Modeling And RSM Optimization Study Of Membrane Distillation For Desalination By Using PTFE Flat Membrane

Membrane distillation is a combination of thermal and membrane separation techniques,which was early used in desalination.Comparing with other membrane separation processes,MD could operate at low pressure,which is lower than the pressure of the reverse osmosis(RO).Moreover,the capability of utilizing solar thermal energy or waste heat from other processes makes MD become an energy-saving and environmental friendly membrane separation technology.However,the membrane distillation technology has not been industrial applied due to many factors.For example,the distillation efficiency is low and the flux is small,also,the heat and mass transfer mechanism is imperfect.Therefore,in order to improve the efficiency and membrane flux,and speed up the industrialization process,it is necessary to optimize the design for membrane distillation,including process optimization and membrane distillation mechanism optimization.There are three main membrane materials for membrane distillation: polytetrafluoroethylene(PTFE),polyvinylidene fluoride(PVDF)and polypropylene(PP).Among them,PTFE is the most widely used membrane material due to its superior physical and chemical properties.PTFE membrane can be made into flat film and hollow fiber membrane.In the experiment,the flat membrane is chosen to study for its easily disassembly and cleaning.In addition,direct contact membrane distillation(DCMD)and vacuum membrane distillation(VMD)is widely studied and used technology in the four forms of membrane distillation.In this paper,experiment and simulation were carried out on DCMD and VMD processes which use the using PTFE flat membranes.Firstly,the response surface methodology(RSM)of DoE was applied for modeling and optimization of operating parameters for the treatment of Nacl solution by DCMD and VMD processes which use the PTFE flat membrane.In order to make RSM effective,the single factor experiment was carried out firstly to determine the factors that significantly affecting the DCMD and the VMD process and their optimal range on the process of DCMD and VMD for treating deionized water.Then,the software Design-Expert8.6 was applied for the design experiment on DCMD and VMD.Five levels were designed for each factor.The second-order polynomial model of membrane flux about each factor was fitted with experimental data.The accuracy of the model is verified by variance analysis.Then,the level between the factors could be determined by response surface analysis,and the DCMD and VMD processes could be optimized based on the analysis.T he results show that it is reasonable to optimize the DCMD and VMD using the fitted model,especially for the VMD process.Furthermore,it is also shown that the maximum flux can be obtained by using the response surface method,which provides a theoretical basis for large-scale integration.Finally,the VMD model was modified by analyzing the mechanism of heat and mass transfer of membrane distillation,and the iterative calculation of DCMD and VMD by EXCEL was carried out.The evaporation enthalpy did not change with temperature and the nussen diffusion model was applied for the DCMD process;the performance of membrane distillation was studied in the case of evaporation enthalpy with temperature change and no change with temperature.In this process,the Nussen-molecular model was used.Also the effect of hot side temperature,hot side liquid flow rate,cold side temperature,cold side liquid flow rate,concentration of Nacl solution and vacuum degree on membrane distillation performance were researched.The accuracy of the model was verified by the experimental results of single factor and RSM design.It was found that the value of simulated flux of DCMD and VMD were larger than the experimental value of flux under the same condition.Under the condition of low flow rate and high concentration,the experimental value of some experimental points would exceed the simulated value,but the overall trend of the two was consistent.The membrane distillation process could be accurately predicted by heat and mass transfer model established in this paper.