Converting VGO HDS units to moderate pressure hydrocracking.pdf

Market demand for middledistillate transportation fuelsis expected to maintain steady

growth in the future. Together with thisincreasing demand, there will be contin-uous pressure to lower sulphur contentand improve burning quality by increas-ing the average cetane index of the gasoil pool. Many refiners have found itprofitable in recent years to modifyexisting vacuum gas oil (VGO)hydrodesulphurisation (HDS) units formild hydrocracking (MHC) service.

This is accomplished through the useof specialised MHC catalysts togetherwith changes in the operations strategyfor the unit which involve running atnear maximum reactor temperatures andaccepting shorter catalyst cycles betweenregenerations. Mobil TechnologyCompany has developed and commer-cialised a new technology calledModerate Pressure Hydrocracking(MPHC) which is now being offered forlicense to the refining industry throughthe Mobil-Akzo Nobel-Kellogg (MAK)hydrocracking alliance.

This process overcomes several of thekey limitations experienced with MHCtype operations and has been applied tothe revamp of a 28 000bpsd VGO HDSunit at Mobil’s joint venture KyokutoPetroleum Industries (KPI) refinery inChiba Prefecture, Japan.

Moderate pressure hydrocracking is asingle-pass hydrocracking process for thepartial conversion of vacuum gas oils tolow-sulphur distillates and unconvertedoil which is highly upgraded relative tothe raw feed. Operating at lower pressuresignificantly reduces capital investmentand results in lower operating costs andsubstantially less hydrogen consump-tion. Furthermore, the process require-ments for MPHC are within the range ofmany existing VGO desulphurisationunits. Typically, the operating condi-tions for MPHC range between 50 and100 bar total pressure, 340–425°C reactor

temperature and between 350 and 1000normal cubic metres of recycle gas percubic metre of feed.

The two companies have been active-ly engaged in moderate pressure hydro-cracking research for more than 10 years.Mobil’s first commercial MPHC installa-tion, a VGO HDS retrofit, was successful-ly started up in 1983. The companyoperates five hydrocrackers, two ofwhich are partial conversion MPHCdesigns that process vacuum gas oils intomiddle distillate products.

This research and operating experi-ence has led to an advanced capability toapply hydrocracking conversion tech-nology to heavy feedstocks under mod-erate pressure conditions. Akzo Nobelhas commercialised a family ofhydrotreating and hydrocracking cata-lysts which are combined to achieve theoptimal balance between activity andselectivity for each specific refiningapplication. Akzo Nobel zeolite-basedhydrocracking catalysts have beenselected for application in both ofMobil’s MPHC units as well as nine addi-tional units around the world.

Process descriptionMAK Moderate Pressure Hydrocrackingis a single stage, single pass process con-figuration consisting of a reactor sectionand fractionation section designed tomeet specific project objectives. Theadvanced Spider-Vortex reactor internalsdesign technology allows the applica-tion of multi-bed reactors while main-taining stable operations and maximis-ing catalyst utilisation. In most cases,catalyst requirements are such that onlya single reactor vessel is needed.

Both high temperature and low tem-perature high pressure separators areutilised in the reaction section toenhance operability and heat integra-tion with the fractionation section. Anamine contactor is utilised in the reactorsection to remove hydrogen sulphide

from the recycle gas enhancing hydro-gen partial pressure and catalyst HDSperformance. Due to the low light endsmake from MPHC, a simple stripper fol-lowed by the fractionating column canbe specified for product recovery. Lowpressure separators upstream of the strip-per tower can be specified if hydrogenrecovery from the flash gas is a designobjective. Lower design pressure and aminimum equipment count result insubstantial capital cost savings.

The general equipment requirementsfor the MPHC unit are the same as formany existing VGO HDS units. The pri-mary difference is the need for increasedreactor volumes to provide sufficient cat-alyst to achieve the desired conversionand cycle life. The advanced reactordesign capabilities developed by Mobiland available through the MAK hydro-cracking technology alliance allow forthe efficient and cost effective additionof catalyst volume while minimisingincremental pressure drop in the reactorsection.Reactor designReactor performance is vital for the safe,reliable and profitable operation of anyhydrocracking process. Multiple catalystbed reactor designs which do not haveadequate quench and redistributioninternals are subject to reactant mal-dis-tribution which can lead to reactor tem-perature instability (temperature excur-sions and runaways), poor catalyst effi-ciency (loss of catalyst cycle life) anddecreased yields.

These issues are as important in mod-erate pressure partial conversion hydroc-racking as they are in high pressure highconversion hydrocracking. Mobil hasmaintained active research and develop-ment programmes in the area ofhydroprocessing reactor design for morethan 30 years. These programmes haveresulted in the commercialisation of theSpider-Vortex quench and redistributionreactor internals together with advanced

Converting VGO HDS units tomoderate pressure hydrocracking

An introduction to the technology of MAK Moderate Pressure Hydrocracking(MPHC), together with details of a successful revamp at the Chiba refinery of

Kyokuto Petroleum Industries, Japan

David A Pappal Michael G Hunter Lucas R Groeneveld

Mobil Technology Company M W Kellogg Technology Company Akzo Nobel Chemicals BV

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design techniques to ensure the opti-mum performance of hydrotreating andhydrocracking process facilities.

The commercially demonstratedimprovements in temperature distribu-tion and catalyst utilisation have trans-lated into better yields, longer catalystcycles and more efficient use of limitedhydrogen resources throughout therefining system. A complete descriptionof the Spider-Vortex development pro-gramme can be found elsewhere.Catalyst selectionAnother key feature of the process is thespecification of an Akzo Nobel catalystsystem. The system consists of a pre-treat catalyst formulated for highhydrodenitrogenation activity followedby a hydrocracking catalyst with bothactivity and selectivity tailored to meetspecific conversion objectives.

Since organic nitrogen compoundsare poisons to acidic hydrocracking cat-alysts, nitrogen must be substantiallyconverted in order to achieve optimalhydrocracking activity. Hydrotreatingcatalysts are also significantly lessexpensive than hydrocracking catalystsand are less sensitive to temperatureinstability. Therefore, it is best to carryout the high heat release desulphurisa-tion and denitrogenation reactions inthe beds that contain only hydrotreat-ing catalyst.

A large family of catalysts are avail-able to optimise pretreat performance,including highly specialised materialsfor metals and carbon residue conver-sion.

Akzo Nobel’s hydrocracking catalystsexhibit a range of activity and selectivi-ty response which can be used to opti-

mise the desired hydrocracking perfor-mance on a case-by-case basis. In gener-al, there is an inverse relationshipbetween catalyst activity for conversionand the selectivity to heavier middle-distillate products. Zeolite crackingcatalysts with the highest activity forgas oil conversion tend to be less selec-tive to the heaviest (middle distillate)products.

The distribution of pretreating andcracking catalyst is an important MPHCdesign consideration. Generally, thepercentage of pretreating catalyst in thetotal fill will range from 40 to 60 percent. The optimal catalyst distributionwill depend on many factors includingfeedstock properties and processingobjectives.

Lower overall catalyst requirementsand the use of less expensivehydrotreating catalysts result in sub-

stantially reduced cost as compared tosingle catalyst hydrocracking systems.

Commercial MPHC experienceMobil’s first application of MPHC tech-nology was for the revamp of an existingVGO desulphuriser located at the jointventure KPI refinery at Chiba, alreadymentioned. Design feed for the MPHCunit there was approximately 4200tpd ofMiddle Eastern vacuum gas oil with sul-phur content of 1.8 to 2.8wt%.Operating pressure at the high pressureseparator is 55 barg and recycle gas isapproximately 645 normal cubic metresof recycle gas per cubic metre of feed.The unit is presently achieving a two-year operating cycle at 35 to 45wt% netconversion, using an Akzo Nobel catalystsystem.

The unit was started up in September1983 and has met or exceeded all designexpectations. Its long-term reliability istestimony to the operating expertise, thequality of the process design and theeffective performance of the advancedSpider-Vortex reactor internals. Thedesign and operation of the KPI MPHCunit has resulted in a leading edge appli-cation of hydrocracking at moderatepressures.

A pilot plant programme was con-ducted at Mobil’s Paulsboro ResearchLaboratory in 1990 to evaluate new cata-lysts which could be used to extend thecapabilities of the Chiba MPHC unit.Experiments were conducted at 54 barhydrogen pressure and 500 normal cubicmetres of recycle hydrogen per cubicmetre of feed, using Akzo Nobel pretreatand zeolitic hydrocracking catalysts.Feedstock properties as well as yields andhydrogen consumptions at two conver-sion levels are shown in Table 1.

Total middle distillate (166–343°C)yields of 31.6 per cent and 37.0 per centwere achieved at 37 per cent and 46 percent net conversion, respectively.


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FFeeeeddssttoocckk pprrooppeerrttiieessSpecific gravity @ 15.5°C 0.907Sim Dis, D-2887 °C

IBP 29550% 429FBP 635

Sulphur wt% 2.1Nitrogen wppm 860CCR, D-4530 wt% 0.4

Net conversion to 343°C-minus % 3377 4466LLiiqquuiidd yyiieellddss vvooll%% ooff ffeeeedd

C5-166°C, Naphtha 9.2 13.7166-227°C, Kerosene 7.8 10,4227-343°C, Lt gas oil 23.8 26.6343-388°C, Hvy gas oil 18.5 17.1388°C-plus, LSFO 44.9 37.1

Hydrogen consumption Nm3/m3 84 103SCFB 500 610

MPHC pilot plant feed properties, yields and hydrogen consumption

Table 1

Figure 1 Normalised temperature vs catalyst age

Hydrogen consumptions were only 84and 103 normal cubic metres per cubicmetre of feed. While these yields do notrepresent the maximum distillate selec-tivity for Akzo catalysts, the high activi-ty and stability of KC-2600 allows for anextended operating cycle and bettercumulative yield as compared with lessactive catalyst alternatives.

All of the kerosene and gas oil frac-tions are good quality low sulphur (lessthan 300wppm) blending componentsfor the distillate pool. The gas oil cutshave cetane indices ranging from 40 to56. The heavy gas oil and unconvertedbottoms are excellent low sulphur (lessthan 500wppm), low viscosity fuel oilcomponents. The low density and nitro-gen content (less than 50wppm) of theHGO and bottoms product also makethem excellent feedstocks for fluid cat-alytic cracking (FCC).

Based on these pilot plant evalua-tions, the Akzo Nobel dual catalyst sys-tem was chosen for the Chiba unit andinstalled in August 1991. The first cycleon KF-843 and KC-2600 was operatedsuccessfully until July 1993.

The average catalyst temperature nor-malised to 45 per cent net conversionand constant feed nitrogen is plottedagainst days on stream in Figure 1. Theleast squares regression line through thedata indicates an aging rate of only 1.2°Cper month. Also shown in Figure 1 is the

actual 343°C-plus net conversion. During the first 100 days of operation,

the conversion was increased from about30wt% in the first week of operation tothe target level of 45wt%. From 100 daysthrough the end-of-cycle at approxi-mately 650 days on stream, the conver-sion was maintained essentially constantat 45 per cent. The distillate yield wasalso very stable throughout the cycle.

A cetane index of 50 was alsoachieved for the gas oil distillate productthroughout the catalyst cycle. The cur-rent cycle at Chiba was started up withregenerated KF-843 and KC-2600 andcontinues to operate successfully.

Following the commercialisation ofMPHC technology in the KPI refinery,Mobil installed a grassroots MPHC unitin 1991 to convert VGO at the JurongRefinery, Singapore – a decision demon-strating Mobil’s belief and financial com-mitment to the technology within itsown refining system.

Design feed for the Jurong MPHC unitwas approximately 4700tpd of MiddleEastern vacuum gas oil with sulphurcontent of 1.8 to 2.8wt%. Operatingpressure at the high pressure separator is83 barg and recycle gas is approximately674 normal cubic metres of recycle gasper cubic metre of feed. The unit ispresently achieving a two-year operatingcycle at 50 to 60wt% net conversion,using an Akzo Nobel catalyst system.

The key design feature of this unit is asingle large diameter multi-bed reactorutilising the MAK Spider-Vortex inter-nals technology.

Hydrocracking costsThe high installed cost for hydrocrack-ing equipment is related to high hydro-gen partial pressure requirements andprocess conditions requiring the use ofexotic materials of construction. Therequired operating pressure level is deter-mined by a complex relationshipbetween feed properties, desired conver-sion level, catalyst life and product qual-ity constraints.

To operate at near 100 per cent totalconversion with extinction recycle,high hydrogen pressures are required tolimit catalyst deactivation to acceptablerates. In general, heavier, higher endpoint feedstocks will necessitate higherdesign pressures for a given conversionlevel and catalyst life. Also, the aro-maticity of hydrocracked products willbe directly proportional to hydrogenpartial pressure.

Designing a hydrocracker with theobjective of producing high smoke pointspecification jet fuel requires very highpressure and can result in significantproduct quality give-away in the naph-tha and diesel fractions and inefficientuse of hydrogen resources. While moder-ate pressure hydrocracking will yield a


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Figure 2 Moderate pressure hydrocracking revamp flowscheme

high quality diesel fuel component, thekerosene fraction in general will nothave high enough smoke point to quali-fy as a specification turbine fuel. Inmany refining situations, however, thekerosene cut will be suitable for blendinginto the jet fuel pool or can be used forother fuel oil blending purposes andfree-up better quality straight runkerosene for jet fuel products.

Designing a hydrocracker for single-pass partial conversion operation miti-gates to a large extent the need for highoperating pressures. The single-passhydrocracker is not subject to fouling orhigh catalyst deactivation rates whichcan result from the buildup of poly-nuclear aromatics in the recycle oilstream.

The kinetic impact of lower hydrogenpressure is compensated by decreasedconversion level and by lowering liquidhourly space velocity (LHSV) if neces-sary. The naphtha, diesel and unconvert-ed bottoms products are not over-satu-rated and hydrogen consumption isthereby minimised. The economic keysto MPHC are the values of the low sul-phur distillate and upgraded bottomsproducts relative to the untreated feed.

For grassroots facilities, the invest-ment required for a moderate pressurehydrocracker is only 50 to 80 per cent ofa full conversion hydrocracker. A grass-roots MPHC unit which utilises availablerefinery hydrogen can require less thanhalf of the capital investment associatedwith a high pressure hydrocracker whichin the majority of cases requires theaddition of hydrogen manufacturingfacilities. Even though the catalyst con-sumption for MPHC can in some casesexceed that of high pressure full conver-sion hydrocracking, the overall operat-ing costs are generally only 60 to 70 percent as high.

As illustrated later in this article, it ispossible to revamp existing moderatepressure hydroprocessing equipment toachieve conversion levels of 30 to 50 percent. In these situations, the capitalinvestment to achieve significant hydro-cracking conversion can be a fraction ofthat required for a new unit.

Many refiners have discovered thatexisting gas oil hydrodesulphurisationunits can be modified to achieve incre-mental conversion to more valuable dis-tillate products. In many cases, a changein catalyst system together with onlyminor modifications to the HDS productrecovery equipment can result in 20 to30 per cent net conversion.

Without the addition of incrementalcatalyst volume, however, this level ofconversion will in most cases result insubstantially shorter operating cycles ascompared to HDS only service. MAK-

MPHC technology can be used to addreactor volume and achieve as much as40 to 50 per cent conversion withoutsacrificing run length.

A commercial example is presentedhere, illustrating the successful applica-tion of this concept at the joint ventureKPI refinery.

Case studyRevamp of existing VGO HDS unit

Kellogg has performed a simple casestudy to make a preliminary assessmentof the economic value of a MAK-MPHCrevamp of existing VGO HDS facilities ata European refinery site.

A VGO hydrodesulphurisation unit isassumed to process 30 000bpsd (4400tons/day) of VGO feedstocks derivedfrom Middle Eastern crudes. The unit isrevamped to MAK-MPHC technologywith the installation of a new reactorvessel sized to achieve 45 per cent netconversion over a two-year operatingcycle. The primary features of therevamp are illustrated in Figure 2.

The existing reactors are converted toparallel instead of series flow and act asthe lead pretreating catalyst bed. A newthree-bed reactor is installed in serieswith the existing reactors and is loaded

with the balance of the pretreating cata-lyst and all of the required zeolitichydrocracking catalyst. Also, a new heatexchanger is added to preheat recycle gasagainst reactor effluent and bypass theexisting combined feed heat exchangersand charge furnace.

This scheme substantially reduces therecycle gas compressor head require-ments and, together with a simple gearchange to speed up the existing com-pressor, can result in a cost effectiveincrease in gas capacity of as much as 25per cent. Additional modificationsinclude a new make-up compressor tosupply the incremental hydrogendemand and the installation of akerosene draw in the fractionator. Theestimated total installed cost of therequired revamp facilities is itemisedbelow:

(Millions)New reactor $9.1Makeup hydrogen compressor $2.9 Bypass exchanger and recyclecompressor revamp $0.8

Fractionator modifications $2.1Estimated total installed cost $14.9

A simple comparison of yields andhydrogen consumption is presented in


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Figure 3 Incremental revenue and payout vs hydrogen cost

OOppeerraattiinngg MMooddee EExxiissttiinngg MMPPHHCC IInnccrreemmeennttaall IInnccrreemmeennttaallVVGGOO HHDDSS rreevvaammpp pprroodduuccttss rreevveennuuee

BBPPSSDD BBPPSSDD BBPPSSDD UUSS$$//ddaayyLiquid products

Naphtha 600 4110 3510 65 848Kero – 3120 3120 69 264Gas oil 1620 7980 6360 138 266LSFO 28 380 16 260 –12 120 –224 583

+48 795Notes:Pricing per ARGUS Fundamentals, May 1995

Naphtha = $18.76/bbl, Kerosene = $22.2/bblGas oil = $21.74/bbl, LSFO = $18.53/bbl

Yield and revenue comparison: VGO HDS vs MAK-MPHC revamp

Table 2

Table 2 together with an analysis of the incremental revenuesbased on European pricing. Total distillate yield is increased by12 990bpd with a corresponding increase in hydrogen demandof 380scfb.

The expected annual increase in product revenues based on350 stream days operation per year is US$17.1 million. Theincremental annual operating costs for catalyst and utilities areestimated to be US$3.5 million. Total hydrogen demand,including chemical consumption and losses, is 370scf/barrel forVGO HDS and 750scf/barrel for MPHC.

The simple payout of the initial revamp investment will be astrong function of the cost of incremental hydrogen, as can beseen in Figure 3. In order to achieve a three-year payout of cap-ital for the study case, the cost of hydrogen should be approxi-mately US$2.0/thousand scf or lower.

ConclusionsMAK Moderate Pressure Hydrocracking provides a profitable,minimum investment route to achieving incremental vacuumgas oil conversion while producing high quality low sulphur dis-tillate products. Mobil has successfully commercialised anddemonstrated this technology in both revamp and grassrootsapplications totalling more than 21 years of operating experience.

Reactor design is a key element in the application of hydro-cracking technology. Mobil’s investment in quench zone tech-nology research and development has resulted in the commer-cialisation of superior reactor internals hardware. The Spider-Vortex system allows for the construction of large diametermulti-bed reactor vessels while ensuring reliable and efficientoperations.

The Mobil KPI joint venture MPHC operation in Chibademonstrates the high level of hydrocracking performance pos-sible by revamping moderate pressure VGO HDS facilities. Thecombination of the two companies’ operating experience anddesign technology, together with Akzo Nobel’s state-of-the-artKF-843/KC-2600 catalyst system, has resulted in a two-yearcycle length and 45wt% conversion at only 54 barg hydrogenpartial pressure.

The technology to achieve the results described here is avail-able for license to the refining industry through the Mobil-Akzo-Kellogg hydrocracking alliance.

David A Pappal is an advanced senior associate at Mobil’sPaulsboro Technical Center, New Jersey, USA, and is hydrocrackingtechnology leader in the hydroprocessing group for MobilTechnology Company. He holds a BS CHE degree from PennsylvaniaState University.Dr Lucas R Groeneveld is a senior application specialist, hydrocracking catalysts, in the hydroprocessing group of the AkzoNobel Catalyst Business Unit at Amersfoort, The Netherlands. Hehas a PhD in physics/inorganic chemistry.Michael G Hunter is product manager of hydroprocessing for The M W Kellogg Technology Company, where his principle experience isin hydrotreating and hydrocracking. He holds BS and MS degrees inchemical engineering from Arizona State University.


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“MAK Moderate Pressure Hydrocracking provides a profitable, minimum investmentroute to achieving incremental vacuum gas oilconversion while producing high quality lowsulphur distillate products”

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Converting VGO HDS units to moderate pressure hydrocracking.pdf