Diesel engines rely on effective dispersion of fuel to ensure efficient combustion. In cold weather regions, maintaining the fluidity flow of fuels can be difficult. Catalytic dewaxing is a selective hydrocracking process that provides a valuable improvement to low temperature performance of middle distillate feedstocks. It greatly improves cloud point (CP) and cold-filter plugging point (CFPP) properties of diesel fuels.
At present, most US refineries are optimized for the production of gasoline, i.e., fluid catalytic cracking (FCC) units. With the growing interest in diesel-powered passenger cars, existing refineries will not be able to serve that new demand for clean diesel. Investments in new technologies to produce high-performance transportation fuels will be necessary. Fuel performance in diesel engines is directly linked to fluidity characteristics in the engine. The highly sophisticated injection technology relies on quick and complete dispersion within the combustion chamber. At low ambient temperatures, the cold-flow properties of typical middle distillate cuts are not adequate.
Several options are typically applied to improve cold-flow properties in diesel transportation fuels, including kerosine blending, undercutting, use of additives (mainly at fuel terminals) and catalytic dewaxing. Combined kerosine blending/undercutting with the addition of cold-flow improvers has some applicability, although it does not work in all cases. As high-value kerosine is mainly used for jet fuel, blending into lower-value diesel fuel is only acceptable if there is no alternative outlet. Seasonal undercutting of middle-distillate fractions will reduce total diesel yield as higher boiling-point fractions end up in the low-value fuel oil (FO) pool.
The application of versatile cold-flow improvement additives, typically done at product terminals or blending sections, is very efficient to tailor flow properties like viscosity index (VI) or pour point (PP). However, the impact on cold-flow filter plugging point is limited, and the impact on CP may even be negative in some cases.
A selective hydrocracking catalyst has proven to be a robust approach for catalytic dewaxinga process that can be used to address all aspects of cold-flow performance.a
For diesel fuel, middle distillates have boiling-point curves in the range of 150°C (300°F) to 400°C (750°F). In addition to environmental specifications regarding sulfur, nitrogen and aromatics impurities, combustion behavior (cetane number and heating value), viscosity and flow behavior specifications are important performance factors for diesel fuels. The top four globally standardized properties are:
Whereas VI and PP primarily describe the quality of the middle-distillate fluidic behavior and its ability to be transported from tank to engine, CP and PP describe the ability to filter and disperse the fuel at lower temperatures. The VI is calculated by the kinematic viscosity at 40°C (100°F) and 60°C (140°F). At higher VIs, the change of kinematic viscosity with temperature is lower. The PP is the lowest temperature at which a liquid will pour or flow under prescribed conditions. It is an approximate indication of the lowest temperature at which the liquid can still be pumped.
The CP is the temperature at which small crystals occur (turbidity) in defined measurement equipment. The CFPP is the temperature at which a filter starts to plug in a defined filtration set-up. If seasonal cold-flow specifications are not met, an unexpected cold snap can lead to equipment damage, as shown in Fig. 1.
| Fig. 1. Solidification of diesel fuel in a |
fuel-filtering device after sudden temperature
drop. Photo courtesy of Fordaq IHB.
Selective cracking of middle-distillate feedstocks
The cracking of middle distillate to select paraffinic and isoparaffinic molecules and their melting points is summarized in Fig. 2. As the melting point of a particular hydrocarbon molecule in the middle-distillate fraction is strongly linked to cold-flow properties, middle distillates with a high content of isoparaffins have some advantages. Therefore, middle distillates with more paraffinic hydrocarbons but poor cold-flow properties can be converted into middle distillates with good cold-flow properties by increasing the isoparaffin-to-paraffin ratio.
| Fig. 2. Simplified molecular structure and |
melting points of selected long-chain
hydrocarbons typically found in
middle-distillate cuts and diesel fuel.
Two types of catalytic conversion are possible: dewaxing by isomerization, and dewaxing by selective cracking. A catalyst system can selectively crack paraffinic hydrocarbons of middle-distillate feedstocks.a The cracking function in this novel catalyst is performed by a solid-acid ingredient based on a medium pore-size zeolite that shape-selectively differentiates between branched isoparaffins and linear normal-paraffins. As shown in Fig. 3, only unbranched normal paraffins (n-paraffins) can enter the pores and be converted into smaller molecules via cracking. The catalyst includes a zeolite with a unique acidity profile that provides outstanding robustness and flexibility for use with a variety of feedstocks.a In addition, a second catalytic-base-metal function allows fast hydrogen transfer for efficient product release and coke prevention.
| Fig. 3. Molecular sieving effect for selective |
cracking of linear paraffinic molecules
in zeolite pores.
The catalyst has been commercially available for nearly 20 years. It can be used as a stand-alone solution, or within an existing middle-distillate hydrotreater or ultra-low-sulfur diesel unit, as shown in Fig. 4. Middle distillates with a wide variety of cut points can be processed. As basic nitrogen has a particular influence on total catalyst activity, the placement of a small bed of cobalt (Co)-molybdenum (Mo) or nickel (Ni)-Mo hydrotreating catalysts in front can be helpful, particularly for a stand-alone operation.b
| Fig. 4. Dewaxing by selective cracking with hydrocracking catalyst in a |
stand-alone unit (a), or within an existing middle-distillate hydrotreating unit (b).
Using selective-hydrocracking catalysts within an existing middle-distillate hydrotreating unit is a very common practice. This catalyst can be used with nearly all types of feedstocks, whether straight-run or converted (such as cracker or visbreaker or coker gasoil) refinery product streams. Its properties are tailored to fit to all hydrotreating catalysts commercially available. Selective cracking is an endothermic process; therefore, the placement of a hydrocracking catalyst bed between two hydrotreating catalyst beds in a hydrodesulfurization (HDS) reactor allows optimum heat integration and very favorable product qualities for cold flow and color. Moreover, commercial experience confirms the control of dewaxing activity according to seasonal demand even without quenching capabilities. In some cases, a two-reactor solution with bypass lines, as represented in Fig. 5, is the most favorable.
| Fig. 5. Combined hydrotreating and |
dewaxing unit for optimal seasonal diesel service.
The use of selective hydrocracking catalysts can moderately reduce diesel production and increase hydrogen consumption, depending on operational severity and cold-flow improvement requirements. However, many selective hydrocracking installations circumvent diesel yield loss by applying feedstock components with higher final boiling point (higher cut point). This action allows the conversion of a portion of non-blendable intermediates into higher-value diesel components to compensate for reduced diesel production.
An additional feature of the selective-hydrocracking catalyst system is the reduced gas-make. This is very important for existing hydrotreating units, as the formation of light hydrocarbons has no big impact on recycle gas density. Therefore, it does not interfere with the recycle-gas compressor operation. Finally, using a selective dewaxing catalyst as a drop-in replacement does not require cost intensive revamp or exchange of recycle compressors. Only minimal modifications of product stabilizers may be necessary to handle higher naphtha volumes in rare cases. HP
The article is a revised and updated version from an earlier presentation at the American Fuel and Petrochemical Manufacturers (AFPM) Annual Meeting, March 1719, 2013, at San Antonio, Texas.
a HYDEX-G is used for selective hydrocracking of long-chain n-paraffins to improve the cold-flow properties of middle distillates. Its most common application is for sulfur-containing diesel streams in combination with HDS catalysts in an integrated system. It is a registered product of Clariant.
b HDMax is a hydrotreating catalyst series developed primarily for severe hydrotreating operation of waxes and lube oil stocks. It is a registered product of Clariant.
1 Koehler, E. O., Catalytic dewaxing with zeolites for improved profitability of ULSD production in from zeolites to porous materials, 40th International Zeolite Conference.
2 Xu, R., Z. Gao, J. Chen and W. Yan, Studies in surface science and catalysis, Elsevier, 2007, p. 7.
3 Weyda, H. and E. Koehler, Proceedings of the 12th Symposium of KFUPM Research Institute, December 2002.
Dr. Rainer Albert Rakoczy is the global product manager for zeolite-based fuel upgrading and fuel production catalysts with Clariant. He started with Süd-Chemie in 2005 and headed the solid-catalyst research department. Dr. Rakoczy studied chemistry at the University of Stuttgart and worked also in the field of PCB production (IBM and Hewlett-Packard) and microprocess engineering (FZ Karlsruhe). Dr. Rakoczy has a deep background in the field of zeolites. He is an elected member of the Zeolite Group board of the German ProcessNet Association (DECHEMA).
Dr. Paige Marie Morse is the global marketing manager for the catalysts business of Clariant; she is based in Munich, Germany. Previously, she held technical and business development roles at Dow and Shell in the US. Dr. Morse holds a PhD in chemistry from the University of Illinois.