Page 209 - Chemical engineering design

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CHEMICAL ENGINEERING
4. Absorption or stripping columns
The amount of a component absorbed or stripped in a column is dependent on the column
design (the number of stages), the component solubility, and the gas and liquid rates.
The fraction absorbed can be estimated using the absorption factor method, attributed to
Kremser (1930) (see Volume 2, Chapter 12). If the concentration of solute in the solvent
feed to the column is zero, or can be neglected, then for the solute component the fraction
absorbed D
L m /mG m sC1 L m /mG m
L m /mG m sC1 1

and for a stripping column, the fraction stripped D
mG m /L m sC1 mG m /L m
mG m /L m sC1 1
1
where G m D gas flow rate, kmol m 2 h ,
1
L m D liquid flow rate, kmol m 2 h ,
m D slope of the equilibrium curve,
s D the number of stages.
For a packed column the chart by Colburn (1939) can be used (see Volume 2, Chapter 11).
This gives the ratio of the inlet and outlet concentrations, y 1 /y 2 , in terms of the number
of transfer units and mG m /L m .
The same general approach can be used for solvent extraction processes.

5. Distillation columns
A distillation column divides the feed stream components between the top and bottom
streams, and any side streams. The product compositions are often known; they may be
specified, or fixed by process constraints, such as product specifications, effluent limits
or an azeotropic composition. For a particular stream, “s”, the split-fraction coefficient is
given by:
x sk r s
x fk

where x sk D the concentration of the component k in the stream, s,
x fk D the concentration component k in the feed stream,
r s D the fraction of the total feed that goes to the stream, s.
If the feed composition is fixed, or can be estimated, the value of r s can be calculated
from a mass balance.
The split-fraction coefficients are not very dependent on the feed composition, providing
the reflux flow-rate is adjusted so that the ratio of reflux to feed flow is held constant;
Vela (1961), Hachmuth (1952).
It is not necessary to specify the reflux when calculating a preliminary material balance;
the system boundary can be drawn to include the reflux condenser.