초록
Experimental investigations were carried out on cross-flow nanofiltration of microfiltrate fermentation broth of lactic acid during membrane-integrated fermentative production of monomer grade lactic acid. Three types of composite polyamide nanofiltration membranes (NF2, NF3 and NF20) manufactured by Sepro Co. (USA) were used in nanofiltration. 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 113Lm<SUP>-2</SUP>h<SUP>-1</SUP> at pH 5.5, temperature 37<SUP>o</SUP>C, transmembrane pressure of 13bar and cross-flow velocity 2.48ms<SUP>-1</SUP>. The study shows that integration of a properly selected nanofiltration 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 nanofiltration 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.