Question
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What options are available for increasing the number of higher octane gasoline components?
Jun-2024
Answers
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Grant Severyn, BASF Refining Catalysts,
Multiple strategies can be utilised to increase FCC gasoline octane. First, increasing the octane of the ‘as-produced’ naphtha is an obvious choice, either by formulating a base catalyst with reduced hydrogen transfer to preserve high octane molecules, adding ZSM-5 additives also, or a combination of both. Of course, process condition adjustments, such as increasing reactor outlet temperature (ROT), can also increase the as-produced octane.
Second, producing high octane blending components is another common choice. The desired blending component is alkylate, having a RON value typically ranging between 92 and 98 (a function of feedstock used between – C₃, C₄, or C₅).
Historically, refiners have used an olefins additive, including ZSM-5, in FCC units to capture value from the market’s demand for increased light olefins for alkylation and higher-octane gasoline components. The ZSM-5 zeolite is designed to selectively crack gasoline-range molecules into propylene (C₃=) and, to a lesser extent, butylenes (C₄=). In general, C₄= alkylate has a higher road octane ([RONC + MONC] / 2) value than C₃= alkylate, and C₄= alkylate is less cost intensive to produce. As an example, alkylate made from C₃= and isobutane can have a road octane of up to 92, whereas alkylate from C₄= can have a road octane of up to 98. Therefore, C₄= alkylation is the preferred method for most refiners operating alkylation units.
Since ZSM-5 zeolite tends to generate more propylene than butylenes (see Figure 1), there are strong incentives for FCCs, which feed to alkylation units, to increase C₄=/C₃= selectivity through the base catalyst technology to generate higher-octane alkylate species. FCC catalysts can be tuned to effect such change. Recent technical advancements have made this even more possible, with some refiners having taken it upon themselves to expand their alkylation units to fully utilise the benefits that improved technologies can offer.
Fourtune and Fourtitude FCC catalysts for vacuum gasoil (VGO) and resid applications, respectively, utilise BASF’s Multiple Framework Topologies (MFT) technology (Figure 1) to maximise butylenes yield and selectivity over propylene.
The multiple zeolite frameworks have optimised acid site density and strength to ensure selective butylenes yield over propylene, as well as enhanced porosity to reduce diffusion limitations and minimise saturation reactions. The mechanism behind the success of such butylenes-maximising catalysts involves both generation and preservation (avoiding saturation) of C₄ olefins while preserving high-octane molecules in the gasoline range.
Fourtune and Fourtitude catalysts have been used in multiple commercial FCC units with stand-out performance in C₄=/C₃= selectivity (up to 1% volume increase in butylenes yield at constant propylene yield) and FCC naphtha octane for gasoline blending (up to 2 RONC increase) compared to alternative suppliers in unit operating data. These changes have allowed refiners to increase their refinery gasoline octane by two methods: improved octane of FCC-generated naphtha and an increase in alkylate production.
Fourtune and Fourtitude are marks of BASF.
Jul-2024
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