Design,Simulation And Optimization Of CO₂ Multi-Stream Heat Exchanger

The increasingly severe issues of resource scarcity,population growth,and environmental pollution have accompanied the rapid development of China’s economy,which has also led to increased energy consumption.CO₂ is the largest greenhouse gas emission,and traditional fossil fuel combustion accounts for 30%of its emissions.To achieve the strategic goal of peaking and achieving carbon neutrality,effective measures are needed to reduce CO₂ emissions.CCS technology is an effective method of emission reduction,as it can capture and store CO₂ emitted from sources,which is particularly necessary for China’s coal-dominated energy supply structure.The multi-stream heat exchanger is a key equipment in the CCS process,but its design requires consideration of multiple variables and parameters,while the non-condensable gas has a significant impact on CO₂ condensation and heat exchange between the fluids inside the heat exchanger.This study aims to investigate the influence of non-condensable gas on the heat transfer performance inside the multi-stream heat exchanger,by evaluating the effects of fluid arrangement,non-condensable gas and its volume fraction,fluid velocity,and other factors on CO₂ condensation.The study also includes an assessment of the variable property models for the mixed gas and an economic analysis comparison,providing a reference for the optimization design and operation of the multi-stream heat exchanger.The main research conclusions are as follows:（1）By comparing the heat transfer of multi-stream heat exchangers under different flow arrangements,it is found that fluid arrangement can significantly affect the performance of multi-stream heat exchangers.Among these arrangements,there is an optimal arrangement that can provide the highest subcooling.（2）The influence of fluid velocity and evaporation on the heat transfer of the heat exchanger was discussed.It was found that increasing the fluid velocity can improve the convective heat transfer coefficient and subcooling.（3）The condensation process of CO₂ mixtures containing non-condensable gases in a multi-stream heat exchanger using a variable property model has been investigated.The simulation results were compared with experimental data to evaluate the performance of three different viscosity models（Wilke,DS and LBC）and three different thermal conductivity models（WD,KM and MS）.The results suggest that the Wilke model should be used as the preferred viscosity model,with an absolute average deviation（AAD）of only2.3%between the simulated values and experimental data.For thermal conductivity models,the KM model is recommended as it has the smallest AAD（8.6%）,followed by the WD and MS models.In addition,the simulation values of variable property models and constant property models were compared under specific operating conditions.The results show that the simulation values of all three viscosity models are higher than those of constant property models,with the Wilke model having the smallest difference with constant property model;among the three thermal conductivity models,the KM model has the highest degree of fit with constant property model,differing by only 1.3%.This is consistent with the recommendations from this study’s variable property model and experimental results.（4）An economic analysis of multi-stream heat exchangers was conducted,and the influence of different factors on heat exchanger design was studied.The results showed that using the KM model can achieve the lowest material cost,so it is recommended to choose this model when designing heat exchangers.In addition,fluid velocity and non-condensable gas volume fraction also have an important impact on material cost.Reducing flow velocity and increasing volume fraction will both lead to an increase in material cost.For example,at a flow velocity of 0.02m/s,the material cost decreased by 8.3%compared to 0.005m/s.At a volume fraction of 15%,the material cost increased by 4.6%compared to 5%.