engleski

Crude Oil and Its Products- a Complex Analytical Matrix for Separation Methods Application

A feature common to most fossil fuels and oils is that they are complex mixtures of different hydrocarbon classes and related compounds. Thousands of individual compounds constituting oil materials vary in molecular structure and size, polarity, functionality and volatility[1]. The composition as well as the occurrence of specific hydrocarbon classes in a given crude oil and its products depend on a type of crude oil and fuels, crude oil and fuels source or history, processing conditions etc. The amount of individual compounds vary from a trace level to significant amounts. Before oil characterization, usually there is a need to analyze source rocks from phisical and chemical points of view[2]. During oil processing some compounds disapear, but some new ones appear. 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 products a number of additives have been used ; detergent, octane number improver, cetane number improver, additives for improving combustion and cold behavior of diesel fuel, antioxidants, modifier of rheological properties, dispersants etc. Environmental protection requirements and producing environment-friendly transportation fuels for new generation of automobile engines will further complicate the oil product matrix. The present compounds, mainly hydrocarbons could be divided in several characteristic classes ;  Saturated hydrocarbons (normal and iso alkanes)  Cyclic alkanes (naphthenes)  Aromatics (mono-, di- and multi-ring)  Unsaturated olefines  Components containing heteroatoms  Combined structure For some purposes there is a need to determine individual components present in oil or oil product sample (MTBE in gasoline or C3 and C4 in natural gas). However, as chemical groups rather than individual molecules determine chemical properties of most oils and fuels, hydrocarbon type (group) analysis by chromatographic techniques has so far been very popular for their chemical characterization. Most of the chromatographic techniques are used for determination of chemical composition of oil and its products, but sometimes they are used as a clean-up or pre-concentration procedure. Very often, they are combined with other analytical or mechanical techniques. For example, in geochemical study for biomarkers determination of source rocks, sample started by grinding, dissolution, liquid-liquid or solid phase extraction, precipitations, chromatographic separation, and finished by GC/MS identification and quantifications. Chromatographic techniques covering oil and oil product samples are: gas chromatography (GC), liquid chromatography (LC), supercritical fluid chromatography(SFC) with all their variants, coupled and hyphenated with different detection techniques. Gas Chromatography (GC) GC has long been vital for refiners and petrochemical analysis where the role ranges from on-line monitoring of process streams and characterization of feeds, intermediates and products of refinery operations, to providing analytical data to meet environmental regulations. For low-molecular weight and volatyle hydrocarbons and heteroatomic molecules having up to 10 or so carbon atoms, capillary GC provides excellent resolutions between major component peaks permtting both identification and quantification of individual components in the sample. Uses of GC stationary phases vary from non polar to highly polar and flame-ionization detector (FID), extensively applied. High resolution capillary GC has been used to separate individual components in gasoline to diesel range samples, and obtained data are then grouped into different hydrocarbon classes according to the carbon number [3]. Multidimensional GC, comprising two or more different columns and operated under a number of valves and traps, allows separation and quantification of hydrocarbon types by carbon number and hydrogen defficiency in light and middle distillates. Determination of biomarkers (important for source, history, genes and migration of oil) in source rocks and crude oil is incredible without GC/MS. Simulated distilation by GC-FID replace time-consuming physical distillation, and it is based on the fact that hydrocarbons are eluted from non-polar column essentially in boiling point order, and that a complete elution of sample can be achieved as the column temperature is increased through programming. High temperature GC is often used in analysis of wax and similar samples. In combination with element specific detector, GC is used for heteroatom compound determination (S, N, O). Fast GC has a growing interest for rapid analysis with an adequate resolution in minutes range. Pyrolisis GC has been applied to obtained useful information on the thermal decomposition of heavy oil products such as asphaltenes and vacuum residues. Supercritical Fluid Chromatography (SFC) As an analytical technique SFC is complementary to both GC and HPLC, with a separation capacity between them, and it is faster than HPLC, but slower than GC. Usually, it is used for materials that are too heavy to meet volatility and thermal stability requirements for GC. SFC has been applied extensively in four major areas ; hydrocarbon-type analysis (light distillates, middle distillates and heavy distillates), simulated distillation (to cover a broader boiling point range than can be achieved by the standard GC-FID method), analysis of polycyclic aromatic hydrocarbons (PAH's) and compound class fractionation. A three-step extraction procedure of SFC has been applied for determination of biomarkers in geological samples. Environmental samples such a soil, sediment, air, diesel particulates have been subjected to SFC to determine total hydrocarbons, PAH's or polychlorinated biphenyls (PCB's). Liquid Chromatography (LC) Application of LC in oil and oil products has started with the open column LC (OCLC) using normal absorbents such as silica gel or alumina and fluorescent-indicator adsorption (FIA) to separate sample in saturates and aromatics mostly for preparative purposes. OCLC has been used for separation of deasphaltene samples (maltenes) after the removing n-heptane insolubles (asphaltene). HPLC has been applied quite extensively to crude oil and its products analyses. Its application includes mostly hydrocarbon-type separation and determination as well as separation, identification and in some cases, quantitative determination of target compounds in many petrochemical samples. All varieties of HPLC i.e. reverse phase (RP), normal phase (NP), thin-layer, size-exclusion (SEC) and ionic chromatography (IC) are important for oil and oil products analyses. Various detection systems and the possibility of column switching allow an analyst to carry out a desired separation with an adequate resolution, rapidity and accuracy. Hydrocarbon- type determination for the samples with boiling range of 150 to 400oC (light fractions to heavy fractions) are mostly carried out by NP HPLC. The detectors included are: refractive index (RI), UV-visible, flourescence, diode array (DAD), infrared (IR), FID, evaporative light scattering (ELSD) and mass spectrometric detectors (MS) [4]. Both analytical and preparative HPLC methods have been developed to replace labor-intensive OCLC. Some preparative HPLC methods have been developed to separate target compounds as subfractions. These subfractions are then analyzed by HR chromatographic methods. They are also subject to spectroscopic and elemental analyses. A hydrocarbon-type determination as a standard HPLC method has been used in light and middle distillates for separation of saturates (n-paraffins, isoparaffins and naphtenes), olefines and aromatics (mono-, di- and polycyclic). In heavy distillates, single column usually is not enough for adequate separations, so multiple columns, column switching and solvent gradients have been used. NP and RP HPLC and combination of both have been used for determination of indigenous compounds, contaminants or additives as target compounds. Thin–layer or planar chromatography (TLC) can be used to obtain a quick class separation of base oils and crude oils. Size exclusion chromatography (SEC) as a separation technique based on the size of present molecules in the sample are important for heavy and very heavy oil fractions. Paraffin wax samples have also been analyzed by HPSEC. HPSEC monitors the change in composition of both oil and additives due to degradation during lubricating oil applications. HPSEC has been applied to the quality control of crude oils by obtaining molecular weight distributions and for evaluation of residue upgrading. There is also a growing interest in multidimensional or coupled or hyphenated chromatographic and detection techniques. Today, it is quite common to connect GC to GC, HPLC to GC, GC and HPLC to SEC. It is also common to connect GC to MS, FID, AED, and HPLC to DAD, MS and FID. HPLC-GC applications have been used for the characterization of saturates and aromatics fractions in gasoline, kerosene and diesel products and for the analysis of PAH's in middle distillates and lubricating oils. A sophisticated on-line coupling of SEC-NPHPLC-GC has been applied to the analysis of atmospheric residue without a prior cleanup. Compounds are separated according to molecular size by SEC, and polarity by NPHPLC. The separation by GC has been carried out according to component boiling point [5]. Suitability of chromatographic technique for a particular oil and oil products application is primarly determined by its selectivity, resolving capability and retention mechanisms. Specific applications include determination of hydrocarbon types, boiling range distribution and molecular mass distribution of petrochemical samples. There are still many opportuinities to develop and adapt methods that will be faster, rugged and automated for both sample preparation and analysis. Development of multitechnique separation methods will be increasingly important to meet the superior product quality control and stricter regulatory requirements. References 1. Barman, B.N. ; Cebolla, V.L. ; Membrado, L. ; Critical rewiews in Analytical Chemistry, 2000, 30(2&3)75 2. Peters, K.E. ; Moldown, J. M. ; in Biomarkers Guide. Interpreting Molecular Fossil in Petroleum and Ancient Sediments (J. Lapidus, Ed.) Prentice-Hall, Englewood Cliffs, NY.1993, 363 3. Johansen, N.G. ; Ettre, L.S., Miller, R.L. ; J.Chromatogr. 1983, 256, 393 4. Pasadakis, N. ; Varotsis N. ; Fuel 2000, 79, 1455 5. Schoenmakers, P. ; Blomberg, J. ; Kerkvliet, S. ; LC-GC 1997, 15(1), 28

crude oil; oil composition; separation techniques

nije evidentirano