Can Feed Sulfur Be Both Good And Bad?
This blog provides some historical context to my previous blog, Metal Catalyzed Coking and feed Sulfur (S) requirements. Moreover, I will also address the flip-side of too little feed S and discuss the impact of too much and its effect on catalyst performance. I’ll also cover how catalyst Surface Area (SA) impacts the recovery from S poisoning.
Background
A common question from those new to CCR catalytic naphtha reforming is… “If we need to inject S to protect against MCC, then why is S also listed as a catalyst poison?” The explanation given in my previous blog on MCC answers the first part of that question, but not the last. I’ll start with some historical background on why feed S guidelines were adopted, and why £1.0 wppm became the official limit for typical guidance to protect against MCC.
In the late 1980’s customers were recognizing a significant boost in aromatics production with these lower pressure units, but the consequences of MCC formation led to extended downtime for some customers to make repairs. Initially, customers were resistant to the idea of passivating steel using sulfur injection to solve this problem, but were also the first of several to adhere to this new guidance. Ideally, feed S should be nil to maximize yield, but there was a need to strike a balance between maximizing yield and keeping these units on-stream for years at a time. This is why the current guidelines were established, and these guidelines will optimize both yield structure and run length.
Determination of High Feed Sulfur and Mechanism for Catalyst Poisoning
High feed S (> 1.0 wppm, or more than your licensor has advised) will result in a decline of selectivity to aromatics. As feed S increases while holding the same Rx Inlet Temperatures (RIT’s) one will observe their Rx dT’s beginning to drop, octane no longer being met and net H2 production dropping. Clearly a problem requiring a quick response, but do not let that response be to simply raise RIT’s! Unfortunately, this has been a response that some customers have made, and is likely the worst thing to do.
When feed S is high it attenuates the metal function by predominately obstructing metal sites (Pt, etc.). This negatively impacts both the stability and selectivity of the catalyst and leaves the catalyst with more accessible acid sites. If raising the RIT’s is the chosen remedy to hit your declining target octane, the increased severity will result in more cracking to light ends (C4-), a drop in recycle gas purity and an increase in coke production. This can dig a deep hole even for a CCR that can burn coke continuously, as at some point you may exceed the coke burning capacity of the regenerator and be forced to reduce feedrate to the unit. So, what should you do?
Response to High Sulfur
Determine the extent of S poisoning by doing much of the following in parallel… Check H2S in your recycle gas. There should always be a trace amount present for MCC protection, but not to exceed 2~3 mol-ppm via gas detection tube (Draeger). As a rule of thumb, you should see 2~3 mol-ppm H2S in your recycle gas for 1 wppm of S in feed. This will help determine how over-sulfided you are. If you have elevated S contamination i.e., 10 ~ 15+ mol-ppm H2S, you should already be seeing a reduction in Rx dT’s and reduced reformate octane.
Note: Licensors will have guidelines on a response for both high and low S poisoning. This blog is intended to cover high S poisoning. You should always contact your licensor to ask for their specific guidance to make that distinction.
If feed S is confirmed high – do not chase octane! In fact, do the opposite. Reduce RIT’s to 900 F (482 C) and as needed route your reformate to off-spec storage. You should already be trying to identify the source of excessive feed S and eliminate it. Once the source of excessive S has been isolated, the clean feed will now begin to desorb the excess S from the catalyst. If desired, one can increase (double) the Cl injection to help desorb the S more rapidly. This is where you should be utilizing the licensors technical support, general operating manual and any other resources available to obtain the best guidance for a full recovery.
Catalyst Surface Area Effect
Always keep in mind the current surface area (SA) of your catalyst. The higher the SA, the more available sites, the longer it will take to desorb both S and Cl from your catalyst. This can stretch out the recovery time from both high S and/or Cl’s. This is important to understand in general, but also while using Cl to speed up the process of S desorption. So, why use Cl’s to help with S contamination? Cl competes and wins the competition with S for catalyst sites. That said, if you overdo the injection of Cl with high SA catalyst, you may be stuck with over-chlorided catalyst for a couple of cycles.
Other Recovery Tips
Can I use the Regenerator to help with recovery from S contamination? Yes, you can increase catalyst circulation but should remain within the general operating curve provided by your licensor, and always control the Burn Zone peak temperature at an acceptable position within the regenerator. You can also increase the feed rate of clean naphtha to help desorb S, provided you’re not constrained by coke production or anything else.
Resuming Normal Operations
Once the source of S has been eliminated and H2S in the recycle gas is at 2 ~3 mol-ppm, one can start bringing the RIT’s back up to meet your target octane. At this point you should already be seeing a reduction in light ends production with other process variables improving across the board. This should allow you to return to normal operating severity with a full recovery from S poisoning.
Introducing New Steel into the Platforming Unit – Fixed Bed or CCR. If you replace any steel – more specifically furnace tubes, even sections – do not startup per normal procedures. You may want to discuss with your licensors technical team to see if they feel you need to pre-sulfide or modify existing procedures. This new steel is not passivated at all and may be a location that is predisposed to MCC formation.
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