Page 259 - Sustainability in the Process Industry Integration and Optimization

236 Cha p te r T e n

is greater than that for low-temperature utilities. The temperature of
flue gas for direct heating ranges from 800°C to 160°C. Steam usage
and generation are isothermal utilities (up to four levels between
100°C and 340°C). A heat pump or heat engine can be introduced.
Additional data for the problem:
• Condensers and reboilers of distillation columns A and B
involve only phase changes.
• In the condenser of distillation column B, total condensation
is assumed (i.e., the outlet is in the liquid phase only).
• An endothermic reaction takes place in reactor R1. A heating
medium (HM) is necessary to keep the isothermal condition
in R1.
Some properties of the streams are as follows:

F1: Boiling point = 120°C, ΔH = 990 kJ/kg,
vap
C = 4.5 kJ/kg·°C
p,vap
F2: Boiling point = 125°C, ΔH = 1250 kJ/kg,
vap
C = 7.5 kJ/kg·°C,
p,vap
C = 15 kJ/kg·°C
p,liq
FR: C = 5.0 kJ/kg·°C
p,vap
HM: C = 25 kJ/kg·°C
p,liq
P1: Boiling point = 270°C, ΔH = 1556 kJ/kg
vap
P2: Boiling point = 200°C
Problem 5: Solutions
Answer to (a). Stream data needed to perform a Pinch Analysis is reported in
Table 10.14.
Answer to (b). For the base case, Q = 4828 kW, Q = 3366.4 kW, and process heat
H C
recovery = 3530 kW. The targets are Q = 2642.6 kW and Q = 1958.6 kW, and
H C
process heat recovery = 5715.8 kW. The base-case design requires much more
utility consumption. This is because some heat is transferred across the Pinch
and also because there is some unnecessary utility use. For example, some
amount of heat is needed in the column B reboiler for heating up P1, and this
amount must be taken out again when P1 needs cooling.
Answer to (c).
• The cold streams below the Pinch are F1, F2, and FR. These streams can
be heated up by P1 (parts below the Pinch), P2, and by the condensers of
columns A and B. In the base case, stream P1 is used to supply heat to
stream F2, resulting in heat transfer across the Pinch. Instead, to heat up
stream F2 at its higher temperature range, heat exchange with stream P2
can be employed; for the lower-temperature part of F2, the heat from the
condenser of column A can be recovered.
• Above the Pinch, P1 can be used to heat up parts of the column A
reboiler or of the heating media of reactor R1.