Study Of The Fouling Process And Its Effects In Membrane Distillation Desalination

Water scarcity is one of the greatest challenges facing modern society,and desalination can effectively solve this crisis.Membrane distillation is a new seawater desalination technology developed in recent years.Compared with the traditional reverse osmosis and multistage flash distillation technology,membrane distillation can effectively reduce the operating temperature and pressure.Membrane distillation is an energy-saving and environmentally friendly seawater desalination technology.In real seawater membrane distillation,a lot of fouling will be produced on the membrane surface,which will seriously affect the normal operation of the membrane distillation system.However,there is little research on membrane distillation fouling.So this paper uses the membrane distillation fouling test system to study the fouling behavior of each composition in seawater.The main innovations of this paper are as follows:(1)real seawater was used to conduct membrane distillation experiments.A ultrasonic time-domain reflectometry test system was used to conduct real-time measurement of membrane distillation fouling;(2)a model for the generation of fouling in membrane distillation was established,and the heat and mass transfer mechanism of fouling in the process of membrane distillation was explored by using computational fluid dynamics(CFD)method.This paper is mainly composed of the following parts:(1)A testing system for fouling in membrane distillation was designed and built.The influence of fouling on membrane distillation was evaluated with different feed fluid states.When only inorganic salt ions are added in the feed solution,the performance of the system declines slowly,and the water quality of the system is good after 127 h.The growth of fouling on the membrane surface is slow,and the fouling are mainly insoluble salts.Organic acids will aggravate the fouling of PVDF membrane.When the feed liquid contains organic acids,the experiment lasted for 81 h.The life of the PVDF membrane was 63.8% of that under mixed salt solution,and the fouling thickness was 1.01 mm,4.59 times that under mixed salt solution.When the real seawater is distilled,the lifetime of the system is 20.5% of that under mixed salt system under the same operating conditions,and the thickness of fouling on the film surface is 10.23 times of that under mixed salt solution.(2)The specific compositions of the seawater from Lantau island in Hong Kong were systematically characterized.In terms of their effects on membrane distillation,they can be divided into three categories: inorganic salts,various organic acids and various biomass.Two organic acids(humic acid and alginate),which have a great influence on seawater desalination were analyzed.Humic acid and alginate concentrations in seawater were 0.8 mg/L and 0.2 mg/L,respectively.Seawater contains fewer microbes than organic acids.The influence of inorganic salts on membrane distillation is mainly to wet the PVDF membrane,which increases the conductivity of production water.When organic acids are contained in the feed solution,a kind of dense gel is formed on the film surface.All kinds of biomass will settle down on the membrane surface to block the membrane pores,thus affecting membrane distillation.(3)A model for seawater desalination fouling is set up.It is a diffusion-reaction model to model the growing up of salt substances such as calcium sulfate during seawater desalination.The effects of fouling on heat and mass transfer mechanism in membrane distillation were discussed with a three-dimensional CFD model.The effects of seawater desalination fouling on membrane distillation were simulated by FLUENT software.The simulation results were consistent with the experimental results.The average relative error of the membrane flux between simulation and experiment is 9.4%.The average relative error of fouling thickness between simulation and experiment is 12.2%.(4)Using the established CFD model,the influence of fouling on membrane distillation at different feed temperature and feed flow rate was simulated.With the increase of inlet temperature,the initial membrane flux will increase,but the loss of flux will also increase after 30 h.Three feeding temperature were simulated.They were 60?,70? and 80?.When the simulation of calcium sulfate fouling run after 30 h,the fouling thickness ratio under the three feed rates was 1:1.251:1.446.When the feed flow rate is higher,the membrane flux is higher,and the fouling formation rate is relatively more quickly.Three feeding flow rates were simulated.They were 300 m L/min,400 m L/min and 500 m L/min.When the simulation of calcium sulfate fouling run after 30 h,the fouling thickness ratio under the three feed rates was 1:1.027:1.052.The growth rate of the fouling is gradually accelerating.When selecting the feed state,not only the flux of the system membrane,but also the influence of membrane fouling should be considered.