Liquid–liquid equilibrium data of water with neohexane, methylcyclohexane, tert-butyl methyl ether, n-heptane and vapor–liquid–liquid equilibrium with methane

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Abstract

Liquid–liquid equilibrium (LLE) data for non-aqueous liquid (neohexane [NH], tert-butyl methyl ether [TBME], methylcyclohexane [MCH], or n-heptane [nC7]) and water have been measured under atmospheric pressure at 275.5, 283.15, and 298.15 K. It was found that TBME is the most water soluble followed by NH, MCH, and nC7. As the temperature increased, the solubility of the non-aqueous liquids (NALs) in water decreased. The solubility of water in the non-aqueous liquid was found to increase in the following order: MCH < nC7 < NH < TBME. It was found to increase with increasing temperature. In addition, vapour–liquid–liquid equilibrium (VLLE) data for the above binary systems with methane were measured at 275.5 K and at 120, 1000, and 2000 kPa. It was found that the vapour composition of water and NALs decreased as the pressure increased. The water content in the non-aqueous phase was not a strong function of pressure. The concentration of methane in the non-aqueous phase increased as the pressure increased. Furthermore, the concentration of the methane and NALs in the water phase increased proportionally with pressure. The solubility of methane in water followed Henry's law. It is noted that the measurements were completed prior to the onset of hydrate nucleation.

Introduction

It is well known that methane and water in the presence of neohexane or methylcyclohexane or tert-butyl methyl ether in water can form a hexagonal hydrate structure (sH) instead of the cubic one (sI and/or sII) that methane forms with water alone [1]. Neohexane and the other substances are conveniently called large-molecule guest substance (LMGS) since they fit into the 20-faced cage (largest) of the structure H crystal [2]. The lower equilibrium pressure of sH hydrate than sI or sII hydrate has brought attention to many researchers in utilizing gas hydrate as a media for transportation and storage of natural gas [3]. The challenge is to find a compound that together with methane can form hydrate at the lowest pressure at the fastest rate. Moreover, n-heptane is not a LMGS but methane hydrate formation in the presence of this hydrocarbon serves as a model hydrate forming system involving three fluid phases.

Hydrate kinetics is a time-dependent hydrate phenomenon which is believed to be stochastic due to the difficulty to predict the induction time. The history of water, the degree of supersaturation, the mixing speed, driving force, guest molecule size to cavity ratio, geometry of the system, surface area and composition of the gases are factors which have been considered to influence the kinetics of hydrate formation [4]. In a recent study it was found that crystal growth is influenced by the amount of dissolved methane gas in the neohexane phase [5]. It was also found that the longer the induction time, the faster the rate of crystal growth due to higher saturation of gas in liquid. Thus, solubility of hydrate former in water and in non-aqueous phase is an important factor in the kinetics of structure H hydrate formation. Other papers also underlined the importance of solubility in determining the rate of hydrate crystal growth [6], [7].

A generalized equation has been established to calculate the solubility of binary alkane–water systems [8]. The mutual solubility of hydrocarbon and water are known to have a dependency on carbon number and temperature [9], [10]. The effect of pressure is small but positive [11]. Tsonopoulos [10] extensively discussed and reviewed the mutual solubility of normal alkanes (C5–C16), alkylcyclohexanes (C6–C12), alkylbenzenes (C6–C12) and 1-alkenes (C5–C12) in water. General equations based on thermodynamic analysis were presented. It was found that the solubility of hydrocarbon in water generally decreases steeply as the carbon number increases and decreases to a minimum at about 298 K before going back up again as the temperature increases. The solubility of water in hydrocarbons is insensitive to carbon number but increases with increasing temperature.

In this work, the mutual solubilities of methylcyclohexane (MCH), tert-butyl methyl ether (TBME), neohexane (NH) and n-heptane (nC7) in water phase as well as their concentration distribution in the presence of methane gas (three-phase system) were investigated. The mutual solubility data were also compared to available experimental data as well as model based calculated values [8].

Section snippets

Experimental procedure

All chemicals used in the experiment are listed in Table 1. Extra dry methanol (water content less than 50 ppm) was used as a solvent for preparing the calibration standard solution.

Results and analysis

All measurements reported here are the average values of two to three injections. The deviations from the mean values are given in parenthesis and shown in each table.

Conclusions

Non-aqueous–aqueous equilibrium (LLE) data at atmospheric pressure and at 275.5, 283.15, and 298.15 K were obtained. The non-aqueous liquid was neohexane (NH), tert-butyl methyl ether (TBME), methylcyclohexane (MCH), or n-heptane (nC7). TBME was found to be the most soluble in water followed by NH, MCH, and nC7. The NAL solubility in water decreases as the temperature increases. The solubility of water in the non-aqueous liquid phase increases with temperature. The distribution of methane in the

Acknowledgements

The financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Institute of Applied Energy (IAE), Japan are greatly appreciated. We thank Dr. Ohmura for his comments.

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