David R. B. Walker

Transport characterization of a polypyrrolone for gas separations

The gas sorption and permeation properties of a polypyrrolone step-ladder polymer have been characterized with a variety of pure gases and one gas mixture at 35°C and for feed pressures up to 60 atm. The transport properties of the soluble prepolymer have also been characterized with pure gases up to 30 atm. The polypyrrolone shows simultaneously increased permeability and selectivity over the analogous polyimide. Wide angle X-ray diffraction measurements of the average segmental spacing of the materials and fractional free volume calculations characterize the packing of the different polymer types. Hindrance to intrasegmental motion is apparent in the polypyrrolone by the lack of a glass transition temperature prior to decomposition and by the limited plasticization by high pressure CO2. The permeability of the polypyrrolone is 89 barrers for He, 7.9 barrers for O2, and 27.6 barrers for CO2.

ANALYSIS OF A THERMALLY STABLE POLYPYRROLONE FOR HIGH TEMPERATURE MEMBRANE-BASED GAS SEPARATIONS

Abstract A rigid polypyrrolone material that exhibits excellent thermal and chemical stability has been studied to determine its potential for novel applications in membrane-based gas separations at elevated temperatures. Operation at high temperatures produces favorably high membrane productivities; however, losses in permselectivity usually occur as well. Gas transport and sorption measurements have been obtained for this polypyrrolone at temperatures up to 200°C. The loss of permselectivity with increasing temperature that occurs for most gas pairs is discussed in terms of both solubility and diffusivity selectivity. The temperature dependences of productivities and permselectivities for various gas paris are also compared to those of other materials thought to be attractive for high temperature gas separations. In most cases, the structurally rigid polypyrrolone out performs these other materials, demonstrating favorable productivities and permselectivities over the entire range of temperatures studied.

Study of ultramicroporous carbons by high-pressure sorption. Part 2.—Nitrogen diffusion kinetics

Sorption–desorption kinetics for nitrogen in the as-received TCM 128 ultramicroporous carbon fibre are reported at 35 °C over a wide range of pressures. The nitrogen sorption kinetics at low pressures follow the Fickian model. As the pressure increases, the deviations from the model become more pronounced, and at sufficiently high pressure a sigmoid kinetic response shape is observed which is indicative of a non-Fickian diffusion process. An additional timescale must be active to account for the non-Fickian transport behaviour. This second timescale process may correspond to adsorbate surface rearrangements leading to locally time-dependent clearing of constrictions at a rate with a characteristic kinetic constant. The nitrogen desorption kinetics are found to be less affected by the non-Fickian transport, thus leading to higher apparent diffusion coefficients for the same average pressures. At high pressures and very long sorption times, slow protracted uptake becomes apparent into restricted regions. The kinetics for this process can be described by a barrier model and are sufficiently slow to allow treatment independent of the processes occurring in the more open pore system.

Study of ultramicroporous carbons by high-pressure sorption. Part 4.—Isotherms and kinetic transport in activated carbons

Low-pressure hystereses, shown to exist for both nitrogen and carbon dioxide on as-received TCM carbon, disappear upon slight activation. These hystereses are related, in the case of nitrogen, to protracted penetration of restricted regions and, in the case of carbon dioxide, to dilation of tiny constrictions. A highly oxygen-activated carbon (19% weight loss) showed minute hysteresis in CO2 sorption experiments, while the more activated carbon (32% weight loss) formed by nitric acid activation showed more extreme hysteresis. These hystereses are related to swelling of the progressively weakened carbon matrix. Nitrogen, which is a poor swelling agent, does not show any hysteresis with these highly activated carbons. A series of TCM carbons which were progressively activated by oxygen showed a maximum in their sorption affinity constant with respect to the degree of activation. This maximum is related to either the greater accessibility of new small-pore regions or to the gradual opening of tiny constrictions that regulate transport into these regions.The lack of internal pore resistance to transport in a slightly activated carbon provides conditions conducive to observation of transient barriers formed in the entrances to the outermost pores immediately after applying a pressure step.

Study of ultramicroporous carbons by high-pressure sorption. Part 3.—Complex transport phenomena as sensed by CO2 and N2 kinetics

High-pressure sorption kinetics for N2 and CO2 in as-received TCM carbon, introduced in Part 2 of this series (J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros, J. Chem. Soc., Faraday Trans. I, 1989, 85, 1545), is elaborated here. Fickian processes are apparent for both gases, and provide the background against which the following complicated transport phenomena are overlaid. The most dramatic of these additional phenomena is CO2-induced constriction-dilation, which causes a decrease over four orders of magnitude in the equilibration time as pressure increases from 0 to 60 atm. Desorption equilibration times at lower pressures following the CO2 exposure also reflect residual dilation. Higher desorption rates compared to adsorption rates were also observed for N2; however, the differences are much less extreme and are believed to be due to immobilization of adsorbate near constrictions instead of constriction-dilation. Evidence is presented to indicate the existence of a weak, transient barrier at the pore entrances at the beginning of sorption runs if appropriate conditions are chosen for each of the two gases.

Study of ultramicroporous carbons by high-pressure sorption. Part 5.—CH4 isotherm and diffusion kinetics

Sorption–desorption kinetics and the isotherm for CH4 in as-received TCM 128 ultramicroporous carbon fibre are reported at 35°C up to 15 atm. The CH4 isotherm exhibits a similar hysteresis to that seen for N2 and CO2 on the carbon but at a much lower pressure. The low-pressure hysteresis might reflect adsorption in restricted regions composed of constrictions analogous to the tiny constrictions which were the cause of the hystereses seen with N2 and CO2. For the larger CH4, however, some of the regions that were easily accessbile to N2 may be behaving as ‘restricted’ to CH4 owing to a molecular-sieving phenomenon. Analysis of the diffusion kinetics indicates that the large CH4 molecules cause a slow barrier build-up over a few constrictions in series at the outer pores of the carbon. The desorption kinetic response, which is much faster than the adsorption, follows the Fickian model and thus reveals diffusion coefficients of the order 10–13 cm2 s–1.