Principle of actual gas and steady-state equilibrium

The prineiple of equilibrated gas predicts conditions where carbon formation is expected (except for noble metals and SPARG). It does not guarantee that carbon is not formed if the principle predicts no potential in the equilibrated gas. 

One example is the formation of carbon in high flux reformers [389] operating far from thermodynamic carbon limits. It means that methane may decompose to carbon instead of reacting with steam to form the required syngas in spite of no potential for carbon in the equilibrated gas. This is of course not possible in a closed system, but in an open system carbon may be stable in a steady state and the accumulation of carbon may continue [389], This risk may be assessed by the so-called criteria of actual gas, which for the methane decomposition reaction as in Equation (5.5) can be written as  

This approach does not apply to the steam reforming of higher hydrocarbons, because in this case the decomposition into carbon is irreversible. 

For tile high activity catalyst, Tcat remains below Tm, i.e. there is no potential for carbon formation. For the low activity catalyst, the mean catalyst temperature (not shown) is less than Tm, but the actual catalyst temperature, Tcat, becomes higher than Tm close to the tube wall, indicating the risk of carbon formation. 

The principle of actual gas predicts carbon at inlet conditions of a conventional steam reformer (500°C). However, when a prereformer is installed, there is no potential for whisker carbon at the catalyst inlet, as illustrated below. 

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