Page 167 - Chemical process engineering design and economics

150 Chapter 4

was considered in the past for power plants, the risk is too great. The other liquid
metals are used for cooling nuclear reactors. Temperatures from 50 to 1000 °C
(90 to 1830 °F) can also be achieved by electrical heating.
Since accidental chemical spills occur occasionally, the effect of the heat-
transfer fluid on the environment and health must be considered. Since the use of
chemicals may be governed by laws, the process engineer must comply. In 1979,
the EPA banned the use of polychlorinated biphenyls (PCBs) because of the con-
cern over environmental contamination [12].
The factors numbered three to six can be reduced to economic considera-
tions. Ultimately, the heat-transfer fluid selected will depend on the total cost,
both capital and operating costs. For example, if a heat-transfer fluid meets the
first two requirements, but it is more toxic than other possibilities, then the heat-
transfer system will have to contain extra safety features, increasing its cost. The
heat-transfer fluid will then need to have other compensating features to reduce the
cost of transferring heat.
Organic heat-transfer fluids require stringent leakage control because they
are all flammable from 180 to 540 °C (356 to 1000 °F) [10], and most of the fluids
irritate eyes and skin [9]. Although a nitrate salt mixture is nonflammable, it is a
strong oxidizing agent and thus should not contact flammable materials.
Organic heat-transfer fluids can degrade somewhat, either by oxidation or
thermal cracking. The primary cause is thermal degradation. In thermal degrada-
tion, chemical bonds are broken forming new smaller compounds that lower the
flash point of the fluid. At the flash point, flammable fluids will momentarily ig-
nite on application of a flame or spark. Organic fluids will also degrade to form
active compounds. The compounds will then polymerize to form large molecules
thereby increasing the fluid viscosity, which reduces heat transfer. Heat-transfer
fluids are usually heated in a furnace and then distributed to several heat exchang-
ers in a process. At high temperatures thermal degradation accelerates, forming
coke at the heater surface in furnaces, which eventually leads to heater failure.
Even the most stable fluids will eventually degrade so that some means must be
provided for removal of the degradation products in the design of the system. Al-
ternatively, the fluid could be replaced periodically and the spent fluid sent back to
the producer for recovery.
Generally, a heat-transfer fluid should be noncorrosive to carbon steel be-
cause of its low cost. Carbon steel may be used with all the organic fluids, and
with molten salts up to 450°C (842 °F) [6]. With the sodium-potassium alloys,
carbon, and low-alloy steels can be used up to 540°C (1000 °F), but above 540°C
stainless steels should be used [6]. Stainless steels contain 12 to 30% Cr and 0 to
22% Ni, whereas a steel containing small amounts of nickel and chromium, typi-
cally 1.85% Ni and 0.80% Cr, is referred to as a low alloy steel [6]. Cryogenic
fluids require special steels. For example, liquid methane requires steels contain-
ing 9% nickel. To aid in the selection of a heat-transfer fluid, Woods [28] has
constructed a temperature-pressure chart for several fluids.

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