Purification of D-(-)-Lactic Acid from Fermentation Broth Using Nanofiltration, Esterification, Distillation, and Hydrolysis Technique

Abstract

Until recently, the potential of biodegradable polymers obtained from agro-resources have long been recognized. In particular, poly-D-lactic acid (PDLA) is a very promising biodegradable polymer because its melting point is 40 °C higher than the melting point of standard PLA, making it as resistant to heat as the petroleum-derived polybutylene terephthalate (PBT). In this work, a fermentation broth containing D-(-)-lactic acid was pretreated with microfiltration and nanofiltration in order to remove cells and color prior to start the esterification reaction with ethanol. A highly efficient fractionating column designed in our laboratory was employed to distill 95 wt% ethanol from the esterification reaction. A vapor permeation technique using NaA zeolite membrane was employed to remove water allowing the dehydrated ethanol to be recycled. This integrated esterification-dehydration process results in a complete conversion of D-(-)-lactic acid to ethyl-D-lactate. For distillation, experimental result showed that more than 98% pure ethyl lactate was obtained by controlling the temperature of ethyl lactate vapor at the exit point of the fractionating column. After hydrolysis, the purity of D-(-)-lactic acid product was obtained at 98.89% with more than 87.8% optical purity.