Study Of The Membrane Module And System For Solar Heated Vacuum Membrane Distillation

Membrane distillation(MD)is an emerging separation technology,which can be applied for seawater desalination.At present,low water flux and high cost are still the main barriers for the application of MD in industry.As a result,it is important to study the MD system to increase the water flux and reduce the cost.The vacuum membrane distillation(VMD)system was integrated with the solar still in this paper.And,in the work a VMD process had been designed,and a mathematic model had been built to describe the effect of the membrane module and the operation conditions on VMD performance.Based on these results,the integrated process of vacuum membrane distillation and solar still was studied in detail.The effects of some facts on the performance of VMD process were studied,including the packing fraction of the hollow fibers,the ratio of length to diameter of the module,the operation conditions and the membrane.The experiment showed that,with the packing fraction increasing,the flux of the VMD system increased to a maximum at the packing fraction of 11%,and then decreased.The flux of the VMD system decreased with the increasing of length to diameter ratio of the module while the yield of the pure water increased with the ratio of the module.Considering the yield and the flux,the optimum ratio was about4.3-5.As the inlet temperature of the feed increased,the flux of the VMD increased. It also increased with the velocity of the feed water and vacuum degree.An excellent membrane for VMD should exhibit large pore size,high porosity and small ratio of thickness to diameter.A model of heat transfer was acquired based on the conservation equation of energy,and solved by the numerical approximation method.The parameters of the membrane module were correlated in the shell-side mass transfer coefficient based on the correlations of the gas phase mass transfer coefficient.And then the model of flux was built based on the overall mass transfer coefficient.It was found that the model values were in qualitative agreement with experiment results.The model could be used for forecast flux and optimize operation of the process.The portable water yield of the integrated system in Hangzhou city was estimated theoretically,and the effects of some facts on the performance of the integrated system were studied,including the intensity of the solar radiation,connecting styles of the modules and the number of membrane modules used.It was found that the solar energy was rich in the costal area of China and the yield of the pure water in a year could reach to 954kg·m² of the solar collector area.The preferred ratio of the area of the solar collector to the area of the membrane was about 11.The flux of the integrated system varied with the solar intensity in a day,and the maximum reached 14L·m^-2·h^-1.At the same time,the performance of the integrated system was influenced little by the connecting styles of the modules.The flux of the system decreased as the number of membrane modules used increased,while the yield of the pure water increased slowly as the number of membrane modules increased.