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i'm working in a LAB plant. For dehydrogenation of paraffins there are 2 standby reactors. After deactivation of the catalyst (about 45 days) we changed to another one. Now we want to use both reactors at the same time in parallel. We increased the paraffin flow rate from 50 t/hr to 60 t/h. The hydrogen flow rate also increased from 10 t/h to 11.5 t/h according to our equipments capacity. I think it'll raise our conversion and increase the catalyst lifetime. Is this possible?

22/06/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I don't have specific knowledge on LAB reactors so am responding based on general principals common in many fixed bed reaction systems. Yes it is possible, but.....

If you increase flow by 20% and then split it between parallel reactors, each reactor will only be seing 60% of the flow that it previously had. Reactor pressure drop will fall off by 60-70% assuming it changes with the square of the flow. Lower feed may create less uniform flow through feed distributors, and the impacts of any catalyst maldistribution will be exacerbated at a low reactor DP condition. If flow distribution becomes a problem, then any yield benefit gains from increased reactor volume could be wiped out by flow maldistribution.

Presumably, with a reactor online and one or more reactors offline, the catalyst can be changed out and the offline reactor can be made ready to come online with only minimal disruptions in feed. There may be some time required to swing blinds and go through a heat up phase to bring the offline reactor up to operating conditions. If both reactors (or 2 out of 3 - not clear from question) are online at the same time, then the unit will have to shutdown completely to replace or regenerate catalyst. This operation will lower the stream factor on the unit and could offset part or all of the benefits of increased rate. The cost of replacing the catalyst with the unit completely shut down is likely to be significantly higher than replacing catalyst in a single reactor while the rest of the unit continues to operate.

Bottom line is that it might work. If it does work, I would anticipate that the benefits are likely to be a lot smaller than you are likely expecting.
22/06/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Well, it depends! The hydrogen to hydrocarbon is lower with parallel reactors, and the residence time is doubled. The mass of dehydrogenation should go up quite a bit, but not 100%. If you lower the reactor inlet temperature, coking should decrease, lengthening the run length. I am not sure of the net result in conversion level though.That depends on what is the limiting factor--kinetics or equilibrium. Doubling the residence time will increase the conversion level if kinetics (reaction time) are the limiting factor. If equilibrium (approach to 100% conversion) was limiting, the slightly lower hydrogen ratio will increase conversion level as the equilibrium conversion is shifted.