Experimental Study On Liquid Desiccant Regeneration Based On Vacuum Membrane Distillation

In the field of heating,ventilation and air conditioning,salt solutions are increasingly used as a medium for heat and mass exchange with air due to their excellent physical and chemical properties.As the heat and mass exchange between the solution and the air proceed,the concentration of the solution will continue to decrease,and the original performance of the solution will change accordingly.Therefore,to restore the original performance of the solution,the solution needs to be regenerated.Existing solution regeneration methods urgently need to be improved or replaced due to the shortcomings of high energy consumption,low thermal efficiency,low regeneration efficiency,and complex equipment.Vacuum membrane distillation technology is widely used in the field of material separation and purification due to its large membrane flux,high thermal efficiency,and simple equipment.This paper attempts to apply it to the solution regeneration in the HVAC field,and the feasibility is verified through theoretical and experimental research.This paper deeply analyzes the heat and mass transfer mechanism of the solution regeneration process by vacuum membrane distillation,determines the heat and mass transfer coefficient in the solution flow boundary layer and Knudsen diffusion-Poiseuille flow as a theoretical model for transmembrane mass transfer,and summarizes the physical properties of the two commonly used liquid desiccant,Li Cl and Li Br.A liquid desiccant regeneration experiment by vacuum membrane distillation for Li Cl and Li Br liquid desiccant was carried out.The experimental results show that the membrane fluxes of the two solutions increase with the increase of the liquid desiccant inlet temperature,flow rate and the vacuum degree of system,and decrease sharply with the increase of the liquid desiccant concentration.In the experimental range,the membrane flux of Li Cl liquid desiccant can reach up to 7.18 kg/(m~2·h),and the membrane flux of Li Br liquid desiccant can reach up to 8 kg/(m~2·h),even if Li Cl and Li Br liquid desiccant is at a high concentration of 40%and 50%,the membrane flux can still reach1 kg/(m~2·h)and 0.998 kg/(m~2·h);The thermal efficiency increases with the increase of the liquid desiccant inlet temperature and the vacuum degree of system,and finally tends to be constant,and decreases with the increase of the liquid desiccant flow rate and concentration.The thermal efficiency in the experimental range is generally higher than 80%,and the thermal efficiency of the Li Cl liquid desiccant can reach up to 94.76%.The thermal efficiency of Li Br liquid desiccant can reach up to 97.12%;The experimental value of the transmembrane mass transfer coefficient increases slightly with the increase of the liquid desiccant inlet temperature and the vacuum degree of system,first increases and then tends to be constant with the increase of the liquid desiccant flow rate,and decreases sharply with the increase of the liquid desiccant concentration;The temperature polarization effect increases with the increase of the liquid desiccant inlet temperature,flow rate,and the vacuum degree of system,and decreases with the increase of the liquid desiccant concentration.When the liquid desiccant flow rate is 0.6 m/s,the performance parameters of the two liquid desiccant can reach a higher level.In the experiment,the conductivity of the distillate under all operating conditions is less than 120μs/cm,and the rejection rate is close to 100%,which effectively guarantees the zero loss of solute during the liquid desiccant regeneration process.By establishing a mathematical model of the vacuum membrane distillation process in the hollow fiber membrane,the membrane fluxes under each experimental operating condition were simulated.The simulation results show that the theoretical and experimental values of the membrane fluxes of the two solutions are in good agreement,and the maximum deviations are both within±15%;The distribution and variation of the temperature of the main body of the liquid desiccant and the temperature of the membrane surface in the hollow fiber membrane along the flow direction of the liquid desiccant under various experimental operating conditions were analyzed by simulation,as well as the influence of the length of the hollow fiber membrane and the structural parameters of the membrane material on the membrane flux.The research results of this paper prove that vacuum membrane distillation is a very potential solution regeneration method.Under optimized operating conditions,each regeneration performance can reach a high level,which is of great significance to solve the current solution regeneration problem in the HVAC field.