Partial, methane using membrane reactors

One example of membrane reactors is oxidation, in which oxygen from one phase diffuses from one side of an oxygen-permeable membrane to react with a fuel on the other side of the membrane. This avoids a high concentration of O2 on the fuel side, which would be flammable. A catalyst on the fuel side of the membrane oxidizes the fuel to partial oxidation products. One important process using a membrane reactor is the reaction to oxidize methane to form syngas,... 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

The present study investigates a different approach. The membrane is used to allow the desired intermediate product to escape from the reaction zone before it is consumed by further reaction. This use of a membrane reactor was first suggested by Michaels [15]. The partial oxidation of methane, which is a challenging reaction of the type propos for this application of membrane reactors, has been analyzed herein. There is no thermodynamic limitation for the production of carbon dioxide and water, actually these products are favored. It is desired to remove any partial oxidation product, for example formaldehyde, before it has a chance to be further oxidized. 

Several profound theoretical and experimental studies performed on the laboratory scale have been reported which focus on the use of various configurations of membrane reactors as a reactant distributor in order to improve selectivity-conversion performances. In particular, several industrially relevant partial oxidations have been investigated, including the oxidative coupling of methane [56], the oxidative dehydrogenations of propane [57], butane [58], methanol [59, 60], the epoxidation of ethylene [61], and the oxidation of butane to maleic anhydride [62]. 

The use of Pd-based membrane reactors can increase the hydrogenation rates of several olefins by more than 10 times higher than those in conventional premixed fixed>bed reactors. Furthermore, it has been pointed out that the type and state of the oxygen used to carry out partial oxidation of methane can significantly affect the conversion and selectivity of the reaction. The use of a solid oxide membrane (e.g., a yttria-stabilized zirconia membrane) not only can achieve an industrially acceptable C2 hydrocarbon yield but also may eliminate undesirable gas-phase reactions of oxygen with methane or its intermediates because oxygen first reaches the catalyst through the solid oxide wall [Eng and Stoukides, 1991]. 

The objective of this study was to determine the conversion and selectivity of the methane partial oxidation reaction when using high feed rates and low methane/oxygen feed ratios in the membrane reactor configuration. 

A Rh supported catalyst was chosen because Rh has been shown to be one of the most active and selective catalysts for methane partial oxidation [6-8]. A 3% Rh/Ti02 was the most active catalyst, which ignited at 320°C in a fixed bed microreactor when using methane and oxygen feed rates of 500 and 250 cc/min respectively. It yielded a methane conversion of 70% and a CO selectivity of 85%[10]. It was also found that 100% oxygen conversion is achieved in all cases and that the ignition temperature could be even lower for lower methane/oxygen feed ratios. Experiments were performed initially in the fixed bed reactor so that results obtained in the membrane reactor could be compared to those obtained in the fixed bed reactor. 

Another partial oxidation reaction that is attracting industrial attention for the application of reactive separations is the production of synthesis gas from methane [Stoukides, 2.127]. The earlier efforts made use of solid oxide solutions as electrolytes. Stoukides and coworkers (Eng and Stoukides [2.200, 2.126], Alqahtany et al. [2.201, 2.202]), for example, using a YSZ membrane in an electrochemical membrane reactor obtained a selectivity to CO and H2 of up to 86 %. They found that a Fe anodic electrode was as active as Ni in producing synthesis gas from methane (Alqahtany et al. [2.201, 2.202]), and that electro-chemically produced O was more effective in producing CO than gaseous oxygen (no ef-... 

M. Schwartz, J.H. White, M.G. Myers, S. Deych, and A.F. Sammels, in "The Use of Ceramic Membrane Reactors for the Partial Oxidation of Methane to Synthesis Gas", Preprints 213th ACS National Meeting, San Francisco, CA, April 13-17, 42, 1997. 

The use of membrane reactors is still in its infancy. In recent years, these reactors using mixed-oxide catalysts have been used in the oxidative coupling and partial oxidation of methane to syngas. There is ample opportunity to extend these studies to achieve a much better understanding of these systems. Besides, there are other applications such as the selective oxidation of both hydrocarbons and oxygenates that deserve attention. It has... 

Packed bed membrane reactors have been used for producing hydrogen via reforming of methane, reforming of alcohols, autothermal reforming, partial oxidation of methane, water gas shift, etc. 

Another measure to increase the hydrogen partial pressure difference between permeate and retenate is to use sweep gas on the retenate side. Because the hydrogen requires humidification for low temperature PEM fuel cells to prevent membrane dry-out, steam is the preferred sweep gas [405]. OHany et al. highlighted the effect of steam as the sweep gas for the permeate in a methane steam reforming membrane reactor [406]. Higher methane conversion was observed, which originated from back-diffusion of steam from the permeate to the reaction side of the membrane, which increased the S/C ratio and consequently the conversion. 

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