The Production Of Organic Acids By Bipolar Membrane Electrodialysis

Organic acids as important feedstock have been widely used in pharmacy, food,cosmatics,etc. Organic acids are traditionally produced by fermentation and chemical synthesis. The fermentation is relatively moderate since the reaction conditions required are generally milder. Besides, renewable resources are used as raw materials and the feedstock supply is guaranteed. Chemical synthesis is fiercer because of the extreme operating conditions. Hence some organic acids should not or are difficult to be produced via chemical synthesis. Both fermentation and chemical synthesis are always associated with intricate post-processing steps,including multifarious separation and purification steps such as extraction,crystallization and distillation.Thus,a large amount of pollution and a high energy consumption are inevitable,which is not conform to the conception of "green chemistry". The newly developed membrane separation technology, especially the application of electrodialysis technology in the production of small organic acids, such as formic acid, acetic acid,citric acid have been reported extensively. However, for organic acids with large molecular weights, the recovery ratio would be decreased greatly during membrane separation technology, since the migration resistance of the large organic anions is higher and the migration through the membrane is more difficult. Therefore we put forward the preparation of porous anion exchange membrane to improve the recovery ratio of organic acids and reduce the energy consumption.Moreover,amino acids as a special kind of organic acids,are widely used in pharmacy, food, fodder, etc. Amino acid molecules contain both carboxyl and amidogen groups and their traditional separation methods are complicated, such as crystallization, ion exchange, special precipitation method, etc. Evaporation consumes high amount of energy during the crystallization process,resin regeneration needs a lot of chemical reagents, while special precipitation method would produce lots of wastewater and recycling the precipitation agent is difficult. In this paper, threonine is taken as an example. Threonine mother liquor is produced during fermentation in which many impurities are intermingled, e.g, threonine, glutamic acid and other components. Bipolar membrane electrodialysis (BMED) can be used to separate the mixed amino acids according to the difference of their isoelectric points. The thesis is divided into four chapters, and the main contents are as follows:Chapter 1 is an introductory part stating the problems in traditional organic acid production processes, and the principals of electrodialysis,especially the ion exchange membrane which is the core component of electrodialysis. Finally, the background,the meanings and main contents of the thesis are presented.In chapter 2, the porous P84 co-polyimide base membrane was prepared by phase inversion method, and then, chemical modifications including amination and quatemization were conducted to introduce positively charged groups. Field emission scanning electron microscopy (SEM), Fourier transform infrared spectrum were used to confirm the micromorphology and composition of the prepared membranes. The result indicated the membrane area resistances were in the range of 0.6-1.8 ? cm2,ion exchange capacities (IECs) were in the range of 0.6-0.9 mmol/g and the water uptake (WR) values were 100-160 %. The prepared membranes were used in BMED process to produce lactnobioic acid (LBA) and sodium hydroxide. The purity of LBA produced by porous membranes was similar to that by the commercial membrane AMX, but the energy consumption was lower and the recovery ratio was almost twice the value of membrane AMX.In chapter 3, BMED was used to separate the mixed amino acids containing threonine and glutamic acid. Three membrane stack configurations were designed to select the optimal configuration. Subsequently, the optimal configuration was used and 50V was considered as the most appropriate operation voltage in consideration of the separation efficiency, energy consumption and current efficiency. Glutamic acid almost migrated into the receiving compartment completely after 4 h running, while threonine retained in the feed compartment. The results showed that the energy consumption was 11.04 kW h/kg, current efficiency was 82.6%, and the product purity was 97.5%. What' more, the membrane fouling also was observed on the surface of the anion exchange membrane,which caused the loss of a slight amount of glutamic acid and a decrease of the overall effectiveness. Overall, the BMED process can achieve a good effect in separation of the mixed amino acids containing threonine and glutamic acid.Chapter 4 is a summary of the whole thesis. The results of above sections show that BMED is highly potential in production of organic acids including lactnobioic acid and amino acids.