Catalysts activity losses

The first-stage effluent temperature has been limited to 560 °C in order to prevent excessive catalyst activity losses. The heat of reaction data is slightly inconsistent with the reported activation energies, but use of this expression demonstrates the ease with which temperature dependent properties may be incorporated in the one-dimensional model. 

Catalyst activity loss was caused by coke formation only. 

Pilot plant results indicated that satisfactory catalyst life could be realized by gradual temperature increase to offset the decrease in activity with time. This decrease in activity was caused mainly by the formation of more refractory aromatic recycle oils. In commercial operation the activity loss of the catalyst was more rapid. The decline in catalyst activity could be slowed down by decreasing the end point of the feed middle oil or by withdrawing small amounts of heavy ends formed. Commercial operation indicated further that the use of recycle hydrogen was a cause of the more rapid loss of catalyst activity. Ammonia and volatile ammonium salts formed by the reduction of tar bases in the feedstock might have been a factor in the accelerated-catalyst-activity loss. 

One of the surprising results of this study Is the ability of the catalyst to operate with concentrated, CO-rich feed gas without catastrophic catalyst deactivation. The present data show that deactivation of methanol-synthesis catalyst in a slurry reactor is within the range of commercial feasibility. Although the experimental data show that catalyst activity loss correlates with the loss of BET surface area, the relationship between catalyst activity and sintering requires more study. Since commercial methanol-synthesis catalysts are composed primarily of Cu and ZnO, with only smalt amounts of AI2O3, the present data do not discriminate between sintering of the overall surface and sintering of the Cu metal surface, which is believed to provide the actual sites for reaction. 

In eqn. 2, e a(T)t j a factor that expresses the extent of catalyst deactivation with time, and may be regarded as a time constant for catalyst activity loss. This form of catalyst deactivation factor derives from the assumption that catalyst activity decays exponentially with time, at constant temperature. A and E are, respectively, the preexponential factor and the activation energy for the rate constant. E can be determined from an Arrhenius plot for k1 under conditions where the quantity e a(T)t is essentially constant. Fig. 4 shows the results of experiments that were conducted at the end of the catalyst life tests at 498°K and 538eK shown in Fig. 3. The greater extent of deactivation at 538°K is clearly evident in Fig. 4, since the data at 538°K fall well below those at 498°K. The activation energies derived from the slopes of the two lines on Fig. 4 average t7.9 kcal/mole and agree to within 0.8 kcal/mole. 

Those who are interested in chromia-alumina catalyst are referred to the detailed review article by Poole and Maclver (44). These authors also studied the sintering of chromia-alumina (45) and found that chromia in the catalyst sinters faster than alumina. Their data suggest that catalyst activity loss may be a serious problem at the temperature levels required for high propane conversions. 

Catalyst deactivation is a common pathological phenomenon in many industrial reactions. In the case of hydrocarbon steam reforming to produce synthesis gas, catalyst activity loss may be due to coke arising from carbon deposition. Carbon lay-down usually occurs via undesired side reaction, namely ... 

Predictions of reforming catalyst activity, such as the reactor inlet temperature (RIT) required to make a specific octane, are determined by the activity kinetics. In the monitoring of Mobil s commercial reformers, inlet temperatures are continually compared to model predicted start-of-cycle (SOC) RlT s to assess commercial catalyst SOC activity and catalyst activity loss over a reformer cycle. Accurate predictions of activity cure therefore essential. 

Idling Membrane pinhole formation Membrane proton conductivity loss Catalyst activity loss Chemical decomposition by peroxide attack Chemical decomposition by peroxide attack Poisoning by membrane fragments... 

Severe environmental Catalyst activity loss condition Membrane proton conductivity loss Gas-diffusion layer gas permeability loss Poisoning by air/fuel impurities Cation contaminants exchanged with protons Gas flow blocked by dust accumulation... 

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