Sublimation salicylic acid purification

Figure 8.30. An entrainer sublimation process used for salicylic acid purification Air To exhaust...

Decarboxylation of salicylic acid takes place with slow heating because of the presence of the electronic configuration of the carboxyl group ortho to the hydroxyl group, but does not occur in the other isomers of hydroxybenzoic acid. On rapid heating, salicylic acid sublimes because of its low vapor pressure. This property allows commercial separation from the other isomers as a means of purification analogous to distillation. The differences in the vapor pressures are shown in Table 4. 

The purification of salicylic acid provides a good example of the use industrially of entrainer sublimation. Air may be used as the carrier gas but as salicylic acid can be decarboxylated in hot air, a mixture of air and CO2 is often preferred. The process shown in Figure 8.30 is carried out batchwise. A 5-10 per cent mixture of CO2 in air is recycled through the plant, passing over heater coils before over the containers, e.g. bins or trays, holding the impure salicylic acid in the vaporizer. The vapours then pass to a series of air-cooled... 

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