Article pubs.acs.org/IECR
Polyethylene Cracking on a Spent FCC Catalyst in a Conical Spouted Bed Gorka Elordi,* Martin Olazar, Pedro Castaño, Maite Artetxe, and Javier Bilbao Department of Chemical Engineering, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain ABSTRACT: The catalytic cracking of HDPE (high density polyethylene) at 500 °C using a spent FCC catalyst agglomerated with bentonite (50 wt %) has been studied in a conical spouted bed reactor. The reaction is carried out in continuous regime (1 g min−1 of HDPE is fed) with no bed defluidization problems. The results obtained, namely, total conversion, and high yields of gasoline (C5−C11 fraction) (50 wt %) and C2−C4 olefins (28 wt %), are explained by favorable reactor conditions and good catalyst properties. These results are compared with those for a catalyst prepared in the laboratory by agglomerating a commercial HY zeolite (SiO2/Al2O3 = 5.2). The conical spouted bed is a suitable reactor for enhancing the physical steps of melting the polymer and coating the catalyst with the melted polymer. Furthermore, high heat and mass transfer rates promote devolatilization, and short residence times minimize secondary reactions from olefins by enhancing primary cracking products. The meso- and macroporous structure of the spent FCC catalyst matrix enhances the diffusion of long polymer chains, whereas the zeolite crystals have micropores that give a proper shape selectivity to form the lumps of gasoline and light olefins. Because of long use in reaction−regeneration cycles, the moderate acidity of the spent FCC catalyst minimizes the secondary reactions of hydrogen transfer, and so restricts the formation of aromatics and paraffins, as well as the reactions of overcracking and condensation and, therefore, the coke formation. The spent FCC catalyst exhibits a low deactivation rate and is regenerable by coke combustion with air at 550 °C. Consequently, the use of a catalyst with the sole cost of a simple agglomeration and the production of value added product streams make the process of polyolefin catalytic cracking a promising option for refinery integration.
1. INTRODUCTION The tertiary recycling of plastic wastes by thermochemical treatments is of increasing interest for addressing the environmental problems associated with the management of these wastes and, at the same time, intensifying the use of oil by recovering monomers and producing fuel and raw materials for the petrochemical industry.1−4 Given that flash pyrolysis technologies are relatively simple, versatile, and have a limited environmental impact, they have reached a considerable level of development with a significant number of pilot-scale and demonstration plants, mainly for the upgrading of polyolefins, which account for two-thirds of waste plastics.5 The versatility of flash pyrolysis means the final upgrading of olefins, fuels, or waxes can be integrated into a conventional refinery. The treatment of feeds derived from the pyrolysis of plastic wastes and other wastes, such as biomass and tires, may be carried out in catalytic cracking (FCC), hydroprocessing, and coker units.6,7 The standard operating conditions of FCC units are suitable for cofeeding waxes (products of the flash pyrolysis of polyolefins) together with the VGO (vacuum gas oil) commonly fed into this unit. Moreover, this cofeed improves the composition of the gasoline and the yield of the C2−C4 olefin fraction.8−10 Furthermore, flash pyrolysis may be carried out in dispersed units that are located close to waste classification plants. In this case, the most interesting products are waxes (C21+), which are obtained with a high yield and reproducible composition by thermal cracking at low temperatures.11 These waxes may then be easily transported to a refinery for centralized upgrading. The operating conditions required for flash pyrolysis of polyolefins are those that minimize the physical limitations of © 2012 American Chemical Society
devolatilization: (i) high heat and mass transfer rates to enhance the melting of the plastic; (ii) high gas−solid contact area.12,13 The fluidized bed reactor allows partial achievement of the aforementioned objectives and, therefore, is the most developed technology.14−18 The conical spouted bed reactor has suitable features to enhance the melting of the plastic and the coating of either the sand (in thermal pyrolysis) or the catalyst with the melted plastic. Moreover, there is no defluidization thanks to the vigorous cyclic movement of the particles and the breakage of any incipient agglomerate in the spout. These properties allow operation in the continuous regime in the polyolefin pyrolysis, with high yields of waxes (67 wt %) at low temperatures (500 °C).19 Catalytic cracking takes place in situ on acid catalysts through a carbocationic mechanism (mainly by carbenium ions),4 instead of the free radical mechanism characteristic of the pyrolysis process (thermal cracking).20,21 Therefore, the following applies: (i) catalytic cracking takes place at a lower temperature and with a lower activation energy than the thermal process;22,23 (ii) fuels with a delimited composition or different product fractions (C2−C4 olefins, BTX) are selectively obtained by using appropriate conditions (temperature, spatial time) and catalysts.5,17,24−27 The acidity and shape selectivity of their porous structure are the properties that determine the Received: Revised: Accepted: Published: 14008
July 11, 2012 September 29, 2012 October 8, 2012 October 8, 2012 dx.doi.org/10.1021/ie3018274 | Ind. Eng. Chem. Res. 2012, 51, 14008−14017
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determined by differential scanning calorimetry (Setaram TGDSC 111) and isoperibolic bomb calorimetry (Parr 1356). The pyrolysis pilot plant has been described elsewhere.27,29 The system for feeding the polyolefin (in the form of chippings) involves a hopper (2 L) with an eccentric vibrator and a three-way ball valve connected to a half-inch pipe cooled by tap water. The spouted bed reactor, 3 L in volume, is of conical shape with a cylindrical section in the upper part for the development of the fountain (Figure 1). The total height of the
kinetic performance of the catalysts. The conical spouted bed has limited segregation, which makes it suitable for using the catalyst together with sand. Furthermore, fast and homogeneous coating of the particles with the melted plastic promotes a higher efficiency of the catalyst as compared to other reactors.28 Hence, product distribution is determined by the shape selectivity of the pores and acidity of the catalyst, without limitations, due to the physical steps of polymer melting and catalyst coating with the melted polymer.29,30 Furthermore, the HZSM-5 zeolite catalyst is suitable for selectively obtaining C2−C4 olefins (primary products of polyolefin cracking), as its shape selectivity and moderate acid strength contribute to minimizing the bimolecular reactions of hydrogen transfer and oligomerization and, therefore, hindering coke formation.31,32 Moreover, coke deposition is restricted owing to the short residence time of the volatiles in the reactor and the three-dimensional structure of the HZSM-5 zeolite, in which coke precursors are easily swept to the outside of the crystals by the high N2 flow-rate inherent to the conical spouted bed (a condition that may also be achieved in a fluidized bed reactor).33 The use of a spent FCC catalyst for waste polyolefin cracking is of special relevance, given that a no-cost waste material removed in the purge of the FCC unit is used. The upgrading of the spent FCC catalyst has been commonly carried out in the cement industry, once the metals that are considered environmentally dangerous have been extracted.34,35 Previous work carried out using FCC catalysts (either fresh or spent) in the cracking of polyolefins or their mixtures with other plastics (such as polystyrene) involves studies performed in different reactors, such as thermobalance,36,37 semibatch,38−43 and fluidized bed reactors.44−49 The results obtained evidence that spent FCC catalysts maintain significant activity for cracking polyolefins, with a high selectivity for the lumps of gasoline and C2−C4 olefins. Therefore, a polyolefin cracking unit would complement the FCC unit, which is of special relevance for integrating the cracking of waste polyolefins into a refinery. This paper studies the performance of a spent FCC catalyst in the catalytic cracking of HDPE carried out in a conical spouted bed reactor and compares it with the performance of a catalyst prepared in the laboratory using an HY zeolite. The HY zeolite is the active phase used in commercial FCC cracking catalysts, being agglomerated in a matrix with Al2O3 and SiO2/ Al2O3 in order to provide the mesoporous and macroporous structure required to enhance the diffusion of the heavy components in the feed, as well as the primary cracking of these components. The most valuable products obtained in the FCC unit (mainly C5−C10 hydrocarbons making up the gasoline pool and C2−C4 olefins within the LPG lump) and their composition is conditioned by the properties of the HY zeolite.
Figure 1. Conical spouted bed reactor diagram.
reactor, HT, is 34.00 cm, with a conical section height, Hc, of 20.05 cm, and a conical zone angle, γ, of 28°. The diameter of the cylindrical section, Dc, is 12.30 cm, the diameter of the base, Di, is 2.00 cm, and that of the gas inlet, Do, is 1.00 cm. The N2 flow rate is 12 NCL min−1, 1.2 times that corresponding to the minimum spouting velocity for catalyst particles with a size between 0.6 and 1.2 mm. The simple design of the conical spouted bed reactor is noteworthy.50−52 The dimensions of the reactor together with the stated reaction conditions guarantee bed stability in a wide range of process conditions, and they have been established in previous hydrodynamic studies of conical spouted beds with different materials.53,54 Two thermocouples are located inside the reactor, one in the bed annulus and the other one close to the wall. The reactor also has a pressure gauge for measuring total and differential pressure, which allows detection of any increase in pressure drop due to the plugging of gas filters, thus prompting their replacement. The condensation system for the volatile stream consists of a stainless-steel condenser cooled by an antifreeze mixture, which allows lowering the temperature to −10 °C and two coalescence filters (made of epoxy-ester), which retain over 99.5% of aerosol particles larger than 0.1 μm.
2. EXPERIMENTAL SECTION 2.1. Equipment and Cracking Conditions. The high density polyethylene (HDPE) has been supplied by Dow Chemical (Tarragona, Spain) in the form of chippings (4 mm) and exhibits the following properties: average molecular weight, 46 200 g mol−1; polydispersity, 2.89; and density, 940 kg m−3. The average molecular weight, Mw, polydispersity (ratio between the weight-average molecular weight and the number-average molecular weight) and density, ρ, have been provided by the supplier. The higher heating value has been 14009
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Table 1. Properties of the Acid Functions (HY Zeolite and Spent FCC Catalyst) and of the Catalysts Prepared by Agglomerating These Functions with Bentonite spent FCC catalyst SBET, m2/g Smicrop (dp < 20 Å), m2/g Vmesop (20 < dp(Å) < 500), cm3/g pore volume distribution dp(Å) < 20/20 < dp < 500/dp > 500, % dp, Å AT, μmolNH3/g Sa, kJ/molNH3
spent FCC catalyst (agglomerated)
HY zeolite
HY zeolite catalyst (agglomerated)
143 103 0.04
84 55 0.06
634 520 0.06
221 127 0.12
5.7/7.6/86.7 101 124 105
3.3/10.0/86.8 63 59 105
70/30/0 22 842 126
8.1/22.3/69.6 45 167 125
2.2. Product Analysis. The online analysis of the outlet volatile stream (very diluted in N2) has been carried out by periodically sending samples to a gas chromatograph (Agilent 6890) provided with a flame ionization detector (FID). The following temperature program has been used: (i) 4.5 min at 35 °C, for obtaining a good separation of C1−C4 hydrocarbons; (ii) a ramp of 15 °C min−1 up to 305 °C, and; (iii) 5.5 min at 305 °C to ensure that all the hydrocarbons are eluted from the HP-PONA column. The yield of each compound has been calculated from the ratio between the corresponding chromatographic area and the sum of the areas for all the compounds. Cyclohexane (0.1 mL min−1) has been used as an internal standard to validate the mass balance. Accordingly, mass balance closure is above 98% in all cases. A GC−MS (Agilent 3000 micro GC and Agilent 5975B inert MSD mass spectrometer) was used to confirm the absence of hydrogen, carbon monoxide, and carbon dioxide; to determine the ethane/ethylene and propane/propylene ratios more accurately; and to identify the lightest components (with a molecular weight lower than 100 g mol−1). Identification of the liquid fraction components (obtained by condensation of the volatile fraction) has been carried out by means of a gas chromatograph coupled with a mass spectrometer (Shimadzu QP2010S). A DB-1MS column of 60 m in length, 0.27 mm in inner diameter, and a thickness of 0.25 μm is used in the gas chromatograph. This column separates the products according to their molecular weight. The temperature sequence of the chromatograph oven has the following steps: (i) 40 °C for 2 min; (ii) a ramp of 4 °C min−1 up to 300 °C; (iii) 300 °C for 6 min. The chromatographic analysis conditions are as follows: detected mass interval, 40− 400; solvent delay, 4.5 min; injector pressure, 100.3 kPa; total flow rate, 93.2 NCmL min−1; column flow rate, 0.89 mL min−1; carrier gas linear velocity, 24.1 cm s−1; split, 1000:1, although it has been reduced in certain cases to increase the peak resolution of the scarcer components. 2.3. Catalysts. Two catalysts have been prepared, one using a spent FCC catalyst (Albemarle) supplied by a Spanish refinery (Petronor, Somorrostro, Spain) and the other, a laboratory-prepared catalyst from a commercial HY zeolite. The performance of the latter is useful as a reference, given that its composition is known in more detail than that of the commercial one. The spent FCC catalyst is collected in the purge, at the exit of the FCC unit regenerator. Therefore, it is considered a waste to be treated (it contains Cu, Ni, V, Sb, Fe, Na, RE2O3, and P2O5), with most of the particles (93 wt %) in the 20−149 μm range, and an average diameter of 81 μm. The HY zeolite is a commercial one (CBV600) supplied by Zeolyst International (Kansas, US) with a molar ratio of SiO2/Al2O3 = 5.2.
To obtain particles of a suitable size (0.6−1.2 mm diameter range) and mechanical resistance for the conical spouted bed reactor, the spent FCC catalyst and HY zeolite particles have been agglomerated by wet extrusion and then ground and sieved. The spent FCC catalyst (50 wt %) has been agglomerated with bentonite (50 wt %). It has previously been regenerated by burning the coke (0.8 wt %) with air at 550 °C for 1 h and then ground, so as to facilitate the agglomeration. The HY zeolite (25 wt %) has been agglomerated with bentonite (Exaloid) (30 wt %) and inert alumina (Martinswerk) (45 wt %). It should be noted that the spent FCC catalyst contains 16 wt % of HY zeolite and, therefore, the zeolite content of the final catalyst is 8 wt %. Moreover, the agglomeration generates meso- and macropores in the particles, which have a positive effect on attenuating deactivation because they promote coke deposition on the outside of the micropores of the HY zeolite (which has three directional channels of 7.4 Å in diameter, whose intersections make up cavities of 12.4 Å in diameter). Consequently, the external blockage of the channels is minimized, as has been verified in the cracking of polyolefins and in other processes.36,55 Prior to use, the catalyst has been calcined at 575 °C for 2 h in a N2 atmosphere. This temperature is suitable for eliminating the strong acid sites of the HY zeolite by dehydroxylation, given that these sites are hydrothermally unstable, recovering their kinetic behavior by coke combustion with air at 550 °C when they are used in reaction−regeneration cycles.56 The spent FCC catalyst is hydrothermally stable, given that it has already been subjected to numerous reaction−regeneration cycles in the FCC unit, at temperatures higher than those required in this process for cracking the polyolefins and for regenerating the catalyst by combustion of the coke formed. Furthermore, it has been verified that the catalyst bed mass remains constant, and so attrition is negligible, which has been proven in consecutive regenerations. The physical properties of the spent FCC catalyst, the HY zeolite, and the catalysts prepared by agglomeration (BET surface area, pore volume, and distribution) (Table 1) were measured by N2 adsorption−desorption (Micromeritics ASAP 2010). The macropore structure of the catalysts was measured by means of Hg intrusion porosimetry (Micromeritics Autopore 9220). Table 1 also shows the values of total acidity and average acid strength. These results and those for the acid strength distribution (Figure 2a, corresponding to the agglomerated catalysts) were obtained by monitoring the differential adsorption of NH3 simultaneously by calorimetry and thermogravimetry in a Setaram TG-DSC 111.57 The curves of NH3 temperature programmed desorption (TPD) (Figure 14010
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Figure 3. Evolution of product fraction yields with the amount of HDPE fed into the reactor: (a) spent FCC catalyst; (b) HY zeolite catalyst. Figure 2. Acid strength distribution (a) and NH3 TPD curves (b) of the catalysts.
(C11‑), heavy liquid fraction (C12−C20), and waxes (C21+). A complete conversion of HDPE is achieved and no solid products are formed in the reaction. The results correspond to the transformation of HDPE by feeding a flow rate of 1 g min−1 into a bed of 30 g of catalyst. A reaction temperature of 500 °C is recommended for avoiding operating problems due to wax formation, although these problems are of lesser significance in the conical spouted bed reactor than in the fluidized bed reactor.19,27 The origin of the x-axis corresponds to zero time on stream (the moment when the catalyst is still fresh and the HDPE starts being fed continuously into the reactor). As the flow rate of this feed is 1 g min−1, the scale for the mass of fed HDPE in g corresponds to the time on stream in min. It should be noted that the spent FCC catalyst (Figure 3a) gives way to a yield of 50 wt % at zero time on stream for the nonaromatic C5−C11 components obtained, which increases slightly with time on stream. A high yield of C2−C4 olefins is also obtained, 28 wt % at zero time on stream, which also decreases slightly with time on stream, as its formation by the cracking of the nonaromatic C5−C11 components of the gasoline fraction is affected by catalyst deactivation. These results are consistent with those obtained by Renzini et al.58 for the cracking of LDPE. They also checked that the yields of the liquid products increase and that the yield of the gas fraction decreases with time on stream due to the deactivation of the ZSM-11 and BETA zeolites. The yield of C12−C20 hydrocarbons, corresponding to a refinery diesel fraction, remains almost constant in the 8−10 wt
2b, corresponding to the agglomerated catalysts) were determined by connecting a Balzers Instruments mass spectrometer (Thermostar) online to a Setaram TG-DSC 111. Furthermore, a comparison between the acidity values for the HY zeolite and spent FCC catalyst and the corresponding values for the catalysts prepared by their agglomeration (Table 1) shows that the total acidity of the catalysts by zeolite mass unit diminishes very slightly ( C3= ≫ C2=, which is a common
% range and the yields also remain constant for the aromatic components (only single-ring), around 5 wt %, and C2−C4 alkanes, < 3 wt %. The yield of waxes (C21+) accounts for 10 wt % at zero time on stream and increases steadily with time on stream. These waxes are the main products in the thermal cracking (without catalyst) of HDPE at the same temperature (500 °C). Therefore, their high conversion (90 wt %) evidence the high activity of this catalyst prepared with 50 wt % of spent FCC catalyst for HDPE cracking, although this catalyst has only 16 wt % of HY zeolite. The attainment of high yields of gasoline fraction hydrocarbons is a feature of FCC catalysts, and although this catalyst has been withdrawn from the unit and replaced by a fresh one, its activity corresponds to the catalyst inventory in the unit. Furthermore, an especially significant advantage concerning the pyrolysis stream is the absence of S. The production of C2−C4 olefins is also a remarkable result, as tackling the increasing demand for the synthesis of polyolefins and other products is a primary aim.59 An analysis of the results obtained with the catalyst prepared in the laboratory with the HY zeolite (Figure 3b) reveals that the yield of waxes at zero time on stream is almost zero, although it increases notably with time on stream due to deactivation. The yield of nonaromatic C5−C11 components increases throughout the first 100 min of reaction to a value of 40 wt %, and then decreases slightly with time on stream, whereas the yield of the C2−C4 olefin fraction decreases slightly from an initial value of 29 wt % at zero time on stream. The yield of C12−C20 components (10 wt %) remains constant with time on stream. The yield of aromatic components initially drops sharply and then more moderately, whereas the yield of C2−C4 paraffins decreases progressively with time on stream from an initial yield of 6 wt %. The reaction conditions corresponding to Figure 3 are suitable for a comparison of the two different catalysts, both based on an HY zeolite. Although the HY zeolite content of each catalyst is significantly different (8 wt % in the catalyst prepared with the spent FCC catalyst, and 25 wt % in the HY zeolite catalyst prepared in the laboratory), the yields of the main lumps are very similar. Furthermore, total acidity and acid strength values are also higher for the catalyst prepared in the laboratory with the HY zeolite. Therefore, the similar activity of the spent FCC catalyst (although waxes are not completely cracked with the amount of catalyst used) is explained by the role of the meso- and macroporous structure. Thus, the mesoand macroporous structure promotes the cracking of HDPE macromolecules because they diffuse easily in the matrix, where they undergo a primary cracking, and the resulting oligomer products are cracked within the crystalline channels of the zeolite. The significance of the FCC catalyst matrix is also considered a key factor by other authors for explaining the high activity of FCC catalyst for polyolefin cracking in experiments carried out in different reactors, such as: thermobalance,36 semibatch reactors,38 and fluidized beds.44 The fact that the catalyst has retained a high amount of metals (mainly Ni and V) does not affect the conversion attained.32,48 Moreover, although the catalyst prepared with the spent FCC catalyst has a lower amount of zeolite, it deactivates more slowly and, therefore, the conversion of waxes and the yields of C5−C11 and C2−C4 olefin fractions are less affected. Given that the composition of the reaction environment is quite similar in the experiments with both catalysts, the aforementioned advantage must be attributed to the differences in the acidity and the porous structure. The moderate acidity of the catalyst 14012
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Figure 4. Evolution of the yields for the gas (C4−) product fraction with the amount of HDPE fed into the reactor when the spent FCC catalyst is used.
result in the cracking of polyolefins (high density and low density polyethylene, and polypropylene) on spent FCC catalysts.44−49 The low yield of C2−C4 paraffins is also wellreported, being explained by the limited activity of the catalyst for hydrogen transfer reactions. Salmiaton and Garforth49 have compared the yields of HDPE cracking products for fresh and equilibrium FCC catalysts in a fluidized bed in a semicontinuous regime, verifying that an equilibrium catalyst is much more efficient for minimizing hydrogen transfer reactions. Furthermore, the insignificant amount of methane obtained is explained by the reduced capacity of the catalyst for overcracking. The yields for the components in the C5−C11 fraction (constituted by nonaromatic and single-ring aromatic components) are shown in Figure 5 when the mass of HDPE fed into the reactor is increased and the spent FCC catalyst is used. Graph a results are displayed according to the number of carbon atoms, and those in graph b according to the type of bond. Figure 5a shows the distribution of products according to their molecular weight. As observed, the yield is higher as the number of carbon atoms in the molecules is lower: C5, 16.0 wt %; C6, 13.0 wt %; C7, 8.0 wt %; C8, 6.5 wt %; C9, 3.5 wt %; C10, 3.0 wt %; and C11, < 0.5 wt %. This trend has also been reported in other studies involving the catalytic cracking of polyolefins with spent FCC catalysts in semibatch38 and fluidized bed reactors.44 A series of diffusion control, first in the meso- and macropores of the catalyst matrix and then in the micropores of the HY zeolite, explains this fact and, therefore, a step-by-step cracking of decreasing polymer chains takes place. As observed in Figure 5b, the lightest components, C5 and C6, are mainly olefins, and the yields of paraffins and isoparaffins increase as their number of carbon atoms is higher. Single-ring aromatics account for 5.5 wt % and they are mainly C9 (with a yield of 2.3 wt %) and C10 (1.8 wt %). The yields of xylenes and toluene are around 1 wt % each, and those of benzene and C11 aromatics are lower. The high concentration of olefins and the limited concentration of aromatics in the gasoline fraction is a characteristic of the spent FCC catalysts,38,39,41 which is explained by the restricted hydrogen transfer capacity of the catalyst. Besides, its reduced capacity for overcracking explains the low yield of paraffins in the gasoline fraction and in the gas fraction (C4‑).
Figure 5. Evolution of the yields for the gasoline (C5−C11) product fraction with the amount of HDPE fed into the reactor when the spent FCC catalyst is used, ordered according to the number of carbon atoms (a) and the type of bond (b).
3.3. Gasoline Fraction Properties. Table 2 shows the properties of the gasoline fraction obtained with the two Table 2. Properties of the C5−C11 Fraction (Gasoline Fraction) catalyst FCC HY required (EU)
RON index
olefins (% vol)
aromatics (% vol.)
benzene (% vol)
96.4 96.7 95
54.2 43.3
