Synthesis loop configurations

If the makeup gas is absolutely free of catalyst poisons, such as water and carbon dioxide (for example, after molecular sieve drying or liquid nitrogen wash), it can be fed directly to the synthesis converter (Fig. 77 A). After the gas leaves the synthesis converter, ammonia is condensed by cooling and the recycle gas is referred to the recycle compressor. This represents the most favorable arrangement from a minimum energy point of view. It results in the lowest ammonia content at the entrance to the converter and the highest ammonia concentration for condensation. 

When the makeup gas contains water or carbon dioxide, advantage is taken of the fact that these materials are absorbed completely by condensing ammonia. This requires the condensation stage to be located partially or wholly between the makeup gas supply point and the converter. This arrangement has the disadvantage that the 

Splitting the cooling step for ammonia condensation also offers advantages when the recycle gas is compressed together with the makeup gas. This applies especially at synthesis pressures above about 25 MPa (250 bar). At these pressures, a greater portion of the ammonia formed can be liquefied by cooling with cooling water or air (see Fig. 92 D). 

When ammonia-containing recycle gas and carbon dioxide containing makeup gas mix together under certain conditions of concentration and temperature, precipitation of solid ammonium carbamate can result. 

The central part of the synthesis system is the converter, in which the conversion of synthesis gas to ammonia takes place. Converter performance is determined by the reaction rate, which depends on the operating variables. The effect of these parameters is discussed briefly in the following (see also Section 4.5.7). 

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Methanol Synthesis. AH commercial methanol processes employ a synthesis loop, and Figure 6 shows a typical example as part of the overall process flow sheet. This configuration overcomes equiUbtium conversion limitations at typical catalyst operating conditions as shown in Figure 1. A recycle system that gives high overall conversions is feasible because product methanol and water can be removed from the loop by condensation. 

Ammonia Synthesis and Recovery. The purified synthesis gas consists of hydrogen and nitrogen in about 3 1 molar ratio, having residual inerts (CH Ar, sometimes He). The fresh make-up gas is mixed with the loop recycle and compressed to synthesis pressures. AH modern synthesis loops recycle the unreacted gases because of equiUbrium limitations to attain high overall conversions. The loop configurations differ in terms of the pressure used and the point at which ammonia is recovered. 

To overcome the pressure drop (5-20 bar) in the synthesis loop re-compression of the recycle gas is required. In practically all modern ammonia plants, the shaft of the final casing also bears the impeller for the compression of the recycle gas. Depending on synthesis configuration, mixing of make-up gas and recycle can be performed inside the casing or outside (three or four-nozzle arrangement Fig. 77). 

A number of different configurations are possible for the synthesis loop. They can be classified according to the location of ammonia condensation and the point at which the makeup gas is introduced. Figure 77 shows the principal possibilities. 

As previously mentioned, two types of ammonia synthesis loops exist. For the inert- containing loop, the inert level in the synthesis loop (most often measured at converter inlet) depends on the inert level in the make-up gas, the production of ammonia per unit make-up gas (the loop efficiency), and the purge rate. The inert level in the make-up gas is solely determined by the conditions in the synthesis gas preparation unit. The ammonia production is determined by conditions around the converter, the gas flow (which may be expressed by the recycle ratio), the inlet temperature and pressure, catalyst volume and activity, and converter configuration. 

Topsoe use the so-called S-200 synthesis loop. However, other converter configurations are also available (39). 

Figure 7.14 Various configurations of the ammonia synthesis loop (a) a simplified version of the traditional BASF synthesis loop (h) a modern arrangement with purge and recompression before separation (c) a modern arrangement with purge and separation before recompression...

The synthesis loop uses a horizontal converter in which the catalyst beds are arranged as horizontal slabs (Fig. 7.8). This configuration results in a very small catalyst pressure drop, and this enables small catalyst particles to be used with a subsequent increase in the effectiveness factor of the catalyst. In this way the process effect is very similar to a radial flow converter. 

The enzyme processes template and newly synthesized strand in such a way that the formation of (plus-minus) double strands is avoided. The experimentally determined rate of RNA chain elongation is highly variable (Mills et al., 1978). The replicase stop temporarily at certain pause sites in order to allow formation or reformation of secondary structures. In particular, hairpin loops are formed in the product strand and reformed in the template strand. Thereby, the formation of RNA double strands is avoided. This is very important for efficient RNA synthesis. Double strands have to melt at least in part before they are recognized as templates by the enzyme. Biebricher et al. (1982) found a polynucleotide which exists in two defined secondary structures. The more stable configuration, presumably a hairpin with a long double stranded region, is very unefficient in replication. 

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