Development of aqueous-based phase change amino acid solvents for energy-efficient CO2 capture: The role of antisolvent
Graphical abstract
Introduction
Over the past few decades, increasing greenhouse gas (GHG) emissions and global temperature increases has become of concern [1], [2]. Anthropogenic carbon dioxide (CO2) emission, primarily from fossil fuel combustion (e.g., coal, oil and natural gas) for power generation, is the second most abundant GHG after water vapor and the main cause of global temperature rise in the atmosphere [3], [4]. Despite international protocols (e.g., Kyoto Protocol) and agreements (e.g., Paris Agreement), the atmospheric CO2 concentration has increased dramatically from the beginning of the industrial revolution (280 ppm in 1750) to date (412 ppm in 2019) resulting in an 0.8 °C elevation in global temperature [5].
Due to the challenges of developing green energy resources, fossil fuels are still the most prevalent source of energy in the world owing to their low cost [6]. As a result of increasing global demand for fossil fuels, particularly in developed countries, a continuing rising trend of CO2 emission is likely in the immediate future which will create further environmental challenges [7]. In this regard, a wide variety of separation and purification technologies (e.g., liquid absorption [8], [9], solid adsorption [10], [11], cryogenic distillation [12], [13] and membrane separation [14], [15]) have been developed to mitigate CO2 emissions. Although each method has its own advantages and disadvantages, CO2 absorption by aqueous monoethanolamine (MEA) solvent has been known as the most efficient, reliable and affordable technique for the majority of industrial applications [8], [16]. Nevertheless, aqueous MEA solvent suffers from a high energy consumption during the regeneration process, as well as high volatility, corrosion, toxicity and low thermal stability [17]. These issues have highlighted the necessity of finding new eco-friendly, benign and energy-efficient solvents.
Aqueous solutions of amino acid salts, containing the same functional groups as alkanolamines, has been identified as a promising alternative for amine solutions due to their physical-chemical properties including negligible volatility, low toxicity and high resistance to oxygen degradation [18], [19]. In recent years, several sodium or potassium amino acid solutions, including glycine [20], [21], proline [21], [22], [23], arginine [20], [21], [24], lysine [21], [25], histidine [21], [26] and sarcosine [20], [21], have been reported as potential candidates for CO2 removal. However, the regeneration of aqueous amino acid solutions is still energy-intensive and poses serious barriers to worldwide large-scale implementation.
While many investigations have been performed to develop novel energy-efficient solvents for commercial applications, phase change solvents have received increasing interest recently as another alternative solvent approach [19], [27], [28]. The replacement of water with organic solutions in non-aqueous phase change solvents can lead to two separate CO2-lean and CO2-rich phases after CO2 absorption. Hence, only the regeneration of the lower CO2-rich phase is required while the upper CO2-lean phase can be directly recycled to the absorption column. This significantly decreases the required energy during the regeneration process. Zheng et al. [29] studied the phase change behavior of a new non-aqueous solvent formed by the dissolution of triethylenetetramine (TETA) in ethanol (TETA/ethanol) for CO2 removal. The authors observed that solid TETA-carbamate precipitants formed during CO2 absorption in the TETA/ethanol solvent, whereas there was no precipitation in the TETA/water solvent. In a similar work, Cheng et al. [30] dissolved N-methyl-1,3-diaminopropane (MAPA) in N,N-dimethylformamide (DMF) and reported that solid precipitates were generated after CO2 capture. The authors established that non-aqueous phase-changing solvents displayed higher absorption capacities, faster absorption rates and lower energy consumptions than their corresponding aqueous solvents. Though a large number of research works have already proposed non-aqueous solutions as the new generation of energy-efficient phase change absorbers for CO2 capture, the serious drawbacks of non-aqueous solutions, including high viscosity, expense and high vapor pressure, make them unlikely choices for commercial applications.
As proline is the only amino acid which can be effectively dissolved in non-aqueous solutions [31], the development of amino acid phase change solvents has been limited. Recently, a handful of research works [32], [33] found that amino acids can be dissolved in aqueous-based solutions and confirmed the phase change feature of some amino acid solvents. However, the aqueous-based phase change amino acid solvent is a recently-emerged concept and further detailed investigations are still required for worldwide acceptance of this energy-efficient technology. Here, for the first time, the CO2 absorption/desorption of aqueous-based phase change amino acid solvents as a more conducive replacement for non-aqueous phase change amine solvents was examined. The potassium glycinate was used as the amino acid, due to its similar structure to MEA and high kinetics of CO2 absorption, for the investigation of aqueous-based amino acid solvents. DMF was chosen as the physical antisolvent (instead of typical alcohols, glycols and ionic liquids) due to its benefits such as low vapor pressure and volatility, high boiling point, low viscosity and comparable price for large-scale implementation. The equilibrium and dynamic CO2 absorption capacity, phase separation and species distribution were also studied. The regeneration and heat duty of aqueous-based amino acid solvents were also determined and compared to aqueous potassium glycinate and MEA solvent as baselines.
Section snippets
Chemicals and materials
Potassium hydroxide (KOH, 85%, Chem-Supply Ltd) and glycine (Gly, 98.5%, Chem-Supply Ltd) were reagent grade and purchased from Australian vendors. N,N-dimethylformamide (DMF, 99.8%, Merck) was used as received without further treatment. Highly purified water (Elix Millipore, resistivity >18.2 MΩ·cm) was utilized as a solvent to prepare aqueous and aqueous-based solvents. Pure carbon dioxide (CO2, 99.9%) and nitrogen (N2, 99.9%) were obtained from BOC Gases Australia Limited and used for
Phase separation and solid precipitation analysis
The phase separation behavior of different aqueous-based GlyK-X solvents at the end of CO2 absorption process is provided in Fig. 2. As seen, GlyK-10 solvent showed no phase change behavior and remained completely homogeneous upon CO2 absorption. By increasing the DMF:H2O ratio from GlyK-10, the system started precipitating in the form of white solid particles. Interestingly, Fig. 2 illustrates that the liquid-liquid-solid separation occurred in the GlyK-40 solvent with a nearly invisible
Conclusions
The mixture of GlyK/DMF/H2O was suggested as an aqueous-based phase change solvent with an excellent CO2 absorption capacity, high regeneration efficiency and comparable energy consumption. To study the role of DMF as an antisolvent in the aqueous-based solution, a series of amino acid salt GlyK-X solvents with different DMF:H2O volume ratios was prepared and tested. Among all GlyK-X solvents, the GlyK-60 solvent with DMF:H2O volume ratio of 60:40 reached the maximum volume of upper liquid
Acknowledgments
The authors would like to acknowledge the University of Melbourne for the Melbourne Research Scholarship, Particulate Fluids Processing Centre (PFPC) for infrastructural support and Peter Cook Centre for financial resources provided for this project.
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