Prediction of Phase Equilibrium of Methane Hydrates in the Presence of Ionic Liquids Huai-Ying Chin, † Bong-Seop Lee, † Yan-Ping Chen, † Po-Chun Chen, ‡ Shiang-Tai Lin,* ,† and Li-Jen Chen* ,† † Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan ‡ Central Geological Survey, P.O. BOX 968, New Taipei City 235, Taiwan * S Supporting Information ABSTRACT: In this work, a predictive method is applied to determine the vapor-liquid-hydrate three-phase equilibrium condition of methane hydrate in the presence of ionic liquids and other additives. The Peng-Robinson-Stryjek-Vera Equation of State (PRSV EOS) incorporated with the COSMO-SAC activity coefficient model through the first order modified Huron- Vidal (MHV1) mixing rule is used to evaluate the fugacities of vapor and liquid phases. A modified van der Waals and Platteeuw model is applied to describe the hydrate phase. The absolute average relative deviation in predicted temperature (AARD-T) is 0.31% (165 data points, temperature ranging from 273.6 to 291.59 K, and pressure ranging from 1.01 to 20.77 MPa). The method is further used to screen for the most effective thermodynamic inhibitors from a total of 1722 ionic liquids and 574 electrolytes (combined from 56 cations and 41 anions). The valence number of ionic species is found to be the primary factor of inhibition capability, with the higher valence leading to stronger inhibition effects. The molecular volume of ionic liquid is of secondary importance, with the smaller size resulting in stronger inhibition effects. 1. INTRODUCTION Gas hydrates are nonstoichiometric crystalline solids of water and gas molecules. Depending on the size, the gas molecules may be encapsulated in the cavities existing in one of the three types of frameworks: structure I (sI), II (sII), and H (sH). 1 Methane hydrates have been attracting much attention because of their abundance in nature 2 and the potential of serving as a source of energy. 3 Gas hydrates may also be used as a media for sequestration of greenhouse gases, such as CO 2 . 4,5 The addition of inhibitors, thermal and pressure stimulation, and the combination of these methods are the primary methods for gas hydrate recovery. 6-9 Inhibitors are often introduced in the pipelines of oil recovery and transportation processes in order to prevent blockage by the formation of gas hydrates. 10-12 The presence of inhibitors in the system shifts the three-phase-coexisting condition to a lower temperature (or higher pressure), and thus prevents the formation of gas hydrates. Organic solvents (e.g., alcohols) and saline solutions are considered as good inhibitors for gas hydrate formation. Maekawa, Mohammadi et al., and Haghighi et al. 13-16 established the data of organic solvent inhibitors for methane, natural gas, propane, and carbon dioxide gas hydrate systems. Mohammadi et al. 17,18 used an isochoric pressure-search method to generate the experimental data of electrolytes added in different kinds of gas hydrate. Considering cost and effectiveness, methanol has been the most widely used thermodynamic inhibitor for gas hydrates. 10,12,16,19-21 In 2009, Xiao and Adidharma 22 discovered a new type of inhibitors for gas hydrates, the ionic liquids (ILs). They found that ILs not only reduce the dissociation temperature of gas hydrates (thermodynamic inhibitor) but also prolong the time for their formations (kinetic inhibitor). 22,23 They concluded that ILs with a shorter alkyl chain substituent and higher electrical conductivity exhibit better inhibition effects. ILs have been regarded as green solvents for chemical processes because they are nonflammable, nonvolatile, and thermally stable. 24 However, their direct use in the natural environment may still be of concern. Some ILs are environ- mentally benign and can be decomposed in wastewater treatments, either through biodegradation 25,26 or electro- chemical treatment. 26 Deng et al. 25 discovered that ILs with the presence of an ester group in the side chain are more easily biodegraded. There is a strong connection between the molecular structure of ILs and their functions as an inhibitor for gas hydrates and their environmental impact when used in the recovery. However, due to the structure diversity and the nearly unlimited combinations from cations and anions, the screen for candidate ILs could be a daunting task. In this work, we examine the prediction of the inhibition effects of ILs on the dissociation condition of gas hydrates based on the molecular structure of ILs. The Peng-Robinson-Stryjek-Vera (PRSV) EOS 27 combined with the predictive COSMO-SAC activity coefficient model 28 through the first order modified Huron- Vidal (MHV1) mixing rule 29 is used for the fluid phase, and the van der Waals-Platteeuw model 30 is used for the hydrate phase. The advantage of this approach is that no parameter fitting is needed for the ILs. The method is first validated using the rather scarce experimental data involving ILs. The effect of Received: August 18, 2013 Revised: October 30, 2013 Accepted: October 31, 2013 Published: October 31, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 16985 dx.doi.org/10.1021/ie4027023 | Ind. Eng. Chem. Res. 2013, 52, 16985-16992