Conversion oxygen

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. 

Equation 1 is referred to as the selective reaction, equation 2 is called the nonselective reaction, and equation 3 is termed the consecutive reaction and is considered to proceed via isomerization of ethylene oxide to acetaldehyde, which undergoes rapid total combustion under the conditions present in the reactor. Only silver has been found to effect the selective partial oxidation of ethylene to ethylene oxide. The maximum selectivity for this reaction is considered to be 85.7%, based on mechanistic considerations. The best catalysts used in ethylene oxide production achieve 80—84% selectivity at commercially useful ethylene—oxygen conversion levels (68,69). 

Figure 6 shows typical results obtained with the plug-flow quartz reactor containing 0.5 g of Sr(lwt%)/La203 catalyst operated in the continuous flow recycle mode. The inlet CH partial pressure was 20 kPa (20% CH in He) at inlet flowrates of 7.1 and 14.3 cm STP/min. A 20% O2 in He mixture was supplied directly, at a flowrate Fog, in the recycle loop via a needle valve placed after the reactor (Fig. 1). The methane conversion was controlled by adjusting Fog, which was kept at appropriately low levels so that the oxygen conversion...

GP 8] [R 7] Dilution with the inert gas argon served to simulate the oxidation behavior when using air. Methane conversion and H2 and CO selectivity remain constant for a long range of dilution until they finally drop at inert gas contents above 50% [CH4/O2 2.0 10 - 57 vol.-% Ar 0.15 MPa 7.8 10 h (STP) 105 W] [3]. Oxygen conversion is near-complete for all experiments. The micro channels outlet temperatures drops on increasing the amount of inert gas. 

Figure3.67 Steep increaseofpropaneand oxygen conversions with increasing reactorinlettemperature. ( ) Fixed-bed reactor ( ) micro-channel reactor. C3Hg/02/Ne=0.3/0.15/0.55, 50mlmin a,b) ...

HRMAS 13c-NMR is a promising method to investigate the nature of carbon-containing residues trapped in zeolitic structures when running hydrocarbons and oxygenates conversion reactions (10). 

Several important chemical reactions for the conversion of coal to methane are shown in Table 2. Steam conversion involves the reaction of coal with steam to produce hydrogen and carbon monoxide. Hydrogen conversion is a reaction in which coal and hydrogen react to form methane. Oxygen conversion produces hydrogen and carbon monoxide by partial oxidation of coal. Methan-ation involves a reaction in which methane and water are produced from carbon monoxide and hydrogen. The water gas shift reaction between carbon monoxide and steam produces carbon dioxide and hydrogen. 

Steam conversion/methanation has a theoretical heat recovery efficiency of 1005L Hydrogen conversion has a theoretical efficiency of about 90%, if the production of hydrogen by steam conversion is taken into account, however, the theoretical efficiency drops to 81%. Oxygen conversion/methanation has a theoretical efficiency of only 61 which is the lowest of the conversion systems. 

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. 

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%. 

In order to compare the activities of the catalysts, the temperature at which 20% oxygen conversion is reached is displayed in Table I, while the oxygen conversion as a function of temperature is shown in Figure la. The increase in activity caused by the addition of vanadium is remarkable good results were also obtained for the chromium-containing material. In both cases, the temperature at which reaction started was 200 °C below that required for the unpromoted Nb205. The addition of molybdenum also increased the activity, but molybdenum was lost... 

Figure 1 Oxygen conversion and propylene selectivity as a function of temperature for niobia with various additives.

It appears that the catalyst containing 0.26 mol% V2O5 is much less active than the other samples. The other catalysts are more or less equally active when oxygen conversion is considered. On a per unit surface area basis, VlNb is less active than V5Nb and VlONb, but this sample also has a much higher surface area. Results not given here showed that the minimum value of selectivity, which usually occurs when the oxygen conversion reaches 100%, became lower for the more active catalysts. 

The results of XPS combined with Ar" -sputtering on a VlNb imp.-sample are shown in Figure 4. In this figure, sputtering profiles are shown for a fresh catalyst and for one which had been used for 300h at 510 °C (at this temperature 100% oxygen conversion was reached, no deactivation was observed). Since XRF showed that no vanadium was lost from the catalyst, it can be concluded that in the fresh catalyst, vanadium is present in a homogeneous layer which is at least 40-80 A thick. Upon use, sub-surface vanadium has diffused into the bulk of the catalyst, while the surface concentration of vanadium remains the same. The vanadium at the surface was found to be mostly V " in both the fresh and the used catalyst. Because of the low concentrations of vanadium at the surface and interference from the Ols peak, the presence of small amounts of reduced vanadium cannot be excluded. 

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