Asymmetric Hollow Fiber Membranes Research Articles

Utilization of nanoplatelets in organic–inorganic hybrid separation materials: Separation advantages and formation challenges

Gas separation demands combined with the need to increase industrial process efficiency have encouraged the use of membranes as attractive alternatives to thermally driven separations. Despite the attractiveness of the membrane platform, intrinsic transport properties of pure polymeric membranes are limited by a tradeoff between productivity and selectivity. This tradeoff can be partially overcome by broadening the spectrum of materials to include hybrid composites (commonly referred to as mixed matrix membranes) of polymer and a dispersed phase of either inorganic or carbon materials. These materials offer processing options that combine the attractive, low cost processing approaches that are already available for the manufacture of pure polymer-based membranes. Hybrid membranes incorporating traditional zeolite materials, with unit aspect ratios ( A r ∼ 1), limit options to create thin selective layers. Recent studies have considered high aspect ratio ( A r > 10) sieving materials ( i.e. flakes or platelets) with significantly higher aspect ratio rather than traditional zeolites. These higher aspect ratios increase the tortuous path of permeation for rejected penetrants, so a low volume fraction of platelet material can be used to produce high performance membranes. In this report we will discuss the advantages of hybrid membranes containing nanoplatelet materials and the challenges associated with creating these advanced materials as well as the unique applications within an asymmetric hollow fiber membrane. We will also report on our initial attempts at producing nanoplatelet-polymer asymmetric hollow fiber membranes.

Preparation of hydrophobic PVDF hollow fiber membranes for desalination through membrane distillation

Fabrication of polyvinylidene fluoride (PVDF) hydrophobic asymmetric hollow fiber membranes was studied by introducing inorganic salt LiCl and water soluble polymer polyethylene glycol (PEG) 1500, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. The membranes properties also were tested and characterized. It is found that the non-solvent additive can increase membranes porosity, ether LiCl or PEG 1500. Because of the addition of PEG 1500, the PVDF membranes obtained a rough topography on the membrane surface and the contact angle of the PVDF membranes increased to 113.50 degrees compared to 89.82 degrees of the PVDF membranes spun without an additive. During direct contact membrane distillation (DCMD) of 0.6 M sodium chloride solution, the PVDF membranes spun with PEG 1500 as a non-solvent additive achieved higher water permeation flux compared to the membranes spun from PVDF/DMAc and PVDF/DMAC/LiCl dopes, but the latter two membranes exhibited higher salt rejection rate.

Development and characterization of novel charged surface modification macromolecule to polyethersulfone hollow fiber membrane with polyvinylpyrrolidone and water

Polyethersulfone (PES) hollow fiber membranes were fabricated using water and polyvinylpyrrolidone as additives and N-methyl pyrrolidinone as solvent. Asymmetric hollow fiber membranes were spun by the dry–wet phase inversion method. A charged surface modifying macromolecule (cSMM) was synthesized and blended into the PES dope. The modification of PES hollow fiber membranes with cSMM was evaluated via contact angle measurement, field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR-ATR) and ionic solute separation performance. Experimental results showed that pure water permeation flux was increased by surface modification with the blended cSMM, which at the same time produced higher ionic solute rejection. From the FESEM, a small increase in the thickness of the hollow fiber outer layer was noticed when the surface was modified by cSMM blending. Nevertheless, there was no significant difference in the overall membrane morphology between the membranes with and without cSMM blending, indicating that the morphology of the base polymer remained practically unchanged. On the other hand, DSC scan and FTIR analysis confirmed the miscibility of cSMM with PES and the presence of cSMM's functional group, respectively.

Dehydration of ethanol–water mixtures using asymmetric hollow fiber membranes from commercial polyimides

Abstract Dehydration of ethanol/water mixtures was performed by a pervaporation process using asymmetric hollow fiber membranes made from commercially available polymers: Matrimid ® (polyimide) and Torlon ® (polyamide–imide). The as-spun membranes showed low separation factors of ethanol and water mixtures. To increase their separation performance, thermal annealing of the fibers was performed. Thermally annealed fibers showed markedly increased separation performance, and this improvement depends upon the annealing conditions (both annealing temperature and time). The calculated intrinsic selectivities of the annealed Matrimid ® fibers are 416 and 275 for 15 wt% and 95 wt% ethanol feeds, respectively. However the observed separation factors for the corresponding 15 wt% and 95 wt% feeds are 40 and 240, respectively. The decrease in the separation factors, compared to the intrinsic selectivities, especially for 15 wt% ethanol feed, is due to the unfavorable relative vapor pressures of the components and their unfavorable relative activity coefficients. The Torlon ® fibers showed separation factors of 130 at 85 wt% ethanol feed. Long term stability tests were performed on the annealed Matrimid ® fibers at low and high ethanol feeds. The annealed Matrimid ® fibers showed stable separations over time at low ethanol concentrations (15 wt% ethanol), however at higher ethanol concentrations (85–95 wt% ethanol), the membranes eventually become swollen and plasticized by ethanol resulting in decreased separation performance. Crosslinking of the polymeric chains, therefore, appears to be necessary to stabilize the structures for long term application.

Preparation and Oxygen Permeation Properties of Highly Asymmetric La0.6Sr0.4Co0.2Fe0.8O3−α Perovskite Hollow-Fiber Membranes

La0.6Sr0.4Co0.2Fe0.8O3−α (LSCF) perovskite powders having a submicrometer size were synthesized by a sol−gel autocombustion method. From these powders, the gastight LSCF hollow-fiber membranes with a highly asymmetric structure comprising a dense layer of thickness ca. 88 μm integrated with a porous substrate were fabricated in a single step through a phase inversion/sintering technique with a mixture of N-methyl-2-pyrrolidone (NMP) and water as internal coagulant. Oxygen permeation fluxes through the obtained hollow-fiber membranes were measured under air/He gradients at different temperatures. The results indicate that the highly asymmetric hollow-fiber membranes possess an oxygen permeation flux of 0.11−2.19 mL cm−2 min−1 in the temperature range of 650−1000 °C, which is 2.6−10.5 times higher than that of the sandwich-structured hollow-fiber membranes. Oxygen permeation in the hollow-fiber membranes is limited primarily by the surface exchange reactions at lower temperatures, but ionic bulk diffusion w...

Polyamide-imide/polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols

To circumvent the common swelling and deteriorated performance of integral asymmetric hollow fiber membranes for pervaporation dehydration, we have developed novel polyamide-imide (PAI)/polyetherimide (PEI) hollow fiber membranes with synergized performance with the aid of dual-layer spinning technology. Dehydration of C1–C4 alcohols has been conducted and the orders of their fluxes and permeances have been analyzed. The hollow fibers spun at 2 cm air gap and annealed at 75 °C exhibit the highest pervaporation performance: separation factors for t-butanol/water and iso-butanol/water binary systems are greater than 50,000 with flux more than 700 g/m 2 h. A comparison with literature data shows that the newly developed membranes outperform most other polymeric membranes for the dehydration of IPA and butanols. The dual-layer hollow fiber membranes also exhibit good long-term stability up to 200 h. The superior performance can be attributed to (1) the balanced properties of PAI as the selective layer for dehydration pervaporation; (2) the low water uptake and less swelling characteristic of the PEI supporting layer; and (3) the desirable membrane morphology consisting of a fully porous inner layer, a porous interface, and an ultrathin dense-selective outer skin.

Defect-free asymmetric hollow fiber membranes from Torlon ®, a polyamide–imide polymer, for high-pressure CO 2 separations

Torlon ®, a polyamide–imide polymer, was used for high-pressure CO 2 separations, as it can form inter- and intra-chain hydrogen bonding that may provide stability against plasticization. Asymmetric hollow fiber membranes with a defect-free selective skin were successfully formed from Torlon ® using a dry–wet spinning process. Dope and spinning parameters were optimized to obtain these fibers, which had CO 2/CH 4 selectivity of 44 and O 2/N 2 selectivity of 7.7. These selectivities are about 85% of the intrinsic (dense film) value of 52 for CO 2/CH 4 and 90% of the intrinsic value of 8.3 for O 2/N 2, respectively. Based on analyses presented, the reduced selectivities are attributed to substructure resistance rather than actual skin layer defects. Macrovoids, which compromise the strength of the fiber, were reduced by increasing the polymer concentration. The resulting fiber could withstand up to 2000 psi of N 2, and a CO 2 permeation study indicates that this fiber can perform selective separations under supercritical conditions of 1100 psi of CO 2 at 35 °C.

Hydrophobic PVDF hollow fiber membranes with narrow pore size distribution and ultra-thin skin for the fresh water production through membrane distillation

Polyvinylidene fluoride (PVDF) hydrophobic asymmetric hollow fiber membrane was fabricated through the dry-jet wet phase inversion process. It is found that the PVDF hollow fiber has an ultra-thin skin layer and a porous support layer from the morphology study. The fully porous membrane structure has the advantage of decreasing the vapor transport resistance and enhancing the permeation flux. The fabricated PVDF membrane has a mean pore size of 0.16 μ m in diameter and a narrow pore size distribution. The rough external surface produces an advancing contact angle of 112 ± 3 ∘ with water. During direct contact membrane distillation (MD) of 3.5 wt% salt solution, PVDF hollow fibers produced a water permeation flux of 41.5 kg m - 2 h - 1 (based on the external diameter of hollow fiber) and a NaCl rejection of 99.99% with a hot salt solution at 79.3 °C and cold distillate water at 17.5 °C. This performance is comparable to or superior to most of commercially available PVDF hollow fiber membranes, indicating that the newly developed PVDF may be suitable for MD applications.

Removal of Chromate Ions from Water using the Polyelectrolyte‐Enhanced Ultrafiltration Process

Polysulfone (PSf) asymmetric hollow fiber membranes with the molecular weight cut‐off (MWCO) of 13,000 Dalton were prepared by the dry‐wet phase inversion method, with which negatively charged chromate ions, CrO4 2− was removed form aqueous solutions using the polyelectrolyte‐enhanced ultrafiltration (PEUF) process. Parameters relating to the chromate removal such as the type of polyelectrolyte, the loading ratio, and the pH value of the aqueous solution were studied. Poly (dially dimethylammonium chloride) (PDADMAC) was found to be the most effective polyelectrolyte. Under the optimal conditions, that is, the polyelectrolyte‐metal loading ratio of 20 and the pH value 8.0 respectively, up to 100% of the chromate rejection was achieved. The used polyelectrolyte may be recovered about 70% by the acidified decomplexation combined with ultrafiltration. The concentration polarization was present but imposed limited effect on the PEUF process in the long‐term operation.

Porous PVDF/TPU blends asymmetric hollow fiber membranes prepared with the use of hydrophilic additive PVP (K30)

Asymmetric blend hollow fiber membranes had been made from a new casting dope containing poly(vinylidene fluoride) (PVDF)/Thermoplastic polyurethane (TPU)/Polyvinylpyrrolidone (PVP) N, N-dimethylacetamide (DMAc). The effect of hydrophilic additive, polyvinylpyrrolidone, on the morphology and crystal structure of PVDF/TPU blends membranes by phase inversion process was studied. The separation property, microstructure and crystalline phase of membranes were characterized by bovine serum albumin (BSA) retention experiments, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) and differential scanning calorimetric (DSC), respectively. The results showed that adding low concentration (less than 3 wt%) of PVP can reduce the hydrophobicity of PVDF and increase its hydrophilicity. With further increment of PVP, the solution demixing is delayed and the kinetic hindrance due to the increase of viscosity. By SEM it was observed that with the increase of PVP the pyriform voids were replaced by the ‘finger-like’ voids, which became longer and widespread. In further increment of PVP (5 wt%), PVP contributes to the suppression rather than the enlargement of macrovoid structure in the membranes. For formed membranes by additive-free casting dope, FTIR–ATR indicated that PVDF crystallized including α and β phase. TPU and low concentration of PVP both can reduce the crystallinity of PVDF membranes. When PVDF/TPU was precipitated from increasing amount of PVP in dopes (e.g. ≥3 wt%), some α phase disappeared and the water flux decreased little by little, the enthalpy of fusion and crystallinity of membranes also declined. When the addition of PVP increased as high as 10 wt%, the heat of fusion and crystallinity of membranes increased slightly compared with membranes adding 3 wt% PVP. In addition the water flux dropped significantly rather than improved.

Auto and mutual plasticization in single and mixed gas C 3 transport through Matrimid-based hollow fiber membranes

This paper reports the gas separation properties of two asymmetric hollow fiber membranes prepared from the polyimide Matrimid. One is based on pure Matrimid, the second one is a blend with the polyimide P84. The hollow fiber membranes are used to separate gas mixtures of propylene and propane and in a particular case methane is present as a third component. The C 3H 6/C 3H 8-mixed gas selectivity of asymmetric membranes P84/Matrimid-blend is stable in time and decreases with increasing propylene partial pressure from 20 to 12. Propylene causes the propane permeance to increase in binary mixed gas experiments: propylene permeance is however not affected. This indicates plasticization of propane transport by propylene. Plasticization effects do influence the separation performance of asymmetric pure Matrimid membranes as well, but the effect is less pronounced as compared to the blend with P84. The presence of 50 vol.% methane in the feed gas diminishes the effect of C 3-plasticization. The comparison with single gas sorption and permeation kinetics leads to a distinction of auto and mutual plasticization.

Effect of internal coagulant on effectiveness of polyvinylidene fluoride membrane for carbon dioxide separation and absorption

Asymmetric hollow fiber membranes with inner skinless structures are favourable for carbon dioxide (CO 2) separation and absorption in gas–liquid membrane contactors by reducing gas transport resistance. In this study, polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated by using different internal coagulants, which include water, ethanol–water, and dimethylacetamide (DMAc)–water solutions. The spinning dope solution was prepared with DMAc as the solvent, lithium chloride and polyvinylpyrrolidone (PVP) as the non-solvent additives. The resultant membranes were examined by scanning electron microscopy (SEM). The examination of membrane cross-sections indicated that the composition of internal coagulants had significant impacts on the thickness and structures of the inner skin layer. Results showed that an inner skinless surface could be formed using a 30 wt.% water–DMAc mixture. Gas permeation tests demonstrated that PVDF hollow fibers with an inner skinless surface had a higher CO 2 absorption rate than other PVDF and polypropylene (PP) membranes and attained performance levels equivalent to a polytetrafluoroethylene (PTFE) membrane. PVDF membranes could be a cost-effective alternative for gas–liquid membrane contactors in applications of CO 2 separation and absorption.

Absorption of SO2 using PVDF hollow fiber membranes with PEG as an additive

In this study, removal of SO2 from gas stream was carried out by using microporous polyvinylidene fluoride (PVDF) asymmetric hollow fiber membrane modules as gas-liquid contactor. The asymmetric hollow fiber membranes used in this study were prepared polyvinylidene fluoride by a wet phase inversion method. Water was used as an internal coagulant and external coagulation bath for all spinning runs. An aqueous solution containing 0.02 M NaOH was used as the absorbent. This study attempts to assess the influence of PEG additive, absorbent flow rate, SO2 concentration, gas flow rate and gas flow direction on the SO2 removal efficiency and overall mass transfer coefficient. The effect of liquid flow rate on SO2 removal efficiency shows that at very low liquid flow rate, the NaOH available at the membrane surface for reacting with SO2 is limited due to the liquid phase resistance. As liquid flow rate is above the minimum flow rate which overcomes the liquid phase resistance, the SO2 absorption rate is controlled by resistance in the gas phase and the membrane. The SO2 absorption rate with inlet SO2 concentration was sharply increased by using hollow fiber membranes compared to a conventional wetted wall column because the former has higher gas liquid contacting area than the latter. The mass transfer coefficient is independent of pressure. When the gas mixture was fed in the shell side, the removal efficiency of SO2 declined because of channeling problems on the shell side. Also, the addition of PEG in polymer dopes increased SO2 removal efficiency.

Preparation of poly(phthalazinone ether sulfone ketone) hollow fiber membrane for gas separation

AbstractPoly(phthalazinone ether sulfone ketone) (PPESK) asymmetric hollow fiber membranes for gas separation were prepared by dry/wet phase inversion technique. The effects of various preparation conditions such as solvent, nonsolvent‐additives(NSA), PPESK concentration, and air gap on the membrane performance were studied. The heat resistance of the PPESK hollow fiber membrane was also examined. The hollow fiber membrane prepared from solvent with stronger solubility showed low gas permeation and high O2/N2 selectivity due to the denser skin layer. Hollow fiber membrane made from PPESK/DMAc/EtOH/THF system had thicker skin layer than that made from PPESK/DMAc/GBL system with the same ratio of near‐to‐cloud‐point of NSA, which resulted in the higher O2/N2 selectivity. Along with the increase of NSA content, the gas permeation increased and the O2/N2 selectivity decreased. The O2/N2 selectivity of hollow fiber membranes made from PPESK/DMAc/GBL and PPESK/DMAc/EtOH/THF systems were 4.9 and 4.8 respectively, when the membrane forming systems contained appropriate content of NSA. The high polymer concentration resulted in low gas permeation and high O2/N2 selectivity. When the air gap was excessively long, the membrane performance dropped because of the damage to the dense skin layer. There was no significant drop on the membrane performance when the operation temperature was elevated to 90°C. The average O2/N2 selectivity was higher than 3.0 at 70°C during a long period of 55 days' test time. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Hydrolysis of palm and olive oils by immobilised lipase using hollow fibre reactor

Lipase from Mucor miehei immobilised by adsorption on microporous, asymmetric hollow fibre membrane reactors was used to hydrolyse two different oils, namely, palm and olive oils. The hydrolysis reaction was carried out at a temperature of 40 °C, an average transmembrane pressure (TMP) of 115 mmHg and oil and aqueous flow rates of 2.5 and 3.0 ml min −1, respectively. It was experimentally proven that adsorption of lipase increased with temperature and was higher on hydrophobic membranes than hydrophilic ones. The effluent concentrations of fatty acid products were measured using gas chromatograph with FID detector. Hydrolysis experimental results were fitted to a multisubstrate kinetic model derived from the Ping Pong Bi Bi mechanism. The final model expression is useful for predicting the free fatty acid profile of the enzymatic hydrolysis of palm and olive oils for different substrate flow rates and enzyme loading.

Mixed matrix hollow fiber membranes made with modified HSSZ-13 zeolite in polyetherimide polymer matrix for gas separation

Organic–inorganic hybrid (mixed matrix) asymmetric hollow fiber membranes were spun via a dry jet-wet quench procedure using surface modified inorganic small pore size zeolite incorporated in an Ultem ® 1000 polyetherimide matrix. The zeolites were modified via two separate techniques and termed as (1) Ultem ® “sized” and (2) Grignard treated. The first technique failed to achieve successful mixed matrix performance, but the second approach gave very attractive results. The Ultem ® “sized” zeolites were prepared by treating the zeolites with a silane coupling agent to allow Ultem ® polymer chains to be grafted to the surface. Poor adhesion was observed between the bulk polymer and most of the zeolite particles in the final membrane using Ultem ® “sized” particles. The post-treated fibers did not display enhanced selectivity over neat polymer with pure gas nitrogen, oxygen, helium, methane and carbon dioxide testing or mixed gas carbon dioxide and methane gas pair. The absence of the mixed matrix effect is hypothesized to be due to the nucleation of hydrophilic solvent and non-solvent around the Ultem ® “sized” zeolite particles during phase separation in the quench bath forming a so called sieve-in-a-cage defect. Although such defects have been reported in dense mixed matrix films, they have not yet been investigated in hollow fibers format which are formed via a phase separation process and thus remain prone to the effects of the non-solvent quenching media. To test the nucleation hypothesis, fibers incorporating 10.3 vol% (with respect to polymer) of zeolites modified with a Grignard reagent were spun. These fibers, after post-treatment, showed significant selectivity enhancement of 10% for O 2/N 2, 29% for He/N 2, 17% for CO 2/CH 4 pure gases and 25% for mixed gas CO 2/CH 4 pairs over neat polymer results. The selectivity properties in these fibers are similar to or exceed the Maxwell model predictions for these hybrid materials.

Thickness and Air Gap Dependence of Macrovoid Evolution in Phase-Inversion Asymmetric Hollow Fiber Membranes

We have studied, for the first time, the effects of spinneret dimension and its coupling effect with air gap distance on macrovoid evolution. Spinnerets with different annulus gaps, i.e., from 0.05 to 0.50 mm, were fabricated. Hollow fiber membranes spun from thinner spinneret annulus gaps show only inward-pointed long, teardrop, and elliptical shape macrovoids, while those spun from thicker annulus gaps have both inward- and outward-pointed macrovoids. On the basis of SEM examination, it is concluded that nonsolvent intrusion due to local surface instability and skin rupture accounts for most inward- and outward-pointed long macrovoid formation, while the diffusion mechanism with the aid of solutocapillary convection possibly accounts for the formation of small teardrop and elliptical shape macrovoids. In addition, the number of inward-pointed macrovoids increases, while the number of outward-pointed macrovoids decreases with an increase in air gap distance. Clearly, many forces affecting macrovoid forma...

Decolorization of dye-containing aqueous solutions by the polyelectrolyte-enhanced ultrafiltration (PEUF) process using a hollow fiber membrane module

Polysulfone (PSf) asymmetric hollow fiber membranes with molecular weight cut-off (MWCO) of 13,000 Da have been prepared and employed for removal of the triphenylmethane dyes including malachite green (MG), brilliant green (BG) and new fuchsin (NF) from aqueous solutions. Several water-soluble polymers such as poly(diallydimethyl ammonium chloride) (PDADMAC), poly(sodium-4-styrenesulfonate) (PSS) and polyvinyl alcohol (PVA) have been examined for the polyelectrolyte-enhanced ultrafiltration (PEUF). The experimental results indicate that all the three triphenylmethane-type dyes can be removed effectively using the PSf hollow fiber ultrafiltration membrane with the aid of the anionic PSS polymer. The enhancement is primarily due to the formation of complexes between the anionic polymer and the cationic dye molecules through electrostatic attraction. The cationic and nonionic polymers such as PDADMAC and PVA are not suitable for the decolorization of MG, BG and NF aqueous solutions. The decolorization performances of the PSf membrane module can be fully restored by the back-washing operation using alcohol after each experiment.

Performance of treated and untreated asymmetric polysulfone hollow fiber membrane in series and cascade module configurations for CO 2/CH 4 gas separation system

This study investigates the effects of one-, two- and three-stage membrane system configurations in series arrangement for the CO 2/CH 4 separation for both untreated and treated membranes. Asymmetric polysulfone hollow fiber membranes were fabricated from 33 wt.% of polysulfone polymer using dry/wet phase inversion process. The produced membranes were characterized by pure gas permeation experiments, Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), density measurement and Thermogravimetric Analysis (TGA). For both untreated and treated membranes, the pressure-normalized flux of CO 2 decreased with increasing of the membrane stages. In addition, the selectivities of asymmetric hollow fiber membrane showed a more constant trend with feed pressure. Treated membrane exhibited lower pressure-normalized flux than untreated membranes due to skin layer densification which increased the transport resistance, thus lead to the reduction in pressure-normalized fluxes. Among all the three configurations studied, two-stage membrane configuration showed the most constant trend in term of selectivity. However, three-stage cascade configuration produced the highest CO 2/CH 4 selectivity especially when tested at low feed pressure range. Effect of stage cut on feed pressure showed an increasing trend with increasing of CO 2 and CH 4 feed pressure for all configurations. This is due to the increase of the permeation driving force, which caused the passage of larger amounts of more permeable gas through the membrane. This study showed that, three-stage cascade configuration exhibited the smallest stage cut values thus produced higher purity of CO 2 in permeate stream.

Performance of natural gas membranes in the presence of heavy hydrocarbons

Intrinsically defect-free asymmetric hollow fiber polyimide membrane modules were studied in the presence and absence of saturated and aromatic components. Under some conditions for the ternary system, the permselectivity of the membrane is scarcely affected, while under other conditions, permselectivity is negatively affected by as much as 20%. In most cases, for the ternary feeds, significant depression in fluxes was observed due to competition between the CO 2, CH 4 and heavier hydrocarbons, but the effect was even more pronounced for the toluene. In addition to steady state tests in the presence and absence of n-heptane and toluene, modules were conditioned for 5 days with ternary mixture of CO 2, CH 4 and one or the other of these heavy hydrocarbons. Following this conditioning process, the modules were studied with a simple binary 10% CO 2/90% CH 4 mixture. These conditioning studies provide insight into the fundamental effects induced in the membrane due to the long-term exposure to the complex mixtures. Following exposure to the ternaries containing n-heptane, negligible CO 2 permeance increase was seen, while significantly increased permeances were seen under some conditions following toluene exposure even at low pressures of the ternary toluene/CO 2/CH 4 conditioning gas mixture. Although a more protracted process occurs in the case of n-heptane/CO 2/CH 4 at 35 °C and 500 ppm, a significant loss in selectivity does still occur in the actual ternary tests after exposure for 5 days. The problem caused by 300 ppm toluene at 35 °C is more immediately apparent than for n-heptane exposure, but the ultimate selectivity loss is similar for both hydrocarbons. In addition to the selectivity, in the presence of toluene the permeability is also depressed significantly, presumably due to a greater capability of toluene to compete for added free volume elements introduced in the conditioning process. The conditioning treatment has negligible effect at 55 °C, suggesting that that the sorption affinity of toluene decreases with increasing temperature. Results also suggest that an essentially defect-free, non-nodular morphology offers advantages in stability under demanding operating conditions. Earlier work [G. Djoekita, Characterization and Analysis of Asymmetric Hollow Fiber Membranes for Natural Gas Purification in the Presence of Hydrocarbons, The University of Texas at Austin, 2000] showed serious losses in performance of membranes comprised of similar materials, when the selective layer had a pronounced fused nodular nature as opposed to the intrinsically defect-free skin layers reported on here.