CC BY
81
https://doi.org/10.29013/AJT-20-5.6-49-53
Makhmudov Mukhtor Jamolovich, PhD in Institute Bukhara of engineering and technology E-mail: makhmudov.mukhtor@mail.ru Axmedov Ulug1 Karimovich, Professor, Institute of the General and Inorganic chemistry Academy of Sciences of the Republic of Uzbekistan, Tashkent
MODERN METHODS OF REDUCING THE CONTENT OF AROMATIC HYDROCARBONS IN GASOLINE
Abstract. Various refining methods are presented to improve the ecological environment and reduce the amount of aromatic hydrocarbons in motor gasolines in the oil refining industry.
Keywords: gasoline, aromatic hydrocarbons, azeotropic distillation, extraction, rectification, adsorption.
the carcinogenic group: 1,2-benzanthracene (CH), 3,4-benzpyrene (C20H12), 1,2-benzpyrene (C20H12), 3,4-benzofluorantene (CH). Particularly danger-
Road transport is the main source of air pollution. Pollution occurs as a result of fuel combustion. About 2 billion tons of petroleum fuel are burned annually in automobile internal combustion engines in the world. Moreover, the average efficiency is 23%, the remaining 77% goes to the environment [1].
The following types of transport are distinguished: light, medium and heavy freight, bus, passenger. At the same time, 90% of cars use gasoline as fuel, and 10% - gas, trucks use diesel fuel and gas, 50% of buses use gasoline, and 50% - gas [2].
About 25% of the world's oil is used to produce gasoline, which is the main type offuel for vehicles [3].
It was found that 30% of urban diseases are directly related to air pollution by exhaust gases. The most dangerous for humans are hydrocarbon compounds of
ous is 3,4-benzpyrene, which is a kind of indicator of the presence of other carcinogens in the mixture [4-6].
The range and quality of gasolines are determined recently by environmental requirements for them. Currently, in order to reduce the toxicity of car exhausts, according to the requirements of European standards, restrictions are set on the content of benzene (up to 1%) and total aromatic hydrocarbons (30-35%) in gasolines (table 1). MPC of gasoline = 300 mg / m3. Reducing the harmful effects of exhaust gases on the environment and humans can be achieved by reducing the content of aromatic hydrocarbons in gasolines, primarily benzene [7].
Table 1.- Modern requirements for the quality of gasoline
Indicators Requirements
Euro 4 (since 2005) Euro 5 (since 2009)
Benzene content, not more than,% 1.0 1.0
Sulfur content,% 0.005 0.001
The content of aromatic hydrocarbons,% 35 35
The content of olefinic hydrocarbons,% 14 14
Oxygen content,% 2.7 2.7
Benzene, the most easily boiling among aromatic compounds, is harmful to people directly working with gasoline, as it contributes to the disease with leukemia [8].
There are various methods for the separation of aromatic hydrocarbons:
- Azeotropic distillation;
- Extractive distillation;
- Extraction;
- Adsorption release.
Azeotropic distillation. As you know, aromatic hydrocarbons form azeotropic mixtures with boiling paraffin-naphthenic hydrocarbons in cases of significant deviations of the mixtures from the behavior of an ideal system. In fig. Figure 1 shows the dependence of vapor pressure on the composition of the mixture of two components A + B for a system with a minimum boiling point.
The dashed line A + B represents the theoretical or ideal vapor pressure of the solution, calculated according to Raoult's law.
o
-ji -ji
0 1,(1 Molar fraction of the component
Figure 1. The dependence of vapor pressure on the composition of the mixture with a minimum boiling point
The upper solid curve shows the actual vapor pressure of the mixture. Point z, where the curve passes through the maximum, corresponds to the composition of the azeotrope. Since this is the point of maximum vapor pressure, it corresponds to the minimum boiling point of mixture A-B, which is lower than the boiling point of pure components A and B. The mixture corresponding to the composition at
point z will be distilled at a constant temperature and without changing the composition.
Aromatic hydrocarbons C6, C7 and C8 form azeo-tropes with paraffin-naphthenic hydrocarbons with only a minimum boiling point. In fig. 2. The composition of the vapor and liquid phases of the cy-clohexane-benzene system is given. These hydrocarbons boiling at 80.0 and 80.1 °C, respectively, form an azeotrope with a benzene content of 51.8% by weight. The boiling point of the azeotrope is 77.5 °C. The vapor pressure curve of this system is similar to the curve in (Fig. 1).
i.o
i? 2 2 c
T = _
E 0,6
"8 > £
^ —■
0,8
0,4
0.2
c
O
/7 //
/
// V
// //
0
0.2 0.4 0.6 0.8 1,0 Content of cyclohexane in the liquid, molar fraction
Figure 2. The cyclohexane-benzene system
Since the azeotrope has a minimum boiling point, then for any ratio of components, the azeotropic mixture will be distilled first. But the nature of the residue will already depend on the ratio of the components. So, if the initial mixture contains 20% cyclohexane and 80% benzene, then after distillation of the azeotrope, pure benzene will remain in the residue; if a mixture consisting of 80% cyclohex-ane and 20% benzene is distilled, pure cyclohexane will remain in the residue. In practice, this method is sometimes used to obtain pure benzene or cyclohexane from mixtures thereof.
When choosing a reagent that forms an azeotropic mixture with one or both of the separated components, the following series of circumstances must be taken into account:
1. When adding the third component, the boiling point of the resulting azeotrope should be quite
different from the boiling points of the other components or mixture.
2. It is desirable that the resulting azeotropic mixture contains the maximum amount of product per unit weight of the distilled reagent and has a low boiling point.
3. The third component must have a low latent heat of evaporation, so that the heat consumption for distilling the resulting product is minimal.
4. The third component should be easily regenerated for further use in the process. For this, several methods are used to separate the reagent and non-aromatic hydrocarbon, such as separation of liquid phases upon cooling, followed by extraction with a solvent, or water washing, distillation at various pressures, and other methods.
5. The third component should be chemically inert, not react with shared hydrocarbons, not corrode the equipment, be thermally stable, non-toxic and available on an industrial scale.
Extractive distillation - characterized by the use
than the boiling points of the separated components. The solvent is fed to the top of the column. Flowing down, it dissolves one of the constituent parts of the mixture - aromatic hydrocarbons. Non-aromatic hydrocarbons free of solvent are removed from the top of the column. The solvent used to separate the hydrocarbon mixture changes the normal relative volatility of the components. In this case, a deviation of the system from the ideal one is observed.
A large number of solvents have been tested to isolate aromatic hydrocarbons by extractive distillation. In the (table 3) shows data on the relative volatility of a mixture consisting of 50 mol%. meth-ylcyclohexane and 50 mol%. toluene, in the presence of various solvents (relative volatility of the toluene -methylcyclohexane 1.37 system).
Table 4 shows the relative volatilities of the toluene-non-aromatic hydrocarbon system (1: 1 ratio) in the presence of various compounds. As a non-aromatic hydrocarbon, a specially dearomatized fraction of direct distillation gasoline boiling in the range of 99-113 °C served.
of a solvent, the boiling point ofwhich is much higher
Table 3.- Relative volatility of methylcyclohexane mixed with toluene in the presence of various solvents
Solvent Solvent boiling point, °C Solvent content, mol%. Relative volatilitymethylcy-clohexane to toluene, a
Aniline 184.4 65.8 2.71
Ethylene glycol monoethyl ether 135.2 65.8 2.29
Diethylene glycol 197.2 66.5 1.88
Formamide 193 66.7 1.42
Ethylene glycol monomethyl ether 124 66.7 2.72
Phenol 182.2 65.8 2.52
Pyridine 115.3 66.7 2.59
Furfural turned out to be the best selective solvent for the isolation of toluene. However, furfural boiling at a temperature of 163 °C can form azeotropic mixtures with non-aromatic hydrocarbons distilled from toluene, which makes it difficult to regenerate it.
Phenol, being the same selective solvent as aniline, is chemically inactive and quite stable. It also meets the requirements for boiling point, availability and cost. For the above reasons, phenol is the main solvent used for the extraction of benzene and toluene mainly consists of aromatic hydrocarbons.
Table 4.- Relative volatility of non-aromatic hydrocarbons mixed with toluene in the presence of various solvents
Solvent Solvent boiling point, 0С The solvent content in the feed,% wt. Relative volatility methylcyclohexane to toluene, a
Furfural 163 50 2.30
Acetonyl acetone 188 50 2.20
Nitrobenzene 212 50 2.16
Nitrotoluene 223 50 2.16
Phenol 182 50 2.10
Aniline 184 50 2.08
Phenol + Cresol (60% + 40%) 193 50 1.98
Phenol + Cresol (40% + 60%) 195 50 1.95
Acetophenone 203 50 1.95
Meta + or paracresol 205 50 1.85
Diacetonglycol 191 50 1.64
Extraction. Extractive separation processes are based on the unequal solubility of aromatic and non-aromatic hydrocarbons in various solvents.
The conditions of the extraction process are determined by the nature of the feedstock and solvent, the amount of solvent, temperature and the required number of stages of extraction and the given selection and properties of the aromatic hydrocarbon concentrate.
The solvent should be characterized by the following qualities:
- high selectivity and high dissolving ability in relation to aromatic hydrocarbons;
- stability during long-term operation;
- the difference in density between the solvent and hydrocarbons to facilitate phase separation during extraction;
- ability to easy regeneration;
- to be economically accessible, not to corrode the equipment and not have a pronounced toxic effect.
The choice of the temperature range of extraction depends on the critical temperature of dissolution of the feedstock. Extraction is carried out in the temperature range at which there are two phases - extract and raffinate.
E = jfo------
<r- 5 -SO -40 -29 -IS -7 +4*1 J
Extraction temperature. °C
Figure 3. The effect of the temperature of extraction and the fractional composition of raw materials on the content of aromatic hydrocarbons in the extract: 1 - average boiling point of the fraction 144 °C; 2 - average boiling point of the fraction 216 °C; 3 - average boiling point of the fraction 276 °C
The amount of solvent is determined by the temperature of the process and a given percentage of extraction of aromatic hydrocarbons from the feedstock. The process must be carried out in such a way as to obtain, possibly, a greater extraction of aromatic hydrocarbons with a minimum consumption of solvent and a practically acceptable number of extraction steps. Diethylene glycol (DEG) and liquid
sulfur dioxide are used as solvents in the evolution of aromatic hydrocarbons in industrial practice.
The influence of the extraction temperature and the fractional composition of the raw material on the content of aromatic hydrocarbons in the extract is shown in (Figure 3).
The curves were obtained on the basis oflabora-tory studies and the operation of an industrial extraction plant using sulfur dioxide.
Adsorption release. Aromatic hydrocarbons are able to adsorb more strongly on specially selected adsorbents than paraffinic and naphthenic hydrocarbons; this is the basis for their isolation from petroleum products.
Separation of complex mixtures using the adsorption method has been used in laboratory practice for a long time - since 1903. Adsorption separation in the liquid phase is used not only to separate aromatic
hydrocarbons, but also to isolate a number of other chemical products [9; 10].
Adsorbents used to extract aromatic hydrocarbons from oil fractions should have high selectivity, mechanical strength, and long service life; they should be easily regenerated, be chemically inert with respect to shared components and economically available.
The industrial design of the process of adsorption of aromatic hydrocarbons is determined by the absorption capacity of the adsorbent, the rate of percolation of the raw material, propellant and desorbent. The necessary amount of propellant and desorbent and the size of the adsorbent granules are also taken into account.
Thus, for the extraction of aromatic hydrocarbons from the composition of automobile fuels in the oil refining industry, the cleaning methods presented in this article can be used.
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