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BED
FOULING
What
about Bed Fouling?
The catalyst is permanently poisoned by chlorine and chlorine compounds,
VOC’s, organics, oil and grease, and oxides of nitrogen. Water
will reduce the reactivity of the catalyst substantially. If the
catalyst gets wet, sometimes it can be dried and reactivity returned
to an acceptable level. In most cases the catalyst must be replaced.
See Replacement Catalyst
Effects of NO/NOx
This section applies primarily to CVD applications.
The catalyst functions as both a sorbent and an oxidizing agent.
When an oxidizable species, such as NO, encounters the surface of
the catalyst, it is oxidized to NOx and then adsorbs or binds to
a site on the surface. A corona discharge ozone generator will produce
oxides of nitrogen that are also adsorbed by the catalyst. A generator
operating on air will produce substantially more NO/NOx than a unit
operating on oxygen.
There are two modes of NOx binding in the catalyst:
Secondary adsorption is a physical binding or molecular entrapment
within the pores of the catalyst. This type of adsorption occurs
at lower temperatures, typically close to ambient temperatures.
This would occur at the start up of the unit.
Primary adsorption is promoted at higher temperatures (200C +) and
is determined by chemical affinity and is very specific. The oxides
of nitrogen do not have a tendency
to desorb, except in specific cases.
When the destruct unit is new, it kills ozone and the inlet end
of the destruct unit runs at a high temperature. This promotes primary
adsorption of the NOx in the first few inches of the bed. This NOx
is permanently adsorbed and in turn reduces the
reactivity of the catalyst.
As the inlet end of the destruct unit "fouls", ozone is
destroyed further down the length of the unit. Because of the change
in the nature of the catalyst (i.e., the inlet layers may not be
totally dead, but less reactive), the destruction of ozone is not
concentrated in the first several inches of the bed. In other words,
the ozone is partially destroyed at the inlet of the bed, but not
completely, due to the loss of reactivity. The completion of the
reaction takes place in the middle of the bed. This would result
in a lower temperature
which promotes the secondary adsorption of NOx.
After a certain amount of time, the NOx that is held in place by
secondary adsorption sloughs off as the pores become plugged and
can "hold" no more NOx. The concentration would come to
some equilibrium with the feed gas to the destruct unit after operating
for a time, based on the inlet concentration of NOx. If primary
adsorption of NOx were occurring throughout the length of the unit,
it would eventually not kill the ozone.
The recommended solution for NOx contamination in the ozone destruct
unit is to replace the catalyst bed. |
