Three-phase fixed-bed reactors

Fig. 4.17. Flow regimes in three-phase fixed-bed reactors, (a) Gas and liquid in co-current downwards flow (trickle-bed operation). (b) Gas and liquid in co-current upwards flow (liquid floods bed), (c) Gas and liquid in countercurrent flow (not often used for catalytic reactors)...

LIquId-llquid-solld three-phase fixed-bed reactors... 

Gas-liquid-solid reactors with a trickle-flow regime are the most widely used type of three-phase reactors and are usually operated under steady-state conditions. The behavior of this kind of reactor under the other three-phase fixed-bed reactors is rather complex due to gas and liquid flow concurrently downward through a catalyst packing. For process intensification it is required to improve some of the specific process steps in such chemical reactors. Figure 4.1 shows an overview of different factors that influenced the trickle-bed reactor performance. 

Bnrghardt, A., Bartehnns, G., Jaroszynski, M., and Kolodziej, A. (1995), Hydrodynamics and mass transfer in a three-phase fixed-bed reactor with cocnrrent gas-liquid downflow, The Chemical Engineering Journal and the Biochemical Engineering Journal, 58(2) 83-99. 

Generally, three phase fixed bed reactors are operated with concurrent downflow of gas and liquid in the trickling regime (trickle-bed reactor). Countercurrent operation is less frequent as in this case, the possible ranges of liquid and gas flow rates are very narrow. It is used when thermodynamic equilibrium limits the extent of reaction. 

Son, S. M., Kimura, H., Kusakabe, K. (2011). Esterification of oleic acid in a three-phase, fixed-bed reactor packed with a cation exchange resin catalyst. Bioresource Technology, 102 2), 2130-2132. 

Two-phase flow in three-phase fixed-bed reactors makes the reactor design problem complex [12], Interphase mass transfer can be important between gas and liquid as also between liquid and catalyst particle. Also, in the case of trickle-bed reactors, the rivulet-type flow of the liquid falling through the fixed bed may result (particularly at low liquid flow rates) in only part of the catalyst particle surface being covered with the liquid phase. This introduces a third mass transfer process from gas to the so-called gas-covered surface. Also, the reaction rates in three-phase fixed-bed catalytic reactors are highly affected by the heat transfer resistances resistance to radial heat transfer and resistance to fluid-to-particle heat transfer. As a result of these and other factors, predicting the local (global) rate of reaction for a catalyst particle in three-phase fixed-bed reactors requires not only... 

Standard two-fluid models for two-phase downflow and upflow in three-phase fixed-bed reactors... 

Reactions carried out in three-phase fixed-bed reactors such as hydrogenation, oxidation, and hydrodesulfurization can be highly exothermic. Such situations require incorporation of an efficient heat removal system in order to avoid hot spots or catalyst deactivation as much as possible [13, 92]. A good knowledge of the packed-bed heat transfer parameters is necessary for the design of the reactor and heat removal system. 

Also, heat transfer in three-phase fixed-bed reactors has been investigated using a two-dimensional homogeneous model with two parameters [94, 96]—the bed radial effective thermal conductivity and the heat transfer coefficient at the wall ... 

As the catalytic reaction taking place inside the pellets is usually accompanied by heat effects, the particle-liquid heat transfer coefficient becomes a fundamental ingredient to be estimated for the assessment of the efficacy of the heat withdrawal from the particle level away to the reactor wall leveL In particular, when highly exothermic reactions are in play, impediment of liquid replenishment over the dried spots on the catalyst surface may favor inception of hot spots that are responsible for reactor runaway. As a result, evacuation of heat across the liquid-covered pellet spots becomes a critical issue. Not many studies in literature deal with particle-liquid heat transfer rates in three-phase fixed-bed reactors. The main reason is probably the difficulty to find an accurate experimental method. The following current trends emanate from the analysis of the particle-liquid heat transfer two-phase downflow fixed-bed literature (i) the transition from trickle to pulsing flow is accompanied by a... 

Catalyst particles in three-phase fixed-bed reactors are usually completely filled with liquid. Then intraparticle temperature gradients are negligible due to the low effective diffusivities in the liquid phase, as pointed out by Satterfield [13] and Baldi [92]. However, if the limiting reactant and the solvent are volatile, vapor-phase reaction may occur in the gas-filled pores, causing significant intraparticle temperature gradients [109, 110]. In these conditions, intraparticle heat transfer resistance is necessary to describe the heat transfer. 

Hydrodynamics, mass, and heat transfer in the commonly used three-phase fixed-bed reactors were briefly outlined. Also, scale-up rules and alternative ways to scale down trickle-bed reactors are discussed. In spite of the extensive studies on the hydrodynamics, mass, and heat transfer in three-phase fixed-bed reactors, clearly, a lot of work remains to be done in providing a fundamentally based description of the effect of pressure on the parameters of importance in three-phase fixed-bed reactors operation, design, and scale-up or scale-down. It is evident that atmospheric data and models/correlations cannot, in general, be extrapolated to operation at elevated pressures. The physics conveyed by the standard two-phase flow models is minimalistic because it insufficiently describes the role and presence of interfaces and their thermodynamic properties. The explicit inclusion of interfaces and interfacial properties is essential because they are known to have a significant role in determining the thermodynamic state of the whole system. 

Bartelmus G. Local solid-liquid mass transfer coefHdents in a three-phase fixed bed reactor. Chem. Eng. Process. 1989 26 111. 

Stationary solid catalyst particles (Three-phase "fixed bed reactors) Freely moving solid catalyst particles (Three-phase "slurry" reactors)... 

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