Catalytic oxidation, process emissions control

A third major group of emissions controlled by envirorunental legislation comprises volatile organic compoimds (VOCs) from all industrial and refinery effluents. The US Clean Air Act amendments of 1990 called for the reduction in the concentration of no fewer than 188 toxic chemical air pollutants before 2000. Many of these chemicals are VOCs and significant reductions have been achieved by the introduction of a range of new catalytic oxidation processes. The magnitude of the problems involved in envirorunental control and the potential demand for catdysts is demonstrated by Table 11.1 that shows the estimated volume of some effluents in the USA during 1995. ... 

Design nd Operation. The destruction efficiency of a catalytic oxidation system is determined by the system design. It is impossible to predict a priori the temperature and residence time needed to obtain a given level of conversion of a mixture in a catalytic oxidation system. Control efficiency is determined by process characteristics such as concentration of VOCs emitted, flow rate, process fluctuations that may occur in flow rate, temperature, concentrations of other materials in the process stream, and the governing permit regulation, such as the mass-emission limit. Design and operational characteristics that can affect the destmction efficiency include inlet temperature to the catalyst bed, volume of catalyst, and quantity and type of noble metal or metal oxide used. 

Once an undesirable material is created, the most widely used approach to exhaust emission control is the application of add-on control devices (6). For organic vapors, these devices can be one of two types, combustion or capture. Applicable combustion devices include thermal incinerators (qv), ie, rotary kilns, liquid injection combusters, fixed hearths, and fluidized-bed combustors catalytic oxidization devices flares or boilers/process heaters. Primary applicable capture devices include condensers, adsorbers, and absorbers, although such techniques as precipitation and membrane filtration are finding increased application. A comparison of the primary control alternatives is shown in Table 1 (see also Absorption Adsorption Membrane technology). 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

The Influence of Phosphorus Poisoning. - Catalytic oxidation using noble metal catalysts has been used to reduce the concentration of unburned hydrocarbons, carbon monoxide pollutants released from internal combustion engines, and similar applications. It is well known that contaminants arising from lubricants, (P, Ca, and Zn) deactivate these catalysts. Phosphorus compounds in printing processes are the source of decay of noble metal catalysts used to control these emissions. 

The catalytic reduction or decomposition of nitrogen oxides, particularly NO to N2, is very important from the viewpoint of environmental air pollution. The selective reduction by hydrocarbons in the presence of air has been extensively studied as a potential process in NO emission control for Diesel and lean-bum engines (see certain recent publications [61]). 

Nitrogen oxides contribute both to ground level ozone formation and acid deposition in the form of acidic particles, fog, and rain [11]. The major source of NOx is from high-temperature combustion processes of fossil fuels in power plants, vehicles, and airplanes. A potentially very large field of application for high-temperature NOx sensors is for emissions control for next-generation lean-bum engines, where catalytic converters are ineffective [12]. 

The release of VOCs into the environment has widespread environmental imph-cations. Pollution by VOCs has been linked to the increase in photochemical smog and ozone depletion. In addition, many VOCs are themselves toxic and/or carcinogenic. The US Clean Air Act of 1990 was one of the first measures to call for a 90% reduction in the emissions of 189 toxic chemicals, with 70% of these classed as VOCs, by 1998. Hence, in recent years, the development of effective technologies for the removal of VOCs from the atmosphere has increased in importance with the introduction of legislation to control their release. Various methods have been proposed, and one of the best is heterogeneous catalytic oxidation. This has the advantage over the more common original thermal oxidation process, since it requires less supplementary fuel and is therefore a less expensive process. However, the characteristics of the catalyst selected for this process are of vital importance for successful operation, and potential problems such as lifetime and deactivation must be solved if catalytic oxidation is to be employed universally. Catalysts currently in use include noble metals, notably platinum and palladium, and those based on metal oxides, however, irrespective of the type of catalyst, the most important characteristics are activity and selectivity for total oxidation. 

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