Separation Processes (CHEG 485)

Dr. Mohammad Abu Zahra

(Associate Professor, Chemical Engineering)

Lecture 2: Evaporation and Flash Distillation

1
 Introduction

• Evaporative separation is based on the

difference in composition between a liquid
mixture and the vapour form from it.
• The composition difference comes from the
different vapour pressure or volatilities of the
component in the liquid mixture.
• The basis of evaporative separation is the
knowledge of the vapour-liquid equilibrium
(VLE) data.

2
 Distillation

• Most common separation method

• Based on difference in volatility
• Distillate enriched in the more
volatile compound
• Vapor phase creation by partial
evaporation of the feed through
adding heat in the reboiler
• Heat can be recovered (albeit at
lower temperature) in the
condenser

3
 Evaporation

• Single-stage distillation
• Selectivity determined by vapor-liquid equilibrium
• Batch-wise and continuous operation are both possible

4
 Vapor-liquid equilibrium
• At thermodynamic equilibrium the
compositions are called “vapour-liquid
equilibria (VLE)”.
• The more volatile (low boiling) component is
used to plot the liquid vapour composition in
the mixture.
• Liquid bubble point at which the liquid on
heating forms the first bubble of vapour.
• The vapour dew point line at which the
vapour on cooling forms the first drop of
condensed liquid.
• The region between both lines is the two-
phase region where vapour and liquid coexist
in equilibrium.
• Liquid composition : xeq
• Vapour composition: yeq 5
 Distribution Coefficient and Relative Volatility

• The distribution coefficient (𝐾𝑖 ): is the ratio between the concentration of

component i in the liquid mixture to its concentration in the vapor phase.

𝑦𝑖 𝑦𝑖 : the mole fraction of component i in the vapour phase

𝐾𝑖 ≡
𝑥𝑖 𝑥𝑖 : the mole fraction of component i in the liquid phase

The relative volatility (∝𝑖𝑗 ): the

𝑦𝑖
𝐾𝑖 𝑥𝑖
selectivity of a component relative ∝𝑖𝑗 = = 𝑦
to a reference component. 𝐾𝑗 𝑗
𝑥𝑗

6
 Separation factor

• Mole fraction in the vapor phase: y

• Mole fraction in the liquid phase: x
• Binary mixture:
y1 + y2 =1 and x1 + x2 = 1
• Index can be omitted:
x1 = x, x2 = 1-x, y1 = y and y2 = 1-y
where x and y are the fractions of the most volatile
compound
• Separation factor: relative volatility

SF 
K1

 y x
 12
K 2 1  y  1  x  7
 Correlations

• Dalton’s law: relates the concentration of a component in an ideal gas or

vapor mixture to its partial pressure

𝑝𝑖 : the partial pressure of component i in the vapor mixture

𝑝𝑖 = 𝑦𝑖 𝑃 𝑁
𝑃: the total pressure of the system (𝑃 = 𝑖=1 𝑝𝑖 )

• Raoult’s law: relates the partial pressure of a component in the vapour

phase to its concentration in the liquid phase

𝑃𝑖𝑠𝑎𝑡 : the saturation pressure/vapor pressure of pure

𝑝𝑖 = 𝑥𝑖 𝑃𝑖𝑠𝑎𝑡 component i at the system temperature.

𝑥𝑖 𝑃𝑖𝑠𝑎𝑡 = 𝑦𝑖 𝑃
8
 Relative volatility

• Relative volatility: 12 

K1

 y x
K 2 1  y  1  x 
• Question: Does the relative volatility depend on the
temperature and/or on the composition?
• Answer: This depends on the type of mixture
• Ideal mixtures: Mixtures of chemically similar molecules (similar
in size, structure and polarity) at low/moderate pressures (< 10 bar)
• The vapor phase is an ideal gas
• The liquid phase is an ideal solution (enthalpy of mixing is zero and volume
change of mixing is zero)
• Non-ideal mixtures: Mixtures of chemically dissimilar molecules
• The vapor phase is still an ideal gas (< 10 bar)
• The liquid phase is a real solution (enthalpy of mixing is non-zero and volume
change of mixing is non-zero) 9
 Ideal mixtures

• Relative volatility: 12 

K1

 y x
K 2 1  y  1  x 

}
• Dalton’s law: pi  yi P
yi P  xi Pi sat

• Raoult’s law: pi  xi Pi sat

N N
• Total pressure: P   pi   xi Pi sat
i 1 i 1
yi Pi sat
• Distribution coefficient: Ki  
xi P
sat
K1 P
  1
sat = only function of temperature
K2 P 2 (composition-independent)
10
 Saturated vapor pressure (Pisat)
Bi
• Antoine equation: ln Pi sat
 Ai 
Ci  T
(Values of A, B and C depend on the units used for P and T)
Psat (mm Hg)

T (oC) 11
 Example 1

• The saturated vapor pressure of benzene and toluene can be

described by the following Antoine equation:
log Pi mmHg   Ai 
10 sat Bi
 
Ci  T o C
• For benzene: A=7.4787, B=1701, C=294.0
• For toluene: A=7.4195, B=1738, C=273.5

• What are the vapor pressures of benzene and toluene at 80 oC

and 110 oC?
• What is the total pressure at 80 oC and 110 oC for a mixture
containing 5 mole% benzene?
• What is the effect of temperature (80 oC or 110 oC) on the
relative volatility? 12
 Example 1
at 80 oC at 110 oC

Vapor pressure benzene


 7.4787 
1701 
 850.9 mm Hg 1852.2 mm Hg
sat
Pbenz mmHg   10

  
294.0 T o C 


Vapor pressure toluene

 1738  318.5 mm Hg 772.2 mm Hg
 7.4195  
Ptolsat mmHg   10

 273.5   
T o
C 


Total pressure at x = 0.05

345.2 mm Hg 826.2 mm Hg
P   xi Pi sat  0.05 Pbenz
sat
 0.95 Ptolsat

Relative volatility
2.67 2.40
P sat
benz /Psat
tol

Relative volatility decreases with increasing temperature! 13

Txy- and xy-diagrams

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 Equilibrium line

• Relative volatility:
P1sat K1
12  sat  
 y x
P2 K 2 1  y  1  x 
• Equilibrium line:

12 x
y
12  1  x  1
= relation between the composition of a liquid
and a vapor at equilibrium

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 xy-diagram of binary system

1-butanol/pentane (α=11.7)
2-butanol/1-butanol (α=3.7)
pentane/hexane (α=2.4)

when α=1, the equilibrium line reduces to y = x, indicating that

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vapor-liquid separation is not feasible!
 Reading out xy-diagrams

• For a binary liquid mixture with α = 4, a liquid composition of

x = 0.32 is in equilibrium with a vapor composition of y = 0.65
(= mole fractions of the most volatile compound)
• The vapor is enriched in the more volatile compound! 17
 Non-ideal mixtures

• Relative volatility: 12 

K1

 y x
K 2 1  y  1  x 
• Dalton’s law:
• Modified
Raoult’s law:
pi  yi P
pi   i xi Pi
N
sat }
N
yi P   i xi Pi sat

activity
• Total pressure: P   pi    i xi Pi sat coefficient
i 1 i 1
yi  i Pi sat
• Distribution coefficient: K i  
xi P
K1  P sat
  1 1
K2  P 2 2
sat = both function of temperature 18
(via Pisat) and composition (via γi)
 Azeotrope

• When the activity coefficient (𝛾𝑖) of a specific component

becomes high enough an azeotrope may be encountered,
meaning that the vapor and liquid compositions are equal
and the components cannot be separated by conventional
distillation. Azeotrope takes place when relative
volatility (⍺) equals 1.

• Minimum boiling azeotrope: boils at lower temperature, the

overhead product is the azeotrope.
• Maximum boiling azeotrope: boils higher than either
components in their pure states, the bottom product of the
distillation.

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 Txy-, Pxy- and xy-diagrams

• Close to ideal activity coefficients (γ ~ 1): no azeotrope

(azeotrope = mixture of which the composition cannot be altered by
simple distillation, because liquid and vapor have identical composition;
x = y and α12 = 1)

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 Txy-, Pxy- and xy-diagrams

• Positive deviations from ideality (γ > 1): minimum azeotrope

• Strong repulsive forces (dissimilar systems)

• Overhead product is azeotrope

21
 Txy-, Pxy- and xy-diagrams

• Negative deviations from ideality (γ < 1): maximum azeotrope

• Strong attraction between the molecules

• Bottom product is azeotrope

22
 Batch-wise vs continuous evaporation

• Single-stage distillation (= evaporation) can be performed as:

• Batch-wise operation

• Continuous operation = flash distillation

23
 Continuous flash distillation

• Flashing = partial evaporation of a liquid feed

• Vapor creation by:
• Heat addition (isothermal flash)
• Pressure release (adiabatic flash)

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Balances over flash drum

Overall mole balance

Mole balance of most

volatile compound:

25
 Equilibrium in flash drum

• Separation factor of flash of binary mixture (where

component 1 is the most volatile compound):
K1 y1 / x1  1 P1sat
12   
K 2 y2 / x2  2 P2sat
• with x1 = x, x2 = 1–x, y1 = y and y2 = 1–y:
y/x
12 
(1  y ) /(1  x)
• Flash equilibrium line:
12 x
y
12  1 x  1
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 Flash calculations

• Overall balance: F = V + L
• Mole balance of most volatile compound: Fz = Vy +Lx
• So far, 6 variables, 2 equations
• y/x ratio is physically determined (K=y/x) and
dependent on temperature
• If F and z are given, one additional equation is needed:
• Two options:
• Choose T (determines K and thus α) and L/V
• Choose T (determines K and thus α) and P

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 Fixing L/V

• Total balance:

• Component balance:

• Result: Operating line

28
 Fixing L/V

Liquid fraction:

Flash operating line:

12 x
Recall, Equilibrium line: y
12  1 x  1
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