Is there a route for post-treating gasoline with minimal octane loss?
Responses to a question in the Catalysis 2021 Q&A feature
Various from Albemarle, Axens, Haldor Topsoe, Hoekstra Trading, Honeywell UOP and W R Grace.
Viewed : 206
Olivier BOISIER, Technology Team Manager, Catalysts & Adsorbents Global Business Unit - firstname.lastname@example.org; Boris HESSE, Technologist, Catalysts & Adsorbents Global Business Unit - email@example.com
For the post-treatment of FCC gasoline, fixed-bed HDT is by far the most trusted solution because of its low capex, its ease of operation, and its flexibility. This technology also allows for excellent octane retention provided there is combined use of smart schemes and highly specific catalysts capable of very high HDS rates with minimum olefins saturation. Axens offers both as part of the Prime-G+ trademark.
A smart scheme should take advantage of the particularities of the FCC gasoline, the most striking being that olefins are more concentrated in the light fraction of the gasoline while sulphur is more concentrated in the heavy fraction (see Figure 1). The combination of the Prime-G+ SHU reactor with the Prime-G+ splitter capitalises on this and allows for the production of an olefins-rich, desulphurised naphtha, bypassing the HDS section and thereby preserving all the valuable light olefins.
The formation of mercaptans in the HDS section through the addition of H2S to olefins is a critical matter when targeting ultra-low sulphur FCC gasoline (see Figure 2). Basically, each ppm of mercaptan that is formed inside the unit requires that an additional ppm of sulphur from the feed be removed, which can dramatically impact octane retention since the octane loss increases exponentially with the HDS rate. Axens offers several solutions, both catalytic and schematic, to tackle this problem.
On the road to high octane, one should also lean on the operation of the FCC as it can have big consequences for both the absolute octane value of the FCC gasoline and octane retention in the post-treating unit. Higher FCC severity for instance generally makes for a more aromatic gasoline, which directionally has a higher octane value (see Table 1). The olefins distribution is modified as well which has an impact on octane preservation in the post-treating unit.
The challenge for catalyst manufacturers is to provide catalytic solutions addressing high octane retention along with high activity/contaminants tolerance as those parameters generally go in opposite directions. High activity/contaminants tolerance is key to long cycle lengths matching the FCC turnaround schedule and to important energy savings. With HR 845 for the SHU and the duo HR 806/HR 841 for the HDS boasting more than 1000 cumulated references, Axens has created benchmark catalysts for FCC gasoline post-treatment. Capitalising on the feedback from nearly 200 running units and relying on extensive R&D work conducted jointly with IFPEN, Axens is now bringing to the market a full range of new Prime-G+ catalysts: HR 855 for higher SHU activity, HR 856 for higher HDS selectivity and HR 866 for higher HDS activity.
These new catalysts will help refiners take on new challenges and enhance further their profitability, allowing them for instance to process more refractory/more contaminated feeds from both FCC and other sources (co-processing of coker naphtha, pyrolysis gasoline, and so on) and to reduce significantly both octane losses (see Figure 3) and hydrogen consumption.
Colin Baillie, Segment Manager, Environmental Additives, W. R. Grace & Co - Colin.Baillie@Grace.com; George Hoekstra, President, Hoekstra Trading - firstname.lastname@example.org
One way to minimise octane loss in post-treated gasoline is through adjusting the FCC process to optimise the properties of FCC gasoline, prior to the subsequent post-treatment.
FCC gasoline typically consists of >300 different molecules, composed of paraffinic, olefinic, and aromatic structures. Portions of these molecules subsequently undergo further hydrogenation reactions when post-treated, which leads to octane loss. The level of octane loss is strongly impacted by the specific olefins that are being saturated, for example saturation of linear olefins, branched olefins, and cyclic olefins will lead to different levels of octane loss, as will the carbon number of the olefins. This is because these olefinic species have different octane values, as do the resulting paraffins that are formed upon post-treatment. The sulphur content of the FCC gasoline and type of sulphur species (mercaptans, thiophenes, benzothiophenes) is also important, as it impacts the post-treatment severity (and therefore octane loss) that is required to achieve a target sulphur level.
W. R. Grace & Co. and Hoekstra Trading LLC have worked together to provide unique insight into how the FCC process can be optimised to reduce octane loss of post-treated gasoline. Detailed hydrocarbon analysis and sulphur speciation of FCC gasoline and the corresponding post-treated gasoline combined with Hoekstra Trading’s software allow the analysis of individual reactions occurring upon post-treatment, and ultimately the octane impact of those reactions. Tailored FCC catalyst and gasoline sulphur reduction technology can then be employed to optimise FCC gasoline properties for minimum octane loss of post-treated gasoline. Such analysis has revealed that improving gasoline octane/sulphur performance can be done quickly, with no capital investment, and with the potential to generate a significant increase in profitability.
Claus Brostrøm Nielsen, Senior Solution Specialist, Haldor Topsoe - email@example.com
It is important to understand the molecular composition of FCC naphtha before looking at desulphurisation of cracked naphtha and octane loss. If we look at boiling points of the sulphur species, like mercaptans, thiophenes, and their derivatives, we can classify cracked naphtha into two broad ranges: light cracked naphtha (LCN) and heavy cracked naphtha (HCN). For better differentiation, it can be said that fraction boiling above 80–85°C (176–185°F) can be considered as HCN. Most of the sulphur in the LCN fraction is mercaptans. The refractive sulphur species are distributed in the HCN fraction.
The LCN fraction contains a large part of the total amount of olefins present in the FCC naphtha. Olefins with carbon number 4, 5, and 6 will be present in the LCN fraction, and olefins with higher carbon numbers will be present in the HCN fraction. This is important to know when going into design strategy for a post-treatment process specific for FCC naphtha. The main challenge in FCC gasoline post-treatment is to selectively remove sulphur molecules while avoiding saturation of olefins, which are a major contributor to octane number of gasoline. Saturated olefins have significantly lower octane than the corresponding unsaturated molecules. When increasing the sulphur removal, the olefin saturation and corresponding octane loss increase almost exponentially the closer you get to 100% HDS conversion.
Typically, post-treatment of FCC naphtha consists of a two-step process: selective hydrogenation of diolefins followed by a hydrodesulphurisation (HDS) step to convert the sulphur species. Selective hydrogenation is undertaken at a low temperature compared to downstream hydrodesulphurisation. Selective hydrogenation of cracked feed also transforms the light sulphur species into heavier sulphur-bearing molecules. With the transformation of lighter sulphur species like mercaptans into heavier sulphide molecules, an almost sulphur-free LCN fraction can be obtained. These heavier sulphide molecules will now end up in the HCN fraction. A typical layout of a post-treatment unit consists of a selective hydrogenation unit, a splitter column to separate LCN and HCN, and then a hydrodesulphurisation unit (HDS). The separation of LCN from HCN in the splitter, which follows the SHU section, ensures that we end up with LCN, which is sulphur-free, and HCN that needs to be hydrodesulphurised. The high octane of the LCN stream can therefore be retained, since no further hydrodesulphurisation is required. The HCN fraction is sent to the HDS section.
In the HDS section of the unit, hydrodesulphurisation of the HCN fraction takes place to meet the required sulphur specifications. Hydrotreatment catalysts with optimised activity and selectivity, which enable removal of sulphur to ultra-low levels while retaining high octane numbers, are the keys to a successful FCC gasoline post-treatment. Haldor Topsoe’s series of HyOctane catalysts is specifically developed for all steps in the FCC gasoline post-treatment applications and meets the requirements for optimised activity and selectivity which enable a profitable production of high-quality gasoline with minimum octane loss. The catalysts are designed with high selectivity, which minimises mono-olefin hydrogenation even at ultra-low sulphur levels down to 10 wtppm. This ensures that a high octane number is maintained in the final gasoline product.
The Haldor Topsoe licensed HOT process (HyOctane Technology) is developed to provide even better octane retention than today’s available technologies. The HOT process is a combination of unit layout, process conditions, and the HyOctane catalysts, and when combined together it can obtain an ultra-low sulphur gasoline product with significantly higher octane value than typically seen from other FCC gasoline post-treatment units.
The route to FCC gasoline post-treatment with minimal octane loss goes through the correct unit layout and selection of the optimum catalysts.
José A. Toledo, Technical Service Engineer, Albemarle - firstname.lastname@example.org
FCC naphtha is the most common gasoline pool blend component, accounting for 30-50% of the overall gasoline blend, and is the biggest contributor with respect to sulphur.
With gasoline regulations focusing on product sulphur levels, post-treating FCC naphtha for reduced sulphur also impacts octane quality. However, because many refiners lack FCC pretreatment capacity, cat naphtha post-treatment remains a commonly applied solution. In any case, the key is to avoid large octane losses that occur due to saturation of olefins during conventional hydrodesulphurisation of full range cat naphtha.
Other processing options are available depending on refinery choice but handling the different cat naphtha fractions (LCN, low boiling and ICN/HCN, intermediate/high boiling) separately is one of the most common approaches. It is widely known that olefin and sulphur content distribution in full range FCC naphtha moves in opposite directions: most of the olefins, which are desired high octane contributors to the gasoline pool, are found in the lighter portion (LCN), while the majority of sulphur is concentrated in the intermediate and high boiling range (ICN/HCN) fractions.
By separating the lighter boiling range naphtha (LCN), a fraction is created wherein most of the sulphur compounds consist of light, low boiling point mercaptans and sulphides. These species are present in LCN in low concentration and are easy to desulphurise. Often, applying just sweetening processes is sufficient and allows for the preservation of olefins compounds and high octane. For deep removal of the refractory and more concentrated sulphur compounds (thiophenes) in heavier fractions of the FCC naphtha (ICN/HCN), a highly selective HDS that can specifically tackle difficult sulphur while minimising olefins saturation is required. Notably, by separating LCN from FCC naphtha, the concentration of sulphur in the heavier fractions (ICN/HCN) increases compared to the full range naphtha itself, which makes it more challenging to treat.
High activity RT catalysts, such as those manufactured by Albemarle and developed for ExxonMobil SCANfining, have demonstrated high selectivity (high octane retention) in ULSG units processing high sulphur feeds ranging from ICN to HCN blends when Tier III product sulphur level is required. In addition, refineries without fractionation capabilities that need to process full range FCC naphtha have also benefitted from the market-leading high selectivity of RT catalysts.
On the process side, high improvements have been obtained in lowering octane losses by applying proficient post-treatment hydrotreating processes such as SCANfining II (second generation SCANfining). Commercial experience demonstrates that at 10 wppm product sulphur, SCANfining II saturates about half of the olefin content compared to the original SCANfining process.
The RT catalysts have also shown commercial success in other post-treatment gasoline hydrotreating processes, such as Axens’ Prime-G+.
Steven Zink, Principal R&D Engineer/Scientist, Honeywell UOP - Steven.Zink@honeywell.com
Assuming FCC pre-treat capacity installation is not an option, sufficiently low octane deltas on post-treatment of FCC gasoline can be achieved via sequential application of several key molecular management operations. Light mercaptan sulphur is catalytically sweetened out of the light-cut, highest octane olefin fraction, which is recovered as a distillate product in a gasoline fractionator, with a recovery rate that is subject to the gasoline pool sulphur specification. The hydrotreating catalyst processing the mid-cut should perform with a very high ratio of desulphurisation rate vs olefin hydrogenation rate, in one or two hydrodesulphurisation stages, depending on the feed sulphur content and the maximum tolerable octane loss. At 10 ppm or less sulphur specification (US Tier 3), this fraction will require polishing, due to olefin recombination with the hydrogen sulphide produced in situ. Catalytic polishing of the deeply desulphurised mid-cut, carried out at relatively higher temperatures where recombination is less thermodynamically favourable, should be leveraged as much as possible to retain more olefins. The hydrogen partial pressure should be kept only just high enough to achieve the desired extent of desulphurisation, to limit octane loss by olefin hydrogenation.
To limit the loss of olefins to recombination, the hydrogen sulphide partial pressure should be kept as low as possible via application of a well-maintained amine scrubber to the recycle gas. The heavy gasoline fraction contains the lowest concentration of olefins (more aromatic than olefinic) and the highest sulphur concentration. Such sulphur is also the slowest to convert, relative to the light-cut and mid-cut sulphur. Considering that the few olefins in the heavy fraction contribute the least to gasoline octane, the heavy fraction is best routed to a conventional naphtha hydrotreater, to enable greater production of high-octane reformate gasoline.
George Hoekstra, President, Hoekstra Trading - email@example.com
Octane loss can be optimised with a good selection of feed properties, splitter cut points, HDS reactor conditions, HDS catalysts, FCC catalyst technology, and capital investments:
• Feed properties: controllable variables are feed source (FCC naphtha and other naphtha streams), sulphur species distribution, olefinicity, iso/normal ratios for paraffins, iso/normal ratio for olefins, FCC naphtha cut points, and heavy tail composition. In one of our commercial field tests, a refiner modified feed composition to reduce octane loss from 8 RON to 5 RON when making 10 ppm sulphur gasoline. The change was implemented for normal Tier 3 operation.
• Splitter cut point: thiophenic sulphur should all be driven down into the HDS reactor while maximising C6- olefins in the overhead. Careful optimisation of this can save 2 RON octane versus ‘blind’ operation.
• HDS reactor conditions: reaction rates of sulphur, olefins, and aromatics are affected differently by changes in reactor operating variables and can by optimised using modern analytical methods and models to improve octane/sulphur performance.
• HDS catalysts: we have tested competitive FCC gasoline post-treating catalysts from Axens, Albemarle, Haldor Topsoe, and a fourth supplier, side-by-side in a multi-client catalyst testing program, and ranked them on activity and octane/sulphur selectivity. Results are available to anyone in Hoekstra Research Report 6.
• FCC catalyst technology: FCC catalyst and additives directly affect the distribution of naphtha sulphur, olefins, and aromatic species. By looking at the FCC and its post-treater as an integrated unit, it is possible to increase the reactivity of rate-limiting sulphur compounds, increase the octane of the FCC naphtha, and reduce feed sensitivity to octane loss on hydrogenation. This can have a bigger effect than selection of the post-treating catalyst.
• Capital investments: one of our refineries performed a field test in 2017 showing unacceptably high octane loss when making 10 ppm sulphur gasoline. They did a $20 million revamp to reduce octane loss by 2.5 AKI with $27 million per year benefit. This small capital investment also enabled them to become a supplier of Tier 3 sulphur credits. Refiners considering investment in new units should consider improved, 21st century gasoline desulphurisation technologies.
The above optimisation steps are enabled by use of modern analytical methods, an industry-best octane model, and a process model that tracks the critical reactions affecting octane/sulphur performance, all of which were developed in a three-year, multi-client research programme and are available to anyone at negligible cost by contacting firstname.lastname@example.org, +1 630 330-8159.
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