Catalyst ignition and extinction: A microkinetics-based bifurcation study of adiabatic reactors for oxidative coupling of methane

Abstract

Understanding ignition and extinction behavior is of crucial importance for oxidative coupling of methane (OCM). Therefore, for the first time the bifurcation behavior of OCM has been investigated while considering both homogeneous gas phase reactions and heterogeneous reactions using a detailed microkinetic model. Three different adiabatic reactor models are considered: a plug flow reactor (PFR), a continuously stirred tank reactor (CSTR) and a lumped thermal reactor (LTR) model. The latter represents the limiting case with zero backmixing (cf. PFR behavior) for species and perfect thermal backmixing (cf. CSTR behavior). For homogeneous processes this reactor type could for example be realized by adding a high thermal conductivity inert to the reactor tubes, for catalytic processes a high thermal conductivity catalyst could be used.The bifurcation behavior in these reactor types is compared with a focus on methane conversion, C2 yields and their dependence on operating conditions such as inlet composition, inlet temperature and space time. Steady state multiplicity is observed for adiabatic CSTR and LTR models. This multiplicity of steady states is not observed for isothermal reactor models, indicating that it is caused solely by thermal backmixing and is not related to chemical feedback features such as autocatalysis. The start-up procedures or initial conditions determine the actual steady state that is obtained. Among the three investigated reactor types, a LTR shows the highest product yields and the lowest extinction temperatures, which allows autothermal operation at a much lower inlet temperature compared to a PFR and CSTR. For OCM without catalyst, autothermal operation on the ignited branch at ambient inlet temperatures and reasonable space times is only possible by using methane-to-oxygen ratios below 3 leading to low selectivities. For catalytic OCM compared to OCM without catalyst, the range for autothermal operation is much broader and it is much easier to find feasible operating conditions allowing autothermal operation at ambient inlet temperatures.By operating a LTR on the ignited branch at ambient inlet temperature of 300 K, methane-to-oxygen ratio CH4:O2 = 6, space time V/FCH4,0 = 0.02 s, bulk density of Sn-Li/MgO = 1000 kgcat/m3 and pressure P = 1 bar, overall C2 selectivities (i.e. sum of ethane, ethylene and acetylene selectivity) of 80% can be obtained at methane conversions as high as 30%.