Catalytic coprocessing of coal and petroleum residues with waste plastics to produce transportation fuels

Fuel Processing Technology 92 (2011) 1109–1120

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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c

Catalytic coprocessing of coal and petroleum residues with waste plastics to produce transportation fuels Mohammad Farhat Ali ⁎, Shakeel Ahmed, Muhammad Salman Qureshi H.E.J. Research Institute of Chemistry, International Center for Chemical and Biological Sciences, University of Karachi, Karachi 75270, Pakistan

a r t i c l e

i n f o

Article history: Received 7 June 2010 Received in revised form 4 January 2011 Accepted 10 January 2011 Available online 12 February 2011 Keywords: Coprocessing Pyrolysis Thermogravimetry Plastic/petroleum residues/coal Catalysts Polypropylene

a b s t r a c t Coprocessing reactions with waste plastics, petroleum residues and coal were performed to determine the individual and blended behavior of these materials using lower pressure and cheaper catalysts. The plastic used in this study was polypropylene. The thermodegradative behavior of polypropylene (PP) and PP/ petroleum residues/coal blends were investigated in the presence of solid hydrocracking (HC) catalysts. A comparison among various catalysts has been performed on the basis of observed temperatures. The higher temperatures of initial weight loss of PP shifted to lower values by the addition of petroleum residues and coal. The catalysts were also tested in a fixed-bed micro reactor for the pyrolysis of polypropylene, petroleum residues and coal, alone and blended together in nitrogen and hydrogen atmosphere. High yields of liquid fuels in the boiling range 100–480 °C and gases were obtained along with a small amount of heavy oils and insoluble material such as gums and coke. The results obtained on the coprocessing of polypropylene with coal and petroleum residues are very encouraging as this method appears to be quite feasible to convert plastic materials into liquefied coal products and to upgrade the petroleum residues and waste plastics. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The environmental issues due to plastic waste arising predominantly due to the throwaway culture that plastic propagates, and also the lack of an efficient waste management system, have attracted widespread attention in Pakistan. Pakistan's estimated population in 2010 is over 170 million. The total waste generated per year in Pakistan is about 31 million metric tons per year or 0.50 kg/cap/day. In big Pakistani cities such as Karachi, about 7 to 8 million metric tons per year of solid waste is generated. It is estimated that about 6–8% of solid waste are post consumer plastic waste, while only 10% of this amount is recycled. Due to lack of integrated solid waste management, most of the plastic waste is neither collected properly nor disposed of in an appropriate manner to avoid its negative impacts on environment and waste plastics have become a main source of littering, causing blockages in sewage system or on road sides [1–3]. On the other hand, plastic waste recycling can provide an opportunity to collect and convert plastic wastes into a cheap energy resource. This resource conservation goal is very important for developing countries like Pakistan where rapid industrialization and economic development is putting a lot of pressure on depleting natural resources. Chemical recovery also known as feedstock recycling or tertiary recycling is the current state of art technology to convert waste polymers into original

⁎ Corresponding author. Tel.: +92 21 4824902x172; fax: +92 21 481901. E-mail address: [email protected] (M.F. Ali). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.01.006

monomers or other valuable chemicals. These products are useful as feedstock for a variety of downstream industrial processes or as transportation fuels [4–11]. The addition of other hydrocarbon sources such as coal, the most abundant hydrocarbon resources in Pakistan would provide a steady supply of feedstock to the processing plant. However, the direct pyrolysis of coal and plastics is difficult due to incompatibility of coal which is mostly aromatics and plastics which are aliphatic in their nature. The petroleum residues, having a composition that includes both aromatic and aliphatic compounds can serve as a vehicle in co-pyrolysis process [12,13]. The coprocessing of plastics and coal or petroleum heavy oils has been reported by other researchers [10,14–16]. However, there is no previous detailed study on pyrolysis of all three materials (coal, petroleum residues and waste plastics) together. This study reports the effect of various catalysts on the pyrolysis of three different materials alone and together: polypropylene (PP), petroleum residues (VR), and coal. PP represents nowadays about 25% of the thermoplastics' market. Also, PP due to its high resistance to degradation and relatively high melting point is thought to be an ideal material for this study rather than using mixed polymer wastes. Thermogravimetry was used to investigate the effects of various catalysts on the reactivity and kinetics of PP/VR/coal pyrolysis. The pyrolysis reactions of single (PP or VR or coal with catalyst) and binary/ternary reactions (PP, VR and coal with catalyst) were also carried out in a batch micro reactor and conversion of solids to liquids and gases was determined. The product distribution under variable reaction conditions is presented. The TGA method developed in this study for the evaluation of different catalysts for coprocessing

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Table 1 Properties of petroleum residues (approx.1000 °F+). Properties

Petroleum residue (VR)

Gravity °API Sulfur (wt.%) Carbon residue (wt.%) Asphaltene (wt.%) Viscosity, cst at 98.9 °C

9.03 3.85 13.2 10.9 39.75

Table 2 Properties of coal from Thar Mines Fields, Pakistan.

2.1.4. Catalysts Fourteen catalyst samples were used. These catalysts were given numbers, C-1 to C-14, for their identification. Catalysts C-1 to C-10 used in this work were synthesized by us according to the method described by Ali and Asaoka [17]. Catalysts C-11, C-12, and C-13 are hydrcracking (HC) catalysts, largely used by petroleum refiners. Catalyst C-14 (RCD-8) is a newly marketed catalyst from UOP for fluidized catalytic cracking of petroleum residues. Catalysts C-11 to C-14 were purchased from commercial sources. The description and characteristics of all catalysts (C-1 to C-14) are given in Table 3. 2.2. Instrumentations

Coal characteristics

Lignite (neat)

Lignite (nitrated)

Moisture (AR) % Ash (AR) % Volatile matter (AR) % Fixed carbon (AR) %⁎ Sulfur (AR) % Heating value (AR), btu/lb Heating value (dry)

6.59 16.60 22.90 53.91 1.25 5870 10,550

5.80 7.80 18.75 67.65 1.05 5900 10,950

⁎ By difference, (AR) = as received.

reactions will help in the development of cheaper catalysts for the conversion of waste plastics, coal and petroleum residues into high value transportation fuels. 2. Experimental 2.1. Materials 2.1.1. Polymers The model plastic polypropylene (PP, Mn = 75,000; d = 978 kg/L) used in this study was obtained from Saudi Basic Industries Corporation (SABIC), Riyadh, Saudi Arabia, and were used as received. 2.1.2. Petroleum residues Vacuum tower residue oil samples (VR) were obtained from National Refinery Ltd., Karachi, Pakistan. The properties and characteristics of VR are shown in Table 1. 2.1.3. Coal samples The coal used was from Thar coal mines, Thar district, Sind, Pakistan. The coal sample was crushed and the ground coal sample was then sieved, dried and stored. The properties of coal are recorded in Table 2.

The following analytical tools and techniques were used during the experiments. 2.2.1. Thermogravimetric experiments Thermal and catalytic cracking experiments were carried out in a TA Instruments SDT Q600 simultaneous TGA–DTA–DSC thermogravimetric analyzer. The analyses were conducted under flowing atmosphere of nitrogen at a purge rate of 200 mL/min. The samples were studied in fine powder form. A quantity of 3.5 ± 0.3 mg was placed in an open ceramic sample pan. The sample was then equilibrated to 200 °C before being heated to 550 °C at different heating rates (3–10 °C/min). The actual heating rate was calculated from temperature measurements made during the period of polymer degradation. In this study, this analysis serves primarily as an assessment tool in the screening of various potential catalysts for polyolefin pyrolysis in a micro reactor. An assessment of the catalyst performance prior to use in a reactor reduces cost. 2.2.2. TGA sample preparation For the catalytic pyrolysis study by TGA, the samples were prepared by mixing dried proportions of PP and catalysts (dry mixing). PP was first frozen in liquid nitrogen and crushed into a very fine powder before mixing with an appropriate amount of very fine catalyst. 2.3. Coprocessing in batch micro reactor Reactions were performed using single components and coprocessed with VR and coal in a 25 cm3 stainless steel tubular micro reactor obtained from Parr. The temperature was kept at 430 °C for 60 min at a gas (N2 or H2) pressure (cold) 8.3 MPa. Gas was introduced at ambient temperature. The micro reactor was agitated by a built-in mechanical stirrer and heated with a temperature

Table 3 Properties of catalysts. Name

Description and composition

Surface area m2/g

Pore volume cc/g

Total acidity (mmol/g)

Wt. loss prior to 100 C

Wt. loss after 100 C

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-11 C-12 C-13 C-14 C-15 C-16

NiMo loaded on [TiO2 + alpha alumina + AP-1 + USY] extrudates NiMo loaded on [TiO2 + alpha alumina + USY] No AP-1 NiW loaded on [TiO2 + alpha alumina + AP-1 + USY] extrudates NiMo (20% MoO3) loaded on [TiO2 + alpha alumina + AP-1 + USY] Extr NiMo (15% MoO3) + USY CT415 + AP1 + gamma alumina (no TiO2) NiMo (15% WO3) + USY CT415 + AP1 + gamma alumina (no TiO2) 10%Mo loaded on mordenite + ZSM-5 extrudates 5%Mo loaded on mordenite zeolite NiMo loaded on titania–alumina only NiMo loaded on USY zeolite only KC-2710 (AKZO Nobel) Z-713 (Zeolyst International) HC-100 (UOP) RCD–8 (UOP) ZSM-5 Silica (SiO2)

359.0 265.5 310.5 369.5 348.9 243.0 176.5 170.0 287.0 257.4 182.0 221 231 210 151.5 550.0

0.42 0.40 0.37 0.39 0.45 0.37 0.20 0.45 0.39 0.42 0.23 0.34 0.25 0.21 0.15 1.2

1.48 1.10 1.80 1.20 1.60 1.10 1.00 1.42 1.65 1.54 0.85 1.15 1.20 1.50 0.99 0.00

3.26 5.20 6.10 3.30 3.39 5.50 4.90 6.79 7.02 6.20 2.96 1.45 3.67 2.19 1.90 1.59

7.36 8.40 7.90 6.50 9.50 10.20 7.50 11.20 10.90 9.80 8.40 4.57 9.73 6.28 3.95 1.10

M.F. Ali et al. / Fuel Processing Technology 92 (2011) 1109–1120 Table 4 Performance of catalysts (our preparations) C-1 to C-10. Catalyst used

1% Conversion T1% (°C)

−ΔT%

T99% (°C)

99% Conversion −ΔT%

Virgin PP C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10

377 298 316 328 323 291 362 374 363 346 339

0 20 16 13 14 23 4 0 4 8 10

490 419 432 433 431 425 437 446 447 437 440

0 15 11 11 11 13 11 9 9 11 10

controlled built-in heater. About 3 to 4 g of polymer or polymer blends with VR/coal was charged to the reactor; VR/coal to polymer ratio for binary reaction was 3:2. All reactions were performed catalytically using 1 wt.% catalyst powder on a total charge basis. All reactions were duplicated. The reactions were followed by the quantitative measurements of masses of products (gas, oil, asphaltenes, proasphaltenes and insolubles) as a function of catalyst, temperature and time. The solubility behavior of these measured products was used in the analysis. The fraction soluble in hexane (HXs) was assumed as oil fraction; the toluene soluble (TOLs) fraction was considered as asphaltenes, THF soluble (THFs) as proasphaltenes and insoluble fraction was the insoluble matter (IM). The boiling range distribution of the hexane soluble fractions was determined by gas chromatograph equipped with a capillary column. The gaseous products were collected in an evacuated glass gas sampler and weighed. 3. Results and discussions 3.1. Determination of catalyst(s) weight loss The thermal stabilities of all catalysts were determined by recording weight loss during TG analysis. Calcium oxalate monohydrate was used

1111

as a calibration standard. All catalysts showed weight loss around 100 °C which indicates water loss. The continual weight loss after 100 °C for both freshly prepared and commercial catalysts may be due to the water molecules trapped within the micro pores of the catalysts, eventually escaping at high temperatures. The weight loss after 100 °C was observed to be more pronounced for most of the freshly prepared catalysts (C-1 to C-10) than the commercial catalysts. This may be due to the reduced pore volume of the commercial catalysts. 3.2. TGA study of catalytic pyrolysis of polypropylene TGA has been used to evaluate the various catalysts (C-1 to C-14) for the decomposition of polypropylene (PP), in 1:1 cat to PP ratio. The following measures were used in assessing the effectiveness of various catalysts. 1. Temperature at 1% conversion (T1%). 2. Temperature at maximum rate of conversion (Tmax). 3. Temperature at 99% conversion (T99%). The results for the catalysts C-1 to C-10 are compared in Table 4. These catalysts were prepared using silica–alumina and/or zeolite supports. The catalytic evaluation results of the catalysts showed effectiveness of all catalysts in reducing the onset temperature. This effect ranged from moderate to good. From Table 4 it is evident that among the catalysts prepared by us during this study (C-1 to C-10), Ni–Mo loaded on gamma alumina/USY support and no titania (CAT-5) catalyst lowered T1% by about 23%. However, when Mo was replaced with W (CAT-6) the effectiveness of the catalyst decreased to a mere 4% only. The catalyst CAT-1, which contained equal parts of titania– alumina and USY zeolite but no gamma alumina, lowered T1% by approximately 21%. The catalyst containing only nanoporous alumina (Cataloid AP-1) and USY zeolite (C-7) did not lower T1% for the reaction. Ni–W loaded catalysts (C-3) showed moderate effect. Titania (C-9) and zeolite (C-10) both lowered T1% by only about 9%. In Ni–Mo loaded formulation when Mo contents were increased to 20% MoO3 (C-4) the catalyst lowered T1% by only 14%. Similar studies were done by other researchers using amorphous silica–alumina (ASA) alone and in combination with alumina and zeolites having NiW, NiMo, and CoMo metal pairs as hydrocracking (HC) and hydrdesulfurization

Fig. 1. TG degradation curves for virgin PP and PP containing 5% catalysts.

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Table 5 Performance of five selected catalysts.

Virgin PP C-1 C-11 C-12 C-13 C-14

T1% (°C)

−ΔT1%

T99% (°C)

−ΔT99%

Tmax

−ΔTmax

Cmax(%)

377 332 320 309 328 350

0 12 15 18 13 7

490 456 446 441 451 482

0 7 9 10 8 1

470 432 437 419 434 468

0 8 7 11 8 0

0 82 86 82 86 74

(HDS) catalysts. In the HC reactions, NiMo showed much higher activity than other metal pairs. However, in HDS reactions, NiW showed higher activity than NiMo and CoMo due to higher H2S tolerance of NiW [18–20]. T99% decreased moderately for all catalysts (C-1 to C-10) by 9–15%. The NiMo catalyst (C-1) performed better than all other catalysts in lowering the onset of degradation. Due to its relatively better performance, as shown in the above results, the catalyst C-1 along with four commercial hydrocracking catalysts (C-11 to C-14), were selected for further TGA evaluation. The effect of catalyst loading was studied using 3, 5 and 8 wt.% of each catalyst. TGA runs at 8 wt.% of all five catalysts were repeated to check for repeatability of the TGA data. It was concluded that the data is repeatable within experimental limits. Fig. 1 shows the results obtained in the runs performed with virgin PP and PP mixed with the catalyst prepared by us (C-1), the hydrocracking (HC) catalysts C-11, C-12 and C-13, and the resid fluid cracking catalyst (RFCC) C-14, all of them with about 95 wt.% of PP (5% catalyst). The temperature at the maximum rate of conversion (Tmax) and conversion maximum weight loss (Cmax) with the addition of the various catalysts and with increasing catalyst weight fractions are shown in Table 5. However, no significant correlation between Cmax and the catalyst type and amount was observed, although a distinct increase in conversion percent occurred with increasing catalyst weight fraction. Fig. 1 and Table 5 also show that all catalysts except the RFCC catalyst (C-14) enhanced the degradation of PP. Zeolyst Z-713, catalyst (C-12) was found to be most effective in reducing the Tmax for degradation of PP. The RFCC catalysts (C-14) had almost no effect. Although AKZO 2710 (C-11) is also a Ni/W catalyst, it is less effective than Z-713 in catalyzing the degradation of PP. The number of acid sites on a solid catalyst plays a key role in the catalytic degradation rate of polyolefins. This number increases with increasing aluminum incorporation into the zeolite crystal. This may explain AKZO 2710's reduced catalytic activity (Al/Si = 1.15) in comparison to the Z-713 catalyst (Al/Si = 6.5). Another influence on polyolefin

degradation using microporous materials is the catalyst pore size. Z713 is a relatively large pore zeolite and with high acidity, whereas the base-zeolite of HC catalysts is characterized by medium pores and relatively low acidity. Given that PP molecules are much larger than the pore size of other catalysts, the degradation of the primary decomposition products (large olefin molecules) occurs preferably over the surface of Z-713 catalysts, forming smaller molecules that can be permitted into the pores of the other HC catalysts for further cracking. Thus, the larger pores of Z-713 will permit further degradation of PP within its pore, unlike medium pore catalysts. The diameter of channels in a catalyst structure is known to play a significant role in the cracking reactions. The greater sized channels allow the largest fragments of polymers to enter the channel to be cracked [21]. The total acidity of the other HC catalysts was found to have lower values (0.84 mmol/g) as compared to the Z-713 (1.3 mmol/g). The metal loading on the titania–alumina in Cat-1 was found to increase the acidity of titania–alumina from 0.84 to1.1 mmol/g. However, impregnation with the metals (Ni/Mo) of silica–alumina base (Catalysts C-1) were less effective than the metals Ni/W (catalysts C-11 and C-12) and Ni/Mo (catalyst C-13). This direct influence of catalyst acidity and pore size on the catalytic degradation of PP was also observed in literature [22–27]. TGA results for virgin PP and PP mixed with 3%, 5% and 8% each of the five catalysts, C-1 and C-11 to C-14 are compared in Table 6. As it can be seen from the table, the catalytic loading effect is remarkable, and Tmax is lowered with increasing catalyst weight for all the five catalysts employed. The number of acid sites that are available for polymer cracking increases with the catalyst amount, thereby, enhancing the degradation reactions. From the results it is evident that all the HC catalysts enhanced the degradation of PP. A more quantitative view of this analysis, considering the temperatures at 1% (T1%), and 99% (T99%) conversion along with the percent decreases (−ΔT%) in the PP degradation temperatures as a result of the respective catalysts are also presented in Table 6. The results show that all hydrocracking (HC) catalysts (C-11, C-12, and C-13) were effective in lowering the onset of degradation. At 3 wt.% catalyst, HC catalysts lowered T1% by approximately 8–9%, whereas titania/alumina catalyst (C-1) lowered T1% by about 5%. Similarly, T99% decreased with increasing catalyst weight fraction for all the catalysts. Thus, the degradation temperature range is significantly reduced with the addition of the HC catalysts. Furthermore, it appears that the Z-713 catalyst is the most effective and RFCC catalyst (C-14) is the least effective. Filho et al. reported similar catalyst loading effects and that the PP degradation temperatures and activation energy diminish when the catalyst concentration is increased from 10 to 30% [21].

Table 6 Effect of catalyst loading on temperatures and conversion % of PP degradation. Materials

T1% (°C)

−ΔT1%

T99% (°C)

−ΔT99%

Tmax (°C)

−ΔTmax

Cmax(%)

Virgin PP (C-1), 3 wt.% CAT-1 (C-1), 5 wt.% CAT-1 (C-1), 8 wt.% CAT-1 (C-11), 3 wt.% AKZOKC (C-11), 5 wt.% AKZOKC (C-11), 8 wt.% AKZOKC (C-12), 3 wt.% Z 713 (C-12), 5 wt.% Z 713 (C-12), 8 wt.% Z 713 (C-13), 3 wt.% UPOHC (C-13), 5 wt.% UPOHC (C-13), 8 wt.% UPOHC (C-11), 3 wt.% RCD-8 (C-11), 5 wt.% RCD-8 (C-11), 8 wt.% RCD-8

377 358 332 305 343 320 271 354 309 268 347 328 283 370 350 334

0 5 12 19 9 15 28 6 18 30 8 13 25 2 7 11

490 470 456 436 465 446 431 450 441 421 465 451 441 485 482 479

0 4 7 11 5 9 12 8 10 14 5 8 10 1 1 2

470 449 432 418 457 437 419 424 419 409 448 434 406 470 468 460

0 4.5 8 11 3 7 11 10 11 13 5 8 14 0 0 2

55 88 82 80 93 86 87 86 82 80 93 86 85 76 74 70

M.F. Ali et al. / Fuel Processing Technology 92 (2011) 1109–1120 Table 7 Activation energy (Ea) for catalytic degradation of PP. Catalyst

Catalysta (% w/w)

Ea (kJ/mol)

Regression for line used for Ea

Nil C-1 C-11 C-12 C-13 C-14

0 5 5 5 5 5

306.8 115.1 116.1 120.8 112.5 230.8

0.993 0.995 0.989 0.987 0.972 0.992

a

Measured weight fraction.

3.3. Estimation of activation energy (Ea) for PP degradation The kinetic parameters, activation energy (Ea) and the preexponential factor (A), were estimated for the catalytic pyrolysis of PP by the various catalysts. A comparison of the activities among the catalysts was thus performed. The Freeman–Carroll differential approach [28] was used to determine kinetic parameters. The kinetic parameters were estimated for the samples containing 5 wt.% of catalysts(C-1 and C-11–C-14) and Ea and regression for line used for Ea calculations were estimated by the Arrhenius Equation. The results are shown in Table 7. The pyrolysis of virgin PP gave overall activation energy as 306.8 kJ/mol. The activation energies for catalyzed decomposition of PP showed considerable lowering (values between 112.5 and 120.8 kJ/mol) for HC catalysts. For RFCC catalyst (C-14), the activation energy was found to be relatively high (230.8 kJ/mol) but still lower than that for PP without any catalyst. 3.4. Studies on coprocessing VR and coal with polypropylene using TGA TGA and DTG curves for PP, petroleum residues (VR), and VR/PP blend are illustrated in Fig. 2. It can be seen that VR decomposed at a lower temperature than PP and decomposition process is also longer (300–515 °C). VR shows relatively higher weight loss in low temperature oxidation region (b300 °C). The bulk of VR sample is volatilized at high temperature, leaving about 5% char yield. This lower decomposition temperature and low ash contents of VR are useful because it should allow VR to act as a solvent media for waste plastics and to intimately mix during coprocessing experiments. Both

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TGA and DTG analysis results for PP/VR blends show that the higher temperatures of initial weight loss for PP and VR are significantly decreased on mixing and that there is a significant interaction between VR and PP. Fig. 3 shows TGA and DTG curves for the Thar coal (top seam, lignite) heated in nitrogen up to 600 °C. The decrease in weight between 50 °C and 150 °C is attributed to the loss of water. The rate of weight loss increased in the temperature range 300–550 °C. The maximum weight loss at 550 °C is also evident in the DTG curve. However, this TGA behavior is in sharp contrast to the PP and VR where the coal sample appears to lose weight over a wide range of temperature and also has a high char yield of some 20–30% under the same conditions. In addition it is interesting to note that the approximate temperature at which coal softens (550 °C) is much higher than the temperatures at which PP volatilizes (350 °C). This behavior can create incompatibility problem during the copyrolysis experiments. Fig. 3 also illustrates TGA/ DTG thermogram for VR, coal and PP blends, the degradation temperatures for this blend are lower (472 °C) than the degradation temperature of PP (480 °C) but higher than that of VR/PP blend (462 °C). Thus the TGA experiment results show a significant interaction effect among PP, VR and coal. It also indicates a synergism effect during copyrolysis of PP with coal and VR. 3.5. Catalytic coprocessing reactions in a micro reactor Coprocessing refers to combined processing of plastics with coal and/or petroleum residues. Reactions were performed using single and double/triple components in a 25 cm3 micro reactor obtained from Parr Instruments, USA. Initially five catalysts namely C-1, C-5 and C-11 to C-14, which were selected for earlier TGA study, were used to evaluate the reactivity and interactive effects among the reactants under nitrogen or hydrogen atmosphere. Two more catalysts, one high acid strength ZSM-5 (C-15), and a non-acidic porous material, silica gel (C-16) were also used. Only three catalysts (C-1, C-12, and C15) were used in later studies. 3.5.1. Catalytic reactions—single components in nitrogen atmosphere The product distribution of PP which reacted at 430 °C with seven different catalysts is compared with the product distribution when no

Fig. 2. Thermograms for vacuum residue + polypropylene.

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Fig. 3. Thermograms for vacuum residue + coal + polypropylene.

catalyst was used for PP degradation in Table 8. The results reflect variations in product yields due to the differing cracking effects of catalyst used. Generally, catalytic reactions yielded higher conversions to liquids as indicated by high percentage amount of HXs. TOLs and THFs (polar organics), which were low melting point solids at ambient temperatures, were also relatively high in yields. IMs, which contained mostly the insoluble organic matter along with coke and catalyst, were significantly low in yields. The gas produced in catalytic reactions was also low for all catalysts except for HZSM-5 (C-15). This can be explained in terms of the nature of the relatively more acidic Bronsted acid sites found in zeolite catalysts as compared to weaker Lewis acid sites with other catalysts. Silica–gel, which is an almost non-acidic porous material, produced HXs product with yields of 38.0%, much less than that of the presulfided NiMo/Al2O3 (C-11) and Zeolyst-713 (C-12). The catalyzed pyrolysis of petroleum residues (VR) produced more volatile products (gas + HXs) as compared to VR pyrolysis in the

Table 8 Effect of catalyst on the product distribution for single components under nitrogen. Reactants

PP

VR

Coal⁎⁎

Catalyst

None C-1 C-5 C-11 C-12 C-13 C-15 C-16 None C-1 C-12 C-15 None C-1 C-12 C-15

Conversion⁎

Product distribution % Gas

HXs

TOLs

THFs

IM

24.1 16.6 17.5 16.0 12.5 14.0 30.6 20.5 4.1 4.5 3.9 7.0 4.5 3.8 5.5 6.4

38.4 43.1 40.5 55.3 53.3 37.5 39.5 38.0 72.5 78.0 80.5 80.0 3.5 4.3 3.9 4.2

4.6 16.5 13.9 9.5 11.5 15.5 14.2 5.8 15.7 9.2 8.8 7.5 8.6 7.9 9.2 6.8

1.9 8.5 9.0 8.7 9.5 13.2 6.5 3.4 7.5 6.5 4.5 3.0 3.9 3.6 3.2 3.3

30.9 15.1 18.7 10.2 18.0 19.5 9.0 31.1 0.2 1.8 2.2 2.0 78.5 79.9 77.7 78.5

%

%

69.0 84.7 80.9 89.5 86.8 80.2 90.8 67.7 99.8 98.0 97.7 97.5 20.5 19.6 21.8 20.7

99.9 99.8 99.6 99.7 99.8 99.7 99.8 98.8 100 99.8 99.9 99.5 99.0 99.5 99.5 99.2

absence of a catalyst. Generally, the catalytic reactions of VR produced more volatile products (gas + HXs) and less hexane insoluble (TOLs + THFs + IM). Coal, however, produced relatively more hexane insoluble (asphaltenes and proasphaltenes) with about 20% total conversion in its pyrolysis behavior either alone or with the catalyst added. The coal sample used is mostly lignite B with a high mineral matter. Suelves et al. [12] in their study on synergetic effects in the copyrolysis of coal and petroleum residues discussed the influences of coal mineral matter on coal conversion process. They concluded that the demineralization of coal greatly improved the liquefaction process. Similarly, Moliner et al. [29,30] also observed an increase in volatile compounds when demineralized coals were co-pyrolyzed with a residue like petroleum residue.

3.5.2. Catalytic reactions—binary and ternary components in nitrogen Reactions containing two components (binary combinations in 1:1 ratio) of the PP, petroleum residues (VR), and or coal were done under nitrogen at 430 °C. The ternary combination (PP/VR/coal = 2:3:2 ratio) was also performed under nitrogen at 430 °C. Table 9 shows the conversions and product distribution for these reactions. The PP conversions in VR environment were higher in terms of volatiles

Recovery

Reaction conditions: 430 °C, 60 min, and 8.3 MPa N2 introduced at ambient temperature with 1% catalyst on total charge basis. ⁎ % conversion calculated as the % of total charge. ⁎⁎ 8.00 g sample.

Table 9 Effect of catalyst on the product distribution for binary/ternary component systems under nitrogen. Reactants

PP/VR

PP/coal

VR/coal

PP/VR/coal

Catalyst

C-1 C-12 C-15 C-1 C-12 C-15 C-1 C-12 C-15 C-1 C-12 C-15

Conversion⁎

Product distribution % Gas

HXs

TOLs

THFs

IM

5.0 4.3 8.7 9.0 8.3 10.5 8.5 6.9 11.2 7.7 9.0 11.2

78.5 80.7 74.6 21.3 20.5 19.8 40.7 47.5 57.5 47.9 52.2 56.6

6.0 5.7 10.3 7.6 8.2 9.5 12.5 11.8 8.7 8.7 9.3 5.3

3.4 2.9 0.8 4.5 5.7 3.9 4.4 3.9 2.3 5.4 4.1 2.3

6.9 6.1 5.0 57.0 56.5 55.9 33.5 29.2 20.0 30.0 25.2 24.2

%

%

92.9 93.6 94.4 42.4 42.7 43.7 66.1 70.6 79.7 69.7 74.6 75.4

99.8 98.7 99.4 99.4 99.2 99.6 99.6 99.8 99.7 99.7 99.8 99.6

Recovery

Reaction conditions: 430 °C, 60 min, 8.3 MPa N2 introduced at ambient temperature. ⁎ % conversion calculated as the % of total charge.

M.F. Ali et al. / Fuel Processing Technology 92 (2011) 1109–1120 Table 10 Effect of catalyst on the product distribution for selected binary/ternary component systems under hydrogen. Reactants

%

Catalyst Product distribution %

PP/VR

C-1 C-12 C-15 PP/coal C-1 C-12 C-15 VR/coal C-1 C-12 C-15 P P / V R / C-1 coal C-12 C-15

Gas

HXs

2.5 3.0 4.7 13.6 9.0 13.0 6.6 5.7 9.0 3.7 3.2 9.6

88.5 2.3 89.7 2.7 91.0 2.9 22.5 7.3 30.2 8.3 28.7 6.5 49.2 11.0 59.1 9.8 63.7 9.5 65.5 4.0 69.7 5.8 72.5 3.5

Conversion⁎⁎

5.8 3.4 1.3 53.9 50.2 49.0 27.0 18.0 11.2 20.9 14.5 8.9

93.9 96.3 98.4 45.7 49.3 50.7 72.8 81.6 88.0 78.9 83.2 90.6

Table 12 Effect of temperature on ternary system (PP/VR/coal). Catalyst

%

Product distribution % at 400 °C

Conversion⁎

%

Recovery

TOLs THFs IM 0.6 0.9 0.8 2.3 1.8 2.5 6.0 7.0 5.8 5.7 6.5 5.0

1115

99.7 99.7 99.7 99.6 99.5 99.7 99.8 99.6 99.2 99.8 99.7 99.5

Gas

HXs

TOLs

THFs

IM

5.6 6.3 7.5

14.5 16.6 12.9

1.8 2.0 2.2

1.9 2.5 1.0

76.2 72.6 76.4

23.8 27.4 23.6

Product distribution % at 430 °C C-1 3.7 65.5 C-12 3.2 69.7 C-15 9.6 72.5

4.0 5.8 3.5

5.7 6.5 5.0

20.9 14.5 8.9

78.9 83.2 90.6

Product distribution % at 500 °C C-1 6.7 69.5 C-12 5.0 70.6 C-15 11.5 67.2

3.8 4.5 7.5

4.2 3.9 5.0

15.8 16.0 8.8

84.2 84.0 91.2

C-1 C-12 C-15

Reaction conditions: 430 °C, 60 min, 8.3 MPa H2 introduced at ambient temperature. ⁎⁎ % conversion calculated as the % of total charge.

Reaction conditions: 60 min, 8.3 MPa H2 introduced at ambient temperature. ⁎ % conversion calculated as the % of total charge.

(HXs), and low for insoluble solids (IMs). For the binary combination of PP and coal, however, the conversion (calculated as the percent of total charge) was generally low for all reactions. This low conversion % for coal–pp mixture was quite unexpected as it was envisaged that PP would cause the product yields to increase. PP is a highly substituted plastic; due to the severe steric hindrances PP molecules may be encountering difficulties in diffusing into the narrow pores of the coal for an intimate contact, thus leading to incomplete reactions and a decrease in yields of total liquid products [31]. The combination of VR and coal facilitated coal conversion to some liquid products. However, the overall product yield for VR was decreased on combining coal with VR. The ternary combination of PP, VR and coal gave relatively higher yield % and conversion %. The addition of VR to PP/coal blend facilitated the conversion of coal to liquid products. As expected, the addition of catalyst to all the above reactions generally promoted the decomposition speed, increased the conversion %, and modified the products to produce more volatiles (gas + liquids) and fewer solids. The catalyzed pyrolysis using zeolite catalysts, C-12 and C-15, resulted in much more volatile hydrocarbons compared with the non-zeolite catalyst C-1.

donate hydrogen. Therefore it was decided to use molecular hydrogen (H2 gas) in the presence of a catalyst as an alternative source of hydrogen, which is required for coal/VR/plastic dissolution and conversion to liquid products. Reactions containing PP, VR, and coal were performed under H2 pressure at 430 °C and product distribution obtained are shown in Table 10. High conversions were obtained for the binary combinations of VR plus PP and VR plus coal. However, the coprocessing of PP plus coal yielded a conversion of 45–50% only. This lower conversion again is indicative of the difference between the chemistry and compatibility of plastics and coal. Also, as pointed out earlier, poor diffusion of the bulky PP molecules into the narrow pores of the coal has significantly reduced its beneficial role [31]. The product distribution for the ternary system (PP/VR/coal) showed some good conversions, which ranged from 78.9 to 90.6%. VR appears to act as a good solvating medium for PP/coal binary combination. The addition of hydrogen promoted the reaction as shown by increased conversion %, and production of more volatiles (gas+ liquids) and fewer solids. Again, the catalyzed hydro-pyrolysis using zeolite catalysts, C-12 and C-15, resulted in much more volatile hydrocarbons compared with the non-zeolite catalyst C-1. Our results are in agreement with previous publications [32,33].

3.5.3. Catalytic reactions—binary and ternary components in hydrogen The above reported reactions under nitrogen were done on the concept that the hydrogen present in many plastics can be used to hydrogenate coal. The low conversion % of reactions under nitrogen for the binary and ternary systems indicated that PP is not providing sufficient hydrogen required for higher yields of HXs, TOLs, and THFs. Unfortunately heavy petroleum residue also have little or no ability to

3.5.4. Effect of catalyst loading on ternary reactions The effect of catalyst loading on the conversion and product distribution of the ternary reaction system of PP, VR and coal (2:3:2

Table 13 Effect of H2 pressure on ternary system (PP/VR/coal).

C-1, 3% C-1, 5% C-1, 8% C-12, 3% C-12, 5% C-12, 8% C-15, 3% C-15, 5% C-15, 8%

%

Product distribution % Gas

HXs

TOLs

THFs

IM

3.7 3.0 2.5 3.2 2.8 2.9 9.6 8.7 7.5

65.5 70.6 72.5 69.7 73.5 75.8 72.5 73.9 74.5

4.0 8.5 7.5 5.8 6.9 8.0 3.5 4.0 5.5

5.7 4.2 3.9 6.5 4.5 3.9 5.0 4.9 6.5

21.1 13.7 13.6 14.5 12.3 11.4 8.9 7.5 7.0

Conversion⁎

78.9 86.3 86.4 83.2 87.7 88.6 90.6 92.5 93.0

Reaction conditions: 430 °C, 60 min, 8.3 MPa H2 introduced at ambient temperature. ⁎ % conversion calculated as the % of total charge.

Conversion⁎

Product distribution at 8.3 MPa H2 pressure Gas

HXs

TOLs

THFs

IM

C-1 C-12 C-15

3.7 3.2 9.6

65.5 69.7 72.5

4.0 5.8 3.5

5.7 6.5 5.0

20.9 14.5 8.9

78.9 83.2 90.6

Product distribution at 6.9 MPa H2 pressure C-1 4.9 55.5 3.0 C-12 6.5 57.7 4.5 C-15 9.8 62.5 3.2

4.6 5.5 5.5

32.0 25.8 19.0

68.0 74.2 81.0

Product distribution at 3.45 MPa H2 pressure C-1 6.7 49.5 3.8 C-12 5.8 53.6 4.9 C-15 11.5 57.2 7.0

4.8 3.7 5.0

35.2 32.0 19.3

64.8 68.0 80.7

Table 11 Effect of catalyst loading on ternary system (PP/VR/coal) under hydrogen. Catalyst % wt.

%

Catalyst

Reaction conditions: 60 min, 8.3 MPa H2 introduced at ambient temperature. ⁎ % conversion calculated as the % of total charge.

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ratio) was studied. Table 11 compares the product distribution and conversion % for catalyst loading of 3, 5 and 8 wt.%, in reactions at 430 °C, 60 min, and 8.3 MPa hydrogen pressure. The conversion % showed a significant increase when catalyst loading was increased from 3% to 5% for all three catalysts. However, the conversion% increase was small when the catalyst loading was increased to 8%. It is concluded from these experiments that 5% catalyst loading is ideal for such reactions. Similar trends were observed in the behavior of zeolites and nonzeolite catalysts for catalytic pyrolysis reactions that zeolites showed better selectivity than non-zeolites in promoting high yields of volatile hydrocarbons [34].

3.5.5. Effect of temperature on ternary reactions The effects of reaction temperature on the conversion % and product distribution were studied for the ternary combinations of PP, VR, and coal (2:3:2 ratio) under 8.3 MPa (cold) hydrogen pressure. The reactions were done for 60 min time at three different temperatures (400, 430 and 500 °C), using 3% catalysts (C-1, C-12, and C-15). The results are compared in Table 12. The conversions % for the reactions ranged from 78.9 to 90.6 at 430 °C, 60 min; and it dropped sharply to 23.6–27.4 at 400 °C, 60 min. At the higher reaction temperature (500 °C) the conversion % was high. The amounts of gaseous products were also high and there was an indication of more coke formation. The influence of operating conditions particularly

Fig. 4. GC profiles of liquid fractions (4a and 4b) compared to a gasoline sample 4c.

M.F. Ali et al. / Fuel Processing Technology 92 (2011) 1109–1120

temperature plays a vital role in the pyrolysis of polymers and petroleum residues. Pyrolysis reactions are accompanied by the formation of free radicals by the scissions or cleavage of weak bonds. The generated radicals are stabilized under controlled temperature conditions. However, the formation of these radicals increases at high temperature due to higher thermal shock, allowing reunion of the radicals leading to coke formation. Moreover, the formation of coke (char) depends on the mobility of thermally generated free radicals. At low temperatures the mobility of free

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radicals is relatively slow leading to higher IM (char + unreacted material). 3.5.6. Effect of H2 pressure on ternary reactions The effect of varying the hydrogen pressure in the coprocesing reactions were studied on PP/VR/coal (2:3:2 ratio) system. The reactions were performed at 430 °C for 60 min using 3% by weight of catalysts. The hydrogen pressures used in the reactions were 3.45, 6.9, and 8.3 MPa. The effect of hydrogen pressure on the product

Fig. 5. GC profiles of liquid fractions (5a and 5b) compared to a diesel sample 5c.

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distribution for these reactions is presented in Table 13. With a decrease in hydrogen gas pressure both HXs and conversion percent have shown a decrease with an increase in gas formation. At 1200 psi (8.3 MPa) HXs and conversion (82.4%) were high and THF solubles were also low at 8.3 MPa. The lowering of hydrogen pressure was found to affect the boiling range of HXs (more N 500 °C material at low H2 pressure) and high IM yields. Hydrogen pressure, 8.3 MPa (cold), proved to be more effective for maximum conversion of reactants into liquid hydrocarbons. 3.6. Analysis of liquid fractions by gas chromatography The liquid fractions of the different reaction systems were extracted with different solvents and analyzed using gas chromatographic (GC) analysis method to determine the carbon number distribution (boiling range) of the liquids. The GC profiles of liquid samples were compared with those of various petroleum distillates (naphtha, gasoline, kerosene, and diesel fuels). n-Hexane was used as a solvent and the volume of sample injected was 1 μL. Fig. 4 compares the chromatograms of HXs fractions (Fig. 4a and b) with the chromatogram of a gasoline sample (Fig. 4c) from local refinery. Similarly, in Fig. 5 the chromatograms of HXs fractions from coprocessing reactions (Fig. 5a and b) are compared with that of a high speed diesel sample (Fig. 5c). The GC profiles were divided into three time zones namely, start (st) to 10 min, 10 to 20 min, and 20 to 30 min. The samples of naphtha, gasoline, kerosene, and diesel fuels from local refinery were used as reference standards. The carbon number distribution for these refinery products was from C5 to C20 mostly, as follows: Naphtha : C5 –C7 ; gasoline : C7 –C11 ; Kerosene : C11 –C15 ; Dieselfuel : C15 –C20 The results in Table 14 show that the liquid products (HXs) derived from PP alone and from cocracking with VR and coal contained mostly gasoline–diesel fractions. The liquids derived from polypropylene cracking did not show much variation with the type of catalyst used. The gasoline fractions in the liquid products were highest for the reaction when catalyst C-12 was used and somewhat lower for catalyst C-5. The gasoline yield for VR/PP mixture was also very high with very low amount of 20–30 min fraction. The liquid derived from coal/VR/PP mixture contained a wide spectrum of hydrocarbon types. The 20–30 min fraction for this reaction was also high (24.8%). 3.7. Analysis of the solids produced (IM) The insoluble solids produced in each of the reaction system after the reaction was completed are reported as insoluble matter (IM). The composition and characteristics of these solids were

Table 14 Product distribution of the liquid fraction obtained from catalytic cracking reaction. S. no.

Sample DESCRIPTION

1 2 3 4 5 6 7 8 9 10

Naphtha Gasoline Kerosene Diesel PP + C-1 PP + C-5 PP + C-15 PP + C-12 PP + VR + C-12 PP + VR + coal + C-12

Area % for retention time (minutes) St–10

10–20

20–30

96.3 70.5 1.2 0.3 54.4 43.7 49.7 50.5 52.8 29.0

3.3 28.4 96.8 70.7 35.8 44.7 37.2 39.5 44.2 46.2

0.3 1.1 2.0 28.9 9.8 11.6 13.3 9.9 3.0 24.8

investigated. Initially it was determined whether these solids were essentially the same as the starting plastic materials or whether the solid materials changed during the reactions. One of the major problems with the analysis of IM was contamination of the plastic residue with coke, asphalted materials and the catalyst. There was no decalin soluble in the IM, indicating the absence of any plastic material. The infrared spectra of all samples were recorded on Shimadzu 8201PC using KBr technique in the wavelength range of 400 and 4000 cm−1. The infrared spectra of insoluble materials (IM) obtained from VR + Coal + PP and catalyst have shown a great degree of conversion of reactants as indicated by the absence of characteristic absorption bands (methyl–CH3 and methene–CH2) in the spectra. Fig. 6 illustrates, respectively, the infrared spectra of model polypropylene polymer, catalyst C-1, and it's IM from catalytic pyrolysis. Four sharp peaks dominate the spectrum of PP (Fig. 6a): The methylene stretches at 2920 and 2850 cm−1 and the methylene deformations at 1462 and 1376 cm−1. The methine peaks are weak and of no analytical value. The IR spectrum of catalyst, C-1 (Fig. 6b) shows a sharp peak with high intensity at 1065.7 cm−1. This strong vibration is assigned to the Si–Al–O asymmetric stretching vibration. The sharp but less intense band at 457.5 cm−1 can be assigned to the Si–Al–O bending mode. The broad band observed at 3421.7 cm−1 is characteristic of OH hydrogen bonded to the oxygen ions of the framework. In addition, an intense band at 1631.9 cm−1, which is characteristic of the bending mode in the water molecule, is also observed. The IR spectrum of IM (Fig. 6c) shows all the characteristic vibration bands of Si–Al–O. Also, this spectrum does not show any characteristic bands for methyl and methane or methine, suggesting the absence of polymer groups in the insoluble matter. This further supports that the IM is mostly recovered catalyst from the reaction. 4. Conclusions Thermogravimetric analysis (TGA) study to evaluate the catalysts prepared during this study and the commercial hydrocracking catalysts showed that all catalysts except the RFCC catalysts (C-14) enhanced the degradation of PP alone and mixed with coal and petroleum residues. The temperatures of onset, Ton, of maximum rate, Tmax and of end, Tend of the pyrolysis reactions shifted to lower values as the acidity of the catalysts increased. Zeolyst Z-713 catalyst (C-12) was found to be most effective in reducing the Tmax for degradation of PP. The RFCC catalysts (C-14) had almost no effect. The titania based catalysts prepared by us (C-1 to C-10) showed moderate to good effect. Thermogravimetric analyses of plastics with coal and petroleum residues showed the reaction chemistry of petroleum residues and model plastics were compatible and yielded higher conversions at relatively low temperatures. The co-pyrolytic behavior for the PP/VR/coal blends was proven to be more complicated. Since the temperature range for the decomposition was broader for coal than PP or VR, this region appeared to be more complex for blends. However, the degradation temperatures for this blend were lower (472 °C) than the degradation endothermic peak of PP (480 °C) but higher than that of VR/PP blend (462 °C). The TGA experiment results showed a significant synergistic effect during copyrolysis of PP with coal and petroleum residue at higher temperatures. The thermal and catalytic pyrolysis of polypropylene, petroleum residue and coal, alone and blended together, has been undertaken in a batch micro reactor using six different catalysts. This study provided some encouraging results. High yields of liquid fuels in the boiling range 100–480 °C and gases were obtained along with a small amount of heavy oils and insoluble material such as gums and coke. Reaction temperature, time of reaction, gas pressure and the amount and type of catalyst strongly affected the conversion and the production of liquid fraction (hexane soluble material). The

M.F. Ali et al. / Fuel Processing Technology 92 (2011) 1109–1120

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Fig. 6. The IR spectra of insoluble matter (IM).

conversion % in the binary system depended upon the compatibility (chemistry and composition) of the reacting materials. The effect of temperature and gas pressure on the yield of volatile products was found to be quite significant. The final products from the initial conversion can be further upgraded to fuels or feedstocks for petrochemicals.

It is concluded that the copyrolysis of polypropylene with coal and petroleum residue is a feasible process to upgrade the low value petroleum residue, coal and waste plastics into high value liquid fuels. The results suggest that a new industry can be developed to convert waste plastics into high quality oil and valuable by-products.

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