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1 This document is downloaded from DR-NTU, Nanyang Technological University Library, Singapore. Title Design and development of novel feed spacers in spiral wound membrane modules with 3D printing Author(s) Citation Tan, Wen See; Chua, Chee Kai; Chong, Tzyy Haur; An, Jia Tan, W. S., Chua, C. K., Chong, T. H., & An, J. (2018). Design and development of novel feed spacers in spiral wound membrane modules with 3D printing. Proceedings of the 3rd International Conference on Progress in Additive Manufacturing (Pro-AM 2018), doi: /d4630p Date 2018 URL Rights 2018 Nanyang Technological University. Published by Nanyang Technological University, Singapore.

2 DESIGN AND DEVELOPMENT OF NOVEL FEED SPACERS IN SPIRAL WOUND MEMBRANE MODULES WITH 3D PRINTING WEN SEE TAN Singapore Membrane Technology Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore CHEE KAI CHUA Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore TZYY HAUR CHONG Singapore Membrane Technology Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore School of Civil and Environmental Engineering, Nanyang Technological University, Singapore JIA AN Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore ABSTRACT: Feed spacer is a mesh-like structure placed between membrane sheets to create channels for fluid flow in a spiral wound membrane module (SWM). It has an important role in the hydrodynamic conditions of a SWM, which serves to facilitate mass transfer in the feed channel by generating vortex and promoting mixing. However, the challenges of commercial feed spacers include the trade-off between mass transfer and pressure drop along the channel that leads to the rise in energy demand as well as their impact on membrane fouling. With the advent of 3D printing, there is greater design freedom for the development of novel spacers. This paper focuses on the design and optimization of a novel spacer for SWM via 3D printing to maximise mass transfer while minimising pressure drop and membrane fouling. Due to the capability of 3D printing to rapidly prototype complicated and intricate structures, a series of existing, modified and innovative spacer structures against commercial feed spacers were designed, printed and examined to identify the basis form of spacer structure with the greatest potential. Eventually, the sinusoidal flutter designs proved to generate higher flux, lower pressure drop and higher mass transport in contrast to the commercial spacer. Therefore, the sinusoidal design is a potential spacer structure that could surpass the performance of the commercial spacer after further investigation by varying the design parameters to obtain the optimal design. KEYWORDS: Feed spacers, 3D printing, mass transfer, pressure drop INTRODUCTION Additive manufacturing, also known as 3 Dimensional (3D) printing or rapid prototyping is a promising technology to prototype and manufacture complex and intricate structures that Proc. Of the 3 rd Intl. Conf. on Progress in Additive Manufacturing (Pro-AM 2018) Edited by Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng Tor Copyright 2018 by Nanyang Technological University Published by Nanyang Technological University ISSN: :: 650

3 Proc. Of the 3 rd Intl. Conf. on Progress in Additive Manufacturing conventional manufacturing techniques cannot achieve. This has attracted the attention of researchers in water and wastewater treatment industries for the fabrication of intricate components in membrane modules, such as membrane (Femmer, Kuehne, Torres-Rendon, Walther, & Wessling, 2015; Femmer, Kuehne, & Wessling, 2014, 2015) and feed spacers (Fritzmann, Hausmann, Wiese, Wessling, & Melin, 2013; Li, Meindersma, De Haan, & Reith, 2005; Schwinge, Wiley, & Fane, 2004; Schwinge, Wiley, Fane, & Guenther, 2000; Tan, Chua, Chong, Fane, & Jia, 2016). Feed spacer is a mesh-like structure placed between membrane sheets to create channels for fluid flow in a spiral wound membrane module (SWM). It has an important role in the hydrodynamic conditions of a SWM, which serves to facilitate mass transfer in the feed channel by generating vortex and promoting mixing. The mass transfer is critical in reducing the concentration polarisation phenomenon in membrane processes, which is often associated with membrane fouling. However, the challenges of commercial feed spacers include the trade-off between mass transfer and pressure drop along the channel that leads to the rise in energy demand as well as their impact on membrane fouling. Many studies via computational fluid dynamics (CFD) simulations have shown that spacers with complex structures have great potential in enhancing the mass transfer. However, conventional method of spacer manufacturing by extrusion could not realise the complicated designs of spacers. In our previous work (Lee et al., 2016; Tan et al., 2016; Tan et al., 2017; W. S. Tan 2014), we have systematically investigated the capabilities and feasibilities of different 3D printing techniques in the printing of spacers, including 1) the different 3D printing techniques that resulted in differences in spacer geometry and surfaces finish and their effects in spacer performance, 2) the printability of net-typed structures with Selective Laser Sintering (SLS), using polypropylene materials as the representative material for commercial feed spacers. Similarly, in this work, 3D printing technology is employed as an enabling tool to fabricate the novel spacers. With the advent of 3D printing, there is greater design freedom for the development of novel spacers. This paper focuses on the design and optimization of a novel spacer for SWM via 3D printing to maximise mass transfer while minimising pressure drop and membrane fouling. A series of existing, modified and innovative spacer structures against commercial feed spacers were examined to identify the basis form of spacer structure with the greatest potential. The spacer designs were selected to replicate and compare the performance of the 3D printed spacers to the commercial spacer. Novel spacer designs in literature including the zigzag spacer (Schwinge et al., 2000) and milled sinusoidal channels spacers (Xie, Murdoch, & Ladner, 2014) that have shown great potential in achieving better performance than commercial spacer were modified and tested. Protrusions were added into the sinusoidal channels to improve shear rate at the concaving part of the sinusoidal wave by introducing dynamic movements. A new design, the saddle design was also proposed in this work considering the potential of sinusoidal/saddle surfaces in generating steady vortices (Ding, Shi, & Luo, 2011; Niavarani & Priezjev, 2009). MATERIALS & METHOD a) 3D Printing- Equipment and design models (i)fabrication via powder based printing: Selective Laser Sintering (SLS) Polyamide 12 based white powder PA2200 (EOS GmbH Electro Optical Systems, Krailling/München, Germany) was used for printing the feed spacer in the EOSINT P395 Selective Laser Sintering (SLS) system (EOS e-manufacturing Solutions). A carbon dioxide (CO2) laser was used to sinter the powder according to the corresponding 2D cross sections at 651

4 Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng Tor (Eds.) 652

5 Proc. Of the 3 rd Intl. Conf. on Progress in Additive Manufacturing The pressure drop and mass transfer study were conducted in a flat channel crossflow test cell with a channel length of 278 mm and width of 35 mm. Feed spacer replicates printed by either SLS, FDM or Polyjet were placed in the feed channel and a commercial net-type permeate spacer was placed in the permeate channel. Supporting inserts with varying thickness were used for adjustments such that the feed spacer height fitted the channel height. In a typical run, fresh feed solution was circulated at the required flow rate and pressure for about 10 minutes for stabilisation at a temperature of 20 C. Measurements of channel pressure drop by pressure sensors were taken. The mass transfer can be obtained from the osmotic pressure model and the film model (Da Costa, Fane, Fell, & Franken, 1991; Schock & Miquel, 1987). RESULTS AND DISCUSSION According to Da Costa (Da Costa, Fane, & Wiley, 1994), pressure drop along the channels can be attributed to viscous drag on spacer, form drag, kinetic losses due to change in flow direction and viscous drag on channel walls. Figure 2a compares the pressure drop along channels for different type of spacers. The saddle spacer portrayed the largest pressure drop as a result of the closely knitted tortuous flow path which increased the viscous drag on spacer and kinetic losses due to continual changes in flow directions. The conventional net type design exhibited the second largest pressure drop. Following next with lower pressure drop were the zigzag and sinusoidal flutter design. It was also noticed that the pressure drop behaviour of Polyjet and SLS replicates matched that of the commercial spacer. SLS fabricated spacers led to slightly higher pressure drop in contrast to Polyjet printed spacer. Subsequently, considering the trade-off between pressure drop and flux behaviour shown in Figure 2b, SLS zigzag and SLS sinusoidal flutter both performed better than the commercial spacer with higher flux and lower pressure drop. Pressure Drop (kpa) a) b) Slit Slit 60 Commercial Polyjet replicate SLS replicate Polyjet zigzag spacer spacer 45 Polyjet regular saddle Polyjet SLS regular regular saddle saddle SLS Polyjet regular sinusoidal saddle flutter Polyjet SLS sinusoidal flutter flutter SLS sinusoidal flutter Flux (L/m 2 h) Slit Commercial Polyjet replicate SLS replicate Polyjet zigzag spacer spacer Polyjet regular saddle Polyjet regular saddle SLS regular saddle SLS regular saddle Polyjet sinusoidal flutter SLS Polyjet sinusoidal sinusoidal flutter flutter SLS sinusoidal flutter Flow Rate (L/min) TMP (kpa) Figure 2: (a) Pressure drop along the channels for different spacer types. (TMP = 20 kpa and C b = 1 g/l) (b) Flux versus transmembrane pressure (TMP) for different spacers. (Q = 1.0 L/min and C b = 1 g/l) The mass transfer and pressure drop results were subsequently plotted in the dimensionless form represented by Sherwood number and Power number. Using the commercial spacer as the benchmark for comparison of 3D printed replicates and novel spacer designs, the spacers fabricated were considered to bring about an improvement if the Sh were higher than the commercial spacer at a fixed Pn value. From Figure 3, SLS replicate and both SLS and Polyjet 653

6 Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng Tor (Eds.) sinusoidal flutter have shown improvement and better performance than the commercial spacer when Pn is more than Increasing pressure drop increased the mass transfer for all of the spacer designs except for the saddle design. The saddle design demonstrated slight decrease in mass transfer with increasing pressure drop. This was an interesting observation that implied the possible elimination of trade-off which needed to be further validated with CFD modelling. In general, SLS printed spacers were found to achieve greater mass transfer at the same pressure drop compared to Polyjet printed spacers. This trend has also been reported in our previous work (Tan et al., 2017) which attributed the differences due to the surface finishes and the randomness of the parts printed by SLS and Polyjet Commercial Polyjet replicate SLS replicate Polyjet zigzag spacer Polyjet regular saddle SLS regular saddle Polyjet sinusoidal flutter SLS sinusoidal flutter Sh Pn Figure 3: Plot of Sherwood number against Power number for different spacers. ( Q = 1.0 L/min and C b = 1 g/l) CONCLUSION A series of existing, modified and innovative spacer structures from prior literature was fabricated by 3D printing technology and tested against commercial feed spacers and its replicate, to preliminarily identify the basis form of spacer structure with the greatest potential. SLS, Polyjet sinusoidal flutter and SLS replicate have shown improvement in terms of higher flux and lower pressure drop. The sinusoidal flutter designs proved to generate higher flux, lower pressure drop and higher mass transport in contrast to the commercial spacer. Therefore, the sinusoidal design is a potential spacer structure that could surpass the performance of the commercial spacer after further investigation by varying the design parameters to obtain the optimal design ACKNOWLEDGEMENTS This work is supported by National Research Foundation (Singapore) under Environment & Water Industry Programme office (EWI) of the PUB. Singapore Membrane Technology Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University is supported by the Economic Development Board of Singapore. Singapore Centre for 3D Printing (SC3DP) is funded by the National Research Foundation (Singapore). 654

7 Proc. Of the 3 rd Intl. Conf. on Progress in Additive Manufacturing REFERENCES Da Costa, A. R., Fane, A. G., Fell, C. J. D., & Franken, A. C. M. (1991). Optimal channel spacer design for ultrafiltration. Journal of Membrane Science, 62(3), Da Costa, A. R., Fane, A. G., & Wiley, D. E. (1994). Spacer characterisation and pressure drop modelling in spacer-filled channels for ultrafiltration. Journal of Membrane Science, 87(1-2), Ding, L., Shi, W., & Luo, H. (2011). Numerical simulation of viscous flow over non-smooth surfaces. Computers & Mathematics with Applications, 61(12), Femmer, T., Kuehne, A. J., Torres-Rendon, J., Walther, A., & Wessling, M. (2015). Print your membrane: Rapid prototyping of complex 3D-PDMS membranes via a sacrificial resist. Journal of Membrane Science, 478, Femmer, T., Kuehne, A. J., & Wessling, M. (2014). Print your own membrane: direct rapid prototyping of polydimethylsiloxane. Lab on a Chip, 14(15), Femmer, T., Kuehne, A. J., & Wessling, M. (2015). Estimation of the structure dependent performance of 3- D rapid prototyped membranes. Chemical Engineering Journal, 273, Fritzmann, C., Hausmann, M., Wiese, M., Wessling, M., & Melin, T. (2013). Microstructured spacers for submerged membrane filtration systems. Journal of Membrane Science, 446, Lee, J.-Y., Tan, W. S., An, J., Chua, C. K., Tang, C. Y., Fane, A. G., & Chong, T. H. (2016). The potential to enhance membrane module design with 3D printing technology. Journal of Membrane Science, 499, Li, F., Meindersma, W., De Haan, A., & Reith, T. (2005). Novel spacers for mass transfer enhancement in membrane separations. Journal of Membrane Science, 253(1), Niavarani, A., & Priezjev, N. V. (2009). The effective slip length and vortex formation in laminar flow over a rough surface. Physics of Fluids, 21(5), Schock, G., & Miquel, A. (1987). Mass transfer and pressure loss in spiral wound modules. Desalination, 64, Schwinge, J., Wiley, D. E., & Fane, A. G. (2004). Novel spacer design improves observed flux. Journal of Membrane Science, 229(1-2), Schwinge, J., Wiley, D. E., Fane, A. G., & Guenther, R. (2000). Characterisation of a zigzag spacer for ultrafiltration. Journal of Membrane Science, 172(1-2), Tan, W. S., Chua, C. K., Chong, T. H., Fane, A. G., & Jia, A. (2016). 3D printing by selective laser sintering of polypropylene feed channel spacers for spiral wound membrane modules for the water industry. Virtual and Physical Prototyping, 11(3), Tan, W. S., Suwarno, S. R., An, J., Chua, C. K., Fane, A. G., & Chong, T. H. (2017). Comparison of solid, liquid and powder forms of 3D printing techniques in membrane spacer fabrication. Journal of Membrane Science, 537(Supplement C), W. S. Tan, C. K. Chua, A. G. Fane, J. AN, T. H. Chong. (2014). Morphological Comparison of 3D Printed Feed Spacers for Spiral Wound Membrane Modules. Paper presented at the 1st International Conference on Progress in Additive Manufacturing (Pro-AM 2014), Singapore. Xie, P., Murdoch, L. C., & Ladner, D. A. (2014). Hydrodynamics of sinusoidal spacers for improved reverse osmosis performance. Journal of Membrane Science, 453,

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