Liquid solid mass transfer in small diameter trickle bed reactors

Abstract

Trickle bed reactors form an integral component of the petrochemical industry. In order to optimise their performance, industrial reactors are scaled down to laboratory scale reactors. It is desired that the laboratory scale reactor ought to operate at the same superficial velocity to replicate the hydrodynamics of the industrial reactor. This can be achieved by scaling down reactors according to the reactor diameter. However, wall effects can severely impact the hydrodynamics of these small diameter laboratory scale reactors The complex distribution of liquid in a trickle bed reactor operated in the trickle flow regime results in partial wetting of the solid particles which impacts the liquid solid mass transfer coefficient and consequently influences the reaction rates in a trickle bed reactor. The distribution of liquid in the trickle flow regime is also affected by prewetting procedures applied to the packed bed, where multiple hydrodynamic states occur. This phenomenon of hydrodynamic multiplicity affects the values of hydrodynamic parameters and complicates the prediction of trickle bed reactor performance. In this study the liquid solid mass transfer was investigated in a variety of columns, with column to particle diameter ratios in the range of 10 to 1, by employing the electrochemical technique. A distinct trend was found where the decrease in column diameter resulted in an increase in the effective liquid solid mass transfer coefficient, even though the values of the effective liquid solid mass transfer coefficients were two orders of magnitude lower than published data. A threefold increase of the effective liquid solid mass transfer coefficient was obtained on decreasing the column to particle diameter ratio from 10 to 3. However, this trend was not sustained for the bead column (D/dp=1) where a decrease in effective liquid solid mass transfer coefficient was observed on decreasing the column to diameter ratio from 3 to 1. The substantial increase in liquid solid mass transfer could not be entirely explained by partial external wetting as well as interstitial velocity variations. The low values of liquid solid mass transfer coefficients also could not be explained despite attempts to validate the measurement technique. The effects of hydrodynamic multiplicity on the effective liquid solid mass transfer coefficients were significant in all the columns The effects of hydrodynamic multiplicity on the effective liquid solid mass transfer coefficients were significant in all the columns. The Kan prewetting outperformed the Levec gas prewetting by as much as 1.5 times, while the super prewetting outperformed Levec prewetting by as much as 1.3 times. The modes without gas outperformed the modes with gas which is contrary to previous findings on the effect of gas on liquid solid mass transfer. Copyright