The term diesel is utilized for the fuel used in compression-ignited engines of motor vehicles. Rudolf Diesel invented the compression-ignition engine in 1892 and Diesel engines and Diesel fuels are called after his name. The Diesel fuel contains C11~-C22 hydrocarbons. Its true boiling point ranges 250~343 oC. In order of economic importance of major oil products in the world, diesel fuel is the most important of all petroleum products. Diesel fuel is used for transportation, manufacturing, power generation, construction, and farming for strengthening the world economy.
In order to comply with, world’s stringent environmental standards, the refiners are currently upgrading their facilities. To meet the specifications diesel fuel is to be hydrotreated. Cetane number, flash point, pour point, cloud point, and sulfur are the most important properties of diesel fuel. The fuel volatility requirements depend upon the engine design and applications. For automotive diesel fuel, more volatile fuels are better while for rail cars, ships, and power stations, heavier fuel is more economical.
1. Cetane Number is the measurement of the ignition quality of diesel fuel the same as the Octane number for Gasoline. A Higher Cetane number means shorter ignition delay periods. The reference fuel is Cetane (n-hexadecane, n-C16H34) with a Cetane number 100 and Alpha-Methyl Naphthalene having Cetane number 0.
High Cetane number of diesel fuel benefits short ignition delay, improved cold start, reduced white smoke during start-up, low emissions, low engine noise, and low fuel consumption.
- Cetane number improves with paraffin and naphthene contents.
- Operating conditions can improve Cetane number due to high saturation of PNA (Parrafins, Naphthenes, Aromatics) high hydrogen partial pressure, high system pressure, and low LHSV.
- Catalyst selection as NiMo show higher rates of PNA saturation at reaction conditions as compare to CoMo
- Use of additives to improve the Cetane number.
2. Wear Scar or Lubricity is the ability to reduce friction between moving parts (fuel pumps and injectors) of a diesel engine. There must be a minimum level of lubricity in the fuel to avoid excessive wear to save the engines from being damaged. Very small amounts of oxygen- and nitrogen-containing compounds and certain classes of aromatic compounds in the diesel fuel play the role of lubricant but after hydrotreatment, these compounds are removed leaving the diesel dry.
- Lubricity additive has to inject in diesel to control the lubricity.
- In addition, controlling the sulfur content at the maximum limit will also reduce the chemical requirement because of natural lubricating agents.
3. Flash Point is the lowest temperature at which diesel produces sufficient vapors that result in the inflammable mixture in air and ignites when the ignition source is provided. It is an important property from a storage and handling point of view. A lower flash point means it will ignite at a lower temperature. High flash point diesel is safe to handle.
- It is improved by increasing striping steam in strippers or reboilers at crude distillation and hydrotreating units.
- The blending of kerosene with diesel reduces the flashpoint which has to be compensated with stripping steam as well.
Both cloud point and pour point described here are not mentioned in the standards but are season or weather base i.e. winter or summer specs.
4. Cloud Point indicates the stability of the fuel at low-temperature operations. It is the temperature below which wax crystals are formed due to the precipitation of paraffin and shows the cloudy appearance of diesel. This is an important parameter because precipitates of wax can clog the fuel filters and atomizers, resulting in poor performance of the engine.
In the winter season, produce the diesel with specifications according to the region’s cold temperature.
- Blend lighter products like kerosene or jet fuel to improve the cloud point.
- Use of additives, solvent dewaxing, and catalytic dewaxing of diesel fuel.
5. Pour Point is also a low-temperature performance indicator of diesel fuel. It is the lowest temperature at which fuel ceases to flow. It is normally, 8 C below the cloud point of diesel.
6. Sulfur, ultra-low sulfur diesel (<10 ppm, sulfur) is being produced in most parts of the world due to the strong negative environmental impacts of sulfur. In Europe, there is a 10 ppm sulfur limit in diesel since 2003.
To control the sulfur in diesel, the hydrotreating process is applied across the globe. The performance of the hydrotreating and hydrocracking catalyst plays a vital role in controlling the sulfur content. Temperature increase across the Hydrotreating reactor is the key parameter to be applied for reducing sulfur in the product. F
For further information, suggestions, or comments contact firstname.lastname@example.org.