Operating conditions, ammonia synthesis loop

The operating conditions in the ammonia synthesis loop are described by a number of parameters, which in some cases may be independent variables, and in other cases a function of other parameters. The relationship between these parameters (and other parameters such as space velocity, inert level, concentrations and temperatures at various points in the synthesis loop, etc.) can be described in mathematical models that are used for design, simulation, and optimisation. 

The conclusion of the above points is that even a quite significant improvement in the catalyst activity would not cause any significant change in operating conditions in an ammonia synthesis loop. This was discussed further elsewhere. ... 

In most processes the reaction takes place on an iron catalyst. The reaction pressure is normally in the range of 150 to 250 bar, and temperatures are in the range of 350°C to 550°C. At the usual commercial converter operating conditions, the conversion achieved per pass is only 20% to 30%53. In most commercial ammonia plants, the Haber recycle loop process is still used to give substantially complete conversion of the synthesis gas. In the Haber process the ammonia is separated from the recycle gas by cooling and condensation. Next the unconverted synthesis gas is supplemented with fresh makeup gas, and returned as feed to the ammonia synthesis converter74. 

Fresh synthesis gas produced by the steam-reforming route is compressed to synthesis pressure in a two-stage centrifugal compressor. The compressor is usually driven by a steam turbine. The compressed make-up gas is chilled and added to the loop before the final loop chiller. After the chiller, the ammonia product is removed in the ammonia separator which operates at a typical temperature of -5 °C. Traces of carbon dioxide and water in the incoming gas are removed with the product ammonia. Topsoe have calculated that with their synthesis conditions, the saving in energy obtained by the alternative system of molecular sieves for carbon dioxide and water removal is small and does not justify either the extra investment or the risk of dust carryover from the molecular sieves. 

A comparison of membrane separation versus cryogenic separation in a typical large ammonia plant was made by Schendel et al. (1983). For the case study described in this paper, the operating conditions were modified to increase the methane content at the entrance to the synthesis loop to about double that allowed without hydrogen recovery. The pertinent process variables for the IS MMsefd feed stream are summarized in Table 15-5. To prevent densiflcation of the membrane or formation of an insoluble phase in the cryogenic system, the ammonia in the feed to both systems is reduced to very low levels in a water scrubber. To prevent formation of a solid phase in the cryogenic unit, molecular sieves are used to remove the water picked up in the scrubber. 

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