Diesel fuels cetane number

D. DIESEL FUEL CETANE NUMBER AND CETANE INDEX DETERMINATIONS... 

Boren, D. S. 1994. (Internal report) Diesel fuel cetane number testing. Sugar Land, Tex. Nalco/Exxon Energy Chemicals, L.P. 

The calculated cetane index methods were developed to help predict the fuel cetane number without extensive engine testing. Since the calculated cetane index is determined from physical property values such as the specific gravity and the 10%, 50%, and 90% distillation points, it is readily measurable from laboratory data. The ASTM methods D-976 and D-4737 are used to determine the diesel fuel cetane index. 

Occasionally, cetane improvers are added to distillate fuel and are capable of increasing the cetane number of diesel fuel from 1 to 14 numbers. The reason for this range of response is due to the differences in the chemical nature of the fuel. Fuel containing a higher level of aromatic compounds will typically respond less effectively to cetane improver treatment than fuels with fewer aromatic compounds. Fuel cetane number determination is evaluated by a standardized engine method ASTMD-613. 

For diesel engines, the fuel must have a chemical structure that favors auto-ignition. This quality is expressed by the cetane number. 

A diesel fuel has a cetane number X, if it behaves like a binary mixture of X% (by volume) n-cetane and of (100 - A) % a-methylnaphthalene. 

The European specifications require a minimum cetane number of 49 for the temperate climatic zones and the French automotive manufacturers require at least 50 in their own specifications. The products distributed in France and Europe are usually in the 48-55 range. Nevertheless, in most Scandinavian countries, the cetane number is lower and can attain 45-46. This situation is taken into account in the specifications for the arctic zone (Table 5.14). In the United States and Canada, the cetane numbers for diesel fuels are most often less than 50. 

The procedure for determining the cetane number in the CFR engine is not extremely widespread because of its complexity and the cost of carrying it out. There also exist several methods to estimate the cetane number of diesel fuels starting from their physical characteristics or their chemical structure. 

The differences between the measured cetane numbers and the cetane indices calculated by the above formula are relatively small for cetane indices between 40 and 55. Nevertheless, for diesel fuels containing a pro-cetane additive, the CCl will be much less than the actual value (refer to paragraph d). 

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 properties of straight run diesel fuels depend on both nature of the crude oil and selected distillation range. Thus the paraffinic crudes give cuts of satisfactory cetane number but poorer cold characteristics the opposite will be observed with naphthenic or aromatic crudes. The increasing demand for diesel fuel could lead the refiner to increase the distillation end point, but that will result in a deterioration of the cloud point. It is generally accepted that a weight gain in yield of 0.5% could increase the cloud point by 1°C. The compromise between quantity and quality is particularly difficult to reconcile. 

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%. 

Hydrocracking makes very good quality diesel fuels concerning the cetane number, cold behavior, stability, and sulfur content. However this type of stock is only available in limited quantities since the process is still not widely used owing essentially to its high cost. 

In the future, European and worldwide refining should evolve toward the production of relatively high cetane number diesel fuels either by more or less deeper hydrotreating or by judicious choice of base stocks. However, it is not planned to achieve levels of 60 for the near future as sometimes required by the automotive manufacturers. 

Diesel fuels, like gasoline, are formulated with additives that affect the process of combustion, in this case to improve the cetane number. Diesel fuels also contain detergents for irijection systems as well as compounds for improving the fuel s low tempierature rheology. Finally, decreasing particulate emissions is a problem of increasing concern, but the mechanism of action to promote this effect is not clearly understood. 

Unlike spark-induced combustion engines requiring fuel that resists autoignition, diesel engines require motor fuels, for vhich the reference compound is cetane, that are capable of auto-igniting easily. Additives improving the cetane number will promote the oxidation of paraffins. The only compound used is ethyl-2-hexyl nitrate. 

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. 

Glavincevski, B., O.L. Gulder and L. Gardner (1984), Cetane number estimation of diesel fuels from carbon type structural composition . SAE paper No. S4- 34, International fuels and lubricants meeting, Baltimore, MD. 

The products could be classified as a function of various criteria physical properties (in particular, volatility), the way they are created (primary distillation or conversion). Nevertheless, the classification most relevant to this discussion is linked to the end product use LPG, premium gasoline, kerosene and diesel oil, medium and heavy fuels, specialty products like solvents, lubricants, and asphalts. Indeed, the product specifications are generally related to the end use. Traditionally, they have to do with specific properties octane number for premium gasoline, cetane number for diesel oil as well as overall physical properties such as density, distillation curves and viscosity. 

We cite isomerization of Cs-Ce paraffinic cuts, aliphatic alkylation making isoparaffinic gasoline from C3-C5 olefins and isobutane, and etherification of C4-C5 olefins with the C1-C2 alcohols. This type of refinery can need more hydrogen than is available from naphtha reforming. Flexibility is greatly improved over the simple conventional refinery. Nonetheless some products are not eliminated, for example, the heavy fuel of marginal quality, and the conversion product qualities may not be adequate, even after severe treatment, to meet certain specifications such as the gasoline octane number, diesel cetane number, and allowable levels of certain components. 

However, this conventional method presents a certain number of limitations. In the first place, the traditional end-use property itself can be difficult to determine. Consider the cetane number for example is it a good characterization of diesel fuel with respect to its behavior in commercial diesel engines In the second place, concern for protecting the environment imposes new specifications which are often specifications linked to the composition of products very low content of certain contaminants, reduced levels of certain families of compounds, or even a specific compound as already discussed. 

Lubricants, Fuels, and Petroleum. The adipate and azelate diesters of through alcohols, as weU as those of tridecyl alcohol, are used as synthetic lubricants, hydrauHc fluids, and brake fluids. Phosphate esters are utilized as industrial and aviation functional fluids and to a smaH extent as additives in other lubricants. A number of alcohols, particularly the Cg materials, are employed to produce zinc dialkyldithiophosphates as lubricant antiwear additives. A smaH amount is used to make viscosity index improvers for lubricating oils. 2-Ethylhexyl nitrate [24247-96-7] serves as a cetane improver for diesel fuels and hexanol is used as an additive to fuel oil or other fuels (57). Various enhanced oil recovery processes utilize formulations containing hexanol or heptanol to displace oil from underground reservoirs (58) the alcohols and derivatives are also used as defoamers in oil production. 

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