Assessing the Long-Term Stability of Fatty Acids for Latent Heat Storage by Studying their Thermal Degradation Kinetics

The thermal degradation kinetics of lauric acid has been studied with the aim of assessing its long-term stability and performance as a latent heat storage medium for low temperature applications. For this purpose, dynamic thermogravimetric (TG) measurements were carried out under different gas atmospheres (N-2 and air) and at various heating rates. The kinetic analysis of TG curves assuming a zero-order evaporation process led to activation energy values very close to the evaporation enthalpy although the simulated d alpha/dT curves could not reproduce the ones experimentally obtained. On the other hand, the analysis of TG measurements with a modelfree isoconversional method has shown that the activation energy significantly varies with the degree of conversion. All these results indicate that thermal degradation of lauric acid is a multi-step process whose single-step mechanisms have to be identified. A deconvolution analysis of d alpha/dT curves was performed by using Fraser-Suzuki functions so that two single-step reaction mechanisms were proposed: the first associated with the evaporation with an activation energy value of 83 +/- 8 kJ/mol, while the other may be related to the emission of volatile compounds. The activation energy value of the second mechanism is lower when lauric acid is heated up in air, which may indicate that O-2 is involved in the reaction. The calculated TG curves, assuming the occurrence of these two mechanisms, fit very well to the corresponding experimental curves. Isothermal degradation curves have been calculated by considering both mechanisms and, according to them, lauric acid should totally disappear after 30 days if it is kept under N-2 at 5 degrees C above its melting temperature. However preliminary isothermal test in an oven with small samples (in the range of few grams) disagree with those predictions. Hence further isothermal experiments under conditions closer to the real service ones are required in order obtain reliable isothermal degradation curves and assess long-term stability of lauric acid.