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Flow diagram of a delayed coking unit:5 (1) coker fractionator, (2)

coker heater, (3) coke drum, (4) vapor recovery column.

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Fluid Coking Heated by the produced coke

Cracking reactions occur inside the heater and the fluidized-bed reactor.

The fluid coke is partially formed in the heater.

Hot coke slurry from the heater is recycled to the fluid reactorto provide the heat required for the cracking reactions.

Fluid coke is formed by spraying the hot feed on the already-formed coke particles. Reactor temperature

is about 520C, and the conversion into coke is immediate,with complete disorientation of the crystallites of product

coke. The burning process in fluid coking tends to concentrate the

metals, but it does not reduce the sulfur content of the coke.

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Characteristics of fluid coke:

high sulfur content,

low volatility, poor crystalline structure, and lowgrindability index.

Flexicoking, integrates fluid coking with cokegasification.

Most of the coke is gasified. Flexicoking gasificationproduces a substantial concentration of the metals in

the coke product.

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Flow diagram of an Exxon flexicoking unit:5 (1) reactor, (2)

scrubber, (3) heater, (4) gasifier, (5) coke fines removal, (6)

H2S removal.

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CATALYTIC CONVERSION PROCESSESCatalytic Reforming

To improve the octane number of a naphtha.

Aromatics and branched paraffins have high octane ratingsthan paraffins and cycloparaffins.

Many reactions: e.g. dehydrogenation of naphthenes and thedehydrocyclization of paraffins to aromatics.

Catalytic reforming is the key process for obtaining benzene,

toluene, and xylenes (BTX).

These aromatics are important intermediates for the productionof many chemicals.

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Reformer Feeds heavy naphtha fraction produced from atmospheric distillation

units.

Naphtha from other sources such as those produced from

cracking and delayed coking may also be used.

Before using naphtha as feed for a catalytic reforming unit, it

must be hydrotreated to saturate the olefins and to

hydrodesulfurize and hydrodenitrogenate sulfur and nitrogen

compounds.

Olefinic compounds are undesirable because they are precursors

for coke, which deactivates the catalyst.

Sulfur and nitrogen compounds poison the reforming catalyst.

The reducing atmosphere in catalytic reforming promotes forming

of hydrogen sulfide and ammonia. Ammonia reduces the acid

sites of the catalyst, while platinum becomes sulfided with H2S.

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Importantis :

Types of hydrocarbons in the feed.

Naphthene content

The boiling range of the feeds

Feeds with higher end points (200C) are favorable because some

of the long-chain molecules are hydrocracked to molecules in the

gasoline range. These molecules can isomerize and dehydrocyclize

to branched paraffins and to aromatics, respectively.

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Reforming Catalysts

Bi-functional to provide two types of catalytic

sites, hydrogenation-dehydrogenation sitesand acid sites.

platinum, is the best known hydrogenation-

dehydrogenation catalyst Alumina, (acid sites) promote carbonium ion

formation

The two types of sites are necessary foraromatization and isomerization reactions.

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Pt/Re catalysts are very stable, active, and selective.

Trimetallic catalysts of noble metal alloys are also used for thesame purpose.

The increased stability of these catalysts allowed operation atlower pressures.

Reforming Reactions

Reforming Catalysts

Aromatization

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The reaction is endothermic i.e. favoured @ higher temp and

lower pressures.

Effect of temp on the conversion and selectivity:

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Catalytic Cracking Catalytic cracking (Cat-cracking): To crack lower-value stocks

and produce higher-value light and middle distillates. To produce light hydrocarbon gases, which are important

feedstocks for petrochemicals.

To produce more gasoline of higher octane than thermal

cracking. This is due to the effect of the catalyst, whichpromotes isomerization and dehydrocyclization reactions.

Feedsvary from gas oils to crude residues

Polycyclic aromatics and asphaltenes peoduce coke.

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Catalytic Catalysts Acid-treated clays were the first catalysts used.

Replaced by synthetic amorphous silica-alumina, which ismore active and stable.

Incorporating zeolites (crystalline alumina-silica) with the

silica/alumina catalyst improves selectivity towards aromatics.

These catalysts have both Lewis and Bronsted acid sites thatpromote carbonium ion formation. An important structural

feature of zeolites is the presence of holes in the crystal

lattice, which are formed by the silica-alumina tetrahedra.

Each tetrahedron is made of four oxygen anions with either an

aluminum or a silicon cation in the center. Each oxygen anion

with a (II) oxidation state is shared between either two silicon,

two aluminum, or an aluminum and a silicon cation.

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Catalytic Catalysts

Bronsted acid sites in HY-zeolites mainly originate from protonsthat neutralize the alumina tetrahedra. When HY-zeolite (X- and

Y-zeolites are cracking catalysts ) is heated to temperatures in

the range of 400500C, Lewis acid sites are formed.

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Zeolite Catalysts Highly selective due to its smaller pores, which allow diffusion

of only smaller molecules through their pores, and to thehigher rate of hydrogen transfer reactions. However, the

silica-alumina matrix has the ability to crack larger molecules.

Deactivation of zeolite catalysts occurs due to coke formation

and to poisoning by heavy metals. Deactivation may be reversible or irreversible.

Reversible deactivation occurs due to coke deposition. This is

reversed by burning coke in the regenerator.

Irreversible deactivation results as a combination of fourseparate but interrelated mechanisms: zeolite dealumination,

zeolite decomposition, matrix surface collapse, and

contamination by metals such as vanadium and sodium.

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Cracking Reactions A major difference between thermal and catalytic cracking is

that reactions through catalytic cracking occur via carbocationintermediate, compared to the free radical intermediate in

thermal cracking.

Carbocations are longer lived and accordingly more selective

than free radicals. Acid catalysts such as amorphous silica-alumina and

crystalline zeolites promote the formation of carbocations.

The following illustrates the different ways by which

carbocations may be generated in the reactor:

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Aromatization Reactions Dehydrocyclizationreaction. Olefinic compounds formed by

the beta scission can form a carbocation intermediate withthe configuration conducive to cyclization.

Once cyclization has occurred, the formed carbocation can lose a proton,and a cyclohexene derivative is obtained. This reaction is aided by the

presence of an olefin in the vicinity (RCH=CH2).

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Cracking Process Most catalytic cracking reactors are either fluid bed or moving

bed. In FCC, the catalyst is an extremely porous powder with an

average particle size of 60 microns.

Catalyst size is important, because it acts as a liquid with the

reacting hydrocarbon mixture.

In the process, the preheated feed enters the reactor section

with hot regenerated catalyst through one or more risers

where cracking occurs. A riser is a fluidized bed where a

concurrent upward flow of the reactant gases and the catalystparticles occurs.

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The reactor temperature is usually held at about 450520C,

and the pressure is approximately 1020 psig.

Gases leave the reactor through cyclones to remove the

powdered catalyst, and pass to a fractionator for separation ofthe product streams. Catalyst regeneration occurs by

combusting carbon

deposits to carbon dioxide and the regenerated catalyst is then

returned

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Typical FCC reactor/regenerator

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Isomerization

Reactions leading to skeltal rearrangements over Pt catalysts

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Hydrocracking

A hydrogen-consuming reaction that leads to higher gas

production

Hydrdealkylation

A cracking reaction of an aromatic side chain in presence of

hydrogen

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Deep Catalytic Cracking Deep catalytic cracking (DCC) is a catalytic cracking process

which selectively cracks a wide variety of feedstocks into lightolefins.

It produces more olefines than FCC.

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Hydrocracking Process It is a cracking process in presence of hydrogen.

The feedstocks are not suitable for catalytic cracking becauseof their high metal, sulfur, nitrogen, and asphaltene contents.

The process can also use feeds with high aromatic content.

Products from hydrocracking processes lack olefinic

hydrocarbons.

The product slate ranges from light hydrocarbon gases to

gasolines to residues.

The process could be adapted for maximizing gasoline, jet

fuel, or diesel production.

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Hydrocracking Catalysts and Reactions

Bifunctional noble metal containing zeolites are used.

This promote carbonium ion formation. Catalysts with strong acidic activity promote isomerization.

The hydrogenation-dehydrogenation is promoted by catalysts

such as cobalt, molybdenum, tungsten, vanadium, palladium,

or rare earth elements. As with catalytic cracking, the main reactions occur by carbonium ion

and beta scission, yielding two fragments that could be

hydrogenated on the catalyst surface.

The first-step is formation of a carbocation over the catalystsurface:

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The carbocation rearrange, eliminate a proton to produce an

olefin, or crack at a beta position to yield an olefin and a new

carbocation.

-Products from hydrocracking are saturated. i.e. gasolines from

hydrocracking units have lower octane ratings. They have a

lower aromatic content due to high hydrogenation activity.

- Products from hydrocracking units are suitable for jet fuel use.

Hydrocracking also produces light hydrocarbon gases (LPG)

suitable as petrochemical feedstocks.

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Hydrocracking Process Mostly single stage, with the possibility of two operation

modes. Once-through and a total conversion of thefractionator bottoms by recyling.

In once-though operation, low sulfur fuels are produced and

the fractionator bottom is not recycled.

In the total conversion mode the fractionator bottom isrecylced to the inlet of the reactor.

In the two-stage operation, the feed is hydrodesulfurized in

the first reactor with partial hydrocracking. Reactor effluent

goes to a high-pressure separator to separate the hydrogen-rich gas, which is recycled and mixed with the fresh feed. The

liquid portion from the separator is fractionated, and the

bottoms of the fractionator are sent to the second stage

reactor.

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Hydrocracking reaction conditions vary widely, depending on

the feed and the required products. Temperature and

pressure range from 400 to 480C and 35 to 170 atmospheres.

Space velocities in the range of 0.5 to 2.0 hr-1 are applied.

Flow diagram of a Cheveron hydocracking unit:29 (1,4) reactors, (2,5)

HP separators, (3) recycle scrubber (optional), (6) LP separator, (7) fractionator.

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Hydrodealkylation Process Designed to hydrodealkylate methylbenzenes, ethylbenzene

and C9+aromatics to benzene. The petrochemical demand for

benzene is greater than for toluene and xylenes.

After separating benzene from the reformate, the higher

aromatics are charged to a hydrodealkylation unit.

The reaction is a hydrocracking one, where the alkyl side

chain breaks and is simultaneously hydrogenated.

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Consuming hydrogen is mainly a function of the number of

benzene substituents.

Dealkylation of polysubstituted benzene increases hydrogen

consumption and gas production (methane).

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Hydrotreatment Processes Hydrotreating is a hydrogen-consuming process to reduce or

remove impurities such as sulfur, nitrogen, and some tracemetals from the feeds.

It also stabilizes the feed by saturating olefinic compounds.

Feeds could be any petroleum fraction, from naphtha to cruderesidues.

The feed is mixed with hydrogen, heated to the proper

temperature, and introduced to the reactor containing thecatalyst.

H d C l d R i

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Hydrotreatment Catalysts and Reactions

The same as those developed in Germany for coal hydrogenation.

The cobalt-molybdenum/alumina is an effective catalyst.

hydrodenitrogenation

Alkylation Process

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Alkylation Process To produce large hydrocarbon molecules in the gasoline fraction

from small moleucles. (branched hydrocarbons).

Normally acid catalyzed using H2SO4or abhydrous HF.

The product is known as the alkylate.

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Some recent research has been devoted to replace the

homogeneous acid catalysts by heterogeneous solid

catalysts employing zeolites and alumina, or zirconia.

I i i

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Isomerization process Small volume but important refinery process.

Acid catalyzed. To produce branched alkanes.

Bifunctional catalysts activated by inorganic chelorides are

used.

Pt/zeolite is a typical isomerization catalyst.

Oligomerization of Olefines (Dimerization)

To produce polymer gasoline with high octane number.

Acid catalyzed. By phosphoric or sulfuric acid.

The feed is Propylne-propane or propykene-butane mixture.

The alkane is used as diluent.