Heterogeneous Gas-Liquid Reactions

Consider a non-catalytic heterogeneous reaction between a reactant A in the gas phase and a reactant B in the liquid phase. 

Reactant A in the bulk of the gas phase at a partial pressure would be moving to the gas-liquid interphase through a resistance that is confined to a gas film of thickness i/g adjacent to the interphase. The flux of A in the gas phase is 

Reactant A on reaching the interphase would dissolve in the liquid phase and attain a concentration Qli at the interphase, which is equal to the equilibrium concentration corresponding to PAi- Assuming Henry s law to hold good 

Similarly, reactant B that is present in the bulk liquid phase at a concentration Cgf, would be moving to the interphase through a liquid film of thickness and attain a concentration Cb, at the interphase. The flux of B in the liquid phase is 

Reactant A that is dissolved in the liquid at the interphase would be diffusing through the liquid film and the flux of A in the liquid film is governed by Pick s law 

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The heterogeneous gas-liquid reaction of singlet oxygen ( A ) with cyclohexa-1,3-diene gave a 33% yield of the bridged peroxide at 223 K, which in turn can be reduced by hydrogen over palladium to cis-cyclohexane-l,4-diol in >99% purity. However, the reaction is not so stereospecific in molecules where alternative additions are possible or when labile allylic hydrogens are present. 

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 Reactions Fluid-Lluid-Solid Reactions, Wiley, 1984) cites another 50 items, and indicates the most suitable land of reactor to be used for each. Estimates of values of parameters that may be expec ted of some types of gas/liquid reac tors are in Tables 23-9 and 23-10. 

Many liquid phase or heterogeneous solid—liquid or gas—liquid reactions result in gaseous products or byproducts. These products may be toxic (refer to Table 4.1) or flammable (refer to Table 5.1), or result in overpressurization of any sealed container or vessel. Unless pressure relief is provided, relatively small volumes of reactants — the presence of which may not be expected — may generate sufficient gas pressure to rupture a container. The causes of pressure build-up may be ... 

Heterogeneous fluid-fluid reactions are made to take place for one of three reasons. First, the product of reaction may be a desired material. Such reactions are numerous and can be found in practically all areas of the chemical industry where organic syntheses are employed. An example of liquid-liquid reactions is the nitration of organics with a mixture of nitric and sulfuric acids to form materials such as nitroglycerin. The chlorination of liquid benzene and other hydrocarbons with gaseous chlorine is an example of gas-liquid reactions. In the inorganic field we have the manufacture of sodium amide, a solid, from gaseous ammonia and liquid sodium ... 

The monolithic stirrer reactor (MSR, Figure 2), in which monoliths are used as stirrer blades, is a new reactor type for heterogeneously catalyzed liquid and gas-liquid reactions (6). This reactor is thought to be especially useful in the production of fine chemicals and in biochemistry and biotechnology. In this work, we use cordierite monoliths as stirrer blades for enzyme-catalyzed reactions. Conventional enzyme carriers, including chitosan, polyethylenimine and different are used to functionalize the monoliths. Lipase was... 

In a heterogeneous gas-liquid reactor system, that is where gas absorption precedes a liquid-phase reaction, the mass transfer rate has to at least equal the reaction rate. This principle can be used to determine mass transfer coefficients and/or reaction rate constants for certain kinetic regimes (see Section B 3.2.1). To determine the mass transfer coefficient, the kinetic regime must be instantaneous, and the place of the reaction must be in the film (Charpentier, 1981 Beltran and Alvarez, 1996). To determine the reaction rate constant, the kinetic regime must be fast and kLa must be known. 

Similarly, the rate of heterogeneous fluid-fluid and gas-liquid reactions depends both on the rate of the chemical reaction and the rate of mass transfer. Since the relative magnitude of the effect of pressure on the two processes can vary greatly we have a large spectrum of possibilities. High pressure can more- or less steeply increase or decrease the overall rate of these reactions. 

Pereda, S., Bottini, S.B. and Brignole, E.A. (2005) Supercritical fluids and phase behavior in heterogeneous gas-liquid catalytic reactions. Appl. Catal. A Gen., 281, 129. 

The autoclave is usually a heterogeneous batch reactor mainly used for high-pressure kinetic studies. The autoclave is typically a solid catalyzed gas-liquid reaction system. 

Heterogeneous reactions involve two or more phases. Examples are gas-liquid reactions, solid catalyst-gas phase reactions and products, and reactions between two immiscible liquids. Catalytic reactions as illustrated in Chapter 1 involve a component or species that participates in various elementary reaction steps, but does not appear in the overall reaction. In heterogeneous systems, mass is transferred across the phase. 

Related to the experimental studies performed in our laboratory, in this review packed-bed membrane reactors were discussed. It should be mentioned that there are significant investigational activities devoted to study catalytically active membranes where the catalyst is deposited in either the membrane pores or on the inner or outer surface of the tubes [11]. Another similarly interesting and promising principle is based on using the Contactor type of membrane reactors, where the reactants are fed from different sides and react within the membrane [79]. Significant efforts have been made to exploit this principle for heterogeneously catalyzed gas-liquid reactions (three-phase membrane reactors) [80, 81]. 

The addition of various surfactants and micelle forming agents on the biphasic hydroformylation of olefins was also considered as a tool for enhancement of the reaction rate. The relation between the extent of emulsification of the reaction mixture and the performance of hydroformylation reaction was also investigated. Mass transfer effects in biphasic hydroformylation of 1-octene in the presence of cetyltri-methylammoniumbromide (CTAB), was studied by Lekhal etal. [33], A mass-transfer model based on the Higbie s penetration theory was proposed to predict the rate of hydroformylation in a heterogeneous gas-liquid-liquid system under... 

The conductivity solvent enters through the solvent inlet and flows through the reference conductivity cell that is formed by the top and outer electrode assemblies. The solvent then flows into the gas-liquid contactor where it is mixed with the gaseous reaction products entering through the gas inlet. The heterogeneous gas-liquid mixture formed in the gas-liquid contactor is separated into gas and liquid phases in the gas-liquid separator. The gas phase exits... 

Solid-eatalyzed reactions can occur in either the liquid or gas phase. Gas-phase reactions are not very common in the production of fine chemieals, beeause eom-plex molecules with limited volatility and thermal stability are usually involved, which makes operation at the high temperatures required for their vaporization impossible. Gas-liquid reactions with a solid catalyst probably encompass the largest number of applications in fine-chemical and pharmaceutical processes [1]. Two other classes of solid-eatalyzed reaction taking place in the liquid phase are liquid-solid reactions and liquid-liquid-solid reactions, but these are much less eommon. We shall, therefore, foeus on gas-liquid-solid reaetions, in which the solid is a heterogeneous catalyst. 

The rate of mass transfer between gas and liquid, determined by the product of the gas-liquid interfacial area, a, and the mass transfer coefficient, kj, is an important parameter many heterogeneously catalyzed gas-liquid reactions are limited by mass transfer of the gaseous reactant. The greater the product a k, the faster is mass transfer, and therefore, the observed rate of reaction for reactions in which mass transfer is the controlling step, i. e. for intrinsically fast reactions. The largest can be achieved in stirred-tank reactors and jet-loop reactors, so... 

However, the simplification should be investigated case by case. The basic concepts of diffusion and reaction will be used as a basis for the treatment for heterogeneous catalytic processes as well as catalytic gas-liquid reactions. 

A heterogeneous gas/liquid reactor illustrates a two-phase effluent (Fig. 7.13). The two phases are already present inside the reaction space. Vapour phase may undergo a new phase-split after condensation. The secondary vapour enters the vapour separation system. Gaseous reactants are recycled to the reactor, but purge may be necessary to eliminate gaseous products or avoid the accumulation of inert. The liquid streams from phase split and vapour recovery are sent to the liquid separation system, from which the liquid reactant is recycled. 

Taking into account the elements discussed before, we may formulate some guidelines for reactor selection. The strategy may be applied also for heterogeneous reactions described by pseudo-homogeneous models, as some gas-solid catalyst reactors, or gas-liquid reactions with reaction in the liquid phase. 

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