Environmental Engineering Reference
In-Depth Information
6.4.2
Procedure for solving absorption problems graphically
The graphical solution to absorption is identical to that for the rectifying section of a
distillation column, except that the operating line is now above the equilibrium line. This
is because the solute is now being transferred from the gas to the liquid instead of from
the liquid to the gas. The “stepping off ” stages procedure is as follows.
1 Plot Y vs X equilibrium data (make sure to convert from mole fractions to mole ratios!).
2 The values X 0 , Y N + 1 (inlet solute concentrations or similar values), and L , G (flowrates)
are usually given. One point on the operating line is represented by the passing streams:
( X 0 , Y 1 ), where Y 1 is the target exit value. Using this point and a slope of L
/
G , plot the
operating line.
3 Step off stages as usual, starting at Stage 1. Start at operating line ( X 0 , Y 1 ) and
move horizontally to equilibrium line ( X 1 , Y 1 ). Move vertically to operating line ( X 1 ,
Y 2 ). Repeat this procedure until you reach the opposite end of column on operating
line.
6.4.3
Limiting liquid / gas ratio
Analogous to the minimum reflux ratio in distillation, there exists a minimum L
G ratio in
absorption. Figure 6.3 compares the operating line to the equilibrium line for absorption.
Because the slope of the operating line is L
/
G , reducing the liquid mass-separating agent
flowrate decreases the slope of the line. Compare lines aA and ab for a separation in which
the gas enters with Y b concentration of solute which must be removed to Y a and the liquid
/
Operating line, ( L G ) min
A
Y b
b
Operating line
Y
Equilibrium
curve
a
Y 0
X a
X A
X b
X
Figure 6.3 Minimum liquid flow.
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