Treatment Of Typically Chemical Wastewater By Bipolar Membrane Electrodialysis (BMED)

At present, a large amount of wastewater is generated during the neutralization reactions in chemical industry, such as brominated butyl rubber wastewater and spent caustic. If this wastewater is directly discharged without any treatment, this will lead to not only resource waste but also environmental pollution. Therefore, whether from viewpoint of environmental protection or process cost, it is necessary to couple treatment of chemical wastewater process with green chemical techniques and achieves resource recovery without second pollution.A bipolar membrane comprises a cation exchange layer and an anion exchange layer, and its well-known function is to split water into H+and OH-under reverse bias in a direct current field. Due to the special characteristics of bipolar membrane, the bipolar membrane electrodialysis can achieve salts dissociate and the corresponding ions form acids and bases. The regeneration acids and based may be used in neutralization reactions, the treated wastewater meets the State’s discharge standard. The thesis is focuses on treating two kinds of chemical wastewater (such as brominates butyl rubber wastewater and spent caustic) by using bipolar membrane electrodialysis. The results and conclusions are as followings:(1) A method is reported to regenerate hydrobromic acid (HBr) and sodium hydroxide (NaOH) from a simulated NaBr wastewater by bipolar membrane electrodialysis (BMED). Sodium hydroxide may be recycled in the neutralization washing process of preparing the brominated butyl rubber. The results indicates that a low energy consumption and a high current efficiency are achieved when the concentration of sodium bromide was in the range of11000-16000mg/L with both the initial concentration of the acid and base at about0.10mol/L. Additionally, it is also demonstrated that a high current density applied to the BMED stack usually results in both higher energy consumption and current efficiency. Finally, a discharged NaBr concentration of133.9mg/L, which is far below the discharge standard of water pollutants for industry, can be obtained from the initial concentration of16000mg/L after the operation of BMED process for3.5h. Therefore, this process cannot only achieve the elimination of environmental pollution, but also the recycling of resources.(2) Under identical conditions, a systematic comparison is made on treating brominated butyl rubber wastewater by using BMED and conventional electrodialysis(ED). BMED has higher current efficiency and desalinization ratio, and much lower voltage drop and energy consumption. The process cost is estimated to be0.82$/kg NaBr for BMED and0.01$/kg NaBr for ED. Therefore, the BMED is a cost-efficient and environmental benignity technology which is used to treatment of brominated butyl rubber wastewater.(3) An experimental study is carried out on regeneration sodium hydroxide (NaOH) from the spent caustic by bipolar membrane electrodialysis (BMED). The results indicate that low energy consumption and high current efficiency can be achieved with the concentration of electrolyte in the range of0.20-0.30mol/L, initial concentration of the base in range of0.10-0.25mol/L and current density in the range of30-60mA/cm2. The process cost is estimated to be USD0.97for regenerating1kg NaOH with the laboratory-scale equipment, showing economically competitive. Finally, the oil in the spent caustic has hardly any effect on NaOH regeneration. The results confirm the feasibility of regenerating sodium hydroxide from spent caustic by BMED.(4) Under identical conditions, a systematic comparison is made on regeneration of sodium hydroxide from spent caustic by using BMED and electro-electrodialysis(EED). The initial concentration of base is in range of0.10-0.30mol/L, BMED has higher current efficiency and base yield, and slightly higher energy consumption. The current density is in range of30-50mA/cm2, BMED has higher current efficiency and base yield, and lower energy consumption. The process cost is estimated to be0.97$/kg NaOH for BMED and0.86$/kg NaOH for ED with the laboratory scale experimental equipment. Therefore, the BMED is a promising technology in regenerating sodium hydroxide from spent caustic with the cost of bipolar membrane decrease.The achievement of this research can provide basic data support for the application of the BMED in chemical wastewater, and lay some basis for green and environmental benign production in treating chemical wastewater as well. This research can also give some references for some other environmental benign and green chemistry production processes.