Applied Research On Ion Exchange Membrane-based Concentration Difference Dialysis

Ion exchange membrane-based concentration difference dialysis technology is based on the concentration gradient instead of external electric field or pressure as a driven force. Due to the operation simplicity, low energy consumption and non-secondary pollution, concentration difference dialysis has attracted more and more attention and has found various applications in resource recovery, clean production, environmental protection and energy conversion. The concentration difference dialysis technology based on ion exchange membrane (IEM) mainly includes of diffusion dialysis (DD) and reverse electrodialysis (RED).A new spiral wound diffusion dialysis (SWDD) membrane module has been developed to overcome the inherent drawbacks of traditional plate-and-frame diffusion dialysis module. The SWDD module has several advantages.1) Compact structure and small volume leads to easy installation and transport; 2) Benefited from the specific spiral structure, it could facilitate the sufficient mass transfer; 3) More importantly, the integration with other reaction and separation technology leads to more industrial applications. However, the study on SWDD, especially the optimization and application, has been rarely reported so far in the literature. Therefore, further investigations on the mass transfer of inorganic acid by SWDD membrane and the connection mode of module is of practical significance, which is conducive to greater industrial applications. RED is another spontaneous IEM-based concentration difference dialysis process. Unlike traditional membrane separation process, RED can extract sustainable clean energy from a mixture of two solution. However, the limitations in RED membrane performance, process mechanism and practical water conditions result in very low energy-conversion efficiency. RED is not used in large-scale application. In the view of these concerns, it is of great significance to improve the self-optimization and application field of RED technology for its development and the social needs.By taking advantage of the spontaneous dialysis of ion exchange membrane, both the self-optimization and integrated application of SWDD and RED were investigated to achieve the sustainable development of environment protection and energy recovery. The main results are concluded as follows:1) A common dicarboxylic acid sulfate was selected to investigate the acid recovery of SWDD membrane module. The effect of operating variables on SWDD diffusion performance were investigated and a mathematical model was established. The experimental results showed that the acid recovery ratio increased with the number of membrane modules but decreased with the flow rate, the initial feed concentration did not affect acid recovery ratio apparently. The diffusion performance of double SWDD membrane modules was depended on connection mode greatly. For double SWDD membrane module systems, mode D-d (series connection in both acid side and water sides) was the best mode in terms of the acid recovery ratio and recovered acid concentration. For multi-modules system, its treating capacity was not the linear additivity of individual module, and much higher than their total sum, for example, the processing ability of the D-d or T system is respectively 3 or 8 times that for S system. In addition, both the recovered acid concentration and the dialysate acid concentration are proportional to feed concentration for different number of SWDD modules.2) Based on the above results, the D-d connection mode of SWDD was integrated with the production process of iron oxide yellow pigment, to remove a large amount of sulfuric acid generated in the reaction process of air oxidation. The linear regression equation as function of by-product sulfuric acid production quantity and time was established. An experiment was successfully implemented with corresponding simulated producing acid treating by SWDD, and the consistency of the flow velocity and feed processing capacity was established. The dissolution of existing problems in the industrial application had also been proposed. Such integration combines advantage of "zero energy" and "zero emissions", which offers a new insight into the application of SWDD for separation.3) In-situ integration of RED and ED was designed to construct a power free electrodialysis (PFED). The related factors for single RED including velocity, concentration difference and water sources of LC compartment were optimized. It was found that PFED demonstrated a very low freshwater conductivity of 72 μS cm-1 under intermittent operation mode, while that of 736 μS cm-1 under continuous operation mode. The desalination performance in simulated seawater-wastewater system had no significant change with NaCl water solution system under intermittent operation mode. Therefore, this PFED desalination process realizes zero discharge of high salinity wastewater as well as economic benefits.4) A bipolar membrane reverse electrodialysis (BMRED) was formed by inserting a bipolar membrane in RED membrane stack. When bipolar membrane replaced the lateral membrane of RED membrane stack, the production rate of acid-base became faster, but it gave rise to base or acid generated in the electrode compartment. When the bipolar membrane was transferred in the middle of RED membrane stack, water chamber concentration difference and current density played a key role in the acid-base production rate. In addition, seawater/river water system was more suitable for acid-base production of BMRED process, and the acid production rate could reach as high as 6.9 m mol h-1.