Scheme

Source: Data from F. Nakajima, 1991, Air pollution control with catalysis-Past, present, and future, Catalysis Today 10:1. Note: aCatalyst: Pd/activated alumina.

Source: Data from F. Nakajima, 1991, Air pollution control with catalysis-Past, present, and future, Catalysis Today 10:1. Note: aCatalyst: Pd/activated alumina.

that oxidation is complete and that partial oxidation products (which can be pollutants) are not formed. Scrubbing with a mild basic aqueous solution removes the acidic component in the tail streams.

The oxidation of a VOC is exothermic. However, because a VOC is present only as a dilute component, the reaction heat is insufficient to maintain the temperature required for oxidation. Therefore, additional heating is required, or a catalyst must be used that enables total oxidation at lower temperatures (see Table 5.20.8).

Most catalytic operations also require elevated temperatures to ensure a complete reaction at a suitably fast rate. Therefore, either the catalyst bed is heated, or the feed stream is preheated. Inlet gas streams are kept 50-150°C higher than the ignition temperatures (Nakajima 1991). When the preheating uses an internal flame, the process is complex, involving both the products of combustion in the flame and processes occurring at the catalyst. The operating parameters (temperature and space velocity) for an application depend on the destructability of the VOC (see Table 5.20.9).

In a typical application, more than one VOC is destroyed. Frequently, oxidation at a catalyst is competitive under stoichiometric or partial oxidation conditions, or one VOC inhibits catalytic oxidation of another. Using excess oxygen in air overcomes these problems. A range of parameters applies in catalytic oxidation for VOC destruction. These parameters depend on the application (Spivey 1987).

Mechanisms

The oxidation of a VOC (S) at a catalyst can involve a species at the surface and in the vapor phase. The

Langmuir-Hinshelwood mechanism (Scheme 1) requires the adsorption of each species at nearby sites and subsequent reaction and desorption.

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