Sulphur poisoning

The stock solution of quinoline-sulphur poison is prepared by refluxing I g. of sulphur with 6 g. of quinoline for 5 hours and diluting the resulting brown liquid to 70 nJ. with xylene which has been purified by distilling over anhydrous aluminium chloride. The addition of the quinoline - sulphur poison ensures that the reduction does not proceed beyond the aldehyde stage it merely slows up the reaction and has no harmful effects. 

In order to enhance the durability of the NSR catalyst, sulphur poisoning should be suppressed by the acceleration of sulphur desorption. To achieve this, the acidity of the... 

If the gasifier product stream is intended for downstream use as the feedstock for further upgrading such as methanation, methanol or Fischer Tropsch synthesis, very thorough desulphuri-sation is essential since the catalysts in these upgrading processes are highly sensitive to sulphur poisoning. The methanation catalysts normally cannot tolerate more than 0.05 ppm of sulphur in the feedstock. In addition to H2S sulphur values in the gasifier product it may contain COS, CS2, mercaptans and thiophenes. These are normally removed by activated carbon or zinc oxide filters ahead of the sensitive synthesis catalyst beds. 

For use in the Rosenmund reduction (Expt 6.120) the catalyst is moderated by the addition of the appropriate quantity of a quinoline-sulphur poison prepared in the following manner. Heat under reflux 1 g of sulphur with 6g of quinoline for 5 hours and dilute the resulting brown liquid to 70 ml with xylene which has been purified by distillation over anhydrous aluminium chloride. Thiourea (about 20% by weight of the palladium-barium sulphate catalyst) may also be used as a catalyst poison. 

Bureau of Standards, Washington. Oudar has also reviewed work on sulphur poisoning of single crystal surfaces.124... 

The deactivation of a Fischer-Tropsch precipitated iron catalyst has been investigated by means of a novel reactor study. After use of the catalyst in a single or dual pilot plant reactor, sections of the catalyst were transferred to microreactors for further activity studies. Microreactor activity studies revealed maximum activity for catalyst fractions removed from the region situated 20 - 30% from the top of the pilot plant reactor. Catalyst characterization by means of elemental analyses, XRD, surface area and pore size measurements revealed that (1 deactivation of the catalyst in the top 25% of the catalyst bed was mainly due to sulphur poisoning (2) deactivation of the catalyst in the middle and lower portions of the catalyst bed was due to catalyst sintering and conversion of the iron to Fe304, Both these latter phenomena were due to the action of water produced in the Fischer-Tropsch reaction. 

The effect of sulphur on the activity profile is shown in Figure 2. The sulphur poisoning arises from sulphur impurities found in the synthesis gas, which is derived from coal. Sulphur is a common F-T catalyst poison [2,11] and it is apparent from the results that the top portion of the reactor is acting as a guard bed" to remove this impurity. The finding that sulphur is only found to be present in the top section of the catalyst bed has been observed previously for F-T and related catalysts [12-15]. 

Sulphur poisoning of nickel catalysts in catalytic hot gas cleaning conditions of biomass gasification... 

Sulphur-poisoning studies of nickel catalysts have been performed with fixed-bed tube reactors at 800 - 950 °C under 1 - 20 bar total pressure with real and simulated gasification gas mixtures containing H2S (50 - 1000 ppmv). The effect of sulphur concentration of the gas and the operation conditions will be presented and discussed in this paper. 

In naphtha reforming, hydrotreatment is always applied to protect the platinum-containing catalyst against sulphur poisoning. The specification of sulphur content for the feed for the reforming unit is less than 1 ppm. 

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. 

Rostrup-Nielsen found that the intrinsic reaction rate, rj, for methane steam reforming is correlated with the sulphur coverage by equation 6 (2). In the adiabatic prereformer, the sulphur acts as a pore mouth poison and as the reactions are restricted by pore diffusion 2,8), the effective activity of the sulphur poisoned catalyst pellet can be described by an empirical relation, equation 7, between the effective pellet reaction rate, rp, and the average sulphur coverage, 0av (7/... 

Gum formation, which is a polymerization of hydrocarbons (especially aromatic compounds) on the catalyst surface, is a deactivation phenomenon that takes place at low temperature. Therefore, an investigation of the appearance of gum on steam reforming catalysts used at prereforming conditions is very relevant. Deactivation by gum formation can proceed several times faster than ordinary sulphur poisoning. 

Figure 6. Intrinsic reaction rate in dependence of sulphur poisoning for a) normal catalysts ( ), b) gum deactivated catalysts (A). The dotted line represents the ideal relation for the average intrinsic activity of a normal shell poisoned catalyst, 0 = 0.9. Steam reforming of ethane, H2O/C = 4.0, P = 1 bar. Activities normalized with activity of unpoisoned catalyst.

The understanding of the interaction of S with bimetallic surfaces is a critical issue in two important areas of heterogeneous catalysis. On one hand, hydrocarbon reforming catalysts that combine noble and late-transition metals are very sensitive to sulphur poisoning [6,7]. For commercial reasons, there is a clear need to increase the lifetime of this type of catalysts. On the other hand. Mo- and W-based bimetallic catalysts are frequently used for hydrodesulphurization (HDS) processes in oil refineries [4,5,7,8]. In order to improve the quality of fuels and oil-derived feedstocks there is a general desire to enhance the activity of HDS catalysts. These facts have motivated many studies investigating the adsorption of S on well-defined bimetallic surfaces prepared by the deposition of a metal (Co, Ni, Cu, Ag, Au, Zn, A1 or Sn) onto a single-crystal face of anodier metal (Mo, Ru, Pt, W or Re) [9-29]. 

In the previous section we have discussed several cases in which bimetallic bonding increases the overall reactivity of a system towards sulphur. If the opposite occurs, such a phenomenon could be useful for the prevention of sulphur poisoning. In practical terms, the idea is to find bimetallic systems that have a good catalytic activity and are less sensitive to the presence of sulphur-containing molecules in the feedstream than pure metals. Sn/Pt and Pd/Rh satisfy these requirements [26-29]. 

Cu, Ag and Sn are added to Pt catalysts as site blockers to improve their selectivity for the reforming of hydrocarbons [4,31,33,64-66]. In this respect the effects of the admetals are more or less similar. From the trends discussed above, it is clear that tin is a much better choice than Cu or Ag when trying to minimize the sensitivity of Pt reforming catalysts toward sulphur poisoning. 

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