Details of this process together with typical yield data are given in the chapters
on “▶ Fluid Catalytic Cracking (FCC) in Petroleum Refining” and “▶ Upgrading
the Bottom of the Barrel.”
The Hydrocracking Process
This process is fairly new to the industry becoming prominent in its use during the
late 1960s. As the title suggests the process cracks the oil feed in the presence of
hydrogen. It is a high-pressure process operating normally around 2,000 psig. This
makes the unit rather costly and, for many years, slowed its expansion in the
industry compared with the FCCU and thermal cracking. However, the process is
very flexible and is capable of making modern, high- quality distillate fuels and
lubes. It can handle a wide spectrum of feeds including straight run gas oils, vacuum
gas oils, thermal cracker gas oils, FCCU cycle oils, and the like. The products it
produces need very little downstream treating to meet finished product specifications. The naphtha stream it produces is particularly high in naphthenes making it a
good catalytic reformer stock for gasoline or aromatic production.
The process consists of one or two reactors, a preheat system, recycle gas section,
and a recovery section. The oil feed (typically a vacuum gas oil) is preheated by heat
exchange with reactor effluent streams and by a fired heater. Make-up and recycled
hydrogen streams are introduced into the oil stream before entering the reactor(s).
(Note: in some configurations the gas streams are also preheated prior to joining the
oil.) The first section of the reactor is often packed with a hydrotreating catalyst to
protect the more sensitive cracking catalyst further down in the reactor from injurious
sulfur, nitrogen, and metal poisoning. Cracking occurs in the reactor(s) and the
effluent leaves the reactor to be cooled and partially condensed by heat exchange.
The stream enters the first of two flash drums. Here, the drum pressure is almost that
of the reactor. A gas stream rich in hydrogen is flashed off and is recycled back to the
reactors as recycle gas. The liquid phase from the flash drum is routed to a second
separator which is maintained at a much lower pressure (around 150–100 psig).
Because of this reduction in pressure, a second gas stream is flashed off. This will
have a much lower hydrogen content but will contain C 3 s and C 4 s. For this reason the
stream is often routed to an absorber column for maximizing LPG recovery. The
liquid phase leaves the bottom of the low-pressure absorber to enter the recovery side
where products are separated by fractionation and sent to storage.
Further details of this process are given in the chapters entitled “▶ Hydrocracking in Petroleum Processing”, “▶ Upgrading the Bottom of the Barrel, and
“▶ Non-energy Refineries in Petroleum Processing”.
Thermal Cracking Units
Thermal cracking processes are the heavy work horses of the oil refining industry.
The processes are relatively cheap when compared with the fluid cracker and the
38
D.S.J. Jones
on “▶ Fluid Catalytic Cracking (FCC) in Petroleum Refining” and “▶ Upgrading
the Bottom of the Barrel.”
The Hydrocracking Process
This process is fairly new to the industry becoming prominent in its use during the
late 1960s. As the title suggests the process cracks the oil feed in the presence of
hydrogen. It is a high-pressure process operating normally around 2,000 psig. This
makes the unit rather costly and, for many years, slowed its expansion in the
industry compared with the FCCU and thermal cracking. However, the process is
very flexible and is capable of making modern, high- quality distillate fuels and
lubes. It can handle a wide spectrum of feeds including straight run gas oils, vacuum
gas oils, thermal cracker gas oils, FCCU cycle oils, and the like. The products it
produces need very little downstream treating to meet finished product specifications. The naphtha stream it produces is particularly high in naphthenes making it a
good catalytic reformer stock for gasoline or aromatic production.
The process consists of one or two reactors, a preheat system, recycle gas section,
and a recovery section. The oil feed (typically a vacuum gas oil) is preheated by heat
exchange with reactor effluent streams and by a fired heater. Make-up and recycled
hydrogen streams are introduced into the oil stream before entering the reactor(s).
(Note: in some configurations the gas streams are also preheated prior to joining the
oil.) The first section of the reactor is often packed with a hydrotreating catalyst to
protect the more sensitive cracking catalyst further down in the reactor from injurious
sulfur, nitrogen, and metal poisoning. Cracking occurs in the reactor(s) and the
effluent leaves the reactor to be cooled and partially condensed by heat exchange.
The stream enters the first of two flash drums. Here, the drum pressure is almost that
of the reactor. A gas stream rich in hydrogen is flashed off and is recycled back to the
reactors as recycle gas. The liquid phase from the flash drum is routed to a second
separator which is maintained at a much lower pressure (around 150–100 psig).
Because of this reduction in pressure, a second gas stream is flashed off. This will
have a much lower hydrogen content but will contain C 3 s and C 4 s. For this reason the
stream is often routed to an absorber column for maximizing LPG recovery. The
liquid phase leaves the bottom of the low-pressure absorber to enter the recovery side
where products are separated by fractionation and sent to storage.
Further details of this process are given in the chapters entitled “▶ Hydrocracking in Petroleum Processing”, “▶ Upgrading the Bottom of the Barrel, and
“▶ Non-energy Refineries in Petroleum Processing”.
Thermal Cracking Units
Thermal cracking processes are the heavy work horses of the oil refining industry.
The processes are relatively cheap when compared with the fluid cracker and the
38
D.S.J. Jones