Stripping Columns: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  1. A stripping column is usually modeled at steady state.
  2. Henry’s law can be used to relate the gas concentration to the amount absorbed for the solute when absorbed concentrations are low.
  3. The number of stages in a stripping column decreases as:
    1. the temperature increases (for most gases, stripping decreases as temperature increases).
    2.  the pressure decreases.
    3. the gas flow rate increases.
  4. On a y-x diagram (solute concentration in gas phase vs. solute concentration in liquid phase) for stripping,
    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 below 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 top of the diagram.
  5. 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).
  6. 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.
  7. 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 a stripping column.
  • Determine the number of stages needed to carry out a process to remove an impurity (solute) from a liquid stream by mass transfer to a gas stream.
  • Explain how changing pressure and temperature affect absorption/desorption and the number of stages needed for separations.
  • Explain where the solute concentrations are high and where they are low in a stripping column.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder.