Hydrocarbon Processing - March 2022 - 87

Water Management
Heat exchangers E-1 through E-4 (TABLE 1) are integral with a
reactor cooling system where it is desirable to maintain isothermal
conditions. Heat exchangers E-5 and E-6 require a higher
supply pressure due to their high (40 m) elevation from grade.
Heat exchangers E-1 through E-6 have a low (< 2°C) temperature
rise; however, the combined cooling water flow for these services
is nearly 50% of the total requirement, thus heat exchangers E-1
through E-6 are suitable candidates as primary users.
Additionally, it is essential that the selection of primary and
cascaded services be coordinated with the thermal design of the
heat exchangers. A cooling water balance based on the cascaded
network is provided in TABLE 3. The accompanying process
schematic (FIG. 6) provides the pressure, temperature and flowrates
for each header in the network.
The cooling water demand in the primary header is generally
greater than (or equal to) the demand in the cascaded
header. A pressure-controlled bypass (FIG. 6), which diverts
flow from the cascaded header directly to the return header,
may be required to maintain the water balance for users on the
cascaded water circuit. In rare cases where the demand for the
cascaded cooling water circuit is higher, it may be necessary to
implement a separate pressure-controlled auxiliary let down
(not shown) from the primary header to the cascaded header
to meet the full water demand for cascaded users.
Optimization. While the cascaded network can significantly
reduce cooling water circulation, the system can still be further
optimized. For example, the inclusion of a dedicated auxiliary
network for heat exchangers at higher elevations (E-5
and E-6) would lower the supply pressure and reduce the
excess cooling water flow to other services. Further improvement
may be possible by rearranging some exchanger services
between the primary and cascaded headers, revising thermal
designs for improved logarithmic mean temperature differences
(LMTDs), and redesigning heat exchanger geometries
for more uniform pressure drops.
On further inspection of the cascaded cooling water balance
(TABLE 3), it can be observed that no excess cooling water remains
in the cascaded header. This may create operational issues
due to minor fluctuations in cooling water flow and or temperature.
As such, any reduction of cooling water to exchangers
E-1 through E-6 will create a shortfall in the cascaded header.
TABLE 5. Cascaded cooling water balance with pressure control
Item No.
Primary header,
kh/hr**
CWS from BL
E-1
E-2
E-3
E-4
E-5
E-6
E-7
E-8
E-9
FIG. 8. Cascaded network with auxiliary pump.
E-10
E-11
E-12
E-13
E-14
E-15
E-16
E-17
E-18
E-19
E-20
E-21
E-22
E-23
E-24
E-25
E-26
E-27
E-28
E-29
Bypass fl ow
CWR to BL
FIG. 9. Cascaded network with pressure control.
11,383,333
-1,936,667
-1,935,688
-1,933,172
1,933,683)
-1,188,100
-113,000
-52,001
-9,949
-52,769
-246,953
-55,600
-299,688
-69,298
-4,567
-10,846
-35,467
-11,778
-23,156
-1,066,051
-486,147
1,066,051
486,147
-2,373,013
-2,780,024
-2,780,024
-715,089
-715,089
-78,048
-167,789
-257,824
-36,725
-494,636
2,373,013
2,780,024
2,780,024
715,089
715,089
78,048
167,789
257,824
36,725
494,634
-11,383,333
Negative values (-) represent outfl ow, while positive values represent infl ow.
Hydrocarbon Processing | MARCH 2022 87
Cascaded header,
kg/hr**
1,936,667
1,935,688
1,933,172
1,933,683
1,188,100
113,000
52,001
9,949
52,769
246,953
55,600
299,688
69,298
4,567
10,846
35,467
11,778
23,156
Return header,
kg/hr**
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