A variety of ammonia converters have been designed and used. Zardi(1821) gives diagrams of 17 diffrent types, plus four types of methanol synthesis converts. Since the reaction is exothernic abd the gas volume decreases on reaction, maximum conversion at equikibrium occurs at high pressure and low temperature. As with methanol synthesis, the temperature profile through the reactor must steer a compromise between inadequate rate if the temperature es too low, and a thernodynamic limitation if the temperature es too high. The maximum catalyst temperature is limited to about 500 C to avoid significant decline in activity with time. The exit gas typically contains about 12 to 14%NH3, which is condensed before the renaining gas is recycled and added to fresh gas .A very high degree of removal of NH3 is not justified economically, so the gas mixture fed to the reactor will typically conatin in the range of 4% NH3In some of the more widely used designs the catalyst is held in two or three baskets in series between which cold-shot or quench cooling (a supply of additional synthesis gas at a lower temperature) is provided. Baskets may de stacked above one another in a somewhat similar fashion to the reactor shown in fig 10.7 except that cooling is provided by quench gas instead of cooling coils. The exact design is dictated to a large extent by mechanical consideration set by operation at a high pressure and temperature. In general cold synthesis gas is passed between the inside of the pressure vessel and the catalyst containers to prevent decarburization (hydrogen embrittlement) of the pressure vessel that might occur at reaction conditions . An ICI quench concerter is similar to that used in the ICI converter for methanol synthesis.A radial-flow reactor may also de used . Because of the shorter path length through the bed, smaller particle sizes e.g. 1.5 to 3 mm may de used without excessive pressure drop. This results in a higher catalyst effectivenessfactor and therefore a redustion in the total volume of catalyst required. Figure 10.12shows a schematic diagram of the design of a Topsoe two-bed radial flow converter, which uses indirect cooling in a heat exchanger for interstage cooling. The accompanying photograph is of a reactor having a capacity of 1500 tons of ammonia per day. The overhead derrick is for instalation and removal of the catalyst basket. In the diagram, inlet gas enters at A, passes through the narrow annulus to the bottom, where it is heated by exchange with product gases leaving al D. This gas un then mixed with cold bypass gas, C passes upward and A radial-flow reactor may also de used . Because of the shorter path length through the bed, smaller particle sizes e.g. 1.5 to 3 mm may de used without excessive pressure drop. This results in a higher catalyst effectiveness factor and therefore a reduction in the total volume of catalyst required. Figure 10.12 shows a schematic diagram of the design of a Topsoe two-bed radial flow converter, which uses indirect cooling in a heat exchanger for interstage cooling. The accompanying photograph is of a reactor having a capacity of 1500 tons of ammonia per day. The overhead derrick is for instalation and removal of the catalyst basket. In the diagram, inlet gas enters at A, passes through the narrow annulus to the bottom, where it is heated by exchange with product gases leaving al D. This gas un then mixed with cold bypass gas, C passes upward and flows radially through the first (top) bed fromthe outside toward the center. The product from this bed is cooled by heat excange with inlet gas B and flows radially through the second, lower bed.A horizontal converter may also be designedto achieve the same goals. Figure 10.13 shows a photograph of a Kellogg reactor , amd Fig. 10.14a shematic drawing of a unit with three beds of catalyst Eschenbrenner & Wagner 1972)The rate equation almost always used for correlation of data and prediction of performance of industrial reactors is based on the formulation of Temkin and Pyzhev (see temkin (1979)). The original form was derived by assuming that the rate limiting step is the dissociative chemisorption of nitrogen to form nitrogen atoms . These are assumed to be the main adbed species ,and their surface concentration is determined by an equilibrium with hydrogen and ammonia in the gas phase. (this is not the equilibrium with the actual concentration of gaseous nitrogen , but instead with the partial pressure of nitrogen that would exist if N2 were present in thermodynamic equilibrium with the gaseous hydrogen and ammonia present. *The sites were further assumed to have a linear distribution of the heat of adsorption of nitrogen. The resulting expression for the rate of reaction is. FORMULAThe first term is for the rate of formation of ammonia and the second for the rate of descomsition. and are both positive, and + =1 .In a detailed review of kinetics of heterogeneous catalytic reactions , Temkin (1979) comments that Eq. (10.23) is unique in chemical kinetics because it holds true for pressures varying from atm to 500 atm- a factor of 2000. No other reaction has been studied over such wide range of pressure.The expression has been modified in various ways .Since operation is at high pressure; fugacities are usually substitutes for pressure. Other derivations have used different assumptions concerning the energy distribution of the sites and the nature of the principal absorbed species .The net effect is that , in essence, the form of the Temkin-Pyzhev equation is retained , but then has a different mechanistic interpretation .The sum of and remains 1 , but the numerical value of may vary . Temkin proposed that =0.5 on all iron catalysts.In various publications Boudart has commented on the kinetics and mechanism of the synthesis .In a later review ( Boudart 1981) , he concludes that the likelythat likely rate-determinig step is dissociative chemisorption of dinitrogen, N2 on a nonuniform iron surface with N* as the most abundant reaction intermediate . in general accordance with earlier views.This dissociative N2 chemisorption is followed by reaction with dissociative hydrogen to form absorbed NH, NH2 and NH3 in succession , all in equilibrium . Ertl (1987) has also evaluated evidence for this mechanism . Stoltse and norskov (1987) derived a kineticexpression with parameters taken from ultrahigh vacuum conditions for the reactants on single crystal surfaces and showed that this could describe the observed synthesis kinetics at 375 to 500C and pressuresup to 30 MPa .Thisis interesting in that rates surface science measurements could predict rates under industrial conditions.Careful experimentation with commercial iron catalysts under practical conditions has led various researchers to the conclusion that the best correlation of the experimental data on various individual catalysts was obtained by use of value of in the range from 0.4 to 0.75( Nielsen 1968; ; Nielsen et al . 1964 ;Guacci et al .1977). To expect that would vary with catalyst composition is not unreasonable , but comparisons of correlations of experimental data using values of from 0.5 to 0.75 show little or no significant difference (Dyson & Simon 1986, Guacci et al . 1977)Equation (10.23) clearly cannot be applicable at zero NH3 concentration.This is generally unimportant for industrial operations , since the feed to the converter contains some NH3 from recycled gas . In tha absence of NH3 Temkin and coworkers (Temkin 1979) hace established that the expression is of the form . FORMULA Where K is the equilibrium constant for the reaction 1.5H2 + 0.5N2=NH3 and K2 is the reaction rate constant for the reverse reaction .(The latter is , of course , proportional to the constant for the forward reaction , K1) . The value of K2 seems to be independent of pressure if fugacities instead of pressures are used in Eq.(10.25). K2 can be expressed in the Arrhenius form , K2=Ae EIRT, but the value of E depends on the value of chosen . Nielsen et al (1964) reported a value of E=177 kJ/mol for their (triply promoted) catalyst, taking =0.64. Dyson and Simon (1989) calculate a value of 169 kJ/mol that is compared to the normally quoted value of 159kJ/mol . Guacci et al ,in their analysis of the activity of several commercial catalysts , aldo show how the comparison may vary depending on the value of chosen and the temperature .An extensive literature exists on ammonia synthesis , which is not surprising in view of its central importance for the manufacture of fertilizer and explosives and the long-time study devoted to it . The book by Nielsen (1968) gives an extensive treatment of industrial catalysts.Composition , characterization , and detailed results of rate measurements under industrial conditions are presented , with extensive references. In a more recent review ( Nielsen 1981), he discusses exploratory and applied research on he synthesis .Much of the earlier understanding of the mechanism of ammonia synthesis is due to P.H. Emmett , who reviewed progress up to 1975 (Emmett 1975)Surveys of various aspects of the subject may also be found in the book by Vancini (1971).Mittash (1950)in a personal memoir describes the development of his early concepts of promoter action and multicomponent catalysts , which led to the first practicable catalyst composition. A detailed treatment of ammonia synthesis catalysts , focusing on promoters, poisons, kinetics, and thertmodynamics is givenby Jennings