| Industrial production is a large consumer of energy and water.Under the background of the current "double carbon" strategy,industrial waste water has attracted extensive attention due to its rich waste heat resources.Membrane distillation is a low grade thermal driven water treatment technology,and the theoretical rejection rate can reach 99.9%,which has certain advantages in the treatment of low temperature and high salt industrial wastewater.After decades of development,membrane distillation technology has made great progress,but low efficiency,low flux and other problems restrict its large-scale development.Therefore,it is necessary to continue to strengthen the research of membrane distillation technology,improving the economic feasibility of membrane distillation technology has become one of the current research hotspots.In this paper,a new optimization mode and superhydrophobic modification technology were proposed from the perspectives of system operation and membrane modification,aiming to improve the performance of membrane distillation system by combining operating conditions and membrane characteristics.The main research contents and results of this paper are as follows:Firstly,this paper takes direct contact membrane distillation as the research object,and establishes a two-dimensional heat and mass transfer model through Matlab programming.In order to verify the reliability of the model,the feed temperature and flow rate(tf、tc、vf、vc)on both sides of the cold and hot sides were used as variables for basic membrane distillation experiment.To evaluate the membrane distillation system comprehensively,objectively and effectively,multi-objective optimization of the membrane distillation system is carried out by combining the entropy-weighted-TOPSIS model,taking permeate flux,thermal efficiency and exergic efficiency as indexes,and quantifying the weight of each indicator under different working conditions.The optimal working conditions were obtained by ranking according to the normalized composite score(RC).multi-objective optimization ranking for different working conditions of the membrane distillation system.It is found that the optimal working conditions were as follows:the inlet temperature of hot side and cold side were 70℃ and 15℃,the flow rate of hot side was 0.29 m·s-1 and 0.424 m · s-1.Under this working condition,the RC value reaches the maximum,which is 0.27.The water flux,thermal efficiency and exergic efficiency were 24.92 kg·m-2·h-1,64.65%and 25.97%,respectively.The water flux and thermal efficiency are also the highest in this condition.Compared with the single objective optimization result of exergic efficiency,although exergic efficiency was 52%lower,water flux and thermal efficiency were 277%and 27.19%higher,respectively.Secondly,based on the advantages of ZIF-8 morphology,controllable particle size,easy preparation and good hydrothermal stability,ZIF-8 material was selected for membrane superhydrophobic modification.Using methanol as solvent,zinc nitrate hexahydrate and 2-methylimidazole as raw materials,the effects of the feeding ratio of Zn2+/2-Hmim,concentration of synthesis solution and synthesis temperature on the particle size of ZIF-8 were studied through the normal temperature synthesis route.By controlling the synthesis parameters,the relative size of nucleation rate and growth rate of ZIF-8 crystal can be changed,thus the size of ZIF-8 particles can be controlled.The results show that the smaller the feed ratio,the higher the concentration of the synthesis solution and the higher the synthesis temperature,the larger the particle size of ZIF-8 prepared.The conditions of the two kinds of ZIF-8 particles are as follows:the concentration of the synthetic solution is 0.1 mol/L,the reaction temperature is 30℃,and the feeding ratio of micron and nano ZIF-8 is 1:3 and 1:6,respectively.According to the DLS characterization results,the particle sizes of micro and nano ZIF-8 particles are about 1310 nm and 266.6 nm,respectively.Finally,ZIF-8 was modified with 1H,1H,2H,2H-perfluorooctyltriethoxysilane(POTS)to reduce its surface energy.The hydrophobically modified micro and nano ZIF-8 particles were deposited on the surface of PTFE membrane by one-step spraying method to construct the micro/nano hierarchical roughness structure.To obtain the preparation conditions for constructing a reasonable micro-nano hierarchical structure of double-scale ZIF-8,the effects of double-size ZIF-8,micro-nano ZIF-8 doping ratio,1H,1H,2H,2H-perfluorooctyltriethoxysilane(POTS)amount and ZIF-8 particle dosage on hydrophobicity were investigated.It was characterized by SEM-EDS,AFM,WCA and other means.The results show that under the conditions of S(膜)=7x7 cm,M(z-6)=1:1,M(POTS):M(ZIF-8)=1:1,M(ZIF-8)=0.8 g,the roughness of modified film reaches the highest(Ra=364 nm),and the contact Angle reaches 165.5°.Finally,membrane distillation experiments were carried out to investigate the properties of modified and original PTFE films.First,using deionized water as raw material,the performance difference between modified film and PTFE original film was investigated under different working conditions.The first five working conditions obtained by entropy-topsis model were used for membrane distillation experiment.It was found that the water flux of the modified film was higher than that of the original PTFE film,and the highest water flux reached 29.11 kg-m-2·h-1,with a growth rate of 16.8%.Different concentrations of NaCI solution were used to simulate industrial wastewater to investigate the performance difference between the modified membrane and the original PTFE membrane in the treatment of high salt water thenn.The water flux of the modified membrane was always higher than that of the original PTFE membrane,with a growth rate of 37.6%-53.8%.With the increase of NaCI solution concentration from 0 wt%to 15 wt%,the water flux of the original PTFE membrane decreased by 33.96%,while that of the modified PTFE membrane only decreased by 19.58%.The results of this study provide theoretical guidance for membrane distillation technology to achieve the balance between low energy consumption and high flux,which helps to improve the economic feasibility of membrane distillation technology and promote the industrial development of membrane distillation technology in the field of industrial wastewater treatment. |
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