Ammonia synthesis chemical equilibrium

Fig ure 6-12. Profiles of equilibrium conversion Xg versus temperature T for ammonia synthesis. (Source Schmidt, L. D., The Engineering of Chemical Reactions, Oxford University Press, New York, 1998.)... 

As an indispensable source of fertilizer, the Haber process is one of the most important reactions in industrial chemistry. Nevertheless, even under optimal conditions the yield of the ammonia synthesis in industrial reactors is only about 13%. This Is because the Haber process does not go to completion the net rate of producing ammonia reaches zero when substantial amounts of N2 and H2 are still present. At balance, the concentrations no longer change even though some of each starting material is still present. This balance point represents dynamic chemical equilibrium. 

Ammonia synthesis is one of the most important processes of chemical industry tens of millions of tons of this product are synthesized annually in various countries of the world. On a commercial scale the reaction is operated on promoted iron catalysts at temperatures near to 500°C and high pressures, mostly at 300 atm. At present K20, A1203, and CaO in amounts of several parts by weight per 100 parts of catalyst are usually employed as promoters. The application of high pressure is caused by the reversibility of the reaction molar fraction of ammonia corresponding to the equilibrium... 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

Some chemical operations, however, demand a supply of pure hydrogen these include ammonia synthesis and fat-hardening, and so it became necessary to find a way of altering the composition of water-gas to achieve this. Its gaseous components can be brought into equilibrium by the water-gas shift... 

To explore the important characteristics of chemical equilibrium, we will consider the synthesis of ammonia from elemental nitrogen and hydrogen ... 

The law of mass action is widely applicable. It correctly describes the equilibrium behavior of all chemical reaction systems whether they occur in solution or in the gas phase. Although, as we will see later, corrections for nonideal behavior must be applied in certain cases, such as for concentrated aqueous solutions and for gases at high pressures, the law of mass action provides a remarkably accurate description of all types of chemical equilibria. For example, consider again the ammonia synthesis reaction. At 500°C the value of K for this reaction is 6.0 X 10 2 F2/mol2. Whenever N2, H2, and NH3 are mixed together at this temperature, the system will always come to an equilibrium position such that... 

It is important to understand the factors that control the position of a chemical equilibrium. For example, when a chemical is manufactured, the chemists and chemical engineers in charge of production want to choose conditions that favor the desired product as much as possible. In other words, they want the equilibrium to lie far to the right. When Fritz Haber was developing the process for the synthesis of ammonia, he did extensive studies on how the temperature and pressure affect the equilibrium concentration of ammonia. Some of his results are given in Table 6.2. Note that the amount of NH3 at equilibrium increases with an increase in pressure but decreases with an increase in temperature. Thus the amount of NH3 present at equilibrium is favored by conditions of low temperature and high pressure. 

The reaction temperature profile is of particular importance because the reaction rate responds vigorously to temperature changes. Figure 82 plots lines of constant reaction rate illustrating its dependence on temperature and ammonia concentration in the reacting synthesis gas. The line for zero reaction rate corresponds to the temperature-concentration dependence of the chemical equilibrium. From Figure 82 it is apparent that there is a definite temperature at which the rate of reaction reaches a maximum for any given ammonia concentration. Curve (a) represents the temperature-concentration locus of maximum reaction rates. To maintain maximum reaction rate, the temperature must decrease as ammonia concentration increases. 

So now we have established that chemical reactions are dynamic and reversible, and, as a result, chemical reactions are an equilibrium mixture of reactants and products. We have also seen that sometimes the reaction strongly favors the products (as in reacting gunpowder) and sometimes the reaction strongly favors the reactants (as in Haber s ammonia synthesis from nitrogen and hydrogen). We haven t answered one question. 

Reaction R-4.7, the water-gas shift reaction, is an exothermic reaction. The water-gas shift reaction has influence on the CO/H2 ratio in the gasification product, which is very important when the gas is used for synthesis purpose. Therefore, the shift process can be found in almost all the ammonia plants and hydrogen generation process in gas plants. The shift reaction can generally be taken into account using thermodynamic chemical equilibrium, since gas-phase temperatures are high. 

During and after World War II, Horiuti continued his research in chemical kinetics and its applications. His results were compiled in a voluminous paper entitled A Method of Statistical-Mechanical Treatment of Equilibrium and Chemical Reactions (1948). This method is applicable both to heterogeneous and homogeneous systems. Horiuti and his co-workers further attempted to apply the method to the study of a number of chemical syntheses and reactions, such as ammonia synthesis and ethylene hydrogenation. Nearly all of his research papers were published in the Journal of the Institute for Catalysis, of which he was the chief editor. 

Knowing the factors that affect chemical equilibrium has great practical value for industrial applications, such as the synthesis of ammonia. The Haber process for synthesizing ammonia from molecular hydrogen and nitrogen uses a heterogeneous catalyst to speed up the reaction (see p. 540). Let us look at the equilibrium reaction for ammonia synthesis to determine whether there are factors that could be manipulated to enhance the yield. 

Chemical equilibrium is a key issue in process design. Chemical equilibrium might set in many cases an upper limit for the achievable conversion, if nothing is done to remove one of the products from the reaction space. Because the equilibrium conversion is independent of kinetics and reactor design, it is also convenient to use it as reference. Note that important industrial reactions take place close to equilibrium, as the synthesis of ammonia and methanol, esterification of acids with alcohols, dehydrogenations, etc, particularly when the reaction rate is fast. Therefore, the investigation of chemical equilibrium should be done systematically in a design project. 

After trying different substances to see which would be most effective, Carl Bosch (see Chemistry Put to Work The Haber Process, page 615) settled on iron mixed with metal oxides, and variants of this catalyst formulation are stiU used today. These catalysts make it possible to obtain a reasonably rapid approach to equilibrium at around 400 to 500 °C and 200 to 600 atm. The high pressures are needed to obtain a satisfactory equilibrium amount of NH3. If chemists and chemical engineers could identify a catalyst that leads to sufficiently rapid reaction at temperatures lower than 400 °C, it would be possible to obtain the same extent of equilibrium conversion at pressures much lower than 200 to 600 atm. This would result in great savings in the cost of the high-pressure equipment used in ammonia synthesis today. 

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