OCM IN A FIXED-BED REACTOR - LIMITS AND PERSPECTIVES

The oxidative coupling of methane (OCM) over a La2O3/CaO catalyst was studied in a polytropic fixed-bed reactor (I.D. = 15 mm, W/F = 0.15 g. s/ml). Reaction conditions for stable operation were determined. (1) A minimum inlet temperature of 580 degrees C was necessary to initiate the reaction. (2) The maximum hot-spot temperature of 1000 degrees C l imited the highest oxygen inlet concentration to 20%. The temperature gradients in the bed amounted to 250 K. The influence of the reaction conditions on the C-2+ selectivity was investigated by testing the eff ects of temperature (T-inlet = 580-860 degrees C), oxygen concentratio n (C-o2 = 5-20%) and particle diameter (d(p) = 250-350 mu m, and pelle ts of h(p) = 4 mm and d(p) = 4 mm). The C-2+ selectivity ran through a maximum with increasing temperature and decreased with rising inlet o xygen concentration. Mass-transfer limitations, which occurred when ap plying pellets, resulted in a drop of C-2+ selectivity. Highest C-2+ y ields amounted to 15.5% (X(CH4) = 31%, S-c2+ = 51%). Distributed feed of oxygen was tested as a means to cope with the high temperature grad ients and to increase C-2+ selectivity. Upon applying this mode of ope ration, oxygen concentrations up to 30% could be converted. However, n o improvement of C-2+ selectivity and yield compared to cofeed operati on was achieved.