Extraction mass transfer rates, with chemical reaction

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. 

For the applications involving multiphase reactions and separations, the mass transfer of a solute from one phase to the other or of a pure phase into another is necessary. The mass transfer rates are different in nonreactive and reactive chemical systems. In nonreactive (separation/extraction) case, the mass is transferred from the phase with higher chemical potential (partial pressure or concentration) to the lower until the equilibrium is reached. In reactive systems, the mass transfer is enhanced because of the consumption of transferring species from one phase to the other. 

If solvent B is a solution that contains a material that will react with solute C, the solubility in the extract phase will increase. If the reaction is rapid, the reactant in solvent B will diffuse toward the liquid film and react there with the solute C diffusing through the interface. In this case, the diffusion of the reactant in solvent B may control the mass transfer rate. However, systems exhibiting fast reactions in the extract phase usually exhibit the main resistance to mass transfer in the raffinate phase. If the reaction is slower, solute C will diffuse some distance into the main body of solvent B before the reaction is completed. In most cases, a chemical reaction in the extract phase increases the rate of mass transfer. 

Preceding chapters have dealt largely with pure substances or with constant-composition mixtures. e.g., air. However, composition changes are the desired outcome, not only of chemical reactions, but of a number of industrially important mass-transfer operations. Thus composition becomes a primary variable in the remaining chapters of tliis text. Processes such as distillation, absorption, and extraction bring phases of different composition into contact, and when tlie phases are not in equilibriimi, mass transfer between the phases alters their compositions. Botli tlie extent of change and tlie rate of transfer depend on the departure of the system from equilibrium. Thus, for quantitative treatment of mass transfer the equilibrium T, P, and phase compositions must be known. 

In the previous sections the use of catalysts dissolved in ionic liquids has been documented with a variety of examples from the most recent literature. They were classified are catalytic systems based on the adoption of Strategies A, B and C, when solvent-less conditions were not adopted. In an ideal liquid-liquid biphasic system, the IL must dissolve the catalytic intermediates and, in part, the substrate to avoid that mass transfer limits reaction rates. Moreover, products should have a limited solubility in the IL to allow a facile product removal or extraction, and, possibly, the recycle of the ionic liquid-trapped catalyst. The separation of the catalyst from the products is made easier if solid support-immobilised ILs are used. The preference for a solid catalyst is dictated not only by the easier separation but also, as outlined by Mehnert in an excellent review article, " by (i) the possible use of fixed bed reactors, and (ii) the use of a limited amount of IL, a generally expensive chemical which can limit the economic viability of the process. In this section attention will be focused only on the most recent examples of solid-phase assisted catalysis using ionic liquids, following Strategy D. Examples prior to 2006 are covered in recent reviews and will not be discussed here. " ... 

This chapter deals with the diffusional transfer of mass to and across a phase boundary. In particular, gas-liquid, gas-solid, and liquid-liquid phase combinations have been considered. Process applications include absorption, stripping, distillation, extraction, adsorption, and the diffusional aspects of chemical reactions on a solid surface. For steady-state transfer operations, the rates of mass transfer can be correlated by variations of Pick s first law, which states that the rate is directly proportional to the concentration driving force and the extent of interfacial area, and inversely proportional to the distance of movement of the mass to the interface. 

In mass-transfer-controlled systems in which extensive complexing or association takes place in the bulk phases, a proper mass transfer model must account for transport of all species. Otherwise, the transport model will not be consistent with a chemical model of phase equilibrium. For example. Fig. 8.4-4 indicates schematically the species concentration profiles established during the extraction of copper from ammonia-ammonium sulfate solution by a chelating agent such as LIX. In most such cases the reversible homogeneous reactions, like copper complexation by ammonia, will be fast and locally equilibrated. The method of Olandei can be applied in this case to compute individual species profiles and concentrations at the interfiice for use in an equilibrium or rate equation. This has been done in the rate analyses of several of the chloride and ammonia systems cited above. ... 

Comings and Briggs (20) studied the extraction of several solutes between benzene and water. For the extraction of benzoic acid, where the distribution favors the benzene, the major resistance to diffusion lay in the water phase. Addition of sodium hydroxide to the water reduced this resistance by causing a rapid chemical reaction, increased the mass-transfer coefficient, and made the effect of benzene rate on the over-all coefficient more pronounced, as would be expected. Similar experiences were obtained in the case of extraction of aniline, but in the case of acetic acid results were contrary to what was expected. The data apparently could be interpreted in terms of Eq. (10.11), with = 0.45 — 0.55,7 = 0 — 0.1, ri = 0.40 — 0.55, T = 0.45 — 0.55. Brinsmade and Bliss (13) extracted acetic acid from methyl isobutyl ketone (core) by water (w all). By making measurements at several temperatures, they were able to investigate the influence of Schmidt number on the rates and by graphical treatment of the data obtained values of the constants of Eq. (10.11) as follows j8 = 1, = 7 = 0,... 

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