Research On CO₂ Absorption Process Using Organic Amine/Glycol Ether Non-aqueous Blends

Chemical absorption using aqueous monoethanolamine?MEA?is the most mature and commercially available method for CO₂ capture.However,large energy consumption required for solvent regeneration and high capture costs still remain major disadvantages which result in delaying the worldwide large-scale deployment of this technology.The use of water in this system is the weakest point because of large specific heat capacity and enthalpy of vaporization of water.These associated energy consumption takes up about50-70%of the total energy for CO₂ desorption.Using organic solvents with high boiling point,low specific heat capacity and low enthalpy of vaporization by replacing the strong polar water to develop non-aqueous absorbents has potential to lower regeneration energy consumption and to reduce thermal degradation and equipment corrosion.Therefore,non-aqueous absorbents have attracted much attention in recent years.In view of the advantages of glycol ethers such as 2-methoxyethanol?EGME?and 2-ethoxyethanol?EGEE?with low viscosity,low volatility as well as low specific heat capacity,several blends of organic amines with glycol ethers were proposed for CO₂ capture in this thesis.CO₂ capture performance,reaction mechanisms,physicochemical properties and regeneration energy consumption for these systems were investigated systematically.The findings will provide important theoretical basis and support for developing next generation energy-efficient absorbents and demonstrating new capture processes,with great scientific value and practical significance.In this thesis,CO₂ absorption behavior and capture performance in various blends of amines and glycol ethers were investigated under gas conditions with similar CO₂ partial pressures to biogas and natural gas in a screening setup.The heat duty for different absorbents was evaluated in an experimental apparatus with electric heating.Physicochemical properties of these absorbents were measured using densimeter and microviscometer at various conditions.Solubility of N₂O and CO₂ in the blends of MEA and glycol ether was also determined in a vapor-liquid equilibrium apparatus using static methos,and the experimental data were interpreted using thermodynamic models.Reaction mechanisms were revealed by the findings of ¹³C NMR,FT-IR and online conductivity.Different phase behaviors were observed during CO₂ absorption.After reaching a specific CO₂ loading,liquid biphasic systems formed for several blends of MEA-diethylene glycol dimethyl ether?DEGDME?,MEA-diethylene glycol monobutyl ether?DGBE?and DEA-DGBE.But for the blends of amines with EGME or EGEE,still remained single phase systems after absorption.These results indicate that the properties of glycol ethers in the blends have great effect on absorption behavior.For homogeneous single phase systems,the CO₂ absorption capacity of MEA-glycol ether blends was about2.1 mol/kg,higher than that of DEA-EGME blends?1.6 mol/kg?at the same conditions.But very similar cyclic capacities of absorption-desorption?about 1.4-1.5 mol/kg?were observed for both absorbent blends.It was found that the cyclic capacity of MEA-glycol ether blends was 70%higher than that of aqueous 30 mass%MEA.Moreover,the mixture of MEA and 2ME or 2EE could significantly reduce the energy consumption by about55%as compared to the benchmark system.The proposed absorbents were stable with an average cyclic capacity of 1.45 mol/kg in several absorption-desorption cycles.MEA carbamate and protonated MEA were identified as the main product species.For liquid-liquid phase change system,about 95%CO₂ was enriched in the lower liquid phase?about 20-30v/v%?.Only a detectable amount of reaction products was found in the top phase.The limited solubility of ionic products in the weak polar solvents is probably the main reason resulting in the phase change.Densities and viscosities of MEA in EGME and EGEE systems decreased with the increasing temperature,but increased sharply with the increase in MEA concentration and CO₂ loading.Excess molar volumes V^E and viscosity deviations??for these blends were both negative over the entire range of mole fraction and temperature,which indicated that there was a volume contraction and decrease in viscosity on mixing,and strong intermolecular forces between MEA molecules.The experimental property data for MEA-glycol ether binary systems were correlated by the Redlich-Kister type equations,and the predictions from the fitting parameters were in good agreement with the experimental data with average absolute deviation?AAD?within 1.5%.Property data of MEA-glycol ether-CO₂ ternary systems were also correlated by the proposed non-dimensional empirical equations.The calculated from the correlations matched well with the experimental with AAD within 4.5%.Henry's coefficients of CO₂ in MEA/glycol ether systems were estimated by N₂O/CO₂ analogy.The predictions from the exponential model and experimental data were in good agreement with each other within 0.8%AAD.Compared with other non-aqueous solvents,glycol ethers with high CO₂ solubility and low viscosity showed advantages for CO₂ absorption process.The CO₂ partial pressures increase with increasing the CO₂ loading at a constant temperature and the CO₂ loadings decrease as the temperature increases at a given CO₂partial pressure.The change of temperature has great effect on the CO₂ solubility,which is favorable for temperature-swing CO₂ capture process.The cyclic absorption capacity from the solubility curves was in accord with the findings above-mentioned.A semi-empirical model and a Kent-Eisenberg model based on the reaction mechanism were proposed to interpret the vapor-liquid equilibrium data for the systems of CO₂/MEA/glycol ether solution and the AADs were within 20%and 10%,respectively.The overall equilibrium constants decrease as the temperature increase.The heat of absorption of CO₂?-?H?in the MEA-glycol ether blends is found to be close to the values in other non-aqueous solvents,but slightly lower than that in aqueous MEA solution.Non-aqueous blends of MEA and glycol ethers have shown low regeneration energy consumption and good capture performance.Therefore,these promising absorbents can be considered for CO₂ capture in several industries such as biogas upgrading and natural gas processing.