Thermodynamic carbon

On both experimental and theoretical grounds there is little doubt of the importance of polarizability as a major factor in determining the commonly encountered, though variable, high RS /RO ratios. Were thermodynamic carbon affinities mainly responsible for the usual reactivity order RS > RO, the peculiar behavior of chloroquinolines would be very difficult to understand. There is some indication, however, that carbon affinities roughly parallel basicities (hydrogen affinities), In the latter case, lower RS /RO ratios could be explained in terms of the intermediate complex mechanism, ... 

A basic premise of solubility considerations is that a solution in contact with a solid can be in an equilibrium state with that solid so that no change occurs in the composition of solid or solution with time. It is possible from thermodynamics to predict what an equilibrium ion activity product should be for a given mineral for a set of specified conditions. As will be shown later in this chapter, however, it is not always possible to obtain a solution of the proper composition to produce the equilibrium conditions if other minerals of greater stability can form from the solution. It shall also be shown that while it is possible to calculate what mineral should form from a solution based on equilibrium thermodynamics, carbonate minerals usually behave in a manner inconsistent with such predictions. 

Fig. 2. Equilibrium chart. Thermodynamic carbon limit. Aged catalyst, CH4 = C + 2H2, 2C0 = C + C02. 400-1000°C, lines show H2/C0 ratio in the reformer exit gas.

The selection of process parameters is limited by the potential for carbon formation. The curve shows the thermodynamic carbon limit considering the deviation of the carbon structure from ideal graphite observed on catalysts (ref. 1). For 0/C and H/C ratios below the values indicated by the curve, there is thermodynamic potential for the formation of carbon. Hence, the position of the carbon limit curve depends on the type of catalyst. 

There are several operational and safety issues to be resolved before this option can be adopted in a major way. Thermodynamically, carbon dioxide should react with carbon (coal) to yield carbon monoxide, which is a highly toxic gas sometimes found in coal mines. In practice, this reaction will not take place at an appreciable rate at ambient temperatures, although it could become significant at depths where the temperature is much higher. If the inadvertent ingress of air to a coal seam that was saturated with carbon dioxide led to a fire, then the potential might exist for vast quantities of toxic carbon monoxide to be formed via the reaction CO2 + C -> 2CO, and subsequently liberated. It is known that the extraction of water from coal beds does allow air to enter and circulate more freely and that this sometimes results in underground fires. Such safety issues should be evaluated carefully. Other factors to be considered are ... 

At a given temperature and for a given hydrocarbon feed, carbon will be formed below a critical steam to carbon ratio (15), the carbon limit A in Fig. 5. It can be shown that this critical steam to carbon ratio increases with temperature. By promotion of the catalyst, it is possible to push this limit to the thermodynamic carbon limit B reflecting the principle of equilibrated gas (4,15) ... 

The thermodynamic carbon limit B is a function of the composition of the feed gas (atomic ratio 0/C and H/C) and total pressure. An example of thermodynamic limits (16) is given in the diagram in Fig. 6. This calculation should apply the thermodynamics of whisker carbon pushing the carbon limits to more critical conditions. It means that in principle the carbon limits depend on the nickel particle size of the crystal (17,18) as illustrated in Fig. 7. 

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 ... 

Limit B is dictated by thermodynamics. Carbon will be formed if the equilibrated gas shows affinity for carbon. The limit is a function of the 0/C, H/C and inert/C ratio of the gas as well as pressure. 

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