Three-phase slurry 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). 

Another current development in the use of F-T chemistry in a three-phase slurry reactor is Exxon s Advanced Gas Conversion or AGC-21 technology (Eidt et al., 1994 Everett et al., 1995). The slurry reactor is the second stage of a three-step process to convert natural gas into a highly paraffinic water-clear hydrocarbon liquid. The AGC-21 technology, as in the Sasol process, is being developed to utilize the large reserves of natural gas that are too remote for economical transportation via pipelines. The converted liquid from the three-step process, which is free of sulfur, nitrogen, nickel, vanadium, asphaltenes, polycyclic aromatics, and salt, can be shipped in conventional oil tankers and utilized by most refineries or petrochemical facilities. 

No systematic research has been done yet on the influence of pressure and gas density on the holdup in three-phase slurry reactors. The only data [45-51] for three-phase bubble columns under pressure suggest rather high values of eq under pressure. Qualitatively, this is in line with the effect of a high density as predicted by Wilkinson [44], The effect might decrease with increased solids concentration [52]. Clearly, additional research is necessary here. 

Little is known about the fluid wall heat transfer in the case of gas -liquid flow in a fixed-bed reactor. Some research on this subject, however, has been carried out for the specific case of cocurrent downflow over a fixed-bed reactor. This is summarized in Chap. 6. Some work on the slurry-wall heat-transfer rate for a three-phase fluidized bed has also been reported. The heat-transfer rate is characterized by the convective heat-transfer coefficient between the slurry and the reactor wall. Some correlations for the heat-transfer coefficient in a three-phase slurry reactor are discussed in Chap. 9. 

If a transport parameter rc — CS/CL is defined, where Cs is the concentration of C at the catalyst surface, then Peterson134 showed that for gas-solid reactions t)c < rc, where c is the catalyst effectiveness factor for C. For three-phase slurry reactors, Reuther and Puri145 showed that rc could be less than t)C if the reaction order with respect to C is less than unity, the reaction occurs in the liquid phase, and the catalyst is finely divided. The effective diffusivity in the pores of the catalyst particle is considerably less if the pores are filled with liquid than if they are filled with gas. For finely divided catalyst, the Sherwood number for the liquid-solid mass-transfer coefficient based on catalyst particle diameter is two. 

For all values of the variables studied, the gas-phase dispersion does not show a significant effect on the steady-state or transient characteristics of the liquid-phase or surface concentration. This means that, in modeling three-phase slurry reactors, the gas phase may be assumed to move in plug flow as long as the performance of the reactor is measured in terms of the change in concentration of the liquid phase. 

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. 

A three-phase slurry reactor is characterized by the holdup of the three phases, satisfying the equation... 

A review of earlier studies on gas, liquid, and solid holdups in a three-phase slurry reactor is given by Ostergaard.97 Kato54 studied the effects of gas velocity, particle size, the amount of solids and liquid in the bed, and the density of the solids on the gas holdup. The gas holdup [defined as volume of gas/(volume of gas + volume of liquid)] decreased with increasing particle size and amount of solids in the bed, and with the decreasing nominal gas velocity. 

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. 

Lee, S. Gogate, M. Kulik, C. A novel single-step dimethyl ether (DME) synthesis in a three-phase slurry reactor from CO-rich syngas. Chem. Eng. Sci. 1992, 47 (13/14), 3769-3776. 

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... 

Hydrogenation studies All reactions were carried out in a three-phase slurry reactor with magnetic stirring (ca. 1000 rpm), at 25-30°C and 70-100 bar. The reactions were run to completion and the conversion was checked by gas chromatography (column OV 101, 2m, 50°C) after esterification of the hydroxyacids (EtOH, HCl, RT). For technical reasons, the time (minutes) required for the uptake of 100% hydrogen is given as a qualitative measure of the catalyst activity. Optical yields were determined by gas chromatography on a chiral capillary column (Chirasil-(L)-Val, 50 m, 150°C) after derivatization of the hydroxyesters with isopropyl-isocyanate. 

Rgure 17.4 The variables and dimensionless groups used in the analysis of continuous three-phase slurry reactors. 

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... 

Three phase slurry reactors are characterized by a gas-liquid (K,a) and liquid-solid (k ) mass transfer coefficient. These coefficients were determined for the rotating disc reactor at the appropriate operation conditions ... 

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. 

The DCL process contains two steps dissolution of coal in the preheater (accompanied by several fast reactions) and subsequent hydrogenation/hydrocracking reactions (slow reactions) in a three phase slurry reactor. 

There are as yet no theoretical correlations capable of predicting the viscosity, pressure drop and heat transfer coefficient in the preheater. Some empirical correlations for this purpose are available in the literature (11,13-16,38). The hydro-dynamic characteristics of three phase slurry reactors have been extensively reviewed (1,39,40,41). Suitable correlations have... 

Fixed bed (ARGE) Sasol I Entrained Fluidized bed Sasol I Three-phase slurry reactor (Rheinpreussen-Koppers)... 

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... 

Stationary solid catalyst particles (Three-phase "fixed bed reactors) Freely moving solid catalyst particles (Three-phase "slurry" reactors)... 

As listed in Tables 6.1 and 6.2, reactors classified as three-phase slurry reactors broadly fall under the following categories ... 

Figure 6.1 Schematic diagrams of industrial three-phase slurry reactors, (a) Slurry bubble column, (b) Three-phase fluidized bed. (c) Three-phase agitated vessel...

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. 

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 shows the macroscopic attributes that may be achieved in these three-phase slurry reactors from a mechanical and configurational standpoint. For instance, in stirred slurry reactors, the action of the stirrer causes chopping of the bubbles emanating from the distributor, and hence the steady-state bubble size achieved in the reactor is largely determined by the breakage caused by the stirrer action. In slurry bubble columns and three-phase fluidized beds, however, fine bubbles emanate from the distributor, and as they rise, coalescence dominates and the bubbles increase in size (causing reduction in interfacial area... 

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