Ammonia synthesis catalyst poison

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Equation (341) solves the task of quantitatively describing the effect of water vapor, and also oxygen gas (the last being rapidly converted to water vapor at the conditions of the reaction), on the activity of commercial ammonia synthesis catalysts. This result is of practical importance for ascertaining the necessary degree of purity of the inlet gas mixture with respect to poisons containing oxygen (122). 

In contrast to poisoning with water vapor, the poisoning of ammonia synthesis catalysts by hydrogen sulfide is irreversible. It was studied using H2S labeled with H 5S (123) that made the radiochemical technique applicable to the determination of sulfur content in catalysts. 

If sulfur is present as H2S or COS, it is a poison for many catalysts and will partly or completely inhibit the catalyst activity46. Carbon monoxide (CO) and carbon dioxide (CO2) can poison the ammonia synthesis catalyst so both of these compounds must be removed53. 

The remaining CO, which poisons the downstream ammonia synthesis catalyst, is removed by methanation using either a Ni or Ru on A1203 catalyst at 300°C... 

Before the synthesis gas enters the ammonia synthesis loop, essentially all of the oxygen compounds must be completely removed to (l) avoid poisoning the ammonia synthesis catalyst and (2) keep C02 from forming carbamates and ammonium carbonate in the synthesis loop. It is also advantageous to remove the inert gases (methane, argon, etc.) to achieve a higher synthesis conversion per pass.74... 

After shift conversion, carbon dioxide, residnal carbon monoxide, and snlfur compounds (only present in the synthesis gas from partial oxidation) have to be removed as they are not only a useless ballast but, more importantly, can poison the ammonia synthesis catalyst. 

After this bulk removal of the carbon oxides, the typical synthesis gas still contains 0.2-0.5vol% CO and 0.005 0.2 vol% CO2. All oxygen-containing compounds have to be reduced to a very low ppm level, as they are poisonous toward the ammonia synthesis catalysts. Methanation is the simplest method to reduce the concentrations of the carbon oxides well below 10 ppm, and is widely used in steam-reforming plants. 

Ammonia synthesis catalysts consist of a-iron with small quantities of different oxides, so-called promoters, which increase the activity of the catalyst, increase its lifetime and decrease its susceptibility to poisoning. 

To protect the ammonia synthesis catalysts from premature poisoning the nitrogen-hydrogen-mixture, after the removal of the acidic components carbon dioxide and hydrogen sulfide,... 

After the water-gas diift reaction, the CO2 is scrubbed from the gas with aqueous monoethanolamine solution followed by NaOH solution. Hie purified gas is compressed from about 0.6 to 1,000 atm (15,000 psi), and residual CO, about 1.5 per cent, is catalytically hydrogenated to methane and water. Methane does not poison the ammonia synthesis catalyst, but as previously explained, it must be purged from the system with other inerts in order to maintain the optimum partial pressure of the reactants. 

The activity of the fresh catalyst is decreased by impurities in the feed gas which block active sites or coat the entire catalyst. In ammonia synthesis, reversible poisoning by oxygen, argon, and methane is known. This can be alleviated by flushing with a pure gas that is free of these components. Irreversible poisoning is caused by... 

If the sample was heated in vacuo for several hours at 973 K, large quantities of sulfur were found to segregate to the surface, presumably forming FeS II). When treated in this way, the sample was totally inactive as an ammonia synthesis catalyst, as might be expected from industrial observations that sulfur is a serious poison of the real catalyst. 

After secondary reforming it is important to reduce the amount of carbon monoxide by way of the shift reaction. A very low level is required because on modern plant any carbon monoxide entering the methanators is converted back to methane to avoid carbon monoxide poisoning of the ammonia synthesis catalyst. This back conversion (a) uses three moles of hydrogen for every mole of carbon monoxide. 

The concentrations of CO (10%-50%) are different in the synthesis gases produced from different feedstock. CO must be removed because it is a poison for ammonia synthesis catalysts. Generally, CO is converted via reaction with steam to form CO2 and H2 over a catalyst, and then CO2 is removed. The reaction between CO and steam over a catalyst is called CO shift reaction as shown in Eq. (1.16). 

In 1920s, the studies on the catalysts for ammonia sjmthesis were performed sporadically in BASF, instead, the company mainly focused on the organic synthesis under high pressm-es and the new fields in heterogeneous catalysis. Dm-ing the development of ammonia synthesis catalysts, researchers provided valuable information about the dm-ability, thermal stability, sensitivity to poisons, and in particular to the concept of promoter. Mittasch smnmarized the roles of various additives as shown in Fig. 1.9. The hypothesis of successful catalyst is multi-component system proposed by Mittasch was confirmed to be very successful. Iron-chromium catalysts for water gas shift reaction, zinc hromium catalystfor methanol synthesis, bismuth iron catalysts for ammonia oxidation and iron/zinc/alkali catalysts for coal hydrogenation were successively developed in BASF laboratories. 

During the manufacture of the catalyst, small amounts of impurities such as silicon, titanium, sulfur, phosphorus and chlorine can be inevitably introduced into the system. These impurities are catalyst poisons. Hence, the total content of impurities in catalysts should be limited in an allowable range, e.g. the content of sulfur must be less than 0.01%, phosphorus less than 0.04%, and chlorine less than (5 — 10) x 10 in the ammonia synthesis catalyst according to Chinese standard. 

For pure Fe304, at 444°C, when Kp = [H2]/[H20] = 5, Fe and Fes04 coexist. However, it is known from the experiment that at 444°C, [H2]/[H20] a 2,000, there are still measurable oxygen remained on the iron surface. It is confirmed from poisoning experiments on ammonia synthesis catalyst that a very low concentration of water vapor in synthesis gas causes the oxygen to be retained on the iron and reduces the catal3+ic activity. 

Chlorine is a serious poison for common metal catalysts including ruthenium-based and iron-based ammonia synthesis catalyst. Therefore, chlorine must be eliminated and its content is limited in the lowest by all means. The exploration of the poisoning effect of chlorine will be beneficial to further increase the activity of the catalyst. 

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. 

S.3.2.4 Poisons and the poisoning phenomena for ammonia synthesis catalysts... 

Water vapor The poisoning effect of water vapor on the ammonia synthesis catalyst is similar to that of O2. The effect of water vapor on the activity of the catalyst relates to the concentration of water vapor, the temperature and pressure as follows. 

The active a-Fe in an ammonia synthesis catalyst is easily poisoned by water. Research shows that the effect of water vapor on the catalytic activity is still reversible when the water vapor content is as high as 27mol9c or more but the activity cannot be restored when the content of water vapor reaches 42mol%. 

Carbon monoxide The amount of carbon monoxide on the catalyst that will initiate poisoning is lower than that of water vapor (Fig. 8.28). The activity of AllO catalysts begins to decline significantly when the content of carbon monoxide reaches 10 m at 370°C. With increasing temperatures, the initial content of poison for threshold poisoning increases. The CO poisoning factor for ammonia synthesis catalyst is related to the content of CO and temperature, as seen in Table 8.30 and equation (8.20). ... 

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