Noura Dawass

6 114 S OLUBILITY OF G ASES IN M ONOETHYLENE G LYCOL 6.1. I NTRODUCTION Monoethylene glycol (MEG) is a colorless, low-volatility, and stable liquid. MEG is fully miscible in water as well as in many organic liquids such as acetone and methanol [172] . In 2020, the global market size of MEG is valued at USD 20 bil- lion [173] . MEG is widely used as an anti-freeze agent, coolant, heat transfer agent, and as a raw material for the manufacturing of polyester fibers. [174] . In the oil–and–gas industry, MEG is widely used for the prevention of gas hydrate formation [174, 175] . In the course of mitigating gas hydrate formation, MEG has been reported to absorb acid gases such as carbon dioxide (CO 2 ) and hydrogen sulfide (H 2 S) [176] . Due to the absorption capability, stability, and miscibility of MEG inmany organic liquids, it is also considered for use in separation processes for acid gases [176– 179] . A number of MEG–based solvents, such as deep eutectic solvents, are consid- ered for CO 2 capture [179– 183] . More recently, mixtures made fromMEG, amines and water are investigated for simultaneously preventing hydrate formation and removing H 2 S in offshore oil–and–gas applications [184] . To achieve these pur- poses, triethylene glycol (TEG)–amine–water mixtures were previously used. Re- placing TEG with the less viscous MEG is expected to improve the absorption capability of glycols–amine–water solvents since absorption rates increase with lower viscosities [185] . To design and optimise processes in which MEG acts as a hydrate formation inhibitor or as an absorbent, knowledge of phase equilib- ria is essential [178, 185] . To this purpose, a number of experimental measure- ments of binary mixtures of MEG and gases, i.e., CO 2 and H 2 S have been per- formed [177, 178, 186] . For a review of experimental studies on the solubility of acid gases in MEG the reader is referred to Refs. [178, 187] . While traditionally phase equilibria data are obtained from experimental measurements, such an approach is not always feasible, especially if high pres- sures and/or temperatures are required, and when dangerous gases, such as H 2 S are involved. For this reason, theoretical approaches for computing the phase equilibria of mixtures of gases and liquids have been widely-used [1, 188– 190] . Unlike classical thermodynamic models, molecular–based methods account for the strong molecular interactions present in associating liquids [176] . The past few decades, molecular simulation has emerged as a powerful tool for using mi- croscopic information of associating liquids to predict their macroscopic behav- ior [22, 23] . In addition to providing thermodynamic and transport data, molec- ular simulation can also be used to investigate the microscopic structure of sol- vents, and to understand absorption mechanisms [5, 148] . The KB theory can be applied to study the absorption of gases in solvents. KBIs relate to fluctuations in the grand–canonical ensemble, from which a num- ber of thermodynamic properties are obtained such as derivatives of the chemi-