Functionalized ionic liquids : absorption solvents for carbon dioxide and olefin separation

L.M. Galan Sanchez

Research output: ThesisPhd Thesis 2 (Research NOT TU/e / Graduation TU/e)

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Abstract

Nowadays one of the most imperative challenges for the industry is to find alternatives that improve the efficiency of processes to make more sustainable use of energy. The processes where gas separations are present normally require a vast use of energy and therefore an improvement in these processes is vital for improving the energy balance in industry. Additionally, improvement of the gas separation processes is positively related with a decrease in the amount of pollutants discharged to the atmosphere. The sectors that carry out a considerable amount of gas separation processes are the oil and gas production, refining industry, chemical industry and energy producers. Among many separations, the CO2/CH4 and olefin/paraffin separation are two of the most crucial and energy intensive separations carried out today. Gas absorption is generally the technology preferred for the CO2/CH4 separation and cryogenic separation, although highly energy demanding is normally applied to olefin/paraffin separations. The CO2/CH4 separation by gas absorption can be improved by finding low volatile solvents that require less energy for regeneration and exhibit a high stability. The olefin/paraffin separation is not yet carried out by absorption mainly due to the lack of a robust and reactive solvent that allows achieving a higher capacity and an olefin separation efficiency without degradation or loss of the separating agent. Ethylene and ethane are the olefin and paraffin selected for this study. Based on their properties, it is expected that Room Temperature Ionic liquids (RTILs) can be used as improved solvents in the targeted gas separations. RTILs are liquid organic salts, which generally consist of an organic cation and either an inorganic or organic anion. Among other properties, the RTILs are non volatile and can be considered as designer solvents. The nature of the cation and the anion determine the physical and chemical properties of the ionic liquid. As result of the existing dependence of properties on the nature of the constituent ions, it is possible to achieve specific properties by choosing the right combination of anion and cation. Using this tailoring process, functional groups can be added to the structure to provide a better performance of the RTIL when chemical reaction or specific affinity and selectivity are required. Commercially available RTILs were initially used to study the relation of the ionic liquid structure with physical properties and the absorption capacity. The physical properties such as density, viscosity and surface tension of standard RTILs are measured at different temperatures. The studied ionic liquids are formed with either an imidazolium, pyridinium or a pyrrolinium cation. The selection of anion includes thiocyanate (SCN), trifluoroacetate (TFA), methylsulphate (MeSO4), tetrafluoroborate (BF4), hexafluorophosohate (PF6), dicyanamide (DCA) and bis(trifluoromethylsulfonyl)imide (Tf2N). The measured ionic liquid densities were between 1.0 g.cm-3 and 1.4 g.cm-3 and viscosities of the order of 102 mPa s. Their surface tensions are between 30 mN.m-1 and 50 mN.m-1. The density, viscosity and surface tension decreased with an increase in temperature. The solubility of CO2, CH4, C2H4, and C2H6 in the selected ionic liquids is measured with a gravimetric balance (IGA 003) at temperatures between 298 K and 343 K and pressures up to 10 bar. The absorption isotherms suggest that gas solubility in the ionic liquids is largely influenced by the nature of the anion. The gas solubility into the standard RTILs increases with an increment in pressure and decreases with increasing temperature. The solubility in the standard ionic liquids is described using the Henry constant. For all studied RTILs, the most soluble gas was CO2, followed by C2H4, C2H6 and CH4. The ionic liquids with NTf2 anion exhibited the highest gas capacity and had a better performance. However, ionic liquids with the NTf2 anion are expensive. Suitable structures for task specific ionic liquids to be used in the targeted gas separation processes are devised based on the results of the physical characterization and gas absorption capacity. Designer RTILs were used as solvents for the separation of CO2/CH4 to improve the absorption of CO2 and increase the CO2 selectivity over CH4. Given the "designer" nature of the ionic liquids, functional groups are incorporated into the structure of a standard ionic liquid to promote the selective absorption of CO2. Structures such as a primary amine, tertiary amine and a hydroxyl group were incorporated to the ionic liquid cation. The individual gas absorption of CO2 and CH4 is measured with a gravimetric balance (IGA-003) at temperatures between 303 K and 343 K and at pressures lower than 10 bar. The performance of the ionic liquids as solvents for the CO2/CH4 separation was improved when functionalized ionic liquids were used. The absorption of CO2 was chemically enhanced and the absorption of CH4 was governed by physical mechanisms only. The absorption of CO2 exhibits simultaneously the behaviour of both physical and chemical absorption mechanisms. The largest enhancement is obtained when primary amine groups were attached to the ionic liquids. The CO2 volumetric capacity of the NH2-functionalized solvents was almost three times higher than that of a similar standard ionic liquid. The CO2 solvent load of NH2- functionalized solvents is between that of the load achieved with a solution 30% MEA and that of 30 % MDEA at 333 K. The CO2/CH4 selectivity calculated from single gas absorption is slightly better for the standard ionic liquids than for the physical solvents. The CO2/CH4 selectivity for the NH2-functionalized ionic liquids is more than twice that of the physical solvents such as Sulfolane and NMP. The NH2-functionalized ionic liquids exhibited a smaller change in enthalpy of absorption than that reported for the aqueous amine solvents. This indicates that less energy is required for the regeneration of the solvent and, therefore, the NH2-functionalized ionic liquids can potentially impact positively on the energy balance of the solvent recovery process. The potential of the standard room temperature ionic liquids as absorption solvents for the olefin/paraffin separation can be expanded due to their designer capability together with their wider range of polarities, low lattice energy and especially their dual organic and ionic character. At the same time, ionic liquids may overcome the drawbacks of the available solvents for olefin/paraffin separations. An RTIL-based solvent, formed by a standard ionic liquid mixed with a salt of a transition metal, was used to boost the C2H4 solubility and enhance the selectivity. Olefins are able to form reversible complexes with metal transition cations via the well known mechanism of metal ion-olefin complexation (p-bond complexation). The RTILbased solvent allowed the stabilization of the metal transition cation; the metal cation forms a reversible complex with C2H4, thereby enhancing the olefin absorption. The silver (I) cation is available from AgNO3, AgBF4, AgTFA, AgOTF and AgNTf2. Standard ionic liquids with a similar anion as the metal salt are used as solution media for the silver (I) salts. The highest C2H4 absorption capacity was obtained with the ionic liquids that contained NTf2 and OTF as anion. The superior capacity exhibited by the solvents with NTf2 and OTF anions can be attributed to the lower degree of ionic association within the ionic liquid and with the silver (I). At 303K, the C2H4 absorption capacity of the Im[NTf2] Ag 1.8 N solvent is five times higher than that of the standard emim[NTf2] and also it is comparable with that of a 6M aqueous silver nitrate solution at 298 K. At 333 K, the average selectivity obtained with Im[NTf2]-Ag 1.8 is around 100 and the C2H4 absorption enthalpy of Im[OTF]-Ag 1.2 N is about -11.2 kJ.mol-1. The RTIL-based solvents containing AgTFA and AgNO3 as source of Ag(I) exhibited the lowest C2H4 absorption capacity and were unstable. However, the RTIL-based solvents with AgOTF and AgNTf2 salts were stable and the absorption loads achieved after the regeneration of the solvent were similar to that obtained with the fresh solvent. To present a broader view of the potential of RTILs as absorption solvents, the kinetics of the CO2 capture in an ionic liquid solvent with primary amine functionalized RTILs were also investigated. A kinetic study was carried out in a stirred cell reactor (1 L), operated in batch mode using the decreasing pressure method. The volumetric mass transfer coefficient of the liquid phase was determined from experiments using bmim[BF4] as the liquid phase and the kinetics of the reaction were studied based on experiments carried out with a liquid phase containing solutions of 1(3-Aminopropyl)-3-methylimidazolium tetrafluoroborate (APMim[BF4]) in bmim[BF4]. The enhancement factor due to the chemical reaction was calculated from the fluxes of CO2 absorbed. The results indicate that the reaction takes place in an intermediate regime and is limited by the diffusion in the ionic liquid. The diffusion limitation of the reaction was anticipated since the diffusion coefficients of CO2 and APMim[BF4] in bmim[BF4] are about 100 times smaller than the coefficients for CO2 and amines in aqueous alkanolamine solutions. The reaction was assumed first order in both CO2 and APMim[BF4] and the calculated kinetic constants (k1,1) are of the same order of magnitude as the ones available for primary amine and CO2 in viscous media. Room Temperature Ionic Liquids (RTILs) are solvents with potential to perform the separation of CO2/CH4 and olefin/paraffin. The absorption capacity of the ionic liquids can be improved by modification of the structure. Designer ionic liquids performed better than the standard RTILs. The ionic liquid can be regenerated by applying high temperature and low pressures and the absorption capacities of the regenerated ionic liquids are similar to that achieved with the fresh liquid. The main disadvantage exhibited for the RTILs is their relatively high viscosity. Lower viscosities may be achieved by finding a counter anion that is associated with a reduced viscosity. The enthalpy of absorption in the designed ionic liquids is lower than that of the traditional reactive solvents; therefore, the regeneration of the ionic liquid solvents is associated with a lower energy demand. The solvent performance of functionalized RTILs combines the selectivity provided by a chemical capture mechanism with the bulk capacity attributed to a physical affinity. The obtained results demonstrate that it is possible to develop industrial solvents based on ionic liquid technology for separation of CO2 from CH4 and olefins from paraffins.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Department of Chemical Engineering and Chemistry
Supervisors/Advisors
  • de Haan, André, Promotor
  • Meindersma, Wytze, Copromotor
Award date17 Dec 2008
Place of PublicationEindhoven
Publisher
Print ISBNs978-90-386-1468-7
DOIs
Publication statusPublished - 2008

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