Electrothermal PVDF Hollow Fiber Membrane And Its Membrane Distillation Process

Membrane distillation?MD?has now been becoming a very promising desalination technology owing to its many advantages such as theoretical 100%rejection and ability of utilizing low-grade waste heat.However,traditional MD performance is offset by temperature polarization?TP?,a phenomenon intrinsically related to water evaporation.There is a temperature difference between the feed bulk and membrane surface where vapor-liquid transition occurs,and the evaporation lowers the temperature at the interface of membrane,which further decreases the driving force of the process.In this paper,we introduced heating elements into hollow braided tube and prepared them into polyvinylidene fluoride?PVDF?hollow fiber membrane with electro-thermal effect,which is to suppress the TP.First of all,poly?tetrafluoroethylene-cohexafluoropropylene??FEP?yarns and heating element--nichrome resistance wire?NRW?were braided into hollow tube via a two-dimensional machine.Then,a PVDF casting solution was homogeneously coated on the outer surface of the braided tube,and the electro-thermal hollow fiber membranes?PVDF/NRW HFMs?were obtained finally by dry-wet spinning method.After that,the electro-thermal performance of PVDF/NRW HFM was characterized.Results showed that the PVDF/NRW HFM surface temperature increased from room temperature to 70? by applying a pretty low direct current?21.0V,0.15 A?.Then,the vacuum membrane distillation?VMD?performances were tested under different feed bulk temperatures and applying currents.In comparison to the membrane without electricity applied,the electro-thermal membrane?a current of 0.15 A?showed up not only about 2.5-times enhancement in permeate flux,but also a half of specific energy consumption(SEC=11.86 k W·h·L^-1),while the salt rejection maintained above99.0%.Meanwhile,the value of temperature polarization factor?TPF?was higher than100%owing to the electro-thermal effect,which demonstrated the effective suppression of temperature polarization.Based on electro-thermal HFM,we introduced carbon fiber?CF?into braided tube owing to its excellent heating effect and prepared PVDF/CF HFMs.In the meantime,the dioctyl-phthalate?DOP?was added into PVDF casting solution to regulate membrane pore structure.The effect of the added DOP on the porosity of membrane was investigated,which was showed that the porosity increased from 33%to 50%when the addition of DOP increased from 0 to 9 wt%.VMD test results showed that a twice increase of permeate flux and still a good rejection?>99%?when the current was 0.20A compared to a current of 0 A.At last,the membranes'stability of VMD performance were tested.When applied a current of 0.15 A,the performances of membrane stayed stable after a 36-hour VMD operation with the DOP addition less than 6 wt%.