Chapter 4: Experimental Procedures and Equipment for Gas Hydrate Phase Equilibrium
4. Experimental Procedures and Equipment for Gas Hydrates Phase Equilibrium
4.6. Hydrate Phase Equilibrium apparatus
The main characteristics of the apparatus and experimental procedures are reviewed in this section which have been mainly derived from the comprehensive review by Sloan and Koh with the addition of illustrative examples from the literature in order to present their main advantages and also some of the experimental problems usually experienced (Sloan and Koh, 2008). Three established experimental techniques representative of recent developments are discussed in the following paragraphs.
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The static technique can be regarded as one of the classical procedures for measuring hydrate phase equilibrium data, especially at high pressures. Deaton and Frost are known as the first developers of static apparatus (Deaton and Frost, 1937). In their apparatus, a glass windowed equilibrium cell (refer to Figure 4.6) was placed in a thermo-regulated bath. The cell was equipped with a valve system to allow for inlet and outlet gas flow regulation. The temperature and pressure of the system were measured using the thermocouple and pressure transducer. The phase equilibrium data were measured using visual observation of hydrate formation and disappearance. There are no significant changes in the above-mentioned principles of the equilibrium cell of Deaton and Frost over the past decades. Recent improvements in the design of hydrate phase equilibrium static equipment incorporate modern measuring devices that enhance experimental uncertainties, designs and materials that extend the operating ranges, precise construction and operation to study unusual conditions and systems, enhancements in the data acquisition and minimization of monotony.
Even though accurate and thermodynamically consistent data can be obtained using static and dynamic procedures, the static method is generally preferred for the measurement of phase equilibrium data due to their main advantages such as (Oellrich, 2004):
Simplicity of the technique and experimental apparatus
Applicability at high pressures and different temperatures
Applicability for single and multiple component systems, enabling reliable evaluation of industrial systems
Easy modification of total compositions and quantities of fluid samples
Small amount of materials are required
Ability to be an automated process enables the measurements to be performed overnight
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Figure 4.6. Static hydrate equilibrium cell used by Deaton and Frost (Deaton and Frost, 1937) as cited in (Sloan and Koh, 2008).
The requirement of a large amount of time to ensure that the system has reached equilibrium conditions (especially when long metastable periods occur) remains a major disadvantage of the static procedure for hydrate phase equilibrium measurement.
The Quartz Crystal Microbalance (QCM) is an example of an alternative experimental technique appropriate for gas hydrate phase equilibrium measurements (Mohammadi et al., 2003). As depicted in Figure 4.7, the QCM includes a thin disk of quartz placed between two electrodes. Crystal oscillation at a certain resonant frequency is activated when an electric current passes through the electrodes. Hydrate formation is then detected by a change in the resonance frequency once the hydrate is attached to the surface of the quartz crystal. The pressure and temperature of the system are measured using a pressure transducer and a thermocouple in a high pressure cell (Sloan and Koh, 2008).
The application of small amounts of samples (approximately one droplet of water) in the QCM method enables a significant reduction in the time required for each experiment
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(Mohammadi et al., 2003). Even though the QCM method has been considered impractical due to the good contact constraint between the surface of the quartz crystal and hydrates, it has been well established that by modifying the droplet size, this method gives reasonable results, especially for rapid and practical gas hydrate application purposes, i.e. selecting a hydrate promoter from among various candidates (Sloan and Koh, 2008).
Figure 4.7. (a) Schematic diagram of the QCM, and (b) the QCM placed within a high pressure cell (Mohammadi et al., 2003, Sloan and Koh, 2008).
Recently, calorimetric approaches such as Differential Scanning Calorimetry (DSC) have also been applied for the measurement of hydrate phase equilibrium and thermal property data for gas hydrates. Dalmazzone et al. developed a micro-DSC analyzer incorporated with special high-pressure vessels. The High Pressure Differential Scanning Calorimetry (HP-DSC) apparatus was used to specify the thermodynamic stability boundaries of methane and natural gas hydrates in solutions of inhibitors (Dalmazzone et al., 2002). Deschamps and Dalmazzone used the same procedure to investigate dissociation enthalpies and phase equilibria of TBAB semi-clathrates with gases (Deschamps and Dalmazzone, 2010). As it is presented in Figure 4.8, the equipment comprises a micro DSC VII, to measure the difference in heat flow between the sample and the reference material. The system can operate between a temperature range of 228.15 to 393.15 K and pressures up to 40 MPa coupled to a pressure multiplicator.
Simultaneous measurement of thermodynamic and thermal data is the main advantage of the micro-DSC technique. This method is also relatively faster than the PVT techniques and
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requires a smaller amount of sample (~ 5mg) (Le Parlouër et al., 2004). However, discrepancies in the measured thermodynamic properties using similar calorimetric techniques can be somewhat greater than the specified experimental uncertainties. For example, different equilibrium temperatures of TBPB semi-clathrates measured using the DSC have been reported by different laboratories (Suginaka et al., 2012).
Figure 4.8. Schematic diagram of a high-pressure micro DSC VII device (a) (Deschamps and Dalmazzone, 2010) (b) (Sfaxi et al., 2014).
Pressure regulator Gas cylinder
Pressure transducer
(a)
(b)
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