CH4 partial oxidation

Several studies suggested that CH4 partial oxidation over Rh and other metals (that is, the combined activation of CH4 by O2. H2O and CO2) involves a network of structure-sensitive reaction steps (including C-H and C-0 bond breaking) [151—... 

A significant recent effort in this area is a collaborative study by Amoco and the Argonne National Laboratory utilizing solid oxide type membranes [112-113]. The newly developed membranes show improved mechanical and thermal characteristics and are reported to remain stable for over 21 days at 900°C under CH4 partial oxidation conditions. The membrane used was tubular in shape. A CH4/Ar mixture was allowed to flow in the tubeside which was packed with a Rh based catalyst. Air was the source of oxygen on the outside... 

The energy balance equations in every region account for energy changes due to the flow and diffusional transport of the various species and the energetic effects associated with the reaction. Tsai et al. [119] describe CH4 partial oxidation to syngas as the direct outcome of the total oxidation of CH4 coupled with CO2 and steam reforming. 

C.Y. Tsai, Y.H. Ma, W.R. Moser and A.G. Dixon, Simulation of nonisothermal catalytic membrane reactor of CH4 partial oxidation to syngas. Paper presented at the 3rd International Congress on Inorganic Membranes, July 10-14,1994, Worcester, MA, USA. 

IshiharaT,Tsuruta Y,TodakaT, Nishiguchi H and Takita Y (2002), Fe doped LaGa03 perovskite oxide as an oxygen separating membrane for CH4 partial oxidation . Solid State Ionics, 152/153,709-714. 

Sazonova NN, Pavlova SN, Pokrovskaya SA, Chumakova NA, Sadykov VA. Structured reactor with a monolith catalyst fragment for kinetic studies. The case of CH4 partial oxidation on LaNiPt-catalyst. Chemical Engineering Journal 2009 154 17-24. 

The particular reactivity of bare Si02 for the production of HCHO is a matter of debate and has not yet been completely rationalized. Parmaliana et al. [113] pointed out that the performance of the silica surface in CH4 partial oxidation is controlled by the preparation method. For several commercial Si02 samples, the following reactivity trend has been established, based on the preparation method precipitation > sol-gel > pyrolysis. The activity of such silicas has been correlated with the density of surface sites stabilized under steady-state conditions acting as O2 activation centers [114], and the reaction rate was the same for all the silicas when expressed as TOF (turnover frequency). Klier and coworkers [115] reported the activity data for the partial oxidation of CH4 by O2 to form HCHO and C2 hydrocarbons over fumed Cabosil and silica gel at temperatures ranging from 903 to 1953 K under ambient pressure. They observed that short residence times enhanced HCHO (and C2 hydrocarbon) selectivity, suggesting that HCHO did not originate from methyl radicals, but rather from methoxy complexes formed upon direct chemisorption. 

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

Effects of Li content on the catalytic behaviors and structures of LiNiLaOx catalysts The dpendence of performance of LiNiLaOx catalysts on Li content at 1073K was shown in Fig.l. When D/Ni mole ratio was 0, the relatively acidic LaNiOx had the highest CH4 conversion(92.0%), but no C2 yielded. The products were CO, CO2 and H2, and CO selectivity was 98.3%. It is not an OCM catalyst but a good catalyst for partial oxidation of methane(POM). With Li content and the baric property of LiNiLaOx catalysts increasing, CH4 conversion and CO selectivity decreased, but there was still no C2 formed imtil Li/Ni mole ratio was 0.4. There was a tumpoint of catalytic behavior between 0.2 and 0.4 (Li/Ni mole... 

Spencer and Pereira (1987) studied the kinetics of the gas-phase partial oxidation of CH4 over a Mo03-Si02 catalyst in a differential PFR. The products were HCHO (formaldehyde), CO, C02, and H20. 

The other two main processes for conversion of methane into synthesis gas are partial oxidation and CO2 reforming. In the 1940s, Prettre et al. (3) first reported the formation of synthesis gas by the catalytic partial oxidation of CH4... 

The reaction pathways for the partial oxidation reaction are still debated. According to one interpretation, CO2 and H20 are the primary products, and CO is formed by the reaction of CO2 or H20 with CH4 according to another interpretation, CO is produced directly by the reaction of CH4 with O2. 

The purpose of this chapter is to provide a critical assessment of the literature regarding the partial oxidation of methane and the C02 reforming of methane, with emphasis on the following challenging areas hot spots, 02 separation cost, and the issues of reaction pathways and catalyst selection we also address the issue of carbon deposition in the C02 reforming of methane. The reason why we review these two reactions together is that they have many common characteristics, including the catalysts, the products, and CH4 as reactant. 

Fig. 1. Relationship between catalyst temperature and reaction time in methane partial oxidation catalyzed by Ni/Si02 (temperature of the gas phase (a) 1019 K, (b) 899 K, (c) 809 K, (d) 625 K). The reaction was carried out in a fixed-bed reactor (a quartz tube of 2 mm inside diameter) at atmospheric pressure. Before reaction, the feed gas was allowed to flow through the catalyst undergoing heating of the reactor from room temperature to 1073 K at a rate of 25 K min-1 to ignite the reaction, and then the reactant gas temperature was decreased to the selected value. Reaction conditions pressure, 1 atm catalyst mass, 0.04 g feed gas molar ratio, CH4/O2 = 2/1 GHSV, 90,000 mL (g catalyst)-1 h-1) (25).

Fig. 6. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane disk was prepared by pressing Bao.5Sro.5Coo.8Feo.2O3-s oxide powder in a stainless steel module (17 mm inside diameter) under a pressure of (1.3-1.9) X 109 Pa. The effective area of the membrane disk exposed to the feed gas (CH4) was 1.0 cm2 (72).

Fig. 7. Methane conversion, CO selectivity, and oxygen flux through the ceramic membrane during the partial oxidation of methane in a ceramic membrane reactor (see Fig. 6). Reaction conditions temperature, 1148 K catalyst, 300 mg of LiLaNi0JC/y-Al203 air flow rate, 300 mL min-1 (NTP) feed gas molar ratio, CH4/He = 1/1 feed flow rate, 42.8 mL min-1 (NTP) (72).

Fig. 8. CH4 conversion as a function of the number of CH4/O2 pulses for partial oxidation of CH4 catalyzed by Ni/La203. Reaction conditions temperature, 873 K catalyst, 20 mg of 20 wt% Ni/La203 loaded in a fixed-bed flow reactor feed gas, 0.9 mL CH4/02 (molar ratio 2/1) in each pulse carrier gas, helium (flow rate, 100 mL min-1) (134).

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