Measurements and modeling of vapor liquid equilibrium of CO2 in amine activated imidazolium ionic liquid solvents
Graphical abstract
Introduction
Owing to the high levels of CO2 concentration in the atmosphere, it has become indispensable to have better and efficient technologies for CO2 capture which ensures no more further effects of CO2 on global climate. Pre-, Post- and Oxy-capture processes have been commercialized successfully through various routes such as chemical and physical absorption, membrane separations, cryogenic separations, molecular sieves, metal organic frameworks, and ionic liquid supported membranes. Majorly, chemical and physical absorption are applied on large scale point sources such as power plants and petroleum or petrochemical plants but still the small growth in efficiency of these technologies have been demanded due to the improvement possibilities available [[1], [2], [3], [4]]. For post combustion processes, amine technology being the most developed one using traditional solvents (MEA, MDEA, AMP, Piperazine, etc.) faces several problems such as high regeneration energy, corrosion products, etc. Hence, the need for better solvents is still a thrust area of research in the discussed field. Few emerging technologies have received good appreciation in research over the past few decades including usage of either pure ILs, blended ILs [[5], [6], [7]], silylated amines [8], and grafted ILs [9,10]. The ILs tend to provide ease of regeneration due to low vapor pressures and better thermal stability. But the counter-disadvantages of using pure ILs being higher cost of processing due to high viscosity of the systems. The regeneration energy costs around 70% of the total cost of the separation process of CO2 capture using absorption. Due to the higher thermal stability and lower vapor pressure of ILs, they are expected to provide the advantage of lower regeneration energy and lower solvent losses. Hence, the amalgamation of ILs and amines is being proposed for post combustion CO2 capture. A brief literature review on the experimental and modeling studies of CO2 solubilities in blends of amines and ionic liquids is presented in Table 1 [[11], [12], [13], [14], [15], [16]].
Amongst the many different ILs, cations which are based on imidazole are somewhat well studied in the literature for two major reasons: one being the high CO2 selectivity due to presence of imidazole group and the other being cost effective in comparison to other cation based ILs. For the present study, effect of inclusion of amine activators on CO2 solubility in ILs, 1-butyl-3-methyl-imidazolium acetate ([bmim] [Ac]), is considered due to its high CO2 solubility [17]. The detailed structural analysis, physicochemical and critical properties such as critical temperature, critical pressure, acentric factor, etc. of [bmim] [Ac] have been studied and reported in the literature [[17], [18], [19], [20], [21], [22], [23], [24]]. Studies on the utilization of [bmim] [Ac] with either binary mixture of MDEA, MEA, DEA, DIPA and, AMP [12,14,16] for CO2 capture has been also reported. The results obtained in the literature indicated a high CO2 solubility along with the benefits of extremely low vapor pressure from IL and high CO2 solubility owing to the presence of amines. However, the reported study were majorly focusing on pure solvent for CO2 capture i.e. non-aqueous system or otherwise, if aqueous phase was considered the total amine solvent was considered to be much higher in comparison to ILs. Hence, present work is focused in choosing amines which should act as an activator at a lower concentration in comparison to IL while keeping the total solvent concentration to be optimum.
Piperazine (PZ) being one of the primarily used activators for the amine technology due to various advantages over other primary, secondary, tertiary or sterically hindered amines such as higher reaction rate with CO2, high equilibrium CO2 solubility and lower regeneration energy [25]. But along with these advantages, one of the major disadvantages is precipitation at a lower temperature and hence formation of slurry in the absorption towers which leads to added mass transfer resistance for CO2 [2,25]. Hence, recently another category of amines which act as activators are being proposed by our research group i.e. AEP and APA [[26], [27], [28], [29]]. The kinetic studies of both these amines indicated as promising solvents towards post combustion CO2 capture.
We conceivably foresee that the combination of [bmim] [Ac] with amines, AEP and APA, may provide better CO2 loadings compared to conventional aqueous amine solvents with the advantages of lower vapor pressures and ease of regeneration of the IL blends. By changing the amine and IL blend composition, a broad spectrum of VLE studies are carried out over a range of composition of ILs and amine activator. This will certainly help in selecting optimum operating conditions of temperature, composition and pressure which are quite useful for CO2 capture process. The present study focuses on the measurement of CO2 equilibrium solubility and physiochemical properties as well as estimation of thermodynamic properties of the novel aqueous blends of imidazolium based 1-butyl-3-methyl-imidazolium acetate IL with AEP and APA.
Section snippets
Materials
CO2 gas (>99% Pure) was procured from Linde India Ltd. and used without further purification. 1-Butyl-3-methyl-imidazolium acetate ([bmim] [Ac], ≥95% Pure), 1-(2-aminoethyl) piperazine (AEP, 99% Pure), and bis (3-aminopropyl) amine (APA, 98% Pure) were purchased from Sigma–Aldrich. AEP and APA are used with no further purification for the CO2 solubility study. For measurement of pure properties of [bmim] [Ac] and solution preparation, the [bmim] [Ac] was first analyzed for its initial water
Modified Kent-Eisenberg modeling
Modeling of CO2 absorption has been done in two different ways in the literature: (1) CO2 capture process modeling using different approaches to optimize the entire process [[35], [36], [37], [38]] and (2) VLE or CO2 solubility modeling. Modeling of CO2 solubility has been much studied in the literature using various approaches such as machine learning through neural networks and neuro fuzzy approach [39], mathematical regression models such as multiple linear and multiple quadratic approaches [
Effect of various reaction parameters on αCO2
The usual trend of decrease in solubility of CO2 while increase in temperature is observed in the absorption studied of aq. [bmim] [Ac], aq. ([bmim] [Ac] + AEP] and aq. ([bmim] [Ac] + APA). This decrease in αCO2 is owing to the exothermic nature of the reaction of proposed solvents and CO2. With rise in activator concentration and keeping the overall concentration of the solvents to be constant, increase in CO2 solubility is also observed in the aqueous blends of aq. ([bmim][Ac]+AEP) and aq.
Conclusions
CO2 solubility in aqueous [bmim] [Ac] enhanced by various amine activators viz. AEP and APA were studied over a wide range of experimental conditions. Qualitative analysis using FTIR and 13C NMR of the unloaded and loaded solvents indicated the formation of intermediate acetic acid, and imidazolium stable carboxylate. The results evidently specify that CO2 solubility increases with respect to rise in both PCO2 and concentrations of activators in solvent blends. Modified KE inclusive of gas
CRediT authorship contribution statement
Sweta Balchandani: Methodology, Investigation, Validation, Software, Writing - original draft. Bishnupada Mandal: Supervision, Conceptualization, Writing - review & editing. Swapnil Dharaskar: Resources, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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