Lecture 6

The chemical composition of petroleum fractions and petroleum products, and methods of their determination. Group hydrocarbon composition, individual composition, structural-group composition of various petroleum fractions and petroleum products and methods of their determination. Specifying a detailed and group composition of gasoline fractions. Detailed definition of the group composition of kerosene-gas oil fraction. Determination of structural-group composition of oil fractions.

Plan of the lecture

1. Chemical composition of oil

2. Group hydrocarbon composition of petroleum fractions

3. Structural and group composition of lube oil

1. Conventional distillation methods can not divide oil and petrochemicals for individual compounds, since they are a complex mixture of hydrocarbon and non-hydrocarbon compounds. Distillation is used only for rough separation of oil on the broad fraction. Hydrocarbon fractions boiling point is not at a certain temperature, but only in certain temperature ranges, i.e. have an initial boiling point and final boiling point. Initial boiling point and final boiling point depends on the chemical composition of the oil fractions.

Indicator fractional composition (FC) oil is of great importance in practice. By fractional composition of oil is judged on what oil and how much of it you can select. For example, the oil fraction which boils in the temperature range of 200 to 350⁰C denote the expression "200-350⁰C fraction".

To determine the FS petroleum oils or oils distilled in a standard unit. A graph in which the ordinate lay boiling point hydrocarbons, and the abscissa - distilled off in time or%. Horizontal sections 1 and 2 broken line corresponds to the temperature of individual hydrocarbons boiling. Oblique lines 3 and 4 show the process of boiling mixtures. In some areas raise the temperature in places of transition are contingent "initial" and "final", and between them "average" boiling point of the mixture (points A, B and C). Thus, a so-called oil distillation curve, i.e. plot of temperature versus volume had been boiled oil.

As the distillation residue is enriched in low-boiling components of high boiling components. Since the vapor pressure of the high-boiling components at a temperature below the external pressure, boiling finally may cease. To be continuous boiling liquid residue is continuously heated. In this case, the vapor space are moving more and more components with ever-increasing boiling points. The exhaust vapor is condensed, the condensate collected at boiling ranges of components in the form of individual fractions of the oil.

Fig. 1 Curve boiling mixture of components:    

A - the beginning of the boiling mixture; B - average boiling point of the mixture; C - the end of the boiling mixture; 1 and 2 - the boiling point of the pure components; 3 and 4 - the boiling point components of the mixture.

Fractional distillation oil is gradual or flash evaporation. With the gradual evaporation of the pair formed continuously withdrawn from the distillation apparatus, they are cooled and condensed in cooler-condenser and collected in a receiver in the form a distillate fraction. If the heat generated in the process is not outputted from the pair of the heating apparatus until the selected temperature is reached, at which at one time (single) separating the vapor phase from the liquid, with the distillation process is called flash evaporation.

With a constant and even more so once the evaporation can not achieve a clear separation of oil into narrow fractions. In connection with this use refluxed distillation or rectification. For this purpose, oil is heated in the flask and the resulting vapors are substantially free of high-boiling components, is cooled in a special unit - a dephlegmator and liquefy - reflux. Phlegm, flowing down, met with the newly formed pairs. As a result, heat reflux boiling components are vaporized and the vapors condense higher boiling components. In this way, a clear separation of oil into fractions.

To determine the fractional composition of petroleum and petroleum products, the following methods of distillation:

1) low-temperature distillation - for liquefied gases and hydrocarbon fractions boiling at a temperature below 20⁰C;

2) medium-temperature distillation - for oil, boiling up to 350⁰C;

3) vacuum distilling liquids boiling above 350⁰C;

4) the molecular distillation - for macromolecular substances (e.g., tar);

5) distillation method flash evaporation.

Low-temperature distillation. Low temperature distillation is carried out to determine the composition of gases dissolved in the oil. To this crude oil is heated for 3-4 hours to 150-200 0 C in the unit for distillation of oil.

The medium and vacuum distillation. The most widely used cell phones two groups. Devices operating without rectification (apparatus for distillation of petroleum products, Claisen flask, distillation apparatus Bogdanov for fuel oil and semi tars).

Vehicles, working with rectification are: ARN-2 (unit petroleum distillate), AChR (clear distillation apparatus). To avoid thermal decomposition of petroleum distillation in some cases lead vacuo.

Higher fractionation unit has the ability to ARN-2, which can be used for the following tasks: data acquisition for the construction of curves true boiling points; establishing the potential content of fractions; obtain narrow fractions in order to study a group of oils.

2. Narrow gasoline and kerosene fractions can be divided into individual hydrocarbons by gas-liquid chromatography, but such mixtures payment chromatograms very laborious.

There are methods for determining the composition of petroleum products in their content of various hydrocarbons. These methods can be divided into types: chemical, physical, chemical, and physical combined.

Determination of group composition of gasoline is carried out by the combined method of aniline points. The method reduces to the calculation of mass A arene content,%, proceed with the change of the mutual dissolution critical temperatures equal volumes of gasoline and aniline (aniline point) before and after the extraction stage. To determine the group composition of gasoline by aniline points must be separated arena contained in the initial product chemical method (sulfonation 98.5-99% sulfuric acid) or physico-chemical method - chromatography on silica gel.

A detailed understanding of the composition of hydrocarbon lubricating oils and kerosene, gas oil fractions give methods of structural-group analysis. When analyzing components as the test mixture not considered atoms and molecules, and structural elements of the molecule aromatic ring, naphthenic ring and paraffin chain.

3. Structural-group analysis of oils in practice is calculated by the direct method, ring analysis and methods n-d-M.

Direct method. The essence of the method proposed by Waterman, Flyugterom and Van Vestenom is to determine the molecular mass and elemental composition of oil fractions before and after hydrogenation. Thus analyte oil should not contain saturated hydrocarbons; hydrogenating the aromatic rings are subjected to. In the hydrogenation the number of carbon atoms in the molecule remains unchanged, but an increase in the content of hydrogen atoms corresponds exactly to the number of carbon atoms contained in aromatic rings of the base oil, since each carbon atom of the aromatic ring attaches one hydrogen atom.

Ring analysis. In the ring structure of the investigated oil analysis are the graphs expressing the relationship between the specific refraction, molecular weight and aniline point for saturated petroleum fractions and the relationship between the specific refraction, molecular weight and percentage of carbon in naphthenic rings saturated oil fraction.

Ring analysis method proposed and Waterman Flyugterom based, on the one hand, on a linear relationship between the specific refraction R (R = ) and fractions saturated in hydrogen,%

Н = 110,48R²⁰ – 22,078.

The essence ring analysis is as follows.

Determine the refractive index of the oil sample n , density  , molecular weight M and aniline point T1. Calculate the specific refraction R1 (on the Lorentz-Lorenz).

According to the schedule determined aniline point, this corresponds to the molecular weight and the specific refraction original oil sample. The difference between the found on schedule aniline point and aniline point of the original oil represent ΔT.

Based on the predicted value can ΔT aniline point T2 is completely hydrated to an oil. It has been established that the increase in aniline point Hydrogenation of different oils 0,85ΔT averages (0.85 value called prediction factor). It is assumed that the molecular weight does not vary in the hydrogenation, based on the detected value for the base oil M and T2 are calculated for specific schedule refraction R2 hydrogenated oil. Then, the graph on the basis of M and R2 determine the total carbon content in the saturated ring (however,%).

n-d-M method is one of the most reliable, which is widespread, both for the study of natural oil fractions, and for oils. The method allows determining the proportion of carbon atoms (%) contained in the aromatic CA, naphthenic rings and paraffinic CH C circuits as well as the total number of rings and the average number of aromatic and naphthenic rings.

Structural-group analysis by the method of refractive index - molecular weight designed Hershey and Fenske. The method provides for the preliminary separation of paraffinic, naphthenic fractions and aromatic hydrocarbons with the definition of the refractive index and molecular weight.