Catalytic oxidation, process emissions

The emission streams from purified terephthalic acid (PTA) plants commonly contain carbon monoxide, methyl bromide, and various volatile organic compounds (VOC s). Before the vent gas (offgas) is exhausted to the atmosphere, these contaminants (often regulated) must be destroyed, normally by the catalytic oxidation process. Currently, most commercially available catalysts are used at an inlet terrperature higher dian 350 C. The improvement of the catalyst activity is desired to increase the catalyst life-time and to reduce the operational cost. Additionally, the catalyst selectivity needs to be improved to minimize or eliminate the formation of polybromobenzenes (PBB s) which can cause plugging or blockage in process lines. 

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. ... 

The additional sulfur for polysulfide pulping can upset the sodium—sulfur balance in the kraft recovery cycle and increase sulfur emission problems. In the MOXY (Mead Corp.) process, polysulfide is formed from kraft white Hquor by catalytic oxidation of sodium sulfide in the white Hquor using air. 

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. 

With chlorinated organic componnds, it is important to know the daily emission limits for hydrogen chloride. Regnlations across the United States vary with respect to this acidic gas, and acid-gas nentralization following catalytic oxidation may or may not be required. This factor has a significant impact on the cost competitiveness of the process (D13106W, p. 4). 

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). 

Ethanol is decomposed to form ether and ethylene on the alumina catalyst. Formaldehyde and formic acid are produced by the catalytic oxidation of ethanol in an atmosphere containing oxygen. In the course of this catalytic oxidation on a certain kind of catalyst, the excited HCHO is produced and the CTL emission is observed in its relaxation process. 

As CTL emission is observed in the course of catalytic oxidation, we must consider the overall reaction process in order to describe the working mechanism of the CTL-based sensor. Figure 8 shows a schematic illustration of the catalyst layer to depict the simplified overall reaction processes involving CTL emission on the CTL-based gas sensor. 

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. 

Possibility of combination of removal by adsorption and catalytic conversion in a separate step. Solid catalysts may act as adsorbent to remove pollutants present in diluted wastewater emissions, and act as catalysts to catalyse the conversion of adsorbed substances during a regeneration step . This allows much better process energy use than wet (catalytic) oxidation for diluted wastewater. Also active carbon can be functionalized with a catalysF to allow its regeneration by oxidative treatment at much lower temperatures (around... 

The detoxification of hydrocarbon pollutants is one of the global environmental challenges [531-533]. The complete catalytic oxidation of hydrocarbons to carbon dioxide and water has received much attention in order to reduce their emission from motor vehicles and processing plants [531,533,534]. 

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. 

---