Development of Statistical Associating Fluid Theory for Aqueous Ionic Liquid Solutions by Implementing Monte Carlo Simulations and Ornstein-Zernike Integral Equation: Application to Describing Gas Hydrate Inhibition Performance of Ionic Liquids.

摘要

A recent version of statistical associating fluid theory (SAFT), namely SAFT2, is coupled with the van der Waals and Platteeuw theory to study the alkane hydrate phase equilibrium conditions. The model is found to provide an accurate representation of the alkane hydrate dissociation conditions with and without inhibitors, such as salts, alcohols, as well as mixed salts and alcohol. Based on SAFT2, a heterosegmented SAFT equation of state is developed to model the thermodynamic properties of aqueous ionic liquid (IL) solutions, which is recently discovered as dual function gas hydrate inhibitors. With transferrable model parameters, the heterosegmented SAFT generally well represents the liquid density, activity coefficient, and osmotic coefficient of aqueous imidazolium IL solutions. The inhibition effects of imidazolium IL on methane hydrate is also studied by the heterosegmented SAFT and the van der Waals and Platteeuw theory. The roles of pressure, anion type, alkyl length of the cation, and IL concentration on the hydrate inhibition performance of the imidazolium ILs are well captured.;The heterosegmented SAFT is then modified to better represent the thermodynamic properties of aqueous IL solutions with the help of Monte Carlo simulation and the solutions of Ornstein-Zernike integral equation. Monte Carlo simulations are conducted on the fluid mixture of charged and neutral hard spheres to obtain its structure and excess energies, the results of which are compared with the thermodynamic properties predicted by solving Ornstein-Zernike equation with the Hypernetted Chain (HNC) and Mean Spherical Approximation (MSA) closures. A simple modification of MSA, referred to as KMSA, is proposed to accurately predict the excess energies of electrolyte system in mixture with neutral component. The KMSA improves the heterosegmented SAFT by taking the effect of neutral alkyl branches on the electrostatic interactions into consideration. Monte Carlo simulations are also conducted on flexible charged hard-sphere chain molecules, and a SAFT model which implements either a dimer or a dimer-monomer approach to account for the charged chain connectivity is proposed. With the SAFT model for charged hard-sphere chain, the cation heads and some anions of ILs are more accurately modeled as charged chains instead of charged spherical segments.;With these improvements to the heterosegmented SAFT, a more accurate representation of activity coefficient and osmotic coefficient is achieved, and the modeling of aqueous IL solutions is extended to ammonium ILs and imidazolium ILs with organic anions. The inhibition effects of ammonium ILs on methane hydrate is investigated using the improved heterosegmented SAFT coupled with the van der Waals and Platteeuw theory, which is demonstrated to be a predictive tool for the screening of effective IL based hydrate inhibitor.