Fluid Phase Equilibria 218 (2004) 235–238

Thermodynamic stability of structure-H hydrates ofmethylcyclopentane and cyclooctane helped by methane

Takashi Makino, Toshiyuki Nakamura, Takeshi Sugahara, Kazunari Ohgaki∗

Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

Received 30 May 2003; received in revised form 7 November 2003; accepted 10 January 2004

Abstract

The four-phase equilibria were measured for the methylcyclopentane+methane+H2O hydrate system (274.28–287.40 K, 1.75–9.34 MPa)and the cyclooctane+methane+H2O hydrate system (274.08–288.57 K, 1.60–9.33 MPa). Each structure-H hydrate has the lower equilibriumpressure than the pure methane structure-I hydrate in the temperature range of the present work. The isothermal equilibrium pressures of bothmethylcyclopentane and cyclooctane hydrates are slightly higher than that of methylcyclohexane hydrate.© 2004 Elsevier B.V. All rights reserved.

Keywords: Clathrate hydrate; Solid–fluid equilibria; Structure-H; Data; Stability; Methane

1. Introduction

Clathrate hydrates are one kind of inclusion compoundsand composed of H2O and guest species. They are stabilizedby the guest molecules enclathrated in the hydrogen-bondedH2O cages. There are three well-known structures, calledstructure-I, II, and H. These structures consist of severaltypes of cages. The structure-H, which was discovered byRipmeester et al.[1], has the U-cage of the icosahedron(51268), the S′-cage of the dodecahedron (435663), and theS-cage of the pentagonal dodecahedron (512). The unit cellof the structure-H consists of one U-cage, two S′-cagesand three S-cages with 34H2O molecules. The heavy guestspecies occupy the largest U-cages and the small molecules(so-called “help gas”, for example, methane and xenon) oc-cupy both the S- and S′-cages.

The structure-H hydrates helped by the methane moleculehave lower stability pressure than the pure methanestructure-I hydrate. In other words, by adding a smallquantity of the heavy guest species that can generate thestructure-H hydrates, we can handle the hydrates contain-ing methane under the moderate conditions. Therefore, thestructure-H hydrates have much attention as a medium ofnatural-gas storage and transport system[2].

∗ Corresponding author. Fax:+81-6-6850-6290.E-mail address: ohgaki@cheng.es.osaka-u.ac.jp (K. Ohgaki).

In the present work, it is the purpose to clarify thermo-dynamic stability of the structure-H hydrates of methyl-cyclopentane or cyclooctane helped by methane. Neithermethylcyclopentane nor cyclooctane can generate gas hy-drates without help gas. We have investigated the four-phaseequilibria for the methylcyclopentane+methane+H2O hy-drate system in the temperature range of 274–288 K and thecyclooctane+ methane+ H2O hydrate system in the tem-perature range of 274–289 K.

2. Experimental

2.1. Experimental apparatus

An experimental apparatus used in this work is the sameas the one reported previously[3]. A high-pressure cell usedin this work is made of SUS-304. The maximum workingpressure and the inner volume are 10 MPa and 150 cm3,respectively. For the observation of phase behavior in thecell, a pair of glass windows is attached to the cell. Thesystem temperature was controlled by immersing the cell ina thermocontrolled bath where the thermostated water wascirculated from a programming-thermocontroller (TaitecCL-80R). The equilibrium temperature was measured byinserting a thermistor probe (TAKARA D 632) into ahole in the cell wall within an accuracy of±0.02 K. The

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236 T. Makino et al. / Fluid Phase Equilibria 218 (2004) 235–238

equilibrium pressure was measured by using a strain pres-sure gage (Valcom VPRT) within an accuracy of±10 kPa.

2.2. Experimental procedures

An excess amount of heavy guest species and distilled wa-ter were introduced into the evacuated high-pressure cell inorder to prevent both phases from disappearing by the forma-tion of s-H hydrate. Then, the methane was introduced intothe cell until the pressure reached a desired value. Here, thesystem pressure must not go over the equilibrium pressureof the pure methane structure-I hydrate. If the pressure wentover, the pure methane structure-I hydrate would generatefirst and it would take a long time to reach the structure-Hequilibrium state. After the introduction of methane, thestructure-H hydrate was formed by continuously agitating.A mixing bar was moved up-and-down by the magnetic at-traction to agitate the contents in the system. The agitationis very important for supplying the fine oil particles intothe aqueous phase. After the four-phase coexisting state wasconfirmed, the phase equilibrium measurement was started.In order to determine the equilibrium pressure precisely, werepeated the formation and dissociation of the structure-Hhydrate with the same pressure control as the previous work[3]. When the pressure distinction was within±0.01 MPa,the system was regarded as the equilibrium state. It gener-ally takes for several days to establish the four-phase equi-librium state. When the system temperature is increased, ittakes the longer time for establishing the equilibrium state.

2.3. Materials

Research grade methane of purity 99.95% was purchasedfrom Takachiho Trading Co., Ltd. Methylcyclopentane(purity 99.5% up) and cyclooctane (purity 98% up) wereobtained from Tokyo Chemical Industry Co., Ltd. andMerck, respectively. The distilled water was purchased formYashima Pure Chemical Co., Ltd. All of them were usedwithout further purification.

3. Results and discussion

The four-phase equilibrium data of the structure-H hy-drates and the enthalpy of hydrate formation,�hydH,are summarized inTable 1 (the methylcyclopentane+methane+ H2O hydrate system) andTable 2 (thecyclooctane+ methane+ H2O hydrate system). The valueof �hydH is calculated from the value of experimentaldp/dT under the assumption of the ideal hydration. Thevalues of dp/dT were regarded as the differential values ofthe fitting line of the phase equilibrium data. The volumet-ric properties of methane and H2O were calculated fromthe IUPAC recommendation[4] and the correlation of Sauland Wagner[5], respectively. The densities of methylcy-clopentane and cyclooctane were obtained from a modified

Table 1The four-phase equilibrium data and the enthalpy of hydrate formationfor the methyl cyclopentane+ methane+ H2O hydrate system

T (K) P (MPa) �hydH (kJ mol−1)

274.28 1.75 373.8275.25 1.98 374.4276.20 2.22 376.3277.08 2.48 376.3277.99 2.77 377.6278.88 3.08 378.9279.78 3.47 375.3280.67 3.88 373.9281.48 4.29 372.8282.27 4.75 369.6283.06 5.25 366.7283.86 5.79 364.8284.66 6.44 358.8285.43 7.12 353.6286.21 7.92 345.8286.77 8.57 339.1287.40 9.34 332.5

Rackett equation[6]. The molar volume of structure-H hy-drate was calculated from the lattice constant of hexagonalstructure-H hydrate (a = 1.226 nm,c = 1.017 nm[7]).

The four-phase equilibrium data of structure-H hydrateof the methylcyclopentane helped by methane are shown inFig. 1 with the literature data[8–10]. The equilibrium pres-sure of the structure-H hydrate of this system is much lowerthan that of the pure methane structure-I hydrate. The presentdata show good agreement with the literatures. The valuesof �hydH are almost constant (about 373± 5 kJ mol−1) in alower temperature range than 282 K.

The four-phase equilibrium data of structure-H hydrate ofcyclooctane helped by methane are shown inFig. 2with theliterature data[9]. Although the melting point of cyclooctane

Table 2The four-phase equilibrium data and the enthalpy of hydrate formationfor the cyclooctane+ methane+ H2O hydrate system

T (K) P (MPa) �hydH (kJ mol−1)

274.08 1.60 357.6275.16 1.84 355.1276.17 2.03 364.8277.15 2.29 364.4278.00 2.53 365.7278.83 2.79 366.6279.78 3.14 364.8280.96 3.57 369.4282.13 4.13 365.9283.07 4.64 362.9284.11 5.28 358.8284.90 5.83 354.9285.90 6.59 350.7286.91 7.50 343.6287.39 7.98 339.9287.87 8.53 334.3288.13 8.84 331.4288.57 9.33 329.0

T. Makino et al. / Fluid Phase Equilibria 218 (2004) 235–238 237

Fig. 1. Four-phase coexisting curves of the methylcyclopentane+methane+ H2O hydrate system.

is 287.6 K, no characteristic behavior of hydration is ob-served in the vicinity of melting temperature. In addition,the four-phase coexisting data look like a smooth curve inthe present experimental conditions. The temperature depen-dency on�hydH shows similar behavior to that of methyl-cyclopentane hydrate. The value of�hydH is about 365±5 kJ mol−1 in a low temperature range and 3% smaller thanthat of methylcyclopentane hydrate. The data of Thomas andBehar [10] shows small difference from the present data,their curve seems to be shifted to the slightly higher tem-perature (lower pressure) side.

Fig. 3 shows the stability boundaries of structure-Hhydrates (methylcyclopentane, cyclooctane, methylcyclo-hexane[3], andcis-1,2-dimethylcyclohexane[3]) helped bymethane. According to Sloan[11], the molecular diameterof methylcyclohexane is 0.859 nm,cis-1,2-dimethylcyclo-hexane is 0.852 nm, cyclooctane is 0.796 nm, methylcy-

Fig. 2. Four-phase coexisting curves of the cyclooctane+methane+H2Ohydrate system.

Fig. 3. Stability boundaries of structure-H hydrates helped by methane.

clopentane is 0.786 nm, and the diameter of cavity of U-cageis 0.862 nm. In the present study, however, the equilibriumpressures for the structure-H hydrate systems of methylcy-clopentane,cis-1,2-dimethylcyclohexane, cyclooctane, andmethylcyclohexane decrease in that order at an isothermalcondition. The structure-H hydrates of C8H16 compounds(cis-1,2-dimethylcyclohexane and cyclooctane) have almostequivalent pressure. These results imply that the molecularshape as well as size has great effect on the thermodynamicstability of s-H hydrate.

4. Conclusion

The four-phase equilibrium data for the methylcyclopen-tane+ methane+ H2O and cyclooctane+ methane+ H2Ohydrate systems were measured. The structure-H hydrateshelped by methane could be generated at lower pressurethan the equilibrium pressure of the pure methane structure-Ihydrate. The present findings suggest that the methylcyclo-hexane molecule has suitable size and shape for the largestU-cage in the structure-H hydrate.

List of symbolsH enthalpy (J mol−1)p pressure (Pa)T temperature (K)

Acknowledgements

This study was supported by the Program for PromotingFundamental Transport Technology Research from the Cor-poration for Advanced Transport and Technology (CATT).The authors are grateful to the Division of Chemical En-gineering, Graduate School of Engineering Science, OsakaUniversity for the scientific support by “Gas-Hydrate Ana-lyzing System (GHAS).”

238 T. Makino et al. / Fluid Phase Equilibria 218 (2004) 235–238

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