Hydrogen plays a foremost role inthe various hydroprocessingoperations within oil refining

and is considered as a critical compo-nent in determining the type and qual-ity of the refinery products slate. In fact,the configuration and complexity of arefinery is often defined by the extentof its usage. The past decade has seen aremarkable growth in hydrogendemand for meeting the ongoing envi-ronmental regulations for cleaner andlighter transportation fuels.

In the coming years, several develop-ments are taking shape for paving theway to the so-called hydrogen economyin relation to hydrogen-based energysystems, with keen deliberations goingon for sustainable hydrogen in thelonger term.

The other major use of hydrogen hasbeen as synthesis gas or syngas (its mix-ture with other gases like nitrogen orcarbon-monoxide) for the manufactureof bulk chemicals like ammonia andmethanol and for speciality chemicalslike oxo-alcohols, MDI/TDI etc. Further,hydrogen finds substantial use in themetal industry as a reducing medium inthe production of direct reduced ironand steel making.

Current global production of hydro-gen, either in its pure form or as syngas,is in the range of 60 million Nm3/h,which is equivalent to about 55 billionscfd or 130 000tpd. Roughly one-thirdof it is generated as on-purpose hydro-gen for refineries and another third isproduced (as contained hydrogen) inthe form of syngas for the chemicalsindustry. The remaining is divided (indecreasing order) over in-situ hydrogenproduced within the refineries (fromcatalytic reforming) and olefins com-plexes, reducing gas for metals/steelindustry and the various consumerindustries like edible fats, textile andpharmaceuticals.

The emerging needs on hydrogengeneration and its management havedriven concerted advances in some ofthe key areas of hydrogen technology,

also setting the ground for some inno-vative developments. Apart from thetypical stimulus for better cost effective-ness and operating efficiency, the majorfocus has been towards enhancing plantreliability, safety and operational flexi-bility, compactness of equipment, easeof operation and, undoubtedly, the gov-erning environmental factors.

Production technologiesOver the years, hydrogen has beenmostly produced from hydrocarbonfeedstocks ranging from natural gas tocoal. A very small portion has been pro-duced from water by electrolysis. Forhydrogen generation, plant capacitiescould range from as low as 100Nm3/hrto over 220 000Nm3/hr (0.1 to 200 mil-lion scfd). In the smaller capacity range,hydrogen production by electrolysis ofwater or dissociation of ammonia andmethanol can be justified and econom-ical based on modular units.

However, as hydrogen capacitiesincrease (above 500Nm3/hr), the hydro-carbon route generally begins to bemore attractive based on overall eco-nomics. Moreover, for higher capacities(above 5000Nm3/h), the modularapproach becomes attractive.

The process for manufacturinghydrogen from hydrocarbons (on-pur-pose hydrogen production) usually con-sists of four major sections, includingfeed preparation and pre-treatment,syngas or raw hydrogen generation,water-gas shift conversion and finalpurification.

The core section from the point ofview of criticality, capital intensity andefficiency relates to the syngas genera-tion. Hence it encompasses differentprocessing routes and technologyoptions whose selection mainlydepends upon available feedstocks andutilities, capacity, site-specific condi-tions/requirements and, of course,overall economics.

The major and commercially provenprocesses available for producing (syn-gas for) hydrogen are steam reforming

(SMR), partial oxidation and auto-ther-mal reforming.

These processes differ mainly in therange of applicable feedstocks as well asthe basic reactants in terms of steamand/or oxygen for producing the rawhydrogen-rich syngas.

Steam reforming (SMR) of hydrocar-bons (ranging from light natural gas upto heavy naphtha) has been the pre-dominant route for the production ofraw hydrogen. It has been the mostwidely employed process and hasremained the technology of choice overthe past five decades, accounting formore than 90% of the global hydrogengeneration capacity, which isattributable to being well-proven, reli-able, simple to operate, and whichoffers significant flexibility in feedstock,apart from being cost-effective.

The basic reactions taking place canbe represented by:

CnHm + nH2O → nCO + (n+m/2) H2

(Endothermic steam reforming; n <7)nCO + nH2O = nH2 + nCO2 (Exothermicwater-gas shift conversion)

For paraffinic feeds, m = 2n + 2 and onlyin the case of methane, the reactionbecomes an equilibrium reaction. Thenet overall reaction is still highlyendothermic and based on its thermo-dynamics, it is favoured by higher steamto carbon ratio, higher temperature andlower pressure.

Based on the intrinsic need for sub-stantial heat input and high tempera-tures, as well as kinetics-limitedcatalytic space velocities, steam reform-ing is typically conducted in a tubularreactor with high level heat input byfuel firing. It typically employs a fire-box type reformer containing catalyst-filled tubes through which preheatedfeed-plus-steam flows to produce anequilibrium mixture of hydrogen, car-bon-oxides along with the residualmethane and steam.

The SMR process, as well as reformertube integrity, is strongly dependant oncatalytic activity and stability. Hence,

Hydrogen technology–an overview

Developments in the area of hydrogen production equipment, configurations,catalysts, materials and automation have brought about improved operationalreliability and effluents curtailment, as well as achieving lower operating costs

Sanjiv RatanTechnip-Coflexip

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proper selection and application of cat-alysts play an important role in the per-formance and reliability of the process.

Partial oxidation (POx) technology isbased on incomplete oxidation ofhydrocarbon, usually using oxygenwith little or no steam:

CnHm + n/2 O2 → m/2 H2 + nCO

The sub-stoichiometric combustionlimits the degree of oxidation of COand H2 to CO2 and H2O, respectively,thus providing syngas with lowerH2/CO ratios. It is a non-catalytic oper-ation, which allows much higher tem-peratures and is therefore applicableparticularly for processing heavy-endfeedstocks.

Oxygen and hydrocarbon are mixedin a burner and allowed to react at con-ditions. The process can use a variety offeeds ranging from natural gas to heavyresidues within the refinery. To enhanceon-stream reliability, usually more thanone train of gasifiers are employed toimprove plant availability.

For heavier feeds, which also tend tocontain higher levels of sulphur andother trace impurities like (heavy) met-als etc, elaborate steps are needed forcarbon/slag handling and downstreamprocessing in terms of H2S and CO2

removal, as well as process condensatetreatment. Further, since the POx pro-cess is based on high-pressure internalcombustion, it does not call for a largeexternal heat input (except for pre-heat-ing of the reactant streams). According-ly, conventional purification schemesbased on machination routes becomemore suitable compared to PSA, toavoid the purge gas fuel (albeit at lowlevels), as well the loss of hydrogen inpurge gas.

The economics of hydrogen genera-tion via POx is governed by the priceand/or availability of oxygen and thecredit for using lower grade (heavier)feedstock. For processing of lighter feed-stocks ranging from natural gas to refin-ery offgas, the POx route can berelatively simpler, more cost effectiveand reliable, especially for hydrogen-carbon monoxide (Hy-CO) production.Heat recovery features either directquench or indirect steam generation.

The auto-thermal reforming (ATR)process is a sort of combination of POxand SMR in one reactor. The feed hydro-carbon mixed with some steam is par-tially combusted with oxygen, followedby steam reforming over the catalystbed within the same vessel. The heat ofcombustion is utilised to conduct theendothermic reforming reaction toalmost complete conversion withoutany external heat input (auto-thermal-ly), hence the name.

The ATR applications are also largely

dependent on the oxygen price andcase-specific economics, especially forHyCO production.

SMR becomes the straightforwardchoice in the case of non-availability ofoxygen and feedstock (up to naphtha).In other cases, especially in relation toHy-CO credits, it could require adetailed study to evaluate the overalleconomics. Generally, factors like econ-omy of scale, low-grade heavy feed,cheap oxygen, credit for CO and addedvalue for captive/export power couldfavour POx/ATR in specific cases. Never-theless, it is generally acknowledged inthe industry that steam reforming is thetechnology of choice for hydrogen pro-duction.

The downstream processing of thereformed gas primarily proceedsthrough the steps of shift conversionand hydrogen purification using pres-sure swing adsorption (PSA) and relatedwaste-heat recovery plus cooling. PSAtechnology completely replaces theconventional CO2 removal plus metha-nation route and has been the industrynorm for over two decades. It provideshigh purity hydrogen (99.99% or evenhigher) in a single system operating atambient temperatures and without anyheat input.

The steam reforming together withPSA purification carries inherent inte-gration of the back-end and the front-end through the use of PSA purge gas asprimary fuel in the reformer. Such linknot only results in the reutilising theunconverted feed as fuel but offers sig-nificant scope for optimising the levelof feed conversion without affectinghydrogen purity and thus helps inachieving the desired objectives of pro-cess optimisation, flexibility and relia-bility. Thus, depending upon therelative pricing of feed and fuel orimposed utilisation of any specific

quantity of low-grade make-up fuel, thefeed conversion can be adapted

Catalyst developmentsThe hydrogen process based on steamreforming is strongly driven by the cat-alytic steps. Hence, proper selection andperformance of catalysts play an intrin-sically important role in optimisationand reliability of the process. Also thefeedstock flexibility of steam reforminghas been widely increased with theapplication of the pre-reforming stepupstream from the reformer.

Notable improvements have beenmade in the catalysts applied in hydro-gen-syngas plants, mainly to respond tothe sought-after needs and desired char-acteristics that include:——Higher resistance to poisons and pro-cess upsets——Catalyst shapes for higher activityand lower pressure drop——Higher space velocities——Longer operating life——Broader range of operating condi-tions——Easier startup (and reduction require-ments)——Better selectivity (reduced formationof undesired by-products).

And specifically for the reformingcatalysts:——Better resistance to carbon formationwhile maintaining activity—— Improved thermal stability andmechanical strength——Better fixation of potash in alkalisedcatalysts (minimised migration)—— Increased resistance to excursions onsteam and temperature.

The typical process configuration of astate-of-the-art hydrogen plant consistsof the following process steps (Figure 1):——Feed pre-treatment——Pre-reforming (optional)——Reforming

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HCfeed Pre-

treatmentPre-

reforming

Recycle H2

Purge gas fuelMake-up fuel

Recycle H2compressor

Steamsystem

Export steam

Hydrogenproduct

Reforming

(optional)

Shiftconversion PSA

30 400 550 900 400 40 Temperature, °C

Figure 1 Generic hydrogen plant flowsheet based on SMR-PSA

——CO Shift conversion ——Purification by PSA

Feed re-treatmentReforming (also pre-reforming) catalyst,is most susceptible to poisoning by sul-phur and chlorine compounds. Hencethese components need to be removedfrom the hydrocarbon feed to very lowlevels (down to <100ppb level) prior toreforming step.

When using heavier feedstocks (suchas SR naphtha) extra provisions areneeded for removal of possible contami-nants like non-reactive organic sulphurand chlorine compounds, unsaturatesand heavy metals etc. Feed pre-treat-ment normally consists of a hydrogena-tion step (Comox or Nimox) to convertorganic sulphur to H2S (and organicchlorides if present, to HCl) followed byH2S absorption using Zinc oxide (ZnO).

For sulphur removal loads (above5–10t/year) two ZnO reactors inlead/lag arrangement are preferred,which apart from maximising sulphurpickup, offer online catalystchangeover, thus allowing extendedcontinuous cycle on-stream runs. Incase of quite high concentrations of sul-phur, generally in naphtha feedstocks(up to 1000ppm and over) a pre-desul-phurisation unit is employed in termsof (primary) hydrogenation followed bythermal stripping for bulk removal ofH2S, in order to minimise the recurringcost of ZnO as well as spent catalystdisposal.

In a refinery environment, severalhydrocarbon streams that do not quali-fy for cost-effective hydrogen recovery,generally end up in the refinery fuelnet. However, after proper techno-eco-nomic assessment based on their com-position (including contaminants),their available pressure and their quan-tity, some of these streams can be effec-tively utilised as feedstock for hydrogenproduction.

It can often involve compression,olefin saturation in combination withhydrogenation of organic sulphur andchlorine compounds, if any. In the caseof high levels of unsaturates, a dedicat-ed recycle loop is usually appliedaround the hydrodesulphuriser (HDS)reactor for diluting the inlet stream andeventually to remove the exothermicheat of reaction. However, the econom-ic attractiveness depends upon theadded value to the “fuel towards feed”(or savings in the hourly operating cost)versus the costs of off-gas conditioning.

In the case of multiple feedstocks, itbecomes critical to define the designbasis and operational modes for thefeed pre-treatment. It mainly involvesestablishing the controlling and/orworst-case design rating, required oper-

ating parameters’ window (mainly levelof recycle hydrogen and temperature)and their effective control philosophyunder various operating cases. More-over, special attention needs to be givenfor proper transition and control duringcapacity and/or feedstock change.

Pre-reformingFeedstock flexibility in SMR has beenwidely enhanced by advances in directreforming catalysts as well as applica-tion of pre-reforming upstream fromthe reformer. Over recent years, pre-reforming has acquired increasingapplication in a refinery environmentfor achieving multiple feedstock flexi-bility, from natural gas to captive liquidhydrocarbon streams such as LPG andnaphtha, in an efficient and reliablemanner.

Pre-reforming is a low-temperature(approximately 400–550ºC) adiabaticreforming step applied upstream fromthe reformer for achieving a variety ofobjectives. To achieve low-temperatureactivity, a highly active catalyst isrequired, converting a wide range of thepreviously mentioned hydrocarbonfeedstocks into a methane-rich gas inthe presence of steam.

The pre-reforming operation is thenet result of two competing equilibriumreactions, namely steam reforming(endothermic), and methanation ofhydrogen with carbon oxides (exother-mic). Hence, the temperature profilesalong the catalyst bed are different fordifferent feedstocks depending uponthe inlet conditions and methane equi-librium. To achieve the required tem-perature window for different operatingcases, part of the process steam is added

as post steam injection, which alsohelps during transient conditions

Incorporation of a pre-reformerallows higher reformer inlet tempera-tures (up to 650 ºC) without the con-cern for thermal cracking due to theabsence of the C2+ fraction. Hence, itinherently offers the potential for fur-ther reheating reformer feed, thus shift-ing the reformer radiant heat duty toconvective mode. This shift can reducethe size of the reformer by 5–15%,depending upon the feedstock and theoperating conditions for the reheatpotential. Another related advantage isthe reduction in steam export againstfuel savings, which can prove quiteattractive in some cases.

Overall, pre-reforming of naphthafeed can improve the net thermal effi-ciency of hydrogen production by1–4%, depending upon the type of feedand selected operating conditions.However, the hourly operating costs aremore sensitive to the relative price ofexport steam against fuel. On the otherhand, pre-reforming is an additionalprocess step and with significant cata-lyst life-cycle costs. Hence its applica-tion needs to be evaluated on acase-to-case basis to ensure overall eco-nomics while satisfying specific plantrequirements.

A simplified scheme of pre-reformerintegration is shown in Figure 2.

Plants using natural gas as one of thefeedstocks have the possibility of on-line catalyst renewal by isolating andbypassing the pre-reformer, while oper-ating on natural gas at reduced capa-city (~70%). For cases where extendedcontinuous operation is required onheavy (liquid) feed, the twin-reactor

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Primaryreformer

Pre-reformer

Process steam

HC feed

Desulphurisation

Figure 2 Pre-reformer integration

configuration in parallel is applied,with one reactor as stand-by. This notonly takes away the limitations on thecontinuous operation run throughonline switching of the exhausted reac-tor, but also offers lower annual catalystcosts and some operational advantagesduring start-up and shutdown.

ReformingSteam reforming of hydrocarbon feedsis generally limited up to heavy naph-tha (C7) in view of the catalytic require-ments and the potential carbonformation. Direct reforming of multipleand/or heavier feedstocks with widelydiffering carbon number (CN) requiresmajor attention in simulating the car-bon margin, which signifies the depar-ture from the carbon equilibrium in thereformer tubes based on thermodynam-ic and kinetic conditions.

There is a range of available reform-ing catalysts that can achieve thedesired carbon margin through specificcombinations of alkalised and non-alka-lised catalyst in order to achieve theproper alkali profile and the related cat-alyst activity.

Important considerations such assteam-to-carbon (S:C) ratio, inlet tem-perature and the applied heat flux needto be addressed for proper design inorder to achieve effective and reliableperformance. The reforming process iscarried out in tubular reactors that arehighly endothermic and catalyticallycontrolled. Heat is supplied by theburners in the furnace box and trans-ferred to the catalyst tubes mainly byradiation.

Water-gas shiftCO shift conversion, also commonlyknown as water-gas shift (WGS) isapplied for increasing hydrogen yieldthrough conversion of residual carbon-monoxide to hydrogen and carbon-dioxide in the presence of steam overiron and/or copper-based catalyst. Ther-modynamically, lower operating tem-peratures are favourable for shiftconversion, and thus has the followingimpact:——Reduction in feed rate (better feedconversion)—— Increase in makeup fuel (reducedheating value of PSA purge gas)——Additional export steam (exothermicheat).

Over the years, advances and devel-opment efforts in shift catalysts havebeen in terms of specific performance,extended activity and operationalaspects, such as resistance to upsets andreduced by-products formation.

The CO in the hydrogen product isconsidered as an undesirable impurityor even as a poison in most of its refin-

ery end-uses. Generally, older hydrogenplants based on conventional purifica-tion route involved the two-stage COshift conversion based on high temper-ature (HT) followed by low temperature(LT), in order to get down to low levelsof CO-slip, which could be handled inthe downstream methanation step.However, later hydrogen plants basedon PSA purification, mostly employedonly HT shift, since the product hydro-gen purity is largely independent of theCO level in the process gas, and all theCO is removed in the PSA purge gas.

The HT shift reactor typically oper-ates on an inlet temperature of320–350°C for bulk conversion of car-bon monoxide to hydrogen, with typi-cal exotherm of 50–80 C and thecorresponding CO-slip between 3–6vol% (dry) in relation to the inletsteam/gas ratio being between 0.4–0.6.The newer catalyst formulations havebeen promoted with some copper,which apart from improving the activi-ty, also helps against the potential Fish-er-Tropsch (F-T) side reactions as well asthe extent of catalyst sintering in rela-tion to steam/gas ratio.

Addition of the LT shift in a hydro-gen plant becomes more of an optimi-sation decision, mainly in cases wherethe feed is more expensive than fuel. Asa relatively recent development, medi-um temperature (MT) shift catalyst hasbeen applied in some of the hydrogenplants but there has been limited expe-rience in view of some of the demeritsin terms of temperature sensitivity v/sactivity and by-product methanol for-mation etc.

The LT shift reactor operates typicallywith inlet temperatures of around200°C and employing copper-based cat-alyst, which is more sensitive to sulphurpoisoning and thermal stability. In viewof allowable temperature of less than250ºC, LT shift needs to be applieddownstream of a HT shift converter tolimit exothermic temperature riseacross the catalyst bed.

For larger hydrogen plants, it is some-times justifiable to install a LT shift con-verter to improve overall efficiencyand/or operating costs, especially incases where the fuel is cheaper thanfeed. The justification for LT shift isplant-specific and needs proper evalua-tion in each case based on relative pric-ing of feed, fuel and steam.

Application of medium temperature(MT) shift step allows combining theHT and LT shift in one step. Anotheradvantage rests in being copper-based,which allows lower S/C ratios in theupstream reformer without the con-cerns of catalyst sintering and unde-sired F-T side-reactions, typical ofFe-based HT shift catalyst. On the other

hand, it suffers from inherent concernsof susceptibility to higher temperature(above 350ºC) based on the largerexotherm due to one step conversion,especially on larger CO loads on heavierfeedstocks for retaining stable activity.

Secondly, it tends to have highermethanol formation due to favourablekinetics, which ends up in the processcondensate. Being sensitive to tempera-ture excursions, it requires extra atten-tion to design, control and operation,especially during transient conditionsof start-up and/or feedstock change.

The CO slip from MT shift is lowerthan HT shift, but still reasonably highcompared to LTS due to much higherexit temperature as well end-of-runapproach to equilibrium. Thus, it resultsonly in a marginal net (feed + fuel)reduction of say between 0.3 to <1.0% .

PSA purificationThe PSA process is based on the princi-ple of relative diffusion and the factthat specific molecular adsorbents arecapable of binding or letting throughdifferent gaseous molecules with differ-ent affinity, depending upon their par-tial pressure, size and polarity.Accordingly, the various impurity com-ponents can be adsorbed at a higherpressure and desorbed by “swinging” toa lower pressure.

As such, a PSA operation is a batchprocess with the adsorber goingthrough various stages of depressurisa-tion in a fixed cycle time. However, byusing multiple adsorbers on asequenced basis, constant on-spec prod-uct flow is maintained. Commercialunits have normally between four and12 vessels, depending upon the capacityand desired hydrogen recovery.

Hydrogen is almost non-adsorbed,which gives PSA system the uniquecapability of producing very high purityhydrogen product of 99.99%+ and on apressure close to the feed pressure. Theeffect of increased H2 purity on hydro-gen recovery is minimal. However, thehydrogen recovery is quite sensitive tothe tail gas pressure, which is usuallydrawn between 20 to 40KPag, providingrecoveries between 84–90%, dependingupon the number of vessels and pres-sure equalisations.

PSA system offers unmatched opera-tional flexibility against upstream fluc-tuations in terms of feed compositionand/or flow swings. It retains producthydrogen purity as well as recovery upto 30% turndown.

The PSA systems are very reliable andversatile, needing very little mainte-nance due to absence of moving parts(except on/off valve stems). On-streamfactors in excess of 99% have been oftendemonstrated. Also, the operating life

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of the adsorbents used is inexcess of 10 years and can lastfor the typical plant life. Inaddition, in case of any mal-function like pressure error ora valve failure, the units aregenerally provided with thecapability to operate withreduced number of beds,bypassing the failing sectionautomatically, while main-taining hydrogen purity withonly slight loss of recovery.

PSA systems operate onfully automatic using micro-processor-based PLC units.The level of automation canbe extended to online puritycontrol, which offersimproved H2 recovery andstabilises sudden fluctua-tions, thus also enhancingthe reliability of PSA units.Considerable advancements have beenmade in the design, operation and inte-gration of the PSA unit in the hydrogenplant based on high performance adsor-bents, automatic purity control, betterstability of the purge gas circuit andsmoother reduced-bed operation.

Reformer designThe steam reforming process is carriedout in tubular reactors that are highlyendothermic and catalytically con-trolled. Heat is supplied by the burnersin the furnace box and transferred tothe catalyst tubes mainly by radiation.

The SMR is essentially a fired reactorconsisting of a multitude of catalyst-filled tubes located in a firebox. Itinvolves simultaneous heat and masstransfer in a non-adiabatic catalytic sys-tem, thus its design incorporates com-plex interactive thermodynamic andkinetic models based on zonal heattransfer for rigorous simulation of bothprocess side as well as combustion side.The models need to be extensively vali-dated and reconciled against the fielddata and feedback from operatingreformers. Typical layout for a top-firedreformer is shown in Figure 3.

Based on the technological advancesin high temperature metallurgy, themicroalloys have permitted higherseverity steam reforming in terms ofhigher outlet temperature and/or high-er pressure together with higher heatflux, while respecting reliability.Though much higher reformer outlettemperatures (say, above 900ºC) tend tooffer slightly improved efficiency, theycarry related concerns on the allowablemetal temperature limits, material creepand potential metal dusting, despiteadvances in the alloy materials based onspecial additives as well as coating ofmetal surfaces (such as aluminising)

and use of non-metallic ferrules in theprocess gas boiler.

Further, in order to have an exhaus-tive analysis of the furnace behaviour interms of fluid dynamics of combustionand heat transfer, advanced computa-tional fluid dynamics (CFD) modellingis employed to assess the performanceof the reformer under different geome-tries and operating conditions. Thesetechniques are used not only for design-ing new reformers but also to evaluatethe performance of existing reformersfor revamping and/or troubleshooting.

Steam philosophySpecific energy efficiency is defined asthe ratio of total heat output to heatinput. For a hydrogen plant, it is thesum of heating value of product hydro-gen and the enthalpy of export steam,divided by the heat of combustion offeed and fuel. In a PSA-based hydrogenplant, the purification does not requireany heat input, apart from the fact thatPSA purge gas-firing requires some extrafuel to heat up its inerts content (main-ly CO2). These factors result in higheramount of export steam.

Export steam philosophy is impor-tant in establishing the specific efficien-cy of the hydrogen plant. A plantdesign could have the least feed plusfuel, based on a minimised amount ofexport steam and still have higher oper-ating costs, if the steam is creditedequivalent or better than the fuel value.Typical net specific energy efficiencies(based on feed + fuel – export steam) liebetween 3.1–3.5 Gcal/kNm3 H2

(340–380Btu/scf) on LHV basis.In cases where there is no effective

use or outlet for additional steam, theamount of export steam needs to beminimised. In such designs, the firedduty of the reformer needs to be

reduced by preheating thecombustion air and further byemploying a pre-reformer,which add to additionalequipment and related capitalcost. Also, some of the processparameters like S:C ratio,excess air level, extent of heatrecovery etc, can also beadjusted to regulate theexport steam amount.

Even if the export steam isvalued as equivalent of fuelcost, a plant that producesless export steam tends tobecome less efficient, sincethe low-level heat recoveryand integration cannot befully utilised. However, theexport steam philosophy ismostly governed/imposed bythe site-wide steam-powerbalance and the specific eco-

nomics in terms of evaluation againstsavings in fuel and emissions. Thus,realistic and long-term cost of feed, fuel,export steam and operational philoso-phy are necessary to tailor a flowscheme that best suits specific needs.

CogenerationFuel and power costs as well as powerreliability dictate the feasibility of inte-grating a power generation unit into ahydrogen plant. The steam generationcan be maximised in the reformer andfurther increased through the auxiliaryboiler for power generation in a steamturbo-generator (STG), especially in thecase of low-credit fuels in the refinery.

The hydrogen plant not onlybecomes self-sufficient in power but canalso export power to improve the overalleconomics depending upon specific siteconditions.

For large hydrogen plants, gas-tur-bine based power integration can alsobe very promising based on fuel-powerstrategies, particularly in places havingpower shortage. The cogenerationschemes can be optimised either oncombined-cycle or using gas turbineexhaust (GTE) as combustion air in thereformer.

Environmental aspectsThough hydrogen usage in refineries isdriven by environmental regulations onthe transport fuels, the hydrogen plantitself also needs to comply with the stip-ulated environmental protection. Itusually entails:——Specific limits on NOx emissions inthe fluegas, which can be achieved usingthe advanced (ultra) low-NOx burnerdesigns based on fuel/air staging etc; for-tunately, for hydrogen plants, the NOx

level is inherently reduced due to theuse of low-caloric purge gas as the pri-

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Flue gas toconvectionsection

Process gasto PG boiler

Transfer line

Multipletube lanes

Outlet pigtails

Feed inletpigtails

Burners

Figure 3 Schematic of typical large top-fired reformer

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mary fuel with its high CO2 content —— Minimisation of plant effluentstreams and safe disposal and release ofundesired component streams, includ-ing fugitive emissions.

With spent catalyst handling and dis-posal, there are now several specialisedcompanies who provide services forunloading, collection and transporta-tion of spent catalysts and possibleresidual value based on their metal (Ni)content. Sacrificial catalysts like ZnOcan also be disposed of as land-dump.

Design optimisationEach hydrogen plant design is usuallyoptimised through evaluation of severalcases for the flow sheet optimisation interms of selecting the process route andsteps, followed by optimisation of theoperating conditions and process vari-ables for the selected flow sheet. To con-duct such an exercise effectively,following reliable information isrequired for the specific plant in ques-tion unit price of feed, fuel and power,credit (and limit) of export steam andeconomic pay-back criteria for incre-mental investment.

In several cases, some specificrequirements and/or criteria can influ-ence the plant configuration and designselection, such as: —— Extent of environmental require-ments——Potential future capacity demand——Reliability, or otherwise, of availablepower supply and extent of desiredsteam-power synergy—— Integration with other units in termsof feed and utilities, as well as backup oralternative feedstocks——Plot space constraints.

Flow sheet optimisationFor the optimisation of the hydrogenplant flow sheet, the following optionsare generally evaluated, as applicablefor the economic analysis:——Direct reforming v/s with pre-reform-ing (especially for liquid feedstocks) ——Level of shift conversion – high tem-perature (HT) only or HT + low temper-ature (LT) shift or sometimes mediumtemperature (MT) shift——Combustion air preheat (yes/no; andif yes, the level of preheat)

Combustion air preheat and its levelis a function of desired export steamand the cost of fuel. By preheating com-bustion air, fired duty in terms of thefuel LHV decreases due to the heat recy-cle from air, and the heat available forgeneration of steam is substantiallyreduced. This is primarily due toreduced firing and secondarily due tothe heat recovery shift from steam gen-eration to air preheating.

In the absence of a target for flow rate

of export steam, the additional invest-ment in an air preheater system is eval-uated against the saving in fuelconsumption.

Often, refinery hydrogen plantsrequire minimising the export steamagainst fuel savings. Hence, the level ofcombustion air preheat has risen overthe recent years up to 550ºC. Thisnecessitates the use of a two-stage airpreheater with an economiser boiler inbetween, to obtain the proper heatrecovery approach and integration inthe convection section.

The consequences of higher levels ofair preheat are towards selection ofproper burner design, along with agreater need for low-NOx features of fuelor air staging, since hotter combustionair causes higher levels of NOx in thefluegas. Also, proper design provisionsare required for covering the cold-endcorrosion of the air preheater in cases ofextreme range of ambient temperaturesand/or high sulphur level in the make-up fuel.

OptimisationBased on the selected flow sheet andcase-specific data, a sensitivity analysisis conducted on the major processparameters. This in turn determines thehourly operating cost as well as theincremental capital investment in theplant to ensure overall economics with-in the given evaluation criteria. Thevariables assessed are usually the S:Cratio, reformer outlet temperature andpressure.

The S:C ratio is set at a level above aminimum that prevents coking in theradiant tubes. For natural gas-basedplants, the S:C ratio ranges between2.7–3.0, while for naphtha-based plantsit is 3.0–3.5, depending upon the qualityof naphtha.

Reformer outlet temperature deter-mines (for a fixed S:C ratio) themethane slip, which indirectly relatesto the feed consumption. The outlettemperature also affects the reformertube design temperature and thereforeits wall thickness. Reformers in hydro-gen plants are designed to operate atoutlet temperatures typically rangingfrom 840 to 920ºC. Thus, the optimisa-tion of this parameter is dependant onthe relative price of feed and fuel as wellas reformer mechanical design.

Lower reforming pressures arefavourable for feed conversion as well asreformer design, but it is establishedeither on the basis of desired producthydrogen pressure, or in relation to theavailable feed pressure. Typically,reformer outlet pressure in hydrogenplants lies between 18–35 barg. Whenboth feed and product compression arenecessary, reformer outlet pressure is

optimised for minimised overall powerconsumption and capital cost with dueconsideration to compression stagingand to the fact that generally it is eco-nomical to maximise feed compressionrather than product hydrogen. In someborderline cases, ejector systems can beemployed to boost the feed pressure by2–4 bar using the higher pressure pro-cess steam as the motive fluid.

Heat recycleGeneration of raw hydrogen fromhydrocarbon feeds involves high tem-perature processing (irrespective of pro-cess route). It is logical that the thermalefficiency of the process is directly relat-ed to the degree of heat recovered fromthe waste heat available within the pro-cess. It is also logical to claim that anyrecycle of the waste heat to the processitself, compared to shifting it to coldutility, will improve the net energy effi-ciency of the process.

In steam reforming technology, fewinnovative concepts and developmentshave been realised to optimise the heatintegration in terms of recycling ofwaste heat to the reforming process.Apart from the basic objective ofimproved efficiency, these advancesneed to maintain cost effectivenesswithout any compromise on reliability,safety and operational flexibility. Someof these concepts are HT heat recycle,MT heat recycle and LT heat recycle.

Advanced concepts in terms of recu-perative or regenerative reforminghave been developed to reduce theduty and size of the reformer as well asof the related steam system. Few inno-vative concepts and related mechanicaldesign have been realised for using thehigh level heat of the reformed gasbetween 600–900°C for reforming partof the feed. While improving the ener-gy efficiency, it also lowers the amountof excess steam against fuel savings,apart from reducing the size of thereformer.

This concept comprises mainly aheat exchanger type reformer havingcatalyst-filled tubes. One of the config-urations, which is proven and is inplace is the “enhanced heat transferreformer” (EHTR). The hot process gasenters the shell side and mixes with thegas coming out of the tubes beforeflowing up for heat recovery in a so-called 2 in, 1 out, configuration. Thisconcept also carries exceptional poten-tial for plant revamping for achieving astep increase in hydrogen capacity (upto 30%), without affecting the existingreformer or posing limitations on thesteam system.

The MT heat recycle concept qualifiesfor the level of pre-reforming, whichutilises waste heat between 400–650°C

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in the convection section, which has been already describedearlier.

The LT heat recycle concept typically finds interest in recov-ering the low-level waste heat of the reformer flue gas in therange of 150–350°C and upgrading to higher-level heat. Itinvolves evaporating the hot condensate to process steam bysaturating the hot (desulphurised) dry feed.

Though it tends to overlap with the other options of com-bustion air or boiler feed water preheating, it allows utilisa-tion of the process condensate without any treatment orprocessing and offers good quality export HP steam, apartfrom its increased rate without increased firing.

Advanced process controlHydrogen plant process control has evolved over the yearsfrom simple pneumatic or semi-automatic control systemsto modern, fully automatic integrated DCS systems, alsoinvolving complex multi-variable control and online plantoptimisation. This advancement has led to improved effi-ciency, reliability, safety and ease of operation.

The advanced plant control systems often use specificdiagnostic routines as well as simulation models and algo-rithms for plant optimisation and parametric control. Theycan conduct direct data reconciliation and multi-variablesensitivity analysis based on time-based measured data. Fur-ther, they can be equipped with a functional decision sup-port system for providing optimised external set pointsagainst a defined objective function or operational targets(best feed for minimum operating cost or maximised hydro-gen production etc).

Also in a multiple feed-based plant, automatic feedstockchangeover systems are often desired/employed, which pro-vide smooth, faster and reliable feed changeover flexibilityby avoiding operator-induced errors as well as any stepreduction in the production.

Hydrogen technology, though quite matured, carries sev-eral options and critical aspects for ensuring and enhancingoperational reliability, flexibility and specific efficiency foreach plant design. Thus, it requires a detailed process opti-misation while applying proven operating conditions anddesign severity. Other factors, in terms of environmentalcompliance for emissions and minimised effluents, as well assophistication of operational control, also become importantin establishing the hydrogen plant design.

This article is based on a paper presented at the AIChE SpringMeeting, New Orleans, USA, March 2003.

Sanjiv Ratan is director syngas technology at Technip Benelux inThe Netherlands. He is responsible for the technological promo-tion, development and consolidation of the hydrogen-syngas product line. He holds aBachelor of Technology degree in chemical engineering from theIndian Institute of Technology, New Delhi. He has over 20 yearsof experience and has held various posts in the design, engineer-ing and state-of-the-art effectiveness of hydrogen-syngas plants.E-mail: sratan@technip.com

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