Petroleum process
The first refining processes were developed
to purify, stabilize, and improve the quality of
oils.
Then, refining methods had to be constantly
adapted and improved to meet the quality
requirements and needs of car and aircraft
engines.
Finally, refineries had to introduce conversion
processes to produce greater quantities of
lighter products from the higher boiling
fractions.
Kerosene
Gasoline
Gasoline &
light naphtha
Refinery processes for crude oil are generally divided into
three categories:
(1) Separation processes, of which distillation is the prime
example.
(2) Conversion processes, of which coking and catalytic
cracking are prime examples.
(3) Finishing processes, of which hydrotreating to remove
sulfur is a prime example.
Conversion Finishing/Treating
Separation • Desalting
• Dewatering
• Atmospheric distillation
• Vacuum distillation
• Low Mw
• Cracking
• Coking
• Hydrocracking
• High quality
• Reforming
• Isomerization
• Polymerization
• Alkylation
• Hydrotreating
• Sweetening treating
• Dewaxing
• Blending
1. Separation process
Field separation, which occurs at a field site near the recovery
operation, is the first attempt to remove the gases, water, and
dirt that accompany crude oil coming from the ground. The
separator may be no more than a large vessel that gives a
quieting zone for gravity separation into three phases: gases,
crude oil, and water containing entrained dirt.
Desalting is a water-washing
operation performed at the
production field and at the
refinery site for additional
crude oil cleanup.
A. Desalting and Dewatering
If the petroleum from the separators contains water and dirt,
water washing can remove much of the water-soluble minerals
and entrained solids.
B. Distillation
Distillation involves the separation of the different hydrocarbon
compounds that occur naturally in a crude oil into a number of
different fractions.
In the atmospheric
distillation process,
heated crude oil is
separated in a
distillation column
into streams.
The atmospheric residuum is then fed to the vacuum distillation unit
at the pressure of 10 mmHg where light vacuum gas oil, heavy
vacuum gas oil, and vacuum residue are
the products.
Operating conditions for vacuum
distillation are usually 50 to 100 mmHg
(atmospheric pressure = 760 mmHg).
2. Conversion process
A. Cracking
The basic processes introduced to bring about thermal
decomposition of the higher boiling streams are known as (1)
thermal cracking. In these processes, the higher boiling
fractions are converted to lower boiling products.
Higher MW Lower MW
(2) Catalytic cracking is the most common cracking process,
in which heavy feedstock or cuts are broken down or changed
by being heated, and reacted with catalysts.
Higher MW Lower MW
Heat
Heat & Catalyst
B. Coking
Coking units convert heavy feedstock into a solid coke and
lower boiling hydrocarbon products that are suitable as
feedstock to other refinery units for conversion into higher value
transportation fuels.
As coke is a by-product of this process, the sulfur and metal
contents of the coke are high (sometimes as high as 8% by
weight).
C. Hydrocracking
Hydrocracking is similar to catalytic cracking, with hydrogenation
superimposed and with the reactions taking place either
simultaneously or sequentially. Hydrocracking was initially used
to upgrade low-value distillate feedstocks, such as cycle oils
(high aromatic products from a catalytic cracker, which are
usually not recycled to extinction for economic reasons), thermal
and coker gas oils, and heavycracked and straight-run naphtha.
These feedstocks are difficult to process by either catalytic cracking
or reforming, because they are characterized usually by a high
polycyclic aromatic content, or by high concentrations, or by both the
two principal catalyst poisons—sulfur and nitrogen compounds.
D. Reforming
Reforming processes are used to change the inherent chemical
structures of the hydrocarbons that exist in distillation fractions
crude oil into different compounds. Catalytic reforming is one of
the most important processes in a modern refinery, altering
straight-run fraction or fractions from a catalytic cracker into new
compounds through a combination of heat and pressure in the
presence of a catalyst. Reforming processes are particularly
important in producing high quality gasoline fuels.
Other processes to maximize the production of gasoline products
include isomerization that is used for reforming or recombining
lighter cuts into new products, polymerization processes, and
alkylation processes.
3. Finishing process
Petroleum contains impurities that have to be removed at different
stages of the refinery. Some of the impurities can be harmful to the
refinery processes, corrosive, odiferous, or degrade the final
product unless removed.
The most common and significant nonhydrocarbon impurity in
petroleum is sulfur. When the sulfur content of the crude oil is low
(usually less than 1% wt.), the crude oil is known as a sweet
crude, while crude oil with higher concentrations of sulfur is called
sour crude.
Generally it is more economical to hydrotreat high-sulfur
feedstocks prior to catalytic cracking than to hydrotreat the
products from catalytic cracking. The advantages are that:
(1) sulfur is removed from the catalytic cracking feedstock, and
corrosion is reduced in the cracking unit;
(2) carbon formation during cracking is reduced so that higher
conversions result; and
(3) the cracking quality of the gas oil fraction is improved.
A. Hydrotreating/hydrodesulfurization
B. Solvent dewaxing
In solvent dewaxing, the oil is diluted with a solvent that has a
high affinity for oil, chilled to precipitate the wax, filtered to
remove the wax, stripped of solvent, and dried. The solvents act
as diluents for the high molecular weight
oil fractions to reduce the viscosity
of the mixture and provide sufficient
liquid volume to permit pumping and
filtering.
Extraction processes to purify oil and its products of impurities,
specific additives are also used to react with corrosive or
odiferous constituents to produce harmless and odorless
substances