487
3.2.2 High C1 Selectivity
The higher yield of methane than the predicted from the ASF law in the FT synthesis product stream has been reported many times and is also evident in Fig. 8 [17,
107, 108]. However, various reasoning has been proposed to explain this anomaly.
Schulz et al. [107, 108] suggested some specific sites exist in the FT synthesis reaction environment to promote methane formation only; however, the author could not
verify it experimentally. The other theory relates to the presence of secondary reactions such as hydrogenolysis to the excess methane formation [12, 109]. Again, the
explanation related to thermodynamics suggests that the heat and mass transfer
limitation increase the methane formation, which seems unreasonable as the cases
where these limitations were absent the methane yield was still high [110]. The high
yield of methane than expected from the ASF model can be explained by using alkyl
and carbide mechanism. The chain initiation is more difficult than the chain propagation as the initiation involves two molecules H 2 and CO. Now, the likelihood of
desorption step after hydrogenation of M=CH 2 intermediate is higher than the
desorption of M=CH 2 -R. Henceforth the hydrogenation and desorption of =CH 2
intermediate as methane increases the selectivity of methane as well as decreases
the probability of chain growth to adjacently adsorbed intermediate.
3.2.3 Low C2 Selectivity
The C 2 hydrocarbon selectivity is predicted by the ASF distribution model to be
much higher than the actual experimental value of C 2 selectivity. Also, the ratio of
ethene to ethane yield is lower and does not fit in the trend for the ratio of alkane to
alkene for other higher carbon number. The higher adsorption rate of ethylene than
other higher olefins can be ascribed to the significantly lower yield of C 2 in the
product [13, 15]. On the detailed investigation on the issue, Cheng et al. reported
that the significantly high chemisorption energy of ethylene is the cause of the C 2
anomaly [104]. The proposed reaction mechanism (Fig. 2) can be a good explanation for this anomaly. The secondary reaction of ethene can increase when reabsorbed in the form of M-CH 2 –CH 2 -M. Henceforth, the adsorbed intermediate
increases the chain growth probability two times as it has two ends to propagate the
chain. The low ratio of ethene to ethane can be attributed to the re-adsorption of
ethene and the relative rate of hydrogenation and desorption.
3.2.4 Variation in Value of α with Carbon Number
Several reported experimental results for FT synthesis confirm the variation in the
value of α with the carbon number [106, 111–115]. A general trend of α is shown in
Fig. 8, wherein the value of α remains almost constant for long-chained hydrocarbon (n > 10). The value α decreases with decreasing chain length for carbon number
10 > n > 3. Also, for lower chain length, the erratic nature of α can be attributed to
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…
3.2.2 High C1 Selectivity
The higher yield of methane than the predicted from the ASF law in the FT synthesis product stream has been reported many times and is also evident in Fig. 8 [17,
107, 108]. However, various reasoning has been proposed to explain this anomaly.
Schulz et al. [107, 108] suggested some specific sites exist in the FT synthesis reaction environment to promote methane formation only; however, the author could not
verify it experimentally. The other theory relates to the presence of secondary reactions such as hydrogenolysis to the excess methane formation [12, 109]. Again, the
explanation related to thermodynamics suggests that the heat and mass transfer
limitation increase the methane formation, which seems unreasonable as the cases
where these limitations were absent the methane yield was still high [110]. The high
yield of methane than expected from the ASF model can be explained by using alkyl
and carbide mechanism. The chain initiation is more difficult than the chain propagation as the initiation involves two molecules H 2 and CO. Now, the likelihood of
desorption step after hydrogenation of M=CH 2 intermediate is higher than the
desorption of M=CH 2 -R. Henceforth the hydrogenation and desorption of =CH 2
intermediate as methane increases the selectivity of methane as well as decreases
the probability of chain growth to adjacently adsorbed intermediate.
3.2.3 Low C2 Selectivity
The C 2 hydrocarbon selectivity is predicted by the ASF distribution model to be
much higher than the actual experimental value of C 2 selectivity. Also, the ratio of
ethene to ethane yield is lower and does not fit in the trend for the ratio of alkane to
alkene for other higher carbon number. The higher adsorption rate of ethylene than
other higher olefins can be ascribed to the significantly lower yield of C 2 in the
product [13, 15]. On the detailed investigation on the issue, Cheng et al. reported
that the significantly high chemisorption energy of ethylene is the cause of the C 2
anomaly [104]. The proposed reaction mechanism (Fig. 2) can be a good explanation for this anomaly. The secondary reaction of ethene can increase when reabsorbed in the form of M-CH 2 –CH 2 -M. Henceforth, the adsorbed intermediate
increases the chain growth probability two times as it has two ends to propagate the
chain. The low ratio of ethene to ethane can be attributed to the re-adsorption of
ethene and the relative rate of hydrogenation and desorption.
3.2.4 Variation in Value of α with Carbon Number
Several reported experimental results for FT synthesis confirm the variation in the
value of α with the carbon number [106, 111–115]. A general trend of α is shown in
Fig. 8, wherein the value of α remains almost constant for long-chained hydrocarbon (n > 10). The value α decreases with decreasing chain length for carbon number
10 > n > 3. Also, for lower chain length, the erratic nature of α can be attributed to
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…