Ammonia axial-radial

The ammonia loop is based on the Ammonia Casale axial-radial three-bed converter with internal heat exchangers. Heat from the ammonia synthesis is used to 1) generate high-pressure steam and 2) preheat feed gas. The gas is then cooled and refrigerated to separate ammonia product. Unconverted gas is recycled to the syngas compressor208 214... 

Figure 6.11. Ammonia Casale Axial-Radial Reactor Design. (Reproduced by permission of Casale Group)...

Megammonia A process for making ammonia. It uses oxygen instead of steam, and novel axial-radial reactors that reduce pressure-drop and catalyst quantities. Developed by Lurgi Oel-Gas-Chemie and Ammonia Casale, for which they received the AstraZeneca Award for Green Chemistry in 2003. 

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... 

Figure 90. Ammonia Casale axial-radial flow pattern...

Casale axial-radial technology for the ammonia converter... 

Ammonia Casale SA Ammonia Natural gas (NG) Process produces anhydrous ammonia from natural gas by applying Casale s high-efficiency secondary reformer design, axial-radial technology for shift conversion, ejector ammonia wash system, axial-radical technology for ammonia converter and advanced waste-heat boiler design in the synthesis loop. 5 2010... 

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). 

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. 

Design of an axial-radial flow catalytic reactor for ammonia synthesis process with major flow radial and minor flow axial through each bed. U. Zandi and G. Pagani (Ammonia Casale SA). EP 293546 (1988). 

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... 

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

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. 

Another type of reactor has one or more annular beds of catalyst with radial flow of gas either inward or outward [5], as shown in Figure 3.12b. This type may be preferred when the diameter of an axial-flow reactor would be much greater than the required bed depth. By putting the same amount of catalyst in a narrower but longer reactor, the wall thickness can be reduced and the reactor cost decreased. This is particularly important for high-pressure reactions, such as the synthesis of ammonia. 

Figure 3 shows the steady-state radial temperature profiles for the two adiabatic catalytic beds operating at conditions of the optimal point. The corresponding axial temperature profiles in the interbed heat exchanger are also included in Fig. 3, for the tube side (Tt) and shell side (Tsh). The simulation results have been compared with industrial data corresponding to a large scale ammonia converter. The deviations at the reactor outlet were less than 0.2% (relative error) in composition and 14 °C in temperature (Toutz)-... 

It was realised by Rowe in the early 1980s that the uniform supersonic flows obtained by the correct design of a Laval nozzle and used for decades in rarefied wind tunnels for aerodynamic studies could provide an ideal flow reactor for the study of chemical reactivity at low and very low temperature. This was the cornerstone around which the CRESU technique has been developed. At the exit of the Laval nozzle, as there is no further expansion downstream of the nozzle exit, the flow parameters (i.e. temperature, density, pressure and velocity) do not exhibit any axial and radial variations at least in the centre of the jet (typically 10 to 20 mm in diameter) where the flow is isentropic for several tens of centimetres. The diffusion velocity is always negligible with respect to the bulk velocity therefore avoiding the major problem of condensation associated with the use of cryogenically cooled cells. As a consequence, in such expansions, heavily supersaturated conditions prevail and condensable species such as water, ammonia or even polycyclic aromatic hydrocarbons (PAHs hereafter), can be maintained in the gas phase at very low temperatures. 

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 ... 

Values of the MDT and HDT terms have been calculated for the two above-mentioned catalyst beds, for axial, centrifugal radial and centripetal radial flow, and are reported in Table 6.6. Some effect of back-mixing of ammonia arises at... 

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