Process for purification of anhydrous hydrogen chloride gas

#The present invention relates to a process for purifying anhydrous hydrogen chloride gas (aHCl), preferably the anhydrous hydrogen chloride gas recovered from an isocyanate production process and to equipment suitable for use in this process. In the process of the present invention, the content of chlor-aromatics may be reduced from up to 1000 ppm to below 10 ppm levels to make the treated hydrogen chloride gas usable in a catalytic oxychlorination process.

#A number of important chemical processes generate anhydrous hydrogen chloride (aHCl) as a byproduct. Examples of such processes include chlorination processes, silane production processes and phosgenation processes. Because large amounts of aHCl can not be disposed of, one of the challenges encountered with each of these processes is purification of the aHCl generated to obtain a usable technical product or raw material for other processes. Several processes for purifying aHCl generated during production processes have been proposed. Thermal treatment of the aHCl at temperatures of up to 800-1600° C. is disclosed in U.S. Pat. No. 5,126,119. Full condensation and distillation under elevated pressure is disclosed in U.S. Pat. No. 4,935,220. However, these processes require high amounts of energy and critical, expensive equipment.

#In the commercial phosgenation processes for the production of isocyanates such as TDI (toluene diisocyanate), MDI (diphenylmethane diisocyanates), and HDI (hexamethylene diisocyanate), two moles of aHCl are formed per isocyanate group produced. This large quantity of byproduct must be used in a secondary process.

#FIG. 1 illustrates an apparatus suitable for carrying out the purification of aHCl in accordance with the present invention.

#The present invention relates to a process and apparatus suitable for the removal of small quantities of high boiling material (i.e., a material having a boiling point which is at least 100° C. at normal atmospheric pressure) from a large volume of anhydrous HCl gas. The process of the present invention is particularly useful for removing chloroaromatic compounds from anhydrous HCl produced as a byproduct in the amine phosgenation process for producing isocyanates.

#In the process of the present invention, an anhydrous HCl gas containing contaminants such as chlorobenzene, o-dichlorobenzene or toluene, is compressed, cooled in a first cooling stage, further cooled in a second cooling stage, returned from the second cooling stage to the first cooling stage, and the condensates from the first and second cooling stages are collected and removed. These collected condensates contain a significant amount of the unwanted high boiling material such as a chloroaromatic. In the compression step of the process of the present invention, the pressure of the gas containing the unwanted high boiling material is increased by at least 7 bar, preferably at least 10 bar, most preferably at least 12 bar. The initial pressure of the hydrogen chloride gas prior to compression is generally in the range of from about 1 to about 6 bar, most preferably from about 1 to about 2 bar. Any compression means known to those skilled in the art may be used to accomplish the desired degree of compression. The optimum amount of compression is dependent upon the initial pressure of the gas and the capability of the processing equipment and may be readily determined by simple preliminary tests. In one particularly preferred embodiment of the invention, the initial pressure of the anhydrous hydrogen chloride gas fed to the compressor is from about 1 to about 5 bar absolute which is increased to from about 8 to about 20 bar absolute.

#The compressed gas is then fed to a first cooling stage. In the first cooling stage, the temperature of the compressed gas is reduced to at least 20° C. above the second stage temperature, preferably at least 12° C. above the second stage temperature. The initial temperature of the compressed anhydrous hydrogen chloride gas fed to the first cooling stage may range from 30 to 150° C., preferably from 30 to 60° C. In a preferred embodiment of the present invention, the temperature of the compressed gas fed to the first cooling stage is from 40 to 60° C. and is cooled to a temperature of from 0 to −20° C. As the gas is cooled, a condensate containing unwanted high boiling material (e.g., chloroaromatics) is formed. This condensate is collected and removed from the cooling stage.