Cetane index

An important application of this type of analysis is in the determination of the calculated cetane index. The procedure is as follows the cetane number is measured using the standard CFR engine method for a large number of gas oil samples covering a wide range of chemical compositions. It was shown that this measured number is a linear combination of chemical family concentrations as determined by the D 2425 method. An example of the correlation obtained is given in Figure 3.3. 

This usefulness of the correlation is twq-fpld first it provides information on the cetane indices that are not aViit le in the literature as in the cases of polynuclear aromatics and sulfur-containing aromatics, and second it helps provide an evaluation of the cetane index based on a few milligrams of sample, instead of the liter or so required for the motor method. 

Some empirical formulas have been devised to link the cetane index to the Diesel Index (D.l.) or even directly to the aniline point. We will cite two such formulas here in order to illustrate their comparative values C ... 

Cetane index ASTM D 4737 ISO 4264 Correlation between density and distillation... 

Cetane number is difficult to measure experimentally. Therefore, various correlation equations have been developed to predict cetane number from fuel properties. One such equation may be found in ASTM D4737 to calculate a cetane index (Cl). ASTM D975 allows use of Cl as an approximation if cetane numbers are not available. 

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. 

Cetane number is measured in a single-cylinder laboratory engine, but cetane index (Cl) is more commonly used. Cetane index is a calculated value and correlates adequately with the cetane number. Most refiners use the ASTM equation (Method D-976-80) to calculate the cetane index. The equation uses the 50% boiling point and API gravity (Example 6-1). 

LCO is highly aromatic (50 wt% to 75 wt%) and has a low cetane index (20-30). The cetane number and sulfur content determine the amount of LCO that can be blended into the diesel or heating oil pool. 

Cetane Number is a numerical indication of a fuel s (kerosene, dic.sel, heating oil) ignition quality. Cetane number is measured in a single-cylinder engine, whereas cetane index is a calculated value. 

Cetane index an approximation of the cetane number (q.v.) calculated from the density (q.v.) and midboiling-point temperature (q.v.). 

FIGURE 2.1 Cetane index correlation with total liquid product aromatics of LCO samples obtained from TBP distillation of pilot riser liquid products. 

LCO quality was determined by two different methods. Before 2-D chromatography was available, liquid product aromatics were measured for all samples using a variation of the ASTMD 5186-96 method. A selected number of samples were cut between 221°C and 343°C using a TBP column to produce an LCO fraction for direct ASTMD 3747 cetane index determination. The cetane values of the LCO cuts were then correlated to the total liquid product aromatics (Figure 2.1) and the correlation was used to estimate the LCO quality of the other samples. 

The cetane index is an estimation of the cetane number from fuel physical properties such as the 10%, 50%, and 90% boiling points and the °API. 

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

Confusion between cetane index and cetane number... 

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. 

As a general rule, it is usually safe to assume that the calculated cetane index will always be higher than the actual cetane number. On rare occasions, the numbers will be the same, but usually not. Because of this, if a calculated cetane index value is known, it is reasonable to believe that the cetane engine number is usually one to two numbers lower than the calculated cetane index. For this reason, refiners often set cetane index specifications higher than the desired number by one to two numbers or more. 

Cetane improver addition will not change the calculated cetane index only the engine cetane number will change. 

A widely accepted relationship exists between the cetane index of a distillate fuel and the total aromatic content. Typically, as fuel aromatic content increases, the fuel cetane index decreases. It is generally believed that distillate fuel containing >35% aromatics will have a cetane index <40. Likewise, fuel having <35% aromatics will have a cetane index >40. 

It is not possible to accurately determine the cetane index for many 1 diesel and kerosene type fuel. This is because the cetane index scale cannot be utilized accurately for fuels with end points <500°F (260°C). 

Cetane index is inaccurate if fuels with end points below 500°F (260°C) are evaluated. 

Unzleman, George H. 1993. Cetane index EPA s control for aromatics. Octane Week, August 1, p. 7. 

Cetane Index A calculated value which incorporates fuel distillation properties and °API which can be used to approximate the cetane number of a fuel. This method, however, does not provide information on the performance provided by a cetane improver. 

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