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Modern Separation Methods in Oil Industry

Crude oil is a very complex mixture of different classes with the size from very small, such as propane, to the very large, such as asphalthenes, and in polarity from alkanes to petroleum acids. The refining process starts with distillation; it separates crude oil into a multiple fraction based on boiling point. Resulting products are gases, gasoline, naphtha, kerosene, light gas-oil, heavy gas-oil and residue. 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. aromatic (mono-, di- and multi-ring) 4. component containing heteroatoms 5. combined structure 6. unsaturated olefines Unsaturated hydrocarbons do not occur in natural oils, but they are formed in substantial amounts in thermal and catalytic cracking processes. Moreover, for improving at least one property of the refinary product a number of additives were used. The additives like detergent, octane number improver, biocide and antistatic in case of gasoline; cetane number improver, additives for improving combustion and improving the cold behavior of diesel fuel, and additive for lubricants, modifier the rheological properties, pour point depressant, antioxidants, dispersants and detergents. The quality requirements for environment-friendly transportation fuels for new generation of automobile engines has compelled the refiners to characterize the cracked oil product from FCC and other conversion processes. Characterization of crude oil and refining products has grown increasingly important because of the need to optimize the feedstock use and evaluate the product performance as a function of chemical composition. Oil industry is a wide field with matrices of thousands of compounds present as traces or in a significant amount. The matrix is too complex to be solved with a single simple method of chemical characterization. Group-type analysis is a widely used procedure for obtaining the information needed to evaluate feedstock and products in oil industry. A general and a widely accepted method of hydrocarbon group-type analysis is the fluorescent indicator adsorption (FIA) procedure which covers the determination of saturates, non-aromatic olefines and aromatics. It has been a standard procedure in oil industry for group-type analysis of gasoline-range materials for many years, although it has certain limitations. Limitations of this technique such as the time required for analysis, poor precision and the fact that most polar compounds are determined as aromatics have led to development of other chromatographic methods for hydrocarbon classes analysis. Parallel with FIA method a classical gas chromatography method has developed in this field. GC is beyond a doubt an excellent tool for finger-printing unknown samples, but has same disadvantages: a) using conventional stationary phase GC separates primarily by boiling point, and individual classes of hydrocarbons cannot be distinguished without an extensive retention calibration b) polar materials, sometimes present as extraneous contaminants, will appear in the chromatogram if it has a suitable boiling point c) even if retention time calibration are made, frequent checking of their constancy is necessary, especially when using high-resolution capillary columns and when peaks in chromatograms are closely spaced d) detectors used are non-specific in terms of classes of hydrocarbons. Supercritical fluid chromatography ( SFC ) has also been used by some investigators for the determination of hydrocarbon types in petroleum fractions using flame ionization detector (FID) and carbon dioxide as a mobile phase for determining saturates, olefines and aromatics in petroleum products boiling below 350o C. The mentioned limitations of classical LC and GC techniques have led to development of more powerful methods so far widely accepted or becoming just a job of research and development laboratories. Most of those 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. Different techniques have been used in the analysis of different oil fractions starting from a low-boiling one like gasoline to heavy fractions like asphalts. HPLC switching techniques utilizing normal phase supports have been primarily limited to backflushing techniques or dual column techniques. Backflushing techniques have been employed for the analysis of polar hydrocarbons especially in the middle or heavier oil fractions (diesel fuels). There has also been reported a coupled column technique utilizing a reverse phase column and size-exclusion column for separation of polynuclear aromatic hydrocarbons ( PAHs ). UV and RI detectors as multidetection systems give complementary information on the overall composition of an analyzed mixture. Asphalts are residues left when practically everything that can be recovered from crude oil by high vacuum, high temperature distillation has evaporized. After removal of a non-solved residue, coupled SEC and NPLC techniques utilize a characterization of this heavy fraction. The development of new LC/MS interfaces as well as improvements in HPLC and column chemistry have greatly assisted in the characterization of the various components in heavy oil fractions. Comprehensive two-dimensional HPLC / CZE separation system utilizing reverse phase chromatography as the first dimension separation and capillary zone electrophoresis as the second dimension separation has been successfully used in characterization of some oil fractions. Detailed and reliable compositional characterization of heavy gas oil fraction 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. Traditional GC analysis has been replaced by the conventional two-dimensional GC, commonly known as heart-cutting, and serially coupling two different chromatographic columns. Heart-cutting is a non-comprehensive hyphenated method, because the secondary instrument cannot be applied to the entire chromatography eluting from the GC inlet. The secondary GC, considered as an independent instrument, is much too slow to serve as a detector for the primary GC column. Meanwhile, a comparison between the standard FIA analysis and multicolumn GC and capillary column GC has been made. The most promising techniques in characterization of complex hydrocarbon mixture are comprehensive multidimensional and hyphenated methods like LCxGC, GCxGC, GCxMS etc.In any comprehensive method, the secondary instrument should make measurements fast enough to preserve the information contained in the primary instrument signal. Comprehensive two-dimensional GC (GCxGC) is a multi-dimensional 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 the necessary separations. Key feature of these techniques is the use of an on-column thermal modulator to collect sample portions from the primary column and transfer them to the secondary column. In summary, there is an increasing need of 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. Coupling of SEC and HRGC can be applied for the analysis of very havy oil products such as residues. 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. For comprehensive characterization of oil samples, the comprehensive coupling of mentioned is the way forward.

Separation methods; Oil industry; cromatography; oil products; crude oil; coupled systems

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