role of dispersed phase in gas-liquid

ROLE OF DISPERSED PHASE IN GAS-LIQUID REACTIONS: A REVIEW* Raminder Kaur and M. Ramakrishna* Chemical Engineering Division Indian Institute of Chemical Technology, Hyderabad-500007, India. K.D.P. Nigam Department of Chemical Engineering, Indian Institute of Technology, Delhi HauzKhas, New Delhi-110016, India

ABSTRACT Gas-liquid-liquid reactions have an important place in the engineering and chemical fields. A review on gas-liquid-liquid reactions is presented, considering the effect of addition of an immiscible (mainly organic) liquid on gas-liquid system. The effect of the presence of immiscible liquid on kt» and a has been reviewed. The dispersed phase has a significant role in mass transfer enhancement. The volumetric mass transfer coefficient, kLa, though initially decreases, it increases with higher loading of the dispersed phase. This behavior has been explained taking into account the nature of dispersed phase and the spreading coefficient, S. The spreading coefficient, bubble and droplet size and turbulence induced by the presence of dispersed phase are •the important parameters in the enhancement of the mass transfer. The enhancement in the mass transfer can be explained by four types of mechanisms, namely: (1) Shuttle effect or grazing mechanism; (2) Bubble covering mechanism; (3) The permeability effect and (4) The hydrodynamic effect. Further, for describing the phenomenon of gas absorption in the presence of dispersed phase, two different types of models, namely homogeneous and heterogeneous models, were reported in the literature. + IICT Communication No.: 070204

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Role of Dispersed Phase in Gas-Liquid Reactions: A Review

Keywords: Gas-Liquid-Liquid Reaction, Enhancement Heterogeneous models, Homogeneous models, Mass Transfer.

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1. INTRODUCTION Reactions involving three-phase systems are frequently encountered in the chemical reaction engineering practice. In general, the dispersed phase (solid or liquid) in a gas -liquid system may either be: a reagent, substrate or a heterogeneous catalyst (pumont and Delmas, 2003). Although gas - liquid - liquid reactions have a significant place in engineering and chemical fields, unfortunately they were abandoned by researchers for a long time. Amongst the literature available on three phase reactions, the bulk is devoted to gas liquid - solid reactions (Beenackers and Van Swaaij, 1993). Gas - liquid liquid systems were able to draw attention in the research field only during the past few years. Gas - liquid - liquid systems have gained interest in -the past decade, due to the introduction of homogeneous biphasic catalysis (Herrmann and Kohlpaintner, 1993; Chaudhari et β/., 1995 and Cornils, 1999) in various reaction systems, e.g. hydroformylation, carbonylation, hydrogenation and oligomerization complexes. For homogeneous catalyzed reactions, the catalyst is usually totally soluble in the liquid phase and the reactions can be classified into the categories of: gas - liquid, liquid - liquid and gas - liquid - liquid reactions. Homogeneous catalysts generally do not suffer from internal mass transfer limitations (as encountered with catalysts fixed on solid carrier materials), but usually they are more expensive. The main advantage of heterogeneous systems over catalysis in one phase is the easy separation of the catalyst and the reactants or products. These classifications can characterize most pharmaceutical and fine chemical processes. Examples of important gas - liquid - liquid reaction systems are: hydroformylation (Kuntz,1987), biochemical processes (Junker et β/., 1990 and Rols et α/., 1990), fine chemicals manufacturing (Mills and Chaudhari, 1997), hydrogenation, alkylation (like the production of ethylbenzene by alkylation of benzene by ethylene), hydroxycarbonylation (Falbe, 1980), hydrometallurgy (Levy et α/., 1981; Gaunand, 1986) and polymerisation reactions, as found in the SHOP process (Freitas and Gum, 1979) and in gas liquid emulsion polymerization (Scott et α/., 1994). Work carried out by different researchers on gas - liquid - liquid reaction system is reported in Table: 1.

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Table 1 Summary of Work Carried Out on Gas - Liquid - Liquid Reactions

Process Biochemical Processes Carboxylation Fine Chemicals Manufacturing Homogeneous Catalysis Hydroformylation

Hydrometallurgy Polymerization reaction Pivalic Acid Synthesis

Reference Junker et al., 1990 Roiset al., 1990 Fable, 1980. Mills and Chaudhari ,1997. Sharma,1998 Kuntz,1987. Chaudhari et al.,1995. Hablotefa/.,1992 Purwantoefa/, 1995. Deshpande et al., 19%. Wachsen ere/, 1998. Lekhal et a/,1999. Mathivetefa/.,2002 Yangefa/,,2002. Zhang et al, 2002 Aghmizera/.,2003 Levy era/., 1981 Gaunand,1986 Freitas and Gum, 1979 Scott etal.,1994 Brilmanefa/.,2001, Cents«/ al., 2001.

Gas - liquid - liquid systems are encountered in reaction systems that inherently consist of three phases due to two (or more) immiscible reactants, reaction products or catalyst For example, in the Koch reaction system, all three reactants originate from different phases (Falbe, 1980). For reactions with two immiscible liquid phases, it is necessary to evaluate the significance of liquid - liquid mass transfer limitations in addition to gas - liquid mass transfer. However, not much work has been reported so far on liquid - liquid mass transfer in gas - liquid - liquid systems. For phase transfer catalytic reactions (Mills and Chaudhari, 1997), such as the carbonylation of benzyl

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chloride to phenyl acetic acid, the catalytic cycle involves reactions in both phases with or without liquid - liquid mass transfer limitations. The overall rate of reaction in gas - liquid - liquid reactions as encountered in phase transfer catalysis would depend on the following parameters: (1) Intrinsic kinetics of the reaction steps, (2) Solubility of gases in two liquid phases, (3) Gas - liquid and liquid - liquid mass transfer coefficients, (4) Drop size of the dispersed phase, and (S) Liquid - liquid equilibrium properties for the reactants and products. Another very important class of the gas - liquid - liquid systems, on which most of the studies have been carried out, are those in which an • additional inert liquid phase is supplemented purposely to a gas - liquid system to increase the mass transfer rate. This latter concept is applied in a few biochemical applications (Rols et α/., 1990). In gas - liquid mass transfer, a very interesting situation may occur when the second liquid is finely emulsified in the continuous liquid phase and the gas solubility in this dispersed liquid phase is higher than that in the continuous phase (solubility ratio mR>l). In such cases, the presence of dispersed liquid droplets can significantly enhance the transfer rate of the reactant gas into the continuous liquid phase (up to a factor 10), due to the increased overall gas solubility in the mixture. However, the addition of a second liquid phase can also retard the gas - liquid mass transfer (Yoshida et α/., 1970). Cent et al., 2001 used the Danckwert's plot in gas -liquid - liquid systems for the analysis of mass transfer parameters, and concluded that two types of systems exist - systems that enhance mass transfer and systems that do not enhance mass transfer. When a third phase (i.e. a dispersed liquid phase) is added to a gas - liquid mixture, the system becomes more heterogeneous and complex. Consequently, one should be careful when applying theories derived for twophase systems, to the gas - liquid - liquid three-phase systems. Reactant species may undergo a chemical reaction. In general, three types of reaction systems have been considered experimentally (Brilman, 1998): (1) Physical absorption experiments, (2) Systems with a "1 ""-order reaction for the diffusing component in the continuous phase, and (3) Systems with a "1M"ordcr reaction in the dispersed phase. No experimental study concerning the influence of an additional second liquid phase on the selectivity for multi reaction systems has been found in the literature.

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2. APPLICATIONS OF GAS - LIQUID - LIQUID REACTIONS: Table 2 Applications of Gas-Liquid -Liquid Reactions. S. No.

1 2 3 4 5 6 7 8 9 10 11

Applications Alkylation. Biochemical Processes. Bioprocess Industry. Carboxylation. Chemical Manufacturing. Fine chemicals manufacturing Homogeneous Catalysis Systems Hydroformylation. Hydrometallurgy. Polymerization. Waste Water Treatment Industry.

Gas - liquid - liquid reaction system is of great scientific and commercial importance and may be encountered in several important fields of application (Table 2), as: 1. Fermentation processes where high oxygen demand is reached by the addition of hydrocarbon in aqueous medium (bubble column reactor and stirred tank reactor). 2. Biological treatment of poorly water-soluble waste gases (bubble column reactor). 3. Koch synthesis for tertiary carboxylic acids from three reactants in three phases (organic liquid phase - olefins, liquid phase - water and gas carbon monoxide) (Brilman et al.,1999 and 2001, Cents et al., 2001) 4. Hydroformylation of propylene to butyraldehyde with homogenous biphasic catalysis (bubble column reactor and stirred tank reactor) (Wachsen et α/.,1998) 5. Hydroformylation of 1-octene using water soluble rhodium complex catalyst (bubble column reactor and stirred tank reactor) (Kuntz,1987; Hablot et al.,1992; Purwanto and Delmas, 1995 and Lakhai et α/,1999) Hydroformylation of 1-octene using perfluorinated solvents. (Aghmiz et al., 2004)

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6. Hydroformylation of 1-dodecene using water-soluble rhodium complex catalyst (Yang et. al, 2002). Hydroformylation of 1-dodecene using phosphate complexes as legends (Mathivet et al., 2002). 7. Hydrometallurgy: oxidation of Cu (I) by oxygen in concentrated NaCl solutions (Levy et al., 1981; Gaunand, 1986) (stirred tank reactor) 8. Fine chemicals manufacturing: (a) small scale production of various vitamin precursors (b) production of phenylacetic acid from benzylchloride and (c) hydrogenation of fine chemicals, e.g., partial hydrogenation of unsaturated aldehydes to unsaturated alchols or saturated aldehydes (stirred tank reactor). Production of ethyl benzene by alkylation of benzene by ethylene (stirred tank reactor). (Mills and Chaudhari, 1997; Sharma,1998) 9. Polymerization reactions: oligomerisation of ethylene to a-olefins (SHOP process (Freitas and Gum, 1979)), Gas - liquid emulsion polymerization: conversion of ethylene to vinyl acetate (Scott et al., 1994) (stirred tank reactor). 10. Biochemical processes (Junker et al., 1990 and Rols et al., 1990).

3. OBSERVATIONS IN GAS - LIQUID - LIQUID SYSTEMS: PARAMETERS AFFECTED DUE TO THE PRESENCE OF THE DISPERSED PHASE In three phase reactors, either gas - liquid - solid or gas - liquid - liquid, frequently the absorption rate of a (sparingly) soluble gas phase reactant is the rate determining step (Beenackers. and van Swaaij, 1993). It has been reported frequently that the addition of an immiscible organic liquid phase to a gas - liquid (aqueous) phase system may significantly increase the gas absorption rate, and consequently increases the mass transfer from gas phase to liquid phase. The addition of the dispersed liquid phase changes the rate of transfer of the solute gas across the boundary layer. Physical properties such as gas solubility, gas diffusivity, density and viscosity of the liquid mixture are changed. The gas - liquid characteristics: droplet distribution inside the boundary layer, possible pathways for mass transfer, mass transfer coefficient and gas - liquid interfacial area, can change due to the interfacial properties of the dispersed liquid. There have been three distinct approaches reported in the literature to explain the change in mass transfer in a gas - liquid - liquid

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. system (Dumont and Delmas, 2003): i) Direct gas and dispersed phase contact forming a 'gas-organic complex' (Brilman, 1998) suggested that the formation of complexes of gas - organic drops is dependent not only on the spreading coefficient, but also on the bubble and droplet size. The probability of formation of these complexes has been expressed from the viewpoint of surface energy change, AES defined as

ii) The shuttle effect of droplets carrying gas solute from the gas - liquid interface to the liquid bulk. iii) The dynamic interaction of the dispersed phase droplets with the concentration boundary layer, causing increased turbulence or mixing in it (Turbulence is associated with the continuous phase, which is assumed to be the dominant phase, and the dispersed phase is present in small quantities). Hence, the dispersed phase can only respond to or modify the continuous phase turbulence). Nishikawa et al. (1994) have shown that for gas - liquid - liquid systems, enhancement of gas absorption can be expected when the solubility of the diffusing component in the dispersed liquid phase exceeds the solubility in the continuous liquid phase. The concentration of gas in the liquid phase increases after the addition of the dispersed organic phase (Figure 1). The presence of an organic phase can alter both the mass-transfer coefficient and the interfacial area (Chaudhari, 1997). In chemical engineering, the rate of mass transfer between two different phases often directly determines the production rate of the process (like the gas absorption rate in gas - liquid systems). This mass transfer rate is directly proportional to both the mass transfer coefficient and the specific interfacial area between the different phases, respectively (Cent et al., 200S). No general trends can be derived from the results reported in the literature. Both parameters (mass transfer rate and specific interfacial area) depend mainly on the (local) hydrodynamic situation inside the system. The studies carried out by different researchers on the effect of dispersed phase on kLa , a and *L are listed in Table 3. To describe the mass-transfer rate theoretically, both effects have to be taken into account.

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3.1. Effect of dispersed phase on k|a The gas liquid volumetric mass transfer co-efficient, *La is generally used to characterize the gas absorption rate in multiphase reactors. Variations in kLa with the addition of a third immiscible phase to gas-liquid system has been studied, taking spreading coefficient, S into consideration. The spreading coefficient, S, for gas-oil-water interaction is: s ff

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