471 / 2018-12-31 07:07:08

Fundamental Understanding of CO2 absorption and regeneration in aqueous PZ-Amines blends

CO2 absorption, PZ blends, Metadynimcs-biased AIMD, CMD

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Fundamental Understanding of CO2 absorption and regeneration in aqueous PZ-Amines blends
Qinlan Luo12, Gyeong S. Hwang2, Qulan Zhou1, Na Li1
1 State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
2 McKetta Department of Chemical Engineering, University of Texas at Austin,
Austin, Texas 78712, USA

Chemical absorption using aqueous amine solutions has considered as one of the most promising technology for CO2 capture. Among aqueous amines, piperazine (PZ), monoethanolamine (MEA), 2-amine-2methly-1-propanol (AMP) and methyldiethanolamine (MDEA) have been widely used with their high absorption rates or low regeneration energy. In same experimental studies, CO2 absorption rate has been found to significantly increase when PZ is added to aqueous MEA or AMP or MDEA solutions, while reducing the regeneration energy. Some theoretical models, such as “shuttle mechanism” and “chemical interactions”, had been developed to explain the enhanced mass transfer in binary PZ blends. The “shuttle mechanism” can describe both the diffusion and chemical reaction, while less research shows the details of diffusion of chemical species and chemical reactions that occur from the gas-liquid interface to bulk solution. Therefore, we attempt to elucidate molecular mechanisms underlying the absorption of CO2 with binary PZ blends using combined metadynamic-biased ab.initial molecular dynamics (AIMD) and classical force field calculation. First, metadynamics-biased AIMD simulations are performed to construct free energy profiles for CO2 absorption and regeneration reaction path in aqueous amine solutions. The identified reaction paths and transition states are analyzed to better understand the behavior of H2O network and stability of absorption productions. Then we use classical molecular dynamics (CMD) to analyze the diffusivity of all chemical species in pure amine aqueous solution and binary PZ blends, and discuss the effect of PZ addition on diffusivity of absorption products. Free energy barrier for absorption and regeneration reactions are shown in Table 1. Comparing to MEA, AMP or MDEA, PZ has a higher absorption rate, and the regeneration energy barrier of PZ is lower than that of MEA. In CMD calculation, the diffusivity of pzcarbamate is lower than that of meacarbamate or bicarbonate, which is mainly due to the high aggregative of pzcarbamate. This study highlights the shuttle role of PZ in binary PZ blends, which can capture CO2 rapidly in the gas-liquid interface, and release CO2 to other amine later, then other absorption product will diffuse quickly in the bulk solution.
Key words: CO2 absorption, PZ blends, Metadynimcs-biased AIMD, CMD