Immiscible phases, mass-transfer operations

Consider any steady-state mass-transfer operation which involves the countercurrent contact of two immiscible phases as shown schematically in Figure 3.10. The two phases will be identified as phase V and phase L, and for the present consider only the case where a single substance A diffuses from phase V to phase L during their contact. 

The purpose of the equipment used for mass-transfer operations is to provide intimate contact of the immiscible phases in order to permit interphase diffusion of the constituents. The rate of mass transfer is directly dependent upon the interfacial area exposed between the phases, and the nature and degree of dispersion of one phase into the other are therefore of prime importance. 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

The treatment of the two-phase SECM problem applicable to immiscible liquid-liquid systems, requires a consideration of mass transfer in both liquid phases, unless conditions are selected so that the phase that does not contain the tip (denoted as phase 2 throughout this chapter) can be assumed to be maintained at a constant composition. Many SECM experiments on liquid-liquid interfaces have therefore employed much higher concentrations of the reactant of interest in phase 2 compared to the phase containing the tip (phase 1), so that depletion and diffusional effects in phase 2 can be eliminated [18,47,48]. This has the advantage that simpler theoretical treatments can be used, but places obvious limitations on the range of conditions under which reactions can be studied. In this section we review SECM theory appropriate to liquid-liquid interfaces at the full level where there are no restrictions on either the concentrations or diffusion coefficients of the reactants in the two phases. Specific attention is given to SECM feedback [49] and SECMIT [9], which represent the most widely used modes of operation. The extension of the models described to other techniques, such as DPSC, is relatively straightforward. 

When a material system, in which liquid phases predominate, is stirred, this action can result in miscible liquid phases forming a molecularly homogeneous mixture ( solution ). In the case of immiscible liquids, on the other hand, a dispersion (possibly an emulsion ) will result. If stirring is performed to increase heat or mass transfer, the purpose is to accelerate this operation and the inherent mixing of the liquid phases is of secondary importance. 

The addition of a second immiscible phase for the enzymatic degradation of poorly-soluble compounds provides several advantages, such as a simpler operation, mainly due to the easy recovery of the solvent depleted of substrate and its reuse in subsequent operations. Mass transfer could be considered a priori as a limitation for this system and to be the determinant of lower efficiencies. However, the selection of the appropriate solvent, as well as the determination of the adequate conditions which lead to the maximum efficiency allowed us to obtain unprecedented degradation rates in enzyme reactors. 

To illustrate how finite-difference equations arise, consider the countercurrent liquid-liquid extraction battery shown in Fig. 5.1. We first assume the phases are completely immiscible (e.g., water and kerosene). The heavy underflow phase has a continuous mass flow L (water, kg/sec), and the light solvent phase flows at a rate V (kerosene, kg/sec). Under steady-state operation, we wish to extract a solute Xq (e.g., acetic acid) from the heavy phase, and transfer it to the light phase (kerosene), using a nearly pure solvent with composition Since the solvent flows are constant, it is convenient in writing the solute balance to use mass ratios... 

In miniaturized channels of microstructured devices, a high surface area to volume ratio leads to enhanced mass transfer between two immiscible phases such as water and oil. Extraction is one of the unit operations for separation using mass transfer between two liquid phases and is often used for the separation of compounds in which the difference in boiling points is small. Several microstructured devices for efficient extraction have been developed. In this chapter, microstructured devices for extraction are introduced with divided into the following three categories according to fiuid operations ... 

Flooding correlations for packed liquid extractors have been developed in a manner similar to those used for gas/liquid systems. Just like the air/water system used to evaluate the maximum hydraulic capacity for a gas absorption operation, the capacities in liquid-liquid systems are based on hydraulic flow rates for immiscible solvents in the absence of any mass transfer. As has been stated, the transfer of a solute can change the properties of the extract and raffinate phases in a significant manner. For this reason, it may be that flooding has been experienced in commercial operations at flow rates well below those predicted by the flooding correlation. Clearly, more research is needed to explain the effect of mass transfer on the capacity of a liquid extractor. Nevertheless, the application of a widely used flooding correlation will be reviewed. However, the designer should consider the limitations of this correlation and apply appropriate safety factors in the specification of equipment. 

The scale of operation of these HT developments has been limited thus far to laboratory-scale experimentation. Much of the work has been performed in small batch reactors from 5 to 1000 mL. In most cases these reactors are agitated either with internal stirring or by a shaker in order to minimize mass transfer limitations considering the several phases involved (hydrogen gas, hydrocarbon, and water vapor solid heterogeneous catalyst aqueous liquid, hydrocarbon liquid, and polar biomass-derived liquids, which are likely immiscible). The HT process is envisioned as a continuous-flow operation, so the limited amount of laboratory-scale HT in continuous-flow systems provides the most useful information for consideration of scale-up and in-process modeling for economic analysis (Jones et al., 2014). 

If the equilibrium curve and the material balance equations are expressed in other than the mass ratio concentrations X and Y and mass of solvent and mass of inert Fs and Ry, the operating line will not be straight, even under conditions of complete immiscibility, since the ratio of total phases is not constant because of transfer of solute between phases. 

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