Process for treating halogenated hydrocarbon containing-gas

Abstract: The present invention provides a process of effectively absorbing and decomposing a halogenated hydrocarbon so as not to exhaust it in the air. The present invention provides a process for treating a gas containing halogenated hydrocarbon, which comprises the following steps; (a) the gas containing halogenated hydrocarbon is contacted with a solution which contains an aprobic polar compound having a 5 or 6 membered ring and a nitrogen at an alpha-position of a carbonyl, to absorb and recover a portion of the halogenated hydrocarbon, and (b) the gas, which is exhausted from the step (a) and still contains halogenated hydrocarbon, is contacted with heating with a mixture solution of an aprotic polar compound and a caustic alkali to decompose the halogenated hydrocarbon.

Description: FIELD OF THE INVENTION The present invention relates to a process for treating a gas containing a halogenated hydrocarbon (e.g. Freon gas or hydrocarbon chloride). BACKGROUND OF THE INVENTION Hydrocarbon chlorides (such as methylene chloride, 1,1,1-trichloroethane, trichloroethylene, tetrachloroethylene and carbon tetrachloride) have been widely used as a dissolving agent of rubber and fatty acids, or as a cleaning agent for dry cleaning and precision machines and elements. The hydrocarbon chlorides have many advantages in low melting point, high solubility or detergency, non-combustibility and so on, but have some disadvantages in toxicity and carcinogenicity. They are therefore used under very limited conditions, e.g. limited concentration in drainage. Air pollution based on the hydrocarbon chlorides is also a big problem, and very strict regulations have been applied since April in 1989 in Japan. The hydrocarbon chlorides are recovered or removed by an active carbon adsorption method which, however, is insufficient because the adsorption rate is poor and the recovery process is complicated. Hydrocarbon fluorides or Freons (such as trichlorofluoromethane (Freon 11), trichlorotrifluoroethane (Freon 113) and dichlorodifluoromethane (Freon 12)) have also been widely used for spraying, and as a refrigerant, foaming agent, solvent, or cleaning agent for precision machines and elements. The hydrocarbon fluorides have a wide boiling point range, within -40 to 50 °C, and very low toxicity. They also have high solubility in oil and organic material and therefore exhibit very high detergency. Accordingly, the hydrocarbon fluorides are very important in recent industries. However, in 1974, Professor Lorland of California University warned that Freon gas which was exhausted into the air reached the stratosphere without being decomposed in the troposphere and was decomposed by strong ultraviolet light in the stratosphere, so as to cause the decomposition or destruction of the ozone layer. He added that the destruction of the ozone layer reduced the ultraviolet light-absorbing capacity of the ozone layer and adversely increased ultraviolet light which reached the earth's surface, so as to adversely affect the ecosystem. This means that, in respect to human beings, bad effects such as increase of skin cancer would be increased. He further warned that the destruction of the ozone layer is a public pollution on the whole earth. After that, much research and many studies have been conducted, to find that there are an ozone hole above the South Pole, and an increase in skin cancer. This serious problem has been considered at many international congresses, including those in Montreal and The Hague. It would be very desirable to inhibit the use of some Freons, and to develop Freon substitutes. The possibility of recovering Freon, in order that it does not pass into the atmosphere, has been intensively studied. US-A-4468297 discloses the decomposition of a halogenated hydrocarbon, by contact with a strong nucleophile selected superoxide and hydroxide, in an aprotic solvent which may be, as one of many examples, N-methyl-2-pyrrolidone. GB-A-0630248 discloses the dehydrohalogenation of halogenated organic compounds by decomposition with caustic alkali in a mutual liquid solvent. US-A-4934149 discloses a process for the absorption and recovery of halogenated hydrocarbons, by contacting them with a sorbent material such as N-methyl-2-pyrrolidone. SUMMARY OF THE INVENTION According to the present invention, a process for treating a gas containing a halogenated hydrocarbon, comprises the steps of: (a) contacting the gas with a solution containing an aprotic polar compound which is a 1,3-di(C₁₋₃ alkyl)-2-imidazolidinone or 1,3-di(C₁₋₃ alkyl)-2-oxohexahydropyrimidine, to absorb and recover a portion of the halogenated hydrocarbon; and (b) contacting the gas exhausted from step (a), and still containing halogenated hydrocarbon, with heating, with a mixture of the aprotic polar compound and a caustic alkali, to decompose the halogenated hydrocarbon. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow chart which shows the process of the present invention. Fig. 2 is a sectional view of an apparatus that is suitable for use in the cooling step of the present invention. Figs. 3 to 5 are respective sectional views of apparatus that is suitable for use in step (a) of the present invention. Fig.6 shows a system for practicing the process of the present invention. DETAILED DESCRIPTION OF THE INVENTION A gas to be treated, which contains halogenated hydrocarbons, is generally sent to a first (absorbing) step (a) via a cooling step, but may be directly sent to the first step without the cooling step. The cooling step is conducted for liquefying a portion of the halogenated hydrocarbons from the gas by cooling it, when the halogenated hydrocarbons are present at a high concentration. The cooling step is carried out with a simple condenser as shown in Fig.2, but various types of coolers or condensers which are known to those skilled in the art can be used. In the first step, the gas is contacted with a solution which contains an aprotic polar compound that may be represented by the following chemical formulae; wherein R is an alkyl group having 1 to 3 carbon atoms. Typical examples of the aprotic polar compounds are 1,3-dimethyl-2-imidazolidinone (DMI), 1,3-dimethyl-2-oxohexahydropyrimidine and a mixture thereof. Preferred are those having a dipole moment of 3.7 to 4.8 D, especially 4.0 to 4.7 D, such as 1,3-dimethyl-2-imidazolidinone (DMI) and 1,3-dimethyl-2-oxohexahydropyrimidine. If the dipole moment is outside the above range, absorption and desorption abilities of the halogenated hydrocarbon are reduced. Although the aprotic polar compounds have been used as reaction solvents, it has been found by the present inventors that the aprotic polar compounds are good absorption solvents for the halogenated hydrocarbons, such as hydrocarbon chloride or Freon gas. In the first step, a conventional gas-liquid contact process is employed. It is preferred that a contacting area of gas and liquid is high. Preferred means include a bubble tower (Fig.3), a plate column (Fig.4) or a packed column. A cooling or pressuring apparatus may be attached to the first step, because a large amount of the halogenated hydrocarbon is absorbed when the absorbing liquid is cooled or the gas is pressured. The absorbed halogenated hydrocarbon may be recovered or desorbed in a recovering step which is shown below the first step in Fig. 1. The recovering step is carried out by distillation, evaporation or introduction of heated air or nitrogen at a high recovery or a high purity. The distillation is generally conducted by a conventional distillation tower. The recovered halogenated hydrocarbon and aprotic polar compound can be used again as an absorbent. The gas, in which a large amount of the halogenated hydrocarbon has been recovered in the first step, is then sent to the second step (a decomposing step). In the second (decomposing) step, the gas which still contains a small amount of the halogenated hydrocarbon is contacted with heating with an agent for decomposing halogenated hydrocarbon which contains the aprotic polar compound and a caustic alkali. The decomposing agent may contain water. In the decomposing step, the halogenated hydrocarbon is reacted with the caustic alkali to decompose to lower molecular weight compounds. It is believed that the decomposition proceeds according to the following chemical reaction equations; In the case of 1,1,1-trichloroethane, CH₃CCl₃ + 4NaOH → 3NaCl + CH₃COONa + 2H₂O In the case of Freon 11, Freon 113 CCl₃F + 6NaOH → 3NaCl + NaF + Na₂CO₃ + 3H₂O CFCl₂CF₂Cl + 8NaOH → 3NaCl + 3NaF + NaOOC-COONa + 4 H₂O Examples of the caustic alkalis are caustic soda, caustic potash, calcium hydroxide or a mixture thereof. The aprotic polar compound is the same as above-mentioned; preferred is 1,3-dimethyl-2-imidazolidinone (DMI). The weight ratio of the aprotic polar compound and the caustic alkali is not limited. The caustic alkali may be dissolved in the aprotic polar compound or an excess amount of it may be present in the aprotic polar compound. When water is added to the decomposing agent, it is preferred that an alkali-resistant dispersant is also added thereto. Water may be contained in an amount of less than about 30 volume %, preferably less than 20 volume %, based on the volume of the aprotic polar compound. A large amount of water reduces the decomposition ability. The second step is also conducted using the same gas-liquid contacting apparatus as is explained in the first step. The treatment may be conducted at a temperature of 10 to 200 °C, preferably 50 to 150 °C. It may be pressurised. The gas, which is exhausted from the second step, contains little of the halogenated hydrocarbon, but if necessary the halogenated hydrocarbon may be further removed by another method, such as plasma decomposition or contact decomposition. One embodiment of apparatus that can be used in the process of the present invention is shown in Fig.6. The gas to be treated 1 is cooled in the cooling step 2 and then sent to an absorbing tower 4 through an air blower 3. The gas is then sent to a decomposing tower 5 from the top of the absorbing tower 4. The halogenated hydrocarbon which is absorbed in the absorbing tower 4 is sent to a continuous distillation tower 7 through a liquid pump 6. In the distillation tower 7, the halogenated hydrocarbon is recovered to obtain a recovered liquid 8. The distilled aprotic polar solvent in the distillation tower is again sent to an absorbing tower 4 through a liquid pump 9. According to the present invention, in the absorbing process the halogenated hydrocarbon is effectively absorbed and in the decomposing process it is decomposed at a high decomposing rate, thus reducing the content of the halogenated hydrocarbon to the degree at which it is not substantially present. Also, the absorbed halogenated hydrocarbon is recovered and employed again. EXAMPLES The present invention is illustrated by the following Examples. Example 1 A pressure-resistant glass autoclave was charged with 1 g of 1,1,1-trichloroethane and as a decomposing agent 100 ml of DMI, 2.4 g of caustic soda powder and 10 ml of water, and packed with a Teflon packing. It was heated at 100 °C for 2 hours with stirring. The amount of caustic soda is 2 times as much as that calculated according to the above formula(1). The inside pressure increased 0.5 Kg/cm² and after 2 hours reduced to 0.2 Kg/cm². It was then cooled to ambient temperature. After cooling, the autoclave was opened and the content was taken out. It separated to two layers. The upper layer was transparent and had a pH of 8, and the lower layer was a concentrated caustic soda solution. A salt crystal was attached to the inside of the autoclave and was washed with methanol. It was dissolved in water and neutralized with diluted nitric acid. It was titrated with 0.1N AgNO₃ to obtain 1.46 g of sodium chloride. The theoretical amount in the formula (1) is 1.314 g, i.e. approximately the same. The recovered DMI could be used as the decomposing agent. Example 2 A pressure-resistant glass autoclave was charged with 1 g of Freon 11 and as a decomposing agent 100 ml of DMI, 3.2 g of caustic potash powder and 15 ml of water, and packed with a Teflon packing. It was heated at 80 °C for 2 hours with stirring. The amount of caustic soda is 1.3 times as much as that calculated according to the above formula(2). The inside pressure increased 0.6 Kg/cm² and after 2 hours reduced to 0.2 Kg/cm². It was then cooled to ambient temperature. After cooling, the autoclave was opened and the content was taken out. A salt crystal was attached to the inside of the autoclave and was washed with methanol. It was dissolved in 100 ml of water and neutralized with 1N diluted nitric acid to precipitate silver chloride or silver fluoride. It was heated and mixed to dissolve the silver fluoride and the remaining silver chloride was filtered and dried to obtain 2.93 g of it. It equals to 93.3 % of the theoretical amount. Example 3 A pressure-resistant glass autoclave was charged with 1 g of Freon 113 and as a decomposing agent 100 ml of DMI, 3.4 g of caustic soda powder and 15 ml of water, and packed with a Teflon packing. It was heated at 100 °C for 2.5 hours with stirring. The amount of caustic soda is 2 times as much as that calculated according to the above formula(3). The inside pressure increased 0.5 Kg/cm² and after 2 hours reduced to 0.2 Kg/cm². It was then cooled to ambient temperature. After cooling, the autoclave was opened and the content was taken out. A salt crystal was attached to the inside of the autoclave and was washed with 30 ml of methanol. It was dissolved in 100 ml of water and neutralized with 1N diluted nitric acid to precipitate silver chloride or silver fluoride. It was heated to 100 °C and mixed to dissolve the silver fluoride and the remaining silver chloride was filtered and dried to obtain 2.24 g of it. It equals the theoretical amount (2.30 g). Example 4 A flask was charged with 100ml of DMI, 10 g of caustic soda powder and 15 g of water, and heated to 90 °C with stirring. Then, air which contained 500 ppm of 1,1,1-trichloroethane was passed through the contents at a flow rate of 0.1 liter/min. Initially, a residual concentration (about 10 ppm) of 1,1,1-trichloroethane was present in the air; after 200 liters of air had been treated, the residual concentration was 30 to 35 ppm. This corresponds to a decomposing rate of 96.5%. Example 5 An flask was charged with the decomposing agent of Example 3 and allowed to stand to separate the DMI layer. The DMI layer was mixed with 5 g of caustic potash and charged in another flask. The content was heated to 130 °C with mixing, through which 200 liter of air containing 500 ppm of 1,1,1-trichloroethane was passed at 0.1 liter/min. The beginning 100 liter had no 1,1,1-trichloroethane and the last 100 liter had less than 10 ppm. The decomposing rate nearly equals 100 %. Example 6 An flask was charged with, as the decomposing agent, 100 ml of DMI, 10 g of caustic soda powder and 10 ml of water, and heated to 90 °C with stirring, through which 90 liter of air containing 1,000 ppm of Freon 11 was passed at 0.1 liter/min. The beginning 30 liter had a decomposing rate of 95 %, the middle 30 liter had a decomposing rate of 94 % and the last 30 liter had a decomposing rate of 91 %. The average decomposing rate is 93.3 %. In this case, the concentration of Freon 11 was less than 70 ppm. Example 7 An flask was charged with, as the decomposing agent, 100 ml of DMI, 10 g of caustic soda powder and 10 ml of water, and heated to 120 °C with stirring, through which 100 liter of air containing 1,000 ppm of Freon 113 was passed at 0.1 liter/min. The decomposing rate was 96.3 %. Then, another mixed gas of 500 ppm Fleon 113 and air was passed therethrough at 0.1 liter/min. The decomposing rate was 93.6 %. Example 8 An flask was charged with, as the decomposing agent, 100 ml of DMI, 5 g of caustic soda powder and 10 ml of water, to which 0.2 g of a perfluorohydrocarbon surfactant (available from Asahi Glass Co., Ltd. as Surflon S-141) was added, and heated to 90 °C with stirring. Through the contents, 300 liter of air containing 1,000 ppm of 1,1,1-trichloroethane was passed at 0.2 liter/min. The beginning 100 liter had a 1,1,1-trichloroethane content of 0 to 10 ppm, the middle 100 liter had 10 to 20 ppm and the last 100 liter had 20 to 30 ppm. The average decomposing rate was 98 %.

Claims: A process for treating a gas containing a halogenated hydrocarbon, comprising the steps of: (a) contacting the gas with a solution containing an aprotic polar compound which is a 1,3-di(C₁₋₃ alkyl)-2-imidazolidinone or 1,3-di(C₁₋₃ alkyl)-2-oxohexahydropyrimidine, to absorb and recover a portion of the halogenated hydrocarbon; and (b) contacting the gas exhausted from step (a), and still containing halogenated hydrocarbon, with heating, with a mixture of the aprotic polar compound and a caustic alkali, to decompose the halogenated hydrocarbon. A process according to claim 1, wherein the aprotic polar compound is 1,3-dimethyl-2-imidazolidinone or 1,3-dimethyl-2-oxohexahydropyrimidine or a mixture thereof. A process according to claim 1, wherein the aprotic polar compound is 1,3-dimethyl-2-imidazolidinone. A process according to any preceding claim, wherein the aprotic polar compound has a dipole moment of 3.7 to 4.8 D. A process according to any preceding claim, wherein the caustic alkali is caustic soda, caustic potash, sodium carbonate or a mixture thereof. A process according to any preceding claim, wherein the decomposition step is conducted at 50 to 150°C. A process according to any preceding claim, wherein the decomposition step is conducted in the presence of an alkali-resistant surfactant. A process according to any preceding claim, wherein the mixture used in the decomposition step is an aqueous solution.

Inventors: ITOYA NAOMI, MASHIO FUJIO, ITOYA, NAOMI, MASHIO, FUJIO
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1991
Main classification: B01D53
IPC codes: A62D3, B01D53