Types of Three-Phase Reactors

Extensive literature is available on the gas-liquid and solid-liquid mass transfer features of MASRs, for example. Levins and Glastonbury (1972a,b), Sano et al. (1974), Yagi and Yoshida (1975), Bern et al. (1976), Teshima and Ohashi (1977), Boon-Long et al. (1978), Patil and Sharma (1984). Mills and Chaudhari (1997) recommend the following correlations of Yagi and Yoshida (1975) (Equation 17.27) and Bern et al. (1976) (Equation 17.28) for gas-liquid mass transfer  

An important relatively recent achievement is the development of the so-called liquid entrained reactor (LER) for liquid phase methanol synthesis. To assess the performance of these reactors, much simultaneous work has been done on MARs, comparing the performance of the two types of reactors for this reaction. A good discussion of this can be found in a paper by Vijayaraghavan et a. (1993). An equation developed for the overall gas-liquid mass transfer coefficient (see Ko, 1987 Lee et al., 1988 Parameswaran et al., 1991) is 

Liquid-liquid mass transfer depends on whether the transfer is from the continuous to the dispersed phase or vice versa. The liquid-liquid interfacial area an can be estimated from an = 6/iL/do where /il is the dispersed liquid phase holdup and c/q is the average size of the dispersed droplets which can be determined from a correlation given by Okufi et al. (1990). 

Solid-liquid mass transfer coefficients can be calculated from the following correlation proposed by Sano et al. (1974)  

This correlation is also recommended for liquid-liquid systems. 

---

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. 

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

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

The well-known advantages of the slurry reactor over other types of three-phase reactors (such as trickle bed or fixed bed upflow reactor) are the following ... 

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. 

Semibatch reactors Design equations for (1,0)- and (l,l)-order reactions Continuous reactors Types of three-phase reactors... 

There are a wide variety of three-phase fluidized bioreactor designs possible. The conventional reactor, shown in Fig. 9, is fluidized by both gas and liquid entering at the bottom of the reactor and leaving at the top and is the most common type of three-phase fluidized bed bioreactor. This reactor may be configured to operate with little axial liquid mixing or in a well-mixed mode by adding a recycle stream. The airlift reactor or draft tube fluidized bed reactor, Fig. 10, is also frequently used. In this reactor, gas is injected at the bottom of a draft tube placed in the center of the... 

In this type of three-phase catalytic reactor, centrifugal force is employed to vary the hydrodynamics and transport characteristics of the conventional gas-liquid-solid reactor. Interphase transport of momentum and mass in such a reactor is governed by the centrifugal forces. Dudukovic and co-... 

A major limitation of the present work is that it deals only with well-defined (and mostly unidirectional) flow fields and simple homogeneous and catalytic reactor models. In addition, it ignores the coupling between the flow field and the species and energy balances which may be due to physical property variations or dependence of transport coefficients on state variables. Thus, a major and useful extension of the present work is to consider two- or three-dimensional flow fields (through simplified Navier-Stokes or Reynolds averaged equations), include physical property variations and derive lowdimensional models for various types of multi-phase reactors such as gas-liquid, fluid-solid (with diffusion and reaction in the solid phase) and gas-liquid-solid reactors. 

There are, in general, five types of three-phase slurry reactor studied in the literature ... 

Of primary interest for the industrial application of monolith reactors is to compare them with other conventional three-phase reactors. Two main categories of three-phase reactors are slurry reactors, in which the solid catalyst is suspended, and packed-bed reactors, where the solid catalyst is fixed. Generally, the overall rate of reactions is often limited by mass transfer steps. Hence, these steps are usually considered in the choice of reactor type. Furthermore, the heat transfer characteristics of chemical reactors are of essential importance, not only due to energy costs but also due to the control mode of the reactor. In addition, the ease of handling and maintenance of the reactor have a major role in the choice of the reactor type. More extensive treatment of conventional reactors can be found in the works by Gianetto and Silveston [11], Ramachandran and Chaudhari [12], Shah [13,14], Shah and Sharma [15], and Trambouze et al. [16], among others. 

Qiaracteristics of three-phase reactor types (adapted from [1])... 

Since three-phase operation is industrially very important and since this is the most challenging catalytic operation. Table 8.2 presents a comparison of the main characteristics of three-phase reactor types, while Table 8.3 gives a number of appreciation criteria. These can serve in the choice of a proper reactor type for a specific reaction. 

In 1979 Chem Systems initiated a program to develop a liquid-entrained catalyst reactor which would provide improved contacting of syngas with the catalyst in a three phase system (ref. 38). This reactor system uses much finer catalyst particles than the fluidized bed reactor, and the catalyst-liquid slurry circulates through the reactor. The syngas can be contacted with the catalyst-liquid slurry either counter currently or co-currently. It appears that this process is more efficient than the original fluidized bed process. However, a major problem with this type of three phase system will no doubt be the development of a suitable catalyst since it is unlikely that conventional co-precipitated Cu-ZnO-A Oj catalysts will have the desired characteristics, particularly mechanical strength. 

Figure 17.5 Schematic diagrams of different types of three-phase slurry reactors.

Three main types of gas-phase reactors are used in the industry fluidized-bed, vertical stirred-bed and horizontal stirred-bed reactors. 

When a three phase system seems to be the best (or the sole) solution for a specific application, there remains the difficult task of selecting the most suitable reactor type among the numerous possibilities of contacting a gas and a liquid in the presence of a solid catalyst. Several papers have been devoted to this problem (see for example references 2,3, and 5) Fundamental characteristics such as residence time distribution are as important as technological aspects such as tightness of pressure vessels. Main features on which can be based a comparison between the two broad classes of three phase reactors - slurry and fixed bed-have been collected in Tables 2 and 3. Of course, such a general comparison is very rough and each mentioned item has to be discussed for every specific case. 

Figure 12.2 Schematic cJiagrams of cJifferent types of three-phase slurry reactors, (a) MASR (b) BCSR (c) jet LSR.

TABLE 3 Comparison of Gas-to-Liquid Mass Transfer in Three Common Three-Phase Reactor Types... 

The pulsating three-phase reactor has been examined only at the laboratory level. The pulsation gives good mixing and l)eat- and mass-transfer characteristics in the column. The first three types of gas-liquid-suspended-solid reactor are the most commonly used in practice. Schematic diagrams for these reactors are shown in Fig. l-3fn), (b), and (c), respectively. The agitated and nonagitated slurry... 

In this chapter, we review the reported studies on the hydrodynamics, holdups, and RTD of the various phases (or axial dispersion in various phases), as well as the mass-transfer (gas-liquid, liquid-solid, and slurry-wall), and heat-transfer characteristics of these types of reactors. It should be noted that the three-phase slurry reactor is presently a subject of considerable research investigation. In some cases, the work performed in two-phase (either gas-liquid or liquid-solid) reactors is applicable to three-phase reactors however, this type of extrapolation is kept to a minimum. Details of the equivalent two-phase reactors are considered to be outside the scope of this chapter. 

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Three phase fluidized bed reactor/slurry reactor FIGURE I l.l Types of bubble column reactors (from Lee and Tsui, 1999). 

New reactor types for three-phase operation are still being developed. An example is the application of structured reactors, which may have certain advantages in three-phase operation, and can be operated with co-, cross- as well as with countercurrent flow. A very recent development is the use of monolith reactors (Fig. 8.9) in three-phase operations. Their advantages are the small pressure drop, the good external mass transfer, the short diffusion distance, and the low adiabatic temperature rise. Disadvantages are the higher catalyst costs, importance of liquid distribution, and moderate catalyst load. 

Despite the experience with batch reactors it may be worthwhile to operate continuous reactors also for fine chemicals. Continuously operated reactors only demand for one start-up and one shut-down during the production series for one product. This increases the operating time efficiency and prevents the deactivation of dry catalysts this implies that the reactor volume can be much smaller than for batch reactors. As to the reactor type for three phase systems an agitated slurry tank reactor [5,6] is not advisable, because of the good mixing characteristics. Specially for consecutive reaction systems the yields to desired products and selectivities will be considerably lower than in plug flow type reactor. The cocurrent down flow trickle flow reactor... 

---