Comparison between static and dynamic methods for sorption isotherm measurements

Sorption isotherms of Microcrystalline Cellulose (MCC), pharmaceutical granules, PolyEtherBlocAmide (PEBA) membrane and sewage sludges were measured at various temperatures using three different experimental gravimetric methods: the saturated salt method, the Dynamic Vapor System (DVS) apparatus and a simultaneous gravimetric and calorimetric measurement techniques using a Setaram TGA-DSC111. A comparison between the salts method and DVS is proposed at ambient temperature for the MCC, the pharmaceutical granules and the PEBA membrane. The comparison between the static method and the DVS shows that the results are consistent as long as the apparent moisture content in the material is high. When the apparent diffusion coefficient becomes low, the difficulty to reach the thermodynamic equilibrium appears in the saturated salts method. Then, DVS and TGA-DSC apparatus were used to investigate the desorption isotherms for the MCC between the ambient temperature and 90degreesC. Finally, an application on biological products is proposed: the desorption isotherms for two sewage sludges coming from the same wastewater treatment plant were determined with the DVS and the TGA-DSC equipments at 45degreesC. The accuracy of the TGA-DSC is good as long as the apparent diffusion coefficient is above 10(-9) m(2)/s. When this threshold value is overstepped, the desorption isotherms are overestimated.