Page 89 - Sustainability in the Process Industry Integration and Optimization

66 Cha p te r F o u r

As plotted in the figure, the nonisothermal utility’s termination
point corresponds to the ambient temperature. The distance from
this point to the zero of the ΔH axis represents the thermal losses
from using the utility. The heat capacity flow rate of the nonisothermal
utility target is determined by making the utility line as steep as
possible, thereby minimizing its CP and the corresponding losses
(Figure 4.21). Its supply temperature is usually fixed at the maximum
allowed by the furnace and the heat carrier composition; the
remaining degree of freedom corresponds to the utility’s exact CP.
Smaller CP values result in steeper slopes and smaller losses.
The placement for a nonisothermal utility (e.g., hot oil) may be
constrained by two problem features: the process Pinch point and a
“kink” in the GCC at the top end of a heat recovery pocket; see
Figure 4.22 for an example.
When fuel is burned in a furnace or a boiler, the resulting flue
gas becomes available to heat up the corresponding cold-stream
medium (for steam generation or direct process duty). Transferring
heat to the process causes the flue gas temperature to drop as it moves
from the furnace to the stack. The stack temperature has to be above
a specified value: the minimum allowed stack temperature, which is
determined by limitations due to corrosion. If flue gas is used directly
for heating, then the Pinch point, if it is higher, may become more
limiting than the minimum allowed stack temperature. If the
analyzed process features both high-temperature and moderate-
temperature utility heating demands, then flue gas heating may not
be appropriate for satisfying all those demands. If steam is cheaper,

(a) ΔT min = 20 (b) T *
T * 290
290 Minimizing
Tsupply = 300
the CP
Minimizing
the CP
The Pinch does not
need to be limiting
CPmin CPmin

T Return, min T Return, min = 150
=130
Pinch Pinch
120 120
ΔH ΔH
Process Pinch limitation Heat recovery pocket limitation

FIGURE 4.22 Constraints for placing hot oil utilities, ΔT = 20 °C.
min