Gas-Liquid-Particle Operations

GAS-LIQUID-PARTICLE OPERATIONS IN CHEMICAL REACTION ENGINEERING ... 

All these gas-liquid-particle operations are of industrial interest. For example, desulfurization of liquid petroleum fractions by catalytic hydrogenation is carried out, on the industrial scale, in trickle-flow reactors, in bubble-column slurry reactors, and in gas-liquid fluidized reactors. 

A considerable amount of information regarding the properties of various gas-liquid-particle operations is available in the chemical engineering literature. This information is of importance both with respect to the design of gas-liquid-particle operations and as a basis for the comparison of and choice between the various types of operation. It is of particular importance with regard to these uses that the available information be compiled and reviewed as comprehensively as possible, and it is intended in the present paper to attempt such a review. 

The area of interest covered by this paper is limited to processes in which chemical conversion occurs, as in the processes noted above. Gas-liquid-particle processes in which a gaseous phase is created by the chemical reaction between a liquid and a solid (for example, the production of acetylene by the reaction between water and carbide) are excluded from the review. Also excluded are physical separation processes, such as flotation by gas-liquid-particle operation. Gas absorption in packed beds, another gas-liquid-particle operation, is not treated explicitly, although certain results for this operation must necessarily be referred to. 

A number of industrially important processes are carried out by gas-liquid-particle operation, as illustrated by the following four examples. 

Catalytic desulfurization is at present carried out industrially by at least three of the major types of gas-liquid-particle operations referred to in Section I trickle reactors, bubble-column slurry reactors, and gas-liquid fluidized reactors. 

Hydrogenation of unsaturated fats and fatty oils is one of the oldest heterogeneous catalytic processes of industrial significance, and is carried out exclusively by gas-liquid-particle operation, the vaporization of the fats being impracticable. Stirred-slurry operation is the normal mode of operation, the suspended catalyst being finely divided by Raney nickel (B2). 

The production of alcohols by the catalytic hydrogenation of carboxylic acids in gas-liquid-particle operation has been described. The process may be based on fixed-bed or on slurry-bed operation. It may be used, for example, for the production of hexane-1,6-diol by the reduction of an aqueous solution of adipic acid, and for the production of a mixture of hexane-1,6-diol, pentane-1,5-diol, and butane-1,4-diol by the reduction of a reaction mixture resulting from cyclohexane oxidation (CIO). 

Gas-phase reactions catalyzed by solid catalysts are normally carried out in gas-particle operation in either fixed or fluidized beds. The possibility of using gas-liquid-particle operations for such reactions is, however, of interest in certain cases, particularly if the presence of a liquid medium for the transfer of heat or mass is desirable. 

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... 

As a final example of the application of gas-liquid-particle operation to a process involving a gaseous reactant and a solid catalyst, the possibility of polymerizing ethylene in, for example, a slurry operation employing a metal or metal oxide catalyst can be cited. It has been suggested that the good control of reaction conditions obtained in a slurry-type operation may be of importance in the production of certain types of polyethylene (Rl). 

A number of important gas-liquid-particle processes fall outside the categories of Sections II,A and II,B. The review of gas-liquid-particle operations in the following sections is written with particular regard to applications in processes of the types already referred to, but may also be of some significance with regard to other types. A few examples of such processes will be briefly mentioned below. 

It may finally be pointed out that certain separation processes in addition to packed-bed gas absorption are gas-liquid-particle operations. Examples are flotation and a special type of fluidized crystallization process (Z2). 

The purpose of this section is to present a general theoretical model of gas-liquid-particle operations, with a number of simplifying assumptions that make possible, at least in principle, the calculation of the conversion and yield from a specified amount of information regarding transport phenomena and reaction kinetics. 

Gas-liquid-particle operations are of a comparatively complicated physical nature Three phases are present, the flow patterns are extremely complex, and the number of elementary process steps may be quite large. Exact mathematical models of the fluid flow and the mass and heat transport in these operations probably cannot be developed at the present time. Descriptions of these systems will be based upon simplified concepts. 

It seems probable that a fruitful approach to a simplified, general description of gas-liquid-particle operation can be based upon the film (or boundary-resistance) theory of transport processes in combination with theories of backmixing or axial diffusion. Most previously described models of gas-liquid-particle operation are of this type, and practically all experimental data reported in the literature are correlated in terms of such conventional chemical engineering concepts. In view of the so far rather limited success of more advanced concepts (such as those based on turbulence theory) for even the description of single-phase and two-phase chemical engineering systems, it appears unlikely that they should, in the near future, become of great practical importance in the description of the considerably more complex three-phase systems that are the subject of the present review. 

A number of theoretical models of gas-liquid-particle operations have recently appeared in the literature. Those considered most significant will be discussed briefly in the following, as introduction to a somewhat more general model that will be proposed in this paper. 

A more general model of gas-liquid-particle processes than those that have so far appeared in the literature would, it seems, be of considerable interest as a basis for comparing the reaction-engineering properties of the several types of gas-liquid-particle operations, and as a means for analyzing operations with finite liquid flow (for example, trickle-flow operation and gas-liquid fluidization). 

The two models commonly used for the analysis of processes in which axial mixing is of importance are (1) the series of perfectly mixed stages and (2) the axial-dispersion model. The latter, which will be used in the following, is based on the assumption that a diffusion process in the flow direction is superimposed upon the net flow. This model has been widely used for the analysis of single-phase flow systems, and its use for a continuous phase in a two-phase system appears justified. For a dispersed phase (for example, a bubble phase) in a two-phase system, as discussed by Miyauchi and Vermeulen, the model is applicable if all of the dispersed phase at a given level in a column is at the same concentration. Such will be the case if the bubbles coalesce and break up rapidly. However, the model is probably a useful approximation even if this condition is not fulfilled. It is assumed in the following that the model is applicable for a continuous as well as for a dispersed phase in gas-liquid-particle operations. 

The simplified concept of a gas-liquid-particle operation used in the following analysis is illustrated in Fig. 1. By considering a differential volume element of height dz, the following material balances may be formulated for one component in the three phases ... 

Discussed in the following section will be such data and other information regarding the elementary process steps in gas-liquid-particle operations as have appeared in the chemical engineering literature. 

The use of even the very simple models for isothermal operation described in Section IV,B requires a substantial amount of information regarding the elementary iate processes occurring in a gas-liquid-particle operation, as discussed in Section IV,A. While a considerable amount of information of this kind is available in the chemical engineering literature, it is widely scattered. It will be attempted in this section to present a comprehensive review of this information in order to facilitate its use. It is hoped that this review will be of value not only to those chemical engineers directly interested in the practical applications of gas-liquid-particle operations, but also, by pointing to the several areas characterized by very limited information, to those interested in research in this field. 

The absorption of reactants (or desorption of products) in trickle-bed operation is a process step identical to that occurring in a packed-bed absorption process unaccompanied by chemical reaction in the liquid phase. The information on mass-transfer rates in such systems that is available in standard texts (N2, S6) is applicable to calculations regarding trickle beds. This information will not be reviewed in this paper, but it should be noted that it has been obtained almost exclusively for the more efficient types of packing material usually employed in absorption columns, such as rings, saddles, and spirals, and that there is an apparent lack of similar information for the particles of the shapes normally used in gas-liquid-particle operations, such as spheres and cylinders. 

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