Study Of Microscale Transport Phenomena In Membrane Distillation Processes

In recent years,the research of membrane distillation(MD)has focused on the improvement of the membrane,the optimization of the experimental conditions,and the module parameters.The high thermal efficiency and easy improvement of air gap membrane distillation(AGMD)make it an important branch in MD technology.The transport behavior of AGMD is determined by the temperature difference across the membrane and the mass transfer coefficient of the membrane.The mass transfer coefficient plays a dominant role in the MD process.The change of the mass transfer mechanism and mass transfer coefficient caused by the optimization of the membrane need to be further explained in the microscale range.The temperature difference across the membrane is the driving force for mass transfer.And the optimization of the temperature difference in the AGMD module still needs investigation.In addition,the microscale interactions and diffusion between molecules in the multi-component diffusion process also need further study.In this study,the mechanism of transport behavior was investigated experimentally and theoretically.The study includes the following aspects:When molecules diffuse in the micropores,Knudsen diffusion,molecular diffusion,and transition diffusion in the microscale range are used to describe the mass transfer process.To facilitate the calculation,according to the mechanism of microscale mass transfer in the membrane pores,a detailed theoretical analysis of the mass transfer coefficient was carried out.It was found that the mass transfer coefficient is a fixed value in the commonly used operating temperature range,which provides a new approach to estimate the permeate flux.The approach was verified by changing the temperature of the feed and the coolant,the feed flow rate,the feed concentration,the width of the air gap,and the pore size.By analyzing the mechanism of microscale mass transfer,it was found that the membrane with higher porosity can provide more diffusion channels for molecules and increase the mass transfer coefficient.The microscale arrangement of the fibers on the surface of the membrane will also affect the micro-flow and change the mass transfer process.To improve the mass transfer coefficient of the membrane,the polyacrylonitrile solution was electrospun into fibrous membranes with nonwoven structure and quasi parallel fibrous structure.The hydrophilic membrane was transformed into hydrophobic by initiated chemical vapor deposition,which expands the choice of the membrane in MD application.It was found that the fiber orientation combing with the feed flow direction would increase the permeate flux by 7%.Membranes with larger pore diameter will decrease the probability of molecular collision thus decrease the mass transfer resistance.But the increase in the pore size of the membrane will cause membrane wetting.It is hard to achieve the optimal combination of porosity,pore size,and fiber arrangement on the single-structure membrane.To reduce the mass transfer resistance and increase the mass transfer coefficient without membrane wetting,a composite membrane composed of a small-pore membrane was electrospun on the surface of the large-pore supporting layer.The theoretical calculation of the mass transfer coefficient of the composite membrane and the results of the AGMD experiment showed that the surface layer has an insignificant effect on the mass transfer process.The composite membrane can increase the mass transfer coefficient of the membrane while ensuring that the membrane is not wetted.The change of membrane structure causes capillary condensation occurred inside the membrane.The capillary condensation inside commercial polytetrafluoroethylene membranes and electrospun fibrous membranes in AGMD was studied experimentally and theoretically.The factors that cause capillary condensation were taken into consideration.The results showed that capillary condensation can take place in the membrane pore.Capillary condensation does not stop MD process but decreases the driving force and permeate flux.It was found that a large decrease in the mass transfer coefficient indicates that capillary condensation has occurred according to the mass transfer coefficient-temperature curve.To increase the permeate flux and avoid capillary condensation in the membrane,the air gap needs to be optimized systematically.By filling the air gap,the influence of the effective thermal conductivity of the air gap on the permeate flux and thermal efficiency was studied.The influence mechanism of air gap parameters on AGMD transport was clarified.The results showed that the transport behavior in AGMD is dominated by the effective thermal conductivity of the air gap,which is determined by the porosity of the air gap and thermal conductivity of the filler material.The increase of the effective thermal conductivity of the air gap will increase the transmembrane temperature difference.When the ratio of the transmembrane temperature difference to the temperature difference between the feed and the coolant is 1.5%-4.5%,the thermal efficiency is optimized.While the permeate flux increases with the increase of the ratio.AGMD technology was used to treat the wastewater with multiple volatile components from hot-dip galvanizing industry.The interaction and transport mechanism of HCl and H2O in a porous membrane were analyzed in detail.The results indicated that the increase of feed temperature increases the permeate flux.The HCl concentration in the feed significantly affects that in the permeate.The presence of FeCl2 will significantly increase the activity of HCl and decrease the activity of water,thus increasing the HCl concentration in the permeate.The transport of HCl and H2O in the membrane promotes each other and follows the Maxwell-Stefan diffusion.