Chapter 6: Results and Discussion
6. Results and discussion
6.3. Summary of the experimental results
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found in the open literature. The parameters optimized for pure refrigerants were applied to estimate the hydrate dissociation conditions of their blends. There were slight differences between the results of the two models and a reasonable agreement was found between the modelling results and experimental dissociation data in different hydrate equilibrium phase boundaries. However, the application of correlation (3.10) is recommended because of the complexity of Kihara potential function approach. Using the developed model in this study the hydrate dissociation conditions of a new refrigerant blend for which there is no experimental data available in literature can be estimated. The upper and lower quadruple point for pure refrigerants was estimated and compared to the experimental data in literature. A good agreement was observed between the model results and experimental data. For some of the refrigerants considered in this study no quadruple points can be found in literature and only the modelling results are reported.
The structure of the gas hydrate was also predicted using the model developed in this study. It was found that except R23, R32 and R152a which are forming structure I of gas hydrate, all the refrigerants studied are forming structure II of gas hydrate.
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Table 6.15. Advantages and disadvantages of the refrigerants studied in this work and those of the literature for the application as a cold storage media.
Refrigerant advantage disadvantage
R404A High enthalpy of dissociation,
Low induction time, promotion effect of SDS solution
high pressure of dissociation, high price, low Kapp,
R406A Low pressure of dissociation,
high enthalpy of dissociation, low induction time, high Kapp, low price, promotion effect of SDS solution
Low temperature of critical decomposition
R408A High enthalpy of dissociation,
promotion effect of SDS solution
high pressure of dissociation, high induction time, low Kapp
R427A Low pressure of dissociation,
high enthalpy of dissociation, high temperature of critical decomposition, promotion effect of SDS solution
high induction time, low Kapp, high price,
R407C Low pressure of dissociation,
high enthalpy of dissociation, low induction time, high Kapp, high temperature of critical decomposition,
inhibition effect of SDS solution, high price
R410A high temperature of critical
decomposition
low enthalpy of dissociation, high price, inhibition effect of SDS solution, high degree of subcooling,
R507C high enthalpy of dissociation,
low price, low induction time
high pressure of dissociation, low Kapp, inhibition effect of SDS solution
R508B high pressure of dissociation,
low enthalpy of dissociation, high price,
110 Table 6.15 continued
R116 high pressure of dissociation,
low enthalpy of dissociation, high price,
R134A Low pressure of dissociation,
high enthalpy of dissociation, promotion effect of alumina or zinc
low temperature of critical decomposition,
R143A Low pressure of dissociation,
high enthalpy of dissociation
low temperature of critical decomposition,
R32 high temperature of critical
decomposition
High pressure of dissociation, Low enthalpy of dissociation, high price
R23 high temperature of critical
decomposition
High pressure of dissociation, Low enthalpy of dissociation,
According to the thermodynamic results (section 6.1.1), R427A and R407C showed the minimum hydrate dissociation pressure between the refrigerants studied in this work and those used in the literature (such as R143a, R32, R23 and R125a) (Akiya et al., 1999, Mooijer-van den Heuvel et al., 2006, Hashimoto et al., 2010b, Hashimoto et al., 2010a) with a high temperatures of critical decomposition (287.6 K and 291.4 respectively) resulting in the high possible hydrate formation temperature. The temperature and pressure range of (272.7-287.6) K and (0.0791-0.830) MPa was found for R427A respectively at Lw-H-V phase equilibrium boundary. The hydrate dissociation pressure and temperature range of (0.106-1.27) MPa and (275.8-291.4) K was found for R407C. However, for R427A and R407C relatively low enthalpy of hydrate dissociation (87.3 and 85.5 kJ/mol respectively) were found which are still higher than that of R32 and R23 used in literature. Form a kinetic perspective, a comparatively high induction time with high degree of subcooling (equal to 6) resulting in a low apparent rate constant were found for R427A hydrate (section 6.1.2). The maximum apparent rate constant of the hydrate formation in the presence of pure water was obtained for R407C. An inhibition effect of the SDS solution was found for R407C gas hydrate.
It was found from the thermodynamic and kinetic results that R406A is an appropriate candidate for use in cold storage systems. The hydrate dissociation temperature and pressure range of (275.8-283.7) K and (0.112-0.536) were found for R406A. At a constant temperature, the pressure of the R406A hydrate dissociation is slightly higher than that of R427A and
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R407C and lower than the refrigerants such as R143a, R32 and R23 used in the literature (Akiya et al., 1999, Mooijer-van den Heuvel et al., 2006, Hashimoto et al., 2010b, Hashimoto et al., 2010a). The relatively high enthalpy of hydrate dissociation equal to 110.75 kJ/mol was found for R406A at its critical decomposition temperature. The induction time and degree of subcooling are zero and 2.2 respectively for R406A which are amongst the lowest values obtained in this study for the refrigerants investigated. Furthermore, a promoting effect of SDS solution was found for R406A at the presence of the 400 and 500 ppm SDS solution. The hydrate formation R406A was observed after 54 minutes in the presence 500 ppm SDS solution with the degree of subcooling equal to 1.2 which is the lowest value obtained in this study.
Furthermore, the highest apparent rate constant was related to the R406A+400 ppm SDS.
Additionally, from an economical perspective R406A has a lower price than R427A, R407C and the refrigerants used in literature such as R125a, R134a and R32.