Direct partial oxidation

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

The Ru/Ti02 catalyst is unique, among many catalysts investigated, in promoting the direct partial oxidation of methane. 

The two mechanisms proposed to account for the partial oxidation of methane to syngas may be dedgnated as the IPO (Indirect Partial Oxidation) mechanism and the DPO (Direct Partial Oxidation) mechanism. The IPO mechanism was proposed by Prette et al [16] and Lunsford et al [12]. They think CO and H2 are the products of indirect reaction, the overall reaction of the POM reaction is composed of three different reactions... 

Direct partial oxidation of methane to produce methanol and other oxygenates... 

Oxidative coupling of methane to yield C2 and higher hydrocarbons 358 Direct partial oxidation of methane to produce methanol and other oxygenates 360... 

From these experiments, one can see that the direct partial oxidation of CH4 to synthesis gas over catalytic monoliths is governed by a combination of transport and luetic effects, with the transport of gas phase species governed by the catalyst geometry and flow velocity and the lanetics determined by the nature of the catalyst and the reactor temperature. Under the conditions utilized here, the direct oxidation... 

The direct catalytic conversion of methane has been actively pursued for many years. Much of the emphasis has been on the direct production of methanol via selective partial oxidation (8), coupling of methane to ethylene (9), or methane aromatization (10). At this time none of these technologies has been demonstrated commercially due to low yields of desired products due to combustion by-products or low equilibrium conversion at reasonable process temperatures and pressures. The potential benefits of a hypothetical process for the direct partial oxidation of methane to methanol (11) are presented as an example. 

Q. Zhang, D. He, Q. Zhu, Recent Progress in Direct Partial Oxidation of Methane to Methanol, 12, Journal of Natural Gas Chemistry, 81-89, (2003). 

The current two-step industrial route for the synthesis of methanol, from coal or methane to synthesis gas and then from synthesis gas to methanol, has certain drawbacks. The economic viability of the whole process depends on the first step, which is highly endothermic. Thus a substantial amount of the carbon source is burned to provide the heat for the reaction. It would be highly desirable, therefore, to replace this technology with a technically simpler, single-step process. This could be the direct partial oxidation of methane to methanol, allowing an excellent way to utilize the vast natural-gas resources. Although various catalysts, some with reasonable selectivity, have been found to catalyze this reaction (see Sections 9.1.1 and 9.6.1), the very low methane conversion does not make this process economically feasible at present. 

At the low temperatures and high pressures that are favorable for direct partial oxidation of methane, methyl peroxy may build up in significant concentrations [254]. The initial... 

The reaction time for direct partial oxidation of methane is of the order of tenths of seconds, which is much longer than the gas residence time in most combustion systems, and the low temperature chemistry for methane is seldom significant in combustion. One exception is the so-called NOv-sensitized oxidation, where presence of nitrogen oxides significantly enhances the oxidation rate of hydrocarbons at lower temperatures. This phenomena is discussed in Section 14.3.1.3. 

It is of interest to assess the process potential of methanol production by a direct partial oxidation of methane. This way the steam reformer and the shift reactor can be saved, and the catalytic methanol reactor can be replaced by a noncatalytic partial oxidation reactor. It is estimated that direct partial oxidation is competitive if a conversion of methane of at least 5.5% can be obtained with a methanol selectivity of at least 80%. 

M. C. Bjorklund and R. W. Carr, Enhanced methanol yields from the direct partial oxidation of methane in a simulated countercurrent moving bed chromatographic reactor. Indust. Engng. Chem. 

Otsuka, K., Wang, Y., Sunada, E., and Yamanaka, I. Direct partial oxidation of methane to synthesis gas by cerium oxide. Journal of Catalysis, 1998, 175, 152. 

Formaldehyde is now made from CH4, but through a series of separate processes steam reforming, methanol synthesis, and methanol oxidation. This complex process route leads to low efficiency and high cost. Direct partial oxidation to formaldehyde is a worthy proce.ss objective. We need a catalyst with high activity and selectivity for reaction (5.1). 

Natural gas, by direct partial oxidation, can provide olefins suitable for oligomerisation using the Mobil Olefin to Gasoline and Diesel process. Alternatively, synthesis gas routes to olefins can be via methanol or Fischer-Tropsch synthesis. In the Fischer-Tropsch option, the hydrogen-rich nature of the synthesis gas requires that the catalyst should have poor shift activity and produce a narrow range of lower olefins. 

Two mechanisms have been proposed for the POM reaction (i) The Combustion and Reforming Reactions mechanism (CRR). In this, the methane is combusted in the absence of oxygen in the first part of the catalytic bed, producing CO2 and H2O. Along the rest of the bed, and after total oxygen conversion, the remaining methane is converted to CO + H2 by SMR and CO2 reforming (reaction (2)). (ii) The Direct Partial Oxidation mechanism (DPO). CO + H2 is produced directly from methane by recombination of CHX and O species at the surface of the catalysts. 

This article provides a comprehensive review of advanced processes for direct conversion of methane. These processes include (1) direct partial oxidation of methane to methanol, (2) catalytic oxidative coupling, and (3) oxyhydrochlorination. The primary goal of this review is to present an overview of the state of the art of these technologies and provide an initiative for advancing the... 

Direct Partial Oxidation to Methanol. - The direct partial oxidation (DPO) route offers the advantage of direct conversion of methane to methanol. The oxidants include air, oxygen, and nitrous oxide. Recently, Pitchai and Klier, Gesser et al., Foster, Scurrell, and Kuo gave comprehensive reviews on this process. Catalytica Associates also discussed this subject in a client-private report. 

Partial/Selective Oxidation. - While publications are numerous concerning direct, partial oxidation to methanol, high yields of commercial interest using only oxygen or air as an oxidant have not been achieved and confirmed. Economically promising conversion rates and selectivities have not yet been proven in large-scale equipment. 

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