CEIC3001 – Advanced Thermodynamics and Separation

Vapour –Liquid Equilibrium Vapour pressure: the pressure exerted by a vapour at equilibrium with a solid or liquid phase

The Phase RuleF = 2 – N

where F = number of independent intensive variables (or degrees of freedom) = number of phases in equilibrium N = number of components

PTxy Phase Diagrams The under surface contains the saturated vapour states: PTy data The upper surface contains the saturated liquid states : PTx data Superheated- vapour lies below the PTy ; subcooled-liquid lies above the PTx Interior space between the two surfaces represents the coexistence of both the saturated

phases 2-phase mixture

Txy diagram P constants BUBBLEPOINT (saturated liquid) curve - LOWER curve represents Tx data DEWPOINT (saturated vapour )curve - UPPER curve represents Ty data Equilibrium between vapour and liquid phases is represented by horizontal tie lines A pure liquid boils at a single T for a given P, as indicated by the end-points of the Txy diagram

Pxy diagram BUBBLEPOINT curve – UPPER curve Px DEWPOINT curve – LOWER curve Py

PT diagram PT diagram represents data for saturated vapour and liquid at constant composition

Azeotropes

azeotrope is a point where the bubblepoint and dewpoint curves intersect in a Txy or Pxy diagram

vapour and liquid = same composition ; liquid does not change composition when evaporated mixtures cannot be separated by distillation

Raoult’s Law – Ideal gas

pi = yiP = x1Pisat

pi = partial pressure of component i yi = mole fraction of component i in the vapour phase P = total pressure xi = mole fraction of component i in the liquid phase Pisat= vapour pressure of component i at the system T

for an ideal gas the total pressure equals the sum of the partial pressures

Dewpoint and bubblepoint calculation with raoult’s lawBUBL P: Calculate {yi} and P, given {xi} and T DEW P: Calculate {xi} and P, given {yi} and T BUBL T: Calculate {yi} and T, given {xi} and P DEW T: Calculate {xi} and T, given {yi} and P

Henry’s Law Estimating the solubility soluble gases in liquids

Pi = yip = xiHi

VLE from k-value correlations

Ki = yi/ xi

K –value is a measure of the volatility of a component