Energy & Fuels 2009, 23, 2743–2749
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Pyrolysis of Polyvinyl Chloride (PVC)-Containing Mixed Plastic Wastes for Recovery of Hydrocarbons N. Miskolczi,* L. Bartha, and A. Angyal Department of Hydrocarbon and Coal Processing, UniVersity of Pannonia, Egyetem u. 10, Veszprem H-8200, Hungary ReceiVed December 23, 2008. ReVised Manuscript ReceiVed March 4, 2009
The pyrolysis of mixed plastic waste (high-density polyethylene, polypropylene, polystyrene, and polyvinyl chloride) using a long residence time (25 min) in a horizontal tube reactor at 530 °C has been investigated. The reactor was fitted with atmospheric and vacuum distillation columns, so that the pyrolysis products could be separated into heavy oil, light oil, gasoline, and gases. The effect of the polyvinyl chloride (PVC) concentration on the properties of products was investigated. Products were characterized using gas chromatography, infrared spectroscopy, and energy-dispersive X-ray spectroscopy, whereas some other standardized methods were used to determine the main properties of hydrocarbons. Results show that the mixed plastic waste samples could be converted into gases, gasoline, and light oil with yields of 36.9-59.6% depending upon the composition of feed polymers, and the conversion of decomposition is significantly increased with an increasing concentration of PVC. It was found that products mostly consisted of paraffin, olefin, and aromatic compounds, with carbon numbers of C1-C4, C5-C17, and C11-C28 in the case of gases, gasoline, and light oil, respectively. Light aromatics (benzene, toluene, styrene, ethyl benzene, and xylenes) could be detected only in gasoline, while oligomers of styrene obtained by principal degradation of polystyrene were in heavy oil. Aromatics consist of mainly ethyl benzene and styrene. Gases and heavy oil had high caloric values of 46-47 and about 41 MJ/kg, respectively; therefore, they could also be used for energy generation. The favorable properties of all fractions were depreciated in consequence of increasing PVC content, because the concentration of chlorine increased with that. The highest chlorine content had been measured in gases, wherein the HCl was the dominant compound, which had succeeded to neutralize in a scrubber filled with a solution of calcium hydroxide.
1. Introduction Recently, the recycling of municipal (or mixed) plastic waste (MPW) has been a major environmental challenge. The worldwide production and application of plastics has grown rapidly over the last few years, and according to forecasts, the consumption of plastics is increasing at 4-5% annually.1-3 The problem is that the growing production of plastics results in an increased mass of waste plastic and causes serious environmental risks. On the basis of data in papers, the average composition of the yearly produced plastics is 35% high-density polyethylene (HDPE), 23% polypropylene (PP), 10% polystyrene (PS), 13% polyvinyl chloride (PVC), 7% poly(ethylene terephthalate) (PET), and 12% other polymers worldwide. Because of the special habits of costumers, polyolefin (PE and PP) and PS are the most dominant plastics inside waste polymers. The dominant mass of waste plastics has been placed in a landfill or incinerated, but disposing of the waste to a landfill or incineration is becoming undesirable because of legal pressures [e.g., European Union (EU) directives]. Those directives try to limit the amount of landfilled or incinerated wastes. The main problems with the above-mentioned waste handling ways are * To whom correspondence should be addressed. E-mail: mnorbert@ almos.vein.hu. (1) TNO Report (STB-99-55). Chemical recycling of plastic waste (PVC and other resins), 1999. (2) Sartorius, I. Development plastics. Manufacturing industry in Europe. ICS-UNIDO Conference, Trieste, Italy, 2002. (3) Department for Environment, Food and Rural Affairs (DEFRA). Waste Strategy 2000: England and Wales Part II; DEFRA: London, U.K., 2000; Chapter 5, pp 64-88.
high cost consumption of suitable waste deposition and greenhouse gas emission or other toxic pollutants (nitrous and sulfur oxides, dust, dioxins, etc.) from incinerators. Therefore, the two main alternatives for recycling of municipal and industrial polymer wastes are pyrolysis and mechanical recycling. Generally, mechanical recycling is a popular way and carried out on single-polymer waste streams, because it is economical where high-purity selectively collected plastics are available. Other problems with mechanical recycling are the difficulties in the type of selective collection of plastic wastes, high-purity requirement, and fluctuating price and quality of wastes. When wastes are pyrolyzed, they can converted into valuable hydrocarbon products. Different types of waste polymers (HDPE, LDPE, PP, and PS) could be converted into hydrocarbons with favorable properties for further application (e.g., fuel-like). The chemical recycling (or pyrolysis) also has a great challenge, because some polymers have unfavorable properties for both pyrolysis and further use of products. For example, the concentration of PVC in MPW is limited in the case of pyrolysis, because the halogen compounds are not desirable. As wellknown, chlorinated compounds are hazardous catalyst poisonings and have acidic character. The problem is that PVC quite often cannot be separated from other plastics as constituents of MPWs. Therefore, it is important to know what correlation is between the PVC content of feed materials for the pyrolysis process and product property, first of all focused to the chlorine content. The thermal degradation of plastic wastes has been realized in both the batch process (e.g., blast furnaces, vessel, autoclave, etc.) and continuous process (e.g., rotary kiln,
10.1021/ef8011245 CCC: $40.75 2009 American Chemical Society Published on Web 03/26/2009
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fluidized-bed reactor, etc.).4-18 Degradation could be divided into catalytic and noncatalytic processes, too. The degradation of polymers requires a high temperature (over 350 °C depending upon the raw materials) in the noncatalytic case, while the required heat and energy of the process could be decreased with catalysts, but its disposal is problematical especially in a continuous system.14-17 Papers have given an excellent summary of the recycle of waste polymers in various circumstances. At higher temperature, significant formation of coke and aromatics was observe by any of the researchers. The degradation of PVC or the mixtures of PVC with other polymers (e.g., HDPE, PP, etc.) had also been studied in several experiments, but they have mainly focused on the distribution of hydrocarbons.15-24 Interactions were found in several papers between components of polymer blends during degradation. The aim of our experimental work was the investigation of the pyrolysis behavior of PVC containing mixed plastic waste (mixtures of HDPE, PP, and PS). According to our preliminary results, the effects of the concentration of unfavorable waste plastics to the product properties were investigated in a horizontal tube reactor, at a temperature of 530 °C, using 25 min residence time.7,8 Different hydrocarbons with similar properties, such as streams of refinery, were produced: gases, gasoline, light oil, and heavy oil. Both the yields and volatile product properties (e.g., olefin content, carbon number distribution, etc.) were investigated. To study the effect of the content of PVC on the properties, the concentration of chlorine was also measured. 2. Experimental Section 2.1. Raw Materials. Four different types of waste polymer were used in this work from different areas of everyday life: HDPE, PP, PS, and PVC. HDPE has a density of 0.962 g/cm3, sulfur content (4) Kim, J. S.; Lee, W. Y.; Lee, S. B.; Kim, S. B.; Choi, K. J. Catal. Today 2003, 87, 59. (5) Seo, Y. H.; Lee, K. H.; Shin, D. H. J. Anal. Appl. Pyrolysis 2003, 70, 383. (6) Seddegi, Z. S.; Budrthumal, U.; Abdulrahman, A. A.; Adnan, M. A.; Sami, A. I. Appl. Catal., A 2002, 225, 167. (7) Miskolczi, N.; Bartha, L.; Dea´k, G.; Jo´ver, B. Polym. Degrad. Stab. 2004, 86, 357. (8) Miskolczi, N.; Bartha, L.; Dea´k, G.; Jo´ver, B.; Kallo, D. J. Anal. Appl. Pyrolysis 2004, 72, 235. (9) Grieken, R.; Serrano, D. P.; Aguado, J.; Garcy´a, J.; Rojo, C. J. Anal. Appl. Pyrolysis 2001, 58, 127–159. (10) Murata, K.; Hirano, Y.; Sakata, Y.; Azhar, M. J. Anal. Appl. Pyrolysis 2002, 65, 71. (11) Aguado, R.; Olazar, M.; Gaisa´n, B.; Prieto, R.; Bilbao, J. Chem. Eng. J. 2003, 92, 91. (12) Moriya, T.; Enomoto, H. Polym. Degrad. Stab. 1999, 65, 373. (13) Karaduman, A.; Simsek, E. H.; Cicek, B.; Bilgesu, A. Y. J. Anal. Appl. Pyrolysis 2002, 62, 273. (14) Bockhorn, H.; Hornung, A.; Hornung, A. J. Anal. Appl. Pyrolysis 1999, 50, 77. (15) Kaminsky, W.; Schlesselmann, B.; Simon, C. M. Polym. Degrad. Stab. 1996, 53, 189. (16) Mastral, F. Y.; Esperanza, E.; Berrueco, C.; Juste, M.; Ceamanos, J. J. Anal. Appl. Pyrolysis 2003, 70, 1. (17) Ali, S.; Garforth, A. A.; Harris, D. H.; Rawlence, D. J.; Uemichi, Y. Catal. Today 2002, 72, 247. (18) Buekens, A. G.; Huang, H. Resour., ConserV. Recycl. 1998, 23, 163. (19) Sørum, L.; Grønli, M. G.; Hustad, J. E. Fuel 2001, 80, 1217. (20) Lingaiah, N.; Uddin, M. A.; Muto, A.; Imai, T.; Sakata, Y. Fuel 2001, 80, 1901. (21) Karago¨z, S.; Karayildirim, T.; Uc¸ar, S.; Yuksel, M.; Yanik, J. Fuel 2003, 82, 415. (22) Bhaskar, T.; Uddin, M. A.; Murai, K.; Kaneko, J.; Hamano, K.; Kusaba, T.; Muto, A.; Sakata, Y. J. Anal. Appl. Pyrolysis 2003, 70, 579. (23) Liu, K.; Pan, W. P.; Riley, J. T. Fuel 2000, 79, 1115. (24) Miranda, R.; Yang, J.; Roy, C.; Vasile, C. Polym. Degrad. Stab. 2001, 72, 469.
Miskolczi et al. Table 1. Composition of Samples (%) sample
HDPE
PP
PS
PVC
S-1 S-2 S-3 S-4 S-5
50.0 49.5 49.0 48.0 47.0
40.0 40.0 40.0 40.0 40.0
10.0 10.0 10.0 10.0 10.0
0.0 0.5 1.0 2.0 3.0
of 11 ppm, and free of other impurities. The density of PP, PS, and PVC was 0.909, 1.045, and 1.076 g/cm3, respectively. The concentration of sulfur was 5, 8, and 7 ppm of PP, PS, and PVC; however, chlorine contains 38.2% chlorine, too. Raw materials were obtained from the communal waste plastic processing plant. All waste plastics have been washed and cut up into small pieces before they were inserted into the reactor feed stage. The average grain size of plastic particles was 4-5 mm. Considering that the main aim of this experimental work was the investigation of the effect of PVC content of MPWs on the properties of end products, different compositions of plastic wastes were used. Those have been made by the mixing of the above-mentioned HDPE, PP, PS, and PVC. The compositions of samples are summarized in Table 1. All samples had 10.0% PS and 40.0% PP. The S-1 model sample was free from PVC, while others consisted of the chlorinated waste polymers in the range of 0.5-3.0%. The largest amount of polymers was HDPE, with a concentration from 47.0 to 50.0%. 2.2. Cracking Apparatus. The thermal degradation of samples was carried out in a horizontal tube reactor at the temperature of 530 °C (Figure 1). The reactor feed capacity was hourly 1 kg in the case of each model MPW sample. Raw materials with suitable grain size were fed in the reactor by an extruder. Because of the special mixing equipment inside the reactor, no coke formation had been found on the internal surface of the reactor. For that reason, each product was free from coke particles. The electrically heated extruder was directly connected to the reactor beginning section, which from the preheated polymer was driven into the reactor and cracked therein at 530 °C using 25 min residence time. Inside the reactor, waste polymers melted and their main carbon frame cracked into smaller fragments. Constant thermocouples and electronic photoionization detector (PID) controllers were used to set the temperature in both the extruder and pyrolysis reactor. The other main part of the cracking apparatus was the separation unit, which consisted of atmospheric and vacuum distillation columns. In the first column, hydrocarbons were separated into gases, gasoline, and heavy residue, in which the latter one was separated into light and heavy oils in the second column. The applied vacuum pressure was 50 mm of Hg inside the second column. The top temperature of atmospheric and vacuum distillation columns was 190 and 210 °C, respectively. Gaseous hydrocarbons had been driven to a scrubber, where the acidity compounds (mainly HCl) were neutralized by a solution of calcium hydroxide. Gases have been collected in the
Figure 1. Apparatus for pyrolysis of MPW: (1) waste storage and blending, (2) feed unit, (3) electric power, (4) tube reactor, (5) atmospheric distillation, (6) vacuum distillation, (7) scrubber, and (8) vacuum pump.
Pyrolysis of PVC-Containing MPWs
Figure 2. Yields of products at 530 °C from different MPWs.
gas bag and analyzed by gas chromatography (GC), while all fractions of gasoline, light oil, and heavy oil were also analyzed by standardized and non-standardized methods. 2.3. Analysis of Products. Each fraction of cracking was analyzed using the following standardized and non-standardized methods. In our analysis, the following standardized methods were used: (1) liquid density measurement (MSZ EN ISO 12185), (2) determination of the distillation curve (ASTM-D 1078), (3) determination of the sulfur content (ASTMD 6428 99), (4) determination of the flash point (ISO 2719:2002 and MSZ 15967:1979), (5) determination of the cold filter plugging point (CFPP) (MSZ EN 116:1997), (6) corrosion test (MSZ EN ISO2160:2000), and (7) determination of the kinematic viscosity (MSZ ISO 3105:1998). Gases were analyzed using a Carlo-Erba Vega Series GC 6000 gas chromatograph provided with a 50 m × 0.32 mm fused silica column with Al2O3/KCl coating, at 40 °C. Liquid products were analyzed by a gas chromatograph with a flame ionization detector using a TRACE GC gas chromatograph. It was provided with a 30 m × 0.32 mm Rtx-1 (Crossbond 100% dimethyl polysiloxane) column. The olefin content of liquids were measured by an infrared technique with a TENSOR 27 type Fourier transform infrared (FTIR) spectrometer too (resolution, 3 cm-1; illumination, SiC Globar light; detector, RT-DLaTGS type), in the 400-4000 cm-1 wavenumber range. The chlorine content of gases was measured by titration of calcium hydroxide solution from the scrubber. The chlorine content of gasoline, light oil, and heavy oil was followed with energy-dispersive X-ray fluorescent spectrometry. A PHILLIPS PW 4025/02 (MiniPal) EDXRF spectrometer was used. The measurement of chlorine content occurred by using 10 kV excitation voltage and 500 µA current. A helium atmosphere and thin aluminum filter were used in the case of measurements. Research octane number (RON), motor octane number (MON), and cetane number were determined with a ZX-101c type instrument based on the infrared spectra of products.
3. Results and Discussion 3.1. Yields. This work has focused the effect of the PVC content of the feed plastics (mixtures of HDPE, PP, PS, and PVC) on the pyrolysis product yields using a tube reactor at temperatures of 530 °C. The reaction temperature and the other reaction parameters, such as residence time (25 min) and pressure, were constant and the same in each case. The quantitative results of the waste plastic decomposition are shown in Figure 2, where it can be seen that the yields of valuable volatile products (gasoline and light oil) increased with an increasing PVC content of samples. According to data, the different compositions of waste plastics have been converted into valuable fractions with yields of 36.9-59.6% depending upon the composition of samples. Yields could be increased from 2.8 to 3.2%, from 18.4 to 24.8%, and from 15.3 to 32.0% in the case of gases, gasoline, and light oil, respectively. The maximum gas yield was below 5%. As Table 1 shows, the concentrations of HDPE, PP, and PS were nearly the same in
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each sample because, basically, the PVC content has increased from 0 to 3%. Therefore, the considerable increasing of volatile products should be attributed to the higher PVC concentration. In the literature, a similar effect of the PS and PVC was given by Miranda et al., when the molecular fragments from PS promoted the degradation of HDPE.24 The breaking of the main chain of polymers is performed via radicals in the thermal case. Generally, the polymer chain is broken in the places where the electron density is different from another part of the molecule or where some chain mistakes are observed. This is the cause that is the highest for the activation energy of the broken main frame of HDPE, while the lowest is that of PP and the first chain broken of PVC, which generally the chlorine has broken.24 On the other hand, the apparent reaction rate of the gross reaction is first-order in the case of HDPE, PP, and PS, while between first- and second-order, in consequence of the two reaction steps, in the case of PVC.24 Generally, DTA curves of HDPE, PP, and also PS contain only one peak, while there are more peaks in the case of PVC because of more degradation steps. The first reaction step of PVC pyrolysis has the lowest activation energy compared to those of HDPE, PP, or PS. Because the dehydrochlorination of PVC is a free-radical chain reaction, it needs a relatively low temperature (280-350 °C). Presumably, PVC pyrolysis has resulted in lighter products at lower temperature, when others are only in the molten phase without a significant chain broken. That reaction has given such radicals, which possibly can promote the decomposition of other polymers (HDPE, PP, or PS). The main cause of that phenomenon is that the C-Cl bond in the PVC structure has a lower bond energy than the C-C and C-H bonds; therefore, the C-Cl bond dissociates first and results in the dehydrochlorination of PVC. This reaction should also be initiated at unsaturated sites (allylic chlorines) or other defect sites along the polymer chains. The possible reaction free chain scission mechanism is shown in Figure 3. Our results suggest that chlorine-containing radicals promoted the degradation of polyolefins at low temperature, which alone do not decompose at this lower temperature. At lower temperature than needed for the degradation of polyolefins with volatile product, chlorine-containing radicals formed from PVC in the initial step. Presumably, these radicals stimulated the decomposition of other polymers inside MPW (e.g., HDPE, PP, or PS). These radicals could be stabilized through the formation of halogen-containing products. 3.2. Gases. The composition of gases formed in the pyrolysis of mixed plastic wastes were analyzed with GC. Results are shown in Figure 4. According to the figure, the PVC content of MPW samples did not considerably affect the hydrocarbon composition of gaseous products but the HCl content of gases has significantly risen from 0.66 to 3.45%, if the PVC content of raw materials increased from 0.5 to 3%. For that phenomenon, the lower bond energy of the C-Cl bond than that of C-C or C-H bonds in the PVC structure is blamed. Therefore, the C-Cl bond has dissociated at first and results in the dehydrochlorination of PVC. It is also clear that the gases consisted of mostly propane and propene, but the C2 and C4 alkanes and alkenes had also represented a determinative amount. As well-known, the thermal cracking of HDPE has resulted in monomers and oligomers with statistical distribution in consequence of the lack of a distinguished part of the carbon framework; therefore, mainly ethane and ethene and butane and butene were produced from HDPE, while the thermal destruction of PP had resulted in a high concentration of propane and propene in gases. For the latest
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Miskolczi et al.
Figure 3. Thermal decomposition of HDPE, PP, PS, and PVC. Table 2. Composition and Main Properties of Gasoline sample
Figure 4. Composition of gases.
mentioned result, the tertiary carbon atoms in the main carbon framework are blamed, because of the chain scission reactions initiated at unsaturated sites or at other defect sites along the polymer chains. According to papers, cracking of PS has been given a different composition of C1-C4 hydrocarbons; however, generally, the quantity of gases from PS is low, and the main components are methane, ethane, and ethane. The calculated heating value of gases was 46-47 MJ/kg, which was quite high for use of gases in energy generation (e.g., to provide the requirement heat for cracking with gas products of degradation). 3.3. Gasoline. The hydrocarbon composition of gasoline obtained by the pyrolysis of model MPWs is shown in Table 2. Data show that the gasoline from the pyrolysis of samples consisted of C5-C17 hydrocarbons. It is important that, because of the decomposition of PS, approximately a third part of aromatic content was observed in the composition of gasoline. The vast majority of the aromatic content of gasoline was ethyl benzene in the range of 16.16-18.71 wt %. It is important to remark that a significant quantity of styrene was also present in the gasoline, most prominently at PVC content of 0.5% (6.99%). Additionally, a small amount of benzene, toluene, xylenes, and other aromatics was detected in the fractions of gasoline, but their concentrations were below 6%. For example, the benzene and R-methyl styrene concentrations in the gasoline were below 2.5 and 1.7%. Because the concentration of PVC
C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 ∑(aliphatic) benzene toluene ethyl benzene styrene xylenes isopropyl benzene R-methyl styrene ∑(aromatic) color density (g/cm3) ROM MON (ROM + MON)/2 M (g/mol) corrosion test IBP 20 50 70 FBP
S-1
S-2
S-3
composition (%) 3.76 3.18 3.83 7.76 7.68 7.68 3.59 3.76 3.42 6.24 6.93 5.70 18.32 15.90 17.21 11.45 10.20 11.40 6.44 7.45 5.48 9.26 10.28 10.19 0.85 1.04 1.00 0.79 0.29 0.33 0.26 0.56 0.42 0.42 0.29 0.16 0.32 0.43 0.09 69.47 67.99 66.92 0.58 0.78 1.24 1.58 1.66 1.82 16.16 17.06 18.35 6.13 6.99 5.80 2.25 3.00 2.61 2.29 0.96 1.61 1.53 1.55 1.66 30.53 32.01 33.08 yellowish yellowish yellowish 0.768 0.767 0.759 87 87 88 74 75 75 78 79 79 120 119 115 group 1a group 1a group 1b distillation data (°C) 38 37 36 55 52 56 138 138 139 147 143 148 210 204 209
S-4
S-5
4.03 7.07 3.81 6.48 17.17 10.10 6.98 9.09 1.44 0.05 0.05 0.05 0.05 66.36 1.56 2.06 18.71 6.09 2.83 0.96 1.42 33.64 yellowish 0.756 89 76 80 114 group 2b
3.79 6.30 3.31 6.69 16.80 10.34 6.32 8.82 1.86 0.04 0.16 0.08 0.08 64.58 2.41 2.46 18.25 6.85 2.98 0.84 1.61 35.42 yellowish 0.750 90 77 81 112 group 2d
35 51 134 146 211
37 54 137 142 205
has risen from 0 to 3%, the aromatic content also increased from 30.53 to 35.42%. A similar tendency was found in the case of benzene and toluene. Presumably, radicals from PVC initiated the degradation of PS, which should have promoted the secondary chain scission of PS. Generally, the substituted groups had been removed in secondary chain scission.
Pyrolysis of PVC-Containing MPWs
As it was discussed earlier, the gas composition suggests that the decomposition of the polymers proceeds by radically initiated chain scission. Seeing Table 2, it is clear that aliphatic compounds with triple sequence could be observed. That is a typical phenomenon of PP pyrolysis; therefore, the non-aromatic hydrocarbons with carbon numbers of 6, 9, and 12 have been presented in the highest concentration in gasoline. The HDPE pyrolysis results statistically distributed quantities of nonaromatic compounds. Each gasoline had nearly half a part of the olefin content, which was the consequence of the radical-initiated chain scission mechanism of the reactions. The olefin content of products was between 45.5 and 49.1%. The appearance of gasoline was a yellowish liquid with densities of 0.768-0.750 g/cm3, which has decreased only to a small degree as a function of the PVC concentration. This result was also supported by the average molecular weights of gasoline, because it has decreased from 120 to 112 g/mol in all probability as a consequence of the more intensive cracking of the main frame of plastics. Gasoline qualities are usually measured in terms of the octane number of the hydrocarbons (RON and MON) and are very important properties. Higher octane numbers are preferred for prevention of early ignition, which leads to cylinder knock. For gasoline production, aromatics, naphthenes, and isoalkanes are highly desirable, whereas olefins and n-paraffins are less desirable. The octane numbers of gasoline were derived from their hydrocarbon composition based on their infrared spectra. Both RON and MON had high values, but the difference between RON and MON (which is named the sensibility of gasoline) was high too. However, both RON and MON have risen from 87 to 90 and from 74 to 77 with an increasing PVC content of raw plastic wastes, for which the increasing aromatic content is blamed. As discussed above, the PVC content of raw materials can significantly affect the aromatic quantity and their concentration; moreover, the disparateness could result from the differences in RON and MON. For example, benzene and toluene have a higher octane number than ethyl benzene. On the other hand, an unbeneficial phenomenon is the change of the corrosion property of gasoline, which shows that, the higher the PVC content of raw material, the more unfavorable (or more acidity) the gasoline corrosion property. The distillation data of gasoline has a boiling point range that shows similarity to that of refinery gasoline (35-211 °C). 3.4. Light Oil. The properties of light oils are shown in Table 3. To compare Tables 2 and 3, it was found that the light oil from model MPWs consisted of mainly non-aromatic compounds and only a trace of aromatics, while the aromatic concentration of gasoline was in the range of 30.53 and 35.42. Aromatics were the substituted benzene rings, such as cumene and 1,3-diphenylpropane. The aromatic content was less than 1% in each case. It is also clear from Table 3 that the nonaromatic compounds in light oils had carbon numbers from 11 to 28 and also their triple sequence was found. The main aliphatic compounds might be characterized with carbon numbers of 12, 15, 18, 21, 24, and 27, but hydrocarbons with the highest concentration possess carbon numbers of 15, 16, and 18. For the triple sequence, the PP content of samples can be blamed. The olefin concentration was a bit smaller (40.2-45.8%) than in the case of gasoline fractions, because the cracking reaction (e.g., β-scission) resulted in olefins that were shorter in length compared to their carbon chain. The color of light oils was yellowish but darker, as found in the case of gasoline. Their density had changed between 0.836 and 0.817 g/cm3, which could be decreased by the PVC content
Energy & Fuels, Vol. 23, 2009 2747 Table 3. Composition and Main Properties of Light Oil sample
S-1
S-2
S-3
composition (%) 0.11 0.12 0.00 C11 C12 6.82 7.02 7.71 C13 6.09 6.76 6.25 C14 6.82 7.67 7.20 C15 16.73 16.82 17.05 C16 14.75 16.16 15.60 C17 9.40 8.39 8.30 C18 14.84 14.29 14.25 C19 5.76 6.37 5.03 C20 2.77 2.54 2.12 C21 2.46 2.72 3.09 C22 2.24 3.52 2.73 C23 1.71 1.33 1.94 C24 2.83 1.93 2.88 C25 0.98 0.64 0.5 C26 2.83 1.81 1.69 C27 0.65 0.53 0.88 C28 0.76 0.7 1.44 C29 0.53 0.52 0.52 ∑(aliphatic) 99.06 99.83 99.18 aromatics (%) 0.94 0.17 0.82 color yellowish yellowish yellowish 0.836 0.831 0.824 density (g/cm3) cetane number 63 64 63 diesel index 70 70 71 viscosity (mm2/s) 2.19 2.17 2.16 CFPP (°C) -1 -2 -3 M (g/mol) 162 161 161 flash point (°C) 86 81 80 corrosion test group 1a group 1a group 1a distillation data (°C) IBP 75 79 76 20 163 161 164 50 190 195 192 70 210 211 213 FBP 251 254 256
S-4
S-5
0.20 8.80 6.99 6.46 18.79 16.50 9.03 15.72 5.51 1.75 2.30 1.80 1.42 1.82 0.06 1.1 0.44 0.77 0.51 99.96 0.04 yellowish 0.821 64 70 2.13 -3 158 79 group 2a
0.16 9.61 7.47 8.24 17.69 16.21 8.42 14.73 5.09 2.22 2.49 2.01 1.27 1.46 0.05 0.76 0.38 0.49 0.53 99.27 0.73 yellowish 0.817 64 71 2.11 -3 155 76 group 2d
73 170 183 218 256
74 165 197 206 253
of raw materials. One of the most important properties of light oil is the cetane number and diesel index. As known, higher values of both the cetane number and diesel index are more favorable. As data show in Table 3, the cetane number and diesel index had changed in the range of 63-64 and 70-71, respectively, which are favorably high for fuel use. The CFPP refers to the so-called cold-side property of diesel fuel, which is preferred in its low value. The CFPP of light oils was measured by the MSZ EN 116:1997 standardized method, and the value was between -1 and -3 °C. It is important to remark that the CFPP, cetane number, and diesel index or viscosity of light oils had not changed notably with the PVC content of raw MPWs. As mentioned in the case of gasoline, the average molecular weight of light oils also decreased (162 f 155 g/mol) as a function of the PVC content of raw materials. Presumably, the preliminary radicals from PVC promoted the more intensive cracking of the main carbon frame of other polymers (e.g., HDPE, PP, or PS), which was discussed above. In the presence of those radicals, “deeper cracking” and destruction of the carbon frame should be reached. The PVC content most of all had modified the corrosion property of the products, because the higher the PVC content of raw material, the more unfavorable (or more acidly) the light oil corrosion property. Distillation data boiling point ranges (73-256 °C) show similarity with middle distillates from refinery. 3.5. Heavy Oils. As Figure 1 shows, heavy oils had been separated as bottom products of the vacuum distillation column. Those products had a boiling point above 300 °C and mainly contained saturated and non-saturated aliphatic hydrocarbons and aromatics (Table 4). The concentration of aromatic compounds had changed from 11.9 to 15.9%, but there was no
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Miskolczi et al. Table 4. Composition and Main Properties of Heavy Oil
a
sample
S-1
S-2
S-3
S-4
S-5
aliphatic paraffin content (%) aliphatic olefin content (%) aromatic content (%) color flash point (°C) melting point (°C) mechanical impurities (ppm) caloric value (MJ/kg) density (g/cm3) M (g/mol)
52.1 33.8 14.1 grayish black 228 106 nda 41.3 0.849 1890
53.3 32.4 14.3 grayish black 225 104 nd 41.4 0.847 1850
51.0 32.5 16.5 grayish black 220 102 nd 41.3 0.841 1830
54.3 33.8 11.9 grayish black 218 101 nd 41.4 0.841 1790
51.8 32.3 15.9 grayish black 215 101 nd 41.3 0.835 1740
nd ) not detectable.
tendency with the increase between the aromatic concentration and PVC content of raw materials. However, aromatics consisted of a significant concentration of oligomers of styrene (from PS decomposition) with a higher carbon number inside heavy oils, which cannot be distilled from the melted polymers under the applied separation parameters. That phenomenon is the characteristic of PS degradation. Lighter aromatics are concentrated in the gasoline, while heavier aromatics are concentrated in the heaviest product (in heavy oil). It is also clear from data that unsaturated aliphatics presented 32.4-33.8%, while saturated aliphatics presented 52.0-54.3%. The latest one was the most dominant type of hydrocarbon in heavy oils. The color was darker than discussed in gasoline and light oil, because heavy oils were grayish black, presumably from the coked particles. The appearance of heavy oils was wax-like, and their melting points were in the range of 101-106 °C. Because of the residue characteristics, they had the highest molecular weight among pyrolysis products (1740-1890 g/mol). The most significant effect of the PVC content of the plastic mixture is the decreasing average molecular weight. The average molecular weight of heavy oil from PVC-free raw materials was 1890 g/mol, which had decreased to 1740 g/mol, when 3% PVC was added to mixtures of HDPE-PP-PS. This phenomenon could be attributed to the greater degradation of MPW, as a consequence of the chain-breaking initiating effect of radicals from, e.g., PVC. Nevertheless, this result was also supported by the values of melting points and flash points. Presumably, the melting point decreased from 106 to 101 °C, owing to the shorter hydrocarbon chains, which were formed by the more significant cracking of main frames in the presence of PVC. The flash point and density of the heaviest fraction had changed from 228 to 215 °C and from 0.849 to 0.835 g/cm3, respectively, when the PVC concentration decreased from 3 to 0%, and those densities were the highest compared to gasoline and light oils. It was also found that mechanical impurities could not be detected in heavy oils. The caloric value was measured by the bomb method, which indicated that it was about 41 MJ/kg, which is quite high for the application of these fractions for energy production. 3.6. Contamination of the Fraction. As discussed above, the concentration of PVC can affect both the quantity and quality of pyrolysis products of model MPW samples. It is well-known that chlorinated compounds are harmful and undesired in products, owing to their unfavorable corrosion and catalystpoisoning properties. To eliminate the unfavorable properties, chlorination of the PVC concentration of raw materials should be limited. Figures 5 and 6 show the sulfur and chlorine content of products obtained by degradation of MPWs with different PVC contents. Products had sulfur below 10 ppm in each case, and correlation was not found between the concentration of sulfur in end products and the PVC content of raw materials. Gases were sulfur-free. The lowest sulfur content had been measured
in gasoline (1-5 ppm) and light oil (2-4 ppm), while the highest sulfur content was found in heavy oil (7-9 ppm). This is advantageous for further fuel-like application, because there are great ambitions to reduce the concentration of heteroatoms (mainly sulfur) in fuels. The hydrodesulphuration occurring at high pressure, owing to its high hydrogen and specific catalyst demand is a very expensive process in refineries. From the application of gasoline and light oil obtained from pyrolysis, a huge mass of conventional gasoline and diesel oil might be replaced, saving lots of crude oil demand upon refinery. The situation was different for the chlorine content. When PVC-free model MPW had been pyrolyzed, all products were free from chlorinated compounds but each product had chlorine content obtained by the pyrolysis of S-2, S-3, S-4, and S-5 samples. The highest chlorine content had been found in gases, where the dominant chlorinated compound was HCl, which had considerably risen from 0.66 to 3.45% when the PVC content of raw material increased from 0.5 to 3%. Other products were lower in chlorine content than gases; e.g., gasoline had Cl of 46-2129 ppm and light oil had Cl of 132-2201 ppm, depending upon the PVC concentration. The increasing chlorine content of products supposed that, the higher the PVC content, the greater the number of Cl radicals, which were able to initiate the further scission of other polymers (e.g., HDPE, PP, or PS). It is necessary to remark that the above-mentioned favorable product properties of gasoline, light oil, and heavy oil were
Figure 5. Sulfur content of products.
Figure 6. Chlorine content of products.
Pyrolysis of PVC-Containing MPWs
depreciated as a consequence of the increasing concentration of PVC of raw material. 4. Summary In this paper, the pyrolysis of PVC-containing MPW had been investigated in a horizontal tube reactor using a long residence time (25 min) at 530 °C. Mainly, the effect of PVC content to the yields and characteristics of products (gases, gasoline, and light and heavy oils) was studied. Results show that the MPW samples could be converted into volatile fractions with yields of 36.9-59.6%, depending upon the composition of feed polymers. The conversion of decomposition significantly increased with the PVC concentration of raw materials. Products consisted of mainly paraffin, olefin, and aromatic compounds, with carbon numbers of C1-C4, C5-C17, and C11-C28 in the case of fuel gases, gasoline, and light oils, respectively, and the main components had triple sequence (e.g., C3, C6, C9, C12, C15, etc.), because of the branched structure of PP constituents of MPW samples. Gasoline had dominant aromatic content with a vast majority of ethyl benzene (16.16-18.71%), but significant quantities of styrene were also present. When the concentration of PVC had changed from 0 to 3%, the concentration of aromatics also increased from 30.53 to 35.42%. Both RON and MON were high and increased with the PVC content. Unfortunately, the corrosion property of gasoline shows more acidity with the PVC ratio. The density of light oil had changed between 0.836 and 0.817 g/cm3, with the cetane number and diesel index
Energy & Fuels, Vol. 23, 2009 2749
in the range of 63-64 and 70-71, respectively, which are favorably high for fuel use of light oils. The CFPP was between -1 and -3 °C. The CFPP, cetane number, and diesel index or viscosity of light oils had not changed noticeably with the PVC content of raw MPWs. Heavy oils had aromatics of 11.9-15.9%, while unsaturated and saturated aliphatics presented 32.4-33.8 and 52.0-54.3%, but there was no tendency of aromatic concentration with the PVC content of raw materials. The average molecular weight of heavy oil from PVC-free raw materials was 1890 g/mol, which had decreased to 1740 g/mol when 3% PVC was added to mixtures of HDPE-PP-PS. According to analysis, no mechanical impurities had been detected in heavy oils and their caloric value was about 41 MJ/ kg, which was high enough to produce energy. All products had less sulfur content than 10 ppm, but the chlorine content was high and increased considerably with the PVC content. The highest chlorine content was found in gases (0.66-3.45%). The chlorine concentration was in the range of 46-2129 ppm in gasoline and 132-2201 ppm in light oil. Acknowledgment. The authors are grateful to the Cooperative Research Centre at the University of Pannonia and the MOL Plc. (Hungarian Oil and Gas Corporation) for financial support received for this work. Special thanks are given to Gyo¨rgy Winkler for manufacturing the reactor. EF8011245
