International Conference on Gas Hydrates (ICGH) (6th : 2008)


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   EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF PROMOTER-STABILIZED CLATHRATE HYDRATES OF NOBLE GASES   Nikolaos I. Papadimitriou1,*  Athanassios K. Stubos1  Louw J. Florusse2  Cor J. Peters2  1 Environmental Research Laboratory National Center for Scientific Research “Demokritos” Patriarchou Grigoriou and Neapoleos, Agia Paraskevi 15310 GREECE  2 Laboratory of Process Equipment, Department of Process and Energy Faculty of Mechanical, Maritime and Materials Engineering Delft University of Technology Leeghwaterstraat 44, 2628 CA Delft THE NETHERLANDS   ABSTRACT It has been found that helium gas can stabilize the THF hydrate (of the sII type). Dissociation temperatures of the binary He-THF hydrates are significantly higher than those of pure THF hydrates at the same pressure. In order to investigate the distribution of He molecules within the cavities of this hydrate, Grand Canonical Monte Carlo simulations have been utilized. A wide pressure range (up to 700 MPa) has been examined. The results of these simulations show that the small cavity of the binary He-THF hydrate is able to accommodate up to three He molecules resulting in a He content of 6.0 wt. % at 700 MPa. Contrary to the similar case of binary H2-THF hydrate, He content as a function of pressure does not present a plateau, at least over the pressure range examined. Moreover, similar simulations on a hypothetical sII hydrate of pure He show that the large cavity of this hydrate could be occupied by up to ten He molecules in the pressure range under question.    * Corresponding author: Phone: +302106503416 FAX: +302106525004 e-mail: INTRODUCTION Helium gas has been experimentally found to fill spaces in ice of type II when high pressures are applied (above 280 MPa) [1]. Contrary to the heaviest of the noble gases (Ar, Kr, Xe) that form clathrate hydrates (of sI, sII or sH type) [2, 3], He cannot form hydrates by itself. However, it has been recently discovered [4] that it can stabilize THF hydrates and increase their dissociation temperature. THF as a single component forms sII hydrates at ambient pressure and temperature up to 277.6 K. Hydrates of the sII type contain two types of cavities [5]: the small, consisting of 12 pentagonal faces (symbol. 512) and the large, consisting of 12 pentagonal and 4 hexagonal faces (51264). There are 16 small and 8 large cavities in the unit cell of the sII hydrate. In THF hydrates, THF molecules occupy only the large cavities as they are too large to fit in the small ones. Consequently, THF forms binary hydrates with small gaseous molecules where THF acts as the promoter [4, 6]. The gas molecules occupy the small cavities but the amount of the enclathrated gas is directly dependent on the type and strength of the interactions between the gas and the water molecules. Helium is a small spherical and non-polar molecule. These properties render He an inert gas that develops weak and very simple interactions with other molecules. Especially in the case of hydrates, the nature of the interactions between the guest gas molecules and the solid lattice of the water molecules has not been completely disclosed. In this concept, He could be the first step towards the investigation of the principles of hydrate formation. The first target of this work is to experimentally determine the stability region of the binary He-THF hydrate and provide the complete phase diagram. This work is currently in progress and the results will be published soon. Next, the He content of this hydrate and the distribution of He within the cavities is thoroughly examined using Grand Canonical Monte Carlo (GCMC) simulations. Regarding the difficulties of a direct experimental measurement of the gas content of a hydrate, such simulations could give valuable information on the hydrate formation process.  SIMULATION DETAILS Simulations are generally performed on one unit cell of sII hydrate with a lattice parameter of 17.31 Å [7]. Some simulations were performed on eight (2×2×2) unit cells in order to examine the effect of the size of the simulation box on the results. Likewise previous works on H2 [8] and Ar [9] hydrates, simulations on one and eight unit cells produced almost identical results. In both cases 3-dimensional boundary periodic conditions were applied. Positions of the oxygen atoms of the water molecules have been taken by the X-ray diffraction measurements of Yousuf et al. [10]. The proton configuration with the minimum dipole moment was selected to be used in the simulations. Water molecules are simulated by the Extended Single Point Charge (SPC/E) model [11]. In this model, Van der Waals interactions are described through one Lennard-Jones site placed on the oxygen atom while electrostatic interactions are accounted for by placing partial charges on the oxygen and hydrogen atoms. All the parameters related to the SPC/E model are shown in Table 1. This model offers sufficient accuracy at an acceptable computational cost [12] and has been widely used in several types of simulations on hydrates [8, 9, 12-14]. He molecules are represented by a Lennard-Jones interaction site with the following parameters: σ = 2.556 Å and ε = 0.0850 kJ/mol. These parameters have been proposed by Lunbeck [15] and have been used in previous works [16, 17] for the description of the interactions between He and water molecules. Due to its spherical shape and non-polar nature, no electrostatic interactions are required for He. They are taken into account only for the interactions between the THF and water molecules. Furthermore, quantum effects of He can be neglected regarding that our simulations are performed at sufficiently high temperatures (270 – 300 K). For the THF molecules, the approach proposed by Alavi et al. [14] that includes a geometry optimization based on the AMBER force field [18] has been followed.  RESULTS AND DISCUSSION Figure 1 presents the He content of a binary He-THF hydrate as a function of pressure. The first notable observation is that the He content clearly exceeds the value of 2.1 wt. % that corresponds to the situation where all the small cavities are singly occupied by He molecules. molecule atom σ (Å) ε (kJ/mol) charge (e) O 3.166 0.6502 -0.8476 H2O H 0.000 0.0000 +0.4238 He He 2.556 0.0850 0.0000 Table .1. Interaction parameters and partial charges for water and helium molecules.    Figure 1. Helium and hydrogen content in a binary THF hydrate as a function of pressure. Dashed lines denote the possible limits of gas content based on the occupancy of the small cavities.   Figure 2. Helium occupancy of the small cavities of the binary He-THF hydrate.  Figure 3. Helium occupancy of the large cavities of the pure He hydrate.  Number of He molecules per large cavity Pressure (MPa) 0 1 2 3 4 5 6 7 8 9 10 50 27.5 47.4 21.6 3.4 0.2 0.0 0.0 0.0 0.0 0.0 0.0 100 6.4 32.7 41.7 16.8 2.3 0.1 0.0 0.0 0.0 0.0 0.0 200 0.2 5.3 26.3 41.6 22.6 3.7 0.2 0.0 0.0 0.0 0.0 500 0.0 0.0 0.0 0.8 10.6 36.5 37.2 13.0 1.8 0.1 0.0 700 0.0 0.0 0.0 0.0 0.1 5.6 28.6 38.1 22.9 4.5 0.3 Table 2. Occupancy ratio of the large cavities of the He hydrate, i.e. fraction (%) of the cavities occupied by the specified number (0 – 10) of He molecules.   This is a clear indication of multiply occupied small cavities and is in stark contrast to what has been experimentally [19] and computationally [8, 20] found in the similar case of hydrogen hydrates though H2 was also initially assumed to doubly occupy the small cavities of the sII hydrate [21]. Indeed, H2 content, also shown in Figure 1, in the H2-THF hydrate follows a Langmuir-type curve that presents a plateau at 1.1 wt. % (all small cavities singly occupied) [8]. More interestingly, He content of the binary hydrate also exceeds the next two possible upper limits that are related to complete double (4.1 wt. %) and complete triple occupancy (5.8 wt. %) of the small cavities. In the pressure range 200 – 700 MPa, He content seems to be linearly correlated with pressure and it does not tend to reach a plateau unless extremely high pressures are applied. In Figure 2, a more detailed investigation of the occupancy of the small cavities is presented. The prevalent occupancy value is highly dependent on pressure. At pressures below 320 MPa, the small cavities are mostly singly occupied while double occupancy becomes dominant in the range 320 – 470 MPa. At even high pressures the majority of the cavities are triply occupied. The fraction of quadruply occupied cavities at pressures up to 700 MPa is negligible (less than 1 %). Consequently, we could conclude that this type of cavity can accommodate three He molecules at most. This is the highest reported value of occupancy for the specific cavity for any guest gas. A series of GCMC simulation has also been performed for a pure He hydrate under the same conditions as the binary He-THF hydrate. Although He gas files the spaces in ice II [1] rather than forms sII hydrates, these simulations can give important information on the behaviour of He as a guest component in hydrates. Regarding the small cavities, the occupancy behaviour is similar as in the case of the binary He-THF hydrate. So, the occupancy profile for the small cavities of the pure He hydrate can be described by the Figure 2 for the binary He-THF hydrate. However, interesting results can occur from the occupancy of the large cavities (Figure 3). This type of cavity seems able to accommodate a large number of He molecules depending on pressure. At 720 MPa, the average occupancy reaches the value of 7.0.  At such high pressures, the large cavities can be occupied by up to ten He molecules. The exact fractions of the large cavity occupancies, as calculated from our GCMC simulations at several pressures, are shown in Table 2.  CONCLUSIONS In this work, binary He-THF hydrates are investigated in order to study the role of He gas in hydrate formation and stability. This work is currently in progress and more results will be available soon. However, the first results from our GCMC simulations have provided important information on the occupancy of the cavities of sII hydrates by He molecules. The small cavities of the binary He-THF hydrate can be occupied by up to three He molecules at pressures up to 700 MPa. Contrary to the binary H2-THF hydrate where the single occupancy of the small cavities sets a maximum limit on the H2 content, He content in the binary He-THF hydrate seems to be linearly dependent on pressure (in the range 200 – 700 MPa) and does not present a plateau at anyone of the possible limits of 2.1, 4.1 or 5.8 wt. % that correspond to the situation where all small cavities are occupied by one, two or three He molecules respectively. Probably, much higher pressures should be applied in order the maximum occupancy of He in this type of cavity to be achieved. Similar simulations on an hypothetical sII hydrate of pure He show that the large cavity of this hydrate can be occupied by up to ten He molecules. For instance, the average occupancy of this cavity at 220 MPa reaches the value of 3.0. REFERENCES [1] Londono D., Kuhs, W.F., Finney J.L. 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