Page 246 - Sustainability in the Process Industry Integration and Optimization

E x a m p l e s a n d Ca s e S t u d i e s 223

cooler through a heat recovery exchanger. The temperatures, which will vary
with the amount of heat shifted, are also indicated in the figure.
The expressions for the variation in the affected heat loads and temperatures
in response to a shifted load of X kW are displayed in Eqs. (10.1)–(10.8) (notation
as in Figure 10.4):
-
Cooler C3 load: Q C3 = 5970 X (10.1)
Temperature of stream C1: T C11 = 247+ X /100 (10.2)
Heater H8 load: Q H8 = 5300- X (10.3)
+
Exchanger E1 heat load: E = 14700 X (10.4)
E1
-
Temperature 1 of stream H1: T = 190 X /100 (10.5)
H11
Output temperature of C2: T = 164 (10.6)
C2,out
Input temperature of C2: C2,in = 35 (10.7)
T
Temperature 1 of stream H1: 99.7 X /100 (10.8)
Answer to (b)(2). The temperature differences should be checked at the affected
heat exchangers, which are E1 and E2. As can be seen, the heat capacity flow
rate (CP) values for both streams in E1 are equal to 100 kW/°C. Exchanger E2’s
hot stream has a higher CP value than that for the cold stream, so the smaller
temperature difference will be at its hot end. This allows one to calculate the
maximum amount of the load X to shift by solving a few inequalities. For
process exchanger E2, the temperature difference should not be less than
ΔT , and this determines the value of the maximum heat load that can be
min
shifted:
T – T ≥ ∆T (10.9)
H11 C2,out min
190 – X/100 – 164 ≥ 10 (10.10)

X≤ 2600 kW (10.11)
For this value of load shift, the exchanger E2 will be pinched at its hot end as
follows: T = 174.0°C and T = 164.0°C. The temperatures at the cold end
H11 C2,out
will be T = 83.7°C and T = 35.0°C.
H12 C2, in
Answer to (c)(1). Total heating requirement for the existing network:
Σ Q = Q + Q = 27,700 kW. Total cooling requirement for the network:
H H8 H9
Σ Q = Q + Q + Q = 21,345 kW. There is no inappropriate use of utilities.
C C3 C6 C10
Compared with the targets, both total utility heating and total utility cooling
are higher by 9920 kW, which is due to cross-Pinch heat transfer.
Answer to (c)(2). The Process Pinch is at 150°C for hot streams and at 140°C for
cold streams. Comparing the network temperatures with the Pinch location
yields a list of exchangers that violate the Pinch; see Table 10.2.