Key Role of Reactor Internals in Hydroprocessing ... - ACS Publications

Energy & Fuels 2007, 21, 1731-1740

1731

Key Role of Reactor Internals in Hydroprocessing of Oil Fractions Anton Alvarez,† Sergio Ramı´rez,† Jorge Ancheyta,*,†,‡ and Luis M. Rodrı´guez§ Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Col. San Bartolo Atepehuacan, Me´ xico D.F. 07730, Escuela Superior de Ingenierıá Quı´mica e Industrias ExtractiVas (ESIQIE-IPN), Me´ xico D.F. 07738, Pemex Refinacio´ n, Gerencia de Ing. de Procesos, Subdireccio´ n de Produccio´ n. Me´ xico D.F. ReceiVed December 21, 2006. ReVised Manuscript ReceiVed February 12, 2007

Several aspects of fixed-bed hydroprocessing reactor internals have been reviewed. Fundamentals of conventional and modern reactor internal hardware such as distributor trays and quench boxes are described, and examples of commercial systems are presented. The methods for detecting maldistribution and cases of successful revamping are also discussed. It was recognized that properly designed reactor internals improve substantially unit performance by increasing product quality and extending catalyst cycle length.

1. Introduction Most of the fixed-bed hydroprocessing reactors currently in operation in worldwide petroleum refineries have been built and designed over the past 30 years.1 Traditionally, it has been of common practice that when refiners acquire hydroprocessing technologies from licensors reactor internal hardware design is also included.2 New design of reactor internals along with the constant catalyst improvements has allowed these units to maintain an acceptable performance to meet the more stringent fuel specifications keeping catalyst cycle life and run length within economically attractive limits. However, those units have been experiencing underperformance with the increasing supply of heavier oils to the refineries and the tightening environmental legislations. The problems of constant changes of feedstock properties and product quality were partially solved with increases of reaction severity, which reduced considerably the catalyst cycle life due to enhanced catalyst deactivation. Mechanical constrains in reactor design and product demand were also other problems that refiners had to face when trying to increase reactor temperature and reduce feed flow rate (i.e., decrease space velocity), respectively. In addition, excessive pressure drops were present due to fouling caused by solids contained in the feed (iron scale, salts, coke fines, etc.) and reaction products (coke and metals).3 All these problems drastically diminished the length of run due to premature shut downs required for replacing the catalyst with a consequent negative impact on the overall economics of the process and refinery.4 Over the years, many strategies have been proposed to meet the product specifications dictated by the clean fuels challenge and at the same time to keep the catalyst cycle life at acceptable * Fax: (01-55) 9175-8429. E-mail: [email protected]. † Instituto Mexicano del Petro ´ leo. ‡ Escuela Superior de Ingenierıá Quı´mica e Industrias Extractivas (ESIQIE-IPN). § Pemex Refinacio ´ n. (1) Swain, J.; Zonnevylle, M. Are You Really Getting the Most from Your Hydroprocessing Reactors? Presented at the European Technology Conference, Rome, November 15, 2000. (2) Jacobs, G. E.; Krenzke, L. D. Insights on Reactor Internals for ULSD - Performance of Existing New Hardware. In Proceedings of the NPRA Annual Meeting, San Antonio, TX, March 23-25, 2003; AM-03-92. (3) Chou, T. Pet. Technol. Q. 2004, 4, 79-85. (4) Sie, S. T. Appl. Catal. A 2001, 212, 129-151.

levels. Those strategies are based on the development of new highly active catalysts, tailoring reaction conditions (e.g., temperature, liquid-hourly space velocity (LHSV), hydrogen partial pressure), and designing new reactor configurations (e.g., multibed reactors with interstage quenching, reactors in series, and counterflow reactors);5-7 for fouling abatement, improved procedures for catalyst loading,8,9 low activity mesoporous materials, and graded bed designs were developed.10 Extensive overviews and study cases of such strategies applied to the production of ultralow sulfur diesel via hydroprocessing have been presented over the past few years.11-16 Figure 1 shows that, in a typical hydrodesulfurization (HDS) unit producing diesel with 2500 ppmw sulfur with a catalyst cycle life of more than 3 years, an increase of reaction temperature for achieving sulfur concentrations of ∼50 ppmw will reduce the catalyst cycle life by a factor of at least 317 or will require more than 4 times the original catalyst volume in order to keep constant the original catalyst cycle life.12 This example gives a clear idea about how expensive modifications of current processes may be in order to produce environmental friendly fuels. All these studies agree that improving catalyst performance and maximizing its volume (5) Sie, S. T. Fuel Proc. Technol. 1999, 61, 149-171. (6) Knudsen, K. G.; Cooper, B. H.; Topsøe, H. Appl. Catal. A 1999, 189, 205-215. (7) Song, C. Catal. Today 2003, 86, 211-263. (8) Sanford, E. C.; Kirchen, R. Oil Gas J. 1988, December 19, 35-41. (9) Criterion Catalyst and Technologies. Technical Bulletin: Hydrotreating Catalyst Reactor Loading Guidelines. http://www.criterioncataysts.com (accessed Oct 2006). (10) Haldor Topsøe. Brochure: Pressure Drop Control. http://www.topsoe.com (accessed Oct 2006). (11) Hamilton, G. L.; Van der Linde, B.; DiCamillo, D. Hydrotreating Revamp Options for Improved Quality Diesel via Cocurrent/Countercurrent Reactor Systems. Presented at the AICHE Spring National Meeting, Atlanta, Georgia, March 5-9, 2000. (12) Bingham, E.; Christensen, P. Revamping HDS Units to Meet High Quality Diesel Specifications. Presented at the Asian Pacific Refining Technology Conference, Kuala Lumpur, Malaysia, March 8-10, 2000. (13) Bharvani, R. R.; Henderson, R. S. Hydrocarbon Process. 2002, 81, 61-64. (14) Torrisi, S.; DiCamillo, D.; Street, R.; Remans, T.; Svendsen, J. Proven Best Practices for ULSD Production, AM-02-35. In Proceedings of the NPRA Annual Meeting, San Antonio, TX, March 17-19, 2002; AM02-35. (15) Palmer, R. E.; Torrisi, S. Pet. Technol. Q. 2003, ReVamps, 15-18. (16) Turner, J.; Reisdorf, M. Hydrocarbon Process. 2004, March. (17) Yeary, D. L.; Wrisberg, J.; Moyse, M. Hydrocarbon Eng. 1997, September, 25-29.

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Figure 1. Effect of required sulfur in product on catalyst cycle life and volume: (s) relative catalyst volume to maintain constant catalyst cycle life,2 (---) catalyst cycle life operating at LHSV ) 1.0 h-1 and P ) 5 MPa.17

within an existing unit are the most cost-effective options for satisfying the current product requirements. Two main key parameters have been identified to be possible solutions of these problems: (1) increasing catalyst activity and (2) efficient distribution of the reactants through the catalytic bed by means of proper reactor internal design.18 The former option has been subject of various reviews19,20 and escapes the scope of the present contribution. In the case of reactor internals, an efficient design is characterized by reduced vertical dimensions which allow for maximizing catalyst volume in an existing reactor. For instance, an evaluation of several staged hydrocracking units designed before 1990 showed that in the first stage (pretreatment) the typical catalyst volume was about 70% of the total reactor volume and as low as 60% in the second stage, the rest of the reactor volumes being occupied by reactor internals.1 Therefore, the general view is that the catalyst and reactor internals are intimately related. Currently, most of the attention is focused on hardware for fixed-bed reactors due their wide use in hydroprocessing of oil fractions. Other reactor technologies such as ebullated-bed and moving-bed reactors also require internals while slurry reactors do not require any; nevertheless, the description of internals for such technologies is beyond the scope of this work. Although interest in the subject has been increasing in the past decade, technical information is rather limited and most of the aspects related to the state-of-the-art hardware are protected by patents. Despite this, some information is available in recent study cases dealing with hydroprocessing unit revamp and in expired patents. The aim of this article is to provide an overview of several aspects of conventional and state-of-the-art fixed-bed hydroprocessing reactor internals along with the role that they play in the pursuit of cost-effective clean fuels production. 2. Reactor Internals 2.1. Statement of the Problem. Since the early stages of hydroprocessing technologies, a great part of research and development (R&D) efforts has been directed toward catalyst development. The advances in this area are really significant; catalysts offered by major licensors in the late 1990s had from (18) Patel, R. H.; Bingham, E.; Christensen, P.; Mu¨ller, M. Hydroprocessing Reactor and Process Design to Optimize Catalyst Performance. Presented at The First Indian Refining Roundtable, New Delhi, India, December 1-2, 1998. (19) Vasudevan, P. T.; Fierro, J. L. G. Catal. ReV. Sci. Eng. 1996, 38, 161-188. (20) Furimsky, E. Appl. Catal. A 1998, 171, 177-206.

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8 to 9 times more activity than those used in the middle 1950s.21 However, none or little attention had been given to reactor internal design until the mid 1990s. The majority of the hydroprocessing units operating at the end of the last century relied on rudimentary reactor internal designs such as sieve trays, chimney trays, conventional bubble cap trays, and impingement quench boxes; even more, in some cases, reactors may not have had any internals at all.18 The design of those distributor trays was strongly influenced by hardware employed in fractionation columns, which is not necessarily adequate for trickle-bed reactors.22 Inappropriate reactor internal design caused poor catalyst utilization due to flow maldistribution of the reactants at the inlet of the catalyst bed. Flow maldistribution was also enhanced by the increase of reaction severity, which eventually led to its detection by the high radial temperature differences measured at catalyst bed outlets.23 The main problems generated by flow maldistribution are the overuse of a part of the catalyst inventory and the formation of hot spots, meanwhile the rest of the catalyst becomes underused, consequently leading to poor product quality and shorter cycle lengths.1 This fact increased the awareness of the importance of the reactor internal design and its role in the efficient catalyst utilization. On the basis of the idea that the efficient catalyst utilization is governed by reactor internals, a proper design must perform the following functions: (1) uniform volumetric and thermal distribution of liquid and gas reactants over the cross-sectional area of the catalyst bed, (2) inter- and intraphase mixing, (3) quench performance, and (4) fouling resistance; all this, together with space-efficiency and ease in maintenance and installation is desirable.24 2.2. Fundamentals. Reactor internal hardware may be located at the reactor inlet, interbed zones, and at the reactor outlet. The hardware at the reactor inlet provides the initial distribution of the reactants and protection against fouling; this is achieved by means of distributor trays together with fouling abatement trays and/or top-bed grading materials. For high hydrogen demanding feeds, where a large delta-T is generated due to reaction exothermality, multibed reactors with interstage quenching are employed to limit the temperature rise; quench zones located between catalyst beds comprise a reactant collection system, a quench fluid injection device, a chamber for mixing the cooling medium with the hot reactants, and a reactant redistribution tray. Finally, the reactor outlet contains hardware for fluid collection along with catalyst retention. Figure 2 shows the hydroprocessing reactor internal fundamentals according to the previous description for a unit with two catalytic beds and one quench.25 Axial and radial delta-T within the reactor are also illustrated. The axial delta-T represents the typical temperature rise caused by the exothermality of the hydroprocessing reactions in the catalytic bed. It allows for establishing the catalytic bed length when reactor temperature reaches a maximum allowable temperature and for determining the number of beds for a required impurity removal.26 On the other hand, the radial delta-T reflects the (21) Simpson, S. G. Refining Catalysts for High Specification Transportation Fuels. Presented at the Institute of Petroleum Meeting: “Improved Catalytic Processes”, London, April 10, 1997. (22) Jacobs, G. E.; Milliken, A. S. Hydrocarbon Process. 2000, NoVember, 76-84. (23) Mayo, S. W. Maximizing Cycle Length with Akzo Nobel’s Guard Bed and Liquid Distribution Technology. Fuel Technol. Manage. 1999, March. (24) Ouwerkerk, C. E. D.; Bratland, E. S.; Hagan, A. P.; Kikkert, B. L. J. P.; Zonnevylle, M. C. Pet. Technol. Q. 1999, 2, 21-30. (25) Ancheyta, J.; Speight, J. Hydroprocessing of HeaVy Oils and Residua; Taylor and Francis Group: New York, 2007.

Hydroprocessing of Oil Fractions

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Figure 2. Hydroprocessing reactor internal fundamentals: (s) axial delta-T, (---) radial delta-T.25

performance of reactor internals.27 The figure shows radial deltaTs for good and poor reactor internal performance; the former is characterized by low radial temperature differences after distribution trays and quench zones, and the latter exhibits gradual widening in radial delta-T which provides evidence of flow maldistribution. It is worthwhile to mention that maldistribution has a cumulative character if the distributor trays and quench boxes are not working adequately; thus, in multibed reactors, the poorest catalyst utilization will be in the last bed which is reflected in the widest radial temperature differences. 2.3. Distributor Trays. Certainly, the most relevant reactor internal feature is the distribution system whose purpose is to establish radial liquid distribution across the catalyst bed, and thus, it determines the performance of a trickle-bed reactor. Most of the hydroprocessing units have been using the original distributor designs such as sieve trays, chimney trays, and bubble cap trays, the last two being the most successful ones. Sieve trays are the most rudimentary systems, being simple and cheap in construction; they comprise a great number of liquid downcomers (perforations) across the tray and sometimes widely spaced chimneys for separating gas flow. This type of tray has been used more as a predistribution system followed by chimney28 or bubble cap29 trays, than as a principal distributor. Chimney trays are basically descendants of sieve trays, their main feature is the evenly spaced chimneys with lateral apertures for the liquid and a top aperture for the gas thus allowing independent flow of both fluids. A great number of chimney designs are available which differ mainly in the (26) Satterfield, C. N. AIChE J. 1975, 21, 209. (27) Bingham, E.; Chan, E.; Mankowski, T.; Hubbard, P. Improved Reactor Internals for Syncrude’s HGO Hydrotreaters. In Proceedings of the NPRA Annual Meeting, San Antonio, TX, March 26-28, 2000; AM00-19. (28) Pedersen, M. J.; Sampath, V. R.; Litchfield, J. F. Method and Apparatus for Mixing and Distributing Fluids in a Reactor. U.S. Patent 5,462,719, 1995. (29) Stangeland, B. E.; Parimi, K.; Cash, D. R. Distributor Assembly for Multi-Bed Down-Flow Catalytic Reactors. U.S. Patent 5,690,896, 1997.

Figure 3. Conventional distributors: (a) Mobil Oil simple chimney,30 (b) Mobil Oil multiaperture chimney,35 (c) Esso notched chimney,33 (d and e) Union Oil bubble caps38 and Fluor modified bubble cap.39

number or type of lateral apertures, such as traditional chimney distributors,30,31 chimneys with triangular notches,32,33 and multiaperture chimneys34,35 as shown in Figure 3. An alternative design of chimney trays comprises gas chimneys and triangularly notched liquid chimneys.36 Bubble cap trays are essentially (30) Aly, F. A.; Graven, R. G.; Lewis, D. W. Distribution System for Downflow Reactors. U.S. Patent 4,836,989, 1989. (31) Koros, R. M.; Wong, Y. W.; Wyatt, J. T.; Dankworth, D. C. Mixed Phase Fixed Bed Reactor Distributor. U.S. Patent 5,403,601, 1995. (32) Halik, R. R.; Hill, D. F. Method for Distributing a Vapor-Liquid Feed and Apparatus Therefor. U.S. Patent 2,898,292, 1959. (33) Effron, E.; Hochman, J. M. Mixed-Phase Flow Distributor for Packed Beds. U.S. Patent 3,524,731, 1970. (34) Grosboll, M. P.; Edison, R. R.; Dresser, T. Apparatus and Process for Distributing a Mixed Phase through Solids. U.S. Patent 4,126,540, 1978. (35) Muldowney, G. P.; Weiss, R. A.; Wolfenbarger, J. A. Two-Phase Distributor System for Downflow Reactors. U.S. Patent 5,484,578, 1996.


similar to those employed in distillation columns, however, with a different function. They are characterized by the contact and mixture of gas and liquid reactants creating a mixed phase that flows through the slots of the bubble cap. The wide range of bubble cap trays goes from the early designs37,38 to the latest high performance designs,39 which are also presented in Figure 3. Many types of the above-mentioned distribution systems have been patented over the years; however, they are simply variations of the original ones delivering little improvement and in many cases promoting liquid maldistribution.16 Even though technical information of these designs is available in expired patents, the reasons for their underperformance were not well understood until recently. Liquid distribution has been lately the object of study for many researchers, a recent review dealing with experimental procedures and mathematical modeling for describing liquid distribution in trickle-bed reactors has been presented by Kundu et al.40 Almost all of these studies focus on aspects such as the bed structure (porosity, D/dp ratio, catalyst bed depth, and method of loading) and the gas-liquid-solid interaction, rather than on the liquid distribution hardware. The study of distribution systems has been more of interest to mayor oil companies resulting in the current state-of-the-art distributors such as Shell GSI’s HD (high distribution) tray,41,42 Topsøe’s Vapor-Lift tray,17,43 Exxon’s Spider Vortex technologies,44,45 Akzo Nobel’s Duplex tray,46 and Fluor’s Swirl Cap tray.39 The development of these high performance distributor trays, using sophisticated high-pressure cold flow units in combination with computational fluid dynamics (CFD), led to the better understanding of the flaws of the original designs. The meticulous evaluation of those designs highlighted the importance of specific parameters such as liquid source layout (i.e., tray spacing and wall coverage), discharge pattern, tray levelness, sensitivity to plugging, and flexibility in operation. 2.3.1. Liquid Source Layout. Liquid source layout is characterized by tray or center-to-center spacing and wall coverage capability. Tray spacing is referred to as the distance between the centers of two drip points. This parameter is directly proportional to the catalyst particle diameter and must be optimized so that radial mixing, provided by the grading material, compensates for maldistribution. The importance of tray spacing is well described by Hoftyzer’s correlation:47 (36) Riopelle, J. E. Bed Reactor with Quench Deck. U.S. Patent 3,353,924, 1967. (37) Zimmerman, M. U. Entrainment-Obviating Bubble Cap. U.S. Patent 2,778,621, 1957. (38) Ballard, J. H.; Hines, J. E. Vapor-Liquid Distribution Method and Apparatus for the Conversion of Hydrocarbons. U.S. Patent 3,128,249, 1965. (39) Jacobs, G. E.; Stupin, W. S.; Kuskie, R. W.; Logman, R. A. Reactor Distribution Apparatus and Quench Zone Mixing Apparatus. U.S. Patent 6,098,965, 2000. (40) Kundu, A.; Nigam, K. D. P.; Duquenne, A. M.; Delmas, H. ReV. Chem. Eng. 2003, 19, 531-605. (41) Den Hartog, A. P.; van Vliet, W. Multi-bed Downflow Reactor. U.S. Patent 5,635,145, 1997. (42) Altrichter, D. M.; Creyghton, E. J.; Ouwehand, C.; van Veen, J. A. R.; Hanna, A. New Catalyst Technologies for Increased Hydrocracker Profitability and Product Quality. In Proceedings of the NPRA Annual Meeting, San Antonio, TX, March 21-23, 2004; AM-04-60. (43) Seidel, T.; Dunbar, M.; Johnson, B. G.; Moyse, B. What a Difference the Tray Made. In Proceedings of the NPRA Annual Meeting, San Antonio, TX, March 17-19, 2002; AM-02-52. (44) McDougald, N. K.; Boyd, S. L.; Muldowney, G. P. Multiphase Mixing Device with Baffles. U.S. Patent 7,045,103, 2006. (45) Davis, T. J. EMRE Hydroprocessing Technologies & Low Sulfur Motor Fuels. Presented at the PEMEX Seminar, Mexico City, December, 2002. (46) Akzo Nobel. Akzo Nobel Duplex Distributor Tray. Presented at the Mexican Institute of Petroleum, Mexico City, May 14, 2003. (47) Hoftyzer, P. J. Trans. Inst. Chem. Eng. 1964, 42, 109-117.

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z)

y2 kHdp0.5

(1)

Equation 1 correlates the bed depth (z) for achieving uniform liquid distribution (plug-flow) with the spacing between drip points (y), catalyst particle diameter (dp), and with a constant (kH) which value must not be greater than 4 in order to achieve uniform distribution. Figure 4a shows the effect of tray spacing on liquid distribution across the catalyst bed, it can be observed that uniform liquid distribution can be achieved closer to the top of the catalyst bed with narrower tray spacing, in other words a larger number of liquid point sources. On the other hand, wide tray spacing reduces catalyst utilization and requires more bed depth to correct liquid distribution by means of radial dispersion. In this matter, original tray designs have not optimal tray spacing as discussed earlier by Patel et al.;18 for instance, bubble cap trays are known for having the worst tray spacing due to the their relative large size (50-100% larger than a chimney). Figure 4b presents a tray spacing comparison of a conventional bubble cap tray against a Topsøe Vapor-Lift tray.48 A higher density of distribution points across the Topsøe tray due to their reduced size in comparison with the bubble caps can be noticed. Wall coverage capability is the other layout parameter that influences reactor performance. Conventional distributors present dead zones without liquid sources near the reactor wall as in the case of bubble caps shown in the same figure. Poor wall coverage together with disc discharge pattern contributes to flow bypassing leaving a great percentage of unused catalyst near the reactor wall vulnerable to hot spot formation. 2.3.2. Discharge Pattern. The most important design parameter of distributor trays is perhaps the liquid discharge pattern. Along with tray spacing, it determines the percent of wetted catalyst across the top of the catalyst bed and consequently overall catalyst utilization. For the past decade, distributor tray development has been focused on providing an efficient discharge pattern, which in this context refers to a uniform distribution closer to the top of the catalyst bed. Conventional distributors, such as chimney trays and bubble cap trays, produce a disc type of discharge pattern which wets only the catalytic surface right beneath the discharge point as shown in Figure 4c. This type of discharge pattern is very inefficient because it leaves a great percentage of unused catalyst at the beginning of the bed. However, commercial state-of-the-art trays provide a very wide spray discharge pattern covering almost 100% of the catalyst bed. The liquid discharge pattern is governed by the hydrodynamics present in the discharge points. Liquid flow in sieve and chimney trays is governed by the overflow principle where the liquid accumulated over the tray drips down through the sieves or the apertures on the chimneys generating the socalled disc type discharge pattern, while gas enters separately through the top of the chimneys as it can be observed in Figure 4d. In addition to the inefficient discharge pattern, these designs provide poor vapor-liquid contacting, and therefore, large temperature gradients may be observed. On the other hand, the gas-assist principle takes advantage of the high gas velocity to drag the liquid held on the tray, forming a dispersed liquid phase which is discharged through a central downcomer as in the case of bubble cap and state-of-the-art distributors, though for the former this does not produce an efficient discharge pattern. This operation principle provides excellent vapor-liquid contacting reducing interphase temperature differences up to 90%.38 (48) Haldor Topsøe. High-Performance Reactor Internals. Presented at the Mexican Institute of Petroleum, Mexico City, May 22, 2003.



Figure 4. Distributor tray design parameters. (a) Effect of tray spacing on liquid distribution.18 (b) Comparison of tray spacing.48 (c) Comparison of discharge pattern of several distributor trays.1 (d) Operating mode of several distributor trays.18

Ouwerkerk et al.24 measured liquid distribution at the top of the packed bed produced by conventional designs and an HD tray from Shell GSI. They determined that for chimney and bubble cap trays from 10% to 20% and around 30% of the bed surface, respectively, receives almost 100% of the liquid while the rest of the surface remains dry as shown in Figure 4c. In each case, the area right beneath each liquid source is heavily dosed with liquid meanwhile the surface between liquid sources is almost dry leading to catalyst underutilization. Also, since in these designs there is low interaction between liquid sources due to poor center-to-center spacing and disc discharge pattern, plugging of an individual liquid source results in an increased maldistribution. Nevertheless, these deficiencies are corrected by the HD nozzles which allow for achieving 100% of liquid distribution at the top of the catalyst bed; additionally, the overlapping of the sprays coming from the nozzles makes these designs less sensitive to plugging due to the compensation effect. Similar results were obtained by Jacobs and Milliken22 when evaluating two conventional distributors (chimney and bubble cap trays) and a proprietary tray design (modified bubble cap)39 using a CFD model which took into account liquid source layout and discharge pattern. CFD modeling is based on the diffusion equation for describing liquid flow through packed beds including radial dispersion. Table 1 presents radial slice catalyst utilization efficiencies at several bed depths for three reactor diameters (2.4, 3.4, and 4.3 m). The first result that can be noticed is the poor catalyst utilization (less than 50%) at the

Table 1. Radial Slice Catalyst Utilization Efficiencies as a Function of Bed Depth22 catalyst utilization efficiency bed depth, m inert-catalyst interphase 1.5 3.0 6.0 12.0 inert-catalyst interphase 1.5 3.00 6.0 12.0 inert-catalyst interphase 1.5 3.0 6.0 12.0

chimney

bubble cap

modified bubble cap

Reactor Diameter ) 2.4 m 24.1 45.5

79.0

66.9 79.8 85.7 88.4

93.5 94.9 95.9 96.5

82.6 87.0 89.0 90.8


81.6

70.5 83.9 90.0 91.5

95.0 96.6 97.5 98.2

85.3 89.9 91.2 92.5


82.5

71.3 84.7 90.8 92.1

95.7 97.1 97.9 98.4

86.3 91.2 92.3 93.3

inert-catalyst interface due to liquid maldistribution generated by conventional designs with large center-to-center spacings and disc type discharge patterns. Though maldistribution is slightly corrected with bed depth, it never reaches uniformity (plug-


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Figure 7. Effect of liquid load on liquid distribution with a tray tilt of 10 mm: (black bars) high liquid load, (grey bars) medium liquid load, (white bars) low liquid load.18

Figure 5. Impact of tray levelness on liquid distribution.17

Figure 6. Effect of tray tilt on liquid distribution:1 (s) HD tray, (‚‚‚) gas chimneys and liquid notched chimneys, (---) sieve tray with gas chimneys.

flow) which according to the authors is indicated by a catalyst utilization of ∼95%. On the contrary, modified bubble caps provide much better catalyst utilization (∼80%) at the inertcatalyst interface and liquid distribution uniformity is reached at 1.5 m of bed depth. Still, the improvement of the modified bubble caps is rather limited in comparison to the HD tray, which is reported to be capable of achieving 100% catalyst utilization at the top of the catalyst and eliminates the necessity of using inert grading material. It is worthwhile to mention that catalyst utilization slightly improves for larger reactor diameters due the decrease of catalyst volume near the reactor wall, no matter the type of distributor tray. 2.3.3 Tray LeVelness. Tray tilt or levelness is another important factor that must be considered during installation. When a tray is not properly leveled, the liquid will gravitate toward the lowest area of the tray resulting in preferential liquid flow causing poor distribution as seen in Figure 5. Swain and Zonnevylle1 evaluated the effect of tray tilt on liquid distribution uniformity for two types of conventional trays and an HD tray (Figure 6). For instance, a tray tilt of 6 mm in the HD tray generates around 4% maldistribution, which means that the catalyst under the lowest region of the tray receives 2% more liquid meanwhile the opposite side receives 2% less. As can be

observed, even at higher tray tilts, maldistribution is lower than 8%, which makes this type of tray very insensitive to levelness. Nevertheless, this is not the case for conventional trays, since the same levelness (6 mm) will cause ∼20% maldistribution in a gas/liquid chimney tray and as high as 40% in a sieve tray. Since the lowest area receives considerable liquid excess, the chimneys at that location pass greater amounts of liquid through their apertures and sometimes through their top aperture, meanwhile the liquid level at the opposite region may be not high enough to reach the chimney apertures; therefore, little or no liquid passes through those chimneys. In the case of gasassisted distributors, since gas drags the liquid retained on the tray, there is less sensitivity to tray tilt. 2.3.4. Liquid Loading SensitiVity. A proper distributor design must be able to provide satisfactory performance over a broad range of liquid loads. The variations of liquid loading, as those presented at start- and end-of-run conditions, may affect the functioning of distributor trays. Figure 7 presents the effect of liquid loading on liquid distribution with a tray tilt of 10 mm for a conventional bubble cap tray and two Topsøe distributors. The measurements were carried out in a cold flow unit described elsewhere,48 which allowed for modifying tray tilt and loading variable amounts of liquid, i.e., variable LHSV, corresponding to commercial operations such as hydrotreating (high liquid load), fluid catalytic cracking (FCC) and hydrocracking pretreatment (medium liquid load), and hydrocracking or ultralow sulfur diesel (ULSD) (low liquid load). As seen, the multiaperture chimney tray is shown to be less sensitive to variable liquid loads generating maldistributions of about 15-21% (one side receives a ∼10% excess liquid while the other receives ∼10% deficit). The Vapor-Lift tray exhibits the lowest maldistribution (∼5%) at high liquid loads; however, at lower liquid loads, it produces higher maldistribution (17-18%) which makes it slightly sensitive to liquid loading. Bubble caps have the inverse behavior with respect to liquid loading; at high liquid loads, they exhibit the highest maldistribution (∼30%) while low liquid loads reduce maldistribution up to ∼23%. These results suggest that chimney trays are more prone to produce maldistribution at low liquid loads while this is true for bubble caps at high loads. The former are designed for keeping the liquid level slightly above the lateral apertures for liquid down-flow, low liquid loads together with excessive vaporization due to elevated temperatures imply a low liquid level on the tray, in the worst cases below the chimney apertures, affecting tray performance. Bubble caps malfunction at high



Figure 8. Quench zones: impingement quench box,57 vortex mixer,24 Shell’s ultraflat quench system,41 Albermarle’s Q-Plex quench mixer.67

liquid loads due to tray flooding which makes the gas-assisted operation difficult. 2.4. Quench Zones. Hydroprocessing fixed-bed reactors require a quench system to control the temperature rise caused by the exothermality of the reactions. Temperature control is essential for achieving economically acceptable catalyst cycle length and obtaining the required product quality.49 The main consequences of temperature runaway are hot spot formation leading to enhanced catalyst aging by coke formation50 and sintering,51 poor product yields due to excessive hydrocracking,52 and sometimes damages to the reactor vessel.53 Controlling reaction temperature in hydroprocessing reactors is achieved by introducing quench fluids, commonly hydrogen, the so-called quench zone located between catalytic beds. This interbed zone allows for: (1) injecting the cooling medium, (2) mixing with the hot reactants from the previous bed, and (3) redistributing the liquid and gas reactants across the following catalyst bed.24 Mechanical aspects of quenching such as quench fluid rate, composition, temperature, position along the reactor, and number of injection points are part of the know-how of each technology which is maintained proprietarily by licensors. Nevertheless, there are a few articles dealing with this type of information.49,52,54,55 A comprehensive review on various aspects of quench systems for hydroprocessing reactors has been published recently.56 (49) Munõz, J. A. D.; Alvarez, A.; Ancheyta, J.; Rodrı´guez, M. A.; Marroquıń, G. Catal. Today 2005, 109, 214-218. (50) Hanika, J.; Sporka, K.; Ruzicka, V.; Pistek, R. Chem. Eng. Sci. 1977, 32, 525-528. (51) Furimsky, E.; Massoth, F. E. Catal. Today 1999, 52, 381-495. (52) Yan, T. Y. Can. J. Chem. Eng. 1980, 58, 259-266. (53) Hsu, C. S.; Robinson, P. R. Practical AdVances in Petroleum Processing; Springer: New York, 2006; Vol. 1. (54) Shah, Y. T.; Mhaskar, R. D.; Paraskos, J. A. Ind. Eng. Chem., Process Des. DeV. 1976, 15, 400-406. (55) Mhaskar, R. D.; Shah, Y. T.; Paraskos, J. A. Ind. Eng. Chem. 1978, 17, 27-33.

Early interbed hardware designs included the so-called impingement quench box57,58 together with a redistribution tray such as the ones discussed in the previous section. Figure 8 shows an impingement quench box according to a Union Oil Co. expired patent. The system comprises a quench tube that allows for injecting cold hydrogen, a liquid collector tray, a mixing box where the fluid impingement occurs, a perforated tray for collecting fluids coming out from the mixing box, and a bubble cap redistributor tray. The operation principle of the quench box is based on: (1) division of the down-flowing fluids (hot reactants and quench gas) into two streams which enter through the openings of the collector tray to separate chambers of the mixing box, (2) directing the flow of the streams by means of baffles located in each chamber causing the fluids to impinge in a turbulent mixing zone, (3) discharge of the mixture toward the redistribution system. However, impingement mixing is known for providing ineffective gas-liquid mixing, due to poor interphase contacting, resulting in large gas-liquid temperature differences. The latter defect along with an inappropriate redistribution tray results in poor quench zone performance. This is characterized by wide radial delta-Ts after each interbed zone, which persists or in the worst case grows as the fluids move down the reactor.59 Nevertheless, the failures of conventional systems were corrected in new interbed designs which are constituted by vortex type mixers and high performance distributors. Examples of these technologies are Shell GSI’s UFQ (ultraflat quench),41,42 ExxonMobil’s Spider Vortex quench zone,44,60 Chevron-Lum(56) Alvarez, A.; Ancheyta, J.; Munõz, J. A. D. Energy Fuels, 2007, 21, 1133-1144. (57) Ballard, J. H.; Hines, J. E. Apparatus for Mixing Fluids in Concurrent Downflow Relationship. U.S. Patent 3,502,445, 1970. (58) Peyrot, C. F. Mixing Device for Vertical Flow Fluid-Solid Contacting. U.S. Patent 4,669,890, 1987. (59) Boyd, S. L.; Muldowney, G. P. Interbed Gas-Liquid Mixing System for Cocurrent Downflow Reactors. U.S. Patent 6,180,068, 2001.


mus’ Nautilus Reactor technologies61,62 and the latest Isomix internals,63 and Fluor’s Swirl Zone Vortex Mixer.2,64 The main feature of these types of systems is the swirling motion of the fluids generated within the mixing box which enhances gasliquid contacting as shown also in Figure 8. The performance of vortex mixers is well explained by Litchfield et al.65 when describing the operation of a proprietary quench zone design.66 The authors stress out the difficulty for achieving effective mixing due the large density differences between the quench gas and process gases, and the importance of the quench zone configuration in order to maximize intra- and interphase contacting. The main parts of a quench zone are the quench fluid injection device, which imparts radial and perpendicular mixing of the process fluids and quench gas, and the vortex mixer arrangement, which provides turbulent swirling motion to the fluids. Injection devices include traditional quench pipes for direct injection into the mixing chamber, concentric manifolds with nozzles that surround the mixing chamber (e.g., UFQ quench ring) for radial inward injection as seen also in Figure 8, or the so-called spider which is a small manifold located at the center of the quench zone (Spider Vortex) for radial outward injection. Vortex mixers vary in the arrangement of vanes and baffles within the chamber that create passageways and constrictions, which impart swirling motion and turbulence to the fluids. A different approach to vortex mixers is the Albermarle Q-Plex quench mixer,67 where the quench gas and process fluids are passed through a single orifice and thus the constriction provides intimate intra- and interphase mixing. The operation is carried out in three mixers in series as detailed in Figure 8. One important aspect of these systems is their reduced height in comparison to conventional ones. In commercial hydroprocessing units, it is extremely important to minimize the vertical dimensions of interbed internals in order to reduce the height of the required reactor vessel, especially in hydrocracking units that may have more than two quench zones. Large reactors with wall thicknesses of 20-40 cm (high-pressure operation) represent considerably heavy reactor vessels, which in return increases the total cost due the larger required supporting structure, and difficult transportation and installation.68 Of all the commercial technologies, perhaps Shell’s technical papers are the ones that emphasize more on this aspect. For instance, after installing the UFQ and HD internals in a conventional hydrocracker where 67% of the reactor vessel volume was occupied by catalyst, catalyst utilization grew up to ∼86% due to the reduced catalyst-to-catalyst distance of the UFQ (1-1.4 m) and the elimination of the grading material due the HD tray (60) Sarli, M. S.; McGovern, S. J.; Lewis, D. W.; Snyder, P. W. Improved Hydrocracker Temperature Control: Mobil Quench Zone Technology. In Proceedings of the NPRA Annual Meeting, San Antonio, TX, March 2123, 1993; AM-93-79. (61) Stangeland, B. E.; Parimi, K.; Cash, D. R. Distributor Assembly for Multi-Bed Down-Flow Catalytic Reactors. U.S. Patent 5,690,896, 1997. (62) Chevron Lummus Global. Brochure: Isocracking (Nautilus Internals). http://www.chevron.com (accessed Oct 2006). (63) Nelson, D. E.; Kuskie, R. W.; Bingham, F. E. Reactor Distribution Apparatus and Quench Zone Mixing Apparatus. U.S. Patent 6,984,365, 2006. (64) Chevron Lummus Global. Brochure: CLG ISOMIX Reactor Internals Technology. http://www.chevron.com (accessed Oct 2006). (65) Litchfield, J. F.; Pedersen, M. J.; Sampath, V. R. Optimization of Interbed Distributors. In Proceedings of the NPRA Annual Meeting, San Antonio, TX, March 17-19, 1996; AM-96-73. (66) Pedersen, M. J.; Sampath, V. R.; Litchfield, J. F. Method and Apparatus for Mixing and Distributing Fluids in a Reactor. U.S. Patent 5,462,719, 1995. (67) Albermarle. Brochure: Maximize reactor performance with stateof-the-art PLEX internals. http://www.albermarle.com (accessed Oct 2006). (68) Grott, J. R.; Bunting, R. L.; Hoehn, R. K.; Goodspeed, R. F. Hydroprocessing Reactor Mixer/Distributor. U.S. Patent 5,837,208, 1998.

AlVarez et al.

Figure 9. Reactor thermometry: (x) horizontal; (o) vertical.45

effectiveness.1 On the other hand, Albermarle’s Q-Plex has been reported to have a total height of ∼0.5 m which is much smaller than most of the vortex mixers (∼1 m).67 However, reduced height internals compromise flexibility for operating at variable liquid/gas loads, especially at high loads where flooding may occur or the residence time in the mixer is not enough for effective fluid mixing.65 It has been also reported recently69 that for optimal fluid mixing over a wide range of liquid/gas loads (33-200%) the vortex mixer should be 0.35-0.65 times the inner reactor diameter, the total quench zone length being less than 1.5 times the inner diameter. 3. Performance of Reactor Internals 3.1. Methods for Detecting Maldistribution. Efficient reactor internals are fundamental for improving hydroprocessing unit performance. When revamping existing units, a careful evaluation of the current reactor performance will highlight the potential improvements that may be generated by installing stateof-the-art hardware. As discussed in the previous sections, maldistribution is the principal driver of unit underperformance followed by ineffective quench mixing. The methods for detecting maldistribution have been briefly mentioned by Mayo,23 among them are radial delta-T measurement, radioactive tracer studies, flow rate tests, and reactor loading and design reviews. 3.1.1. Radial delta-T. Radial delta-T measurement perhaps is the most commonly applied method for detecting maldistribution and also for evaluating quench performance. Radial delta-T may be defined as temperature differences on a radial plane of the reactor, generally inlet and outlet, as a result of liquid maldistribution. A well-accepted guideline is that radial delta-T should not exceed ∼5 °C.24 Another criterion states that radial temperature gradient should not be greater than the axial temperature rise;65 however in processes where the temperature increases 20-30 °C along the reactor (e.g., hydrocracking and residue hydrotreatment), a radial delta-T of such a magnitude may have serious implications. The radial and axial temperature profiles in a reactor are documented by thermocouple readings. Conventional thermometry systems comprise vertical and horizontal thermocouples while advanced ones have series of evenly spaced horizontal thermocouples as presented in Figure 9. Though, it is well-known that the former systems provide inadequate readings due to the incomplete coverage of the thermocouples across the catalytic bed and also vertical thermocouples promote channeling which is a precursor of maldistribution. A well-documented revamp case is the Syncrude heavy gasoil hydrotreater report.27 The unit had a three-bed reactor with (69) Van Vliet, W.; Den Hartog, A. P.; Den Hartog-Snoeij, M. Mixing Device Comprising a Swirl Chamber for Mixing Liquid. U.S. Patent 7,078,002, 2006.



Figure 11. Effect of liquid distribution on catalyst cycle length (s) before revamping and (---) after revamping.23

Figure 10. Syncrude reactor temperature profiles (s) before revamping and (---) after revamping.27

reaction with a reaction order of two (n ) 2), which by the way can be determined with the same information of LHSV vs HDS conversion, the following power law model is commonly used:

(

k ) LHSVi

1 1 Cs,pi Cs,f

)

(2)

conventional bubble cap distributors, impingement quench boxes, and a set of conventional thermocouples like those presented in Figure 9. The readings of the thermometry systems are presented in Figure 10, in which poor agreement between the vertical and horizontal thermocouples is observed as well as significant radial delta-T measured by the vertical thermocouples at the outlet of the catalytic beds, especially the second one (∼30 °C). This is a clear evidence of maldistribution which means that the distribution system is underperforming; also, the wide radial delta-Ts measured at the entrance of the second and third catalytic beds (∼10 °C) indicate that the quench boxes are not working properly. Nevertheless, after retrofitting the unit with Topsøe internals, the radial temperature gradients decreased substantially (2-4 °C) thus decreasing the average bed temperature of the reactor. 3.1.2. RadioactiVe Tracer Studies. Radioactive tracer scanning is another technique for diagnosing maldistribution.70 It has been successfully applied by Tru-Tec (Koch Engineering) in a diesel hydrotreater as reported by Yeary et al.17 The method involves adding an isotope to the reactor feed so that it adheres on the catalyst surface in a consistent pattern with the liquid flow. Next, the reactor is scanned in order to collect the signals radiated by the isotope, and then with the obtained information, a set of distribution polar plots at several bed depths is generated. In this case, the method allowed for detecting severe maldistribution in the unit. 3.1.3. Flowrate Tests. The use of flowrate tests for detecting maldistribution is well-known and practiced. Old hydrotreaters or even new ones without radial thermocouples are still using this approach. The method is based on determining the variation of kinetic constant values (k) for a certain reaction (e.g., HDS) at different liquid-hourly space velocities for a given reaction temperature. This principle is widely known and used in experimental studies for determining the magnitude of mass transfer limitations in order to define conditions to work under a kinetic regime. If the reaction temperature is indeed constant (or almost constant), any variation of LHSV will yield almost the same kinetic constant value. For instance, for an HDS

where Cs,f and Cs,pi are the concentrations of sulfur in the feed and products, respectively. Then, maldistribution may be diagnosed when the difference between kinetic constant values is considerable.23 Since variations in LHSV are directly dependent on the feed flowrate and HDS conversion on sulfur analyses, acceptable differences would be within the experimental errors given by plant mass balance and sulfur analyses equipment. If k values are out of this permitted error, it means that liquid maldistribution may be high. Flowrate tests may also be employed for detecting and correcting maldistribution caused by fouling or nonuniform catalyst loading, as suggested by Edgar et al.71 Elevated pressure drop is a clear symptom of bed fouling and inadequate catalyst loading, in such a case, the authors recommend to increase at least 25% the feed rate of liquid and gas, as well as temperature in order to keep the conversion constant. A common guideline for increasing temperature is that it takes around a 20 °C increase when LHSV is doubled. A product quality improvement with the increased feed rate will indicate the presence of maldistribution. 3.2. Examples of Successful Internal Revamping. The correction of liquid distribution in hydroprocessing reactors improves substantially unit performance. Some of the benefits are the reduced radial delta-T (

Key Role of Reactor Internals in Hydroprocessing ... - ACS Publications

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