Reactor designs for ethylene production via ethane oxidative dehydrogenation: Comparison of performance

Abstract

The implementation of ethane oxidative dehydrogenation (ODH) toward ethylene production in two different reactor configurations is studied here by means of a mathematical model of the reactors. A conventional liquid-cooled multitubular reactor and a multitubular membrane reactor are considered for comparison. Both reactor designs use a Ni-Nb-O catalyst washcoated over raschig-rings inside the tubes; molten salts flow in the shell side of the conventional reactor whereas pure oxygen is assumed for the shell of the membrane reactor. Industrial-scale ethylene production is the aim. Results show that the variation of the bed density (different thickness of the catalytic washcoat over the pellets) shows opposite effects on both reactor designs. For the conventional reactor, the increase in bed density leads to more pronounced hot spots as well as to an undesired oxygen depletion inside the tubes. Conversely, for the membrane reactor, higher bed densities prevent oxygen accumulation along the tube length leading to lower oxygen partial pressures and, consequently, higher selectivities. In this way, higher ethylene production rates are feasible. Although molten salts provides enhanced heat removal, the oxygen injection at only the tube mouth in the conventional reactor leads to lower global selectivities and higher heat generation rates. In the membrane reactor design, the heat generation rate proves to be efficiently controlled by the permeation flow of oxygen through the membrane.