Analysis Of Some Key Scientifical Issues In The Organic Acids Production By Electrodialysis With Bipolar Membranes

The electro-membrane processes based on ion exchange membrane are novel separation technologies, which have high potential for application in green chemistry and cleaning production. Due to the special charge characteristics of ion exchange membrane, the eletro-membrane processes can achieve the separation and classification of the ions, and can also obtain some new chemical synthesises with the largest use of the resources and alleviate the pollution problem as well. Electrodialysis with bipolar membranes (EDBM) is developed based on ion exchange membrane technology. The most significant characteristic of the EDBM technology is that water can be splitting into H+ and OH- (or alcohol splitting into CH3O-) under a current field but without the introduction of any other salts. Because of the technology advance, cost-efficient and environmental benignity of the EDBM process, this technology is playing more and more important role in green chemisity and cleaning production. However, some hurdles such as less recongintion of this EDBM technology, lack research in process intergration and difficulty in simulating the process restrict the industrical application of the EDBM technology. Therefore, this thesis chooses one of the most important applications of the EDBM technology, the organic acid as the research object. The gluconic acid was choosen as a model agent of the organic acid. The thesis was focuses on the solutions to some problems related to the application of electrodialysis with bipolar membranes for organic acids production. The main conclusions are as followings,(1) The process cost of gluconic acid produced from gluconic sodium by using electrodialysis with bipolar membrane and ion exchange resin were compared by introduce environmental factor of the ion exchange (a) and price factor of the bipolar ion-exchange and properly control the conversion during EDBM running.(2) The Box-Behnken design of the response surface methodology was employed to determine the optimum conditions for production of organic acids by using EDBM. The Box-Behnken center united experimental design was used to quantify the effects of current density, electrolyte concentration, and feed concentration on the process cost. According to the ridge and canonical analysis, the optimum conditions were as follows: current density, 44.83mA cm-2; electrolyte -3 -3. Moreover, the experimental results were in good agreement with the predictions, suggesting that RSM was a good tool for modeling the process of EDBM.(3) Conventional electrodialysis (CED) was integrated with EDBM, i.e., CED supplied concentrated organic salts as the feed to EDBM. Results indicated that due to concentrated effect of gluconate and electrode reactions in CED, this kind of in-situ integration could achieve a very high current efficiency a low energy consumption.The process cost of CED-EDBM was less than that of a single EDBM process. This integration not only made the production cost-effective but also kept the operation of EDBM stable.(4) An overall description of the concentration, potential, and resistance distribution across a Neosepta CMX type cation exchange membrane in 1:2 type electrolytes was developed based on Nernst-Planck equations, Donnan equilibrium and electroneutrality assumption. Electrochemical impedance spectroscopy (EIS) was applied to correlate with the developed model. Results from the model indicated that the concentrations of ions in the diffusion layers were high dependent on the current density. The potential in the diffusion layers were significantly dependent on the applied current, while the influences of current on the Donnan potential were insignificant. The resistances come from the Donnan interfacial layers were the dominant component at low current density, whereas the diffusion layers became the dominant one at high current density. The model resistances were mainly in consistent with the electrochemical impedance spectroscopy measurement. The little deviation between the model and the EIS measurement was might due to the neglect of the electroconvection in the diffusion layers.(5) Based on the concentration, resistance, and potential distribution across the monopolar membrane and the water-splitting characteristic of the bipolar membrane as well, a mathematic model of a typical three-compartment EDBM process was developed to calculate the energy consumption and total cost of the process. Results indicated that the resistances of the solutions, diffusion layers and Donnan interfaces were high dependent on the applied current. The resistances in the diffusion layers were the dominant resistances, while the resistances due to Donnan interfaces and resistances of the membranes were neglected. The energy consumption of the EDBM process was increased with the increase in current. The energy consumption in the validation experiment with domestic membranes was in good agreement with the prediction, suggesting the reliability of the model.The achievement of this research can promote the improvement of EDBM technology, and lay some basis for green and environmental benign production in organic acids field as well. This research can also give some references for some other green chemistry and environmental benign production processes.