Page 474 - Advanced thermodynamics for engineers

19.6 PROBLEMS 465

P19.5 Figure P19.5 shows an existing design of a process plant, containing two exothermic
processes. These require streams of reactants as shown in the diagram, and produce products
at the temperatures shown. The plant achieves the necessary conditions by providing 480 kW
of heat from a steam source, and rejects a total of 560 kW of energy to cold water utilities;
only 460 kW is transferred between the streams. Show
(a) that there is a pinch point, and evaluate the temperature;
(b) that the existing plant is inefficient in its use of the energy available;
(c) calculate the energy targets for DT min ¼ 20 C and show a design that achieves these

targets.
+ +
[(a) T C pinch ¼ 110 C; T H pinch ¼ 130 C; (b) Q C ¼ 560 kW; Q H ¼ 480 kW; (c)
¼ 130 kW]
Q C min ¼ 210 kW; Q H min
P19.6 Recalculate the problem in P19.5 using a DT min ¼ 10 C. Comment on the effect of reducing

the minimum temperature difference.
+ +
[(a) T C pinch ¼ 110 C; T H pinch ¼ 120 C; (b) Q C ¼ 560 kW; Q H ¼ 480 kW; (c)
¼ 40 kW]
Q C min ¼ 120 kW; Q H min

Steam
60 kW
30 o 150 o 180 o 70 o 50 o
Process Process
190 o
Heat exchanger
Cooling water
420 kW
660 kW
o
110 160 o
Steam
500 kW
FIGURE P19.7
Network for problem P19.7.

P19.7 A network for a process plant is shown in Fig P19.7.
(a) Calculate the energy targets for DT min ¼ 10 C and show a design that achieves these

targets.
(b) Explain why the existing network does not achieve the energy targets.
¼ 90 kW; (b) there is transfer across the pinch]
[(a) Q C min ¼ 190 kW; Q H min