Development and characterisation of polyaniline-based composite membranes for gas-separation
Polyaniline-based membranes were developed during this work. The combination of polyaniline with PVDF offers the opportunity of creating large, dense and stable membrane areas in a simple and reproducible way. New production techniques were tested to meet the requirements of large scale applications and to overcome the poor mechanical properties of polyaniline. In order to fulfil industrial requirements, with PVDF a material was found that combines well with polyaniline and forms stable and thin composite membranes having high flux and selectivity in gas-separation applications. By simultaneous reduction of the film thickness the permeation rate could be increased by at least 5 times compared to the data in other publications. The thickness of the polyaniline layer could be reduced down to 1.6μm. The separation factors for some interesting gas pairs are: H2/CO2 (3.5), CO2/CH4 (40) and 02/N2 (7). In the case of nitrogen, diffusion coefficients were determined that are by some orders higher than those observed by other authors. Also the impact of factors such as the exchange state, membrane thickness, temperature and feed pressure on selectivity and permeability were tested Thinnest polyaniline layers on PVDF supports allowed the observation of permeation change during doping and dedoping the membranes. It has also been illustrated in how far the gas flux responds to doping. In these tests the change of permeability was observed in situ. Smallest microporous defects can be plugged without affecting the polyaniline layer itself, and still maintaining selectivity. The combination of polyaniline / PVDF with siloxanes allows plugging microscopic defects without restricting the permeant flux. In sorption tests in the case of oxygen, the Dual-mode model was found to be appropriate to describe sorption, whereas sorption on nitrogen could satisfactorily be described by the Langmuir isotherm. Isotherms of carbon dioxide indicate that the Dual-mode model applies at pressures up to 35bar. Multi-layer sorption and condensation at still higher pressures can be described by the BET model.