PTQ Q3 2026 Issue

selective to linear hydrocarbons; it is a shape-selective zeo - lite.5 Due to their shape and molecular size, the cracking of naphthenes and aromatics by ZSM-5 is restricted by diffu - sion. This understanding is applicable to naphthenes with a molecular size above 5.5-6Å.6 , 7 Nevertheless, conversion remains feasible at the ZSM-5 external surface and pore mouth. C₅ naphthenes have a molecular size closer to that of the ZSM-5 micropore; they are more likely to diffuse, and conversion remains governed by active-site chemistry. FCC catalyst manufacturers have to assess the treated C₅ cut’s crackability with used catalysts. The assessment will allow industrials to compare this catalytic alternative with the traditional non-catalytic way: rerouting treated C₅ cut (rich with olefins) to the steam cracker for further cracking. Naphthenes in FCC promote hydrogen-transfer reactions and generate n-paraffins and aromatics.5 Cyclopentane and cyclopentene, if introduced in the FCC, may potentially lead to cyclopentadiene formation through hydrogen transfer, which may generate gums through secondary reactions. When dealing with a treated C₅ cut processed through a full hydrogenation process (two-stage hydrogenation) in the steam cracker, C₅ olefins will no longer exist, with the resulting C₅ cut mainly composed of n-pentane, i-pentane, and cyclopentane. Rerouting the C₅ cut in this configuration will involve co-processing an olefin-free cut, which is incon - sistent with the principle of LCN recycle. Pygas in gas crackers As the pygas yield from gas crackers is low, SHU is often not installed. Similar to liquid crackers, due to pygas’ nota - ble content of reactive diolefins and styrenics, gas cracker pygas requires treatment for any further upgrade. Untreated pygas from gas crackers can be integrated if an FCC unit is located nearby, fed with a high-sulphur content feed, when the WCN HDT process is existing. Based on the design of a WCN HDT process, untreated pygas can be par - tially or fully integrated with FCC naphtha. The degree of integration depends on the maximum benzene content that LCN-HCN can absorb with respect to gasoline pool specifications. The integration limit based on benzene content in finished products may be relaxed by implementing a benzene recovery mode. Moreover, C₅ con - jugated diolefins and styrenics content is a key parameter for the integration assessment with FCC naphtha. WCN HDT units exist in different configurations. The main difference lies in whether a naphtha splitter is installed downstream of the FCC debutaniser. Having a naphtha splitter enables the removal of WCN heavy aromatics, pre - venting their presence in HCN. Additionally, the naphtha splitter allows the overhead light cut, rich in light olefins, to be sent to the FCC riser for further cracking and maximisa - tion of propylene. This stream is highly rich in light linear ole- fins, suitable for ZSM-5 cracking. It is also rich in untreated diolefins; their impact must be monitored carefully. In the case of a treated FCC feed with low sulphur con - tent, where no WCN SHU-HDS exists, recycled LCN to the FCC riser may be rich in diolefins. FCC naphtha diolefins differ from those in pygas, diolefins speciation is highly rec - ommended. When pygas from gas crackers is considered

C unsaturated species

56%

C olens C diolens

40%

39%

24%

13%

C p y g as (Liquid crackers)

C p y g as (Gas crackers)

LCN (VGO FCC)

and mercaptans and the removal of sulphur to generate cracked naphtha streams, free of diolefins and sulphur, highly rich in aromatics and olefins, with a high octane num - ber, ready for blending into the gasoline pool. Two distinct cuts are recovered from the unit: LCN and heavy cracked naphtha (HCN). A medium cut can be addi - tionally generated, called medium cracked naphtha (MCN). In FCC units processing clean feed (as hydrotreated vac - uum gas oil), where the WCN HDT process is not needed, WCN is routed through a simple splitter. Figure 3 shows the simplified FCC scheme. Based on its content of olefins and iso-paraffins, LCN is blended into the gasoline pool. Due to its low motor octane number, it is carefully blended while complying with gasoline specifications. With unstable gasoline demand affected by the electric vehicle market, refiners can switch FCC units to propyl - ene mode production by recycling LCN to the FCC riser. Propylene yield will improve with the addition of ZSM-5, a specific zeolite catalyst that further cracks FCC gasoline- range linear olefins to LPG olefins.4 Linear olefins are the most suitable feed for using ZSM-5 to boost propylene pro - duction in FCC units. If gasoline demand drops, the C₅ cut from pygas may not be needed for blending. If not sold into the C₅ naphthenes market, it can then be recycled back to the cracker. C₅ olefins will exist based on the configuration of the pygas HDT pro - cess, with or without a second hydrogenation stage. It is relatively rare to find a steam cracker integrated with a refinery. An integrated FCC-steam cracker complex could consider additional synergies. One alternative would be to reroute the treated pygas C₅ cut to the FCC riser as a co-feed for the propylene mode, similarly to LCN. The treated C₅ cut coming from the steam cracker is free of diolefins, as it was processed through the pygas SHU. In comparison to LCN from the FCC, the main difference is C₅ naphthenes in the pygas C₅ cut. Introducing these naph - thenes into the ZSM-5 catalyst in the FCC raises questions regarding catalyst micropore limitations versus the effective molecular size of C5 naphthenes. It is well known in FCC chemistry that ZSM-5 is highly Figure 2 Content of C5 unsaturated species in light cracked naphtha (LCN) and pygas C5 cut

PTQ Q3 2026