Numerical Simulation Study Of Water Recovery Process With The PVDF Hollow Fiber Membrane

In many industrial production processes,a large amount of moisture-containing gases are emitted directly into the atmosphere,resulting in the waste of water resources.In coal-fired power plants,for example,the flue gas after wet desulfurization contains a large amount of moisture,and the direct emission of this highly humid flue gas causes not only the waste of water resources,but also some pollution to the atmosphere.Membrane separation method has the features of simple process,high reliability,small footprint and no secondary pollution,etc.It is a feasible technical way to recover the moisture in flue gas by membrane separation method.Compared to conventional methods of recovering moisture from flue gases,the most important feature of the membrane method for recovering moisture is the presence of a transmembrane transport process of the gas,which intensifies the condensation of water vapor in the moisture-bearing gas.The condensation process of water vapor is not only driven by temperature difference,but also influenced by the pressure difference-driven transport across the membrane.In this paper,the process of moisture recovery by membrane method for moisture-containing gas is mainly studied by numerical simulation,and the influencing factors of water recovery characteristics of PVDF membrane are analyzed,and the heat transfer process is also analyzed.The specific studies are as follows.1)In this paper,the hydrophobic polyvinylidene fluoride(PVDF)hollow fiber membrane is used to study the mechanism of condensation of highly humid gases on the membrane surface.It is concluded from the analysis that the condensation process on the membrane surface is driven by the temperature difference,and also the transmembrane transport of gas has a corresponding effect on the condensation process.In this paper,a three-dimensional PVDF hollow fiber membrane module model is established,and the water recovery characteristics and influencing factors of PVDF hollow fiber membrane are investigated by numerical simulation method.2)The effects of temperature difference between the fluids on both sides of the membrane,flow rate ratio,high moisture gas parameters,and membrane porosity on the water recovery performance of hydrophobic hollow fiber membrane modules were analyzed by numerical simulations.The results show that increasing both the flow ratio and temperature difference between purge gas and high humidity gas can promote heat transfer,however,the water recovery rate does not increase monotonically with the increase of heat across the membrane.The optimal sweep gas to high moisture gas flow ratio exists to maximize the water recovery,and the optimal flow rate ratio increases with the increase of membrane porosity.Under the conditions studied in this paper,the water recovery rate showed a trend of decreasing and then increasing with the increase of membrane porosity.3)The heat transfer characteristics of the membrane module water recovery process were analyzed.The simulation data show that the convective heat transfer resistance on the high moisture gas side of the heat transfer process accounts for the largest proportion of the total thermal resistance across the membrane,up to 50%of the total thermal resistance across the membrane.On one hand,increasing the purge gas flow rate strengthens the convective heat transfer between the purge gas and the inner surface of the membrane tube,which can reduce the convective heat transfer resistance on the purge gas side;on the other hand,increasing the purge gas flow rate decreases the trans-membrane pressure difference,which inhibits the transmembrane gas transport process,which weakens the convective heat transfer process between the high moisture gas and the outer surface of the membrane,and makes the convective heat transfer resistance on the high moisture gas side increase continuously.The ratio of the heat carried by the transmembrane gas to the total heat across the membrane was investigated,and this ratio was defined as γ.It was found that there exists an optimum γ value that makes the water recovery of the membrane module reach a high level,and the optimum γ value corresponds to different porosities.Under the conditions studied in this paper,the γ values that allowed the water recovery to reach the maximum value were 14.7%,17.8%and 25.5%when the porosity was 40%,60%and 80%.