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  and  is,

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

with In general Mathematical Relationships

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

Bubble Point P 2  P 1  T x 1,y 1 z i =x i

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 P 2  P 1  T x 1,y 1 z i =y i

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