Ammonia catalyst poisons Sulfur

Raw Fuel Cleaning - Removal of sulfur, halides, and ammonia to prevent fuel processor and fuel cell catalysts poisoning. 

We can readily understand these setbacks today if we consider the high sensitivity of iron as an ammonia catalyst toward numerous catalyst poisons. In those early years, this fact was unknown to us. Specifically, no one suspected the harm which is done to the catalyst by substances such as sulfur and sulfur compounds. Even Haber had never discussed a catalyst poisoning by impurities, because he had been able, apparently to avoid the presence of catalyst poisons in his small scale experiments. 

Other substances decrease or annihilate, even in traces, the catalytic properties of iron. Such catalyst poisons had already been known as a nuisance in the catalytic oxidation of sulfur dioxide. With the ammonia catalysis several elements, particularly sulfur proved to be harmful, even in amounts of 4oo of one per cent. Chlorine, phosphorus and arsenic showed a similar behavior (30), just as certain metals, such as lead, tin and zinc. 

Poisoning of iron catalysts during ammonia synthesis by sulfur compounds has received relatively little attention (154, 240-244). Nevertheless, the previous work provides information on the poisoning mechanism and interesting examples of how oxide promoters may influence the sulfur poisoning behavior of a catalytic metal. 

With regard to the sulfur bound on the catalyst surface, differences exist between the various types of ammonia catalysts, especially between those that contain alkali and alkaline earth metals and those that are free of them. Nonpromoted iron and catalysts activated only with alumina chemisorb S2N2 and thiophene. When treated with concentrations that lie below the equilibrium for the FeS bond, a maximum of 0.5 mg of sulfur per m2 of inner surface or free iron surface is found this corresponds to monomolecular coverage [382], [383], The monolayer is also preserved on reduction with hydrogen at 620 °C, whereas FeS formed by treatment above 300 °C with high H2S concentrations is reducible as far as the monolayer. For total poisoning, 0.16-0.25 mg S/m2 is sufficient. Like oxygen, sulfur promotes recrystallization of the primary iron particle. 

The main drawback in the catalytic hydrogenolysis of Z-protected peptide derivatives is caused by the presence of sulfur-containing amino acid residues (Cys, Pen, or Met) due to catalyst poisoning. Attempts to overcome these restrictions by addition of either tertiary basesf or boron trifluoride-diethyl ether complext to the hydrogenation mixture were of limited usefulness. More efficient appears to be the use of liquid ammonia (at —33°C) as the solvent to prevent poisoning of the Pd/C catalyst.f °l... 

These catalysts are extremely sensitive to catalyst poisons, which reduce chemisorption of hydrogen and nitrogen on the active surfaces of the catalyst and thereby reduce its activity. Gaseous oxygen-, sulfur-, phosphorus-and chlorine compounds, such as water, carbon monoxide, carbon dioxide, the latter being reduced to water under ammonia synthesis conditions, are particularly troublesome in this regard. Catalyst poisoned with oxide compounds can be reactivated by reduction with pure synthesis gas. 

Enders and Hundertmark [9] recently reported that debenzylation of 5 with calcium in liquid ammonia gives the corresponding alcohol 6 in quantitative yield [10]. Similar debenzylation of 7 produces 8 in 83% yield. The use of calcium, in contrast with the more reactive lithium, circumvents reduction of the existing phenyl ring. They also found that the calcium method is far more reliable than palladium-catalyzed hydrogenolysis, which is very sensihve to catalyst poisoning by traces of sulfur and tin by-product. 

Sulfur and chlorides (and other catalyst poisons) can enter the ammonia plant in the steam or in the air to the secondary reformer thus, precautions should be taken to eliminate such impurities insofar as is practical. In addition, a layer of guard absorbent may be placed on top of the catalyst, particularly in the case of the low temperature shift catalyst. 

During the manufacture of the catalyst, small amounts of impurities such as silicon, titanium, sulfur, phosphorus and chlorine can be inevitably introduced into the system. These impurities are catalyst poisons. Hence, the total content of impurities in catalysts should be limited in an allowable range, e.g. the content of sulfur must be less than 0.01%, phosphorus less than 0.04%, and chlorine less than (5 — 10) x 10 in the ammonia synthesis catalyst according to Chinese standard. 

Hydrocarbons that can be fed to ammonia plants include natural gas, associated gas, liquid petroleum gas, and naphthas boiling up to 220 = C. Higher hydrocarbons are not used in primary steam reforming because it would lead to coke formation on the catalysts. Hydrocarbons are usually contaminated with variable quantities of different sulfur compotmds and often contain chlorides. These catalyst poisons must be removed before the other catalysts in the plant can operate in a satisfactory manner. 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

Figure 8.3.1 is a typical process diagram for tlie production of ammonia by steam reforming. Tlie first step in tlie preparation of tlie synthesis gas is desulfurization of the hydrocarbon feed. Tliis is necessary because sulfur poisons tlie nickel catalyst (albeit reversibly) in tlie reformers, even at very low concentrations. Steam reforming of hydrocarbon feedstock is carried out in tlie priiiiiiry and secondary reformers. 

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