Chemical Engineering January 2017 - 31

bination with a higher yield during the service
lifecycle, resulted in a significant 20%
increase in productivity over the graphite
heat exchanger.
Smaller tube dimensions
The thickness of tube or pipe walls in
phosphoric-, nitric- and sulfuric-acid
applications is determined primarily by
corrosion allowance, and less so by mechanical
strength. This is because high
pressures occur less in these processes
than in other fertilizer-production methods,
such as urea applications, where
material strength is more of a factor.
Whereas the brittle structure of graphite
necessitates thick walls, high-chromium
alloys make smaller tube dimensions
possible due to the material's enhanced
anticorrosion properties, thereby enabling
more tubes and thinner walls in
heat-exchanger applications.
It should be noted that redesign by a
specialist is necessary in order to take
full advantage of high-alloyed materials,
such as high-chromium austenitic
stainless
steels. Although there have
been a limited number of cases where
high-chromium tubing has experienced
increased corrosion, with proper design
and operation of the heat exchanger,
it has proven possible to diminish
this concern.
Nitric acid processes
In nitric acid (HNO3) processes, chromium
also has a predominant effect
on the corrosion resistance of stainless
steels. Much of the nitric acid
produced worldwide is used in the
production of fertilizers, which the industry
typically uses at a concentration
F-, %
1.5
H3PO4 = 70%
H2SO4 + 4%
Fe3+ = 0.45%
Corrosion rate below the curves is less
than 0.3 mm/a (12 mils/yr)
1.0
High-chromium alloy
0.5
Alloy 825
Alloy 904L
Alloy 20Cb3
200
400
acid condensates, so almost all equipment
that comes into contact with
the process
600
800
CI-, ppm
media is made from
stainless steels.
The most commonly used stainless
steel for the wetted parts of nitric-acid
service is standard-grade 304L - often
regarded as the " workhorse " of the industry.
Despite the grade's popularity,
its performance in service can vary
considerably between different producers,
mainly because standard specificaFIGURE
5. The combined effect of chloride
ion content and free fluoride content on
alloys' corrosion resistance at 100°C
indicates the increased tolerance levels of
alloys with higher chromium content
It should be noted that redesign by a specialist is necessary in order to take full
advantage of high-alloyed materials, such as high-chromium austenitic stainless steels
of 60-65%. Nitric acid is a strong acid,
yet only becomes extremely corrosive
when it is processed in high concentrations
or at high temperatures. During
the nitric acid process (Figure 7),
ammonia is oxidized to produce nitric
oxide, then oxidized further to form nitrogen
dioxide and then absorbed in
water to finally give a solution of HNO3.
Nitric acid service is very corrosive,
due mainly to the presence of nitric
tions allow for very generous limits of
critical impurities like carbon, sulfur and
phosphorus. These impurities can segregate
in grain boundaries in standard
304L heat-exchanger tubing and shells,
thereby increasing the risk of intergranular
corrosion. This has led to instances
of costly downtime and maintenance in
nitric acid plants. In order to avoid these
consequences, good process controls
are necessary. Controls can help to enCHEMICAL
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Chemical Engineering January 2017

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