(195f) Membrane Based Purification Process for Lactic Acid Production

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.