Reverse Osmosis Hollow Fiber Membranes Research Articles

Numerical analysis of hollow fiber membranes for desalination

Water desalination by reverse osmosis hollow fiber membrane has been widely used to produce fresh water. This work numerically characterizes flux performance of the membrane, concentration polarization and potential fouling sites in the reverse osmosis desalination module containing hollow fiber membranes arranged in an inline and staggered configuration. Steady k-ω SST turbulence model is utilized to study membrane performance. An accurate membrane flux model, the solution-diffusion model, is employed. Hollow fiber membrane surface is treated as a functional boundary where the rate of water permeation is coupled with local concentration along the membrane surface. The rate of water permeation increases and concentration polarization decreases as the feed flow rate is increased. Hollow fiber membranes in the staggered geometry perform better than those in the inline geometry. It is proven by the present study that desalination modules containing hollow fiber membranes should be designed and optimized by careful consideration of their configurations. It is demonstrated here that flows in the hollow fiber bank becomes strongly time dependent at high flow rates and that transient effects could profoundly influence hollow fiber membrane flux performance and characterization of concentration polarization.

Effect of Rheological Behaviour of Cellulose Acetate Spinning Solutions on Performance of Reverse Osmosis Hollow Fiber Membranes

Kajian ini bertujuan untuk menyelidik kesan reologi semperitan terhadap membran gentian geronggang osmosis balikan. Ini dilakukan dengan penilaian reologi larutan semperitan selulosa asetat menggunakan sebuah reometer putaran dan sel optik ricih. Sifat hukum kuasa, daya normal dan profil aliran yang terjana memberi penjelasan mengenai fasa balikan dan orientasi molekul. Keputusan reologi ini kemudiannya dihubungkaitkan dengan prestasi membran gentian geronggang osmosis balikan dari segi kadar buangan dan kadar fluks. Keputusan kajian menunjukkan bahawa kadar buangan dan kadar fluks meningkat apabila kadar semperitan larutan meningkat dan ini mungkin disebabkan oleh orientasi molekul. Kata kunci: reologi semperitan; osmosis balikan; gentian geronggang; orientasi molekul Rheological assessment of cellulose acetate spinning solution using a rotational rheometer and an optical shear cell in carried out so as to study the cause and effect relationship between membrane preparation, polymer morphology and membrane performance. The power law behaviour, normal force and flow profiles generated provided clues regarding phase inversion and molecular orientation. These rheological results are then related to the separation performance of cellulose acetate reverse osmosis (RO) hollow fibers membranes; both the rejection rate and the flux rate increased with increasing dope extrusion arate, possibly due to the molecular orientation. Key words: spinning rheology; reverse osmosis; hollow fiber; molecular orientation

Formation and structural evolution of biphenyl polyamide thin film on hollow fiber membrane during interfacial polymerization

A novel reverse osmosis hollow fiber membrane was prepared by interfacial polymerization from 3,3′,5,5′-biphenyl tetraacyl chloride and m-phenylenediamine on a polysulfone hollow fiber membrane. The structural evolution of biphenyl polyamide thin film during interfacial polymerization was monitored by attenuated total reflectance infrared, X-ray photoelectron spectroscopy and atom force microscopy. The relationship between the structure and separation properties of the membranes was investigated. Results show that the biphenyl polyamide thin film had a three-layer structure: a loose initial layer with a low cross-linked structure, a dense middle layer with a high cross-linked structure and a loose surface layer with a low cross-linked structure. The dense middle layer had an intrinsic cross-linked structure with over 86.0% amide bonds (–CONH–) and below 14.0% carboxylic groups (–COOH), mainly responsible for separation. A growth model of the biphenyl polyamide thin film was proposed to describe the structural evolution process during interfacial polymerization.

Solvent treatment of CTA hollow fiber membrane and its pervaporation performance for organic/organic mixture

Cellulose triacetate (CTA) hollow fiber membrane used to separate methanol/methyl tert-butyl ether (MTBE) by pervaporation (PV) has been prepared from CTA hollow fiber reverse osmosis (RO) membrane for desalination of brackish water with high salinity. Acetone was selected as a modification agent of CTA membrane. PV performance depended on the solvent concentration, the treatment time and modification temperature of CTA RO hollow fiber membrane soaked in the aqueous acetone. The results show that CTA hollow fiber membrane modified with the solvent has a superior performance both the separation factor and the permeate flux in the PV experiment conditions.

Measurement of rheologically induced molecular orientation using attenuated total reflection infrared dichroism in reverse osmosis hollow fiber cellulose acetate membranes and influence on separation performance

The objective of this study is to investigate the pure influence of shear and its capacity to increase the separation performance of cellulose acetate reverse osmosis hollow fiber membranes, produced using a dry/wet spinning process with forced convection in the dry gap. In order to achieve this, the experiments are designed to decouple the effect of extrusion shear from forced convection residence time by fixing the residence time at 0.615 s. RO hollow fibers were spun at various dope extrusion shear rates ranging from 3.0 to 4.5 ml/min corresponding to shear rates of around 13,479–20,219 s −1, respectively, at the outer spinneret wall. Fourier transform attenuated total reflection (ATR) also known as internal reflection spectroscopy was used to probe the membrane active layer so as to examine if high shear rates may induce anisotropy at the molecular level in the active layer. The rejection rates of the membranes were evaluated using salt water. Results revealed that: (i) ATR can be used to determine qualitatively and quantitatively—the degree of molecular orientation in the sheared membranes; (ii) molecular orientation is enhanced as the dope extrusion rate increases; and (iii) there exist an optimum shear rate that can induce a certain degree of molecular orientation to yield membrane morphology with optimum separation performance.

Rheology assessment of cellulose acetate spinning solution and its influence on reverse osmosis hollow fiber membrane performance

Cellulose acetate spinning solution used to produce reverse osmosis (RO) hollow fiber membranes was rheologically assessed using a rotational rheometer and an optical shear cell. Rheology measurements which involved flow curves were carried out so as to obtain the values of power law coefficients, n and k. The power law behaviour, normal force and flow profiles generated provided clues regarding phase inversion and molecular orientation. These rheological results are then correlated to the performance of cellulose acetate RO hollow fibers spun at different extrusion shear rates. The results suggest that extrusion shear is linked indirectly to phase inversion through induced molecular orientation, which in turn, affects the subsequent dry/wet precipitation stages in spinning. As the extrusion shear rate increases, the level of shear experienced at the walls of the spinneret also increases, thus leading to greater molecular orientation, resulting in membranes with higher rejection and flux rates.

The deduction of fine structural details of reverse osmosis hollow fiber membranes using surface force–pore flow model

In order to elucidate the relationship between the dope extrusion shear rate and membrane performance, sodium chloride transfer through the asymmetric cellulose acetate reverse osmosis hollow fiber membranes is modeled, allowing fine details of the fiber structure to be deduced from the NaCl-H2O rejection characteristics. The structural information such as the pore size radius and skin thickness of the active layer deduced from the sodium chloride separation experimental data and surface force-pore flow model (SF-PF) is then used to interpret the relationship between the rheological conditions during spinning and membrane performance. The modeling results revealed that increased extrusion shear rate would decrease both pore size and thickness of the active layer, thus increasing the separation performance of the RO hollow fiber membranes.

Study of shear rate influence on the performance of cellulose acetate reverse osmosis hollow fiber membranes

The effect of shear rate on the separation performance of reverse osmosis hollow fiber membrane is discussed. Experiments involving six different dope extrusion rates (DERs) (ranging 2.5–5 ml/min) are performed with the other process factors set at the optimum conditions determined by the Taguchi analysis. This will enable an assessment to be made on the relationship between the DER and the rejection rate. The regression method is used to analyse the experimental results and an empirical model has been developed. Simultaneously, it is found that there is a fairly strong correlation between extrusion shear rate and the rejection rate of the membranes, whereby as the shear rate increases, the rejection rate increases until a critical level of shear is achieved, beyond which reverse osmosis membrane performance deteriorates, suggesting that there exists an optimum shear rate which yields optimal membrane morphology for reverse osmosis hollow fiber membranes.

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The objective of this study is to investigate the pure influence of shear and its capacity to increase the separation performance of cellulose acetate reverse osmosis hollow fiber membranes, produced using a dry/wet spinning process with forced convection in the dry gap. In order to achieve this, the experiments are designed to decouple the effect of extrusion shear from forced convection residence time by fixing the residence time at 0.615 s. RO hollow fibers were spun at various dope extrusion shear rates ranging from 3.0 to 4.5 ml/min corresponding to shear rates of around 13,479–20,219 s−1, respectively, at the outer spinneret wall. Fourier transform attenuated total reflection (ATR) also known as internal reflection spectroscopy was used to probe the membrane active layer so as to examine if high shear rates may induce anisotropy at the molecular level in the active layer. The rejection rates of the membranes were evaluated using salt water. Results revealed that: (i) ATR can be used to determine qualitatively and quantitatively—the degree of molecular orientation in the sheared membranes; (ii) molecular orientation is enhanced as the dope extrusion rate increases; and (iii) there exist an optimum shear rate that can induce a certain degree of molecular orientation to yield membrane morphology with optimum separation performance.

The structure of reverse osmosis hollow fiber membrane of cellulose triacetate (CTA)

The structure of the reverse osmosis hollow fiber membrane of CTA made by gel-spinning method is described here. As everyone know, the structure of a chemical fiber not only depends on the polymer material, but also on the fabrication technology. It is also the same for reverse osmosis membrane. The membrane in this study has been made by the gel-spinning technology, therefore, the morphological structure of the membrane possesses the characteristics of gel-spinning fibers. First, there is a remarkable difference in the structure across the traverse section of the hollow fiber membrane, that is so called “core-skin” structure; secondly, there is a network structure in the hollow fiber. The experimental results showed that the flux of the membrane is closely related to the meltdraw ratio of the fiber. Based on the performance of the reverse osmosis membrane, this paper discusses the effect of the network structure on the membrane performance.