Industrial, ammonia synthesis plants, 253

Knowledge of the reaction kinetics is important for designing industrial ammonia synthesis reactors, for determining the optimal operating conditions, and for computer control of ammonia plants. This means predicting the technical dependence on operating variables of the rate of formation of ammonia in an integral catalyst volume element of a converter. 

Industrially urea is only produced from ammonia and carbon dioxide. Since carbon dioxide is a byproduct in the production of hydrogen for use in the synthesis of ammonia from natural gas or crude oil (in the case of natural gas only in 90% of the required amount), a urea plant is often coupled with an ammonia synthesis plant. 

An outstanding example is the sunflower seed. The industrial ammonia synthesis process mentioned under Example 0.1 is still technologically too primitive in comparison to the millimetersized sunflower seed. This seed, after being planted and exposed to sufficient nutrients and solar energy, establishes an oil factory, a cellulose factory, and a paint factory within six months. It has several process control units, but the most explicit one makes the sunflower follow the sun. It is the chemist/chemical engineer s dream to emulate this unique feature of the sunflower and be able to synthesize on demand chemicals, as much as, when and where needed. As we lamented earlier, it is still a dream. 

Ammonia synthesis is one of the most important processes operated by the chemical industry. Modern ammonia synthesis plants can produce up to 1800 tons of ammonia per day. Clearly, the design of the large reactors requires powerful and reliable calculation methods and the availability of a sound kinetic equation is an essential requirement. It is not only necessary for reactor design, but is also required to express catalyst performances generated from either laboratory or pilot plant experiments. 

Figure 1.1(a) shows four of the five practically-identical ammonia synthesis plants at the Donaldsonville, LA plant of CF Industries. Each of these produces 1500 tons/day of ammonia, for use in fertilizer. These five plants produce a total of about 5 billion pounds per year of ammonia, equivalent to 16 pounds per year for each person in the United States. That fertilizer contributes in a major way to the abundance, variety, and low cost of food in the United States. Such plants are vitally important to the human race. They produce the fixed nitrogen used in fertilizers throughout the world. Roughly 80 pounds of a variety of fertilizers are produced per year for each person on earth. If we lost this supply of synthetic fertilizers and then all stopped eating meat, we would be able to feed about 80% of the world s... 

Industries. (Courtesy of CF Industries.) (b) Very simplified flow diagram of an ammonia synthesis plant. Only the synthesis section is discussed in the text. The feed preparation section is more complex and expensive than the synthesis section. The seemingly illogical placement of the chiller and separator so that they process the fresh feed plus the recycle is dictated by the fact that some feed impurities dissolve in the hquid ammonia, and thus are prevented from entering the reactor. The reactor converts only about 15% of the feed on each pass [2]. 

The industrial catalyst is prepared by fusion. The catalyst may be supplied in the unreduced state after crushing and screening to the desired particle size or the catalyst may be reduced and subsequently stabilized by controlled oxidation in the catalyst factory. Although the reduced catalyst is pyrophoric, the prereduced catalyst can be safely handled. In the ammonia synthesis plant the catalyst is activated by reduction with a mixture of hydrogen and nitrogen as the final step in the start-up procedure for the plant. The reduction of the prereduced catalyst is faster and simpler than the start-up of the unreduced catalyst. 

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 ammonia-hydrogen plant requires a large source of ammonia or hydrogen. Synthesis gas, used for producing ammonia on an industrial scale, is an ideal source. But this limits the production capacity of HWP to about 100 Mg D20/yr corresponding to 1500 Mg NH3/day in the ammonia plant. 

To date, the largest capacity plants have utilized pipe reactors due to the relatively straightforward design of the pressure envelope. Large, high-pressure vessel reactors exist in industry for ammonia synthesis (2.4 to 3 m diameter x 18... 

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