Study On Process Enhancement And Simulation Of Vacuum Membrane Distillation

Membrane distillation (MD) is a kind of membrane separation technology, which is used in many areas more and more widely. It is a special effective process which applies for seawater desalination by membrane distillation technology. But the studies on MD are still in the form of laboratory and the industrialized level is very low just because of its lower permeation flux over a long period of times. So flux enhancement by effective measures and preparations of the excellent property of high permeability hydrophobic porous membrane become the focus of research in membrane distillation process.The studies on vacuum membrane distillation (VMD) NaCl aqueous solutions are carried out experimentally using the tubular carbon membrane. Experiments are conducted to compare the performances of inside/out and outside/in tubular carbon membrane module. The influences of operating conditions such as feed liquid temperature, feed liquid flow and permeate-side vacuum degree on permeation flux are investigated. The results show the permeation flux of inside/out is higher than that of outside/in at the same operating conditions. The inside/out operational modes is preferred for the following application. Two methods are performed respectively for membrane distillation process enhancement. One is with turbolator, and the other with two-phase flow. Compared with no enhancement, the flux increased by 7%-50% and 16%-66% respectively. For the operation with two-phase flow, nitrogen as the secondary phase companied with feed liquid flows through the membrane module. The influences of feed liquid temperature, gas flow, permeate-side vacuum degree and membrane orientation on permeation flux are also studied experimentally.Based on the experiment results, a heat and mass transfer model for vacuum membrane distillation is modified. The obtained results are compared with experiment results. The heat transfer coefficient and temperature polarization phenomenon of vacuum membrane distillation are analyzed. Then a model for describing the heat and mass transfer of MD enhancement process is presented. The fluid flow channel has been changed when the turbolator is filled. The equivalent diameter of flow channel is calculated. Results are obtained combined with operation parameter. A two-phase flow MD model is described that combined two-phase flow and MD heat transfer process. The heat transfer coefficient and temperature polarization coefficient of MD enhancement process are computed and analyzed. At last a novel concept, "enhancement coefficient" of MD process is proposed to describe the effect of different enhancing method. The analysis results show that the transfer process in the membrane pores is the control step.