Industrial process for the separation and recovery of chlorine

Abstract: A process for the removal of chlorine from a gaseous mixture comprising chlorine gas and carbon dioxide gas, which comprises feeding an aqueous solution or suspension containing an alkali metal sulfite and/or an alkaline earth metal sulfite and optionally an alkali metal hydroxide and/or an alkaline earth metal hydroxide in a proportion not greater than twice in molar ratio the alkali metal sulfite and/or alkaline earth metal sulfite, and washing the gaseous mixture with the solution or suspension while controlling the pH of the solution or suspension within a range of 1.9-6.3, whereby chlorine is solely removed from the gaseous mixture.

Description: The present application has been divided out of application 89301394.6 (EP-A-0 329 385) which relates to a process for the separation and recovery of chlorine from a gaseous mixture comprising chlorine, carbon dioxide and further gas, which comprises compressing and cooling the gaseous mixture thereby to separate the gaseous mixture into a residual gas formed principally of a major portion of the further gas which is non-condensable under the conditions employed, and a condensate formed primarily of chlorine, and feeding the condensate to a stripping column to desorb carbon dioxide and a minor portion of the non-condensable gas dissolved in the condensate. The present invention is concerned with a process for removing chlorine from a gaseous mixture containing chlorine and carbon dioxide by washing under controlled conditions. The invention can be applied to absorbing and removing chlorine gas contained in a mixture containing carbon dioxide which is formed upon production or utilization of chlorine. A variety of processes have already been proposed for the recovery of chlorine from a gaseous mixture containing chlorine, including the following patent publications: (1) JP-A-49062378 (WPI Abstract 75-06381W) discloses the removal of small amounts of chlorine from waste gases by washing with alkali metal sulfite solutions. (2) RO-A-62211 ( Chemical Abstracts 89 (12) 1978 abstract 94428h) discloses the removal of chlorine from waste gases by absorption with solutions of NaOH containing Na 2 SO 3 . (3) U.S. Patent No. 2,765,873 discloses a process for recovering chlorine in a form substantially free of non-condensable gas. The process comprises causing a solvent to absorb a gaseous mixture, which is composed of 30-50 wt.% of chlorine and air, under a pressure of 2.0-14.3 atm at a column top temperature of from -22.8°C to 32.2°C and a column bottom temperature higher by from 27.8°C to 52.8°C than the column top temperature. (4) West German Patent No. 2413358 discloses a process to absorb chlorine alone from a mixture of chlorine and carbon dioxide gas as the hydrochlorite of an alkali metal and/or alkaline earth metal by using a multi-stage counter current absorption apparatus making use of an alkali metal hydroxide and/or an alkaline earth metal hydroxide and operating the apparatus while controlling the pH of the final stage of the liquid side at about 7.5. Chlorine is a useful raw material produced industrially on a large scale primarily by brine electrolysis, and is used widely. Gaseous components are formed as byproducts upon production of chlorine. These components are accompanied by chlorine, so that due to the toxicity of chlorine, they cannot be released into the atmosphere without further treatment. Accordingly, chlorine contained in such a gas is removed usually by its absorption with an alkaline substance. However, when another gas is contained besides chlorine and said other gas is acidic (like carbon dioxide gas) it is not only chlorine but also carbon dioxide that is absorbed in an alkaline solution. As a corollary to this, an alkali is required in an amount equal to the sum of the carbon dioxide and chlorine. In particular, where a gas contains carbon dioxide at a high level and chlorine at a trace level, an inconvenience arises in that a large amount of an alkali is required for the removal of such a trace amount of chlorine. It is accordingly desired to develop a process for the selective absorption and removal of chlorine which is contained in carbon dioxide. The process (4) has already been proposed as a process for removing chlorine from a mixture of carbon dioxide gas and chlorine. In this process, pH 7.5 is however a value far higher than 6.35, which is the first dissociation constant (pKa) of carbonic acid, as shown in FIG. 4 and falls within the range where carbon dioxide gas reacts with an alkali metal hydroxide and/or alkaline earth metal hydroxide to form the corresponding bicarbonate. To absorb chlorine alone from the mixture of carbon dioxide and chlorine in the above range, it is necessary to use an alkali exactly in an equimolar amount as chlorine. Where the concentration of chlorine varies, it is difficult to balance the alkali in quantity with chlorine. In addition, pH 7.5 is close to the dissociation constant of hypochlorous acid. When the pH becomes lower than the above level, hypochlorous acid takes the form of a free acid so that it becomes prone to decomposition. Precise control of pH is therefore needed. Even if chlorine is removed successfully, the resulting hypochlorite solution has strong oxidative effects and offensive odor and cannot thus be discarded as is. An additional step is therefore required for its reduction. According to the present invention, there is provided a process for the removal of chlorine from a gaseous mixture comprising chlorine gas and carbon dioxide gas. The process comprises feeding an aqueous solution or suspension containing an alkali metal sulfite and/or an alkaline earth metal sulfite, and washing the gaseous mixture with the solution or suspension while controlling the pH of the solution or suspension within a range of 1.9-6.3, whereby chlorine is solely removed from the gaseous mixture. Some embodiments of the invention will now be described with reference to the accompanying drawings, in which: FIG. 1 illustrates ionic forms of absorbents, which are useful, in the practice of the present invention, at various pH levels in their aqueous solutions; FIG. 2 is a flow chart depicting the absorption of chlorine in Example 3; and FIG. 3 is a flow chart of a plant which is suitable for use in practicing a process which includes a process according to the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The process of the invention may be applied to a gaseous mixture composed of carbon dioxide and non-condensable gas and containing chlorine or to carbon dioxide containing chlorine. The following gases may be mentioned by way of example as applicable gases: a) a gaseous mixture which occurs in the production of chlorine by oxidizing hydrogen chloride and contains chlorine, carbon dioxide and non-condensable gas; b) a gaseous mixture which is obtained by removing chlorine to a substantial extent from the gaseous mixture a) and contains a still-remaining portion of the chlorine along with the carbon dioxide and non-condensable gas; and c) a mixture containing chlorine gas and carbon dioxide. Upon chlorination or phosgenization of organic compounds, a great deal of hydrogen chloride is formed as a byproduct. Since its quantity has been much greater than the demand for hydrochloric acid, a lot of hydrogen chloride has been unutilized and disposed wastefully. A substantial cost has also been required for its disposal. Accordingly, it has been desired to develop a process for efficiently recovering chlorine from hydrogen chloride, which has been discarded in a large quantity as mentioned above, and industrially producing chlorine. It has been known for many years to produce chlorine by oxidizing hydrogen chloride. However, no industrial production process has been completed. Recently, it has been found that a chromium oxide catalyst obtained by calcining chromium hydroxide in particular has high activity even at relatively low temperatures, leading to the provision of an industrial process for producing chlorine by oxidizing hydrogen chloride. This invention is useful as an effective process for the recovery of chlorine or as a process for the removal of chlorine from a waste gas in the above industrial production process of chlorine. A description will next be made of a process for the removal of chlorine from a gaseous mixture containing chlorine gas and carbon dioxide gas, which comprises feeding an aqueous solution or suspension containing an alkali metal sulfite and/or an alkaline earth metal sulfite or containing an alkali metal sulfite and/or an alkaline earth metal sulfite along with an alkali metal hydroxide and/or an alkaline earth metal hydroxide in a proportion not greater than twice in molar ratio the alkali metal sulfite and/or alkaline earth metal sulfite, and washing the gaseous mixture with the solution or suspension while controlling the pH of the solution or suspension within a range of 1.9-6.3, whereby chlorine is removed from the gaseous mixture. Examples of the alkali metal hydroxide useful in the practice of the above process include the hydroxides of alkali metals such as lithium, sodium and potassium, while illustrative alkaline earth metal hydroxides may be the hydroxides of magnesium, calcium, barium and the like. On the other hand, the metal in the form of the sulfite may be an alkaline earth metal such as magnesium, calcium or barium. An alkali metal having a high solubility in water is however preferred. When an alkali metal sulfite and/or an alkaline earth metal sulfite is used along with an alkali metal hydroxide and/or an alkaline earth metal hydroxide in the above process, it is essential to use the hydroxide in a proportion not greater than twice in molar ratio the sulfite. Since this process requires an alkali for neutralizing chlorine and also a sulfite for reducing chlorine, at least one equivalent of a reductive sulfite is required per 2 equivalents of an alkali required for the neutralization. If the proportion of the alkali exceeds twice in molar ratio the sulfite, the reductive sulfite becomes scarce and chlorine cannot be absorbed even if the pH is controlled within the range of 1.9-6.3. If the proportion of the alkali is not greater than twice in molar ratio the sulfite, the sulfite acts as a substitute for the hydroxide. Further, the greater the sulfite, the more the bissulfite ions in the absorbent solution. The pH control is thus facilitated owing to their buffer action. Accordingly, the washing solution may be that containing neither alkali metal hydroxide nor alkaline earth metal hydroxide. Chlorine in a waste gas can be absorbed with an aqueous solution or suspension of an alkali metal and/or an alkaline earth metal sulfite alone. The pH range suitable for the chlorine-absorbing reaction is 1.9-6.3. A pH higher than 6.3 converts carbon dioxide gas into bicarbonate ions as shown in FIG. 1, so that the alkali is consumed. More alkali is therefore consumed to make the process uneconomical. When the pH becomes lower than 1.9, free sulfurous acid is formed and decomposed whereby sulfur dioxide gas tends to occur more easily. Such a low pH is accordingly not preferred. This pH control of the aqueous solution within the range of 1.9-6.3 can be achieved by adjusting the amount of the alkali metal sulfite and/or alkaline earth metal sulfite to be added as well as the amount of the alkali metal hydroxide and/or alkaline earth metal hydroxide to be added in a proportion not greater than twice in molar ratio the sulfite. In this case, the hydroxide and sulfite may each be added in a solid form or as an aqueous solution, the latter being preferred. They may be fed together as a single solution. Needless to say, they may be added as separate aqueous solutions. The concentration of each aqueous solution may preferably be in a range that in the case of an alkali metal salt or alkaline earth metal salt, the raw materials and the resulting salt and sulfate are dissolved. Operation is however still feasible even when they are each added in the form of a slurry. When the operation is carried out in a homogeneous system at a concentration near the solubility of the salt, the reaction temperature should be determined taking the solubility into parallel consideration. Since the solubilities of alkali metal and alkaline earth metal sulfites and chlorides are all not dependent too much on the temperature, use of a higher temperature cannot bring about any substantial effect for increasing their concentrations. The reaction velocity between chlorine and a sulfite increases as the temperature becomes higher. A reaction temperature of from 0°C to 70°C is however preferred in view of the potential problems of corrosion and deterioration of the material of the reactor. Regarding the manner of the reaction, the gas may be absorbed by causing it to bubble in a stirring tank or by treating it in a washing column. Although the stage number varies depending upon the tolerable level of chlorine in the treated gas, it is not necessary to increase the stage number too much because the reaction or absorption velocity is high. The above-described embodiment will next be described with reference to FIG. 2. A gaseous mixture 401 which contains carbon dioxide gas and chlorine gas and may also contain nitrogen, oxygen and the like in some instances is introduced into a lower part of an absorption column 407, whereby the gaseous mixture 401 is washed with a washing liquid 402 recirculated from an upper part of the column. The washing liquid 402 is an aqueous solution or suspension which as described above, contains an alkali metal sulfite and/or alkaline earth metal sulfite or an alkali metal sulfite and/or alkaline earth metal sulfite along with an alkali metal hydroxide and/or alkaline earth metal sulfite and has a pH controlled within the range of 1.9-6.3. The washing liquid is diluted with water to a desired concentration, and is recirculated while controlling its pH and maintaining its flow rate constant by means of an overflow 404. After the treatment, a waste gas 406 from which chlorine gas alone has been removed with the washing liquid is released out of the system. The process according to the present invention can be used as one part of a chlorine separation process which itself constitutes an important part of an industrial production process for chlorine. Such a separation process will be described with reference to FIG. 3. The process permits highly-efficient recovery of chlorine from a gaseous mixture composed of carbon dioxide and non-condensable gas and containing chlorine and further almost complete removal of chlorine from a waste gas to be released out of the system, so that the pollution problem can be solved. A gaseous mixture composed of carbon dioxide and non-condensable gas and containing chlorine is fed through a line 531 to a compressor 501, where it is compressed to a predetermined pressure. The gaseous mixture is cooled by a cooler 502 to a predetermine temperature and then liquefied by a condenser 503, so that a portion of the chlorine is liquefied. The resultant gas-liquid mixture is fed to a separator 504, where it is separated into a residual gas formed principally of a major portion of the non-condensable gas and a condensate formed primarily of chlorine. The condensate is fed to an upper part of a stripping column 505. While falling down through the column 505, the condensate is brought into contact with chlorine vapor boiled up by a reboiler 506 so that carbon dioxide and non-condensable gas dissolved in the condensate are caused to evaporate. Chlorine is collected in the bottom of the stripping column as liquid chlorine having a high purity and is then drawn as product chlorine through a line 511. The purity of chlorine obtained by such an operation is usually 99% by volume or higher. The carbon dioxide and non-condensable gas which have evaporated from the condensate are allowed to flow from the top of the stripping column and then through a line 509, and join the residual gas fed from the separator through a line 508. At least a portion of the mixed gas thus joined is fed to an absorption column 512. From the mixed gas charged into the absorption column, almost the whole chlorine and portions of the carbon dioxide and non-condensable gas are absorbed in a solvent 513 falling from the top of the column. Accordingly, the mixed gas can be converted into a waste gas 514 which is composed of carbon dioxide and non-condensable gas and contains not greater than 1% by volume of chlorine. A solvent 515 with chlorine, carbon dioxide and non-condensable gas absorbed therein is drawn out from the bottom and is fed by a pump 516 to a heat exchanger 517. After being preheated by the heat exchanger 517, the solvent is fed to a distillation column 518. The solvent charged into the distillation column 518 is boiled up by a reboiler 519, condensed by a condenser 533 and distilled by a reflux 520 to the distillation column, so that the gases absorbed are caused to evaporate to separate a recovered gas composed primarily of chlorine. A solvent 521 having a chlorine concentration not higher than 5,000 ppm by weight, preferably, not higher than 500 ppm by weight is drawn out through the bottom of the column and is then fed by a pump 522 to the heat exchanger 517. After heat is removed from the solvent by the heat exchanger 517, a fresh supply 523 of the solvent is added to replenish any loss. The solvent is thereafter cooled by a cooler 524 and is then fed back to the absorption column 512. A waste gas 514 composed of carbon dioxide and non-condensable gas and containing not more than 1% by volume of chlorine is introduced into a lower part of ban absorption column 525, in which the waste gas 514 is washed with a washing liquid 526 recirculated from an upper part of the column. The washing liquid is an aqueous solution or suspension which as described above, contains an alkali metal sulfite and/or alkaline earth metal sulfite or an alkali metal sulfite and/or alkaline earth metal sulfite along with an alkali metal hydroxide and/or alkaline earth metal hydroxide and has a pH controlled within the range of 1.9-6.3. The washing liquid is diluted with water to a desired concentration, and is recirculated while controlling its pH and maintaining its flow rate constant by means of an overflow 527. After the treatment, a waste gas 528 from which chlorine gas alone has been removed with the washing liquid is released out of the system. The washing liquid is prepared from water 530 and an aqueous solution of sodium sulfite. [Examples] This invention will hereinafter be described in detail by the following Examples. Example 1: In a flask charged with 1 ℓ of a 10 wt.% aqueous solution of sodium sulfite and equipped with an overflow tube opening at the lower end thereof, a mixture of 1 ℓ/min of carbon dioxide gas and 100 mℓ/min of chlorine gas was allowed to bubble while tracing pH variations. In the beginning, the blown gas was absorbed in its entirety. The pH dropped as the blowing was continued. When the pH dropped to 6 and lower, a portion of the gas started passing through. When the pH reached 4, the feeding of a 10% aqueous solution of sodium sulfite was started at a rate of 15.7 cc/min to maintain the pH at 4. In the above state, the gas passed through without absorption in the solution was caused to flow through a trap of an N/2 solution of potassium iodide and then through another trap of an N/2 solution of caustic soda. Those solutions were titrated with N/10 sodium thiosulfate while using an aqueous solution of starch as an indicator, whereby the chlorine levels in the traps were determined by titration. The concentration of chlorine in the waste gas was found to be not higher than 1 ppm by volume. In addition, the carbonic acid absorbed in the N/2 solution of potassium iodide and that absorbed in the N/2 solution of caustic soda were titrated respectively with N/10 NaOH and N/10 HCl while using methyl orange as an indicator. As a result, it was confirmed that the carbon dioxide gas blown was not absorbed in its entirety and was allowed to pass to an extent through the flask. Example 2: A similar reactor as in Example 1 was charged with an aqueous solution which contained 8.6% by weight of sodium sulfite and 1.4% by weight of caustic soda. Similarly, 1 ℓ/min of carbon dioxide gas and 100 cc/min of chlorine gas were mixed and caused to flow. When the pH reached 4, the feeding of an aqueous solution of the same composition as the feed solution was started at a rate of 12.1 mℓ/min so as to maintain the pH at 4. A gas passed through the flask in the above state was analyzed by a similar method as in Example 1. As a result, it was confirmed that the chlorine level was not higher than 1 ppm by volume and the carbon dioxide passed in its entirety. Example 3: In accordance with the flow chart of Fig. 2, a gaseous mixture 401 containing 30% by volume of carbon dioxide gas, 70% by volume of nitrogen plus oxygen and 1,000 ppm by volume of chlorine gas was introduced at a rate of 500 Nm 3 /hr from a lower part of the absorption column 407. The absorption column had a diameter of 0.8 m and was packed with 1-inch Raschig rings to a height of 2.2 m. The gaseous mixture 401 was washed with an absorbent liquid 402 recirculated at 10 m 3 /hr from the top of the column. In the column, the pH was maintained at 4.0 with about 94.5 kg/hr of an aqueous solution 403 containing 10% by weight of sodium sulfite while adding water 405 at a rate of 278 kg/hr. At the same time, the liquid level was maintained constant by means of an overflow line 404. A treated gas 406 flowed out of the column through an upper part thereof was sampled. As a result of its absorption and analysis with an N/10 aqueous solution of potassium iodide, the chlorine level was found to be 1 ppm by volume or lower. Example 4: The flow sheet of an apparatus used in the present Example is illustrated in FIG. 3. Fed to the apparatus was a gaseous mixture composed of 38.9% by volume of chlorine, 12.9% by volume of carbon dioxide and 48.2% by volume of nitrogen plus oxygen. When the compression pressure of the compressor and the cooling and liquefying temperature of the condenser were set at 7 kg/cm 2 -G and -24°C respectively, 27.3% of the gaseous mixture, namely, 67.5% of the chlorine in the gaseous mixture was condensed. The condensate was only fed to the upper part of the stripping column. The stripping column was operated by controlling its top pressure and bottom temperature at 7 kg/cm 2 -G and 25.4°C respectively, thereby obtaining liquid chlorine containing 99.5% of chlorine. A distilled gas from the upper part of the stripping column contained 18.6% by volume of chlorine, 17.1% by volume of carbon dioxide, and 64.3% by volume of nitrogen plus oxygen. Carbon tetrachloride in an amount 3.6 times by weight the feed gas to the absorption column was cooled to -15°C and used as an absorbent solvent. The absorption column was operated so as to maintain the top pressure at 7 kg/cm 2 -G. The composition of a waste gas from the top of the absorption column was 47 ppm by volume of chlorine, 19.7% by volume of carbon dioxide, 80.1% by volume of nitrogen plus oxygen, 0.2% by volume of carbon tetrachloride. Carbon tetrachloride with chlorine absorbed therein was fed by the pump from the bottom to the distillation column. The distillation column was operated at a top pressure of 1.3 kg/cm 2 -G, a condenser temperature of -15°C and a bottom temperature of 105.6°C. The composition of a gas recovered from the top of the distillation column was 89.4% by volume of chlorine, 7.3% by volume of carbon dioxide, 3.3% by volume of nitrogen plus oxygen, and 500 ppm by volume of carbon tetrachloride. The chlorine concentration in the solvent was 5 ppm by weight. The recovery rate of chlorine was 99.99%. A gaseous mixture discharged from the top of the absorption column and composed of 47 ppm by volume of chlorine, 19.7% by volume of carbon dioxide, 80.1% by volume of nitrogen plus oxygen and 0.2% by volume of carbon tetrachloride was introduced into the waste gas washing column to remove chlorine. The gaseous mixture was introduced from the bottom, while the absorbent liquid recirculated was caused fall down from the top. Accordingly, the chlorine in the gaseous mixture was absorbed. As the absorbent liquid, an aqueous solution containing 10% by weight of sodium sulfite was caused to flow down while controlling its pH at 4.2. The absorbent liquid was brought into contact with the gaseous mixture to have it absorb chlorine. The chlorine concentration in the gas released from the top was not higher than 1 ppm by volume.

Claims: A process for the removal of chlorine from a gaseous mixture comprising chlorine gas and carbon dioxide gas, which comprises feeding an aqueous solution or suspension containing an alkali metal sulfite and/or an alkaline earth metal sulfite, and washing the gaseous mixture with the solution or suspension while controlling the pH of the solution or suspension within a range of 1.9-6.3, whereby chlorine is solely removed from the gaseous mixture. A process for the removal of chlorine from a gaseous mixture comprising chlorine gas and carbon dioxide gas, which comprises feeding an aqueous solution or suspension containing an alkali metal sulfite and/or an alkaline earth metal sulfite along with an alkali metal hydroxide and/or an alkaline earth metal hydroxide in a proportion not greater than twice in molar ratio the alkali metal sulfite and/or alkaline earth metal sulfite, and washing the gaseous mixture with the solution or suspension while controlling the pH of the solution or suspension within a range of 1.9-6.3, whereby chlorine is solely removed from the gaseous mixture.

Inventors: AJIOKA MASANOBU, ITOH HIROYUKI, KIKUCHI ISAO, KONO YOSHITSUGU, KUDOH MITSUO, TAKENAKA SHINJI, AJIOKA, MASANOBU, ITOH, HIROYUKI, KIKUCHI, ISAO, KONO, YOSHITSUGU, KUDOH, MITSUO, TAKENAKA, SHINJI
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1989
Main classification: C01B7
IPC codes: B01D53, C01B7