Polylactic Acid Production from Glucose

The process under analysis comprises three major sections: (1) Fermentation; (2) Oligomerization & Lactide Formation; (3) Lactide Polymerization.

Fermentation. The culture media used in fermentation is prepared by mixing process water, glucose and corn steep liquor. The fermentation is carried out in continuous mode and under anaerobic process conditions in an agitated, jacketed fermenter. The pH control of the fermentation is made by addition of recycled sodium carbonate. The fermentation mass yield of lactate on glucose ranges from 85 to 95 wt%. The lactate concentration obtained in the fermentation broth is about 10 wt%. Fermentation broth is continuously drawn out of the fermenter and passed through filtration, evaporation and extraction steps, after which an 88 wt% lactic acid is obtained.

Oligomerization & lactide formation. Lactic acid is fed to the oligomerization reactor from which water vapor is removed. The resultant oligomer stream is mixed with catalyst and fed to the depolymerization reactor, which consists in a packed column operating under vacuum. Water, lactic acid and lactide are removed in the gaseous overhead stream. Most lactide and some impurities are condensed and sent to the drying column, where water is removed from the crude lactide. The remaining contaminants are removed in the purification column, leading to a 99.9% pure lactide.

Lactide polymerization. The polymer grade lactide undergoes a melt-phase polymerization. Molten lactide, a stabilizer and catalyst are fed to the reactor. The polymer melt is then pumped to a devolatizer operating under vacuum to remove unreacted lactide in the gaseous stream, which is condensed and recycled to the purification column. The devolatilized melt polymer is extruded, pelletized, and dried, resulting in the polylactic acid product.