Pyrolysis paraffin

Note that steps 19 and 20 go through transition states involving relatively unstrained 5-membered rings. Related isomerizations are well known in paraffin pyrolysis. 

T he expansion of the petrochemical industry and the accompanying increase in the demand for ethylene, propylene, and butadiene has resulted in renewed interest and research into the pyrolytic reactions of hydrocarbons. Much of this activity has involved paraffin pyrolysis for two reasons saturates make up most of any steam cracker feed and since the pioneering work of Rice 40 years ago, the basic features of paraffin cracking mechanisms have been known (1). The emergence of gas chromatography as a major analytical tool in the past 15 years has made it possible to confirm the basic utility of Rice s hypotheses (see, for example, Ref. 2). 

It has been assumed that the remaining products are formed by some sort of free radical chain mechanism, but no generalized mechanism like that of Rice s for paraffin pyrolysis has been proposed. Tanaka et al. have been able to simulate product distributions for shorter olefins—up to hexene (10). We shall describe a model for higher alpha-olefin pyrolysis and use it to account for the products from the cracking of several olefins. 

The qualitative features of paraffin pyrolysis, on the other hand, are reasonably well understood. The decomposition is, in general, initiated by rupture of C-C bonds, carried by chains of hydrogen atoms, methyl radicals, and to some extent ethyl radicals, and terminated by assorted radical recombinations. Product inhibition occurs through the reaction... 

Leathard, D.A. Purnell, J.A. "Paraffin Pyrolysis". Annual Review Phys. Chem. 1970. 21, 197. 

The pyrolysis of olefins has not been as well studied as for paraffin pyrolysis. The lesser interest in olefins can be attributed to both more complex product mixtures and lower rates of pyrolysis for the olefins. 

For the pyrolysis of paraffinic hydrocarbons at 700- 800 C, yields of olefins such as ethylene, propylene, butenes, butadiene and cycloolefins increase during the initial stage of the reaction, pass through their maxima, and later decrease yields of aromatics, hydrogen and methane however increase monotonically throughout the reaction course. Sakai et al. (1 ) reported previously the result of a kinetic study on thermal reactions of ethylene, propylene, butenes, butadiene and these respective olefins with butadiene at the conditions similar to those of paraffin pyrolysis, directing their attention on the rates of formation of cyclic compounds. Kinetic features of the thermal reactions of these olefins are sunnnarized in Table I combined with the results obtained in later investigations for thermal reactions of cycloolefins ( 2) and benzene O). 

The original method for the manufacture of ethyne, the action of water on calcium carbide, is still of very great importance, but newer methods include the pyrolysis of the lower paraffins in the presence of steam, the partial oxidation of natural gas (methane) and the cracking of hydrocarbons in an electric arc. 

Pyrolysis of alkanes is referred to as eraeking. Alkanes from the paraffins (kerosene) fraetion in the vapor state are passed through a metal ehamher heated to 400-700°C. Metallie oxides are used as a eatalyst. The starting alkanes are broken down into a mixture of smaller alkanes, alkenes, and some hydrogen. 

Pyrolyses of Nl- or N3-substituted derivatives of compounds 4 and 5 have continued to find use as routes to azacarbazoles, although the yields are often indifferent and there are no recent examples. The photochemical reactions are dealt with in Section IV.G. Pyrolysis media are paraffin (P) or PPA, and examples of products are compounds 247 (P, cytostatic) (83MI2), 248 (P) (84MI1), and 249 (from a 1-substituted derivative) (86MI2). Indications of diradical intermediates are provided by the thermolysis of compound 250 (P) (83MI2) where one product is a dimer. 

TABLE 5 Pyrolysis of Paraffin Wax (C2 -CJ2) Typical Composition of the Reaction Product... 

The carbon number distribution of technical secondary alkanesulfonates determined by pyrolysis gas chromatography and mass spectrometry (GC-MS) is shown in Fig. 13 together with the corresponding carbon number pattern of the raw material paraffins obtained by GC [16]. Pyrolysis was performed in a crucible-modified SGE pyrojector after covering the mixture with quartz wool. The presence of up to 10 wt % of disulfonates in technical alkanesulfonates is demonstrated by fast atom bombardment and mass spectrometry (FAB-MS) (Fig. 14) [24],... 

FIG. 13 Carbon number distribution of alkanemonosulfonates by pyrolysis gas chromatography (GC)/mass spectrometry (paraffin raw material by GC). 

The pyrolysis yields of + paraffins obtained from biomass samples using different percentages of additives at different temperatnres ate presented in Table 6.5 (Caglar and Demiibas, 2002a, 2002b). The chemicals (ZnClj, Na2C03 and K COj) were nsed as additives in the experiments. The yields of hydrogen -i- paraffins... 

Figures 6.5 to 6.7 show the effect of temperature on yields of + paraffins obtained from biomass samples by pyrolysis. As can be seen in Figs. 6.5 to 6.7, the percentage of + paraffins in gaseous products obtained from the samples of hazelnut shell, tea waste and spmce wood increased, while the final pyrolysis temperature was increased from 700 to 950 K.

Table 6.5 Pyrolysis yields of Hj + paraffins obtained from cotton cocoon shell and olive husk by using different percents of additives at different temperatures (% by volume)...

Fig. 6.5 Pyrolysis yields of H, + paraffins from hazelnut shells at different temperatures...

Fig. 6.7 Pyrolysis yields of Hj+ paraffins from spruee wood at different temperatures...

For each coal, at the maximum in hydrogen content, or H/C atomic ratio, the aliphatic hydrogen content (determined by H NMR analysis) accounted for over 90% of the total hydrogen. The aliphatic hydrogen contents were 10.5% for the subbituminous coal,PSOC-1403, and 6.9% for the bituminous coal, PSOC-1266. The high aliphatic hydrogen content was associated with the presence of polymethylene chains. The early release of paraffinic material, as n-alkanes and as long chain substituents to aromatic structures, under conditions of mild pyrolysis has been observed in other research (13-15. ... 

The principal source of toluene is catalytic reforming of refinery streams. This source accounts for ca 79% of the total toluene produced. An additional 16% is separated from pyrolysis gasoline produced in steam crackers during the manufacture of ethylene and propylene. The reactions taking place in catalytic reforming to yield aromatics are dehydrogenation or aromatization of cyclohexanes, dehydroisomerization of substituted cyclopentanes, and the cyclodehydrogenation of paraffins. The formation of toluene by these reactions is shown. 

In addition to the polymer and facilitated transport membranes, novel materials are being proposed and investigated to achieve membranes with economically attractive properties. Carbon molecular sieve (CMS) membranes prepared by pyrolysis of polyimides displayed much better performance for olefin/paraffin separation than the precursor membranes [39, 46, 47]. Results obtained with CMS membranes indicated properties well beyond the upper-bond trade-off curve, as shown in Figure 7.8. Nonetheless, this class of materials is very expensive to fabricate at the present time. An easy, reliable, and more economical way to form asymmetric CMS hollow fibers needs to be addressed from a practical viewpoint. 

Propylene is the major olefin obtained during isobutane pyrolysis however, there is no known industrial unit that uses it as the feedstock. Propylene yields are often in the 12-16% range when propane, heavier normal paraffins, napthas, gas oils, and heavier petroleum feeds are pyrolyzed. 

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