Ammonia synthesis partial pressures

Because the ammonia synthesis reaction is an equiUbrium, the quantity of ammonia depends on temperature, pressure, and the H2 to-N2 ratio. At 500°C and 20.3 MPa (200 atm), the equiUbrium mixture contains 17.6% ammonia. The ammonia formed is removed from the exit gases by condensation at about —20° C, and the gases are recirculated with fresh synthesis gas into the reactor. The ammonia must be removed continually as its presence decreases both the equiUbrium yield and the reaction rate by reducing the partial pressure of the N2—H2 mixture. 

The choice of a specific CO2 removal system depends on the overall ammonia plant design and process integration. Important considerations include CO2 sHp required, CO2 partial pressure in the synthesis gas, presence or lack of sulfur, process energy demands, investment cost, availabiUty of solvent, and CO2 recovery requirements. Carbon dioxide is normally recovered for use in the manufacture of urea, in the carbonated beverage industry, or for enhanced oil recovery by miscible flooding. 

In almost all modem plants, the ammonia is recovered by condensation and at modern synthesis pressures, ammonia is usually the source of refrigeration required. In order to maintain a high partial pressure of reactants, inerts entering with the make-up gas are normally removed using a purge stream. 

The equilibrium eonstant for ammonia synthesis is expressed as a funetion of the partial pressure as... 

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 ... 

We will list the elementary steps and decide which is rate-limiting and which are in quasi-equilibrium. For ammonia synthesis a consensus exists that the dissociation of N2 is the rate-limiting step, and we shall make this assumption here. With quasi-equilibrium steps the differential equation, together with equilibrium condition, leads to an expression for the coverage of species involved in terms of the partial pressures of reactants, equilibrium constants and the coverage of other intermediates. 

Figure 7.21 shows the results for the ammonia synthesis on real catalysts in a reactor. The surface is predominantly covered by atomic nitrogen and by NH intermediates. This actually limits the rate of the reaction as soon as an appreciable partial pressure of ammonia has built up. In fact, ammonia poisons the reaction. 

This synthesis of amino acids, called the Strecker synthesis, requires the presence of NH4+ (and NH3) in the primitive ocean. On the basis of the experimental equilibrium and rate constants it can be shown16 that equal amounts of amino and hydroxy acids are obtained when the NH4+ concentration is about 0.01 M at pH 8 and 25°C with this NH4+ concentration being insensitive to temperature and pH. This translates into a pNH3 in the atmosphere of 2 x 1(U7 atm at 0° and 4 x 10-6 atm at 25°C. This is a low partial pressure, but it would seem to be necessary for amino acid synthesis. Ammonia is decomposed by ultraviolet light, but mechanisms for resynthesis are available. The details of the ammonia balance on the primitive earth remain to be worked out. 

Calculate the free-energy change for ammonia synthesis at 25°C (298 K) given the following sets of partial pressures ... 

The reactor can operate with either a liquid-phase reaction or a gas-phase reaction. In both types, temperature is very important. With a gas-phase reaction, the operating pressure is also a critical design variable because the kinetic reaction rates in most gas-phase reactions depend on partial pressures of reactants and products. For example, in ammonia synthesis (N2 + 3H2 O 2NH3), the gas-phase reactor is operated at high pressure because of LeChatelier s principle, namely that reactions with a net decrease in moles should be mn at high pressure. The same principle leads to the conclusion that the steam-methane reforming reaction to form synthesis gas (CH4 + H20 O CO + 3 H2) should be conducted at low pressure. 

Since the partial pressure is the mole fraction in the vapor phase multiplied by the total pressure, (i.e., p, = y, P), the equilibrium constant Keq is expressed as Keq = Ky PAn, where An = (2 - 1 - 3), the difference between the gaseous moles of the products and the reactants in the ammonia synthesis reaction. 

A frequent reason for the dependence of catalyst structure on the chemical potential in the gas phase containing all the reactants is the incorporation of molecules or atoms from the reaction mixture into the catalyst phases. Formation of subphases, often only in the near-surface region of the solid, fails to create phases with individual reflections but modifies the reflections of the starting precatalyst phase notably (see previous sections). This complication presents a massive problem in the analysis of working catalysts when significant partial pressures of products are important to the phase formation and when the necessary conversions cannot be reached in the experimental cell. The investigation of ammonia synthesis catalysts when insufficient partial pressures of the product ammonia prevent the formation of the relevant nitride phases is a prominent example of this limitation (Herzog et al., 1996 Walker et al., 1989). 

The basic equation most widely accepted to express the kinetics of ammonia synthesis is that of Teml and Pyzbev (1940). It expresses the reaction rate as a function of the partial pressures of the reactants and products ... 

At 450°C, Kp = 6.5 X 10-3 atm-2 for the ammonia synthesis reaction. Assume that a reaction vessel with a movable piston initially contains 3.0 mol H2(g) and 1.0 mol N2(g). Make a plot to show how the partial pressure of NH3(g) present at equilibrium varies for the total pressures of 1.0 atm, 10.0 atm, 100. atm, and 1000. atm (assuming that Kp remains constant). (Note Assume these total pressures represent the initial total pressure of H2(g) plus N2(g), where PNHl = 0.)... 

Contradictory data on the kinetics of ammonia synthesis, especially in the earlier literature, in some circumstances may reflect a lack of attention to the influence of impurities in the gas. If oxygen compounds are present in the synthesis gas, reversible poisoning of the adsorbing areas, in accordance with an equilibrium depending on the temperature and the water vapor-hydrogen partial pressure ratio, must be taken into account when developing rate equations (see also Section 3.6.1.5). 

In applying these equations, the authors assumed that the adsorption of nitrogen on the iron catalyst in the presence of an ammonia-hydrogen mixture is the same as it would be when at a nitrogen pressure equivalent to the existing partial pressure of ammonia and hydrogen in the gas mixture. Thus, since the equilibrium constant for ammonia synthesis is... 

The addition of potassium to industrial Fe catalysts leads to an increase in activity for ammonia synthesis (N2 -I- 3H2 - 3NH3) (136). This promotion effect has been the subject of considerable attention from the surface science community, particularly with regard to the coadsorption of K or K + O and N2 (136-139). Ertl and co-workers have shown that potassium addition to single-crystal Fe surfaces can lead to a 10- to 100-fold enhancement in the rate of dissociative N2 adsorption, which is thought to be the rate-determining step in NH3 synthesis (136-139). However, Bare et al. (140) were unable to promote the activity of Fe(l 11), (100), or (110) surfaces for this reaction at 20-atm pressure with either K, K + O, or K + AlO, addition. They interpreted this result to indicate that the promotional role of K in industrial catalysis may be cooperation with other promoters, such as the support material, to cause structural rather than electronic promotion. These results were for very low conversions, however, so that the product (NH3) partial pressure was low. Strongin and... 

Figure 1323 illustrates the principle of enriching deuterium by monothermal ammonia-hydrogen exchange in conjunction with a synthetic ammonia plant. The flow quantities have been developed from partial information reported for the Mazingarbe plant [LI, El, N2]. Feed, at point (1), consists of ammonia synthesis gas, SHj/lNj, which has been purified to reduce its content of water, CO, and COj to less than 1 ppm and compressed to the pressure of ammonia... 

This limiting rate law has been found only at very low values of Pnh3 (106). When the synthesis of ammonia proceeds, i.e., the partial pressure of NHg increases, the rate of formation of NH3 decreases substantially even under conditions where the rate of the backward reaction is negligible. This has been explained by assuming that the surface is partially blocked by adsorbed nitrogen atoms whose concentration is determined by the equilibrium... 

---