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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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Cellulose acetate hollow fiber membranes for reverse osmosis (RO) were spun using a forced convection technique. In this study, a systematic experimental design based on Taguchi’s method (which is a fractional factorial method) has been employed for discussing the relationship between the rejection rate coefficient, permeation rate and the dry–wet spinning conditions for making cellulose acetate hollow fibers for RO. The factors considered in the experimental design included the polymer contents (PCs), the ratio of the solvent (acetone) to swelling agents (formamide) in the dope solution, the dope extrusion rate (DER), the type of bore fluid (BF), the residence time (RT) and the nitrogen gas flushing rate (GR). The results indicate that the BF and the DER are the two most important factors in determining the performance of the RO membranes. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]