Coke gasification reaction rate

Effect of Substrate on Coke Gasification Rate. The effect of substrate variations on the steam-carbon reaction rate at 1500°F is shown in Table II and in Figure 3. The substrates can be classified into two categories according to their effect on coke gasification reaction rates ... 

Gasification Kinetics of Coke Deposited on Silica-Alumina. Within the temperature range 1400 to 1600°F and in the presence of excess steam, the gasification reaction of coke deposited on the silica-alumina cracking catalyst closely followed first-order kinetics with respect to unreacted carbon (Figure 1). First-order rate constants were calculated from the slopes of these plots (Table III), and yielded an activation energy of 55.5 Kcal/mole. 

The gasification of coke with CO2 can be regarded as a first-order reaction with regard to CO2. The effective reaction rate in a fixed bed reactor is given by ... 

Tabie 6.5.2 Parameters used to calculate the effective reaction rate of coke gasification with carbon dioxide. Partly taken from Hedden (1976) and Heynert and Hedden (1961). 

Gasification of coke by hydrogen, then, can be seen to be a complex process in which a surface reaction is important at low temperatures (< 550 °C). In the range 550-600 °C, the surface reaction is also important but the rates seem to be affected by pore diffusion. Above 700 °C the apparent activation energy is negative and this seems to result from the approach of the system to equilibrium. 

The steam-carbon reaction is known to be catalyzed by metals, particularly transition metals (3,4.). In an effort to improve the rate of gasification, separate samples of the silica-alumina (Durabead) catalyst were impregnated with one of various metals prior to coke deposition, and the results for the subsequent steam-carbon reaction at 1500°F over these materials are shown in Figure 2 and Table IV. The effects of the deposited metal oxides can be summarized as follows ... 

At 1500°F, the steam-coke reaction on alumina occurs approximately three to six times faster than on silica-alumina type substrates. Comparison of results from Runs 56 and 58 indicates the extent of reproducibility of the experiments. In comparison with this reproducibility and in consideration of the observed consistency, the increased gasification rates on the alumina substrates are significant. All alumina-based materials tested, viz., y-alumina, calcined Catapal, and bauxite, showed equally high activities. 

This comparison shows that there is a potential to form carbon from the methane-cracking reaction in the inside of the reformer wall at this location in the reformer tube. Detailed, proprietary kinetic expressions for the reactions 8-10 indicate that while reaction 9 will form carbon, the coke will be gasified by steam and CO2 (reactions 8 and 10), so there is no net accumulation of carbon in this example. However, as the steam-to-hydrocarbon feed ratio is reduced further there will be a point where there is an accumulation of carbon because the coking rate of reaction 9 will be greater than the combined gasification rates of reactions 8 and 10. 

Figure 6.5.16 shows a comparison of the effective rate constants for coke combustion and CO2 gasification, indicating that the rate constant of the chemical reaction of combustion is by several orders of magnitude faster than the Boudouard reaction. For a particle diameter of 4 cm, the combustion rate is then already controlled by film diffusion for temperatures above about 900 °C. This confirms that at the much higher temperatures reached in the tuyeres (about 2000 °C) the combustion is almost instantaneous, that is, only a length of ten particle diameters are needed for complete oxygen conversion. 

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