Modelling of a Catalytic Micro-Reactor Coupling Endothermic Methane Reforming and Combustion

Abstract

In this study the mathematical modelling of a catalytic microstructured plate reactor for the production of hydrogen was performed in 2D and 3D geometry. The proposed reacting system uses the heat generated by an exothermic reaction (combustion) to sustain endothermic reforming reactions. Therefore, it pertains to those devices useful for producing the feed for fuel cell system for the remote generation of electrical power. However, because of the compactness of the reacting system it can also be considered in the context of apparatus aiming at process intensification. Within this frame the catalytic contribution of both exothermic and endothermic reactions was modelled considering the classic Langmuir-Hinshelwood surface kinetic theory. The advantage of using a real 3D geometry configuration consists in the possibility of considering the importance of the entering and boundary effects with particular attention to fluid stagnation and heat hot spots. The trade off of such a choice is certainly the huge increase of computing time and/or of the power of the computing facility. With respect to other works performed with similar reactor geometry and reacting systems this does not use simplifying assumptions such as catalyst layers modeled by one-dimensional approach, fully developed laminar flow or transverse heat and mass transfer taken into account through lumped heat and mass transfer coefficients. Results of simulations presented here concentrates on the comparisons between results of: countercurrent (CTC) and concurrent (CNC) flow patterns of the reactant streams; of simulations carried out with 2D and 3D models and of the influence of the thickness of the catalytic layers on the reactor performance. Simulations indicates that CNC flow pattern of reactants streams allows a better performance of the reactor since positive temperature differences between the catalyst layers and the gas in the channels maintain along the whole reactor and, consequently, there are not heat flux inversions, which occur under CTC flow pattern. Results also showed that as concerns an adiabatic reactor, whatever the operating conditions, 2D and 3D models yield substantially the same results. Finally, modelling demonstrated that for a realistic catalyst layer configuration thicknesses larger than 50 µm are useless for enhancing the reactor performance. The feasibility of the model proposed may show its potential in fast and easy implementation of several combustion and reforming fuels so to significantly enhance the performance prediction of real processes.

Catalytic Combustion for Supplying Energy for Endothermic Reaction

“…The former are designed as cylindrical devices where heat produced by an inner combustor sustains the endothermic reaction occurring in a thin annular space surrounding it [25][26][27] or, may be constituted by the joining of several rectangular micro-channels arranged in such a way to form a network where the reforming channels are interspersed with the combustion ones [28,29]. The micro-structured plate reactor configuration ( Figure 2) is, instead, generally formed by parallel thin plates delimiting chambers where exothermic and endothermic reactions occur simultaneously but separately in space [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. Micro-structured reactors have, generally, inner dimension lower than one millimetre, and more specifically between ten and a hundred micrometers.…”

“…Today methane steam reforming represents the principal way to produce hydrogen rich synthesis gas [72][73][74][75][76][77][78][79][80] fulfilling about 50% of the world demand of H 2 [81,82]. It has been reported that the heat required for the endothermic reaction can be supplied through several exothermic reactions in different reactor configurations such as hydrogenation of nitrobenzene for the production of aniline in a tube and shell device [3], combustion of H 2 in a multi-plate reactor [40] or combustion of CH 4 in a variety of systems including honeycomb monolith [50,53], tubular [16], membrane-assisted [67] or catalytic plate reactor [30,33,34,[36][37][38][39]41,[43][44][45][46]. Some examples of couplings of endothermic and exothermic reactions are listed in Tables 1 and 2 in the cases of the absence and of the presence of a selective membrane, respectively.…”

“…These may include optimization of the relative position of the catalyst layers at endothermic and exothermic sides, optimization of catalyst distribution in the combustion channel, or optimization of fuel distribution along the reactor [93]. Reactants conversion changes according to the flow orientation being generally lower for the countercurrent mode [38]. The higher conversions of co-current reactors with respect to the counter-current mode are consequence of the behaviours reported above that give rise to higher average temperatures and, then, to higher reaction rates [38,57].…”

A CFD study on the effect of the characteristic dimension of catalytic wall microreactors

“…Two‐dimensional models of microreactors consisting of parallel catalytic plates coupling the SMR and the catalytic combustion of a gaseous fuel in cocurrent flow configuration have been developed also by Stefanidis and Vlachos17–19 to investigate the effects on the microreformer performance of the characteristic length scale, contact time, wall material, fuel, and reforming catalyst. Finally, 3‐D models and computational fluid dynamics (CFD) simulations have been performed by Vaccaro et al24, 25 and Mettler et al12, 26 to study the issue of external heat losses and their influence on the stability and scale‐out of microreactors coupling the SMR and methane combustion reactions.…”

Optimizing the catalyst distribution for countercurrent methane steam reforming in plate reactors

“…However, producing balanced countercurrent designs that match the rates of heat generation in the exothermic channels and heat consumption in the endothermic channels remains an important issue. [11][12][13][14][15] Previous studies 11,13,15 indicate that the coupling of methane steam reforming with methane combustion in a countercurrent flow arrangement is difficult: the reactant depletion occurs in opposite directions relative to the reactor length. This complicates the efficient synchronization of the reaction rates and therefore the rate of heat generation and heat consumption, with the reactor thermal behavior typically drifting to one of two extremes:…”