14 Oilfield Review

Refining Review—A Look Behind the Fence

David AllanConsultantHouston, Texas, USA

Paul E. DavisChevronRichmond, California, USA

For help in preparation of this article, thanks to Dr. Douglas Harrison, Louisiana State University, Baton Rouge, USA.

One end product of production activity is crude oil. Produced crude gains significantly

in value once it is converted into finished products. Like upstream activities, refining

involves operation at extreme conditions and application of advanced technology.

Drilling and production are only the beginning—complex refining processes, often conducted atextreme temperatures and pressures, arerequired to turn produced crude oil into theproducts that power a global society. Frommultibillion-dollar deepwater platforms to drag -lines scooping oil sands from permafrost, oil

producers hold the attention of the public. Thecrude oil from this effort disappears behindrefinery fences at some 658 locations worldwide.These plants range from a Venezuelan facilityrunning 149,000 m3/d [940,000 bbl/d] to loca -tions running less than 160 m3/d [1,000 bbl/d].1

Despite the huge disparity in size, these

> Global liquids supply and demand. Global liquids demand (transportation, industrial, residential,commercial and power generation) is expected to rise from the current 13.5 million m3/d [85 million bbl/d]to about 18.3 million m3/d [115 million bbl/d] in 2030 (left). Most of this demand will be satisfied bycrude and condensate. As conventional crude reserves become scarce, increasing reliance will beplaced on heavy oil to meet liquids demand. Recoverable heavy oil (22°API or less) (right) is nearly50% of conventional oil reserves. Contributions from oil sands will grow throughout this period,increasing from about 320,000 m3/d [2 million bbl/d] to nearly 1.1 million m3/d [7 million bbl/d] in 2030.(Global liquids demand adapted with permission from ExxonMobil’s Energy Outlook, 2006, reference 2.Recoverable resources adapted from Meyer and Attanasi, reference 2.)

01980 1990 2020 20302000 2010

Year

20

40

60

80

100

120

Oil e

quiv

alen

t, m

illio

n bb

l/d

Conventional oil,952 billion bbl

Oil sands,651 billion bbl

Recoverableresources

Liquidsdemand

Heavy oil,434 billion bbl

Biofuels

Other

Natural gas liquids

Oil sands

Crude and condensate

59819schD5R1.qxp:p 7/19/07 3:56 PM Page 14

Summer 2007 15

refineries share a common goal of convertingcrude oil into valuable and usable finishedproducts. That makes the refining story animportant one—economically and technologi -cally. It is also a story of scientific achievementand continuous improvement.

Refining is a vital link in the world economy.Rising income levels and growing populationsexert continuous pressure on transportation fuelsand all chemical products made from oil (previouspage).2 This pressure to produce a growing supplyof fuels and chemicals coincides withincreasingly stringent worldwide environmentalstandards. To meet these demands, refiners areliterally digging deeper into the bottom of eachbarrel and processing more heavy oils asconventional crude supplies become scarce.

This article discusses refining and its evolu tion from simple beginnings using batchequip ment to today’s highly automated plantsoperating around the clock. We will also examinethe growing presence of heavy oils in refineryfeedstocks and the trend toward achieving nearlyzero contaminant levels in transportation fuels.

From Simple Beginnings to a Key Global IndustryAlthough historians have noted the use ofpetroleum and tar in ancient times, the firstreported refinery was built in 1860 in Titusville,Pennsylvania, USA, at a cost of $15,000.3 Then, as now, the refiner’s challenge was to converthigh-boiling-point viscous crude oil into lower-boiling-point products. Early refiners employedbatch rather than continuous systems and usedthermal cracking as the conversion process (see“Refining Glossary,” next page). In this type ofcracking, large oil molecules are thermallydecomposed to molecules of lower-boiling-pointsubstances. The lower-boiling-point materialsthat are stable leave the system as cracked gas,gasoline and distillate in the kerosene-dieselboiling range. Other components that are lessstable polymerize to form products heavier thanthe original crude.

Thermal cracking to produce motor gasoline,or petrol, was the primary conversion processduring the first part of the 20th century. Use ofthermal processes peaked in the 1930s andsubsequently declined as fluid-bed catalytic

cracking was introduced during World War II.Catalytic cracking eventually displaced thermalcracking as the primary conversion process,although mild thermal cracking is still in use atmany small refineries. This displacement of thethermal process is due to catalytic cracking’sgreater yields of high-octane gasoline with less of the heavy fuel oil and no coke by-product.Following the war, refining continued to matureand expand through use of sophisticated cata -lysts and automated process control. Theseimprovements increased conversion levels andimproved selectivity to desired products.

1. “Global Refining Capacity Increases Slightly in 2006,” Oil & Gas Journal 104, no. 47 (December 2006): 56–60.McKetta JJ Jr (ed): Petroleum Processing Handbook.New York City: Marcel Dekker, 1992.

2. ExxonMobil’s Energy Outlook 2006. http://www.exxonmobil.co.uk/Corporate/Citizenship /Imports/Energy Outlook06/slide_9.html (accessed February 10, 2007). Meyer RF and Attanasi ED: “Heavy Oil and Natural Bitu-men—Strategic Petroleum Resources,” USGS (August2003), http://pubs.usgs.gov/fs/fs070-03/fs070-03.html(accessed February 10, 2007).

3. Nelson WL: Petroleum Refinery Engineering, 4th ed. New York City: McGraw-Hill, 1958.

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Refiners today confront the same challengethat their predecessors faced over a hundredyears ago—refinery products must matchmarketplace demands. The current marketdemands a transportation-fuel product that boilsbelow 345°C [650°F] with nearly zero sulfurcontent. The problem is that crude oil rarelyoccurs in that form. Light, sweet crude such asBrent and West Texas Intermediate with sulfurlevels below 1.0% by weight have become scarceand expensive as the market has moved towardheavier crude with sulfur levels in the range of1.0 to 3.0% by weight. Increased use of heavy oilwith sulfur levels above 3.0% by weight has putadditional demands on refineries.4 The qualitydifference between light and heavy oil shows upin the marketplace price that refiners pay forfeedstock. The price differential between lightcrude (>40°API) and heavy oil (<20°API) varieswith the market, with $9.00 per bbl being atypical value.5 With high demand for lighttransportation fuels, 70 to 90% of the productbarrel now boils below 345°C.6

Refiners have met these challenges byinstalling more conversion and product-finishingcapacity. In this article, we will examine how arefinery takes crude and converts it to finishedproducts for the market.

Separation Is the First StepAll refineries have three primary sections:separation, conversion and finishing.7 Beforeprocessing crude, refiners must physically sepa -rate it into various molecular-weight ranges. Thisallows for tailored and selective conversion stepsto operate efficiently. Products from these con -ver sion steps are then treated in several finishing

16 Oilfield Review

4. Dealing with heavy oil has presented producers withadditional challenges as well. Heavy-oil physicalproperties make most varieties very difficult to transportto the refinery by conventional means. Producers mustdecide whether to prepare heavy oil for shipping bydilution or by partial or full upgrading on site.

5. Davis NC: “Overview of Domestic Petroleum Refiningand Marketing,” (February 5, 2007), http://www.eia.doe.gov/emeu/finance/usi&to/downstream/update/index.html(accessed February 13, 2007).

6. Davis P, Reynolds J, O’Neal A and Simmons K: Crude Oiland Its Refining. Richmond, California, USA: ChevronTechnical University, 2005.

7. The terms product treating and finishing are often usedinterchangeably.

8. Speight JG: The Desulfurization of Heavy Oils andResidua. New York City: Marcel Dekker, 2000.Gary JH and Handwerk GE: Petroleum RefiningTechnology and Economics, 4th ed. New York City:Marcel Dekker, 2001.

9. In a reference to early refinery operations when thermalcracking was prevalent, these distillation units are stillcalled the atmospheric and vacuum pipestills in many refining publications.

10. Hsu CS and Robinson PR (eds): Practical Advances inPetroleum Processing. New York City: Springer, 2006.

Aromatics—a general term for petroleum hydrocarbons containing at least one ring with alternating double bonds.

Batch processing—a production method in which the ingredients are mixed in a vessel and the requiredconditions applied; after a designated amount of time, the process is shut down and the vessel emptied.

Bubble cap—a slotted cap placed on top of a vapor riser in a distillation column to promote vapor-liquid contact.The bubble cap-riser assemblies are arranged on horizontal trays in the column. Other common tray-contactingassemblies in the refinery include sieve trays and valve trays.

Coke—a carbonaceous material formed by condensation reactions at high temperatures.

Coking—a severe thermal cracking process on vacuum resid to produce coke and lighter products.The most common variant is delayed coking in which long residence times at high temperature are used todrive the process to nearly complete conversion.

Continuous processing—a production method with the ability to produce a product without interruption.

Cracking—splitting a carbon-carbon bond either by thermal means (coking) or with the aid of a catalyst(catalytic cracking, hydrocracking).

Deasphalting—using a light hydrocarbon to bring asphalt out of solution.

Distillate—a refinery stream that has been vaporized and condensed.

Distillation—the separation of components based on differences in their volatility.

Endothermic reaction—a chemical reaction that absorbs heat.

Fouling—restricted flow in refinery lines or vessels as a result of coke formation, sludge accumulation orparticle accumulation.

Fractionation—a separation process based on concentration gradients.

Fuel oil—a broad classification for liquid fuels produced in the refinery that range from distillates to heavy fuel oil.

Gas oil—any distillate stream having a boiling point higher than that of heavy naphtha.

Heavy oil—heavy, high-sulfur oil. Heavy oils are difficult to refine due to high levels of sulfur, condensed aromatics(asphaltenes) and metals. There is no universal definition for heavy oils but sulfur values are usually higher than 3.0%by weight, and gravity is usually at or below 20 to 22°API.

Isomerization—transformation of a molecule into another form (the isomer) with the same molecular weight buta different structural arrangement.

Naphtha—a distillation cut in the range of 32 to 220°C [90 to 430°F]; naphthas are usually classified according toprocess and boiling range.

Naphthenes—a general term for petroleum hydrocarbons containing at least one saturated ring.

Octane number—a measure of the resistance to autoignition (knocking) of the fuel. The octane number is thevolume percentage of iso-octane in a mixture of n-heptane and iso-octane that has the same knock characteristicsas the fuel in question.

Olefin—a general term for petroleum hydrocarbons containing at least one carbon-to-carbon double bond.

Paraffin—a general term for saturated petroleum hydrocarbons that contain no rings and have a carbon numbergreater than about 20.

Petrochemicals—the generic name given to a broad range of products produced using by-product refinery streamsas feed. Petrochemical building blocks such as ethane may come directly from refinery streams or be produced by aprocess such as naphtha cracking.

Silica-alumina—the name given to the amorphous catalytic cracking catalyst base material. Silica-alumina forcracking catalyst is synthetically produced by combining sodium silicate, sodium aluminate and sodium hydroxide.

Steam ejection—the passage of steam through a jet ejector to generate a vacuum.

Straight run—refinery fluid streams cut directly from the crude oil.

Three-way catalytic converter—a canister in automobile exhaust systems used to reduce emissions.The converter acts by reduction of nitrogen oxides to nitrogen and oxygen, oxidation of carbon monoxide to carbondioxide and oxidation of unburned hydrocarbons to carbon dioxide and water.

Vacuum resid—the bottom or heaviest stream from the crude-oil vacuum distillation tower.

Visbreaking—mild thermal cracking.

Zeolite—a silica-alumina mineral that has an open porous structure. Zeolites usually undergo further treatment byexchange with rare earths to produce the desired catalytic cracking catalyst properties.

Refining Glossary

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Summer 2007 17

steps to make them ready for sale. All these stepsoperate in concert to turn crude into thousandsof products (below).8

The first step in any refinery is separation ofthe crude oil into component streams in adistillation unit. Crude oil contains thousands ofindividual compounds and at atmospheric

pressure these components can boil anywherebetween 0°C [32°F] to more than 540°C[1,000°F]. Distillation is used to separate thecrude into different boiling-range fractions forefficient conversion and cleanup downstream.Modern conversion refineries typically have twodistillation towers in series—a tower operating

close to atmospheric pressure followed by avacuum unit.9 These large towers often have thehighest feed rate of any unit in the refinery. Alarge unit can process 32,000 m3/d [200,000 bbl/d]or more of crude oil. The towers contain trays orstructured packing for vapor-liquid contacting,and heights of 45 m [150 ft] are common.10

> Typical refinery process flow showing separation, conversion and finishing sections with end products (left). The products from most refineries numberin the hundreds and may number in the thousands from conversion refineries if lubricating oil, wax and grease production are present. In some facilities,refinery gas and naphtha are sent to petrochemical plants for further upgrading—where the eventual end products number in the tens of thousands(right). (Adapted with permission from Speight, reference 8.)

Separation

Deasphalter

Coking

Catalytic cracking

Catalytic cracking

Crudeoil

Hydrocracking

Hydrocracking

Hydrotreating

Hydrotreating

Hydrotreating

Hydrotreating

Reformer

Alkylation

Liquefied petroleumgas, fuel gas,petrochemical feedstock

Gasoline

Petrochemicalfeedstock(gas, naphtha)

Hydrogen

Kerosene

Kerosene,mid-distillate

Naphtha, kerosene

Naphtha, kerosene

Heating oil

Fuel oil

Fuel oil

Light ends

Gasoline, kerosene,diesel oil

Gasoline, kerosene

Coke

Asphalt

Aromatic oil

Lubricating oil,wax, grease

Visbreaking

SolventExtraction

Gasplant

Vacuumdistillation

660 to 880°F

580 to 650°F

450 to 580°F

320 to 450°F

320°F

Butanes, butylenes

880

to 1

,050

°F 1,050°F

650°F

Soluble

Insoluble

Atmosphericdistillation

Conversion Finishing Products

Ink

Paintbrushes

Telephones

Fishing lures

Deodorant

Floor wax

Electrical tape

Food preservatives

Safety glass

Synthetic rubber

Vitamin capsules

Insect repellant

Paint

Beach umbrellas

Garden hoses

Nail polish

Antihistamines

Tennis shoes

False teeth

Shoe polish

Fan belts

Aspirin

Lipstick

Parachutes

Toothpaste

Yarn

Anesthetics

Ballpoint pens

Heart valves

Products

Petrochemical OperationsRefining Operations

Petrochemical products> 10,000

Refinery products100s to 1,000s

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The flow scheme through the atmosphericand vacuum towers is straightforward. Afterwater and salt are removed, the crude oil is fed tothe atmospheric tower (left). In the atmospherictower, it is separated into several componentstreams—gas, naphtha, distillate and residue.The residue from this unit is sent to the vacuumdistillation tower to recover addi tional liquids inthe higher boiling-point ranges that will be usedas feed for the critical con version units. Vacuumdistillation towers use steam ejection to generatea vacuum of 50 to 100 mm of mercury. This keepstemperatures low enough to avoid fouling on theinternal sec tions of the tower. Although allrefineries have distillation as the primaryseparation process, some locations haveadditional separation steps such as deasphaltingand other extraction steps.11

The units downstream of the towers trans -form the separated fractions into products; thisis where the real work takes place. Some unitsuse sophisticated catalysts while others do theirwork using brute-force thermal methods.Temperatures in these units range from 4°C[40°F] in an alkylation reactor to 700°C[1,300°F] in a catalytic cracking regeneratorvessel. Pressures may be as high as 20.7 MPa[3,000 psi] in a hydrocracker to nearatmospheric level in a delayed-coking unit.

The Conversion WorkhorseAmong the unique collection of elements in arefinery, the most crucial are the conversionunits—for catalytic cracking, hydrocracking andcoking. These units convert the high molecular-weight oil fractions from separation into com -ponents that become finished products. Of theseconversion units, the premier process is fluidcatalytic cracking (FCC). Catalytic cracking wasdiscovered in the 1920s using treated clay as acatalyst; Exxon commercialized the first fluid-bed unit at its Baton Rouge, Louisiana, refinery in1942.12 Since then, catalytic cracking has becomethe most widely used process for convertinghigher boiling fractions into gasoline andother products.13

The catalytic cracking process has a toler -ance for a wide variety of feeds. A common feedis a nominal 340 to 540°C [650 to 1,000°F]fraction from the vacuum distillation tower. TheFCC feed is preheated and injected into a movingstream of fluid catalyst from the regenerator atthe reactor entrance (next page).14 The tempera -ture of the catalyst stream is about 700°C[1,300°F] and cracking reactions happenquickly. The kinetics for breaking carbon-carbon

18 Oilfield Review

> Distillation process. Crude-oil distillation is a thermal, physical separationinto product boiling-range components. In an atmospheric crude-oildistillation unit, the feedstock is heated in exchangers and furnaces to about370°C [700°F] (top). At this temperature, a significant portion of the crudevaporizes and moves up the distillation column, while the remaining liquidsmove down toward the bottom of the column. Vapors that escape to the top ofthe column are condensed to a liquid, and a portion is returned to the columnas reflux. Throughout the column, liquid and vapors are in contact—internalelements such as trays or packing assist in making that contact (bottom).Inside the column, thermal layers are established that match the boiling-pointranges of the product streams. These product streams are withdrawn as sidestreams. Nitrogen level in the crude affects the color of the product streams.From the top of the column down, these product streams start as clear fluidsand become progressively darker in color. The bottom stream from theatmospheric crude distillation tower is sent to a vacuum tower if that tower ispresent. (Adapted from Davis et al, reference 6.)

V A P O R

LI

QU

ID

LI

QU

ID

LI

QU

ID

V A P O R

V A P O R

Liquid

Tray

Bubble caps

Liquid

Tray

Bubble caps

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Summer 2007 19

bonds are complex and may involve multiplepathways and several secondary reactions. As aresult of the various reactions, the catalystleaving the reactor is deactivated from carbon deposition.

The temperature at the top of the reactoroutlet is typically in the range of 480 to 550°C [900to 1,020°F].15 Operators control this temperaturecarefully, as it has an important effect on productdistribution; lower tempera tures favor distillateyield, while higher temperatures favor gasolineand light olefins. Although secondary reactionscan be controlled to some degree by quenching atthe top of the reactor, enough occur that thecatalyst leaving the reactor is deactivated fromcarbon deposition.

After separation, the deactivated catalystpasses to the regenerator vessel where thecarbon deposits are burned off in controlledfluidized combustion using air or oxygen-enriched air. The regenerated catalyst passesfrom the regenerator to the reactor to begin theprocess again.

Temperatures in cracking regenerators canapproach 730°C [1,350°F], and the vessel wallsmust be lined with refractory material to protectthe steel shell. Large vessel diameters keep gasvelocities low to minimize entrainment of thefine catalyst particles in the flue gas. The energypresent in the regenerator flue gas is toovaluable for discharge to the atmosphere. It istypically used to generate steam in a carbonmonoxide boiler.

Catalytic cracking units are heat balanced—the combustion of the carbon on the spentcatalyst provides the energy required to preheatthe feed and supply the endothermic reaction.Typically, about 70% of the energy from thecombustion is required for feed preheating andthe reaction. The other 30% is consumed by heatlosses, preheating air to the regenerator andsteam generation. A large catalytic cracking unitcan have 545 tonnes [600 tons] of catalystcirculating at mass rates of tons per second.Tuning of the simultaneous mass and energybalances is the key to successful operation.

Although the catalytic cracking process hasseen many improvements during the last75 years, none has had more effect thanimprovements in the catalyst itself. The keyproperty of any cracking catalyst is the presenceof an active acid site on a solid surface. EarlyFCC catalysts were synthetic silica-alumina.These catalysts had a random distribution ofcatalyst pores, and pore diameters were muchlarger than molecular sizes.

The breakthrough discovery in crackingcatalysis occurred in the 1960s with theintroduction of zeolites to the silica-aluminacatalyst base. Zeolites allowed preparation ofcatalysts with a controlled three-dimensionalstructure having molecule-sized pores.16 Controlof pore size allows high molecular-weight aro -

matic compounds to be excluded, thereby reduc -ing undesirable reactions to carbon. Theseimprovements sharply increased catalyst activityand improved selectivity to desirable products.Additional improvements to cracking catalystactivity and selectivity continue to the present day.

11. Some refineries use light hydrocarbons such as propaneto precipitate asphaltenes in a solvent extraction step.The resultant deasphalted oil may be used in conversionsteps or to produce lubricants.

12. The first use of treated clays to catalytically crackpetroleum fractions to gasoline is attributed to EugeneHoudry. Commercial implementation during the late1930s employed a bead catalyst that shuttled betweenthe reactor and the regenerator (moving beds).

Magee JS and Dolbear GE: Petroleum Catalysis inNontechnical Language. Tulsa: PennWell PublishingCompany, 1998.

13. Gary and Handwerk, reference 8.14. Gary and Handwerk, reference 8.

Hsu and Robinson, reference 10.15. Hsu and Robinson, reference 10.16. Venuto PB and Habib ET Jr : Fluid Catalytic Cracking with

Zeolite Catalysts. New York City: Marcel Dekker, 1979.

> Fluid catalytic cracking unit at a Chevron refinery (left). The process uses a catalyst with an averageparticle size of about 70 microns—similar in size to flour or talcum powder. The catalyst is fine enoughthat it behaves as a fluid when aerated by a gas. In a typical operation (right), feed is injected into astream of fluidized catalyst from the regenerator and the resultant mass moves through the reactor.The reaction is fast and the products, spent catalyst and unconverted feed move into the disengagingvessel where hydrocarbons and catalyst are separated. Products and unconverted feed go tofractionation. The spent catalyst goes to the regenerator vessel, and regenerated hot catalyst movesthrough a large slide valve to the feed injection point where the process begins again. (Adapted fromDavis et al, reference 6.)

Regenerator

Reactor

Regeneratedcatalyst

Feed

Steam

Steam

Spentcatalyst

Products

Flue gas

Disengagingvessel

Air, or air + O2

59819schD5R1.qxp:p 7/19/07 3:34 PM Page 19

Another important refinery conversion processis hydrocracking. Hydrocracking combines break -ing of carbon-carbon bonds with the addition ofhydrogen. The process was originally developedby I.G. Farben in 1927 to convert coal intogasoline.17 Hydrocracking may take many formsdepending on the application. These range frommild hydrocracking of heavy vacuum gas oils at5.5 to 10.4 MPa [800 to 1,500 psi] hydrogenpartial pressure to severe hydrocracking ofresidual oil at 20.7 MPa [3,000 psi].

Hydrocracking is a flexible process that canbe designed to maximize gasoline or dieselproduction, to pretreat catalytic cracker feed, orto produce base oils for manufacturing lubri -cants. Regardless of the application, hydro -cracking reactors typically use a shaped catalystloaded in downflow, fixed beds.18 Nearly allhydrocracking catalysts use a silica-alumina basewith a metal component such as platinum orpalladium. The hydrocracking catalyst becomesdeactivated over time as carbon deposits buildup and cover active sites. This necessitates acorresponding gradual increase in temperatureto maintain desired conversion targets. After twoor three years, hydrocracking catalyst activitydecreases to a point at which the unit must beshut down and the catalyst regenerated orreplaced. Regeneration is accomplished byburning off the carbon deposits in situ.Hydrocracking catalysts may go through severalcycles before they must be replaced and preciousmetals recovered.

Although hydrocrackers are expensive tobuild, recent interest in them has been spurredby high demand for light motor fuels and theirability to produce specialty products such aslubricant base oils. Environmental concernsabout refinery product contaminant levels havealso contributed to hydrocracking growth.

The last important refinery conversionprocess is coking. Coking deals with the heaviestpart of the barrel—those components withboiling points exceeding 540°C [1,000°F] knownas vacuum resid. Catalytic cracking can accom -modate some vacuum resid, and direct sale offuel oil and asphalt is another outlet. However,because of increased demand for light productsand more reliance on heavy oils, vacuum-residsupply often exceeds demand. The refiner mustuse a process like delayed coking to convert the excess vacuum resid into useful products(above right).

Unlike the majority of other refinery processsteps, no catalyst is used in coking. Time andtemperature are used to convert the vacuumresid by means of two reaction pathways—

thermal cracking and condensation. Liquidproducts from a delayed-coking unit span theentire range from naphtha to heavy gas oil.Because of the high concentration of olefins andother contaminants, coker product liquids mustundergo hydrotreating so they can be blendedinto finished products. Depending on the feedquality and coke-drum temperature, severalvarieties of solid coke may be produced.

Heavy, high-sulfur crudes tend to producefuel-grade coke, a low-value solid fuel that can beblended with coal. Lower-sulfur crudes canproduce higher-value anode-grade coke that isconverted to anodes for aluminum manu fac -turing. Although delayed coking is the mostcommon coking variant, some process capacity isinstalled as fluid coking—similar to fluidcatalytic cracking but without the catalyst.

20 Oilfield Review

> Delayed coking unit at a Chevron refinery (top). Residuum feed plusrecycled material is heated to more than 480°C [900°F] in the feed preheatfurnaces (bottom). The temperature is high enough and residence time in the furnaces long enough that thermal cracking of the feed occurs as thedischarge enters the coke drums. About 70% of the thermally crackedproduct vaporizes; gas leaves the coke drum and moves to the productfractionation tower. The other 30% undergoes condensation reactions and istransformed into a solid, carbon-rich coke that eventually fills the drum. Pairsof drums are employed and when a drum nears capacity, the drums areswitched and coke is physically removed. Initially, the drum is steam-strippedto remove additional hydrocarbons and then quenched with water. When thedrum has cooled, the top and bottom heads are removed, and the coke isdrilled out using high-pressure water jets. Cycle time on delayed cokers isusually 18 to 24 hours. The solid coke is discharged into a pit and handled asa solid with grinders and a conveyor for moving the material to shipping.(Adapted from Davis et al, reference 6.)

Heavy coker gas oil

Heavy coker naphtha

Light coker naphtha

Process gas

Residuumfeed

Cokedrums

Coke(+ water)

Drilling water

Feed plus recycle

Furnace

Fuel

Light coker gas oil

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Summer 2007 21

Finishing Completes the PictureAs important as the conversion units are to therefinery, the story does not end with them. Oncethe large molecules in the crude oil have beenconverted to a range of smaller ones, they mustundergo one or more finishing steps. The mostwidely used process in the finishing arsenal ishydrotreating—a generic name given to a widerange of hydrogenation or hydrogen-additionsteps. The most common reason for usinghydrotreating on any refinery stream is sulfurremoval. In addition to removing a substantialamount of sulfur, hydrotreating may also targetother compounds containing metals andnitrogen—and occasionally olefins andaromatics may be hydrogenated.

Hydrotreaters exhibit a wide range ofoperating conditions. These units span the rangefrom simple kerosene units operating at 1.7 MPa[250 psi] hydrogen partial pressure to unitsoperating at 10.4 MPa [1,500 psi] and treating340 to 540°C [650 to 1,000°F] material.Hydrotreating can be used either in a stand-alone mode to make a product ready for sale or as a pretreatment for molecular rearrangement.

The demand for ever-increasing volumes ofhigh-quality motor gasoline has heightened the

focus on molecular rearrangement processes—reforming, alkylation and isomerization. Thisrearrangement is necessary because thegasoline-boiling-range streams from conversionand hydrotreating are rich in straight-chainparaffins and naphthenes that have low octanenumbers. Rearrangement transforms these low-octane components into higher octane branchedparaffins and aromatics. All of these processesare catalytic. Reforming and isomerization areconducted in the gas phase using either preciousmetals on alumina (reforming) or zeolite onalumina (isomerization) as a catalyst. Alkylation,on the other hand, is conducted in the liquidphase using either sulfuric or hydrofluoric acidas a catalyst.

The products produced in all of the conversionand finishing steps are almost ready for sale.Straight-run products may need additionaltreating steps for drying and sulfur removal, andmany products will require some blending andadditives to meet final sales specifications.Depending on the location, gasoline usuallyrequires additives for oxidation, metals andcorrosion inhibition plus anti-icing. Depending onthe complexity and product requirements, somerefineries may have units to produce asphalt, wax,lubricating oils and greases.

Refining Present and FutureThere has never been a more challengingsituation for refineries than the one theycurrently face. Those that survived the profitmargin collapse and capacity glut of the late1980s and early 1990s now see their capacitylimits stretched. No new refineries have been

built in the USA since 1976 despite a 45%increase in gasoline usage over the same period.19

Refiners have dealt with capacity constraints byinstalling numerous debottlenecking projects. Insome cases, these projects are new constructionin an existing refinery while others may add anew catalyst or improved process control.

In addition to the challenges presented bycapacity constraints, refineries worldwide mustnow cope with greater quantities of heavy oil asconventional sweet crude becomes scarce.Refining heavy oil requires significantly moresevere conditions for a given set of productspecifications. This puts an additional strain onrefinery costs.

Refineries prosper or fail on the profit marginthey make on each barrel of crude oil processed.However, refineries are caught between thedesire of the public for low fuel prices and theproducers who want to sell their crude oil at thehighest price. Both the oil producers and thepublic believe that prices are set by the refineryand that refinery profits are high. In reality,neither the producer nor the refiner sets theprice, and average worldwide refining marginsare generally modest.20 Prices are set on thevarious worldwide financial exchanges that tradecontracts on crude oil and refined products.Those prices are a minute-to-minute reflection ofhow investors view future needs for energy andpetroleum products. All parties are at the mercyof those estimates.

Like crude-oil producers, refiners have alsohad to deal with the challenges presented byenvironmental regulatory legislation. Environ-mental regulations started to tighten in 1970,and the trend has accelerated in recent years.During this period, refineries have maderemarkable progress in cleaning up their directand indirect emissions. Refineries have reduceddirect emissions by flue-gas scrubbing andcontrol systems, optimization of furnaces andincreased monitoring to reduce hydrocarbonemissions from valves and fittings.21 Refinerieshave also become more energy efficient, therebyreducing production of carbon dioxide. Nowherehas the environmental push been stronger thanthe trend toward clean transportation fuels. Thistrend has quickly spread worldwide (above left).

Refiners are a vital part of the team thatconverts crude oil into useful products. As moreof that crude oil comes from heavy oil and higher-sulfur sources, refineries will have tocontinue to develop new technologies to supplythe world with products that are both clean and affordable. —DA

> Gasoline-sulfur targets. Reduction of sulfur in motor gasoline is an important environmental goal.Sulfur in gasoline is converted to sulfur dioxide in the automobile exhaust, and that acts as a poisonto the three-way catalytic converter. Reducing gasoline-sulfur content increases exhaust-gasconverter efficiency thereby decreasing toxic emissions. During the past few years, governmentalagencies worldwide have targeted motor gasoline as a candidate for large reductions in sulfurcontent. For example, Japan reduced sulfur in gasoline from 100 ppm in 2004 to 10 ppm in 2006, andGermany achieved 10 ppm in 2004. The concept of low sulfur levels in gasoline is becoming the normrather than the exception. Sulfur levels in diesel fuel are following a similar path. (Adapted from Hsuand Robinson, reference 10.)

1,000 100 10

Gasoline-sulfur target, ppm

1

Australia

USA

Canada

Other EU

Korea

Japan

Germany 2004

2004

2004

2004

2004

2004

2004

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2008

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2005

2006

2006

17. Gary and Handwerk, reference 8.18. Fixed-bed reactors are vertical, cylindrical vessels filled

with catalyst particles of controlled size, surface areaand pore distribution.

19. Mouawad J: “No New Refineries in 29 Years, But ProjectTries to Find a Way,” The New York Times (May 9, 2005),http://select.nytimes.com/search/restricted/article?res=F30611FC39540C7A8CDDAC0894DD404482(accessed January 31, 2007).

20. http://omrpublic.iea.org/refinerysp.asp (accessedApril 16, 2007).

21. Hsu and Robinson, reference 10.

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