Meeting Euro IV fuel specifications
Pretreating technology and special catalysts can help refiners meet the current and forthcoming European environmental regulations and improve their FCCU performance.
Georgy Andonov, Stefan Petrov, Dicho Stratiev, Lukoil Neftochim Bourgas AD
Per Zeuthen, Haldor TopsÃ¸e
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Lukoil Neftochim Bourgas AD (LNB) is currently the only refinery operating in Bulgaria, supplying the Bulgarian market with motor fuels in accordance with the Euro III specifications, which will be in force until 31 December 2006. Bulgaria is expected to enter into the European Union on 1 January 2007, and from that date on all EU regulations will be mandatory in Bulgaria. LNB is a fluid catalytic cracking (FCC) based refinery. The FCC unit processes one-fourth of the crude oil distilled (heavy vacuum gas oil, or HVGO) and provides about 50% of the refinery gasoline pool and 15% of the diesel pool. The FCC unit consists of a feed hydrotreating section, a FCC reactor-regenerator section and a vapour-recovery section.
In order to meet the Euro III specifications for gasoline, the LNB FCC pretreater was until November 2004 operated in HDS mode, which achieved about 90% HDS (0.2% FCC feed sulphur) and about 5% net conversion. However, the rapid penetration of Euro IV motor fuels into the Bulgarian market along with the increasing demand for diesel posed a challenge to the refinery. To meet this, LNB changed the mode of its FCC pretreater operation from hydrodesulphurisation (HDS) to mild hydrocracking (MHC). The net conversion was increased from 5–11% and FCC feed sulphur was reduced from 0.2–0.04% (FCC gasoline sulphur dropped from 140–20ppm). The FCC feed nitrogen level was also reduced, resulting in an increase in the gasoline precursors and a 1 wt% rise in FCC conversion and FCC gasoline yield. The FCC gasoline composition and RON remained unchanged.
A test run was conducted in the FCC pretreater to assess the feasibility of producing FCC gasoline with less than 10ppm sulphur. Within the context of this discussion, the results from this test run are reported with respect to the required increase in weight average bed temperature (WABT), increased conversion, sulphur and nitrogen content in the product, as well as the effect on the unit cycle length. An evaluation is made of the feasibility of improving the FCC feed quality and the consequent increase in FCCU conversion and profitability obtained by using Topsøe’s Aroshift process.
The increased demand for diesel fuel in Europe and a narrow sulphur specification for diesel and petrol force refiners to make decisions that influence their future investment plans. Hydrocracking technologies play an essential part in meeting these challenges. In particular, medium-pressure hydrocracking can convert heavy distillates and fuel oils into high-quality middle distillates that meet the new specifications. This opportunity is especially attractive to refiners who already have a FCC pretreater in their process scheme.
Results and discussions
The LNB FCC pretreater HVGO feedstock is produced from a mixture of Ural oil, reduced crude, VGO, low-sulphur crude oil and naphtha. Table 1 presents the typical HVGO properties. Topsøe’s proprietary TK-558 BRIM catalyst has been used in the FCC pretreater since 1 January 2004. The catalyst has been employed in two modes of operation: HDS and MHC. In order to meet the Euro III specifications for gasoline, the FCC pretreater was first operated in HDS mode, which achieved about 90% HDS (0.2% FCC feed sulphur), which allowed the FCC gasoline sulphur to not exceed 140ppm. The HDS mode of operation was applied for ten months with a start-of-run (SOR) temperature of 319°C and an end-of-run (EOR) temperature of 333°C (a catalyst deactivation rate of 1.4°C/month).
LNB changed its FCC pretreater operating mode from HDS to MHC to meet the previously mentioned rapid penetration of Euro IV motor fuels along with increased diesel demand. After ten months of operation, the temperature of the first FCC pretreater reactor increased from 333–360°C. Table 2 summarises operating conditions in the hydrotreating reactors plus yield distribution in the pretreater and FCC reactor/ regenerator in the HDS and MHC modes of operation.
It is evident from the data that the change in mode of operation from HDS to MHC led to an increase in gross pretreater conversion from 14–19% (net conversion from 5–11%). The diesel (180–360°C) yield was increased from 11.7–15.9%. The increased pretreater severity improved FCC feed quality, resulting in an FCC conversion increase from 76.2–77.4%.
The physical and chemical properties of hydrotreated HVGO (HTHVGO, unconverted pretreater product, feed for FCC), pretreater diesel and FCC gasoline are given in Table 3. The table shows that the content of FCC sulphur and nitrogen in the feed was reduced from 0.2–0.04% and from 0.09–0.07% respectively as a result of the change from HDS to MHC mode. The FCC feed gasoline precursor level was increased from 78.5–81.1%. The higher FCC gasoline precursor level along with the lower nitrogen content may explain the higher FCC conversion observed when the pretreater is operated in MHC mode. The FCC gasoline sulphur dropped from 140–20ppm as a result of increased pretreater severity. The FCC gasoline composition and octane number remained unchanged. The data also show that it was not possible to directly blend the pretreater diesel into the Euro III diesel commodity in HDS mode. Neither was it possible to directly blend it into the Euro IV diesel commodity in MHC mode of operation. The MHC diesel is typically used for production of diesel commodity and needs additional hydrotreatment.
In this study, data obtained indicated that the pretreater diesel sulphur is about three times lower than the unconverted product sulphur, whereas the FCC gasoline sulphur is about 20 times lower than the FCC feed sulphur. The distribution of FCC feed sulphur and the numbers in parentheses given in Figure 1 illustrate nitrogen in the products. The data show that 8% of FCC feed sulphur accounts for regenerator SOx emissions. Thus, the regulation of 400mg/Nm3 (140ppm) may be achieved at a feedstock sulphur level not higher than 0.1%. More than a half of the feedstock nitrogen species end up in the coke (60%) and 8% of the coke nitrogen contributes to NOx — 450mg/Nm3 (180ppm). The regulation of 200mg/ Nm3 is therefore difficult to achieve by hydrotreatment of FCC feed alone. It might, however, be obtained by the use of NOx additives.1
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