Ammonia converter design

In the early days the synthesis gas was produced at atmospheric pressure, and the synthesis gas was compressed in reciprocating compressors to pressures as high as 100 MPa in some cases. Capacities were limited to around 300 - 400 MTPD due to limitations in reciprocating compressors. However, with the development of steam reformer based front-ends and the introduction of centrifugal compressors, the ammonia plant capacities suddenly increased to 1000 MTPD with ammonia synthesis loop pressures typically around 15 MPa. Since the 1960 s new developments have been in the ammonia converter designs, such as introduction of radial flow converters and introduction of converters with multiple catalyst beds to increase ammonia conversion. 

Dybkjffir, I. Jarvan, J.E. Advances on Ammonia Converter Design and Catalyst Loading. Nitrogen 97 Conference, 9-11 Feb. Geneva, Switzerland. 

Ammonia Converter Design - Today all low-energy designs use indirectly cooled converters. The design and layout offered by various ammonia plant designers differ considerably. Kellogg s Horizontal Converter and Topsoe s Series 200 Converter use two catalyst layers with an intermediate heat exchanger for... 

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. 

For many years into the future there will be a continuing requirement for the large-scale production of ammonia, principally for fertilizer use. The main objectives for commercially sized plants are high efficiency, low capital cost, and high reliability. The trends in plant design to achieve these objectives can be considered on a number of different levels, ranging from broad issues such as choice of feedstock to details of the ammonia converter design. 

Dehydration. Use of molecular sieve driers for final clean-up of the carbon oxides and water in the synthesis gas to less than 1 ppm levels has gained prominence in low energy ammonia plant designs. The sieves are usually located at the interstage of the synthesis gas compressor to reduce volume requirements. The purified make-up gas can then be combined with the recycle and routed direcdy to the converter. 

Figure 17.22. Representative ammonia converters operating at various pressures and effluent concentrations (Vancini, 1971). (a) Original Uhde design operating at 125 atm typical dimensions, 1.4 x 7 m. (b) Haber-Bosch-Mittasch converter operating at 300 atm typical dimensions, 1.1 x 12.8 m. (c) Claude converter operating at 1000 atm typical dimensions 1.2 x 7 m. (d) Fauser-Montecatini (old style) converter operating at 300 atm with external heat exchange, showing axial profiles of temperature and ammonia concentration.

As discussed above, several different types of ammonia converters are available. These types include axial quench converters (e.g., standard Kellogg reactors), tube cooled converters (e.g., TVA and Synetix designs), axial-radii designs (e.g., Ammonia Casale retrofit) and Kellogg s horizontal design. Typical operating data for different types of ammonia converters are shown in Table 6.4204. 

KBR has designed a 4,000 tonne/day plant in which all the equipment is single-train except for the primary reformer and the ammonia converter, each of which consists of two identical, parallel units. The design uses the KAAPplus technology, which includes the KBR Reforming Exchanger system (KRES) and the KBR Purifier. These technologies are discussed earlier in this chapter and in Chapter 5215. 

Haldor Topsoe s ammonia synthesis technology is based on the S-200 ammonia converter. This is a two-bed radial flow converter with indirect cooling between the beds. This converter concept has been used extensively to upgrade existing converters (Topsoe or other designs) in modification projects to achieve higher capacity (up to 20%) and/or better energy efficiency.85... 

For the particle sizes used in industrial reactors (> 1.5 mm), intraparticle transport of the reactants and ammonia to and from the active inner catalyst surface may be slower than the intrinsic reaction rate and therefore cannot be neglected. The overall reaction can in this way be considerably limited by ammonia diffusion through the pores within the catalysts [211]. The ratio of the actual reaction rate to the intrinsic reaction rate (absence of mass transport restriction) has been termed as pore effectiveness factor E. This is often used as a correction factor for the rate equation constants in the engineering design of ammonia converters. 

For a long time efforts to improve the efficiency of industrial ammonia production concentrated on synthesis gas production, and major progress was achieved over the years. In ammonia synthesis itself considerable progress was made in converter design and recovery of the reaction enthalpie at high temperature, but there has been no substantial improvement in the catalyst since the 1920s. The standard commercial iron... 

It has been already mentioned briefly, that compared to the synthesis section itself, where of course some progress has been made in converter design and optimization of heat recovery, the more fundamental changes over the years have occurred in synthesis gas preparation and gas compression. It is therefore appropriate to discuss the various methods for the synthesis gas generation, carbon monoxide shift conversion, and gas purification in some detail. Figure 29 shows schematically the options for the process steps for ammonia production. 

A) Synthesis loop for pure and dry makeup gas B) Product recovery after recycle compression C) Product recovery before recycle compression (four-nozzle compressor design) D) Two stages of product condensation a) Ammonia converter with heat exchangers b) Ammonia recovery by chilling and condensation c) Ammonia recovery by condensation at ambient temperature d) Synthesis gas compressor e) Recycle compressor... 

Converter Design. Design of ammonia synthesis reactors is not just the calculation of the required catalyst volume other parameters have to be considered, too, and for... 

The ammonia converter is a demanding engineering and chemical engineering task. To calculate the parameters for the design, including dimensions and number of catalyst beds, temperature profiles, gas compositions, and pressure drop, a suitable mathematical model is required. 

Kellogg has developed for its ruthenium catalyst based KAAP ammonia process [404], [478] a special converter design. Four radial flow beds are accommodated in a single pressure shell with intermediate heat exchangers after the first, second and third bed. The first bed is loaded with conventional iron catalyst, the following ones with the new ruthenium catalyst. Figure 95 is a simplified sketch of the converter and the synthesis loop of the KAAP for a new plant. For revamps Kellogg has also proposed a two-bed version completely loaded with ruthenium catalyst to be placed downstream of a conventional converter [398]. 

Anonymous. Radial flow ammonia converter—New ammonia synthesis design. Nitrogen 31 12 (1962). 

Ammonia converters TVA design with an internal heat exchanger. 

Due to the large capacities of modem ammonia production lines (>1200 MTPD), small improvements in the performance of the production line result in large economic benefits (e.g. it was estimated recently that 1% increase in the ammonia concentration in the effluent of the ammonia converter with a capacity of 1200 MTPD will have a dollar value of 1.2x10 US /year (Elnashaie et al., 1988b). Mathematical models are very powerful tools for the accurate design. 

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