Studies On Heat Recovery In Vacuum Membrane Distillation

The fresh water is so important to human beings in the21st century just as the oilto the human in the20th century and water treatment are regarded to be the biggestindustry. Unfortunately fresh water resources are scarce and even worse, wasunevenly distributed, furthermore the exploitation of groundwater resources resultedthe destruction of the geological environment, followed by many environment issueswhich cannot be restored. All the conditions above urged us to find new low-cost andeco-friendly approaches to obtain the fresh water, and the desalination of seawater andindustrial wastewater, as well as wastewater treatment have become an effective wayto expend the fresh water resources.Vacuum membrane distillation technology, an emerging membrane separationtechnology, has been being of great concern due to the following advantages:Theoretically100%desalination rate, available of low grade heat energy, compactmembrane module structures and high thermal efficiency. This thesis focuses onprocess improvement of vacuum membrane distillation(VMD) system to achieve therecycling of system heat, and also the effect of operating conditions on permeate fluxand heat recovery such as, feed temperature, feed linear velocity, vacuum degree. Theexperiment results reveal that there are good consistency on the influence of threeoperation parameters on the permeate flux between the level one, level two and levelthree heat recovery type VMD distillation system equipped with the correspondingpolymer hollow fiber heat exchangers.The permeate flux increased with the increasing of feed temperature, feed linearvelocity and vacuum degree abide by different increasing curve rules. The system heatrecovery efficiency was analyzed comprehensively based on the experiment results oflevel-one, level-two and level-three heat-recovery VMD systems. At high flow rate,the feed temperature will increase after flowing through heat-exchange modules, andan amount of water can be obtained also, which demonstrate the heat-recoveryvacuum membrane distillation system we designed can effectively recycle the steamlatent heat in vacuum membrane distillation process. The values of heat recovery areused to evaluate the effect of level-one, level-two and level-three heat-recovery VMDsystem. The increase of the level can improve the heat recovery, while the permeate flux will decrease. Experiment results reveal permeate flux and heat recovery is aninterrelated and mutually contradictory unity, and the optical conditions deservedfurther research.This thesis laid the foundation for the study of heat recovery in VMD system,accomplished preliminarily reclamation of the vapor latent heat energy in vacuummembrane distillation process, and reduced heat consumption rate of water productionfinally.