Axial-flow ammonia converters

Axial-flow ammonia converters should use a larger particle size of catalysts. For the multi-bed axial-flow converter with direct heat-exchange between beds (cold-quench) having a diameter of 1600-3200 mm and a production capacity of 1000 t d" or more, and the height of catalyst bed 10-12m, choose large particles with a diameter of 6.7-9.4mm and 9.4-13 mm to minimize the pressure drop. For an axial-converter with a diameter of 800-1300 mm, height of catalyst bed 7-8 m, use 4.7-6.7mm or 9.4 mm particles to keep the low pressure drop. For an axial-converter with a diameter of 500-600 mm, the height of catalyst bed is only about 5 m, 2.2-3.3 mm, 3.3-4.7mm, and smaller particles may be used to increase the ammonia production. 

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.)...

As mentioned above, Haldor Topsoe A/S, KBR, and Uhde GmbH share the major part of the market for new ammonia synthesis technology. For plant retrofits also others are active and especially for ammonia converter revamps, Ammonia Casale offer their ammonia synthesis technology. The main feature of the Casale technology is the so-called axial-radial flow synthesis converter (55, 56), and (57). 

In ammonia production, the space velocity is often (0.6—2) x 10 h. In axial flow converters, the velocity of gas flow can be up to several m s the effect of external diffusion should be absent, but the effect of intraparticle diffusion cannot be ignored. The gas flow area In a radial flow converter is very large, the gas velocity is very low, and thus both effects of external and intraparticle diffusion cannot be ignored. Among the many factors that influence diffusion, the particle size of catalyst is the most significant and can be easily adjusted. 

In Haldor Tops0e s ammonia and methanol synthesis processes a series of adiabatic beds with indirect cooling between the beds is normally used, at least in large plants. In smaller plants internally cooled reactors are considered. In ammonia synthesis, the Tops0e solution is today the so-called S-200 converter (Fig. 7) and L6j. This converter type, which is a further development of the S-100 quench-type converter, was developed in the mid seventies the first industrial unit was started up in 1978, and today about 20 are in operation or on order. Both the S-100 and the S-200 reactors are radial flow reactors. The radial flow principle offers some very specific advantages compared to the more normal axial flow. It does, however, also require special catalyst properties. The advantages of the radial flow principle and the special requirements to the catalyst are summarized in Table 5. 

In methanol synthesis, the case for radial flow converters is less obvious. Tops0e has earlier proposed, the use of one-bed radial flow converters in large methanol plants. Later analyses, partly based on a change of catalyst type, have, however, led to the conclusion that axial flow should be preferred even in very large methanol synthesis converters. The reasons for this difference in reactor concept between ammonia synthesis and methanol synthesis are in the differences between the properties of the catalysts. As mentioned above, the ammonia synthesis catalyst is ideally suited for the radial flow principle. This is not true for the methanol synthesis catalyst. The reasons for not using the radial flow principle in methanol synthesis are the following ... 

The ammonia converter in the purifier process is the Braun 2-stage adiabatic synthesis converter. In process engineering terms this is a two-bed, axial-flow converter with heat exchange between the beds. However, the mechanical design differs from the majority of modem converters in that each catalyst bed is enclosed in its own separate pressure shell (see Fig. 7.9). 

As far as ammonia converter design is concerned. Ammonia Casale s technology has already been introduced in Section 7.2.3. Ammonia Casale converter designs using the axial-radial flow concept have achieved considerable success in recent years, particularly for modification of existing axial-flow converters. These converter retrofits can be achieved by in situ changes to the converter internals in the case of converters where the catalyst basket cannot be removed from the pressure shell. 

An axial-radial flow converter has been introduced by Ammonia Casale [490,492, 551-553]. The special feature of this design is that gas can enter each catalyst bed both from the top (in axial direction) and from the side through perforations (in radial direction), see Fig. 6.11 (from [492]). The gas leaves the catalyst bed through perforations in the inner wall. There are no perforations in an upper part of the inner wall, and the gas entering at the top of the bed is therefore forced to flow through part of the catalyst in partially axial flow before it can leave the catalyst bed. This flow principle can be used in quench cooled... 

Synthesis of ammonia in a dry synthesis loop (with addition of make-up gas after the ammonia separator) at approximately 180 kg/cm g with the Braun converter system consisting of two or three single-bed, adiabatic, axial flow converters. Cooling between the converters is by synthesis gas preheating and/or by steam production. 

Axial-radial flow pattern was introduced by Ammonia Casale. Converters with strictly radial gas flow require mechanical sealing of the top of each catalyst bed... 

The calculated values of temperature and ammonia concentration in the bulk gas and at the catalyst external surface are reported in Table 6.7 for the first catalyst bed of Fig. 6.2 in both axial and radial centrifugal flow. Since radial flow converters are usually filled with smaller-size catalyst particles than the catalyst considered here, from this point of view they are equivalent to axial converters, which are always filled with large-size catalysts to contain pressure drops. It also appears that the effects of mass and heat transfer at the external surface of the catalyst particle are oppositely directed so that they partly compensate for each other. We may conclude that in industrial converters their combined influence on the reaction rate is negligible compared to the inaccuracies inherent in the experimental determination of the intrinsic activity of the catalyst. In any case, interparticle phenomena can be readily incorporated as boundary conditions in the intraparticle problem ... 

Nielsen [100] also solved the simultaneous differential equations numerically and reported the results for a hypothetical reactor operating at 450 °C, 214 atm, and with a 3 1 H2/N2 feed gas containing 3% ammonia and 12% inerts. The concentration profiles calculated at the bed inlet and outlet are shown in Figs. 4.2 and 4.3. Nielsen used t = 2.2, based on a detailed, electron-microscopic study of the pore system. Figs 4.4 and 4.5 show the calculated ammonia concentration in the bulk at half pellet radius and the effectiveness factor for particle diameters of 5.7 mm and 1.5 mm as a function of axial distance. For the 1.5 mm particles, the effectiveness factor very quickly approaches 1. The higher reaction rates of the 1.5 mm particles are utilized in modern radial flow converters. 

Fig. 6.11. Principle of axial-radial flow converter. Ammonia Casale design (from [492])...

Fig. 6.19. Four bed Kellogg quench cooled converter after modification to Ammonia Casale s four bed quench cooled axial-radical flow concept (from [591])...

Ammonia Casale Axial-radial flow through three catalyst beds with inter-bed heat exchange or quench cooling. A second converter holding the third bed has been used. 

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