Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a purification of
chlorine gas. More particularly, it relates to a process for
preparing chlorine gas of high purity by purifying a crude
chlorine gas which contains at least one of oxygen, hydrogen and
carbon dioxide gas admixed with the chlorine.
Chlorine gas has been used :in various industrial fields.
When chlorine gas is used for the chlorination of organic compounds,
if the chlorine gas contains another gas such as oxygen, the
oxygen content increases with the consumption of the chlorine
in the reaction forming an explosive gas with the organic compound
whereby certain problems such as explosions may be caused and
water is formed in the reaction of the organic compound corroding
the apparatus. Accordingly, it is necessary to use the chlorine
gas having a high purity.
Chlorine gas has been prepared industrially by the
electrolysis of sodium chloride, by the Weldon method using
hydrochloric acid with manganese chloride or by the Deacon-Hurter
method oxidizing hydrochloric acid. The crude chlorine gas
produced by these methods contains relatively large amoun~s of
impurities. The crude chlorine gas prepared by the electrolysis
of sodium chloride has relatively higher purity in comparison
with other methods, however the crude chlorine gas contains about
2 to 10% of oxygen and other gases, such as carbon dioxide gas,
hydrogen gas, as impurities. When the electrolysis of sodium
chloride is carried out using a metallic electrode, the crude
chlorine gas may contain relatively high oxygen gas content.
Accordingly, when the chlorine gas is to be used in the fields
where chlorine gas having high purity is used, it is necessary to
purify the crude chlorine gas.
It is known to purify the crude chlorine gas by the
recovery of chlorine gas after liquefication of the chlorine
gas. Such a method as the compression method of recovery of
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8~9;~:Z
chlorine gas by liquiefying chlorine gas after drying the crude
chlorine gas compressing the gas at about 8 to 10 atm. pressure
cooling the gas at about 15 to 20C and then, vaporizing the
liquefied chlorine. Another such method is the cooling method
of recovery of chlorine gas by lique~ying chlorine gas after
drying the crude chlorine gas, cooling the gas at about -40C
under about 1 atm.pressure and then, vaporizing the liquefied
chlorine. However, these methods have disadvantages because a
large amount of a drying agent is needed for completely drying
the gas to liquefy the gas and a large power consumption of the
apparatus for compression and cooling is needed providing a high
cost for the purification. When the purification is carried out
in the compression condition, the condition of the operation
may be especially severe.
When hydrogen gas is present with oxygen as the impurity
gases, the hydrogen content in the residual gas increases with
the separation of chlorine gas and an explosive gas may be formed
by the residual chlorine gas with the hydrogen gas and oxygen gas.
Accordingly, it is required that the crude chlorine gas is diluted
with a gas inert to hydrogen before liquefying the crude chlorine
gas by cooling. The efficiency of the purification of chlorine
gas has been disadvantageously lowered.
In another conventional method of purification of the
crude chlorine gas, only chlorine gas is absorbed into a solvent
such as sulfur chloride or carbon tetrachloride to separate the
chlorine gas from the impurity gases and then, chlorine gas is
removed. The latter method could be economically carried out
however, the solvent disadvantageously may be incorporated in the
resulting chlorine gas and the separa-tios of the solvent has not
been easy.
The present invention provides a method of purification
of chlorine gas without liquefying it with drying and without
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absorbing the chlorine gas into a solvent to yield high purity
chlorine at low cost.
According to the present invention there is provided
a process for the purification of chlorine gas which comprises
cooling a crude chlorine gas in the presence of water sufficient
to form chlorine hydrate crystals and a heat transfer medium so
as to crystallize chlorine in the crude chlorine gas as chlorine
hydrate crystals; separating the other components of the crude
chlorine gas from chlorine hydrate crystals and then, decomposing
the chlorine hydrate crystals to obtain chlorine gas.
Thus according to the present invention the crude
chlorine gas is cooled in the presence of wa-ter to crystallize
chlorine from the crude chlorine gas as chlorine hydrate crystals,
and the other components of the crude chlorine gas separated
frGm chlorine and then the chlorine hydrate crystals are decomposed
to obtain chlorine gas.
The method of the present invention has the following
advantages.
The chlorine in the crude chlorine gas is crystallized
as chlorine hydrate crystals so that it is unnecessary to compress
it to a too high pressure or to keep it in too low temperature
or to completely dry the crude chlorine gas in the process.
Accordingly, the~process and the apparatus for the purification
can be simplified. Moreover, it is possible to carry out the
process under the atmospheric pressure and to decrease the power
consumption for cooling the gas.
The crude chlorine gas is cooled in the presence of
water so that the chlorine in the crude chlorine gas is crystallized
to form the chlorine hydrate crystals. The other components in
the crude chlorine gas such as oxygen, hydrogen or carbon dioxide
gas are in gaseous state under the conditions of forming the
chlorine hydrate crystals, and can be easily separated by removing
these gases such as by suction.
Z2
The method o~ the present invention will be illustrated
in detail.
The crude chlorine gas used ln the method of the presènt
invention contains chlorine and o-ther impurity gases which are
not solidified under the conditions of crystallizing chlorine
as the chlorine hydrate crystals, such as oxygen gas, hydrogen
gas and carbon dioxide gas.
The erude ehlorine gas eontaining impurities sueh as
oxygen ean be obtained by the electrolysis of sodium ehloride -
using a metallic electrode. Thus, the crude chlorine gas can
be obtained by mercury electrolysis, asbestos diaphragm type
electrolysis and ion-exchange membrane type electrolysis. In
these cases, the crude chlorine gases may contain 90 to 98 vol.%
of ehlorine; 1.5 to 10 vol.% of oxygen; 0.1 to 0.2 vol.% of
hydrogen and 0.5 to 0.6 vol.% of carbon dioxide gas. These eontents
may vary depending upon the kind of the anode.
The ehlorine hydrate erystals are mainly CQ2-6H2O or
CR2-8H2O. Other chloride hydrate crystals include those between
six to eight hydrates or less than six hydrate or more than eight
hydrate. The ehlorine hydrate crystals can be formed by crystalli-
zing chlorine gas under various conditions of a temperature and
a pressure in the presence of water. When CQ2-6H2O is prepared,
it can be obtained by cooling chlorine gas at the chlorine partial
pressure of 1 atm. at a temperature lower than 9.6C. When the
ehlorine hydrate crystals are formed by cooling chlorine gas in
the presence of water, various methods can be employed.
For example, the erude ehlorine gas is eooled in the
presenee of water in an amount suffieient to form the ehlorlne
. hydrate erystals from chlorine gas in the crude chlorine gas such
as at least 6 moles of water per 1 mole of chlorine for CQ2-6H2O.
Indirect cooling with a system for cooling with a heat exchange
or the other methods can be employed.
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For -the formation of the chlorine hydrate crystals,
the method of coollng the crude chloxine gas and water for forming
the chlorine hydrate crystals in the presence of a heat transfer
medium ~r-~ used. The method of using -the heat transfer medium
is ~Ye~aab~¢t because of the following reasons.
The heat transfer medium while being as the medium
for heat transfer may also be used as the medium for transferring
chlorine hydrate crystals (hereinafter heat transfer medium is
referred as a transfer medium). Accordingly, the transfer
medium should be substantially inert to chlorine and it is
preferably the liquid~from the crystallization of the chlorine
hydrate to the decomposition thereof and it is further preferable
to have low-vaporizing properties.
Suitable transfer media include halohydrocarbons such
as dichlorofluoromethane, chlorodifluoromethane and water. It is
preferable to use water from the viewpoint of vaporizing property.
Water can be the transfer medium as well as the source of water
for forming chlorine hydrate crystals.
In this case, the method of cooling the crude chlorine
gas can be the method of contacting the crude chlorine gas with
the transfer medium which is previously cooled. In this method, -
water can be previously mixed with chlorine gas or water can be
also mixed with the transfer medium. In the other method, the
mixture of the crude chlorine gas, water and the transfer medium
can be directly or indirectly cooled. For example, when the
cooled transfer medium is contacted with the crude chlorine gas,
the crude chlori.ne gas can be countercurrently contacted with the
cooled transfer medium in the presence of water or the crude
chlorine gas can be fed into the cooled transfer medium or i-t
can be contacted by the other methods.
Whenchlorine gas is crystallized to form thechlorine
hydrate crystals, the cooling temperature is depending upon the
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pressure in the cooling operation. It is preferable to form the
chlorine hydrate crystals under the Eollowing condition.
log P _ 16.043 - (P : mmHg; T : K),
270 K < T < 310K.
The temperature T is preferably in a range of 271K
to 285K. When it is cooled under such temperature condition,
the chlorine hydrate crystals can be formed under the chlorine
partial pressure of about 1 atm. ~-. 760 mmHg). Accordingly,
it can be carried out under substantailly atmospheric pressure.
The chlorine in the crude chlorine gas is crystallized
as the chlorine hydrate crystals by cooling. The other gases in -
the crude chlorine gas are in gaseous state whereby the other
gases can be separated such as by the suction. The separated
chlorine hydrate crystals are decomposed to generate chlorine
gas having high purity. The decomposition of the chlorine hydrate
crystals is a reverse process to the crystallization with respect
to the chlorine hydrate crystals. Accordingly, the step of
decomposing the chlorine hydrate crystals can be attained by
. .
decreasing the pressure to less than the decomposition pressure ~-
of the chlorine hydrate crystals or increasing the temperature
to higher than the temperature for the decomposition pressure.
~or example, when the crude chlorine gas is cooled under
1 atm. to a temperature lower than 9.6C to form the chlorine
hydrate crystals, the temperature is raised to higher than 9.6C
or the pressure is decreased to lower than 1 atm. However, it
is preferable to decompose the crystals by controlling -the pressure
because of the following reason.
- The crystallization for forming the chlorine hydrate
crystals and the decomposition of the chlorine hydrate crystals
can be carried out in one reactor. However, it is preferable in
the practical process to separate the step of formation of the
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chlorine hydrate crystals and the step of decomposition of the
chlorine hydrate crystals.
In the latter process, it is especially effective to
use the transfer medium as described. That is, in the step of
formation of the chlorine hydrate crystals, the crude chlorine
gas is contacted with the caoled trans~er medium in the presence -
of water whereby most of chlorine in the crude chlorine gas is ;~
crystallized as the chlorine hydrate crystals. The other gases
in the crude chlorine gas are in gaseous state and are separated
from the chlorine hydrate crystals. The chlorine hydrate crystals
with the transfer medium as a slurry are fed to the decomposition
step of the chlorine hydrate crystals.
In the decomposition step of the chlorine hydrate
crystals, the pressure is kept in lower than the decomposition
pressure of the chlorine hydrate crystals, whereby the chlorine
hydrate crystals are decomposed to generate chlorine gas.
However, the transfer medium is cooled by the
endothermic decomposition reaction of the chlorine hydrate
crystals, and the cooled transfer medium is recycled to the
formation of the chlorine hydrate crystals wherein the transfer
medium is used for cooling the crude chlorine gas. Accordingly,
the cooling energy in the step of formation of the chlorine
hydrate crystals can be substantially decreased.
When the transfer medium is used, the transfer medium
imparts the heat transfer effect and it is used for transferring
the chlorine hydrate crystals and it is used to control the
pressure in the step of decomposition of the chlorine hydrate
crystals. Accordingly, efEective utiliza-tion of heat can be
. attained.
The following effects can be also expected by using
the transfer medium.
When the crude chlorine gas contains an ac-tive component
~869~
such as hydrogen which reacts with chlorine or oxygen, the
ratios of hydrogen and oxygen in the gas are increased relatively
by crystallizing chlorine gas as the chlorine hydrate cyrstals,
whereby the composition of the gas may become the expensive gas
range. However, when the crude chlorine gas is fed into the
cooled transfer medium in the presence o~ water, the explosion
can be prevented and effective operation can be attained. The
crude chlorine gas is dispersed into the cooled transfer medium
wherein the chlorine hydrate crystals are formed by reacting
chlorine with water. Thus, the content of hydrogen and oxygen
in the crude chlorine gas are increased with the conversion of
chlorine gas to the chlorine hydrate crystals whereby an explosive
gas may be formed. However, the crude chlorine gas is cooled
by the transfer medium so as to prevent the explosion. The crude --
chlorine gas is dispersed into the transfer medium whereby a
chain explosion can be prevented. The explosive gas is floated
through the transfer medium and is discharged. When it is
discharged, it can be diluted by feeding an inert gas such as
air, nitrogen gas and other inert gas whereby the explosion can
be prevented. In accordance with this process, it is unnecessary
to decrease the formation of the chlorine hydrate crystals so
as to change the composition of the residual gas out of the
explosive gas range.
As described, most of chlorine in the crude chlorine
gas is crystallized as the chlorine hydrate crystals and then
the chlorine hydrate crystals are decomposed to obtain chlorine
gas having high purity. When it is necessary to effectively
crystallize chlorine in the crude chlorine gas as the chlorine
hydrate crystals, it is preferable to carry out the step of form-
ation of the chlorine hydrate crystals by multi-steps. The
purpose can be at-tained by combining the first and second steps
of formation of the chlorine hydrate crystals.
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That is, the crude chlorine gas is countercurrently
contacted with the cooled transEer medium in the presence of
water under about the atmospheric pressure whereby most of
chlorine in the crude chlorine gas is crystallized to form the
chlorine hydrate crystals in the first crystallization step and
the residual chlorine gas and other gases, such as oxygen
and hydrogen are taken out from the first crystallization step
andthese gases are fed to the seconcl crystallization step. The
chlorine partial pressure in the gases, is low whereby the gases
are preferably slightly compressed. In the second crystallization
step,the partial pressures of the bther gases such as oxygen
and hydrogen beside chlorine in the crude chlorine gas are higher.
When the chlorine gas is crystallized as the chlorine hydrate
crystals, the explosive gases may be generated as described
above. Accordingly, the crude chlorine gas is fed into the
cooled transfer medium to form the chlorine hydrate crystals.
The chlorine hydrate crystals crystallized in the first
and second crystallization steps, are fed, with the thermal medium
in the Eorm of a slurry, to the decomposition step of the chlorine
- hydrate crystals.
Thus, the crude chlorine gas is cooled under about
atmospheric pressure to crystallize most of chlorine as
the chlorine hydrate crystals in the first crystallization step
and the residual chlorine is cooled under the pressure slightly
higher than 1 atm. in the second crystallization step, whereby
chlorine in the crude chlorine gas can be substantially
crystallized as the chlorine hydrate crystals in high efficiency.
The case using water as the transfer medium will be
illustrated.
The advantages of the use of water as the transfer
medium are as follows:
(1) Water can be the source of crystal water for forming
the chlorine hydrate crystals.
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(2) Water is less vaporizable and a large amount of water
will not be incorporated in the chlorine gas when the
chlorine hydrate crystals are decomposed.
(3) Water can be easily separated even though water is
incorporated in chlorine gas.
(4) Chlorine is not substantially absorbed by water.
(5) Oxygen and hydrogen are not substantially absorbed by
water.
The crude chlorine gas is contacted with a cold water
so as to crystallize chlorine in the crude chlorine gas as the
chlorine hydrate crystals. The other components of the crude
chlorlne gas are in the gaseous state and are separated. The -
chlorine hydrate crystals with water are removed as a slurry and
the slurry is fed to the decomposition step of the chlorine
hydrate crystals, wherein the pressure is lowered to lower than
the decomposition pressure of the chlorine hydrate crystals so
as to generate chlorine gas. The water is cooled by the endo-
thermic decomposition of the chlorine hydrate crystals and is
recycled to the step of formation of the chlorine hydrate crystals ,
so as to cool the crude chlorine gas. Thus, the chlorine gas
having high purity can be obtained. In -this case, the cold
- water can be advantageously maintained to the constant temperature
in the presence of an ice. Strictly speaking, a small amount of
chlorine gas in the crude chlorine gas is dissolved in water
whereby chlorine water is formed.
When water is used as the transfer medium, the step
of formation of chlorine hydrate crystals is a-t least two steps
and the pressure in the second crystallization step is higher than
~ the pressure in the first crystallization step, it is preferable
to lower the pressure applied to the slurry of the chlorine hydrat~
crystals discharged from the second crystalliza-tion step so as to
discharge -the dissolved gas before the slurry is fed to the step
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of decomposition of -the chlorine hydrate crystals because oxyyen
may be dissolved in water. In particular, the slurry of the
chlorlne hydrate crystals discharged from the second crystalliza-tion
step is returned to the first crystallization step and it is
fed to the decomposition step of the chlorine hydrate crystals.
The present invention will be further illustrated
by the following Examples in conjunction with the accompanying
drawings in which Figs. 1, 2 or 3 are flow diagrams of various
embodiments of the method of the present invention.
Example 1:
A crude chlorine gas (CQ2 : 97 vol.%; 2 : 2.1 vol.%;
H2 : 0.2 vol.%; CO2 : 0.6 voI.%) obtained by an asbestos
diaphragm type electrolysis of sodium chloride was fed into a
crystallization vessel containing 5.0 m3 of water at -0.3C (ice:
35 wt.~) at a volume of 139 Nm and it was kept in 1 atm. The
chlorine gas in the crude chlorine gas was crystalllzed and
precipitated as CQ2-6H2O. The gas discharged from the water in
the crystallization vessel and the residual water were discharged
from the vessel.
The pressure in the crystallization vessel was then
kept at 0.2 atm. so as to generate chlorine gas. The chlorine
gas was dried and analyzed to find the chlorine concentration
of 99.8% The recovery rate was 98.5%.
Example 2:
In the crystallization vessel (1) shown ln Figure 1,
water at -0.3C (ice: 35 wt.%) was fed a-t a rate of 3.2 m /hr.
A crude chlorine gas (3) (CQ2 : 97 vol.%; 2 : 2.1 vol.%; H2 : 0.2
vol.%; CO2 : 0.6 vol.~) obtained by an asbestos diaphragm type
electrolysis of sodium chloride was fed at a rate of i39 Nm3/hr.
and the pressure of the crystallization vessel (1) was kept in
1 atm. Most of chlorine gas in the crude chlorine gas was
crystallized and precipitated as the hydrate of CQ2-6H2O. Ox~gen
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gas, hydrogen gas and carbon dioxide gas (4) were discharged out
of the vessel. The chlorine hydrate crystals were then removed
as a slurry and was fed to a crystal decomposition vessel (2).
The pressure of the crystal decomposition vessel (2)
was kept in 0.2 at~. to generate chlorine gas. Water was cooled
by the endothermic reaction of the chlorine hydrate crystals and
water was recycled to the crystallization vessel by a pump. Thus,
in the recycle of water, a small amount of chlorine was dissolved
in water. The chlorine gas (5) generated in the crystal decomposi-
tion vessel was dried and analyzed to find the chlorine concentra~
tion of 99.8~. The recovery rate was 98%.
Example 3:
In the first crystallization vessel (1) shown in Figure
2, water at -0.3C (ice : 35 wt.%) was fed at a rate of 2.4 m3/hr. '~
A crude chlorine gas (3) (CQ2 : 97 vol.%; 2 : 2.1 vol.%; H2 : 0.2
vol.%; CO2 : 0.6 vol.%) obtained by an asbestos diaphragm type
electrolysis of sodium chloride was fed at a rate of 139 Nm3/hr.
and the pressure of the first crystallization vessel (l) was kept
in l atm. The chlorine gas was crystallized and precipitated as
the hydrate of CQ2-6H2O. The gases (CQ2 : 35 vol.%; 2 : 48 vol.%;
H2 : 4 vol.%; CO2 : 13 vol.%) were discharged from the first
crystallization vessel (l) and they were compressed to about '
1.9 atm, and fed ,to the second crystallization vessel ~l').
In the second crystallization vessel, the water at -0.3C
(ice : 35 wt.%) was fed at a rate of 0.8 m3/hr. and the pressure
of the vessel was kept in 1.7 atm. The gases were fed into the
water in the second crystallization vessel from many distributed
inlets. In the second crystallization vessel, the chlorine hydrate
crystals CQ2-6H2O was crystallized. The slurry of the crystals
was fed to the first crystallization vessel. Air was fed to dilute
the gas discharge from the water in the second crystallization
vessel and the diluted gas (4') was discharged out of the vessel.
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The chlorlne hydrate crystals collected inthe first crystallization
vessel were fedto the crystal decomposition vessel (2)as the
slurry.
The pressure of the crystal decomposition vessel was
kep-t at 0.2 atm. so as to generate chlorine gas. Water was cooled
by the endothermic decomposition reaction of the chlorine hydrate
crystals and water was recycled to the first and second crystalliza-
tion vessels (1), (l') by a pump. A small amount of chlorine was
dissolved in the recycled water. The chlorine gas (5) generated
in the crystal decomposition vessel was dried and analyzed to
find the chlorine concentration of 99.8%. The recovery rate was
99.5%.
E ample 4
In the crystallization vessel (1) shown in Figure 3, water
at 4C was fed at a rate of 2.4 m3/hr. A crude chlorine gas (3)
(CQ2 : 97 vol.~; 2 : 2.1 vol.%; H2 : 0.2 vol.%; CO2 : 0.6 vol.%)
obtained by an asbestos diaphragm type electrolysis of sodium
chloride was fed at a rate of 139 Nm3/hr., and the gas was counter-
currently contacted with water under the pressure of 1.5 atm. of
20- - the crystallization vessel. Most of chlorine in the crude chlorine
gas was crystallized to precipitate the chlorine hydrate of CQ2-6H2O.
Oxygen hydrogen and carbon dioxide gases (4") were discharged out
of the vessel. The chlorine hydrate crystals were then removed
as a alurry and the slurry was fed to an aeration vessel (6) kept
in 1 atm. wherein the aeration was carried out with the crude
chlorine gas. From the vessel oxygen etc. dissolved in water
was removed.
Thus, the chlorine hydrate crystals were taken out from
.the vessel as a slurry with water, and the slurry was fed to a
crystal decomposition vessel (2). The pressure of the crystal
decomposition vessel was kept in 0.4 atm. so as to generate chlorine
gas. Water was cooled by the endothermic reaction of the chlorine
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hydrate crystals and was recycled to the crystallization vessel
by a pump. A small amount of chlorine was dissolved in the
recycled water. The chlorine gas (5) generated in the crystal
decomposition vessel was dried and analyzed to find the chlorine
concentration of 99.8%. The recovery rate was 99~.
Example 5: -
In accordance with the process of Example 2 a crude
chlorine gas (CQ,2 : 97-5 vol.%; 2 : 2.2 vol.~; CO2 : 0.3 vol.~)
which was obtained by the ion-exchange membrane type electrolysis
of sodium chloride was purified. As the result, the chlorine
concentration was 99.9% and the recovery rate was 98.5~.
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