Analysis Of Osmosis-driven Membrane Process Based On The Continuity Of Water Inside The Membrane

The continuous water transport in the membrane during osmosis is the prerequisite for effective osmotic pressure.The destruction of continuous water transport may be another reason for restricting water flux.In this paper,it was established a mathematical model of water flux and water pressure on both sides of the membrane based on the continuous water transfer process of permeation.The effects of water pressure distributed on both sides of the membrane on the water flux were analyzed in the osmotically driven membrane processes(ODMPs),and permeation and pressure experiments were performed to verify the fitting effect.Secondly,the ultrasonic method was used to analyze the characteristics of water continuity mass transfer during ODMPs.The effects of osmotic pressure and feed pressure on water continuity mass transfer were studied.Combined with the energy loss of pressurization,it provided guidance for ODMPs process optimization.(1)To a certain extent,the suction effect of osmotic pressure causes cavitation of water in the membrane,which affects the continuous water transport,will make it difficult for the osmotic pressure to exert a driving force at the cavitation place.(2)Since the retarded pressure applied to the draw solution can changes the the osmotic pressure,it is also affected by the continuity of water.The feed pressure applied on the feed solution side is always an effective driving force for water transport,because it pushes water directly from the feed side without interacting with the osmotic pressure.Therefore,the effect of the two hydraulic pressures on the water flux is independent,(3)The air pores generated by cavitation can cause significant attenuation of ultrasonic waves.Using ultrasonic waves to analyse the cavitation process,it is found that the greater the osmotic pressure,the more serious the cavitation.Although increasing the osmotic pressure improves the water flux to a certain extent,it also seriously affects the continuous water transport,causing the continuous improvement of water flux to encounter a "bottleneck"(4)After the feed pressure P_E is applied in ODMPs,the degree of cavitation decreases and the rise speed of flux increases,so P_Eand osmotic pressure play a good synergistic action to promote the continuous water transport.This is because the application of the feed pressure helps to convert the metastable water in the membrane to a stable liquid and is not easily cavitate.(5)In order to better promote the continuous water transport in ODMPs and reduce the effect of cavitation,the application of P_E is optimized by combining the energy utilization effect and the flux improvement effect.In the conditions of this thesis,when the osmotic pressure is less than 69 bar,the selected P_E value should be greater than 1.68 bar to obtain better ODMPs performance;when the osmotic pressure is more than 69 bar,P_E is less than 1.68 bar to obtain better ODMPs performance.The optimal parameter selection area was obtained through optimization operations to assist in the selection of P_E and osmotic pressure according to the working conditions to achieve the best mass transfer process of ODMPs.