ADVANCES IN UNICRACKING CATALYSTS Suheil F. Abdo UOP LLC Des Plaines, IL USA ABSTRACT Increased demands for clean transportation fuels has made hydrocracking one of the key processes in modern refineries. In order to meet this increased demand, hydrocracking catalyst development must continue at a rapid pace and catalyst performance must be precisely tailored to deliver the desired product slate and yield characteristics. Application of catalyst science and technology which involves not only materials synthesis and modification but also an intimate understanding of catalyst-feed interactions under the conditions of the process is critical to accomplishing this objective. Examples of successful catalyst design and development at UOP will be given in this paper to illustrate the performance achieved with state of the art catalysts and identify needs for future performance improvements. INTRODUCTION The hydrocracking process has been in wide use in refineries throughout the world due to its ruggedness and flexibility. It is able, in its various configurations, to process a wide variety of feedstocks ranging from vacuum gas oils to light distillates and cycle oils to produce an array of products ranging from high quality distillates and naphthas for transportation fuels to petrochemical feedstocks and even to LPG-s. Its flexibility makes it very well suited to help refiners meet the growing worldwide need to adjust the diesel to gasoline ratios and to accommodate regional variations in the product mix. This flexibility derives from a wide choice of process configurations and catalyst options available. Many process configurations are in use today and they include the single stage once-through and recycle designs, separatehydrotreat designs, with separation of gas and liquids between the treating and cracking reactors, and two-stage designs with separation of gas and liquid after the first cracking stage and recycle of the fractionator bottoms to the second stage. Figure 1 shows examples of the single-stage and two-stage designs Unicracking designs licensed by UOP, LLC. Each of these offers a specific set of advantages, balancing unit cost with the needs for flexibility to handle changing feed properties while still achieving the desired product objectives and adequate catalyst life. When examined closely, one observes that hydrocrackers are highly customized units each designed to suit its users specific objectives. In order to fulfill the needs of such a wide variety of configurations, a range of hydrocracking catalysts must be available and each catalyst must have the flexibility to function efficiently in more than one narrow set of operating conditions. On the molecular level, the environments in which these catalysts operate ranges widely in the level and severity of deactivating influences exerted by 1
refractory feed components, water, ammonia, hydrogen sulfide and other intermediate byproducts. H 2 H 2 R F Products Recycle Oil R F Products Once Through To other units Single Stage H 2 H 2 H 2 R Two Stage R F Recycle Oil Products R Separate Hydrotreat F R Recycle Oil Products Figure 1. Unicracking Flow Schemes These catalysts are bifunctional in nature, containing an acidic and a hydrogenation function (1-6). The acidic function catalyzes cracking of longer chain hydrocarbons to lower boiling shorter chains, while the hydrogenation function is required to initiate the cracking reactions and for reduction of aromatics. The latter is important for improving product properties and maintenance of catalyst stability by protecting it from the deactivating influence of polynuclear aromatics which are either present in the feed or formed as reaction byproducts and eventually lead to formation of carbonaceous deposits. On a fundamental level, hydrocracking has elements in common with two other major refining processes, namely, fluidized catalytic cracking (FCC) and catalytic reforming. In one sense, it occupies a position on a spectrum intermediate between these two processes. On one end of this spectrum is FCC with its total reliance on acid catalysis while reforming is on the other with its primary emphasis on strong, hydrogenation-dehydrogenation function provided by noble metals and weaker acidity provided by alumina supports. Hydrocracking catalysis relies strongly on both the acid and metal functions and it has been previously discussed that successful design of hydrocracking catalysts must achieve the right balance between these two functions (6-9). This balance may vary from one type of catalyst to another, depending on the specific characteristics of the feed, the process conditions and the intended performance objectives as will be discussed in this paper, vide infra. A Brief History of the Unicracking Process: A Catalytic Perspective The advent of modern hydrocracking began with the use of amorphous silica-aluminas and acidic clays as cracking components. In the early days of hydrocracking, acidic clays and 2
amorphous silica aluminas were employed as the acid function. Synthesis of the first manmade analogue of the natural mineral Faujasite was first accomplished with introduction of synthetic X and Y zeolites by the workers at Union Carbide Corporation in the 1940 s and early 1950 s (10). The subsequent discovery of their potential to provide high acidity eventually led to their use in FCC and hydrocracking catalysis. However, they were initially considered unsuitable due to thermal instability. Discoveries of zeolite stabilization techniques by workers at Mobil and WR Grace are well recognized and credited with the wide spread application of zeolites in refining primarily in the FCC process. However, it is less well recognized that discovery of their utility and subsequent application in hydrocracking proceeded simultaneously on a parallel but less noticed path, possibly, due to the lesser penetration of hydrocracking into refining processes during that era. This was first done in the early to mid 1960 s in the North American market, which was primarily a naphtha market dominated then by Chevron s Isocracking technology based on amorphous silicaalumina catalysts (11). Their very strong acidity, compared to amorphous silica-aluminas, rendered them much more tolerant to the deactivating influence of basic compounds. This in turn led to designs of single stage units where the catalyst operated in ammonia-rich first and single stage configurations without the need for gas-liquid separation prior to the feed entry into the cracking reactor. High zeolite acidity and the resulting high catalyst activity allowed for the production of naphthas by hydrocracking distillates and light gas oils to feed the demand of the North American market and established the Unicracking Process as a major hydroprocessing technology. The technology was originally co-licensed by The Standard Oil of New Jersey (Exxon) and Union Oil Company of California (Unocal) with Union Carbide as the catalyst supplier and was called the Unicracking-JHC process. The joint licensing arrangement was terminated by mutual consent of the parties in the mid 1970 s and the Unicracking process continued to be marketed by Unocal. Simultaneously, UOP was a major licensor in the distillate market due to the much larger demands of its overseas client base. In the late 1980 s Carbide s strength in zeolite technology and UOP s strength in process design and engineering were combined to form the new UOP. Today s package of Unicracking technologies was completed with the acquisition of Unocal s technology in 1995 thereby consolidating all the pieces of this strong offering and covering the full spectrum of hydroprocessing. Another major milestone in the development of hydrocracking catalysis was reached with the discovery of zeolite modification methods to enable a significant shift in the selectivity of zeolite-based hydrocracking catalysts to produce high quality middle distillates. Zeolite dealumination, as measured by the shrinkage in the unit cell size (UCS), was discovered by workers at Union Carbide to reduce acidity with a consequent reduction in catalyst activity and increase in selectivity to jet and diesel products (12). This improvement in selectivity to heavier products is attributed to a reduction in the number of acid sites and an increase in the distance between them as a result of severe hydrothermal treatment. Yet the residual level of activity still exceeded that of amorphous catalysts providing the advantages of longer cycle lengths or smaller reactor volumes. This work, which took place in the early 1980 s, culminated with the introduction of the first high activity catalyst for production of middle distillate, HC-22, in the mid 1980 s in the Neste-Oy hydrocracker in Finland. It has been replaced today by the widely used DHC-32 catalyst, which was developed to combine the best elements of HC-22, and its then competitive product, DHC-100, for production of high quality jet and diesel products. It is interesting to note that the discovery of the 3
performance characteristics of low unit cell size zeolites and their application in FCC for production of high octane gasoline through reduced hydrogen transfer was much more recognized in the refining world (13). Introduction of these low UCS catalysts into middle distillate hydrocracking service served to dispel the prevailing belief at that time that zeolites, due to their narrow pore geometry, could not be employed for processing heavy gas oils to produce distillates. The Current Generation of Unicracking Catalysts In the intervening years, advances in zeolite science have led to a much better understanding of the structure-function requirements of zeolites. It allowed for much more precise tailoring of their properties to more precisely match their acidity and framework composition to their functional requirement. An illustration of these advancements can be found in the breadth of application of Y zeolites in hydrocracking, ranging from maximum production of naphthas all the way to production of high quality distillates. More importantly, the art of hydrocracking catalyst development has also progressed due to an improved understanding of how to optimize the use of zeolitic and amorphous acidic components and their interaction with the hydrogenation metals to deliver the desired performance. Thus the range of commercial Unicracking catalysts has expanded to not only cover the full spectrum of performance and product objectives, but also to accurately fit specific niches within this spectrum. On the one end of this range is the high activity catalyst, HC-28, which is the most active hydrocracking catalyst currently in use for maximum naphtha production. It is based on a highly active proprietary form of Y zeolite. Near the other end of this range is the spherical, all-amorphous, DHC-8 catalyst which has been the dominant catalyst used in maximum production of distillates, especially, diesel fuels. Its neighbors DHC-32, and DHC-39 add the flexibility to shift the product slate to jet from heavy diesel while providing high activity and cycle length should the unit economics favor such a shift. A new generation of catalysts for the maximum distillate side of this spectrum has also been developed, including HC-115 and HC-215 that are intended to deliver improved activity over DHC-8 at equal or better yields. A flexible family of catalysts designed for production of jet and heavy naphthas is in the middle of this spectrum and is represented by catalysts such as HC-43 and HC-150. A new base-metal high activity catalyst, HC-29, designed for naphtha service has also been introduced. A particularly attractive feature of this catalyst is its ability to maintain high activity on the presence of high nitrogen in the feed. The wide-ranging performance characteristics of the Unicracking family of catalysts derives from a very large underlying knowledge of materials properties, process fundamentals and the ability to couple the two to achieve target performance. The balance of this paper will discuss some key elements of our approach to hydrocracking catalyst design and illustrate the successful outcome that can be achieved from judicious acquisition and application of fundamental knowledge. Fundamental Elements of Hydrocracking Catalysts Numerous discussions and treatises have appeared in the literature describing the fundamentals of hydrocracking catalysis (1-3) and the mechanistic underpinnings of the 4
59.0 57.0 Produ ct A PI Gravity 55.0 53.0 Noble Metal-Y1 Fresh Noble Metal Y1 Spent 51.0 Base Metal-Y2 Fresh 49.0 47.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 Conversion to 165 C minus, vol% process are well understood. Briefly, these bifunctional catalysts require an acidic component to catalyze scission of the carbon-carbon bonds and a metal function for the formation of olefinic intermediates, which are thought to initiate the cracking reactions, and for saturation of aromatics. Another key requirement of the catalyst is the appropriate porosity to allow diffusion of reactants and product to and from the active sites, respectively. Maintenance of each of these functions not only at the start of the operating cycle but also throughout the operating cycle is a key to successful performance. Much discussion has taken place in the hydrocracking open literature about the importance not only of having the right levels of each of these functions in the catalyst, but also of achieving the right balance between them (7-9). The optimum ratio of these two functions varies depending on unit design, feedstock composition and processing objective. In general, increased strength of the hydrogenation function results in much more positive than negative performance attributes including catalyst stability and product quality. But as a practical matter, these must be balanced against catalyst cost and constraints on hydrogen availability. An example of how differences in the hydrogenation strength of a catalyst can affect product properties is given in Figure 2. The API gravity of the total liquid product from two catalysts differing significantly in their hydrogenation capability as a function of conversion to naphtha cut point. Both catalysts are based on a strong acid function with a noble metal hydrogenation function in catalyst A and a base metal hydrogenation function for catalyst B. The noble metal catalyst yields a higher API gravity (lower density) product at equal conversion compared to the base-metal catalyst with the weaker hydrogenation function. Higher API gravity, of course, results from the lower aromatics content in the product. Catalyst C in this figure is a spent sample of the noble metal catalyst discharged from commercial service. Attenuation of the hydrogenation function by coking of this catalyst is evidenced by the significantly lower product API gravity compared to its fresh counterpart at all conversion levels. At higher conversion levels this catalyst shows even lower product gravity than the base metal catalyst. The parallel slopes from catalysts A and B suggest that the relative rates of the hydrogenation to cracking reactions are higher for the noble metal catalyst and evolve similarly with temperature for both of these fresh catalysts. On the other hand, the lower slope of the spent catalyst indicates a higher temperature response for the cracking function compared to the hydrogenation function and suggests that the latter is more severely hindered by coking. Detailed studies of this type shed valuable insight into mechanisms of catalyst deactivation and provide invaluable information for directed catalyst design. Figure 2. Influence of Hydrogenation Function Strength on Product Properties While a strong hydrogenation function is most often desirable in distillate production due to improvement in product properties and improved catalyst life, it must be tempered by the limits of hydrogen availability due to cost and design pressure limitations. Thus, selective use of available hydrogen by hydrogenating specific feed components is an important objective to keep in mind in the catalyst design phase. To illustrate, a hydrocracker producing middle and heavy naphtha to be further processed in a downstream reformer could lose efficiency in hydrogen utilization if excessive saturation of aromatics to naphthenes were to take place in 5
the hydrocracker only to have the naphthenes reconvert to aromatics in the reformer. Thus, less hydrogenation of the C-11 minus aromatics in the hydrocracker to the corresponding naphthenes would be desirable. On the other hand, a unit processing vacuum gas oils, deasphalted oils and similar refractory feeds must employ catalysts with the strongest hydrogenation function suited for this service in order to maintain catalyst stability by hydrogenation to prevent formation of heavy polynuclear aromatics (HPNA s) and excessive coking. A proper choice of the type and level of hydrogenation metals is critical to address the challenge of HPNA control. Design Tools and Approaches Rational design of hydrocracking catalysts requires a systematic assessment of the intended service and selection of the catalytic materials most likely to deliver the desired performance. The major steps of such an assessment involve:!"determination of the functional requirements of the catalyst!"selection of best materials and material properties including the acid component, metal function and the diffusion characteristics.!"material modification sequences required to achieve these properties!"pilot plant testing and evaluation of prototype catalysts!"scale up and manufacturing of finished product From a catalyst research and design perspective, the first three steps above are research intensive and require knowledge of catalysis and materials fundamentals. Attaining this knowledge requires use of the appropriate characterization tools to help assess the key properties of catalysts and catalyst components. With zeolites, for example, a deeper look beyond the unit cell size measurements to other key properties such as framework composition, acid site distribution and porosity characteristics is critical to successful catalyst design. Other characterization techniques such as Infrared spectroscopy (IR) and nuclear magnetic resonance spectroscopy (NMR), acidity measurements and selective adsorption are all utilized and their effectiveness will be presented in the following section s. A. Interrelationship Between Catalyst Properties and Process Conditions In order to achieve the correct balance between catalyst functions to meet the performance objectives, it is very important to match the catalyst properties to the feedstock and process conditions. For example, ammonia presence in the reactor environment suppresses the catalyst acidity and cracking activity as illustrated in Figure 3. A steep increase in the temperature required to achieve constant conversion occurs with increasing ammonia content of the recycle gas. Catalyst acidity must be adequate to withstand the deactivating influence of ammonia and other basic molecules in the feed. Basic nitrogen heterocyclic compounds can exert an even more severe and deactivating influence on the catalyst before they are converted to ammonia and the hydrocarbon fragment due to their higher adsorption strength. Conversion of these heterocyclic compounds is thought to occur via a bifunctional mechanism (14) requiring some level of acidity and hydrogenation on the catalyst and is known to be hydrogenation limited. Thus, allowing access of these molecular types in the 6
feed to the correct ensemble of active sites requires not only correct matching and placement of the active sites, but also the appropriate porosity to allow ingress and egress of reactants and products to and from these sites. It is especially critical to prevent excessively long residence times of these molecules at the sites, which may lead to side reactions such as coking and secondary cracking. Optimizing catalyst performance requires a detailed knowledge of the properties of catalytic materials such as aluminas, amorphous silica-aluminas, zeolites and hydrogenation components including base metals and noble metals and the ability to manipulate them. For example, to achieve optimum performance when processing heavier gas oils to produce distillate products, amorphous silica-aluminas are employed, at as in part, as acidic cracking components. However, to maintain the proper balance between the cracking and hydrogenation functions, the type as well as the strength of each of these functions must be carefully selected. The acidity of amorphous silica-aluminas varies with composition going through a maximum. However, the distribution of strength and location of the acid sites varies widely depending on the manner of preparation and the maximum can shift as a result of the manner of preparation. This results in widely different performance depending on the exact synthesis route selected. Other widely employed sources of acidity are zeolites, which offer regular geometrically defined micropores and channels, in addition to strong acidity. Zeolites are well known to possess much stronger acid strength than amorphous silicaaluminas, hence they are employed in processing options where a high degree of cracking is required, such as in the production of naphthas. Required Catalyst ABT, deg. C +78 +67 +56 +44 +33 +22 +11 78 ppm 51 ppm 41 ppm 33 ppm 6 ppm 17 wppm 0 ppm Base 0 50 100 150 200 250 300 350 400 Time on Stream, hours Figure 3. Effect of NH3 on Hydrocracking Catalyst Activity Even though the scientific and technical literature is replete with information related to the characterization of catalytic materials, the knowledge embodied therein is devoid of value unless it is utilized in the proper context. The choice of starting material characteristics, selection of the appropriate modification procedure and the eventual steps of metal 7
incorporation and forming are critical in controlling catalyst performance characteristics. For example, the importance of zeolite unit cell size and silica-to-alumina ratios in determining the performance of FCC and hydrocracking catalysts are well recognized (13, 15). However, advanced characterization tools are required to more accurately assess their critical properties. Among these are various acidity measurements and spectroscopic techniques such as infrared measurements (IR) and solid state NMR. An example of the level of detailed characterization required to completely define zeolite properties and project their performance is given in Figure 4. Here the hydroxyl group region in the infrared spectra of several Y zeolites prepared by a variety of modification methods including steam stabilization, framework and extra-framework aluminum removal by a variety of chemical treatments and acid washing. Even though the basic geometry of the zeolite structure remains the same, such changes in the exact nature and distribution of hydroxyl groups reflect key differences in acidity function among these samples which in turn results in major differences in performance. It illustrates how varying the combination of treatments applied can result in a large variation in the acidic OH groups. Description Si/Al2 Si/Al2 a0 Chem NMR USY -1 5.2 9.0 24.56 USY -2 8.6 11.0 24.49 USY -3 9.6 17.1 24.43 USY -4 13.9 23.0 24.38 USY -5 8.9 9.3 24.52 0.60 USY-3 USY-5 0.50 USY-2 USY-1 0.40 0.30 Re l. A 0.20 USY-4 0.10 3800 3700 3600 3500 0.00 3400 w ave num ber 9560-88A 9951-10A 9951-10B 9951-10C 9951-10D Figure 4. Infrared Spectra of Y Zeolites Stabilized by Different Methods The influence of the sequence of stabilization and acid-washing treatments can be seen to alter not only the total spectral intensity but also the relative distribution of the various OH species, which represent different acid strengths and local geometries. To get a closer look at acidity, pyridine adsorption can be employed to monitor the strength and distribution of Bronsted and Lewis acid sites. This is carried out at UOP in an in-situ appratus where pyridine adsorption and sample treatment can be carried out at the IR spectrometer under a controlled atmosphere and without exposure of the sample to air. In a typical experiment, pyridine is adsorbed on the samples at room temperature and then desorbed in a stream of flowing helium as the sample is heated in the infrared apparatus and the spectra are recorded at successfully higher temperatures. The amount of pyridine remaining on the sample at a given temperature is related to the adsorption strength and by inference to the strength of the 8
acid site at which it is adsorbed. Thus, acid strength distribution between weak, medium and strong acid sites can be assessed by measuring the amount of residual pyridine after desorption at successively higher temperatures. Figure 5 gives these results for the same set of samples illustrating how the differences observed in the OH spectra correspond to differences in sample acidity. Variations within the envelop of OH band in the infrared spectra reflect changes in acid site distribution reflected in pyridine adsorption work. Description Si/Al2 Si/Al2 a0 Chem NMR USY -1 5.2 9.0 24.56 USY -2 8.6 11.0 24.49 USY -3 9.6 17.1 24.43 USY -4 13.9 23.0 24.38 USY -5 8.9 9.3 24.52 B-wk B-med B-str B total L total USY-1 USY-2 USY-3 USY-4 USY-5 Bronsted acidity: B-wk = weak, B-med = medium, B-str = strong B total and L total are total Bronsted and Lewis acidities Figure 5. Acid Site Distribution of Stabilized Y Zeolites by Pyridine Adsorption Manipulation of zeolites to alter the aluminum distribution between Bronsted sites of various strengths and Lewis sites is one of the common tools employed to control catalyst activity and selectivity. However, this can only be carried out successfully if its impact on the remaining ensemble of changing catalyst properties discussed above is also controlled. Table 1 shows the performance data of three catalysts prepared using three zeolites with different silica-toalumina ratios as they were evaluated in a naphtha-hydrocracking pilot plant test. The catalysts were evaluated with highly hydrogenated recycle oil derived from a Unicracking unit processing a light gas oil of straight run and coker derived origins. Doping with the appropriate amounts of NH 3 and H 2 S precursors was used to simulate ammonia-free second stage reactor environment and the ammonia-rich first stage environment. The tests feed stock properties are given in Table 2. 9
Table 1. Influence of Zeolite Framework Composition on Hydrocracking Activity Catalyst SiO 2 /Al 2 O 3 1 st Stage Activity* 2nd Stage Activity* A 6 Base Base B 9 Base -5 Base + 8 C 12 Base -8 Base + 14 * Temperature required to achieve 50% conversion to naphtha cutpoint (165 C minus). 1 st stage environment contains 2000 ppmv NH 3 and 5000 ppmv H 2 S. 2nd Stage is ammonia free and contains 5000 ppm H 2 S. Table 2 Feedstock Characterization and Reaction Conditions for Hydrocracking Activity Test Feedstock: Atm+Coker Gas Oil Sulfur: var. Nitrogen: var. API Gravity: 35.7 Distillation: IBP 160 C 5% 182 10% 199 20% 218 30% 235 40% 260 50% 285 60% 310 70% 332 80% 357 90% 391 95% 416 EP 454 Test Conditions: Pressure LHSV Temperature H 2 /HC ratio 1400 psig 1.7 hr -1 Varied 8000 scfb Careful consideration of these data illustrates the complexity of attempting to link catalyst performance to a single catalyst property, even one as important as zeolite acidity. Under ammonia-rich conditions, catalyst activity increases (lower required temperature) as the zeolite silica-to-alumina ratio increases, consistent with our understanding of the role of acidity in cracking. However, in the absence of ammonia, an opposite trend of decreasing catalyst activity is observed as the silica-to-alumina ratio increases. In order to rationalize 10
these seemingly contradictory trends, we briefly consider the underlying bifunctional hydrocracking mechanism described in the literature and summarized as follows (1-3) Dehydrogenation of Paraffins # Olefinic Intermediate Metal Sites (1) Protonation of Olefin #Carbenium Ion Intermediate Acid Sites (2) Carbenium Ion Rearrangement and Cracking to Products Acid Site (3) Hydrogenation of Olefin Intermediates and Desorption Metal Sites (4) According to this mechanism, the critical first step is the dehydrogenation of reactants to form olefinic intermediates (1). These are subsequently protonated at the acid site to form very reactive carbenium ions (2). These subsequently undergo rearrangement to the most stable tertiary carbenium ions (3) and are either followed by beta scission to yield a smaller chain product and another carbenium ion or combine with a hydride ion at the metal sites and desorb as products (4). Under second stage conditions and without the poisoning influence of ammonia, the acidity of all three zeolites provides an adequate supply of acid sites to catalyze the protonation (2) and subsequent carbon-carbon bond scission steps. One can then hypothesize that the reaction rate is controlled by another step not involving the acid sites. Possibly, the dehydrogenation or hydrogenation of reactants and intermediates, respectively, which takes place at the metal functions, step (1) or (4) is rate determining under these conditions. A hypothesis invoking an increased deficiency in the hydrogenation function follows a trend opposite to that of the acid function would be consistent with these results. The change in the composition of the zeolite framework could then exert a secondary influence on the ability to effect good dispersion of the metals. One may conclude from this experiment that steps taken in the control and modification of the acid function of the catalyst can also influence disposition of the metal function. They are intimately interrelated and must be approached in a combined fashion. Clearly, more than one simple property determines catalyst performance in the complex and varied environments of hydrocracking and the gross oversimplification often encountered in published literature rarely describes these systems adequately. Thus, correctly matching the ensemble of properties in the catalyst while keeping to the service requirement of the catalyst are of paramount importance to successful design of hydrocracking catalysts. B. Importance of Materials Modification and Manufacturing Procedures It is often impossible to define the key performance-related characteristic of a given material without knowledge of the precise path followed to arrive at the target properties. This is illustrated in the following example where two catalysts derived from zeolite samples having similar unit cell sizes and chemical composition are shown to give significantly different activities and selectivities. Table 3 compares the key properties of two samples with similar chemical compositions and similar zeolite unit cell sizes and shows the properties of their parent material and intermediates. The properties shown here are often discussed in the literature as being the key determinants of derived catalyst performance. However, the performance data reveal a significant difference in their activity and selectivity. These data were obtained from pilot plant testing using a light Arabian gas oil feed with temperature 11
adjusted to achieve 80% conversion to 371 C minus distillate. Despite the apparent similarity in the zeolite properties, the selectivity data for sample S-1 shows a shift towards jet and diesel boiling ranges compared to sample S-2. Sample S-2 on the other hand displays a higher make of heavy naphtha and a higher activity and a lower required temperature to achieve the same conversion level. Even though catalysts S-1 and S-2 were derived the same parent zeolite, differences in the exact conditions and sequence of intermediate steps followed in the preparation of each material resulted in a significant shift of activity and selectivity in hydrocracking service. The underlying causes of this difference in performance can be understood only after closer examination of the zeolite properties. Application of advanced characterization tools such as IR, 29 Si and 27 Al NMR was necessary to clearly identify the key differences in the properties of these materials, which underlie their performance differences. Figure 6 shows the OH stretching region of the infrared spectrum of zeolite samples produced by the same methods used for the zeolites in catalysts S-1 and S-2. The spectra clearly reveal significant differences in the type of OH vibrations between these two samples indicating a difference in the distribution of acid sites which could not be discerned by simply looking at the gross properties shown in Table 3..35.3.25 Absorbance.2.15 S-1.1.05 S-2 0 3800 3750 3700 3650 3600 3550 3500 3450 3400 Wavenumber (cm -1 ) Figure 6. Influence of Zeolite Stabilization Route on the Hydroxyl Infrared Spectra This example shows that commonly employed tools such as the zeolite unit cell size (UCS) are not adequate for predicting performance in hydrocracking and other hydrocarbon conversion processes. Detailed characterization by advanced techniques is an important part of the process of catalyst design. Use of cutting edge tools such as Multiple Quantum Magic Angle Spinning (MQ-MAS) NMR can reveal very important differences in the aluminum distribution in zeolites that are not observable by other methods. This is illustrated in the 12
MQ-MAS spectrum of a modified zeolite in Figure 7 which shows clear evidence that aluminum has three different coordination geometries including tera-, penta- and hexacoordinated aluminum. Use of such tools serves to guide researchers in proper selection of catalyst components to meet the needed performance target. Table 3. Influence of Zeolite Modification Route on Performance in Hydrocracking Parent/Sample P S-1 S-2 Chemical SiO 2 /Al 2 O 3 ~5.5 ~9 ~10 UCS, angstroms 24.68 24.36 24.36 HC Testing Activity, C 363 358 Yields, vol% 85-149 13.3 14.2 149-288 41.5 39.5 288-371 11.4 8.0 6 coordinate Al 5 coordinate Al 4 coordinate Al Figure 7. MQ-MAS Spectrum of Stabilized Y Zeolite C. Matching Catalyst Properties to Feeds and Operating Objectives Having illustrated the great potential for adjustment of hydrocracking catalyst performance by judicious selection and adjustment of key properties, we can now focus on a more detailed assessment of the reactivity of feed components over catalysts with differing compositions. It 13
is widely accepted that as the feed becomes heavier and the average hydrocarbon chain becomes longer, diffusion in and out of catalyst pores becomes more hindered. This leads to lower reactivity due to difficulty of accessing active sites and loss of selectivity due to higher rates of secondary cracking if the primary reaction products must diffuse through a tortuous path before escaping the catalyst surface (4,5). This is illustrated in Figure 8 where a trace of the simulated distillation results for a light Arabian gas oil feed is compared to the portion products of boiling above 205 C from two different catalysts. Catalysts A and B both have the same level of NiW hydrogenation metals but catalyst B contains twice the zeolite content of catalyst A and therefore has narrower pores. It is seen here that conversion of the heavier 205 C plus fraction of the product is higher over catalyst A compared to the higher zeolite catalyst B indicating that the very high zeolite content of catalyst B is unsuitable for processing this type of heavy feed. This trend, of course, is expected to become worse as catalysts age in commercial service due to the further narrowing of small zeolite pores of by deposition of coke. Processing lighter feedstocks, however, requires a different catalyst design strategy. Here, high zeolite catalysts are more beneficial because of their better ability to handle the relatively shorter chain hydrocarbons present. This is exemplified on the molecular scale by the data shown in 110 100 90 80 70 Vol % Off 60 50 40 30 20 10 0 150 200 250 300 350 400 450 500 550 600 650 Temperature, C Feed Catalyst A Catalyst B Figure 8. Impact of Catalyst Porosity on Conversion of Heavy Feed Components Figure 9 where the same two catalysts discussed above along with a third, high zeolite, catalyst possessing a stronger hydrogenation function are compared. The feed of this experiment is significantly lighter than above and the testing was conducted under clean conditions simulating a second stage hydrocracking environment, where the catalysts are unhindered by the presence of highly refractory molecules or ammonia poisons. The feed 14
endpoint is about 285 C. The data here are presented in terms of the carbon number distribution of the feed and products to better. 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Carbon Number Feed Catalyst A Catalyst B Catalyst C Figure 9. Impact of Catalyst Porosity and Metal Function on Conversion of Light Feeds illustrate the results on the molecular level. Examination of the trends illustrated in this data shows that, for a constant metals level, increasing the zeolite content from catalyst A to B gives better conversion of the larger molecules in the C-11+ (~195 C plus) range and produces a higher content of C-9 minus naphtha range product. However, the combination of high zeolite and a much stronger hydrogenation function represented by catalyst C results in a much more drastic increase in conversion of the longer chain hydrocarbons while yielding a significantly higher content of light products under the conditions of this test. Clearly, this experiment illustrates the critical importance of control and modification of more than one component in designing hydrocracking catalysts for different applications. Together with the data in Figure 8 they illustrate the critical importance for careful matching of various catalyst components to the feed properties and process conditions. Conclusions The discussion presented here attempted to illustrate some of the key steps and considerations involved in successful design of hydrocracking catalysts. It demonstrates that the path to successful catalysts includes careful selection of materials with the right acidity, porosity and hydrogenation coupled with careful matching of their catalytic properties to the process conditions where they are expected to operate. Discussion of some of the key principles in hydrocracking catalyst design has demonstrated the breadth of the underlying, world-class tools and methodologies that are put to use at UOP in support of continued developments of the Unicracking process. As a major licensor of refining process and catalyst technology, UOP has focussed on the widely varied needs of refiners throughout the world. This focus combined with the wealth of core skills in all hydrocarbon-processing technologies places 15
UOP in a primary position to contribute to the refiners goals of efficient and economic production of high quality transportation fuels and, generally, to the upgrading of low value feedstocks. Acknowledgements The NMR measurements were carried out in UOP s Advanced Characterization Group by Dr. Scott Blackwell, Ms. Linda Laipert and Mr. Alfred Le Comte. The IR experiments were condcuted by Dr. Thomas Mezza and Ms. Padma Tota. Their contribution to the work described here and their valuable insight in discussion of the work are gratefully acknowledged. REFERENCES [1] H.L. Coonradt, and W.E. Garwood, Ind. Eng. Chem. Process Des. Develop., 3, 38 1964). [2] J. Weitkamp, Preprints, Div. Petro. Chem., A.C.S., 489(1975). [3] J. Weitkamp, ACS Symp. Ser., 20, 1(1975). [4] B. Debrabandere and G.F. Froment, Influence of the Hydrocarbon Chain Length on the Kinetics of the Hydroisomerization and Hydrocracking of n-paraffins, G.F. Froment et al. (eds.), 1997, Elsevier Science B.V., Hydrotreatment and Hydrocracking of Oil Fractions, Proceedings of the 1 st International Symposium/6th European Workshop, Oostende, Belgium, February 17-19, 1997, p.379. [5] E.J.P. Feijen, J.A. Martens and P.A. Jacobs, Isomerization and Hydrocracking of Decane and Heptadecane on Cubic and Hexagonal Faujasite Zeolites and Their Intergrowth Structures, 11 th International Congress on Catalysis, Studies in Surface Science and Catalysis, J.W. Hightower, et al (eds.) Elsevier Science B.V., 101, 721(1996). [6] A.J. Gruia and J. Scherzer, Hydrocracking Science and Technology Marcel Dekker, Inc., New York, 1996, p.135. [7] G.J. Antos, Catalyst Design and Performance Development of a Catalyst for Gas Oil Desulfurization presented at the 7 th Annual Symposium KFUPM Research Institute, November 30 December 2, 1997 [8] G.J. Antos and J.P. Marks, Predicting the Stability of Maximum Distillate Hydrocracking Catalyst from Feedstock Analysis presented at the 8 th Annual Saudi-Japanese Symposium KFUPM Research Institute, November 29-30, 1998. [9] G.J. Antos and Li Wang, Predicting the Stability of Maximum Distillate Hydrocracking Catalyst from Feedstock Analysis presented at the 9 th Annual Saudi-Japanese Symposium KFUPM Research Institute, 1999. [10] R. M. Milton, U. S. Patent, 2,882,243 [11] R. F. Sullivan, J. W. Scott, Heterogeneous Catalysis: selected American Histories, A. C. S. Symposium Series 222, Washington, D. C., 293 (1983) [12] R. D. Bezman, J. A. Rabo, U. S Patent, 4,401,556 [13] L. A. Pines, F. K. Maher, W. A. Wachter, J. Cat. 85, 466 (1984) [14] H. Topsoe, B. S. Clausen, F. E. Massoth, Hydrotreating Catalysts-Science and Technology, Springer- Verlag, 1996 and references there in. [15] J. Scherzer, in J. S. Magee, M. M. Mitchell, (Eds.), Fluid Catalytic Cracking-Science and Technology, Elsevier (1993) 16