catalyst breakagein reformer tubes

8/10/2019 Catalyst Breakagein Reformer Tubes

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Topic: Understanding Catalyst Breakage inReformer Tubes

Catalyst Breakage in Reformer Tubes

Introduction

Catalyst breakage is a well known phenomena that occurs during operation and transients suchas reformer trips, whether this be due to,

Normal in service breakage, Breakage due to carbon formation/removal, Breakage due to steam condensation or carry over, Breakage during a trip.

The effect of catalyst breakage can be observed in a number of ways,

Hot bands, Speckling and giraffe necking, Catalyst breakage and settling.


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These effects are illustrated below,

Figure 1 Tube Appearance

In the worst case, catalyst breakage will lead to carbon formation and hence a deterioration of theobserved problem.

This document intends to detail the following,

The types and causes of breakage that can occur,

The effect on tube appearance,

The effect of changing out the worst affected tubes and what this means in terms of theperformance of the rest of the reformer.

This document will restrict itself to catalyst breakage and damage in primary reformers only.


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Types of Breakage

There are a number of different types of breakage that can be observed in a steam reformerwhich are detailed below.

Effect of Catalyst DesignIf the catalyst has been designed such that on breakage, it forms a large number of smallfragments, the pressure drop will rise rapidly. An example of this below.

GBHE C2PTsVULCANSeries VSG-Z101/102 Primary Reforming catalyst;

VSG-Z101 4 Holecatalyst, similarto the JMC 4-Hole catalyst Is anexample of a catalyst with goodbreakage characteristics, in thatwhen it does break it forms large

fragments which mean that thepressure drop is relatively small.

This is because pressure drop isinversely proportional to effectivepellet diameter therefore if thefragments formed are large, thenthe effective pellet diameter onlyincreases marginally.

Furthermore, pressure drop is related to voidage by the following term (1-e)/e and therefore anydecrease in voidage will cause large increases in pressure drop

Figure 2 Good Breakage Characteristics


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An example of a catalyst with poor breakage characteristics if that of the Sud Chemie WagonWheel (the extended Wagon Wheel EW, with thicker ligaments may be better) and HaldorTopses seven hole catalyst,

Figure 3 Poor Breakage Characteristi cs

Breakage of the catalyst in a tube will lead to a high resistance to flow and therefore, the flowthrough the tube will be low. This will cause the tube to operate hot a similar effect is caused byvariability in the loaded voidage.

Causes of Breakage

Trips

Excessive trips cause expansion and contraction of the tubes; the contraction of the tubes causeslarge stresses to build up on the pellets and these stresses can only be relieved by movement ofthe catalyst axially in the tube or pellet breakage. In reality, only the catalyst at the top of thetubes can move and the catalyst towards the bottom of the tube, where the temperature changeswill be the greatest, are locked in position. Therefore, the only possibility is for the catalyst tofracture.

Figure 4 Example of Catalyst Breakage


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Settling

Care should be taken to allow for the effect of tube expansion. Sufficient catalyst must becharged into the reformer tube when cold to make sure that when operating, and therefore hot,the catalyst does not settle down so far as to expose empty space at the top of the reformer tube.

Figure 5 Catalyst Settling

Insufficient Catalyst Loaded

In some cases, it is possible that insufficient catalyst is loaded into the reformer tube and whenthe plant is started up, due to the radial tube expansion (see above), the total fired volume of thetube increases. Under such circumstances, it is possible that the top of the catalyst falls below

the bottom of the roof refractory and this section of tube will become hot since there is no catalystto support the steam reforming reaction to keep the tubes cool.

Milling

Milling typically occurs within a primary reformer at the tube inlet where the high gas velocities atthe inlet of the reformer tube can cause movement of the catalyst pellets and hence attrition. Thishas been observed on a number of plants a Eastern European Plant. In this instance, thecatalyst had been installed too high up the tube and the jet from the inlet pigtail had moved thecatalyst such that they were turned in sphere.

Carbon Lay down and Removal

There are a number of forms of carbon, but the most serious form in terms of catalyst damage ispolymeric since this carbon forms within the pore structure of the pellet. As the carbon structuregrows within the pellet, stresses are generated and once these become sufficiently great, thenthey can cause the pellet to fracture.


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As is well known, once carbon has been laid down, it is possible to remove it by conducting whatis known as a steam out. Although a steam out does have the beneficial effect of removing thecarbon by gasification, this does have an effect on the catalyst. As the carbon is gasified, it isconverted from a solid to a gas there is a huge volume expansion which can lead to some pellet

breakage due to the large stresses generated within the pellet.

Hydration

Some catalysts (Haldor Topse) suffer from hydration of the catalyst support (MgO) if subjectedto steaming conditions between 450-650C. The hydration of the MgO to Mg(OH)2causes avolume expansion within the pellet structure and this generates stresses which can lead toexcessive catalyst breakage.

Effects of Water

Water can affect the primary reforming catalyst in a number of different ways; these are detailedbelow,

Water Carry Over - One problem associated with water is the carryover from the steamdrum, where the liquid is not fully dis-engaged from the steam. If this liquid is notvaporized in the steam superheater, then it is possible for boiler salts to be carried over tothe reformer where it can be poisoned or a crust of salts can be formed on the catalyst.

Water Soaking On a Brazilian Ammonia plant, the operator managed to fill the bottomsection of the reformer tubes with water. Upon restart, the pressure drop across thereformer was high and this lead to a shut down. After discharging the catalyst it was foundto have had the edges sheared off as shown below,

Figure 6 Catalyst Damage


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The cause of this was when the catalyst was heated up, the water could not escape fromthe centre of the ligaments, which represents the thickest part of the catalyst pellet, beforeit was vaporized. As soon as the water vaporized, there was a huge volume expansion

which caused these sections to break away from the rest of the pellet.

Condensation - On a plant trip it is very possible that steam can condense and the sit indead legs or low points in the feed header system. On a plant restart, it is possible that thewater is carried forward on to the catalyst. The catalyst is normally hot at this stage, andas the cold water hits the hot catalyst, the catalyst will be rapidly cooled and the stressesinduced can shatter the catalyst.

This problem can be prevented by eliminating low points and dead legs during the designof the plant it is usual that this kind of problem will be picked up during the plant HAZOPreview. Suitable positioning of drains and correct start up procedures will also help inminimizing the risk.

The inlet headers and associated pipe work from the mixing tee to the tube inlet shall bedesigned such that there are no dead legs where condensate (feed or steam) can collect.If there are low points then drains should be installed such that this condensate can beremoved. Operations procedures should clearly state that these drains are opened duringstart ups.

Passing Steam Valve - If the process steam valve passes during a shut down or whilst theplant is shut down, then it is possible for water to condense on the catalyst. On restart thiscan lead to a number of problems such as shattering of the catalyst and potential formationof concrete.

Effect of Water Carry Forward - If water is carried forward either from a saturator or fromthe process steam, it is possible to generate an extreme thermal shock due to the

quenching of the inside of the reformer tubes. This creates both a high tensile stress onthe inside of the tubes, and reduced ductility leading to sudden, deep cracking, or evenshattering of tubes.

Effect on the Catalyst and Tube - In some cases where the catalyst has been wetted, thesupport material can be leached out and deposited on the inside of the tube walls. Whenthis residue is dried out, a hard coating is formed on the inside of the tube wall which isvery difficult to remove. A device known as a frapper can be used to remove this coating;this device consists of a pear shaped metal head attached to a high speed rotating shaft bya hinge. This problem occurred at a Gulf Coast Plant in the late 1990s and took threedays to clean out.


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Effects of Liquid Hydrocarbons

There have been a number of instances of liquid hydrocarbons being passed to the reformer without steam. This causes gross carbon formation which can lead to a loss of activity and alsosignificant catalyst breakage.

Localized Overheating

There are a number of causes of flame impingement on the tubes, for example, mis-alignedburners, flue gas mal-distribution and poor burner maintenance. The effect of these problems isto rapidly cycle the tube and catalyst temperatures up and down and in so doing causes catalystbreakage

Up Flow Fluidization Problems

The majority of reformers have the process gas flowing downwards and hence there are noissues associated with fluidization of the catalyst, however, there are a number of up flow circularreformer. If the design of the reformer is poor or the plant has been up-rated, then is it possible toachieve process side velocities that are sufficiently high to fluidize the catalyst. This will lead tocatalyst attrition and breakage which will cause excessively high pressure drop and fouling ofdownstream equipment by catalyst dust. A potential solution to this problem is to install a holddown device with sufficient mass to resist the fluidization force. A typical design is shown below.

Figure 7 Hold down Plant


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Poor Catalyst Loading

Ensuring a good catalyst loading is fundamental in ensuring efficient operation of the primary

reformer.

Any deviations in resistance to flow throughthe tubes will result in differential flowsbetween tubes and this in turn will lead totube wall temperature differences asillustrated to the right.

A good catalyst loading will cause evenprocess gas distribution and hence even tubewall temperature distribution as shown below,

Figure 9 Good Catalyst Loading

Another effect is that there will be process gas exit temperature spreads on the reformer whichwill artificially increase the methane slip from the reformer. The effect of this effect is illustratedbelow.

Figure 8 Poor Catalyst Loading


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Figure 10 Effect on Flow and Pressure Drop of Poor Loading

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-5

0

5

10

15

20

25

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Pressure Variation (%)

Flow

Variation(%)

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-10

-5

0

5

10

15

20

25

Flow Variation (%)

TWT Variation (C)

GBHE C2PT Catalyst Process Technologyrecommends the use of a pressure drop

measurement device, which allows for tubes pressure drops to be measured at various pointsduring catalyst loading. The results of this allow the operator to determine which tubes have alow resistance to flow (a low pressure drop) which need further vibration and those with a highresistance to flow (a high pressure drop) which need reloading.

Also the method of loading is very important. The traditional sock loading, can when appliedcorrectly, give a very good catalyst loading. However, the more modern dense loading methodscan give a loading where little or in some cases no remedial action is required during and aftercatalyst loading to achieve a uniform catalyst loading.

Voids

Furthermore, a poor loading can give rise to localized voids within the tube which will be seen ashot spots on the tube. This can then limit the reformer performance since to keep these tubescool, the firing around these tubes with hot spots has to be reduced. This will lead to highmethane slip from the affected tubes and therefore a high overall methane slip from the reformer.


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Effect on Tube Appearance

The above problems can cause a wide variety of tube appearances; these are defined in the

following table,

Parameter Tube Appearance

Poor breakage characteristics- Minor damage- Severe damage

Hot bandsHot patches & high P

Trips- Minor damage- Severe damage

Hot patchesHot bands & high PWhole tube hot & high P

Settling Top of the tube is hot

Insufficient catalyst loaded Top of the tube is hotMilling High P

Carbon Lay down- Minor- Severe

Hot bandsLong hot bands & high P

Hydration Hot bands & high P

Water- Carry over- Water soaking- Condensation

Poisoning and hot bandsHigh PCatalyst breakage, hot bands and high P

Liquid hydrocarbons Carbon formation, hot bands and severe breakage onsteaming

Localized overheating Catalyst breakage and hot bands

Up flow and fluidization Catalyst breakage, carryover of dust to WHBPoor catalyst loading Variety of effects including hot bands through to whole

tubes appearing hot


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Options for Rectification

The key option for rectification of all of the above problems is to replace the affected catalyst.However, this is not as simple as it first appears since deciding which tubes to replace is not just

a matter of considering those tubes that appear hot. Why is this? Well this will be explained inthe sections below.

Pressure Drop Theory

In order to fully understand the effect of breakage, it is important to understand the theory behindthe calculation of pressure drop through a catalyst bed and more specifically the pressure dropthrough a reformer tube. The pressure drop across a reformer tube is defined by the equation(Eqn 1),

( )3

2x

2

1QRePDCP

= Eqn 1

Where,

P is the pressure drop,PDC is a constant representing the performance of a particular catalyst,Re is the Reynolds number which is defined as,

( )1du

Re c

= Eqn 2

is the gas density,Q is the gas flow rate,is the catalyst voidage,

u is the superficial gas velocity,dcis the effective channel diameter which is inversely proportional to the pellet size,is the gas viscosity

This can be rearranged to give,

( )1RePDCP2

Qx

3

= Eqn 3

Which in turn can be simplified (after inclusion of the Reynolds number) to the following,

( ) ( )( )x+ =

11duPDC

P2Qx

c

xx1

x3

Eqn 4


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Which can then be simplified to (by removing parameters that can be considered constant andreplacement of the channel diameter as inversely proportional to the pellet diameter),

( )( )x

= 11

dP

CQ

x

p

3

Eqn 5

Where C is a constant representing parameters that for a single point in a tube can be consideredconstant, that is to say,

( )x

x1

x

uPDC

2C

=

+ Eqn 6

What does this tell us? As we change the resistance to flow (due to catalyst breakage), i.e.:reduced equivalent pellet diameter and voidage, the flow rate through a tube will be decreased.Equation 1 can also be rewritten to give,

( )( )x

p

32

x12

dC

1QP

=

Eqn 7

From this a resistance to flow term can be defined as follows,

( )( )

p

x

d

ce to Flowsis

=

3

11

tanRe Eqn 8

And also,

2

2

C

Qce to flowsisP

=

tanRe Eqn 9

When the resistance to flow increases, due to catalyst breakage so does the pressure drop.Why is this, well firstly the effective pellet diameter is decreased since there are now some smallfragments of pellets as well as whole pellets and so the effective pellet diameter is reduced.Since resistance to flow is inversely proportional to the effective pellet diameter. Also these smallfragments tend to fill the void spaces between the pellets and thereby reduce the voidage againincreasing the resistance to flow and hence the pressure drop.

At this point a common fallacy will be put to rest; it often said that the pressure drops through the

reformer tubes is different when there has been some breakage of the catalyst. This is in factuntrue. Given that the pressure at the end of the feed header when is passes process gas intothe sub headers is the same and that the pressure at the collection points on the transfer headersis the same, the differential pressure drop across the reformer is the same.


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Therefore, if there is a variation in breakage, and hence voidage and effective pellet diameter, theonly variable that can change is the flow rate.

Conceptualization

To understand more fully the problem associated with catalyst breakage and the effect it has onthe performance of the reformer, consider a reformer that has been loaded well, and thenconsider the distribution of tubes with respect to deviation of the PD Rig pressure drop from theaverage. The following figure illustrates this relationship assuming that there are a large numberof tubes and that the relationship can be approximated by a Normal Distribution,

Figure 11 Normal Distr ibut ion Frequency Chart

What does this Conceptualization Mean?

We can take the above conceptualization and apply it too many different situations; for example,

Poor catalyst loading,

Gross carbon formation,

A slug of water,

Excessively fast trips, Catalyst milling,

Localised overheating.


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In the next sections, the above conceptualization will be applied to each of these situations. Bearin mind that the pressure drop deviation is as measured by a PD Rig (see below for anexplanation) and that the actual pressure drop across all of the reformer tubes during normaloperation will always be the same.

Poor Catalyst Loading

If the loading was deemed to be poor then the frequency plot would be as follows,

Figure 12 Poor Distribut ion Frequency

So what does this difference mean in terms of real performance? In order to understand thisquestion, we need first to understand what the above plot really means in terms of the reformeroperation under normal conditions.

Although the PD rig does actually report a pressure drop through the tested tubes, what this reallyrepresents is in fact a resistance to flow. Why is this true? Well, due to the design of the PD Rig,the flow through the orifice plate is sonic and therefore for a fixed upstream pressure, the actualflow rate will be constant. Since pressure drop is proportional to the square of the flow rate timesby a term that could be called a resistance to flow, it can be inferred that the PD Rig actuallymeasures resistance to flow.

Therefore, tubes with a high pressure drop actually have a high resistance to flow whilst those

with a low pressure drop have a low resistance to flow. Tubes with a high resistance to flow willtherefore under normal operating conditions have,


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A low process gas flow rate which if the heat flux to the tubes (i.e.: the firing surroundingthe affected tubes) is not reduced will lead to higher tube wall temperatures,

A higher potential for carbon formation since the process gas temperature within the

tubes is higher.

This therefore means that the carbon potential of the process gas has increased which in turnmeans that the rate of carbon lay down increases and therefore the rate of carbon formationincreases.

Gross Carbon Formation

Under these circumstances it is assumed that all the tubes have suffered from carbon formationand that this has then been steam off. The following graph illustrates what will happen to thepressure drop variation post carbon formation,

Figure 13 Effect on Pressure Distribution ofCarbon

Frequency Distribution

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Pressure Drop Deviation

FractionalNumberofTubes

Base Case

Carbon Formation

As can be seen the distribution has moved to the right through a decrease in both effective pelletdiameter and voidage and therefore the total pressure drop across the reformer will rise.


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A steam out can be conducted to remove this carbon, however, during a steam out, any carbonwithin the pores of a pellet will be gasified and this leads to a huge volume expansion leading to ahigh stress on the pellet and breakage of the pellets.

The effect of this is highlighted below,

Figure 14 Effect of a Steam Out on Frequency Distribution


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FractionalNum

berofTubes

Base Case

Carbon FormationPost Steam Out

A Slug of Water

Slugs of water can be passed to the reformer either due to condensation on the feed header ordue to carry over of water from the steam drum. The effect of this on the reformer depends onhow the water is distributed to the tubes; the first way is when the water affects all the tubeswithin the reformer. In this case, the catalyst at the top of the tubes will all suffer significantbreakage. The effect is that all tubes are affected the same and as such the resistance to flowand hence the pressure drop across all the tubes rises by the same amount. In reality, due to theinherent variation in such an effect, there will always be an increase in the pressure dropvariation. Overall, the effect is the same as detailed in figure 13.

This is typical of the effect of condensation within the tubes or back flow of water into the tubesdue to a waste heat boiler failure. The second alternative is that the water is passed to a smallsection of the reformer; the location of this depends on the gas velocity through the feed headersand the droplet size. In terms of velocity, as the process gas passes down the feed header andportions of the process gas enter the tubes, the gas velocity is gradually reduced. Whether adroplet enters a particular tube, is a function of the gas velocity and the droplet size.


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The relationship between droplet size and gas velocity is complex, however, it can be explainedas follows. Small droplets have a relatively low momentum and therefore are relatively easilyturned into a tube, no matter what the process gas velocity. Large droplets have a relatively high

momentum and therefore are relatively difficult to turn into a tube when the gas velocity is highbut will turn more easily when the gas velocity is low.

If the droplets are small, then they will tend to affect the tubes at the feed end of the reformer(i.e.: that end of the reformer where the feed enters the reformer). Whereas if the droplets arelarge then they will tend to pass to the non feed end (i.e.: the opposite end). The effect both ofthese on the distribution plot is the same, in that the normal distribution as defined in figure 12, ischanged such that a double hump is formed as highlighted below,

Figure 15 Double Hump due to Localized Water Effects

Frequency Distribut ion

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Base Case

Effect of Small WaterDroplets

As can be seen there is now a double hump in the distribution which highlights that some of thetubes now have a higher resistance to flow than they did have. This flags up that only a portion ofthe reformer has been affected by the water droplets. If (almost) all the tubes had been affectedthen the whole distribution profile will have moved to the right.

Excessively Fast TripsSpecific shut downs such as loss of MP steam, tripping of the flue gas (ID fan) or combustion air(CA fan) can lead to fast plant shut downs, which in turn leads to rapid temperature transients ofthe reformer tubes. During these transients, the tubes contract rapidly, causing rapid reductionsin tube diameter and this leads to very high stresses on the pellets and hence a high degree of

catalyst breakage. Such breakage is exemplified by figure 13.

In this case we see that (almost) all the tubes now have a higher pressure drop that the basecase.


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Catalyst Milling

Catalyst milling is seen on only a few plants and typically affects all the tubes since the root causeis a consistent one that, is that the jet from the pigtail impacts on the catalyst and rolls the pelletsaround. Since there will always be a variation in the outage (or distance between pigtail inlet and

catalyst surface), some tubes will be affected more than others and so not only is the frequencydistribution moved to the right, but the distribution is smeared.

This is as per figure 13 above.

Localized Overheating

Localized heating due to poor burner design, maintenance, and installation or flue gas mal-distribution effects will lead to some tubes being affected by flame impingement. In some cases,the flame itself does not impinge on the tube, but the hot jet of gas associated with the flameimpinges on the tube.

Although this latter effect is not as bad as the former, it still leads to rapid process gastemperature cycles which in turn lead to catalyst temperature cycling. This can then lead tocatalyst breakage and an increased resistance to flow and hence increased pressures drop (asmeasured by a PD rig). The magnitude of the effect depends on what the root cause is andhence how many tubes are affected.

Clearly a single burner that is mal-performing will affect only one or two tubes whilst flue gas mal-distribution will affect both rows along the side wall of the reformer.

In principle the effect will be seen on the frequency chart as highlighted as below,

Figure 16 Localized Overheating


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Pressure Drop Deviati on

Frac

tionalNumberofTubes

Base Case

Poor Burner Maintainence

Fluegas Maldistribution


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As can be seen, for the case with poor burner maintenance, only a few tubes are affected and wesee the formation of a small hump with high pressure drop deviations. It should be noted that ifwe could measure this effect with time, we would see this small hump moving from left to right asthe breakage in the affected becomes worse and worse.

For the flue gas mal-distribution effect, the number of tubes affected is much greater since we areconsidering both outside rows of tubes (on a 10 row reformer this is 20% of the tubes). Again wesee a double hump formed and this again will move to the right with time.

Replacing Catalyst

Once catalyst breakage has been identified as a root cause of the observed visual effects in thereformer, then it is important to consider what actions to take. In some cases, it is possible tocontinue to run the reformer until the problem becomes so severe that action has to be taken. Inother cases, the damage is already so severe that action has to be taken immediately.

The classic action to take when suffering from excessive catalyst breakage is to shut the plantdown, measure the pressure drops (using a PD rig) across all tubes and cross reference the

measured PDs against the visual observations (i.e.: which tubes appeared hot during operation)made whilst the reformer was on line. This cross check is performed to ensure that on restart allthe problem tubes will have been recharged. Once these problem tubes are identified then theyshould be discharged (partially or completely); the effect on the frequency distribution ishighlighted below,

Figure 17 Tubes to Change Out

So if we change out the tubes that have a high breakage (i.e.: were hot during normal operation),what happens? The tubes with the fresh catalyst will have little or no breakage and therefore willhave a relatively low resistance to flow (see equation 8 above). This is illustrated below,


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Figure 18 Effect on Frequency Distr ibuti on of Replacing Catalyst


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Base Case

Carbon Formation

Post Change Out

Since the pressure drop across all tubes has to be the same, then the tubes with a low resistanceto flow will see a higher gas flow rate. Now these tubes will receive the gas that was flowingthrough the tubes before the change out, but this is insufficient to fulfill the requirement to give thesame pressure drop across all tubes. So these tubes will take some flow from the rest of thetubes. Unless the firing is reduced around these tubes, then they will be hotter as there is lessflow through the tubes and therefore less heat sink (in terms of both the sensible heat load andthe reaction heat required). It will therefore appear that the hot tubes have moved and that theproblem has not been eliminated by changing out the catalyst. This effect has been seen on anumber of plants who have performed a partial catalyst charge out.


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Tube Appearance

How can we tell what has happened within the tube based on the appearance of the outside tubeFor reference purposes, here is figure one again.

Figure 19 Tube Appearance

Hot Bands

Hot bands typically appear in Top Fired reformers around one third of the way down the tube.Their formation is associated with carbon formation due to poisoning, operating the catalyst pastits effective end of life or catalyst bridging.

Carbon formation and the effect it has on tube appearance is more fully discussed in Basics ofReforming, Shapes and Carbon .

The first effect noted above, of operation of catalyst poisoning, occurs because as the catalystactivity is reduced, there is less reforming. This causes a rise in process gas temperature andmore hydrocarbons slipping further down the tube. Both of these effects contribute to an increasein the carbon forming potential of the process gas. Once the carbon pinch point is reachedcarbon formation will start to occur and this leads to hot bands as illustrated below,


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Figure 19b Hot Bands

The second effect noted above, of operation of a catalyst past its useful end of life, is essentiallythe same as the first effect in that the activity of the catalyst is too low to prevent carbonformation.

The third effect is discussed below under giraffe necking.


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Speckling

Speckling is a very mild form of giraffe necking and operators often comment on it and worry

about it since they think it is the start of hot band formation. Speckling occurs with all types ofcatalyst and on all types of reformers. It occurs when there is a small void formed near the insidewall of the tube. In this zone there is no reaction and so the tube is not cooled by transfer of heatto the process gas and hence the outside tube wall appears hot. The following figure illustratesthe effect,

Figure 20 Tube Speckling


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Giraffe Necking

Giraffe necking occurs when there are large voids close to the inside of the tube wall. This is

similar to speckling as noted above, but is somewhat more severe. In the worst case, giraffenecking can lead to catalyst bridging which causes a localized hot spot on the tube since in thiszone there is no catalyst to support the reforming reactions and therefore reduce the process gastemperature. This is illustrated below,

Figure 21 Giraffe Necking and Catalyst Bridging

If the void is sufficiently large, then the hot patch will become a hot band all the way around thetube.

Catalyst Settling

Catalyst settling leads to a large void at the top of the tube which since there is no catalyst tosupport the reforming reaction, means that this section of the tube becomes hot.


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Excessive Breakage

Excessive breakage has two effects, the first is that the resistance to f low has increased and so

there is less flow through the tube and hence the tube appears hot. The second effect is thatsince the voidage has been reduced, then the catalyst will tend to settle and the loaded heightdrops giving the problems detailed above.

Tube Hot on one Side

There are a number of reasons why one side of a tube may appear hotter than the other side,

Misaligned burner the flame deviated from the vertical and either the flame or the jetassociated with the flame impinges on the tube surface. The opposite side of the tube isshaded and therefore does not become hot and hence change colour.

Flue gas mal-distribution here the flame is moved from the vertical and as noted in theprevious bullet point can impinge on the tube leading to a hot and cold face.

Over firing in one row if one row is being over fired compared to the adjacent row, thenthe tube surface on one side of the tube will appear to be hotter due to the higher heat flux.

Insufficient combustion air if one row of burners has too little combustion air, then thiscan lead to excessively high flame temperatures which in turn cause the outside tubesurface to appear hot.

On many furnaces, the tube coloration varies very rapidly, cycling from appearing hot toappearing cold. This is normally due to the impingement of the jet associated with the burnerflame or the flame itself.

Afterburn ing

Afterburning can also cause excessive catalyst breakage since the tube temperature is rapidly

cycled as combustion occurs, increasing the temperature and then stops as the combustionstops. This can lead to temperature transients within the tube and hence damage to the catalyst.

After burning is normally observed as flames flickering on the tube surface.

Terminology

In order to be able to describe problems on a reformer, it is important to be clear as to what thevarious colors of reformer tubes mean.

Brown/Black if a tube appears brown or black then the tube/catalyst is okay,

Brown with a slight orange colouration in this case the tube appears to be brown but witha slight hint of orange. This is typical of tubes were problems are just starting to occur.

Brown with significant orange colouration in this case the tube appear to be an

orange/brown. This is typical of tubes were there are problems. Orange with a hint of white this is typical of tubes where there are significant problems.

White this is typical of a tube with a very serious problem.


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catalyst breakagein reformer tubes

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