Analysis Of Thermal Transpiration Effect And Its Application On Gas Separation

Thermal transpiration is a gas flow phenomenon driven by the temperature difference between both sides of the microchannel.This phenomenon of pure gas has a certain capacity of conveying and boosting.while miArd gas also has a separation effect tp some extent.Taking the advantages of the thermal transpiration into consideration,it can be applied to gas separation driven by industrial process waste heat and without mechanical loss because of no moving parts.Furthermore,owing to the flexible structures,it can not only meet different concentration demand but also be integrated with different industrial processes organically.In this paper,the thermal transpiration effects of 13 kinds of pure gases in microchannels are discussed.The influence of microchannel structures,gas molecular characteristics,and temperature difference et.al.on flow are analyzed.The results show that the net mass flow rate is larger in the initial stage of the transition flow field and the gas pressure difference gets the peak in the free molecular flow regime.Knudsen number and microchannel geometrical coefficient are coupled with each other,And 1000 or so,the net mass flow decrease by 99%at Knudsen number or geometrical coefficient is about 2.4 or 1000,respectively.Increasing temperature difference of both sides of the microchannel can help enlarge the mass flow and pressure difference.Then,taking He-Ar miArd gas as an example to further discuss the thermal transpiration effect of binary gas mixture,and to analyzes the influence of mixture mole fraction,flow regime,microchannel structures,and temperature difference et.al.on the flow of mixture and each component.The results show that with the same mole fraction,the mixture flow law is similar to that of the pure gas flow pumped by the thermal transpiration effect.The couple between Knudsen number and the geometric coefficient makes the net mole flow reduced by 99%when the two reaches around 1.6 or 1000,respectively.Besides,there are some flow differences between He and Ar components.He whose molecular is lighter flows more to the hot side,and Ar flows more to the cold.The greater of the mixture initial mole fraction and the temperature difference is,the greater of the flow difference between He and Ar is.In addition,the temperature difference in the free molecular flow regime has slightly stronger influence than that of in the transition flow regime.Based on the analysis of the thermal transpiration effect for single-stage microchannel,gas separation model of the multi-stage series of the thermal transpiration effect is analyzed.It is pointed out that the improvement of temperature difference can improve the separation ability of single-stage separation unit,however,it can not guarantee the purity of more than 99%at last.In addition,the separation degree changes stage by stage but the variation is small.Finally,combined with the conventional heat-driven refrigeration system,taking obtaining H2 from the coke oven gas as an example,a residual heat-driven heat-flow gas separation system is designed,and the unit energy consumption model of the gas separation system is proposed.The results show that the unit energy consumption of the gas separation system is influenced by the factors such as the number of separation units,the temperature difference between the mixture and heat exchanger in the hot and cold chamber,the initial mixture concentration et.al.