Ammonia synthesis converter design

A very significant number of different ammonia synthesis converter designs have been used in industrial practice. In the following survey the different designs have been characterized, first by the cooling principle applied, and thereafter mainly by flow direction through the catalyst bed(s). 

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.)...

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. 

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).

Early types of ammonia synthesis converters are described in [4, 50, 385]. More recent developments are discussed in [23,490-492]. Good overall reviews of different converter designs may be found in [493, 494]. 

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... 

A limited number of companies possess the basic knowledge and the experience which makes them able to prepare the basic engineering information-process flow diagrams with heat and mass balances, equipment specifications, specification of instrumentation and process control including safety precautions, operating manuals etc. -which is required for the design of an ammonia plant. These companies have also developed their own proprietary design of critical equipment, most often the ammonia synthesis converter and the primary reformer. 

Proprietary items are the primary reformer, the ammonia synthesis converter, and certain catalysts. Operating experience in plants designed by Topsoe has been described in [720,771-774,956,966,967]. In [967J a net energy consumption of 6.97 Gcal/MT ammonia was reported for a plant located in a warm climate. For the most energy efficient process concept, which has so far not been demonstrated in an industrial installation, a net consumption of 6.67 Gcal/MT ammonia is claimed for a stand alone plant [920]. 

Two Uhde design concepts are available and are compared in Table 6.3 and Figure 6.12211. They can provide 1) a single converter that contains three ammonia synthesis beds (see Figure 6.13) or 2) a two converter system that also contains three synthesis beds (Figure 6.14). 

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... 

Description Natural gas or another hydrocarbon feedstock is compressed (if required), desulfurized, mixed with steam and then converted into synthesis gas. The reforming section comprises a prereformer (optional, but gives particular benefits when the feedstock is higher hydrocarbons or naphtha), a fired tubular reformer and a secondary reformer, where process air is added. The amount of air is adjusted to obtain an H2/N2 ratio of 3.0 as required by the ammonia synthesis reaction. The tubular steam reformer is Topsoe s proprietary side-wall-fired design. After the reforming section, the synthesis gas undergoes high- and low-temperature shift conversion, carbon dioxide removal and methanation. 

Knowledge of the reaction kinetics is important for designing industrial ammonia synthesis reactors, for determining the optimal operating conditions, and for computer control of ammonia plants. This means predicting the technical dependence on operating variables of the rate of formation of ammonia in an integral catalyst volume element of a converter. 

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. 

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... 

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). 

Like the ammonia synthesis reaction (discussed in section 3.2.4), the shift reaction although known to take place according to a CSD mechanism, power law kinetics are adequate for accurate design and simulation of industrial shift converters. The most successful rate equation is that of Rase (1977) obtained from industrial data (and therefore includes diffusional limitations). 

The equilibrium constant for the third, deuterium exchange, reaction is around 2 at the temperature at which the second, water-gas shift, reaction is carried out. Because an excess of water is used to convert CO completely to COj, the deuterium content of hydrogen will be less than that of the methane and water fed, unless the excess water is fully recycled. Because water recycle is usually not practiced at ammonia synthesis plants, the deuterium content of synthesis gas at operating plants is sometimes as low as 0.009 percent [M7]. If the ammonia plant were specifically designed for deuterium recovery from its synthesis gas, the deuterium content could be increased to the average of the methane and water feeds by recycling aU water and preventing losses. 

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