Electric Field-Enhanced Catalytic Conversion of Methane: AN Experimental Study on the Effects of Corona Discharge on Methane Reactions
The oxidative coupling of methane (OCM) is currently being actively studied for the production of higher hydrocarbons from natural gas. The present study concentrates on the oxidative conversion of methane in a high-pressure (one atmosphere), nonthermal plasma formed by corona discharge. Here, methyl radicals are formed by the reaction of methane with negatively-charged oxygen species created in the corona discharge. The results of methane conversion in the presence of both AC and DC corona discharges revealed that ethane and ethylene product selectivity is affected by electrode polarity, frequency, and oxygen partial pressure in the feed. Higher C_2 yields were obtained with the AC corona. All of the AC corona discharges specified here were initiated at room temperature (i.e., no oven or other heat source used), with temperature increases from 300 to 500^circC due to the exothermic gas discharge and exothermic reaction. A reaction mechanism is presented to explain the observed phenomena. The results suggest that AC and/or DC gas discharge techniques should be further studied for improved economics of methane conversion. The oxidative dehydrogenation of ethane in DC corona discharges was investigated. The atomic oxygen radicals initiated by corona discharges are thought to be active for the OXD of ethane. The selectivity to ethylene is affected by the reaction temperature, the DC applied voltage, voltage polarity, and the C_2H _6/O_2 ratio. The results of this study suggest the corona discharge process to be very efficient and selective in the oxidative dehydrogenation of ethane. The effects of DC corona discharge were examined in the presence of a typical OCM catalyst, Sr/La _2O_3. Experimental investigations have correspondingly been conducted, in which all reactive gases passed through a catalyst bed situated within the corona-induced plasma zone. The methane conversion and C_2 yield increased (with O_2 partial pressure) during the corona-enhanced catalytic reactions, while the C _2 selectivity slightly decreased with increasing O_2 partial pressure. Experiments were conducted with Sr/La_2O _3, Sm_2O _3, Li/MgO and NaOH/CaO catalysts (under identical operating conditions) to evaluate the catalytic performance in the presence of a DC corona discharge. The results show that the Sr/La_2O _3 catalyst yields are the best results. This suggests that the corona heterogeneous technique has potential for improving the process economics. Methane conversion to higher hydrocarbon was achieved with temperatures from 100 to 500^circ C with DC corona-discharge heterogeneous reactions. In the absence of the corona discharge, the catalytic process over NaY zeolite produces no C_2 hydrocarbons over the entire range of temperatures. The corona discharge heterogeneous reaction, however, resulted in both high C_2 yield (6-15%) and high methane conversion (14-53%) 100^circ C. Increasing the gas temperature was found to have a negative effect on both C_2 formation and C_2H_4 /C_2H_6 ratio. Experiments conducted in both remote and direct corona discharges showed that the remote corona showed better enhancement than the direct corona under the identical experimental conditions. A study of the zeolite stability shows only slight changes in the methane conversion and C_2 selectivity over a period of two hours. The present methane conversion results (over NaY zeolite in the presence of DC corona discharges) suggests that the yield of CO and C_2H _4 are high. (Abstract shortened by UMI.).
- Pub Date:
- OXIDATIVE CONVERSION;
- Engineering: Chemical; Physics: Electricity and Magnetism; Physics: Fluid and Plasma