Preparation Of Hollow Fiber Membrane For Seawater Desalination And Improvement Of Performance Testing Methods

With the needs of production and life,the contradiction between the existing amount of fresh water and the demand of human beings is increasing day by day.Desalination can alleviate this contradiction to a certain extent.Membrane distillation seawater desalination(MD)is a rapidly developing technology in recent years.It is a heat-driven membrane seawater desalination technology.Comparing with traditional seawater desalination,membrane distillation can use low-grade energy and effectively reduce operating temperature and pressure.It has received extensive attention by scholars.In the process of seawater desalination operation,not only the pollutants in the seawater will reduce the performance of the membrane material itself and reduce the permeation flux,but also because the membrane material is damaged during the operation,the components need to be maintained or even out of service,so the performance of itself is a key factor restricting the efficient and stability of MD.Comparing with flat membranes,hollow fiber membranes have a larger packing density and are self-supporting,but current membrane materials have some defects such as poor mechanical properties,small contact angle and small diameter control range.Although at the present stage,many scholars have modified for poor mechanical properties and small contact angle,most of the modification is by external means instead of targeting the base film.In addition,the control range of pipe diameter is less improved,which limits the application of hollow fiber membrane.Therefore,it is necessary to change the structure and performance of base membrane and improve the regulation range of tube diameter.Permeation flux is an important parameter to measure the performance of membrane distillation seawater desalination,but the accuracy of the existing measurement methods is poor,so it is necessary to optimize and modify the existing test methods to improve the test accuracy.The main work of this article is developed from the following aspects:(1)An ultra-thin hollow fiber membrane for seawater desalination was prepared.The membrane thickness of the membrane is about 0.07 mm,which greatly reduces the mass transfer resistance in the membrane distillation process.Therefore the hollow fiber membrane has a relatively large permeation flux,and on this basis,it also maintains relatively good mechanical strength.The influence of casting liquid parameters and operating parameters on the structure and performance of hollow fiber membranes during the preparation process is described in detail,which provides guidance for the preparation of medium fiber membrane materials.The self-made membrane was compared with the commercial hollow fiber membrane in mechanical properties,contact angle,permeation flux and other aspects.In addition,a hollow fiber membrane with a tube diameter of 4.2 mm was prepared by adjusting the ratio of the core liquid and the membrane liquid and matched with the speed of the runner,which improved the control range of tube diameter and increased the application range of hollow fiber membrane module.(2)Permeation flux is one of the main parameters that characterizing the system's seawater desalination performance,and its measurement method is the most important.At present,there are two commonly used methods for measuring permeation flux: condensing the permeate side material and calculating the difference in humidity between the inlet and outlet.Both of these traditional methods have certain defects.For example,they cannot reflect the local permeation flux of the system.When the area is small or the flow is unstable,an accurate permeation flux cannot be measured.Therefore,a new non-invasive membrane distillation permeation flux measurement method is proposed based on energy conservation.This method can achieve non-invasive real-time measurement,using the latent heat in the energy to reflect the permeation flux,and can reflect the local permeation flux in the module.In the case of low flow rate,it can maintain a relatively high accuracy,and the measuring device and principle of this method are described in detail.(3)A single-tube hollow fiber membrane model was established,and the heat and mass transfer mechanism in the model was analyzed in detail.The heat and mass transfer process was simulated by computational fluid dynamics(CFD),and the newly proposed permeation flux test method was verified.The simulation and experimental value were more consistent,and the maximum error between the outlet temperature simulation and the experiment is0.61%,and the maximum error between permeation flux simulation and experiment is 9%.(4)Using the established CFD model,more physical details of the membrane distillation process are revealed,and the applicable condition of the new test method is that the Reynolds number on the air side is less than 250.When it is greater than 250,the experiment should be optimized to improve the test accuracy.Further experimental investigations revealed the influence trend of feed temperature and feed flow rate on membrane distillation under the new test method.