EXPERIMENTAL EVALUATION OF DEHYDROGENATIONS USING CATALYTIC MEMBRANE PROCESSES

Many industrially important dehydrogenation reactions are operated under conditions where the equilibrium conversion is limited by the production of hydrogen. Ceramic membrane reactors offer the potential for increased conversion at existing operating temperatures or reduced operating temperature for the same conversion level by removal of product hydrogen. This paper reports the results of recent efforts to develop catalytic membrane reactors for the dehydrogenation of ethylbenzene to styrene. The focus of this study was to compare the performance of a hybrid reactor, consisting of a packed bed followed by a membrane reactor, with that of a traditional two-stage packed-bed reactor under industrially relevant conditions. The hybrid configuration mimics the simplest implementation of a ceramic membrane reactor, simulating the use of the membrane reactor as an add on stage to the existing reactor train. A benchscale system has been developed that is capable of experimentally simulating the industrial operation. Features of this system include syringe pumps from which an ethylbenzene liquid hourly space velocity of 0.4 hr-1 is attainable with a water.ethylbenzene molar ratio of 9, a 7-zone furnace in which isothermal catalyst bed temperature profiles within +/- 1-degrees-C are achieved, and two dual FID/TCD on-line gas chromatographs for simultaneous analysis of the entire spectrum of compounds in the permeate and reject effluents from the reactor with 30 minute analysis turnaround time. The membrane module incorporates a four-point thermocouple in the catalyst bed to insure isothermal operation and three single-point thermocouples on the permeate side for monitoring purposes. Results obtained with this system showed a 4% yield enhancement to styrene in the hybrid reactor compared to the traditional two-stage packed bed. This enhancement was achieved with no loss in styrene selectivity. Carbon deposition on the membrane was observed during reaction which rapidly reduced the permeability from 70 ms/m2/hr/atm for the fresh membrane to a value of 2 m3/m2/hr/atm under reaction conditions. This 2 m3/m2/hr/atm permeability was a steady state value representing a dynamic equilibrium between coke formation from organic compounds and coke removal due to the presence of steam in the reaction mixture and was constant for run times in excess of 100 hours.