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.

 

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