DECONTAMINATION OF LAND USING ELECTROCHEMICAL TREATMENT

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THERMODYNAMICS OF SEPARATION OPERATIONS

• Aseotropes

The increased repulsion between molecules can
result in the formation of an azeotrope, which is
a liquid mixture whose equilibrium vapor has the
same composition as the liquid ( i.e. xi yi for
an azeotrope).

1. Minimum-Boiling Homogeneous Azeotropes

This type of azeotropes occurs due to repulsion
between the molecules
Dr Saad Al-Shahrani
ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
xy diagram, ether P or T constant
Xy
45 o line
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
b) Maximum-Boiling azeotropes
This type of azeotropes occurs due to attraction
between the molecules.

• lt 1
• (-) deviation
• from ideality

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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
xy diagram
xy
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
Example Ethanol and n-hexane from a minimum
boiling point azeotrope at 3.2 mole ethanol at
58.68 oC and 760 mmHg pressure. The vapor
pressure of ethanol and n-hexane are 6 psia and
12 psia respectively, at 58.68 oC, determine ?iL
for ethanol and n-hexane at the azeotropic
condition
solution
At azeotrope xy
For methanol
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
For methanol
Note foe ethanol and n-hexane ?Lgt 1.0,
indicating repulsion (positive deviation from
ideality
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
DePriester Charts For Light Hydrocarbons

• Figures (a,b) give K-value charts for some Iight
  hydrocarbons. These arts do not assume ideal
  vapor-phase behavior. Some corrections for
  pressure effects are included.

• Figure (a) is used for low temperatures and
  Figure (b) high temperatures.

• To find the appropriate K-values, a straight line
  is drown on the diagram connecting the
  temperature and pressure of the system.
  intersection of this line with the K-value curve
  for each hydrocarbons its K-value at this
  temperature and pressure.

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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
RELATIVE VOLATILITY

• The relative volatility is the ratio of K-values

• For two component j and k

• If the system is ideal (i,e. obeys Raoults law,
  i.e. no attraction or repulsion between molecules
  or ?1.0)

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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
For component j
For component k
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
Relative volatility for binary system

• For two components system under equilibrium
  conditions (j,k)

Solve for yj

• This equation is very important in distillation
  operation

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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS

• Relative volatilities (are essentially constant.
  In general, they are functions of temperature and
  composition.
• ?jk f ( T and composition)
• In most systems, (?) decreases as temperature
  increases, which means that separation of
  components becomes more difficult.
• Therefore, It is often desirable to keep
  temperatures as low as possible (use low
  pressure) to reduce energy consumption.

• The following figure shows some VLE curves on an
  xy diagram for various values of ?.
• The bigger the relative volatility, the fatter
  the VLE curve and the easier the separation (low
  number of stages required).

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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS

• As ? ? 1.0, the VLE curve approaches the 45o line
  x y.
• It is impossible to separate components by
  distillation if the value of ? is too close to
  unity. Distillation is seldom used if ? lt 1.0 5.

X
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
Relative volatility For a multicomponents system.

• For a multi-component system, the relative
  volatilities are defined with respect to some
  component, typically the heaviest one.

• If we have multi-components system containing
  components (1,2,3, H), H is the heaviest one and
  (1) is the lightest one.

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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
By the same manner
. . . .
. . . .
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THERMODYNAMICS OF SEPARATION OPERATIONS
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THERMODYNAMICS OF SEPARATION OPERATIONS
Substitute (6) in (4)
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
Example A multi-component liquid mixture has
the compositions and relative volatilities given
in the table below. Calculate the composition of
the vapor phase.
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
The lever rule
F L V
F zi
zi F xi L yi V
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ChE 334 Separation Processes
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THERMODYNAMICS OF SEPARATION OPERATIONS
The ratio of the product flows (L,V) is the
inverse of the ratio of the lengths of the lines
connecting the feed mole fraction of each of the
products. This is known as Lever Rule
T2sat
y
x
T
Temperature
T1sat
zi
1.0
yi
xi
0
Note the two phases must be under equilibrium
conditions
Dr Saad Al-Shahrani
ChE 334 Separation Processes