Natural gas conversion to light olefins in low-density plasmas

The value that may be added to natural gas, by its direct conversion to olefins, has prompted the low-density plasma approach. A geometry was chosen in which conversion takes place in a large-surface-area electrode assembly, where CH₄ ions combine with CH₄ neutrals to produce C₂₊ gases. Bench-level experiments were carried out on the direct, one-step conversion of CH₄ to ethylene, propylene, and other light hydrocarbons, in a low-pressure plasma glow-discharge. Relative percentages of useful higher hydrocarbons in the output mixture were ≤ 42% for ethylene, ≤ 68% for ethane, and ≤ 48% for acetylene, respectively. The relative proportions depended on pressure, current, and voltage across the plasma discharge. Conditions favoring the formation of one or two of these desirable compounds at the expense of the other light hydrocarbon products were established. The production operations for which this technology is intended would include a separation of the CH₄ component in the output mixture and its recycling in a feedback loop, to extinction, if no other uses were planned. Results relating the yields of the preferred gases to the pressure and the electrical conditions were presented.