The need to reduce international reliance on oil and the increased access to natural gas reserves due to improved technological developments have simultaneously elevated recent interest in utilizing natural gas and its main component, methane, to invaluable products. Ethylene, formed from the steam cracking of petroleum, and ethane are critical for the petrochemical industry. Oxidative coupling of methane (OCM) has exhibited potential in producing these higher hydrocarbons since its conception in the early 1980s. Traditional OCM schemes use gaseous oxidants from energy-intensive air separation units to catalytically convert methane to C2 products known as the co-feed approach. An alternative approach is the chemical looping technology that poses an interesting technique to convert hydrocarbons to flexible products utilizing an intermediate, metal-based oxygen carrier that provides oxygen and catalytic activity towards higher hydrocarbons.
This study compares the traditional co-feed approach to the chemical looping approach for selectively controlling the OCM reactions. For both approaches, reducible manganese-based oxide catalysts or oxygen carriers were synthesized and compared. Co-feed and chemical looping experiments were conducted in a high-pressure, fixed bed reactor with pressures between atmospheric and 5 atm and at varied gas-hourly space velocities. The oxidation and reduction reactions are also investigated in a high-pressure thermogravimetric analyzer (TGA). The effects of pressure on the oxygen carriers were characterized by scanning electron microscopy (SEM) and nitrogen physisorption (BET). The experimental results are used in process simulations in order to model the theoretical commercial OCM co-feed system and the commercial OCM chemical looping system. Previous work demonstrated that pressure and steam have an effect on methane conversion and C2+ selectivity with the OCM co-feed and chemical looping processes. Initial simulations demonstrate that with proper heat integration, the OCM chemical looping process exhibit promising reactor and process operations for an alternative direct approach to utilize methane. The processes are further proposed and examined for various final products such as ethylene and liquid fuels.