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Cracking kp

Date post: 08-Aug-2018
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    Reactions To Convert Heavy Oils

    HYDROCRACKINGH2+ heavy oil gasoline + diesel

    550 F 300 F 450 F


    heavy oil gasoline + propylene, butane,

    other light ends

    550 F 300 F

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    Reactions To Convert Resid

    Resid is the bottom of the barrel - the material that

    is left in the bottom of the crude/vacuum distillation



    resid + heat coke + heavy oil

    > 900 F solid 550 F

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    a. Because the simple distillation of crude oil produces amounts and types of productsthat are not consistent with those required by the marketplace, subsequent refinery

    processes change the product mix by altering the molecular structure of the

    hydrocarbons. One of the ways of accomplishing this change is through "cracking," a

    process that breaks or cracks the heavier, higher boiling-point petroleum fractions intomore valuable products such as gasoline, fuel oil, and gas o ils. The two basic types ocracking are thermal cracking, using heat and pressure, and catalytic cracking.

    b. The first thermal cracking process was developed around 1913. Distillate fuels andheavy oils were heated under pressure in large drums until they cracked into smaller

    molecules with better antiknock characteristics. However, this method produced largeamounts of solid, unwanted coke. This early process has evolved into the followingapplications of thermal cracking: visbreaking, steam cracking, and coking.

    Visbreaking Process

    Visbreaking, a mild form of thermal cracking, significantly lowers the viscosity of heavycrude-oil residue without affecting the boiling point range. Residual from the atmospheric

    distillation tower is heated (800-950 F) at atmospheric pressure and mildly cracked in aheater. It is then quenched with cool gas oil to control overcracking, and flashed in a

    distillation tower. Visbreaking is used to reduce the pour point of waxy residues andreduce the viscosity of residues used for blending with lighter fuel oils. Middle distillatesmay also be produced, depending on product demand. The thermally cracked residue tar,

    which accumulates in the bottom of the fractionation tower, is vacuum flashed in a

    stripper and the distillate recycled.

    Coking Processes

    Coking is a severe method of thermal cracking used to upgrade heavy residuals intolighter products or distillates. Coking produces straight-run gasoline (coker naphtha) and

    various middle-distillate fractions used as catalytic cracking feedstock. The process socompletely reduces hydrogen that the residue is a form of carbon called "coke." The twomost common processes are delayed coking and continuous (contact or fluid) coking.

    Three typical types of coke are obtained (sponge coke, honeycomb coke, and needle

    coke) depending upon the reaction mechanism, time, temperature, and the crudefeedstock.

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    Delayed coking

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    a. Catalytic cracking breaks complex hydrocarbons into simpler molecules in order toincrease the quality and quantity of lighter, more desirable products and decrease theamount of residuals. This process rearranges the molecular structure of hydrocarbon

    compounds to convert heavy hydrocarbon feedstock into lighter fractions such askerosene, gasoline, LPG, heating oil, and petrochemical feedstock.

    b. Catalytic cracking is similar to thermal cracking except that catalysts facilitate the

    conversion of the heavier molecules into lighter products. Use of a catalyst in thecracking reaction increases the yield of improved-quality products under much lesssevere operating conditions than in thermal cracking. Typical temperatures are from

    850-950 F at much lower pressures of 10-20 psi. The catalysts used in refinery crackingunits 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 extrudites.

    c. There are three basic functions in the catalytic cracking process:

    Reaction: Feedstock reacts with catalyst and cracks into different hydrocarbons;

    Regeneration: Catalyst is reactivated by burning off coke; andFractionation: Cracked hydrocarbon stream is separated into various products.

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    a. The most common process is FCC, in which the oil is cracked in the presence of a

    finely divided catalyst which is maintained in an aerated or fluidized state by the oilapors. The fluid cracker consists of a catalyst section and a fractionating section that

    operate together as an integrated processing unit. The catalyst section contains the reactorand regenerator, which, with the standpipe and riser, forms the catalyst circulation unit.

    The fluid catalyst is continuously circulated between the reactor and the regenerator usingair, oil vapors, and steam as the conveying media.

    b. A typical FCC process involves mixing a preheated hydrocarbon charge with hot,regenerated catalyst as it enters the riser leading to the reactor. The charge is combined

    with a recycle stream within the riser, vaporized, and raised to reactor temperature (900-1,000 F) by the hot catalyst. As the mixture travels up the riser, the charge is cracked at10-30 psi. In the more modern FCC units, all cracking takes place in the riser. The

    "reactor" no longer functions as a reactor; it merely serves as a holding vessel for thecyclones. This cracking continues until the oil vapors are separated from the catalyst in

    the reactor cyclones. The resultant product stream (cracked product) is then charged to afractionating column where it is separated into fractions, and some of the heavy oil isrecycled to the riser.

    c. Spent catalyst is regenerated to get rid of coke that collects on the catalyst during the

    process. Spent catalyst flows through the catalyst stripper to the regenerator, where mostof the coke deposits burn off at the bottom where preheated air and spent catalyst aremixed. Fresh catalyst is added and worn-out catalyst removed to optimize the cracking


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    In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such

    as kerogens or heavy hydrocarbons are broken down into simpler molecules (e.g. light hydrocarbons)

    by the breaking of carbon-carbon bonds in the precursors.The rate of cracking and the end products are

    strongly dependent on the temperature and presence of any catalysts. Cracking, also referred to as

    pyrolysis, is the breakdown of a large alkane into smaller, more useful alkenes and an alkane. Simplyput, cracking hydrocarbons is when you separate long chain hydrocarbons into short ones.

    Fluid catalytic cracking (FCC) is the most important conversion process used in petroleum refineries.

    It is widely used to convert the high-boiling hydrocarbon fractions of petroleum crude oils to more

    valuable gasoline, olefinic gases and other products. Cracking of petroleum hydrocarbons was

    originally done by thermal cracking which has been almost completely replaced by catalytic crackingbecause it produces more gasoline with a higher octane rating. It also produces byproduct gases that are

    more olefinic, and hence more valuable, than those produced by thermal cracking.

    The feedstock to an FCC is usually that portion of the crude oil that has an initial boiling point of 340

    C or higher at atmospheric pressure and an average molecular weight ranging from about 200 to 600 or

    higher. The FCC process vaporizes and breaks the long-chain molecules of the high-boiling

    hydrocarbon liquids into much shorter molecules by contacting the feedstock, at high temperature and

    moderate pressure, with a fluidized powdered catalyst.

    In effect, refineries use fluid catalytic cracking to correct the imbalance between the market demand for

    gasoline and the excess of heavy, high boiling range products resulting from the distillation of crude oil.

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    Reactor and Regenerator

    The preheated high-boiling petroleum feedstock (at about 315 to 430C) consisting of long-chain

    hydrocarbon molecules is combined with recycle slurry oil from the bottom of the distillation column

    and injected into the catalyst riserwhere it is vaporized and cracked into smaller molecules of vapor by

    contact and mixing with the very hot powdered catalyst from the regenerator. All of the cracking

    reactions take place in the catalyst riser. The hydrocarbon vapors "fluidize" the powdered catalyst and the

    mixture of hydrocarbon vapors and catalyst flows upward to enter the reactorat a temperature of about

    535C and a pressure of about 1.72 barg.

    The reactor is in fact merely a vessel in which the cracked product vapors are: (a) separated from the so-calledspent catalystby flowing through a set of two-stage cyclones within the reactorand (b) thespent

    catalystflows downward through a steam stripping section to remove any hydrocarbon vapors before the

    spent catalyst returns to the catalyst regenerator. The flow of spent catalyst to the regenerator is

    regulated by aslide valve in the spent catalyst line.

    Since the cracking reactions produce some carbonaceous material (referred to as coke) that deposits on

    the catalyst and very quickly reduces the catalyst reactivity, the catalyst is regenerated by burning off the

    deposited coke with air blown into the regenerator. The regenerator operates at a temperature of about

    715C and a pressure of about 2.41 barg. The combustion of the coke is exothermic and it produces a

    large amount of heat that is partially absorbed by the regenerated catalyst and provides the heat required

    for the vaporization of the feedstock and the endothermic cracking reactions that take place in the

    catalyst riser. For that reason, FCC units are often referred to as being heat balanced.

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    The hot catalyst (at about 715 C) leaving the regenerator flows into a catalyst withdrawal wellwhere

    any entrained combustion flue gases are allowed to escape and flow back into the upper part to the

    regenerator. The flow of regenerated catalyst to the feedstock injection point below the catalyst riser is

    regulated by a slide valve in the regenerated catalyst line. The hot flue gas exits the regenerator after

    passing through multiple sets of two-stage cylones that remove entrained catalyst from the flue gas.

    The amount of catalyst circulating between the regenerator and the reactor amounts to about 5 kg per kg

    of feedstock which is equivalent to about 4.66 kg per litre of feedstock. Thus, an FCC unit processing

    75,000 barrels/day (12,000,000 litres/day) will circulate about 55,900 metric tons per day of catalyst.

    Distillation column

    The reaction product vapors (at 535 C and a pressure of 1.72 barg) flow from the top of the reactor to

    the bottom section of the distillation column (commonly referred to as the main fractionator) wherethey are distilled into the FCC end products of cracked naphtha, fuel oil and off-gas. After further

    processing for removal of sulfur compounds, the cracked naphtha becomes a high-octane component of

    the refinery's blended gasolines.

    The main fractionator off-gas is sent to what is called a gas recovery uni twhere it is separated into

    butanes and butylenes, propane and propylene, and lower molecular weight gases (hydrogen, methane,

    ethylene and ethane). Some FCC gas recovery units may also separate out some of the ethane and


    Although the schematic flow diagram above depicts the main fractionator as having only one sidecut

    stripper and one fuel oil product, many FCC main fractionators have two sidecut strippers and produce

    a light fuel oil and a heavy fuel oil. Likewise, many FCC main fractionators produce a light cracked

    naphtha and a heavy cracked naphtha. The terminology lightand heavy in this context refers to theproduct boiling ranges, with light products having a lower boiling range than heavy products.

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    The bottom product oil from the main fractionator contains residual catalyst particles which were not

    completely removed by the cyclones in the top of the reactor. For that reason, the bottom product oil is

    referred to as a slurr y oil. Part of that slurry oil is recycled back into the main fractionator above the entry

    point of the hot reaction product vapors so as to cool and partially condense the reaction product vapors asthey enter the main fractionator. The remainder of the slurry oil is pumped through a slurry settler. The

    bottom oil from the slurry settler contains most of the slurry oil catalyst particles and is recycled back into

    the catalyst riser by combining it with the FCC feedstock oil. The so-called clarified slurry oil is

    withdrawn from the top of slurry settler for use elsewhere in the refinery or as a heavy fuel oil blending


    Regenerator flue gas

    Depending on the choice of FCC design, the combustion in the regenerator of the coke on the spent

    catalyst may or may not be complete combustion to carbon dioxide (CO2). The combustion air flow is

    controlled so as to provide the desired ratio of carbon monoxide (CO) to carbon dioxide for each specific

    FCC design.

    In the design shown in Figure 1, the coke has only been partially combusted to CO2. The combustion flue

    gas (containing CO and CO2) at 715 C and at a pressure of 2.41 barg is routed through a secondary

    catalyst separator containingswirl tubes designed to remove 70 to 90 percent of the particulates in the flue

    gas leaving the regenerator. This is required to prevent erosion damage to the blades in the turbo-expander

    that the flue gas is next routed through.

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    The expansion of flue gas through a turbo-expander provides sufficient power to drive the

    regenerator's combustion air compressor. The electrical motor-generator can consume or

    produce electrical power. If the expansion of the flue gas does not provide enough power to

    drive the air compressor, the electric motor/generator provides the needed additional power.

    If the flue gas expansion provides more power than needed to drive the air compressor, than

    the electric motor/generator converts the excess power into electric power and exports it to

    the refinery's electrical system.

    The expanded flue gas is then routed through a steam-generating boiler (referred to as a CO

    boiler) where the carbon monoxide in the flue gas is burned as fuel to provide steam for usein the refinery as well as to comply with any applicable environmental regulatory limits on

    carbon monoxide emissions.

    The flue gas is finally processed through an electrostatic precipitator (ESP) to remove

    residual particulate matter to comply with any applicable environmental regulations

    regarding particulate emissions. The ESP removes particulates in the size range of 2 to 20microns from the flue gas.

    The steam turbine in the flue gas processing system is used to drive the regenerator's

    combustion air compressor during start-ups of the FCC unit until there is sufficient

    combustion flue gas to take over that task.

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    a. Hydrocracking is a two-stage process combining catalytic cracking and

    hydrogenation, wherein heavier feedstocks are cracked in the presence of hydrogen toproduce more desirable products. The process employs high pressure, high temperature, a

    catalyst, and hydrogen. Hydrocracking is used for feedstocks that are difficult to processby e ither catalytic cracking or reforming, since these feedstocks are characterized usually

    by a hi gh pol ycycl i c aromati c contentand/or hi gh concent rati ons of the two pri ncipalcatal yst poi sons, sul fu r and nit rogen compounds.

    b. The hydrocrack ing process largely depends on the nature of the feedstock and therelative rates of the two competing reactions, hydrogenation and cracking. Heavyaromatic feedstock is converted into lighter products under a wide range of very highpressures (1,000-2,000 psi) and fairly high temperatures (750-1,500 F), in the presenceof hydrogen and special catalysts. When the feedstock has a high paraffinic content, theprimary function of hydrogen is to prevent the formation of polycyclic aromaticcompounds. Another important role of hydrogen in the hydrocracking process is to

    reduce tar formation and prevent buildup of coke on the catalyst. Hydrogenation alsoserves to convert sulfur and nitrogen compounds present in the feedstock to hydrogensulfide and ammonia.

    c. Hydrocracking produces relatively large amounts of isobutane for alkylation

    feedstock. Hydrocracking also performs isomerization for pour-point control and smoke-

    point control, both of which are important in high-quality jet fuel.

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    Hydrocracking Process

    a. In the first stage, preheated feedstock is mixed with recycled hydrogenand sent to the first-stage reactor, where catalysts convert sulfur and nitrogen

    compounds to hydrogen sulfide and ammonia. Limited hydrocracking alsooccurs.

    b. After the hydrocarbon leaves the first stage, it is cooled and liquefied and

    run through a hydrocarbon separator. The hydrogen is recycled to the

    feedstock. The liquid is charged to a fractionator. Depending on the productsdesired (gasoline components, jet fuel, and gas oil), the fractionator is run tocut out some portion of the first stage reactor out-turn. Kerosene-range

    material can be taken as a separate side-draw product or included in thefractionator bottoms with the gas oil.

    c. The fractionator bottoms are again mixed with a hydrogen stream andcharged to the second stage. Since this material has already been subjected

    to some hydrogenation, cracking, and reforming in the first stage, theoperations of the second stage are more severe (higher temperatures and

    pressures). Like the out-turn of the first stage, the second stage product is

    separated from the hydrogen and charged to the fractionator.

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    a. High-purity hydrogen (95%-99%) is required for hydrodesulfurization, hydrogenation,hydrocracking, and petrochemical processes. Hydrogen, produced as a by-product orefinery processes (principally hydrogen recovery from catalytic reformer product gases),

    often is not enough to meet the total refinery requirements, necessitating themanufacturing of additional hydrogen or obtaining supply from external sources.

    b. In steam-methane reforming, desulfurized gases are mixed with superheated steam

    (1,100-1,600 F) and reformed in tubes containing a nickel base catalyst. The reformedgas, which consists of steam, hydrogen, carbon monoxide, and carbon dioxide, is cooled

    and passed through converters containing an iron catalyst where the carbon monoxidereacts with steam to form carbon dioxide and more hydrogen. The carbon dioxide is

    removed by amine washing. Any remaining carbon monoxide in the product stream isconverted to methane.

    c. Steam-naphtha reforming is a continuous process for the production of hydrogen from

    liquid hydrocarbons and is, in fact, similar to steam-methane reforming. A variety onaphthas in the gasoline boiling range may be employed, including fuel containing up to

    35% aromatics. Following pretreatment to remove sulfur compounds, the feedstock ismixed with steam and taken to the reforming furnace (1,250-1,500 F) where hydrogen

    is produced.

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