CA1283771C - Manufacturing process of chlorine - Google Patents

Abstract

ABSTRACT
In the process of producing chlorine by oxidizing hydrogen chloride with molecular oxygen, activity of the catalyst can be maintained for a long period with a high conversion ratio under high space velocity of gaseous raw materials without using additives as conventional methods, in the presence of a crystalline chromic oxide catalyst obtained by support-ing relatively large amount of chromic oxide on a silicon oxide carrier having a specified pore volume.

Description

l'Z83771 SPECIFICATION

TITLE OF THE INVENTION
MANUFACTURING PROCESS OF CHLORINE
BACKGROUND OF THE INVENTION
a) Field of the Invention: -This invention relates to a manufacturingprocess of chlorine, and more particularly to an improvement for the process of producing chlorine by oxidizing hydrogen chloride gas with an oxygen contain-ing gas.

b) Description of the Prior Art:
Chlorine is produced in large amounts by the electrolysis of sodium chloride. The electrolysis supplies a chlorine demand which has recently been in a sharp increase. The electrolysis of sodium chloride, however, co-produce sodium hydroxide which has a smaller demand than that of chlorine. Therefore it is difficult to successfully adjust the supply-demand imbalance, though the demand of chlorine and sodium chloride is tried to be met by the electrolysis of sodium chloride.
On the other hand, a great quantity of hydrogen chloride is produced as a by-product in the lX83~771 chlorination or phosgenation reaction of organic com-pounds. The amount of by-produced hydrogen chloride is far greater than the market demand of hydrochloric acid. Thus a large quantity of hydrogen chloride is discarded without utilization and moreover considerable expenses are required for its disposal.
Therefore, if chlorine can be efficiently recovered from by-produced hydrogen chloride which is abandoned as aforesaid in a large amount, the supply-demand balance of chlorine and sodium hydroxide willeasily be able to be maintained by the combination of sodium chloride electrolysis and oxidation of the by-produced hydrogen chloride.
The reaction of oxidizing hydrogen chloride to produce chlorine has been known as Deacon reaction for many years. In the reaction of preparing chlorine by oxidizing hydrogen chloride, the so-called "Deacon catalyst" which is a copper-base catalyst invented in 1868 has conventionally been considered to exhibit the highest activity. Since then, it has been proposed a number of catalysts which are added with various compounds as a third component to copper chloride and potassium chloride. The reaction tempera-tures, however, are required to be 450C and above in order to oxidize hydrogen chloride at a practical applicable reaction rate by use of these catalysts.

lZ83~

The high temperature causes problems such as reduction of catalyst life accompanied by the vaporizing of catalyst ingredients. The use of catalysts other than copper-base including, for example, iron-base etc.
have been proposed in order to eliminate these problems, and yet no catalysts have been known to exhibit satisfactory performance for practical application. For example, chromic-oxide-base catalysts have been suggested to be generally superior in high-temperature stability and lifetime to the copper-base catalysts. No results, however, have been reported to show enough activity. That is, U.K. Patent No.
584,790 discloses an intermittent process. In this process, hydrogen chloride is introduced to form chlorine at about 400C on the catalyst obtained by impregnating a suitable carrier with an aqueous solu-tion of chromic acid or chromium nitrate and subjecting the impregnated carrier to thermal decomposition.
When the catalyst is inactivated, the feed of hydrogen chloride is stopped and the catalyst is regenerated by introducing the air. Then the air flow is stopped and the feed of hydrogen chloride is started again to prepare chlorine.
Besides U.K. Patent No. 676,667 discloses another process using a carrier-supported catalyst prepared from bichromate or dark-green chromic oxide, ~Z83771 that is, unglowed chromia. In this process, hydrogen chloride and the oxygen containing gas are fed to react at a reaction temperature of 420 to 430C.
Chlorine is obtained with a conversion ratio of 67.4 of the equilibrium value at a space velocity of 380 Hr 1 and 63~ at 680 Hr 1, When the reaction tempera-ture is lowered to 340C, the reaction can also proceed and yet the conversion ratio obtained is only 52% even by maintaining the space velocity at a low level such as 65 Hr 1, The prior art further discloses that chromia can not necessarily be active as the oxidizing catalyst of hydrogen chloride. That is, amorphous chromia is active for the oxidation of hydrogen chloride. It is required to carry out the heat-treatment of chromic anhydride at a temperature of 400C or less in order to prepare the amorphous chromia catalyst. Chromia is clearly illustrated to crystallize by heating above 500C and to loose its catalitic activity for the hydrogen chloride oxidation.
Furthermore, the chromia catalyst has a short life in the oxidation of hydrogen chloride and cannot be employed for industrial operation. As a means of overcoming the drawback, U.K. Patent No.
846,852 (corresponding to U.S. Patent No. 3,006,732) discloses that the catalyst life can be extended by incorporating a small amount of chromyl chloride l~a37ql (CrO2C12) in the raw material. As shown above, the intact chromia catalyst cannot be employed in continuous operation for a long period as a result of its short life. In addition, the patent also describes on the chromia catalyst that high activity is exhibited by the amorphous chromia prepared by calcining ammonium bichromate or chromic anhydride at a temperature of 500C and less, preferably 350 to 400C.
As aforementioned, the conventionally known processes could not improve the drawbacks of short catalyst life, high reaction temperature and low space velocity, even though chromic oxide is used as the catalyst. Therefore these processes could not withstand the industrial operation unless new reacting agents, such as chromyl chloride were incorporated.
That is, the traditional chromic oxide catalyst did not specifically exhibit excellent properties as compared with the copper-base catalyst.

SUMMARY OF THE INVENTION
In view of the aforesaid problems, the present inventors have investigated for providing a catalyst which has a high activity and a long lifetime and also efficiently accelerates the oxidizing reaction of hydrogen chloride with molecular oxygen at relatively lower temperatures. Consequently, it has been found lZ8377~

that such catalyst can be obtained by selecting and controlling the silicon oxide carrier to be used, the amount of catalyst to be supported, and the tempera-ture of heat-treatment to be conducted. And thus the present invention has been achieved.
That is, a production process of chlorine in this invention by the oxidation of hydrogen chloride with molecular oxygen is characterized by oxidizing in the presence of a chromic oxide catalyst obtained by supporting chromic oxide in an amount of 20 to 90 wt.% as chromia on a silicon oxide carrier having a pore volume in the range of 0.3 to 1.8 cc/g and conducting a heat-treatment at a temperature of 450 to 700C.
The chromic oxide catalyst for use in the present invention is particularly preferable for the employment on the oxidizing reaction at a relatively low temperature and a high space velocity of gaseous raw materials. The oxidizing reaction is carried out in such conditions that the reaction temperature is 350 to 430C, molar ratio of the molecular oxygen in an oxygen containing gas to the hydrogen chloride is 0.25 to 5 feed amount of the hydrogen chloride is 200 to 1800 Nl/kg catalyst per hour. The fluidized bed reaction is suitable for present invention.
The chromic oxide catalyst used in the present ~Z83'77~

invention is preferably prepared by contacting an aqueous solution of chromic anhydride or a chromium salt with a silicon oxide carrier and successively subjecting the resultant carrier to drying and heat-treatment.
The silicon oxide carrier for use in the catalyst has preferably a surface area of 100 to 600 m2/g, an average pore diameter of 20 to 300 A, and a sodium content and an iron content of 0.5 wt.% or less respectively. Furthermore, the chromic oxide catalyst thus obtained has preferably a microcrystal size of 200 to 500 A.

BEST MODE OF THE INVENTION
The carrier of the catalyst for use in the process of this invention is silicon oxide and usually applied in the form of amorphous silica gel.
Silica may also be used in the presence of a small amount of alumina, titania, zirconia, etc.
Silica gel as a preferred carrier has generally a pore volume ranging 0.3 to 1.8 cc/g and more preferably ranging 0.5 to 1.5 cc/g, a surface area in the range of 100 to 600 m2/g, and an average pore diameter of 20 to 300 A range. In these cases, when the pore volume is less than 0.3 cc/g, chromic oxids covers the surface of silicon oxide and is liable 1~8377~

to release from the surface. On the other hand, silica gel having the pore volume of 1.8 cc/g and more is difficult to prepare and even if it can be prepared, the catalyst cannot be applied because of its low mechanical strength.
Silicone oxide usually contains sodium, iron etc. as contaminants. The catalysts for use in this invention employs silicon oxide having a sodium content of 0.5 wt.% or less and an iron content of 0.5 wt.% or less, more preferably 0.1 wt.% or less.
When the content of above contaminants exceeds 0.5 wt.%, all of the resulting catalysts have low activity.
The silicon oxide carrier employed in this invention is preferably heat-treated at a temperature of 300 to 500C for 1 to 10 hours generally prior to supporting chromia.
As the raw materials of chromic oxide used in the process of this invention, normally employed are the aqueous solution of chromic anhydride (CrO
or chromium salts. The chromium salts which may be used include, for example, inorganic chromium salts such as chromic sulfate, chromic nitrate, chromic chloride etc., and chromium salts of organic acids such as chromic acetate etc. which provide chromia (Cr2O3) by thermal decomposition. Normally, it is most convenient to dip the carrier into the aqueous solution lZ83~7~

g containing these compounds. The carrier is then separated and dried to prepare the catalyst. The catalyst can be prepared with a low cost by this method.
The supported amount of chromia on the carrier is relatively in a high level and in the range of 20 to 90 wt.%. For example, it is supported on the carrier by 1 to 3 impregnating operations with the aqueous solution of chromic anhydride having a concentration of 20 to 60 wt.%. When the supported amount is 60 wt.%
and more in particular, dipping procedures are preferably performed at least twice for supporting the catalyst. At the first operation, impregnation and supporting are applied in a quantity corresponding to 1/3 - 1/2 of the desired amount. After drying the impregnated carrier, the operation for impregnating the aqueous solution of chromic anhydride and drying the carrier is preferably repeated again to obtain the prescribed amount of supported chromia.
After supporting the prescribed amount of chromia, the heat-treatment is conducted normally for about 1 to 10 hours at a temperature of 450 to 700C, preferably 490 to 700C, more prèferably 500 to 700C, and exceeding 500C up to 700C under certain conditions.
When the heat-treatment temperature is lower than 450C, the catalyst has an amorphous form and a short ~ 83~

lifetime. In other words, the catalyst exhibits high initial activity and yet the activity drops rapidly. Furthermore, the ingredients of the catalyst are liable to vaporize. On the other hand, when the heat-treatment is carried out at a temperature of 700C and more, crystals are grown by the heat-treatment.
As a result, the size of microcrystals becomes too large, which leads to the decrease of catalyst activity.
In addition, when the catalyst has a supported amount of chromia of less than 20 wt.~ after the heat-treatment, the activity of catalyst is low, the quantity of hydrogen chloride converted is small and moreover the catalyst lifetime is short. Accordingly it is necessary to support in an amount of 20 wt.% and more, preferably 40 wt.% and more, which results in a long lifetime of the catalyst. On the other hand, even though the supported amount of chromia exceeds 90 wt.%, further improvement of the activity and lifetime can not be expected and the mechanical strength of the catalyst is reduced. Therefore the catalyst can not be applied in a supported amount of exceeding 90 wt.%.
Since the amount of attrition loss is particularly increased in employing for the catalyst of fluidized bed process, the supported amount is preferably about 80% or less and more preferably maintained at about 60 to 70 wt.%.

83~

The chromic oxide catalyst heat-treated in the above-mentioned temperature range is crystals.
The microcrystals have a size of normally ranging 200 to 500 A and are not amorphous.
In the process of this invention, the reaction temperature for oxidizing hydrogen chloride with molecular oxygen is preferably maintained in the range of 350 to 430C and particularly suitable in the range of 370 to 420C. The higher reaction temperature leads to the higher conversion velocity of hydrogen chloride to chlorine, whereas it accompanies lowering of the equilibrium conversion ratio and increasing the vaporization of the catalyst. Accordingly, above-mentioned temperature range is selected in the reaction.
Besides, the oxidizing agent which may be used in the process of this invention is molecular oxygen, and oxygen gas or the air is usually employed.
The molar ratio of molecular oxygen in an oxygen containing gas to the hydrogen chloride is in the range of preferably 0.25 to 5, and more preferably 0.3 to 0.75. The amount of oxygen less than the stoichiometric amount of 0.25 results in a low conversion ratio. On the other hand, excess of oxygen in the molar ratio is economically unfavorable.
The amount of hydrogen chloride fed to the catalyst bed is suitably in the range of 200 to 1800 Nl/kg ~.~8377'1 catalyst, preferably 300 to 1000 Nl/kg catalyst per hour. sesides the reactors which may be used are fixed bed or fluidized bed, and the fluidized bed reactors are particularly suitable for the practice of the process of this invention.
According to the conventional information as stated above, chromia which exhibits catalytic activity for the oxidation of hydrogen chloride is in the amorphous form obtained by the heat-treatment at a temperature of 500C and less, preferably 350 to 400C. Chromia treated at high temperatures of above 500C has been considered to crystallize and eliminate the catalytic activity.
According to the process of this invention, however, the chromic oxide catalyst can be used in the conditions of high space velocity of gaseous raw materials and high conversion ratio. In addition, the catalyst can maintain its activity for a long period without feeding the raw materials of the reac-tion together with the additives such as described inU.K. Patent No. 846,852.
That is, the present invention provides a technically advantageous process which can produce chlorine from hydrogen chloride stably and efficiently for a long period.
The process of this invention will hereinafter ~.~8~

described in detail with respect to the following examples and comparative examples.
Besides the surface area, average pore diameter and pore volume of the silicon oxide carrier were measured by the nitrogen gas adsorption method, that is, according to the method for the determination of nitrogen gas adsorption at a temperature of liquefied nitrogen [Refer to CLYDE ORR Jr. and J.M. DALLVALLE, "Fine Particle Measurement", The Macmillan Co., NY.
(1959)].

Example 1 Silica gel microspheres having a diameter of 80 to 250 meshes (pore volume 0.75 cc/g) were dipped into a 20 wt.% aqueous solution of chromic anhydride, filtered and excess solution was removed. After drying at 120C, the resultant spheres were heat-treated in air at 350 to 400C for 2 hours. The above procedures were repeated 3 times and finally heat-treatment was carried out at 500C for 3 hours to prepare the catalyst. The catalyst thus obtained exhibited crystalline structure according to the X-ray diffraction analysis. The size of microcrystals calculated from the broadening of diffraction pattern was 276 A in face 104 and 280 A in face 110. Analytical values of the catalyst were 48 wt.~ chromia and 52 wt.% silica.

1~8377~

In a fluidized bed reactor having an internal diameter of 4 inches was charged with 2 kg of the catalyst obtained above. Hydrogen chloride gas and oxygen gas were fed with a velocity of 800 Nl/hr and 400 Nl/hr respectively, and reacted at a temperature of 400C. The resulting gas was absorbed into an aqueous solution of potassium iodide. The absorbed solution was titrated with sodium thiosulfate to obtain the conversion ratio of 78%. The reaction was conducted continuously. The conversion ratio of hydrogen chloride obtained was 73% after 10 days, 70% after 30 days, and the latter value was maintained thereafter for a long period.

Example 2 Silica gel microspheres having a diameter of 20 to 150 ~m (pore volume 1.28 cc/g) were dipped into a 50 wt.% aqueous solution of chromic anhydride, filtered and excess solution was removed and dried at 120C. The above procedures were repeated 3 times and finally heat-treatment was carried out at 500C
for 3 hours to prepare the catalyst. The catalyst thus obtained exhibited crystalline structure according to the X-ray diffraction analysis. The size of micro-crystals calculated from the broadening of diffraction O O
pattern was 280 A in face 104 and 290 A in face 110.

l~a3~

Analytical values of the catalyst were 68 wt.%chromia and 32 wt.% silica.
In the same fluidized bed reactor as in Example 1 was charged with 2 kg of the catalyst obtained above. Hydrogen chloride gas and oxygen gas were reacted by use of the same feeding conditions and reaction temperature as in Example 1. The resulting gas was absorbed into an aqueous solution of potassium iodide. The absorbed solution was titrated with sodium thiosulfate to obtain the conversion ratio of 80%.
The reaction was conducted continuously. The conversion ratio of hydrogen chloride was 74% after 10 days and 72% after 30 days, and the latter value was maintained thereafter for a long period.

Examples 3 - 9, Comparative examples 1 - 2 Catalysts were prepared by the same procedure as in Example 2 so as to have various supported amounts of chromia. The reaction in Example 1 was repeated by using the catalysts thus obtained. The results are illustrated in Table 1.

~,~83771 _ _ NLll __ S~l~ ~ 11~ ~
~ X ~O I~ O
E~ ~ -1 ~r t~
~) ~:1 1/~ N N ~D
_ r~ O N N

2'~ h ~d d ~-- ~ h ~ h ,~ ~.~ ~ ~ ~ O
.~ a~ ~ ~ _ ~ c~
~ ~ O ~ O d~ ~ m u, U~ h h 11~ t) ,1-- h td O ~ r~ O ~ Q) O ~ ~ rl ~
~1 ~ ) ~ ~ >~ ~
~m ~ o ~ o V~ O H-- h ~ ra O

lZ837 7~

Examples 10 - 14, Comparative examples 3 - 4 Carriers were changed in the procedures of Example 1 and catalysts were prepared to have a supported amount of 38 to 43 wt.% and 55 to 60 wt.% as chromia.
The activity of the catalysts obtained was measured by the same method as in Example 1. Results are illustrated in Table 2.

0 ~ ~r ~ o d' O
~o ~, ~ coc~ ~OD 1n ~1 r~l ~ r~l ~
,_i ~ O

~ X _l r ~, ~ o ,_ E~ ~3 ,~ o ~ d' ~
,o~ u~ a~ lo~ ~D
. o ~ O dP ~0, ~0 .

rl ~ U Q) O ~,_ ~ ,1_~
~ ~ ~ ~ ~ ~ ~ 8 ~oQ ~ ~ ~
In a~ u, ~ ~ O ~ ,~ ~1 ,~ ~ > ,1 ,~ ~ o ~ .,, o~ ~ ~ ~ Uo o 1~ 0 U~ f~ U H-- 51 ~¢--I U S~

~'~8377~

Examples 15 - 17, Comparative examples 5 - 6 Silica gel having a pore volume of 1.2 cc/g was dipped into the aqueous solution of chromic anhydride to prepare catalyst having a supported amount of 70 wt.% as chromia. Heat-treatment was conducted independently at 400, 500, 600, or 700C
for each part of the catalyst. Activity of the heat-treated catalysts was measured by the same method as in Example l. The results are illustrated in Table 3.

Table 3 Example Comparative example Classification temperature (C) 500 600 700 400 800 Initial activity (Hcl conversion 79 76 73 78 64 ratio %) Activity after 10 days (HCl74 73 70 37 60 conversion ratio %) Comparative example 7 A catalyst was prepared by repeating the procedure of Example l except the final heat-treatmet was conducted at a temperature of 410C. X-ray dif-fraction result of the resultant catalyst exhibited ~ ,~83 77~

ZO --no crystallinity and the catalyst was amorphous.
The same reaction as in Example 1 was carried out by using above obtained catalyst.
The hydrogen chloride conversion ratio was 76~ after 10 hours from the start of reaction, 62%
after 2 days, and 49% after 5 days. Besides evaporation of chromium from the catalyst was found during the reaction and chromium compounds were deposited around the outlet of the reactor.

Claims (8)

1) A manufacturing process of chlorine by the oxidation of hydrogen chloride with molecular oxygen which comprises oxidizing in the presence of a chromic oxide catalyst obtained by supporting chromic oxide in an amount of 20 to 90 wt.% as chromia on a silicon oxide carrier having a pore volume in the range of 0.3 to 1.8 cc/g and conducting a heat-treatment at a temperature of 450 to 700°C.

2) The manufacturing process of chlorine as claimed in claim 1 wherein the chromic oxide catalyst is obtained by contacting an aqueous solution of chromic anhydride or a chromium salt with a silicon oxide carrier and successively subjecting the resultant carrier to drying and heat-treatment.

3) The manufacturing process of chlorine as claimed in claim 1 wherein temperature of the oxidation is 350 to 430°C, the molar ratio of the molecular oxygen in an oxygen containing gas to the hydrogen chloride is 0.25 to 5,feed amount of the hydrogen chloride is 200 to 1800 Nl/kg catalyst per hour, and reaction type is a fluidized bed reaction.

4) The manufacturing process of chlorine as claimed in claim 1 wherein surface area of the silicon oxide carrier is 100 to 600 m2/g.

5) The manufacturing process of chlorine as claimed in claim 1 wherein average pore diameter of the silicon oxide carrier is 20 to 300 .ANG..

6) The manufacturing process of chlorine as claimed in claim 1 wherein the silicon oxide carrier contains 0.5 wt.% or less of sodium and 0.5 wt.% or less of iron.

7) The manufacturing process of chlorine as claimed in claim 1 wherein the silicone oxide carrier is subjected to a heat-treatment at a temperature of 300 to 500°C prior to supporting the chromia.

8) The manufacturing process of chlorine as claimed in claim 1 wherein a microcrystal size of the chromic oxide catalyst is 200 to 500 .ANG..