Catalytic Cracking of Waxes Produced by the Fast Pyrolysis of

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Catalytic Cracking of Waxes Produced by the Fast Pyrolysis of Polyolefins Jose´ M. Arandes,* Iker Torre, Pedro Castanõ, Martin Olazar, and Javier Bilbao Departamento de Ingenierıá Quı´mica, UniVersidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain ReceiVed September 20, 2006. ReVised Manuscript ReceiVed January 2, 2007

The cracking of the waxes obtained in the flash pyrolysis of polypropylene has been studied in laboratory FCC units under the standard conditions in FCC (fluid catalytic cracking) units. The reaction equipment is provided with a riser simulator reactor, and the experiments have been carried out using a commercial equilibrium catalyst, with a catalyst/feed ratio of C/O ) 5.5, in the 500-550 °C range and for contact times between 3 and 12 s. The effect of these operating conditions on the yields of products and on the composition of gas and gasoline lumps has been studied. The results have been compared with those of VGO (vacuum gas oil) cracking, which is the standard FCC feed in refineries, and with those of a mixture of VGO (80 wt %) and waxes (20 wt %).

1. Introduction The economic viability of waste plastic valorization processes is a subject that needs to be addressed in developed countries. Concerning the problems that delay their large-scale implementation, there are those inherent to the process of transforming plastic into monomers and fuels and to the complexity of adapting product conditioning steps to market requirements. Pyrolysis is a transformation technology with good perspectives for treating both polyolefins (2/3 of waste plastics) and remaining waste plastics, and it has undergone important development. Pyrolysis is efficient for recovering monomers and obtaining fuels with a significant reduction in gases and volatile compounds compared to gasification, with a low emission of pollutants. The processes proposed for plastic waste pyrolysis are flexible and may treat both mixtures of plastics and mixtures of these with residual materials (such as wood and agroforest wastes and tire derived fuel).1-9 It may also operate autothermally, under a controlled O2 content.10 The use of acid catalysts in the pyrolysis reactor itself effectively decreases the temperature required for cracking and for modifying product distribution.11-20 This second objective * Author to whom all correspondence should be addressed. Tel.: +3494-6012511. Fax +34-94-6013500. E-mail address: josemaria.arandes@ ehu.es. (1) Williams, P. T.; Williams, E. A. J. Inst. Energy 1998, 71, 81-93. (2) Williams, P. T.; Williams, E. A. Energy Fuels 1999, 13, 188-196. (3) Kaminsky, W.; Schmidt, H.; Simon, C. M. Macromol. Symp. 2000, 152, 191-199. (4) Kaminsky, W.; Predel, M.; Sadiki, A. Polym. Degrad. Stab. 2004, 85, 1045-1050. (5) Kaminsky, W.; Kim, J. S. J. Anal. Appl. Pyrolysis 1999, 51, 127134. (6) Predel, M.; Kaminsky, W. Polym. Degrad. Stab. 2000, 70, 373385. (7) Mastral, F. J.; Esperanza, E.; Garcıá, P.; Juste, M. J. Anal. Appl. Pyrolysis 2002, 63, 1-15. (8) Mastral, F. J.; Esperanza, E.; Berrueco, C.; Juste, M.; Ceamanos, J. J. Anal. Appl. Pyrolysis 2003, 70, 1-17. (9) Faravelli, T.; Bozzano, G.; Colombo, M.; Ranzi, E.; Dente, M. J. Anal. Appl. Pyrolysis 2003, 70, 761-777. (10) Wey, M. Y.; Lo, C. S.; Wu, S. Y.; Lee, Y. T. Waste Manage. Res. 1998, 16, 72-82. (11) Aguado, J.; Sotelo, J. L.; Serrano, D. P.; Calles, J. A.; Escola, J. M. Energy Fuels 1997, 11, 1125-1231.

is also attained through the catalytic reforming of the thermal pyrolysis product stream.21-27 Nevertheless, the economic viability of upgrading plastic waste requires large-scale operation and overall optimization of the steps involved; namely, the collection and separation of plastic waste, the pyrolysis process itself (highly endothermal), adjusting the product stream to the requirements of the fuel market and the marketing of these products. Considering these objectives, the implementation of a pyrolysis unit in a refinery already possessing an fluid catalytic cracking (FCC) unit, is proposed in this paper. The availability of the equipment for this process and for the subsequent operations of product (12) Garforth, A.; Fiddy, S.; Lin, Y. H.; Ghanbari-Siakhali, A.; Sharratt, P. N.; Dwyer, J. Thermochim. Acta. 1997, 294, 63-69. (13) Marcilla, A.; Beltrań M. I.; Conesa, J. A. J. Anal. Appl. Pyrolysis 2001, 58-59, 117-126. (14) Marcilla, A.; Beltrań, M. I.; Hernańdez, F.; Navarro, R. Appl. Catal., A.: Gen. 2004, 278, 37-43. (15) Marcilla, A.; Beltrań, M. I.; Navarro, R. J. Anal. Appl. Pyrolysis 2005, 74, 361-369. (16) Marcilla, A.; Garcıá-Quesada, J. C.; Sańchez, S.; Ruiz, R. J. Anal. Appl. Pyrolysis 2005, 74, 387-392. (17) Cardona, S. C.; Corma, A. Catal. Today 2002, 75, 239-246. (18) Ali, S.; Garforth, A. A.; Harris, D. H.; Rawlence, D. J.; Uemichi, Y. Catal. Today 2002, 75, 247-255. (19) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, J. M.; Rodrı´guez, J. M.; Morselli, L.; Palazzi, G.; Orsi, R. Appl. Catal., B.: EnViron. 2004, 49, 257-265. (20) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, J. M. J. Anal. Appl. Pyrolysis 2005, 74, 353-360. (21) Uemichi, Y.; Kashiwaya, Y.; Tsukidate, M.; Ayame, A.; Kanoh, H. Bull. Chem. Soc. Jpn. 1983, 56, 2768-2773. (22) Uemichi, Y.; Hattori, M.; Itoh, T.; Nakamura, J.; Sugioka, M. Ind. Eng. Chem. Res. 1998, 37, 867-872. (23) Mordi, R.; Fields, R.; Dwyer, J. J. Chem. Soc. Chem. Commun. 1992, 4, 374-375. (24) Ohkita, H.; Nishiyama, R.; Tochihara, Y.; Mizushima, T.; Kakuta, N.; Morioka, Y.; Ueno, A.; Namiki, Y.; Tanifuji, S.; Katoh, H.; Sunazuka, H.; Nakayama, R.; Kuroyanagi, T. Ind. Eng. Chem. Res. 1993, 32, 31123116. (25) Takuma, K.; Uemichi, Y.; Ayame, A. Appl. Catal., A.: Gen. 2000, 192, 273-280. (26) Ukei, H.; Hirose, T.; Horikawa, S.; Takai, Y.; Taka, M.; Azuma, N.; Ueno, A. Catal. Today 2000, 62, 67-75. (27) Bhaskar, T.; Uddin, M. A.; Murai, K.; Kaneko, J.; Hamano, K.; Kusaba, T.; Muto, A.; Sakata, Y. J. Anal. Appl. Pyrolysis 2003, 70, 579587.

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

separation and adjustment to market requirements is a considerable advantage. The requirements of an FCC feed are fulfilled by the waxes obtained by the pyrolysis of polyolefins. These waxes are the products obtained by incipient pyrolysis at low temperature and, consequently, are obtained with minimum energy consumption. Furthermore, they may be obtained at small scale near the points where waste is collected and classified. Removing waxes to the refinery would avoid the problems involved in the transportation of waste plastics, which are of irregular texture and low density. Likewise, centralizing the waxes transported from different geographical points to a refinery will allow for controlling and adapting their composition to the requirements of an FCC unit feed. The conical spouted bed reactor (CSBR) has suitable characteristics for obtaining waxes by flash pyrolysis of polyolefins.28-30 The cyclic movement of particles and of the vigorous gas-solid avoids bed defluidization, which is a serious problem in bubbling fluidized beds due the sticky nature of sand particles coated with polymer.31-35 The design of a CSBR is more straightforward than other reactors proposed for avoiding defluidization, such as the circulating fluidized bed reactor,36,37 the screen reactor,38,39 the screw reactor,19,20,40,41 the rotary conical reactor,42,43 the circulating sphere reactor,44,45 and the sphere stirring reactor.46 Moreover, the residence time of the product stream in a CSBR is very short (around 0.01 s), which minimizes secondary reactions and allows for recording yields of waxes of more than 92%.29 The valorization by catalytic cracking of polyolefin pyrolysis waxes has been studied in the literature under several conditions.6,47-50 In this paper, this reaction is studied under (28) Aguado, R.; Olazar, M.; Gaisań, B.; Prieto, R.; Bilbao, J. Ind. Eng. Chem. Res. 2002, 41, 4559-4566. (29) Aguado, R.; Olazar, M.; San Jose´, M. J.; Gaisań, B.; Bilbao, J. Energy Fuels 2002, 16, 1429-1437. (30) Aguado, R.; Prieto, R.; San Jose, M. J.; Olazar, M.; Alvarez, S.; Bilbao, J. Chem. Eng. Process. 2005, 44, 231-235. (31) Arena, U.; Mastellone, M. L. Chem. Eng. Sci. 2000, 55, 28492860. (32) Arena, U.; Mastellone, M. L. Powder Technol. 2001, 120, 127133. (33) Mastellone, M. L.; Arena, U. AIChE J. 2002, 48, 1439-1447. (34) Mastellone, M. L.; Arena, U. Polym. Degrad. Stab. 2004, 85, 10511058. (35) Mastellone, M. L.; Perugini, F.; Ponte, M.; Arena, U. Polym. Degrad. Stab. 2002, 76, 479-487. (36) Sodero, S. F.; Berruti, F.; Behie, L. A. Chem. Eng. Sci. 1996, 51, 2805-2810. (37) Lovett, S.; Berruti, F.; Behie, L. A. Ind. Eng. Chem. Res. 1997, 36, 4436-4444. (38) Darivakis, G. S.; Howard, J. B.; Peters, W. A. Combust. Sci. Tech. 1990, 74, 267-281. (39) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 1997, 36, 1955-1964. (40) Aguado, J.; Serrano, D. P.; Escola, J. M.; Garagorri, E. Catal. Today 2002, 75, 257-262. (41) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, E. J. Anal. Appl. Pyrolysis 2001, 58-59, 789-801. (42) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 1998, 37, 2293-2300. (43) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 1998, 37, 2316-2322. (44) Bockhorn, H.; Hornung, A.; Hornung, U. J. Anal. Appl. Pyrolysis 1998, 46, 1-13. (45) Bockhorn, H.; Hentschel, J.; Hornung, A.; Hornung, U. Chem. Eng. Sci. 1999, 54, 3043-3051. (46) Masuda, T.; Kushino, T.; Matsuda, T.; Mukai, S. R.; Hashimoto, K.; Yoshida, S. Chem. Eng. J. 2001, 82, 173-181. (47) Kaminsky, W.; Ro¨ssler, H. Chemtech 1992, 22, 108-113. (48) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Energy Fuels 1994, 8, 131-135. (49) Isoda, T.; Nakahara, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1998, 12, 1161-1167.

Arandes et al. Table 1. Vacuum Gas Oil Properties properties density, g cm-3 (at 15 °C) Conradson C, wt %

0.9007 0.61

Simulated Distillation, °C initial point 10 wt % 30 wt % 50 wt % 70 wt % 90 wt % end point

264 378 434 469 505 553 627

Type of Compound, wt % paraffins naphthenes aromatics sulfur compounds

6.8 35.6 41.1 16.5

Table 2. Simulated Distillation of Waxes wt %

T, °C

IP 5 10 30 50 70 90 95 EP

182 241 275 362 440 512 591 618

conditions similar to those of an FCC unit and, consequently, the results will be useful in refineries. The results are supplementary to those obtained in previous papers in which a study carried out on the catalytic cracking under FCC conditions of polyolefins dissolved in VGO (vacuum gas oil),51 either in pure aromatics52,53 or in an FCC stream of secondary interest such as LCO (light cycle oil).54 The main advantage of a feed made up of polyolefin waxes lies in the uniformity required in refineries, and furthermore, the plastic dissolution process is avoided. Considering that a high yield of waxes is obtained in the pyrolysis of polyolefins (between 92 and 97 wt % in the 400-500 °C range),28-30 the level of valorization of polyolefins is only slightly lower than that of dissolved polyolefins. In this paper, the effect of wax feed on the yields and compositions of FCC products is analyzed and a comparison is carried out between the results corresponding to two strategies used for feeding wax into the FCC, namely, a feed of pure waxes or a feed made up of 20 wt % waxes in a VGO (standard feed in a refinery). Likewise, operation by feeding waxes is compared to the standard one involving VGO. 2. Experimental 2.1. Feed. The waxes have been obtained by flash pyrolysis of polypropylene (PP) at 500 °C. The pyrolysis equipment is provided with a conical spouted bed reactor.28-30 Vacuum gas oil (VGO) has been used as a reference feed, which is the standard feed in an FCC unit at a Repsol S.A. refinery (Spain). The simulated distillation of VGO (Table 1) and of the waxes (Table 2) has been carried out in a GC Perkin-Elmer 8500, provided with a metallic semicapillary column, WCOT Unimetal of 5 m × 0.53 mm × 0.17 (50) Masuda, T.; Kuwahara, H.; Mukai, S. R.; Hashimoto, K. Chem. Eng. Sci. 1999, 54, 2773-2779. (51) Ng, S. H. Energy Fuels 1995, 9, 216-224. (52) de la Puente, G.; Sedran, U. Appl. Catal., B.: EnViron. 1998, 19, 305-311. (53) de la Puente, G.; Klocker, C.; Sedran, U. Appl. Catal., B.: EnViron. 2002, 36, 279-285. (54) Arandes, J. M.; Erenã, J.; Bilbao, J.; Lo´pez-Valerio, D.; de la Puente, G. Ind. Eng. Chem. Res. 2003, 42, 3952-3961.

Catalytic Cracking of Waxes

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Table 3. Catalyst Properties particle size, µm 0-20 0-40 0-80 bulk density, g cm-3 BET surface area, m2 g-1 zeolite area, m2 g-1 matrix area, m2 g-1 UCS, Å Ni, ppm V, ppm Al2O3, wt % Re2O3, wt % acidity, mmolNH3 (gcatalyst)-1 acid strength, kJ (molNH3)-1

wt % 0 7 58 0.84 122 88 34 24.27 890 2186 40.1 2.70 0.020 120

µm, and with an flame ionization detector (FID). A number average molecular weight of 363 and a weight average molecular weight of 2405 have been determined for the waxes by gel permeability chromatography. The density is 0.89 g cm-3. A clearly olefinic nature has been determined by Fourier transform infrared (FTIR) spectrophotometry. 2.2. Catalyst. CAT-1, whose properties are set out in Table 3, is an equilibrated commercial catalyst supplied by Repsol S.A. (Spain) and used in an FCC unit. The porous structure has been determined by N2 adsorption-desorption in a Micromeritics ASAP 2010 surface analyzer. The acidity has been measured by NH3 adsorption at 150 °C and by temperature programed desorption (TPD), in an SDT 2960 Simultaneous DTA-TGA (TA Instruments) thermobalance. The acid strength has been determined by combining the calorimetric and thermogravimetric measurements of NH3 ammonia differential adsorption.55 2.3. Cracking Equipment. The equipment used in this work is a riser simulator reactor (Figure 1). It is an internal recycle reactor specially designed for catalytic cracking and has been previously described.56 The equipment is easy to operate and its main characteristics are the following: (1) the ability to operate with low and precise values of contact time in the 1-10 s range; (2) a suitable feed-catalyst contact, as the reaction occurs in a dilute fluidized bed regime with perfect mixing for the catalyst and reaction mixture. The catalyst is in a basket and the gases circulate through the basket, driven by a turbine located in the upper part. At zero time, the established amount of feed is injected and a timer is activated. Once the programmed time has elapsed, a valve is opened and the reaction products pass to a vacuum chamber maintained at 300 °C. These products are sent through a thermostated line to a gas chromatograph by means of a six-port valve. The runs have been carried out under 1 atm, in the 500-550 °C range, with a catalyst/feed ratio C/O ) 5.5 by weight and with a value of contact time of 3-12 s. These conditions correspond to those of industrial FCC units. The low value of residence time minimizes the contribution of thermal cracking. 2.4. Product Analysis. Product analysis has been carried out by means of a device for reaction product sampling connected to a Hewlett-Packard 6890 chromatograph. The sampling is activated by a timer that controls the desired value of contact time. Product identification was carried out on-line by GC-FTIR using a Nicolet/ Aldrich library, by means of a FTIR Nicolet 740 SX spectrophotometer connected to a Hewlett-Packard 5890 II chromatograph. The results were verified by GC-MS (HP 5890-MS Engine with electronic ionization). In order to check and assign retention times in the chromatographic analysis of compounds in the C5-C12 range, pure compounds and mixtures were used as standards. The assignment of the retention times of the gasoline lump components has been carried out by using Alphagaz PIANO (Air Liquide) calibration standards, which consist of 19 paraffinic components, 35 isopar(55) Aguayo, A. T.; Gayubo, A. G.; Erenã, J.; Olazar, M.; Arandes, J. M.; Bilbao, J. J. Chem. Tech. Biotechnol. 1994, 60, 141-146. (56) de Lasa, H. I. Riser Simulator. US Patent 5,102,628, 1992.

affinic, 39 aromatic, 30 naphthenic, and 25 olefinic. The amount of the C5-C12 lump has been determined as that corresponding to the components with a boiling point between n-C5 paraffins (npentane) and n-C12 (n-dodecane). The coke deposited on the catalyst was measured by thermogravimetric analysis in a Setaram TG-DSC 111 calorimeterthermobalance. The deactivated catalyst from the reactor is dried at 110 °C in a nitrogen stream and subsequently subjected to combustion with air at a programmed temperature ramp (5 °C min-1) up to 700 °C.

3. Results and Discussion 3.1. Comparison of Conversions and Yields in the Cracking of VGO, Waxes, and a Mixture of VGO (80 wt %) + Waxes (20 wt %). The conversion is defined in weight percent and calculated as the sum of the yields in weight of dry gases (C1-C2), liquid petroleum gases (LPG) (C3-C4), gasoline (C5C12), and coke. Each yield is calculated as:

Yield of lump i )

mass of lump i 100 total mass in the feed

(1)

The conversions of three feeds are compared in Figure 2 for different temperatures and contact times. The conversion order observed is as follows: waxes > mixture > VGO. This order is fulfilled in the whole range of temperatures and contact times, except at the highest temperature (550 °C) and the shortest contact time (3 s), which are the conditions under which conversions are similar. This result is explained by the suitable composition of waxes, whose olefinic composition is more favorable for cracking under FCC conditions than that of vacuum gas oil, whose aromatic content is 41.1 wt % (Table 1). Figure 2 also shows that the conversion of the mixture is intermediate between those corresponding to the pure feeds. Under certain conditions, particularly at 550 °C and t ) 9 s, the increase in the conversion when 20 wt % of waxes is in the feed is a consequence of a synergetic effect of waxes on the conversion of VGO in the mixture (80 wt %). This effect is similar to that observed in the catalytic cracking of polyolefins dissolved in VGO,51 and in LCO,54,57,58 and is explained by the higher reactivity of the waxes. Consequently, the VGO in the mixture is cracked with a higher effective C/O ratio than that corresponding to pure VGO. This reactivity of the waxes contributes to a different product distribution in the cracking of the mixture, compared to the cracking of the pure components (VGO and waxes). A comparison of the yields of the different product fractions is shown in Figure 3, at 500 (graph a) and 550 °C (graph b). It is observed that the cracking of waxes produces a higher yield of gasoline and that this yield is higher in the cracking of the mixture than in the cracking of VGO. This high yield of gasoline in the cracking of waxes has already been observed by Songip et al.48 in the cracking of a similar feed, albeit for nonequilibrated HY zeolites (and consequently more active than the catalyst under FCC conditions) and at a considerably lower temperature than in this study. At 500 °C (Figure 3a), the main fraction in the product stream is LCO for the three feeds. This result is explained by the fact that the composition of waxes obtained by simulated distillation corresponds to a 20 wt % in the LCO range. Furthermore, part of the HCO fraction of the waxes is cracked to LCO. Under (57) Arandes, J. M.; Abajo, I.; Lo´pez-Valerio, D.; Fernańdez, I.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. Ind. Eng. Chem. Res. 1997, 36, 4523-4529. (58) Arandes, J. M.; Erenã, J.; Azkoiti, M. J.; Lo´pez-Valerio, D.; Bilbao, J. Fuel Proc. Technol. 2004, 85, 125-140.


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Figure 1. Diagram of the riser simulator reactor.

Figure 2. Comparison of conversions in the cracking of different feeds, for different temperatures and contact times: (black symbols-solid lines) waxes; (gray symbols-dashed lines) VGO; (white symbolsdotted lines) VGO + waxes mixture.

conditions of higher severity in the cracking (550 °C and contact times longer than 6 s), the main fraction in the product stream is gasoline, with yields of 42 wt % in the cracking of waxes (Figure 3b). On the basis of these results, there is no significant overcracking of the gasoline in the operating ranges studied. At 500 °C (Figure 3a), the yield of LPG is much lower in the cracking of waxes. These results are consistent with those of Ng,51 who also obtained a lower yield of LPG and a higher yield of gasoline in the cracking of polyethylene dissolved in VGO than in the cracking of VGO, for high contents of polyethylene in the mixture. Nevertheless, under more severe cracking conditions (550 °C, Figure 3b), the yields of LPG are closer for the three feeds, which is explained by an incipient overcracking to LPG of the gasoline obtained by wax cracking. The cracking of waxes produces lower yields (approximately 1 wt % lower) of dry gases and coke than vacuum gas oil cracking. In the case of dry gases, the reason is the low reactivity of waxes in thermal cracking,58 and in the case of the coke, the composition of the waxes (without aromatics) is less prone to generating coke precursor components. The coke yield of the waxes is within the 2.8-5.0 wt % range, which is acceptable for satisfying the energy balance of reactor and regenerator sections in an FCC unit The aforementioned comments on the comparison of cracking yields for the different feeds are reinforced when the yields for the same levels of conversion are compared (Figure 4). In addition to that detailed, Figure 4 shows that there is a synergetic effect when mixtures of VGO + waxes are cracked. This effect explains that, although the content of waxes in the mixture is 20%, the yields of gasoline and LPG for the same conversion are the average of those corresponding to the cracking of waxes and gas oil. The results in Figure 4 concerning the yields of LPG and gasoline confirm this synergetic effect. The experimental yields

Figure 3. Comparison of the yields of the different product fractions obtained by cracking different feeds: (black symbols-solid lines) waxes; (gray symbols-dashed lines) VGO; (white symbols-dotted lines) VGO + waxes mixture. (a) 500 °C. (b) 550 °C.

of gasoline are higher than the theoretical ones by a range of 5-10%. Nevertheless, the experimental yields of LPG are lower than those expected with a linear contribution of the cracking of mixture components. An explanation for this synergetic effect may lie in the increase of reactive components due to the cracking of waxes, particularly C2-C4 olefins, which react between each other and with VGO cracking products, mainly with LPG olefins, to give olefins in the gasoline range. This effect of gasoline generation is the opposite to the overcracking of standard gasoline produced in the cracking of VGO. 3.2. Comparison of Gases Composition. Table 4 shows the results of olefinicity for the LPG components corresponding to the cracking of the three feeds. The olefinicity of C3 components, and especially those of C4, obtained by cracking of waxes is considerably higher than that obtained by VGO cracking. The results for the mixture are intermediate. The increase in olefinicity as temperature is



Figure 4. Comparison of the yields of the different product fractions obtained at different conversions and temperatures by cracking different feeds: (black symbols-solid lines) waxes; (gray symbols-dashed lines) VGO; (white symbols-dotted lines) VGO + waxes mixture.

increased is due to the fact that cracking to olefins is favored by β scission.59 This is less significant in the cracking of waxes than in the cracking of VGO. Contact time has a small influence on C3 olefinicity, although this increase gives way to a decrease in the olefinicity of C4 compounds. It is noteworthy that reaction conditions, and especially temperature, have a significant effect on the composition of gases. The composition obtained at 550 °C, with propene as the main product, is similar to that obtained by Arandes et (59) Aitani, A.; Yoshikawa, T.; Ino, T. Catal. Today 2000, 60, 111117.

al.54,57,58 at the same temperature in the cracking of polypropylene dissolved in LCO. Furthermore, the effect of contact time is similar to that observed by de la Puente et al.53 in the cracking of polyethylene dissolved in toluene. Given that temperature favors the formation of propene by cracking of olefins in the gasoline fraction, the fact that there is only a small increase in its concentration as temperature is increased in the cracking of waxes is explained by its transformation into aromatics, via Diels-Alder condensations, within the gasoline fraction.54,60 C4 olefins also take part in these condensations.


Arandes et al.

Table 4. Effect of Temperature and Contact Time on the Olefinicity of the LPG Components Obtained in the Cracking of the Three Feeds 500 °C C3)/C3Total

C4)/C4Total

525 °C

550 °C

t, s

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

3 6 9 12 3 6 9 12

0.74 0.70 0.72 0.69 0.52 0.48 0.48 0.45

0.64 0.60 0.64 0.65 0.33 0.29 0.29 0.36

0.74 0.66 0.67 0.69 0.40 0.35 0.35 0.39

0.74 0.72 0.74 0.75 0.59 0.55 0.57 0.49

0.68 0.66 0.66 0.66 0.35 0.40 0.41 0.36

0.71 0.69 0.72 0.71 0.43 0.42 0.43 0.43

0.77 0.75 0.74 0.73 0.67 0.60 0.60 0.57

0.72 0.72 0.70 0.69 0.52 0.56 0.53 0.51

0.79 0.77 0.76 0.73 0.55 0.57 0.53 0.50

As temperature is increased, the concentration of linear and branched paraffins in the C4 fraction of the gases obtained from the cracking of waxes decreases considerably, whereas there are no significant changes in the concentration of olefins. There are only small changes in cis- and trans-butenes (mainly due to an increase in the production of other compounds) and an increase in butadiene concentration. In the cracking of waxes, contact time has little influence on the concentration of the C1-C3 fraction. Concerning C4 composition, it is observed that an increase in contact time and the fact that it favors hydrogen transfer reactions produces a higher concentration of paraffins at the expense of a decrease in the concentration of olefins, especially 1-butene, i-butene, and butadiene. 3.3. Gasoline Composition. Figure 5 compares the composition of the gasolines. Graph a corresponds to mild cracking conditions (500 °C and t ) 3 s), and graph b, to severe cracking conditions (550 °C and t ) 12 s). A comparison of these graphs shows that the severity in the cracking does not affect the concentration of aromatic, naphthenic, and n-paraffin compounds. Nevertheless, severity favors an increase in the con-

Figure 5. Comparison of the gasolines obtained by cracking different feeds at two severity levels. (a) Mild cracking conditions (500 °C, t ) 3 s). (b) Severe conditions (550 °C, t ) 12 s).

centration of olefins and a decrease in isoparaffins. It is observed that, comparing cracking of VGO with cracking of waxes, the latter gives way to a gasoline with a higher content of olefins, naphthenes, and paraffins, with a lower content of aromatics and a higher content of isoparaffins (at 550 °C). This is a consequence of wax composition, with olefinic linear chains,61 and of VGO composition, which is conditioned by its aromatic nature (Table 1). Moreover, it is surprising that at 500 °C the results for isoparaffins and olefins obtained in the cracking of the mixture are intermediate to those corresponding to the cracking of waxes and VGO. At 550 °C, the cracking of the mixture gives way to higher concentrations of aromatics and to lower concentrations of naphthenic compounds, n-paraffins, and olefins than the cracking of pure feeds. This result supports the hypothesis of higher reactivity of waxes, and consequently, two circumstances are forthcoming that contribute to the formation of aromatics in the gasoline range. On the one hand, the light olefins produced in the cracking of waxes react with each other to generate aromatics by Diels-Alder condensation. On the other hand, as a consequence of the high reactivity of waxes, the higher effective C/O ratio contributes to the higher cracking capacity (mainly dealkylation and breakage of bonds between aromatic rings) of LCO and HCO fractions of the VGO, which produce aromatics in the gasoline range. This increase in the concentration of aromatics decreases the concentration of other fractions. Figure 6 shows the effect of temperature (graph a, corresponding to a contact time of 6 s) and of contact time (graph b, corresponding to 550 °C) on the composition of the gasoline (wt % of aromatics, naphthenes, n-paraffins, olefins, and isoparaffins) obtained by cracking waxes. An increase in temperature from 500 to 525 °C leads to a significant amount of olefin concentration. This increase preferably favors cracking reactions by β scission of heavy olefins contained in the LCO and HCO fractions and internal reconversion reaction of the olefins contained in the gasoline fraction over the cracking of these olefins to give C3-C4 olefins in the LPG range. It is well-documented that the reactivity of olefins is higher than that of the remaining components in the FCC feed and that a size increase favors the cracking of olefins.62,63 Consequently, the LCO and HCO fractions of the waxes in the feed and the components of the product fractions formed by primary cracking of waxes, particularly olefins, are easily cracked under the conditions studied. On the basis of these (60) Negelein, D. L.; Lin, R.; White, R. L. J. Appl. Polym. Sci. 1998, 67, 341-348. (61) Chapus, T.; Cauffriez, H.; Marcilly, C. Influence of the nature of FCC feed on the production of light olefins by catalytic cracking. Presented at the Symposium on Advances in FCC, Conversion Catalysts, 211th National Meeting of the American Chemical Society, New Orleans, LA, March 24, 1996. (62) Buchanan, J. S. Catal. Today 2000, 55, 207-212. (63) den Hollander, M. A.; Wissink, M.; Makkee, M.; Moulijn, J. A. Appl. Catal., A.: Gen. 2002, 223, 85-102.


Figure 6. Effect of temperature (a) and of contact time (b) on the composition of the gasoline obtained by cracking waxes.

hypotheses, a decrease in the concentration of olefins in the gasoline as temperature is increased from 525 to 550 °C is due to the fact that an increase in the cracking severity under the conditions studied gives way to the cracking of gasoline olefins to LPG to a greater extent than their generation from LCO and HCO. It is also observed in Figure 6a that, as temperature is increased, the concentration of both paraffins (both linear and branched) and aromatics decreases. This is due to the fact that hydrogen transfer reactions are hindered. The cracking of naphthenes does not seem to be favored in the temperature range studied. The aforementioned effect of temperature is consistent with the results of Songip et al.48 at lower temperatures and using a more active catalyst (nonequilibrated). Likewise, the trends are similar to those determined by Arandes et al.54,57 in the cracking of polypropylene dissolved in LCO using different catalysts based on HY zeolite and in the same temperature range as in this paper. Figure 6b shows that an increase in reaction time gives way to a decrease in the concentration of olefins, which is partly attributed to the fact that they are cracked to LPG components and to the fact that hydrogen transfer reactions take place, which consume both olefins and naphthenes to give aromatics and paraffins (in this case branched). Aromatics are formed via Diels-Alder condensation of olefins. Table 5 shows the values of the olefinicity of C5 and C6 fractions in the gasolines. As is observed, contact time has a small effect on olefinicity (it decreases as contact time is increased). The effect of temperature increase is more significant and gives way to an important increase in the concentration of olefins. The high content of C5 and C6 olefins in the gasoline obtained by the cracking of waxes (higher than that correspond-


ing to the cracking of VGO in the case of C6 olefins) is especially important from a commercial point of view, given that these olefins are the raw material for the synthesis of oxygenated additives for gasoline (such as TAME) and in alkylation.63 The olefinicity of the two fractions is lower for the gasoline obtained by cracking the mixture, which is a consequence of two effects; on the one hand, the heavy olefins in the gasoline range are not overcracked to C5 and C6, and on the other, there are sufficient free acid sites in the catalyst for an efficient hydrogen transfer. Table 6 shows that the branching of the olefins contained in C5 and C6 fractions obtained in the cracking of waxes is significantly lower than that corresponding to the cracking of VGO, which reduces the commercial interest of this fraction. Taking into account the branching degree of C5 and C6 olefins, an increase in temperature decreases the iso-olefin/linear olefin ratio, which is explained by the attenuation of isomerization reactions.64 The effect of contact time is the opposite. The branching degree of the C5) fraction in the gasoline obtained from the mixture is similar to that obtained from VGO, and the C6) fraction is intermediate between those obtained from waxes and VGO. As is observed in Table 7, the branching degree of C5-C7 paraffins contained in the gasoline obtained by wax cracking is lower than that corresponding to the gasoline obtained by VGO cracking under mild cracking conditions, which is an inconvenience for the marketing of this gasoline due to environmental regulations concerning isoparaffinicity. The mixture of values is close to those of VGO. Furthermore, the effect of increasing contact time is noteworthy, given that it gives way to a significant rise in the paraffinicity of the gasoline obtained by cracking waxes. This effect and the lesser one corresponding to temperature increase contribute to the fact that, at long contact times and 550 °C, the isoparaffinicity of the gasoline obtained by cracking waxes is even higher than that obtained by cracking VGO. These results are explained by the selectivity of higher molecular weight olefins (those in the LCO and HCO range) in the waxes. Consequently, the C/O ratio studied (5.5), the number, and the activity of the catalyst active sites are insufficient for other isomerization and hydrogen transfer reactions, which in this case are secondary or “delayed” by the reactivity of a high concentration of heavy olefins. This situation is compensated by the increase in contact time and cracking temperature. The octane number (RON) of the gasoline obtained by the cracking of waxes and mixtures (Table 8) is similar to that that obtained by cracking VGO and is, consequently, suitable for adding to the refinery gasoline pool. These values of RON have been calculated with the correlation proposed by Anderson et al.65 on the basis of lump composition. Table 8 shows that, although contact time does not have a significant impact on the RON, an increase in temperature gives way to a considerable improvement in it. It is noteworthy that temperature increase also gives way to an improvement in the composition of the gasoline obtained by cracking waxes, given that it decreases the percentage of aromatics and increases the proportion of C5 and C6 fractions in the lump of olefins. 4. Conclusions The waxes obtained by pyrolyzing polypropylene and other olefins (given that their composition is similar) are a homoge(64) Stokes, G. M.; Wear, C. C.; Suarez, W.; Young, G. W. Oil Gas J. 1990, July 2, 58-63. (65) Anderson, P.; Sharkey, J.; Walsh, R. J. Inst. Pet. 1972, 58, 83-94.


Arandes et al.

Table 5. Olefinicity of C5 and C6 Fractions in the Gasoline Obtained by Cracking the Three Feeds, at Different Temperatures and Contact Times 500 °C C5)/C5Total

C6)/C6Total

525 °C

550 °C

t, s

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

3 6 9 12 3 6 9 12

0.38 0.36 0.36 0.35 0.46 0.44 0.43 0.42

0.32 0.30 0.33 0.33 0.28 0.26 0.25 0.24

0.31 0.27 0.25 0.27 0.27 0.24 0.24 0.22

0.54 0.50 0.53 0.53 0.56 0.52 0.55 0.48

0.45 0.45 0.41 0.39 0.38 0.37 0.35 0.34

0.35 0.35 0.35 0.35 0.32 0.32 0.32 0.34

0.58 0.49 0.51 0.46 0.59 0.54 0.51 0.49

0.59 0.56 0.54 0.52 0.55 0.52 0.50 0.49

0.46 0.48 0.41 0.40 0.41 0.41 0.39 0.38

Table 6. Branching of C5 and C6 Olefins Contained in the Gasoline Obtained by Cracking the Three Feeds, at Different Temperatures and Contact Times 500 °C i-C5)/n-C5)

i-C6)/n-C6)

525 °C

550 °C

t, s

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

3 6 9 12 3 6 9 12

0.35 0.49 0.46 0.43 0.29 0.36 0.35 0.40

0.89 0.92 0.75 0.78 1,03 0.99 0.83 0.89

1.00 1.07 1.06 1.05 0.73 0.70 0.69 0.85

0.29 0.31 0.34 0.36 0.24 0.31 0.31 0.34

0.77 0.78 0.84 0.75 0.96 0.70 0.74 0.85

0.90 0.93 0.86 0.84 0.65 0.56 0.60 0.51

0.24 0.27 0.29 0.35 0.23 0.29 0.26 0.30

0.75 0.76 0.81 0.84 0.62 0.51 0.58 0.61

0.78 0.75 0.82 0.75 0.49 0.45 0.46 0.47

Table 7. Isoparaffinicity Index of the Gasoline Obtained by Cracking the Three Feeds, at Different Temperatures and Contact Times 500 °C i-C5/n-C5

i-C6/n-C6

i-C7/n-C7

525 °C

t, s

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

waxes

VGO

VGO +waxes

3 6 9 12 3 6 9 12 3 6 9 12

3.62 4.13 4.43 4.64 3.03 3.20 3.41 3.57 2.88 4.45 4.62 4.66

9.12 9.20 9.77 9.76 5.99 5.81 6.32 4.83 2.82 3.24 3.77 2.48

7.55 8.45 8.90 9.69 5.04 6.55 7.45 7.86 2.24 2.57 2.87 2.26

2.67 3.34 3.44 3.46 2.08 2.45 2.54 2.92 4.33 3.23 5.11 5.06

8.53 7.02 6.80 7.08 5.38 4.67 4.69 5.36 3.29 2.29 3.09 3.06

7.06 7.17 7.28 7.39 5.10 5.75 5.22 5.69 2.45 2.45 2.04 2.38

3.04 3.74 4.01 4.60 2.16 3.96 4.22 4.59 3.58 3.66 5.02 5.12

6.38 4.98 5.27 5.38 4.25 2.93 3.22 3.45 3.91 2.64 3.05 2.79

6.71 6.85 7.71 7.11 4.12 5.18 7.04 4.79 2.58 2.59 3.01 2.80

Table 8. Effect of Temperature and Contact Time on the Octane Number (RON) of the Gasoline Obtained by Cracking the Three Feeds

waxes

VGO

VGO + waxes

550 °C

t, s

500 °C

525 °C

550 °C

3 6 9 12 3 6 9 12 3 6 9 12

95.1 95.5 95.4 95.2 94.0 95.1 94.2 95.7 95.4 95.2 94.9 94.6

96.8 96.4 97.8 95.6 95.6 97.1 96.6 96.6 96.6 96.8 96.6 97.5

97.9 97.9 98.5 98.5 99.1 99.4 99.5 99.1 98.2 99.4 97.9 98.1

neous feed that is easy to characterize and could be used as feedstock into FCC units. Given that the compounds present in the waxes are more easily cracked than that of VGO, its conversion is higher in the whole range of temperatures and contact times, except for the higher temperature (550 °C) and lower contact time (3 s) studied, for which conversion is similar. For high values of temperature (550 °C) and contact time (9 s), a feed made up of a mixture of VGO + waxes is especially interesting, because a synergetic effect that increases the yields of gasoline and LPG is produced, probably due to the reactivity of C2-C4 radicals generated in the primary cracking of waxes.

The conversion of waxes and the yield of the different product fractions (dry gases, coke, LPG, gasoline) are very sensitive to the values of contact time and temperature in the ranges used in catalytic cracking in FCC units. The high yield of gasoline is noteworthy, which accounts for 42 wt % for a contact time of 12 s at 500 °C and above 9 s at 550 °C. The yield of coke is within acceptable ranges for the unit and varies from 2.8 wt % at 3 s and 500 °C to 5.0 wt % at 12 s and 550 °C. This yield is slightly lower than that in VGO cracking. The LPG obtained in the cracking of waxes contains propene as the main component and the olefinicity (particularly C4)) increases as the temperature is raised, which is a consequence of the fact that cracking reactions are favored over those of hydrogen transfer. The cracking temperature has a significant effect on the composition of the gasoline obtained by cracking waxes, which compared to the cracking of the standard VGO has a higher content of olefins, naphthenes, and paraffins and less aromatic content but a higher isoparaffin one (at 550 °C). Thus, the content of olefins in the gasoline obtained by cracking waxes reaches a maximum at an intermediate temperature of 525 °C, and above this temperature, cracking of LPG is favored. A high content of olefins in the gasoline is also noteworthy, given that C5 and C6 account for approximately 50 wt % of the total olefin amount, and consequently, it is of great commercial interest. Furthermore, the content of aromatics decreases as the temper-


ature is increased, except for the content of benzene, which has a maximum value at 525 °C. An increase in contact time contributes to favoring hydrogen transfer reactions, and consequently, paraffin and aromatic concentration in the gasoline increases at the expense of a decrease in olefins, except of light C5. Likewise, the branching degree of paraffins increases considerably. The octane number (RON) of the gasoline obtained in the cracking of waxes (between 95 and 98) is adequate for its addition to the refinery gasoline pool. Although contact time does not have a significant impact on the RON (given that it does not give way to significant changes in the distribution of lumps), an increase in temperature considerably improves the


RON and composition, as it decreases the percentage of aromatics and increases the proportion of olefins (especially C5 and C6), although it also decreases the concentration of paraffins (linear and branched). Acknowledgment. This work was carried out with the financial support of the University of the Basque Country (Project 9/UPV 00069.310-13607/2001) and of the Ministry of Science and Technology of the Spanish Government (Project PPQ2003-07822). We are indebted to Petronor S.A. and Dow Chemical S.A. for their generous provision of materials EF060471S

Catalytic Cracking of Waxes Produced by the Fast Pyrolysis of

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