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  Enhanced process for the purification of anhydrous hydrogen chloride gasUSPTO Application #: 20070261437Title: Enhanced process for the purification of anhydrous hydrogen chloride gas Abstract: The present invention relates to a process for purifying anhydrous hydrogen chloride gas (“aHCl”), and preferably the anhydrous hydrogen chloride gas recovered from an isocyanate production process. In the process of the present invention, the content of chlorinated organics may be reduced from up to 1000 ppm by volume to below 10 ppb by volume levels. Generally, the process of the invention allows for chlorinated organic levels to be reduced to from 1 to 100 ppb, rendering the treated hydrogen chloride gas usable in a catalytic oxychlorination process or a Deacon process. The treated gas is also suitable for absorption in water or dilute hydrochloric acid. (end of abstract)
Agent: Bayer Material Science LLC - Pittsburgh, PA, US Inventors: Eric F. Boonstra, John M. Teepe, Renae M. Vandekamp, Anke Hielscher, Kaspar Hallenberger USPTO Applicaton #: 20070261437 - Class: 062617000 (USPTO) Related Patent Categories: Refrigeration, Cryogenic Treatment Of Gas Or Gas Mixture, Separation Of Gas Mixture The Patent Description & Claims data below is from USPTO Patent Application 20070261437. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] The present invention relates to a process for purifying anhydrous hydrogen chloride gas ("aHCl"), and preferably the anhydrous hydrogen chloride gas recovered from an isocyanate production process. In the process of the present invention, the content of chlorinated organics may be reduced from up to 1000 ppm by volume to below 10 ppb by volume levels. Generally, the process of the invention allows for chlorinated organic levels to be reduced to from 1 to 100 ppb, rendering the treated hydrogen chloride gas usable in a catalytic oxychlorination process or a Deacon process. The treated gas is also suitable for absorption in water or dilute hydrochloric acid. [0002] A number of important chemical processes generate 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 to 1600.degree. 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. The processes disclosed in these patents require high amounts of energy and expensive equipment. [0003] Treatment of aHCl at pressures of 5 to 20 bar absolute and final temperatures below -20.degree. C. is disclosed in U.S. Pat. No. 6,719,957. The process disclosed in the '957 patent results in contaminant levels occasionally unacceptable for use in vinyl chloride production. The contaminant level achieved is always unacceptable for use in Deacon processes. [0004] In the commercial phosgenation processes for the production of isocyanates such as TDI (toluene diisocyanate, MDI (diphenylmethane diisocyantes) and HDI (hexamethylen diiscocyanate), two moles of aHCl are formed per isocyanate group produced. This large quantity of by-product must be used in a secondary process. [0005] One such secondary process is the production of muriatic acid. However, the volume of HCl byproduct produced often exceeds the market demand. Another alternative is to use the aHCl in a catalytic oxychlorination process with ethylene to produce ethylene dichloride and finally vinyl chloride as the commercial product. This catalytic process is very sensitive to traces of organic compounds, particularly (chloro-) aromatic compounds which can deactivate the catalyst employed. [0006] Another secondary process is the Deacon process, which produces chlorine and water by passing gaseous HCl and oxygen over a transition metal catalyst. This process is very sensitive to traces of some contaminants, such as sulfur and some organic compounds, which over time can lead to catalyst deactivation and/or plugging of reactors, which in turn can lead to unwanted by-product formation. [0007] The most commonly used solvents in isocyanate production are chlorobenzene and dichlorobenzene (See G. Oertel, Polyurethane Handbook, page 66 (Carl Hanser Verlag, Munich (1985)). The aHCl recovered from the phosgenation process is saturated with these chloro-aromatics. Deep chilling of the aHCl gas can reduce the chloro-aromatic content, but not to the necessary level. Another complicating factor is the high melting point of dichlorobenzene (o-isomer: -17.5.degree. C., p-isomer: +52.8.degree. C.), which limits the usefulness of this approach. Low pressure phosgenation processes such as those described in G. Oertel, Polyurethane Handbook, p. 66 (Carl Hanser Verlag, Munich (1985)), which yield aHCl gas at pressure ranging from atmospheric to below 5 bar, will, even with deep chilling, contain chloro-aromatics in a concentrations of from several hundred ppm to 1000 ppm. [0008] The present invention has several objects: i) a process for the removal of one or more contaminants from hydrogen chloride gas, ii) a process for separating small quantities of high boiling material, e.g., (chloro) aromatic compounds from large volumes of anhydrous HCl gas; and, iii) a process for reducing the concentration of contaminants such as (chloro)aromatic compounds in anhydrous HCl gas to <100 ppb. BRIEF DESCRIPTION OF THE DRAWINGS [0009] FIG. 1 schematically illustrates a flow diagram for the present invention. [0010] FIG. 2 schematically illustrates a second embodiment of the present invention. [0011] FIG. 3 schematically illustrates a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [0012] The present invention is broadly directed to a cooling and distillation process to remove contaminants having boiling points higher than hydrogen chloride from a hydrogen chloride-containing gas comprising: [0013] a) compressing said hydrogen chloride-containing gas, [0014] b) cooling the resultant compressed gas in a first heat exchanger resulting in a first condensate stream and a first gas stream, wherein said compressed gas is cooled to a temperature low enough to partially condense said contaminants and at a rate sufficiently low that fog formation is prevented, [0015] c) feeding said first gas stream from said first heat exchanger to a distillation column having a top portion and a bottom portion to a point between said top portion and said bottom portion, to cause mass transfer between liquid and gas and to thereby concentrate the contaminants in the bottom portion of said column and hydrogen chloride gas in the top portion of said column, [0016] d) feeding said hydrogen chloride gas from said top portion to a second heat exchanger whereby the hydrogen chloride gas is partially condensed to form a second condensate stream and a second gas stream, [0017] e) feeding said second condensate stream to said top portion of said column to provide reflux to said column, [0018] f) feeding said first condensate stream to said distillation column below the point where said first gas stream is fed, [0019] g) feeding said second gas stream from step d) to said first heat exchanger as cooling medium, [0020] h) recovering purified hydrogen chloride gas from said first heat exchanger, and [0021] i) feeding said contaminants from the bottom portion of said column to a collection vessel. [0022] In the compression step (step a)), the gas is preferably compressed to a pressure of from 5 to 30 bars absolute. [0023] The contaminants contained in the gas stream are preferably chlorinated aromatic compounds. In one preferred embodiment, the gas stream also contains a contaminant with an intermediate boiling range between the hydrogen chloride boiling point and the chlorinated aromatic compound boiling point. The intermediate contaminant is removed from the distillation column, is subsequently depressurized, and is discarded. In one especially preferred embodiment, the intermediate contaminant is phosgene. [0024] In the cooling step (step b)), the incoming contaminated gas is cooled slowly. The temperature difference between the cooling wall of the first heat exchanger and the inlet gas temperature is preferably between 0.5 and 40.degree. C., and most preferably in the range of from 5 to 25.degree. C. The temperature of the compressed gas is preferably reduced to a temperature of from +10 to -25.degree. C. in the cooling step (step b)). [0025] In one embodiment of the invention, in step f), the condensate stream (of step b)) is fed to a separation vessel (or vessels) used to trap solids. If multiple vessels are used, one vessel can be used to collect solids while overflowing condensate from the vessel being fed to the distillation column at a point below the point where the first gas stream is fed, while the other vessel is depressurized to enable collected solids to be purged to waste. [0026] In another preferred embodiment, step e) comprises: [0027] e1) feeding said second condensate stream to one or more separation vessels used to trap any solids present and to form a solids stream and a third condensate stream, [0028] e2) feeding said third condensate stream to said top portion of said column to provide reflux to said column. The solids collected in the solids stream can be purged to waste. [0029] In an other preferred embodiment, step i) comprises i1) feeding liquid from the bottom portion of said column to a reboiler to generate stripping vapors for the bottom portion of the column, and wherein the reboiler heats the said liquid at low heat flux so as to prevent foaming action and i2) removing any remaining liquid from the reboiler to a collection vessel for disposal. Preferably, from 5% to 95% of the liquid reaching the reboiler is evaporated. Most preferably, the reboiler design prevents the formation of foams and has a heat flux of from 500 to 20,000 BTU/hr/ft.sup.2 as a lower limit and from 3,000 to 30,000 BTU/hr/ft.sup.2 as a higher limit. In any even more preferred embodiment, a portion of the liquid removed from the reboiler comprises hydrogen chloride and contaminants and is sprayed into the gas stream being fed to the first heat exchanger, most preferably in amount of from 1 to 25% by weight of the weight of the incoming gas stream. [0030] The temperature of the gas being fed into the distillation column is preferably reduced to a temperature of from 0 to -35.degree. C. during the distillation step. [0031] In another preferred embodiment, the purified hydrogen chloride gas from the first heat exchanger is further purified by treatment with activated charcoal. [0032] In a second broad embodiment, the invention comprises [0033] a) compressing said hydrogen chloride-containing gas, [0034] b) cooling the resultant compressed gas in a first heat exchanger resulting in a first condensate stream and a first gas stream, wherein said compressed gas is cooled to a temperature low enough to partially condense said contaminants and at a rate sufficiently low that fog formation is prevented, [0035] c) feeding said first gas stream from said first heat exchanger to a distillation column having a top portion and a bottom portion to a point between said top portion and said bottom portion, to cause mass transfer between liquid and gas and to thereby concentrate the contaminants in the bottom portion of said column and hydrogen chloride gas in the top portion of said column, [0036] d) feeding said hydrogen chloride gas from said top portion to one side of a third heat exchanger and feeding said contaminants from the bottom portion of said column to the other side of said third heat exchanger to flash against and cool the hydrogen chloride gas passing through said third heat exchanger, whereby the following streams are formed: [0037] 1) a second gas stream containing contaminants, [0038] 2) a contaminant stream, [0039] 3) a third cooled gas stream, and [0040] 4) a second condensate stream, [0041] e) feeding said third gas stream to a second heat exchanger whereby the hydrogen chloride gas is partially condensed to form a third condensate stream and a fourth gas stream, [0042] f) combining said second condensation stream and said third condensation stream and feeding the resulting combined stream to said top portion of said column to provide reflux to said column, [0043] g) feeding said first condensate stream to said distillation column below the point where said first gas stream is fed, [0044] h) feeding said fourth gas stream from step d) to said first heat exchanger as cooling medium, and [0045] i) recovering purified hydrogen chloride gas from said first heat exchanger. 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