Oxygen conversion rates

Effect of Pressure. The effect of pressure in VPO has not been extensively studied but is informative. The NTC region and cool flame phenomena are associated with low pressures, usually not far from atmospheric. As pressure is increased, the production of olefins is suppressed and the NTC region disappears (96,97). The reaction rate also increases significantly and, therefore, essentially complete oxygen conversion can be attained at lower temperatures. The product distribution shifts toward oxygenated materials that retain the carbon skeleton of the parent hydrocarbon. 

Figure 3.44 Conversion rates and product selectivity of partial methane oxidation performed under constant heating power as a function of the methane/oxygen ratio [112. ...

Key points that limit the industrialization of the process were recently illustrated by researchers from Sumitomo. Since the selectivity to methacryhc acid plus methacrolein typically decreases with temperature as the conversion increases, this implies that the rate of production of useful products increases only until the higher conversion compensates for the fall of selectivity. As a consequence of this, the maximum productivity value is reached at a specified temperature. For instance, when a selectivity of 45% is reached at 22% isobutane conversion, with a residence time of 5.4 s, a temperature of 370°C, and a feed containing 25% isobutane, 25% oxygen, and 15% steam, a productivity equal to 0.72 mmol/h/gcat is obtained, which is one order of magnitude lower than the one needed to make the process industrially viable. However, the productivity is limited by the oxygen conversion, the maximum concentration of which is dictated by the flammability limits (see Figure 14.1), and by temperature, since the POM decomposes above 380°C. 

Regime I prevails at low volume fluxes of primary air. The overall conversion rate is controlled by the diffusion rate of oxygen to the oxidation of the solid char. Regime I is characterized by the fact that the overall conversion rate is slower than the macroscopic ignition rate, which implies that solid fuel convertibles are accumulated behind the macroscopic ignition front. In other words, the conversion zone is growing thicker. [9,33,40]... 

The batch reactor, above described, permits both to operate at quasi-zero conversion per pass and to evaluate the cat ytic activity at finite values of the reagents conversion. A typical test performed on Si02 catalyst at 600°C is presented in Figure 1. It is remarkable how in our approach the product selectivity is unaffected by the methane conversion. A special care was taken to avoid oxygen-limiting conditions and, hence, methane conversion data obtained for oxygen conversions below 20% only have been used for the calculation of reaction rates. 

Figure 6. Rate of oxygen conversion on Si02, 5%V205/Si02 and 4%Mo03/Si02 catalysts.

The highest formation rate for C2 hydrocarbons was found over the K promoted catalysts. The ratio of oxygen conversion to methane conversion at equal residence times revealed an increased amount of oxygen utilized by both Li and Na catalysts over MnMo04, while the oxygen consumption for the potassium promoted catalyst decreased below that of MnMo04 catalyst (Fig. 4c). 

The results of XRF, BET surface area measurements and catalytic testing are summarized in Table I, while the results of catalytic testing are also shown in Figure 1. The rates displayed in Table I are the rates of reaction of propane per unit surface area as calculated from the interpolated propane conversion. Propane conversion was 3% or less, while oxygen conversion was less than 30%. 

The effect of various oxygen-2-methylpropene ratios in the feed (R) on the conversion, rate of formation, and selectivity for Z = 0.02 at 425°C. is shown in Figure 3. While the conversion of 2-methylpropene and rate of formation of methacrolein increased steadily with feed ratios, the rates of formation of water and carbon dioxide increased rapidly with increasing R. However, the selectivity decreased with increased oxygen-2-methylpropene ratios in the feed. 

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