(419ar) Performance of multi-tube palladium membrane reactor and its CFD analysis

Global energy and environmental issues such as exhaustion of fossil fuel and carbon dioxide emissions to the atmosphere, which were warned late 1980’s, have been not yet solved, although being urgent and important in the view of establishing sustainable human society. It is certain that there are many technological, sociological and political approaches to solve such enormous and complicated problem. Principal technological measures should be saving energy and finding new energy resource. Hydrogen can be employed as a fuel for fuel cells, vehicle engines and power plant etc.  However, there is no well spouting hydrogen on the earth differently from natural gas. In this sense, it is the fact that hydrogen is a secondary energy and therefore is necessary to be produced via water electrolysis. Nevertheless, a great expectation has been concentrating on hydrogen because of its cleanness, that is, the product by combustion is water only. Further, if water could be decomposed to hydrogen by any help of natural energy like solar, wind, hydro and geothermal power, a closed recycle system with energy would be completed, thereby constructing a hydrogen-based energy society.

Cyclohexane and methylcyclohexane promise to be chemical hydrides because of having high hydrogen content (7.19wt% and 6.16wt%) and being a liquid state at room temperature. The hydrogen recovery by the dehydrogenation of the hydrides, however, is essential to be limited by chemical equilibrium. To release from the equilibrium limitation, an application of membrane reactor has been concerned. However, one must pay attention to a large endothermic heat of dehydrogenation, without which the reaction cannot proceed at all.

A multi-tube membrane reactor using palladium, which could produce about 1L/min of hydrogen, was designed and used for evaluating the performance for cyclohexane dehydrogenation. Physical and chemical phenomena occurring inside the membrane rector was analyzed employing a CFD (Computational Fluid Dynamics) code, whereby visualizing the cross-sectional concentration and temperature profiles in the catalyst-packed bed. As a comparison between experimental and calculated results, it was found that effects of heat transfer rate on the conversion of cyclohexane were very large. By changing the heating media for the reactor, it is shown that the reactor performance can be much improved.