(214f) Steady-State and Transient Optimization of Monolith Reactor Design for Autothermal Oxidative Dehydrogenation of Ethane over a MoVTeNbOx Catalyst

Ethylene is an essential building block of the petrochemical industry and is used to produce a variety of chemical intermediates and polymers. Conventional ethylene production relies on the steam cracking of ethane which is highly energy intensive due to its endothermicity. The cumulative CO₂ emissions from steam cracking of ethane amount to about 1.0 kg CO₂ per kg of ethylene produced. Oxidative dehydrogenation of ethane (ODHE) is an attractive route to produce ethylene from ethane which can have similar performance to that of steam cracking of ethane. A significant body of work exists in the literature to develop a stable and active catalyst that maximizes the ethylene selectivity; however, the process is yet to be commercialized due to heat management (exothermicity) concerns, i.e. those associated with maintaining optimum temperature such that catalyst deactivation and reactor runaway are avoided. Autothermal operation without external heat input is a potential solution which utilizes heat generated by the reaction to drive the reaction forward along with higher productivity and lower energy consumption. In this work, we demonstrate the feasibility of carrying out ODHE in an autothermal monolith reactor through comprehensive bifurcation analysis.

We use a multi-scale reduced order model that accounts for pore diffusion to analyze ignition-extinction behavior during ODHE in a monolith reactor coated with a MoVTeNbO_x (M1) catalyst. We determine the optimal catalyst layer thickness and monolith substrate properties so as to maximize the region of autothermal operation (or per pass conversion of ethane) as well as selectivity to ethylene. Our modeling results indicate that metallic monoliths with intermediate length, high substrate conductivity and high cell density (or small hydraulic radius) are optimally suited to approach the so called “homogeneous lumped thermal reactor (LTR) limit” which leads to the best reactor performance (92% ethylene selectivity at 25% ethane conversion). It is also shown that operation of the reactor in the external mass or heat transfer controlled regime with strong interphase gradients can lower ethylene selectivity. We also report the feasibility of carrying out ODHE at high pressure (5 bar) with fixed linear velocity and examine the impact of feed dilution on reactor performance and compare the same with that obtained in the steam cracking of ethane. To sustain stable reactor operation near the extinction point, a startup protocol is proposed wherein the reactor is initially heated to a temperature that is slightly above the operating temperature, and inlet conditions are carefully adjusted such that the reactor is maintained nearly at constant temperature, thereby preventing runaway or quenching.