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CHAPTER SEVEN: CONCLUSIONS
CHAPTER 7………..………... CONCLUSIONS
153 0.20 mass fractions, the promotion effect of TBAB on the Ar and Kr hydrate phase equilibria increases.
The results obtained for the hydrate dissociation data related to the system of Xe + TBAB + water showed that aqueous TBAB solutions, depending on the pressure range, have a twofold effect on the Xe gas hydrate phase equilibria. So, at pressures below 0.73 MPa, the aqueous TBAB solution with a concentration of 0.1 mass fraction, has a drastic promotion effect on the xenon hydrate dissociation conditions. A similar behaviour was observed for the semi-clathrate hydrate dissociation conditions for the system of xenon + 0.2 and 0.3 mass fraction of aqueous TBAB solution in which, aqueous TBAB solutions with concentrations of 0.2 and 0.3 mass fraction, showed a promotion effect for xenon hydrate dissociation conditions at pressures below (1.4 and 1.57) MPa respectively. The results indicated that the promotion effect of TBAB on Ar hydrate is greater than on the Kr and Xe hydrate.
Obtained results of hydrate dissociation data for the CF4 + water system revealed a discrepancy between the experimental data reported in literature. The possible reason for such discrepancies may be attributed to some parameters such as a small gas leak, fast heating, error in the calibration and low-accuracy of the measurements. The results showed that 0.05, 0.10, and 0.20 mass fractions of aqueous TBAB solutions have no promotion effect on the CF4 hydrate phase equilibrium. The aqueous TBAB solution with 0.30 mass fraction showed a significant promotion effect on the CF4 hydrate formation with a temperature difference, ∆T, approximately equal to 10 K. Some crystallographic studies such as H-NMR, Raman Spectrometry, C13-NMR or X-Ray Diffraction are needed to explain the strange behaviour of this hydrate system.
The maximum value for average absolute deviation percentage (AAD%) between the experimental and the results of three thermodynamic models (namely fugacity approach (Approach 1), Chen and Guo approach (Approach 2) and a simple method based on vapour pressure calculations (Approach 3)) was approximately 0.3% which confirms the accuracy of the used thermodyamic models. The binary interaction parameters (BIPs) of the VPT EoS combined with the NDD mixing rules for Ar/Ke/Xe/ or CF4 + water, new Kihara parameters, Antonine constants, and Langmuir constants for Ar, Kr, Xe and CF4 were derived.
A thermodynamic model based on Joshi et al’s work was extended in this work for representation of semi-clathrate hydrates dissociation conditions for the systems of Ar/ Kr/ Xe and CF4 + TBAB +water. A reasonable agreement between the measured data and the model results with the maximum AAD% of 0.3% was observed.
154 The kinetic study on the CF4 hydrate demonstrated that with an increase in the initial pressure from 7.08 MPa to 11.83 MPa, the induction time decreases, consequently the CF4 hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion increase. Similar results were found with decreasing the initial temperature at a constant initial pressure. The values obtained for the apparent rate constants, rate of hydrate formation, water to hydrate conversion and gas consumption can be useful to design a cost effective separation process based on clathrate/semi-clathrate hydrate formation.
The kinetic results on the semi-clathrate hydrate of Ar + TBAB + water showed that with an increase in the initial pressure, the induction time decreases significantly which means the rate of hydrate nucleation increases. The same trends were detected with a decrease in the initial temperature. With a decrease in the initial temperature, the amount of the pressure drop during the semi-clathrate hydrate formation increases resulting in the increase of the gas consumption.
With an increase in the TBAB concentration form 0.1 to 0.3 mass fraction, the rate of semi- clathrate hydrate nucleation increases and the induction time decreases, significantly. With an increase in the TBAB concentration, the amount of the pressure drop increases resulting in more gas consumption during the semi-clathrate hydrate formation. The results show the positive kinetic and thermodynamic effect of TBAB on Ar hydrate which makes TBAB as a reliable promoter to decrease the pressure of argon hydrate formation and increase the rate of argon hydrate formation. The results indicated that SDS in the concentration of 100, 200, 400 ppm increases the induction time of the semi-clathrate hydrate formation for the system of Ar + TBAB + water. Therefore, the recommendation from this study is not to use SDS for the separation of noble gases. An alternative kinetic promoter should be explored for the semi- clathrate hydrate of Ar + aqueous TBAB solutions.
The results demonstrated that Xe hydrate in the presence of aqueous TBAB solution has much lower pressure conditions compared with Kr, and Ar + aqueous TBAB solutions. These differences of the gas hydrate dissociation pressure confirm the capability of the separation of the mixture of Ar + Kr + Xe using gas hydrate method. Semi-clathrate hydrate of Xe + TBAB + water form at ambient temperatures and very low pressures which decreases the cost of separation with hydrate method compared to the cryogenic distillation which is costly and quite energy intensive. In order to design a hydrate based process for gas separation (Kr, Xe, Ar) and estimate the number of stages necessary, further studies considering the equilibrium composition data in liquid, vapour and hydrate phases are required.
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