CFD Simulation For Nanofiltration Concentration Polarization And Its Influential Factors

Concentration polarization?CP?is one of the most important causations of membrane fouling in nanofiltration?NF?,which leads to increasing of membrane surface concentration,declination of membrane flux and elevation of applied driving pressure.In the present study,laser induced fluorescence?LIF?and micro particle image velocimetry?Micro-PIV?were used to characterize the CP at the vicinity of membrane surface,according which the characteristics of CP layer could be well obtained.Based on these CP characteristic parameters,computational Fluid Dynamics?CFD?was used to simulate the polarization process in NF,and to analyze the influential factors affecting CP.The experiments results provide direct scientific basis for understanding the polarization process in NF.In the visualized crossflow nanofiltration system,the LIF and micro-PIV technology was used to visualize the development process of the concentration polarization layer near nanofiltration membrane surface.With the assistant of tracer particles and fluorescence reagents,the velocity field and concentration field distribution during CP development were clearly visualized,and then the thickness of polarization layer was determined accurately.The results show that the thickness of concentration polarization layer was ranged from 47 to 69?m and the solute concentration near CP layer was ranged from 20.53 to 20.64 mg/L,during the filtration of 20 mg/L HA.An increased cross flow velocity reduced the CP thickness,reflected by that the thickness of the concentration polarization layer decreased from 69 to 47?m and the solute concentration in the CP layer decreased from 20.64 mg/L to 20.53 mg/L,when the cross flow velocity increased from 0.83 cm/s to 2.50 cm/s.These phenomena were mainly caued by the increased hydraulic shear force and solute diffusion velocity under a high cross flow velocity.By combination of the concentration polarization characteristic parameters into the dynamic control equation,the CFD model was calibrated to simulate the concentration polarization behavior during NF.As the concentrations of rejected solutes at different membrane locations were simulated in real-time,the varied process of concentration polarization was simulated dynamically.In addition,the influential factors affecting the concentration polarization were also analyzed.The simulation results showed that the increasing of the solution concentration led to a high mass transfer coefficient,while much more solutes perferentially deposited to the membrane surface and form thick CP layer.Moreover,the increased transmembrane pressure contributed to higher diffusion coefficient that would enhance the solute deposition rate onto membrane surface,and then result to a thick polarization layer.The results showed that in 20 mg/L HA solution,the cross flow velocity increased from 0.86 cm/s to 2.50 cm/s,and the mass transfer coefficient of the concentration polarization layer decreased from 4.0×10^-5m/s to5.5×10^-5m/s.The established CFD model was employed to simulate the impact of nanofiltration membrane morphologies to CP formation and development profiles under different operational conditions.The membrane morphological characteristics and changes would affect or destroy the polarization layer,and the circular surface morphology could improve the mass transfer coefficient in the concentration polarization layer significantly.