4
difficult still to desulfurize substituted dibenzothiophenes such as 4,6-dimethyl
dibenzothiophene (4,6 DMDBT) due to the refractory nature of these substituted
compounds. Lower HDS reactivity of 4,6 DMDBT is ascribed to the steric hindrance of the substituent groups for accessing the active sites by sulfur species.
Although sulfur present in the crude oil is distributed among the various distillation
fractions, most of the sulfur tends to concentrate in the higher boiling fractions.
During hydrotreating, HDS reactions take place concurrently along with other reactions such as hydrodenitrogenation, saturation of aromatics/olefins, hydrodeoxygenation, etc. As the specification of sulfur becomes tougher in fuels and
desulfurization requirements change continuously, it is of paramount importance to
remove sulfur from refractory species such as benzothiophenes, dibenzothiophenes,
and substituted dibenzothiophenes. Sulfur removal from such refractory compounds
usually requires saturation of adjacent aromatic ring to obtain catalytic access of the
sulfur molecule with the consumption of hydrogen. The type of hydrotreating catalyst plays an important role in achieving such deep desulfurization in order to produce ultralow sulfur fuels such as gasoline and diesel.
Hydrodesulfurization reaction occurs mainly through the following two
pathways:
• Direct desulfurization (DDS) pathway.
• Hydrogenation (HYD) pathway.
In the direct desulfurization pathway, the sulfur compound is removed directly
from the hydrocarbon molecule by hydrogenolysis reaction, and in the hydrogenation pathway, hydrogenation reaction occurs as the first step followed by hydrogenolysis. Cobalt molybdenum-type catalysts mostly desulfurize hydrocarbons
through the DDS pathway, while nickel molybdenum catalysts desulfurize through
the hydrogenation pathway. The actual reaction pathway for desulfurization of a
particular sulfur compound depends mainly on the type of catalyst and process conditions, especially hydrogen partial pressure employed during the reactions.
Hydrodesulfurization through the DDS pathway consumes less hydrogen compared
to the HYD pathway since the latter requires prior hydrogenation of one of the aromatic rings before the actual HDS reaction. The structure or shape of the sulfurcontaining molecule also plays a role in determining the HDS reaction pathway.
The reaction network for the HDS of 4,6-DMDBT over NiMo catalysts supported
on meso-microporous Y zeolites through different reaction pathways such as direct
desulfurization, hydrogenation, and isomerization is presented in Fig. 1 [1].
Hydrodesulfurization reactions are inhibited by certain compounds such as
hydrogen sulfide (H 2 S), nitrogen compounds, and polynuclear aromatics. Each of
the reaction pathways is being inhibited by specific compounds. Hydrodesulfurization
by the DDS pathway is inhibited by H 2 S and basic nitrogen compounds. The HYD
pathway is inhibited by all types of nitrogen compounds and polynuclear aromatics.
Tao et al. [2] studied the inhibiting effects of nitrogen compounds such as quinolone and indole on the HDS of thiophenic sulfur compounds in straight-run gas oil
over a NiW/Al 2 O 3 catalyst and showed that quinolone has a stronger inhibiting
effect on HDS than indole has. The nitrogen inhibiting effects on the thiophenic
G. Valavarasu and B. Ramachandrarao
difficult still to desulfurize substituted dibenzothiophenes such as 4,6-dimethyl
dibenzothiophene (4,6 DMDBT) due to the refractory nature of these substituted
compounds. Lower HDS reactivity of 4,6 DMDBT is ascribed to the steric hindrance of the substituent groups for accessing the active sites by sulfur species.
Although sulfur present in the crude oil is distributed among the various distillation
fractions, most of the sulfur tends to concentrate in the higher boiling fractions.
During hydrotreating, HDS reactions take place concurrently along with other reactions such as hydrodenitrogenation, saturation of aromatics/olefins, hydrodeoxygenation, etc. As the specification of sulfur becomes tougher in fuels and
desulfurization requirements change continuously, it is of paramount importance to
remove sulfur from refractory species such as benzothiophenes, dibenzothiophenes,
and substituted dibenzothiophenes. Sulfur removal from such refractory compounds
usually requires saturation of adjacent aromatic ring to obtain catalytic access of the
sulfur molecule with the consumption of hydrogen. The type of hydrotreating catalyst plays an important role in achieving such deep desulfurization in order to produce ultralow sulfur fuels such as gasoline and diesel.
Hydrodesulfurization reaction occurs mainly through the following two
pathways:
• Direct desulfurization (DDS) pathway.
• Hydrogenation (HYD) pathway.
In the direct desulfurization pathway, the sulfur compound is removed directly
from the hydrocarbon molecule by hydrogenolysis reaction, and in the hydrogenation pathway, hydrogenation reaction occurs as the first step followed by hydrogenolysis. Cobalt molybdenum-type catalysts mostly desulfurize hydrocarbons
through the DDS pathway, while nickel molybdenum catalysts desulfurize through
the hydrogenation pathway. The actual reaction pathway for desulfurization of a
particular sulfur compound depends mainly on the type of catalyst and process conditions, especially hydrogen partial pressure employed during the reactions.
Hydrodesulfurization through the DDS pathway consumes less hydrogen compared
to the HYD pathway since the latter requires prior hydrogenation of one of the aromatic rings before the actual HDS reaction. The structure or shape of the sulfurcontaining molecule also plays a role in determining the HDS reaction pathway.
The reaction network for the HDS of 4,6-DMDBT over NiMo catalysts supported
on meso-microporous Y zeolites through different reaction pathways such as direct
desulfurization, hydrogenation, and isomerization is presented in Fig. 1 [1].
Hydrodesulfurization reactions are inhibited by certain compounds such as
hydrogen sulfide (H 2 S), nitrogen compounds, and polynuclear aromatics. Each of
the reaction pathways is being inhibited by specific compounds. Hydrodesulfurization
by the DDS pathway is inhibited by H 2 S and basic nitrogen compounds. The HYD
pathway is inhibited by all types of nitrogen compounds and polynuclear aromatics.
Tao et al. [2] studied the inhibiting effects of nitrogen compounds such as quinolone and indole on the HDS of thiophenic sulfur compounds in straight-run gas oil
over a NiW/Al 2 O 3 catalyst and showed that quinolone has a stronger inhibiting
effect on HDS than indole has. The nitrogen inhibiting effects on the thiophenic
G. Valavarasu and B. Ramachandrarao