Purification of lactic acid from microfiltrate fermentation broth by cross-flow nanoflltration

摘要

Experimental investigations were carried out on cross-flow nanoflltration of microfiltrate fermentation broth of lactic acid during membrane-integrated fermentative production of monomer grade lactic acid. Three types of composite polyamide nanoflltration membranes (NF2, NF3 and NF20) manufactured by Sepro Co. (USA) were used in nanoflltration. Lactic acid fermentation broth produced by fermentation of sugar cane juice by Lactobacillus plantarum in a membrane-integrated hybrid reactor system was filtered by NF membranes after preliminary filtration through microfiltration membranes for cell separation and recycle. Effects of cross flow rate, transmembrane pressure and pH on flux and rejection of both acid and unconverted sugars for recycle were analyzed. NF3 membrane was successful in retaining 94% unconverted sugars while allowing 32% lactic acid to permeate at a flux of 113 Lm~(-2) h~(-1) at pH 5.5, temperature 37 °C, transmembrane pressure of 13 bar and cross-flow velocity 2.48 ras~(-1). The study shows that integration of a properly selected nanoflltration membrane with a cell-recycle fermentation unit and lactate conversion unit can yield L (+) lactic acid with an optical purity of 85.6% through an economically attractive route by recovering and recycling of unconverted sugars. The nanoflltration combine with bipolar electrodialysis in downstream purification can replace the multiple purification steps by two steps while yielding a monomer grade lactic acid from a mixture of unconverted sugars and lactic acid.