Sulphur compounds, poison

Other catalysts are poisoned and inactivated by sulphur compounds. 

The allylic ion (XI) is probably too stable to propagate the reaction. The inhibiting (poisoning) effect of oxygen, nitrogen, and sulphur compounds is also due to formation of stable ions (in so far as it is not due to sequestration of metal halide by complex formation), e.g., the reaction [98] ... 

In selective poisoning or selective inhibition, a poison retards the rate of one catalysed reaction more than that of another or it may retard only one of the reactions. For example, there are poisons which retard the hydrogenation of olefins much more than the hydrogenation of acetylenes or dienes. Also, traces of sulphur compounds appear selectively to inhibit hydro-genolysis of hydrocarbons during catalytic reforming. 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

Sulphur compounds in the fuel (0.05-0.01 %) must be removed because they would poison the reformer catalyst. Sulphur compounds are removed by using a hydrodesulphurization catalyst (HDS), typically noble metals on alumina, to convert the organic sulphur to hydrogen sulphide. Then a zinc oxide adsorbent bed is used to trap the hydrogen sulphide ... 

Balali-Mood, M., Navaeian, A. (1986). Clinical and paraclinical findings in 233 patients with sulphur mustard poisoning. In Proceedings of the Second World Congress on New Compounds in Biological and Chemical Warfare, Vol. 1 (B. Heyndrickx, ed.), pp. 464-73. Rijksuniversiteit, Ghent, Belgium. 

Sulphur Poisoning. Sulphur is the most common poison for steam reforming catalysts. Sulphur is a natural component of all hydrocarbon feedstocks, but the sulphur contents of the feed is reduced to a few ppb by hydro-desulphurization followed by absorption over zinc oxide. The remaining sulphur, normally below the analytical detection limit, will slowly poison the catalyst (7). The mechanisms of sulphur poisoning are described in detail in the literature 2,8), The sulphur compounds are chemisorbed dissociatively on the nickel surface equation 4. 

Three different Cr-Co spinels were prepared and tested as catalysts for the oxidation of methane in the presence of SO2, a typical catalyst poison. The spinels were prepared from nitrate precursors using a co-precipitation method, followed by calcining at three different temperatures, (400, 600 and 800 °C) for 5 hours. Characterisation results indicate that the catalyst calcined at 800 C presents a structure of pure spinel, whereas the presence of single oxides is observed in the catalyst calcined at 600 C, and the catalysts calcined at 400 C presents a very complex structure (probably a mixture of several single and binary oxides). Experiments show an important influence of calcining temperature on the catalyst performance. In absence of SO2, catalysts calcined at 400"C and 600 C performs similarly, whereas the activity of the catalysts calcined at 800 C is worse. When sulphur compounds were added to the feed, catalyst calcined at 600"C deactivated faster than the other two catalysts. 

The effect of catalysts based on noble metals on the aromatics hydrogenation have been well documented in literature. These catalysts are very sensitive to poisoning by very small amounts of sulphur compounds in the feedstocks [6 ]. As the sulphur level tolerable by such catalysts ranges from 1.5 ppm to 600 ppm, a deep HDS has to be performed. 

The reactor feed consisted of 5000 ppmV methane in N-50 synthetic air (Air Liquid). To study the poisonous effect of sulphur compounds, 40 ppmV of SO2 were added to the feed in some experiments. Gas flow rate (1 NL/min) was controlled by a mass flow controller (Brooks 5850 TR), and the exhaust gas was analysed by gas chromatography (Hewlett Packard HP 5890 Series II). CO was not detected in any experiment. Methane and SO2 concentrations have been selected because they are values representative of industrial emissions, such as coke oven emissions. 

Depending on the purity of the gas feedstock, there may be pre-treatment of natural gas to remove impurities such as sulphur compounds that will poison catalysts used in subsequent processes. There are three common technologies in use to convert natural gas into syngas steam methane reforming (SMR), partial oxidation (POX) and auto-thermal reforming (ATR). 

One of the advantages of a TFBR is the ease with which the product separates from the catalyst the liquid waxes simply flow out of the catalyst bed. Also, if the reactor becomes contaminated with sulphur compounds, only the part of the catalyst bed closest to the reactant entrance will be de-activated, whereas with the SPR then, all the catalyst will be susceptible to poisoning. This disadvantage can be overcome by ensuring that sulphur removal processes for the syngas feedstock are wholly efficient. However, in contrast, there are a number of advantages of SPRs over TFBRs, which are as follows ... 

Gas Phase Poisons. The effectiveness was examined of four sulphur compounds, thiophene, hydrogen sulphide, sulphur dioxide and carbonyl sulphide, as gas phase poisons. 

This procedure cannot be recommended for the analysis of samples that contain sulphurous compounds and dienes with conjugated bonds, as these compounds poison the catalyst. 

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. 

Poisoning of sulphur compounds Pd-coated membranes could rapidly be destroyed after exposure to a gas stream containing hydrogen sulphide and the poisoning effects are irreversible [43,44]. Palladium becomes palladium sulphide, whose lattice constant is twice than of pure Pd and, thus, the structural stress leads to the formation of cracks. 

As in the case of homogeneous catalysis, poisons can also lead to deactivation of heterogeneous catalysts. Soluble or volatile metal or nitrogen compounds can destroy acid sites, while carbon monoxide and sulphur compounds almost invariably poison nickel and noble metal hydrogenation catalysts by bonding strongly with surface metal atoms. These considerations often lead to the selection of less active, but more poison-resistant, catalysts for industrial use. 

Fish raw material is primarily the whole fish, although fish oil is also extracted from the waste material from fish markets and canning factories. The enzymic breakdown of fish tissue is even more rapid than from the viscera of animals and has more pronounced effects on the oil and its downstream processing. Fish oils are highly unsaturated and are therefore more liable to oxidative breakdown. Virtually all fish oil for human food is hydrogenated and therefore it is particularly important that oxidation products and sulphur compounds, both of which are hydrogenation catalyst poisons. 

Another method for determination of the nickel area is based on chemisorption of hydrogen sulphide [389], Hydrogen sulphide is the stable sulphur compound at conditions for tubular reforming. It is the most severe poison for the reaction (refer to Section 5.4). The adsorption of hydrogen sulphide on nickel is rapid even below room temperature [376] [381]. At temperatures of industrial interest hydrogen sulphide is chemisorbed dissociatively on nickel. 

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 ... 

Resistance to poisons in the fuel. Most raw hydrocarbon fuels that may be used in MCFC systems (including natural gas) contain impurities (e.g. sulphur compounds) that are harmful for both the MCFC anode and the reforming catalyst The tolerance of most reforming catalysts to sulphur is very low, typically in the parts per billion (ppb) range. 

Natural gas and petroleum liquids contain organic sulphur compounds that normally have to be removed before any further fuel processing can be carried out. Even if suphur levels in fuels are below 0.2 ppm, some deactivation of steam reforming catalysts can occur. Shift catalysts are even more intolerant to sulphur (Farrauto, 2001), and to ensure adequate lifetimes of fuel processors the desulphurisation step is very important. Even if the fuel processor catalysts were tolerant to some sulphur, it has been shown that levels of only 1 ppb are enough to permanently poison a PEM anode catalyst. 

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