The ion exchange system uses cation, anion, and mixed bed exchangers to remove catalyst and other impurities. The removal of ionic substances by ion exclusion chromatography is the concentration step. Due to their charge, the ionic substances are repelled from the resin surface which stays in the liquid volume. The non-ionic substances are accommodated in the resins and pores. Anionic and cationic ion exchangers are exchanged for wash water, which first removes the ionic substances in the liquid and later the non-ionic substances. Negative anionic ion exchangers are exchanged for hydroxide ions whereas positive cationic ion-exchanged for hydrogen ions. The purification step is the next step which uses ion exchangers. The removal of odour and colour, inorganic salts, soap and fat components are done by the purification process.
For smaller capacity plants, ion exchange purification of glycerol is a good alternative to vacuum distillation. However, for this process ion-exchange is not economical since high salt content of glycerol issued from biodiesel production. When the salt content is around 5-7 per cent range the chemical regeneration costs becomes extremely high. The disadvantage of the ion- exchange is that it obstructs the process of obtaining high purity glycerol and also the system is fouling by soaps and fatty acids. The other shortcoming is the necessity for water evaporation after purification, which results in additional losses of glycerin, carried over by water steam.
Vacuum distillation with steam injection, followed by activated carbon bleaching is the commonly practised method for the final purification of glycerol. Evaporation of components can be accomplished in vacuum distillation. Vacuum distillation is also known as low-temperature distillation. Vacuum distillation is used as a separator in some separation techniques because glycerol is sensitive to heat and the compound splits into the water and decomposes. Due to the high boiling point of glycerol an extreme deep vacuum should be used to distil glycerol from inorganic salt.
The advantages of vacuum distillation are that it is a commonly well-established technology as it produces high purity glycerol in high yield. Another advantage is it is the reduced temperature requirement at lower pressures. Vacuum distillation could be used without heating the solution. The number of equilibrium stages needed can be reduced by utilizing the vacuum distillation. The disadvantage of this process is that distillation of glycerol has high capital cost and it is energy-intensive. This is because glycerol heat capacity is high which demands a high energy input for vaporisation. The vacuum distillation cannot proceed out continuously and is accompanied by considerable losses of glycerol. It has been suggested that vacuum distillation of glycerol is best suited to operations > 25 tons per day.