Synthesis Loop Operation

A commercially available ammonia synthesis catalyst is usually supplied with the iron phase in the form of magnetite, which first must be reduced to metallic iron before the catalyst is used. The reduction time is typically from three to five days, although the actual time required is dependent on the plant design and on limitations of equipment, such as the start-up heater. The general principles of reduction are outlined below. More detailed information to suit a specific plant can be obtained from catalyst suppliers. The principal factors governing a plant reduction are the water content of the circulating gas, the gas flowrate, the reduction pressure, and the reduction temperature. 

The water content of the gas leaving the catalyst bed during reduction influences the final activity of the reduced catalyst. In general, the lower the water concentration the better. In practice, an exit water concentration of 5000 ppm is a good operating limit and 10,000 ppm should be regarded as a maximum. In a multibed converter, the beds must be reduced in sequence to avoid the reduction water re-oxidizing catalyst that has already been reduced. 

Good gas distribution is important during reduction as uneven flows can lead to uneven reduction of the catalyst. This can result in local high concentrations of water and high local temperatures when the synthesis reaction starts. This could lead to some loss of activity. 

Uneven distribution can be overcome by increasing the flowrate through the converter. 

As far as the catalyst is concerned, the reduction pressure is not critical. Higher pressure increases the rate of the ammonia reaction and hence increases the evolution of heat once the first part of the catalyst is reduced. This heat liberation enables the flowrate to be increased as explained above. On the other hand, a high rate of ammonia production can make temperature control more difficult. The practical solution to these conflicting effects is to perform the reduction at a pressure rather lower than the normal operating pressure, for example, using a reduction pressure of 100 bar in a loop with a normal operating pressure of 150 bar. 

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Gas leaving the C02 removal plant is methanated, cooled, dried and fed to an ammonia synthesis loop operating at 80 to 110 bar. 

Methanol Synthesis. AH commercial methanol processes employ a synthesis loop, and Figure 6 shows a typical example as part of the overall process flow sheet. This configuration overcomes equiUbtium conversion limitations at typical catalyst operating conditions as shown in Figure 1. A recycle system that gives high overall conversions is feasible because product methanol and water can be removed from the loop by condensation. 

Provided that stable and active catalysts are available, water contents ranging between 2.5-5 wt % can be obtained while operating with reasonable (1.2-1.5) H / CO ratios in the synthesis loop. 

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 first application in 1992 used a two-bed, hot-wall KAAP reactor that featured a low pressure drop and radial flow. Because of the KAAP catalyst s high activity, thin beds are necessary to keep operating temperatures within the desired range203. In 2002 the KAAP reactor had evolved to a four-bed design. A magnetite catalyst is used in the first bed of the synthesis loop when the ammonia concentration is below 2% of the feed. Then the ruthenium catalyst is used in the next three beds to bring the ammonia level up to 18% or more215. 

After final cooling, the synthesis gas is compressed (7) and sent to the synthesis loop. The loop can operate at pressures between 70 to 100 bar. The converter design does impact the loop pressure, with radial-flow designs enabling low loop pressure even at the largest plant size. Low loop pressure reduces the total energy requirements for the process. 

The synthesis loop comprises a circulator (8) and the converter operates around 200°C to 270°C, depending on the converter type. Reaction heat from the loop is recovered as steam, and is used directly as process steam for the reformer. 

A purge is taken from the synthesis loop to remove inerts (nitrogen, methane), as well as surplus hydrogen associated with non-stoichiometric operation. The purge is used as fuel for the reformer. Crude methanol from the separator contains water, as well as traces of ethanol and other... 

Methanol synthesis section. The synthesis loop is comprised of a circulator combined with compressor (6), "MRF-Z" reactor (7), feed/effluent heat exchanger (8), methanol condenser (9) and separator (10). Currently, MRF-Z reactor is the only reactor in the world capable of producing 5,000-6,000 t/d methanol in a single-reactor vessel. The operation pressure is 5-10 MPa. The syngas enters the MRF-Z reactor (7) at 220-240°C and leaves at 260-280°C normally. 

II) A typical synthesis loop will consist of a circulator (3), the methanol converter (4), heat recovery and coolers and a methanol separator (5). The methanol synthesis catalyst is copper-based and works at pressures between 50 to 100 bar and temperatures between 200°C and 290°C. Larger plants have an operating pressure range of 80 to 100 bar. 

Unconverted methane present in the refonmng effluent behaves in the successive operations like an inert diluent To prevent its buiid-up in the recycle, which constitutes the methanol synthesis loop", a purge is necessary. 

Ammonia-Casale, which proposes a technology that remodels the processes with carbamate decomposition by gas stripping the SRR (Split Reaction Recycle) process. This involves adding to the synthesis loop in place a side reaction section operating at 20 to 22.10 Pa absolute, with an N/C ratio of 4 to 5, making it possible to reach the... 

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. 

With increasing pressure, ammonia formation increases (Fig. 78). This results not only from the more favorable equilibrium situation for the reaction, but also from the effect on the reaction rate itself. In industrial practice, there are plants that operate at about 8 MPa (80 bar), but there are also those that operate at more than 40 MPa (400 bar). Today, plants are built mainly for synthesis pressures of 150-250 bar. Typical operating parameters for modern synthesis loops with different pressures are listed in Table 34. 

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. 

The previous sections mainly considered the individual process steps involved in the production of ammonia and the progress made in recent years. The way in which these process components are combined with respect to mass and energy flow has a major influence on efficiency and reliability. Apart from the feedstock, many of the differences between various commercial ammonia processes lie in the way in which the process elements are integrated. Formerly the term ammonia technology referred mostly to ammonia synthesis technology (catalyst, converters, and synthesis loop), whereas today it is interpreted as the complete series of industrial operations leading from the primary feedstock to the final product ammonia. 

The earlier plants operated at deficit, and needed an auxiliary boiler, which was integrated in the flue gas duct. Auxiliary burners in tunnels or flue gas duet were additionally used in some instances. This situation was partially caused by inadequate waste heat recovery and low efficiency in some energy consumers. Typically, the furnace flue gas was discharged in the stack at rather high temperature because there was no air preheating and too much of the reaction heat in the synthesis loop was rejected to the cooling media (water or air). In addition, efficiency of the mechanical drivers was low and the heat demand for regenerating the solvent from the C02 removal unit (at... 

ICI AMV Process. The ICI AMV process [1034], [1083], [1111] - [1122], also operates with reduced primary reforming (steam/carbon ratio 2.8) and a surplus of process air in the secondary reformer, which has a methane leakage of around 1 %. The nitrogen surplus is allowed to enter the synthesis loop, which operates at the very low pressure of 90 bar with an unusually large catalyst volume, the catalyst being a cobalt-enhanced version of the classical iron catalyst. The prototype was commissioned 1985 at Nitrogen Products (formerly CIL) in Canada, followed by additional plants in China. A flow sheet is shown in Figure 110. 

Synthesis operates at 82 bar in a proprietary tubular converter loaded with a cobalt-enhanced formulation of the classical iron catalyst. Purge gas is recycled to the PSA unit, and pure COz is recovered from the PSA waste gas by an aMDEA wash. Very little steam (60 bar) is generated in the synthesis loop and from waste gases and some natural gas in the utility boiler in the utility section, and all drivers are electric. 

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