Fischer–Tropsch Synthesis: Kinetics and Water Effect on Methane Formation over 25%Co/γ-Al₂O₃Catalyst

Abstract

The kinetics and the effect of indigenous and externally added water on methane formation during Fischer–Tropsch synthesis (FTS) was studied over Co based catalysts using a 1 L continuously stirred tank reactor (CSTR). The water cofeeding study (10% water) was conducted over a 0.27%Ru–25%Co/Al2O3 catalyst at a low CO conversion level of 19% at 220 °C in order to lessen the effect of catalyst aging during the addition of water, while the kinetic experiment was conducted over 25%Co/γ-Al2O3 at the conditions of 205–230 °C, 1.4–2.5 MPa, H2/CO = 1.0–2.5, and 3–16 (NL/gcat)/h (X CO < 60%). Indigenous and externally added water decreases methane formation by a kinetic effect. The addition of 10% water led to a decrease in the CH4 rate by 12% (3.5 → 3.0 (mmol/gcat)/h), while little catalyst deactivation was observed during water addition. Increases in indigenous water partial pressure also lowered the CH4 rate and its selectivity. Kinetic analysis was performed using a group of 220 °C data collected between 365 and 918 h when the deactivation rate was very low. An empirical CH4 kinetic model, with a water effect term (P H2O/P H2 ), (r CH4 = kP CO a P H2 b /(1 + mP H2O/P H2 )) was used to fit kinetic data. The CH4 kinetic results suggest a negative water effect on CH4 formation during FTS on the unpromoted cobalt catalyst, consistent with the water effect results. The final methane kinetics (r CH4 ) equation obtained at 220 °C over 25%Co/γ-Al2O3 is as follows: r CH4 /[(mol/gcat)/h] = 0.001053{P CO –0.86 P H2 1.32/[1 + 0.46(P H2O/P H2 )]}. Meanwhile, a methane selectivity model at 220 °C for the 25%Co/Al2O3 catalyst was also developed: S CH4 = 0.0792P CO –0.55 P H2 0.44[(1 – 0.24P H2O/P H2 )/(1 + 0.46P H2O/P H2 )]. The CH4 selectivity model provided a good prediction of CH4 selectivities under the experimental conditions used. Furthermore, our empirical CH4 kinetic results on the cobalt catalyst are consistent with literature kinetic models that were derived from carbide mechanisms; high CH4 selectivity from the cobalt catalyst is found to be mainly due to a high CH4 reaction rate constant.

Catalytic performance of cobalt–silica catalyst for Fischer–Tropsch synthesis: Effects of reaction rates on efficiency of liquid synthesis

“…The results showed that a higher CH 4 selectivity of the Co catalyst is primarily correspondent to a higher CH 4 rate constant, compared with other lighter HCs' formation (C 2 -C 4 ). This conclusion is consistent with the previous report from the literature [25]. Figure 8 illustrates the trend of changes in CO and H 2 conversion at different methanation reaction rates along the normalized axial dimension.…”

“…Jarosch et al [23] developed a model using a similar approach for a Co:Re (21:1) catalyst on ߛ-Al 2 O 3 . Another relevant example is the work done by Ma et al [24,25] that was applied to a 25% Co/Al 2 O 3 catalyst to fit the CH 4 kinetic data. Overall, the kinetic study based on the power-law rate model can be a powerful method to provide some insight into FT synthesis, without the need to represent a complex reaction network.…”

On the performance of a Co-based catalyst supported on modified γ-Al2O3 during Fischer–Tropsch synthesis in the presence of co-fed water

“…Considering the effect of water on the reaction rate, both positive and negative effects have been reported. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Most authors have observed a negative effect of H 2 O on the catalyst activity, and have attributed such an effect to the oxidation of the Co 0 active sites to inactive CoO or Co 3 O 4 species, [1][2][3][4][5][6][7][8] and/or to the formation of FTS inactive cobalt-support mixed spinels. 1,9 These latter species are particularly favored in the case of γ-Al 2 O 3 supported catalysts due to the strong interaction of the support with highly dispersed cobalt (SMSI, Strong Metal Support Interaction).…”

Thermodynamics of oxidation of an alumina-supported cobalt catalyst by water in F-T synthesis

“…The amount of water formed depends upon the CO conversion, reactor system and the catalyst used. Although cobalt catalyst does not exhibit significantly WGS activity, the effects of water on the performance of cobalt catalysts have been examined in both fixedbed and slurry reactors by many researchers [7,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In the absence of other substances involved in the case, CoO, Co 3 O 4 and Co(OH) 2 are the main possible forms for the direct oxidation of cobalt metal by water, and thus the involved chemical equations are as follows:…”