Phase Behaviour, Thermodynamics and Kinetics of Clathrate Hydrate Systems of Carbon Dioxide in Presence of Tetrahydrofuran and Electrolytes

In view of the possibilities for new development of carbon dioxide hydrate processes, this study focused on experimental measurements to obtain fundamental insight into the phase behaviour and the kinetic of formation of carbon dioxide hydrate forming systems. These data are essential for the development of process design of any carbon dioxide hydrate based processes such as separation of carbon dioxide via hydrate formation and refrigeration processes. In addition, a modelling approach is also being used to predict the phase behaviour of the systems. In this work, special attention is paid to the influence of tetrahydrofuran, a hydrate promoter, and electrolytes on the phase behaviour and the kinetics of formation of these systems and their implications for practical applications. The phase behaviour has been measured with the Cailletet apparatus. Significant pressure reduction or temperature increment at a specific temperature or pressure has been observed in the presence of tetrahydrofuran, when the equilibria in the ternary system are compared to the corresponding equilibria in the binary system of carbon dioxide and water. A liquid-liquid phase split, which is frequently observed to occur in the ternary system, creates a four-phase equilibrium line of H-LW-LV-V in the hydrate forming region. For this equilibrium line, the equilibrium conditions are independent of the composition of tetrahydrofuran in the system. The pseudo-retrograde behaviour, in which the equilibrium temperature is decreasing with an increase of the pressure, is frequently observed for the four-phase equilibrium line. The presence of electrolyte in the mixed hydrate forming system reduces the hydrate stability region. Moreover, the liquid-liquid phase split in the systems is further enhanced by the presence of these electrolytes. The hydrate inhibiting effect of the metal halides (electrolyte) is increasing in the following order: NaF Cl- > F-. Based on the results, it is suggested that the probability of formation and the strength of the ionic-hydrogen bond between an ion and water molecule and the effects of this bond on the surrounding network of water molecules are the major factors that contribute to hydrate inhibition by electrolytes. The phase behaviour is modelled with the combination of the Van der Waals – Platteeuw model and the Peng-Robinson-Stryjek-Vera Equation of State (PRSV EoS) with Huron-Vidal-Orbey-Sandler (HVOS) mixing rule to describe the clathrate hydrate phase and the fluid phases respectively. In the presence of an electrolyte, the effect of the electrolyte is taken into account by a Debye-Huckel electrostatic term. For the equilibria of the fluid phases (LW-LV-V ? LW-LV and LW-LV-V ? LW-V), excellent agreement between experimental and modelling data is achieved. On the other hand, for the hydrate equilibria for the ternary and quaternary systems, good agreement is achieved for the three phase H-LW-V equilibrium while acceptable agreement is obtained for the H-LW-LV-V and H-LW-LV equilibria. By taking advantage of the availability of the measured three-phase H-LW-V data, the enthalpy of dissociation of simple carbon dioxide and mixed carbon dioxide and tetrahydrofuran hydrates is estimated by using the Clausius-Clapeyron equation. The estimated value is found to be significantly influenced by the compressibility factor of carbon dioxide in the system. Therefore, it is shown that this method can be used for the estimation of the enthalpy of hydrate dissociation in a small range where the compressibility factor is almost constant. Moreover, it is also found that the enthalpy of dissociation of the mixed hydrate is significantly higher than that of simple carbon dioxide hydrate. The presence of electrolyte is observed to slightly reduce the value of the enthalpy of dissociation of both the simple carbon dioxide and the mixed hydrates. The kinetics of formation of simple carbon dioxide and mixed carbon dioxide and tetrahydrofuran hydrates are studied in a 150 ml batch reactor with the T-cycle method. The clathrate hydrate formation is divided into two parts, hydrate nucleation and hydrate growth processes. From the measured induction time, it is shown that the hydrate nucleation process is more readily to occur in the mixed hydrate system. The presence of sodium chloride is found to slightly prolong the induction time in the mixed hydrate systems. For the hydrate growth process, the activation energy of mixed carbon dioxide and tetrahydrofuran hydrates is found to be significantly lower than that of simple carbon dioxide hydrate, implying that the mixed system is more susceptible for hydrate growth. The presence of tetrahydrofuran results in a strongly reduced storage potential of carbon dioxide in mixed clathrate hydrates. Furthermore, the presence of sodium chloride is found to slightly reduce the carbon dioxide consumption and increases the activation energy for hydrate formation in the mixed hydrate systems. The experimental and modelling results provide a better understanding of the phase behaviour and hydrate formation kinetics of simple carbon dioxide and mixed carbon dioxide and tetrahydrofuran in water or aqueous electrolyte solutions. In general, this allows better assessment of practical hydrate applications with tetrahydrofuran. The reduction of the pressure for the hydrate equilibria, the increase of enthalpy of dissociation and the decrease in the induction time achieved with the inclusion of tetrahydrofuran might allow the hydrate process becoming more attractive for applications such as a cooling medium or secondary refrigerant. However, the reduction of storage potential of carbon dioxide in the hydrate formed may cause it to be less attractive for storage applications. Additionally, the pseudo-retrograde behaviour at which the shift of the equilibrium to lower temperatures is restricting the clathrate hydrate stability region and may hinder the mixed hydrates based process to operate close to ambient temperatures.