Absorption Title

GAS ABSORPTION & DESORPTION

Gas absorption (also known as scrubbing) is an operation in which a gas mixture is contacted with a liquid for the purpose of preferentially dissolving one or more components of the gas mixture and to provide a solution of them in the liquid.

Sample plant picture no. 08

Therefore we can see that there is a mass transfer of the component of the gas from the gas phase to the liquid phase. The solute so transferred is said to be absorbed by the liquid.

In gas desorption (or stripping), the mass transfer is in the opposite direction, i.e. from the liquid phase to the gas phase. The principles for both systems are the same.

We will focus on the analysis for gas absorption, for the simple case whereby only one component of the gas solute is being absorbed. The other components of the gas are assumed to be non-soluble in the liquid (i.e. the other gas components are inert components), and the liquid is non-volatile, which means that there is no transfer of molecules from the liquid to the gas phase.

In addition, we assume there is no chemical reaction in the system and that it is operating at isothermal condition.

 

The process of gas absorption thus involves the diffusion of solute from the gas phase through a stagnant or non-diffusing liquid.

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The Table below showed representative commercial absorption applications.

Examples of gas absorption processes

From: Table 6.1 "Separation Process Principles", J.D. Seader and E.J. Henley, p.272

Click here for more information on gas absorption operation.

Gas absorption and desorption (stripping) can often integrated. Click here for an example.

 

Physical vs. Chemical Absorption

There are 2 types of absorption processes: physical absorption and chemical absorption, depending on whether there is any chemical reaction between the solute and the solvent (absorbent).

When water and hydrocarbon oils are used as absorbents, no significant chemical reactions occur between the absorbent and the solute, and the process is commonly referred to as physical absorption.

When aqueous sodium hydroxide (a strong base) is used as the absorbent to dissolve an acid gas, absorption is accompanied by a rapid and irreversible neutralization reaction in the liquid phase and the process is referred to as chemical absorption or reactive absorption.

More complex examples of chemical absorption are processes for absorbing CO2 and H2S with aqueous solution of monoethanolamine (MEA), diethanolamine (DEA), diethyleneglycol (DEG) or triethyleneglycol (TEG), where a reversible chemical reaction takes place in the liquid phase. Chemical reactions can increase the rate of absorption, increase the absorption capacity of the solvent, increase selectivity to preferentially dissolve only certain components of the gas, and convert a hazardous chemical to a safe compound.

 

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