Ammonia synthesis effects

Fig.10. Ammonia Synthesis. Effectiveness Factor (n) end NH3 Concentration in the Bulk (NH3,b) and in Centre of Catalyst Pellet (NH3,c) in First Catalyst Bed with 2.5-3mm Particles (cfr. Table 2, Case 1) ...

The addition of potassium to Fe single crystals also enliances the activity for ammonia synthesis. Figure A3.10.19 shows the effect of surface potassium concentration on the N2 sticking coefficient. There is nearly a 300-fold increase in the sticking coefficient as the potassium concentration reaches -1.5 x 10 K atoms cm ... 

Bare S R, Strongin D R and Somoqai G A 1986 Ammonia synthesis over iron single crystal catalysts—the effects of alumina and potassium J. Phys. Chem. 90 4726... 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

These processes have been used in many plants to remove carbon dioxide from ammonia synthesis gas or natural gas. They are most effective if the... 

These pioneers understood the interplay between chemical equiUbrium and reaction kinetics indeed, Haber s research, motivated by the development of a commercial process, helped to spur the development of the principles of physical chemistry that account for the effects of temperature and pressure on chemical equiUbrium and kinetics. The ammonia synthesis reaction is strongly equiUbrium limited. The equiUbrium conversion to ammonia is favored by high pressure and low temperature. Haber therefore recognized that the key to a successful process for making ammonia from hydrogen and nitrogen was a catalyst with a high activity to allow operation at low temperatures where the equiUbrium is relatively favorable. 

By-product power does not give enough power to match the demand for many processes such as ammonia synthesis, and designs have historically incorporated condensing turbines for incremental power with heat rejection to cooling water. A more effective response is use of the gas turbine combined cycle shown by Figures 5c and 6c. 

Since theoretical calcination of effectiveness is based on a hardly realistic model of a system of equal-sized cylindrical pores and a shalq assumption for the tortuosity factor, in some industrially important cases the effectiveness has been measured directly. For ammonia synthesis by Dyson and Simon (Ind. Eng. Chem. Fundam., 7, 605 [1968]) and for SO9 oxidation by Kadlec et aJ. Coll. Czech. Chem. Commun., 33, 2388, 2526 [1968]). 

The effect of alkali additives on N2 chemisorption has important implications for ammonia synthesis on iron, where alkali promoters (in the form of K or K20) are used in order to increase the activity of the iron catalyst. 

These effects profoundly influence the ammonia, as shown in Fig. 8.28 where the ammonia synthesis rate is plotted for two basal planes of iron and the same iron surfaces modified with 0.1 Ml of potassium. 

Figure 6.3 The effect of temperature on the ammonia synthesis reaction.

The range of reactions which have been examined is wide (248) and includes hydrogenations (256), ammonia synthesis (257), polymerizations (257), and oxidations (258). Little activity has occurred in this area during the past few years. Recent reports of the effects of sonication on heterogeneous catalysis include the liquefaction of coal by hydrogenation with Cu/Zn (259), the hydrogenation of olefins by formic acid with Pd on carbon (260), and the hydrosilation of 1-alkenes by Pt on carbon (261). 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

N2 and CO, respectively [31,32], Empirical knowledge about the promoting effect of many elements has been available since the development of the iron ammonia synthesis catalyst, for which some 8000 different catalyst formulations were tested. Recent research in surface science and theoretical chemistry has led to a fairly complete understanding of how a promoter works [33,34],... 

Since theoretical calculation of effectiveness is uncertain and is moreover sensitive to operating conditions, for industrially important cases it is determined by such reaction tests. Common types of curve fits may be used. For ammonia synthesis catalyst, for instance, an equation is provided by Dyson Simon (IEC Fundam 7 605, 1968) in terms of temperature and... 

In contrast, the use of carbonyl-derived ruthenium catalysts on different supports has been explored in ammonia synthesis [120-122], The use of K2[Ru4(CO)i3] as ruthenium precursor on MgO or carbon yields especially effective catalysts for low-temperature ammonia synthesis [120, 122],... 

One or more steps may form a dead end in the form of an intermediate formed through an elementary reaction and consumed exclusively by the reverse of this step. Although the dead-end will not contribute to the overall reaction rate, the step may affect the kinetics if the intermediate is strongly adsorbed on the surface. The poisonous effect of H2O in ammonia synthesis is an example. 

As an example we will consider the mechanism for ammonia synthesis. We include Ar in the gas phase to illustrate the effect of inerts. This mechanism is rich enough to illustrate most of the features discussed below. 

If the gas mixture is considered to be an ideal gas mixture then all fugacity coefficients are 1 and since K is a constant, the effect of increasing pressure is an increase of the equilibrium mole fraction of ammonia and a decrease of the mole fractions of nitrogen and hydrogen. However, since the ammonia synthesis is a high pressure process the gas mixture is not an ideal gas and the fugacity coefficients have to be taken into account. 

---