Cracking Technology

7/27/2019 Cracking Technology

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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF CRACKING

Introduction:In context to refinery processes for petroleum, “cracking” is defined as decomposition induced byelevated temperatures (>350° C, >660° F), whereby the higher molecular weight constituents of petroleum are converted to lower molecular weight products. Cracking reactions involve carbon-carbon bond rupture & are thermodynamically favored at high temperatures.

Although cracking basically involves converting large oil molecules into low boiling materials,during the actual process some smaller molecules may combine to give a product of higher molecular weight.

A number of products may be formed during cracking of the petroleum feedstock such asgasoline, coke & fuel oil. Some material obtained during cracking & having a boiling rangeintermediate between gasoline & fuel oil is referred to as “recycle” stock, which is recycled backinto the cracking equipment until conversion is complete.

Chemistry of Cracking:Two general types of reaction occur during cracking:

1. The decomposition of large molecules into small molecules (primary reactions):

CH3 –CH2 –CH2 – CH3 CH4 + CH3 –CH=CH2

Butane methane propeneOr

CH3 –CH2 –CH2 – CH3 CH3 –CH3 + CH2=CH2

Butane ethane ethylene2. Reactions by which some of the primary products react to form higher molecular weight

materials (secondary reactions):

CH2=CH2 + CH2=CH2 CH3 –CH2 –CH=CH2

Ethylene buteneOr

R –CH= CH2 + R’ –CH= CH2 tar, heavy oil, coke, etc.

Methodology of cracking:There are several methods of performing cracking reactions & are described below:1. Thermal Cracking : This involves the noncatalytic conversion of higher-boiling petroleum

stocks into lower-boiling products by application of temperatures above 350° C. From thereaction point of view thermal cracking is a free radical chain reaction, a free radical beingdefined as an atom or group of atoms with an unpaired electron. For free radical reactions of various kinds involving hydrocarbons, refer page 255-257 of “The Chemistry & Technology of

Petroleum “ by James G. Speight or any standard organic chemistry textbook.

2 Catalytic Cracking : This is nothing but thermal decomposition similar to thermal crackingexcept that the cracking process occurs in the presence of a catalyst, which is not (in theory)consumed in the process & directs the course of the cracking reactions to produce more of the desired higher-octane hydrocarbon products. Nowadays most gasoline fractions areproduced by this method superseding the older thermal cracking process. The chemistry of catalytic cracking is an ionic process involving carbonium ions which are hydrocarbon ionshaving a positive charge on a carbon atom. For details of chemical reactions involving

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catalytic cracking, refer page 255-257 of “The Chemistry & Technology of Petroleum“ byJames G. Speight or any standard organic chemistry textbook.

3. Visbreaking : Viscosity breaking or visbreaking is a mild thermal cracking operation used toreduce the viscosity & pour point of residual or asphaltic feed stocks. This operation producesa large amount of partially cracked gas oil which could then be processed in a conventionalcatalytic cracking plant.

4. Coking : Coking is again a variation of the thermal cracking process in which the time of cracking is so long that coke is produced as the bottom product. It is used for the continuousconversion of heavy, low-grade oils into lower products. The feedstock can be material suchas reduced crude, straight-run residua, or cracked residua, & the products are gases,naphtha, fuel oil, gas oil, & coke. The coke obtained is usually used as fuel, but also findsspecialty uses such as electrode manufacture, production of chemicals, & metallurgical coke,thus increasing its value.

5. Hydrocracking : This is an advancement in catalytic cracking process (>350° C) in whichhydrogenation accompanies cracking. It is characterized by the cleavage of carbon-to-carbonlinkages accompanied by hydrogen saturation of the fragments to produce lower-boilingproducts. Relatively high hydrogen pressures (100 to 2000 psi) are required to minimize

polymerizations & condensations leading to coke formation.

6. Hydrotreating : This is again a variation of the catalytic cracking process, except that,although catalysts are employed for hydrotreating, there is very little cracking involved & theprocess actually is used for selective hydrogen addition to olefins & aromatics in order tosaturate them. Another important purpose of hydrotreating is removal of sulfur & nitrogencompounds present in the feedstock by selective hydrogenation. The temperatures &pressures employed are generally moderate compared to hydrocracking.

Description of Cracking processes:

1. Thermal Cracking : Thermal cracking of higher-boiling materials to motor or high-octanegasoline is now becoming an obsolete process, since these days the requirement of high-

octane & low levels of deleterious sulfur & nitrogen compounds has proved to be a seriouslimitation for this process. New units are now practically not installed & many of the older operating refineries have either shutdown these units or have gone for revamping the older units to the more modern catalytic processes. Nevertheless a brief description of thecommercial thermal cracking processes is given for better understandingThe Dubbs Process: This is a typical application of the thermal cracking process. Thefeedstock (reduced crude) is preheated by direct exchange with the cracked products in thefractionating columns. Cracked gasoline & heating oil are removed from the upper section of the column. Light & heavy distillate fractions are removed from the lower section & arepumped to separate heaters. Higher temperatures are used to crack the more refractory lightdistillate fraction. The streams from the heaters are combined & sent to a reaction chamber where a certain residence time allows the cracking reactions to be completed. The crackedproducts are then separated in a low-pressure flash chamber where a heavy fuel oil is

removed as bottoms. The remaining cracked products are sent to the fractionating column.

Low pressures (<100 psi) & temperatures of 500° C or greater, produce low molecular weighthydrocarbons than those produced at higher pressures (400-1000 psi) & at temperaturesbelow 500° C. The reaction time is also important; light feeds (gas oils) & recycle oils requirelonger reaction times than the readily-cracked heavy residues. Mild cracking conditions, witha low conversion per cycle, favor a high yield of gasoline components, with low gas & cokeproduction, but the gasoline quality is not high, whereas more severe conditions giveincreased gas & coke production & reduced gasoline yield (but of higher quality).

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Gas

Feedstock (topped crude)

Gasoline

FurnaceOil

Residuum

Recycle

Mixed-Phase Cracking: Mixed-phase cracking (also called liquid-phase cracking) is a continuousthermal decomposition process for the conversion of heavy feedstocks to products boiling in thegasoline range. The process generally employs rapid heating of the feedstock (kerosene, gas oil,reduced crude, or even whole crude), after which it is passed to a reaction chamber & then to aseparator where the vapors are cooled. Overhead products from the flash chamber arefractionated to gasoline components & recycle stock, while flash chamber bottoms are withdrawnas a heavy fuel oil. Coke formation, which may be considerable at the process temperatures (400to 480° C), is minimized by use of pressures in excess of 350 psi.

Gas

Gasoline

FeedStock

Residuum

Mixed-phase thermal cracking

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F r a c t i o n a t o r

Heater R e a c t i o n

c h a m b e r

F l a s h e r


F l a s h e r

Heater R e a c t i o n

C h a m b e r




Vapor-Phase Cracking: Vapor-phase cracking is a high temperature (545 to 595° C), low-pressure (< 50 psi) thermal conversion process which favors dehydrogenation of feedstock(gaseous hydrocarbons to gas oils) to olefins & aromatics. Coke is often deposited in heater tubes causing shutdowns - relatively large reactors are required for these units.

Gas

Gasoline

FeedStock

Heavy fuel oil

Recycle

Vapor-phase thermal cracking

Selective Cracking: This is a thermal conversion process, which utilizes optimum conditions of temperature & pressure for maximum product yield solely depending on the nature of thefeedstock. For example, heavy oil might be cracked at 495 to 515° C & 300 to 500 psi, whilelighter gas oil may be cracked at 510 to 530° C & 500 to 700 psi. It eliminates the accumulation of

stable low-boiling material in the recycle stock & also minimizes coke formation from high-temperature cracking of the higher-boiling material. The end result is the production of fairly highyields of gasoline, middle distillates, & olefinic gases. Gas

Feedstock(topped crude)

Gasoline

MiddleDistillate

ResiduumLight oil (recycle) Heavy oil (recycle)

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s h e r

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b e r

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Naphtha Cracking: The thermal cracking of naphtha involves the upgrading of low-octanefractions of catalytic naphtha to higher-quality material. The process is designed for upgradingheavier portions of naphtha, which contain uncracked virgin feedstock & to remove napthenes, aswell as paraffins. During the process, small quantities of heavier aromatics are formed bycondensation reactions, & the product stream contains substantial quantities of olefins.

2. Catalytic Cracking : As mentioned earlier, catalytic cracking differs from thermal cracking byuse of catalyst & also is a much more efficient process compared to the non-catalytic thermalcracking. The catalytic & thermal methods are compared in the table below:

Sr.No.

Catalytic Cracking Thermal Cracking

1 The gasoline produced by this methodhas a higher octane number.

Comparatively lower octane number gasolineproduced.

2 The gasoline produced consists largelyof isoparaffins & aromatics, whichcontribute to the higher octane number & also are chemically more stable.

Gasoline contains more mono-olefins &diolefins which are relatively less stable.

3 Catalytic cracking produces substantialquantities of olefinic gases suitable for polymer gasoline manufacture alongwith small amounts of methane, ethane,& ethylene.

Quantity of olefinic gases is smaller inthermal cracking.

4 Catalytic cracking is a more selectivecracking process & gives lesser endproducts.

Not a very selective process & end productsare more.

5 Gives a more economically salablecoke.

Coke quality is not very high.

6 It has greater capability to accept high-sulfur feedstock. Also gasolineproduced by this method has a lower sulfur content.

High-sulfur feedstocks can prove to be alimitation.

Catalytic cracking, as a commercial process thus involves contacting a gas oil fraction with anactive catalyst under suitable conditions of temperature, pressure, & residence time so that asubstantial part (> 50%) of the gas oil is converted into gasoline & lower-boiling products, usuallyin a single pass operation.

A limitation of the catalytic cracking process is the deposition of carbonaceous material on thecatalyst, reducing the catalyst activity. The removal of the coke or carbonaceous deposit istherefore an important factor in the design of such units, one method being the burning of thecatalyst bed or layer in the presence of air for regenerating the catalyst.

A brief description of the various commercial catalytic cracking processes including the early ones

is given below for better understanding: a. Houdry Fixed-Bed Catalytic Cracking : This was the first of the modern catalytic processes &

went into commercial operation in 1936. In this fixed-bed process, the catalyst in the form of small lumps or pellets was made up of layers or beds in several (four or more) catalyst-containing drums called converters. Feedstock vaporized at about 450°C & under 7 to 15-psipressure passed through one of the converters where the cracking reactions took place. After a short time, deposition of coke on the catalyst rendered it ineffective, and, using asynchronized valve system, the feed stream was diverted to the adjacent converter while the

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catalyst in the first converter was regenerated by carefully burning the coke deposits with air. After about 10 minutes, the catalyst was ready to go on stream again.

In the present age, fixed-bed processes have been replaced with the more versatile moving-bed or fluid-bed processes.

Productfractionation

Flue Gas

Air Feedstock(heated)

Houdry fixed-bed catalytic cracking

b. Fluid-bed Catalytic Cracking : This is presently the most widely used catalytic crackingprocess & is characterized by the use of a finely divided silica/alumina based catalyst, whichis moved through the processing unit. The catalyst particles are of such a size that when“aerated” with air, or hydrocarbon vapor, the catalyst behaves like a liquid & can be movedthrough pipes. Thus, vaporized feedstock & fluidized catalyst flow together into a reaction

chamber where the catalyst, still dispersed in the hydrocarbon vapors, forms beds in thereaction chamber & the cracking reactions take place. Because of the even flow distributionof the catalyst & because of its high specific heat in relation to the vapors reacting, the entirereaction can be maintained at a remarkably constant temperature. The cracked vapors passthrough cyclones located in the top of the reaction chamber, thereby removing the catalystfrom the vapors by centrifugal action. The cracked vapors out of the reaction chamber enter the fractionating towers where fractionation into light- & heavy-cracked gas oils, crackedgasoline, & cracked gases takes place.

Due to the contamination of the catalyst in the reaction chamber with coke, its activity isreduced, & it has to be regenerated. Thus the separated spent catalyst flows via steamfluidization from the reaction chamber to the regenerator vessel, where the coke is removedby controlled burning. In the course of burning the coke, a large amount of heat is liberated.

Most of this heat of combustion is absorbed by the regenerated catalyst, & is sufficient tovaporize the fresh feed entering the reaction chamber.

The fluid-bed catalytic cracking units abbreviated as “FCCU” are large-scale processes & unitthroughputs are typically in the range of about 10,000 to 130,000 barrels per day whichcorresponds to catalyst circulation rates of 7 to 130 tons per minute. The large circulationrates of hot, abrasive catalyst constitute a very significant challenge to the mechanicalintegrity of the reactor, the regenerator & their associated internal equipment.

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Gases &gasoline

Flue gas

SteamWater Products Light gas

Oil

Heavy gasOil

Bottoms

Catalyst Steammake-up

Regenerated

Air catalyst

Spent catalystFeedstock

Flowsheet for fluid-bed catalytic cracking

c. Model IV Fluid-Bed Catalytic Cracking Unit: This unit involves a process in which the catalystis transferred between the reactor & regenerator by means of U-bends, & the catalyst flowrate can be varied in relation to the amount of air injected into the spent-catalyst U-bend.Regeneration air, other than that used to control circulation, enters the regenerator through agrid, & the reactor & regenerator are mounted side by side. This design was preceded by theModel III balanced pressure design, the Model II downflow design, & the original Model I

upflow design.

Flue gas Productfractionation

Feedstock

Regenerated catalyst SteamAir

Spent Catalyst

Flowsheet for Model IV fluid-bed catalytic cracking

d. Orthoflow Fluid-Bed Catalytic Cracking: This process uses the unitary vessel design, whichprovides straight-line flow of catalyst & thereby minimizes erosion encountered in pipe-bends.

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R e a c t o r

R e g e n e r a t o r

F r e s h C a t a l y s t

R e a c t o r





Commercial Orthoflow designs are of three types: Models A & C, with the regenerator beneaththe reactor, & model B, with the regenerator above the reactor. In all cases the catalyst-stripping section is located between the reactor & the regenerator; all designs employ theheat- balanced principle incorporating fresh feed–recycle feed cracking.

Flue gas

Air

Product fractionation

Recycle

Steam

Feedstock

Air

Model B Orthoflow fluid-bed catalytic cracking process


Steam

Flue gas

Air

Feedstock Recycle

Model C Orthoflow fluid-bed catalytic cracking unit

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R e a c t o r

Regenerator

Reactor




e. Universal Oil Products (UOP) Fluid-bed Catalytic Cracking: This process is adaptable to theneeds of both large & small refineries, & its important distinguishing features include 1)elimination of the air riser with its attendant large expansion joints, 2) elimination of considerable structural steel supports, & 3) reduction in regenerator & in air-line through useof 15 to 18 psi pressure operation.


Flue gas Catalyst stripper

Steam

Air

FeedstockFlowsheet for UOP fluid-bed catalytic cracking

f. Shell Two-Stage Fluid-Bed Catalytic Cracking : This two-stage fluid catalytic process allowsgreater flexibility in shifting product when dictated by demand. Thus, virgin feedstock is firstcontacted with cracking catalyst in a riser reactor, that is, a pipe in which fluidized catalyst &vaporized oil flow concurrently upward, & the total contact time in this first stage is of theorder of seconds.

Light products Light products

Middle distillate

Heavy distillate

Air

Steam

Regenerated catalystFeedstock

Flowsheet for Shell two-stage catalytic cracking

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High temperatures 470 to 565° C are employed to reduce undesirable coke lay-down oncatalyst without destruction of gasoline by secondary cracking. Other operatingconditions in the first stage are a pressure of 16 psi & catalyst-oil ratio of 3:1 to 50:1, &volume conversion ranges between 20 & 70% have been recorded.

All or part of the unconverted or partially converted gas-oil product from the first stage isthen cracked further in the second-stage fluid-bed reactor. Operating conditions are 480to 540° C & 16 psi with a catalyst-oil ratio of 2 to 12/1. Conversion in the second stagevaries between 15 & 70% with an overall conversion range of 50 to 80%.

g. Airlift Thermofor Catalytic Cracking (Socony Airlift TCC process) : This process is amoving-bed, reactor-over-generator continuous process for conversion of heavy gas oilsinto lighter high-quality gasoline & middle distillate fuel oils. Feed preparation may consistof flashing in a tar separator to get vapor feed, & the tar separator bottoms may be sent toa vacuum tower from which the liquid feed is produced.

The gas-oil vapor-liquid flows downward through the reactor concurrently with theregenerated synthetic bead catalyst. The catalyst is purged by steam at the base of thereactor, & gravitates into the kiln or regeneration is done by the use of air injected into thekiln. Approximately 70% of the carbon on the catalyst is burned in the upper kiln burning

zone & the remainder in the bottom burning zone. Regenerated, cooled catalyst enters thelift pot, where low-pressure air transports it to the surge hopper above the reactor for reuse.

Regeneratedcatalyst

(liquid)

Feedstock

(vapor) Productfractionation

Steam

Flue gas

Air (hot)

Air (lift)

Flowsheet for airlift thermofor catalytic cracking

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K i l n

K i l n

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h. Houdriflow Catalytic Cracking : This is a continuous, moving-bed process employing anintegrated single vessel for the reactor & regenerator kiln. The charge stock, sweet or sour,can be any fraction of the crude boiling between naphtha & soft asphalt. The catalyst istransported from the bottom of the unit to the top in a gas lift employing compressed flue gas& steam.

The reactor feed & catalyst pass concurrently through the reactor zone to a disengager section, in which vapors are separated & directed to a conventional fractionation system.The spent catalyst, which has been steam purged of residual oil, flows to the kiln for regeneration, after which steam & flue gas are used to transport the catalyst to the reactor.

FeedstockSteam

ProductsFlue gas

CatalystLift

Air

Flowsheet for Houdriflow catalytic cracking

i. Houdresid Catalytic cracking : Houdresid catalytic cracking is a process that uses a variationof the continuous-moving catalyst bed designed to get high yields of high-octane gasoline &light distillate from reduced crude charge.

Residuum cuts ranging from crude tower bottoms to vacuum bottoms, including residua highin sulfur or nitrogen can be employed as the feedstock, & the catalyst is synthetic or natural.Though the equipment employed is similar in many respects to that used in Houdriflow units,novel process features modify or eliminate the adverse effects on catalyst & productselectivity usually resulting when heavy metals –iron, nickel, copper, & vanadium – arepresent in the fuel. The Houdresid catalytic reactor & catalyst-regenerating kiln are contained

in a single vessel. Fresh feed plus recycled gas oil are charged to top of the unit in a partiallyvaporized state & mixed with steam. Refer flowsheet below:

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Lightproducts

Middledistillates

Feedstock Redu cedcrude

Fuel stocks

Recycle

Flowsheet for Houdresid catalytic cracking

j. Suspensoid Catalytic Cracking : This was developed from the thermal cracking processcarried out in tube & tank units. Small amounts of powdered catalyst or a mixture with thefeedstock & the mixture are pumped through a cracking coil furnace. Crackingtemperatures are 550 to 610° C with pressures of 200 to 500 psi. After leaving the furnace,the cracked material enters a bubble tower where they are separated into two parts, gas oil& pressure distillate. The latter is separated into gasoline & gases. The spent catalyst isfiltered from the tar, which is used as a heavy-industry fuel oil.

The process is a compromise between catalytic & thermal cracking. Here the catalystallows a higher cracking temperature & assists mechanically in keeping coke from

accumulating on the walls of the tubes. The normal catalyst employed is spent clayobtained from the contact filtration of lubricating oils (2 to 10 lb. per barrel of feed).

Products (gasoline, gases)

Catalyst

Feedstock

Heavy fueloil

Catalyst

Flowsheet for Suspensoid catalytic cracking

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H o u d r e s i d u n i t




H e a t e r

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3. Visbreaking : As explained earlier visbreaking is a mild thermal cracking operation employedfor viscosity reduction of residual or asphaltic feed stocks.

Visbreaking conditions range from 455 to 510°C & from 50 to 300 psi at the heating-coiloutlet. Liquid-phase cracking takes place at these low-severity conditions. Besides fuel oil(major product), material in the gas oil & gasoline boiling range is also produced. The gas oilproduced may be diverted to catalytic cracking units or used as heating oil.

Thus, a crude oil residuum is passed through a furnace where it is heated to a temperature of 480°C, the outlet pressure controlled at about 100 psi. The furnace is designed in a manner such that it contains a soaking section of low heat density, where the charge can be held untilthe visbreaking reactions are completed. The cracked products are then passed into a flash-distillation chamber. The overhead material from this chamber is then fractionated to producea low-quality gasoline as an overhead product & light gas oil as bottoms. The liquid productsfrom the flash chamber are cooled with a gas oil flux & then sent to a vacuum fractionator.This yields a heavy gas oil distillate & a residual tar of reduced viscosity.

GasolineFeedstock

Heavygas oil

Tar

Light gas oil

Flowsheet for Visbreaking operations

4. Coking : As mentioned earlier coking processes generally utilize longer reaction times thanthermal cracking processes. To accomplish this, drums or chambers (reaction vessels) areemployed. Normally two or more such vessels are provided, in order to simultaneouslydecoke the off-line vessel without interrupting the semicontinuous type of process. Thevarious type of coking processes are described below:

a. Delayed Coking : This is a semicontinuous process in which the heated charge istransferred to large soaking (or coking) drums, which provide the long residence timeneeded to allow the cracking reactions to be completed. The feed to these units isnormally an atmospheric residuum although cracked tars & heavy catalytic oils may also

be used.

The process flow is as follows:The feedstock enters the product fractionator where it is heated & lighter fractions areremoved as side streams. The fractionator bottoms, including a recycle stream of heavyproduct, are then heated in a furnace whose outlet temperature varies from 480 to 515°C.The heated material enters one of a pair of coking drums where the cracking reactions arecompleted. The cracked products leave as overheads, & coke deposits form on thethe inner surface of the drum. Two drums allow for continuous operation, with one on streamwhile the other is being cleaned. The temperature in the coke drum ranges from 415 to 450°C

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at pressures from 15 to 90 psi. The overheads from the coking drum go to the fractionator,where naphtha & heating oil fractions are recovered. The heavy recycle material is material iscombined with preheated fresh feed & returned to the reactor.

The coke drum is usually on stream for about 24 hours before becoming filled with porouscoke, & the following procedure is used to remove the coke: 1) the coke deposit is cooled withwater; 2) one of the heads of the coking drum is removed to permit the drilling of a holethrough the center of the deposit; & 3) a hydraulic cutting device, which uses multiple high-pressure jets, is inserted into the hole, & the wet coke is removed from the drum. Thesecleaning operations normally require 24 hours before the drum can be put into reuse.

GasGasoline (naphtha)Gas oil

Feedstock

Coke

Operative Nonoperative

Flowsheet for Delayed coking

b. Fluid Coking : Fluid coking is a continuous process, which uses the fluidized-solids techniqueto convert residua, including vacuum pitches, to more valuable products. The residuum iscoked by being sprayed into a fluidized bed of hot, fine coke particles, which permit thecoking reactions to be conducted at higher temperatures & shorter contact times than can beemployed in delayed coking. Moreover, these conditions result in decreased yields of cokewith greater quantity of more valuable liquid product being recovered.

Fluid coking uses two vessels, a reactor & a burner; coke particles are circulated betweenthese to transfer heat (generated by burning a portion of the coke) to the reactor. The reactor holds a bed of fluidized coke particles, & steam is introduced at the bottom of the reactor to

fluidize the bed. The pitch feed at, for example 260 to 370°C is injected directly into thereactor. The temperature in the coking vessel ranges from 480 to 565°C, & the pressurenearly atmospheric causing the incoming feed to partly vaporize & partly deposit on thefluidized coke particles. The material on the particle surface then cracks & vaporizes, leavinga residue, which dries to form coke. The vapor products pass through cyclones, whichremove most of the entrained coke.

The vapor is discharged into the bottom of a scrubber where the products are cooled tocondense a heavy tar, which contains substantial quantity of coke dust & is recycled back tothe reactor. The upper part of the scrubber tower is a fractionating zone from which coker gas

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C o k i n g d r u m

( s o a k e r )

C o k i n g d r u m

( s o a k e r )




oil is withdrawn for feeding to a catalytic cracking unit. The naphtha & gas from thefractionating zone are taken overhead to condensers.

In the reactor, the coke particles flow down through the vessel into a stripping zone at thebottom. Steam displaces the product vapors between the particles & the coke then flows intoa riser, which leads to the burner. Steam is added to the riser to reduce the solids loading &induce upward flow. The average bed temperature in the burner is 590 to 650°C, & air isadded as needed to maintain the temperature by burning part of the product coke. Thepressure in the burner may range from 5 to 25 psi. Flue gases from the burner pass throughcyclones & discharge to the stack. Hot coke from the bed is returned to the reactor through asecond riser assembly.

Coke is one of the products of the process, & it must be withdrawn from the system in order to keep the solids inventory from increasing. The net coke produced is removed from theburner bed through a quench elutriator drum, where water is added for cooling & cooled cokeis withdrawn & sent to storage. During the course of the coking reaction the particles tend togrow in size. The size of the coke particles remaining in the system is controlled by a grindingsystem within the reactor.

Fuel gas

Cracked gasoline

Gas oil Flue gas

Coke

By-product coke

FeedstockAir

Coke

Steam

Flowsheet for Fluid coking

c. Decarbonizing : The decarbonizing thermal process is designed to minimize coke & gasolineyields but, at the same time, to give maximum yield of gas oil. The process is essentially the

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r




same as the delayed coking process, but lower temperatures & pressures are employed. For example, pressures range from 10 to 25 psi while heater outlet temperatures maybe 485°C &coke drum top temperatures maybe of the order of 415°C.

Gas

Gasoline

Gas oil

Feedstock

Flowsheet for Decarbonizing

d. Low-Pressure Coking : Low-pressure coking is a process designed for a once-through, lowpressure operation. The process is similar to delayed coking except that recycling is notpracticed & the coke chamber operating conditions are 435°C, 25 psi. Excessive coking isinhibited by the addition of water to the feedstock.

Gas andgasoline

Gas oil

Feedstock

Fuel oil

Flowsheet for Low-pressure coking

e. High-Temperature Coking : This is a semicontinuous thermal conversion process designed for high-melting asphaltic residua which yield coke & gas oil as the primary products. The coke

may further be treated to remove sulfur to produce a low-sulfur coke (≤ 5%), even though the

feedstock could have as much as 5% wt/wt sulfur.The process flow is as follows:

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C o k e d r u m


F l a s h e r

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The feedstock is fed into the pitch accumulator, then to the heater (370°C, 30 psi), & finally tothe coke oven where temperatures may be as high as 980 to 1095°C. Volatile materials arefractionated, and, after the cycle is complete, coke is collected for sulfur removal & quenchingprior to storage.

Gas

Gasoline

Gas oil

Coke

Flowsheet for High-temperature coking

5. Hydrocracking : Hydrocracking is similar to catalytic cracking, with hydrogenationsuperimposed & with the reactions taking place simultaneously or sequentially. The purposehydrocracking is to convert high-boiling feedstocks to lower-boiling products by cracking thehydrocarbons in the feed & hydrogenating the unsaturated materials in the product streams.The polycyclic aromatics are first partially hydrogenated before cracking of the aromaticnucleus takes place. Also the majority of sulfur & nitrogen is converted to hydrogen sulfide &ammonia. The reaction rates are facilitated by use of catalysts.

Large quantities of hydrogen sulfide & ammonia are formed when using high sulfur & nitrogenfeedstocks for hydrocracking units. These are removed by the injection of water in which,under the high pressure conditions employed, both hydrogen sulfide & ammonia are very

soluble compared with hydrogen & hydrocarbon gases.

6. Hydrotreating : The purpose of the process is the removal of sulfur & nitrogen compoundswithout appreciable alteration in the boiling range or in other words it is selectivehydrogenation of the feedstock for removal of sulfur & nitrogen with very little crackinginvolved.

Hydrotreating catalysts are usually cobalt plus molybdenum or nickel plus molybdenum in thesulfide forms, impregnated on an alumina base.

The operating conditions of 1000 to 2000 psi hydrogen pressures & 370°C temperatures aresuch that appreciable hydrogenation of aromatics will not occur.

Commercial Processes for Hydrocracking & Hydrotreating : Since commercial processes for hydrocracking & hydrotreating operate essentially in the same manner i.e. feedstock ispassed along with hydrogen gas into a tower or reactor filled with catalyst pellets thecommercial processes have not been classified separately as hydrocracking & hydrotreating.The processing conditions i.e. the temperature & pressures decide whether a lot of crackingreactions are taking place along with the hydrogenation or just removal of nitrogen & sulfur istaking place.

Hydrofining: This process can be applied to lubricating oils, naphthas, & gas oils. Thefeedstock is heated in a furnace & passed with hydrogen through a reactor containing a

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a c c u m u l a t o r




suitable metal oxide catalyst, such as cobalt & molybdenum oxides on alumina. Reactor operating conditions range from 205 to 425°C & from 50 to 800 psi, depending on thefeedstock & the degree of treating required.. Higher-boiling feedstocks, high sulfur content, &maximum sulfur removal require higher temperatures & pressures.

After passing through the reactor, the treated oils cooled & separated from the excesshydrogen, which is recycled through the reactor. The treated oil is pumped to a stripper tower where hydrogen sulfide, formed by the hydrogenation reaction, is remove by steam, vacuum,or flue gas, & the finished product leaves the bottom of the stripper tower. In this process thecatalyst is usually not regenerated & is replaced after about a year’s use.

This process is used to upgrade low-quality, high-sulfur naphthas. The sulfur content of kerosenes can be reduced with improved color, odor, & wick-char characteristics. Thetendency of kerosene to form smoke is not affected since aromatics, which cause smoke., arenot affected by the mild hydrofining conditions. Cracked gas oils with high sulfur content canbe converted to excellent furnace fuel oils & diesel fuel oils by reduction in sulfur content & byremoval of components that form gum & carbon residues.

Hydrogen

Water (vapor)

FeedstockProduct stream

Hydrogen

Flowsheet for Hydrofining

Unifining: This is regenerative, fixed-bed, catalytic process to desulfurize & hydrogenaterefinery distillates of any boiling range. Contaminating metals, nitrogen compounds, & oxygencompounds are eliminated, along with sulfur. The catalyst is a cobalt –molybdenum-aluminatype which may be regenerated in situ with steam & air.

Ultrafining: It is a regenerative, fixed-bed, catalytic process to desulfurize & hydrogenaterefinery stocks from naphthas through lube stocks. The catalyst is cobalt-molybdenum on

alumina & may be regenerated in situ using an air-stream mixture. Regeneration requires 10to 20 hours & may be repeated 50 to 100 times for a given batch of catalyst; catalyst life is 2to 5 years depending on the feedstock.

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Autofining: The autofining process differs from other hydrorefining processes in that an externalsource of hydrogen is not required. Sufficient hydrogen to convert sulfur to hydrogen sulfide isobtained by dehydrogenation of naphthenes in the feedstock.

The processing equipment is similar to that used in Hydrofining. The catalyst is cobalt &molybdenum oxides on alumina, & operating conditions are usually 340 to 425°C at pressures of 100 to 200 psi. Hydrogen formed by dehydrogenation of naphthenes in the reactor, is separatedfrom the treated oil & is then recycled through the reactor. The catalyst is regenerated with steam& air at 200 to 1000 hour intervals, depending on whether light or heavy feedstocks have beenprocessed. The process is used for the same purpose, as Hydrofining but is limited to fractionswith end points not higher than 370°C.

Feedstock Flue gas(preheated)

Productstream

Heavy fuel oil

Flowsheet for Autofining

Isomax: The Isomax process is a two-stage, fixed-bed catalyst system which operates under hydrogen pressures from 500 to 1500 psig in a temperature range of 205 to 370°C, for examplewith middle distillate feedstocks. Exact conditions depend on the feedstock & productrequirements, & hydrogen consumption is of the order of 1000 to 1600 SCF per barrel of feedprocessed. Each stage has a separate hydrogen recycle system. Conversion may be balanced to

provide products for variable requirements, & recycle can be taken to extinction if necessary.Fractionation can also be handled in a number of ways to yield desired products.

RecycleHydrogen Fuel gas hydrogen Fuel gas

Feedstock recycle Fuel gasButanes

Light gasolineHeavy gasolineDiesel fuel

Bottoms

Feedstock recycle

Flowsheet for Isomax hydrocracking process

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u l a t o r

K n o c k

o u t

d r u m

R e a c

t o r

S t a b i l i z e r

R e a c t o r

R e a c t o r




H-Oil: The H-Oil process is basically a catalytic hydrogenation technique in which, during thereaction, considerable hydrocracking takes place. The process is used to upgrade heavy sulfur-containing crudes & residual stocks to high-quality sweet distillates, thereby reducing fuel oilyield. A modification of H-Oil called Hy-C cracking will convert heavy distillates to middledistillates & kerosene.

Oil & hydrogen are fed upward through the reactors as a liquid-gas mixture at a velocity such thatcatalyst is in continuous motion. Catalyst of small particle size can be used, giving efficientcontact among gas, liquid, and solid with good mass & heat transfer. Part of the reactor effluent isrecycled back through the reactors for temperature control & to maintain the requisite liquidvelocity. The entire bed is held within a narrow temperature range, which provides essentially anisothermal operation with an exothermic process. Because of the movement of catalyst particlesin the liquid-gas medium, deposition of tar & coke is minimized, & fine solids entrained in the feedwill not lead to reactor plugging. The can also be added & withdrawn from the reactor withoutinterrupting the continuity of the process.

The reactor effluent is cooled by exchange & separates into vapor & liquid. After scrubbing in alean-oil absorber, hydrogen is recycled, and the liquid product is either stored directly or fractionated prior to storage & blending.

Hydrogenrecycle

Hydrogen

Gas recycle

Lean oil

Rich oil

Distillate

ProductFeedstock stream

Hydrogen make-up

Flowsheet for H-Oil Process

Unicracking-JHC: This is a fixed-bed catalytic process that employs a high-activity catalyst with a

high tolerance for sulfur & nitrogen compounds & can be regenerated. The design is based upona single-stage or a two-stage system with provisions to recycle to extinction.

A two-stage reactor system receives untreated feed, make-up hydrogen, and a recycle gas at thefirst stage in which gasoline conversion may be as high as 60% by volume. The reactor effluent isseparated to recycle gas, liquid product, and unconverted oil. The second-stage oil may be either once through or recycle cracking; feed to the second stage is a mixture of unconverted first-stageoil & second-stage recycle.

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A b

s o r b e r

F l a

s h e r

S e p

a r a t o r

R

e a c t o r

R

e a c t o r




Hydrogenmake-up Gas and light

gasolineFeedstock

Hydrogen Gasolinerecycle

Diesel fuel

Feedstock recycle

Flowsheet for Unicracking-JHC process

Gulf HDS: This is a regenerative fixed-be process to upgrade petroleum residues by catalytichydrogenation to refined fuel oils or to high-quality catalytic charge stocks. Desulfurization &quality improvement are the primary purposes of the process, but if the operating conditions &catalysts are varied, light distillates can be produced & the viscosity of heavy material can belowered. Long on-stream cycles are maintained by reducing random hydrocracking reactions to aminimum, and whole crudes, virgin, or cracked residua may serve as feedstock.

The catalyst is a metallic compound supported on pelleted alumina & may be regenerated in situwith air & steam or flue gas through a temperature cycle of 400 to 650°C. On-stream cycles of 4to 5 months can be obtained at desulfurization levels of 65 to 75% & catalyst life may be as longas 2 years.

Hydrogenmake-up

Hydrogen recycle

LeanGas

Feedstock Diethanolamine

Rich LightGas gasoline

Heavy

naphtha

Light

gas oil

Heavygas oil“Light”

bottoms“Heavy”bottoms

Flowsheet for Gulf HDS process

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F l a s h e r ( s )

S e p a r a t o r

( l o w p r e s s . )

S e p a r a t o r

( h i g h p r e s s . )

A b s o r b e r

R e a c t o r

R e a c t o r




H-G Hydrocracking: This process may be designed with either a single- or a two-stage reactor system for conversion of light & heavy gas oils to lower-boiling fractions. The feedstock is mixedwith recycle gas oil, make-up hydrogen, and hydrogen-rich recycle gas, and then heated &charged to the reactor. The reactor effluent is cooled & sent to a high-pressure separator wherehydrogen-rich gas is flashed off, scrubbed, then recycled to the reactor. Separator liquid passesto a stabilizer for removal of butanes & lighter products, & the bottoms are taken to a fractionator for separation; any unconverted material is recycled to the reactor.

C2 gases

Hydrogen C3, C4 gasesFeedstock recycle

(Heavy gas oil) Gasoline

(Recycle) ProductStream

Hydrogen

(Make-up)

Feedstockrecycle

Flowsheet for H-G hydrocracking process

Ferrofining: The mild hydrogen-treating process was developed to treat distilled & solvent-refinedlubricating oils. The process eliminates the need for acid & clay treatment. The catalyst is a three-component material on alumina base with low hydrogen consumption & life expectancy of 2 yearsor more. Process operations include heating the hydrogen-oil mixture & charging to a downflowcatalyst-filled reactor. Separation of oil & gas is a two-stage operation whereby gas is removed to

the fuel system. The oil is then stripped to control the flash point, dried in vacuum, & a finalfiltering step removes the catalyst fines.

HydrogenFeedstock Fuel

Gas Gas andsteam

Product

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R e a c t o r

R e a c t o

r

S e p a r a t o r

( h i g h p r e

s s . )

S e p a r a t o r

( l o w p r e s s . )

S t r i p p e r / v a c u u m d r i e r

F i l t e r




Flowsheet for Ferrofining processList of know-how suppliers for Cracking Processes: Most oil majors have developed &modified technologies to suit their own particular requirements. There have been manymodifications to the older technologies & many of these processes are patented with the patentsowned by the worlds leading refining companies. Some of the major technology licensors are:

Universal Oil Products (UOP): They are one of the leading technology licensors & provideintegrated refinery solutions including catalytic cracking, hydroprocessing & reforming. “ReliancePetroleum Ltd.”, the largest single-stream refinery in Asia has major processing units designed byUOP.

Shell Development Company (Shell): Shell is a leading company providing technology for visbreaking units & crackers.

ABB-Lummus Global/Chevron: They are among the top technology & engineering solutionsprovider for FCC units, Hydrocrackers & Hydrotreaters.

Stone & Webster Engineering Corporation (S & W): They specialize in FCC units & have a major share of the world market.

Institut Francais Petole (IFP): IFP is recognized the world over for its Hydrotreating technologiesfor light distillates & majority of new installations are opting for IFP technology. One of their specialties is a new selective hydrotreating technology named “Prime G” for ultra low sulfur gasoline.

Haldor Topsoe AS (Topsoe): They are leading contenders for supply of Hydrotreating technology.

Kellogg, Brown & Root Inc. (KBR): Most of the older FCC units in North America were designedby KBR & they are doing a lot of revamp/ technology upgradation jobs on their older units.

BASF: BASF is a Germany based multinational & they have developed certain commercialHydroprocessing technologies.

ExxonMobil: They have developed new selective hydrotreating technologies for ultra low sulfur gasolines called “Scanfining”, “Octgain 125” & “Octgain 220” which shall become very relevantwhen international norms for sulfur content in motor fuels are brought down to 50 ppm.

Some other companies offering Cracking & Hydroprocessing technologies include CD tech,Petrobras (Brazil), Akzo, Criterion (mainly catalyst suppliers) etc.

It is important to note that of the various technologies available, the selection has to be madebased basically on three factors: a) feedstock to be processed b) end-product or its mix required& c) economics of a particular process vis-à-vis its competitive technology.

List of reference books:

1. The Chemistry and Technology of Petroleum – James G. SpeightPublisher: Marcel Dekker, Inc.

2. Petroleum Refinery Engineering – W. L. NelsonPublisher: McGraw-Hill Kogakusha, Ltd.

3. Chemical Process Industries – R. Norris Shreve & Joseph A. Brink, Jr.Publisher: McGraw-Hill International Book Company

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