Development of Predictive Techniques for Estimating Liquid Water-Hydrate Equilibrium of Water-Hydrocarbon System

Amir H. Mohammadi,Dominique Richon

doi:10.1155/2009/120481

Abstract

In this communication, we review recent studies by these authors for modeling the -H equilibrium. With the aim of estimating the solubility of pure hydrocarbon hydrate former in pure water in equilibrium with gas hydrates, a thermodynamic model is introduced based on equality of water fugacity in the liquid water and hydrate phases. The solid solution theory of Van der Waals-Platteeuw is employed for calculating the fugacity of water in the hydrate phase. The Henry's law approach and the activity coefficient method are used to calculate the fugacities of the hydrocarbon hydrate former and water in the liquid water phase, respectively. The results of this model are successfully compared with some selected experimental data from the literature. A mathematical model based on feed-forward artificial neural network algorithm is then introduced to estimate the solubility of pure hydrocarbon hydrate former in pure water being in equilibrium with gas hydrates. Independent experimental data (not employed in training and testing steps) are used to examine the reliability of this algorithm successfully.

Highlights

Gas hydrates are ice-like structures in which water molecules, under pressure, form structures composed of polyhedral cages surrounding gas molecule “guests” such as methane and ethane [1,2,3,4]
Among the liquid water-hydrate (LW -H) equilibrium data reported in the literature for the solubility of methane in pure water being in equilibrium with gas hydrates, those reported by Yang [16], Servio and Englezos [10] and Kim et al [11] seem to be the most reliable
As can be seen, (13a) with no adjustable parameter shows encouraging results. The predictions of this thermodynamic model for the solubility of methane in pure water being in equilibrium with gas hydrates show less than 18% absolute deviation and the average absolute deviation (AAD) among all the experimental and predicted data is 7.3% [4]

Summary

Introduction

Gas hydrates are ice-like structures in which water molecules, under pressure, form structures composed of polyhedral cages surrounding gas molecule “guests” such as methane and ethane [1,2,3,4]. It has been proved that gas hydrates occur in staggering abundance in cold subsea, sea floor, and permafrost environments where temperature and pressure conditions ensure their stability [1,2,3,4,5]. It is believed the amount of natural gas trapped in these deposits is much higher than the amount of natural gas existing in classical reserves [1, 2, 5]. It is believed that the potential storage of gas in hydrate is comparable to gas storage in the form of liquefied natural gas (LNG) and compressed natural gas (CNG) [2, 6]

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Conclusion