ExxonMobil Catalytic Dewaxing - A Commercial Proven Technology

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1 ExxonMobil Catalytic Dewaxing - A Commercial Proven Technology Anna Gorshteyn, Paul Kamienski, Tim Davis, William Novak, Matthew Lee ExxonMobil Research ad Engineering Company, Fairfax, VA, USA Abstract Distillate fuels have specifications on flow properties at low temperature. Catalytic dewaxing is an important process to upgrade fuels to meet these specifications. Mobil Oil Corporation, a predecessor of ExxonMobil Corporation, patented the zeolite ZSM-5 catalyst in ZSM-5 catalyzed distillate dewaxing was first introduced after intensive pilot tests in 1978 and has been continuously improved upon. Today, the ExxonMobil Distillate Dewaxing (MDDW) process is used worldwide more than 30 licensees who have a combined dewaxing capacity in excess of 160,000 BPD. A more recent Distillate dewaxing process via isomerization, which uses proprietary bifunctional molecular sieve metal catalys. ExxonMobil catalyst was first introduced in ExxonMobil s Jurong refinery in This process gives higher distillate yield by isomerizing waxy materials in a feedstock, rather than removal by cracking. The technology lends itself to use with other catalysts in units to produce low pour point, ultra-low sulfur distillate products with premium qualities. Details of the ExxonMobil Isomerization Dewaxing (MIDW) process are proprietary. Cold Flow Properties Distillate fuels typically have property specifications at low temperatures. These specifications ensure a sustained supply of fuel to an engine or burner at conditions of low ambient temperature. As a fuel is cooled, wax compounds that are present will start to crystallize and affect the flow characteristics of the fuel. There are three main specifications that are used to define flowability at low temperatures: Pour Point (ASTM D97): Pour point is the temperature at which there will be no movement of a chilled sample in a test jar. As the sample is chilled, one can often observe the formation of a layer of gelatinous wax crystals. As the temperature is lowered further, this layer increases and at some point, the sample will no longer flow. Cloud Point (ASTM D2500): Cloud point is the temperature at which a haze or "cloud" in fuel appears. It is an indication that wax crystals have begun to form. Therefore, the cloud point temperature is higher than that of pour point. The specification temperature varies with the intended use of the fuel.. CFPP (IP309): Cold filter plugging point is defined as the temperature at which a sample of oil fails to pass through a wire mesh filter in a specified amount of time. ExxonMobil's experience is that this temperature is often midway between the pour and cloud points. Methods for Improvement of Cold Flow Properties The cold flow properties of a distillate are affected by its original source (crude type) and any subsequent processing. Improvement of cold flow properties can be achieved in several ways: 1. Additives: Chemical additives exist to improve pour point. These are less effective for cloud point and CFPP. 2. Addition of lighter material: Lighter distillate typically has better cold flow properties than heavier diesel. The addition of lighter material, such as kerosene, can improve the properties of the blend. 1

2 The quantity is limited by the flash point specification. Lighter material can also have negative impact on other properties, such as cetane. It also may be disadvantageous in that some kerosene is lost to diesel fuel or home heating oil. 3. Solvent Dewaxing: Use of a solvent to remove wax is possible, but is seldom employed for distillates. It has the disadvantage of producing a low-grade wax by-product that must be disposed of. 4. Catalytic Dewaxing: Catalytic dewaxing uses a catalyst to remove waxy material either by cracking or by isomerization. Development of Catalytic Dewaxing and Introduction of MDDW Process In 1972, Mobil obtained its first patent on the zeolite ZSM-5. Much has been published about this catalyst, including its structure, shape, selectivity and activity. Zeolites can be visualized as repeating crystals that contain pores of a very specific size. Figure 1. Only normal paraffin and slightly branched isoparaffins, which define cold flow properties, can enter the pores having such dimensions and are cracked into lower molecular weight hydrocarbons, leaving aromatics, naphthenes and highly branched paraffin unchanged. As a result, products are distillate with excellent low temperature fluidity. There are variants of the ZSM-5 crystal and different ways that the ZSM-5 crystal can be employed in a finished catalyst. ExxonMobil continues to improve catalysts for the MDDW process, based on ZSM-5. Listed below are some important aspects in obtaining an optimal dewaxing catalyst: Silica/Alumina ratio: Zeolites, such as ZSM-5, are composed of repeating units of silicon and aluminum oxides. The silicon/alumina ratio can be varied, within a certain range, to optimize the number and strength of the catalytic acid sites. ExxonMobil has continued to do this to maximize dewaxing with minimum cracking. Incorporation of ZSM-5 crystal into the catalyst particle: Historically, ZSM-5 crystal is usually incorporated into a catalyst pellet using a binder. ExxonMobil has perfected a method where a finished catalyst of much higher activity can be produced. 2

3 Metals: ZSM-5 can be employed alone or impregnated with certain metals. The exact catalyst to be used is selected based on the proposed application. Process Description Based on extensive pilot work, Mobil introduced the first commercial dewaxing process, called MDDW, in MDDW is a fixed bed catalytic process. Hydrogen is circulated to maintain satisfactory catalyst activity, but little is consumed. Reactor temperature is adjusted to control product fluidity and compensate catalyst aging. Typical equipment requirements, as illustrated by an MDDW process schematic are compatible with HDS and in fact almost half of MDDW applications were converted from HDS units. Figure 2. Typical operating conditions also are the same as for HDS units Temperature, o C 260 to 455 Inlet H2 Partial Pressure, Kg/cm2 20 to 50 LHSV, hr to 2.5 H2 Circulation, Nm3/m3 250 to 425 MDDW Process Options MDDW unit configuration is very flexible and has been tailored to individual refinery situations. Hydrogen-rich makeup gas sources include reformers, H2 purge gas from hydrotreaters and hydrogen plants. Hydrogen circulating systems include both recycle and once through. Oil and hydrogen are heated to reaction temperature together in some units, in separate furnaces in other. Distillate, gasoline and gas product separation metods include steam stripping and reboiled fractionation. The ability for amine scrubing hydrogen sulfide from recycle gas allows producing sweeter gasoline. 3

4 Initially, MDDW was often employed alone. Its high resistance to sulfur & nitrogen enabled its use on raw feeds, where the feed sulfur already met the required product specification. When product sulfur specifications began to be reduced, different configurations that combined dewaxing catalyst with hydrotreating catalysts were introduced. Figure 3 below shows different possible process applications. In combining dewaxing catalysts with hydrotreating, it is very important to consider the temperature requirements of both catalysts throughout the cycle. All MDDW applications have been commercialized and proved their feasibility. MDDW catalyst improvement allows also it to work with a cascade HDS/MDDW mode with two reactors in series. To show the capability of the process, below are represented commercial data of MDDW applications for case A, when MDDW was employed alone. Feedstocks A refiner's optimal MDDW feedstock depends on crude properties, product markets and existing units - HDS, hydrocracker, catalytic cracker an/or visbreaker capability. Various feedstock types have been successfully processed, including virgin atmospheric and vacuum, desulfurized and hydrocracked gas oils from Persian, Gulf, African, North Sea, Southeast Asian, Canadian and United States crude. Feedstock 95vol% point has ranged from 340 to 485 o C, sulfur content from 0.2 to 2.5wt% and our point from -10 to +33 o C. Example of feedstock from three commercial unit illustrate this variety. Yields Commercial MDDW catalyst performance has been quite satisfactory in all units. Total MDDW gasoline plus distillate yield usually exceeds 94wt% for any feedstock. Distillate yields are 80 to 90wt% for most feedstock. However, distillate yields decreasing with feedstock paraffin content to as low as even 60wt% for some highly paraffinic feedstock. Table 2 shows example of yields, which respond to the feedstock represented in Table1. 4

5 While distillate yield are feedstock dependent, MDDW catalyst selectivity for producing gasoline and lighter products is relatively constant: generally 70 to 80wt% of converted paraffin is C5+ gasoline, 12-22wt% -C4's 4-8wt% - C3's and 0-2wt% C2-C1. MDDW distillate and gasoline products yields are essentially constant through each cycle for the life of the catalyst. Table1. Commercial MDDW Feedstock properties Example Relative Feedstock Paraffin Content Low Medium High Distillate Products Properties Specific Gravity, 15/4 C Sulfur, wt % Pour Point, C Cloud Point, C Paraffins, wt % 15 Naphthenes, wt % 39 Aromatics, wt % 46 Flashpoint, C Aniline Point, C C, cs C, cs C, cs Distillation, C - Type D-86 D-1160 D-1160 IBP vol % " " EP " Product quality MDDW distillate product has proven an attractive blend stock for diesel fuels and heating oils worldwide. It has excellent low temperature fluidity, and has been used in fuels with very broad range specifications. Distillate product properties (Table 3) depend strongly on feedstock properties (Table 1). Due to paraffin removal by shape selective cracking, MDDW gas oil product has lower API gravity and paraffin content than feedstock, as a result cetane index usually decline by 1-2 numbers. As there were no feed or product hydrotreating, sulfur content is also higher than in the feed. MDDW gasoline product typically has 88 to 91 research octane number (C 6 + basis, Table 4) and thus is a valuable gasoline blending component after sweetening. It usually contains 60 to 80wt%, mostly high octane C 5 - C 7 olefins, and this makes it a good pool front end octane improver. The clear motor octane is about numbers lower than research octane number. 5

6 Table 2.Commercial MDDW Yields. Example Relative Feedstock Paraffin Content Low Medium High Feedstock Pour Point, C Distillate Product Pour Point, C Yields,%MDDW Charge Wt. Vol Wt. Vol Wt. Vol H H 2 S NH 3 <0.01 <0.01 <0.01 C < C 2 <0.01 < C 2 = < <0.01 C C 3 = ic nc C 4 = ic nc C 5 = C 6 + Gasoline Distillate Total H 2 Consumption, nm 3 /m Reliability Refinery processes have to be reliable. With fixed bed catalytic processes it means long cycle, long catalyst life, minimum selectivity shifts as the catalyst ages. These reliability factors have been demonstrated in MDDW commercial applications. Below in Figure 4 is shown aging plot for one of this application in Germany, which covers 44 months of operation. During that time five catalyst regenerations have been done. Neither this refinery nor any other well-operated MDDW unit has shown any significant shifts in yield as catalyst aged. The yield/pour point curve has been constant for each given feed irrespective of catalyst age. 6

7 Table 3. Commercial MDDW Distillate Product Quality Example Relative Feedstock Paraffin Content Low Medium High Distillate Products Properties Specific Gravity, 15/4 C Sulfur, wt % Pour Point, C Cloud Point, C Paraffins, wt % 15 Naphthenes, wt % 39 Aromatics, wt % 46 Flashpoint, C Aniline Point, C C, cs C, cs C, cs Distillation, C - Type D-86 D-1160 D-1160 IBP vol % " " EP " Table 4.Commercial MDDW Gasoline Product Quality Example Relative Feedstock Paraffin Content Low Medium High Gasoline Product Properties Cut Type C6+ C6+ As Cut As Cut Specific Gravity, 15/4 C Sulfur, wt % Mercaptans, ppm Bromine Number Paraffins, wt % 24 Olefins, wt % 70 Naphthenes, wt % 4 Aromatics, wt % 2 Octanes Research Clear Motor Clear Distillation, C - Type D-86 D-86 D-86 D-86 IBP vol %

8 50 " " EP " Figure 4. MDDW Catalyst Performance Actual Average Reactor Temperature Today ExxonMobil Disstillate Dewaxing technology is used worldwide in ExxonMobil refineries and by more than 30 licensees who have a combined dewaxing capacity in excess of 160,000 BPD. Today, the ExxonMobil Distillate Dewaxing (MDDW) process is used worldwide at EXONMobil refineries and more than 30 licensees who have a combined dewaxing capacity in excess of 160,000 BPD. Table 5. The reference list by country of some running dewaxing units: Location Start Up Location Start Up South Africa 1980 Italy 1992 China 1982 USA 1993 Italy 1982 USA 1994 South Africa 1982 Egypt 1995 Italy 1984 Japan 1996 Japan 1985 Italy 1997 Italy 1985 France 1996 USA 1986 Germany 1992 USA 1988 UK 1994 Italy

9 Location Capacity Number of units BPSD Grassroots Retrofit Africa Asia Europe North America ExxonMobil Isomerization Dewaxing Technology (MIDW) To obtain even higher distillate yield than the in original MDDW process, the MIDW process is used. First announced in 1995, the MIDW process uses a proprietary, dual-functional noble metal-molecular sieve catalyst to hydro-isomerize and selectively crack waxy gas oils to produce premium, low pourpoint distillates. The catalyst is distillate selective. The isomerized paraffins and selectively cracked paraffins remain in the distillate range, thus the distillate yield is very high. Typical distillate yield is greater than 90%. MIDW is a low investment, very flexible, fixed-bed reactor process capable to operate in hydrodesulfurization conditions. The process is very flexible to feedstock choices. MIDW feedstock can be straight run and hydrotreated vacuum gas oil and hydrocracker bottoms. High pour point atmospheric gas oil, distillate and high freeze point kerosene can be processed to improve the cold flow and other distillate properties. As for products, MIDW is able to produce high quality kerosene, jet and diesel fuel and low pour point fuel oil. The basic MIDW configurations can be used, as shown in Figure 5. It can be integrated into, and is compatible with distillate desulfuization units. The MIDW catalyst is relatively sulfur and nitrogen tolerant. MIDW feeds more than 10 different crude sources having nitrogen contents up to 1200ppm and sulfur up to 3wt% have been processed. So the first processing configuration (Figure 5) can be used. Feed nitrogen determines processing temperature, the more nitrogen the higher temperature is required. High sulfur content can reduce the effectiveness of the metal function. This can be reduced substantially by feed hydrotreating. Several MIDW reactor configurations can be used depending upon the level of feed pretreatment required. Cascade HDT followed by MIDW is effective with moderate feed heteroatom levels, lowering reactor temperatures by o C. Commercial experience with such cascade single stage operation in the very severe conditions with pour point improvement from +11 o C to -64 o C showed outstanding results in catalyst stability and excellent distllate properties improvement in density, cetane index and aromatic content at distillate yield ( see Table 6). Interstage removal of H 2 S and NH 3 can improve activity by 50 o C or more. The MIDW reactor can operate with relatively low hydrogen pressure. Hydrogen-rich gas is recycled back to the MIDW reactor to maintain catalyst activity. Operating conditions and processing objectives are very flexible. Typical operation conditions are: Temperature, o C 260 to 440 Inlet H2 Partial Pressure, Kg/cm2 25 to 50 H2 Circulation, Nm3/m3 180 to 900 9

10 Figure 5. Basic MIDW Configurations. Low Sulfur Low Nitrogen High Pour Moderate Sulfur Moderate Nitrogen High Pour High Sulfur Very High Nitrogen High Pour HDT MID W MID W HDT MID W Low Sulfur Low Nitrogen Low Pour Low Sulfur Low Nitrogen Low Pour H 2 S, NH 3 Low Sulfur Low Nitrogen Low Pour Depending on refinery feedstock, product requirements and unit configuration the cycle length between oxygen regenerations is typically between one year and six years. Table 6. Results of Commercial Experience for HT/MIDW Cascade Single Stage at High and Low Severity Operation High Severity Severity Low Feed Product Feed Product Specific Gravity, g/ml Sulfur, ppm * Nitrogen, ppm PNA, wt% * Pour point, o C Yield 180 o C *sulfur and PNA in product can Be adjusted By HDT MIDW versus MDDW MIDW is more distillate selective comparing with MDDW process because it retains paraffins in distillate increasing distillate yields and quality. Especially it is important with high paraffin content in the feed (Figure 6), where the distillate yields may grow substantially with MIDW process. 10

11 Figure 6. Yields versus Feed Paraffin Content Low Pour Distillate Yields, wt% Feed Paraffin Content, wt% For example, the very waxy South-East Asian LVGO with 63% total paraffins has been comparatively dewaxed by both MIDW and MDDW in identical conditions with exception of the reactor temperatures required to achieve the target point. No feed pretreatment has been used in this case. The results of the test which are represented in Table 6 show substantial distillate yield and quality improvements comparing with conventional dewaxing process. Table 6. MIDW and MDDW comparative test with high paraffin LVGO Properties Feed MDDW MIDW Pour Point, o C 32-7 (target) -7(target) Sulfur, ppm Cetane Index 52 6 TBP 95%, o C KV, cs@40 o C Yield, wt% ExxonMobil has several versions of the MIDW catalysts, which can be applied in different process configurations. have different conversion activity. This may allow refineries to process more heavy feed and increase their total conversion. OMV Burghausen HDT/MIDW Commercial Process The kick-off meeting for the design of the OMV Burghausen MAKFining HDT/MIDW unit took place in September 1999 and the engineering design by Halliburton KBR was completed by the end of 11

12 December of Construction of the new unit was completed by the 3rd quarter of 2001, in time for ultra-low sulfur diesel to be marketed by the 4th quarter of The raw diesel feed in the OMV Burghausen case is a straight run gas oil containing 2300ppmw sulfur with cloud point about +5 o C. The hydrotreating (HDT) catalyst was selected based on the need for the highest possible HDS activity, with sufficient HDN activity to optimize the downstream MIDW catalyst performance. One of Akzo Nobel's commercially available, high activity STARS HDT catalyst formulations was chosen for this service. The STARS catalysts are base metal (NiMo, CoMo) HDT catalysts. In order to achieve optimal activity, the STARS catalysts are specifically sulfided according to procedures developed by Akzo Nobel. Distillate selective MIDW catalyst was chosen for the dewaxing catalyst. The OMV Burghausen Phase-1 construction was completed on schedule and the unit successfully started-up in August The HDT/MIDW stacked bed start-up progressed smoothly as expected, and after a 2-day catalyst activation, feed was introduced. The OMV MIDW unit came on-line with excellent HDS, dewaxing activity, and yield selectivity as was expected from pilot plant simulations. The OMV Burghausen low sulfur diesel /MIDW unit has been on-stream for about a year. HDS activity has been excellent - the unit has been producing a diesel fuel generally <10ppmw sulfur. Cloud point reduction has averaged about -18 C consistent with OMV diesel product needs and design expectations. Catalyst stability is also being excellent. Reactor Internals are critical for good performance Poor flow distribution is often a result of reactor internals design and can result in Poor catalyst utilization Poor selectivity performance Poor reactor stability and control The successful and profitable operation of dewaxing technologies very dependent upon choice of all hardware, but the most important area frequently overlooked is reactor internals. Reactor internals can have as large an effect on reactor performance as the catalyst choice. ExxonMobil offers the combined experience of both Exxon and Mobil in providing the latest technology in reactor internals. ExxonMobil Research and Engineering Company's (EMRE) Spider Vortex reactor internals provide the leading edge technology solution for owners who want to optimize grassroots or revamped hydroprocessing reactor performance and reliability. Spider Vortex technology assists with enhanced reactant distribution and uniform radial temperatures for dewaxing technologies. Radial delta temperature is a measure of uniformity of catalyst bed temperature in the reactor and is being defined as the maximum difference between the hottest and coldest temperature along the horizontal thermowell. Relatively high radial delta temperature's (>8oC) indicate poor gas/liquid flow distribution. Properly engineered reactor internals have many benefits. Poor distribution can lead to catalyst utilization issues. For example, bed channeling can occur and, if so, will cause hotspots. Through channeling, portions of the catalyst bed will essentially be bypassed, thereby using less than the whole catalyst bed. This can have several cascading effects. Yields may be lower because stagnant material in the bed may be over cracked, creating less distillate-range material. Because some of the catalyst is bypassed, bulk temperature will be higher than it should be if the entire catalyst bed were utilized. As a result, catalyst run lengths will be shorter. Also, stagnant catalyst portions will show higher radial delta temperatures and portions of the bed will deactivate faster than will others. Temperature control will be more difficult because of the varying catalyst activities in the bed. All these problems can be corrected 12

13 with properly designed reactor internals. Through the use of the latest reactor internals, ExxonMobil can design very large diameter reactors and take advantage of economies of scale for large units. In addition to improved inlet vapor-liquid distributors, ExxonMobil also provides spider vortex quench technology. This technology provides many of the same benefits above, including maximum utilization of quench in the reactor. Also, it contributes to a high uniformity of mixing in the quench zone to obtain maximum heat transfer. Similar distribution trays to those used at the reactor inlet are also used to provide the benefits listed above. ExxonMobil has extensive test facilities that have been used to optimize these reactor internals designs. Research to improve the design of these internals continues. This research also allows for easy scale up to any grassroots or revamp reactor design. EMRE has established a leading position in the application of next generation reactor internals technology in both revamped and new dewaxing reactors using MDDW and MIDW catalysts. More than thirty Spider Vortex systems have been installed, nine of which were revamped reactors and internals designed by ExxonMobil as part of the EMRE alliance. The audit of one refinery application showed that improved performance associated with the revamp connected with internals installation has a Net Present Value (NPV) of $15 million with an investment of only $500,000. Final Summary ExxonMobil invented catalytic dewaxing using zeolite catalyst in the early 1970s. Since that time, company has been an industry leader in providing shape-selective catalytic processes, of which the MDDW and MIDW dewaxing technologies have been very successful in industry. The MDDW process has successfully been implemented with the current offering using a fourth generation catalyst with almost 30 years of commercial experience. MIDW is another commercially-proven technology, with over 10 years of commercial experience. ExxonMobil's MDDW and MIDW processes provide a staged investment strategy that will produce premium distillate fuels in response to regulatory challenges for refining industry. REFERENCES 1. Chen, N.Y., Gorring, R.L., Ireland, H.R., and Stein, T.R., "New Process Cuts Pour Point of Distillates", Oil & Gas Journal, Vol. 75, No. 23, June 6, 1977, pp Graven, R. G. and Green, J.R., "Hydrodewaxing of Fuels and Lubricants Using ZSM-5 Type Catalysts", Australian Institute of Petroleum 1980 Congress, Sydney, Australia, September 15-17, Donnelly, S. P., "MDDW Commercial Experience", Japanese Petroleum Institute Symposium, November 15-16, Pappal, D.A., "A New Conversion Process for Premium Distillate", PETROTECH 96, Middle East Refining & Petrochemicals Conference & Exhibition, Manama, Bahrain, June 10-12, Chitnis, G.K., Helton, T.E., Nagel, U., Novak, W.J., Macris, A., Pappal, D.A., Tracy, W.J., "Innovative Solutions for Production of Low Sulfur Distillates Using Selective Dewaxing and Advanced Hydrotreating Catalyst", 3 rd European Catalyst Technology Conference, Amsterdam, The Netherlands, February 26-27,