Application and modeling of pressure-concentration double-driven diffusion dialysis

Abstract

Diffusion dialysis (DD) is an ion exchange membrane separation process driven by concentration differences. DD is simple to operate and has excellent stability, low energy consumption, and a low installation cost. To overcome the limitations of the traditional DD process, such as low processing capacity and serious water osmosis, the authors propose a pressure-concentration double-driven (PCDD) DD process. This novel DD is double driven because the concentration difference is the main mass transfer force, and the pressure difference is the assistant mass transfer force. The common FeSO4/H2SO4 mixed solution system is used to investigate the PCDD DD process. Our first area of interest is the operating parameters' effects (the pressure difference as well as the initial H2SO4 and FeSO4 concentrations in feed) on the PCDD DD process performance. Results show that the high pressure difference can significantly promote the increment of the H+ dialysis coefficient from 0.0012 to 0.0039 m/h and the Fe2+ dialysis coefficient from 1.25 × 10−5 to 5.95 × 10−5 m/h. Pressure differences in high value can also change the direction of water osmosis (from the dialysate to diffusate). All these are employed to maintain the separation factor in an acceptable range (∼65). Moreover, the increment of the initial FeSO4 concentration in the feed can improve the H+ dialysis coefficient and reduce the Fe2+ dialysis coefficient to increase the separation factor further. However, increasing the initial H2SO4 concentration in the feed is not a commonly accepted method to improve the PCDD DD process performance. A mathematical model based on non-equilibrium thermodynamics is established to quantify the PCDD DD mass transfer process. Notably, the fitting results of the calculated fluxes based on the model and the experimental fluxes are very satisfactory. The research presented here indicates that the PCDD DD process can increase the DD's processing capacity, maintain a good separation effect, and inhibit the water osmosis simultaneously.