visbreaker furnace

[rgrokkam]

I read something like this "....unsuccessful in steam air decoking of visbreaker furnace. An iron oxide layer had formed on top of coke and shielded it. Reduction step with
hydrogen is now often used prior to final steam air decoking."
Can anyone explain-
1. how iron oxide forms above the coke layer
2. How can we ascertain ironoxide layer formation.
3. Will the reduction step help?

Many thanks

 

[25362]

Some thoughts:

When referring to air/steam coke burnoff for catalyst reactivation in dehydrogenation reactors, I've read that it is not rust powder (rouge) Fe2O3, but its reduced cousin Fe3O4, that is capable of encapsulating coke.

Following this reasoning one may rationalize as follows:

a. Partial oxidation of the tube walls may render Fe3O4 instead of Fe2O3.

b. Hydrogen may reduce Fe2O3 to Fe3O4 compounding the problem.

 

[rgrokkam]

thanks for the response
Well there's no reducing atmosphere(hydrogen) in a Visbreaker furnace. But coke layer is shielded. If rust is carried over by the feed (upstream) and deposited on coke in tubes...is this feasible? Can we ascertain this shielding by any method?

 

[25362]

You asked for the possible effect of a hydrogen reduction step, didn't you ?

I referred to the partial oxidation of the hot tubes on air/steam decoking.

Your idea of the feed carrying these oxides is worthy of checking.