Ammonia synthesis temperature profiles

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.)... 

The effect of feed composition cycling on the time-average rate and temperature profile was explored in the region of integral conversion in a laboratory fixed bed ammonia synthesis reactor. Experiments were carried out at 400°C and 2.38 MPa over 40/50 US mesh catalyst particles. The effect of various cycling parameters, such as cycle-period, cycle-split, and the mean composition, on the improvement in time-average rate over the steady state were investigated. 

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. 

Temperature profiles, reactors ammonia synthesis, 582, 584 cement kiln, 590 cracking of petroleum, 595 endo- and exothermic processes, 584 jacketed tubular reactor, 584 methanol synthesis, 580 phosgene synthesis, 594 reactor with internal heat exchange, 584 sulfur dioxide oxidation, 580... 

Figure 17.21. Some recent designs of ammonia synthesis converters, (a) Principle of the autothermal ammonia synthesis reactor. Flow is downwards along the wall to keep it cool, up through tubes imbedded in the catalyst, down through the catalyst, through the effluent-influent exchanger and out. (b) Radial flow converter with capacities to l tons/day Haldor Topsoe Co., Hellerup, Denmark), (c) Horizontal three-bed converter and detail of the catalyst cartridge. Without the exchanger the dimensions are 8 x 85 ft, pressure 170 atm, capacity to 2000 tons/day (Pullman Kellogg), (d) Vessel sketch, typical temperature profile and typical data of the ICI quench-type converter. The process gas follows a path like that of part (a) of this figure. Quench is supplied at two points (Imperial Chemical Industries).

Fig. 13-2 Variation in temperature profile with on-stream time in fixed-bed ammonia-synthesis reactor [by permission from A. V. Slack. H. Y. Allgood, and H. E. Maune, Chem. Eng. Progr., 49. 393 (/95J)]...

The NEC (Nitrogen Engineering Co.) and TVA (Tennessee Valley Authority) —ammonia synthesis reactors are practical realizations of the above principles. Figure 11.3-5 of Sec. 11.3 schematically represents a TVA reactor. "ITie corresponding temperature profiles inside the tubes and in the catalyst bed section, calculated by Baddour, Brian, Logeais, and Eymery [26] are shown in Fig. 11.5.e-9. Reactor dimensions for the TVA converter simulated by Baddour et al. and also by Murase, Roberts and Converse [27] are... 

Figure ll.5.e-9 Temperature profiles inside TVA. ammonia-synthesis reactor. 1 = gas in heat e.xchanger tubes 2 = gas in catalyst bed full curve 2 = simulated dashed curve 2 = plant (from Baddour, et al. [26J). 

This figure clearly illustrates that the range within which multiple steady states can occur is very narrow. It is true that, as Hlavacek and Hofmann calculated, the adiabatic temperature rise is sufficiently high in ammonia, methanol and oxo-synthesis and in ethylene, naphthalene, and o-xylene oxidation. None of the reactions are carried out in adiabatic reactors, however, although multibed adiabatic reactors are sometimes used. According to Beskov (mentioned in Hlavacek and Hofmann) in methanol synthesis the effect of axial mixing would have to be taken into account when Pe < 30. In industrial methanol synthesis reactors Pe is of the order of 600 and more. In ethylene oxidation Pe would have to be smaller than 200 for axial effective transport to be of some importance, but in industrial practice Pe exceeds 2500. Baddour et al. in their simulation of the TVA ammonia synthesis converter found that the axial diffusion of heat altered the steady-state temperature profile by less than 0.6°C. Therefore, the length of... 

Exothermic equilibrium reactions like methanol or ammonia synthesis have the disadvantage that a low temperature is needed to reach a favorable high equUibrium conversion of the reactants. Conversely, a sufficiently high temperature is required with respect to kinetics to carry out the reaction at an acceptable rate. Unfortunately, the temperature increases towards the exit of the fixed bed due to the exothermicity of the reaction (if we do not use intensive cooling), which additionally lowers the obtainable equilibrium conversion. Thus, the temperature profile is exactly the wrong way round, and the feed has to be preheated and the product stream has to be cooled, usually by feed-effluent heat exchangers. In addition, heat has to be removed between reaction stages, if the reaction temperature increases too much. 

Ammonia synthesis converters with radial flow in tubular cooled catalyst beds have been suggested by Toyo Engineering Corp. [520] and in [502]. The Toyo concept has, so far, not been used industrially, while the concept described in [502] has, as mentioned above, been demonstrated in revamps of converters originally designed by SBA. It is claimed that the cross flow makes it possible -through proper design of the cooling tube bundles - to optimize the temperature profile so that it follows very closely the maximum reaction rate curve. It is furthermore reported that the heat transfer coefficients obtained in practice in... 

If the objective in design or operation were optimizing catalyst utilization, then Figure 82 shows that the converter temperature-composition profile should follow curve (a), which corresponds to maximum reaction rate at all points. It is also obvious that in reality this ideal temperature - concentration profile cannot be achieved. For example, a synthesis gas with about 3 % ammonia concentration entering the converter cannot be heated to the ideal temperature by heat exchange because the very high temperature required does not exist in the converter system. To reach the ideal temperature, the first portion of the catalyst must initially operate adiabatically. Consideration of the service life of the catalyst requires that this maximum initial temperature not exceed that recommended by the manufacturer, usually 530 °C (cf. Section... 

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