Absorption is a thermal separation operation in which a selective liquid (scrubbing liquid, absorbing agent) absorbs a component (absorbed substance) of a gas mixture. The objectives of absorption are :-
In addition, however absorption also finds application in the extraction of a commercial product (e.g. absorption of NH3in water, production of HCL and HNO3etc. Depending on the nature of the absorption process a distinction is made between scrubbing liquids which have a physical and chemical action.
Desorption is the reverse of absorption and involves the transfer of a gas dissolved in the scrubbing liquid to a gas stream. It is frequently used to regenerate the scrubbing liquid for absorption processes in order to make it possible to circulate the scrubbing liquid or perhaps to obtain a pure gas. In relation to absorption the term scrubbers is generally used if it is only a matter of removing harmful components from waste gases and the term absorbers if the objective is also to extract a valuable substance.
In many cases absorption plants mean high investment costs. It is all the more critical that such plants have a long service life, low maintenance and service costs and as great a flexibility as possible in relation to changing applications.
In collaboration with the chemical and pharmaceutical industries processes and plants have been specifically developed with a view to environmental engineering which because of the materials used can be safely operated even with highly corrosive media.
To produce HCL containing more than 33% HCL the absorption process must run at the lowest possible temperature (<30 deg C). The inlet temperature of the cooling water is usually of this order so that large heat exchanger surfaces are required to remove the heat of absorption. It is therefore often uneconomic to carry out the entire process isothermally. More often an adiabatically operated column and an isothermally operated column are linked together. The main quantity of heat is removed in the adiabatically operated absorber and the hydrochloric acid produced in it (approximately 30% by mass) is used as the scrubbing liquid for an isothermally operated absorber situated upstream. The small amount of heat of absorption still produced in the isothermally operated absorber is removed by intermediate cooling of the scrubbing water.
For purely isothermal absorption a falling film absorber is offered. In this case the scrubbing liquid runs down the inside surfaces of the tube in a shell and tube heat exchanger which is arranged vertically, the heat of absorption liberated being transferred through the tube wall to the cooling water flowing in the jacket. The gas containing HCL flows through the equipment concurrently with the liquid. In order to compensate for the disadvantage of concurrent operation from the point of view of process engineering, the falling film absorber should logically only be employed if the raw gas contains a very small portion of inert gas.