Selective FCC naphtha desulphurisation
By combining specially designed catalyst and process technology, the SCANfining process selectively removes sulphur from FCC naphthas while preserving olefins, so avoiding undesirable octane loss and H2 consumption
K L Riley, J L Kaufman, S Zaczepinski, Exxon
P H Desai, Akzo Nobel Chemicals
A R Gentry, Kellogg Brown & Root (Now KBR Technology)
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Beginning in 1995, the US Environmental Protection Agency (EPA) required gasoline sold in areas with below-standard air quality to meet certain reformulated gasoline (RFG) standards. Initially, the sulphur content of gasoline was tied to historic company levels. On 1 January 1998 the baseline sulphur content was changed to an historical industry average and gasoline emissions of RFG were defined by a Phase 1 complex-model. This resulted in many refiners making modest reductions in gasoline sulphur levels.
When the Phase 2 complex-model RFG performance standards are introduced on 1 January 2000, many refiners will probably reduce sulphur levels again in order to comply.
Recent gasoline quality proposals for the European Union for the years 2000 and 2005 require significant reductions in sulphur level. In the Asia Pacific region Japan already has a low-level specification. It is anticipated that Canada will follow the US lead in higher quality gasoline, most likely with lower sulphur levels.
Sulphur in gasoline comes mainly from three refinery streams: FCC naphtha, coker naphtha and straight-run naphtha. Of these streams the FCC naphtha is the largest contributor, being responsible for 40–90 per cent of the sulphur in the final product. To achieve lower sulphur levels, the refiner must produce lower-sulphur FCC naphtha.
Although refiners have a number of process options for reducing the sulphur level in FCC naphtha, two that are generating the most interest are: hydrotreating the feed to the FCC unit and hydrotreating the FCC naphtha.
Hydrotreating the FCC feed improves naphtha yield and quality and reduces the SOx emissions from the FCC unit, but it is typically a high-pressure process and is therefore very capital-intensive. Furthermore, manipulation of process feed sulphur alone may not be sufficient to meet future gasoline performance standards. Refineries wishing to process heavier, less expensive crudes will be forced to desulphurise the resulting higher-sulphur naphthas.
Hydrodesulphurisation of FCC naphtha is a low-pressure process and is therefore not as capital-intensive as FCC feed hydrotreating. The selection of an optimum hydrotreating process option for reducing sulphur in FCC naphtha is determined by economic factors specific to a refinery.
In response to a reformulated gasoline and high-sulphur crudes environment, Exxon Research & Engineering Co. (ER&E) has developed the SCANfining (proprietary process name of ER&E) process. This process uses the RT-225 catalyst jointly developed and commercialised with Akzo Nobel, and a combination of process design and operating conditions that maximise catalyst performance.
The RT-225 catalyst is specially designed to selectively remove sulphur from FCC naphthas while preserving olefins in order to avoid undesirable octane loss and higher H2 consumption. This combined catalyst/ process technology package is suitable for retrofit and grassroots units and is now available for license. The process was commercialised in 1994. SCANfining is marketed by both ER&E and Kellogg Brown & Root (KBR).
The EPA introduced reformulated gasoline standards in the U.S. to help certain areas meet the National Ambient Air Quality Standards for atmospheric ozone concentrations. These standards require a reduction in NOx, volatile organic compounds (VOC), and other compound emissions according to a phased-in schedule. Table 1 summarises the emission reductions scheduled for the USA along with the most sensitive gasoline parameters that affect these emissions.
In 1995, the EPA required gasoline sold in non-compliance areas to satisfy RFG standards defined by the simple model which included just a few variables such as oxygen and benzene contents, and Reid Vapour Pressure (RVP). This simple model controlled VOC emissions by limiting summer regional RVP, required a reduction of other regulated compound emissions by a minimum of about 16 per cent relative to 1990 baseline values, and ensured that NOx levels would not increase.
In these regulations, sulphur, olefins, and T90 (90 per cent boiling point) in gasoline were limited to the 1990 company levels [Rhodes A K, Oil & Gas Journal, 17 January 1994]. Beginning 1 January 1998, the quality of RFG was defined for the first time by a complex model that calculates emissions, using eight gasoline quality parameters, including sulphur, benzene, aromatics, and olefin concentrations, RVP, and evaporative properties.
In order to comply with the regulations and meet emission limits, a refiner’s gasoline properties must fall within accepted ranges for all eight parameters.
The structure of the complex model indicates that the gasoline parameters that most affect NOx emissions are sulphur, RVP, aromatics, and olefins. After considering the nature of the complex model, it appears that reducing sulphur will be the primary means refiners will use to comply with the NOx reduction standards scheduled for 1998 and 2000 [Rhodes A K, Oil & Gas Journal, 5 January 1998].
The difficult tradeoffs that the refiner will have to make in adjusting gasoline parameters are illustrated by olefin saturation. Although reducing olefins leads to decreasing NOx emissions, it also increases VOC emissions. Since the complex model indicates that reducing sulphur in gasoline has the largest impact on reducing NOx emissions, this process objective has become very important to the US refiner.
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