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A Basic Overview of a Petroleum Refinery

A petroleum refinery is a chemical plant that processes crude oil and produces several valuable

products. A refinery contains many different types of units that perform a variety of different

operations. The main goal is to take the undesirable components of the crude oil and upgrade

them into more valuable products. Gasoline, diesel, and jet fuel are among the most valuable

products, whereas fuel oils and lubricants are sometimes sold at a loss. Below is a list describing

the different types of units found in a refinery, followed by a system overview from the

beginning of the refining process to the end.

Separation Units

These units take an incoming stream and separate it into different components. No chemical

reactions occur in these units.

Desalter: Ions in the crude oil will corrode the pipes in the refinery and may deactivatethe catalysts. It is important to remove these salts from the crude before any otherprocesses are started. The process involves forcing water into the crude oil feed stream.

This pulls out the salts and prevents corrosion.

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Atmospheric Distillation: The distillation is performed at atmospheric pressures. Theoutputs of the distillation unit include light ends, kerosene, diesel, heavy gas oil, and

atmospheric residue.

Vacuum Distillation: This unit distills the atmospheric residue and produces lightvacuum gas oil, heavy vacuum gas oil, and vacuum residue. The distillation occurs

because the pressure inside of the unit is decreased to nearly zero, allowing the

components of the atmospheric residue to boil at a lower temperature.

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Light Ends Unit: The light ends unit consists of many different fractionators thatseparate the different components of the light ends fraction from atmospheric distillation.

The separated components consist of methane and ethane, which are used for fuel to heat

operations throughout the refinery; propane and butane, which are mixed and compressed

to be sold as Liquid Petroleum Gas (LPG); light straight run (lsr) naphtha (C5 and C6),

which is used in gasoline pools; and heavy straight run (hsr) naphtha, which is used as a

feed stock for the catalytic reformer.

Deasphalter: This unit takes the vacuum residue and pulls out all of the heavy particlesleaving heavy gas oil that can be further refined or used as fuel oil, and asphalt, which is

used in paving.

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Dewaxer: This unit precipitates long n-paraffins out of heavy vacuum gas oil creatinglubricating oils that will withstand low temperatures without solidifying.

Finishing Units

These units add the final touches before the product can be sold. Some chemical reactions may

occur, but none that significantly alter the final product.

Blending: These units take a variety of streams and mix them to meet certain criteria andcompositions.

Hydrotreater: Most crude oils today have a high sulfur content. Sulfur is a strongpollutant and must be removed to meet emission standards. Also, sulfur can deactivate

catalysts in further refining units. Sulfur is removed by pumping hydrogen gas into any

stream. The hydrogen reacts with a molecule and extracts the sulfur to produce hydrogen

sulfide.

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Conversion

These units change the composition of input streams through chemical reactions. In the reactors,

a low grade product is converted into a higher grade product.

Catalytic Reformer: This unit takes HSR naphtha and unsaturates the hydrocarbons toproduce aromatic rings and other various olefins. These aromatic rings and olefins are

used in premium gasolines because of their high octane numbers.

Catalytic Cracker: This unit transforms heavy gas oil into light distillates, such asgasoline and LPG, as well as light cycle oil. The unit utilizes a catalyst to take long chain

hydrocarbons and break (crack) them into shorter, more valuable hydrocarbons.

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Hydrocracker: This unit performs the same operation as the catalytic cracker, but useshydrogen gas instead of a catalyst to break long hydrocarbon chains into shorter ones.

Also, the feedstock is light vacuum gas oil and the products are light and middle

distillates.

Thermal Cracker: This unit uses heat to break larger molecules into smaller ones, but israrely used today.

Visbreaker: This unit employs a mild version of thermal cracking to convert vacuumresidue into light and middle distillates, fuel oil, and coke or heavy gas oil into slightly

lighter fuel oil.

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Alkylater: This unit converts short olefins (ethylene, propylene, and butenes) andisobutane chains into branched C7 and C8 chains via strong acids and heat.

Delayed Coker: This unit uses a very severe version of thermal cracking to convertvacuum residue into light and middle distillates, as well as coke.

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Refining Process Overview

The crude oil first enters the desalter to remove any salts that may corrode the processing

units. From there, the desalted crude enters the atmospheric distillation unit. The separated

components are as follows: light distillates, kerosene, light gas oil (diesel), heavy gas oil (fuel

oils), and atmospheric residue.

The light distillates enter the light ends unit, where they are further separated into several

more distinct components. The methane and ethane is captured and burned to provide heat for

other processes. Propane and butane are first hydrotreated before being sold as LPG. The light

straight run naphtha is sent to the gasoline blending pool and the heavy straight run naphtha is

hydrotreated and then sent to the catalytic reform to produce high octane gasoline.

The straight run kerosene is hydrotreated before being sold as a final product, which is

mostly jet fuel. The straight run diesel is also hydrotreated before being sold to the public.

Hydrotreating the diesel remains extremely important due to the increasing governmental

restriction on its sulfur content. The heavy gas oil is either sold as low grade fuel oil or more

often it is upgraded using fluid catalytic cracking to produce more desirable products.The atmospheric residue is sent to vacuum distillation. The light vacuum gas oil is

further refined using a visbreaker and then sold as a fuel oil. The heavy vacuum gas oil is

hydrocracked to produce gasoline and other products. The vacuum residue is sent to a

deasphalter, producing asphalt and a deasphalted oil, which is further treated to produce fuel oils,

or to a coker.

Finally, various processes, such as the fluid catalytic cracker and coker provide the

feedstocks for the alkylater.

ConclusionA refinery is a complex chemical plant that utilizes several different techniques to take a very

rough feedstock, crude oil, and converts it into desirable products, such as gasoline. Oil

companies invest large sums of capital into these refineries in hopes of making a large profit.

This has only been a simple overview and further instruction will be needed to completely

understand all aspects of a petroleum refinery.