By Winston Robbins, Ph.D.
Mercaptans (i.e., sulfur compounds with the generic formula of R-SH) have a long history of generating problems for refiners. Control of mercaptans is necessary because these compounds have an objectionable odor, high toxicity, and high corrosivity. Although mercaptans are easily reduced by hydrotreating, control has historically been accomplished by application of wet chemical techniques.
In the 1920's, the “Doctor process” (i.e., lead catalyzed air-oxidation to disulfides) was used to reduce the objectionable mercaptan odor in gasoline. In the late 1950's, UOP developed the Merox process for eliminating mercaptans from light ends, and this process is still widely used today. In that process, patented catalysts are used to oxidize mercaptans in caustic, which is used to extract the low molecular weight (MW) mercaptans. After separation of a disulfide phase, the regenerated caustic is recycled. Numerous variations of the Merox approach were developed by the late 1970's. For example, Merichem applied its thin-film contactor technology for mercaptan control, and UOP developed a catalytic carbon bed that extended the oxidation to the higher MW mercaptans in jet fuel.
Until 2000, these technologies were adequate for mercaptan control. Since then, however, mercaptans have reappeared to create process and product quality problems. These problems arise from both world-wide changes in crude slates and stringent fuel sulfur specifications.
Mercaptans in new crudes are creating problems in crude unit corrosion and jet fuel quality. In traditional crudes, mercaptan levels in crudes generally drop rapidly with MW (and boiling point). For these crudes, mercaptans distill into naphtha cuts with only trace ppm levels of mercaptans in higher boiling fractions. However, some crudes, especially some condensates, contain higher MW mercaptans that distill into higher boiling fractions. Published crude assay data for some crudes show high ppm concentrations of mercaptans in higher boiling fractions (Figure 1). For some of these crudes, the total mercaptan concentration is greater than the concentration in any of the fractions shown, i.e., for these crudes, there may be even higher mercaptan concentrations in the vacuum cuts (i.e., mercaptans are not routinely assayed in 650+ cuts).
Figure 1. Boiling Point Distribution For High Mercaptan Crude Oils
A recent NACE paper (#07565) describes the potential for crude unit corrosion by mercaptans. Under the conditions tested, the maximum mercaptan corrosion rate was observed around 527°F (275°C), i.e., on the high end of the jet range. Mercaptans in this boiling range may also present a challenge for control with catalytic Merox technology. This may be especially true if a high mercaptan crude is blended with an acidic crude. Because the catalytic process involves the reaction of mercaptides with air, the charcoal bed must be kept alkaline. Consequently, the feed to a jet fuel Merox treater must be pre-treated with caustic not only to convert the mercaptans to mercaptides, but also to neutralize and extract low MW acids. Successful operation requires careful control of the pre-treater, avoiding soap carry-over while providing protection of the catalytic bed.
Not all the new mercaptan problems are related to high mercaptan crudes. In fact, one of the bigger challenges arises from trace mercaptans formed in retro reactions that occur in cat cracking. Mercaptans are formed in sufficient concentrations to exceed the 2006 low sulfur fuel specifications. Even with hydro-treated feed, cat cracking generates some H2S that reacts with olefins in the cracked products before the separator. Although mercaptans in cat naphtha can be readily reduced with hydrogen, such a treatment would also saturate the high-octane olefins. Several schemes have been developed to remove these trace mercaptans. One of the most successful is ExxonMobil's SCANFINING technology that optimizes conditions to avoid retro-reactions, removes mercaptans from the light cat naphtha using a Merichem thin film technology, and hydrotreats intermediate and heavy naphtha with selective catalysts to reduce thiophenes with minimal olefin reduction.
Every refinery is engineered to different requirements. As the low
sulfur specifications continue to force process changes and new crudes
challenge operations, understanding the role of mercaptans is becoming
increasingly important. If you have any questions about mercaptans or
mercaptan control processes, please contact us.