Increasing diesel production from the FCCU
FCC naphtha desulphurisation technology produces low-sulphur heavy catalytic naphtha as a separate product for blending into the diesel pool
Maurice Korpelshoek, Gary Podrebarac, Kerry Rock and Rajesh Samarth, CDTech
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Sulphur concentration in the gasoline pool is being reduced in many parts of the world. In Europe, Euro V regulations, implemented in 2009, require less than 10 ppm sulphur in gasoline. Since 2008, the gasoline sulphur specification in the US and Canada has been less than 30 ppm on average, and a further reduction to 10 ppm is under consideration in the US. In Europe and North America, diesel with a sulphur content in this range is referred to as ultra-low sulphur diesel (ULSD). In Russia, the gasoline sulphur content will be reduced to 10 ppm by 2015. It is likely the sulphur specification in the rest of the world will also become more stringent over time.
Gasoline is a complex product, with many important parameters determining its quality, aside from sulphur concentration; these include vapour pressure, benzene concentr-ation, boiling range and octane rating. In some regions, such as the EU and India, demand for diesel is higher than for gasoline. In others, including the US, demand for diesel is projected to grow while gasoline demand declines, as shown in Figure 1, which also illustrates the future projected change in the global gasoline-to-distillate ratio.
To reduce gasoline product and increase diesel product, some refiners produce lower end-point gasoline, routing the heavier cut of the full-range gasoline to the diesel pool. This results in higher capacity requirements for the diesel hydro-treaters, which are often already fully utilised.
To remove sulphur from FCC gasoline, CDTECH offers the commercially proven LCN (light catalytic naphtha) CDHydro and CDHDS processes. These processes, which employ the principle of catalytic distillation, conduct selective hydro-desulphurisation in a distillation environment. The full-range gasoline is split into three different cuts, giving the refiner blending flexibility. It is possible to use the CDHDS process to tailor the sulphur content of the distillate and bottom product, making it feasible to produce 10 ppm low end-point gasoline and a separate heavier cut that meets the less than 10 ppm sulphur specification for blending into the diesel pool. This flexibility can provide significant value by debottlenecking the diesel hydrotreater.
Properties of FCC gasoline
Figure 2 shows a plot of the concentrations of total sulphur, mercaptan sulphur (RSH) and olefins (measured via bromine number) as a function of the boiling point of the FCC gasoline. At the light end of the gasoline, the olefin concentration is high and the total sulphur concentration is relatively low. Nearly all the total sulphur is in the form of RSH. As the boiling point increases, the sulphur concentration begins to increase quite significantly, while the RSH concentration actually declines. At the heaviest end, most of the sulphur is contained in compounds such as benzothiophene and methyl benzothiophene. Conversely, the olefin concentration profile follows the opposite trend: the lighter end of gasoline is olefin-rich, while the heavier end contains very few olefins.
The data plotted in Figure 2 illustrate the challenge involved in treating the light end of the gasoline through to the heavy end. To meet a 10 ppm sulphur specification, the light cut requires only 90% conversion of sulphur; the middle cut (near 135°C) requires 99% conversion, while the heaviest cut requires 99.8% sulphur conversion.
The preservation of olefins is vital to reducing hydrogen consumption and minimising octane loss. Olefin saturation, although inevitable in a hydrodesulphurisation process, is minimised when sulphur conversion is kept to a minimum. With this fact in mind, an ideal desulphurisation process would provide an environment where the highest severity is applied only to the heavy fraction of the gasoline, which has high sulphur and low olefin concentrations. The olefins in the heavy fraction also have a lower octane content than the olefins in the light fraction. Reaction severity would be decreased for the lighter fractions, which have lower sulphur and higher olefin concentra-tions. Treating the lighter fraction at lower severity limits the saturation of valuable olefins in this olefin-rich region.
CDTECH has developed a selective treatment method for full-range FCC gasoline, which optimises the severity of treatment for different cuts of gasoline to maximise sulphur removal while minimising olefin loss. As Figure 3 shows, the first step is to treat the lightest fraction of the gasoline in a LCN CDHydro unit, where the RSH is non-destructively removed. The LCN CDHydro unit is not a hydro-desulphurisation step. It operates at very mild conditions, resulting in no measurable olefin loss.
Part of the rectification section of the LCN CDHydro column contains catalyst packed in a distillation structure. The rest of the column contains conventional distillation trays. The LCN CDHydro unit works by performing an additional reaction between the RSH and the contained diolefins over the catalyst to form a heavier sulphide (RSR). The heavy sulphide goes to the bottom of the LCN CDHydro column and exits with the bottom product.
The light product from the LCN CDHydro column has a very low RSH content, very high olefin concentration, a high octane rating and a relatively high Reid vapour pressure (RVP). Some refiners choose to isolate this fraction in order to increase flexibility in the blending operation.
The LCN CDHydro column bottoms go to the CDHDS column. This is the hydrodesulphurisation step that converts sulphur in the gasoline to H2S. The CDHDS column also contains a hydrodesulphur-isation catalyst packed in a distillation structure. Since the process is based on distillation, the catalyst structure below the feed point operates at much higher temperatures than the catalyst structure above the feed point. This has the effect of increasing reaction severity for the heavy portion of the gasoline, where the requirement for sulphur conversion is the highest and the olefin content is the lowest. Simultaneously, the medium catalytic naphtha (MCN), which contains higher olefin levels and fewer refractory sulphur com-pounds, goes up the column, where conditions are less severe than they are at the bottom. Thus, the reaction conditions in the CDHDS unit are ideally suited to the goals of preserving olefins and minimising octane loss.
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