Developments in Natural GasReforming Technology for Syngas

Natural gas is the main feedstock for the production of ammonia, methanol, hydrogenand many other catalytic syntheses, requiring

syngas of various H2 /ratios. Conventional and state-of-the-art reforming processesare described. New developments are under way to reduce investment cost and pollu-

tion and improve efficiencies and operational safety.

Pan OrphanidesOrphanides Consultants, Agia Paraskevi, Greece

INTRODUCTION

Hydrogen and CO are two of the most importantbuilding blocks of the chemical industry. Hydrogenis mainly used in ammonia and methanol synthesisand petroleum refining. Mixtures of Hydrogen andCO are used in the OXO synthesis for theproduction of higher alcohols and of syntheticfuels. CO is a major component in the productionof paints, plastics, foams, pesticides andinsecticides. The production of hydrogen and CO ,called usually Synthesis Gas or Syngas, is carriedout mainly by the following processes, whennatural gas or other light hydrocarbons are used asfeed stock:

* Steam reforming (primary or primary/secondary)* Autothermal - oxygen-enhanced reforming,Partial oxidation

Steam reforming is the process most widelyapplied for the generation of syngas andhydrogen. Utilising conventional supported nickelcatalyst, the highly endothermic reaction betweennatural gas and steam is usually carried out in adirect fired reformer. Secondary or autothermalreforming is a type of steam reforming that utilise

the heat of partial combustion, by air or oxygen offeed stock to supply the heat required to sustainthe endothermic steam reforming reaction on anadiabatic catalyst bed. The feed stock to thesecondary or autothermal reformer is either naturalgas or partly steam reformed natural gas or amixture of both.

Partial oxidation (POX) does not utilise catalyst anddepend on partial combustion, usually by oxygenof the feed stock to internally supply the heat ofreaction.

Although heavy oils and coal are less expensivefeed stocks, the capital cost for heavy fuel oil POXunits, or for coal gasification can be two to threetimes more expensive than for a natural gasreforming plant. Coal in conventional gasificationplants is used in countries without natural gasavailability or for political reasons. From the otherside a dozen of advanced coal gasificationprocesses are under development around theworld and few of them have reached industrialmaturity [1 ].This development stands in sharp contrast topractically no further development in the field of

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partial oxidation. Reforming of natural gas isexpected to remain the most cost effective way togenerate syngas for at least this decade andprobably beyond year 2000.

Beyond this time frame, new advanced technologyreforming processes will have an increasing sharein the syngas production. Technologies usingregenerative or nuclear energy sources willbecome also available mainly for hydrogenproduction. These technologies will all involvewater splitting by one or the other energy source.Nuclear energy may be used for water electrolysisas a mean of storing electricity produced during off-peak hours. Thermochemical and hybridprocesses will be technologically proven and solar-, geothermal-, biomass-, wind-, sea water thermalgradient- hydrogen production will still be indevelopment, whereas fusion energy the last oneto become available, could be used for directthermal decomposition of steam.[ 2 ]

In this report we will focus on the key factors(efficiency, operational safety, cost andenvironmental impact) affecting today's state ofthe art and the new developments in the naturalgas reforming systems design only. Within the timeframe considered, this feed stock and thistechnology, will continue to supply more than80% of syngas generated world wide.[ 3 ]

SYNGAS GENERATION BYCONVENTIONAL DESIGN REFORMERS

Steam and oxygen reforming technologies fornatural gas and light hydrocarbons are reviewedalong with their respective advantages anddisadvantages

Conventional and state of the art steamreforming [Primary Reforming]

Primary Reformer Basics and reforming Catalysts

In a conventional steam reforming processhydrogen and CO/CO2 are produced by reactingmethane with steam over a nickel catalyst at hightemperatures. The catalyst is contained in thereformer tubes, which are located in a box typefired furnace that provides the large endothermicheat of reaction.The primary reformer catalyst activeelement is metallic nickel (15-25%) finelydispersed on suporting material, which is either a-alumina, or Ca-aluminate, or magnesia - aluminaspinel[ 4 ]. The basic reactions of the steam

reforming of methane are expressed by theequations shown in Table I

Reaction (1) is known as steam - methanereforming, while reaction (2) is referred to as thewater gas shift reaction. Both reactions arereversible and approach equilibrium. In addition tothe desired reactions (1) and (2), other sidereactions (3, 4, and 5) are under certain conditionsalso possible. Selection of suitable catalyst andappropriate operating conditions will promotereactions (1) ana (2) and suppress reactions 3 to 5.Reaction (6) is taking place in steam reformerswhere CO2, available downstream the CO2removal section is recycled back to the reformer inorder to increase CO formation in the syngas{methanol, oxo syngas production}.

The conversion is favoured by high steam tocarbon ratio (in excess of the stochiometricquantity), high outlet temperature, low pressureand high catalyst activity. There are restrictions andlimitations for a maximum conversion:

Too high steam to carbon ratio will make theprocess inefficient energy wise , largequantities of excess steam have to becondensed and high volumetric steam flowwill increase the equipment cost. Low steamto carbon ratio will improve the energyefficiency, but can form carbon in thecatalyst pores and other undesirable by-products.! 5]

Too high operating temperature posesproblems of heat transfer and lowers themechanical strength of the tube material(metal temperatures up to 960°C underpressure of about 35 bar). Low operatingtemperature reduces the conversion, butsome times this offers an attractive possibilityfor conversion completion under optimumconditions down stream the primaryreformer[6].

Too low operating pressure increases thecost of equipment and makes the reformingprocess inefficient, as there is alwaysnecessity to compress the produced syngasat higher pressures for further processing(purification and synthesis). There aremechanical limits for higher reformingpressures, but in general higher reformingpressure reduces the overall operating andinvestment cost.

Catalyst with too high activity could at firststage improve the conversion rate butbecause of its sensitivity, it may lose easily

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activity due to impurities in the feed stock, orbecause of maloperation. Small size catalystin a given total reformer tube volume willpresent a higher surface to catalyst volumeratio and thus a higher conversion rate and abetter approach to equilibrium, but alsogives a higher pressure drop. Loss of activitymay lead to tube overheating, as reducedconversion means less heat absorption forthe endothermic reforming reaction.! 7 ]

The today offered nickel based catalysts can ingeneral fulfil the requirements for: a) enhancedactivity, b) reasonable life duration withoutappreciable deactivation, c) good heat transfer,acceptable pressure drop, good thermal stabilityand sufficient mechanical strength to withstandsevere conditions during start up and shut downs,d) in situ regeneration from accidental poisoning orcarbon formation, e) operation at low steam tocarbon ratio, and f) heavier than methanehydrocarbons (catalyst alkalised with potash)[8].Often some of the above catalyst characteristicshave to traded-off against some others, as the "allpurpose" catalyst optimised for all serviceconditions is not yet invented.

Primary Reformer Configurations

Several configurations of reforming furnaces are inuse today, characterised mainly by the dispositionand position of the burners. As a fired reformer isheat transfer limited, it is a main concern to designthe reforming furnace in such a way, that theburner flame do not reach thehot tube wall and that the highest heat flux can begiven only to zones of low process temperatureand high feed stock partial pressure. The mostcommon types of fired reformer configurations arethe Top Fired and the Side Fired Reformer .In Fig. 1 and Fig. 2 the respective profiles of tubeskin temperature , heat flux, and methaneconversion are shown.

Top fired reformer use multiple rows of tubes withburners located in the arch on each side of thetubes. The heat to the tubes is supplied by theradiating products of combustion. Main advantageof this configuration is the few burners relative tothe tubes, the higher radiant efficiency, thepresence of the high heat flux zone in the "cold"inlet of the feed stock and the very large tubenumber which can be accommodated in oneradiant box. Main disadvantages is the limitation inthe heat input control and the hot operating level atthe top.

The Side fired reformer has multiple radiant wallburners along both side walls and one row of tubes

in the middle of the box. The heat to the tubes isemitted from the radiant walls. The main advantageis the uniform heat distribution and the very goodheat input control. Disadvantages are the requiredlarge number of burners, the lower radiantefficiency and the size limitation of the fired box(single box for 100 to 150 tubes).For critical reformer applications, i.e. CO2 recycle aside fired reformer is preferred, due to the higherrisk of carbon formation on the catalyst-! 91

Other type of fired reformer are the Bottom firedand the Terraced wall Reformer.

The high heat flux and the high tube skintemperature at the upper part of the reformer is themain characteristic of top fired reformers . In theside fired and the terraced wall reformer the heatflux along the reformer tube and the conversionare more uniform.

Reformer efficiency and Environmental impact

In a conventional reforming process only 40% offurnace duty is absorbed by the endothermic heatof reaction. About 35% are recovered in the formof waste heat export steam , by utilising part of thelatent heat of the syngas for CO2 removal and bypreheating feed stock-combustion air or BFW inthe convection section of the reformer, where fluegas is cooled from 900 to 950 °C to about 125 to150 °C. 20 to 25% of the heat applied are lost inthe stack, in cooling water and in heat losses.

Depending on the burner configuration (low NOxburners) and the combustion air preheatingtemperature, the NOx content in the flue gas canbe kept below 200 mg/Nm3[10], but many highefficiency reforming plants with combustion airpreheating temperatures above 450 °C havehigher NOx emission levels, despite the use of lowNOx burners. High NOx values occur in somereforming plants for ammonia production, whennon scrubbed purge gas from the synthesissection containing NH3 are used as fuel in thereformer. Depending on the content of sulphur inthe fuel gas , there is a small SO2 emission fromthe stack. Even 800 ppm of sulphur in the fuel gas,which is for natural gas or LPG a very high value,the SO2 emission is relatively low with regard toemission regulations in USA or Germany. It isevident that large quantities of CO2 are released inthe atmosphere from the reformer stack . Noisewith the today forced draft burners is not any morea major problem, as it was with the old generationinduced draft burners, but the addition of a fan willincrease capital cost, plant complexity anddowntime.

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Reformer Tubes

The most critical item in a fired reformer are thetubes in which the catalyst is placed. Tubesconstitute up to the 30% of the total reformer cost.The most common materials are :

25Cr/20Ni(HK40)

25 Cr / 35 Ni, Nb (HP with Nb)

For mechanical and process reasons , a typicalreformer tube is 11 to 14 m long, with ID of 100 to130 mm and wall thickness between 8 and 15 mm.Tubes are usually centrifugally casted andmachined in the internal borehole.

The well known HK 40 material is suitable for steamreformers operating at low pressure. For higherreformer pressures, application of more expensivematerials like HP with Nb prove to be more costeffective. Despite these improvements reformertube materials work inevitably under creepconditions.

Consequently their service life is limited. Atemperature increase of only 20 °C above designtemperature can reduce service life by 50% (from10 to 5 years)[11]. Several methods are tried toestimate residual tube life. Non destructive testsare not very reliable. Destructive tests on a regularbasis in combination with dimensional checksprovide better basis for tube replacement decision,while minimising the risk of unexpected tubefailures.(12]

Many Reformer plant shut downs are attributed toreformer tube failures.[13] The major reasons ofthe failures are:

Poor catalyst performance, uneven flow intubes

Thermal and pressure cycling due to manyshut downs and start ups

Overfiring during start up, or loss of steamsupply during shut down

Poor burner operation, uneven heating oftube

Condensation at the bottom of the tubeThermal shock because of water carry over.Catalyst aging" or loss of acivity.

Conventional and state of the art ofCombined Reformer, Oxygen EnhancedReformer - Oxygen/ AutothermalReformer - Gas Heated Reformer(GHR)

Ammonia production is the most importantreforming operation. There are more than 400reforming plants in operation around the globeproducing syngas for ammonia synthesis. Thenitrogen required for the ammonia synthesis isusually supplied in the form of process air injectedtogether with steam reformed gas in an otherreformer called Secondary or AutothermalReformer. The secondary reformer consists of arefractory lined vessel housing in the upper part amixing and burner assembly, a combustion zone inthe middle and a catalyst bed in the lower part. Thecatalyst is operating under high temperatures in theupper part of the bed and must be able towithstand temperatures up to 1370°C andabrasion resulting from the highly turbulent gasflow. Usually a shield layer from refractory material islaid at the top of the catalyst bed. The catalyst usedin the secondary reformer is also nickel based, withthe same supporting materials as the primaryreformer catalyst, but because of the higheroperating temperatures the nickel content is lower(risk of nickel crystallite sintering), thus the activityof the catalyst is much lower.

In Fig. 3 a typical reforming plant with Primary-Secondary Reformers, Waste heat reccovery fromflue gas and process gas, for Ammonia productionis shown. In Fig. 4a and 4b sections of typicalsecondary reformers are shown. The refractorylined Incoly central tube in the Uhde design ischaracteristic for Autothermal Reformers operatingin serie with a Primary Reformer, in order to keepthe Transfer Line from the Primary to theSecondary as short as possible. This advantage isin some cases off-set by breakages of the freestanding part of the central tube, leading to gasbypass and consequently increase of the methaneslip. In the last reference of UHDE's Secondaryreformer the typical air distributor, consisting of twoconcentrical, ring - shaped Incoloy tubes, isreplaced by two row s ofperipheral nozlles injectingair in a way, which creates a double vortex,providing so an intensive mixing with the downflowing gas.

As it is shown in Table II, oxygen is partlyconverting methane, hydrogen, and carbonmonoxide of the steam reformed gas in the upperpart of the secondary reformer. Principallyhydrogen is consumed in the combustion zone,together with partly converted methane and CO,supplying the necessary heat for the almostcomplete conversion of the remaining methane inthe catalytic steam reforming zone at the lowerpart of the secondary reformer in conformity withreactions (1) and (2).

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In order to supply the at least stochiometricallyrequired quantify of nitrogen for the ammoniasynthesis, a minimum amount of residual methaneshall be left at the exit of the primary steamreformer. From the other side the heat suppliedfrom the partial combustion (oxidation) ofhydrogen at the upper part of the reformer shall besufficient to complete the catalytic steam reformingreaction till a very small fraction of residual methane(methane slip) of about 0.6%. To achieve that, thegas is heated in the partial oxidation zoneabovel300°C and leaves the reformer at about960 to1020°C.

The gas is cooled in special design waste heatboiler followed usually by a steam superheater.

Oxygen - Enhanced Reforming (OER)

A consequent development of the combinedreforming for ammonia production, is the use ofoxygen enriched air as oxidant in the autothermalsecondary reformer . In this mode of operationmore load is shifted from the primary to thesecondary reformer reducing the required size ofthe primary and the severity under which itoperates, but more hydrogen from the steamreformed gas has to be oxidised in the combustionzone of the autothermal secondary reformer. Byinjection of oxygen in the process air oxygencontent can be increased to 32 to 35%, primaryreformer firing can be reduced by 25% and gastemperature can by lowered from 860 to730°C. Tohave reformer tubes under similar mechanicalstress conditions, reformer pressure can beincreased from 20 bar to 36 to 37 bar, thusachieving substantial saving in compressionpower. Further more fuel consumption andemissions are reduced, as the conversionefficiency of the autothermal reformer is 65 to 70 %against 40 to 45 in a fired reformer. It is selfunderstood that in the overall savings the cost ofproducing or buying oxygen has to be considered.

The same overall improvements can be obtainedwhen air is used in excess in the secondaryreformer. It is evident that , in this case an overstochiometric for ammonia synthesis syngas will beproduced. Excess nitrogen has to be removed,preferably cryogenically either prior the syngascompression ( C F Braun Purifier process [ 14 J),or together with inert gases at loop pressure (ICIAMV process [15]), partly off-setting theadvantages of shifting reforming load from primaryto secondary. In the ICI/LCA process excessnitrogen is removed by a PSA unit [16].

Oxygen reforming

92% of the methanol in North America andEurope is produced in units with conventionalsteam reforming, operating at a relatively moderatepressure of 15 to 24 bar and relatively hightemperature ( 860 to 890°C), using almostexclusively natural gas (93 to 97% CH4) as feedstock. The main reason for the low pressure is acompromise for higher methane conversion.Given the high H/C ratio of methane and theadditional hydrogen produced by thedecomposition of the process steam, the so calledstochiometric number (SN), defined as

SN = (H2 - CO2 )/(CO + CO2) (10)

is between 2.7 and 3.0, much higher than theoptimum value for methanol synthesis, which is2.05 and much higher H2/CO ratio than required inmany other synthesis processes ( Oxo alcohols,reducing gas, synthetic fuels) . This hydrogen inexcess has to be compressed at the methanolsynthesis pressure, to be purged from thesynthesis for use as fuel in the reformer, or asadditional hydrogen in an adjacent ammonia unit. I n

An interesting application of oxygen blownreformer is in an existing conventional methanolplant. This oxygen blown autothermal SecondaryReformer installed down strehm the fired reformer,is burning sufficient Hydrogen to complete themethane conversion to about 98%, but leavingsufficient excess hydrogen in the syngas, so thatthe purge gas from the methanol synthesis can beused together with nitrogen from the air separationunit for additional ammonia production[ 17 ]. Thesystem can be further optimised by shifting firingload from the Primary to the Oxygen blownreformer, and so reduce heat losses andcompression energy to compress syngas tomethanol synthesis pressure, by increasingreforming pressure.

When CO2 is available, it can be recycled to thereformer, to reduce the SN. CO2 recyclingincreases the fire duty and the investment cost ofthe reformer. CO2 reforming prevents carbonformation at steam to carbon ratio as low as 1.5, asCO2 reforming reaction (5) is faster than the carbonforming reaction (3). This advantage of CO2reforming can not be put to full service inconventional steam reformers for methanol, as theyoperate usually at SIC ratios of 2.5 and above.

By combining steam reforming and oxygenreforming, as shown in the arrangement of Fig.5 it is possible to generate a syngas with a

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stochiometric number of 2.05. The PrimaryReformer operates at a much higher pressure, asthe firing duty can be reduced, because areformed gas at the outlet of the steam reformer,with higher methane slip can be tolerated. Theremaining methane is further reformed withoxygen and steam in the autothermal reformer.Part of natural gas feed can even supplied directlyto the autothermal reformer, provided sufficienthydrogen is produced in the primary reformer tosupply the necessary heat for the endothermicreaction on the secondary reformer catalyst. Thespecific oxygen consumption is about 0.41 per tonof produced methanol. The fired reformer duty isreduced by 45 to 50% and the makeup gascompression requirements by more than 50%.

The advantages/disadvantages of combinedreforming for methanol production againstconventional steam reforming are the following:

low residual methane in the syngas,optimum SN , higher reforming pressure =low compression energy, almost offsettingenergy for air and oxygen compression.

less natural gas consumption per ton ofmethanol because of better thermalefficiency of auto thermal reforming

less NOx and CO2 emission. For 50% ofmethane steam reformed and oxygenreformed together with bypass methanedirectly oxygen reformed, the emissionsare reduced by about 75 and 35%respectively.[ 18 ]

single train for capacities up to5000 tpd.because of reduced equipment size.

for plant size of about 2500 tpd capacitytotal investment cost including airseparation unit are up to 15% higheraccording to some estimates^ 9] or less bysome others[20], than the case ofconventional single train steam reforming

the combined reforming process is viableand commercially proven, but only limitedreferences are available and the operationof these plants with oxygen burners is stillless reliable than the well proven steamreforming

high natural gas prices and environmentalrestrictions could make combinedreforming a viable solution for very largescale single train methanol plants

reduction of S/C ratio to increase thermalefficiency in connection with high COpartial pressure may result in severecorrosion phenomena of enhancedcarburisation of high austenitic metallicsurfaces, phenomena known as metaldusting.

Autothermal oxygen reforming alone can also beused to produce stochiometric makeup gassuitable for methanol synthesis, or for syntheticfuels. In this case the heat required for theendothermic catalytic steam reforming is suppliedby partial oxidation (substochiometric combustion)of methane and higher hydrocarbons in thecombustion chamber on top of the catalystbed.

The reaction mechanism of methane with oxygenat an overall O2/C ratio of 0.55-0.6, at first stepproceeds according to the highly exothermicreaction (10) of Table II, with an O2/CH4 ratio of 1.5until all the oxygen has been consumed. Excessof methane is then further converted in the socalled thermal zone of the combustion chamberby homogeneus gas phase reactions, the mostimportant being thermal methane reformingand water- gas shift reaction, according toreactions (1) and (2) respectively of Table I. Thereaction mechanism of methane with oxygen at anoverall O2/C ratio of 0.55-0.6, at first stepproceeds according to the highly exothermicreaction (10) of Table II, with an O2/CH4 ratio of 1.5until all the oxygen has been consumed. Excessof methane is then further converted in the socalled thermal zone of the combustion chamber byhomogeneus gas phase reactions, the mostimportant of which are thermal methane reformingand water- gas shift reaction, according toreactions (1) and (2) respectively of Table I. In thecatalyst bed the hydrocarbon conversion iscompleted through heterogeneus catalyticreactions as in a conventional SecondaryReformer.

The very fast and highly exothermicsubstochiometric reaction of oxygen withhydrocarbons always involves the risk ofincomplete combustion and soot formation, whenthe gas stream mixing and the temperaturedistribution are not homogeneous. The burnerdesign is a key element in the smooth operation ofAutothermal Reformer processing hydrocarbons.To avoid carbon formation a special design forcomplete mixing of oxygen with the feedgas isrequired. Lower temperatures at the inlet have tobe used so that only a small fraction of the reactioncan start at the mixing point, the bulk of

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combustion being completed in the combustionchamber.

Some special design burners are operatingsuccessfully in few small size oxygen reformers[21], but their operating life is relatively short.Usual causes are burner tip overheating due torefractory shield failure, or cooling water problems.In Fig. 4c & 4d examples ofAutothermalOgygen Blown Reformers and in Fig. 4e aflow sheet with Autothermal Reformer and CO2recycling for syngas production with low H2 /COratio, are shown.

Instead of a fired Primary Reformer, an adiabaticreactor can be placed in front of the AutothermalOxygen Blown Reformer filled with a specialcatalyst promoting steam reforming at lowertemperatures. This reactor is an adiabatic steamreformer, called Prereformer and operates attemperatures of about 550 to 600 °C[22].Prereformer is also used in connection withconventional steam reformers in order to reducethe firing duty of the reformer and to hydrogenateand convert heavier hydrocarbons. Catalyst costwill be higher with Prereformer, since the catalyst issusceptible to fouling and must be replacedfrequently, but the pre-reforming catalyst acts as aguard bed, so run lengths of the primary reformingcatalyst will be increased.

As the reaction in the prereformer is endothermictoo, reheating of the prereformed gas is requiredto take full advantage of the operation, but this iscomplicating the whole prereformer set-up, (seeFig. 5a for various installation options ofPrereformer).

Assessment of the today State of the Art in naturalgas Reforming Technology

A drastic development has taken place in thereforming system design in the last fifteen years.This development is achieved in a series ofprocess improvements and by the introduction ofnew materials, including mechanicalandmanufacturing innovations in the area of burners,reformer tubes, convection systems, catalyst,instrumentation and automatic process controldesign, insulation materials. In the area of energyconservation the concepts of combustion airpreheating, lower S/C ratio, improved energycycles, i.e. the use of gas turbines, high efficiencysteam turbines and compressors, the use of lowlevel heat recovery systems, va'pourrecompression systems, PSA and membraneseparation technology, have led to an increase in

the energy efficiency, the plant safety andreliability, as well as environmental improvementsto levels considered highly uneconomical in thepast. In Table III these achievements analysed.

Despite all the above improvements, many weakpoints are still inherent in today's state of the artreforming technology: Improved efficiency createda considerable amount surplus waste energywhich had to be exported outside the reformingplant and this is not always feasible or desirable.Preheating the combustion air to temperatures ashigh as 505 to 550 °C increase the preheater costsignifically and also the NOx content in the flue gasto levels requiring denitrification. Furthermore highcombustion air temperatures in connection withhigh feed stock- steam mixture temperatures andlower steam to carbon ratios increase the heat fluxat the upper part of the tubes and the risk of "hotbands" and carbon formation there [23]. This inconnection with higher reformer pressure bringsreformer tubes closer to the creep limits.

Special techniques were successfully applied toshift reforming load and severity away from firedreformers (prereformer, combined reforming, andoverstochiometric secondary reforming [24]), butstill today's direct fire reformers are somewhatcomplicated and require close attention to operatesafely. Heavy investment in bulky equipment areneeded to efficiently recover the large amount ofproduced waste heat.

The limitations of the conventional ReformerTechnology are summarised in Table IV.

An interesting development to reduce load andreforming severity in fired reformers is the use ofthe adiabatic Prereformer [25] and theExchanger Type Reformer (ETR) [26],developed by Air Products and shown in Fig. 6.The ETR is a pressurised refractory lined vesselcontaining free standing open end reformer tubessupported from a tube plate located at the bottomcooler side of the reformer. About 25% of the totalfeed stock mixed with steam and preheated toabout 380 °C is entering the ETR from the bottomand flows up through the tubes and it is steamreformed by the convection heat supplied from therest 75% of the feed stock already steam reformedin a conventional fired reformer and entering at thetop of the ETR, where mixed with the alreadyreformed 25% portion of the syngas is flowingdownwards along the tubes.

The ETR technology in existing Reform inig plantsallows for plant capacity debottlenecking withoutincreasing reformer fuel and consequently

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reformer emissions (NOX) and without increasingsteam production.

This last development is the breakthrough point,which opened the way to the completereplacement of the fired steam reformer by moreefficient advanced reformer designs, which we willdiscuss in the following paragraphs.

ADVANCED DESIGN REFORMINGSYSTEMS

In the Fig. 7 the schematic diagram of a reformerconfiguration (LCA) is shown, developed by ICIand in operation at Severside UK since April 1988as a dual 450 tpd ammonia plant. In this type ofreformer configuration, called Gas HeatedReformer (GHR), the mixed feed gas with steampass through central bayonet type tubes filled inthe annulus space with catalyst, while the reformedgas from the secondary reformer, at about 960 to1000° C flows on the shell side through thin tubesurrounding the catalyst tubes, providing the heatfor the primary steam reforming. Each of the twoGHR in Severside contains 78 tubes filled with 6m3 of the standard ICI 4 hole reforming catalyst,57-4M. The 2.2 m diameter and 12m high,refractory lined and water jacketed pressurevessel of the GHR contains the removable bundle.

To complete reforming to acceptable methane sliplevels, when air is used as oxidant, anoverstochiometric gas is produced, requiring theelimination of the excess nitrogen from the syngasbefore the final synthesis to ammonia .[27]

The basic chemistry and the underlying principlesof steam reforming are unchanged from that in aconventional steam reforming for ammoniaproduction. The secondary reformer catalyst was inthe original version a monolithic type of extremelyhigh activity having a volume of only 1/8 of aconventional secondary reformer catalyst, butbecause of the rapid pressure drop build up, dueto blockage of the gas passage, it has beenreplaced in early 1992 by a conventional typesecondary reformer. Increased pressure drop hasbeen also observed in the primary, Gas HeatedReformer (GHR) due to breakage of the lowerpart of the catalyst and was directly related to thelarge number of thermal cycles (stops, restarts)during the early operation period. With animproved bayonet tube design and with less shutdowns it is hoped that this problem is resolvednow. In Fig. 8 a section through GHR is shown.

Despite the rather sophisticated mechanicalarrangement of the GHR no major problems havebeen encountered. Only some minor corrosion onthe tubes which are eliminated by selection ofmore suitable materials and by improving themanufacturing methods. The secondary reformerburner operates at slightly higher temperaturesthan in a conventional plant, due to the relativelyhigher air rate. The ICI LCA reformer configurationhas not yet operated with oxygen enriched air forproduction of a stochiometric syngas compositionfor ammonia synthesis, nor with pure oxygen formethanol synthesis suitable syngas. A 164 MTDdemonstration methanol plant is underconstruction near Melbourn, Australia for BHPPetroleum, based on Id's GHR technology andusing pur oxygen.

Another type GHR is in operation in a 600 MTPDammonia unit since 1988 in Grodno- Belorussiya,near the Polish frontier, developed by GIAP[28].The so called Tandem GHR is shown in Fig.9.The plant is producing hydrogen for down streamammonia and caprolactam plants , the secondaryreformer operating with oxygen. The tubes are hotrolled, not centrifugally casted, containing 23% Crand 18% Ni. In spite of relatively low alloy tubematerial, no metal dusting has been observed. Theplant capacity is brought to the equivalent of 750MTPD ammonia.

In August 1992 Brown & Root Braun (B&R-B)acquired rights to use commercially the TandemReformer. It is B&R-B intention to incorporate theTandem Reformer in the Braun Purifier process bythe use of atmospheric air in the SecondaryReformer and to increase capacity to 1000 MTD byextending the tube length, keeping the samereactor configuration.

An other development announced by MW Kelloggis schematically shown in Fig.10.[21] The effluentof an autothermal reformer is supplying the heat forcompletion of the conversion in a open tubereforming exchanger. The autothermal reformer isdesigned to operate either with enriched air or withoxygen. Kellogg Reforming Exchanger System(KRES) is not yet in operation, but MWK is claimingthat the concept is based on proven componentsand in their patented open tube reformingexchanger having a mechanically simple design. InFig. 11 the Reforming Exchanger is shown.

The flow split between autothermal reformer andreforming exchanger is 75/25.In case enriched air or pure oxygen will be used,Kellogg's proprietary water cooled burner will beapplied, shown in Fig. 12, which has already hadcommercial reference in operation with pure

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oxygen and steam. The enriched air stream entersthe top of the reformer through a refractoryprotected injector tube and flows out at theperiphery of the lower part of the injector. Mixedfeed enters tangentially to the reformer getting avortex like flow. Because of the lack of hydrogen inthe feed to the Autothermal Reformer the designof the burner is very critical in order to ensureconversion free from carbon formation. Steam tocarbon ratio is 3.3 to3.8. This is a high S/C ratioselected to minimise carbon formation risk andcarburization corrosion(metal dusting).

In this design, as well as in all other reformingsystems without fired reformer there is no wasteheat available to produce steam, to preheat fuel,BFW and process air. This is usually supplied by aseparate heater fuelled with natural gas, or from theexhaust of the turbine driving the air/oxygencompressor. Feed and steam is heated beforeentering the reforming exchanger by the effluentof this exchanger. The reforming exchanger isdesigned to operate at 38 bar pressure.

An other interesting reformer configuration isdeveloped by UHDE[ 29 ].

Figure 13 gives in section the demonstrationReactor of Uhde's CAR (Combined AutothermalReforming). In this design the reformingexchanger and the autothermal reformer arehoused in a common refractory lined vessel. Opentubes, filled with conventional nickel catalyst onalumina rings are hanging free from a sandwichtype tube-plate . The total feed (desulfurizednatural gas) is split up into two parts, a primary and asecondary. The primary feed is mixed with steamand it is steam reformed flowing down in thecatalyst tubes. These reformer tubes are externallyheated bypartly reformed hot gas, returning upfrom the partial oxidation chamber below. In thischamber secondary feed and partly steamreformed gases are partly oxidised with oxygen oroxygen enriched air, supplied through peripheralwater cooled injectors. Secondary feed andoxidant injectors are creating a strong vortexmovement resulting in a high turbulent flow patternwith the gas blown out of the catalyst tubes. This isensuring a temperature and reactionhomogenisation. Unlike Id's LCA system CAR isdesigned to use also oxygen, or oxygen enrichedair.

The fact that part of the feed gas is only partiallyoxidised, without undergoing a subsequentsecondary steam reforming, deteriorates slightlythe efficiency, as POX is a reaction, againstcatalytic reforming taking place away fromequilibrium. From the other side the arrangement

of the oxidation chamber at the bottom, eliminatessome risks for catalyst deterioration inherent toautothermal oxygen, or even air secondaryreformers.

A demonstration CAR reformer is in operation inStrazske Czechoslovakia since December 1990.The reformer has 19 tubes OD 150 mm and 12mlong. Material is HK 40 and HP modified. 6 oxygenand 3 secondary water cooled nozzles arepositioned superposed around the upper part ofthe partial oxidation chamber. The block diagram isshown in Fig. 14. The CAR is designed to producea gas containing 67% H2, 24% CO, 7.6% CO2 .Oxygen consumption is 0.5 mol/mol of C. Steamto carbon ratio is 1.6 total and 2.5 in the primarypart. Operating pressure 17 bar. The reformer isfed with 2600 Nm3/ h natural gas in the primaryand with 1508 Nm3/h in the secondary. 2340Nm3/h of 95% oxygen are consumed. After 9months of trouble free operation, even at a slightlyhigher S/C ratio enhanced carburization (metaldusting) was found on HK and Incoloy 800material[30]. The total S/C ratio has beenincreased to 2.3 and this stopped the metaldusting. New more appropriate materials (Inconel601 and others), after long term testing underoperating conditions, have been installed during amodification implemented in September 1992,with the objective to operate the CAR at lower S/Cratio. However on Operating Company's decisionCAR continues to operate at higher S/C ratio andat slightly lower pressure.

UHDE is designing the CAR also as a simplereforming exchanger without the partial oxidationchamber, to be operated in a similar way like theETR of Air Products. The purpose is to be able tooperate it in connection with an authothermal orconventional reformer. ICI, MWKellogg and othersare following the same route. [31]

The open tube type GHR is certainly simpler in itsconstruction, than the closed ,or bayonet tubetype. But the advantage of closed tube GHR,when in operation in connection with anautothermal reformer, is that the autothermalreformer is receiving a partly steam reformed gas-stream together with a mix of feed and steam. Thisarrangement certainly facilitates the operation andreduces the reforming severity in the autothermalreformer. Any closed tube type GHR would be asuitable reforming exchanger type. Suitableexchanger type reformers are the TandemReformer, as well as Id's GHR. In the case of opentube type GHR arrangement, like that shown onFig. 10, the autothermal reformer has to beinstalled in front of the GHR. Higher steam tocarbon ratio, high thermal stability catalyst for the

300

top layer, and special design burners are requiredin this case, as we have seen in the previoussection.

DISCUSSIONS AND CONCLUSION

Reforming technology of natural has progressedin the last ten years impressively. Autothermalreformers in connection with gas heated reformersis certainly a proven process scheme with alreadygood industrial-commercial references. Specificenergy consumption against state of the artconventional reformers is only slightly better,when oxygen has to be produced in cryogenic airseparation plant and no credit is given for thecoproduced nitrogen. Plant complexity willdecrease, due to the elimination of the firedreformer with all its large waste heat recoveryequipment, despite the added air separation unitand the oxygen compressor, but oxygen reformeroperation will be a source of concern until theoperation reliability of the various designs isproven.

Large scale single train standing alone OxygenBlown Aytothermal Reformers for syngas suitablefor methanol production will have certainly aneconomic advantage against conventional plants,especially when natural gas prices increase above3.5 US$/MMBTU, and if there are not any hidden,unknown yet factors, which may hamper the scaleup from the existing size demonstration units tolarge plants. For instance no operating experienceexist with an oxygen burner in a large combustionchamber on topof catalystbed and ComputationalFluid Dynamic (CFD) programs simulating withsufficient exactitude the conditions in largecombustion chambers are not yet available.Furthermore the conditions and mechanism underwhich metal dusting is occurring are still not verywell known, and the construction of oxygenreformers operating at methanol synthesispressure (elimination of syngas compression) maybe not without problems.

In environmentally sensitive areas the operation ofnon fired reformers has certainly big advantagesagainst conventional reformers.

Even today medium size oxygen reformers fedwith low cost surplus oxygen to enrich air, is anattractive alternative for revamp of old, lessefficient ammonia plant steam reformers.An interesting application of the GHR, with orwithout Autothermal reformer could be the retrifitof1360 MTPD, multitrain ammonia units in eastEuropean countries (Russia, Ukrania, Poland,

etc.). Replacing the oldest, very often badlymaintained direct fired Reformer section andseverely damaged waste heat boilers, with a GHRof 1000 MTPD equivalent capacity, could be aneconomically attractive solution. Other positivepoints for such applications are the low hardcurrancy requirements for such a replacement andthe minimum shut down time.

The selection of the most suitable syngasreforming system design depends from severalkey factors, which must be carefully considered.Either steam reforming or oxygen reforming orcombination of both could be the optimumdepending on the syngas end use requirements,feed stock - utilities (oxygen, CO2, power)availability and cost. Process optimisation requiresconsideration of all main process parameters suchas reforming pressure, temperature, steam tocarbon ratio, CO2 to carbon ratio, as well asefficient waste heat requirements and integration.

Mechanical design considerations are equallyimportant in achieving a reliable and economicadvanced design reforming plant. Some of themore critical areas are the oxygen reformerconfiguration, the burner design, and theExchanger Type Reformer mechanical design.

An energy price increase will certainly accelerateworld-wide the development and multiply theapplications of the Advanced Design ReformingSystem .

LITERATURE CITED

[1] "Clean Coal Technology" Intrenational PowerGeneration. (May 1990)[2] W. Balthasar "Hydrogen production andTechnology: today, tomorrow and beyond". Int. J.Hydrogen Energy Vol. 9, No 8, pp 649-668, 1984[3J K.S. Raghuraman "Steam Hydrocarbonreforming technology - A review", pp 5, KTINetherlands Symposium , Jan 1984 ZoetermeerNL[4] Dr. Max Appl "Modern Ammonia Technology",Nitrogen No 199, Sept-Oct 92[5] L. Storgard "Evaluation of CatalystPerformance in Natural Gas Steam Reforming"Nitrogen 91 Conference Copenhagen June 91.[6] "Reforming catalysts for the production ofammonia",Nitrogen No174, pp 23-24, Jul-Aug. 88[7] B.J. Cromatry, "The development andapplication of shaped Reforming Catalysts,

301

Nitrogen 91 Conference, Copenhagen, June 51991.[8] British Patents 953,877; 1,003,707;1,040,066 (ICI).[9] R. Vanaby and Ch. Stub-Nielsen , "SteamReforming for Co Production", HydrocarbonTechnology International, 1993.[10] J. Claes'Today techniques for emissoncontrol", Uhde Ammonia Symposium, Dortmund,June 1992.[11] "Reforming the front end" , Nitrogen No 195,pp 22 - 31, Jan-Feb. 92.[12] "Ammonia plant failure statistics", AlChEAmmonia Plant Safety Symposium, Paneldiscussion 1986.[13] "High-temperature High pressure service",UHDE GmbH publication Nov. 91.[14] B.J. Grotz, K.C. Wilson, "The Status of BraunPurifier Process",Nitrogen No 151, Sept.-Octob.1984.[15]"C-I-L a successful debut for the AMVprocess", Nitrogen 162 , Jul.-Aug. 1986.[16] J.M. Halstead and A. Pinto, "Design andOperating Experience of the ICI-LCA AmmoniaProcess", FAI Seminar, New Delhi India, Dec.1988.[17] J.M. Lee and E.J. Cialkowski, Ammonia andMethanol Coproduction", Ammonia Plant SafetySymposium, San Antonio, Texas, Sept. 27-301992.[18] D. Kitchen "Energy Efficient Small AmmoniaPlants", AlChE Ammonia Plant Safety Symposium,San Diego, Aug. 1990.[19] R.V. Schneider , and J.R. LeBlanc "Chooseoptimal syngas route", Hydrocarbon Processing,March 1992.[20] G.L.Farina and E. Supp "Produce syngas formethanol" .Hydrocarbon Processing March 92.[21] R.V. Schneider III, "Advances in ReformingSystem Design", AlChE Ammonia Plant SafetySymposium, San Diego August 1990.[22] R. Vannby and W. Madsen "Adiabatic pre-reforming" , Ammonia Plant Safety SymposiumLos Angeles , Nov 1991.[23] J. Rostrup-Nielsen "Catalytic SteamReforming", Catalysis Science and Technology,Vol. 5, Springer Verlag 1984.[24] K.G. Christensen, J.H. Gosnell and B.J. Grotz"Flexible design provides economical ammoniaplant", Nitrogen (191), May-June 1991.[25] D.N. Clark, and W.G.S. Henson,"Opportunities for savings with pre-reformers,AlChE Ammonia Plant Safety Symposium ,Minneapolis August 1987.[26] S.I Wang and N.M. Pattel, "Hydrogenproduction by Enhanced Heat TransferReformer", AlChE Summer national meeting,Session I, Denver Colorado, 22 Aug 1988.

[27] K.J.EIkins, I.C.Jeffery, D. Kitchen and D.Pinto " The ICI Gas-Heated reformer (GHR)system", Nitrogen 91 conference, CopenhagenJune 91.[28] i) "A cost effective, new Reforming Process",Nitrogen No 179, May-June 1989, pp 16-19. ii) M.Sosna, I. Bondar, K. Gunko, B. Grotz"Development History of Tandem ReformingProcess. AlChE Ammonia Plant SafetySymposium, Orlando 1993[29] N. Tiagaradjan , "CAR - Future Prospects",UHDE Ammonia Symposium , Dortmund June1992.[30] H.Marsch"CARoperation experience", UhdeAmmonia Symposium, Dortmund, June 1992.[31] R.N. Udengaard , L.J.Christiansen, " A HeatExchanger Reformer for Hydrogen Production",AlChE Meeting, Orlando, March 1990.

302

DISCUSSIONIjaz Chaudhary, Sabic Basic Industries, SaudiArabia: I especially would like to know, in youropinion, what is the advantage in converting con-ventional reforming technology to the new heatexchange technology?! would like to have thespecific benefits for the new advanced technologywhich is replacing the old conventional steamreforming technology.Pan: The advantages of heat exchange technolo-gy would take a long time to discuss. If you areinterested, I can give you a copy of the informa-tion I have to avoid reading this long page. Is thisOK?Chaudhary: Is the investment a low-cost invest-ment and energy-efficient? What are the realadvantages?Pan: There are no large-scale advance designreforming plants built yet As far as investment isconcerned, there are only speculations at best youcan make. These guesses have a certain tolerance.Some people think the investment will be 15-20%less and other say 10-15% more. There is notremendous difference in investment cost in myjudgment.Chaudhary: There is no tremendous difference?Pan: No. There certainly would not be a big dif-ference in investment cost.Chaudhary: Then what is the benefit of goinginto the advance technology in your opinion?Pan: It is to make the plant more efficient andavoid the limitation and shortcomings of conven-tional reforming technology. As the presentationshows, the energy consumption is reduced and,with higher energy prices, these advantages willbe more obvious.Luc Guns, BASF Aktiengesellschaft: You men-tioned some disadvantages of conventionalreforming such as waste heat recovery system,tube failures, hot gas corrosion due to low steamto carbon and metal dusting. These are all factorsinfluencing our operability and the on-streamtime. Although we have some of these problems,we still have very good on-stream times at mostmajor plants. Some plants are running as much astwo or three years without a shutdown. What isthe longest time that an advanced technologyplant has been on-line? Do you have any idea?Pan: You cannot compare the two since there is

not enough experience in running time with con-ventional reforming to obtain reliable statisticalresults. I don't think that three-year running timeis the average. This is an excellent result in con-ventional reforming. On the other hand, there isvery limited operating time with advanced tech-nology plants. They are only in the infant stage ofdevelopment. As you heard, the mechanicaldesign of the gas-heated reformer is simple andless risky to operate as a reformer.Guns: Do you think that all the problems weresolved? When I looked at one of the pictures youshowed, the inlet temperature was about 450°C,and the outlet temperature of the heat exchangerwas 580°C. I think this is right in the middle ofthe metal dusting zone. We know from conven-tional plants that we have some problems there.It's not mainly pressurized parts where we havethe problems. Problems exist where you enterprocess exchangers or equipment.Pan: Do not forget that the pressure differentialof a few boxes is not very great. Secondly, thereare gas-heated reformers already in operationwhich have shown very good behavior in metaldusting, like the tandem reformer. Certainly,improvements in the ICI reformer and the UHDEreformer have resolved the problem of metaldusting.Max Appl, BASF (retired): Mr. Pan, with respectto energy consumption, I have a little differentview. From my knowledge and from what I heardup to now, energy consumption, at least as far asammonia is concerned, is not better. It is actuallya little higher, and to my understanding the beau-ty of the new technology is really more in equip-ment investment costs. This savings allows inte-gration into modern traditional plants. This is anadvantage in a highly industrialized place.However, this may not be an advantage in a lessindustrialized area. In these cases, it is some-times better to have a plant that is not so highlyintegrated with some energy brought in by sepa-rate installments, separate heaters, or even fromelectric power. That is my understanding of theadvantages of new technology. Energy wise, atleast for ammonia, I don't expect to cut downenergy consumption.Pan: Thank you very much for your remarks, andI have to say that I agree with you.

303

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Figure I. Top fired reformer profiles. Figure 2. Side fired reformer profiles.

Figure 3. Syngas generation and heat recovery.

304

ÉW

cojocepT

TYPICAL SECOND/\ßV—UHD6

Rg.^a

ALU M

\

a

S

Figure 4a. Typical secondary reformer—UHDEdesign.

A LU M l NB A LUS

Figure 4b. Typical secondary reformer MWKdesign.

305

Gas inlet

Oxygen inlet

Water jacket

Vault

C.W. return A C.W. supply

Enrichedair stream

Catalyst bed

Mixed feed(hydrocarbons

plus steam)

Gas outlet

Figure 4c. Oxygen blown secondary reformer.

Partially combustedfeed gas

Jacket

Carbon Dioxide

Figure 4d. M.W. Kellogg oxygen blown secondaryreformer.

Steam Export

Natural Gas

Heat Recovery forSteam Production

Figure 4e. Process diagram of syngas production,H2/CO = 2.O, by autothermalreforming.

306

Desulfurizedfeedstock

Processsteam o

Flue gas )

^

k __

1m. Ài \y/ \

Ductpreheater Com,

pressed O;

Hj/ (2 CO + 3 COj) » 0.985 to 1.05

To K.O. and syngascompression

Figure 5. Combination reforming.

desulphurizedhydrocarbon

X adlabatfcprerelonner

CASE Aflue gas channel

fuel

CASES

. desulphurizedhydrocartion

fuel

adlabatlcpraratormer

tubularreformer

flue gas channel

synthesis gasto heat recovery,purification andsynthesis

tubularreformer

synthesis gasto heat recovery,purification andsynthesis

CastableRefractory

From Primary (?*# M*JReformer

Catalyst Tube»

Reformed GasCollection

To ProcessHeat Recovery

Figure 6. Exchanger type reformer (ETR) of AP.

CASE C

desulphurizedhydrocarbon

tubularreformer

adlabatlcprereformer

flue gas channel

lsynthesis gasto heat recovery,purification anasynthesis

Figure Sa. Options for installing a prereformer.

START UPIGNITOR

HAS AND STEAM-»—

REFORMED QAS <

r

•— '

1

,

I Ir 1 '

k i

CATALYSTS^^J* ^**~:*r

'PRIMARY REFORMER SECONDARY REFORMER

Figure 7. LCA gas heated reforming area.

307

Methane andsteam

Cartridge

Hot secondarygas

Process oxygen

Primary effluent

Cooled secondarygas exit

Scabbard Tube

Primary catalyst

Fin

Sheath Tube

Bayonet Tube

Refractory

Figure 8. Gas heated reformer.[To Secondary Reformer. t=67QO cCH4 =13.24, COa =5.3, CO =1.53, H226.84, H2O =51.57, N2 =1.23

Feed, t=450° C,Steam/Carbon ratio 2.5

•*• « *• Product, p=33 bar, t=S80o C

Nr of Tubes 132I/O-OO 113-133Tube material

From Secondary Reformer10000 c, CH4 = 0.24, CO=7.75, HZ = 34,29 r Ha 0 =38,44, N « 13,30, C02 = 5.7

Naturalgas

ToMUQcompression& synthesis

Reforming Autothermalexchanger reformer

Figure 10. Reforming exchanger.

Feed + steam

Cylindricaldistributer

Reformereffluent

To heatrecovery

Waterjacket

Catalysttube

Perforateddistributer

Figure 9. Tandem type reformer of GIAP.Figure 11. Open tube reforming exchanger.

308

C.W. return ; t C.W. supply

Enrichedair stream

Mixed feed(hydrocarbons

plus steam)

Jacket

Figure 12. Proprietary Kellogg burner.

SECTION A-A

REFRACTORY\ LINING

ENVELOPING TUBE

REFORMER TUBESTUBE SHEET DETAIL X

WATER COOLEDOXYGEN NOZZLES, AND SEC.

•f FEED NOZZLES

REFRACTORY LINING

WATER COOLED• START-UP BURNER

- WATER JACKET

EXISTING FACILITIES ADDITIONAL FACILITIES

HYDROGEN PRODUCT13000 Nm'/h

DRY GAS COMPOSITION EX CARHi 97.0 Voir.CO 24.2 VoIXCOl 7.2 Voir.CH« 0.5 Vol%Nl»Ar 1.1 Vol»

Figure 14. Tie-in of CAR demonstration unit.

Figure 13. CAR demonstration reactor.

309

Table I : Basic reactions of methane Steam Reforming

CH4 + H2O = CO + 3H2 ( 1 )

CO + H2O = COa +H2 (2)

2CO = C + CO2 (3)

CH4 = C + 2H2 (4)

CO + H2 = C + H2O (5)

CH4 + CO2 = 2CO + 2H2 (6)

Table II: Basic reactions of oxygen with gas components in the upperpart of a Secondary Reformer

CH4 + 1/2O2 = CO + H2 (7)

H2 + 1/2 O2 = H2O (8)

CO + 1/2O2 = CO2 (9)

CH4 +3/2O2 = CO + 3 HaO (10)

310

o Large capacity single line reformers <up to soo tubes and methanol capacities upto2600MTP)

o Reformer tube materials with improved mechanica!Characteristics (présure up to 40 bar and metal temperatures up to 960 C, extended tube life).

0 Catalysts with higher activities and improved mechanical-thermal behavior (enhanced activity, reasonable life without deactivation, good heat transfer,acceptable pressure drop, good thermal and mechanical stability, insitu regeneration, operation at low S/Cratio and with heavier hydrocarbons. But the all purpose catalyst is not yet invented)

0 Improved design burners (lOW NOx-Sllent) (Three stage burners canreduceNOx down to 125 ppm with air preheated up to 350 C).

o Higher energy utilization, i.e. lower S/C ratio, highercombustion air temperature, Prereformer, shifting ofReformer heat load

o Improved energy cycles, i.e. use of gas turbines, higherefficiency steamturbioes-compressors,vapour recompressionsystems

o Advanced automatic process controls (expert systems),more reliable instrumentation (smart transmitters, 2 out of 3voting systems, fully redundant hard/soft ware),sophisticated monitoring devices

Table III : ACHIEVEMENTS IN THE CONVENTIONAL REFORMER DESIGN

311

Limited radiant heat transfer efficiency of fired reformer(Low efficiency. Only 40% ofsupplied heat is absorbed

Large quantities of recovered steam which often can not beUsed Within the prOCeSS plant (As down stream units get more energy efficient-nitricacid, urea, ammonium nitrates, sulphuric acid, NPK and other chemical plants-, it becomes very difficult to findconsummers for export of surplus steam).

Huge waste heat recovery systems increasing investmentCOSt and affecting Operabilîty (HP steam recovered at 520 C and 11S bar requiresexpensive equipment, special metalurgy materials- superheated HP steam and Hydrogen /ammonia/CO, andvery high quality BFW).

Bulky structures hampering easy overview and monitoring (Anoperator to go around for a routine check on a large Reforner plant has to walk approximately one km up anddown)

Risk of carbon formation or tube overheating leading toCatastrophic failures (InspUe of improved tube material. Reformer tubes operate inevitablyunder creep conditions and non destructive tests are costly and not very reliable).

Close attention from the operators and complicatedinstrumentation to maintain safe conditions during operation,start up and shut down

Long duration and COStly Start UP (Heating up of large refractory surfaces requiringslow preheating, not easy firing control = risk of condensation, or overfiring)

Increased NOx in flue gas due to higher combustion airpreheating temperatures (Even low WOx burners are not able to reach the very lowemmission values required in Nox sensitive areas. DeNox equipment may be required).

Hot gas corrosion phenomena due to low S/C ratio and useOf Special'materials (Metal Dusting phenomena, mainly on Process Gas Cooler and SteamSuperheater )

Table IV LIMITATIONS-SHORTCOMINGS IN CONVENTIONALREFORMER DESIGN

312