Environmental Engineering Reference
In-Depth Information
Now to calculate the cold stream C1 outlet temperature from H1-C1 heat exchanger we
divide the load/duty of this hea exchanger, 1900 kW, on cold steam C1 CP(FCp) to get the
raise in temperature of this stream upon matching it with the hot stream H1. Adding this raise
in temperature to the the cold stream C1 inlet temperature to H1-C1 heat exchanger renders
the cold stream C1 outlet temperature equal to 412.5 ºC. Upon the completion of defining the
matches for the two hot streams H1 and H2 at the pinch cold stream C1, we need now to
decide the amount of hot utility needed to allow the cold stream C1 reaches its target
temperature, 550 ºC.
The Grid Diagram for the Step -By-Step HEN Design
Pinch
CP
(kW/ ºC)
310 ºC
QH_min = 2870 kW
Q= 1900 kW
520 ºC
330 º
C
H1=10
Q= 350 kW
380 ºC
300 ºC
H2=5
Q= 2750 kW
550 ºC
300 ºC
412.5 ºC
317.5 ºC
C1=20
380 ºC
320 ºC
C2=2
QC_min = 50 kW
300 ºC
Figure 24. Grid Diagram for C1 matching with hot utility away from the pinch
Figure 24 above shows that; the hot utility unit has a heating load/duty of 2750 kW. Bear in
mind that in order to reach the minimum heating utiltity target the cold stream C2 should not
require more than 2870-2750 kW of heating utility. Otherwise, the network streams matching
to realize the minimum heating utility target will not be adequate.
To enable the cold stream C2 reaches its target temperature we need a hot utility heater with
duty equal to 120 kW as shown in figure 25 below.
Counting the number of units and adding the duties of the two utilities units, we see that
the estimated minimum number of units, above the pinch, and minimum heating utiltity
requirements are both satisfied. Before we move to the network design below the pinch, it is
instructive to note that same result obtained in figure 25 above for the estimated minimum
number of units and minimum heating utility equal to 2870 kW, may be obtained again using
different HEN structure and process engineers are always encouraged to explore; synthesize
and generate more than one HEN structure to study other process objectives satisfaction
