Determination and modelling of the change in heat capacity during the melting of THF - CO2 hydrates
Determination et modélisation de la variation de capacité calorifique lors de la fusion d'hydrates de THF - CO2
Résumé
The study is related to the use of CO2 clathrates slurries for secondary refrigeration. For this application, it is important to be able to get slurries at moderate CO2 partial pressures as well as to have the possibility to adapt the hydrate melting temperature to the refrigeration need. This target could be achieved using various additives. Tetrahydrofuran (THF) is one of the additives studied in our laboratory. The study of mixed CO2 - THF clathrates formed has the type II structure. In relation with the application, it is important to obtain experimental values of the stability conditions as well as the values of the heat of melting and the change in heat capacity associated to the melting process. These three properties have been obtained using the differential scanning calorimetry (DSC) method. Equilibrium properties and heat of melting have been reported previously and modelled using the classical Van der Waals and Platteeuw model for the clathrate phase. The calculation of the THF and CO2 fugacity as well as the water activity in the liquid phase was made applying MHV2 model, the Excess enthalpy model being the Larsen version of UNIFAC, the aim being to minimize the number of parameters to be ajusted. In the present paper, we focus on the measurement of the heat capacity and its modelling using the model described above. The heat flux signals obtained in the case of hydrate as well as in the case of the solution produced by the melting process have been compared with the values obtained in the same conditions for sapphire. This allows getting, at various CO2 partial pressures (from 0 to 12 bar) the Cp values of clathrate and of the corresponding liquid solution. The obtained values were then corrected taking into account water-THF mixing heat capacity. These experimental values have been included in a data treatment together with the experimental values of dissociation temperature and heat of melting. In the modelling work, we have compared the parameter sets of Holder et al. and Parrish and Prausnitz. As, in the model, we use the second derivative with temperature of the chemical potentials, it is possible to get new information about, especially, the temperature effects on the Langmuir constants and the heat capacity difference between empty hydrate and pure water. The use of Parrish and Prausnitz's parameter give better results. However we needed to adjust few parameters to get a representation of all experimental values that is consistent with experimental deviation.