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In our plant, DSN tank exists for cold feeding to CCR unit in case of interruption in u/s hot feed from NHT unit or during total power failure start-up of Refinery. Cold DSN is fed to NHT stripper and thereby to CCR unit via Depentanizer and Re-run columns to produce Hydrogen for NHT start-up. Once Hydrogen is produced, NHT feed in is done. Stripper bottom is recycled back to FSD till DSN is on spec. When DSN is on-spec, feed from DSN tank is isolated. How to avoid poisoning of CCR catalyst while keeping NHT running with off-spec DSN? Further, can DSN from Tank be sent directly to Depentanizer bypassing Stripper in absence HP nitrogen input in Stripper? What are the precautions to be taken for operation of CCR unit?

Mar-2024

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Raghu Nee, HPCL, neeraghu@gmail.com

It is recommended to route cold feed (DSN naphtha from tank) through stripper to strip-off not only the dissolved H2S and other lighter HC but also the water. You never know, Naphtha from storage tanks may contain some traces of water. Moisture and oxygenates in CCR feed would lead to rust formation in colder regions of CCR and eventual iron deposits on catalyst. This would affect metal active sites.

Aim to achieve <0.1 ppm S in stripper bottom that is an indirect measure of N2 in Stripper R/D. Lower the Sulfur better the feed in terms of Nitrogen contaminant. Then, make up the Sulfur by injecting DMDS in CCR ffed to avoid MCC.

Mar-2024
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

Thanks for the additional information, well it's possible to observe a rise in the contaminants content of the DSN while the off-spec naphtha is recycled to the DSN tank. It's necessary to carry out a mass balance to control the total nitrogen content in the feed below 0,5 ppmw considering the tank inventory and the off-spec naphtha recycle flow rate, this is another reason to keep the processing flow rate as low as possible in this step. Again, the main concern with the high concentration of nitrogen in the feed is related to the salt deposition in the cold and separation sections of the catalytic reforming unit.

Mar-2024
VIKRAM BABA, INDIAN OIL CORPORATION LIMITED, vikram.baba@gmail.com

Thanks Marcio for the feedback. DSN mentioned in the Re-run column bottom naptha which is the normal hot feed to CCR unit. Excess quantity is stored in nitrogen blanketed DSN tank for usage during requirement/exigency. DMDS is added prior feeding to CCR reactor to maintain sulphur concentration. PERC is dosed in Regeneration section to maintain catalyst activity.

Total Sulphur of DSN is around 0.3-0.6 ppmw and Total nitrogen is < 0.3 ppmw. Will there be increase in nitrogen content in DSN from nitrogen blanketed DSN tank due to level changes? At what range of nitrogen, it is detrimental to CCR catalyst?

Mar-2024
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

Well, to respond adequately the question would be fundamental to know the contaminants level of the desulfurized naphtha (DSN) and desulfurized off-spec naphtha.

Some licensors recommend that the naphtha fed to the catalytic reforming units should be hydrotreating aiming to achieve a zero concentration of sulphur in order to ensure a very low concentration of other contaminants like nitrogen, oxygenates, etc. It's not clear in the question if this is carried out in this case, but a side effect of this deep desulfurization is that the absence of sulfur in the catalytic reforming feed can led to metal catalyzed coke (MCC) in the tubes of fired heater which migrate to the reactors and can affect the catalyst circulation and the performance of the unit. To avoid this, the licensors recommend the injection of a controlled amount of organic sulphur (like DMDS) to the feed aiming to ensure minimum sulphur concentration (between 0,25 to 0,50 ppm). It's fundamental to know the concentration of sulphur and nitrogen in the off-spec feed quoted in the question to conduct an adequate decision making process, the contamination by sulphur is reversible and the catalyst performance can be recovered after some regeneration cycles after the contamination stops. Regarding the nitrogen, the high concentration can affect the acid function of the catalyst which can be compensated through a higher chloride dosage in the regeneration section but the main concern here is the salt formation (NH4Cl) in the cold areas of the unit like condensers, separation and compression sections.

Among the actions to protect the catalyst against temporary high contaminants concentration it's possible to reduce the process severity and the flow rate of the unit as well as raise the dosage of chlorination agent (normally Perchloroethylene), as previously mentioned, to maintain the acid function of the catalyst.

Regarding the stripper bypass, I don't have knowledge about this operation mode once this action will add sour gas (high H2S concentration) to the catalytic reforming feed and will overload the depentanizer column with a very low initial boiling point naphtha which can lead to higher gas production as well as raise the benzene concentration in the reforming naphtha. Furthermore, a higher concentration of H2S in the feed can produce deleterious effects over the catalyst as mentioned above.

Mar-2024