Absorption Columns: Summary

The answers to the ConcepTests are given below and will open in a separate window.
Key points from this module:
• An absorption column is usually modeled at steady state.
• Henry’s law can be used to relate the gas concentration to the amount absorbed for the solute when absorbed concentrations are low.
• The number of stages in an absorption column decreases as:
1. the temperature decreases (for most gases, absorption increases as temperature decreases).
2.  the pressure decreases.
3. the liquid flow rate increases.
• On a y-x diagram (solute concentration in gas phase vs. solute concentration in liquid phase) for absorption,
1. the slope of the equilibrium line is the Henry’s law constant, which has units of atm in this module.
2. the operating line is above the equilibrium line.
3. the slope of the operating line is L/V, where L is the liquid molar flow rate and V is the gas molar flow rate.
4. the top of the column is at the bottom of the diagram.
• The liquid stream in an absorption column flows down the column and its molar flow rate is much larger than the gas molar flow rate (the gas flows up).
• For a desired separation in an absorption column at a given temperature and pressure and gas low rate, a minimum liquid flow rate is required; otherwise an infinite number of stages is required.
• The liquid and gas streams leaving a stage in an absorption column are assumed in equilibrium.
From studying this module, you should now be able to:
• Carry out mass balances on an absorption column.
• Determine the number of stages needed to carry out a process to remove an impurity (solute) from a gas stream using absorption into a liquid stream.
• Explain how changing pressure and temperature affect absorption and the number of stages needed for separations.
• Explain where the solute concentrations are high and where they are low in an absorption column.

After finishing this module, another good module to study is Stripping Columns.

Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder.