Study On Energy Efficiency Of Molecular Exchange Separation System

The temperature gradient and pressure gradient are applied in the micro-channel/micropore,and a molecular exchange flow phenomenon is constructed by adjusting the relative size of the two for gas separation.This method is dif-ferent from the traditional membrane separation method which only relies on pressure drive.When the temperature gradient is applied under the same pres-sure,the Knudsen diffusion separation flow rate and separation factor of the mi-crochannel/micropore are less than the separation effect of the molecular ex-change flow.It can be considered that the molecular exchange constructed by the superposition of the heat flow escape flow generated by the temperature gra-dient and the Poiseuille flow is a method to enhance membrane separation.Fur-ther,the gas separator based on molecular exchange flow can produce high con-centration target product gas by adjusting the separator structure.The product gas flow rate of the single-stage separator is small and the product rate is low.Therefore,a multi-stage molecular exchange separation system is designed.By using low-grade thermal energy（industrial waste heat）to drive the separator and Knudsen pump in the separation system,multi-stage separation is realized to improve the product gas flow rate and product rate.This thesis first discusses the flow and separation differences between thermal transpiration flow,Poiseuille flow and concentration driven flow and Knudsen diffusion in microchannels.The results show that the separation per-formance of the corresponding molecular exchange flow is better than that of Knudsen diffusion at the same pressure and different molecular mass ratios.In-creasing the temperature difference can enhance the separation ability of the mo-lecular exchange flow.Increasing the Kn number within a certain range can en-hance the molecular exchange capacity,but not monotonically positive correla-tion.The energy consumption of the multi-stage molecular exchange separation system changes with the flow rate of the separated product gas.The size of the product gas flow rate is related to the target concentration.The higher the target concentration,the smaller the product gas flow rate,the more the flow rate into the next stage Knudsen pump and separator,and the higher the energy consump-tion.The increase in the number of separation stages will increase the total flow of the separated product gas,that is,the product rate,but the energy consumption will increase and the smaller the gas flow and concentration to be separated,the greater the separation difficulty.The energy conversion of the separation process is more converted into gas internal energy,and the proportion of conversion into effective chemical potential energy is decreasing.Therefore,it is necessary to comprehensively consider the selection of target concentration and gas product rate.Based on the separation characteristics and energy consumption character-istics of molecular exchange flow,a flue gas carbon dioxide separation system for cascade utilization of flue gas waste heat is designed.The flue gas of a typical1000 MW coal-fired power plant was treated by a 24-stage molecular exchange separator in series in the system.Under ideal conditions,the concentration of CO₂ was up to 98.89%,the recovery rate was 72.53%,the exergy efficiency was64.8%,and the unit energy consumption was 0.047 GJ·（t CO₂）^-1.It is a new method to realize zero carbon gas separation in the future.