Poisons of ammonia synthesis

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. 

P. E. H0jlund Nielsen, The Poisoning of Ammonia Synthesis Catalysts by Oxygenic Compounds Lecture given at ACS Meeting, Washington D.C. (Sept. 1983). 

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. 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

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. 

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... 

Contradictory data on the kinetics of ammonia synthesis, especially in the earlier literature, in some circumstances may reflect a lack of attention to the influence of impurities in the gas. If oxygen compounds are present in the synthesis gas, reversible poisoning of the adsorbing areas, in accordance with an equilibrium depending on the temperature and the water vapor-hydrogen partial pressure ratio, must be taken into account when developing rate equations (see also Section 3.6.1.5). 

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. 

It may be noted that catalyst poisons which owe their toxic properties to the presence of a multiple bond, in general lose this toxicity if they pass into saturated derivatives during hydrogenation. Thus, in the purification of ammonia synthesis gases in which the hydrogen contains small concentrations of carbon monoxide, the gas is sometimes, before its passage to the main catalyst chamber, passed through a precatalyst by means of which the traces of carbon monoxide are converted into nontoxic... 

The make-up gas is added to the loop upstream of the last chiller, where most of the ammonia has already been condensed. Traces of carbon dioxide and water vapour are removed by co-condensation in the last chiller so that the risk of poisoning of the synthesis catalyst with these compounds is eliminated. In case of inert free make-up gas the make-up gas is mixed directly with the recirculating gas in the synthesis gas compressor. 

It was found during studies of ammonia synthesis on iron that the incorporation of a condenser downstream of the sample valve in the external circulation loop of the HPLP apparatus (Fig. 7), enabled the system to be run as a flow rather than a batch reactor. This is true for any reaction system where the reactants are more volatile than the products, since the condenser temperature can be adjusted to trap the products almost exclusively, allowing a nearly pure stream of reactants to impinge on the catalyst. In the case of ammonia synthesis, (where, next to the product, nitrogen at a partial pressure of 5 atm was the most condensable species) a slurry of isopentane (— 159.9 °C) was found to be the ideal condenser medium. During a study of rhenium-catalyzed ammonia synthesis the isopentane condenser was switched in periodically to reduce the ammonia partial pressure to below that at which it appeared to poison the catalyst. In this way, the rhenium was able to produce ammonia in excess of the amount usually leading to poisoning. 

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. 

In the process of ammonia synthesis, the common toxic compounds that lead to poisoning and loss of catalytic activity are oxygen and oxygenous compounds (CO, CO2, H2O), and non-metallic compounds such as sulfur, phosphorus, arsenic and chlorine, etc. Toxic metals may be present in the catalysts themselves, while metallic compounds are rarely present in the reaction gas. Oxygen and oxygenous compounds are reversible poisons which cause temporary poisoning but sulfur, phosphorus, arsenic, chlorine and their compounds are irreversible poisons which cause permanent poisoning. 

The most important poisons in ammonia synthesis are oxygen-containing compoiinds (confer paragraph 4.1) and -especially in older plants- sulphur compounds. In methanol synthesis the sulphur and chlorine compounds are most often responsible for poisoning in industrial plants. A discussion of ageing and poisoning mechanisms in ammonia and methanol synthesis is given in [l ... 

Sulfur is the most common of the permanent poisons. In plants based on gasification of sulfur-containing feedstocks, in particular coal, traces of carbonyl sulfide and hydrogen sulfide may reach the synthesis converter. In plants based on natural gas, the sulfur content of the feed is very low, and furthermore the copper-based low-temperature shift catalyst and the nickel-based clean-up metha-nation catalyst act as efficient guards by irreversibly absorbing these traces of sulfur. In the earlier days of ammonia synthesis, the lubrication oil for the compressors was a common source of sulfur, but today this problem is widely recognized and examples of sulfur poisoning in the industry are rare. 

A temporary poison lowers the activity of an ammonia synthesis catalyst by reversible adsorption onto the catalyst surface. Unsaturated hydrocarbons like ethylene may also react as a temporary poison according to studies by Nielsen, but generally, it is the oxygenic compounds that constitute the single most important poison for ammonia synthesis. 

Brill et examined the water poisoning of an unpromoted iron catalyst at ambient pressure, where water levels up to 95.8 ppm were applied. The activation energy of ammonia synthesis was found to be independent of the degree of poisoning, which is shown in Fig. 8.4, using results from a backmix reactor. Neither... 

An important step in heterogeneous catalysis is the adsorption and desorption of reactants and products of the reaction. Important information on the mechanism of ammonia synthesis has come from the study of the adsorption of H2, N2 and NH3. The adsorption of H2O and O2 is interesting because of the role of H2O as a poison for NH3 synthesis. 

H2S is very poisonous for NH3 synthesis [14, 18, 721, 722] and N2 adsorption [513]. The poisoning is irreversible [14, 590]. The poisonous effect is due to S surface blockage [590, 723]. The activation energy of ammonia synthesis is unchanged during H2S poisoning [723] After exposure to large partial pressures of H2S, FeS may be observed by X-ray powder diffraction [590]. 

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