Absorption in packed towers: Rich gases case

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Absorption in packed towers Rich gases case
Rich gas case V?const, L?const
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Absorption in packed towers Rich gases case
Rich gas case V?const, L?const
Consider an absorption column Consider a mass
transfer process in a section of the column dZ
(cross-section of the column is S)
Va,ya
La,xa
Z
dZ
Vb,yb
Lb,xb
S
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Absorption in packed towers Rich gases case
Va,ya
La,xa
Z
Example McCabe, Smith, Harriott Chapter
18 Seader Hanley Chapter 6
Vb,yb
Lb,xb
S
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Absorption in packed towers Rich gases case
Prescriptive design process 1) Establish
equilibrium relation ymx
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Absorption in packed towers Rich gases case
Prescriptive design process 1) Establish
equilibrium relation ymx 2) Operating line. In
case of rich gases both V and L change and it is
more convenient to work in terms of pure carrier
gas V and pure solvent stream L
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Absorption in packed towers Rich gases case
Prescriptive design process 1) Establish
equilibrium relation ymx 2) Operating line. In
case of rich gases both V and L change and it is
more convenient to work in terms of pure carrier
gas V and pure solvent stream L plot
y vs x
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Absorption in packed towers Rich gases case
3) For several values of y on the operating line
find
(mole fraction of A in V)
y
equilibrium line
yb
y
y
xa
xb
xb
x
(mole fraction of A in L)
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Absorption in packed towers Rich gases case
3) For several values of y on the operating line
find
(mole fraction of A in V)
y
equilibrium line
yb
y
y
xa
xb
xb
x
(mole fraction of A in L)
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Absorption in packed towers Rich gases case
3) Continued 4) Find the term
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Absorption in packed towers Rich gases case
5) Integrate
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Absorption in packed towers
Seader, Hanley
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Design considerations Pressure drop and flooding
dry
Loading point
Flooding point
G mass flow per unit area (GV-gas, GL-liquid)
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Design considerations Pressure drop and flooding

• Some flooding description
• A visual build-up of liquid on the upper
• surface of the packed bed
• A rapid increase in liquid hold-up with
• increasing gas rate
• Formation of a continuous liquid phase above
• the packing support plate
• A considerable entrainment of liquid in
• the outlet vapour
• Filling of the voids in the packed bed with
  liquid

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Design considerations Pressure drop and flooding
GL
GV
(McCabe, Smith, Harriott)
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Design considerations Diameter of packed towers
Pressure drop analysis Eckert graph
Flooding line
Pressure drop in inH2O/ft of packing (brackets
mm H20/ m of packing)
L/VGL/GV
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Design considerations Diameter of packed towers
McCabe, Smith Harriott
Sinnott
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Design considerations Diameter of packed towers
Moderate to high pressure distillation 0.4
to 0.75 in water / ft packing 32 to 63 mm
water / m packing Vacuum Distillation 0.1
to 0.2 in water / ft packing 8 to 16 mm water /
m packing Absorbers and Strippers 0.2 to
0.6 in water / ft packing 16 to 48 mm water / m
packing
Given L, V (mass flow rates) Select
pressure drop
GV
D
select packing
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Design considerations Diameter of packed towers
at flooding velocity
Given L, V (mass flow rates)
GVGV(flooding)/2
D
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Design considerations Diameter of packed towers
Example of using pressure drop correlations to
estimate geometrical parameters of the packed
column
Specify the packing type and column dimensions
for a column that will be used to remove
chlorine from a gas stream using an organic
solvent.  The vapor flow is 7000 kg/h, the
average vapor density is 4.2 kg/m3.  The liquid
flow is 5000 kg/h, the average liquid density is
833 kg/m3.  The liquid's kinematic viscosity is
0.48 centistokes (4.8 x 10-7 m2/s)
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Design considerations Diameter of packed towers
First, we evaluate the x-axis of the graph
above(L/V)(vapor density/liquid density)0.5
(5000/7000)(4.2/833)0.5 0.0507
Moderate to high pressure distillation 0.4
to 0.75 in water / ft packing 32 to 63 mm
water / m packing Vacuum Distillation 0.1
to 0.2 in water / ft packing 8 to 16 mm water /
m packing Absorbers and Strippers 0.2 to
0.6 in water / ft packing 16 to 48 mm water / m
packing
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Design considerations Diameter of packed towers
Fp packing factor, accounts for structure of
packing and decreases with increasing void
fraction
Fp24 for 2 inch Pall rings
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Correlations for HTU
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Correlations for HTU
Cornell
Sinnott
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Correlations for HTU
Cornell
Sinnott
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Correlations for HTU
Sinnott
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Correlations for HTU
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Correlations for HTU
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Design of packed towers
1) Calculate NTU
distillation/ absorption from dilute gas
absorption from rich gas
2) Select packing defines size, packing factor
Fp, specific surface area, void fraction etc
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Design of packed towers
3) Pressure drop, diameter of the column
D
4) HTU from Cornell, or Onda methods 5)
At this stage, height, diameter of the column and
the type of packing is known