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CHAPTER 5

C R A C K I N G

CHAPTER 5 OUTLINE• Cracking• Fluid catalytic crackingI. PrinciplesII. Recent developmentsIII. FeedstockIV. Product yields and qualitiesV. Catalyst and operating parameters

• HydrocrackingI. PrinciplesII. Process requirementsIII. Product yields and qualitiesIV. Residue cracking

OIL REFINING

• Method by which crude oil convertedtopetroleum products

– (I think that a barrel (42 gal—produces 44gal of

petroleum products)• Distillation (fractionation)

– At high temperature the lightest fractions rise to the top of a tower, heavier fractions condense at bottom

OIL REFINING

Typical Oil– Gasoline C4 to C10 27%– Kerosene C11 to C13 13%– Diesel C14 to C18 12%– Heavy gas oil C19 to C25 10%– Lubricating oil C26-C40 20%– Residue >C40 18%

OIL REFINING

• Thermal Cracking

• Catalytic Cracking

OIL REFINING

• What we get out of oil now with modernrefineries:– 50% gas– 30% fuel

oil– 7.5% jet

fuelHOW??

CRACKING• Crude oil contains many large molecules. If these are to

be used as fuels or feedstock for the chemical industry then they have to be cracked into smaller molecules.

• When hydrocarbons burn they are reacting with oxygen in the air. In general, the smaller the molecule the better it will mix and then react with the air.

Fuel gas Naphtha Diesel

Petrol KerosineFuel Oil and bitumen

CRACKINGInvolves the breaking of C-C bonds in alkanes

Converts heavy fractions into higher value products

• THERMALproceeds via a free radical mechanism

• CATALYTICproceeds via a carbocation (carbonium ion) mechanism

• High Pressure ... 7000 kPa• High Temperature ... 400°C to 900°C• Free Radical Mechanism• Homolytic fission• Produces mostly alkenes ...e.g. ethene

formaking polymers and ethanol

• Produces Hydrogen ... used in theHaber Process and in margarine manufacture

• Bonds can be broken anywhere in the moleculeby C-C bond fission or C-H bond fission

THERMAL CRACKING

• Slight pressure• High Temperature• Use catalyst toCatalysts include

speed up the crackingreaction. zeolite,aluminium hydrosilicate,bauxite and silica alumina.

• Carbocation Mechanism• Heterolytic fission• Produces branched and cyclic alkanes,

naromatichydrocarbons used for motor fuels

**ZEOLITES are crystalline aluminosilicates; clay like substances

CATALYTIC CRACKING

•Catalytic cracking is similar to thermal cracking except that catalystsfacilitate the conversion of the heavier molecules into lighter products.

•Use of a catalyst (a material that assists a chemical reaction but does not take part in it) in the cracking reaction increases the yield of improved-quality products under much less severe operating conditions than in thermal cracking.

•Typical temperatures are from 450°-510° C at much lower pressures of 10-20 psi.

•The catalysts used in refinery cracking units are typically solid materials (zeolite, aluminum hydrosilicate, treated bentonite clay, fuller's earth, bauxite, and silica-alumina) that come in the form of powders, beads, pellets or shaped materials called extrudates.

CATALYTIC CRACKING

There are three basic functions in thecatalyticcracking process:

I. Reaction: Feedstock reacts with catalystandcracks into different hydrocarbons;

II. Regeneration: Catalyst is reactivatedby burning off coke; and

III. Fractionation: Cracked hydrocarbon stream isseparated into various products.

BASIC FUNCTIONS INCATALYTIC CRACKING

molecules• Large hydrocarbons are broken into smaller using heat and a catalyst.

• This process is known as catalytic cracking.• The small molecules produced are then separated by

distillation.

CATALYTIC CRACKING PROCESS

CatalyticcrackerHeat to

vaporise

Distillationtower

pressure

Big Molecules

Smal

ler m

olec

ules

Molecules break up

In the catalytic cracker long chain molecules are‘cracked’. An example of such a reaction is:

C8H18 C6H14 + C2H4

CC

H

H

H

H+

ethene

H H H H H H H HH C C C C C C C C H

H H H H H H H H

Octane

HH H H H H H C C C C C C H H H H H H

H

hexane

Ethene is used to make plastics

Heat pressure catalyst

Used as a fuel

CATALYTIC CRACKING REACTION

CATALYTIC CRACKING REACTION• Products formed are the result of both

primaryand secondary reactions.• Primary rxns – involve the initial C-C bondscission and the immediate neutralization of the carbonium ion.

• Primary rxns as below:Paraffin paraffin + olefin

Alkyl napthene napthene + olefin Alkyl aromatic aromatic + olefin

CLASSIFICATION OFCATALYTIC CRACKING

• Catalytic cracking processes can be classified as either moving-bed (Thermafor catalytic cracking - TCC) or fluidized-bed units (FCC).

• Very few TCC units in operation today, FCC unit has taken over the field – where the major fraction of the cracking reaction occurs.

PROCESS FLOW OFCATALYTIC CRACKING

• Process flows for FCC and TCC are similar.• The hot oil feed is contacted with catalyst in either the

feed riser or the reactor.• The catalyst is progressively deactivated by

theformation of coke on the surface of the catalyst.• Catalyst and hydrocarbon vapors are

separatedmechanically, oil remaining on the catalyst is removedby steam stripping before catalyst enterstheregenerator.• The oil vapors are taken overhead to afractionationtower for separation into streams having thedesired boiling ranges.

CATALYST REGENERATEDWhat happen to the catalyst then?• It flows into the regenerator and is activated by burning

off the coke deposits with air.• Regenerator temperatures are carefully controlled to

prevent catalyst deactivation by overheating and to provide the desired amount of carbon burn-off. – by manipulating the air flow in the exit flue gas.

• Flue gas & catalyst are separated by cyclone separatorsand electrostatic precipitators.

• Important to make sure the catalyst is steam-stripped as it leaves the generator, to remove the adsorbed oxygen before it is contacted with the oil feed.

FLUIDIZED –BED CATALYTIC CRACKING

Introduction - FCC

• The fluidized catalytic cracking (FCC) unit is the heart of the refinery and is where heavy low-value petroleum stream such as vacuum gas oil (VGO) is upgraded into higher value products, mainly gasoline and C3/C4 olefins, which can be used in the alkylation unit for production of gasoline (C7– C8 alkylates).

• Major developments have occurred in areas of newcatalysts and new reactor and regenerator designs.

Role of FCC in the Refinery

• The role of the FCC is to take heavy desulphurised feedstock and crack it into lighter, mainly high octane gasoline.

• The FCC also produces olefins (C5 = and C4 =) and LPG.

FCC Process Flow Diagram

Introduction - FCC

• FCC employs a catalyst in the form of very fine particles (70 microns), that can behave as a fluid when aerated with a vapor.

• Two type of FCC units:I. Side-by –side type, where the reactor and

regenerator are separate vessels adjacent to each other

II. Orthoflow/stacked type, reactor is mountedon top of the regenerator.

Zeolite as Catalyst in FCC

• Early attempts to increase production of light olefins from the FCC were based primarily on process variables.

• Poor selectivity of this approach resulted in excess production of dry gas and coke.

• By 1970s, researchers found that non-Y zeolites could also co-produce light olefins (C2= to C5=), often at the expense of gasoline.

Zeolite as Catalyst in FCC

• Thedevelopment

chronology offor

catalyst and additivesthe

lightto enhance production of olefins in FCCs.

Feedstock

• The main feedstock used in a FCC unit is the gas oil , which can be considered mixtures of aromatic, naphthanic and paraffinic molecules.

• There are also varying amounts of contaminants such as sulphur, nitrogen and metals. To protect thecatalyst, required

feed pre-treatment byhydrotreating is in order toremove contaminants(especially sulphur) and improve

cracking characteristics and yields.

• Nitrogen tends to poison the catalyst by neutralising its acid sites. However, the FCC process is unaffected if the nitrogen content level is controlled below 0.2%.

• The acidity and unique porous structures of zeolites play an important role in controlling the activity and selectivity of many zeolite-based catalysts.

• Some possible feedstocks atmospheric distillates, coking distillates, visbreaking distillates, VGO, atmospheric residue (desulphurised) and vacuum residue (desulphurised, deasphalted).

Feedstock

FCC products