Studies on polyethylene glycol/polyethersulfone composite membranes for FCC gasoline desulphurization by pervaporation

A polyethylene glycol (PEG)/polyethersulfone (PES) composite membrane that can be applied on a commercial (or scale up) plant for fluid catalytic cracking (FCC) gasoline desulphurization was prepared through pre-wetting combined with double-layer coating methodology. Preparation methodology, morphologies characterization and performance test for the composite membranes were conducted. The results indicated that the pre-wetting method effectively confined the intrusion of PEG solution to porous PES support layer in coating process. The composite membrane had a clear-cut boundary surface between the dense active layer and the porous support layer, which was examined by scanning electron microscope (SEM). Pervaporation (PV) experiments indicated that the membrane, with the crosslinking agent amount of 17% and solids content in active layer solution of 16%, had a stable performance for FCC desulphurization. The sulphur enrichment factor came to 3.63, and the total permeation flux was 3.37 kg/m² h. It was found that the PV performance of the composite membrane changed slightly when the thickness of active layer varied from 4.25 μm to 33.26 μm.     

Pervaporation separation of n-heptane/thiophene mixtures by polyethylene glycol membranes: Modeling and experimental

Gasoline desulfurization by membrane processes is a newly emerged technology, which has provided an efficient new approach for sulfur removal and gained increasing attention of the membrane and petrochemical field. A deep understanding of the solution/diffusion of gasoline molecules on/in the membrane can provide helpful information in improving or optimizing membrane performance. In this study, a desulfurization mechanism of polyethylene glycol (PEG) membranes has been investigated by the study of sorption and diffusion behavior of typical sulfur and hydrocarbon species through PEG membranes. A solution–diffusion model based on UNIFAC and free volume theory has been established. Pervaporation (PV) and sorption experiments were conducted to compare with the model calculation results and to analyze the mass transport behavior. The dynamic sorption curves for pure components and the sorption experiments for binary mixtures showed that thiophene, which had a higher solubility coefficient than n-heptane, was the preferential sorption component, which is key in the separation of thiophene/hydrocarbon mixtures. In all cases, the model calculation results fit well the experimental data. The UNIFAC model was a sound way to predict the solubility of solvents in membranes. The established model can predict the removal of thiophene species from hydrocarbon compounds by PEG membranes effectively.     

Pervaporation performance of crosslinked polyethylene glycol membranes for deep desulfurization of FCC gasoline

An crosslinked polyethylene glycol (PEG) membrane was prepared for fluid catalytic cracking (FCC) gasoline desulfurization. Sulfur enrichment factor come to 4.75 and 3.51 for typical FCC gasoline feed with sulfur content of 238.28 and 1227.24 μg/g, respectively. Pervaporation performance of membranes kept stable within the long time run of 500 h, which indicated that crosslinked PEG membranes had the property of resisting pollution. Judging from chromatographic analysis, the membranes were more efficient for thiophene species. Effects of operation conditions including permeate pressure, feed temperature, feed flow rate and feed sulfur content level on the pervaporation performance were investigated. Permeation flux decreased with increasing permeate pressure while increased with the operating temperature increase. Sulfur enrichment factor increased firstly and decreased then when permeate pressure and temperature rose. The peak value occurred at 10.5 mm Hg and 358 K for model compounds feed (378 K for FCC gasoline feed). Arrhenius relationship existed between flux and operating temperature. Both sulfur enrichment factor and flux were shown to increase with increasing feed flow rate. Permeation flux increased while sulfur enrichment factor decreased as the feed sulfur content increased, but the influence of increasing sulfur content on pervaporation performance weakened when sulfur content come to 600 μg/g.     

Pervaporation performance of PDMS-NiY zeolite hybrid membranes in the desulfurization of gasoline

PDMS-Ni²⁺Y zeolite hybrid membranes were fabricated and used for the pervaporation removal of thiophene from model gasoline system. The structural morphology, mechanical stability, crystallinity, and free volume characteristics of the hybrid membranes were systematically investigated. Molecular dynamics simulation was employed to calculate the diffusion coefficients of small penetrants in the polymer matrix and the zeolite. The effect of Ni²⁺Y zeolite content on pervaporation performance was evaluated experimentally. With the increase of Ni²⁺Y zeolite content, the permeation flux increased continuously, while the enrichment factor first increased and then decreased possibly due to the occurrence of defective voids within organic–inorganic interface region. The PDMS membrane containing 5.0 wt% Ni²⁺Y zeolite exhibited the highest enrichment factor (4.84) with a permeation flux of 3.26 kg/(m² h) for 500 ppm sulfur in feed at 30 °C. The effects of operating conditions on the pervaporation performance were investigated in detail. It has been found that the interfacial morphology strongly influenced the separation performance of the hybrid membrane, and it was of great significance to rationally modify the interfacial region in order to improve the organic–inorganic compatibility.     

Kinetics of Sorption,Desorption, and Diffusion in Zeolites

Sorption and desorption in zeolites (molecular sieves) have to be considered as complex processes, involving simultaneous diffusion in zeolite crystals, mass transfer in the intercrystalline void of a pellet, and heat transfer between the zeolitic sorbent and its surroundings. The kinetics of sorption and desorption, respectively, of n-C₄H₁₀ in zeolite X and of CO₂ in zeolite A have been investigated: only the initial rates of uptake or release of the sorbet are controlled by mass transfer alone whereas ultimately they also depend on the rate of heat transfer from the sorbent to its surroundings or vice versa. Diffusivities of the sorbate in the zeolite crystals can be obtained from the kinetics of mass transfer, provided the resistance due to viscous or Knudsen flow between the crystals of the zeolitic sorbent can be eliminated. A sample consisting of a monolayer of single crystals had to be used for this purpose in the n-butane/zeolite X system; the intracrystalline diffusivity obtained in this way is not in conflict with data obtained by NMR spectroscopy. The intracrystalline diffusivities obtained in this way—taking into account the coupling of several processes during sorption — are higher than values reported in the literature.     

Pervaporation characteristics of cross-linked poly(dimethylsiloxane) membranes for removal of various volatile organic compounds from water

The permeation and separation characteristics of volatile organic compounds (VOCs), such as chloroform, benzene, and toluene, from water by pervaporation through cross-linked poly(dimethylsiloxane) membranes prepared from poly(dimethylsiloxane) dimethylmethacrylate macromonomer (PDMSDMMA) and divinyl compounds, such as ethylene glycol dimethylmethacrylate (EGDM), divinyl benzene (DVB), divinyl siloxane (DVS), and divinyl perfluoro-n-hexane (DVF) are described. When aqueous solutions containing 0.05 wt.% VOCs were permeated through cross-linked PDMSDMMA membranes, these membranes showed high VOC/water selectivity and permeability. Both VOC/water selectivity and permeability were affected significantly by the divinyl compound. Furthermore cross-linked PDMSDMMA membranes showed the highest chloroform/water selectivity. The VOC/water selectivity was mainly governed by the sorption selectivity rather than the diffusion selectivity. However, the difference in the selectivity between different types of VOCs depended on differences in the diffusivity of permeants. With increasing downstream pressure, the VOC/water selectivity of all cross-linked PDMSDMMA membranes increased, but the permeability decreased. A PDMSDMMA–DVF membrane exhibited a normalized permeation rate of 1.9 × 10⁻⁵ kg m/m² h and a separation factor for chloroform/water of 4850, yielding a separation index of 9110. The pervaporation characteristics of the cross-linked PDMSDMMA membranes are discussed based on their chemical and physical structures as well as the chemical and physical properties of the permeants.     

Pervaporation of benzene/cyclohexane mixtures using ion-exchange membrane containing copper ions

The sorption, diffusion, and pervaporation (PV) properties of benzene/cyclohexane (Bz/Cx) mixtures on cation-exchange membranes containing copper ions (Cu(II)) were investigated. The equilibrium sorption isotherms of pure vapors in the membranes and the partial solubility of binary solutions in the membranes were described using the UNIQUAC model. The τ_iM and τ_Mi values were 0.978 and 0.591 for Bz, and 0.922 and 0.475 for Cx. The transient regimes of vapor sorption were employed to calculate the concentration-dependent diffusion coefficients. Long’s model sufficiently explained the diffusivity of Bz and Cx in the membranes. The pre-exponential factors were 3×10⁻¹³ m²/s and the plasticization factors were 3.0 and 3.6 for Bz and Cx, respectively. Excellent agreement was found with the experimental results applying the solubility and diffusivity data to simulate the pervaporation performance (flux and selectivity) using the modified Maxwell–Stefan equation. The membrane containing Cu(II) demonstrates better facilitating capability for Bz transport than that with Na(I), mainly due to its preferential sorption property toward Bz. Replacing Na(I) with Cu(II) into a Neosepta membrane resulted in better separation efficiency and higher Bz flux throughout the entire Bz concentration range.     

Gas transport properties of poly(ethylene oxide-co-epichlorohydrin) membranes

Gas permeation properties of crosslinked membranes prepared from a series of poly(ethylene oxide-co-epichlorohydrin) (P(EO/EP)) copolymers with different contents of ethylene oxide are determined by using the constant-volume and pressure-increase method. In addition to the chemical composition, the transport properties are related to the main characteristics of copolymers like the glass transition temperature, crystallinity and crosslinking ratio. Permeation measurements of He, H₂, N₂, O₂, CO₂ and CH₄ show that the permeabilities are nearly constant up to an EO content of about 75–80 mol%, then increase rapidly up to a maximum around 90 mol% of EO in the copolymers. The same behavior is observed for the diffusion coefficient and the CO₂ sorption coefficient. The presence of an optimal EO composition is explained by the competition between crystalline and amorphous EO sequences. The copolymers present very high CO₂ permeability and selectivity respect to other permanent gases even in gas mixtures and under high pressures.     

Analysis of transient permeation as a technique for determination of sorption and diffusion in supported membranes

Systematic membrane selection, process design as well as elucidation of structure–property relationships for pervaporation and vapor permeation require knowledge of sorption and diffusion properties. Direct measurement of sorption is not possible in the case of commercial membranes due to the presence of a support layer. Sorption measurements may also be difficult if the polymer is synthesized or crosslinked directly on the support and its properties are different from the bulk polymer. This work describes a technique to obtain sorption as well as diffusion parameters for supported membranes using transient permeation data. Computer simulations for transient permeation were carried out using sorption and diffusion data from the literature. It was demonstrated that the desired parameters could be estimated using data having a reasonable degree of error (±2%) by the least squares method. Alternatively, a time-lag analysis may be used instead of direct regression of the parameters by the least squares method. A general method for estimating the sorption as well as diffusion parameters using the time-lag and steady-state flux is described. Analytical solutions are derived for the various transport models, wherever possible.     

Water and methanol permeation through short-side-chain perfluorosulphonic acid ionomeric membranes

The water and methanol transport into a short-side-chain perfluorosulphonic acid ionomeric (PFSI) membrane suitable for application in proton exchange membrane fuel cells (PEMFC), namely Hyflon^® Ion, was studied between 35 and 65 °C. In particular, the permeabilities of pure water, pure methanol and their mixtures at different temperatures were measured through pervaporation experiments, at various values of feed composition. Due to the presence of mutual interactions between permeants as well as among penetrants and polymeric matrix, the composition of the feed solution affects the membrane permeability in a way which cannot be predicted on the basis of permeability data of the pure liquid components alone. It has been found in particular that the presence of the water in the mixture enhances the methanol permeability, due to the positive effects of matrix plasticization and favourable energetic interactions. In turn, by considering water permeability data in the presence of a poorly permeating component such as glycol, it can be concluded that also water permeation is enhanced by the presence of methanol, although to a lower extent.     

Simulations of gas transport in membranes based on polynorbornenes functionalized with substituted imide side groups

This paper studies the diffusive and sorption steps of several gases across membranes cast from poly(N-phenyl-exo,endo-norbornene-5,6-dicarboximide) chloroform solutions. Chains packing effects on gas transport was investigated by conducting a parallel study on the permeation characteristics of membranes cast from hydrogenated poly(N-phenyl-exo,endo-norbornene-5,6-dicarboximide) chloroform solutions. The permeability coefficients of several gases in the two membranes were measured finding that hydrogenation of the norbornene moieties decreases gas permeability. The transition states approach was used to determine the trajectories of the gases in the two types of membranes from which the diffusion coefficients were obtained. Monte Carlo techniques based on the Widom method were used to simulate gas sorption process as a function of pressure. The values of the solubility coefficients thus obtained undergo a relatively sharp drop at low pressures approaching to a constant value as pressure increases. With the exception of carbon dioxide, pretty good agreement between the experimental and simulated values of the permeability coefficient is found for the gases studied.     

Sorption and transport mechanism of gases in polycarbonate membranes

The transport phenomena of oxygen and nitrogen across a pure polycarbonate (PC) and a cobalt(III) acetylactonate (Co(acac)₃) containing PC membrane was studied. Co(acac)₃ was added into a polycarbonate membrane to enhance its oxygen solubility. The oxygen sorption isotherms was measured. It was found that the oxygen solubility decreased sharply as pressure increased, especially at low pressure region. On the contrary, the oxygen permeability increased slightly with respect to pressure. Both the solution-diffusion model and traditional dual mobility model were unable to explain the inconsistent pressure dependency between solubility and permeability. Instead of adopting Langmuir-Henry sorption model, a modified dual mobility model which incorporates BET-type isotherm to describe oxygen sorption. The diffusivity of molecules moving at the first adsorbed layer was assumed to be different from those moving at higher layers. This modified dual mobility model satisfactorily described both the pressure dependency of oxygen solubility and permeability. It was also found that the increase of oxygen/nitrogen selectivity was not due to the elevation of oxygen to nitrogen solubility ratio but due to the mobility ratio of oxygen to nitrogen at the higher adsorption layers.     

Sorption and diffusion of ethanol vapor in polybutadiene/acrylonitrile, polybutadiene/styrene and polybutadiene based polyurethanes

The sorption and diffusion behavior of ethanol vapor in series of polyolefine based polyurethanes (PU) made from hydroxyl-terminated polybutadiene/acrylonitrile (HTBN), hydroxyl-terminated polybutadiene/styrene (HTBS) and hydroxyl-terminated polybutadiene (HTPB) were investigated by using the quartz-spring, DSC, FTIR and AFM. The equilibrium absorption reduced with increasing content of hard segments for all the three types of PUs. The values of the maximum absorption were in the order of HTBN > HTBS > HTPB based PU and related to their composition. The non-Fickian diffusion was confirmed and the sorption was discovered mainly in the hard segments. The HTBN based PU revealed different sorption and diffusion behavior from the other two, which was resulted from its hydrogen bonding not only between ethanol and hard segments but also soft segments. The morphologies of PUs before and after ethanol absorption were also compared. The HTBN based PU showed the most evident phase re-congregation after ethanol absorption.     

Permselectivity, solubility and diffusivity of propyl propionate/water mixtures in poly(ether block amide) membranes

This work is concerned with the separation of propyl propionate/water mixtures by pervaporation using PEBA membranes, which is relevant to aroma compound recovery from dilute aqueous solutions. The solubility and diffusivity pertinent to the permselectivity were investigated. The effects of feed concentration and the operating temperature on the separation performance were studied. Under the experimental conditions tested, the permeate concentration was much higher than the solubility limit, and upon phase separation substantially pure propyl propionate could be achieved. The diffusivity of propyl propionate through the membrane from its dilute aqueous solutions was affected by the solution concentration exponentially. It was shown that the permselectivity of the membrane for propyl propionate/water separation was mainly derived from its sorption selectivity due to the organophilicity of the membrane. The diffusivity of pure propyl propionate in the membrane was about 28 times higher than pure water diffusivity.     

Transport parameters of carbon dioxide in poly(etheretherketone) membranes

Sorption and permeability measurements have been performed to determine the transport parameters of carbon dioxide through dense homogeneous PEEKWC membranes. The enthalpy of solution of CO₂ is exothermic. The concentration dependence of diffusion has been measured. Permeability coefficients obtained from sorption data have been compared with the experimental values, obtaining a good agreement.     

Measurement of sorption and diffusion in nonporous membranes by transient permeation experiments

The knowledge of sorption and diffusion of liquids or vapors and their mixtures in nonporous membranes facilitates systematic membrane selection and process design. A novel experimental technique for the measurement of sorption and diffusion in nonporous membranes is described. An experimental apparatus has been designed for carrying out the transient permeation experiments. A general time-lag analysis procedure has also been developed in order to obtain the sorption and diffusion parameters from the transient permeation data. The effects of concentration dependent diffusivities, polymer swelling and a thermodynamic correction factor have been included in the analysis. Transient permeation experiments have been carried out to determine the sorption and diffusion of acetone in polydimethylsiloxane (PDMS). The results obtained are in good agreement with data from the literature, indicating that the proposed technique is suitable for the measurement of sorption and diffusion in nonporous membranes.     

Sorption and permeation in elastic solids: applicability to gas transport in glassy polymeric materials

A thermodynamic analysis of sorption and transport in elastic solids is presented. The penetrant's chemical potential in the solid is calculated from the free energy changes that accompany deformation of the solid to accommodate the penetrant and mixing of the deformed solid and penetrant. Sorption isotherms are obtained by equating this chemical potential to that of that of the gas phase. The penetrant diffusivity is determined from a statistical mechanical analysis described in the literature. Both pure and mixed gas solubility, diffusivity and permeability are predicted. Mixed gas behavior is predicted using material parameters obtained from pure gas data only. The results suggest certain relationships between the dual mode model parameters that are found experimentally. Comparison with several experimental gas–polymer systems indicate the elastic solid analysis can reproduce pure gas data and predict mixed gas data well with physically realistic material parameters. The results provide a basis for investigating the relationships between the mechanical, volumetric, gravimetric and thermodynamic processes that give rise to transport.     

Effects of pH and Concentration on Cs Ions Sorption and Diffusion in Crushed Granite by Using Batch and Modified Capillary Method

A comprehensive evaluation of Cs ions sorption to and diffusion in crushed granite was conducted in this study. The sorption capacity of crushed granite suggested by the Langmuir model was 5.48 × 10^?6 mol‐Cs/g‐granite. The distribution coefficient (Kd) was around 7.5 mL/g and pH independent. By using an in‐diffusion method with a modified capillary column, some diffusion relevant parameters of Cs ions in crushed granite were derived. The apparent diffusion coefficient (Da) seemed unaffected by Cs concentration (1.15 × 10^?10 to 2.82 × 10^?10 m²/s, at 10^?7 and 10^?3 M, respectively). The determined effective diffusion coefficients (De) were located in the window from 8.59 × 10^?10 (10^?7 M) to 1.69 × 10^?9 (10^?3 M) m²/s. Under various pH environments, pH independent Da (9.0 × 10^?9 m²/s) and De (1.0 × 10^?9 m²/s) values were observed. Under current systems, consistently higher De than Da implied the diffusion of Cs ions was governed by surface diffusion phenomenon. Whereas the pH insensitive feature indicated the Cs sorption to crushed granite was mainly through ion‐exchange reaction. Moreover, further SEM/EDS mapping clearly showed the adsorbed Cs ions were highly concentrated on the fracture surface of biotite.     

Sorption and transport of linear alkane hydrocarbons in biaxially oriented polyethylene terephthalate

Equilibrium sorption and uptake kinetics of n‐butane and n‐pentane in uniform, biaxially oriented, semicrystalline polyethylene terephthalate films were examined at 35 °C and for pressures ranging from 0 to approximately 76 cmHg. Sorption isotherms were well described by the dual‐mode sorption model. Sorption kinetics were described either by Fickian diffusion or a two‐stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at long times. Diffusion coefficients increased with increasing penetrant concentration. n‐Butane solubility was lower than that of n‐pentane, consistent with the more condensable nature of n‐pentane. However, n‐butane diffusion coefficients were higher than those of n‐pentane. Infinite‐dilution, estimated amorphous phase diffusion and solubility coefficients were well correlated with penetrant critical volume and critical temperature, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1160–1172, 2001     

Identification and quantification of polyethylene glycol types in polyethylene glycol methyl ether and polyethylene glycol vinyl ether

Quantitative determination of polyethylene glycol (PEG) impurities in two monofunctional polyglycol types, PEG methyl ether (M-PEG) and PEG vinyl ether (V-PEG), has been carried out by reversed-phase liquid chromatography with evaporative light scattering detection (ELSD). In addition to optimizing the resolution between PEG and monofunctional PEG peaks, the major focus has been to determine the molecular weights of PEG impurities in M-PEG and V-PEG of diverse molecular weights. The latter is achieved by examining liquid chromatography–mass spectrometry (LC–MS) mass spectra of both monofunctional PEG and PEG in several cases, and matching peak retention times with those of available PEG standards for all M-PEG and V-PEG sample types. This information is helpful in selecting the appropriate PEG standard to determine PEG content in each sample type. ELSD response factors for various PEG standards have also been compared. It has been found that PEG standards with molecular weights from 1000 Da to 8000 Da show responses that are within 10% of each other. However, a low molecular weight PEG such as PEG 400, provides approximately 30% less response compared to its higher molecular weight counterparts.