| Bipolar membrane electrodialysis takes advantages of the water splilliting performance of bipolar membrane and ion separation of conventional electrodialysis, which optimizes the traditional industrial process and offers a promising strategy on the solution in environment, resources and energy issues, etc.As known to us, bipolar membrane serves as the core component of bipolar membrane electrodialysis, playing a key role in the bipolar membrane electrodialysis production. Therefore, the performance of bipolar membrane electrodialysis crucially depends on the properties of bipolar membrane such as water splilliting. To date, the technologies on the hydrolysis investigation of bipolar membrane mainly contain Ⅰ-Ⅴ curve and AC impedance spectroscopy etc. Bipolar membrane electrodialysis develops electrodialysis and deals with technical problems in aqua-ethanol system those are difficult to be dissolved for conventional electrodialysis. However, the influence of mixed systems on the performance of bipolar membrane has rarely been investigated. The water splilliting in BPM shows an electric field-enhanced (EFE) phenomenon and its magnitude is critically determined by the specific structure and composition of the bipolar intermediate layer. By varying the content of ethanol, three commercial bipolar membranes (Neosepta BP-1, Fumasep FBM and CJ-BPM) are examined by AC impedance spectra, and their bipolar membrane solvent resistance is also evaluated. The experimental results indicate that water splilliting phenomenon occurs in the LiCl water-ethanol mixed solutions. In addition, the local effective value of the BPM resistance increases with in the increasing ethanol, meanwhile the water splilliting becomes less obvious. This phenomenon could be attributed to the weaker dissociative ability of alcohol than that of water. Thus, the existed ethanol decreases the concentration of water in the intermediate layer of the bipolar membrane, leading to decreased water splilliting effect. Moreover, the influence of ethanol content on the water splilliting is more intuitive when simplifying the calculating algorithm of the intermediate layer thickness for the BP-1 and FBM membranes.Based on the above results, aqua-ethanol mixture serves as the solution medium in the BMED technique to overcome the low solubility of salicylic acid (SAH) in the production process. To evaluate the parameters including ethanol content, current density, sodium salicylate concentration and stack configurations on the performance of BMED process, a series of experiments are conducted. It indicates that highest acid concentration can be obtained as ethanol percentage is 50v/v%, and the current efficiency reaches 98.2%. Furthermore, the BMED stack of BP-C (BP, bipolar membrane; C, cation exchange membrane) configuration is found to be most cost-effective and achieve the highest current efficiency of 99.6%. For industrialization, a 4-unit BMED stack of BP-C configuration is assembled as a scale-up experiment. After 180 min operation, the current efficiency reaches 88% and the energy consumption is 1.78 kw h kg-1.In conclusion, it is feasible to produce salicylic acid by EDBM in aqua-ethanol system, which increases the solubility of water-insoluble organic acids by using aqua-ethanol solution in EDBM. Such technique will play a significant role in the cleaner production of water-insoluble aromatic acids. |
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