Trade-off between hydrogen production and temperature hot spots in the design of a membrane reactor
Advancing the chemical engineering fundamentals
Membranes and Membrane Science (T2-8P)
Keywords: non-isothermal conditions, modelling, water gas shift reaction, catalyst mass distribution
Hydrogen worldwide demand is increasing rapidly for its potential as energy carrier and clean fuel, meeting the environmental restrictions for a sustainable development.
Hydrogen is mainly produced from a variety of fossil fuels by either steam-hydrocarbon reforming or autothermal reforming for light feedstocks or partial oxidation using pure oxygen for heavy feedstocks, coal and coke [1].
The water gas shift reaction (WGSR) represents a fundamental step in the main industrial routes to produce hydrogen [2], specially where the CO content must be minimized as for fuel cell applications or for adjusting the CO/H2 ratio of the syngas produced.
The opportunity to carry out this exothermic reaction, limited by thermodynamic equilibrium, in a membrane reactor (MR) offers important advantages with respect to a conventional packed bed reactor if an appropriate design of the unit is performed. Pd-based membranes, that present the highest selectivity values to hydrogen, are adequate to withstand the high operative temperatures (of about 600 K for low-temperature shift catalyst and 700 K for the high-temperature shift catalyst). Therefore, the effect of some important parameters (e.g. sweep gas temperature and flow rate, reaction temperature and pressure) on CO conversion and temperature hot spot managing has been analysed in order to improve the MR performance.
Mass and heat transport phenomena in MR have been investigated by means of a two-dimensional pseudo-homogeneous model that considers both axial convective and radial diffusive contributions.
Computer simulations showed as a proper choice of sweep gas temperature and flow rate makes the membrane reactors inherently safer than conventional systems in carrying out exothermic reactions since they allow to control temperature hot spots without modifying the global yield and the hydrogen recovery. In order to minimize the hot spot intensity, an effective heat exchange is suggested. This result can be achieved operating on the effective radial thermal conductivity or in alternative on the catalyst distribution, without reducing the hydrogen recovery. For what concerns the effect of membrane surface and reactor volume (Sm/Vr) ratio on the reactor performance, it is more important at low feed pressures.
[1] M. Conte, A. Iacobazzi, M. Ronchetti, R. Vellone, J. Power Sources, 100, 2001, 171-187.
[2] D.S. Newsome, P. Kellog, Catal. Rev. Sci. Eng., 21(2), 1980, 275-318.
Presented Tuesday 18, 13:30 to 15:00, in session Membranes and Membrane Science (T2-8P).
