Studies with inorganic precipitate membranes. IV. Evaluation of apparent fixed charge on membranes

Membrane potentials arising across four parchment-supported ferrocyanide membranes of manganese, cobalt, silver, and cadmium when they separate 1:1 electrolyte solutions of concentration C₁ and C₂ such that C₁ = 10C₂, have been measured. The data have been used according to the procedure prescribed by one of the theories of membrane potential due to Teorell and Meyers and Sievers to derive values for the quantity of charge present on the membranes. An alternative procedure employed by Altug and Hair has been considered and found to overestimate the value for the charge on the membranes.     

Studies with inorganic ion-exchange membranes

The pyridinium molybdoarsenate membrane shows a response to pyridinium ions and can be used to determine the concentration of these ions in the range 10(-3)-1M. The potentials generated across the membrane are reproducible and the response time is less than 1 min. There is no interference from certain inorganic and organic ions. The electrode can be used in the pH range 3-6 as well as in non-aqueous medium. Small additions of cetyltrimethylammonium bromide cause large shifts in the membrane potentials. A membrane, after being treated with this surfactant, shows a wider range of response to pyridinium ions. Precipitation titration of pyridinium nitrate has been monitored by using this membrane electrode.     

Studies on the permeation of aromatic hydrocarbons through liquid surfactant membranes

The effect of operating parameters on the batch scale permeation of hydrocarbons from benzene—heptane mixtures and a straight run naphtha through liquid membrane is reported. The thirteen operating parameters studied include: mixing intensity, surfactant concentration, treat ratios, contact times, temperature and additives. The variations observed in the two key properties of selectivity and aromatic recoveries as well as in product compositions with change in operating parameter is discussed. Surfactant concentration contact time during permeation, type and concentration of additive used appear to exert a marked effect on the enrichment obtained. The careful optimization of operating parameters give selectivities as high as 50 and aromatic recoveries of 75% in one stage at 30°C. Comparison of data with batch liquid—liquid extraction data from extraction of similar feed mixtures with the most widely used solvent, sulpholane, under typical industrial conditions, has shown that selectivities and aromatic recoveries in liquid membrane permeation (LMP) are much higher. Batch scale LMP experiments with straight run naphtha as feed show that under optimum conditions of membrane stability and operating parameters the dearomatization of naphtha from an initial aromatic level of 22 vol.% to 10.5 vol.% is possible in one stage at 30°C with a raffinate yield of 63%. The results obtained on benzene—heptane model mixture compare fairly well with those obtained on naphtha feed.     

Studies on phenol permeation through supported liquid membranes containing functionalized polyorganosiloxanes

In this study, the functionalized, linear, hydrophobic fluid organosiloxane polymers, namely, methylhydrosiloxane–dimethylsiloxane copolymers supported on a polypropylene microporous flat sheet membrane (Celgard 2502 and 2402) have been tested as supported liquid membranes (SLMs) for phenol recovery from aqueous phases into a 0.1 M NaOH phase. The functionalized polymers include, Me₃SiO[MeSi(OR)O]_x[Me₂SiO]_ySiMe₃ (containing x = 15–18, 25–35 and 50–55 mol% of R, where R is –(CH₂)_nNMe₂ (n = 3 or 4 or 6) or –(CH₂)₂OEt pendent organofunctional groups. The functionalities, R, tested were derived from the commercially available 3-dimethylamino-1-propanol and 2-ethoxyethanol as well as newly synthesized 4-dimethylamino-1-butanol and 6-dimethylamino-1-hexanol which have been made for the purpose of this study.

The study showed that phenol permeation expressed as permeate flux through the membranes increases with the larger number of carbon spacers in the alkyl chain of the aminoalcohol pendent, larger porosity of the polypropylene support films, higher mol% of the methylhydrosiloxane portion functionalized and faster flow rates of both the feed and the receiving phases. Phenol permeation was enhanced significantly when the mol% of the methylhydrosiloxane portion was 50–55 or 25–35 with 6-dimethylamino-1-hexanol functionality supported on Celgard 2502.     

Studies on permeation of hydrocarbons through liquid membranes in a continuous contactor

This paper reports experimental studies, in a continuous cocurrent packed column, on the removal of aromatics from hydrocarbon streams by making use of selective transfer through aqueous surfactant membranes. Two types of feed mixtures were used: (i) benzene—heptane and (ii) straight run naphtha from Bombay High crude oil. The receiving phase in both cases was kerosene. The height of a mass transfer unit (HTU) was determined and found to largely lie in the range 2-2.5 m, irrespective of feed composition and solvent/feed ratio. The experimental study confirms the feasibility of a liquid membrane process for dearomatization and provides HTU data for further feasibility and scale-up studies.     

Gas permeation of porous organic/inorganic hybrid membranes

Selective gas permeation of porous organic/inorganic hybrid membranes via sol-gel route and its thermal stability are described. Separation performance of the hybrid membrane was improved compared with porous membranes governed by the Knudsen flow, and gas permeability was still much higher than that through nonporous membranes. Additionally, it was shown that these membranes were applicable at higher temperatures than organic membranes.SEM observation demonstrated that the thin membrane was crack-free. Nitrogen physisorption isotherms showed the pore size was in the range of nanometers. Gas permeability through this membrane including phenyl group was in the range of 10^–8 [cc(STP) cm/(cm² s cmHg)] at 25°C. The ratios of O₂/N₂ and CO₂/N₂ were 1.5 and 6.0, respectively, showing the permeation was not governed by the Knudsen flow. The permeability decreased as the temperature increased. Furthermore, the specific affinity between gas molecules and surface was observed not only in the permeation data of the hybrid membranes but in the physisorption data. These results suggested that the gas permeation through the hybrid membrane was governed by the surface flow mechanism.Thermal analysis indicated that these functional groups were still stable at higher temperatures. The phenyl group especially remained undamaged even at 400°C.     

Inert gas permeation through homopolymer membranes

New data are reported for the permeation of inert gases through polyethylene, polytetrafluoroethylene, and silicone and natural rubbers. Additional data are compiled from the literature. The relative solubilities of these gases are practically insensitive to chemical variations in the homopolymer. Hence variations in structure at the glass transition (T_g) and melting (T_m) temperatures that affect diffusion also unambiguously affect permcation. Consequently an equivalence results between permeation at a given temperature for different polymers and permeation at different temperatures for a given polymer. Although the diffusion coefficient changes continuously with temperature, the Arrhenius parameters D_o and E_d apparently change discontinuously at T_g and T_m. Their magnitudes and variations with atomic weight reach maxima at about T_g. These data indicate a dependence of the classical correlation between D_o and E_d on polymer properties. A perturbed diameter for the permeant, specific for each polymer, is proposed for correlating the D_o and E_d data. This correlation makes the changes observed at T_g and T_m more perceptible.     

Gas permeation through water-swollen hydrogel membranes

This work deals with water-swollen hydrogel membranes for potential CO₂ separation applications, with an emphasis on elucidating the role of water in the membrane for gas permeation. A series of hydrogel membranes with a wide range of water contents (0.9–10 g water/g polymer) were prepared from poly(vinyl alcohol), chitosan, carboxyl methyl cellulose, alginic acid and poly(vinylamine), and the permeation of CO₂, H₂, He and N₂ through the membranes at different pressures (200–800 kPa) was studied. The gas permeabilities through the dry dense membranes were measured as well to evaluate the resistance of the polymer matrix in the hydrogel membranes. It was shown that the gas permeability in water-swollen membrane is lower than the gas permeability in water, and the selectivity of the water-swollen membranes to a pair of gases is close to the ratios of their permeabilities in water. The permeability of the water-swollen membranes increases with an increase in the swelling degree of the membrane, and the membrane permeability tends to level off when the water content is sufficiently high. A resistance model was proposed to describe gas permeation through the hydrogel membranes, where the immobilized water retained in the polymer matrix was considered to form transport passageways for gas permeation through the membrane. It was shown that the permeability of hydrogel membranes was primarily determined by the water content in the membrane. The model predictions were consistent with the experimental data for various hydrogel membranes with a wide range of water contents (0.4–10 g water/g polymer).     

Modeling permeation through anisotropic zeolite membranes with nanoscopic defects

We have modeled permeation through anisotropic zeolite membranes with nanoscopic defects that create shortcuts perpendicular to the transmembrane direction (x). We have found that the dimensionless ratio D_y/(k_dΔy) can be used to estimate whether the shortcuts contribute significantly to the overall flux. Here D_y is the diffusion coefficient for motion in the plane of the membrane, k_d is the rate of desorbing into defect voids, and Δy is the spacing between adjacent defects. For values of D_y/(k_dΔy)⪢1, we find that shortcuts increase the flux by significant amounts. The magnitude of the flux is increased as the imperfection spacing Δy is decreased. For small values of Δy, permeation through shortcuts becomes sorption-limited so that decreasing Δy further does not increase the flux through a single shortcut. However, as Δy is decreased, the concentration of shortcuts increases, thereby increasing the total contribution of the shortcuts to the flux. We have found regimes where increasing Δy or decreasing D_y decreases the overall flux, showing that permeation can be diffusion-limited by motion perpendicular to the transmembrane direction.     

Temperature and concentration effects on electrolyte transport across porous thin-film composite nanofiltration membranes: Pore transport mechanisms and energetics of permeation

The influence of temperature and concentration on nanofilter charge density and electrolyte pore transport mechanisms is reported. Crossflow filtration experiments were performed to measure transport of several electrolytes (NaCl, NaNO3, NaClO4, CaCl2, MgCl2, and MgSO4) across two commercially available thin-film composite nanofiltration membranes in the range 5-41 degrees C. Experiments were also performed with selected salts in the range 1-50 meq/L to quantify concentration effects. Three different approaches, irreversible thermodynamics, extended Nernst-Planck formulation, and theory of rate processes, were employed to interpret retentions of these symmetric and asymmetric electrolytes at varying temperature and concentration. Increasing feed water temperature slightly increased electrolyte reflection coefficients and only weakly increased permeability compared with neutral solutes. Electromigration and convection tended to counteract each other at high fluxes explaining the weak temperature dependence of the reflection coefficient. Changes in membrane surface charge density with temperature were attributed to increased adsorption of electrolytes on the polymer constituting the active layer. Activation energy of permeation for charged solutes was primarily determined by the Donnan potential at the membrane-feed water interface. Electrolyte permeation was shown to be an enthalpy-driven process that resulted in small entropy changes. Increasing sorption capacity with temperature and low sorption energies indicated that co-ion sorption on polymeric membranes was an endothermic physicosorption process, which appears to determine temperature dependence of electrolyte permeation at increased feed concentrations.     

Gas permeation through hydrogels : I. Gel cellophane membranes

Permeabilities and diffusion coefficients of various gases, Ar, N₂, O₂, CO₂, CH₄, C₂H₄, C₃H₈ and C₄H₁₀, were measured for Water-swollen gel cellophane membranes. No dependence of permeabilities on gas pressure below 1 atm was found. It was observed that the permeability coefficients were not related linearly to the coefficients in bulk water. For the two states of water in the membrane, an analytical method is presented to estimate the diffusion coefficients and the solubilities in free water and non-freezing water. It was found that the diffusion coefficients in non-freezing water were lower than those in free water, and the solubilities in non-freezing water were higher than in free water for all gases studied except CO₂ and C₂H₄, which gave reverse results.     

Gas permeation through PSF-PI miscible blend membranes

The permeation rates of He, H₂, CO₂, N₂ and O₂, are reported for a series of miscible polysulfone-polyimide (PSF-PI) blend membranes synthesized in our laboratory. For gases which do not interact with the polymer matrix (such as He, H₂, N₂ and O₂), gas permeabilities in the miscible blends vary monotonically between those of the pure polymers and can be described by simple mixture equations. In the case of CO₂, which interacts with PI, blend permeabilities decrease somewhat, compared to pure PSF and PI. This, however, is accompanied by a two-fold improvement in the critical pressures of plasticization vs. polyimide. Permselectivities of CO₂/N₂ and H₂/CO₂ in the blends deviate from mixing theory predictions, in contrast to selectivities of gas pairs which do not interact with PI. Differential scanning calorimetry measurements of pure and PSF/PI blend membranes show one unique glass transition temperature, supporting the miscible character of the PSF/PI mixture. Optical micrographs of the blend membranes clearly indicate perfect homogenization and no phase separation. Frequency shifts and absorption intensity changes in the FTIR spectra of the blends, as compared with those of the pure polymers, indicate mixing at the molecular level. This compatibility in mixing PSF and PI, results essentially in a new blend polymer material, suitable for the preparation of gas separation membranes. Such membranes combine satisfactory gas permeation properties, reduced cost, advanced resistance to harsh chemical and temperature environments, and improved tolerance to plasticizing gases.     

Transient permeation of organic vapors through elastomeric membranes

The permeation of benzene and acetone vapors through sulfur-cured natural rubber was studied by the time-lag method. The experimental results were analyzed by a method suggested by Meares. The zero concentration diffusion coefficient D₀ was obtained by the early-time method. The Frisch time-lag equation was utilized to estimate both the solubility coefficient s and the additional parameter b required to define the concentration dependence of the diffusion coefficient: D(c) = D₀ exp {bc}. This form of concentration dependence was manifested by the corresponding permeability coefficient values. At low entering penetrant pressure, where the transport coefficients are constant, indirect evidence was obtained that D₀ is the mechanistically correct diffusion coefficient. The solubility coefficient values calculated for benzene vapor in natural rubber are in reasonable agreement with published equilibrium sorption data for a similar rubber compound. At higher entering penetrant pressures, average diffusion coefficients obtained at steady state tended to be larger than the corresponding average diffusion coefficients derived from the time lags. This same effect has been detected by other experimental approaches. Permeation experiments designed for this rapid method of analysis appear capable of yielding information consistent with that obtained by more time-consuming traditional methods.     

Surface processes in hydrogen permeation through metal membranes

The phenomenon of hydrogen permeation through metals is considered as a complex of interphase and diffusion processes. Basic principles of application of the hydrogen permeation technique to studies of various stages of hydrogen interaction with metals are formulated. An analysis is made of possible errors of the permeation technique when used to measure the characteristics of hydrogen diffusion through metals originating from the finite rate of the interphase processes. The permeation of nonequilibrium and corrosion-produced hydrogen through metal membranes, and the hydrogen permeability of multilayer systems are considered.     

Studies with model membranes. IX. Evaluation of thermodynamic parameters from the transition state theory of rate processes for electrolyte diffusion through silver chloride parchment-supported membranes

Diffusion rate of biologically important electrolytes through parchment-supported silver chloride membrane have been determined by the use of Nernst-Planck equation at various temperatures taking into account membrane resistance R_m, membrane potential E_m, etc. The diffusion rates were found to be primarily dependent upon the difference in the hydration energies of the counterion in the external solution. On the basis of Eisenman-Sherry theory the diffusion rate sequence of alkali metal cations point towards the weak field strengths of the fixed charge groups. The various thermodynamic parameters, namely, ΔH^?, ΔF^?, and ΔS^? were evaluated. The values of ΔS^? found to be negative, indicating that the diffusion is taking place with partial immobilization in the membrane phase. A formal relation between ΔH_hydration, ΔF_hydration, and ΔS_hydration of cations with the corresponding values of ΔH^?, ΔF^?, and ΔS^? for diffusion was also found to exist for the investigated system.     

Studies of inorganic precipitate membrane. Part XXX. Membrane selectivity from electrical potential and conductivity measurements

Bi-ionic and multi-ionic potentials across parchment-supported mercuric sulfide membrane with various combinations of 1:1 electrlytes at different concentrations were measured. Membrane conductivity in contact with a single electrolyte was experimentally determined to evaluate selectivity of the membrane with the predetermined values of intramembrane mobility ratio. The selectivity sequence of the membrane was K⁺>Na⁺>Li⁺, which on the basis of the Eisenman–Sherry model of membrane selectivity, points toward the weak field strength of the charge groups attached to the membrane matrix. The permeability ratio of ions within the membrane was also evaluated by use of the equation based on the macroscopic linear laws of nonequilibrium thermodynamics derived by Sandblom and Eisenman. Three different methods of various integrated forms of the Nernst-Plank flux equation were used to derive the potentiometric selectivity constant K_ij^pot of the membrane. These values were close to one another.     

Thermoosmosis of various electrolyte solutions through anion-exchange membranes

Thermoosmosis through anion-exchange membranes was measured for 10^-3 to 2 mol/kg of aqueous KCl, LiCl, and NH₄Cl and for 10^-3 to 3 x 10^-1 mol/kg of aqueous KIO₃ and K₂SO₄. For all electrolytes used the direction of thermoosmosis was from the cold side to the hot side over the whole range of concentrations. For KCl and LiCl the experimental results were analyzed with an extension of a previously published theory, using additional data for transport numbers of ions in membranes and for electroosmosis. The agreement between theory and experiment is satisfactory. The absolute value of thermoosmosis for KIO₃ is larger than that for other electrolytes because the pore volume fraction of the membrane for KIO₃ is larger than that for the other electrolytes.