RU2164907C1 - Method of purifying benzene - Google Patents

Abstract

FIELD: organic chemistry. SUBSTANCE: method comprises removing coal-tar chemical industry benzene and benzene resulting from fractions of pyrolysis of petroleum products, from impurities of saturated and unsaturated hydrocarbons, thiophene and carbon disulfide. Benzene is removed from saturated, unsaturated nonaromatic hydrocarbons, thiophene and carbon disulfide by extractive rectification using mixture of dimethylformamide and toluene as extracting agent at weight ratio of dimethylformamide and toluene as extracting agent at weight ratio of dimethylformamide, 90-95 and toluene, 5-10. EFFECT: more efficient purification method. 4 ex, 4 tbl, 1 dwg

Description

 The invention relates to the petrochemical industry, to the purification of benzene obtained by coking from coal, and benzene obtained from liquid fractions of the pyrolysis of petroleum products.

 In a number of petrochemical plants using benzene as a raw material for synthesis, its purity requirements are very high. For example, in the production of cyclohexane, the starting material for the production of caprolactam, adipic acid and hexamethylene diamine, all impurities of saturated, unsaturated and sulfur-containing compounds are harmful, since they pollute the resulting cyclohexane.

 Nickel catalysts used in the hydrogenation of benzene to cyclohexane are very sensitive to impurities of sulfur-containing and unsaturated hydrocarbons, since the former are adsorbed, the latter polymerize, closing the active catalytic centers.

 Upon receipt of benzenesulfonic acid by sulfonation of benzene in the vapor phase, all sulfur-containing impurities, saturated and unsaturated hydrocarbons adversely affect the process, as they cause deoxidation of sulfuric acid and resinous sulfomass.

 Similar circumstances in the above and other petrochemical syntheses using benzene make it necessary to limit the content of saturated, unsaturated hydrocarbons, thiophene, and carbon disulfide in it.

Despite the fact that benzene coke undergoes a double stage of distillation purification from impurities of hydrocarbons and carbon disulfide, the resulting product, benzene brand "for nitration", contains up to 0.30% wt. impurities of non-aromatic hydrocarbons (marginal and unsaturated), thiophene - up to 0.04% wt. carbon disulfide - up to 0.005% wt. , its crystallization temperature is 5.20 ^o C, the sulfuric acid stain is up to 0.15 (GOST 8448-78), which indicates the presence of unsaturated compounds in it.

 Another source of benzene for petrochemical syntheses is benzene, obtained from the liquid fractions of the pyrolysis of petroleum products by isolating two-stage distillation from the bulk of the limiting, unsaturated and part of sulfur-containing impurities, contains at least 94.0% wt. the main substance, from sulfur-containing impurities - only thiophene up to 0.01% wt., up to 0.1-0.12% wt. toluene, the rest are impurities of non-aromatic saturated and unsaturated hydrocarbons.

 In order to obtain benzene of the brand “for synthesis” from the above-mentioned raw materials, corresponding to GOST 8448-78 and GOST 9572-77 for benzene of coke and oil production, additional cleaning methods are required, since the less impurities in benzene, the more problematic their separation.

 The complex composition and formation of azeotropic mixtures of non-aromatic hydrocarbon impurities with benzene make high purity benzene separation processes using clear and azeotropic distillation very energy-intensive, requiring highly efficient technological equipment.

A known method of purification of benzene obtained from pyrocondensate from impurities of non-aromatic unsaturated hydrocarbons and thiophene on natural adsorbents at a temperature of 150-250 ^o C, a pressure of 2-2.8 atm, followed by the isolation of benzene by distillation [A.D. Berents and other liquid Processing pyrolysis products. - M .: Chemistry, 1985, p. 216]. Purified benzene contains 99.90% wt. and above the main substance
The disadvantages of the method are the relatively short service life of adsorbents (up to 6 months), their high consumption per unit of raw material, the need for periodic disposal of spent adsorbents at landfills.

 A known method of purification of benzene obtained from petroleum feedstock or by coking coal containing up to 98% wt. the main substance on natural adsorbents from unsaturated non-aromatic compounds and then from sulfur compounds on catalysts based on metals of groups VI and VIII of the Periodic system [US patent N 3642927, C 07 C 7/00, 15/04, decl. 02/07/68, publ. 02/15/72). The method is not without the drawbacks of the previously described and additionally includes an energy-intensive stage of hydrotreating.

 A combination of adsorption and catalytic purification of benzene of pyrolysis origin is known, containing up to 0.0072% by weight. non-aromatic saturated and unsaturated hydrocarbons and 0.002% wt. thiophene, in a one-step process in hydrogenation plants using specially designed catalysts [V.A.Dvinin et al. Fine purification of the benzene fraction of pyrocondensate from olefins and thiophene. Oil refining and petrochemicals. 1988, N 9, p. 19-21].

 The method allows one to isolate benzene from the purification products by subsequent distillation that meets the requirements of GOST 9572-77 for “higher purity” brand benzene — the content of the main substance is 99.90% wt., Total sulfur — 0.00005% wt.

 The disadvantages of the method are restrictions on the content of thiophene in the starting benzene at the level of 0.002% wt., Non-aromatic hydrocarbons - 0.01% wt. and problems associated with the use of adsorbents, in addition, the degree of conversion of unsaturated is 84.3-93.7% wt.

 The use of chemical bonding techniques for impurities present in benzene, for example carbon disulfide, with alkali in methyl alcohol [V.E. Privalov. Coke and chemistry. 1961, N 3, p. 361], thiophene, unsaturated compounds - sulfuric acid [A.P. Kolesov, V.Z. Sokolov. Coke and chemistry. 1962, N 5, p. 41], thiophene - chlorine, formaldehyde, hypochlorite - is associated with the use of large quantities of reagents, with the problems of disposal of condensation products of impurities with the reagent, and the loss of the target product [L.Ya. Kololyndr. Getting pure benzene. - M.: Metallurgy, 1966, p. 64-69].

 Liquid extraction is used to separate benzene from hydrocarbon impurities, then benzene is then removed from the solvent by distillation.

As extractants use glycols, dimethyl sulfoxide, sulfolane [A. A. Miroshnichenko et al. Coke and chemistry. 1974, N 8, p. 35-37], N-methylpyrrolidone mixed with ethylene glycol and water [A.A. Miroshnichenko et al. Coke and chemistry. 1958, N 3, p. 61]. Get benzene with a basic substance content of 99.90% wt. and higher, with a crystallization temperature of 5.40 ^° C. and higher.

 The disadvantages of the method are the need to process a large amount of product, increased losses of benzene. Therefore, extractive purification is used to remove the bulk of impurities from benzene, followed by purification from them by other methods — extractive distillation, adsorption, crystallization, and chemical bonding.

 The method of extractive distillation is used to achieve deep purification of benzene, for this the method requires the preliminary separation of most of the impurities.

 A known method of purification of benzene from impurities of non-aromatic hydrocarbons by extractive distillation using a binary solvent - a mixture of N-methylpyrrolidone and water with a mass ratio of 3: 1 [V.Z. Sokolov, GD Kharlampovich. Production and use of aromatic hydrocarbons. - M.: Chemistry, 1980, p. 240-241].

 The ratio of extractant / raw material is 4.5: 1 (wt.). As a raw material, a benzene fraction of a medium-temperature hydrotreating raffinate containing 0.73% wt. non-aromatic saturated hydrocarbons. After distillation separation of the bottoms product, benzene containing 99.97% of the basic substance is isolated.

 However, the method is aimed only at the purification of saturated hydrocarbons, since the raw materials are previously subjected to hydrotreatment.

 It is known that along with saturated hydrocarbons during the extractive distillation of benzene using N-pyrrolidone as a solvent, thiophene and carbon disulfide are removed [A.A. Miroshnichenko et al. Coke and chemistry. 1975, N 8, p. 21-25. A.A. Miroshnichenko et al. Coke and chemistry. 1975, N 10, p. 35-40].

 Carbon disulfide from coke-benzene brand "for nitration" is completely eliminated due to the formation of condensation products with amines formed during partial hydrolysis of N-methylpyrrolidone. The residual thiophene content, however, is significant, not reaching the level required for benzene brand "for synthesis".

 Closest to the technical nature of the claimed is a method for purifying benzene by extractive distillation using dimethylformamide as an extractant [French patent 2224425, cl. C 07 C 7/02, claimed 04/06/73, publ. 10.31.74].

As raw materials, benzene of catalytic rifming of gasoline fractions containing 70% of the basic substance and 30% of non-aromatic hydrocarbons C ₆ -C ₈ with boiling limits of 65-102 ^o C is used. Purification is carried out in a 5 cm diameter column with 60 practical Aldershaw plates, top temperature columns - 80 ^o C, cubic - 160 ^o C. On the 25th plate from the bottom serves a mixture of raw materials with dimethylformamide, on the 55th plate serves dimethylformamide. The ratio of dimethylformamide (DMF) entering the column and the feed is 4.5-5.5: 1 (mass). Distillate containing up to 98% wt. saturated hydrocarbons and 2% wt. DMF, together with vapors condensing at the outlet of the column, is supplied for irrigation.

 VAT product containing 78.1% wt. DMF and 21.0% wt. benzene, sent to the allocation of purified benzene and the regeneration of DMF in 2 columns with resin XE 284 type K.

 Purified benzene contains 99.99% wt. and above the basic substance and less than 0.01% wt. non-aromatic saturated hydrocarbons.

 The method relates to the purification of benzene from saturated non-aromatic hydrocarbons. The initial benzene, obviously, in the process of catalytic reforming passed the stage of purification (adsorption or catalytic hydrogenation) from thiophene and unsaturated hydrocarbons, otherwise, their presence in the feedstock is inevitable.

 Since the selectivity of dimethylformamide with respect to the thiophene-benzene mixture is quite high - 1.93-1.50 at a solvent concentration of 98-80 mol.% [A. from. USSR 245134, cl. C 07 D 63/18, publ. 06/04/1969], then with appropriate extractive distillation modes with dimethylformamide, thiophene can be separated from benzene, which is confirmed by the practical experience of the authors of this application. However, its residual content exceeds that required for “for synthesis” grade benzene according to GOST 9572-77 and GOST 8448-78 for petroleum and coke chemical benzene, respectively. As for the purification of benzene from unsaturated compounds by extractive distillation with dimethylformamide as a solvent, in practice their complete separation from benzene is achieved by removing the head fraction with saturated hydrocarbons. The content of benzene in the head fraction increases. If in the separation of saturated non-aromatic hydrocarbons during extractive distillation with DMF, the benzene / saturated hydrocarbons ratio is 0.40-0.45: 1, then in the case of the allocation of unsaturated hydrocarbons this ratio increases to 3.5-3.8: 1.

 During the extractive purification of benzene with dimethylformamide as a solvent, carbon disulphide is removed quite easily, its residual content does not exceed 0.00005% by weight, which meets the requirements for benzene grades "for synthesis". The removal of carbon disulfide occurs due to its chemical interaction with dimethylamine, which is invariably associated with DMF as an impurity and additionally formed from DMF due to moisture present in DMF and raw materials.

 Thus, the disadvantages of the method for purifying benzene from impurities of saturated and unsaturated hydrocarbons, thiophene and carbon disulfide by extractive distillation using DMF as a solvent are the insufficient degree of purification of benzene from thiophene and the increased loss of benzene in the head fraction during the evolution of saturated and unsaturated hydrocarbons.

 The aim of the invention is to improve the quality of benzene purification, to reduce benzene losses during purification.

This goal is achieved by the fact that the purification of benzene from impurities of non-aromatic saturated and unsaturated hydrocarbons, thiophene and carbon disulfide is carried out by extractive distillation using a binary solvent as a extractant - a mixture of toluene and dimethylformamide (DMF) with a ratio of components (% wt.):
toluene - 5-10
DMF - 90-95
The ratio (wt.) Extractant / raw material is 4.5-6: 1.

As raw materials, benzene coke-chemical brands “for nitration” and benzene isolated by distillation distillation of “heavy” and “light” hydrocarbons from liquid hydrocarbon C ₆ -C ₇ pyrolysis fractions of ethylene-propylene production corresponding to TU 33.402-62-144-93 are used.

 Impurities of non-aromatic saturated and unsaturated hydrocarbons during extractive rectification with dimethylformamide are removed from benzene by the upper product in a mixture with benzene. The head fraction is 9.80-10.00% wt. from the feedstock, the benzene content in it is 51-53% wt. Thiophene is excreted in a cubic product with DMF and toluene. With a continuous cleaning process, the bottoms product is periodically cleaned from it by distillation. The purified benzene removed from the column contains 0.00005% wt. carbon disulfide, the decomposition products of which are removed by filtration on the output line of the bottom product.

 Part of the toluene contained in the starting benzene is also excreted by the bottoms product, therefore, as it accumulates in DMF in an amount of more than 10% wt. the extractant is subjected to "regeneration" - distillation distillation from excess toluene.

 Purified from impurities of non-aromatic saturated and unsaturated hydrocarbons, thiophene and carbon disulfide, benzene contains 99.99% wt. the main substance and according to indicators corresponds to the brand "purified" in accordance with GOST 9572-77 "Petroleum benzene", and coke chemical benzene - to the brand "for synthesis" of the highest grade according to GOST 8448-78.

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

Raw materials are fed into the extractive rectification column with an efficiency of 140-160 practical plates (valve) to the 70th plate (counting from the bottom of the column), an extractant is fed to the top of the column at a temperature of 90 ^o C - a mixture of dimethylformamide (DMF) and toluene, toluene content 5 -10% wt.

 The ratio of extractant / raw material (wt.) Is 4.5-6: 1.

The operation mode of the column: reflux number (fl.h.) 2, top temperature (t ^o _top ) 80 ^o C at 1 ata, cube temperature (t ^o _cube ) 125-130 ^o C.

 From the 40th plate, purified benzene is removed in the vapor phase.

 The head fraction contains non-aromatic saturated and unsaturated hydrocarbons, benzene.

 The extractant is recycled from the bottom of column 1 or through stripper 2 to the top of the extractive distillation column.

 The head fraction of the stripper is used as a fuel component or it is separated into thiophene and benzene in distillation column 3, directing the recovered benzene by recycling to column 1.

 In a continuous process, the extractant is purified from thiophene in an adsorber either continuously or periodically, as thiophene accumulates up to 25-30 kg per ton of extractant (after about 200 or more revolutions of the extractant).

 As accumulation of toluene in the extractant (over 10% by weight), an excess of toluene is isolated in distillation column 4.

 The following are examples of benzene purification by the present method and using an extractant by a known method.

 Example 1 (using a known extractant).

 1000 g of benzene isolated from the liquid pyrolysis fractions of petroleum products corresponding to TU 38.402-62-144-93, with two-stage clear distillation to remove the main amount of impurities of saturated and unsaturated hydrocarbons, are sent to the 70th plate of extractive distillation column 1 (drawing) with an efficiency of 150 practical plates (etc.). The content of the main substance in benzene is 95.15% wt., Saturated hydrocarbons 2.55% wt., Unsaturated 2.23% wt., Toluene 0.06% wt., Thiophene 0.01% wt. (Table 1).

The column works in the mode: fl. h. 2, t ^o _top 80 ^o C at 1 ata, t ^o _cube 130 ^o C.

In the upper part of the column, dimethylformamide heated to 90 ^° C is fed to the 140th plate. The mass ratio of extractant / raw material is 5: 1.

The purified benzene removed from the 40th plate has the composition (% wt.):
benzene - 98.630
saturated non-aromatic hydrocarbons - 0,006
toluene - 0.046
thiophene - 0.002
The yield of benzene 86.80% wt.

 The head fraction is 14.10% wt. the content of benzene in it is 66.18% wt.

 The compositions of the starting benzene and fractions during extractive distillation are shown in table 1.

 Example 2

 1000 g of benzene composition according to example 1 is fed into the extractive distillation column 1 (drawing), operating in the mode of example 1.

In the upper part of the column 1 (on the 140th plate) at 90 ^o C serves extractant composition (% wt.):
DMF - 95.0
toluene - 5.0
The ratio of extractant / raw material is 5.0: 1 (wt.).

From the 40th plate of the column in the vapor phase, purified benzene of the composition (% wt.) Is removed:
benzene - 99,990
toluene - 0.002
n-heptane - 0.002
methylcyclohexane - 0.003
thiophene - 0.0001
The crystallization temperature of 5.52 ^o C.

 Staining with sulfuric acid less than 0.05 (GOST 2706-74).

 The yield of benzene 90.07% wt.

 The head fraction is 9.81% wt. from the feedstock, the benzene content in it is 51.37% wt.

 VAT product containing extractant, 0.002% wt. thiophene, 0.009% wt. benzene recycle through desorber 2 sent to the column extractive distillation 1.

The desorber works in the following mode: pr. 40; fl.h. 1.0; t ^o _top 80.2 ^o C, pressure 0.7-1 atm, temperature on the 5th plate (from the bottom) 140 ^o C.

 The head fraction of the stripper is sent to distillation column 3, from the top of which the isolated benzene is recycled to column 1 with a feed stream.

 The compositions of the initial benzene and fractions during extractive distillation are shown in table 2.

 Example 3

 1000 g of benzene by-product coke production brand "for nitration" containing 99.69% wt. basic substance, 0.015% wt. thiophene, 0.003% wt. carbon disulfide, the rest is non-aromatic saturated and unsaturated hydrocarbons, sent to the extractive distillation column 1 (drawing) on the 70th plate.

The effectiveness of the column 140 pr. T., the operating mode: fl.ch. 2.0; t ^o _top 80 ^o C at 1 ata, t ^o _cubic 125 ^o C.

In the upper part of the column on the 130th plate at 90 ^o C serves a solvent - a mixture of DMF and toluene composition (% wt.):
DMF - 90.0
toluene - 10.0
The ratio of extractant / raw material (mass) is 6: 1.

From the 40th plate of the column, the purified benzene composition (% wt.) Is removed in the vapor phase:
benzene - 99,990
toluene - 0.003
n-heptane - 0.002
cyclohexane - 0,062
methylcyclohexadiene - 0.002
carbon disulphide - 0.00005
thiophene
0.00018
The crystallization temperature of 5.50 ^o C.

 Staining with sulfuric acid less than 0.1.

 The yield of benzene 99.44% wt.

 The yield of the head fraction is 4.21% wt., The benzene content in it is 50.95% wt.

 The compositions of the starting benzene and fractions during purification are shown in table 3.

 The bottoms product of column 1 is sent for regeneration purification to column 4 for distillation of excess toluene, from which the bottoms product enters the extractant supply line, the toluene content in the extractant when fed to column 1 is at least 5.0 wt%.

 Example 4

 1000 g of benzene by-product coke production brand "for nitration" containing 99.78% wt. benzene, sent to the 70th plate of the column of extractive distillation 1 efficiency 160 pr. The column operates in the mode of example 1.

In the upper part of the column on the 150th plate serves at 90 ^o C extractant composition (% wt.):
DMF - 92.50
toluene - 7.50
The ratio of extractant to raw material is 4.5: 1 (wt.).

From the 40th plate of the column, purified benzene is removed in the vapor phase of the composition (% wt.):
benzene - 99,993
toluene - 0.002
n-heptane - 0.002
cyclohexane - 0.002
methylcyclohexene - traces
carbon disulphide - 0.00005
thiophene - 0.0001
The crystallization temperature of 5.52 ^o C.

 The yield of benzene 99.54% wt.

 The head fraction is 3.92% wt. the content of benzene in it is 52.67% wt.

 The extractant is recycled to the top of column 1.

 The cleaning results are shown in table 4.

 Analysis of the results of benzene purification using the inventive extractant and the known one shows the advantage of the former with respect to the best degree of purification from thiophene (its residual content in purified benzene is 0.00010-0.00018% by weight, compared with 0.002% by weight using the known extractant) .

 According to the claimed method of purification of 4.52% wt. (based on the starting benzene) benzene losses in the head fraction are reduced with a sufficiently complete release of all non-aromatic hydrocarbons. The benzene content in the bottoms product is approximately 4 times less than when using the known extractant. The residual content of carbon disulfide in coke benzene is 0.00005% wt. Apparently, the removal of carbon disulfide occurs due to its reactions with dimethylamine, which is formed under the action of moisture and is present in DMF.

 So, TU 6-09-3720-74 on dimethylformamide provides a moisture content of up to 0.07% wt., Dimethylamine - up to 0.02% wt.

 When testing the proposed method for cleaning in a semi-industrial installation, the extractant is periodically (after 200-250 revolutions) subjected to regeneration - distillation purification from thiophene and toluene, while no corrosive products of DMF decay were detected in it.

 The toluene contained in the extractant and in the feed does not reduce the relative volatility coefficients of the benzene-n-heptane, benzene-methylcyclohexane systems, as is the case when using known extractants, for example N-methylpyrrolidone [L. Ya. Kolyandr. New ways to process crude benzene. - M.: Metallurgy, 1976, p. 104].

 When the content of toluene in the extractant in an amount of less than 5.0 wt.%, The loss of benzene in the head fraction and still product and the thiophene content in purified benzene increase. When the content of toluene in the extractant in an amount of more than 10% wt. the quality of purification from non-aromatic saturated and unsaturated hydrocarbons is deteriorating.

 When the extractant is supplied with a mass ratio of less than 4.5: 1 raw materials, the required quality of benzene purification from impurities is not achieved, with a ratio greater than 6: 1, benzene losses in the bottoms product increase and energy consumption increases unjustifiably from the point of view of the achieved improvement.

 Modes of operation of the column provide the required completeness of separation of impurities in the overhead stream and minimal losses of benzene in the bottoms product and in the head fraction.

 Thus, the advantage of the proposed method for purification of benzene is carried out in the possibility of simultaneous purification from non-aromatic and sulfur-containing impurities with lower losses of benzene in the head fraction and bottoms product using a technologically simple and less energy-intensive process compared, for example, with catalytic hydrotreating.

 Used in examples 1-4, dimethylformamide corresponds to TU 6-09-3720-74, toluene - GOST 14710-78.

Claims (1)

The method of purification of benzene from impurities of non-aromatic saturated and unsaturated hydrocarbons, thiophene and carbon disulfide by extractive distillation using dimethylformamide as a solvent, characterized in that a solvent additionally containing toluene is used in the following ratio, wt.%:
Dimethylformamide - 90.0 - 95.0
Toluene - 5.0 - 10.0

Images