Note: Descriptions are shown in the official language in which they were submitted.
83~
BACKGR0UND 0F THE INVENTI0N
The present invention relates to a new, economical, very
I efficient process for generating chlorine dioxide.
¦ Inasmuch as chlorine dioxide is of considerable commercial
importance in the field of pulp bleaching, water purification,fat bleaching, removal of phenols from industrial wastes, textile
bleaching, and the like, it is very desirable to have a process
by wh;ch it can be economically generated.
One means for the generation of chlorine dioxide is by way of
reaction of a chlorate, a chloride, and sulfuric acid providing an
acid normality of about 2 to 12. The reactions which occur are ex-
emplified below, wherein, for the sake of illustration, the chlorate
¦ used is sodium chlorate and the chloride used is sodium chloride.(1) NaC103 ~ NaCl + H2S04 > C102 + 1/2 C12 + Na254 + H20
(2) NaC103 + 5NaCl + 3H2So4 --~ 3C12 + 3Na254 + 3H20
This technique for chlorine dioxide production is used on a com-
mercial scale, with the reactants continuously being fed into a
reaction vessel and the chlorine and chlorine dioxide produced con-
tinuously being removed from the reaction vessel with large quan-
tities of sodium sulfate (saltcake) by-product also being generated
! for use in kraft paper mill processes.
'I Another means for the generation of chlorine dioxide is by
the reaction of a chlorate with hydrochlor;c acid at an acid
normality of about 0.05 to about 1. The reactions which occur are
exemplified below, wherein, for the sake of illustration the chlor-
ate used is sodium chlorate.
(la) 2NaC103 + 4HCl ~ 2C102 + C12 + 2NaCl + 2H20
(2a) NaC103 + 6HC1 ~ 3C12 + NaCl + 3H20.
The combined use of the reagents sodium chloride and sulfuric
acid to convert sodium chlorate to chlorine dioxide has generally
~ 6~33iL
been equated in the prior art with the use o~ hydrochloric acid to
convert sodium chlorate to chlorine dioxide (Sepall et al. U.S.
3~347,628, column 2, lines 39-52 and column 3, lines 1-2). In
theory, sodium chloride and sulfuric acid react to produce hydro-
chloric acid in situ in accordance with the following equation:
(3) 2NaCl ~ H2S04 -~ 2HCl + Na2S04
In actuality, however, an equilibrium exists between Na2S04 and
sulfuric acid to produce NaHS04 per the following reaction seq-
uence
(4) 504 + 2H ~ 2 HS04
(5) NaCl + H2S04 ~ HCl + NaHS04
Thus, though the acidity range generally considered applicable
in chlorine dioxide generation is about 2 to 12 normal in sulfuric
acid, when sulfuric acid is employed as the strong acid, at temp-
eratures above about 30 degrees centigrade, one will recover the
neutral sodium sulfate (Na2S04) if the acidity of the reaction
solution is maintained between about 2-4.8 normal (Winfield et al.,
U. S. Patent 3,864,456). The production of sodium sulfate and
sodium bisulfate occurs at higher acid concentrations.
The acidity range generally considered most desirable in chlo-
rine dioxide generation in a hydrochloric acid system (mode) is
about 0.05 to about l.0 without a catalyst. Beyond about l.0 nor-
mality the chlorine production increases at the expense of chlorine
dioxide production so as to render the process inefficient (Canadian
Patent 956,784 to Winfield). The use of catalysts, however,
extends the acidity range which might be efficiently utilized up
to about l.9 normal (British Patent l,347,740 to Partridge et at).
Thus, heretofore to operate either a sulfuric acid-sodium
- sulfate system or a hydrochloric acid-sodium chloride system in a
single vessel chlorine dioxide reactor-crystallizer-evaporator it
-
1~l683l
was necessary to operate at two widely different acidity levels.
In changing from one acid system to the other using the same re-
actor-crystallizer it was necessary to evacuate the first gen-
erator liquor to a holding tank and then refill the generator with
the second liquor before continuing generation of chlorine dioxide.
It has now been discovered that if chlorine dioxide generator
liquor is saturated with solùtions of both alkali metal sulfate
. and alkali metal chloride it is permissable to operate under the
same reaction conditions regardless of whether sulfuric acid or
hydrochloric acid etc., is being used in the same generator. Thus,
depending on what the particular salt by-product needs of a paper
mill are at a given time, and the ready availability of acid, sul-
furic or hydrochloric, it is now possible to operate under the
same conditions at the same acid normality (about 2 to about 11)
by simply introducing one acid or the other to the generator. This
more flexible process eliminates required evacuation of the gen-
erator in switching from one system or mode to the other, and
furthermore, offers a more satisfactory means of achieving
balanced inventories of both sodium sulfate saltcake and sodium
chloride by-products required in pulp bleaching processes and in
producing sodium chlorate.
Accordingly, it is the principal object of the present in-
vention to provide a more flexible process for product;on of
chlorine dioxide and chlorine wherein various mineral acids may
be used interchangeably in the same reactor without modification
of operating conditions or changing generating liquors.
It is a further object of the present invention to provide
a versatile process for generating chlorine dioxide and chlorine
which enables greater control over salt by-product inventories.
A still further object of the instant invention is to provide
a versatile high acidity hydrochloric acid system which will also
permit highly efficient production of both chlorine dioxide and
~` chlorine in a single vessel type generator-crystallizer-evaporator.
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These and other objects, ~eatures and advantages will
become more apparent from a reading of the following summary
and more detailed description of the invention.
SUMMARY OF THE INVENTION
In accordance with this invention there is provided a
process for the production of chlorine dioxide and chlorine
permitting the use of either hydrochloric acid or sulfuric
acid interchangeably as strong mineral acids in the same
reactor under substantially the same reaction conditions,
- lO said process comprising the steps of (a) feeding sulfuric
: acid into a reactor containing an aqueous reaction solution
. comprising an alkali metal chlorate, an alkali metal chloride,
:~ and an alkali metal sulfate, the acid normality of said re-
action solution being in the range of from about 2 to about
11, said reaction solution being continuously saturated with
the alkali metal chloride and alkali metal sulfate, (b) con-
tinuing to feed sulfuric acid into the reactor to generate
chlorine, chlorine dioxide and an alkali metal salt product,
(c) changing the acid feed from sulfuric acid to hydrochloric
acid, and (d) continuing to feed hydrochloric acid into the
reactor without substantial dification of reaction conditions
to generate chlorine, chlorine dioxide and an alkali metal salt
product.
The aqueous reaction suitably contains said alkali
metal chlorate in a concentration of from O.2 to about 5
: molar, suitably 0.5 to about 5, and the process is suit-
ably carried out at a temperature from about 25 to 90
: degrees centigrade and a pressure of about 20 to about
400 millimeters mercury absolute, with coordination of the
r
v
~116831
- 5a -
temperature and pressure to effect the withdrawal of water
vapor from the reaction solution in admixture with the
chlorine dioxide and chlorine to maintain a substantially
constant volume, the substitution of the strong acid being
allowed to produce the desired alkali salt of the acid.
Optionally the aqueous solution contains at least one
catalyst selected from the group consisting of vanadium
pentoxide, silver ions, manganese ions, dichromate ions and
arsenic ions.
m e process of this invention provides for the production
of chlorine dioxide, chlorine, and an alkali metal chloride or
sulfate saltcake, and results in the substantial absence of
acid effluent from the vessel. The process also allows
maximum flexibility in the control of the alkali metal salt
produced therefrom. By simply changing the acid feed, without
adjusting acidity, the salt produced can be either the sulfate
or the chloride as may be needed to provide reactants for other
portions of the bleaching process.
1~16831
~` ~C~IPII ~ [~[D_ BODIME;TS
The acid concentration of the generating liquor of this
invention may vary from about 2 to about 11 normal, and more
specifically from about 2 to about 5 normal. Higher acid con
S centrations generally result in lower sodium chlorate concent- ¦
rations in the generator liquor and increased solubility of sodium
sulfate (if sulfuric acid is employed) and sodium chlor;de (if
hydrochloric acid is employed).
Unexpectedly, within the normality range of about 2 to about
11, the presence of the saturated alkali metal chloride solution
does not adversely effect the chlorine dioxide generating effi-
ciency. The presence of the saturated alkali metal sulfate
solution increases the efficiency.
When hydrochloric is the strong acid, the saturated solution
of alkali metal sulfate increases the efficiency of chlorine di-
oxide generation such that the system will efficiently produce
chlorine dioxide at normalities well beyond the 2 normal range.
This phenomenon may be explained as a result of an ionic-strength
mechanism, possibly occuring due to the presence of sodium sulfate
-- 20 with hydrogen ion causing an equilibrium between the sulfate andacid sulfate salts thereby permitting the system to perform effi-
ciently at this higher acidity even with hydrochloric acid. How-
ever, applicant does not wish to be bound by any theory or mechanism
of operation.
In any event, this increase in efficiency of hydrochloric acid
systems at normalities of about 2 and above is surprising in view of
previously known hydrochloric acid systems (British Patent 1,347,740),
and presents a highly desirable reaction parameter permitting opti-
mum operation of such hydrochloric acid systems at an acidity level
equivalent to optimally operating sulfuric acid systems. The result
is a continuous system which may utilize as acid feed either sulfuric,
hydrochloric, or ixed acids, whichever is desired, without costly
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shutdowns for ~xtensive changes in generator acidity and reactant
composition. Thus, the commercial QperatOr has at his control an
efficient but versatile continuous chlorine dioxide generating
system which allows easy and non-interruptive switching of acid
feeds as the market and chanying by-product requirements dictate.
The rate of chlorine dioxide generation in the process of this
I invention increases with the concentration of alkali metal chlorate
present in the reaction solution. Therefore, the concentration of
alkali metal chlorate is preferably maintained on the high side of
the applicable concentration range of about 0.2 to about 5 molar.
This is especially true during operation in the region from approxi-
mately 75 to 90 degrees centigrade and at pressures in the region
of 400 millimeters mercury absolute, which conditions favor the
solubility of large amounts of alkali metal chlorate.
As the temperature is reduced in coordination with the develop-
ment of a vacuum over the reaction solution to withdraw water vapor,
the concentration of chlorate is necessarily reduced to prevent
crystallization of chlorate from solution which would negate any
advantage derived from an increased reaction rate. Thus, when
operating at the preferred pressure from about 100 to about 300
millimeters mercury absolute and temperatures between about 50 to
85 degrees centigrade, the concentration of alkali metal chlorate
should be between about 0.2 to about 3 molar.
The reaction of the alkali metal chlorate with the strong
acid may be conveniently performed in, but not limited to, a
unilocular vessel (a single chamber reaction vessel) into which
the reactants may be fed in separate streams and from which the
gaseous mixture of chlorine dioxide, chlorine and water vapor is
continuously removed by coordinating the reaction solution temp-
erature with the pressure in the vessel so that water is evaporated
from the reaction solution in an amount sufficient to maintain a
substantially constant volume of reaction solution. The water
~116831
!
-- 8 --
removed from the reaction solution is an amount substantially equal
to the amount of water introduced to the vessel in addition to the
water produced in the reaction, and serves several functions.
Among these are dilution of the chlorine dioxide gas to prevent
development of explosive concentrations of gas, sweeping of the
gases from the head-space above the reaction solution to assist
in gas disengagement from the liquid medium, thereby avoiding the
less desirable use of gaseous diluents with their attendant sep-
aration problems, and maintaining the saturation of alkali metal
chloride and sulfate.
According to a preferred embodiment of the instant process,
reactants comprising an aqueous solution of an alkali metal chlorate
and a second solution comprising an aqueous strong acid solution are
continuously fed to the reaction solution in a vessel and reacted
in the presence of one or more catalysts, with the chloride ion
being provided as either alkali metal chloride or hydrogen chloride.
t A vacuum is applied to the reaction vessel and coordinated with the
temperature of the reaction solution to remove an amount of water
from the reaction solution sufficient to maintain a substantially
constant volume within the vessel. The vacuum may be applied by
any known means, e.g. by a venturi eductor type vacuum device such
as that produced by high pressure water, steam, or air, or with a
vacuum pump. The products, chlorine dioxide and chlorine in ad-
mixture with water, are withdrawn from the reaction vessel and are
further processed to separate the chlorine dioxide, water vapor,
and chlorine.
As the reaction proceeds in the vessel, crystals of alkali
metal salt appear from whence they are withdrawn as a slurry. The
slurry may be removed by such well-known means as centrifuging,
filtering, or other solid-liquid separation techniques.
Operation of the vessel under vacuum at a reaction solutian
temperature corresponding to the boiling point is not essential.
Instead, the temperature of the reaction solution and the vacuum
1~1683~
under which the vessel is maintained may be so coordinated as to
be slightly below the boiling temperature of the react;on solution.
Then, with the assistance of a stream of dry inert gas, such as
nitrogen, or air, passing through the reaction solution, a suitable
amount of water may be removed from the reaction solution as vapor
to maintain a substantially constant volume of reaction solut;on.
This latter method, however, has the disadvantage of causing the
chlorine dioxide to be heavily diluted with inert gas. Therefore,
it is preferred to so coordinate the vacuum and react;on solution
temperature that the amount of water vapor necessarily removed from
the vessel is flashed off without the necessity for the introduction
of additional agents to remove water vapor.
The alkali metal chlorate, chloride and strong acid may be intro-
duced into the reaction vessel as aqueous solutions containing any
desired concentration to ultimately provide any desired ratio of the
two reactants in the reaction solution. The concentration of alkali
metal chlorate and strong acid influences the ratio of chlorine
dioxide to chlorine evolved, as well as the solubility of the alkali
metal sulfate and alkali metal chloride.
Through the use of at least one catalyst selected from the
group consisting of vanadium pentoxide, silver ions, manganese ions,
dichromate ions, palladium ions and arsenic ions, in conjunction
with the strong acid to convert an alkali metal chlorate to chlorine
dioxide, it has been found that the chlorine dioxide efficiency
and the reaction rate increases at reaction solution average acid-
ities from about 2 to about 11 normal. Due to the continuous eva-
poration of water from the reaction solution, a saturated solution
with respect to the alkali metal sulfate and chloride is maintained,
thereby causing selective crystallization in the reaction solution
within the vessel of the salt formed by the reaction. The alkali
metal salt is withdrawn from the vessel periodically or contin-
uously, and the chlorine dioxide and chlorine evolved together
with water vapor are continuously withdrawn from the vessel.
831
-- 10 --
In operation, the concentrat;on of alkali metal chlorate in
the aqueous reaction solution depends upon the acid normality range
being used and generally will vary within the range of 0.2 to 5
moles per liter, preferably within the range of 0.2 to 3 moles per
liter. By continuous addition of alkali metal chlorate to the re-
action solution during the process operation, the desired concen-
tration can be maintained and coordinated with the concentration
of strong acid to provide the optimum chlorine dioxide generating
conditions. The strong acid may be continuously introduced into
the reaction solution to afford an average acidity level between
about 2 to about 11 normal, or most preferably, within the range
of between about 2 to about 5 normal. The addition of the reagents,
alkali metal chlorate and strong acid, as well as any additional
make-up catalyst is coordinated with the removal of water from the
reaction solution, to provide a desired continuous state concent-
ration of reagents and catalysts. The rate of heat input and the
! reduced pressure is also coordinated to provide for the removal of
water equivalent to that being added to the system and being formed
by the reaction taking place within the reaction solution.
The process described herein utilizes in the reactor a
saturated solution of alkali metal chloride and alkali metal sul-
fate. When the strong acid selected is sulfuric acid, additional
alkali metal sulfate crystals will be formed in the reactor, and
likewise if hydrochloric acid is used as the strong acid, addi-
tional alkali metal chloride crystals would be formed. The
crystals produced in the reactor may be removed from time to time
and would be removed in a slurry form.
The slurry which will be predominately the alkali metal salt
of the strong acid being used in the reactor will also contain aminor amount of the other alkali metal salt which saturates the
reaction solution.
1~6~33~
When hydrochloric acid is used as the strong acid, the process
slurry may be introduced into the top of a separating column as
disclosed in U.S. Patent 4,045,542, to obtain substantially pure
alkali metal chloride salt crystals on the bottom of the column
while continuously returning the washing liquid containing chloride,
chlorate and acid values to the generator.
Where the generator is utilizing sulfuric acid, or mixtures
thereof with hydrochloric acid, the process slurry may be intro-
duced to the top of a separating or methathesis column as described
in U.S. Patent 3,976,758, U.S. Patent 4,049,785, and U.S. Patent
4,156,713, to obtain separation and purification of the resultant
salt, or to produce other desirable products by metathesis with or
without the return of usable values to the generator. The process
permits the return to the reaction vessel of any liquid removed
from the vessel with solid alkali metal salt as a slurry. If
substantially all of this liquid removed from the vessel is not
returned, the rate at which the water is evaporated from the reaction
solution will be adjusted relative to the amount returned to the
system.
It should be understood that alkali metal chlorates other than
sodium chlorate may be employed. Thus, potassium, lithium and
calcium chlorate may be used. Mixtures of these sales may be em-
ployed if desired.
It should also be understood that the sulfuric acid employed
in this invention may be derived from a variety of sources, such
as the "split acid" normally discharged as effluent liquor from
conventional chlorine dioxide generating processes, and more fully
described in U.S. Patent 3,446,584.
The preferred process is conducted in the presence of a
catalyst selected from the group consisting of silver ions,
manganese ions, dichromate ions, arsenic ions, palladium ions, vana-
dium pentoxide, and mixtures thereof.
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- 12 -
The silver ion is a preferred catalyst. From about 0.0~01 to
about 1.5 grams of silver ion per liter of reaction solution should
be used. Although more than about 1.5 grams of silver ion may be
used, one does not obtain significant increased efficiency with the
5 excess amount of said ;on.
Manganese ion is also one of the preferred catalysts. From
about 0.001 to about 4 grams of the manganous ion per liter of
reaction solution should be used. Again, although one may use
more than 4 grams of manganous ion per liter of reaction solution,
one does not obtain any significant increased efficiency in chlorine
dioxide generation due to the use of the excess amount of said ion.
The dichromate ion, especially in the form of an alkali metal
dichromate, such as sodium and potassium dichromate, is the most
preferred catalyst. It should be used at concentrations of from
about 0.5 to about 25 grams per liter~ it again being understood
that one can use more than 25 grams per liter if so desired.
The arsenic ion, vanadium pentoxide and palladium ions are
also useful catalysts.
EXAMPLE 1
The reaction was effected by establishing a generator-evapor-
ator-crystallizer unit charged with an aqueous solution of sodium
chlorate, sodium chloride and sodium sulfate. An aqueous solut;on
of silver nitrate was added to provide silver ion as a catalyst.
This solution was saturated with sodium chloride. Sulfuric acid
was introduced into the generator and chlorine dioxide and chloride
were generated until the solution became saturated with sodium
sulfate. During operation, after sulfate saturation, the gram
atom efficiency of chlorine dioxide produced ranged from 47.6% to
49.6%. The generator composition during this period varied within
the following ranges:
H2S04 2. 7 - 3. lN
NaC103 1.43 - 1.54M
~1~683~
The temperature ranged from 61.5 to 64 C and the pressure
was maintained at 164-170 mm rnercury absolute and chlorine dioxide
chlorine and water vapor were removed and the rates of production
and amounts of chlorine dioxide and chlorine were determined. The
rate of production of chlorine dioxide varied from 0.9 to 1.6 gram/
minute and the chlorine was produced at the rate of 0.5 to 0.87
gram/minute.
During operation the acid feed was switched to hydrochloric
acid, the gram atom efficiency of chlorine dioxide ranged from 42.9
to 46.4%. The generator composition during the HCl operation varied
within the following ranges:
HCl 2.69 - 3.19N
NaC103 1.47 - 1.63M
The temperature ranged from 62-63.5 C and the pressure was
t5 maintained at 164-168 mm mercury absolute, and chlorine dioxide,
chlorine and water vapor were removed and the rate of production
of chlorine dioxide and chlorine were determined. The rate of pro-
duction of chlorine dioxide varied from 1.64 to 1.91 grams/minute
and the chlorine was produced at the rate of 0.85 to 1.6 grams/minute.
Operation under each type of acid feed continued for a period
of greater than 6 hours.
It is appreciated that the instant specification and descrip-
tion are set forth by way of illustration and not limitation, and
that various modifications and changes may be made without depart-
ing from the spirit and scope of the present invention.
