Page 85 - Sustainability in the Process Industry Integration and Optimization

62 Cha p te r F o u r

cheapest and most effective combination of the available utilities
(Figure 4.17).
To assist with this choice and to enhance the information
derived from the HCC and CCC, another graphical construction
has been developed, known as the Grand Composite Curve (GCC)
(Townsend and Linnhoff, 1983). The heat cascade and the PTA
(Linnhoff and Flower, 1978) offer guidelines for the optimum
placement of hot and cold utilities, and this allows one to determine
the heat loads associated with each utility. For the previous sections,
the assumption has been that only one cold and one hot utility are
available—albeit with sufficiently low and high temperatures to
satisfy the cooling and/or heating demands of the process. However,
most industrial sites feature multiple heating and cooling utilities
at several different temperature levels (e.g., steam levels, refrigeration
levels, hot oil circuit, furnace flue gas). Each utility has a different
unit cost. Usually the higher-temperature hot utilities and the
lower-temperature cold utilities cost more than the ones with
temperatures closer to the ambient. This fact underscores the need
to choose a mix that results in the lowest utility cost. The general
objective is to maximize the use of cheaper utilities and to minimize
the use of more expensive utilities. For example, it is usually
preferable to use low-pressure (LP) instead of high-pressure (HP)
steam and to use cooling water (CW) instead of refrigeration. The
Composite Curves plot in Figure 4.7 provides a convenient view for
evaluating the process driving force and the general heat recovery
targets. However, the CCs are not useful for identifying targets
when multiple utility levels are available; the GCC is used for
this task.

Boiler House and Power Plant
Fuel
Fuel HP
Power Power
Steam MP Steam Gas
Levels turbine
turbine
LP
BFW
preheat Heating Heating Heating Heating
Heating
(Q+W)
Processes, building complexes
Heating Heat
pump
Cooling Cooling Power
(W)
Cooling (Q)
Furnace Refrigeration
Air
Fuel preheat Heating Power
FIGURE 4.17 Choices of hot and cold utilities (amended after CPI 2004 and
2005).