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OIL & GASJOURNAL 1455WestLoopSouth,Suite400 Houston,TX 77027 www.ogjonline.com ® Any Project, Any Size. Concept Studies FEED Detailed Design Project Management Startup Revamps www.mustangeng.com Must_OS aps_0903 1 /18/09 4:11:56PM Modern Refinery Editorial d irectionand coordinationfrom DavidNakamura,Refining/PetrochemicalEditor WarrenR. True,ChiefTechnology Ed itor Modern Refinery: Crude Distillation 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. Atmo spheri cresidue. 750+°F. Atmo sphericresi dueisfurtherseparat edintodieselfuel,ligh tvacuumgas oil,heavyvacuumgasoil andvacuumresidueinthe vacuu mdistillat ion column.It isdifficulttorecoverallof thedieselfuelin theatmosphe ric disti llati oncolumn.Therefore , mostnew design s includ e a provis ionto recov era portio nof thisfract ioninthe vacuumcol umn.Thisisconsisten t withthelargeincr easeindemandfordieselfueldurin gthelastfewyears. Mostofthe abovepetrol eumfractio nscan besold direc tlyto endusers, altho ughfurtherproc essinginrefine rydownstrea munitsusuallyoccu rsto incre asethevalueof thefinalproduct .Crudequalitydictate sthe levelof furthe rprocessingnec essarytoachie vetheoptimalmixofproductout put. Process description Crude oftencontains water,inorganic salts,suspend edsolids, and water- solubl e trace metals.Electrosta tic desalt ing remove s most of these contaminant s to reduce downstream corros ion,plugging,and foulin g ofequipmentand to preventpoisonin g ofcatalysts inthe downstream conversionunits. Toremo vesaltsandsuspen dedsolids ,crudeismixedwith3-10vol%water andheate dto215-300°F.inwhatistypical lycalledthe“co ldpreheattrai n” 1 .Thisistheheat-exch angenetworkupstrea mofthedesalters.Hotcru de oilandwaterareforc edtomixbyspecialmixingval ves 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. Surfac tants 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. Coll ectedwater(brine)contai ns95-99% ofthe crude’ s saltcontentand flowstothe wastewat er-tr eatmentplant.Depe ndingoncrudetypeandthe requi reddegreeofdesalt ing,twostag esofdesaltin gmaybe neces saryand mayutilizemultiple typesof electricfields. Desaltedcrude entersanother heat-exchangernetwork,typically calledthe “hotpreheattrain” 4 .Bothhotandcoldprehea ttrainsusewastehea tfrom multipleproductand “pumparound”streams (definedto theright) thatneed cool ingaspartofthe proce ss. Preheatedcrude enters the crude furnace 5 ,where itis furtherheatedto about650-700° F.before enteringthe crude tower 6 . Parti allyvaporiz edcrude isfed intothe feedregionof thecrudetower, knownasthe“flashzone” 7 ,wherevaporandliquidseparat e.Thevapor leavi ng theflash zoneincludesall thecomponent s thatcomprisethe produc ts,whiletheliquidcontainstheatmospher icresiduewitha small amoun tof compo nentsintheatmospheri cgas-oilproductrange .Someof thesecomponent sareremovedfromtheresiduebysteamstrippin gatthe 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 smal l amou ntof highe r-bo ilingcompon entsin thevapor cond ense,whil ea smallamou ntof lowe r-boi lingcompo nentsintheliquid vaporize.Thisprocess(distillation)causesthelightcomponentsto concentrate inthevaporand heavycomp onent sto conce ntrat ein theliquid.Theliqui d 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“atmosph erictowerbottoms”(ATB).Atsuccessivel y higherpoints in the tower,majorproduct s 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 arrange ments inwhich some of the column liqui d is withdrawn,cooled,and pumpedback to the columnare calle d “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 marketabl e produc ts.The naphtha or gasoli ne range cut is producedfromthe tower as a vaporand conden sedvia 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 l owerthe naphth a vaporpressur e so thatit canbe storedinatmosp herictanks . Overhead liquid fromthe naphtha stabilizer consists mainly of methane, ethane,propane and butanes, which flow to the refinery gas plant. The naphth a stabilize r may have a vapor produc t,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 strea mlow in hexane s.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 reco ver these components atatmospheric pressure is higher thanthat where t hermal decomp ositi on occurs , furtherheating and distillationare conductedunder vacuumconditions. Toppedcrude is heatedinthe vacuumfurnace 12 to about750° F.Because the vacuumconditions create very low vapor density andcorresponding highvaporvolumeflows,thevacuumtower 13 has a distinctively large diameterto provide sufficientcross sectional area for the vaportraffic. Simil ar 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 thecrudecolumnbutata muchlowerpress uredrop. The vacuumtowerdoes not have a tradit ional overhea d conden serand does nottypically use side strippers.A vacuumis maintainedwithsteam jetejectors 14 follo wedby water -cool edsteamcondensers .In some cases a liqui d-ringvacuumpumpis used forthe thirdstage of the vacuumsystem to reduce st eam use and waste wa ter generation. Because the heavy crude fraction contains metal complexes (asphaltenes andporphyrines) that are catalyst poisons f or downstreamprocesses, circulating“wash oil”above the flash zone minimi zes entrainmentofheavy fractiondroplets up the column .Althougha single cutof vacuu mgas 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,hydrocrac kingunit,or both.As notedprevious ly, mostnew designs include provisions to yield a diesel product above the LVGO draw.The vacuumtowerbottoms (VTB) can be blendedinto residual fuels orusedas feedto a delayed coker , visbre aker,vacuum residhydroprocess ing, or asphaltunit. Contentby the Mustangtechnical team:Ed Palmer,Process EngineeringManager , JulianMigliavacca,Sr.Technical Professional,Shih-HsinKao,Sr.Technical Professiona l, Nicholas Perry,Piping Design,andTara Johnson,MarketingCommunications Coordinator DistributedinpartnershipwithNationalPetrochemical&RefinersAssociation(NPRA) Artwork&rendering:BeauBrown, Industrial3d. com Graphiccoordination:Chris Jones,XenonGroup| xenongroupd esign.com www.npra.org Noot OGJpos 090316 1 / 3/09 9:5 5:49AM Independent Advice. Global Reach. Independent Advice. Global Reach. AMER+12812938200EMEA+441932242424ASIA+6567355488 www.kbcat.com EngineeringConsultingBusinessStrategyTraining Software K C_OGJpos_090316 1 / /09 4:19:15PM A SMART Refinery: Your Pathway to the Top Quartile SMART Fractionation • SMART Rotating Equipment Reliability • SMART Process Safety Learn more at www.SmartRefinery.com E ePro_OGJPos_090316 1 /1 /09 11:3:37AM illb_OGJpos_090316 1 /1 /09 11:04:38AM a ox4u_OGJpos_090316 1 / /09 4:16:58PM Techn_ Jpos_090316 1 2/11/09 3:33:14 NPRAspeaksforthe petrochemi caland reningindustriesonissuesimportantto their business . We seektoinformpolicy- makersandthepublichow theseindustries help improve their lives, strengthenthe economy, protect the environment and promote nationalsecurit y. Shaping a Secure Energy Future visitus at: www.npra.org NRA_OGJpos_090316 1 /1 /09 11:35:45AM Better desalting for challenging crudes Producing Solutions www.NATCOGroup.com 713.849.7500 Natco_OGJpos_090316 1 /17/09 11:56: 8AM www.recip.com/api618 urCo _OGJpos_090316 1 /16/09 9:50:54AM EveEne_OGJpos_090316 1 /1 / 09 11:37:36AM 1 6 11 10 4 3 3 12 13 14 14 5 5 Back copies of this postermay be obtainedfromPennWell.To order:call 713/963-62 10;fax 713/963-6228; e-mail posters@pennw ell.com;orsubmitrequestto 1455WestLoopSouth,Suite 400,Houston,TX 77027. Reproductio nofthe contents ofthis poster,inany manner,is prohibitedwithoutthe consentofPennWell Corp.©2009 6 www.mustangeng.com Crude/water mixtureinlet Desalted crude outlet Water/brine outlet Crude outletcollectorheader Crude/waterinletdistributor Brine outlet Transformer Transformer Upper electrodegrid Lower electrodegrid 3 2 9 8 7 www.burnsmcd.com For moreinformation contact Warren Kennedy• (816)822-3384• wkenned@burn smcd.com Design and EPC Services for the Rening Industry urMc_OGJpos_090613 1 1/ 3/09 : 05:30PM Image fromKoch-Glitsch,LP
