Pyrolysis of paraffins

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

T he literature on pyrolysis of paraffin hydrocarbons is extensive. A recent review (1) of propane pyrolysis lists 103 references covering a period from 1928 to 1976. The study indicates a remarkable lack of quantitative agreement on energies of activation and on the effect of product inhibition on rate of decomposition. Energies of activation, from reputable experimental efforts, cover a range from 40 to 80 kcal/mol. 

Sulfur hexafluoride accelerates the pyrolysis of paraffin hydrocarbons (164), lowers the octane number of gasoline containing lead tetraethyl (189), removes silicon from a platinum catalyst when heated to 800 to 1000° (206) and catalyzes the reaction of ammonia with a ketone and aldehyde to give a substituted pyridine (196). It may be used at high pressure to fill a fuse. When the fuse blows an arc is prevented (210). 

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. 

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.

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. 

The kinetics associated with the reactions shown in Figure 7 are summarized in Table n. Detailed mechanistic studies on the pyrolysis of alkylaromatics (12,13,15), alkylnaphthenes (14) and alkyltetralins (14) have allowed for the formulation of the Arrhenius parameters and stoichiometric coefficients shown. The kinetics for paraffin and olefin pyrolyses were extracted from the abundant literature data (16-18). Finally, the issue of kinetic interactions have been both theoretically and experimentally addressed (11,19). These interactions in general cause the reaction of the mixture to be different then the linear combination of the pure component rates. 

Sundaram, K.M. and G.F. Fioment, Modeling of Thermal Cracking kinetics. 3. Radical Mechanisms for the Pyrolysis of Simple Paraffins, Olefins, and Their Mixtures., Ind. and Eng. Chem. Fund., 17,174—182,1978. 

Abstract. A variety of pyrocarbon/silica gel adsorbents were prepared using commercial mesoporous silica gels Si-40, Si-60, and Si-100 as matrices modified by carbon deposits from pyrolysis of several organic precursors. The second type of hybrid carbon-mineral adsorbents was synthesized using spent natural palygorskite utilized in paraffin purification. The adsorbents were then heated, hydrothermally treated, or modified by additional deposition of carbon. Changes in the structural and adsorption characteristics of hybrid adsorbents before and after treatments were analyzed by microscopy, p-nitrophenol and nitrogen adsorption isotherms, and TG, TEM, XRD, and XRF methods. 

Examples include pyrolysis of an alkylbenzene homogeneous aldehyde hydrogenation olefin hydroformylation to alcohol with paraffin by-product formation, aldehyde condensation to heavy ends, and olefin isomerization cyclo-addition reactions and hydrogen-halide reactions. 

Y.-H. Seo and D.-H. Shin, Determination of paraffin and aromatic hydrocarbon type chemicals in liquid distillates produced from the pyrolysis process of waste plastics by isotope-dilution mass spectrometry. Fuel, 81, 2103-2112 (2002). 

Pyrolysis was carried out on a feed composed of a 50/50 mixture by weight of low-density polyethylene (LDPE) and hydrotreated FT wax. Yields are given in Table 13.2, showing a 385°C- - yield of 57.5 wt%. The yield for a broader lube feed, 343°C- -, was 66.0 wt%. While there was considerable 538°C- - in the feed to the pyrolyzer, there was little 538°C- - in the product, which is believed here to be advantageous for low cloud point. Oleflnicity in the pyrolysis overhead was 76 wt% by PONA analysis. The olefinic overhead liquids from the pyrolysis of both FT wax and LDPE/FT wax were analyzed using gas chromatography. This showed the cracked product to be almost entirely 1-normal olefins and normal paraffins. 

Different types of polymer degrade into liquid products at different temperature and form hydrocarbons with structural differences according to the structure of the parent polymer. The liquid hydrocarbons obtained by pyrolysis of PE are widely distributed from C3 to C25 and are composed of linear olefins and paraffins. The liquid hydrocarbons obtained from pyrolysis of PP are also distributed in the range C3-C25, but the gasoline fraction obtained in PP pyrolysis has a higher octane number compared with gasoline obtained from PE pyrolysis. 

When no reforming process is carried out in the pyrolysis of PP, 90.50% of the gasoline fraction in the products is olefin, and the yield fractions of isomerized paraffins, cycloalkanes and aromatics are very low. The gasoline has a RON of no more than 80 and is very unstable [99]. However, after reforming and fractionation [100], the results improved significantly, as shown in Table 28.11. Two kinds of molecular sieve catalysts were adopted for the process. 

The chapters in this volume are grouped in several sequences as follows Chapters 1-6 relate to epbba production directly. Specific subjects covered are pyrolysis of alpha-olefins, butenes, paraffins, unsubsti-... 

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). 

Results of pyrolysis of propane, n-butane, and n-hexane at a wide range of temperatures and conversions, including the range of commercial operation, are presented. Extensive product inhibition is evident in all cases. The rates of decomposition can be characterized by two pseudo energies of activation E, calculated by comparing data at constant decomposition, variable time and temperature, and E (always less than E) at constant time, variable decomposition and temperature. Both E and E are relatively constant over the conversion range studied. Near atmospheric pressure data fit the equation X = exp —sl0 [1 + 1-3 (Nc — 2)] t1/re — E /RT where X = fraction feed paraffin undecomposed, Nc = carbon number of feed paraffin, r = E/E = 1.68, t = reaction time (seconds), T = reaction temperature (K), a0 = 2.85 109 (from propane data) or 2.66 109 (better to n-butane and n-hexane data), and E = 46.0 kcal/ mol. 

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... 

Naphtha feed is treated as a single pseudo species. Naphthas, used as pyrolysis feedstocks, are mainly composed of paraffins and naphthenes, with lesser amounts of aromatics. Olefin content is usually very small. Consistent with observed pyrolytic behavior of paraffins and naphthenes (15,16,26,27,28), feed decomposition is assumed to follow first-order kinetics. Equation 3 of the reactor model can be simplified as follows. 

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