unsw ceic3001 lecture w3 advance thermo

Unsw ceic3001 - SCHOOL OF CHEMICAL ENGINEERING CEIC3001 Advanced Thermodynamics and Separation NON-IDEALITY IN THE LIQUID PHASE CONTENTS VLE by

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1 SCHOOL OF CHEMICAL ENGINEERING CEIC3001 Advanced Thermodynamics and Separation NON-IDEALITY IN THE LIQUID PHASE CONTENTS VLE by Modified Raoult's Law . ...................................................................................... 2 Partial Molar Properties. ................................................................................................... 3 The Ideal Solution . ........................................................................................................... 5 The Activity Coefficient: Species i in a Solution of Liquids . .......................................... 6 Models for the Excess Gibbs Energy . .............................................................................. 7 Margules Equations. ..................................................................................................... 7 Non-ideal gas and non-ideal solution: the Gamma/Phi Formulation of VLE. ............... 10 HIGHLIGHTS 1. The Modified Raoult’s Law describes an equilibrium system where the vapour behaves as an ideal gas and the liquid mixture is a non-ideal solution . 2. The deviation of each species i from non-ideal solution behaviour is described by an activity coefficient i : sat i i i i P x P y 3. The deviation of a a liquid mixture from ideal solution behaviour is measured by the excess Gibbs energy G E 4. Several models can be used to correlate and predict activity coefficients, of which the Margules equation is one of the simplest, with only two parameters for a binary system. 5. When the vapour is not an ideal gas and the liquid mixture is not an ideal solution, the Gamma-Phi formulation may be used to describe VLE: sat i i i i i P x P y 6. i and i depend on T, P and composition in a complex manner and are usually calculated using computer simulation software.
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2 VLE by Modified Raoult's Law a modified form of Raoult's law is expressed as sat i i i i i P x P y p (1) where i = activity coefficient of component i in the liquid phase the activity coefficient is a function of T and liquid-phase composition and accounts for nonideal solution behaviour modified Raoult's law provides a satisfactory description of VLE data for a wide range of non-electrolyte mixtures such mixtures typically include solutions of organic solvents, e.g. MEK/toluene, and solutions of water and water-miscible organic solvents, e.g. H 2 O/MeOH if values of the activity coefficient are known, Eq. 1 can be used to generate Pxy and Txy diagrams via bubblepoint and dewpoint calculations it is still assumed that the vapour phase behaves as an ideal gas N i i i i P x P 1 sat (2) Eq. 2 can also be solved for x i and summed over all species as follows   N i N i i i i i P P y x 11 sat 1 (3) rearrangement of Eq. 3 leads to an alternative expression for P N i i i i P y P 1 sat / 1 (4) Eqs 1, 2 and 4 are used for bubblepoint and dewpoint calculations but the iteration procedures are more complex than those for the standard form of Raoult's law (see Example 10.3 in the 6th ed. text) given a set of experimental values of Pxy data (or Txy data), Eq. 1 also provides a means of determining experimental values of i sat i i i i P x P y (5) the more general problem though is that we need to develop equations which allow us to calculate i the excess Gibbs energy can be used for this purpose
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3 Partial Molar Properties in the previous lecture the fugacity of a pure gas was introduced using the
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Unsw ceic3001 - SCHOOL OF CHEMICAL ENGINEERING CEIC3001 Advanced Thermodynamics and Separation NON-IDEALITY IN THE LIQUID PHASE CONTENTS VLE by

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