Applied Fundamental Research On CO₂ Capture Using Potassium Prolinate/Ethanol Phase Change Absorbent

High energy consumption of solvent regeneration is one of the main disadvantages for CO₂ capture using aqueous monoethanolamine（MEA）.Phase change absorbents have a feature that CO₂ can be absorbed and enriched in one of liquid-solid or liquid-liquid phases.Only CO₂-rich phase needs to be heated and regenerated,which has great potential in reducing heat duty.Compared with aqueous amino acid salt（AAS）solutions for phase changing,potassium prolinate（ProK）/ethanol water-lean solution has showed some better features such as fast absorption rate and wide operation window of liquid-solid phase transition.However,limited information is available on phase-changing characteristics during CO₂ absorption,reaction mechanism and reaction kinetics in the open literature.Therefore,the research on these areas for the novel ProK/ethanol absorbent will have important theoretical and practical significance for developing highly effective-energy CO₂ capture processes using water-lean solvent.In this thesis,CO₂ absorption and desorption performances in ProK/ethanol solutions were characterized in a screening setup under the simulated flue gas conditions and CO₂distribution in the liquid and solid phases was also analyzed.Physicochemical properties of these absorbents were measured using density and viscosity meters.N₂O and CO₂solubility in the system of ProK-ethanol-H₂O were determined in a static vapor-liquid equilibrium apparatus.Reaction mechanisms and the compositions of solid phase were identified by the findings of ¹³C NMR,XRD,etc.Reaction kinetics of CO₂ absorption into ProK/ethanol solution was also measured in a magnetic-drive stirred reactor.ProK/ethanol solution has shown a fast absorption rate.The value for 5.954 m₁（3.0M）ProK/ethanol solution is three-time higher than that for aqueous 30 mass%MEA solution.ProK/ethanol solution can undergo liquid-solid phase change from a homogeneous liquid during CO₂ absorption.About 55-60%CO₂ is enriched in the solid phase.Carbamate,bicarbonate and ethyl carbonate were identified as the main products.Potassium hydrogen carbonate is the major composition in the solid precipitation,which can decompose at temperature about 383K.Several absorption and desorption cycles showed that the absorbent was stable and could be reused.The heat of CO₂ absorption varies with the change of CO₂ loading,and a sharp decrease was observed when CO₂ loading was in the range of 0.35-0.60 mol·mol^-1.Densities and viscosities of the systems studied decreased with the increasing temperature,but increased with the increase in ProK concentration.The predictions from the proposed empirical correlations are in good agreement with the experimental data.CO₂ physical solubility was estimated by N₂O/CO₂ analogy.The use of ethanol as a solvent can enhance greatly the CO₂ physical solubility,which is beneficial to the absorption process.The effect of temperature on CO₂ solubility into ProK/ethanol solution is greater than that into aqueous ProK.A larger cyclic absorption capacity was observed within narrower changes in temperature under a constant CO₂ partial pressure.The formation of solid precipitation can increase the total CO₂ solubility in the solution.Under a fast pseudo-first-order behavior of CO₂,it was found that the overall reaction rate constants(k_ov)increased noticeably with the increasing temperature and the molality of ProK.The organic solvents have great effect on the reaction rate and the partial reaction order for ProK.The k_ov at 280.2 K varied in different systems as follows:ProK/EG>ProK/ethanol>ProK/water.The kinetic results for ProK/ethanol systems were interpreted using two models based on zwitterion mechanism and termolecular mechanism for the reactive absorption of CO₂.The model parameters were also correlated.The partial reaction order is about 2.0 with respect to ProK.For the zwitterion mechanism model,the rate of zwitterion deprotonation is relatively slower than that of zwitterion formation and ProK plays a major role in this step.The reaction activation energy was estimated about16.0 kJ·mol^-1.In summary,ProK/ethanol solution has shown good capture performace with liquid-to-solid phase change characteristics such as highly CO₂-enriched in the solid phase,high CO₂ absorption rate and cyclic capacity.Therefore,ProK/ethanol solution can be considered a promising phase change absorbent in the practical application.