The Research On Mechanism Of Heat And Mass Transfer Model In Conductive Gap Membrane Distillation

Membrane distillation technology is a widely used membrane separation technology,which is often used to treat saline solution.During its operation,the vapor pressure difference of the components caused by the temperature difference between the two sides of the hydrophobic membrane is used as the driving force to make the volatile components pass through the membrane pores to the cold side to complete the separation process.Membrane distillation technology is suitable for industrial wastewater treatment and seawater desalination due to its low temperature operation,waste heat utilization,good water quality recovery,and high concentration of raw materials.In this paper,an experimental study was carried out on a flat membrane module,and the effects of various parameters on the flux of the membrane module were observed.Firstly,the heat and mass transfer process of a gap membrane distillation module is theoretically analyzed.Based on the three mass transfer mechanisms of viscous flow,Knudsen diffusion and Molecular diffusion,the application of various mixed models of the three mechanisms in trans-membrane mass transfer is analyzed,and the influence of temperature on the selection of mixed models of trans-membrane mass transfer is analyzed.Secondly,in the experiment part,experiments are carried out through parameters such as hot material liquid concentration,temperature,flow rate and cold material liquid inlet flow rate of cross-flow flat membrane module,and the influences of different pore sizes and different forms of metal sheet on flux are also studied.The experimental results show that:(1)Among all the test parameters,the temperature has the greatest influence on the permeation flux.Under the condition that the inlet flow rate of cold and hot feed liquid and the inlet temperature of cold feed liquid are constant,the permeation flux increases by about 4.00kg/(m~2·h)for every 10°C increase in the inlet temperature of hot feed liquid.(2)The trans-membrane mass transfer model in membrane distillation is a combination of two mixed models.According to the change trend of flux with temperature,the dominant mass transfer model in different temperature ranges is determined.And with the change of temperature,the mass transfer model will change from one to another.(3)The difference of permeation flux between NaCl solution with 5%and 10%concentration of feed solution and pure deionized water under the same working condition is not big,i.e.the concentration has little influence on permeation flux.In addition,the influence of different metal sheet on the permeation flux was investigated,of which type a metal sheet had better effect and larger gain on the flux.The effective volume of the membrane module used in this paper is 10cm×4cm×2cm,and the numerical simulation of the working condition is carried out based on the experimental parameters.The turbulence model was used in the simulation process.The initial velocity of hot material liquid was 0.07m/s,and the initial velocity of cold material liquid was 0.1 m/s.The turbulence intensity and hydraulic diameter were used.UDF was used to compile the source terms involved in heat and mass transfer and the physical parameters of membrane material and feed liquid.Finally,according to theoretical analysis,experiment and simulation,the improvement of membrane module is proposed,and the design of a new type of gap membrane distillation module is completed.This module can effectively weaken the polarization phenomenon in the transmission process of membrane distillation in theory.