Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 14

Absorption is an approach in which the absorbed gas is ultimately distributed throughout the absorbent (liquid). Gas absorption (sometimes also known as gas 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 the components in the liquid phase. 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.

The process involves a mass transfer of the component of the gas from the gas phase to the liquid phase in which the solute so transferred is absorbed by the liquid. In gas desorption (also known as stripping), the mass transfer is in the opposite direction, i.e., from the liquid phase to the gas phase. Typically, there is no chemical reaction involved in the absorption process and it operates under isothermal conditions. Common absorbing media used are water, aqueous amine solutions, caustic, sodium carbonate, and nonvolatile hydrocarbon oils, depending on the type of gas to be absorbed. Usually, the gas-liquid contactor designs which are employed are plate columns or packed beds.

Chemical adsorption processes adsorb sulfur dioxide onto a carbon surface where it is oxidized (by oxygen in the flue gas) and absorbs moisture to give sulfuric acid impregnated into and on the adsorbent. 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 acid gas removal from gas streams. In the process, carbon dioxide (CO₂) and hydrgoen sulfide (H₂S) are absorbed by treating the gas stream (invariably by passing the gas stream though the liquid absorbent) with an aqueous solution of monoethanolamine (MEA), diethanolamine (DEA), diethylene glycol (DEG) or triethylene glycol (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.

See also: Absorption, Gas Cleaning, Gas Processing, Gas Treating, Scrubbing, Stripping.