Note: Descriptions are shown in the official language in which they were submitted.
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HIGH EFFICIENCY CHLORINE DIOXIDE PROCESS
The present invention relates to the production of
chlorine dioxide.
It is known to produce chlorine dioxide by
reduction of an acid aqueous sodium chlorate solution
using methanol, as described in U.S. Patent No.
2,881,052. The process, however, is quite slow,
involves the handling of a large volume of liquid
effluent and the efficiency of the process is quite
low. More recently there issued U.S. Patent No.
4,081,520, assigned to the applicant herein, wherein
the problems of the prior process were overcome by the
use of a single vessel generator-evaporator-
crystallizer. The latter process operates at high
efficiency, produces no liquid effluent and has an
acceptable production rate.
In the commercial implementation of the
above-noted process, complete loss of chlorine dioxide
production has been observed from time-to time. This
phenomenon is thought to arise from complete exhaustion
of the trace quantities of chloride ions, which must be
present for reaction to occur, as a result of some
change in the process conditions of the reaction
medium.
It is well known that all chlorine dioxide
generating processes involving reaction with chlorate
proceed in accordance with the reaction:
C103 + Cl + 2H ~ C1O2 + ~C12 + H2O
In the processes of U.S. Patents Nos. 2,881,052 and
4,081,520, the chloride ions are formed in si-tu by
reaction of methanol with the co-produced chlorine, so
that significant quantities of chlorine do rlot result
as a by-product and hence the chlorine dioxide contains
little or no chlorine, depending on the overall
efficiency of the process. If, for some reason,
therefore, all the chloride ions are consumed by the
above reaction, then production of chlorine dioxide
will cease until chloride is produced by reduction of
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chlorate by methanol. This periodic random loss of
chlorine dioxide production is termed a "wnite-out".
The present invention relates to an improvement in
a continuous process for the production of chlorine
dioxide at high efficiency by reducing sodium chlora-te
with methanol in an aqueous acid reaction medium and
which exhibits periodic random loss of production,
wherein aqueous sodium chlorate solution and sulphuric
acid are continuously fed to a boiling aqueous acid
reaction medium in a reaction zone maintained under a
subatmospheric pressure and haviny a total acid
normality of at least 9 normal; methanol is
continuously fed to the reaction medium in sufficient
quantity to form chlorine dioxide from the reaction
medium; chlorine dioxide is continuously removed from
the reaction zone in gaseous admixture with ste.am and
dissolved in water to form an aqueous solution thereof;
and sodium acid sulphate is continuously deposited from
the aqueous reaction medium in the reaction zone.
The improvement of the present invention is one
wherein chloride ions are also continuously fed- to the
reaction medium in the reaction zone in an amount at
least sufficient to prevent the periodic random loss of
production and no more than that which results in
chlorine being present in the gaseous admixture in an
amount which permits dissolution of the chlorine in the
aqueous chlorine dioxide solution.
In the present invention, therefore, the problem
of white-outs in the process of U.S. Patent No.
4,081,520 is eliminated while the other useful
attributes thereof, most significantly high efficiency,
are retained, by the continuous addition of chloride
ions -to the reaction medium. By purposely adding
chloride ions to the reaction medium, the presence of
chloride ions at all times is ensured and hence the
possibility of white-outs is eliminated.
The beneficial result of continuous addition of
chloride ions is not accompanied by any signifi.cant
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adverse result. The high efficiency of production of
chlorine dioxide from chlorate ions enjoyed by the
process of U.S. Patent No. 4,081,52Q is maintained,
along with an acceptable rate o~ chlorine dioxide
production. The addition of the chloride ions to the
reaction medium results in the production of some
chlorine along with the chlorine dioxide, but usually
such coproduction of chlorine can be tolerated, and
often is desirable, in a pulp mill environment.
In a preferred embodiment of the invention, there
is provided a continuous process for the production of
chlorine dioxide, which comprises con-tinuously feeding
an aqueous sodium chlorate solution to a reaction zone
containing an aqueous acid chlorine dioxide-generating
reaction medium to provide a concentration of sodium
chlorate in the reaction medium of about 0.2 to about
1.5 molar; continuously feeding sulphuric acid to the
reaction medium to provide a total acid normality of 9
to about 12 normal in the reaction medium; continuously
feeding methanol to the reaction medium in sufficient
quantity to effect formation of chlorine dioxide from
the reaction medium at high efficiency; continuously
feeding an aqueous solution of chloride ions to the
reaction medium to maintain a concentration of chloride
ions in the reaction medium of about 0.002 to about 0.3
molar and to eliminate the incidence of periodic random
loss of production otherwise present in the absence of
the added chloride ions; continuously maintaining the
reaction medium at its boiling point at a temperature
in the range of about 60 to about 90C while a
subatmospheric pressure of about 60 to about 400 mm Hg
is applied to the reaction zone and the partial
pressure of chlorine dioxide is maintained below about
90 mm Hg; continuously withdrawing a gaseous mixture of
chlorine dioxide, chlorine and steam Erom the reaction
zone; and continuously depositing a sodium acid
sulphate from -the reaction medium after the reaction
medium becomes saturated thereby after the initial
start up of the process.
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The general operating parameters of the chlorine
dioxide generation process of the invention may vary
over a wide range. Concentrations of reactants are
generally controlled by flow rates of aqueous sodium
chlorate solution, sulphuric acid and methanol to the
reaction zone, which typically takes the form of a
unilocular single vessel
generator-evaporator-crystallizer.
The total acid normality of sulphuric acid in the
reaction medium is maintained at at least 9 normal and
may vary up to about 12 normal. The efficiency of
conversion of chlorate ions to chlorine dioxide tends
to decrease with decreasing total acid normalities. It
is preferred to employ a total acid normality of
sulphuric acid in the range of 9 to about 10 normal.
Sulphuric acid generally is fed to the reaction medium
in the form of concentrated (93%) sulphuric acid.
The concentration of sodium chlorate in the
reaction medium usually varies from about 0.2 to about
1.5 molar, preferably about 0.9 to about 1.1 molar.
Sodium chlorate is fed to the reaction medium in the
form of an aqueous solution thereof, usually having a
concentration of about 5 to about 7 molar.
As mentioned above, under normal operating
conditions, chloride ions are present in the reaction
medium as a result of in situ reduction of chlorine by
the methanol. When sodium chloride is continuously fed
to the reaction medium in accordance with this
invention, the concentration of chloride ions present
in the reaction medium is not slgnificantly greater
than in the absence of such added sodium chloride,
since the added chloride ions are converted to chlorine
ln the reaction zone. Usually, the chloride ion
concentration in the reaction medium varies from about
0.002 to about 0.3 molar.
The chloride ions are added -to the reaction medium
in the form of an aqueous sodium chloride solution,
usually having a concentration of about 5 molar. The
sodium chloride may be added as part of the sodium
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chlorate solution. Hydrochloric acid or hydrogen
chloride also may be used to provide the chloride ions
to the reaction ~edium.
The methanol may be fed to the reaction medium in
the form of 100% methanol or as an aqueous solution of
methanol containing greater than 1% by weight of
methanol, although at least about 30% by weight is
preferred to avoid excessive water feed to the process.
The reaction temperature usually varies from about
60 to about 90C, preferably about 70 to about 75C.
Higher temperatures generally lead to faster reaction
and hence production rates, but decomposition of
chlorine dioxide at excessively high temperatures
decreases the yield of chlorine dioxide.
The chlorine dioxide which is present in the
gaseous stream produced from the reaction medium in the
chlorine dioxide generator is formed into an aqueous
solution of chlorine dioxide for use as a bleaching
agent, usually by an initial cooling of the gaseous
stream to condense a substantial proportion of the
steam and a subsequent contact with a water stream in
sufficient volume to dissolve all the chlorine dioxide.
In the two~stage condensation and dissolution
operation, the initial condensation may be effected by
cooling to a temperature of about 3 to about 60C,
preferably about 7 to about 60C while the subse~uent
dissolution may be effected by contact of the cooled
gas stream from the condensation step with water having
a temperature of about 0 to about 22C, preferably
about 3 to abou-t 10C. Depending on the flow rate of
water relative to chlorine dioxide production and the
temperatures of condensation and dissolution water, a
chlorine dioxide solution is formed having a chlorine
dioxide concentration ranging from about 6 to about 20
grams per litre, preferably about 10 to about 15 grams
per litre.
As noted above, in the present invention sodium
chloride is continuously fed to the reaction medium to
result in formation of chlorine along with the chlorine
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dioxide. This chlorine is dissolved in the chlorine
dioxide solution and is present in an amount from about
0.1 to about 2.0 grams per litre, preferably about 0.1
to about 0.5 grams per litre.
The quantity of sodium chloride or other source of
chloride ion, such as, hydrochloric acid, which is
added to the reaction medium should not exceed that
quantity which coproduces chlorine with the chlorine
dioxide beyond the solubility limit of chlorine in the
chlorine dioxide solution.
The sodium acid sulphate, which is deposited from
the reaction medium, usually is in the form of sodium
bisulphate (NaHSO4) or sodium sesquisulphate
(Na3H(SO4)2). The acid values of this sodium acid
sulpha~e may be recovered therefrom by converting the
acid sulphate to neutral sodium sulphate by treatment
with water and methanol, as described in U.S. Patent
No. 4,325,93~, assigned to the applicant herein, with
the sulphuric acid recovered thereby being recycled to
the reaction zone. Alternatively, the sodium acid
sulphate may be added to the reaction medium of another
chlorine dioxide producing process in which sodium
chlorate and sodium chloride and/or hydrogen chloride
are reacted in an acid aqueous medium at a total acid
normality of less than about 4.8 normal, the sodium
acid sulphate being used to provide all or park of the
acid requirement of such process, as described in U.S.
Patent No. 3,789,108, assigned to the applican-t herein.
Chlorine dioxide is known to be spontaneously
explosive at high partial pressures. In the process of
U.S. Pa-tent No. 4,081,520, chlorine dioxide is diluted
with steam generated by the boiling of the reaction
medium and this steam, combined with a low pressure of
operation, typicall~ around 100 mm Hg, maintains the
chlorine dioxide below explosive concentrations. At
these low pressures, the concentration of chlorine
dioxide at the base of the absorption tower wherein the
chlorine dioxide is dissolved in water to form the
aqueous chlorine dioxide solution, following
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condensation of the bulk of the steam, remains at a
safe level.
The process, however, may be operated at higher
but still subatmospheric pressures by introducing
sufficient purge air to maintain the partial pressure
S of chlorine dioxide below about 90 mm Hg. The actual
pressure of operation will depend largely on the
temperature of the reaction medium, but may vary widely
from about 60 to about 400 mm Hg, preferably abou-t 90
to about 190 mm Hg. The ability to modify the pressure
of operation by the utilization of a controlled amount
of pur~e air is advantageous in situations where a
chlorine dioxide generating plant designed to use the
higher subatmospheric pressure, such as when large
quantities of chlorine are coproduced with the chlorine
dioxide, is used to effect chlorine dioxide formation
by reduction of sodium chlorate with methanol.
The invention is illustrated further by the
following Example:
Example:
0 A 15 tons per day capacity chlorine dioxide
generator was run wherein acid sodium chlorate solution
was reduced with methanol while the reaction medium was
boiled under a subatmospheric pressure. Sodium
chlorate was continuously fed to the reaction medium as
a 5M aqueous solution formed from crystal sodium
chlorate at a flow rate of 9.2 USGPM sufficient to
maintain a chlorate concentration of lM in the reaction
medium. Sulphuric acid was also continuously fed to
the reaction medium as 93~ H2S0~ at a ~low rate of 2.0
USGPM sufficient to maintain an acidity of reaction
medium of about 9 to lON. Methanol was continuously
fed to the reaction medium as a 50~ w/w aqueous
solution at a flow rate of 0.8 USGPM. The average
temperature for the generator liquor was about 80C and
sodium sesquisulphate crystals were removed from the
generator.
The generator was run under substantially steady
state conditions to produce chlorine dioxide, first in
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the absence of deliberately added sodium chloride and
subsequently with the con~inuous addition of 0.5 USGPM
of 5M sodium chloride solution. Offyases from the
generator were cooled to a temperature of 30C to
condense the steam and the cooled gases were dissolved
in water in an absorption tower using water having a
temperature of 10 C. The chemical efflciency of
conversion of chlorate ions to chlorine dioxide was
determined in each case.
A chemical e~ficiency of about 96% was observed
both for the run when deliberately added sodium
chloride was omitted and for the run when sodium
chloride was dellberately added. During the period of
time that sodium chloride was deliberately added, the
chlorine dioxide solution obtained contained about 10
gpl chlorine dioxide and about 0.7 to 0.8 gpl chlorine,
while during the initial run when sodium chloride
addition was not effected, the chlorine dioxide
solution contained about 10 gpl chlorine dioxide and
about 0.5 to 0.6 gpl chlorine. In addition, an
increase in production rate of chlorine dioxide
production was observed during the period when sodium
chloride was deliberately added to the reaction medium.
In summary of ~his disclosure, the present
invention relates to improvements in the operability of
highly efficient chlorine dioxide processes without
adversely affecting that efficiency. Modifications are
possible within the scope of the invention.
