Study On The Effect Of Impurity On Two-stream Heat Transfer In CO₂ Low-temperature Purification Process

Energy utilization and environmental protection has been two major issues in our international society,and a large amount of CO₂emissions have led to the greenhouse effect.According to the World Energy Statistical Yearbook published by Britain BP in 2020,recently,global carbon emissions have had sustained growth,reached 34.169 billion tons in2020.The Intergovernmental Panel on Climate Change（IPCC）pointed out that rise of global temperature was limited to 1.5℃by the end of the century.In order to reduce global CO₂emissions and achieve our country’s"carbon peak and carbon neutral"strategic goals,CO₂capture,utilization and storage technology（CCUS）which is considered to be a very effective method was proposed.The oxy-fuel combustion carbon capture technology is relatively low-cost,easy to scale,and can be retrofitted,which is considered the most promising technology.The burned flue gas needs to be purified,dehydrated and removed non-condensable gases.This process is called cryogenic purification of CO₂.The CO₂multi-stream heat exchanger is a key equipment in the low-temperature,physical purification process of CO₂,and it is extremely sensitive to operating temperature,pressure and impurities.This paper intends to combine numerical simulation and experimental research to study the effect of impurity on the condensation heat transfer of two-stream CO₂in the horizontal tube.Firstly,this paper uses Fluent to conduct numerical simulation research on the condensation heat transfer characteristics of CO₂and CO₂/O₂in a horizontal tube,then,establish a physical model and complete the meshing,select the appropriate multi-phase flow model,condensation source-term model,turbulence model and use UDF（user-defined function）,obtains the velocity contours,temperature contours,pressure contours and volume fraction contours in the horizontal tube,and also obtains the gas-liquid two-phase flow phase interface distribution law under different dryness conditions,that is the two-phase flow pattern.Lastly,this paper analyze the influence on condensation heat transfer of different inlet mass flow rate G=100～300kg/（m²·s）,heat flux q=5～8k W/m²,dryness（0.1～0.9）and impurities（O₂）.The following conclusions are drawn:when the mass flow rate,the heat flux density and the dryness rise,the condensation heat transfer coefficient will be increase,and the presence of impurities（O₂）will reduce the condensation heat transfer coefficient of CO₂.The condensation heat transfer coefficient of CO₂decreases with the increase of impurity components.In order to further verify the accuracy of the simulation and observe the gas-liquid two-phase flow pattern,an experimental study on the heat transfer characteristics of the two-stream CO₂flow condensation was conducted.The test section adopts a double-pipe heat exchanger with an inner diameter of 2mm and a length of 250mm.Using average-dryness method,this paper conducted test experiments in different work conditions（G=100,200kg/m²·s,x=0.1～0.9）,obtained the heat transfer coefficient,and observed the gas-liquid two-phase flow pattern.Comparing the experimental results with the simulation results,and the absolute error is within 20%.It is considered that the model is reasonable and the simulation results are relatively accurate,and a reasonable analysis of the experimental uncertainty is carried out.