Yang Jun ¹, Liu Ying^1,2, Zhang Yuanming¹, Tang Yu¹
(¹Department of Chemistry, Jinan University, Guangzhou 510632, China; ²State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China)

Abstract The detailed kinetics of Fischer-Tropsch synthesis (FTS) over an industrial Fe-Mn catalyst during early period of reaction (200 h Time on stream (TOS)) was studied in a continuous fixed bed reactor under operation conditions of the low temperature and low H₂/CO ratio. 39 combined kinetic models of FTS based on the LHHW type mechanism were scanned with non-linear regression method. The conclusion in this work is consistent with that obtained in our previous work under higher reaction temperature and H₂/CO ratio of syngas. The FTS model based on alkylidene mechanism could produce a good fit of experimental data. In this work, the estimated activation energy for chain growth is 60.55 kJ.mol^-1, which is lower than that of our previous study (75.52kJ.mol^-1), indicating that the energy barrier for the chain growth is lower for the catalyst with shorter TOS. This can well interpret the trend to higher selectivity of the lower molecular hydrocarbon, especially the methane, with longer TOS observed in our previous work, which probably results from phase change of catalyst in the course of the synthesis reaction.
Keyword  Fischer-Tropsch synthesis, Fe-Mn catalyst, Kinetics

2. EXPERIMENTAL SECTION
4.01 g (3cm³) catalyst particles in the diameter of 0.18-0.25 mm (60-80 ASTM mesh), was packed in a fixed bed reactor with a length of 1.0 m and an internal diameter of 0.012 m. The catalyst had a composition of Fe-Mn (atomic ratio of Fe/Mn =3) and 1% of K₂O, and a BET surface area of 117.8 m²/g. Prior to reaction, the catalyst was activated at 548K, 0.25-0.30MPa, 1000 h^-1 with syngas of 2.0 (H₂/CO) ratio for 36 h. After the activation, the temperature was adjusted to desired reaction temperature. In this work, the catalyst was initially treated at 523 K under FTS reaction conditions with syngas for 200 h to ensure that the relatively stable catalytic phases were established. For each kinetics experiment run, before collecting products, it took at least 10 hour to ensure the steady-state behavior of catalyst after a change of the reaction conditions. The analytical methods of products are the same as that of our previous paper.¹² The products of the synthesis were separated into three portions: gas phase, oil phase (from ice trap), and wax phase (from the hot trap). Both the purified syngas and the tail gas were analyzed on a gas chromatography (GC). H₂, O₂, N₂, CH₄, and CO were separated on 13x molecular sieve packed column (1.5 m×3 mm i.d., Ar carrier flow) and detected with a Thermal Conductivity Detector (TCD). C₁-C₅ hydrocarbons in gas phase were analyzed on C₂₂⁰/C_-22 (170-250m m) packed column (7.2 m×3.2 mm i.d.), flame ionization detector (FID) with N₂ as carrier. CO₂ was measured on a silica gel packed column (1 m×3 mm i.d, H₂ carrier) with TCD and quantified by external standard method. The oil product from the cold trap was separated on a 60 m×0.25 mm (i.d) OV-101 capillary column (N₂ carrier, FID) with temperature programmed from 333(maintained for 20 min) to 563K at the rate of 3K/Min. The wax product from the hot trap was firstly dissolved in CS₂ (0.5-1.0 mass%) and then analyzed on OV-101 capillary column, FID, and N₂ carrier with temperature programmed (1K/min) from 343 to 563K. Water phase was separated by using BD-wax (GW, US) capillary column (N₂ carrier, FID).

3. RESULTS AND DISCUSSION
3.1 Experimental results
The experiments results are given in Table 3. An orthogonal composite experimental design was used to arrange temperature, pressure and H₂/CO ratio for all experiments, enabling efficient and optimal distribution of experiment points. Finally, 26 experimental points, which include 16 orthogonal points (according to an orthogonal table of L₁₆ (4³)) and 10 additional points were applied for the kinetic studies. One point (point 15 in Table 3) is used as reference point (point 12 in Table 3) to check for deactivation and reproducibility of the experimental data. The results show that the conversion of reactants at No. 15 well reproduced those at No.12, indicating that the catalyst did not suffer permanent deactivation during the course of kinetic experiments and catalytic phase had reached a relatively stable state.

Table 1 Experimental conditions and results

    where a_n represents the chain growth factor for carbon number n (), a_n represents the chain growth factor without olefin readsortion, b_n represents the readsorption factor of 1-olefin with carbon number n (), [s₁] represents the concentration of free site for the hydrocarbon formation, represents .
    For different assumptions of the rate-determining step in WGS, the rate expressions of the CO₂ of WGS reaction can be derived, the details are described in our previous work.¹² Take model WGS3 as an example, it is assumed that the desorption of CO₂ via formate intermediate (step IV) is slower than others in WGS, the remaining steps are at equilibrium, and the rate expression of CO₂ formation can be derived as follows

Table 3 The comparison of the values of the parameters for different kinetics experiment

3.3 Discussions
The model FT III WGS3 fit well with the experimental data. This is consistent with that of our previous work performed under higher temperature (553-601 K) and high H₂/CO ratio (1.0-3.0) after 1000 h TOS. The comparison of the parameters obtained in these two kinetic tests, Run A (after 200 h TOS) and Run B (after 1000 h TOS), is presented in Table 3, and all of parameters are significantly different from zero.
    In this work, the estimated activation energy for chain growth is 60.55 kJ.mol^-1 (Run A), and is lower than that of Run B (75.52kJ.mol^-1), indicating a lower energy barrier for the chain growth with shorter TOS. The activation energies for methane, olefin and paraffin formation are 100.14 kJ.mol^-1, 118.24 kJ.mol^-1, and 103.17 kJ.mol^-1, respectively, which are slightly different from those of Run B (97.39 kJ.mol^-1, 111.48 kJ.mol^-1, and 97.37 kJ.mol^-1, respectively). The results show that the rates of the chain growth, the methane, olefin and the paraffin formation of Run A are greater than those of Run B, indicating that the catalysts in Run B suffered modest deactivation. The trend to higher selectivity of the lower molecular hydrocarbon, especially the methane, observed in previous work is probably attributed to the higher energy barrier for chain growth during the later period of reaction (after 1000 h TOS ), which probably results from phase change of the catalyst during the reaction. The shift in hydrocarbon distribution towards lower molecular hydrocarbon with catalyst aging has also been observed in several other studies with iron catalyst.^[13-14]
    The comparison between the experimental and the predicted values of conversion of CO, selectivity of CO₂ are presented in Figures 1&2. The results show that the relative residuals between the model predicted and experimental are mostly within 20%.

Figure 1. Experimental and predicted CO conversions

Figure 2. Experimental and predicted CO2 selectivities

Figure 3 Prediction of product distribution
Reaction condition: T=543.0K, P=2.0MPa, H₂/CO=0.67, G_HSV=3.7 l(STP)/g-cat·h

4. CONCLUSION
FTS kinetics experiments over an industrial Fe-Mn UFP catalyst are carried out during the different reaction periods for a wide range of industrially relevant conditions. Based on different LHHW and ER type mechanisms for the FTS reactions and the WGS reaction, 39 models for FT synthesis were constructed. The rival models were used to fit the experimental data sequentially by global parameter optimization. The result is consistent with our previous finding, namely the model based on alkylidene mechanism for the FTS gives a good fit of the experimental data. The estimated activation energy for chain growth in this work is 60.55 kJ·mol^-1, which is lower than that of our previous study (75.52kJ.mol^-1), indicating that the energy barrier for the chain growth is lower for the catalyst with shorter TOS. This can well interpret the trend to higher selectivity of the lower molecular hydrocarbons, especially the methane, with increasing of TOS observed in our previous work, which probably results from phase change of catalyst in the course of the synthesis reaction.

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