Study On Mechanism Of Vacuum Membrane Distillation

Membrane separation is a kind of new technology which can be used for separation, concentration, purification and cleanse. Membrane distillation is an important branch of membrane separation technology which employees a hydrophobic microporous membrane for the separation process. During MD process, feed contacts with one side of membrane, after water evaporates on hot side, vapor transfers to cold side under pressure drive, where vapor is condensed on cooling wall and then collected or removed. Vacuum membrane distillation (VMD) which possesses of vacuum pump to provide enhanced drive force and air gap similar to air gap membrane distillation is a kind of separation technology of membrane distillation. A brief introduction of VMD process is presented. Mass and heat transfer behavior through hot side, membrane as well as cold side during VMD process is analyzed and derived. Theory calculation and derivation demonstrate that the heat conduction of membrane exerts little influence on performance. The characteristics of mass transfer through membrane are mixed flow including Knudsen flow and Poiseuille flow, which moreover, vary with operational variables.Air permeation experiment, VMD of distilled water and VMD of solution containing Cr3+ are carried out. The results indicate that the dominant resistance during membrane process comes mainly from evaporation of water on hot side and mass transfer through membrane, and that mass transfer and heat transfers during VMD process are interdependent and interactive; The results also indicate that the phenomenon of temperature polarization and concentration polarization is inevitable during VMD process. Results of VMD experiments indicate that flux increases when feed temperature, feed rate and vacuum degree increase. But the increase of flux becomes slowly when feed rate reaches 60L.h-1; Flux increases with pore size rapidly, but rejection approaches zero when pore size reaches 0.5m. On the basis of above theoretical derivation and analysis on experimental results, by correlating solvent (water) transfer with solute (salt) transfer on hot side and correlating mass transfer with heat transfer during VMD and introducing a contribution coefficient which can reflect the characteristics of mass transfer through membrane, a new model of mixed flow is presented. In order to examine the model, a computerizing program with witch can realize cycle calculation is programmed in VB6.0, The comparison between experimental results and theoretical calculations indicates that the computerizing results agree with experimental results well in a large scope.