(560iz) Bifurcation Analysis of Coupled Homogeneous-Heterogeneous Reactions in Monolith Reactors

Bhaskar Sarkar and Vemuri Balakotaiah

Chemical Engineering Department, University of Houston, Houston, TX 77024, USA

Keywords:

Bifurcation, homogeneous ignition, partial oxidation, OCM

Abstract:

Catalytic partial oxidation is an attractive technology for meeting future energy demands and production of intermediate chemicals. The models describing this process typically involve both catalytic and homogeneous reactions and strong species as well as thermal coupling. There have been numerous experimental and computational studies on these systems in the past twenty-five years. Most of these studies have used micro-kinetic models for homogeneous and catalytic reactions or focused on hydrodynamic aspects. Such analyses lead to models that are not amenable to a bifurcation analysis in the parameter space as they are computationally complex due to existence of exponentially thin boundary layers in space and/or time. Consequently, a clear picture of the essential/qualitative features of the process as the parameters are changed is not entirely clear. The objective of this study is to determine how the various operating and design parameters such as the space time, inlet fluid temperature, inlet mole ratio, pressure and channel hydraulic radius impact the bifurcation (ignition and extinction) behavior of coupled homogeneous-heterogeneous reactions in monoliths. We use the oxidative coupling of methane (OCM) as test example and use a hierarchy of reactor and chemistry models to determine their impact on the predicted bifurcation behavior.

First, we investigate a twelve step global kinetic model with seven reactions in heterogeneous phase (wall reactions) and five reactions in homogeneous phase using a two-phase ‘Short Monolith’ or ‘Gauze Reactor Model’. We calculate various bifurcation diagrams and bifurcation sets for different state variables (gas temperature, solid temperature, conversion, selectivities etc.) vs. feed inlet temperature (T_in) and determine a complete phase diagram in the plane of oxygen to methane ratio and space time. Our calculations show that it is possible to obtain ~80% C₂ selectivity with ~20% CH₄ conversion for inlet CH₄/O₂ mole ratio of 6-8 and space time of 0.1-1s with La₂O₃/CaO catalyst. Higher CH₄/O₂ mole ratios lead to higher selectivities at the cost of lower CH₄ conversion and productivity. It is found that operations on mass transfer controlled regime result in higher selectivities but lead to lower conversion and require higher space times, thereby again lowering the productivity. Further, on the ignited branches the fluid exit temperatures are found to be low enough (~1000K) to conduct stable autothermal operation of OCM.

Next, we move towards a more realistic multi-scale coarse-grained model for the monolith reactor, which includes diffusion of species and heat through the washcoat (catalyst). Both the steady state and dynamic hysteresis curves are calculated with this low dimensional model using flat and parabolic velocity profiles. The qualitative bifurcation features of both the models are found to be similar and the coarse-grained model approaches the short monolith model in the limit of small washcoat thickness.