Construct an x-y Diagram for an Absorption Column

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This Demonstration leads you through a step-by-step procedure to construct an - diagram for a trayed absorption column that is used to remove an impurity from a gas feed by absorbing the impurity in a liquid solvent. The number of trays/stages needed to obtain an outlet concentration of ppm in the gas stream is calculated. Click the "new problem" button to create a different set of conditions and reset the Demonstration. Click "next" to go to the next step. Check "hint" for a hint, and check "solution" to see the solution before selecting the next step. Once "solution" is checked, your answer cannot be changed, and once "next" is selected, you cannot go back. To restart, press "new problem". For steps 2 and 3, check "show column" to display a diagram of the absorption column.

Contributed by: Rachael L. Baumann (May 2018)
Additional contributions by: John L. Falconer
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA



The operating line (magenta) represents the mass balance:


Rearranging the mass balance to solve for :


where is the mole ratio of impurity in the inlet liquid stream (ppm); is the mole ratio of impurity (ppm) in the gas stream that corresponds to , which is the mole ratio of impurity in the liquid stream (ppm); is the mole ratio of impurity in the outlet gas stream (ppm); is the liquid molar flow rate (Mmol/h); and is the gas molar flow rate (MMol/h).

The equilibrium line (orange) is calculated using Henry's law:


where is Henry's constant (atm/mole fraction) and is pressure (atm).

Henry's constant is calculated from , its value at standard temperature :


where is the activation energy (J/mol), is the gas constant (J/(mol K)) and has units of atm/mole fraction.


[1] P. C. Wankat, Separation Process Engineering: Includes Mass Transfer Analysis, 3rd ed., Upper Saddle River, NJ: Prentice Hall, 2012.

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