Energy Evaluation of Dynamically Operated Reverse Osmosis Systems

Water desalination by Reverse Osmosis is energy intensive and the reduction of its energy demand is one of the main topics in current research. A disadvantage of membrane processes is the formation of a concentration boundary layer at the membrane, resulting in an increased osmotic pressure that needs to be overcome. Applying pulsating flows is a possible approach to disturb this boundary layer. The purpose of this work is to investigate the impact of pulsating flows on the mass transfer in spacer-filled channels and thus on the energy demand of Reverse Osmosis systems. To evaluate the energy demand of applying pulsating flows compared to conventionally operated Reverse Osmosis systems, three main issues have to be investigated: damping, pressure loss and mass transfer across the membrane. The first two parameters were experimentally determined. Computational Fluid Dynamics simulations were used to identify the predominant phenomena for mass transfer enhancement. A correlation between the Sherwood number and excitation frequency, amplitude ratio, average Reynolds number, and permeate flux was developed. This correlation was used for a developed quasi-2D simulation tool for spiral-wound modules to transfer the findings to an entire Reverse Osmosis system. A key finding of this work is that pulsating flows in spiral-wound modules can significantly improve the mass transfer due to strong perturbations of the concentration boundary layer. The enhancement increases with the amplitude and excitation frequency applied. However, damping rates and pressure losses also increase. These tendencies have opposite impacts on the specific energy demand. Therefore, optimum dynamic parameters needed to be found. For brackish water systems, the increased mass transfer could not compensate the additional energy demand. Therefore, pulsating flows can be considered as a potential way to improve the performance of Reverse Osmosis systems.