Computational Study of Zigzag Spacer Design with Elliptical Cross-Section Filaments

Gohar Shoukat,Farhan Ellahi,Emad Uddin,Muhammad Sajid,M El Ganaoui,B Liu,R Bennacer,M Darcherif,S Morsli

doi:10.1051/matecconf/202030701047

Abstract

The large energy consumption of membrane desalination process has encouraged researchers to explore different spacer designs using Computational Fluid Dynamics (CFD) for maximizing permeate per unit of energy consumed. In previous studies of zigzag spacer designs, the filaments are modeled as circular cross sections in a two-dimensional geometry under the assumption that the flow is oriented normal to the filaments. In this work, we consider the 45° orientation of the flow towards the three-dimensional zigzag spacer unit, which projects the circular cross section of the filament as elliptical in a simplified two-dimensional domain. OpenFOAM was used to simulate the mass transfer enhancement in a reverse-osmosis desalination unit employing spiral wound membranes lined with zigzag spacer filaments. Properties that impact the concentration polarization and hence permeate flux were analyzed in the domain with elliptical filaments as well as a domain with circular filaments to draw suitable comparisons. The range of variation in characteristic parameters across the domain between the two different configurations is determined. It was concluded that ignoring the elliptical projection of circular filaments to the flow direction, can introduce significant margin of error in the estimation of mass transfer coefficient.

Highlights

Reverse Osmosis desalination technique has gained pace over the years due to advancement in technology and abundance of sea-water, especially in the Middle East
The process utilizes high pressure fluids being pumped into devices that employ hydrophilic membranes to filter out the water into a permeate channel and retaining the concentrated water in the retentate channel
Among several forms of modules used to carry out this process, spiral wound membrane (SWM) stands quite popular for industrial use because of its high membrane packing density compared to rectangular channels

Summary

Introduction

Reverse Osmosis desalination technique has gained pace over the years due to advancement in technology and abundance of sea-water, especially in the Middle East. The use of turbulence promoters was initiated in 1965 [5] and used for the first time a ladder type configuration of spacers in the channel He reported significant gains in permeate flux and that the increased energy requirement due to enhanced pressure drop across the channel because of the spacers is outweighed by the increased mass transfer and permeate flux obtained as reported by Thomas et al [5, 6] and Da Costa et al [7]. Da Costa et al [11] conducted experiments on several different types and orientation of spacer configuration and reported that incoming flow at an angle to the horizontal of the spacers has greater mass transfer compared to one that comes at zero angle They utilized circular strands instead of rectangular ones in their studies and concluded that the zigzag configuration shown in Figure 1 is the most efficient configuration of spacer designs. This results in the minor axis length equal to the original radius, but the major axis is a function of the radius and the cosine of the projection angle

Geometry

Mathematical modelling

Results and discussion

Conclusion