8/7/2019 Crude Distil
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Modern Refinery
Editorial directionand coordinationfrom
DavidNakamura,Refining/Petrochemical Editor
WarrenR. True,ChiefTechnology Editor
Modern Ref inery:Crude Dis t i l la t ion
Crude units are the firstto process petroleumin anoil refinery.Crude oil, as
producedin the oil field,is a complex mixture ofhydrocarbons rangingfrom
methane to asphalt,with varyingpr oportions ofparaffins, naphthenes,and
aromatics.The objective ofcrude atmosphericdistillation is to separate,
orfractionate, crude oil into several components ofmaterials withsimilar
properties according to boiling point. Most commonly,these groups and
theirboiling ranges are:
LPG Lighter gases
Naphtha andgasoline 90-400°F.
Kerosine 330-540°F.
Diesel 420-700°F.
Atmosphericgas oil 550-830°F.
Atmosphericresidue. 750+°F.
Atmosphericresidueisfurtherseparatedintodieselfuel,l ightvacuumgas
oil,heavyvacuumgasoil andvacuumresidueinthe vacuumdistil lation
column.It isdifficulttorecoverallof thedieselfuelin theatmospheric
distil lationcolumn.Therefore, mostnew designs include a provisionto
recovera portionof thisfractioninthe vacuumcolumn.Thisisconsistent
withthelargeincreaseindemandfordieselfuelduringthelastfewyears.
Mostofthe abovepetroleumfractionscan besold directlyto endusers,
althoughfurtherprocessinginrefinerydownstreamunitsusuallyoccursto
increasethevalueof thefinalproduct.Crudequalitydictatesthe levelof
furtherprocessingnecessarytoachievetheoptimalmixofproductoutput.
Process descriptionCrude oftencontains water,inorganic salts,suspend edsolids, and water-
soluble trace metals.Electrostatic desalting removes most of these
contaminants to reduce downstream corrosion,plugging,and fouling
ofequipmentand to preventpoisoning ofcatalysts inthe downstream
conversionunits.
Toremovesaltsandsuspendedsolids,crudeismixedwith3-10vol%water
andheatedto215-300°F.inwhatistypicallycalledthe“coldpreheattrain”
1 .Thisistheheat-exchangenetworkupstreamofthedesalters.Hotcrude
oilandwaterareforcedtomixbyspecialmixingvalves 2 orstatic mixers.
The mixture thenenters the desaltervessel 3 where anelectric fieldaids
inseparating the oil andwater phases.The electricfieldcauses the charged
saltwaterparticles to agglomerate andfall by gravity to the vessel’s bottom.
Surfactants canbe added to aidseparation,suchas whenthe crude has
a large amountof suspended solids orcertain chemical compounds that
encourage emulsionformation. Chemical surfactants (demulsifiers) aid
salts andother impurities to dissolve into the wateror attachto the water
to make theirremoval easier.
Collectedwater(brine)contains95-99% ofthe crude’s saltcontentand
flowstothe wastewater-treatmentplant.Dependingoncrudetypeandthe
requireddegreeofdesalting,twostagesofdesaltingmaybe necessaryand
mayutilizemultiple typesof electricfields.
Desaltedcrude entersanother heat-exchangernetwork,typically calledthe
“hotpreheattrain” 4 .Bothhotandcoldpreheattrainsusewasteheatfrom
multipleproductand “pumparound”streams (definedto theright) thatneed
coolingaspartofthe process.
Preheatedcrude enters the crude furnace 5 ,where itis furtherheatedto
about650-700° F.before enteringthe crude tower 6 .
Partiallyvaporizedcrude isfed intothe feedregionof thecrudetower,
knownasthe“flashzone” 7 ,wherevaporandliquidseparate.Thevapor
leaving theflash zoneincludesall thecomponents thatcomprisethe
products,whiletheliquidcontainstheatmosphericresiduewitha small
amountof componentsintheatmosphericgas-oilproductrange.Someof
thesecomponentsareremovedfromtheresiduebysteamstrippingatthe
bottomofthecolumn 8 .
Thecrudecolumncontainshorizontaltrays 9 forseparatingandcollectingthe
varioushydrocarbonsthatcomprisetheend productliquids.Liquidflowsdown
thetoweracrosseach tray,whilevaporrises inthetower throughperforations
(valves,holes,bubble caps)ineach tray.Theseperforationspermit thevapors
tobubblethrough theliquid,causing heatandmass transfer.
Ateachtray,a small amountof higher-boilingcomponentsin thevapor
condense,whilea smallamountof lower-boilingcomponentsintheliquid
vaporize.Thisprocess(distillation)causesthelightcomponentsto concentrate
inthevaporand heavycomponentsto concentratein theliquid.Theliquid
drainsfromeach trayvia“downcomers”to thetray below,wheretheprocess
isrepeatedover enoughtrays tomeet product-purityrequirements.
The crude towerbottoms product is knownby several names including
“toppedcrude,” “atmosphericresid,” “atmosphericreduced crude”(ARC) ,
and“atmospherictowerbottoms”(ATB).Atsuccessively higherpoints in
the tower,majorproducts suchas gas oil,diesel,kerosine,naphtha,and
uncondensedgases are withdrawn.
Some products are sent to side strippers to reduce the content of the
lightest components in each product. In addition, for more effective
heat integration, liquid is withdrawn at strategic locations,co oled by
exchange with cold crude and returned to the column a few trays above
the draw location. These arrangements inwhich some of the column
liquid is withdrawn,cooled,and pumpedback to the columnare called
“pumparounds.”These provide muchof the duty neededto heat the crude
oil fromstorage temperature to the crude column’s feed temperature.
Usually the kerosine,diesel,and gas-oil components fromthe crude tower
are cooledandsent to otherrefinery units forupgradingto marketable
products.The naphtha or gasoline range cut is producedfromthe tower
as a vaporand condensedvia exchange withcoldcrude andby airand/or
wateroverhead condensers.
Some ofthis condensedliquidr eturns to the towertoptray as reflux;the
remainder(overhead liquidproduct) flows to a naphtha stabilizer 10 .The
stabilizerremoves nearly all the butanes andlighter materials to lowerthe
naphtha vaporpressure so thatit canbe storedinatmospherictanks.
Overhead liquid fromthe naphtha stabilizer consists mainly of methane,
ethane,propane and butanes, which flow to the refinery gas plant. The
naphtha stabilizer may have a vapor product,whichis sentto the sour
fuel-gas system.Common practice is to send the stabilizednaphtha to a
naphtha splitter 11 to create a lightnaphtha stream low inheptanes and
a heavy naphtha streamlow in hexanes.Lightnaphtha canbe sentto an
isomerization unit, and the heavy naphtha is typically desulfurized and
usedas catalyticreformer feed.
Toppedcrude leavingthe bottomofthe atmospherictower contains a large
amount of valuable components. Because the distillation temperature
required to recover these components atatmospheric pressure is higher
thanthat where thermal decomposition occurs, furtherheating and
distillationare conductedunder vacuumconditions.
Toppedcrude is heatedinthe vacuumfurnace 12 to about750° F.Because
the vacuumconditions create very low vapor density andcorresponding
highvaporvolume flows,the vacuumtower 13 has a distinctively large
diameterto provide sufficientcross sectional area for the vaportraffic.
Similar to the crude tower, the vacuumtoweruses pumparounds to
condense liquidproducts while recovering valuable heatto the crude oil
charge.Packing inthe vacuumtower serves a similarpurpos e as trays in
the crude columnbutata muchlowerpressure drop.
The vacuumtowerdoes not have a traditional overhead condenserand
does nottypically use side strippers.A vacuum is maintainedwithsteam
jetejectors 14 followedby water-cooledsteamcondensers.In some
cases a liquid-ringvacuumpumpis used forthe thirdstage of
the vacuumsystem to reduce steam use and waste water
generation.
Because the heavy crude fraction contains metal complexes
(asphaltenes andporphyrines) that are catalyst poisons for
downstreamprocesses, circulating“wash oil”above the flash
zone minimizes entrainmentofheavy fractiondroplets up the
column.Althougha single cutof vacuumgas oil (VGO) is used
insome cases,drawinglightvacuumgas oil (LVGO) and heavy
vacuum gas oil (HVGO) separately allows forincreased heat
recovery to the crude oil because the HVGO draw temperature
is 200-250°F.higher thanthe draw temperature ofa single VGO
cut. It is commonto draw separate LVGO andHVGO products
fromthe vacuumtoweran dimmediately combine these streams
whenthey leave the unit.
BothLVGO andHVGO typically feedthe fluidcatalyticcracking
(FCC) unit,hydrocrackingunit,or both.As notedpreviously,
mostnew designs include provisions to yield a diesel product
above the LVGO draw.The vacuum towerbottoms (VTB) can
be blendedinto residual fuels orusedas feedto a delayed
coker, visbreaker,vacuum residhydroprocessing, or
asphaltunit.
Contentby the Mustangtechnical team:Ed Palmer,Process EngineeringManager,
JulianMigliavacca,Sr.Technical Professional,Shih-HsinKao,Sr.Technical Professional,
Nicholas Perry,Piping Design,andTara Johnson,MarketingCommunications Coordinator
DistributedinpartnershipwithNational Petrochemical &Refiners Association(NPRA)
Artwork &rendering:BeauBrown, Industrial3d.com
Graphiccoordination:Chris Jones,XenonGroup| xenongroupdesign.com
www.npra.org
Noot_OGJpos_090316 1 / 3/09 9:55:49 AM
Independent Advice. Global Reach.Independent Advice. Global Reach.
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A SMART Refinery: Your Pathway to the Top QuartileSMART Fractionation • SMART Rotating Equipment Reliability • SMART Process Safety
Learn more at www.SmartRefinery.com
E ePro_OGJPos_090316 1 /1 /09 11:3 :37 AM
il lb_OGJpos_090316 1 /1 /09 11:04:38 AM a ox4u_OGJpos_090316 1 / /09 4:16:58 PM Techn_ Jpos_090316 1 2/11/09 3:33:14
NPRA speaksfor the petrochemicalandreningindustriesonissuesimportant totheir business. We seektoinformpolicy-makersand the public how these industrieshelp improve their lives, strengthentheeconomy, protect the environment andpromote nationalsecurity.
Shaping a Secure Energy Future
visitus at: www.npra.org
NRA_OGJpos_090316 1 /1 /09 11:35:45 AM
Better desalting for
challenging crudes Producing Solutions
www.NATCOGroup.com 713.849.7500
Natco_OGJpos_090316 1 /17/09 11:56: 8 AM
www.recip .com/ ap i618
urCo _OGJpos_090316 1 /16/09 9:50:54 AM EveEne_OGJpos_090316 1 /1 /09 11:37:36 AM
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Reproductionofthe contents ofthis poster,inany manner,is prohibitedwithoutthe consentofPennWell Corp.©2009
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Crude/watermixture inlet
Desaltedcrude outlet
Water/brineoutlet
Crude outletcollectorheader
Crude/waterinletdistributor
Brine outlet
Transformer Transformer
Upperelectrode grid
Lowerelectrode grid
3
2
9
8
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www.burnsmcd.com
For moreinformation contact Warren Kennedy• (816)822-3384• [email protected]
Design and EPC Services for the Rening Industry
urMc_OGJpos_090613 1 1/ 3/09 :05:30 PM
Image fromKoch-Glitsch,LP
