Carbon steam equilibria

Figure 10. Carbon-steam equilibria (T = 1100°K, no CHh output, indirect...

Figure 13. Carbon-steam equilibria (no CH,t output, indirect heating demand)...

Figures 1 to 3 present calculated equilibrium molar ratios of products to reactants as a function of temperature and total pressure of 1 and 100 atm. for the gas-carbon reactions (4), (7), and (5), (6), (4), (7), respectively. Up to 100 atm. over the temperature range involved, the fugacity coefficients of the gases are close to 1 therefore, pressures can be calculated directly from the equilibrium constant. From Fig. 1, it is seen that at temperatures above 1200°K. and at atmospheric pressure, the conversion of carbon dioxide to carbon monoxide by the reaction C - - COj 2CO essentially is unrestricted by equilibrium considerations. At elevated pressures, the possible conversion markedly decreases hence, high pressure has little utility for this reaction, since increased reaction rate can easily be obtained by increasing reaction temperature. On the other hand, for the reaction C -t- 2H2 CH4, the production of methane is seriously limited at one atmosphere pressure and practical operating temperatures, as seen in Fig. 2. Obviously, this reaction must be conducted at elevated pressures to realize a satisfactory yield of methane. For the carbon-steam reaction.

Reactions 17.5, 17.6, and 17.7 illustrate the gasification of char by reaction with various gases. The carbon-steam Reaction 17.5 is an endothermic reversible reaction. Steam undergoes a side reaction, Reaction 17.8, called the water-gas shift reaction. This reaction, which is very rapid, is catalyzed by various impurities and surfaces. The carbon-C02 reaction, Reaction 17.6, is favored at high temperatures and low pressures, whereas the carbon-H2 reaction, Reaction 17.7, is favored at low temperatures and high pressure. Since only three of Reactions 17.5-17.9 are independent, if the equilibrium constants for Reactions 17.6, 17.7, and 17.8 are known, the... 

The water gas shift reaction is considered to be in equilibrium. However, the heterogeneous reactions are influenced by both chemical kinetics and diffusive transport of reactants. Further, in the case of the carbon-steam reaction, the inhibition by both carbon monoxide and hydrogen is also included. 

FIGURE 9.1 Change in the equilibrium composition of carbon-steam systems with pressure at 1200 K. From Parent and Katz (1948). 

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.

Figure 6. Approach to steam-carbon reaction equilibrium for pretreated Pittsburgh seam bituminous coal...

The rate at which the steam-carbon reaction proceeds depends greatly on temperature (9), requiring heat above 2000°F. to approach equilibrium (II). Since hydrogasification tests are conducted at less than 2000°F. to preserve the methane formed, the carbon-steam reaction is expected to be substantially removed from equilibrium. This is shown by Figure 6 where calculated equilibrium ratios are plotted against maximum bed temperature. The curve represents true equilibrium for comparison. 

Figure 1.14 Equilibrium mol fractions for the carbon-steam reaction as a function of temperature. [After S.I. Sandler, Chemical and Engineering Thermodynamics, reprinted by permission of John Wiley and Sons, New York, NY (1989).]...

The minimum amount required to achieve complete conversion of the hydrocarbon feedstock is 0.5 mol 02 per mol carbon. Steam is added to control the reaction temperature, which leads to additional H2 generation via CO shift [Eq. (3)]. The final partial oxidation effluent gas composition is governed by the following chemical equilibrium expressions ... 

When operating with naphtha as feed, the endothermic reforming reaction is dominant at the top of the bed. As the concentrations of both hydrogen and carbon monoxide increase, the exothermic methanation reaction becomes the dominant reaction so that the overall reaction is exothermic. Gradual catalyst deactivation leads to the temperature profile moving down the bed and catalyst fife can be as long as two years. With natural gas feed any hydrocarbons heavier than methane are reformed, the methane steam equilibrium is established, and the overall reaction is endothermic. Operating details are shown in Table 9.16. 

The carbon conversion increases with increasing temperature as shown in Figure 4.8. Since Boudouard reaction and carbon-steam reaction are endothermic, at higher temperature the equilibrium of these reactions is shifted to the right and, therefore, carbon conversion increases. 

A promoted nickel type catalyst contained in the reactor tubes is used at temperature and pressure ranges of 700-800°C and 30-50 atmospheres, respectively. The reforming reaction is equilibrium limited. It is favored at high temperatures, low pressures, and a high steam to carbon ratio. These conditions minimize methane slip at the reformer outlet and yield an equilibrium mixture that is rich in hydrogen. ... 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

In the various laboratory studies when the outlet gas composition was not at equilibrium, it was observed that the steam-to-gas ratio (S/G) significantly affected the hydrogen leakage while the carbon monoxide still remained low. On the assumption that various reactions will proceed at different rates, a study was made to determine the effect of S/G on the reaction rate. The conditions for this test are presented in Table VII the findings are tabulated in Table VIII. 

Partial equilibrium isotherms (°F) at 270 psia partial pressure of hydrogen, carbon monoxide, carbon dioxide, and steam... 

From these numbers, a large number of calculations of technical interest can be made. Further, if we divide the equilibrium constant of carbon dioxide by that of steam we obtain the equilibrium constant of the water-gas equilibrium ... 

Direct thermal decomposition of methane was carried out, using a thermal plasma system which is an environmentally favorable process. For comparison, thermodynamic equilibrium compositions were calculated by software program for the steam reforming and thermal decomposition. In case of thermal decomposition, high purity of the hydrogen and solidified carbon can be achieved without any contaminant. 

A change in the amount of any substance that appears in the reaction quotient displaces the system from its equilibrium position. As an example, consider an industrial reactor containing a mixture of methane, hydrogen, steam, and carbon monoxide at equilibrium ... 

The reactor residence time is about 45 minutes, a 95 per cent approach to equilibrium being achieved in this time. The ammonia is fed directly to the reactor, but the carbon dioxide is fed to the reactor upwardly through a stripper, down which flows the product stream from the reactor. The carbon dioxide decomposes some of the carbamate in the product stream, and takes ammonia and water to a high-pressure condenser. The stripper is steam heated and operates at 180°C, whilst the high-pressure condenser is at 170°C and the heat released in it by recombination of ammonia and carbon dioxide to carbamate is used to raise steam. Additional recycled carbamate solution is added to the stream in the high-pressure condenser, and the combined flow goes to the reactor. 

At 900 °F the equilibrium constant for this reaction is 5.62 when the standard states for all species are taken as unit fugacity. If the reaction is carried out at 75 atm, what molal ratio of steam to carbon monoxide is required to produce a product mixture in which 90% of the inlet CO is converted to C02 ... 

A mixture was being stirred and steam heated when power failure interrupted stirring, and heating was turned off for a hour before power was restored. When stirring was restarted, the hot contents of the pan erupted immediately. Carbon dioxide is evolved from warm aqueous solutions of the base, and absence of stirring and presence of the carbon adsorbent would lead to non-equilibrium retention of the gas, which would be released instantaneously on stirring. 

An addition of the supplemental steam shifts the reforming reaction equilibrium away from carbon formation. 

---