Article pubs.acs.org/EF
Evaluation of Organic and Ionic Liquids for Three-Phase Methanation and Biogas Purification Processes Manuel Götz,*,† Felix Ortloff,† Rainer Reimert,† Omar Basha,‡ Badie I. Morsi,‡ and Thomas Kolb† †
DVGW-Forschungsstelle at the Engler-Bunte-Institut of the Karlsruhe Institute of Technology (KIT), Engler-Bunte-Ring 1, 76131 Karlsruhe, Germany ‡ Department of Chemical and Petroleum Engineering, University of Pittsburgh, 1249 Benedum Hall, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *
ABSTRACT: Three ionic liquids {butyl-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide [N1114][BTA], 1-methyl-1propyl-piperidinium bis(trifluoromethylsulfonyl)imide [PMPip][BTA], and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][Tf]} and two heat-transfer oils [dibenzyltoluene (DBT) and polydimethylsiloxane (trade name X-BF)] were evaluated for use in the three-phase methanation and the biogas purification processes. The density, viscosity, and surface tension of these liquids were measured and modeled as a function of the temperature. The solubilities of H2, CO, CO2, and CH4 in these five liquids were also obtained under different pressures and temperatures. Additionally, the criteria required for each of the two processes considered were identified: the three-phase methanation process requires a thermally stable liquid with a low vapor pressure and a high H2, CO2 and CO solubility, while the biogas purification process requires a highly selective CO2 solubility liquid at ambient temperature. From the evaluation of both the experimental data and the process requirements, the most suitable liquid for each of the aforementioned processes was identified. For the three-phase methanation process, the two ionic liquids [N1114][BTA] and [PMPip][BTA] and the two heat-transfer oils DBT and X-BF met the minimum requirements, while [EMIM][Tf] showed promising potential for the biogas purification process.
1. INTRODUCTION One option for reducing global CO2 emissions is to replace conventional coal-fired power plants with an integrated gasification combined cycle (IGCC) process with subsequent CO2 capture and sequestration. Another option is to efficiently use renewable energy sources, such as substitute natural gas (SNG), which can be produced by biochemical conversion (fermentation + biogas purification) or thermochemical conversion (gasification + methanation).1 The IGCC would benefit from CO2 capture at elevated temperatures, if suitable solvents or sorbents could be found,2−4 whereas the biogas purification process allows for CO2 removal at ambient temperature.5−7 It should be emphasized that these two options involve several processing steps, which require the use of a liquid with specific properties and high capture efficiency for CO2 (see Figure 1). The focus of this paper is on thermochemical SNG production using a novel catalytic threephase methanation reactor, where a liquid phase with high thermal stability is required for the solid catalyst suspension.8,9 Apart from that, the applicability of several liquids for the biogas purification process is discussed. Ionic liquids (ILs)2,10 and other novel solvents3,11 were recently proposed for gas purification processes. In this study, five liquids, comprising three ILs and two heattransfer oils, were investigated as potential liquid phase for the design of the three-phase reactor. The ILs include butyltrimethyl-ammonium bis(trifluoromethylsulfonyl)imide [N 1 1 1 4 ][BTA], 1-methyl-1-propyl-piperidinium bis(trifluoromethylsulfonyl)imide [PMPip][BTA], and 1-ethyl-3methylimidazolium trifluoromethanesulfonate [EMIM][Tf], © 2013 American Chemical Society
Figure 1. Requirements for solvents applied for three-phase methanation (blue) and biogas purification (green).
which were provided by IoLiTec GmbH, Germany, and the heat-transfer oils include dibenzyltoluene (DBT), trade name Jarytherm DBT, manufactured by Total, Germany, and polydimethylsiloxane (PDMS), trade name X-BF, manufactured by Fragol GmbH, Germany. The density, viscosity, surface tension, and selectivity of the five liquids were obtained. Received: February 26, 2013 Revised: June 24, 2013 Published: June 27, 2013 4705
dx.doi.org/10.1021/ef400334p | Energy Fuels 2013, 27, 4705−4716
Energy & Fuels
Article
Table 1. Overview of the Heat-Transfer Oils and the ILs Used DBT heat-transfer oil molecular formula molecular weight, M (kg/kmol) current application
X-BF heat-transfer oil
C21H20 (C2H6OSi)n 272 building block = 74 heat-transfer media, bath liquid for thermostats
2. CHARACTERIZATION OF THE FIVE LIQUIDS [N1114][BTA] and [PMPip][BTA] are ILs based on the bis(trifluoromethylsulfonyl)imide anion, which leads to comparatively high thermal stability and low miscibility in water. [EMIM][Tf] is an IL with a trifluoromethanesulfonate anion, which exhibits higher selectivity for CO2 separation from CH4 than the other ILs based on the [BTA] anion. In general, it should be mentioned that ILs have been reported to exhibit negligible vapor pressures at high temperatures. DBT is an aromatic molecule and is the only one among the five used liquids that consists only of C and H atoms. X-BF is a siloxane-based synthetic polymer with a high molecular weight, which includes additives to improve its thermal stability. Both liquids are commonly used as indirect heat-transfer media for several hightemperature applications and as a liquid bath for thermostats (see Table 1). 2.1. Density, Viscosity, and Surface Tension. Figure 2 shows the densities of DBT, X-BF, and [N1114][BTA]12−14 as a function of
σ = Aσ + Bσ T
(3)
(4)
(5)
2.2. Thermal Stability and Vapor Pressure. Because the threephase methanation reactor requires a thermally stable liquid, Table 3 that DBT has the highest application temperature (Tmax = 623 K); however, it exhibits the highest vapor pressure (37 mbar). Also, the nickel-based catalyst commonly used in the three-phase methanation may promote hydrogenation and, consequently, degradation of DBT, resulting in low boiling products, such as benzene29 or cyclohexane. On the other hand, X-BF appears to be less stable, but it has a lower vapor pressure. In general, ILs have been reported to exhibit negligible vapor pressures, minimizing the contamination of the products; however, most ILs do not feature long-term thermal stability at temperatures higher than 473 K.30−33 Isothermal thermogravimetric analysis (TGA) measurements performed in collaboration with IoLiTec GmbH, Germany, have identified the two ILs [N1114][BTA] and [PMPip][BTA] to be thermally stable up to 573 K. Also, isothermal temperature exposure for over 6 h at 573 K resulted in a mass loss of
