455
Refining Chemistry
liquid fuels. Nevertheless, the decomposition of asphaltenes is, again, an issue, and just as models
consisting of large polynuclear aromatic systems are inadequate to explain the chemistry of visbreaking, they are also of little value for explaining the chemistry of hydrocracking.
Deposition of solids or incompatibility is still possible when asphaltenes interact with catalysts,
especially acidic support catalysts, through the functional groups, for example, the basic nitrogen
species just as they interact with adsorbents. And there is a possibility for interaction of the asphaltene with the catalyst through the agency of a single functional group in which the remainder of the
asphaltene molecule remains in the liquid phase. There is also a less desirable option in which the
asphaltene reacts with the catalyst at several points of contact, causing immediate incompatibility
on the catalyst surface.
There is evidence to show that during the early stages of the hydrotreating process, the chemistry of the asphaltene constituents follows the same routes as the thermal chemistry (Ancheyta et al.,
2005). Thus, initially, there is an increase in the amount of asphaltene constituents followed by a
decrease indicating that, in the early stages of the process, resin constituents are being converted
to asphaltene material by aromatization and by some dealkylation. In addition, aromatization and
dealkylation of the original asphaltene constituents yields asphaltene products that are of higher
polarity and lower molecular weight that then original asphaltene constituents. Analogous to the
thermal processes, this produces an overall asphaltene fraction that is more polar material and
also of lower molecular weight. As the hydrotreating process proceeds, the amount of asphaltene
constituents precipitated decreases due to conversion of the asphaltene constituents to products. At
more prolonged on-stream times, there is a steady increase in the yield of the asphaltene constituents. This is accompanied by a general increase in the molecular weight of the precipitated material.
These observations are in keeping with observations for the thermal reactions of asphaltene constituents in the absence in hydrogen where the initial events are a reduction in the molecular weight
of the asphaltene constituents leading to lower molecular weight by more polar products that are
derived from the asphaltene constituents but are often referred to as carbenes and carboids. As the
Fresh gas
First
stage
Second
stage
Products
Recycle
Fractionation
Quench gas
Recycle gas
compressor
HP
separator
LP
separator
Feed
FIGURE 16.11 A two-stage hydrocracking unit. (From Occupational Safety and Health Administration,
United States Department of Labor, Petroleum refining processes, In OSHA Technical Manual, Section IV,
Chapter 2, Washington, DC, 1999, http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.)
Refining Chemistry
liquid fuels. Nevertheless, the decomposition of asphaltenes is, again, an issue, and just as models
consisting of large polynuclear aromatic systems are inadequate to explain the chemistry of visbreaking, they are also of little value for explaining the chemistry of hydrocracking.
Deposition of solids or incompatibility is still possible when asphaltenes interact with catalysts,
especially acidic support catalysts, through the functional groups, for example, the basic nitrogen
species just as they interact with adsorbents. And there is a possibility for interaction of the asphaltene with the catalyst through the agency of a single functional group in which the remainder of the
asphaltene molecule remains in the liquid phase. There is also a less desirable option in which the
asphaltene reacts with the catalyst at several points of contact, causing immediate incompatibility
on the catalyst surface.
There is evidence to show that during the early stages of the hydrotreating process, the chemistry of the asphaltene constituents follows the same routes as the thermal chemistry (Ancheyta et al.,
2005). Thus, initially, there is an increase in the amount of asphaltene constituents followed by a
decrease indicating that, in the early stages of the process, resin constituents are being converted
to asphaltene material by aromatization and by some dealkylation. In addition, aromatization and
dealkylation of the original asphaltene constituents yields asphaltene products that are of higher
polarity and lower molecular weight that then original asphaltene constituents. Analogous to the
thermal processes, this produces an overall asphaltene fraction that is more polar material and
also of lower molecular weight. As the hydrotreating process proceeds, the amount of asphaltene
constituents precipitated decreases due to conversion of the asphaltene constituents to products. At
more prolonged on-stream times, there is a steady increase in the yield of the asphaltene constituents. This is accompanied by a general increase in the molecular weight of the precipitated material.
These observations are in keeping with observations for the thermal reactions of asphaltene constituents in the absence in hydrogen where the initial events are a reduction in the molecular weight
of the asphaltene constituents leading to lower molecular weight by more polar products that are
derived from the asphaltene constituents but are often referred to as carbenes and carboids. As the
Fresh gas
First
stage
Second
stage
Products
Recycle
Fractionation
Quench gas
Recycle gas
compressor
HP
separator
LP
separator
Feed
FIGURE 16.11 A two-stage hydrocracking unit. (From Occupational Safety and Health Administration,
United States Department of Labor, Petroleum refining processes, In OSHA Technical Manual, Section IV,
Chapter 2, Washington, DC, 1999, http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.)