Cetane number

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. 

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. 

In practice, the reference base is usually taken not as a-methylnaphthalene but as heptamethyinonane (HMN), a branched isomer of n-cetane. The HMN has a cetane number of 15. In a binary system containing Y% of n-cetane, the cetane number CN vyOl be, by definition (./ - V ... 

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). 

Other techniques for predicting the cetane number rely on chemical analysis (Glavinceski et al., 1984) (Pande et al., 1990). Gas phase chromatography can be used, as can NMR or even mass spectrometry (refer to 3.2.1.l.b and 3.2.2.2). 

The spectroscopic methods, NMR and mass spectrometry for predicting cetane numbers have been established from correlations of a large number of samples. The NMR of carbon 13 or proton (see Chapter 3) can be employed. In terms of ease of operation, analysis time (15 minutes), accuracy of prediction (1.4 points average deviation from the measured number), it is... 

The additives for improving the cetane number, called pro-cetane, are particularly unstable oxidants, the decomposition of which generates free radicals and favors auto-ignition. Two families of organic compounds have been tested the peroxides and the nitrates. The latter are practically the only ones being used, because of a better compromise between cost-effectiveness and ease of utilization. The most common are the alkyl nitrates, more specifically the 2-ethyl-hexyl nitrate. Figure 5.12 gives an example of the... 

Effectiveness of DtI3 additive in improving the cetane number. Source (Umelman, 1984). 

Improving the cetane number by additives results in better engine behavior, as would be predicted by the combustion mechanisms in the diesel engine (noise reduction, better operating characteristics, particularly when cold). Nevertheless, concerning certain items such as pollution emissions, it may be better to obtain a higher cetane number rather by modification of the... 

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%. 

Hydrotreating the LCO increases its cetane number to around 40 (Table 5.16), but this technique needs large amounts of hydrogen for rather mediocre results, the aromatics being converted into naphthenes which are still not easily auto-ignited. That is why LCO is sent to the domestic heating oil pool. 

The gas oils from visbreaking and coking have better cetane numbers than LCO but they are unstable and need hydrotreatment before they can be used. 

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 it will be difficult to avoid deterioration of certain characteristics such as viscosity, asphaltene and sediment contents, and cetane number. The users must employ more sophisticated technological means to obtain acceptable performance. Another approach could be to diversify the formulation of heavy fuel according to end use. Certain consuming plants require very high quality fuels while others can accept a lower quality. 

Table 5.28 gives the modifications in physical/chemical characteristics resulting from deeper and deeper hydrotreatment (Martin et al., 1992). The sulfur contents could thus be reduced to first as low as a few hundred ppm, then to a few ppm. The level of aromatics in the selected example drops from 39% to 7% while the cetane number increases from 49 to 60. Note here that such a treatment, possible through experimental means, does not correspond to current industrial practice because of its high cost and its very high hydrogen consumption. 

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. 

Calculational methods. Associating the analysis, the knowledge of the property-structure relationships, and the calculation methods has made possible the replacement of costly and arduous test methods by quicker tests whose results are linked by calculations to the characteristic under study. Some examples are the cetane number, in some cases, the octane number, or the characteristics of LPG (refer to Chapter 3). 

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