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In mid-2006, thanks to the USEnvironmental Protection Agency(EPA), 80% of refiners’ on-road diesel
pool will have to meet a new ultra-lowsulphur diesel (ULSD) specification ofno more than 15wppm sulphur. Toensure this target is not exceeded at theultimate point of distribution, and toallow for some manufacturingflexibility, revamped or new facilitieswill need to achieve a desulphurisedproduct sulphur level significantlybelow the mandated target. Sulphurlimits and regulations of similar severityare being implemented in other areas ofthe world such as Europe and certainparts of Asia.
For an existing diesel hydrotreater, anumber of options will directionallyimprove sulphur removal, including:— Higher activity catalyst— Increased reactor temperature— Installation of high-efficiency reactorfeed distributors — Increased catalyst volume— Recycle gas scrubbing to removehydrogen sulphide.
Revamp design basisThe feed basis for this study is identicalto the original design and consists of ablend of two-thirds straight-run gas oiland one-third light-cycle oil (LCO).Table 1 shows the feed constituents andblended properties. Key design criteriafor the original facility are shown inTable 2. This includes existing reactor
operating pressure, catalyst volume andtreat gas rate. The design productsulphur for the original design was470wppm.
A simplified flow diagram of the basecases is shown in Figure 1. Other studybasis criteria were:— Product sulphur content of 8wppm— Utility systems such as steam, cooling
water, power, instrument/plant air, andthe pressure relief system have sufficientcapacity for the revamp— The existing DCS has capacity to
Hydrotreater revamps for ULSD fuel
Scope and capital investment for revamping an existing diesel hydrotreater to meetthe 15wppm sulphur standard. The base design is typical of hydrotreaters
commissioned in the early 1990s to meet on-road specifications of 500wppm sulphur
R E (Ed) Palmer Mustang Engineers and Constructors Inc, Houston, Texas, USASalvatore P Torrisi Criterion Catalysts & Technologies
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Diesel product
Strippingsteam
Naphtha
Sourwater
Tofuel gas
Purge
Reformerhydrogen
Ste
am s
trip
perA
min
e co
ntac
tor
Feed
Leanamine
Heater
Washwater
Existingreactor
Existing recycle andMU compressors
Rich amine
CHPSHHPS
Figure 1 Base case process flow diagram
Feed property SRGO LCO BlendBPSD 20 000 10 000 30 000API 34.5 20.0 29.4Sulphur, wt% 0.8 2.0 1.2N, wppm 100 500 242Total aromatics, vol% 20.0 65.0 35.0Mono-aromatics, vol% 12.8 23.0 16.2Poly-aromatics, vol% 7.2 42.0 18.7Bromine number 1.0 10.0 4.2D86 90 vol% Temp, °F 640 640 640Cetane index 49.9 30.3 42.6
Base design condition Base case Charge rate, BPSD 30 000Number of reactors 1Reactor inlet pressure, psig 760LHSV, hr-1 1.5Catalyst volume, ft3 4680Reactor outlet H2, psia 348Chemical H2, SCF/bbl 250SOR WABT, °F 670Cycle length, months 18Make-up hydrogen, SCF/bbl 290Treat gas rate, SCF/bbl 1400Quench rate, SCF/bbl 0Product sulphur content, wppm 470Recycle gas scrubber Yes
Table 1
Feed properties
Base case reactor conditions
Table 2
accommodate all new instrumentation— Plot space is available near thehydrotreater unit for new equipment — The refinery is currently consumingall available reformer hydrogen.Incremental hydrogen will be purchasedfrom a pipeline — Hydrotreating kerosene for cloudpoint control in the winter is notrequired— The existing hydrotreators are notoperating at charge rates significantlyabove the original design.
The original design treat gas rate of1400 SCFB was assumed for this study.Higher recycle rates, if hydraulicallyfeasible, would extend the catalystscycle or allow higher throughputs. Forthis study, Criterion Catalysts providedthe reaction conditions in Table 3 forseveral options required for theproduction of ULSD. All reactionconditions are based on an assumeddistribution of higher boiling sulphurcompounds. For a definitive design,pilot testing of the feedstock isrecommended. The base case reactionparameters are also shown forcomparison.
In all revamp cases, the currentcatalyst was replaced with Criterion’smost active desulphurisation catalyst,DC-2118. Three options wereconsidered. The first (Revamp A)consisted of purifying the reformerhydrogen with a pressure swing
adsorption (PSA) unit. Combined withthe purchased hydrogen, this resulted inthe total make-up volume having a
purity of 99.9 vol%. Revamp cases B andC consisted of adding incrementalcatalyst volume to the base case design.The incremental hydrogen volume wasalso supplied by pipeline purchases.Purification of the reformer make-upwas not considered.
Using the reaction conditionsspecified, computer simulations weredeveloped for each revamp alternative.Hydrogen make-up rates were calculatedbased on the sum of chemical hydrogenconsumption, solution losses and purgegas, if any, from the simulations.Additional process information for eachrevamp is presented in Table 4.
The simulation results were used torate existing equipment and determinewhat modifications were required aswell as the size and other designinformation for new items. All shell andtube exchangers, air coolers, pumps andfired heaters were rigorously rated.Pressure drop across each catalyst bedwas calculated using the Ergun(1)
equation with the Larkin(2) two-phasecorrection parameter. An allowance wasalso included for fouling. The resultinginformation was used to develophydraulic calculations across the reactorloop to establish an operating/designpressure and temperature profile, checkthe performance of the recyclecompressor, and to identify otherlimitations within the system.
Table 5 summarises the detailed
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Diesel product
Strippingsteam
Naphtha
Sourwater
Tofuel gas
Purge
PSAReformerhydrogenPurchased
hydrogen
Ste
am s
trip
perA
min
e co
ntac
tor
Feed
Leanamine
Heater
Washwater
Existingreactor
NewPSA
Existing recycleand MU
compressors
New boostercompressors
Rich amine
CHPSHHPS
Figure 2 Revamped hydrotreater with two new packed beds added to existing reactor,new MU compressor and introduction of purchased hydrogen
Reaction condition Base 1 Revamp A Revamp B Revamp CCharge rate, BPSD 30 000 30 000 30 000 30 000Reactor inlet, psig 760 760 760 760LHSV, hr-1 1.50 1.70 1.00 0.75Catalyst volume, ft3 4680 4150 7020 9360Catalyst type (Criterion) 448 DC-2118 DC-2118 DC-2118SOR WABT, °F 670 709 688 674Cycle length, months 18 10 18 27Chemical H2, SCF/bbl 250 33 284 323Treat gas rate, SCF/bbl 1400 1400 1400 1400Product sulphur content, wppm 470 8 8 8Recycle gas scrubber Yes Yes Yes Yes
Simulation results Base case Revamp 1A Revamp 1B Revamp 1CReactor outlet H2 PP, psia 348 575 403 430Make-up as 100 % H2, SCF/bbl 290 364 334 374Make-up purity, mole % H2 83.9 99.9 88.0 87.9Treat gas purity, mole % H2 70 93 76 76Quench rate, SCF/bbl 0 400 400 400Purge gas rate, SCF/bbl 14 0 38 38Reformer hydrogen, SCF/bbl 346 346 346 346Purchased hydrogen, SCF/bbl 0 74 44 84Total make-up, SCF/bbl 346 420 390 430
Table 3
Base case and revamp reaction conditions
Simulation results
Table 4
revamp scope for each case. Process flowdiagrams of the hydrotreater revampsare shown in Figures 2 and 3(equipment and lines shown in bold arenew or modified).
Revamp requirementsEach revamp case shares the followingcommon requirements, including state-of-the-art high-activity catalyst,increased hydrogen make-upcompression capacity and the additionof reactor quench.
All cases take advantage of the latestadvances in catalyst technology and arebased on using Criterion’s DC-2118CoMo catalyst. Catalyst beds wereassumed to be dense loaded with 1/20”catalyst to provide maximum cyclelength and to ensure even contactingbetween the catalyst and oil. Also toensure maximum cycle length, theexisting feed distributors were upgraded.Good feed distribution and propercatalyst installation is critical for theproduction of ULSD. Design and costinformation for new feed distributorsand inter bed quench internals wasprovided by Shell Global Solutions.
Producing ULSD requires a significantincrease in chemical hydrogenconsumption compared to what isneeded to make diesel meeting the500wppm specification. This is due toadditional, marginal saturation ofaromatics and the removal of sulphurfrom aromatic sulphur compounds. Toprovide the incremental make-upcapacity, a new spared reciprocatingbooster compressor is added upstreamof the existing first-stage make-upcylinders. Due to design pressurelimitations, the existing first-stagemake-up compressor intercooler andknock-out drum were re-piped toservice the new booster compressor.A new intercooler and knock-outdrum designed for a higher pressurewere specified to replace this
equipment. The existing make-upcylinders also required replacementbecause of the higher capacity andoperating pressure.
Another option to consider forproviding increased make-up is to addnew compression capacity in parallelwith the existing make-up compressors.The new machine would handlepurchased hydrogen, while the existingcompressors would still be dedicated toreformer off-gas. The new compressorwould require three stages, however, tocomply with API Standard 618, whichrecommends a maximum dischargetemperature of 275°F for reciprocatingcompressors in high-purity hydrogen
service. This is a result of the increasedheat capacity ratio associated with thehigher-purity hydrogen.
This would also be an issue for theexisting make-up compressors for CaseA, where the reformer hydrogen ispurified in a PSA unit. With the newcompressor installed in series with theexisting make-up machines, the totalsupply to the hydrotreater could befurnished with high-purity purchasedhydrogen when the reformer is out ofservice for catalyst regeneration ormaintenance. Due to the dischargetemperature limitations previouslynoted, this flexibility would be lost witha parallel make-up compressorinstallation.
The higher hydrogen consumptionresults in a significant increase inreaction heat for all cases and, thus,quench was added to limit thetemperature rise in the reactor andextend catalyst cycle life. In revampCase A, the existing reactors weremodified to include inter-bed quench bysplitting the catalyst volumes into twobeds. In the other cases where a newreactor is added in series, quench gas isadded between the reactors. Modifyingthe existing reactors for inter-bedquench requires four vertical feet thatotherwise would have been occupied bycatalyst. The loss of catalyst volume ismore than offset by quenching thereaction and limiting the overalltemperature rise of the reactor.
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Figure 3 Base revamp Case B and C flow diagram with modified flow from recyclecompressor into existing reactor
Diesel product
Strippingsteam
Naphtha
Sourwater
Tofuel gas
Purge
ReformerhydrogenPurchased
hydrogen
Ste
am s
trip
perA
min
e co
ntac
tor
Feed
Leanamine
Heater
Washwater
Existingreactor
Newreactor
Existing recycleand MU
compressors New boostercompressor
Rich amine
CHPSHHPS
Case A * Install Criterion DC-2118 catalyst* Add hydrogen booster compressor and replace cylinders in the existing
make-up compressor* Add quench to reactor* Add PSA Unit with tail gas surge drum and compressor* Revamp cost: $108 000 000
Case B * First three items in Revamp 1A plus* 50% more catalyst in a new reactor* Revamp cost: $8 100 000
Case C * First three items in Revamp 1A plus* 100% more catalyst in a new reactor* Revamp cost: $8 660 000
(Items in Italics are unique to that revamp option)
Table 5
Revamp scope
Utilising reactor quench directionallylowers the reactor loop pressure dropand heater duty for all revamp cases,because quench gas bypasses thefeed/effluent exchangers and chargeheater. This may allow for some increasein the recycle gas rate by operating thespare recycle compressor and furtherextending the cycle length. A moredetailed analysis of the reactor loophydraulics and unloading capabilities ofthe existing compressors would berequired to determine the magnitude ofthis improvement.
Upgrading the catalyst alone is notsufficient to meet the lower sulphurspecification. As noted previously, inrevamp case A the reformer make-uphydrogen is purified using a PSA unit,which concentrates the hydrogenfrom 83.9–99% with a hydrogen yield of86%. High-purity hydrogen from thepipeline is used to satisfy theincremental demand. The combinedmake-up gas has a hydrogenconcentration of 99.9 vol%, whichincreases the purity in the treat gas(compared to the base case) from 70–93vol%. As a result, the hydrogen partialpressure at the reactor outlet increasesfrom 348–575psia. The resulting catalystcycle length, however, is reduced to10 months.
The PSA unit could be installedoutside the hydrotreater plot area,which may have particular appeal ifspace inside the unit is limited.Installation at a remote location allowsfor construction to be completed whilethe hydrotreater is operating, leavingonly minimal piping tie-ins for a unitshutdown. In addition to the PSA unit, aknock-out drum and tail gascompression system are required. Also,additional purchased hydrogen isrequired to make-up for hydrogen yieldlosses across the PSA.
The catalyst volumes are increased inrevamp options B and C in lieu ofpurifying the reformer make-uphydrogen. New reactors in series withthe existing reactors were sized toincrease the catalyst volume by 50% and100% in cases B and C, respectively.Direct hydrogen quench is addedbetween reactors. The additionalvolume of catalyst more thancompensates for the less pure make-uphydrogen and increases the cycle life to18 and 27 months for cases B and C,respectively.
Other considerationsManufacturing ULSD will require theconsideration of a number of otherfactors, including(3) :— Dedicated feed storage to minimisefeed quality variability— Rapid response to routine
operational upsets, including productstripper performance— Segregation of offsite producthandling systems to minimisecontamination with other streams— Handling and reprocessing of off-spec product— Equipment reliability.
The specific revamp scope costs forthese issues have not been addressed inthis article.
ConclusionsMost hydrotreaters that were installedto meet the 1993 low sulphur dieselrequirements (500wppm) can bemodified to meet the futurespecification for ULSD of 15wppm for amodest on-site capital investment. Thekey alternatives to consider for therevamp design are:— Installation of the most activecommercially available hydrotreatingcatalyst— Increased catalyst volume— Removal of hydrogen sulphide viarecycle gas amine scrubbing— Improved reactor hydrogen partialpressure.
For all designs, the use of high-efficiency reactor vapour/liquid feeddistributors is essential.
For revamp case A, even though thehydrogen partial pressure was increasedsubstantially, the loss of catalyst volumeassociated with the reactor quenchrequired an unusually high start-of-runWABT, which in turn limited the cyclelength to just 10 months. This may notbe acceptable to many refiners.
The revamp designs presented in thisstudy, although sufficient to meet thefuture requirements of ULSD, onlymarginally improve other productproperties such as cetane number,polynuclear aromatics, total aromaticsand gravity. Improvement of thesequality specifications, which are thesubject of regulations in certain areas ofNorth America and Europe, requiresanother stage of treating for thereduction of aromatics and possiblypartial destruction of polycyclicsaturated compounds. It may bepossible to integrate this equipmentinto the existing hydrotreater;otherwise, a new grass roots facility isrequired.
The offsite revamp scope and costwas not addressed in this work, butcould substantially increase totalproject cost. This includes modificationsto product rundown, storage andshipping systems to preventcontamination with other middledistillate streams. Also, infrastructuresystems for hydrogen supply andutilities could be impacted. Further, therevamp designs, although theoretically
capable of meeting the more stringentsulphur specification, will lack some ofthe robust design features of a newgrass-roots facility custom designed forthis purpose.
AcknowledgementsThe PSA POLYBED unit budgetary yieldand cost estimate was provided by UOPProcess Plants division.
The authors also wish to thank ShellGlobal Solutions for verifying thefeasibility of modifying an existinghydrotreater reactor for inter-bedhydrogen quench and providing costestimates for the same.
R E (Ed) Palmer is currently the managerof downstream process engineering forMustang Engineering. He is responsible forprocess design and marketing support forall refining, petrochemical and chemicalprojects. At Mustang, he has led numerousstudies, technology evaluations andprojects relating to clean fuels production.He has been the author/co-author ofnumerous articles and industry meetingpresentations relating to petroleumrefining. Prior to assuming his currentposition at Mustang, he spent 23 yearswith Litwin Engineers and Constructors ina variety of assignments in process design.Palmer spent the first five years of hiscareer as a refinery process engineer forConoco in Oklahoma. E-mail: [email protected] P Torrisi, clean fuels technicalspecialist for Criterion Catalysts &Technologies, has responsibility for cleanfuels project coordination in the Americas.He joined the CRI International family in1996, becoming lab manager within theresearch and commercial developmentgroup where he was responsible forsupporting regeneration andpresulphurisation technologies. Morerecently, with Criterion he has beeninvolved with new product marketintroduction, particularly with applicationof CENTINEL catalysts. Prior to joining CRI,Torrisi spent 10 years in a variety ofassignments including process engineering,technical service and R&D with Exxon inBaton Rouge and Florham Park. He holdsa BS degree in chemical engineering fromthe Virginia Tech.
References1 Ergun S, Fluid Flow through packed
columns, Chem Eng Progr, 48, 89, 1952.2 Larkin R P, White R R and Jeffery D W, Two-
phase concurrent flow in packed beds,AIChE J., 7231, 1961.
3 Bjorklund B L, Heckel T L, Howard N D,Lindsay D A and Piasecki D J, The lower itgoes, the tougher it gets (The practicalimplications of producing ultra-low sulphurdiesel), presented at the 2000 NPRA AnnualMeeting.
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