of any chemic21
whatsoever in connection with bleaching. Furthermore, the procedure
and the apparatus may be used in connection with any kind of meth-
ods of determination, and apparatus, for determining degree of
bleaching and residual chemicals.
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