The solubility of carbon dioxide (CO2) in five tetraalkylphosphonium superbase ionic liquids, namely the trihexyltetradecylphoshonium phenoxide, trihexyltetradecylphoshonium benzotriazolide, trihexyltetradecylphoshonium benzimidazolide, trihexyltetradecylphoshonium 1,2,3-triazolide and trihexyltetradecylphoshonium 1,2,4triazolide was studied experimentally under dry and wet conditions at 22 °C and at atmospheric pressure using a gravimetric saturation technique. The effects of anion structure and of the presence or absence of water in solution on the carbon dioxide solubility were then deduced from the data. 1 H and 13 C-NMR spectroscopy and ab initio calculations were also conducted to probe the interactions in these solutions, as carbon dioxide and water can compete in the ionic liquid structure during the absorption process. Additionally, the viscosity of selected superbase ionic liquids was measured under dry and wet conditions in the presence or absence of CO2 to evaluate their practical application in carbon dioxide capture processes. Finally, the recyclability of the trihexyltetradecylphoshonium 1,2,4-triazolide under dry and wet conditions was determined to probe the ability of selected solvents to solubilize chemically a high concentration of carbon dioxide and then release it within a low energy demand process.

Biogas from anaerobic digestion of sewage sludge is a renewable resource with high energy content, which is composed mainly of CH 4 (40−75 vol %) and CO 2 (15−60 vol %). Other components, such as water (H 2 O, 5−10 vol %) and trace amounts of hydrogen sulfide and siloxanes, can also be present. A CH 4 -rich stream can be produced by removing the CO 2 and other impurities so that the upgraded biomethane can be injected into the natural gas grid or used as a vehicle fuel. The main objective of this paper is to assess the technical and economic performance of biogas upgrading processes using ionic liquids that physically absorb CO 2 . The simulation methodology is based on the COSMO-SAC model as implemented in Aspen Plus. Three different ionic liquids, namely, 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, 1-hexyl-3methylimidazoliumbis[(trifluoromethyl)sulfonyl]imide, and trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)sulfonyl]imide, are considered for CO 2 capture in a pressure-swing regenerative absorption process. The simulation software Aspen Plus and Aspen Process Economic Analyzer is used to account for mass and energy balances as well as equipment cost. In all cases, the biogas upgrading plant consists of a multistage compressor for biogas compression, a packed absorption column for CO 2 absorption, a flash evaporator for solvent regeneration, a centrifugal pump for solvent recirculation, a preabsorber solvent cooler, and a gas turbine for electricity recovery. The evaluated processes are compared in terms of energy efficiency, capital investment, and biomethane production costs. The overall plant efficiency ranges from 71 to 86%, and the biomethane production cost ranges from $9.18−11.32 per GJ (LHV). A sensitivity analysis is also performed to determine how several technical and economic parameters affect the biomethane production costs. The results of this study show that the simulation methodology developed can predict plant efficiencies and production costs of large scale CO 2 capture processes using ionic liquids without having to rely on gas solubility experimental data.

The miscibility of monoethanolamine (MEA) in five superbase ionic liquids (ILs), namely the trihexyl-tetradecylphosphonium benzotriazolide ([P 66614 ][Bentriz]), trihexyl-tetradecylphosphonium benzimidazolide ([P 66614 ][Benzim]), trihexyl-tetradecylphosphonium 1,2,3-triazolide ([P 66614 ][123Triz]), trihexyl-tetradecylphosphonium 1,2,4-triazolide ([P 66614 ][124Triz]), and trihexyl-tetradecylphosphonium imidazolide ([P66614][Im]) was determined at 295.15 K using 1 H NMR spectroscopy. The solubility of carbon dioxide (CO 2 ) in equimolar (IL + MEA) mixtures was then studied experimentally using a gravimetric technique at 295.15 K and 0.1 MPa. The effect of MEA on the CO 2 capture ability of these ILs was investigated together with the viscosity of these systems in the presence or absence of CO 2 to evaluate their practical application in CO 2 capture processes. The effect of the presence of MEA on the rate of CO 2 uptake was also studied. The study showed that the MEA can enhance CO 2 absorption over the ideal values in the case of [P 66614 ][123Triz] and [P 66614 ][Bentriz] while in the other systems the mixtures behave ideally. A comparison of the effect of MEA addition with the addition of water to these superbase ILs showed that similar trends were observed in each case for the individual ILs studied.