Primary Reformer Riser Liner Collapse

To avoid problems of this type, proper feed desulfurization must beaccomplished and the furnace should not be operated with hightemperatures and high methane leakage over an extended periodof time.

B.P. Ennis and U.R. Le BlancThe M. W. Kellogg Co.Houston, TX 77046

In December of 1982, a Kellogg designed ammo-nia plant owned and operated by P.T. Pupuk Ku-jang in Cikampek, West Java, Indonesia, re-ported a fire in the primary reformer pent-house. Kellogg was contacted and we respondedwith assistance to get the plant back on-linein a relatively short time.

While investigating the cause of the fireit was determined that a riser liner had col-lapsed, causing a failure of the radiant coiloutlet manifold. The cracked outlet manifoldleaked process gas into the radiant box,which, in turn, ignited and damaged radiantarch steel and piping. Further investigationby Kujang revealed a large deposit of carbona-ceous material at the point of the riser linercollapse.

In view of the fact that Kellogg's com-mercial steam reformers number in excess of150, and that the design has proven to be veryreliable over many years of operation, our in-terest in this problem was very high. A pro-gram was undertaken to determine the probablecauses and solutions. Our overall effort con-sisted of: site visits, laboratory investiga-tion and engineering analysis. This paper re-ports on our program and the conclusions wereached.

PRIMARY REFORMER DESCRIPTION

Before initiating this discussion, a review ofthe basic reformer furnace configuration isappropriate. The radiant section of a typicalKellogg steam reforming furnace is shown inFigure 1. The'natural gas/steam feed flowsthrough the inlet manifold to the catalysttubes. The catalyst packed tubes are suspendedvertically, and are arranged in parallel, sin-gle width rows. Burners are located in the ra-diant arch and are arranged in parallel rowson either side of the reformer tubes. The re-formed gases exit the catalyst tubes into theoutlet manifolds. It is one of the outlet man-ifolds that failed and leaked- process gascausing the fire. The gas from the outlet man-ifold flows to the riser tubes and then to theeffluent chamber. The collapsed riser lineris in the area between the furnace arch andthe effluent chamber. This part of the furnaceis called the riser transition assembly (seeFigure 2).

In this assembly, the transition is madefrom cast riser tubes to a water jacketedpressure vessel, internally lined with cast-able refractory. The assembly is a series offorged Alloy 800H pieces. An internal stain-less steel liner acts as a shroud to preventerosion of the castable lining by the processgas. Pressure is retained by the cone-shapedAlloy 800H piece, marked FC on Figure 2 andthe 16 inch (406 mm) diameter carbon steelpipe in the water jacketed area.

92

The stainless steel liner extends fromthe top end of the forged transition piece,marked FA on Figure 2, to a point inside thehorizontal effluent chamber. The lower end ofthe liner is wrapped with ceramic fiber mate-rial (approximately 1/2 inch(13 mm) thick),which extends from the top of the transitionpiece to the water jacket. Above the ceramicfiber the liner is wrapped with a 1/8 inch(3 mm) thick layer of cardboard. On initialstart-up, this cardboard decomposes to provideclearance and free expansion of the liner up-ward into the effluent chamber. Also the voidleft after the cardboard decomposition createsa vent for the process gas, equalizing inter-nal and external pressure on the liner.

Figure 3 shows the general location andshape of the riser collapse.

U.S. PLANT VISITS

In an effort to find other operators witha problem such as Kujang's, in-house fileswere reviewed and U.S. operators of Kelloggplants were contacted. Ten plants were locatedwhich had experienced liner problems of sometype. These ten plants were visited to obtainall available information, including the caus-es to which they were attributed, and repairsor changes which had been made to prevent re-current problems.

It should be noted that some of theplants included in this investigation havebeen on stream much longer than Kujang. Someof the liner problems occurred ten to twelveyears ago, and information was limited.

Eight of the plants visited had experi-enced collapsed liners which appeared to becaused by external pressure. These failureswere discovered on routine inspections of therisers during turnarounds. The degree of re-pairs made ranged from mechanically openingthe collapsed liner from instde the transferline, to the complete replacement of the tran-sition assembly. The collapses were attributedto various causes, including restricted ther-mal movement of the liner and settling of thecastable refractory.

Two plants were located which had collapsedliners similar to Kujang's; that is, collapsesin the cone section with carbon surroundingthe outside of the liner. At one plant, con-siderable amounts of carbon were reportedlyfound when three liners were replaced in 1980.At the second plant, carbon was found in thecone section when a transition assembly with a

collapsed liner was recently cut open by Kel-logg. Figure 4 is a photograph showing thenature and extent of the observed liner col-lapse and carbon formation phenomenon.

Operating conditions at these two plantsprior to the discovery of the collapsed linerswere very similar. Both experienced high a-mounts of sulfur in the feed gas. The highsulfur content reduced catalyst activity,which, in turn, resulted in higher methaneleakage and reforming temperature. This condi-tion, which probably resulted in the carbonformation at these plants, will be discussedin more detail later in this report.

LINER COLLAPSE MECHANISMS

During our review of the riser transitionassembly design, a number of failure mechan-isms were considered. The most notable ofthese are:

1. Restricted thermal expansion of the lin-er, either vertically or diametrically.

2. External pressure resulting from trappedsteam.

3. External pressure from carbon formation.4. A combination of two or more of the

above.

A possible source of restricted verticalexpansion is the "bell" connection near thetop of the liner, reference Figure 2. If thecardboard is not properly placed around theliner during construction, it is possible forthe bell to compress the castable refractoryas the liner expands upward. The axial loadresulting from this restriction is dependenton the strength of the castable and the areaof contact. In addition to possibly bucklingthe riser liner, such a force would likelybuckle the liner of the transfer line as thecastable is pushed upward.

Restriction of diametrical thermal expan-sion results in an external pressure typeforce. Such restriction could be caused bycarbon around the liner or crushed castablesettling into the cone section. Once eithermaterial is compacted around the liner, a tem-perature increase can result in very highstresses. The magnitude of these stresseswould depend on the mechanical properties, ofany material around the liner. The castablerefractory in the cone section cut open byKellogg was found to be in excellent conditionand we concluded that the maximum crushingstrength of the castable was therefore not ex-ceeded.

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"Trapped" steam has been mentioned as apossible source for the pressure to collapsethe liners. It is possible for steam to becometrapped in the transition assembly duringstart-up if the annular space between the lin-er and refractory becomes blocked. Such an oc-currence might result in a sufficient differ-ential pressure to collapse the liner, espe-cially if it were slightly elliptical inshape. However, considerable damage to thecastable refractory would also be expectedthroughout the transition assembly. It isthought very unlikely the collapsed linerswere caused by trapped steam.

Based on the strength of the liner, thecollapse was likely caused by a combination ofcarbon formation and restricted expansion. Ascarbon accumulated around the liner and in theceramic fiber packing, the liner was restrict-ed from expanding both diametrically and ver-tically. An upset in temperature would subjectthe liner to external pressure and an axialcompressive force. The combination of theseforces could cause ovalization and correspond-ing weakening of the pipe. Temperature excur-sions and formation of additional carbon wouldtend to gradually collapse the liner. Highprocess gas temperatures, such as reported byKujang and the two aforementioned U.S. plants,would make these effects more pronounced, asthe expansion of the liner increases and itsstrength decreases at elevated temperatures.

An apparent solution to prevent a recur-rent liner collapse is to use stronger, heavi-er wall pipe for the liner. This modificationis not recommended since it may result in thecone section being exposed to excessive inter-nal pressure if carbon formation continues tooccur.

SOURCE OF CARBON FORMATION

In the immediate area of the collapsedliner, there are reactive compounds, favorableconditions, and potential catalysts whichcould conceivably lead to the formation ofcarbonaceous materials.

We have examined all of the reactionmechanisms proposed and concluded that undernormal reformer operating conditions, none ofthese mechanisms are favored. However, docu-mented long term operating data indicate thatabnormal conditions have existed in the failedrisers which may have produced an environmentfavoring several of the possible reactionmechanisms.

For a chemical reaction to occur and forit to influence the mechanical system, thereare several requirements which must be satis-fied;Requirement 1: The reaction must be thertno-dynamically possible. Basic laws of chemis-try which determine the possibility of aparticular reaction must be met.

Requirement 2: Favorable conditions must bepresent. There are a large number of pos-sible reactions which can occur in manysystems. Favorable conditions of tempera-ture, pressure and concentration must be metfor the particular reaction being consid-ered.

Requirement 3: The reaction must proceed ata significant rate. In order to be of con-sequence, the reaction rate must fit thetime frame being considered. Reaction rateis controlled by temperature, pressure andconcentration. It can be dramatically ac-celerated by the presence of a catalyst.

Requirement 4: The reaction must have signi-ficant magnitude. The magnitude of the ef-fects of the reaction must be sufficient toinfluence the system. Mechanical effectsare typically controlled by the active orpassive removal of by products from the re-action zpne.

The following discussion analyzes eachof the proposed mechanisms with respect to theconditions specified above:

I. Fiberfrax DecompositionII. Carbon Monoxide Disproportionati on

(Boudouard's Reaction)III. Fischer-Tropsch Cracking ReactionIV. Hydrocarbon Dehydrogenation

Each mechanism is examined with respectto the requirements listed above and a conclu-sion reached concerning its probability.

Mechanism I - Fiberfrax Decomposition

Background: Figure 2 illustrates the ar-rangement of the riser transition cone in thearea of the liner failure. As noted in anearlier section, the lower segment of the lin-er is wrapped with a special insulating mate-rial called "Fiberfrax".

In general, Fiberfrax is a woven cloth-like material composed of alumina and silicon.A nickel/chrome alloy wire is added to the

94

weave to give adequate physical strengtn athigh temperatures. The Fiberfrax fabric, asproduced, contains about 20% to 25% rayon fab-ric, which assists its manufacture. This crudeFiberfrax material is known as L-144 and iswhite in appearance. The L-144 can be heattreated at the factory to remove 97% of thevolatile organic material and the resultingcharcoal gray product is known as L-144T.

Kellogg specifies the use of the heattreated version (L-144T) in the manufactureof its riser subassemblies. A sample of theactual material used in the Kujang as-builtassemblies was supplied to Kellogg by the cli-ent for analysis. The results indicate that a-bout 4% of the sample is carbon, which indi-cates incomplete heat treating.

Proposed Mechanism: It has been proposedthat the organic material contained in the"as-built" Fiberfrax material has decomposedinto carbon under the operating conditionspresent in the liner/cone assembly.

Analysis: Requirements 1, 2 and 3 are allsatisfied for this mechanism. However, Re-quirement 4 is not satisfied directly. Theamount of carbon which could be produced bythis mechanism alone is far less than requiredto account for the observed carbon.

In addition, the amorphous carbon whichwould result has almost twice the density ofthe compressed Fiberfrax. Thus, a net shrink-age, not expansion, would occur.

Two other points are worth noting becausethey may influence any reaction mechanism:

a. the exposed nickel surface area ofthe ni chrome wire (1.45 cm^/cm^) andthe liner itself (0.2 cm^/cmZ) canhave a catalytic effect on crackingreactions, and

b. it is known that carbon itself canproduce active sites for further car-bon decomposition.

Conclusion: The Fiberfrax material usedin the construction of the riser cone subas-semblies probably contained more organic ma-terial than allowed under the manufacturer'sspecification. It is possible that in the hot,oxygen free environment of its application,some carbon may have been formed. However, byitself this quantity of carbon would not ac-count for the observed amounts, nor would itspresence produce any compressive forces on the

liner. It is possible that carbon producedfrom this material acted as a seeding mechan-ism for carbon generation from another source.

Mechanism II - CO Disproportionati on(Boudouard's Reaction)

Background: Carbon monoxide dispropor-tionation under favorable conditions can pro-duce solid carbon deposition. This reactionis sometimes called the Boudouard Reaction:

2CO

2 molesgas

1 molesolid

C02

1 molegas

There are parallel reactions which mayalso occur such as the well-known water gasshift:

CO H,

and methanation:

CO

All of these compounds are present inthe normal reformer outlet gas.

Proposed Mechanism: At some point in orimmediately adjacent to the Fiberfrax zone,the Boudouard Reaction will deposit solidcarbon and the reduced gas volume will pullmore reactant CO into the reaction zone.

Analysis:

Requirement 1: All of the proposed re-actions are theoretically possible but onlyunder favorable conditions.

Requirement 2: Our analysis of the condi-tions under which the Boudouard Reaction willoccur indicates that normal reformer outlettemperatures are too high. However, at somepoint removed from the liner skin itself, inthe insulation, the conditions may be favor-able.

A catalyst can increase the rate of re-action; however, it cannot cause a reactionto occur if conditions are not favorable.Catalysts which promote the rate of reactionof CO disproportionate are iron, cobalt,nickel and carbon black. The decomposition ofthe organic matter in the Fiberfrax producescarbon which may catalyze the formation ofmore carbon from the process gas. Nickel,present in the wire used to give strength to

95

the Fiberfrax, may also contribute. Carboncould form on the wire and the formed carbonwould continue to act as a catalyst even afterthe nickel containing wire is covered. Itshould also be noted that mixtures of cata-lysts seem more effective at promoting theBoudouard Reaction than a single catalystalone.

Kellogg included the Boudouard mechanismin our laboratory study. The results indicateno carbon growth could be stimulated at thetwo conditions studied which were at the highand low temperature end of the favored range.

Requirements 3 and 4: The reaction rate,particularly if catalyzed as mentioned above,can be sufficient over a period of months tobuild up carbon in the quantities found in thefailed riser cone area.

Conclusion: Based on laboratory results,and on the study of this reaction system, weconclude that the conditions in the immediatevicinity of the liner are not conducive forcarbon formation by the Boudouard Reaction. Ifso, all risers in all reformers would experi-ence these problems repeatedly. This reactioncould take place on the cooler outer layers ofa large carbon deposit caused by another me-chanism, thus contrubuting to the ultimatefailure of the liner. Nevertheless, it is prob-ably not the primary mechanism for carbongrowth.

Mechanism III - Fischer-Tropsch Synthesis

Background: There are well-known reactionroutes which synthesize heavy hydrocarbons andalcohols from carbon monoxide and hydrogen.These reactions are generally known as Fischer-Tropsh Synthesis and are catalyzed by iron,nickel and other metals.

Proposed Mechanism: The reformer effluentcontains sufficient hydrogen and carbon monox-ide to form heavy hydrocarbons and alcohols inthe stagnant zone behind the liner, in the areaof the Fiberfrax containing nichrome wire.These heavy compounds decompose to form coke.("Coke" is used here to indicate carbonaceousmaterial having some hydrogen content, as op-posed to pure carbon.)

Analysis:

Requirement 1: The mechanism of carbonmonoxide and hydrogen to "heavies and "heavies"to coke is theoretically possible. These heavi-er compounds will form coke with a significant

hydrogen content. However, laboratory analy-sis found the actual material found at theriser/liner to be almost entirely carbon,with an almost undetectable hydrogen content.

Requirement 2: F-T reactions are (at thepressure conditions present in the reformer)favored by temperatures lower than present inthe area where the carbon was found. Thesetemperatures exist in the outer areas of in-sulation but beyond the radius where the ne-cessary catalyst exists (the nichrome wire oriron in the liner).

Requirements 3 and 4: These requirementsare satisfied.

Conclusion: Based on the unfavorable re-action conditions and the absence of signifi-cant hydrogen in the observed deposit, wehave concluded that F-T reactions are not thecause of the carbon deposition.

Additionally, this Fischer-Tropsch pos-sibility exists in essentially all of theKellogg ammonia plant reformers in operation.If such a mechanism were applicable here, itin effect should be apparent in all of thereformers. This is definitely not the case.

Mechanism IV - Hydrocarbon Dehydrogenation

Background: The reformer effluent con-tains low concentrations of unreacted hydro-carbons. Normally less than ten mole percentis present as methane, along with substantialquantities of steam and hydrogen.

Heavier hydrocarbons may also be presentbut usually for brief periods only. Sometimesduring upsets in the natural gas separationplant which produces the methane feed for theammonia plant, quantities of ethane and heavi-er hydrocarbons appear in the reformer feed.Some of these components will show up in thereformer effluent.

Hydrocarbons, particularly those with agreater molecular weight than methane, candehydrogenate to form carbon. Normally, theriser temperature and the concentration ofmethane or other hydrocarbons present in thereformer effluent are not high enough to pro-duce a reaction. However, there are severalsituations which could produce the necessaryenvironment.

a. During shutdowns, natural gas can leak intothe reformer through feed inlet valves (notblocked or blinded) or through sample purge

96

connections to the reformer system. Thismethane rich gas could fill the voids a-round the liner area. As the reformer isrestarted, the hydrocarbon concentrationin the high temperature area would be suf-ficient to produce a carbon forming en-vironment.

b. During operation, deactivated catalystcould produce a situation which wouldfavor carbon deposition by causing simul-taneously high methane leakage and highoutlet temperatures. Figure 5 illustratesthe effect these conditions have on thepotential for carbon formation.

Both of these situations, while not nor-mally anticipated conditions, ere feasible.

Proposed Mechanism: Hydrocarbons presentin the reaction zone dehydrogenate over a pe-riod of time to produce carbon which accumu-lates to crush the liner.

Analysis:

Requirement 1: Mechanisms for the dehy-drogenation of methane and higher molecularweight hydrocarbons to form carbon and hydro-gen are well known and documented.

Requirement 2: For the typical composi-tion at the exit of the primary Deformer list-ed below, methane cracking will not commenceuntil the gas temperature exceeds 1775°F(968°C) at 450 psia (31.63- kg/cm2g).

TYPICAL ?RIMARY REFORMER EFFLUENTCOMPOSITION

Met Mol '% Dry Mol %

Ho 38.8 68.6

CO

C02

CH4

Ar

5.8

6.2

5.7

0.1

43.4

100.0

10.1

10.9

10.1

0.2

100.0

Under normal operating conditions, dehy-drogenation reactions are not favored by theconcentration of hydrocarbons in the effluentgas which could come in contact with the riser

liner and Fiberfrax material. In fact, thechemical equilibrium considerations tend toretard, or reverse the reactions necessary forcarbon formation.

However, as the percent methane increas-es, the required reaction temperature decreas-es as shown in Figure 5. The calculations alsotake into account that the water gas shift isin equilibrium.

High methane levels in the reformer ef-fluent can occur when the primary reformingcatalyst is losing its activity. Typically,one major cause is sulfur poisoning; anotheris advanced age. Decreasing activity resultsin higher methane leakage and higher tempera-ture. As previously noted, both of these fac-tors increase the chance of hydrocarbon crack-ing in the stagnant area behind the liner.

During our search for similar failures inthe United States, we found only two ammoniaproducers who experienced a riser collapse as-sociated with carbon formation. Both U.S. pro-ducers received a heavier natural gas feed,and noted an increase in sulfur compounds.Since they were not equipped to handle thehigh amounts of sulfur encountered, the sul-fur compounds passed through to the primaryreforming catalyst resulting in an increasedmethane leakage (15 vol% dry) and increasedoutlet temperature. During turnarounds, bothproducers increased their desulfurization fa-cilities. After these improvements were made,they did not experience additional catalystdeactivation or poisoning.

Kujang indicated that hydrocarbon surgesor "blips" do periodically show up in the feedgas. These heavy hydrocarbons (mostly ethane)contain more sulfur compounds than in the nor-mal feed gas. Such conditions, if undetected,could lead to sulfur breakthrough from thesulfur removal unit, which can partially de-activate the reforming catalyst.

Kujang has also reported high gas temper-atures exiting several outlet manifolds for anextended period of time.

Partial collapse of one or more of theriser liners will produce high temperaturesin only the affected risers, while the averagereformer outlet temperature appears to be nor-mal. The reformer firing rate is controlledat the combined outlet as is methane leakage.At these temperatures, increases in methanelevel in any particular riser could produce afavorable condition for cracking in the stag-

97

nant area behind the liner.

Since nickel catalyzes methane cracking,the location that has the greatest surfacearea of nickel should have the largest depo-sits of carbon. The surface area of nickel inthe Fiberfrax is 1.45 cm̂ /cnr of Fiberfrax,while the surface area of the nickel in theriser is 0.2 cmVcmZ of the tube. This helpsto explain why carbon is concentrated wherethe Fiberfrax is located.

Therefore, favorable conditions for hy-drocarbon dehydrogenation, including methanedecomposition, were probably present at sometime during the period of operation beingconsidered.

Requirement 3: The reaction rates willbe fast enough to produce significant carbondeposition if the temperature and hydrocarbonconcentration behind the liner are in the re-gion which is favorable for reaction. Thetime period considered extends over a periodof months. There are also present in the areaof the reaction two materials which couldpossibly have a catalytic effect. The firstmaterial is the ni chrome wire support in theFiberfrax insulation material. The second isany carbon material which may have depositedwhen the organic binder in the Fiberfrax de-composed on the initial heatup of the furnace.

Requirement 4: The magnitude of the car-bon laydown by hydrocarbon cracking can bevery significant and produce large mechanicalstresses. It is well known that the quantityof carbon or coke laydown in thermal and cata^-lytic hydrocarbon cracking processes is sig-nificant, and when growth occurs in stagnantareas, the resulting mechanical stresses cancause deformation of mechanical equipment.

The rate and magnitude of carbon deposi-tion in this system will be controlled by therate at which reactants (hydrocarbon from thereformer effluent gas) enter the reactionzone in the Fiberfrax and the rate at whichthe unreacted components and reaction bypro-ducts leave the reaction zone.

In other words, if the area where thecarbon formed were truly "stagnant", the hy-drocarbon initially present would react, thesystem would come to equilibrium and the me-chanism would stall with very little carbondeposit. Consequently, a means of supplyingreactants and moving byproducts is requiredto account for the quantity of carbon present.Two means are possible: a. cyclic start-up

conditions, and b. diffusion during normaloperation.

Cyclic Startup Conditions: When the re-former is shutdown after being purged withsteam, and as the reformer cools, condensa-tion of the remaining steam will pull gasesinto the system to replace the condensingsteam. If hydrocarbons (natural gas feed) en-ter the reformer through leaking valves orinstrument purge connections, they will pene-trate all void spaces as the steam condensesand be present in very high concentration.The entire insulation system around the riserliner may become filled with methane and oth-er components in the reformer feed. On start-up, the minor amounts of condensed steam willvaporize and quickly exit the system throughlarger areas such as the gap between the lin-er and insulation. The hydrocarbons left be-hind in the smaller voids could be the sourceof carbon deposits, as this concentrated ma-terial enters the region of the Fiberfrax in-sulation. This mechanism would produce asmall amount of coke at each shutdown wherethe reformer was not adequately isolated fromnatural gas feed. Over a period of time, nu-merous shutdown cycles could produce a signi-ficant amount of carbon deposit.

Continuous Diffusion: Molecules of gasmove becayse of driving forces. Pressure dif-ferences are commonly understood drivingforces because they produce rapid, easily de-tected bulk movements of gases. Concentrationdifferences are also significant drivingforces but are often overlooked in flowingsystems because pressure driving forces al-most always overpower them.

However, the carbon has been formed ina stagnant area and concentration differencesof the different compounds will cause move-ment of gas molecules (diffusion) in and outof reaction zones. Diffusion is a molecularmotion, not a bulk gas movement. It is slowbut definite. Molecules of different com-pounds can be moving in opposite directionsnext to each other at the same time. Theyare responding to differences in their indi-vidual concentrations at different locations,trying to equalize their distribution.

For this particular system, a moleculeof methane can react in the Fiberfrax areawhere the temperature is high and can be cat-alyzed by the presence of the Nichrome wire.CH4 ~>- C 2H2

1 mole gas 1 mole solid 2 moles gas

98

There is a very slight volume of gas re-sulting from this reaction, carbon is pro-duced as a solid, methane disappears, thequantity of steam, nitrogen and other com-pounds remain unchanged.

As a result of this reaction, hydrogenwill diffuse away from the reaction site be-cause its concentration is now higher thanthe reformer effluent. At the same time meth-ane will diffuse from the process gas intothe reaction zone where its concentration isnow lower.

The exact path of diffusion is difficultto establish: Figure 6 illustrates one pos-sibility. Hydrogen is known as a "fast gas".It has a very high rate of diffusion and willmove quickly through even the porous aluminarefractory around the liner. Methane can alsodiffuse through this area or down the vaporspace around the liner. The process is slow;nevertheless, it is present if the reactionoccurs.

Conclusion: Dehydrogenation of hydrocar-bon is a very probable mechanism for carbondeposition.

Favorable conditions can develop as aresult of shutdown practices, or, more prob-ably, as a result of operation with a pro-gressivley deactivated reforming catalyst.

Conditions in the Fiberfrax area are en-hanced by a stagnant void region, high tem-perature and the existence of possible cata-lytic activity in the extended Nichrome wiiesurface in the Fiberfrax itself.

The reaction rates are sufficiently ra-pid and the magnitude of the expected resultsagree with those observed in the field. Dif-fusion of reactants and products to and fromthe Fiberfrax area is plausible during opera-tion.

LABORATORY INVESTIGATION

An experimental program was carried outin Kellogg's laboratory to test the influenceof several ceramic fiber materials on twopossible carbon-producing reactions. The re-actions tested were the disproportionateof carbon monoxide and the cracking of meth-ane. This program concluded that at the tem-peratures tested, no difference in carbonforming activity exists among the insulatingmaterials exposed to the test conditions.

CONCLUSION

From the total body of work done, thefollowing conclusions were reached:

1. The collapse of the liner most likelyresulted from a combination of carbonformation, restricted thermal expan-sion and increasing process gas tempera-ture.

2. Decomposition of the Fiberfrax is notthe source of a significant portion ofthe carbon found to be present, althoughit may have contributed to the amount ofcarbon and acted as a catalyst.

3. Based on available information, the mostprobable source of carbon is methane de-composition. Thjs reaction may be cata-lyzed by nickel contained in the Ni-chrome wire of the Fiberfrax and is pro-moted by the relatively higher tempera-ture in the cone area of the riser tran-sition.

4. To avoid problems of this type in thefuture, proper feed desulfurization mustbe accomplished and the furnace shouldnot be operated with high temperaturesand high methane leakage over an extend-ed period of time.

As a further precautionary measure, careshould be taken to ensure isolation ofthe furnace from feed leakage into theprocess stream during shutdown,

ACKNOWLEDGEMENT

The witers wish to thank P.T. Pupuk Ku-jang, International Minerals and Chemicals,Agrico, DuPont, C.F. Industries, Triad, Al-lied Chemical, Monsanto and Farmland Indus-tires for their cooperation and assistancewith this investigation.

LeBLANC, J.R. ENNIS, B.P.

99

INLET MANIFOLD

EFFLUENT CHAMBER

VALVE.

FUEL GAS HEADER

SERVICE PLATFORM

VERTICAL FIRINGBURNERS

PEEP DOOR

PLATFORM

MIXED FEEDHEADER

FLUE GAS DUCT TOCONVECTION SECTION

CATALYST TUBESPRING SUPPORT

CATALYST LOADINGFLANGE

FLUE GAS TUNNELS

OUTLET MANIFOLD

Figure 1. Radiant section of box-type steam reforming furnace.

100

FIELD INSTALLEDBUBBLED ALUMINA^?

INSULATION TOTHESE LINES PRIORTO FIELD WELD

LINER (TP 310 SS)

PRESSURE SHELL (CS)

WATER JACKET (CS)

CARDBOARD INSTALLEDPRIOR TO LINING

CEFIELD W'ELD'EDINTO LINER

•BELL"CONNECTION

PIECE MARKFC

PIECE MARKFB

PIECE MARKFA

PRESSURE PIPE (CS)

• WATER JACKET (CS)

INTERNAL PIPE(TP 310 SS)

CERAMIC FIBERLAYERS)

Figure 2. Riser to transfer line connection.

"BELL*CONNECTION

Figure 3. Riser to transfer line connection.

LJNER

PRESSURE SHELL

WATER JACKET

VOIDAFTER START-UP

PRESSURE PIPE

WATER JACKET

4" PIPECARBONMATERIALBUILD-UP

CONE

101

Figure 4. Sectioned transition assembly fromammonia plant with collapsed riser and carbonbuild-up.

940

020

900

880

860

840

820

800

WC

CH4 * C(S) + 2H2

(PRESSURE * 31.63 KG/CM2J

NO CARBONFORMATION

NOTE: CALCULATIONSASSUME WATER-GAS-SHIFTK AT EQUILIBRIUM

CARBON FORMATIONREGION

NORMAL OPERATING POINTs• %CH4 (MOL %, DRY)i i t i i

10 12 14 16 18 20

Figures. Methane cracking temperature vs. methaneconcentration exit the primary reformer.

PROCESS GAS(CH4. C02, CO. H2. H20

DIFFUSIONTO & FROMFAILURE AREA

VOID AFTERCARDBOARDDISINTEGRATIONAT START-UP

CARBON FORMATIONAT THE NICHROMEWIRE BYCH4 - C,s) + 2H2

RISERLINER"

PROCESS GASRICH IN HYDROGEN(CHj. CO,. CO, Hj, H2O)

WATERJACKET

H2 DIFFUSIONTHROUGHBUBBLED ALUMINA

POROUSBUBBLED ALUMINAREFRACTORY

FIBERFRAX INSULATIONW/ NICHROME WIRE

Figure 6. System of carbon formp.*if*n via methane cracking and hydrogen diffusion.

102

DISCUSSION

JOHN LIVINGSTONE, ICI: We did have some failures inearlier days of our operation with loose loop. On each ofthose occasions of failure, there was no sign whatsoever.We were operating with naphtha around it and thereforehad highest in hydrocarbons available for this sort ofreaction and we were running up to slightly high exitmethane concentrations and slightly higher temperatures.Therefore, I find this situation somewhat at variance withyour suggestion. Do we know whether or not there wasthe formation of carbon on the catalyst? In fact during thetime when this pressure drop was occurring, I believethere was a catalyst replacement.

DARADJAT, Ujang Fertilizer Co.: Catalyst was quitenew—only 2 months in service—when the fail ure was firstnoted. The type of the catalyst was the same as the firstcharge catalyst. There was no carbon formation on thecatalyst.

ENNIS: This carbon did fill the gaps and the voids in thecollapsed section. It was not as though we had a cylinder

of carbon with a gas space and then a collapsed liner;rather, the carbon did completely fill the area. It is difficultto analyze all these things. The focus of our redesignefforts is to eliminate that dead space; regardless of whatthe mechanism is, our concern goes away if we can have adifferent design. Quite a number of plants and theoperators we surveyed commented that there was nocarbon present.

After this paper was prepared, we have uncovered twoother cases where liners have been examined in detail.Unless the situation as that castable is coming out isclosely looked at, one is just chipping out that castableand replacing the liner. The presence of this carbonmaterial is hard to detect and is easily ignored, unless youknow exactly what you are looking for. When one examinesa nice clean cross section, it is very easy to see that it isthere. When you do this kind of repair work withsubcontracted crews on a 24-hour basis, that castablecomes out and a new liner goes in. It could be easilyoverlooked.

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