Comparison of gas absorption and distillation :
1. Gas absorption deals with the separation of the constituents of a gas mixture.
1. Distillation deals with the separation of the constituents of a liquid mixture.
2. In gas absorption, the constituents of a gas mixture are separated by using liquid solvent.
2. In distillation, the constituents of a liquid mixture are separated by using thermal energy.
3. Liquid solvent used in gas absorption is below its boiling point.
3. Liquid in distillation operation is at its bubble point.
4. In gas absorption, a gas phase contacts a liquid phase and mass (solute) is transferred from the gas phase to liquid phase (i.e., mass transfer takes place in one direction). It is a case of steady state diffusion of A through a nondiffusing B.
4. In distillation, a vapour phase contacts a liquid phase and mass is transferred both from the liquid phase to the vapour phase and from the vapour phase to the liquid phase (i.e., mass transfer takes place in both the directions simultaneously). It is a case of steady state equimolar counter diffusion.
5. Gas absorption does not yield a pure product. It needs further processing for the recovery of the solute/solvent (usually by distillation).
5. Distillation yields almost pure product in a single processing step.
6. In gas absorption, heat effects are attributed to the heat of solution of the dissolved gas.
6. In distillation, heat effects are attributed to the latent heats of vaporisation and condensation.
7. Gas in absorption operation is well below its dew point.
7. Vapour in distillation is always at its dew point.
8. Example – the removal of SO2 from flue gases by alkaline solutions.
8. Example – separation of petroleum crude into gasoline, kerosene, fuel oil, etc.
9. Packed towers are widely used for absorption operation.
9. Plate towers are widely used for distillation operation.
10. The difference in solubility of gases in a given solvent is responsible to effect the separation by absorption.
10. The difference in vapour pressures (volatilities) of different components at the same temperature solution is responsible to effect the separation by distillation.