Qh

Settler ig

Raffinate

Figure 3. Sequential contact and separation.

352 Fermentation and Biochemical Engineering Handbook 2.0 DISTRIBUTION DATA

Although data for many systems are available in the literature,'11 in many cases it will be necessary for the engineer to obtain the distribution information for his own specific application.

The simplest method is to mix solvent and feed liquors containing varying quantities of solute in a separatory funnel, and analyze each phase for solute after settling. Where feed and solvent are essentially immiscible, the binary plot, such as shown in Fig. 4, is useful. For later ease of calculation, it is desirable to express concentrations on a solute-free basis. If there is extensive miscibility, a ternary plot (Fig. 5) would be preferable. Tie lines represent the equilibrium between the coexisting phases.

Figure 4. Binary plot of distribution data.

Plotting the data on log-log graphs may be helpful in understanding some of the underlying phenomena and interpolating or extrapolating meager data. An example is shown in Fig. 6 for the distribution of phenol between water and various chlorinated methanes. In the dilute region, the limiting slope is generally always unity. However, as the solute becomes more concentrated, there may be a tendency for solute molecules to associate with each other in one of the phases. Thus, the equilibrium data in Fig. 6 suggest that the phenol molecules form a dimer in the organic phase, probably by hydrogen bonding, leading to a slope of 2 in the distribution plot.

The possibility of complex formation in one of the phases illustrates the concern that many industrial extraction processes involve not only the physical transfer of molecules across an interface but, also, that there may be a sequence of chemical steps which have to occur before the physical transfer can take place, and which may be rate limiting.

Figure 6. Distribution of phenol between water and chlorinated methanes.

Whenever the distribution coefficient is greatly different than unity, there is an implication that there exists an affinity of the solute for that specific solvent, and this affinity may involve some loose chemical bonding.

Examples of computer programs for predicting and correlating equilibrium data are described by Lo, Baird, and Hanson.'21

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