Determination of thermophysical properties of cyclopentane hydrate using a stirred calorimetric cell
Graphical abstract
Introduction
Clathrates are crystalline compounds composed of molecules (named “hosts”) whose crystal lattices develop cavities that encage other molecules (named “guests”), thereby stabilizing the structure. Clathrate hydrates are formed by water molecules and C1–C4 hydrocarbon gases up to methylcyclopentane or benzene [1]. Gas hydrates form at low temperature (typically a few degrees above 273 K) and high pressure (typically a few MPa). They are often encountered in oil and gas production where, in certain conditions, they can block flow lines, valves and wellheads, causing serious production losses and safety problems [2]. In studies on hydrate properties, e.g. to understand complex interactions between hydrates and surfactants [3], [4], [5], the cyclopentane (CP) hydrate is often used as a model system because it can form at atmospheric pressure and moderately cold temperatures (lower than 280.1 to 280.8 K [1], [6]). Interestingly, the CP-hydrate is believed to be a good analog of the natural gas hydrates that form under pressure [7] owing to the low solubility of CP in water. The CP-hydrate has a crystallographic structure termed “structure two” (sII): each unit cell is composed of 16 small cavities forming pentagonal dodecahedrons and 8 large cavities forming hexakaidecahedrons. The unit cell is formed of a network of 136 hydrogen-bonded water molecules, and the CP molecules are enclosed in the large cavities (Fig. 1) of this structure with a stoichiometric water/CP molar ratio of 17:1 [8]. The molar weight of the CP-hydrate (with a stoichiometric water/CP molar ratio of 17:1) is 376.39 g.mol−1.
Calorimetry techniques are commonly used to detect phase transitions, measure the kinetics of thermal events, and determine the heat capacity of materials [9]. Many calorimetric studies have been performed on gas hydrates since the 1980’s, starting with those of Handa, Dalmazzone and co-workers who studied the thermal properties of a variety of clathrate hydrates [10], [11], [12], [13]. Later on, it was shown that reliable hydrate dissociation temperature data, and consequently practically applicable hydrate stability curves, could be obtained using calorimetric measurements [14], [15]. For the gas hydrate analyses, the Differential Scanning Calorimetry (DSC) or microcalorimeters used were able to perform measurements in controlled pressure conditions. This technique, applied with classical high-pressure calorimetric cells (i.e. where the fluids are not stirred) still has some limitations due to the fact that the gas hydrate generally forms at the interface (water/gas or water/oil) [16]. The low interfacial surface area between CP and water, in addition to the lack of in-situ stirring, leads to: (i) long induction times (i.e. a long period before the start of crystallization), and (ii) the formation of a hydrate crust covering the CP/liquid interface which prevents complete water-to-hydrate conversion within a reasonable time. Accordingly, and despite the common uses of CP-hydrate, the determination of the phase change enthalpy (also called formation or dissociation enthalpy, ΔHd) is rather difficult and challenging using DSC.
Karanjkar et al. (2012) [17] used a µDSC to study a pure system with excess CP, applying a cooling ramp from 293 to 228 K. They demonstrated that only ice formed and that the use of an emulsion stabilized with surfactant instead of a pure system resulted in the formation of an ice/hydrate mixture. To overcome these limitations and obtain phase transition properties – such as the CP-hydrate dissociation enthalpy – studies were conducted on CP/water emulsions stabilized by the addition of surfactants. Nakajima et al. (2008) [18] emulsified water and CP with surfactants to form the CP-hydrate in a stirring reactor, removed some hydrate particles and analyzed them by DSC. They reported a value of ΔHd = 106,900 J.mol−1 of CP. Zhang et al. (2008) [8] proceeded quite similarly: in a DSC they applied a heating ramp to a previously formed CP-in-water or water-in-CP emulsion, and from the dissociation peak determined a value of ΔHd = 82,300 ± 4000 J.mol−1 for the CP-hydrate. Although other studies involving both calorimetry and cyclopentane-based hydrate systems can be found [19], [20], these two ΔHd values are the only ones we have found in literature. The discrepancy between them (about 25 kJ.mol−1 of CP) requires new experiments to more accurately determine the true value.
In this work, the enthalpy of dissociation of pure CP-hydrate was determined using a customized stirred high-pressure calorimetric cell mounted on a heat-flow calorimeter. This cell overcomes some of the existing limitations encountered with conventional high pressure calorimetric cells in which the fluids are left in static conditions. Therefore, this stirring cell, developed initially for experimentation with gas hydrates under pressure, was used here to accurately determine the thermophysical properties of the CP-hydrate.
Section snippets
Calorimeter and stirred calorimetric cell
The development was based on a Tian-Calvet heat-flow calorimeter (model BT 2.15) manufactured by SETARAM Instrumentation, France. The main feature of Calvet-type calorimeters is their network of thermocouples surrounding the calorimetric cell in which the samples are placed, making it possible to measure the heat-flow in all directions and thereby reliably determine the enthalpies of transition. The sensor is differential, meaning that the heat-flow data of the sample are constantly compared
Results
To illustrate the effect of the agitation on water-to-hydrate conversion, Fig. 7 shows a series of thermograms obtained for experiments performed at various stirrer speeds varying from 0 to 200 RPM. The thermograms show the endothermic peak that corresponds to CP-hydrate dissociation. For these experiments, only one formation cycle was performed at 274.1 K before the dissociation step from 274 to 298 K at 0.1 K.min−1 (no decrease to 263 K at the end of the formation). As no peak was observed on
Conclusion
The use of a mechanically stirred calorimetric cell resulted in the unprecedented formation of pure CP-hydrate in a calorimetric cell, with 100% water-to-hydrate conversion without having to resort to either a previous step of ice formation or emulsification of CP in water using surfactant(s). In-situ agitation contributes to reducing the induction time before hydrate crystallization, increasing the contact surface between water and CP liquid phases, breaking the hydrate crust that generally
Acknowledgments
The authors wish to acknowledge the “Atelier de Physique” of the University of Pau for their help in developing and manufacturing the elements of the calorimetric cell prototypes. We extend our thanks to J. Diaz for technical help, to E. Normandin, F. Guerton for technical assistance in instrumentation and to M. Conde for the illustration of CP-hydrate. The authors are also grateful to Aquitaine Science Transfert, to Carnot institute ISIFoR (Institute for the Sustainable engIneering of Fossil
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