The work presented in this thesis aims at developing a new method for characterising the multi-component solute transport through dense membranes, both in the transient and in the steady state of gas separation and pervaporation systems, using a Mass Spectrometer (MS) as an on-line, real-time, monitoring tool.
The study of the transient period of mass transport through a membrane, although more complex than the steady-state period, has attracted the attention of researchers because it may offer a route for a better understanding of the membrane material under study and how it interacts with the permeating species. In fact, noticeable structural membrane adjustments may occur during the transient period, from when the solute starts permeating, impacting directly on the membrane intrinsic transport properties in a structure-transport relationship. The greater the affinity of the solute to the membrane, the greater the modification it may cause in the membrane matrix and, consequently, the greater the impact on the transport properties. Therefore, estimation of diffusion coefficients during the time-course of the whole permeation process is critical.
The goal of the work developed in this PhD thesis was to study the transport properties of different multi-component feed streams through different polymeric membrane materials and different permeation systems. This work includes a study ranging from โnon-interactingโ solutes, such as inert gases, to more complex systems where the solutes have strong affinity to the permeated material, such aroma compounds or water vapour. The transient behaviour of the selected membranes was followed when exposed to penetrating solvents and solutes through the on-line monitoring of the permeating species using mass spectrometry, which offers the possibility to acquire one data point per second.
The transport properties (sorption and diffusion coefficients) were assessed for mixed gas permeation systems through the development of a novel time lag measurement, where both parameters can be determined in a single step. In this system, solute-membrane interactions are not relevant and a constant diffusion coefficient can be considered during the whole permeation process, because the membrane structure is not significantly altered when in contact with these gases.
Otherwise, several phenomena may occur inside the membrane in non-ideal processes, leading to a change of the diffusivity of the permeant with its own local concentration and, consequently, the change of its diffusivity with time. From the on-line MS monitoring tool, a method for calculating time-dependent diffusion coefficients in non-ideal systems was developed, both for gas separation, humidified gas streams, and pervaporation systems, where the solute presented affinity to the membrane. Time-dependent diffusion coefficients of permeating solutes through different membranes were calculated, considering that the membrane structure is potentially modified, due to solute-membrane interactions. During solute transport in the transient period, permeating solutes with high affinity to the membrane may extensively solubilise within the membrane structure, causing membrane rearrangements. As a consequence, longer transient periods may be observed. Finally, based on the information acquired by mass spectrometry, namely the estimation of time-dependent diffusion coefficients, a mathematical model was developed in order to obtain solute concentration profiles inside the membrane and their evolvement along time. Two case-studies were selected, corresponding to different systems, using permeating solutes with different affinities towards the membranes under study. The transport properties of two different membrane materials were compared: a polymeric membrane, which may be prone to potential material rearrangements and a ceramic membrane with a rigid structure, where material rearrangements are not anticipated
