Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
HIGH EFFICIENCY CHLORINE DIOXIDE PROCESS
The present in~ention relates to the producti-on- of
chlorine dioxide.
It is known to produce chlorine dïoxidè 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 U.S. Patent No. 4,081,520, the minimum total
acid normality of operability disclosed is 9 normal,
since it had previously been found that total acid
normality values below such minimum did not give rise
to high efficiency. The experiments which lead to such
a conclusion were done on a laboratory scale and
involved an evaporation rate of 10 to 20 1~ of gas
phase (water vapour, chlorine dioxide and chlorine)
/hr/sq. ft. of surface area of reaction medium.
It has now surprisingly been found that, in the
process of U.S. Patent 4,081,520 for the production of
commercial quantities of chlorine dioxide, the total
acid normality can be decreased below 9 normal and high
efficiency of chlorine dioxide production may be
maintained.
In accordance with the present invention,
therefore, there is provided a continuous process for
the production of chlorine dioxide in commercial
quantities at high efficiency by reclucing sodium
chlora-te with methanol in an aqueous acid reaction
medium, which comprises continuously feeding aqueous
sodium chlorate solution and sulphuric acid to a
. boiling aqueous acid reaction medium in a reaction zone
f~o2~
maintained under a subatmospheric pressure; the
sulphuric acid being fed to the reaction medium to
provide a total acid normality in the reaction medium
below 9 and down to about 7 normal; continuously
feeding methanol to the reaction medium in sufflcient
quantity to form chlorine dioxide from -the reaction
medium; continuously removing chlorine dioxide from the
reaction zone in gaseous admixture with steam and
dissolving the removed chlorine dioxide in water to
form an aqueous solution thereof; and continuously
depositing sodium acid sulphate from the aqueous
reaction medium in the reaction zone.
In U.S, Patent No. 4,081,5209 there is data for an
experiment conducted at 8 normal sulphuric acid, under
laboratory conditions using small volumes of generator
liquor, and it was only above about 9 normal that
highly efficient chlorine dioxide production was
achieved. Contrary to this data, it has now been
surprisingly found that, under commercial-scale plant
conditions, chlorine dioxide may be produced at high
efficiency at total acid normality values in the range
of about 7 up to 9 normal. At the same time as the
increased efficiency at lower acidity was observed,
there was also observed a significant increase in the
concentration of sodium chloride present in the
reaction medium, -typically to about 0.2 molar. The
difference in behaviour is not satisfactorily
explained, but is thought to arise from the
substantially larger volume of reaction medium
available for reaction under the plant conditions and
hence the longer effective residence time and hence
more efficient use of the methanol in the reaction
medium, leading to decreased evapora-tive losses.
The Larger scale of operation which gives rise to
the ability to produce ch]orine dioxide highly
efficiently at a lower total acid normality also
results in a increased evaporation rate in the range of
about 50 to about 500 lb gases/hr/s~. ft. of surface
area.
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In accordance wi-th a preferred embodiment of the
inventlon, there is provided a continuous process for
the production of chlorine dioxide in commercial
quantities, which comprises continuously 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
reacti.on medium to provide a total acid normality of
below 9 normal down to about 7 normal in the reaction
medium; continuously feeding methar.ol to the reaction
medium in sufficient quantity to effect formation of
chlorine dioxide from the reaction medium at high
ef~iciency; 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 chiorine
dioxide and steam from the reaction zone; boiling the
reaction medium at an evaporation rate of about 50 to
about 500 lb of gaseous mixture/hr~sq.ft. of surface
area of reaction medium; 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.
The general operating parameters of the chlorine
dioxide generation process used in the present
invention may vary over a wide range. Concen-trations
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 7 normal and
may vary up to 9 normal. Sulphuric acid generally is
..3
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 abou-t 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.
Under normal operating conditions, chloride ions
are present in the reaction medium as a result of in
situ reduction of chlorine by the methanol. Sodium
chloride may be continuously fed to the reaction
medium, if desired, as disclosed in our copending
Canadian patent application Serial No. ~30,111 filed
June 10, 1983. The concentration of chloride ions
present in the reaction medium when such sodium
chloride feed is made is not significantly greater than
in the absence of such added sodium chloride, since the
added chloride ions are converted to chlorine in the
reaction zone. Usually, the chloride ion concentration
in the reaction medium varies from about 0.1 to about
0.3 molar.
The chloride ions, when added to the reaction
medium, are 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 chlorate solution. Hydrochloric acid or
hydrogen chloride also may be used to provide the
chloride ions to the reaction medium.
The me-thanol may be ~ed 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 reacti.on 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
s
chlorlne dioxide at excessively high temperatures
decreases the yield o 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-skaye condensation and dissolution
operation, the initial condensation may be effected by
cooling ko a temperature of about 3 to about 60C,
preferably about 7 to about 60C while the subsequent
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 about 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.
When sodium chloride is continuously fed to the
reaction medium, chlorine is formed along with the
chlorine 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 li-tre, 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
Z~
(Na3H(SO4~2). The acid values of this sodium acid
sulphate 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 hydroyen chloride
are reacted in an acid aqueous medium at a total acid
normality of less than about 4.8 normal, the sodiu~
acid sulphate being used to provide all or part of the
acid requirement of such process, as described in U.S.
Patent No~ 3,789,108, assigned to the applicant herein.
The sodium acid sulphate usually is removed from
the reaction vessel as a slurry with reaction medium,
the sodium acid sulphate is separated from the reaction
medium, and the reaction medium is recycled to the
reaction zone, usually after addition of fresh
reactants thereto.
The volume of liquid in the reaction zone and the
rate o recycle determine the evaporation rate of
gases. In the present invention, the process is
effected at an evaporation rate of about 50 to about
500 lb gases (water vapour,~chlorine dioxide and
chlorine) ihr/sq. ft. of surfàcè"~area of reaction
medium.
Chlorine dioxide is known to be spontaneously
explosive at high partial pressures. In the process of
U.S. Pa-tent No. ~,0~1,520, chlorine dioxide is diluted
with steam generated by the boiling of the reaction
medium and this steam, combined wi-th a low pressure of
operation, typicall~ around 100 mm Hg, maintains the
chlorine dioxide below explosive concentrations. At
these low pressures, the concentration o~ 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
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 about 90
to about 190 mm Hg. The ability to modify the pressure
of operation by the utilization of a controlled amount
of purge 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 present invention is illustrated further by
the following Example:
Bxample:
A 14 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 suf~icient to
maintain a chlorate concentration of lM in the reaction
medium. Sulphuric acid was also continuously fed to
the reaction medium as 93~ H2SO~ at a flow rate
sufficient to maintain the desirecl acidity of reaction
medium. Methanol was continuously Eed 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, at
varying total acid normality levels. Offgases from the
~L8~
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 efficiency of
conversion of chlorate ions to chlorine dioxide was
determined in each case.
The results are reproduced in the following Table
I:
Table I
Run No.
1 2 3
Total Acid
Normality (N) 7.3 to 8.7 9 to lO 7.8 to 8.8
Length of run (hrs) 12 12 8.5
Efficiency (%) 98% ~99% ?99%
15It will be seen from the results of the above
- Table I, that high efficiency operation was maintained
at total acid normality values below 9 normal.
In summary of this 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.
5l~
