Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas–liquid downflow, trickle-beds with countercurrent gas–liquid flow, and packed-bubble columns where gas and liquid are contacted in coc...

Multiphase catalytic reactors: a perspective on current knowledge and future trends

Trickle-bed reactors are frequently used in bench scale and pilot plant experiments to determine reaction kinetics and generate data for commercial scale-up. Depending on experimental conditions, these reactors sometimes cannot produce accurate data if the influences of three main factors (plug flow deviation, external wetting efficiency, and reactor wall effect) on reactor performance are not properly minimized or eliminated. Therefore, it is highly desirable to operate a trickle-bed reactor at conditions at which these effects can be neglected so that reliable and repeatable data can be obtained. There have been various criteria reported in the literature to theoretically determine the influence of these factors. The objectives of this work are to conduct an extensive review on the studies related to these criteria and to further make recommendations as how to use these criteria to provide guidelines for designing experiments.

Review on criteria to ensure ideal behaviors in trickle-bed reactors

Hydrodynamics of trickle-bed reactors involve complex interactions of gas and liquid phases with packed solids. Such complex interactions manifest in different flow regimes occurring in trickle-bed reactors. Knowledge of prevailing flow regime, pressure drop, and liquid holdup is essential for design and performance evaluation of the reactor. Detailed knowledge of fluid dynamics is essential for “a priory” predictions as well as for interpretation of available data. In this study, we have used wall pressure fluctuation measurements to identify prevailing flow regime in trickle beds. Experiments were carried out on two scales of columns (of diameter 10 cm and 20 cm) with two sets of particles (3 mm and 6 mm diameter spherical particles). Effects of prewetted and unwetted bed conditions on pressure drop and liquid holdup were reported for a range of operating conditions (VG = 0.22−0.44 kg/m2s, VL = 2−24 kg/m2s). A comprehensive CFD model was developed to predict measured hydrodynamic parameters. The model w...

Hydrodynamics of Trickle-Bed Reactors: Experiments and CFD Modeling

A two-fluid hydrodynamic model for the transition between trickle and pulse flow in a cocurrent gas–liquid packed-bed reactor

The Eulerian k-fluid CFD model was used to simulate the macroscale multiphase flow in packed beds. The geometric complexity of the bed structure is resolved by statistically describing the porosity distribution. The complicated multiphase interactions are computed using the Ergun type of formula developed based on bench-scale hydrodynamic experiments. The work is presented in two sequential articles. Part I discusses implementation issues of the k-fluid CFD model for packed beds. The drag exchange coefficients are obtained from the model of Holub et al. for the particle-fluid interfaces Xks and from the model of Attou et al. (1999) for the gas–liquid interface, Xgl. The effect of particle external wetting on flow distribution was incorporated into the model through the capillary pressure evaluated by either the J-function of Leverett (1941) for air–water or by the expression of Attou and Ferschneider (1999) for other fluids. In the framework of CFDLIB, the choice of the grid size and boundary conditions are discussed. An appropriate relationship between the section size and variance of the sectional porosity distribution was used for flow simulation. Part II discusses the extensive numerical results, and the CFD model is compared with experimental data in the literature.

A. Attou

Papers

Modelling of the hydrodynamics of the cocurrent gas–liquid trickle flow through a trickle-bed reactor

A two-fluid model for flow regime transition in gas–liquid trickle-bed reactors

A simple model for pressure drop and liquid hold-up in packed-bed bubble reactors