Note: Descriptions are shown in the official language in which they were submitted.
~()90~85
Background of the Invention
The present invention relates to a new, economical, very efficient
process for generating chlorine dioxide.
Inasmuch as chlorine dioxide is of considerable commercial importance
in the fields of pulp bleaching, water purification, fat bleaching, re-
moval of phenols from industrial wastes, textile bleaching, and the like,it is very desirable to have a process by which 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. The reactions
which occur are exemplified below, wherein, for the sake of illustration,
the chlorate used is sodium chlorate and the chloride used is sodium
chloride.
(1) NaC103 + NaCl + H2SO4 C102 + 1/2 C12 + Na2so4 + HzO
(2) NaC103 + 5NaCl + 3H2SO4 3C12 + 3 Na2SO4 + 3H20.
This technique for chlorine dioxide production is used on a commercial
scale, with the reactants continuously being fed into a reaction vessel
and the chlorine and chlorine dioxide produced continuously being removed
from the reaction vessel.
Another means for the generation of chlorine dioxide is by the
reaction of a chlorate with hydrochloric acid. The reactions wh;ch occur
are exemplified below, wherein, for the sake of illustration the chlorate
used is sodium chlorate.
(la) 2 NaC103 + 4 HC1 2 C102 + C12 + 2 NaCl + 2 H20
(2a) NaC103 + 6 HCl 3 C12 + NaCl + 3 H20.
Typically, in such processes, the chlorine, chlorine dioxide and
water produced are removed ;n the vapor state and the alkal; metal salt
109058S
is removed from the reaction solution by crystallization.
Various processes and systems have been employed for the generation
of chlorine dioxide, chlorine, and an alkali metal salt based in general
on the reactions shown in the above equations. Such systems include for
example, systems comprising a multiplicity of generators operating in
cascade flow (U.S. Patent No. 3,446,584, to W. A. Fuller) or a combination
of a poly-zoned apparatus or plural apparatuses in which various distinct
chemical and/or physical operations have been separated into chlorine
dioxide generation, water evaporation, by-product salt crystallization,
and reactant introduction into the system (U.S. Patent No. 3,516,790,
June 23, 1970i U.S. Patent No. 3,341,288, Partridge, September 12, 1967).
More recently, it has been proposed to provide for the generation of
chlorine dioxide, chlorine, and an alkali metal salt in a single reaction
vessel. Thus, for example, in U.S. Patent 3,816,077, Fuller et al., June
11, 1974, there is disclosed a system wherein the generation of chlorine
dioxide and chlorine, the evaporation of water, and crystallization of an
alkali metal salt, are effected in a single reaction Yessel. Although
substantial improve~lent in efficiency is achieved in the conlbining of
- these processes into a single reaction Yessel, the system requires
external conduits, pump, and heating apparatus for the mixing, heating,
circulating and recycling of the reaction mixture. It will be appreciated
by those skilled in the art that still further improvements are desirable
to provide a system wherein a single process vessel is employed for the
mixing and temperature control of reactants, the generation of chlorine
dioxide, evaporation of water, and the crystallization of alkali metal
salt, as well as the continuous and efficient circulation and re-circul-
ation of reaction mixture.
Summary of the Invention
This invention proYides a process for the production of chlorine
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1()90~85
dioxide, chlorine, and an alkali metal salt which comprises:
(A) separately feeding an alkali metal chlorate solution and a strong
acid solution selected from the group sulfuric acid, hydrochloric ac;d,
phosphoric acid and mixtures thereof into an integral forced circulation
crystallizing reaction evaporator comprising, in vertical disposition:
a) an upper reaction crystallizing evaporation chamber having a substantially
vertically disposed cylindrical partition which divides at least the lower
portion of the reaction crystallizing evaporation chamber into a first and
second substantially concentric cylindrical sections; b) a heat exchange
chamber comprising a multiplicity of vertically disposed tubular elements for
transm;ssion of fluids through said heat exchange chamber; and c) a lower pump
chamber, hav;ng an intake section and a discharge section and having a pump
means disposed therein to prov;de forced c;rculation of liquid between the
intake and discharge sections;
(B) mixing the solutions therein and circulating the resultant reaction
mixture, in response to the pump means, in sequence, from the d;scharge section
of the lower pump chamber, upwardly through a firstjportion of the tubular
elements, through the first cylindrical section of the upper reaction crystal-
l;zing evaporat;on chamber, over the cyl;ndrical partition; downwardly through
a second portion of tubular elements ;nto the intake section and discharge
section of the lower pump chamber;
(C) maintaining the reaction mixture ;n sufficient volume to at least
partially fill the evaporation chamber and provide a vapor-liquid separation
space in the upper region thereof;
(D) withdrawing an aqueous slurry of alkali metal salt of said acid
from the circulating reaction mixture,
(E) withdrawing chlorine dioxide, chlorine, and water vapor from the
vapor liquid separation space of the upper reaction crystallizing evaporation
chamber.
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The reaction of the alkali metal chlorate with the strong acid is
preferably achieYed by feeding the reactants in separate streams into the
reaction vessel from which the gaseous mixture of chlorine dioxide,
chlorine and water Yapors are continuously removed by coordinating the
reaction solution temperature 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 removal of water
from the reaction solution is in an amount substantially equal to the
amount of water introduced to the reaction evaporator plus the water
produced in the reaction. The removal of water Yapor serves seYeral
functions. Among these are dilution of the chlorine dioxide gas to prevent
development of explosive concentrations of gas; sweeping of the gases from
the vapor space above the reaction solution to assist in gas disengagement
from the liquid mediumi the proYision of a readily condensable diluent for
the chlorine gas produced in the reaction, thereby aYoiding the less
desirable use of gaseous diluents with their attendant separation problems;
and maintaining saturation of the alkali metal salt.
The process that is inYolved in the generation of chlorine dioxide,
chlorine, formation of an alkali metal salt and eYaporation of water in
the reaction solution, all occur within the same circulating solution, the
necessary reactants being continuously introduced into the reaction solution
and the gaseous products of reaction being continuously withdrawn in
admixture with water vapor until the solid alkali metal salt simultaneously
and continuously crystallizes from the reaction solution.
The rate of chlorine dioxide generation in the process of this invention
increases with the concentration of alkali metal chlorate present in the
reaction solution. Therefore, the concentration of alkali metal chlorate
is preferably maintained within the concentration ran~e of about 0.2 to
about 5 molar. This is especially true during operation in the range of
~09098~
60 to 110C and at pressures in the range of up to about 400 millimeters
of mercury absolute, which conditions favor the solubility of large amounts
of alkali metal chlorate. As the temperature is reduced in coordination with
the development of a vacuum over the reaction solution to withdraw water
vapor, the concentration of chlorate is necessarily reduced to prevent the
crystallization of chlorate from solution which would negate any advantage
derived from an increased reaction rate. Thus, when operating at a preferred
pressure of about 100 to about 300 millimeters of mercury àbsolute and at
temperature between about 65 to 100C, the concentration of alkali metal
chlorate should be about 0.2 to about 3 molar.
The acidity of the reaction solution can be maintained anywhere
between about 0.~ N to about 12 N, the strong acid used affecting the
efficiency at the proposed amount in ranges. For example, if an HCl
system is utilized the preferred normality is in the range of about 0.5 N
to about 3.5 N, wherein the alkali metal salt produced will be an
alkali metal chloride. When H2S04 is the preferred acid used or m;xtures
of H2S04 and HCl are used, a preferred normality is about 1.5 N up to
about 12 N. In operating H2S04 system in ranges from about 1.5 to about
6 N the product will be an alkali metal sulfate salt and as the normality
increases beyond about 6 N the alkali metal salt precipltated will be an
alkali metal acid sulfate salt. Furthermore, as is known in the art, there
may be employed in the process of this invention, a reducing agent such-as
methanol, oxalic acid, sulfur dioxide and the like, which react w;th chlorine
to enhance the production of chlorine dioxide.
Through the use of at least one catalyst selected from the group
cons;sting of vanadium pentoxide, silver ions, manganese ions, dichromate
ions, and arsenic ions, in coniunction with the strong acids to convert
1090985
an alkali metal chlorate to chlorine dioxide, applicants have found
that the chlorine dioxide eff;ciency and the reaction rate increases
at reaction solution average acidities from about 2 N to about 5 N.
The silver ion is a preferred catalyst. From about 0.001 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 excess amount of
said ion.
Managanese ion is also one of the preferred catalysts. From about
0.001 to about 4 grams of manganese ion per liter of reaction solution
should be used, again, although one may use more than 4 grams of man-
ganese ion per liter of reaction solution one does not obtain a signi-
ficantly increased efficiency in chlorine dioxide generation due to
the use of an excess amount of said ion.
The dichromate ion, especially in the form of an alkali metal di-
chromate such as sodium potassium dichromate is another preferred cata-
lyst. It should be used at concentrations from about 0.5 to about 25
grams per liter, it again being understood that one could use more than
25 grams per liter if so desired.
The arsenic ion and vanadium pentoxide may also be used as catalysts.
They should be in a concentration of about 0.05 to about 25 grams per
liter.
As the reaction proceeds in said vessel, crystals of alkali metal
salt appear withdrawn as a slurry. The solids may be removed from the
25 slurry by such well known means of centrifuging, filtering, or other salt
and liquid separation techniques.
When hydrochloric acid is employed as the strong acid, the process
slurry may be introduced to the top of a separation column as disclosed
in Canadian Patent application SN 245,890, filed February 17, 1976, to
30 obtain substantially pure alkali metal chloride salt crystals on the
bottom of the column while continuously returning chloride, chlorate
and acid values to the reaction evaporator.
1~90985
Where the generator is utilizing sulfuric acid and mixtures thereof
with hydrochloric acid, as the strong acid, the process slurry may be
introduced at the top of a separating or metathesis column as described
in U.S. Patent 3,976,758; U.S. Patent 3,975,505; U.S. Patent 3,974,266;
and Canadian Patent application SN 278,407, filed May 13, 1977, to obtain
separation purification of resultant salts or to produce other desirable
products by metathesis with or without the return of usable values to
the reaction evaporator.
The mother liquor from the crystals can be used again as wash water,
discarded, or saturated with sodium chloride and passed to an electrolytic
cell for the production of additional alkali metal chlorate.
The partial pressures and the resulting temperatures prevailing in
the reaction chamber may be varied by the admission of a stream of dry
inert gas, such as nitrogen or air. This, in turn, makes possible the
variation of the amount of water removed from the reaction solution as
vapor. This latter method, however, has the disadvantage of causing
the chlorine dioxide to be heavily diluted with inert gas. Therefore,
it is preferred ~o so coordinate the vacuum and reaction 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.
For a more complete understanding of the invention, reference is
made to the accompany drawing wherein:
Fig. I represents a vertical section of a preferred embodiment an
integral forced circulation crystallizing reaction evaporator employed
in the process of this invention.
The apparatus employed in the process of this invention is con-
structed so that a reaction crystallizing evaporation chamber, a heat
exchange chamber, and a pump chamber are formed as an integral vertical
unit into which the reactants may be continuously fed and the reaction
mixture continuously circulated.
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~09098S
The process of the present invention is best described with specific
reference to the drawing of Fig. I wherein the integral forced circulation
crystallizing reaction evaporator employed comprises three chambers:
an upper reaction crystallizing evaporation chamber 11, a heat exchange
chamber 12, and a lower pump chamber 13.
Within the reaction crystallizing evaporation chamber 11, a sub-
stantially vertically disposed cylindrical partition 20, serves to divide
at least the lower portion of the chamber into an outer section 21 and an
inner section 22, the two sections being substantially concentric cylindrical
sections. This inner cylinder has at its upper end a means of distributing
the reaction mixture evenly across the interface between the liquid reaction
mixture and the vapor space.
The lower pu~p chamber 13 is provided with partition means 14 which
serve to separate the chamber into a discharge section 15 and an intake
section 16 which are in direct fluld communication respectively, with an
inner portion 17 and outer portion 18 of the tubular elements of the heat
exchange chamber 12 and in turn, respectively, with the first and second
cylindrical sections of the crystallizing evaporation chamber 11. Within
the lower pump chamber 13, a pump means 23 is provided to direct the flow
of liquids from the intake section 16 to the discharge section 15 with the
result that the reaction mixture may be efficiently continuously circulated
through the heat exchange chamber 12 and into the reaction crystallizing
evaporation chamber 11 where the cylindrical partition 20 and the distribution
member 33 serves to direct the flow of heated reacting liquid to a level near
the liquid-vapor interface where the desired evaporation of water and evolution
of chlorine dioxide and chlorine into the vapor-liqu;d separation space 19
will take place. The upper portion of cylindrical partition 20 is preferably
funnel-shaped, diverging upwardly, and distribution member 33 is preferably
funnel-shaped and centrally and concentrically positioned with respect to the
funnel shaped portion of partition 20. ~later vapor, chlorine dioxide and
chlorine are withdrawn through outlet means 26 from the vapor-liquid
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1 0 9 0 3 8 5
separation space 19. The liquid reaction mixture will flow from the
vapor-liquid interface above the top of the cylindr;cal partition 20 and
downwardly through the outer cylindrical section 21, the outer port;on 18
of tubular elements in the heat exchange chamber 12 and to the intake section
16 of the pumping chamber.
The alkali metal chlorate solution is introduced preferably into the
pumping chamber 13, through an inlet means 24 preferably positioned in the
d;scharge section 15 and, ;n response to the pump means 23, is c;rculated
and mixed in the manner described above. The strong acid is preferably
introduced into the rising liquid, through inlet means 25, preferably
positioned at a location in the lower part of the inner cylindrical section
22 o, the evaporation chamber 11 so that substantial mixing and reaction
occur as the reaction mixture is circulated to the top of the cylindrical
partition 20. Inlet means 25 is preferably constructed of or protected by
a strong acid resistant material, such as Teflon.* It is preferred to
introduce the strong acid through inlet means 25 into the rising liquid,
in close proximity above the tubular elements of the heat exchanger, since
turbulence occurring as the liquids rise out of the tubular elements into
the reaction crystallizing evaporation chamber 11 increases the rapid
mixing of the reactants. As the reaction occurs in the crystallizing
evaporation chamber 11, with the generation of chlor;ne dioxide and chlorine,
an alkali metal salt of the strong acid is formed and crystallizes in the
evaporating liquid. The alkali metal salt crystals may be removed as an
aqueous slurry through outlet means 27. The aqueous slurry may be then
treated as described aforesaid, and the mother liquor may be returned to the
reaction mixture.
The pump means 23 may be of the mechanical type such as a rotary pump
or propeller pump. Alternatively, a gas lift utilizing a stream of air or
other inert gas may be employed as the pump means, either soley or in
combination with mechanical pumping means.
* Trademark
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1090~8~
In a preferred embodiment an entrainment separator 28 is positioned
within the vapor-liquid separation space 19 in the upper region of the
reaction evaporation chamber 11 to effect the separation of any entrained
liquids from the vapors to be withdrawn through outlet means 26. The
entrainment separator 28 may be any of the types known in the art such as a
mesh type entrainment separator.
Preferably the reaction crystallizing evaporation chamber is provided
with an explosion relief valve 32 to provide an emergency release ofi~gases
in the event of a sudden increase in pressure.
Although the reactions of the present process may be carr;ed out at
atmospheric pressure, it is preferred to employ subatmospheric pressures,
most preferably in the range of about 50 to about 500 mm Hg absolute. The
vacuum producing means may be any conventional device such as a mechanical
vacuum pump, a water eductor, steam siphon, or the like (not shown).
The heat exchange chamber 12 includes a heat exchange fluid inlet 29 and
outlet 30 to permit the flow of a heat exchange fluid, such as steam, through
the extra-tubular space. In addition, in the extra-tubular space, there is
preferably provided a multiplicity of horizontally disposed baffles 31 adapted
to direct the flow of heat exchange fluid in a horizontally alternating
path ~o maximize the efficiency of heat exchange. The flow of heat exchange
fluid is adjusted to achieve the desired reaction temperature. To adjust
the reaction mixture to the desired temperature, the degree of vacuum
applied to the evaporator chamber is adjusted until the reaction solution
will be at its boiling point when at the desired temperature, and the rate
of heat input to the heat exchange chamber is adjusted to raise the temper-
ature of the reaction to the boiling point and to evaporate water at arate sufficient to maintain a substantially constant volume of liquid.
The evaporation of water at the aforementioned rate causes the
formation of crystalline product in the reaction solution in the reaction
1090985
crystallizing evaporation chamber 11. The rate of energy input into
the system from all sources after steady state conditions have been
reached is such that all of the water being added to the system and being
formed by the reactions taking place therein, less any water of crystal-
lization in any of the crystalline alkali metal salts and water from the
aqueous slurry withdrawn from the system, is evaporated from the reaction
solution in the reaction crystalliz;ng evaporation chamber and withdrawn as
water vapor from the system. This rate of energy input is related to the
temperature chosen, the corresponding vacuum, the rate at wh;ch water is
being added to the system after steady state conditions have been reached
and the rate at which water is being removed as water of crystallization
and as aqueous slurry.
the temperature of the reaction mixture may vary considerably
depending on the rate of evaporation desired and the operating pressure.
It is preferred,-however, that the reaction mixture be maintained at the
boiling temperature of about 65 to about 100 Celsius, at the operating
pressure which is preferably a pressure of about 150 to about 300 mm Hg
and most preferably about 200 to about 250 mm Hg.
The integral forced circulation crystallizing reaction evaporator
employed in the process of this invention is subiect to both corrosive
and corrasive actlon from the reaction mixture and thus must be constructed
of materials capable of withstanding such attack. Furthermore, the mat-
erials of construction must be resistant to deterioration at operating
temperatures and have suff;cient strength to withstand the effect of
vacuum operation. It is known that titanium is an excellent material of
construct;on for such purposes. However, the cost of titanium is such
that, for economic considerations, other materials have been used in the
prior art, as a partial or complete substitute for titanium. In one
io90~8~j
such prior art process for the generation of chlorine dioxide and
chlorine, a preferred material of construction is a polyester resin
which is imperv;ous to the corrosive attack of the chloride ion and
is not subject to oxidation by the action of chlorine dioxide nor
5 subject to attack by the chlorine through addition or substitution,
and is not subject to corrosive attack by the mineral acid employed in
the reaction solution. A typical polyester resin that meets these re-
quirements is the polyester resin disclosed in U.S. Patent 3,816,077,
comprising about 0.5 mole fraction of chlorendic acid and maleic
10 anhydride and about 0.5 mole fraction neopentyl glycol, and about 45
parts of styrene per 100 parts of resin. The resin itself may be
prepared in accordance with the procedures disclosed in U.S. Patent
No. 2,634,251, which illustrates the techniques for the formulation
of the resin from the components set forth.
Polyester resins of the type described satisfy the requirements of
temperature resistance, mechanical strength, and corrosion resistance.
However, such materials are subject to deterioration from corrosion
where contact with the alkali metal salt crystals occurs. With the
above considerations in mind, it is preferred to construct the integral
20 forced circulation crystallizing reaction evaporator of a combination
of materials whereby that portion of the evaporator in contact with the
liquid reaction mixture is constructed of titanium and that portion of
the evaporator in contact with the vapors above the liquid is constructed
of a suitable polyester resin.
In a typical operation, the process of the invention is effected
utilizing a flow scheme as illustrated in the drawing to produce a
gaseous mixture of chlorine dioxide and chlorine and a sodium sulfate
salt. An aqueous solution of 3.2 M sodium chlorate and 3.36 M sodium
chloride is fed into the discharge side of the pump chamber (13) through
- 13 -
~)90985
inlet 24 and circulated upwardly through the inner portion (17) of the
tubular elements of the heat exchange chamber (12). As the solution
enters the inner portion (22) of the evaporation chamber (ll), an aqueous
solution of 50% sulfuric acid is added through inlet means (25) and
mixes with the rising solution. The pressure of the system is adjusted
to about 200 millimeters of mercury absolute. Steam is passed through the
heat exchange chamber (12) by means of inlet (29) and outlet (30) at a
rate and temperature sufficient to maintain the circulating reaction
mixture at a temperature of about 78 Celsius. The rate of input of
reactants is adjusted such that the volume of reaction mixture is
maintained at a constant level, above the top of the cylindrical partition
(20) and distribution member 33 as the excess water added with the feed
solutions is boiled out, exiting as a vapor from the evaporation chamber (ll)
through outlet (26), together with the chlorine dioxide and chlorine generated
by the reaction mixture. In response to the pump (23) the reaction mixture
circulates throughout the chambers of the reaction evaporator in the
manner indicated in the accompanying drawing. As the reaction proceeds,
the chlorine dioxide and chlorine are generated and withdrawn together
with water vapor. Anhydrous sodium sulfate is crystallized from the
solution and withdrawn from the reaction mixture through outlet (27) of
the pump chamber (13).
It will be appreciated that, although various specific embodiments
of the process of this invention have been described hereinabove, various
modifications and combinations of these specific embodiments may be employed
without departing from the spir1t and scope of the invention. Thus, for
example, although the preferred method of circulating the reaction mixture,
to achieve maximum efficiency of mixing and flow across the liquid vapor
interface, is in the direction shown by the arrows in Fig. 1, the direction
of flow may be reversed. Furthermore, although the reaction crystallizing
- 14 -
109098S
evaporation chamber 11 has been described, in the preferred embodiment
as comprising two substantially concentric cylindrical sections 21 and
22, these sections may vary from cylindrical shape without substantial
adverse effect on the desired flow of reaction mixture. Fruthermore,
although the heat exchange chamber 12 has been illustrated as being
formed of a multiplicity of tubular elements 17 and 18, the exact number
and diameter of the tubular elements may be varied, depending on the
anticipated flow-rate and the amounts of reaction mlxture to be circulated.
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