Page 137 - Design of Simple and Robust Process Plants

122 Chapter 4 Process Synthesis and Design Optimization
. A decrease in utility requirement can be realized by the application of the so
called Plus/Minus principle by:
± An increase the total hot stream heat duty above the pinch
± A decrease the total cold stream heat duty above the pinch
± A decrease the total hot stream heat duty below the pinch
± An increase the cold stream heat duty below the pinch (Smith, 1995)

Optimization of the process design to result in lower energy targets can be achieved
by exploring several design options (Smith and Linnhoff, 1988), including:

. Adapting temperature conditions of the process streams by changing pres-
sure conditions or compositions to enable cross-exchange of process streams
around the pinches (utility as well as process pinches). Shift hot streams
from below the pinch to above the pinch, and shift cold streams from above
the pinch to below the pinch.
. Energy system guidelines:
± Placement of heat pumps, including refrigeration cycles, across a pinch.
± Consider that multi-level cooling, like multi-level heating, is applied.
± Maximize power generation by installation of a gas turbine in front of a fur-
nace or fired boiler or recovering power at pressure let down stations.
± Placement of absorption cooling across a utility pinch but the applied heat
source should low level heat from below the process pinch. Comparison of
an absorption and compression refrigeration systems in a grand composite
curve are shown in Figure 4.32, illustrated with permission of IChemE 1997.

Temp
Process heat
source Q abs
Ambient Pinch
Absorption sink
ref. system Q abs
T amb
Q ref Q ref Enthalpy
Temp
Power
Ambient Pinch
Compression sink
ref. system
T amb
Q ref
Q ref
Enthalpy

Fig. 4.32. Comparison of an absorption and
compressor refrigeration system in a grand composite
curve (IChemE 1997).