James G. Speight - Encyclopedia of Renewable Energy

See also: Adsorption, Gas Cleaning, Gas Processing, Gas Treating.

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 (CO 2) and hydrgoen sulfide (H 2S) 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.

An example of absorption dehydration is known as glycol dehydration and diethylene glycol, the principal agent in this process, has a chemical affinity for water and removes water from the gas stream. In this process, a liquid desiccant dehydrator serves to absorb water vapor from the gas stream.

Essentially, glycol dehydration involves using a glycol solution, usually either diethylene glycol (DEG) or triethylene glycol (TEG), which is brought into contact with the wet gas stream in a contactor . The glycol solution will absorb water from the wet gas and once absorbed, the glycol particles become heavier and sink to the bottom of the contactor where they are removed. The gas stream, having been stripped of most of its water content, is then transported out of the dehydrator. The glycol solution, bearing all of the water stripped from the gas stream, is put through a specialized boiler designed to vaporize only the water out of the solution. The boiling point differential between water (100°C, 212°F) and glycol (204°C, 400°F) makes it relatively easy to remove water from the glycol solution, allowing it to be reused in the dehydration process.

As well as absorbing water from the wet gas stream, the glycol solution occasionally carries with it small amounts of methane and other compounds found in the wet gas. In the past, this methane was simply vented out of the boiler. In addition to losing a portion of the gas stream that was extracted, this venting contributes to air pollution and the greenhouse effect. In order to decrease the amount of methane and other compounds that are lost, flash tank separator-condensers work to remove these compounds before the glycol solution reaches the boiler. Essentially, a flash tank separator consists of a device that reduces the pressure of the glycol solution stream, allowing the methane and other hydrocarbons to vaporize ( flash ).

The glycol solution then travels to the boiler, which may also be fitted with air or water cooled condensers, which serve to capture any remaining organic compounds that may remain in the glycol solution. The regeneration (stripping) of the glycol is limited by temperature: diethylene glycol and triethylene glycol decompose at or before their respective boiling points. Such techniques as stripping of hot triethylene glycol with dry gas (e.g., heavy hydrocarbon vapors, the Drizo process ) or vacuum distillation are recommended.

In practice, absorption systems recover 90 to 99% by volume of methane that would otherwise be flared into the atmosphere.

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

Absorption oil (also called absorber oil, scrubbing oil, wash oil) is low-boiling liquid hydrocarbon used to absorb or remove the higher-boiling liquid hydrocarbons from a gas stream. Typically, absorption oil is a hydrocarbon-based liquid that is contacted with a gas stream to remove higher boiling components, such as occurs in the recovery of natural gasoline from the gas stream (often referred to as the wet gas stream).

An absorption plant is a facility used to recover the condensable portion of the gas stream to remove acid gas stream. Typically, the absorption takes place in a counter-flow (countercurrent) vessel, where the gas stream enters the bottom of the vessel and flows upward through the absorption oil. The plant also includes the necessary ancillary thermal units to distill the absorbed hydrocarbons from the higher boiling absorption oil.

See also: Absorption, Absorption Processes, Gas Cleaning, Gas Processing, Gas Treating, Olamine Processes.

The absorption method of extraction is similar to using absorption for dehydration. The main difference is that in the absorption of liquids from the gas stream, absorbing oil is used as opposed to glycol. This absorbing oil has an affinity for gas liquids in much the same manner as glycol has an affinity for water. Before the oil has picked up any gas stream liquids, it is termed lean absorption oil.

The oil absorption process involves the countercurrent contact of the lean (or stripped) oil with the incoming wet gas with the temperature and pressure conditions programmed to maximize the dissolution of the liquefiable components in the oil. The rich absorption oil (sometimes referred to as fat oil), containing gas liquids, exits the absorption tower through the bottom. It is now a mixture of absorption oil, propane, butanes, pentanes, and other higher-boiling hydrocarbons. The rich oil is fed into lean oil stills, where the mixture is heated to a temperature above the boiling point of the gas liquids but below that of the oil. This process allows for the recovery of around 75% by volume of the butane derivatives, and 85 to 90% by volume of the pentane derivatives, and higher-boiling constituents from the gas stream.

The basic absorption process above can be modified to improve its effectiveness, or to target the extraction of specific gas stream liquids. In the refrigerated oil absorption method, where the lean oil is cooled through refrigeration, propane recovery can be upwards of 90% by volume, and approximately 40% by volume of the ethane can be extracted from the gas stream. Extraction of the other, higher-boiling liquids can be close to 100% by volume using this process.