Methanation sulfur poisoning

These tests demonstrated that the Lurgi Rectisol process provides an extremely pure synthesis gas which can be charged directly to the metha-nation plant without problems of sulfur poisoning of the nickel catalyst. However, in order to cope with a sudden sulfur breakthrough from Rectisol as a result of maloperation, a commercial methanation plant should be operated with a ZnO emergency catchpot on line. 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

Alifanti, M Auer, R Kirchnerova, J Thyrion, F Grange, P Delmon, B. Activity in methane combustion and sensitivity to sulfur poisoning of LauxCexMni.yCoyOs perovskite oxides. Appl. Catal, B Environmental, 2003, Volmne 41, Issues 1-2, 71-81. 

In spite of its obvious practical importance, sulfur poisoning has received only moderate attention in the literature. In fact, the most recent comprehensive review of the literature dealing with poisoning of metals was by Maxted in 1951 (/). More recently, Madon and Shaw (2) reviewed the pre-1970 literature describing the effects of sulfur in Fischer-Tropsch synthesis and methanation. 

Sulfur poisoning is a key problem in hydrocarbon synthesis from coal-derived synthesis gas. The most important hydrocarbon synthesis reactions include methanation, Fischer-Tropsch synthesis, and methanol synthesis, which occur typically on nickel, iron, or cobalt, and ZnO-Cu catalysts, respectively. Madon and Shaw (2) reviewed much of the early work dealing with effects of sulfur in Fischer-Tropsch synthesis. Only the most important conclusions of their review will be summarized here. 

Based on the results of Dalla Betta and co-workers, it is clear that the steady-state activity of a completely sulfur-poisoned Ni or Ru methanation catalyst is 102-104 times lower than that of the fresh catalyst. However, a typical industrial methanation process would more probably involve a catalyst only partly poisoned by sulfur. Bartholomew and co-workers (23, 113, 157) attempted to assess how sulfur poisoning of only a portion of the catalyst would affect its activity/selectivity properties in fixed-bed and fluidized-bed reactors. Data in Table XII show the effects on specific activity and product distribution of partially presulfided Co/A1203 and Ni/Al203 catalysts in a fixed bed. Catalysts were presulfided with 10 ppm H2S at 725 K, and reaction was carried out with sulfur-free feedgas. Corresponding data are listed in Table XIII for catalysts partially presulfided and then studied in a fluidized-bed reactor under the same conditions. The decrease in H2 uptake... 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

Gravimetric (Adsorption and Desorption) Measurements of Surface Species Formed during Methanation on Fresh and Sulfur-Poisoned 14/ Ni/Al203 at 675 K°... 

Rostrup-Nielsen and Pedersen (209) recently studied sulfur poisoning of supported nickel catalysts in both methanation and Boudouard reactions by means of gravimetric and differential packed-bed reactor experiments. In their gravimetric experiments a synthesis mixture (H2/CO/He = 5/7/3) containing 1-2 ppm H2S was passed over a catalyst pellet of 13% Ni/Al203-MgO at 673 K and 1 atm. The rates of Boudouard and methanation reactions were determined from weight increases and exit methane concentrations respectively. In the presence of 2 ppm H2S a factor of 20 decrease was observed in both methanation and Boudouard rates over a period of 30-60 min. However, the selectivity or ratio of the rates for Boudouard and methanation reactions was constant with time. From these results the authors concluded that the methanation and Boudouard reactions involve the same intermediate, carbon, and that sulfur blocks the sites for the formation of this intermediate. 

The transient methanation activity behavior of Ni, Co, Fe, and Ru during in situ sulfur poisoning was more or less identical in all cases. Hence, only the results for Ni will be discussed in detail those for Co, Fe, and Ru will be discussed only where significant differences were observed. 

That the activation energies and rate concentration dependences for methanation over sulfur-poisoned Ni and Ru are the same as those observed under sulfur-free conditions (99-100,140,147,209) provides further evidence that sulfur poisoning of Ni and Ru involves geometric blockage of active metal sites. However, the observations that (i) the activation energies for methanation over sulfur-poisoned and carbon-deactivated Co are both 50 kJ/mol lower than that for fresh Co and (ii) the rate dependence on CO partial pressure is positive order for both carbon-deactivated Co and sulfur-poisoned Co, suggest that the deactivations of Co by sulfur and carbon are similar and may involve electronic as well as geometric effects. 

Deviations from the Schulz-Flory distribution arc possible if secondary reactions such as cracking on acidic supports or insertion of product olefins into the growing chain occur [42]. It has been reported recently that the Schulz Flory constant a has a tendency to increase from C3 to C, [45]. This may be the reason why the values found are usually higher for methane and lower for Cj and C) j.)., as would be expected for an ideal Schulz-Flory distribution [40]. Investigations by Madon et at. on partly sulfur-poisoned iron/copper catalysts revealed a dual product distribution. This was explained by the assump tion of > 2 types of active sites for hydrocarbon chain formation, each with a slightly different value of the chain growth probability [46]. 

Reversibility of these poisons depends on process conditions. Sulfur-poisoning of nickel catalysts, for example, is irreversible at lower temperatures. Methanation catalysts beds cannot be regenerated even with... 

---