Oxidative Conversion of Methane to Syngas

The POM is a weakly exothermic process that requires no additional heat. Developed in the early 1950s [316], by the end of the 1960s, it was used to produce about a quarter of the s)mgas for ammonia production in the U.S. [93]. The POM is usually conducted at a pressure of 30—100 atm with the use of pure oxygen, which is taken in a small excess over the stoichiometry of reaction (12.5) to provide a more complete conversion and to achieve the desired temperature. This, however, leads to the formation of some amount of deep oxidation products, CO2 and H2O. Typical temperatures of the noncatalytic POM that provide a complete conversion of methane and minimize soot formation are 1300—1500 °C, with the thermal efficiency of the process reaching 60—75%. 

The H2/ CO ratio for syngas produced from natural gas, close to 2, makes the process of partial oxidation of natural gas to s)mgas very attractive from the point of view of the s)mthe-sis of methanol and synthetic hydrocarbons. However, since part of the hydrogen is oxidized to water, the actual H2/CO ratio is typically 1.5—1.6. The situation can remedied by combining POM with steam reforming, for which the H2/CO ratio is 3. 

These calculations make it possible to estimate the yields of H2 and CO for the heavier hydrocarbons, such as propane and butane, direct kinetic calculations for which are difficult to perform because of the lack of reliable kinetic models of their oxidation under these conditions. Assmning that the dependences displayed in Fig. 12.4 hold for other hydrocarbons, one estimate the )delds of H2 and CO from propane and butane oxidation by using H C = 2.67 for propane and H C = 2.5 for butane. Thus, the main factor determining the )delds of H2 and CO in the homogeneous partial oxidation of hydrocarbons to syngas is the mixture composition. The optimal composition of the mixture and the corresponding maximum yields of the conversion products are determined by the specific conditions of the partial oxidation of the hydrocarbon. 

An increase in the total pressme increases the partial pressures of CH4, CO2, and H2O at the equilibrium gas composition i.e., high pressme is unfavourable for the POM. However, high temperatures achieved in the noncatalytic process compensate for this effect, because selectivities to CO and H2 increase with the temperature (Fig. 12.5) [318]. 

According to the above data, the methane—oxygen and methane—air systems produce practically interesting )delds of hydrogen and carbon monoxide only at 02 CH4 1, which 

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Fig. 9. Influence of temperature on conversion, H2/CO ratio and net heat of reaction in oxidative conversion of methane to syngas in presence of steam and C02 in the feed. Reaction conditions CH4/02 = 2.1, CH4/H20 = CH4/C02 = 12.0, GHSV=43 100cm3 g 1 h . With permission from Choudhary et al. (1996a).

Choudhary, V.R., Uphade, B.S., and Belhekar, AA. (1996) Oxidative conversion of methane to syngas over LaNiOa perovskite with or without simultaneous steam and CO2 reforming reactions influence of partial substitution of La and Ni. /. Catal., 163, 312 318. 

Chen, C. et al., Conversion of methane to syngas by a membrane-based oxidation reforming process, Angew. Chem. Int. Ed., 42, 5196, 2003. 

Methanol is an important multipurpose intermediate traditionally used for production of various chemicals [57], It is currently produced from syngas, which is industrially generated via catalytic steam or autothermal reforming of methane [13-15]. Figure 23.7 schematically illustrates commercial and alternative routes for methanol formation from methane. Despite the fact that syngas production and methanol synthesis are highly optimized processes, strong economic and environmental interests exist in direct oxidative conversion of methane to methanol. 

Partial oxidation of methane to syngas over Ni and Co catalysts was effected by use of microwave irradiation, and compared with conventional heating [73]. Although the same conversion levels and H2/CO ratio (2.0 0.2) were observed, the temperature of the catalyst bed was much lower (200 K) when microwave irradiation was used than with conventional heating. Under both activation modes the Ni-based... 

The ITM Syngas process involves the direct conversion of methane to synthesis gas (see Fig. 22.7). The process utilizes a mixed, conducting ceramic membrane and partial oxidation to produce the synthesis gas.40 The goal of this advanced reformer technology is to reduce the cost of hydrogen production by over 25 percent.59,79... 

The catalytic partial oxidation of methane to syngas was first reported by Pettree as early as 1946 [1]. More recently, several investigators have studied the reaction mostly on Ni supported catalysts [2-4] and on Ru oxides [5]. In the past several years, Schmidt and coworkers [6-8] have studied the reaction using various noble metals in a monolith reactor. In this configuration, high methane conversions and syngas selectivities were achieved in a reactor operated under autothermal conditions... 

This paper reviews research and development In several areas of methanol synthesis technology. Alternatives to the co-precipitated Cu-ZnO-A O and Cu-ZnO-CrgOj catalysts are considered first. Novel processes for syngas conversion are then reviewed, and the paper ends with a discussion of direct conversion of methane to methanol by partial oxidation of natural gas. 

The partial oxidation reaction of methane to syn is mildly exothermic, in contrast to hi fy endothermic steam reforming. It could produce stoichiometric syngas for methanol synthesis in one step. It is an ideal process for producing methanol syn. Effective catalysts are needed to carry the reaction selective at mild temperatures. A recent finding by researchers at the University of Oxford indicated that the reaction could be carried out selective at 775°C (97+% selectivity at 94% conversion) using lanthanide ruthenium oxide or alumina-supported ruthenium catalysts, in contrast to more than 1200°C in conventional processes [62],... 

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