Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method for automatic
control of the dosageing of chlorine dioxide and chlorine for
controlling the quality of pulp by instrumentation in a continu-
ous pulp bleaching process, which process involves at least one
chlorination step preceded by pretreatment of the pulp with
chlorine dioxide and in which process the hardness of the un-
bleached pulp (e.g., Kappa number), washing losses, and other
factors affecting the total demand on active chlorine, or the
variation of these factors, are observed by means of laboratory
methods and/or by using continuous analyzers (e.g., Polarox*).
The use of chlorine dioxide for pretreatment of the pulp
to be bleached before chlorination is at present known per se.
The use of chlorine dioxide gives, among other things, the follow-
ing advantages:
* Polorox is a trademark which refers to the Polarox
Analyzer developed by the Finnish Company Rauma-Repola Oy and
patented in many countries including Canada. The Polorox Analyzer
measures the concentration of oxidizing or reducing chemicals.
The device comprises a measuring cell with four electrodes: two
metallic electrodes (measuring and current electrodes~, a refer-
ence electrode, and a temperature electrode.
The solution to be measured flows through the measuring
cell. Between the measuring electrode and the reference electrode,
a constant potential is maintained by feeding a current through
the circuit consisting of the measuring and the current electrode.
The current is proportional to the concentration of free chlorine
in the solution. In the electronic unit of the device, the
electrode current is transformed into a standard signal and a
temperature compensation is effected.
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- the chlorination can be r1m at a higher temperature,
- the total quantity of active chlorine to be dosaged
(chlorine dioxide and chlorine added together) can
be larger without risking the strength of the pulp
than when chlorine alone is used, and
- the amount of waste-water is lower.
In the methods known at present, the dosageing of
chlorine dioxide before chlorination has been performed by usual-
ly giving the ratio between the chlorine dioxide flow and the
production rate a certain constant value (e.g., l). If necessary,
active adjustment has been performed by means of dosageing
chlorine, for example by using a redox measurement. Despite the
above general advantages, these methods have involved, among
other things, the following drawbacks:
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- the quality of the pulp after chlorination has not
been uniform, and
- on the average, it has been necessary to dosage more
of the expensive chlorine dioxide than what would
have bePn necessary for the process situation or for
the purpose of reaching a certain total chlorination
goal - the ratio of the required minimum dosage of
chlorine dioxide to the production rate and to the
chlorine dosage lwhen a certain total chlorination is
aimed at) usually varies from moment to moment when
the process situation varies; overlooking of this
circumstance has, in a way, implied additional
expenses.
The purpose of the present invention is to eliminate the
above drawbacks. More precisely, by means of the present
invention it is possible to control the dosageing of chlorine
dioxide ahead of the chlorination stage, as well as the
dosageing of chlorine, and to control ths quality of the pulp
after the chlorination stage without the above drawbacks.
The method in accordance with the invention is mainly
characterized in L,lat the relative share of the chlorine
dioxide dosaged in connection with the pretreatment in the
total quaniity of act;ve chlorine that is dosaged in the pre-
treatm~nt and in the chlorination stage together is made to
follow the hardness of the unbleached pulp, the desired hard-
ness after the chlorination stage or after the alkali stage
following after the chlorination stage, and possibly the
washing losses of the unbleached pulp, the chlorination
temperature,and the prcduction rate, which following takes
place in accordance with a mathematical empiric function that
is being constantly calculated. -~
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Below, the invention will be examined by means of a
detailed example, wherein reference is made to the attached
drawings.
Figure 1 is a diagrammatic view of an industrial process
controlled in accordance with the invention.
Figure 2 shows as a block diagram an equipment to be used
for controlling the process in accordance with Figure 1. -~
Figurs 3 shows a nomogram that indicates the operations
connected with the controlling of the process of Figure 1.
EXAMPLE
The example case is a control method for a pulp bleaching
process, which method is directed at the chlorination stage and
at the preceding chlorine dioxide treatment. In the process
~Figure 1) a mixture of unbleached pulp and water and chlorine
dioxide solution are supplied into a mixer MIX, wherein the ~ -
chlorine dioxide starts reacting with the pulp. After the mixer
the pulp passes into the pretreatment stage proper DE, in which
the reactions between chlorine dioxids and pulp continue. After
stags DE, chlorine is dosaged aheaG of stage C, whereupon the
pulp comes into the chlorination stage C. As a result of the
chlorine dioxide treatment and the chlorination treatment, a
considerabl portion of the ligrin contained in the pulp is
transformed into a form soluble in water, on one hand, and into
a form extractable by alkali. Th~ chlorination stage is usually
followed by washing with water and by an alkali stage, in which
the extraction of the chlorinated lignin component extractable
with alkali, mentioned above, takes place. Normally, the
trsatment of the pulp continues further by means of some
oxydizer, e.g., sodiumhypochlorite or chlorine dioxide; these
steps will, however, not be discussed here.
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The on-line computer CP (Figure 2) recsives, as
measurement signals from the prncess, through an analog-digital
converter ADM, a reading Vs indicating ths total flow of pulp,
a reading vclO indicating the flow of chlorine dioxide, a
reading fCl indicating the flow of chlorine, a reading sC
indicating the thickness of the pulp, and readings ccl and tc
indicating the concentration of free chlorine in the pulp and
the temperature in the bottom part of the chlorination column C
le.g., Polarox-measurement value). The computer CP is programmed
to make use of these values as well as of values that can be set
by means of the questioning and control station INFO and that
are connected with the process running situation, the mariginal
tests concerning measurement results, and with the intermediate
aims of the cellulose quality tfor example, the preset Kappa
numbEr KIE after the chlorination stage or after the subsequent
alkali stage). The comDuter CP is Drogrammed so that it here-
upon, by simulation (by predicting by means of mathematical
models) gives the control devices Sl to S4 connected with the
process, by means of the digital-analog converter DAM and
through the converters Ml to M4, set values which are,
correspondingly, concerned with the dosageing of chlorine dioxide
solution and chlorine and, if necessary, with the chlorination
temperature. The final maintenance of the set values is
accomplished by said control devices. The set values given
remain in force for a period which is (for e~ch 07 them
separately) determined by the frequencies of performance of the
programs of calculation and putting into effect of the set
values. The frequencies of performance of said programs can be
selected so that the changes in the process situation are taken
into account sufficiently nrecisely and rapidly. The methods of
calculation of the set values will be discussed more closely
later.
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~ elow, the abbreviations and reference numerals
appearing in Figures 1 and 2 are explained:
1. Automatic process control equipment
CP Central computer
AOM Analog-digital converter
DAM Digital-analog converter
TPR Report printer
INFO ~uestioning and control station
2. Process instruments
2.1 Analog measurements
VS Measurement indicating the total flow of pulp
tm3/h)
vclO Measursment indicating the flow of chlorine dioxide
solution (m3/h)
fcl2 Measurement indicating the flow of chlorine (kg/h~
sC Measurement indicating the thickness of the pulp
(per cent)
ccl2 Measurement indicating the concentration of freP
chlorine in the pulp in the bottom part of the
chlori:~'ion column te.g., Polarox) (per cent of the
range of O to 400 mg/l)
tC Measurement of the temperature of the pulp in Ihe
bottom part nf the chlorination column (C)
2,2 Converters
Ml, M2, M3, M4 Flow-pressure converters
M6, M7, M8, M9 Prsssure-flow converters
2.3 Set values given by the comput~r, and the corresponding
analog control devices (in parentheses)
VclO (Sl) Flow of chlorine dioxide solution (m3/h)
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Fcl (S2) Flow of chlorine (kg/h)
TC ~S3) Temperature of puln (C)
SC ~S4~ Thickness of pulp tper cent)
3. Tanks and reaction columns
MIX Chlorins dioxide mixsr
DE Pretreatment stage
C Chlorination column
The demand on active chlorine of the cellulose entering
the bleaching process, in the chlorination per unit of mass,
depends on the hardness ~Kappa number) of the pulp and on the
desired chlorination effect, i.e., the desired Kappa number after
the chlorination stage or the subsequent alkali stage. The
washing losses accompanying the pulp also affect the total
demand on chlorine. Moreover, successful chlorination requi^es
correct reaction conditions, temperature and thickness, which
in part affect the efficiency of chlorination and the quantity
of active chlorine required per mass unit of pulp to be
chlorinated. The demand on active chlorine per mass unit of the
pulp to be bleachsd, presented in Figure 3, is calculatEd out ;~
of an empiric function ~1) which is based on observation of the
reaction speed and reaction condit;.ns and, thereby also on
nbservation of the hardn~ss of the pulp to be bleached at the
beginning of the chlorination stage.
F
~11 tot fl ~VCl02' CCl02' fC12' tot' sC tc~ CCl2' ~E)
A
wherein ;
FC n the new required total flow of chlorine
tot
~kg of active C12tt)
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A = productior. rate ~t/h)
fl ~ the function concerned
VCl02 ~ volumetric flow of chlorine dioxide solution
Cm /h3
cclO = concentration of chlorine dioxide solution ~g/l)
fcl2 = chlorine flow ~kg/h)
VS ' total flow of pulp ~m3/h)
8C = thickness of pulp (per cent)
tC = temperature in the bottom part of chlorination
column (C)
ccl2 ~ concentration of free chlorine in the bottom
part of the chlorination column (mg~l)
KIE ' the desired Kappa number after the alkali stage
following the chlorination stage.
Among the variables of the above function, vclO is by -
the delay between the dosageing of chlorine dioxide and the -
measurement of the free chlorine concentration older and fC
i5 by the delay between the dosageing of chlorine and the
measurement of the concentration of free chlorine older than the
other variables. The total chlorine demand of the pUlP to be
bleached can also be followed by determining the hardness of the
pulp in the laboratory; a drawback is, however, the long delay
involved in the determination. In Figure 3 the brok~n line
provided with arrow marking shows the proceeding ~T the
calculation operations starting from the total demand on
chlorine determined above. On the basis of the prevailing
production rate, the required total flow of chlorine Fc i5
obtained. The distribution of the total flow of chlorine
between chlorine dioxide and chlorine is hereupon determined
from the function (2), which takes into account the desired
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Kappa number after the alkali stage following the chlorination
stage (Figure 3).
(21 1 2 = f2 tFC ' KIE) = d
Fcl2 c
wherein
FC10 = the new set value of the chlorine dioxide flow
(kg/h)
Fcl2 = the new set value of the chlorine flow tkg/h)
f2 ~ the funct.ion concerned
FC = the required total flow of chlorine (kg of active
tot
C12 per ton)
KIE = desired Kappa after the alkali stage
d/c = ratio of the above chemical flows. .~-
The final set values of the chemical flows are determined,~
possibly by filtering, on the basis of the ratio calculated
above.
C102 cld CtOt ccl02 and FCl = d FC
whersin
Vc~O ~ the new set value of the volumetric flow of the
chlorine dio,:ide solution (m3/h)
CC ~ = concentration of tlle chlorine dioxide solution (g/l)
and the rest of the variables are the same as those indicated in
connection with equation (2).
Thus, the new set values of chlorine dioxide and chlorine
dosage as well as their ratio depend on the desired Kaopa and,
through the variable FC ~ also on the total chlorine demand
tot
on the pulp to be bleached (hardness, washing losses, etc.), on
the production rate, and on the reaction conditions.
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By means of the method in accordance with the invention,
the quality of pulp can be controlled by giving the desired
Kappa KIE after the alkali stage different values, whereby, -
correspondingly, the factual Kappa number of the pulp after
the alkali stage is also developed to the desired level.
The computer system sends electrical signals corresponding~
to calculated new set values Vcl02 and FCl2
dioxide flow and chlorine flow through the converters Ml and M2
to the analog control devices Sl and S2, which adjust the system
in view of said set values.
In the example case, set values are also calculated
separately for the chlorination temperature ~Tc) and
chlorination thickness (Sc). The bases of calculation of these
will not be discussed here any further; the goal is a
sufficiently high reaction speed and efficient use of chlorine
dioxide and chlorine. The computer system sends electrical
signals corresponding to said set values through the converters
M3 and M4 to the analog control devices S3 and S4, which adjust
the system in view of said set values.
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