Studies on hydrodynamics and liquid-solid contacting in trickle-bed reactors

Abstract

In trickle-bed reactors, gas and liquid flow concurrently downwards over fixed beds of catalysts. The reactors find wide application for three-phase contacting in chemical, petrochemical, petroleum and fermentation industries, and in pollution control. The packings used are, in general, porous particles. The performance of these reactors is strongly influenced by hydrodynamics and liquid-solid contacting efficiency. Proper design and scale-up of trickle-bed reactors require careful modelling of these phenomena, and accurate estimation of the model parameters. Dynamic tracer testing can be successfully used for determining the parameters. The objective of the present work is to develop models and efficient methods for determining the hydrodynamic and liquid-solid contacting parameters in trickle-beds by tracer techniques. The present work proposes a two-response method—pulse response for determining the mixing parameters, and step-elution response for determining the liquid-solid contacting efficiency. The step response data is interpreted in terms of a comprehensive model that is developed taking into account the liquid holdup, dispersion, exchange with the stagnant zones, external mass transfer, internal mass transfer and partial wetting effects operating in the trickle-bed reactor. The piston-dispersion-exchange (P.D.E.) model, a special case of the comprehensive model, helps in interpreting the pulse response data where the mass transfer effects are not significant. The method is evaluated experimentally in a laboratory trickle-bed reactor (made of 10 cm i.d. and 91.44 cm long glass column) with an air-water-alumina (6 mm spheres) system. Potassium chloride is used as the tracer, and the responses are recorded online by means of flow-type conductivity cells. All the experiments are carried out in the trickling-flow regime. Drainage, external static, internal static, and total liquid holdups are measured by weighing method. Gas flow rate is shown to have no significant effect on these holdups. The pulse-measured liquid holdup is found to be equal to the external liquid holdup measured by the weighing method, though the dynamic liquid holdup is significantly different from the drainage liquid holdup. The liquid phase Peclet number is independent of the liquid velocity. Its value indicates that the trickle-bed operates in the finite dispersion region. The effects of the liquid velocity on the drainage, dynamic, and external static liquid holdups and on the coefficient of exchange between the flowing liquid and the stagnant zones are evaluated, and are found to be, in general, similar to those of the earlier reports. The Peclet number, the ratio of external static to dynamic liquid holdups, and the mass exchange parameter determined from the pulse response are used to estimate liquid-solid contacting efficiency directly from the moments of the comprehensive model and the step-elution response. The liquid-solid mass transfer coefficient required for this purpose is estimated from standard correlations, while the internal diffusivity is determined by dynamic testing in a rotating catalyst basket reactor. The contacting efficiencies thus determined are found to be close to unity (about 0.94) and independent of the liquid velocity.