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Multicomponent Distillation, Mass transport theories , Principles of adsorption , Principles of humidification , Principles of drying are main topics covered in this Unit Operations course. This lecture covers following points: Absorption Operations, Gas Absorption, Stripping, Equilibrium Relations, Solubility, Henry's Law, Design Considerations, Mass Transfer, Mccabe-Thiele Graphical Construction, Gas-Liquid Ratio
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Gas
absorption
is a unit operation in which one or more
soluble components of a gas mixture are dissolved in aliquid Stripping
is an inverse operations performed when it is
desired to transfer a volatile component from a liquid into a gas
Examples:
solubility
A
A
A^
Henry’s law is valid for dilute solutions,where A does not ionize, dissociate orreact in the liquid phase
For water H [atm/mole fraction]
Va, y
a
L^ a, x
a Vb, y
b
L^ b, x
b Plate 1 Plate 2 Plate n Plate N
L1 x
V2 y L2 x
V3 y
Plate 3
L(^ n-1)
x(^ n-1)Vn yn L^ n^ xn V(
n+1)^
y(^ n+1)
y
x
(mole fraction of A in L)
(mole fraction of A in V)
xa^
xb
yb ya
equilibrium line
x*^ b
(^1) n
a a a a n n^1 n
V
x L y V x L V 1 yn
^
A
A^
x f y^
P x T H y^
A
A^
/ ) (
*^
Equilibrium line
y
x
(mole fraction of A in L)
(mole fraction of A in V)
xa^
xb
yb ya
equilibrium line
x*^ b
(^1) ^
n e^
y The driving force for mass transfer y
xn
y
x
(mole fraction of A in L)
(mole fraction of A in V)
xa^
xb
yb ya
equilibrium line
x*^ b
McCabe-Thiele graphicalconstruction
(^1) n
a a a a n n^1 n
V
x L y V x L V 1 yn
^
In general this is not a straight line because mass isconstantly transferred from phase V to phase L (soL^ <La^
and Vb^
<Va
and Lb
/Vn^ n+
is not constant)
If we decrease L, y
does not change but overallb^
concentration of A in L increases. We can continue thisprocess until the operating line crosses the equilibriumline. At this point (x*
, y^ b^ b
) the driving force of mass transfer
y-y* is equal to zero. This means we need infinite numberof stages to reach this separation.
y
x
(mole fraction of A in L)
(mole fraction of A in V)
xa^
xb
yb ya
equilibrium line
x*^ b
docsity.com
: L, V constant -> L/V constant
This is possible for very dilute (<5% mole fraction) mixturesso change in total number of moles of each flow isinsignificantA) Limiting (L/V)
value:min^
y
x
(mole fraction of A in L)
(mole fraction of A in V)
xa^
xb
yb ya
equilibrium line
x*^ b
B) Number of ideal stages: the actual L/V ratio iscalculated as a multiple of the limiting value (f* (L/V)
)min^
this gives a steeper slope of the operating line.The number of ideal stages can be then constructed usingMcCabe-Thiele method.
a b
a b
a a b b^
x x
y y
L V
Lx V Vy x L V y^
min
*^
docsity.com
: L, V constant -> L/V constant, y
=mxe^
e
This is possible for very dilute (<5% mole fraction) mixturesso change in total number of moles of each flow isinsignificant and the region of interest is in the Henry’s law regimeA) Limiting (L/V)
value:min^
y
x
(mole fraction of A in L)
(mole fraction of A in V) xa
xb
yb ya
equilibrium line
x*^ b
B) Number of ideal stages: can be calculatedanalytically
a b
a b a b
a b
a a b b
x m y
y y x x
y y
L V
Lx V Vy Lx V y
^
/
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Design of absorbers: simplified cases Let’s consider the following:
)
...
( )
...
(^1) (
)
...
( )
...
(^1) (
2
2
2
2
1
N a N a b N n
a n
a
n
A A A y A A A y y y A A A y A A A y y
Inside the brackets we have geometric series:
r r a s
n
n^
^
1
) (^1) ( 1
A A
A y
A A
y y
N
a
N
a b^
1
)
(^1) (
1 1
1
Therefore:
This is
Kremser
equation
Design of absorbers: simplified cases Let’s consider the following:
)
(^
a b
b a
a a b a a N b
y y A
y y
y
Ay
Ay y
Ay
Ay y
b a a b a a N N
a
N
a
b
N
a
N
a b
y y y y A y y A A A y A y A y A A A y
A A
y y
)
( )
(
)
(
1
) (^1) (
1
)
(^1) (
1 1
1
1
*^1 1
From operating line:
VN
,yN
= V,yb
b
L^ ,xN
=N L^ ,xb
b
