Optimization of membrane area and catalyst distribution in a permeative-stage membrane reactor for methane steam reforming

Methane steam reforming to produce high purity hydrogen is conveniently carried out in membrane reactors. The interest on this process has led to several studies about the influence of the key operating variables on the system performances. In the present paper we focus on the optimization of the catalyst and membrane area axial distribution by computer simulation. To this purpose, a numerical model of a permeative-stage membrane reactor (PSMR), a system composed of traditional reactive and membrane permeative stages laid out in alternated series, has been developed. This is based on 1D mass, momentum and energy balances including thermal effects on all the system properties. First, an equi-sized nine-stage PSMR is analyzed at various temperatures and membrane thicknesses, in comparison with a conventional membrane reactor, showing that improved performances, especially in terms of methane conversion, can be obtained. Then, considering the stage lengths as design variables, the optimal system performances are determined by maximizing two objective functions, namely the methane conversion and the hydrogen recovery factor. Different optimal length distributions are found utilizing the two criteria, corresponding to privileged kinetic or permeative aspects. In comparison to the conventional MR, the PSMR leads to a higher conversion and lower recovery factor when the conversion is maximized, whilst the opposite situation occurs when maximizing the recovery factor. Furthermore, the role of the heat exchange area between the furnace and the reactor is studied, considering a PSMR with adiabatic permeative stages. From this case, it is possible to see that the greater advantage of this apparatus (having the same membrane area, catalyst amount and heat-exchange area as the MR) consists in a much higher recovery factor (+21% ca.), which at the end demonstrates to be the most convenient objective function by means of the introduction of an overall performance index, identified as the hydrogen recovery yield. Therefore, the analysis proposed in the present paper shows that the reactive/permeative stage distribution has to be considered an important reactor design parameter, which can be opportunely modified to improve the performances of the process.