Ammonia synthesis deactivation

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. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

The ammonia synthesis process consists of a series of catalytic reactions that aim to make a mixture of N2 and H2 without components that would deactivate the catalyst. Ammonia is formed only in the last reactor. 

Carbon monoxide, which is formed in the steam reforming reaction, deactivates the ammonia synthesis catalyst and must be removed by means of the exothermic water-gas shift reaction, which also maximises hydrogen production. To this end, CO is converted first to more easily removable CO2 ... 

Another source of deactivation is Fe contamination originating from the corrosion of upstream equipment depositing on the gauze resulting in decomposition of the NH3 to N2. Another source is from the Fe-containing ammonia synthesis catalyst. 

In order to prolong the time between regenerations and shutdowns, the reactor tube may be made longer than required for the reaction itself. For example, suppose a length of 3 ft is necessary to approach the equilibrium conversion with fresh catalyst of high activity. The reactor may be built with tubes 10 ft long. Initially, the desired conversion will be obtained in the first 3 ft. As the catalyst activity falls off, the section of the bed in which the reaction is mainly accomplished will move up the bed, until finally all 10 ft are deactivated. This technique can be used only with certain types of reactions but it has been employed successfully in the ammonia synthesis. 

Potassium is used as a dopant on catalysts for the methanation reaction and ammonia synthesis. Its purpose is to increase the rate of the reaction. Potassium is also used on the steam reforming catalyst, not as a promotor but as a dopant that inhibits catalyst deactivation by coke formation (ref. 1). It is reasonable that the role of potassium as a promotor of reaction rates is to lower some barrier to bond dissociation. Since molecular beam techniques afford a convenient means of measuring changes in barrier heights as well as in shapes of the barrier through measurements of the dissociation probability versus energy, the possible effect of potassium on the dissociation of CH4 is investigated. 

All catalysts, operated either in laboratory or conmiercially, are deactivated during their use. Deactivation is very important in commercial operation because it influences the choice of the operational conditions and fixes the cycle length between regenerations and the total life of the catalyst. Some catalysts remain active for a decade (catalysts for oxidation of SO2 and for ammonia synthesis) whereas others must be regenerated after a few minutes of operation (catalysts for fluidized bed hydrocarbon cracking). 

The majority of catalysts are subject to deactivation, e.g. to changes (deterioration) of activity with operation time. The time scale of deactivation depends on the type of process and can vary from a few seconds, as in fluid catalytic cracking (FCC), to several years, as in, for instance, ammonia synthesis. Due to the industrial importance, the modelling of deactivation was mainly developed for heterogeneous catalysis. Although the reasons for deactivation (inactivation) of homogeneous and enzymes could differ from solid catalysts, the mathematical approach can sometimes be very similar. 

Figure 7.18. Deactivation of the restructured Fe(l 10) surface occurs within 1 hr while the restructured AltO /Fe(l 10) surface maintains its activity under ammonia synthesis conditions [38].

During the course of operation, the activity of the catalyst gets reduced and it will not be able to provide the desired performance. The achvihes of a catalyst normally decrease with time. In the development of a new catalyhc process, the life of the catalyst is usually a major economic consideration. Shuthng down a process for regenerating or replacing the catalyst at frequent intervals is economically prohibitive. The rate at which the catalyst is deactivated may be very fast, such as for hydrocarbon-cracking catalysts, or may be very slow, such as for the promoted iron catalysts used for ammonia synthesis, which may remain on-stream for several years without appreciable loss of activity. 

A few days production stop because of a catalyst failure may be crucial for the plant economy. It means that secondary phenomena such as catalyst deactivation are important issues. For large-scale operation, economic arguments will limit the minimum space time yield to approximately 0.1 tonne product/m at a typical catalyst life of 5 years [289]. This corresponds to a catalyst consumption of less than 0.2 kg cat/t product. For ammonia synthesis a typical figure is 0.03 kg eat/t NHj... 

Egyhazi, J., Scholtz, J., 1983a, Simulation of the operation of the new and partially deactivated ammonia synthesis catalyst, part 2 Calculation of the concentration and temperature distributions, Hungarian chemical Soc., Hung, Vol.2,13. 

Pig. 8.22 Typical deactivation curve of ammonia synthesis catalysts ... 

Generally ammonia synthesis catalysts have a long life time of 5 to 15 years. A typical deactivation curve with time is shown in Fig. 8.22. The main reasons of deactivation are due to thermal sintering and poisoning. 

The concentration effect of poisons. There exists a range of concentration of poison that can make the catalyst inactive. This range varies with different catalysts, the chemical reactions involved and the reaction conditions. For instance, a 0.63% of sulfur adsorbed on an industrial ammonia synthesis iron catalyst will completely deactivate the catalyst. A typical cmve of the activity associated with the concentration of poison on an iron catalyst is shown in Fig. 8.24. Even if the concentration of poisons is very low, the activity of catalyst decreases linearly with the increase of the content of poisons. 

Fig. 10.15 shows the influence of potential (Uwr) catalyst on ammonia synthesis rates. This reaction demonstrates a strong electrophilic effect, i.e. negative potential can promote reaction while positive potential can envenom reaction. The catalyst will deactivate after prolonged treatment at positive potential (3 h), suggesting the existence of hysteresis phenomena. The treatment of H2 can restore the activity of catalysts. 

The clean Fe(llO), Fe(lOO), and Fe(lll) surfaces restructured with 20ton of water vapor produce the same TPD spectra as the Al O restructured surfaces. Deactivation of the (100) and (110) clean restructured iron surfaces is rapid under the conditions of ammonia synthesis, and the P2 peaks become equivalent in intensity to those on the respective clean surfaces within one hour of ammonia synthesis. 

Promoted magnetite in its reduced state, used as an ammonia synthesis catalyst, undergoes deactivation for a number of reasons. Like other high-surface area structures, it may sinter. Species present in the gas stream may chemisorp upon the catalytic surface and thereby poison it, and furthermore, access to the pore system may be blocked by deposits. These processes of sintering, poisoning, and fouling are discussed below. 

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