Non-conventional analysis of hydrogen upgrading in membrane reactor by process intensification metrics

The always increasing necessity arose in the last decennia in redesign the industrial processes with new unit operations more compact, efficient and, thus, able to address various environmental concerns, has led to the definition of new unit operations able to overcome the limitations imposed by conventional units. Among these the use of membrane reactors for hydrogen production is becoming more and more a reality with various pilot installations all over the world. However, to make the use of a new technology more attractive, it is fundamental to define a new way of analysing its performance and highlighting its potentialities with respect to the well consolidated traditional technologies. Hand in hand with the redesign of new processes comes, thus, the identification of new indexes, so-called, metrics, that together with the traditional parameters usually used to analyse a process can supply additional and important information to decide on the type of operation and the identification of the operating condition windows that makes a process more profitable. In this work, some sustainability indexes, mass and energy intensities [1,2], volume and conversion indexes were used in a no conventional evaluation of the up-grading stage in hydrogen production, i.e. the water gas shift reactor, by means of membrane reactors. Defined as the ratio between the total inlet mass and total energy involved in the reactor, with respect to the hydrogen fed and produced by the reactor, mass and energy intensity provide useful information about the material exploitation and the energy efficiency of this new technology. On the other side, Volume and conversion index provide an indication on the gain offered by membrane reactor in terms of catalyst volume required and reactants conversion achieved, at the same operating conditions of a traditional unit. The comparative study of membrane reactor performance with respect to the conventional reactor was analysed as function of the main process variables, such as temperature, pressure, feed molar ratio and space velocity. In the comparison between the two unit operations, the membrane reactor resulted always more material and energy intensive than a traditional reactor. The advantage offered by a membrane reactor was also quantified in terms of ratios referred to the equilibrium condition of the traditional reactor. The membrane reactor resulted always more intensified than a traditional reactor operated in similar conditions and exceeded also the ideal performance achievable by a traditional reactor, at a temperature higher than 350°C. At the highest temperature (450°C) and gas hourly space velocity (40000 h-1) the indexes for the membrane reactor were quite the same of the ideal values (the ones at equilibrium) of the traditional reactor; the membrane reactor can, thus, be operated with different combinations of operating conditions, achieving the same performance in terms of material exploitation and energy efficiency (Figure 1). Mass and energy intensities demonstrated, in line with the process intensification strategy, the assets of the membrane reactor technology also in terms of better exploitation of raw materials (reduction up to 40%) and higher energy efficiency (up to 35%). In addition, the catalyst volume required by membrane reactor was more than three times lower than the one of traditional unit, for achieving the same conversion [3,4].