Crude oil refiniring

8/14/2019 Crude oil refiniring

1/96

CRUDE OIL PROCESSING

AND REFINING


2/96

Introduction

Why crude oil has to be processed

before it is used?

Because crude oils is just too thick to be of

any use , it also contains many

contaminants that have to be removed.


3/96

Introduction

Crude oil as it is found in nature consists of

complex mixtures of compounds containing

hydrogen and carbon (hydrocarbons).

In addition to the hydrocarbons, compounds of

sulphur, nitrogen and oxygen are present in

small amounts.

Furthermore; there are usually traces of

metallic compounds.


4/96

Introduction

These compounds are harmful unless removed

from crude oil by refining.

In this topic we are going to discuss the various

methods of refining crude oils which include

atmospheric distillation, vacuum distillation

and other methods such as solvent extraction,

absorption, adsorption and conversions.


5/96

Petroleum Refining

What is it?


6/96

PHYSICALSEPARATION PROCESSES

Physical separation techniques separate a

mixture such as a crude oil without changing

the chemical characteristics of the

components.

The separation is based on differences of certain

physical properties of the constituents such as

the boiling and melting points, adsorptionaffinities on a certain solid, and diffusion

through certain membranes.


7/96

The important physical separation processes,

discussed here, are distillation, absorption,

adsorption, and solvent extraction.

PHYSICAL SEPARATION PROCESSES


8/96

ATMOSPHERIC DISTILLATION

(Primary Fractional Distillation)

Atmospheric distillation separates the crude oil

complex mixture intodifferent fractions with

relatively narrow boiling ranges.

In general, separation of a mixture into fractions

is based primarily on the difference in the

boiling points of the components.

In atmospheric distillation units, one or more

fractionating columns are used.


9/96

Distilling a crude oil starts by preheating the

feed by exchange with the hot product

streams. The feed is further heated to about

320C as it passes through the heater pipe.

The hot feed enters the fractionators, which

normally contains 3050 fractionation trays.

Steam is introduced at the bottom of the

fractionators to strip off light components.




10/96

The efficiency of separation is a function of the

number of theoretical plates of the

fractionating tower and the reflux ratio.

Reflux is provided by condensing part of the

tower overhead vapors.

Reflux ratio is the ratio of vapors condensing

back to the still to vapors condensing out of

the still (distillate).




11/96

Reflux ratio is the ratio of vapors condensing

back to the still to vapors condensing out of

the still (distillate).

The higher the reflux ratio the better the

separation of the mixture




12/96

Products are withdrawn from the distillation

tower as side streams, while the reflux is

provided by returning a portion of the cooled

vapors from the tower overhead condenser.

From the overhead condenser, the uncondensed

gases are separated, and the condensed light

naphtha liquid is withdrawn to storage.




13/96

Heavy naphtha, kerosene, and gas oil are

withdrawn as side stream products.

The residue is removed from the bottom of the

distillation tower and may be used as a fuel

oil.

It may also be charged to a vacuum distillation

unit, a catalytic cracking or steam cracking

process.




14/96


15/96

Products of primary distillation

Product name No of C atoms Boiling range in C Uses

Petroleum gas 14 Less than 40 heating, cooking, making plastics

Naphtha 59 60 - 100 intermediate that will be further processed to

make gasoline and chemicals

Gasoline 5

12 40

205 motor fuelKerosene 1018 175 - 325 fuel for jet engines and tractors; starting material

for making other products

Gas oilor Diesel 1420 270 - 350 diesel fuel and heating oil; starting material for

making other products

Lubricating oil 2050 300 - 370 motor oil, other lubricants

Heavy gasor Fuel oil 20 - 70 370 - 600 industrial fuel; starting material for making other

products

Residuals- > 70 > 600 coke, asphalt, tar, waxes; starting material for

making other products


16/96

Vacuum Distillation

Vacuum distillation increases the amount of the

middle distillates and produces lubricating oil

base stocks and asphalt.

The feed to the unit is the residue from

atmospheric distillation.

In vacuum distillation, reduced pressures are

applied to avoid cracking long-chain

hydrocarbons present in the feed.


17/96

Vacuum Distillation

The feed is first preheated by exchange with the

products, charged to the vacuum unit heater,

and then passed to the vacuum tower in an

atmosphere of superheated steam.

Using superheated steam is important: it

decreases the partial pressure of the

hydrocarbons and reduces coke formation inthe furnace tubes.


18/96

Vacuum Distillation

Products obtained as side streams are vacuum

gas oil (VGO), lube oil base stocks, and asphalt

(bitumen).

Asphalt may be used for paving roads.


19/96

Vacuum Distillation


20/96

Absorption and Stripping

Absorption and stripping are processes used to

obtain valuable light products such as propane

and butane from the gasoline vapors that pass

out of the top of the fractionating tower.

In the absorption process gasoline vapors are

bubbled through absorption oil such as

kerosene or heavy naphtha in equipmentresembling a fractionating column.


21/96


The light products dissolve in the oil while dry

gases such as hydrogen, methane, ethane, and

pass through undissolved.

The light products are separated from the

absorption oil in the stripping process.


22/96


The solution of the absorption oil and light

products is boiled by steam and passes to

stripping column where the light product

vapor pass upward and are recovered bycondensation by water cooling under

pressure.

The unvaporised oil passes from the base of thecolumn for reuse.


23/96

ADSORPTION PROCESS

Adsorption processes use a solid material

(adsorbent) possessing a large surface area

and the ability to selectively adsorb a gas or a

liquid on its surface.

Examples of adsorbents are silica (SiO2),

anhydrous alumina (Al2O3), and molecular

sieves (crystalline silica/alumina).


24/96

ADSORPTION PROCESS

Adsorption processes may be used to remove

acid gases from natural gas and gas streams.

For example, molecular sieves are used to

dehydrate natural gas and to reduce its acid

gases.

Adsorption processes are also used to separate

liquid mixtures. For example, molecular sieve

5A selectively adsorbs n-paraffins from a low-

octanenaphtha fraction.


25/96

ADSORPTION PROCESS

Branched paraffins and aromatics in the mixture

are not adsorbed on the solid surface.

The collected fraction containing mainly

aromatics and branched paraffins have a

higher octane number than the feed.

Desorbing n-paraffins is effected by

displacement with another solvent or by using

heat.


26/96

Solvent extractionSolvent extraction process is used primarily for

the removal of constituents that would havean adverse effect on the performance of the

product in use.

Solvent extraction processes use a liquid solventthat has a high solvolytic power for certain

compounds in the feed mixture.

For example, ethylene glycol has a greateraffinity for aromatic hydrocarbons and

extracts them preferentially from a reformate

mixture (a liquid paraffinic and aromatic

roduct from catal tic reformin .


27/96

Other solvents that could be used for this

purpose are liquid sulfur dioxide and sulfolane

(tetramethylene sulfone). The sulfolane

process is a versatile extractant for producinghigh purity BTX aromatics (benzene, toluene,

and xylenes).

It also extracts aromatics compounds fromkerosene to produce low-aromatic jet fuels.

Solvent extraction


28/96

On the other hand, liquid propane also has a

high affinity for paraffinic hydrocarbons.

Propane deasphalting removes asphaltic

materials from heavy lube oil base stocks.

The asphaltic materials reduce the viscosity

index of lube oils. In this process, liquid

propane dissolves mainly paraffinic

hydrocarbons and leaves out asphaltic

materials.

Solvent extraction


29/96

Deasphalted oil is stripped to recover propane,

which is recycled.

Solvent extraction may also be used to reduce

asphaltenes and metals from heavy fractions

and residues before using them in catalytic

cracking.

Solvent extraction


30/96

Solvent extraction is used extensively in the

petroleum refining industry.

Each process uses its selective solvent, but, the

basic principle is the same.

Solvent extraction


31/96

CONVERSION PROCESSES

(CRACKING)

The separation processes described above are

based on differences in physical properties of

the components of crude oil.

By chemically changing their molecular

structure, it is possible to convert less valuable

hydrocarbon compound into more valuable .

Conversions are therefore chemical processes

which result into new compounds with

different chemical properties as the feed.

Conversion processes in the petroleum


32/96


industry are generally used to:

1. Upgrade lower-value materials such as heavy

residues to more valuable products such as

naphtha and LPG. Naphtha is mainly used to

supplement the gasoline pool, while LPG isused as a fuel or as a petrochemical

feedstock.



33/96

2. Improve the characteristics of a fuel. For

example, a lower octane naphtha fraction is

reformed to a higher octane reformate

product.


industry are generally used to:



34/96

3.Reduce harmful impurities in petroleum

fractions and residues to control pollution and

to avoid poisoning certain processing

catalysts. For example, hydrotreatment ofnaphtha feeds to catalytic reformers is

essential because sulfur and nitrogen

impurities poison the catalyst.

Conversion processes in the petroleumindustry are generally used to:


35/96

Conversion (cracking) processes are either

thermal, where only heat is used to effect the

required change, or catalytic, where a catalyst

lowers the reaction activation energy.

The catalyst also directs the reaction toward a

desired product or products (selective

catalyst).


36/96

Thermal Cracking

In thermal cracking, high temperatures (typically

in the range of 450C to 750C) and pressures

(up to about 70 atmospheres) are used to

break the large hydrocarbons into smallerones.

Thermal cracking gives mixtures of products

containing high proportions of hydrocarbonswith double bondsalkenes.


37/96

The three important thermal cracking

techniques are:

coking,

viscosity breaking and

steam cracking

Thermal Cracking


38/96

Coking processes

Coking is a severe thermal cracking process

designed to handle heavy residues with high

asphaltene and metal contents. These

residues can not be fed to catalytic crackingunits because their impurities deactivate and

poison the catalysts.

Products from coking processes varyconsiderably with feed type and process

conditions.


39/96

Cocking processes

These products are hydrocarbon gases, cracked

naphtha, middle distillates, and coke.

The gas and liquid products are characterized by

a high percentage of unsaturation.

Hydrotreatment is usually required to saturate

olefinic compounds and to desulfurize

products from coking units.


40/96


41/96

Viscosity Breaking (Vis-breaking)

Viscosity breaking aims to thermally crack long-

chain feed molecules to shorter ones, thus

reducing the viscosity and the pour point of

the product.

In this process, the feed is usually a high

viscosity, high pour point fuel oil that cannot

be used or transported, especially in coldclimates, due to the presence of waxy

materials.


42/96

Wax is a complex mixture of long-chain paraffins

mixed with aromatic compounds having long

paraffinic side chains.

Vis-breaking is a mild cracking process that

operates at approximately 450C using short

residence times. Long paraffinic chains break

to shorter ones, and de-alkylation of thearomatic side chains occurs.

Viscosity Breaking (Vis-breaking)


43/96

Steam cracking

Steam cracking is a petrochemical process in

which saturated hydrocarbons are broken

down into smaller, often unsaturated,

hydrocarbons.

It is the principal industrial method for

producing the lighter alkenes (or commonly

olefins), including ethene (or ethylene) andpropene (or propylene).


44/96

Steam cracker units are facilities in which a

feedstock such as naphtha, liquefied

petroleum gas (LPG), ethane, propane or

butane is thermally cracked through the use ofsteam in a bank of pyrolysis furnaces to

produce lighter hydrocarbons.

Steam cracking


45/96

The products obtained in steam cracking depend

on the composition of the feed, the

hydrocarbon-to-steam ratio, and on the

cracking temperature and furnace residencetime.

Steam cracking

CATALYTIC CONVERSION


46/96

CATALYTIC CONVERSION

PROCESSES

Catalytic conversion processes include naphtha

catalytic reforming, catalytic cracking,

hydrocracking, hydrodealkylation,

isomerization, alkylation, and polymerization.

In these processes, one or more catalyst is used.

Other important catalytic processes are those

directed toward improving the product qualitythrough hydrotreatment.


47/96

Catalytic Reforming

The aim of this process is to improve the octane

number of a naphtha feedstock by changing

its chemical composition.

Octane number (rating )is a value used to

indicate the resistance of a motor fuel to

knock. Octane numbers are based on a scale

on which isooctane is 100 (minimal knock)and heptane is 0 (bad knock).


48/96

Hydrocarbon compounds differ greatly in their

octane ratings due to differences in structure.

In general, aromatics have higher octane ratings

than paraffins and cycloparaffins. Similar to

aromatics, branched paraffins have high

octane ratings.

Catalytic Reforming


49/96

The octane number of a hydrocarbon mixture is

a function of the octane numbers of the

different components and their ratio in the

mixture.

Increasing the octane number of a low-octane

naphtha fraction is achieved by changing the

molecular structure of the low octane numbercomponents.

Catalytic Reforming


50/96

Many reactions are responsible for this change,

such as the dehydrogenation of naphthenes

and the dehydrocyclization of paraffins to

aromatics.

Catalytic reforming is considered the key process

for obtaining benzene, toluene, and xylenes

(BTX).The BTX are important intermediates for the

production of many chemicals.

Catalytic Reforming


51/96

Aromatization

The two reactions directly responsible for

enriching naphtha with aromatics are the

dehydrogenation of naphthenes and the

dehydrocyclization of paraffins.

The first reaction can be represented by the

dehydrogenation of cyclohexane to benzene.


52/96

+ 3H2H = 221 kJ/mol.

Kp = 6 x 105@ 500 C

This reaction is fast; it reaches equilibrium quickly.

The reaction is also reversible, highly endothermic,and the equilibrium constant is quite large (6 l05@

500C).

It is evident that the yield of aromatics (benzene) is

favored at higher temperatures and lower pressures.

The effect of decreasing H2partial pressure is even

more pronounced in shifting the equilibrium to the

right.


53/96

Aromatization

The second aromatization reaction is the

dehydrocyclization of paraffins to aromatics.

For example, if n-hexane represents this

reaction, the first step would be to

dehydrogenate the hexane molecule over the

platinumsurface, giving 1-hexene (2- or 3-

hexenes are also possible isomers).


54/96

This is also an endothermic reaction, and theequilibrium production of aromatics is favored

at higher temperatures and lower pressures.

However, the relative rate of this reaction is

much lower than the dehydrogenation of

cyclohexanes

CH3(CH2)3CH=CH2 + 3H2

H = 266 kJ/mol.

Kp = 7.8 x 104@ 500 C


55/96

Isomerization

Reactions leading to skeletal rearrangement of

paraffins and cycloparaffins in a catalytic

reactor are also important in raising the

octane number of the reformate product.

Isomerization reactions may occur on the

platinum catalyst surface or on the acid

catalyst sites. In the former case, the reactionis slow.


56/96

The example these reactions is the

isomerization of n-heptane to 2-methylhexane.

Isomerization

CH3CH2CH2(CH2)3CH3 CH(CH2)3CH3H3C

CH3


57/96

Isomerization of alkylcyclopentanes may also

occur on the platinum catalyst surface or on

the silica/alumina.

For example, methylcyclopen-tane isomerizes to

cyclohexane:

CH3

+ 3H2

Isomerization


58/96

Hydrocracking

Hydrocracking is a catalytic cracking process

assisted by the presence of an elevated partial

pressure of hydrogen gas.

The function of hydrogen is the purification of

the hydrocarbon stream from sulfur and

nitrogen hetero-atoms.


59/96

The products of this process are saturated

hydrocarbons; depending on the reaction

conditions (temperature, pressure, catalyst

activity) these products range from ethane,LPG to heavier hydrocarbons consisting mostly

of isoparaffins.

Hydrocracking


60/96

The following represents a hydrocracking

reaction:

RCH2CH2CH2R' + H2 RCH2CH3 + R'CH3

Bond breaking can occur at any position alongthe hydrocarbon chain.

Hydrocracking


61/96

Hydrodealkylation

Hydrodealkylation is a cracking reaction of an

aromatic side chain in presence of hydrogen.

Like hydrocracking, the reaction consumes

hydrogen and is favored at a higher hydrogenpartial pressure.

This reaction is particularly important for

increasing benzene yield whenmethylbenzenes and ethylbenzene are

dealkylated.


62/96

Hydrodealkylation may be represented by the

reaction of toluene and hydrogen.

+ H2 + CH4

CH3

Hydrodealkylation


63/96

As in hydrocracking, this reaction increases the

gas yield and changes the relative equilibrium

distribution of the aromatics in favor of

benzene.

Hydrodealkylation


64/96

Catalytic Cracking

Catalytic cracking (Cat-cracking) is a remarkably

versatile and flexible process.

Its principal aim is to crack lower-value stocks

and produce higher-value light and middledistillates.

The process also produces light hydrocarbon

gases, which are important feed stocks forpetrochemicals.


65/96

Catalytic cracking produces more gasoline of

higher octane than thermal cracking.

This is due to the effect of the catalyst, which

promotes isomerization anddehydrocyclization reactions.

Catalytic Cracking


66/96

Products from catalytic cracking units are also

more stable due to a lower olefin content in

the liquid products.

This reflects a higher hydrogen transfer activity,which leads to more saturated hydrocarbons

than in thermally cracked products.

Catalytic Cracking


67/96

A major difference between thermal and

catalytic cracking is that reactions through

catalytic cracking occur via carbocation

intermediate, com-pared to the free radicalintermediate in thermal cracking.

Carbocations are longer lived and accordingly

more selective than free radicals.

Catalytic Cracking


68/96

Alkylation Process

Alkylation in petroleum processing produces

larger hydrocarbon molecules in the gasoline

range from smaller molecules.

The products are branched hydrocarbons havinghigh octane ratings.


69/96

Alkylation Process

The term alkylation generally is applied to the

acid catalyzed reaction between isobutane

and various light olefins, and the product is

known as the alkylate.Alkylates are the best of all possible motor fuels,

having both excellent stability and a high

octane number.


70/96

Alkylation Process

Either concentrated sulfuric acid or anhydrous

hydrofluoric acid is used as a catalyst for the

alkylation reaction.

These acid catalysts are capable of providing aproton, which reacts with the olefin to form a

carbocation.


71/96


72/96

Th f d b ti f th l t t b t t


73/96

The formed carbocation from the last step may abstract a

hydride ion from an isobutane molecule and produce 2,2-

dimethylpentane, or it may rearrange to another carbocation

through a hydride shift.

The new carbocation can rearrange again through a


74/96

The new carbocation can rearrange again through a

methide/hydride shift as shown in the following

equation:

The rearranged carbocation finally reacts with


75/96

The rearranged carbocation finally reacts with

isobutane to form 2,2,3-trimethylbutane

The final product contains approximately 6080%

2,2-dimethylpentane and varying amounts of

2,2,3-trimethylbutane.


76/96

Hydrotreatment Processes

Hydrotreating is a hydrogen-consuming process

primarily used to reduce or remove impurities

such as sulfur, nitrogen, and some trace

metals from the feeds.It also stabilizes the feed by saturating olefinic

compounds.


77/96

Feeds to hydrotreatment units vary widely; they

could be any petroleum fraction, from

naphtha to crude residues.

In this process, the feed is mixed with hydrogen,heated to the proper temperature, and

introduced to the reactor containing the

catalyst.The conditions are usually adjusted to minimize

hydrocracking.


78/96

Reactions occurring in hydrotreatment units are

mainly hydrodesulfurization and

hydrodenitrogenation of sulfur and nitrogen

compounds.In the first case H2S is produced along with the

hydrocarbon. In the latter case, ammonia is

released.

The following examples are hydrodesulfurization


79/96

The following examples are hydrodesulfurization

reactions of some representative sulfur compounds

present in petroleum fractions and coal liquids.SH + H2 RH + H2SR

S RR 2H2 2RH H2S+

RS SR 3H2 2RH + 2H2S

+

+

Examples of hydrodenitrogenation of two types of


80/96

Examples of hydrodenitrogenation of two types of

nitrogen com-pounds normally present in some

light and middle crude distillates are shown asfollows:

l id l i ki ( CC)


81/96

Fluid catalytic cracking (FCC)

FCC is used to convert the high-boiling, high-

molecular weight hydrocarbon fractions of

petroleum crude oils to more valuable

gasoline, olefinic gases and other productsFCC uses a catalyst in the form of a very fine

powder which flows like a liquid when

agitated by steam, air or vapour.

FCC


82/96

FCC process

Feedstock entering the process meets a streamof very hot catalyst and vaporizes.

The resulting vapours keep the catalyst fluidized

as it passes into the reactor, where thecracking takes place and where it is fluidizedby the hydrocarbon vapour.

FCC


83/96

FCC process

The catalyst next passes to a steam strippingsection where most of the volatilehydrocarbons are removed.

It then passes to a regenerator vessel where it isfluidized by a mixture of air and the product ofcombustion which are produced as the cokeon the catalyst is burnt off.

FCC


84/96

FCC process

The catalyst then flows back to the reactor. Thecatalyst thus undergoes acontinuous

circulation between the reactor, stripper and

regenerator sections.

P i C d Oil f P i


85/96

Preparing Crude Oil for Processing

Crude oil often contains water, inorganic salts,suspended solids, dissolved gases and water-

soluble trace metals.

As a first step in the petroleum refining process,to reduce corrosion, plugging, and fouling of

equipment and to prevent poisoning the

catalysts in processing units, thesecontaminants must be removed.

D i


86/96

Degassing

Degassing is the initial separation of associatedgas from crude oil.

By removing dissolved gases and hydrogen

sulfide, crude is stabilized and sweetened theprocesses which diminish safety and corrosion

problems.

Gases are removed by a stabilizer (vacuumpumps or compressors).

D lti


87/96

Desalting

Salt in the crude stream presents seriouscorrosion and scaling problems, and must be

removed.

Salt is dissolved within the remnant brine of thecrude oil.

Desalting removes both salt and the residual

free water.

T f lt i d il


88/96

Types of salts in crude oil

Salts in the crude oil are mostly in the form ofdissolved salts in fine water droplets

emulsified in the crude oil. This is called

water-in-oil emulsion, where the continuousphase is the oil and the dispersed phase is the

water.

The water droplets are so small that they cannotsettle by gravity.

T f lt i d il


89/96

Types of salts in crude oil

The salts can also be present in the form of saltcrystals suspended in the crude oil.

Salt removal requires that these salts be ionized

in the water. Hence wash water is added tothe crude oil to facilitate the desalting

process.

C lt i d il


90/96

Common salts in crude oil

The e types of salts mostly found in crude oil areMg, Ca, and Na chlorides with NaCl being the

abundant type.

These chlorides, except NaCl, hydrolyze at hightemperature to hydrogen chloride.

HCl dissolves in the water producing HCl acid, an

extremely corrosive acid.


91/96

Desalting process


92/96

Desalting process

Caustic or acid may be added to adjust the pH ofthe water wash.

Wastewater and contaminants are discharged

from the bottom of the settling tank to thewastewater treatment facility.

The desalted crude is continuously drawn from

the top of the settling tanks and sent to thecrude distillation (fractionating) tower.

Pre flashing


93/96

Pre-flashing

The crude flow rate to the crude distillation unit(CDU) determines the capacity of the whole

refinery.

The capacity of distillation column is limited bythe vapour flow rate. The vapor flow rate

increases as the vapors rise from the flash

zone to the overhead.

Pre flashing


94/96

Pre-flashing

To keep the vapour velocity within requiredlimits, the pumparounds, which are installed

at several points along the column, extract

heat from the column.This result in condensing the rising vapours and

reducing their vapor velocity.

Pre flashing


95/96

Pre-flashing

To expand crude capacity, the most usedtechnique is to introduce a pre-flash column

before the crude heater.

The crude oil after preheating in the hotproducts and pumparound heat exchangers is

flashed into a column where the lightest

products are removed.

Pre flashing


96/96

Pre-flashing

The botoms from the pre-flash column areintroduced into the crude heater and then to

the crude column.

The amounts of light ends in the crude are nowless, and this reduces the vapour loading up

the column.

Documents

Crude oil refiniring