Modem Ammonia Plants

Up to eight different catalysts and absorbent materials are regularly used in a modem ammonia plant to produce synthesis gas, and the design of any given process is determined by the type of feedstock available. A wide range of different catalyst compositions, shapes and sizes is now available to make operation more reliable and economic. The selection of the best choice of catalysts for a given process is made more complicated by the fact that several major suppliers 

TABLE 9.2. Ammonia Synthesis Gas Process Developments since 1920. 

1940-55 Steam reforming process used in ammonia plants. Natural gas feed. Sulfur absorption on activated carbon secondary reforming high temperature shift catalysts used. Reformer-pressure increasing from atmospheric to 9bar. Plant capacity increasing from 150 tpy to 300 tpy. 

1960-63 Reforming process improvements and alkalized catalysts introduced. Use of naphtha feed. Two-stage HTS conversion lowered CO slippage to 1.0% and methanation included. Plant capacity increased from 300 tpy to 600/1000 tpy. LTS with methanation replaced need for old CO scrubbing processes. Operating pressure at least 30 atm. 

TABLE 9.3. Introduction of Ammonia Plant Catalysts and Typical Operating Life. 

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This process for production of synthetic ammonia by catalytic steam reforming of natural gas is a relatively clean process and presents no unique environmental problems. To assess the environmental impacts of a modem ammonia plant on air, water, and soil, each step in the ammonia synthesis namely, desulfurization, reforming, shift conversion, carbon dioxide removal, final purification, ammonia synthesis, and refrigeration should be examined. The sources of pollutants need to be identified and matched with cost-effective solutions for minimization/elimination by using the best available pollution control measure. 

In the shift conversion step, carbon monoxide reacts with steam to form equivalent amounts of hydrogen and carbon dioxide. Upon cooling of the effluent gas, most of the unreacted steam is condensed and separated as process condensate. Modem ammonia plants utilize a two-step, in-series shifting, carried out at high and then low temperatures to increase conversion efficiency. Use of the dual-shift conversion system lowers overall plant steam requirements, and the lower CO leakage results in reduction in plant feed requirements due to more complete conversion of CO to hydrogen. Under normal operating conditions there is no emission from the shift converters. 

Modem ammonia plants operate at 200-300 atm and around 673 K (400.°C), and the catalyst consists of small iron crystals fused into a mixture of MgO, AI2O3, and Si02. The reactant gases in stoichiometric ratio (N2 H2 =1 3) are injected into the reaction chamber and over the catalyst beds. The emerging equilibrium mixture, which contains about 35% NH3 by volume, is cooled to condense and remove the NH3 the remaining N2 and H2, which are still gaseous, are recycled into the reaction chamber. 

Hansen, J-H. B.. Storgaard, L.. and Pederson, P. S., Aspects of modem reforming technology and catalysts, Ammonia Plant Safety, 32 52-62 (1992). 

The steam turbines t5T)ically operate at an effieiency of the steam cycle slightly above 30%. One might eonsider taking advantage of the higher efficiency (45—48%) of modem air derivative gas turbines by integrating the ammonia plant with a power plant operating with steam turbines [406], However, there is rarely a need for more electric power at loeations where cheap natural gas is available. 

The simplest way to recover the product from an ammonia plant is by condensation followed by separation of liquid ammonia from the circulating gas. The optimum temperature for the separator is a function of the synthesis pressure. At high pressure, such as 300-350 bar, simple water cooling can be utilized with a separator temperature just above ambient. At lower pressures, however, the dewpoint of the converter effluent is reduced significantly and refrigeration must be used to condense the product ammonia. Typical separator temperatures in modem plants are in the range --10°C to -25 C. 

From 1940, when synthesis gas first was prodnced from natural gas rather than coal, single-stream ammonia plants were developed and the process was subject to an ongoing series of improvements. Improved catalysts based on the same natural magnetite were made as the internal structure of magnetite and the function of the promoters could be investigated with modem analytical procedures. Catalyst life with purer synthesis gas can now exceed 15 years. 

In modem, single stream ammonia plants there is little scope in the design to make significant changes to the operating conditions in any of the individual catalyst reactors. Operating conditions for the carbon monoxide conversion reaction are shown in Table 9.14. The only practical variable is operating temperature which can be slowly increased as catalyst loses activity. 

Operational Constraints and Problems. Synthetic ammonia manufacture is a mature technology and all fundamental technical problems have been solved. However, extensive know-how in the constmction and operation of the faciUties is required. Although apparendy simple in concept, these facihties are complex in practice. Some of the myriad operational parameters, such as feedstock source or quaUty, change frequendy and the plant operator has to adjust accordingly. Most modem facihties rely on computers to monitor and optimize performance on a continual basis. This situation can produce problems where industrial expertise is lacking. 

In almost all modem plants, the ammonia is recovered by condensation and at modern synthesis pressures, ammonia is usually the source of refrigeration required. In order to maintain a high partial pressure of reactants, inerts entering with the make-up gas are normally removed using a purge stream. 

The modem process for manufacturing nitric acid depends on the catalytic oxidation of NH3 over heated Pt to give NO in preference to other thermodynamically more favour products (p. 423). The reaction was first systematically studied in 1901 by W. Ostwald (Nobel Prize 1909) and by 1908 a commercial plant near Bochum. Germany, was producing 3 tonnes/day. However, significant expansion in production depended on the economical availability of synthetic ammonia by the Haber-Bosch process (p. 421). The reactions occurring, and the enthalpy changes per mole of N atoms at 25 C are ... 

Many fertilizers are based on ammonia compounds. Modem agriculture requires more nitrogen in soils than is normally replaced by the nitrogen cycle, hghtning, decaying plants and animals, and other natural means... 

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