Q What could be causing rapid catalyst deactivation in the hydrogenation of heavier fractions downstreamof our steam cracker furnace? A Yoeugourthen Hamlaoui, Global Market Manager, Yoeugourthen.HAMLAOUI@axens.net and Edouard Barange, Olefins Product Line Manager, Edouard.BARANGE@axens. net, Axens Several cuts are valorised downstream of the steam cracker furnace. The heavier fraction corresponds to the pyrolysis gasoline, also known as pygas. Raw pygas contains highly valuable components such as BTX (ben- zene, toluene, xylenes), unsaturated compounds like diolefins, styrenics and olefins, and sulphur-containing compounds. Depending on the feed quality, the catalyst used in the pygas first stage could either be palladium based or nickel based. For a feed highly contaminated with metal such as arsine, mercury, silicon or lead, nickel based catalyst is recommended as the metal contaminant resistance is higher than the resistance allowed by palladium based catalyst. However, in operation, some rapid catalyst deactiva- tion episodes can be observed in pygas hydrogenation units. Let us define first the different types of contaminant that could be present: • Inhibitors or activity moderators that compete with reactants for catalyst active surface. As the adsorption is reversible, the catalyst activity is recovered once the contaminant is no more present in the feed without any specific treatment • Temporary poisons with strong adsorption on the active surface of the catalyst. The catalyst activity is recovered with specific treatment (hot H 2 stripping/ regeneration), which requires a shutdown of the unit • Permanent poisons with very strong adsorption on the active surface of the catalyst. The catalyst activity cannot be recovered. Among these contaminants, arsine, silicon, sulphur species, oxygenates compounds, free water, gums may often be found in the pygas feed. These contaminants may be carried out by the crack- ing furnace feed or/and the process itself. For example: • Free water could come from an issue relative to the operation around the raw pygas storage tank or the operation of the caustic tower • Silicon can be brought by injection of anti-fouling chemical agent upstream • Metal contaminants come mainly with the steam crackers feed.
Contaminants that could have a drastic impact on catalyst activity, causing a rapid deactivation, are free water (free water combined with caustic soda is a tem- porary poison) and sulphur species. Indeed, pygas feed may contain up to several hundred parts per million of sulphur. Speciation of sulphur, including CS 2 , has high- lighted the presence of thiophenes (80 wt% of the total sulphur species), mercaptans/sulphides/disulphides (15 wt%), CS 2 (5 wt%), and H 2 S/COS (below 0.5 wt%). These sulphur species have different poisoning effects on the catalyst. Among the sulphur species described here, thiophenes present the lowest poisoning effect, followed by mercaptans, sulphides, and disulphides in ascending order. H 2 S/COS has the strongest poisoning effect, and CS 2 has the second strongest. Another source of contamination is the H 2 make-up used in the pygas first stage, where CO is the most com - mon one acting as a strong inhibitor. High CO could occur with methanator upsets. At a glance, rapid catalyst deactivation in pygas units is often explained by contamination issues that can be mitigated by a better understanding of the operational constraints and closed monitoring of the feed quality and upstream operation. These could be combined with a well-adapted loading diagram including adsorbent, grading, and catalyst to ensure the longest cycle length possible. Q What is the latest progress in FCC catalysis to boost bottoms upgrading? A Heather Blair, Senior FCC Technical Service Engineer, firstname.lastname@example.org, Rick Fisher, Senior FCC Technical Service Engineer, email@example.com , Todd Hochheiser, Global Technical Service Manager, todd. firstname.lastname@example.org Johnson Matthey There are multiple catalytic options available for improving FCC bottoms upgrading, but one of the most effective methods of improving bottoms upgrading is using a separate particle additive. This gives the refiner ultimate flexibility to quickly change bottoms upgrad - ing based on changing economics, feed, and/or unit constraints. Johnson Matthey’s BCA-105 is a very effective bot - toms cracking additive. The additive is made from highly selective matrix that provides the first cracking sites for larger FCC feed molecules. By doing this, the larger hydrocarbons (C 20 -C 40 ) are cracked into smaller, more easily crackable hydrocarbons that can be fur- ther cracked on the Y-Zeolite of the FCC catalyst. This function allows lower Z/M base catalysts to continue
Additional Q&A can be found at www.digitalrefining.com/qanda
Catalysis 2022 7
Powered by FlippingBook