July 2022

Process Optimization

MAA refinery’s experience producing Europe’s most-stringent diesel specification

The European climate has one of the harshest winter weather conditions—therefore, diesel products’ cold flow properties [i.e., cloud point (CP) and cold filter plugging point (CFPP) parameters] are key to ensure diesel products’ usability during these climate conditions.

Matar, M. B., Al-Mane, A., Layri, A. Y., Kuwait National Petroleum Co.

The European climate has one of the harshest winter weather conditions—therefore, diesel products’ cold flow properties [i.e., cloud point (CP) and cold filter plugging point (CFPP) parameters] are key to ensure diesel products’ usability during these climate conditions. Kuwait National Petroleum Company’s (KNPC’s) Mina Al-Ahmadi (MAA) refinery has conducted studies and field trials to establish its capabilities to produce ultra-low sulfur diesel (ULSD) that adheres to a CFPP of –22°C and to achieve the ARAL (owned by bp, ARAL has the largest filling station network in Germany) test’s requirements. Wax anti-settling flow improver (WAFI) additives were procured from different suppliers during two separate periods to carry out the studies while modifying the units’ mode of operations. The MAA refinery’s revamped gasoil desulfurization unit and new diesel hydrotreating (DHT) unit were utilized in the trials. The DHT unit was one of the processing units commissioned for KNPC’s Clean Fuels Project (CFP), which was awarded Hydrocarbon Processing’s Top Projects award in 2016.

This article provides a comprehensive analysis of cold flow properties of the ULSD product and the MAA refinery’s experience in producing ULSD to adhere to the European Union’s (EU’s) most stringent diesel specification.

A guide for ULSD

ULSD was mandated by the U.S. Environmental Protection Agency (EPA) in 2006 by phasing in stringent regulations for diesel fuel with a sulfur content of 15 mg/kg parts per million (ppm).1 In Europe, the European Parliament and the Council of the European Union mandated diesel fuel production with a maximum sulfur content of 10 mg/kg ppm. This mandate went into effect on January 1, 2009, with the aim of reducing emissions of conventional pollutants from the existing fleet of vehicles to improve air quality.2

Classification for Grade F diesel fuels for temperate climates and Class 0 diesel fuels for arctic zones in accordance with EN 590 standard specifications3 are depicted in TABLES 1 and 2, respectively. Grade F diesel fuel is the most stringent limit for temperate climates, while Class 0 diesel fuel is the lowest limit for arctic climates.

Cold flow properties of diesel fuel are typically represented by CP and CFPP parameters. CP is the temperature at which wax precipitation occurs4 and CFPP is like a low-temperature flow test (LTFT), which is used to estimate the filterability of diesel fuels in some automotive equipment at low temperatures with the following exceptions:

  1. The fuel is cooled by immersion in a constant temperature bath, making the cooling rate nonlinear but comparatively much more rapid.
  2. CFPP is the temperature of the sample when 20 ml of the fuel first fails to pass through a wire mesh in less than 60 sec.4

To improve cold flow properties in diesel fuel, a few techniques may be considered by refiners, such as reducing the distillation endpoint by considering the presence of n-paraffins in the heaviest fraction; reducing the initial point for better overlap with the kerosene cut; selecting more naphthenic and aromatic fractions than paraffinic fractions (mainly affected by crude oil origin); operating a dewaxing bed in the hydrotreating unit; and injecting additives in the form of middle-distillate flow improvers (MDFIs), along with waxy anti-settling additives (WASAs) or wax anti-settling flow improvers (WAFIs). The latter is a combination of MDFIs and WASAs, which are used to meet the cold flow property requirements and to ensure that diesel fuel passes the KPI 130 ARAL sediment test, as mentioned in Research Report 787 of the DGMK German Society for Sustainable Energy Carriers, Mobility and Carbon Cycles.

Background and challenges

Kuwait Petroleum Corporation (KPC), which is KNPC’s parent company and marketing entity, wanted to market low-sulfur diesel fuels that adhere to the most stringent European specifications. In December 2017, a study was carried out to establish the company’s capabilities for this goal. The scope of the study covered DHT units at both the MAA and Mina Abdullah (MAB) refineries.

The units can achieve the required CFPP limit with dewaxing and CFPP additives. However, the study did not elaborate on the specific requirements of feed and product needed for the additive to be effective. Furthermore, the study concluded that the additive’s vendors will be the final entity in determining the quantity required to achieve the desired final specification. The study utilized kinetic programs to estimate the requirement for both DHT units at the MAA and MAB refineries, based on estimated unit design parameters.

The following crucial details were essential for chemical selection:

  • Feed and product specific parameters and the distillation range required for the additives to be effective
  • The feed blend and difficulty to meet the specific gravity limit in accordance with the EN590 standard
  • The expected properties of the individual feed streams to the MAA refinery’s DHT units
  • Required additives
  • Approval requirements for the additives
  • Information about the type of additive (not limited to WASAs, MDFIs and WAFIs)
  • A clear representation of economics (inclusive of the requirement details for kerosene to achieve final specifications).

Previously, KNPC had designed CFP units to meet a mild cold properties specification that was achievable, as tabulated in TABLE 3. The challenge was the ability to produce diesel with 10 ppm sulfur and adhering to a CFPP limit of –22°C. The study concluded that the MAA refinery has different options, including revamping the DHT unit, operating the dewaxing bed or testing a specialized chemical additive—or a combination of these options.

The CFP units have been designed for a 6-pt CP upgrade. To comply with the CFPP requirements as per EN590 Euro 5 diesel, a further 6-pt upgrade by means of cold flow improvers is required. The primary challenge is to investigate the consequences on the CFP units. The study provides insights on the capability of the CFP units’ diesel product to either achieve a CFPP of –20°C (base case Euro 5 specification) or –22°C (stringent specification) based on the original design feed or with a change in the catalyst bed operations. Also, the study evaluated the product properties for these cases to quantify any revamp scope. Furthermore, it also explored the use of additives [CFPP, lubricity, anti-static additive (ASA) and WASA], including the quantity required for the changed mode of operation. This also necessitated laboratory requirements for diesel specification based on new analytical methods.

All units are single-stage catalytic dewaxing DHT units. The hydrotreating process scheme is an integrated stripper concept with a vacuum dryer. To limit the diesel yield loss, the CFP units have been designed for a 6-pt CP upgrade.

The hydrotreater dewaxing (catalyst bed) scheme impacts product properties—some properties are intentionally changed, and others can be corrected due to the stripper and/or dryer design. Conversely, some properties are changed (improved or worsened) because of hydrotreating reactions, while some are feed-related properties that are determined by the inside cleanliness of the unit. However, most important are the properties that affect fluidity, like CP, which relates to catalyst bed operations and specification, including CFPP.

For most of the units, the feed is a mix of raw diesel and cracked and atmospheric residue desulfurization (ARD) units’ distillate. TABLE 4 summarizes the main parameters of the feed composition. The original blended design feed had a CP of –6°C. The product received a ∆ improvement of 6°C for the winter cases. Without additives, the CFPP product in the winter case equaled –14°C. Therefore, the original CFPP specification of –20°C can only be met if CFPP additives are applied. The DHT and gasoil desulfurization feed properties are detailed in TABLE 5.

CFPP can be achieved by dewaxing bed operations and partly by CFPP additives. Furthermore, a lubricity additive, WASA and ASA must be added to achieve product specifications. The unit should be revamped to meet specifications, as the dewaxing capabilities are utilized to reach the CFPP specification for the base and the stringent specifications. However, the lubricity additive, WASA and ASA are still required to meet the specification.

To increase catalytic dewaxing capabilities, the weighted average bed temperature (WABT) in the dewaxing bed must be increased compared to the original design. For a single-stage catalytic dewaxer, this means that the hydrogen desulfurization catalyst must be operated at higher WABTs, as well. There will be product giveaway with respect to sulfur. The product stripper overhead air cooler is too small in these cases. In addition, the reactor effluent trim cooler is too small, so the cold low-pressure separator offgas cooler in the hydrogen sulfide removal (HSR) unit, which is dedicated to the DHT unit offgas stream, was proposed to be replaced.

Due to the higher production of naphtha in these cases, the increase in vapor and liquid traffic in the product stripper might cause flooding. Currently, the product stripper is designed with conventional trays. The advice was to re-tray the stripper using proprietary trays. Some nozzles (naphtha streams) exceeded the maximum impulse momentum. A risk-based inspection was deemed an effective barrier to mitigate this. The gas load (impulse momentum) on the high-pressure absorber increased by approximately 22%. The high-pressure amine absorber is fitted with conventional trays. It was advised to re-tray the column with high-capacity trays. The product stripper offgas is treated in the HSR unit. The low-pressure absorber for the HSR unit is likely to be replaced. Vacuum dryer packing should be increased without revamping the dryer. This will generate deteriorated slops quality. This is indicated by the fact that ASTM D86 T98 changes from 172°C to 226°C. The specific gravity changes slightly. The quantity will slightly increase (2.18 × the normal design rate), which will affect the crude distillation unit (CDU) if not designed for this function. Otherwise, the dryer slops can be safely routed to a slops tank, as the dryer slops have been stabilized. Vessels handling more naphtha will see lower holdup times as the capacity increases. The recycle gas compressor is a turbine-driven centrifugal compressor. As the megawatts (MW) increase by 22%, the compressor should be evaluated by the vendor if it cannot perform its duty.

Calculations show that increasing the dewaxing in the unit to meet the CFPP of –20°C or –22°C results in yield losses vs. the base case, as shown in TABLE 6.

The main issue is diesel yield loss vs. the costs of CFPP additive injection. Increasing the dewaxing abilities of the catalyst to –20°C or –22°C results in substantial diesel yield loss, as diesel is converted to naphtha (slops) and offgas, as well as a reduced cycle length for the stringent CFPP –22°C case. For the dewaxing cases, a significant economic loss is incurred. Therefore, the costs for the revamp compound the financial losses; it is financially beneficial to achieve the required CFPP specification of –20°C or –22°C by using additives. The original design of the MAA units was promising, so a partial product upgrade by dewaxing and by CFPP additives was adopted.

A pilot plant was initiated with a licensor and a division of KNPC. However, due to the unavailability of a location and some other disputes, the Kuwait Institute of Scientific Research (KISR) was engaged to address some of the concerns. Unfortunately, due to confidentiality issues between KISR and the licensor, the pilot plant remained on hold and was subsequently canceled. After discussions with Kuwait Petroleum Corporation and obtaining required details/approvals for the additives considered, the MAA refinery decided to perform its own analysis and studies on producing winter-grade ULSD.

The process began by communicating with various vendors, performing initial unit trials, conducting analytical testing in both the vendors’ and the MAA refinery’s laboratory facilities to pinpoint the requirements based on reports from the vendors, procuring sample additives from selected vendors, and performing the field trial. The field trial is the final hurdle for the MAA refinery to confirm its capabilities to produce winter-grade ULSD in the actual plant environment.

Unit configuration

KNPC has two refineries—MAB and MAA—that can produce clean-burning fuels conforming to Euro 5 standards. It has a major share in boosting Kuwait’s global position in the oil refining industry. In the MAA refinery, three processing units can produce ULSD: the revamped gasoil desulfurization unit, the CFP DHT unit and the hydrocracking (HCR) unit.

As per design, KNPC will produce Euro 5 diesels for the local market and export requirements from all units. The gasoil desulfurization unit will consume the diesel stream from the CDU and eocene (EOC) unit as its primary feed, while the feed streams going to the DHT unit are a combination of multiple diesel/distillate streams from the CDU, the EOC unit, the ARD unit, the delayed coker unit (DCU) and the fluidized catalytic cracking unit (FCCU). Therefore, feed blending is one of the critical considerations for DHT operation in the MAA refinery. Meanwhile, HCR consumed vacuum gasoil and produced 10 ppm sulfur diesel as one of its products. FIG. 1 details the diesel configuration at the MAA refinery. No additive injection facilities are available at the HCR unit. This is compensated by higher injection at the gasoil desulfurization unit or offsite facilities (finished product site) in case of any off-specification situation. Simplified block flow diagrams of the MAA refinery’s gasoil desulfurization and DHT plants are shown in FIGS. 2 and 3,  respectively.

FIG. 1. Diesel configuration at the MAA refinery.
FIG. 2. Simplified block flow diagram of the MAA refinery’s gasoil desulfurization plant.
FIG. 3. Simplified block flow diagram of the MAA refinery’s DHT plant.

Lab-scale analysis

Understanding diesel characteristics is essential in determining suitable treatments for improving cold flow properties. Narrow boiling diesel is a fuel with a ∆ (T90%–T20%) lower than 100. Such diesel is unresponsive to cold flow additives and requires high-performing WAFI solutions to meet both CFPP European winter-grade specifications and ARAL pass requirements. Ultra-narrow boiling diesel is a fuel with a ∆ (T90%–T20%) lower than 80. This diesel is extremely unresponsive to cold flow additives and requires the latest top-tier additive technology to meet both CFPP European winter-grade specifications and ARAL pass requirements. In such cases, kerosene blending is a common option used by refiners.

Based on the results of the analytical testing conducted at the additive vendors’ and MAA laboratories, the following characteristics are observed for both ULSD products from the gasoil desulfurization and DHT units. The results for the DHT sample are provided in the absence of dewaxing operations. The characteristics are divided into three main categories:

  1. Cold flow properties—represented by CP and CFPP
  2. Boiling distillation range—indicated by the actual distillation temperature at 90% recovery minus the actual distillation temperature at 20% recovery ∆ (T90%–T20%)
  3. Wax content.

FIG. 4 depicts the typical n-alkane distribution in diesel fuel. Note: The carbon numbers C23–C31 represent diesel fuel tail and normally characterize the wax content.

FIG. 4. Distribution of n-alkane in diesel fuel.

Cold flow properties. The gasoil desulfurization sample base characteristics do not meet the desired CP limit of –7°C; the CP for the product stream is –5°C. The DHT sample base characteristics meet the desired CP limit of –7°C. The dewaxing bed for the DHT unit is not activated for the sample collection.

Boiling distillation range ∆ (T90%–T20%). In gasoil desulfurization, the diesel fuel has a narrow boiling distillation range of 90°C. In the DHT unit, the fuel has an ultra-narrow boiling distillation range of 75°C.

Wax content. In the gasoil desulfurization unit, the amount of wax at the CFPP target is in the average area, and the wax anti-settling should be achievable. In the DHT unit, the wax amount at the CFPP target is remarkably high, and the wax anti-settling test is challenging to pass.

Two improvement techniques are considered for the cold flow properties during the laboratory analysis stage. These improvements are interrelated and consist of (1) adding kerosene, which is important to achieve a better additive treating rate, and (2) injecting additives by varying the dosage rate. The kerosene-blended sample provided a different treating rate than the original diesel fuel sample. Typical CFPP response curves are shown in FIG. 5, considering the CFPP target as per the CFP design and stringent targets.

FIG. 5. CFP response curves with additives treatment.

Kerosene addition analysis. Adding kerosene will widen the distillation breadth between carbon number 8 (C8) and C13. This will slow the wax precipitation rate by reducing the steepness of the fuel tail (represented by C23–C31).

In the gasoil desulfurization unit, kerosene is blended to achieve a lower CP by considering the minimum flash point constraint. Furthermore, the CFPP target is achieved at a lower treating rate.

In the DHT unit, kerosene blending widens the distillation breadth of the sample. The CP, which is already meeting the target, is lower than the CP limit, causing quality giveaway. In addition, the CFPP target of –22°C is achieved only with this sample.

Additive response curves. In the gasoil desulfurization unit, a CFPP of –22°C can be attained through additive injection. For the gasoil desulfurization sample diluted with kerosene, the CFPP target is achieved at a lower treating rate.

In the DHT unit, the CFPP target is not achievable for the DHT sample without kerosene dilution; however, this target can be achieved when using a sample with diluted kerosene.

KNPC field test

To diversify the target markets for the ULSD product, the MAA refinery evaluated WAFI additives from multiple suppliers to meet the target cold flow properties limit.

Preliminary studies conducted by two different suppliers concluded that ULSD produced from the MAA refinery’s DHT unit has an ultra-narrow distillation range, which is less than 80°C. ASTM Test Method D86 determines the distillation temperature considered for the diesel fuel product. The target endpoint for the feed stream is between 350°C–355°C, with an initial point target of less than 197°C. Based on the narrow distillation range and wax content profiles, both suppliers offered a specific type of WAFI additive for a field trial (i.e., Additive A and Additive B). Upon completion of the lab-scale analysis and chemical procurement process, the WAFI field trial was scheduled to establish the MAA refinery’s capabilities in actual refinery operation.

Preparation stage. A multidisciplinary team composed of operational planning (coordinator), process engineering, operations and the laboratory was formed to implement the field trial. The field trial was conducted in two phases with Additive A and with Additive B at different periods. The selection of finished tanks was significantly important to ensure that there was no interruption to the supply of committed diesel product for local and export customers.

In terms of diesel-producing units (i.e., gasoil desulfurization and DHT units) and the preparation of upstream units, the distillation initial point and endpoint for the feed streams were controlled to achieve a ∆ (T90%–T20%) above 80°C. The operations and process engineering divisions monitored unit conditions closely prior to the target date of the field trial by adjusting the unit parameters, such as the column separation temperature and severity of the units. Subsequently, the dewaxing bed of the DHT unit was activated to achieve a preliminary CFPP of approximately –15°C to –17°C. These actions are to ensure a better additive response to the diesel product.

During the field trial. The additive was injected through the unit’s injection facilities to achieve the desired CFPP temperature of less than –22°C. The injection dosage rate was closely monitored and controlled as per the recommended rate by each supplier. Samples were collected during every shift to test the key parameters and to ensure additive effectiveness and product qualities. Other important parameters that were monitored were the density at 15°C, which must be less than 845 kg/m3, as well as a CP of less than –7°C. Furthermore, other additives injected to meet lubricity and electrical conductivity parameters were controlled as per existing practice to meet the final specification. Once the tank filling was completed, the existing sampling and analysis procedure was undertaken against the winter-grade specification agreed upon with KPC. The final finished tank results for each field trial are shown in TABLE 7.

Various blends were used during both trials to ensure diversity in future conditions and to establish the additives response and behavior during these conditions. Blends consisted of gasoil desulfurization, and DHT and HCR products, while other blends considered only dewaxed DHT products and gasoil desulfurization and HCR products separately.

Post field trial. The challenges faced during the field trial were consolidated and then discussed by the team members. Some of the key challenges were resolved during the first phase of the field trial, such as injection pump failure and off-specification in one of the parameters during tank filling. Other challenges (e.g., a wild kerosene stream that was routed to slop, which was observed during the first phase, and which affected crude unit operations) were resolved prior to Phase 2 by routing the same to a kerosene desulfurizer unit. Furthermore, an economic assessment was carried out of the additional cost that was incurred during the field trial to evaluate the MAA refinery’s competitiveness in selling winter-grade ULSD product.

Takeaways

To adhere to Europe’s most stringent diesel specifications, KNPC performed two field trials with two different WAFI additives to achieve a CFPP limit of less than –22°C. This was accomplished by adjusting the upstream units’ stream qualities to meet the required distillation range, the operation of the DHT unit’s dewaxing bed and the controlled additives’ dosing rate. Still, overall optimization is critical to ensure economic viability.

The CFPP limit can be achieved by controlling the feed stream’s condition to achieve a ∆ (T90%–T20%) of more than 80°C for better WAFI additive response toward the final diesel product. Further dosing rate controls of other additives are equally important to achieve the desired product specifications.

With this accomplishment, the MAA refinery is in the position to diversify its target market, strengthen KNPC’s position in the export market and produce clean diesel products that conform to the latest European standards. HP

ACKNOWLEDGMENTS

The authors would like to acknowledge Nik Mohn Ridhwan, Operational Planning, KNPC, and Raghu Kutikuppala, Section Head of Operations, KNPC, for their help in authoring this article.

LITERATURE CITED

  1. U.S. EPA, “Final rule for control of air pollution from new motor vehicles: Heavy-duty engine and vehicle standards and highway diesel fuel sulfur control requirements,” January 18, 2001, online: https://www.govinfo.gov/content/pkg/FR-2001-01-18/pdf/01-2.pdf
  2. EU, “Directive 2003/17/EC of the European Parliament and of the Council of 3 March 2003 amending Directive 98/70/EC relating to the quality of petrol and diesel fuels,” March 2003, online: https://eur-lex.europa.eu/legal-content/EN/TXT/?qid=1435618704689&uri=CELEX:32003L0017
  3. European Committee for Standardization, “EN590: Automotive fuels and diesel requirements and test methods,” October 2021.
  4. Rand, S. J. and A. W. Verstuyft, Significance of Tests for Petroleum Products, 9th Ed., ASTM International, West Conshohocken, Pennsylvania, January 2018.

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