Carbon catalyst deactivation

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

The heat released from the CO—H2 reaction must be removed from the system to prevent excessive temperatures, catalyst deactivation by sintering, and carbon deposition. Several reactor configurations have been developed to achieve this (47). 

Regardless of the hydrogen generation technique used, tlie unpurified syii gas contains the oxides of carbon, which deactivate tlie aimnonia syntliesis catalyst and must be removed. In tlie sliift converters, embon monoxide is calaljlically converted to carbon dioxide, wliich is removed more easily Ilian... 

As is indicated in Figure 1, the heat liberated in the conversion of carbon monoxide to methane is 52,730 cal/mole CO under expected reaction conditions. Also, the heat liberated in the conversion of carbon dioxide is 43,680 cal/mole C02. Such high heat releases strongly affect the process design of the methanation plant since it is necessary to prevent excessively high temperatures in order to avoid catalyst deactivation and carbon laydown. Several approaches have been proposed. 

Four pilot plant experiments were conducted at 300 psig and up to 475°C maximum temperature in a 3.07-in. i.d. adiabatic hot gas recycle methanation reactor. Two catalysts were used parallel plates coated with Raney nickel and precipitated nickel pellets. Pressure drop across the parallel plates was about 1/15 that across the bed of pellets. Fresh feed gas containing 75% H2 and 24% CO was fed at up to 3000/hr space velocity. CO concentrations in the product gas ranged from less than 0.1% to 4%. Best performance was achieved with the Raney-nickel-coated plates which yielded 32 mscf CHh/lb Raney nickel during 2307 hrs of operation. Carbon and iron deposition and nickel carbide formation were suspected causes of catalyst deactivation. 

As was noted for experiments HGR-12, HGR-13, and HGR-14, the rate of catalyst deactivation increased as the fresh gas feed rate increased. It is possible that higher rates of carbon deposition and metal sintering occur at the higher feed rates with resultant higher deactivation rates. 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

Transition metal-mediated phosphorus-carbon bond cleavage and its relevance to homogeneous catalyst deactivation. P. E. Gorrou, Chem. Rev., 1985,85,171 (109). 

No attempt was made to measure CO2 in these experiments. By increasing the temperature to 320°C, catalyst deactivation was prevented, and no carbon residue could be detected on the spent catalyst. Thus, temperature can be expected to significantly shift the reaction pathways of organic contaminants. In this study, and in all other studies, excellent corrosion resistance was observed for the corrosion coupons. 

Fig. 3 showed the catalyst stability of Ni-Mg/HY, Ni-Mn/HY, and Ni/HY catalysts in the methme reforming with carbon dioxide at 700°C. Nickel and promoter contents were fixed at 13 wt.% and 5 wt.%, respectively. Initial activities over M/HY and metal-promoted Ni/HY catalysts were almost the same. It is noticeable that the addition of Mn and Mg to the Ni/HY catalyst remarkably stabilized the catalyst praformance and retarded the catalyst deactivation. Especially, the Ni-Mg/HY catalyst showed methane and carbon dioxide conversions more thrm ca. 85% and 80%, respectively, without significant deactivation even after the 72 h catalytic reaction. 

Although the process is of significance, it has not well studied. Since the initial development of the CTA hydropurification process in 1960s , only a few papers have been published, mainly regarding catalyst deactivation [2]. Recently, Samsung Corporation, in collaboration with Russian scientists, developed a novel carbon material-CCM supported palladium-ruthenium catalyst and its application to this process [3]. However, pathways and kinetics of CTA hydrogenation, which are crucial to industrialization, are not reported hitherto. 

The accumulation of carbonates is another reason for gold catalyst deactivation [9]. The in-situ FTIR experiments in Fig. 6 show that the carbonate build-up is slower in the 03/02-treated gold catalyst (Fig. 6a) compared to the air-treated sample (Fig. 6b). Also, the air-treated catalyst displays a strong band at 1435 cm" corresponding to the non-coordinated carbonate. Although our understanding of the process is incomplete, it is clear from the results that O3 pretreatment inhibits the deactivation of gold catalyst. 

How relevant are these phenomena First, many oscillating reactions exist and play an important role in living matter. Biochemical oscillations and also the inorganic oscillatory Belousov-Zhabotinsky system are very complex reaction networks. Oscillating surface reactions though are much simpler and so offer convenient model systems to investigate the realm of non-equilibrium reactions on a fundamental level. Secondly, as mentioned above, the conditions under which nonlinear effects such as those caused by autocatalytic steps lead to uncontrollable situations, which should be avoided in practice. Hence, some knowledge about the subject is desired. Finally, the application of forced oscillations in some reactions may lead to better performance in favorable situations for example, when a catalytic system alternates between conditions where the catalyst deactivates due to carbon deposition and conditions where this deposit is reacted away. 

Cracking is an endothermic reaction, implying that the temperature must be rather high (500 °C), with the consequence that catalysts deactivate rapidly by carbon deposition. The fluidized catalytic cracking (FCC) process, developed by Standard Oil Company of New Jersey (1940) (better known as ESSO and nowadays EXXON), offers a solution for the short lifetime of the catalyst. Although cracking is... 

The long-term stability of the Ru/Ti02 catalyst was studied under various reaction conditions and the spent catalysts were characterized for assessing the reasons of deactivation. It was observed that the rate exhibits a rapid reduction at the initial several hours of reaction, followed by a slow and continuous deactivation. Analysis of the spent catalyst, by H2 adsorption after removing surface carbon, showed that the initial rapid reduction of activity is mainly due to metal sintering, while the continuous and slow deactivation is related to the occurrence of the SMSl phenomenon at the later part of the catalyst bed, where reducing conditions prevail. In order to avoid these processes which lead to catalyst deactivation, Ti02... 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Carbon species covered partly the catalyst [60, 62]. Also, cracks in the plate and void areas with catalyst loss were observed which are responsible for catalyst deactivation. 

In order to study the catalyst deactivation, the recycle of the carbon supported 5%Pd-0.3%Sn catalyst was attempted. The catalyst was filtered after reaction under nitrogen, dried under vacuum, and again charged with fresh solvent and substrate under the same reaction conditions as the first cycle. The results are shown in Table 15.3. 

Also, manganese added to cobalt on activated carbon catalysts resulted in a decrease in bulk carbide formation during reduction and a decrease in the subsequent deactivation rate.84 Magnesium and yttrium added to the support in alumina-supported cobalt catalysts showed a lower extent of carburization. This was explained by a decrease in Lewis acidity of the alumina surface in the presence of these ions.87... 

Gruver, V., Young, R., Engman, J., and Robota, H. J. 2005. The role of accumulated carbon in deactivating cobalt catalysts during FT synthesis in a slurry-bubble-column reactor. Prepr. Pap.-Am. Chem. Soc. Div. Pet. Chem. 50 164—66. 

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