Ultra Low Sulfur Diesel 21
Advanced Refining Technologies - Hydroprocessing Catalysts from Chevron and Grace Davison Advanced Refining Technologies (ART), is the joint venture of Chevron Products Company and W. R. Grace & Co. s Grace Davison catalysts business unit, created to develop, market and sell a comprehensive line of state-of-the-art hydroprocessing catalysts. Since our formation in March 21, we have firmly established ourselves as a leading supplier of premium hydroprocessing catalysts and technical service to the petroleum refining industry worldwide. ART brings new standards of product innovation and customer service to the global refining industry by harnessing the technical expertise of both Chevron and Grace Davison in catalyst development and their broad experience in supporting refiners. With the combination of Grace Davison s material science, manufacturing, marketing and sales strength, and Chevron s extensive experience from operating its own refineries and leadership in design and process licensing, ART offers refiners one-stop access to hydroprocessing knowledge and technical service unparalleled in the industry. Product Overview Chart Crude Tower Naphtha Kerosene & Jet ART offers refiners the unique synergies derived from our parent joint venture partners being leaders in hydroprocessing catalyst technologies. Chevron, and now Chevron Lummus Global (CLG) represent over 35 years of experience designing and licensing RDS and VRDS units, as well as operating resid hydroprocessing and distillate hydrotreating units at Chevron refineries. Grace Davison has over 5 years of experience in extruding hydroprocessing catalysts. Leveraging the refining and hydroprocessing experience of Chevron with Grace Davison s expertise in specialty materials and catalyst technologies makes ART uniquely equipped to quickly commercialize and deliver innovative products to the dynamic hydroprocessing marketplace. To keep pace with the high demand for ART catalysts driven by heavy resid hydroprocessing applications and more stringent clean fuels standards, ART has expanded manufacturing capacity at its existing North American plants. ART acquired Orient Catalyst Company s (OCC) hydroprocessing catalyst technologies, and the HOP catalyst product line in 22; and obtained an ownership position in Kuwait Catalyst Company (KCC) which has manufactured HOP resid hydroprocessing catalysts under license since 21. ART s comprehensive line of hydroprocessing catalysts deliver maximum sulfur removal and upgrading for a wide range of feedstocks. ART offers hydroprocessing catalysts for the resid segment of the hydroprocessing market, including the fixed-bed Onstream Catalyst Replacement (OCR) resid process and ebullating bed applications, as well as a complete product line of premium catalysts for distillate hydrotreating of heavy VGO/DAO, diesel, kerosene and light naphtha applications. Diesel Hydrocracker Pretreat Vacuum Tower Coker Naphtha AT TM Series AT TM Series DX TM Series AT TM Series DX TM Series ICR Series FCC Pretreat ICR Series HOP Series GR Series LS TM Series HSLS TM Series StART TM Catalyst System AT Series SmART Catalyst System DX Series ApART TM Catalyst System AT, DX Series Fixed Bed Resid Fixed Bed Resid Ebullating Bed Resid for LC-Fining and H-Oil processes
Contents Understanding Ultra Low Sulfur Diesel By Gerianne D Angelo and Charles Olsen This article discusses the importance of feedstock properties and operating conditions to successfully producing ultra low sulfur diesel. The SmART Catalyst System : Meeting the Challenges of Ultra Low Sulfur Diesel By Charles Olsen A disucssion of the various components of the SmART catalyst system and the factors that go into designing customized systems to meet a variety of ULSD processing needs. No Need to Trade ULSD Catalyst Performance for Hydrogen Limits: SmART Approaches By Charles Olsen and Geri D Angelo The SmART Catalyst System provides the optimum balance between activity and hydrogen consumption. This article discusses the chemistry and kinetics involved in designing a system, and reviews the commercial performance observed in several ULSD applications. 2 8 15 ART Excels In ULSD Service: Update on Sulfur minimization By ART By Greg Rosinski, Dave Krenzke, and Charles Olsen ULSD production with the first SmART Catalyst System Series began early in 24 at a North American refinery processing a feed containing 4% of a high endpoint LCO. Since that time DX Platform Catalysts have been selected for over 7 ULSD applications as either stand-alone catalysts or as components in SmART System. The technology has been a great success since its introduction with millions of pounds installed in commercial units around the world. 23 Distillate Pool Maximization by Exploiting the use of Opportunity Feedstocks Such as LCO and Syncrude By Brian Watkins and Charles Olsen The use of opportunity feeds such as FCC LCGO, diesel streams from other hydroprocessing units and feeds from synthetic crude sources has helped refiners to maximize their diesel pool. In this article we highlight differences in feed reactivity for various feed components and explore the impact of various Hydrotreating Catalysts and operating conditions on diesel production. Cetane Improvement In Diesel Hydrotreating by Greg Rosinski and Charles Olsen This article discusses the importance of cetane in ULSD. The SmART Catalyst System, which utilizes both the CoMo and NiMo catalyst, results in a cetane uplift which is nearly two numbers higher than an all-como system with only a small increase in hydrogen consumption. For H 2 constrained refiners this is an ideal solution for improving the product cetane. 3 38 Factors Influencing ULSD Product Color By Greg Rosinski, Charles Olsen and Brian Watkins Product Color of petroleum products such as kerosene, jet fuel, diesel fuel and lube oils is a concern. Unit cycle length can be shortened due to product color degradation. In this paper we identify components that contribute to color degradation and report on the effects of feedstocks and operating conditions on ULSD color. 43 1
Understanding Ultra Low Sulfur Diesel Charles Olsen New Product Development Manager Gerianne D Angelo Senior Technical Services Engineer ADVANCED REFINING TECHNOLOGIES Chicago, IL USA Very tight specifications on sulfur content in diesel fuel have recently been announced by the European Union, the U.S. EPA and others. Refiners will be required to produce transportation diesel fuel with sulfur levels below ten ppm (Ultra Low Sulfur Diesel or ULSD). These new specifications will place a severe burden on diesel hydrotreaters as refiners struggle to keep up with diesel fuel demand and quality. As the diesel sulfur specification drops to the low ppm range, it is important to look at the types of sulfur compounds that need to be removed in order to meet the lower sulfur levels. The sulfur compounds can be classified into different groups commonly termed easy and hard sulfur, based on the ease of sulfur removal. The easy sulfur compounds are represented by dibenzothiophenes (DBT) and lighter. The hard sulfur species include multi-substituted dibenzothiophenes commonly represented by 4,6 dimethyl-dibenzothiophene (DMDBT), and these compounds are extremely difficult to desulfurize. The reason for the reactivity differences can be explained by the different reaction pathways for sulfur removal from DBT s and substituted DBT s. DBT s are more effectively desulfurized via a direct sulfur abstraction route, and it is the generally accepted reaction pathway for HDS of diesel to 5 ppm sulfur. Substituted DBT s, on the other hand, are not effectively desulfurized via direct abstraction because the methyl group(s) adjacent to the sulfur atom effectively shield the sulfur from the catalyst active sites. However, hydrogenation of an aromatic ring allows the molecule to flex enabling access to the sulfur atom, and C-S bond scission readily follows. Direct abstraction is catalyzed more effectively by CoMo catalysts, while the hydrogenation-abstraction route is typically more facile over NiMo catalysts. This forms the basis for ART s 2 www.artcatalysts.com
Figure 22 Fraction Hard Sulfur Correlates with Feed End Point.7.6.5 Fraction Hard Sulfur.4.3.2.1. 232 26 288 316 343 371 399 Feed D86 9% Point, C SR LCGO LCO SmART Catalyst System which is tailored to optimize both reaction pathways. The relative amounts of easy and hard sulfur in a feed are critical parameters to consider when choosing a catalyst and operating conditions for production of ULSD. Their concentration can vary significantly from feed to feed depending on crude source, the boiling range, and the treatment the feed has been subjected to whether it be thermal (coker) or catalytic treating (cycle oil). Analysis of a wide array of diesel feeds shows that the fraction of hard sulfur correlates quite well with the end point of the feed. Figure 22 shows the fraction of hard sulfur as a function of D86 9% point for a variety of feeds. The figure clearly shows a strong correlation with the 9% point of the feed. The fraction of hard sulfur increases rapidly for feed T9 between about 316 C and 343 C. Interestingly, the fraction of hard sulfur does not correlate strongly with feed type; whether it is straight run, light cycle oil, or light coker gas oil the fraction of hard sulfur is largely determined by the feed endpoint. The amount of nitrogen in the feedstock is another critical parameter that must be considered. Figure 23 demonstrates the detrimental effects nitrogen has on the removal of hard sulfur. The relative rate constant for hard sulfur removal decreases significantly in going from a feedstock containing 5 ppm nitrogen to a feedstock containing 15 ppm nitrogen. The relative rate constant continues to decrease as nitrogen content increases beyond 15 ppm, but the decrease is at a somewhat slower rate. This inhibiting effect of nitrogen is a result of poisoning of the acid sites needed for aromatic ring saturation, and as discussed above, the saturation-abstraction route is the preferred reaction pathway for hard sulfur removal. Thermodynamic equilibrium is another issue related to poly nuclear aromatic (PNA) saturation which must be taken into account. The saturation of PNA s does reach a thermodynamic constraint on conversion at high enough temperatures. Figure 24 shows the expected PNA conversion as a function of temperature for a variety of LHSV s. The equilibrium constraint is readily apparent for the higher temperatures in the chart. Decreasing the LHSV serves to increase conversion in the kinetically controlled region (i.e. low temperatures), but has no effect on conversion in the thermodynamically controlled regime (i.e. high temperatures). This suggests that adding catalyst volume will help in the removal of hard sulfur, but a point is reached where decreases in sulfur do not occur due to unfavorable thermodynamic equilibrium at those conditions. This raises the potential for situations where significant increases in temperature will not result in appreciable decreases in product sulfur at very low levels. reprinted from Catalagram 95 24 3
Figure 23 Nitrogen Inhibits Hard Sulfur Removal A number of operating parameters take on greater importance in ultra low diesel HDS due to the issues raised above. ART has conducted extensive pilot plant work to define appropriate operating conditions for ULSD and to define the influence of cracked stocks. One study utilized a straight run feed and a 2% LCO blend with the same straight run component, and selected properties of the two feeds are shown in Table X. The operating conditions used in the work are representative of the pressure and H 2 /oil ranges typically found in diesel hydrotreaters. Figure 25 summarizes the influence of H 2 /oil ratio on ULSD production. It is a plot of the relative HDS rate constant as a function of the relative excess hydrogen (H 2 /oil ratio divided by the hydrogen consumption). Data for both high and low pressure operation are shown for each feed. For the high pressure operation, it is apparent that increasing H 2 /oil is beneficial for both feeds up to a certain point. The reactivity of the straight run feed increases from about 6 at low H 2 /oil (i.e. 6% of the highest activity achievable for that feed) to nearly 1 at a H 2 /oil ratio / H 2 consumption ratio of about five. Further increases in H 2 beyond that provide little additional benefit. Similarly, the 2% LCO feed shows a reactivity of about 2 at low hydrogen rate, and steadily increases towards 1 near a H 2 /oil ratio / H 2 consumption ratio of about six. Further increases in hydrogen rate again do not add much additional benefit. These data suggest that for high pressure more excess hydrogen is required for ULSD compared to conventional diesel hydrotreating (i.e. 5 ppm sulfur) where H 2 /oil ratio consumption ratios of between three and four are generally recommended. Table X Feed Properties Straight Run 2% LCO/Straight Run Specific Gravity.847.868 Sulfur, wt.% 1.21 1.56 Nitrogen, ppm 95 2 D86 Dist., C, IP/5/FP 212/288/354 28/286/353 4 www.artcatalysts.com
Figure 24 Thermodynamic Equilibrium Can Limit Conversion The effect of H 2 /oil ratio at lower pressure is quite different. In this case, the relative HDS rate constant for both feeds shows a steady increase with increasing hydrogen rate. Note that the activity is relative to the straight run feed at high pressure and high H 2 rate. The benefit of increasing H 2 /oil never reaches a plateau as observed in the high pressure case, suggesting that more hydrogen is always better at low pressure. The effect of hydrogen pressure is also readily apparent in Figure 25. At relative excess hydro- Figure 25 Excess Hydrogen Improves Reactivity reprinted from Catalagram 95 24 5
Figure 26 H 2 S is Detrimental to Hard Sulfur Removal gen values around three to four, the straight run feed at low pressure has a relative rate constant of about 2 compared to the high pressure case, and the difference for the LCO feed is even greater. Figure 26 summarizes these data in another way. This chart shows the relative activity plotted as a function of H 2 S partial pressure at reactor outlet conditions. Again, quite different behavior is observed for the low and high pressure cases. At high pressure, the data suggest some tolerance for low H 2 S pressure and a steady decline in activity as H 2 S pressure increases beyond that. The straight run feed also appears more forgiving, as it has a slower activity decline compared to the LCO containing feed. Looking at the low pressure data, it is apparent that even a small amount of H 2 S is detrimental to activity. There is a rapid initial decrease in activity with the first increments of H 2 S, and thereafter a slow steady drop-off in activity as H 2 S level increases further. These data are from the pilot plant where there is no H 2 S in the treat gas to the unit. A hydrogen stream containing H 2 S will exacerbate the activity loss due to H 2 S especially at low pressure. This study also investigated the effects of adding catalyst volume and the potential tradeoffs between catalyst volume, operating pressure and H 2 /oil ratio. Figure 27 summarizes the relationship between 1/LHSV or residence time (i.e. catalyst volume), H 2 /oil ratio and hydrogen pressure at 1 ppm product sulfur for the straight run feed. Not surprisingly, there is a significant difference in required catalyst volume between high and low pressure operation at constant H 2 /oil ratio. The catalyst volume at low pressure is nearly three times higher than required for the high pressure operation at typical H 2 rates. Figure 28 is a similar plot, but in this case it is comparing the performance of the straight run feed with the 2% LCO blend at an intermediate pressure. The intermediate pressure is the practical lower limit for ULSD production for the 2% LCO feed. Lower pressures required unreasonably high gas rates and or long residence times (very large reactors). This chart clearly shows the difficulty in treating cracked stocks relative to a straight run feed. At high H 2 rates, 1.5 times higher catalyst volume is required for the LCO, and this increases to nearly two times higher at the low H 2 rate. Of course, increasing the pressure helps somewhat, but even at high pressure the LCO case requires 1.3-1.7 times higher catalyst volume depending upon the H 2 rate. 6 www.artcatalysts.com
Figure 27 Trade-offs Between Pressure, H 2 /Oil and LHSV In summary, it is possible in some cases to trade off pressure, H 2 /oil and catalyst volume, when designing for ULSD production. However, there is a limit which depends strongly upon the feedstock. For example, cracked stocks require higher pressure and higher H 2 /oil ratio compared to a straight run feed. To learn more about producing ULSD at your refinery, contact your ART technical or sales representative. Figure 28 Cracked Stocks Change The Rules reprinted from Catalagram 95 24 7
The SmART Catalyst System TM : Meeting the Challenges of Ultra Low Sulfur Diesel Charles Olsen New Product Development Manager ADVANCED REFINING TECHNOLOGIES Chicago, IL USA In 21, ART introduced the SmART Catalyst System TM to help refiners deal with the severe demands of ultra low sulfur diesel (ULSD). The SmART Catalyst System utilizes state-of-the-art catalyst technology which is staged in the proper proportions to provide the best performance while at the same time meeting individual refiner requirements. The catalyst staging is designed to take advantage of the different reaction mechanisms for sulfur removal; ART CDX, a high activity CoMo catalyst, efficiently removes the unhindered, easy sulfur via the direct abstraction route and ART CDY, a high activity NiMo catalyst, then attacks the remaining sterically hindered, hard sulfur. Pilot plant work has proven that the properly configured SmART Catalyst System provides higher activity than either the CoMo or NiMo catalyst alone. ART CDX and ART CDY, individually or as part of a SmART Catalyst System, were selected for 14 diesel units in 24, and most of these applications aim to evaluate ULSD capability and/or produce ultra low sulfur fuels in advance of the regulations for economic benefit. Optimizing the SmART Catalyst System An important aspect of the SmART Catalyst System is determination of the optimum proportions of the CoMo and NiMo catalysts that will deliver the best performance. This is dependent upon a number of factors, including the refiners requirements, and selected feed properties and operating conditions as discussed in detail previously in Catalagram No. 95 (March 24). One clearly important parameter which must be considered is the boiling range of the feedstock. Sulfur speciation on a wide variety of feedstocks has shown that there is a strong correlation between the fraction of multi-substituted dibenzothiophenes (hard sulfur) and the feed endpoint. Once the D86 endpoint increases beyond about 329 C there is a rapid increase in the fraction of hard sulfur contained in the feed. This has a large impact on catalyst activity as shown in Figure 21. The figure shows pilot plant data comparing results from treating two feeds with different endpoints over the same catalyst under identical conditions. At ultra low sulfur levels there is about 17 C difference in reactivity of the two feeds with the lower endpoint feed more reactive. Clearly, feed endpoint and the amount of hard sulfur are critical parameters that influence the optimum SmART configuration. Another critical feed property that must be accounted for is the nitrogen content. It is generally accepted that nitrogen inhibits aromatic saturation reactions through poisoning of acidic sites on the catalyst. Recall that the primary reaction pathway for removal of hard sulfur is via hydrogenation of an aromatic ring, and it is not surprising that feed nitrogen content has a serious, negative impact on HDS activity. The magnitude of the impact can be seen in Figure 22 which summarizes data for NiMo and CoMo catalyst activity on an SR feed 8 www.artcatalysts.com
Figure 21 Feed Endpoint Impacts HDS Activity 357 C D86 EP 39 C D86 EP Temperature, C 17 C 31 F 1 C 1 1 1 1 Product Sulfur, ppm before and after selectively removing the nitrogen via an adsorption process. The difference in activity on the two feeds is quite large. Increasing the nitrogen content from 25 to 16 ppm results in a loss in HDS activity of 22-28 C for both catalysts. Comparing the catalysts on the low nitrogen feed shows that the NiMo catalyst has about 8 C higher activity relative to the CoMo, and that decreases to an advantage of about 3 C or less on the higher nitrogen feed. This suggests the impact of nitrogen is different for NiMo and CoMo catalysts with the CoMo catalyst more tolerant of nitrogen. This is another important consideration when designing the optimum SmART Catalyst System. Hydrogen availability, in terms of hydrogen pressure and hydrogen circulation, also takes on greater importance in ULSD. Figure 23 is a chart showing how the relative HDS rate constant changes as a function of the excess hydrogen (H 2 /Oil ratio divided by the hydrogen consumption) for both high and low pressure operation. Note the range in operating pressure from low to high represents that typically encountered in diesel Figure 22 Nitrogen Impacts Catalysts Differently 28 Increase in required Temperature, C 22 17 11 6 Base 16 ppm Nitrogen 25 ppm Nitrogen NiMo CoMo reprinted from Catalagram 97 25 9
Figure 23 Hydrogen Availability is Critical hydrotreating. At high pressure, increasing the H 2 /Oil is beneficial for both SR and 2% LCO feeds up to a point, after which further increases in H 2 rate provide little additional benefit. At low pressure, the effect of H 2 /Oil ratio is quite different. In that case, the relative rate constant for both feeds shows a steady increase with increasing hydrogen rate. The benefit of increasing H 2 /Oil never reaches a plateau as observed in the high pressure case indicating that more hydrogen is always better at low pressure. The effect of pressure is also readily apparent in the figure. Comparing the relative rate constant for high and low pressure at a typical H 2 /Oil ratio reveals that the activity at low pressure is only 1-2% of that at high pressure for 2% LCO and SR feed respectively. Controlling Hydrogen Consumption One of the key advantages of the SmART Catalyst System is the efficient use of Hydrogen. Figure 24 illustrates how the system can be tailored to provide the best balance of high HDS activity while minimizing H 2 consumption. The figure shows that as NiMo catalyst is added to the system there is a large increase in HDS activity relative to the all CoMo reference, and eventually, a minimum in the product sulfur curve is reached (i.e. maximum HDS activity). The position and magnitude of this minimum varies with feed and operating conditions such as those discussed above. The figure also shows the relative H 2 consumption, and again, as the percentage of the NiMo component increases, the H 2 consumption relative to the base CoMo system increases. In the region where the system shows the best activity, the hydrogen consumption is only slightly greater than that for the all CoMo system, and well below that for the all NiMo catalyst. This is a result of the different kinetics for sulfur and aromatics removal and is a critical consideration when customizing a SmART Catalyst System. To help understand the differences in kinetics it is useful to compare the performance of CoMo and NiMo catalysts alone. Figure 25 shows a comparison of the hydrogen consumption over a NiMo and CoMo catalyst for a straight run feed at ULSD conditions. The amount of hydrogen consumed by sulfur, nitrogen and olefins removal is essentially the same for each catalyst and is not shown. What separates the two catalysts is the amount of aromatics saturation which occurs, and in particular, the amount of mono ringed aromatics which are hydrogenated. In this case, an additional 12 Nm 3 /m 3 of hydrogen is consumed with the NiMo catalyst due to mono ringed aromatics saturation. This represents excess hydrogen consumption above that required for the removal of sulfur. Figure 26 is a similar chart for LCGO feed. In this example, the NiMo catalyst hydrogenates more PNA (2 rings and greater) and mono ringed aromatics compared to the CoMo catalyst accounting for an additional 19 Nm 3 /m 3 of hydrogen consumption above that required for sulfur, nitrogen and olefins removal. Clearly, in cases where hydrogen consumption needs 1 www.artcatalysts.com
Figure 24 Optimizing HDS and H 2 Consumption Figure 25 Excess H 2 Consumption with SR Feed in ULSD NiMo CoMo HDS HDN Olefins HDS HDN Olefins 28 Nm 3 /m 3 32 Nm 3 /m 3 28 Nm 3 /m 3 2 Nm 3 /m 3 Poly aromatics Mono aromatics Poly aromatics Mono aromatics to be minimized, a NiMo catalyst for ULSD is the wrong choice. Unfortunately, in many of these cases a CoMo catalyst does not provide the best activity for ULSD, and it is precisely these units where the SmART Catalyst System is ideal as it offers the highest activity and is more efficient with hydrogen compared to an all NiMo system. It perhaps appears contradictory that a NiMo catalyst is included in the SmART Catalyst System due to its high hydrogenation activity making it preferred for hard sulfur removal, and yet, under some conditions the hydrogenation activity is too high and excess aromatics saturation occurs. This highlights one of the keys to designing the proper system. The design involves increasing the hydrogenation selectivity of the system to provide highest HDS activity, while at the same time minimizing hydrogen consumption (i.e. minimizing excess aromatics saturation). Figure 27 shows a schematic of the reaction pathway for poly aromatics saturation. Naphthalene, a two ringed aromatic molecule, is featured at the top of the figure. The reaction begins with the hydrogenation of one of the aromatic rings to form tetralin, a mono ring aromatic. The next reaction in the sequence is hydrogenation of the remaining aromatic ring to produce decalin, a fully saturated molecule. Each step is reversible and subject to equilibrium constraints. The slowest reaction in the sequence is the saturation of the mono ring aromatic. The reaction sequence for a substituted biphenyl is also shown at the bottom of the figure. The sequence is essentially the same. In both examples the hydrogenation of the mono aromatic is reprinted from Catalagram 97 25 11
Figure 26 Excess H 2 Consumption with LCGO Feed in ULSD NiMo CoMo HDS HDN HDS HDN Olefins Olefins 46 Nm 3 /m 3 22 Nm 3 /m 3 42 Nm 3 /m 3 8. Nm 3 /m 3 Poly aromatics Mono aromatics Poly aromatics Mono aromatics the slowest step and also consumes more hydrogen compared to the other hydrogenation reactions shown; three moles of hydrogen per mole of aromatic are consumed by the saturation of the mono ring aromatic compared with two moles of hydrogen/mole for the multi ringed aromatics. The reaction sequences shown in Figure 27 can be treated as a series of first order reversible reactions where the intermediate (the mono ring aromatic) is the desired product. For these sorts of reaction systems the intermediate often tends to be favored at shorter contact times (see Chemical Reaction Engineering by Octave Levenspiel [Wiley, 1998], for example), and it this tendency which is exploited when customizing a SmART Catalyst System. The effective residence time in both the CoMo and NiMo beds of a system is short compared to the overall reactor residence time, and this helps minimize the hydrogenation of the mono ring aromatic (i.e. minimize hydrogen consumption). Figure 28 shows the effect of residence time, as indicated by 1/LHSV, on aromatics saturation. For PNA saturation, the two ringed aromatic going to the mono ring aromatic, there is a fairly steep curve for conversion as a function of residence time below about 1 hr. Above that point, which represents space velocities of 1 hr -1 or less there is very little change due to equilibrium constraints. For mono ring aromatic saturation there is a steady increase in conversion as the residence time is increased indicating more and more saturation as the residence time is increased (i.e., LHSV is decreased). Both sets of curves suggest aromatic saturation can be limited by appropriate choice of LHSV, or in the case of a SmART Catalyst System, by adjusting the relative quantities of CoMo and NiMo catalyst. SmART Catalyst System Experience The SmART Catalyst System is the culmination of an extensive effort put towards understanding the chemistry and process conditions required for ultra low sulfur fuels. In addition, properly designed high activity Figure 27 Reaction Sequence for Poly Aromatics Hydrogenation 12 www.artcatalysts.com
Figure 28 Limiting Aromatics Saturation Table VII Commercial Experience with ART CDX, CDY and the SmART Catalyst System Refiner 1 2 3 4 5 6 7 8 Pilot Testing Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Startup Date 4Q4 4Q4 4Q4 2Q5 2Q4 3Q4 1Q5 26 26 26 26 26 26 Catalysts CDX CDX CDX CDX CDX CDX CDX CDX CDX CDX CDX CDX CDX CDY CDY CDY CDY CDY CDY CDY CDY CDY CDY Feedstock: % cracked stock 4 1-2 3 1 1 36 6 Sp Gravity.8628.8514.8468.8665.8735.8534.8448.8956.8649.8198.8676.8933.8328 Sulfur, wt.% 1.66.99.7 1.26.34 1.26 1..9.86.82.87.89.83 EP D86 Dist, C 371 377 335 396 354 38 36 36 285 545 346 363 349 Product : Sulfur, ppm 7 7 8 1 1 35 5 7 7 7 7 7 7 Conditions LHSV.7 1.5 2.8.7 1.1 1.6 1.9 1. 1.1 2.6 1.3 1. 1.6 Inlet P, BARG 54 64 48 56 56 51 44 88 56 41 73 88 5 reprinted from Catalagram 97 25 13
catalysts must be used in order to take full advantage of the SmART System concept. ART has devoted significant resources to designing the best ULSD catalysts for use in the SmART Catalyst System, and this effort has lead to the commercialization in 24 of a new CoMo catalyst CDX, and a new NiMo catalyst CDY. These new technologies benefit from the optimization of the alumina chemistry to give the right surface area and pore size distribution, as well as providing the right surface chemistry (i.e., acidity). ULSD production with the CDX/CDY SmART System began early in 24, in a North American refinery processing a feed containing 2-4% of a high endpoint LCO. Since that time, ART CDX and ART CDY have been selected for 14 different diesel applications either as components in SmART Systems or as a complete charge. A list of commercial applications of CDX, CDY and SmART Catalyst Systems is shown in Table VII. It is apparent from the table that the SmART catalyst technology has been successfully applied to a wide range of feeds and conditions. In most cases it has been selected based on the high activity demonstrated in pilot plant testing. As these and other recent successes demonstrate, Advanced Refining Technologies has developed stateof-the-art technologies aimed at helping refiners meet the challenges of clean fuels. These successes are the result of harnessing the unique heritage of ART which includes a collective expertise in material science, catalyst formulation, and manufacturing knowhow. The science of designing specific catalyst components to operate in an optimum system is a fundamental part of ART's catalyst technology. With the advent of clean fuels, ART seized the opportunity to extrapolate their catalyst system expertise from resid to lighter feedstocks. As a result, ART has been able to deliver high performance technologies ranging from StART Catalyst Systems for Si tolerance in coker naphtha applications, SmART Catalyst Systems for ULSD, and ApART Catalyst Systems for cat feed hydrotreating. ART strives to continuously improve the performance of its catalysts, and the current focus of that effort is on maximizing the effectiveness of the catalytic metals. As described in Catalagram No. 96 (October 24), ART's new CoMo catalyst, CDX, benefits from improved metals utilization through the application of novel metals chemistry and a unique impregnation technique. These same techniques have been successfully applied to a new NiMo catalyst called ART NDXi. As shown in Figure 26, this catalyst has significantly higher activity for ULSD than ART CDY and the conventional reference NiMo catalyst. ART NDXi will be commercialized in early 25 and is slated to become the latest NiMo catalyst component of the SmART Catalyst System. Figure 29 ART NDXi has Step Out Activity 14 www.artcatalysts.com
No Need to Trade ULSD Catalyst Performance for Hydrogen Limits: SmART Approaches Charles Olsen New Product Development Manager Gerianne D Angelo Senior Technical Services Engineer ADVANCED REFINING TECHNOLOGIES Chicago, IL USA I t has been widely discussed that desulfurization of dibenzothiophene and substituted dibenzothiophenes occurs through two reaction pathways, the direct sulfur abstraction route and the hydrogenation-abstraction route. The former involves adsorption of the molecule on the catalyst surface via the sulfur atom followed by C-S bond scission. This is the primary pathway over cobaltmolybdenum (CoMo) based hydrotreating catalysts. The second pathway involves saturation of one aromatic ring of the dibenzothiophene species followed by the extraction of the sulfur atom. Nickel-molybdenum (NiMo) catalysts have a higher activity for desulfurization via this route. It has become a fairly common practice to model ULSD applications by lumping the various sulfur species into easy sulfur and hard sulfur. The so-called easy sulfur reprinted from Catalagram 99 26 15
Figure 9 Optimizing HDS and Hydrogen Consumption Product Sulfur, ppm All CoMo Reference is made up of compounds which are readily desulfurized via direct abstraction and boil below about 363 CF whereas hard sulfur is made up of compounds which are more readily removed via hydrogenation followed by abstraction. These compounds include 4,6 dimethyl-dibenzothiophene and other di- and tri-substituted dibenzothiophenes. The relative amounts of easy and hard sulfur in a feed are a critical property to consider since the concentration of each can vary significantly from feed to feed depending on crude source, boiling range and the prior thermal or catalytic treatment of the feedstock. ART first introduced the SmART Catalyst System in 21 (1,2) in anticipation of the stringent new demands required for ULSD. The SmART system concept is based on a staged catalyst approach designed to exploit the fact that there are two reaction pathways for desulfurization. The system utilizes a high activity CoMo catalyst like ART CDXi for efficient removal of sulfur via the direct abstraction route and a high activity NiMo catalyst like ART NDXi which effectively removes the multisubstituted dibenzothiophenes via the hydrogenation route. Product Sulfur H 2 Consumption SmART Systems All NiMo Reference 1.15 Relative H 2 Consumption 1. A number of factors need to be considered when designing the optimum SmART system for a given application. These have been reviewed in detail previously (3, 4), and include feed endpoint (amount of hard sulfur), feed nitrogen and H 2 availability. One of the big advantages of the SmART System is illustrated in Figure 9. The figure shows that as NiMo catalyst is added to the SmART system there is a large increase in HDS activity relative to the all CoMo reference, and eventually, a maximum in HDS activity is reached. The position and magnitude of this maximum varies with feed and operating conditions, especially H 2 partial pressure. The figure also includes the relative H 2 consumption, and again, as the percentage of the NiMo component increases, the H 2 consumption relative to the base CoMo system increases. Notice, however, that in this case the relationship between H 2 consumption and the fraction of NiMo catalyst is nonlinear. In the region where the system shows the highest activity the hydrogen consumption is only slightly greater than that for the all CoMo system, and well below that for the all NiMo catalyst. Again, the nature of this relationship varies with feed and operating conditions, with a strong correlation to hydrogen availability. It is this ability to balance HDS activity and H 2 consumption to meet individual refiner requirements that sets SmART apart. The balancing of hydrogen consumption and HDS activity is possible because of the different kinetics and reaction pathways for sulfur removal and aromatic hydrogenation. The reaction pathways for sulfur removal were discussed above with the main point being the hydrogenation pathway is critical for ULSD production. The multisubstituted dibenzothiophenes (hard sulfur) are polynuclear aromatic species containing two aromatic rings, one of which must be hydrogenated for efficient removal of the sulfur atom. Thus, the catalyst system needs good hydrogenation activity and selectivity in order to minimize the use of hydrogen. To more fully understand how this works, it is useful to review polynuclear aromatic hydrogenation in general. To begin with, hydrogenation of aromatics is reversible, and equilibrium conversion is less than 1% under practical hydrotreating conditions. The equilibrium conversion decreases with increasing temperature, and therefore, increasing temperature to get higher hydrogenation rates may ultimately result in lower conversion. These reactions are also exothermic which can have an impact on conversion in adiabatic systems. The hydrogenation of a number of poly aromatic species such as naphthalene and biphenyl have been studied by a number of investigators (5) and the work has lead to the reaction networks presented in Figure 1. In the case of naphthalene, the reaction begins with the hydrogenation of one of the aromatic rings to form tetralin, a mono ring aromatic. The next reaction is hydrogenation of the remaining aromatic ring to produce decalin, the fully saturated species. As indicated in the figure, the reactions proceed sequentially with the rate of hydro- 16 www.artcatalysts.com
genation of the final aromatic ring (tetralin) at least an order of magnitude lower than saturation of the first aromatic ring (naphthalene). Interestingly, the rate of tetralin hydrogenation is about the same as that observed for benzene hydrogenation. A variety of substituted naphthalene s have also been shown to follow a similar reaction network with the rate of hydrogenation of the first aromatic ring roughly the same as that observed for naphthalene. Figure 1 Reaction Pathways for Poly Aromatics Hydrogenation naphthalene tetralin decalin k1 biphenyl cyclohexylbenzene bicyclohexyl k3 k2 k4 The hydrogenation of biphenyl proceeds similarly. The hydrogenation is a stepwise reaction with the rate of hydrogenation of the first aromatic ring about an order of magnitude faster than that of the mono ring compound. An important difference is that the rate of the first hydrogenation reaction in naphthalene is about and an order of magnitude faster than the rate of hydrogenation of the first ring in biphenyl. Thus, there is a significant difference between the hydrogenation of an aromatic with two fused rings compared to a two ring aromatic where the rings are not fused. This is an important difference when considering hard sulfur removal since these species do not contain two fused aromatic rings and as such, can be expected to behave more like a biphenyl species. Concentration 1..8.6.4.2. The challenge when designing a SmART system is to provide enough hydrogenation activity to efficiently saturate the first ring on the two ring aromatic (biphenyl type, sulfur containing) molecule, but not so much as to catalyze the final hydrogenation step in the reaction pathway discussed above. The poly aromatic hydrogenation reaction networks shown in Figure 1 can be modeled as first order reversible reactions in series. Figure 11 shows the species concentration profiles as a function of residence time for a hydrogenation reaction sequence such as that for naphthalene. In the example shown, the rate Figure 11 First Order Reversible Reactions in Series: Concentration Profile mono di Sat d Contact time k1=1*k2 k1>k2, k3>k4, and k1>k3 of the first hydrogenation reaction in the series is an order of magnitude faster then the rate of the second hydrogenation reaction. There is a rapid decrease in the concentration of the two ringed aromatic at short times, and a corresponding increase in the mono ringed species. As time increases however, the mono ring aromatic concentration begins to decrease and the fully saturated species begin to build up. This type of concentration profile suggests that there is a window of residence times corresponding to a maximum in the mono ringed aromatic concentration. The way to minimize hydrogen consumption is by avoiding saturation of the mono ringed compounds. This can be accomplished by adjusting the residence time in the CoMo and NiMo beds of a SmART system through varying the amounts of each catalyst. This effectively tunes the hydrogenation activity of the overall catalyst bed to maximize hydrogenation of the poly aromatics and minimize hydrogenation of the mono ringed species. This is explored further on a diesel feed comparing a CoMo and NiMo catalyst in a pilot plant test. Figure 12 compares the HDS activity of the two catalysts as a function of residence time at constant reactor temperature. It is apparent that the NiMo catalyst is more active than the CoMo catalyst at the indicated reprinted from Catalagram 99 26 17
Figure 12 HDS Activity Comparison at High Pressure 5 Product Sulfur, ppm 4 3 2 1 CoMo NiMo..2.4.6.8 1. 1.2 1/LHSV, hrs 4 Figure 13 Aromatics Concentration Profile for CoMo Catalyst at High Pressure Concentration, % 3 2 1 Mono's Sat'd Poly's Concentration, % 4 3 2 1..2.4.6.8 1. 1.2 1/LHSV, hrs Figure 14 Aromatics Concentration Profile for NiMo Catalyst at High Pressure Mono's Sat'd Poly's..2.4.6.8 1. 1.2 1/LHSV, hrs 18 www.artcatalysts.com
Cetane Index Increase Product Sulfur, ppm 6.5 6. 5.5 5. 4.5 4. 3.5 3. 2.5 2..5 CoMo NiMo 1. 1.5 2. 2. 3. 3.5 Relative H 2 Consumption Figure 16 HDS Activity Comparison at Moderate Pressure 1 9 CoMo 8 NiMo 7 6 5 4 3 2 1 Concentration, % Figure 15 Cetane Index Increase with H 2 Consumption 3 2 1...2.4.6.8 1. 1.2 1/LHSV, hrs Figure 17 Aromatics Concentration Profile for CoMo Catalyst at Moderate Pressure 4 Mono's Poly's Sat'd.2.4.6.8 1. 1.2 1/LHSV, hrs conditions, although both catalysts are able to achieve <1 ppm product sulfur if the residence time is long enough. Figures 13 and 14 compare the concentration profiles for poly aromatics, mono ringed aromatics and the saturates as a function of residence time for the CoMo and NiMo catalyst respectively. Both charts show the same concentration trends as shown in Figure 11. There is a rapid decrease in poly aromatics concentration and a corresponding increase in mono ringed aromatics for both catalysts. Clearly, however, the NiMo catalyst is much more efficient at hydrogenating the final aromatic ring as evidenced by a lower maximum mono ringed aromatic concentration and a faster increase in saturates with increasing residence time compared to the CoMo catalyst. At the longest residence time (lowest LHSV), the NiMo catalyst has about 15 numbers (absolute) higher saturate concentration (and about 15 numbers lower mono ringed aromatic concentration) than the CoMo catalyst. That translates to about 53 Nm 3 /m 3 higher hydrogen consumption compared to the CoMo catalyst. This difference decreases rapidly as the residence time decreases. Of course, if there is sufficient H 2 available, the incremental increase in aromatics saturation and the correspondingly higher hydrogen consumption can offer some benefits such as cetane improvement. Figure 15 summarizes the cetane index increase observed for this example. The cetane index increases linearly with increasing H 2 consumption, and ultimately, a cetane index improvement of six numbers was achieved by the high activity NiMo catalyst. Not surprisingly, pressure has a dramatic effect on the performance of NiMo and CoMo catalysts in ULSD. Figure 16 compares the HDS activity of the NiMo and CoMo catalyst for the same feed and conditions as above, but at lower pressure. At reprinted from Catalagram 99 26 19
these conditions the CoMo catalyst is more active at the higher LHSV s (shorter residence times), while at the lower LHSV s the catalysts have essentially the same activity. A comparison of the aromatic species concentration profiles is shown in Figures 17 and 18. In this case, the difference between the catalysts is much less than observed for the higher pressure case, and the degree of saturation achieved by either catalyst is lower. For the CoMo catalyst in Figure 17, the concentration of Mono ringed aromatics never begins to decrease, and the concentration of saturates remains very low. The NiMo catalyst data shown in Figure 18 looks similar, although at long residence time there is a decrease in mono ringed aromatics, the poly aromatic level is lower and the saturates concentration is higher than for the CoMo catalyst case. These data are a good example of the flexibility of the SmART system. In applications where there is sufficient H 2 partial pressure, a NiMo catalyst is likely the most active system for HDS. However, it will consume significantly more hydrogen due to its efficiency at catalyzing hydrogenation reactions. If the incremental hydrogen consumption cannot be tolerated, a SmART system can be designed which will deliver high HDS activity and minimize hydrogen consumption. In cases where the hydrogen pressure is lower, the SmART system is often more active than either component alone without increasing the H 2 consumption significantly over the all CoMo system. SmART Performance Since introducing the SmART system for ULSD, ART has conducted a significant amount pilot work to demonstrate both the activity and stability of these systems. Figure 19 summarizes some of that work comparing ART CDXi and a SmART system consisting of CDXi and NDXi. This particular study 2 www.artcatalysts.com Temperature, C Concentration, % Product Aromatics, vol.% 4 3 2 1 388 382 377 371 366 36 354 349 343. 3 28 26 24 22 2 18 16 14 12 1 Figure 18 Aromatics Concentration Profile for NiMo Catalyst at Moderate Pressure Mono's Poly's Sat'd.2.4.6.8 1. 1.2 1/LHSV, hrs Figure 19 Pilot Plant Aging of a SmART System Normalized to 1.25 LHSV and 55 BARG 1.26 wt.% feed sulfur and 1 ppm product sulfur 25 5 75 1 125 15 175 2 1.25 LHSV and 55 BARG Hours on Stream CDXi SmART Figure 2 Pilot Plant Aging of a SmART System Aromatics Hydrogenation 25 5 75 1 125 15 175 2 Hours on Stream CDXi SmART
spanned about 2 hours at the conditions shown in the chart processing a straight run feed from the West Coast. The H 2 partial pressure is quite good in this example which explains the large activity advantage for the SmART system relative to the all CoMo system. Notice that the slopes of the lines through each data set are essentially the same indicating the ART CDXi and a SmART system of CDXi/NDXi have the same relative stability when producing ULSD. Figure 2 shows how the aromatic saturation activity changed over the course of the 2 hour run. The total aromatics saturation stayed essentially constant for the SmART system throughout the run, while for CDXi there is a suggestion of some slight loss in saturation activity at the end of the run. Although not shown in the chart, the PNA content in the product was also analyzed, and the data tell the same story. Another thing to note is the similarity in aromatics (and PNA) conversion between the SmART system and CDXi. This is an indication that the hydrogen consumption is similar for both systems, even though the SmART system has a substantial activity advantage. The SmART catalyst system or its components have been selected for over 2 applications since being introduced to the market. The technology has been successfully applied to a wide range of feeds Temperature, C 427 399 371 343 3-5% LCO blend with 36-393 C end point 316 2 4 6 8 1 12 14 16 and conditions, and in most cases has been chosen based on the high activity demonstrated in pilot plant testing, both in ART labs and refiner testing facilities. A few of these applications are discussed in the paragraphs that follow. ULSD production with the SmART System began early in 24 in a North American refinery processing a feed containing 3-5% of a refractory, high endpoint LCO. The LCO end point can gat as high as 391 C, and sulfur speciation indicates that the concentration of hard sulfur often exceeds 4 ppm. The performance of the SmART system is shown in Figure 12. Despite processing a feed which was more difficult than anticipated, the SmART system provided excellent performance. The typical deactivation period following start up was exacerbated by the fact that the Table V Commercial ULSD Performance for Refiner A Months on stream 1.5 LHSV.75.75 H 2 /Oil, Nm3/m3 166 166 Inlet pressure, BARG 65 65 WABT, C 334 336 Figure 21 Commercial SmART System Performance 15.75 166 65 346 Days on Stream 18 refiner had to run off a tank of LCO which had built up during the unit turnaround. Even when processing a blend of 5-6% LCO at SOR, the SmART system had no problems making the <1 ppm sulfur target. Once the feed and operating conditions lined out the operation was very stable with essentially no deactivation apparent over the last several months. In another example, Refiner A conducted in-house testing for their ULSD catalyst selection. ART proposed a SmART catalyst system which ultimately tested substantially more active than other catalysts in the program including one from the perceived market leader. As a result, this refiner selected a CDX/CDY SmART system for their ULSD unit. The commercial unit began operation in the fourth quarter of 24, and a snapshot of the performance thus far is shown in Table V. The feed is a blend of up to 4% LCO and light coker gas oil with a sulfur content of about 1.3 wt.%. The unit has decent pressure, but the H 2 /Oil ratio is lower than optimum especially given that the H 2 consumption is typically around 98 Nm 3 /m 3. The performance of the SmART system has met the high expectations set by the pilot plant testing. Product sulfur 7 7 7 reprinted from Catalagram 99 26 21