A pilot plant was designed, constructed and commissioned to study the axial distribution of products and reactants in a fluidised bed reactor (0.15 m I.D. x 1.5 m). The dehydration of vaporised ethanol to ethylene and diethyl ether, catalysed by Laporte type 13X zeolite, was selected as typical of high temperature, complex, industrial chemical reactions. Kinetic data were obtained from experiments in laboratory fixed bed and stirred gas-solid reactors. The influences of side reactions, catalyst deactivation and transport limitations on the observed reaction rates were accounted for in a consistent manner. Theoretical considerations and experimental evidence from this study demonstrated that the effects of intracrystalline (surface) diffusion in the 13X zeolite were substantially different from those of transport processes in amorphous, non-crystalline porous catalysts. Both the individual product formation rates and the ethylene-to-ether selectivity were satisfactorily correlated by simplified Hougen-Watson (Langmuir-Hinshelwood) type rate expressions, which assumed that the rate-controlling step was a dual site surface reaction. However, the rate expressions were also adequate at the higher temperatures (350 - 375°C), where intracrystalline (surface) diffusional restrictions were significant. It is therefore suggested that a Langmuir adsorption isotherm can adequately represent the concentration dependency of both surface reaction and surface diffusion. Existing models of fluidised bed reactors, originally involving simple power law kinetics, were developed to incorporate the complex Hougen-Watson type rate expressions applicable to this study. The unique product distribution data obtained from the pilot plant experiments was not adequately represented by the existing models, which implicitly assumed negligible gas-solid transport limitations in the emulsion phase. Because of the fluidisation and catalytic properties of the zeolite and the nature of multiorifice distributors in deep beds, it was proposed that better predictions would be possible if gas-solid transport restrictions or bubble channelling could be accounted for in the models. Future experimental work has been outlined to permit further evaluation and development of the most appropriate models as proposed in this work.
|Date of Award||1983|