Mechanism And Application Study On High Flux Air Gap Membrane Distillation System

Air gap membrane distillation (AGMD) is an innovative membrane distillation technique. AGMD separation process not only has the advantages common in membrane distillation system, such as using low grade energy, operating under low temperature and pressure, yielding highly purified permeate and no chemical action in distillation solution, but also has its own advantages that is distillation solution can be collected alone and its thermal efficiency is high because its permeate vapor is not direct contact with cooling liquid. Consequently its application is more widespread than other membrane distillation and separation technique, especially in corroding solution extraction, radiate wastewater treatment, sea water desalination, subterranean water treatment and drinking water production. The main disadvantage of AGMD is low membrane permeate. If we can develop a new high flux air gap membrane distillation module in which membrane flux is close to or exceeds the flux of direct contact membrane distillation, it will be more valuable to commercial application of AGMD.There are two methods to improve the flux, one is to develop new membrane material, the other is to design new membrane module which can provide high heat and mass transfer efficiency. In this paper, a new high flux air gap membrane distillation module is developed. Its characteristics are adopting a rotation and tangential inlet pipe in the module to improve the temperature and concentration polarization, and lessening the air gap width to decrease the resistance of mass transfer in the air gap and improve the efficiency of heat transfer. The mechanism of heat and mass transfer of the high flux membrane distillation module is studied in this paper and the mathematic model and optimum design methods for this new membrane distillation module are also researched with experiment demonstration.1.Improve and perfect the high flux membrane distillation experiment system. Based on the former experiment system, the flow regulation range is enlarged in this new experiment system. At the same time, the refrigeration cycle and control system for experiment system is developed to solve the problem of unstable temperature in cooling side. The adjustable multipoint temperature test devices that install the intelligent measure instrument and computer system to collect and manage data are adopted to improve the temperature test and the whole experiment test precision.2.Develop a new style high flux membrane distillation module. The rotation and tangential inlet flow membrane module is designed and manufactured. The temperature polarization and concentration polarization in this module are reduced thus the heat and mass transfer efficiency and flux are increased. The V grooves are machined in the surface of cooling plate to make vapor condense rapidly and condensed water flow out alone grooves more rapidly. Lessen air gap width to make membrane contact partly with cooling plate so the mass transfer resistance is reduced and flux is improved distinctly.3.Experiments proved that the ideal experiment result will be obtained when 70%-80% membrane surface contact with cooling plate under some rotation and tangential inlet conditions. The permeate flux of the new module was about 120kg/m2.h with water as feed solution, 1mm air gap width and temperature difference up to 65℃. However the maximum flux of conventional air gap membrane distillation system is 28 kg/m2.h with air gap width 4mm. In this paper the same results are obtained when using 15 wt% salt solutions. The ultrasonic technique is also adopted to enhance the membrane distillation process and improve membrane permeate flux. The experiment results showed that ultrasonic technique is of research and application value to membrane distillation. The flow field in hot solution cavity is simulated by software of Fluent. The influence of tangential inlet pipe and position of inlet pipe on flow field near the membrane are compared. It indicated that tangential inlet flow increased the scour for membrane surface therefore the temperature and concentration boundary layer were destroyed and membrane flux was increased.4.A new heat and mass transfer model for high flux membrane is proposed in this paper. The characteristic of this model is to taking the effect of vapor condenses in air gap and membrane contact with cooling plate on heat and mass transfer into consideration. The error between flux predicted by this model and experiment results is less than 10% under six different air gap width and membrane and cooling plate contact degree. The prediction of condensate in air gap agrees with the measurement data. That indicates the heat and mass transfer model is correct. 5.The solar driven membrane distillation system for desalination is also prepared beforehand in this paper. In order to make membrane distillation technique enter the commercial market from laboratory and to solve the problem of freshwater shortage in western area where there is no electricity, the solar driven membrane distillation system is proposed which can use low grade energy as driven power. The assemble system applied solar photovoltaic system, solar heating system and solar refrigeration system to membrane distillation system. The mathematic model and simulation model of every system are also proposed in this paper.The simulation system of solar driven membrane distillation is studied by software Labview. The comparison result showed that the error between simulation and experiment data is less than 10%. It is proved that the simulation system has good prediction and generality. The design idea and design method for the real assemble system are perfected through building the simulation software for solar driven membrane distillation system. So the exploratory research was done for the future work.