FCC-Ch-21

554 Adv. in Pet. Engg. I: Refining

21

Fluidized Catalytic CrackingOMPRAKASH H. NAUTIYAL1*

ABSTRACT

As the heart of a refinery, the Fluid Catalytic Cracking (FCC) unit iscontinuously pushed to the limit. Refiners are continually evaluating potentialFCC modifications to increase capacity and improve product yields, as wellas to maximize on-stream factor and mechanical reliability in order to bemore profitable while simultaneously meeting stringent environmentalregulations. The oil crisis of recent times has caused a drastic decrease in thetotal consumption of oil and changed the demand pattern for the products ofpetroleum refining. The demand for heavier fractions or residual oils hassteadily decreased, making it imperative to convert these into gasoline, dieseland such lighter fractions. Fluid catalytic cracking (FCC) of these heavierfractions, however, poses several serious problems, caused mainly by theirmuch higher hetero-atom concentration, metal contents and coking tendency,as compared to earlier feed stocks. Several process and catalyst innovationshave been made to tackle these problems. A new generation of FCC catalysttechnology has emerged with tailor-made catalysts for higher structuralstability and attrition strength, more complete CO combustion duringregeneration, reducing SOx emissions from FCC stacks, enhancing the gasolineoctane number, passivation of the harmful effects of metals like Ni and Vaccumulating on the catalyst, etc., These developments contain valuablelessons for the science and technology of catalysis.

Key words: Liquid fuels, Oil/petroleum, Refining, Technology, Fluid catalyticcracking

1 Shubh Building, 102, Shivalik II, Canal Road, Chhani Jakat Naka, Vadodara 39002, India*Corresponding author: E-mail: [email protected]

555Fluidized Catalytic Cracking

INTRODUCTION

The Fluid Catalytic Cracking (FCC) process from Lummus Technology is aproven technology used to convert gas oils and residual stocks to lighter, higher-value products such as gasoline. Combining an advanced reaction system designwith an efficient catalyst regeneration system, the process achieves highconversion and selectivity to light products. While it can be used to maximizethe production of gasoline, the flexibility of the process allows conversion andselectivity to vary from maximum distillate production at one extreme, tomaximum propylene production at the other. This FCC technology can beapplied fully in grassroots units or partially, as applicable, in the revamp ofexisting units to increase throughput and/or residue processing capability,improve selectivity, and reduce operating costs.

Refineries vary by complexity; more complex refineries have moresecondary conversion capability, meaning they can produce different types ofpetroleum products. Fluid Catalytic Cracking (FCC), a type of secondary unitoperation, is primarily used in producing additional gasoline in the refiningprocess.

Unlike atmospheric distillation and vacuum distillation, which are physicalseparation processes, fluid catalytic cracking is a chemical process that uses acatalyst to create new, smaller molecules from larger molecules to make gasolineand distillate fuels.

The catalyst is a solid sand-like material that is made fluid by the hotvapor and liquid fed into the FCC (much as water makes sand into quicksand).Because the catalyst is fluid, it can circulate around the FCC, moving betweenreactor and regenerator vessels (see photo). The FCC uses the catalyst andheat to break apart the large molecules of gas oil into the smaller moleculesthat make up gasoline, distillate, and other higher-value products like butaneand propane.

After the gas oil is cracked through contact with the catalyst, the resultingeffluent is processed in fractionators, which separate the effluent based onvarious boiling points into several intermediate products, including butaneand lighter hydrocarbons, gasoline, light gas oil, heavy gas oil, and clarifiedslurry oil.

The butane and lighter hydrocarbons are processed further to separatethem into fuel gas (mostly methane and ethane), propane, propylene, butane,and butene for sale, or for further processing or use. The FCC gasoline must bedesulfurized and reformed before it can be blended into finished gasoline; thelight gas oil is desulfurized before blending into finished heating oil or diesel;and the heavy gas oil is further cracked in either a hydrocracker (using hydrogenand a catalyst) or a Coker. The slurry oil can be blended with residual fuel oilor further processed in the Coker.


Carbon is deposited on the catalyst during the cracking process. Thiscarbon, known as catalyst coke, adheres to the catalyst, reducing its ability tocrack the oil. The coke on the spent catalyst is burned off, which reheats thecatalyst to add heat to the FCC process. Regeneration produces a flue gas thatpasses through environmental control equipment and then is discharged intothe atmosphere.

Process of Fluidized Catalytic Cracking

In the petroleum refining system, an atmospheric distillation column orreduced-pressure distillation column is used to refine crude oil into gasoline,kerosene, and lubrication oil. In addition, the petroleum refining systemincorporates an FCC to distill high-octane gasoline and LPG from the heavycontents of the crude oil. In many refineries the FCC unit serves as the primaryunit, converting, or cracking low-value crude oil heavy ends into a variety ofhigher value, light products. In the US, the primary function of the FCC unitis to produce gasoline. Modern FCC units can process a wide variety offeedstock and can adjust operating conditions to maximize production ofgasoline, middle distillate olefins (LCO) or light olefins to meet differentmarket demands. The top gas generated in the fraction column of the FCCgoes through a heat exchanger and is then pumped to a high pressure. Theresulting gas content is transferred to the LPG recovery system and the liquidcontent to the gasoline generation system. In this process, it is important tomeasure the density (specific gravity) of the gas because the data areessential as a critical parameter in controlling the operation of the FCC.In addition to being used to monitor the system and the quality of theproduct, this measurement can also help prevent pump pressure surges.The GD402 Gas Density Meter has been introduced for this explosionprotected application. It features an intrinsically safe and explosion-proofdesign, fast response, and a dustproof, anti-corrosive, and flame-proofconstruction. GD402 will ensure stable and rapid measurement of gasdensity under hazardous conditions. It is capable of displaying specificgravity and molecular weight readings derived from the density data, andit will greatly reduce the workload by ensuring continuous and accuratemeasurement. Fig. 1 depicts the structure of Fluidized catalytic cracking.

Expected benefits

• Ensures stable and rapid measurement of gas density under hazardousconditions

• Capable of displaying specific gravity and molecular weight readingsderived from the density data

• Greatly reduces the workload by ensuring continuous and accuratemeasurement


Field Data

Process conditions

Measurement point: Outlet of the top fraction column in the FCC

Temperature: 34ºC

Pressure: 75 kPa to 180 kPa

Humidity: Wet

Gas Composition: O2, N2, CO, H2, H2 S, C1 to C5

Dust: None

Measurement Range: 1,600 to 1,800 kg/Nm3

Gas Processing Technology

Patented process technology and proprietary know-how developed byLummus Technology are used in more than 200 natural gas plants aroundthe world. Many of the innovations we have developed, like the use of plate-fin exchangers and packing in cryogenic columns, remain today as standarddesigns in the industry. We have a wide portfolio of patented designsincluding deep ethane and propane recovery, NicheLNGSM, mitigation ofCO2 recovery in NGL processing, and hydrocarbons from refinery streams,enabling us to expand our technology positions through the natural gas valuechain. Fig. 2 depicts the Petrochemical plant.

Fig. 1: Structure of fluid catalytic cracking (courtesy http://www.yokogawa.com/an/index.htm)


Fluid catalytic cracking (FCC) is one of the most important conversionprocesses used in petroleum refineries. It is widely used to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oil tomore valuable gasoline olifinic gases, and other products. Cracking of petroleumhydrocarbons was originally done by thermal cracking, which has been almostcompletely replaced by catalytic cracking because it produces more gasolinewith a higher octane rating. It also produces by product gases that are moreolefinic, and hence more valuable, than those produced by thermal cracking.

The feedstock to an Fluid Catalytic Cracking is usually that portion ofthe crude oil that has an initial boiling point of 340°C or higher at atmosphericpressure and an average molecular weight ranging from about 200 to 600 orhigher. This portion of crude oil is often referred to as heavy gas oil orvacuum gas oil (HVGO). The Fluid Catalytic Cracking process vaporizes andbreaks the long-chain molecules of the high-boiling hydrocarbon liquids intomuch shorter molecules by contacting the feedstock, at high temperatureand moderate pressure, with a fluidized powdered catalyst.

In effect, refineries use fluid catalytic cracking to correct the imbalancebetween the market demand for gasoline and the excess of heavy, highboiling range products resulting from the distillation of crude oil.

As of 2006, Fluid Catalytic Cracking units were in operation at 400petroleum refineries worldwide and about one-third of the crude oil refinedin those refineries is processed in an Fluid Catalytic Cracking to producehigh-octane gasoline and fuel oils.[2][4] During 2007, the Fluid CatalyticCracking units in the United States processed a total of 5,300,000 barrels(834,300,000 litres) per day of feedstock[5] and FCC units worldwide processedabout twice that amount.

To maintain the catalyst activity at a useful level, it’s necessary toregenerate the catalyst by burning off the coke with hot air. As a result, thecatalyst is continuously moved from the reactor to regenerator and back toreactor. Remaining oil on the catalyst is removed by steam stripping beforecatalyst enters the regenerator. The steam supply to the reactor takes placeat a temperature at dry saturated steam. The cracking process producescarbon (coke) which remains on the catalyst particle and rapidly lowers its

Fig. 2: CB and I petrochemicals plant (courtesy CB&I http://www.cbi.com/technologies/petrochemicals-technology)


activity. To maintain the catalyst activity at a useful level, it’s necessary toregenerate the catalyst by burning off this coke with air. Regeneration is akey part of the FCC process. It’s critical to control the regeneratortemperature carefully to prevent catalyst deactivation by overheating andto provide the desired amount of burn-off. This is done by controlling the airflow. A typical air temperature is around 600°C.

Continuously catalyst regenerating makes it possible to manage thehigh catalyst coking rate. The constancy of the yields is achieved by catalystcycling between reaction and regeneration, which ensures the reactor, iscontinuously supplied with freshly regenerated catalyst, and product yieldsare maintained at fresh catalyst levels. Catalyst handling valves play animportant role in ensuring proper FCC performance, reliable and accuratecontrol, on-off and ESD-valve performance is important for total processefficiency. Each day, several tons of fresh catalysts are added to replacelosses through the cyclones and to maintain the activity of the unit’s inventoryat an acceptable level. A typical temperature here is ambient.

The feedstock is vaporized by the hot regenerated catalyst, the crackingbegins, and the resultant vapour carries the catalyst upward through theriser. The heat of combustion raises the catalyst temperature to (620–845°C),and most of this heat is transferred by the catalyst to the oil feed in the feedriser. The regenerator/reactor cycle continues until catalyst is spent andremoved from process through extraction valve. A typical temperature hereis 760°C. Hot flue gases exit the regenerator through cyclones where catalystis separated and recycled to reactor. A typical temperature here is 760°C.

The remaining heavy residual oil, together with any catalyst carryover,collects at the bottom of the fractionators and recycles back to the reactorfor the catalyst to be used in the reactor. Bottom recycle is used to recoverheat for feed preheat through kettle boilers and exchangers. The fluid isknown as catalyst oil slurry and its control and isolation, due to its highlyabrasive nature and temperature, provide a demanding valve application.The typical temperature of bottom slurry is 370°C.

The flue gas from the FCC process exiting the regenerator hassignificant pressure, temperature and volume, and it is a source of usefulenergy that represents an energy cost-saving opportunity to a refinery.Using an expander could maximize recovery of available energy from theflue gas. This energy can then be used to drive the compressor that providesair to the regenerator (the main air blower) or an electric generator.

Flow Diagram and Process Description

The modern Fluid Catalytic Cracking units are all continuous processeswhich operate 24 hours a day for as long as 2 to 3 years between scheduledshutdowns for routine maintenance.


There are several different proprietary designs that have been developedfor modern FCC units. Each design is available under a license that must bepurchased from the design developer by any petroleum refining companydesiring to construct and operate a Fluid Catalytic Cracking of a given design.Fig. 3 shows typical fluid catalytic cracking units in a petroleum refinery.

There are two different configurations for a Fluid Catalytic Crackingunit: the “stacked” type where the reactor and the catalyst regenerator arecontained in a single vessel with the reactor above the catalyst regeneratorand the “side-by-side” type where the reactor and catalyst regenerator arein two separate vessels. These are the major Fluid Catalytic Crackingdesigners and licensors

Side-by-side configuration:

• CB and I• Exxon mobil research and engineering (EMRE)• Shell global solutions• Stone and webster process technology — currently owned by Technip• Universal oil products (UOP) — currently fully owned subsidiary

of HoneywellStacked configuration:

• Kellogg Brown and root (KBR)Each of the proprietary design licensors claims to have unique features

and advantages. A complete discussion of the relative advantages of each ofthe processes is beyond the scope of this article. Suffice it to say that all ofthe licensors have designed and constructed FCC units that have operatedquite satisfactorily.

Fig. 3: A typical fluid catalytic cracking units in a petroleum refinery (courtesy http://en.wikipedia.org/wiki/fluid_catalytic_cracking)


Reactor and Regenerator

The reactor and regenerator are considered to be the heart of the fluidcatalytic cracking unit. The schematic flow diagram of a typical modernFluidized Catalytic Cracking unit in Fig. 1 below is based upon the “side-by-side” configuration. The preheated high-boiling petroleum feedstock (at about315 to 430°C) consisting of long-chain hydrocarbon molecules is combinedwith recycle slurry oil from the bottom of the distillation column and injectedinto the catalyst riser where it is vaporized and cracked into smallermolecules of vapour by contact and mixing with the very hot powderedcatalyst from the regenerator. All of the cracking reactions take place inthe catalyst riser within a period of 2–4 seconds. The hydrocarbon vapours“fluidize” the powdered catalyst and the mixture of hydrocarbon vapoursand catalyst flows upward to enter the reactor at a temperature of about535 °C and pressure of about1.72 brag.

The reactor is a vessel in which the cracked product vapours are: (a)separated from the so-called spent catalyst by flowing through a set of two-stage cyclones within the reactor and (b) the spent catalyst flows downwardthrough a steam stripping section to remove any hydrocarbon vapours beforethe spent catalyst returns to the catalyst regenerator. The flow of spent catalystto the regenerator is regulated by a slide valve in the spent catalyst line.

Since the cracking reactions produce some carbonaceous material (referredto as catalyst coke) that deposits on the catalyst and very quickly reduces thecatalyst reactivity, the catalyst is regenerated by burning off the depositedcoke with air blown into the regenerator. The regenerator operates at atemperature of about 715°C and a pressure of about 2.41 barg.The combustion of the coke is exothermic and it produces a large amount ofheat that is partially absorbed by the regenerated catalyst and provides theheat required for the vaporization of the feedstock and the endothermic crackingreactions that take place in the catalyst riser. For that reason, FCC units areoften referred to as being ‘heat balanced’.

The hot catalyst (at about 715°C) leaving the regenerator flows intoa catalyst withdrawal well where any entrained combustion flue gases areallowed to escape and flow back into the upper part to the regenerator. Theflow of regenerated catalyst to the feedstock injection point below the catalystriser is regulated by a slide valve in the regenerated catalyst line. The hot fluegas exits the regenerator after passing through multiple sets of two-stagecyclones that remove entrained catalyst from the flue gas (Fig. 4).

The amount of catalyst circulating between the regenerator and thereactor amounts to about 5 kg per kg of feedstock, which is equivalent toabout 4.66 kg per litre of feedstock. Thus, an FCC unit processing 75,000barrels per day (11,900 m3/d) will circulate about 55,900 MT per day ofcatalyst.


The thermal cracking process functioned largely in accordance with thefree-radical theory of molecular transformation. Under conditions of extremeheat, the electron bond between carbon atoms in a hydrocarbon molecule canbe broken, thus generating a hydrocarbon group with an unpaired electron.This negatively charged molecule, called a free radical, enters into reactionswith other hydrocarbons, continually producing other free radicals via thetransfer of negatively charged hydride ions (H”). Thus a chain reaction isestablished that leads to a reduction in molecular size, or “cracking,” ofcomponents of the original feedstock.

Use of a catalyst in the cracking reaction increases the yield of high-quality products under much less severe operating conditions than in thermalcracking. Several complex reactions are involved, but the principalmechanism by which long-chain hydrocarbons are cracked into lighterproducts can be explained by the carbonium theory. According to this theory,a catalyst promotes the removal of a negatively charged hydride ion from

Fig. 4: A schematic flow diagram of a Fluid Catalytic cracking unit as used in petroleumrefineries (courtesy http://en.wikipedia.org/wiki/fluid_catalytic_cracking)


a paraffin compound or the addition of a positively charged proton (H+) toan olefin compound. This results in the formation of a carbonium ion, apositively charged molecule that has only a very short life as an intermediatecompound which transfers the positive charge through the hydrocarbon.Carbonium transfer continues as hydrocarbon compounds come into contactwith active sites on the surface of the catalyst that promote the continuedaddition of protons or removal of hydride ions. The result is a weakening ofcarbon-carbon bonds in many of the hydrocarbon molecules and a consequentcracking into smaller compounds.

Olefins crack more readily than paraffin, since their double carbon-carbon bonds are more friable under reaction conditions. Isoparaffins andnaphthenes crack more readily than normal paraffins, which in turn crackfaster than aromatics. In fact, aromatic ring compounds are very resistantto cracking, since they readily deactivate fluid cracking catalysts by blockingthe active sites of the catalyst. The table illustrates many of the principalreactions that are believed to occur in fluid catalytic cracking unit reactors.The reactions postulated for olefin compounds apply principally tointermediate products within the reactor system, since the olefin content ofcatalytic cracking feedstock is usually very low. Reactions in fluid catalyticcracking are shown in Table 1.

Typical modern catalytic cracking reactors operate at 480–550°C (900–1,020°F) and at relatively low pressures of 0.7 to 1.4 bars (70 to 140 KPa), or10 to 20 psi. At first natural silica-alumina clays were used as catalysts, but bythe mid-1970s zeolite and molecular sieve-based catalysts became common.Zeolitic catalysts give more selective yields of products while reducing theformation of gas and coke.

A modern fluid catalytic cracker employs a finely divided solid catalystthat has properties analogous to a liquid when it is agitated by air or oil vapours.The principles of operation of such a unit are shown in the figure. In thisarrangement a reactor and regenerator are located side by side. The oil feed isvaporized when it meets the hot catalyst at the feed-injection point, and thevapours flow upward through the riser reactor at high velocity, providing afluidizing effect for the catalyst particles. The catalytic reaction occursexclusively in the riser reactor. The catalyst then passes into the cyclone vessel,where it is separated from reactor hydrocarbon products.

As the cracking reactions proceed, carbon is deposited on the catalystparticles. Since these deposits impair the reaction efficiency, the catalystmust be continuously withdrawn from the reaction system. Unit productvapours pass out of the top of the reactor through cyclone separators, butthe catalyst is removed by centrifugal force and dropped back into thestripper section. In the stripping section, hydrocarbons are removed fromthe spent catalyst with steam, and the catalyst is transferred through thestripper standpipe to the regenerator vessel, where the carbon is burned


with a current of air. The high temperature of the regeneration process (675–785°C, or 1,250–1,450°F) heats the catalyst to the desired reaction temperaturefor recon acting fresh feed into the unit. In order to maintain activity, a smallamount of fresh catalyst is added to the system from time to time, and a similaramount is withdrawn.

The cracked reactor effluent is fractionated in a distillation column. Theyield of light products (with boiling points less than 220°C, or 430°F) is usuallyreported as the conversion level for the unit. Conversion levels average about60 to 70 percent in Europe and Asia and in excess of 80 percent in many catalytic

Table 1: Reactions in fluid catalytic cracking (courtesy Encyclopaedia Britannica) (courtesyhttp://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)


cracking units in the United States. About one-third of the product yield consistsof fuel gas and other gaseous hydrocarbons. Half of this is usually propyleneand butylenes, which are important feed stocks for the polymerization andalkylation processes discussed below. The largest volume is usually crackednaphtha, an important gasoline blend stock with an octane number of 90 to 94.The lower conversion units of Europe and Asia produce comparatively moredistillate oil and less naphtha and light hydrocarbons. Fluid catalytic crackingunit is depicted in Fig. 5.

The light gaseous hydrocarbons produced by catalytic cracking are highlyunsaturated and are usually converted into high-octane gasoline componentsin polymerization or alkylation processes. In polymerization, the lightolefins propylene and butylenes are induced to combine, or polymerize, intomolecules of two or three times their original molecular weight. The catalystsemployed consist of H3PO4 on pellets of kieselguhr, a porous sedimentaryrock. High pressures, on the order of 30 to 75 bars (3 to 7.5 MPa), or 400 to1,100 psi, are required at temperatures ranging from 175 to 230°C (350 to450°F). Polymer gasoline’s derived from propylene and butylenes have octanenumbers above 90. The various features during the fluidized catalyticcracking are shown in the figures 3, 4, 5 and 6. The hydrocarbons, crude oil(product contents), fractional distillation and fluid catalytic cracking unitmay be understood in an elaborative manner. Crude oil (product contents)is shown in Fig. 6.

Fig. 5: Fluid catalytic cracking: fluid unit (courtesy Encyclopaedia Britannica) (courtesyhttp://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)


The alkylation reaction also achieves a longer chain molecule by thecombination of two smaller molecules, one being an olefin and the other anisoparaffin (usually isobutane). During World War II, alkylation became themain process for the manufacture of isooctane, a primary component in theblending of aviation gasoline.

Two alkylation processes employed in the industry are based upondifferent acid systems as catalysts. In sulphuric acid alkylation, concentratedsulphuric acid of 98 percent purity serves as the catalyst for a reaction thatis carried out at 2 to 7°C (35 to 45°F). Refrigeration is necessary because ofthe heat generated by the reaction. The octane number of alkylates producedrange from 85 to 95.

Hydrofluoric acid is also used as a catalyst for many alkylation units.The chemical reactions are similar to those in the sulphuric acid process,but it is possible to use higher temperatures (between 24 and 46°C, or 75 to115°F), thus avoiding the need for refrigeration. Recovery of hydrofluoricacid is accomplished by distillation. Stringent safety precautions must beexercised when using this highly corrosive and toxic substance.

The demand for aviation gasoline became so great during World War IIand afterward that the quantities of isobutane available for alkylation feedstockwere insufficient. This deficiency was remedied by isomerization of the moreabundant normal butane into isobutane. The isomerization catalyst is aluminiumchloride supported on alumina and promoted by hydrogen chloride gas.

Fig. 6: Crude oil: product contents (courtesy Encyclopaedia Britannica) (courtesy http://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)


Isomerisation

Commercial processes have also been developed for the isomerization oflow-octane normal pentane and normal hexane to the higher-octane is paraffinform. Here the catalyst is usually promoted with platinum. As in catalyticreforming, the reactions are carried out in the presence of hydrogen.Hydrogen is neither produced nor consumed in the process but is employedto inhibit undesirable side reactions. The reactor step is usually followed bymolecular sieve extraction and distillation. Though this process is anattractive way to exclude low-octane components from the gasoline blendingpool, it does not produce a final product of sufficiently high octane tocontribute much to the manufacture of unleaded gasoline.

Distillation Column

The reaction product vapours (at 535°C and a pressure of 1.72 brag) flowfrom the top of the reactor to the bottom section of the distillation column(commonly referred to as the main fractionators) where they are distilledinto the FCC end products of cracked naphtha, fuel oil, and off gas. Afterfurther processing for removal of sulphur compounds, the cracked naphthabecomes a high-octane component of the refinery’s blended gasoline.Fractional distillation (crude oil column) is depicted in Fig. 7.

The main fractionators’ off gas is sent to what is called a gas recoveryunit where 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 andethylene.

Although the schematic flow diagram above depicts the main fractionatorsas having only one side cut stripper and one fuel oil product, many FCCmain fractionators have two side cut strippers and produce a light fuel oiland a heavy fuel oil. Likewise, many FCC main fractionators produce lightcracked naphtha and a heavy cracked naphtha. The terminology light andheavy in this context refers to the product boiling ranges, with light productshaving a lower boiling range than heavy products.

The bottom product oil from the main fractionators contains residualcatalyst particles which were not completely removed by the cyclones in thetop of the reactor. For that reason, the bottom product oil is referred to asslurry oil. Part of that slurry oil is recycled back into the main fractionatorsabove the entry point of the hot reaction product vapours so as to cool andpartially condense the reaction product vapours as they enter the mainfractionators. The remainder of the slurry oil is pumped through a slurrysettler. The bottom oil from the slurry settler contains most of the slurry oilcatalyst particles and is recycled back into the catalyst riser by combining it


with the FCC feedstock oil. The so-called clarified slurry oiler decant oil iswithdrawn from the top of slurry settler for use elsewhere in the refinery,as a heavy fuel oil blending component, or as carbon black feedstock.

Depending on the choice of FCC design, the combustion in theregenerator of the coke on the spent catalyst may or may not be completecombustion to carbon dioxide CO2. The combustion air flow is controlled soas to provide the desired ratio of carbon monoxide (CO) to carbon dioxidefor each specific FCC design.

Regenerator fuel gas

In the design shown in Fig. 1, the coke has only been partially combusted toCO2. The combustion flue gas (containing CO and CO2) at 715°C and at apressure of 2.41 brag is routed through a secondary catalyst separatorcontaining swirl tubes designed to remove 70 to 90 percent of the particulatesin the flue gas leaving the regenerator[8]. This is required to prevent erosiondamage to the blades in the turbo expander that the flue gas is next routedthrough.

Fig. 7: Fractional distillation: crude-oil column (courtesy Encyclopaedia Britannica)(courtesy http://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)


The expansion of flue gas through a turbo-expander provides sufficientpower to drive the regenerator’s combustion air compressor. The electricalmotor generator can consume or produce electrical power. If the expansionof the flue gas does not provide enough power to drive the air compressor,the electric motor/generator provides the needed additional power. If theflue gas expansion provides more power than needed to drive the aircompressor, than the electric motor/generator converts the excess powerinto 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 isburned as fuel to provide steam for use in the refinery as well as to comply withany 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 applicableenvironmental regulations regarding particulate emissions. The ESP removesparticulates in the size range of 2 to 20 microns from the flue gas.

The steam turbine in the flue gas processing system (shown in the abovediagram) is used to drive the regenerator’s combustion air compressor duringstart-ups of the FCC unit until there is sufficient combustion flue gas totake over that task.

Chemistry

Before delving into the chemistry (Fig. 8) involved in catalytic cracking, itwill be helpful to briefly discuss the composition of petroleum crude oil.Petroleum crude oil consists primarily of a mixture of hydrocarbons withsmall amounts of other organic compounds containing sulphur, nitrogen andoxygen. The crude oil also contains small amounts of metals such as copper,iron, nickel and vanadium. The elemental composition ranges of crude oilare summarized in Table 2 and the hydrocarbons in the crude oil can beclassified into three types:

• Paraffin or alkanes: Saturated straight-chain or branched hydrocarbons,without any ring structures

• Naphthalene or cycloalkanes: Saturated hydrocarbons having one ormore ring structures with one or more side-chain paraffin

• Aromatics: Hydrocarbons having one or more unsaturated ring structuressuch as benzene or unsaturated polycyclic ring structures suchas naphthalene or phenanthrene, any of which may also have one ormore side-chain paraffin.

Olefins or alkanes, which are unsaturated straight-chain or branchhydrocarbons, do not occur naturally in crude oil.

The elemental composition ranges of crude oil are summarized in Table 2.


Table 2: Elemental composition of crude oil

Carbon 83–87%Hydrogen 10–14%Nitrogen 0.1–2%Oxygen 0.1–1.5%Sulphur 0.5–6%Metals < 0.1%

Technically, the fluid catalytic cracking process breaks large hydrocarbonmolecules into smaller molecules by contacting them with powdered catalystat a high temperature and moderate pressure which first vaporizes thehydrocarbons and then breaks them. The cracking reactions occur in thevapour phase and start immediately when the feedstock is vaporized inthe catalyst riser. Fig. 9 is a very simplified schematic diagram that exemplifieshow the process breaks high boiling, straight-chain alkanes (paraffin)hydrocarbons into smaller straight-chain alkanes as well as branched-chainalkanes, branched alkenes (olefins) and cycloalkanes (naphthenes). The breakingof the large hydrocarbon molecules into smaller molecules is more technicallyreferred to by organic chemists as scission of the carbon-to-carbon bonds.

As depicted in Fig. 8, some of the smaller alkanes are then broken andconverted into even smaller alkenes and branched alkenes such as thegases ethylene, propylene, butylenes, and isobutylene. Those olefin gasesare valuable for use as petrochemical feed stocks. The propylene, butylenesand isobutylene are also valuable feed stocks for certain petroleum refiningprocesses that convert them into high-octane gasoline blending components.

Fig. 8: Carbon: hydrocarbons (courtesy Encyclopaedia Britannica) (courtesy http://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)


As also depicted in Fig. 8, the cycloalkanes (naphthenes) formed by theinitial breakup of the large molecules are further converted to aromaticssuch as benzene, toluene, and xylenes, which boil in the gasoline boiling rangeand have much higher octane ratings than alkanes.

In the cracking process there is also produced carbon that deposits onthe catalyst (catalyst coke). The carbon formation tendency or amount ofcarbon in a crude or FCC feed is measured with methods such as Micro CarbonResidue, Conrad son Carbon Residue or Rams bottom Carbon Residue.

By no means does Fig. 8 include all the chemistry of the primary andsecondary reactions taking place in the fluid catalytic process. There are agreat many other reactions involved. However, a full discussion of the highlytechnical details of the various catalytic cracking reactions is beyond thescope of this article and can be found in the technical literature.

Modern FCC catalysts are fine powders with a bulk density of 0.80 to0.96 g/cc and having a particle size distribution ranging from 10 to 150 mand an average particle size of 60 to 100 m. The design and operation of anFCC unit is largely dependent upon the chemical and physical properties ofthe catalyst. The desirable properties of an FCC catalyst are:

• Good stability to high temperature and to steam

Fig. 9: Diagrammatic example of the catalytic cracking of petroleum hydrocarbonscatalysts (courtesy http://en.wikipedia.org/wiki/fluid_catalytic_cracking)


• High activity• Large pore sizes• Good resistance to attrition• Low coke production

A modern FCC catalyst has four major components: crystalline zeolite,matrix, binder, and filler. Zeolite is the primary active component and canrange from about 15 to 50 weight percent of the catalyst. The zeolite used inFCC catalysts is referred to as fauja site or as Type Y and is composedof silica and alumina tetrahedral with each tetrahedron having eitheran aluminium or silicon atom at the centre and four oxygen atoms at thecorners. It is a molecular sieve with a distinctive lattice structure that allowsonly a certain size range of hydrocarbon molecules to enter the lattice. Ingeneral, the zeolite does not allow molecules larger than 8 to 10 nm (i.e., 80 to90 Å) to enter the lattice.

The catalytic sites in the zeolite are strong acids (equivalent to 90% H2SO4)and provide most of the catalytic activity. The acidic sites are provided by thealumina tetrahedral. The aluminium atom at the centre of each aluminatetrahedral is at a +3 oxidation state surrounded by four oxygen atoms at thecorners which are shared by the neighbouring tetrahedral. Thus, the net chargeof the alumina tetrahedral is -1 which is balanced by a Na+ during the productionof the catalyst. The sodium ion is later replaced by ammonium ion, which isvaporized when the catalyst is subsequently dried, resulting in the formationof Lewis and Brønsted acidic sites. In some FCC catalysts, the Brønstedsites may be later replaced by rare earth metals such as cerium andlanthanum to provide alternative activity and stability levels.

The matrix component of an FCC catalyst contains amorphous aluminawhich also provides catalytic activity sites and in larger pores that allows entryfor larger molecules than does the zeolite. That enables the cracking of higher-boiling, larger feedstock molecules than are cracked by the zeolite.

The binder and filler components provide the physical strength andintegrity of the catalyst. The binder is usually silica sol and the filler is usuallyclay Kaolin).

Nickel, vanadium, iron, copper and other metal contaminants, present inFCC feed stocks in the parts per million ranges; all have detrimental effects onthe catalyst activity and performance. Nickel and vanadium are particularlytroublesome. There are a number of methods for mitigating the effects ofthe contaminant metals:

• Avoid feed stocks with high metals content: This seriously hampers arefinery’s flexibility to process various crude oils or purchased FCCfeed stocks.


• Feedstock feed pre treatment: Hydro desulfurization of the FCC feedstockremoves some of the metals and also reduces the sulphur content of theFCC products. However, this is quite a costly option.

• Increasing fresh catalyst addition: some of the circulating equilibriumcatalyst as spent catalyst are withdrawn by FCC units and replaces itwith fresh catalyst in order to maintain a desired level of activity.Increasing the rate of such exchange lowers the level of metals in thecirculating equilibrium catalyst, but this is also quite a costly option.

• De metallization: The commercial proprietary Demet Process removesnickel and vanadium from the withdrawn spent catalyst. The nickeland vanadium are converted to chlorides which are then washed outof the catalyst. After drying, the de metalized catalyst is recycled intothe circulating catalyst. Removals of about 95 percent nickel removaland 67 to 85 percent vanadium have been reported. Despite that, theuse of the De metallization process has not become widespread, perhapsbecause of the high capital expenditure required.

• Metals passivation: Certain materials can be used as additives whichcan be impregnated into the catalyst or added to the FCC feedstock inthe form of metal-organic compounds. Such materials react with themetal contaminants and passivation the contaminants by forming lessharmful compounds that remain on the catalyst. For example,antimony and bismuth are effective in passivation nickel and tin iseffective in passivation vanadium. A number of proprietary passivationprocesses are available and fairly widely used. The role of catalysts inconversion process is shown in Fig. 10.

Fig. 10: The role of catalysts in conversion processes (Courtesy CB and I) (courtesy CB&Ihttp://www.cbi.com/technologies/catalysts-refining-petchem-polymer)


Since World War II the demand for light products (e.g., gasoline, jet, anddiesel fuels) has grown, while the requirement for heavy industrial fuel oils hasdeclined. Furthermore, many of the new sources of crude petroleum (California,Alaska, Venezuela, and Mexico) have yielded heavier crude oils with highernatural yields of residual fuels. As a result, refiners have become even moredependent on the conversion of residue components into lighter oils that canserve as feedstock for catalytic cracking units.

As early as 1920, large volumes of residue were being processedin visbreakers or thermal cracking units. These simple process units basicallyconsist of a large furnace that heats the feedstock to the range of 450 to500°C (840 to 930°F) at an operating pressure of about 10 bars (1 MPa), orabout 150 psi. The residence time in the furnace is carefully limited to preventmuch of the reaction from taking place and clogging the furnace tubes. Theheated feed is then charged to a reaction chamber, which is kept at a pressurehigh enough to permit cracking of the large molecules but restrict cokeformation. From the reaction chamber the process fluid is cooled to inhibitfurther cracking and then charged to a distillation column for separationinto components.

Visbreaking units typically convert about 15 percent of the feedstock tonaphtha and diesel oils and produce a lower-viscosity residual fuel. Thermalcracking units provide more severe processing and often convert as muchas 50 to 60 percent of the incoming feed to naphtha and light diesel oils.

Coking is severe thermal cracking. The residue feed is heated to about475 to 520°C (890 to 970°F) in a furnace with very low residence time and isdischarged into the bottom of a large vessel called a coke drum for extensiveand controlled cracking. The cracked lighter product rises to the top of thedrum and is drawn off. It is then charged to the product fractionators forseparation into naphtha, diesel oils, and heavy gas oils for further processingin the catalytic cracking unit. The heavier product remains and, because ofthe retained heat, cracks ultimately to coke, a solid carbonaceous substanceakin to coal. Once the coke drum is filled with solid coke, it is removed fromservice and replaced by another coke drum.

Decoking is a routine daily occurrence accomplished by a high-pressurewater jet. First the top and bottom heads of the coke drum are removed.Next a hole is drilled in the coke from the top to the bottom of the vessel.Then a rotating stem is lowered through the hole, spraying a water jetsideways. The high-pressure jet cuts the coke into lumps, which fall out thebottom of the drum for subsequent loading into trucks or railcars forshipment to customers. Typically, coke drums operate on 24-hour cycles,filling with coke over one 24-hour period followed by cooling, decoking, andreheating over the next 24 hours. The drilling derricks on top of the cokedrums are a notable feature of the refinery skyline.


Cokers produce no liquid residue but yield up to 30 percent coke byweight. Much of the low-sulphur product is employed to produce electrodesfor the electrolytic smelting of aluminium. Most lower-quality coke is burnedas fuel in admixture with coal. Coker economics usually favour the conversionof residue into light products even if there is no market for the coke.

Before petroleum products can be marketed, certain impurities mustbe removed or made less obnoxious. The most common impurities are sulphurcompounds such as hydrogen sulphide (H2S) or the mercaptans (“R”SH), thelatter being a series of complex organic compounds having as many as sixcarbon atoms in the hydrocarbon radical (“R”). Apart from their foulodour, sulphur compounds are technically undesirable. In motor and aviationgasoline they reduce the effectiveness of antiknock additives and interferewith the operation of exhaust-treatment systems. In diesel fuel they causeengine corrosion and complicate exhaust-treatment systems. Also, manymajor residual and industrial fuel consumers are located in developed areasand are subject to restrictions on sulfurous emissions.

Most crude oils contain small amounts of hydrogen sulphide, but theselevels may be increased by the decomposition of heavier sulphur compounds(such as the mercaptans) during refinery processing. The bulk of thehydrogen sulphide is contained in process-unit overhead gases, which areultimately consumed in the refinery fuel system. In order to minimize noxiousemissions, most refinery fuel gases are desulphurized.

Other undesirable components include nitrogen compounds, which poisoncatalyst systems, and oxygenated compounds, which can lead to colour formationand product instability. The principal treatment processes are outlined below.

Sweetening processes oxidize mercaptans into more innocuous disulfides,which remain in the product fuels. Catalysts assist in the oxidation.The doctor process employs sodium plum bite, a solution of lead oxide incaustic soda, as a catalyst. At one time this inexpensive process was widelypracticed, but the necessity of adding elemental sulphur to make the reactionsproceed caused an increase in total sulphur content in the product. It haslargely been replaced by the copper chloride process, in which the catalystis slurry of copper chloride and fuller’s earth. It is applicable to both keroseneand gasoline. The oil is heated and brought into contact with the slurrywhile being agitated in a stream of air that oxidizes the mercaptans todisulfides. The slurry is then allowed to settle and is separated for reuse. Aheater raises the temperature to a point that keeps the water formed in thereaction dissolved in the oil, so that the catalyst remains properly hydrated.After sweetening, the oil is water washed to remove any traces of catalystand is later dried by passing through a salt filter.

Hydrogen processes, commonly known as hydro treating, are the mostcommon processes for removing sulphur and nitrogen impurities. The oil iscombined with high-purity hydrogen, vaporized, and then passed over a catalyst


such as tungsten, nickel, or a mixture of cobalt and molybdenum oxides supportedon an alumina base. Operating temperatures are usually between 260 and 425°C(500 and 800°F) at pressures of 14 to 70 bars (1.4 to 7 MPa), or 200 to 1,000 psi.Operating conditions are set to facilitate the desired level of sulphur removalwithout promoting any change to the other properties of the oil.

The sulphur in the oil is converted to hydrogen sulphide and the nitrogento ammonia. The hydrogen sulphide is removed from the circulating hydrogenstream by absorption in a solution such as diethanolamine. The solution canthen be heated to remove the sulphide and reused. The hydrogen sulphiderecovered is useful for manufacturing elemental sulphur of high purity. Theammonia is recovered and either converted to elemental nitrogen and hydrogen,burned in the refinery fuel-gas system, or processed into agricultural fertilizers.

Molecular sieves are also used to purify petroleum products, since theyhave a strong affinity for polar compounds such as water, carbon dioxide,hydrogen sulphide, and mercaptans. Sieves are prepared by dehydration ofan alumina silicate such as zeolite. The petroleum product is passed througha bed of zeolite for a predetermined period depending on the impurity to beremoved. The adsorbed contaminants may later be expelled from the sieveby purging with a gas stream at temperatures between 200 and 315 °C (400and 600°F). The frequent cycling of the molecular sieve from adsorb to desorbs operations is usually fully automated.

PETROLEUM PRODUCTS AND THEIR USES

Gases

Gaseous refinery products include hydrogen, fuel gas, ethane, propane,and butane. Most of the hydrogen is consumed in refinery desulfurizationfacilities, which remove H2S from the gas stream and then separate thatcompound into elemental hydrogen and sulphur; small quantities of thehydrogen may be delivered to the refinery fuel system. Refinery fuel gasvaries in composition but usually contains a significant amount of methane;it has a heating value similar to natural gas and is consumed in plantoperations. Periodic variability in heating value makes it unsuitable fordelivery to consumer gas systems. Ethane may be recovered from the refineryfuel system for use as a petrochemical feedstock. Propane and butane aresold as liquefied petroleum gas (LPG), which is a convenient portable fuelfor domestic heating and cooking or for light industrial use.

Gasoline

Motor gasoline, or petrol, must meet three primary requirements. It mustprovide an even combustion pattern, start easily in cold weather, and meetprevailing environmental requirements.


In order to meet the first requirement, gasoline must burn smoothly inthe engine without pre mature detonation, or knocking. Severe knockingcan dissipate power output and even cause damage to the engine. Whengasoline engines became more powerful in the 1920s, it was discovered thatsome fuels knocked more readily than others. Experimental studies led tothe determination that, of the standard fuels available at the time, the mostextreme knock was produced by a fuel composed of pure normal heptanes,while the least knock was produced by pure isooctane. This discovery led tothe development of the octane scale for defining gasoline quality. Thus, whena motor gasoline gives the same performance in a standard knock engine asa mixture of 90 percent isooctane and 10 percent normal heptanes, it isgiven an octane rating of 90.

There are two methods for carrying out the knock engine test. Researchoctane is measured under mild conditions of temperature and engine speed(49°C [120°F] and 600 revolutions per minute, or RPM), while motor octaneis measured under more severe conditions (149°C [300°F] and 900 RPM).For many years the research octane number was found to be the moreaccurate measure of engine performance and was usually quoted alone. Sincethe advent of unleaded fuels in the mid-1970s, however, motor octanemeasurements have frequently been found to limit actual engine performance.As a result a new measurement, road octane number, which is a simpleaverage of the research and motor values, is most frequently used to definefuel quality for the consumer. Automotive gasolines generally range fromresearch octane number 87 to 100, while gasoline for piston-engine aircraftranges from research octane number 115 to 130.

Each naphtha component that is blended into gasoline is testedseparately for its octane rating. Reformate, alkylates, polymer, and crackednaphtha, as well as butane, all rank high (90 or higher) on this scale, whilestraight-run naphtha may rank at 70 or less. In the 1920s it was discoveredthat the addition of tetraethyl lead would substantially enhance the octanerating of various naphtha. Each naphtha component was found to have aunique response to lead additives, some combinations being found to besynergistic and others antagonistic. This gave rise to very sophisticatedtechniques for designing the optimal blends of available components intodesired grades of gasoline.

The advent of leaded, or ethyl, gasoline led to the manufacture of high-octane fuels and became universally employed throughout the world afterWorld War II. However, beginning in 1975, environmental legislation beganto restrict the use of lead additives in automotive gasoline. It is now bannedin the United States, the European Union, and many countries around theworld. The required use of lead-free gasoline has placed a premium on theconstruction of new catalytic reformers and alkylation units for increasingyields of high-octane gasoline ingredients and on the exclusion of low-octanenaphtha from the gasoline blend.


High-Volatile and Low-Volatile Components

The second major criterion for gasoline is that the fuel be sufficiently volatileto enable the car engine to start quickly in cold weather is accomplished bythe addition of butane, very low-boiling paraffin, to the gasoline blend.Fortunately, butane is also a high-octane component with little alternateeconomic use, so its application has historically been maximized in gasoline.Another requirement, that a quality gasoline have high energy content, hastraditionally been satisfied by including higher-boiling components in theblend.

However, both of these practices are now called into question onenvironmental grounds. The same high volatility that provides good startingcharacteristics in cold weather can lead to high evaporative losses of gasolineduring refuelling operations, and the inclusion of high-boiling componentsto increase the energy content of the gasoline can also increase the emissionof unburned hydrocarbons from engines on start-up. As a result, since the1990 amendments of the U.S. Clean Air Act, much of the gasoline consumedin urban areas of the United States has been reformulated to meet stringentnew environmental standards. At first these changes required that gasolinecontain certain percentages of oxygen in order to aid in fuel combustion andreduce the emission of carbon monoxide and nitrogen oxides. Refiners metthis obligation by including some oxygenated compounds such as ethylalcohol or methyl tertiary butyl ether (MTBE) in their blends.

However, MTBE was soon judged to be a hazardous pollutant ofgroundwater in some cases where reformulated gasoline leaked fromtransmission pipelines or underground storage tanks, and it was banned inseveral parts of the country. In 2005 the requirements for specific oxygenlevels were removed from gasoline regulations, and MTBE ceased to beused in reformulated gasoline. Many blends in the United States containsignificant amounts of ethyl alcohol in order to meet emissions requirements,and MTBE is still added to gasoline in other parts of the world.

One of the most critical economic issues for a petroleum refiner isselecting the optimal combination of components to produce final gasolineproducts. Gasoline blending is much more complicated than a simple mixingof components. First, a typical refinery may have as many as 8 to 15 differenthydrocarbon streams to consider as blend stocks. These may range frombutane, the most volatile component, to a heavy naphtha and include severalgasoline naphtha from crude distillation, catalytic cracking, and thermalprocessing units in addition to alkylates, polymer, and reformate. Moderngasoline may be blended to meet simultaneously 10 to 15 different qualityspecifications, such as vapour pressure; initial, intermediate, and final boilingpoints; sulphur content; colour; stability; aromatics content; olefin content;octane measurements for several different portions of the blend; and otherlocal governmental or market restrictions.


Since each of the individual components contributes uniquely in each ofthese quality areas and each bears a different cost of manufacture, theproper allocation of each component into its optimal disposition is of majoreconomic importance. In order to address this problem, most refinersemploy linear programming, a mathematical technique that permits the rapidselection of an optimal solution from a multiplicity of feasible alternativesolutions. Each component is characterized by its specific properties andcost of manufacture and each gasoline grade requirement is similarly definedby quality requirements and relative market value. The linear programmingsolution specifies the unique disposition of each component to achievemaximum operating profit. The next step is to measure carefully the rate ofaddition of each component to the blend and collect it in storage tanks forfinal inspection before delivering it for sale. Still, the problem is not fullyresolved until the product is actually delivered into customers’ tanks.Frequently, last-minute changes in shipping schedules or production qualitiesrequire the re blending of finished gasoline or the substitution of a high-quality (and therefore costlier) grade for one of more immediate demandeven though it may generate less income for the refinery.

Though its use as an illuminate has greatly diminished, kerosene is stillused extensively throughout the world in cooking and space heating and isthe primary fuel for modern jet engines. When burned as a domestic fuel,kerosene must produce a flame free of smoke and odour. Standard laboratoryprocedures test these properties by burning the oil in special lamps. Allkerosene fuels must satisfy minimum flash-point specifications (49°C, or120°F) to limit fire hazards in storage and handling.

Jet fuels must burn cleanly and remain fluid and free from wax particlesat the low temperatures experienced in high-altitude flight. The conventionalfreeze-point specification for commercial jet fuel is “50°C (“58°F). The fuelmust also be free of any suspended water particles that might cause blockageof the fuel system with ice particles. Special-purpose military jet fuels haveeven more stringent specifications.

Diesel Oils

The principal end use of gas oil is as diesel fuel for powering automobile,truck, bus, and railway engines. In a diesel engine, combustion is inducedby the heat of compression of the air in the cylinder under compression.Detonation, which leads to harmful knocking in a gasoline engine, is anecessity for the diesel engine. A good diesel fuel starts to burn at severallocations within the cylinder after the fuel is injected. Once the flame hasinitiated, any more fuel entering the cylinder ignites at once.

Straight-chain hydrocarbons make the best diesel fuels. In order tohave a standard reference scale, the oil is matched against blends of cetane(normal hexadecane) and alpha methylnaphthalene, the latter of which gives


very poor engine performance. High-quality diesel fuels have cetane ratings ofabout 50, giving the same combustion characteristics as a 50-50 mixture of thestandard fuels. The large, slower engines in ships and stationary power plantscan tolerate even heavier diesel oils. The more viscous marine diesel oils areheated to permit easy pumping and to give the correct viscosity at the fuelinjectors for good combustion.

Until the early 1990s, standards for diesel fuel quality were not particularlystringent. A minimum cetane number was critical for transportation uses,but sulphur levels of 5,000 parts per million (ppm) were common in mostmarkets. With the advent of more stringent exhaust emission controls,however, diesel fuel qualities came under increased scrutiny. In the EuropeanUnion and the United States, diesel fuel is now generally restricted tomaximum sulphur levels of 10 to 15 ppm, and regulations have restrictedaromatic content as well. The limitation of aromatic compounds requires amuch more demanding scheme of processing individual gas oil componentsthan was necessary for earlier highway diesel fuels.

Fuel Oils

Furnace oil consists largely of residues from crude oil refining. These areblended with other suitable gas oil fractions in order to achieve the viscosityrequired for convenient handling. As a residue product, fuel oil is the onlyrefined product of significant quantity that commands a market price lowerthan the cost of crude oil.

Because the sulphur contained in the crude oil is concentrated in theresidue material, fuel oil sulphur levels are naturally high. The sulphurlevel is not critical to the combustion process as long as the flue gases donot impinge on cool surfaces (which could lead to corrosion by thecondensation of acidic sulphur trioxide). However, in order to reduce airpollution, most industrialized countries now restrict the sulphur content offuel oils. Such regulation has led to the construction of residual desulfurizationunits or Cokers in refineries that produce these fuels.

Residual fuels may contain large quantities of heavy metals suchas nickel and vanadium; these produce ash upon burning and can foul burnersystems. Such contaminants are not easily removed and usually lead tolower market prices for fuel oils with high metal contents.

Olefins

The thermal cracking processes developed for refinery processing in the1920s were focused primarily on increasing the quantity and quality of gasolinecomponents. As a by-product of this process, gases were produced thatincluded a significant proportion of lower-molecular-weight olefins, particularlyethylene, propylene, and butylenes. Catalytic cracking is also a valuable


source of propylene and butylenes, but it does not account for a very significantyield of ethylene, the most important of the petrochemical building blocks.Ethylene is polymerized to produce polyethylene or, in combination withpropylene, to produce copolymers that are used extensively in food-packagingwraps, plastic household goods, or building materials.

Ethylene manufacture via the steam cracking process is in widespreadpractice throughout the world. The operating facilities are similar to gas oilcracking units, operating at temperatures of 840°C (1,550°F) and at lowpressures of 165 kilopascals (24 pounds per square inch). Steam is added tothe vaporized feed to achieve a 50-50 mixture, and furnace residence timesare only 0.2 to 0.5 second. In the United States and the Middle East, ethaneextracted from natural gas is the predominant feedstock for ethylene crackingunits. Propylene and butylenes are largely derived from catalytic crackingunits in the United States. In Europe and Japan, catalytic cracking is lesscommon, and natural gas supplies are not as plentiful. As a result, both theEuropeans and Japanese generally crack a naphtha or light gas oil fraction toproduce a full range of olefin products.

Aromatics

The aromatic compounds, produced in the catalytic reforming of naphtha,are major sources of petrochemical products. In the traditional chemicalindustry, aromatics such as benzene, toluene, and the xylenes were madefrom coal during the course of carbonization in the production of coke andtown gas. Today a much larger volume of these chemicals are made as refineryby-products. A further source of supply is the aromatic-rich liquid fractionproduced in the cracking of naphtha or light gas oils during the manufactureof ethylene and other olefins.

Polymers

A highly significant proportion of these basic petrochemicals are convertedinto plastics synthetic rubbers and synthetic fibres. Together these materialsare known as polymers because their molecules are high-molecular-weightcompounds made up of repeated structural units that have combinedchemically. The major products are polyethylene, polyvinyl chloride andpolystyrene, all derived from ethylene, and polypropylene, derived frommonomer propylene. Major raw materials for synthetic rubbers includebutadiene, ethylene, benzene, and propylene. Among synthetic fibres thepolyesters comprised of ethylene glycol and terephthalic acid (madefrom xylenes) are the most widely used. They account for about one-half ofall synthetic fibres. The second major synthetic fibre is nylon, its mostimportant raw material being benzene. Acrylic fibres, in which the majorraw material is the propylene derivative acrylonitrile, make up most of theremainder of the synthetic fibres.


Power Recovery in Fluid Catalytic Cracker

The combustion fuel gas from the catalyst regenerator of a fluid catalyticcracker is at a temperature of about 715°C and at a pressure of about 2.4brag (240 k Pa gauge). Its gaseous components are mostly carbonmonoxide (CO), carbon dioxide (CO2) and nitrogen (N2). Although the flue gashas been through two stages of cyclones (located within the regenerator) toremove entrained catalyst fines, it still contains some residual catalyst fines.

Figure 5 depicts how power is recovered and utilized by routing theregenerator flue gas through a turbo expander. After the flue gas exits theregenerator, it is routed through a secondary catalyst separator containingswirl tubes designed to remove 70 to 90 percent of the residual catalystfines. This is required to prevent erosion damage to the turbo expander.

As shown in Fig. 8, expansion of the flue gas through a turbo expanderprovides sufficient power to drive the regenerator’s combustion air compressor.The electrical motor generator in the power recovery system can consume orproduce electrical power. If the expansion of the flue gas does not provide enoughpower to drive the air compressor, the electric motor-generator provides theneeded additional power. If the flue gas expansion provides more power thanneeded to drive the air compressor, than the electric motor-generator convertsthe excess power into electric power and exports it to the refinery’s electricalsystem. The steam turbine shown in Fig. 5 is used to drive the regenerator’scombustion air compressor during start-ups of the fluid catalytic cracker untilthere is sufficient combustion flue gas to take over that task.

The expanded flue gas is then routed through a steam-generatingboiler (referred to as a CO boiler)) where the carbon monoxide in the fluegas is burned as fuel to provide steam for use in the refinery.

The flue gas from the CO boiler is processed through an electrostaticprecipitator (ESP) to remove residual particulate matter. The ESP removesparticulates in the size range of 2 to 20 micrometers from the flue gas. As aunit operation being very crucial for the manufacturing of petro productsthe power recovery system in a fluid catalytic cracking unit is shown in theFig. 11.

Two prominent inorganic chemicals, ammonia and sulphur, are alsoderived in large part from petroleum. Ammonia production requires hydrogenfrom a hydrocarbon source. Traditionally, the hydrogen was produced froma coke and steam reaction, but today most ammonia is synthesized fromliquid petroleum fractions, natural gas, or refinery gases. The sulphurremoved from oil products in purification processes is ultimately recoverableas elemental sulphur or sulphuric acid. It has become an important sourceof sulphur for the manufacture of fertilizer.


The most versatile refinery configuration is known as the conversionrefinery. A conversion refinery incorporates all the basic building blocksfound in both the topping and hydro skimming refineries, but it also featuresgas oil conversion plants such as catalytic cracking and hydro crackingunits, olefins conversion plants such as alkylation or polymerization units,and, frequently, coking units for sharply reducing or eliminating theproduction of residual fuels. Modern conversion refineries may produce two-thirds of their output as gasoline, with the balance distributed betweenhigh-quality jet fuel, liquefied petroleum gas (LPG), diesel fuel, and a smallquantity of petroleum coke. Many such refineries also incorporate solventextraction processes for manufacturing lubricants and petrochemical unitswith which to recover high-purity propylene, benzene, toluene, and xylenesfor further processing into polymers.

Processing Configuration

Each petroleum refinery is uniquely configured to process a specific rawmaterial into a desired slate of products. In order to determine whichconfiguration is most economical, engineers and planners survey the localmarket for petroleum products and assess the available raw materials. Sinceabout half the product of fractional distillation is residual fuel oil, the localmarket for it is of utmost interest. In parts of Africa, South America, andSoutheast Asia, heavy fuel oil is easily marketed, so that refineries of simpleconfiguration may be sufficient to meet demand. However, in the UnitedStates, Canada, and Europe, large quantities of gasoline are in demand, andthe market for fuel oil is constrained by environmental regulations and theavailability of natural gas In these places, more complex refineries arenecessary.

Fig. 11: A schematic diagram of the power recovery system in a fluid catalytic crackingunit Inorganic Chemicals (courtesy http://en.wikipedia.org/wiki/turboexpander)


Fluid catalytic cracking (FCC) plants are used to convert heavy distillatesinto lighter ones like gasoline and diesel. The feedstock is primarily vacuumgas oil, often mixed with refinery residues.

The main products are:

• Gas fraction (mainly C3/C4) • Liquid fraction • Coke (solid formation on the catalyst).

FCC units produce sulphur dioxide and nitrogen oxides (NOx) and theseparticulates are tightly regulated in the petro chemical companies. Thisplaces refinery operator under pressure to mange NOx emissions andensuring, that these impurities do not impair air quality.

Linde offers the various solutions to enrich regeneration air with oxygenand boost capacity. These include LoTOx technologies to help controlparticulates, sulphur dioxide and NOx emissions from the FCC.

LoTOx technology is a patented innovation that uses ozone to selectivelyoxidize insoluble NOx to highly soluble species that can be easily removed ina wet scrubber. The benefits include increased capacity, greater flexibilityin the choice of feeds, increased conversion rates and reduced emissions.Test plants results explained by Linde Industrial gases is as follows. It maybe seen in Fig. 12.

Fig. 12: Test Plants results by Linde IG

Theory of Heat Balance

Catalytic cracking reactions are endothermic; they create products withhigher heat contents than the reactants and they absorb heat from theenvironment. In the cracking of paraffin by the beta scission mechanism,


high molecular weight paraffin is cracked to form a lower molecular weightolefin and paraffin. Table 3 uses the cracking of normal decane and normalheptanes as examples of this beta scission mechanism. Lower molecularweight hydrocarbons and higher molecular weight hydrocarbons both requireapproximately equal BTU’s of heat to crack a carbon-carbon bond by betascission, but the energy on a per pound basis increases as the molecularweight of the feed decreases. Thus Table 3 shows a higher endothermic heatof cracking for heptanes than for decane.

Table 3: Endothermic reactions of scission cracking (Ernest L.Leuenberger and Linda J.Wilbert)

Reaction 1: N Decane Npentane + 1 - PenteneEquation: C10H22 N C5H12 + C5H10Heat of Reaction inBTU/LB Feed = 249 BTU/LB FeedReaction 2: N Heptane Propane + 1 - ButeneEquation: C7H16 C3H8 + C4H8Heat of Reaction inBTU/LB Feed = 366 BTU/LB Feed

Table 4: Exothermic reactions of hydrogen transfer (Ernest L. Leuenberger and Linda J.Wilbert)

Reaction 1: Cyclohexane + Benzene + 3N Butane3 C-2 Butene

Equation: C6H12 + 3 C4H8 C6H6 + 3N C4H10

Heat of Reaction inBTU/LB Feed = -259 BTU/LBReaction 2: Benzene +

3C-2 Butene Coke + 3 N ButaneEquation: C6H6 + 3 C4H8 6C + 3 C4H10Heat of Reaction inBTU/LB Feed = -772 BTU/LB

Rare earth exchange in a zeolite catalyst promotes hydrogen transferreactions in competition with beta scission. One effect of hydrogen transferis to limit the production of C3 and C4 gases by beta scission. When thenumber of cracking reactions that form light gases is inhibited, theendothermic heat of reaction is reduced.

The hydrogen transfer reactions promoted by rare earth exchange alsotend to reduce the endothermic heat of cracking because they are exothermic.Table 4 gives examples of two such exothermic hydrogen transfer reactions.In the first example, a naphthenes and lower molecular weight olefins reactto form aromatic and light paraffin. When the olefins saturated by hydrogentransfer are in the gasoline boiling range, this reaction is responsible forreducing gasoline octane. The second reaction shows how hydrogen transfercan form a carbonaceous deposit on the catalyst from a heavy aromatic.When this type of hydrogen transfer is eliminated, coke make is reduced.


Table 5: Lower Heats of cracking by rare earth exchanged catalysts (Ernest L. Leuenbergerand Linda J. Wilbert)

Catalyst type Heat of cracking BTU/LB Fresh feed

Rare earth exchanged y Faujasite 80Partially rare earth exchanged y Faujasite 140Hydrogen exchanged y Faujasite 160Ultrastable hydrogen exchanged y Faujasite 180

Values for Commercial units Reported by J.L. Mauleon and J.C. Courcelle, Oil and GasJournal, October 21, 1985.

FCC octane catalysts maximize the octane of the cracked gasoline byminimizing the hydrogen transfer reactions that saturate gasoline olefins.Mauleon et al., observed that reducing hydrogen transfer must also increasethe endothermic heat of cracking. The results of his work, which arepresented in Table 5, show that heat of cracking can be correlated withcatalyst rare earth content. Rajagopalan and Peters observed that reducedhydrogen transfer reduces coke make may be referred in Table 6.

Table 6: Higher cokes make with rare earth exchange catalysts (Ernest L. Leuenbergerand Linda J. Wilbert)

Catalyst type Rare earth Coke make atWT% 70% conversion

Rare earth exchanged y Faujasite 3.0 2.1Partially rare earth exchanged y Faujasite 1.5 2.2Hydrogen exchanged y Faujasite 0.0 2.1

Values for mat runs on Mid continent feeds reported by K. Rajagopoalan and A.W. preprintof the A.C.S. division of petroleum chemistry, Vol. 30, No. 3, Page 538 (1985).

Methods of Determining Heats of Cracking

Two different approaches to determining the heat of cracking are possible:

1. Determine the heat absorbed by the cracking reaction through heatbalance methods.

2. Analyze the reaction products and assign each one a heat of combustion,then add up the heat of combustion of the products. For constantfeedstock, the reaction with the highest product heat of combustionhas the highest endothermic heat of reaction.

The first method was not attempted for laboratory heat of crackingmeasurements, but was found useful for commercial data analysis. The secondmethod was found to be applicable for both laboratory and commercial dataanalysis.

The first technique, which is called the heat balance technique fordetermining heats of cracking, involves calculating the heat of combustion


of coke in a commercial FCC operation, then subtracting all other heatrequirements. The remaining heat is then assumed to be the endothermicheat of cracking. The accuracy of this measurement technique depends onthe accuracy of the measurements that determine the coke heat ofcombustion and the completeness of the heat balance information recordedat the commercial plant.

The second technique, which we call the product analysis technique,was applied by Dart and Oblad in their classic work on measuring heats ofcracking. Each product was assigned a heat of combustion from the literature.For the liquid products, a correlation from the API data book was used todetermine the heat of cracking from the API gravity and the K factor. Thiswas a modification of Dart’s procedure, where these liquid heats ofcombustion were determined by calorimeter. The heats of combustion usedin the product analysis technique are presented in Table 7.

Table 7: Heat of reactions calculation for heats of combustion (Ernest L. Leuenberger andLinda J. Wilbert)

Reaction product Heat of combustion BTu/LB

H2 51980CH4 21580C2= 20350C2 20480

C3= 19750C3 19990

C4= 19480C4 19670

NC4 19720Gasoline (58 API, 11.85k) 18780

LCO (24, API, 11.01k) 18030Bottoms (11 API, 10.64k) 17630

Coke (7% Hydrogen) 16120

Net or Lower heating values are used.

The procedure described in the preceding paragraph is also applicable tocommercial data and was used to confirm the heat balance heat of reactioncalculations.

It was further examined that the heat of cracking and coke makedifferences between rare earth exchanged catalysts and FCC octane catalysts.Our objective is to quantify the heat balance effects and to use the resultingcorrelation to model catalyst changes in commercial FCCU’s.

Significance of Heat Cracking and Delta Coke Differences onCommercial FCCU Operation

When a catalyst change increases the heat of cracking in a commercial FCCU,the unit must either increase its heat generation or reduce its heat


requirements to stay in heat balance. Since the catalyst change out usuallyoccurs over a period of several weeks, the changes will be gradual and theunit will have sufficient time to respond without a crisis. A unit with normalslide valve controls will respond to the increase in heat requirement bygradually increasing catalyst circulation. According to one published modelthe coke yield will increase proportional to the catalyst circulation raised tothe 65 exponent. Thus higher circulation will increase the coke burned inthe regenerator and bring the unit back into heat balance. If the unit doesnot have the air blower capacity to burn more coke, changes must be madein the unit operation to reduce the heat requirements. The simplified overallheat balance in Table 8 lists these other requirements as the heat requiredto increase the feed temperature to the riser outlet temperature, the heatto vaporize the feed, and the sensible heat of the air. When no more cokecan be burned, it is most economical to raise the feed preheat in order toreduce the feed sensible heat requirement. The last resort would be to lowerthe feed rate to reduce all three feed enthalpy terms in the heat balance.The air heat requirement cannot be lowered if the unit is at a coke burninglimit.

Table 8: Simplified overall heat balance (Ernest L. Leuenberger and LInda J. Wilbert)

Heat generated Heat required to Heat required Heat required to Heat requiredby coke increase feed to to vaporize crack feed + to increase airburning = reactor temp + feed + to fuel gas temp

HCOKE COKE = F CPF (TREACTOR – TFEED)+ FHVAP+F HCRACK + Air CPA (TFLUE – TAMB)

The reduction in delta coke make caused by introducing an octane catalystwill also create an imbalance in the overall heat balance. Again the standardFCCU control system would raise catalyst circulation to increase the coke makeback to its original level. This control strategy for handling lower delta cokewill fail only if the catalyst circulation is at its maximum. Alternate strategiesto increase coke make include introducing a feed to the FCCU with a highercarbon residue such as vacuum reside or slurry recycle. Spraying torch oil intothe regenerator can be used as a last resort.

The increases in catalyst circulation caused by lowering the delta coke andraising the heat of cracking tend to reduce the regenerator temperature. Thesimplified regenerator heat balance in Table 8 shows that the heat transferredto the reactor plus the air sensible heat is equal to the heat released by burningcoke. The coke burned in the table is represented by the previously sited model.

Coke = K Cat Circ•65

When the heat balance is solved for the regenerator temperature minusthe reactor temperature and the air sensible heat is ignored, thistemperature difference is inversely proportional to the catalyst circulationraised to the .35 exponent. At constant reactor temperature, the heat balance


Table 9: Projected heat balance effects of FCC octane catalysts (Ernest L. Leuenberger andLinda J. Wilbert)

Case description Gasoline cat Octave cat at Octane cat atbase case constant activity constant conversion

Temperatures, FReactor 968 968 968Regenerator 1281 1262 1245Combined feed 300 300 300

CatalystActivity 72 72 68Cat ti OL 7.0 8.0 8.5

YieldsConversion, vol% 70 72 70Coke make, Wt% 5.9 6.3 6.2

then requires the regenerator to cool as the catalyst circulation increases.

More complete versions of the simplified coke make kinetics and heatbalances discussed above have been incorporated into an FCCU computersimulation program. This program was used to model the effects of a 50BTU per pound increase in the heat of cracking and a 10 relative percentdrop in delta coke make on a commercial FCCU. The results of one simulationare presented in Table 9. According to the model, if enough octane catalystis added to the unit to maintain catalyst activity, then a 20°F loss inregenerator temperature, a 15 relative percent increase in cat to oil, a 7relative percent increase in coke make, and a 2 volume percent increase inconversion would be expected. Correlations show this catalyst would alsoincrease gasoline octane by 2 RON. A second projection has also been includedwhere only enough octane catalyst is used to hold conversion constant. Thiscase is more usual for commercial octane catalyst trials, since the octanecatalysts often do not hold activity in the unit as well as rare earth exchangedcatalysts. For this alternate case, a 35°F drops in regenerator temperatureand a 20 relative percent increase in cat to oil ratio are predicted.

Table 10 shows the heat balance results of an octane catalyst trial that

parallels the simulation model’s predictions. When catalyst P1 replaced a rareearth exchanged gasoline catalyst, the regenerator temperature equilibrated70°F lower. The cracked gasoline octane increased by 2.5 RON. The octanecatalyst was not able to hold MAT activity; it dropped by 6 numbers. Even withthat activity loss, the conversion loss was minimized because of a 24 percentincrease in cat to oil ratio. The refiner could have held conversion constant ifhe had the coke burning capacity; but he was forced to increase the feed preheatto keep the coke make constant. Thermal verses catalytic cracking yields onsimilar toped feed is presented in Table 11 and comparison of fluid Thermafor,Houdry and catalytic cracking units may be referred in Table 12.


Table 10: Commercial heat balance data (Ernest L. Leyebverger abd Linda J. Wilbert)

Catak Yst R5 P1

Reo, Wt% 3.9 0.9Matrix Sa, M2/Gm 130 117Fresh unit cell size, A 24.7 24.6Equilibrium mat activity 75 69Unit operationReactor temperature, F 980 979Feed temperature, F 430 477Regen temperature, F 1355 1284Cat to Oil ratio 5.5 6.8Conversion, vol% 75 73Coke make Wt% 5.1 5.1

Table 11: Thermal Vs catalytic cracking yields on similar toped feed

Thermal cracking Catalytic cracking

Wt% vol% wt% vol%

Fresh feed 100.0 100.0 100.0 100.0Gas 6.6 – 4.5 –Propane 2.1 3.7 1.3 2.2Propylene 1.0 1.8 2.0 3.4Isobutane 0.8 1.3 2.6 4.0n-Butane 1.9 2.9 0.9 1.4Butylene 1.8 2.6 2.6 3.8C5 + gasoline 26.9 32.1 40.2 46.7Light cycle oil 1.9 1.9 33.2 32.0Decant oil – – 7.7 8.7Residual oil 57.0 50.2 – –Coke 0 – 5.0 –Total 100.0 96.5 100.0 102.0

Table 12: Comparison of fluid Thermafor, Houdry catalytic cracking units

FCC TCC HCC

Reactor space velocity 1.1–13.4b 1–3b 1.5–4b

C/O 5–16c 2–7d 3–7d

Recycle/fresh feed, vol 0–0.5 0–0.5 0–0.5Catalyst requirement, ib/bbi feed 0.15–0.25 0.06–0.13 0.06–0.13Cat. crclt. rate. ton cat./bbi total feed 0.9–1.5 0.4–0.6 0.4–0.6On-stream efficiency, % 96–98 – –Reactor temp., F 885–950c 840–950 875–950Regenerator temp., F 1200–1500 1100–1200 1100–1200Reactor pressure, psig 8–30c 8–12 9–12Regenerator pressure, psig 15–30 – –Turndown ratio – – 2:1Gasoline octane, clearRON 92–99 88-94 88–94MON 80–85 – –a ib/hr/ib.; b v/hr/v.; c wt.; d vol. * One company has operated at 990°F and 40 psig to producea 98 RON (clear) gasoline with a C3-650F liquid yield of 120 vol% on feed (once-through);there was approximately 90% yield of the C5-650F Product.


REFERENCES

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1997. Passivation vanadium on FCC catalysts for improved Refinery Profitability. (Annual NationalPetrochemical and Refiners Association (NPRA) Meeting).

2002. Editorial Staff, Refining Processes, Hydrocarbon processing, pp. 108–11, ISSN 0887-0284.Alex Hoffmann, C. and Lewis Stein, E. 2002. Gas cyclones and swirl tubes: Principles, design and

operation (1st ed.). Springer. ISBN 3-540-43326-0.Amos Avidan, A., Edwards, Michael and Owen, Hartley 1990. Mobil research and development.

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Mauleon, J.L. and Courcelle, J.C. 1985. Oil and Gas Journal, p. 64.Murphee Eger and the Four Horsemen: FCC, Fluid Catalytic Cracking (North American Catalysis

Society website).Palucka, Tim. 2005. The wizard of octane eugene houdry. Invention and Technology, 20(3).Pioneer of catalytic cracking: Almer McAfee at Gulf Oil (North American Catalysis Society website).Rajagopalan, K. and Peters, A.W. 1985. Preprint of the ACS Division of Petroleum Chemistry,

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8415-289-8.Scherzer Julius, 1990. Octane-enhancing zeolitic FCC catalysts: Scientific and technical aspects.

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