Converter types Quench converters

Fig. 7. Methanol converter types (a) quench, (b) multiple adiabatic, (c) tube-cooled, and (d) steam-raising.

Figure 8 shows the characteristic sawtooth temperature profile which represents the thermodynamic inefficiency of this reactor type as deviations from the maximum reaction rate. Catalyst productivity is further reduced because not all of the feed gas passes through all of the catalyst. However, the quench converter has remained the predominant reactor type with a proven record of reflabiUty. 

Retrofitting features of the more efficient reactor types have been the principal thmst of older methanol plant modernization (17). Conversion of quench converters to radial flow improves mixing and distribution, while reducing pressure drop. Installing an additional converter on the synthesis loop purge or before the final stage of the synthesis gas compressor has been proposed as a debotdenecking measure. 

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. 

In this type of converter only a fraction of the recycle gas enters the first catalyst layer at about 400 °C. The catalyst volume of the bed is chosen so that the gas leaves it at ca. 500 °C (catalyst suppliers specify a maximum catalyst temperature of 530 °C). Before it enters the next catalyst bed, the gas is quenched by injection of cooler (125-200 °C) recycle gas. The same is done in subsequent beds. In this way the reaction profile describes a zig-zag path around the maximum reaction rate line. A schematic drawing of a quench converter together with its temperature/location and temperature/ammonia concentration profiles is presented in Figure 86. 

Fig. 2 shows I.C.I. cold-shot quench converter methanol loop which operates similar to the warm-shot loop. This adiabatic type quench reactor with its associated equipment is used when the syngas entering the methanol loop is stoichiometric or slightly carbon rich. It has the same adiabatic reactor profile as shown in Fig. 3. The main features of the cold-shot reactor loop are ...

Figure 3.14 for a three-stage converter with two quenches. This type of converter is used for Id s low-pressure methanol synthesis [6]. 

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 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. 

Fig. 6.6. Schematic drawing, typical temperature profile, and operating curve (temperature/am-monia concentration plot) for four important converter types, (from [471]) a Internal cooling, countercurrent flow (TVA-converter) b Internal cooling, cocurrent flow (NEC-converter) c Quench cooling d Indirect cooling (heat exchange)...

The most important example of this converter type is the M. W. Kellogg 3- or 4-bed Quench Converter which was used in a large number of plants. The converter is described in [490, 491, 527, 528]. A simplified drawing of a 3-bed... 

This converter type has given excellent service in industry. It is, however, due to the inherent weaknesses of the quench cooling system, not very efficient, and the performance has in many cases been improved by revamping, see Sect. 6.4.3.4. 

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