Process Optimization And Scaling Potential Prediction Of Pilot Nanofiltration Softening System

With the rapid development of the domestic economy, freshwater resources has become an important part of the economic developing. While the water interbasin transfer and traditional water saving measures can no longer adapt to the present stage of the actual production and national development needs. Nowadays seawater desalination technology was an effective way to solve the problem of water shortage. In the field of seawater desalination, how to reduce energy consumption as well as the investment of desalination technology as far as possible decides the further extension of desalination application.Seawater is a kind of very complex electrolyte solution with high salt concentration, it is easy to scale in the desalination process, and the caused membrane fouling is inevitable. Scaling increases the membrane cleaning frequency, reduces the membrane life and increases operating costs.In this work, Pitzer theory of high concentration electrolyte solution was used to calculate activity coefficient of different ions at high concentration. We also use the concentration polarization model to predicte the scaling tendency of sparingly soluble salt at various operating conditions. We investigated the effect of nanofiltration membrane modules arrangement, flow rate, operating pressure, et. al, on organic removal rate, rejection of different ions, energy consumption, supersaturation index (SI); the influence of circulating flow rate on the performance of nanofiltration system were also discussed. The results are as follows:1. The activity coefficients of scaling ions were calculated by using Pitzer’s theory, considering seawater as mixed solution of sodium, potassium, calcium, magnesium, sulfate, and chlorine ions. The prediction model of the scaling tendency of the ions on the membrane surface was established combined with the concentration polarization model2. Removal rate of organic substance of 2 NF modules,4 NFmodules is relatively high, mostly about 90%, while that of 6 modules membrane is lower than other modules relatively. The ion rejection of 2 NF modules is higher than 4 NF modules, and the ion rejection of 6 NF modules is lowest. The divalent ion rejection rate of nanofiltration membrane is relative high, so the nanofiltration could remove divalent ion efficiently. The energy consumption of 2 modules membrane is significantly higher than 4 and 6 modules nanofiltration membrane.3.Two ways of improving recovery was investigated, one way is by increasing the pressure (Recovery mode 1), another way is by improving circulation flow (Recovery mode 2). In Recovery mode 1, the permeate flow rate is increasing with recovery. But in Recovery mode 2, the permeate flow rate is decreasing with recovery. The concentration polarization factor in Recovery mode 1 is higher than Recovery mode 2; the ions rejections in Recovery mode 1 are higher than those in Recovery mode 2; energy consumption in Recovery mode 1 is higher than that in Recovery mode 2 at low recovery section, however, it showed a reverse trend at high recovery.4. Scale tendency of the end module. For Recovery mode 1 and Recovery mode 2, the scaling trend prediction is increasing with recovery, the scaling trend prediction of 4 module is larger than 6 module at same recovery.