The FCC gasoline hydrotreater allows olefinic and cracked naphtha to be hydrodesulphurised to meet gasoline blending specifications. In order to produce products for gasoline blending, the unit’s secondary objectives are to minimise reductions in product RVP and octane.

In order to minimise the octane loss across the process, the selective desulphurisation unit is often paired with a Selective Hydrogenation Unit (SHU) which contains a di-olefin saturation reactor to minimise the potential for polymerisation.

The SHU reactor product is separated by a naphtha splitter to allow olefin-rich light naphtha to be fed to the gasoline pool or to act as alkylation unit feed. The heavy cracked naphtha from the bottom of the splitter is mixed with additional hydrogen to undergo further desulphurisation. The final reactor catalyst targets mercaptan desulphurisation.

FCC Naphtha cannot be routed to the NHT there for multiple reasons, the key one is that olefins in cracked naphtha provide a large part of the octane when it is blended into gasoline. When hydrotreated (rather than selectively hydrotreated in a dedicated FCC naphtha treatment unit), these are saturated and there is a very large associated loss in octane number.