Temporary poisons oxygen compounds

Its content of oxygen compounds, which represent temporary poisons, must satisfy certain limitations, such as O, CO2 < 1 ppm CO, H1O, CO CO2 < 2 ppm. The requisite purity levei is reached by repeating the liquid nitrogen scrubbing or methanation treatment, as required. Moisture is then removed by drying or by cryogenics. 

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. 

Temporary Poisoning of the Ammonia Catalyst by Oxygen-Containing Compounds... 

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. 

If there is no poison in the synthesis gas, the poison will be removed from the surface and the catalyst will regain its activity. An oxygenic compound reacts in that manner with the catalyst surface [1]. Unsaturated hydrocarbons may also react as a reversible poison [1], but usually oxygenic compounds are the most important, temporary poisons for ammonia synthesis catalysts. 

Although the effect of an oxygenic compound is temporary in the sense that the catalyst regains its activity when the poison is removed from the gas stream, a severe poisoning for an extended period of time may result in a permanent loss of activity, probably due to recrystallization, see [1] and [11]. 

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