Catalytic Reactions Involving Three Phases

Two different systems can be identified depending on the porosity nonporous or porous catalytic system. In case of nonporous systems, it is possible to discriminate among (i) chip-like MSR with thin film coating, (ii) chip-like MSR with 

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Reactions involving gas, liquid, and solid are often encountered in the chemical process industry. The most common occurrence of this type of reaction is in hydroprocessing operations, in which a variety of reactions between hydrogen, an oil phase, and a catalyst have been examined. Other common three-phase catalytic reactions are oxidation and hydration reactions. Some three-phase reactions, such as coal liquefaction, involve a solid reactant. These and numerous other similar gas-liquid solid reactions, as well as a large number of gas-liquid reactions, are carried out in a vessel or a reactor which contains all three phases simultaneously. The subject of this monograph is the design of such gas-liquid -solid reactors. 

Ordinarily two phases are involved gas/solid, gas/liquid, or liquid/solid. Other important applications can involve three phases, and some of these will be treated in Chapter 8. The most important case of heterogeneous reaction must be the gas/solid system, which is typical of most catalytic processes fortunately, the same general principles pertain to the analysis of all two-phase systems of interest here, so separate developments on a case-by-case basis are not required. 

The reliability of a model is the function of the validation method such as testing it with independent data and experimental transport and thermodynamic properties at the reaction conditions. For the case of hydrotreating of heavy petroleum, the reactor involves three phases the nonvaporized hydrocarbon (liquid), the vaporized hydrocarbon plus the hydrogen (gas), and the fixed-bed catalyst (solid). Hence, the system to be modeled is a three-phase fixed-bed catalytic heterogeneous reactor. Some assumptions can be made in order to represent the real experimental reactor. 

Describe the various mass transfer and reaction steps involved in a three-phase gas-liquid-solid reactor. Derive an expression for the overall rate of a catalytic hydrogenation process where the reaction is pseudo first-order with respect to the hydrogen with a rate constant k (based on unit volume of catalyst particles). 

Crossed reactions of the two aldehydes under phase-transfer catalytic conditions with the intermediate thioacetates, which can be isolated under controlled reaction conditions [14], leads to the formation of three products [13], as result of retro-Michael reactions (Scheme 4.18). In the case of the reactions involving crotonaldehyde, the major product results from the reaction of the aldehyde with the released thiolacetic acid, with lesser amounts of the expected crossed reaction products (Table 4.23). In contrast, the reaction of acrolein with the thioacetate derived from crotonaldehyde produces, as the major product, the crossed cycloadduct. These observations reflect the relative stabilities of the thioacetates and the relative susceptibilities of acrolein and crotonaldehyde to the Michael reaction. 

When components in solid and fluid phases react, the sequence of steps must be similar to those for fluid-solid catalytic reactions. In Sec. 8-2 catalytic reactions were explained in terms of a three-step process adsorption of the fluid molecule on the sohd surface reaction on the surface involving the adsorbed molecule, and desorption of product. We shall not be concerned here with the mechanism of these processes instead we shall start out on the basis that the rate equation is known from experimental measurements. Often observed data agree with a rate equation which is first order in concentration of the reactant injthe-fluid..phase and directly proportional to the surface of the reactant in the solid phase. For example, despite the stoichiometry of the reaction... 

Multi-phase catalysis performed in ILs can lead to various phase systems where the catalyst should reside in the IL. Prior to the reaction, and in cases where there are no gaseous reactants, two systems can usually be formed a monophase, that is, the substrates are soluble in the IL and biphasic systems where one or all the substrates reside preferentially in an organic phase. If a gas reactant is involved, biphasic and triphasic systems can be formed. At the end of the reaction, three systems can be formed a monophasic system a biphasic system where the residual substrates are soluble in the ionic catalytic solution and the products reside preferentially in the organic phase and triphasic systems, formed, for example, by ionic catalytic solutions, with an organic phase containing the desired product and a third phase containing the byproducts. In most cases, catalysis performed in ILs involves two-phase systems (before and after catalysis). 

The thrust of this chapter is on reactions and reactors involving a gas phase, a liquid phase, and a solid phase which can be either a catalyst (but not a phase-transfer catalyst) or a reactant, with greater emphasis on the former. The book by Ramachandran and Chaudhari (1983) on three-phase catalytic reactions is particularly valuable. Other books and reviews include those of Shah (1979), Chaudhari and Ramachandran (1980), Villermaux (1981), Shah et al. (1982), Hofmann (1983), Crine and L Homme (1983), Doraiswamy and Sharma (1984), Tarmy et al. (1984), Shah and Deck wer (1985), Chaudhari and Shah (1986), Kohler (1986), Chaudhari et al. (1986), Hanika and Stanek (1986), Joshi et al. (1988), Concordia (1990), Mills et al. (1992), Beenackers and Van Swaaij (1993), and Mills and Chaudhari (1997). Doraiswamy and Sharma (1984) also present a discussion of gas-liquid-solid noncatalytic reactions in which the solid is a reactant. 

In two-phase catalytic reactions (gas-solid or liquid-solid, where the catalyst is the solid phase), all concentrations are expressed in terms of concentrations in the liquid bulk. Because concentrations at the catalyst surface are used to formulate the rate equation and these are seldom known, they are related to the corresponding bulk concentrations (which are observable values) through appropriate mass transfer coefficients. In extending this procedure to a three-phase reaction, it must be noted that more mass transfer steps are involved here, as shown in Figure 17.1. Thus let us consider a gas-liquid reaction such as... 

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