CS252248B1 - A method for removing undesired olefinic substances from benzene, toluene and xylene-rich fractions - Google Patents

Abstract

Method for removing unwanted olefinic hydrocarbons from hydrocarbon fractions rich in benzene, toluene and xylene before their distillation separation into pure aromatics by percolation at a temperature of 50 to 250 C, a pressure of 0.5 to 5.0 MPa and space velocities of 0.5 to 2.5 vol./vol. h on bentonite aluminosilicate with a mass ratio of SiO2 : AljOj of 2.9 to 3.2, a TiO2 content above 3.5 i wt., a mass ratio of Fe2O3 : Al2O2 of 0.8 to 1.1, a bound water content of 4 to 10 wt. % and a grain size of 0.1 to 5 mm.

Description

The invention relates to a method for removing undesirable olefinic substances from fractions containing high concentrations of monoaromatics, which are used to produce pure aromatic hydrocarbons, in particular benzene.

Benzene is one of the most important aromatic hydrocarbons produced. The production of benzene from pyrolysis gasoline, which is a mixture of mainly aromatic hydrocarbons boiling in the range of 30 to 215 °C resulting from the pyrolysis of hydrocarbon raw materials, is one of the most widespread methods of its production. The procedure is to separate the heaviest fractions boiling above 215 °C (so-called pyrolysis oils) and lighter products from the pyrolysis products, which are further divided into gaseous products and pyrolysis gasoline in the ethylene unit.

The so-called C^-fraction is separated from the pyrolysis gasoline by distillation and the residue after distillation is subjected to hydrogenation in order to remove diolefins. Sometimes the entire pyrolysis gasoline is hydrogenated in this way. The stabilized hydrogenated pyrolysis gasoline is separated by distillation into the so-called BTX-fraction (benzene-toluene-xylene fraction) and the C^-fraction (residue after distillation).

The obtained BTX fraction is then processed by catalytic hydrodealkylation at temperatures of 540 to 660 °C, pressures of 3.0 to 10.0 MPa on a catalyst containing chromium oxide on alumina.

Hydrogen and low-boiling cleavage products are removed from the benzene-rich reaction products obtained. This product still contains a certain amount of unwanted olefins, which cannot be completely separated from the desired benzene by distillation.

Another widespread method of producing pure aromatic hydrocarbons is extraction from their concentrates. Extraction agents are mainly diethylpyrrolidone or other substances.

Typically, mixtures of benzene, toluene and xylenes and saturated hydrocarbons, as represented in reformer petroleum gasoline or hydrotreated pyrolysis gasoline, are extracted. Both reformers and pyrolysis gasolines contain certain small olefinic hydrocarbons which are partitioned between the extract and the raffinate. The level of partitioning depends on the selectivity of the extraction agent.

Very strict requirements are placed on the pure aromatic hydrocarbons produced, especially benzene: in particular, the olefin content should be extremely low. The olefin content is specified by various colorimetric or titration methods, and the contamination of aromatic olefins is indicated either by the bromine index or by refining tests with sulfuric acid.

To remove unwanted olefins from aromatic mixtures obtained by the above procedures, percolation on clays - natural or synthetic - is most often used. These are usually magnesium aluminosilicates of montmorillonite character. Usually, the operation is carried out at temperatures of 50 to 250 °C, pressures that ensure the fluidity of the percolated fraction and a volumetric load of 1 to 2 vol./vol. h.

To ensure high refining efficiency and durability, clays must be specially treated, which mainly consists of changing their composition by various chemical processes and subsequent adjustment of grain size and water content. Chemical processes mainly consist of the action of mineral acids, e.g. HF, on the chemical structure of the initial original aluminosilicate with the aim of increasing the concentration of acid centers of the final adsorbent. Chemical treatment is followed by other necessary operations - leaching, drying and adjustment of grain size. These treatments aim to prepare an effective adsorbent with high activity and durability. These processes significantly increase the cost of the resulting product for perlocation. Natural clays with an untreated composition generally show lower activity and significantly lower durability.

These shortcomings are now being eliminated by finding that the aromatic concentrates mentioned, from which pure aromatic hydrocarbons - benzene, toluene and xylenes - are further obtained by distillation, can be qualitatively freed from undesirable olefinic hydrocarbons to a bromine number value of no more than 12 mg Br/100 g according to the invention by a method where these fractions are percolated on natural bentonite aluminosilicate characterized by a mass ratio of SiO ₂ in the range

3.0 to 3.4 with a TiO ₂ content above 3.0 wt. % mass ratio: ΑΙ,,Ο^ - 0.9 to 1.1 with a water content of 4 to 10 wt. % and a grain size of 0.1 to 5 mm. Percolation takes place at 50 to 250 °C, a pressure of 1.0 to 4.0 MPa and a space velocity of 0.5 to 2.5 vol./vol. h.

The advantages of the proposed procedure are shown in the examples.

Example 1

a) On bleach clay about oxidic composition (% mass) sio ₂ ai ₂ o ₃ ^Fe 2°3 TiO ₂ CaO MgO At ₂ ° ^To 2° 64.2 15.3 3.2 0.4 0.2 1.5 0.2 0.8 grain size 0.1 up to 4 mm has not yet processes BTX fraction from catalytic hydrodealkylation

It is a highly refined and expensively processed product with a high price. Refining is achieved to a bromine index of the produced benzene below 10 mg/100 g of benzene. Percolation takes place at a temperature of 180 °C, a pressure of 3.0 MPa and a volume velocity of 1 vol./vol. h.

b) When percolating to ordinary in oxide composition dried and sorted clay (bentonite localities Obrnice) SiO ₂ Al ₂ O ₃ Fe ₂ O _{3 TiO2CaO} MgO At ₂ 0 ^To 2° 48.7 20.8 6.6 1.5 1.7 2.4 0.2 0.3

With a grain size of 0.1 to 2 mm, a bromine index of the produced benzene of 15 mg Br/100 g and a capacity of 2.5 t BTX per 1 kg bentonite are achieved. The working conditions during percolation are the same as during percolation on bleaching clay.

Example 2

Natural bentonite edited to a water content of 8.4% by weight had the following composition (% by weight): yes ₂ Al^0 ₃ ^Fe 2°3 ^Ti0 2 CaO MgO On ₂ OK ₂ O ^With °3 48.5 15.27 15.44 4.08 7.41 2.46 0.30 0.71 0.41

loss on annealing 10.39 grain size 0.2 to 1 mm.

The product of catalytic hydrodealkylation of pyrolysis BTX fraction separated from pyrolysis gasoline was percolated on this fresh clay using the method according to the invention. The percolation was carried out at a temperature of 180 °C, a pressure of 3.1 MPa and various clay volumetric loadings (LHSV vol./vol. h).

Bromine index of the raw material and its percolation product

Raw material Perlokat LHSV=1 Perlokat LHSV-4

36.9 3.7 16.8 distilled benzene 0.9 4.8

The capacity of bentonite of this composition is 5 t BTX per 1 kg of bentonite.

Example 3

The same raw material was percolated on bentonite as in Example 1 during a long-term experiment. After 250 days of operation (T = 190 °C, p = 3.0 MPa, LHSV = 1.0 vol./vol. h) benzene was obtained from the percolation product in an amount of 62% relative to the original BTX fraction with a purity of 96 wt. and a Br-index of 3.5. The percolate itself had a Bromine index of 10.9.

In comparison, the hydrodealkylation product before percolation had a Bromine Index of 36.9 and the benzene distilled from the unpercolated hydrodealkylation product had a Bromine Index of 25.

Claims (1)

Method for removing undesirable olefinic substances from fractions rich in benzene, toluene and xylene prior to their distillation into pure aromatics, to a residual Br-index value of not more than 12 mg Br per 100 g perlocations of these mixed fractions at temperatures of 50 to 250 ° C, pressures 0, 5 to 5.0 MPa and volumetric rates of 0.5 to 2.5 vol / vol. characterized in that the aromatic hydrocarbon mixture is applied to a bentonite alumosilicate characterized by a weight ratio of SiC > by weight ratio θθ 2 ° 3 ^{: A4} 2 ° 3 " ^{0.8 and} % ¹ ' ¹ ' ^{with a} bound water content of 4 to 10% by weight. and a grain size of 0.1 to 5 mm.