1-Alkene/n-alkane ratio

Figure 6. Effect of the extent of cracking (condensed-oil basis) on three 1-alkene/ n-alkane ratios. The ratios were determined by capillary column chromatography with an FID detector. C8 ) C12 (O) C18 ([J).

Oil coking can be minimized by decreasing the liquid-phase residence time, which, in turn, can be accomplished by using high pyrolysis temperatures and/or an inert sweep gas (9). Because 1-alkene/n-alkane ratios depend on pyrolysis temperature and heating rate, they are good indicators of oil coking (16, 25,... 

J. H. "Correlation of Shale Oil 1-Alkene/n-Alkane Ratios with Process Yield," Anal. Chem. 1978, 50, 958. 

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). 

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. 

Figure 6. Effect of inert sweep gas during retorting on the 1 -alkene/n-alkane ratios in shale oil. The ratios at the peaks were determined on samples from capillary column GC by total-ion MS.

We first consider the oil from the 1972 operation of the TOSCO-II semi-works. Figure 8 shows the FID chromatogram of this oil. In comparison to Fischer assay oil, significantly higher concentrations of aromatics are evident. We determined 1-alkene/n-alkane and naphthalene/(C i + C12) ratios from the FID chromatogram. We obtained Cg, Ci2> and Cig ratios of 1.36, 1.22, 1.05, and a naphthalene/(Cyy + C12) ratio of 0.047. These ratios also indicate a yield of from 75 to 85% on a condensed-oil basis and 80 to 85% on a C5+ basis. In contrast, TOSCO reports a 93% yield for its 1972 run (22). 

Unlike polycondensation polymers, polymers of addition polymerization such as polyethylene and polypropylene when depolymerized in inert atmosphere (39) or in supercritical water (37) do not convert to just the monomer, but a homologous series of oligomers (alkanes and alkenes). Compared to pyrolysis in argon, for polyethylene, the portion of the lighter products increases in supercritical water depolymerizations conducted at 693 K and water densities of 0.13 and 0.42 g/cm. The 1-alkene to n-alkane ratio also increases in supercritical water and with density. These are shown in Figure 11. These results are attributed to the fact that in argon pyrolysis, the reaction proceed in the molten state of the polymer, whereas in supercritical water, some of degradation products... 

F/gwre 7/. Comparison of the 1-alkene to n-alkane ratio and product distribution inpyrolysis reactions of polyethylene at 693 K in argon (filled triangles), and in supercritical water at a density of 0.13 g/cm (filled circles) and at a density of 0.42 g/cm (open circles) [from ref 37]. 

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. 

The second gc technique determined the individual n-alkanes and 1-alkenes in the pyrolyzed sample. A 100 m wall-coated glass capillary gave the required resolution and the n-alkanes and 1-alkenes stood out as distinct, well resolved peaks. OV-101 or OV-17 wall coatings provide adequate separation. A carrier gas flow of one cc/min was combined with an inlet split ratio of 50 1 and a 310°C injector temperature. The column temperature was raised to 250°C at 4°/min after an 8.0 min initial hold at 80°C. Peak identification was based on retention time matching with n-alkane and 1-alkene standards. 

The distribution can be changed, however, by using different gas-water ratios. Table I shows the partitioning of various classes of hydrocarbons for selected gas-water ratios. Increasing the gas-water ratio partitions a higher percentage of the individual hydrocarbons to the gas phase. However, the concentration per unit volume of gas decreases with increase in gas-water ratios for n-alkanes, alkenes, and cycloalkanes. Aromatic hydrocarbons, by coincidence, partition to give approximately the same concentration per unit volume of gas over gas-water ratios from 1 10 to 10 1. 

TcAvari et al. [66] plotted the average interaction parameters of normal and branched hydrocarbons, 1-alkenes, mono- and di-substituted derivatives of benzene in n-tetracosane, n-triacontane and n-hexatriacon-tane versus the Tii V Y ratio they obtained distinct straight lines for n-alkanes, mono-branched alkanes, di-branched alkanes, 1-alkenes and mono- and di-substituted derivatives of benzene. The slope of the first four lines is negative, which points to lower densities of cohesive energies than those of the solvent. For mono- and di-substituted derivatives of benzene the slopes are positive, i.e. the densities of cohesive energies are higher than those of the solvent. 

Cracking catalysts using combinations of medium and large-pore zeolites in order to maximize the production of products to be used in reformulated gasoline has been r orted [23]. In a study of the craclmg of n-heptane over MCM-22, ZSM-5 and Beta it was shown that the yield of propene was greatest in the case of MCM-22 and the overall alkane/alkene ratio of the products lay between ZSM-5 (0.94) and Beta (1.17). 

Alkyl iodides " and perfluoroalkyl iodides react with Zn-Cu couples. Use also is made of mixtures of the alkyl iodide and bromide (e.g., a 1 3 ratio of n-BuI n-BuBr gives n-BujZn in high yield). Byproducts, such as alkenes (R—H) and alkanes, RH, can become problems for secondary alkyl halides, RX, and for these care and control of the conditions and workup are vital. Success is possible as shown by yields of 85% for i-PrjZn, and 72% for s-Bu Zn. Tertiary alkyl iodides do not produce organozincs. 

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