Direct production of L (+) lactic acid in a continuous and fully membrane-integrated hybrid reactor system under non-neutralizing conditions

Experimental investigations were carried out on continuous and direct production of L (+) lactic acid by fermentation of sugarcane juice in a novel two stage membrane-integrated fermentor under non-neutralizing conditions. Novelty of the integrated system lies in high degree of purity, yield and productivity of lactic acid from a relatively cheap and renewable carbon source in a very compact, flexible yet eco-friendly plant. Integration of cross flow microfiltration and nanofiltration membrane modules in two stages with the fermentor helped achieve reasonably high purity, productivity and yield by continuous recycling of cells and unconverted sugars. The system could be operated at steady state for a constant fermentor capacity and also at different set flux values ranging from 631 m(-2) h(-1) to 761m(-2) h(-1). Cross flow microfiltration membrane modules ensured almost complete separation and recycle of cells without much fouling problem whereas composite nanofiltration membrane module helped achieve 96% recovery and recycle of unconverted sugars along with high degree of purity. When operated at a dilution rate of 0.15 h(-1), the yield and volumetric productivity of L (+) lactic acid were 0.96 gg(-1) and 12.40 gl(-1) h(-1) respectively. Volumetric productivity could be increased further to 13.4 gl(-1) h(-1) with increase of dilution rate to 0.18 h(-1). Flat sheet cross flow modular design permitted continuous production of lactic acid in a highly flexible, largely fouling-free and environmentally benign plant as direct lactic acid was obtained instead of lactate salts of a conventional process. (C) 2011 Elsevier B.V. All rights reserved.