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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
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