Poisoning deactivation, methanation

Exposure to reactant mixtures containing HMDS caused a decreased in catalytic activity. The magnitude of the decrease in catalytic activity was dependent on the HMDS concentration, and deactivation was much larger for methane than for butane (Fig.4). For the purpose of recovery, the deactivated catalyst was purged with the mixture of hydrocarbon. It was found that the catalysts were almost irreversibly poisoned for methane oxidation, but much of the activity was recovered for butane oxidation, (Fig. 4). 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

Methanation catalysts are not usually deactivated by thermal sintering. The principal reason for any loss of activity is poisoning. Sulfur compounds will poison methanation catalysts, but sulfur is not present unless the low temperature shift catalyst is by-passed. The poisons most likely to occur under normal operating conditions are those originating from the carbon dioxide removal system that precedes the methanator. Carry-over of a small amount of liquid into the methanator is not serious. Large volumes of liquid will have a... 

That the activation energies and rate concentration dependences for methanation over sulfur-poisoned Ni and Ru are the same as those observed under sulfur-free conditions (99-100,140,147,209) provides further evidence that sulfur poisoning of Ni and Ru involves geometric blockage of active metal sites. However, the observations that (i) the activation energies for methanation over sulfur-poisoned and carbon-deactivated Co are both 50 kJ/mol lower than that for fresh Co and (ii) the rate dependence on CO partial pressure is positive order for both carbon-deactivated Co and sulfur-poisoned Co, suggest that the deactivations of Co by sulfur and carbon are similar and may involve electronic as well as geometric effects. 

Rates of deactivation of Ni and Ni bimetallic catalysts as a result of poisoning by 10-ppm H2S during methanation were investigated in a series of studies by Bartholomew and co-workers (23, 113, 161, 194). Effects of catalyst composition and geometry, gas composition and reaction temperature on the rate of deactivation were considered. Deactivation rates were found to be relatively insensitive to temperature and quite sensitive to gas and catalyst composition (194). In fact, the rates of deactivation were 2-3 times more rapid in a H2-rich mixture (H2 /CO = 99), compared to a normal synthesis (H2/CO = 3-4) mixture. 

Three different Cr-Co spinels were prepared and tested as catalysts for the oxidation of methane in the presence of SO2, a typical catalyst poison. The spinels were prepared from nitrate precursors using a co-precipitation method, followed by calcining at three different temperatures, (400, 600 and 800 °C) for 5 hours. Characterisation results indicate that the catalyst calcined at 800 C presents a structure of pure spinel, whereas the presence of single oxides is observed in the catalyst calcined at 600 C, and the catalysts calcined at 400 C presents a very complex structure (probably a mixture of several single and binary oxides). Experiments show an important influence of calcining temperature on the catalyst performance. In absence of SO2, catalysts calcined at 400"C and 600 C performs similarly, whereas the activity of the catalysts calcined at 800 C is worse. When sulphur compounds were added to the feed, catalyst calcined at 600"C deactivated faster than the other two catalysts. 

Catalytic total oxidation of volatile organic compounds (VOC) is widely used to reduce emissions of air pollutants. Besides supported noble metals supported transition metal oxides (V, W, Cr, Mn, Cu, Fe) and oxidic compounds (perovskites) have been reported as suitable catalysts [1,2]. However, chlorinated hydrocarbons (CHC) in industrial exhaust gases lead to poisoning and deactivation of the catalysts [3]. Otherwise, catalysts for the catalytic combustion of VOCs and methane in natural gas burning turbines to avoid NO emissions should be stable at higher reaction temperatures and resists to thermal shocks [3]. Therefore, the development of chemically and thermally stable, low cost materials is of potential interest for the application as total oxidation catalysts. 

Ageing experiments were carried out at 450°C (Fig. 4). The operation tenperature was chosen because in previous experiments it was observed that self-deactivation of palladium catalysts is important at this temperature. Space time was 4.5 g h/mol CH4 in all the experiments. The gas feed consisted of 5000 ppmV methane in synthetic air. In order to study the poisonous effect of sulphur compounds, 40 ppmV SO2 were added to the feed in some experiments. 

It must be emphasized that high purity syngas is essential for the methanol synthesis catalyst, which is easily poisoned or deactivated by components such as chlorides, sulfur and heavy metals, i.e., mercury. Also, large quantities of inerts, namely, nitrogen and methane, which have no effect on the methanol catalyst, can reduce the overall carbon efficiency which will increase the amount of syngas required per unit make of methanol. 

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