Designing a crude unit heat exchanger network - CB&I

CDU/VDU run length as well asdownstream unit reliability. A highdesalted crude chloride contentincreases crude unit corrosion and,in some cases, can reduce thedownstream hydrotreater and cokerrun length, as well as increasemaintenance costs. In the short run,it is possible to have poor desalterperformance and be profitable;however, unscheduled outagesand/or loss of containment cancause major profit losses and possiblymuch worse.The cold train heats the crudefrom the storage tanks to thedesalter through seasonal changesin raw crude temperatures. Forexample, Canadian crude oil pipelinetemperatures vary seasonallyfrom 20-40°C, with the optimumdesalter temperature varying from120-140°C, depending on the crudeblend. The amount of cold trainduty that needs to be shifted tomeet the wide range of desaltertemperatures, while also handlingthe variable raw crude temperature,is very large. This is a major challengebecause of the large amountof swing heat that must be movedbefore and after the desalter.Identifying services that allowheat adjustments upstream anddownstream of the desalter is critical.Typically, the crude columnkerosene pumparound, vacuum gasoil product, diesel product andsometimes vacuum bottoms whenit is being run down to storage atlow temperatures are good candidates.Exchanger services thatprovide swing heat should haveflow-controlled bypasses so thatadjustments can be made as needed.Some Canadian crude blends havelarge middle distillate productyields, whereas others produce highpercentages of VGO. Column heatremoval must be sufficient to dealwith these variations while meetingdesalter and product rundowntemperatures. Preheat system flexibilityis essential.Low-fouling exchanger designshould be an objective, while theoutdated practice of designing forallowable pressure, which leads tolow velocity and high fouling,should be discarded. Old rules-ofthumbthat require the dirtierservice on the tube side for ease ofcleaning no longer apply. For example,placing vacuum residue on thetube side will result in poorexchanger design with a high pressuredrop and high fouling. Placingvacuum residue on the highvelocityshell side of theHelixChanger heat exchanger willprovide a higher heat transfer coefficient,less surface area, lowerfouling and less cleaning.CDU/VDU process flow schemeSelecting the right process flowscheme for a crude unit can have alarge impact on preheat train designoptimisation and unit reliability.Unfortunately, there is no computerprogram that determines the optimumflow scheme. It is determinedfrom experience and thoughtfulevaluation of distillation columnheat and material balance requirementsdictated by crude blends andtheir variability. Other factors, suchas the mitigation of a high-risk,high-corrosion surface area andcrude tower stability, are alsoimportant.Three areas in which the designerpositively influences the preheattrain are: energy recovery from thetop of the crude tower, the numberof crude tower pumparounds andtheir location, and the number ofvacuum column product draws.Selecting the right flow scheme, inmany cases, can significantly reduceexchanger surface area requirements.In other instances, selectingthe wrong flow scheme for the sakeof network optimisation can destroyunit reliability and profitability.Refiners processing opportunitycrudes have learned this the hardway. Many have experienced shortrun lengths or periodically mustreduce the crude charge rate toclean rapidly fouling exchangers.Others have had extremely highcorrosion rates in the crude overheadsystem, causing unscheduledoutages. Clearly, the flow schemematters.Many heavy Canadian crudescontain large portions of naphthaused as diluent. In these cases, it isnecessary to recover some low-levelheat in the crude column overhead,where raw crude is heated withoverhead vapour. However, thecrude overhead exchanger is ahigh-fouling and often severe corrosionservice. Since this is a high-riskexchanger, the design shouldmaximise heat recovery with minimalsurface area.When crude overhead exchangersare used, viscous crude should berouted through the shell side tominimise the surface area. This israrely done, because many designersstill believe raw crude shouldalways be in the tubes for ease ofcleaning. However, it is nearlyimpossible to get a reasonableheat transfer coefficient with thehighly viscous crude on the tubeside. In fact, the flow regime isoften laminar, resulting in largesurface area requirements with ahigh pressure drop. Shell-sidedesign can be further improved byusing helical baffles to minimisedead areas and maximise theconversion of pressure drop to heattransfer. To keep the exchangerclean, it is important to targetvelocities of 1.5-2.4 m/s on the shellside of the exchanger.When crude is put on the shellside, the exchanger must bemounted vertically so that the crudeoverhead can be water washedeffectively to minimise corrosion.Crude overhead exchanger corrosionis one of the most commoncauses of unscheduled outages dueto tube failures. Good desalting andan effective water wash system areessential for crude overhead corrosioncontrol.Horizontal exchangers, withcrude routed through the tubesand crude overhead condensing onthe shell, have a proven trackrecord of high fouling and arevirtually assured of high corrosionrates when processing heavycrudes. It is simply not possible tothoroughly water wash the shellside of conventional exchangerbundles, with inherent “dead”areas that are nearly impossible toreach with water wash. It isalso very difficult to effectivelywater wash the crude overheadstream when it is on the tube sideof a horizontal exchanger.However, a vertical exchanger withcrude overhead condensing on thewww.eptq.com Sour & Heavy 2012 5

CondensersCondensersTop PATop PAKeroseneproductKerosenePAKerosenePAKeroseneproductDieselproductAGOproductDieselPAVery lowreflux rateAGO PADieselPAHigh refluxRemove PAHigher dieselproduct yieldMorefractionationtraysLoweroverflashCrudechargeLow efficiencylow strtippingsteamCrudechargeImprovedefficiencyHigherstripping steamHigh amountof 200-316ºChydrocarbonVery low amountof 200-316ºChydrocarbonFigure 2 Crude tower pumparoundstube side has no dead areas andcan be effectively water washed.A top pumparound can be addedto further reduce the amount ofhigh-risk surface area in the crudeoverhead system. This is especiallyimportant with heavy Canadiancrude processing. Without a toppumparound all of the reflux forthe top tray must be condensed inthe crude overhead exchangers.With a top pumparound to supplythe reflux the crude overheadexchangers will only condense theproduct. To be effective, the toppumparound return temperaturemust be high enough to avoidsublimation of amine salts andwater condensation in the top ofthe crude tower.Crude column pumparound andproduct streams can be drawn fromthe same location or from differentlocations in the column. Forexample, some designers will drawdiesel product and diesel pumparoundfrom different locations.While this provides some benefits indraw temperature, it can adverselyaffect fractionation. Low liquid rateson fractionation trays can ultimatelylead to product draws that dry outat certain conditions, resulting inunstable operation. When thisoccurs, as it frequently does, thebenefits of split draw are diminished.Crude units designed toprocess a wide range or variablecrude blends should draw productand pumparound from the samelocation in the column.Atmospheric gas oil (AGO)pumparounds (see Figure 2) areonly warranted when the crudeblend is light enough to generatehigh lift in the crude tower andthen provide sufficient refluxbetween diesel and AGO productfor good fractionation. If an AGOpumparound is used with heavycrude, the diesel/AGO productfractionation is poor, resulting inexcessive diesel boiling range materialin the AGO product. However,this is often overlooked in favour ofthe high draw temperature associatedwith AGO pumparounds.Most vacuum towers are designedwith light (LVGO) and heavyvacuum gas oil (HVGO) products,which sets the amount of heat availableat a given temperature level.Two-product vacuum towersproduce draw temperatures ofapproximately 145°C and 270°C forLVGO and HVGO streams, respectively.Most, if not all, of thelow-level LVGO pumparound heatis lost to air or cooling water becausethere are not many heat sinks for thelow draw temperature. HVGO productheat can be recovered into crude,waste heat steam generation or toreboil light ends towers. Therequired HVGO pumparound rate6 Sour & Heavy 2012 www.eptq.com

CWCWCrude250ºC270ºCCrudeCrude320ºCReducedcrudeReducedcrudeFigure 3 Vacuum tower MVGO pumparoundand exchanger surface area are typicallyvery high with two productvacuum towers due to a relativelylow temperature and high duty.Adding medium vacuum gas oil(MVGO) product produces threetemperature levels (see Figure 3),minimises heat loss to air andwater, maximises the heat recoveredinto crude preheat, andreduces the overall surface area.MVGO and HVGO pumparoundstreams are approximately 250°Cand 320°C, respectively, dependingon the product split betweenMVGO and HVGO. The higherHVGO draw temperature reducesthe number of exchanger shells andsurface area compared to only anHVGO pumparound.Figure 4 shows a fully optimisedvacuum tower producing diesel,LVGO, MVGO and HVGO productstreams. This maximises theproduction of high-value dieselboiling range material from theCDU/VDU and optimises both theMVGO and HVGO pumparounddraw temperature for a givenamount of recoverable heat.Drawing LVGO product from thebottom of the fractionation bedfurther increases the MVGO drawtemperature, reducing the capitaland operating costs to recover it.Low-fouling designFouling is a layer that accumulateson the inside and outside of thetubes, reducing heat transfer. Thehigher the fouling resistance, thelower the heat transfer. The foulingresistance can be between 50-85%of the total resistance for a heavilyfouled exchanger. To compensatefor high fouling, more area isneeded; however, adding more areacan be counterproductive because itgenerally results in lower velocityand higher fouling.Crude preheat exchangers can besusceptible to very high fouling.Fouling factors as high as 0.01hr-m 2 -°C/Kcal have been backcalculatedfrom operating data.Exchanger design and selection areimportant and can have a significantimpact on fouling. Lowvelocity designs result in high fouling,even for light crudes. Awww.eptq.com Sour & Heavy 2012 7

FeedStrippingsteamFigure 4 Optimised four product vacuum towerproperly designed exchanger withhigh tube- and shell-side velocitieswill minimise fouling. Certainheavy crude oils are incompatiblewhen mixed together, causingasphaltenes to precipitate. Thesecrude oils are highly unstable, so itis very important to design theexchangers for high velocity.Exchanger design guidelinesThe designer cannot control thecrude fouling tendency, only theexchanger design and exchangertype. For new designs, exchangerscan be designed with high velocityand reasonably low fouling factors,which results in a minimal excesssurface area. However, for revamps,it is not always possible to rectifyall low-velocity designs andCrudeCrudeCrudeCrudeEjectorsDieselLVGOMVGOHVGOVacuum towerbottomsrealistic fouling factors must bedetermined from actual plant dataso that future performance can bepredicted.VelocityLow tube velocities result in highfouling. To minimise fouling, velocitiesare ideally kept above 2.4 m/sand sometimes higher on the tubeside and between 1.2-2.4 m/s onthe shell side. High shell-side velocitiesare achievable with advancedbundle designs, such as that of theHelixChanger heat exchanger,whereas a high shell-side velocity isnot practical with standard segmentalbaffles.For a new design, when crude isplaced on the tube side, a two-passexchanger with 1in tubes should beused. The number of tubes neededto obtain 2.4 m/s sets the shell’sinside diameter. The length of theexchanger or number of shells isadjusted to meet surface arearequirements. Increasing the shell’sinside diameter and the number oftubes to meet the surface arearequirements reduces the tube- andshell-side velocities and increasesfouling. Designers will sometimesenlarge the shell’s inside diameterto reduce the number of shells toreduce costs. However, this designresults in lower velocity, more foulingand higher lifecycle costs thatare greater than the originalsavings.For a new design, a 1in tube withtwo passes minimises the pressuredrop. A typical two-pass crudeexchanger with crude on the tubeside at 2.4 m/s will have less than0.7 kg/cm 2 pressure drop per shell.One-inch tubes are preferred over0.75in tubes because the pressuredrop per metre of tube is lower atthe same velocity.For grassroots crude units, a lowfoulingdesign can be incorporatedinto the pump head specifications.In revamps, exchanger velocitiesare often limited by existing pumpsize, pipe flange ratings andexchanger design pressure. Pumpsystem hydraulics must be evaluatedcarefully for each circuit todetermine opportunities to increasevelocity and reduce fouling. A lowfouling design is not always possiblein a revamp because of existingconstraints.Shell-side velocity is limited bythe bundle’s inside diameter, andbaffle geometry and spacing. Forexample, velocities much higherthan 0.62 m/s are not practical in asegmental baffled exchanger. Toincrease shell-side velocity, bafflespacing and cut must be reduced.A higher pressure drop generallyincreases flow in the leakage andbypass areas of the bundle. Poorlymatched baffle spacing and cuts canalso lead to large eddies and “dead”zones.Advanced baffle designs such asthat used in the LummusTechnology HelixChanger designuse quadrant-shaped baffles at anangle to create a helical flow pattern8 Sour & Heavy 2012 www.eptq.com

through the bundle. This flowpattern reduces dead areas andresults in a lower pressure drop forthe same velocity compared with asegmental baffled exchanger. Thebenefit is that the shell-side velocitycan be designed for 1.5-2.48 m/s.Crude preheat exchangers designedwith high velocity on both the shelland tube side of a HelixChangerhave demonstrated extremely lowfouling in heavy crude service.Is the pressure drop really lower?Fouling begins as soon as anexchanger starts up and continuesuntil the terminal velocity isreached. After the terminal velocityis reached, fouling continues but ata much slower rate. Low-velocityexchangers foul rapidly, sometimesreaching the asymptotic foulinglevel within the first 6-12 months ofoperation. However, most crudeunits are being operated for four tosix years between planned turnarounds.A fouled pressure drop issignificantly higher than a cleanpressure drop for low-velocityexchangers. It is not uncommon forthe fouled pressure drop to be twoto three times higher than the cleanpressure drop.An exchanger designed for highvelocity results in a higher initialclean pressure drop. However, ahigh-velocity exchanger’s fouledpressure drop is less than 1.5 timesthe clean pressure drop. So, doesdesigning a low velocity reallyresult in less pressure drop?This is especially true for crudepreheat trains with multipleexchangers in series. Designing forhigh velocity will appear to add topumping costs at first; however,this is not necessarily true afterfactoring in the significantly higherfouled pressure drop of the lowvelocitydesign. The problem is thatthere is no way to calculate fouledpressure drop. Most designers donot get feedback from their designsand they do not go to the field tomeasure pressure drop. If they did,they would appreciate the differencesin fouled pressure dropbetween a low-velocity and highvelocitydesign.It is slowly becoming acceptedthat a high-velocity (high shearFigure 5 HelixChanger tube bundle in fabricationstress) design is the main variableneeded to produce a low-foulingdesign; however, there is still reluctanceto specify exchangers thisway because of a higher initial pressuredrop and corresponding higherpumping costs. What is notnormally factored in is the fuelcosts of a low-velocity, high-foulingdesign, not to mention the highermaintenance costs associated withmore frequent cleanings.HelixChanger heat exchangeradvantageThe inherent deficiencies of conventionalsegmental baffle shell-and-tubeexchangers are widely understood.The key deficiencies are:• The shell-side region is compartmentalised.Pressure energy iswasted in expansions, contractionsand turnarounds in multiple bendsrather than in generating heat transfer.The pressure gradient acrossthe baffles drives a significantamount of flow through the tubeto-baffleand shell-to-baffleclearances that escapes heat transfer.The result is inefficientconversion of shell-side pressuredrop to heat transfer• The flow leakage streams distortthe temperature profile, reducingthe effective mean temperaturedifference (MTD) for heat transfer• The perpendicular baffles encouragedead spots or recirculationzones where fouling or corrosioncould occur.The HelixChanger design removesmost of the above deficiencies.Courtesy: Lummus Technology Heat TransferQuadrant-shaped baffle segments,arranged at an angle to the tubeaxis in a sequential pattern, guidethe shell-side fluid in a helical paththrough the tube bundle. Figure 5shows a tube bundle in fabrication.The baffle segments serve as guidevanes without any compartmentalisation,and the flow traverses onboth sides of the baffles. The helicalflow path through the bundleprovides the necessary characteristicsto reduce flow dispersion andgenerate near plug-flow conditions,resulting in high thermal effectiveness.It ensures a certain amount ofcross-flow to the tubes to achievehigh heat transfer coefficients.Uniform flow velocities areachieved through the tube bundle,and the smooth helical flow eliminatesunnecessary pressure lossesin the exchanger. There is alsonegligible dead volume in the helicalshell space.Reduced fouling characteristicsThe design offers reduced foulingcharacteristics for the followingreasons:• There are few dead spaces withinthe helical shell space• The helical flow velocitiesachieved are significantly higher(1.5-3 times higher) than the averagecross-flow velocities achievedin equivalent segmental baffledesigns. Thus, the shear forcesacting on the tube wall are significantlyhigher in the HelixChangerdesigns• Uniform flow velocities throughwww.eptq.com Sour & Heavy 2012 9

pipelines, is also required.Designing for high velocity andusing advanced exchanger technologyhas proven that low-foulingdesigns are possible (see Figure 6).HELIXCHANGER is a mark of LummusTechnology Heat Transfer.Figure 6 HelixChanger bundle after two years of operation in hot crude vs hot resid servicethe tube-bundle are achieved due tothe relatively constant helical flowarea. This also translates into moreuniform tube-wall temperatures.ConclusionPreheat train design for heavyCanadian crudes can be verychallenging. Different requirementssuch as the need to vary desaltertemperature dictate a differentapproach not normally requiredwith other crudes. A well-conceiveddesign, with the flexibility to handlethe variable composition of diluentsbeing used to transport the crude inTony Barletta is a Chemical Engineer withProcess Consulting Services, Inc, in Houston,Texas. His primary responsibilities areconceptual process design and process designpackages for large capital revamps.Email: tbarletta@revamps.comSteve White is a Chemical Engineer withProcess Consulting Services, Inc, in Houston,Texas. He has more than 30 years of processdesign experience for refinery revamps andgrassroots units. Email: swhite@revamps.comKris Chunangad is Director of Heat Exchangersat Lummus Technology Heat Transfer, a CB&Icompany in Bloomfield, New Jersey. He hasmore than 18 years of experience in the designand development of advanced heat exchangerequipment, as well as their specialisedapplication in the oil and gas, refining,petrochemical and chemical industries.Email: kchunangad@cbi.comHELICAL BAFFLES AND hiTRAN TUBESIDEENHANCEMENT REDUCES CRUDE SHELLAND TUBE COUNT FROM 9 TO 2 ON FPSOImage courtesy of Total. - Gonsalez ThierryDesigning in partnership, Calgavin and Lummus Technologydeliver high performance compact shell and tube exchangersreducing weight from 400 tonnes to just 130 tonnes.Challenged with minimum space requirement and maximumefficiency, the 2x 4MW exchangers operate at 8x tube-sideheat transfer co-efficient and deliver long run times throughminimal fouling.CONVENTIONALSHELL AND TUBECOMPACT, ENHANCEDSHELL AND TUBE‘HELI-TRAN’EXCHANGERSPLAIN / SINGLESEGMENTAL BAFFLEhiTRAN /HELICAL BAFFLEGAINOHTC [W/m 2 k] 59.9 242.7 4XTUBE SIDEHTC [W/m 2 k] 95 770 8XDP [bar] (ALLOWED 1.50) 1.40 1.50 -SHELL SIDEHTC [W/m 2 k] 455 789 ~ 2DP [bar] (ALLOWED 1.50) 1.50 1.25 -GEOMETRYTOTAL NO. OF SHELLS [-] 9 2 -7TOTAL HT AREA [m 2 ] 6420 1704 ~ 1/4PLOT SPACE [m 2 ] 104.5 26.2 ~ 1/4WEIGHT WET [kg] 401148 130716 ~ 1/3EXCHANGER COSTS [%] 100 35 ~ 1/3RESEARCH. EVALUATION. INNOVATION. SOLUTION.talk to our engineers +44 (0) 1789 40040110 Sour & Heavy 2012 www.eptq.com