Ethylene epoxidation in low-temperature alternating current plasma

Abstract

In this research, ethylene epoxidation reaction was investigated in low temperature plasma systems: parallel plate dielectric barrier discharge (DBD), cylindrical DBD, corona discharge, and DBD jet. The combined catalytic and plasma process was initially investigated in the parallel plate DBD system, cooperating with silver catalysts loaded on two different supports (silica and alumina particles) for ethylene oxide production. From the results, the presence of silver catalysts improved the ethylene oxide production performance. The silica support interestingly provided a higher ethylene oxide selectivity than the alumina support. The optimum Ag loading on the silica support was found to be 20 wt.%, exhibiting the highest ethylene oxide selectivity of 30.6%. Next, the separate C2H4/O2 feed was investigated in the cylindrical DBD system in order to improve the epoxidation performance. The C2H4 feed position of 0.25 was considered to be an optimum position and other operating conditions, including O2/C2H4 feed molar ratio, applied voltage, input frequency, and total feed flow rate, were subsequently investigated to find out the best conditions. In comparisons with the mixed feed, the separate feed of C2H4 and O2 could provide a superior ethylene epoxidation performance, resulting in higher EO selectivity and yield, and lower power consumption. These results can be explained by the fact that the C2H4 separate feed can reduce all undesired reactions of C2H4 cracking, dehydrogenation, and oxidation reactions; therefore, the separate C2H4/O2 feed was used to further study in corona discharge and DBD jet. The effects of C2H4 feed position, O2/C2H4 feed molar ratio, applied voltage, input frequency, total feed flow rate, and electrode gap distance on ethylene epoxidation were investigated in the corona discharge reactor. The optimum operating conditions; a distance between plate electrode and C2H4 feed position of 0.2 cm, an O2/C2H4 feed molar ratio of 1:2, an applied voltage of 18 kV, an input frequency of 500 Hz, a total feed flow rate of 100 cm3/min, and an electrode gap distance of 10 mm; provided the highest EO yield of 1.8%. The DBD jet, modified from the corona discharge and cylindrical DBD was employed for the ethylene epoxidation. The highest EO selectivity of 55.2% and yield of 27.6%, as well as the lowest power consumption were obtained at a total feed flow rate of 1,625 cm3/min, an O2/C2H4 feed molar ratio of 0.25:1, an applied voltage of 9 kV, an input frequency of 300 Hz, and an inner electrode position of 0.3 mm.