Mixed gas separation with thermally rearranged polymer membranes: mass transport properties, aging and restoring

Thermally rearranged polymer membranes offer great advantages in gas separation applications owing to the presence of micropores, appropriately tuned cavities size and distribution, which make them highly permeable. Their use in CO2 separation streams is an important field of application currently under development. In this work, the transport properties of a thermally rearranged hollow fibre membrane modules [1,2], also functionalized with carbon nanotubes [3], were evaluated feeding gas mixtures, also in presence of water vapour. The mutual influence of mixed-gas permeation in TR polymer membranes was also evaluated, analyzing how and how much CO2 (the most soluble gas) and H2 (the fastest diffusing gas among the gas species tested) affect each other permeation as well as the permeation of the other gases, and a discussion based on sorption and diffusion contributions was also included [4]. Most of the CO2-containing streams are water saturated. The presence of water vapour in the mixture showed great influences on the permeation behavior. Considering a mixed gas having a typical flue gas composition a drop in permeance of about 50% was measured for CO2 and ca. 30 and ca. 40%, for N2 and O2, respectively. This was translated in a 20% reduction in CO2/N2 selectivity from 20 (dry condition) to 16 (wet condition) at 25°C. In addition, the membrane module, tested continuously for about six months, showed a certain drop in permances that can be attributed to aging effects induced by physical tendency to equilibrium state. To address this issue, a strategy for restoring initial mass transport properties was proposed. The permeance of CO2 and N2 were tracked for more than 500 days, divided in two cycles, under continuous exposure to the fed gas. After each cycle of observation lasted 190 and 285 days, the membrane module was restored by means of an appropriate procedure, foreseeing the injection of methanol. The separation properties, permeance and selectivity, of the membranes were fully recovered since both reached their earliest values (day 1). After the first restoring, the membrane monitoring, for further 285 days feeding single and mixed dry gases, again showed aging, similarly to that in the first cycle of observation. The second restoring carried out on the again aged module confirmed the repeatability of this procedure for recovering separation properties, option particularly attractive from an applicative point of view.