Gas-liquid-solid multiphase reactions

Many gas-liquid-solid multiphase reactions, such as the hydration of propene, catalytic hydrogenation of nitrate, chloroform dehalogenation, and H2O2 synthesis by H2 + O2 reaction in solution, are generally limited by the diffusion of the volatile reactant. Retention of homogeneous catalysts and efficient gas-liquid mass transfer are the key properties of such reaction systems. Both can be well achieved in the contactor-type MRs. 

Reaction Membrane/ configuration Catalyst Operating conditions Ref. 

+ H20 C3H70H Carbon/flat disk Pt/AbOj T=130°C P = 2.1MPa [60] 

Dehalogenation of chloroform CHCl3 + 3H2 CH4 + 3HCl H2+ 02(dissolved) 0 H2O2 a-Al203 tube Pd/AbOs T=20°C [61] 

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Table 2.10 Gas-liquid-solid multiphase reactions in porous membrane reactors...

Table 2.10 summarizes the application of porous MRs in gas-liquid-solid multiphase reactions. Operation of the high-pressure membrane contactor requires precision measurement and control. The activity and selectivity of the process may be controlled through controlling the dosage of gas reactant. 

Liu, W., Mini-structured Catalyst Bed for Gas-Liquid-Solid Multiphase Catalytic Reaction, AIChEJ, 2002, July, pp. 1519-1532. 

Photocatalytic oxidation of p-chlorophenol and toluene under gas-liquid-solid multiphase flow conditions was investigated using a photocatalytic microreaction system [160]. By loading both gaseous and liquid samples simultaneously into a microchannel with a photocatalytic titanium dioxide thin layer therein, a gas-liquid-solid multiphase annular flow was generated. The reaction yield was greatly enhanced with decreasing thickness of liquid layer because of improved efficiency of interaction and mass transfer between different phases. 

Kreutzer, M.T. et al. (2005) Multiphase monolith reactors chemical reaction engineering of segmented flow in microchannels. 7th International Conference on Gas-Liquid and Gas-Liquid-Solid, 2005,... 

This reaction is an example of a heterogeneous reaction with a solid catalyst with one reactant principally in solution and another in the gas phase the gas-liquid-solid mixture has to be mixed thoroughly to promote conversion (see Chapter 5 for more detailed consideration of multiphase reactions). Compared with the examples above, the measurement of the hydrogen uptake delivers an additional signal, which can also be used for the determination of reaction parameters. 

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

Classification by Phase Despite the generic classification by operating mode, reactors are designed to accommodate the reactant phases and provide optimal conditions for reaction. Reactants may be fluid(s) or solid(s), and as such, several reactor types have been developed. Singlephase reactors are typically gas- (or plasma- ) or liquid-phase reactors. Two-phase reactors may be gas-liquid, liquid-liquid, gas-solid, or liquid-solid reactors. Multiphase reactors typically have more than two phases present. The most common type of multiphase reactor is a gas-liquid-solid reactor however, liquid-liquid-solid reactors are also used. The classification by phases will be used to develop the contents of this section. 

Multiphase reactors include, for instance, gas-liquid-solid and gas-liq-uid-liquid reactions. In many important cases, reactions between gases and liquids occur in the presence of a porous solid catalyst. The reaction typically occurs at a catalytic site on the solid surface. The kinetics and transport steps include dissolution of gas into the liquid, transport of dissolved gas to the catalyst particle surface, and diffusion and reaction in the catalyst particle. Say the concentration of dissolved gas A in equilibrium with the gas-phase concentration of A is CaLt. Neglecting the gas-phase resistance, the series of rates involved are from the liquid side of the gas-liquid interface to the bulk liquid where the concentration is CaL, and from the bulk liquid to the surface of catalyst where the concentration is C0 and where the reaction rate is r wkC",. At steady state,... 

The analysis and design of multiphase reactors is probably the most widely researched subject in the area of chemical reaction engineering at the present time. While the subject of two-phase reactor design (i.e., gas-solid and gas-liquid) has been extensively reviewed in numerous texts, no similar treatment of three-phase (i.e., gas-liquid-solid) reactor design is available. 

Multiphase Reactors Reactions between gas-liquid, liquid-liquid, and gas-liquid-solid phases are often tested in CSTRs. Other laboratory types are suggested by the commercial units depicted in appropriate sketches in Sec. 19 and in Fig. 7-17 [Charpentier, Mass Transfer Rates in Gas-Liquid Absorbers and Reactors, in Drew et al. (eds.), Advances in Chemical Engineering, vol. 11, Academic Press, 1981]. Liquids can be reacted with gases of low solubilities in stirred vessels, with the liquid charged first and the gas fed continuously at the rate of reaction or dissolution. Some of these reactors are designed to have known interfacial areas. Most equipment for gas absorption without reaction is adaptable to absorption with reaction. The many types of equipment for liquid-liquid extraction also are adaptable to reactions of immiscible liquid phases. 

In the case of multiphase reactions, such as those involving gas-liquid, gas-liquid-solid and gas-liquid-liquid systems, microreaction technology is still in an early stage of development with respect to single-phase applications. However, this is also a rapidly developing area, but the fluidodynamics in microchannels have to be better understood. 

For carrying out multiphase reactions (gas-solid, gas-liquid, gas-liquid-solid, liquid-liquid, gas-liquid-solid, liquid-liquid-solid,...), the number of reactor configurations that are possible is extremely large. There is therefore a need to give careful consideration to the choice of the ideal reactor configuration that meets fully with all the process musts and, to the maximum possible extent, the process wants. The process musts could be ... 

Multiphase reaction engineering has developed into a very active field of research, with international symposia held at frequent intervals. We have only considered a few common reactor types currently in use. A few others of potential importance in relatively large volume intermediates production are the gas-liquid-solid fluidized beds, liquid entrained reactors, and rotating drum reactors. 

The most catalytic or noncatalytic processes involving reactions in multiphase systems. Such processes include heat and mass transfer and other diffusion phenomena. The applications of these processes are diverse and its reactors have their own characteristics, which depends on the type of process. For example, the hydrogenation of vegetable oils is conducted in a liquid phase slurry bed reactor, where the catalyst is in suspension, the flow of gaseous hydrogen keeps the particles in suspension. This type of reaction occurs in the gas-liquid-solid interface. 

L Homme,G.A. "Introduction to gas-liquid-solid systems" (Proceedings of NATO ASI on "Mass transfer with chemical reaction in multiphase systems", Turkey, 1981)... 

Kinetics are readily determined in the laboratory under carefully controlled conditions such that the effects of diffusional resistances are entirely eliminated. The intrinsic kinetics so determined yield the rate constant, the orders of the reaction with respect to the reactants, catalyst, and the activation energy. These are unique to a given gas-liquid or gas-liquid-solid catalyst system and do not vary with the type or size of the multiphase reactor. This matter is briefly discussed later in Section 2.5. 

Multiphase reactions may involve gas-liquid, gas-liquid-solid (solid as catalyst or reactant), liquid-liquid, liquid-liquid-solid reactions, etc. The reactions may vary from very slow to very fast, endothermic to highly exothermic. Based on the reaction characteristics, different types of multiphase reactors are used in industrial practice. A number of texts deaUng with design of multiphase reactors are available (Satterfield 1970 Shah 1979 Ramachandran and Chaudhari 1983 Westerterp et al. 1988 Deckwer 1992). Considerable information on theoretical, hydrodynamics, and mass transfer aspects of different multiphase reactors has become available since the publication of the above texts. This recent information is likely to allow rational, simple and yet reliable designs of many industrially important multiphase reactors. In this book, different types of multiphase reactors falUng under two categories—(1) gas-liquid and (2) gas-liquid-solid— are considered. The basic aim is to provide user-friendly, simple, and reasonably accurate design procedure for each multiphase reactor. 

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