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8/12/2019 Coker National

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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 thefluidized-bed reactor.

The fluid coke is partially formed in the heater.

Hot coke slurry from the heater is recycled to the fluidreactor to provide the heat required for the crackingreactions.

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

is about !"#C$ and the conversion into coke is

immediate$ %ith complete disorientation of the crystallitesof product coke.

The burning process in fluid coking tends to concentratethe metals$ but it does not reduce the sulfur content ofthe coke.


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Characteristics of fluid coke& high sulfur content$

lo% volatility$ poor crystalline structure$ and lo%grindability inde'.

Fle'icoking$ integrates fluid coking %ith cokegasification.

(ost of the coke is gasified. Fle'icoking

gasification produces 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) eater, (!) gasifier, (5) coke fines remo"al, (#)

$2% remo"al&


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CATALYTIC CONVERSION

PROCESSESCatalytic Reforming

To improve the octane number of a naphtha.

)romatics and branched paraffins have high octaneratings than paraffins and cycloparaffins.

(any reactions& e.g. dehydrogenation of naphthenes andthe dehydrocyclization of paraffins to aromatics.

Catalytic reforming is the key process for obtainingbenzene$ toluene$ and 'ylenes *+T,.

These aromatics are important intermediates for theproduction of many chemicals.


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Reformer Feeds heavy naphtha fraction produced from atmospheric

distillation units.

aphtha from other sources such as those produced from

cracking and delayed coking may also be used.

+efore 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.

/lefinic compounds are undesirable because they are

precursors for coke$ %hich deactivates the catalyst.

0ulfur and nitrogen compounds poison the reforming

catalyst.

The reducing atmosphere in catalytic reforming promotes

forming of hydrogen sulfide and ammonia. )mmoniareduces the acid sites of the catal st %hile latinum


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1mportantis &

'Types of hydrocarbons in the feed.

'aphthene content'The boiling range of the feeds

Feeds %ith higher end points *2!""#C are favorable becausesome 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 +i-functional to provide t%o types of

catalytic sites$ hydrogenation-dehydrogenation sites and acid sites.

platinum$ is the best kno%n

hydrogenation-dehydrogenation catalyst )lumina$ *acid sites promote carbonium

ion formation

The t%o types of sites are necessary foraromatization and isomerization reactions.


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3t4Re catalysts are very stable$ active$ and selective.

Trimetallic catalysts of noble metal alloys are also usedfor the same purpose.

The increased stability of these catalysts allo%edoperation at lo%er pressures.

Reforming Reactions

Reforming Catalysts

Aromatiation


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The reaction is endothermic i.e. favoured 5 higher tempand lo%er pressures.

6ffect of temp on the conversion and selectivity&


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Catalytic Cracking Catalytic cracking *Cat-cracking& To crack lo%er-value

stocks and produce higher-value light and middledistillates.

To produce light hydrocarbon gases$ %hich are important

feedstocks for petrochemicals.

To produce more gasoline of higher octane than thermalcracking. This is due to the effect of the catalyst$ %hich

promotes isomerization and dehydrocyclization

reactions.

Feedsvary from gas oils to crude residues 3olycyclic aromatics and asphaltenes peoduce coke.


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Catalytic Cataly!t! )cid-treated clays %ere the first catalysts used.

Replaced by synthetic amorphous silica-alumina$ %hichis more active and stable.

1ncorporating zeolites *crystalline alumina-silica %ith the

silica4alumina catalyst improves selectivity to%ards

aromatics. These catalysts have both 7e%is and+ronsted acid sites that promote carbonium ion

formation. )n important structural feature of zeolites is

the presence of holes in the crystal lattice$ %hich are

formed by the silica-alumina tetrahedra. 6achtetrahedron is made of four o'ygen anions %ith either an

aluminum or a silicon cation in the center. 6ach o'ygen

anion %ith a *11 o'idation state is shared bet%een either

t%o silicon$ t%o aluminum$ or an aluminum and a silicon

cation.


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Catalytic Cataly!t!

ronsted acid sites in $*+eolites mainl originate from -rotonstat neutrali+e te alumina tetraedra& .en $*+eolite (/* and

*+eolites are cracking catalsts ) is eated to tem-eratures in

te range of !00'500, ewis acid sites are formed&


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"eolite Cataly!t! Highly selective due to its smaller pores$ %hich allo%

diffusion of only smaller molecules through their pores$and to the higher rate of hydrogen transfer reactions.

Ho%ever$ the silica-alumina matri' has the ability to

crack larger molecules.

8eactivation of zeolite catalysts occurs due to cokeformation and to poisoning by heavy metals.

8eactivation may be reversible or irreversible.

Reversible deactivation occurs due to coke deposition.

This is reversed by burning coke in the regenerator. 1rreversible deactivation results as a combination of four

separate but interrelated mechanisms& zeolite

dealumination$

zeolite decomposition$ matri' surface collapse$ andcontamination b metals such as vanadium and sodium.


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Cracking Reactions ) ma9or difference bet%een thermal and catalytic

cracking is that reactions through catalytic crackingoccur via carbocation intermediate$ compared to the free

radical intermediate in thermal cracking.

Carbocations are longer lived and accordingly more

selective than free radicals. )cid catalysts such as amorphous silica-alumina and

crystalline zeolites promote the formation of

carbocations. The follo%ing illustrates the different %ays

by %hich carbocations may be generated in the reactor&


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)romatization Reactions #e$ydrocycliationreaction. /lefinic compounds

formed by the beta scission can form a carbocationintermediate %ith the configuration conducive to

cyclization.

4nce ccli+ation as occurred, te formed carbocation can lose a -roton,and a ccloexene deri"ati"e is obtained& is reaction is aided b te

-resence of an olefin in te "icinit (6'$7$2)&


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Cracking Process (ost catalytic cracking reactors are either fluid bed or

moving bed. 1n FCC$ the catalyst is an e'tremely porous po%der %ith

an average particle size of :" microns.

Catalyst size is important$ because it acts as a liquid %ith

the reacting hydrocarbon mi'ture. 1n the process$ the preheated feed enters the reactor

section %ith hot regenerated catalyst through one or

more risers %here cracking occurs. ) riser is a fluidized

bed %here a concurrent up%ard flo% of the reactantgases and the catalyst particles occurs.


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The reactor temperature is usually held at about ;"

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