EFFECTS OF MOISTURE ON SULFURIC ACID CATALYSTS
Prepared by: Dr. Atis Vavere
Catalyst Consultant
MECS, Inc.
Vanadium-based sulfuric acid catalysts are subject to a variety of chemical poisons. Moisture
(liquid water, water vapor, or acid mist) serves as a significant source of catalyst deactivation
and degradation. As moisture attacks the catalytic material, it reacts with the excess SO3 present,
forming sulfuric acid. Therefore, it is not necessary for actual acid mist to be present to generate
acid damage in sulfuric catalysts. This report will document the effects of moisture attack on the
activity of sulfuric acid catalysts.
Moisture will generally not affect sulfuric acid catalysts if exposure occurs above the typical dew
points of sulfuric acid (150-200oC) and the water concentration is low1,2. However, if moisture
or acid exposure takes place at low temperatures and/or high concentrations, severe chemical and
physical changes in the catalyst will occur which will result in catalyst deactivation and
structural degradation1. Duecker and West have also indicated that catalyst will generally have
long life “…if not otherwise abused, i.e., overloaded or exposed to acid spray or condensation”3.
There is a strong body of evidence that supports the deactivation of sulfuric acid catalysts due to
moisture exposure.
The deactivated catalyst can be characterized in several ways. If fully-activated, gold sulfuric
acid catalyst is exposed to high humidity at room temperature, some pale green color will slowly
appear on the catalyst surface. This color is due to the reduction of a small portion of the
vanadium from V+5 (gold) to V+4 (green). This minimal reduction does not structurally change
the catalyst and hence is fully reversible. The V+4 species is undesirable as it provides less
conversion capability than the V+5 vanadium and hence its concentration should be minimized.
A pale green catalyst can be fully generated by simply exposing the rings/pellets to a converter
gas environment (SO2/SO3/O2) at temperatures greater than 400oC. If, however, moisture or acid
attack is significant, a variety of detrimental mechanisms begin to operate on the catalyst.
One of the most dramatic effects of large “doses” of moisture (acid) is the generation of low
valent vanadium which is very difficult to re-oxidize and re-incorporate into the molten salt.
These catalysts have a dark blue or black appearance. The dark color is due to V+3 species in the
melt environment. This type of vanadium compound is quite insoluble in the molten salt and is
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nearly inactive toward the SO2 oxidation reaction. Under converter conditions, it is very
difficult to re-oxidize this low valent species back to the desired V+5 moiety.
There are also observed structural changes in the “heavily moisture-dosed” sulfuric catalysts.
Electron microscopy photos of both fresh (yellow/gold) catalysts and severely deactivated
(blue/black) rings show actual catalyst support changes due to the moisture/acid exposure.
Based on these electron micrographs, it is clear that the deactivated ring material shows much
less support fine structure than the fresh catalyst, indicating a severe loss of external surface
area. Large dark areas in the “deactivated” photos are pools of “blue” solidified molten salt. At
reaction temperatures, this molten salt will not be as active as the fresh material because of the
aforementioned reduction in vanadium valence to the +3 and/or +4 states. Examination of the
fresh and deactivated ring catalyst in the standard activity reactor showed an irreversible 25%
loss in activity for the moisture-damaged catalyst. In fact, a literature report of moisture effects
on sulfuric catalysts indicated a 50% decline in activity which was not recoverable even by firing
to 600oC4. Extensive examination of numerous plant catalyst samples over several decades
verified this deactivation due to moisture exposure.
There is another chemical effect of moisture/acid on the sulfuric acid catalyst. The presence of a
significant amount of moisture or acid mist will result in the leaching of both potassium (K) and
vanadium (V) from the internal pore structure to the surface. This leaching can hence expose
more of the active species to an environment where the vanadium can become deactivated.
Furthermore, the Alkali (K,Na,Cs) is leached more readily than V and hence, the internal molten
salt stoichiometry is upset to where insufficient potassium is available to produce the optimally
active molten salt. Also, when the actives are leached to the surface of the catalyst pellet/ring,
there is a greater possibility for loss of K and V to abrasion or volatilization via reaction with
contaminants in the gas stream. All of these chemical effects of moisture and acid attack lead to
a less active sulfuric acid catalyst. Therefore, it is imperative that exposure of the converter
catalysts to high levels of moisture or acid mist be strictly avoided. If severe exposure does
occur, either catalyst replacement or supplementing the existing catalyst with additional fresh
product will regenerate the overall activity of the converter bed.
It is also important to consider the effects of low temperature (ambient) exposure to humidity in
the surrounding air as would be experienced under poor storage conditions. The sulfuric acid
catalyst color changes from bright gold to green, then pale green, then tan, and finally pale blue
with some red crystals “sprinkled” on the surface. Under these slow-acting conditions, even the
“tan” stage (which may be pale yellow or off-white) catalyst activity is recoverable by
resulfation in service as long as the mechanical strength is maintained. The catalyst durability
can be roughly tested by crushing a ring between the thumb and index finger. Once the
humidity-exposed catalyst reaches the pale blue or white stage, the catalyst is destroyed and
should be discarded.
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Finally, it has been shown in a number of instances that moisture and/or acid mist attack on
sulfuric acid catalyst severely degrades the structural strength of the catalyst1,2. The attacking
acid degrades the chemical bonds which hold the various small support particles (5-20 microns)
together and hence weakens the pellet/ring. With large moisture/mist loadings, severe screening
losses have been observed. Again, in order to preserve both the activity and structural integrity
of the sulfuric catalyst, the moisture/acid exposure must be minimized.
1
Donovan, J. R., Stolk, R. D., and Unland, M. L., Applied Industrial Catalysis, Academic
Press; Volume 2 (1983), p. 275.
2
Sander, U. H. F., Fischer, H., Rothe, U., and Kola, R., Sulfur, Sulfur Dioxide and
Sulfuric Acid, British Sulfur Corporation (1984), p. 286.
3
Duecker, W. W. and West, J. R., The Manufacture of Sulfuric Acid, Reinhold
Publishing (1959), p. 183.
4
Michalek, J., Sbornik vysoke skoly chickotechnologicke v Praze, B25 (1980), pp. 57-65
(Czech.).
January, 1999
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