Thermal Degradation of LDPEâVacuum Gas Oil Mixtures for Plastic

870

Energy & Fuels 2007, 21, 870-880

Thermal Degradation of LDPE-Vacuum Gas Oil Mixtures for Plastic Wastes Valorization Antonio Marcilla,* A Ä ngela N. Garcıá, and Maria del Remedio Hernańdez Department of Chemical Engineering, UniVersity of Alicante, P.O. Box 99, Alicante, Spain ReceiVed October 23, 2006. ReVised Manuscript ReceiVed December 20, 2006

The thermal degradation of blends of different percentages of low-density polyethylene (LDPE) in vacuum gas oil (VGO) has been carried out in a fluidized-bed reactor. A wide range of percentages of the polymer has been selected for the study, including 0, 5, 25, 75, and 100% LDPE by weight. The strategy of mixing plastics with a solvent to be degraded avoids the possible heat-transfer limitations and other problems present in the cracking of solid plastics. A temperature of 500 °C has been selected as the degradation temperature in order to simulate industrial conditions of a refinery and, in this way, to evaluate the applicability of the products generated. The results show a significant increase of the volatile yields by increasing the amount of LDPE present in the blend, dry gas, and liquefied petroleum gas, increasing with the percentage of polymer mixed with VGO. Condensable products exhibit a decrease of their yield with this variable. The influence of the temperature at the exit of the reactor is also evaluated in the present work. Significant differences have been observed in the product distribution as a function of this parameter, showing the importance of the temperature profile on the results obtained in this type of reactor. The methodology shown in the present work could suggest the possibility of using a refinery stream of secondary interest (such as the VGO employed) and, simultaneously, recycling plastics, reducing the environmental problems they cause.

1. Introduction Nowadays, plastics are one of the most used materials due to their characteristics. The world production of plastics rose to a value of 169 MT in 2003, while materials such as aluminum, very much used in many applications for years, only showed a world production of 28 MT in the same year.1 This fact has caused the development of different techniques for the elimination of plastics, taking advantage of this type of material. Basically, four methods for the elimination of plastics can be distinguished: mechanical recycling, landfilling, incineration, and chemical recycling,2-5 although they present several disadvantages. In one way, mechanical recycling (or secondary recycling) can only be applied to thermoplastic materials;4 in the case of landfilling, the space is limited and most of the plastic wastes are resistant to environmental degradation;4,5 incineration (or quaternary recycling) is an interesting alternative because of the energy production but less attractive from an environmental point of view.2,4,5 Finally, chemical recycling (or tertiary recycling) implies the conversion of polymers into more valuable chemicals or fuels.2-4 In this type of recycling method, pyrolysis is included as an interesting alternative for the elimination of plastics. In Western Europe, the main components of municipal polymer waste are polyethylene (low-density and high-density, * Corresponding author e-mail: [email protected]. (1) An Analysis of Plastics Consumption and RecoVery in Europe, 20022003; Association of Plastics Manufactures in Europe (APME): Brussels, Belgium. (2) Miskolczi, N.; Bartha, L.; Antal, F.; Dudas, Cs. Talanta 2005, 66, 1264-1271. (3) Uç ar, S.; Karago¨z, S.; Karayildirim, T.; Yanik, J. Polym. Degrad. Stab. 2002, 75, 161-171. (4) De la Puente, G.; Sedran, U. Appl. Catal., B 1998, 19, 305-311. (5) Yanik, J.; Uddin, Md. A.; Sakata, Y. Energy Fuels 2001, 15, 163169.

Table 1. Composition of Vacuum Gas Oil elemental analysis % nitrogen % carbon % hydrogen % sulfur % oxygen

0.05 85.35 12.11 2.08 0.41

components (mass fraction) lineal paraffins branched paraffins olefins naphthenes aromatics sulfured compounds

0.66 0.10 0.004 0.14 0.08 0.03

LDPE and HDPE, respectively) and polypropylene, the lowerdensity polyethylene being the polymer which shows the highest consumption.1 The thermal degradation of LDPE has been widely studied.6-10 Among the volatile products obtained in the pyrolysis of this polymer, C3-C5 olefins are the most valuable products since they are the basic building blocks for the manufacturing of petrochemical products, and their demand is steadily increasing.11 Furthermore, light olefins such as ethylene and propylene are raw materials for the production of polymers and alkylbenzenes.12,13 Other valuable products present in the volatile compounds generated during LDPE pyrolysis are propane and (6) Aguado, J.; Serrano, D. P.; Escola, J. M.; Garagorri, E. Catal. Today 2002, 75, 257-262. (7) Van Grieken, R.; Serrano, D. P.; Aguado, J.; Garcıá, R.; Rojo, C. J. Anal. Appl. Pyrolysis 2001, 58-59, 127-142. (8) Williams, P. T.; Williams, E. A. J. Anal. Appl. Pyrolysis 1999, 51, 107-126. (9) Bagri, R.; Williams, P. T. J. Anal. Appl. Pyrolysis 2002, 63, 2941. (10) Williams, E. A.; Williams, P. T. J. Anal. Appl. Pyrolysis 1997, 40-41, 347-363. (11) Wallenstein, D.; Harding, R. H. Appl. Catal., A 2001, 214, 1129. (12) Bortonovsky, O.; Sazama, P.; Wichterlova, B. Appl. Catal., A 2005, 287, 203-213. (13) Den Hollander, M. A.; Wissink, M.; Makke, M.; Moulijn, J. A. Appl. Catal., A 2002, 223, 85-102.

10.1021/ef0605293 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007

Degradation of LDPE-Vacuum Gas Oil Mixtures

Energy & Fuels, Vol. 21, No. 2, 2007 871

Figure 1. Carbon number distribution of n-paraffins in the vacuum gas oil.

butane, which are very useful products that constitute a nonrenewable source of energy. On the other hand, the condensable products generated contain valuable gasoline-range hydrocarbons.14 All these compounds have interesting industrial applications. These valuable compounds can be compared with those obtained in the thermal cracking of petroleum.15 In this process, low-value heavy oils are converted into more valuable products like gasoline, light cycle oil, and lighter products.15-18 When this is taken into account, a more interesting alternative may be to introduce polymer wastes into the usual feedstock of the thermal cracking of petroleum units. The underlying aim of this alternative is to reduce the high viscosity of the molten plastics favoring the treatment of this type of materials. Under standard process conditions, a large number of plastics, including polyolefins such as LDPE, can be mixed with vacuum gas oils (VGO) for thermal cracking, because both materials are constituted by molecules formed by carbon and hydrogen.19,20 In this way, it is possible to merge a recycling plastic technique (chemical recycling) with the standard petrochemical or petroleum refining industry operation. The objective of the present work is to evaluate the product distribution obtained when LDPE-VGO blends are thermally degraded. The study has been performed in a wide interval of percentages, using a fluidized-bed reactor. In order to simulate the conditions used in the petroleum refinery industry where the thermal cracking is carried out between 420 and 550 °C, the degradation temperature selected in this work has been 500 °C. Possible applications of the products obtained have been evaluated. 2. Materials and Methods 2.1. Materials. Polyethylene employed in this work is a lowdensity polyethylene (LDPE) obtained from Dow with a density of 918 kg/m3. The vacuum gas oil used was supplied by Repsol YPF and corresponds to the vacuum distillation of the residue coming from the atmospheric distillation of the petroleum crude. Table 1 shows the composition of the VGO used. The carbon number distribution of the n-paraffin fraction is shown in Figure 1. (14) Buekens, A. G.; Huang, H. Resour. ConserV. Recycl. 1998, 23, 163-181. (15) EÄ rij, V.; Ra´sina M.; Rudin, M. Quı´mica y Tecnologıá del Petro´ leo y del Gas; Mir: Moscow, 1988. (16) Arandes, J. M.; Erenã, J.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. J. Anal. Appl. Pyrolysis 2003, 70, 747-760. (17) Ng, S.; Yang, H.; Wang, J.; Zhu, Y.; Fairbridge, C.; Yui, S. Energy Fuels 2001, 15, 783-785. (18) Atias, J. A.; De Lasa, H. Chem. Eng. Sci. 2004, 59, 5663-5669. (19) De la Puente, G.; Klocker, C.; Sedran, U. Appl. Catal., B 2002, 36, 279-285. (20) De la Puente, G.; Arandes, J. M.; Sedran, U. Ind. Eng. Chem. Res. 1997, 36, 4530-4534.

The inert fluidized bed is sand, 70-210 µm particle size, supplied by Resacril s.l., with the following composition: 98-99% SiO2, 0.19% CaO, 0.016% MgO, 0.008% Na2O, 0.25% Al2O3, 0.05% Fe2O3, 0.30% K2O, and 0.05% TiO2. In all of the experiments, the static bed depth is maintained at around 14.8 cm (approximately 460 g of sand). 2.2. Methods. 2.2.1. Blend Preparation. LDPE-VGO mixtures used in this work show relative proportions of LDPE of 0, 5, 25, 75, and 100 (%, w/w). For their preparation, LDPE and VGO are weighted and heated at a controlled temperature (90-100 °C) in separated containers. When the LDPE is melted, both components are mixed (total weight equals 2 g) and the blend is stirred. The blend is heated at 80-85 °C over approximately 3 h in order to obtain an intimate mixture between the polymer and VGO. The blend is then withdrawn from the heating, and around 10 g of sand is added to the LDPE/VGO blend and mixed thoroughly. The sand allows the mixture to be dropped inside the reactor from the feed hopper in spite of its high viscosity. 2.2.2. Equipment and Experimental Procedure. The equipment used for the flash pyrolysis of LDPE/VGO blends is a sand fluidized-bed reactor. A diagram of the reactor employed is shown in Figure 2. As can be seen, the reactor, glass traps, gasometer, and sampling bag are connected in-line. The body of the reactor is a 71-cm-high cylinder with 5.8 cm of internal diameter. At 46 cm from the bottom of the reactor, a lateral exit for the volatile compounds is located. A porous plate at the bottom of the reactor supports the bed and uniforms the fluidizing gas at the entrance. The reactor is heated by a cylindrical refractory oven. The process temperature selected (T1) is 500 °C. The exit of the reactor is heated at 350 or 400 °C (T2), depending on the run. The inert bed is sand, and the fluidization agent is nitrogen. The flow used inside the reactor is 3700 mL/min, measured at the process temperature, which is assumed to be 2.9 times the minimum fluidization velocity of the sand at 500 °C. The reactor is programmed to the set temperature (T1), and the exit reactor heating system is switched on at the decided temperature (T2). A valve allows the flow direction to change (to the gassampling bag or to the exit). During the heating time, a nitrogen flow circulates through the system to the exit, in order to purge it. Prior to the experiment, the sample (2 g of LDPE-VGO blend mixed with 10 g of sand) is placed into the feed hopper, which is purged with nitrogen to guarantee an inert atmosphere inside the reactor during the pyrolysis. When the reactor reaches the selected temperature, the nitrogen flow is adjusted to the appropriate value; the sand bed, being fluidized previously to the sample, is dropped onto it. The experiment begins by changing the valve, to allow the nitrogen flow to enter into the sampling bag, and connecting the chronometer. The sample (i.e., the sand impregnated with the VGO/polymer mixture) is dropped onto the hot sand fluidized bed. The agglomerate LDPE-VGO-sand has a high minimum fluidization velocity. However, the fluidization of the bed is enough to guarantee the shaking and sinking of the sample. The heat transfer in the degradation process can take place by two mechanisms, convection (fluidization-gas-sample contact) and conduction (sand-bedsample contact and sand-bed-sand of the agglomerate-sample

872 Energy & Fuels, Vol. 21, No. 2, 2007

Marcilla et al.

Figure 2. Experimental system: (A) manometers, (B) oven, (C) reactor, (D) feed hopper, (E) top reactor heating system, (F) ice-salt bath, (G) gasometer, (H) stainless steel Dixon rings, (I) glass traps, (J) gas-sampling bag of 25 liters.

contact). Since the LDPE-VGO mixtures are deposited on the sand and the vigorous movement of the bed allows a good contact between the bed and sample, a conduction heat-transfer mechanism is favored versus convection. When the sample reaches the degradation temperature, the pyrolysis process begins. Volatiles generated can undergo secondary reactions until they leave the reactor. Condensable products generated are trapped in the glass traps while gases are collected in a sampling bag. Two sampling bags are collected in order to ensure that all volatile compounds generated are analyzed. The experiment is finished when no more gas or condensable products are generated after approximately 40 min. 2.2.3. Analysis of Products. The gas fraction is collected in Tedlar bags. The volatile compounds are identified and quantified using standard gases by an Agilent 6890N gas chromatograph with a flame ionization detector and a GS-Alumina column (30 m × 0.53 mm i.d.). The column program includes several ramps of heating: Tinitial ) 35 °C, Timeinitial ) 2.5 min; heating rate 1 ) 12 °C/min, Tfinal ) 75 °C; heating rate 2 ) 5 °C/min, Tfinal ) 80 °C; heating rate 3 ) 12 °C/min, Tfinal ) 92 °C; heating rate 4 ) 5 °C/min, Tfinal ) 170 °C, Timefinal ) 15 min; Timetotal ) 38.43 min; injector temperature ) 150 °C, detector temperature ) 210 °C, carrier gas ) helium, split ratio ) 5:1, total flow column ) 6.8 mL/min. The stainless steel Dixon rings of the glass traps are washed with hexane to collect the C10-C30 condensable fraction. Heavier hydrocarbons (> C30) are not dissolved in hexane. Both fractions are separated by filtering. Liquid compounds (C10-C30) are identified and quantified by an Agilent GC-MS (GC 6890N-MD 5973N) with a HP-5MS column (30 m × 0.25 mm i.d.). The column program is as follows: Tinitial ) 40 °C, timeinitial ) 5 min; heating rate ) 12 °C/min, Tfinal ) 320 °C, Timefinal ) 25 min; Timetotal ) 53.33 min; injector temperature ) 250 °C, carrier gas ) He 1 mL/min, average velocity ) 38 cm/s, solvent delay ) 6 min. Library Wiley 275 is used for the identification of the saturation degree of the liquid compounds (paraffins, olefins, or aromatics). In the degradation of LDPE, the presence of typical triplets (nparaffins, 1-alkenes, and diolefins) with the same carbon number are clearly observed and easily identified; meanwhile, in the VGO degradation, this distribution does not appear. The LDPE/VGO blends show the presence of triplets when the percentage of polymer is high, but their resolution is worse than in the pure polymer degradation. Because of this fact, the chromatogram corresponding to the cases of the pure polymer experiment is used, in addition to the standards injected (1-decene, 1-hexedecene, n-hexadecane, 1-eicosene, n-eicosane, n-docosane, n-tricosane, naphtalene, 5-tert-

Table 2. Yield of the Fractions Obtained in the Degradation of LDPE/VGO Blends (T2 ) 400 °C) gases

liquidsa

% LDPE

C1-C8 wt %

C10-C30 wt %

> C30 wt %

solid residue

% total

0 5 25 75 100

30.0 32.4 37.7 61.8 67.1

53.0 46.9 33.4 12.3 8.7

13.8 7.1 3.6 2.9 0.6

6.9 13.0 7.5 7.1 3.4

104 99 82 84 80

a Heavy hydrocarbons can remain adhered to the reactor walls without being quantified.

butyl-m-xylene, isobutylbenzene, and 1,3-diisopropenylbenzene), for the identification of the carbon number of the hydrocarbons detected in each case. Paraffins and olefins different from those present in the LDPE thermal pyrolysis (branched paraffins and nonterminal olefins) are quantified by groups. Thus, paraffins or olefins located between a retention time at which paraffins Cn and Cn+1 are obtained in the thermal pyrolysis of LDPE are quantified together and assigned to a Cn carbon number. The solid residue is calculated by the difference of weight by burning the reactor bed at 800 °C for 2 h. This residue contains the blend which has not been degraded as well as the coke formed.

3. Results and Discussion 3.1. Fraction Yields. The experiments carried out show that the liquefaction of the plastic by mixing it with a hot solvent prior to its thermal degradation is possible, and it can be considered as an alternative to recycle the mixture, converting it into lighter combustibles. Table 2 shows the yields of the fractions obtained in the pyrolysis of the blends studied. As can be seen, the gas yield shows practically a linear increase with the percentage of LDPE in the blend. On the other hand, the amount of liquid compounds (C10C30) shows a significant decrease by increasing the percentage of LDPE present in the blend. While the pyrolysis of VGO leads to high yields of condensable compounds, the presence of polyethylene generates higher percentages of gas, reducing the liquid fraction obtained. Apparently, the yield of condensable compounds with a carbon number higher than 30 shows a gradual decrease with the incorporation of polymer into the blend. A low percentage of polymer (5%) produces the most significant reduction in the



Figure 3. Yields of the volatile compounds obtained in the degradation of LDPE/VGO blends.

Figure 4. Comparison between experimental and theoretical values of yields obtained (LDPE/VGO 5%).

yield of this fraction. A similar behavior is observed in the solid residue. However, by observing the mass balance obtained, a significant discrepancy between the experimental results and the theoretical value (100%) is detected by increasing the percentage of LDPE in the sample. This can be due to the fact that the heaviest hydrocarbons can remain adhered to the walls of the reactor, without being collected at the exit and, therefore, without being quantified. These hydrocarbons were found mainly in the upper part of the reactor, near the exit. Thus, the experimental value of the liquid fraction at a high-LDPE percentage is lower than the real one. Ng21 used a fixed bed reactor for the cracking of HDPE resin pellets dissolved in VGO at 510 °C, and the experiments were developed in a much smaller scale than those performed in this work. In that study, blends only containing 5 and 10 wt % HDPE were evaluated. In that case, vaporization of the feed took place (21) Ng, S. H. Energy Fuels 1995, 9, 216-224.

in a preheated zone, prior to the cracking in the bed. Analogously, Serrano et al.24 studied the LDPE-lubricating oil mixtures’ degradation in a screw kiln reactor. High percentages of polymer in the blend were evaluated in this case (40-70 wt %). In both cases, the increase of gas yield with the percentage of LDPE in the blend is observed. However, the yields of the fractions obtained are very different from those shown in this paper. In both cases, gas fraction yields are much lower than those obtained in the fluidized-bed reactor, while condensables reached very high percentages. Thus, for example, Ng indicates a gas yield of 1% in the thermal degradation of VGO versus 30% obtained in the present paper and a value close to 97% for (22) Speight, J. G. Handbook of Petroleum Products Analysis; John Wiley & Sons, Inc.: New York, 2002. (23) Meyers, R. A. Handbook of Petroleum Refining Processes, 3rd ed.; McGraw-Hill: New York, 2004. (24) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, E. Appl. Catal., B 2003, 44, 95-105.


Marcilla et al.

Table 3. Comparison of the Gases Obtained in the Degradation of LDPE/VGO Blends (T2 ) 400 °C)

Table 4. Yields Obtained in the Degradation of LDPE/VGO Blends (T2 ) 350 °C)

% LDPE

0

5

25

75

100

% LDPE

0

25

100

dry gas methane ethane ethene LPG propane propene i-butane n-butane trans-butene 1-butene isobutene

11 2.9 2.0 6.1 8.7 0.4 5.4 0.02 0.05 0.3 1.6 0.9

11.9 3.3 1.6 7.1 9.8 0.3 5.9 0.05 0.06 0.4 1.9 1.1

14.4 3.1 1.5 9.8 11.3 0.3 6.8 0.03 0.1 0.4 2.7 1.1

20.4 4.7 2.4 13.3 18.0 0.6 10.2 0.1 0.3 0.9 4.0 1.9

21.4 4.6 1.8 14.9 19.5 0.7 10.3 0.2 0.3 0.9 4.5 2.7

gases (C1-C8 wt %) liquids (C10-C30 wt %) liquids (>C30 wt %) dry gas methane ethane ethene LPG propane propene i-butane n-butane trans-butene 1-butene isobutene

7.9 72.9 10.9 2.6 0.7 0.5 1.5 2.2 0.1 1.3 0.0 0.0 0.01 0.5 0.2

14.8 71.8 10.0 4.2 1.0 0.7 2.5 4.1 0.3 2.1 0.0 0.2 0.2 0.9 0.4

50.6 10.7 1.2 3.7 0.8 0.6 2.3 22.7 0.7 10.7 0.2 0.5 3.3 2.9 4.4

the condensable fraction in the pyrolysis of a blend with 5% polymer versus the 54% shown in Table 2. Clearly, differences in the type of sample, operating conditions, and the characteristics of the reactor used lead to different results, making the comparison between them difficult. On one hand, the lower reactivity of a linear polyethylene (HDPE used by Ng) can lead to a lower cracking and, consequently, lower volatile generation. In the same way, the lubricating oil used by Serrano et al. was formed by heavier hydrocarbons than the VGO employed in the present work, which favors the generation of heavier fractions. On the other hand, the previous vaporization of the feed before its cracking together with a fixed-bed reactor, or the screw kiln reactor used by Serrano et al., favors the generation of condensable compounds, as compared with flash pyrolysis in a fluidized-bed reactor, where the generation of volatiles is favored. Differences in the heat-transfer effectiveness of the reactors as well as in their temperature profiles can cause the differences observed. Similar aspects must be taken into account to extrapolate the results obtained in a laboratory-scale reactor to a larger scale. In that way, the fluidized-bed reactor used in this paper tries to approach the industrial conditions of a refinery in order to check the viability of this type of process. 3.2. Composition of the Gas Products Obtained. 3.2.1. Influence of the LDPE Percentage in the Blend. Figure 3 shows the composition of the gas fraction analyzed (C1-C8) as a function of the percentage of polymer. As can be observed, in general, the compounds generated show a linear increase of their yields with the proportion of LDPE in the blend. In all of the cases studied, the main volatile products generated are ethene and propene, the first one reaching the highest yields for all of the blends analyzed. In order to check if the gas distribution obtained results from the weighted average of the blend components or if there is a special effect induced by the mixture of the polymer with the gas oil, theoretical yield values were calculated using the following expression:

[

% theoretical i ) (% i)VGO

]

(%VGO)blend + 100 (% LDPE)blend (% i)LDPE (1) 100

[

]

These results were compared with the experimental ones obtained. An example of these comparisons is shown in Figure 4, where the results obtained for the blend with 5% LDPE are represented. As can be seen, a good agreement between theoretical and experimental values is observed. This fact confirms that, as was expected, the increase of the gas yield is proportional to the amount of polymer in the blend, without

any additional matrix effect. All the blends studied in the present work show a similar behavior. In this way, the yields of volatile products generated in the pyrolysis of different blends can be predicted from the results obtained in the degradation of LDPE and VGO separately. The highest difference observed between theoretical and experimental values for all the cases evaluated is 1.2. In a petroleum refinery process, volatile compounds are usually grouped in two different fractions: dry gas and liquefied petroleum gas (LPG). The first fraction is composed by the lighter hydrocarbons such as methane, ethane, and ethene. Hydrogen is usually included in this group. The LPG fraction, extracted from crude oil and natural gas, is composed of paraffins and olefins with three and four carbon atoms.13,21-23 Table 3 shows the yields reached for these two fractions in the samples studied. Hydrogen was not analyzed in these runs. As can be observed, a clear increase of both fractions (dry gas and LPG) is produced when the percentage of LDPE present in the blend increases. According to the results obtained, dry gas represents a percentage between 32 and 40% from the total gas produced, while the percentage of the LPG is in the range 2931%. Thus, although both fractions reach very similar yields, dry gas always shows higher values. The ratio dry gas/LPG can give us an idea of the cracking extension. Thus, under the conditions used in this paper, where a high percentage of volatiles are produced, dry gas/LPG is always higher than 1; that is, light hydrocarbons reach higher yields than the heavier ones. By comparing these results with those given by Ng,21 using a fixed-bed reactor where the main fraction corresponds to the condensable compounds, the value of the dry gas/LPG ratio is in the range 0.7-0.8, indicating the higher percentage of heavier volatiles versus C1-C2 hydrocarbons. 3.2.2. Influence of the Temperature at the Exit of the Reactor. The importance of the secondary reactions on the results obtained in the thermal degradation processes is well-known. Obviously, these reactions are produced in a major extension by increasing the temperature or the residence time of the volatiles generated in the hot zone of the reactor. The temperature of the reactor used in this work is not uniform. The upper part of the reactor is not covered by the furnace, and it has its own heating system (Figure 2). The role of this heating system is to avoid the condensation of heavier volatiles before reaching the exit of the reactor. However, the important effect of the temperature selected at the exit of the reactor (T2) on the different yields obtained has been checked, even when T2 is always lower than the reactor temperature (T1). Three runs were carried out with this objective, feeding 100% VGO, 100% LDPE, and 25% LDPE in VGO, with T2 ) 350 °C,



Figure 5. Comparison of the gas composition at different T2 values: (a) pure VGO and (b) pure LDPE.

comparing the results with the previous study where T2 was equal to 400 °C. In all of the cases, the reactor temperature was 500 °C. Obviously, the value of T2 has an influence on the temperature profile in the upper part of the reactor, and this fact influences the cracking suffered by the volatiles. Table 4 shows the gas and condensable volatile yields obtained in the new runs (T2 ) 350 °C). Similarly to the results obtained in the previous experiments, with T2 equal to 400 °C, when the percentage of LDPE in the blend increases, a higher difference between the experimental mass balance results and theoretical ones is detected. This fact, commented on previously, can be due to the nonquantification of the heaviest hydrocarbons obtained that can remain adhered to the walls of the reactor. In the case of using a low temperature at the exit of the reactor, this fact is more evident since the generation of condensable compounds is favored. Thus, the experimental value of the liquid fraction in the pure LDPE degradation is lower than the real one. As can be observed by comparing the results shown in Tables 2 and 4, an increase of the top reactor temperature from 350 to 400 °C leads to a very significant increment in the gas yield obtained. This increase is more significant in runs that generate low percentages of volatiles in the primary cracking process,

such as the cases of nil or low percentages of LDPE in the sample. Thus, in the case of the thermal degradation of VGO, the total gas obtained at T2 ) 400 °C is 3.5 times that produced at T2 ) 350 °C. This ratio is reduced by increasing the percentage of LDPE in the sample, that is, by increasing the gas fraction generated during the cracking in the fluidized bed. It seems that, at high percentages of volatiles, the temperature profile influences the gas product distribution more than it does the global yields of the fractions produced. This aspect can be observed by comparing the values of dry gas and LPG fractions obtained. As was previously explained, at 400 °C (Table 3), the dry gas fraction always shows higher yields than the LPG fraction. However, at 350 °C (Table 4), this trend is not observed. Both fractions show very similar values at low percentages of LDPE in the blend, while for the degradation of pure LDPE, that is, at high gas production, the LPG yield is much higher than that of the dry gas. It seems that a low temperature in the upper part of the reactor prevents C3-C4 compounds from suffering secondary cracking that would increase the dry gas fraction. Thus, in this case, while ethene is the main product at 400 °C, propene reaches the highest yield at T2 ) 350 °C.


Marcilla et al.

Figure 6. Detail of the GC/MS chromatograms obtained in the analysis of the condensable fraction obtained from pure VGO, 25% LDPE and VGO, and pure LDPE.

Figure 5 shows a comparison of the gas fraction obtained at 350 and 400 °C for both extreme compositions. It can be noticed that the compounds whose yields show a higher increase with the exit temperature are the lighter hydrocarbons. It seems that in VGO pyrolysis (Figure 5a) the increase in T2 produces an increase in the yields of all the hydrocarbons obtained, proving that all come from secondary reactions. In the case of LDPE

(Figure 5b), this behavior is not as clear, and products such as propene, trans-butene, isobutane, cis-2-butene + isopentane, or 1-hexene decrease their yields rather than increase them, proving that they undergo cracking reactions or that other products are more favored by the primary reactions. By comparing Tables 2 and 4, it is observed that, as was expected, condensable compounds (C10-C30) show a decrease



Figure 7. Yields of the compounds analyzed in the condensable fraction (T2 ) 400 °C).

in their yield when the temperature T2 increases, because of the higher cracking produced. One more time, this fact is less evident at high percentages of LDPE, where the gas fraction is the major fraction produced. No research papers of polymer-oil blend pyrolysis where the temperature profile is analyzed to check its effect on the secondary gases have been found. Only the influence of the primary temperature on the yields reached in this type of blends has been previously studied. In this way, Arandes et al.16,25 car-

ried out the degradation of light cycle oil (LCO) and blends of LCO with 5% PP, 25% PS, 25% PS-PBD, and 10% PE in the range 450-550 °C in a riser simulator, degrading a small amount of blend in each experiment (about 125 mg). Other researchers, such as Lovett et al.,26 worked at higher temperatures (740-880 °C). As was expected, in all cases, light hydrocarbons increase by increasing the temperature, although differences with this parameter are lower than the cases presented in this paper. Serrano et al.24 modified the screw speed rate of the reactor to

(25) Arandes, J. M.; Erenã, J.; Azkoiti, M. J.; Lo´pez-Valerio, D.; Bilbao, J. Fuel Process. Technol. 2003, 85, 125-140.

(26) Lovett, S.; Berruti, F.; Behie, L. A. Ind. Eng. Chem. Res. 1997, 36, 4436-4444.


Marcilla et al.

Figure 8. Carbon number distribution of n-paraffins (T2 ) 400 °C).

Figure 9. Yields of the compounds analyzed in the condensable fraction (T2 ) 350 °C).

Figure 10. Carbon number distribution of n-paraffins (T2 ) 350 °C).

vary the residence time of the volatiles in it. The results obtained agree with the effect of the temperature shown in this paper. This study remarks that even few deviations from the set temperature in the reactor significantly affect the yield of the

fractions obtained in each case, having important consequences in the reactor operation and efficiency. 3.3. Composition of the Condensable Products Obtained. 3.3.1. Influence of the Percentage of LDPE in the Blend.


According to the yield of condensable compounds presented in Table 2, the yield of this fraction shows a different behavior from the linear tendency, and condensable compounds are obtained in a lower proportion than that expected as a weight average of the contributions of the blend components (eq 1). This behavior is different from that commented on for the gas fraction, whose behavior agrees with the linear tendency. On the other hand, for solid residues [calculated as 100% (% gas) condensable compounds], a tendency above the linear one is observed, obtaining higher values than those predicted by eq 1. For each run carried out (blends LDPE/VGO 0, 5, 25, 75, and 100%; T2 ) 400 °C), the composition of the condensable fraction obtained has been analyzed. The groups of compounds present in this fraction are n-paraffins, branched paraffins, 1-olefins, “nonterminal” olefins, diolefins, aromatics, benzothiophenes, and naphthenes. Figure 6 shows the GC/MS chromatograms obtained in the analysis of condensable compounds (C10-C30) for three of the cases studied. As can be observed, in the VGO degradation, a nonregular peak distribution is obtained. On the contrary, the chromatogram corresponding to the LDPE shows the typical triplets obtained in the thermal degradation of polymers, composed of diolefins, 1-olefins, and n-paraffins. Obviously, the chromatograms of the LDPE/VGO blends will be a combination of both, as a function of the LDPE/VGO ratio. Figure 7 shows the yield reached by each group as a function of the percentage of LDPE in the cracked blend. As can be observed, in general, all compounds analyzed show a tendency near to the linear behavior in blends with a high content of polymer. Results obtained in the degradation of blends with a low percentage of LDPE present a significant change in the general behavior observed. The yield of benzothiophenes decreases with the percentage of polyethylene down to 0% for pure LDPE, since the presence of sulfur is due to the VGO exclusively. Diolefins show an opposite behavior, their yield increasing with the amount of polymer in the blend. No diolefins are formed in the degradation of pure VGO. n-Paraffins and 1-olefins decrease their yield with the amount of LDPE present in the blend, but the yield reached by 1-olefins is higher than that reached by n-paraffins in all of the ranges evaluated. The n-paraffins obtained comprise a range of carbon numbers between C10 and C31 (Figure 8). While in the case of pure LDPE, it is not possible to detect a maximum in the carbon distribution, the presence of VGO increases the paraffins between C20 and C27. This maximum is more remarkable at low percentages of LDPE in the blend. The relative maximum at C16 detected in all of the samples studied must be noticed. Branched paraffins, nonterminal olefins, and naphthenes are the compounds obtained at the lowest yields, in the range 0.011.2%. From the results obtained, it can be affirmed that the addition of polymers in a VGO stream for its thermal cracking makes possible the elimination of this type of wastes, reducing the environmental problem that they can generate. The presence of LDPE in the VGO stream hardly modifies the condensable products obtained. 3.3.2. Influence of the Temperature at the Exit of the Reactor. Similarly to the analysis of the volatile compounds, the influence of the temperature at the exit of the reactor on the condensable fraction has also been studied. Thus, the yields of the compounds


present in the liquid fraction as well as the carbon number distribution for n-paraffins are shown in Figures 9 and 10, when T2 equals 350 °C. By comparing Figures 7 and 9, some conclusions can be reached. All the compounds analyzed show similar trends versus the percentage of polyethylene, independently of the temperature in the upper part of the reactor, decreasing their yields by increasing the amount of LDPE used. Only the olefins (1-olefins and nonterminal olefins) show a slight maximum when T2 equals 350 °C. As was expected, practically all the compounds analyzed in the condensable fraction show a higher yield by reducing the value of T2. Only aromatic compounds follow the opposite trend. This behavior can be due to the thermal stability of aromatic compounds, being accumulated in higher extension by increasing the temperature of the cracking processes.15 A similar increase of yield of aromatic compounds with temperature has been shown in the thermal cracking of some hydrocarbons. Thus, Mastellone et al.28 degraded a recycled polyethylene in a fluidized-bed reactor in a range of temperatures between 500 and 800 °C. In all of the cases, aromatic compounds obtained increased their yields with temperature. The same behavior was observed by Mastral et al.29 and Demirbas30 in the degradation of HDPE in a range of temperatures between 640 and 850 °C and 402 and 602 °C, respectively. While 1-olefins are the main products in the liquid fraction when runs are carried out at T2 ) 400 °C (except for the case of pure VGO), n-paraffins reach the highest yields by decreasing the T2 value. The carbon number distribution of n-paraffins at 350 °C (Figure 10) is very similar to that obtained at 400 °C (Figure 8). In both cases, the carbon number distribution for n-paraffins is in the range C10-C32. Pure LDPE degradation produces a distribution without any noticeable maximum, while the presence of VGO in the blend generates a maximum in the range C20-C28. Conclusions In the present work, the thermal cracking of LDPEVGO blends, with relative proportions of LDPE of 0, 5, 25, 75, and 100 wt %, is carried out in a fluidized-bed reactor at 500 °C. From the results obtained, some conclusions can be reached: The yields of gas products obtained show an increase with the percentage of LDPE present in the blend, while liquid compounds and solid residue exhibit a decrease with this variable. The composition of the volatile fraction produced from the thermal degradation of the blends evaluated in the present work agrees with the theoretical value estimated as the weighted average of the results obtained from the thermal cracking of VGO and LDPE, confirming the results exposed and discussed in this study. Dry gas and LPG fractions show an increase of yields by increasing the LDPE percentage in the blends. Dry gas presents yields slightly higher than those reached by LPG in all of the ranges of LDPE evaluated. (27) Karago¨z, S.; Yanik, J.; Uç ar, S.; Song, C. Energy Fuels 2002, 16, 1301-1308. (28) Mastellone, M. L.; Perugiri, F.; Ponte, M.; Arena, U. Polym. Degrad. Stab. 2002, 76, 479-487. (29) Mastral, F. J.; Esperanza, E.; Berrueco, C.; Juste, M.; Ceamanos, J. J. Anal. Appl. Pyrolysis 2003, 70, 1-17. (30) Demirbas, A. J. Anal. Appl. Pyrolysis 2004, 72, 97-102.


The main products present in the liquid compounds are aromatics for the pure VGO degradation and 1-olefins when the blends are composed of polyethylene and VGO. n-Paraffins show a range of carbon numbers between C10 and C31. The temperature profile in the upper part of a fluidizedbed reactor causes a significant effect on the product distribution obtained. The grade of influence of this parameter

Marcilla et al.

on the results obtained depends on the extension of the previous cracking developed in the lower part of the fluidized bed. Acknowledgment. The authors wish to thank CICYT CTQ200402187; FEDER; GV (ACOMP06/162); and the Ministry of Education, Culture and Sport for financial support. EF0605293

Thermal Degradation of LDPEâVacuum Gas Oil Mixtures for Plastic

Recommend Documents

Thermal Degradation of LDPEâVacuum Gas Oil Mixtures for Plastic