Alkene/alkane ratios

Krishnamoorty et al.4S studied FTS over a 12.7Co/Si02 and reported alkene alk-ane ratios for the C5 and C8 fractions during kinetic experiments and with water addition. Water addition increased the alkene alkane ratio at both carbon numbers, and the authors point to the rates of the formation of the various hydrocarbons ... 

The yield of the alkane-alkene alkylation in homogeneous HF-TaF5 depending on the alkene-alkane ratio has been investigated by Sommer et al.149 in a batch system with short reaction times. The results support direct alkylation of methane, ethane, and propane by the ethyl cation and the product distribution depends on the alkene-alkane ratio (Figure 5.14). 

An increase in the alkene-alkane ratio results in a significant decrease in single-labeled propane ethylene polymerization-cracking and hydride transfer become the main reaction. This labeling experiment carried out under conditions where side reactions were negligible is indeed unequivocal proof for the direct alkylation of an alkane by a very reactive carbenium ion. 

An important factor in commercial operation is the relative amounts of alkene produced, relative to alkanes. Alkene/alkane ratios for the Cl to C5 range are presented in Fig. 3 for n-hexadecane and for 1% and 10% additions of quinoline and phenanthrene to the n-hexadecane feedstock. In all cases the ratio was greater than unity, with 1% addition of additives having relatively little effect on this ratio. However, at 10% addition, phenanthrene enhanced this ratio, whilst quinoline showed a corresponding decrease. Thus, although these additives diminished the individual yields of the gaseous components, with a marked reduction in the case of quinoline, small concentrations had little effect on the alkene/alkane ratio. 

FIGURE 3. ALKENE/ALKANE RATIO FOR MAT 16 CATALYST WITH OUINOLINE AND PHENANTHRENE. 

A plot of the alkene and alkane yields and the alkene/alkane ratio for these three catalysts and the MAT 16 catalyst (the model for component material) is given in Fig. 5. Again the 100% zeolite catalyst LZYl produces by far the greatest yield of alkane whereas all the other materials produce more alkene than alkane and thus producing values of the alkene/alkane ratio in excess of unity. On the basis of product yield the catalyst Z-A6 is seen to be superior. 

ALKENE/ALKANE RATIO HU ALKENE, NJ7.. ALKANE. WTjJ. 

FIGURE 5. ALKENE/ALKANE RATIOS FOR LZY1, Z-A6, BPM1 AND MAT16. 

Table II gives the product distribution for thermal cracking of shale oil. We defined oil as the sum of condensed oil and C5-C9 hydrocarbons in the gas. The amount of each gaseous product was determined from the slope of the curve plotting gas production versus cracking loss (conversion) (14). The amount of coke produced was determined by difference, but it agreed well with the measured value for the few experiments in which carbon was analyzed in the shale from the bottom reactor. The alkene/alkane ratios in the gas depended more strongly on the cracking temperature than on the extent of conversion. This topic is discussed in greater detail in another paper published in these proceedings (20).

Figure 9. Chromatogram of shale oil from Rock Springs No. 9, a true in situ experiment. The alkene/alkane ratios are very low (coking) and the naphthalene content are very high (combustion and associated cracking). The naphthalene/methylnaphthalene ratios are high compared with the...

Raley, J. H. "Monitoring Oil Shale Retorts by Off-gas Alkene/Alkane Ratios," Fuel 1980, 59, 419. 

Our objective was to clarify the reaction mechanisms that determine the observed alkene/alkane ratios under various conditions, and the results are reported here. When oil shale is pyrolyzed either isothermally or nonisothermally, the hydrocarbon and hydrogen concentrations are all time dependent. To determine if the alkene-alkane-hydrogen system is at equilibrium, we heated oil shale at a constant rate and measured the C to C3 hydrocarbons and hydrogen over time. We also measured the effect of an inert sweep gas on the time-dependent ethene/ethane and propene/propane ratios and the integral 1-alkene/n-alkane ratios in the oil. We determined that the C2H4-C2H6-H2 system is not at thermal equilibrium and interpret our results in terms of a nonequilibrium free-radical mechanism proposed by Raley (8). 

If the ethene/ethane ratios are combined with the hydrogen partial pressures, we can demonstrate that the ethene-hydrogen-ethane system is far from thermal equilibrium under the conditions of the experiment shown in Figure 2. The experimental value of c2h6 Pc2h4 Ph2 s comPare< Figure 4 with the value of Keq. Only at temperatures near and above 600°C, at which the C2 evolution rate is negligible, does the ethene/ethane ratio approach equilibrium. Therefore, a nonequilibrium explanation of the observed alkene/alkane ratios is required. 

The 1-alkene/n-alkane ratios in the oil, measured by capillary-column gas chromatography/mass spectroscopy, also increase with the addition of inert diluent (Figure 6). This effect and the previously demonstrated dependence on heating rate are consistent with a free-radical mechanism. In addition, we noted that alkene/alkane ratios for even-numbered hydrocarbons are significantly higher the ratios for odd-numbered ones. We do not understand this effect at this time but suspect that it is related to the structure of kerogen and the mechanism of its pyrolysis. 

When oil shale is heated at a constant rate, the alkene/alkane ratios in the evolved hydrocarbon gases change with time. In addition, the alkene/alkane ratios in both the gas and the oil are affected by an inert sweep gas. The ethene/ethane ratio is not determined by equilibrium with hydrogen, and we interpret this phenomenon in terms of a free-radical cracking mechanism. The implication is that alkene/alkane ratios, especially the ethene/ethane ratio, can be used as an indicator of retort performance only if the correct relationships are used for each set of retort conditions. 

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