- Email: [email protected]
Copyrolysis of naphtha with polyalkene cracking products; the influence of polyalkene mixtures composition on product distribution
J. Anal. Appl. Pyrolysis 79 (2007) 196–204 www.elsevier.com/locate/jaap
Copyrolysis of naphtha with polyalkene cracking products; the influence of polyalkene mixtures composition on product distribution Elena Ha´jekova´ *, Bozˇena Mlynkova´, Martin Bajus, Lenka Sˇpodova´ Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinske´ho 9, 812 37 Bratislava, Slovakia Received 18 July 2006; accepted 18 December 2006 Available online 23 December 2006
Abstract The steam cracking (copyrolysis) of naphtha with oils/waxes from thermal decomposition of polyalkenes has been investigated as a process for chemical recycling of plastic wastes. High-density polyethylene (HDPE), two-component mixture (LDPE/PP) and three-component mixture (HDPE/LDPE/PP) were thermally decomposed in a batch reactor at 450 8C, thus forming oil/wax products. Subsequently, these products were dissolved in heavy naphtha in the amount of 10 mass% to obtain steam cracking feedstock. The composition of gaseous and liquid products during copyrolysis was studied at 780 8C and 820 8C in dependence on residence time from 0.08 s to 0.51 s. The obtained results were compared with the product composition from steam cracking of naphtha at identical experimental conditions. The decomposition of polyalkene oils/waxes during copyrolysis was confirmed on the basis of analysis of liquid products. It was shown that more ethene and propene was formed during copyrolysis of oil/wax from HDPE in comparison with naphtha and both mixtures and so oil/wax from HDPE seems to be favourable component of steam cracking feedstock. There were slight differences between product compositions from copyrolysis of two- and three-component mixtures. The presence of HDPE in three-component mixture supported formation of gas and ethene. The presence of oil/wax form PP enhanced formation of propene and branched alkenes. For both type of polyalkenic mixtures the yields of desired low molecular alkenes and alkanes were higher or approximately the same as from naphtha. The results confirm suitability of oils/waxes from polyalkenes as a co-feed for steam cracking units. # 2007 Elsevier B.V. All rights reserved. Keywords: Copyrolysis; Polyalkene waxes; Recycling; Plastics; Thermal decomposition; Steam cracking; HDPE; LDPE; PP
1. Introduction Plastics are important components of municipal solid waste. Numbers of reports have been published on different approaches of feedstock and/or chemical recycling of plastics, mainly polyalkenes, including thermal [1–5] and catalytic degradation [6,7], and hydrocracking [8]. These processes enable transformation of waste plastics into valuable petrochemicals or fuels [9]. Several petrochemical companies have considered possible recycling of plastic wastes in existing refinery facilities, which would avoid the need to invest and build new processing plants [10–12]. This alternative is based on the similarity of elementary composition of plastics and petroleum fractions. There exist two main approaches to co-processing of plastic
* Corresponding author. Tel.: +421 2 59325401; fax: +421 2 52493198. E-mail address: [email protected] (E. Ha´jekova´). 0165-2370/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2006.12.022
wastes in refinery units. The plastics can either be added directly into the petroleum fractions [13–15] or the plastic waste is first thermally decomposed into smaller molecules. Thermal decomposition of polymers leads to a complex mixture of products. In general terms, up to four product fractions can be recovered: gases, oils and/or solid waxes and a solid residue. Depending on the process temperature (cracking <600 8C, pyrolysis 600 8C), a variety of products and applications can be envisaged from thermal decomposition of polymeric materials: fuel gases, olefinic gases useful in chemical synthesis, naphtha and middle distillates, oil fraction, long-chain paraffins and olefins, coke, etc. [10]. In order to minimize investments for the recycling of municipal waste, the co-processing of light thermal cracking oil (LTCO) derived from municipal waste plastics has been studied and demonstrated by a Japanese oil refinery company. The cohydrotreating of vacuum gas oil and LTCO has been selected as a favourable LTCO upgrading method [11]. A different approach to processing of oils derived from waste plastics
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
was proposed by Tsuji et al. [16]. They studied steam reforming of oils derived from LDPE and polystyrene (PS) with the aim to produce hydrogen rich synthesis gas. Research of different scientists [1,3,4] showed the possibility of thermal decomposition of mixed-plastics waste in different reactor types using mild conditions (450–550 8C) to gain high yields of oil-waxy hydrocarbons suitable as a cofeed for many downstream processing units. Production of the oil/wax as an intermediate product provides flexibility because the oil/wax may be easily transported as a solid, a liquid or in a solution [4]. The condensed oil/wax can be mixed with liquid petroleum fractions. Such a mixture may be used as a refinery feedstock; for example, it may be fed to a steam cracker to produce reusable olefins [4,5] or it may be fed to a catalytic cracker to produce gasoline or it may be even upgraded in a hydrocracker [4]. The advantage of above-mentioned approach is much higher solubility of the oils/waxes in comparison with the initial plastics and also a better manipulation with the mixture of oils/waxes in liquid petroleum fractions. Also important is the fact that potential impurities present in plastic waste are removed during thermal decomposition. The predominant mechanism in cracking of hydrocarbons is a free-radical chain. In case of binary or multicomponent mixtures the radical intermediates produced by one compound can interact with those produced by another, or with the reactant itself [17]. Most of plastic waste is in the form of mixed plastic waste. Depending on mixture composition some polymers may either enhance or inhibit the degradation of other polymers within mixed plastic waste [18] and product distribution can differ from that obtained by simple summation of product yields obtained from individual polymers. Different types of polyalkenes are produced (LDPE, HDPE, PP and their copolymers) and hence all these types in different forms can be found as individual or mixed in waste. It is well known that ternary carbons decrease thermal stability in polymers of similar nature. In case of PE, the higher the branching degree the lower the density, and the lower decomposition temperature [19]. Polypropylene molecule is characterized by the presence of a side methyl group at every second carbon and so PP is thermally degraded at a faster rate than both LDPE and HDPE [10]. Although several authors studied thermal decomposition of polyalkene mixtures, there is still lack of knowledge about interactions between PP and PE and obtained results are in some cases contradictory. It can be due to the fact that interactions between compounds largely depend on mixture composition, on degradation temperature, on reactor type, on phase, in which reaction occurs (liquid or gaseous), etc. Westerhout et al. [20] studied kinetics and product spectrum during pyrolysis of PE/PP mixture using different reactor types. No significant effect due to mixing of different polymers was observed in reactors with a short gas-phase residence time. Predel and Kaminsky [2] did not find the product-changing interactions between PE and PP during pyrolysis in a fluidised-bed reactor and on pyro-GC/MSdevice.
197
However, Ciliz et al. [21] during slow pyrolysis of PE/PP mixture at temperature up to 600 8C observed mutual influence of these polyalkenes reflected in product distribution. Also Ohmukai et al. [22] studied binary interactions of LDPE and PP during pyrolysis in a high-pressure batch reactor at 0.6 MPa and at 450 8C and in a low-pressure gas-flowing reactor at 600 8C. The authors confirmed the interactions between LDPE and PP both in liquid and in gaseous phase during pyrolysis. Dolezˇal et al. [23] found that the relative amount of the pyrolysis products of LDPE/PP blends were exponentially dependent on their composition in pyro-GC/MS experiments. Wong and Lam [24] confirmed changes in induction time for degradation of PE/ PP blends. From the foregoing results it is evident that polyalkenes can interact during thermal decomposition what results in different quality of gaseous and oil/wax products obtained by thermal decomposition of PE/PP mixtures. In case of copyrolysis we add oils/waxes obtained from PE/PP mixtures, which are composed mostly of higher linear or branched saturated and unsaturated hydrocarbons, into mixture of lighter hydrocarbons creating naphtha. Complex mutual influence of components is expected, because various authors confirmed influence of branched and linear hydrocarbons during radical decomposition in steam-cracking conditions. For example, inhibitingaccelerating effect was observed during steam cracking of 2,2,4-trimethylpentane mixed with hexadecane [25] or with heptane [26]. In the background literature, there is no work dealing with copyrolysis of naphtha with oils/waxes obtained by cracking of mixed polyalkenes. In a recent work [5], we have studied steam cracking of naphtha with addition of oils/waxes from individual LDPE or PP. The objective of the present study was to investigate (i) the thermal decomposition of individual HDPE and of two- and three-component mixtures of polyalkenes (LDPE/PP; HDPE/LDPE/PP) in a batch reactor at 450 8C as a process for preparation of oil/wax–feedstock; (ii) the influence of residence time and temperature on conversion and product distribution during steam cracking (copyrolysis) of prepared oils/waxes with naphtha in a tubular reactor at 780 8C and 820 8C. 2. Experimental 2.1. Materials The feedstocks used in the experiments were obtained from Slovnaft, a.s. /Inc./ (Slovak Republic). Heavy straight-run naphtha was a fraction from the atmospheric distillation unit with distillation range between 98 and 181 8C and with bromine number 1.8 g Br2/100 g. The molar mass of the naphtha was 116 g mol1 and its density was 726 kg m3 (at 25 8C). The polyalkenes used were as follows: high-density polyethylene ¯ w ¼ 33; 800; M ¯ n ¼ 6; 950; r = 950 kg m3), lowHDPE (M ¯ w ¼ 292; 000; M ¯ n ¼ 22; 000; density polyethylene LDPE (M ¯ w ¼ 200; 000; r = 919 kg m3) and polypropylene PP (M ¯ n ¼ 65; 000; r = 903 kg m3), all of them were virgin M plastics. The diameter of pellets ranged from 3.3 to 4.4 mm.
198
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
2.2. Solutions of oils/waxes from polyalkenes in naphtha The oil/wax products were prepared by thermal cracking of individual (HDPE) and mixed polyalkenes: two-component mixture LDPE/PP (1:1 mass) and three-component mixture HDPE/LDPE/PP (1:1:1 mass). The thermal decomposition of plastics was carried out in a batch reactor at temperatures up to 450 8C in nitrogen atmosphere. The prepared oils/waxes are predominantly composed of linear or branched alkenes and alkanes. The bromine numbers of prepared oils/waxes expressed in g Br2/100 g of the sample were for HDPE, twocomponent, and three-component mixtures as follows: 49.8, 64.7 and 57.0, respectively. Contrary to the virgin plastics, the oils/waxes are well soluble in naphtha. Three 10 mass% solutions of oils/waxes in naphtha were prepared and consequently used in copyrolysis experiments. A detailed description of the cracking apparatus and of the procedure was made in [5]. 2.3. Copyrolysis We performed a series of steam cracking experiments (copyrolysis) with 10 mass% solutions of oils/waxes in naphtha. The oils/waxes were obtained from HDPE, from LDPE/PP and from HDPE/LDPE/PP. The laboratory tubular stainless-steal reactor was used. The experiments were made at 780 8C and 820 8C, residence times from 0.08 s to 0.51 s and at steam to feedstock mass ratio of 0.5. In individual experiments different residence times were obtained by changing the feedstock flow rates. The flow of naphtha and wax solutions varied from 15 g h1 to 70 g h1. Before feeding it into the reactor, the feedstock was warmed up to 60 8C, to ensure homogeneous solution of oil/wax in naphtha. A detailed description of the steam cracking apparatus as well as the procedure was given in [5]. In order to find out about the influence of polyalkene oil/wax on product distribution, we conducted steam cracking experiments with pure heavy naphtha under the conditions similar to those during copyrolysis. We were not able to perform steam cracking experiments with pure oil/wax fractions from polyalkenes due to problems with charging semi-solid feedstock and due to a high coking tendency of these raw materials. Copyrolysis offers the possibility to overcome these shortcomings [5]. 2.4. Analysis of gaseous and liquid steam cracking products The qualitative and quantitative analyses of gases and liquids formed during copyrolysis were performed. Gas chromatograph HP 6890 + equipped with three PLOT chromatographic columns, FID and TCD detectors and with three switching valves was used for analysis of gases. This system allows for a detailed separation of the mixture of C1–C5 gaseous hydrocarbons, saturated and unsaturated, hydrogen and carbon oxides in one sample. The types of columns and analysis conditions were listed in [5].
The analyses of liquids formed during copyrolysis were performed on a CHROM V gas chromatograph equipped with a FID detector. Helium was used as carrier gas. A DB-PETRO (50 m 0.2 mm i.d. 0.5 mm) polydimethylsiloxane capillary chromatographic column was operated under following oven temperature programme: 40 8C followed by a heating rate of 2 8C min1 up to 60 8C, then an increase up to 240 8C at a heating rate of 5 8C min1, and finally 30 min at 240 8C. Solutions of the samples in n-hexane (5 mass%) were prepared and injected into the chromatograph in amounts of 0.3 ml. 3. Results and discussion 3.1. Thermal decomposition of polyalkenes The original plastics were solid at laboratory temperature and in the form of pellets. During thermal decomposition they produced a hydrocarbon gas, a light yellow oil/wax fraction collected in the separator, and a brown waxy solid residue. The yields of gases, oils/waxes and residues on the bottom of the batch reactor are listed in Table 1. The following profiling products were identified in the gases from HDPE cracking in decreasing order: propene, propane, butane, 1-butene, ethane and ethene. In case of two-component (LDPE/PP) and threecomponent (HDPE/LDPE/PP) mixture of polyalkenes, the prevailing products were: propene, pentane, methylpropene, propane and ethane. A detailed composition of the formed gases is given in Table 2. Also traces of hydrogen, benzene, and toluene were detected in the gases from the thermal decomposition of plastics. Comparing the yields of components in gases from thermal decomposition of HDPE and from decomposition of LDPE [5], we can see that HDPE yielded more ethene, propane and propene. The yields of methane and ethane were equal from both polyethylenes. On the contrary, lower amounts of C4 to C6 hydrocarbons were formed during thermal decomposition of HDPE. Williams and Williams [27] have also observed approximately the same content of methane and ethane from HDPE as from LDPE. They have also declared a higher content of ethene and propane and a lower content of butenes in the gases from HDPE. The mentioned differences in product distribution could be related with the fact, that HDPE is a highly linear polymer, whereas LDPE possesses a certain degree of branching providing a higher proportion of reactive tertiary carbons for the initiation step of degradation and LDPE degradation takes place at lower temperatures than in case of HDPE [10]. Table 1 Yields of products (Y) from thermal cracking of HDPE, LDPE/PP and HDPE/ LDPE/PP at 450 8C Feedstock
HDPE LDPE/PP HDPE/LDPE/PP
Y (mass%) Oil/wax
Gas
Residue
79.8 79.4 82.4
17.0 17.2 13.8
3.2 3.4 3.8
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
199
Table 2 Comparison of experimental and theoretical product distributions (mass%) in gases from thermal cracking of HDPE and polyalkene mixtures at 450 8C Component (mass%)
Methane Ethane Ethene Propane Propene Methylpropane Butane Methylpropene 2-Butenes 1-Butene Pentane 1,3-Butadiene 1,3-Pentadiene 1-Pentene 2-Methyl-1-butene Hexane 2-Methyl-1-pentene 1-Hexene
PP [5]
LDPE [5]
HDPE
LDPE/PP
Exp.
Exp.
Exp.
Exp.
Theo.
Exp.
Theo.
2.3 6.4 0.5 3.4 43.1 0.3 0.2 10.0 0.1 0.2 23.3 tr. 1.1 0.3 0.5 – 5.2 –
2.7 8.3 4.5 14.9 14.2 0.2 12.6 1.2 3.2 12.2 6.0 1.3 0.6 7.4 tr. 1.3 – 3.4
2.6 8.7 7.3 19.1 25.5 0.1 10.0 0.7 3.7 8.8 2.4 1.5 0.2 3.3 – 0.2 – 0.8
3.1 8.1 2.2 8.6 36.9 0.4 3.6 10.0 0.7 4.1 15.8 0.4 0.5 1.2 0.3 0.1 1.7 0.3
2.5 7.3 2.5 9.1 28.7 0.2 6.4 5.6 1.7 6.2 14.7 0.7 0.8 3.8 0.2 0.7 2.6 1.7
3.3 8.5 3.4 10.5 34.7 0.4 4.4 9.7 1.0 4.8 13.7 0.7 0.4 1.3 0.3 0.1 1.2 0.2
2.5 7.8 4.1 12.5 27.6 0.2 7.6 3.9 2.4 7.1 10.6 0.9 0.6 3.7 0.2 0.5 1.7 1.4
In order to be able to determine mutual influence of feedstock components on formation of gaseous thermal cracking products, we have calculated theoretical product distributions based on product distributions obtained during pure HDPE, LDPE, or PP thermal decomposition. The theoretical distribution of products from thermal decomposition of two- and three-component mixtures is listed in Table 2. Comparing experimental and theoretical product distributions in gases, it is evident that there exist some differences. In gases from thermal decomposition of both mixtures actually more propene, methylpropene and pentane were detected, which are prevailing components of gaseous products from cracking of polypropylene. The experimental yields of propane, butane, butenes, 1-pentene, hexane and 1-hexene were lower than theoretical yields. These components are prevailing in gases from thermal decomposition of polyethylenes. From this comparison it results, that polypropylene and polyethylenes influence each other when they are thermally decomposed in the mixture. In the product distribution in gases this influence manifest itself mainly in higher formation of light products from polypropylene degradation. We have also confirmed differences between experimental and theoretical product distribution in liquid cracking products using GC—analysis of oils from thermal decomposition of polyalkene mixtures [31]. 3.2. Copyrolysis of oils/waxes from plastics with naphtha The selection of the optimal process for treating of oils and waxes derived from thermal decomposition (cracking) of plastics depends to a great extent on the composition of oils and waxes. Oil/wax products from polyethylene cracking are composed predominantly of linear alkanes and 1-alkenes [3,28]. The oil/ wax fraction formed during PP cracking is a complex mixture of
HDPE/LDPE/PP
branched alkenes and alkanes [28,29]. In steam cracking straight-chain paraffins produce high yields of straight-chain alkenes (mainly ethene). Branched hydrocarbons yield some branched products (mainly propene) and are valuable components of steam cracking feedstock. Similar paths to paraffins decompose higher a-olefins and the products are similar to those observed in paraffin steam-cracking [30]. The main products from PS cracking are aromatic hydrocarbons, namely styrene, toluene, ethyl-benzene and other styrene derivatives [10]. However, aromatics are undesirable in steam-cracking feeds, because they greatly contribute to coke formation and give small yields of alkenes. That is why we have not used PS as a component of mixtures studied in present work. Following-up our work [5] in which we had copyrolysed oils/waxes from LDPE and PP, we made experiments at identical experimental conditions with solutions of oils/waxes from HDPE and from mixtures of LDPE/PP and HDPE/LDPE/ PP in naphtha. We chose identical experimental conditions, as in case of naphtha alone, to be able to clearly establish how the replacement of naphtha by a non-petroleum material, in the amount of 10 mass%, influences the overall steam cracking process. The 10 mass% allowance of oil/wax into naphtha as a co-feed is well dissolved. Such a substitution of crude oil raw material in large-scale ethylene units can bring considerable economic benefits. The price of feedstock substantially influences the steam cracking economy, mainly at present, when on the world market the crude oil price is rather high. The results obtained from steam cracking of naphtha and from copyrolysis of oils/waxes at 780 8C are given in Tables 3 and 4 and the results at 820 8C are given in Tables 5 and 6. The steam cracking under used experimental conditions led to gaseous (Figs. 1 and 2) and liquid products. Alkenes are the main gaseous products while ethene (Figs. 3 and 4) and propene (Figs. 3 and 4) are the prevailing components in the gases.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
200
Table 3 Conversion to gas (X), molar mass of gas (M) and yields of gaseous components (Y) from steam cracking of different feedstocks at 780 8C depending on the residence time (t) Feedstock Naphtha X (%) t (s) Mw (g mol1)
51.2 0.11 30.0
Naph/HDPE 67.4 0.17 28.7
Component
Y, mass%
Hydrogen Carbon oxides Methane Ethane Ethene Propane Propene Propadiene Acetylene trans-2-Butene 1-Butene Methylpropene cis-2-Butene 1,3-Butadiene 3-Methyl-1-butene trans-2-Pentene 1,3-Pentadiene 2-Methyl-2-butene 1-Pentene 2-Methyl-1-butene Isoprene Unknown in gas
0.3 tr. 6.2 1.9 16.8 0.3 11.7 0.1 0.1 0.5 2.7 1.7 0.4 3.6 0.1 0.3 0.2 0.1 0.4 0.4 0.5 2.4
0.4 tr. 9.4 2.8 23.3 0.4 15.1 0.1 0.2 0.6 2.9 2.2 0.5 4.8 0.2 0.3 0.2 0.1 0.3 0.4 0.6 2.2
73.6 0.38 26.5
0.5 0.2 13.0 3.3 27.8 0.4 14.5 0.2 0.4 0.5 1.5 2.0 0.4 4.9 0.3 0.1 0.1 0.1 0.1 0.2 0.5 2.0
56.0 0.09 31.1
0.2 tr. 6.1 2.1 18.3 0.3 12.7 0.1 0.1 0.5 3.2 1.7 0.4 3.9 0.1 0.4 0.2 0.1 0.6 0.5 0.4 3.2
Naph/LDPE/PP
72.3 0.16 29.5
0.4 tr. 9.4 2.8 24.8 0.4 15.8 0.1 0.3 0.6 3.2 2.1 0.5 5.4 0.2 0.3 0.2 0.1 0.4 0.4 0.5 3.2
78.7 0.41 25.8
0.5 0.1 16.9 5.0 29.3 0.5 14.0 0.1 0.3 0.4 1.1 1.7 0.3 3.8 0.2 0.1 0.1 0.1 0.2 0.1 0.2 2.1
51.9 0.11 29.0
0.4 tr. 5.9 1.9 16.9 0.3 12.2 0.1 0.1 0.5 2.8 2.2 0.4 3.6 0.2 0.3 0.2 0.1 0.4 0.5 0.5 1.9
66.0 0.16 27.5
0.7 tr. 8.8 2.7 22.2 0.4 14.9 0.1 0.2 0.6 2.8 2.6 0.4 4.6 0.2 0.2 0.2 0.1 0.3 0.5 0.6 2.2
Naph/HDPE/LDPE/PP 76.4 0.40 23.8
1.0 0.2 15.9 4.1 29.1 0.4 13.7 0.1 0.4 0.4 1.1 2.0 0.3 3.9 0.3 0.1 0.1 0.1 0.1 0.2 0.4 1.6
52.7 0.10 30.4
0.3 tr. 6.1 2.0 17.1 0.3 12.3 0.1 0.1 0.5 2.9 2.0 0.4 3.7 0.2 0.3 0.2 0.1 0.4 0.5 0.5 2.4
68.1 0.16 29.6
77.4 0.41 25.6
0.3 tr. 8.9 2.7 23.1 0.4 15.0 0.1 0.3 0.6 2.8 2.4 0.4 5.0 0.2 0.3 0.2 0.1 0.3 0.4 0.6 3.0
0.5 0.2 16.4 4.4 29.3 0.4 14.1 0.1 0.3 0.4 1.1 2.0 0.3 4.1 0.3 0.1 0.1 0.1 0.1 0.1 0.4 1.9
tr.—traces.
The influence of steam cracking severity (i.e. prolongation of residence time and temperature increase) on the yields of products is important. The yields of ethene varied for different feedstocks from 16.8 to 29.3 mass% at 780 8C and at 820 8C they reached values from 27.5 to 34.6 mass%. The ethene yields increased with increasing residence time and at 820 8C they passed through moderate maximum. The propene yields passed through maximum in the dependence on residence time at both
temperatures and the decrease in propene yields was stronger at 820 8C. The yields of propene varied from 11.7 to 15.8 mass% at 780 8C and at 820 8C they reached values from 8.1 to 15.4 mass%. From the dependences it is evident that a decrease in propene yields is caused by the secondary (mainly splitting) reactions on forming ethene and methane. From the C4 hydrocarbons, the most prevailing is 1,3-butadiene followed by 1-butene and 2-butenes (Tables 3 and 5). In the C5
Fig. 1. Influence of residence time on gas formation during copyrolysis of oils/ waxes in comparison with steam cracking of naphtha at 780 8C.
Fig. 2. Influence of residence time on gas formation during copyrolysis of oils/ waxes in comparison with steam cracking of naphtha at 820 8C.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
201
Table 4 Yields of liquid components (Y) from steam cracking of different feedstocks at 780 8C depending on residence time (t) and on feedstock conversion to gas (X) Naphthab
Feedstock Naphtha
X (%) t (s)
0 –
51.2 0.11
Naph/HDPE 67.4 0.17
73.6 0.38
Component
%mass
Y, mass%
Benzene Heptane Methylcyclohexane Toluene + 2-methylheptane Dimethylcyclohexane Ethylcyclopentane Octane 2,6-Dimethylheptane Ethylbenzene p + m-Xylene Styrene o-Xylene Nonane Ethyltoluene Methylstyrenes Decane Methylindenes Naphthalene Methylnaphthalenes Anthracene + Phenanthrene Unknown in liquid
0.2 5.6 4.9 1.4 + 2.6 2.4 1.0 6.0 3.3 1.6 2.2 – 0.8 5.7 1.0 – 3.8 – – – – 57.5a
4.3 2.9 0.1 6.1 0.8 0.6 2.2 0.2 0.9 2.4 0.8 1.4 2.2 1.5 1.8 1.7 0.2 0.1 0.2 0.1 18.4
56.0 0.09
3.0 0.7 0.7 4.5 0.4 0.3 1.2 0.1 0.7 2.0 1.2 1.2 0.9 2.2 0.5 1.4 0.4 0.4 0.2 0.1 10.8
Naph/LDPE/PP
72.3 0.16
3.5 – – 4.7 – – – – 0.6 1.9 2.3 1.3 0.2 1.8 0.3 0.9 0.4 2.0 0.7 0.1 4.2
78.7 0.41
1.9 1.5 1.8 4.1 0.9 0.8 3.2 0.8 0.7 1.6 0.8 1.3 2.4 1.1 1.0 0.3 0.2 0.2 0.1 0.1 19.9
3.3 0.7 0.8 4.5 0.4 0.3 1.3 0.4 0.7 2.1 1.2 1.1 1.0 1.6 0.5 0.3 0.3 0.4 0.2 0.1 7.2
51.9 0.11
4.2 tr. 0.1 4.1 0.3 0.1 0.3 tr. 0.4 1.4 1.1 1.4 0.7 1.6 0.3 1.2 0.5 1.7 0.6 0.2 2.6
66.0 0.16
4.7 2.0 2.1 6.8 1.0 0.7 3.1 0.9 0.9 2.1 1.3 1.4 2.3 1.9 0.8 0.3 0.2 0.2 0.1 0.1 15.8
Naph/HDPE/LDPE/PP 76.4 0.40
4.6 0.9 0.9 6.2 0.5 0.3 1.5 0.4 0.9 2.5 1.7 1.4 1.1 1.6 0.5 0.5 0.4 0.5 0.2 0.1 7.9
52.7 0.10
5.2 – – 5.2 – – – – 0.5 1.6 2.1 1.0 0.2 1.3 0.3 0.9 0.6 1.9 0.8 – 3.2
68.1 0.16
2.1 2.1 1.6 3.0 0.7 0.6 1.9 0.4 0.9 2.1 1.0 0.9 2.0 1.6 1.8 1.6 1.1 0.8 – – 21.5
6.3 1.1 0.8 4.6 0.3 0.2 1.2 0.1 0.7 1.8 1.3 1.1 0.8 0.8 1.4 0.4 0.4 1.0 0.7 – 7.8
77.4 0.41
5.7 0.2 0.2 4.0 0.1 0.1 0.3 tr. 0.4 1.4 1.7 0.8 0.2 0.4 1.3 0.2 0.5 1.3 0.5 – 3.9
tr.—traces. a A sum of other identified and unknown hydrocarbons in feedstock naphtha. b Composition of feedstock naphtha.
hydrocarbons, isoprene seems to be the dominant hydrocarbon (around 0.5 mass%). The higher gas formation and the higher yield of smaller molecules (e.g. methane, acetylene) witnesses deeper splitting of the feedstock and the course of secondary reactions when the severity of pyrolysis increases. Formation of carbon oxides and hydrogen was higher at 820 8C (Table 5) and it reflects the
course of secondary reactions leading to a formation of carbonaceous solid products (coke). Carbon oxides and hydrogen are products of gasification reactions between precursors of carbonaceous deposits or coke, and steam. Oils/waxes obtained by thermal decomposition of polyalkenes are predominantly composed of linear and branched alkenes and alkanes depending on the polymer type. Alkane-alkenic
Fig. 3. Influence of residence time on formation of ethene and propene during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 780 8C.
Fig. 4. Influence of residence time on formation of ethene and propene during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 820 8C.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
202
Table 5 Conversion to gas (X), molar mass of gas (M) and yields of gaseous components (Y) from steam cracking of different feedstocks at 820 8C depending on the residence time (t) Feedstock Naphtha X (%) t (s) Mw (g mol1)
73.9 0.12 27.0
Naph/HDPE 79.4 0.19 25.3
Component
Y, mass%
Hydrogen Carbon oxides Methane Ethane Ethene Propane Propene Propadiene Acetylene trans-2-Butene 1-Butene Methylpropene cis-2-Butene 1,3-Butadiene 3-Methyl-1-butene trans-2-Pentene 1,3-Pentadiene 2-Methyl-2-butene 1-Pentene 2-Methyl-1-butene Isoprene Unknown in gas
0.6 0.3 11.5 2.5 28.1 0.4 14.2 0.2 0.5 0.5 2.0 1.8 0.4 5.5 0.4 0.2 0.1 0.1 0.1 0.2 0.5 2.7
0.8 0.7 14.9 2.9 31.8 0.3 13.1 0.2 0.7 0.3 0.9 1.5 0.3 5.3 0.5 0.1 tr. tr. 0.1 0.1 0.4 2.6
72.1 0.40 22.3
0.9 1.3 18.8 3.3 31.2 0.2 8.1 0.1 0.7 0.1 0.2 0.7 0.1 2.9 0.3 tr. tr. tr. tr. tr. 0.2 1.2
78.5 0.12 28.1
0.5 0.1 11.4 2.8 29.6 0.4 15.4 0.2 0.5 0.5 2.5 1.8 0.4 5.7 0.4 0.1 0.2 0.1 0.3 0.3 0.5 3.6
Naph/LDPE/PP
84.4 0.18 27.0
0.6 0.1 14.7 3.2 32.8 0.4 14.5 0.2 0.6 0.4 1.7 1.7 0.3 5.6 0.4 0.2 0.1 0.1 0.2 0.2 0.4 3.3
80.4 0.35 23.4
0.8 0.5 20.1 4.1 34.6 0.3 10.6 0.1 0.6 0.2 0.4 0.9 0.2 3.3 0.3 0.1 tr. tr. tr. tr. 0.2 1.4
73.5 0.11 26.6
0.5 0.1 11.4 2.6 27.5 0.4 14.4 0.2 0.5 0.4 2.1 2.1 0.4 5.3 0.4 0.1 0.1 0.1 0.2 0.3 0.6 2.7
80.4 0.20 24.8
0.6 0.1 15.7 3.3 32.0 0.4 13.4 0.2 0.6 0.3 1.4 1.7 0.3 4.3 0.4 0.2 0.1 0.1 0.2 0.3 0.5 2.6
Naph/HDPE/LDPE/PP 70.0 0.37 22.5
0.8 0.5 17.1 3.3 29.9 0.3 9.7 0.1 0.6 0.2 0.4 1.0 0.1 3.1 0.3 0.1 tr. tr. tr. 0.1 0.2 1.2
75.9 0.12 28.1
0.5 0.1 11.3 2.7 28.0 0.4 14.7 0.2 0.5 0.5 2.1 2.1 0.4 5.6 0.4 0.1 0.1 0.1 0.2 0.3 0.5 3.6
80.6 0.16 27.1
73.0 0.37 23.9
0.5 0.1 13.8 3.1 31.0 0.4 14.6 0.2 0.6 0.4 1.5 2.0 0.3 5.4 0.4 0.1 0.1 0.1 0.1 0.2 0.5 2.9
0.6 0.2 17.8 3.5 30.8 0.3 10.0 0.1 0.6 0.2 0.4 1.1 0.2 3.3 0.3 0.1 tr. tr. tr. tr. 0.2 1.7
tr.—traces.
feedstock decomposes preferably into low molecular alkenes and alkanes [7]. Our results also support this observation (Tables 3 and 5). The most noticeable differences between the yields of low-molecular alkenes formed at copyrolysis in comparison to steam cracking of naphtha are observed for the system naphtha/ HDPE. During copyrolysis of oil/wax from HDPE with naphtha the yields of ethene were higher up to 2.6 mass% (what represents 8% relat.) in comparison with the ethene yields from pure naphtha. The yields of propene were higher up to 1.2 mass% (i.e. 12.7% relat.) than the propene yields from naphtha. The described differences are observed especially at severe steam cracking conditions (long residence time, 820 8C). The yields of low-molecular alkenes obtained during copyrolysis of HDPE were even higher than those from copyrolysis of LDPE [5] at identical experimental conditions. It is obvious, that the presence of HDPE during recycling of polyalkene mixtures via copyrolysis would be favourable. From comparison of product yields obtained during copyrolysis of two- and three-component mixtures following observations resulted. The oil/wax from pure PP provided during copyrolysis less gas and ethene than pure naphtha, but more propene and methylpropene [5]. Now in two-component mixture of PP with LDPE the yield of gas from copyrolysis increases to the level of gas creation from naphtha (Figs. 1 and 2). The same is observed for ethene yields (Figs. 3 and 4). The higher splitting of oil/wax from LDPE into ethene probably compen-
sates low ethene and gas formation from polypropylene oil/wax. As concerns propene production, this is a bit higher from twocomponent than from three-component mixture and also higher than that from naphtha alone due to higher proportion of branched molecules coming from PP decomposition (Fig. 3). In case of three-component mixture the oil/wax from HDPE, which give high yields of gas and ethylene, was added in the system. The presence of HDPE reflects in slightly higher
Fig. 5. Influence of residence time on content of unreacted alkanes during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 780 8C.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
Fig. 6. Influence of residence time on formation of naphthalene during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 820 8C.
formation of gas and ethene during copyrolysis of threecomponent mixture at both temperatures in comparison with two-component mixture (Figs. 2 and 4). It is important to point out that presented tendencies are very moderate. Bigger differences between product distribution from copyrolysis of different polyalkene mixtures could manifest themselves much more at higher content of oils/ waxes in naphtha than was the content of 10 mass% used in our experiments. The construction of our experimental apparatus did not allow us to agitate the warmed feedstock at charging it into the reactor and that is why we could not work with more concentrated solutions of oils/waxes. But in industrial steam cracking units liquid feedstock is usually preheated and dilution of oils/waxes from polyalkenes in higher amounts could not cause problems at charging. These results confirm, that after replacing 10 mass% of conventional liquid steam cracking feedstock by the oils/waxes from polyalkenes, the yields of valuable light olefins are comparable to those from steam cracking of naphtha, or in some cases are higher than from pure naphtha. Goossens et al. [32] reported that in a typical 1 billion-pounds/year ethylene plant, a 0.8 wt.% increase in ethylene selectivity could correspond to a net profit gain of $1 million. But also at unchanged yields of desired alkenes replacement of a part of petroleum feedstock by the non-petroleum feed in high capacity steam cracking units could be economically advantageous. The total world ethylene capacity in 2002 was close to 110 million metric tons per annum and naphtha steam cracking represents about 45% of this production capacity [33]. Together with gases, liquids are also formed during steam cracking. The yields of components in liquids from copyrolysis and from pyrolysis of naphtha are listed in Tables 4 and 6. The composition of naphtha feedstock is also given in Table 4. At both temperatures the total content of unreacted alkanes determined in pyrolysis liquids decreases with prolongation of residence time (Fig. 5). The content of aromatics increases with increase of pyrolysis severity (Tables 4 and 6). From the described relation new formation of aromatics is evident. This is valid for benzene, toluene, styrene, ethyltoluene, methyl-
203
styrene, naphthalene with its derivatives and anthracene with phenantrene. The presence of small amount of unreacted wax in pyro-liquid was detected only during experiments with oil/wax from HDPE at the mildest conditions (780 8C, residence time 0.09 s). At more severe conditions the wax did not occur in products, which proves its decomposition under experimental conditions at all used feedstocks. Comparing the yields of liquid products from copyrolysis with those from steam cracking of naphtha, we can state that the formation of benzene was in case of copyrolysis of two- and three-component mixture slightly higher than from steam cracking of naphtha. Yields of toluene, C8 aromatics and C9 aromatics are comparable to the yields from naphtha. Naphthalene formation at 820 8C (Fig. 6) is at the copyrolysis conditions for all feedstocks lower than naphthalene formation during steam cracking of individual naphtha. Olefins, diolefins, aromatics, polyaromatics and acetylenic compounds are defined as the coke precursors in the process of steam cracking [34]. We observed comparable yields of main gaseous coke precursors such as 1,3-butadiene, propadiene, isoprene and acetylene during copyrolysis in comparison with steam cracking of naphtha. From these results we could expect approximately equal coke formation during copyrolysis of oil/ wax solutions, as is the coke formation during steam cracking of naphtha alone. Acknowledgement We would like to thank the VEGA Scientific Grant Agency of the Slovak Republic, the commission for chemistry and chemical technology, for its financial support for this work through research project No. 1/3587/06. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13]
J. Walendziewski, Fuel Process. Technol. 86 (2005) 1265. M. Predel, W. Kaminsky, Polym. Degrad. Stabil. 70 (2000) 373. P.T. Williams, E.A. Williams, J. Anal. Appl. Pyrol. 51 (1999) 107. K.C. Kirkwood, S.A. Leng, W. David, Polymer Cracking, US Patent 5364995 (1994). E. Ha´jekova´, M. Bajus, J. Anal. Appl. Pyrol. 74 (2005) 270. S. Karago¨z, J. Yanik, S. Uc¸ar, M. Saglam, C. Song, Appl. Catal. A-Gen. 242 (2003) 51. D.P. Serrano, J. Aguado, J.M. Escola, E. Garagorri, J. Anal. Appl. Pyrol. 58/59 (2001) 789. C.M. Simon, W. Kaminsky, B. Schlesselmann, J. Anal. Appl. Pyrol. 38 (1996) 75. A.G. Buekens, H. Huang, Resour. Conserv. Recycl. 23 (1998) 163. J. Aguado, D. Serrano, Feedstock Recycling of Plastic Wastes, The Royal Society of Chemistry, Cambridge, 1999, p. 192. T. Kawanishi, N. Shiratori, H. Wakao, E. Sugiyama, H. Ibe, M. Shioya, T. Abe, in: M. Muller-Hagedorn, H. Bockhorn (Eds.), Upgrading of Light Thermal Cracking Oil Derived from Waste Plastics in Oil Refinery, Proceedings of the International Symposium on Feedstock Recycling of Plastics, Karlsruhe, Germany, September 25–29, 2005, Universita¨tsverlag Karlsruhe, 2005. A. Tukker, Rapra Rev. Rep., 13 (2002) 122. J. Aguado, D.P. Serrano, J.M. Escola, in: J. Scheirs, W. Kaminsky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley and Sons, Chichester, 2006, , p. 102, Chapter III.
204
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
[14] J. Walendziewski, in: J. Scheirs, W. Kaminsky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley and Sons, Chichester, 2006, , p. 119, Chapter IV. [15] J. Yanik, T. Karayildirim, in: J. Scheirs, W. Kaminsky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley and Sons, Chichester, 2006, , p. 217, Chapter VIII. [16] T. Tsuji, S. Okajima, T. Masuda, in: M. Muller-Hagedorn, H. Bockhorn, (Eds.), Gasification of Waste Plastics by Steam Reforming, Proceedings of the International Symposium on Feedstock Recycling of Plastics, Karlsruhe, Germany, September 25–29, 2005, Universita¨tsverlag Karlsruhe, 2005. [17] Ch. Rebick, in: L.F. Albrigt, B.L. Crynes, W.H. Corcoran (Eds.), Pyrolysis: Theory and Industrial Practice, Academic Press, New York, 1983,, p. 82, Chapter 4. [18] T.M. Kruse, S.E. Levine, H.W. Wong, E. Duoss, A.H. Lebovitz, J.M. Torkelson, L.J. Broadbelt, J. Anal. Appl. Pyrolysis 73 (2005) 342. [19] A. Marcilla, J.C. Garcı´a-Quesada, S. Sa´nchez, R. Ruiz, J. Anal. Appl. Pyrol. 74 (2005) 387. [20] R.W.J. Westerhout, J. Waanders, J.A.M. Kuipers, W.P.M. van Swaaij, Ind. Eng. Chem. Res. 37 (1998) 2293. [21] N.K. Ciliz, E. Ekinci, C.E. Snape, Waste Manage. 24 (2004) 173. [22] Y. Ohmukai, K. Kubo, I. Hasegawa, K. Mae, in: M. Muller-Hagedorn, H. Bockhorn (Eds.), Change in Interactions by Pressure During the Pyrolysis of Plastics Mixture, Proceedings of the International Symposium on
[23] [24] [25] [26] [27] [28] [29] [30]
[31]
[32] [33] [34]
Feedstock Recycling of Plastics, Karlsruhe, Germany, September 25– 29, 2005, Universita¨tsverlag Karlsruhe, 2005. Z. Dolezˇal, V. Paca´kova´, J. Kova´rˇova´, J. Anal. Appl. Pyrol. 57 (2001) 177. A.C.-Y. Wong, F. Lam, Polym. Test. 21 (2002) 691. E. Bartekova´, M. Bajus, Petrol. Coal 41 (1999) 91. V. Ille´s, I. Pleszka´ts, L. Szepesy, Acta Chim. 80 (1974) 267. P.T. Williams, E.A. Williams, J. Inst. Energy 71 (1998) 81. L. Soja´k, R. Kubinec, H. Jurda´kova´, E. Ha´jekova´, M. Bajus, J. Anal. Appl. Pyrol. 78 (2007) 387. E. Kiran, J.K. Gillham, J. Appl. Polym. Sci. 20 (1976) 2045. Ch. Rebick, in: L.F. Allbright, B.L. Crynes, W.H. Corcoran (Eds.), Pyrolysis: Theory and Industrial Practice, Academic Press, New York, 1983, , p. 78, Chapter 4. L. Sˇpodova´, E. Ha´jekova´, M. Bajus, B. Mlynkova´, Cs. Ho´ka, Separation and characterization of products from thermal liquefaction of individual and mixed polyalkenes, in: Procedings of the International Symposium on Analytical and Applied Pyrolysis, Budapest, Hungary, May 21–26, (2006), p. 190. A.G. Goossens, M. Dente, E. Ranzi, Hydrocarbon Process 57 (9) (1978) 227. S. Kapur, in: R.A. Meyers (Ed.), Handbook of Petrochemicals Production Processes, McGraw-Hill, USA, 2005, , p. 6.3, Chapter 6.1. E. Ranzi, M. Dente, S. Pieruci, S. Barendergi, P. Cronin, Oil Gas J. 9 (1985) 49.
Copyrolysis of naphtha with polyalkene cracking products; the influence of polyalkene mixtures composition on product distribution Elena Ha´jekova´ *, Bozˇena Mlynkova´, Martin Bajus, Lenka Sˇpodova´ Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinske´ho 9, 812 37 Bratislava, Slovakia Received 18 July 2006; accepted 18 December 2006 Available online 23 December 2006
Abstract The steam cracking (copyrolysis) of naphtha with oils/waxes from thermal decomposition of polyalkenes has been investigated as a process for chemical recycling of plastic wastes. High-density polyethylene (HDPE), two-component mixture (LDPE/PP) and three-component mixture (HDPE/LDPE/PP) were thermally decomposed in a batch reactor at 450 8C, thus forming oil/wax products. Subsequently, these products were dissolved in heavy naphtha in the amount of 10 mass% to obtain steam cracking feedstock. The composition of gaseous and liquid products during copyrolysis was studied at 780 8C and 820 8C in dependence on residence time from 0.08 s to 0.51 s. The obtained results were compared with the product composition from steam cracking of naphtha at identical experimental conditions. The decomposition of polyalkene oils/waxes during copyrolysis was confirmed on the basis of analysis of liquid products. It was shown that more ethene and propene was formed during copyrolysis of oil/wax from HDPE in comparison with naphtha and both mixtures and so oil/wax from HDPE seems to be favourable component of steam cracking feedstock. There were slight differences between product compositions from copyrolysis of two- and three-component mixtures. The presence of HDPE in three-component mixture supported formation of gas and ethene. The presence of oil/wax form PP enhanced formation of propene and branched alkenes. For both type of polyalkenic mixtures the yields of desired low molecular alkenes and alkanes were higher or approximately the same as from naphtha. The results confirm suitability of oils/waxes from polyalkenes as a co-feed for steam cracking units. # 2007 Elsevier B.V. All rights reserved. Keywords: Copyrolysis; Polyalkene waxes; Recycling; Plastics; Thermal decomposition; Steam cracking; HDPE; LDPE; PP
1. Introduction Plastics are important components of municipal solid waste. Numbers of reports have been published on different approaches of feedstock and/or chemical recycling of plastics, mainly polyalkenes, including thermal [1–5] and catalytic degradation [6,7], and hydrocracking [8]. These processes enable transformation of waste plastics into valuable petrochemicals or fuels [9]. Several petrochemical companies have considered possible recycling of plastic wastes in existing refinery facilities, which would avoid the need to invest and build new processing plants [10–12]. This alternative is based on the similarity of elementary composition of plastics and petroleum fractions. There exist two main approaches to co-processing of plastic
* Corresponding author. Tel.: +421 2 59325401; fax: +421 2 52493198. E-mail address: [email protected] (E. Ha´jekova´). 0165-2370/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2006.12.022
wastes in refinery units. The plastics can either be added directly into the petroleum fractions [13–15] or the plastic waste is first thermally decomposed into smaller molecules. Thermal decomposition of polymers leads to a complex mixture of products. In general terms, up to four product fractions can be recovered: gases, oils and/or solid waxes and a solid residue. Depending on the process temperature (cracking <600 8C, pyrolysis 600 8C), a variety of products and applications can be envisaged from thermal decomposition of polymeric materials: fuel gases, olefinic gases useful in chemical synthesis, naphtha and middle distillates, oil fraction, long-chain paraffins and olefins, coke, etc. [10]. In order to minimize investments for the recycling of municipal waste, the co-processing of light thermal cracking oil (LTCO) derived from municipal waste plastics has been studied and demonstrated by a Japanese oil refinery company. The cohydrotreating of vacuum gas oil and LTCO has been selected as a favourable LTCO upgrading method [11]. A different approach to processing of oils derived from waste plastics
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
was proposed by Tsuji et al. [16]. They studied steam reforming of oils derived from LDPE and polystyrene (PS) with the aim to produce hydrogen rich synthesis gas. Research of different scientists [1,3,4] showed the possibility of thermal decomposition of mixed-plastics waste in different reactor types using mild conditions (450–550 8C) to gain high yields of oil-waxy hydrocarbons suitable as a cofeed for many downstream processing units. Production of the oil/wax as an intermediate product provides flexibility because the oil/wax may be easily transported as a solid, a liquid or in a solution [4]. The condensed oil/wax can be mixed with liquid petroleum fractions. Such a mixture may be used as a refinery feedstock; for example, it may be fed to a steam cracker to produce reusable olefins [4,5] or it may be fed to a catalytic cracker to produce gasoline or it may be even upgraded in a hydrocracker [4]. The advantage of above-mentioned approach is much higher solubility of the oils/waxes in comparison with the initial plastics and also a better manipulation with the mixture of oils/waxes in liquid petroleum fractions. Also important is the fact that potential impurities present in plastic waste are removed during thermal decomposition. The predominant mechanism in cracking of hydrocarbons is a free-radical chain. In case of binary or multicomponent mixtures the radical intermediates produced by one compound can interact with those produced by another, or with the reactant itself [17]. Most of plastic waste is in the form of mixed plastic waste. Depending on mixture composition some polymers may either enhance or inhibit the degradation of other polymers within mixed plastic waste [18] and product distribution can differ from that obtained by simple summation of product yields obtained from individual polymers. Different types of polyalkenes are produced (LDPE, HDPE, PP and their copolymers) and hence all these types in different forms can be found as individual or mixed in waste. It is well known that ternary carbons decrease thermal stability in polymers of similar nature. In case of PE, the higher the branching degree the lower the density, and the lower decomposition temperature [19]. Polypropylene molecule is characterized by the presence of a side methyl group at every second carbon and so PP is thermally degraded at a faster rate than both LDPE and HDPE [10]. Although several authors studied thermal decomposition of polyalkene mixtures, there is still lack of knowledge about interactions between PP and PE and obtained results are in some cases contradictory. It can be due to the fact that interactions between compounds largely depend on mixture composition, on degradation temperature, on reactor type, on phase, in which reaction occurs (liquid or gaseous), etc. Westerhout et al. [20] studied kinetics and product spectrum during pyrolysis of PE/PP mixture using different reactor types. No significant effect due to mixing of different polymers was observed in reactors with a short gas-phase residence time. Predel and Kaminsky [2] did not find the product-changing interactions between PE and PP during pyrolysis in a fluidised-bed reactor and on pyro-GC/MSdevice.
197
However, Ciliz et al. [21] during slow pyrolysis of PE/PP mixture at temperature up to 600 8C observed mutual influence of these polyalkenes reflected in product distribution. Also Ohmukai et al. [22] studied binary interactions of LDPE and PP during pyrolysis in a high-pressure batch reactor at 0.6 MPa and at 450 8C and in a low-pressure gas-flowing reactor at 600 8C. The authors confirmed the interactions between LDPE and PP both in liquid and in gaseous phase during pyrolysis. Dolezˇal et al. [23] found that the relative amount of the pyrolysis products of LDPE/PP blends were exponentially dependent on their composition in pyro-GC/MS experiments. Wong and Lam [24] confirmed changes in induction time for degradation of PE/ PP blends. From the foregoing results it is evident that polyalkenes can interact during thermal decomposition what results in different quality of gaseous and oil/wax products obtained by thermal decomposition of PE/PP mixtures. In case of copyrolysis we add oils/waxes obtained from PE/PP mixtures, which are composed mostly of higher linear or branched saturated and unsaturated hydrocarbons, into mixture of lighter hydrocarbons creating naphtha. Complex mutual influence of components is expected, because various authors confirmed influence of branched and linear hydrocarbons during radical decomposition in steam-cracking conditions. For example, inhibitingaccelerating effect was observed during steam cracking of 2,2,4-trimethylpentane mixed with hexadecane [25] or with heptane [26]. In the background literature, there is no work dealing with copyrolysis of naphtha with oils/waxes obtained by cracking of mixed polyalkenes. In a recent work [5], we have studied steam cracking of naphtha with addition of oils/waxes from individual LDPE or PP. The objective of the present study was to investigate (i) the thermal decomposition of individual HDPE and of two- and three-component mixtures of polyalkenes (LDPE/PP; HDPE/LDPE/PP) in a batch reactor at 450 8C as a process for preparation of oil/wax–feedstock; (ii) the influence of residence time and temperature on conversion and product distribution during steam cracking (copyrolysis) of prepared oils/waxes with naphtha in a tubular reactor at 780 8C and 820 8C. 2. Experimental 2.1. Materials The feedstocks used in the experiments were obtained from Slovnaft, a.s. /Inc./ (Slovak Republic). Heavy straight-run naphtha was a fraction from the atmospheric distillation unit with distillation range between 98 and 181 8C and with bromine number 1.8 g Br2/100 g. The molar mass of the naphtha was 116 g mol1 and its density was 726 kg m3 (at 25 8C). The polyalkenes used were as follows: high-density polyethylene ¯ w ¼ 33; 800; M ¯ n ¼ 6; 950; r = 950 kg m3), lowHDPE (M ¯ w ¼ 292; 000; M ¯ n ¼ 22; 000; density polyethylene LDPE (M ¯ w ¼ 200; 000; r = 919 kg m3) and polypropylene PP (M ¯ n ¼ 65; 000; r = 903 kg m3), all of them were virgin M plastics. The diameter of pellets ranged from 3.3 to 4.4 mm.
198
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
2.2. Solutions of oils/waxes from polyalkenes in naphtha The oil/wax products were prepared by thermal cracking of individual (HDPE) and mixed polyalkenes: two-component mixture LDPE/PP (1:1 mass) and three-component mixture HDPE/LDPE/PP (1:1:1 mass). The thermal decomposition of plastics was carried out in a batch reactor at temperatures up to 450 8C in nitrogen atmosphere. The prepared oils/waxes are predominantly composed of linear or branched alkenes and alkanes. The bromine numbers of prepared oils/waxes expressed in g Br2/100 g of the sample were for HDPE, twocomponent, and three-component mixtures as follows: 49.8, 64.7 and 57.0, respectively. Contrary to the virgin plastics, the oils/waxes are well soluble in naphtha. Three 10 mass% solutions of oils/waxes in naphtha were prepared and consequently used in copyrolysis experiments. A detailed description of the cracking apparatus and of the procedure was made in [5]. 2.3. Copyrolysis We performed a series of steam cracking experiments (copyrolysis) with 10 mass% solutions of oils/waxes in naphtha. The oils/waxes were obtained from HDPE, from LDPE/PP and from HDPE/LDPE/PP. The laboratory tubular stainless-steal reactor was used. The experiments were made at 780 8C and 820 8C, residence times from 0.08 s to 0.51 s and at steam to feedstock mass ratio of 0.5. In individual experiments different residence times were obtained by changing the feedstock flow rates. The flow of naphtha and wax solutions varied from 15 g h1 to 70 g h1. Before feeding it into the reactor, the feedstock was warmed up to 60 8C, to ensure homogeneous solution of oil/wax in naphtha. A detailed description of the steam cracking apparatus as well as the procedure was given in [5]. In order to find out about the influence of polyalkene oil/wax on product distribution, we conducted steam cracking experiments with pure heavy naphtha under the conditions similar to those during copyrolysis. We were not able to perform steam cracking experiments with pure oil/wax fractions from polyalkenes due to problems with charging semi-solid feedstock and due to a high coking tendency of these raw materials. Copyrolysis offers the possibility to overcome these shortcomings [5]. 2.4. Analysis of gaseous and liquid steam cracking products The qualitative and quantitative analyses of gases and liquids formed during copyrolysis were performed. Gas chromatograph HP 6890 + equipped with three PLOT chromatographic columns, FID and TCD detectors and with three switching valves was used for analysis of gases. This system allows for a detailed separation of the mixture of C1–C5 gaseous hydrocarbons, saturated and unsaturated, hydrogen and carbon oxides in one sample. The types of columns and analysis conditions were listed in [5].
The analyses of liquids formed during copyrolysis were performed on a CHROM V gas chromatograph equipped with a FID detector. Helium was used as carrier gas. A DB-PETRO (50 m 0.2 mm i.d. 0.5 mm) polydimethylsiloxane capillary chromatographic column was operated under following oven temperature programme: 40 8C followed by a heating rate of 2 8C min1 up to 60 8C, then an increase up to 240 8C at a heating rate of 5 8C min1, and finally 30 min at 240 8C. Solutions of the samples in n-hexane (5 mass%) were prepared and injected into the chromatograph in amounts of 0.3 ml. 3. Results and discussion 3.1. Thermal decomposition of polyalkenes The original plastics were solid at laboratory temperature and in the form of pellets. During thermal decomposition they produced a hydrocarbon gas, a light yellow oil/wax fraction collected in the separator, and a brown waxy solid residue. The yields of gases, oils/waxes and residues on the bottom of the batch reactor are listed in Table 1. The following profiling products were identified in the gases from HDPE cracking in decreasing order: propene, propane, butane, 1-butene, ethane and ethene. In case of two-component (LDPE/PP) and threecomponent (HDPE/LDPE/PP) mixture of polyalkenes, the prevailing products were: propene, pentane, methylpropene, propane and ethane. A detailed composition of the formed gases is given in Table 2. Also traces of hydrogen, benzene, and toluene were detected in the gases from the thermal decomposition of plastics. Comparing the yields of components in gases from thermal decomposition of HDPE and from decomposition of LDPE [5], we can see that HDPE yielded more ethene, propane and propene. The yields of methane and ethane were equal from both polyethylenes. On the contrary, lower amounts of C4 to C6 hydrocarbons were formed during thermal decomposition of HDPE. Williams and Williams [27] have also observed approximately the same content of methane and ethane from HDPE as from LDPE. They have also declared a higher content of ethene and propane and a lower content of butenes in the gases from HDPE. The mentioned differences in product distribution could be related with the fact, that HDPE is a highly linear polymer, whereas LDPE possesses a certain degree of branching providing a higher proportion of reactive tertiary carbons for the initiation step of degradation and LDPE degradation takes place at lower temperatures than in case of HDPE [10]. Table 1 Yields of products (Y) from thermal cracking of HDPE, LDPE/PP and HDPE/ LDPE/PP at 450 8C Feedstock
HDPE LDPE/PP HDPE/LDPE/PP
Y (mass%) Oil/wax
Gas
Residue
79.8 79.4 82.4
17.0 17.2 13.8
3.2 3.4 3.8
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
199
Table 2 Comparison of experimental and theoretical product distributions (mass%) in gases from thermal cracking of HDPE and polyalkene mixtures at 450 8C Component (mass%)
Methane Ethane Ethene Propane Propene Methylpropane Butane Methylpropene 2-Butenes 1-Butene Pentane 1,3-Butadiene 1,3-Pentadiene 1-Pentene 2-Methyl-1-butene Hexane 2-Methyl-1-pentene 1-Hexene
PP [5]
LDPE [5]
HDPE
LDPE/PP
Exp.
Exp.
Exp.
Exp.
Theo.
Exp.
Theo.
2.3 6.4 0.5 3.4 43.1 0.3 0.2 10.0 0.1 0.2 23.3 tr. 1.1 0.3 0.5 – 5.2 –
2.7 8.3 4.5 14.9 14.2 0.2 12.6 1.2 3.2 12.2 6.0 1.3 0.6 7.4 tr. 1.3 – 3.4
2.6 8.7 7.3 19.1 25.5 0.1 10.0 0.7 3.7 8.8 2.4 1.5 0.2 3.3 – 0.2 – 0.8
3.1 8.1 2.2 8.6 36.9 0.4 3.6 10.0 0.7 4.1 15.8 0.4 0.5 1.2 0.3 0.1 1.7 0.3
2.5 7.3 2.5 9.1 28.7 0.2 6.4 5.6 1.7 6.2 14.7 0.7 0.8 3.8 0.2 0.7 2.6 1.7
3.3 8.5 3.4 10.5 34.7 0.4 4.4 9.7 1.0 4.8 13.7 0.7 0.4 1.3 0.3 0.1 1.2 0.2
2.5 7.8 4.1 12.5 27.6 0.2 7.6 3.9 2.4 7.1 10.6 0.9 0.6 3.7 0.2 0.5 1.7 1.4
In order to be able to determine mutual influence of feedstock components on formation of gaseous thermal cracking products, we have calculated theoretical product distributions based on product distributions obtained during pure HDPE, LDPE, or PP thermal decomposition. The theoretical distribution of products from thermal decomposition of two- and three-component mixtures is listed in Table 2. Comparing experimental and theoretical product distributions in gases, it is evident that there exist some differences. In gases from thermal decomposition of both mixtures actually more propene, methylpropene and pentane were detected, which are prevailing components of gaseous products from cracking of polypropylene. The experimental yields of propane, butane, butenes, 1-pentene, hexane and 1-hexene were lower than theoretical yields. These components are prevailing in gases from thermal decomposition of polyethylenes. From this comparison it results, that polypropylene and polyethylenes influence each other when they are thermally decomposed in the mixture. In the product distribution in gases this influence manifest itself mainly in higher formation of light products from polypropylene degradation. We have also confirmed differences between experimental and theoretical product distribution in liquid cracking products using GC—analysis of oils from thermal decomposition of polyalkene mixtures [31]. 3.2. Copyrolysis of oils/waxes from plastics with naphtha The selection of the optimal process for treating of oils and waxes derived from thermal decomposition (cracking) of plastics depends to a great extent on the composition of oils and waxes. Oil/wax products from polyethylene cracking are composed predominantly of linear alkanes and 1-alkenes [3,28]. The oil/ wax fraction formed during PP cracking is a complex mixture of
HDPE/LDPE/PP
branched alkenes and alkanes [28,29]. In steam cracking straight-chain paraffins produce high yields of straight-chain alkenes (mainly ethene). Branched hydrocarbons yield some branched products (mainly propene) and are valuable components of steam cracking feedstock. Similar paths to paraffins decompose higher a-olefins and the products are similar to those observed in paraffin steam-cracking [30]. The main products from PS cracking are aromatic hydrocarbons, namely styrene, toluene, ethyl-benzene and other styrene derivatives [10]. However, aromatics are undesirable in steam-cracking feeds, because they greatly contribute to coke formation and give small yields of alkenes. That is why we have not used PS as a component of mixtures studied in present work. Following-up our work [5] in which we had copyrolysed oils/waxes from LDPE and PP, we made experiments at identical experimental conditions with solutions of oils/waxes from HDPE and from mixtures of LDPE/PP and HDPE/LDPE/ PP in naphtha. We chose identical experimental conditions, as in case of naphtha alone, to be able to clearly establish how the replacement of naphtha by a non-petroleum material, in the amount of 10 mass%, influences the overall steam cracking process. The 10 mass% allowance of oil/wax into naphtha as a co-feed is well dissolved. Such a substitution of crude oil raw material in large-scale ethylene units can bring considerable economic benefits. The price of feedstock substantially influences the steam cracking economy, mainly at present, when on the world market the crude oil price is rather high. The results obtained from steam cracking of naphtha and from copyrolysis of oils/waxes at 780 8C are given in Tables 3 and 4 and the results at 820 8C are given in Tables 5 and 6. The steam cracking under used experimental conditions led to gaseous (Figs. 1 and 2) and liquid products. Alkenes are the main gaseous products while ethene (Figs. 3 and 4) and propene (Figs. 3 and 4) are the prevailing components in the gases.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
200
Table 3 Conversion to gas (X), molar mass of gas (M) and yields of gaseous components (Y) from steam cracking of different feedstocks at 780 8C depending on the residence time (t) Feedstock Naphtha X (%) t (s) Mw (g mol1)
51.2 0.11 30.0
Naph/HDPE 67.4 0.17 28.7
Component
Y, mass%
Hydrogen Carbon oxides Methane Ethane Ethene Propane Propene Propadiene Acetylene trans-2-Butene 1-Butene Methylpropene cis-2-Butene 1,3-Butadiene 3-Methyl-1-butene trans-2-Pentene 1,3-Pentadiene 2-Methyl-2-butene 1-Pentene 2-Methyl-1-butene Isoprene Unknown in gas
0.3 tr. 6.2 1.9 16.8 0.3 11.7 0.1 0.1 0.5 2.7 1.7 0.4 3.6 0.1 0.3 0.2 0.1 0.4 0.4 0.5 2.4
0.4 tr. 9.4 2.8 23.3 0.4 15.1 0.1 0.2 0.6 2.9 2.2 0.5 4.8 0.2 0.3 0.2 0.1 0.3 0.4 0.6 2.2
73.6 0.38 26.5
0.5 0.2 13.0 3.3 27.8 0.4 14.5 0.2 0.4 0.5 1.5 2.0 0.4 4.9 0.3 0.1 0.1 0.1 0.1 0.2 0.5 2.0
56.0 0.09 31.1
0.2 tr. 6.1 2.1 18.3 0.3 12.7 0.1 0.1 0.5 3.2 1.7 0.4 3.9 0.1 0.4 0.2 0.1 0.6 0.5 0.4 3.2
Naph/LDPE/PP
72.3 0.16 29.5
0.4 tr. 9.4 2.8 24.8 0.4 15.8 0.1 0.3 0.6 3.2 2.1 0.5 5.4 0.2 0.3 0.2 0.1 0.4 0.4 0.5 3.2
78.7 0.41 25.8
0.5 0.1 16.9 5.0 29.3 0.5 14.0 0.1 0.3 0.4 1.1 1.7 0.3 3.8 0.2 0.1 0.1 0.1 0.2 0.1 0.2 2.1
51.9 0.11 29.0
0.4 tr. 5.9 1.9 16.9 0.3 12.2 0.1 0.1 0.5 2.8 2.2 0.4 3.6 0.2 0.3 0.2 0.1 0.4 0.5 0.5 1.9
66.0 0.16 27.5
0.7 tr. 8.8 2.7 22.2 0.4 14.9 0.1 0.2 0.6 2.8 2.6 0.4 4.6 0.2 0.2 0.2 0.1 0.3 0.5 0.6 2.2
Naph/HDPE/LDPE/PP 76.4 0.40 23.8
1.0 0.2 15.9 4.1 29.1 0.4 13.7 0.1 0.4 0.4 1.1 2.0 0.3 3.9 0.3 0.1 0.1 0.1 0.1 0.2 0.4 1.6
52.7 0.10 30.4
0.3 tr. 6.1 2.0 17.1 0.3 12.3 0.1 0.1 0.5 2.9 2.0 0.4 3.7 0.2 0.3 0.2 0.1 0.4 0.5 0.5 2.4
68.1 0.16 29.6
77.4 0.41 25.6
0.3 tr. 8.9 2.7 23.1 0.4 15.0 0.1 0.3 0.6 2.8 2.4 0.4 5.0 0.2 0.3 0.2 0.1 0.3 0.4 0.6 3.0
0.5 0.2 16.4 4.4 29.3 0.4 14.1 0.1 0.3 0.4 1.1 2.0 0.3 4.1 0.3 0.1 0.1 0.1 0.1 0.1 0.4 1.9
tr.—traces.
The influence of steam cracking severity (i.e. prolongation of residence time and temperature increase) on the yields of products is important. The yields of ethene varied for different feedstocks from 16.8 to 29.3 mass% at 780 8C and at 820 8C they reached values from 27.5 to 34.6 mass%. The ethene yields increased with increasing residence time and at 820 8C they passed through moderate maximum. The propene yields passed through maximum in the dependence on residence time at both
temperatures and the decrease in propene yields was stronger at 820 8C. The yields of propene varied from 11.7 to 15.8 mass% at 780 8C and at 820 8C they reached values from 8.1 to 15.4 mass%. From the dependences it is evident that a decrease in propene yields is caused by the secondary (mainly splitting) reactions on forming ethene and methane. From the C4 hydrocarbons, the most prevailing is 1,3-butadiene followed by 1-butene and 2-butenes (Tables 3 and 5). In the C5
Fig. 1. Influence of residence time on gas formation during copyrolysis of oils/ waxes in comparison with steam cracking of naphtha at 780 8C.
Fig. 2. Influence of residence time on gas formation during copyrolysis of oils/ waxes in comparison with steam cracking of naphtha at 820 8C.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
201
Table 4 Yields of liquid components (Y) from steam cracking of different feedstocks at 780 8C depending on residence time (t) and on feedstock conversion to gas (X) Naphthab
Feedstock Naphtha
X (%) t (s)
0 –
51.2 0.11
Naph/HDPE 67.4 0.17
73.6 0.38
Component
%mass
Y, mass%
Benzene Heptane Methylcyclohexane Toluene + 2-methylheptane Dimethylcyclohexane Ethylcyclopentane Octane 2,6-Dimethylheptane Ethylbenzene p + m-Xylene Styrene o-Xylene Nonane Ethyltoluene Methylstyrenes Decane Methylindenes Naphthalene Methylnaphthalenes Anthracene + Phenanthrene Unknown in liquid
0.2 5.6 4.9 1.4 + 2.6 2.4 1.0 6.0 3.3 1.6 2.2 – 0.8 5.7 1.0 – 3.8 – – – – 57.5a
4.3 2.9 0.1 6.1 0.8 0.6 2.2 0.2 0.9 2.4 0.8 1.4 2.2 1.5 1.8 1.7 0.2 0.1 0.2 0.1 18.4
56.0 0.09
3.0 0.7 0.7 4.5 0.4 0.3 1.2 0.1 0.7 2.0 1.2 1.2 0.9 2.2 0.5 1.4 0.4 0.4 0.2 0.1 10.8
Naph/LDPE/PP
72.3 0.16
3.5 – – 4.7 – – – – 0.6 1.9 2.3 1.3 0.2 1.8 0.3 0.9 0.4 2.0 0.7 0.1 4.2
78.7 0.41
1.9 1.5 1.8 4.1 0.9 0.8 3.2 0.8 0.7 1.6 0.8 1.3 2.4 1.1 1.0 0.3 0.2 0.2 0.1 0.1 19.9
3.3 0.7 0.8 4.5 0.4 0.3 1.3 0.4 0.7 2.1 1.2 1.1 1.0 1.6 0.5 0.3 0.3 0.4 0.2 0.1 7.2
51.9 0.11
4.2 tr. 0.1 4.1 0.3 0.1 0.3 tr. 0.4 1.4 1.1 1.4 0.7 1.6 0.3 1.2 0.5 1.7 0.6 0.2 2.6
66.0 0.16
4.7 2.0 2.1 6.8 1.0 0.7 3.1 0.9 0.9 2.1 1.3 1.4 2.3 1.9 0.8 0.3 0.2 0.2 0.1 0.1 15.8
Naph/HDPE/LDPE/PP 76.4 0.40
4.6 0.9 0.9 6.2 0.5 0.3 1.5 0.4 0.9 2.5 1.7 1.4 1.1 1.6 0.5 0.5 0.4 0.5 0.2 0.1 7.9
52.7 0.10
5.2 – – 5.2 – – – – 0.5 1.6 2.1 1.0 0.2 1.3 0.3 0.9 0.6 1.9 0.8 – 3.2
68.1 0.16
2.1 2.1 1.6 3.0 0.7 0.6 1.9 0.4 0.9 2.1 1.0 0.9 2.0 1.6 1.8 1.6 1.1 0.8 – – 21.5
6.3 1.1 0.8 4.6 0.3 0.2 1.2 0.1 0.7 1.8 1.3 1.1 0.8 0.8 1.4 0.4 0.4 1.0 0.7 – 7.8
77.4 0.41
5.7 0.2 0.2 4.0 0.1 0.1 0.3 tr. 0.4 1.4 1.7 0.8 0.2 0.4 1.3 0.2 0.5 1.3 0.5 – 3.9
tr.—traces. a A sum of other identified and unknown hydrocarbons in feedstock naphtha. b Composition of feedstock naphtha.
hydrocarbons, isoprene seems to be the dominant hydrocarbon (around 0.5 mass%). The higher gas formation and the higher yield of smaller molecules (e.g. methane, acetylene) witnesses deeper splitting of the feedstock and the course of secondary reactions when the severity of pyrolysis increases. Formation of carbon oxides and hydrogen was higher at 820 8C (Table 5) and it reflects the
course of secondary reactions leading to a formation of carbonaceous solid products (coke). Carbon oxides and hydrogen are products of gasification reactions between precursors of carbonaceous deposits or coke, and steam. Oils/waxes obtained by thermal decomposition of polyalkenes are predominantly composed of linear and branched alkenes and alkanes depending on the polymer type. Alkane-alkenic
Fig. 3. Influence of residence time on formation of ethene and propene during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 780 8C.
Fig. 4. Influence of residence time on formation of ethene and propene during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 820 8C.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
202
Table 5 Conversion to gas (X), molar mass of gas (M) and yields of gaseous components (Y) from steam cracking of different feedstocks at 820 8C depending on the residence time (t) Feedstock Naphtha X (%) t (s) Mw (g mol1)
73.9 0.12 27.0
Naph/HDPE 79.4 0.19 25.3
Component
Y, mass%
Hydrogen Carbon oxides Methane Ethane Ethene Propane Propene Propadiene Acetylene trans-2-Butene 1-Butene Methylpropene cis-2-Butene 1,3-Butadiene 3-Methyl-1-butene trans-2-Pentene 1,3-Pentadiene 2-Methyl-2-butene 1-Pentene 2-Methyl-1-butene Isoprene Unknown in gas
0.6 0.3 11.5 2.5 28.1 0.4 14.2 0.2 0.5 0.5 2.0 1.8 0.4 5.5 0.4 0.2 0.1 0.1 0.1 0.2 0.5 2.7
0.8 0.7 14.9 2.9 31.8 0.3 13.1 0.2 0.7 0.3 0.9 1.5 0.3 5.3 0.5 0.1 tr. tr. 0.1 0.1 0.4 2.6
72.1 0.40 22.3
0.9 1.3 18.8 3.3 31.2 0.2 8.1 0.1 0.7 0.1 0.2 0.7 0.1 2.9 0.3 tr. tr. tr. tr. tr. 0.2 1.2
78.5 0.12 28.1
0.5 0.1 11.4 2.8 29.6 0.4 15.4 0.2 0.5 0.5 2.5 1.8 0.4 5.7 0.4 0.1 0.2 0.1 0.3 0.3 0.5 3.6
Naph/LDPE/PP
84.4 0.18 27.0
0.6 0.1 14.7 3.2 32.8 0.4 14.5 0.2 0.6 0.4 1.7 1.7 0.3 5.6 0.4 0.2 0.1 0.1 0.2 0.2 0.4 3.3
80.4 0.35 23.4
0.8 0.5 20.1 4.1 34.6 0.3 10.6 0.1 0.6 0.2 0.4 0.9 0.2 3.3 0.3 0.1 tr. tr. tr. tr. 0.2 1.4
73.5 0.11 26.6
0.5 0.1 11.4 2.6 27.5 0.4 14.4 0.2 0.5 0.4 2.1 2.1 0.4 5.3 0.4 0.1 0.1 0.1 0.2 0.3 0.6 2.7
80.4 0.20 24.8
0.6 0.1 15.7 3.3 32.0 0.4 13.4 0.2 0.6 0.3 1.4 1.7 0.3 4.3 0.4 0.2 0.1 0.1 0.2 0.3 0.5 2.6
Naph/HDPE/LDPE/PP 70.0 0.37 22.5
0.8 0.5 17.1 3.3 29.9 0.3 9.7 0.1 0.6 0.2 0.4 1.0 0.1 3.1 0.3 0.1 tr. tr. tr. 0.1 0.2 1.2
75.9 0.12 28.1
0.5 0.1 11.3 2.7 28.0 0.4 14.7 0.2 0.5 0.5 2.1 2.1 0.4 5.6 0.4 0.1 0.1 0.1 0.2 0.3 0.5 3.6
80.6 0.16 27.1
73.0 0.37 23.9
0.5 0.1 13.8 3.1 31.0 0.4 14.6 0.2 0.6 0.4 1.5 2.0 0.3 5.4 0.4 0.1 0.1 0.1 0.1 0.2 0.5 2.9
0.6 0.2 17.8 3.5 30.8 0.3 10.0 0.1 0.6 0.2 0.4 1.1 0.2 3.3 0.3 0.1 tr. tr. tr. tr. 0.2 1.7
tr.—traces.
feedstock decomposes preferably into low molecular alkenes and alkanes [7]. Our results also support this observation (Tables 3 and 5). The most noticeable differences between the yields of low-molecular alkenes formed at copyrolysis in comparison to steam cracking of naphtha are observed for the system naphtha/ HDPE. During copyrolysis of oil/wax from HDPE with naphtha the yields of ethene were higher up to 2.6 mass% (what represents 8% relat.) in comparison with the ethene yields from pure naphtha. The yields of propene were higher up to 1.2 mass% (i.e. 12.7% relat.) than the propene yields from naphtha. The described differences are observed especially at severe steam cracking conditions (long residence time, 820 8C). The yields of low-molecular alkenes obtained during copyrolysis of HDPE were even higher than those from copyrolysis of LDPE [5] at identical experimental conditions. It is obvious, that the presence of HDPE during recycling of polyalkene mixtures via copyrolysis would be favourable. From comparison of product yields obtained during copyrolysis of two- and three-component mixtures following observations resulted. The oil/wax from pure PP provided during copyrolysis less gas and ethene than pure naphtha, but more propene and methylpropene [5]. Now in two-component mixture of PP with LDPE the yield of gas from copyrolysis increases to the level of gas creation from naphtha (Figs. 1 and 2). The same is observed for ethene yields (Figs. 3 and 4). The higher splitting of oil/wax from LDPE into ethene probably compen-
sates low ethene and gas formation from polypropylene oil/wax. As concerns propene production, this is a bit higher from twocomponent than from three-component mixture and also higher than that from naphtha alone due to higher proportion of branched molecules coming from PP decomposition (Fig. 3). In case of three-component mixture the oil/wax from HDPE, which give high yields of gas and ethylene, was added in the system. The presence of HDPE reflects in slightly higher
Fig. 5. Influence of residence time on content of unreacted alkanes during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 780 8C.
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
Fig. 6. Influence of residence time on formation of naphthalene during copyrolysis of oils/waxes in comparison with steam cracking of naphtha at 820 8C.
formation of gas and ethene during copyrolysis of threecomponent mixture at both temperatures in comparison with two-component mixture (Figs. 2 and 4). It is important to point out that presented tendencies are very moderate. Bigger differences between product distribution from copyrolysis of different polyalkene mixtures could manifest themselves much more at higher content of oils/ waxes in naphtha than was the content of 10 mass% used in our experiments. The construction of our experimental apparatus did not allow us to agitate the warmed feedstock at charging it into the reactor and that is why we could not work with more concentrated solutions of oils/waxes. But in industrial steam cracking units liquid feedstock is usually preheated and dilution of oils/waxes from polyalkenes in higher amounts could not cause problems at charging. These results confirm, that after replacing 10 mass% of conventional liquid steam cracking feedstock by the oils/waxes from polyalkenes, the yields of valuable light olefins are comparable to those from steam cracking of naphtha, or in some cases are higher than from pure naphtha. Goossens et al. [32] reported that in a typical 1 billion-pounds/year ethylene plant, a 0.8 wt.% increase in ethylene selectivity could correspond to a net profit gain of $1 million. But also at unchanged yields of desired alkenes replacement of a part of petroleum feedstock by the non-petroleum feed in high capacity steam cracking units could be economically advantageous. The total world ethylene capacity in 2002 was close to 110 million metric tons per annum and naphtha steam cracking represents about 45% of this production capacity [33]. Together with gases, liquids are also formed during steam cracking. The yields of components in liquids from copyrolysis and from pyrolysis of naphtha are listed in Tables 4 and 6. The composition of naphtha feedstock is also given in Table 4. At both temperatures the total content of unreacted alkanes determined in pyrolysis liquids decreases with prolongation of residence time (Fig. 5). The content of aromatics increases with increase of pyrolysis severity (Tables 4 and 6). From the described relation new formation of aromatics is evident. This is valid for benzene, toluene, styrene, ethyltoluene, methyl-
203
styrene, naphthalene with its derivatives and anthracene with phenantrene. The presence of small amount of unreacted wax in pyro-liquid was detected only during experiments with oil/wax from HDPE at the mildest conditions (780 8C, residence time 0.09 s). At more severe conditions the wax did not occur in products, which proves its decomposition under experimental conditions at all used feedstocks. Comparing the yields of liquid products from copyrolysis with those from steam cracking of naphtha, we can state that the formation of benzene was in case of copyrolysis of two- and three-component mixture slightly higher than from steam cracking of naphtha. Yields of toluene, C8 aromatics and C9 aromatics are comparable to the yields from naphtha. Naphthalene formation at 820 8C (Fig. 6) is at the copyrolysis conditions for all feedstocks lower than naphthalene formation during steam cracking of individual naphtha. Olefins, diolefins, aromatics, polyaromatics and acetylenic compounds are defined as the coke precursors in the process of steam cracking [34]. We observed comparable yields of main gaseous coke precursors such as 1,3-butadiene, propadiene, isoprene and acetylene during copyrolysis in comparison with steam cracking of naphtha. From these results we could expect approximately equal coke formation during copyrolysis of oil/ wax solutions, as is the coke formation during steam cracking of naphtha alone. Acknowledgement We would like to thank the VEGA Scientific Grant Agency of the Slovak Republic, the commission for chemistry and chemical technology, for its financial support for this work through research project No. 1/3587/06. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13]
J. Walendziewski, Fuel Process. Technol. 86 (2005) 1265. M. Predel, W. Kaminsky, Polym. Degrad. Stabil. 70 (2000) 373. P.T. Williams, E.A. Williams, J. Anal. Appl. Pyrol. 51 (1999) 107. K.C. Kirkwood, S.A. Leng, W. David, Polymer Cracking, US Patent 5364995 (1994). E. Ha´jekova´, M. Bajus, J. Anal. Appl. Pyrol. 74 (2005) 270. S. Karago¨z, J. Yanik, S. Uc¸ar, M. Saglam, C. Song, Appl. Catal. A-Gen. 242 (2003) 51. D.P. Serrano, J. Aguado, J.M. Escola, E. Garagorri, J. Anal. Appl. Pyrol. 58/59 (2001) 789. C.M. Simon, W. Kaminsky, B. Schlesselmann, J. Anal. Appl. Pyrol. 38 (1996) 75. A.G. Buekens, H. Huang, Resour. Conserv. Recycl. 23 (1998) 163. J. Aguado, D. Serrano, Feedstock Recycling of Plastic Wastes, The Royal Society of Chemistry, Cambridge, 1999, p. 192. T. Kawanishi, N. Shiratori, H. Wakao, E. Sugiyama, H. Ibe, M. Shioya, T. Abe, in: M. Muller-Hagedorn, H. Bockhorn (Eds.), Upgrading of Light Thermal Cracking Oil Derived from Waste Plastics in Oil Refinery, Proceedings of the International Symposium on Feedstock Recycling of Plastics, Karlsruhe, Germany, September 25–29, 2005, Universita¨tsverlag Karlsruhe, 2005. A. Tukker, Rapra Rev. Rep., 13 (2002) 122. J. Aguado, D.P. Serrano, J.M. Escola, in: J. Scheirs, W. Kaminsky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley and Sons, Chichester, 2006, , p. 102, Chapter III.
204
E. Ha´jekova´ et al. / J. Anal. Appl. Pyrolysis 79 (2007) 196–204
[14] J. Walendziewski, in: J. Scheirs, W. Kaminsky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley and Sons, Chichester, 2006, , p. 119, Chapter IV. [15] J. Yanik, T. Karayildirim, in: J. Scheirs, W. Kaminsky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley and Sons, Chichester, 2006, , p. 217, Chapter VIII. [16] T. Tsuji, S. Okajima, T. Masuda, in: M. Muller-Hagedorn, H. Bockhorn, (Eds.), Gasification of Waste Plastics by Steam Reforming, Proceedings of the International Symposium on Feedstock Recycling of Plastics, Karlsruhe, Germany, September 25–29, 2005, Universita¨tsverlag Karlsruhe, 2005. [17] Ch. Rebick, in: L.F. Albrigt, B.L. Crynes, W.H. Corcoran (Eds.), Pyrolysis: Theory and Industrial Practice, Academic Press, New York, 1983,, p. 82, Chapter 4. [18] T.M. Kruse, S.E. Levine, H.W. Wong, E. Duoss, A.H. Lebovitz, J.M. Torkelson, L.J. Broadbelt, J. Anal. Appl. Pyrolysis 73 (2005) 342. [19] A. Marcilla, J.C. Garcı´a-Quesada, S. Sa´nchez, R. Ruiz, J. Anal. Appl. Pyrol. 74 (2005) 387. [20] R.W.J. Westerhout, J. Waanders, J.A.M. Kuipers, W.P.M. van Swaaij, Ind. Eng. Chem. Res. 37 (1998) 2293. [21] N.K. Ciliz, E. Ekinci, C.E. Snape, Waste Manage. 24 (2004) 173. [22] Y. Ohmukai, K. Kubo, I. Hasegawa, K. Mae, in: M. Muller-Hagedorn, H. Bockhorn (Eds.), Change in Interactions by Pressure During the Pyrolysis of Plastics Mixture, Proceedings of the International Symposium on
[23] [24] [25] [26] [27] [28] [29] [30]
[31]
[32] [33] [34]
Feedstock Recycling of Plastics, Karlsruhe, Germany, September 25– 29, 2005, Universita¨tsverlag Karlsruhe, 2005. Z. Dolezˇal, V. Paca´kova´, J. Kova´rˇova´, J. Anal. Appl. Pyrol. 57 (2001) 177. A.C.-Y. Wong, F. Lam, Polym. Test. 21 (2002) 691. E. Bartekova´, M. Bajus, Petrol. Coal 41 (1999) 91. V. Ille´s, I. Pleszka´ts, L. Szepesy, Acta Chim. 80 (1974) 267. P.T. Williams, E.A. Williams, J. Inst. Energy 71 (1998) 81. L. Soja´k, R. Kubinec, H. Jurda´kova´, E. Ha´jekova´, M. Bajus, J. Anal. Appl. Pyrol. 78 (2007) 387. E. Kiran, J.K. Gillham, J. Appl. Polym. Sci. 20 (1976) 2045. Ch. Rebick, in: L.F. Allbright, B.L. Crynes, W.H. Corcoran (Eds.), Pyrolysis: Theory and Industrial Practice, Academic Press, New York, 1983, , p. 78, Chapter 4. L. Sˇpodova´, E. Ha´jekova´, M. Bajus, B. Mlynkova´, Cs. Ho´ka, Separation and characterization of products from thermal liquefaction of individual and mixed polyalkenes, in: Procedings of the International Symposium on Analytical and Applied Pyrolysis, Budapest, Hungary, May 21–26, (2006), p. 190. A.G. Goossens, M. Dente, E. Ranzi, Hydrocarbon Process 57 (9) (1978) 227. S. Kapur, in: R.A. Meyers (Ed.), Handbook of Petrochemicals Production Processes, McGraw-Hill, USA, 2005, , p. 6.3, Chapter 6.1. E. Ranzi, M. Dente, S. Pieruci, S. Barendergi, P. Cronin, Oil Gas J. 9 (1985) 49.