July 2019

Catalysts

Next-generation catalyst improves reliability of LPG mercaptan extraction

For decades, sulfur reduction processes have been an integral part of refinery operations.

Ahmed, S., Motiva Enterprises; Mulvaney, W., Nair, M., Rios, R., Honeywell UOP

For decades, sulfur reduction processes have been an integral part of refinery operations. The need for reliable and effective treating technologies is even greater due to increasingly strict product specifications. A commercially proven sulfur reduction extraction unita is used to treat light feeds, such as natural gas, LPG and light naphtha. It uses a caustic solution to remove mercaptan sulfur in the extraction portion of the unit. The caustic is regenerated by oxidizing the extracted mercaptan to a disulfide, using an organometallic catalyst, and removing the disulfide from the caustic solution via a separator. The type of oxidation catalyst used in this process is extremely effective, but many customers have experienced plugging in their catalyst injection systems. These plugging events cause unexpected downtime and lost revenue for refineries.

As a result, a next-generation catalystb has been developed. This catalyst improves treating performance compared to previous generations and eliminates plugging issues in the catalyst injection system due to its improved flow properties.

Mercaptan oxidation extraction catalyst injection plugging

With a processing capacity of 630,000 bpd, Motiva Enterprises is the largest refinery in the US and the sixth-largest refinery in the world. Located in Port Arthur, Texas, this refinery has the largest mercaptan oxidationa unit. It can treat 8,760 bpd of coker LPG with 3,359 wppm of feed mercaptan sulfur. During normal operations, the feed also has an average of 17 wppm of non-extractable sulfur. The product of the mercaptan oxidation unit is fractionated to a propane and butane cut, then routed to a propylene recovery unit and an alkylation unit.

Over the past several years, Motiva has used two different catalysts—one being a previous-generation catalystc—to regenerate caustic used in mercaptan extraction. Motiva experienced catalyst injection system reliability issues when using these liquid catalysts. Specifically, residual catalyst material was plugging the injection pump’s discharge and small instrumentation lines. The resulting low catalyst concentration in the caustic led to incomplete mercaptide oxidation and off-spec product mercaptans. These plugging events forced the refinery to route LPG to low-value fuel gas instead of high-value propylene product. Increased product LPG sulfur concentration also caused higher acid consumption in the alkylation unit.

Frequent intervention and maintenance by the unit operations team were required to address injection system plugging and avoid inconsistent catalyst injections. As a result, operational efficiency decreased, and operational costs increased.

Motiva consulted the authors’ company to identify and eliminate the cause of the catalyst injection plugging and to improve unit reliability. The catalyst injection plugging was completely resolved due to two main factors. First, a next-generation catalyst was introducedb into the unit. Unlike previous generations of oxidation catalysts, this new catalyst has a lower viscosity, thus providing improved flow properties.

Second, a technical services team went to the refinery to conduct daily monitoring, troubleshooting and unit optimization. The new catalyst, combined with the technical service team’s recommendations, successfully resolved the catalyst injection system’s reliability issues, while maintaining effective mercaptan removal during long-term commercial operation.

Technical background

The mercaptan oxidation process is designed to remove mercaptans from light hydrocarbon streams. Product mercaptan levels of less than 1 part per million (ppm) by weight (wppm) may be achieved. The mercaptan oxidation unit comprises a liquid-liquid extraction process. A simplified flow diagram is shown in FIG. 1.

FIG. 1. Simplified flow diagram of the mercaptan oxidation operating unit.
FIG. 1. Simplified flow diagram of the mercaptan oxidation operating unit.

 

The process first extracts mercaptan sulfur (RSH-S) by contacting the hydrocarbon stream with a caustic solution. The aqueous sodium mercaptide is then catalytically oxidized at low temperatures (100°F–110°F) to a disulfide (RSSR) in a caustic environment. The disulfide is then separated from the caustic in the regeneration section of the unit. The reaction sequence is summarized in FIG. 2.

FIG. 2. Reaction sequence in the mercaptan oxidation unit.
FIG. 2. Reaction sequence in the mercaptan oxidation unit.

 

Motiva frequently experienced catalyst injection system plugging problems prior to using the next-generation catalyst. Plugging is a serious concern because the concentration of the catalyst in the circulating caustic is vital to properly oxidize the sodium mercaptide, regenerate the circulating caustic and produce on-spec product. Without the proper concentration of oxidizing catalyst in the system, the caustic cannot regenerate and the feed mercaptans cannot be effectively extracted.

Motiva isolated the catalyst addition system and took it offline during plugging events, forcing operations to manually inject the catalyst in batches. The treated product did not meet specifications every 3 mos–6 mos because insufficient catalyst makeup resulted in low catalyst concentration in the caustic. This unreliable operation forced the refinery to incur costs of high acid consumption from sulfur contamination in downstream alkylation and propylene recovery units.

The next-generation catalyst

The authors’ company developed a solution for these challenges by introducing its next-generation mercaptan oxidation catalystb. This catalyst is a seamless drop-in replacement with a lower viscosity. Other benefits include:

  • High catalyst activity and lower freeze point
  • Selective for mercaptan oxidation
  • Easy handling and ready to use.

While Motiva’s coker LPG unit was one of the first to demonstrate the effectiveness and benefits of the new catalyst, it has been successfully implemented in more than 45 operating commercial units on lighter-molecular-weight hydrocarbon streams, such as condensate LPG, FCC LPG and coker LPG.

Demonstration test and commercial operation

Motiva and the authors’ company conducted a 25-d demonstration test of the new catalyst. The authors’ company provided onsite assistance for the duration of the test, which included daily monitoring of unit operation, troubleshooting and optimization.

FIG. 3 depicts an onstream comparison of product mercaptan for the previous-generation catalyst (gray) vs. the latest-generation catalyst (blue). The gray line’s data shows the effects of inconsistent catalyst injection. Total product mercaptan levels spiked three times to levels in the 40 wppm–80 wppm range. It took days to bring the product back on spec to less than 5 wppm. However, once the new-generation catalyst was introduced through the pumped catalyst injection system, the refinery immediately reported stable operation and a reduction in product mercaptan. Note: The only spike in total product mercaptan during the latest-generation catalyst demo occurred for a reason completely unrelated to catalyst injection plugging. A combination of cold weather and a lack of insulation on the oxidizer caused the spike. Based on the results, Motiva decided to purchase the new catalyst for continued use.

FIG. 3. Commercial demonstration: Previous mercaptan oxidation catalyst (gray) vs. the new-generation catalyst (blue).
FIG. 3. Commercial demonstration: Previous mercaptan oxidation catalyst (gray) vs. the new-generation catalyst (blue).

 

Process optimization during a seamless turnaround without downtime

Motiva wanted to complete modifications on its regeneration section without shutting down and losing valuable production. By working closely with the authors’ company’s technical service engineers, minor piping modifications were designed and implemented—as an interim solution—that did not lose or downgrade the desired product. This new piping allowed fresh makeup caustic to be added to the extraction section, thus isolating the regeneration section. The turnaround was accomplished without taking the mercaptan oxidation extraction unit offline. 

By Motiva’s estimates, the modification prevented approximately $2.3 MM of potential loss in alkylate, propane and propylene product. 

The technical service personnel were onsite to provide hands-on training and share valuable optimization methods to reduce sulfur carryover from the regeneration section to the extraction tower. This optimization (TABLE 1) resulted in lowering total product sulfur of 35%, which has been sustained for more than 2 yr. Combined, the latest-generation catalyst and continuing technical services have helped maximize unit performance.

 

Long-term operation

Motiva completed its conversion to the new catalyst in August 2016. Since implementation, the new catalyst has produced reliable operations without any plugging issues.

After initial stabilization during the new catalyst reintroduction, the product mercaptan level was maintained consistently at or below 5 wppm, and was typically less than 1 wppm (FIG. 4). Comparative results of the previous-generation mercaptan oxidation catalyst baseline operation, the latest-generation catalyst demonstration test and the long-term new catalyst operation are shown in TABLE 1. The last column in TABLE 1 summarizes the improved performance of the unit with the new catalyst after the turnaround restart and through the optimization period.

FIG. 4. By using the new mercaptan oxidation catalyst, the total mercaptan levels were kept below the product specification of 5 wppm.
FIG. 4. By using the new mercaptan oxidation catalyst, the total mercaptan levels were kept below the product specification of 5 wppm.

 

Both the demonstration test and optimization period data show improved performance of the new-generation mercaptan oxidation catalyst vs. the previous-generation catalyst. The implementation of the new catalyst allows for higher unit throughput and lower, on-spec product mercaptan levels. More importantly, the new catalyst shows reliability for constant mercaptan conversion with less than 1 wppm product mercaptan and no increase in catalyst consumption. The technical service team’s best practices guidelines led to a constant improvement in total sulfur (bottom row in TABLE 1).

Having successfully used the new catalyst, Motiva has noticed improved catalyst performance and operating stability of the mercaptan oxidation unit, and was able to successfully operate the original catalyst injection system without plugging.

Takeaway

After 4 yr of experiencing catalyst injection plugging issues, Motiva resolved the issue by utilizing a new generation of mercaptan oxidation catalysts. The new catalyst’s improved flow properties enable reliable catalyst injection and consistency in meeting product specifications.

With more than 2 yr of uninterrupted operations, this catalyst is the reliable choice for mercaptan oxidation units seeking to reduce product mercaptan sulfur to ultra-low levels. In addition, it will alleviate catalyst injection reliability issues, while providing superior activity for long-term commercial application. HP

NOTES

  a Refers to Honeywell UOP’s MeroxTM extraction unit

  b Refers to Honeywell UOP’s WS-2TM catalyst

  c Refers to Honeywell UOP’s Merox WSTM catalyst

  d Refers to Honeywell UOP’s Extractor PlusTM technology

The Authors

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