Coke gasification activation energy

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. 

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 activation energies for coke gasification on the three substrates shown in Figure 4 were 33 Kcal/mole for the three alumina-based materials and 54 Kcal/mole, for the silica-alumina catalysts. The increased activity and lower activation energy for the coke deposited on the aluminas (compared to that on the silica-aluminas) cannot be due to a direct catalytic effect of alumina on the gasification reaction, but rather to an indirect effect of the alumina that controls the nature and structure (surface area and structural disorder) of the coke during its deposition. 

Figure 4. Arrhenius plots for steam gasification of coke deposited on American Cyanamid alumina (A), fresh bauxite (O. and silica-alumina (O). Corresponding activation energies are 32.6, 33.8, and 53.1 kcal/mol, respectively.

Finally, is observed that the rcw increases with the pcH4 [16-18] and with the reduction temperature, while it diminishes with the reaction temperature. The influence of the ph2 has no clear effect. The effect of the reaction temperature is due to the fact that high reaction temperatures favor gasification over the growth of coke filaments [15,19,26,27] which would explain the negative value of the activation energy apparent for the filament growth. 

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