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Liquid Phase Properties from VLE Data SVNA 12.1

Liquid Phase Properties from VLE Data SVNA 12.1. Purpose of this lecture : To illustrate how activity coefficients can be calculated from experimental VLE data obtained at low pressures Highlights

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Liquid Phase Properties from VLE Data SVNA 12.1

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  1. Liquid Phase Properties from VLE Data SVNA 12.1 • Purpose of this lecture: • To illustrate how activity coefficients can be calculated from experimental VLE data obtained at low pressures • Highlights • For our calculations we take advantage of the fact that as P->0 the vapour phase molecular interactions in a mixture at VLE become very weak, hence the vapour behaves as an ideal gas. In thermodynamic terms this can be written as • The modified form of Raoult’s law can then be used for the estimation of the activity coefficients from experimental low P VLE • Reading assignment: Section 12.1 (pp. 430-432) Lecture 15

  2. 7. Liquid Phase Properties from VLE Data SVNA 12.1 • The mixture fugacity of a component in non-ideal liquid solution is defined by: • (11.46) • We also define the activity coefficient: • (11.91) • which is a measure of the departure of the component behaviour from an ideal solution. • Using the activity coefficient, equation 11.46 becomes: • How do we calculate/measure these properties? Lecture 15

  3. Liquid Phase Properties from VLE Data • Suppose we conduct VLE experiments on our system of interest. • At a given temperature, we vary the system pressure by changing the cell volume. • Wait until equilibrium is established (usually hours) • Measure the compositions of the liquid and vapour Lecture 15

  4. Liquid Solution Fugacity from VLE Data • Our understanding of molecular dynamics does not permit us to predict non-ideal solution fugacities, fil . We must measure them by experiment, often by studies of vapour-liquid equilibria. • Suppose we need liquid solution fugacity data for a binary mixture of A+B at P,T. At equilibrium, • The vapour mixture fugacity for component i is given by, • (11.52) • If we conduct VLE experiments at low pressure, but at the required temperature, we can use • by assuming that iv = 1. Lecture 15

  5. Table 12.1 Liquid Solution Fugacity from Low P VLE Data • Since our experimental measurements are taken at equilibrium, • What we need is VLE data at various pressures (all relatively low) Lecture 15

  6. Activity Coefficients from Low P VLE Data • With a knowledge of the liquid solution fugacity, we can derive activity coefficients. Actual fugacity • Ideal solution fugacity • Our low pressure vapour fugacity simplifies fil to give: • and if P is close to Pisat: • leaving us with Lecture 15

  7. Table 12.2 Activity Coefficients from Low P VLE Data • Our low pressure VLE data can now be processed to yield experimental activity coefficient data: Lecture 15

  8. Activity Coefficients from Low P VLE Data Lecture 15

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