Recycle loop, ammonia synthesis

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. 

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. 

As an example, consider ammonia synthesis. In an ammonia synthesis loop, hydrogen and nitrogen are reacted to ammonia. The reactor effluent is partially condensed to separate ammonia as a liquid. Unreacted gaseous hydrogen and nitrogen are recycled to the reactor. A purge on the... 

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. 

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... 

Inerts and excess nitrogen from the ammonia synthesis loop are removed by a purge from the circulator delivery and treated in a hydrogen recovery unit. Recovered hydrogen is recycled to the circulator suction. 

Recovery of ammonia by water scrubbing offers the advantage of achieving a very low residual ammonia content, but the drawback is that the whole recycle gas has to be dried afterwards and in addition distillation of aqueous ammonia is necessary to yield liquid ammonia. Nevertheless the scrubbing route was again proposed for a synthesis loop to be operated under extremely low pressure (around 40 bar) [938]. Snam Progetti [280], [939], [940] has proposed removing the ammonia from the loop gas at ambient temperature down to 0.5 mol% by absorption in dilute aqueous ammonia. 

Gas recirculation in the ammonia synthesis section is necessary because only 9-30% conversion is obtained per pass over tlie catalyst, There are two tj jes of synthesis loops. One type reeovers 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, Sinee anliydrous ammonia is readily available, it is normally used as tlie refrigerant. 

Since the ammonia reaction is limited by equilibrium with a conversion per pass of reactants of 25 - 35% as mentioned earlier, the synthesis of ammonia takes place in a recycle loop. 

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. 

Compression of synthesis gas. The synthesis gas composed by hydrogen and nitrogen is compressed to required pressures, usually 10-30 MPa, by piston-type or centrifugal compressors. During ammonia synthesis, single-pass conversion is only 10%-20%, and therefore, most of the synthesis gas must be recycled, compressed and returned to the synthesis loop again. 

In both cases only partial conversion can be obtained at realistic conditions, and in order to approach full conversion a recycle system is used. The detailed lay-out is, of course, different in different cases. As illustrations, the lay-out of Haldor Tops0e Low Energy Ammonia Synthesis Loop is shown schematically in Fig. 1 and of a Haldor Tops0e Low Energy Methanol Synthesis Loop in Fig. 2. 

Synthesis gas from the purifier is compressed to a pressure in the range 140 to 200 bar before addition to the synthesis loop. In the synthesis loop, recycle gas is mixed with fresh synthesis gas before the final compressor wheel (recycle wheel) and, because this gas is pure, it can be fed directly to the converter. Gas leaving the converter is cooled, ammonia is condensed and separated, and the remaining gas is recycled to the converter. A small purge stream is necessary to control the level of argon in the loop. This purge gas is scrubbed with water to recover ammonia and recycled to the purifier. 

The synthesis of ammonia takes place in a recycle loop which always contains... 

The Isobaric Double-Recycle (IDR) urea process was developed by Montedison. In this process most of the unconverted material leaving the reactor is separated by heating and stripping at synthesis pressure using two strippers in series. The unconverted ammonia, C02, and carbamate in the urea solution are recycled to the synthesis loop.108110... 

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