7/27/2019 Crude Distillation Poster
1/1
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Modern Refinery
Editorialdirectionand coordinationfrom
DavidNakamura,Refining/PetrochemicalEditor
WarrenR. True,Chief Technology Editor
Modern Refinery:Crude Distillation
Crude units are the first to process petroleuminanoil
refinery.Crude oil,as
producedin the oilfield,is a complex mixture of hydrocarbons
rangingfrom
methane to asphalt,with varyingpr oportions of
paraffins,naphthenes, and
aromatics.The objective of crude atmosphericdistillation is to
separate,
orfractionate, crude oilinto severalcomponents of materials
withsimilar
properties accordingto boilingpoint. 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.
Atmosphericresidue isfurther separatedinto dieselfuel,
lightvacuum gas
oil,heavyvacuumgasoil andvacuumresidueinthe
vacuumdistillation
column.It isdifficulttorecoverallof thedieselfuelin
theatmospheric
distillationcolumn.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
of equipment andto prevent poisoningof catalysts in the
downstream
conversionunits.
Toremovesaltsandsuspendedsolids,crudeismixedwith3-10vol%water
andheatedto215-300F.inwhatistypicallycalledthecoldpreheattrain
1
.Thisistheheat-exchangenetworkupstreamofthedesalters.Hotcrude
oilandwaterareforcedtomixbyspecialmixingvalves 2 orstatic
mixers.
The mixture thenenters the desaltervessel 3 where anelectric
fieldaids
inseparating the oiland waterphases. The electricfieldcauses the
charged
saltwaterparticles to agglomerate andfall by gravity to the
vessels bottom.
Surfactants canbe added to aidseparation,suchas whenthe crude
has
a large amount of suspended solids orcertain chemicalcompounds
that
encourage emulsion formation. Chemical surfactants
(demulsifiers) aid
salts andother impurities to dissolve into the wateror attachto
the water
to make theirremoval easier.
Collectedwater(brine)contains95-99% ofthe crudes
saltcontentand
flowstothe
wastewater-treatmentplant.Dependingoncrudetypeandthe
requireddegreeofdesalting,twostagesofdesaltingmaybe
necessaryand
mayutilizemultiple typesof electricfields.
Desaltedcrude entersanother heat-exchangernetwork,typically
calledthe
hotpreheattrain
4 .Bothhotandcoldpreheattrainsusewasteheatfrom
multipleproductand pumparoundstreams (definedto theright)
thatneed
coolingaspartofthe process.
Preheatedcrude enters the crude furnace 5 ,where it is
furtherheated to
about 650-700 F.before enteringthe crude tower 6
.
Partiallyvaporizedcrude isfed intothe feedregionof
thecrudetower,
knownastheflashzone 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.
At each tray,a small amountof higher-boilingcomponents in the
vapor
condense,whilea smallamountof
lower-boilingcomponentsintheliquid
vaporize.Thisprocess(distillation)causesthelightcomponentsto
concentrate
inthevaporand heavycomponentsto concentratein
theliquid.Theliquid
drainsfromeach trayviadowncomersto thetray
below,wheretheprocess
isrepeatedover enoughtrays tomeet
product-purityrequirements.
The crude towerbottoms product is knownby several names
including
toppedcrude, atmosphericresid, atmosphericreduced crude(ARC)
,
andatmospheric towerbottoms (ATB).At successively higher points
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 much of the duty neededto heat the
crude
oilfrom storage temperature to the crude columns
feedtemperature.
Usually the kerosine,diesel, andgas-oil components fromthe crude
tower
are cooledandsent to otherrefinery units for upgradingto
marketable
products.The naphtha or gasoline range cut is producedfrom the
tower
as a vaporand condensedvia exchange withcoldcrude andby
airand/or
wateroverhead condensers.
Some of this condensedliquid returns to the towertop tray as
reflux;the
remainder(overhead liquidproduct) flows to a naphtha stabilizer
10
.The
stabilizerremoves nearly allthe butanes andlightermaterials to
lowerthe
naphtha vaporpressure so that it canbe storedin
atmospherictanks.
Overhead liquid fromthe naphtha stabilizerconsists mainly of
methane,
ethane,propane and butanes, which flow to the refinery gas
plant. The
naphtha stabilizermay have a vaporproduct, whichis sent to the
sour
fuel-gas system.Common practice is to send the stabilizednaphtha
to a
naphtha splitter 11 to create a light naphtha streamlow in
heptanes and
a heavy naphtha streamlow in hexanes. Light naphtha canbe sent
to an
isomerization unit, and the heavy naphtha is typically
desulfurized and
usedas catalyticreformer feed.
Toppedcrude leavingthe bottom of the atmospherictowercontains a
large
amount of valuable components.Because the distillation
temperature
required to recoverthese components at atmosphericpres sure is
higher
thanthat where thermaldecomposition occurs, furtherheating
and
distillationare conductedunder vacuumconditions.
Toppedcrude is heatedinthe vacuumfurnace
12 to about 750 F.Because
the vacuumconditions create very low vapor density
andcorresponding
highvaporvolume flows,the vacuumtower
13 has a distinctively large
diameterto provide sufficient cross sectionalarea for the
vaportraffic.
Similar to the crude tower, the vacuumtoweruses pumparounds
to
condense liquidp roducts while recoveringvaluable heat to the
crude oil
charge.Packing inthe vacuumtower serves a similarpurpos e as
trays in
the crude columnbut at a muchlower pressure drop.
The vacuumtowerdoes not have a traditionaloverhead
condenserand
does not typically use side strippers.A vacuumis maintainedwith
steam
jet ejectors
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 fractioncontains metal complexes
(asphaltenes and porphyrines) that are catalyst poisons for
downstreamprocesses, circulatingwash oilabove the flash
zone minimizes entrainment of heavy fractiondroplets up the
column.Althougha single cut of vacuumgas oil (VGO)is used
insome cases,drawinglight vacuumgas oil(LVGO)andheavy
vacuumgas oil (HVGO)separately allows forincreasedheat
recovery to the crude oil because the HVGO draw temperature
is 200-250 F.higherthan the draw temperature of a single VGO
cut. It is common to 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 noted previously,
most new designs include provisions to yielda dieselprodu ct
above the LVGO draw.The vacuumtower bottoms (VTB)can
be blendedinto residualfuels orused as feedto a delayed
coker, visbreaker,vacuum residhydroprocessing, or
asphalt unit.
Contentby the Mustangtechnicalteam:Ed Palmer,Process
EngineeringManager,
JulianMigliavacca,Sr.TechnicalProfessional,Shih-HsinKao,Sr.TechnicalProfessional,
Nicholas Perry,Piping Design,andTara
Johnson,MarketingCommunications Coordinator
DistributedinpartnershipwithNationalPetrochemical&Refiners
Association(NPRA)
Artwork &rendering:BeauBrown,Industrial3d.com
Graphiccoordination:Chris Jones,XenonGroup|
xenongroupdesign.com
www.npra.org
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public how these industrieshelpimprove their
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Back copies of this postermay be obtainedfromPennWell.To
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West LoopSouth,Suite 400,Houston,TX 77027.
Reproductionof the contents of this poster,inany manner,is
prohibitedwithout the consent of PennWellCorp.2009
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Crude/watermixtureinlet
Desaltedcrudeoutlet
Water/brineoutlet
Crudeoutlet collectorheader
Crude/waterinlet distributor
Brineoutlet
Transformer Transformer
Upperelectrodegrid
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Formoreinformation contactWarren Kennedy (816) 822-3384
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