4. Review of experimental methods and equipment
4.3. Available experimental methods for the determination of hydrate dissociation
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4.3. Available experimental methods for the determination of hydrate
CHAPTER 4………....REVIEW OF THE EXPERIMENTAL METHODS AND EQUIPMENT
53 In this section the aforementioned experimental methods for the determination of gas hydrate phase equilibrium conditions are explained.
4.3.1. Visual isothermal pressure search method
At the start of the experimental measurement by the isothermal pressure search method, the temperature of the system is set at the constant value. After the evacuation of the cell to eliminate any contamination, the pressure of the system is set at a pressure above the estimated region of hydrate formation by introducing hydrate former inside the equilibrium cell (approximately 20-30 kPa above the estimated pressure). Then the agitation of the system is initiated and the system is allowed to reach to equilibrium conditions at that temperature. At the start of hydrate nucleation, the temperature at the hydrate interface increases because of the translational energy output due to the movement of the molecules from the gas and liquid phases to the hydrate phase. However, it is necessary to discharge this kind of energy from the system to the bath and neighbouring phases using agitation or conduction/ convection. During hydrate formation, the system pressure decreases because of gas encapsulation. The pressure during the experiment is controlled by the exchange of a gas or liquid such as mercury from an external reservoir. After the formation of gas hydrates, the pressure is decreased gradually by withdrawing fluids (gas or liquid) from the external reservoir until the disappearance of the last crystal of hydrate. This procedure is valid only for pure liquid or gas. This point is considered as the visual equilibrium pressure point of hydrate at a constant temperature. In order to reduce the inaccuracy when using the visual effect, the experimental procedure for hydrate formation and dissociation should be performed twice (Sloan and Koh, 2008, Englezos and Bishnoi, 1991).
4.3.2. Visual isobaric temperature search method
In the isobaric temperature search method, addition or withdrawal of fluid (gas or liquid) from an external reservoir is used to maintain a constant pressure. After the evacuation of the cell to remove any impurities in the cell, the equilibrium cell is pressurised by introducing the hydrate former to reach to the desired and constant pressure. The temperature of the system is then decreased about 5 K below the expected temperature of the hydrate equilibrium conditions.
After the temperature of the system reached a constant value, the stirrer is switched on and the formation of gas hydrate begins. Hydrate formation is determined by the addition of a
54 significant fluid (gas or liquid) from an external reservoir in to the cell. After hydrate formation is complete and the pressure of the system has reached to constant value, the temperature of the cell is increased slowly to dissociate the gas hydrate. In order to maintain the pressure at the constant value in this step, some fluid must be withdrawn from the cell. This procedure is valid only for pure liquid or gas. The decrease in the temperature continues until the last hydrate crystal disappears. This point is considered as the visual equilibrium temperature point of hydrate at a constant pressure. In order to reduce the inaccuracy using the visual effect, the experimental procedure for hydrate formation and dissociation should be performed twice (Sloan and Koh, 2008).
4.3.3. Isochoric pressure search method
The isochoric or constant volume method is appropriate for gas hydrate measurements at high pressure conditions. As the isochoric method does not require viewing of the cell contents to distinguish the final hydrate dissociation point, this procedure can be used as an alternative to the visual isothermal and isobaric methods. This method was applied in the experiments conducted in this study. A pressure- temperature diagram generated during the hydrate formation and dissociation conditions in the isochoric procedure is presented in Figure 4-7. As seen in this figure, measurements commence with a mixture of water and gas and conditions of pressure and temperature outside the hydrate stability zone, point A. Then the gradual cooling to the point B provides the system to reach to the hydrate formation conditions. After point B, crystals of gas hydrate start to form and consequently the pressure of the system is decreased until the system reaches point C, owing to gas encapsulation. The amount of the pressure reduction in this step depends on the amount of gas molecules filling the hydrate cavities and other thermodynamic restrictions. Once the gas hydrate formation is complete and the system has reached a constant pressure, point C, the temperature is increased slowly to dissociate the gas hydrate crystals. Heating of the system continues until all of the encapsulated gas is released from the crystals and the pressure and temperature of the system reaches an equilibrium, known as the dissociation condition, at point D. After complete decomposition of the hydrate crystals, with temperature increasing, the pressure is altered by considering the relationship between temperature, volume and pressure change. According to Sloan and Koh in 2008, the hydrate equilibrium point is considered as the intersection between the hydrate dissociation curve and the initial cooling curve (point D) (Sloan and Koh, 2008).
CHAPTER 4………....REVIEW OF THE EXPERIMENTAL METHODS AND EQUIPMENT
55 Figure 4-7. Primary cooling and heating curve for formation and dissociation of simple hydrate in the isochoric method (Sloan and Koh, 2008).
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