Membrane-based purification of optically pure D-lactic acid from fermentation broth to poly(D-lactide) polymer

Abstract

Fermentation of optically pure D-lactic acid was carried out using cassava starch as the main substrate. For downstream processing, membrane-based processes consisted of microfiltration, nanofiltration, and pervaporation-assisted esterification were successfully employed for the production of high purity D-lactic acid. The spiral wound microfiltration and nanofiltration were performed as high potential pretreatment steps for removal of bacterial cell, protein, and color compounds. The pervaporation dehydration of esterification reaction between D-lactic acid and ethanol was investigated in order to enhance the conversion to ethyl lactate. Up to 95% of water was separated out of the system resulting in a lactic acid conversion yield of 0.93. Mathematical modeling of each membrane system was investigated. In microfiltration, the major cause of membrane fouling was determined by breaking down resistances by a cleaning method. For nanofiltration, the reflection coefficient (σ) and the solute permeability (Ps) were obtained using the best-fit method, and allowed the prediction of lactate retention and permeate flux. In the pervaporation-assisted esterification, the kinetic of reaction, as well as separation factor, pre-exponential factor (Qmemb), and the activation energy (Eperm) of pervaporation were modelled using NRTL to model the liquid phase non-idealities. The results with 8.77% error were obtained in comparison to experimental data. High purity D-lactic acid was obtained after distillation and hydrolysis with deionized water. Finally, direct polycondensation followed by ring-opening polymerization was successfully employed to synthesize a high purity poly(D-lactide) polymer.