621
Hydrocracking
separator system. The synthetic crude may be sent to stabilization and fractionation unit as required.
The gases are sent to a lean oil scrubbing system for contaminant removal and are recycled.
The HDH process (hydrocracking/distillation/hydrotreating process) was developed by IntevepPDVSA in the 1980s and its scheme follows a similar pattern to the VEBA Combi Cracking process.
Compared with the latter, the HDH works at relatively less severe conditions, that is, at a temperature of 420°C–480°C (790°F–895°F), 2000 psi, and using an iron sulfate catalyst.
In the most recent version (HDHPlus), the catalyst also contains molybdenum incorporated on
coke used as a support, so as to improve the degree of dispersion and to make it easier to separate it
from the high-boiling residuum after the reaction.
22.4 CATALYSTS
The early 1960s saw increasing demand for high-octane gasoline for the high-compression-ratio
engines in new high-performance cars. Demand also grew for diesel fuel for diesel-electric locomotives and low-freeze-point jet fuel. These needs were met by rapid growth in hydrocracking of
the more-refractory crude fractions that were not converted to gasoline and lighter products in the
catalytic cracking units. This growth in demand was accompanied by the pioneering development
of new zeolite-based hydrocracking catalysts that had improved activity and selectivity (Degnan,
2000). The need to develop catalysts that can carry out cracking and hydrogenation has become
even more pressing in view of recent environmental regulations limiting the amount of sulfur and
nitrogen emissions. The development of a new generation of catalysts to achieve this objective
of low nitrogen and sulfur levels in the processing of different feedstocks presents an interesting
challenge for catalyst development (Wade et al., 2009).
Amorphous and zeolite-based catalysts (microporous, crystalline aluminosilicates with ion
exchange, sorption, and molecular sieving properties) are generally are used in hydrocracking reactors. The zeolite-based catalysts are high-activity catalysts with high ammonia tolerance and offer
higher gasoline selectivity than do amorphous catalysts. Most zeolites are synthesized from a mixture of silica and alumina sources and caustic. Active catalysts are obtained by modifying the synthesized zeolite with ionic exchange and thermal or chemical treatment. High zeolite content catalysts
rely primarily on the zeolite for their hydrocracking function. In low zeolite content catalysts, both
the zeolite and acidic amorphous content are responsible for the cracking activity. Zeolite-based
hydrocracking catalysts have certain advantages over amorphous catalysts such as greater acidity
which results in greater cracking activity. They also possess better thermal/hydrothermal stability,
naphtha selectivity, and resistance to nitrogen and sulfur compounds than amorphous catalysts. In
addition to these advantages, the zeolite-based catalysts also have a low coke-forming tendency and
can be more easily regenerated.
The deposition of coke and metals on to the catalyst diminish the cracking activity of hydrocracking catalysts. Basic nitrogen plays a major role because of the susceptibility of such compounds for
the catalyst and their predisposition to form coke (Speight, 2000; Speight and Ozum, 2002). The
reactions of hydrocracking require a dual-function catalyst with high cracking and hydrogenation
activities (Ho, 1988). The catalyst base, such as acid-treated clay, usually supplies the cracking
function or alumina or silica–alumina that is used to support the hydrogenation function supplied
by metals, such as nickel, tungsten, platinum, and palladium. These highly acid catalysts are very
sensitive to nitrogen compounds in the feed, which break down the conditions of reaction to give
ammonia and neutralize the acid sites. As many heavy gas oils contain substantial amounts of nitrogen (up to approximately 2500 ppm), a purification stage is frequently required. Denitrogenation
and desulfurization can be carried out using cobalt–molybdenum or nickel–cobalt–molybdenum on
alumina or silica–alumina.
Hydrocracking catalysts typically contain separate hydrogenation and cracking functions.
Palladium sulfide and promoted group VI sulfides (nickel molybdenum or nickel tungsten) provide the hydrogenation function. These active compositions saturate aromatics in the feed, saturate
Hydrocracking
separator system. The synthetic crude may be sent to stabilization and fractionation unit as required.
The gases are sent to a lean oil scrubbing system for contaminant removal and are recycled.
The HDH process (hydrocracking/distillation/hydrotreating process) was developed by IntevepPDVSA in the 1980s and its scheme follows a similar pattern to the VEBA Combi Cracking process.
Compared with the latter, the HDH works at relatively less severe conditions, that is, at a temperature of 420°C–480°C (790°F–895°F), 2000 psi, and using an iron sulfate catalyst.
In the most recent version (HDHPlus), the catalyst also contains molybdenum incorporated on
coke used as a support, so as to improve the degree of dispersion and to make it easier to separate it
from the high-boiling residuum after the reaction.
22.4 CATALYSTS
The early 1960s saw increasing demand for high-octane gasoline for the high-compression-ratio
engines in new high-performance cars. Demand also grew for diesel fuel for diesel-electric locomotives and low-freeze-point jet fuel. These needs were met by rapid growth in hydrocracking of
the more-refractory crude fractions that were not converted to gasoline and lighter products in the
catalytic cracking units. This growth in demand was accompanied by the pioneering development
of new zeolite-based hydrocracking catalysts that had improved activity and selectivity (Degnan,
2000). The need to develop catalysts that can carry out cracking and hydrogenation has become
even more pressing in view of recent environmental regulations limiting the amount of sulfur and
nitrogen emissions. The development of a new generation of catalysts to achieve this objective
of low nitrogen and sulfur levels in the processing of different feedstocks presents an interesting
challenge for catalyst development (Wade et al., 2009).
Amorphous and zeolite-based catalysts (microporous, crystalline aluminosilicates with ion
exchange, sorption, and molecular sieving properties) are generally are used in hydrocracking reactors. The zeolite-based catalysts are high-activity catalysts with high ammonia tolerance and offer
higher gasoline selectivity than do amorphous catalysts. Most zeolites are synthesized from a mixture of silica and alumina sources and caustic. Active catalysts are obtained by modifying the synthesized zeolite with ionic exchange and thermal or chemical treatment. High zeolite content catalysts
rely primarily on the zeolite for their hydrocracking function. In low zeolite content catalysts, both
the zeolite and acidic amorphous content are responsible for the cracking activity. Zeolite-based
hydrocracking catalysts have certain advantages over amorphous catalysts such as greater acidity
which results in greater cracking activity. They also possess better thermal/hydrothermal stability,
naphtha selectivity, and resistance to nitrogen and sulfur compounds than amorphous catalysts. In
addition to these advantages, the zeolite-based catalysts also have a low coke-forming tendency and
can be more easily regenerated.
The deposition of coke and metals on to the catalyst diminish the cracking activity of hydrocracking catalysts. Basic nitrogen plays a major role because of the susceptibility of such compounds for
the catalyst and their predisposition to form coke (Speight, 2000; Speight and Ozum, 2002). The
reactions of hydrocracking require a dual-function catalyst with high cracking and hydrogenation
activities (Ho, 1988). The catalyst base, such as acid-treated clay, usually supplies the cracking
function or alumina or silica–alumina that is used to support the hydrogenation function supplied
by metals, such as nickel, tungsten, platinum, and palladium. These highly acid catalysts are very
sensitive to nitrogen compounds in the feed, which break down the conditions of reaction to give
ammonia and neutralize the acid sites. As many heavy gas oils contain substantial amounts of nitrogen (up to approximately 2500 ppm), a purification stage is frequently required. Denitrogenation
and desulfurization can be carried out using cobalt–molybdenum or nickel–cobalt–molybdenum on
alumina or silica–alumina.
Hydrocracking catalysts typically contain separate hydrogenation and cracking functions.
Palladium sulfide and promoted group VI sulfides (nickel molybdenum or nickel tungsten) provide the hydrogenation function. These active compositions saturate aromatics in the feed, saturate