The study of elongation and shear rates in spinning process and its effect on gas separation performance of Poly(ether sulfone) (PES) hollow fiber membranes

doi:10.1016/j.ces.2003.10.023

Chemical Engineering Science

Volume 59, Issue 5, March 2004, Pages 1053-1062

https://doi.org/10.1016/j.ces.2003.10.023 Get rights and content

Abstract

The influence of elongation and shear rates induced by the geometry of spinnerets on gas performance of PES hollow fiber membranes has been studied. Different elongation and shear rates were introduced in various spinnerets with flow angles of 60°, 75° and 90° by changing the flow rate of dope solution. The PES hollow fiber membranes were fabricated under the wet-spun condition without extra drawing force and their gas performances were tested by using O₂ and N₂. The flow profiles of dope solution and the elongation and shear rates at the outermost point of the outlet of spinnerets were simulated by the computational fluid dynamics model. A hypothetic mechanism is assumed to explain the effects of elongation and shear rates on the changes of conformation of polymer chain. While trying to correlate the elongation and shear rates with the gas performance of hollow fibers, we have come to some preliminary conclusions that the elongation rate has more contribution portion in permselectivity than in permeance and the shear rate has more contribution portion in permeance than in permselectivity.

Introduction

Hollow fiber membrane modules have been often used in the application of commercialized gas separation membrane products owing to their larger ratio of membrane area over volume as compared to spiral-wound modules (Ho and Sirkar, 1992; Koros and Fleming, 1993; Paul and Yampol'skii, 1994; Matsuura, 1994; Mulder, 1996; Ho, 2003; Bao and Lipscomb, 2002). However, the fabrication of a polymer hollow fiber membrane with consistently high performance remains a complex challenge due to the fact that their morphology, mechanical properties and gas performances are influenced by various spinning conditions, such as shear rate (Chung et al., 2000; Ismail 1997, Ismail 1999; Shilton et al., 1997; Wang and Chung, 2001), air gap (Niwa et al., 2000; Chung and Hu, 1997), precipitation (Pinnau and Koros, 1992; Khayet et al., 2002), solvent exchange (Clausi and Koros, 2000), and other factors.

The shear rate induced when a polymer dope is pumped through the thin concentric annulus of a spinneret plays an important role on membrane separation performance. Chung et al. (2000) studied the effects of shear rate on gas separation performance of 6FDA-durene polyimide membranes, in which they suggested that there exists an optimal shear rate to fabricate hollow fiber membranes with optimal separation performance and morphology. A V (down and up) pattern for permeance and a Λ (up and down) pattern for permselectivity were observed with increasing shear rate. Their work implies that, in the low shear rate range, permeances decrease and permselectivities increase with increasing shear rate. Once above a threshold shear rate, permeances will instead increase and the permselectivities decrease with increasing shear rate. Possible explanations for these patterns may be understood from the relationship between the fiber morphology and shear stress: a better molecular orientation and chain packing induced by shear rate are the dominant factors for the enhanced permselectivity in the low shear rate range, while a relatively defective skin structure resulted from the shear thinning nature of a spinning fluid contributes the enhanced permeance in the high shear rate range. The enhancement in gas permselectivity of hollow fiber membranes as the result of an increase in shear rate has also been reported. (Shilton et al., 1997; Ismail and Shilton, 1998; Sharpe et al., 1999). The rheologically induced molecular orientation in hollow fiber membranes has been directly confirmed by means of plane-polarised infrared spectroscopy (Ismail et al., 1997; Shilton et al., 1997; Ismail et al., 1999) and attenuated total reflection infrared dichroism (Idris et al., 2003). Compared with the study on shear rate effects, there are not many reports on the effects of elongation rate. Though it is well known that elongation rate induced by the gravity of nascent hollow fibers in the air gap is unavoidable, its effect on gas separation performance cannot be extensively explored due to the difficulty in quantitative simulation of elongation rate induced by the geometry of a spinneret. As a result, Chung et al. utilized a long straight spinneret plus a wet phase inversion process to fabricate the hollow fibers in order to minimize the complicated coupling effect of elongation rate (Chung et al., 1998).

The introduction of computational fluid dynamics (CFD) technology makes it possible to quantitatively study the elongation rate and shear rate induced in spinnerets. Using the CFD method, one should first build a computational model to represent the researched system, then apply the fluid dynamics laws to simulate the virtual prototype, finally a stable solution can be obtained which can be used to make performance predictions. The CFD technology has already been applied to various membrane processes, which include: building a two-dimensional membrane reactor model for the adsorption of CO₂ into amines (Hoff et al., 2003); simulating the concentration polarization and flow distribution in membrane modules used for gas and vapor permeation (Staudacher et al., 2002); calculating the flow and concentration polarization in pressure driven membrane processes (Wiley and Fletcher, 2003); studying the turbulent effects, pressure-related boundary conditions, and flexible module geometry inside a membrane module (Pellerin et al., 1995); designing a thin channel cross-flow module for the characterization of flat ceramic membranes with a criterion of flow uniformity and low pressure drop (Darcovich et al., 1997); estimating the pressure drop and shear rate in a membrane module with rectangular channels filled with several commercially available spacers (Karode and Kumar, 2001); modeling the slug flow ultrafiltration process to enhance the permeate flux by gas sparging (Taha and Cui, 2002). However, no study has been reported on the use of the CFD method to simulate the flow profile of dope solution in spinnerets during the fabrication of hollow fiber membranes.

The objective of this paper is to explore the effects of elongation and shear rates induced in spinnerets on the gas separation performance of PES hollow fiber membranes. Along this direction, we have carried out the following works: (1) designed spinnerets with various flow angles, which can introduce various elongation rates even for a fully-developed dope fluid; (2) simulated the flow and stress profiles in various spinnerets by the CFD method; (3) prepared the PES hollow fiber membranes by using various spinnerets and evaluated their separation performance; (4) studied the correlation between the gas separation performance of hollow fiber membranes with the corresponding elongation and shear rates.

Section snippets

Simulation of the rate-of-strain profile in spinnerets by the CFD model

For a spinneret with a concentric annulus structure, the rate-of-strain profile is caused by the motion of dope solution in the spinneret. In cylindrical coordinates (z, r, θ), a rate-of-strain tensor can be written as follows: ${∇ V}_{z,r,θ} = ∇ V_{z,z} ∇ V_{z,r} ∇ V_{z,θ} ∇ V_{r,z} ∇ V_{r,r} ∇ V_{r,θ} ∇ V_{θ,z} ∇ V_{θ,r} ∇ V_{θ,θ} = ∂V_{z} ∂z ∂V_{r} ∂z ∂V_{θ} ∂z ∂V_{z} ∂r ∂V_{r} ∂r ∂V_{θ} ∂r 1 r ∂V_{z} ∂θ 1 r ∂V_{r} ∂θ − V_{θ} r 1 r ∂V_{θ} ∂θ + V_{r} r,$ where V represents the local velocity in the unit of m/s for a dope solution.

Supposing the flow profile of dope solution in the spinneret is ideally axially

Dope preparation and viscosity measurements

Poly(ether sulfone)(PES) Radel A-300 with a weight-average molecular weight of about 15,000 was purchased from Amoco Performance Product Inc., OH, USA. The dope solution with a density of $1186.5 kg / m^{3}$ was prepared with $37 wt %$ PES in N-methyl-2-pyrrolidone (NMP) as a procedure published elsewhere (Chung and Kafchinski, 1997; Chung et al., 1997), whose rheological properties were determined by using an ARES Rheometric Scientific Rheometer with a $25 mm$ cone-plate fixture under dynamic mode. The

Simulation of flow profiles of dope solution in spinnerets by CFD model

For a straight spinneret with a flow angle of 90°, the fully-developed flow profile of a dope solution within the spinneret with L/ΔD⩾10 can be analyzed by a series of general flow equations and boundary conditions deduced elsewhere (referred as the Shilton method for simplicity) (Freeman et al., 1986, Bird et al., 1987; Shilton, 1997; Qin et al., 2000). For this geometry, the elongational rate along the major axis is nonexistent. However, this analytical method cannot be completely applied in

Conclusion

We summarize the investigation, observations, and preliminary conclusions in the present study as follows:

1.
We have studied, for the first time, the effects of elongation and shear rates within a spinneret on gas separation performance of asymmetric hollow fiber membranes by using straight and conical spinnerets with flow angles of 60° and 75°. Elongation rates in the major axis were introduced unavoidably in these conical spinnerets.
2.
A CFD method was employed to model the flow profiles of dope

Acknowledgements

The authors would like to thank NUS for funding this research with the grant number of R-279-000-108-112. Special thanks are due to Dr. S.J. Shilton for provide his excellent software to calculate the flow profile induced in a conventional spinneret, and due to Prof. Takeshi Matsuura, Prof. Xijun Fan, Mr. Chun Zhou and Dr. Yujie Wang for their supportive comments on this research.

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