Absorption processes remove chemicals from gas or liquid streams by mixing them with another stream. It is used in chemical processing, refrigeration, digestion, and plant nutrient absorption. Natural gas is purified using absorption to remove impurities. Adsorption using oil-based products can remove organic molecules. Dewatering uses desiccants or glycol-based chemicals to remove water. Sulfuric acid production and carbon dioxide removal also use absorption processes. Dual absorption systems minimize air pollution.
An absorption process occurs when a chemical is removed from a gas or liquid stream by mixing it with another stream. This process can be used in chemical processing, refrigeration to cool foods, human digestion of vitamins and minerals, and when plants absorb nutrients. In chemical processing, sorption is used to remove one or more valuable products from a gaseous or liquid stream of mixed chemicals.
The natural gas supplied to homes and businesses is mostly methane, a molecule of a carbon and hydrogen, or CH4. The gas taken from the ground during oil or gas drilling, however, is a mixture of several gases. The various products may have value in other chemical operations, but are not used for heating homes. Absorption is used in several stages to purify methane and remove impurities such as sulfur and water.
Many other organic or carbon-based molecules can be removed in an adsorption process using an oil-based product. Raw natural gas is mixed with oil in a processing tank to remove unwanted gases that can be separated and used later. The methane-rich stream then passes through other tanks or vessels for further processing.
Dewatering is an absorption process that often uses a desiccant, which is a silica or alumina-based product usually produced as round beads. The solid desiccant is loaded into a vertical tank called a tower and the water-containing gas is passed through it. Water absorbs to the surface of the desiccant because it is hydrophilic, meaning it likes water to stick to its surface. The natural gas leaves the tower dry and the desiccant can be heated afterwards to remove the water and be reused.
A glycol-based chemical, similar to the antifreeze used in automobiles, can also be used to remove water from natural gas or other streams. Glycol can absorb water due to a molecular bond in its chemical structure, so it will easily remove water from the gas stream. A tower can be used to bubble the gas through the glycol and the water-rich glycol can be processed later and reused in many cases.
One method of making sulfuric acid uses an absorption process. The sulfur is first burned with oxygen to form sulfur dioxide (SO2), then a secondary reaction creates sulfur dioxide or SO3. This product mixes with a stream of sulfuric acid and the SO3 is absorbed into the acid, making it more concentrated.
Beginning in the late 20th century, acid manufacturers began using a dual absorption system to minimize the amount of SO20 that is released into the air. Sulfur dioxide is thought to contribute to air pollution which can cause acid rain, which is harmful to plants and animals. A dual absorption system repeats the combustion and absorption process to minimize the amount of SO2.
Another use for an absorption process is the removal of carbon dioxide, or CO2, from industrial waste gases. Power plants and other industrial operations that burn coal or gas create large amounts of CO2, which can create environmental problems. The exhaust gases can be treated in an absorption scrubber, where the CO2 is absorbed into a liquid or in some cases onto a carbon-based solid, which can subsequently be treated for disposal. The carbon dioxide removed in this way is eventually landfilled, which is thought to be a better environmental option than leaving it in the air.
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