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Separation Methods in Fuel Characterization

Crude oil is a very complex mixture of different classes with the size from very small, such as propane, to very large, such as asphalthenes, and in the polarity range from alkanes to petroleum acids. The refining process starts with distillation; it separates crude oil into a multiple fraction based on boiling point. After distillation, refiners feed the distillation fraction to other process units for further separation and upgrading. Result is a mixture of hydrocarbons of the following classes: 1. saturated hydrocarbons (normal and iso alkanes) 2. cyclic alkanes (naphthenes) 3. aromatics (mono-, di- and multi-ring) 4. components containing heteroatoms 5. combined structures 6. unsaturated olefines Moreover, for improving at least one property of the refinery product, numerous additives have been used. The additives like detergent, octane number improver, biocide and antistatic in case of gasoline; cetane number improver, additives for improving the combustion and the cold behavior of diesel fuel. Generally and widely accepted methods of hydrocarbon group-type analysis are the fluorescent indicator adsorption (FIA) in LC, classical and heart-cutting GC and SFC. The developed powerful methods mainly comprising a coupled technique, hyphenated technique or multidimensional analysis, involving LC, GC, SFC or capillary zone electrophoresis separation, and RI, UV, FID, MS, NMR as detectors have been employed. HPLC backflushing techniques have been used for the analysis of polar hydrocarbons especially in the middle or heavier oil fractions (diesel fuels). Detailed and reliable characterization of heavy gas oil fraction composition has been obtained using the HPLC and diode array detector with a powerful software allowing the overlapping components to be distinguished by their individual spectrum patterns. The most promising techniques in characterization of complex hydrocarbon mixtures are comprehensive multidimensional and hyphenated methods like LCxGC, GCxGC, GCxMS etc. Comprehensive two-dimensional GC (GCxGC) is a multidimensional method of analysis. It is one of a large number of possible two-dimensional couplings of separation techniques. It is also a member of classes of hyphenated analytical methods in which the coupled techniques are not necessarily separations. In summary, there is an increasing need for highly sensitive, highly specific analyses required not only for the complex oil mixture but also for the environmental issues. Generally speaking, a technique of comprehensive 2D GCxGC promises to be an excellent tool for the extensive characterization of very complex volatile samples including fuels such as gasoline or kerosene. On-line coupling of SEC-NPLC-GC allows the separation of very complex oil fractions according to the size, polarity and volatility in a single run.

Separation methods; Fuel; Characterization

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