Homogeneous gas-phase oxidation of methane using oxygen as oxidant in an annular reactor

The gas-phase partial oxidation of methane with oxygen has been investigated in a high-pressure quartz-lined annular reactor. The work undertaken consists of a systematic investigation of the effects of reactor tube wall temperature, pressure, feed oxygen concentration, and gas flow rate on methane conversion and methanol yield and selectivity. Methanol yields in the range of 1.5-2.3 mol % and selectivities in the range of 23-47 mol % have been observed, depending on the process parameters used. Increasing the oxygen concentration in the feed is found to decrease methanol selectivity dramatically, while yield exhibits a trade-off between decreasing selectivity and increasing conversion. The influence of pressure is most noticeable between 1.5 and 3.0 MPa, where substantially more methanol is produced at the higher pressure. The effect is less pronounced as the pressure is increased further. The most significant outcome of this study is the recognition of the importance of the interaction of the chemistry of the system and the heat-transfer properties of the reactor system. The system is very sensitive to heat release rate and exhibits a discontinuity in methane conversion, with hysteresis being observed under process conditions employing high feed oxygen concentrations and total gas flow rates. More importantly, highest methanol yields are observed on the downward sweep of reactor wall temperature, reinforcing the concept that the reaction is most sensitive to temperature and that low temperatures favor methanol production.