Page 142 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 127
Exergy analysis provides a good insight of where exergy is lost, the losses often
being caused by:

. combustion of fuel, conversion of chemical energy to thermal energy during
combustion with air results in a large exergy losses:
. a large temperature difference between streams, as in furnaces or boilers
where the flue gas temperature is much higher than the cold stream;
. friction, as in throttling valves or piping;
. efficiency term in turbo machinery;
. mixing of streams at different concentrations and or temperatures; and
. separation efficiencies (low number of effective stages).

The following example shows alternative ways of minimizing exergy losses in a fired
reboiler (Figures 4.36 and 4.37) (Note that a steep temperature curve in the bottom
of the reactor would favor a solution). The following opportunities can be exploited:

Opportunities for exergy /
energy improvement for fired FEED
reboiler
•Install gas turbine in front of
fired reboiler furnace
•Install air preheater
• Replace fired reboiler by
conventional steam reboiler
by: lower pressure operation
Allow more lights in bottom
•Install side reboiler with
electric heater in bottom
•Install side reboiler with
superheated steam reboiler in Air
bottom
Fuel
Fig. 4.36. Alternative options for a fired reboiler to save energy.

Side steam
FEED FEED Side steam
reboiler reboiler

~
Electric Super heated
reboiler steam reboiler

Fig. 4.37. Side reboiler complemented with electrical heater or
superheated steam heater as bottom reboiler.