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Azeotropic Mixtures SVNA 10.5. Large deviations from ideal liquid solution behaviour relative to the difference between the pure component vapour pressures result in azeotrope formation. In CHEE 311, we are interested in:
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Azeotropic Mixtures SVNA 10.5 • Large deviations from ideal liquid solution behaviour relative to the difference between the pure component vapour pressures result in azeotrope formation. • In CHEE 311, we are interested in: 1. Describing azeotropic mixtures both physically and in thermodynamic terms. 2. Detecting azeotropic conditions and calculating their composition. Lecture 24
Azeotropic Mixtures Water / Hydrazine, P=1atm Water / Pyridine, P=1atm Lecture 24
Azeotropes - Impact on Separation Processes • Separation processes that exploit • VLE behaviour (flash operations, • distillation) are influenced greatly • by azeotropic behaviour. • An azeotropic mixture boils • to evolve a vapour of the • same composition and, • conversely,condenses to • generate a liquid of the • same composition. • Ethanol(1)/Toluene(2) at P=1 atm Lecture 24
Predicting Whether an Azeotrope Exists • To determine whether an azeotrope will be encountered at a given pressure and temperature, we define the relative volatility. For a binary system, a12 is • 10.8 • where xi and yi are the mole fractions of component i in the liquid and vapour fractions, respectively. • At an azeotrope, the composition of the vapour and liquid are identical. Since, y1=x1 and y2=x2 at this condition, • To determine whether an azeotropic mixture exists, we need to determine whether at some composition, a12can equal 1. Lecture 24
Predicting Whether an Azeotrope Exists • We can derive an expression for a12 using modified Raoult’s Law as our phase equilibrium relationship, • which when substituted into the relative volatility, yields • 10.9 • a12 is therefore a function of T (Pisat, gi) and the composition of the liquid phase. Calculation of a12 therefore requires: • Antoine’s equation • an activity coefficient model (Margule’s, Wilsons, …) • a liquid composition • Our goal is to determine whether an azeotrope exists. • At some composition, cana12=1? Lecture 24
Predicting Whether an Azeotrope Exists • One means of determining whether a12=1 is possible is to evaluate the function (Eq’n10.9) over the entire composition range. • This is plotted for the ethanol(1)/toluene(2) system using Wilson’s equation to describe liquid phase non-ideality. • According to this plot, a12=1 at x1 = 0.82, meaning that an azeotrope exists at this composition. Lecture 24
Predicting Whether an Azeotrope Exists • Because equation 12.22 is continuous and monotonic, we do not need to evaluate a12 over the whole range of x1. • It is sufficient to calculate a12 at the endpoints, x1=0 and x1=1 • At x1 = 0, we have • and at x1 = 1, we have • If one of these limits has a value greater than one, and the other less than one, at some intermediate composition we know a12 =1. • This is a simple means of determining whether an azeotrope exists. Lecture 24
Determining the Composition of an Azeotrope • For an azeotropic mixture, the relative volatility equals one: • at an azeotrope. • To find the azeotropic composition, two methods are available: • trial and error (spreadsheet) • analytical solution • Rearranging 12.22 as above yields: • The azeotropic composition is that which satisfies this equation. • Substitute an activity coefficient model for g1, and g2. • Solve for x1. Lecture 24
Example • While azeotropes are undesirable from a processing point of view, we can still benefit from this phenomenon in obtaining parameters for GE models. • For example, the binary mixture of 1,4-dioxane (1)/water (2) exhibits an azeotrope at x1=0.554 at T= 50 oC and P=0.223 bar. How can we use this information in the estimation of the Margules parameters A12 and A21 for the mixture? • Given: P1sat(50 oC)=0.156 bar; P2sat(50 oC)=0.124 bar. ‘ • Set all fugacity coefficients equal to 1.00. Lecture 24