(544al) Bifurcation Analysis of Coupled Homogeneous-Heterogeneous Reactions in Monoliths

Bifurcation, homogenous 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 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 chemistry models to determine their impact on the predicted bifurcation behavior.

First, we investigate the simplest OCM model with one homogeneous and one catalytic reaction with a high activity (La-Ce) and a low activity (Mn-Na₂WO₄/SiO₂) catalyst in an adiabatic monolith reactor. For both catalysts, we calculate various double-S shaped bifurcation diagrams for the state variables (gas and solid temperature, conversion etc.) vs T_in and determine a complete phase diagram in the plane of oxygen to methane ratio and space time. For higher O₂/CH₄ ratio and space times our bifurcation diagrams typically have two ignitions and two extinctions, and we interpret the first ignition as being due to catalytic reaction and the second due to the thermally coupled homogenous reaction. It is found that when transverse Peclet no. P >> 1, there is an intermediate stable state between the catalytic extinction and homogeneous ignition where temperatures are relatively low. But in such stable branches, the CH₄ conversion and C₂H₆ yield is poor (~10%) because of mass transfer limitations. When P << 1 and inlet O₂/ CH₄ mole ratios are high (0.25-0.5), the thermally coupled ignition happens to be left of catalytic ignition and hence the system suddenly jumps to a high temperature state bypassing the intermediate region which results in very poor yield of C₂H₆. But at low inlet O₂/ CH₄ mole ratios (0.08-0.16) and space times (~0.1s), the ignition-extinction behavior is majorly dominated by the catalytic reaction alone, with homogenous reaction assisting only in the late stages of the O₂ conversions. In this region of parameters, ignition-extinction temperatures due to catalytic reaction are found to be lowest (~650K for La-Ce and ~980K for Mn-Na₂WO₄/SiO₂) whereas yields of C₂H₆ are found to be highest.

As the channel hydraulic radius R_Ω is reduced from 1.32mm to 250μm, the region of hysteresis of the catalytic reaction expands whereas that of thermally coupled reaction shrinks and moves more towards higher O₂/CH₄ mole ratios. This means that as the channel diameter decreases, the gradients between the two phases becomes smaller which eventually lead to a parameter space where catalytic chemistry primarily dominates. We also observe that the ignition temperature is not a monotonic function of channel hydraulic radius R_Ω. With the decrease of channel hydraulic radius, a significant drop in the ignition temperature of coupled homogenous reaction is observed.

With these basic understandings we next move towards including micro-kinetic oxidation, reforming and dehydrogenation reactions that are known to occur in a high temperature combustion process like OCM and study the ignition-extinction behavior. Most of the qualitative bifurcation features observed in our simplistic model are found to be retained in these more complicated micro-kinetic models. The C₂ product selectivities and C₂H₄/C₂H₆ product ratios are studied against various operating conditions and the results obtained provide an operating window where the ignition temperatures are low, and yield is high. Further, the results are compared with available data in literature for quantitative validation.