DE2647757A1 - PROCESS FOR SEPARATION OF AROMATIC HYDROCARBONS BY EXTRACTIVE DISTILLATION - Google Patents

Description

The present invention relates to a method of separation aromatic hydrocarbons, in particular of benzene and / or toluene, from mixtures with more saturated hydrocarbons by means of an extractive distillation in the presence of a polar compound, in particular dimethylformamide or Dimethylacetamide, characterized in that the distillate to remove traces of the polar compound with silica gel brings into contact, the final regeneration of which is effected by contact with at least one hydrocarbon,

The extraction of aromatics by means of an extractive distillation in the presence of a polar compound is a known procedure; in particular compounds from the family of aliphatic

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see alkylamides ("especially the first members of this family: Dimethylformamide and dimethyl acetamide) have proven to be "special proven suitable for cleaning benzene and / or toluene. Their low boiling point allows you to be at relatively works at low temperatures, which are favorable for the effectiveness of the separation, since the selectivity of a solvent is rapidly decreased as the temperature rises.

On the other hand, they also have the property of forming azeotropes with certain more saturated hydrocarbons of 6, 7 and 8 carbon atoms, and their use in conventional extractive distillation for the purification of benzene and / or toluene is partially neutralized in performance by the inevitable loss of solvent which is dragged along at the top of the column by these more saturated hydrocarbons.

The present process is suitable for the treatment of, for example, benzene fractions and / or toluene fractions from reforming products or the effluents from steam cracking, v / which before were subjected to partial hydrogenation. In this way the impurities are practically on the more saturated ones Hydrocarbons are limited, with the loss of solvent as a result of the entrainment of the same in the distillate of the extractive Distillation can be avoided with an effective technique that is tedious and costly to simply perform Avoids manipulation.

Benzene fractions are a mixture of benzene and hydrocarbons understood, whose lower boiling point is at least about 65 C and whose upper boiling point is at most about 102 C. For example, it is a mixture of benzene and more saturated linear or cyclic hydrocarbons having essentially 6 to 8 carbon atoms. However

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the invention is applicable to benzene fractions containing lighter hydrocarbons.

Under toluene fraction is meant a mixture of toluene and more saturated hydrocarbons whose upper and lower boiling point (about 1O2 ° C) and about 120 ⁰ C between the final boiling point of the Beiizolfraktionen.

The new process has the advantage that it is efficient and economical to use dimethylformamide and Dimethylacetamide, because of their physical and physicochemical properties Properties "remarkable solvents ^ as enables industrial means for the separation of aromatic and non-aromatic hydrocarbons.

It has been found that the extractive distillation of benzene and / or toluene can be carried out under extremely effective conditions can, if n: to recover the polar compound entrained in the distillate by means of silica gel, which has the property of selectively retaining the polar molecules from the mixture with which they are brought into contact, and regenerable using any hydrocarbon solvent to be.

According to the present invention, pure benzene and / or toluene is obtained from a batch that contains benzene and / or toluene and at least contains a more saturated hydrocarbon, internally one this batch of extractive distillation in the presence of a polar compound, especially dimethylformamide or Diinethylacetamidj subject, being at the lower part of the column a! fraction is obtained which contains the greater part of the polar compound and the benzene and / or toluene, while a fraction is obtained at the upper part which contains the greater part of the more saturated hydrocarbon (s) and a smaller part Contains part of the polar compound.

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By "more saturated hydrocarbons" is meant one non-aromatic hydrocarbon, i.e. an aliphatic or cycloaliphatic, saturated or unsaturated, straight-chain or branched hydrocarbon, e.g., a paraffinic and / or olefinic hydrocarbon.

The lower fraction is distilled: this is practically obtained at the top pure benzene and / or toluene and below the polar solvent that is circulated in the extractive distillation returns. The upper fraction of the extractive distillation is brought into contact with silica gel, which the polar compound adsorbed; the process is characterized in that the silicon dioxide gel is then regenerated, irvdem you do it with at least part of the batch that is then subjected to extractive distillation, brings into contact and the silica gel is then reabsorbed the polar compound of the upper fraction of the extractive distillation is used.

The process works without the addition of external products, their removal inevitably leads to additional investments and sometimes to non-negligible losses of the desired Products leads.

The silica gel is a solid adsorbent that can selectively and quickly retain polar molecules. It is very often used in industry and laboratories for drying, separating and cleaning liquids, air and other gases are used. It is readily available on the market and inexpensive.

When it is used in the process according to the invention, it has been found that a particularly rapid fixation and very easy desorption of polar molecules with granular

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Silica gel is obtained. A silicon dioxide gel is preferred, the particles of which have a size between 0.05 and 7 mm, in particular between 0.5 and 3 mm. The surface of this gel is usually between 400 and 800 m ² / g, mostly 750 m 2 / g.

During the stage of fixation of the polar molecules, the extractive distillate is passed through the silica gel Distillation, which contains the above-mentioned more saturated hydrocarbons, also the polar solvent and in in some cases a small amount of aromatic hydrocarbons - substances, so that an effluent is obtained which consists essentially of hydrocarbons.

The fixation of the polar molecules takes place quickly and throughputs of up to 10 to 50 parts by volume per volume of the bed per hour can be used effectively if, in order to achieve saturation of the silica gel, the amount of distillate and gel in a weight ratio between 1: 1 and 100: 1, generally between 2: 1 and 20: 1. However, the invention is not intended to be restricted to these quantitative ratios.

Under the conditions of use, it has been found that the adsorption capacity of the silica gel (g weight of polar compound / 100 g silica gel) is relatively large, up to 35 ° and more, usually between 5 and 30 % as the case may be , that is, depending on the granulometry of the silica gel or after the content of the distillate of the polar compound. Values close to $ 14 have often been observed during the course of the proceedings.

The benzene or toluene fraction may contain a small amount of water, less than 1% by weight, generally about 0.05 % by weight. "

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The capacity of the silicon dioxide gel with respect to polar solvents is not affected by the presence of the water which is brought into contact with it. The silica gel adsorbs - if it originally anhydrous v / ar - during the first regeneration cycle water to a level of several percent (approximately 4 to 5 ° / ° of its weight, v / as but not categorically); then it remains inactive with respect to the water that has been brought into contact with it;

In general, the distillate is brought into contact in the liquid state, after. one from the extractive distillation condensed vapors. One can compare comparable Results obtained with the distillate in the vapor state, but the capacity of the silica gel is then less.

The distillates resulting from such extractive distillations contain small amounts of the polar solvent, e.g. 0.1 up to 10% by weight, usually 1 to 6% by weight. A residual concentration is obtained of a few ppm by passing through silica.

Once you notice that the saturation level of the silica gel is reached, i.e. as soon as a polar compound content of more than the desired value is noticed in the effluent, the regeneration of the silica gel is carried out; i.e. at least a portion is passed through the silica gel the hydrocarbon fraction subjected to extractive distillation.

This step is carried out at a slightly elevated temperature, e.g. at the temperature at which the benzene or toluene fraction is available, i.e. between room temperature and the boiling point of the fraction; the silica gel can also be mixed with vapor or even slightly overheated hydrocarbons

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generate up to a temperature that diirch the economic Conditions of the procedure are limited.

The effluent obtained after this stage essentially contains those contained in the hydrocarbons used at the outset Hydrocarbons and the polar solvent; it is sent to the entrance of the column for extractive distillation, which presents a very important advantage of the process.

The invention is suitable for removing any other polar solvent commonly used for solvent separation of hydrocarbon compounds other than benzene and / or toluene. As an example are mentioned ET-methylpyrrolidone, lormylmorpholine, sulfolane, dimethylsulfoxide, ! Furfural and, in general, any polar solvent that is used as an extractant. Under "polar solvent" or "polar compound" means any organic liquid the carbon and at least 1 heteroatom such as 0, S, P and / or N contains.

The prerequisite for the continuous operation of the extractive distillation is the use of at least two with silicon dioxide G-el filled columns, each column operating alternately in the stage of fixation and regeneration.

In a preferred embodiment, three columns filled with Sill ciusadi oxy dG-el are used, two of which work in series in the fixation and one in the regeneration. In this way one can use the capacity of the silica gel to the maximum and ensure greater adaptability of the puncture.

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The scheme shown in FIG. 1 shows how the device can function without the invention being restricted thereto shall be.

The device consists of an extractive distillation column Cl, a distillation column C2 and two identical columns Rl and R2, which contain Siliciuiüdioxyd gel; Rl works in the fixation stage, while R2 is in the regeneration stage is located.

The mixture to be separated is introduced into column C1 through line 1, and dimethylformamide (DMF) through line 2. The fraction from the distillation head is passed through line 3 and condensed in exchanger 17; part is circulated as reflux through line k and another part, which exits through line 5, is sent into column Rl. A fraction is obtained through line 6 which consists of the non-aromatic hydrocarbons of the initial gas.

At the same time, through line 7, a fraction is obtained which essentially contains DMF and benzene and / or toluene; this is sent via line 7 into column C2, where it is separated into its components. After passing through the condenser 18, a distillate is obtained via the line 8, which is practically pure benzene and / or toluene (the reflux takes place via the line 9)? ^{At the} lower end of the column, DMF is obtained, which is returned to column C1 via line 2. Line Io is provided for the addition of DMF.

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As soon as you notice that the content of the line 6 obtained The polar solvent fraction has reached the acceptable limit, the column Rl must be regenerated. The valves (not shown) in the lines are then closed

5, 6, 12 and 16.

Then the distillate of the column C 1 is duroh the Line 11 passed into the column R 2, which is located on the Line 15 is located; this passed, until then the mixture to be treated, which had been introduced through line 12 was. The mixture is fed into the column through line 13 R 1 sent, which it leaves through line 14 after it has entrained the polar molecules from the gel with it; the mixture then combines with line 1 and is introduced into column C 1 without any disadvantages.

During this time, a DMF is obtained via line 15 freed distillate, which is identical to that obtained through line 6.

If the column R2 is working in the regeneration stage, contains the mixture to be treated, which is introduced into the column Cl through line 1, likewise DMF. The valves on lines 5 »

6, 12 and l6 are then open, those in lines 11 ", 13, I ^ and 15 closed.

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As shown in the figure, the "regeneration liquid is preferably left in R1 and R2 in the opposite direction to that of the distillate.

After a modification of the procedure, in particular "at Batches with a high aromatic content, one can get the first fraction from a bed of silica gel at the beginning of the Return the absorption cycle that follows a regeneration to the extractive distillation in the circuit. This faction namely contains a certain amount of aromatic carbons, originating from the batch used as the regenerant. The above recirculation allows the recovery this amount of aromatic carbons.

The systems of pumps, column heating, column head cooling, which are required for the functioning of the device are not shown in the scheme.

EeiSOiel 1

According to the scheme described above is subjected to a benzene fraction, v / hich 70 wt -.% Benzene and 30 i ° nonaromatic Kohlenv / asserstoffe (paraffins and naphthenes having 6, 7 and 8 carbon atoms, boiling point between about 65 and 102 C) contains an extractive distillation with dimethylformamide (DMF).

The column is of the Oldershaw type, contains 60 real trays, and has a diameter of 5 centimeters.

The mixture to be treated (line 1) is introduced from below at the 25th floor; the DMF is at the level of the 55th floor (line 2) introduced.

The column C1 vyird initially with an average amount of

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500 g / hour of the benzene fraction, then at 503 g / hour (of which 3 g are DMF) of the benzene fraction from the regeneration the columns R1 or R2 charged.

The amount of DMF is 1250 g / hour.

The condensed vapor at the top of the column is returned to the column under reflux, and zv / ar in an amount of 280 g / hour. 153 g / hour of a distillate is obtained (line 5), which is composed as follows:

- $ 97.95 weight non-aromatic hydrocarbons

- 1.98 "DMF

- 0.07 "benzene.

At the lower end of the column, a mixture is drawn off (line 7) which essentially consists of DMF and benzene, and in an amount of about 78.1% by weight or 21.9% by weight. The amount of entrained non-aromatic hydrocarbons is less than $ 0.01.

The columns R1 and R2 each contain 30 g of silicon dioxide gel with a granulometry of 0.5 to 1 mm; its temperature is kept at 60.degree.

If the device works continuously, then one wins via the lines 6 or 15 after the fixation stages non-aromatic hydrocarbons and traces of benzene from the distillate, which is almost completely derived from the DMF it contains is exempt. The DMF content is less than 20 ppm (parts by volume).

On the other hand, after the regeneration stage, the line

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line 14 or 16 is the benzene fraction, which is 0.61 wt .- $ 3DMF en. holds, sent as a feed to column 01.

The alternating puncture of columns R1 and R2 will open effected in the following way;

After an hour! Works in the fixation level of R1 "or a regeneration stage of R2, the role of the two columns with silica gel is reversed; while, d the The following hour, R2 works in the fixation stage, while R1 is in regeneration.

The mixture obtained at the lower end of the column (line 7) is introduced into the middle of the second distillation column; this is 2 m high, consists of Oldershaw elements and has 5 cm in diameter. Benzene is recovered at the top of this second column in an amount of 700 g / hour; the half of this is returned under reflux and the remainder consists of aromatics (line 8), their content of non-aromatic Hydrocarbons is no greater than 0.02% by weight. At the lower end of the column, the DMF is taken off, v / elches in the circuit the first column is returned.

Example 2

The procedure of Example 1 is repeated, but only half of the benzene fraction is used for the regeneration of the silica gel, while the other half is passed directly into the extractive distillation. The alternation of the fixation and regeneration cycles is effected every 55 minutes. The amount of the non-aromatic distillate is comparable to that according to Example 1, ie the DMF content is less than 20 ppm. The mean concentration of DMF in the benzene fraction after the regeneration of the silicon dioxide gel is 1.20% by weight.

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Example 3

The example 1 is repeated, but with a benzene fraction, which was previously saturated with water. The alternation period the cycle remains fixed at one hour and the total amount of the benzene fraction is used to regenerate the silica gel directed. After three days of implementation (essentially about an equilibrium saturation of the silica gel in water to achieve) is the DMF content of the non-aromatic distillate apparently always below 20 ppm, i.e. the capacity of the silicon dioxide gel has not been substantially changed by the presence of water in the benzene fraction.

Example 4

The procedure of Example 1 is repeated, except that previously the silicon dioxide oil is moistened so that its water concentration tration 5.6 wt. all other conditions remain same.

The results are similar to those of Example 1 if one keeps the alternation period of the cycles to one hour. This means that the initial wetting of the silica gel its capacity towards DMF does not affect.

Example 5

The procedure of Example 1 is repeated, but with the regeneration step of the silica gel with an evaporated

and benzene fraction superheated to 100 ° C is carried out; the

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The pixelation level is always effected at 60 ° C. The alternation of the fixation-regeneration cycles is carried out every hour. After passing through the silica gel, the benzene fraction is condensed and introduced into the extraction column. Their average DMF content is 0.6 wt .- ^ as in Example 1, The DMF content of the non-aromatic distillate is -

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less than 20 ppm after passage over the silica gel.

Example 6

According to the scheme described is subjected to a benzene fraction, 76 wt. -Fo benzene and 24 i ° non-aromatic hydrocarbons (paraffins and naphthenes with 6.7 and 8 carbon atoms), an extractive distillation with dimethylacetamide (DMAC).

The column is of the same type as that described in Example 1. The mixture to be treated is introduced at the level of the 25th floor from below in an amount of 500.6 g / hour (of which 0.6 g / hour DMAC comes from the regeneration of the silica gel); the DMAC is introduced at the 55th floor level at a rate of 1350 g / hour. The vapors condensed at the top of the column are returned to the column under reflux, in an amount of 248 g / hour; 124 g / hour of a distillate are obtained _; which is composed as follows:

- 96.78 -fo non-aromatic hydrocarbons wt.

- 0.48 wt. DMAC

- 2.74 wt .- $ benzene

At the lower end of the column, a mixture is withdrawn which consists essentially of DMAC and benzene, in an amount of about 78.2 and 21.8% by weight, respectively. The amount of entrained non-aromatic hydrocarbons is less than 0.05 f °.

After the alternating passage through columns R1 and R2, which each contain 30 g of silicon dioxide gel (granulometry

O exempt from .DMAC pt »

0.5 to 1 mm, temperature 60 C), you get the'nichi-aromatic Hydrocarbons and the benzene of the distillate.

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i *

After 4.5 hours of operation in the fixation stage at R1 "or the regeneration stage at R2 one turns the role of the two columns with silica gel around; during the following 4.5 hours R2 works in the fixation stage, while R1 works through the Batch stream is regenerated.

The mixture obtained at the lower end of the column (line 7) is treated as described in Example 1.

Example 7

The procedure of Example 1 is repeated with a benzene fraction containing 70 wt -.% Benzene, 28 wt .- ^ paraffinic and naphthenic hydrocarbons having 6 to 8 carbon atoms and 2 fo monoolefinic hydrocarbons having 6 to 7 carbon atoms includes.

The functioning of the process is not modified and monoolefinic hydrocarbons are recovered in the distillate an overall composition of

- $ 97.95 weight non-aromatic hydrocarbons

- 1.98 & ew .- # DMP

- 0.07 G-ew.-ji benzene

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Claims (10)

Claim, e Process for the treatment of a hydrocarbon fraction containing benzene and / or toluene and at least one more saturated Contains hydrocarbons, for the separation of the benzene and / or toluene by means of extractive distillation in the presence a polar solvent, with a fraction being withdrawn at the lower end of the column, which fraction is the larger Contains part of the polar solvent and the benzene and / or toluene, while a fraction at the top of the column removes which is the more saturated hydrocarbons and a smaller amount of the polar solvent contains, whereupon the fraction taken off below is fractionated, the benzene and / or toluene isolated and the polar Sends solvent back into the extractive distillation column, characterized in that the fraction brings the top of the column into contact with silicon dioxide gel, so that smaller amounts of the polar solvent contained in this top fraction are adsorbed subsequently separates the top fraction from the silica gel and from the Process peeled off, and periodically regenerated the silica gel during a desorption stage in which it was with at least a portion of the hydrocarbon charge brought into contact, which is then subjected to extractive distillation whereupon the regenerated silicon dioxide gel is brought into contact with a new top fraction to make the polar Adsorb solvent. 2. The method according to claim 1, characterized in that only a part of the hydrocarbon fraction, which then is subjected to extractive distillation, used to regenerate the silica gel, while a 709818/1106 ORIGINAL! NSFBCT'EO Claims the other part is sent directly to the extractive distillation. 3. The method according to claim 1, characterized in that one the entire hydrocarbon charge, which is then stored in the extractive distillation is used to regenerate the silica gel. 4. The method according to claims 1 Ms 3 »characterized in that that a silica gel is used, its particles Now have a granulometry between 0.05 and 7. 5. The method according to claims 1 Ms 3, characterized in that that a silica gel is used, the particles of which have a granulometry between 0.5 and 3 mm. 6. Process according to Claims 1 Ms 5, characterized in that the polar solvent used is dimethylformamide. 7. Process according to Claims 1 to 5, characterized in that the polar solvent used is dimethylacetamide. 8. The method according to claims 1 to 7, characterized in that the silicon dioxide-G-el is regenerated inx-by it with the hydrocarbon fraction in the liquid state in Brings contact. 9. The method according to claims 1 to 7, characterized in that the Siliciurndioxyd-G-el regenerated in ^ by it in contact with the hydrocarbon fraction "in the vapor state brings. 709818/1106 Claims 10. The method according to rench 1 "to 9, characterized in that man
that the first fraction of the effluent from the silica gel, which was obtained at the beginning of an adsorption stage following a desorption stage, is sent back to the extractive distillation in order to isolate the benzene and / or toluene contained therein. 709818/1 106