Converters, ammonia synthesis

Figure 11.5 Examples of ammonia synthesis converters (a) tube-cooled, axial-flow converter (Twigg, 1996, p. 438 reproduced with permission from Catalyst Handbook, ed. M.V. Twigg, Manson Publishing Company, London, 1996.)...

Figure 8. Designs of ammonia synthesis converters (a) Principle of the autothermal ammonia synthesis reactor (b) Radial flow converter with capacities of 1,800 tpd (c) Horizontal three-bed converter and detail of the catalyst cartridge. (Source Walas, M. S., Chemical Process Equipment, Selection and Design, Butterworth Series in Chemical Engineering, 1988.)...

If the makeup gas to the ammonia synthesis loop is absolutely free of catalyst poisons, such as H2O and C02, it can flow directly to the ammonia synthesis converter. This leads to the most favorable arrangement from a minimum energy point of view. This can be accomplished by allowing the gas that leaves the methanation step to pass through beds of molecular sieves to remove water and traces of C02 74... 

For revamps of ammonia synthesis converters, Ammonia Casale offers 1) an in-situ modification of bottle-shaped converters of the Kellogg type, and 2) a three-bed intercooled configuration. The intercooled design is similar in some ways to the Uhde design discussed below213. 

Many ammonia synthesis converters operate at 150 to 200 bar and around 515°C. Under these conditions, nitriding and hydrogen embrittlement can occur. The pressure shell is a multi-layer or multi-wall carbon steel vessel. The internal catalyst baskets, contained in the shell, are made of SS 321 material88. 

The reaction normally takes place on an iron catalyst. The reaction pressure is in the range of 100 to 250 bar, and temperatures are in the range of 350°C to 550°C. At the usual commercial converter operating conditions, the conversion achieved per pass is 20 to 30 percent.53 In most commercial ammonia plants, the Haber recycle loop process is still used to give substantially complete conversion of the synthesis gas. In this process the ammonia is separated from the recycle gas by cooling and condensation. Next the unconverted synthesis gas is supplemented with fresh makeup gas, and returned as feed to the ammonia synthesis converter.74... 

The practical application of kinetic equations to the mathematical calculation of ammonia synthesis converters is described in [196], [217], [223]-[229]. 

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).

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... 

Baddour et al. [26] in their simulation of the TVA ammonia-synthesis converter, already discussed in Sec. 11.5.e, found that in steady-state operation the temperature difference between the gas and the solid at the top, where the rate of reaction is a maximum, amounts to only 2.3°C and decreases as the gas proceeds down the reactor to a value of 0.4°C at the outlet. In the methanol reactor simulated in Sec. 11.9.b the difference between gas and solid temperature is of the order of 1 C. This may not be so with highly exothermic and fast reactions involving a component of the catalyst as encountered in the reoxidation of Fe and Ni catalysts used in ammonia synthesis and steam reforming plants or involving material deposited on the catalyst, coke for example. 

The heart of the ammonia synthesis loop is the ammonia converter. The ammonia converter normally comprises a basket (internals) enclosed in a pressure shell. In the ammonia synthesis converter(s), due to the exothermal nature of the reaction taking place, it is normally not possible to go from the inlet to, the outlet conditions in the reactor in one adiabatic step, if a reasonable conversion is desired. Therefore, some type of cooling is required. In principle, the cooling can be applied in three different ways ... 

More detailed reviews of the various types of ammonia synthesis converters can be found in (2, 3). 

As an example of an ammonia synthesis converter, the Haldor Topsoe S-300 converter is illustrated in Figure 5. The ammonia synthesis converter consists of a pressure shell and a basket. The basket consists of three catalyst beds and two interbed heat exchangers placed in the centre of the first and second catalyst bed respectively. 

Figure 6.5 Composition-temperature space for ammonia synthesis converter (a) reaction rate as a function of ammonia mole percent and temperature (b) suboptimal cold-shot composition-temperature traj ectory, plotted over reaction rate contours, with bypasses set to = [0.1,0.1]. ...

Figure 1.12 Radial flow ammonia synthesis converter by Haldor-Topsoe. (Source Couper et al. [12]. Reproduced with permission of Elsevier.)...

A gas leaving an ammonia synthesis converter contains 12 mol % of NH,. It circulates through a cooler at 250atm and passes out of the cooler at 30 Use the data below to estimate the fraction of the entering... 

Heat exchange in multi-bed direct heat exchange reactor is by directly adding a cold gas to the reaction gas to lower the reaction temperature this is the so-called cold-quench . If the quenching gas is the feed gas, it is called feedgas quench , such as the Topspe S-100 type ammonia converter, and ICI cold-quencher ammonia synthesis converter. If the quenching gas is not the feed gas, it is called non-feedgas quench , such as the steam cold-quench carbon monoxide shift reactor. 

In plants with a methanation process, before coming into the ammonia synthesis converter, the fresh syngas first passes through the Ni methanation catalyst. Ni catalyst is sensitive to sulfur in the same way as Fe catalyst. The Ni catalyst becomes a scapegoat to protect the Fe catalyst. Therefore, the life of a Fe catalyst in these plants could be as long as 10 to 15 years. 

S-200 Radial Flow Ammonia Synthesis Converter without Lower Heat Exchanger... 

Ammonia synthesis converter internals, hot ammonia piping, H-P steam generator up to 550 °C nitriding, hydrogen attack... 

W. A. Glover and J. P. Yoars, Two-Stage Adiabatic Ammonia Synthesis Converter, AIChE Safety Symposium, Minneapolis (Aug 29, 1972). 

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