Lecture 19 Refinery.pptx

CNPC Niger Petroleum S.A., Niamey, Niger
Petroleum Engineering English
Lecture 19 : Refinery
CNPC Niger Petroleum S.A.

Contents
• Lecture 19: Refinery
• Lesson 1: Crude oil composition
• Lesson 2: Types of hydrocarbon
• Lesson 3: Crude Oil Distillation Unit
• Lesson 4: Vacuum Distillation Unit
• Lesson 5: Advanced Refinery Process
• Lesson 6: Product Specifications
• Lesson 7: Gas composition
• Lesson 8: Gas treatment processes

Refinery
The objective of refining petrochemical and gas industries is to
transform the crude oil and gas, into final commercial products such
as gasoline , diesel, plastics or natural gas to deliver to the
consumers. This segment is also called the “downstream” segment
of the oil and gas industry.

Crude Oil Composition
The composition of crude oil varies with geographical location,
age, and depth of the well.
The raw material of a refinery is crude oil. In a barrel of crude, we
have approximately 85% of carbon elements plus 10% of hydrogen.
A barrel of crude is composed mainly of hydrocarbons up to 95%,
consisting of Carbon and Hydrogen. The other 5% are called
impurities. On average, after the first treatments, and depending on
the origin of the crude oil we find approximately 2% of sulfur
compounds.

Sulfur Content
The sulfur level in the crude-oil is a key parameter for crude selection
and crude prices. The higher the sulfur level in the crude, the more difficult
it will be to treat the crude to obtain products with a low sulfur specification.
In general, the higher the sulfur content of a crude the lower the price.
We find other impurities including Nitrogen typically around 2% ,water, salts
and sediments which could be present in the crude oil. Water and salts must
be removed from the crude at the inlet of the refinery, to avoid corrosion
problems and fouling by salt depositions in the units of the refinery.

Types of Hydrocarbons
In crude-oil, you can find naturally 4 types of Hydrocarbon.

Paraffins
Paraffins are linear hydrocarbons made up of Carbon and
Hydrogen atoms. Long linear paraffins are also called waxes.
General formula of paraffins is C n H 2n+2

Iso-paraffins
Iso-paraffins, which are non-linear paraffins, with one or multiple
small groups of carbon and hydrogen, attached to them.

Naphthene Family
These hydrocarbons are composed of carbon and hydrogen,
linked together in a ring shape.

Aromatics
Aromatics are also hydrocarbons composed of rings of carbon and
hydrogen, but, with double bonds between the carbon elements. Compared to
Naphthenes, aromatics have, for a same number of carbons, less Hydrogen.
The first Aromatic is called benzene with six atoms of carbon. It is an important
compound for petrochemical and chemical industries.

Conclusion
In conclusion, a crude-oil is characterized by its Paraffin, Iso-
Paraffin, Naphtene and Aromatic content. It is important to know
the type of hydrocarbon you have in a crude, because these
molecules will directly influence the quality of the different
products, you will obtain from this crude-oil.

Crude Distillation Unit (CDU)
The first unit of a refinery is called the
Crude Distillation Unit or CDU. This unit
is operated at high temperatures, around
360°C at the bottom, and at a pressure of 2
barg. This first unit divides the crude oil
into different smaller petroleum cuts,
used as bases for everyday commercial
products.

Crude Distillation Unit (CDU)
At the top of the crude column, we have the lighter compounds,
with the lowest carbon number, and the lowest boiling
temperature

Gas
First we have the Gas with carbon numbers between one and
four. We find in this cut, gases such as, methane, ethane, propane
and butane, which are used as fuel.

Naphtha
The naphtha cut has a carbon number range between five and
six. This cut is the raw material used in the petrochemical industry, to
produce different types of plastics with different properties.

Gasoline
The gasoline cut is composed of hydrocarbons with seven to
eleven carbon numbers. This cut is the base of the gasoline fuels,
used for spark ignition engines.

Kerosene
The Kerosene cut is the main base used to produce jet fuel -
called Jet A1 - delivered to all international airports. Typically, the
carbon number of this cut is between eleven and thirteen.

Diesel
The Diesel cut is the base of diesel fuel, for diesel engines of
cars and trucks. The hydrocarbon chains contain between thirteen
and twenty five carbon numbers. This cut is also the base for heating
oil, used to heat buildings, houses and offices.

Vacuum Distillation Unit (VDU)
The atmospheric residue obtained at the bottom of the CDU is
treated in a second distillation column called the Vacuum
Distillation Unit or VDU. This column is operated under vacuum (80
mm Hg), and at a temperature of 360°C at the bottom of the column.

Lube oils & Paraffins
From this VDU, we obtain distillates, as bases for lube oils and
paraffins. Lube oils are used for the lubrication of car and truck
engines, as well as for the lubrication of equipment in industry.

Hydro-De-Sulfurization(HDS) Unit
On the diesel cut, depending on the origin on the crude-oil, the
cetane number obtained is high enough to be within the
specifications. It’s the same for the cold flow properties.
But the sulfur content of the diesel cut from the crude
distillation is too high to meet the target specifications.
To decrease the sulfur content below 10 ppm weight, to comply
with European specifications, we need to treat this cut in a unit
called the hydro-desulfurization unit (or HDS unit). At the outlet
of the HDS unit, the diesel fuel complies with specifications such as
density, cetane number, cold flow properties and sulfur content. The
diesel can be stored in the final product tank, ready to be sold to
customers of the refinery.

Reforming Unit
For gasoline fuels, remember that the main specifications to comply
with are : density, volatility, sulfur content and RON. Density and
volatility are regulated by operating the distillation column. At the
outlet of the CDU, the sulfur content is higher than the European
specification, of 10 ppm weight. And, typically the RON, of this cut, is
really low, in a range of 20 to 50. To decrease the sulfur content of
the gasoline cut, we install another HDS unit.
To increase the RON, the gasoline cut is treated in a new unit,
called the Reforming unit.

Fluid Catalytic Cracking(FCC) Unit
At the bottom of the CDU, the atmospheric residue is distilled under
vacuum in a VDU to obtain a Vacuum distillate. The vacuum distillate
is introduced into a Fluid Catalytic Cracking unit, called FCC The
vacuum distillate is introduced at the bottom of the reactor, in a big
pipe called a riser. In the riser, the feed is in contact with a specific
catalyst. The reaction occurs in the gas phase at high temperature.

Distillate Hydro-Cracker(DHC) Unit
The vacuum distillate cut can also be treated in a Distillate
Hydro-Cracker unit, called DHC. The hydrocarbon molecules react
at the surface of the catalyst with hydrogen. For example, aromatic
molecules are saturated with hydrogen, to form naphthene.
Naphthene cracks into linear compounds, under high pressure and
high temperature. And long chain molecules can again be cracked
into smaller molecules. Typical operating conditions are : temperature
of the reactor is around 360°C , and pressure around 180 bar.
In conclusion at the outlet of the DHC unit, we obtain mainly
diesel, jet and gasoline.

Delayed Coker Unit
To convert the heavy residue, we can treat it in a delayed coker
unit. The two main pieces of equipment of a delayed coker are
the furnace, and the coker drum. Inside the tubes of the furnace,
the residue is heated, to around 500°C. At this temperature, the long
chain molecules are thermally cracked into smaller molecules. This
chemical reaction also gives aromatics, olefins, and a lot of solid
coke, as around 30 % of the feed of the unit is converted into solid
coke.

Summary
The first step of a refinery is to cut the crude oil into different
bases. This operation takes place in the crude distillation unit
(or CDU). All the cuts obtained, at the outlet of this distillation
column, are not within the desired quality specifications, and need
extra treatment, in additional refining units.
The second step is to treat the cuts in order to comply with the
specifications, and market demands. We have seen, the HDS
units used to remove sulfur compounds from gasoline,
kerosene and diesel. And, the reforming unit, to increase the
RON of the gasoline. We added the FCC which treats the
vacuum distillate, to produce mainly extra gasoline. And the
Hydrocracking unit (or DHC) to produce mainly diesel fuel and
JetA1. At the end of these treatments, we obtain several bases ready
to be blended to formulate the final commercial products.

Summary
In conclusion, the third and last step of the refinery, is to blend all the
bases, to obtain the final commercial products, complying with the
specifications.

Product Specifications
To sell a product, it is important to check if it respects all of a
series of technical specifications. Per product, the number of
specifications to be met can be really high: for instance, for Jet fuel,
there are more than 30 technical specifications to check before being
able to sell the Jet fuel to an airport.
We will see together five important specifications

Density
It is a specification widely used to characterize a petroleum product.
It is expressed as the weight of the product per cubic meter of
the same product. When the carbon number of a cut increases, the
density has a higher value. For instance, the density of gasoline is
lower than the density of Jet A1.

Octane number
This specification is used for gasoline engines. The Octane
number characterizes the knock resistance of the gasoline.
In a spark ignition engine, after mixing air and fuel in the combustion
chamber, the ignition is controlled by a spark. The knock
phenomenon is an abnormal combustion. It consists of autoignition of
the fuel, at a nonoptimum position of the combustion chamber, before
the spark. This uncontrolled autoignition creates pressure waves,
which lead to vibration. This vibration sounds like a metallic noise,
and it is called “knock”. The knock phenomenon can induce several
engine failures.

Octane number
The specification which controls and limits the knock
phenomenon is the Octane number. Octane is a comparative
measurement carried out in the lab, with a specific engine. There are
2 methods: one for RON (Research Octane Number ) and one for
MON (Motor Octane Number). The principle is the same, with the
same engine. Only the engine set-up changes

Octane number
In terms of specifications, both RON and MON can be required (as in
Europe). RON is always higher than MON. The anti-knock index is
sometimes used, as in the United States. It is the average of RON
and MON. When the Octane number increases, the gasoline has a
higher knock resistance.

Cetane number
The Cetane number is the opposite of the Octane number.
Cetane characterizes the ability of Diesel fuel to auto-ignite.
A Diesel engine is a compression engine, where the air-fuel mixture
auto-ignites.
To control the combustion, it is important to master the auto-ignition
delay, and consequently the cetane value. Two types of cetane
requirements exist: the Cetane number, which is measured on a
specific engine, like for the Octane number, and the Cetane index,
obtained by calculus. The Cetane index is lower than the Cetane
number

Sulfur
The fuel specifications can contribute to the reduction of
pollutants in the atmosphere. One of the main improvements
made over the past few years concerns the decrease of sulfur in
the fuels.
Indeed, in less than 15 years only, the sulfur limit was divided by 50,
and now in Europe the specification is less than 10 ppm (or parts per
million) weight. Sulfur from fuels has a direct impact on
environmental emissions. It contributes to sulfuric emissions, such as
sulfur oxides. Moreover, sulfur is a poison for the after-treatment
systems, which are used in many vehicles.

Cold flow behaviour
The last specification we will see together is the cold flow behaviour
of petroleum products. In order to ensure cold start and operation of
a diesel vehicle at low temperatures, it is crucial to master the
behavior of Diesel fuels at low temperatures. Some Diesel fuel
compounds, called waxes, may crystallize at low temperatures, and
consequently, clog the diesel filter.
Cold flow properties of Diesel fuel are described by the Cloud
point. For the Cloud point, a Diesel fuel sample is slowly cooled
down, and its visual aspect is observed. When a sort of cloud is
noticed, this is the Cloud point. When the diesel becomes solid and
cannot flow, this is the Pour point.

Gas Composition
Gas is mainly composed of methane and ethane. In addition to
these two major compounds, you can find some impurities such as
heavier hydrocarbons, like propane and butane, acid compounds like
CO2 and H2S, but also water and mercury. The level of impurities will
mainly depend on the origin of the gas field production.

Why impurities should be eliminate?
It is important to eliminate these impurities because:
• Heavy hydrocarbon can condensate in the gas depending on
pressure and temperature conditions,
• Acid compounds are corrosive.
• It is important to eliminate water especially when you transport the
gas in liquid form at a very low temperature, otherwise you will have
a problem of plugging because of the formation of hydrates and ice.
• Mercury is really corrosive especially for aluminium metallurgy
used in the liquefaction unit.

Gas treatment processes
Here, we will see the treatment units used to eliminate all these
impurities. Of course, the treatment processes to be installed will
depend on the types and quantities of impurities in the gas at the inlet
of the plant.

LPG Extraction
From 36°C for pentane to -161°C for methane. We use these
differences of temperature from C1 to C5 to extract the heavy
hydrocarbons present in the natural gas. First we eliminate the
condensate with a carbon number higher than C5. Then we
eliminate the propane and butane called LPG for liquified
petroleum gas. The processes of removing LPG is done by
adjusting the dew point, by cooling and separation, in combination
with a pressure drop. Finally we obtain the Liquified natural gas
called LNG.

Amine Unit
The two main acid gas compounds in Natural gas are Hydrogen
Sulfide (called H2S) and Carbon dioxide (called CO2) .
To eliminate these 2 acids, the main process used is amine
absorption. The gas containing H2S and CO2 is put in contact with a
chemical liquid absorbent. For H2S and CO2 removal, the solvent is
an amine, such as Diethanol Amine called DEA. A chemical
reaction occurs between the H2S and the DEA.The sour gas
containing CO2 and H2S enters at the bottom of an absorber column.
The DEA enters at the top of the column. The sweet gas, without
H2S and CO2, is recovered at the top of the absorber. The DEA,
with the H2S, flows to a regenerator column.

Claus Unit
This unit transforms H2S into liquid or solid sulfur used by
chemical industries. The first equipment of the Claus unit is a
furnace where H2S is mixed and burnt with air, under controlled
operating conditions, to give sulfur and water. The sulfur is
condensed and recovered in liquid form.
Because this reaction is not complete, we add 2 or more catalytic
stages to the process. The reactors containing the catalyst are
operated at around 250°C at 1 bar. Liquid sulfur is again recovered
at the outlet of each reactor.

Gas dehydration
Gas dehydration is the removal of water from the gas stream in
order to meet pipeline specifications.
There are 3 common processes of dehydration:
• Physical absorption using Glycols like Triethylene glycol (called
TEG).
• The second process is based on adsorption on solids such as a
molecular sieve, activated alumina or silica gel.
• The third dehydration process is membrane permeation.

Mercury adsorption
The chemical reaction takes place again in a reactor full of another
type of solid. The mercury is adsorbed and stays on the adsorbent.
From time to time, it is necessary to change the adsorbent when is
full of mercury. The natural gas flows, free of mercury, to the bottom
of the reactor.

Liquefaction Unit
To transport the gas in liquid form, it is necessary to add a
liquefaction unit. Then, the LNG is transported by tanker at -160°C
and then re-vaporized for the different consumers. The natural gas is
cooled down to -160°C to condensate methane and obtain LNG at
the outlet of the liquefaction unit.

Lecture 19 Refinery.pptx

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