Experimental study of hydrocarbon structure effects on the composition of its pyrolysis products

J. Anal. Appl. Pyrolysis 87 (2010) 207–216

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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap

Experimental study of hydrocarbon structure effects on the composition of its pyrolysis products Petr Za´mostny´ *, Zdeneˇk Beˇlohlav, Lucie Starkbaumova´, Jan Patera Department of Organic Technology, Faculty of Chemical Technology, Institute of Chemical Technology, Prague, Technicka´ 5, 166 28 Praha 6, Czech Republic

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 August 2009 Accepted 22 December 2009 Available online 11 January 2010

Pyrolysis experiments aimed at measuring product yields and conversion of various hydrocarbons were carried out in a micro-pyrolysis laboratory reactor. Standard pyrolysis conditions were used for all samples—810 8C, 400 kPa and 0.2–0.4 s residence time in the reaction zone. Analysis of products was performed by on-line GC/FID/TCD. All pyrolysis products from hydrogen to pyrolysis oil were detected either individually or by fractions. Detailed compositions of the pyrolysis products of 56 hydrocarbons were obtained and are included completely in the article. The study involved hydrocarbons, which represent the wide structural variability, including linear, branched, cyclic, aromatic and unsaturated. Measured data were used to identify important relationships between structural parameters and product yields. Trends of important product yields, depending on the carbon number, molecule saturation, substitution, ring presence and substitution or a substituent size are presented. ß 2010 Elsevier B.V. All rights reserved.

Keywords: Pyrolysis Thermal cracking Hydrocarbons Hydrocarbon structure Ethylene

1. Introduction The pyrolysis of hydrocarbon species under laboratory conditions has been used since the middle of the last century as a tool to investigate the behavior of hydrocarbons at high temperatures. The main-stream papers were more or less related to the hydrocarbons’ role as a feedstock in the steam-cracking process. The steam-cracking units use assorted hydrocarbon mixtures and crude oil fractions as their feedstock base and therefore, they can greatly benefit from information concerning the relationship between the feedstock structure and the composition of obtained product. Such information is extremely hard to derive from fullscale steam-cracker operation data because the process regime is generally governed by requirements for economy and stability and any deviation towards the more extreme conditions, either in terms of feedstock composition or operating conditions is highly improbable. Therefore, studying the relationship between the feedstock structure and the cracked product quality has become largely the domain of laboratory experiments, mathematical modeling and the combination thereof. The pyrolysis of hydrocarbons under laboratory conditions was studied in reactors of various kinds, as well as under various operation conditions. The most frequently used reactors generally aim to simulate industrial reactors and work under similar conditions, however, there are still significant differences. They

* Corresponding author. Tel.: +420 2 20444222. E-mail address: [email protected] (P. Za´mostny´). 0165-2370/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2009.12.006

possess a substantially higher ratio of the tube internal surface to the reaction volume and the relatively low flow rate of a reaction mixture creates different axial and radial temperatures and thus pressure profiles. Thus, radicals disappear by an order of magnitude faster on metal surfaces (high thermal conductivity and redox properties) than they do on the walls of laboratory quartz reactors. On the other hand, the wall effect in laboratory metal reactors is stronger than in their industrial counterparts because of the relatively large reactor-wall surface area and small reaction volume. Laboratory experiments, apart from studying the behavior of industrial feedstocks under laboratory conditions, are used to study the behavior of their individual components, so as to determine the cracking mechanisms and develop mathematical models. Bajus et al. published several papers (e.g. [1–4]) containing results obtained in a stainless steel reactor, where steam was used as a diluting medium in order to maximize the similarity to the steam-cracking process. A similar reactor was later employed in works by Billaud et al. (e.g. [5–9]), where both low and high temperature pyrolysis of several selected n-alkanes, iso-alkanes, cyclohexane and substituted aromatics are studied. They are generally aimed at developing mechanistic models of selected compound pyrolysis. Depeyre et al. [10–12] focused solely on the cracking of atmospheric gas oils and suitable model compounds, such as the n-nonane or n-hexadecane. Pant and Kunzru presented several papers aiming at experimental pyrolysis of several individual hydrocarbons [13–15]. They complemented the experimental data by deriving an empirical model of pyrolysis for each studied compound. Comparison of the pyrolysis behavior of

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different hydrocarbons was published by Safarik and Strausz [16– 18]. It was aimed at n-alkanes, aromatics and polycyclic hydrocarbons. High paraffin cracking was recently studied by Pinte´r et al. [19]. The kinetics of decalin cracking and oxidation was studied by Oehlschlaeger et al. [20]. Other authors published studies aimed mainly at investigating the detailed mechanisms of pyrolysis reactions. Guselnikov et al. [21,22] focused on the first initiation step of the pyrolysis reactions and Heuts et al. [23] studied hydrogen transfer reactions. Extensive amount of information regarding the overall rate constants of hydrocarbon decomposition, the distribution of primary products (the first generation of products forming by thermal cracking) was obtained in laboratory conditions. Reaction mechanisms hypotheses were formed as well as rigorous mathematical models were developed upon the observed yields and selectivities of different products. Rate constants of radical reactions were usually estimated using techniques of thermochemical kinetics or using linear-free relations [3,7,8,13,16,24–27] and subject to further modifications in the process of developing mathematical models that would match the experimental data. It is therefore clear that the pyrolysis of individual hydrocarbons has been experimentally studied quite intensively in past years and the results of such studies were used to study mechanisms of selected hydrocarbon cracking and for the derivation of mathematical models of varied complexity. The research was focused mainly on the pyrolysis of n-alkanes and some iso-alkanes, and to a lesser extent on the pyrolysis of non-substituted monocyclic hydrocarbons. Studies dealing with substituted cycloalkanes are rather scarce and studies involving unsaturated hydrocarbons are limited to the most common compounds, as were those involving hydrocarbon species containing more than one cycle per molecule. Published experimental papers are mostly aimed at studying selected hydrocarbon species, not at systematic studying of a structurally related hydrocarbon series. Moreover, each study was carried out in quite different experimental apparatus and more often than not at different conditions as well—different temperature and pressure profiles, different residence times in reactor coils and outlet coolers, different diluents a dilution ratios, and also the different approach to gas/liquid products separation and balance have significant effect on the experimental results. Therefore, the possibilities of cross-study comparison of results and their discussion are still limited due to the complexity of radical mechanisms and their sensitivity to reaction conditions and environment. One example of the cross-study disagreement may be provided by the comparison of Arrhenius parameters for the first-order overall thermal decomposition of selected n-alkanes reported by different authors (Table 1), where the differences are unreasonably large. Completely different values reported for the ethyl radical decomposition (Table 2) present another good example. Yet another is demonstrated in Table 3, which compares the results of two different n-alkanes pyrolysis. Separately, the results are reasonable, but their comparison violates even the generally accepted rule that ethylene yield increases with increasing temperature and n-alkane chain length. Those examples indicate that it is very difficult or even impossible to develop structure–reactivity or structure–selectivity relationships from separate studies published on a few individual hydrocarbons, albeit they are very detailed.

Table 1 Arrhenius parameters for the first-order overall thermal decomposition of selected n-alkanes by different authors. E, activation energy; k, rate constant. Substrate

E (kJ mol

n-hexane n-nonane n-hexadecane

221 264 162

1

)

1

k700 8C (s 3.3 1.4 7.0

)

Ref. [3] [28] [29]

Table 2 Arrhenius parameters for the ethyl radical decomposition (CH3–CH2 ! C2H4 + H) by different authors. A (s1) 9

3.2  10 3.2  1013 1.0  1014 4.0  1013 2.0  1013 3.8  1013

E (kJ mol1)

k800 8C (s1)

Ref.

167.5 167.5 197.3 168.7 160.0 159.1

2.3.101 2.3.102 2.5.104 2.4.105 3.2.105 6.8.105

[11] [30] [31] [7] [32] [33]

Studies of homological hydrocarbons were performed mainly for linear and non-linear alkanes [29,35–38], but they were not aimed at developing the structure–selectivity relationships. Their goals were mainly the research of radical reactions mechanism including the confirmation of Rice–Kosiakoff theory [3,16,26,29,34,35,39], the estimation of reaction order and reaction rates of pyrolysis reactions [3,7,13,16,24,26,29,34,39–42], or the effect of temperature on the selectivity of products formation [3,13,16,24,26,29,34,35,38,41]. In our previous work, it was demonstrated that laboratory pyrolysis experiments can be carried out via a simpler and faster technique employing a micro-pulse reactor coupled with a gas chromatograph [43]. This technique, similar to pyrolysis gas chromatography (Pyr-GC), was used to assay cracking yields of assorted steam-cracking feedstocks [44,45]. It was found [46] that it can provide very reliable results, the absolute standard deviation of product yields varying in the range of 0.05–0.75% (wt.) for 27 evaluated products or product fractions, while the range of detected products extends from hydrogen to pyrolysis oil. A mathematical model based on the artificial neural network was presented [47] showing that there is a good correlation between the product yields obtained with laboratory apparatus and corresponding yields observed in the data from an industrial unit [48]. The mathematical model was also able to predict yields of major products obtained by an industrial scale steam-cracking of a feedstock from the yields observed by Pyr-GC experiment using the same feedstock. Therefore, in order to study feedstock structure effect on the steam-cracking product composition it seems unnecessary to strive for maximum resemblance of an industrial unit. More likely, there is more benefit in processing as broad a range of compounds as possible at uniform conditions and in uniform equipment. Such a study can provide a solid base of data that can provide reference for industrial feedstocks screening as well as for validating industrial scale pyrolysis models [49]. Therefore, the work presented in this paper is aimed at carrying out laboratory micro-pyrolysis experiments at selected standard conditions for the series of hydrocarbon species that would, as completely as possible, represent all structure elements possibly present in the components of industrially pyrolysed feedstocks. It comprised saturated and unsaturated, cyclic and acyclic, branched and non-branched hydrocarbons with broad structure variability and various numbers of carbon atoms per molecule. The detailed list of all compounds included in this study is provided in Table 4. IDs provided in this table are used to improve legibility of other tables and figures. Table 3 Product yields for n-heptane and n-decane pyrolysis by different authors. Parameter

n-heptane [13]

n-decane [34]

Conversion (%) Temperature (8C) Methane yield (wt.%) Ethylene + ethane yield (wt.%) Propylene yield (wt.%)

89 750 12.4 44.9 18.6

88 810 5.9 41.1 16.7

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Table 4 Overview of hydrocarbon species covered in this paper by groups—aliphatic saturated (AS), cyclic saturated (CS), aliphatic unsaturated (AU), cyclic unsaturated (CU), aromatics and hydroaromatics (AR). GRP

ID

Name

AS AS AS AS AS AS AS AS AS AS AS AS AS AS AS AS AS CS CS CS CS CS CS CS CS CS CS CS CS CS CS CS CS AU AU AU AU AU AU AU AU AU AU CU CU CU CU AR AR AR AR AR AR AR AR AR AR AR

nPen nHex nHep nOct nDod nHxd 2MBu 2MHx 3MHx 2MHp 3MHp 22DMP 23DMP 24DMP 33DMP TMB 3EHx CP MCP ECP PCP CH MCH ECH PCH BCH 11DMCH 12DMCH 13DMCH 14DMCH 124TMCH CHCH DHN 1Pen 2Pen 1Hen 1Hpen 1Oen 1Dden 2MBen 2MPen 13Pden 14Pden CPen CHen 13CHden 14CHden TOL EB PB BB o-X m-X p-X 124TMB Ian Ien THN

n-pentane n-hexane n-heptane n-octane n-decane n-hexadecane 2-Methylbutane 2-Methylhexane 3-Methylhexane 2-Methylheptane 3-Methylheptane 2,2-Dimethylpentane 2,3-Dimethylpentane 2,4-Dimethylpentane 3,3-Dimethylpentane 2,2,3-Trimethylbutane 3-Ethylhexane Cyclopentane Methylcyclopentane Ethylcyclopentane Propylcyclopentane Cyclohexane Methylcyclohexane Ethylcyclohexane Propylcyclohexane Butylcyclohexane 1,1-Dimethylcyclohexane 1,2-Dimethylcyclohexane 1,3-Dimethylcyclohexane 1,4-Dimethylcyclohexane 1,2,4-Trimethylcyclohexane Cyclohexylcyclohexane Decahydronaphthalene 1-Pentene 2-Pentene 1-Hexene 1-Heptene 1-Octene 1-Dodecene 2-Methyl-2-butene 2-Methyl-2-pentene 1,3-Pentadiene 1,4-Pentadiene Cyclopentene Cyclohexene 1,3-Cyclohexadiene 1,4-Cyclohexadiene Toluene Ethylbenzene Propylbenzene Butylbenzene o-xylene m-xylene p-xylene 1,2,4-Trimethylbenzene Indane Indene Tetrahydronaphthalene

2. Experimental 2.1. Materials All feedstocks presented in this paper were obtained commercially (Sigma Aldrich) without further treatment. The purity of all samples was tested chromatographically using GCMS and GC/FID. The vast majority of samples contained more than 99% (w/w) of the nominal compound, the impurities being largely isomers of identical carbon number. The conversion values reported are based on the experimentally determined purity of the feedstocks.

Fig. 1. Scheme of the experimental unit (R, reactor; C1–C4, capillary columns; V1– V3, switching valves; I1 and I2, auxiliary injectors; FID1–FID3, flame ionization detectors; TCD, thermal conductivity detector; Q, auxiliary carrier-gas source; H, permanent oven-independent heating; positions of valves are shown as they are the start of the analysis).

Experiments were carried out using the micro-pyrolysis unit (Shimadzu Pyr-4A), attached on-line to a chromatographic system comprising of two gas chromatography units (Shimadzu GC-17A). The pyrolysis reactor was a narrow quartz tube (18 cm in length and 3 mm in maximum inner diameter). An electric furnace with controlled temperature (up to 820 8C) was used to heat the reactor. The temperature profile in the reactor was determined by a measurement using sensitive thermocouple. The profile was peak-shaped with maximum positioned in the middle of the reaction zone, defined as a portion of the reactor where the temperature exceeded 550 8C [46]. The reactor was operated in the state of steady continuous flow, but the actual samples were introduced as pulses into the carrier-gas flow. The quartz tube reactor was filled by carbide silica pellets to induce turbulent flow of the reaction mixture and minimize the temperature fluctuations after the sample injection. The liquid samples (0.2 ml) were injected directly into the pyrolysis unit, into the stream of carriergas (nitrogen) using a special micro-syringe. The very small volume of the sample combined with a relatively high thermal capacity of the reactor prevented the need for any active quenching of the reaction mixture. The gas chromatographic analytical system consisted of four valve-switched elution channels using two independent temperature programs during the analytical procedure. The scheme of the experimental arrangement is shown in Fig. 1, a further description regarding the experimental procedure is provided in detail in Ref. [43]. The analytical procedure enabled the assessment of hydrogen, C1–C4 hydrocarbons, as well as most C5–C8 hydrocarbons as individual compounds, assessment of the other reaction products was performed on a group basis. The carrier-gas flow rate and the reactor temperature were optimized on experiments studying the pyrolysis of industrial naphtha feedstocks [44] in order to achieve similar cracking severity (estimated in terms of the attained ethylene/propylene mass ratio) as under industrial conditions and the maximal resemblance of a flow regime in the pulse laboratory reactor to the plug-flow mode. Since the optimization of laboratory conditions were limited by the maximum attainable temperature of the pyrolysis reactor ( 810 8C), the only parameters to be further optimized were the carrier-gas flow rate and the pressure in the reactor. Thus, laboratory tests were conducted at a temperature of 810 8C, a pressure of 400 kPa and a carrier-gas flow rate of

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100 Nml min1. The residence time of a sample in the reaction zone was 0.3 s under those conditions. The on-line connection between the reaction and the analytical part of the equipment enables the acquisition of very reliable information about the reaction product as any need for hot-gas sampling that is usually prone to discrimination errors between mixture components with different boiling points or due to analysis by fractions is avoided. Additionally, the on-line arrangement, combined with the back-flush operation of the C2 column (i.e. the heaviest portion of products is eluted from the column in countercurrent direction), ensures that the entire range of products is detected and accounted for in the mass balance. The detailed analysis of the experimental procedure’s repeatability [46] showed that the overall (including the random errors of both the reaction and analytical part of the experiment) relative standard uncertainties of product content assay range from 1% to 4% (rel).

ward relationship to be observed. Since the increasing number of carbon atoms also means more hydrogen atoms susceptible to the hydrogen abstraction reaction, the rate of the hydrogen transfer increases and thus the conversion attained at standard conditions increases as well. Data in Table 5 indicates that the conversion increased from 78% for n-pentane to 99% for n-hexadecane. Longer hydrocarbon chains promote ethylene yields due to their ability to remove more ethylene molecules by successive C–C bonds b-scissions, regardless of the original position of the unpaired electron in the radicals formed by the hydrogen abstraction. On the other hand, a very long chain increases the probability that the C–C bonds b-scissions sequence would be interrupted by a C–H bond b-scission, leading to a higher 1-alkene. Therefore, the ethylene yield vs. carbon number relationship (see Fig. 2) exhibits a maximum and the yields of C5–C6 alkenes slightly increase with increasing chain length. The yields of propylene, and to some extent also those of methane, increase towards the low carbon numbers of the original feedstock due to the more pronounced effect of alkyl radicals with odd carbon numbers formed in the process of cracking. The case of propylene is also similar due to lower cracking severity attained for smaller less reactive molecules.

3. Results and discussion The products of the laboratory micro-pyrolysis of hydrocarbon species comprise hydrogen and a variety of hydrocarbons reported in Tables 5–8. The composition of products is represented, according to common industrial practice, by the content of individual products for those containing up to four carbon atoms per molecule, for important aromatics and by the content of the typical fraction for other products. The content of feedstock remaining in the product mixture was always determined on the basis of individual products, so that the conversion could be accurately determined. The data reported in the tables are calculated as averages of three independent measurements.

3.2. Effect of carbon chain branching Hydrocarbons with a branched carbon chain have a higher proportion of tertiary (or quaternary) and primary carbon atoms to secondary ones, compared to n-alkanes of the same carbon number. The effect of relatively weakly bonded hydrogen atoms on the tertiary carbon atom usually outweighs the effects of those more strongly bonded to the primary carbon atoms on the overall rate of hydrogen abstraction. Therefore, the branched hydrocarbons usually reach higher conversion at comparable conditions than their linear counterparts. For example, data of residual feedstock concentrations in Tables 5 and 7 show that the

3.1. Effect of hydrocarbon chain length The effect of hydrocarbon chain length (or that of carbon number) on the pyrolysis behavior of the n-alkanes is the most straightfor-

Table 5 Pyrolysis of different saturated aliphatic hydrocarbons at 810 8C. Composition of reaction mixture after pyrolysis (in columns—n-pentane; n-hexane; n-heptane; n-octane; ndecane; n-hexadecane; 2-methylbutane; 2-methylhexane; 3-methylhexane; 2-methylheptane; 3-methylheptane; 2,2-dimethylpentane; 2,3-dimethylpentane; 2,4dimethylpentane; 3,3-dimethylpentane; 2,2,3-trimethylbutane; 3-ethylhexane). Feedstock w (wt.%)

Hydrogen Methane Ethane Ethylene Propane Propylene Acetylene Isobutane Propadiene n-butane 2-Butene 1-Butene Isobutene Propyne 1,3-Butadiene Cyclopentadiene Other NA C5–C6 Benzene Feedstock Toluene Ethylbenzene Xylenes Styrene Naphthalene Other C7–C12 C12+

n

n

n

n

n

n

2

2

3

2

3

22

23

24

33

Pen

Hex

Hep

Oct

Dod

Hxd

MBu

MHx

MHx

MHp

MHp

DMP

DMP

DMP

DMP

TMB

EHx

3

1.2 9.3 3.4 33.6 0.5 18.6 0.3 0.1 0.1 tr. 0.5 4.2 0.1 0.3 3.1 0.3 1.6 0.3 22.1 0.1 tr. tr. tr. tr. 0.1 tr.

1.3 10.2 3.4 39.1 0.6 17.6 0.4 0.2 0.1 tr. 0.6 4.6 0.1 0.2 4.2 tr. 2.3 0.7 13.8 0.2 tr. tr. tr. tr. 0.2 tr.

1.6 7.1 2.1 47.8 0.5 17.3 0.6 0.2 0.1 tr. 0.5 4.6 0.1 0.3 4.3 0.3 3.7 0.4 12.2 0.1 tr. tr. tr. tr. tr. tr.

1.6 8.5 3.7 43.2 0.6 17.0 0.4 tr. 0.2 tr. 0.6 4.7 0.1 0.2 5.1 0.7 4.7 tr. 7.3 0.3 tr. tr. 0.1 tr. 0.8 0.1

1.8 7.7 3.4 42.5 0.6 18.4 0.4 tr. 0.1 tr. 0.7 5.7 0.1 0.3 6.7 tr. 7.2 tr. 2.7 0.5 0.1 0.1 0.2 tr. 0.8 0.1

1.9 7.5 3.7 41.0 0.6 18.7 0.4 tr. 0.1 tr. 0.8 5.6 0.1 0.3 7.5 tr. 8.7 tr. 0.9 0.8 0.1 0.1 0.2 tr. 1.0 tr.

1.1 12.1 2.0 16.2 0.4 19.6 0.3 0.7 tr. tr. 4.1 2.6 9.9 tr. 3.6 1.0 3.2 0.5 21.8 0.3 tr. 0.1 tr. tr. 0.4 tr.

1.5 11.0 2.3 27.7 0.6 23.1 0.4 0.6 0.1 tr. 0.8 3.6 7.4 0.6 5.0 1.4 3.3 1.2 8.5 0.6 tr. 0.1 0.1 tr. 0.2 tr.

1.5 12.2 3.4 30.0 0.7 21.0 0.6 0.5 0.1 tr. 2.1 3.4 2.4 0.7 6.6 2.0 4.3 1.5 5.7 0.7 0.1 0.1 0.1 tr. 0.3 tr.

1.7 9.0 2.2 34.0 0.5 22.1 0.5 0.6 0.1 tr. 0.7 3.5 6.0 0.6 5.3 1.2 3.1 1.1 5.9 0.5 0.1 0.1 0.1 tr. 1.0 0.1

1.6 10.1 3.2 36.5 0.6 19.1 0.7 0.6 0.1 tr. 1.8 4.1 2.3 0.7 6.4 1.7 3.4 1.0 5.3 tr. 0.1 0.1 0.1 tr. 0.5 tr.

1.4 13.5 2.3 19.8 0.4 11.2 0.4 1.4 tr. tr. 0.4 1.3 24.9 1.2 1.8 4.1 5.9 2.0 4.4 1.8 0.1 0.5 0.1 tr. 0.7 0.2

1.5 15.8 2.7 17.4 0.7 26.5 0.6 0.5 0.1 tr. 3.8 3.5 2.1 0.9 6.9 5.1 5.0 2.5 1.4 1.5 0.1 0.4 0.2 tr. 0.7 0.2

1.5 11.7 1.1 9.8 0.5 36.3 0.4 1.0 tr. tr. 0.7 3.4 13.3 0.8 3.6 1.7 4.0 1.4 7.0 0.8 0.1 0.2 0.1 tr. 0.4 tr.

1.4 14.6 4.2 23.9 0.8 5.7 0.4 1.4 0.1 tr. 0.5 0.8 20.3 1.5 2.1 6.0 7.9 2.2 1.7 2.1 0.2 0.6 0.2 0.1 1.2 0.3

1.6 12.6 1.6 11.1 0.5 22.2 0.4 1.8 tr. tr. 0.5 1.8 23.8 1.4 2.0 4.1 6.3 2.3 1.8 1.9 0.1 0.6 0.2 tr. 1.1 0.2

1.7 11.2 4.0 39.2 1.0 12.4 1.0 0.4 0.1 tr. 0.9 5.3 0.7 0.7 10.4 1.9 4.0 1.6 2.0 0.5 0.1 0.1 0.1 tr. 0.6 tr.

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Table 6 Pyrolysis of different saturated cyclic hydrocarbons at 810 8C. Composition of reaction mixture after pyrolysis (in columns—cyclopentane; methylcyclopentane; ethylcyclopentane; propylcyclopentane; cyclohexane; methylcyclohexane; ethylcyclohexane; propylcyclohexane; butylcyclohexane; 1,1-dimethylcyclohexane; 1,2dimethylcyclohexane; 1,3-dimethylcyclohexane; 1,4-dimethylcyclohexane; 1,2,4-trimethylcyclohexane; cyclohexylcyclohexane; decahydronaphthalene). Feedstock 11

12

13

14

124

w (wt.%)

CP

MCP

ECP

PCP

CH

MCH

ECH

PCH

BCH

DMCH

DMCH

DMCH

DMCH

TMCH

CHCH

DHN

Hydrogen Methane Ethane Ethylene Propane Propylene Acetylene Isobutane Propadiene n-butane 2-Butene 1-Butene Isobutene Propyne 1,3-Butadiene Cyclopentadiene Other NA C5–C6 Benzene Feedstock Toluene Ethylbenzene Xylenes Styrene Naphthalene Other C7–C12 C12+

0.4 0.1 0.1 6.7 tr. 8.0 tr. 0.1 tr. tr. tr. 0.1 0.2 tr. 0.2 1.8 3.0 0.1 78.8 tr. tr. tr. tr. tr. 0.2 tr.

1.2 4.7 0.6 10.7 0.1 14.9 0.2 tr. 0.4 tr. 0.5 1.2 2.9 0.4 4.8 6.4 6.6 3.5 37.5 0.9 tr. tr. tr. 0.3 2.1 tr.

1.4 4.3 1.8 18.3 0.1 8.6 0.2 0.3 tr. tr. 0.5 3.9 0.4 0.3 5.5 6.8 8.8 3.6 30.3 0.9 0.1 0.2 0.3 0.3 2.2 0.9

1.6 4.6 1.6 23.9 0.2 13.4 0.3 0.3 tr. tr. 0.4 2.5 0.4 0.3 5.2 6.5 8.3 4.4 21.3 0.9 0.1 0.2 0.4 0.3 1.9 0.9

1.2 1.1 0.7 16.7 tr. 3.5 0.6 tr. tr. tr. 0.4 0.4 tr. 0.2 18.1 1.3 3.0 1.5 50.6 0.1 tr. tr. tr. tr. 0.6 tr.

1.5 6.3 0.7 18.0 0.1 8.9 0.6 0.4 tr. tr. 0.6 1.0 0.7 0.7 16.0 2.9 7.2 4.3 25.6 1.3 0.1 0.2 0.2 tr. 2.1 0.5

1.9 6.9 1.2 30.3 0.2 5.8 1.2 0.3 tr. tr. 0.4 1.7 0.2 0.7 16.3 3.0 7.1 6.5 9.3 1.5 0.2 0.4 0.4 0.1 3.9 0.3

1.9 7.5 1.3 33.3 0.2 9.9 1.1 0.3 tr. tr. 0.4 1.5 0.3 0.6 14.5 2.9 6.5 7.1 5.3 1.6 0.2 0.2 0.4 0.2 2.3 0.2

1.9 8.4 3.1 32.6 0.3 10.3 0.6 0.2 0.1 tr. 0.8 1.7 0.4 0.4 10.5 3.0 5.1 9.2 2.5 3.1 0.3 0.4 0.8 0.4 3.1 0.9

1.6 8.4 1.0 17.2 0.1 6.1 1.0 1.2 tr. tr. 0.3 1.1 7.7 1.9 8.4 2.1 12.7 5.7 15.8 3.2 0.4 0.5 0.4 0.1 3.1 tr.

1.7 12.8 2.0 17.8 0.2 9.5 0.5 tr. 0.2 tr. 1.5 1.2 0.6 0.5 8.9 3.4 8.6 10.4 5.6 5.8 0.4 1.1 1.0 0.6 4.3 1.5

1.5 8.4 0.8 11.1 0.1 13.9 0.4 0.4 tr. tr. 0.7 1.7 1.7 0.8 tr. 6.4 6.8 5.3 22.8 2.9 0.2 0.6 0.3 0.2 2.6 0.7

1.7 10.5 1.1 11.8 0.2 16.8 0.5 tr. 0.4 tr. 1.1 1.5 2.5 0.6 10.1 3.7 7.8 7.8 10.6 4.6 0.3 0.9 0.7 0.3 3.6 1.0

1.6 13.6 1.3 11.8 0.2 15.2 0.5 0.4 tr. tr. 1.9 1.5 1.4 0.8 8.1 3.8 9.7 9.2 6.7 6.6 0.5 0.2 0.3 0.2 3.3 1.1

2.3 3.5 0.8 31.7 0.1 4.9 1.9 0.3 tr. tr. 0.4 1.4 0.2 0.4 22.9 3.5 4.7 9.3 1.9 3.1 0.5 0.8 0.8 0.2 4.6 tr.

1.9 4.7 0.5 20.0 0.2 4.5 0.9 0.2 tr. tr. 0.2 0.8 0.1 0.4 7.8 2.8 8.0 14.0 12.1 6.0 1.0 0.8 1.3 0.6 10.7 0.5

impossible for a radical with an unpaired electron in the immediate vicinity of a tertiary or quaternary carbon atom. Therefore, a good measure of the ethylene producing ability is the number of –CH2– groups in the molecule, as shown in Fig. 3. The yield profile shows that even fully branched hydrocarbons with

conversion of n-heptane is 87.8%, while those of methyl hexanes are 91.5–94.2% and those of dimethyl pentanes are 93.0–98.6%, under standard conditions. Branched carbon chains have a significant effect on the yields of pyrolysis products. The formation of ethylene by b-scission is

Table 7 Pyrolysis of different unsaturated aliphatic and cyclic hydrocarbons at 810 8C. Composition of reaction mixture after pyrolysis (in columns—1-pentene; 2-pentene; 1-hexene; 1-heptene; 1-octene; 1-dodecene; 2-methylbutene; 2-methylpentene; cyclopentene; cyclohexene; 1,3-pentadiene; 1,4-pentadiene; 1,3-cyclohexadiene; 1,4-cyclohexadiene). Feedstock w (wt.%)

Hydrogen Methane Ethane Ethylene Propane Propylene Acetylene Isobutane Propadiene n-butane 2-Butene 1-Butene Isobutene Propyne 1,3-Butadiene Cyclopentadiene Other NA C5–C6 Benzene Feedstock Toluene Ethylbenzene Xylenes Styrene Naphthalene Other C7–C12 C12+

1

2

1

1

1

1

2

2

13

14

13

Pen

Pen

Hen

Hpen

Oen

Dden

Mben

Mpen

CPen

CHen

Pden

Pden

Chden

14 Chden

1.7 6.8 4.7 31.8 0.5 20.6 0.3 0.2 0.2 tr. 1.2 3.8 0.3 0.3 10.0 2.1 3.0 6.3 0.4 2.1 0.2 0.3 0.7 0.3 1.5 0.9

1.7 14.6 2.4 14.6 0.3 9.3 1.1 0.2 tr. tr. 2.1 2.7 0.2 0.6 23.6 3.9 8.1 6.9 0.2 2.9 0.4 0.6 0.8 0.3 2.2 0.2

1.7 8.3 2.8 29.9 0.5 23.1 0.3 0.2 0.1 tr. 1.2 5.3 0.2 0.2 9.8 2.0 3.1 5.6 tr. 2.0 0.2 0.3 0.6 0.3 1.5 0.8

1.8 6.5 4.5 35.9 0.5 18.7 0.3 0.2 0.2 tr. 1.2 3.8 0.2 0.2 8.8 2.2 3.1 5.9 0.1 2.0 0.2 0.3 0.7 0.3 1.6 0.9

1.8 7.2 3.6 35.7 0.5 19.4 0.3 0.2 0.1 tr. 1.1 3.6 0.2 0.2 8.7 2.2 3.4 5.8 0.1 2.0 0.2 0.3 0.7 0.3 1.7 0.8

2.0 6.5 2.1 37.4 0.4 20.3 0.4 0.2 0.1 tr. 1.0 4.9 0.2 0.3 10.6 2.0 3.2 4.6 tr. 1.4 0.1 0.2 0.5 0.2 1.1 0.5

1.3 9.4 0.5 4.2 tr. 3.5 0.2 0.4 0.3 tr. 2.2 0.4 12.7 1.3 3.5 20.8 5.4 2.1 21.8 3.0 0.2 1.9 0.3 0.1 3.3 1.0

1.7 17.2 3.3 10.8 0.3 4.7 0.3 0.5 tr. tr. 0.7 0.6 6.3 1.1 4.2 13.5 6.0 7.3 0.9 6.6 0.5 3.0 1.0 0.7 5.0 3.7

1.8 0.8 0.1 4.5 tr. 5.0 0.1 tr. tr. tr. 0.1 0.1 tr. 2.5 1.2 60.3 4.0 1.1 10.3 0.4 0.1 0.1 0.3 1.5 5.2 0.4

2.1 1.4 0.3 32.3 tr. 1.5 1.0 tr. 0.1 tr. 0.5 0.3 tr. 0.3 35.5 3.1 3.6 13.4 0.1 1.4 tr. tr. tr. tr. 2.9 tr.

1.7 5.6 0.4 3.5 tr. 1.9 0.5 0.1 tr. tr. 0.6 0.5 tr. 0.3 19.6 14.1 32.4 5.0 5.2 2.1 0.2 0.6 0.6 0.6 3.9 0.5

1.8 3.4 0.4 7.1 tr. 8.0 0.7 0.1 tr. tr. 0.6 0.9 0.1 0.3 14.9 24.1 19.6 4.9 5.4 1.8 0.1 0.3 0.5 0.7 3.8 0.4

2.4 3.8 0.7 4.2 tr. 1.5 0.1 tr. tr. tr. 0.4 0.2 tr. 0.1 2.7 5.3 1.2 66.3 tr. 2.5 0.1 0.3 0.9 1.5 2.8 3.0

2.7 0.1 tr. 0.1 tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. 0.1 0.1 tr. 96.6 tr. tr. tr. tr. tr. tr. 0.1 tr.

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212

Table 8 Pyrolysis of different aromatic species at 810 8C. Composition of reaction mixture after pyrolysis (in columns—toluene; ethylbenzene; propylbenzene; butylbenzene; orthoxylene; meta-xylene; para-xylene; 1,2,4-trimethylbenzene; indane; indene; tetrahydronaphthalene). Feedstock w (wt.%)

TOL

EB

PB

BB

o-X

m-X

p-X

124 TMB

Ian

Ien

THN

Hydrogen Methane Ethane Ethylene Propane Propylene Acetylene Isobutane Propadiene n-butane 2-Butene 1-Butene Isobutene Propyne 1,3-Butadiene Cyclopentadiene Other NA C5–C6 Benzene Feedstock Toluene Ethylbenzene Xylenes Styrene Naphthalene Other C7–C12 C12+

tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. 0.1 99.7 tr. tr. tr. tr. tr. 0.1 tr.

1.5 2.2 0.4 6.5 tr. 0.2 0.2 tr. tr. tr. tr. tr. tr. tr. 0.1 tr. 0.1 19.6 20.0 8.9 tr. 1.2 35.5 0.5 3.5 2.5

1.8 2.0 0.6 17.1 0.1 0.6 0.3 tr. tr. tr. tr. tr. tr. tr. 0.2 0.1 0.1 11.5 0.3 18.1 9.7 0.9 29.5 0.5 7.8 2.5

1.8 3.8 1.6 16.2 0.2 3.1 0.1 tr. tr. tr. tr. 0.2 tr. tr. 0.4 tr. 0.1 7.4 2.0 17.1 7.1 0.4 26.2 2.0 6.5 7.0

0.1 0.1 tr. 0.1 tr. tr. 0.1 tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. 0.1 96.5 0.3 tr. 0.4 1.6 tr. 0.8 0.1

tr. tr. tr. tr. tr. tr. 0.1 tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. 97.9 0.2 tr. tr. 1.3 tr. 0.3 0.2

tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. 98.4 0.6 tr. 0.1 tr. tr. 0.5 0.3

0.1 0.2 tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. 0.1 91.5 0.1 tr. 1.6 0.1 0.1 4.9 1.4

0.7 0.2 0.1 2.0 tr. 0.1 tr. tr. tr. tr. tr. tr. tr. tr. 0.1 0.1 0.4 0.3 39.8 5.9 tr. 0.4 0.2 0.2 50.4 0.3

0.1 0.2 tr. 0.4 tr. 0.2 tr. tr. tr. tr. tr. tr. tr. tr. 0.1 tr. tr. 0.9 87.0 0.3 0.3 0.2 0.1 0.4 8.4 1.3

1.4 1.8 0.1 3.7 tr. 0.2 0.1 tr. tr. tr. tr. tr. tr. tr. 0.2 tr. 0.1 1.8 17.9 1.0 0.1 0.9 5.5 38.7 27.4 1.7

N CH2  ¼ 0 (e.g. 2,2,3-trimethylbutane) produce about 12% of ethylene due to secondary reactions. Pyrolysis of branched hydrocarbons leads to significantly higher yields of methane. However, methane yield is not directly related to the proportion of methyl groups in the molecule (see methane yields of different C7isomers in Table 5), but rather to the number of methyl groups in the b-position to the most probable position of radical center formation by hydrogen transfer. Two neighboring tertiary carbon atoms, or a quaternary carbon (with two methyl substituents), amidst the carbon chain are the configurations most favoring high methane yields. The lower the yield of ethylene with decreasing N CH2  , the higher is the cumulative propylene and butene yields. Nevertheless, the propylene/butane ratio is independent of the N CH2  and depends on the exact configuration of the pyrolysed

Fig. 2. Pyrolysis of linear alkanes: yields of hydrogen (&), methane (~), ethylene (*), propylene (*), butenes (&), butadienes (^), and C5–C6 non-aromatics (5), obtained by laboratory pyrolysis at 810 8C related to the linear alkane carbon number.

hydrocarbon molecule. The presence of a quaternary carbon atom strongly favors high yields of butenes (namely isobutene) as does the 2-monomethyl substitution of the main carbon chain, albeit to a much lesser extent. 3.3. Effect of double bonds The induction effect of the double bond in a hydrocarbon weakens the C–H bond on carbon atoms in b-positions to the double bond substantially and thus formation of allyl-type radicals by hydrogen abstraction is made very easy. Since hydrogen abstraction is the rate controlling step of the pyrolysis reaction

Fig. 3. Pyrolysis of branched alkanes: yields of methane (~), ethylene (*), propylene (*), and butenes (&), obtained by laboratory pyrolysis of 2-methylbutane, 2-methylhexane, 3-methylhexane, 2-methylheptane, 3-methylheptane, 2,2dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 2,2,3-trimethylbutane, and 3-ethylhexane at 810 8C related to the number of –CH2– groups per molecule (data of different isomers with identical N data are slightly shifted on X-axis to improve readability).

P. Za´mostny´ et al. / J. Anal. Appl. Pyrolysis 87 (2010) 207–216

system, unsaturated hydrocarbons reach higher conversion than their saturated counterparts. All tested samples reached almost total conversion at standard conditions, regardless of the carbon number, with the only exception being 2-methyl-2-butene where the lower conversion is attributed to two factors. There are only primary-carbon-bonded hydrogen atoms available for abstraction and forming radicals, blocking the only possible C–C bond bscission opportunity by the double bond presence in that position. The most basic effect of the double bond on the pyrolysis product yields consists in the lower H/C ratio in unsaturated feedstocks and hence the products reflect that fact, containing more dienes and aromatics. As long as there is only a single double bond in the feedstock molecule, its relative importance decreases as the carbon number of the molecule grows bigger. As a consequence, higher olefins lead to similar yields of most products as alkanes. In a more detailed view, the double bond works like a bscission blocker and its effect depends on its position within the hydrocarbon. Obviously, the more C–C bond connections, which exist to the double bonded carbons, the more b-scissions the double bond is able to block. The most desirable products (light alkenes) require the greatest number of scissions per feedstock molecule to occur and hence their yields are most hampered if the double bond occupies the central position in the carbon chain or the branching nodes in case of branched hydrocarbons. The detailed composition of products formed by the pyrolysis of various alkenes is provided in Table 7. Important trends in product yields can be related to the number of –CH2– groups in the molecule that represents the portion of the carbon chain available for splitting (see Fig. 4). The yield of ethylene rises steeply from NCH2  ¼ 0 to N CH2  ¼ 2 due to the fact that the alkene must contain at least two neighboring –CH2– groups to be able to form a radical with the unhindered possibility of splitting off ethylene via b-scission (e.g. 1-penten-5-yl). The further increase in ethylene yield is much slower and can be attributed to the decreased importance of the unsaturated part of the molecule. The trend of propylene yield mostly follows that of ethylene. The low number of –CH2– groups per molecule increase the probability that any formed radical is stabilized by C–H, rather than by C–C bond bscission. Hence, such compounds produce high yields of dienes, e.g. 1,3-butadiene or higher, and as a consequence also substantial yields of aromatics formed either by the intra-molecular cyclization of higher dienyl radicals or the Diels–Alder additions. Hydrocarbons with no –CH2– groups constitute the exception from the trend as their carbon chain tends to be too short or too

Fig. 4. Pyrolysis of linear and branched alkenes: yields of ethylene (*), butenes (&), C5–C6 non-aromatics (5), and aromatics (^) obtained by laboratory pyrolysis at 810 8C related to the number of –CH2– groups per molecule.

213

branched to produce a conjugated diene suitable for aromatization. Those hydrocarbons produce C5–C6 branched alkenes and dienes in high yields instead. 3.4. Effects of cyclopentane and cyclohexane rings The composition of cyclic hydrocarbon pyrolysis products (Table 6) shows that unsubstituted saturated cyclic hydrocarbons attain much lower conversions at standard conditions in comparison to acyclic ones. The conversion for cyclohexane is about 50% and only slightly above that of 20% for cyclopentane. Though it can be partially explained [50] by slightly higher values of C–H bond dissociation energies in cyclic structures (especially in five-membered rings) and thus more difficult hydrogen transfer reactions, the observed magnitude of the conversion difference between the cyclic and the acyclic molecules is probably beyond that explanation. More likely this behavior can be attributed to the initiation phase. Biradicals that are products by initiation homolytic splitting can undergo b-scission, which shortens the distance between the two radical centers and thus brings about the possibility of intra-biradical recombination preventing the bi-radical from starting a propagation chain. The shorter original distance between the unpaired electrons in pent-1,5-diyl is the reason for the lower reactivity of cyclopentane. Cycloalkenes exhibit much higher conversion values as the double bond weakens certain C–H bonds and also blocks the bi-radical recombination. Products yields are not influenced by the initiation significantly and therefore, the main products can be devised from the decomposition of cyclopentyl or cyclohexyl radicals respectively. Cyclopentane produces mainly ethylene, propylene and cyclopentane, while cyclohexane produces ethylene and 1,3-butadiene. 3.5. Effects of ring substitution and the substituent Monomethyl substitution of a cycloalkane increases the attained conversion substantially. Quite logically, the conversion further increases with increasing substituent size, i.e. increasing the aliphatic portion of the molecule. The detailed composition of the products formed by the pyrolysis of various substituted cycloalkanes is provided in Table 6. Fig. 5 shows the relationship between the size of the substituent and the yields of important products for the pyrolysis of n-alkyl-cyclopentanes. While there is only one kind of cyclopentyl radical that can

Fig. 5. Pyrolysis of n-alkyl substituted cyclopentanes: yields of ethylene (*), propylene (*), butenes (&), butadienes (^), C5–C6 non-aromatics (5), and aromatics (^) obtained by laboratory pyrolysis at 810 8C related to the substituting n-alkyl carbon number.

214

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produce ethylene, there are four different radicals originating from methylcyclopentane, only three of which can produce ethylene directly. Therefore the ethylene yield profile in Fig. 5 shows a sharp minimum for methylcyclopentane. The ethylene yield increases for higher values of N due to the larger n-alkyl substituent’s ability to produce ethylene. A similar relationship can also be observed for propylene yields at N = 2. Yields of C5 and C6 non-aromatic products are attributed to b-scission of C–H bonds in the cyclopentane ring and hence there is only a small difference among various alkylcyclopentanes corresponding to the portion of the aliphatic part of the molecule. Butadienes, butenes and aromatics can form only by secondary reactions during cyclopentane pyrolysis. Substituted cyclopentanes offer primary reactions leading to butenes and dienes, the yields of which are increased as is the yield of aromatics. Again increasing the size of the substituent (i.e. the linear part of the molecule) decreases the yields of those products. The effect the n-alkyl length of the substituent of cyclohexane has on the pyrolysis yields of ethylene (Fig. 6) is similar to that for cyclopentanes. While unsubstituted cyclohexane provides a very high yield of 1,3-butadiene, substituted cyclohexanes exhibit decreasing yields, proportional to the substituent alkyl length. The typical reaction pathway of the cyclohexane ring, which leads, via cyclohexyl, to ethylene and 1-buten-4-yl, is very likely to produce 1,3-butadiene because the C–C bond in the b-position to the radical center is strengthened by the immediate vicinity of the double bond. If the formed radical is substituted, there is often another C–C bond available for splitting. The products of such further reactions are often propylene, isobutene or a higher (iso)alkene, depending on the exact feedstock. Therefore, profiles of those compounds are influenced by the exact substituent. Increasing the length of the substituted alkyl chain increases the possibility that the initial reactions take place completely on the substituent and produce cycloalkenyl. Such a radical has an increased probability to split off hydrogen radicals and produce (substituted) cyclohexadiene, which rapidly forms aromatic compounds either by disproportionation or by the abstraction– scission mechanism. This increased probability is evident from the high benzene yield by pyrolysis of cyclohexene, shown in Table 7.

Fig. 7. Pyrolysis of multi-methyl substituted cyclohexanes: yields of ethylene (*), propylene (*), butenes (&), butadienes (^), C5–C6 non-aromatics (5), and aromatics (^) obtained by laboratory pyrolysis at 810 8C related to the number of methyl substituents (different dimethyl cyclohexane isomers data are slightly shifted from N = 2 to improve readability).

di-, and trimethyl cyclohexanes. The detailed product compositions (Table 6) show that the exact configuration of substituents (see different dimethylcyclohexanes) has a moderate effect; however, there are still trends that can be attributed to the number of substituents regardless of their position (Fig. 7). The most marked trends are represented by the ethylene and butadiene yields rapidly decreasing with increasing degree of substitution. Like in the case of acyclic branched hydrocarbons, it is caused by the fact that the radicals formed during the reactions often have the unpaired electron in a position other than terminal, so that propylene, isobutene or even 2-butene are produced rather than ethylene. In addition, the radicals formed after breaking the ring are always unsaturated, so that if they are branched, they can produce relatively large structures that cannot be easily split further, as was observed for acyclic substituted alkenes.

3.6. Effect of multiple ring substitutions 3.7. Effect of substituted benzene ring The effect of multiple substitutions on a monocyclic compound was studied on a series of cyclohexanes and mono-,

Fig. 6. Pyrolysis of n-alkyl substituted cyclohexanes: yields of ethylene (*), propylene (*), butenes (&), butadienes (^), C5–C6 non-aromatics (5), and aromatics (^) obtained by laboratory pyrolysis at 810 8C related to the substituting n-alkyl carbon number.

Aromatic ring is very stable and does not undergo scission reactions. However, its presence in the molecule has significant impact on the behavior of the substituting hydrocarbon chains. Methyl substituted benzenes are also very stable, because the benzyl type radicals do not offer any feasible possibility of a scission reaction. The negligible conversion of toluene attained at 810 8C and 0.2 s residence time corresponds to the observations made by Pant at al. [15]. There is only one at least slightly significant reaction pathway involving formation of –CH2–CH2– chain by recombination or isomerization, further reactions involving that aliphatic chain. Since there are very little olefinic products, the addition reactions do not occur. Conversely, monoalkyl substituted benzenes starting from C2 substituent are reactive, actually more reactive than their saturated counterparts (compare data for ethyl, propyl, and butyl benzene in Table 8 with ethyl, propyl, and butyl cyclohexane in Table 6). It can be explained by very easy formation of a benzyl type radical by hydrogen abstraction. The dominance of the radicals with unpaired electron in the immediate vicinity of the aromatic ring leads to very high yields of styrene, compared to benzene or toluene yields, because the b-scission sequence starts preferably from that end of the aliphatic chain attached to the aromatic ring. Yields of alkenic products that increase with the length of aliphatic chain also subscribe to the formation of condensation product by their

P. Za´mostny´ et al. / J. Anal. Appl. Pyrolysis 87 (2010) 207–216 Table 9 Pyrolysis of methyl cyclohexane—comparison with published data. Za´mostny´

Pant [14]

Bajus [2]

Conversion

74.4

72.5

50

Yields (wt.%) Methane Ethylene Propylene 1,3-Butadiene Benzene Toluene

8.5 24.2 12.0 21.5 5.8 1.7

13.0 25.1 15.3 16.2 7.7 1.8

9.3 22.6 13.3 21.1 4.5 3.0

reactions with aromatic rings or with styrene. Hydro-aromatics can form radicals by abstraction quite easily, like substituted benzenes. However, the presence of the aromatic ring influences the C–H bond strength on the attached ring and thus the C–H bond b-scission is preferred much more than in saturated rings. Therefore, very high yields of indene from indane and naphthalene from tetrahydronaphthalene were observed. The results for tetrahydronaphthalene are in general agreement with the results of theoretical study published by Poutsma [41]. 3.8. Comparison with published data Owing to the declared aim of this work to compare pyrolysis behavior of individual hydrocarbons related to their structure, it is not easy to provide a suitable comparison of our results with published data. As it was already shown in the introduction, the variability of experimental conditions and equipment used in published studies affect not only the obtained yields, but lead also to substantial differences in published kinetic data. However, it is possible to validate results of this study semi-quantitatively with the findings of other authors. For example, the observed trend of product yields of n-alkane pyrolysis showing maximum for nheptane corresponds to compared results published by Bajus et al. [1] and Bartekova and Bajus [3]. Nevertheless, more detailed comparison is hardly possible due to quite different conditions used for our study. It is possible to compare obtained experimental data for methyl cyclohexane pyrolysis with values published by Pant [14] (Table 9). Similar conversion was observed 72.4 vs. 74.4 at relatively close temperatures (820 8C vs. 800 8C) and residence time compensating for the temperature effects. Experiments yielded close to 25% ethylene yield (wt.), our apparatus slightly favoring 1,3-butadiene and propylene over methane (by approximately 2%) owing to the higher pressure used by our experimental setup. The results of the study published by Bajus [2] are also added to the comparison. Further evidence that our results match with those obtained in narrowly oriented studies can be found, e.g. the results for tetrahydronaphthalene are in general agreement with the results of theoretical study published by Poutsma [41], or the dominant yields of styrene obtained by alkyl benzenes pyrolysis correspond to the results published by Billaud [51]. 4. Conclusions The results of this study present the detailed composition of pyrolysis products obtained by laboratory pyrolysis of 56 hydrocarbons. Hydrocarbons included in this study represent many actual compounds forming the real naphtha feedstocks in industrial scale steam-cracking. They also represent all important aspects of structural variability that is encountered in higher boiling industrial feedstocks. Measured data were used to identify important trends in product yields in relation to the feedstock structure. Observed trends are provided in the graphical form and

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