Badanie procesu nieodwracalnej dezaktywacji stopowych katalizatorów żelazowych do syntezy amoniaku

• Autorzy: Narkiewicz U.

Warianty tytułu

EN

Study of the irreversible deactivation process of fused iron catalysts for ammonia synthesis

• Języki publikacji: PL

Abstrakty

EN

The process of irreversible (chemical and thermal) deactivation of fused iron catalyst has been studied. Laboratory simulation of the poisoning process under industrial conditions has been carried out. The process may take place either during manufacture, transport, manipulation and storage (catalyst precursor - oxidised form), or during ammonia synthesis (active catalyst reduced form). In the latter case, the deactivating effect can be observed directly after the introduction of poison, while the same effect for the oxidised catalyst requires a long work period. This difference is because the catalyst in its active form adsorbs the poison from the gas phase directly on its active surface. In the case of the catalyst precursor, the poison may form compounds with promoters, sometimes of exceptional stability (e.g. calcium sulphate), which remain in the intergranular space and decompose only to a minor degree during reduction of the catalyst. As a result, the diffusion of poison to the active surface is very slow. It may thus be concluded that traces of poisons in raw materials used for preparation of the catalyst, or contamination during improper transport or charging of the catalyst, may lead to gradual loss of catalytic activity, albeit at a much slower pace than in the case of poisoning of the active catalyst from the gas phase. The effect of three poisons: sulphur, chlorine and phosphorus, on the activity of the catalyst has been compared. It was found that these non-metals are very strong poisons of the iron catalyst, with even minor quantities causing a marked and irreversible loss of activity. The deactivating effects of sulphur and chlorine are similar, and phosphorus is less potent. The process of poisoning continues with two phases: during the first (low poison concentration) significant loss of catalytic activity occurs with increasing poison concentration; during the second (high poison concentration) increasing poison concentrations have little effect on activity and even at very high concentrations some catalytic activity is still observed. Measurements of catalytic activity of the poisoned catalyst have been interpreted using data from a model system (modified surface of monocrystalline iron sample) under ultra-high vacuum conditions. A method for estimating the number of adsorption sites in defects of the monocrystalline sample based on measurements of segregation kinetics has been developed. A mechanism of permanent deactivation of the iron catalyst for ammonia synthesis has been proposed, explaining chemical deactivation at low poison concentrations by simple blocking of active sites, and at higher poison concentrations by reconstruction of the catalyst surface. In the latter case, segregation of poison atoms from the bulk grains or from the intergranular space to the active surface becomes more prominent. Interactions between poison atoms and oxygen atoms (occupying part of the adsorption sites and responsible for thermal stability of the catalyst), as well as between poison atoms and potassium atoms become increasingly important. leading to reconstruction of the surface and loss of catalytic activity. Deactivation is greater for higher poison concentrations and for more negative Gibbs free energy of poison segregation. Thermal deactivation of the iron catalyst is caused by sintering of iron crystallites and elimination of potassium from the active surface. A method for partial regeneration of the iron catalyst has been proposed, based on the addition of potassium into the catalyst bed. Regeneration may be effected through impregnation of the passivated catalyst in the ammonia synthesis reactor using a solution of potassium hydroxide. Before impregnation it is necessary to remove the most poisoned part of the catalyst bed (near the gas inlet where the majority of poisons is adsorbed) and to sieve the catalyst before recharging, in order to avoid excessive pressure drop on the catalyst bed.

Słowa kluczowe

PL

katalizator żelazowy; synteza amoniaku; dezaktywacja

• Wydawca: Wydawnictwo Uczelniane Politechniki Szczecińskiej
• Czasopismo: Prace Naukowe Politechniki Szczecińskiej. Instytut Technologii Nieorganicznej
• Rocznik: 2000
• Tom: Nr 554(6)
• Strony: 9--148
• Opis fizyczny: Bibliogr. 150 poz., rys.

Twórcy

autor

Narkiewicz U.