The kinetics of the reactions of the steam-carbon system

Abstract

The kinetics of the various individual reactions which may occur in the steam-carbon system have been studied. The pressures of the separate gases have been varied in the range 10-760 mm. Essentially similar results have been obtained with coconut shell charcoal at 700° C and coal charcoal at 800° C. The steam-carbon reaction, the primary product of which is carbon monoxide, is of fractional order with respect to steam and strongly retarded by hydrogen. The carbon dioxide-carbon reaction is of fractional order with respect to carbon dioxide and strongly retarded by carbon monoxide. The rates of both these reactions can be represented closely by an expression of the form rate = k 1 p 1 /1 + k 2 p 2 + k 3 p 1 where p 1 and p 2 are respectively the pressures of steam and hydrogen for the steam reaction, and of carbon dioxide and carbon monoxide for the carbon dioxide reaction. This kinetic scheme provides a consistent interpretation of the apparently conflicting results of previous work under a variety of conditions with many different types of carbon. Further experimental work, however, is necessary to elucidate without ambiguity the mechanisms of these reactions. The water-gas reaction, CO + H 2 O = CO 2 + H 2 , takes place predominantly on the charcoal surface, and the approach to equilibrium has been studied from both sides. The forward reaction is of nearly the first order with respect to carbon monoxide and of fractional order with respect to steam; it is retarded by hydrogen and to a lesser extent by carbon dioxide. The reverse reaction is of fractional order with respect to both carbon dioxide and hydrogen, retarded by steam and unaffected by carbon monoxide. The kinetic expressions for the forward and reverse components of this heterogeneous reversible reaction combine to give the thermodynamically correct form of the equilibrium constant.