# 13-Stripping: process design ```Stripping process design
Prof. Dr. Marco Mazzotti - Institut f&uuml;r Verfahrenstechnik
1. The Column
Given Data
G
L
y1
x0
Specification
Unknown
Simplifications
T, p
n
T,P constant value
Raoults law
x *m  y
m=distribution coefficient
Material balance:
y n1 
L
L 

x n   y1  x 0 
G
G 

G
yn+1
L
xn
2. Diagram
The known data is plotted in a x,y Diagram
The equilibrium line is plotted
The operating line is plotted for n 
The slope of the operating line =
L
 
 G max
Equilibrium line
L
 
 G max
Operating line
yn+1
x*n+1
xn
x0
3. Dimensioning of the Column
3.1 Calculating the necessary gas amount
In the practice, the slope for the operating line is taken as 1 to 2 times the minimum slope:
L
G  a *   *L
 G max
Good values of a are between 1.2 and 2
3.2. Number of stages by graphical Construction
The known data is ploted in a x,y Diagram
The Equilibrium line is plotted
The Operating line is plotted
The number of stages n are constructed using the proceedure seen in the countercurrent stage configuration
Equilibrium line
n-2
n-1
n
yn+1
xn
x0
Goal (here 3 Stages)
3.3. Number of stages by Kremser Equation
Fractional stripping:
σ
x x
amount stripped
 0 *n
max amount strippable x 0  x n1
x n* 1 
y n1
m
Stripping Factor:
S
mG
L
Kremser Equation (as shown in Counter current cascade)
Sn1  S
σ  n1
S 1
Solving this equation for n gives the number of stages
4. Final Procedure
x n:
As it is not possible to build decimal stages the number resulting from the Kremser equation
has to be round up to the next integer.
From this the exact stripping fraction  and the concentration x0 can be calculated
Sn1  S x 0  x n
σ  n1

S  1 x 0  x n* 1
y1:
The gas concentration y1 can be calculated by using the total mass balance
y n1 
L
L 

x n   y1  x 0 
G
G 

Goal:
Liquid stream purification using a gas to a certain
specification
xn
Unknown:
Given data:
Gas flow rate
G
Liquid flow rate
L
Number of stages
n
Liquid composition,
x0
Temperature
T
Composition of gas stream
yn+1
Pressure
P
Gas composition at outlet
y1
```