Three-phase slurry reactors types

In terms of industrial use, each of the aforementioned reactors and their contacting modes offer different advantages and disadvantages. Therefore, the reactor selection for a particular chemistry or process needs to be done after careful consideration of the operating factors and contacting options that the different reactor configurations provide. Table 6.2 provides some qualitative ratings to these factors for some of the important and industrially common reactors from the list presented in Table 6.1. Clearly, the three-phase slurry reactor types classified within the box, which are the scope of this chapter, represent a class of reactors... 

Three-phase slurry reactors are commonly used in fine-chemical industries for the catalytic hydrogenation of organic substrates to a variety of products and intermediates (1-2). The most common types of catalysts are precious metals such as Pt and Pd supported on powdered carbon supports (3). The behavior of the gas-liquid-sluny reactors is affected by a complex interplay of multiple variables including the temperature, pressure, stirring rates, feed composition, etc. (1-2,4). Often these types of reactors are operated away from the optimal conditions due to the difficulty in identifying and optimizing the critical variables involved in the process. This not only leads to lost productivity but also increases the cost of down stream processing (purification), and pollution control (undesired by-products). 

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

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. 

Here, P0 is the impeller power, s0 is the impeller speed, d, is the impeller diameter, Pl and v l are the density and kinematic viscosity of the liquids, respectively. The term tf Myr adjusts the actual impeller speed to the speed at which a fan-disk turbine would rotate for the same power input per unit mass. Although no gas was used in this study, the correlation should be useful as a first estimate for Ks in various types of stirred three-phase slurry reactors. 

Fig. 30. Contacting patterns and contactor types for gas-liquid-solid reactors, (a) Co-current downflow trickle bed. (b) Countercurrent flow trickle bed. (c) Co-current downflow of gas, liquid, and catalyst, (d) Downflow of catalyst and co-current upflow of gas and liquid, (e) Multi-tubular trickle bed with co-current flow of gas and liquid down tubes with catalyst packed inside them coolant on shell side, (f) Multi-tubular trickle bed with downflow of gas and liquid coolant inside the tubes, (g) Three-phase fluidized bed of solids with solids-free freeboard, (h) Three-phase slurry reactor with no solids-free freeboard, (i) Three-phase fluidized beds with horizontally disposed internals to achieve staging, (j) Three-phase slurry reactor with horizontally disposed internals to achieve staging, (k) Three-phase fluidized bed in which cooling tubes have been inserted coolant inside the tubes. (1) Three-phase slurry... 

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

If energy needed for the suspension of solid is small, three phase slurry reactors behave very similar to twophase gas-liquid reactors and design methods developed for those types of reactors can be used as first approximation. 

An examination of gas absorption mechanisms of three phase slurry reactors indicates that three major points,which are not normally considered in reviews concerning this type reactors(see,for in-stance(l) - (3))deserve further attention.These are the slurry reactors involving reactive solids,the possible gas absorption... 

Table 6.3 is an illustrative list of various applications in which three-phase slurry reactors are used today and could potentially be used, detailing the system chemistry and process, catalyst types, and application sector of the economy. While this is not an exhaustive list, it is instructive to see the variety of existing and potential application areas of three-phase slurry reactors. Details about these processes may be found from the references cited in Table 6.3. 

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

Experiments were carried out in a three phase slurry type reactor with aqueous sugar solutions. Hydrogen was fed into the reactor through a flow regulation device (A). The stainless steel reactor was heated electrically and the temperature controlled by a powerstat. It was equipped with a magnetically driven variable speed stirrer. 

H-Coal Process The H-Coal process, based on H-Oil technology, was developed by Hydrocarbon Research, Inc. (HRI). Depending on the type of products desired, the process can be operated in either a fuel-oil mode or a syncrude mode by adjusting operating severity. The heart of the process is a three-phase fluidized reactor (ebullated-bed) in which catalyst pellets are fluidized by the upward flow of slurry and gas through the reactor. Catalyst activity in the reactor is maintained by the withdrawal of small quantities of spent catalyst and the addition of fresh catalyst. The reactor contains an internal tube for recirculating the reaction mixture through the catalyst bed. 

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. 

FIGURE 3.2 (Cont d) (b) Slurry reactor or bubble column with a draft tube cum heat exchanger, (c) Airlift (external downcomer type) slurry or three-phase sparged reactor. (Reprinted from de Deugd et al. (2003) with kind permission from Springer Science + Business Media. 2003.)... 

Hydrodynamics of slurry reactors includes the study of minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large number of slurry reactor types in use (see fig. 1) and the fact that most correlations available are at least partially of an empirical nature. We will therefore restrict ourselves to sparged slurry columns and slurries in stirred vessels. A second problem is the difference with three phase fluidization. To avoid too much overlap we will only consider those cases where superficial liquid velocities are so low that its contribution to suspension of the particles is relatively unimportant. 

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. 

Epoxides such as ethylene oxide and higher olefin oxides may be produced by the catalytic oxidation of olefins in gas-liquid-particle operations of the slurry type (S7). The finely divided catalyst (for example, silver oxide on silica gel carrier) is suspended in a chemically inactive liquid, such as dibutyl-phthalate. The liquid functions as a heat sink and a heat-transfer medium, as in the three-phase Fischer-Tropsch processes. It is claimed that the process, because of the superior heat-transfer properties of the slurry reactor, may be operated at high olefin concentrations in the gaseous process stream without loss with respect to yield and selectivity, and that propylene oxide and higher... 

Slurry Reactors. Slurry reactors are commonly used in situations where it is necessary to contact a liquid reactant or a solution containing the reactant with a solid catalyst. To facilitate mass transfer and effective catalyst utilization, the catalyst is usually suspended in powdered or in granular form. This type of reactor has been used where one of the reactants is normally a gas at the reaction conditions and the second reactant is a liquid, e.g., in the hydrogenation of various oils. The reactant gas is bubbled through the liquid, dissolves, and then diffuses to the catalyst surface. Obviously mass transfer limitations can be quite significant in those instances where three phases (the solid catalyst, and the liquid and gaseous reactants) are present and necessary to proceed rapidly from reactants to products. 

The reaction rate that is most commonly used in the analysis of the corresponding reactor type or model. Three- (slurry) and two-phase systems. 

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 ... 

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... 

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. 

In our discussion of surface reactions in Chapter 11 we assumed that each point in the interior of the entire catalyst surface was accessible to the same reactant concentration. However, where the reactants diffuse into the pores within the catalyst pellet, the concentration at the pore mouth will be higher than that inside the pore, and we see that the entire catalytic surface is not accessible to the same concentration. To account for variations in concentration throughout the pellet, we introduce a parameter known as the effectiveness factor. In this chapter we will develop models for diffusion and reaction in two-phase systems, which include catalyst pellets and CVD reactors. The types of reactors discussed in this chapter will include packed beds, bubbling fluidized beds, slurry reactors, and trickle beds. After studying this chapter you will be able to describe diffusion and reaction in two- and three-phase systems, determine when internal pore diffusion limits the overall rate of reaction, describe how to go about eliminating this limitation, and develop models for systems in which both diffusion and reaction play a role (e.g., CVD). 

An SBC is a vertical, tubular column in which a three-phase (gas-solid-liquid) mixture is used. The slurry phase consists of FT catalysts and FT wax. The syngas flows though the slurry phase in the form of bubbles, as shown in Figure 12.12. The effective heat and mass transfer, low intraparticle diffusion, low pressure drop, and design simplicity are important advantages of this type of reactor. However, considerable problems arise in separating the liquid-phase synthesis products from the catalyst. With their attractive features, the SBC reactors are receiving extensive investment in both R D and commercialization. The concept of SBC is not new. 

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

Liquid-solid reactors of the fluid bed or slurry type do exist but are much less frequently encoimtered than their gas-solid analogues. In most cases a gas phase is involved, too, resulting in three-phase operation. 

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