RU2157360C1 - Method of removing unsaturated hydrocarbons from benzene - Google Patents

Abstract

FIELD: petrochemical processes. SUBSTANCE: benzene produced from petroleum product pyrolysis C₆-C₇-fractions is freed of unsaturated hydrocarbon impurities by extractive distillation with dimethylformamide, after which unsaturated hydrocarbons separated from benzene as head fraction are oligomerized at 150-170 C in presence of 0.0001-0.001% (based on the weight of benzene) of benzenesulfonic acid. Benzene distilled off from resulting oligomers is combined with benzene obtained as bottom fraction from extractive distillation column and freed of thiophene by known techniques. Purity of thus obtained benzene is at least 99.90 wt %. EFFECT: simplified process and increased purity of product. 1 dwg, 3 tbl, 3 ex

Description

 The invention relates to the field of petrochemistry, in particular to methods for purifying benzene from impurities of unsaturated hydrocarbons.

 The organic synthesis industry places high demands on the quality of petrochemical benzene, especially in the content of unsaturated compounds and thiophene. For benzene grade "purified", produced in accordance with GOST 9572-77, coloring with sulfuric acid should not exceed 0.1 in the numbers of the standard scale, and the thiophene content - 0.0001% wt.

 A known method of purification of benzene obtained by distillation from the benzene-toluene fraction of the pyrolysis condensate of ethylene-propylene production from unsaturated, including diene, non-aromatic hydrocarbons, by treating them with a cleaning complex based on aluminum and carbon tetrachloride (ed. Certificate of the USSR N 615053, C 07 C 7/148, 1978). The method allows purification from unsaturated compounds to their residual content of 0.05% wt. at the source of 1.5 - 2.0% wt.

 Modification of the cleaning complex due to its deposition on the activated surface of aluminum oxide can improve the quality of treatment at a higher content of impurities of unsaturated hydrocarbons in the feed - up to 5.0% wt. (ed. certificate of the USSR N 1825355, class С 07 С 7/148, 15/04, decl. 04/01/91, publ. 06/30/93).

 The disadvantages of adsorption purification methods are the large specific consumption of adsorbent per unit of product, the recurring problem of disposal of spent adsorbent, the duration of the cleaning process. In the case of thiophene in benzene, the life of the cleaning agent is drastically reduced.

 Similar problems arise during the purification of benzene from unsaturated compounds on natural adsorbents, for example, on active clays, with the subsequent isolation of the target product by distillation [A.D. Berenz et al. Processing of liquid pyrolysis products. M., Chemistry, 1985, p. 216; author testimonial. USSR 1065392, class C 07 C 15/04, 7/148, 1984].

In the industrial production of benzene, the most effective method for purifying benzene from unsaturated compounds is catalytic hydrotreating, which includes the processing of raw materials with hydrogen (hydrogen-containing gas) in the presence of catalysts - metal oxides of groups VIII, VI of the Periodic Table at a temperature of 140-180 ^o C, pressure 3-5 at I stage of purification, at a temperature of 350-480 ^o C and a pressure of 3-5 ata at stage II, followed by purification from impurities of unsaturated compounds on active clays and the release of saturated aromatic hydrocarbons, count the number of which increases during hydrotreating, by azeotropic or extractive distillation [Composition and processing of liquid pyrolysis products in domestic plants. Thematic review. M., TsNIITEneftekhim, 1977, p. 76-77]. The method provides high-purity benzene (content of the main substance of 99.90% wt. And above) with a high yield.

 However, a significant drawback that does not allow its use in small-tonnage semi-industrial plants is its technological complexity, high capital costs, problems associated with providing hydrogen, high energy intensity of the process - factors that make the process of obtaining high-purity benzene unprofitable.

 The method of catalytic hydrotreating during its development provided for instead of the first stage of hydrohydrogenation for purification from unsaturated compounds their thermal polymerization, used, for example, at facilities of the company Sholvan-Khemi, Lurga [L.Ya. Kolyandr. Obtaining pure benzene for synthesis. M., Metallurgy, 1966, p. 120-122].

The polymerization of unsaturated compounds in the crude benzene coke production (later - benzene-toluene-xylene fraction) was carried out at 200 ^o C, a pressure up to 60 atm in the presence of coke-oven gas, followed by distilling off benzene from the polymer, which was the only waste during cleaning.

 However, the thermal polymerization method turned out to be insufficiently effective, despite the severe polymerization mode, did not provide a sufficiently complete separation of unsaturated compounds, which led to the coating of the equipment at the next stage of hydrotreating with tarry deposits, which complicated the operation of the plants. Therefore, thermal polymerization has been universally replaced by catalytic pre-hydrogenation (forhydrogenation).

 However, if it is necessary to obtain high-purity benzene, including from liquid pyrolysis products, after hydrotreating, along with purification from saturated non-aromatic hydrocarbons, fine purification of unsaturated compounds and thiophene by known methods — adsorption, crystallization, catalytic purification using special catalysts, and chemical purification [V. A. Dvinin et al. Fine purification of the benzene fraction of pyrocondensate from olefins and thiophene. Oil refining and petrochemistry, N 9, 1988, p. 19-21].

 Thus, all known methods for purification from unsaturated non-aromatic hydrocarbons are based on adsorption processes, hydrogenation, or chemical interaction with a cleaning agent that is also adsorptive (author certificate 615053, author certificate 1825355, class C 07 C 15/04, 7/148).

 Information on fundamentally different methods of purification from unsaturated hydrocarbons that are not based on the above processes was not found in the technical and patent literature.

A method is proposed for purifying benzene from unsaturated non-aromatic hydrocarbons by extractive distillation using dimethylformamide as a solvent, the unsaturated hydrocarbons removed with the top product are oligomerized at a temperature of 150-170 ^o C in the presence of 0.001-0.0001% wt. benzenesulfonic acid followed by distillation of benzene from oligomerizate. The ratio (mass) of dimethylformamide (DMF) to feed is 4.5 - 6: 1.

 The bottoms product of the extractive distillation column, which does not contain impurities of unsaturated hydrocarbons, after separation of dimethylformamide is sent for dry cleaning from thiophene, benzene purified from thiophene is combined with benzene, which is isolated after purification of the head fraction from unsaturated hydrocarbons.

 Highly purified benzene is obtained with a basic substance content of at least 99.90% wt.

As the feedstock, benzene is used, which is separated by two-stage distillation from liquid C ₆ -C ₇ pyrolysis fractions of ethylene-propylene production (EPP) and after-treatment of saturated and unsaturated non-aromatic hydrocarbons by azeotropic distillation with methanol.

 As the main impurities, benzene contains thiophene - 0.01% wt. and cyclodiene aromatic hydrocarbons (2-2.6% wt.), not separated upon purification by azeotropic distillation using known solvents (methanol, acetone).

 The method is carried out according to the following scheme (see drawing).

Benzene is sent to the middle part of the extractive distillation column 1. The efficiency of the column is 140 practical plates (etc.), reflux ratio (fl.h.) - 2, top temperature (t _top ^o ) - 80 ^o C, pressure - 0.8 -1 ata, the temperature of the cube (t _cube ^o ) - 160 ^o C.

At the temperature of 90 ^° C., dimethylformamide (DMF) is fed to the top of the column at a weight to feed ratio of 4.5-6: 1.

The top product of the column containing 75-80% wt. benzene and a component of 9.6 - 12% wt. from the source, sent to the polymerization unit 2, in which at a temperature of 150-170 ^o C and in the presence of 0.001-0.0001% wt. sulfonic acids carry out oligomerization of unsaturated hydrocarbons.

In this case, unsaturated compounds form mainly dimers, co-dimers, trimers, the boiling point of the bottoms product is 210-240 ^° C. The oligomerizate is sent to distillation column 3 to recover purified benzene, which is combined with benzene isolated from the bottoms product.

 The bottoms product of the extractive distillation column 1 is sent to stripper 4, from which dimethylformamide is recycled to column 1, and the head fraction to the thiophene purification unit.

 Thiophene is purified by the formaldehyde method [L.Ya. Kolyandr. Obtaining pure benzene for synthesis. M., Metallurgy, 1966, p. 66-67].

The benzene purified from thiophene is combined with the benzene of the column 1 top product, purified from unsaturated hydrocarbons. Benzene is obtained with a basic substance content of at least 99.90% by weight, with a crystallization temperature of at least 5.45 ^° C, a density of 0.8778 g / cm ³ at 20 ^° C, and a sulfuric acid stain of less than 0.1 (GOST 2706.3 -74).

 Benzene corresponds to the brand "purified" in accordance with GOST 9572-77 "Petroleum benzene."

 The yield of benzene 95-95.6% wt. calculated on the original product or 97.5 - 97.8% wt. calculated on the benzene contained therein. In this case, the main losses of benzene occur at the stage of chemical purification of the bottoms product from thiophene (more than 2.0% wt.).

 The following are examples of the practical implementation of the purification of benzene according to the claimed method.

 Example 1

10000 g of benzene containing 2.09% wt. non-aromatic unsaturated hydrocarbons, 97.84% wt. of the main substance, sent to the middle part of the extractive distillation column 1 (Fig. 1), dimethylformamide heated to 90 ^o C. is fed to the upper part of the column. Efficiency of the column is 140 practical plates (etc.), reflux ratio (ph.) -2, the temperature of the top of the column (t _top ^o ) -80 ^o C, the temperature of the cube (t _cube ^o ) -160 ^o C. The mass ratio of dimethylformamide / benzene is 4.5: 1. The top product of the column, comprising 9.62% wt. . from the source, sent to the polymerizer 2. The benzene content in the upper product is 77, 75% wt. Oligomerization is carried out at a temperature of 150 ^o C in the presence of 0.00096 g (0.0001% wt.) Of sulfonic acid, pre-mixed with 300 g of benzene, purified from impurities of unsaturated hydrocarbons (for uniform distribution of sulfonic acid in the charge of the polymerization unit).

 Unsaturated non-aromatic hydrocarbons contained in the top product of column 1, in this case, quantitatively form compounds representing mainly dimers, codimers and trimers of diene hydrocarbons with a small amount (about 1.0-1.5%) of higher molecular masses.

 The oligomerization time is 30 minutes with a stirring intensity of 900 rpm.

 The oligomerizate is sent to a distillation column 3 to distill off benzene, which is combined with benzene isolated from the bottoms product of column 1. From the bottoms product of column 1, benzene is first isolated by the upper product of stripper 4, then it is sent to the thiophene 5 purification unit.

 Dimethylformamide recovered by the bottoms product of the stripper is recycled to column 1.

Operation mode of the stripper: 40 ave., Fl.ch. - 1.0, t _top ^o -80.2 ^o C, pressure 0.8-1 atm, temperature on the 5th plate (from the bottom of the column) - 150 ^o C.

The thiophene stripping head fraction is purified by the formaldehyde method in the presence of 178.5 g of 28% formalin (0.52% in terms of 100%), 1048.0 g of 94% sulfuric acid at a temperature of 80 ^o C, mixing speeds of 900 rpm for 40 minutes, followed by sediment until the reagent is drained for 45 minutes.

 Purified thiophene-free benzene is combined with purified benzene oligomerization of the top product of extractive distillation column 1.

The content of the main substance in the purified product is 99.94% wt., Saturated hydrocarbons 0.06% wt., Unsaturated non-aromatic hydrocarbons, including cyclodiene, are absent. The crystallization temperature is 5.46 ^o C, density 0.8787 g / cm ³ at 20 ^o C, staining with sulfuric acid in the numbers of the standard scale is less than 0.05 (GOST 2706.3-74).

 The yield of benzene is 95.51% wt. in terms of the original product or 97.58% wt. calculated on the benzene contained therein.

 The content of components in the starting benzene and at all stages of purification are shown in table 1.

 Refined benzene in terms of performance corresponds to the brand "refined" according to GOST 9572-77.

 Example 2

 10000 g of benzene containing 2.69% wt. non-aromatic unsaturated hydrocarbons, 97.21% wt. the main substance is sent to the extractive distillation column 1 (Fig. 1), operating in the mode of example 1.

Dimethylformamide heated to 90 ^° C. is fed to the top of the column at a weight ratio of 6.0: 1.0 to charge. The top product of the column, comprising 12.11% wt. from supplied for cleaning, sent to the polymerization device 2.

 The benzene content in the upper product is 77.37% wt.

Oligomerization is carried out at 170 ^o C in the presence of 0.012 g (0.001% wt.) Benzenesulfonic acid, pre-mixed with 300 g of purified benzene. The oligomerization time is 30 minutes, the stirring speed is 900 rpm. Oligomerizate is fed to distillation column 3, the top product of which is distilled off pure benzene, which is then combined with purified benzene bottoms product of column 1.

 From the bottoms product of column 1, benzene is isolated by the upper product of stripper 4 operating in the mode of Example 1.

 The upper product of the stripper is sent to the thiophene purification unit under the conditions of Example 1 in the presence of 165 g of 28% formalin and 95.7 g of 94% sulfuric acid. Purified benzene containing 0.0001% wt. thiophene, combined with benzene, purified by oligomerization from unsaturated hydrocarbons.

Get benzene containing 99.90% wt. the main substance, not containing unsaturated non-aromatic hydrocarbons, having a crystallization temperature of 5.45 ^o C, a density of 0.8787 g / cm ³ at 20 ^o C, the color of sulfuric acid is less than 0.10.

 The yield of benzene 95.27% wt. calculated on the original product or 97.91% wt. on benzene contained in it.

 Refined benzene in terms of performance corresponds to the brand "refined" according to GOST 9572-77.

 The content of the components in the starting benzene and at all stages of purification are shown in table 2.

 Example 3

10000 g of benzene of the composition of example 1 is subjected to purification in an extractive distillation column 1 (Fig. 1), operating in the mode of example 1. The ratio of dimethylformamide / purified benzene 5.5: 1.0. The top product of column 1, comprising 9.98% wt. from the source, sent to the polymerizer 2. The benzene content in the upper product is 78.46% wt. Oligomerization is carried out at 160 ^° C. in the presence of 0.006 g (0.0006% by weight) sulfonic acid under the conditions of Example 1.

 The oligomerizate is sent to distillation column 3 to isolate purified benzene, which is combined with benzene isolated from the bottoms product of column 1.

 The bottoms product of column 1 is sent to a stripper, in which, under the conditions of Example 1, benzene is isolated, which is then sent for purification from thiophene.

Purification from thiophene is carried out under the conditions of example 1 in the presence of 163 g of 28% formalin and 96 g of 94% sulfuric acid. Purified thiophene-free benzene is combined with purified benzene of the column 1 top product. Benzene is obtained containing 99.94% wt. the main substance that does not contain unsaturated hydrocarbons and thiophene, having a crystallization temperature of 5.46 ^o C, a density of 0.8778 g / cm ³ at 20 ^o C, the color of sulfuric acid is less than 0.10.

 The yield of benzene 99.57% wt. from the original product or 97.70% wt. on benzene contained in it.

 The content of components in the starting benzene and at all stages of purification are shown in table 3.

 Thus, the inventive purification method provides a deep purification of benzene obtained from liquid pyrolysis fractions of EPP from impurities unsaturated, including cyclodiene hydrocarbons. The yield of benzene in this case is 97.6 - 97.9% wt. calculated on the benzene contained in the feedstock.

 From known cleaning methods, these hydrocarbons are separated by methods including adsorption processes or catalytic hydrotreatment. These methods are unprofitable in the case of benzene of the highest purification grades in small-tonnage, semi-industrial plants, especially if they do not have available sources of hydrogen in the case of hydrotreating.

 The inventive method allows for a simpler and more economical technology that requires significantly less investment and energy consumption than catalytic hydrotreatment, and with a more efficient cleaning process and less waste technology than adsorption treatment, which is important in the volume of industrial production of benzene, to obtain high-purity benzene, containing at least 99.90% wt. the main substance.

 The separation of the purified benzene in the extractive distillation column into two streams allows for deep purification of thiophene by known methods with relatively small losses of benzene, since there are no unsaturated compounds capable of gum formation or polymerization.

 Since the selectivity of dimethylformamide with respect to the mixture of cyclodiene hydrocarbons - benzene is low, when they are removed from benzene with the head fraction, the benzene / cyclodiene hydrocarbons ratio increases to 3.5-3.75: 1 compared with its value of 0.40-0.50 : 1 during the allocation of saturated hydrocarbons. That is why the problem arises of extracting benzene from a mixture with cyclodiene hydrocarbons and returning it to the target product. Carrying out the oligomerization of cyclodiene hydrocarbons followed by distillation of benzene from oligomers allows you to return to the final product of 7.6 -9.6% wt. benzene based on benzene contained in the original product, that is, significantly reduce the loss of benzene in the process of purification by extractive distillation.

The oligomerization reaction of olefin and diene non-aromatic hydrocarbons isolated from benzene under the action of sulfonic acid at a temperature of 150-170 ^o C proceeds according to a mechanism that combines the mechanism of cationic and thermal polymerization with a significant prevalence of the cationic polymerization mechanism. The resulting oligomeric structures include both cyclodiene and olefin hydrocarbons. The relatively low boiling point of the resulting mixture of oligomers (210 - 240 ^o C) confirms the practical absence of higher molecular masses in it than dimers, trimers.

 Olefin unsaturated hydrocarbons also enter into oligomerization reactions with diene hydrocarbons, so they can not be removed from the purified benzene before it is sent for purification by extractive distillation.

 However, they are completely removed from benzene when it is purified from saturated non-aromatic hydrocarbons by extractive distillation with methanol or acetone. It should be noted that oligomerization under the claimed conditions proceeds in the absence of olefinic hydrocarbons at a higher rate, which is important for a continuous purification process.

When the oligomerization temperature is below 150 ^o C, the polymerization rate slows down, its time is significantly increased.

At oligomerization temperatures above 170 ^o C, partially crosslinked oligomeric structures are formed, the boiling point of the oligomer mixture rises to 310-320 ^o C, the bottom residue after distillation of benzene acquires a darker color and higher viscosity, its transportation is difficult, especially at low temperatures.

 With the introduction of sulfonic acid in an amount less than 0.0001% wt. based on benzene removed by the head fraction of column 1, part of the unsaturated hydrocarbons does not enter into oligomerization reactions, benzene distilled from oligomers contains 15.0-20.0% wt. them from the original quantity. A slight excess of the amount of sulfonic acid in excess of 0.001% wt. it is impractical, since this does not affect the rate and completeness of polymerization of unsaturated hydrocarbons, with a significant excess of the amount of sulfonic acid, the polymerization rate increases, along with linear ones, crosslinked structures are formed, including diene, olefin hydrocarbons and benzene, that is, part of the benzene is lost with oligomers.

 1-3 dimethylformamide used in practical examples corresponds to TU 6-09-3720-74, technical sulfuric acid - GOST 2184-77, formaldehyde - GOST 1625-89.2

Claims (1)

The method of purification of benzene from unsaturated hydrocarbons, characterized in that unsaturated hydrocarbons are separated from benzene by the top product during extractive distillation with dimethylformamide, then they are subjected to oligomerization at 150 - 170 ^o C in the presence of 0.0001 - 0.001 wt.% Benzenesulfonic acid based on benzene with the subsequent combination of benzene distilled from oligomers with benzene isolated from the bottom product of the extractive distillation column and purified from thiophene by known methods.

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