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Fluidized bed pyrolysis of a recycled polyethylene
Polymer Degradation and Stability 76 (2002) 479–487 www.elsevier.com/locate/polydegstab
Fluidized bed pyrolysis of a recycled polyethylene M.L. Mastellone, F. Perugini, M. Ponte, U. Arena* Dipartimento di Scienze Ambientali, Seconda Universita` di Napoli, Via Vivaldi, 43-81100 Caserta, Italy Received 24 October 2001; received in revised form 16 January 2002; accepted 25 January 2002
Abstract Fluidized bed pyrolysis is a special class of feedstock recycling processes for plastics recycling which provides for a rather uniform spectrum of products, reduces the maintenance time and cost and gives the possibility to apply the process even on a relatively small scale. The present study describes investigations carried out by injecting recycled polyethylene in a laboratory scale fluidized bed reactor, batchwise or continuously operated. The batchwise experiments allowed to investigate the polymer-to-particles interactions inside the fluidized bed reactor, which strongly affect the phenomenology and the rate of polymer degradation. The continuous experiments indicated how the yield and the composition of products are affected by two key operating variables of the process: the bed temperature and the residence time in the reactor. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Feedstock recycling; Plastics; Pyrolysis; Fluidized bed
1. Introduction The term feedstock recycling describes a family of advanced recycling technology for plastics that breaksdown the solid polymeric materials into a spectrum of basic chemical components. These latter can be used as raw materials in the production of new petrochemicals and plastics, without any deterioration in their quality and without any restriction regarding their application [1,2]. A special class of feedstock recycling processes is pyrolysis, i.e. the thermal decomposition performed in the total absence of air that produces oils and gases for further treatment by standard petrochemical processes. In the plant configuration and under the operating conditions indicated by Kaminsky et al. [3], the fluidized bed pyrolysis appears a particularly attractive technology for feedstock recycling of plastics. The very good heat and material transfer, and the consequent almost constant temperature, provides for a more uniform spectrum of products and allows shorter residence times at moderate temperatures; the absence of a moving part in the hot region reduces the maintenance time and cost;
* Corresponding author. Tel.: +39-0823-274414; fax: +39-0823274605. E-mail address: [email protected] (U. Arena).
the possibility to apply the process on a relatively small scale makes wider the range of investment alternatives. In the last 5 years, some Authors focused their attention on chemical and engineering aspects of this attractive recycling process. It is well known the wide and appreciated research activity carried out by the group of Professor Kaminsky [3,4] which defined the best operating conditions (reactor temperature, type of fluidizing gas, etc.) for the fluidized bed pyrolysis of several plastics, rubber and tyres. The behaviour of the fluidized reactor was investigated by Arena and Mastellone [5,6] which also defined the operating ranges under which it is possible to obtain the optimal process conditions without the risk of bed defluidization. Luo et al. [7] recently explored the possibility of carrying out the process with bed material having catalytic properties. Limited information is instead available about the yield and the composition of plastic pyrolysis products: Conesa et al. [8] and Mastral et al. [9] provided some data for HDPE, while Williams and Williams [10] data for LDPE. The present study describes a series of investigations aimed to study the sequence of phenomena occurring in the fluidized pyrolysis process and to quantify how the yield and the composition of pyrolysis products are affected by two key operating variables: the bed temperature and the residence time in the reactor. To this end a laboratory scale bubbling fluidized bed pyrolyser was batchwise and continuously operated with pellets of
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00052-6
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recycled polyethylene at different temperatures and gas velocities.
2. Experimental 2.1. Materials, apparatus and diagnostics The tested material was recycled polyethylene (PE), obtained from municipal collection of plastic wastes. The proximate and ultimate analysis of the PE cylindrical pellets are reported in Table 1. The experiments were carried out in a bubbling fluidized bed reactor made of stainless steel having an internal diameter of 55 mm and a height between the gas distributor plate and the outlet of sampling gas stream of 0.7 m (Fig. 1). An electric furnace driven by a PID controller was used to heat the reactor and to keep the bed temperature at a desired value. At the bottom of the bed was located an electronic pressure transducer interfaced whit a data acquisition unit and a paper recorder. A sampling line was specifically designed and installed. This special auxiliary equipment was formed by: (1) a three-way valve; (2) a cold trap to cool the gas and promote the condensation of its heavy fraction; (3)
Table 1 Properties of the tested material Material Proximate analysis, % (as received) Moisture Volatile matter Ash Fixed carbon Ultimate analysis, % C H Particle density, kg/m3 Softening temperature, C Melting temperature, C Cylindrical pellet size diameter, mm height, mm
PE — 99.97 0.03 — 86 14 920 100 137 5 2.5
an active carbon phial to recover the remaining heavy hydrocarbons (in particular benzene); (4) a Teflon bag having a volume of 10 dm3 to collect gas for the chromatographic analyses; (5) a pump able to keep fixed the suction flow rate also when the pressure drop of the entire line increased; (6) a series of gas analyzers to measure the concentration of methane and some inorganic species (Fig. 1). All the line connections were
Fig. 1. Bubbling fluidized bed reactor made of stainless steel and the special auxiliary equipment for gas pre-treatment and sampling.
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made of Teflon, which is a material insoluble in the solvents used for the extraction of the condensed phase. Different gas chromatographic techniques were used for the characterization of hydrocarbon species contained in the gas and condensed phases of the pyrolysis products. Gas chromatography in isothermal conditions on a molecular sieve/polymer column was carried out for the analysis of inorganic gases: O2, CO2, CO, H2, N2 by using a HP5890 gas chromatograph equipped with a thermal conductivity detector. C1–C4 (from methane to butadiene) hydrocarbons contained in the gas phase were quantified by gas chromatography on a Al2O3/ KCL capillary column by using a HP5890 gas chromatograph equipped with a flame ionization detection. The quantitative analysis of benzene, toluene and polycyclic aromatic hydrocarbons contained in the condensed phase, collected in the cold trap and on the carbon filter, and dissolved in dichloromethane was carried out by gas chromatography/mass spectrometry on a cross linked 5% phenyl methyl silicone capillary column. A Leica MZ8 optical microscope and a Philips XL 30 scanning electron microscope was used to investigate the microstructure of polymer particles retrieved from the bed after different residence times. The SEM was coupled with an energy disperse X-ray apparatus able to provide for the elemental analysis of the different zones of the examined samples. 2.2. Procedures Batchwise tests were utilized in order to understand the sequence of phenomena during the pyrolysis process. They were carried out by injecting single polymer particles into the bed, kept at a fixed temperature and gently fluidized by nitrogen. The bed was made of silica sand having a size range 300–400 mm. After a fixed residence time the particle was retrieved, quenched and analyzed by means of the optical and scanning electronic microscope. This procedure was repeated at two different temperatures, 300 and 450 C, and different residence times, from 60 to 180 s. The same reactor was coupled with a sampling device and used to carry out the continuous runs aimed to investigate the effect of operating conditions on the yield and composition of produced volatiles. In this case, a fixed amount of 360 g of silica sand (having a size range 90–200 mm) was charged into the reactor at the beginning of each run, fluidized by air at a given velocity and heated up by means of the electrical furnace. When the desired temperature was reached, nitrogen was used as the fluidizing gas and a fixed feed rate (about 1 g/min) of the recycled PE pellets was injected into the pyrolyser. The pressure at the bottom of the bed was continuously measured in order to monitor the quality of fluidization [5] and the occurring of plugging in the sampling line. A series of preliminary runs were
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used to define a layout able to guarantee from any risk of obstruction along the feeding pipe. The partial softening of the plastic pellets, due to the contact with the outgoing gases, can in fact induce the plugging of the feeding device. The flue gas stream was then in part sent to the stack by means of a preferential way, while the remaining part was sent to the sampling line. The data acquisition unit continuously recorded the values of gas concentrations as well as those of temperature and bed bottom pressure, all being processed by means of a LabVIEWTM software. When the signal of methane concentration became sufficiently stable (i.e. the amplitude and frequency of its fluctuations were constant) a fraction of the sucked gas was collected inside a Teflon bag for a time as long as 10 min. Just after the gas sampling, the feeder was turned off and the data acquisition stopped. The sampled gas was analyzed by means of the above mentioned chromatographic system. The results obtained from diagnostic apparatus were elaborated on the basis of the constant flow rate of nitrogen at the entrance and the exit of the reactor. The reasonable assumption that nitrogen concentration was equal in the stack and in the sampling lines, allowed to determine the volume of pyrolysis gas produced during the experiment. A couple of additional checks were made in order to obtain a reliable material balance: (i) the value of of the sampling flow rate was further measured by means of a gas meter located downstream the sampling line; (ii) the effective amount of PE pellets fed into the reactor was verified by weighing the plastics contained in the feeding hopper before and after the experiment.
3. Results and discussion 3.1. Polymer-particle interactions in the fluidized bed reactor The yields of the products obtained from a pyrolysis process in the fluidized bed reactor are due to two contributions. The first, called primary cracking, is that of raw material decomposition, i.e. the cracking of solid (or, better, liquid) polymer, which occurs in the dense bed. The second contribution, referred as secondary cracking, is that of reactions suffered by the primary volatiles, which can occur partially inside the bed and, for a large part, along the freeboard [8]. A possible sequence of steps that a plastic polymer undergoes during the fluidized bed pyrolysis is schematically reported in Fig. 2 on the basis of a series of batchwise experiments, carried out as described above. Just after the injection into the hot fluidized bed, a very fast heat transfer mechanism leads the polymer pellet external surface up to the softening temperature. Sand particles, therefore, stick on the plastic surface, forming
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Fig. 2. Different stages which a polymer particle undergoes after the injection into a fluidised bed during a pyrolysis process. The graph indicates the carbon paths inside the reactor.
a polymer–sand aggregate that has the external shell made of sand particles and the internal core made of polymer not yet molten. When the temperature further increases, the surface of the pellet reaches the melting temperature and the polymer flows throughout the bed particles of the external shell, so forming a uniform coating over and between them. The described progress of heating leads to the cracking of the carbon–carbon bonds of the polymer chain, i.e. to the beginning of the pyrolysis process, that starts when the polymer has already covered the bed particles. Therefore, it is not related to the whole molten pellet, but to a layer of polymer which coats and adheres on the external surfaces of single sand particles. Moreover, if the polymer does not produce a sticky carbon residue, the flow throughout the bed particles leads to a fast crumbling of the aggregates. The risk that the so-generated aggregates may grow so large as to lead to bed defluidization has been investigate in other papers [5,6]. Note that Fig. 2 also reports a graph that indicates the carbon patterns inside the reactor. Square-shaped blocks represent the ‘‘phases’’ in which the carbon can
Fig. 3. Photographs of a PE pellet which remained 60 s in the fluidized bed reactor taken at 300 C. (A) SEM picture of the external surface (magnification=70); (B) detail of picture (A) (magnification= 600).
Fig. 4. Photographs of a PE pellet which remained 15 s in the fluidized bed reactor taken at 450 C. (A) SEM picture of the external surface (magnification=40); (B) detail of picture (A) (magnification= 157).
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Fig. 5. Photographs of a PE pellet which remained 60 s in the fluidized bed reactor taken at 450 C. (A) SEM picture of the external surface (magnification=81); (B) detail of picture (A) (magnification= 650).
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be assumed to be present in the bed and in the freeboard (fixed carbon, FC, carbon fines, F, volatile matter in the bed and freeboard, V). Circle shaped blocks represent some of the processes occurring in the reactor, as above mentioned and as listed on the right side of the same figure. The general scenario schematically described above is the result of the observations on samples obtained by the batchwise experiments. The Figs. 3–5 report SEM pictures related to samples taken at 300 and 450 C for different residence times of the pellet inside the bed. The photos at the lower temperature show several sand particles that start to cover the external surface of the polymer. The latter is not yet molten and appears (Fig. 3A) as a set of irregular solid pieces of degraded polymer which are progressively decomposing (Fig. 3B). At the higher temperature and for the shorter residence time, the polymer appears more and more degraded and in large part already molten (Fig. 4A). The flowing over and throughout the sand particle is in progress (Fig. 4B). Fig. 5A and B are taken at the same temperature of the latter set but at a residence time equal to 60 s. Fig. 5A shows the external aggregate surface by highlighting the connection between the sand particle covered by a very small thickness (about 5–10 mm) of the polymer. This connection is more evident in the Fig. 5B that, at larger magnification, shows the polymer bridge that keeps together the sand particles. The degradation of the polymer proceeds when time (or the temperature) increases, so that the primary cracking of the polymer is completed when the polymer is under the form of coating layer uniformly distributed on each particle present in the bed.
Fig. 6. Normalized mass loss as a function of temperature for the tested PE in a thermogravimetric balance.
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The volatiles formed during the primary reactions pass throughout the dense bed and proceed along the freeboard region. The secondary cracking occurs in these two regions of the reactor and produces a large spectra of hydrocarbon compounds. An investigation of the yield and the composition of these compounds is reported in the following paragraph. 3.2. Experimental determination of the yield of pyrolysis products
Fig. 7. Process yield as a function of temperature at fluidizing velocity of 0.15 m/s.
This latter feature led to investigate the rate of the primary cracking step by means of thermal gravimetric analyses, carried out on samples of inert material covered by the molten polymer. These latter were retrieved from the reactor before the beginning of devolatilization by keeping the bed at a temperature smaller than the starting degradation temperature and by verifying the complete absence of volatiles evolution. The thermal gravimetric analyses provided for the temperature profile of mass loss (Fig. 6), which in turn gives the kinetic parameters of the primary cracking step (reported inside the figure), and demonstrated that the kinetics is well described by a first order equation.
3.2.1. Influence of bed temperature The effect of the reactor temperature and fluidizing gas velocity on the yield and composition of the feedstock produced by pyrolysis of the recycled PE was investigated. The first series of runs was carried out at temperatures ranging between 550 and 750 C and at fluidizing gas velocity, U, equal to 0.15 m/s. In the second series of tests the value of gas velocity was increased up to 0.28 m/s, while the temperature ranged between 550 and 700 C. In all the experiments, the quality of fluidization remained always good (Arena and Mastellone, [5]) and gases, liquids and solids were obtained in different yield and composition. Fig. 7 describes the variation of the gas, liquid and solid yield (expressed as grams of pyrolysis products for each gram of injected PE) in the tested range of bed temperature when the fluidizing velocity was fixed at 0.15 m/s. In particular, solids were found under two forms: a large fraction of polymer only partially degraded was collected in the cold trap in the range 550–650 C; tar residue was instead collected from reactor walls in the whole range of tested temperatures. Fig. 8 shows a SEM picture of the partially unconverted polyethylene. It should be noted that the values of tar yield reported in the Fig. 7
Fig. 8. SEM pictures of degraded PE collected in the cold trap during a continuous run at 450 C (magnifications=11,709).
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yield had its maximum at 700 C in correspondence to the minimum yield of PAH.
(and in the following Fig. 11) were estimated as the difference between the quantity of the polymer fed in the reactor and the sum of not degraded PE and of the total gaseous and liquid products. A check of these values was made in some of the runs by collecting and chemically analyzing tar from the reactor walls and by switching, at the end of the run, the fluidizing gas from nitrogen to air. The latter procedure allowed to close the material balance by processing the data of CO, CO2 and CnHm concentrations in the flue gases. Fig. 7 also shows that, in agreement with other studies [4], the yield of obtained products, as well as their composition, remarkably depended on the reaction temperature. In particular, the gaseous phase presented a maximum, equal to about 70%, at bed temperature of 650 C. On the contrary, the liquid yield was substantially negligible below 600 C and appreciable only when the process was carried out at a temperature as high as 650 C. Fig. 9 shows the detailed composition of gaseous pyrolysis products obtained in the same series of runs. The ethene concentration resulted the highest at each temperature and reached its maximum at 650 C. The shape of propene curve appeared similar. The methane yield continuously increased with the reaction temperature, whereas hydrogen yield decreased. All these findings appear in agreement with those obtained at the University of Hamburg by pyrolysing polyethylene in a similar range of temperature [3]. The liquid products were mainly composed of aromatics like benzene and toluene (Fig. 10), other compounds being styrene, xylene and polycyclic aromatic hydrocarbons, from naphthalene to pyrene. The BTX
Fig. 10. Composition of liquid fraction as a function of temperature at a fluidizing velocity of 0.15 m/s.
Fig. 9. Composition of gaseous fraction as a function of temperature at a fluidizing velocity of 0.15 m/s.
Fig. 11. Process yield as a function of temperature at fluidizing velocity of 0.28 m/s.
3.2.2. Influence of gas-phase residence time The gas residence time inside the reactor, i.e. the time necessary to gases for moving through the bed and the freeboard and then leaving the reactor, was changed by increasing the fluidizing gas velocity from 0.15 to 0.28 m/s. The residence times inside the reactor consequently decreased from about 4 to 2 s, respectively, as reported
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M.L. Mastellone et al. / Polymer Degradation and Stability 76 (2002) 479–487 Table 2 Calculated residence times for the two hydrodynamic conditions tested
Fig. 12. Composition of gaseous fraction as a function of temperature at a fluidizing velocity of 0.28 m/s.
U, m/s
T, C
Qmf, m3/s
Qpyr, m3/s
Vgas, m/s
tres,bed, s
tres,fb, s
tres, s
0.13 0.13 0.15 0.15 0.15 0.28 0.28 0.28 0.28
550 600 650 700 750 550 600 650 700
2.8*105 2.7*105 2.6*105 2.4*105 2.5*105 2.8*105 2.7*105 2.6*105 2.5*105
3.45*105 5.9*105 4.7*105 4.4*105 3.1*105 3.4*105 3.4*105 4.2*105 4.4*105
0.44 0.44 0.43 0.42 0.39 0.59 0.56 0.53 0.52
0.13 0.14 0.14 0.14 0.15 0.10 0.11 0.11 0.12
4.02 4.02 3.83 3.89 4.26 2.02 2.14 2.26 2.31
4.15 4.15 3.97 4.03 4.41 2.12 2.25 2.37 2.43
analysis of all the above cited figures allows some considerations. The amount of not-degraded polymer was larger for shorter residence times (Fig. 11) even though it again decreased as the temperature increased. The liquid products are again appreciable only at higher temperatures. Therefore, a process devoted to obtain a high yield in BTX should be carried out at temperatures higher than 650 C and it should carefully take into account the formation of tars. These latter increased in this temperature range as consequence of the promoted cyclization. They also increased as a consequence of a reduced residence time (Figs. 7 and 11) that likely limited the progress of their degradation. The analysis of gas composition in the two hydrodynamic conditions also indicates that there is not a substantial effect of the gas residence time. Only the hydrogen content in the pyrolysis gas appears to be higher in the runs at lower fluidizing velocity, i.e. at longer residence time.
4. Conclusions
Fig. 13. Composition of liquid fraction as a function of temperature at a fluidizing velocity of 0.28 m/s.
in Table 2 together with the partial residence times in the bed and in the freeboard. The calculation procedure is detailed in [11] and here schematically reported in the Appendix. It is noteworthy that the procedure utilized in order to change the gas-phase residence time mainly affected the time spent in the freeboard. The residence time in the bed is instead affected by another operating variable, the height of expanded bed, which was kept fixed in these experiments due to the small scale of the reactor used. Figs. 11–13 reported the same information of Figs. 7, 8 and 10 but with reference to the experiments carried out at the higher fluidizing velocity. The comparative
A preliminary series of experiments was carried out to investigate the polymer-to-particles interactions inside the pyrolyser and the effect of the main operating variables on the yield and composition of products of the fluidized pyrolysis of a recycled polyethylene. The results appeared in agreement with the limited information available in the literature and validate the proposed experimental procedure. The temperature played a crucial role in defining yield and composition of the different phases of products. In the range below 650 C, the amount of BTX and other aromatics was just appreciable even though the yield in gaseous products was of interest. The aromatic content of products increased at the higher temperatures but the production of tar became so remarkable that it could lead to operating problems. In the investigated range, the gas residence time mainly affected the progress of cyclization and the aromatic content in the products as well as that of tars. More work is necessary for a deeper understanding of the reported data and to extend the investigation on
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other operating variables like the size of inert materials, the bed height, the polymer feed rate.
Acknowledgements The authors are grateful to Dr. A. Cjaiolo, Dr. B. Apicella and Mrs. R. Barbella of the Institute for Combustion Research (National Research Council of Italy) who performed the chemical analyses.
Appendix. Calculation procedure for the gas-phase residence time The overall gas-phase residence time has been determined as the sum of two contributions, the residence time inside the fluidized bed and that along the freeboard region. The latter was immediately evaluated by means of the relationship tres;freeboard ¼
h hbed Qf þ Qpyr Abed
ð1Þ
where h and hbed are the reactor and the expanded bed heights, Qf is the flow rate of nitrogen used for fluidization, Qpyr is the overall flow rate of pyrolysis products (gases and liquids) and Abed is the area of transversal section of the bed. A little more complicated was the evaluation of the residence time in the bed. In order to take into account the axial distribution of PE particles along the bed only a half of the overall bed height is considered, so that tres;bed ¼
hbed =2 Vgas
ð2Þ
where Vgas is the effective gas velocity along the bed. If the two-phases fluidization theory [12] is assumed to be valid, the gas flow rate inside the bed emulsion phase is equal to that necessary for the minimum fluidization, Qmf. The same theory suggests that the bed volume is occupied by dilute (or bubble) phase for a fraction Eb, while the remaining part (1"b) is occupied by emulsion (or dense) phase, which in turn has a voidage "mf. It is possible to write Vgas ¼
Qmf ðQf Qmf Þ þ Qpyr þ Abed "mf ð1 "b Þ Abed "b
ð3Þ
The variables "mf and "b must be calculated by means of relationships available in the literature. The Carman– Kozeny equation [13] for the minimum fluidization velocity can be rearranged as:
"3mf 150Umf ¼ 2 ð1 "mf Þ d g s g
487
ð4Þ
where and g are the viscosity and the density of gas, g is the acceleration due to the gravity, d and s are the diameter and the density of bed material. Eq. (4) was used for "mf determination, being Umf evaluated by means of Geldart’s relationship [14]. The fraction "b of bubble phase is instead defined as: U Umf "b ¼ ð5Þ UB þ U Umf where U is the fluidization velocity and UB is the bubble velocity. The latter is given by the relationship from Allahwala and Potter [15]: 2 3 0:5 !1:8 pffiffiffiffiffiffiffi 1=1:8 D e 43:6 5 UB ¼ 0:35 gDtgh ð6Þ D where D is the reactor diameter and De the equivalent bubble diameter. The latter is in turn given by a relationship proposed by Darton et al. in 1977 [16]. " 0:5 #0:8 AD 0:4 0:54ðU Umf Þ hþ4 N De ¼ ð7Þ g0:2 where AD and N are the transversal section and the holes number of the grid, respectively.
References [1] Brophy JH, Hardman S. In: Brandrup J, et al., editors. Recycling and recovery of plastics. New York: Hanser Publishers; 1996. [2] Scheirs J. Polymer recycling science, technology and applications. New York: J. Wiley & Sons; 1998. [3] Kaminsky W, Schiesselmann B, Simon CM. J Anal Appl Pyrolysis 1995;32:19–27. [4] Kaminsky W, Schiesselmann B, Simon CM. Polym Degrad Stab 1996;53:189–97. [5] Arena U, Mastellone ML. Chem Eng Sci 2000;55:2849–60. [6] Arena U, Mastellone ML. Powder Technol 2001;120(1–2):127–33. [7] Luo G, Suto T, Yasu S, Kato K. Polym Degrad Stab 2000;70:97– 102. [8] Conesa JA, Font R, Marcilla A, Garcia AN. Energy and Fuels 1994;8:1238–46. [9] Mastral FJ, Esperanza E, Garcia P, Berrueco C Juste, M. In: Proceedings of 6th Int Conf on Technologies and Combustion for a Clean Environment, Portugal, 9–12 July 2001. p. 133–44. [10] Williams PT, Williams EA. J Anal Appl Pyrolysis 1999;51:107– 26. [11] Ponte, M. Thesis in chemical engineering, University ‘‘Federico II’’ of Naples, 2000. [12] Toomey RD, Johnstone HF. Chem Eng Progr 1952;48:220–5. [13] Carman PC. Trans I Chem Engrs 1937;15:150. [14] Geldart D. Gas Fluidizat Techn 1986:24. [15] Allahwala SA, Potter OE. Ind Eng Chem Fund 1979;18(2):112. [16] Darton RC, LaNauze RD, Davdson JF, Harrison D. Trans I Chem Engrs 1977;55:274.
Fluidized bed pyrolysis of a recycled polyethylene M.L. Mastellone, F. Perugini, M. Ponte, U. Arena* Dipartimento di Scienze Ambientali, Seconda Universita` di Napoli, Via Vivaldi, 43-81100 Caserta, Italy Received 24 October 2001; received in revised form 16 January 2002; accepted 25 January 2002
Abstract Fluidized bed pyrolysis is a special class of feedstock recycling processes for plastics recycling which provides for a rather uniform spectrum of products, reduces the maintenance time and cost and gives the possibility to apply the process even on a relatively small scale. The present study describes investigations carried out by injecting recycled polyethylene in a laboratory scale fluidized bed reactor, batchwise or continuously operated. The batchwise experiments allowed to investigate the polymer-to-particles interactions inside the fluidized bed reactor, which strongly affect the phenomenology and the rate of polymer degradation. The continuous experiments indicated how the yield and the composition of products are affected by two key operating variables of the process: the bed temperature and the residence time in the reactor. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Feedstock recycling; Plastics; Pyrolysis; Fluidized bed
1. Introduction The term feedstock recycling describes a family of advanced recycling technology for plastics that breaksdown the solid polymeric materials into a spectrum of basic chemical components. These latter can be used as raw materials in the production of new petrochemicals and plastics, without any deterioration in their quality and without any restriction regarding their application [1,2]. A special class of feedstock recycling processes is pyrolysis, i.e. the thermal decomposition performed in the total absence of air that produces oils and gases for further treatment by standard petrochemical processes. In the plant configuration and under the operating conditions indicated by Kaminsky et al. [3], the fluidized bed pyrolysis appears a particularly attractive technology for feedstock recycling of plastics. The very good heat and material transfer, and the consequent almost constant temperature, provides for a more uniform spectrum of products and allows shorter residence times at moderate temperatures; the absence of a moving part in the hot region reduces the maintenance time and cost;
* Corresponding author. Tel.: +39-0823-274414; fax: +39-0823274605. E-mail address: [email protected] (U. Arena).
the possibility to apply the process on a relatively small scale makes wider the range of investment alternatives. In the last 5 years, some Authors focused their attention on chemical and engineering aspects of this attractive recycling process. It is well known the wide and appreciated research activity carried out by the group of Professor Kaminsky [3,4] which defined the best operating conditions (reactor temperature, type of fluidizing gas, etc.) for the fluidized bed pyrolysis of several plastics, rubber and tyres. The behaviour of the fluidized reactor was investigated by Arena and Mastellone [5,6] which also defined the operating ranges under which it is possible to obtain the optimal process conditions without the risk of bed defluidization. Luo et al. [7] recently explored the possibility of carrying out the process with bed material having catalytic properties. Limited information is instead available about the yield and the composition of plastic pyrolysis products: Conesa et al. [8] and Mastral et al. [9] provided some data for HDPE, while Williams and Williams [10] data for LDPE. The present study describes a series of investigations aimed to study the sequence of phenomena occurring in the fluidized pyrolysis process and to quantify how the yield and the composition of pyrolysis products are affected by two key operating variables: the bed temperature and the residence time in the reactor. To this end a laboratory scale bubbling fluidized bed pyrolyser was batchwise and continuously operated with pellets of
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00052-6
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recycled polyethylene at different temperatures and gas velocities.
2. Experimental 2.1. Materials, apparatus and diagnostics The tested material was recycled polyethylene (PE), obtained from municipal collection of plastic wastes. The proximate and ultimate analysis of the PE cylindrical pellets are reported in Table 1. The experiments were carried out in a bubbling fluidized bed reactor made of stainless steel having an internal diameter of 55 mm and a height between the gas distributor plate and the outlet of sampling gas stream of 0.7 m (Fig. 1). An electric furnace driven by a PID controller was used to heat the reactor and to keep the bed temperature at a desired value. At the bottom of the bed was located an electronic pressure transducer interfaced whit a data acquisition unit and a paper recorder. A sampling line was specifically designed and installed. This special auxiliary equipment was formed by: (1) a three-way valve; (2) a cold trap to cool the gas and promote the condensation of its heavy fraction; (3)
Table 1 Properties of the tested material Material Proximate analysis, % (as received) Moisture Volatile matter Ash Fixed carbon Ultimate analysis, % C H Particle density, kg/m3 Softening temperature, C Melting temperature, C Cylindrical pellet size diameter, mm height, mm
PE — 99.97 0.03 — 86 14 920 100 137 5 2.5
an active carbon phial to recover the remaining heavy hydrocarbons (in particular benzene); (4) a Teflon bag having a volume of 10 dm3 to collect gas for the chromatographic analyses; (5) a pump able to keep fixed the suction flow rate also when the pressure drop of the entire line increased; (6) a series of gas analyzers to measure the concentration of methane and some inorganic species (Fig. 1). All the line connections were
Fig. 1. Bubbling fluidized bed reactor made of stainless steel and the special auxiliary equipment for gas pre-treatment and sampling.
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made of Teflon, which is a material insoluble in the solvents used for the extraction of the condensed phase. Different gas chromatographic techniques were used for the characterization of hydrocarbon species contained in the gas and condensed phases of the pyrolysis products. Gas chromatography in isothermal conditions on a molecular sieve/polymer column was carried out for the analysis of inorganic gases: O2, CO2, CO, H2, N2 by using a HP5890 gas chromatograph equipped with a thermal conductivity detector. C1–C4 (from methane to butadiene) hydrocarbons contained in the gas phase were quantified by gas chromatography on a Al2O3/ KCL capillary column by using a HP5890 gas chromatograph equipped with a flame ionization detection. The quantitative analysis of benzene, toluene and polycyclic aromatic hydrocarbons contained in the condensed phase, collected in the cold trap and on the carbon filter, and dissolved in dichloromethane was carried out by gas chromatography/mass spectrometry on a cross linked 5% phenyl methyl silicone capillary column. A Leica MZ8 optical microscope and a Philips XL 30 scanning electron microscope was used to investigate the microstructure of polymer particles retrieved from the bed after different residence times. The SEM was coupled with an energy disperse X-ray apparatus able to provide for the elemental analysis of the different zones of the examined samples. 2.2. Procedures Batchwise tests were utilized in order to understand the sequence of phenomena during the pyrolysis process. They were carried out by injecting single polymer particles into the bed, kept at a fixed temperature and gently fluidized by nitrogen. The bed was made of silica sand having a size range 300–400 mm. After a fixed residence time the particle was retrieved, quenched and analyzed by means of the optical and scanning electronic microscope. This procedure was repeated at two different temperatures, 300 and 450 C, and different residence times, from 60 to 180 s. The same reactor was coupled with a sampling device and used to carry out the continuous runs aimed to investigate the effect of operating conditions on the yield and composition of produced volatiles. In this case, a fixed amount of 360 g of silica sand (having a size range 90–200 mm) was charged into the reactor at the beginning of each run, fluidized by air at a given velocity and heated up by means of the electrical furnace. When the desired temperature was reached, nitrogen was used as the fluidizing gas and a fixed feed rate (about 1 g/min) of the recycled PE pellets was injected into the pyrolyser. The pressure at the bottom of the bed was continuously measured in order to monitor the quality of fluidization [5] and the occurring of plugging in the sampling line. A series of preliminary runs were
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used to define a layout able to guarantee from any risk of obstruction along the feeding pipe. The partial softening of the plastic pellets, due to the contact with the outgoing gases, can in fact induce the plugging of the feeding device. The flue gas stream was then in part sent to the stack by means of a preferential way, while the remaining part was sent to the sampling line. The data acquisition unit continuously recorded the values of gas concentrations as well as those of temperature and bed bottom pressure, all being processed by means of a LabVIEWTM software. When the signal of methane concentration became sufficiently stable (i.e. the amplitude and frequency of its fluctuations were constant) a fraction of the sucked gas was collected inside a Teflon bag for a time as long as 10 min. Just after the gas sampling, the feeder was turned off and the data acquisition stopped. The sampled gas was analyzed by means of the above mentioned chromatographic system. The results obtained from diagnostic apparatus were elaborated on the basis of the constant flow rate of nitrogen at the entrance and the exit of the reactor. The reasonable assumption that nitrogen concentration was equal in the stack and in the sampling lines, allowed to determine the volume of pyrolysis gas produced during the experiment. A couple of additional checks were made in order to obtain a reliable material balance: (i) the value of of the sampling flow rate was further measured by means of a gas meter located downstream the sampling line; (ii) the effective amount of PE pellets fed into the reactor was verified by weighing the plastics contained in the feeding hopper before and after the experiment.
3. Results and discussion 3.1. Polymer-particle interactions in the fluidized bed reactor The yields of the products obtained from a pyrolysis process in the fluidized bed reactor are due to two contributions. The first, called primary cracking, is that of raw material decomposition, i.e. the cracking of solid (or, better, liquid) polymer, which occurs in the dense bed. The second contribution, referred as secondary cracking, is that of reactions suffered by the primary volatiles, which can occur partially inside the bed and, for a large part, along the freeboard [8]. A possible sequence of steps that a plastic polymer undergoes during the fluidized bed pyrolysis is schematically reported in Fig. 2 on the basis of a series of batchwise experiments, carried out as described above. Just after the injection into the hot fluidized bed, a very fast heat transfer mechanism leads the polymer pellet external surface up to the softening temperature. Sand particles, therefore, stick on the plastic surface, forming
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Fig. 2. Different stages which a polymer particle undergoes after the injection into a fluidised bed during a pyrolysis process. The graph indicates the carbon paths inside the reactor.
a polymer–sand aggregate that has the external shell made of sand particles and the internal core made of polymer not yet molten. When the temperature further increases, the surface of the pellet reaches the melting temperature and the polymer flows throughout the bed particles of the external shell, so forming a uniform coating over and between them. The described progress of heating leads to the cracking of the carbon–carbon bonds of the polymer chain, i.e. to the beginning of the pyrolysis process, that starts when the polymer has already covered the bed particles. Therefore, it is not related to the whole molten pellet, but to a layer of polymer which coats and adheres on the external surfaces of single sand particles. Moreover, if the polymer does not produce a sticky carbon residue, the flow throughout the bed particles leads to a fast crumbling of the aggregates. The risk that the so-generated aggregates may grow so large as to lead to bed defluidization has been investigate in other papers [5,6]. Note that Fig. 2 also reports a graph that indicates the carbon patterns inside the reactor. Square-shaped blocks represent the ‘‘phases’’ in which the carbon can
Fig. 3. Photographs of a PE pellet which remained 60 s in the fluidized bed reactor taken at 300 C. (A) SEM picture of the external surface (magnification=70); (B) detail of picture (A) (magnification= 600).
Fig. 4. Photographs of a PE pellet which remained 15 s in the fluidized bed reactor taken at 450 C. (A) SEM picture of the external surface (magnification=40); (B) detail of picture (A) (magnification= 157).
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Fig. 5. Photographs of a PE pellet which remained 60 s in the fluidized bed reactor taken at 450 C. (A) SEM picture of the external surface (magnification=81); (B) detail of picture (A) (magnification= 650).
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be assumed to be present in the bed and in the freeboard (fixed carbon, FC, carbon fines, F, volatile matter in the bed and freeboard, V). Circle shaped blocks represent some of the processes occurring in the reactor, as above mentioned and as listed on the right side of the same figure. The general scenario schematically described above is the result of the observations on samples obtained by the batchwise experiments. The Figs. 3–5 report SEM pictures related to samples taken at 300 and 450 C for different residence times of the pellet inside the bed. The photos at the lower temperature show several sand particles that start to cover the external surface of the polymer. The latter is not yet molten and appears (Fig. 3A) as a set of irregular solid pieces of degraded polymer which are progressively decomposing (Fig. 3B). At the higher temperature and for the shorter residence time, the polymer appears more and more degraded and in large part already molten (Fig. 4A). The flowing over and throughout the sand particle is in progress (Fig. 4B). Fig. 5A and B are taken at the same temperature of the latter set but at a residence time equal to 60 s. Fig. 5A shows the external aggregate surface by highlighting the connection between the sand particle covered by a very small thickness (about 5–10 mm) of the polymer. This connection is more evident in the Fig. 5B that, at larger magnification, shows the polymer bridge that keeps together the sand particles. The degradation of the polymer proceeds when time (or the temperature) increases, so that the primary cracking of the polymer is completed when the polymer is under the form of coating layer uniformly distributed on each particle present in the bed.
Fig. 6. Normalized mass loss as a function of temperature for the tested PE in a thermogravimetric balance.
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The volatiles formed during the primary reactions pass throughout the dense bed and proceed along the freeboard region. The secondary cracking occurs in these two regions of the reactor and produces a large spectra of hydrocarbon compounds. An investigation of the yield and the composition of these compounds is reported in the following paragraph. 3.2. Experimental determination of the yield of pyrolysis products
Fig. 7. Process yield as a function of temperature at fluidizing velocity of 0.15 m/s.
This latter feature led to investigate the rate of the primary cracking step by means of thermal gravimetric analyses, carried out on samples of inert material covered by the molten polymer. These latter were retrieved from the reactor before the beginning of devolatilization by keeping the bed at a temperature smaller than the starting degradation temperature and by verifying the complete absence of volatiles evolution. The thermal gravimetric analyses provided for the temperature profile of mass loss (Fig. 6), which in turn gives the kinetic parameters of the primary cracking step (reported inside the figure), and demonstrated that the kinetics is well described by a first order equation.
3.2.1. Influence of bed temperature The effect of the reactor temperature and fluidizing gas velocity on the yield and composition of the feedstock produced by pyrolysis of the recycled PE was investigated. The first series of runs was carried out at temperatures ranging between 550 and 750 C and at fluidizing gas velocity, U, equal to 0.15 m/s. In the second series of tests the value of gas velocity was increased up to 0.28 m/s, while the temperature ranged between 550 and 700 C. In all the experiments, the quality of fluidization remained always good (Arena and Mastellone, [5]) and gases, liquids and solids were obtained in different yield and composition. Fig. 7 describes the variation of the gas, liquid and solid yield (expressed as grams of pyrolysis products for each gram of injected PE) in the tested range of bed temperature when the fluidizing velocity was fixed at 0.15 m/s. In particular, solids were found under two forms: a large fraction of polymer only partially degraded was collected in the cold trap in the range 550–650 C; tar residue was instead collected from reactor walls in the whole range of tested temperatures. Fig. 8 shows a SEM picture of the partially unconverted polyethylene. It should be noted that the values of tar yield reported in the Fig. 7
Fig. 8. SEM pictures of degraded PE collected in the cold trap during a continuous run at 450 C (magnifications=11,709).
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yield had its maximum at 700 C in correspondence to the minimum yield of PAH.
(and in the following Fig. 11) were estimated as the difference between the quantity of the polymer fed in the reactor and the sum of not degraded PE and of the total gaseous and liquid products. A check of these values was made in some of the runs by collecting and chemically analyzing tar from the reactor walls and by switching, at the end of the run, the fluidizing gas from nitrogen to air. The latter procedure allowed to close the material balance by processing the data of CO, CO2 and CnHm concentrations in the flue gases. Fig. 7 also shows that, in agreement with other studies [4], the yield of obtained products, as well as their composition, remarkably depended on the reaction temperature. In particular, the gaseous phase presented a maximum, equal to about 70%, at bed temperature of 650 C. On the contrary, the liquid yield was substantially negligible below 600 C and appreciable only when the process was carried out at a temperature as high as 650 C. Fig. 9 shows the detailed composition of gaseous pyrolysis products obtained in the same series of runs. The ethene concentration resulted the highest at each temperature and reached its maximum at 650 C. The shape of propene curve appeared similar. The methane yield continuously increased with the reaction temperature, whereas hydrogen yield decreased. All these findings appear in agreement with those obtained at the University of Hamburg by pyrolysing polyethylene in a similar range of temperature [3]. The liquid products were mainly composed of aromatics like benzene and toluene (Fig. 10), other compounds being styrene, xylene and polycyclic aromatic hydrocarbons, from naphthalene to pyrene. The BTX
Fig. 10. Composition of liquid fraction as a function of temperature at a fluidizing velocity of 0.15 m/s.
Fig. 9. Composition of gaseous fraction as a function of temperature at a fluidizing velocity of 0.15 m/s.
Fig. 11. Process yield as a function of temperature at fluidizing velocity of 0.28 m/s.
3.2.2. Influence of gas-phase residence time The gas residence time inside the reactor, i.e. the time necessary to gases for moving through the bed and the freeboard and then leaving the reactor, was changed by increasing the fluidizing gas velocity from 0.15 to 0.28 m/s. The residence times inside the reactor consequently decreased from about 4 to 2 s, respectively, as reported
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M.L. Mastellone et al. / Polymer Degradation and Stability 76 (2002) 479–487 Table 2 Calculated residence times for the two hydrodynamic conditions tested
Fig. 12. Composition of gaseous fraction as a function of temperature at a fluidizing velocity of 0.28 m/s.
U, m/s
T, C
Qmf, m3/s
Qpyr, m3/s
Vgas, m/s
tres,bed, s
tres,fb, s
tres, s
0.13 0.13 0.15 0.15 0.15 0.28 0.28 0.28 0.28
550 600 650 700 750 550 600 650 700
2.8*105 2.7*105 2.6*105 2.4*105 2.5*105 2.8*105 2.7*105 2.6*105 2.5*105
3.45*105 5.9*105 4.7*105 4.4*105 3.1*105 3.4*105 3.4*105 4.2*105 4.4*105
0.44 0.44 0.43 0.42 0.39 0.59 0.56 0.53 0.52
0.13 0.14 0.14 0.14 0.15 0.10 0.11 0.11 0.12
4.02 4.02 3.83 3.89 4.26 2.02 2.14 2.26 2.31
4.15 4.15 3.97 4.03 4.41 2.12 2.25 2.37 2.43
analysis of all the above cited figures allows some considerations. The amount of not-degraded polymer was larger for shorter residence times (Fig. 11) even though it again decreased as the temperature increased. The liquid products are again appreciable only at higher temperatures. Therefore, a process devoted to obtain a high yield in BTX should be carried out at temperatures higher than 650 C and it should carefully take into account the formation of tars. These latter increased in this temperature range as consequence of the promoted cyclization. They also increased as a consequence of a reduced residence time (Figs. 7 and 11) that likely limited the progress of their degradation. The analysis of gas composition in the two hydrodynamic conditions also indicates that there is not a substantial effect of the gas residence time. Only the hydrogen content in the pyrolysis gas appears to be higher in the runs at lower fluidizing velocity, i.e. at longer residence time.
4. Conclusions
Fig. 13. Composition of liquid fraction as a function of temperature at a fluidizing velocity of 0.28 m/s.
in Table 2 together with the partial residence times in the bed and in the freeboard. The calculation procedure is detailed in [11] and here schematically reported in the Appendix. It is noteworthy that the procedure utilized in order to change the gas-phase residence time mainly affected the time spent in the freeboard. The residence time in the bed is instead affected by another operating variable, the height of expanded bed, which was kept fixed in these experiments due to the small scale of the reactor used. Figs. 11–13 reported the same information of Figs. 7, 8 and 10 but with reference to the experiments carried out at the higher fluidizing velocity. The comparative
A preliminary series of experiments was carried out to investigate the polymer-to-particles interactions inside the pyrolyser and the effect of the main operating variables on the yield and composition of products of the fluidized pyrolysis of a recycled polyethylene. The results appeared in agreement with the limited information available in the literature and validate the proposed experimental procedure. The temperature played a crucial role in defining yield and composition of the different phases of products. In the range below 650 C, the amount of BTX and other aromatics was just appreciable even though the yield in gaseous products was of interest. The aromatic content of products increased at the higher temperatures but the production of tar became so remarkable that it could lead to operating problems. In the investigated range, the gas residence time mainly affected the progress of cyclization and the aromatic content in the products as well as that of tars. More work is necessary for a deeper understanding of the reported data and to extend the investigation on
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other operating variables like the size of inert materials, the bed height, the polymer feed rate.
Acknowledgements The authors are grateful to Dr. A. Cjaiolo, Dr. B. Apicella and Mrs. R. Barbella of the Institute for Combustion Research (National Research Council of Italy) who performed the chemical analyses.
Appendix. Calculation procedure for the gas-phase residence time The overall gas-phase residence time has been determined as the sum of two contributions, the residence time inside the fluidized bed and that along the freeboard region. The latter was immediately evaluated by means of the relationship tres;freeboard ¼
h hbed Qf þ Qpyr Abed
ð1Þ
where h and hbed are the reactor and the expanded bed heights, Qf is the flow rate of nitrogen used for fluidization, Qpyr is the overall flow rate of pyrolysis products (gases and liquids) and Abed is the area of transversal section of the bed. A little more complicated was the evaluation of the residence time in the bed. In order to take into account the axial distribution of PE particles along the bed only a half of the overall bed height is considered, so that tres;bed ¼
hbed =2 Vgas
ð2Þ
where Vgas is the effective gas velocity along the bed. If the two-phases fluidization theory [12] is assumed to be valid, the gas flow rate inside the bed emulsion phase is equal to that necessary for the minimum fluidization, Qmf. The same theory suggests that the bed volume is occupied by dilute (or bubble) phase for a fraction Eb, while the remaining part (1"b) is occupied by emulsion (or dense) phase, which in turn has a voidage "mf. It is possible to write Vgas ¼
Qmf ðQf Qmf Þ þ Qpyr þ Abed "mf ð1 "b Þ Abed "b
ð3Þ
The variables "mf and "b must be calculated by means of relationships available in the literature. The Carman– Kozeny equation [13] for the minimum fluidization velocity can be rearranged as:
"3mf 150Umf ¼ 2 ð1 "mf Þ d g s g
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ð4Þ
where and g are the viscosity and the density of gas, g is the acceleration due to the gravity, d and s are the diameter and the density of bed material. Eq. (4) was used for "mf determination, being Umf evaluated by means of Geldart’s relationship [14]. The fraction "b of bubble phase is instead defined as: U Umf "b ¼ ð5Þ UB þ U Umf where U is the fluidization velocity and UB is the bubble velocity. The latter is given by the relationship from Allahwala and Potter [15]: 2 3 0:5 !1:8 pffiffiffiffiffiffiffi 1=1:8 D e 43:6 5 UB ¼ 0:35 gDtgh ð6Þ D where D is the reactor diameter and De the equivalent bubble diameter. The latter is in turn given by a relationship proposed by Darton et al. in 1977 [16]. " 0:5 #0:8 AD 0:4 0:54ðU Umf Þ hþ4 N De ¼ ð7Þ g0:2 where AD and N are the transversal section and the holes number of the grid, respectively.
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