Study On Interface Effect Of Nano-confined Fluid Transport

Separation technology is an important process in the chemical industry.Compared with traditional separation methods,membrane separation technology has the advantages of high separation efficiency and low energy consumption,and has received much attention in recent years.Due to the strong surface interaction on the fluids in the membrane micropores,fluid transport dynamics and structure are difficult to study with only macroscopic theory,and it is difficult to observe the molecular structure of the fluid through experimental methods.Understanding the mechanism of nano-confined fluid transport can provide theoretical guidance for the future development of membrane technology.By using molecular dynamics simulation,this dissertation studies the interface effect of fluid transport and separation through nano-confined space.We develop a quantitative prediction model to accurately calculate the fluid flux across the membranes and reveal the key factors of transport and separation efficiency under the interface effect.This dissertation is divided into the following four parts:(1)For non-equilibrium molecular dynamics simulation of membrane transport,the thermostats may have an artificial effect on flux prediction.Here,we use the Langevin thermostat to study the water transport through membranes,and explore the effect of thermostat strategies on mass transfer by adjusting the thermostat coupling time and object.The research shows that in the simulation of non-equilibrium fluid transport,the choice of thermostat has a significant effect on the simulation results.The Langevin thermostat method has a strong negative impact on the transport dynamics of fluids.By adjusting its coupling time and applying a thermostat to the membrane material alone,it can effectively weaken the artificial effects of the thermostat and reduce the difference between experimental results and simulation.(2)To further study the huge quantitative discrepancy which is up to several orders of magnitude between simulation and experiment,we develop a quantitative model based on vertically aligned carbon nanotube membrane by using non-equilibrium molecular dynamics simulation.The effect of wall flexibility is analyzed.It is found that the mass transfer efficiency of flexible membrane channels is about 20%higher than that of rigid channels.The enhanced effect of fluid mass transfer is more pronounced in membranes of small pore sizes.The effect of pore wettability on flux is explored,and hydrophobic pores are found to have higher fluid flux.Coupling these three factors,and combining experimentally measured membrane pore size distribution and effective pore size in the calculation,for the first time,simulation results that quantitatively agree with experimental observations are obtained.(3)The fluid transport through membranes is not only related to the characteristics of the nano-pores,but also affected by the surface properties of the membranes.These two mechanisms are often interrelated and work together for ultra-thin membranes.We study the wettability effect of water transport through ultra-thin membranes.We find that hydrophilic interior and exterior surfaces are not conducive to the mass transfer of water molecules,but the mechanisms are different.First,a layer of water molecules adsorbed on the wall surface exhibits a solid-like structure,which intensifies the friction between water and the wall surface.Second,the adsorption layer and the water in the center of the channel delaminate.There is a gas-like phase between them,which reduces the synergistic effect of the two flows.These two effects together decrease the water flux.Near the hydrophilic exterior surface,due to the increased adsorption amount of water molecules,the diffusivity of water molecules decreases,and the entrance/exit resistance increases.(4)There is a trade-off between permeability and selectivity of membrane which determines the separation upper bound.This competition mechanism is a statistical law discovered through a large number of experimental measurements,and the internal physical mechanism is not clear.In this chapter,the theoretical analysis of Poiseuille flow is used,and the two characteristics,fluid flux and selectivity,are decoupled as an adsorption-transport mechanism.The Monte Carlo simulation is used to calculate the adsorption structure of the mixed components in the membrane pores,and the fluid flux is described by the modified Hagen-Poiseuille equation.We analyze the intrinsic correlation between the permeability and selectivity of the mixture and the system temperature,pressure,surface wetting,and other factors.The theoretical results agree well with the Robeson upper limit of the polymer membrane in the experiment.The innovations of the dissertation are summarized as follows:Through non-equilibrium molecular simulation,a quantitative model for the calculation of fluid flux through carbon nanotube membranes is proposed for the first time.By tuning the surface properties,the role of interface effects on nano-confined fluid transport is revealed.The effects of system temperature,pressure and surface wettability on the membrane separation upper bound are analyzed by combining theory and simulation method.