Research On Heat And Mass Transfer Enhancement By Turbulence For Membrane Distillation And Optimization Of Multi-Effect Membrane Distillation System

Water shortage is a global problem faced by the international community.Efficient and low-cost wastewater recycling is one of the important ways to solve this problem.Membrane distillation,as a new type of water treatment technology which combines membrane separation and evaporation process,can be widely used in fields such as wastewater recycling and seawater desalination.Membrane distillation has advantages of using low-grade heat,working at the atmospheric pressure,and simple configuration.However,there are also disadvantages such as limited material selection,low permeate flux,and high energy consumption.The calculation models of membrane distillation are commonly based on empirical formulas.The influence of transmembrane vapor on the flow and temperature fields is seldom considered,resulting in deviations between the calculated and actual results.Spacers are mostly linear structures,which have a limited turbulence effect.It is commonly believed that the more disturbed flow field caused by spacers lead to higher pressure drop.The studies involving the comprehensive evaluation of permeate flux and pressure drop are seldom investigated.Research on membrane distillation systems is mostly experimental.There is a lack of theoretical analysis and economic analysis of system integration.To address these issues,membrane distillation module and multi-effect membrane distillation systems are investigated from three aspects:model construction,module optimization,and system integration.In terms of model construction,a computational model coupling steam transmembrane transport and fluid flow in membrane distillation modules is established,which improves the accuracy of the numerical simulation.In terms of module optimization,the effects of membrane properties and the co-current and counter-current operating modes on the heat and mass transfer performance are investigated.The curved spacers are proposed to enhance the turbulence inside the membrane distillation module and improve energy efficiency.The heat transfer enhancement coefficient αc is introduced to assess the heat transfer performance improvement of the curved spacers compared to the straight spacers.The comprehensive evaluation index Pec is introduced to evaluate the combined effect of heat transfer and pressure loss.In terms of system integration,a multi-effect membrane distillation system is designed and integrated.The corresponding calculation program is developed.The effects of the inlet steam temperature,inlet water flowrate,inlet NaCl concentration,cooling water temperature,and cooling water flowrate are analyzed from three evaluation factors,which are permeate flux,specific energy consumption,and concentration ratio.Additionally,an economic calculation model of the system is established to study the water treatment cost under different steam prices and concentration ratios.Addressing to membrane distillation module.the results showed that the temperature polarization coefficient increased with the decrease of feed temperature or the increase of feed flow rate.the temperature polarization coefficient decreased significantly along the feed flow direction.The optimal membrane thickness was found in the range of 13 to 18 μm which corresponded to the highest permeate flux for the selected materials.Membrane material with lower thermal conductivity and larger porosity was prone to get higher permeate flux and had larger optimal membrane thickness.The average temperature difference of the counter-current operation was higher than that of the co-current operation.The temperature gradient along the flow direction of the counter-current operation was lower than that of the co-current operation.The counter-current operation performed better than co-current operation in most cases.the efficiency of both co-current and counter-current operation decreased significantly along the feed flow direction,which suggested heat transfer loss increased.It was essential to add spacers into the membrane distillation module.Addressing to spacers,a new type of curved spacers was invented.Different distances between the spacer and the top wall were defined as d and varied at values of 0,1.5,3,and 4.5 mm.Flow distribution,temperature polarization coefficient,convective heat transfer coefficient,and permeate flux were obtained for 4 straight spacers(Sd0,Sd1.5,Sd3,Sd4.5)and 4 curved spacers(Cd0,Cd1.5,Cd3,Cd4.5).The curved spacer performed better in terms of heat flux,but at a considerable cost in terms of pressure drop.When d was 1.5 mm,ac was 1.33,which was the maximal among the studied cases.Pec varied with Reynolds number(Re)and spacer configurations.When Re was around 200,the maximal value of Pec was achieved by Sd0 spacer.When Re was in 200-550,the maximal value of Pec was achieved by Cd1.5 spacer.When Re was in 500-920,the maximal value of Pec was achieved by Cd3 spacer.Suitable spacers configuration can be selected for membrane distillation according to the range of Re.Addressing to multi-effect membrane distillation system,the results showed that as the inlet steam temperature increased,the permeate flux and the concentration ratio of 2,4,and 8 effects systems increased while the specific energy consumption decreased.The 2-effect system was most affected by the inlet steam temperature.When the inlet water flow rate ranged from 50 to 500 kg/h,the permeate flux significantly increased while the specific energy consumption and concentration ratio decreased.When the inlet water flow rate ranged from 500 to 2500 kg/h,the amplitude of increase in permeate flux was decreased while the specific energy consumption remained almost unchanged.As the inlet NaCl concentration increased,the permeate flux of the 2,4,and 8-effect systems decreased,and the specific energy consumption increased.The 8-effect system showed the largest increase in average specific energy consumption.As the cooling water flow rate increased,the permeate flux of all three systems increased,with the 2-effect system showing the greatest amplitude of increase.There were different critical cooling water flow rate values,beyond which the permeate flux no longer increased.The critical cooling water flow rate was 5000 kg/h for the 2-effect system,3000 kg/h for the 4-effect system,and 2000 kg/h for the 8-effect system.As the cooling water temperature increased,the permeate flux and concentration ratio of all three systems decreased,while the specific energy consumption kept constant.The steam price was an important factor that affected the water production cost.The cost of system using waste heat was significantly lower than that of system using steam heat sources,and the cost of the 2effect system using waste heat was 22 yuan/t.With the increase of steam price,the cost of all multi-effect system increased.When the steam price ranged from 25 yuan/t to 150 yuan/t,the cost of 4-effect system was lowest.When the steam price exceeded 150 yuan/t,the cost of the 8-effect system was the lowest.