Study On Characteristics And Performance Optimization Of Direct Contact Membrane Distillation

The global water crisis has become one of the major risk factors affecting social stability.In order to reduce the adverse effects of freshwater resources on social stability,population growth and socio-economic activities In order to reduce the huge negative impact of freshwater resources on social stability,population growth,and economic activities,as one of the most important treatment technologies for seawater desalination,the membrane distillation has received widespread attention.Membrane distillation,as one of the most promising water treatment technologies,has outstanding advantages such as low operating temperature,high rejection rate,and corrosion resistance.However,compared with existing mature technologies,it has lower flux and low energy efficiency.So far,membrane distillation has not been practically applied on a large scale.Therefore,systematic research on membrane distillation technology should be carried out,and it is of great significance to provide important theoretical guidance and application foundation support for practical engineering applications.This paper takes direct contact hollow fiber membrane module as the research object,establishes its mathematical model and calculation model,and verifies the correctness of the model.The effects of membrane distillation operating conditions and physical structure parameters on membrane distillation performance parameters were calculated and analyzed.The calculation results show that the membrane flux in the counter-current mode greatly exceeds that in the smooth mode;within the range of the working conditions studied,as the feed-side inlet temperature Tf,in increases from 50?to 9?,membrane flux increased by 446%,GOR increased by 91%,thermal efficiency increased by 48.7%,and the increase of Tf,in greatly improves the three performance indicators of membrane distillation;As the membrane area Am increased from 0.597 m2 to 2.388 m2,the GOR increase was as high as 175.6%,and it can be seen that the increase in membrane area can significantly improve GOR;When the average surface porosity ? of the membrane increased from 0.3 to 0.8,the membrane flux increased by 218.8%,the GOR increased by 152.3%,and the thermal efficiency increased by 202.99%.The porosity ? greatly improved the three performance indexes of the membrane distillation.In addition to the mass flow ratio,the membrane distillation performance parameters show a monotonous increase or decrease with the influencing factors.As the mass flow ratio MFR increases from 0.4 to 16,GOR increases first and then decreases.The energy consumption of the membrane distillation process is high,and single-stage independent membrane modules cannot achieve thermal energy recovery,resulting in lower overall energy efficiency.By adding an independent heat exchanger outside,the waste heat of the permeate-side outlet product is recovered,and the feed-side inlet solution is preheated,thereby improving the energy efficiency of the membrane distillation process.The results show that:within the range of working conditions studied,when the feed-side inlet temperature Tf,in,the inlet flow min,the feed-side inlet salinity Sf,in,and the membrane area increase,the GOR of the DCMD system with an external independent heat exchanger can be greatly improved than the GOR of the independent DCMD module.The maximum increase of GOR is 379.42%,and the minimum increase is 35.26%;With the membrane area of 1.2m2 as the cut-off point,the influence of the membrane area and the inlet temperature on the feed side on the growth of GOR changed from slightly larger to the same effect.The membrane distillation balance control method can adjust the heat capacity rate on both sides by changing the mass flow rate on the feed side and the permeate side.The balance control methods used include:mass flow ratio MR=1,heat capacity flow rate ratio HCR=1,critical mass flow ratio MRc.By applying three balanced control methods to the independent DCMD membrane module for simulation calculation and analysis,the improvement of GOR by the balanced control method under different parameter changes was studied.The results show that based on the MR=1 balance control method,the heat capacity flow rate ratio HCR=1 and the critical mass flow ratio MRc can effectively improve the GOR of the DCMD process;When the inlet salinity on the MRc feed side increased from 35 ‰ to 175 ‰,the maximum GOR increase of the HCR method reached 15.11%.When the membrane area increased from 0.597 m2 to 1.791 m2,the maximum GOR increase of the MRc method reached 6.2%.The research in this paper provides basic data and theoretical reference for the development of membrane distillation model,systematic optimization research of membrane modules and membrane materials,and matching optimization research of energy supply system and membrane distillation module.