(222d) A Comparison of Monoliths and Short Contact Time Supports for Selective Catalytic Oxidation: Performance and Steady-State Multiplicity

The reaction rate in catalytic reactors is highly temperature dependent. Typically the transition from kinetic to mass transfer control of the reaction rate is driven by temperature, however in selective reactors such as the ones for trace contaminant removal, it can be affected significantly by species concentration. To understand transport and kinetic properties, experiments have been performed on Short Contact Time (SCT) and Monolith supports, as they have opposite heat and mass transfer properties. Their performance was studied with the PROX process, consisting of a system of 3 chemical reactions, designed to remove traces of CO from a H2 rich stream. For the base case the reactor was loaded with 1 gcat on both catalyst supports, resulting in a space velocity of 17,000 h⁻¹ for the monolith and 200,000 h⁻¹ for SCT bed. Two different formulations were used (P t/Al2O3 and a commercial PROX catalyst formulation). Furthermore a space velocity sweep was performed (while keeping constant linear velocities) in order to understand the transport behavior. This study was conducted by operating the reactor over a range of O2-to-CO ratios (with constant reactor inlet temperature) and measuring CO conversion, selectivity and temperature rise. Experimental data shows that both geometries achieve the same maximum conversion of 96% at the same O2-to-CO ratio. However their behavior at low conversion differs: at low O2-to-CO ratio the SCT reactor has very low conversion (2030%) and high selectivity (up to 60%) up to the point where there is a dramatic conversion swell (to 96%). Conversely the monolith exhibits a nearly constant conversion slope, with a gradual performance swell up to 96% conversion. The behavior with respect to selectivity is very similar, as selectivity reaches its maximum value of 60% at a O2-to-CO ratio of 1.5. Both geometries were tested to understand the impact of the reverse water gas shift reaction and it was found that this reaction is only promoted by catalyst formulation and loading, not by the catalyst geometry. For both catalyst supports steady-state multiplicity was observed. The monolith exhibits steady state multiplicity at a very low O2-to-CO ratio, whereas the SCT bed showed this behavior in the middle of the considered O2-to-CO window, close to maximum performance. The effect of water concentration on reactor stability was also studied and it was found that water reduces the operating window at which the reactor exhibits multiple steady states. In order to understand this behavior, both reactors were analyzed with an in-house kinetic/transport model. This comparison allowed to draw some conclusions about the effect that species diffusion/reaction and heat generation/removal interferences have on reactor multiplicities. This work in progress will show the results to date and next steps to better understand the impact of reactor geometry on selective catalytic reactors.

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