There are three main catalytic failure modes:
- Poisoning, which is usually irreversible.
- Fouling, which is usually reversible.
- Thermal degradation, which is irreversible.
Catalytic Poisoning. Several substances -- antimony, arsenic, bismuth, calcium, chromium, copper, iron, lead, magnesium, mercury, nickel, phosphorous, potassium, silicon, sodium, sulfur, tin and zinc -- are known to poison precious metals catalyst. If any catalyst poisons are in the effluent stream, the useful life of the catalyst will be reduced.In the process streams from the energy-reduction study, none of these catalyst poisons were present.
Catalyst Fouling. Most air-quality jurisdictions require that the oxidizer be at incineration temperature before the process can be started. However, there are some known installations where the process can be started before the oxidizer is at incineration temperature. Fouling usually occurs when a catalytic assembly is started up with dirty effluent flowing and the catalyst temperature is below the prescribed incineration temperature.
If the process can be started before the oxidizer reaches incineration temperature, the routine must be changed so that the oxidizer is at incineration temperature before the process starts effluent flowing This procedure essentially eliminates the catalyst fouling problem.
In the East Coast plant fuel usage study, the thermal oxidizers were at incineration temperature before the process was started.
Catalyst Thermal Degradation. Thermal degradation of the catalyst occurs when the temperature rises to the point where molecules of the catalyst agglomerate. The total amount of catalyst remains the same, but the surface area decreases, which in turn decreases the catalyst activity level. The higher the temperature, the greater the molecular agglomeration. This cannot be reversed.
To guard against thermal degradation, the inlet temperature to the catalyst bed is monitored on a continuous basis. If the outlet temperature from any online thermal oxidizer exceeds the catalyst thermal degradation temperature minus 50°F (28°C), the controller will switch the diverter gate at that oxidizer to its existing outlet stack.