Industrial 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. 

Fig. 22.9. Schematic flow diagrams of typical ammonia synthesis loops.74 (Courtesy of Wiley-VCH. Bakemeier, H., Huberich, , et al. "Ammonia" in Ullmann s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A 2, VCH Verlagsgesellschaft, Weinheim 1985, pp. 143-242.

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. 

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 industrial scale reaction of synthesis gas to ammonia in pressure reactors takes place in a cyclic process in which the ammonia formed is removed from the reaction gas and the unreacted synthesis gas returned to the reactor. In addition to the ammonia formed, inert gases and the liberated reaction heat have to be continuously removed from the cyclic process. The excess heat of the product gas is used to heat the feed synthesis gas to the reaction temperature in a heat exchanger integrated into the reactor. Additional waste heat can be utilized for steam generation. The pressure loss in the synthesis gas due to its passage through the synthesis loop is compensated for and the fraction of synthesis gas converted replaced by fresh compressed synthesis gas ( fresh gas ). 

The conversion to ammonia is limited by thermodynamics. Since the gas volume decreases with reaction, very high pressures must be used in order to push equilibrium to the ammonia side. Although the reaction is exothermic, and therefore conversion increases with decreasing temperature, the reaction temperature must be high so as to achieve industrially acceptable reaction rates. Due to the application of more active catalysts, most modem synthesis loops nowadays are operated at 100-300 bar and 450-500°C, with equilibrium conversion in the order of 10-15%. 

The reduction program in an industrial converter, besides the above-mentioned common basic principles, should be considered synthetically according to the structure of reactor, catalyst type and their properties, processes and equipments of synthesis loop and their actual operation conditions. But the basic principle is consistent with the requirements of a catalyst. Therefore, the reduction of different catalysts has different methods and procedures. The scheme of reduction should be designed according to the scheme supplied by R D researcher and manufacturer of the catalyst, and combined with the actual conditions of the plant. 

The adverse effect caused by a high concentration of water has also been demonstrated on an industrial scale. In a partial oxidation plant with a liquid nitrogen wash, about 100 ppm of oxygen leaked into the synthesis loop, giving an [0] content in the inlet gas of about 20-30 ppm. This resulted in an immediate activity decline. The pressure was increased as well as the temperature. However, the production declined somewhat. After some months of operation, the oxygen contaminant was removed and the catalyst regained its former activity. 

The external-loop slurry airlift reactor was used in a pilot plant (3000 t/a) for one-step synthesis of dimethyl ether (DME) from syngas. Specially designed internals were used to intensify mass transfer and heat removal. This new technology is highly efficient and easy to scale-up to industrial. 

With the hydrogen-rich feed typical of the recycling loop in an industrial plant for the low temperature methanol synthesis, only methanol was observed. Appreciable productivities of H.M.A. were obtained for H /CO ratios 2, with the maximum for every alcohol progressively displacec towards the lower values of the H /CO ratio when che chain length increases. / t the same time a li-... 

Fluidized bed reactors (FBRs) are chemical reactors in which (catalytic) particles interact with a gas stream that is fed from the bottom, such that the mixture (emulsion phase) behaves as a fluid. This type of reactors is often used in the chemical and process industries, where they have gained their popularity due to their excellent heat and mass transfer characteristics. FBRs are used for instance for gas-phase polymerization reactions for polyolefin production (polyethylene, polypropylene), chemical looping combustion or reforming processes, and gas-phase Fischer—Tropsch synthesis. 

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