Elucidating the use of pressure-recovery diagrams for analyzing energy consumption in reverse osmosis desalination

Abstract

The pressure-recovery (PY) diagram used in reverse osmosis (RO) literature to compare energy consumptions in different RO configurations has a flaw of not holding the design flux constant. In this work, the PY diagrams are constructed with the aid of transport models. It is shown that the area underneath the PY curve represents the specific energy consumption (SEC) imposed by design flux and thermodynamics, which may be reduced by improving spatial uniformity in flux. The trend generally observes the equipartition of entropy production theorem. For seawater RO (SWRO) in which pressure drop relative to feed osmotic pressure is small and operation is near the thermodynamic limit, staged designs with interstage booster pumps enable a more uniform flux, thus reducing the SEC. However, for low-salinity brackish water RO (BWRO), improving flux uniformity may lead to a higher SEC as the increased friction loss often outweighs the reduced energy requirement imposed by system flux.