The process of absorption, which is a chemical process, is to be distinguished from that of adsorption, which is a purely physical process.
A detailed explanation of adsorption is beyond the scope of this book, but Robinson and Quellet (1999) have provided a useful account of the process, which is sometimes termed gas-phase filtration and is also known as adsorption condensation. This relates to the fact that some gaseous pollutants, with suitably high boiling points, diffuse onto the suraces of
Fig. 17.9 A cyclone for collecting industrial dust.
Certain materials and condense. The temperature, molecular weight, and vapour pressure of the pollutant play a part, as also does the nature of the adsorbing surface. Pollutants with high boiling points diffuse onto the surfaces of the adsorption material and further diffusion into the surfaces within the material also occurs. The pollutants exist in the liquid phase at room temperatures. As air and pollutants flow through the filter the adsorption process continues but its effectiveness diminishes and eventually all the available receiving surface area is covered with condensed pollutant. The process is reversible and the filter can be removed and reactivated by heating.
Moisture in the air also condenses onto the surfaces, lessening their effectiveness for adsorbing the pollutant. It follows that high relative humidities are undesirable. If necessary, the air onto the adsorption filter might be warmed to reduce the humidity.
The principal adsorption filter used in air conditioning is the activated carbon filter. It is most effective in removing smells from the atmosphere, and in removing poisonous gases such as sulphur dioxide. The capacity of an activated carbon cell is expressed as an efficiency, but for sulphur dioxide, as an example, a typical cell with an efficiency of 95 per cent cannot adsorb 95 per cent of its own weight in sulphur dioxide. In fact, for the product of one particular manufacturer, a cell with a quoted efficiency of 95 per cent can adsorb 10 per cent of its own weight in sulphur dioxide. The cell contains 20 kg of activated carbon and can adsorb 2 kg of S02. (Activated carbon, incidentally, is prepared from the shells of coconuts and has a structure which offers an enormous surface area to any stream of gas passing over it.)
The adsorption capacity of activated carbon for ammonia, ethylene, formaldehyde, hydrogen chloride and hydrogen sulphide, which boil at between -104° and -21°C, is inadequate for practical purposes. On the other hand, substances such as butyric acid (‘body odour’), petrol, putrescine and the common mercaptans etc., which boil at the higher temperatures of 8° to 158°C, are very effectively adsorbed. The activated carbon used during the 1939— 45 war for adsorbing the poisonous low boiling point substances such as arsine, hydrocyanic acid and other war gases was specially impregnated with other chemicals to increase the adsorption. This is sometimes called chemisorption. A chemical reaction occurs between the reagent added and the receiving surfaces of the adsorbent, forming a new chemical compound. Such filters cannot be regenerated but must be safely disposed of at the end of their useful lives.
On the other hand, when activated carbon has reached saturation it is removed from the filter for re-activation. This is accomplished by heating the carbon to a high temperature, of the order of 600°C or more. It is customary to return saturated cells to the supplier for re-activation, using spare replacement cells in the meantime.
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