1. Field of the Invention
The instant invention pertains to a process for the removal of sulfur oxides and oxidizable sulfur compounds from a waste gas and comprises the steps of oxidizing the waste gas with molecular oxygen to convert the oxidizable sulfur compounds and elemental sulfur which may be present to sulfur dioxide or trioxide, absorbing the sulfur oxides with a metal oxide absorbent, and regenerating the metal oxide absorbent by contacting it with a hydrocarbon in the presence of a hydrocarbon cracking catalyst under conditions such that the absorbent is substantially converted to a sulfur-free state. During regeneration, the spent absorbent is reconverted to the original metal oxide, and the absorbed sulfur oxides are removed as hydrogen sulfide.
2. Prior Art
A major industrial problem involves the dvelopment of efficient methods for reducing the concentration of sulfur containing air pollutants, such as hydrogen sulfide and sulfur oxides, in the waste gas streams from a variety of processes. By way of example, the well-known Claus process for the conversion of hydrogen sulfide to elemental sulfur produces an effluent gas which may contain up to 2 or 3 percent by weight of sulfur compounds, substantial proportions of which are hydrogen sulfide, and sulfur dioxide. Although a variety of methods have been developed for the control of these emissions, the prior art methods have not been entirely satisfactory.
Four fundamental approaches have been suggested for the removal of sulfur oxides from a waste gas. One approach involves scrubbing the waste gas with an inexpensive alkaline material, such as lime or limestone, which reacts chemically with the sulfur oxides to give a non-volatile product which is discarded. Unfortunately, this approach requires a large and continual supply of the alkaline srubbing material, and the resulting reaction products can create a solid waste disposal problem of substantial magnitude.
A second approach to the control of sulfur oxide emissions has been to employ various catalysts to promote the oxidation of the sulfur dioxide in a waste gas stream to sulfur trioxide. This sulfur trioxide may then be utilized for the production of sulfuric acid.
A third approach to the removal of sulfur compounds from a waste gas exemplified by the SCOT, Beavan and Cleanair processes involves catalytic reduction of the sulfur compounds to hydrogen sulfide with hydrogen present in the waste gas or with supplemental hydrogen followed by selective absorption of the hydrogen sulfide.
A fourth approach to the removal of sulfur oxides from a waste gas involves the use of sulfur oxide absorbents which can be regenerated either thermally or chemically. The process of the subject invention is representative of this approach.
U.S. Pat. No. 4,001,375, J. M. Longo, discloses and claims a process for the removal of sulfur oxides from a gas. The process of this patent involves absorbing the sulfur oxides with cerium oxide followed by regenerating the spent cerium oxide through reaction with hydrogen gas. The regeneration step results in the production of a regeneration gas which contains a 1:1 ratio of hydrogen sulfide to sulfur dioxide. The patent does not, however, suggest that the spent cerium oxide could be regenerated by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst to convert the absorbed sulfur oxides to hydrogen sulfide. Moreover, the process of this patent is not suitable for a waste gas containing hydrogen sulfide or other reduced forms of sulfur.
An article, entitled "Bench-Scale Investigation on Removing Sulfur Dioxide from Flue Gases," by Bienstock et al. in the Journal of the Air Pollution Control Associated, Vol. 10, No. 2, April 1960, pp. 121-125, discusses the ability of various metal oxides to absorb sulfur dioxide from a simulated flue gas at elevated temperatures. The article indicates that the spent metal oxides may be thermally regenerated and that reducing gases such as producer gas, hydrogen, and carbon monoxide are effective in lowering the temperature of regeneration. This article does not, however, suggest that a spent metal oxide absorbent could be regenerated by contacting it with a hydrocarbon in the presence of a hydrocarbon cracking catalyst.
An article, entitled "Making Sulphur from Flue Gas," by J. E. Newell in Chemical Engineering Progress (Vol. 65, No. 8) August 1969 proposes utilization of the Bienstock process to remove sulfur oxides from power station flue gases. Alkalized alumina is suggested as the absorbent and regeneration of the absorbent is effected by contacting it in a fluidized bed with the effluent gases from an autothermic reformer to which methane, steam and air is fed. The sulfur containing gas from the regenerator is sent to a Claus plant for sulfur recovery and the Claus tail gas is combusted with methane and air before being fed to the flue gas adsorber reactor. This article does not suggest that a spent metal oxide absorbent could be regenerated by contacting it with a hydrocarbon in the presence of a hydrocarbon cracking catalyst, nor does it suggest that a Claus tail gas may be oxidized in the presence of the cracking catalyst and the absorbent to react therewith.
A more recent article, entitled "Selection of Metal Oxides for Removing SO.sub.2 from Flue Gas," by Lowell et al. in Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971, is also addressed to the use of various metal oxides to absorb sulfur dioxide from flue gas. This article suggests that the absorbents should be regenerated thermally and does not discuss their regeneration under reducing conditions.
U.S. Pat. No. 4,071,436, W. A. Blanton, et al., discloses in one embodiment that sulfur oxides can be removed from various stack and tail gas streams including Claus plant tail gas by reacting the sulfur oxides with alumina contained in a particulate solid reactant and contacting the solid reactant with hydrocarbon to form hydrogen sulfide in a regeneration zone. The described embodiment does not make use of a hydrocarbon cracking catalyst in regeneration, does not utilize steam to form hydrogen sulfide and does not recycle the thus formed hydrogen sulfide to the Claus plant producing the tail gas.
The cyclic, fluidized, catalytic cracking of heavy petroleum fractions is one of the major refining operations involved in the conversion of crude petroleum oils to valuable products such as the fuels utilized in internal combustion engines. Such a process involves the cracking of a petroleum feedstock in a reaction zone through contact with fluidized solid particles of a cracking catalyst. Catalyst which is substantially deactivated by non-volatile coke deposits is then separated from the reaction zone effluent and stripped of volatile deposits in a stripping zone. The stripped catalyst particles are separated from the stripping zone effluent, regenerated in a regeneration zone by combustion of the coke with an oxygen containing gas, and the regenerated catalyst particles are returned to the reaction zone. In the application of this process to sulfur-containing feedstocks, catalyst is deactivated through the formation of sulfur-containing deposits of coke. In conventional processes, the combustion of this sulfur-containing coke results in the release of substantial amounts of sulfur oxides to the atmosphere. U.S. Pat. No. 3,835,031, R. J. Bertolacini et al., however, discloses a method for the reduction of these sulfur oxide emissions through the use of a molecular sieve type cracking catalyst which is impregnated with one or more Group IIA metal oxides. These metal oxides react with sulfur oxides in the regeneration zone to form non-volatile inorganic sulfur compounds. These non-volatile inorganic sulfur compounds are then converted to the starting metal oxides and hydrogen sulfide upon exposure to hydrocarbons and steam in the reaction and stripping zones of the cyclic cracking process unit. The disclosure of this patent does not, however, suggest that the sulfur oxides and oxidizable sulfur compounds contained in a waste gas, generated separately from catalyst regeneration, can be either controlled or eliminated by contacting the gas with a metal oxide modified cracking catalyst.
U.S. patent applications Ser. Nos. 748,555, I. A. Vasalos et al.; and 748,556, I. A. Vasalos, both assigned to Standard Oil Company (Indiana), now U.S. Pat. Nos. 4,153,535 and 4,153,534, respectively, disclose processes similar to that set forth in U.S. Pat. No. 3,835,031, which involve the removal of sulfur oxides from the regeneration zone flue gas of a cyclic, fluidized, catalytic cracking process through the use of a molecular sieve type cracking catalyst in combination with a regenerable metallic reactant which absorbs sulfur oxides in the regeneration zone and is converted back to starting material and hydrogen sulfide in the reaction and stripping zones of the cracking process. The disclosure of these applications is also limited to the removal of sulfur oxides from the regenerator effluent gas stream of a cyclic, fluidized, catalytic cracking process.
Our U.S. patent application Ser. No. 731,949, H. D. Radford et al., assigned to Standard Oil Company (Indiana), now U.S. Pat. No. 4,146,463 discloses a process in which a waste gas containing sulfur oxides and/or carbon monoxide is conveyed to the regeneration zone of a cyclic, fluidized, catalytic cracker and is there contacted with catalyst particles modified with a metal oxide which reacts with the sulfur oxides. The application does not suggest that a Claus plant tail gas in which hydrogen sulfide is the predominant sulfur compound would be susceptible to removal of sulfurr contaminants by contacting with cracking catalyst and a metal oxide absorbent. Moreover, the application does not suggest that the effluent gases containing hydrogen sulfide and organic sulfur compounds obtained when the catalyst and the absorbent are contacted with a hydrocarbon under cracking conditions may be recycled to the Claus plant, effectively to extinction.