The Ammonia Synthesis Loop

A liquid nitrogen wash used to remove inerts from the make-up gas which then contained less than 240 ppm methane was incorporated into some of the early plants. However, it was found that the inerts were preferentially dissolved in the product auunonia and they did not accumulate in the recycle gas in 

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 

Notes 1. Make up gas equivalent to methanator outlet gas containing 5 ppm carbon oxides. 2. Purge gas is used for hydrodesulfurization and primary reformer fuel. 

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If the makeup gas to the ammonia synthesis loop is absolutely free of catalyst poisons, such as H2O and C02, it can flow directly to the ammonia synthesis converter. This leads to the most favorable arrangement from a minimum energy point of view. This can be accomplished by allowing the gas that leaves the methanation step to pass through beds of molecular sieves to remove water and traces of C02 74... 

Ammonia synthesis is normally carried out at a pressure higher than that for synthesis gas preparation. Therefore the purified synthesis gas that is fed to the ammonia synthesis loop must be compressed to a higher pressure. Synthesis loop pressures employed industrially range from 8 to 45 MPa (80 to 450 bar). However, the great majority of ammonia plants have synthesis loops that operate in the range of 15 to 25 MPa (150 to 250 bar)74. 

The gas that leaves the PSA unit is methanated, cooled and dried. The dried gas enters the ammonia synthesis loop at the circulator suction. In the synthesis loop, gas from the circulator is heated and passed over low pressure ammonia synthesis catalyst to produce ammonia17. 

Assuming the feedstock is methane, which is the major component of natural gas, the theoretical feed requirement would be equivalent to one-fourth of the potential hydrogen production or 16,713 SCF CH /ST NH3(15.2 MM BTU/ST). However, the actual process consumes on the order of 22,420 SCF CHi+/ST NH3 or about 20.4 MM BTU/ST NH3 (LHV). The required quantity of feed depends on the process design criteria chosen for the methane conversion in the reforming section, the efficiency of CO conversion, degree of CO2 removal and the inerts (CHi+ + Ar) level maintained in the ammonia synthesis loop. Thus, the potential hydrogen conversion efficiency of the feedstock in the steam reforming process is about 75%. Table 3 shows where the balance of the feed is consumed or lost from the process. 

PSA would be used as a front-end purifier for the synthesis of ammonia. The PSA system would produce, in a single purification step, an amnonia synthesis gas, free of the usual argon and methane inerts and, as a further advantage, would eliminate the need for the purge stream from the ammonia synthesis loop. 

Before the synthesis gas enters the ammonia synthesis loop, essentially all of the oxygen compounds must be completely removed to (l) avoid poisoning the ammonia synthesis catalyst and (2) keep C02 from forming carbamates and ammonium carbonate in the synthesis loop. It is also advantageous to remove the inert gases (methane, argon, etc.) to achieve a higher synthesis conversion per pass.74... 

Ammonia synthesis is normally carried out at a pressure that is higher than that for synthesis gas preparation. Therefore the purified synthesis gas to the ammonia synthesis loop must be compressed to a higher pressure.74... 

The ammonia synthesis loop uses two ammonia converters with three catalyst beds. Waste heat is used for steam generation downstream the second and third bed. Waste-heat steam generators with integrated boiler feedwater preheater are supplied with a special cooled tubesheet to minimize skin temperatures and material stresses. The converters themselves have radial catalyst beds with standard small grain iron catalyst. The radial flow concept minimizes pressure drop in the... 

The ammonia synthesis loop consists of two stages. Makeup gas is compressed in a two-stage inter-cooled compressor, which is the low-pressure casing of the syngas compressor. Discharge pressure of this compressor is about 110 bar. An indirectly cooled once-through converter at this location produces one third of the total ammonia. Effluent from this converter is cooled and the major part of the ammonia produced is separated from the gas. 

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. 

The potential for ruthenium to displace iron in new plants (several projects are in progress [398] of which two 1850 mtpd plants in Trinidad now have been successfully commissioned [1488]) will depend on whether the benefits of its use are sufficient to compensate the higher costs. In common with the iron catalyst it will also be poisoned by oxygen compounds. Even with some further potential improvements it seems unlikely to reach an activity level which is sufficiently high at low temperature to allow operation of the ammonia synthesis loop at the pressure level of the synthesis gas generation. 

In the second step, the remaining syngas is compressed to the operating pressure of the ammonia synthesis loop (approx. 210 bar) in the HP casing of the syngas compressor. This HP casing operates at a much lower than usual temperature. The high synthesis loop pressure... 

Basically two different layouts of the ammonia synthesis loop exist, depending upon the quality of the make-up synthesis gas feeding the loop. In most cases the make-up gas also contains (apart from H2 and N2 in proper ratios) some inerts (CH4, Ar and minor traces of rare gases), which have to be purged from the loop to avoid build-up of inerts. (Inerts containing loop layout). This layout is primarily used in plants where the synthesis gas is generated from reformer based frond-ends with primary and secondary reforming. 

Generally speaking the ammonia synthesis loop will contain the following main elements ... 

The heart of the ammonia synthesis loop is the ammonia converter. The ammonia converter normally comprises a basket (internals) enclosed in a pressure shell. In the ammonia synthesis converter(s), due to the exothermal nature of the reaction taking place, it is normally not possible to go from the inlet to, the outlet conditions in the reactor in one adiabatic step, if a reasonable conversion is desired. Therefore, some type of cooling is required. In principle, the cooling can be applied in three different ways ... 

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 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... 

As Fig.2 and Table 1 show, increasing the flow rate of Ammonia produced by 66.6% resulted in a decrease in total plant Carbon Footprint (= 2.8 - 6.6%) considering all scenarios. Carbon Footprint of each unit increased with increased production rate except for the Ammonia Synthesis loop where this decrease occurred. A total decrease of ( 3.8 - 6%) of plant Carbon Footprint occurred when using sweet gas instead of sour gas that has to be treated first. Moreover, the utilization of HP and LP steam reduces the overall Carbon Footprint (= 12.5 - 15%). LP steam is utilized in boilers of coliimns in both Acid Gas Sweetening and CO2 Removal units. 

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...

Methanol can be produced as co-product in the ammonia synthesis [239] [406]. The CO and CO2 for the methanol synthesis is obtained by a partial bypass of the shift converters and the CO2 wash. The methanol synthesis takes place in an adiabatic reactor or a simple once-through boiling water reactor and the unconverted S5mgas is passed through a methanator back to the ammonia synthesis loop. 

This low level of inerts coupled with the low level of oxygen-containing poisons influences the design of the ammonia synthesis loop. 

With natural gas as the raw material, plants depend on steam reforming to produce hydrogen and carbon monoxide. Air is bled into a secondary reformer to supply nitrogen. The carbon monoxide is then shift-reacted with steam to produce hydrogen and carbon dioxide. The carbon dioxide is removed, and the last traces of carbon monoxide and carbon dioxide are converted to methane, which is not a poison in the ammonia synthesis step. The purified hydrogen and nitrogen are compressed and fed to the ammonia synthesis loop. 

Innovations for the ammonia synthesis loop include the recovery of argon and hydrogen. 

The product gas from steam reforming, catalytic partial oxidation, partial oxidation, or gasification processes contains in all cases significant amounts of carbon monoxide and carbon dioxide. Since the feed gas for the ammonia synthesis loop must be completely free of these compounds, they must be removed in the gas preparation part of the plant. Carbon monoxide is removed in a conversion step by the so-called water-gas shift reaction or just the shift reaction, in some cases followed by further conversion by selective oxidation. 

The conditions in the ammonia synthesis loop can be described by a number of parameters, which may each be in some cases independent variables, and in other cases a function of other parameters. Important parameters are ... 

The main revamp options of the ammonia synthesis loop are the installation of molecular sieves for drying of the make-up gas, installation of a purge gas hydrogen recovery system, and improvements in the ammonia synthesis converter system. 

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