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Measurement and prediction of gas hydrate equilibrium conditions in the presence of inhibitors Wu, Huijie

Abstract

Gas hydrates are crystals which are formed by water and small gas molecules at low temperature and high pressure. Hydrate for many years have been a problem in o i l and gas industries because hydrate formation may plug the pipelines or valves and might also cause blowout in the drilling operations. In order to avoid hydrate formation, inhibitors are introduced to increase the pressure needed at a given temperature for hydrates formation. Generally used inhibitors in oil and gas industries are methanol, glycerol, ethylence glycol and triethylene glycol. Knowledge of the equilibrium hydrate-forming conditions is necessary for the rational and economic design of processes in the chemical, oil, gas, and other industries where hydrate formation is encountered. It is important to measure the incipient hydrate formation conditions for the systems containing different inhibitors, and also it is important to have available reliable methods for calculating the impact of the addition of these chemicals into the aqueous phase on the equilibrium hydrate formation conditions (inhibiting effect). In this work, the inhibiting effects of triethylene glycol (TEG) and glycerol in methane-ethane and methane-propane gas mixture hydrate formation systems were measured. The data showed that TEG (20.0wt% and 30.0wt %) and glycerol (20.0wt %) have considerable inhibiting effect on hydrate formation. These data are also valuable for validating the hydrate prediction models. Several models have been published based on cubic equations of state. In this work the Trebble-Bishnoi equation was used. The results were found to be in very good agreement with the data. The statistical associating fluid theory (SAFT) equation of state was also employed for the prediction of the thermodynamic inhibiting effect of methanol, glycerol, ethylene glycol and triethylene glycol on single gas hydrate formation. The results were found to be in satisfactory to excellent agreement with the experimental data. The SAFT equation takes into account hard sphere repulsion, hard chain formation, dispersion and association. This enables this model to be able to correlate and predict successfully systems containing water, alcohols and hydrocarbons.

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