Ethylene production through pyrolysis (steam cracking) produces several byproducts, including a naptha range C5+ stream. The C5+ cut is referred to in the industry as pyrolysis gasoline or pygas. Left in its raw form, the stream has limited commercial value owing to its high reactivity and low stability. However, the stream contains many high value components, such as isoprene, benzene, toluene and xylenes. Extracting these components can be highly economical. To do so requires multiple steps, including distillation, hydrotreating and extraction. Although multiple processing arrangements are possible (depending on the desired final product slate), pygas processing typically involves two hydrotreatment stages. The first stage involves hydroprocessing the feed to remove diolefins, alkenylaromatics (styrene) and some olefins in preparation for optimal downstream processing. The second stage involves olefin and sulfur removal from all or a specific cut of the first stage product. This processing prepares the feed for aromatic extraction and/or motor gasoline pool addition. The two hydrotreating steps can be problematic and negatively impact process reliability, forcing production losses as well as additional operating and maintenance costs. Chronic problems such as reactor plugging (pressure drop build up), short catalyst cycles and/or off spec product have led to frequent shut downs, catalyst regenerations and catalyst changes.
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