Three-phase slurry reactors reactor design

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. 

In recent years, the use of coal as a raw material for the productions of hydrocarbons, liquid transportation fuels, chemical feedstocks and solid fuel is gaining importance. Tliree important processes for the achievement of this goal are (1) direct (2) removal of sulfur from coal by oxydesul-indirect coal liquefaction or the Fischer-All of these processes employ three-phase slurry reactors. In this overview, a present state of the art for the models, scaleup, design and other operational problems associated with these processes are briefly evaluated. 

In terms of industrial use, the aforementioned three-phase slurry reactors are in themselves amenable for qualitative comparison in terms of their physical attributes and the various operating parameters. While the specifics of these attributes are determined by the process chemistry and detailed design (guidelines to which is discussed later in this chapter), Table 6.4 provides at a glance qualitative comparison of these attributes. 

Table 6.4 Overview of vessel designs and performance attributes of three-phase slurry reactors.

In this chapter, we have attempted to summarize the philosophy and procedural details for the design and scale-up of three-phase slurry reactors. First, the widespread use of three-phase slurry systems in the petroleum processing, chemicals, and process industry has been highlighted with suitable examples. The factors governing the performance of three-phase slurry... 

In the design of upflow, three phase bubble column reactors, it is important that the catalyst remains well distributed throughout the bed, or reactor space time yields will suffer. The solid concentration profiles of 2.5, 50 and 100 ym silica and iron oxide particles in water and organic solutions were measured in a 12.7 cm ID bubble column to determine what conditions gave satisfactory solids suspension. These results were compared against the theoretical mean solid settling velocity and the sedimentation diffusion models. Discrepancies between the data and models are discussed. The implications for the design of the reactors for the slurry phase Fischer-Tropsch synthesis are reviewed. 

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. 

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. 

The MRH process is a hydrocracking process designed to upgrade heavy feedstocks containing large amount of metals and asphaltene, such as vacuum residua and bitumen, and to produce mainly middle distillates (Sue, 1989). The reactor is designed to maintain a mixed three-phase slurry of feedstock, fine powder catalyst and hydrogen, and to promote effective contact. 

The design of conventional biological reactors is very similar to those of gas-liquid, slurry, and polymerization reactors outlined in other chapters. As a matter of fact, biological reactors are the most versatile of all reactors, since such a reactor can carry two or three phases, the liquid can be Newtonian or non-Newtonian, the solids can be heavy or light, and the reaction mixture can be simple or complex. A biological reactor, however, carries certain distinct features ... 

An important aspect of the design of three phase bubble columns is the variation of catalyst distribution along the reactor height, and its effect on reactor performance. Many factors influence the degree of catalyst distribution, including gas velocity, liquid velocity, solid particle size, phase densities, slurry viscosity, and, to a lesser extent, column diameter, solid shape and chemical affinity between the solid and liquid phases. 

Cybulski et al. [39] have studied the performance of a commercial-scale monolith reactor for liquid-phase methanol synthesis by computer simulations. The authors developed a mathematical model of the monolith reactor and investigated the influence of several design parameters for the actual process. Optimal process conditions were derived for the three-phase methanol synthesis. The optimum catalyst thickness for the monolith was found to be of the same order as the particle size for negligible intraparticle diffusion (50-75 p.m). Recirculation of the solvent with decompression was shown to result in higher CO conversion. It was concluded that the performance of a monolith reactor is fully commensurable with slurry columns, autoclaves, and trickle-bed reactors. 

A number of industrially important reactions are carried out in three-phase reactors. Typically, a gas and a liquid-phase reactant are converted to products in the presence of a solid catalyst. There are two basic concepts for designing such reactors, the fixed-bed reactor and the slurry reactor [31]. 

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. 

Precombustion of sulfur from coal by selective oxidation is an important step in coal cleaning, without the need for a postcombustion cleanup step to remove sulfur oxides. This process, known as oxydesulfurization of coal, is a three-phase gas-liquid-solid system. As an example, a sparged reactor wiU be designed to oxydesulfurize 100 tons/day of coal in an aqueous slurry of coal with air. 

Oxidation in a three phase system is a very complicated process. The rate of oxidation can depend on mass transfer of oxygen from the gas to the liquid, on the rate of solid dissolution, and on chemical kinetics. These three processes are closely interrelated and the relative importance of any one depends on the conditions in the scrubber or hold tank. The individual steps of these processes include the gas to liquid transport of oxygen, the solid to liquid mass transfer of sulfite, the precipitation of sulfates, and the chemical reaction. The rates of these steps depend on the concentrations of reacting and nonreacting species in the slurry, on pH, on slurry density, and on the design of the reactor. 

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. 

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