Novel green non-aqueous solvents for enhanced CO2 absorption

Carbon Dioxide capture from natural gas and flue gas is an important process for reducing the greenhouse gases emissions as well as avoiding operational challenges. Natural gas (NG) is consumed in a larger pace compared to other fossil fuels. However, one of the serious environmental, economical, and technological problems of utilising NG resources is the capture of accompanying CO2. The depletion of sweet natural gas fosters pressure for using the lower grade NG (sour gas) as an alternative. Consequently, the admixed CO2 in this grade of NG lowers its fuel value and contributes to global warming crises. In addition, CO2 is emitted from combustion and other chemical processes. This urges more advanced technologies for the removal of CO2. Production of natural gas involves either the gas-to-liquid (GTL) or liquefied natural gas (LNG) technologies. For both technologies, raw gas conditioning facility close to the natural gas reservoirs is needed in order to reduce acid gases [1]. The conditioning is necessary to avoid important operational challenges such as CO2 crystallization and equipment corrosion. Crystallization of CO2 present in transportation pipelines leads to many operational problems. Other problems associated with corrosion have to be taken into consideration when dealing with acid gas systems. Apart from operational issues, CO2 is one of the greenhouse gases which contribute to the global warming problem. Due to these and other problems, new stringent regulations require very high percentage recovery solutions for the sour/acid gases for new gas projects. In addition to NG, other important source of CO2 is fossil-fueled power plants. These plants dispose huge quantities of flue gas with paramount proportions of acid gases which introduce complex technological and economical challenges and cause global climate change. Figure 1.a shows the global relative amount of the main greenhouse gases. CO2 represents more than three quarters of all emitted gases. The most important source of CO2 is energy-related emissions with a proportion of 61% (Figure 1b). Moreover, CO2 emissions from fossil fuel combustion (coal, oil, and natural gas) for energy production are also the main source of energy-related emissions (Figure 1.c and Figure 1.d). Fossil fuels currently supply over 67% of the electricity used worldwide (Figure 1d).This is mainly because of their low cost, availability, existing reliable technology for energy production, and energy density. World CO2 energy-related emissions are expected to increase at a rate of 2.1% per year [2]. This is in agreement with the forecasted consumption of fossil fuels for electricity generation (natural gas will increase from 19.4% in 2005 to 25% in 2030, and coal will increase from 41.4% in 2005 to 46% in 2030) [2]. Figure 1. World GHG emissions indicating [2] (a) Gases, (b) CO2 emissions origin, (c) CO2 energy-related emissions, and (d) World electricity generation by fuel. Conventional CO2 Capture Techniques Removal of acid gas is a standard process in the natural gas processing and the liquefied natural gas (LNG) plants. Conventionally this is accomplished using amine solvents to meet the acid gas content limitations for products (e.g., sales gas, natural gas liquid (NGL) and LNG). If the concentrations of some acid gas components are likely to exceed the limitations prescribed in local environmental regulations, the separated acid gas is processed in acid gas incinerators to dispose the H2S and hydrocarbon gases in an environmentally friendly manner in order to meet the regulations. In most plants, the acid gas containing CO2 is released to the atmosphere either directly or after processing in the acid gas incinerators. Table.1 shows the typical natural gas composition [3]. CO2 content depends on the production source and is typically in the range of 0 - 8 vol%. For post combustion flue gases, CO2 content varies with fuel type, boiler age, design, and generating load. Most modern coal fired boilers produce a flue gas that contains approximately 12-14 vol% CO2 at about atmospheric pressure [2]. Large scale approaches proposed for CO2 capture and post combustion flue gases include cryogenic distillation, purification with membranes, absorption with liquids, adsorption with solids and biological ?xation [4-8]. Chemical absorption process is generally recognized as the most effective technology [9]. Efficient capture of CO2 at this low concentration and pressure will require chemical sorption. A variety of amines have been studied, which chemically react with CO2. Some of these are; monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), and di-2-propanolamine (DIPA), proprietary amines (KS-1), and mixed amines. Additives, such as piperazine and 2-amino-2-methyl-1-propanol (AMP) have also been studied to improve the reaction kinetics [10-13]. One of the highly desired features of favorable gas capture is the ability to undergo minimal loss of the capture agent or solvent into the gas stream [14]. Achieving this characteristic requires solvents with very low vapour pressure. Moreover, CO2 capture processes involving aqueous amine solutions suffer from other technical difficulties. The primary thermodynamic limitation with amine-based scrubbing technologies is the energy required to decompose the carbamate (reaction product) at high temperatures (383-403 K) during regeneration. The current capital and operating costs are high and do not meet the target of the Department of Energy to remove 90% of CO2 from post-combustion flue gas with no more than a 35% increase in the cost of electricity [15]. Furthermore, recent studies on amine-based scrubbing systems revealed that the corrosivity of the solvents employed is very high, and in the case of primary amines like MEA, corrosion rates may exceed 1 mm/year for carbon steel. This adds to the operational difficulties involved in running these plants [16]. Ionic Liquids for CO2 Capture Ionic liquids (ILs) are materials composed of ions and have melting points below 100 oC. ILs are considered good organic solvents, dissolving both polar and nonpolar species. In many cases, they perform better than commonly used solvents [2]. In addition, ionic liquids are non-flammable and non-volatile. The wide and readily accessible range of ionic liquids with corresponding variation in physical properties offers the opportunity to design an ionic liquid solvent system optimized for a particular process [17]. In this context, ionic liquids (low-temperature molten salts) have been proposed as solvent reagents for gas separation processes. They exhibit negligible volatility due to the Coulombic attraction between the ions of these liquids. Their corrosive behavior is controllable and more importantly, their absorption capabilities and selectivities are tunable by carefully selecting the individual ions and their substituents. These favorable properties and the urgent need for better solvents motivated researchers to study the solubility of different emission gases in ionic liquids. The choice of ILs as possible solvents was based on the fact that CO2 is soluble in many ILs. This incited many researchers to explore the rich synthetic landscape provided by ILs. Early studies of ILs involved combinations of few cations and anions, some of these are shown in Figure 2. The high affinity of imidazolium-based ILs toward CO2 was reported by several studies [18-20]. These results showed that CO2 solubility could be enhanced by changing the substituents in the IL cation. Furthermore, Scovazzo et al. [21] tested CO2 solubility of emim-based ILs with different anions. Their study showed that the nature of the anion has the key role in CO2-IL interactions. One approach to boost the CO2 solubility is to incorporate amine functional groups to ILs to mimic the use of amine based processes for CO2 absorption. This is a clear application of the Task-Specific ILs (TSILs) because they are specifically designed to increase the CO2 -IL interaction in this case. Bates et al. [22] prepared an imidazolium-based IL with an amine functional group attached to one of the alkyl chains. For this family of ILs, the absorption of CO2 is carried out through chemisorption, the proposed reaction, in which the studied IL reacting with CO2 in a 2:1 stoichiometric ratio is in good agreement with experimental results, showing that the mole ratio of CO2/TSIL approaches a maximum of 0.5 over the course of 3 h. The improved ionic liquid reacts reversibly with CO2, sequestering the gas as a carbamate salt. Another approach for enhancing the CO2 capture efficiency of ILs is by combining them with commercially available amines. This approach was found to be effective for the capture of CO2 as carbamate salts. Camper et al. [23] used IL solutions containing 50 mol % (16% v/v) MEA and DEA to capture 1 mol of CO2 per 2 moles MEA to give an insoluble MEAandminus;carbamate precipitate that helps to drive the capture reaction (as opposed to aqueous amine systems). These RTILandminus;amine solutions are claimed to behave similarly to their water-based counterparts but may offer many advantages, including increased energy efficiency, compared to current aqueous amine technologies. This success story of ILs as CO2 capture agents resulted in a pioneering industrial commercialization of power plant carbon emissions capture facility. The implemented technology involves a process that utilizes imidazolium-based ionic liquid to absorb gases like CO2, N2 and CH4 [24]. Novel Green Solvents for CO2 Capture One of the serious limitations preventing the widespread usage of ILs in industrial applications is their high cost. Deep Eutectic Solvents (DESs) have been recognized as low cost alternatives of ILs. DESs are mixtures of a salt and a hydrogen-bond donor (HBD), in which a new compound is formed, usually having much lower freezing temperature than their constituting components. DESs share common solvation properties with ILs. In addition, DESs have the advantage that they can be prepared easily with high purity at low cost compared to ILs. Other advantages include its relative non-toxicity, non reactivity with water and being biodegradable which made them considered as green solvents. Currently, DESs have mediated in many research areas and became attractive for many applications because of their potential use as alternative environment friendly solvents [25]. Recently, Li et al. [26] measured CO2 solubility in DESs composed of various mole ratios of urea and choline chloride (commercial name andldquo;relineandrdquo;) at pressures andgt;1 MPa. From their findings, they extrapolated Henryandrsquo;s law constants. This study showed that the DESs are capable of efficiently absorbing CO2. In a subsequent study, Su et al. [27] studied the solubility of CO2 in the same reline DES with different, moisture content at temperatures of (303, 308, and 313) K. The results showed that CO2 solubility in reline decreases with an increase in moisture content. Hence, water may serve as an antisolvent to drive out CO2 dissolved in reline. Their work showed that the solubility of CO2 in reline ranges from 6-8 times the solubility of CO2 in water. Moreover, they found that the enthalpy of absorption of CO2 in reline at low pressure is endothermic and if water content increases to beyond 76.9 %, the absorption becomes exothermic. They concluded that these interesting temperature and concentration dependencies make these novel solvents possible alternatives for future green and energy-saving CO2 capture processes. Previous studies on DESs were done based on a single DES (reline) with a fixed salt to HBD ratio. The effect of DES structure has a profound effect on its physical and thermodynamic properties [28]. Consequently, considerable investigation should be done to explore this factor and its effect on the solubility of CO2. In addition, other DESs (with different molecular structures) need to be investigated and their CO2 capture performance be compared to that of reline based systems. In this proposed work, novel ammonium and phosphonium based deep eutectic solvents (DESs) will be synthesized at different composition ratios. The DESs will be selected based on their physical properties, toxicity and the chemical structure of the constituting salt and hydrogen bond donor. Experimental investigation of the solubility and selectivity of CO2 in the selected DESs will be conducted. The static absorption experimental investigation will result in the selection of the most appropriate DES- CO2 systems for further investigations. The effectiveness and economy of DES recyclability will be evaluated. The developed novel process will be compared to conventional amine based technologies and further recommendations will be suggested for the scale-up of the bench scale process to the pilot plant scale.