Hysteresis in Trickle-Bed Reactors:  A Review

A concise review of the hysteresis in cocurrent down-flow trickle-bed reactors (TBRs) is presented. The effects of several factors on the hysteresis, such as the type of particles (porous/nonporous), the size of the particles, the operating flow ranges, and the start-up conditions (wet/dry) are described. Also, some limited information that is available about the effects of other factors, such as addition of wetting agents (surfactants) and inlet liquid distribution, are also discussed. Empirical and theoretical models developed to predict hysteresis are briefly described. Most of the studies have reported an appreciable amount of hysteresis in key hydrodynamic parameters such as pressure drop, liquid holdup, and wetting efficiency, especially with small packing material (∼1.6−3.5 mm). The hysteresis is attributed to the different degrees of wetting, which occurs because of the flowing of liquid in two different modes, such as film and rivulet. The hysteresis behavior of a packed bed with porous particles is observed to be significantly different than that of nonporous particles. These differences are linked to the dual nature of pores, i.e., the pores act both as an accelerator and brakes on the spreading drop edge over the porous surface. This pore-level concept is further enhanced by the concept of participating and nonparticipating particles to understand the different hysteretic behavior of TBRs. Few mathematical modeling efforts have been attempted to capture the trend that is observed in hysteresis behavior with nonporous particles only. Recently, an attempt has been made to understand the comprehensive hysteretic behavior of both porous and nonporous particles with the conceptual framework of hysteresis. The present review focuses the attention to enhance the further understanding of the physics of the flow of fluids through TBRs, which may be responsible for the phenomena of hysteresis, especially in porous particles.