The Mathmatical Simulation And Experiment Research Of Multi-effect Vacuum Membrane Distillation

Membrane distillation(MD)is a novel separation technology which combines low-temperature evaporation with membrane separation technology.Hydrophobic microporous membrane is used as medium.The volatile components permeate through the membrane bulk under the vapor pressure difference and are separated.MD technology has many attractive advantages,such as high separation efficiency,mild operating condition and so on.It can be used to remove volatile substances and concentrate salts.However,MD has not been widely used in industry up to now.One of the primary obstacles is high energy consumption.Multi-effect vacuum membrane distillation(VMD)processes were studied in this dissertation to recover the latent heat of vapor.The research focused on the following four aspects.1.Mathematical simulation of the VMD process based on computational fluid dynamics(CFD)The feed flowed in the tube-side of hollow-fiber membrane,while the shell-side was vacuumed in VMD.The mass transfer obeyed the combination mechanism of viscous flow-Knudsen diffusion.The conservation laws of mass,energy and momentum were used in the simulation of VMD process via CFD.The three-dimensional heat and mass transfer based on hollow-fiber membrane was modeled and calculated via Fluent software.Three-dimensional and two-dimensional distributions of temperature and flow velocity of the feed were obtained respectively.Moreover,the local temperature difference,local heat transfer coefficient,local heat flux,local mass flux and local temperature polarization coefficient along the fiber length were calculated.The influence of feed temperature,feed flow velocity,pressure in the permeate side and module length on the average mass flux and thermal efficiency were deeply discussed.The average mass flux and overall thermal efficiency increased with the increase of feed temperature.When the feed inlet temperature increased,the temperature polarization became more serious,and the mass transfer was mainly controlled by the heat transfer within the boundary of the hot feed.Improving the feed flow rate was conductive to obtain high mass flux,but the overall thermal efficiency would decrease.The heat transfer resistance mainly existed in the feed side.The mass transfer driving force could be improved by decreasing the pressure in the permeate side,and further the mass flux and thermal efficiency increased,while the energy consumption of the vacuum pump increased.The average mass flux and thermal efficiency decreased with the increase of module length,and therefore the short module was choosen for the VMD process.Finally,the VMD experimental was conducted based on PTFE hollow-fiber membrane to verify the feasibility of mathematical model.2.Mathematical simulation and economic analysis of multi-effect VMDThe equipment that a membrane module cascaded with a heat exchanger to recover the latent heat of vapor was called a single-effect VMD unit.Various multi-effect VMD processes could be set up while n single-effect VMD units cascaded together.Three multi-effect VMD processes were modeled and compared including the first-stage heating,each-stage heating and parallel flow multi-effect VMD systems.Although the each-stage heating multi-effect VMD system had a relatively high water recovery rate,gained output ratio(GOR)was low,and the system complexity and operating cost increased due to n heaters in each circle.Therefore,this dissertation focused on the first-stage heating and parallel flow multi-effect VMD systems.A first-stage heating 30-effect VMD system was simulated coupling with the mathematic model of single module in previous chapter,and an economic evaluation method was established.The effects of feed temperature,flow rate and effect number on the mass flux,water recovery rate and GOR were deeply discussed.The results demonstrated that the unit product cost mainly depended on the heat exhaustion which could be zero if there was available waste heat,and the total product cost could be decreased to 0.59$/m~3.This result could be completely competitive to the RO technique.The unit product costs for the first-stage heating4-effect VMD system was the most economic process.3.Experimental research of the parallel flow 4-effect VMD systemA parallel flow 4-effect VMD system was designed on the basis of multi-effect evaporation,latent heat was recovered to heat the feed.The feed in this process only flowed in each effect and could not flow to next effect.The permeate sides of the hollow fibers cascaded together.The produced vapor of each effect was used as the heating vapor of next effect.The operating parameters of each unit were same to that of the single-effect unit.The experimental results demonstrated that the temperature difference between each effect increased with the increase of feed temperature and decreased with the effect number;the higher the feed temperature,the more the optimal effect number.High feed temperature,low flow rate and pressure in the permeate side were conductive to improve the mass flux,water production rate and GOR.The heat efficiency increased with the increase of feed temperature,and decreased with the increase of pressure in the permeate side and the flow flow rate.4.Design and fabrication of multi-effect vacuum membrane distillation deviceOn the basis of multi-effect VMD simulation in previous chapters,a pilot plant of 4-effect vacuum membrane distillation was designed and fabricated.The device control system consists of on-site inspection instrumentation,programmable logic controller(PLC)and industrial control computer(IPC)monitoring mode.In addition,it can control logity,collect data and save information.The effect of different operating conditions on the MD process and performances of hollow fiber module can be tested in this device,which also can be used to optimize the design and manufacture and provide technical validation and basic data for large-scale membrane distillation plant.