Separation of CO₂ and H₂S Using Room-Temperature Ionic Liquid [bmim][MeSO₄]

Abstract

We have developed a ternary equation of state (EOS) model for the CO2/H2S/1-butyl-3-methylimidazolium methylsulfate ([bmim][MeSO4]) system to understand separation of these gases using room-temperature ionic liquids (RTILs). The present model is based on a modified RK (Redlich−Kwong) EOS, with empirical interaction parameters for each binary system. The interaction parameters have been determined using our measured VLE (vapor−liquid equilibrium) data for H2S/[bmim][MeSO4] and literature data for CO2/[bmim][MeSO4] and CO2/H2S. Due to limited VLE data for H2S/[bmim][MeSO4], we have also used VLLE (vapor−liquid−liquid equilibrium) measurements to construct the EOS model. The VLLE for H2S/[bmim][MeSO4] is highly asymmetric with a narrow (mole fraction H2S between 0.97 and 0.99) LLE gap which is the first such case reported in the literature and exhibits Type V phase behavior, according to the classification of van Konynenburg and Scott. The validity of the ternary EOS model has been checked by conducting VLE experiments for the CO2/H2S/[bmim][MeSO4] system. With this EOS model, solubility (VLE) behavior has been calculated for various (T, P, and feed compositions) conditions. For large (9/1) and intermediate (1/1) CO2/H2S feed ratios, the CO2/H2S gas selectivity is high (10 to 13, compared with <4.5 in the absence of ionic liquid) and nearly independent of the amount of ionic liquid added. For small CO2/H2S mole ratios (1/9) at 298.15 K, increasing the ionic liquid concentration increases the CO2/H2S gas selectivity from about 7.4 to 12.4. For high temperature (313.15 K) and large CO2/H2S feed ratios, the addition of the ionic liquid provides the only means of separation because no VLE exists for the CO2/H2S binary system without the ionic liquid.

Equation of state modelling of systems with ionic liquids: Literature review and application with the Cubic Plus Association (CPA) model

“…Yokozeki and Shiflett have used a modified generic Redlich-Kwong EoS in many studies since 2005 [65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82]. They used a generic Redlich-Kwong type of cubic EoS [83], with a modified van der Waals-Berthelot mixing rule.…”

“…The critical properties of ionic liquids were estimated by Vetere's method [84], assuming a critical compressibility factor of 0.256. Systems studied by this group include CO 2 in several different ionic liquids [65][66][67][68][69], hydrofluorocarbons and hydrofluoroethers in some ionic liquids [70][71][72][73], ammonia mixtures with ionic liquids [74,75] [77], water solubility in different ionic liquids [78] and ternary systems with ionic liquids, CO 2 and H 2 [79], SO 2 [80], and H 2 S [81,82]. The model has a maximum of four binary interaction parameters, although in some of the studies mentioned, some of the parameters are equal to zero.…”

Solubilities of hydrofluorocarbons in ionic liquids: Experimental and modelling study

“…They used a generic Redlich-Kwong type of cubic EoS [30], with a modified van der Waals-Berthelot mixing rule and critical properties of ILs estimated by Vetere's 4 method [31]. Systems studied by this group include CO 2 in several different Ils [8][9][10][11][12], hydrofluorocarbons and hydrofluoroethers in some Ils [13][14][15][16], ammonia mixtures with Ils [17,18] [23], and H 2 S [24,25]. The authors present most of their results in graphical form and conclude that the model yields a good correlation of the experimental values.…”

The solubility of gases in ionic liquids

“…One system where this has been done is gas mixtures of N 2 O and CO 2 in the IL (1‐butyl‐3‐methylimidazolium acetate [C 4 C 1 im][C 1 CO 2 ]), but there are other systems . Ternary VLE measurements were performed using a GC method to analyze the vapor composition which confirmed the validity of the ternary EOS model . Figure is a PTx phase diagram for N 2 O + [C 4 C 1 im][C 1 CO 2 ] which shows physical absorption with a large immiscibility (LLE) region that suggests this binary system likely exhibits Type III phase behavior.…”

“…An EOS model for a ternary system has been developed to evaluate the effectiveness of ILs for gas separation . Pure component and binary interaction parameters can be determined using the same procedures as discussed for the RK EOS model; however, the phase behavior prediction of the ternary system may not always be guaranteed based on the binary interaction parameters alone.…”