Methanation reaction sulfur effect

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

Based on the studies on the KD306-type sulfur-resisting methanation catalyst, the non-isothermal one-dimensional and two-dimensional reaction-diffusion models for the key-components have been established, which were solved using an orthogonal collocation method. The simulation values of the effectiveness factors for the methanation reaction ch4 and the shift reaction fco2are in fair agreement with the experimental values, which indicates that both models are able to predict intraparticle transport and reaction processes within catalyst pellets. 

These observations are all consistent with the proposed tandem-catalysis mechanism shown in Fig. 8. Central to this proposal is that the reaction proceeds via several parallel reaction steps as might be expected for formal eight-electron coupling of two methane molecules to acetic acid. The observation that the reaction is effectively stopped by high concentrations of added CO but proceeds at low levels of CO can be explained if the rate-limiting step is oxidation of Pd(0) by sulfuric acid. From independent experiments we have found that under high CO pressure... 

The production of carbon disulfide from methane and sulfur vapor can be carried out homogeneously or with a solid catalyst. Also, some solid materials act as a poison, retarding the reaction. The following data were obtained on a flow basis at a constant temperature of 625°C and with an initial reactants ratio of 1 mole of CH4 to 2 moles of sulfur vapor (considered as S2). The first set of data was obtained with the reactor empty (effective volume 67.0 ml), and the second set was obtained after packing the reactor with a granular material (7 mesh) which reduced the void volume to 35.2 ml. 

Effect of Moisture and Sulfur on Methane Decomposition Rate. The presence of small amounts of moisture and H2S (<3 v.%) in the hydrocarbon feedstock is not detrimental to the process efficiency. However, these impurities result in contamination of hydrogen with carbon oxides and H2S, which could be removed from the product gas using methanation and H2S scrubbing steps, respectively. Figure 2 demonstrates the removal of carbon oxides from hydrogen-rich gas via methanation reactions using Ru-catalyst. 

Several exothermic reactions may occur simultaneously within a methanation unit (Table 20.2). A variety of metals have been used as catalysts for the methanation reaction the most common, and to some extent the most effective methanation catalysts, appear to be nickel and ruthenium, with nickel being the most widely used (Seglin, 1975 Cusumano et al., 1978 Tucci and Thompson, 1979 Watson, 1980). The synthesis gas must be desulfurized before the methanation step since sulfur compounds will rapidly deactivate (poison) the catalysts (Cusumano et al., 1978). A problem may arise when the concentration of carbon monoxide is excessive in the stream to be methanated since large amounts of heat must be removed from the system to prevent high temperatures and deactivation of the catalyst by sintering as well as the deposition of carbon (Cusumano et al., 1978). To eliminate this problem, temperatures should be maintained below 400°C (750°F). 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

Reaction Studies. Despite the large body of work on the effect of sulfur on Ni-based and precious metal catalysts for reforming of methane, there is relatively little work on sulfur deactivation in the reforming of liquid fuels. 

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... 

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