Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Halogenous component
Reexamination Certificate
2002-04-20
2002-10-15
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Halogenous component
C423S24000R, C502S305000, C502S306000, C502S307000, C502S308000, C502S309000, C502S313000, C502S319000, C502S321000, C502S344000, C502S345000, C502S349000, C204S158200, C204S157300, C204S157480, C204S157150, C204S158210, C210S748080
Reexamination Certificate
active
06464951
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for treating a contaminated gas stream and to a catalyst for reducing the phosgene content in the gas stream by conversion of the contaminant into less harmful products.
In spite of decades of effort, a significant need remains for an advanced technology to control stationary source emissions of volatile organic compounds (VOCs) as for example benzene, chlorinated volatile organic compounds (CVOCs) as for example trichloroethylene, and toxic air pollutants (TAPs) as for example acrylonitrile. A particular need exists for technology which controls emissions from industrial processes and other applications where low concentrations of VOCs and TAPs are present in high flow rate air streams.
Dilute air stream pollution control is becoming recognized as a major environmental control issue for the United States industrial community at large. For example, the control of indoor air pollution associated with solvent degreasing operations is necessary, including the dilute emissions associated with exhaust ventilation fans. Also, air stripping of contaminated groundwater produces dilute air emissions for which current technology provides no satisfactory solution. Catalytic combustors are available, but require processing tremendous volumes of air and result in uneconomic performance. Thermal incinerators require excessive supplemental fuel for dilute mixtures, and exhibit uncertain selectivity when CVOCs are involved. Gas membrane processes are only now emerging for gas separation, and are ill-suited for dilute mixtures. Pressure swing adsorption using zeolites or resins is not applicable to dilute mixtures, and rotating wheel adsorbers are uneconomic for such dilute concentrations of organics. Packed bed activated carbon adsorption is widely practiced, but creates a hazardous solid waste which is increasingly difficult to manage. Carbon regeneration by steam is costly, and is generally economic only for very large scale operations. Landfill options for spent carbon will become more limited, as it involves transportation and disposal of hazardous wastes, particularly for CVOC applications.
Control of indoor air pollution is also of growing importance, with the objective of enhancing workplace environmental health and safety protection. The Occupational Safety and Health Administration (OSHA) is promulgating new regulations to reduce workplace exposure to indoor air contaminants such as CVOCs. Many CVOCs are particularly toxic. Certain CVOCs are suspected carcinogens, others are linked to possible birth defects, and still others are suspected active precursors in the destruction of the stratospheric ozone layer. Of the 189 targeted air toxics in the Clean Air Act Amendments of 1990, about one-third of the compounds are chlorinated. By the standards of conventional air pollution control technology, indoor air pollution is at exceedingly dilute concentrations. An effective and economic pollution control technology in dilute air systems is the objective of this invention.
In spite of considerable efforts of researchers in the field, most UV photolytic and photocatalytic systems exhibit shortcomings in performance which limit their commercial utility for air pollution control. When treating air to remove CVOC contaminants, one shortcoming of note is the propensity of such processes to produce undesirable byproducts of incomplete oxidation, both by the photolytic treatment of CVOC contaminated air and by the photocatalytic treatment of CVOC contaminated air. Indeed, some such byproducts can be more harmful than the original contaminant being removed, such as the formation of phosgene byproduct through both photolytic and photocatalytic oxidation of trichloroethylene. As used herein, photolytic oxidative destruction is defined as the reaction of contaminants in an oxygen-containing gas stream as a result of the action of the ultraviolet radiation, by oxidation reactions, decomposition reactions, bond-scission reactions, and the like. Photocatalytic oxidative destruction is defined as the reaction of contaminants from an oxygen-containing gas stream as a result of the action of the ultraviolet radiation on the surface of a photocatalyst, by oxidation reactions, decomposition reactions, bond-scission reactions, and the like.
The photocatalytic oxidation of chlorinated hydrocarbon emissions often produces phosgene as a byproduct. U.S. Pat. No. 4,966,665 discloses the use of titanium dioxide as a photocatalyst for destruction of chlorine-containing organic compounds in an oxygen bearing vent gas. When a vent gas containing 30 ppm TCE was contacted with titanium dioxide for 26 seconds, a 90% destruction of TCE was obtained. However, the analysis of the reactor products revealed the formation of 4 ppm phosgene as a byproduct of the reaction. This phosgene production must be controlled, so as not to be emitted into the atmosphere.
Phosgene also is a contaminant in a number of industrial process emissions, and economic and simple means to control these emissions is required. Duembgen, et al., in U.S. Pat. No. 4,301,126, teach that phosgene can be decomposed catalytically in the presence of active carbon by hydrolysis with water vapor, at temperatures of 10° C. to 80° C. However, tests in the examples that follow show that the effectiveness of this system declines with time of usage, probably because of the difficulty of replacing the OH
−
groups on the carbon which have been consumed through hydrolysis of the phosgene.
German Patent No. 961,681 teaches that phosgene can be catalytically decomposed by contact with activated carbon and liquid water. However, with this technique, it is difficult to control the correct flow and proper dispersion and contacting of water with the carbon. Further, it requires a long residence time of operation.
Sauer et al., in U.S. Pat. No. 4,764,308, teach that phosgene can also be formed from a mixture of chlorine and carbon monoxide over activated carbon, at temperatures from 50° C. to over 250° C.
German Application DASA 1,667,438 discloses a process where phosgene is destroyed by contacting phosgene and water vapor with an alkali-activated alumina at temperatures from 95° C. to 190° C. However, substantial energy is consumed to raise the gas stream to these high temperatures, and the alkali would be consumed by the phosgene and must be replaced at considerable cost.
Schwarz, et al., in U.S. Pat. No. 5,362,399, teach removal of trace amounts of water from a phosgene stream containing at least 60% phosgene by contacting the stream with a strongly basic macroreticular anion exchange resin which catalyzes the hydrolysis of phosgene and the trace of water. The residence time required for this operation is measured in minutes, which may be economic for the liquid streams mentioned in the '399 patent, but not for the gaseous streams which the present invention is directed to.
Scholz, et al., in U.S. Pat. No. 4,064,218, disclose that phosgene can be removed from an off-gas stream, by washing with an aqueous solution of alkali and ammonia, at a temperature of 10° C. to 100° C. Bicker, et al., in U.S. Pat. No. 4,900,523, disclose that phosgene can be removed from waste gases by contact with an aqueous solution of at least one amine. Gross, in U.S. Pat. No. 4,493,818, discloses that phosgene can be removed from an off-gas by contacting with an aqueous solution containing alkali metal hydroxide and a tertiary amine compound. The use of such aqueous scrubbers is more complex to operate and more costly than the technology of the present invention.
Doughty, et al., in U.S. Pat. No. 5,492,882, disclose a metal-promoted activated carbon adsorbent for removing noxious gases from a contaminated air stream. The activated carbon is impregnated with at least one compound selected from each of the groups of sulfuric acid and sulfuric acid salts, molybdenum compounds, and copper and zinc compounds. Adsorption processes are uneconomic because they require a means to regenerate the adsorbent or replacement of th
Gerrish David A.
Kittrell James R.
Milazzo Michael C.
KSE Inc.
Strickland Jonas N.
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