Improving cold flow in diesel fuel

Hydrotreating arrangements that overcome challenges to diesel’s cold flow properties meet current specifications more economically than blending and fuel additives.

Honeywell - UOP

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Article Summary

Normal paraffins are the main factor affecting cold flow properties in diesel fuel. The refiner has several options such as adjusting the fractionation in process units, blending with kerosene, and addition of cold flow property improvers into diesel fuel pools. However, these options must account for the feasibility and penalty on refinery economics. Integration of hydroprocessing based cold flow improvement solutions can offer more technically and economically effective options. The integration of hydrocracking and hydroisomerisation catalysts with cold flow-improving components can significantly improve cold flow properties. Hydroprocessing units can be configured to target the portion of a diesel blend component feed that does not meet cold flow property requirements or the distillation requirements of a diesel fuel.

Diesel fuel is a fuel widely used by passenger vehicles, buses, tractor-trailers, farm equipment, locomotive engines, boats, power generators, and other high-horsepower equipment. Diesel must meet fuel specifications such as ASTM D 975, EN 590, BIS 1460, and GB 19147 in the US, the EU, India, and China, respectively. Recent specifications are summarised in Table 1. Worldwide specifications require hydroprocessing of all petroleum based diesel blending components to achieve ultra-low sulphur concentrations, and may require more severe hydroprocessing conditions — such as higher hydrogen partial pressure and lower catalyst space times — to saturate aromatics and improve their cetane index and number.

Diesel fuel also must be suitable for use in engines over a range of ambient temperatures. For example, ASTM D 975 provides the tenth percentile minimum ambient temperatures for each state on a month-by-month basis from October through March in the US. For these colder temperatures, lower cold flow properties in the diesel are required to prevent fuel line, filter, and engine fuel injector plugging. The main properties are cloud point, cold filter plugging point (CFPP), and pour point.

The cloud point occurs when the fuel temperature is low enough to start forming crystals, which comprise n-paraffins and make the fuel appear cloudy. ASTM D 2500, ASTM D 5771, IP 219, and IP 444 methods can determine the cloud point. The reported reproducibility for ASTM D 2500 and ASTM D 5771 is 4°C.

Based on the minimum flash point and maximum D 86 distillation temperatures shown in Table 1, the n-paraffin carbon numbers can range from 9 to 25. The cloud point can be calculated using a fundamental, thermodynamic model when the distribution of n-paraffin concentrations as a function of carbon number is known.1 The concentration distribution of n-paraffin carbon numbers can be converted into a single equivalent concentration at a reference n-paraffin carbon number. All other component classes such as iso-paraffins, naphthenes, and aromatics are considered solvent molecules.

Crystallisation of a single n-paraffin in a solution can be calculated using a thermodynamic equation, knowing the enthalpy of fusion and the melting temperature of the reference n-paraffin. The methodology1 was applied to calculate the cloud point of nearly 200 diesel samples with measured n-paraffin carbon number concentration distribution. More than 95% of the calculated cloud points (see Figure 1) were within the reproducibility of the measured cloud points, which is typically +/-4°C, and experimentally confirmed the fundamental relationship of cloud point and n-paraffins in diesel.

The CFPP is closely related to cloud point. Plugging occurs when a fuel sample crystallises enough wax crystals on a standard size 45-micron mesh in a pipette to slow or stop flow of the fuel. The CFPP is measured using ASTM D6371, IP 309, SH/T 0248, and other related laboratory methods. A database of 300 diesel samples with measured cloud and CFPPs indicates a near one-to-one correspondence of the cloud point with the CFPP — which is on average 1°C lower. Analysing a diesel fuel sample with a one- or two-dimensional gas chromatography method, and ascertaining the n-paraffin carbon number distribution, allows accurate calculation of the cloud point and approximate value of the CFPP.

The pour point is the lowest temperature for movement of the fuel, and is measured using ASTM D97, D 5949, IP 15, and other similar methods. At the pour point, a network of n-paraffins crystals forms a gel that prevents the flow of the fuel.2 Whereas the first incident of crystal formation in the diesel fuel is the cloud point, the crystallisation of 0.5-1 wt% of n-paraffins is sufficient to create the onset of the pour point. Lower n-paraffin concentrations in diesel produce greater differences between cloud and pour points.

The refinery intermediate streams typically blended into diesel fuel are hydrotreated distillate streams, and kerosene and heavy diesel from a hydrocracking unit if one is present. The oil feeds to one or more distillate hydrotreating units may comprise the following sources:
- Kerosene, heavy diesel, heavy atmospheric gasoil, vacuum diesel from crude distillation units, or ‘straight-run’ distillates
- Light coker gasoil from a delayed coking unit
- Light cycle oil from a fluidised catalytic cracking unit

All of these streams are produced from distillation columns that determine the boiling point range of hydrocarbons in the streams. The higher boiling portion of these streams largely determines their cold flow properties. The melting temperatures of n-paraffins, which is closely related to cold flow properties, logarithmically increases with the increasing true boiling points of n-paraffins (Figure 2). Inclusion of higher carbon number, higher boiling point n-paraffins from the distillation of refinery intermediates used for diesel fuel blends adversely affect cold flow properties.

Based on the more typical concentrations and carbon number distributions of n-paraffins, kerosene intermediates with a boiling point range of about 150°C to about 250-280°C will have cloud points between -30°C and -40°C. Heavy diesel intermediates with a boiling point range of 250-280°C to 370-400°C will have cloud points between -15°C and +15°C. The cold flow properties are dependent on the crude sources, distillation true boiling cut points, distillation efficiencies, processing conditions in catalytic units, and catalyst compositions.2

Diesel fuel pools can be segregated into summer, winter, and arctic cold flow grades, and may have +5°C to -5°C, -15°C to -20°C and -30°C to -40°C cloud or CFPPs requirements, respectively. There are several ways to produce these seasonal grades. The most common is to reduce the distillation cut points in crude distillation units and conversion unit distillation columns until the blend of the hydroprocessed intermediates meets the required cold flow properties.

For example, crude units produce a distillation cut between heavy diesel or vacuum diesel and vacuum gasoil. Delayed cokers produce a distillation cut between light coker gasoil and heavy coker gasoil. Fluidised catalytic crackers produce a distillation cut between light cycle oil and clarified slurry oil. Hydrocrackers produce a distillation cut between heavy diesel hydrocrackate and unconverted oil.  Summer grade cut points in these fractionation units are typically 360-380°C. Winter grade cut points may vary between 345°C and 360°C, while arctic grade cut points are even lower and may range from 285°C to 345°C. The cut points are reduced to exclude n-paraffins, which typically represent a low proportion of the total hydrocarbons.

When cut points of the crude unit, delayed coker, or hydrocracking unit are reduced, the excluded portion of higher boiling range heavy diesel typically goes to a conversion unit such as a fluidised catalytic cracking unit or a hydrocracking unit. Heavy diesel in a fluidised catalytic cracking unit is extensively converted to gasoline. The reduction of cut point in the hydrocracking unit at the same gross conversion will increase reactor severity and reduce diesel selectivity. In both cases, less diesel and more naphtha are produced from the refinery.

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