| Membrane distillation,as a sewage treatment and seawater desalination technology,has a great perspect of development due to its high water quality.However,because of the low thermal efficiency and small permeation flux in the membrane distillation process,the commercial application of membrane distillation technology is seriously restricted.At present,the experimental study of membrane distillation only takes the permeation flux as the index,and there are few studies on energy consumption.Moreover,the theoretical study of membrane distillation mostly focuses on the study of heat and mass transfer mechanism,yet it is still not perfect.In this paper,we studied the permeation flux,specific energy consumption,mass transfer mechanism and operation stability of DCMD(direct contact membrane distillation)high salt water system,which provides a theoretical reference for the development and commercial application of membrane distillation technology.Firstly,a DCMD experimental platform for high brine content was built by using PTFE flat plate hydrophobic membrane and membrane module with spoiler plate as the core.Based on the CCD(central composite design)method in RSM(response surface method)with membrane flux and specific energy consumption as response values,hot side temperature,hot side flow rate,cold side flow rate and material concentration as influencing factors,4 factors and 5 levels of experiments were designed to carry out DCMD experiments.In addition,Design-Expert software was used for data processing to explore the relationship between each operating parameter and response value.The results showed that the R~2 values of the fitted polynomial of membrane flux and specific energy consumption were 0.9746 and 0.9738,respectively,and the P value was less than 0.0001,indicating that the fitted equation had a high degree of fit.The DCMD process was optimized according to the response surface model,and the optimal operating conditions were obtained as follows:hot side temperature 55?,hot side flow rate 0.8 m/s,cold side flow rate 0.4 m/s,and concentration 2.5 wt%.Under these conditions,the membrane flux is predicted to be10.89 g/(m~2·s)and the specific energy consumption is 523.55 kW·h/m~3.The experimental verification of the conditionsshows that the relative errors are 1.47%and6.79%,respectively,which prove that the response surface model has high accuracy.Secondly,the DCMD system was tested by timing water replenishment for a long time.No flux change and membrane wetting occurred during the experiment,which proved the feasibility of long-term operation of the whole DCMD system.Moreover,the permeation flux decreased with the increase of operation time,from 7.7g/(m~2·s)to 5.8 g/(m~2·s).Finally,three mass transfer models were selected to perform the analog calculations of DCMD under different membrane pore sizes.It has been found that pure Knudsen diffusion models independent of pore size hardly exist,so various models should be considered comprehensively when selecting membrane distillation mass transfer models.The membrane flux of DCMD shows an increasing trend with increasing pore size,and the corresponding mass transfer model can be found at different pore sizes.In this membrane distillation system,when the pore size is 0.22?m,it is recommended to use KMP(Knudsen-Molecular-Poiseuille)model or mixed model with small?value(<0.1);when the pore size is 0.45?m,it is recommended to use?-KM(?-Knudsen-Molecular)model(?value is 0.2-0.3)or KMP model;when the pore size is 1?m,it is recommended to use?-KM model(?value is 0.45-0.55).The results show that the perturbed plate membrane module can effectively improve the fluid disturbance,but the pressure loss is high,and the inlet and outlet pressure difference increases with the increase of flow rate,showing a parabolic trend. |
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