In the naphtha hydrotreating unit, the sulphur is removed, and then in the CCR Platformer feed DMDS is dosed again. Why can't we maintain the required sulphur level in the CCR Platformer directly from the NHT unit (suppose in the NHT unit feed Sulus is 786 wppm and we have to maintain 0.75 wppm in CCR PF feed)? Why it is necessary to remove sulphur first and then add sulphur (DMDS) again?Jul-2021
David Chen, Formosa Chemicals & Fibre Corporation, email@example.com
Because we want to ensure that other contaminants (nitrogen, metals, oxygenates, etc.) are also removed from the feed to the extent achievable by the NHT. The sulphur is then added to the Platforming unit feed with a chemical injection system pumping in a specific and controlled amount of organic sulphur compound to achieve the target recommended.
This provides you with independent control of the sulphur in the feed to the unit that can be changed as needed if feed rate or operating conditions change.
Richard Rhead, Refining & Aromatics Consulting, firstname.lastname@example.org
As previously mentioned that NHT upstream of a CCR Platformer does more than just remove sulfur, with some typical other concerns being nitrogen, metals, water. In the typical NHT reactor operating temperature range sulfur is the easiest to remove (lower temperature). If you just run the reactor at a temperature just high enough to remove sulfur to 0.5 wppm then most likely you are letting nitrogen and trace metals slip through the reactor. Trace metals needs a higher temperature, whereas nitrogen is a function of temperature but also hydrogen partial pressure and catalyst type (Co/M or Ni/M). Downstream fractionation will take care of water removal, but oxygenate can be an issue removal is a combination of catalyst reaction + fractionation.
As mentioned is you run to 0.5 wppm sulfur other contaminants will slip through. So you have to run with a higher catalyst temperature to ensure efficient contaminant removal. In this case the sulfur levels will be much lower than 0.5 wppm. Hence the need to add-back sulfur (typically DMDS). If feed naphthas change frequently, it would be hard to control catalyst performance to obtain a consistent 0.75 wppm sulfur (feed sulfur, feed sulfur types, feed rate).
Tofan Santoso, PT indofarma, email@example.com
I agree, in theoretical it can be done, but please remember that NHT is not only removing sulfur. It also removing nitrogen, water and metal. In operational usually when you want to add more sulfur in your treated naphtha, you can lower down your RIT or adjusting your stripper, but this action will also adding other impurities to your treated naphtha.
In my experience in operating NHT-Naphtha Reforming Unit, we keep the sulfur 0.5 and monitor the nitrogen content under 0.5 ppm, when we optimized the NHT operation we also monitor moisture content in the recycle gas naphtha reforming.
In actual condition its hard to optimized the sulfur with tight monitoring other impurities, it is more easy and safer to add DMDS and condensate in naphtha reforming feed. That's why licensor usually design DMDS and Condensate injection in front of Naphtha reforming unit.
In my experience operating NHT-CCR atmospheric, we usually keep 0.5 ppm S and N and we found no metal dusting or MCC phenomena.
I hope this information will help you.
Jake Gotham, InSite Technical Services, firstname.lastname@example.org
You are right, this could be done, in theory at least. In practice, it is usually challenging to control the sulphur content at such a low level with sufficient consistency. The repeatability & reproducibility of the analysis method and variation in feed rate & feed quality make this difficult. In most cases it is more reliable to over-hydrotreat and inject DMDS to achieve the required sulphur in the reformer. The cost of DMDS is small compared to the potential impact of sulphur slip on the reformer.
In some units (particularly if processing FCC naphtha or coker naphtha), the NHT reactor temperature is set to achieve the reformer feed nitrogen spec, and the sulphur content is below the reformer requirement.