Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies

Abstract

One of the best-known uses of methanol is as antifreeze. Methanol is used in large quantities in industrial applications to prevent methane clathrate hydrate blockages from forming in oil and gas pipelines. Methanol is also assigned a major role as antifreeze in giving icy planetary bodies (e.g., Titan) a liquid subsurface ocean and/or an atmosphere containing significant quantities of methane. In this work, we reveal a previously unverified role for methanol as a guest in clathrate hydrate cages. X-ray diffraction (XRD) and NMR experiments showed that at temperatures near 273 K, methanol is incorporated in the hydrate lattice along with other guest molecules. The amount of included methanol depends on the preparative method used. For instance, single-crystal XRD shows that at low temperatures, the methanol molecules are hydrogen-bonded in 4.4% of the small cages of tetrahydrofuran cubic structure II hydrate. At higher temperatures, NMR spectroscopy reveals a number of methanol species incorporated in hydrocarbon hydrate lattices. At temperatures characteristic of icy planetary bodies, vapor deposits of methanol, water, and methane or xenon show that the presence of methanol accelerates hydrate formation on annealing and that there is unusually complex phase behavior as revealed by powder XRD and NMR spectroscopy. The presence of cubic structure I hydrate was confirmed and a unique hydrate phase was postulated to account for the data. Molecular dynamics calculations confirmed the possibility of methanol incorporation into the hydrate lattice and show that methanol can favorably replace a number of methane guests.flow assurance | outer planets | X-ray crystallography | NMR spectra | molecular dynamics simulation M ethanol is the quintessential antifreeze. It works by altering thermodynamic conditions of aqueous solution to suppress or delay the formation of icy phases when the temperature is reduced to below the normal freezing point of water. The action in aqueous solution is readily understood by considering strong hydrogen-bonding interactions between water and methanol that lower the chemical potential of the aqueous phase, leading to strongly nonideal solution behavior (1). The low-temperature phase diagram of water-methanol is well known (2), with a single solid 1:1 methanol hydrate compound identified (3) in addition to the pure solid phases. The eutectic in the water-methanol system occurs at rather low temperatures (between 155 and 160 K), which has led to speculations that liquid aqueous oceans may exist on icy planetary bodies, such as Titan and Enceladus (2-5). For the formation of clathrate hydrates, which are ice-like host lattices composed of water molecules so as to form cages that encapsulate guest molecules, the behavior of methanol as an antifreeze has always been assumed to parallel that in watermethanol solutions. Even after some 70 y of applied research of hydrate inhibition by methanol as part of gas-pipeline flow assurance programs in the oil and gas industry (6) and, more recently, applie...

Molecular Dynamics Simulations of Hydrogen Bonding in Clathrate Hydrates with Ammonia and Methanol Guest Molecules

“…Experimental observations show that the THF + CH 3 OH and THF + NH 3 clathrate hydrates are at least stable up to the 233 K and 263 K, respectively. 9,10 The stability of the THF + CH 3 OH and THF + NH 3 simulations at 263 K for both the TIP4P/ice and TIP4P potentials are consistent with these experimental results. A recent experimental work states that the THF clathrate hydrates synthesized in the presence of NH 3 in concentrations ranging from 0 to 0.25 mass fraction begin to decompose at 203.6 K. 28 Further study is need to determine if the exact conditions of the THF binary hydrate decomposition and how these experimental results can be used to distinguish between the predictions of the different force fields used in the simulation.…”

“…9 Under similar conditions, pure methanol hydrate did not form, but a complex mixture of sI and a mixed layer form of sII and structure H (sH) binary clathrate hydrates of methanol and methane are formed when vapor codeposited methanol and water are exposed to pressures of methane gas. 10 The presence of ammonia and methanol in clathrate hydrate Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday cages has been demonstrated by single-crystal X-ray diffraction studies, powder X-ray diffraction (PXRD), NMR studies, and molecular dynamics simulations. As well, complex ternary sI clathrate hydrate phases have been synthesized from water/ NH 4 F/methanol system.…”

“…However, the small polar NH 3 and CH 3 OH molecules can form nonstoichiometric hydrate phases from ice in the presence of methane or from aqueous solutions of THF. 9,10 The synthetic routes include clathrate hydrate synthesis via vapor deposition of water and gaseous guests, direct synthesis of binary hydrates from aqueous THF solutions with either ammonia or methanol, and synthesis of hydrates from frozen aqueous methanol or ammonia solutions and gaseous guests in the presence of methane or propane.…”

STRUCTURE AND DYNAMICS OF COLD WATER SUPER-EARTHS: THE CASE OF OCCLUDED CH4AND ITS OUTGASSING

“…It is interesting to note that molecular dynamics simulations show that it is energetically favorable to replace the methane molecules trapped in the large cages of a SI clathrate hydrate with methanol molecules (Shin et al 2013). Only in the case in which the ice experienced melting along its history and the methanol is in an aqueous solution will the reaction above become unfavorable.…”

“…It is commonly believed that since ammonia and methanol can create hydrogen bonds with water they will force the formation of stoichiometric compounds. Recent experiments and molecular dynamic simulations, however, are starting to reveal the true nature of the effect that ammonia and methanol have on a water system (Shin et al 2012(Shin et al , 2013). It appears that the role ammonia and methanol play changes dramatically when clathrate hydrate forming molecules are present.…”