thermo ii lecture 3 vaporliquideuilibria

8/9/2019 Thermo II Lecture 3 VaporLiquidEuilibria

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Phase Equilibria

Dr. M. Subramanian

Associate Professor

Department of Chemical Engineering

Sri Sivasubramaniya Nadar College of Engineering

alava!!am " #$% &&$' anchipuram (Dist)

*amil Nadu' +ndia

msubbu.in,A*-gmail.com

Jan-2012

CH2351 Chemical Engineering Thermodynamics II

Unit – I, II

www.msubbu.in

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Contents

Criteria for vaporliquid equilibrium bet/een phases in multi

component nonreacting systems in terms of chemical potential andfugacity' estimation of fugacity of liquid' 0aoult1s la/' constant

temperature and constant pressure 23E (P4y' *4y diagrams)' effect

of temperature and pressure on 23E' P* diagram' deviations from

ideality " positive and negative deviations' models for activitycoefficient' a5eotropes " minimum and ma4imum boiling a5eotropes

Jan-2012 M Subramanian



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+deal Solution



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Deviations from +deal Solution



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Minimum 6oiling A5eotrope



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Ma4imum 6oiling A5eotrope



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x y Diagrams



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P = x A P A

sat + xBP

B

sat

y A = x A P A

sat/ P

0aoult1s 3a/



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Effect of *emperature



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Effect of Pressure



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P* Diagram



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Deviation from +deal Solution

7 *he activity coefficients e4press the departure from ideality andthus define the e4cess 8ibbs energy of the solution.

7 Deviation from ideality is said to be positive /hen γ > 1 (ln γ ispositive) and negative /hen γ < &(ln γ is negative).w w w .m s u

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Positive Deviation from 0aoult1s 3a/

P = γ Αx A p A sat+ γ BxBpBsat

γ i greater than 1

ln γ i positive

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Negative Deviation 9rom 0aoult1s 3a/

P = γ Αx A p A sat+ γ BxBpBsat

γ i less than 1

ln γ i negative

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A5eotropes

7 +f a5eotropes are not present' a fractional distillation can eventuallyseparate the mi4ture into the pure components' /ith the component

/ith the higher vapor pressure ending up as the distillate and the lessvolatile component (!no/n as the residue) left in the distillation pot.

7 9or a minimum boiling a5eotrope' a fractional distillation can produce adistillate /ith the a5eotropic composition and a residue that is one of

the pure components' depending on the composition of the startingmi4ture.

7 9or a ma4imum boiling a5eotrope a fractional distillation can produce

one of the pure components as the distillate' and a residue /ith thea5eotropic composition.



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Positive homogeneousazeotropes in mixtures ofethanol(1) and benzene(2).The Pxy diagram is at 370 K,and the Txy diagram is at 2.5

bar. Filled circles locate theazeotropes.



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Negative homogeneousazeotropes (dots) in

mixtures of acetone(1)and chloroform(2). ThePxy diagram is at 50°C;the Txy diagram is at 0.75bar.



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Double A5eotrope



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23E Calculations of Nonideal Solutions

7 :btain γ i from a5eotropic composition data

7 Evaluate model parameters of γ i x i models based on the abovedata

7 Ma!e 23E calculation /ith y iP ; γ i x iPisat



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The equilibrium flash of a multi-component liquid is also anisenthalpic process and may bevisualized as a simple distillationprocess using a single equilibriumstage.



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Problem

Name

Knowns Unknowns

to find

6ubble P *' 4i P' yi

De/ P *' yi P' 4i

6ubble * P' 4i *' yi

De/ * P' yi *' 4i

9lash *' P' 5i 4i' yi' nv


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=enry1s 3a/ > 0aoult1s 3a/

7 As early as in &?$% @illiam =enry sho/ed empirically that the vapor

pressure of a solute i is proportional to the concentration of solute i

/here x i is the mole fraction solute and k ='i is !no/n as the Henry’slaw constant.

7 More than ?$ years later 9ranBois 0aoult demonstrated that at lo/concentrations of a solute' the vapor pressure of the solvent is simply

/here x i is the mole fraction solvent and pi is the vapor pressure of the

pure solvent.

7 0aoult1s and =enry1s la/s are often termed limiting la/s1. *his usereflects that real solutions often follo/ these la/s at infinite dilutiononly.



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Comparison of Raoult’s law and Henry’s law



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Schematic illustrations of the saturatedvapor pressures above two differentnon-ideal solutions.

The continuous solid curves representthe behavior of the real solutions, thedashed lines represent Raoult’s law andshort solid lines represent Henry’s law.



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thermo ii lecture 3 vaporliquideuilibria

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