Mass Transfer Intensification And Scaling Mitigation In Bubbling-enhanced Membrane Distillation

Membrane distillation process is a membrane separation technology driven by vapor pressure difference,which has broad application prospects in the fields of high-salt wastewater treatment,fruit juice concentration and seawater desalination.Compared with the traditional pressure driven membrane processes,membrane distillation technology has the advantages of low energy consumption,high rejection rate and low operating pressure.However,the phenomena of temperature polarization and concentration polarization in the process of membrane distillation will increase the operation energy consumption,and the problems such as membrane surface scaling in the concentration of high concentration liquid will also reduce the stability of the system and limit its industrial application.By introducing gas into the membrane distillation system and using bubbling to generate shear force on the membrane surface,it can effectively destroy the boundary layer,slow down the polarization phenomenon and delay the crystallization of the membrane surface.This paper aims to explore the working mechanisms of gas bubbling on mass transfer intensification and scaling alleviation for direct contact membrane distillation high brine concentration.Firstly,the mass transfer enhancement process of direct contact membrane distillation by bubbling was studied.Discuss the enhancement effect of bubbles on membrane distillation mass transfer under different feed temperature,feed solution concentrations,gas flowrate and feed solution flowrate.The results showed that the mass transfer enhancement effect of bubbling on the system with low feed temperature and high feed concentration is more significant under the same bubbling intensity.The size of bubbles increases with the gas flowrate,while the rising speed of the bubbles first increases and then decreases with the bubble size,and the flux enhancement ratio first increases and then decreases.The bubbles with area range of 50 to100 mm² have the fastest rising speed the strongest disturbance to the membrane surface stagnant layer,the best mass transfer enhancement effect,and the best enhancement effect is 2.5 times.With the increase in feed flowrate,the shearing effect of the screen on the bubbles increases,bubbles become smaller,and the optimal gas flowrate increases.The results of the study on the mass transfer enhancement effect of bubbling on both sides of the permeable membrane distillation membrane showed that the bubbling enhancement effect on the feed side is better than that on the permeate side.Because of the existence of concentration polarization on the permeate side,the flux enhancement ratio of bubbling on the feed side of permeate membrane distillation is lower than that of direct contact membrane distillation.The flux enhancement ratio of direct contact membrane distillation decreased from 2.50 times to 1.97 times that of osmotic membrane distillation.The scale inhibition effect of bubbling on the concentration of high salt solution was investigated.The results demonstrated that bubbling can improve the water flux in the concentration process of salt solution,delay the time of crystal deposition on the membrane surface,and significantly slow down the growth rate of scaling layer.By comparing the scale inhibition effect of bubbling in nearly saturated CaSO₄ and Na Cl systems,it is found that the shear force produced by bubbling has a better effect on slowing down the formation of large-size needle like CaSO₄ crystals.However,for Na Cl cubic crystals with small particles,a relatively large gas flow rate is required to achieve the scale inhibition effect.