Two-Phase Fixed Bed Catalytic Reactors with

2 Two-Phase Fixed Bed Catalytic Reactors with Cocurrent Downflow. Trickle Bed Reactors and Packed Downflow Bubble Reactors 

This section deals with problems that bear considerable relation to those dealt with in Chapter 11 on fixed bed catalytic reactors with a single fluid phase, the main difference being in the hydrodynamics, because of the existence of two 

Concentration and partial pressure profiles in fluid phase and catalyst. 

The advantage of downflow operation with respect to upflow lies in the fact that there is no limitation on the flow rates imposed by flooding limits. The flow rates are only limited by the available pressure head at the inlet. Furthermore, the liquid is much more evenly and thinly distributed than with upward flow. Depending on the respective flow rates of gas and liquid, different flow regimes may be obtained. 

The trickle flow regime corresponds to rather low flow rates of gas and liquid the gas phase is continuous and the liquid phase dispersed. Increasing the gas flow rate leads to pulsed flow. If, for a given liquid flow rate, the gas flow rate is increased too much, spray flow will be obtained, however. For higher liquid throughputs, the liquid phase may be continuous and the gas phase dispersed this is called bubble flow. Increasing the gas flow rate will lead to dispersed bubble flow and then to pulsed flow. 

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Two-Phase Fixed Bed Catalytic Reactors with Cocurrent... 

This section deals with problems that bear considerable relation to those dealt with in Chapter 11 on fixed bed catalytic reactors with a single fluid phase, the main difference being in the hydrodynamics, because of the existence of two fluid phases. In addition, the mass and heat transfer phenomena are more complex, since resistances in the gas phase, the liquid phase, and the solid catalyst, where the reaction takes place, have to be considered. Figure 14.3.b-l illustrates concentration and partial pressure profiles around a catalyst particle and defines the notation. A is the reacting component of the gas phase, B that of the liquid phase. 

As indicated in Table 17.1, there are essentially three main classes of three-phase fixed-bed catalytic reactors. The class of reactors characterized by cocurrent downflow of gas and liquid is called the trickle bed reactor (TBR). We shall be concerned here only with these reactors, for they are more commonly used in organic technology than the other two variations. 

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... 

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... 

Fixed- or packed-bed reactors refer to two-phase systems in which the reacting fluid flows through a tube filled with stationary catalyst particles or pellets (Smith, 1981). As in the case of ion-exchange and adsorption processes, fixed bed is the most frequently used operation for catalysis (Froment and Bischoff, 1990 Schmidt, 2005). Some examples in the chemical industry are steam reforming, the synthesis of sulfuric acid, ammonia, and methanol, and petroleum refining processes such as catalytic reforming, isomerization, and hydrocracking (Froment and Bischoff, 1990). 

When the selectivity of a catalytic reaction is liable to be bad because of transport limitations, fixed-bed catalysts cannot be used with liquid-phase reactants. When selectivity is less important, fixed-bed catalysts have some advantages. Firstly, the catalyst need not be separated from the reaction products-a flow of reactants can simply be passed through the reactor. Furthermore, the catalyst can be readily thermally pretreated in a gas flow. Hence, deactivated catalysts can be regenerated in situ in the reactor. Exchange of the catalyst of a fixed-catalyst bed by another catalyst is, however, usually a tedious procedure. Fixed-catalyst beds are therefore used only within dedicated reactors in which only one or a limited number of products is produced. Also the lifetime of catalysts employed in fixed-bed reactors must usually be long, viz., two to five years. 

Ethylbenzene (EB) is currently produced by alkylation of benzene with ethylene, primarily via two routes liquid-phase with AlCl, catalyst, or vapour-phase in catalytic fixed bed reactor (Ullmann, 2001). Examine the differences, as well as advantages and disadvantages of these routes. List pros and cons in selecting suitable reactors. 

This review paper is concentrated on problems in scaling-up multiphase catalytic fixed bed reactors such as trickle-bed or packed bubble column reactors, in which two fluid phases (gas and liquid) pass concurrently through a bed of solid (usually porous) catalyst particles. These types of reactors are widely used in chemical and petrochemical industry as well as in biotechnology and waste water treatment. Typical processes are the hydrodesulphurization of petroleum fractions, the butinediol syntheses in the Reppe process for synthetic rubber, the anthrachinon/hydrochinon process for H202 production, biochemical processes with fixed enzymes or the oxidative treatment of waste water under pressure. 

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