FCC gasoline and C4 streams for BTX production
An extractive distillation process provides direct recovery of aromatics from FCC gasoline to rebalance refinery production towards higher levels of petrochemicals
Joseph Gentry and Weihua Jin
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Declining demand for gasoline in many markets has produced a surplus of FCC gasoline. With new FCC technologies used to maximise the production of propylene, more benzene, toluene and xylenes (BTX) are generated in the FCC gasoline. However, increasing restrictions through government regulations are being imposed on the aromatics and sulphur content of gasoline. Table 1 shows the historical and current gasoline regulations in the US and Europe. We anticipate that the requirements for lower aromatics and sulphur content in fuels in developing regions will over time match the currently more stringent levels in Europe and the US.
Aromatics is a fast-growing market. Chemical Market Associates forecast the growth in demand for benzene at 4.1% per year between 2000 and 2020, representing a new demand of 24.3 million tonnes.1 Capacity for mixed xylenes is likely to double by 2020 to meet the strong anticipated growth in demand.1 This growth will in turn be driven by a strong demand for polyester, for which Strategic Business Analysis reports historical growth rates of 7% per year between 1997 and 2007, and predicts annual growth of more than 6% over the next ten years.2 Since demand for gasoline is generally declining, and the specifications for gasoline limit the level of aromatics, it is logical to convert gasoline components to aromatics.
Significant amounts of aromatics can be recovered from refinery cracked naphtha streams for use as petrochemical feedstocks. This can be accomplished while offering the option to produce incremental, low-cost aromatics and light olefins from alternative feed sources. The new approach also reduces the sulphur and olefin contents of gasoline, while moving towards an environmentally acceptable specifi-cation. Focusing on aromatics production also offers the opportunity to rebalance gasoline supply/demand in certain regions. This frees up naphtha reformer capacity to accept fresh feed naphtha, so increasing the production of aromatics and hydrogen.
This is an especially attractive proposition in Europe. The region has limited availability of aromatics but a relative surplus of gasoline. Environmental pressures are leading to tighter gasoline specifications, which mandate reduced sulphur and olefin contents, while severely limiting the benzene content of gasoline.
Driven by a fast-growing demand for propylene, high-severity FCC is intended to increase olefin yields. Propylene yields can be increased from 3–5% in conventional FCC to 15–28% in the high-severity process.
In a high-severity FCC operation, the aromatic content of cracked naphtha is also increased to the level of 50–70%, but this contains significant amounts of thiophenic sulphur impurities and is high in olefin content. Thus, a special process is needed to recover aromatics from this stream. The typical sulphur content of FCC gasoline ranges from 1000–2000 ppm. This is the dominant source of sulphur in a typical gasoline pool. Desulphurisation of FCC gasoline is required to meet regulations. To reduce the sulphur content effectively, and to minimise octane loss, it is necessary to separate the olefins from the sulphur.
Table 2 shows sulphur distribution and type, with corresponding carbon number, in a typical FCC gasoline. The light ends are relatively low in sulphur, with mercaptans being the major species. Heavy FCC gasoline contains predominately thiophenic sulphur. Typically, the olefins are concentrated in the light fraction, with few olefins in the heavy ends.
The sulphur content can be reduced considerably by hydrodesulphurisation (HDS), but the reactions are, inherently, not perfectly selective. Thus, some olefins are saturated, which results in octane loss and excessive consumption of hydrogen. Figure 1 shows a traditional three-stage FCC gasoline desulphuri-sation process to remove sulphur. FCC naphtha is first separated into three fractions by distillation. Since the primary source of sulphur in the light cut naphtha (LCN) is mercaptans, a caustic extraction process is an effective way to remove them. Alternatively, mild HDS can be used. Therefore, most of the octane value of the light fraction is retained.
For middle cut naphtha (MCN), the increased content of thiophenic sulphur requires medium-severity HDS to remove it. This will unavoidably result in saturation of C6-C9 olefins, with resulting octane loss. A decrease in octane numbers of approximately 20 occurs as a result of saturating an olefin to the corresponding paraffin. Heavy cut naphtha (HCN) is processed by severe HDS, but the low olefin content of this stream means octane loss is minimal.
Some refiners believe that if they process FCC gasoline through a naphtha reformer, they can obtain more aromatics. In theory, reformed FCC gasoline does have a high aromatic content, but this material is not a particularly good reformer feed. The aromatics simply take a free ride through the unit, while the olefins will consume hydrogen in the naphtha hydrotreater unit before being reformed. Fresh naphtha, on the other hand, will create more aromatics and hydrogen through the reformer unit. This is why uncracked naphtha is always a better choice of feed to the reformer.
Aromatics cannot be directly recovered at high purity by conventional distillation because of the close boiling components and azeotropes that form with aromatics. Therefore, they are typically recovered by extraction with a selective solvent. This can be accomplished through liquid-liquid extraction or by extractive distillation. Extractive distillation offers better plant economics and flexibility, and is generally preferred for BTX purification.
Until recently, refiners did not consider recovering aromatics from FCC gasoline, because the extraction technology would not function with olefinic or sulphur impurities in the feed. GT-BTX PluS technology is designed to achieve this operation by extractive distillation. This permits the direct recovery of aromatics, while rejecting the olefin-rich fraction as raffinate. Sulphur species are also extracted into the aromatic fraction and removed by hydrotreatment in the absence of olefins. Thus, there is very little hydrogen consumption and no octane loss. The hydrogenation unit is much smaller than a conventional unit and can be a simple HDS design. The raffinate from the GT-BTX PluS unit can be sweetened in a conventional caustic unit or used directly in gasoline.
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