Because of their low thermal inertia, catalytic systems heat up
quickly and are therefore useful for applications in which the plant
regularly runs for less than 24 hours per day.
For catalytic oxidation, the exhaust gas is preheated to 300 to 400°C
depending on the solvent type and the degree of conversion required.
Catalysts, however, have maximum operating temperatures of typically
600 to 650°C. The permissible rise in temperature due to the oxidation
reaction therefore limits the maximum concentration of solvent which
can be handled.
The activity of the catalyst can be quickly destroyed by catalyst
poisons, which include phosphorus, silicon and heavy metals. Dust
and sulphur compounds can also reduce the activity and, although
they can be removed by washing, they introduce an unwelcome additional
maintenance requirement.
The quantity of catalyst in an oxidation system is directly proportional
to the total volume of exhaust gas being handled. On large plants (handling
more than 10,000 m3/h), the proportion of the capital cost attributable
to the catalyst is very significant. Consequently, there is often a
reluctance to use a catalytic system for large flows because:
- the risk of poisoning or contamination may not always be
quantifiable, and
- the system may not be suitable for a future change in solvent
concentration or solvent type.
The catalyst also has a limited life of 5 to 10 years, and its replacement
represents a significant operating cost.
A catalytic system is therefore applicable in situations where:
- the flow rate is fairly low, and
- the solvent concentration is fairly low, and
- the solvents are clearly identified and suitable
for catalytic treatment, and
- there is no dust or catalyst poison present in the
exhaust gas.
