CO2 separation plays an important role to mitigate the CO2 emissions due to burning of fossil fuels, and it is also of importance in biofuel production (e.g. biogas upgrading and bio-syngas purification and conditioning). The solvent-based absorption is the state-of-art technology for CO2 separation, where various solvents, e.g. amine solutions, Selexol (i.e. dimethyl ethers of polyethylene glycol), and propylene carbonate, have been introduced. However, these solvent-based technologies meet challenges such as high solvent degradation, high corrosion rate to equipment, high construction cost, high energy demand for solvent regeneration and high solvent make-up rate. Therefore, the development of novel solvents to overcome the challenges of the currently available solvents is essential.

Recently, ionic liquids (ILs) have gained great interest as new potential solvents for CO2 separation, mainly due to their very low vapor pressure and relatively high CO2 solubility. In addition, ILs have lower corrosive characteristic, lower degradation rate and lower energy requirement for solvent regeneration compared with the conventional organic solvents. However, the main challenge of the application of ILs is their higher viscosity than the conventional solvents, which can be solved by adding co-solvents such as water.

The overall objective of this thesis work was to develop low-cost IL based technologies for CO2 separation. To achieve this objective, the deep eutectic solvent (DES) of choline chloride (ChCl)/Urea with molar ratio 1:2 as a new type of IL was selected as an absorbent and H2O was used as co-solvent for CO2 separation from biogas. The conceptual process was developed and simulated based on Aspen Plus, and the effect of water content on the performance of ChCl/Urea for CO2 separation was evaluated. It was found that the optimal proportion of aqueous ChCl/Urea was around 50 wt% (percentage by weight) of water with the lowest energy usage and environmental effect.

The performance of aqueous ChCl/Urea was further compared with the commercial organic solvents in this thesis work. The rate-based process simulation was carried out to compare the energy usage and the cost for CO2 separation from biogas. It was found that aqueous ChCl/Urea achieved the lowest cost and energy usage compared with other commercial solvents except propylene carbonate. The performance comparison proved that CO2 solubility, selectivity and viscosity were three important parameters which can be used as criteria in the development of novel physical solvents for CO2 separation.

ILs with acetate anions normally show high CO2 solubility and selectivity, and the ILs with alkylmorpholinium as cations have low toxicity leading to lower environmental effect. Therefore, in this thesis work, a series of N-alkyl-N-methylmorpholinium-based ILs with acetate as counterpart anion were investigated, and water was added as co-solvent to adjust the viscosity. The CO2 solubility in these aqueous ILs was measured at different temperatures and pressures. It was found that the increase of alkyl chain length in the cation led to an increase of CO2 solubility of the ILs with the same anion. Aqueous N-butyl-N-methylmorpholinium acetate ([Bmmorp][OAc]) had the highest CO2 solubility, and it was selected to further carry out thermodynamic modeling and process simulation. The energy usage and the size of equipment of using aqueous [Bmmorp][OAc], aqueous ChCl/Urea, water, Selexol, and propylene carbonate for CO2 separation from biogas were compared. It was found that this novel IL mixing with water had better performance, that is, with lower energy usage and smaller size of equipment than the other solvents. This result suggests that using this aqueous [Bmmorp][OAc] has the potential to decrease the cost of CO2 separation.

CO₂ separation plays a vital role in reducing CO₂ emissions to combat climate change, in which solvent-based absorption is widely considered the most promising technology. Many conventional chemical and physical solvents have been introduced for CO₂ separation, still facing challenges. The critical challenges for the absorption process based on conventional chemical solvents are the volatility, corrosivity, and degradation of the solvents and high regeneration energy demand in solvent regeneration. For the absorption process based on conventional physical solvents, the main challenges are the absorption capacity at low CO₂ partial pressure, selectivity of the solvents, and the discharge of volatile organic compounds. Therefore, it is critical to develop an energy-efficient, cost-effective, and environmentally benign technology for CO₂ separation based on new solvents. Ionic liquid/deep eutectic solvent (IL/DES) has drawn significant attention as a “green” alternative to conventional solvents in CO₂ separation. The reason is that they are relatively nonvolatile, nonflammable, environmentally benign, tunable, and can exhibit good thermal stability and high CO₂ solubility. But the main drawback of ILs/DESs is their much higher viscosity than conventional solvents. Several research works have proved that adding cosolvents in ILs/DESs to form hybrid solvents can overcome the disadvantage of the high viscosity of pure IL/DES. However, most work focuses on studying the physicochemical properties, and the research to develop hybrid solvents for CO₂ separation covering the whole chain from properties to process is very limited.

This thesis aims to develop hybrid solvents based on ILs/DESs for CO₂ separation and study their potential from energy and economic perspectives, where cosolvents were used to adjust the high viscosity of IL/DES. A systematic methodology of IL-/DES-based hybrid solvents for CO₂ separation spanning from properties determining, thermodynamic modeling to process design and evaluation, was built up. Several specific DESs/ILs that interacted with CO₂ chemically and/or physically aiming for the gas streams with different CO₂ concentrations (biogas or flue gas) were studied in this thesis work. A detailed comparison of the performances of IL-/DES-based hybrid solvents in terms of energy and cost with respect to conventional solvents was provided to evaluate their potential as alternatives. Thermodynamic models play an important role to precisely describe and predict the properties of the hybrid solvents. Therefore, the model development considering the micro mechanism was carried out. The main results are summarized below.

The cosolvent (water) greatly affects the properties, energy usage, and environmental impact in the study of using aqueous DES (choline chloride/urea, ChCl/Urea) solution for CO₂ separation from biogas, and this aqueous ChCl/Urea with 50 wt.% water shows the lowest energy usage and environmental impact. Compared to three other conventional physical solvents, aqueous ChCl/Urea achieves the lowest cost and energy usage in the scenario of building up a new process for CO₂ separation. At the same time, aqueous ChCl/Urea shows the second-lowest cost and energy usage in the scenario of retrofitting an existing process. The solvent properties, including CO₂ solubility, selectivity, and viscosity, are the three key properties in developing novel physical solvents for CO₂ separation. 

The investigation of a series of novel N-alkyl-N-methylmorpholinium-based ILs with acetate as counterpart anion for CO₂ separation shows that their CO₂ solubilities increase with the increase of alkyl chain length in the cation, even in their aqueous solutions. The use of this novel aqueous N-butyl-N-methylmorpholinium acetate IL solution for CO₂ separation from biogas shows the lowest energy usage and the smallest equipment size compared to other conventional physical solvents. The water acting as a cosolvent decreases the viscosity significantly, leading to a comparable mass transfer rate to the low viscous solvent. The modified process using this novel aqueous IL exhibits a 24.7% lower cost than the original water scrubbing. A new solvent screening index linking solvent properties and the cost is further formulated, providing a fast and quantitative criterion to screen solvent for CO₂ separation from biogas, free from the steps of process simulation and cost estimation. 

The hybrid solvents formed by IL (1-butyl-3-methylimidazolium acetate) and cosolvents were investigated in CO₂ capture from post-combustion flue gas and compared with the amine-based process. The techno-economic analysis of the new IL-based process integrated with waste heat recovery, when the CO₂ capture rate is 90% and the CO₂ purity in the recovered gas reaches 94%, shows that, compared with the aqueous amine solution, this new process exhibits a 45% decrease in utility cost and a 10% reduction in the total CO₂ capture cost.

In the model development, the model based on excess Gibbs free energy was developed to describe the macro properties of IL-H₂O systems (enthalpy of mixing, osmotic coefficient) and interpret their microstructures (real ionic strength, IL-dissociation, ionic hydration). This study clarifies the role of association and hydration in model development. The model reflecting the intrinsic mechanism of dissociation and hydration competition gives the best modeling results, and the predicted real ionic strength can be used to reliably estimate the electrical conductivities.