CATALYST DEACTIVATION

 

The following is a partial list of airstream constituents that can reduce the lifespan of precious metal catalysts.
This discussion is restricted to poisons and masking agents that have a possibility of occurring in soil vapor extraction applications.  It is usually not cost effective to test for poisons having a low probability of occurrence. 

Poisoning and masking
Catalyst poisons and masking agents may be transported to the catalyst if groundwater is run into the equipment. 
It is important to separate water from the airstream.

Masking agents (may be reversible)

  • Silt
  • Mineral salts (may also be poisons)
  • Coke deposits
  • Sulfur

Coke deposits may be produced with certain hydrocarbons when operating at relatively low temperature. 
Sometimes this masking can be reversed by raising catalyst temperature (thermal regeneration).
Sulfur masking may be reversible by operating at temperatures above 400 deg C.
Mineral salts can sometimes be washed off.

Poisons (usually permanent)

  • Iron
  • Zinc
  • Phosphorus
  • Lead
  • Halogenated hydrocarbons

Poisoning is the deactivation of the catalyst due to chemicals in the airstream which cause active catalytic sites to become inactive.  Poisoning is usually permanent.
Deactivation rates are dependent on species, concentration, temperature and time.

Fast acting poisons that may be present at SVE sites are:

  • Chlorinated solvents and other halogen compounds            
  • Lead

Chlorinated solvents:
The effect that Chlorinated solvents have on a precious metal catalyst depends on concentration in the vapor stream. Without any chlorinated solvents, average catalyst lifespan of five years in SVE applications treating Gasoline can be expected. If 1-5 ppm of chlorinated is present in this airstream, catalyst lifespan is minimally reduced.
Above 10 ppmv chlorinated solvents attack the catalyst substrate reducing the number of sites available for reaction, and life-span is significantly reduced. Specially designed catalysts are available for the destruction of chlorinated solvents.

Lead
Catalyst deactivation by lead is typically caused by Tetraethyl Lead (TEL) (organic).
Inorganic lead is less of a problem as it is more readily captured in the blower filter and with effective water separation.

The massive packed bed catalysts in the FALCO 300/600 are less susceptible to deactivation than monolithic catalysts. Catalyst lifespan in soil vapor extraction applications is typically longer than five years.