Aging study of carbon molecular sieve membranes

Further insight into the impact of air and humidity exposure on carbon molecular sieve membranes (CMSM) is provided. A CMSM was exposed to water vapor and to different dry environments (air, oxygen, nitrogen, propylene, etc.) for several months and the performance stability with time was analyzed periodically. In an effort to better understand the effect of oxygen on the fresh CMSM, regeneration at high temperature, used to remove some oxygen surface groups, was conducted 1 year after fabrication. The impact of oxygen removal from the surface on adsorption equilibrium, kinetic transport and pore structure is considered. Membrane aging as a result of oxygen adsorption was investigated on the heat-treated sample. This is the first time that oxygen chemisorption uptake rates have been measured on CMSM. Passivation methods and regeneration procedures on CMSM are also discussed.     

Asymmetric molecular sieve carbon membranes

An asymmetric molecular sieve carbon membrane is obtained by conventional pyrolysis of a thermosetting polymeric film, followed by unequal oxidation. Morphology, pore size distribution, and gas separation characteristics of the membrane are discussed. The transport mechanism for gas permeation is clearly non-Knudsen diffusion since heavier oxygen permeates faster than lighter nitrogen. The proposed mass transfer mechanism is that of a molecular sieve.     

Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyimide hollow fiber

Asymmetric carbon hollow fiber membranes were prepared by pyrolysis of an asymmetric polyimide hollow fiber membrane, and their mechanical and permeation properties were investigated. The carbon membrane had higher elastic modulus and lower breaking elongation than the polyimide membrane. Permeation experiments were performed for single gases such as H₂, CO₂, and CH₄, and for mixed gases such as H₂/CH₄ at high feed pressure ranging from 1 to 5 MPa with or without toluene vapor. The permeation properties of the carbon membranes and the polyimide membrane were compared. There was little change in the properties of the carbon membranes with a passage of time. The properties were hardly affected by the feed pressure, whether the feed was accompanied with the toluene vapor or not, because the carbon membranes were not affected by compaction and plasticization.     

Appealing sheath-core spun high-performance composite carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a “Pseudo Wheel+Hub & Spoke” asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO₂ removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof-of-concept membrane for this application. Facile formation of the sheath-core spun precursor with a 6FDA-DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO₂/CH₄ (50 : 50) mixed gas feed with an attractive CO₂/CH₄ selectivity of 64.3 and CO₂ permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA-DAM in a composite sheath-core CMS membrane with the “Pseudo Wheel+Hub & Spoke” structure.     

Performance and pore characterization of nanoporous carbon membranes for gas separation

The performance of a novel nanoporous carbon membrane for separation of hydrogen-hydrocarbon gas mixtures is described. The membrane selectively adsorbs hydrocarbons from hydrogen at the high pressure side and the adsorbed molecules then diffuse along the pore walls to the low pressure side. Pressure levels at thigh gh and low pressure sides of the membrane and the type and flow rate of the sweep gas at the low pressure side of the membrane were varied. The effects of these variables on the hydrogen recovery and hydrocarbon rejection by the membrane were investigated.     

Characterization of microporous membranes for use in membrane contactors

Methods of selecting applicable membranes for use in membrane contactors for flue gas desulfurization are proposed in this paper. The mass transfer mechanism for SO₂ diffusion through gas filled pores is explored by simple measurements in order to identify suitable membrane structures for use in contactors for flue gas cleaning. It is attempted to correlate the experimentally determined membrane mass transfer coefficient to intrinsic physical properties of the membrane by applying theoretical and empirical correlations for the porosity-tortuosity relationship of the porous structure. Thereby limiting fluxes can be predicted with good accuracy from data quoted in the manufactures catalogue.     

Thermodynamic properties affect the molecular motion of novolac type phenolic resin blended with polyamide

The effect of association reaction length on the substantial increase of molecular motion as well as entropy (−TΔS_m) of phenolic-polyamide blends is investigated with the ¹³C solid-state NMR and DSC. The H-bonding strength by forming the phenolic-polyamide interaction is great enough to overcome the breaking off the self-association of phenolic. With respect to decreasing the association reaction, the polyamide resonance intensity of ¹³C solid-state NMR spectra is weakened due to the reduction of the cross-polarization efficiency at a high mobile sample. The glass transition temperature of phenolic-polyamide blend as well as T^H_1ρ value from NMR experiments is also decreased. The decreasing strength of H-bonding resulting from blending causes higher entropy (−TΔS_m) and higher molecular mobility of the phenolic-polyamide blends. Accordingly, the polyamide-66 possesses higher H-bonding force and exhibits more mobile role in this phenolic/polyamide blends family. It can be concluded that the molecular segmental motion and entropy are progressively decreased while increasing the inter-association force of the polyamide within the miscible window.     

Transport of organic vapors through poly(1-trimethylsilyl-1-propyne)

Poly(1-trimethylsilyl-1-propyne) [PTMSP], a high-free-volume glassy polymer, has the highest gas permeability of any known synthetic polymer. In contrast to conventional, low-free-volume, glassy polymers, PTMSP is more permeable to large, condensable organic vapors than to permanent gases. The organic-vapor/permanent-gas selectivity of PTMSP based on pure gas measurements is low. In organic-vapor/permanent-gas mixtures, however, the selectivity of PTMSP is much higher because the permeability of the permanent gas is reduced dramatically by the presence of the organic vapor. For example, in n-butane/methane mixtures, as little as 2 mol% n-butane (relative n-butane pressure 0.16) lowers the methane permeability 10-fold from the pure methane permeability. The result is that PTMSP shows a mixed-gas n-butane/methane selectivity of 30. This selectivity is the highest ever observed for this mixture and is completely unexpected for a glassy polymer. In addition, the gas mixture n-butane permeability of PTMSP is considerably higher than that of any known polymer, including polydimethylsiloxane, the most vapor-permeable rubber known. PTMSP also shows high mixed-gas selectivities and vapor permeabilities for the separation of chlorofluorocarbons from nitrogen. The unusual vapor permeation properties of PTMSP result from its very high free volume - more than 20% of the total volume of the material. The free volume elements appear to be connected, forming the equivalent of a finely microporous material. The large amount of condensable organic vapor sorbed into this finely porous structure causes partial blocking of the small free-volume elements, reducing the permeabilities of the noncondensable permanent gases from their pure gas values.     

Adsorption of Cu(II) on porous chitosan membranes functionalized with histidine

The ability of chitosan to form complexes with bivalent metal ions has been broadly explored in the literature. The present work investigates the influence of functionalization of macroporous chitosan membranes with histidine on their ability to remove copper ions from aqueous solution in the range of pH 4–6. The maximum adsorption capacity for Cu(II) ion was 2.5 mmol metal/g pristine chitosan membranes. Under this condition, no influence of membrane porosity was observed. However, for membranes with immobilized histidine, the porosity was shown to be a factor that affects the maximum adsorption capacity, with values ranging from 2.0 to 3.0 mmol metal/g chitosan. These results indicate that the immobilization of histidine on porous chitosan membranes presents synergy with porosity in the ability to complex Cu(II) ions. This synergy may be negative or positive, depending on the initial membrane porosity.     

Chemorheological study of the curing kinetics of a phenolic resol resin gelled

The changes in the resin viscosity, conductivity, mass, and enthalpy during curing reactions have been studied to obtain kinetic parameters that allow modeling of the resin behavior throughout its industrial application. In this work, isothermal rheological tests of a phenolic resol resin were performed in order to study its complex viscosity during crosslinking reactions. Samples were prepared by a precuring treatment in a heated plate press to reach gel point of the resin. Rheological analyses of resol resin curing were carried out at five different temperatures (80-100 °C), and the kinetic models of Arrhenius and Kiuna were applied. The resol resin curing presented an activation energy of 72.1 kJ/mol according to the Arrhenius model. The Kiuna model was proposed to fit the non-linear evolution of the resin’s complex viscosity at the highest temperatures. This kinetic model was suitable for predicting the changes in the complex viscosity of the resol resin after its gelation, and the process activation energy obtained for the second order polynomial applied in this model was 88.1 kJ/mol. In addition, the profile for the degree of curing of resol resin was determined from measurements of the material’s elastic modulus.     

The use of moment theory to interpret diffusion and sieving measurements in terms of the pore size distribution in heteroporous membranes

Moment theory has been applied to model porous membranes to show that one can place reasonable bounds on the cumulative pore size distribution, the hindered diffusivity or the reflection coefficient of large solutes in a heteroporous membrane by measuring the diffusive permeability to a small solute, the hydraulic permeability and one or two additional transport characteristics. These additional measurements involve either the flux of a small solute at Pe1, the hindered diffusivity of a large solute or the reflection coefficient of a large solute at Peå1. Membrane heteroporosity is incroporated in the predicted bounds without requiring one to make any a priori assumptions about the nature of the pore size distribution. In this paper, the results from calculations performed with different model membranes containing log-normal pore size distributions are reported. A comparison of the results obtained with three different membranes shows that one can distinguish between membranes with the same average pore size but different pore size distributions by measuring either the hindered diffusion coefficient or the reflection coefficient of two different sized solutes. A comparison of the bounds on D and the bounds on σ predicted from different types of transport measurements shows that, under certain conditions, one can place tighter bounds on one transport characteristic by measuring a different one.     

Tertiary amine-mediated synthesis of microporous carbon membranes

Microporous carbon membranes were prepared on an -alumina support by a pyrolysis of cationic tertiary amine/anionic polymer composites. The precursor solutions contain a thermosetting resorcinol/formaldehyde (RF) polymer and a cationic tertiary amine. Three types of cationic tertiary amines with different chain lengths were used, such as tetramethlammonium bromide (TMAB), tetrapropylammonium bromide (TPAB) and cetyltrimethylammonium bromide (CTAB). A porous structure was produced by a decomposition of the amine and the resulting pores assisted the further gasification of the RF polymer at high temperature. The carbon/alumina membranes have thin and continuous carbon top layers with a thickness of 1 μm. Gas permeation tests were performed using single gases of CO₂, O₂, N₂, CF₄, n-C₄H₁₀ and i-C₄H₁₀, as well as binary mixtures of CH₄/n-C₄H₁₀ and N₂/CF₄ at different temperatures between 23 and 150 °C. The carbon membrane prepared using TMAB showed separation factors higher than 650 for the CH₄/n-C₄H₁₀ mixtures and higher than 8100 for the N₂/CF₄ mixture. From the permeation of pure gases with different molecular sizes, the pore sizes of the carbon membrane prepared using TMAB, TPAB and CTAB are estimated to be 4.0, 5.0 and larger than 5.5 Å, respectively, indicating that the micropore size of the carbon membranes is controllable by using different amines.     

Magnetic phenolic resin core-shell structure derived carbon microspheres for ultrafast magnetic solid-phase extraction of triazine herbicides

In this study, monodisperse magnetic carbon microspheres were successfully synthesized through the carbonization of phenolic resin encapsulated Fe₃O₄ core-shell structures. The magnetic carbon microspheres showed high performance in ultrafast extraction and separation of trace triazine herbicides from environmental water samples. Under optimized conditions, both the adsorption and desorption processes could be achieved in 2 min, and the maximum adsorption capacity for simazine and prometryn were 387.6 and 448.5 μg/g. Coupled with high-performance liquid chromatography-ultraviolet detection technology, the detection limit of triazine herbicides was in the range of 0.30–0.41 ng/mL. The mean recoveries ranged from 81.44 to 91.03% with relative standard deviations lower than 7.47%. The excellent magnetic solid-phase extraction performance indicates that magnetic carbon microspheres are promising candidate adsorbents for the fast analysis of environmental contaminants.     

High carbon yield thermoset resin based on phenolic resin,hyperbranched polyborate,and paraformaldehyde

Hyperbranched polyborate (HBPB) is a new hyperbranched polymer with excellent thermal resistance, which can be used to improve the thermal stability, especially carbon yield, of phenolic resin (PR). In the present paper, thermal properties and curing degrees of the blends of PR and HBPB (PR–HBPB blends) are investigated. It is found that the curing degrees of PR–HBPB blends would be decreased, and the decrease of curing degree can be attributed to the improper ratio of phenol groups of HBPB to hydroxymethyl groups of PR. Paraformaldehyde (PFM) is introduced to remedy the deficiency of hydroxymethyl groups in PR–HBPB blends. The curing degrees of PR–HBPB blends are improved and the carbon yields of the blends of PR, HBPB, and PFM (PR–HBPB–PFM blends) can be up to 75–80% at 800°C in nitrogen. PR–HBPB–PFM blends can be explored as novel precursors for carbon materials with excellent properties, ease of preparation and low cost. Copyright © 2010 John Wiley & Sons, Ltd.     

Negatively charged poly(vinylidene fluoride) microfiltration membranes by sulfonation

Negatively charged PVDF microfiltration membranes were prepared using direct sulfonation with chlorosulfonic acid. The effect of sulfonation on the surface chemical properties, morphology, pore size distribution, hydrophilicity, water uptake, pure water flux, fouling and rejection were investigated. As the sulfonation reaction time was furthered, the degree of sulfonation and ion-exchange capacity increased and the membranes became more hydrophilic due to introduction of sulfonyl groups to the membrane surface. Using X-ray photoelectron spectroscopy, the composition of sulfonyl group with respect to sulfur concentration increased with time. From the SEM and porosity measurements, both the untreated and treated membranes did not reveal a substantial change in its morphology. The pure water flux increased significantly having a decreasing intrinsic resistance trend with degree of sulfonation. Both fouling phenomena and rejection were enhanced, with fouling of charged poly(styrene sulfonic acid) molecules on the surface-modified membrane decreased and rejection values increased with increasing degree of sulfonation mainly due to the effective electrostatic repulsion between the negatively charged PSSA and the negatively charged membrane.     

Are pore size distributions in microfiltration membranes measurable by two-phase flow porosimetry?

The issue of evaluating equivalent pore diameter distributions in membrane microfilters from gas-liquid (g-l) porosimetry data has been critically examined. Experiments performed with one isotropic and one composite anisotropic membrane in both possible orientations revealed conspicous dependence of the obtained (g-l) porosimetry peaks on imposed pressure ramp rates, p. Interference of this kinetic effect can be eliminated from the measured data by extrapolation to p = 0. The ramp rate effect is most likely caused by tortuous pore length distribution, and relatively long times required for liquid expulsion. For two experiments, the observed effects of p could be reconciled with predictions of the Schlesinger-Bechhold theory [Bechold et al., Kolloid Z., 55 (1931) 172–198]. The data obtained with the thin top layer of the composite membrane facing intruding air directly did deviate somewhat from the theory. Pores characterized by (g-l) porosimetry are likely of the “throat type”, and their size distribution is considerably more narrow than that obtained for the “node-type” pores by SEM-image analysis [Zeman and Denault, J. Membrane Sci., 71 (1992) 221–231]. A single bivariate distribution function was constructed for these two distinct pore populations. Flow-weighted or number fraction distributions can be calculated from the extrapolated porosimetry data. For narrow ranges of “throat” diameters, these distributions are fairly similar.     

Effects of modified multi-walled carbon nanotubes on the curing behavior and thermal stability of boron phenolic resin

One kind of boron phenolic resin (BPR) was prepared from the solvent-less reaction of resoles with boric acid. X-ray photoelectron spectroscopy (XPS) showed that the reaction degree of boric acid was 83.8%. Multi-walled carbon nanotubes (MWCNTs) were modified by nitric acid, 4,4′-Diaminodiphenyl methane and boric acid. The effect of modification was determined by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA) techniques and XPS. The cure kinetics and thermal behavior of BPR and modified multi-walled carbon nanotubes (m-MWCNTs)/BPR were studied. It was found that the curing apparent activation energy (E_a) decreased with the increasing amount of m-MWCNTs. But there was no obvious change in the orders of curing reactions. The results of TGA showed that 1.0 wt% of the m-MWCNTs could increase the thermal decomposition temperature (T_d) and the char yield of m-MWCNTs/BPR nanocomposites by 36.7 °C and 6.2%. These critical enhancements will definitely help to attract more researches on this area.     

Turbulent flow technique for the estimation of oxygen diffusive permeability of membranes for the oxygenation of blood and other cell suspensions

When transport-efficient membrane modules (such as those where the liquid flows outside hollow fibre membranes) or membranes with prolonged resistance to wetting are used for the oxygenation of blood or other cell suspensions, membrane contribution to the overall oxygen transfer resistance into the liquid may become significant. Thus, estimation of membrane diffusive permeability towards relevant gases (e.g., oxygen) is important to develop new membranes and to ensure reproducible commercial membrane performance.

In this paper, we report on a turbulent flow technique for the estimation of the oxygen diffusive permeability of membranes used in outside-flow oxygenators. Water is re-circulated under turbulent flow conditions in a closed-loop from a reservoir to the shell of lab-scale membrane modules. The overall oxygen transfer to water coefficient is estimated at increasing water flow rates from the time the change of dissolved oxygen tension in the stream leaving the water reservoir occurs. Oxygen diffusive permeability is estimated as the reciprocal overall transfer resistance at infinitely high water flow rates, for negligible gas-side oxygen transport resistance. The technique was used to estimate oxygen diffusive permeability of commercial Oxyphan^® polypropylene membranes for blood oxygenation and of two laboratory polypropylene membranes, the one featuring a microporous wall structure with smaller-than-standard pore size, the other featuring an outer thin, dense layer supported by a thick spongy layer. The turbulent flow technique yields oxygen diffusive permeability estimates consistent both with membrane hydraulic permeability towards gaseous nitrogen, membrane wall structure, and with values in literature obtained using a liquid reactive with oxygen, but without the complications associated with reaction and physical transport kinetic characterisation. We conclude that the turbulent flow technique is a useful tool in the development and quality control of membranes for the oxygenation of blood and other cell suspensions.     

Plasticization of ultra-thin polysulfone membranes by carbon dioxide

Plasticization of gas separation membranes by carbon dioxide permanently alters their performance and increases the possibility of membrane failure. This is amplified in ultra-thin composite membranes, where the active polymeric layer is less than 2 μm. Here, the plasticization influence of CO₂ is measured on ultra-thin polysulfone composite membranes for a range of active layer thicknesses, at four temperatures. The resulting permeability–pressure isotherms demonstrate plasticization occurs for all thicknesses at pressures lower than has been reported for dense membranes. These isotherms were quantitatively fitted with an expanded dual-sorption model that takes into account plasticization of the membrane. The plasticization potential of CO₂ for polysulfone was found to increase with reduced active layer thickness. Similarly, the plasticization potential of CO₂ was found to decrease with temperature. These results are consistent with similar research that shows that thin films behave differently to dense membranes.     

Removal of VOCs from air by membrane-based absorption and stripping

Atmospheric emission of volatile organic compounds (VOCs) such as toluene, xylene, acetone etc. from industrial facilities causes serious environmental problems and financial losses. Existing technologies for VOC emission abatement have many strengths as well as considerable limitations. A regenerative absorption-based process for removal of VOCs from N₂ in an inert, nonvolatile, organic liquid flowing in compact hollow fiber devices has been studied here. These devices eliminate flooding, loading and entrainment encountered in conventional absorption units. Detailed experimental results and theoretical analyses for absorption studies were communicated elsewhere. The overall performance of the combined absorption-stripping process is described here; it appears to be controlled by stripping due to the low temperature and the lower membrane surface area in the stripper. The difference between only absorption and combined absorption-stripping results was more pronounced for VOC-absorbent systems having higher Henry's law constant and diffusivity. A theoretical model has been developed from first principles to simulate the behavior of the membrane stripper; this has been combined with the model for the membrane absorber to determine the overall process performance. Simulated results obtained from the mathematical models agree well with the experimental results for combined absorption-stripping. Simulation results suggest that higher stripping temperature and larger stripper area enhance the performance considerably.