Dr Saad Al-Shahrani

1. BINARY VAPOR-LIQUID EQUILIBRIUM • Nonideal Liquid Solutions • If a molecule contains a hydrogen atom attached to a donor atom (O, N, F, and in certain cases C), the active hydrogen atom can form a bond with another molecule containing a donor atom. two water molecules coming close together • Table 2.7 shows qualitative estimates of deviations from Raoult’s law for binary pairs when used in conjunction with Table 2.8. • Positive deviations correspond to values of iL > 1. Nonideality results in a variety of variations of (iL)with composition, as shown in Figure 2.15 (Seader & Henely) for several binary systems, where the Roman numerals refer to classification groups in Tables 2.7 and 2.8. ChE 334: Separation Processes Dr Saad Al-Shahrani

2. BINARY VAPOR-LIQUID EQUILIBRIUM ChE 334: Separation Processes Dr Saad Al-Shahrani

3. BINARY VAPOR-LIQUID EQUILIBRIUM ChE 334: Separation Processes Dr Saad Al-Shahrani

4. BINARY VAPOR-LIQUID EQUILIBRIUM • Figure 2.15a: Normal heptane (V) breaks ethanol (II) hydrogen bonds, causing strong positive deviations. n-heptane(v)-Ethanol (II) system (Semi-log paper) Note: Ethanol molecules form H-bonds between each other and n-heptane breaks these bond causing strong (+) deviation. ChE 334: Separation Processes Dr Saad Al-Shahrani

5. BINARY VAPOR-LIQUID EQUILIBRIUM • In Figure 2.15b, • Similar Figure 2.15a but less positive deviations occur when acetone (III) is added to formamide (I). iL>1 • In Figure 2.15c, • Hydrogen bonds are broken and formed with chloroform (IV) and methanol (II) resulting in an unusual positive deviation curve for chloroform that passes through a maximum. ChE 334: Separation Processes Dr Saad Al-Shahrani

6. BINARY VAPOR-LIQUID EQUILIBRIUM In Figure 2.15d, Chloroform (IV) provides active hydrogen atoms that can form hydrogen bonds with oxygen atoms of acetone (III), thus causing negative deviations • Non-ideal solution effects can be incorporate into K-value formation into different ways. Non-ideal liquid solution at near ambient pressure 1. Non-ideal liquid solution at moderate pressure and TC. 2. ChE 334: Separation Processes Dr Saad Al-Shahrani

7.   BINARY VAPOR-LIQUID EQUILIBRIUM • Repulsion • Molecules that are dissimilar enough from each other will exert repulsive forces Component(1) x1 + e. g: polar H2O molecules – organic hydrocarbon molecules. i> 1 Component(2) x2 When dissimilar molecules are mixed together due to the repulsion effects, a greater partial pressure is exerted, resulting in positive deviation from ideality. + ChE 334: Separation Processes Dr Saad Al-Shahrani

8. BINARY VAPOR-LIQUID EQUILIBRIUM • Fore the last two figures, as the mole fraction x1 increases its 1 →1, as its mole fraction x1 decreases 1 increases till it reaches to 1 (activity coefficient at infinite dilution) ChE 334: Separation Processes Dr Saad Al-Shahrani

9. BINARY VAPOR-LIQUID EQUILIBRIUM • Attraction When dissimilar molecules are mixed together, due to the attraction effects, a lower partial pressure is exerted, resulting in negative deviation from ideality. i< 1 are called negative deviation from ideality. Component(1) x1 Component(2) x2 - - ChE 334: Separation Processes Dr Saad Al-Shahrani

10. Vapor phase ym = 0.665 yw = 0.33 Liquid phase xm = 0.3 xw = 0.7 BINARY VAPOR-LIQUID EQUILIBRIUM • Example: calculateij of methanol – water system for the following data 760 mmHg Vapor phase ym = 0.665 yw = 0.33 Liquid phase xm = 0.3 xw = 0.7 Vapor Pressure Data at 78 oC (172.1°F) Methanol: Pmsat= 1.64 atm Water: Pwsat= 0.43 atm ChE 334: Separation Processes Dr Saad Al-Shahrani

11. BINARY VAPOR-LIQUID EQUILIBRIUM solution For methanol For water ChE 334: Separation Processes Dr Saad Al-Shahrani

12. BINARY VAPOR-LIQUID EQUILIBRIUM • How to calculate iL of Binary Pairs Many empirical and semi-theoritical equations exists for estimating activity coefficients of binary mixtures containing polar and/ or non-polar species. These equations contain binary interaction parameters, which are back calculated from experimental data. Table (2.9) show the different equations used to calculate iL. ChE 334: Separation Processes Dr Saad Al-Shahrani

13. BINARY VAPOR-LIQUID EQUILIBRIUM ChE 334: Separation Processes Dr Saad Al-Shahrani

14. THERMODYNAMICS OF SEPARATION OPERATIONS Table (2.10) shows the equations used to calculate excess volume, excess enthalpy and excess energy. ChE 334: Separation Processes Dr Saad Al-Shahrani

15. THERMODYNAMICS OF SEPARATION OPERATIONS Example. (problem 2.23 ( Benzene can be used to break the ethanol/water azeotrope so as to produce nearly pure ethanol. The Wilson constants for the ethanol(1)/benzene(2) system at 45°C are A12 = 0.124 and A21 = 0.523. Use these constants with the Wilson equation to predict the liquid-phase activity coefficients for this system over the entire range of composition and compare them, in a plot like Figure 2.16, with the following experimental results [Austral. J. Chem., 7, 264 (1954)]: ChE 334: Separation Processes Dr Saad Al-Shahrani

16. THERMODYNAMICS OF SEPARATION OPERATIONS Let: 1 = ethanol and 2 = benzene The Wilson constants are A12 = 0.124 and A21 = 0.523 From Eqs. (4), Table 2.9, Using a spreadsheet and noting that  = exp(ln ), the following values are obtained, ChE 334: Separation Processes Dr Saad Al-Shahrani

17. THERMODYNAMICS OF SEPARATION OPERATIONS ChE 334: Separation Processes Dr Saad Al-Shahrani

18. THERMODYNAMICS OF SEPARATION OPERATIONS ChE 334: Separation Processes Dr Saad Al-Shahrani

19. THERMODYNAMICS OF SEPARATION OPERATIONS • Activity coefficient at infinite dilution Modern experimental techniques are available for accurately and rapidly determining activity coefficient at infinite dilution (iL ) Appling equaion(3) in table (2.9) (van Laar (two-constant)) to conditions: Xi = 0 and then xj = 0 ChE 334: Separation Processes Dr Saad Al-Shahrani

20.   THERMODYNAMICS OF SEPARATION OPERATIONS Component(1) x1 Component(2) x2 + + Repulsive > 1.0 ChE 334: Separation Processes Dr Saad Al-Shahrani