Of gas-liquid reactors

Lo, S., Application of population balance to CFD modelling of gas-liquid reactors . Conference on Trends in Numerical and Physical Modelling for Industrial Multiphase Flows , Cargese, Corse 27-29 September (2000). 

The design of gas-liquid reactors requires the consideration of four sets of basic data ... 

In all cases, a limiting reactor size may be found on the basis of mass transfer coefficients and zero back pressure, but a size determined this way may be too large in some cases to be economically acceptable. Design procedures for mass transfer equipment are in other chapters of this book. Data for the design of gas-liquid reactors or chemical absorbers may be found in books such as those by Astarita, Savage, and Bisio (Gas Treating with Chemical Solvents, Wiley, New York, 1983) and Kohl and Riesenfeld (Gas Purification, Gulf, Houston, TX, 1979). 

In a number of important industrial processes, it is necessary to carry out a reaction between a gas and a liquid. Usually the object is to make a particular product, for example, a chlorinated hydrocarbon such as chlorobenzene by the re action of gaseous chlorine with liquid benzene. Sometimes the liquid is simply the reaction medium, perhaps containing a catalyst, and all the reactants and products are gaseous. In other cases the main aim is to separate a constituent such as C02 from a gas mixture although pure water could be used to remove CO2, a solution of caustic soda, potassium carbonate or ethanolamine has the advantages of increasing both the absorption capacity of the liquid and the rate of absorption. The subject of gas-liquid reactor design thus really includes absorption with chemical reaction which is discussed in Volume 2, Chapter 12. 

The individual mass transfer and reaction steps occurring in a gas-liquid-solid reactor may be distinguished as shown in Fig. 4.15. As in the case of gas-liquid reactors, the description will be based on the film theory of mass transfer. For simplicity, the gas phase will be considered to consist of just the pure reactant A, with a second reactant B present in the liquid phase only. The case of hydro-desulphurisation by hydrogen (reactant A) reacting with an involatile sulphur compound (reactant B) can be taken as an illustration, applicable up to the stage where the product H2S starts to build up in the gas phase. (If the gas phase were not pure reactant, an additional gas-film resistance would need to be introduced, but for most three-phase reactors gas-film resistance, if not negligible, is likely to be small compared with the other resistances involved.) The reaction proceeds as follows ... 

Overall and Phase Balances for Mass. The examples so far in Chapter 11 were designed to be simple yet show some essential features of gas-liquid reactors. Only component balances for the phases, Equations (11.11) and (11.12), have been used. They are reasonably rigorous, but they do not provide guidance regarding how the various operating parameters can be determined. This is done in Section 11.1.2. Also, total mass balances must supplement the... 

Cyclohexene hydrogenation is a well-studied process that serves as model reaction to evaluate performance of gas-liquid reactors because it is a fast process causing mass transfer limitations for many reactors [277,278]. Processing at room temperature and atmospheric pressure reduces the technical expenditure for experiments so that the cyclohexene hydrogenation is accepted as a simple and general method for mass transfer evaluation. Flow-pattern maps and kinetics were determined for conventional fixed-bed reactors as well as overall mass transfer coefficients and energy dissipation. In this way, mass transfer can be analyzed quantitatively for new reactor concepts and processing conditions. Besides mass transfer, heat transfer is an issue, as the reaction is exothermic. Hot spot formation should be suppressed as these would decrease selectivity and catalytic activity [277]. 

Some of this theoretical thinking may be utilized in reactor analysis and design. Illustrations of gas-liquid reactors are shown in Fig. 19-26. Unfortunately, some of the parameter values required to undertake a rigorous analysis often are not available. As discussed in Sec. 7, the intrinsic rate constant kc for a liquid-phase reaction without the complications of diffusional resistances may be estimated from properly designed laboratory experiments. Gas- and liquid-phase holdups may be estimated from correlations or measured. The interfacial area per unit reactor volume a may be estimated from correlations or measurements that utilize techniques of transmission or reflection of light, though these are limited to small diameters. The combined volumetric mass-transfer coefficient kLa, can be also directly measured in reactive or nonreactive systems (see, e.g., Char-pentier, Advances in Chemical Engineering, vol. 11, Academic Press, 1981, pp. 2-135). Mass-transfer coefficients, interfacial areas, and liquid holdup typical for various gas-liquid reactors are provided in Tables 19-10 and 19-11. 

The discussion is centered around gas-liquid reactors. If the dissolved gas content exceeds the amount needed for the reaction, the liquid may be first saturated with gas and then sent through a stirred tank or tubular reactor as a single phase. If the residence times for the liquid and gas are comparable, both gas and liquid may be pumped in and out of the reactor together. If the gas has limited solubility, it is bubbled through the reactor and the residence time for gas is much smaller. Figure 19-29 provides examples of gas-liquid reactors for specific processes. 

This study, which contributes towards the understanding of hydrodynamic behaviour of gas-liquid reactors at elevated pressure, has shown the influence of pressure on the gas flow in a packed column through the axial dispersion coefficient. The gas flow diverges from plug flow when the pressure increases. As for the gas hold-up, an important parameter for the calculation of the reactional volume of a reactor, the pressure has no effect on this parameter in the studied range. This result allows to extrapolate gas hold-up values obtained... 

It is important to recognize that results of this study are based on models which involve several assumptions. The validity of some of these assumptions may be questionable under certain circumstances. However, gas-liquid reaction systems involve complex interacting events, which are difficult to describe precisely. There is also a paucity of experimental data on the behavior of gas-liquid reactors. In view of all this, we believe that studies of this type can be valuable qualitatively for the purpose of model discrimination. 

Two lists of gas/liquid reactions of industrial importance have been compiled recently. The literature survey by Danckwerts (Gas-Liquid Reactions, McGraw-Hill, 1970) cites 40 different systems. A supplementary list by Doraiswamy and Sharma (Heterogeneous Reaetions FluidrRluid-Solid Reaetions, Wiley, 1984) cites another 50 items, and indicates the most suitable kind of reactor to be used for each. Estimates of values of parameters that may be expected of some types of gas/liquid reactors are in Tables 23-9 and 23-10. 

The specific surface area of contact for mass transfer in a gas-liquid dispersion (or in any type of gas-liquid reactor) is defined as the interfacial area of all the bubbles or drops (or phase elements such as films or rivulets) within a volume element divided by the volume of the element. It is necessary to distinguish between the overall specific contact area S for the whole reactor with volume Vr and the local specific contact area 51 for a small volume element AVi- In practice AVi is directly determined by physical methods. The main difficulty in determining overall specific area from local specific areas is that Si varies strongly with the location of AVi in the reactor—a consequence of variations in local gas holdup and in the local Sauter mean diameter [Eq. (64)]. So there is a need for a direct determination of overall interfacial area, over the entire reactor, which is possible with use of the chemical technique. 

Mass transfer parameters of gas-liquid reactors. Adapted from [1,2]... 

Lo S (2000) Application of population balance to CFD modeling of gas-liquid reactors. Proc of Trends in numerical and physical modelling for industrial multiphase flows. Corse, 27-29 September... 

This case study on oxidation of sodium sulfide illustrates the design of a variety of gas-liquid reactors and compares their performances. Bubble column reactors are particularly attractive, as they offer advantages such as simplicity of construction and operation, but they suffer from such drawbacks as high pressure drop and backmixing in the liquid phase. To reduce the pressure drop, two modifications have been considered an external-loop air-lift reactor and a horizontal sparger reactor. Both result in substantial energy savings (because of low AP) under similar conditions of capacity and conversions in the gas and liquid phases. 

A practically useful table, presented in Section 11.4 (Table 11.26), provides valuable guidelines for the selection of gas-liquid reactors. Other reactor configurations should be considered and a table similar to Table 11.26 prepared. 

The mathematical models for different kinds of gas-liquid reactors are based on the mass balances of the components in the gas and liquid phases. The bulk gas and liquid phases are divided by thin films where chemical reactions and molecular diffusion occur. The flux of component i from the gas bulk to the gas film is, and the flux from the liquid film to the liquid bulk is, Vf(. The fluxes are given with respect to the interfacial contact area (A)... 

Another way to approach the topic of gas-liquid reactor design is just to state the basic phase balances in very general form, and then simplify according to the particular situation. A possible drawback is that there is a seemingly endless number of these individual situations, e.g., is there plug flow or CSTR behavior (in one or both phases), is bubble size constant, is the equilibrium according to Henry s law. 

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