January 13, 2014
Separate hydrogen from methane electrochemically. The US petrochemical industry is undergoing a complete turnaround because of the sudden availability of inexpensive methane and ethane from shale gas. This renaissance is evidenced by the large number of new ammonia, methanol, and ethylene plants under construction. In addition to new investments in conventional plants, access to shale gas is prompting a renaissance of petrochemical process innovation for using methane in unconventional ways.
Aromatics are conventionally made from petroleum-based naphtha by catalytic reforming or as byproducts of naphtha-fed ethylene steam crackers. If techniques can be developed that use inexpensive natural gas to make aromatics, the developer would have a substantial competitive advantage.
J. Coelho Tsou and co-inventors disclose a technique that facilitates the conversion of natural gas to aromatics. One impediment to achieving this goal is that the conversion of methane to aromatics is thermodynamically limited. For example, the inventors state that the equilibrium conversion in the nonoxidative dehydroaromatization of methane to benzene at 1 bar and 750 ºC is only ≈17%.
One way to push the conversion to higher levels is to remove the evolved hydrogen from the product effluent. Separating hydrogen from unreacted methane, however, is not straightforward. The inventors developed a technique for this separation that is based on removing hydrogen electrochemically by using a gas-tight membrane–electrode assembly in which hydrogen is transported through the membrane in the form of protons.
One example in the patent describes a membrane based on H3PO4-filled polybenzimadazole. The anode and cathode contained 1 mg/cm2 platinum. The experiments were performed at 160 ºC and atmospheric pressure. A test gas mixture that consisted of 11.4 mol% H2, 88.10 mol% CH4, 0.5 mol% ethylene, 100 mol ppm benzene, and 50 mol ppm ethane was prepared and run through the membrane at varying flow rates, voltages, and current densities.
At a 100 mL/min flow rate, 88 mV voltage, and 0.52 A/cm2current density, hydrogen conversion was a very high 88%. When the flow rate was increased to 1 L/min, the conversion dropped to 26%. (BASF SE [Ludwigshafen, Germany]. US Patent 8,609,914, Dec. 17, 2013; Jeffrey S. Plotkin)