Article pubs.acs.org/EF
Cocracking Kinetics of PE/PP and PE/Hydrocarbon Mixtures (I) PE/PP Mixtures Xiaodong Jing,†,‡ Guoxun Yan,†,§ Yuehong Zhao,*,† Hao Wen,† and Zhihong Xu† †
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ University of the Chinese Academy of Sciences, Beijing 100049, China § Hi-Tech Institute for Petroleum and Chemical Industry, Qingdao University of Science and Technology, Qingdao 266042, Shandong China ABSTRACT: Cocracking behaviors of polyethylene/polypropylene and polyethylene/hydrocarbon mixtures have been investigated using a closed batch reactor/tube reactor, followed by Thermogravimetric Analysis (TGA) at certain heating rate from ambient temperature to 873 K. In part I, the cocracking of HDPE, LDPE, and PP was divided into two stages for intensive study: polyolefin cracking at closed batch/tube reactor was performed using nonisothermal/isothermal method, and further, the thermogravimetric study of polyolefin and the cracked products from polyolefin cracking was realized using nonisothermal method. Presence of synergistic effect is observed for cocracking of polyolefin mixtures, and such synergistic effect not only exists in mild cracking using closed batch reactor/tube reactor but also exists in the thermal degradation process of thermogravimetric analysis. The kinetic studies were performed according to Coats and Redfern method for first-order reaction. It is found that the pyrolysis process can be described by one first-order reaction and PE/PP mixture can decompose at a lower temperature than PE, where the activation energy of PE/PP can significantly decreased compared with that of HDPE. The results also indicate that interactions between HDPE and PP are mainly related to experimental conditions: temperature, residence time, and the degree of mixing for PE/PP. Thus, closed batch reactor with agitation equipment is useful for thermal cracking of polyolefin to produce more light oils.
1. INTRODUCTION High and low density polyethylene (HDPE and LDPE) and polypropylene (PP) belong to polyolefin and generally make up a majority of the plastic waste.1,2 Pyrolysis or cracking of polyolefin, as well as their mixtures, in absence of air or oxygen has been widely studied in recent years,3−6 since thermal degradation is the simplest form of tertiary recycling of addition polymers. The degradation of polyolefin molecules takes place through a complex free-radical mechanism7 and yields hydrocarbon waxes and oils, aromatics and gaseous olefins (ethylene, propylene, etc.) at different conditions.8,9 The cocracking of polyolefin mixtures or polyolefin/hydrocarbon mixtures10,11 have also been discussed. Our previous12 studies show that the effect of PP in cocracking of HDPE/PP or HDPE/ LDPE/PP mixtures may significantly decrease the cracking temperature and the viscosity of liquid product. Results from Chowlu et al.13 suggest that presence of synergistic effect is anticipated for all the mixture compositions (LDPE/PP) and is prominent for mixture samples with PP composition >40 wt %. Meanwhile, Ballice,14 Albano et al.,15 Miranda et al.,16 and Waldman et al.17 have also found there were some interactions between PP and PE in the degradation of PE/PP blends. However, no obvious product-changing interaction between PE and PP was observed by Predel et al.18 and Westerhout et al.,19 and no interactions were found between the degradation of PP and PE by Costa et al.20 On the other hand, the heavier fractions of liquid product can be returned to mix with polyolefin as the feedstock of cocracking, in order to generate more light fractions in liquid product.21 It is, therefore, important for understanding the interactions between different polyolefin, and between © 2014 American Chemical Society
polyolefin and cracked fractions in cocracking process. However, these results about interactions between PE and PP are still contradictory. Furthermore, most of studies have mainly focused on the whole cocracking process. The cocracking of polyolefin mixtures or polyolefin/hydrocarbon mixtures, emphatically studying the interactions in the different stages, was seldom discussed. In this work, the cocracking kinetics of PE/PP and PE/hydrocarbon mixtures at inert atmosphere were studied using closed batch or tube reactor at lower temperature, and thermogravimetric analysis (TGA) at different heating rates from ambient temperature to 873 K. In part I, the cocracking of HDPE, LDPE, and PP was divided into two stages for intensive study: polyolefin cracking at closed batch/tube reactor was performed using nonisothermal or isothermal method, meanwhile, the thermogravimetric study of polyolefin and the cracked products from polyolefin cracking was realized using nonisothermal method. Our object is to determine the apparent activation energy and the pyrolysis reaction model, to discuss the interactions between HDPE, LDPE, and PP in different reaction phase, and further to determine the feasibility of cocracking of polyolefin mixtures.
2. EXPERIMENTAL SECTION 2.1. Experimental Materials. High density polyethylene (HDPE, Type 5200B), Low density polyethylene (LDPE, Type LD100AC) and polypropylene (PP, Type K7726) in form of 2−4 mm grains, manufactured by SINOPEC Beijing YanShan Company (Beijing, China), Received: April 14, 2014 Revised: July 7, 2014 Published: July 8, 2014 5396
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Figure 1. Schematic diagram of batch reactor (a) and horizontal quartz tube reactor (b) system. and HDPE/PP mixtures were used as polyolefin materials in this study. Their cracked products were, generated by the mild cracking system in Figure 1, also used as experimental materials for thermogravimetric study. The descriptions of the polyolefins are presented in Table 1.
Beijing Xinranda Company, is used to complete the data acquisition. The cooling water coil is arranged in the vessel for quick cooling as cracking experiment finished. About 200 g polyolefin sample were used in each cracking experiment. The closed batch reactor was purged by nitrogen to prevent the presence of air before the cracking experiment started. High purity nitrogen (>99.999%) and Mass Flow Controller (CS200) were, supplied by Beijing Sevenstar Electrnics Co. Ltd., used to completed the air purging. In cracking experiments, the polyolefin samples were heated with average heating rate of 3−5 K·min−1, and the timing of the cracking started from 473 K. The pressure of reactor was recorded, and the reactor was cooled by cooling water until the temperature inside the vessel reached room temperature, as cracking experiments finished. For all the experiments, the initial pressure is ordinary pressure (gauge pressure = 0.00 MPa). On the other hand, the tube reactor system, supplied by Heifei Ke Jing Materials Technology CO, Ltd. as shown in Figure 1b. The length of quartz tube is 1000 mm. The heating zone is about 440 mm (the thermal temperature zone is about 140 mm with a adiabatic stopper in the left). The temperature for experiments using tube reactor in this study is the position outside of the quartz tube. The vessel used to contain the feedstock is 60 × 30 × 15 mm. A 4−5 g polyolefin sample was used in each cracking experiment. The volatile products were discharged from outlet or deposited on the position A and B due to lower temperature of the tube ends. Two series of experimental conditions, as shown in Table 2, were designed in order to study the cocracking of polyolefin during the initial phase. Especially, series A was for cocracking of polyolefin mixtures with different PE/PP mass ratios, while series B was for the cocracking of HDPE, PP, and 50/50 HDPE/PP at different temperatures. The heating rate, residence time and cracking temperature were determined mainly based on our previous studies.12 2.4. Volatile Yields Calculation and Theoretical Noninteraction Yield for Series B. After the tube reactor had cooled, the reactor was opened, and vessel were carefully brought out and weighed. The yield of volatile products was calculated as shown in eq 1.
Table 1. Experimental Samples Used in This Study sample
type
d (293 K) (kg·m−3)
MFI (g·(10 min)−1)
HDPE LDPE PP
5200B LD100AC K7726
0.956 0.9225 0.905
3 2 27
2.2. Experimental Scheme. This study is designed to combine mild cracking in a closed batch reactor/tube reactor and thermogravimetric analysis (TGA): polyolefin is mildly cracked in closed batch or tube reactor, then both the cracked product (marked as P* or P#) and polyolefin are used to thermogravimetric analysis (TGA) at certain heating rate from ambient temperature to 873 K. It is noted that the P* only denotes the cracked products by mild cracking polymer in batch reactor, while P# is the cracked products by mild cracking polymer in tube reactor. The P* or P# in this study are solid products in room temperature. 2.3. Mild Cracking System and Experimental Procedure. The mild cracking of PE, PP, and PE/PP mixtures was performed in a closed batch reactor or tube reactor under nitrogen atmosphere. The batch reactor system, supplied by Beijing Xinranda Company, consists of four main parts, reactor system, gas removal and collection system, data acquisition and control system, and cooling system, as shown in Figure 1a. The reactor system consists of a 1000 mL stainless steel vessel externally heated by an electric ring furnace, and fitted with a purging gas inlet valve, two outlet valves for gas sampling, liquid sampling, or purging gas discharge, respectively. The reaction vessel is stirred using a stirrer with rotor blade attached to an inner magnetic drive rotor powered through an outer magnetic drive rotor. The reactor is equipped with a pressure gauge to measure the internal pressure while the experiments are performed. The controller is used to control the reaction temperature and stirring apparatus. The software, developed by
yield of volatile product = 1 −
mass of residue in vessel 100 mass of raw material
(1)
Table 2. Experimental Series and Experimental Conditions in This Study series A B: B1/110a B2/40a B2/ 110a B3/30a B3/40a
polyolefin sample LDPE, HDPE, HDPE/PP, PP HDPE/PP HDPE PP
reactor
experimental conditions
batch The average heating rateb is 3−5 K·min−1 and timing of the cracking starts from 473 K. When the time is 30 min after 473 K, the reactor is cooled by water to stop the cracking reactions. reactor tube B1: K/min 110 min by air reactor 293 K ⎯4⎯⎯⎯⎯⎯⎯⎯ → 623 K ⎯⎯⎯⎯⎯⎯⎯→ 623 K ⎯⎯⎯⎯→ 293 K B2: 4 K/min 40 or 110 min by air 293 K ⎯⎯⎯⎯⎯⎯⎯⎯→ 673 K ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 673 K ⎯⎯⎯⎯→ 293 K B3: 4 K/min
30 or 40 min
by air
293 K ⎯⎯⎯⎯⎯⎯⎯⎯→ 703 K ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 703 K ⎯⎯⎯⎯→ 293 K a
This symbol denotes the experimental condition: B1, B2, and B3 correspond to the cracking temperature and 30, 40, and 110 are the isothermal time (min). bFor each experiment, the heating rate was kept constant before the temperature was up to about 473 K; then, it became distinguishing due to the different thermal cracking characteristics of plastics. So, it was an average value for all heating-up time. 5397
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The theoretical noninteraction yield in mild cracking of HDPE/PP in tube reactor was calculated by the following equation:
theoretical yield =
mHDPEYHDPE + mPPYPP mHDPE + mPP
Table 4. Mild Cracking Results of Polyolefin and Polyolefin/ Cracked Product Samples at Series B feedstock
(2)
HDPE 50/50 HDPE/PP
where mHDPE is the mass percent of HDPE in HDPE/PP mixture, %; mPP is the mass percent of PP in HDPE/PP mixture, %; YHDPE is the yield of volatile product when HDPE is cracked alone, %; YPP is the yield of volatile product when PP is cracked alone, %. 2.5. Nonisothermal Thermogravimetric Measurements. Experimental samples presented in Table 1 and their cracked products were subjected to thermogravimetric analysis (TGA) in an inert atmosphere of nitrogen, where grinding in an agate mortar was performed for 30 min in preparing the sample of 50/50 LDPE*/PP*. Mettler-Toledo TGA/DSC was used to measure the mass change of sample as a function of increasing temperature over the course of cracking reaction. The TGA curves were obtained at heating rates of 5, 10, 15, and 20 K·min−1 between room temperature (293 K) and 873 K. The differential scanning calorimetry (DSC) measurements were also performed in this study. Nitrogen was used as an inert purge gas to displace air in the pyrolysis zone, thus avoiding unwanted oxidation of the sample. A flow rate of 40 mL·min−1 was fed to the system from a point below the sample and a purge time of 60 min to be sure the air was eliminated from the system and the atmosphere is inert.
gage pressure (MPa)
a
max.
at 293 K
final temp.a (K)
HDPE 70/30 HDPE/PP 50/50 HDPE/PP 30/70 HDPE/PP PP LDPE
0.07 0.82 1.00 0.63 0.62 0.58
0.02 0.37 0.31 0.11 0.11 0.07
619 647 626 650 619 630
offwhite solid offwhite/white solid white solid white solid white solid
B2/40
PP HDPE 50/50 HDPE/PP
B2/110
white solid white solid light yellow solid
PP HDPE 50/50 HDPE/PP
B3/30
yellow solid white solid light yellow solid
PP HDPE 50/50 HDPE/PP
B3/40
dark yellow solid white solid yellow solid dark yellow solid
0.23 (±0.00) 1.07 (±0.76)/ 1.80 (±0.13) 3.37 (±0.39) 1.44 (±0.42) 19.73 (±0.40)/ 21.88(±0.59) 42.32 (±0.76) 4.71 (±0.04) 48.23 (±1.03)/ 46.32 (±0.05) 87.93 (±0.05) 16.92 (±1.58) 68.23 (±1.49)/ 57.09 (±0.78) 97.26 (±0.11) 27.77 (±0.82) 78.27 (±0.45)/ 63.66 (±0.25) 99.32 (±0.08)
with the residue being white or yellow solid for PP mild cracking. For HDPE/PP mild cracking, the volatile yield is lower than the theoretical yield at B1/110 and B2/40 but higher than the theoretical yield at B2/110, B3/30, and B3/40. This result indicates that there are some interactions between HDPE and PP in liquid phase that restrain the volatile formation during cracking at B1/110 and B2/40 but improve the volatile formation at B2/110, B3/30, and B3/40. Compared with the (HDPE/PP)*, the (50/50 HDPE/PP)#/B1/110 obtained 50/50 HDPE/PP mild cracking is still off-white and white solid, as shown in Figure 2, suggesting that HDPE and PP are not miscible at 623 K/110 min without external stirring.
Table 3. Mild Cracking Results of Polyolefin Samples at Series A polyolefin sample
B1/110
PP HDPE 50/50 HDPE/PP
PP
3. RESULT AND DISCUSSION 3.1. Mild Cracking Result in Closed Batch Reactor/Tube Reactor. The mild cracking results of polyolefin samples at cracking conditions in Table 2 are presented in Tables 3 and 4.
experimental volatile yield (wt %)/ conditions residue in vessel theoretical yield (wt %)
The temperature was recorded when the time is 30 min after 473 K.
Series A gives gaseous product and solid product. As shown in Table 3, the final temperature for cracking of polyolefin samples (HDPE, LDPE, PP, and HDPE/PP mixtures) has reach to the temperature range of 613−653 K, in the case of setting 673 K as upper limit of cracking temperature. The maximum gage pressure and those at room temperature (293 K) is about 0.07− 1.00 MPa and 0.02−0.37 MPa, respectively, indicating the generation of gaseous products in cracking experiments. Meanwhile, the 70/30 and 50/50 HDPE/PP samples exhibit relatively higher pressure, suggesting that the HDPE/PP mixtures generate more volatile product, in other words, HDPE/PP mixtures give chain scissions in these experiments. It should be noted that the main reactions are in the liquid phase at this temperature range since our former study shows that the final pressure is about 4 MPa with the higher temperature and longer cracking time. In these experiments, polyolefin thermally decomposed to white solid product and it is very hard only for HDPE*. On the other hand, series B gives volatile product and solid residue, as shown in Table 4. For HDPE mild cracking, the volatile yield is relatively low, with the residue being off-white or white solid, while the volatile yield varies from 3.37% to 99.32%,
Figure 2. Residues obtained from HDPE (A), 50/50 HDPE/PP (B), and PP (C) mild cracking at B1/110.
Comparing series A (batch reactor) and series B (tube reactor), it can be seen that the volatile produced (gases such as C1−C4 are discharged when the reactor is opened) in series A is not discharged and will be collected with nonvolatiles as cracked product P*, while the volatile produced in series B is discharged in the cracking process and only nonvolatiles are collected as solid product P#. As a result, volatile product is generated with high yield especially at 673 K (series B2) or 703 K (series B3) in tube reactor, while only a very small amount of gaseous product is produced in closed batch reactor. 5398
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Figure 3. TG (left) and DTG (right) curves of HDPE at different heating rates.
Figure 4. TG (left) and DTG (right) curves of PP at different heating rates.
Figure 5. TG (left) and DTG (right) curves of HDPE* at different heating rates.
Figure 6. TG (left) and DTG (right) curves of PP* at different heating rates.
3.2. TG Analysis. 3.2.1. Polyolefin and P* TG Analysis. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves for the thermal degradation of HDPE and PP, and of their mild cracked products, HDPE* and PP*, at different
heating rates are shown in Figures 3−6. The characteristic temperatures are summarized in Table 5. In this study, TI, TP, and TF, determined from the thermogravimetric data, are temperature of initial weight loss, temperature corresponding to the 5399
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Table 5. Characteristic Temperatures of Polyolefin and Their Cracked Products Determined by TGA and DSC Measurements at Different Heating Rates TI (K)
TP (K)
TF (K)
sample
heating rate/K·min−1
TGA
DSC
TGA
DSC
TGA
DSC
HDPE
5 10 15 20 5 10 15 20 10 5 10 15 20 5 10 15 20 10
703 716 716 721 676 686 691 695 706 704 713 714 720 664 672 672 679 672
653 709 714 722 664 676 681 692 706 722 705 720 712 690 653 684 675 691
727 740 743 748 704 715 721 726 734 729 738 737 748 697 709 714 719 709
699 741 744 751 701 715 721 728 728 728 736 732 753 691 707 717 723 727
742 756 764 772 721 735 744 752 752 743 753 764 772 719 732 740 749 732
716 758 765 772 723 736 746 752 755 745 756 744 771 720 732 740 752 754
PP
LDPE HDPE*
PP*
LDPE*
Figure 7. TG (left)/DTG (right) curves of HDPE*, PP* and (HDPE/PP)* at heating rate of 10 K·min−1.
maximum rate of weight loss, and temperature at the end of weight loss, meanwhile, TI, TP, and TF, measured by the differential scanning calorimetry (DSC) measurements (for the peak corresponding to the decomposition reaction), are temperature of initial thermal decomposition, temperature corresponding to the maximum rate of thermal decomposition, and temperature at the end of thermal decomposition. The lateral shift of TG and DTG curves to higher temperature can be observed in Figures 3−6, indicating the appearance of higher characteristic temperatures (TI, TF, and TP) due to the increasing heating rate. This lateral shift of TG and DTG curves has also been observed by other workers22,23 in the pyrolysis study of plastics using TGA. The thermal degradation of polyolefin and their cracked products can be considered as a onestep process which will be accelerated at higher heating rate, because only one rising peak with the increasing heating rate appears in DTG. Table 5 shows that the thermal degradation of polyolefin starts at 675−721 K, and ends at 720−763 K, while that of their cracked products starts at 645−720 K, and ends at 720−779 K. Two peaks, corresponding to the endothermic occurrence, will generally be observed in DSC measurements. The peak at lower temperature, without weight loss occurred, corresponds to the melting of polyolefin, while the peak at higher temperature to
the decomposition reaction. It is noted that the thermal degradation of LDPE and PP commences at higher temperature than that of LDPE* and PP* does, while the thermal degradation of HDPE and HDPE* at close temperatures. A similar situation can also be observed on the onset temperature of melting of polyolefin and their corresponding cracked products. These results state that more scission reactions occur in mild cracking of LDPE and PP than that of HDPE. 3.2.2. (PE/PP)* and PE*/PP* Mixture TG Analysis. TG and DTG curves for the thermal degradation of (HDPE/PP)* and HDPE*/PP* mixture at heating rate of 10 K·min−1 are shown Figure 7. The characteristic temperatures TI, TF, and TP determined from TGA and DSC measurements are summarized in Table 6. Thermal degradation of the cracked products of polyolefin mixtures (HDPE/PP)* can be considered as a one-step process, which can be concluded by the appearance of only one peak in DTG as observed in Figure 7. The TG and DTG curves of HDPE* are lateral shifted to higher temperature compared with those of (HDPE/PP)* and PP*, while the TG and DTG curves of (HDPE/PP)* and PP* are closed each other, suggesting that PP will significantly improve the thermal cracking of HDPE when the mass fraction of PP is more than 30% in HDPE/PP mixture. The behavior of TG and DTG curves is consistent with the results of our previous study. 5400
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Table 6. Characteristic Temperatures of Cracked Products of Polyolefin Mixtures and LDPE*/PP* Determined by TGA and DSC Measurements at Different Heating Rates TI (K)
TP (K)
TF (K)
sample
heating rate (K·min−1)
TGA
DSC
TGA
DSC
TGA
DSC
(70/30 HDPE/PP)*
5 10 15 20 5 10 15 20 5 10 15 20 10
676 686 692 680 666 670 684 686 661 692 692 701 633
648 682 670 720 720 681 674 682 690 681 675 676 645
712 724 731 731 709 716 728 731 701 719 731 741 706
704 720 731 737 737 691 728 735 699 712 722 726 700
733 746 756 760 733 742 755 761 726 739 756 760 735
733 751 760 756 756 707 760 763 728 740 752 768 754
(50/50 HDPE/PP)*
(30/70 HDPE/PP)*
50/50 LDPE*/PP*
On the other hand, the TG and DTG curves of (50/50 HDPE/PP)#/B2/40 are very close to those of PP#/B2/40 and laterally shift to lower temperature compared with HDPE. Such result is possibly related to the fact that the presence of PP has improved HDPE cracking in tube reactor. In addition, the TG and DTG curves of (50/50 HDPE/PP)#/B3/40 are very close to those of HDPE#/B3/40. Meanwhile, Table 4 shows the volatile yields of PP and HDPE/PP at B3/40 are about 99.32% and 78.27%, respectively. So, it is concluded that the (50/50 HDPE/ PP)#/B3/40 mainly consists of HDPE cracked products. 3.3. Kinetic Analysis. The rate of the kinetic process can be described by eq 4 as
A theoretical TG curve is calculated in order to investigate the interaction between LDPE* and PP*, where the weight loss of LDPE*/PP* mixture is calculated as the arithmetic summation of the individual weight loss of LDPE* and PP* wsum = x1wLDPE * + x 2wPP *
(3)
where x1 and x2 are mass fraction of LDPE* and PP* in LDPE*/ PP* mixture, wLDPE* and wPP* are weight loss of LDPE* and PP*. The calculated and experimental TG curves of 50/50 LDPE*/PP* mixture at heating rate of 10 K·min−1 are illustrated in Figure 8.
dx dx =β = K (T )f (x ) dt dT
(4)
where K(T) is a temperature-dependent reaction rate, f(x) is a component-dependent kinetic model function, β = dT/dt is a constant heating rate. There is an Arrhenius type dependency between K(T) and temperature T, according to eq 5. ⎛ E ⎞ ⎟ K (T ) = A exp⎜ − ⎝ RT ⎠
(5)
where A is the pre-exponential factor, E is the apparent activation energy, and R is the gas constant. The conversion x can be calculated by w −w x= 0 w0 − wf (6)
Figure 8. TG curves of LDPE*, PP*, and 50/50 LDPE*/PP* mixture at heating rate of 10 K·min−1.
where w is the mass of a sample at time t, w0, and wf refer to the mass of at the beginning and the end of the weight loss event of interest. The kinetic parameters, the apparent activation energy E, and pre-exponential factor A, of the cracking of polyolefin and polyolefin mixtures were determined by the integral method. The cracking kinetic of polyolefin and polyolefin mixtures may be simply expressed as eq 7, as the plastic pyrolysis is reported to be a first-order reaction.
A significant interaction between LDPE* and PP* is observed in the temperature range 573−773 K and both cocomponents can improve the thermal degradation of LDPE*/PP*. Thus, it is concluded that the interactions between PE and PP exist not only in the lower temperature but also in the higher temperature. 3.2.3. P# and (HDPE/PP)# TG Analysis. TG and DTG curves for the thermal degradation of HDPE#, PP#, and (HDPE/PP)# at heating rate of 10 K·min−1 are shown Figures 9−11. The characteristic temperatures TI, TF, and TP determined from TGA measurement are summarized in Table 7. As observed from Figure 9, the TG and DTG curves of two samples for (50/50 HDPE/PP)#/B1/110 are very different each other: one group is near to HDPE, while the other is close to PP. This result shows that the thermal cracking of HDPE and PP still performs separately at B1/110.
⎛ E ⎞ dx ⎟ (1 − x) = A exp⎜ − ⎝ RT ⎠ dt
(7)
For nonisothermal conditions, when the temperature varies with a constant heating rate β, rearranging eq 7 and integrating gives ⎡ AR ⎛ ⎡ −ln(1 − x) ⎤ E 2RT ⎟⎞⎤ ⎜1 − ln⎢ ⎥− ⎥ = ln⎢ 2 ⎝ ⎠ ⎣ ⎦ ⎣ βE E ⎦ RT T 5401
(8)
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Figure 9. TG (left)/DTG (right) curves of P#/B1 at heating rate of 10 K·min−1 (110-1 and 110-2 denote the two samples for (50/50 HDPE/ PP)#/B1/110).
Figure 10. TG (left)/DTG (right) curves of P#/B2 at heating rate of 10 K·min−1.
Figure 11. TG (left)/DTG (right) curves of P#/B3 at heating rate of 10 K·min−1.
It is noted that the thermal degradation of HDPE, LDPE, and PP is reasonably described by a first-order reaction. However, for P*, (HDPE/PP)*, HDPE*/PP*, P#, and (HDPE/PP)#, this process can be described by two or three consecutive first-order reactions, respectively. However, we only apply eq 8 to the stage related to the main thermal degradation. Figures 12−14 show the plots of ln ⌊−ln(1 − x)/T2⌋ vs 1/Tfor thermal degradation of polyolefin and their cracked products. Table 8 shows the kinetic parameters of all samples that were determined by this method. The good correlation coefficient indicates that the corresponding independent first-order reaction model fits the experimental data very well. As observed from Table 8, the activation energy of polyolefin can be sequenced as HDPE > LDPE > PP, suggesting that HDPE degrades at higher temperature than LDPE and PP do, which is in line with the results from others.24,25 The cracked products such as HDPE*, LDPE*, PP*, HDPE#, and PP#, obtained from
Table 7. Characteristic Temperatures of P# Determined by TGA Measurements at Heating Rate of 10 K·min−1 sample HDPE#/B1/110 PP# /B1/110 HDPE#/B2/40 (50/50 HDPE/PP)#/B2/40 PP#/B2/40 HDPE#/B3/30 (50/50 HDPE/PP)#/B3/30 HDPE#/B3/40 (50/50 HDPE/PP)#/B3/40
TI/TP/TF (K) 736/759/774 706/735/755 723/755/774 712/739/766 684/725/751 702/734/751 686/740/764 721/750/771 717/745/768
Plotting of ln ⌊−ln(1 − x)/T2⌋ vs 1/T may present a straight line when the cracking process is assumed as a first-order reaction, because 2RT/E ≪1 exists for most of E and the temperature range of cracking and ln ⌊AR(1−2RT/E)/βE⌋ in eq 8 is essentially constant. 5402
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only in the mild cracking (in closed batch reactor) but also in the pyrolysis of TG. On the other hand, series B has provided three cracking temperatures to study the cracking mechanism of HDPE/PP. The obtained activation energies of two samples from (50/50 HDPE/PP)#/B1/110 are very different each other, suggesting that the thermal cracking HDPE and PP occurs separately and the interactions between HDPE and PP are still minor in such experimental condition. In addition, the activation energy of (50/50 HDPE/PP)# is much lower than that of HDPE# at the B2/40 and B3/30. This result shows that in these conditions the interactions between HDPE and PP not only improve the volatiles formation (as shown in Table 4) but also encourage the chain scission reactions of the residues. Furthermore, the activation energy of (50/50 HDPE/PP)#/B3/40 is very close to that of HDPE#/B3/40, suggesting that most of PP (at B3/40) has degraded into volatiles and (50/50 HDPE/PP)#/B3/40 mainly consists of HDPE cracked products. Comparing the kinetic analysis of P* (series A) and P# (series B) will give great differences. First, the activation energy of HDPE#/ B1/110 is very close to that of HDPE but higher than that of HDPE*, though these two products are obtained at similar cracking temperature (619 K for HDPE* and 623 K for HDPE#). Second, the activation energy of (50/50 HDPE/PP)* is lower than those of HDPE* and PP* series A, but the activation energies of two samples from (50/50 HDPE/PP) #/B1/110 are very different each other. Third, the activation energies of (HDPE/PP)* are much lower than that of HDPE*, while the activation energies of (HDPE/PP)# are lower than or close to that of HDPE*. It is noted that these results can be contributed to the two differences: on one hand, two reactors have different reaction characteristics; on the other hand, magnetic stirring in closed batch reactor (series A) has not only greatly improved the heat transfer but also enhanced the mass transfer, which is very important for the interactions between the HDPE and PP in the cocracking of polyolefin. Consequently, it is important for cocracking of polyolefin mixture with the aim of producing more light oils under mild conditions, to perform longer residence time and adequate mixing. The activation energy of the pyrolysis of polyolefin and polyolefin cracked products, reported in the literature, varied over a wide range. Zhou et al.27 and Cai et al.28 found activation energies of HDPE, LDPE, and PP as 457.2 kJ mol−1, 300.4 kJ mol−1, and 319.7 kJ mol−1. Again, other studies gave lower values: Ballice29 found activation energies of pyrolysis of HDPE and LDPE as 238.4 kJ mol−1 and 209.3 kJ mol−1, and Encinar et al.30obtained 259.7 kJ mol−1 for LDPE and 215.8 kJ mol−1 for PP. Meanwhile, Costa et al.31 obtained the activation energies of HDPE cracked product P1 to gas and to light oil as 185.0 kJ mol−1 and 156.4 kJ mol−1. As for the polyolefin mixture, Chowlu et al.13 found that activation energy was highest for the decomposition of the waste LDPE sample and much lower for the mixtures (50/50 PP/LDPE, 65/35 PP/LDPE, and 80/20 PP/LDPE). Compared with the values in the literature, the observed activation energies of cracking of polyolefin, P* and P# in this work are higher, though those of HDPE and PP are consistent with the values from Cai et al. and Zhou et al. Such result is possibly related to polymer samples used, experiment conditions and the method used to calculate the activation energy.
Figure 12. Plots of ln ⌊− ln(1 − x)/T2⌋ vs 1/T for thermal degradation of HDPE, LDPE, and PP.
Figure 13. Plots of ln ⌊− ln(1 − x)/T2⌋ vs 1/T for thermal degradation of P*, (HDPE/PP)*, and LDPE*/PP*.
Figure 14. Plots of ln ⌊− ln(1 − x)/T2⌋ vs 1/T for thermal degradation of P# and (HDPE/PP)#.
the series A and B experiments, give the lower activation energy accordingly, compared with the polyolefin. The obtained activation energies of (HDPE/PP)* are much lower than that of HDPE* and lower than or close to that of PP*, indicating that the presence of PP can improve the thermal cracking of HDPE at series A and such improvement mainly exists in the initial reaction stage (
