GSR Products Maintain Excellent Performance When Used with Olefins Additives

Transcription

1 G Products Maintain Excellent Performance When Used with Olefins Additives Wu-Cheng Cheng Grace Davison uizhong Hu Grace Davison Introduction T o maximize profitability from the FCCU and comply with environmental regulations, many refineries have turned to the use of FCC additives. For example, a refiner may use D-PriM or G -5 additives to lower gasoline sulfur, while using an olefins additive, such as OlefinsMax to increase propylene and gasoline octane. For units using multiple additives, it is logical to inquire whether the action of one additive may interfere with the effectiveness of another. This is particularly true with the increased focus on sulfur in fuels. It is well known that ZM-5 cracks gasoline range olefins into LPG olefins, thus limiting the extent of hydrogen transfer reactions that form gasoline range paraffins. [1] As a consequence of decreased gasoline volume, the concentration of aromatics and sulfur compounds in gasoline are expected to increase. Others [3] have suggested that due to the increase in product olefinicity, there would also be a greater possibility of recombination reactions involving olefins and diolefins with H 2 and sulfides to form thiophenic compounds, thus increasing the concentration of sulfur compounds in gasoline. In this article, we will show that although ZM-5 increases gasoline sulfur by a concentration effect, it does not diminish the effectiveness of catalytic sulfur reduction technologies, either in laboratory testing or in commercial applications. 12

2 Figure 17 Gasoline ulfur Formation through ecombination Historical Understanding H-- or -- H-- or -- The origin of sulfur molecules in FCC gasoline and their interaction with FCC catalyst have been studied extensively [1-4]. Harding et al. determined that only about 4% of feed sulfur ended up in gasoline boiling range (430 F-) products, while the majority of feed sulfur cracked into H 2. The gasolinerange sulfur compounds were primarily thiophene derivatives and benzothiophene. Thiophene compounds could be removed primarily via hydrogen transfer to tetrahydrothiophenes and subsequent ring opening to form H 2. Thus, increasing the hydrogen transfer activity of the base FCC catalyst or using D- PriM additive, which enhances the rate of cracking of tetrahydrothiophene to H 2, can both effectively lower gasoline sulfur. the rate of recombination reactions (such as those in Figure 17) due to the increase in light olefins present was also suggested as a possibility. The most likely explanation is that the addition of 10% ZM-5 additive to the base catalyst, which had inherently low activity, diluted the cracking and hydrogen transfer activity. This reduced cracking and hydrogen transfer activity could very well have led to the modest increase in gasoline sulfur observed. In many commercial applications where higher levels of ZM-5 additive are used, the base catalyst is typically reformulated to maintain constant cracking activity. 1 An attempt to quantify the potential for recombination reactions in the FCC unit was conducted by Leflaive et al. [4] using pure olefin and diolefin compounds in the presence of H 2 and FCC catalyst. The study showed that at very long contact times, the transformation of olefins or diolefins into thiophenic compounds was possible, and diolefins were shown to be more likely than olefins to transform. The proposed mechanism by Leflaive et al. involves first the formation of a thiol through a nucleophilic addition of H 2, then a cyclization into tetrahydrothiophene derivatives that can be dehydrogenated into thiophenic compounds. However, as pointed out by Harding et al. and Corma et al. [5], the thiol and tetrahydrothiophene sulfur compounds are among those most easily converted in the FCC process. Moreover, many gasoline sulfur reduction technologies (e.g. D-PriM additive) are specifically designed to drive these easy to crack sulfur compounds to H 2, and make them unavailable for recombination reactions. Hence, though the recombination reactions are mechanistically possible, their contribution to gasoline sulfur formation in actual FCC commercial operation has not been quantified. Figure 18 Effect of ZM-5 on Gasoline Olefins ZM-5 does not Increase gasoline olefins concentration DC Evaluation [3] Lappas et al. showed that the addition of a 10% ZM-5 additive to a low metals (vanadium and nickel), low unit cell size Ecat increased the concentration of FCC gasoline sulfur. However, most of this increase could be attributed to the concentration effect caused by shifting of gasoline to LPG. Calculated on a feed basis, gasoline sulfur increased by only 4%. The authors attributed this modest gasoline sulfur increase to the change in relative rates of chain cracking, hydride transfer and cyclization. Increasing Gasoline Diolefins, wt.% wt.% OlefinsMax wt.% OlefinsMax 4 wt.% OlefinsMax Catalagram 102 Fall

3 ecent Laboratory Examples d on our laboratory testing and commercial experience, we have found that the use of ZM-5 additives does not increase gasoline sulfur beyond the above mentioned concentration effect for two significant reasons. First, while diolefins are much more reactive than olefins and are expected to be more likely to undergo recombination reactions, ZM-5 does not increase the concentration of diolefins (Figure 18), which are mainly a product of thermal cracking (Figure 19). Next by examining the possible pathways of a gasoline range olefins molecule, it is evident that recombination reactions are not as favorable as alternative reactions. A gasoline olefin molecule can undergo hydrogen transfer to form a paraffin. It can further crack to form smaller olefins or it can recombine with H 2 or sulfides to form thiophenes. The ability of ZM-5 to alter FCC yields indicates that the rate at which Gasoline Diolefins, wt.% Figure 19 Effect of Temperature on Gasoline Olefins Gasoline olefins are a result of thermal cracking DC Evaluation 970 F 1010 F 1050 F gasoline olefins crack on ZM-5 is the same or faster than the rate at which gasoline olefins undergo hydrogen transfer reactions on base FCC catalyst. In other words, ZM-5 does not decrease the intrinsic rate of hydrogen transfer; it merely offers a faster alternative pathway for gasoline olefins. Thus, the relative rates of the three reactions are: Cracking Table VII Effect of ZM-5 on Yields 3000 ppm Ni, 3000 ppm V/CP DC Evaluation on esid Feed - 75 wt.% Conversion 1% OlefinsMax Cat to Oil H 2 Yield, wt.% C 1 + C 2 s, wt.% Total C 3, wt.% C 3 =, wt.% Total C 4, wt.% Total C 4 =, wt.% 2% OlefinsMax 4% OlefinsMax 8% OlefinsMax Gasoline, wt.% G-Con P, wt% G-Con I, wt% G-Con A, wt% G-Con N, wt% G-Con O wt% G-Con ON ET G-Con MON ET LCO, wt.% Bottoms, wt.% Coke, wt.%

4 Table VIII Gasoline ulfur Concentration 3000 ppm Ni, 3000 ppm V/CP DC Evaluation on esid Feed - 75 wt.% Conversion ZM-5 concentrates gasoline sulfur by cracking gasoline into LPG. Gasoline sulfur concentration on a feed basis is constant 1% OlefinsMax 2% OlefinsMax 4% OlefinsMax 8% OlefinsMax ulfur Concentration ppm in Gasoline Mercaptans Thiophene MethylThiophenes TetrahydroThiophene C 2 - Thiophenes C 3 - Thiophenes BenzoThiophene AlkylBenzoThiophenes ulfur Concentration ppm on Feed Basis Mercaptans Thiophene MethylThiophenes TetrahydroThiophene C 2 - Thiophenes C 3 - Thiophenes BenzoThiophene AlkylBenzoThiophenes on ZM-5 > Hydrogen Transfer > ecombin-ation, and the addition of ZM-5 should actually lower, not increase the relative contribution of recombination. To support this discussion, a pilot plant study was commissioned to show the effect on yields and gasoline sulfur of increasing levels of ZM-5 additive combined with a high rare earth base FCC catalyst. Tables VII and VIII list hydrocarbon yields and the concentration of various gasoline sulfur species, respectively, at constant conversion resulting from cracking a resid feedstock over the various samples in the Davison Circulating iser (DC). The samples were deactivated with 3000 ppm nickel and 3000 ppm vanadium, using the Cyclic Propylene teaming (CP) protocol. Table VII exhibits the effect of OlefinsMax additions, at 0, 1, 2, 4 and 8%, where the hydrocarbon yields clearly show the expected trend when an increasing amount of ZM-5 additive is used. The C/O at constant conversion was essentially unchanged for these blends with 1%, 2% and 4% OlefinsMax and increased slightly for the blend with 8% OlefinsMax due to a slight activity dilution effect. The C 3 and C 4 olefins and paraffins increased at the expense of gasoline olefins, and there was an overall decrease in gasoline yield. Table VIII shows the major gasoline-range sulfur species produced for the catalyst blends tested, and are listed first on a gasoline basis and then on a feed basis. It is evident that on a gasoline basis, the concentrations of most sulfur species increased with the increase in OlefinsMax additive concentration. However, on a feed basis, the yields of most sulfur species were unchanged (fluctuations were within the measurement error). It is interesting to note that the concentration of mercaptans (sum of all gasoline range sulfur species with a boiling point below thiophene) appears to decrease with increasing concentrations of OlefinsMax additive. This trend is consistent with the observations of Corma et al. Compared to the earlier example by Lappas et al., we believe that the higher activity of the base catalyst and the lower concentrations of ZM-5 additive used in this study resulted in a lower impact on the overall cracking and hydrogen transfer activity, and therefore no gasoline sulfur increase was observed on a feed basis. In addition to this study where laboratory deactivated samples were used in the testing, we also commissioned a study using commercial equilibrium catalysts (Ecats) containing both ZM-5 and gasoline sulfur reduction additives. Table IX lists three Ecats from one commercial unit that were collected at different times. The first two Ecats contained 0% and 2.0% OlefinsMax, respectively, while the third Ecat contained 1.5% OlefinsMax but with a different base catalyst. The first two Ecat samples contained an alu- Catalagram 102 Fall

5 Table IX Properties of Ecat amples Ecat No OlefinsMax Ecat 2.0% OlefinsMax uca Ecat 1.5% OlefinsMax Al 2 O 3, wt.% e 2 O 3, wt.% Na 2 O, wt.% Fe, wt.% Ni, ppm V, ppm ABD, cc/gm AP, m urface Area, m 2 /gm Zeolite Matrix Unit Cell, Å mina-sol catalyst with moderate rare earth, while the third Ecat sample is the uca version of that catalyst, designed to provide similar yields as the base catalyst but to also reduce gasoline sulfur. The surface area and unit cell size of the three Ecats are essentially identical. All three Ecats were tested in an ACE [6] pilot plant unit using the refiner's hydrotreated feed. The hydrocarbon yields and sulfur in gasoline at constant conversion are listed in Table X. The increase in propylene yield and octane from the 0% OlefinsMax sample to either the 2.0% or the 1.5% OlefinsMax samples is proportional to the increase in OlefinsMax in the Ecat. The decrease in gasoline yield is also proportional to the increasing ZM-5 additive concentration. In spite of the concentration effect from the decreasing gasoline yield, the gasoline sulfur concentration of the uca containing Ecat is about 30% lower than the two base Ecats. Table X ACE Evaluation of and uca Ecat amples uca lowers gasoline sulfur, even in the presence of ZM-5 additive Conversion 70 Ecat No OlefinsMax Ecat 2.0% OlefinsMax uca Ecat 1.5% OlefinsMax Catalyst to Oil atio Dry Gas Propylene Total C 3 s Total C 4 = s Total C 4 s Gasoline ON MON LCO Bottoms Coke Gasoline ulfur, on gasoline basis eduction Light Cut ulfur, ppm % Heavy Cut ulfur, ppm % Cut Gasoline ulfur, ppm % Total ulfur, ppm % 16

6 G - Con ON ET C 3 = wt.% Figure 20 Propylene Yield Trends for Ecat ample Blends 4% OlefinsMax produces expected increase in propylene G-5 does not negatively impact ZM-5 ZM-5/G Interaction DC tudy Ecat Al 2 O 3 (wt.%) 42.9 E 2 O 3 (wt.%) 3.2 Na2O (wt.%) 0.36 Fe 2 O 3 (wt.%) 0.72 P 2 O 5 (wt.%) 0.09 Ni (ppm) 40 V (ppm) 60 urface Area (m 2 /g) Zeolite Ecat Ecat + 4% OlefinsMax Ecat + 25% G-5 Ecat + 4% OlefinsMax + 25% G-5 Table XI Properties of Ecat and Feed Matrix 37 Unit Cell (Å) Feed API 24.7 Aromatic ing Carbons, Ca (wt.%) 20.5 Napthenic ing Carbons, Cn (wt.%) 17.1 Paraffinic Carbons, Cp (wt.%) 62.4 ulfur (wt.%) 0.82 Concarbon (wt.%) 0.77 Distillation ( F), IBP % ( F) % ( F) % ( F) % ( F) % ( F) 1002 FBP ( F) 1177 Figure 21 Gasoline Octane Trends for Ecat ample Blends 4% Olefins-Max produces expected increase in octane G-5 does not negatively impact ZM-5 ZM-5/G Interaction DC tudy Ecat Ecat + 4% OlefinsMax Ecat + 25% G-5 Ecat + 4% OlefinsMax + 25% G-5 To further decouple the ZM-5 and sulfur reduction effects, a DC study with commercial Ecat was conducted. A high rare earth, low metals base Ecat was blended separately with 4% OlefinsMax and with 25% G-5, as well as with both 4% OlefinsMax and 25% G-5. G- 5 is an additive based on the sulfur reduction functionality of uca that also provides base cracking functionality. The samples were lab deactivated to match typical commercial performance. A fairly paraffinic feedstock with 0.8 wt% sulfur was cracked over each of the samples. The properties of the base Ecat sample and the feed used in the study are shown in Table XI. The addition of 4% OlefinsMax provided similar shifts in propylene (Figure 20) and gasoline octane (Figure 21) as the same amount of the additive produced in Table VII. The addition of the G-5 additive did not cause any change in olefins or octane. Additionally, because G-5 imparts cracking activity while providing gasoline sulfur reduction, the addition of G-5 to the base Ecat did not significantly change the yields of other FCC products. Figure 22 shows the cut gasoline sulfur concentration on a gasoline basis for the samples tested. Cut gasoline sulfur is the sum of the sulfur species that boil through 428 ºF (mercaptans through C 4 thiophenes). G-5 by itself reduces gasoline sulfur by 30% at 78% conversion, and the sulfur reduction performance is similar with the addition of 4% OlefinsMax. This is consistent with the performance observed in Table X. The addition of 4% OlefinsMax yields 8% higher gasoline sulfur at 78% conversion on a gasoline basis, which is comparable to the 7% increase in gasoline sulfur observed for the commercial comparison of the base Ecat to the 2% OlefinsMax Ecat in Table X. Normalizing the results in Figure 20 to a feed basis, the increase in Catalagram 102 Fall

7 Cut Gasoline ulfur, ppm Figure 22 Gasoline ulfur Trends for Ecat ample Blends G-5 reduces gasoline sulfur by 30% alone or with ZM-5 Higher gasoline sulfur is due to concentration effect of ZM-5 ZM-5 / G Interaction DC tudy Ecat Ecat + 4% OlefinsMax Ecat + 25% G-5 Ecat + 4% OlefinsMax + 25% G-5 gasoline sulfur with 4% OlefinsMax in inventory is only 3%, again indicating that it is a concentration effect that causes higher apparent gasoline sulfur with ZM-5 additive use. Commercial Example The gasoline sulfur reduction performance of uca catalyst and the G-5 additive in the above studies are similar to what we have observed with Ecats that do not contain ZM-5 additives. In 2004, the Alon UA, Big pring, TX refinery co-authored an article with Grace Davison summarizing their successful experience with the use of uca catalyst to reduce their FCC gasoline sulfur by 20% [7]. The refinery also used OlefinsMax on an opportunity basis to make incremental refinery grade propylene. The performance of uca was quantified both with and without the ZM-5 additive. As Figure 23 indicates, the 20% reduction in gasoline sulfur was consistent whether or not ZM-5 was present. Conclusion ecent pilot plant analysis and commercial data indicates that any increase in gasoline sulfur observed with the use of ZM-5 additives is due to a concentration effect from the cracking of gasoline molecules into LPG, as opposed to recombination reactions. Furthermore, ZM-5 additives used in combination with gasoline sulfur reduction technologies do not show increased gasoline sulfur from recombination reactions. efiners have utilized both technologies simultaneously and achieved performance comparable to the use of each independently. Those refiners considering the use of both technologies can be confident in the ability of each product to achieve its individual product performance goals. Gasoline lbs/feed lbs Figure 23 uca Performance at Alon UA, Big pring, Texas % Lower Gasoline ulfur electivity % Point, F eferences 1. P.H. chipper, F.G. Dwyer, P.T. parrell,. Mizrahi, and J.A. Herbst, Fluid Catalytic Cracking ole in Modern efining, M.L. Occelli (Ed.), AC ymposium eries, Vol. 375, American Chemical ociety, Washington D.C., 1988, p H. Harding... Gatte, J.A. Whitecavage, and.f. Wormsbecher, eaction Kinetics of Gasoline ulfur Compounds, J.N. Armor (Ed.), AC ymposium eries, Vol. 552, American Chemical ociety, Washington D.C., 1994, p A.A. Lappas, J.A. Valla, I.A. Vasalos, C. Kuehler, J. Francis, P. O'Connor, and N.J. Gudde, The Effect of Catalyst Properties on the In itu eduction of ulfur in FCC Gasoline, Applied Catalysis A: General 262 (2004) p P. Leflaive, J.L. Lemberton, G. Pérot, C. Mirgain, J.Y. Carriat and J.M. Colin, On the Origin of ulfur Impurities in Fluid Catalytic Cracking Gasoline - eactivity of Thiophene Derivatives and of Their Possible Precursors Under FCC Conditions, Applied Catalysis A: General 227 (2002) p A. Corma, C. Martínez, G. Ketley, and G. Blair, On the Mechanism of ulfur emoval During Catalytic Cracking, Applied Catalysis A: General 208 (2001) p J.C. Kayser, Versatile Fluidized Bed eactor, U.. Patent No. 6,069,012 (2000). 7. M. Gwin (Alon), E.J. Udvari, and D.A. Hunt, uca Catalyst educes FCC Gasoline ulfur and More at the Alon UA, Big pring efinery, Catalagram 96 (2004) p (published by Grace Davison, a business unit of W.. Grace & Co.). Division Catalyst uca uca with OlefinsMax 18