Experimental And Modeling Investigations Of Ammonia-based CO₂ Capture Activated By Piperazine

Due to the advantages of low cost,low regeneration duty and the potential of removing varieties of pollutants simultaneously,ammonia-based CO₂ capture processes have a promising prospect in the CO₂ removal field.Nevertheless,the industrial application of these processes still faces several severe obstacles such as the slow absorption rate,the high volatility of ammonia and so on.To solve these problems,piperazine?PZ?was proposed as an additive to activate aqueous ammonia solution,and fundamental studies on this blend solution were performed in this paper,including the reaction mechanisms of ammonia-based C02 capture,the activated mechanisms of PZ,the mass transfer characteristics and chemical dynamics,and the absorption experiments in spray tower.On the basis of the fundamental studies,a comprehensive three-dimensional numerical model was developed to simulate C02 capture by PZ activated ammonia solution,providing a potential tool and guidance for the future industrial application.To study the reaction mechanisms of ammonia-based CO₂ capture and the activated mechanisms of PZ,the method of combining the implicit and explicit solvent model was employed to investigate the possible reaction pathways for NH₃,H₂0 and C02 in aqueous solution according to the quantum chemistry theory.Subsequently,the influence of the addition of PZ on the prior reaction pathways and the reaction mechanisms between PZ and C02 in aqueous solution were also studied.The calculation results showed that there existed two main reaction pathways in ammonia-based CO₂ capture,namely,the formation of ammonium carbamate pathway and the formation of ammonium bicarbonate pathway.According to the AIM?Atoms in Molecules?analysis and the NPA?Natural Population Analysis?,it was found that the formation of ammonium carbamate was a two-step reaction.Firstly,NH₃ attacked COo2 to form a zwitterion intermediate NH₃+CO₂-,and then this intermediate proceeded a barrierless proton transfer process reacting with another NH₃ to form the final product of NH4+ and NH₂COO-.However,when the intermediate reacted with H₂O instead of NH₃ in the proton transfer process,it was found that the activated free energy was much higher than that of both NH₃ and the decomposition process of the intermediate,indicating this pathway is unfavorable.Further,in the presence of PZ in the above reaction pathways,there was no evidence to support that the formation of NH₃+CO₂-could be catalyzed by PZ,but it was found that PZ could also react with CO₂ to form a zwitterion intermediate PZ+CO₂-with a much lower activated free energy.In the subsequent proton transfer process,both PZ and NH₃ could react with the intermediate PZ+CO₂-,respectively,to form the product of PZCOO-,PZH+ or NH4+ with a negligible activated free energy.This suggested the activated effect was due to the reduced activated free energy of the zwitterion formation process.For the ammonium bicarbonate pathway,although the inclusion of PZ or NH₃ could reduce the activated free energy of the pathways to form HCO3-,NH4+ or PZH+,the energy was still much higher than that of the ammonium carbamate pathways,indicating the ammonium carbamate pathways is the main reaction channel.A wetted wall column system was established to study the mass transfer characteristics and chemical dynamics of CO₂ capture by the blend solution.According to the quantum calculation results above,new models for ammonia-based CO₂ capture and PZ-based CO₂ capture were proposed,respectively.And the key parameters in these models were regressed against the experimental data obtained in the wetted wall column system.The regression models showed a satisfactory agreement with previous experimental results.For the blend solution,it was found that a combination of the prior regression models was enough to describe the absorption characteristics well.It's believed that the activated mechanism of PZ and the addition of small amount of PZ should be responsible for the finding.A laboratory-scale spray absorption system was established to investigate the influence of operation parameters on the CO₂ removal efficiency,the NH₃ concentration in the outlet gas,the volumetric overall CO₂ mass transfer coefficient,and ammonia escape rate.The operation parameters included the amount of added PZ,the concentration of ammonia solution,the flux of the blend solution,the temperature of the solution,the flux of simulated gas,and the temperature of simulated gas.The experimental results showed that the amount of added PZ and the concentration of ammonia solution promoted the CO₂ removal efficiency and the volumetric overall CO₂ mass transfer coefficient,and the concentration of ammonia solution and the temperature of the blend solution have a significant influence on the NH₃ concentration in the outlet gas.Besides,there existed the optimal solution temperature for the CO₂ removal efficiency.The ammonia escape rate increased as the flux of the solution increased firstly,and then decreased slightly.On the basis of the above study,a comprehensive three-dimensional numerical model was developed to simulate the spray absorption system using the discrete phase model integrated with user-defined functions.The phenomenon of gas-liquid flow,mass transfer,heat transfer,and chemical reactions were all considered in the new model.The simulation results could illustrate the gas-liquid flow characteristics,temperature distributions,species distributions,CO₂ removal rate,and ammonia escape rate in the spray column.Comparing with the experimental results,the simulation results showed a good agreement.Furthermore,this model could be used to investigate the effects of the operation parameters and promote the CO₂ capture process using the blend solution,achieving the supplement and extension of the experimental studies.In addition,this model could also be beneficial to the design and optimization of absorption column serving as an important tool and guide.