Analysis Of Molecular Exchange Flow Effect And Its Application On Gas Separation

Based on the flow characteristics of binary gas mixtures in microchannels,a molecular exchange flow phenomenon can be formed by setting the driving forces for flow appropriately.This special flow phenomenon can be used for gas separation.A counter flow setup in a novel gas separator is used to accumulate the separation effect of the microchannels where the molecular exchange flow occurs,which allows the concentration of the target component in feed gases to reach a certain set value.The operation mechanism of the separator determines that it can be driven by low-grade thermal energy,which makes the separator significantly different from the traditional gas separation methods.In this paper,the flow characteristics of the thermal transpiration flow,the Poiseuille flow and the concentration-driven flow in microchannels have been discussed.The results illustrate that the separation coefficients for the thermal transpiration flow and the concentration-driven flow remain almost the same in the transitional flow regime and the free molecular flow regime,in contrast to that the separation coefficient for the Poiseuille flow varies remarkably in the transitional flow regime and remain relatively stable in the free molecular flow regime.In the transitional flow regime,the separation coefficient for the Poiseuille flow is smaller than that for the thermal transpiration flow.In addition,the separation coefficient is also related to the molecular mass ratio of binary gas mixtures:an increasing molecular mass ratio leads to a higher separation coefficient.Based on the differences of the separation coefficients for the different flows in the transitional flow regime,the molecular exchange flow was established,and the effects of the Knudsen number,molecular mass ratio as well as component concentration on the intensity?molar mass flow?and the formation conditions?ratios of temperature gradient and pressure gradient?of the molecular exchange flow were discussed.The results demonstrate that these factors have significant influences on the formation condition as well as the intensity of the molecular exchange flow.A smaller Knudsen number and a larger molecular mass ratio result in a wider conditional range.However,the formation condition become more stable with the increase of the Knudsen number.For the gas mixture of He-Ne,with the component concentration of 0.5and the Knudsen number at about 1,the intensity of the ideal molecular exchange flow reaches its maximum.Taking the gas mixtures of He-Ar,He-Ne and Ne-Ar as an example,the performance of the novel gas separator based on molecular exchange flow was analyzed.The study highlights that increasing the driving temperature difference?and the corresponding driving pressure difference?can enhance the separation performance of the separator.A less molar concentration difference between components leads to a higher separation efficiency for the separator.Moreover,the separation performance can also be affected by varying the Knudsen number and the species of gas mixtures.The gas separator has a great advantage in increasing the concentration of the target component in feed gases.However,its product rate?product gas flow rate?is small.Finally,taking the above-mentioned gas separator as one separation unit,a gas separation system composed of modular gas separation units in series was proposed,and a mathematical model with corresponding algorithms was established to describe the gas separation process and the optimal design for the proposed gas separation system.On this basis,this separation system was designed to separate the component He from the binary mixture of He-Ar with structural arrangements,components and specified operating parameters.The purity of He obtained from the system is more than 99%and the target component recycling rate is over 80%when 20 gas separation units are set in series.The energy consumption of the system is 6327k J?m^-3when the separation purity is satisfied.The gas separation system conforms to the principle of energy cascade utilization since it can directly be driven by low-grade heat energy.