| Acetylene (C2H2) and ethylene (C2H4) are important basic chemical feedstocks, so the research of their separation method is very significant. The traditional volatile absorbents in absorption lead solvent loss and environmental pollution. Ionic liquids (ILs) have unique properties, such as nearly non-volatility, tunable structures and properties, so IL absorbents can avoid solvent loss, environmental pollution and enhance C2H2/C2H4selectivity through tuning structures. We studied the interaction mechanism and separation performance between ILs and C2H2/C2H4, and developed a new kind of ionic liquid absorbents with high selectivity and capacity in separating C2H2and C2H4.This work used quantum chemistry calculation (QC) and molecular dynamics simulation (MD) to study the microstructure and interaction mechanism between ILs and C2H2/C2H4and to offer a deep insight into the difference of C2H2and C2H4solubilization mechanism in ILs. The calculated results indicate that the hydrogen bonding interaction between C2H2and anion is the dominant factor in determining the solubility of C2H2in ILs. The weak interactions, such as van der Waals forces, p-π interaction and π-π interaction, affect the solubility of C2H4in ILs. The diffusion coefficients calculation in MD shows that the diffusion coefficients of cations and anions have a significant increase after dissolving C2H2, indicating the viscosity of IL will decrease along with absorbing C2H2.On the basis of theoretical research above, we designed420ILs which are potential in separating C2H2and C2H4. An optimized COSMO-RS method to predict Henry’s law constants of C2H2, C2H4and their separation selectivity in ILs was developed for the first time. The prediction results indicate that the anion mainly determines C2H2selectivity and capacity. The cation with flexible structure and large molecular size can improve C2H2capacity.According the COSMO-RS prediction results, a kind of tetrabutylphosphonium-based long-chain carboxylate IL with proper free volume, strong basicity and availablity for separating C2H2and C2H4, were selected to determine the solubility of C2H2and C2H4in298.1K～313.1K and20kPa～180kPa. The experimental results show that [P4444][C5COO] has the highest C2H2capacity and high selectivity, Henry’s law constants of C2H2, C2H4and their separation selectivity are2.1bar,44.9bar and21.4at298.1K respectively, which is better than any other IL. Compared with traditional absorbents N-methylpyrrolidone (NMP) which has been applied in industry, C2H2mole and mass capacity in [P4444][C5COO] is0.476bar-1and2.425mol·kg-1·bar-1respectively, which are four times and two times as large as those in NMP. In the research of absorption rate, the capacity of C2H2in [P4444][C5COO] was99%of equilibrium capacity when absorption went on for10minutes. Moreover, the recycle experiment demonstrates that the C2H2-IL binding is reversible and [P4444][C5COO] can be regenerated without loss of IL and C2H2capacity.Besides, the solubility experiments of CH4, C2H6, C3H6, C3H8, CO2, H2and N2in [P4444][C5COO] indicate that [P4444][C5COO] has remarkable C2H2selectivity.298.1K, the selectivities of C2H2to C3H6, C2H6and H2are5.7,18.6and574.8respectively. Moreover,[P4444][C5COO] also displays its potential in recycling C2, C3hydrocarbon and separating different hydrocarbon.298.1K, the selectivity of C2H6/H2, C3H8/H2, C2H6/CH4and C3H8/CH4can reach31.0,92.9,5.6and16.8respectively. Thus, the tetrabutylphosphonium-based long-chain carboxylate IL can be used not only in separating C2H2from pyrolysis gas, but also in light hydrocarbon recovery from natural gas, dry gas and industrial exhaust gas. |
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