Coke deposit oxidation catalysts

It should be noted that TPR is not limited to the reduction of oxides. Many species react with hydrogen and can be studied by TPR. Examples are the study of coke deposits on catalysts ... 

Examples of the application of Eq. (3.3.17) to spherical particles include the work of Carter [9] for the oxidation of nickel, Kawasaki et al. [10] for the reduction of iron oxides, and Weisz and Goodwin [11] for the combustion of coke deposits on catalysts. In the latter two cases, the solid was initially a porous pellet when diffusion controls the overall rate, however, the relationships above may be used. We will discuss this in more detail when the system of a porous reactant solid is presented. Many studies on the oxidation of metals have been made in one-dimensional geometries [12,13]—hence the term parabolic law for the rate of progress of such oxidation reactions under conditions of diffusion control [see Eq. (3.3.14)]. Hutchins [14] has verified Eq. (3.3.16) experimentally using a cylindrical system. 

Regeneration of noble metal catalysts to remove coke deposits can successfully restore the activity, selectivity, and stabiUty performance of the original fresh catalyst (6—17). The basic steps of regeneration are carbon bum, oxidation, and reduction. Controlling each step of the regeneration procedure is important if permanent catalyst damage is to be avoided. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

The TEM images of deposits observed on Catalyst I used for the steam reforming of naphthalene are shown in Fig. 5. Two types of deposits were observed and they were proved to be composed of mainly carbon by EDS elemental analysis. One of them is film-like deposit over catalysts as shown in Fig. 5(a). This type of coke seems to consist of a polymer of C H, radicals. The other is pyrolytic carbon, which gives image of graphite-like layer as shown in Fig. 5(b). Pyrolytic carbon seems to be produced in dehydrogenation of naphthalene. TPO profile is shown in Fig. 6. The peaks around 600 K and 1000 K are attributable to the oxidation of film-like carbon and pyrolytic carbon, respectively [11-13]. These results coincide with TEM observations. 

TEM-EDS and XPS analyses were conducted on Co/MgO catalysts. The results of surface analyses showed that Co metal is not supported on the MgO as particles, but covers MgO surface in the case of 12 wt.% Co/MgO calcined at 873 K followed by reduction. After the reduction of catalyst at 1173 K, both cobalt oxide and CoO-MgO solid solution are observed on the surface of catalyst. In the steam reforming of naphthalene, two types of coke deposited on the surface of catalyst are observed. These are assigned to film-like and graphite type carbon by TPO analysis. 

In the regenerator (830°C), the coke deposited on the catalyst is partially oxidized to CO, thus reducing magnetite to wustite. Sulfur dioxide produced by POx of sulfurous coke is fixed into the catalyst as follows ... 

The steam also reacts with coke deposits on the iron oxide catalyst, forming CO2, giving the catalyst a longer, more active lifetime. The onstream factor of the styrene plant is extended by reducing the shutdown frequency for catalyst regeneration or replacement. 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

The catalytic coke produced by the activity of the catalyst and simultaneous reactions of cracking, isomerization, hydrogen transfer, polymerization, and condensation of complex aromatic structures of high molecular weight. This type of coke is more abundant and constitutes around 35-65% of the total deposited coke on the catalyst surface. This coke determines the shape of temperature programmed oxidation (TPO) spectra. The higher the catalyst activity the higher will be the production of such coke [1],... 

Coke Deposition on a Commercial Nickel Oxide Catalyst During the Steam Reforming of Methane... 

The steam reforming of methane cycle suffers from the problem of coke deposition on the catalyst bed. The primary objective of this project was to study the stability of a commercial nickel oxide catalyst for the steam reforming of methane. The theoretical minimum ratios of steam to methane that are required to avoid deposition of coke on the catalyst at various temperatures were calculated, based on equilibrium considerations. Coking experiments were conducted in a tubular reactor at atmospheric pressure in the range of 740-915°C. 

The quantities of coke deposited on the catalyst were determined by oxidation of coke to CO2, and adsorption on Ascarlte. The experimental minimum ratios were obtained graphically from these data. The quantities of coke obtained experimentally were less than the theoretical values, whereas the experimental minimum steam to methane ratios were higher than the theoretical. A simple model of the Voorhies type described the coking data reasonably well. In the course of the coking runs the catalyst did not deactivate to a great extent, the conversion decreasing by not more than 15 percent. 

The experimental results are presented in Figure 3. In this figure the amount of coke deposited on the catalyst has been plotted versus the volume of methane (at STP) fed into the reactor. Both absolute amounts of coke on a 5 g charge of catalyst, and percent coke by weight are reported. Since the feed flow rate of methane was maintained constant at 0.31 liters (STP)/min, the abscissa also represents time. Each point on Figure 3 represents an experimental run of approximately 12-15 hours duration including the reduction and subsequent oxidation of the catalyst. 

Following an experiment the reactor was unloaded and the catalyst was extracted using toluene first and then pentane. The coke content of the thus extracted catalyst was determined using Combustion Mass spectrometric Element analysis (CME). Since the total catalyst charge of the reactor was mixed at the end of an experiment, the coke data thus obtained are average data over the reactor. In other words, no coke profiles have been established by experiments. The amount of coke deposited is reported on the basis of the catalyst mass (fresh oxidic catalyst basis, the total amount of feed processed (percent weight on feed, %wof),... 

The ciystalJine structure of the catalyst and the oxidation state of surface nickel are specially relevant in this case due to the fact that coke deposition as well as acetylene hydrogenation occur on the metallic nickel sites [2]. Therefore, the pretreatments carried out on the catalyst with the aim of obtaining the active species have a great influence on the relationship between coke deposition and the main reaction kinetics. 

The hydrocarbon gas from each cycle is collected in a gas holder, mixed, and sampled. Gas composition is determined chromatographically. The coke deposited on the catalyst is determined for each cycle by passing an aliquot of the regeneration gases over hot cupric oxide to convert carbon monoxide to carbon dioxide and then through an Ascarite absorption bulb. 

Coke, deposited on a catalyst, may be removed by one of several reactions oxidation, reaction with water to form carbon monoxide and hydrogen methanation, and the Boudouart reactions. 

Non-stationary operations have found large scale industrial application. An important classical example is catalytic cracking, where oil is exposed with a short residence time to a rapidly deactivating zeolitic catalyst, which is regenerated in a second step by removal of deposited coke. A novel non-stationary process is selective butane oxidation over a regenerable oxidation catalyst (see Chapter 2). Undoubtedly we will see more examples of this type of process, in which the proper catalytic step and the regeneration of the catalytic sites occur in different compartments under different conditions. A nice application involves... 

It is clear that a behavior such as the one shown in Fig. 6-4 is caused by plugging of the catalyst bed by solids deposition. The plugging causes the bed void fraction to decrease. Some speculations on the modes of deposition can be made from analysis of the plugging material. This material consists primarily of oxides of iron in the form of loose particles and is densely deposited in the inlet portion of the reactor. The material is probably carried by the fluid while it is flowing through rusted pipelines. As the reaction proceeds, bed plugging in an HDS reactor also occurs as a result of metal deposits, both nickel and vanadium, and coking of the catalyst. 

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