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Title: 3D printing of feed channel spacers for spiral wound membrane modules
Authors: Tan, Wen See
Keywords: DRNTU::Engineering::Mechanical engineering::Prototyping
DRNTU::Engineering::Environmental engineering::Water supply
Issue Date: 2018
Source: Tan, W. S. (2018). 3D printing of feed channel spacers for spiral wound membrane modules. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Research in the field of 3D printing and water industry, in particular, on the fabrication and evaluation of feed spacers produced via 3D printing, rapid prototyping or additive manufacturing (AM) techniques is presented in this thesis. 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 this work, the 3D printing technology is employed as a tool to fabricate novel feed spacers with structures that maximise the mass transfer while minimise the channel pressure drop. It is nevertheless important to note that there are limitations in current AM technology. A link between the design and fabrication i.e. printability and surface finish properties of spacers was established. The printability of polypropylene (PP) which is the representative material of spacers was also investigated. The fabricated spacers were evaluated in terms of flux and pressure drop. Direct observation through the membrane (DOTM) technique was also used to study the impact of different 3D printed spacers on the critical flux of particles. Among the 3 broad classes of solid, liquid, powder based 3D printing techniques, the liquid based Polyjet technique was found to give the most accurate representation of the intended design while the solid based FDM (Fused Deposition Modeling) technique produced spacers that showed the greatest deviation from the specifications. On the other hand, the improvement in mass transfer by the FDM printed spacer was unexpectedly the greatest among the three 3D printed spacers, due to the ‘randomness’ in the strand of spacer. In addition, the surface finish of the spacers was found to have an effect on the critical flux, specifically, the anisotropic surface finish of FDM and semi-anisotropic surface finish of Polyjet. Biofouling potential was discovered to be correlated to the surface roughness of samples. Among which, Polyjet and SLS printed samples have greater bacteria attachment due to their higher microscale roughness. Eventually, three design considerations for developing 3D printed novel spacers were recommended. A series of existing (i.e., taken from the literatures), modified and innovative spacer structures, compared with commercial feed spacer, were examined to identify the basis form of spacer structure with the greatest potential. Further optimization of the selected structure was conducted to improve the spacer performance. With the primary structure made up of sinusoidal curves and secondary structures consisting cylindrical protrusions along the curve, the novel spacer S12 showed an improvement of 18% at power number, Pn=106, and 25% at Pn= 105 in mass transfer compared to commercial spacer. With this optimal design, a smart multi-material spacer was developed by designing the primary structure with a rigid material and the secondary structures with flexible materials. The multi-material responsive spacer with Tangoblackplus as the flexible material was able to lower the flux decline due to bentonite fouling by 7.6% as compared to the commercial spacer and 5.2% when compared to the single rigid material spacer. This indicates that a spacer with flexible filaments has a beneficial effect on fouling reduction. The flexible properties of the selected section on the spacer allow dynamic movement under varying flow rate, which creates a “sweeping” effect that disrupts the boundary layers and prevents foulants from accumulating near spacer filaments and on the membrane surface, thereby mitigating fouling. These favourable findings have contributed as one of the pioneering work in this interdisciplinary field.
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