Study On Water-Lean Amino Acid Salts-Based Absorbents And Their Application For CO₂ Capture

High energy consumption for solvent regeneration and high capture costs are still the main obstacles that prevent the implementation of large-scale CO₂ capture using aqueous amines globally.The use of water in these systems is probably the weakest point due to large specific heat capacity and huge enthalpy of vaporization of water.Using water-lean or water-free solvents has attracted much attention recently,which has potential to lower regeneration energy consumption.In this thesis,amino acid salts?AAS?based water-lean absorbents were proposed for CO₂ capture.These novel systems have advantages over the conventional aqueous alkanolamines by introducing amino acid salts and organic solvents with low volatility and low specific heat capacity.However,limited information is available on their capture performance,physicochemical properties,kinetics and regeneration energy.Applied fundamental research on water-lean absorbents can offer an alternative strategy to develop energy-efficient capture processes with great practical and academic significance.In this thesis,water-lean systems including potassium prolinate?ProK?,potassium lysinate?LysK?and potassium sarcosinate?SarK?in the ethylene glycol?EG?,2-methoxyethanol?EGME?and 2-ethoxyethanol?EGEE?were selected and the CO₂absorption and desorption performance was investigated in a screening setup using a mixture of N₂/CO₂ to simulate flue gases.Physicochemical properties of these absorbents were measured using densimeter and microviscometer.Solubility of N₂O and CO₂ in the system of AAS/EG or EGME/water were determined using a static method in a vapor-liquid equilibrium apparatus.The rate of CO₂ absorption into ProK/EG/water solution with ProK concentrations ranging from?0.5 to 2.0?M was measured over the temperatures range of?283.2-313.2?K in a magnetic-drive stirred cell reactor.The main products after CO₂ absorption were identified by NMR,XRD and TGA.The relative heat duty of the proposed phase change absorbents was also evaluated and compared with the conventional aqueous monoethanolamine?MEA?as a baseline case.For CO₂ absorption by AAS/EG water-lean absorbents,different phase behavior was observed.Similar absorption behavior and desorption efficiency were found between the system of ProK/EG/water and aqueous MEA.However,liquid-solid phase change took place at about CO₂ loading of 1.4mol/kg from a homogeneous solution of LysK/EG/water during CO₂ absorption.Absorption capacity showed quite stable after eight runs of absorption-desorption cycle,about 0.88 mol/kg and 1.35 mol/kg for ProK/EG/water and LysK/EG/water respectively.Carbamate and bicarbonate were identified as the main products in the CO₂-loading samples.Densities and viscosities of the systems above-mentioned decreased with the increasing temperature,but increased greatly with the increase in AAS concentration as well as the overall CO₂ loading.The calculated values from the proposed empirical correlations are in good agreement with the experimental data and the average absolute deviations?AADs?are within 2.5%.It was found from the vapor-liquid equilibrium data that,solubility of N₂O and CO₂ in EG solvent is greater than that in water.The addition of water can enhance the Henry's law constants.Henry's law constants of CO₂ in water-lean solvents were estimated and correlated by N₂O/CO₂ analogy.Compared with the aqueous systems,high solubility of CO₂ in water-lean solvents will be beneficial for chemical reaction,while the high viscosity of the solutions has adverse effect on mass transfer between phases during CO₂absorption.The heat of CO₂ absorption for 2.0M ProK/EG/water was estimated to be about 32-70 kJ/mol CO₂ from the solubility curves at temperatures of?313-353?K.The overall reaction rate constants were obtained from the measured absorption flux against CO₂ partial pressure under the pseudo-first-order regime conditions.Results show that the overall rate constants for ProK/EG/water system are greater than those for their aqueous solutions.The reaction order with respect to ProK?about 1.86?is found to increase with increasing the temperature,which is higher than that observed in aqueous ProK systems?about 1.40?.This indicates that zwitterion deprotonation is the rate-determining step.The kinetic results for ProK/water/EG systems were interpreted using two models based on zwitterion mechanism and termolecular mechanism.The proposed models could satisfactorily describe the experimental kinetic data with AADs within 10%.All the AAS/EGME/water absorbents can undergo liquid-solid phase change during CO₂ absorption.Absorption capacity for the systems of 2M SarK/EGME/water and 3M ProK/EGME/water was observed to be lower than that for the aqueous 30%MEA,however about 50-80%CO₂ absorbed can be enriched in the solid phase after absorption.The cyclic capacity for 3M ProK/EGME/water system was observed about 0.82mol/kg and kept stable after several absorption-desorption cycles.Carbamate and bicarbonate species were also the main products,similar to the findings for the system of ProK/EG/water.The variation of density and viscosity on temperature and compositions for the water-lean glycol ether systems is similar to that of water-lean EG systems.The predictions from the empirical correlations match well with the experimental data with AADs within 4.0%.It was observed that CO₂ loading increased with the increasing CO₂partial pressures.The Soft model can well represented the experimental solubility data for phase change systems.Under the thermal dsorption at 373 K,the reboiler heat duty for the CO₂-loaded absorbents above-mentioned was estimated to be in the range of 165-255kJ/mol CO₂,which only accounts for about 45-70%of the baseline case.Therefore,the proposed AAS/EGME/water systems show huge reduction in regeneration energy consumption and good stability,which are considered to be promising absorbents with excellent capture performance.