Simultaneous thermal cracking and oxidation of propane to propylene and ethylene

Abstract

Propane conversion by its simultaneous endothermic thermal cracking and exothermic noncatalytic oxidative conversion topropylene and ethylene in the presence of steam and limited oxygen was investigated at various process conditions (temperature, 635-800°C; C₃H₈/O₂ ratio in feed, 2.0-16.0; H₂O/C₃H₈ ratio, 0.0-2.5; space velocity, 1,170-7,150 h^-1; and sulfur additive/C₃ H₈ ratio, 10^-3- 10^-5). Influence of a sulfur additive (thiophene, CS₂, or dimethyl sulfide) in the feed on process performance was also studied. The propylene/ethylene mole ratio in the products is decreased with increasing the temperature but increased with the C₃H_S/O₂ and H₂O/C₃H₈ ratios and space velocity. At all the process conditions, the selectivity for CO, a useful byproduct, is much more than that for CO₂. Because the thermal cracking of propane is carried out in the presence of limited O₂, not only the total conversion of propane but also the conversion of propane by its thermal cracking alone is increased greatly. Hence this process occurs at a much lower temperature or contact time than that required for achieving the same conversion in the thermal cracking process. Since the endothermic thermal cracking and exothermic oxidative conversion reactions of propane occur simultaneously, both reactions are coupled to make it highly energy-efficient, with a drastic reduction in external energy requirement and coke formation, and safe to operate. The overall process can be made almost thermoneutral, mildly exothermic, or mildly endothermic by manipulating the temperature and C₃H₈/O₂ ratio in the feed.