Effect of carbon monoxide in causing nonuniform gasification of graphite by carbon dioxide



Existing theories on the effect of mass transport on the rate (uniformity) of chemical reaction within a porous solid assume that the reaction is of integer order. However the carbon-carbon-dioxide reaction has the kinetic rate form, rate = k1pmath image/(1 + k2pCO + k3pmath image). For conditions where the retarding effect of carbon monoxide is very pronounced, the buildup of small concentrations of carbon monoxide within the porous graphite can lead to appreciable nonuniformity of gasification. Thus the criteria normally used to predict uniformity of gasification break down. A numerical integration of the combined differential equation of mass transport and chemical reaction has been performed, with rate constants for the carbon-carbon dioxide reaction taken from the literature. The results indicate that extreme nonuniformity of reaction can exist even when the change in carbon dioxide concentration from the exterior of the sample to the interior is small. The results are similar to experimental determinations of non-uniformity of gasification, obtained by determining the porosity of reacted graphite samples as a function of depth from the reacting face. Experimentally nonuniformity of reaction was observed for gasification rates a hundredfold lower than the usual Thiele criteria would predict, probably because of carbon monoxide inhibition.