Three-phase reactor

Catalytic three-phase processes are of enormous industrial importance. Catalytic three-phase processes exist in the oil and petrochemical industry, in the manufacture of synthetic fuels, as intermediate steps in the processing of organic compounds, in the production of 

FIGURE 6.1 Various phases in a three-phase reactor. 

FIGURE 6.2 Typical three-phase reactors (a) a bubble column, (b) a tank reactor, (c) a packed bed reactor, and (d) a fluidized bed reactor. 

TABLE 6.1 Examples of Catalytic Three-Phase Processes 

Hydrogenation of aromatic compounds (dearomatization), that is, hydrogenation of henzene, toluene, and polyaromatics Hydrogenation of anthraquinone in the production of H2O2 

Some reactors involve three or even more phases. This section discusses the fairly specific situation of a gas phase, a liquid phase, and a solid phase. 

With the above relations for three phase reactors with gas, liquid and solid catalyst phases we can determine the mass and heat transfer coefficients for the transport to and from the catalyst particle, as it is suspended in the liquid phase. The same holds for transfer in the liquid phase surrounding catalyst particles through which gas and liquid flow. 

The thorough mixing of the solid leads to effective gas-solid heat exchange with an excellent heat-transfer characteristic and hence a uniform temperature distribution in the reaction space. Heat-transfer coefficients are typically 100-400 kJm h K and for small particles can be as high as 800 kJm h K. For fine particles and at high reaction rates, circulating fluidized-bed reactors with separation and recycling of the soUd are particularly suitable. 

To give a conversion comparable to that of a fixed-bed reactor, a fluidized-bed reactor must be considerably larger. Disadvantages are the broad residence-time distribution of the gas, which favors side reactions attrition of the catalyst particles and the difficult scale-up and modeling of this t5 e of reactor. 

The generally low reaction temperatures allow the production of heat-sensitive compoimds and the use of thermally less stable but particularly active or selective catalysts such as  

Liquid-phase processes generally give higher space-time yields than gas-phase processes. The higher heat capacity and thermal conductivity of liquids leads to bet- 

Depending on the arrangement of the catalyst, three-phase reactors can be classsi-fied as  

Since af is small for sparingly solnble gases, the enhancement factor can be quite large. 

Piston flow is a reasonable approximation for the liquid and gas phases. The design equations of Section 11.1.3 can be applied by adding an effective pseudoho-mogeneous reaction rate for the liquid phase  

Compare Equation 11.42 to the Equation 9.1. The standard model for a two-phase packed-bed reactor is a PDE that allows for radial dispersion. Most trickle-bed reactors have large diameters and operate adiabatically so that radial gradients do not arise. They are thus governed by ODEs. If a mixing term is required, the axial dispersion model can be used for one or both of the phases. See Equations 11.33 and 11.34. 

The pseudohomogeneous reaction term in Equation 1L42 is analogous to that in Equation 9.1. We have explicitly included the effectiveness factor r] to emphasize the heterogeneous nature of the catalytic reaction. The discussion in Section 10.5 on the measurement of intrinsic kinetics remains applicable, but it is now necessary to ensure that the liquid phase is saturated with the gas when the measurements are made. The void fraction e is based on relative areas occupied by the liquid and solid 

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Some contrasting characteristics of the main lands of three-phase reactors are summarized in Table 23-15. In trickle bed reactors both phases usually flow down, the liquid as a film over the packing. In flooded reactors, the gas and hquid flow upward through a fixed oed. Slurry reactors keep the solids in suspension mechanically the overflow may be a clear liquid or a slurry, and the gas disengages from the... 

Single-or three-phase Single-phase reactors are used in the neutral circuit either to limit the ground fault currents or as arc-suppression coils (Section 20.5). Similarly, three-phase reactors are used for three-pha.se applications. 

Figure 27.6 A gapped iron three-phase reactor...

Consider a three-phase reactor having the following specifications ... 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

Ueyama, K. (ed.), 1993. Handbook of bubble columns and three phase reactors. IPC, Tokyo. 

Dudukovic, M. P., Trends in catalytic reaction engineering, Catal. Today, 48, 5-15 (1999). A comprehensive review of the complex hydrodynamic issues associated with three-phase reactors is given in... 

Batchwise operating three-phase reactors are frequently used in the production of fine and specialty chemicals, such as ingredients in drags, perfumes and alimentary products. Large-scale chemical industry, on the other hand, is often used with continuous reactors. As we developed a parallel screening system for catalytic three-phase processes, the first decision concerned the operation mode batchwise or continuous. We decided for a continuous reactor system. Batchwise operated parallel sluny reactors are conunercially available, but it is in many cases difficult to reveal catalyst deactivation from batch experiments. In addition, investigation of the effect of catalyst particle size on the overall activity and product distribution is easier in a continuous device. 

Fan (1989) provided a detailed historical development of three-phase fluidization systems in reactor applications. Only a brief review of the significant accomplishments and the economic factors affecting the development of three-phase reactors will be provided here. Table 1 provides the important contributions in the application of three-phase fluidization systems for the past several decades. The direct liquefaction of coal to produce liquid fuels was the first commercial reactor application of three-phase fluidization systems, with development having occurred from the mid-1920 s throughout the 1940 s. A large effort was put forth at this time in Europe for the production of liquid fuels from coal as a direct... 

The development of three-phase reactor technologies in the 1970 s saw renewed interest in the synthetic fuel area due to the energy crisis of 1973. Several processes were developed for direct coal liquefaction using both slurry bubble column reactors (Exxon Donor Solvent process and Solvent Refined Coal process) and three-phase fluidized bed reactors (H-Coal process). These processes were again shelved in the early 1980 s due to the low price of petroleum crudes. 

New applications and novel reactor configurations or operational modes for three-phase systems are continually being reported. These include the operation of a three-phase fluidized bed in a circulatory mode (Liang et al., 1995), similar to the commonly applied gas-solid circulating fluidized bed the application of a three-phase fluidized bed electrode that can be used as a fuel cell (Tanaka et al., 1990) magnetically stabilized three-phase fluidized beds centrifugal three-phase reactors and airlift reactors. 

Three-phase reactor systems are ideally suited for methanol production because of the ability to provide intimate contact between the gaseous phase reactants and the solid phase catalysts and to remove the large amounts of heat created by the high heats of reaction. In the three-phase system, an inert liquid phase circulates between the reactor and an external... 

The catalytic ethylene oligomerization was performed in a 0.3 L well-mixed three-phase reactor operating in semi-batch mode, at constant temperature (70 or 150 °C) and pressure (4 MPa of ethylene) in 68 g of n-heptane (solvent). Prior to each experiment, the catalyst was successively pretreated, firstly in a tubular electrical furnace (550 °C, 8 h) and then in the oligomerization autoclave (200 °C, 3 h), under nitrogen flow at atmospheric pressure. After 30 min of reaction, the autoclave was cooled at -20 °C and the products were collected, weighted and analyzed by GC (FID, DB-1 60 m capillary column). 

There are three main types of three-phase reactors in which the catalyst particles move about in the fluid. 

Numerous studies have been made of the hydrodynamics and other aspects of the behavior of gas-liquid-solid systems, in particular of trickle beds, and including absorption and extraction in packed beds. A selection of correlations of these parameters is presented in problem P8.03.02. They tell something of what is going on in three-phase reactors. 

In Chapter 12 we will consider multiphase reactors in which drops or bubbles carry one phase to another continuous fluid phase. In fact, these reactors frequently have a sohd also present as catalyst or reactant or product to create a three-phase reactor. We need the ideas developed in this chapter to discuss these even more complicated reactors. 

Figure 12-1 Sketch of two- and three-phase reactors. Variables must be specified in each phase a, and y.

In this case the reaction occurs only in the hquid phase (the oils have negligible vapor pressure), but reaction requires a solid catalyst such as finely divided Ni, which is suspended in the hquid. Thus we see that this is in fact a three-phase reactor containing H2 gas, organic hquid reactants and products, and sohd catalyst. 

This is a three-phase reactor with aqueous and organic liquid phases and a gas phase. The C3 and C4 alkanes are fed into the reactor as gas and liquid, the catalyst is sulfuric or hydrofluoric acid in water, and the products are liquid organics. 

Three-phase reactors are generally needed in cases where there are both volatile and nonvolatile reactants, or when a liquid solvent is necessary with all reactants in the gas-phase (Smith, 1981). Some examples are... 

The employment of three-phase reactors is mostly desirable when there are some reactants that are too volatile to liquefy, whereas some others are too nonvolatile to vaporize. Hence, the situation where a gaseous component reacts with another reactant in the liquid-phase is of great interest. The following reaction represents this case (Smith, 1981) ... 

In complex systems such as three-phase reactors, the methods of mathematical modeling cannot provide the required information for process design and scale-up since it is practically impossible to take into account all existing phenomena and safely predict the influence of hydrodynamics, heat and mass transfer, or kinetics on each other (Datsevich and Muhkortov, 2004). Thus, models are almost always approximate in nature. They are based on a number of assumptions that cannot be met during scale-up. So, it is not surprising that industrial unit designers do not completely trust the results obtained from mathematical modeling. Thus, several systems cannot be fully modeled mathematically and other methods for scale-up are followed. 

Film diffusion resistance All bubbling reactors such as Catalytic fixed-bed G/S reactors BFB and three-phase reactors All fast, noncatalytic G/S reactions such as combustion and gasification ... 

The cocurrent gas-liquid downflow fixed-bed reactors well known as trickle-bed reactors (TBR) are among the most widely used three-phase reactors. 

Figure 5.3-1. Schematic representation of some types of three-phase reactors [8].

The individual mass transfer and reaction steps occurring in a gas-liquid-solid reactor may be distinguished as shown in Fig. 4.15. As in the case of gas-liquid reactors, the description will be based on the film theory of mass transfer. For simplicity, the gas phase will be considered to consist of just the pure reactant A, with a second reactant B present in the liquid phase only. The case of hydro-desulphurisation by hydrogen (reactant A) reacting with an involatile sulphur compound (reactant B) can be taken as an illustration, applicable up to the stage where the product H2S starts to build up in the gas phase. (If the gas phase were not pure reactant, an additional gas-film resistance would need to be introduced, but for most three-phase reactors gas-film resistance, if not negligible, is likely to be small compared with the other resistances involved.) The reaction proceeds as follows ... 

Three-phase reactors 2 can thus be divided into two main classes A. Suspended-bed reactors, and B. Fixed-bed reactors. 

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