Arsenic permanent poison

The catalyst is poisoned by CO, C02, and H20 so they must be rigorously removed upstream in the hydrogen synthesis process. Oxygen molecules are permanent poisons. Other poisons such as sulfur, arsenic, halides, and phosphorous must be carefully removed upstream in as much as they too are permanent poisons. 

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. 

The catalyst is sensitive to sulfur and arsenic poisoning (the Utter being a permanent poison). Natural gas must, therefore be desulfurized. Carbon and coke deposits also damage the catalyst and must be removed by steam or by burning off with air. 

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]. 

Notes. (1) For elementary students, it is sufficient to weigh out accurately about 1.25 g of arsenic(III) oxide, dissolve this in 50 mL of a cool 20 per cent solution of sodium hydroxide, and make up to 250 mL in a graduated flask. Shake well. Measure 25.0 mL of this solution by means of a burette and not with a pipette (caution — the solution is highly poisonous) into a 500 mL conical flask, add 100 mL water, 10 mL pure concentrated hydrochloric acid, one drop potassium iodide solution, and titrate with the permanganate solution to the first permanent pink colour as detailed above. Repeat with two other 25 mL portions of the solution. Successive titrations should agree within 0.1 mL. 

Realgar is employed as a depilatory in tanning, its red colour being a desirable feature in the treatment of hides and skins. It has been used as a paint pigment under the name of arsenic orange, but it is not too permanent and is now seldom used. Its early use as a remedy for asthma, etc., has been mentioned (p. 23). Unless carefully prepared, the commercial product is liable to contain white arsenic, and the poisonous nature of the latter tends to limit the applications of realgar. 

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. 

The steam reforming catalysts are very sensitive to some impurities in the feedstock such as sulfur, arsenic, halogens, phosphorous and lead etc., even with very low contents. Generally, sulfur content is required to be below 0.5 ml m . Halogen such as chlorine, poisoning role is similar to sulfur, has the same limited content. Arsenic poisoning is permanent and irreversible. Thus, the restriction for arsenic is very strict. The steam reforming catalysts must be replaced when they are seriously poisoned by arsenic. 

Permanent deactivation can occur when the catalyst is poisoned by arsenic or flumine ot damaged due to exposure to moisture or high gas temperatures. 

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