Ammonia synthesis loop pressure

In the early days the synthesis gas was produced at atmospheric pressure, and the synthesis gas was compressed in reciprocating compressors to pressures as high as 100 MPa in some cases. Capacities were limited to around 300 - 400 MTPD due to limitations in reciprocating compressors. However, with the development of steam reformer based front-ends and the introduction of centrifugal compressors, the ammonia plant capacities suddenly increased to 1000 MTPD with ammonia synthesis loop pressures typically around 15 MPa. Since the 1960 s new developments have been in the ammonia converter designs, such as introduction of radial flow converters and introduction of converters with multiple catalyst beds to increase ammonia conversion. 

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. 

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... 

Application To produce ammonia from natural gas, LNG, LPG or naphtha. Other hydrocarbons—coal, oil, residues or methanol purge gas— are possible feedstocks with an adapted front-end. The process uses conventional steam reforming synthesis gas generation (front-end) and a medium-pressure (MP) ammonia synthesis loop. It is optimized with respect to low energy consumption and maximum reliability. The largest single-train plant built by Uhde with a conventional synthesis has a nameplate capacity of 2,000 metric tons per day (mtpd). For higher capacities refer to Uhde Dual Pressure Process. 

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. 

Application The ICIAMV process produces ammonia from hydrocarbon feedstocks. The AMV process concept offers excellent energy efficiency together with simplicity and reduced capital cost for plant capacities between 1,000 tpd and 1,750 tpd. Key features include reduced primary reformer duty, low-pressure synthesis loop and hydrogen recovery at synthesis loop pressure. 

Krupp Udhe Ammonia Natural gas, LPG, LPG, naphtha Process uses conventional steam reforming synthesis gas generation and a medium pressure ammonia synthesis loop 15 NA... 

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. 

Table VII presents the estimated investments. The methanol case investment reflects the same gasifier type as used for the IBG and SNG cases. A conceptual Chem Systems methanol synthesis step is used. EPRI is sponsoring the development of the Chem Systems technology (5 ). The ammonia case investment reflects the same wood gasification concepts, employs pressure swing adsorption for hydrogen gas purification (based on information provided by the Linde Division, Union Carbide Corporation), and uses a conventional high-pressure ammonia synthesis loop.

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... 

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 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 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. 

Figure 8.17 shows that with increasing particle size, the ammonia concentration at the outlet decreased. For example, when the particle size increases from 0.6-0.9 mm to 4.0-6.7mm, the ammonia concentration at the outlet at 15.0 MPa and 400°C decreases from 23.42% to 19.43%, representing a 17.0% reduction in ammonia concentration. At a low pressure of 7.0 MPa, the net value of ammonia is not so high, and the percentage of decrease in ammonia concentration is especially considerable. Taking the bed resistance and other factors into consideration, low pressme ammonia synthesis loop should as far as possible use smaller catalyst particles. 

The quality of the recovery heat is related to the temperature or pressure of the steam. In ammonia synthesis loop, the reaction heat is forepart applied to heat the feed water of waste heat boiler. With the use of turbocompressor, the reaction heat is commonly used to get high pressure steam as power to drive it, and use in rundle as shown in Fig. 9.26. 

Figure 7.4. High-pressure ammonia synthesis loop.

The corrosion phenomena causing the most problems in ammonia synthesis loop equipment are hydrogen attack, nitriding, and stress corrosion. A survey of the types of corrosion is given in [618, 619]. Steel for service at different hydrogen partial pressures and temperature may be chosen on the basis of the so-called Nelson-Curves [620, 621]. A survey of materials problems in ammonia plants is given in [622]. 

M. W. Kellogg has also suggested a process (the KAAP process) based on a new catalyst [928, 949]. This technology has been used in a revamp project where a new reactor was installed downstream of the existing reactor in an ammonia synthesis loop (see Sect. 6.4.3.4). In addition, more radically new process schemes deviating from the traditional route have been described. An example is a scheme based on so-called parallel reforming [30,32,120,121] and a low pressure loop with ammonia recovery by water absorption [769]. None of these new developments have been implemented in practice. 

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. 

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