Sigmoid Correlations for Gas Solubility and Enthalpy Change of Chemical Absorption of CO₂

Abstract

Knowledge of the relationship between gas solubility and enthalpy change of chemical absorption of CO 2 is very important for exploring energy-efficient absorbents for CO 2 capture. To this end, equations that can directly correlate gas solubility with absorption enthalpy were derived through combining the van't Hoff equation with the reaction equilibrium thermodynamic model (RETM). Two typical reaction mechanisms for chemical absorption of CO 2 (1:1 and 1:2) were considered for RETM. The variations of gas solubility with enthalpy change were found to be distinctively sigmoid functions, regardless of the investigated temperature and pressure or assumed reaction forms between CO 2 and the absorbent molecule. Theoretically calculated variation curves of gas solubility vs enthalpy change agreed well with experimental results reported in literature. On the basis of the trade-off relationship between gas solubility and enthalpy change, criterions for evaluating energyefficient chemical absorbents for CO 2 capture were proposed. ■ INTRODUCTIONAs the concentration of atmospheric CO 2 continuously increases, the resulting global warming is attracting widespread attention across academic and industrial communities because of its potentially destructive impact on the environment and ecosystems. CO 2 is mainly emitted in flue gases, which results from the combustion of fossil fuels. 1 A widely used strategy to eliminate CO 2 from industrial gas streams is reversible absorption in liquids. 2 For the purpose of capturing CO 2 from flue gases, chemical absorption is preferred over physical absorption, as the concentration of CO 2 in flue gas is low (10− 15 v/v %). 3 However, the chemical absorption method typically consumes high amounts of energy during the desorption process. 4 Gas solubility and absorption enthalpy are two vital thermodynamic properties for evaluating the performance of chemical absorbents for CO 2 capture. The former indicates how much gas an absorbent can accommodate, and the latter reflects the strength of interaction between the gas and absorbent.For chemical absorbents designated for CO 2 capture, those with high absorption capacity and low enthalpy change are particularly attractive. 5,6 High CO 2 loading is required to reduce the volume of circulation of the absorbents, and low enthalpy change is meaningful for reducing the energy consumed during desorption. However, there is a contradictory relationship between gas solubility and absorption enthalpy since a low enthalpy change in an absorbent is generally accompanied by a low absorption capacity. 7,8 Therefore, finding a proper balance between gas solubility and absorption enthalpy is very important in selecting energy-efficient chemical absorbents for CO 2 capture.Despite the significance of gas solubility and absorption enthalpy, they have usually been treated separately in previous research. Notably, Deshmukh et al. 9 proposed a thermodynamic model based on the extended Debye−Huckel theory of electrolyte solutions for the solubility of CO 2 in alk...

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