Abstract
Abstract A new one-dimensional predictive model for spiral wound modules (SWMs) applied to reverse osmosis membrane systems is developed by incorporating a detailed description of the geometric features of SWMs and considering flow in two directions. The proposed model is found to capture existing experimental data well, with similar accuracy to the widely-used plate model in which the SWM is assumed to consist of multiple thin rectangular channels. However, physical parameters that should in principle be model-independent, such as membrane permeability, are found to differ significantly depending on which model is used, when the same data sets are used for parameter estimation. Conversely, when using the same physical parameter values in both models, the water recovery predicted by the plate-like model is 12–20% higher than that predicted by the spiral model. This discrepancy is due to differences in the description of geometric features, in particular the active membrane area and the variable channel heights through the module, which impact on predicted performance and energy consumption. A number of design variables – the number of membrane leaves, membrane dimensions, centre pipe radius and the height of feed and permeate channels – are varied and their effects on performance, energy consumption and calculated module size are analysed. The proposed spiral model provides valuable insights into the effects of complex geometry on the performance of the SWM as well as of the overall system, at a low computational cost.
Highlights
Reverse osmosis (RO) processes have been widely used in many applications, especially for producing nearly pure water from seawater in desalination plants, and have seen a dramatic increase in their market share in recent years (Elimelech and Phillip, 2011; Fritzmann et al, 2007; Ghaffour et al, 2013; Greenlee et al, 2009; Kim et al, 2009; Malaeb and Ayoub, 2011; Semiat, 2008)
Water passes through a semi-permeable RO membrane at a rate that is proportional to the difference between the external pressure gradient and the osmotic pressure gradient, while salts dissolved in concentrated solution are rejected
The plate-like and spiral models are compared under various operating conditions with respect to the spatial variations obtained from each model as well as overall performance and specific energy consumption
Summary
Reverse osmosis (RO) processes have been widely used in many applications, especially for producing nearly pure water from seawater in desalination plants, and have seen a dramatic increase in their market share in recent years (Elimelech and Phillip, 2011; Fritzmann et al, 2007; Ghaffour et al, 2013; Greenlee et al, 2009; Kim et al, 2009; Malaeb and Ayoub, 2011; Semiat, 2008). In an RO process external hydraulic pressure that exceeds the osmotic pressure difference between two solutions is applied to the side where the more concentrated solution is placed. Water passes through a semi-permeable RO membrane at a rate that is proportional to the difference between the external pressure gradient and the osmotic pressure gradient, while salts dissolved in concentrated solution are rejected. Using RO membrane systems, seawater can be separated into pure water and concentrated brine containing the rejected salts. In SWMs, several sheets of RO membranes and feed and permeate spacers are alternately stacked and wrapped around a perforated centre pipe, forming separate feed and permeate channels. Cross-sectional area of feed channel [m2] Cross-sectional area of permeate channel [m2] Total cross-sectional area of feed channel at entrance [m2] Trans-membrane area [m2] Total membrane area [m2]
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