Cracking of Coker Naphtha with Gas−Oil. Effect of HZSM-5 Zeolite

Energy & Fuels 2007, 21, 11-18

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Cracking of Coker Naphtha with Gas-Oil. Effect of HZSM-5 Zeolite Addition to the Catalyst Iker Torre, Jose´ M. Arandes,* Miren J. Azkoiti, Martıń Olazar, and Javier Bilbao Departamento de Ingenierıá Quı´mica, UniVersidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain ReceiVed July 28, 2006. ReVised Manuscript ReceiVed October 24, 2006

A study has been carried out on the effect of operating conditions (temperature and catalyst/feed ratio) on the yields of products and on the compositions of the lumps of gases and gasoline in the cracking of a mixture consisting of heavy coker naphtha and vacuum gas-oil (20 wt % in naphtha). The runs have been carried out in a MAT (microactivity test) reactor (500-550 °C; C/O ) 4-8). The results, obtained for an equilibrated commercial catalyst in a refinery fluid catalytic cracking (FCC) unit, have been compared with those corresponding to the gas-oil feed. The presence of the naphtha in the feed has an inhibiting effect on the cracking of gas-oil and on the overcracking of the gasoline. This effect is avoided by using a sufficiently high catalyst/ feed ratio (C/O > 6), and under these conditions, the yields of products and the composition of the gasoline are similar to those corresponding to the cracking of gas-oil, although the gasoline is more aromatic. The presence of HZSM-5 zeolite in the catalyst causes a significant increase in the amount of LPG (liquefied petroleum gases) (especially propene and i-butene) and efficiently contributes to decreasing the aromaticity of the gasoline in the cracking of the mixture. Furthermore, the HZSM-5 zeolite is efficient in increasing the concentration of olefins in the C5 and C6 fractions of the gasoline obtained by cracking the mixture.

1. Introduction FCC (fluid catalytic cracking) is a key process in the financial viability of a refinery. This process was originally conceived in order to maximize the production of gasoline, and it subsequently proved to be efficient in meeting the increasing demand for light olefins.1,2 It has undergone continuous technological development for over more than 70 years, which has led to substantial modifications in the design of reaction and regeneration sections, and at the same time, the composition and properties of the catalyst have greatly evolved.3 FCC units face new challenges as a consequence of crude market instability and the increasing severity in the environmental requirements that regulate fuel composition. In view of this situation, refinery profitability requires optimizing the combined use of units for thermal cracking, FCC units and modern hydrocracking units, by a suitable transformation of streams. These need to be transformed in order to fulfill the requirements of FCC or hydrocracking feeds or because they are of low interest and the valorization to products of commercial interest, such as fuel or petrochemical raw materials, renders them profitable. Thermal cracking is applied for the valorization of atmospheric distillation residue and barrel bottom material.4 Nevertheless, the naphtha obtained by thermal cracking processes (visbreaker or coker) has a limited use as fuel due to its high content of olefins, and consequently, selective hydrogenation * Author to whom all correspondence should be addressed. Tel.: +3494-6012511. Fax: +34-94-6013500. E-mail address: [email protected]. (1) Zhao, X.; Harding, R. H. Ind. Eng. Chem. Res. 1999, 38, 38543859. (2) Buchanan, J. S. Catal. Today 2000, 55, 207-212. (3) Al-Khattaff, S.; de Lasa, H. Ind. Eng. Chem. Res. 1999, 38, 13501356. (4) Singh, J.; Kumar, M. M.; Saxena, A. K.; Kumar, S. Chem. Eng. Sci. 2004, 59, 4505-4515.

treatments are required. Moreover, the amount of this naphtha that can be fed into the hydrogenation reactor together with the naphthas from the atmospheric distillation of the crude is limited, because they are responsible for the rapid deactivation of the catalyst. An interesting option for upgrading these naphthas consists in their incorporation into the feed of FCC units.5 Knowledge on the viability for treating coker naphtha is interesting in order to ascertain the limits of FCC versatility. The results will allow for determining the viability of this transformation compared to other new alternatives, such as hydrocracking, which require new equipment and consequently major investment. Furthermore, and although implementation of hydrocracking units is growing fast, the high demand for fuels and olefins augurs a period in which FCC units that have already been amortized will be used for both the treatment of coker naphtha and other feeds of secondary interest in refineries. Likewise, other naphthas arise as possible feeds for FCCs, as they are secondary interest streams in processes, such as the hydrovisbreaking of heavy crudes (when these must be pumped long distances),6 or new streams, such as the liquid products of residual material pyrolysis (e.g., waxes obtained by pyrolysis of waste plastics).7 Hsing and Pratt8 determined that the reactivity of coker naphtha is higher than that of FCC naphtha, due to its higher content of olefins. Passamonti et al.9 propose recirculating the (5) Fernańdez, M. L.; Lacalle, A.; Bilbao, J.; Arandes, J. M.; de la Puente, G.; Sedran, U. Energy Fuels 2002, 16, 615-621. (6) Larachi, F.; Dehkissia, S.; Adnot, A.; Chornet, E. Energy Fuels 2004, 18, 1744-1756. (7) Torre, I.; Arandes, J. M.; Azkoiti, M. J.; Castanõ, P.; Bilbao, J.; de Lasa, H. I. Int. J. Chem. Reactor Eng. 2006, 4, A31. (8) Hsing, L. H.; Pratt, R. E. Cracking of FC and coker naphthas by ZSM-5 catalyst and equilibrium FC catalyst. Presented at the Symposium on Hydrogen Transfer in Hydrocarbon Processing, 208th National Meeting of the American Chemical Society, Washington, D.C., August 24, 1994.

10.1021/ef060344w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

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Table 1. Properties and Composition of the Vacuum Gas-Oil (VGO) and of the Coker Naphtha (HCN) poperties

VGO

HCN

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

0.9082

0,7737

331 399 444 477 510 552 607 0.69 0.28 85 12.5

67 109 134 152 166 183 201 1.22

26.6 47.7

40.2 20.6 9.0 30.1

C6+ fraction of FCC naphthas given its olefinic nature. Other authors have proposed the injection of naphthas into an FCC reactor at a lower position than that of the gas-oil feed in order to favor conversion to light olefins.10,11 Thus, the cracking of naphthas is produced selectively under severe conditions and the naphthas do not inhibit the cracking of gas-oil. In this paper, which is a continuation of that by Fernańdez et al.,5 the valorization of a mixture of heavy coker naphtha and gas-oil has been studied. Fernańdez et al.5 determined that the cracking of coker naphtha together with vacuum gas-oil (with 20 wt % of naphtha) is efficient for attenuating the decrease in the global yield of gasoline (as a consequence of overcracking to LPG) that is produced in the separate cracking of naphtha and gas-oil. Here, a more detailed study is carried out on the composition of product fractions and they are compared with the results for the standard feed. The effect on this transformation of modifying the catalyst by addition of HZSM-5 has also been studied. In this case, an increase in the yield of light olefins is pursued.12-14 2. Experimental Details 2.1. Feed. Table 1 shows the properties and composition of the vacuum gas-oil (VGO) and of the heavy coker naphtha (HCN), which have been supplied by the company Repsol S.A. The results of the simulated distillation have been obtained from gas chromatography data (Hewlett-Packard 6890 provided with a metallic semicapillary column, WCOT Ultimetal of 5 m × 0.53 mm × 0.17 µm, and a flame ionization detector (FID)) by means of a program written in Matlab following the ASTM D-2887 standard for the VGO and the ASTM D-3710 standard for the HCN. The composition of the VGO has been provided by Repsol S.A. (ASTM D-2786-91). The composition of the HCN has been obtained by means of the aforementioned chromatograph, with a high-resolution column (Tracer, methy silicone of 60 m × 0.20 mm × 0.50 µm). 2.2. Reaction and Analysis Equipment. The scheme of the reaction equipment provided with a MAT reactor (ASTM D-3907) is shown in Figure 1. The 400 mm reactor consists of two pieces (9) Passamonti, F. J.; de la Puente, G.; Sedran, U. Ind. Eng. Chem. Res. 2004, 43, 1405-1410. (10) Tiscornia, I. S.; de la Puente, G.; Sedran, U. Ind. Eng. Chem. Res. 2002, 41, 5976-5982. (11) Corma, A.; Melo, F. V.; Sauvanaud, L.; Ortega, F. J. Appl. Catal., A: Gen. 2004, 265, 195-206. (12) Arandes, J. M.; Abajo, I.; Fernańdez, I.; Azkoiti, M. J.; Bilbao, J. Ind. Eng. Chem. Res. 2000, 39, 1917-1924. (13) den Hollander, M. A.; Wissink, M.; Makkee, M.; Moulijn, J. A. Appl. Catal., A: Gen. 2002, 223, 85-102. (14) den Hollander, M. A.; Wissink, M.; Makkee, M.; Moulijn, J. A. Appl. Catal., A: Gen. 2002, 223, 103-119.

of 12.5 mm internal diameter coupled together. In the lower part, there is a pressure indicator (Trafag) in the 0-2.5 bar range. It is heated by three independently controlled resistances, with the middle one controlling the temperature in the reactor. This temperature is measured at three positions in the bed with Omega K type thermocouples. The flowrate meters for N2 and for the synthetic air used for in situ coke combustion (Bronkhorst HI-TEC, model EL-FLOW) have capacities of 100 and 50 mL min-1, respectively. The feed is impelled by a pulse pump (Gilson 307 pump 5SC) that feeds 2 mL min-1 with a sensitivity of 10-4 mL min-1. Data acquisition and control of the equipment is carried out with a customized program (Adkir), which is sequenced in three different sessions. Mass balance closure is carried out by measuring the amount of liquid products condensed in a Peltier and the volume of noncondensable gases. The coke deposited on the catalyst is quantified by combustion with synthetic air at 600 °C and by measuring the amounts of water and carbon dioxide, which are adsorbed on ascarite and drierite, respectively. Product identification has been carried out by means of two techniques: (1) gas chromatography-mass spectrometry (HewlettPackard-5890 II and MS-Engine, Hewlett-Packard, respectively) on-line and (2) injection of pure patterns. The components of the gasoline fraction have been identified by injecting calibration standards (Alphagaz PIANO from Air Liquide), which consist of 29 n-paraffins, 35 isoparaffins, 38 aromatic components, 30 naphthenic, and 25 olefinic. Product quantification has been carried out by gas chromatography (Hewlett-Packard 6890) provided with an FID and a high-resolution capillary column (Tracer, methyl silicone of 60 m × 0.20 mm × 0.50 µm). The carrier gas is He (20 cm s-1). The research octane number (RON) of the gasoline has been determined from the chromatographic results by means of the correlation of Anderson et al., who defined 29 groups with different molecular ranges.15 2.3. Catalysts. Two catalysts have been used. CAT-3 is a commercial cracking catalyst that has already been used and equilibrated in a refinery FCC unit (Petrobras S.A.) and is appropriate for maximizing olefin yield. CAT-H32 is a hybrid catalyst made up of a physical mixture of 75 wt % of CAT-3 and 25 wt % of a catalyst (CAT-Z2) prepared based on a HZSM-5 zeolite (SiO2/Al2O3 ) 50), which has been agglomerated (25 wt % zeolite) by wet extrusion with bentonite as binder (45 wt %) and Al2O3 as inert charge (30 wt %). CAT-Z2 is subsequently calcined and subjected first to milling and sieving and then to a hydrothermal equilibration process by steaming. The physical properties and acidity data of the two catalysts are set out in Table 2. The physical properties have been determined by adsorption-desorption of N2 (ASAP 2100, Micromeritics). Clearly, the micropore sizes are those corresponding to HY and HZSM-5 zeolites, around 7.5 and 5.5 Å, respectively. The mesopores are those corresponding to the matrix in CAT-3 and those of the bentonite used as binder in CAT-Z2. The total acidity and the distribution of the acid strength (around 100 kJ molNH3-1 for a significant proportion of sites) (Figure 2a) are similar for both catalysts. These properties have been determined by combining thermogravimetry and calorimetry for the monitoring of the differential adsorption of ammonia at 150 °C. These results are in agreement with those obtained by TPD (temperature programmed desorption) of NH3 (Figure 2b). The equipment used for the acidity measurement is a thermobalance (Setaram SDT 2960) connected on-line to a mass spectrometer (Thermostar Balzers Instruments). The Bro¨nsted/Lewis ratio of the acid sites has been determined from the ratio of Fourier tranform infrared (FTIR) band areas of pyridine adsorbed, at 1445 cm-1 (pyridine coordinate to Lewis sites) and 1555 cm-1 (Bro¨nsted-pyridinium ion bonds), in a Nicolet 740 SX spectrophotometer. (15) Anderson, P.; Sharkey, J.; Walsh, R. J. Inst. Pet. 1972, 58, 83-94.

HZSM-5 Zeolite Addition to the Catalyst

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Figure 1. Scheme of the reaction equipment. Table 2. Catalysts Properties catalyst

CAT-3

CAT-Z2

Physical Properties particle size 0-20 µm (wt %) 0-40 µm (wt %) 0-80 µm (wt %) average particle size (µm) surface area (BET, m2g-1) micropore surface area (m2g-1) average mesopore size (matrix, Å) unit cell size (UCS, Å)

0 0 39 87 102 72 208 24.23

52 74 98 38 182 63 103

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

12 0.026 5.4

25 0.024 1.9

3. Results 3.1. Yields of the VGO+HCN Mixture Cracking and Comparison with Those of Separate Cracking of VGO and Naphtha. The yields of dry gases (C1-C2), LPG (C3-C4), gasoline (C5-C12), and coke obtained for the feeds of VGO, HCN, and the mixture of 80 wt % VGO+20 wt % HCN are compared in Figure 3. Each graph corresponds to a temperature. The yield of each product fraction i is defined:

Yield of (i) )

mass fraction of (i) × 100 total mass in the feed

(1)

The results show that the feed with 20 wt % of HCN leads at 500 °C (Figure 3a) and 525 °C (Figure 3b) to an increase in the yield of gasoline over that corresponding to the feed of gasoil. This increase is more relevant for high values of C/O ratio

(catalyst/feed, in mass). Above C/O ) 6, the yield of gasoline increases, as does naphtha in the feed. The explanation is that naphtha components are cracked more selectively than the heavy components of the gas-oil, and only when the number of acid sites is high (high C/O) are these heavy gas-oil components cracked to give gasoline fraction components. This interpretation is confirmed when the yields of LPG are compared for the different feeds. It is observed that for high values of C/O the yield of LPG obtained by cracking the mixture at 500 and 525 °C is even lower than that corresponding to the cracking of pure gas-oil and naphtha. This shows that, in the cracking of the mixture, the cracking of gas-oil components to give gasoline is delayed and this hinders the subsequent cracking of part of this gasoline to give LPG. Moreover, the high yield of LPG in the cracking of gas-oil, higher than in the cracking of naphtha, is evidence that LPG is obtained in significant quantities from the cracking of the components within the ranges of LCO (light cycle oil) and HCO (heavy cycle oil). The decrease in the yield of LPG in the cracking is the basis for establishing the hypothesis that there is an inhibiting effect of the naphtha also in the direct cracking of the gas-oil LCO and HCO to LPG. It is noteworthy that the results of yield to coke in the cracking of the mixture are similar to those of gas-oil cracking, which is a feature of the experimentation in the MAT reactor, where the generation of both thermal and catalytic coke are favored.16 The inhibiting effect of naphtha in the generation of gases is shown in Figure 4, in which the mass ratio between the fractions (16) ) Ng, S.; Yang, H.; Wang, J.; Zhu, Y.; Fairbridge, C.; Yui, S. Energy Fuels 2001, 15, 783-785.


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Figure 2. (a) Distribution of the acid strength of the catalysts. (b) TPD of NH3.

of gases (dry gases + LPG) and gasoline are plotted for the cracking of the three feeds. The results correspond to 525 °C. As is observed, gas-oil tends to generate gases, and this generation is even more evident for high values of the C/O ratio, which is a consequence of gasoline overcracking. In the case of mixture cracking, the effect of the C/O ratio is similar to naphtha cracking, which again is evidence of that mentioned above concerning the preferable cracking of naphtha and the inhibition in the cracking of the gasoline generated in the cracking of the gas-oil present in the mixture. 3.2. Composition of Gases. Figure 5 shows the results of gas composition (dry gases + LPG) obtained in the cracking of the three feeds, for the different temperatures. The results correspond to C/O ) 4. As is observed in Figure 5a, the concentration of propene is similar for the three feeds and is lower in the cracking of the mixture of VGO and HCN. The concentration of the other main product, i-butane (Figure 5b), is much higher in the cracking of naphtha than in gas-oil, and the result of the mixture cracking is only slightly higher than that corresponding to gas-oil. The effect of the feed on the concentration of n-butane is qualitatively similar. The concentrations of the isomers cis-, trans-, and i-butene (Figure 5c) follow the same order: gas-oil > mixture > naphtha. The fact that the composition of LPG obtained by cracking the mixture is very similar to that corresponding to the cracking of gas-oil confirms the above comment whereby most of the LPG obtained by cracking, either from gas-oil or from the

Figure 3. Comparison of product fraction yields obtained in the cracking of the different feeds: CAT-3. (a) 500. (b) 525. (c) 550 °C: black points-solid line ) HCN; gray points-dashed line ) VGO; empty points-dotted line ) mixture 80% VGO + 20% HCN.

mixture, is generated in the intermediate cracking of HCO range components to lower fractions. As is observed in Table 3, the olefinicity of the LPG obtained by cracking the mixture is very similar to that corresponding to the cracking of gas-oil, and both have a higher olefinicity than the LPG obtained in the cracking of naphtha.



Figure 4. Comparison of gases/gasoline ratio as percentage for different feeds and C/O ratios: 525 °C; CAT-3.

3.3. Gasoline Composition. The composition of the gasolines obtained for the three feeds is compared in Figure 6. The results correspond to 525 °C and C/O ) 4. A comparison of the different group compositions shows that, except in the case of aromatics, which is slightly higher in the gasoline from the mixture than in that from the naphtha, in the remaining fractions the concentrations of this gasoline are intermediate between those obtained from naphtha and VGO. Nevertheless, the effect of cracking the naphtha (20% in the mixture) has a greater influence on the result than that corresponding to its percentage in the feed. This effect of the naphtha is very evident in the composition of n-paraffins and olefins, which shows that when the mixture is cracked these components are the main ones, due to the fact that cracking of naphtha predominates over that of gas-oil. A more detailed comparison of the composition of the gasoline produced from the three feeds is obtained by analyzing the distribution of components with different carbon atoms. Thus, concerning the concentration of aromatics, C11 components have a higher concentration in the gasoline of the mixture than in the gasoline of the naphthas. Concerning the other aromatics in the gasoline of the mixture, the aromatic nature of the naphthas predominates. This result is evidence, on the one hand, of the refractory nature of naphtha aromatics and, on the other, of the fact that the cracking of the mixture generates aromatics from the gas-oil, either by dealkylation of LCO range aromatics or by cyclization-dehydrogenation of gasoline range olefins. Concerning n-paraffins, most of the paraffins present in the gasoline obtained from the mixture come from the naphthas, given that they have a high concentration of paraffins. This is supported by the following hypotheses: (1) In the cracking of naphtha, as has been mentioned above and is supported in the literature,17 paraffins of high molecular weight are those that preferably react to give olefins, whereas light paraffins do not crack. (2) In the cracking of pure gas-oil, the formation of n-paraffins is not significant. (3) In the cracking of the mixture, olefins are preferably cracked, which has a pronounced inhibiting effect on the cracking of high molecular weight n-paraffins.5 Concerning olefins, the gas-oil is a major generator of olefins, especially of light olefins. The presence of olefins in the naphthas and their preferable cracking hinders olefin (17) Chapus, T.; Cauffriez, H.; Marcilly, C. Influence of the nature of FCC feed on the prodution of light olefins by catalytic cracking. Presented at the Symposium on Advances in FCC, 211th National Meeting of the American Chemical Society, New Orleans, LA, March 24, 1996.

Figure 5. Comparison of gas concentration in the cracking of the three feeds for different temperatures: C/O ) 4; CAT-3; black points-solid line ) HCN; gray points-dashed line ) VGO; empty points-dotted line ) mixture 80% VGO + 20% HCN.

generation in the cracking of the mixture due to the aforementioned inhibiting effect on gas-oil cracking. Likewise, the effect of the feed on i-paraffin composition of the corresponding gasoline is very small. Table 4 shows the values of olefinicity of C5 and C6 fractions of the gasolines obtained with the different feeds for different values of the C/O ratio. It is observed that the content of olefins in the gasoline obtained from the mixture decreases as the C/O ratio is increased and approaches the value corresponding to


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Table 3. Olefinicity of LPG Components for Different Feeds and C/O Ratios at 525 °C with CAT-3 C3)/C3Total C4)/C4Total

Feed

C/O ) 4

C/O ) 6

C/O ) 8

HCN VGO VGO + HCN HCN VGO VGO + HCN

0.75 0.79 0.78 0.37 0.55 0.50

0.66 0.77 0.77 0.26 0.47 0.45

0.64 0.72 0.71 0.27 0.42 0.37

Table 4. Olefinicity of C5 and C6 Components of the Gasolines for Different Feeds and C/O Ratios at 525 °C with CAT-3 C5)/C5Total C6)/C6Total

T (°C)

C/O ) 4

C/O ) 6

C/O ) 8

HCN VGO VGO + HCN HCN VGO VGO + HCN

0.19 0.49 0.33 0.07 0.30 0.18

0.12 0.34 0.25 0.05 0.22 0.17

0.05 0.13 0.12 0.05 0.19 0.14

Figure 6. Composition of the gasolines obtained with different feeds: 525 °C; C/O ) 4; CAT-3.

Table 5. Isoparaffinicity of the Gasolines for Different Feeds and C/O Ratios at 525 °C with CAT-3 i-C6/n-C6 i-C7/n-C7 i-C8/n-C8

feed

C/O ) 4

C/O ) 6

C/O ) 8

HCN VGO VGO + HCN HCN VGO VGO + HCN HCN VGO VGO + HCN

3.18 8.44 5.84 1.05 8.39 3.69 0.62 4.80 2.00

3.23 7.64 5.97 0.97 9.31 3.70 0.53 5.77 1.91

2.84 7.75 4.73 0.84 8.50 3.11 0.45 5.52 1.65

Table 6. Values of RON of the Gasolines for Different Feeds and C/O Ratios at 525 °C with CAT-3 feed

C/O ) 4

C/O ) 6

C/O ) 8

HCN VGO VGO + HCN

83.1 95.4 91.3

83.4 94.6 91.0

83.9 93.8 91.8

the gasoline from the cracking of gas-oil. This result again indicates that as the C/O ratio is increased most of the acid sites mitigate the inhibition of the naphtha in the cracking of the gasoline LCO range to give olefins in the gasoline range. Nevertheless, the results in Figure 4 show that this number of acid sites is insufficient to overcrack these C5-C6 olefins to gases. When the isoparaffinicity of the gasolines is studied (Table 5), it is observed that results for the gasoline obtained from the mixture are highly influenced by the cracking of the naphtha, given that there is a large amount of n-paraffins in the naphtha, which are of low reactivity, especially when there are olefins liable to be cracked in the gas-oil LCO.5 The RON values of the gasolines obtained in the cracking of the mixture (Table 6) are intermediate between those obtained by cracking naphtha and gas-oil separately, but they are closer to those of the gasoline obtained from the gas-oil. The reason for this is the high concentration of aromatics obtained when the mixture is cracked. 3.4. Effect of HZSM-5 Zeolite in the Catalyst. The incorporation of HZSM-5 zeolite in the catalyst has a significant effect on the yields of the different fractions. As is observed in Figure 7, the hybrid catalyst CAT-H32 (CAT-3+CAT-Z2) produces a lower yield of gasoline and a higher yield of LPG (by gasoline overcracking) than CAT-3. The explanation is the combination of a higher shape selectivity and a more moderate acid strength of the HZSM-5 in the catalyst CAT-Z2 (which has been severely equilibrated) than the HY zeolite in CAT-3. The shape selectivity of the HZSM-5 zeolite limits the access of voluminous molecules that contain the LCO and HCO ranges

Figure 7. Effect of hybrid catalyst use on the product fraction yields in the cracking of the mixture of 80% VGO + 20% HCN, for different temperatures: black points-solid line ) CAT-3; empty points-dashed line ) CAT-H32; C/O ) 6.

of the feed.18-20 Furthermore, the moderate acid strength of HZSM-5 zeolite is efficient for cracking the accessible fraction of the gasoline to LPG but limits cracking to dry gases and coke generation.2,12-14 Nevertheless, this effect of limiting the internal transfer of LCO and HCO in the HZSM-5 zeolite has a lower importance regarding the transformation of HCO, whose diffusion is also severely limited in the HY zeolite and whose cracking takes place mainly on the catalyst matrix. Figure 8 shows the effect of hybrid catalyst use on gas composition. This effect is important, given that it significantly increases the concentration of all the olefins, especially propene and i-butene. This result is relevant because CAT-3, having a unit cell size (UCS) of 24.23 Å (determined by X-ray difractometry), is a commercial catalyst designed for promoting the production of light olefins. The increase in olefins is at the expense of a decrease in the concentration of methane, propane, butane, and i-butane, whereas the concentration of ethene is similar for both catalysts. (18) Nalbandian, L.; Lemonidou, A. A.; Vasalos, I. A. Appl. Catal., A: Gen. 1993, 105, 107-125. (19) Adewuyi, Y. G.; Klocke, D. J.; Buchanan, J. S. Appl. Catal., A: Gen. 1995, 131, 121-133. (20) Aitani, A.; Yoshikawa, T.; Ino, T. Catal. Today 2000, 60, 111117.



Figure 9. Effect of hybrid catalyst use on the composition of the gasoline produced in the cracking of the mixture of 80% VGO + 20% HCN; 525 °C; C/O ) 6. Table 7. Effect of the Hybrid Catalyst on the Olefinicity of LPG Components for Different Temperatures with C/O ) 6 )/C

C3

3Total

C4)/C4Total

catalyst

500 °C

525 °C

550 °C

CAT-3 CAT-H32 CAT-3 CAT-H32

0.76 0.84 0.42 0.53

0.77 0.85 0.45 0.57

0.70 0.84 0.43 0.58

Figure 8. Effect of hybrid catalyst use on the gas composition produced in the cracking of the mixture of 80% VGO + 20% HCN, for different temperatures: black points-solid line ) CAT-3; empty points-dashed line ) CAT-H32; C/O ) 6.

interest of these olefins is increasing for the production of oxygenate additives in order to raise the RON index, such as methyl tert-butyl ether (MTBE) from methanol and i-butene,21,22 ethyl tert-butyl ether (ETBE) from ethanol and i-butene,23 and di-isopropyl ether (DIPE) from propene and water (with diisopropyl ether as intermediate).24 Figure 9 shows the effect of the hybrid catalyst in the composition of the gasoline. The results correspond to 525 °C and C/O ) 6. It is observed that the concentration of aromatics is lower with the hybrid catalyst, whereas higher concentrations of olefins and naphthenes and a lower concentration of iparaffins are obtained. The concentration of n-paraffins is also slightly higher with the hybrid catalyst. The lower concentration of aromatics is explained by the hybrid catalyst characteristics that limit the three routes for aromatic formation in the gasoline: first, hydrogen transfer limitations in the HZSM-5 zeolite; second, the diffusional limitations of heavier aromatics (contained in LCO and HCO ranges), which generate the gasoline aromatics by breakage of the bonds between rings; and third, in spite of the welldocumented ability of the HZSM-5 zeolite for activating olefin oligomerization reactions and aromatic generation by DielsAlder condensation,25,26 the acid strength limitation of the HZSM-5 zeolite in the hybrid catalyst (Figure 2), due to the severe equilibration of the catalyst, which is insufficient for oligomerization reactions and aromatic generation. With respect to the composition of the gasoline obtained by cracking the mixture, although benzene and toluene have slightly

This result whereby the HZSM-5 zeolite favors the formation of olefins is well-documented in the literature2,12-14,19,20 and is attributed first to the steric hindrance of hydrogen transfer reactions in the HZSM-5 zeolite, due to its shape selectivity, and second to its isomerization ability. The above results concerning olefinicity improvement of LPG components when HZSM-5 zeolite is incorporated in the catalysts are quantified in Table 7. It is noteworthy that the

(21) Ali, M. A.; Brisdon, B. J.; Thomas, W. J. Appl. Catal., A: Gen. 2000, 197, 303-309. (22) Dıáz, R.; Hernańdez, A.; Quintana, R.; Cabrera, C.; Dıáz, E. Catal. Today 2001, 65, 373-380. (23) Collignon, F.; Poncelet, G. J. Catal. 2001, 202, 68-77. (24) Ancillotti, F.; Fattore, V. Fuel Process. Technol. 1998, 57, 163194. (25) Chen, C. S. H.; Bridger, R. E. J. Catal. 1996, 161, 687-693. (26) Dutta, P.; Roy, S. C.; Nandi, L. N.; Samuel, P.; Pillai, M.; Bhat, B. D.; Ravindranathan, M. J. Mol. Catal. A: Chem. 2004, 223, 231-235.


Torre et al.

Table 8. Effect of the Hybrid Catalyst on the Isoparaffinicity of C5-C8 Fractions of the Gasolines Obtained by Cracking the Mixture of VGO + HCN at 525 °C with C/O ) 6 i-C5/n-C5 i-C6/n-C6 i-C7/n-C7 i-C8/n-C8

CAT-3

CAT-H32

7.11 5.97 3.70 1.91

10.29 6.62 1.74 1.11

higher concentrations with the hybrid catalyst, the presence of heavier aromatics is favored by the greater ability of CAT-3 for cracking (mainly dealkylation) compounds in the LCO range (and to a lesser degree within HCO, with breakage of bonds between rings) of the gas-oil contained in the mixture. The n-paraffins obtained with the hybrid catalyst have a high molecular weight, which is due to the fact that they are mainly those remaining in the naphtha fraction of the mixture without being cracked and this cracking is more efficient with CAT-3, given that the HY zeolite has a larger pore size than the HZSM-5 catalyst. The concentration of C5 and C6 paraffins is higher in the gasoline obtained with CAT-3 as they are less crackable than the heavy ones,17 and their continuance in the reaction medium is favored by the hydrogen transfer ability of CAT-3. The hybrid catalyst significantly promotes the production of C5 and C6 olefins as a consequence of a decrease in the hydrogen transfer capacity and of the contribution of the HZSM-5 zeolite to increasing the cracking of C10-C12 olefins in the gasoline range.2,27 This result is particularly relevant because when the hybrid catalyst is used C5-C6 olefins represent approximately 2/ of the whole amount of olefins in the gasoline range. C 3 5 and C6 olefins are of great commercial interest, given that they are raw materials in the alkylation and production reactions of TAME (tert-amyl methyl ether).9,28 The C5)/C5 ratio is 0.40 with the hybrid catalyst, whereas it is 0.25 with CAT-3. Likewise, the C6)/C6 ratio is 0.25 with the hybrid catalyst and 0.17 with CAT-3. The higher concentration of C5 and C6 i-paraffins (Table 8) is due to a more selective activity of the HZSM-5 zeolite for isomerization of these olefins than for cracking. This selectivity is a consequence of a high SiO2/Al2O3 ratio (50) and the consequent moderation of HZSM-5 zeolite acid strength.13,14 For the heavier i-paraffins, the higher hydrogen transfer ability of CAT-3 is evident. These results agree with those of the literature for hybrid catalysts used in the cracking of gas-oil.18,27 The octane index (RON) obtained in the cracking with the hybrid catalyst is higher (92.2 at 525 °C) than that obtained with CAT-3 (91.0). This is a consequence of the above results concerning a greater amount of olefins and light i-paraffins, naphthenes, and aromatics. 4. Conclusions The feed made up of heavy coker naphtha and standard gasoil produces a more selective cracking of naphtha components than that made up of VGO. Nevertheless, with a sufficiently high catalyst/feed ratio (above 6) and due to catalyst availability for cracking heavy gas-oil fractions under these conditions, the yield of gasoline with the mixture is higher than that in the cracking of a gas-oil feed. As a consequence of the inhibiting effect of the naphtha, overcracking of gasoline is reduced in the crack(27) Buchanan, J. S.; Adewuyi, Y. G. Appl. Catal., A: Gen. 1996, 134, 247-262. (28) Stokes, G. M.; Wear, C. C.; Suarez, W.; Young, G. W. Oil Gas J. 1990, 88 (27), 58-63.

ing of the mixture and the yields of LPG and dry gases decrease compared to those corresponding to the cracking of gas-oil. The olefinicity of the LPG obtained by cracking the mixture is similar to that corresponding to the cracking of gas-oil, with a slightly lower concentration of propene. The composition of the gasoline obtained by cracking the mixture reflects its origin from the cracking of naphtha heavy components, and although it is slightly more aromatic than the gasoline from gas-oil cracking, its concentration of naphthenes, n-paraffins, i-paraffins, and olefins is intermediate between those corresponding to the cracking of gas-oil and naphtha separately (although closer to the gasoline obtained by naphtha cracking than that corresponding to a feed with 20% naphtha in the mixture). This conclusion on the intermediate nature of the gasoline obtained by cracking the mixture (intermediate between those corresponding to the cracking of the individual feeds) may also be extended to other key aspects concerning gasoline quality, such as the olefinicity and branching degree of either olefins or C5 and C6 paraffins. Consequently, the RON index of the gasoline from the mixture is only slightly lower than that from the cracking of the standard gas-oil feed, with the inconvenience of a slightly higher concentration of aromatics. The use of a hybrid catalyst, prepared by physical mixture of a commercial catalyst with one prepared based on HZSM-5 zeolite, shows that in spite of the severity in the equilibration of the latter (and of its low acidity and acid strength), the HZSM-5 zeolite is very active in the overcracking of gasoline to LPG and gives way to an increase in the concentration of olefins compared to that corresponding to the base catalyst. The presence of HZSM-5 zeolite gives way to a significant increase in the concentration of propene and i-butene, which is especially attractive from the commercial perspective of these products (i-butene for the synthesis of oxygenate gasoline additives, such as MTBE and ETBE). It is noteworthy that the use of HZSM-5 zeolite as additive efficiently contributes to decreasing gasoline aromaticity, by attenuating this problem in the cracking of naphtha. This result is explained by the shape selectivity of the HZSM-5 zeolite, by its reduced ability for activating hydrogen transfer reactions, and by its moderate acid strength (as a consequence of the severe hydrothermal equilibration). The hybrid catalyst significantly promotes the production of C5 and C6 olefins in the cracking of the mixture, and in fact, they account for as much as 2/3 of the whole amount of gasoline olefins. The C5)/C5 ratio is 0.40 for the hybrid catalyst, whereas it is 0.25 for the commercial catalyst (CAT-3). Furthermore, the C6)/C6 ratio is 0.25 for the hybrid catalyst and 0.17 for CAT-3. This result is relevant from a commercial perspective, given that these olefins are especially interesting as raw materials in the alkylation and production reactions of TAME. As a consequence of the predominant effect of increasing olefins, naphthenes, and C5 and C6 i-paraffins over that of decreasing aromatics, the RON index of the gasoline increases when the hybrid catalyst is used. Consequently, the results (particularly those obtained with the hybrid catalyst) are encouraging concerning the commercial viability of cracking naphtha coker together with standard gas-oil in the ratio such as that studied in this paper (20 wt % of naphtha). 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. for its generous provision of materials. EF060344W

Cracking of Coker Naphtha with Gas−Oil. Effect of HZSM-5 Zeolite

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