Reactivity in the system carbon-hydrogen-methane

Abstract

Experimental work has shown that the rate constant k* = R/p₁ (where R is the rate of methane formation and p₁ is the hydrogen partial pressure) for the forward reaction, C + 2H₂ ↔ CH₄, is a function of the temperature of char preparation and reaction and can be expressed by                        k* = w exp(b/Tp)exp(-c/Tp) where w, b, and c are constants; T_p and T_g are the temperatures, in °K, of char preparation and gasification respectively; and T_g≤T_p. Chars which have been gasified at a given temperature and then exposed to a higher temperature do not show their initial reactivity when returned to the lower temperature. The deactivation of the char, as evidenced by a measurable decrease in active surface area, is a non-reversible process. Approximate values of the active surface area per unit mass of char have been calculated, and a linear relationship between reactivity and active surface area has been found experimentally. Both the forward and reverse processes proceed by chemisorption of either methane or hydrogen on the same type of active site, and the nature of the sites is unchanged by temperature, although their number decreases with increasing temperature and as a result, the equilibrium is independent of char type. The value of the heat of adsorption of hydrogen on carbon, calculated from the experimental results, was approximately 7 kcal mole^-1.