Rice-Kossiakoff

The product yields by carbon number are plotted in Figure 6. At all except the most severe conditions studied,the major component in the gas phase was n-butane. This observation is consistent with the a-ring opening and dealkylation mechanism proposed for tetra-and octa-hydrophenanthrene cracking. At the most severe conditions ethane was present in the greatest quantities. This can be explained by side chain cracking of n-butylbenzene according to the Rice-Kossiakoff mechanism or by secondary reactions of the n-butane. 

The liquid products of the pyrolysis of PP contain primarily olefins that resemble the molecular skeleton of PP (i.e. branched hydrocarbons). A distinguishing feature of PP pyrolysis is the predominant formation of a particular C9 olefin in the pyrolysis product. The level of this C9 compound identified as 2,4-dimethylhept-l-ene can be as high as 25%. Also present are C5 olefin, Cs olefin, several C15 olefins and some C21 olefins [2]. The tertiary carbon sites in PP allows for the facile chain cleavage and rearrangements according to the Rice-Kossiakoff cracking mechanism shown in Figure 15.2. The noncondensable gas from PP pyrolysis contains elevated levels of propylene, isobutylene and n-pentane. 

Figure 15.2 Rice-Kossiakoff cracking mechanism for polypropylene showing that the pyrolysis products of PP retain a branched structure... 

The relative contributions of the three proposed reaction paths remain to be determined. The products from both the retro-ene and the addition reactions can be predicted with some quantitative certainty. For dodecene, the former produces only 1-nonene and propylene. The product distributions from the addition paths should be identical to those from the cracking of the corresponding paraffins. These product distributions can be predicted using the method of Rice and Kossiakoff (16). This approach parallels excellently with experiments (16,17). (From available data we estimate its accuracy as d=10%.) For dodecene, the addition of hydrogen atoms, methyl radicals, and ethyl radicals will produce Ci2H25, Ci3H27, and Ci4H29 parent radicals. The product distributions predicted by the Rice-Kossiakoff method for the decomposition of these radicals at 525°C are shown in Table IV. 

The fate of 1-olefins formed in the cracking process was not treated in the original Rice-Kossiakoff hypothesis (7-10) and such compounds were apparently considered stable products at low conversion. It was... 

Mechanism of Decalin Hydropyrolysis. On the basis of some principles of the Rice-Kossiakoff free-radical theory (7-JO), the compositional changes summarized in Table VI and Figure 10 can be rationalized in terms of the following suggested mechanism for hydropyrolysis of 2 (Scheme 2) ... 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

Kossiakoff A, Rice FO (1943) Thermal decomposition of hydrocarbons, resonance stabilization and isomerization of free radieals. J Am Chem Soc 65 590-594 Kowalewski I, Vandenbroucke M, Hue AY, Taylor MJ, Faulon JL (1996) Prehminary results on molecular modeling of asphaltenes using structure elucidation programs in conjunction with molecular simulation programs. Energy Fuels, 10 97-107... 

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