Lactic acid has a wide range of applications in the food and pharmaceutical industries. Especially, lactic acid is being used in chemical and plastic industries nowadays due to its renewable characteristics. Fermentation by adding CaCO3 is the general method to produce lactic acid with reacidification by a strong acid [1]. Therefore, some impurities including residual sugar, bacteria, protein and pigment exist in the fermentation broth, which resulted in the poor quality of the final product. Furthermore, the process was high energy and substance cost. To develop a new clean method, the membrane separation (microfiltration and nanofiltration) and electrodialysis were investigated.
In the new production of lactic acid, the acidity of the fermentation broth was neutralization by alkali (NaOH). Then, the sodium lactate fermentation was filtrated by ceramic membrane with pore size of 200 nm. The permeate from the filtration of ceramic membrane was clarified by nanofiltration to remove the residual sugar and multivalent metal ions. At last, the obtained clean sodium lactate was treated by electrodialysis with bipolar membranes to prepared the lactic acid and alkali. In this work, the membrane processes including ceramic membrane filtration, nanofiltration and electrodialysis with bipolar membranes were studied.
The ceramic membranes with four pore sizes were used to clarify the fermentation broth and the effects of velocity on the permeate flux using the four ceramic membranes were obtained. The ceramic membrane with pore size of 200nm had the better performance and the crossflow velocity had little influence on the filtration. The tubidity data of the permeates using ceramic membranes with pore sizes of 20, 50, 200 and 500nm were 0.06, 0.1, 0.1 and 0.3, respectively. Therefore, the ceramic membrane with pore size of 200 nm was more suitable to clarify the fermentation broth. The total sugar in the fermentation system decreased from 2.5% to 2.0%. And then, the permeate was fed to the nanofiltration unit. The nanofiltration membrane with the molecular weight cut-off of 200 Da was run at the pressure of 2.5MPa and the temperature of 30 oC. After nanofiltration, the permeate was purified sodium lactate solution with the concentration of 12%. In the permeate solution, the content of Ca2+/Mg2+, Fe2+ and Zn2+ were 3, 2 and 1 mg/L, respectively.
In the electrodialysis process, the three-apartment equipment was used to investigate the function of the bipolar membranes. The yield and current efficiency increased with the increasing voltage from 27 to 40 V. The conversion rates of sodium lactate by bipolar membrane electrodialysis were over 99%. Under the voltage of 40 V and the current of 8 A, the current efficiency of the bipolar membrane electrodialysis arrived at the maximum (61%).
The experimental results of the membrane based purification process for lactic acid production showed the good functions for clarification of fermentation broth, purification and conversion of soldium lactate by the integrated membrane processes including ceramic membranes, nanofiltration and bipolar membranes. The new clean process for lactic acid producing is promising in the future.
Keywords: Microfiltration; Fermentation; electrodialysis with bipolar membrane; Ceramic membrane
Acknowledge: This work is supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732) , National Hi-tech Research Development Program of China (No. 2007AA03Z437) .
References
[1] S. Kascak, J. Kominek, M. Roehr, H.J. Rehm, G. Reed, A. Puhler, P. Stadler(Eds.), Biotechnology, Products of Primary Metabolism,Vol.6, VCH, Weinheim Germany, 1996, Chapter 8.