591
22
Hydrocracking
22.1 INTRODUCTION
Hydrocracking is a refining technology that, like hydrotreating (Chapter 21), also falls under the
general umbrella of hydroprocessing. The outcome is the conversion of a variety of feedstocks to
a range of products, and units to accomplish this goal can be found at various points in a refinery
(Dolbear, 1998; Billon and Bigeard, 2001; Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary
et al., 2007; Liu et al., 2009; Speight, 2011).
The history of the process goes back to the late 1920s when it was realized that there was a need
for gasoline of a higher quality than that obtained by catalytic cracking, which led to the development of the hydrocracking process. One of the first plants to use hydrocracking—commissioned
for the commercial hydrogenation of brown coal—was at Leuna in Germany. Tungsten sulfide was
used as a catalyst in this one-stage unit, in which high reaction pressures, 2900–4350 psi, were
applied. The catalyst displayed a very high hydrogenation activity: The aromatic feedstock, coal,
and heavy fractions of oil, containing sulfur, nitrogen, and oxygen, were virtually completely converted into paraffins and iso-paraffins. In 1939, Imperial Chemical Industries in Britain developed
the second-stage catalyst for a plant that contributed largely to Britain’s supply of aviation gasoline
in the subsequent years.
During World War II, two-stage processes were applied on a limited scale in Germany, Britain,
and the United States. In Britain, feedstocks were creosote from coal tar and gas oil from petroleum. In the United States, Standard Oil of New Jersey operated a plant at Baton Rouge, producing gasoline from a Venezuelan kerosene/light gas oil fraction. Operating conditions in those
units were comparable: approximate reaction temperature 400°C (750°F) and reaction pressures of
2900–4350 psi. After the War, commercial hydrocracking was very expensive, but by the end of the
1950s, the process had become economic. The development of improved catalyst made it possible
to operate the process at considerably lower pressure, viz. 1000–2200 psi. This in turn resulted in a
reduction in equipment wall thickness, whereas simultaneously, advances were made in mechanical
engineering, especially in the field of reactor design and heat transfer. These factors, together with
the availability of relatively low-cost hydrogen from steam-reforming process, brought hydrocracking back on the refinery scene. The first units of the second generation were built in the United States
to meet the demand for conversion of surplus fuel oil in the gasoline-oriented refineries.
The older hydrogenolysis type of hydrocracking practiced in Europe during and after World War II
used tungsten sulfide (WS 2 ) or molybdenum sulfide (MoS) as catalysts. These processes required
high reaction temperatures and operating pressures, sometimes in excess of about 3000 psi for
continuous operation. The modern hydrocracking processes were initially developed for converting
refractory feedstocks to gasoline and jet fuel, process and catalyst improvements and modifications have made it possible to yield products from gases and naphtha to furnace oils and catalytic
cracking feedstocks. The zeolites most frequently used in commercial hydrocracking catalysts are
partially de-aluminated and low sodium, or high silica, type Y zeolites in hydrogen or rare-earth
forms. Other zeolites and mixtures of zeolites also are used. The zeolites often are imbedded in a
high-surface-area amorphous matrix, which serves as a binder. The metals can reside inside the
zeolite and on the amorphous matrix.
Thus, hydrocracking is a more recent process development compared with the older thermal
cracking, visbreaking, and coking. In fact, the use of hydrogen in thermal processes is perhaps the
22
Hydrocracking
22.1 INTRODUCTION
Hydrocracking is a refining technology that, like hydrotreating (Chapter 21), also falls under the
general umbrella of hydroprocessing. The outcome is the conversion of a variety of feedstocks to
a range of products, and units to accomplish this goal can be found at various points in a refinery
(Dolbear, 1998; Billon and Bigeard, 2001; Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary
et al., 2007; Liu et al., 2009; Speight, 2011).
The history of the process goes back to the late 1920s when it was realized that there was a need
for gasoline of a higher quality than that obtained by catalytic cracking, which led to the development of the hydrocracking process. One of the first plants to use hydrocracking—commissioned
for the commercial hydrogenation of brown coal—was at Leuna in Germany. Tungsten sulfide was
used as a catalyst in this one-stage unit, in which high reaction pressures, 2900–4350 psi, were
applied. The catalyst displayed a very high hydrogenation activity: The aromatic feedstock, coal,
and heavy fractions of oil, containing sulfur, nitrogen, and oxygen, were virtually completely converted into paraffins and iso-paraffins. In 1939, Imperial Chemical Industries in Britain developed
the second-stage catalyst for a plant that contributed largely to Britain’s supply of aviation gasoline
in the subsequent years.
During World War II, two-stage processes were applied on a limited scale in Germany, Britain,
and the United States. In Britain, feedstocks were creosote from coal tar and gas oil from petroleum. In the United States, Standard Oil of New Jersey operated a plant at Baton Rouge, producing gasoline from a Venezuelan kerosene/light gas oil fraction. Operating conditions in those
units were comparable: approximate reaction temperature 400°C (750°F) and reaction pressures of
2900–4350 psi. After the War, commercial hydrocracking was very expensive, but by the end of the
1950s, the process had become economic. The development of improved catalyst made it possible
to operate the process at considerably lower pressure, viz. 1000–2200 psi. This in turn resulted in a
reduction in equipment wall thickness, whereas simultaneously, advances were made in mechanical
engineering, especially in the field of reactor design and heat transfer. These factors, together with
the availability of relatively low-cost hydrogen from steam-reforming process, brought hydrocracking back on the refinery scene. The first units of the second generation were built in the United States
to meet the demand for conversion of surplus fuel oil in the gasoline-oriented refineries.
The older hydrogenolysis type of hydrocracking practiced in Europe during and after World War II
used tungsten sulfide (WS 2 ) or molybdenum sulfide (MoS) as catalysts. These processes required
high reaction temperatures and operating pressures, sometimes in excess of about 3000 psi for
continuous operation. The modern hydrocracking processes were initially developed for converting
refractory feedstocks to gasoline and jet fuel, process and catalyst improvements and modifications have made it possible to yield products from gases and naphtha to furnace oils and catalytic
cracking feedstocks. The zeolites most frequently used in commercial hydrocracking catalysts are
partially de-aluminated and low sodium, or high silica, type Y zeolites in hydrogen or rare-earth
forms. Other zeolites and mixtures of zeolites also are used. The zeolites often are imbedded in a
high-surface-area amorphous matrix, which serves as a binder. The metals can reside inside the
zeolite and on the amorphous matrix.
Thus, hydrocracking is a more recent process development compared with the older thermal
cracking, visbreaking, and coking. In fact, the use of hydrogen in thermal processes is perhaps the