Comparative study of the separation of methanol–methyl acetate mixtures by pervaporation and vapor permeation using a commercial membrane

In the present study, the permeation behavior of methanol and methyl acetate in the pervaporation (PV) experiments are compared with those in vapor permeation (VP) experiments using a PVA-based composite membrane. Experiments have been carried out to study the selectivity and mass transport flux of the systems under varying operations conditions of feed temperature (40–60 °C) and feed methanol concentrations (2–34 wt%). The selected membrane was found to be methanol selective. Results show higher permeation flux but a similar separation factor for methanol in PV than in VP. For PV operation, the resulting separation factor at 60 °C shows a monotonous decrease (6.4–4.1) as the alcohol concentration in the feed mixture increases (2.3–34 wt%), whereas the total flux increases from 0.97 to 7.9 kg m⁻² h⁻¹. Based on the solution-diffusion theory, a mathematical model that describes satisfactorily the permeation fluxes of methanol and methyl acetate in both the PV and VP processes has been applied. The fluxes of both permeants can be explained by the solution-diffusion model with variable diffusion coefficients dependent on MeOH concentration in the membrane. Both PV and VP processes can be described with the same model but using different fitting parameters.     

Detection of traces of formaldehyde in pure air by gas chromatography and helium ionization detection

The sensitive helium ionization detector (HID) was used for the direct determination of ppm to ppb levels of formaldehyde in air. Two methods to generate formaldehyde in an air stream were evaluated. The first method utilized a paraformaldehyde permeation tube and the second utilized a motor driven syringe and a dilute solution of formaldehyde. The two methods were evaluated using gas chromatography with HID and spectrophotometry. The paraformaldehyde permeation tube generates only about 60% of the theoretical value, while the motor driven syringe was accurate for levels below 2 ppm; however, at a concentration of 13 ppm or above, oligomers or other chemical forms of CH2O are formed to decrease the concentration of gaseous CH2O produced.     

Ti–Ni–Pd dense membranes—The effect of the gas mixtures on the hydrogen permeation

In this experimental work the influence of co-existing gases on the hydrogen permeation through a Ti–Ni–Pd membrane was studied. It was found that nitrogen, carbon dioxide and helium do not influence the hydrogen permeation through the dense membrane. However, carbon monoxide influences the hydrogen flux at each temperature investigated (400–500 °C). The results show that for low CO concentration (i.e. at H₂ upstream >80%), the hydrogen flux through the membrane decreases faster than linearly, while, at H₂ upstream <80%, the slope is linear but smaller than the theoretical one.     

Synthesis of industrial scale NaY zeolite membranes and ethanol permeating performance in pervaporation and vapor permeation up to 130 °C and 570 kPa

NaY zeolite tubular membranes in an industrial scale of 80 cm long were synthesized on monolayer and asymmetric porous supports. The quality of synthesized membranes were evaluated by pervaporation (PV) experiments in 80 cm long at 75 °C in a mixture of water (10 wt.%)/ethanol (90 wt.%), resulting in higher permeation fluxes of 5.1 kg m⁻² h⁻¹ in the monolayer type membrane and of 9.1–10.1 kg m⁻² h⁻¹ in the asymmetric-type membranes, respectively. The uniformity with small performance fluctuation in longitudinal direction of the membranes were observed by PV for 10–12 cm long samples at 50 °C in a mixture of methanol (10 wt.%)/MTBE (90 wt.%). The ethanol single component permeation experiments in PV and vapor permeation (VP) up to 130 °C and 570 kPa were performed to determine the relations between the ethanol flux and the ethanol pressure difference across the membrane which is represented by permeance (Π, mol m⁻² s⁻¹ Pa⁻¹) for estimate of potential of ethanol extraction through the present NaY zeolite membranes applying feasible studies. Results indicate that (1) the permeation fluxes are linearly proportional to the driving force of vapor pressure for each sample in VP and PV. The permeances through an asymmetric support type membrane were rather constant of 0.6–1.2 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ in the wide temperature range of 90–130 °C in PV and VP, indicating that the ethanol permeances have weak temperature dependency with the feed at the saturated vapor pressure.

The results of superheating VP experiments showed that ethanol permeation fluxes are increased with increasing of the degree of superheating at a given constant feed vapor pressure. The ethanol permeances are increased with increasing of temperature at a given feed vapor pressure. The superheating VP could be a feasible process in industry.     

Pervaporation separation of aqueous chlorophenols by a novel polyurethane urea–poly (methyl methacrylate) interpenetrating network membrane

A novel polymer membrane system consisting of interpenetrating network (IPN) of hydroxy terminated polybutadiene (HTPB) based polyurethane urea (PUU)–poly (methyl methacrylate) (PMMA) has been designed and developed as highly permselective membrane for pervaporation separation of toxic p-chlorophenol and 2,4-dichlorophenol from their dilute aqueous solutions. It was observed that 3 ppm 2,4-dichlorophenol in water could be reduced to 0.3 ppm 2,4-dichlorophenol using a PUU–PMMA IPN membrane of 28 cm² area and 150 μm thickness. This membrane has shown high selectivity towards p-chlorophenol and 2,4-dichlorophenol at very low concentration in feed. Feed concentration of p-chlorophenol was varied from 1000 to 7000 ppm and that of 2,4-dichlorophenol was varied from 3 to 4000 ppm. Fifty seven percent 2,4-dichlorophenol in permeate was obtained from 3 ppm concentration in feed compared to 87% 2,4-dichlorophenol in permeate from 1000 ppm in feed. Pervaporation studies were carried out by varying the temperature of feed, membrane thickness and PMMA content in the membrane. The results of this investigation have revealed that these membranes would be suitable for separation of chlorophenols from industrial effluents.     

Fluorimetric flow injection analysis of trace amount of formaldehyde in environmental atmosphere with 5,5-dimethylcyclohexane-1,3-dione

A simple and sensitive flow injection method with fluorimetry and 5,5-dimethylcyclohexane-1,3-dione (dimedone) was developed for the determination of formaldehyde. Formaldehyde reacted with dimedone in the presence of ammonium acetate to form a fluorescence compound, which has an excitation wavelength at 395 nm and an emission wavelength at 463 nm. A two-channel flow system was assembled. Distilled water and 0.3% dimedone buffered at pH 5.5 were delivered at 0.7 ml min⁻¹ and 100 μl of sample was injected into the carrier stream. The reaction was done in the reaction system designed newly, which consists of heating and cooling devices. The chemical reactivity with formaldehyde was excellent in the reaction system and selective. The calibration graphs were linear in the range of 25–100 and 5–10 ppb. RSDs (n=10) for 50 and 10 ppb formaldehyde were 0.6 and 3.4% and the LOD (S/N=3) was 0.9 ppb. The sample throughput was 20 h⁻¹. The method was applied to the determination of formaldehyde in gas sample evolved from adhesive agents and in living environmental indoor. The sensitive and selective method is useful for monitoring trace of formaldehyde in the environmental atmosphere.     

Determination of lead in water using laser ablation–laser induced fluorescence

The detection sensitivity of laser-induced breakdown spectroscopy (LIBS) is improved by coupling it with a laser-induced fluorescence method. A waterjet sample containing 500 ppm of Pb as an analyte was ablated by a 266 nm, frequency-quadrupled Q-switchedNd:YAG laser at an energy of ~ 260 μJ. After a short delay the resulting plume was re-excited with a 283.306 nm, nanosecond pulse dye laser at energies ranging from 45 to 100 nJ. The limit of detection (LOD) of lead in water was determined both by the single-pulse LIBS technique and Laser Ablation coupled with Laser-Induced Fluorecence (LA–LIF) method. It was found to be 75 ppm in the case of single-pulse LIBS and 4.3 ppm for LA–LIF. When the resonant pulse was detuned from the transition wavelength the LA–LIF signal disappeared demonstrating the resonant selectivity of this technique.     

Removal of pollutants from indoor air using zeolite membranes

MFI-type zeolite membranes prepared by liquid phase hydrothermal synthesis on tubular commercial supports were used to remove model pollutants n-hexane, formaldehyde and benzene present at very low concentration levels (2–230 ppmv) in indoor air. The influence of several operating parameters was studied both in batch and continuous separation experiments. Depending on the operation conditions, permeation fluxes of the organic compound up to 3300, 130 and 30 mg/(m² h) and organic/air separation factors of 250, 6.3 and 38 were achieved for n-hexane, formaldehyde and benzene, respectively.     

Development of formaldehyde sensing element using porous glass impregnated with beta-diketone

We have developed a sensor element for detecting formaldehyde. The sensor element is made of porous glass impregnated with both β-diketone and ammonium ions. We used three kinds of β-diketone; acetylacetone, 1-phenyl-1,3-butanedione, and 1,3-diphenyl-1,3-propanedione. The three kinds of sensor element, which are initially colorless, turn yellow after exposure to formaldehyde, and absorption with a peak wavelength of 407–424 nm appears. There is a linear relationship between the 407–424 nm absorbance of the sensor element after exposure to formaldehyde, and the formaldehyde concentration. The sensor element also works cumulatively, and the absorbance changes of the three kinds of sensor elements are acetylacetone > 1-phenyl-1,3-butanedione > 1,3-diphenyl-1,3-propanedione, when exposed to the same concentration of formaldehyde in the atmosphere. We also found that both the formation and decomposition reactions of lutidine derivative (yellow dye) occur on the acetylacetone element at 20 °C, and the kinetic constant of the former is 100,000 times larger than that of the latter. On the other hand, only the formation reaction occurs on the 1-phenyl-1,3-butanedione element at 20 °C. Therefore, the acetylacetone element would be suitable for short-term measurements and the 1-phenyl-1,3-butanedione would be suitable for long-term measurements.     

A photocatalytic membrane reactor for VOC decomposition using Pt-modified titanium oxide porous membranes

Porous titanium oxide membranes with pore sizes in the range of 2.5–22 nm were prepared by a sol–gel procedure, and were applied for decomposition of methanol and ethanol as model volatile organic compounds (VOCs) in a photocatalytic membrane reactor, where oxidation reaction occurs both on the surface and inside the porous TiO₂ membrane while reactants are permeating via one-pass flow. Methanol was completely photo-oxidized by black-light irradiation to CO₂ when methanol at a concentration of 100 ppm was used at a feed flow rate of 500 × 10⁻⁶ m³/min, but the conversion decreased when the MeOH concentration in the feed was increased. Pt-modification was carried out by photo-deposition, and led to a decrease in pore diameter. Using Pt-modified membranes, a nearly complete oxidation of methanol up to 10,000 ppm at a feed flow rate of 500 × 10⁻⁶ m³/min was observed. Thus, such membranes would be effective for purifying a permeate stream after one-pass permeation through the TiO₂ membranes. The decomposition of ethanol is also discussed.     

A sensitive microwave cavity spectrometer: Direct detection of formaldehyde in automobile exhaust

A newly constructed sensitive microwave cavity spectrometer of the Stark-dc-voltage sweep type has revealed that formaldehyde in concentrations as small as 0.2 ppm can be directly detected by using the 7₂₅ ← 7₂₆ rotational line. The cavity resonator is operated in the rectangular TE_1,0,20 mode. A sample of automobile-engine exhaust showed a strong spectrum indicating a concentration of 24 ppm of formaldehyde.     

Supported liquid membranes for the determination of vanillin in food samples with amperometric detection

A supported liquid membrane system has been developed for the extraction of vanillin from food samples. A porous PTFE membrane is impregnated with an organic solvent, which forms a barrier between two aqueous phases. The analyte is extracted from a donor phase into the hydrophobic membrane and then back extracted into a second aqueous solution, the acceptor. The determination (100–1400 μg ml⁻¹ vanillin) was performed using a PVC-graphite composite electrode versus Ag/AgCl/3MKCl at +0.850 V placed in a wall-jet flow cell as amperometric detector. The solid sample is directly placed in the membrane unit without any treatment, and the analyte was extracted from the sample, passes through the membrane and conduced to the flow cell by the acceptor stream. The limit of detection (3σ) was 44 μg ml⁻¹. The method was applied to the determination of vanillin (9–606 μg g⁻¹) in food samples.     

Flow injection determination of iodide ion in a photographic developing solution using iodide ion-selective electrode detector

A potentiometric flow injection determination method for iodide ion in a photographic developing solution was proposed by utilizing a flow-through type iodide ion-selective electrode detector. The sensing membrane of the electrode was Ag₂S–AgI membrane. The response of the electrode detector as a peak-shape signal was obtained for injected iodide ion in a photographic developing solution. A linear relationship in the subnernstian zone was found to exist between peak height and the concentration of the iodide ion in a photographic developing solution in a concentration range from 0 to 6.0×10⁻⁵ mol l⁻¹. The relative standard deviation for ten injections of 2×10⁻⁵ mol l⁻¹ iodide ion in a photographic developing solution was 0.96% and the sampling rate was approximately 12–13 samples h⁻¹. The iodide ion could be determined under coexisting of an organic reducing reagent and inorganic electrolytes of high concentration in a photographic developing solution sample solution by the present method.     

Effect of copolymer type and composition on separation characteristics of pervaporation membranes—A case study with separation of acetone–water mixtures

Five different copolymer membranes, i.e. copolymer of acrylonitrile with 2-hydroxyethyl methacrylate (PANHEMA), vinyl acetate (PANVAC) and methyl methacrylate (PANMMA) and styrene with vinyl acetate PSTYVAC) and methyl methacrylate (PSTYMMA) were synthesized, each with two different copolymer compositions (i.e. PANHEMA-1, PANHEMA-2, etc.). The copolymer membranes were synthesized on the basis of their relative solubility parameters with respect to acetone and hydrophilicity with respect to water. These membranes were used for pervaporative dehydration of acetone over the entire concentration range of 0–100 wt% water as well as acetone separation over 0–44 wt% acetone in feed. The acrylonitrile copolymers showed water selectivity with maximum water flux and selectivity for PANHEMA-2 copolymer (29.3 g/(m² h), 16.73, respectively, for 2.5 wt% water in feed) while the styrene copolymers showed maximum acetone selectivity with reasonable acetone flux for PSTYMMA-1 copolymer (7.12 g/(m² h), 12.61, respectively, for 1.6 wt% acetone in feed) membrane. The influence of one permeant on permeation of the other permeant was also studied in terms of permeation factor.     

The determination of aldehydes in the exhaust gases of LPG fuelled engines

Summary The exhaust gas of a LPG fuelled engine is drawn through two bubblers in series in an ice bath, and filled with saturated 2,4-dinitrophenylhydrazine in 2M HCl. After heating the derivatives are extracted with toluene-cyclohexane and 1l samples injected on-column on a OV1 capillary column. Using an FID the lower limit of detection is 15–18 pg for formaldehyde (about 8–10 ppbv for a 16l exhaust sample). Taking the blank into account, the limit is about 40 ppbv.The exhaust gases of a LPG-fuelled engine contain formaldehyde, acetaldehyde, propionaldehyde, acrolein and acetone. Carbonyl compounds of more than 3 C-atoms were not found in detectable amounts. The engine was rund under stoichiometric, lean and rich air/fuel conditions. Under rich conditions the concentrations of the aldehydes were: formaldehyde 2.8 ppm, acetaldehyde 1.3 ppm, propionaldehyde 0.06 ppm, acrolein 0.03 ppm, acetone 0.17 ppm; under stoichiometric conditions: 4.5, 1.6, 0.10, 0.03 and 0.18 ppm respectively; under lean conditions 17.0, 2.9, 0.13, 0.07 and 0.27 ppm respectively. These figures demonstrate the necessity of measuring aldehydes in exhaust gases of LPG-fuelled engines.     

Prediction of the concentration polarization in the nanofiltration/reverse osmosis of dilute multi-ionic solutions

The aim of this work was to develop a simple and accurate model for predicting the concentration polarization index in the nanofiltration (NF)/reverse osmosis (RO) of dilute multi-ionic solutions. On the grounds of this model, the total flux of the ion i at the feed-solution/membrane interface consists of the sum of the diffusion, convection and migration fluxes, the former of which is determined by conventional mass-transfer correlations duly corrected to take into account the permeation through the membrane (suction effect). The coupling of the ionic fluxes is enforced by the electroneutrality requirement at the feed-solution/membrane interface. The model developed dispenses with the arbitrary assumption of the thickness of a film layer in the vicinity of the membrane surface.

Assessing the accuracy/validity of this model with multi-ionic solutions would be rather harsh, thus the model accuracy and ranges of validity were ascertained for a benchmark case: NF/RO of single salt solutions. The model predicts approximate concentration polarization indexes of the salts A⁺B⁻, A⁺₂B²⁻ and A⁺₃B³⁻ (or A²B⁻₂ and A³⁺B⁻₃) with positive deviations lower than 10% with respect to the benchmark concentration polarization index, for ions diffusivities ratios, D₁/D₂ (or D₂/D₁), in the range 0.16–5.5 and ≡J_v/k_c<3, where J_v is the permeation flux and k_c is the mass-transfer coefficient of the salt for vanishing mass-transfer rates at impermeable walls. The main assumption of the model – the individual mass-transfer coefficients of the ions are independent of each other – appears to hold in a broad range of conditions, for single salt solutions.

The model developed was expeditely applied to predict the concentration polarization in the nanofiltration of solutions containing Na⁺, Cl⁻ and a dye³⁻ (experimental data of Bowen and Mohammad [AIChE J. 44 (8) (1998) 1799–1812]), and its predictions are in fair agreement with the predictions of the extended Nernst–Planck equations in the film layer of the “slowest” ion.     

Membrane purification of Cl2 gas: I. Permeabilities as a function of temperature for Cl2, O2, N2, H2 in two types of PDMS membranes

Permeability (P) of Cl₂, O₂, N₂ and H₂ was measured in polydimethylsiloxane (PDMS) composite membranes with two different degrees of cross-linking. The permeability was measured in the low pressure range (1–3 bar absolute) over a fairly large temperature range 35–120°C. The functionalities of the membranes were compared on the basis of permeation rate and ability to separate the gases Cl₂–O₂. These results are part of an extensive survey where perfluorinated and carbon membranes are also included (not reported here). The purpose of the project is to develop an industrial membrane with high permselectivity for either O₂ or Cl₂ (depending on the type of membrane) at temperatures preferably above 70°C. Process conditions are set in an industrial project. The PDMS membranes are good candidates for this separation, having a high permeation rate for Cl₂ and a selectivity of Cl₂/O₂ in the range of 8–25 depending on temperature. Durability of the PDMS membranes in this aggressive environment is found to be very dependent on process conditions and on how the material is polymerized and cured. For documentation of durability, various silicones were tested; these results are to be reported separately.     

Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition

A thin, gas-tight palladium (Pd) membrane was prepared by the counter-diffusion chemical vapor deposition (CVD) process employing palladium chloride (PdCl₂) vapor and H₂ as Pd precursors. A disk-shaped, two-layer porous ceramic membrane consisting of a fine-pore γ-Al₂O₃ top layer and a coarse-pore -Al₂O₃ substrate was used as Pd membrane support. A 0.5–1 μm thick metallic membrane was deposited in the γ-Al₂O₃ top layer very close to its surface, as verified by XRD and SEM with a backscattered electron detector. The most important parameters that affected the CVD process were reaction temperature, reactants concentrations and top layer quality. Deposition of Pd in the γ-Al₂O₃ top layer resulted in a 100- to 1000-fold reduction in He permeance of the porous substrate. The H₂ permeation flux of these membranes was in the range 0.5–1.0 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at 350–450°C. The H₂ permeation data suggest that surface reaction steps are rate-limiting for H₂ transport through such thin membranes in the temperature range studied.     

New universal,portable and cryogenic sampler for time weighted average monitoring of H2S,NH3, benzene,toluene, ethylbenzene,xylenes and dimethylethylamine

A new cryogenic integrative air sampler (patent application number 08/00669), able to overcome many of the limitations in current volatile organic compounds and odour sampling methodologies is presented. The sample is spontaneously collected in a universal way at 15 mL/min, selectively dried (reaching up to 95% of moisture removal) and stored under cryogenic conditions. The sampler performance was tested under time weighted average (TWA) conditions, sampling 100 L of air over 5 days for determination of NH₃, H₂S, and benzene, toluene, ethylbenzene and xylenes (BTEX) in the ppm_v range. Recovery was 100% (statistically) for all compounds, with a concentration factor of 5.5. Furthermore, an in-field evaluation was done by monitoring the TWA inmission levels of BTEX and dimethylethylamine (ppb_v range) in an urban area with the developed technology and comparing the results with those monitored with a commercial graphitised charcoal diffusive sampler. The results obtained showed a good statistical agreement between the two techniques.     

Fast flow-injection fluorimetric determination of amiloride by using a solid sensing zone

A rapid and simple flow-through solid phase spectrofluorimetric system is described in this paper for the determination of the diuretic amiloride in physiological fluid (serum) and pharmaceuticals. The sensor was developed in conjunction with a monochannel flow-injection analysis system with fluorimetric transduction. Amiloride was transitorily retained on cationic exchanger gel Sephadex SP-C25 placed in the detection area into the cell. The determination is carried out without any derivatization reaction, by measuring directly the intrinsic fluorescence of the analyte and using the peak height as analytical signal. The wavelengths of excitation and emission were 291 and 419 nm, respectively. Amiloride could be determined in the concentration ranges of 10–600 and 4–250 μg l⁻¹ at a sampling rate of 24 and 30 h⁻¹, respectively with detection limits of 0.92 and 0.33 μg l⁻¹ for 100, and 600 μl of sample volume injected, respectively. The relative standard deviations for ten independent determinations were better than 0.65%. The method was satisfactorily applied to the determination of amiloride in spiked biological fluids (serum) and pharmaceutical preparations without any pretreatment of the samples.