Synthesis loop refrigeration

Gas recirculation in the anunonia synthesis section is necessary because only 9-30% conversion is obtained per pass over tlie catalyst." There are two tj pes of synthesis loops. One type recovers anunonia product before recycle compression. Inerts entering witli tlie makeup gas are removed with a purge stream. The ammonia is recovered by condensation, which requires refrigeration. Since airliydrous anunonia is readily available, it is normally used as tlie refrigerant. 

Synthesis gas is compressed to the synthesis pressure, typically ranging from 140 to 220 kg/cm2g and converted into ammonia in a synthesis loop using radial flow synthesis converters, either the two-bed S-200, the three-bed S-300, or the S-250 concept using an S-200 converter followed by a boiler or steam superheater, and a one-bed S-50 converter. Ammonia product is condensed and separated by refrigeration. This process layout is flexible, and each ammonia plant will be optimized for the local conditions by adjustment of various process parameters. Topsoe supplies all catalysts used in the catalytic process steps for ammonia production. 

Although ammonia condensation was already used in Haber s lab-scale ammonia plant and in early pilot plants of Bosch, the first commercial units of BASF used ammonia absorption in water to remove the product from the cool synthesis loop gas, because various technical problems were encountered with refrigeration at that time. It was only in 1926 that ammonia condensation was introduced in the Haber-Bosch... 

The loop pressure has an important influence on the performance of the ammonia synthesis loop because of its influence on the reaction equilibrium, reaction kinetics, and gas/liquid equilibrium in the product separation. Actual selection of loop pressure is in many cases a compromise between selecting a high pressure to favour the ammonia synthesis reaction, and on the other hand selecting a reasonable pressure to minimise the compression power of the synthesis gas compressor, which compresses the synthesis gas to the desired loop pressure. The loop pressure also has a significant impact on the ammonia refrigeration system, since a high loop pressure favours condensation of the ammonia product in the loop water cooler and saves compression power on the refrigeration compressor. On the other hand, a low loop pressure saves compression power on the synthesis gas compressor, but increases the... 

The final energy input is power for the three gas-compression duties synthesis gas compression, synthesis loop circulation, and ammonia refrigeration. The power required for these duties depends, for example, on synthesis pressure and on converter design. Refrigeration power is strongly dependent on ambient temperature. However, typical figures for a modern 1000 ton per day low-pressure... 

Furthermore, motive power is required for compression of feed streams and make-up gas for the synthesis loop, for recovery and refrigeration of product ammonia, and for certain other purposes, mainly pumps and blowers. 

Synthesis at about 125 kg/cm g in a dry synthesis loop using two converters, the first a two-bed quench-cooled converter, the second a one-bed converter. Absorption refrigeration for ammonia recovery. Cryogenic pure gas recovery. 

Synthesis in an inert-free, dry synthesis loop at 350 kg/cm g without refrigeration. 

As the synthesis loops became simplified, and were operated at lower pressures, it became necessary to cool the converter exit gas with air or water, and then to incorporate a refrigeration stage using liquid ammonia. The system operated with two separators and more product ammonia was removed from the... 

When produced from natural gas the synthesis gas will be impure, containing up to 5 per cent inerts, mainly methane and argon. The reaction equilibrium and rate are favoured by high pressure. The conversion is low, about 15 per cent and so, after removal of the ammonia produced, the gas is recycled to the converter inlet. A typical process would consist of a converter (reactor) operating at 350 bar a refrigerated system to condense out the ammonia product from the recycle loop and compressors to compress the feed and recycle gas. A purge is taken from the recycle loop to keep the inert concentration in the recycle gas at an acceptable level. 

The ammonia loop is based on the Ammonia Casale axial-radial three-bed converter with internal heat exchangers. Heat from the ammonia synthesis is used to 1) generate high-pressure steam and 2) preheat feed gas. The gas is then cooled and refrigerated to separate ammonia product. Unconverted gas is recycled to the syngas compressor208 214... 

The separator temperature, together with operating pressure and location of make-up gas addition point, determines the ammonia concentration at the converter inlet. A low temperature means a low ammonia concentration at the converter inlet, which again means either a low catalyst volume or high conversion. But low separator temperature can only be obtained by increased power consumption and cost of the refrigeration units. In the end the selection of loop pressure as well as the choice of separator temperature is a compromise between energy consumption and capital costs in various parts of the synthesis unit. 

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