Elsevier

Journal of Membrane Science

Volume 389, 1 February 2012, Pages 137-147
Journal of Membrane Science

The development of novel micro-capillary film membranes

https://doi.org/10.1016/j.memsci.2011.10.023Get rights and content

Abstract

This paper reports the experimental results for the processing of micro-capillary film (MCF) membranes. MCFs are films with embedded multiple hollow capillaries and can be considered as a hybrid geometry between flat sheet and hollow fibres. Compared to the conventional membrane geometries, MCFs potentially provide a higher surface area per unit volume, better mechanical strength, ease of handling and more efficient module fabrication. MCF membranes were fabricated out of ethylene vinyl alcohol (EVOH) copolymer through a solution extrusion followed by a nonsolvent induced phase separation process (NIPS). Single capillary EVOH membranes were also fabricated prior to the fabrication of MCFs to establish a base process. Fluid flow observations were carried out at different regions in the process in order to better understand the dynamics of extrusion and phase inversion. It was observed that polymer solution and bore fluid rheology as well as processing conditions including air-gap distance and take up rate have significant effects on the flow dynamics and consequently the macrostructure of the membranes. Furthermore, the flow observations led to the identification of an extrudate expansion phenomenon in the external coagulation bath which strongly influenced the membrane formation process. The micro-structure characterization of the fabricated membranes showed the formation of a highly porous, interconnected and macrovoid-free microstructure in the membranes. MCF membranes can be applied with a potentially promising performance in a variety of membrane applications such as micro and ultrafiltration, membrane bioreactors (MBR), porous microfluidic devices as well as membrane contactors for process intensifications.

Highlights

► Development of membranes in a novel micro-capillary film (MCF) geometry is reported. ► Flow observations were carried out to better understand the process. ► The effect of processing conditions on the MCF membrane macrostructure was studied. ► Air-gap distance and take-up rate significantly influence the MCF fabrication process. ► An extrudate expansion phenomenon was discovered in the process and explained.

Introduction

Membranes for separation processes are commonly produced in two types of geometries: flat sheets and circular single capillaries (sometimes called tubular membranes or hollow fibres depending on their internal diameters). Flat sheet membranes are usually stacked in parallel to form plate-and-frame modules or rolled around a central tube to form spiral wound modules, while capillary membranes are normally formed into shell and tube-like configurations. However, in addition to flat sheets and circular single capillaries other geometries can also be imagined for membranes, which could provide advantages over the conventional ones. Recently there have been a few developments in this area. Inge watertechnologies GmbH patented and commercialized Multi-bore® membrane technology that combines seven single capillaries in a fibre. The multi-bore structure provides an improved mechanical strength required for applications such as ultrafiltration with backwash cleaning [1]. In another development, Nijdam et al. [2] produced single capillary membranes in noncircular forms using dies with micro-engineered orifice and annulus. In this process characteristic patterns were formed in the inner and outer surfaces of the capillaries which improved the separation performance of the membranes in comparison to the circular ones. The authors attributed the improved performance to the higher surface area provided by the patterned capillaries [2].

This paper is concerned with the development of a different and potentially advantageous geometry for membranes called micro-capillary film (MCF). MCFs are films with embedded multiple hollow capillaries and can be considered as a hybrid geometry between flat sheets and hollow fibres. Compared to flat sheet membranes, MCFs are self supported and provide a higher surface area per unit volume. Compared to single capillary membranes, MCFs provide an improved mechanical strength, ease of handling and more efficient module fabrication.

The concept of solution processed microporous membranes with an MCF type geometry was disclosed through a Japanese patent by Toray Ind. Inc. in 1999 [3]. Later, the melt processing of MCFs was developed by Hallmark et al. [4] which led to the fabrication of non-porous MCFs applied for micro-fluidics in flow chemistry [5], heat exchange [6], [7] and fluid delivery systems [6]. The manufacture of MCFs was achieved by entraining air within a molten polymer from injection nozzles during the extrusion of polymer out of a slit die followed by drawing down the extrudate in an air-gap and finally cooling it down between chilled rolls. Fig. 1 shows the cross-section of a melt extruded polyethylene MCF. Recently, Chung et al. [8] reported preliminary studies on the potential manufacture of rectangular membranes with multiple holes using a similar die design used by Hallmark et al. [4]

In this paper the development of nonporous melt-extruded MCFs is extended into porous membranes using a solution extrusion followed by nonsolvent induced phase separation (NIPS) process. NIPS developed by Loeb and Sourirajan in 1963 [9] is still a common technique in industry for fabrication of asymmetric membranes. It is a versatile process that provides the possibility for fabrication of a wide variety of membrane microstructures for applications ranging from microfiltration to gas separations through the control of the processing parameters [10]. This paper particularly describes the experimental results on the solution processing of ethylene-co-vinyl alcohol (EVOH) MCF membranes. The solution processing of single capillary EVOH membranes is also reported to establish base conditions for processing MCFs and obtain a base for comparison.

Section 2 of this paper describes the materials, equipment and procedures used in the development of single capillary and MCF membranes. Section 3 details the experimental results and discussions followed by the conclusions. Rheological characterization of the polymer solutions used in this paper has been included as an Appendix.

Section snippets

Materials

Ethylene-co-vinyl alcohol (EVOH) was used as an example polymer to form the membrane matrix and supplied by Kuraray. EVOH was chosen due to the growing interest in this polymer in fields of biomedical science and water treatment because of its good biocompatibility and considerable hydrophilicity. To prepare EVOH polymer solutions as a processing liquid, N-methyle-2-Pyrrolidone (NMP) was used as the solvent. NMP is a polar solvent which is commonly used for solution processing of membranes by

Single capillary membrane processing

In order to establish the base processing of EVOH membranes, single capillary EVOH hollow fibres were fabricated prior to processing MCFs, using a single capillary die with a design shown in Fig. 3. The single capillary die end consisted of an annular channel for polymer solution and a nozzle in the middle for the bore fluid.

Initially a polymer solution consisting of 20 wt% EVOH in NMP was used as the processing fluid; however, the process using this solution resulted in the formation of surface

Conclusions

This paper has established conditions for solution processing of microporous single capillary EVOH membranes and shows that a novel geometry for membranes called micro-capillary films (MCFs) can be manufactured. It also shows that successful manufacturing of MCF membranes is very sensitive to polymer solution and bore fluid rheology, polymer shrinkage rate as well as processing conditions such as air-gap distance and take up rate. A high viscosity solution is desirable to dampen process

Acknowledgements

Sina Bonyadi would like to thank Trinity College at University of Cambridge and especially Mr. Viswanathan Krishnan and Mrs. Tzo Tze Ang for their generous financial support of his PhD studies in Cambridge. The authors would like to thank Dr. Simon Butler at Cambridge University Chemical Engineering department for his assistance. Thanks are due to Dr. Bart Hallmark at Cambridge University Chemical Engineering department and Prof. W. B. Krantz at University of Colorado at Boulder for their

Cited by (28)

  • Development of a disposable micro-capillary film grafted with peptide ligands for immunoadsorption

    2019, Journal of Chromatography B: Analytical Technologies in the Biomedical and Life Sciences
    Citation Excerpt :

    Other reagents were supplied by Shanghai Wokai Biotechnology Co., Ltd. (Shanghai, China). The preparation processes of MMCFs can be found in literature reported previously [22]. Briefly, polymer solutions consisting 15/10/75 wt% EVOH/PVP/NMP were extruded through a 19-nozzle die with capillary size of 0.5 mm and then non-solvent induced phase separation (NIPS) was applied to the extruded membranes in a water bath.

  • Recent progresses of ultrafiltration (UF) membranes and processes in water treatment

    2018, Membrane Separation Principles and Applications: From Material Selection to Mechanisms and Industrial Uses
View all citing articles on Scopus
View full text