possible to convert low-grade blend stocks, such as FCCU or thermal cracker
product streams, to good diesel precursors. Among the more important parameters
to be met both from the performance and the emission aspects of the fuel are
discussed below.
Increasing cetane value. The simplest way to improve cetane number is to use an
appropriate ignition improvement additive. These are mainly alkyl nitrates. The
effectiveness of cetane improvement additives tends to be linear with addition rate;
however, the improvement can vary with different diesel blend stocks. Paraffins which
already have high cetane value respond best to the additives. Aromatics, on the other
hand, which have a low cetane number, have a poorer response. About 500 wppm of
standard alkyl nitrate will increase the cetane value by 2–5 numbers. Another way to
increase cetane number is to modify the hydrocarbon type in the diesel blend. Figure 2
shows the relationship between molecular types and cetane number.
Straight chain paraffin molecules have the highest cetane numbers, while multiple ring aromatics have the lowest. Cetane number improvement also occurs
during the hydrotreating of the gas oil fractions used as blend components in the
diesel product. During hydrotreating to remove the organic sulfur and nitrogen
species found in these untreated gas oil fractions, the molecules containing these
items are opened and saturated to form paraffins and naphthenes. At the same time,
some of the aromatic molecules contained in the gas oils are also opened and
saturated increasing the paraffin/naphthene content of the hydrotreated product. As
a result of this ring opening and saturation, a cetane number improvement of
between 3 and 5 numbers can be expected. Hydrocracking of heavy gas oil fractions
and hydrotreating of thermal cracked gas oils and FCC gas oils significantly
increase product cetane numbers from those processes.
0
0
10
20
30
Cetane Number
40
50
Is o p a ra ff in s
60
70
80
n - P a r a f f in s
90
100
110
10
15
Na ph tha len es
A r o m
a t i c s
O le fi n s
T e tr a lin s
Decalins
M o n o c y c lo n a p h th e n e s
Carbon Number
20
25
Fig. 2 Cetane number of hydrocarbon types
80
D.S.J. Jones and S.A. Treese
product streams, to good diesel precursors. Among the more important parameters
to be met both from the performance and the emission aspects of the fuel are
discussed below.
Increasing cetane value. The simplest way to improve cetane number is to use an
appropriate ignition improvement additive. These are mainly alkyl nitrates. The
effectiveness of cetane improvement additives tends to be linear with addition rate;
however, the improvement can vary with different diesel blend stocks. Paraffins which
already have high cetane value respond best to the additives. Aromatics, on the other
hand, which have a low cetane number, have a poorer response. About 500 wppm of
standard alkyl nitrate will increase the cetane value by 2–5 numbers. Another way to
increase cetane number is to modify the hydrocarbon type in the diesel blend. Figure 2
shows the relationship between molecular types and cetane number.
Straight chain paraffin molecules have the highest cetane numbers, while multiple ring aromatics have the lowest. Cetane number improvement also occurs
during the hydrotreating of the gas oil fractions used as blend components in the
diesel product. During hydrotreating to remove the organic sulfur and nitrogen
species found in these untreated gas oil fractions, the molecules containing these
items are opened and saturated to form paraffins and naphthenes. At the same time,
some of the aromatic molecules contained in the gas oils are also opened and
saturated increasing the paraffin/naphthene content of the hydrotreated product. As
a result of this ring opening and saturation, a cetane number improvement of
between 3 and 5 numbers can be expected. Hydrocracking of heavy gas oil fractions
and hydrotreating of thermal cracked gas oils and FCC gas oils significantly
increase product cetane numbers from those processes.
0
0
10
20
30
Cetane Number
40
50
Is o p a ra ff in s
60
70
80
n - P a r a f f in s
90
100
110
10
15
Na ph tha len es
A r o m
a t i c s
O le fi n s
T e tr a lin s
Decalins
M o n o c y c lo n a p h th e n e s
Carbon Number
20
25
Fig. 2 Cetane number of hydrocarbon types
80
D.S.J. Jones and S.A. Treese