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An absorption tower (also known as a scrubbing tower) is a long vertical column that typically contains packed bed which is used for absorbing gases from gas streams. The gas is introduced at the bottom of the column and the absorbing liquid, often water, passes in at the top and falls down against the countercurrent of gas. In the tower, the higher-boiling (gasoline-range) hydrocarbon derivatives are partially absorbed by a liquid in the form of falling droplets.

The gas stream is introduced into the lower portion of the tower and flows upwards while making contact with a washing fluid. The tower is provided with one or several washing stages, each comprising one or several venturi tubes through which the gas is given a speed increase and the washing fluid is discharged through fluid nozzles in the gas flow path. Liquid distribution in the tower occurs in the gas flow in directions forming such angles to the vertical that the fluid flow will not fall backwards and it is preferential that the gas flow is not interrupted.

There are many devices that fall into the category of absorption tower – packed towers (packed columns) are most frequently used to remove contaminants from a gas stream. However, packed towers can also be used to remove volatile components from a liquid stream by contacting it with an inert gas (stripping). They are also used in distillation applications where the separation is particularly difficult due to close boiling components.

The packed tower consists of a vertical hollow column that is filled with small solid pieces (the packing) which occupy the space but leave voids between the pieces. Thus a liquid stream can be introduced into the top of the tower and flow down over the surface of the pieces, creating a large surface area for a given volume of liquid. The gas to be treated is introduced into the tower and flows through the voids where it comes into intimate contact with the surface of the liquid. The packings are divided into three principal types: (i) dumped packings, which is the loose packing method, (ii) stacked packings, which is the dense packing method, and (iii) structured packing, which is the ordered packing method.

The diameter of a packed absorption tower depends on the quantities of gas and liquid handled, their properties, and the ratio of one stream to the other. The height of the tower, and hence the total volume of packing, depends on the magnitude of the desired concentration changes and on the rate of mass transfer per unit of packed volume.

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