We are operating a Naphtha Hydrotreater with two reactors. The first reactor is for diolefin saturation. We are facing high DP issues in the 2nd reactor. What could be the cause?
Jake Gotham, InSite Technical Services, email@example.com
There are many potential causes, including:
• Debris left in the reactor or heat exchangers at the turnaround. How long has this equipment been in service since it was last opened for cleaning / inspection?
• Poor attention to detail in the catalyst loading procedure. Some refineries leave the catalyst loading contractor to manage the task. Others assign a junior engineer to monitor. Others assign a senior engineer because they understand the impact poor loading can have on the reliability of the next cycle.
• Loss of activity in the first reactor, meaning the saturation reactions are taking place in the second reactor where they could be forming gums. Has the exotherm in the first reactor reduced?
• Usually the furnace is after the first reactor, but before the second reactor. Corrosion of furnace tubes causes scale to build up in the reactor. Even if the corrosion rate is low enough to not be a concern to your inspection/integrity colleagues, it can still cause premature pressure drop in the reactor. This can be minimised at the next catalyst load by adjusting the topping layers in the reactor. Some units have upgraded furnace tube metallurgy to stabilised 347 purely to manage reactor pressure drop.
• Mechanical failure of the catalyst. If catalyst is handled and loaded correctly, this is rare, but not unheard of.
• I assume from the fact that the unit has a diolefin saturation reactor that the unit is processing coker naphtha.
• Is the catalyst particle size smaller in the second reactor than the first? If so, it’s possible the second reactor is filtering out material which passed through the first reactor (e.g. coke fines). Talk to your catalyst vendor about using 2.5mm catalyst particles at the next catalyst change. The diffusion penalty on activity from a larger particle size is less significant in a naphtha hydrotreater.
• Are there filters on the unit feed or coker naphtha stream? If so, check the bypass is closed. Also check the backwash frequency and pressure drop trend for signs that a filter element is damaged.
Investigating these possibilities may help you diagnose the problem and put in place measures to prevent a repeat on the next cycle. If it is not possible to identify the cause, you should ensure the unloading procedure at the next outage includes entering the reactor before dumping to inspect the top of the bed and get samples from each topping layer and the top of the main bed.
I hope that helps.
Manan Joshi, L&T-Chiyoda, firstname.lastname@example.org
Please share more details such as API gravity of your feed, catalyst details and operating parameter.