The cost effectiveness of many large-scale chemical reactions is often determined by the marketability of the various reaction products. At the very least, the efficient utilization of reaction products produced during industrial reactions is highly desirable. Similarly, the effective and judicial use of energy and its conservation within a system produces significant economic and environmental advantages. Hence, methods and apparatus for achieving favorable product utilization and energy conservation are of great importance in the design and operation of large-scale reactions.
One reaction which has been shown by the assignee of the present invention to be economically viable is the large-scale wet oxidation of organic waste products such as municipal waste. Originally, wet oxidation reaction apparatus typically included massive, thick-walled, high pressure, above ground reactors with complex mechanical stirring mechanisms. As an alternative to these above ground reactors, attempts were made to design low-profile, subterranean or "down-hole" reaction apparatus. As a result, the first known successful subterranean wet oxidation reaction apparatus was constructed and operated by the assignee of the present invention according to the principles set forth in U.S. Pat. No. 4,272,383 to McGrew which is assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference. This down-hole reaction apparatus has a vertical configuration which utilizes gravitational force and thermodynamic relationships to provide a high-pressure reaction environment in which thermal energy is conserved.
The down-hole reaction apparatus is particularly useful in breaking down organic matter present in municipal waste through aqueous-phase combustion, generally referred to as wet oxidation. As will be known to those skilled in the art, wet oxidation of combustible matter is an exothermic reaction which proceeds quite rapidly at temperatures above 350.degree. F. The reduction of chemical oxygen demand (COD) of the waste is the primary goal of municipal waste destruction, and by reducing the COD of the waste, eutrophication of receiving waters is prevented. In addition, the wet oxidation process degrades potentially toxic hydrocarbons which would otherwise serve as a source of pollutants. Thus, wet oxidation is a proven method for the destruction of municipal waste and industrial organic waste.
Generally, a down-hole reaction apparatus comprises a vertically oriented, large subsurface chamber defined by the casing of a subterranean shaft. The subterranean shaft extends about 3,000 to 10,000 feet and preferably about 5,000 feet into the earth. Suspended in the chamber and spaced apart from the casing is a tubular reaction vessel. The tubular reaction vessel has a closed-end waste containment tube in which a waste pipe is centrally disposed. The containment tube and waste pipe are arranged concentrically to form an external passage or annulus defined by the inner wall of the containment tube and the outer wall of the waste pipe. The bore of the waste pipe and the external passage are in flow communication at the lower end of the reaction vessel. Also suspended in the chamber is a conduit which is substantially parallel to, but spaced apart from, the reaction vessel. Through this conduit, a heat transfer medium is preferably flowed into the chamber. Thus, an externalized heat exchanger is provided.
In order to rapidly oxidize large quantities of organic matter found in municipal waste, it is necessary to supply an oxidant. Hence, in operation, municipal or industrial waste containing organic matter is flowed into the downgoing reaction passage along with an oxidant, typically air and preferably oxygen enriched air or gaseous oxygen. Generally, liquid oxygen is supplied on site which must then be converted into the gaseous state. The rate of flow of the diluted municipal waste and the gaseous oxidants through the reaction apparatus are regulated to provide a mixed flow velocity which promotes intense mixing. Mixing enhances the mass transfer between the oxygen and the combustible components of the municipal waste. The gaseous oxygen is preferably injected into the diluted municipal waste through one or more gas supply lines which are suspended in the downgoing and/or upcoming reactant passage. Hence, it will be appreciated by those skilled in the art that the injection of gaseous oxygen or an oxygen enriched gas into the diluted municipal waste is an important aspect of the down hole wet oxidation process.
As the concentration of available oxygen and the temperature of the waste increase, the rate of the wet oxidation reaction increases. The exothermic oxidation reaction generates substantial heat which, in turn, further elevates the temperature of the reactants. When the temperature of the reactants exceeds about 350.degree. F. to about 400.degree. F., the reaction becomes autogenic. The hydrostatic head of the column of diluted waste prevents the reaction mixture in the downcomer from boiling. Typically, the column of diluted waste mixed with gaseous oxygen will extend the entire length of the mile long reaction apparatus. The temperarure of the reaction mixture is allowed to increase to about 500.degree. F. to 700.degree. F. in a reaction zone in the lower part of the downgoing reactant passage. The diluted municipal waste is thus oxidized in the wet oxidation reaction. The reaction products, or effluent, include a low-volume, sterile ash, a liquid effluent and off-gases. Supercritical reaction conditions are also possible.
These off-gases in the wet oxidation of municipal sludge, for example, include carbon dioxide, carbon monoxide, and short chain hydrocarbon gases along with relatively smaller concentrations of other constituents. Nitrogen and hydrogen are also produced during supercritical operation. When the wet oxidation process is operated to obtain very high COD reductions, the off-gases may contain an excess of 90% by volume CO.sub.2. The off-gases generated by the wet oxidation reaction may also have an obtrusive odor. It would therefore be desirable to devise a convenient method by which the compounds producing these undesirable odors can be efficiently eliminated.
As a chemical commodity, carbon dioxide is one of the highest volume chemicals produced in the United States. CO.sub.2 is used widely in greenhouses to promote photosynthesis. CO.sub.2 is used in various industrial chemical processes, including coke gasification processes. Liquid oxygen and liquid nitrogen are used as coolants in many applications such as the cooling of superconductors. As will be appreciated by those skilled in the art, to be a marketable commodity, these gases must be in the liquid state for ease of handling and shipment. It will also be appreciated that the liquefaction of gases generally requires a considerable expenditure of energy and is thus quite costly using conventional methods. Although the wet oxidation of municipal waste is an excellent source of gaseous CO.sub.2, the CO.sub.2 off-gas is generally not marketable unless it is liquified. Therefore it would be desirable to devise an economical method for liquefying the off-gas, and in particular the CO.sub.2 generated during wet oxidation.
The present invention provides such a method and apparatus for liquefying wet oxidation off-gases which at the same time gasifies liquid oxygen for injection into the wet oxidation reaction mixture. In another aspect, the present invention provides a method and apparatus for removing odor-producing compounds from the off-gases.