Phase behaviour of gas hydrates of carbon dioxide in the presence of tetrahydropyran, cyclobutanone, cyclohexane and methylcyclohexane

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Abstract

This paper presents experimental data on gas hydrate phase behaviour of ternary systems containing water+carbondioxide+additive. The four additives studied are tetrahydropyran, cyclobutanone, cyclohexane and methylcyclohexane. Since all organic additives are very poorly soluble in water, they will be referred to as insoluble. Compared to the additive-free binary carbon dioxide system, reduction of the gas hydrate equilibrium pressure was observed in all systems. The pressure reducing effect of the additives on the hydrate equilibrium was similar for ternary systems with water+methane+additive and water+carbondioxide+additive. However, the phase behaviour of the latter type of systems is more complex, due to the occurrence of a liquid carbon dioxide phase. Quintuple points (H–Lw–La–Lhc–V) are reported for these systems. In the presence of tetrahydropyran, cyclobutanone and cyclohexane sII hydrates are assumed to be formed, while the hydrate phase with methylcyclohexane may be either sI or sH hydrate.

Introduction

Gas hydrates are molecular structures forming a lattice of hydrogen bonded water molecules, which are orientated in such a way that cavities are created. These cavities are large enough to host small molecules, e.g. methane (CH4), nitrogen (N2), xenon (Xe) and propane (C3H8). The encapsulation of these molecules stabilises the ice-like structure and enables the existence of gas hydrates at temperatures higher than the melting point of ice and at elevated pressures. Occupation of a considerable number of the available cavities is of major importance for stabilisation of the gas hydrate structure. Three different structures are currently known to exist; i.e. structure I (sI), structure II (sII) and structure H (sH). All structures are composed of cavities with different sizes [1].

For a long time, the main interest in gas hydrates came from the oil and gas industry, originally, due to blockages of transportation pipelines for oil and gas. Since it became apparent that in subsea sediments on the ocean floor and in permafrost regions [1], [2], [3] large natural gas reserves in the form of gas hydrates are present, industry showed a renewed interest in natural gas hydrates, albeit from a different point of view this time. The ability to enclose different sized molecules in large amounts also raises opportunities for technological applications of gas hydrates [4].

To judge whether various technological applications of gas hydrates are attractive and feasible or not, knowledge of the phase behaviour of systems in which gas hydrates occur is of major importance. Over the years, gas hydrate phase behaviour has been investigated experimentally for many binary systems, systems mainly of interest for the oil and natural gas industry. Examples are: H2O+CH4, H2O+CO2, H2O+C3H8, etc. Sources of experimental data on the system H2O+CO2 are numerous and have been summarised by Sloan [1].

This study reports experimental data on the gas hydrate phase behaviour of ternary systems water+carbondioxide+additive. The additives are tetrahydropyran, cyclobutanone, cyclohexane and methylcyclohexane. These organic additives are very poorly soluble in water and will therefore be referred to as water-insoluble. The phase behaviour and the pressure reducing effect of the additives will be compared with the additive-free system H2O+CO2, and with corresponding ternary systems H2O+CH4+additive.

Recently, it was established that the presence of some organic additives has the capability to reduce the hydrate equilibrium pressure significantly [4], [5], [6], [7], [8], [9], [10]. This phenomenon might provide such attractive conditions that gas hydrates become worthwhile to (re)consider for various technological applications. The hydrate equilibrium pressure for pure CO2 gas hydrates is substantially lower than the hydrate equilibrium pressure for CH4 and N2. In comparison with the latter two hydrate formers, the temperature range of CO2 hydrate existence is more limited, i.e. up to as high as approximately 283.3 K. This is because of the formation of a liquid CO2 phase. For technological application of gas hydrates it is an attractive condition if the gas hydrate equilibrium pressure could be reduced and/or the range of gas hydrate existence could be extended to higher temperatures. Addition of selected components might procure one or both of these conditions.

Section snippets

Theory

Gas hydrates can considered to be a separate solid phase that consists of at least two components, contrarily to other solid phases that may appear. It is composed of water, which forms the hydrate lattice, and at least one enclathrated component, which is in general a small size molecule. For gas hydrate applications, the hydrate equilibrium curve, H–Lw–V for a binary system, is the most important equilibrium in the phase diagram.

From previous hydrate research, it is known that the presence of

Experimental equipment and method

All measurements have been performed using Cailletet equipment, which has been described in detail elsewhere [13], [14]. A major advantage of the equipment is the combination of visual observation of the number and kind of phases present in the system and the precise and accurate measuring devices attached to it. The pressures are measured with a dead weight pressure gauge (de Wit) which has an accuracy of 0.005 MPa. The temperature is measured by a Platinum resistance thermometer (AΣΛ), which

Experimental results

This investigation concentrates on additives that are insoluble in water, because in certain gas hydrate applications it is important that the additive phase easily can be separated from the other phases. Systems of the nature H2O+CH4+additive, with the additives used here, have been reported previously [4], [7], [9]. At first, the hydrate equilibrium data for the binary system H2O+CO2 have been determined in order to compare them to data from literature [15], [16], [17], [18], [19], [20], [21]

Discussion

Measurements on the systems with H2O+CH4+water-insoluble additive showed that significant reductions of the hydrate equilibrium pressure are possible, mainly with cyclic organic components used as additives. The cyclic organic components are believed to cause reduction of the hydrate equilibrium pressure because they fit nicely into the large cavities of sII and simultaneously the small cavities are occupied to a larger extent by CH4. Thus, the additive causes a reduction of the hydrate

Conclusion

Experimental data for the hydrate equilibria H–Lw–La–V and H–Lw–La–Lv are collected as well as for the quintuple point (H–Lw–La–Lv–V) of the systems H2O+CO2+water-insoluble additive. The results show that the hydrate equilibrium pressure for systems with CO2 can be reduced significantly by water-insoluble additives. The additives are cyclic organic components, like, for instance, tetrahydropyran, cyclobutanone and cyclohexane. Compared to the additive-free system, for the latter two additives

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