Description
1. Flue gas from a process plant contains 15% CO2, 6% O2 and 79% N2. This gas is to be treated
in a tray column absorber operating at 25°C and 1.2 atm pressure with 30% (wt) aqueous
ethanolamine (C2H4OHNH2) solution as the solvent (absorbent) which enters the column at
25°C. The solvent is recycled from a stripper and contains 0.058 mol of CO2/mol of solution.
The gas leaving the absorption column is to contain only 2% CO2. Assume isothermal
operation and that only CO2 gets absorbed in the solution.
a) Determine the minimum liquid to gas ratio (mol/mol)
b) Determine the number of kilograms of liquid solution to enter the absorber per cubic metre of
entering gas for a liquid to gas ratio of 1.2 times the minimum.
c) Determine the number of theoretical trays for the condition defined in part 2 (round off to
nearest higher integer).
Equilibrium partial pressure of CO2 over 30% (wt) aqueous solution of ethanolamine at 25°C is
given below.
Mol CO2/mol solution Partial pressure of CO2 (mmHg)
0.058 5.6
0.060 12.8
0.062 29
0.064 56
0.066 98.7
0.068 155
0.070 232
2. In a bioprocess, molasses is fermented to produce a liquor containing ethyl alcohol. A
CO2-rich vapor with a small amount of ethyl alcohol is evolved. The alcohol is
recovered by absorption with water in a sieve-tray tower.
Determine the number of
equilibrium stages required for counter current flow of liquid and gas, assuming
isothermal, isobaric conditions and that only alcohol is absorbed. Entering gas is 180
kmol/h; 98% CO2, 2% ethyl alcohol; 30°C, 110 kPa. Entering liquid absorbent is
100% water; 30°C, 110 kPa. Required recovery (absorption) of ethyl alcohol is 97%.
The gaseous mixture can be assumed to be ideal, but the ethyl alcohol-water mixture
is not. Vapour pressure of ethyl alcohol at 30°C is 10.5 kPa and the liquid phase
activity coefficient of ethyl alcohol is 6. (Hint: modified Raoult’s law may be used to
estimate the vapour liquid equilibrium distribution constant).