Environmental Engineering Reference
In-Depth Information
3.1.1 Light Feeds
According to the flowsheet of conventional refinery shown in Fig. 2.4 , atmospheric distillates,
coking distillates, and vacuum gas oil (VGO) are among the fractions requiring
hydroprocessing. Coking distillates are usually fractionated to naphtha and heavy gas oil
(HGO) fractions. Figure 2.4 shows that VGO is subjected to hydroprocessing to obtain fuels.
But, VGO can also be the feed for fluid catalytic cracking (FCC), particularly if gasoline is a
preferred product. These feeds are free of contaminants, such as metals, resins, and
asphaltenes. However, depending on the origin of crude, the composition of the virgin
fractions having a similar boiling range may exhibit a great variability. Because of a higher
temperature employed, a higher content of olefinic and aromatic structures is expected for
coking and FCC distillates. From the processing point of view, the content of sulfur and
nitrogen is of primary interest. These heteroatoms are in the form of heterocyclic rings. The
stability and/or refractory nature of these rings increase with increasing molecular weight. The
distribution of hydrocarbon groups must be also adjusted to meet the specifications of
commercial products. For example, the content of n-paraffins must be low to ensure desirable
cold flow properties of fuels and lubricants. The content of aromatics in diesel fraction must be
kept below specified limits as well.
The primary products from FT synthesis contain no metals and little of sulfur and nitrogen.
Heteroatom containing compounds is dominated by oxygenates and small concentrations of
water, which is the product of FT synthesis. Most of the oxygenates have aliphatic structures.
Hydrocarbon groups of the FT synthetic crude are dominated by straight chain paraffins and
olefins. Small quantities of aromatic and naphthenic structures can also be present. The
principal objective of the upgrading FT products is the hydroisomerization (HIS) of n-paraffins
and n-olefins to i-paraffins. However, these reactions are affected by the presence of
oxygenates. Therefore, hydroprocessing, as the primary step during upgrading of the FT
products, must be conducted to extend the life of HIS catalysts, which usually contain noble
metals.
3.1.2 Medium Heavy Feeds
For the purpose of this topic, a medium heavy crude is characterized as one having less than
100 ppm of V+Ni and less than 10 wt.% of Conradson carbon residue (CCR). According to
Table 2.1 [27] , the atmospheric residues (340+ C) derived from Arab Light crude and North
Sea Ekofisk crude containing 49 and 12 ppm of V+Ni, respectively. For the latter crude, even
vacuum residue (VR) (33 ppm of V+Ni) is classified as a medium heavy feed. Therefore, with
respect to metal content, the VR (565+ C) derived from North Sea Ekofisk crude represents a
rather unique case of VR. Some DAO can also be classified as medium heavy feed. The
 
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