Minimum Boiling Point Azeotrope

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Minimum Boiling Point Azeotrope
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Maximum Boiling Point Azeotrope
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Single Equilibrium Stage
Ethanol-Water, P 1 atm
Vapor
V, yEtOH
F, zEtOH
L, xEtOH
Liquid
F is the total moles of ethanol and water fed to
the stage. V is the total moles in the vapor
stream exiting the stage. L is the total moles in
the liquid stream exiting the stage. zEtOH is the
mole fraction of ethanol in the feed. yEtOH is
the mole fraction of ethanol in the vapor
stream. xEtOH is the mole fraction of ethanol in
the liquid stream.
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Feed Mole-Fraction Relationships

• Note that a feed mole-fraction, zF,can be a
  subcooled liquid, a saturated liquid, a two-phase
  mixture, a saturated vapor, or a superheated
  vapor.
• The feed phase is dependent upon the temperature,
  pressure, and the composition (mole fraction).

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zF and x,y Relationships

• Assuming that the equilibrium stage is at the
  same temperature and pressure of the feed
• If zF is a subcooled liquid, then zF is simply xF
  and there is no y.
• If zF is a superheated vapor, zF is yF and there
  is no x.
• If zF is a saturated liquid, zF is essentially xF
  with a single vapor bubble formed of new mole
  fraction y.
• If zF is a saturated vapor, zF is essentially yF
  with a single liquid drop formed of new mole
  fraction x.
• If zF is in the two-phase region, the system will
  separate into a liquid and vapor of new mole
  fractions x and y, respectively. zF is not equal
  to either x or y, but x and y be determined from
  the T vs. x,y data or plot.

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An Initial Way to Investigate this
Equilibrium Behavior

• Another way to look at this behavior is to first
  look at a closed system at a particular pressure
  and initial temperature, and heat or cool the
  system at constant pressure and z mole fraction.
• We can do this using the T vs. x,y plot

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Mass Balance Lever Rule

• Assume a two-phase mixture of known mole fraction
  is allowed to separate at constant temperature
  and pressure.
• After a suitable period of time, the system will
  reach equilibrium.
• The two-phase mixture at point A separates into
  the liquid at point B and vapor at point C along
  the isotherm, Tsys, as illustrated by

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Mass Balance Lever Rule
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Mass Balance Lever Rule
Vapor
V, yC
F, zA
L, xB
Liquid
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Mass Balance Lever Rule
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Mass Balance Lever Rule
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Mass Balance Lever Rule
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Mass Balance Lever Rule
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Temperature-Composition Diagram
for Ethanol-Water, P 1 atm
100
95
Two Phase
Superheated Vapor Phase
90
C)
o
T(
85
80
Subcooled Liquid Phase
75
zEtOH
0.0
0.2
0.4
0.6
0.8
1.0
x
or y
EtOH
EtOH
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Temperature-Composition Diagram
for Ethanol-Water, P 1 atm
100
V
95
Two Phase
Superheated Vapor Phase
V
90
L
V
C)
o
V
T(
L
85
L
80
L
Subcooled Liquid Phase
75
zEtOH
0.0
0.2
0.4
0.6
0.8
1.0
x
or y
EtOH
EtOH
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Mass Balance Further Relationships
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Note for a Two-Phase Mixture

• At vapor-liquid equilibrium, the temperatures of
  the vapor and liquid are equal.
• When a two-phase mixture separates at
  vapor-liquid equilibrium conditions, the vapor
  phase will be at saturated vapor conditions and
  the liquid phase will be at saturated liquid
  conditions.
• We will use this assumption when we do our
  multi-stage solutions the vapor and liquid
  streams exiting a stage will be assumed to be at
  saturated conditions.
• Thus, the liquid fed from one stage to another
  stage can be assumed to be a saturated liquid and
  the vapor feed to another stage can be assumed to
  be a saturated vapor.

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Enthalpy vs. Composition Ponchon-Savarit
Plot

• Presents the temperature equilibrium relationship
  for enthalpy vs. x and y.
• Pressure is constant (note the units).
• One normally plots the more volatile component.
• Enthalpy will be required in future problems
  utilizing energy balances.
• Note the units of concentration!

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Enthalpy vs. Composition Ponchon-Savarit Plot
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Tips For Reading Charts

• Use a clear ruler.
• Photocopy and enlarge the diagrams.
• Draw lines on the diagram to find intersection
  points.
• Measure the scale in mm and convert to chart
  units.
• E.g., What is the enthalpy of a (two phase) feed
  stream at 1 kg/cm2 pressure, 82C, 0.6 wt
  ethanol?
• Measure 21 mm/100 kcal/kg. Intersection at 8.5 mm
  above 200 kcal/kg. H 200 kcal/kg(8.5mm/21mm)10
  0 kcal/kg240.5 kcal/kg