Refining has developed to give us fuels, lube oils and chemical feedstock from crude oil
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1 PETROLEUM REFINERY ENGINEERING Petroleum is a combustible oily liquid of reddish brown to almost black colour, produced from oil wells. It is a complex mixture of hydrocarbons and their derivatives containing oxygen, sulphur, nitrogen and minor quantities of some other materials. The importance of petroleum crude oil and natural gas has been realized with the development of its numerous applications as fuel and feedstock. The invention of the internal combustion engines in the last quarter of the nineteenth century gave an impetus to the development of petroleum processing. The most basic refining process is aimed at separating the crude oil into its various components. Crude oil is heated and put into a still -- a distillation column -- and different hydrocarbon components boil off and can be recovered as they condense at different temperatures. Additional processing follows crude distillation, changing the molecular structure of the input with chemical reactions, some through variations in heat and pressure, and some in the presence of a catalyst. The main constituents of petroleum _hydrocarbons _ may differ in the number of carbon and hydrogen atoms in the molecular structure. The hydrocarbons are present in the following groups or homologous series: paraffins (saturated st. chain hydrocarbons, alkanes), naphthenes (cycloalkanes), and benzene hydrocarbons (aromatics). In most grades of petroleum, paraffins and naphthenes prevail mostly. Based on the chemical composition of the crude (1) Paraffin-Base Crude Oils These contain higher molecular weight paraffins which are solid at room temperature, but little or no asphaltic (bituminous) matter. They can produce high-grade lubricating oils. (2) Asphaltic-Base Crude Oils Contain large proportions of asphaltic matter, and little or no paraffin. Some are predominantly naphthenes so yield lubricating oil that is more sensitive to temperature changes than the paraffin-base crudes. (3) Mixed-Base Crude Oils The "gray area" between the two types above. Both paraffins and naphthenes are present, as well as aromatic hydrocarbons. Most crude fit this category. CRUDE OIL PHYSICAL PROPERTIES The physical properties of crude are as follows Specific Gravity: 0.669 to 0.99 API Gravity: 10 – 50
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PETROLEUM REFINERY ENGINEERING
Petroleum is a combustible oily liquid of reddish brown to almost black colour, produced from oil
wells. It is a complex mixture of hydrocarbons and their derivatives containing oxygen, sulphur,
nitrogen and minor quantities of some other materials. The importance of petroleum crude oil and natural
gas has been realized with the development of its numerous applications as fuel and feedstock. The
invention of the internal combustion engines in the last quarter of the nineteenth century gave an
impetus to the development of petroleum processing. The most basic refining process is aimed at
separating the crude oil into its various components. Crude oil is heated and put into a still -- a distillation
column -- and different hydrocarbon components boil off and can be recovered as they condense at
different temperatures. Additional processing follows crude distillation, changing the molecular structure
of the input with chemical reactions, some through variations in heat and pressure, and some in the
presence of a catalyst.
The main constituents of petroleum _hydrocarbons _ may differ in the number of carbon and
hydrogen atoms in the molecular structure. The hydrocarbons are present in the following groups or
homologous series: paraffins (saturated st. chain hydrocarbons, alkanes), naphthenes (cycloalkanes),
and benzene hydrocarbons (aromatics). In most grades of petroleum, paraffins and naphthenes prevail
mostly.
Based on the chemical composition of the crude
(1) Paraffin-Base Crude Oils These contain higher molecular weight paraffins which are solid at
room temperature, but little or no asphaltic (bituminous) matter. They can produce high-grade lubricating
oils.
(2) Asphaltic-Base Crude Oils Contain large proportions of asphaltic matter, and little or no
paraffin. Some are predominantly naphthenes so yield lubricating oil that is more sensitive to temperature
changes than the paraffin-base crudes.
(3) Mixed-Base Crude Oils The "gray area" between the two types above. Both paraffins and
naphthenes are present, as well as aromatic hydrocarbons. Most crude fit this category.
CRUDE OIL PHYSICAL PROPERTIES
The physical properties of crude are as follows
Specific Gravity: 0.669 to 0.99
API Gravity: 10 – 50
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Viscosity: 1 – 4 centipoises
The American Petroleum Institute (API) has developed the term Degrees
API Gravity (°API) which is widely used as another general characterization of
the density of crude oils. The relationship is as follows:
°API = (141.5/Specific Gravity at 60 degrees Fahrenheit) - 131.5
“ Specific Gravity at 60 degrees Fahrenheit” is the density of the crude oil
measured at 60°F divided by the density of water at 60°F.
Therefore, when comparing two crude oils, the higher density crude (i.e., the one
with the highest specific gravity) will have a correspondingly lower °API. For
example, the 34.5°API West African crude oil Bonny Light is heavier than the
40.4°API North Sea crude oil Forties.
Chemical composition
On an average crude oil is [ultimate analysis] made up of the following components:
· General formula: C6H5 - Y (Y is a longer, straight molecule that connects to the benzene ring)
· They are unsaturated ring type (cyclic) compounds which react because they have carbon
atoms that are deficient in hydrogen.
· ringed structures with one or more rings. They have at least one benzene ring.
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rings contain six carbon atoms, with alternating double and single bonds between the carbons
· typically liquids and are found in heavier fractions of crude oil.
· examples: benzene, naphthalene
3. Napthenes or Cycloalkanes
· General formula: CnH2n (n is a whole number usually from 1 to 20)
· ringed structures with closed rings (cyclic)
Found in all fractions of crude except the very lightest.
· rings contain only single bonds between the carbon atoms
· typically liquids at room temperature
· examples: cyclohexane, methyl cyclopentane
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Nonhydrocarbons.
1. Sulfur Compounds. Sulfur may be present in crude oil as hydrogen sulfide (H2S), as
compounds (e.g. mercaptans, sulfides, disulfides, thiophenes, etc.) or as elemental sulfur. Each crude oil
has different amounts and types of sulfur compounds, but as a rule the proportion, stability, and
complexity of the compounds are greater in heavier crude-oil fractions. Hydrogen sulfide is a primary
contributor to corrosion in refinery processing units. Other corrosive substances are elemental sulfur and
mercaptans.
Moreover, the corrosive sulfur compounds have an obnoxious odor.
2. Oxygen Compounds. Oxygen compounds such as phenols, ketones, and carboxylic acids
occur in crude oils in varying amounts.
3. Nitrogen Compounds. Nitrogen is found in lighter fractions of crude oil as basic compounds,
and more often in heavier fractions of crude oil as non basic compounds that may also include trace
metals such as copper, vanadium, and/or nickel. Nitrogen oxides can form in process furnaces. The
decomposition of nitrogen compounds in catalytic cracking and hydrocracking processes forms ammonia
and cyanides that can cause corrosion.
4. Trace Metals. Metals, including nickel, iron, and vanadium are often found in crude oils in small
quantities and are removed during the refining process. Burning heavy fuel oils in refinery
furnaces and boilers can leave deposits of vanadium oxide and nickel oxide in furnace boxes,
ducts, and tubes. It is also desirable to remove trace amounts of arsenic, vanadium, and nickel
prior to processing as they can poison certain catalysts.
Fractionation Processes:
Process name Action Method Purpose feedstocks products
Atmospheric
distillation
separation thermal Separate
fractions
Desalted
Crude oil
Gas, Gas oil,
distillate,
residue
Vacuum
distillation
separation thermal Separate w/o
cracking
Atm. tower
residue
Gas oil, lube
stock, residue
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An oil refinery is an industrial process plant where crude oil is processed and
refined into more useful products. Oil refineries are quite large industrial complexes with
extensive pipelines carrying streams of fluids between large chemical (thermal and
catalytic) processes.
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Temperature
increases
down the column
(Petroleum Gas)
Petrol
Naphtha
Kerosene
Diesel
Lubricants
Bitumen
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Catalytic Reforming
Although motor gasolines have numerous specifications that must be
satisfied to provide the performance demanded by our high-performance motor
vehicles, the most widely recognized gasoline specification is the octane number.
Gasolines are typically retailed in grades of regular, mid-grade and premium,
which are differentiated by the posted octane number.
The Octane Number of a test fuel refers to the percentage by volume of
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isooctane in a mixture of isooctane and heptane in a reference fuel that when
tested in a laboratory engine, matches the antiknock quality, as measured by a
knockmeter, of the fuel being tested under the same conditions. The octane
number posted at the gasoline pump is actually the average of the Research
Octane Number (RON) and Motor Octane Number (MON), commonly referred to
as (R+M)/2. RON and MON are two different test methods that quantify the
antiknock qualities of a fuel. Since the MON is a test under more severe
conditions than the RON test, for any given fuel, the RON is always higher than the MON. Unfortunately, the desulfurized light and heavy naphtha fractions of crude
oils have very low octane numbers. The heavy naphtha fraction is roughly 50
(R+M)/2. Catalytic Reforming is the refinery process that reforms the molecular
structure of the heavy naphtha to increase the percentage of high-octane
components while reducing the percentage of low-octane components.
The hydrocarbon compounds that constitute heavy naphtha are classified
into four different categories: paraffins, olefins (a very low percentage of olefins
occur in the heavy naphthas from crude), naphthenes and aromatics. In lieu of a
complete course in organic chemistry, simplistically the paraffins and olefins are
compounds with straight or branched carbon chains, whereas the naphthenes
and aromatics are carbon rings. The paraffins and naphthenes are saturated
hydrocarbons. Saturated means that they have the maximum number of
hydrogen atoms attached to the carbon atoms. The olefins and aromatics,
however, are unsaturated hydrocarbons because the compounds contain carbon
atoms that are double bonded to other carbon atoms. The straight chain,
saturated compounds exhibit very low octane numbers, the branched, saturated
compounds exhibit progressively higher octane numbers, while the unsaturated
compounds exhibit very high octane numbers.
Catalytic Reforming uses a precious metal catalyst (platinum supported by
an alumina base) in conjunction with very high temperatures to reform the
paraffins and napthenes into high-octane components. Sulfur is a poison to the
Catalytic Reforming catalyst, which requires that virtually all the sulfur must be
removed from the heavy naphtha through Hydrotreating prior to Catalytic
Reforming. Several different types of chemical reactions occur in the Catalytic
Reforming reactors.olefins are converted to paraffins, paraffins are isomerized
to branched chains and to a lesser extent to naphthenes, and naphthenes are
converted to aromatics. Aromatic compounds are essentially unchanged. The
resulting reformate product stream from Catalytic Reforming has a RON from 96-
102 depending on the reactor severity and feedstock quality. The
dehydrogenation reactions which convert the saturated naphthenes into
unsaturated aromatics produce hydrogen. This hydrogen is available for
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distribution to other refinery processes which consume hydrogen.
The Catalytic Reforming process consists of a series of several spherical
reactors which operate at temperatures of approximately 900°F. The reforming
reactions are .endothermic. meaning that the reactions cool the hydrocarbons.
The hydrocarbons are re-heated by direct-fired furnaces in between the
subsequent reforming reactors. As a result of the very high temperatures, the
catalyst becomes deactivated by the formation of .coke. (i.e., essentially pure
carbon) on the catalyst which reduces the surface area available to contact with
the hydrocarbons. A simplified process flow for the Catalytic Reforming process
is presented above.
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Fluidized Catalytic Cracking
The Fluidized Catalytic Cracking (FCC) process unit is considered by many refiners to be the heart of the
petroleum refinery. This derives from the fact that the FCC is a key tool to correct the imbalance reflected
by the markets demand for predominantly lighter, lower boiling petroleum products, whereas fractionated
crude oils typically provide an excess of heavy, high boiling range oils. The FCC process converts heavy
gas oils into lighter products which are then used as blend stocks for gasoline and diesel fuels. The
olefinic FCC catalytic naphtha product exhibits a very high-octane value for gasoline blending. The FCC
process cracks the heavy gas oils by breaking carbon-to-carbon bonds in the large molecules comprising
the gas oils and splitting them into multiple smaller molecules which boil at a much lower temperatures.
The FCC may achieve conversions of 70-80% of the feed hydrocarbons boiling above the gasoline range
(i.e., 430°F) to products boiling below 430°F. The lower density of the FCC products relative to the gas oil
feedstocks has the added benefit of producing a volume gain in which the combined volume of the liquid
product streams is greater than the volume of the unit feed stream. Since most petroleum products are
bought and sold on a volume basis, the volume gain aspect of the FCC process is a key aspect in how it
enhances refinery profitability. The resulting FCC product hydrocarbons are highly olefinic (i.e.,
unsaturated). Virgin is a term used to distinguish straight-run (i.e., crude distillation and possibly
hydrotreated only) hydrocarbons stocks from those that are products of conversion units such as the
FCC.
The FCC cracking reactions are catalytically promoted at very high temperatures of 950-1,020°F. At these
temperatures, coke (i.e., essentially pure carbon) formation deactivates the catalyst by blocking catalyst
surface area which prevents intimate contact between the catalyst and the hydrocarbons. To retain
catalyst activity, the FCC utilizes a very fine powdery, zeolite catalyst that behaves like a fluid (i.e., is able
to flow). The fluidized catalyst is continuously circulated in the FCC from the reactor to a regenerator
vessel and then returned to the reactor. Coke is removed from the catalyst in the regenerator vessel
through the controlled incomplete combustion of the carbon with oxygen to form carbon monoxide and
