Читать книгу Poly(lactic acid) - Группа авторов - Страница 79

The differences in the boiling point of the different lactic acid species can be utilized in the purification of lactide. The most volatile compounds are water and LA; meso‐ and racemic pure lactides are less volatile, and lactic acid oligomers are often in the liquid phase. One purification method employs a gas stream for purification of an impure cyclic ester by passing a gaseous inert substance through the impure cyclic ester in a molten state and removing the gas stream, whereby the purified cyclic ester is recovered from the gas stream [104]. There are several references on lactide distillation processes applied on an industrial scale, and even if the processes are similar, they all have different detailed technical solutions. Some basic differences between the processes described are in many cases found in how the outlet for the purified lactide is arranged. In one method, the liquefied gaseous impurities are separated from the solidified lactide, after which the liquefied impurities are returned to the lactide synthesis step [105]. Another process involves producing pure lactide using a final step of purifying meso‐lactide from L‐lactide and/or D‐lactide by distillation to give one meso‐enriched purified lactide stream and one meso‐depleted purified lactide stream [106]. A further and improved process uses a partial condensation of the rectified lactide, whereby the low boiling point gaseous fraction remains as a vapor and is discarded. The lactide fraction is condensed and passed in the liquid phase to a distillation column [107].