Carbon burnoff

The reaction is carried out at a bed temperature of 400-500 °C and a gas contact time of 1-15 s [76] or 5-20 s [2]. Figure 17 is a schematic of the reactor. Air is fed to the bottom of the fluidized-bed vessel. The reactants ammonia and propene are fed in through a separate distributor (b). Catalyst regeneration by carbon burnoff occurs in the space between the air distributor and the feed-gas distributor. The heat of reaction is removed by bundles of vertical tubes (a) inside the bed (horizontal tubes are used in other designs [77]). 

Carbon burnoff experiments were also used with the Type lib samples to determine the carbon and inorganic oxide contents of these materials. In the titania series the metal oxide content ranged from 46 to 61 wt%, the zirconia series from 32 to 60 wt%, and in the titania-zirconia series from 38 to 44 wt% (see Table 3). In this preparative procedure the addition of the oxidic powders did nothing to improve the yield of carbon from the resin. 

TABI.E 21-10. Effect of Charge Stock on Carbon Burnoff ... 

The writers have found in their laboratory that invariably after a certain burnoff (depending upon the reactor, temperature, and sample), a subsequent extended period of constant reaction rate, expressed in grams of carbon reacting per unit time, is attained. In this bumoff region, there obviously is equilibrium between the rate of formation of the surface-oxygen complex and its removal with a carbon atom. It is felt that this is the reaction rate most characteristic of a given temperature and should be used in kinetic calculations. In principle, Wicke (31) concurs with this reasoning and reports reactivity data only after the sample has attained a total surface area which is virtually constant. 

Fig. 11. Plot of weight loss vs. time for reaction of spectroscopic carbon rod with carbon dioxide at 1305° (Zone II) to 11% burnoff.

Fio. 13. Porosity profiles through spectroscopic carbon rods before and after ca. 11% burnoff at different temperatures. 

Char nitrogen enrichment which occurs at low burnoff at 773 K, has been observed during the partial combustion of shale particles (4). The results at 798 and 823 K are in agreement with the results of Song 05) for the oxidation of a 1750 K lignite char at 1250 K. It appears that the rates of oxidation of char carbon and nitrogen are equal at the temperatures of interest in practical combustors. 

Figure 4 Dependence of burnoff time on initial carbon level, for diffusion controlled combustion (silica-alwnina cracking catalyst, 700°C) (from Weisz and Goodwin [11]).

For carbon fiber systems, there is some loss of fiber with the resin burnoff method, so an alternative method based on chemical digestion, ISO/DIS 14,127 [13], has been developed. This standard also includes void fraction determination, but for high-quality aerospace materials, where the voidage is less than l%, the method is not sufficiently accurate. However, a voidage level of less than l% is generally acceptable and need not be measured. 

The calculations discussed above do not include the effects of graphite corrosion caused by impurities such as water, carbon dioxide, and other oxidants in the coolant. This corrosion, or "burnoff", of the graphite will result in some loss of strength and changes to the elastic modulus and other mechanical and thermal properties. The maximum burnoff occurs in the hottest element. 

The results of a very recent study by Attia [3], summarized in Fig. 1, are a good illustration. Carbons of varying surface area were produced by carbonization and activation of date pits in steam, air, or carbon dioxide (at different temperatures and to different extents of burnoff). The correlations of water vapor and pyridine uptakes with the total surface area are seen to be very poor. The author concludes that water vapour adsorption is related to the chemistry of the surface rather than to the extent of the surface area but does not identify, let alone quantify, this chemistry. Regarding pyridine adsorption, the author notes that activation of carbons at low temperatures created acidic sites while treatment at high temperatures led to the generation of basic sites on the surface, but she does not identify these sites. 

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