Phosphorus, permanent poison

The make-up gas must be free of sulfur, arsenic and phosphorus compounds, as well as chlorine and. in general halogenated derivatives which constitute permanent poisons. 

Steam reforming catalysts are poisoned by sulfur, arsenic, chlorine, phosphorus, copper and lead. Poisoning results in catalyst deactivation however, sulfur poisoning is often reversible. Reactivation can be achieved by removing sulfur from the feed and steaming the catalyst. Arsenic is a permanent poison therefore, feed should contain no more than 50 ppm of arsenic to prevent permanent catalyst deactivation by arsenic poisoning 13]. 

Sulphur compounds, halides, phosphorus and arsenic are permanent poisons to ammonia catalysts (22). However, in most plants the upstream low-temperature shift catalyst and the Ni-based methanation catalyst both serve as efficient guards by irreversibly adsorbing traces of such compounds. Thus, permanent poisons are normally not a severe problem. 

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. 

Compounds of sulfur, chlorine, phosphorus and arsenic. The compounds of sulfur, chlorine, arsenic and phosphorus will cause permanent poisoning of catalysts. The sulfides in the synthesis gas are commonly larger than the compounds of phosphorus, chlorine and arsenic, and the toxic effect of sulfides to catalysts is more serious than that of phosphorus, chlorine and arsenic. The following discussion will focus on the poisoning effect of sulfur and its relation to the life of catalyst. 

In general, any compound that can lower the surface tension of iron may be considered as a poison [9]. Phosphorus and arsenic compounds are also known as permanent poisons [1]. However, in the natural gas-based plants they are rare and only found in exceptional cases. A catalyst prepared with 1-3 wt% P2O5 was examined and found to be severely poisoned [10]. Chlorine [1] is a serious poison that reacts with the potassium promoter and forms potassium chloride which is slightly volatile and consequently removed from the catalyst surface. It is expected that other halogens behave similarly [1]. Calculations show that ppb amounts in the synthesis gas should result in a marked deactivation [2]. 

Electronic promoters, for example, the alkali oxides, enhance the specific activity ofiron-alnmina catalysts. However, they rednce the inner snrface or lower the thermal stability and the resistance to oxygen-containing catalyst poisons. Promoter oxides that are rednced to the metal during the activation process, and form an alloy with the iron, are a special group in which cobalt is an example that is in industrial use. Oxygen-containing compounds such as H2O, CO, CO2, and O2 only temporarily poison the iron catalysts in low concentrations. Sulfur, phosphorus, arsenic, and chlorine compounds poison the catalyst permanently. 

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