Q & A > Question Details
During precommissioning a diesel unit why we can not start the recycle gas compressor with 100 % hydrogen before catalyst sulfiding as per the licensor: 50% nitrogen with 50% hydrogen?
20/04/2021 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
There are two issues here:
1. While the reactor and separator are cold, you have to operate at <25% of design pressure to avoid the risk of brittle fracture. If the recycle gas compressor is a centrifugal machine, the density of the recycle gas may be too low at this pressure for the compressor to provide adequate flow. Circulating a mixture of hydrogen and nitrogen improves the performance of the compressor.
2. The catalyst supplier may have hydrogen partial pressure limits that need to be respected until the catalyst has been sulphided. Hence there may be a requirement to dilute the hydrogen in the recycle gas with nitrogen.
Depending on your situation, either or both of these factors may be relevant, but the licensor should be able to explain the reason for their recommendation.

Regardless of why or how you introduce nitrogen to the high pressure system, it is very important to ensure all nitrogen systems are positively isolated (i.e. blinded or physically disconnected) before the unit pressure is increased above the nitrogen supply pressure to avoid a situation where hydrogen flows into the nitrogen header. At best this would be a contamination problem, at worst it could be a loss of containment.
14/04/2021 A: Mukesh Patel, Reliance Industries Limited, mukesh1000@yahoo.com
If RGC design cases specify 100% Hydrogen i.e. ~ 2.1 MW case, then RGC can be operated. Suggest to check RGC data sheet provided by vendor. Since MW of 50-50% Nitrogen / Hydrogen case is higher than 100% hydrogen, suitability of RGC to be checked before decision to operate. Normally hydrotreating units are design to handle 95-100% hydrogen.