Oxygen equilibria, carbon-steam

Figure 12. Carbon—steam-oxygen equilibria (T = 1100°K, no CHk output)...

Figures 10 - 12 present calculated equilibrium compositions at 1100°K neglecting CHz, for the carbon-steam system with various ways of providing the needed heat. The reactants are indirectly heated for Fig. 10, heated by addition of a stoichiometric mixture of air and methane for Fig. 11, and by consumption by oxygen of some of the carbon for Fig. 12. The results for heating by an air-carbon reaction are given in Fig. 7.

In this example we have assumed a sufficient depth for the coal bed equilibrium is approached by the gases while they are in contact with the incandv carbon. This need not be the case if oxygen and steam are supplied at too rate, the reactions may not attain equilibrium or may reach equilibrium after have left the coal bed. In this event, carbon is not present at equilibrium, and problem must again be reformulated. 

This objection, however, is not valid, for, whatever the temperature, the law of mass action requires that definite, even if small, amounts of carbon dioxide and hydrogen shall exist in equilibrium with the other gases in the system. Hence, if for any reason the partial pressure of the carbon dioxide or hydrogen falls below that required for equilibrium, it is always possible for reaction (i) to proceed, even at high temperatures, in the direction of left to right. Traube explained the reaction, however, on the assumption that the function of the steam is to unite with one atom of the oxygen molecule, the second atom being occupied in the oxidation of the carbon monoxide. Thus... 

At 800°C with excess carbon, Kshift is approximately 1.0. Invariably the oxygen exchange reactions never reached equilibrium. The apparent or pseudo values for K8hift were generally less than 0.2. Ergun and Menster found similar low values when steam conversion was low. 

TABLE 9.3 Oxygen-Steam Ratios Yielding Equilibrium Products with Zero Net Change in Enthalpy in the Carbon-Oxygen-Steam Reaction"... 

FIGURE 9.4 Variation of equilibrium composition and enthalpy change with oxygen-steam ratio for the carbon-oxygen-steam system at atmospheric pressure and 900 K. From Parent and Katz (1948). 

Parent, J. D., and Katz, S. (1948). Equilibrium Compositions and Enthalpy Changes for the Reactions of Carbon, Oxygen, and Steam, Bulletin No. 2. Institute of Gas Technology, Chicago. 

The thermodynamics of the gasification reactions has been considered in detail in reference 177. Gasification by oxygen and hydrogen is exothermic, but reaction with steam or carbon dioxide is endothermic. The reaction between carbon and hydrogen may be restricted by equilibrium at normal working temperatures and pressures, but the other reactions are not. 

Both the thermodynamic equilibrium and the heat of reaction are reasonably well known for the heterogeneous gasification of carbonized residue with oxygen, steam, carbon dioxide and hydrogen. 

Parent, J.D. Equilibrium compositions and enthalpy changes for the reactions of carbon, oxygen and steam. Inst. Gas Technol. Res. Bull. 1948, 1. 

Higher hydrocarbons do not exist at equilibrium and any risk of whisker formation from these compounds can be disregarded at these conditions. Nevertheless whiskers may still form from higher hydrocarbons because at nonequilibrium conditions a potential for the irreversible carbon formation [e.g.. Reaction (11) in Table 3] may exist. The formation of whisker carbon at these conditions depends on a kinetic balance between the rate of the carbon forming and steam-reforming reactions. A simplified reaction sequence outlining the kinetic balance is shown in Fig. 8. The key step is whether the adsorbed hydrocarbon species will react to form adsorbed carbon and whiskers or react with oxygen species to produce gas. ... 

In the third zone, slow secondary reactions take place. The carbon particulates formed react with C02 and steam to form synthesis gas. However, these slow reactions do not reach equilibrium due to low residence time in the reactor, leaving some carbon formation in the reactor. The final composition of the synthesis gas is determined by the water-gas-shift reaction (Reaction 2.2). The key variables to control outlet gas composition are the oxygen fuel ratio and steam fuel ratio. 

In Eqs. (22.24) and (22.25), ideal gas law is assumed for the determination of molar flow. The desired product gas of a steam reformer for hydrocarbons C Hm consists of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and steam (H2O) that is added to the mixture in excess. Partial oxidation uses air for fuel conversion, leading to nitrogen (N2) as part of the product gas. It can be assumed that oxygen (O2) reacts completely. Methane (CH4) can always be found in reformates due to chemical equilibrium. Finally, the product gas of an autothermal reformer contains H2, CO, CO2, H2O, N2, and CH4. The carbon balance (C) for an idealized reforming process of any C Hm without byproducts results in... 

Figure 32.2 Hydrogen yield from 1 mol C Hm (here C14H30) in relation to the steam-to-carbon ratio and oxygen-to-carbon ratio for reforming. Methane concentrations were determined using an equilibrium calculation assuming a WGS reaction and methanation at 1023 K. In order to extend the hydrogen yield, a WGS reactor operated in equilibrium at 573 K was considered. An...

It is critical to determine and control the steam-to-carbon (S/C) and/or oxygen-tointernal reforming) to avoid carbon deposition. Thermodynamic analysis is commonly used to estimate the minimum ratios. For example. Figure 33.18 shows the equilibrium number of moles of carbon per mole of methane introduced into an ATR as a function of S/C and O/C at two reformer inlet temperatures of 150 and 400 °C [8]. It can be seen that for aU values of O/C between 0 and 1.5, carbon deposition should not be a concern if an S/C > 1.2 is maintained in the fuel gas mixture entering the ATR (fiiUy mixed inlet stream). It should be noted that many thermodynamic calculations (as in this example) assume adiabatic equilibrium reactions and do not take into account reaction kinetic effects. The inclusion of reaction kinetics in the analysis may lead to different results. 

Methane reforming Eq. (2.36) is the simplest example of steam reforming (SR). This reaction is endothermic at MCFC temperatures and over an active solid catalyst the product of the reaction in a conventional reforming reactor is dictated by the equilibrium of Eq. (2.36) and the water-gas shift (WGS) reaction Eq. (2.37). This means that the product gas from a reformer depends only by the inlet steam/ methane ratio (or more generally steam/carbon ratio) and the reaction temperature and pressure. Similar reaction can be written for other hydrocarbons such as natural gas, naphtha, purified gasoline, and diesel. In the case of reforming oxygenates such as ethanol [125, 126], the situation is in some way more complex, as other side reactions can occur. With simple hydrocarbons, like as methane, the formation of carbon by pyrolysis of the hydrocarbon or decomposition of carbon monoxide via the Boudouard reaction Eq. (2.38) is the only unwanted product. 

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