Waste Heat Recovery Performance From Stripped Gas In CO₂ Chemical Absorption Process By Using Membrane Technology

Carbon capture,utilization and storage（CCUS）is a key technology to address the global climate change and achieve the goal of carbon neutrality.As one of the most promising CO₂ capture technologies for large-scale application,CO₂ chemical absorption process still suffers from a gigantic energy penalty with the high heat input for CO₂regeneration.In the regeneration stage of traditional CO₂ chemical absorption process,the hot stripped gas leaving the CO₂ stripper（i.e.,the mixture of water vapor and CO₂）has abundant waste heat（mainly the latent heat of water vapor）,implying that the energy saving potential if this waste heat can be efficiently recycled back to the stripper.In the existing rich solvent-split（RS）regeneration process,a partial cold CO₂ rich solvent was split to the top of stripper or the steel heat exchanger before the lean-rich heat exchanger（LRHE）to recover the waste heat from hot stripped gas.However,the waste heat recovery performance of the traditional RS process is poor.Therefore,this study proposed to build an energy efficient and reliable CO₂ regeneration system,via the integration of an advanced transport membrane condenser（TMC）and principle of rich solvent-split（RS）,namely the TMC-based RS system.The coupled mass and heat transfers of stripped gas through the TMC can realize the efficient recovery of the waste heat in stripped gas,thereby reducing the heat consumption of CO₂ regeneration.This study has explored the feasibility of TMC application in waste heat recovery from stripped gas,the mechanism of waste heat recovery,the optimization of waste heat recovery characteristics and parameters,the consumption reduction performance under natural regeneration conditions and economic analysis in terms of the specific heat recovery.The main conclusions are as follows:（1）When using the rich solvent as the waste heat recovery medium,the feasibility of TMC application in waste heat recovery from stripped gas was verified.The results showed that the condensation of H₂O（g）blocked the mass transfer of CO₂.TMC can offer better performance in heat recovery since both mass and heat transfer occurs,especially the recovery of latent heat of condensation via the mass transfer of water vapor.Comparing the heat and mass transfer fluxes of the three TMCs with the same pore size,it was found that the inner-side coated mono-channel membrane（the membrane layer was coated on the inner surface of the membrane tube）had the best water and heat recovery flux,which were 11.1 kg/（m²·h）and 21.5 MJ/（m²·h）.However,the maximum consumption reduction potential reached 1084.8 k J/kg-CO₂ for the inner-side coated 19-channel membrane due to the high membrane heat exchange area.The study of the heat and mass transfer mechanism showed that the gas-phase mass transfer mechanism of water vapor was Knudsen diffusion,accounting for up to 5.5%of the total mass transfer flux.Capillary condensation was happened in the pore of ceramic membrane,and the maximum amount of water vapor captured by the multi-layer diffusion（include Knudsen diffusion）and capillary condensation mechanism accounted for 22.6%of the total mass transfer flux.Thermal conduction contributed to more than 73.8%of the total heat transfer,dominating the heat transfer through the TMC.The convective heat transfer was determined by the latent heat released through the direct condensation of the mass transferred water vapor on the rich solvent side,accounting for up to 89.1%of the total convective heat transfer flux.（2）The multi-channel TMC（19-channel TMC）with different pore sizes was used to explore the consumption reduction potential after the waste heat recovery.The results showed that a large pore-sized multi-channel TMC had a low mass transfer flux but a high heat transfer for the mesoporous ceramic membrane.The maximum consumption reduction potential reached 1423.3 k J/kg-CO₂ by using 30 nm multi-channel TMC.Increasing the bypassed rich solvent flow rate,and increasing the mole fraction of water vapor in the stripped gas,and raising the pressure on the gas phase side can improve the waste heat recovery performance.Conversely,an increase in the stripped gas flow rate and the rich solvent temperature resulted in a reduction in the consumption reduction potential.When the heat exchange area was fixed,the consumption reduction potential increased first and then decreased with the stripped gas flow rate.When the consumption reduction potential was set as 800 k J/kg-CO₂（regeneration heat consumption reduced by20%）,the maximum stripped gas flow rate matched with 30 nm multi-channel TMC,12nm multi-channel TMC and 2 nm multi-channel TMC was 1.66（Nm³/h）/m²,1.57（Nm³/h）/m² and 1.26（Nm³/h）/m²,respectively.（3）A CO₂ regeneration system with a flue gas capture capacity of～3 Nm³/h has been built.The regeneration system reached a stable state after running for about 160 minutes,the material balance deviation was within±5%,and the energy balance deviation was within±7%,showing good operation stability and reliability.As compared to the traditional regeneration mode without RS modification,the reboiler duty（i.e.CO₂regeneration heat consumption）of the conventional RS process decreased up to 4.8%at the optimal split fraction of 10%;while the TMC-based RS process showed up to 21.7%saving in the reboiler duty at the optimal split fraction of 30%.As compared to other operation modes,the TMC-based RS mode achieved a least sensitive to the changes in regeneration conditions,showing a lower and relatively stable reboiler duty.An additional benefit for the TMC-based RS regeneration mode was that the total cooling water consumption required in the whole carbon capture system can be saved.TMC showed better consumption reduction performance than the stainless-steel heat exchanger with the same dimensions.The Config-B（one core multi-channel TMC in series）,Config-C（three core multi-channel TMC）and Config-E（seven core mono-channel TMC）can achieve the highest regeneration consumption reduction of 21-22%,while the maximum regeneration consumption reduction of the stainless-steel heat exchanger was only 14.3%.（4）The consumption reduction performance of N₂-assisted stripping coupled with waste heat recovery system were preliminarily explored.The results showed that the introduction of N₂ would increase the latent heat of water evaporation in the regeneration heat consumption,increasing the energy consumption of CO₂ regeneration.However,after controlling the water content in the stripped gas as a constant,the regeneration energy consumption can be reduced by 7.9%.If TMC was used to recover the waste heat from stripped gas coupled with N₂ stripping,the regeneration energy consumption can be further reduced.When the N₂ flow rate was 1 L/min,the maximum regeneration consumption reduction can reach up to 26.8%.（5）The economic analysis was carried out by the consumption reduction performance of the TMC in a natural CO₂ regeneration system in term of CO₂ capture cost saving.The results showed that the TMC-based RS process achieved a reduction in CO₂ capture cost with a net profit（NPC）range of 15.69-27.99 RMB/t-CO₂.Among all the additional investment and operation and maintenance costs,the replacement cost of the ceramic membrane tube was the main component,accounting for more than 40%.In the sensitivity analysis,the steam price had the greatest impact on the net profit,indicating that the waste heat recovery system was more suitable for promotion and application in a high fuel price scenario.When the membrane unit price exceeded 4600RMB/m²,most configurations of TMC no longer had an economic advantage over stainless steel heat exchangers.For membranes with more than five years lifetime,developing low-cost membranes would be more commercially valuable than studying the antifouling properties of membranes.With the innovation of membrane technology,increasing the consumption reduction potential was more effective than reducing the required area of the membrane in terms of the CO₂ capture cost saving.This study innovatively proposed an energy efficient and reliable CO₂ regeneration system,via the integration of an advanced transport membrane condenser（TMC）and principle of rich solvent-split（RS）.A reliable CO₂ regeneration system was built to achieve a significant reduction in the energy consumption of CO₂ regeneration,which will provide an important reference for the low energy consumption carbon capture process of flue gas,biogas and other gases.