Attempts To Improve the Product Slate Quality ... - ACS Publications

Several catalysts with increasing initial coke content were processed in the Microdowner unit in order to assess the activity and selectivity of coked...
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Attempts To Improve the Product Slate Quality: Influence of Coke-on-Catalyst Content Avelino Corma,* Francisco V. Melo, and Laurent Sauvanaud Instituto de Tecnologı´a Quı´mica, UPV-CSIC, UniVersidad Polite´ cnica de Valencia, AVenida de los Naranjos s/n, 46022-Valencia, Spain

Several catalysts with increasing initial coke content were processed in the Microdowner unit in order to assess the activity and selectivity of coked catalyst. Tests were carried out at both constant operating conditions and changing catalyst-to-oil ratio to maintain conversion. It was found that coke on regenerated catalyst (CRC) has very little influence on the main product selectivity, while some effect on product quality was detected. CRC changed the hydrogen transfer to cracking relative rate as it lowered site density, resulting in a dramatic increase in the isobutene-to-isobutane ratio. Gasoline was more olefinic and less paraffinic, while sulfur content increased. LCO yield increased slightly, producing a distillate with better quality (fewer aromatics and more paraffins). When ZSM5 is used, it has been observed that propylene selectivity increases at constant conversion at a rate around 1 point of propylene per percent coke on catalyst. Apart from the cracking selectivity changes in the riser, the significant increase in catalyst circulation that is necessary to maintain conversion may have some beneficial effects for feed vaporization and reduced thermal cracking at the injection point. 1. Introduction The actual operation of a fluid catalytic cracking unit (FCCU) is aimed at obtaining a level of coke on the catalyst as low as possible after the regeneration phase, in order to restore the maximum activity of the catalyst. In a modern FCC design, values under 0.1% carbon on regenerated catalyst (CRC) are common,1 and can be lower than 0.05% in two-stage regenerator units.2,3 In old FCC units, however, catalyst and operational limits did not allow reaching total combustion in the regenerator bed, resulting in partial regeneration of the catalyst and high CO emissions that could originate overburn in the regenerator cyclones and require a CO boiler. Air blower capacity and low temperature limits for mechanical and catalyst resistance were usual limits, which derived in constraints in the processing of high coke-make feeds or increasing throughput. Typically, values in the range 0.3-0.8% CRC were observed in such partial combustion modes.4 Nonetheless, these units were still able to reach an appreciable conversion per pass, even though an important part of the feed was recycled to the reactor. Micro Activity Test (MAT) studies and pilot plant studies have shown that a partially coked catalyst can still have appreciable activity,5,6 and that gasoline selectivity decreases with increasing CRC.7,8 Also, various patents claimed that partially coked catalyst may offer selectivity advantages, especially referring to dry gas and coke selectivity.9-11 Coked catalyst is proposed as a choice catalyst in the processing of heavy gasoline for its reduced hydrogen transfer activity, resulting in a low coke yield.12 Others issues in FCC such as propylene increase and sulfur reduction in gasoline have recently focused the attention of researchers. Coke on catalyst may have an influence on both issues. A previous laboratory study showed an increase in the olefinicity of the gasoline at low vacuum gas oil (VGO) conversion,13 which may result in an enhanced propylene yield under favorable conditions. It has been demonstrated that sulfur reduction in gasoline within the FCC riser is governed by hydrogen transfer14 and * To whom correspondence should be addressed.Tel.: +34 96 387 78 00. Fax: +34 96 387 78 09. E-mail: [email protected].

Figure 1. Conversion vs coke on regenerated catalyst (CRC) for FCC1 catalyst at 550 °C, various CTOs. ([, b, 9) Our data, CTO 11, 15, and 19, respectively; (2) Upson data.21 Lines are to guide the eyes. Table 1. Catalyst Physicochemical Properties

rare earth content (wt %) unit cell size (nm) % zeolite in catalyst Si/Al ratio in zeolite BET surface area (m2/g)

FCC1

ECat

Add B

1.0 2.430 11 24 170

1.0 2.427 10 45 112

15 50 50

Table 2. VGO Properties density at 288 K (g/cm3) sulfur (wt %) N2 (ppm) CCR (wt %) average molecula r weight (g/mol) K (UOP) vol % temp (K)

5 638

0.9081 0.35 1614 0.15 464 12.11

distillation curve D-1160 10 30 50 673 704 729

70 763

90 824

catalyst metal content, especially Ni and V.15 Gatte et al.16 showed that FCC catalysts with differing acid site density such as REY and USY grades may convert sulfur in slightly different

10.1021/ie061067j CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006

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Figure 2. Product yields vs coke on regenerated catalyst (CRC) for FCC1 at 550 °C, various CTOs. Symbols as in Figure 1.

ways, with hydrogen transfer being preferred when the acid site density is higher. It has been discussed in various articles that the activity of the metals in FCC largely depends on their oxidation states. Tangstad et al. have shown that the +4 and +5 states of vanadium have similar dehydrogenation activity, and deduced that the +3 state is almost inactive.17 Inversely, Long et al. claimed that the most effective state for sulfur reduction was the +2 state.18 It has been shown by temperature programmed reduction (TPR) experiments12 that coke deposited on a catalyst is effective in reducing the oxidation state of the metals on the surface of the catalyst, and it has been proposed that vanadium is reduced mainly at the +4 state with traces of the +3 state with the typical levels of carbon on catalyst encountered in FCC operation. Thus, it is very likely that coke on catalyst will have an influence on sulfur conversion, through both hydrogen transfer rate change and metal oxidation state change.

In this study, both activity influence and selectivity will be studied, from the point of view of the riser chemistry. Gasoline and light cycle oil (LCO) chemical compositions, as well as sulfur in gasoline and propylene selectivity will also be presented. Implications for the unit operation will also be discussed. 2. Experimental Section A. Catalyst and Feed. A commercial equilibrium FCC catalyst (Ecat), a fresh laboratory-deactivated FCC base catalyst (FCC1), and a commercial additive based on ZSM5 (Add B) have been used in this study. Physicochemical characteristics are given in Table 1. Ecat has a low BET area and a metal content of 3100 ppm vanadium and 1700 ppm nickel. FCC1 has a higher BET area and no metal load. This catalyst was deactivated in the laboratory by steam treatment at 1089 K for

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Figure 3. Product selectivity of coked catalyst compared with regenerated catalyst, FCC1 at 550 °C. (4) Regenerated catalyst, residence time of 0.8 and 0.3 s, respectively. (b, 9, [) Coked catalyst, residence time 0.8 s, CRC 0.4, 0.8, and 1.0 wt %, respectively. Lines are to guide the eyes and refer to yields with regenerated catalyst.

4 h. The feedstock was a partially hydrotreated vacuum gas oil whose properties and distillation curve are given in Table 2. B. Reaction Procedure. Testing Unit. The hardware and detailed operation of the Microdowner unit have been described previously.19,20 The main features of the unit comprise a catalyst preheater where the catalyst is stored before the test, a oncethrough reactor where the feed and the preheated catalyst are fed continuously during the test while their residence time is very short, ranging from 0.3 to 2 s, and a separator which stores the catalyst used during the test for regeneration and coke determination. The unit simulates a steady-state regime during the length of the test, which usually takes between 1 and 2 min. The catalyst separated from the reaction products is continuously

stripped during the reaction and 60 s more after the end of the reaction. Liquids and gaseous products are recovered by known methods (cold traps and water displacement burette), while catalyst is regenerated after the test with 500 mL/min air at 850 K for 3 h. Alternatively, the coked catalyst can be withdrawn from the unit after the stripping step, thus ignoring the regeneration step. A flow of nitrogen is used for solid transportation and feed dispersion. Procedure for Preparation of Coked Catalyst. A series of experiments at constant reactor temperature (550 °C), constant solid residence time (0.8 s), and various initial coke contents have been carried out. The coke loading of the catalyst was gradually increased, beginning from an initially completely

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Figure 4. Isobutene/isobutane ratio with coked and regenerated catalyst, FCC1 catalyst at 550 °C. Symbols as in Figure 3.

regenerated catalyst. After each reaction, the catalyst was stripped but not regenerated. Then, the recovered catalyst was loaded in the previously cleaned preheater and was used for the next test. It has been checked that coke on catalyst does not vary substantially when coked catalyst is heated under nitrogen flow before testing begins. Only a small emission of H2 and methane has been observed, which represents less than 3% of the weight of the coke on catalyst. This is probably due to thermal decomposition of part of the coke (probably the most recently deposited) at high temperature. It was observed that after a short period of time these emissions stopped and that coke still retains hydrogen in amounts between 4 and 6% of total coke on catalyst.

Analysis of the Cracked Products and Mass Balance. The gases were analyzed using a Varian 3800-GC gas chromatograph equipped with three detectors, two thermal conductivity detectors (TCD) for analysis of H2 and N2 after separation in 6 ft long MS 5A and 2.5 ft long MS 13X molecular sieves, respectively, and a flame ionization detector (FID) for C1-C6 hydrocarbons separated in a 30 m long Plot/Al2O3 column. Simulated distillation of the liquids was carried out with a Varian 3800GC following the ASTM-2887-D procedure. Cuts were made at 210 °C for light gasoline and 345 °C for LCO. The coke content of catalyst was measured with a LECO apparatus when coked catalyst samples were available and by adsorption of CO2 and H2O on ascarite and drierite when catalyst was regenerated in the separator. In this study, “coke on catalyst” means the weight percent of carbon plus hydrogen remaining on the catalyst. Conversion, labeled as X, is defined as the sum of gases, gasoline, and coke. In some figures, the kinetic conversion, defined as X/(1 - X), is used. Mass balances were considered acceptable in the range 100 ( 5% of the feed introduced. Gasoline composition was determined using PIONA analysis. The separation was carried out on a 3900 Varian chromatograph equipped with a Petrocol-100 fused silica column connected to a FID detector. Sample composition was determined up to C12, using DHA 5.0 Varian software. LCO polyaromatics content was determined using the same chromatogram, by identification of mono-, di-, tri-, and tetramethylnaphthalene isomers and phenanthrene. Also, n-paraffins from n-C13 to n-C20 were identified with reference sample.

Figure 5. Gasoline composition with coked and regenerated catalyst, for FCC1 at 550 °C. (4) Regenerated catalyst; (b, 9, [) coked catalyst, CRC at 0.4, 0.8, and 1.0 wt %, respectively. Lines are to guide the eyes and refer to component yield at same catalyst-to-oil ratio varying coke on catalyst. Dashed lines refer to yields at constant coke on catalyst, varying CTO.

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Figure 6. LCO composition with coked and regenerated catalyst for FCC1 at 550 °C. Symbols as in Figure 5.

The analysis system includes a pulsed flame photometric detector (PFPD) for determining the sulfur compounds separated in a 30 m GS-Q megabore column. Some TPR experiments were carried out on a Micromeritics Autochem 2910, on both regenerated and coked samples, at a heating rate of 10 °C/min in 50 cm3/min of a mixture of 10% H2 in Ar carrier gas (volume percent). 3. Results and Discussion A. Conversion and General Yields. In sections A and B, the FCC1 catalyst, a fresh laboratory-deactivated catalyst, was used. The first consequence of the presence of coke on regenerated catalyst is the decrease in the conversion, with all other variables constant. A decrease rate of 1.2-1.8 wt % conversion loss per 0.1 wt % carbon on regenerated catalyst is observed (Figure 1), with a higher rate at lower catalyst-to-oil ratio (CTO) and conversion. Others observed higher deactivation rates (2.4-3.2%21,22), but this effect may be due to the lower conversion because of the lower CTO used. Similar deactivation rates have been encountered in other laboratory units that were designated to properly reflect FCC operation,13,23 in contrast with a traditional MAT unit. It must be noted that, even with 1% CRC, the catalyst still has remarkable activity. If CTO is increased enough, it is possible to recover the conversion level of the completely regenerated catalyst. For example, a catalyst with 0.8% CRC operated with a CTO of 19 reaches the same activity as the same regenerated catalyst at a CTO of 8.24 That means that, with 0.8% CRC, roughly half of the cracking sites are completely deactivated. The influence of CRC on the yields of the principal products is clear: more LCO, less gasoline, less gas, and less coke (Figure

2). However, this effect must be related to the activity decrease of the catalyst.25 A comparison with the yields obtained from regenerated catalyst at several CTOs and residence times clearly shows that the changes in the yield of the gasoline and gas products are only due to the activity decrease of the catalyst (Figure 3). Also, it can be noted that the picture is slightly different if the residence time is changed in order to compare at the same conversion. A lower LCO yield and a higher coke yield are observed with regenerated catalyst and shorter residence time compared with coked catalyst and longer residence time (Figure 3). This may be due to the different time frame of coke formation and feed cracking. Although the conversion is lower at a shorter time on stream, because of a higher space velocity, coke yield may be similar at both times and same CTO, because at this time scale, coke yield depends mainly on the strong adsorption of feed/LCO molecules on the catalyst surface and this does not change appreciably between 0.3 and 0.8 s of time on stream. This may change when the time on stream is long enough (some seconds) so that catalytic coke formed from olefin addition on aromatic cores begins to play a significant role in coke yield. Thus, the influence of coke on catalyst on the main selectivities can be interpreted as the blockage of a certain number of active sites by coke molecules, while the other sites remain active for cracking. Such a diminution in acid site density should have an effect on the hydrogen transfer rate relative to cracking, in the same way as an increase in the Si/Al ratio for a Y zeolite lowers this relative rate.26,27 Such effects should be reflected in the product quality. B. Product Quality and Distillate Composition. The observed change in LPG olefinicity supports the hypothesis presented above. The isobutene/isobutane ratio (Figure 4), which is an indicator of hydrogen transfer relative rate to cracking, shows a clear increase over the regenerated catalyst case when CRC increases. Also, gasoline composition presents some changes with increasing CRC: olefin content increases with respect to the base case with regenerated catalyst, while paraffin content decreases in a similar amount (Figure 5). These changes are explained by the typical hydrogen transfer reaction:

1 naphthene + 3 olefins f 1 aromatic + 3 paraffins The difference in the olefin and paraffin content is around 4% weight of gasoline, and due to the reaction stoichiometry, a 1.3% weight change in aromatic and naphthene content should be observed. Such a change, however, is not clearly observed in our experiments (Figure 5). It has also been suggested that hydrogen transfer may also occur with the concourse of LCO molecules or coke molecules as hydrogen donors, rather than naphthenes from gasoline fraction.28 Hydroaromatics from the LCO fraction are proposed as good candidates for such a reaction. This also implies that the aromatic character of the LCO would increase with the increase of the hydrogen transfer relative rate. Within the range of conversion explored, the LCO yield decreases with increasing conversion and increases with increasing CRC. Meanwhile, the polyaromatic content of the LCO increases with conversion, while the n-paraffin content decreases (Figure 6). A slight decrease in the selectivity toward polyaromatics with coked catalyst could be detected, thus showing that there is some interaction between LCO and gasoline through hydrogen transfer reactions. Such an interaction may be more readily observed in a MAT unit, since it is well-known that this laboratory unit magnifies the hydrogen transfer relative

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Figure 7. General yields with 5% ZSM5 based additive in base catalyst FCC1. (4) Regenerated catalyst; ([) coked catalyst, CRC from 0.4 to 1.2 wt % on catalyst. Table 3. Main Yields and Gas Yields with Increasing Coke on Regenerated Catalyst (CRC), Catalyst Ecat, 550 °C, Residence Time around 1 s test number operating conditions catalyst/oil ratio (g/g) CRC (wt %) conversion (wt %) general yields (wt % of feed) gases gasoline LCO HCO coke detailed gas yield (wt %) hydrogen methane ethane ethylene total dry gas propane propylene isobutane n-butane isobutene linear butanes ratio of interest: isobutene/isobutane

1

2

3

11.0 0.0 68.4

16.3 0.5 67.1

32.6 1.2 68.4

17.6 45.8 20.4 11.2 4.9

16.7 45.6 21.4 11.1 4.8

17.8 45.5 20.5 11.5 5.1

0.29 0.61 0.43 0.70 2.0 0.6 4.9 2.4 0.5 2.3 4.6 1.0

0.37 0.54 0.45 0.68 2.0 0.5 4.6 2.0 0.4 2.4 4.6 1.2

0.48 0.54 0.46 0.73 2.2 0.6 5.2 1.9 0.4 2.7 4.8 1.5

reaction rate28 due to a combination of higher catalyst deactivation and lower space velocity compared with transported bed reactors. Also, coke may participate in these hydrogen transfer reactions to a greater extent than LCO. Another very small difference in the selectivity toward linear paraffins was observed. Then, the difference is less than 1 wt % of LCO, or around 0.2% referred to the feed. Thus, although the use of coked catalyst is beneficial for LCO quality, this represents only a small increase in practice.

Table 4. Sulfur Content of the Gasoline with Varying Coke-on-Catalyst Contents, Ecat at 550 °C, Residence Time around 1 s test number CRC (wt %) S in gasoline distribution (ppm) thiophene 2-methylthiophene 3-methylthiophene tetrahydrothipophene C2-thiophene C3-thiophene C4-thiophene benzothiophene total S in gasoline (ppm)

1 0.0 89 145 159 13 296 152 73 564 1491

2 0.5 71 145 170 21 370 233 126 752 1888

3 1.2 72 143 175 28 414 272 155 783 2042

C. Effect of Coke on Catalyst with ZSM5 Additive. The base catalyst FCC1 was mixed with 5% of an additive based on ZSM5 in order to boost the production of propylene. The increased content of olefins in the gasoline that is obtained with coked catalyst may also help to increase the propylene yield obtained from the addition of ZSM5. In this set of experiments, the catalyst-to-oil ratios have been increased as the coke on regenerated catalyst increased, so that the conversion with coked samples was maintained between 64 and 68%, a range that allowed a proper comparison with the yields obtained with totally regenerated catalysts at CTOs between 7 and 9. As a result, the CTO used for coked samples ranged from 10.5 to 31. Yields for the main product components are presented in Figure 7. As observed with FCC1 catalyst, heavy cycle oil (HCO), LCO, coke, and dry gas selectivities hardly changed with the increasing catalyst-to-oil ratio. However, a significant difference appeared with the base catalyst: while gasoline and LPG selectivities remained unchanged with FCC1 catalyst, the

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Figure 8. LPG yield changes vs coke on catalyst, 5% ZSM5 additive in FCC1, 550 °C, in % of component referred to VGO feed. (a) (b) Propylene, (]) isobutene, (2) isobutane; (b) (2) linear butanes; (c) (b) propane; (]) n-butane.

Figure 9. TPR results on regenerated and coked Ecat. From top to bottom: regenerated catalyst; 0.4% CRC; 1% CRC.

presence of the ZSM5-based additive resulted in a 1-3 point decrease in gasoline yield and a 1-3 point increase in LPG gas yield when coked samples were used. The LPG yield changes with increasing coke on catalyst are presented in Figure 8. The yield of each component obtained with coked catalyst has been compared with the yield of this component obtained at the same conversion and regenerated catalyst. A major increase in propylene and isobutene is observed as coke

on catalyst increases, resulting in a 20% yield increase with a coke on regenerated catalyst of 1.2 wt %. Meanwhile, isobutane yield decreased, linear butane yield remained stable, and propane and n-butane suffered moderate increases. These figures are typical of the addition of ZSM5 to the FCC catalyst inventory.29 Thus, we can deduce that the relative activity of base catalyst and additive changes with increasing coke on catalyst because ZSM5 additive is less sensitive to coke deactivation. This may

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Figure 10. Hydrogen yield and wet/dry gas volumetric yield with coked and regenerated catalyst. Symbols as in Figure 3.

allow the refiner to maximize the effect of the ZSM5 additive while ZSM5 addition may be maintained low in order to limit the dilution of the base catalyst by the additive. D. Effect of Coke on Catalyst on Sulfur Content in Gasoline. In this section a commercial catalyst with a high metal content has been used (Ecat), as it has been demonstrated that metal plays an important role in sulfur reduction chemistry.15 Several tests with increasing contents of coke on regenerated catalyst were carried out, and the catalyst-to-oil ratio was increased with the content of coke on regenerated catalyst in order to maintain the same level of conversion. TPR results (Figure 9) have shown an effective reduction of the metals at the surface of the catalyst with coked samples, even with 0.4% coke on catalyst. The peaks at 476 and 535 °C detected with the regenerated catalyst disappeared with both coked samples. Main cracking yields and detailed gas yields are given in Table 3. As we observed before with the FCC1 catalyst, CRC has little effect on the general cracking selectivity. The isobutene/isobutane ratio increased significantly as an indication of a decrease of the hydrogen transfer rate compared with the cracking rate. Surprisingly, we observed that there was no reduction of the dry gas yield as we expected from the reduction of the metals on the catalyst. Moreover, the hydrogen yield experienced an increase in the same way that we observed with the FCC1 catalyst (which has no metal load), although the amplitude of this increase was lower for Ecat (+65% at 1.2 wt % CRC) than for FCC1 (+200% at 1.2 wt % CRC). The sulfur content is detailed in Table 4. It is evident that

coke on catalyst has a negative effect on sulfur reduction, as the sulfur total increases. In particular, large increases are observed for the heaviest compounds, including C3-thiophenes, C4-thiophenes, and benzothiophene. This is typical of a reduction of sulfur conversion activity, where larger compounds are converted to lighter (thiophene), and ultimately reduced to H2S and hydrocarbons.14 As a consequence, fewer alkylthiophenes are converted into thiophene, which explains the thiophene yield decrease. It has been described in the literature that under catalytic cracking conditions thiophene is removed mainly through hydrogen transfer on catalyst with low metal loads,16 and that metal load significantly affects the removal rate, although it is not clear what the dominant mechanism is. Our TPR results have shown an effective reduction of the metals at the surface of the catalyst, at both 0.4% CRC and 1.2% CRC. At the same time, the sulfur content increased from 1500 to 1900 ppm with a coke on catalyst from 0 to 0.4%, and a similar value was observed with 1% coke on catalyst (2050 ppm). Thus we propose that at lower values of coke on catalyst the fast metal reduction is responsible for the increased sulfur content in the gasoline, and at higher coke-on-catalyst contents the lower hydrogen transfer rate also contributes slightly to the increase in gasoline sulfur. E. Implications for the Commercial FCC Unit Operation. Even if the selectivity changes may be small from the point of view of the reaction chemistry, the activity decrease of the catalyst may have an important impact on the operation of the

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unit, thus entraining a large change in FCCU yields. As an example, although the coke selectivity may not change, the increase in the unit circulation that arises from CTO increase may lead to a temperature drop in the regenerator. Thus, the temperature of the returned catalyst to the riser may be lower, thus reducing the thermal shock between the hot catalyst and colder feed, resulting in a lower dry gas yield, which may balance the increase in hydrogen yield observed with coked catalyst (Figure 10). This late entrains a significant increase in the volume of dry gas generated, while it has much less influence on the total volume of wet gas to be compressed. On the other hand, an excessive catalyst circulation may decrease the stripper efficiency, thus resulting in higher coke selectivity. Also, it would be a challenge to operate the regenerator in such a way that a high level of CRC is maintained, while CO combustion is as complete as possible. A careful use of combustion promoters with low regenerator temperature has been shown to be effective in lowering CO in flue gas while maintaining the CRC at levels higher than 0.3% weight of the catalyst.30 A collateral and beneficial effect of the presence of CO is the great reduction in the NOx emissions through the reaction31,32

CO + NO f CO2 + (1/2)N2 It is an industrial experience in old FCC that the change from partial combustion mode to complete combustion mode entrained a significant increase in the NOx emission, showing that CO may reduce the NOx concentration even without the presence of an adequate catalyst.33 Working with a high level of CRC but with complete combustion would make CO unavailable for this reaction, but NOx reduction can also proceed through the reaction

C + 2NO f CO2 + N2 with carbon on catalyst as a reductant, with the help of an adequate catalyst.34 4. Conclusion Although coke significantly reduces the activity of the catalyst, it retains enough activity to be used in the FCC process. With base catalyst, the selectivity for the main products will remain unchanged, but product quality suffers some changes. Gas and gasoline olefinicities increase, while LCO becomes slightly more paraffinic and less aromatic. Also, gasoline isoparaffin content may also slightly decrease due to a lower hydrogen transfer relative rate, thus resulting in only a marginal increase of the RON number of the gasoline. The use of a ZSM5 additive, which is less sensitive to coke deactivation, allows increasing the light olefins yield without increasing the ZSM5 additive load in the catalyst inventory. Although not an effect of catalyst selectivity, the dry gas mass yield may be reduced in the commercial unit with coked catalyst due to a lower thermal shock and a better vaporization of the feed. From the operational point of view, the partial regeneration of catalyst allows reducing the regenerator temperature, which reduces regenerator mechanical stress and catalyst deactivation. Also, it may help to reduce NOx emissions in FCC flue gas. All these features rely on a large increase of catalyst circulation to compensate for a lower catalyst activity. Acknowledgment The authors thank CICYT (Project No. MAT 2003-07945C02-01) for financial funding.

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ReceiVed for reView August 14, 2006 ReVised manuscript receiVed November 13, 2006 Accepted November 14, 2006 IE061067J