Innovative use of the Zero Length Column for the measurement of water vapor adsorption equilibrium and kinetics on mesoporous solids at high relative humidity

The present work is an experimental and modeling study aimed to develop the use of the zero-length column (ZLC) technique for adsorption equilibrium and kinetic measurements of condensable vapors on solid adsorbents. Water vapor was used as adsorptive. Two silica-based materials with different pore structure were used as solid adsorbents. The ordered SBA-15 characterized by cylindrical independent pores and the amorphous silica-gel characterized by a disordered pore network. The decision to use two solid adsorbents, with completely different pore structure and substantially different shape of the isotherm, was dictated by two factors. The first factor is related to the equilibrium and it is aimed to test the capabilities of the ZLC system for the measurement of adsorption-desorption isotherms characterized by different shapes. Due to the complex shape of the water vapor adsorption isotherms on mesoporous materials and due to the presence of a hysteresis loop, the use of these two solids represents a challenging and reliable benchmark for the evaluation of the ZLC capabilities. The second factor is related to the kinetic measurements. SBA-15 permits to study the kinetic transport mechanism of a condensing vapor in a well-defined straight cylindrical pore, meanwhile silica-gel represents a good example for the study of kinetics adsorption in a pore network. The adsorption-desorption equilibrium isotherms were measured using the ZLC system and validated against a commercial gravimetric system designed for water vapor adsorption measurements. The ZLC system has shown innovative and extraordinary advantages for the measurement of the equilibrium isotherms: a) the possibility to obtain accurate and continuous adsorption-desorption isotherms for complexly shaped isotherms; b) the extremely small amount of time needed to complete an experiment. The possibility to measure continuous adsorption-desorption isotherms is extremely useful, especially for isotherms characterized by nearly vertical branches as water on SBA-15. In a traditional discontinuous measurement system as the gravimetric or the volumetric systems, an optimal number of single step points is to be chosen to obtain an accurate isotherm. More points are needed in the nearly vertical condensation-evaporation parts of the isotherm and an extremely stable concentration of the gas phase is required to avoid irreversible change of the adsorbed amount. The ZLC is a continuous measurement system which can potentially be fully automated. A continuous adsorption-desorption isotherm is obtained when switching the feed line from inert to adsorptive or vice versa. The only requirement to be fulfilled is given by the assessment of the equilibrium control regime. The calculation of the adsorption-desorption isotherm is based on a simple and robust integration of the mass balance in the column which is assumed to be a perfectly mixed cell. The advantages of the ZLC extend to the measurement of the scanning curves. Continuous adsorption and desorption scanning curves were obtained. The possibility to accurately observe the shape of a scanning curve represents an important advantage for the structural characterization of porous solids. The scanning curves are easily obtained if one considers that the shape of the isotherm can be continuously observed from the response concentration curves of the ZLC. The scanning curves can be measured by switching the feed line at the gas phase concentration chosen as starting point for the scanning curve. The ZLC is intrinsically faster than the traditional discontinuous techniques. The water adsorption-desorption isotherm on SBA-15 was measured in 36 hours, meanwhile, 9 days were required by the commercial gravimetric system. Consequently, the ZLC technique is one order of magnitude less time consuming than the gravimetric system. A theoretical analytical model for the adsorption-desorption equilibrium isotherm of vapors on mesoporous solids was formulated. The equilibrium model was designed to consider several possible adsorption mechanisms of vapors in a distribution of cylindrical pores from zero loading up to full saturation of the solid. The model was validated against the experimental data obtained with the different experimental systems. The model can be used for kinetics studies or for process simulations in adsorption columns. The kinetics of water adsorption and desorption on SBA-15 and silica-gel was studied using the ZLC system. The study was aimed to understand the transport mechanism of condensing water in mesoporous solids. Large concentration steps going from full saturation up to dry solid and vice versa were used for the kinetic study. The adsorption and desorption response concentration curves, measured at different flowrates, were used for the study and the modeling of the adsorption kinetics. A simple theoretical procedure is suggested to assess if the kinetic can be well represented by a linear driving force model or if a more accurate kinetic model is needed. A linear driving force (LDF) model and several diffusive models were formulated to predict the kinetic response curves of the ZLC for SBA-15 powder and silica-gel pellets. All the models have a concentration-dependent mass transfer coefficient and only one fitting parameter. The Darken equation and the derivative of the equilibrium isotherm were used to correlate the mass transfer coefficients with the amount adsorbed. In this view, the possibility given by the ZLC to obtain a continuous and accurate isotherm is crucial for the correct prediction of the mass transfer coefficient. Only one parameter is needed to regress the experimental response curves measured at different flowrates for high concentration steps. The LDF model fails to predict the kinetic response curves for water vapor on SBA-15 and silica-gel. The diffusive models can better predict the adsorption kinetics for water on SBA-15 and silica-gel pellets