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What is the need of using three reactors in Platforming instead of a single one? Plus, do different reactions take place in each reactor? If so, what's the feed of the second and subsequently the third reactor?
 
Answers
26/03/2018 A: AMIT KALYAN CHANDRA, INDIAN OIL CORPORATION LIMITED, chandraak@indianoil.in
1) The reforming reactions are in general endothermic. A single reactor would have led to a drop in the reaction temperature to the extent that substantial forward reaction could not be achieved.
2) All the reforming reactions take place in all the reactors. But Dehydrogenation Reaction being the fastest takes place in the first followed by Isomerisation Reaction in the 2nd and the most difficult Dehydrocyclisation Reaction in the last reactor respectively.
14/02/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The net effect of the various reactions in a reforming reactor is endothermic. That requires reheating at defined temperatures to achieve the desired reaction results. It is interesting to compare the application used to the way a defined temperature range is maintained in a hydrotreating and hydrocracking units where the net reactions are exothermic. In the hydrotreater and hydrocracker, relatively cool hydrogen is injected at some elevation(s) of a single vessel to cool the hydrocarbon. In a reforming reactor, it would be feasible to inject hot hydrogen at defined elevations of a taller single reactor. However, that method is considered feasible but not practical. Instead, the hydrocarbon and hydrogen mix is passed through coils in the furnace and into multiple reactor vessels. It would be feasible but not practical to return the reheated hydrocarbon and hydrogen mix to the same but taller reactor vessel. In the reformer, yes, the reactions are different as the hydrocarbon “sees” more and more catalyst and for a longer period of time. The third reactor is usually the largest because more of the hydrocracking occurs there, laying down more coke on the catalyst. Older units had four reactors.
14/02/2018 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The main reforming reactions are highly endothermic, so the temperature drops across the reactor. As the temperature drops, the rate of reaction drops. To keep the reaction rates high, multiple reactors are employed to allow the feed (reactor effluent from the previous reactor) to be reheated and get the reaction rate back up.
The same reactions are occurring in each reactor but the extent to which they are occurring changes. The faster reactions happen mainly in the first reactor, and the slower reactions requiring more residence time happen to a greater extent in the last reactor, which is much larger than the other reactors. By the time the feed gets to the last reactor, the faster reactions have gone mostly to completion so are not happening to any great extent any longer.