Is there an alternative to conventional hydrotreating of FCC naphtha that will reduce octane loss?
Responses to a question in the Q1 2021 issues Q&A feature
Various from Sulzer GTC, BASF Catalysts, Unicat and G. W. Aru LLC.
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Charlie Chou, Business Segment Leader Licensing & Technology, Sulzer - firstname.lastname@example.org
The octane loss from FCC naphtha hydrodesulphurisation is due to the unavoidable saturation of olefins going through the HDS unit. Sulzer GTC’s GT-BTX PluS technology can separate the sulphurs from olefins in the FCC naphtha, allowing sulphur compounds to go to the HDS while low sulphur (<10ppm) olefins can skip the HDS and go directly into the gasoline pool. By preserving the olefins from skipping the HDS, GT-BTX PluS can preserve almost the entire octane number of FCC naphtha, regardless which HDS catalyst is used. Also thanks to olefins skipping the HDS, hydrogen consumption in the HDS will be reduced by more than half, and catalyst cycle length will be extended.
An alternative to hydrotreating the FCC naphtha (which results in sometimes significant octane loss) is to produce cleaner naphtha from the FCC directly. An initial attempt to do this involves a look at the crude diet to see if lower sulphur crude feedstocks are an option. If, given the crude diet, the naphtha sulphur from the FCC is still higher than desired, another effective method to do so is to use sulphur reduction technology in the FCC to generate lower sulphur FCC naphtha.
In refinery trials with this product, we have seen significant gasoline sulphur content decreases, around 35%. We have employed FCC applications targeting naphtha sulphur ranging from hundreds of ppm to ~10 ppm sulphur (depending on the region and depending on the regional gasoline sulphur specifications). The technology works in conjunction with the base catalyst and is integrated into the catalyst without sacrificing any catalyst activity (as opposed to FCC additives like ZSM-5). The technology works by cracking or coking the gasoline-range sulphur containing molecules to push them to other product streams, ultimately leaving the FCC as either H2S or SOx. By employing a change in crude diet to lower sulphur or the aforementioned gasoline sulphur reduction technology, you can either bypass the naphtha hydrotreater completely or reduce its severity (thereby reducing any loss in octane) to preserve naphtha octane and quality. Then, after this is all in place, it is still possible to further reduce the gasoline sulphur by acting on the fractionation overlap between light and heavy naphtha (or heavy naphtha and LCO), since often small amounts of the next heavier stream brings significant sulphur in. So, acting on the naphtha splitter by increasing the overhead reflux ratio, or enhancing the tray efficiency (going from single to double pass trays, to high efficiency trays or even to packing) can significantly reduce the sulphur of the gasoline going to the hydrotreater.
Tom Ventham, Sales & Technical Europe and Africa, Unicat - email@example.com, CJ Farley, Senior Technical Services Engineer, G. W. Aru LLC - firstname.lastname@example.org, Natalie Herring, Director of Technology and Business Development, G. W. Aru LLC - email@example.com, Kate Hovey, Senior Technical Services Engineer, G. W. Aru LLC - firstname.lastname@example.org
Octane loss through cat gasoline hydrotreaters is an unfortunate side effect of the objectives of these units to reduce the sulphur content of the final product to meet road fuel specifications. In general, a two-point road octane (R + M)/2 loss can be expected when FCC naphtha is hydrotreated. This octane loss can be higher if deeper hydrotreatment is required due to greater sulphur content of the cat naphtha or tighter final product specification.
Two options are presented here for the operator to consider. Both aim to improve cat naphtha properties before hydrotreatment through modification of FCC catalytic reactions. The first option is to reduce the sulphur content of cat naphtha by up to 30% through use of a gasoline sulphur reducing additive, such as GSBuster from G. W. Aru and Unicat. By doing this, hydrotreatment severity can be reduced, minimising loss of octane, as well as reducing hydrogen consumption. If the sulphur content can be reduced sufficiently using a gasoline sulphur reducing additive, a slip-stream of cat naphtha can bypass the hydrotreater and be sent directly to the gasoline pool whilst still meeting sulphur specification and further maximising octane.
The typical distillation range of FCC naphtha contains what is often referred to as an ‘octane dip’, with higher octane value material concentrated in both the low and high boiling point gasoline ranges. There is a reduction in octane observed in the middle boiling point region. Sulphur, on the other hand, concentrates in the heavier boiling point range of FCC naphtha. For this reason, many refineries opt to divert light FCC naphtha to the gasoline pool and bypass the hydrotreater. This allows high octane material in light gasoline to be retained whilst concentrating efforts to reduce sulphur content in the remaining portions of FCC naphtha.
The second option presented is to further upgrade the octane value of cat naphtha prior to hydrotreating, assuming constant octane loss through the hydrotreater for a given sulphur removal. FCC gasoline octane can be effectively improved using ZSM-5 based additives, such as Ultra C3Booster or Ultra C4Booster from G. W. Aru and Unicat. There are two parallel mechanisms that boost the octane value of FCC gasoline when ZSM-5 is used. Low octane value molecules are upgraded to LPG olefins by ZSM-5, resulting in a higher octane value of the remaining material. The second mechanism involves the isomerisation of gasoline range molecules, as promoted by the shape selective crystal cage of ZSM-5. Branched molecules have a higher octane rating than straight-chains, hence increased isomerisation within the gasoline range will boost FCC naphtha octane value.
Should this methodology be of interest but additional LPG components are of little value due to limited export potential, increased yields of LPG cannot be handled due to downstream processing capacity limitations, or it is not economic to sacrifice gasoline yield, other ZSM-5 family additives, such as Ultra C8Booster, can be used. This additive minimises cracking reactions to LPG but retains shape selectivity properties to increase octane in the cat naphtha product through increased branch formation.
Through 2020, refiners have been employing ZSM-5 chemistries more than in previous years. This year, optimisation of refinery flowsheets is driving increased production focus on propylene and butylene, and less on transportation fuels, such as gasoline. These higher value LPG species can be alkylated or esterified to dramatically improve the overall refinery naphtha pool properties. Consequentially, this results in a lower volume of naphtha requiring hydrotreating with a corresponding major net gain to the overall octane balance of the entire naphtha pool.
Gasoline Production Technology and Methods, and an Evaluation of Their Economic Viability, Takashi Hagiwara, Technology Department, Petroleum Energy Center (PEC) https://fisicoquimicaciencias.files.wordpress.com/ 2019/11/ana.pdf
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