Cascaded design of desalination systems: A graphical approach

Abstract

A graphical technique to design staged desalination systems is introduced in this work by utilizing the concept of composite curves. Material recycle networks (MRNs), which are very popular graphical tools, also rely on the use of composite curves. MRNs involve sources and sinks that are independent and, hence, require two separate curves. The use of composite curves for designing cascaded desalination systems is very different, since it has to be handled as a "composite loop", due to the interdependence between the streams involved. A standard desalination stage behaves as one process sink (the feedwater stream), which in turn is automatically associated with two different process sources (the potable water stream and the brine stream). In a cascaded design, a sink may then become a source, as more stages are added. The use of the terms "source"and "sink"in this work is merely to indicate that a stream is flowing in or out of a stage. Various factors that affect the design and performance of desalination systems, such as water recovery and salt rejection, must be carefully considered when plotting a composite loop. The ability to visualize all those aspects using graphical tools provides great advantages for cascaded desalination systems. Moreover, identifying the point at which adding an extra stage no longer enhances system performance, in terms of permeate and/or brine quality, can be easily identified.