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Gas scrubbing, also known as gas absorption, is a process in which a gas is removed from a mixture via contact with a liquid solvent. Gas scrubbing is a purification technique common in chemical process industries and is most commonly performed in structures known as packed towers or packed columns. This technique may be used for removing impurities or for reclaiming gases.
Gas scrubbers are commonly used to separate gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2) from natural gas, syngas and flue gases. In these instances, the solute materials must be removed from the product gases because of performance, health and environmental issues. In other cases, the solute gas itself may be the intended final product and, after being absorbed in the liquid solvent, it is recovered.
The underlying theory of gas scrubbing lies within the solubility of the solute gas in the liquid solvent. Solvents are chosen specifically to pull the solvent gas out of a gaseous mixture. For example, CO2 is soluble in an aqueous solution of ethanolamine. If a gaseous stream of nitrogen and CO2 were to come in contact with ethanolamine, the CO2 would be more likely to dissolve in the solution, leaving behind a gaseous mixture composed primarily of nitrogen.
For gas scrubbing to work efficiently, there must be good contact between the liquid and gas phases. Temperature and pressure also can affect the solubility of the solute gas in the liquid phase. The proper values for these process variables may be determined through experimentation or determined with process simulation software. The balance between temperature and pressure will depend on the capabilities of the packed column and the solubility of the other gases in the mixture.
Gas scrubbing is usually carried out in packed towers or packed columns. These columns are similar in shape to process distillation columns but, instead of trays, they contain tower packings. These packings fill the middle of the column and come in several different shapes, materials and sizes. Raschig rings, Pall rings, Berl saddles and Intalox® saddles are common types of packings. The packings are used to increase surface area and ensure good contact between the gas and the fluid.
In general, the solute-containing gas, or rich gas, enters the bottom of the column and travels upward through the packing. Fresh liquid enters the top of the column and goes through a liquid distributor, which evenly spreads the liquid across the packing. After scrubbing, the solute-free — or lean — gas exits through the top of the column. The solute-rich liquid, or strong liquor, exits the column at the bottom.