Experimental Study On The Kinetics Of CO₂ Capture Using Semi-clathrate Hydrates Formed In Quaternary Ammonium Salt Solutions

Unconventional natural gases(coalbed methane,shale gas,etc.)have attracted attention worldwide due to their rich reserves,clean and efficient.Injecting supercritical CO₂ to extract unconventional natural gases can improve gas recovery as well as sequester CO₂underground,it is an effective and prospective technology.However,the method will make the recovered gas a mixture of CH₄+CO₂.As a result,CO₂ must be removed from the recovered gas before transfer them for further industrial application.Hydrate-based CO₂ capture and separation is more environmentally friendly and has higher CO₂selectivity,simpler operation conditions and lower energy consumption.However,it faces technical challenges of rigorous hydrate formation conditions and low hydrate growth rate,gas consumption,CO₂ separation efficiency.Aiming at these two key technical problems,in this paper,quaternary ammonium salt(QAS)was used to mild the hydrate formation conditions,and the formation and decomposition characteristics of semi-clathrate hydrates formed with CH₄+CO₂ in different QAS solutions were systematically studied.The main contents and conclusions of this work are as follows:(1)The phase equilibrium temperature of semi-clathrate hydrate formation in 2.57mol%TBAB and 2.57 mol%TBPB solutions with CO₂ were measured by DSC.The experimental results showed that both TBAB and TBPB can greatly improve the formation conditions of CO₂ hydrate.Based on the phase equilibrium data,the kinetics of semi-clathrate hydrates formed with CO₂ in presence of 2.57 mol%TBAB,TBPB and TBAB+TBPB were investigated,the initial experimental pressure was fixed at 2.8MPa and three subcoolings(?T=6 K?9 K?12 K).It was found that adding TBPB into TBAB solution can promote hydrate nucleation,and this promotion effect was stronger at a lower subcooling(?T=6 K).The gas consumption obtained in TBAB,TBPB and TBAB+TBPB solutions decreased with the increase of subcooling,despite the increase of the rate of hydrate growth.At a given subcooling,the gas consumption obtained in TBAB+TBPB solution was larger than that obtained in TBAB and TBPB solutions.The gas consumption improved with the increase of V_gas/V_liquid,but it decreased while the volume of reactor was increased.Therefore,a combination of TBAB+TBPB will be a promising option to improve the kinetics of semi-clathrate hydrate formation with CO₂.(2)The effects of TBAB concentration(0.29 mol%?0.62 mol%?1.38 mol%?2.57mol%)on the induction time,gas consumption,CO₂ recovery rate and separation factor for TBAB+CH₄+CO₂ hydrate formation were studied,and the effects of subcooling(?T=6 K?9 K?12 K)and SDS concentration(300 ppm?500 ppm?1000 ppm)on semi-clathrate hydrate formation kinetics and CO₂separation efficiency at 2.57 mol%TBAB were investigated.The initial pressure was fixed at 2.8 MPa.It was found that hydrate formed in 2.57 mol%TBAB solution had the highest equilibrium temperature,the fastest nucleation rate and the best CO₂ separation efficiency,but the lowest gas consumption.The rate of hydrate growth for CH₄+CO₂ hydrate was increased while increasing subcooling,however,the gas consumption and the CO₂ separation factor were decreased dramatically.A lower subcooling(?T=6 K)showed a better CO₂selectivity for CO₂ capture from CH₄+CO₂gas mixture.Adding SDS into 2.57 mol%TBAB solution can promote semi-clathrate hydrate nucleation and growth and also can improve gas consumption.But SDS reduced the CO₂ separation efficiency,the larger the SDS concentration,the smaller the CO₂ separation factor.The optimum results in this study was obtained in 2.57 mol%TBAB solution at a subcooling of 6 K.(3)The results obtained in this work were compared with different systems(THF?THF/SDS?TBPB)in the literature.It was found that 2.57 mol%TBAB showed the highest operating temperature and CO₂ separation efficiency,but the lowest gas consumption in these systems.Therefore,TBAB is a promising chemical promoter,which has a significant promotion effect on reducing energy consumption and increasing the CO₂ separation factor,but has room in increasing gas consumption.