MERICAT J: A New Kerosene Treating Technology to Meet Jet Fuel Specifications
MERICAT J is a new wet treating technology that Merichem has developed for treating kerosene to meet jet fuel mercaptan specifications. It does not use either hydrogen or sodium hydroxide (caustic) followed by a catalytic carbon bed. MERICAT J’s robust technology maintains mercaptan treating efficiencies over a wide range of feedstock qualities. Further, it offers the capability to increase unit capacity using the existing equipment. This paper reviews the operating experience of the first commercial MERICAT J installation at Alon USA’s Krotz Springs refinery, and describes the advantages of MERICAT J over other wet treating technologies which include lower capital investment, reduced operating expenses, and other technical benefits.
Jet fuels must meet stringent international specifications because they are used by airlines world-wide. Regardless of where planes land, airlines need safe, high quality fuels for re-fueling. Today’s jet fuels must meet numerous specifications including ranges for acidity, aromatics, naphthalenes, distillation range, density, viscosity, flash point, smoke point, total sulphur, mercaptan sulfur, freeze point, color/appearance, copper corrosion, water separation index, and thermal stability properties. Many publications, including ASTM, list the jet fuel specifications and test methods for each jet fuel grade.
Jet fuel production begins with crude oil distillation which separates the kerosene cut within a specific boiling range from the whole crude. Some conversion units within the refinery produce kerosene directly or indirectly. Further treating of the kerosene to meet jet fuel product specifications varies based on the quality and impurities in the kerosene.
Kerosene treating processes can be grouped into three categories:
1) No additional processing or minimal cleanup, like clay filtration.
2) Wet treating using caustic (NaOH) solution. Wet treating reduces acids content and total mercaptan sulphur level to meet corrosion and sulphur tests specifications without altering other kerosene properties. These processes typically use a NaOH solution with a catalytic carbon bed to oxidize mercaptans to disulphide oil (sweetening), followed by water washing, salt tower drying, and clay filtration.
3) Hydrotreating Based on the crude, some kerosene feed stocks will require hydrotreating to meet specifications. Compared to wet treating processes, hydrotreating capital costs are likely to be many times higher with operating costs up to 20 times higher. As hydrotreating can be used to produce jet fuel from almost any kerosene, it offers more flexibility lowering the total sulfur content in the final product. Despite some flexibility over wet treating, hydrotreating may alter many properties of the kerosene while wet treating only reduces the acid and mercaptan sulfur contents. Of the total jet fuel produced about 45-55% is by hydrotreating and 35-45% by wet treating, with a small percentage falling into a “minimal cleanup” category.
MERICAT J falls into a “wet” treating process category. Unlike other wet treating processes, MERICAT J uses no NaOH or catalytic carbon bed. Instead, the process uses Merichem’s proprietary treating solution JeSOLTM-9 to remove mercaptans from the jet fuel in a single reactor followed by water washing, salt tower drying and clay filtration. MERICAT J has many advantages over NaOH treating which are discussed later in this paper. Prior to further discussion of the MERICAT J process it is valuable to describe the wet treating process called MERICATTM II that Merichem began licensing in 1986. Currently, Merichem has more than 40 MERICAT II units operating worldwide. In addition there are many more wet treating (NaOH) processes operated by owners/operators.
Description of MERICAT II
FIGURE 1 contains a block flow diagram of a Merichem Kerosene Treating Unit (KTU) with NAPFININGTM, MERICAT II, and AQUAFININGTM. The NAPFINING step reduces strong acids (mostly naphthenic acids) to meet Total Acid Number (TAN) specifications and protect the carbon bed in MERICAT II from naphthenic emulsion formations. NAPFINING uses a dilute NaOH solution with Merichem’s proprietary contacting technology called the FIBERFILM® Contactor (FFC). Merichem’s FFC is a patented interfacial contacting design that has been successfully used in over 800 operating plants worldwide since the late 1970’s. The FFC is an innovative design developed as an improvement to conventional caustic/hydrocarbon dispersive mixing designs like mix valves. The design creates a large contacting surface area between the hydrocarbons and the aqueous treating reagents (E.g. NaOH, water, etc.) without vigorous dispersive mixing that form emulsions. Merichem’s FFC reduces NaOH carryover with the hydrocarbon after the contacting and reduces the size of separation vessels. Please see reference 1 for a more detailed description of NAPFINING and Merichem’s FFC.
Upon exiting NAPFINING, the kerosene stream flows to MERICAT II. MERICAT II reduces mercaptans down to Jet Fuel product specifications, which are typically < 0.003 wt. % mercaptan sulphur. Mercaptans are a family of organic compounds in which sulphur and hydrogen atoms (SH) are bonded to a carbon atom that is commonly abbreviated as RSH. Mercaptans exude a very bad odor, are toxic, and cause corrosion. MERICAT II consists of two discrete stages. The first stage uses an FFC to contact the kerosene with caustic and a homogenous catalyst to oxidize lighter mercaptans. In the second stage, heavier or longer chain mercaptans are treated across a fixed bed of activated carbon.
The two MERICAT II stages first extract mercaptans with a caustic solution to form sodium mercaptides (NaSR) and then oxide them using injected air. The oxidation produces disulphides, commonly referred to as DSO (disulphide oil). The overall reaction is commonly referred to as “sweetening”. The DSO produced in the process remains with the kerosene. Although the total sulfur content in the jet product does not change from the feed kerosene total sulfur, the kerosene has been “sweetened” after the mercaptans were removed.
Upon exiting MERICAT II, the treated kerosene passes through a water washing step with an FFC called AQUAFINING to remove any residual caustic, a salt tower to remove free water, and a clay filter to remove surfactants, particulates, and other trace contaminants that may negatively impact jet fuel product specifications.
The general mercaptan reactions in MERICAT II are: Extraction: 2RSH + 2NaOH 2NaSR + 2H2O Oxidation: 2NaSR + ½ O2 + H2O + (catalyst) RSSR + 2NaOH Overall Reaction: 2RSH + ½ O2 + (catalyst) RSSR + H2O Where RSH = mercaptan and RSSR = Disulphide Oil (DSO) Note: In the reactions above, the NaOH is regenerated in the oxidation reaction to form RSSR and H2O. Also, some kerosene feeds contain H2O that can dilute the NaOH strength with the H2O formed in the MERICAT II process. Over time, the NaOH strength lessens until it becomes “spent” due to reactions with contaminants (E.g. nap acids, sulfides, carbon dioxide, etc.). Spent NaOH must be replaced.
Description and Benefits of MERICAT J
MERICAT J was developed by Merichem as a new wet treating process for treating kerosene to meet jet fuel mercaptan specifications. MERICAT J is truly a new “step out” process technology that can replace the long used caustic / catalytic carbon bed processes (which include MERICAT II). It offers many advantages that include lower capital and operating costs and more robust performance characteristics. MERICAT J is now proven with the successful operation of the first commercial unit at the ALON USA Refinery in Krotz Springs, Louisiana. A more complete list of the advantages of MERICAT J will be given after the process is described in more detail below.
Figure 2 is a block flow diagram of the MERICAT J process for treating kerosene. It should be noted that the Alon USA Krotz Springs refinery installation is not identical to this standard Merichem process design. As shown in the figure, MERICAT J replaces MERICAT II in Merichem’s KTU process. Downstream of the NAPFINING stage, the kerosene stream contacts the JeSOL-9 solution and FFC design in the MERICAT J process. The kerosene then goes directly to the AQUAFINING stage, through the salt tower and clay filter, and exits the KTU.
Merichem’s JeSOL-9 solution is a specially formulated treating reagent with higher activity than the caustic / carbon bed process. Similar classes of reactions occur in the MERICAT J process as in the MERICAT II process, but at a higher reaction rate. Although the JeSOL-9 solution is internally regenerated after reacting with mercaptans, some reactions with contaminants eventually cause the JeSOL-9 solution to gradually become “spent” over time. Eventually, the JeSOL-9 solution will become fully spent and require replacement with fresh JeSOL-9 solution.
Merichem offers a total package service agreement to its MERICAT J licensees. This service includes supplying all fresh JeSOL-9 and taking back all spent JeSOL-9. As used by Merichem, the spent JeSOL-9 is not a waste material under the Basel Convention. This results in the refiner having no waste streams to dispose of in the MERICAT J process.
Advantages of MERICAT J compared to MERICAT II include:
1). Significantly lower capital investment
Merichem’s evaluations show about 20 - 30% lower capital costs for MERICAT J compared to a MERICAT II unit having the same design basis. These savings are realized primarily because the carbon bed vessel is relatively large compared to the MERICAT J vessel. Since the MERICAT J vessel is significantly smaller than any vessels required for carbon bed wet caustic treating technologies, operators can use existing equipment to further reduce capital investment. Additionally, MERICAT J offers the opportunity to retrofit existing units with carbon beds to higher capacities using the existing carbon bed vessel.
2). Operating costs are significantly lower
Merichem’s evaluations show that MERICAT J operates with 30 - 40% lower costs compared to MERICAT II. These savings are realized by eliminating the carbon bed, improving on-stream operating time, reducing consumption of JeSOL-9 compared to NaOH, and lowering catalyst consumption. Another large cost associated with operating a carbon bed is the cost to downgrade the jet fuel product values into the diesel pool during routine carbon bed maintenance.
3). Higher Reliability
MERICAT J eliminates the catalytic carbon bed that is susceptible to temporary fouling and permanent poisoning by certain impurities in the kerosene. In some cases, the carbon bed is poisoned quickly which may not be recognized until the carbon bed unit starts producing off specification jet fuel. Operating without a carbon bed eliminates all of these issues and enables MERICAT J to have a longer on-stream factor that other carbon based treating units. The ALON operating MERICAT J has demonstrated on specification jet product that is significantly higher than many caustic treating units operating with carbon beds.
4). Elimination of Carbon Bed Maintenance
All maintenance activities associated with catalytic carbon beds have been eliminated. These include recausticization, water-washing, catalyst re-impregnation, and carbon bed replacement.
5). Elimination of Waste Stream
Since Merichem supplies fresh JeSOL-9 solution and takes back all spent JeSOL-9 solution, a waste stream associated with treating the kerosene has been eliminated for Merichem’s licensees. The operator does not have to process a JeSOL-9 waste stream or any spent carbon material. Merichem assumes title and risk of loss upon safe loading by the licensee at its facility or upon safe delivery and transfer back to Merichem’s plant. All JeSOL-9 materials (fresh and spent) can be shipped by railcar, tank wagon, or ISO totes depending on the quantity.
6). Revamping for Higher Capacity
MERICAT J offers refiners the opportunity to use existing vessels when they need to increase capacity of an existing jet fuel wet treating unit. In most cases, Merichem can retrofit existing carbon bed vessels with the MERICAT J internals to achieve increased jet fuel capacity.
The ALON Krotz Spring Refinery MERICAT J Unit
ALON started up the MERICAT J unit in December of 2011. With the startup of this unit, the refinery was able to make jet fuel from lower cost crude slates. In order to reduce capital and meet an aggressive project schedule, the kerosene treating unit used equipment for the water washing, salt drying and clay treaters that were already in place at the refinery.
The Alon MERICAT J unit return on capital employed was greater than 100%, resulting in a payout of less than 12 months. The kerosene treating unit design included NAPFINING and MERICAT J technologies. Some of the basic design and normal operating parameters for Alon’s MERICAT J unit are shown below in Table 1.
TABLE 1: ALON MERICAT J Primary Design/Operating Parameters
|Design Capacity, Kerosene Flow||20,000 BPD|
|Normal Operating Capacity, Kerosene Flow||13,000 BPD|
|Normal Operating Temperature||100 deg F|
|Normal Operating Pressure||70 psig|
|Kerosene Inlet Mercaptan Sulfur, (max)||130 ppm-wt.|
|Jet Fuel Product Mercaptan Sulfur||< 30 ppm-wt.|
This MERICAT J unit project was implemented quickly to meet the fast track project scope using the existing vessels as described earlier. The refinery elected to start the unit up instead of delaying the project until long lead time parts for the JeSOL-9 water removal system were delivered.
Soon after startup, Alon confirmed that the kerosene feed to the MERICAT J unit contained more free water than was originally specified in the design feed basis. Without the water removal system in place at startup, adequate water balance could not be maintained and the JeSOL-9 solution was weakened more quickly than expected. As a result, the JeSOL-9 had to be replaced more often than planned until the water removal system came on line several months after startup. Despite the higher water content in the unit, the MERICAT J operated very profitably.
After the water removal system was installed and became operational, the JeSOL-9 replacement frequency returned to predicted levels. A second equipment modification to mitigate some of the design limits from reusing existing vessels was identified shortly after startup but was not installed until March 2013. Following installation of both the water removal system and the equipment modification, unit profitability increased to new levels that have been maintained to the present.
Chart 1 shows the monthly averaged jet fuel percent on specification mercaptan levels for the first 24 months of the MERICAT J operation following initial startup. Of the first 17 months of operation prior to May 2013, product mercaptans met specification from 82% to 97% of the time for 11 months and 100% for 6 months. The equipment modification was made in March 2013. From May 2013 through March, 2014, product mercaptans were on specification 100% of the time. The data are a simple average during the month of each mercaptan measurement taken once per day in the jet fuel product run down stream.
Chart 2 shows additional mercaptan removal performance on a daily basis over several months following the equipment modification in March 2013. The kerosene inlet mercaptan is on the x axis and jet fuel product mercaptan is on the y axis. The horizontal red line is the mercaptan specification limit of 30 ppm wt. During this time, the kerosene feed rate averaged 14,080 BPD compared to normal operation design of 13,000 BPD and feed mercaptans exceeded the design basis. During these periods, the MERICAT J process demonstrated its flexibility by maintaining product specifications despite the wide range of feed mercaptan levels.
Merichem’s MERICAT J jet fuel treating technology has been successfully demonstrated at the Alon Krotz Springs refinery. The process can be designed to treat jet fuel mercaptans containing up to several hundreds of ppm (wt.). MERICAT J requires lower capital investment and operates at a lower operating cost than other NaOH treating unit designs using carbon beds. Additionally, MERICAT J can be used to revamp existing units quicker and for lower total installed capital by reusing existing vessels. MERICAT J is a true step out in jet fuel wet treating that has application throughout the world.
1) Forero, Patricia and Suarez, Felipe J (Merichem Company), duPont, Abe J (National Petroleum Refiners of South Africa) “Caustic Treatment of Jet Fuel Streams”, PTQ Q1 1997.
2) Mohamadbeigy, Kh, Bayat, M., and Forsat, Kh. (Research Institute of Petroleum Industry, Tehran), “Studying on the Jet Fuel Treating Plant for Maximum Efficiency”, Petroleum and Coal 48(3), 36-41, 2006.
3) ASTM D1655, “Standard Specifications for Aviation Turbine Fuels”, and additional relevant ASTM specifications and methods.
4) “Aviation Fuels Technical Review”, by Chevron Corporation, 2006.
5) Internal Merichem Company Reports and Documents.
6) Data and operating information supplied and used with permission by Alon USA Krotz Springs Refining