PRIMARY AND SECONDARY

PROCESSING IN PETROLEUM

REFINERY

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Learning outcome

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Classify and differentiate the primary processing

and secondary processing in petroleum refinery

processing.

Outline and sketch the operating processes in

petroleum refinery.

- downstream processing

- cracking

- thermal cracking

Introduction

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Primary refining process

- the distillation of the feedstock into its basic fractions,and then the re-distillation of these in separate towersinto highly concentrated intermediates.

- In a simple refinery, the split would probably consist ofsix basic cuts: petroleum gas (refinery gas and LPG),gasoline, naphtha, kerosene, gas oil and residue.

Introduction (cont.)

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Source: http://www.galpenergia.com/EN/agalpenergia/Os-nossos-negocios/Refinacao-Distribuicao/ARL/Refinacao/Paginas/Didatico-Refinacao.aspx

Introduction (cont)

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Primary processing unit

The purpose of primary unit is to separate the crude into

different fractions by distillation

Know as mother unit of the refinery, consist of

•CRUDE DISTILLATION UNIT (CDU)

•VACUUM DISTILLATION UNIT (VDU)

Commonly referred as Atmospheric and Vacuum Distillation

unit (AVU)

Introduction (cont.)

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Secondary Processes (downstream)

Scenario

- When automobile industry was in its infancy, it become

obvious that then production of large quantities of

straight run gasoline from crude distillation would

involve production of much larger quantities of residual

fuels than the market could absorb.

- The demand for gasoline thus spurred on scientists to

look for means of obtaining a higher yield of this product

from crude oil.

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Modern refining

- Essentially involves 2 categories of processing:

1) The physical separation of the raw material into

a range of homogeneous petroleum fractions,

include distillation and blending.

2) The subsequent chemical conversion of certain

fractions to alter the product yield and improve

product quality, include cracking, coking,

reforming, alkylation, polymerization,

isomerization and hydrogen treatment.

Introduction (cont.)

Physical and Chemical Processes8

PHYSICAL

Distillation

Solvent extraction

Propane deasphalting

Solvent dewaxing

Blending

THERMAL

Visbreaking

Delayed coking

Flexicoking

CATALYTIC

Hydrotreating

Catalytic reforming

Catalytic cracking

Hydrocracking

Catalytic dewaxing

Alkylation

Polymerization

Isomerization

CHEMICAL

Secondary Processing

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Secondary processes (downstream)

After crude oil is separated into its fractions, each

stream is further converted by changing the size and

structure of molecules through cracking, reforming

and other conversion processes.

Significance of Secondary Processing

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• The significance of secondary processing

(1) to remove any impurities and undesirable constituents from

the distilled fractions.

(2) to convert some of the distilled hydrocarbons into different

molecular forms.

(3) to improve product quality.

(4) to improve the refinery profit margins by converting low

value heavy ends to high value products like LPG/ gasoline/

kerosene/ diesel.

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(1) to remove any impurities from the distilled fractions

- All crudes contain organic sulphur compounds (eg. H2S-

hydrogen sulfide, mercaptan) which will be carried out over from

the column into the resulting gases, distillates and residues.

- The higher the density of a crude, the greater its sulphur content.

- Sour cuts are corrosive and possess an extremely objectionable

odor.

- The secondary process for the treatment of toxic, corrosive and

evil-smelling sulphur-compound impurities is known as

“sweetening”.

- Caustic washing and hydrodesulphurization are finishing

processes designed to remove H2S and mercaptan impurities.

Significance of Secondary Processing

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(2) to convert some of the distilled hydrocarbons into different

molecular forms.

- in conversion processes, the structures of natural hydrocarbon

molecules are changed.

- cracking process, large hydrocarbon molecules are cracked or

broken to form two or more smaller molecules.

- cracking can be done by the action of heat and pressure alone

(thermal cracking) or by heat in the presence of suitable catalyst

(catalytic cracking).

- the main purpose of cracking is to increase the yield of lighter,

more valuable fractions from medium and residual cuts.

Significance of Secondary Processing

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Classification of Secondary Processing Units

Factors for Selecting Secondary

Processes for a Refinery14

Type of Crude

Product Slate

Product specifications

Investment cost

Operating cost

Sophistication of technology

Chemical Processing15

We can change one fraction into another by one of

three methods:

CRACKING

• Breaking large hydrocarbons into smaller pieces

UNIFICATION

• Combining smaller pieces to make larger ones

ALTERATION

• Rearranging various pieces to make desired hydrocarbons

Cracking16

Cracking takes large hydrocarbons and breaks them

into smaller ones.

After various hydrocarbons are cracked into smaller

hydrocarbons, the products go through another

fractional distillation column to separate them.

CRACKING

THERMAL CRACKING

CATALYTIC CRACKING

Chemical Processing - Cracking17

There are several types of cracking:

(1) THERMAL – heat large hydrocarbons at high temperature

(sometimes high pressure as well) until they break apart.

Steam – high temperature steam is used to break ethane, butane and

naptha into ethylene and benzene, which are used to manufacture

chemicals.

Visbreaking – residual from distillation tower is heated, cooled with gas

oil and rapidly flashed in a distillation tower. This process reduces the

viscosity of heavy weight oils and produces tar.

Coking – residual from the distillation tower is heated to temperatures

above 482 degree C until it cracks into heavy oil, gasoline and naphtha.

When the process is completed, a heavy and almost pure carbon residue is

left (coke).

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Chemical Processing – Cracking

(cont.)

(2) CATALYTIC – use catalyst to speed up the cracking reaction.

Catalysts include zeolite, aluminium hydrosilicate, bauxite and silicaalumina.

Fluid catalytic cracking – a hot, fluid catalyst at 538 degree C cracksheavy oil into diesel oils and gasoline.

Hydrocracking – similar to fluid catalytic cracking, but uses a differentcatalyst, lower temperature, high pressure and hydrogen gas. It takes heavyoil and cracks it into gasoline and kerosene (jet fuel).

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Fluidized Catalytic Cracking Unit

Objective: to convert heavy vacuum gas oil to valuable distillates like LPG, Gasoline,Diesel by catalytic cracking in fluidized bed.

Hydro Cracker Unit

Objective: to convert heavy vacuum gas oil to valuable distillates like LPG, Naptha,Automatic transmission fluid (ATF), Kerosene and Diesel

Visbreaker Unit

Objective: to reduce viscosity of heavy ends i.e. RCO/ Vacuum residue by thermalcracking.

Coking Unit

Objective: to produce valuable distillate from heavy ends by thermal cracking

Cracking Process Units

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Combine smaller hydrocarbons to make larger ones

Major unification process is called catalytic

reforming, and catalyst as platinum, platinum-

rhenium mix to combine low weight naptha into

aromatics.

A significant by-product of this reaction is

hydrogen gas, which is then either used for

hydrocracking or sold.

Chemical Processing - Unification

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The structures of molecules in one fraction are

rearranged to produce another.

Alkylation – The process combines an unsaturated

light hydrocarbon (such as olefins, propylene or

butene) with isobutane to produce alkylate in the

presence of a catalyst such as hydrofluoric acid or

sulfuric acid ( a by product from removing

impurities from many oil products).

Product of alkylation are high octane hydrocarbons.

Chemical Processing - Alteration

THERMAL PROCESSES

When a hydrocarbon is heated to a sufficiently high temperature thermal

cracking occurs. This is sometimes referred to as pyrolysis (especially

when coal is the feedstock). When steam is used it is called steam

cracking. We will examine two thermal processes used in refineries.

• Visbreaking

• Delayed coking

Visbreaking

• Visbreaking is a mild form of thermal cracking that lowers the

viscosity of heavy crude-oil residues without affecting the boiling

point range.

• Residuum from the atmospheric distillation tower is heated (425-

510ºC) at atmospheric pressure and mildly cracked in a heater.

• It is then quenched with cool gas oil to control over-cracking, and

flashed in a distillation tower.

• Visbreaking is used to reduce the pour point of waxy residues and

reduce the viscosity of residues used for blending with lighter fuel oils.

Middle distillates may also be produced, depending on product

demand.

• The thermally cracked residue tar, which accumulates in the bottom

of the fractionation tower, is vacuum-flashed in a stripper and the

distillate recycled.

Alternatively, vacuum residue can be cracked. The severity of the

visbreaking depends upon temperature and reaction time (1‐8 min).

• Usually < 10 wt% of gasoline and lighter products are produced.

The objective is to reduce the viscosity as much as possible without

significantly affecting the fuel stability.

Soaker Visbreaking Process

The furnace operates at a lower outlet temperature and a soaker drum is

provided at the outlet of the furnace to give adequate residence time to

obtain the desired conversion while producing a stable residue product,

thereby increasing the heater run and reducing the frequency of unit shut

down for heater decoking . The products from soaker drum are quenched and

distilled in the downstream fractionator.

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Delayed Coking

• Coking is a severe method of thermal cracking used to upgrade heavy

residuals into lighter products or distillates.

• Coking produces straight‐run gasoline (Coker naphtha) and various

middle‐distillate fractions used as catalytic cracking feedstock.

• The process completely reduces hydrogen so that the residue is a form

of carbon called "coke."

• Three typical types of coke are obtained (sponge coke, honeycomb

coke, and needle coke) depending upon the reaction mechanism, time,

temperature, and the crude feedstock.

• In delayed coking the heated charge (typically residuum from

atmospheric distillation towers) is transferred to large coke drums which

provide the long residence time needed to allow the cracking reactions to

proceed to completion.