“The SRU is only as good as what it receives in terms of amine acid gas and SWS acid gas”.
When evaluating the performance of an SRU, the first things to check are the feed stream quality and consistency. Since the SRU is only as good as what it receives, there are many potential areas for troubleshooting with regards to upsets with the feed stream units. All SRUs have an Amine Acid Gas feed stream, and many refineries have an additional Sour Water Stripper Acid Gas feed stream. The first indication that there maybe an issue with your feed streams is swings within your air demand signal. Large and uncontrollable swings in the combustion air demand signal occur due to fluctuations in the acid gas feed stream compositions and flow rates. Typically, when these fluctuate by more than 10%, the Air Demand Signal will fall outside of the optimal range and result in an immediate loss in recovery efficiency.
While plant swings can be unpredictable, they can be prepared for, new gas streams should be introduced slowly for a smoother transition. This can, for example, give the ADA time to adjust to the changing H2S content in the feed stream. Plant upsets can never be fully eliminated, so the SRU instrumentation must be regularly calibrated in order to be prepared for feed stream swings. Full performance testing of the upstream amine and SWS units is the best way to optimize performance and minimize swings in the feed stream compositions and flows.
Another common issue with SRU feed streams is when they have high hydrocarbon contents. Hydrocarbons require much more oxygen to oxidize in the reaction furnace, which throws off the air demand. Also, more hydrocarbons means more CS2 production, which will hurt the recovery efficiency if it isn’t hydrolyzed. If sufficient amounts of hydrocarbon make it into the converters, catalyst poisoning will occur and deactivate the catalyst.
Excessive accumulation of hydrocarbons in the feed streams can be due to:
Over circulation of amine, or amine temperatures falling below inlet gas temperatures, which results in HC condensation.
Insufficient flash tank residence time or skimming operation can also increase hydrocarbons in the amine or SWS acid gas. The same goes for the reflux drum.
Sometimes the Reflux Drum purge rate is too low, or sometimes the pump does not start, resulting in LPG carryover to the RF.
High levels of other contaminants can be problematic as well; BTEX, mercaptans, and methanol are all catalyst poisoning compounds that should be kept below threshold levels in order to ensure their full destruction in the reaction furnace. High CO2 contents will increase COS production and reduce efficiency if not fully hydrolyzed in the first converter.
The best way to minimize contaminants in the feed streams is to perform regular maintenance, testing, and optimization of the amine and sour water units.