Coke deposit oxidation

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. 

Hydrocarbons and carbonized or coke deposits can be removed by chromic acid. The chromic acid oxidizes the binders holding the deposits together. Use a 10 to 20% solution for 12 to 24 hours at 190 to 200 °F. Chromic acid cannot be effectively inhibited and is not suitable for cleaning copper, brass, aluminum, zinc, or cast iron because these are all rapidly attacked. 

Last but not least, one should check the inertness of the auxiliary electrodes in single-pellet arrangements, both under open and closed circuit conditions and, also, via the closure of the carbon balance, the appearance of coke deposition. This is especially important in systems with a variety of products (e.g. selective oxidations), where the exact value of selectivity towards specific products is of key interest. This in turn points out the importance of the use of a good analytical system and of its careful calibration. 

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. 

On the basis of the analysis presented in Tables II, III, and IV and measurements of the mass of C02 evolved during oxidation, Figure 1 was constructed to display the fraction of original carbon mobilized by heating, the fraction of the remaining (available) carbon mobilized as incompletely oxidized hydrocarbon by oxidation, and the fraction of available carbon deposited as coke by oxidation. The distribution of available carbon between the mobile and non-mobile products of oxidation lends additional support to our proposed "two-reactions" mechanism. 

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. 

Water and organic molecules occluded during the synthesis were removed from the intracrystalline volume as follows. The solids were slowly heated (5°C/min) in a N2 flow up to 550°C and held at this temperature for 2h. The coke deposit resulting from the non-oxidative degradation of the organics was then... 

Additives have been shown to effect decreases in coke deposits. Small amounts of organometallic compounds and commercial fuel-oil additives have been shown to decrease coke weight. Metallic compounds in excess can result in increased deposition because of the metal oxide, and some of these compounds have also been reported to cause metal corrosion, as have sulfur compounds. Results of investigations with additives to inhibit coke formation are not conclusive enough at present to justify their acceptance. 

It is desirable to operate the molecular sieve bed in the adsorption mode at the same temperature as in the desorption mode. Sista and Srivastava (16) show that temperatures in excess of 533 K are needed to desorb by vacuum C12 to C32 n-paraffins from type 5A molecular sieves at a pressure of 13 Pa (0.1 mm Hg). Only 5% of 2-C32 is removed at 636 K. Asher e al. (14) show that, whereas it is possible to remove 98% of Cg/C g n-paraffins from type 5A molecular sieves with ammonia at 589 K, only 79% removal is attained with C15/C33 n-paraffins even though the temperature is higher (658 K). Some of the retained material over a long period of exposure to high temperature gradually forms a carbonaceous deposit which reduces the adsorption capacity of the molecular sieve this coke deposit must be occasionally removed by a controlled oxidation step which eventually reduces molecular sieve life. Desorption rates increase with... 

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),... 

In this paper, the bumoff of coke deposited on Pt-Re/Al203 by the use of oxygen and ozone is studied and their oxidative capacities are compared. 

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 H/C ratio of the coke deposits was quantified by temperature programmed oxidation (TPO) in a 1 % oxygen helium mixture. Temperature was raised to 850° C at a heating rate of 10° min 1. The calculations of the H/C ratio involved the results from the measurements of carbon dioxide production and oxygen uptake (according to Ref. [8]). Coke deposits were also characterized by thermogravimetry and transmission electron microscopy. 

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. 

Results of experiments varying the ratio of uranium to nickel showed that the ratios giving the largest surface area and catalyst volume were in the range 0.45-0.76 (U Ni). These two characteristics were the most important for activity for these reactions. The catalysts were in a reduced state, which could explain the addition of the reduction-oxidation step in the previous patent A further reason for using catalysts in the 0.45-0.74 U Ni range was that the catalyst demonstrated greatest resistance to coke deposition at a raho of 0.71 1. 

---