CS276386B6 - Process for removing chlorine from a gaseous mixture - Google Patents

Abstract

PURPOSE:To absorb and remove Cl2 alone, by washing gas contg. Cl2 and CO3 with an aq. mixed soln. of alkali(earth) metal hydroxide and alkali(earth) metal sulfite while regulating the pH value of the aq. mixed soln. to a specified range. CONSTITUTION:The Cl2 alone in the gas contg. Cl2 and CO2 is removed, by using the aq. soln. or suspension contg. alkali(earth) metal sulfite (e.g., Na2SO3) and alkali(earth) metal hydroxide (e.g., NaOH) in <=2times molar ratio to the sulfite, as washing liq., as occasion demands, while supplying the liq. having the compsn. same to one of the aq. soln. or suspension initially charged, so as to regulate the pH value of the washing liq. to 1.9-6.3.

Description

The invention relates to a process for removing chlorine from a gas mixture.

A large number of processes have already been proposed for isolating chlorine from a chlorine-containing gas mixture. Examples of these procedures are described in particular in the following publications:

1) U.S. Pat. No. 3,972,691 discloses a process for isolating liquid chlorine from a gas mixture containing 20 to 90% by volume of chlorine, 10 to 80% by volume of carbon dioxide, nitrogen, oxygen and carbon monoxide. In this process, the gas mixture is compressed to 0.4 to 0.8 MPa, then cooled and liquefied in a completely reflux-type rectification column, adjusting the temperature of the liquid chlorine collected at the bottom of the rectification column to evaporate the carbon dioxide dissolved in liquid chlorine.

2) U.S. Pat. No. 2,199,797 discloses a process in which organic impurities that are present in chlorine gas at a concentration of at most about 1 weight percent can be removed from chlorine gas. The process is carried out by bringing said chlorine gas containing said impurities into countercurrent contact with liquid chlorine in the scrubbing column, the organic impurities being absorbed in liquid chlorine.

3) GB 938 073 describes a process for separating a non-condensable impurity having a lower boiling point than chlorine and which forms an explosive deer with chlorine from a gas mixture containing chlorine and said non-condensable impurity. The process is carried out by gradually reducing the temperature of the chlorine gas and the non-condensable impurities until finally the final gas residue is brought into countercurrent contact with liquid chlorine whose temperature has been reduced to a condensation temperature of chlorine gas or even lower in a liquefaction column whereby the liquefied chlorine gas is liquefied while the non-condensable impurity remains in the gaseous phase since it has a substantially lower boiling point.

4) U.S. Pat. No. 3,443,902 discloses a method of compressing chlorine gas obtained by contacting a chlorine gas containing an impurity with countercurrent contact with liquid chlorine in a scrubbing column to remove said impurity by absorption in liquid chlorine, wherein a portion of the chlorine gas liquefies in as a result of its heat exchange with liquid chlorine in the scrubbing column and the liquefied portion of the originally chlorine gas is used for the same purpose as the primary liquid chlorine in the role of an absorption agent.

(5) GB 1 164 069 describes a process in which a gas mixture consisting of a non-condensable gas, including nitrogen and chlorine, can be separated into liquid chlorine and a non-condensable gas by compressing said gas mixture to a pressure of 0.6 to 1.0 MPa, this compression being carried out in two steps, and then further cooling the cooled gas mixture by heat exchange to a temperature of -120 to -15 ° C.

6) U.S. Pat. No. 2,540,905 discloses a process for isolating carbon dioxide-free chlorine. In this process, chlorine is absorbed from the residual liquefaction gas obtained after the electrolysis of the brine and containing 5 to 10 weight percent chlorine together with carbon dioxide, carbon monoxide, hydrogen, nitrogen, oxygen and other gaseous components, in a chlorinated solvent. Thereafter, at a temperature that is higher than the temperature of the absorption in the chlorinated solvent, carbon dioxide, which has also been absorbed in the chlorinated solvent, is evaporated.

This is achieved, for example, by heating the bottom of the absorption column.

7) U.S. Pat. No. 2,765,873 discloses a process for isolating chlorine in a form substantially free of non-condensable gas. The process is carried out by absorbing a gaseous mixture of 30 to 50 percent by weight of chlorine and air

CS 276386 BS in a solvent at a pressure of 0.2 to 1.43 MPa at a column head temperature of -22.8 to 32.2 ° C and a column bottom temperature of 27.8 to 52.8 ° C higher than temperature at the top of the column.

8) Patent DB 2 413 358 discloses a method for absorbing 3-mass chlorine from a mixture of chlorine and carbon dioxide in the form of an alkali metal and / or alkaline earth hypochlorite using a multi-stage countercurrent or absorption device, using alkali metal hydroxide and / or and the pH of the liquid side of the last stage of the counter-flow absorber is about 7.5.

The methods disclosed in publications 1) to 4) are used in cases where chlorine or chlorine and carbon dioxide as the only one or more condensable components are present at relatively high concentrations. In the case of the isolation of chlorine from a chlorine-containing gas mixture containing about 50% by volume or possibly more non-condensable gas, these processes are associated with certain problems resulting from the presence of a large amount of non-condensable gas.

All these processes utilize countercurrent contact of the descending liquid chlorine and the ascending raw gaseous chlorine in the column, although the individual columns used in these methods differ from each other (liquefaction, shut-off or distillation columns).

If the rising gaseous chlorine contains a high concentration of non-condensable components, then it is virtually impossible to prevent the entrainment of a certain portion by the rising gas and to reduce the efficiency of the gas-liquid contact due to the channel effect of the decreasing liquid or the like.

If, in order to maintain the efficiency of the separation process, the amount of descending liquid is increased, then the cooling and heating loads of the distillation column are also increased, and therefore larger dimensions of individual parts of the distillation column, such as column body, condenser and digester, must be used. This solution is therefore not economical.

In the case of method 4), the circulating amount of non-condensable gas becomes enormous and the dew point of the compressed gas decreases. If the compression pressure is low, the problem is that chlorine cannot liquefy after its heat exchange with the scrubbing column. Even if the liquefaction of chlorine occurs, there is an increase in energy costs due to the increased compression ratio of the compressor and an increase in the circulating amount, thereby losing the benefit of this procedure.

Methods 1) and 5) are also intended mainly for high chlorine gas concentrations. The chlorine-containing gas mixture to be treated is compressed and cooled to separate the chlorine by liquefying it. However, these processes are intended for the isolation of high purity chlorine. The residual chlorine-containing off-gas, consisting essentially of a non-condensable gas, therefore contains chlorine at a concentration of 5-9% by volume in the case of process 1) or even at a concentration of even 10% by volume in the case of process 5).

The industrial isolation of chlorine causes problems with atmospheric pollution due to discharge into the atmosphere of waste gases containing chlorine at the relatively high concentrations mentioned above. It is therefore necessary to remove such waste gases from their chlorine content before they are discharged into the atmosphere. However, this requires the use of at least one chemical in tremendous amounts, notwithstanding the need for equipment necessary for such a chlorine-containing waste gas purification. In addition, this cleaning results in loss of chlorine, or? the chlorine contained in the off-gas is converted into its chemical compound during this chemical treatment. These methods are therefore not economical.

In order to reduce the chlorine content of the waste gas to such an extent that the chlorine content can be neglected, it is necessary to further reduce the compression ratio and the compression pressure and temporarily reduce the cooling and liquefaction temperature even further. However, this leads to increased energy costs as well as increased costs associated with cooling. In addition, it is not advantageous, in view of the reliability of the apparatus used, to compress the chlorine-containing gas mixture to high pressure. In addition, it is not possible to lower the cooling and liquefaction temperature down to the freezing point of carbon dioxide (-56.6 ° C at 0.52 MPa) or even to a temperature even lower, or? Doing so would block and block the device with frozen dry ice (solid carbon dioxide). Therefore, such liquefaction methods cannot prevent the waste gas leaving these processes from yet containing chlorine.

Both methods 6) and 7) use solvents to which the impurity is also absorbed, which is then desorbed to isolate the chlorine. To this end, in the process 6), the lower part of the absorption column is heated, whereby a part of the chlorine and a major part of the carbon dioxide absorbed in said solvent is evaporated so that the chlorine gas obtained in the evaporation column can be of higher purity.

Accordingly, the exhaust gas drawn in the top of the absorption column does not contain a certain amount of chlorine. Especially when the carbon dioxide is present in a high concentration, it is necessary to intensify the heating of the lower part of the absorption column in proportion to the content of said carbon dioxide. As a result, the concentration of chlorine in the off-gas also increases, leading to significant losses of chlorine.

Method 7) requires that the evaporation be carried out at a substantially higher pressure in the evaporation column, since the chlorine released during evaporation from the solvent is isolated by liquefaction. If the solvent absorbs more air than necessary in the absorption column, chlorine of reduced purity is isolated. It is therefore necessary to reduce the amount of absorption solvent. As a result, if the carbon dioxide and the non-condensable gas are contained in a high concentration, a sufficient degree of chlorine absorption cannot be achieved, while the proportion of chlorine and the solvent accompanying the off-gas increases sharply.

In order to reduce the chlorine concentration in the waste gas of the solvent absorption method, the cell may be increased in solvent, or the temperature of the solvent may be reduced, or the pressure in the absorption column further increased, but in this case it is difficult to isolate high purity chlorine; absorption of carbon dioxide and non-condensable gas is promoted.

Chlorine is an important raw material, which is industrially produced in large quantities, primarily by electrolysis of brine, and which is widely used. During this chlorine production, by-products are formed in the form of gaseous components. These by-products are always accompanied by a certain proportion of chlorine and thus cannot be discharged into the atmosphere, given the toxicity of the chlorine, without pretreatment to remove their chlorine content.

The chlorine contained in such gases is generally removed from these gases by absorption of chlorine in the alkali. However, if another gas is present in addition to chlorine, and if the other gas is acidic (such as gaseous carbon dioxide), then the alkali is absorbed by the chlorine but also by the acid gas, which is mostly represented by the oxide. carbon dioxide. Consequently, it is necessary to use alkali in an amount corresponding to the total amount necessary for the absorption of chlorine and carbon dioxide.

The disadvantage of this process is further emphasized in cases where the gas mixture contains a high concentration of carbon dioxide and a trace amount of chlorine, since a large amount of alkali is required to remove the trace amount of chlorine. It is therefore desirable to find a method for selectively absorbing and removing chlorine from a mixture containing chlorine and carbon dioxide.

Process 8) has already been proposed as a process for removing chlorine from a mixture of chlorine and carbon dioxide. However, in this method a pH of 7.5 is used, which is well above the value of 6.35, which represents the first dissociation constant (pKa) of carbonic acid, as can be seen from Figure 4. 5 falls within the range in which carbon dioxide reacts with an alkali metal and / or alkaline earth metal hydroxide to form the corresponding bicarbonate.

In order to absorb only chlorine with a mixture of carbon dioxide and chlorine within the above range, it is necessary to use alkali in an amount that is exactly equimolar and the amount of chlorine present. However, in cases where there is a constant change in the chlorine concentration in the carbon dioxide / chlorine mixture, it is very difficult to continuously charge the alkali in an amount still corresponding to the instantaneous chlorine content of the split gas mixture.

In addition, the pH of 7.5 lies in the immediate vicinity of the dissociation constant of chromic acid. At a pH lower than this value, hypochlorous acid becomes a free acid state in which it is susceptible to decomposition. In this case, precise pH control is therefore required. Even if the removal of chlorine has been achieved, the resulting hypochlorite solution is obtained, which has a strong oxidizing effect and aggressive inflammation, so that it cannot be exported in this form as waste and stored. Therefore, an additional step is required in which the reduction of this product is carried out.

The above-mentioned drawbacks have no method of removing chlorine from a gas mixture containing chlorine gas and carbon dioxide gas according to the invention, which comprises washing the gas mixture with a solution or suspension comprising an alkali metal sulphite and / or an alkaline earth metal sulphite, regulates the pH of the solution or suspension in the range of 1.9 to 6.3, thereby removing only chlorine from the gas mixture.

Preferably, the gas mixture comprising said chlorine and carbon dioxide is scrubbed with an aqueous solution or suspension comprising an alkali metal sulfite and / or an alkaline earth metal sulfite together with an alkali metal and / or alkaline earth metal hydroxide in an amount not exceeding twice the molar amount of alkali metal sulfite; and an alkaline earth metal sulfite, controlling the pH of the solution or suspension in the range of 1.9 to 6.3, thereby removing only chlorine from the gas mixture.

From the foregoing, it has been found that a method has been found which is suitable for separating and isolating chlorine from a gas mixture containing chlorine, carbon dioxide and a non-condensable gas, in particular for recovering substantially all of the chlorine from said gas mixture, of chlorine present in the various waste gases before these waste gases can be discharged into the atmosphere without risk.

In the following, the process of the invention will be described in more detail as the final step of removing chlorine from a gas mixture comprising chlorine, carbon dioxide and non-condensable gas, with reference to the figures.

-Giant. 1 shows a flow diagram of an apparatus in which chlorine is separated from a gas mixture containing chlorine, carbon dioxide and a non-condensable gas 1) by compressing and cooling the gas mixture, 2) dividing the resulting gas mixture into residual gas, which is mainly composed of a predominant portion of non-condensable gas and to the condensate, which is predominantly chlorine, and then introducing 3) the condensate itself into a stripping column in which carbon dioxide and a minor part of the non-condensable gas which have been dissolved in said condensate are desorbed;

-giant. 2 illustrates a flow diagram of an apparatus in which ee implements a conventional process corresponding to the process implemented in the apparatus shown in FIG. 1;

-giant. 3 is a flow diagram illustrating operation for implementing a process for industrial chlorine separation and isolation, which comprises 1) compressing a gas mixture containing 10 to 60 percent by volume chlorine, carbon dioxide and a non-condensable gas, and then cooling and liquefying the gas mixture; dividing the resulting mixture into residual gas, which is mainly composed of a major part of the non-condensable gas, and into a condensate, which is mainly composed of chlorine, 3) introducing the obtained condensate into a stripping column to desorption carbon dioxide and a minor part of the non-condensable gas condensate, thereby separating and isolating chlorine, 4) mixing the stripping gas thus obtained, which is discharged from the stripper head and which is mainly composed of chlorine and sulfur dioxide, with the residual gas of step 1), 5) introducing at least a portion of the talc obtained mixed gas d absorption columns for the absorption of predominantly a portion of the remaining chlorine in the halogenated hydrocarbon, 6) separating off the waste gas consisting of carbon dioxide and non-condensed gas and containing no more than 1% chlorine by volume, and discharging the waste gas from the system; absorbed chlorine into the distillation column to separate the solvent into an isolated gas essentially consisting of chlorine and a halogenated hydrocarbon, 8) recirculating the recovered solvent to use the solvent as an absorption agent in the absorption column, and 9) recycling the isolated gas to the compression zones in step 1);

-giant. 4 illustrates the ionic forms of absorbents which are useful in the process of the invention at different pH values in their aqueous solutions;

-giant. 5 shows a flow diagram of chlorine absorption in an exemplary embodiment of the method of the invention; and

-giant. 6 is a flow diagram illustrating an exemplary operation for implementing a process for industrial separation and isolation of chlorine from a gas mixture containing carbon dioxide and a non-condensable gas comprising 1) compressing the gas mixture and then cooling and liquefying it to split the gas mixture into a residual gas that is mainly 2) introducing said condensate into a stripping column for desorption of carbon dioxide and a minor part of the non-condensable gas dissolved in said condensate, thereby isolating chlorine, 3) mixing the stripper obtained therefrom a gas which is withdrawn from the head of the stripping column and which is mainly made up of carbon dioxide, with the residual gas of step (1), (4) introducing at least a portion of the mixed gas thus obtained into an absorption column in which it is used as a halogenated hydrocarbon to absorb most of the remaining chlorine and reduce the chlorine content to 1 volume percent or less, separating off the carbon dioxide and non-condensable gas waste gas, 5) introducing the chlorine-absorbed solvent into the distillation 6) introducing an aqueous solution or suspension of an alkali metal sulfite and / or an alkaline earth metal sulfite or an aqueous solution or suspension containing an alkali metal sulfite, and / or

CS 276386 B alkaline earth metal sulphite together with an alkali metal and / or alkaline earth metal hydroxide in an amount not exceeding twice the molar amount of the alkali metal sulphite and / or the alkaline earth metal sulphite and scrubbing the waste gas of step 4) with said solution or suspension; maintaining the pH of said solution or suspension in the range of 1.9 to 6.3, thereby removing only chlorine from the gas mixture and discharging the gas mixture from the system.

Examples of gases which can be treated by the process according to the invention are the following gases:

(a) a gaseous mixture from the chlorine production stage by oxidation of hydrogen chloride containing chlorine, carbon dioxide and non-condensable gas;

(b) a gaseous mixture obtained by substantially removing chlorine from the gaseous mixture (a) and still containing the remaining proportion of chlorine together with carbon dioxide and non-condensable gas;

and

(c) a gaseous mixture containing gaseous chlorine and gaseous carbon dioxide.

During the chlorination and phosgenisation of organic compounds, a large amount of hydrogen chloride is produced as a by-product. Since the amount of hydrogen chloride generated exceeds the demand for hydrochloric acid many times, part of this amount of hydrogen chloride is unusable and constitutes a difficult waste. Up to now, converting this waste hydrogen chloride into a usable product has required considerable costs.

Accordingly, it has been desirable to develop a process for effectively recovering chlorine from said hydrogen chloride, which accumulates as waste in the above-mentioned treatment of organic compounds, thereby producing chlorine industrially. It has long been known that chlorine can be produced by oxidation of hydrogen chloride. However, to date, no chlorine production utilizing this oxidation of hydrogen chloride has been operated industrially. It has recently been found that a chromium oxide catalyst, obtained by calcination of chromium hydroxide, has high efficiency even at relatively low temperatures in the chlorine production process by oxidation of hydrogen chloride.

In the process according to the invention, it is possible to treat a gaseous mixture comprising chlorine, carbon dioxide and a non-condensable gas. Thus, the mixture may also be a gaseous mixture which is produced in the above-mentioned, recently developed process for producing chlorine by oxidation of hydrogen chloride. The process according to the invention thus proves to be an efficient process for the isolation of chlorine or for the removal of chlorine from the off-gas leaving the above mentioned industrial chlorine production.

The method of the invention will be further described with reference to the figures.

Referring to Fig. 1, there is shown a flow diagram comprising a compressor 101, a stripping column 102, which may be a conventional type column, i.e., a tray column or packed column, a heat exchanger 103, a separator 104 and a reboiler 105.

The gas mixture, which generally contains 10 to 60 volume percent chlorine and 40 to 90 volume percent carbon dioxide and non-condensable gas, is fed via line 106 to a compressor 101 where it is compressed to a predetermined pressure.

Examples of non-condensable gas include nitrogen, oxygen or carbon monoxide,

CS 276386 Βδ

The compressed gas mixture is cooled to a predetermined temperature in the heat exchanger 103, whereby a portion of the chlorine is liquefied. The resulting mixture of liquid and gas is then passed through line 112 to a separator 104 where the mixture is separated into a residual gas, which is mainly composed of the bulk of the non-condensable gas, and into a condensate, which is mainly composed of chlorine.

The condensate obtained is then passed through line 107 to the top of the stripper column 102, and as the condensate sinks through the stripper column 102, the condensate is contacted with the chlorine vapor vaporized by the digester 105. Consequently, carbon dioxide and non-condensable gas evaporate from the condensate collecting condensate as liquid chlorine at the bottom of the stripper column; the liquid chlorine collected is of high purity and is then withdrawn as px-degassed chlorine via line 111.

If the gaseous mixture containing chlorine, carbon dioxide and non-condensable gas is compressed, then, when introduced into the stripper column to effect liquefaction and distillation of the mixture in the stripper column, the stripper column must have a larger diameter if conventional surface speeds are used. However, if a substantial amount of non-condensable gas is first separated from the treatment mixture and only the condensate containing only a minor amount of non-condensable gas is introduced into the stripping column, as in the process according to the invention, then stripping can be carried out about 30% of the diameter of the otherwise used stripper column.

The carbon dioxide and non-condensable gas thus separated from the condensate are removed from the stripper head via line 109 which flows into line 110 where these gases are combined with the residual gas removed from the separator 104 via line 1 (¾). is then discharged as waste gas from the system.

In order to isolate the chlorine from the gas mixture as efficiently as possible, it is desirable to use as high as possible a compression pressure in the compressor and as low as possible a cooling temperature in the heat exchanger. For safety reasons, however, it is not recommended to increase the compression pressure of a chlorine-containing gas mixture without any restriction. In addition, there is another limitation that the liquefaction temperature of the carbon dioxide-containing mixture must be higher than the melting point of the carbon dioxide (-56.6 ° C at a partial pressure of carbon dioxide of 0.52 MPa)

If the melting point of carbon dioxide was exceeded during liquefaction, the device would be blocked by dry ice (solid carbon dioxide).

Compressor compression pressure and liquefaction temperature in the heat exchanger should therefore be appropriately determined taking into account economic process factors such as compressor energy demand, cooling load in heat exchanger, heat load of the digester and the cost of waste gas treatment before it is discharged into the atmosphere. .

In general, it is preferable to select a compression pressure of the gaseous mixture in the range of 0.3 to 1.5 MPa and a cooling and liquefaction temperature in the range of -10 to -50 ° C and to operate the stripping column at a pressure of 0.3 to 1.5 MPa and column bottoms in the range of 20 to 45 ° C.

The purity of the chlorine obtained by such a process is generally equal to or even higher than 99% by volume.

FIG. 3 shows a flow diagram of a device including a compressor 301, a condenser 3.02, a condenser 303 &quot; gas-liquid separator 304. a stripping column 305. a reboiler 306. an absorption column 307. a heat exchanger 308. a reduction valve 309. a distillation column 310. a condenser 311, gas-liquid separators 46, digester 313, condenser 314 and pumps 315 and 316. The stripper column 305, the absorption column 307, and the distillation column 310 may be stacked or packed columns.

The gas mixture fed via line 317 and containing chlorine, carbon dioxide and non-condensable gas is mixed with the isolated gas recycled via line 326 and fed from the gas-liquid separator 312. The mixed gas is then compressed to a predetermined compression pressure in the compressor 301.

In this way, a gas mixture containing chlorine at a concentration of 10 to 60% by volume can be treated. Since gaseous mixtures with such a chlorine concentration are also formed in the production of chlorine by oxidation of hydrogen chloride with the use of a specific chromium oxide catalyst, these gaseous mixtures can also be treated in this way.

Examples of the non-condensable gas present in the gas mixture include nitrogen, oxygen, carbon monoxide and / or similar non-condensable gases.

Preferably, a compression pressure of 0.3 to 1.5 MPa is chosen. A particularly preferred compression pressure is 0.5 to 1.2 MPa. The compressed gas mixture is then cooled in condenser 302 and condensed in a subsequent condenser 222. At this condenser 3.03 at least two thirds of the chlorine contained in the gas mixture is liquefied.

The cooling and liquefaction temperature can be selected from -10 to -50 ° C, with the preferred cooling and liquefaction temperature being between -20 and -40 ° C,

Thereafter, the resulting gas-liquid mixture is separated in a separator 304 into the residual gas discharged via line 319, which is mainly composed of the bulk of the non-condensable gas, and into the condensate discharged via line 318, which is mainly composed of chlorine. »»

The stripping column 222 can be operated at a pressure of 0.3 to 1.5 MPa, preferably at a pressure of 0.5 to 1.2 MPa.

The condensate which has been introduced into the head of the stripping column 305 drops downwards through the column. During this descent, carbon dioxide and non-condensable gas evaporate from the condensate as a result of the countercurrent contact of the condensate with increasing vapors, consisting predominantly of chlorine and the vaporizer 306 having a temperature at the bottom of the column of 20-45 ° C.

Thus, at the bottom of the stripping column 30, condensate is collected in the form of liquid chlorine, which is discharged as the chlorine produced via line J21.

The carbon dioxide and the non-condensable gas that have been evaporated from the condensate are removed together with chlorine as stripped gas via line 320 from the top of the gOgi stripper column. Then, the stripped gas is mixed with the residual gas through line 319.

At least a portion of the mixed gas thus obtained is introduced into the absorption column 3 ° 7 via line 323. It is preferable to operate the absorption column [307] at the same pressure as that used in the stripper column operation, the degree of absorption increases with increasing pressure in the absorption column. provided that the temperature of the absorbent solution is constant.

Almost all of the chlorine and a portion of carbon dioxide and non-condensable gas are absorbed from the gas supplied via line 222 ^{to the} absorption isolone 307 into the absorption solution supplied to the top of the absorption column via line 3.30. The gas so purified is then discharged from the system as the waste gas discharged from the top of the absorption column 307 through line 324. This waste gas consists of carbon dioxide and a non-condensable gas and contains at most 1 volume percent of chlorine.

The solvent used in the above absorption step is a halogenated hydrocarbon such as carbon tetrachloride or chloroform. The solvent may be used in an amount of 2 to 100 times, preferably 3 to 30 times the weight of the gas fed to the absorption column.

The solvent is then removed from the absorption column 307 at its bottom vedení® 325 ^{to e} ° F the pump 315 is pumped into the heat exchanger £ 08th After preheating the solvent in the heat exchanger 308, the preheated solvent is fed to the distillation column 310.

The distillation column can be operated at a pressure of 0.01 to 1.5 MPa, preferably at a pressure of 0.1 to 1 MPa.

The solvent introduced into the distillation column 310 is evaporated with a digester 313. Condensed in condenser 311 and recycled via line 327 back to the distillation column 310, evaporating gases absorbed into the solvent during absorption in the absorption column. These gases are then removed via line 326. At the bottom of the absorption column 310, recovered solvent is collected. The vaporized gas is predominantly chlorine.

Said recovered solvent whose chlorine concentration does not exceed 5,000 ppm by weight, preferably does not exceed 500 ppm by weight, is discharged from the bottom of the absorption column 310 through the pump 316 back to the heat exchanger 308. After removal of heat by the heat exchanger 308 a solvent to replace solvent losses during solvent absorption and recovery.

The solvent is then cooled in the condenser 314, then fed back into the head of the absorption column 307. Although the absorbency of the absorption column 307 increases with decreasing temperature at the entrance to the absorption column 307. it is not possible to lower this temperature beyond the melting point of the solvent. carbon tetrachloride, this temperature is, for example, -22.6 ° C). It is desirable to lower the temperature of the condenser 310 of the distillation column 310 to such an extent that the solvent does not remain in the gaseous phase and thus not be entrained by the off-gas. On the other hand, the excessively low condenser temperature of the distillation column 310 results in the liquefaction of the chlorine in said condenser 31 so that the recovered solvent then contains more chlorine, reducing its absorption power in the absorption column 307. Therefore, such an excessively low temperature is not preferred.

It is therefore necessary to determine in an appropriate manner the optimum conditions for the temperature of the solvent at the entrance to the absorption column and the temperature of the condenser 311 of the distillation column 310, taking into account the above mentioned facts. S vj ^f HODO temperature of the solvent at the inlet of the absorption column may be from -20 to 0 ° C, the temperature of the condenser 311 of the distillation column 310 may preferably range from -20 to -10 ° C.

In the following, a process for removing chlorine from a gas mixture comprising chlorine gas and carbon dioxide gas according to the invention is described, characterized in that said gas mixture is washed in an aqueous solution or suspension containing an alkali metal sulphite and / or an alkaline earth metal sulphite or containing an alkali metal sulphite and / or an alkaline earth metal sulphite together with an alkali metal and / or an alkaline earth metal hydroxide in an amount not exceeding twice the molar amount of an alkali metal sulphite and / or an alkaline earth sulphite, maintaining the pH of the solution or suspension between 1.9 and 6.3, thereby separating only chlorine from said gas mixture.

Examples of alkali metal hydroxides useful in the above process are lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides which can be used in the process according to the invention are, in particular, magnesium hydroxide, calcium hydroxide and barium hydroxide.

The alkaline earth metal in said sulphite may in particular be magnesium, calcium or barium. However, the preferred sulfite is an alkali metal sulfite relative to its

CS 276386 Βδ of high solubility.

If the alkali metal sulfite and / or the alkaline earth metal sulfite is used together with an alkali metal hydroxide and / or an alkaline earth metal hydroxide, then it is necessary to use the hydroxide in an amount not more than twice the molar amount of the sulfite.

Since the process requires an alkali to neutralize chlorine as well as a sulfite to reduce chlorine, at least one equivalent of a reducing sulfite to two equivalents of alkali necessary for said chlorination is required.

If the amount of alkali exceeds twice the molar amount of sulphite, the sulphite will not suffice and chlorine will not be effectively absorbed even if the pH is maintained in the range of 1.9 to 6.3.

If the amount of alkali is not more than twice the molar amount of the sulfite, the sulfite acts as a proxy for the hydroxide. It should further be noted that the more sulfite is present, the more bisulfite ions are present in the absorption solution. PH control is then facilitated by their buccal effect.

However, the absorption solution need not contain either an alkali metal or alkaline earth metal hydroxide in the process of the invention. The chlorine in the waste gas may be absorbed by an aqueous solution or suspension containing only alkali metal sulfite and / or alkaline earth metal sulfite.

A suitable pH range for chlorine absorption ranges from 1.9 to 6.3. At pH higher than 6.3, the conversion of carbon dioxide to bicarbonate ions occurs, as shown in the graph in Figure 4, so that alkali is consumed. Therefore, more alkali is consumed and the absorption process thus conducted is uneconomical.

If the pH is less than 1.3, sulfuric acid is formed and decomposed, which means that the absorption solution tends to more easily form sulfur dioxide. A process conducted at such a low pH is therefore also not advantageous.

Said pH control in the range of 1.9 to 6.3 can be achieved by adjusting the amount of alkali metal sulphite and / or alkaline earth metal sulphite added and the amount of alkali metal hydroxide and / or alkaline earth metal hydroxide added in an amount not more than twice the molar amount of sulphite.

In this case, the hydroxide and sulphite may be added in solid form or as an aqueous solution, the addition of the hydroxide and sulphite in the form of an aqueous solution being more preferred. Both of these may be added together as a single solution. These substances can of course also be added as separate aqueous solutions.

The concentration of each of the aqueous solutions should be in the case of an alkali metal salt or alkaline earth metal salt such that both the starting material and the resulting salt remain in solution. However, the operation is feasible even when said substances are added in the form of a slurry.

If said operation is carried out in a homogeneous system at a concentration close to said salt, then the reaction temperature should be determined with respect to said solubility. Since the solubility of sulphites and chlorides is not significantly temperature dependent, the use of higher temperatures cannot induce any significant increase in the concentrations of said solutions.

The reaction rate between chloride and sulfite increases with increasing temperature. However, the preferred reaction temperature is in the range of 0 to 70 ° C due to the potential corrosion problems of the absorber reactor material.

For practical absorption, the gas may either be bubbled through the absorption solution in a stirred tank or may be contacted with the absorption solution in the scrubbing column.

Although the number of absorption steps will vary depending on the permissible chlorine concentration in the off-gas effluent, it is not necessary to use too many of the absorption steps given that the reaction or absorption rate is high.

In the following part of the description, the above-described embodiment of the method according to the invention will be described in more detail with reference to the attached Fig. 5.

The gaseous mixture, which contains carbon dioxide in gaseous form and chlorine gas, and which in some cases may also contain nitrogen, oxygen and the like, is fed to the absorption column 407 through line 401 and is washed in the absorption column 407 by washing liquid recirculated to the absorption column The wash liquid is an aqueous solution or suspension as described above which comprises an alkali metal sulfite and / or an alkaline earth metal sulfite or an alkali metal sulfite and / or an alkaline earth metal sulfite together with an alkali metal hydroxide; and an alkaline earth metal hydroxide, and the pH of the solution or suspension ranges from 1.9 to 6.3.

The wash liquid is diluted with water to the desired concentration, controlling its pH and maintaining a constant flow rate through the absorption column 407 through the overflow through line 404.

After treatment in the absorption column 407, the waste gas from which only chlorine has been removed by absorption in the absorption liquid is discharged from the system through a 4.06 line.

The process according to the invention, in which a combination of all the essential features of the invention is utilized, represents an important industrial process for producing chlorine. Such a process is illustrated in FIG. 6. This combined process allows for a highly efficient isolation of chlorine from a gas mixture of carbon dioxide and non-condensable gas and also containing chlorine, as well as almost complete removal of chlorine from the off-gas before such off-gas This method solves the problems associated with the pollution of the environment by the traces of chlorine contained in the discharged waste gases.

It can be seen from FIG. 6 that the gaseous mixture of carbon dioxide and non-condensable gas and containing chlorine is passed via line 531 to the compressor g1, where the mixture is compressed to a predetermined pressure. The gas mixture is then cooled in a condenser 502 to a predetermined temperature and then liquefied in condenser 503 to liquefy a portion of the chlorine.

The resulting mixture of gas and liquid is passed to separators 504, where the mixture is separated into a residual gas, which is mainly composed of the predominant part of the non-condensable gas, and into a condensate, which is mainly composed of chlorine. The condensate obtained is fed to a stripping column 505 where the condensate is contacted with the chlorine vapors vaporized by the digester 506, resulting in the evaporation of the carbon dioxide and non-condensable gas dissolved in the condensate. Chlorine is collected at the bottom of stripper column 505 as high purity liquid chlorine. This liquid chlorine is withdrawn from the bottom of the stripper column 505 as produced by the chlorine via line 511. The purity of the liquid chlorine is generally equal to or greater than 99% by volume.

The carbon dioxide and non-condensable gas that has been evaporated from the condensate is removed from the top of the stripper column 505 via line 509 and then combined with the residual gas supplied from the separators 504 via line 508.

At least a portion of the mixed gas thus obtained is introduced into the absorption column 512.

From this feed gas, almost all amounts of chlorine and a portion of carbon dioxide and non-condensable gas are absorbed in the absorption column 512 into the absorption solution fed to the head of the absorption column 512 via line 52. In this way, said mixed gas is converted to off-gas from the absorption column (top) by line 514 containing chlorine at a concentration not exceeding 1 volume percent.

The solvent with absorbed chlorine, carbon dioxide and non-condensable gas is discharged from the bottom of the absorption column 512 via line 513. Through this line, the solvent is introduced via pump 516 to heat exchanger 517. After preheating in this heat exchanger 517 the solvent is fed to distillation column 518.

The solvent introduced into the distillation column 518 is vaporized by the digester 519, condensed in the condenser 533 and recycled through the separator 532 via line 520 back to the distillation column 518, evaporating the absorbed gas and recovering an isolated gas consisting mainly of chlorine.

The solvent removed from the distillation column 518 of conduit 521 has a chlorine concentration not exceeding 5000 ppm by weight, preferably not exceeding 500 ppm by weight. This solvent is fed to the bottom of the distillation column 518 via line 521 through the pump 522 to the heat exchanger 517. After heat has been removed from the solvent by the heat exchanger 517, a certain amount of fresh solvent is added to the solvent. during the absorption process and solvent recovery. The solvent is then cooled in a cooler 524 from which the cooled solvent is recycled to the absorption column 512. The waste gas discharged from the absorption column 512 via line 51.4 is comprised of carbon dioxide and a non-condensable gas and contains at most 1 volume percent chlorine. The off-gas is introduced into the bottom of the absorption column 525, in which the off-gas is washed with a washing liquid supplied to the head of the absorption column 525 via line 526.

The wash liquid is recirculated to the bottom of the absorption column 525 by a pump.

Said washing liquid consists of an aqueous solution or suspension as described above and which comprises an alkali metal sulphite and / or an alkaline earth metal sulphite or contains an alkali metal sulphite and / or an alkaline earth metal sulphite together with an alkali metal and / or metal hydroxide alkaline earths; this washing liquid has a pH controlled in the range of 1.9 to 6.3. It is diluted with water to the desired concentration, controlling its pH and maintaining its flow rate constant by the overflow of line 527.

After the absorption treatment, the off-gas removed from the top of the absorption column 525 via line 528 from which only chlorine was removed by the scrubbing liquid.

The wash liquid recirculated in the absorption column 525 is prepared from aqueous sodium sulfite solution and water supplied via line 530.

In the following part of the description, the process for industrial chlorine separation will be described in more detail on specific exemplary embodiments.

CS 276386 Βδ

Example 1

A 1 L / min gaseous mixture of carbon dioxide and 100 ml / min of chlorine gas was introduced into a 1 liter 10% (w / v) aqueous sodium sulfite solution flask equipped with an overflow pipe. The gaseous mixture was bubbled through the solution while monitoring the pH changes of the absorption solution. At the beginning of the bubbling, all of the feed gas was absorbed. The pH of the absorbing solution dropped during continued bubbling. As the pH of this solution dropped to 6 or even lower, some of the feed gas began to pass through the absorption solution. When the pH of the solution reaches 4, a 10% aqueous sodium sulfite solution of 15.7 cnP / min is added to the solution to maintain the pH of the solution at 4.

Under the above conditions, gas passing through the absorption solution and not absorbed by said absorption solution was passed through a flask containing an R / 2 potassium iodide solution and then through a flask with an R / 2 sodium hydroxide solution. These solutions are then titrated with H / 10 sodium thiosulfate solution using an aqueous starch solution as an indicator. These titrations determine the chlorine concentration in the potassium iodide and sodium hydroxide solutions. This determined that the concentration of chlorine in the waste gas leaving the absorption system did not exceed 1 ppm by volume.

In addition, carbonic acid was absorbed in K / 2 potassium iodide solution and absorbed in K / 2 potassium hydroxide solution was titrated with N / 10 sodium hydroxide solution respectively. The titration determined that the bubbled carbon dioxide was not completely absorbed by the absorption solution and that it was somewhat absorbed through the solution.

Example 2

To the same flask as used in the previous example was added an aqueous solution containing 8.6 weight percent sodium sulfite and 1.4 weight percent sodium hydroxide. As in Example 1, a gas of 1 l / min of carbon dioxide gas and 100 µm / min of chlorine gas was introduced into this solution.

When the pH of the absorption solution reached 4, an aqueous solution having the same composition as the present solution was added to the absorption solution at a rate of 12.1 ml / min to maintain the pH of the absorption solution 4.

The gas passing through the absorption solution was analyzed in the same manner as in the previous Example 1. It was determined that the chlorine content of the gas did not exceed 1 ppm by volume, while carbon dioxide passing through the solution was not absorbed.

Example 3

In this example, the flow diagram shown in FIG. 5 was followed.

A gas mixture containing 30 volume percent carbon dioxide, 70 volume percent nitrogen and oxygen and 1000 volume ppm chlorine is passed through line 401 to the bottom of the absorption column 407 at a rate of 500 N @ ³ / h. filled with Raschig rings with a diameter of 25.4 mm to a height of 2.2 meters. The gaseous mixture fed through line 401 ee is scrubbed by the absorption liquid recirculated to the head of the absorption column via line 402 at a rate of 10 m ³ / h.

A pH of 4.0 was maintained in the absorption column by feeding 94.5 kg / h of aqueous solution through line 403, said aqueous solution containing 10 weight percent sodium sulfite, and water being fed to absorption absorber 407 through line 405 at a rate of 278 kg / h. At the same time, the absorption column 407 maintains a constant level of absorption solution by means of a flow line 404.

The treated gas discharged from the top of the absorption column through line 406 is analyzed. By absorption in Ν / 1Ό aqueous potassium iodide solution and titration of this solution, it was determined that the concentration of chlorine in said waste gas did not exceed 1 ppm by volume.

Example 4

In this example, the flow diagram shown in Figure 6 was followed.

A gas mixture containing 38.9 volume percent chlorine, 12.9 volume percent carbon dioxide, and 48.2 volume percent nitrogen and oxygen was fed to the apparatus shown. At a compression pressure of 0.7 MPa and a cooling and liquefaction temperature of -24 ° C, a condensation of 27.3% by volume of the gas mixture was condensed, of which 67.5% by volume of chlorine was condensed from the total amount of chlorine originally present in the feed mixture.

The condensate was then fed to the top of the stripper column. The stripping column was operated at a head pressure of 0.7 MPa and a bottom temperature of 25.4 ° C. The liquid chlorine obtained had a purity of 99.5% by volume.

The distilled gas from the top of the stripper column contained 18.6 volume percent chlorine, 17.1 volume percent carbon dioxide, and 64.3 volume percent nitrogen and oxygen.

In the absorption column, carbon tetrachloride was used as the absorption solvent, which was cooled to -15 ° C and then fed to the absorption column in an amount equal to 3.6 times the amount of feed gas. The absorption column was operated to maintain a pressure of 0.7 MPa in the top of the column. The off-gas effluent from the top of the absorption column contained 47 ppm by volume of chlorine, 19.7 volume percent carbon dioxide, 80.1 volume percent nitrogen and oxygen, and 0.2 volume percent carbon tetrachloride.

Chlorine-absorbed carbon tetrachloride is then pumped to the bottom of the distillation column. The distillation column was operated at a head pressure of 0.13 MPa, a condenser temperature of -15 ° C and a bottom temperature of 105.6 ° C. The isolated gas from the top of the distillation column contained 89.4 volume percent chlorine, 7.3 volume percent carbon dioxide, 3.3 volume percent nitrogen and oxygen, and 500 ppm v / v carbon tetrachloride. The concentration of chlorine in the recovered solvent was 5 ppm by weight. The degree of chlorine recovery was 99.99%.

The overhead gas mixture containing 47 ppm by volume of chlorine, 19.7 volume percent carbon dioxide, 80.1 volume percent nitrogen and oxygen, and 0.2 volume percent carbon tetrachloride is then introduced into the scrubbing column to remove the chlorine contained. . The gaseous mixture is fed to the bottom of the column while recirculated absorption liquid is fed to the top of the column. The chlorine present in the feed gas mixture is absorbed in the absorption liquid.

As the absorption liquid, an aqueous solution containing 10 weight percent sodium sulfite was used, maintaining the pH of the solution at 4.2 during the absorption. The absorption liquid in the absorption tower was contacted with the gas mixture to be treated to absorb the chlorine present in the mixture.

The concentration of chlorine in the gas withdrawn from the top of the absorption column did not exceed 1 ppm by volume.

Claims (2)

PATENTOVÉ SŠÍROKY Process for removing chlorine from a gas mixture containing chlorine gas and carbon dioxide gas, characterized in that the gas mixture is scrubbed with a solution or suspension comprising an alkali metal sulphite and / or an alkaline earth metal sulphite, controlling the pH of the solution or suspension in the range of 1 , 9 to 6.3. 2. A process according to claim 1, wherein the gas mixture is scrubbed with an aqueous solution or suspension comprising an alkali metal sulfite and / or an alkaline earth metal sulfite together with an alkali metal and / or alkaline earth metal hydroxide in an amount not exceeding twice the molar amount of alkali metal sulfite. and / or an alkaline earth metal sulfite, wherein the pH of the solution or suspension is controlled to be in the range of 1.9 to 6.3.