Cetane number/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. 

The products manufactured are predominantiy paraffinic, free from sulfur, nitrogen, and other impurities, and have excellent combustion properties. The very high cetane number and smoke point indicate clean-burning hydrocarbon Hquids having reduced harmful exhaust emissions. SMDS has also been proposed to produce chemical intermediates, paraffinic solvents, and extra high viscosity index (XHVI) lubeoils (see Lubrication and lubricants) (44). 

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. 

Other methods are also available for the estimation of diesel fuel quaUty. For example, the diesel index (DI) is defined by the relation DI = (A° F X ° API)/100, where A °F is the aniline point in degrees Fahrenheit and °API is the American Petroleum Institute gravity. A high aniline point corresponds to a high proportion of paraffins in the diesel fuel such a fuel has a high diesel index and, therefore, a high cetane number. 

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. 

Dan Resasco (with colleagues Phuong Do, Steven Crossley, Malee Santikuna-porn University of Oklahoma) examine strategies for improving important fuel properties catalytically—e.g., cetane number and threshold soot index. They show that proper choice of catalysts and reaction conditions can significantly improve these widely used measures of fuel performance. 

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

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. 

It is impractical to determine the cetane number of residual fuels in the ASTM D-613 cetane engine. Because of this, the Calculated Carbon Aromaticity Index and the Calculated Ignition Index were respectively developed by Shell and BR These values can be determined from the following equations where d = Density in kg/m3 59°F (15°C) and v = Viscosity in cSt 122°F (50°C). 

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. 

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. 

The catalytic Diesel fuel has a lower cetane number than the corresponding component of the virgin charge because of the changes in chemical composition effected in the cracking operation. However, in mild cracking conditions, the drop may amount to only a few numbers in Diesel index. 

The fraction of diesel-oil-like hydrocarbons had also a triple sequence and the main aliphatic componnds may be characterized with carbon numbers 12, 15, 18, 21, 24, 27. In contrast with the experimental results of naphtha diesel oil had considerably lower concentration of aromatics. In the case of each MPW sample its concentration was not more than 1%, because aromatics with lower boiling point stayed in the naphtha-like fraction. Similarly, as mentioned above, the diesel-oil-like fraction also had favourable properties for further fuel-like application. The olefin content was a bit smaller than in case of naphtha-like fractions, because cracking reaction resulting olefins (e.g. P-scission) produced hydrocarbons with a shorter length of carbon chain. Both cetane numbers and diesel indexes of products were high enough, while the CFPP was rather low. 

Cetane index is an estimation of the cetane number, which measures the anti-knoclc properties of the fuel. This index is more used than the cetane number because it can be calculated by a four-variable equation (density, BP 10%, BP 50% and BP 90%) according to ASTM D4737 standard method, instead of the test engine needed to measure cetane number. In our case, the relatively low amount of product obtained in these initial experiments does not allow determination of the cetane number, so we have used the cetane index to evaluate and compare our products, an approximation perfectly valid at this initial stage of investigation. 

Results show that rosin acid produces fuels with low cetane indexes, between 28 and 42 for catalyst A and between 34 and 43 for catalyst B. According to ASTM D975 (Standard Specifications for Diesel Fuel Oils), diesel fuels must have a minimum cetane number of 40 (diesel no. 2). Therefore, some of our products seem to fall into specification for diesel fuels. However, it must be taken into account that cetane index is just an estimation of cetane number, and this estimation may be inaccurate. According to ASTM D4737 standard method, this equation possesses some limitations ... 

A study about different diesel fuels for example, showed that cetane index underestimates cetane number when it is lower than 43, sometimes more than 5 units (C.N.=40, C.I. = 35.1). Therefore, the actual cetane numbers for our products can be higher than the measured ones. In the future, producing higher amounts of product to measure actual cetane numbers appears to be necessary to confirm their suitability as diesel fuel. 

The Calculated Cetane Index (CCI), though not an accurate predictor of Cetane Number for biodiesel, since it is based on a calculation using specific gravity and the distillation curve, was used to estimate the Cetane Number of the biodiesel and two diesel fuels. From the results it can be seen that biodiesel has a CCI value higher (55) than that of diesel No.2 (49). This accords the biodiesel with better cold-start properties, minimizes the formation of white smoke (emissions), and leads to less engine noise and hence improved engine durability and reduced fuel consumption. ... 

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