theory of carbon formation in steam reforming

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Theory of Carbon Formation in Steam

Reforming

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Contents

1 Introduction

2 Underpinning Theory 2.1 Conceptualization 2.2 Reforming Reactions 2.3 Carbon Formation Chemistry 2.3.1 Natural Gas 2.3.2 Carbon Formation for Naphtha Feeds 2.3.3 Carbon Gasification 2.4 Heat Transfer 3 Causes 3.1 Effects of Carbon Formation 3.2 Types of Carbon 4 What are the Effects of Carbon Formation? 4.1 Why does Carbon Formation Get Worse? 4.1.1 So what is the Next Step? 4.2 Consequences of Carbon Formation 4.3 Why does Carbon Form where it does? 4.3.1 Effect on Process Gas Temperature 4.4 Why does Carbon Formation Propagate Down the Tube? 4.4.1 Effect on Radiation on the Fluegas Side 4.5 Why does Carbon Formation propagate Up the Tube? 5 How do we Prevent Carbon Formation 5.1 The Role of Potash 5.2 Inclusion of Pre-reformer 5.3 Primary Reformer Catalyst Parameters 5.3.1 Activity 5.3.2 Heat Transfer 5.3.3 Increased Steam to Carbon Ratio 6 Steam Out 6.1 Why does increasing the Steam to Carbon Ratio Not Work? 6.2 Why does reducing the Feed Rate not help? 6.3 Fundamental Principles of Steam Outs



TABLES 1 Heat Transfer Coefficients in a Typical Reformer 2 Typical Catalyst Loading Options FIGURES

1 Hot Bands

2 Conceptual Pellet

3 Naphtha Carbon Formation

4 Heat Transfer within an Reformer

5 Types of Carbon Formation

6 Effect of Carbon on Nickel Crystallites

7 Absorption of Heat

8 Comparison of "Base Case" v Carbon Forming Tube

9 Carbon Formation Vicious Circle

10 Temperature Profiles

11 Carbon Pinch Point

12 Carbon Formation

13 Effect on Process Gas Temperature

14 How does Carbon Propagate into an Unaffected Zone?

15 Movement of the Carbon Forming Region

16 Effect of Hot Bands on Radiative Heat Transfer

17 Effect of Potash on Carbon Formation

18 Application of a Pre-reformer

19 Effect of Activity on Carbon Formation



Theory of Carbon Formation 1 Introduction Carbon formation is a major problem for many operators of steam methane reformers, most typically reformers of the Top Fired design. This document will discuss the reasons why carbon formation occurs, how it develops and what can be done to eliminate carbon formation. The questions that hopefully will be answered are, • How does carbon form? • What are the causes of carbon formation? • What are the effects of carbon formation? • Why does carbon formation get worse? • Why does carbon form where it does? • Why does carbon formation propagate down the tube? • Why does carbon formation propagate up the tube? • How can carbon formation be prevented? • Why does increasing the steam to carbon not remove carbon? • Why does reducing the feed rate not remove carbon? • How can we remove carbon? If carbon is formed then eventually “hot bands” will be observed within the reformer around one third of the way down the tube, or in the case of a bottom fired design, one third up the tube. This is illustrated in the following picture,

Figure 1 – Hot Bands



2 Underpinning Theory The following sections will detail some of the underpinning theory behind carbon formation, and as such will include details on, • The conceptualization of the pellet,

• The reaction chemistry for both reforming and cracking reactions including

details on the reactions occurring with natural gas and naphtha as a feedstock,

• The reaction chemistry for carbon gasification, • Heat transfer

2.1 Conceptualization For the purpose of this document, it is assumed that the reforming catalyst is a pellet covered with nickel crystallites as shown below,

Figure 2 – Conceptual Pellet

The nickel crystallites in the “active” form are represented by the grey shapes siting on the surface of the catalyst pellet. It is these crystallites that support the reforming reaction.



2.2 Reforming Reactions The reforming reaction for methane can be described as, CH4 + H2O CO + 3H2 Eqn 1 In parallel to this reaction is the water-gas shift reaction, CO + H2O CO2 + H2 Eqn 2 And if these two reactions are combined, then the following reaction is the result, CH4 + 2H2O CO2 + 4H2 Eqn 3 Clearly similar equations exist for the reforming of ethane, propane and butane, C2H6 + 2H2O → 2CO + 5H2 Eqn 4 C3H8 + 3H2O → 3CO + 7H2 Eqn 5 C4H10 + 4H2O → 4CO + 9H2 Eqn 6 Or in the most general form, CnH2m + nH2O nCO + (m+n)/2 H2 Eqn 7 The rate of methane steam reforming can be described by the following equation,

[ ] [ ]( ) [ ]4TRΔE/4 CHPexpGSAAct

dtCHd

×××∝ ×− Eqn 8

Where, • [CH4] is the methane concentration, • t is time • Act is the relative catalyst activity, • GSA is the geometric surface area of the catalyst, • ∆E is the activation energy for methane steam reforming, • R is the universal gas constant, • T is the temperature (in Kelvin), • P[CH4] is the partial pressure of methane.



Which can again be written in the more general form,

[ ] [ ]( ) [ ]2mnTRΔE/2mn HCPexpGSAAct

dtHCd

×××∝ ×− Eqn 9

2.3 Carbon Formation Reaction Chemistry 2.3.1 Natural Gas Carbon can be formed by hydrocarbon cracking or CO disproportionation (Boudouard reaction). In reformers for ammonia, hydrogen and methanol it is hydrocarbon cracking that is the most likely to occur. The simplistic reaction for the formation of carbon from methane can be written as, CH4 C + 2H2 Eqn 10 From this the reaction rate equation can be defined as,

[ ] ( )[ ] [ ]4TRΔE/4

4 CHPexpdt

CHdcracking CH of rate ×∝= ×− Eqn 11

Where, • [CH4] is the methane concentration, • t is time • ∆E is the activation energy for carbon cracking, • R is the universal gas constant, • T is the temperature (in Kelvin), • P[CH4] is the partial pressure of methane. The key issues to note are that, • As the pressure rises, the rate of carbon formation increases, • As the concentration of methane rises, the rate of carbon formation also

rises; this is really only true if the concentration of inerts (CO2 and N2 is reduced); see below for the effect of increasing the higher hydrocarbon content of the feed gas whilst reducing the methane content,



• The rate of carbon formation rises with temperature; this is the classic Arrehnius equation.

• When the hydrogen partial pressure is high enough the cracking reaction will stop/reverse

Carbon forms in the region one third down the tube because at this point the temperature is high enough for cracking reactions to proceed at a fast rate. Carbon will form at this point if the hydrogen partial pressure from reforming is not yet high enough to prevent the reactions from an equilibrium viewpoint. Similarly for higher hydrocarbon, the following carbon cracking equations can be written, C2H6 → C2 + 3H2 Eqn 12 C3H8 → C3 + 4H2 Eqn 13 C4H10 → C4 + 5H2 Eqn 14 Which can be summarized as a general equation that says, CnH2m → nC + mH2 Eqn 15 And in turn this can be translated into a reaction rate equation,

[ ] ( )[ ] [ ]2mnTRΔE/2mn

2mn HCPexpdtHCdcracking HC of rate ×∝= ×− Eqn 16

2.3.2 Carbon Formation for Naphtha Feeds Naphtha feeds due to the inherent high carbon to hydrogen ratio and the fact that conversion of long chain alkanes require that the hydrocarbons are first cracked (before being reformed) mean that the carbon formation potential is greater than for natural gas. The cracking process produces olefins which can then be reformed to carbon oxides and hydrogen, however, in parallel to this there is also the polymerization of these olefins to form carbon. Heavy hydrocarbons can also thermally crack to produce carbon directly. These competing reactions are illustrated below,



Figure 3 – Naphtha Carbon Formation

2.3.3 Carbon Gasification The above sections detail the various carbon forming reactions that can occur with the various hydrocarbon feeds seen on steam reformers. There is, however, a reverse reaction that removes carbon from the reforming catalyst and that is the reaction of carbon with steam. C + H2O CO + H2 Eqn 17 This is the carbon gasification reaction. Gasification can also occur with the reaction of carbon with carbon dioxide as detailed below, C + CO2 2CO Eqn 18 These reactions operate in tandem with the carbon lay down reactions and provided that the total carbon removal reaction rate is faster than the lay down rates of reaction, then there will be no net carbon laydown. It should be noted that both these reactions are reversible and therefore, if the operating conditions are suitable, carbon can be formed from these two reactions.



2.4 Heat Transfer Although the activity of the installed catalyst is important in determining the reaction rate within a primary reformer, a second parameter, heat transfer is just as important and in the case of reformers that are on the carbon pinch point [q.v.], the heat transfer is even more important. This is because the hottest point inside the tube is the tube inside wall. Within a primary reformer, the most important heat transfer mechanism is radiation; however, within the tube convection and conduction are also important. This is key since any improvement in the heat transfer properties of the catalyst will lead to a reduction in carbon formation potential. This is illustrated in the figure below,

Figure 4 – Heat Transfer within an Reformer



3 Causes There are a number of causes of carbon formation; these include, • Insufficient catalyst activity, either due to significant poisoning or the

catalyst is at the end of life,

• Operation at excessively low steam to carbon ratios, either during a transient such as a start up or shut down or during normal operation due increases in higher hydrocarbon content of the feed gas or incorrect steam to carbon ratios,

• Poor catalyst heat transfer properties leading to high inside tube wall and

process gas temperatures, • Condensation of liquid feeds in low points or dead legs upstream of the

reformer which is transferred to the steam methane reformer during start up or when switching between available feed options,

• Hot spots formed due to either poor catalyst loading (e.g.: formation of

bridges) or zones of severe breakage (e.g.: crushing of catalyst during excessively fast shut downs) which in there is low gas flow leading to hot spots.

• Localized flame impingement either due to poor burner maintenance,

fluegas recirculation etc which leads to a localised hot spot. This is a particular issue if the plant is operating at low rate.



3.1 Effects of Carbon Formation 3.2 Types of Carbon The following figure illustrates the two types of carbon formation,

Figure 5 – Types of Carbon Formation

• Graphitic – this is a hard layer of carbon that forms on the surface of the

catalyst and prevents the process gas from accessing the active nickel sites. This type of carbon formation is usually generated from the thermal cracking of hydrocarbons.

• Polymeric carbon – this is generated in the pore structure of the pellet and does to some extent block off some of the active sites. A more serious effect is that this carbon exerts a stress on the pellet and can if severe enough lead to pellet breakage and associated pressure drop rise. Alternatively, this form of carbon can lead to cracking of the pellets but the pellets can remain intact with the carbon acting as a binder. Subsequent steaming removes this carbon and it is at this stage that the pressure drop rises significantly. Also, when polymeric carbon is gasified, the sudden volume expansion can over stress the pellet and lead to pellet breakage, usually giving the pellets a pock marked effect. This type of carbon is usually generated from olefin polymerization.



Whisker carbon – this is formed from the Boudouard reaction. The carbon lifts a crystal of nickel off the catalyst support and grows as a whisker from behind the nickel crystal. This is a similar mechanism to metal dusting and can have the similar effect of spalling off the outer layer of a catalyst particle.

4 What are the Effects of Carbon Formation? Once some carbon has been formed, then a number of processes occur, • Loss of Activity – Firstly, the carbon coats the nickel crystallites on the

surface and within a small element of the pellet (as illustrated below); this leads to a loss of inherent activity since there is less nickel (since it is covered in carbon and is therefore inaccessible to the process gas) available to support the reforming reactions.

Figure 6 – Effect of Carbon on Nickel Crystallites

A consequence of this is that since there is no reaction but there is still heat transfer to the process gas from the furnace side, the process gas temperature rises.



Since the outside tube wall temperature is directly related to the process gas temperature, any increase in process gas temperature will result in an increase in outside tube wall temperature. Since the process gas temperature has increased, the rate of carbon lay down (equations 10 and 12 through 14) increases.

• Resistance to Flow – On a simplistic level, the carbon forms a layer over the catalyst surface and therefore should not affect the resistance to flow over the catalyst within that tube1

. However, once a significant amount of carbon has been laid down in a tube, the resistance to flow increases; this is not just because the flow area has been reduced (due to the blockage of flow area due to carbon particles) but also the inherent friction has increased (due to the roughness of the carbon particles).

Once the flow through a tube is reduced, then for a constant fluegas side firing rate, the tube will still be receiving the same amount of heat (heat flux) but there is less gas flow to receive this flow. Part of this additional heat will be absorbed as heat of reaction, but a significant portion will increase the process gas temperature as detailed in the following diagram,

Figure 7 – Absorption of Heat

1 Note that pressure drop is not mentioned here; the pressure drop during

normal operation across all tubes is the same, however, the resistance to flow may vary between tubes. What does change is the flow rate through tubes; a tube with a “relative” high resistance to flow will have a lower flow than a tube with a “relative” low resistance to flow.



Once carbon formation starts to occur, as more of the heat supplied to the tubes is converted to sensible heat (i.e.: a temperature rise of the process gas), this causes a rise in process gas temperature. Since the tube wall outside temperature increases, the typical hot spots as observed will occur. It should be noted that this additional resistance to flow through the affected tubes will cause a re-distribution of flow of process gas throughout the reformer. In the early stages of carbon formation, it is typical that only a few tubes are affected (with the exception of carbon formation due to low steam to carbon ratio or a large amount of heavy hydrocarbons), the flow redistribution is relatively small.

• Heat Transfer Resistance – The carbon coats the surface of the pellets (and in the worst case can also form between the pellets) and this increases the heat transfer resistance. This increase in heat transfer resistance means that the heat is not supplied as quickly (to provide the required heat of reaction), the rate of reaction will drop. Why is this, well consider the following heat transfer coefficients2

from a “typical” reformer,

Table 1 – Heat Transfer Coefficients in a Typical Reformer

Location Relative Heat Transfer Coefficient

Units W/(m2.K) Fluegas 210 Tube Outside Laminar Layer 160 Tube Wall 2700 Tube Inside Laminar Layer 1200 Bed 100

So what does this mean? Well the “furnace side” and “outside tube laminar layer” represent the greatest resistance to heat transfer (by this it is meant that they have the lowest heat transfer coefficient). However, as carbon is laid down, the “inside laminar layer” and the “bed” heat transfer coefficient will be reduced. This will in turn reduce the overall heat transfer coefficient, as defined by,

2 From here on referred to as “htc”.



bitwotfg0 h

1h1

h1

h1

h1

U1

++++= Eqn 19

Where, Uo is the overall heat transfer coefficient, hfg is the furnace side heat transfer coefficient, hot is the outside tube laminar layer heat transfer coefficient, hw is the wall heat transfer coefficient, hit is inside wall heat transfer coefficient, hb is the bed heat transfer coefficient. This can be rewritten as,

bitwotfg h1

h1

h1

h1

h1

++++=

1Uo Eqn 20

As can be seen any decrease in the “inside tube wall” or “bed” heat transfer coefficient will result in a decrease in the overall heat transfer coefficient. One minor point is that although carbon formation will reduce the flow rate through the tube, there is also an effect on the heat transfer coefficient. In terms of the flow through the tube, there are two effects; the first is that the capacity for sensible heat is reduced since this is defined by, ΔTCmQ psensible ××= Eqn 21 Where, Qsensible is the sensible heat load transferred to the process gas, m is the mass flow of gas through the tube, Cp is the specific heat capacity of the gas, ∆T is the resultant rise in temperature.



The second effect is that as the flowrate through a tube is reduced, there is less reactants to be reformed and therefore there is less heat that can be absorb as heat if reaction – this is defined as follows,

[ ]dtH2CdΔHΔHr mn

N

1rn ×=∑ Eqn 22

Where, ∆Hr is the total heat of reaction, ∆Hrn is the heat of reaction for a hydrocarbon which has the formulae CnH2m, N is the largest hydrocarbon in the feed, t is time. 4.1 Why does Carbon Formation Get Worse? There are a number of reasons why carbon formation always gets worse with time (this is a summary of the effects detailed above), • Loss of activity – As the catalyst activity is reduced (due to the coating of

the active nickel sites), the process gas temperature will rise and this means that the reaction rate of carbon formation increases as per equation 16 above. That is to say the temperature dependent part of the reaction rate equation increases and therefore so does the overall reaction rate.

• Flow resistance – Since the flow through an affected tube is decreased

due to the resistance generated by the carbon already formed, this leads to increased process gas temperatures which in turn leads to an increase in the rate of carbon formation (see equation 16).

• Heat transfer resistance – As the overall heat transfer coefficient is

reduced, the process gas temperature rises, thereby increasing the rate of carbon formation.

It could be assumed that the reduction in reforming and consequent increase in methane (and alkane) concentration in the next part of the tube would (as per equation 16) lead to an increase in the rate of reforming, this is unfortunately not the case. Furthermore, since the process gas temperature is higher once carbon formation starts, it again could be assumed that the rate of reforming reaction would increase; again this is not the case. Why is this?



Well the reasons are complex but in summary, • As the alkane content of the feed gas increases, the carbon formation

reaction rate increases faster than the reforming reaction rate for that alkane. This is dictated by the thermodynamics of the cracking and reforming reactions.

• The activation energy of the carbon forming reactions is greater than for the

reforming reactions and so in broad terms, the reforming reactions should be more selective than the carbon forming reactions.

However, as the process gas temperature is increased due to carbon lay down, the relative selectivity is changed marginally in favor of the carbon forming reactions.

In real terms, this means that the rate of carbon forming reactions increases faster than the increase in reforming reactions and therefore on a relative basis, the carbon forming reactions will be preferred over the reforming reactions. It should be noted that the rate of reforming reactions during the initial phases of carbon formation is still orders of magnitude greater than the rate of carbon forming reactions.

In summary the result of carbon formation can be illustrated by the following figure which considers a small element of the tube (highlighted in orange),



Figure 8 – Comparison of “Base Case” v Carbon Forming Tube

It should be noted that “Base Case” and “Base” refers to the situation where there is no carbon formation (the left hand figure). The with carbon formation case is named “With Carbon Formation”. Text in red highlights the key changes that will occur when carbon formation sets in. 4.1.1 So what is the Next Step? In summary, all of the above effects of carbon formation have two consequences, • The process gas temperature increases (due to the loss of activity,

reduction in flow through the effected tube and the reduction in heat transfer) and as defined by equation 16, this will increase the rate of carbon deposition,

• Since there is less reforming (due to the reduction in activity), there is an increased concentration of alkanes which means that the rate of carbon formation increases.

Both of these factors cause an increased rate of carbon formation which then in turn causes,



• An further reduction in catalyst activity and hence less reforming, so higher

temperatures and more hydrocarbon slippage down the tube,

• An increased resistance to flow, so that the flowrate through an effected tube is reduced and hence the process gas temperature increases,

• A reduction in the heat transfer coefficient which raises the outside tube wall

temperature. These then all cause the rate of carbon formation to increase and a vicious circle is form as highlighted below,

Figure 9 - Carbon Formation Vicious Circle

It is also important feature of the two factors noted above that the rate of carbon formation will always increase as the catalyst ages (it should also be noted that as the catalyst ages, the activity drops due to the sintering of the nickel



4.2 Consequences of Carbon Formation The main consequence of carbon formation is an increase in process gas temperature. Since the outside tube wall temperature is inherently linked to the process gas temperature (due to the heat transfer coefficient of the tube wall and inside laminar layer), any increase in the process gas temperature will lead to an increase in the outside tube wall temperature. As is well known, reformer tubes operate in the creep regime, and therefore any increase in tube wall temperature will increase the rate of creep. Since the rate of creep defines the expected tube life, any increase in the rate of creep will reduce the tube life. For instance, hot banded tubes are between 30-50°C hotter than a tube that is not affected. As is known, a 20°C rise in tube wall temperature will reduce the tube life, hot bands will typically reduce the tube life by between 33-75%; i.e.: rather than achieving 100,000 hours (12 years) life, the expected life will range between 25,000 and 66,000 hours (3 and 5.5 years respectively). A second effect is that the resistance to flow through the tube is increased and therefore there will be less flow through the tube. This is not a major problem if only a few tubes become hot since the majority of tubes will accept the additional flow and there will be little overall pressure drop increase across the reformer. However, if all the reformer tubes are affected by hot banding, then a pressure drop rise will be observed. 4.3 Why does Carbon Form where it does? This question can be translated as “Why does carbon form one third of the way down the tube?” As we have seen in equation 16, there are two key factors when carbon will start to form; these are the hydrocarbon content and the process gas temperature. The effect of both of these parameters is inter-linked; as the gas proceeds down the tube, it is heated up and reforming reactions start to convert hydrocarbons to carbon oxides and hydrogen. For carbon formation to occur, the process gas temperature and hydrocarbon content must be sufficiently high that the rate of carbon formation is greater than the rate of carbon gasification; i.e.: the net rate of carbon formation is positive. In a top fired furnace, the conditions for forming carbon (a high enough temperature and sufficient hydrocarbons) are co-incident at around one third of the way down the tube.



In terms of understanding this process, a comparison of the carbon formation equilibrium line and the process gas temperature should be made as illustrated below.

Figure 10 – Temperature Profiles

The key feature of this graph is that the two lines do not cross and there is a positive difference between the two lines; this difference is called “the carbon formation margin”. Some key conclusions can be drawn from this figure, • Provided this margin is positive, the rate of carbon formation is less than the

rate of carbon gasification.

• If the margin is zero, then the process gas at this point can be described at being at the “carbon pinch point”; that is to say the rate of carbon formation is precisely matched by the rate of carbon gasification. This is illustrated in the figure below,



Figure 11 – Carbon Pinch Point

However, a small increase in the heavies in the feed, or a small increase in the process gas temperature will mean that the balance is shifted to favor carbon formation and hence carbon will be laid down. This is illustrated in the following figure,

Figure 12 – Carbon Formation

As we can see, the change in heavies in the feed gas has moved the carbon forming equilibrium line down such that it now crosses the process gas temperature line.



Furthermore, the margin is now negative and this indicates that there will be net carbon formation. Furthermore, as the margin becomes more negative, the rate of carbon formation increases and therefore more carbon will be laid down more quickly. 4.3.1 Effect on Process Gas Temperature As noted above, carbon formation will increase the process gas temperature due to the loss of activity, reduction in heat transfer coefficient and flow through the tube. The following figure illustrates the effect on the process gas temperature in the affected zone,

Figure 13 – Effect on Process Gas Temperature

What does this mean – firstly, the small increase in process gas temperature means that the carbon formation line and the process gas temperature line now do cross and so we have a negative margin for carbon formation which means that we now have net carbon formation. We shall return to this concept in the next section which details why carbon formation propagates down the tube.



4.4 Why does Carbon Formation propagate Down the Tube? When carbon formation starts, it is always observed to grow down the tube. In order to understand this effect, consider the following illustration (see figure 8 above),

Figure 14 – How does Carbon Propagate into an Unaffected Zone ?

The gas leaving the affected zone is now at a higher temperature than if the tube was unaffected (see figure 8) and has a high methane content; within the unaffected element more heat is transferred from the flue gas thereby raising the temperature even further. Although there will be some reforming reaction within this unaffected element, this is insufficient to reduce the process gas temperature and hydrocarbon content of the process gas such that the carbon margin is positive. Since the carbon formation margin is negative, carbon will start to form in this element. This process is illustrated below,



Figure 15 – Movement of the Carbon Forming Region

This process is then repeated down the tube so that the carbon forming region grows down the tube; this is observed as an increase in the hot band on the tube. This continues until the situation arises that the hydrocarbon content has been reduced (by the reforming reaction and the cracking reaction) such that carbon formation margin is again positive. 4.4.1 Effect on Radiation on the Fluegas Side There is also a second order effect of carbon formation and the consequent increase in outside tube wall temperature (observed as hot bands) in terms of the radiative heat transfer on the fluegas side of the reformer. As the outside tube temperature is increased, the rate of radiation emitted by the tube increases; it should be noted that the rate of radiative heat transfer is governed by the Stefan-Boltzman law which is,

)T(TσdtdQ 4

B4 −×= Eqn 22



Where, Q is the radiative heat transferred t is time σ is the Stefan-Boltzmann constant T is the outside tube wall temperature TB is the fluegas bulk temperature Although the fluegas temperature is much hotter than the outside tube wall temperature, heat is still transferred in both directions. Since this is a 4th power relationship, a small increase in the outside tube wall temperature will lead to a significant increase in the rate of heat transfer to the fluegas. The fluegas will re-emit this heat back towards the tube, but some of this heat will not return back to the element of the tube it came from, some will be transferred to a higher portion (this will be discussed below) and some to a lower portion. This is illustrated below,

Figure 16 – Effect of Hot Bands on Radiative Heat Transfer

As can be seen, the element of the tube below the affected zone will now receive more radiation and therefore the outside tube wall temperature will increase.



This will then lead to an increase in the inside tube wall temperature and therefore the process gas temperature which as is illustrated above, may lead to the carbon formation margin becoming negative hence leading to carbon formation in this previously un-affected element. This is a second mechanism for the progression of carbon formation down the tube. 4.5 Why does Carbon Formation propagate Up the Tube? It would initially seem impossible for carbon formation to propagate back up the tube. However, as is illustrated in figure 16 above, the element of the tube above an affected zone will receive more radiation than normal and therefore the outside tube wall, inside tube wall and process gas temperature will rise. Since it is implicit that as the zone below this unaffected zone is forming carbon, then this unaffected zone must be close to forming carbon. Therefore, the small temperature change that occurs will be sufficient to cause carbon formation in this zone. This will cause the outside tube wall temperature to rise and hence more radiation will pass back to the fluegas and some of this will be re-emitted to the zone above this one. Hence there is a mechanism for hot bands to propagate back up the tube. However, it is typical that the hot band will grow more than 5% upward. If hot bands are observed above the 25% mark, then it is most likely to be due to another effect (catalyst bridging which will lead to carbon formation) rather than carbon formation growth.



5 How do we Prevent Carbon Formation

5.1 The Role of Potash One way of preventing the formation of carbon is to include a promoter in the catalyst to help increase the rate of carbon gasification; one such promoter is potash (others include lanthanum). The way that potash helps is as illustrated below,

Figure 17 – Effect of Potash on Carbon Formation

Depending on the feedstock, there are a number of options for the inclusion of potash within the reformer; the following table details some typical catalyst loading options,



Table 2 – Typical Catalyst Loading Options Plant Conditions Loading Light natural gas Plant at design rate High steam to carbon

100% Comp A or Comp B

Heavy natural gas Plant above design rates Feedgas composition changes Low steam to carbon

40% Comp A over 60% Comp B or Comp C

LPG Light Naphtha

30% Comp D over 20% Comp B over 50% Comp D or Comp A

Heavy Naphtha 50% Comp D over 50% Comp C In order to prevent the formation of carbon on heavier feeds, the level of potash is increased from 0% (Comp A/C- series) to 2% (Comp C - series) to 7% (Comp D - series). Note: Comp = Proprietary Catalyst (Competitor Designation) 5.2 Inclusion of Pre-reformer Installation of a pre-reformer allows for the conversion of all higher hydrocarbons to methane, carbon dioxide and hydrogen. This effectively reduces the carbon forming potential of a feed stock such that in the majority of cases, carbon formation within the primary reformer can be eliminated. The pre-reformer is a simple adiabatic bed with a highly active nickel bases steam reforming catalyst. The application of a pre-reformer is illustrated below,



Figure 18 – Application of a Pre-reformer

5.3 Primary Reformer Catalyst Parameters Other than the addition of promoters to prevent carbon formation, there are two catalyst parameters that can be altered to prevent carbon formation, activity and the inherent heat transfer coefficient. 5.3.1 Activity Increasing the catalyst activity, say for instance, by the use of a high surface area catalyst, has a two fold effect, • There is more reforming reaction higher up the tube which reduces the

process gas temperature due to the increased heat of reaction requirement.

• Also this reduces the hydrocarbon content of the process gas. Both of these reduce the carbon potential of the process gas as illustrated below,



Figure 19 - Effect of Activity on Carbon Potential

Again we see that the margin between the equilibrium line and the process gas temperature line is increased when installing a highly active catalyst. 5.3.2 Heat Transfer By improving the heat transfer characteristics of the reforming catalyst, for instance making the pellet smaller, the rate of heat transfer from the fluegas side to the process gas can be increased. Intuitively this would appear to increase the process gas temperature thereby making the carbon forming potential worse, however, since the reforming reaction with a primary reformer is heat transfer limited, the additional heat transferred will increase the reaction rate such that the carbon forming potential is reduced. Furthermore, the additional reaction reduces the hydrocarbon content of the process gas and this lifts the carbon forming equilibrium line away from the process gas temperature. The effect as exactly the same as for installing a highly active catalyst as illustrated above.



5.3.3 Increased Steam to Carbon Ratio By increasing the steam to carbon ratio, two effects occur, • The mass flow through the tubes increases and since the majority of the

additional steam does not react, this acts essentially as an inert and therefore will reduce the process gas temperature such that carbon formation is less likely. Furthermore, the inside tube wall heat transfer coefficient is increased as the process gas velocity is increased which reduces the outside tube wall temperature.

• Since there is a higher steam partial pressure, the rate of reforming increases and therefore the concentration of higher hydrocarbons is reduced which reduces the carbon forming potential of the process gas.

The effect as exactly the same as for installing a highly active catalyst as illustrated above. 6 Steam Out Once carbon has been laydown, then the only option to remove this carbon is to conduct a steam out. 6.1 Why does increasing the Steam to Carbon Ratio Not Work? Many people believe that increasing the steam to carbon ratio during normal operation will remove carbon laid down on the reforming catalyst. Unfortunately this is not true since, • Once hot bands are observed (the normal method for identification that

carbon formation has occurred), the activity of the catalyst has already been reduced. As such the rate of the reforming reaction is also reduced. Increasing the steam rate will increase this rate of reaction such that the hydrocarbon content of the gas is reduced. However, the activity suppression reduces the reaction far more than the increase in steam to carbon ratio causes. Therefore on balance, the reforming reaction rate is still suppressed.



• Increasing the steam to carbon ratio will also provide more driving force to

gasify the carbon already laid down. However, since there is less reforming occurring, the hydrocarbon content of the process gas is raised and therefore this increases the rate of carbon deposition. Again on balance, the rate of carbon deposition is increased far more than the increase in the rate of carbon gasification caused by the steam to carbon ratio increase.

It should be noted that if the steam to carbon is increased, carbon formation and gasification will still occur. The key is whether sufficient additional steam has been added such that the rate of gasification is increased such that the net carbon formation rate is zero. It should also be noted that increasing the steam to carbon ratio will not remove any carbon that has already been laid down. 6.2 Why does reducing the Feed Rate not help? Reducing the feed rate has exactly the same effect as increasing the steam to carbon ratio (provided the total rate of steam is maintained), in that although this will reduce the rate of carbon deposition, it will not eliminate it. 6.3 Fundamental Principles of Steam Outs The key method to removing carbon is to conduct what is known as a “steam out”. A steam out consists of operating the reformer at high temperature with no feed gas being passed to the reformer whilst maximizing the steam rate to the reformer. Under these conditions, carbon will be gasified and is therefore removed from the surface and the pores of the catalyst. When conducting a steam out, it is important that the process is monitored very closely since the potential for a burn down of the reformer tubes is very high. Under these conditions, the majority of the heat sink available during normal operation (the reforming reactions) is not available and therefore small changes in fuel rate can have a dramatic effect on tube wall temperatures. It is recommended that regular visual inspections of the reformer tubes are conducted. It is not sufficient to monitor the exit reformer header temperatures since under these conditions; these will not give a true representation of what is happening within the reformer.



To ensure that a steam out has been effective, the effluent from the reformer should be sampled and tested for carbon dioxide and methane; it is typical that these will start off high and gradually reduces as more of the carbon is gasified. Once the levels have dropped to an acceptable level, the feed can be introduced back into the plant. It is also worth checking the reformer effluent and the process condensate for hydrogen sulfide and sulfates / sulfites respectively. This will identify whether sulfur was the root cause of the carbon formation. In some cases, a steam out is not sufficiently aggressive enough to remove carbon, and then an air burn may be required. It is important to note that it must be possible for steam to be passed through the tubes at the same time as the air or else there is no temperature control.