Ionic liquids in carbon capture

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The use of ionic liquids in carbon capture is a potential application of ionic liquids as absorbents for use in carbon capture and sequestration. Ionic liquids, which are salts that exist as liquids near room temperature, are polar, nonvolatile materials that have been considered for many applications. The urgency of climate change has spurred research into their use in energy-related applications such as carbon capture and storage.

Contents

Carbon capture using absorption

Ionic liquids as solvents

Amines are the most prevalent absorbent in postcombustion carbon capture technology today. In particular, monoethanolamine (MEA) has been used in industrial scales in postcombustion carbon capture, as well as in other CO2 separations, such as "sweetening" of natural gas. [1] However, amines are corrosive, degrade over time, and require large industrial facilities. Ionic liquids on the other hand, have low vapor pressures . This property results from their strong Coulombic attractive force. Vapor pressure remains low through the substance's thermal decomposition point (typically >300 °C). [2] In principle, this low vapor pressure simplifies their use and makes them "green" alternatives. Additionally, it reduces risk of contamination of the CO2 gas stream and of leakage into the environment. [3]

The solubility of CO2 in ionic liquids is governed primarily by the anion, less so by the cation. [4] The hexafluorophosphate (PF6–) and tetrafluoroborate (BF4–) anions have been shown to be especially amenable to CO2 capture. [4]

Ionic liquids have been considered as solvents in a variety of liquid-liquid extraction processes, but never commercialized. [5] Beside that, ionic liquids have replaced the conventional volatile solvents in industry such as absorption of gases or extractive distillation. Additionally, ionic liquids are used as co-solutes for the generation of aqueous biphasic systems, or purification of biomolecules.

Process

A typical amine gas treating process flow diagram. Ionic liquids for use in CO2 capture by absorption could follow a similar process. AmineTreating.png
A typical amine gas treating process flow diagram. Ionic liquids for use in CO2 capture by absorption could follow a similar process.

A typical CO2 absorption process consists of a feed gas, an absorption column, a stripper column, and output streams of CO2-rich gas to be sequestered, and CO2-poor gas to be released to the atmosphere. Ionic liquids could follow a similar process to amine gas treating, where the CO2 is regenerated in the stripper using higher temperature. However, ionic liquids can also be stripped using pressure swings or inert gases, reducing the process energy requirement. [3] A current issue with ionic liquids for carbon capture is that they have a lower working capacity than amines. Task-specific ionic liquids that employ chemisorption and physisorption are being developed in an attempt to increase the working capacity. 1-butyl-3-propylamineimidazolium tetrafluoroborate is one example of a TSIL. [2]

Drawbacks

Selectivity

In carbon capture an effective absorbent is one which demonstrates a high selectivity, meaning that CO2 will preferentially dissolve in the absorbent compared to other gaseous components. In post-combustion carbon capture the most salient separation is CO2 from N2, whereas in pre-combustion separation CO is primarily separated from H2. Other components and impurities may be present in the flue gas, such as hydrocarbons, SO2, or H2S. Before selecting the appropriate solvent to use for carbon capture it is critical to ensure that at the given process conditions and flue gas composition CO2 maintains a much higher solubility in the solvent than the other species in the flue gas and thus has a high selectivity.

The selectivity of CO2 in ionic liquids has been widely studied by researchers. Generally, polar molecules and molecules with an electric quadrupole moment are highly soluble in liquid ionic substances. [6] It has been found that at high process temperatures the solubility of CO2 decreases, while the solubility of other species, such as CH4 and H2, may increase with increasing temperature, thereby reducing the effectiveness of the solvent. However, the solubility of N2 in ionic liquids is relatively low and does not increase with increasing temperature so the use of ionic liquids in post-combustion carbon capture may be appropriate due to the consistently high CO2/N2 selectivity. [7] The presence of common flue gas impurities such as H2S severely inhibits CO2 solubility in ionic liquids and should be carefully considered by engineers when choosing an appropriate solvent for a particular flue gas. [8]

Viscosity

A primary concern with the use of ionic liquids for carbon capture is their high viscosity compared with that of commercial solvents. Ionic liquids which employ chemisorption depend on a chemical reaction between solute and solvent for CO2 separation. The rate of this reaction is dependent on the diffusivity of CO2 in the solvent and is thus inversely proportional to viscosity. The self diffusivity of CO2 in ionic liquids are generally to the order of 10−10 m2/s, [9] approximately an order of magnitude less than similarly performing commercial solvents used on CO2 capture. The viscosity of an ionic liquid can vary significantly according to the type of anion and cation, the alkyl chain length, and the amount of water or other impurities in the solvent. [10] [11] Because these solvents can be “designed” and these properties chosen, developing ionic liquids with lowered viscosities is a current topic of research. Supported ionic liquid phases (SILPs) are one proposed solution to this problem. [5]

Tunability

1-butyl-3-propylamineimidazolium tetrafluoroborate is a task-specific ionic liquid for use in CO2 separation. 1-butyl-3-propylamineimidazolium-tetrafluoroborate-balls.png
1-butyl-3-propylamineimidazolium tetrafluoroborate is a task-specific ionic liquid for use in CO2 separation.

As required for all separation techniques, ionic liquids exhibit selectivity towards one or more of the phases of a mixture. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) is a room-temperature ionic liquid that was identified early on as a viable substitute for volatile organic solvents in liquid-liquid separations. [12] Other [PF6]- and [BF4]- containing ionic liquids have been studied for their CO2 absorption properties, as well as 1-ethyl-3-methylimidazolium (EMIM) and unconventional cations like trihexyl(tetradecyl) phosphonium ([P66614]). [3] Selection of different anion and cation combinations in ionic liquids affects their selectivity and physical properties. Additionally, the organic cations in ionic liquids can be "tuned" by changing chain lengths or by substituting radicals. [5] Finally, ionic liquids can be mixed with other ionic liquids, water, or amines to achieve different properties in terms of absorption capacity and heat of absorption. This tunability has led some to call ionic liquids "designer solvents." [13] 1-butyl-3-propylamineimidazolium tetrafluoroborate was specifically developed for CO2 capture; it is designed to employ chemisorption to absorb CO2 and maintain efficiency under repeated absorption/regeneration cycles. [2] Other ionic liquids have been simulated or experimentally tested for potential use as CO2 absorbents.

Proposed industrial applications

Currently, CO2 capture uses mostly amine-based absorption technologies, which are energy intensive and solvent intensive. Volatile organic compounds alone in chemical processes represent a multibillion-dollar industry. [12] Therefore, ionic liquids offer an alternative that prove attractive should their other deficiencies be addressed.

During the capture process, the anion and cation play a crucial role in the dissolution of CO2. Spectroscopic results suggest a favorable interaction between the anion and CO2, wherein CO2 molecules preferentially attach to the anion. Furthermore, intermolecular forces, such as hydrogen bonds, van der Waals bonds, and electrostatic attraction, contributes to the solubility of CO2 in ionic liquids. This makes ionic liquids promising candidates for CO2 capture because the solubility of CO2 can be modeled accurately by the regular solubility theory (RST), which reduces operational costs in developing more sophisticated model to monitor the capture process.

Related Research Articles

Solubility Capacity of a substance to dissolve in a solvent in a homogeneous way

Solubility is the property of a solid, liquid or gaseous chemical substance called solute to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure and presence of other chemicals of the solution. The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begins to precipitate the excess amount of solute.

Adsorption Process resulting from the attraction of atoms, ions, or molecules from a gas, liquid, or solution sticking to a surface

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. Adsorption is a surface phenomenon, while absorption involves the whole volume of the material, although adsorption does often precede absorption. The term sorption encompasses both processes, while desorption is the reverse of it.

Ionic compound Chemical compound involving ionic bonding

In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as the sodium (Na+) and chloride (Cl−) in sodium chloride, or polyatomic species such as the ammonium (NH+
4) and carbonate (CO2−
3) ions in ammonium carbonate. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network. Ionic compounds usually form crystalline structures when solid.

A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It can effuse through porous solids like a gas, overcoming the mass transfer limitations that slow liquid transport through such materials. SCF are much superior to gases in their ability to dissolve materials like liquids or solids. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned".

Ionic liquid

An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as 100 °C (212 °F). While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.

Piperazine Chemical compound

Piperazine is an organic compound that consists of a six-membered ring containing two nitrogen atoms at opposite positions in the ring. Piperazine exists as small alkaline deliquescent crystals with a saline taste.

Membrane gas separation Technology for splitting specific gases out of mixtures

Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials.

Ion chromatography

Ion chromatography separates ions and polar molecules based on their affinity to the ion exchanger. It works on almost any kind of charged molecule—including large proteins, small nucleotides, and amino acids. However, ion chromatography must be done in conditions that are one unit away from the isoelectric point of a protein.

Amine gas treating, also known as amine scrubbing, gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines (commonly referred to simply as amines) to remove hydrogen sulfide (H2S) and carbon dioxide (CO2) from gases. It is a common unit process used in refineries, and is also used in petrochemical plants, natural gas processing plants and other industries.

Liquid–liquid extraction (LLE), also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an organic solvent (non-polar). There is a net transfer of one or more species from one liquid into another liquid phase, generally from aqueous to organic. The transfer is driven by chemical potential, i.e. once the transfer is complete, the overall system of chemical components that make up the solutes and the solvents are in a more stable configuration. The solvent that is enriched in solute(s) is called extract. The feed solution that is depleted in solute(s) is called the raffinate. LLE is a basic technique in chemical laboratories, where it is performed using a variety of apparatus, from separatory funnels to countercurrent distribution equipment called as mixer settlers. This type of process is commonly performed after a chemical reaction as part of the work-up, often including an acidic work-up.

Deep eutectic solvents are systems formed from a eutectic mixture of Lewis or Brønsted acids and bases which can contain a variety of anionic and/or cationic species. They are classified as types of ionic solvents with special properties. They incorporate one or more compound in a mixture form, to give a eutectic with a melting point much lower than either of the individual components. One of the most significant deep eutectic phenomenon was observed for a mixture of choline chloride and urea in a 1:2 mole ratio. The resulting mixture has a melting point of 12 °C, which makes it liquid at room temperature.

1-Butyl-3-methylimidazolium hexafluorophosphate Chemical compound

1-Butyl-3-methylimidazolium hexafluorophosphate, also known as BMIM-PF6, is a viscous, colourless, hydrophobic and non-water-soluble ionic liquid with a melting point of -8 °C. Together with 1-butyl-3-methylimidazolium tetrafluoroborate, BMIM-BF4, it is one of the most widely studied ionic liquids. It is known to very slowly decompose in the presence of water.

A multiphasic liquid is a mixture consisting of more than two immiscible liquid phases. Biphasic mixtures consisting of two immiscible phases are very common and usually consist of an organic solvent and an aqueous phase.

Hexafluorophosphate Anion with the chemical formula PF6–

Hexafluorophosphate is an anion with chemical formula of PF−
6. It is an octahedral species that imparts no color to its salts. PF−
6 is isoelectronic with sulfur hexafluoride, SF6, and the hexafluorosilicate dianion, SiF2−
6, and fluoroantimonate SbF−
6. Being poorly nucleophilic, hexafluorophosphate is classified as a non-coordinating anion.

A carbon dioxide scrubber is a piece of equipment that absorbs carbon dioxide (CO2). It is used to treat exhaust gases from industrial plants or from exhaled air in life support systems such as rebreathers or in spacecraft, submersible craft or airtight chambers. Carbon dioxide scrubbers are also used in controlled atmosphere (CA) storage. They have also been researched for carbon capture and storage as a means of combating global warming.

Superheated water

Superheated water is liquid water under pressure at temperatures between the usual boiling point, 100 °C (212 °F) and the critical temperature, 374 °C (705 °F). It is also known as "subcritical water" or "pressurized hot water." Superheated water is stable because of overpressure that raises the boiling point, or by heating it in a sealed vessel with a headspace, where the liquid water is in equilibrium with vapour at the saturated vapor pressure. This is distinct from the use of the term superheating to refer to water at atmospheric pressure above its normal boiling point, which has not boiled due to a lack of nucleation sites.

Methyl diethanolamine Chemical compound

Methyl diethanolamine, also known as N-methyl diethanolamine and more commonly as MDEA, is the organic compound with the formula CH3N(C2H4OH)2. It is a colorless liquid with an ammonia odor. It is miscible with water, ethanol and benzene. A tertiary amine, it is widely used as a sweetening agent in chemical, oil refinery, syngas production and natural gas.

Calcium looping (CaL), or the regenerative calcium cycle (RCC), is a second-generation carbon capture technology. It is the most developed form of carbonate looping, where a metal (M) is reversibly reacted between its carbonate form (MCO3) and its oxide form (MO) to separate carbon dioxide from other gases coming from either power generation or an industrial plant. In the calcium looping process, the two species are calcium carbonate (CaCO3) and calcium oxide (CaO). The captured carbon dioxide can then be transported to a storage site, used in enhanced oil recovery or used as a chemical feedstock. Calcium oxide is often referred to as the sorbent.

Nitroapocynin Chemical compound

Nitroapocynin is a mono-nitrated form of apocynin.

Dioxide Materials was founded in 2009 in Champaign, Illinois, and is now headquartered in Boca Raton, Florida. Its main business is to develop technology to lower the world's carbon footprint. Dioxide Materials is developing technology to convert carbon dioxide, water and renewable energy into carbon-neutral gasoline (petrol) or jet fuel. Applications include CO2 recycling, sustainable fuels production and reducing curtailment of renewable energy(i.e. renewable energy that could not be used by the grid).

References

  1. ↑ Arthur Kohl and Richard Nielson (1997). Gas Purification (5th ed.). Gulf Publishing. ISBN   978-0-88415-220-0.
  2. 1 2 3 Bates, Eleanor D.; Mayton, Rebecca D.; Ntai, Ioanna; Davis, James H. (2002). "CO2 Capture by a Task-Specific Ionic Liquid". Journal of the American Chemical Society. 124 (6): 926–927. doi:10.1021/ja017593d. ISSN   0002-7863. PMID   11829599.
  3. 1 2 3 Zhang, Xiangping; Zhang, Xiaochun; Dong, Haifeng; Zhao, Zhijun; Zhang, Suojiang; Huang, Ying (2012). "Carbon capture with ionic liquids: overview and progress". Energy & Environmental Science. 5 (5): 6668. doi:10.1039/c2ee21152a. ISSN   1754-5692.
  4. 1 2 Ramdin, Mahinder; de Loos, Theo W.; Vlugt, Thijs J.H. (2012). "State-of-the-Art of CO2 Capture with Ionic Liquids". Industrial & Engineering Chemistry Research. 51 (24): 8149–8177. doi: 10.1021/ie3003705 . ISSN   0888-5885.
  5. 1 2 3 Rodríguez, Héctor (2016). Ionic Liquids for Better Separation Processes. Green Chemistry and Sustainable Technology. doi:10.1007/978-3-662-48520-0. ISBN   978-3-662-48518-7. ISSN   2196-6982.
  6. ↑ Weingartner, H (2008). "Understanding ionic liquids at the molecular level: facts, problems, and controversies". Angew. Chem. Int. Ed. 47 (4): 654–670. doi:10.1002/anie.200604951. PMID   17994652.
  7. ↑ Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. (2002). "Solubilities and thermodynamic properties of gases in the ionic liquid 1-n-butyl-3- methylimidazolium hexafluorophosphate". J. Phys. Chem. B. 106 (29): 7315–7320. doi:10.1021/jp020631a.
  8. ↑ Ramdin, M.; de Loos, T. W.; Vlugt, T. J. H (2012). "State-of-the-Art of CO2 Capture with Ionic Liquids". Ind. Eng. Chem. Res. 51 (24): 8149–8177. doi: 10.1021/ie3003705 .
  9. ↑ Maginn, E. J. (2009). "Molecular simulation of ionic liquids: current status and future opportunities". J. Phys.: Condens. Matter. 21 (37): 373101. doi:10.1088/0953-8984/21/37/373101. PMID   21832331.
  10. ↑ Jacquemin, J; Husson, P.; Padua, A. A. H.; Majer, V. (2006). "Density and viscosity of several pure and water-saturated ionic liquids" (PDF). Green Chemistry. 8 (2): 172–180. doi:10.1039/b513231b.
  11. ↑ Gardas, R. L.; Coutinho, J. A. P. (2009). "Group contribution methods for the prediction of thermophysical and transport properties of ionic liquids". AIChE J. 55 (5): 1274–1290. CiteSeerX   10.1.1.619.2109 . doi:10.1002/aic.11737.
  12. 1 2 Huddleston, Jonathan G.; Willauer, Heather D.; Swatloski, Richard P.; Visser, Ann E.; Rogers, Robin D. (1998). "Room temperature ionic liquids as novel media for 'clean' liquid–liquid extraction". Chem. Commun. (16): 1765–1766. doi:10.1039/A803999B. ISSN   1359-7345.
  13. ↑ Freemantle, Michael (1998). "Designer Solvents". Chemical & Engineering News. 76 (13): 32–37. doi:10.1021/cen-v076n013.p032. ISSN   0009-2347.

Further reading

  1. Blanchard, Lynnette A.; Hancu, Dan; Beckman, Eric J.; Brennecke, Joan F. (1999). "Green processing using ionic liquids and CO2". Nature. 399 (6731): 28–29. Bibcode:1999Natur.399...28B. doi:10.1038/19887. ISSN   0028-0836.
  2. Camper, Dean; Bara, Jason E.; Gin, Douglas L.; Noble, Richard D. (2008). "Room-Temperature Ionic Liquid−Amine Solutions: Tunable Solvents for Efficient and Reversible Capture of CO2". Industrial & Engineering Chemistry Research. 47 (21): 8496–8498. doi:10.1021/ie801002m. ISSN   0888-5885.
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