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

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Multiphase reactors – revisited

Hydrodynamics of gas-solid fluidization

Discrete characterization tools for cohesive granular material

Experiments of pressure effects on gas holdup and bubble size in slurry bubble columns at 5.6 MPa and at gas velocities up to 45 cm/s indicate that the gas holdup increases with an increase in pressure, especially at high slurry concentration. At ambient pressure, a higher solids concentration significantly lowers gas holdup over the entire gas-velocity range, while at 5.6 MPa, the effect of solids concentration on gas holdup is relatively small at gas velocities above 25 cm/s. An empirical correlation was developed based on these data and those in the literature to predict gas holdup in bubble and slurry bubble columns over a wide range of operating conditions. An analysis of bubble flow characteristics during dynamic gas disengagement indicates that large bubbles play a key role in determining gas holdup due to the large bubble and wake volumes that induce the acceleration of small bubbles. Direct measurement of bubble size shows that elevated pressures lead to smaller bubble size and narrower bubble-size distributions. Bubble size increases significantly with increasing solids concentration at ambient pressure, while at high pressures this effect is less pronounced. A theoretical analysis of circulation of gas inside the bubble yields an analytical expression for maximum stable bubble size in high-pressure slurry bubble columns. Based on this internal circulation model, the maximum stable bubble size at high pressures is significantly smaller due to the high gas inertia and low gas-liquid surface tension. The smaller bubble size and its reduced bubble rise velocity account for the observed pressure effect on gas holdup.

Maximum stable bubble size and gas holdup in high-pressure slurry bubble columns

This paper reports on baffle effects on the performance of a catalytic circulating fluidized bed reactor which were examined experimentally. The circulating fluidized bed reactor or riser was 102 mm in diameter and 6.32 m in height. Reaction was the catalytic decomposition of ozone using FCC particles with a mean diameter of 89 [mu]m, impregnated with ferric oxide as catalysts. Four ring-type baffles, mounted horizontally around the riser wall, were used in this study. Ozone concentrations were measured in both axial and radial directions under various operating conditions in a riser with and without baffles. Experimental results showed that in a riser with baffles, the ozone concentration in the radial direction was more uniform and the ozone conversion was higher than that in a riser without baffles except at the lowest gas velocity used. A mathematical model developed accounted for the gas-phase ozone conversion under various operating conditions. The gas-solid contact efficiency in the riser was discussed in light of the model.

Baffle effects on performance of catalytic circulating fluidized bed reactor

High-pressure operations are common in industrial applications of gas-liquid-solid fluidized-bed reactors for resid hydrotreating, Fischer-Tropsch synthesis, coal methanation, methanol synthesis, polymerization, and other reactions. The phase holdups and the heat-transfer behavior were studied experimentally in three-phase fluidized beds over a pressure range of 0.1--15.6 MPa. Bubble characteristics in the bed are examined by direct flow visualization. Pressure effects on the bubble coalescence and breakup are analyzed mechanistically. The study indicates that the pressure affects the hydrodynamics and heat-transfer properties of a three-phase fluidized bed significantly. The average bubble size decreases and the bubble-size distribution becomes narrower with an increase in pressure. The bubble-size reduction leads to an increase in the transition gas velocity from the dispersed bubble regime to the coalesced bubble regime, an increase in the gas holdup, and a decrease in the liquid and solids holdups. The pressure effect is insignificant above 6 MPa. The heat-transfer coefficient between an immersed surface and the bed increases to a maximum at pressure 6--8 MPa and then decreases with an increase in pressure at a given gas and liquid flow rate. This variation is attributed to the pressure effects on phase holdups and physical properties of the gas and liquid phases.more » A mechanistic analysis revealed that the major heat-transfer resistance in high-pressure three-phase fluidized beds resides in a liquid film surrounding the heat-0transfer surface. An empirical correlation is proposed to predict the heat-transfer coefficient under high-pressure conditions.« less

High-pressure three-phase fluidization: Hydrodynamics and heat transfer

Bubble characteristics in three-phase systems used for pulp and paper processing

A systematic study conducted explores the hydrodynamics of a circulating fluidized bed with polymeric particles employing polyethylene resins with a particle density of 660 kg/m3 and size ranging from 90 to 500 μm. The study indicates that polyethylene resins can be fluidized smoothly in the fast fluidization regime. However, the operating range of the fast fluidization regime for these particles is smaller than that for FCC particles. The deviation of the fluidization behavior of polyethylene particles from that of common Group-A particles is explained considering the interparticle forces. Experiments with fine polyethylene particles are also conducted with coarse particles added in a circulating fluidized bed. Axial profiles of solid holdups in a bed with and without coarse particles, as well as overall fine particle holdup, are studied. The results show a significantly wider operating range of the fast fluidization regime and enhancement of fine particle holdups in a bed with the presence of coarse particles. For comparison, fluidization with FCC particles is also conducted. A mechanistic model considering particle-particle collision is proposed. The model accounts for the momentum exchange rate between a coarse particle and a cloud of fine particles, which explains the enhancement of fine particle holdups observed experimentally.

Peijun Jiang

Papers

Baffle effects on performance of catalytic circulating fluidized bed reactor

High-pressure three-phase fluidization: Hydrodynamics and heat transfer

Bubble characteristics in three-phase systems used for pulp and paper processing

Hydrodynamic behavior of circulating fluidized bed with polymeric particles

Coarse-particle effects in a multisolid circulating fluidized bed for catalytic reactions