Author: admin
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Plotting Ternary LLE Data
Graphical representation of ternary LLE data is important for design of separation processes. For ternary systems, triangular coordinates simultaneously represent all three mole fractions, or alternatively, all three weight fractions. Triangular coordinates are shown in Fig. 14.6(a), with a few grid lines displayed. The fraction of component A is represented by lines parallel to the BC axis: Along , the composition…
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Binary Phase Diagrams
Liquid-liquid mutual solubilities in partially miscible systems change with temperature at a given pressure. Whether the solubilities increase or decrease can be due to a number of factors including hydrogen bonding. When one species H-bonds and the other does not, then as the temperature is raised and hydrogen bonds are broken, the fluids become more “similar,”…
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VLLE with Immiscible Components
A special case of VLLE is obtained when one of the liquid-phase components is almost entirely insoluble in other components, and all other components are essentially insoluble in it, as occurs with many hydrocarbons with water. When a mixture forms two liquid phases, the mole fractions sum to unity in each of the phases. When…
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LLE Using Activities
Usually we require higher precision than obtained by graphing the Gibbs energy. Furthermore, we may encounter multicomponent mixtures, for which the extension of the above method is not straightforward. We can develop an entirely general method for computing the phase partitioning given relative activities in Eqn 14.1. In Fig. 14.4 are plotted the activities for the water +…
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Binary LLE by Graphing the Gibbs Energy of Mixing
shows the contributions to the Gibbs energy of a mixture for A12 = 3 of Fig 14.1. The pure component Gibbs energies do not contribute to the curvature in the Gibbs energy of a mixture, and therefore are not needed for LLE calculations—we need just ΔGmix. In principle, all that is required to make predictions of LLE partitioning…
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Stability and Excess Gibbs Energy
Expressions for activity coefficients are the same for LLE as they are for VLE. The difference is that multiple liquid compositions can give the same activities or total pressure at a given temperature. This behavior is implied in Fig. 11.10, where we commented that the calculated lines indicate LLE. The time has come to analyze why…
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The Onset of Liquid-Liquid Instability
Our common experience tells us that oil and water do not mix completely, even though both are liquids. If we consider equilibria between the two liquid phases, we can label one phase α and the other β. For such a system we can quickly show that the equilibrium compositions are given by Equations for LLE.…
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Homework Problems
13.1. Show that Wilson’s equation reduces to Flory’s equation when Aij = Aji = 0. Further, show that it reduces to an ideal solution if the energy parameters are zero, and the molecules are the same size. 13.2. The actone(1) + chloroform(2) system has an azeotrope at x1 = 0.38, 248 mmHg, and 35.17°C. Fit the Wilson equation, and predict the P-x-y diagram. 13.3. Model the behavior of…
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Practice Problems
P13.1. The following lattice contains x’s, o’s, and void spaces. The coordination number of each cell is 8. Estimate the local composition (Xxo) and the parameter Ωox based on rows and columns away from the edges. (ANS. 0.68,1.47)
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Important Equations
The UNIFAC method receives broad application throughout thermodynamic modeling. In fact, occasional applications of UNIFAC may be too broad in the sense that experimental data specific to a particular binary system are ignored and UNIFAC predictions are not validated with actual measurements. A literature search should be conducted for experimental data pertaining to every molecular interaction in…