Lecture 2.

General principles of the oil chemical composition research. Physical methods of separation, isolation and analysis of hydrocarbon mixtures. Types of chromatography and analysis. The separation of hydrocarbons using molecular sieves, and the thermal diffusion method complexation. Adsorbents in gas chromatography. The principle of operation of the gas chromatograph. Types of chromatography columns, and the phase detector. Physico-chemical constants of petroleum hydrocarbons and their role in the identification and analysis of hydrocarbons and hydrocarbon mixtures. Spectroscopic methods of analysis and identification of hydrocarbons and hydrocarbon mixtures.

Plan of the lecture

1. Physico-chemical methods of analysis of petroleum and petroleum products

2. Chromatographic analysis of the oil and gas

3. Physical and chemical properties of oil

4. Spectral analysis methods of oil and petroleum products

1. To facilitate the analysis of petroleum and petroleum products use a variety of methods for their pre-separation as the molecular weight and chemical composition. For oil separation and isolation of different groups of hydrocarbons and heteroatomic components used chemical and physical methods. Chemical methods are based on unequal reactivity separated components, and the physical (or physico-chemical) - on the difference in the concentrations of the components of the equilibrium phases coexisting.

Physical methods of oil components separation

Phase states	Simple methods	Sophisticated methods
Gas - gas	Diffusion through the membrane	Diffusion with a carrier gas
Gas - liquid	Distillation and rectification	Steam distillation absorption azeotropic distillation extractive distillation
Gas - solid phase	Distillation	Adsorption
Liquid-liquid	Diffusion through the membrane Thermal diffusion	Extraction
Liquid - solid phase	Crystallization	adsorption extractive crystallization Adduction crystallization

By physico-chemical separation methods also include various types of chromatography, different aggregate states mobile and stationary phases.

2. Chromatography - separation method of substances based on the difference in their distribution coefficients between two phases, one stationary and the other direction relative to the first moving (along the column or thin layer stationary phase).

On technology performance distinguishes the separation, columnar, paper, thin-layer and gas-liquid chromatography.

In chromatographic separation material repeatedly separation occurs in a single step. The substance is distributed between the stationary and mobile phases. The stationary phase - it is a solvent (usually water) adsorbed on a solid material. The latter may be paper or aluminum oxide, silica gel, filled in a column, or coated on a glass plate. Mobile phase - a second solvent percolates through the stationary phase.

Adsorption is called substance concentration (adsorbent) phase volume of the interface between them, for example gas or liquid on the surface of the solution. Distinguish physisorption, which displays the results of electrostatic forces disperse. Chemical adsorption (chemisorption) is accompanied by a chemical reaction of the adsorbate with the adsorbent. Adsorbents distinguish disordered crystal structure and an inhomogeneous porosity (silica gel, alumina, activated carbons) and adsorbents with uniform pores - zeolites and molecular sieves. Zeolites are selective sorbents, which differ strongly regular structural pores, which in normal conditions are filled with water molecules. If water removes from the zeolite, pores can be refilled either by water or other substances. The absorption of the substance occurs in the adsorption cavities, different inputs windows. Zeolites are also called molecular sieves which adsorb only those molecules smallest critical diameter to the molecular axis less than the effective diameter of the windows.

Zeolites - polar adsorbents, in connection with which the separation of hydrocarbons on them can be carried out using both the difference in molecular size, and various levels of saturation and polarity. Zeolites are widely used for drying gases and liquids, in addition, they are excellent sinks mercaptans.

On technology performance distinguishes the separation, columnar, paper, thin-layer and gas-liquid chromatography (GLC).

GLC separation based on differences in the distribution ratios of mixture components between stationary and mobile phases. Detector device continuously measures the concentration of the components at the exit of the column, and converts it into an electric signal detected by potentiometer. On the tape recordercurve is obtained - the chromatogram.

3. The most important physical properties of the oils should include the density, molecular weight, optical activity.

Density - value, defined as the ratio of the mass of a substance to the volume occupied by them. Density units in the International System of Units (SI) - kilogram per cubic meter (kg/m³). Relative density is the density of the substance in relation to the density of a standard substance (water at most ≈ 4°C).

In the CIS, the density of oil and oil products is determined at 20°C and referred to the density of water at 4°C. Relative density in this case is denoted ρ₄²⁰ (can be found in the literature refer to d₄²⁰, rarely - γ₄²⁰). However, determination of the density may be performed at any temperature, and then calculate the value ρ₄²⁰ formula:

wherein - the density at the test temperature;

          γ - coefficient of thermal expansion (its values are given in the literature);

          t - the temperature at which the density was determined, the ⁰C.

The molecular weight is the most important physical - chemical characteristics of the substance. For oil, this figure is especially important for giving "average" molecular weight substances that are part of one or another faction of oil. The molecular weight of oil is widely used for the calculation of equipment refineries.

Molecular weight (M) of the oil has only an average value and varies with the mixture. The first representative of liquid hydrocarbons pentane is M = 72. We resinous substances M reaches 1500-2000. M is greater than the higher boiling point fraction.

For the calculation of molecular weight can be used formula Voinov:

М_ср= 60 + 0,3t_ср + 0,001 ,

where t_ср – average boiling temperature

Voynov formula applies mainly to rich fraction alkanes.

Optical properties. Optical properties such as refractive index, molecular refraction and dispersion in practice is used to quickly determine the composition of petroleum products, as well as control over the quality of products during their production.

When light beams passing from one medium to another their speed and direction vary. This phenomenon is called refraction.

The ratio of sines of the angles of incidence and refraction for the environment - a constant, independent of the angle of incidence:

sin r/sin i = n = const,

where r - the angle of incidence; i - the angle of refraction; n - coefficient (index) index.

The refractive index depends on the temperature at which the determination is carried out, and the wavelength of light. Therefore, always specify the conditions in which we measured. Usually the definition of lead on the most striking of the Fraunhofer lines (mostly yellow line of the sodium spectrum D = 589,3 nm) at 20°C. Hence the designation of the refractive index is . The effect of temperature is taken into account by the formula:

where t₀ - conditional temperature (20°C); t - temperature of the experiment; a - 0.0004.

Refractive index is a very important constant, not only for individual substances, but also for oil is a complex mixture of different compounds. It is known that the refractive index is smaller, the more hydrocarbons relative to hydrogen content.

Another characteristic of the derivative of the refractive index of refraction is intertsept (refractometric difference). Intertsept refraction (RI) is the difference between the values of the refractive index and half the density of matter (this value is constant for a homologous series of hydrocarbons):

The viscosity of the oil and its products is the result of intermolecular forces shear resistance of the fluid layer with respect to the other layer, and hence is the group function of the chemical composition and molecular weight of the oil.

Distinguish dynamic, kinematic viscosity and conditional. Dynamic viscosity () is measured in poise, poise dimension - g/sm·sek.

Kinematic viscosity is the ratio of the dynamic (absolute) viscosity of the liquid to its density at a given temperature.

The calculations use the kinematic viscosity (), equal to the ratio of the dynamic viscosity of the oil to its density

 = ,

where  – the dynamic viscosity, g/(smsec);

 – the density of the oil, g/sm³.

The unit of kinematic viscosity is Stokes (v), the dimension of the Stokes - cm²/sec. Use as conventional viscosity (RD), which is defined as the ratio of the flow time of a standard viscometer certain amount (eg, 200 ml) of the test fluid at the time of the expiry of the same amount of distilled water at 20⁰C. Relative viscosity is expressed as the expiration time (in seconds) a certain volume of liquid from the standard viscometer Saybolt, Redwood.

Viscosity index (VI), proposed by Dean and Davis - the viscosity ratio . Viscosity Index evaluates the performance characteristics of lubricating oils.

Pour point is considered the temperature at which the cooling tube in the fraction does not change the level of the tubes at an inclination to 45 °. Pour the fuel to some extent characterizes the behavior of fuel in the power supply system. For it is increasingly judged on refueling capabilities, transportation, loading and unloading of fuel. The pour point of fuel - is the amount of the notional and serves only as a well known landmark to determine the possible conditions of use of fuel.

Flashpoint is called the lowest temperature oil (under standard conditions), in which over its surface to form pairs can flare up in the air from the source of ignition, but the rate of their formation is still insufficient for later burning.

Fuel ignition temperature is the temperature at which the product is heated in the standard conditions is illuminated when held close to it and a flame burns at least 5 seconds. The ignition temperature is always above the flash point, often quite dramatically - a few tens of degrees.

Auto-ignition temperature is called the minimum temperature at which the oil vapors mixed with air to ignite without an external source of ignition. On this property, petroleum based diesel operation of internal combustion engines. Auto-ignition temperature is higher than the flash point of a few hundred degrees. Auto-ignition temperature is the lowest temperature at which dramatically increases the speed of the exothermic oxidation reaction of fuel vapor in the air, ending with the appearance of the flame "burning."

4. Spectral analysis - is a physical method of analysis is used to study the chemical composition of its optical spectrum. Depending on what material the optical spectrum - the emission spectrum, absorption or scattering of light - is used for the study of spectral analysis is divided into three different as the procedure and in the region of its application, the type of optical spectral analysis.

Spectral analysis of the spectra of radiation or emission spectral analysis is used to study the composition of the line spectrum emitted luminous vapor substance. This spectrum belongs to the atoms of matter, so emission spectral analysis is used to study the elemental composition of the substance.

The second type of spectral analysis is an analysis of the absorption spectra or absorption spectral analysis. Analyte is passed through a beam of light from the light source. In the spectrum of the light transmitted through the substance, rays, due to absorption of light by matter, are reduced in comparison with the spectrum of light coming directly from the source. The absorption bands are determined by the molecular and atomic structure of matter, and the absorption rate - the number of absorbing atoms and molecules. Thus, the absorption spectra can be seen as a qualitative and a quantitative (atomic and molecular) composition of matter. Absorption spectral analysis is also called spectrophotometry. A special case of spectrophotometry is a colorimetric spectrophotometry.

The third type of optical spectrum analysis is the spectral analysis of the spectra of Raman scattering. In this case, illuminating the material being studied by light from the source line spectrum (for example, a mercury lamp) are observed spectrum of light scattering material. Viewing direction is perpendicular to the direction of the illuminating light beam. In addition the spectrum of the scattered light of the spectral lines from the light source, there are still additional satellite lines, accompanying each of the lines of the primary light. Hence the name of the phenomenon is a combination scattering of light.