Published on IDRA Global Connections Winter 2021 Issue

By Yehia M. El-Sayed and Robert S. Silver Yehia M. El-Sayed and Robert S. Silver, 1980, “Fundamentals of Distillation,” in Principles of
Desalination, 2nd ed., K. S. Spiegler and A. D. Laird, eds., Academic Press, New York, Chapter 2.

Nominated by: John H. Lienhard V, PhD, PE Department of Mechanical Engineering Massachusetts Institute of Technology lienhard@mit.edu

El-Sayed and Silver gave me an excellent introduction to thermodynamic analysis of seawater distillation systems and the rationale for multi-stage designs. I found the technical framework surprisingly accessible, and, after brief investigation, I was not surprised to discover that both authors were distinguished mechanical engineers. So, of course the approach made sense to me—they spoke in my native language. Robert S. Silver was the James Watt Professor of Mechanical Engineering at the University of Glasgow, a recipient of the ASME Heat Transfer Memorial Award, and renowned as one of the fathers of MSF technology. Yehia M. El-Sayed had conducted research at many universities (including MIT) and twice received the ASME Edward F. Obert Award for outstanding research in thermodynamics.

I have used this book chapter directly in my desalination class to teach mechanical engineering students about distillation and energy efficiency.
El-Sayed and Silver point out the levers for raising the performance of distillation. First, try to minimize the entropy produced by heat transfer across temperature differences. Second, especially, avoid discarding exergy with the leaving streams. Multistage designs target the latter issue. In a multi-effect evaporator, heat released during condensation at one pressure drives evaporation in a lower pressure section, so that a given quantity of heat can distill vapor over and over again. In multi-stage flash distillation, liquid vaporizes as brine is throttled into successive stages, while the condensing vapor preheats counterflowing feed. The irreversibility of heat transfer across the evaporator stages
of MEE is replaced by throttling irreversibilities in MSF.

The exergetic assessment of water/power coproduction, as discussed in this chapter, remains a topic of research today. Does thermal desalination have an advantage over RO from a primary energy perspective? The challenge in such an analysis is to correctly apportion the
power loss that accompanies steam extraction. El-Sayed and Silver had the basic framing right, but they did not address the likelihood that
extraction would change a turbine’s second-law efficiency. Under their inspiration, my group has recently looked at that question.