Richard Casady wrote:
snip

The phase change from liquid to gas
takes place at a constant temperature, the boiling point,

Which isn't a constant temperature, as you address below..

and and the
last degree of heating below that point, is the same as any other
degree, one BTU per pound.

This is true from a physics perspective, but not, unfortunately from an
applications engineering perspective. Since the effectiveness of
whatever heat exchange mechanism you use is proportional to the delta-T
between the process and the exchange medium, each degree of process rise
requires more heat input into the system than the previous one. Not into
the 'process', but into the 'system'.

This, IMO, is the crux of the issue of trying to use engine heat for
evaporation (i.e. distillation), versus just preheating. For an
efficient process, the engine-to-transfer medium exchanger needs to run
with a significant delta-t, and so to does the transfer
medium-to-process exchanger. This two-step cascade would likely require
much higher engine operating temperatures than normal, with all the
attendant maintenance and longevity issues.

In round figures, seven eights of the
energy goes into the evaporation, one eighth to temperature change.

Boiling point varies greatly with pressure, but I assume that we are
all talking about ordinary sea level, 14.7 psi type air.[give or take
changes with the weather]

Casady

Keith Hughes