Chemical Engineering March 2023 - 34

ChemTreat
FIGURE 5. Shown here is a sudden failure of a
boiler tube from hydrogen damage. Notice the
thick-lipped fracture, which indicates failure with
little metal loss
important factor in
treatment selection
for these systems.
Activatedcarbon
filtration
may be effective
for organics removal.
Several
are possible for
particulate control,
including direct
filtration. The aforementioned boiler-makeup alkalinity
reduction can reduce CO2 transfer to steam and
the potential for condensate carbonic-acid corrosion.
Alternatively, carbon-steel corrosion is minimized by operating
within a mildly basic pH range. Alkalizing amine
feed for condensate and feedwater pH conditioning is
common. These are small-chain organic molecules with
an ammonia group attached to, or embedded within the
compound (Table 2).
Selecting the best product or product blend is sometimes
difficult, as each compound has a different basicity
and distribution ratio (the tendency for the product
to depart with steam or remain dissolved in the boiler
water). It is desirable to have comprehensive pH control
throughout the network, but a single compound
may not be sufficient to achieve broad results. Recently
commercialized blended amine products can provide
wide-ranging coverage. A thorough analysis of system
design, metallurgy, current chemistry, operating temperatures,
and other parameters is a prerequisite for
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Applications:
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Biologics ∙ Catalysts
Ceramics ∙ Chemicals
Food Ingredients
Herbicides ∙ Minerals
Nutraceuticals ∙ Pesticides
Pharmaceuticals ∙ Pigments
Polymers ∙ Powdered Metals
Proteins ∙ Resins ∙ Vitamins
TABLE 2. COMMON ALKALIZING AMINE
proper program selection, as it is for any other condensate
polishing or treatment choice. Filming amines that
directly protect metal surfaces have also been successfully
applied in some applications, but space limitations
prevent a discussion at this time.
Also note the low feedwater dissolved oxygen (D.O.)
guidelines in Table 1. Depending on the application, D.O.
control can be a thorny issue. For power-generating
units with high-purity makeup and feedwater, research
has shown that a small amount (5-30 ppb) of dissolved
oxygen at the economizer inlet is necessary to minimize
flow-accelerated corrosion (FAC). FAC has been an extremely
concerning issue since the first observed FAC
failure killed four power plant personnel in 1986 [3].
However, in industrial units with lower purity makeup
and condensate return, D.O. must be maintained at
near-zero concentrations to minimize iron and copper
corrosion.
A key piece of equipment is a mechanical
deaerator, which, when functioning properly, will reduce
D.O. to approximately 7 ppb. Additional oxygen reduction
is achieved with reducing agent/oxygen scavenger
feed. Over time, deaerator trays and spray nozzles can
become misaligned or damaged, which may be overlooked
by plant personnel. On-line dissolved oxygen
monitoring is just one part of a comprehensive sampling
and analysis program [4].
Boiler-water treatment
The boiler has the harshest conditions within a steamgenerating
system, and chemistry excursions can easily
lead to rapid corrosion or other problems. In power
boilers, which typically receive consistent high-purity
makeup, the primary issue is pH control to minimize
carbon-steel corrosion. Table 1 outlines the typical mildly
basic ranges that are optimal for many boilers.
Tri-sodium phosphate (Na3PO4) is still commonly
employed for pH control in both industrial and many
power boilers.
Na3PO4 + H2O  Na2HPO4 + NaOH (8)
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Knowledge beyond this basic concept can be lacking
at plants without trained water chemists. A prime
example is in systems where hardness and silica may
periodically or regularly enter the boiler via contaminated
condensate. Without proper chemistry control,
these constituents can form very tenacious calcium and
magnesium silicate scales that are extremely difficult to
remove. In large measure, it was this chemistry that influenced
the development of phosphate treatment with
accompanying hydroxide alkalinity. Phosphate and hydroxide
alkalinity react directly with calcium to produce
calcium hydroxyapatite:
Q ual
191
ity &
Since
1
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Chemical Engineering March 2023

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