Trickle-bed reactors: Liquid distribution and flow texture

A concise review of the liquid distribution and flow texture in cocurrent down flow trickle-bed reactors is presented. The following topics are considered: inlet liquid distribution, wall flow, wettability, catalyst dilution with inerts, periodic flow modulation, gas-liquid interfacial area, multiplicity of hydrodynamic states, i.e., hysteresis and effect of the method of packing the bed. The effects of particle geometry and physical properties of the operating fluids on the liquid distribution / maldistribution at both the micro and macro level with the use of non-invasive techniques are discussed. Empirical and theoretical models developed to account for the effect of various parameters relevant to above topics are briefly described. There are several factors that affect the macro and microlevel liquid distribution and flow textures. Macro level flow distribution is mainly affected by inlet liquid distribution, particle shape and size of the particle, fluid velocity and packing method. Liquid distribution and flow textures at the micro level are affected by start up procedures, fluid velocity, wettability, flow modulations and particle coordination number. Significant wall flow which occurs due to the presence of more void space near the wall can be reduced by maintaining the ratio of column diameter to particle diameter (D/d(p)) at more than 25. Changing the wettability of the particle (by coating the surface/ liquid flow) affects the partial wetting characteristics of the particle. This behavior leads to presence of multiple hydrodynamic states in the bed. The present review focuses attention on enhancing the further understanding of microlevel phenomena to demystifying the hydrodynamic structure in TBR. It also focuses the future directions of work in trickle bed reactors to facilitate the further understanding of the design and scale-up of TBR to meet the challenges of stringent sulphur specification in transportation fuel (5-10 ppmw in EURO-III & -IV).