Permeation and separation of benzene/cyclohexane mixtures through liquid-crystalline polymer membranes

The side-chain liquid-crystalline polymer (LCP) was synthesized by the addition of the mesogenic monomer to poly(methylsiloxane) with Pt catalyst. When the benzene/cyclohexane mixtures were permeated through the LCP membranes by pervaporation at various temperatures, the permeation rate increased with increasing benzene concentration in the feed solution and permeation temperature. Though the LCP membranes exhibited a benzene permselectivity, a mechanism of the permeation and separation for the benzene/cyclohexane mixtures was different in the glassy, liquid-crystalline and isotropic state of the LCP membranes. These results suggested that the permselectivity was fairly influenced by the change of the LCP membrane structure, that is, a state transformation. It was found that a balance of the orientation of mesogenic groups and flexibility of siloxane chains is very important for the permeability and selectivity. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 699–707, 1997     

Effect of permeation temperature on permeation and separation of a benzene/cyclohexane mixture through liquid-crystalline polymer membranes

When a benzene/cyclohexane mixture of 10 wt % benzene was permeated through side-chain liquid-crystalline polymer (LCP) membranes by pervaporation at various temperatures, the permeation rate increased with increasing permeation temperature. The LCP membranes also exhibited a benzene permselectivity. The permselectivity for the benzene/cyclohexane mixture through the LCP membrane was different in the glassy, liquid-crystalline, and isotropic states. The LCP membrane had different apparent activation energies for permeation at each state. LCP membrane in the liquid-crystalline state had the highest apparent activation energy of the three states. Results suggest that the benzene permselectivity was influenced by changes in the LCP membrane structure, i.e., a state-transformation. It was found that a balance of the orientation of mesogenic groups and the flexibility of the siloxane chains was very important for benzene permselectivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 281–288, 1998     

Selective separation of aromatic hydrocarbons through supported liquid membranes based on ionic liquids

Ionic liquids are emerging as alternative solvents for volatile organic compounds traditionally used in liquid–liquid extraction and liquid membrane separation. In this paper, we examine whether room-temperature ionic liquids as a membrane solution can be utilized for hydrocarbon separation by using a supported liquid membrane. Aromatic hydrocarbons, benzene, toluene and p-xylene were successfully transported through the membrane based on the ionic liquids. Although the permeation rates through the membrane based on the ionic liquids were less than those of water, the selectivity of aromatic hydrocarbons was greatly improved. The maximum selectivity to heptane was obtained using benzene in the aromatic permeation and 1-n-butyl-3-methylimidazolium hexafluorophosphate in the liquid membrane phase.     

Separation of benzene/cyclohexane mixtures using polyurethane–silica hybrid membranes

Mixed sols were prepared by dissolving polyurethane (a 30 wt% solution in n-propanol, PU) and tetraethylorthosilicate (TEOS) in ethanol at PU:TEOS mass ratios of 1:2, 1:1, 2:1 and 3:1. Each of the sols was coated on a porous α-alumina support tube by the dipping method, and green membranes were heat-treated at 200°C for 1 h in an atmosphere of nitrogen. A PU membrane was also prepared with PU alone. The membranes were 5–6 μm thick. The polyurethane–silica membranes were swollen in benzene but only slightly in cyclohexane at room temperature. The degree of swelling in benzene decreased with increasing fractions of TEOS in the hybrid sols. The selectivity of benzene to cyclohexane was improved due to the suppression of swelling as a result of hybridization with TEOS. The total permeation flux and benzene/cyclohexane selectivity in the membrane prepared with a sol of PU:TEOS=1:1 were 3×10⁻⁵ kg m⁻² s⁻¹ and 19, respectively.     

Selective separation of copper with Lix 864 in a hollow fiber module

The transfer and separation of Cu(II), Co(II), Ni(II) and Zn(II) ions across a hollow fiber supported liquid membrane containing LIX 864 as the mobile carrier dissolved in kerosene solvent has been investigated. The flux and selectivity for copper has been studied as a function of the feed flow, the carrier concentration in the liquid membrane and the extraction solution acidity. A maximum copper recovery at 30% of LIX (v/v) in the diluent was obtained. The permeation experiments showed that at pH 2 in the extraction solution a highly selective separation of Cu over the other cations can be achieved. Increasing the acidity of the extraction solution copper selectivity decrease and the grade of recuperation sequence is Cu>Co>Ni>Zn. These results suggest that in selected situations, this membrane system can be competitive with the conventional liquid-liquid extraction process, in particular in leaching solutions with low metal concentration.     

Permeation and separation characteristics for benzene/cyclohexane mixtures through tosylcellulose membranes in pervaporation

Tosylcelluloses (TosCells) with different degrees of tosylation were synthesized as membrane materials for the separation of benzene/cyclohexane (Bz/Chx) mixtures. TosCell membranes showed a high benzenepermselectivity for the Bz/Chx mixtures in pervaporation (PV). An increase in the benzene concentration in the feed mixtures increased permeation rate but decreased the benzenepermselectivity of the TosCell membranes. The increase in the permeation rate was attributed to the increase of the degree of swelling of the TosCell membranes by the feed mixtures and the decrease in the benzenepermselectivity was mainly caused by the decrease of sorption selectivity. With low benzene concentrations in the Bz/Chx mixtures, the permeation rate of a TosCell membrane with a higher degree of tosylation was greater than that with a lower degree of tosylation, but was vice versa with a high benzene concentration. The benzenepermselectivity of the former TosCell membrane was higher than that of the latter membrane. Differences of the permeation rate and benzenepermselectivity with changes in the benzene concentration in the feed mixture and degree of tosylation of the TosCell membrane were significantly influenced by the degree of swelling of the TosCell membrane and the benzene concentration sorbed into the TosCell membrane. Mechanism of separation for the Bz/Chx mixtures through the TosCell membranes is discussed by the solution–diffusion model.     

Performance and stability of supported liquid membranes using LIX 984N for copper transport

The transport of copper through supported liquid membranes (SLM) using Celgard and Accurel membranes as supports and a novel commercial extractant LIX 984N as a carrier was investigated. LIX 984N provides good overall transport performance for copper from the acidic and dilute solution to the concentrated copper sulfuric acid. The instability of the supported liquid membranes using Celgard 2500 as the membrane support has been studied. It has been demonstrated that initially the surface shear forces due to stirring are a main cause for membrane liquid loss leading to SLM instability. However, during long term permeation no single instability mechanism dominants. The instability of long term operation involves a complex interaction of a number of factors, including surface shear forces, Marangoni effects, changes to membrane morphology, Bernard instabilities and membrane preparation protocal.     

Separation of organic binary mixtures by pervaporation

Pervaporation is a membrane separation process which has the inherent advantage of excellent selectivity for a number of mixtures that are otherwise difficult to separate (e.g. azeotropic mixtures or mixtures of liquids with only small differences in vapor pressures). Important parameters for pervaporation are the operating pressure on the permeate side and the temperature drop at the membrane interface caused by the phase change from liquid to vapor. This paper presents a mathematical model for the mass transport of binary mixtures within the membrane. The calculations are based on the transport equations of Shelden and co-workers [1]. These equations were solved without any simplifying assumptions and tested experimentally with benzene—cyclohexane mixtures. The limitations of the mathematical model and possible methods of improving it are discussed. The temperature drop at the membrane interface and its influence on the rate of permeation are also examined.     

Hybrid liquid membrane (HLM) system in separation technologies

A novel liquid membrane system, denoted hybrid liquid membrane (HLM), was developed for the separation of solutes (metal ions, acids, etc.). It utilizes a solution of an extracting reagent (carrier solution), flowing between membranes. The membranes, which separate the carrier solution from feed and receiving (strip) solutions, enable the transport of solutes, but block the transfer of the carrier to the feed or to the strip. Blocking the carrier is achieved through membranes hydrophilic/hydrophobic or ion exchange properties, or through their rentention abilities, due to pore size.     

Induction time in metal permeation processes through supported liquid membranes

The origin of the induction time observed in the permeation process of cadmium species through trilaurylammonium chloride in triethylbenzene supported liquid membranes is discussed. A model for the non-steady state transference process, where aqueous film diffusion coupled to an interfacial chemical reaction are the main rate determining processes, was developed. By comparison with experimental transference data, the rate constant of the interfacial reaction between cadmium chloride aqueous complexes and the membrane carrier, trilaurylammonium chloride, was evaluated. The time evolution of the concentration profiles through the aqueous diffusion film is also described.     

Selective Recognition of Calcium Ion through Liquid Membrane

The mole transport rate of alkaline earth metal ions through a bulk liquid membrane and a supported liquid membrane using a series of proton diionizable acyclic polyethers was measured. Among alkaline earth metal ions, the calcium ion was observed to be selectively transported in both single and competitive transport experiments. Potentiometric titration and solution calorimetric titration also gave calcium selectivity over other alkaline earth metal ions. Acyclic polyether bearing a diethylene glycol unit andn-tetradecyl lipophilic chain at the α position of carboxylic acid affords the best selectivity for the calcium ion in bulk and supported liquid membranes.     

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.     

Application of 2-(octylsulphanyl)benzoic acid as Pb<Superscript>2+</Superscript> selective ionophore in hybrid membrane system

A solution of 2-(octylsulphanyl)benzoic acid in 1,2-dichloroethane was used as a liquid membrane for selective pertraction of Pb²⁺ cations. Transport processes were carried out in a multi-membrane hybrid system (MHS) consisting of two cation-exchange membranes (CEM) and a flowing liquid membrane (FLM) in the following order: CEM | FLM | CEM. The liquid membrane phase was dehydrated continuously using a pervaporation method (PV). The system was capable of transporting Pb²⁺ ions selectively from a multi-cation aqueous solution composed of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Pb²⁺ nitrates. A comparative study of the carrier efficiency under various feed pH conditions was performed. It was found that the carrier exhibited sufficient selectivity and transport efficiency under a broad range of operational conditions, with a maximum transport rate of Pb²⁺ ions attaining the value of (1.09 ± 0.03) × 10⁻¹⁰ mol cm⁻² s⁻¹ and the selectivity coefficient of up to 40.     

Comparison of permeability coefficients of organic vapors through non-porous polymer membranes by two different experimental techniques

The permeability coefficients of saturated and non-saturated vapors of benzene, hexane and cyclohexane through flat polymer membranes (low density polyethylene BRALEN FB2-30 and polyether-block-amide PEBA 4033-PE) by two different experimental techniques at 298.15 K are reported. The permeation data have been obtained using the differential flow permeameter and sorption ones by glass sorption apparatus with McBain’s spiral balance. The so-called stationary (steady) diffusion theory has been applied for evaluating the permeability coefficients from sorption (equilibrium) data and obtained values have been compared with the permeability coefficients from permeation (steady-state) measurements. In the case of relative lower vapors sorption in polymers (hexane and cyclohexane) good agreement between permeability coefficients from sorption and permeation is obtained. Hence, this paper proves the possibility to estimate the permeability coefficients of organic vapors from sorption data without need of performing the permeation experiments.     

Membrane Filtration with Liquids: A Global Approach with Prior Successes,New Developments and Unresolved Challenges

After 70 years, modern pressure‐driven polymer membrane processes with liquids are mature and accepted in many industries due to their good performance, ease of scale‐up, low energy consumption, modular compact construction, and low operating costs compared with thermal systems. Successful isothermal operation of synthetic membranes with liquids requires consideration of three critical aspects or “legs” in order of relevance: selectivity, capacity (i.e. permeation flow rate per unit area) and transport of mass and momentum comprising concentration polarization (CP) and fouling (F). Major challenges remain with respect to increasing selectivity and controlling mass transport in, to and away from membranes. Thus, prediction and control of membrane morphology and a deep understanding of the mechanism of dissolved and suspended solute transport near and in the membrane (i.e. diffusional and convective mass transport) is essential. Here, we focus on materials development to address the relatively poor selectivity of liquid membrane filtration with polymers and discuss the critical aspects of transport limitations. Machine learning could help optimize membrane structure design and transport conditions for improved membrane filtration performance.     

Membrane Filtration with Liquids: A Global Approach with Prior Successes,New Developments and Unresolved Challenges

After 70 years, modern pressure‐driven polymer membrane processes with liquids are mature and accepted in many industries due to their good performance, ease of scale‐up, low energy consumption, modular compact construction, and low operating costs compared with thermal systems. Successful isothermal operation of synthetic membranes with liquids requires consideration of three critical aspects or “legs” in order of relevance: selectivity, capacity (i.e. permeation flow rate per unit area) and transport of mass and momentum comprising concentration polarization (CP) and fouling (F). Major challenges remain with respect to increasing selectivity and controlling mass transport in, to and away from membranes. Thus, prediction and control of membrane morphology and a deep understanding of the mechanism of dissolved and suspended solute transport near and in the membrane (i.e. diffusional and convective mass transport) is essential. Here, we focus on materials development to address the relatively poor selectivity of liquid membrane filtration with polymers and discuss the critical aspects of transport limitations. Machine learning could help optimize membrane structure design and transport conditions for improved membrane filtration performance.     

Combination of a Mid-infrared Hollow Waveguide Gas Sensor with a Supported Capillary Membrane Sampler for the Detection of Organic Compounds in Water

A Fourier transform infrared (FT-IR) spectroscopy based gas sensor for continuous analysis of liquid phase samples has been developed, coupling a short hollow waveguide (HWG) gas cell with a supported capillary membrane sampler (SCMS) probe. Passing an inert carrier gas through the thin-walled tubular silicon membrane enables the permeation of volatile organic compounds (VOCs) present in aqueous solution and facilitates their continuous and quantitative detection in the infrared hollow fiber by multiple internal reflection spectroscopy. The sensitivity of the sensor system has been determined at the ppb (μg/L) concentration level and the response time ranges from few minutes to 30 min, depending on the analyte and the permeation properties of the sampling membrane.

The experimental set-up consists of Bruker Vector 22 FT-IR spectrometer with an externally aligned 50 cm long silica HWG coupled to the SCMS, which is immersed into a glass flask filled with analyte solution and kept under constant stirring.

Aqueous solutions of benzene, toluene, xylene isomers and chloroform were qualitatively and quantitatively analyzed confirming the feasibility of this sensor approach for environmental analysis.     

Intensification of gadolinium(III) separation by effective utilization of nanoliquids in supported liquid membrane using Aliquat 336 as carrier

A supported nanoliquid membrane was developed to improve the separation of rare metal ion gadolinium (Gd) from nitrate solution medium. The nanoliquid membrane was prepared by dispersion of nanoparticles in organic phase and Aliquat 336 was applied as the carrier. TiO₂ and SiO₂ as hydrophobic and hydrophilic nanoparticles were effectively incorporated in the supported liquid membrane (SLM) system and the effect of size, concentration, and type of nanoparticle in the SLM were evaluated. A membrane phase of 0.015 M Aliquat-336 in kerosene and 0.04 wt% of SiO₂ with the size of 15 nm was found to have the highest permeability coefficient of 12.57?×?10^?5 m/s and enhanced the permeability coefficient by 28.2%. Hydrophobicity and hydrophilicity of the nanoparticles were observed to have remarkable effects on the permeation of the SLM system and concluded that the hydrophobic nanoparticle was more desirable. Results showed that the solid supported pore’s blockage and aggregation of nanoparticles could bring adverse effects at a high nanoparticle concentration at this SLM configuration. The stability tests were conducted over ten cycles of separation and the supported nanoliquid membrane had slight reduction of permeation during the test.     

Experimental and theoretical study on propylene absorption by using PVDF hollow fiber membrane contactors with various membrane structures

Five kinds of asymmetric poly(vinylidene fluoride) (PVDF) hollow fiber membranes with considerable different porosities at the inner and outer surfaces of the membrane were prepared via thermally induced phase separation (TIPS) method and applied for propylene absorption as gas–liquid membrane contactors. A commercial microporous poly(tetrafluoroethylene) (PTFE) hollow fiber membrane was also used as a highly hydrophobic membrane. Experiments on the absorption of pure propylene into silver nitrate solutions were performed and the effects of membrane structure, inner diameter, silver nitrate concentration and absorbent liquid flow rate were investigated at 298 K. PVDF membranes prepared by using nitrogen as bore fluid had lower inner surface porosity than the membranes prepared with solvent as bore fluid. Except the membrane with a skin layer at the outer surface, propylene absorption flux was inversely proportional to the inner diameter of the hollow fiber membrane, and propylene absorption rate per fiber was almost the same. Propylene flux increased with increasing the silver nitrate concentration and also with increasing the absorbent flow rate.A mathematical model for pure propylene absorption in a membrane contactor, which assumes that the membrane resistance is negligibly small and the total membrane area is effective for gas absorption, was proposed to simulate propylene absorption rates. Experimental results were satisfactorily simulated by the model except for the membrane having a skin layer. The model also suggested that propylene is absorbed in silver nitrate solutions accompanied by the instantaneous reversible reaction. This paper may be the first experimental and theoretical study on propylene absorption in membrane contactors.     

Chemical modification of poly(substituted-acetylene): II. Pervaporation of ethanol / water mixture through poly(1-trimethylsilyl-1-propyne) / poly(dimethylsiloxane) graft copolymer membrane

Poly(1-trimethylsilyl-1-propyne)/poly(dimethylsiloxane) (PTMSP/PDMS) graft copolymer was prepared to evaluate the permeation characteristic at pervaporation of aqueous ethanol solution through the graft copolymer membrane. For the preparation of PTMSP/PDMS graft copolymer, an improved synthetic procedure was released in this paper, which comprised a one-pot reaction of PTMSP in lithium bis(trimethylsilyl)amide followed by treatment with hexamethylcyclotrisiloxane and trimethylchlorosilane. PDMS content of the graft copolymer was controlled in the range 5–74 mol%. Very tough and thin membranes could be prepared from these copolymers having various PDMS content by the solvent casting method. The permselectivity of the membranes was investigated by pervaporation of ethanol/water mixture at 30°C. Preferential permeation of ethanol was observed for the membranes. It was also found that the selectivity of every copolymer membrane was higher than that of the PTMSP membrane. Moreover, the selectivity depended on the PDMS content of the graft copolymer. The separation factor and permeation rate assumed the maximum values at 12 mol% PDMS content. At the maximum point, 7 wt% aqueous ethanol solution was concentrated to about 70 wt% ethanol solution, and the separation factor and permeation rate were 28.3 and 2.45 × 10^?3g · m/m² · h, respectively. Such a high permselectivity for ethanol might be due to a delicate alteration of membrane structure, which was induced by the introduction of a short PDMS side chain into a PTMSP backbone.