Ph.D Thesis

Ph.D StudentSznejer Gabriel
SubjectThe Application of Membrane Reactors to Equilibrium Limited
DepartmentDepartment of Chemical Engineering
Supervisor PROFESSOR EMERITUS Moshe Sheintuch


Membrane reactors couple catalytic units with separation through a selective membrane that envelops them in order to improve performance, e.g., to remove a product in equilibrium-limited reactions. Carbon membranes are molecular sieves that contain pores of molecular dimensions. This research was aimed at developing membrane-reactors with a molecular-sieve carbon membrane.

The studied reaction was isobutane dehydrogenation. In the first phase the selectivity and permeability of a hollow-fiber module were studied, in a temperature range of 25-400 oC for a series of relevant light hydrocarbons along with their mixtures with hydrogen and nitrogen. The individual transport coefficients were calculated from the differential mass balances over the module, knowing the concentrations of the exiting flows.  The selectivity factor was calculated and was found to be lower in single-component systems than in multi-component systems. The selectivity of hydrogen with any of the hydrocarbons in a multi-component system was higher than 100 at 400 oC, thus, carbon membranes should be appropriate for carrying out a dehydrogenation reaction in a membrane reactor system. Nitrogen transport through the membrane was observed in all the systems.

The reaction was carried out in the membrane reactor at two types of operation modes: a membrane reactor with either nitrogen as sweeping gas in counter current flow or with vacuum as driving force for membrane separation. The conversion achieved in the counter-current flow operation method is higher than in all other modes achieving a maximum of 85% at 500 oC . While this result is much higher than  in PFR, the obtained conversion improvement in the sweeping gas mode is a result of membrane transport and nitrogen dilution. The maximum conversion in the vacuum mode is 40% at 500 oC that is 10% above the PFR conversion. In the vacuum system no dilution occurs so all the conversion improvement is due the membrane separation of hydrogen. These results were compared with simulations that used the experimentally determined transport parameters.