Effect of Atmospheric Residue Incorporation in the Fluidized Catalytic

Energy & Fuels 2008, 22, 2149–2156

2149

Effect of Atmospheric Residue Incorporation in the Fluidized Catalytic Cracking (FCC) Feed on Product Stream Yields and Composition Jose´ M. Arandes,* Iker Torre, Miren J. Azkoiti, Javier Eren˜a, and Javier Bilbao Departamento de Ingenierı´a Quı´mica, UniVersidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain ReceiVed January 15, 2008. ReVised Manuscript ReceiVed April 1, 2008

A study has been carried out on the cracking of a mixture of atmospheric residue (20 wt %) and VGO (vacuum gas-oil) from a refinery, in a riser simulator reactor under industrial unit conditions in the 525-575 °C range, with contact times between 3 and 12 s and using a commercial catalyst for residues. The yields and compositions of the product streams (dry gas, LPG, gasoline, LCO, HCO, and coke) have been compared to those corresponding to a standard feed in a fluidized catalytic cracking (FCC) unit. Catalyst accessibility renders it efficient for maintaining conversion, although overcracking of LCO and gasoline is significant above 550 °C, whereas the cracking of the HCO fraction in the residue is significantly limited. The temperature and contact time (especially the former) have a considerable effect on the gasoline composition, because of the significance of overcracking under process conditions. As the temperature is increased, the olefin concentration increases and that of the other fractions in the gasoline decreases, particularly in the case of C6-C9 aromatics.

1. Introduction The interest in improving refining increase by valorizing heavy fractions and secondary-interest streams in refineries is a priority strategy. This trend is due to several circumstances: (a) intensification in the demand for medium distillates (diesel) for transportation, light olefins for petrochemical production, and oxygenate compounds, which are used in reformulated gasoline; (b) drop in the demand for heavy distillates (fuel-oil) as a consequence of environmental regulations concerning the content of aromatics and sulfur in fuels; (c) exhaustion of crude oil resources and their substitution by heavier crude, with lower American Petroleum Institute (API) gravity and greater quantities of metals and sulfur, which requires expensive treatments (such as hydrovisbreaking) for its pumping; and (d) globalization of crude oil, instability of this market, and the uncertainty of the barrel price.1 Fluidized catalytic cracking (FCC) units are essential for converting heavy fractions into light components. Accordingly, they have been subjected to considerable innovations in the design of the feed injection, reaction-regeneration sections,2–4 and the catalyst.5–7 The progress made in the simulation, control, and optimization of the unit is also significant.8,9 * To whom correspondence should be addressed. Telephone: +34-946012511. Fax: +34-94-6013500. E-mail: [email protected]. (1) Karachi, F.; Dehkissia, S.; Adnot, A. Energy Fuels 2004, 18, 1744– 1756. (2) Marcilly, C. R.; Bonifay, R. R. Arabian J. Sci. Eng. 1996, 21, 627– 652. (3) Chen, Y. M. Powder Technol. 2006, 163, 2–8. (4) Gauthier, T.; Andreux, R.; Verstraete, J.; Roux, R.; Ross, J. Int. J. Chem. Reactor Eng. 2005, 3, A47. (5) O′Connor, P.; Imhof, F.; Yanik, S. J. Stud. Surf. Sci. Catal. 2001, 134, 299–310. (6) Kuehler, C. W.; Jonker, R.; Imhof, F.; Yanik, S. J.; O’Connor, P. Stud. Surf. Sci. Catal. 2001, 134, 311–332. (7) Siddiqui, M. A. B.; Aitani, A. M. Pet. Sci. Technol. 2007, 25, 299– 313. (8) Arandes, J. M.; Azkoiti, M. J.; Bilbao, J.; de Lasa., H. I. Can. J. Chem. Eng. 2000, 78, 111–123.

Furthermore, on the basis of the good results obtained at the laboratory and pilot plant scale, FCC unit versatility will in the future allow for treating secondary-interest streams in refineries, such as coker and visbreaker naphthas,10,11 and valorizing residual feeds, such as dissolved plastics,12,13 plastic pyrolysis waxes, either pure or dissolved in vacuum gas-oil (VGO),14,15 waxes from Fischer-Tropsch synthesis,16 and biomass-derived oxygenates.17 The term residues accounts for heavier fractions, with boiling points higher than 530 °C, which have different origins and compositions. Certain residues fed into the FCC come directly from the distillation section, such as atmospheric or vacuum distillation residues, and they have high concentrations of sulfur and nitrogen components and metals. Other residues are subjected to several treatments (thermal cracking, hydrotreatment, or deasphalting) prior to their feed into the FCC, to improve their purity and quality. Although they have a very similar composition (C, 85 ( 2 wt %; H, 12 ( 2 wt %; N and O, 1 wt %), the physical and chemical properties of the residues are very different to those of the standard FCC feed (VGO). The residues have higher concentrations of very heavy hydrocarbons, such as asphaltenes (aromatics with high contents of (9) Souza, J. A.; Vargas, J. V. C.; Von Meien, O. F.; Matignoni, W.; Amico, S. C. AIChE J. 2006, 52, 1895–1905. (10) Ferna´ndez, M. L.; de la Puente, G.; Lacalle, A.; Bilbao, J.; Sedran, U.; Arandes, J. M. Energy Fuels 2002, 16, 615–621. (11) Torre, I.; Arandes, J. M.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. Energy Fuels 2007, 21, 11–18. (12) Arandes, J. M.; Abajo, I.; Lo´pez-Valerio, D.; Ferna´ndez, I.; Azkoiti., M. J.; Olazar, M.; Bilbao, J. Ind. Eng. Chem. Res. 1997, 36, 4523–4529. (13) Arandes, J. M.; Eren˜a, J.; Bilbao, J.; Lo´pez-Valerio, D.; de la Puente, G. Ind. Eng. Chem. Res. 2003, 42, 3952–3961. (14) Torre, I.; Arandes, J. M.; Azkoiti, M. J.; Castan˜o, P.; Bilbao, J.; de Lasa, H. I. Int. J. Chem. Reactor Eng. 2006, 4, A31. (15) Arandes, J. M.; Torre, I.; Castan˜o, P.; Olazar, M.; Bilbao, J. Energy Fuels 2007, 21, 561–569. (16) Dupain, X.; Krul, R. A.; Schaverien, C. J.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 2006, 63, 277–295. (17) Corma, A.; Huber, G. W.; Sauvanaud, L.; O’Connor, P. J. Catal. 2007, 247, 307–327.

10.1021/ef800031x CCC: $40.75  2008 American Chemical Society Published on Web 05/23/2008

2150 Energy & Fuels, Vol. 22, No. 4, 2008

Arandes et al.

Table 1. Properties and Composition of VGO and Residue properties

VGO

residue

density (g/cm3) (15 °C) simulated distillation (°C) initial point 10 wt % 30 wt % 50 wt % 70 wt % 90 wt % final point sulfur (wt %) nitrogen (wt %) hydrogen (wt %) carbon (wt %) Conradson C (wt %) aniline point (°C) paraffins (wt %) olefins (wt %) naphthenes (wt %) aromatics (wt %)

0.9172

0.9263

264 378 434 469 505 553 627

0.61 85.0 6.8 35.6 41.1

381 435 474 514 0.97 0.64 11.19 86.28 9.40 3.5 48.9 42.1

S, N, and O and containing the larger molecules of the original crude, with more than 50 carbon atoms and 350 Å in size).1 This composition gives way to problems in accessing the porous structure of the catalyst,18 causes poisoning of acid sites, and favors coke generation.19 The high concentration of heavy metals (V, Ni, and Fe) and alkaline metals (Na and K) contributes to their deposition on the catalyst.20,21 Likewise, residue feeding contributes to generating SOx and NOx in the regenerator.22 The general characteristics mentioned above explain that residues are considered problematic feeds that are directly processed in a conventional FCC only if they meet certain conditions, such as Conradson carbon lower than 3 wt % and metal content lower than 30 ppm.2 Higher contents imply the need for treatments, such as thermal cracking, deasphalting, or hydrotreatment, prior to their feed in the FCC.23–27 This paper deals with the cracking of a mixture of the residue obtained by atmospheric distillation in a Spanish refinery and a VGO, which is the standard feed of a commercial FCC unit. The aim is to determine the effect of cofeeding the residue on the different product fraction yields and compositions. Accordingly, these results are compared to those for a VGO, when a specific commercial catalyst for cracking residues is used and the operation is carried out under similar conditions to those of a FCC reactor. 2. Experimental Section 2.1. Feed. The compositions of the VGO and residue (Table 1) have been determined by the supplier (Repsol YPF). The results of simulated distillation have been obtained from gas chromatog(18) Lu, Y.; Mingyuan, H.; Song, J.; Shu, X. Stud. Surf. Sci. Catal. 2001, 134, 209–217. (19) Cerqueira, H. S.; Sievers, C.; Joly, G.; Magnoux, P.; Lercher, J. A. Ind. Eng. Chem. Res. 2005, 44, 2069–2077. (20) Jeon, H. J.; Park, S. K.; Woo, S. H. Appl. Catal., A 2006, 306, 1–7. (21) Tangstad, E.; Myrstad, T.; Myhrvold, E. M.; Dahl, I. M.; Stocker, M. Appl. Catal., A 2006, 313, 35–40. (22) Chen, Y. M. Abstr. Pap. Am. Chem. Soc. 2006, 231–238. (23) Kataria, K. L.; Kulkarni, R. P.; Pandit, A. B.; Joshi, J. B.; Kumar, M. Ind. Eng. Chem. Res. 2004, 43, 1373–1387. (24) House, P. K.; Saberian, M.; Briens, C. L.; Berruti, F.; Chan, E. Ind. Eng. Chem. Res. 2004, 43, 5663–5669. (25) Gray, M. R.; McCaffrey, W. C.; Huq, I.; Le, T. Ind. Eng. Chem. Res. 2004, 43, 5438–5445. (26) Song, X.; Grace, J. R.; Bi, H.; Lim, C.; Chan, E.; Knaper, B.; McNight, C. A. Ind. Eng. Chem. Res. 2005, 44, 6067–6074. (27) Chen, J.; Ring, Z.; Dabros, T. Ind. Eng. Chem. Res. 2001, 40, 3294– 3300.

Table 2. Catalyst Properties Physical Properties particle size 0-20 µm (wt %) 0-40 µm (wt %) 0-80 µm (wt %) average particle size (µm) surface area (BET) (m2 g-1) micropore surface area (m2 g-1) average mesopore size (matrix) (Å) unit cell size (UCS) (Å)

2 4 53 83 250 159 96 24.32

Chemical Properties zeolite percentage (wt %) total acidity [mmol of NH3 (g of catalyst)-1] Bro¨nsted/Lewis ratio

23 0.042 1.7

raphy data (Hewlett-Packard 6890 provided with a metallic semicapillary column, WCOT Ultimetal of 5 m × 0.53 mm × 0.17 µm, and flame ionization detector) using a Matlab program. 2.2. Catalyst. The catalyst used is a commercial cracking catalyst supplied by Akzo Nobel and has been subjected to steaming (100% steam) in a fluidized bed for 5 h at 760 °C under atmospheric pressure. The physical properties and acidity data are set out in Table 2. The physical properties have been determined by adsorption-desorption of N2 (ASAP 2010, Micromeritics) and Hg porosimetry (Autopore 9220, Micromeritics). The acidity has been determined by combining thermogravimetry and calorimetry for the monitoring of the differential adsorption of ammonia at 150 °C in a thermobalance (Setaram SDT 2960) connected online to a mass spectrometer (Thermostar Balzers Instruments).28,29 The Bro¨nsted/Lewis ratio of the acid sites has been determined from the ratio of FTIR band areas of pyridine adsorbed at 1445 cm-1 (pyridine coordinated to Lewis sites) and 1555 cm-1 (Bro¨nsted-pyridinium ion bonds), in a Nicolet 740 SX spectrophotometer. X-ray diffraction (Phillips PW 1710 diffractometer) has allowed for determining the crystalline structure characteristic to HY zeolite, which has a unit cell size of 24.32 Å. Macropores greatly contribute to the porous structure and their distribution is shown in Figure 1. These macropores of 0.05-0.5 µm size correspond to the SiO2/Al2O3 matrix and provide capacity for cracking large molecules. The acid strength distribution (Figure 2) is evidence of a uniform site acidity, with a value of adsorption heat between 120 and 125 kJ (mol of NH3)-1. 2.3. Reaction Equipment. The equipment used in this work is a riser simulator reactor. It is an internal recycle reactor specially designed for catalytic cracking and has been previously described.30 The equipment is easy to operate, and its main characteristics are (1) the capability for operating with low and precise values of contact time in the range of 1-10 s and (2) a suitable feed-catalyst contact, because the reaction occurs in a dilute fluidized bed regime with a perfect mix for the catalyst and the reaction mixture. The catalyst is in a basket, and the gases circulate through the basket, impelled by a turbine located in the upper part. The established amount of feed is injected at zero time, 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 thermostatted line to a gas chromatograph by means of a six-port valve. The runs have been carried out at 1 atm in the 525-575 °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 (28) Aguayo, A. T.; Gayubo, A. G.; Eren˜a, J.; Olazar, M.; Arandes, J. M.; Bilbao, J. J. Chem. Technol. Biotechnol. 1994, 60, 141–146. (29) Gayubo, A. G.; Benito, P. L.; Aguayo, A. T.; Olazar, M.; Bilbao, J. J. Chem. Technol. Biotechnol. 1996, 65, 186–192. (30) de Lasa, H. I. U.S. Patent 5,102,628, 1992.

FCC Feed on Product Stream Yields and Composition

Energy & Fuels, Vol. 22, No. 4, 2008 2151 times of the components of the gasoline lump has been carried out using Alphagaz PIANO (Air Liquide) calibration standards, which consist of 19 paraffinic components, 35 isoparaffinic components, 39 aromatic components, 30 naphthenic components, and 25 olefinic components. The amount of the C5-C12 lump has been determined as that corresponding to the components with a boiling point between n-C5 paraffins (n-pentane) and n-C12 (n-dodecane). The coke deposited on the catalyst was measured by thermogravimetric analysis in a Setaram TG-DSC 111 calorimeter-thermobalance. 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 Figure 1. Macropore volume distribution of the fresh catalyst and once it has been used at 575 °C with VGO and VGO plus residue mixture feeds.

3.1. Conversion and Yields. The results for the conversion of VGO plus the residue mixture are compared in Figure 3 to those corresponding to VGO for different contact times and three temperatures, 525, 550, and 575 °C. The conversion is defined as the sum of the yields for dry gases (C1-C2), LPG (C3-C4), gasoline (C5-C12), and coke. The yield of each fraction is given by yield of lump i )

Figure 2. Acid strength distribution of the fresh catalyst and once it has been used at 575 °C with VGO and VGO plus residue mixture feeds.

Figure 3. Effect of the feed on the conversion, for different temperatures and contact times. Black points and solid lines, VGO; gray points and dashed lines, VGO plus residue. C/O ) 5.5.

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 online by gas chromatography-Fourier transform infrared spectroscopy (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 gas chromatography-mass spectrometry (GC-MS) (HP 6890-MS Engine with electronic ionization). To check and assign the 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

mass of lump i total mass in the feed

100

(1)

As observed in Figure 3, the addition of 20 wt % residue to VGO has no unfavorable effect on the conversion at temperatures lower than 550 °C but there is a considerable decrease in conversion at 575 °C, which is evidence of the higher refractory nature of HCO range components in the residues compared to the same range in the VGO. Below 550 °C, the catalyst (specific for residues and provided with active matrix) efficiently cracks the atmospheric residue. Feeding residues considerably affects the yields of product fractions. Figure 4 shows a comparison of the yields for the same values of conversion, and as observed, the cracking of the mixture produces lower yields of gasoline and LCO, whereas the yields of dry gases, LPG, HCO, and coke are higher than those corresponding to the cracking of VGO. The decrease in the gasoline yield and the increase in that of coke have also been observed by Abul-Hamayel.31 These results are explained by the high concentration of polyaromatic compounds and asphaltenes in the residue. These compounds are refractory to cracking, because their structure and high molecular volume hinder diffusion in the crystals of the HY zeolite or even in the matrix.32,33 Consequently, the actual catalyst/crackable hydrocarbons ratio is higher than in VGO, and these products undergo a more severe overcracking than in the cracking of VGO, giving way to the formation of LPG and dry gases, which is more significant at 575 °C. The greater amount of Conradson carbon in the residue explains the higher yield of coke when the mixture is cracked. Nevertheless, the higher content of coke in the catalyst compared to VGO feed has little effect on catalyst activity. Thus, as observed in Figure 3, at 525 and 550 °C, there is no difference between the results of conversion corresponding to the two feeds for different contact times. At 575 °C, the difference in conversion is maintained constant as the contact time is increased above 3 s. Nevertheless, the higher deposition of coke is expected to have an influence on selectivity, given that, in addition to deactivation of acid sites, it will contribute to the partial blockage of both micro- and (31) Abul-Hamayel, M. A.; Siddiqui, M. A. B.; Ino, T.; Aitani, A. M. Appl. Catal., A 2002, 237, 71–80. (32) Al-Khattaf, S.; de Lasa, H. I. Ind. Eng. Chem. Res. 1999, 38, 1350– 1356. (33) Al-Khattaf, S.; de Lasa, H. I. Appl. Catal., A 2002, 226, 139–153.

2152 Energy & Fuels, Vol. 22, No. 4, 2008

Arandes et al.

Figure 4. Effect of the feed on the yields of the different product fractions, for different conversions and temperatures. Black points and solid lines, VGO; gray points and dashed lines, VGO plus residue. C/O ) 5.5.

macropores. The higher accessibility limitation to the porous structure because of higher coke deposition when the residue is in the feed will be more significant at 575 °C. In fact, the yield of coke at this temperature is 10 wt %, and there is a major drop in acidity (Figure 2). Total acidity decreases from 0.042 mmolNH3 (gcatalyst)-1 for the fresh catalyst to 0.012 mmolNH3 (gcatalyst)-1 for the catalyst used with VGO and to 0.009 mmolNH3 (gcatalyst)-1 for the catalyst used with the mixture. Figure 1 shows that coke deposition largely affects mesopores, which become partially blocked. The effect is more important for the feed with the residue. Micropore blockage is significant and, consequently, the micropore surface decreases from 159 m2 g-1 for the fresh catalyst to 62 m2 g-1 for the catalyst used with VGO and 55 m2 g-1 for the catalyst used with the mixture.

These results concerning the effect of coke on the catalyst porous structure are evidence of a significant diffusional limitation, which selectively affects the conversion of the components in the HCO range and, to a lesser extent, of those in the LCO range. 3.2. Composition of Gases. The addition of the residue gives way to significant changes in the composition of the gases. The trend is to obtain less olefinic and more paraffinic compounds (Figure 5). This result has been explained in the literature by the presence of metals (mainly Ni and V) in the residue.31,34 These metals are deposited on the catalyst (by accumulation in the successive reaction cycles) and are active in dehydrogenation. The H2 produced may take part efficiently in the reactions

FCC Feed on Product Stream Yields and Composition

Figure 5. Effect of the feed on the olefinicity of C3 (graph a) and C4 (graph b) fractions of LPG, for different conversions and temperatures. Black points and solid lines, VGO; gray points and dashed lines, VGO plus residue. C/O ) 5.5.

Figure 6. Effect of the feed on the composition of the gasoline. C/O ) 5.5, 575 °C, and t ) 9 s.

of hydrogen transfer under reaction conditions. Nevertheless, the catalyst in this study has been used in only one reaction cycle, and the metal deposited is insufficient to explain the result. Figure 5 compares the olefinicity of the LPG fractions in the cracking of both VGO and the mixture. As observed, the addition of the residue decreases the concentration of propene (Figure 5a) in the whole temperature range, whereas for butenes (Figure 5b), the difference is only significant at 575 °C. This result shows that hydrogenation of butenes by hydrogen transfer (34) Alkemade, U.; Paloumbis, S. Stud. Surf. Sci. Catal. 1996, 100, 339– 354.

Energy & Fuels, Vol. 22, No. 4, 2008 2153

Figure 7. Effect of the temperature (graph a, for t ) 6 s) and contact time (graph b, for 525 °C) on the composition of the gasoline obtained by cracking the mixture VGO plus residue. C/O ) 5.5.

requires a high temperature (575 °C). Figure 5a also shows that, as the temperature is increased, the paraffinicity of C3 fraction is favored. Nevertheless, diffusional limitation in the cracking of gasoline fraction components because of high coke yields at high temperature contributes to this result. 3.3. Gasoline Composition. The concentrations of the gasoline lumps obtained in the cracking of both VGO and the mixture are compared in Figure 6. The results correspond to 575 °C and a contact time of 9 s. As observed, the gasoline from the mixture has a higher concentration of aromatics and isoparaffins and the concentration of olefins is lower with that of naphthenes and n-paraffins being similar. The difference in results is explained by an increase in hydrogen transfer, which leads to a decrease in olefin concentration and an increase in that of paraffins. In this case, the higher concentration of aromatics in the gasoline obtained by cracking the mixture is due to the dealkylation of heavy aromatics, mainly those in the LCO range. In fact, this fraction of residue in the feed is highly aromatic. The yield and composition of the gasoline fraction are primary objectives in the valorization of the residue. Figure 7 shows that both temperature and contact time have an influence on gasoline composition. The effect of these variables is very important, and the results of compositions correspond to wide ranges. The effect of the temperature increase (Figure 7a, corresponding to t ) 6 s) is a consequence of favoring β-scission reactions to give olefins. The effect of the temperature is much more pronounced than that of the contact time, and the increase in the concentration of olefins is highly significant. Moreover, as the temperature is increased, the paraffin concentration (both linear and

2154 Energy & Fuels, Vol. 22, No. 4, 2008

Arandes et al.

Figure 9. Effect of the feed on the olefinicity of C5 (graph a) and C6 (graph b) components in the gasoline, for different conversions and temperatures. Black points and solid lines, VGO; gray points and dashed lines, VGO plus residue. C/O ) 5.5.

Figure 8. Effect of the feed on the composition of the components in the gasoline, according to their carbon atom number. C/O ) 5.5, 575 °C, and t ) 9 s.

branched) decreases, as does that of aromatics, because the exothermal reactions of hydrogen transfer are hindered.

The effect of contact time is contrary to that of the temperature (Figure 7b, corresponding to 525 °C), for the above reasons (in this case, it favors hydrogen-transfer reactions). Thus, as contact time is increased, the concentration of aromatics and isoparaffins increases (the latter very slightly) at the expense of a decrease in the concentration of olefins. The compositions of the gasolines obtained by cracking both feeds under given cracking conditions are compared in Figure 8, according to the carbon atom number of the lumps. As observed, residue cofeeding produces a higher concentration of C6-C8 aromatics. These aromatics are formed by oligomerization of LPG olefins and the cracking of aromatics in the LCO-HCO range. In the case of linear paraffins, the increase in hydrogentransfer reactions gives way to higher concentrations of C5-C10 n-paraffins. The concentration of C5-C10 olefins diminishes as a result of hydrogen transfer. The more significant effect of a higher hydrogen-transfer capacity is an increase in C5 isoparaffins. Concerning the C6 fraction, the temperature has a significant effect. Thus, a feed of residues at 525 and 550 °C leads a paraffinicity decrease, whereas at 575 °C, paraffinicity increases (Figure 9). This result is explained because high temperatures favor the generation of C6 olefins by cracking heavier olefins over the hydrogen transfer. This situation is less favorable for VGO cracking, for which the ratio of these crackable olefins to the total fraction liable to cracking is lower. As observed in Figure 10, cofeeding a residue means that C5 (Figure 10a) and C6 (Figure 10b) olefins are slightly more branched than those obtained in the cracking of gas-oil. The explanation lies in the presence of refractory components that

FCC Feed on Product Stream Yields and Composition

Figure 10. Effect of the feed on the branching of C5 (graph a) and C6 (graph b) olefins in the gasoline, for different conversions and temperatures. Black points and solid lines, VGO; gray points and dashed lines, VGO plus residue. C/O ) 5.5.

give way to an increase in the C/O ratio of crackable components, and consequently, the catalyst can activate other types of reactions, apart from those of cracking, such as hydrogen-transfer reactions and olefin isomerization. Given that hydrogen transfer in linear olefins is more favorable than in branched ones, Figure 11 shows that a residue in the feed gives way to a increase in C5 (Figure 11a) and C6 (Figure 11b) paraffin branching. Table 3 shows that the octane number (RON) of the gasoline decreases by cofeeding the residue. The difference is more significant when cracking severity increases, and consequently, the greater difference is for the longest contact time (t ) 12 s) and the highest temperature studied (575 °C). This result is because, when the mixture is cracked, the decrease in olefins and the increase in paraffins are more important than the increase in aromatics and isoparaffins. The information provided in this paper on the crackability of the residue is qualitatively consistent with that obtained by de la Puente et al. in the cracking of the mixture of a residue with pure compounds (toluene and methylnaphthalene).35 Nevertheless, in the cracking of the residue plus VGO mixture, the results obtained are influenced by the complex composition of both the real refinery feeds. (35) de la Puente, G.; Devard, A.; Sedran, U. Energy Fuels 2007, 21, 3090–3094.

Energy & Fuels, Vol. 22, No. 4, 2008 2155

Figure 11. Effect of the feed on the branching of C5 (graph a) and C6 (graph b) paraffins in the gasoline, for different conversions and temperatures. Black points and solid lines, VGO; gray points and dashed lines, VGO plus residue. C/O ) 5.5. Table 3. Effect of the Feed on Gasoline RON for Different Temperatures and Contact Times (C/O ) 5.5) feed VGO

VGO plus residue

t (s)

525 °C

550 °C

575 °C

3 6 9 12 3 6 9 12

95.6 95.5 95.3 96.1 94.4 95.4 95.4 94.7

98.0 98.1 98.3 98.4 97.4 97.4 98.5 97.8

100.4 100.7 100.7 101.2 99.4 100.3 100.1 100.1

4. Conclusions Accessibility to the catalyst matrix is essential for the cracking of the residue in the mixture with the same conversion level as in the cracking of VGO up to 550 °C. Nevertheless, lower yields of gasoline and LCO and higher yields of dry gases, LPG, HCO, and coke are obtained. This result is explained by the higher actual catalyst/crackable hydrocarbons ratio in the mixture than in the VGO, and these components undergo overcracking, with the formation of dry gases and LPG, which is more significant at 575 °C. Likewise, the macropores of the matrix favor the condensation of polyaromatic structures, and more coke is formed. Despite the accessibility and acidity of the matrix, this is only efficient for cracking the LCO range but is not significant for cracking the polyaromatic structures of HCO in the atmospheric residue.

2156 Energy & Fuels, Vol. 22, No. 4, 2008

As a consequence of overcracking of the gasoline and LCO products in the cracking of the mixture, a high concentration of C3 and C4 olefins are obtained in the LPG and the temperature should be kept below 575 °C, because significant concentrations of methane and ethene are obtained at this temperature. In the cracking of the mixture, temperature and contact time (especially the former) have a significant effect on gasoline composition. Thus, as the temperature is increased in the 525-575 °C range, the concentration of olefins increases,

Arandes et al.

whereas that of the main aromatics (C6-C9) and C5 and C6 paraffins, both linear and branched, decreases. Acknowledgment. This work was carried out with the financial support of the University of the Basque Country (project GIU06/ 21) and the Ministry of Education and Science of the Spanish Government (project CTQ2006-03008). We are indebted to Petronor S. A. for its generous provision of materials. EF800031X