Aromatics content diesel fuel

The cetane index of distillate fuel can be related to the aromatic content of the blend. As fuel aromatic content increases, the cetane index will typically decrease. A general relationship exists which relates a cetane index of 40 with a diesel fuel aromatic content of about 35%. 

Heavy residue conversion is linked to the demand for high quality diesel motor fuel (aromatics content 10%, cetane number 55) as well as to the demand for production of light fuel-oil having very low sulfur, nitrogen and metal contents. 

As we have shown previously, obtaining both good cold operation characteristics and sufficient cetane numbers constitutes the principal objective for the refiner in the formulation of diesel fuel. To this is added the need for deep desulfurization and, perhaps in the future, limitations placed on the chemical nature of the components themselves, e.g., aromatics content. 

The gas oil cut from catalytic cracking called Light Cycle Oil (LCO), is characterized by a very low cetane number (about 20), high contents in aromatics, sulfur and nitrogen, all of which strongly limit its addition to the diesel fuel pool to a maximum of 5 to 10%. 

Aromatics Content. Aromatic compounds have very poor ignition quahty and, although they are not specifically limited in ASTM D975, there are practical limitations to using high aromatic levels in highway diesel fuel. In the United States, where gasoline demand represents about one-half of the... 

Diesel Fuel. Eederal diesel specifications were changed to specify a maximum of 0.05% sulfur and a minimum cetane index of 40 or a maximum aromatics content of 35 vol % for on-road diesel. Eor off-road diesel, higher sulfur is allowed. CARB specifications require 0.05% sulfur on or off road and 10% aromatics maximum or passage of a qualification test. Process technologies chosen to meet these specifications include hydrotreating, hydrocracking, and aromatics saturation. 

Irrespective of the fuel supply chain, alternative fuels have generally lower tailpipe emissions in terms of local pollutants (such as NO, CO, S02, VOC and particle emissions) than conventional gasoline and diesel engines for instance, natural gas completely eliminates particle emissions synfuels are manufactured with very low sulphur and aromatic contents alcohol-based fuels have high octane numbers, which... 

An alternative method for fractionating and purifying petroleum hydrocarbons prior to GC or HPLC separation has been developed (Theobald 1988). The method uses small, prepacked, silica or Cjg columns that offer the advantage of rapid separation (approximately 15 minutes for a run) good recovery of hydrocarbons (85% for the Cjg column and 92% for the silica column) reusability of the columns and for the silica column in particular, good separation of hydrocarbon from non-hydrocarbon matrices as may occur with environmental samples. Infrared analysis and ultraviolet spectroscopy were used to analyze the aromatic content in diesel fuels these methods are relatively inexpensive and faster than other available methods, such as mass spectrometry, supercritical fluid chromotography, and nuclear magnetic resonance (Bailey and Kohl 1991). 

A two-dimensional supercritical fluid chromotography (SFC) system has been developed for the determination of saturates, alkenes, and mono-, di-, and tri-aromatics in diesel fuel. This technique results in a short analysis time (less than 8 minutes) and good relative standard deviations at low alkene content (Andersson et al. 1992). 

If a diesel fuel is reported to have a lower than expected cetane number, the first place to look for an explanation would be the total aromatic content of the fuel. A comparison of the approximate cetane number of various pure hydrocarbons is provided in TABLE 5-2. 

Identifying the paraffin, olefin, naphthene, and aromatic content of fuel good for gasoline not as accurate for jet, diesel, and other mid-distillates... 

Because of the low-sulfur, -nitrogen, and -aromatic content of Fischer-Tropsch fuels, there is renewed interest in these products. In two recent studies, Fischer-Tropsch diesel was evaluated and compared to an ultra-low sulfur California diesel and to a 49 cetane number, low-sulfur diesel. In the two studies, carbon monoxide, nitrogen oxide, hydrocarbon, particulate, and carbon dioxide emissions were reduced in vehicles fuels by Fischer-Tropsch diesel. The fuel economy, however, was also reduced. The low aromatic content and high concentration of waxlike hydrocarbons in Fischer-Tropsch diesel can lead to the need for special handling and treatment of the fuel to prevent gelling when used in cold-temperature conditions. 

Carbon formation is also different for diesel and gasoline. The long chain hydrocarbons present in diesel or kerosene fuel are more difficult to reform than the shorter chain hydrocarbons present in gasoline, while aromatics in gasoline hinder the overall reaction rate. An example is found in the results of Ming et who showed that SR of n-Ci required a higher steam/ carbon ratio to avoid coke formation than i-Cg. The cetane number of the feed had little effect on carbon formation. Carbon formation can often be attributed to fuel pyrolysis that takes place when the diesel fuel is vaporized. This is considerably decreased when the steam content in feed increases. 

The new specifications not only limit the concentration of sulfur to 0.05% but also specify that the fuel should have the combustion properties of a 10% or lower aromatics-containing fuel and have a minimum cetane number of 40. Fuels that have more than 10% aromatics are now able to meet these specifications through additives (22). However, as smoke emission standards become more restrictive, the aromatics content of diesel fuels may have to be lowered to a true value of 10% or less. A very thorough review of the consequences of this potential problem has recently been written by Stanislaus and Cooper, which covers in detail aromatic hydrocarbon hydroprocessing kinetics, thermodynamics, catalyst compositions, and mechanisms (109). There is little need to repeat the details of that report... 

These results show that, in equimolar concentrations, naphthalene would not be considered as a strong inhibitor toward direct sulfur extraction (A Do) for PASCs. However, as discussed earlier, the content of di- and trinuclear aromatics in diesel fuels and gas oils can be as high as 20-30%, whereas the level of sulfur compounds in today s diesel fuels is only 0.2% sulfur, or about 1 wt% PASCs. So the competition for the active site by aromatic hydrocarbons is very strong. Their effect on the direct desulfurization route will lower the rate to about one-third of the noninhibited rate in the case of dibenzothiophene and would lower that of 4,6-DMDBT even more. 

---