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Xie et al. / Applied Energy 175 (2016) 69–81 List of symbols ai a constant that corrects for attractive potential of molecules of component i bi a constant that corrects for volume of component i Cp heat capacity, J mol1 K1 Hi Henry’s constant of component i Pc critical pressure, bar Tb normal boiling point, K Tc critical temperature, K Vc critical volume, m3/kmol xi mole fraction of component i in the liquid phase yi mole fraction of component i in the vapor phase Zc critical compression factor LCH4 CH4 loss ratio Nstage number of theoretical stage of absorber Pabsorber pressure of absorber, bar Pbiogas pressure of biogas, bar Pflash1 pressure of flash-1, bar Pflash2 pressure of flash-2, bar CO2 removal efficiency temperature of biogas, K temperature of desorber, K CO2 mole flow rates of biogas, kmol/h VCO2-SG CO2 mole flow rates at the top exit of Flash-2, kmol/h VCH4-BIOGAS CH4 mole flow rates of biogas, kmol/h VCH4-SG CH4 mole flow rates at the top exit of Flash-2, kmol/h YCH4 CH4 product yield Greek Letters c activity coefficient g viscosity, Pa s q density, kg m3 r surface tension, N m1 u fugacity coefficient x acentric factor RCO2 Tbiogas Tdesorber V CO2 -BIOGAS whereas this technology is energy intensive, combining with defi- ciencies of volatility, degradation and corrosion. Therefore, it is necessary to explore an efficient and low-cost technology. Nowadays, ionic liquids (ILs) have been paid much attention to be used as green solvents to separate CO2 due to the advantages of non-volatility, design ability, high thermal stability and high acid gas solubility. The conventional imidazolium-based ILs, such as [Cnmim][Tf2N], [Cnmim][BF4] and [Cnmim][PF6], have been investi- gated [9–12] intensively. The comparisons on the properties of ILs and gas solubilities in ILs have been conducted [11–15]. However, how the difference in properties and gas solubilities will further influence the performance of CO2 separation process has not yet been studied well. Process simulation via the commercial software Aspen Plus pro- vides a tool to conduct this research [16–19]. ILs, as a new type of sol- vent, has not been included in the databank, which makes it unready to conduct relevant research. Xu et al. [20] investigated the energy consumption and environmental impacts of three biogas upgrading technologies, i.e., high pressurized water scrubbing, aqueous mono- ethanolamine scrubbing and IL scrubbing. In our previous work [21], the biogas upgrading process using aqueous choline–chloride/urea (ChCl/Urea) has been studied, and the influence of water content on the amount of recirculated solvent and energy consumption was evaluated. However, the comparison with the performance of ILs on biogas upgrading has not yet been conducted. In this work, three imidazolium-based ILs, [hmim][Tf2N], [bmim][Tf2N] and [bmim][PF6], were chosen as liquid solvents to upgrade biogas because of the available experimental data on the properties and gas solubilities in these ILs. The experimental phys- ical properties of ILs were fitted by semi-empirical equations, and the experimental gas solubilities in ILs were represented by the non-random two-liquid (NRTL) model and Redlich–Kwong (RK) equation of state. After the implementation of model results, the process simulation for biogas upgrading using these ILs was per- formed in Aspen Plus. The simulation results were further com- pared with the processes using aqueous ChCl/Urea and pure water. 2. Thermo-physical properties and phase equilibria The knowledge of thermo-physical properties and phase equi- libria is requested to conduct the process simulation. The requested properties include the normal boiling temperature, crit- ical properties, molecular weight and temperature-dependent properties, such as density, viscosity, surface tension, heat capacity and vapor pressure. In this work, it was assumed that the biogas contains CO2 and CH4. The other impurities, such as H2S and H2O, have been removed by the pretreatment. In Aspen Plus, the properties of CO2 and CH4 can be directly obtained from Aspen property data- bank. However, neither conventional ILs nor novel liquid solvents have been included, thus the properties and phase equilibria need to be studied and then implemented before performing the process simulation. 2.1. Thermo-physical properties of ILs The group contribution method proposed by Valderrama et al. [22,23] was used to estimate the critical properties (TC, PC, VC, ZC) together with the normal boiling temperature (Tb) and the acentric factor (x) of ILs. The estimated properties of [hmim][Tf2N], [bmim] [Tf2N] and [bmim][PF6] are listed in Table 1. The properties of density, viscosity, surface tension and heat capacity have been measured experimentally. The available sources of experimental data for [hmim][Tf2N], [bmim][Tf2N] and [bmim][PF6] at different temperatures and atmospheric pressure were collected and listed in Table 2. The comparisons of available experimental data show that the experimental density and viscos- ity from different sources are consistent with each other, while the experimental surface tension and heat capacity of [hmim][Tf2N] and [bmim][PF6] from different sources show considerable dis- crepancies. As shown in Figs. 1 and 2, the surface tensions of [hmim][Tf2N] and [bmim][PF6] measured by Kilaru et al. [24] and Ghatee et al. [25] were higher than those from other sources. The heat capacities of [hmim][Tf2N] and [bmim][PF6] measured by Crosthwaite et al. [26] and Holbrey et al. [27] were much lower comparing to the experimental data from other sources. Therefore, the experimental data from these Refs. [24–27] was excluded in the further investigation. Table 1 Estimated properties of ILs. IL [hmim][Tf2N] [bmim][Tf2N] [bmim][PF6] Tb, K Tc, K Pc, bar Vc, cm3/mol Zc x 908.2 1297.8 23.89 1104.4 847.0 1261.9 27.65 990.1 0.261 0.300 554.6 719.4 17.28 762.5 0.220 0.792 0.245 0.389PDF Image | CO2 Separation with Ionic Liquids
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