Three-phase slurry reactors applications

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. 

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. 

Table 6.3 Some illustrative applications of three-phase slurry reactors.

What is proposed earlier is also in line with the three levels of reactor engineering discussed by Krishna and Sie [50] and indeed is eminently applicable in the context of three-phase slurry reactors. Naturally, the goal here is to decide on a flow pattern that optimally utilizes the catalyst. In other words, the catalyst has a certain intrinsic activity, and the contacting pattern should try and realize that activity in all parts of the reactor. Thus, level [I] design explained earlier must establish the effective performance metric at the catalyst level, which will be the major topic of discussion in this section. 

In connection with the engineering content of the book, a large number of reactors is analyzed two- and three-phase (slurry) agitated reactors (batch and continuous flow), two-and three-phase fixed beds (fixed beds, trickle beds, and packed bubble beds), three-phase (slurry) bubble columns, and two-phase fluidized beds. All these reactors are applicable to catalysis two-phase fixed and fluidized beds and agitated tank reactors concern adsorption and ion exchange as well. 

The general difficulties in design and scale-up of bubble column reactors concern reaction specific data, such as solubilities and kinetic parameters as well as hydrodynamic properties. The paper critically reviews correlations and new results which are applicable in estimation of hydrodynamic parameters of two-phase and three-phase (slurry) bubble column reactors. 

The concept of three-phase sparged reactor or bubble column with a draft mbe (BCDT) can be advantageously applied in this oxidation process. Section 10.9 presents a detailed discussion of various aspects of BCDT. The BCDT (Fig. 3.2b) is a simple variadon of the conventional slurry or three-phase sparged reactor. The major conclusions that can be drawn with respect to the present application are as follows (i) the overall gas holdup in a BCDT is approximately the same as that in a conventional bubble column. Further, the gas holdup is independent of sohd loading, (ii) There is a well-directed Uquid circulation—upward in the draft tube (riser) and downward in the... 

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. 

In some applications such as catalytic hydrogenation of vegetable oils, slurry reactors, froth flotation, evaporative crystallisation, and so on, the success and efficiency of the process is directly influenced by the extent of mixing between the three phases. Despite its great industrial importance, this topic has received only limited attention. 

Typical properties of slurry reactors, and of packed bed co-current downflow trickle flow reactors, are summarized in Table 1. Most properties indicated for slurry reactors also hold for three-phase fluidized beds. These properties can be advantageous or disadvantageous, depending on the application ... 

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 order to illustrate the application of the developed hybrid algorithm, the optimization of a three-phase hydrogenation catalytic slurry reactor is considered. The study aims to determine the optimal operating conditions that lead to maximization of profit. 

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. 

The design, scaleup and performance prediction of slurry reactors require models which must consider not only the hydrodynamic and mixing behavior of the three phases, but also the mass transfer between the phases along with the intrinsic kinetics. In the DCL and FTS processes, an axial dispersion model is applicable, with the solid phase assumed to follow sedimentation or dispersed flow model. However, in the CCC, where the solid particles take part in the reaction, dispersion model is no longer applicable. 

In WAO with solid catalysts, three-phase reactors are used trickle bed, bubble slurry column, and bubble fixed-bed (monolith) or three-phase fluidized-bed reactors. When the catalyst is present in the liquid phase (homogeneous) or absent, two-phase reaetors such as bubble columns, jet-agitated reactors, and mechanically stirred reactor vessels are used. The limitations and advantages of these reactors for the application to WAO are listed in Table 10.7. 

Multi-environment systems with two flowing phases. These systems are perhaps of most interest in reaction engineering applications since they include the most frequently used multiphase reactors. Gas-liquid bubble columns, ebullated beds, three-phase fluidized beds, gas-lift slurry reactors, trickle-bed reactors, pneumatic transport reactors, etc. fall into this category. Some of the developments presented in Section 6.1.1 can be extended to treat these systems. The multivariable joint p.d.f. has to be defined taking into the account that the system has multiple inlets and outlets, i.e. by following the rules established in Section 3 by the appropriate extension of eqs. (9) and (10). However, this approach has not been presented or used to date. The main reason is that the transforms do not have a readily useable analytical form and are functions of many system... 

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