Ind. Eng. Chem. Res. 2007, 46, 3497-3504
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Enhanced Production of r-Olefins by Thermal Degradation of High-Density Polyethylene (HDPE) in Decalin Solvent: Effect of the Reaction Time and Temperature J. Aguado,* D. P. Serrano, G. Vicente, and N. Sa´ nchez Department of Chemical and EnVironmental Technology, Escuela Superior de Ciencias Experimentales y Tecnologı´a (ESCET), Rey Juan Carlos UniVersity, Mo´ stoles 28933, Madrid, Spain
The thermal degradation of high-density polyethylene (HDPE) in decalin has been investigated as a method for the feedstock recycling of plastic wastes. The study is focused on the effects of the reaction time and temperature. The solvent increased the HDPE conversion and had a significant effect on the product distribution, which may be assigned mainly to the decalin participation on the reaction mechanism. Short reaction times and low temperatures in the presence of decalin contributed to an enhanced production of gaseous hydrocarbons. Longer reaction times and high temperatures in the presence of this solvent improved the production of C5-C55 hydrocarbons. The gaseous olefins and C5-C32 R-olefins increased with time and temperature; however, the best selectivity to olefins (96.5%) in the gaseous fraction was obtained at 5 h and 375 °C, whereas the higher selectivity to R-olefins (76%) within the C5-C32 fraction was achieved after 5 h and 425 °C. 1. Introduction Polyolefins (low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP)) are plastic materials used extensively in containers and packaging. They represent ∼60% of the total amount of plastics in municipal solid waste.1 In western Europe (the European Union (EU), Norway, and Switzerland), the current strategies that involve these wastes are still based in landfilling to a great extent (∼53% of the total available plastic waste collectable in 2004).2 In this context, the EU launched the 94/62/CE directive on packaging and packaging waste in 1994, which has been recently amended through Directive 2004/12/CE. According to the new directive, between 55% w/w, as a minimum, and 80% w/w, as a maximum, of packaging waste must be recycled no later than 2009. A promising alternative for the reprocessing of waste plastics is feedstock recycling, which involves the conversion of plastics residue into raw chemicals, monomers for plastics or hydrocarbon feedstocks. In this way, thermal degradation has been used to convert different polyolefins into hydrocarbon mixtures.3-5 The thermal decomposition of polyolefins at temperatures of 400 °C or higher produce a mixture of hydrocarbons that is formed by a gas fraction (C1-C4), a liquid fraction (C5-C18), and a solid residue (C19-C70). For each number of C atoms, three main components are produced: the corresponding n-paraffin, R-olefin, and R,ω-diene. The relative proportion of these products is dependent on the thermal degradation operating conditions, which are determined mainly by the temperature. The thermal degradation of polyolefins involves complex reactions through a radical mechanism, and their extension is very dependent on temperature, pressure, reactor geometry, and heat- and mass-transfer rates, as well as mixing intensity. Polymerssand particularly polyolefinsshave high viscosity, which hinders mass- and heat-transfer phenomena.6,7 The * To whom correspondence should be addressed. Tel.: 34-914887005. Fax: 34-91-4887068. E-mail:
[email protected].
addition of solvents can improve the mass- and heat-transfer rates, decreasing, in turn, the required temperature and increasing the conversion and the yield of the hydrocarbon products. In this context, many works have studied the thermal degradation of polymers, using several solvents. However, most of them have been focused on plastics that are different from polyolefins, mainly polystyrene.6-9 On the other hand, the use of determined solvents in the thermal cracking of polyolefins may have another important benefit, in comparison to the conventional degradation, because some solvents are able to modify the thermal degradation mechanism, promoting the production of specific hydrocarbons. In particular, solvents with a low hydrogen donor capability (e.g., decalin) modify the thermal degradation of polyolefins, enhancing the production of C5-C32 R-olefins.10,11 The use of these solvents has been extensively investigated for coal liquefaction.12-15 The terminal double bond of R-olefins presents a high reactivity, in regard to a wide variety of chemicals, and, therefore, they are used to produce any derivative requiring an even-numbered, straight carbon chain. In this sense, R-olefins are utilized as intermediates in the manufacture of many commercial products, including plastics (e.g., HDPE and LDPE), synthetic lubricants (e.g., poly-R-olefins, polyol esters), surfactants (e.g., R-olefin sulfonates, alkyl benzene sulfonates, alkyl dimethyl amines), additives (e.g., alkenyl succinic anhydrides, and polyvinylchloride lubricants and stabilizers) and specialty chemicals (e.g., epoxides, halogenated R-olefins). The thermal degradation of polyethylene is particularly important, because this polyolefin is the major component of plastics waste. In the present work, the thermal cracking of HDPE is investigated, using decalin as a solvent to increase the yields of R-olefins. The study is focused on the effect of time and temperature on the product distribution. The results obtained are also compared with those obtained in the absence of solvent. Decalin solvent has a high thermal stability at the reaction temperature and a relatively low vapor pressure.11 As a consequence, it can be easily separated by distillation after the
10.1021/ie060975d CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 11, 2007
Figure 1. Effect of reaction time on the yields for hydrocarbon groups: (a) reactions with decalin, and (b) reactions without solvent. Conditions: temperature ) 400 °C, HDPE/decalin ratio ) 1/10, and initial pressure ) 20 bar.
HDPE degradation and used iteratively in subsequent reaction cycles, which is very important from an economical point of view. 2. Experimental Section 2.1. Materials. The polyolefin used in this work was HDPE, which was provided by REPSOL-YPF. The solvent (decalin) had a purity of 98% and was purchased from ACROS. 2.2. Equipment. The thermal cracking reactions were performed in a 100-mL stainless steel autoclave provided by a mechanical stirrer and surrounded by an electric blanket that was used to transfer heat to the reaction mixture. The reactor was equipped with appropriate temperature and pressure controls. A thermocouple, which was in contact with the reaction mixture, was capable of maintaining the reaction temperature within 1 °C of the set value. The pressure gauge allowed us to follow the reaction progress and keep the pressure below 150 bar, which is the safety limit for the autoclave. 2.3. Experimental Conditions. The reactions were conducted by varying the time (1, 3, 5, and 7 h) at 400 °C and also the temperature (350, 375, 400, and 425 °C) for 5 h. In all cases, the HDPE/decalin ratio was 1/10, and the initial pressure was 20 bar. During the reaction, an increase in the pressure was observed, because of the production of gaseous hydrocarbons. The corresponding solventless reactions were also performed under the same operating conditions, to compare the results. 2.4. Experimental Procedure. Initially, the HDPE beads were ground cryogenically to a particle size of 32 could not be separated by hydrocarbon type using this type of gas chromatography analysis. 3. Results and Discussion 3.1. Influence of Reaction Time. Four reactions were performed by varying the duration of the reaction (1, 3, 5, and 7 h) at 400 °C, with an initial pressure of 20 bar and using a plastic/solvent ratio of 1/10. These reactions were compared with the corresponding experiments that were conducted in the absence of the decalin solvent. Figure 1 shows the yields for different hydrocarbon groups, based on the number of C atoms, as a function of reaction time for the runs with decalin (Figure 1a) and without the solvent (Figure 1b). The total yield (C1-C55 hydrocarbons) increased with time in the reactions with decalin and also in the reactions in its absence. However, these yields were ∼3 times greater in the reactions that used the solvent. A total conversion of HDPE
Ind. Eng. Chem. Res., Vol. 46, No. 11, 2007 3499 Table 1. Effect of Reaction Time on the Selectivities to Gaseous Hydrocarbons in the Reactions with Decalin and without Solventa Selectivity (%) description
1h
3h
5h
7h
reaction with decalin reaction without solvent
78 45
54 21
37 25
32 29
a Conditions: temperature ) 400 °C, HDPE/decalin ratio ) 1/10, initial pressure ) 20 bar.
to C1-C55 hydrocarbons was achieved in the reaction with decalin at 7 h. The yield of the gaseous fraction (C1-C4) improved with time in the thermal degradations using decalin and also in the corresponding reactions in its absence, because of the extension of scission reactions over time. As can be observed, the gaseous hydrocarbon yields were remarkably higher using decalin. In all the reactions, the yields of the other fractions (C5-C20, C21-C32, C32-C55) also increased with time in real terms, because of the increase in the number of polymer scissions with time. However, the presence of decalin slightly affected the C5-C20, C21-C32, and C32-C55 yields for 1 and 3 h, compared to the solvent-free reactions. In this sense, the thermal degradation in the presence of decalin produced mainly gaseous hydrocarbons at short reaction times. In fact, the selectivities to the C1-C4 hydrocarbons decreased with the reaction time from 78% at 1 h to 32% at 7 h in the reactions using decalin (Table 1). The yields of the C5-C20, C21-C32, and C32-C55 fractions were significantly higher in the experiments that were performed with decalin for 5 and 7 h, in comparison to the corresponding solventless reactions. There seems to be an induction time for the solvent to have an effect on the production of these hydrocarbons. Nevertheless, the yield of C5-C20 hydrocarbons was lower in the reaction using decalin at 7 h than in the corresponding at 5 h, because these hydrocarbons were cracked to gaseous products at longer reaction times. Therefore, the solvent has a significant effect on the HDPE thermal degradation, mainly in the reactions for 5 and 7 h. In previous works,10,11 we have concluded that the presence of decalin has a promoting effect on mass- and heat-transfer rates and also modifies the thermal degradation mechanism, increasing the HDPE conversion into low-molecular-weight hydrocarbons. The second decalin effectsthe contribution of decalin in the thermal degradation mechanismsis more significant, because the use of other solvents in this reaction did not produce this important increase in the HDPE conversion. This solvent participates in the termination stage of the degradation mechanism, especially at long reaction times (5 and 7 h). Under these operating conditions, decalin, as a hydrogen-donating solvent, is able to terminate the chain reactions, giving hydrogen to the HDPE primary radicals, according to reaction 1:
However, decalin is known to be a poor hydrogen-donating solvent, in comparison to other compounds, such as tetralin or 9,10-dihydroantracene.6 Hence, decalin donates hydrogen to the radicals at a moderate rate, allowing for the plastic degradation into relatively low-molecular-weight hydrocarbons. This, in turn, leads to high levels of plastic conversion.
The distribution of gaseous hydrocarbons, broken down by hydrocarbon type, is shown in Figure 2. Much greater amounts of olefins were observed in the gaseous fraction of the HDPE thermal degradation using decalin, in comparison to the corresponding solventless reaction. In all cases, the main gaseous olefin was propylene. In addition, the olefin yields increased with reaction time. However, the paraffin yield values were significantly lower in these reactions. The olefin and paraffin yields were 23% and 2%, respectively, in the reactions in the presence of decalin at 5 h. However, the selectivity to olefins was only somewhat greater in the reactions with decalin, in comparison to the solventless reactions (Figure 3). These results also indicate that the presence of this solvent modifies the thermal cracking mechanism, increasing the HDPE conversion. This led to an enhancement in the production of gaseous olefins, useful as raw chemicals for the petrochemical industry. The selectivity to gaseous olefins increased with time in the reactions using decalin and also in the solventless reactions, achieving a maximum at 5 h. At this reaction time, the selectivities to gaseous olefins were 93% for the reaction with decalin and 83% for the solventless reaction. The C5-C55 hydrocarbons have been broken down into linear and nonlinear hydrocarbons, and the corresponding yields are represented versus the reaction time for the reactions with decalin (Figure 4a) and for the corresponding solventless reactions (Figure 4b). In both cases, the linear C5-C55 yields were notably higher than the corresponding nonlinear hydrocarbon yields. The presence of decalin increased the linear C5C55 yields, compared to those in the solvent-free reactions. Nevertheless, this increase was significantly higher in the solvent thermal degradation at 5 and 7 h. The nonlinear hydrocarbons (branched, aromatic, and cyclic compounds) yields were
