Lecture # 26 Chapter 12 Gas-Liquid Equilibrium Non-ideal VLE PETE 310 Lecture # 26 Chapter 12 Gas-Liquid Equilibrium Non-ideal VLE

Gas-Liquid Equilibrium Non Ideal Behavior Applications to low & high pressures Use models for the equilibrium ratios (ki-values) Simpler models ki = f (P,T) More complex models ki = f(P,T,compositions) use equations of state EOS, convergence pressure

VLE general (same) The equilibrium criteria between 2 phases a and b is,

Statement of Equilibrium Most general case is that Ki is a function of P, T, and compositions 1 P 2 3 T

Mathematical Relationships with In general

Bubble Point Evaluation (Non-Ideal Behavior Model) The bubble point pressure at a given T is

Bubble Point T s P 1 zi=xi s P 2 x ,y 1 1

Dew Point Calculation At the dew point the overall fluid composition coincides with the gas composition. That is.

Dew Point Calculation (Non-Ideal Behavior Model) Find DP pressure and equilibrium liquid compositions

Dew Point T s P 1 s P zi=yi 2 x ,y 1 1

Flash Calculations In this type of calculations the objective is to: find fraction of vapor vaporized (fv) and equilibrium gas and liquid compositions given the overall mixture composition, P and T.

Flash Calculations (Non-Ideal Behavior Calculations) Start with the equilibrium equation Material balance

Flash Calculations Now replace either liquid or gas compositions using Ki values (exercise derive the expression)

Flash Calculations Objective function (flash function) is

Flash Calculations There are several equivalent expressions for the flash function (a) (b) (c)

One of the Models for Equilibrium Ratios 1. Require critical properties & acentric factors

Example calculations in excel file VLE_310