What Is Gas Absorption & How Does It Work?

The Engineer's Perspective
The Engineer's Perspective

Table of Contents

What Is Gas Absorption & How Does It Work? Gas Absorption process

Gas Absorption (“scrubbing”) is the process of removing a gas solute from a gas mixture by dissolving it with an absorbent liquid, usually referred to as scrubbing liquid or solvent. As the solvent comes in contact with the gas mixture, select components of the gas mixture dissolve — effectively separating the mixture.

Principles of Gas-Liquid Absorption

In gas absorption, an appropriately chosen solvent must be relatively pure and has low volatility. In a gas scrubber, a closed space/container is filled partially with the solvent while letting the gas fill the remaining space causing contact between the two phases. As long as the gas dissolves only in the solvent and the solvent is not volatile, mass transfer of components from gas to liquid will occur.

What is Gas Absorption and How Does It Work gas to liquid in Gas Absorption
Behavior of solute transfer from gas to liquid in Gas Absorption Source: www.researchgate.net

Temperature affects the gas-liquid mass transfer since only a specific amount of gas components will be dissolved by the liquid at a certain temperature. This amount of gas dissolved in the liquid is called equilibrium solubility and is expressed in units of concentration or composition. 

What is Gas Absorption and How Does It Work Absoption chart
Solubility of Various Solutes in Relation with Temperature Source: Perry’s Chemical Engineering Handbook

However, a significant increase in temperature may cause gas to escape from the liquid. In this case, absorption can be enhanced by adjusting the gas amount or gas pressure and determining how much gas is dissolved in the liquid.

How Does Gas Absorption Work?

What is Gas Absorption and How Does It Work Gas Absorption Work process
Gas Absorption Process Diagram

Gas (solute, usually mixed with air) feed enters at the bottom of the absorber column while the liquid solvent is pumped at the top and is distributed through a nozzle or spray. Gas and liquid streams come in countercurrent or co-current contact, allowing the gas solute to dissolve in the liquid solvent. The resulting absorber flue gas is then passed through a condenser and gas analyzer to test the final concentration of the gas components after scrubbing. Flue gas composition will determine if the gas needs to be recycled as feed or can be allowed to be released into the environment. 

The solute-rich solvent is recovered at the bottom of the absorber column and subsequently processed in a stripping process to separate the rich gas and recover the solvent. Stripping is just a desorption process to recycle the solvent stream.

What Happens In An Absorption Tower?

To increase mass transfer phenomena, liquid distribution mechanisms (nozzles, spray, etc.) are used to maximize the area that the liquid will cover. Furthermore, column internals (e.g. trays and packings) are installed inside the columns to increase the contact area, thus increasing mass transfer significantly.

Though gas absorption setups are designed ideally, common unexpected issues occur:

Flooding inside the column happens when the velocity of the gas phase is greater than that of the liquid, causing the liquid not to drain correctly across the column internals. Adjusting the gas feed parameters (pressure, feed rate) as well as using the appropriate column internals will help in addressing this issue.

Foaming happens when the liquid expands causing high resistance between the gas and liquid phases. This is primarily caused by impurities present in the gas feed and can be solved by subsequent absorption processes. A thorough analysis and consideration of the feed should be conducted to optimize scrubbing.

Industrial Applications of Gas Absorption

Gas absorption (either physical or chemical) is used in gas purification (e.g. removal of pollutants from the exhaust or from flue gas streams that need to be further processed), gas recovery (e.g. recovery of ammonia from air), and in production of gas solutions for various processes (e.g. absorption of CO2 gas in making carbonated beverages). The choice of which shall then depend on the desired product.

In physical gas absorption, the mass transfer, which increases linearly with pressure following Henry’s Law, happens without a chemical reaction. The absorbing liquid is regenerated (desorption) through the reduction of pressure and temperature. 

Chemical gas absorption focuses on the reaction between the solvent and the gas (solute) where the solute is selectively dissolved. The solute-rich solution is sent to a stripping column to recover the solvent, while the desorbed gas is compressed to a supercritical condition for storage. Chemical absorption is used commonly in the petroleum industry (removal of impurities in the product stream) and in the chemical industry where a high level of product purity is required.


What is the difference between absorption and stripping?

Gas absorption is the mass transfer of components from gas to liquid while stripping (desorption) is the mass transfer of components from liquid to gas.

How tall is an absorption column?

The height of an absorption column depends on the feed conditions, solvent to be used, product requirements, and the rate of absorption at equilibrium conditions. In some conditions, column internals also contribute to column sizing. McCabe-Theile Method is used in the calculation of stages as well as column height.

What is the minimum gas-liquid ratio in gas absorption columns?

Minimum gas-liquid (MLG) ratio is the least amount of liquid required to dissolve the required amount of solute at a certain temperature and pressure; the liquid flow is expressed as a function of the gas flow rate to be absorbed.


  1. Beil, M. and Beyrich, W. “The Biogass Handbook”. Elsevier. 2013
  2. LANYU Gustawater Treatment. “The Ultimate Guide to Gas Absorption.” https://www.gustawater.com. Accessed 18 August 2022
  3. Panchmatiya, A. “Gas Absorption.” 10 October 2015. https://www.slideshare.net. Accessed 18 August 2022
  4. Plaza, M.G., Pevida, C., Rubiera, F. “Ongoing Activity on CO2 Capture in the Power Sector: Review of the Demonstration Projects Worldwide.” Instituto Nacional del Carbon, INCAR-CSIC, Oviedo Spain. 15 June 2016
  5. Perry, R.H. Perry’s Chemical Engineer’s Handbook 7th Ed. McGraw-Hill, 1997

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