Pure- and mixed-gas permeation of CO2 and CH4 in thermally rearranged polymers based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

Abstract

a b s t r a c tPermeability coefficients for pure CO 2 , pure CH 4 , and CO 2 /CH 4 mixtures containing 50% CO 2 are reported for a polyimide synthesized from 3,3 0 -dihydroxy-4,4 0 -diamino-biphenyl (HAB) and 2,2 0 -bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and for three thermally-rearranged (TR) derivatives thereof. Permeability measurements were made at 35 1C for fugacities ranging from 4 to 25 atm. The permeability of CO 2 and CH 4 increased as the degree of TR conversion increased. For example, CO 2 permeability at 10 atm increased by a factor of 30 between the unconverted polyimide and its TR analog converted at 450 1C. In pure-gas experiments, CO 2 was observed to plasticize the unconverted polyimide, but it did not appear to plasticize the TR polymers. In mixed-gas experiments, dual-mode competitive sorption caused a depression in CH 4 permeability, with very little change in CO 2 permeability. In addition, plasticization by CO 2 was evident in the CH 4 mixed-gas permeability trends, but its impact was small in contrast with dual-mode competitive effects. Consequently, CO 2 /CH 4 mixed-gas permeability selectivity was higher than the ideal selectivity, calculated as the ratio of pure gas permeability coefficients. The dual-mode sorption and permeation model was fit to the experimental data. Dual-mode model parameters and model predictions are reported, along with their confidence intervals. By comparing the dual-mode model predictions with the experimental mixed-gas data, the degree of CO 2 -induced plasticization was observed to decrease as the degree of TR conversion increased and was completely absent (within experimental uncertainty) for the TR polymer converted at 450 1C.

Separation of CO2 from humidified ternary gas mixtures using thermally rearranged polymeric membranes

“…The trend observed in this study was different from other polymers commonly used for the same applications where the performance measured in the mixture was often lower than that of the single gas. However, it was in agreement with the results of Gleason et al [24] who used HAB-6FDA polyimides and TR derivatives, and can be mainly attributed to the lower condensability of N2 and O2 compared to that of CO2, which was, therefore, favored over the incondensable gases and thus had a higher affinity for the Langmuir sites. Owing to the greater adsorption of CO2 in the Langmuir sites, the permeances of N2 and O2 in gas mixture were decreased as a result [34,35,36].…”

“…Across the pressure range considered, comparing the single gas behavior with that of the gas mixture, a reduction in permeance was observed for all gas species within the entire temperature range investigated (Fig. 5), which was in agreement with that observed by Gleason et al [24]. For the permeation of a gas mixture, the dual mode competitive sorption caused a permeance reduction of less permeable gases (N2, O2), and a marginal change in CO2 permeance.…”

“…In this case, membranes exhibiting high permeabilities such as polymers with intrinsic microporosity (PIMs) and thermally rearranged (TR) polymers [17,18,19,20,21,22] has become an important field of study. Membranes prepared with thermal rearranged polymers show excellent separation performance due to their complex microstructure whose cavity size and distribution can be opportunely tuned by the choice of template molecules and thermal treatment protocols [23,24,25,26,27].…”

Nonequilibrium Lattice Fluid Modeling of Gas Solubility in HAB-6FDA Polyimide and Its Thermally Rearranged Analogues

“…Due to their unusual combinations of high gas permeability and selectivity, thermally rearranged (TR) polymers have attracted interest for membrane gas separation 1 . These materials, first reported by Park et al 2 , exhibit transport properties that, in some cases, surpass the upper bound defined by Robeson in 2008 3,4,5 .…”

“…The transport properties exhibited by TR polymers are ascribed to a favorable size and distribution of free volume elements 1,6,7,8 . For example, the increase in gas solubility with increasing extent of thermal conversion has been attributed to increases in excess fractional free volume 6 .…”