Ammonia synthesis plant

Preferably, an ammonia plant is constructed at a geographical location where plenty of energy (e.g. as methane) and water are available, and where easy transport of the ammonia by ship is feasible. An ammonia plant is shown in Fig. 8.20, which produces roughly 2x1350 tons ammonia per day over 2x150 tons of catalyst. The facility is visually dominated by the two steam reforming plants, which are easily recognized. 

Oxygen-containing molecules cannot be tolerated in the ammonia synthesis, primarily because they form iron oxide that blocks the active surface. First the CO2 is removed, through a scrubber, by reaction with a strong base. The remaining CO (and CO2) is then removed by the methanation reaction, converting the CO into methane and water. Finally the water is removed by, for example, molecular sieves. Methane does not present problems because it interacts weakly with the catalyst surface. The gas mixture (Tab. 8.6) is compressed to the roughly 200 bar needed for ammonia synthesis and admitted to the reactor. 

The gas reacts over the ammonia catalyst in an exothermic process at 450-500 °C, leading to an exit concentration of ammonia of about 15-19%. The ammonia is extracted by condensation and the unreacted gas recycled to the reactor. A fraction is purged to prevent the accumulation of inert components. The ammonia condensation is not complete, meaning that the real inlet gas of the reactor already contains several percent of ammonia. 

---

Urea is produced from liquid NH and gaseous CO2 at high, pressure and temperature both reactants are obtained from an ammonia-synthesis plant. The latter is a by-product stream, vented from the CO2 removal section of the ammonia-synthesis plant. The two feed components are deUvered to the high pressure urea reactor, usually at a mol ratio >2.5 1. Depending on the feed mol ratio, more or less carbamate is converted to urea and water per pass through the reactor. 

Ammonia from coal gasification has been used for fertilizer production at Sasol since the beginning of operations in 1955. In 1964 a dedicated coal-based ammonia synthesis plant was brought on stream. This plant has now been deactivated, and is being replaced with a new faciUty with three times the production capacity. Nitric acid is produced by oxidation and is converted with additional ammonia into ammonium nitrate fertilizers. The products are marketed either as a Hquid or in a soHd form known as Limestone Ammonium Nitrate. Also, two types of explosives are produced from ammonium nitrate. The first is a mixture of fuel oil and porous ammonium nitrate granules. The second type is produced by emulsifying small droplets of ammonium nitrate solution in oil. 

Ammonium nitrate is manufactured by reacting ammonia with nitric acid. Consider the process shown by Fig. 9.19. First, namral gas is reformed and converted into hydrogen, nitrogen and carbon dioxide. Hydrogen and nitrogen are separated an fed to the ammonia synthesis plant. A fraction of the produced ammonia is employed in nitric acid formation. Ammonia is first oxidized with compressed air then absorbed in water to form nitric acid. Finally nitric acid is reacted with anunonia to oduce ammonium nitrate. 

Comparable to IGT technology for syngas generation is the high-temperature Winkler (HTW) gasification process. A commercial HTW unit was installed in 1988 at a Finnish ammonia synthesis plant and has operated successfully using peat as a feedstock.51... 

BYAS [Bypass ammonia synthesis] An economical process for expanding existing ammonia synthesis plants by introducing the additional natural gas at an intermediate stage in the process. The additional nitrogen in the air, which has also to be introduced, is removed by PSA. Developed and offered by Humphreys and Glasgow, UK. 

The present treatment of the conventional process will be based on the process diagram presented in Figure 1, which represents a steam reformer coupled with an ammonia synthesis plant [6], This is one of the two cases, which were considered in the project. The other was the use of the produced hydrogen as fuel in combined cycle gas turbines. In this chapter, the steam reforming part will be treated only, but some comments on the ammonia plant will be made, in view of the composition of the product stream leaving the steam reformer. 

A design for an ammonia-synthesis plant includes a step that takes ammonia vapor at I and 300(psia) and changes its state to saturated liquid ammonia at 14.71(psia). For ammonia ... 

The BYAS (Bypass Ammonia Synthesis) process can be used for economical expansion of existing ammonia synthesis plants. It is described in Reference 1. 

The most important part of an ammonia synthesis plant is the pressure reactor, which is filled with catalyst and in which ammonia formation takes place at temperatures between 400 and 500°C. A maximum temperature of 530°C must not be exceeded, otherwise catalyst damage will ensue. 

Different reactor types are used in the individual ammonia-synthesis plants. They have in common, that the catalyst mass is to be found in a separate container inside the reactor chamber. Between the catalyst holder and the reactor wall there is a gap through which the cold synthesis gas can be fed in, such that the reactor wall is not heated to the same temperature as the catalyst holder. 

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. 

FIGURE 11.2 Outline of the main components of an ammonia synthesis plant using reforming and secondary reforming as the principal sources of hydrogen. Electrolysis of water is used to supplement this. 

Integration of HWP with the ammonia synthesis plant Purified synthesis gas after the pressure adjustment is sent to HWP, which acts as a by-pass to the ammonia synthesis unit. ... 

Mazingarbe HWP. The Mazingarbe plant was started in 1968. The basic design described above was followed. The final concentration of 99.8% was achieved by ammonia distillation. The Mazingarbe plant was shut down in 1972 because of an explosion in the ammonia synthesis plant. 

The Talcher HWP is coupled with a 900Mg/day ammonia synthesis plant of the Fertilizer Corporation of India to produce 62.5 Mg D20/yr. After purification, syngas enters the enrichment section, which consists of a three-stage cascade. The first stage is superimposed by two cold- and hot-stripping columns. Each stage consists of one pair of cold and hot columns. Deuterium concentration in ammonia is high... 

This route requires considerable separation equipment to remove the water and by-product higher alcohols formed during the reaction. Virtually all plants using this process have been replaced by newer technology. Du Font s first methanol plant was based on a by-product carbon monoxide stream from an existing ammonia synthesis plant ... 

Urea is produced commercially by reacting NH3 and C02 at high pressure and temperature. The process is usually near an ammonia synthesis plant which produces both reactants. The two-step process is shown in Reactions 3.9 and 3.10 [5]. 

The equilibrium constant for the third, deuterium exchange, reaction is around 2 at the temperature at which the second, water-gas shift, reaction is carried out. Because an excess of water is used to convert CO completely to COj, the deuterium content of hydrogen will be less than that of the methane and water fed, unless the excess water is fully recycled. Because water recycle is usually not practiced at ammonia synthesis plants, the deuterium content of synthesis gas at operating plants is sometimes as low as 0.009 percent [M7]. If the ammonia plant were specifically designed for deuterium recovery from its synthesis gas, the deuterium content could be increased to the average of the methane and water feeds by recycling aU water and preventing losses. 

Clark uses that report to argue that at the early stages of development of a process a plant is best run under manual control - though with a large number of instruments, as was done with the ammonia synthesis plant - until the behaviour of the plant becomes understood and then automatic control can be used. 

TABLE 1.1 Typical exit syngas compositions (%) of the ammonia synthesis plant and auto-thermal reforming for H2 production... 

Then, the significant improvement in the CO conversion was obtained by introducing multi-stage water-gas shift reactor in the ammonia synthesis plant. The first stage is characterized by working at higher temperatures, favouring... 

---