Catalyst deactivation Common causes

 

Catalyst deactivation Common causes

Authors:
MOHAMED ABDEL AATY AHMED, MOPCO, Egypt

This article was first published at Nitrogen and Syngas Conference, 5-8 March 2013, Berlin, Germany

The causes of deactivation are basically three-fold: chemical, mechanical and thermal. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms, catalysts have only a limited lifetime. Some lose their activity after a few minutes, others last for more than ten years. The maintenance of catalyst activity for as long as possible is of major economic importance in industry. A decline in activity during the process can be the result of various physical and chemical factors like Blocking of the catalytically active sites and Loss of catalytically active sites due to the following mechanisms.

The mechanisms of catalyst deactivation in ammonia plant can be classified into different common types: (i) poisoning, (ii) coking, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (vi) crushing. The most common causes are: Poisoning of the catalyst, Typical poisons are H2S, Pb, Hg, S, P in primary reformer and low shift, coke and Deposits on the catalyst surface block the active centers and change the pore structure, Thermal processes and sintering of the catalyst lead to a loss of active surface area and catalyst losses by evaporation of components (e. g., formation of volatile metal carbonyls with CO in methanation reaction). These processes are shown schematically in Figure 1. We a brief discussion about these phenomena to understand these deactivation mechanisms.

Fig. 1: Mechanism of catalyst deactivation (M=metal) Keywords: Catalyst deactivation; Catalyst poisoning; Catalyst sintering; Catalyst coking; Kinetics of catalyst deactivation.

WHAT DOES CATALYST DEACTIVATION MEAN?

One of the major problems related to the operation is the catalyst loss of activity with time-on-stream; this process is both of chemical and physical nature and occurs simultaneously with the main reaction. Deactivation is inevitable, but it can be slowed or prevented and some of its consequences can be avoided.

Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes. Costs to industry for catalyst replacement and process shutdown total billions of dollars per year. Time scales for catalyst deactivation vary considerably; for example, in the case of cracking catalysts, catalyst mortality may be in the order of seconds, while in ammonia synthesis the iron catalyst may last for 5–10 years. But it is inevitable that all catalysts will decay. Steam reforming of methane or naphtha great care must be taken to avoid reactor operation at excessively high temperatures or at steam to hydrocarbon ratios below a critical value. Indeed, these conditions can cause formation of large quantities of carbon filaments which plug catalyst pores and voids, pulverize catalyst pellets, and bring about process shut down all within a few hours. The maintenance of catalyst activity for as long as possible is of major economic importance in industry.

Catalyst deactivation, also known as ageing, is expressed by the decrease in catalyst activity with time Fig. (2). Catalyst activity a is the ratio of the reaction rate at a given time t to the reaction rate at the time that use of the catalyst began t = 0

Activity (t) = r (t) / r (t = 0)

Not only does the decreasing catalyst activity lead to a loss of productivity, it is also often accompanied by a lowering of the selectivity. Therefore, in industrial processes great efforts are made to avoid catalyst deactivation.

Fig. 2: Deactivation behavior of catalysts

 

COMMON CAUSES FOR CATALYSTS DEACTIVATION

Catalysts are evaluated for their activity and stability. The performance of the catalyst is determined by parameters like lower pressure drop, lower tube wall temperature and longer operation close to equilibrium methane conversion. These parameters can be achieved by optimizing the properties of catalyst like — better coke resistance, easy reducibility, higher crushing strength, higher metal dispersion, higher surface area, higher pore volume, higher geometric surface area, resistance to thermal shocks, better heat transfer properties. Earlier catalyst average life used to be 2–3 years but because of the latest improvement in the preparation of the catalyst the life of the catalyst was increased to 5–6 years. All catalysts deactivate with time, although the spent material must then be processed, the nature of the processing depends markedly on the means of deactivation. The three most common causes of catalyst decay are coking, poisoning or thermal degradation. Coke deposition is the most common process [Furimsky, 1996], Poisoning involves strong chemical interaction of a component of the feed or products with active sites on the catalyst surface. Sulphur poisoning of metals is the most widely quoted example [McCulloch, 1983 and Dowden, 1978]. Catalysis involves interfaces, and heterogeneous catalysts are prepared with high surface areas, a condition that is thermodynamically unstable. If a suitable condition arises such as high temperatures in the absence or presence of a suitable chemical environment catalysts will rearrange to form the more favorable lower surface area agglomerates a process known as sintering. The relative importance of the different deactivation processes has been reviewed by many researchers [Al-Dalama, 2006 and Marafi, 2007]. These processes will now discuss these effects in more detail and examine some examples from the catalysts in ammonia industry.

Deactivation processes

Catalyst Poisoning

Catalyst poisoning is a chemical effect and involves strong interaction between a component of the feed or products and the active sites of the catalyst. Therefore, even very small quantities of catalyst poisons can influence the adsorption of reactants on the catalyst. The term catalyst poison is usually applied to foreign materials in the reaction system. Reaction products that diffuse only slowly away from the catalyst surface and thus disturb the course of the reaction are referred as inhibitors. Table (2) lists some catalyst poisons and inhibitors and the way in which they act. The poisons involve molecules that can chemisorb strongly to a catalyst and are entirely specific. Thus, for example, carbon monoxide poisons iron, but has little effect on copper or silver. Prevention or removal is often reasonably facile. Thus, for example, the most active sites on a catalyst may be prepoisoned or an additive may be used that preferentially adsorbs the poison. Removal may involve only increasing temperature, or may involve treatment with a chemical that reacts with the poison or competes with the poison for active sites.

The critical question in the context of a poisoning process is whether it is reversible. If it is, the catalyst may be re-usable. If not, the catalyst must be discarded. As a result, it is useful to consider the general classes of poison while remembering that any strong chemisorptions or interaction can poison a catalytic surface. However, prevention is the preferred option either by using a guard process (such as hydrodesulphurization), a guard bed (such as ZnO) or by including an additive in the catalyst that will selectively adsorb sulphur. The total removal of poisons is often difficult, and even residual traces may decrease activity as the catalyst is brought back on line.

Poisons can be also classified as “reversible” or “irreversible”. In the first case, the poison is not too strongly adsorbed and accordingly regeneration of the catalyst usually occurs simply by poison removal from the feed. This is the case, for example, of oxygen containing compounds (e.g. H2O and COx) for the ammonia synthesis catalysts. These species hinder nitrogen adsorption, thus limiting the catalyst activity, but elimination of these compounds from the feed and reduction with hydrogen removes the adsorbed oxygen to leave the iron surface as it was before. However, gross oxidation with oxygen leads to bulk changes which are not readily reversed: in this case poisoning is “irreversible”, and irreversible damages are produced.

Poisoning of Metals [Hagens, J., 2005]

Metal catalysts are highly sensitive to small amounts of certain impurities in the reaction medium. Catalytically active metals make their d orbitals available for adsorption, and this is the key to understanding both their catalytic activity and their sensitivity to poisons.

• Poisons for metals can be classified in three groups:

– Nonmetallic ions

– Metal ions

– Unsaturated molecules

Particularly strong catalyst poisons are the ions of elements of groups 15 (P, As, Sb, Bi) and 16 (O, S, Se). The poisoning activity depends on the presence of electron lone pairs, which have been shown to form dative bonds with transition metals on chemisorption. If these are involved in bonding to other elements, then the ions are nonpoisons:

Poisons : H2S, thiophene, NH3, PH3, AsH3                                     

Non poisons: SO2- , NH+, PO3, AsO3, sulfones

The poisoning effect of metal ions depends on the number of d electrons. Metals with an empty d shell, such as alkali and alkaline earth metals, and those with less than three d electrons are non poisons, as shown in the following for the example of platinum:

Poisons : Zn2+, Cd2+, Hg2+, Sn2+, Pb2+, Cu+, Cu2+, Fe2+, Mn2+, Ni2+, etc.

Non poisons: Na+, Be2+, Mg2+, Al3+, La3+, Ce3+, Zr4+, Cr2+, Cr3+

Metals readily adsorb unsaturated molecules such as CO and olefins. If they are adsorbed irreversibly in molecular form, then they act as poisons. If dissociation or decomposition occurs, then this can lead to deactivation by coking. Catalyst poisoning can be reversible or irreversible, depending on the reaction conditions. For example, sulfur poisoning of nickel catalysts is irreversible at low temperatures, and methanation catalysts cannot be regenerated, even by treatment with hydrogen. At higher temperatures sulfur can be removed by hydrogenation and steam, so that nickel catalysts for steam reforming are considerably more resistant to sulfur – containing poisons.

 Poisoning of metal catalysts can best be avoided by pre-treatment of the reactants by: Chemical treatment (expensive; can lead to other impurities), Catalytic treatment (very effective for organic poisons) and Using of adsorbers (e. g. ZnO to remove sulfur-containing compounds in natural gas reforming) the incorporation of promoters can also neutralize catalyst poisons.

Thus the sulfur poisoning of nickel catalysts is reduced by the presence of copper chromite since copper ions readily form sulfides. The appropriate treatment method and the decision whether the catalyst or the process should be modified requires detailed knowledge of the cause of deactivation.

Thus, poisoning has operational meaning; that is, whether a species acts as a poison depends upon its adsorption strength relative to the other species competing for catalytic sites to physically blocking of adsorption sites, adsorbed poisons may induce changes in the electronic or geometric structure of the surface [Hegedus, 1980 and Bartholomew, 1987].

Sulphur poisoning

Sulphur is a severe poison for steam reforming catalysts of group VIII metals. Nickel is most susceptible to sulphur poisoning of the group VIII metals as shown by Wise et al. (1985). Sulphur must be removed to a very low level from the feed before it enters the reformer. Under steam reforming conditions all sulphur compounds will be converted into H2S, which is chemisorbed on the nickel surface through the reaction:

H2S + Ni surface  →  Ni surface-S + H2

The adsorbed sulphur forms a well defined 2-dimentional surface structure with a stoichiometry of approximately 0.5 [Rostrup-Nielsen et al., 2002]. This corresponds to a sulphur uptake of 440 mg S per m2 nickel surface [Rostrup-Nielsen, 1984; Rostrup-Nielsen et al., 2002]. The surface coverage of sulphur on nickel depends on the temperature and the partial pressures of H2S and H2.

The uptake of sulphur correlates with the Ni-surface area. The low H2S equilibrium pressure is also reflected in the sulphur uptake of a catalyst pellet as illustrated in Fig. (3). A sharp sulphur profile is seen and only the interior of the pellet furthest away from the exterior surface and the holes are unpoisoned. The poisoning by sulphur takes place as shell poisoning due to pore diffusion. The average coverage of sulphur in the particle will be lower than in the shell and it may take years before the chemisorption front has moved to the centre of the particle [Christensen, 1996].

Fig. 3: Conceptual model of poisoning by sulfur atoms of a metal Surface.

Sulphur has a strong impact on the reaction rate of the reforming catalyst and will decrease the rate significantly [Rostrup-Nielsen, 1984]. It was shown that the intrinsic activity of a catalyst decreases rapidly with the coverage of unpoisoned sites. Other poisons reported are arsenic, lead, phosphorous, silica and alkali metals [Rostrup-Nielsen, 1984]. Silica may substantially reduce the activity of the catalyst by acting as a pore mouth poison [Christensen and Rostrup-Nielsen, 1996]. The alkali metals reduce the reaction rates in some cases by orders of magnitude.

Mechanisms by which a poison may affect catalytic activity are multifold as illustrated by a conceptual two-dimensional model of sulfur poisoning on a metal surface shown in Fig. (3). To begin with, a strongly adsorbed atom of sulfur physically blocks at least one three- or four-fold adsorption/reaction site (projecting into three dimensions) and three or four topside sites on the metal surface. Second, by virtue of its strong chemical bond, it electronically modifies its nearest neighbour metal atoms and possibly its next nearest neighbour atoms, thereby modifying their abilities to adsorb and/or dissociate reactant molecules, although these effects may not extend beyond about 5 a.u. [Bartholomew,1987]. A third effect may be the restructuring of the surface by the strongly adsorbed poison, possibly causing dramatic changes in catalytic properties, especially for reactions sensitive to surface structure. In addition, the adsorbed poison blocks access of adsorbed reactants to each other and finally prevents or slows the surface diffusion of adsorbed reactants. Toxicity increases with increasing electro negativity [Bartholomew, 1987].

Another approach to prevent poisoning is to choose proper catalyst formulations and design. For example, both Cu-based methanol synthesis and low-temperature shift catalysts are strongly poisoned by S-compounds. In these catalysts significant amounts of ZnO are present that effectively trap sulfur leading to the formation of ZnS. The catalyst design (e.g. surface area, pore size distribution, pellet size) can also modify the poison resistance. Finally, it is noted that the operating conditions also affect the poison sensitivity of several catalysts: for example 5 ppm sulfur in the feed poison a Ni/Al2O3 steam reforming catalyst working at 800◦C, less than 0.01 ppm poison a catalyst working at 500◦C, due to the increased strength of S adsorption.

Sulfur adsorbs so strongly on metals and prevents or modifies the further adsorption of reactant molecules, its presence on a catalyst surface usually effects substantial or complete loss of activity in many important reactions. Erekson and Bartholomew (1983), study the methanation activities of Ni, Co, Fe, and Ru relative to the fresh, unpoisoned surface activity as a function of gas phase H2S concentration. The results indicate that Ni, Co, Fe, and Ru all suffer 3–4 orders of magnitude loss in activity at 15–100 ppb of H2S, i.e. their sulfur tolerances are extremely low! Moreover, the sharp drop in activity with increasing H2S concentration suggests highly selective poisoning. Nevertheless, the rate of sulfur poisoning and hence sulfur resistance varies from catalyst to catalyst and is apparently a function of catalyst composition and reaction conditions Indeed, it is possible to significantly improve sulfur resistance of Ni, Co and Fe with catalyst additives such as Mo and B which selectively adsorb sulfur.

Thermal degradation and sintering

Thermally induced deactivation of catalysts results from (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse and of catalytic surface area due to pore collapse on crystallites of the active phase, and/or (iii) chemical transformations of catalytic phases to non-catalytic phases. The Sintering processes generally take place at high reaction temperatures (e.g. >500◦C) and are generally accelerated by the presence of water vapor. Most of the previous sintering and redispersion work has focused on supported metals. Experimental and theoretical studies of sintering and redispersion of supported metals published before 1997 have been reviewed fairly extensively [Bartholomew, 1997].

Three principal mechanisms of metal crystallite growth have been advanced: (1) crystallite migration, (2) atomic migration, and (3) (at very high temperatures) vapor transport. The processes of crystallite and atomic migration are illustrated in Fig.4. Crystallite migration involves the migration of entire crystallites over the support surface followed by collision and coalescence. Atomic migration involves detachment of metal atoms from crystallites, migration of these atoms over the support surface and ultimately, capture by larger crystallites. Redispersion, the reverse of crystallite growth in the presence of O2 and/or Cl2, may involve (1) formation of volatile metal oxide or metal chloride complexes which attach to the support and are subsequently decomposed to small crystallites upon reduction and/or (2) formation of oxide particles or films that break into small crystallites during subsequent reduction. In general, sintering processes are kinetically slow (at moderate reaction temperatures) and irreversible or difficult to reverse. Thus, sintering is more easily prevented than cured.

Fig. 4: Two conceptual models for crystallite growth due to sintering by (A) atomic migrationor (B) crystallite migration.

Effects of important reaction and catalyst variables on sintering rates of supported metals

Promoters Some additives decrease atom mobility, e.g. C, O, CaO, BaO, CeO2, GeO2,Cs; others Increase atom mobility, e.g. Pb, Bi, Cl, F, or S; oxides of Ba, Ca, or Sr are “trapping agents” that decrease sintering rate.

Pore size Sintering rates are lower for porous vs. non-porous supports; they decrease as crystallite diameters approach those of the pores

Alkali metals, for example, accelerate sintering; while calcium, barium, nickel, and lanthanum oxides form thermally stable spinel phases with alumina. Steam accelerates support sintering by forming mobile surface hydroxyl groups that are subsequently volatilized at higher temperatures. Chlorine also promotes sintering and grain growth in magnesia and titania during high temperature calcinations

As a general rule, the rearrangement of most solids will start to occur due to sintering of metal at ca. 0.3–0.5 times the melting point of the material, and will be accelerated in particular chemical environments. Thus, for example, moist atmospheres accelerate structural changes in oxide supports. The resulting loss in surface area decreases the catalytic activity. Component interaction can also occur on overheating. The formation of nickel aluminate from the reaction between nickel and alumina is a good case in point, with the catalytic activity of Ni-aluminate being much lower than that of the metal [Ponec, 1995]. Alloy formation or phase separation can also occur [Chinchen, 1985] which could lower overall catalytic activity.

For example, the Sintering of small Ni-particles in size and thereby loss in surface area, which will reduce the activity. It is a complex process influenced by several parameters including chemical environment, catalyst structure and composition, and support morphology. Factors that promote sintering include high temperature and high steam partial pressure [Sehested, 2003, 2006; Sehested et al., 2001, 2004, 2006 and Rasmussen et al., 2004]. The sintering mechanisms have been followed by in situ electron microscopy [Sehested et al., 2001]. Particle movement is associated with diffusion of Ni2-OH dimers on the nickel surface; A simple model was proposed to account for the Ni-particle size growth with time as a function of exposed gaseous environment and temperature [Sehested, 2006]. The sintering of the Ni-particles is initially fast and will slow down as the Ni-particles growing size. The increase in the sintering rate in H2O/H2 atmospheres is seen at temperatures above 600◦C [Sehested, 2006]. Furthermore, the dependence of H2 partial pressure is seen to be stronger. This is interpreted as a change in sintering mechanism from particle migration and coalescence via atom migration at the support.

Accordingly, over high-surface area catalysts it is desirable to minimize the water vapor concentration at high temperatures during both operation and activation procedures as well. The presence of specific additives is known to reduce the catalyst sintering. For example BaO, CeO2, La2O3, SiO2 and ZrO2 improve the stability of γ-alumina towards sintering [Bartholomew, 2001], whereas Na2O enhances its sintering. In addition to a decrease in the surface area, sintering may also lead to a decrease in the pore openings, and eventually the pores close completely making the active species inaccessible to the reactants.

Gas/vapor–solid reactions

Reactions of gas/vapor with solid to produce volatile compounds

Shen et al.,1981 found that Ni/Al2O3 methanation catalysts deactivate rapidly during methanation at high partial pressures of CO (>20 kPa) and temperatures below 425◦C due to Ni(CO)4 formation, diffusion and decomposition on the support as large crystallites; under severe conditions (very high PCO and relatively low reaction temperatures) loss of nickel metal occurs. Thus, loss of nickel and crystallite growth are serious problems at the entrance to methanation reactors where the temperature is low enough and PCO high enough for metal carbonyl formation. Agnelli et al., (1994), investigated kinetics and modelling of sintering due to formation and migration of nickel carbonyl species. They found that the initially sharp crystallite size distribution evolved during several hours of sintering under low temperature (230◦C) reaction conditions to a bimodal system consisting of small spherical crystallites and large faceted crystals favoring (111) planes. The sintering process was modelled in terms of mass transport of mobile subcarbonyl intermediates (Fig. 5). For example is the use of nickel catalysts in the methanation of synthesis gas. If the temperature of the catalyst bed drops below 150◦C, catalyst is lost by formation of highly toxic nickel tetracarbonyl. Long term simulations were found to predict reasonably well the ultimate state of the catalyst. Based on their work, they proposed two solutions for reducing loss of nickel: (1) increasing reaction temperature and decreasing CO partial pressure in order to lower the rate of carbonyl formation, and (2) changing catalyst composition, e.g. alloying nickel with copper or adding alkali to inhibit carbonyl species migration [Bartholomew, 2001].

Fig. 5: Formation of volatile tetra-nickel carbonyl at the surface of nickel crystallite in CO atmosphere.

Deposits on the catalyst surface (Coking)

The blocking of catalyst pores by polymeric components, especially coke, is another widely encountered cause of catalyst deactivation. Chemical aspects of coking for catalytic reactions involving hydrocarbons (or even carbon oxides) side reactions occur on the catalyst surface leading to the formation of carbonaceous residues (usually referred to as coke or carbon) which tend to physically cover the active surface. Coke deposits may amount to 15% or even 20% (w/w) of the catalyst and accordingly they may deactivate the catalyst either by covering of the active sites, and by pore blocking.

The mechanisms for carbon formation from carbon monoxide over Ni catalysts have been reviewed by Bartholomew. The formation of such species depends on the operating conditions, catalyst formulation, etc. In the case of the steam reforming of hydrocarbons on Ni-based catalysts, three different kinds of carbon or coke species were observed [Forzatti, 1999]. It appears that under certain conditions the coke is very non-uniform, with preferential deposition of carbon in the exterior of the particle. The non-uniform coke deposition inside the catalyst pores may be related to the existence of intra particle diffusional limitation, as reported by Levinter et al., (1967). It is noted that as coke accumulates within the catalyst pores, the effective diameter of the pores decreases, leading to an increase of the resistance to the transport of reactants and products in the pores. If coke is concentrated near the pore mouth it will be more effective as a barrier than the same amount evenly distributed on the pore wall, and eventually pore blockage can occur [Richardson, 1972]. The catalyst composition does also affect significantly the coke deposition. Promoters or additives that enhance the rate of gasification of adsorbed carbon or coke precursors and/or depress the carbon-forming reactions minimize the content of carbon on the catalyst surface. For this reason alkali metal ions, e.g. potassium, are incorporated in several catalysts (e.g. Ni-based steam reforming catalysts). Potassium has several effects: it neutralizes acid sites which would catalyze coke deposition via the carbonium ion mechanism and catalyzes the gasification of the adsorbed carbon deposits, thus providing an in situ route for catalyst regeneration.

Coke deposition mechanism

According to Froment; (2000), the filamentous carbon formation involves the following three processes:

(1) Dissolution of adsorbed-carbon through Ni: **C ↔ CNi,f

(2) Diffusion of carbon through Ni : CNi,f ↔ CNi,r

(3) Precipitation / Dissolution of carbon : CNi,r ↔ Cf

CNi,f – Carbon concentration below the Ni surface (mol C/m3 Ni)

CNi,r – Carbon concentration on the support side (mol C/m3 Ni)

The adsorbed surface carbon, **C, at first dissolves in the Ni particle, therefore it diffuses and precipitates at rear of the nickel crystallite. The continuous precipitation of this adsorbed carbon will form filamentous carbon. For this process to take place, the carbon concentration in the layer just below the Ni surface, CNi, f must exceed the carbon solubility in Ni, Csat. The higher the difference between CNi, f and Csat, the bigger the driving force for this process of filamentous carbon formation. One carbon atom occupies two active sites on the catalyst.

However, the critical problem in this process is catalyst deactivation caused by carbon deposition (coking) and It has been proposed that catalyst deactivation by carbon deposition depends on the amount, type and location of carbon formed. Basically, there are two types of carbon formation in this reaction, i.e. encapsulating carbon and filamentous, CaO-modified Ni catalysts present high stability and good activity with respect to Ni/Al2O3 [Ding and Yan, 2001]. Quincoces et al. (2001) suggested that catalyst promoted with 3% of Ca presents lesser amount of whisker (filamentous) carbon with respect to Ni/Al2O3 catalyst. The CaO addition inhibits the whisker carbon formation and is therefore more stable. This stability is related to the formation of more reactive carbonaceous residues that act as reaction intermediate during methane reforming. According to Kim et al. (2000) the minimum metal particle diameter needed to form filamentous carbon is 6 nm.

According to Wang and Lu; 1998, who evaluated the catalyst performance of various Ni catalysts with different supports, it was discovered that both types of Ni/Al2O3 catalysts, namely Ni/γ-Al2O3 and Ni/α-Al2O3, give very high CO2 and CH4 conversions. Recent research catalyst Deactivation Simulation through Carbon Deposition in Carbon Dioxide Reforming over Ni/CaO-Al2O3 [Istadi and Anggoro, 2011].

Fig. 6: Schematic of encapsulating and filamentous Carbon on nickel catalyst

1. CH4 + 2* → *CH3 + *H
2. *CH3 + *H → CH4 + 2*
3. *CH3 + * → *CH2 + *H
4. *CH2 + *H → *CH3 + *
5. *CH2 + * → *CH + *H
6. *CH + *H *CH2 + *
7. *CH + 2* → **C + *H

Catalyst indicates that the fraction of vacant sites decrease drastically at beginning of the CO2 reforming reaction. This result shows that the catalyst has a high activity. The fraction of vacant sites becomes stagnant after a short time in the reaction. The maximum amount of encapsulating carbon is treated as a monolayer of carbon on the Ni surface, and this monolayer is sufficient to block the active sites and consequently deactivate the catalyst. Any multiple layers of carbon are treated as filamentous carbon in this model. It is observed that the rate of encapsulating carbon formation increases initially at the reaction. This is because the reaction takes place very fast in the beginning, and the site coverage of adsorbed carbon is high. Therefore, this will increase the rate of encapsulating carbon formation. However, after a certain period of time, the rate starts to decrease until the end of the simulation time. According to Mieville, (1991), who studied the kinetics of coking for a reforming process, there is an inverse relationship between the coking rate and the amount of coke formed.

Studying the carbon formation for Nickel catalysts

(i) Steam reforming reaction

Carbon formation is a challenge in steam reforming processes. The potential for carbon formation is highest when the steam-to-carbon ratio is low or under CO2 reforming. In steam reforming processes, carbon formation is avoided through proper design of the catalyst and steam reforming process. The reactions leading to carbon formation are given in Table 4. Reaction (1) in Table 4 is commonly referred to as “the Boudouard reaction”, reaction (2) as “CO reduction”, and reaction (3) as “methane cracking”. Reaction (4) describes how hydrocarbons polymerise into long-chain hydrocarbons. The reaction product is often referred to as “encapsulating carbon” or “gum” as mentioned. Different types of carbon may be formed by the carbon forming reactions as illustrated in Fig. (7, 8), i.e. whisker carbon, encapsulating carbon also called gum and pyrolytic carbon (Rostrup-Nielsen, 1984; Rostrup-Nielsen et al., 2002 and Sehested, 2006).

Carbon formation from higher hydrocarbons (reaction 4 in Table 4) is an irreversible reaction that can only take place in the first part of the reactor with the highest concentration of C2 compounds. The criterion for carbon formation can be described as a kinetic competition between the carbon forming and steam reforming reactions. A thorough kinetic analysis, both with fresh catalyst, and towards end-of-run at each point in the reactor, is required to accurately evaluate this criterion. In general the limits for carbon formation from higher hydrocarbons are approached with reduced ratio of steam to higher hydrocarbons and with increased temperature [Rostrup-Nielsen, 1994 and Christensen, 1996]. The knowledge of the carbon limits is imperative for optimal design. Examples of pilot plant experiments at low pressure performed to gather information about these limits are given in Table 5.

Whisker carbon formation is the most destructive form of carbon as mentioned before. It is characterised by long filamentous nanofibres formed from the decomposition of carbon monoxide, methane or higher hydrocarbons on the Ni-particles in gas mixtures where the steam to – hydrocarbon ratio is too low and the temperature above a certain limit. Carbon whiskers grow by the reaction of hydrocarbons at one side of the nickel particle and nucleation of carbon as a whisker on the other side of the nickel particle. Continued growth may cause catalyst disintegration and increase the pressure drop. The carbon whisker has a higher energy than graphite (Rostrup-Nielsen, 1984). This means that operation under conditions at which thermodynamics predict formation of graphite may be feasible without carbon formation of the catalyst.

Smaller nickel crystals are more resistant towards carbon formation. The temperature at the onset of whisker carbon formation was approximately 100◦C higher for the catalyst with small nickel crystals (around 7 nm) than for that with large crystals (around 100 nm) (Rostrup-Nielsen et al., 2002).

In tubular reformers the formation of pyrolytic carbon is seen as reddish zones known as ‘hot bands’ on the walls of the tubes. The pyrolytic carbon is a result of carbon formed by thermal cracking of higher hydrocarbons, often related to loss of catalyst activity due to sulphur poisoning. Detailed insight into the mechanism of carbon formation has emerged from a combination of in situ electron microscopy studies and density functional calculations (Helveg et al., 2004; Abild- Pedersen et al., 2006 and Saadi et al., 2010). Adsorbed atomic carbon is much more stable at steps than at terrace sites (Fig. 7D), and steps are therefore much better nucleation sites for carbon. When carbon atoms cover step sites, a single graphite layer can grow from the step as illustrated in Fig.8A. After a graphene is land has nucleated, the growth may continue by surface or bulk transport of carbon atoms or carbon containing fragments to the island. In this case gum has formed. Alternatively new layers may nucleate below the first graphene layer and grow by addition of carbon atoms. This growth is accompanied by surface transport of nickel to the free nickel surface resulting in the growth of carbon whiskers from the Ni particle (Fig. 8B and C).

Step sites thus play an important role both in having a higher turnover frequency of the steam reforming reaction but also in carbon formation. Potassium and gold are known to retard carbon formation (Bengaard et al., 2002; Rostrup-Nielsen et al., 2002). DFT calculations have shown that these species are preferentially located at step sites, thus explaining their retarding effect on carbon formation.

 

 

Fig. 8: Schematic illustration of the process by which carbon whiskers are formed at the nickel particle during steam reforming. (A) Illustration of a graphene island nucleated from a Ni (211) step at a Ni (111) surface (Bengaard et al., 2002). (B) Schematic illustration of whisker formation. (C) In situ electron microscopic picture of ‘lift off’ for a nickel particle from the carrier due to whisker carbon formation (Helveg et al., 2004 )

 

(ii) During Methanation Reaction

By Characterization and studying of surface processes at the Ni-based catalyst during the methanation reaction [Czekaj, et al. 2007] investigated by X-ray photoelectron spectroscopy (XPS), it was found that, publications on the methanation reaction over supported Ni highlighted the general formation and the early steps of carbon whiskers detaching Ni clusters from the support [Helveg et al., 2004 and Bartholomew, 2001]. Especially an identification of certain nickel carbide compounds like Ni3C formed only after extended exposure times is of importance, which will be shown in the present work, because the initiation of the detachment mechanism of the nickel clusters from alumina support could be closely related to the appearance and existence of Ni3C. Before only ‘‘general’’ NiCx and nickel carbide were described on pure metallic nickel using XPS technique [Klink, et al., 1995; Tanaka, et al., 1992 and Nakano and Nakamura, 1995].

Globally, the methanation process can be described as proceeding between two stoichiometries:

CO + 3H2 → CH4 + H2O

2CO + 2H2 → CH4 + CO2

Ideally, the methanation reaction requires a H2/CO ratio of more than three to avoid carbon deposition at the surface [Seemann, et al., 2006]. There are several interesting theoretical studies (mostly DFT calculations) of the pure Ni system [Sehested, et. al., 2004; Bengaard, et al., 2002 and Abild-Pedersen, 2005]. They include the stability and diffusivity of surface species, methane activation, and graphite / graphene formation at the surface of nickel particles (mainly whiskers formation). Also numerous theoretical studies about pure and modified γ-Al2O3 surfaces were performed [Handzlik, et al., 2005; Digne, et al., 2004 and C. Arrouvel, et al., 2005]. Helveg et al. (2004), proposed a mechanism of carbon nanofibre formation, where the graphene–Ni interface is responsible for carbon fibres/whiskers formation.

Another important issue that has been extensively debated is the nature of catalytically active species. It is well known that the hydrogenation process occurs only at metallic Ni0 sites [Ertl, 1997] and that other Ni-species, like oxides, hydroxides or carbides are not active in the methanation reaction. Catalytically inactive Ni-species are generated under specific conditions, especially over γ-Al2O3 support. Generally, small lattice differences at the interface are energetically favorable (see Table 1). This is due to better structural compatibility between the γ -Al2O3 support and these nickel compounds (Ni3C, NiO or Ni(OH)2), whose interface with the γ -Al2O3 support is more stable than the one of pure metallic nickel particles as illustrated in Table (6) and Fig. (9).

Fig. 9: Sketch of the catalyst structure and form of nickel carbid during methanation reaction

 

Mechanism of surface processes in formation of the carbon whiskers

The surface becomes decorated with carbon species (working catalyst, phase 1; 10C). Longer methanation under fixed bed-conditions results in a severe carbon deposition and C-whiskers formation, followed by a detachment of Ni particles from the support (working catalyst, phase 2; Fig. 10d and TEM image; Fig. 9). It was possible to remove the deposited carbon easily, which results in partial loss of the Ni particles (C-removal; Fig. 10e). Hansen et al. (2002) described a mechanism of a carbon migration to the support. The study support this and in addition showed that the Ni3C and/or the NiCx are observed described compatibility of the Ni compounds with the γ- Al2O3 support (see Table 6), we can speculate that one of the most probable mechanism for the detachment of the Ni particles from the surface appears due to the lattice mismatch between the smaller Ni0 particles and the γ -Al2O3 forming an additional thin interface composed by mixed Ni–NiCx or Ni3C. The existence of the M–M3C phase was already shown by Tanaka et al. (1992). The binding between nickel particles and the support is weakened, and smaller Ni-clusters can be easily removed from the surface, leading to the carbon whiskers formation.

Fig. 10: The suggested mechanism for carbon deposition on Ni/Al2O3 catalyst surface (a) fresh catalyst; (b) reduced catalyst; (c) initial state of working catalyst(d) working catalyst with carbon and C-whiskers and (e) catalyst after C removal.

Catalytic Physical properties to avoid carbon formation

In order to ensure good performance and a long lifetime of the catalyst in the plant, optimal physical properties of the catalyst are just as important as optimal catalytic properties (Aasberg-Petersen et al., 2004 and Rostrup-Nielsen, 1984). Key aspects to consider are pore size distribution and pellet shape, size and mechanical strength. The pore size distribution must be optimised for large surface area and good access to the active sites. The pellet shape is important with respect to packing density in the reactor and thereby the void fraction. The pressure drop over the reactor strongly depends on the void fraction: the higher the pellet diameter the lower is the pressure drop.

In tubular reformers the pressure drop can be large and a compromise between pellet size and void fraction is made. The result is catalyst pellets with large external diameters and high void fraction achieved by rings or cylinders with several holes. The shape of the catalyst is also important with respect to ensuring a high heat transfer. This is important in tubular reformers where a high heat transfer coefficient results in a lower tube wall temperature, thereby increasing the Life time of the tubes. A catalyst pellet with high external surface is also desirable to maximize the effective activity. Good mechanical pellet strength is of importance since deterioration of the pellets will increase the pressure drop in the reactor, may create hot spots and eventually require shutdown and reload of the reactor. This means that the catalyst support material must be stable under process conditions and under the conditions during start-up and shutdown of the plant. The initial catalyst pellet strength should be high, but also the strength under operating conditions should be high. Fig. (8) shows two typical shapes of commercial reforming catalyst.

Fig. 11: Examples of commercial reforming catalyst.

Mechanical failure

Attrition / crushing forms and mechanisms of failure

Mechanical failure of catalysts is observed in several different forms, including (1) crushing of granular, pellet or monolithic catalyst forms due to a load, (2) attrition, the size reduction and/or breakup of catalyst granules or pellets to produce fines, especially in fluid or slurry beds, and (3) erosion of catalyst particles or monolith coatings at high fluid velocities. Attrition is evident by a reduction in the particle size or a rounding or smoothing of the catalyst particle easily observed under an optical or electron microscope. Wash coat loss is observed by scanning the wall of the honeycomb channel with either an optical or electron microscope. Large increases in pressure drop in a catalytic process are often indicative of fouling, masking or the fracturing and accumulation of attired catalyst in the reactor bed. Commercial catalysts are vulnerable to mechanical failure in large part because of the manner in which they are formed; that is catalyst granules, spheres, extrudates, and pellets ranging in diameter from 50 μm to several centimetres are in general prepared by agglomeration of 0.02–2 μm aggregates of much smaller primary particles having diameters of 10–100 nm by means of precipitation or gel formation followed by spray drying, extrusion, or compaction [Pham, 1999].

These agglomerates have in general considerably lower strengths than the primary particles and aggregates of particles from which they are formed. Two principal mechanisms are involved in mechanical failure of catalyst agglomerates: (1) fracture of agglomerates into smaller agglomerates and (2) erosion (or abrasion) of aggregates of primary particles having diameters ranging from 0.1 to 10mm from the surface of the agglomerate. While erosion is caused by mechanical stresses, fracture may be due to mechanical, thermal and/or chemical stresses. Mechanical stresses leading to fracture or erosion in fluidized or slurry beds may result from (1) collisions of particles with each other or with reactor walls or (2) shear forces created by turbulent eddies or collapsing bubbles (cavitations) at high fluid velocities.

Thermal stresses occur as catalyst particles are heated and/or cooled rapidly (start up and shutdown); they are magnified by temperature gradients across particles and by differences in thermal expansion coefficients at the interface of two different materials, e.g. catalyst coating/monolith interfaces; in the latter case the heating or cooling process can lead to fracture and separation of the catalyst coating.

Chemical stresses occur as phases of different density are formed within a catalyst particle via chemical reaction; for example, carbiding of primary iron oxide particles increases their specific volume and micro-morphology leading to stresses that break up these particles [Kalakkad, et al., 1995]. A further example occurs in supported metal catalysts when large quantities of filamentous carbon overfill catalysts pores generating enormous stresses which can fracture primary particles and agglomerates.

Metal dust

Metal dusting corrosion can be a challenge in all process equipment involving synthesis gas operating with metal temperatures in the range of 400◦C – 800◦C. In particular, in all process concepts using heat exchangers heated by process gas, e.g. gas heated reformer applications; the problem of avoiding metal dusting corrosion on the heat transfer surfaces is a significant challenge. Metal dusting corrosion results in loss of material, in some cases as ‘metal dust’, a mixture of metal, carbides, and/or carbon. In severe cases the material wastage can be very fast, leading to catastrophic failure of equipment as well as plugging of downstream equipment.

The attack is most often seen as shallow pits, but in other cases the attack is over the entire surface. The corrosion product is a mixture of carbon, metal oxides and metal particles. The mechanism behind metal dusting involves the formation of carbon from CO and, more rarely, hydrocarbons. The carbon forming reactions are the Boudouard reaction, the CO reduction reaction and the methane cracking reactions (see Table 3). Carbon atoms are believed to adsorb on the metal surface, dissolve in the base metal and form carbides (iron carbides if the base metal is carbon steel, chromium carbides if the base metal is stainless steel or a nickel alloy).

Carbides decompose into solid carbon and metal particles that on one hand further catalyse the formation of carbon and on the other hand oxidise to inhomogeneous scales on the surface. The most current theories of the mechanisms behind metal dusting are described in Agüero et al. (2011). It is well known that some alloys are more prone to attack by metal dusting than others. This is ascribed to the fact that some alloys are better at forming and maintaining sound and stable chromium oxide scales (alternatively alumina scale) that restrict the carbon diffusion into the material. Industrial experience has demonstrated that commercial alloys like Inconel 690, Alloy 602 CA, Inconel 693 and most recently Sumitomo 696 all have significant resistance to metal dusting attack [Baker et al., 2002; Agarwal et al., 2001; Nishiyama and Otsuka, 2009].

In severe synthesis gas environments, the afore-mentioned alloys are not immune but do exhibit longer incubation times (for the first pits to appear) and low rates of material wastage compared to other materials. Apart from alloy composition, many factors impact whether metal dusting will be seen or not in a specific synthesis gas environment. The pre-treatment of the alloy is of the utmost importance. A surface with a mixed oxide or a surface depleted of Cr will tend to corrode rapidly. The severity of the gas composition is critical. It is, however, clear that the partial pressure of CO plays a major role but also the presence of steam and hydrogen is determining for the gas aggressively. Carbon penetration into the material can also be prevented by application of a coating on the metal surface. Various coating systems have been proposed and investigated (Agüero et al., 2011).

Fig. 12: Micrograph showing typical (severe) metal dusting attack

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