1997: postcommissioning operating experience at the .../fileser… · the lurgi combined reforming...

Postcommissioning OperatingExperience at the Mossgas Plant

Operational experiences gained on the gas reforming unit since commissioning are discussed. Thecause of problems evaluated and subsequent corrective actions taken include carbon formation and

metal dusting control, revised startup and shutdown procedures, and catalyst during prevention.

Henry de Wet, Rudie O. Minnie, and Anthony J. DavidsMossgas Reforming Plant, Mossel Bay, South Africa

Introduction

In 1987 the South African government boldlyapproved the construction of a synthetic fuelsfacility on the coast of Mossel Bay. The strategic

purpose of the facility was to allow South Africagreater self-sufficiency in terms of its fuel require-ments. Commissioning of the plant compared favor-ably with international standards for plants of a similarsize. The first gas arrived onshore on March 31, 1992,and full production was achieved on January 2, 1993.The front end of the plant layout incorporated a gasreforming unit that was designed on the principle ofthe Lurgi combined reforming process. A schematicdiagram of the Mossgas onshore plant is shown inFigure 1.

Gas reforming unit

The Lurgi combined reforming process is welldocumented in the literature and includes both pri-mary tubular steam reforming and oxygen-blownsecondary reforming. The novelty of this patentedprocess is that a portion of the lean natural gas feed

is bypassed around the tubular reformer and direct-ly to the oxygen-blown secondary reformer. In thecase of Mossgas, the design provided for threeidentical trains that under normal running condi-tions could comfortably sustain 850,000 Nm3/hsynthesis gas to the Synthol plant, at a CO concen-tration of 23 mol % (dry) to the downstream units(equivalent to 2,500 metric ton/d methanol pertrain). A simplified flow diagram is depicted inFigure 2.

Commissioning overview

Although the commissioning period went remark-ably well compared with international standards,reforming plant startup did not proceed free of prob-lems. The major problems were the need to rebuildpoorly installed refractories hi the secondary reformer,shorter than expected secondary reformer burner life-times, sodium carbonate fouling of the wasteheat boil-ers, and metal dusting of the secondary reformer neckliner and wasteheat boiler bypass valves. Specificdetails of the above are contained in a previous article(Shaw et al., 1995).

AMMONIA TECHNICAL MANUAL 64 1998

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MCL FRACnONATION

• ALCOHOLS

• GASOLINE

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Figure 1. Mossgas onshore plant.

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Figure 2. Lurgi combined reforming process.


Postcommissioning operating experience

During the last four years, valuable, but in somecases costly, operating lessons concerning the gasreforming unit were learned that form the basis of thispaper. These lessons include:

• Primary reformer catalyst deactivations with cor-rective actions.

• Carbon formation and metal dusting control.• High-pressure wasteheat boiler-related problems.• Revision of reformer startup and shutdown proce-

dures.• Secondary reformer burner evaluations and devel-

opment.

Primary Reformer Catalyst Deactivations

In the last 4 years, Mossgas experienced severalincidents of catalyst damage that caused substantialfinancial losses. In each instance, in-depth investiga-tions were conducted to determine the causes of theseincidents. Once identified, remedial steps of both anengineering and an operational nature were imple-mented to ensure that the budgeted catalyst lifespan offive years would be achieved. We discuss specificcases of catalyst deactivation in sequential order, thecauses of catalyst damage, and the corrective actionsimplemented.

Case 1: Iron sulfide carryover to primary reformer

Both zinc oxide (ZnO) desulfurization reactors wereunloaded in September 1994 because of high differen-tial pressure problems. The catalyst was screened andreloaded. Iron sulfide dust inside the reactor fellthrough the lower ceramic ball layers into the outletpiping. On restarting the desulfurization reactor, pri-mary reformer catalyst degradation was immediatelyapparent because "giraffe-neck" hot spots on the tubeswere visible throughout the furnace. The procedurenow calls for thorough vacuuming and washing out ofthe reactor outlet piping before recommissioning. Inaddition, an kon scavenger was installed in the top ofthe upstream reactor. These steps, together with someadditional steps outlined in Case 3, have fixed theproblem.

Case 2: Low steam-to-carbon ratio to primaryreformer

During a trip of train 2 in March 1995, another batchof primary reformer catalyst was deactivated with car-bon because the steam-to-carbon ratio was too low. Atthat stage the steam flow configuration to the primaryreformer provided for two flow valves, one for normaloperation that closed on a primary reformer trip andone BSD valve that should immediately open to 35ton/h on closing of the other valve to prevent carbonlaydown on the catalyst. Unfortunately, the latter valvedid not open immediately as designed and dry hydro-carbons entered the primary reformer. The postmortemon this incident revealed the following:

• The slow response of the BSD valve was caused byhysterisis of internal components resulting in a slug-gish response to the control instrument air impulse.

• The steam-to-carbon ratio of the primary reformerwas well below the minimum of 2.2 (based on thetotal carbon number) for only 20 s. This, however, waslong enough to destroy the entire top 50% of the cata-lyst in the tubes. The most severe region of catalystdisintegration was approximately one third of the waydown the length of the tubes, which corresponds to thearea of highest heat flux.

Both valves have now been made controllers. Theyare operated in split range with auto/manual stations.On a reformer trip, the steam flow controller remainsin cascade and lags with the lean natural gas flow con-troller downwards until the steam flow controllerreaches a set point of 35,000 kg/h. Furthermore, a"double-block & auto-vent" system was installed onthe LNG feed to the primary reformers. These changeshave on several unit trips sufficiently protected prima-ry reformer catalyst against carbon laydown.

Case 3: Mercaptan sulfur and olefins in primaryreformer feedstock

Slugs of methanol are injected at the offshore plat-form into the condensate line to prevent hydrate for-mation in the onshore feedstock pipeline. The over-heads from the condensate stabilizer, which containthe methanol, are compressed into the lean natural gasline onshore. This methanol reacts with hydrogen sul-


fide in the lean natural gas feed during preheating, andwith sulphur in the zinc oxide desulfurization beds, toform organic sulphur. This is not adsorbed by the zincoxide. In April 1995, poisoning of train 1 primaryreformer catalyst resulted from this, as the originalplant design did not provide for hydrogénation of thefeedstock (this not being considered necessary at thetime). The two ZnO desulfurization reactors were inef-fective in the removal of organic sulfur, olefins, andoxygenates. Cracking of the olefins (originating hi therecycled streams from the refinery and the molecularsieves of the Synthol tailgas processing plant) alsoresulted in coke formation and led to unacceptablyhigh pressure differentials over these beds. As a shortterm solution, the leading ZnO desulfurization reactorwas dual-loaded with a batch of cobalt molybdenum(CoMo) catalyst and a batch of ZnO catalyst, includ-ing a 3 mol % hydrogen recycle. This not only result-ed in the successful elimination of the organic sulfursbut also hydrogenated the olefin material in the prima-ry reformer feed. The top fifty percent of the primaryreformer tubes were loaded with a shaped, lightlyalkalized catalyst as an additional precautionary mea-sure to resist the resulting carbon formation.

Case 4: Hydrocarbon contamination of heat-upnitrogen

The primary reformer catalyst in train 3 wasdestroyed in May 1996 when hydrocarbons fromanother unit inadvertently contaminated the high pres-sure nitrogen system used for heating up a reformertrain. These hydrocarbons cracked inside the primaryreformer where the inlet temperature during that partof the heating cycle was around 430°C (806°F, 703 K).Coke deposited onto the catalyst manifested a highpressure drop. Corrective steps included the following:

• Additional check valves were installed to limithydrocarbon backflow into the low pressure nitrogencircuit.

• A hydrocarbon analyzer was installed to monitorfor hydrocarbons at the suction of the nitrogen com-pressors.

• The process pressure of relevant equipment through-out the plant is now monitored and compared with thepressure in both the low-pressure and high-pressure

nitrogen circuits. Should the nitrogen pressure decreaseor the process pressure increase, resulting in a low dif-ferential pressure, an alarm is generated to warn theoperator of possible contamination. This is repeated atthe operator consoles of other relevant units.

After these measures were implemented, no catalystdeactivation or primary reformer tube ruptures havebeen experienced since the implementation of thesemeasures.

Carbon Formation and Metal Dusting

Serious metal dusting problems were predicted dur-ing the design phase of the unit. These were mainlyaddressed by the use of refractory materials to protectequipment exposed to high temperature gas environ-ments. In areas where this was not feasible, theprocess design was such that equipment was operatedbelow the critical metal dusting temperature. This waspredicted to be above 480°C (896°F, 753 K) of metalsurface temperature.

Both Boudouard carbon formation and metal dustingoccurred during commissioning. The onset of carbonformation is usually first evident in the turbid color ofthe process condensate samples on the exit batterylimits of each train. Concurrently, turbidity analysis onthe process condensate then shows an increasing trendand frequency of nitrogen rumbling to dislodge thesolid cakes from the process condensate filter car-tridges, and the process condensate filter cycle timecorrespondingly decreases.

Previous experience

The reforming unit was severely plagued by carbonhi the process condensate in 1995. A number of con-trolled process changes, including changing wasteheatboiler bypass valve openings to reduce the wasteheatboiler outlet temperatures below 500°C (932°F, 773 K)and compositional variations to allow more carbon tothe secondary reformer feed, were implemented. Thepurpose of these changes was an attempt to decreasethe kinetics of what was then solely attributed to metaldusting of the process equipment. No success wasachieved. Subsequent investigations with assistancefrom Lurgi have, however, revealed that metal dusting


of the process equipment did not occur in the afore-mentioned cases. The carbon formation was catalyzedby fine iron particles entering the reforming unit viathe offshore feed and Fischer-Tropsch recycle gasstreams. The problem was finally solved (or the symp-toms treated) by the addition of one hundred parts perbillion (volume) of hydrogen sulfide to the secondaryreformer feed (Murdoch and Still, 1996). It was con-tended that hydrogen sulfide quantities in excess ofone hundred parts per billion would poison the iron-based catalyst in the downstream Fischer-Tropschunit. Prior to the installation of the CoMo catalyst inthe leading desulfurizer, minute quantities of organicsulfur had unbeknownst to us poisoned the fine iron inthe reformer feed streams. With the installation of aCoMo bed, this silent source of sulfur was no longeravailable. This caused the aforementioned carbonproblems.

Current experience

Despite the addition of hydrogen sulfide to the sec-ondary reformer feed, another problem occurred withcarbon in process condensate. The problem essentiallyonly manifested itself on one train where the waste-heat boiler outlet temperature was operating at 545 °C(1,013°F, 818K). The temperature could not be low-ered to below 500°C (932°F, 773K), as was done pre-viously, because the boiler bypass valves were alreadyfully closed. (The boiler tubes were known to befouled with catalyst dust and sodium from a primaryreformer catalyst reload performed earlier.) A test runwas conducted in which the hydrogen sulfide wasremoved from the other two trains and all the sulfur(i.e. 300 parts per billion) charged to the secondaryreformer feed of the problematic train. The processcondensate cleared in a matter of hours.

Subsequent inspections showed that both the waste-heat boiler bypass valves (310 stainless steel) and sev-eral plugged steam superheater tubes were badly dam-aged by metal dusting. It was therefore concluded that,in the Mossgas case, carbon in the process condensatecould be ascribed to two primary causes:

« Free fine iron particles in the feed gas that catalyzecarbon formation reactions.

• Rapid metal dusting of the wasteheat boiler bypass

valves and the plugged superheater tubes at tempera-tures in excess of 520°C (968°F, 793 K).

Work done by Grabke (Grabke and Muller-Lorenz,1995) shown in Figure 3 confirmed that at wasteheatboiler outlet temperatures hi excess of 520°C (968°F,793 K), one hundred parts per billion of hydrogen sul-fide per train in the secondary reformer feed is insuffi-cient to prevent the onset of metal dusting on thewasteheat boiler bypass valves and superheater tubes.Unfortunately, due to catalyst poisoning constraints inthe downstream Fischer-Tropsch plant, no additionalhydrogen sulfide can be added.

Figure 3 shows the thermodynamics of the sulfureffect on metal dusting, log p(Hß)/p(K^) versus l/T,including the regions of sulfur-free iron surface, thetransition from the surface to sulfur monolayer

„1000 900 800 700 600 500 'C

8 9 10 11 12 13

Figure 3. Thermodynamics of the sulfur effect onmetal dusting.

(hatched), and the region with sulfur monolayer onbron and FeS stability range. The region of metal dust-ing corresponds to the region with little or no sulfuradsorption.

The situation is best managed at present by main-taining the wasteheat boiler outlet temperature below


520°C (968°F, 793 K) where possible, after which asteam-out of the wasteheat boiler tubes is consideredto improve heat exchange. However, it must be pointedout that fouling of the wasteheat boiler tubes on thegas side is particularly severe after fresh catalyst isloaded in the primary or secondary reformers, or whennew ceramic balls are loaded into the bottom of thesecondary reformer. After the first steam-out (generallythree months after the loading event) the fouling prob-lem disappears.

High Pressure Wasteheat Boiler RelatedProblems

Numerous failures have occurred on the wasteheatboilers since 1993, making the boilers the prime con-tributor to unit downtime. The reasons for the failureshave been extensively investigated and are well docu-mented. The failure mechanisms are summarized inthis article.

Wasteheat boiler tube-to-tubesheet cracks

It is believed that the event initiating these crackswas due to the running dry of the boilers hi 1993. Thisincident severely weakened the tube-to-tubesheetwelds, which resulted in several unit outages, tubesheetleaks, and subsequent weld repairs. Considerable timewas spent developing a pattern for these failures, withlittle success. In some cases weld repairs were con-ducted several tunes on the same welds during differ-ent unit outages, suggesting that other failure mecha-nisms may have also played a significant role, as bothcircumferential and radial cracks were detected.

Specialized ultrasonic testing of welds revealed thepresence of incomplete weld root penetration, whichcreated a crevice in the weld on the water side. Causticstress corrosion cracking was detected in some weldmetal removed for metallurgical investigation.

Serious metal dusting damage resulting in carbonpenetration into the tubesheet material has also causedcracking associated with repair welding (metal dustingcaused by failure of tubesheet refractory installation).

The final resolution was to install six new wasteheatboilers (three sets) hi the three reforming trains.

To prevent the wasteheat boilers from being run dry

again, an automated wasteheat boiler dry-run protec-tion system was installed. This system is activatedwhen

• Both BFW pumps are offline for more than thirtyseconds, which results in the tripping of all threereformer trains. Steam from the internal letdown sta-tions is fed to the primary and secondary reformers atreduced rates as per a normal trip, for a period of 5minutes, after which all steam valves are trippedclosed.

« A low level occurs in the boiler feed water deaera-tor drums that results in the tripping of all threereformer trains. Steam is fed to the primary and sec-ondary reformers at reduced rates as per a normal tripfor a period of five minutes. All steam valves are thentripped closed.

• A train specific low steam drum level occurs,which results in a train trip. All steam valves are thenimmediately tripped closed. (It was deemed prudent tojeopardize the primary reformer catalyst inventoryrather than the wasteheat boilers.)

The wasteheat boiler dry-run protection system canalso be activated from the operator console if neces-sary. The above system has protected the wasteheatboilers from being run dry on several occasions.Furthermore, quality control of repair welding is con-ducted by utilizing a patented ultrasonic testing probe.This ensures that the problem associated with water-side crevices does not recur.

Increases in tubesheet refractory thickness, as wellas several other improvements in refractory installa-tion, have minimized tubesheet damage associatedwith metal dusting. Thermal sprayed coatings werealso applied to the tubesheets on an experimentalbasis. A 50% Cr-50% Ni coating is considered suc-cessful, but application difficulties need to be addressed.Coating density is extremely important because carbideformation can occur under the coating in areas wherecoating voids are present.

Hot water corrosion on the wasteheat boilertubes

Severe metal loss occurred on the waterside of thewasteheat boiler tubes on one of the reformer trainsdue to the phenomena of hot water corrosion, devia-


tion from nucleate boiling caused by lengthy operationat heat fluxes in excess of design limits. This mode ofoperation was caused by closed wasteheat boilerbypass valves due to fouling. Deviation from boilerfeed water quality, not supporting these high heatfluxes, was also contributing.

The operating philosophy now is to prevent thewasteheat boiler heat fluxes from exceeding the designof 460 kW/m2. This is done by reducing the openingof the wasteheat boiler bypass valves. When this is nolonger possible, reformer load reductions are institutedto achieve the desired effect. Furthermore, the moni-toring of online boiler feed water quality has beenimplemented and the steam drum continuous blow-down manifolds were lowered. This ensures that it isalways below the water level and that the system is ridof contaminants.

Cracks on the wasteheat boiler manways

Magnetic particle inspection inside the manway noz-zle of train 2 revealed extensive cracking in the man-way flange-to-nozzle welds (Cr-Mo low alloy steel).Similar cracking was also found in the inlet manway-to-flange weld. All four manway flanges wereremoved and the mends of the welds were investigat-ed.

Cracking occurred mostly in localized transverserepair welds that were not documented in code databooks, but smaller cracks were also found in themain manway-to-flange welds. A metallurgicalinvestigation suggested that the transverse weldswere most probably performed after postweld heattreatment of the wasteheat boilers during construc-tion. Cracking can be ascribed to wet CO/CO2 stresscorrosion cracking (Baartman and de Bruyn, 1996).This mechanism, as concluded by de Bruyn, is initi-ated by water condensation in the presence ofreformed gases under the refractory insulation of thewasteheat boilers.

The manways were rewelded and stress relieved.A physical barrier between the steel and the wetgas environment was installed by coating the man-ways on the inside with an inorganic zinc-richprimer. Subsequent inspections revealed theabsence of cracking.

Revision of Reformer Startup andShutdown Procedures

In view of several inlet chamber tube-to-tubesheetcrack-related problems on the wasteheat boilers, boththe startup and shutdown procedures were revised tomake them more wasteheat boiler "friendly".Wasteheat boiler-related unit outages were the primecontributor to unit onstream factors that were lowerthan planned. The startup and shutdown proceduresare written in a mixed instructional and checklist for-mat to ensure that steps taken are followed up by phys-ical checks in the field. Areas where the unit startup andshutdown procedures were revised, and the reasoningbehind the revisions, are highlighted below.

Unit startup

• In the past the steam drum and associated equip-ment were pressure tested with firewater, which wasconsidered to be of reasonable quality. This practicewas stopped, and cold-polished water is now usedinstead. The chlorides in the firewater are believed tohave contributed to tube failures in the wasteheat boil-ers. To ensure that correct boiler water quality is main-tained hydrazine and ammonia are now added to thesteam drum's inventory prior to startup.

• At the train battery limit only the fuel gas, HPnitrogen, and 40 bar steam blinds are swung open. Allother hydrocarbon blinds are kept closed. These blindsare only swung open once the primary reformer outlettemperature is above 600°C (1,120°F, 873 K) andsteam has already been introduced into the reformer.This prevents catalyst deactivation by the hydrocar-bons originating from leaking feed gas valves.

• A pressure ramp function was installed on the gaspath that limits the pressurization and depressurizationto 100 kPa/min. This also applies when the HP nitro-gen leak test is performed. Previously, pressurizationand depressurization rates were suspected to be toohigh, damaging the newly installed refractory on thewasteheat boiler tubesheets.

• A pressure ramp function was also installed on thesteam drum pressure controller to ramp the set point at100 kPa/minute. The system will stop the set pointramp function when the rate of temperature increase or


decrease is more than 50°C/h (122°F/h, 323 K/h) andprevent the steam pressure controller set point rampfunction from increasing when the steam production isless than the minimum required. This system will pre-vent the primary reformer duct heat exchangers fromoverheating. This was necessary to prevent suppres-sion of boiling in the wasteheat boilers with conse-quential undue thermal stresses.

• Forced manual opening of the wasteheat boilerbypass valves to 10% was installed when the primaryreformer BSD group is reset. This ensures even heat-ing up of metal parts of boilers on startup. Previouslythe bypass valves were only opened when the trainwas almost fully online; this resulted in thermalshocks on the bypass tube.

• A display of the burners that are lit is indicated onthe operator console and nominated burners are dedi-cated for starting up the primary reformer. Thisensures standardized startups between the variousoperating shifts.

• It is important to note that the correct circulationroute in the boiler is ascertained. Once the secondaryreformer outlet temperature is above 50°C (122°F, 323K), 40 bar startup steam is slowly added to all the ris-ers of the wasteheat boilers to induce the correct circu-lation route in the boiler.

• Process steam is now only introduced to thereforming train when the primary reformer inlettemperature is above 350°C (662°F, 623 K) and thesecondary reformer outlet above 400°C (752°F, 673K). This is to prevent condensation in the wasteheatboilers and rehydration of the tubesheet refractory.Evidence of hydrates on the cold rear surface of thetubesheet was found. Previously, steam was intro-duced to the reformer when the secondary reformeroutlet temperature was at 250°C (482°F, 523 K).

• To minimize undue temperature gradients acrossequipment during the heating up cycle, a temperaturechange rate of maximum 50°C/h (122°F/h, 323 K/h) isrigorously adhered to on the primary and secondaryreformer outlets, and the inlet to the primary reformerduct. A graphic display on the plant information systemallows plant operating and management personnel to pin-point and investigate deviations from the standard.

Unit shutdown

A similar approach as outlined hi the startup proce-dure is employed in the shutdown procedure. Againthe emphasis is on reducing pressure and thermalstresses on process equipment. Moreover, the hydro-carbon and hydrogen blinds are swung closed beforesteam is totally removed from the primary reformer,and while the primary reformer outlet temperature isstill above 600°C ( 1120°F, 873K).

Secondary Reformer Burner Evaluationsand Development

Since the initial startup of the Mossgas reformers,several problems were experienced with the burners hithe oxygen-blown secondary reformers. The two mainproblems encountered were mechanical integrity andunstable flame pattern.

The burner flame pattern is easily skewed by achange in feedgas composition and minor productionupsets, which allows suboptimum mixing between thegas, oxygen, and steam. This sometimes results inflame "impingement" on the refractory walls. In anattempt to rectify the unstable flame patterns, severaldesign changes were made to the mixer assembly. Themodifications included changing from welded con-struction mixers to cast mixers, as well as changed gasand oxygen vane angles. Considerable successes wereachieved in eliminating most of the problems.

The burner problems Mossgas has encountered andlargely resolved ARE:

• Syngas quality below optimum.• General overheating of the shell and melting of

cone refractory, resulting in secondary reformer conerefractory replacements and downtime every 30-120days for inspection.

• Regular replacement of oxygen mixers due tocracked oxygen vanes.

Although the improvements helped achieve goodprocess performance, the mechanical reliability of thecast oxygen tip remains problematic.

As a backup measure, Mossgas embarked on a parallelroute of burner development with an independent compa-


ny. Towards the end of 1994 a decision was taken to optfor a completely new and different burner/mixer design.

Mossgas launched a thorough investigation into thereplacement of the burner assembly of one trahi withthis design. The new burner assembly was installed hione secondary reformer during the October 1995 shut-down. Since then this train had to be decommissionedon several occasions to repair wasteheat boiler leaks.These occasions were used to inspect the overall con-dition and mechanical integrity of the new burnerassembly. No defects were detected and the sameburner was reinstated every time.

This burner has now been in service for more than520 days. Its overall mechanical integrity proved to besuperior to the original burners, and similar processperformance is achieved. Some of the noted improve-ments include more stable and even reactor top bedtemperatures, lower reactor cone and shell tempera-ture, unproved ease of operation and predictability ofperformance, and no scheduled shutdowns to checkburner and/or refractory for defects.

Conclusion

The production of synthesis gas for the productionof ammonia or methanol is well documented.Application of the technology discusses in this articlehas shown that utilization of synthesis gas for a differ-ent application is unique. Production of synthesis gas

for processes other than ammonia and methanol has itsown pitfalls and challenges that need to be recognizedup front. We are convinced that Mossgas can make asignificant contribution towards the safe operation ofthe growing synthesis gas business.

Acknowledgment

The authors wish to thank their colleagues atMossgas who have either directly or indirectly con-tributed to the contents of this paper.

Literature Cited

Baartman, C.A., and H de Bruyn, "Examination ofCracked Manway Weldments on Make Gas Boilers06-ES201 A/B," Mossgas Internal Report, 7,(1996).

Grabke, H.J., and E.M. Muller-Lorenz, "Effect ofSulphur on the Stability of Cementite," Steel Res.66, No. 6, (1995).

Murdoch, R, and K. Still, "Elimination of CarbonFormation in Secondary Reformer System,"Ammonia Technical Manual, Vol. 36, AIChE, NewYork (1996).

Shaw, G, H. de Wet, and F. Hohmann, "Commissioningof the World's Largest Oxygen Blown SecondaryReformers," Ammonia Technical Manual, Vol. 35,AIChE, New York (1995).

DISCUSSIONJ. Gosneil, Brown & Root: I think you said the mate-rials at the cold end of the waste heat boiler weremade out of 300 stainless steel and that, when youhad the metal dusting problem, you solved it byputting some sulfur in the feed. Did you also changethe material from 300 stainless steel?Minnie: Yes, we did change the material. We havethree trains. On one trahi we have Inconel 601 alloy, onthe second one we have Inconel 800, and on the thirdtrain I think we still have 310 stainless. We plan toinstall 50 chrome-50 nickel bypass valves during thenext shut down.Gosneil: Are any of these plants with these three dif-ferent metallurgies having any metal dusting at themoment?Minnie: We only have metal dusting when we goAMMONIA TECHNICAL MANUAL

above our magic 520°C temperature.Gosnell: Even with sulfur in the feed?Minnie: We restrict sulfur hi the feed to one hundredparts per billion. We can go higher, but then we have aconstraint on our catalyst in the downstream units.Gosneil: Are the metal dusting problems of the sameseverity with all three metallurgies or is there a dif-ference?de Wit: We still get metal dusting, but it can be con-trolled with good water all the time. I don't think thatwe've resolved the problem, we've just been able tocontrol or treat the symptoms. However, with the50-50 nickel-chrome casting that we intend to installbased on other tests we've conducted during the lasttwo years, we're convinced that we will be able to stopmetal dusting completely.

72 1998

1997: postcommissioning operating experience at the .../fileser… · the lurgi combined reforming...

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