Vapor-Liquid Equilibria - Chemical Engineering Learning

CH2351 Chemical Engineering Thermodynamics II
Unit – I, II
www.msubbu.in
PhasewwwEquilibria
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Dr. M. Subramanian
Associate Professor
Department of Chemical Engineering
Sri Sivasubramaniya Nadar College of Engineering
Kalavakkam – 603 110, Kanchipuram (Dist)
Tamil Nadu, India
msubbu.in[AT]gmail.com
Jan-2012
Contents
Criteria for vapor-liquid equilibrium between phases in multi
component non-reacting systems in terms of chemical potential and
fugacity, estimation of fugacity of liquid, Raoult’s law, constant
temperature and constantwpressure
VLE (Pxy, Txy diagrams), effect
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of temperature and pressure .on
ms VLE, P-T diagram, deviations from
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ideality – positive and negative udeviations,
models for activity
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coefficient, azeotropes – minimum and maximum boiling azeotropes
Jan-2012 M Subramanian
Ideal Solution
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Deviations from Ideal Solution
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Minimum Boiling Azeotrope
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Maximum Boiling Azeotrope
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x-y Diagrams
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Raoult’s Law
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P = xAPAsat + xBPBsat
yA = xAPAsat/ P
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Effect of Temperature
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Effect of Pressure
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P-T Diagram
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Deviation from Ideal Solution
• The activity coefficients express the departure from ideality and
thus define the excess Gibbs energy of the solution.
• Deviation from ideality is said to be positive when γ > 1 (ln γ is
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positive) and negative when
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Positive Deviation from Raoult’s Law
P=
γΑxA pA sat+ γBxBpBsat
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γi
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greater than 1
ln γi positive
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Negative Deviation From Raoult’s Law
P=
γΑxA pA sat+ γBxBpBsat
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γi
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less than 1
ln γi negative
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Azeotropes
• If azeotropes are not present, a fractional distillation can eventually
separate the mixture into the pure components, with the component
with the higher vapor pressure ending up as the distillate and the less
volatile component (known as the residue) left in the distillation pot.
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• For a minimum boiling azeotrope,
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distillate with the azeotropic composition
and a residue that is one of
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in composition of the starting
the pure components, depending on the
mixture.
• For a maximum boiling azeotrope a fractional distillation can produce
one of the pure components as the distillate, and a residue with the
azeotropic composition.
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Positive homogeneous
azeotropes in mixtures of
ethanol(1) and benzene(2).
The Pxy diagram is at 370 K,
and the Txy diagram is at 2.5
bar. Filled circles locate the
azeotropes.
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Negative homogeneous
azeotropes (dots) in
mixtures of acetone(1)
and chloroform(2). The
Pxy diagram is at 50°C;
the Txy diagram is at 0.75
bar.
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Double Azeotrope
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VLE Calculations of Non-ideal Solutions
• Obtain γi from azeotropic composition data
• Evaluate model parameters of γi-xi models based on the above
data
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• Make VLE calculation with yiP = γbixu.iPisat
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The equilibrium flash of a multicomponent liquid is also an
isenthalpic process and may be
visualized as a simple distillation
process using a single equilibrium
stage.
VLE Problems
Problem
Name
Bubble P
Knowns
T, xi
Unknowns
to find
P, yi
Dew P
T, yi www.
P, xi
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Bubble T
P, xi
Dew T
P, yi
T, xi
Flash
T, P, zi
xi, yi, nv/n
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T, yi
Henry’s Law & Raoult’s Law
• As early as in 1803 William Henry showed empirically that the vapor
pressure of a solute i is proportional to the concentration of solute i:
where xi is the mole fraction solute and kH,i is known as the Henry’s
law constant.
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bb demonstrated that at low
• More than 80 years later François Raoult
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concentrations of a solute, the vapor pressure
of the solvent is simply
where xi is the mole fraction solvent and pi* is the vapor pressure of the
pure solvent.
• Raoult’s and Henry’s laws are often termed ‘limiting laws’. This use
reflects that real solutions often follow these laws at infinite dilution
only.
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Comparison of Raoult’s law and Henry’s law
Schematic illustrations of the saturated
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non-ideal
solutions.
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The continuous solid curves represent
the behavior of the real solutions, the
dashed lines represent Raoult’s law and
short solid lines represent Henry’s law.
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