Study On Hollow Fiber Braided Structure Air Gap Membrane Distillation

Membrane distillation（MD）is a technique that combines the traditional distillation process and membrane separation technology,which has the advantages of high rejection rate,moderate operating conditions,and can treat high concentration wastewater,and has great potential in the fields of high salt wastewater treatment and deep concentration.However,high energy consumption has been an important factor limiting its development.Therefore,in this paper,air-gap membrane distillation（AGMD）has been studied in terms of both enhanced feed flow state and steam condensation heat transfer as follows:The feed flow state has a great influence on the mass and heat transfer in the air-gap membrane distillation（AGMD）process.For the conventional parallel-arranged hollow fiber AGMD module,the feed is in the laminar flow state at the conventional flow rate,lacking disturbance and having thick boundary layer.In this paper,a new membrane/tube braided structure is designed by braiding a PVDF hollow fiber hydrophobic membrane and a PP hollow fiber heat exchange tube with each other,and it is introduced into the AGMD module.The results showed that,the braided structure significantly reduces the critical Reynolds number for the feed’s unstable flow from2300 to 148,improves the feed flow state at a low flow rate,and weakens the temperature polarization,concentration polarization,and membrane fouling in the AGMD process.The specific electrical energy consumption（SEEC）and specific thermal energy consumption（STEC）of the AGMD process decreased by 12.7%and61.3%,respectively.In addition,to address the problem that conventional single-factor experiments cannot reflect the interaction between AGMD performance and operating parameters,a quadratic regression model was developed for the relationship between the comprehensive heat and mass transfer performance equivalent membrane distillation flux(J_AGMD)and operating parameters(hot-feed temperature（T₁）,heat exchange temperature difference（ΔT）and feed flow rate（F）of the AGMD process using the Response surface methodology（RSM）based on the braided structure AGMD module.The results showed that the interactions between T₁ and F,ΔT and F affected the J_AGMD significantly,and the best combination of operating parameters obtained was T₁=90°C;ΔT=12°C;F=10.7 L/h,corresponding to an optimal J_AGMD of 34.1 kg/（m²?h）.The vapor condensation state on the surface of the heat-exchange tube within the AGMD module affects the heat transfer process of the AGMD,which in turn affects the water production and energy efficiency of the AGMD.For this purpose,a hybrid hydrophilic/hydrophobic heat transfer surface structure is proposed in this paper.First,aqueous solutions of diethylenetriamine（DETA）were used to react with the hydrophobic PVDF-CTFE hollow fiber heat-exchange tubes.By changing the reaction conditions（amine concentration and reaction time）,hydrophilic（water contact angle of52.2°）PVDF-CTFE heat-exchange tubes were obtained with a certain mechanical strength.Then,the heat-exchange surface with a hybrid hydrophilic/hydrophobic structure was constructed by braiding the hydrophilic and the hydrophobic heat-exchange tube,and introduced into the AGMD module.Through the synergistic control of droplet migration by the hybrid surface structure,the condensation heat transfer was enhanced.The results showed that the total heat transfer coefficient,flux,GOR and J_AGMD of AGMD module with hybrid heat transfer surfaces improved by 39.4%,25.7%,78.9%,125%and 14.9%,12.2%,36.5%,53.2%compared to modules with hydrophilic surface or hydrophobic surface,respectively.In the experimental range,the maximum J_AGMD of the AGMD module with the hybrid surface can reach 51.6 kg/（m²?h）.This paper also solves the problem of not being able to directly compare the performance of the modules due to the differences in feed composition and temperature in different studies by the modified calculations.