Azeotropic Mixtures SVNA 10.5

1. 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

2. Azeotropic Mixtures Water / Hydrazine, P=1atm Water / Pyridine, P=1atm Lecture 24

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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