The Synthesis of Ammonia

Let s design a chemical process based on the chemical change of N2 and H2 into ammonia, NH3, used primarily as a fertilizer. The U.S. chemical industry produced 35 billion pounds of ammonia in 1995, the sixth largest production of all chemicals. The chemical reaction is 

Indeed, reaction (2.1) is the key to the process. Before a catalyst was developed to promote this reaction, large-scale production of nitrogenous fertilizer was not feasible. The device that conducts this reaction, the reactor, is the core of the process. The details of this reactor are key to the viability of the process, but these are not important now. We will assume a viable reactor is available - that s our incentive for starting the design. Our task is to design a process around this reaction. 

We need a means of describing our design. We could describe the process with words, such as a mixture of N2 and H2 is piped into the reactor. This would be cumbersome. A chemical process is a situation where a picture is worth a thousand words. A chemical process is represented by a diagram known as a voctss flowsheet, a key tool in chemical engineering design. 

A process flowsheet comprises units, represented by simple shapes, such as rectangles or circles  

The pipes that conduct material between units are called streams. The streams are represented by arrows  

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Resources for Nitrogen Fertilizers. The production of more than 95% of all nitrogen fertilizer begins with the synthesis of ammonia, thus it is the raw materials for ammonia synthesis that are of prime interest. Required feed to the synthesis process (synthesis gas) consists of an approximately 3 1 mixture (by volume) of hydrogen and nitrogen. 

The reverse reaction to ammonia synthesis, the decomposition to nitrogen and hydrogen, is used in die nitriding of iron and canied out industiially at temperatures around 800 K and atmospheric pressure to produce surfacehardening. This dissolution reaction must also play a part in the synthesis of ammonia by the industiial process. The attempt to ninide non by reaction with nin ogen gas is vety slow under atmospheric pressure, presumably due to the stability of the nitrogen molecule. 

In 1838, Frederic Kuhlmann discovered die formation of nitrogen oxide (NO) during die catalytic oxidation of ammonia. Wilhelm Ostwald developed die production mediods in 1902 and established die base for today s major commercial processes. However, industrial production began only after Haber and Bosch developed the synthesis of ammonia around 1916. 

Not all reaetions ean be expressed to have a speeifie order. A speeifie example is the synthesis of ammonia expressed as... 

For the synthesis of ammonia, Nj -i- 3H2 —> 2NH3, over an iron catalyst, develop the rate expression for the following mechanism... 

For example, eonsider the synthesis of ammonia at two different temperatures. Table 13-1 shows values for the heat of reaetion. The negative enthalpy ehanges upon reaetion show that the produets of the reaetion eontain less enthalpy than did the reaetants. This implies that heat must have been generated, whieh is eharaeteristie of an exotliermie... 

F. Haber s catalytic synthesis of NH3 developed in collaboration with C. Bosch into a large-scale industrial process by 1913. (Hater was awarded the 1918 Nobel Prize in Chemistry for the synthesis of ammonia from its elements Bosch shared the 1931 Nobel Prize for contributions to the invention and development of chemical high-pressure methods , the Hater synthesis of NH3 being the first high-pressure industrial process.)... 

F. Haber (Berlin-Dahlem) the synthesis of ammonia from its elements. 

The products of this electrolysis have a variety of uses. Chlorine is used to purify drinking water large quantities of it are consumed in making plastics such as polyvinyl chloride (PVC). Hydrogen, prepared in this and many other industrial processes, is used chiefly in the synthesis of ammonia (Chapter 12). Sodium hydroxide (lye), obtained on evaporation of the electrolyte, is used in processing pulp and paper, in the purification of aluminum ore, in the manufacture of glass and textiles, and for many other purposes. 

Promotion We use the term promotion, or classical promotion, to denote the action of one or more substances, the promoter or promoters, which when added in relatively small quantities to a catalyst, improves the activity, selectivity or useful lifetime of the catalyst. In general a promoter may either augment a desired reaction or suppress an undesired one. For example, K or K2O is a promoter of Fe for the synthesis of ammonia. A promoter is not, in general, consumed during a catalytic reaction. If it does get consumed, however, as is often the case in electrochemical promotion utilizing O2 conducting solid electrolytes, then we will refer to this substance as a sacrificial promoter. 

Self-Test M.3 A In the synthesis of ammonia, what is the percentage yield of ammonia when 100. kg of hydrogen reacts with 800. kg of nitrogen to produce 400. kg of ammonia ... 

The Haber process for the synthesis of ammonia is one of the most significant industrial processes for the well-being of humanity. It is used extensively in the production of fertilizers as well as polymers and other products, (a) What volume of hydrogen at 15.00 atm and 350.°C must be supplied to produce 1.0 tonne (1 t = 10 kg) of NH3 (b) What volume of hydrogen is needed in part (a) if it is supplied at 376 atm and 250.°C ... 

In an exothermic reaction, such as the synthesis of ammonia or a combustion reaction, the heat released by the reaction increases the disorder of the surroundings. In some cases, the entropy of the system may decrease, as when a gaseous reactant is converted into a solid or liquid. However, provided that AH is large and negative, the release of energy as heat into the surroundings increases their entropy so much that it dominates the overall change in entropy and the reaction is spontaneous (Fig. 7.18). 

FIGURE 9.2 (a) In the synthesis of ammonia, the concentrations of N, and H, decrease with time and that of NH1 increases until they finally settle into values corresponding to a mixture in which all three are present and there is no further net change, (bi If the experiment is repeated with pure ammonia, it decomposes, and the composition settles down into a mixture of ammonia, nitrogen, and hydrogen. (The two graphs correspond to experiments at two different temperatures, and so they correspond to different equilibrium compositions.)... 

STRATEGY The synthesis of ammonia is exothermic, and so we expect the equilibrium constant to be smaller at the higher temperature. To use the van t Hoff equation, we need... 

The reactant is adsorbed on the catalyst s surface. As a reactant molecule attaches to the surface of the catalyst, its bonds are weakened and the reaction can proceed more quickly because the bonds are more easily broken (Fig. 13.36). One important step in the reaction mechanism of the Haber process for the synthesis of ammonia is the adsorption of N2 molecules on the iron catalyst and the weakening of the strong N=N triple bond. 

Stoichiometric coefficients describe the relative numbers of molecules involved in the reaction. In any actual reaction, immense numbers of molecules are involved, but the relative numbers are always related through the stoichiometric coefficients. Further, these coefficients describe both the relative numbers of molecules and the relative numbers of moles involved in the reaction. For example, the Haber reaction always involves immense numbers of molecules, but the equation describing the synthesis of ammonia tells us the following ... 

In this example, only one of the reagents has a concentration that can vaiy, and each stoichiometric coefficient is one. What happens for a more complicated reaction Consider the synthesis of ammonia carried out in a pressurized reactor containing N2, H2, and NH3 at partial pressures different from 1 bar ... 

The detailed chemistry describing the synthesis of ammonia Is complex, so we Introduce the principles of equilibrium using the chemistry of nitrogen dioxide. Molecules In a sample of nitrogen dioxide are always colliding with one another. As described in Chapter 15, a collision in the correct orientation can result In bond formation, producing an N2 O4 molecule 2 NO2 N2 O4... 

Figure 6.40. Catalytic activity of various supported metals for the synthesis of ammonia. [Adapted from A. Ozaki and K. Aika, in Catalysis Vo. 1 (1981), Eds.

D.A. Rudd, L.A. Apuvicio, J.E. Bekoske and A.A. Trevino, The Microkinetics of Heterogeneous Catalysis (1993), American Chemical Society, Washington DC]. Ideally, as many parameters as can be determined by surface science studies of adsorption and of elementary steps, as well as results from computational studies, are used as the input in a kinetic model, so that fitting of parameters, as employed in Section 7.2, can be avoided. We shall use the synthesis of ammonia as a worked example [P. Stoltze and J.K. Norskov, Phys. Rev. Lett. 55 (1985) 2502 J. Catal. 110 (1988) Ij. 

The first step in constructing a micro-kinetic model is to identify all the elementary reaction steps that may be involved in the catalytic process we want to describe, in this case the synthesis of ammonia. The overall reaction is... 

Figure 7.22. NH3 concentration as a function of reactor length in the synthesis of ammonia with a potassium-promoted iron catalyst. The exit concentration is 19 % and corresponds to...

Why is the synthesis of ammonia so important Nitrogen is an essential component of biological systems, for which amino acids are fundamental building blocks. Although nitrogen accounts for 80% of the air, N2 is among the most stable molecules and is therefore not easily activated. 

In this paper we will discuss the results that we have obtained studying two important processes, the synthesis of ammonia over Fe and Re catalysts and the hydrodesulfurization of thiophene on Mo. These examples illustrate two problems in catalysis to which our methods are particularly sensitive. The first is the effect of the structure of the catalyst on reaction rates. The sensitivity of the... 

High pressures are required for many commercial chemical processes. For example, the synthesis of ammonia is carried out at reactor pressures of up to 1000 bar, and high-density polyethylene processes operate up to 1500 bar. 

M Fritz Haber first used osmium for the synthesis of ammonia. The rare and expensive element was soon replaced by the cheapest iron. 

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