DE2263344A1 - PROCESS FOR OBTAINING PURE AROMATS FROM HYDROCARBON MIXTURES BY EXTRACTIVE DISTILLATION - Google Patents

Description

MEOiALIGESELLSOHAPT Frankfurt a.M., December 20th, 1972

Aktiengesellschaft '-κ _τ /haben.r·

Prankfurt a.M. 'Dr La/ffi/i

Reuterweg H 2263344

prov.No, 6 8 2 7 LÖ

Process for the production of pure aromatics from coal only mixed off by extractive distillation. ____

The invention relates to a method for obtaining pure aromatics from hydrocarbon mixtures by extractive distillation with a selective solvent in an extractive distillation column.

In such processes, careful work is required to ensure continuous operation and to achieve a high degree of purity of the aromatics to be separated off.

It is known to be pure to meet these needs Aromatics from mixtures with non-aromatics by extractive distillation to win by introducing the mixture to be separated in the middle of the column and using the selective Solvents is brought into contact, this being divided into two or more substreams and with one is introduced at such a temperature that the rising vapors essentially condense and the remainder split up and at a lower temperature than the temperature prevailing on the relevant feed tray of the column is introduced (DT-OS 1 54-5 104).

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It is also known to use N-methylpyrrolidone as a selective solvent in extractive distillation and in this way pure xylene from a hydrocarbon step to be separated (DT-OS 1 808 758).

It has also been proposed for extractive distillation of benzene to maintain a certain ratio between solvent circulation and supplied thermal energy, in order to optimize the process (BeIg. Patent 767 693).

When separating aromatic hydrocarbons, the aim is to achieve the greatest possible purity to use the lowest possible expenditure on operating resources and investment costs. The known procedures however do not achieve this goal. In most cases, a higher solvent circulation is required, which is a high one Requires energy.

In the known processes, the solvent flows in the extractive distillation column with the rising vapors against, whereby the solvent dissolves the aromatics and the vapor pressure of the non-aromatics is fully retained. Since the solvent also dissolves traces of non-aromatics in addition to aromatics, this is what occurs in the bottom of the column Solvent-aromatic mixture free of non-aromatics only if the amount of steam that is released from the sump of the Column must rise is high in order to remove the non-aromatics from the solvent. The higher the solvent circulation however, the higher the amount of non-aromatics dissolved in the solvent increases.

In general, the selective solvent is about 5 trays below the top of the extractive distillation column given up. However, proportions of the solvent in the overhead vapors correspond to the equilibrium

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and present as a result of the entrainment effect. Accordingly, in order to reduce the solvent losses reduce, work with high reflux amounts. As a result, non-aromatics take the place of the solvent to be applied, as a result of which the selective value of the solvent drops.

The invention is based on the task, these and others Disadvantages of the St.d.T. to avoid and a procedure to make available that with the most economical solvent circulation an optimization of the extractive distillation allowed and does not affect the selectivity of the solvent.

In particular, the process should be economical and enable the aromatics to be obtained to be obtained in high yield and free from non-aromatics.

According to the invention, this task is solved in that one the extractive distillation column is preceded by a stabilizing column, the product to be separated from this column gives up, withdraws high-boiling residue from the bottom of this column, the mixture of aromatics and non-aromatics withdraws in vapor form from the lower part or from some trays below the inflow of the product to be separated, then to influence the evaporation ratio, in vapor form or in whole or in part liquefies an extractive distillation column at one or more points and supplies the pure aromatics together with the selective solvent from the sump withdraws and then separates.

By this measure the task of the mixture of "aromatics and non-aromatics in vapor or liquid or Partly vaporous, partly liquid form can do that

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Evaporation ratio in the upper and lower part of the Extractive distillation column affects v / ground.

To further improve the purity of the aromatics obtained especially if it is simultaneous It is a matter of obtaining several aromatics, according to the invention, some of the pure aromatics separated off can be used return to the bottom of the extractive distillation column.

According to the invention it is also possible to improve the purity of the aromatics obtained by the extractive distillation column is followed by one or more columns and the aromatics with the lowest Recirculates boiling point in the bottom of the extractive distillation column.

In the context of the invention, preference is given to the raffinate the extractive distillation column drawn off overhead or on the side.

According to a development of the invention, the withdrawn raffinate can be in vapor and / or liquid form Feed in the raffinate distillation column, remove the solvent-free raffinate overhead and from the bottom Remove the remaining solvent and return it to the extractive distillation column.

The bottom product of the raffinate distillation column can have any high content of raffinate.

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The invention is schematic and simplified in FIG Drawing shown 'and is described in more detail below.

The mixture to be processed, a fully hydrogenated, non-stabilized coke oven benzene or pyrolysis gasoline, consisting of aromatics and non-aromatics, is fed into the stabilizing column 2 via line 1. The top product is returned to the stabilizing column 2 via line 3, condenser 5, reflux tank 6, pump suction line 7> reflux pump 8 and reflux line 9. Gases containing hydrogen sulfide are withdrawn via line 4 as the gaseous top product (off gas).

The stabilizing column 2 is heated with the reboiler 49. From the bottom of the stabilizing column 2 is over Line 10 drawn off a residue. This residue contains high boilers, some of which are formed in the hydrogenation and which were not separated from the solvent by distillation in the downstream extractive distillation as they boil in the same boiling range as the solvent or, in some cases, have a higher boiling point. These high boilers could only be removed from the solvent by downstream water heating.

Below the inflow to the stabilizing column 2, the aromatic-non-aromatic mixture becomes vaporous via line 11 deducted, which ensures that higher boiling point. Residue is no longer present in the vapors and, there these vapors rise from the bottom of the stabilizing column, also free of gaseous components the hydrogenation is.

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Line 11 branches into lines 12 and 12. The Line 12 leads into a condenser H, in which some of the vapors can be precipitated. This Condensate flows into a container 15 and is over Line 16, PUnipe 17, line 18, cooler 19 and Line 20 as a liquid task to the extractive distillation column 21 abandoned.

Through line 13 are aromatic-non-aromatic vapors fed into the extractive distillation column 21 above the liquid feed 20.

The stabilizing column 2 operates under a higher pressure than the extractive distillation column 21.

Due to the partly vaporous and liquid task, one is able to control the evaporation ratio in the upper and to influence the lower column of the extractive distillation column 21. Is the entire product via line 20 abandoned in liquid form, then the evaporation ratio for the lower column is a maximum. Y / is the feed mix Abandoned via line 13 only in vapor form, then that is Evaporation ratio for the upper column a maximum, a minimum for the lower column. Since the purity of the aromatics obtained depends on the evaporation ratio, which represents the amount of steam rising in the column to the amount of aromatics obtained, is dependent, leaves The evaporation ratio changes by varying the product streams discharged through lines 20 and 13 influence by an optimal operation of the extractive distillation.

The top of the extractive distillation column 21 has a selective solvent applied to it. Via line 22 the non-aromatics are drawn off in vapor form, wherein

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Traces of solvent are taken away. These vapors are in a raffinate distillation column 23 Further processing via line 24, condenser 25, reflux tank 26, reflux pump 27, reflux line 28 and raffinate line 29, the non-aromatics are partly abandoned as reflux to the column 23, in part as Final product won. This raffinate is solvent-free.

In the bottom of column 23, which is heated with reboiler 48 ″, solvent is drawn off via line 30, the mode of operation of the column 23 being adjusted in this way can that this solvent still contains a part of raffinate, the raffinate content set as high as desired can be. The line 30 opens into line 45, via which the cooled selective solvent of: Extractive distillation column 21 is fed * Instead of allowing line 30 to open into line 45, can Line 30 can also be routed separately to the top tray of column 21.

The extractive distillation column 21 is equipped with a reboiler 46 and V / ärmeaustauscherauf boiler 42 heated. The bottom product which consists of solvents and aromatics is fed to the solvent stripper 32 via line 31 fed.

The solvent stripper 32 is heated with a reboiler 47. Via line 33, condenser 34, reflux tank 35, reflux pump 36, reflux line 37, line 38 and The aromatics are recovered in line 39 or, in some cases, given to the stripper 32 as reflux. As a bottom product the solvent stripper 32 is the solvent, which may contain a residual load of aromatics, via line 41, Heat exchanger reboiler 42, line 43, cooler 44 and

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Line 45 withdrawn.

In many cases it is advisable to first add the hot solvent from line 43 via the heating system 48 lead to heating the column 23 partially or completely and then via cooler 44 and line the column 21 to be fed. This makes better use of the heat of the solvent.

It is expedient to feed a substream from line 38 via line 40 into the bottom of the extractive distillation column 21 of the pure aromatics obtained in order to be able to influence the purity of the aromatics obtained. It is there possible, if several aromatics are to be obtained at the same time, from the subsequent aromatics distillation attributed to the lightest-boiling aromatics.

By this measure, the evaporation ratio of the Lower column increased, which may be necessary if the concentration of aromatics in the feed mixture is low.

At the same time, the temperature of the product flowing off from the bottom tray of the column 21 is lowered as a result of a higher concentration of the lower-boiling aromatics, whereby the heat exchange in the heating system 42 is enlarged.

The advantages achieved with the invention exist in particular in that it succeeds in an optimal way, aromatics with high purity from hydrocarbon mixtures with The help of extractive distillation.

The proposed method enables the simultaneous separation of several aromatics and is easy to carry out and very economical.

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By giving up the mixture to be separated in part vaporous and partially liquid state, it is possible to adjust the evaporation ratio in the upper and To influence the lower column of the extractive distillation column. By simply "varying the abandoned This ensures optimal operation of the product streams.

Since the invention greatly reduces the amount of solvent in circulation, only small amounts get there Nautical aromatics in the sump of the extractive distillation column. The rising damping constrictions can through Introduction of a partial stream of pure aromatics obtained can be increased, which has the advantage that the purity of the aromatics obtained is improved.

According to the invention, the raffinate is separated from the solvent in a separate column. This has the advantage that, contrary to the usual procedures, any high return flow can be run, which causes the in the raffinate contained amounts of solvent can be kept as small as desired. Since there are no Hxchtaromats in addition with the If the solvent comes into contact, the solvent retains its full selectivity at the feed tray.

Since up to now that accumulated in the bottom product of the stabilizing column Aromatic-Uichtaromatic mixture to be extracted, higher boiling point, tending to polymerisation reactions Contained constituents and these go into the extraction, the circulating solvent was always contaminated and had to be cleaned by washing or by distillation.

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This disadvantage is eliminated by the invention. The high boilers are in the bottom of the stabilizing column deducted. The aromatic fraction is withdrawn in vapor form. As a result of this measure, the aromatic fraction "does not contain any or only small amounts of low-boiling polymer formers. These can be done very easily either along with obtained from the raffinate, or by means of an easy-to-operate, parallel in the solvent cycle installed, small distillation device can be removed.

Execution example

This example describes the extraction of pure benzene from a pyrolysis gasoline with the help of extractive distillation. A pyrolysis gasoline cut, which is fully hydrogenated but not yet stabilized, is fed into the column via line 1 introduced. The composition of the pyrolysis gasoline is the following:

Non-aromatics 3718 kg / h

Benzene 14044 kg / h

Toluene 20 kg / h

High boiling point 60 kg / h.

The upper part of the column 2 works as a stabilizing column; their operating pressure is 5 »5 ata. Via line 3 HPS-containing light product is a temperature of 85 ⁰ C removed from the column 2bai. The condensable is condensed in condenser 5 and collected in container 6. Via line 7 with pump 8 and via line 9, 1,871 kg / h is returned to the top of column 2 as reflux. From the degassing line 4, which is attached to the container 6, HpS-containing gas-vapor mixture at a temperature of 35 ⁰ C and a pressure of 4.8 ata as

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Off-Ga ^ s (exhaust gas) discharged. In the bottom of the column 2 traps 130 kg / h. Residue, which is discharged via isolation 10 with a pressure of 5-7 ata and a temperature of 195.degree. The column receives the required heat of distillation via a heating system 49 into which, for example with heating steam, 1,681,000 kcal / h are supplied. The benzene fraction consisting of benzene and non-aromatics is withdrawn from column 2 in vapor form via line 11. The temperature is 155 ° C, the amount 17-423 kg / h. A portion of this vaporous amount is fed via line 12 into the condenser. 1,600,000 kcal / h are destroyed in the condenser 14 and 12,000 kg / h condensate is collected in the container 15. This is then conducted at a temperature of 80 ^° C. via line 16 with pump 17 via line 18 and ^{cooled to 50 °} C. in the cooler 19, with 180,000 kcal / h being discharged in the cooler 19. This cooled substream of the side offtake of the column 2 is fed to the extractive distillation column 21 on the 22nd tray.

Via line 13, 5,423 kg / h of the side take-off of column 2 are fed in vapor form at a temperature of 155 ^° C. to the 25 · tray of column 21.

The column 21 receives its heat of distillation via the heating system 46 and heating system 42. In the heating system 46, heating steam for the required heat of distillation can be supplied. In the present case, the required amount of heating steam is 0 kg / h. In the heating system 42, the hot solvent, which is drawn off from the bottom of the stripper 32 via line 41, is supplied with an amount of heat of 2,280,000 kcal. The solvent N-methylpyrrolidone (NMP) occurs with a liquid Temperatiir of 180 ° C in the heating system 42 and with 11O ⁰ C out.

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The amount of solvent is 52,000 kg / h.

The solvent is cooled in cooler 44 of 11O ⁰ G 60 ⁰ C. 1,300,000 kcal / h are destroyed in the cooler. The solvent is applied to the top of the extractive distillation column 21 via line 45.

Overhead vapors from column 21 are drawn off via line 22. Since only the cold solvent stream serves as the reflux for the column 21 and no further trays are present between the solvent feed and the vaporous top take-off, is used as the distillate the top vapor flowing off the column 21 $ 2 N-Methy! Pyrrolidon taken along. The via line 22 the flowing vaporous amount is 3 · 950 kg / h. To free the raffinate from the solvent is Column 23 is provided. The column 23 receives the required heat of distillation from the heating system 48. The bottom temperature this column is about 98 G, the pressure in the swamp 0.69 ata. There are 150 kg / h consisting of

N-methylpyrrolidone, traces of benzene and raffinate withdrawn via line 30 and, together with the solvent, which flows via line 45, to column 21 given up.

The required amount of heat to be supplied to the column 23 in the heating system 48 is 158,000 kcal / h. In the example, the supplied Y / arm is intended to be steam, it is easily possible by using the appropriate Dimensioning of the heating system, which is 110 C To send solvent before entering the cooler 44 in heat exchange through the heating register 48, the The amount of heat to be destroyed in the cooler 44 is correspondingly reduced.

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Via the top of column 23 is conducted at a temperature of 6O ⁰ O and a pressure of 0.61 ata via line 24 a Kopfdämpfmenge of 5,700 kg / h in condenser 25, in which 600,000 keal / h destroyed v / ground. The liquefied vapors are passed at a temperature of 50 ⁰ G in container 26 and applied with a pump 27 via line 28 to an amount of I.9OO kg / h as Rückflm3 of column 23 on the uppermost tray, while via line 29 3 * 800 kg / h of raffinate is obtained as a finished product.

The column 21 operates via the column 23 under one slight negative pressure in a vacuum system that is here is not described in detail is generated.

The bottom product, consisting of pure benzene and N-methylpyrrolidone, is fed from column 21 via line 31 into column 32. The bottom product of the column 32 is, as already described, the solvent. The required amount of heat from distillation is expediently fed to the column 32 with the heating system 47 with heating steam. An amount of heat of 3,358-000 kcal / h 'is required. Via the top of the column 32, pure benzene in vapor form in an amount of 20,500 kg / h is passed into condenser 34 at a pressure of 0.5 ata and a temperature of 58 ^{0 O.} In the condenser 34, an amount of heat of 2,180,000 kcal is destroyed. The amount of distillate flowing out of the condenser 34 into the container 35 has a temperature of 50 ^° C. With. Pump 36 - the reflux at 6,877 kg / h of column 32 is fed via line 37. 13-623 kg / h of benzene are obtained via line 39. In order to improve the purity of the pure benzene, it is possible to increase the evaporation ratio in the column .21 with the aid of a benzene slip stream (partial stream).

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This partial flow can be 4,000 kg / h, for example. A pure benzene of the following purity is obtained:

Methylcyclohexane ) 0, 0014 Weight benzene 99 9892 11 toluene 0, 0024 Il i-heptane 4 0070 11 i-octane

The above embodiment for obtaining

themselves
Pure benzene can be extended to the production of several aromatics. For this purpose, the aromatic mixture, which can consist of two or three aromatics, for example benzene and toluene or toluene and xylene or benzene, toluene und.Xylene, is drawn off at the top of column 32 via line 39 and fed to a downstream plant, in v / elcher Aromatics are separated, the lightest-boiling aromatic being expediently circulated via line 40. The downstream system can be a distillation system, for example.

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Claims (5)

Claims 1) Process for obtaining pure aromatics from hydrocarbon mixtures by extractive distillation with a selective solvent in an extractive distillation column, characterized in that a stabilizing column is provided for the extractive distillation column, the product to be separated from this column is given up, high-boiling residue is withdrawn from the bottom of this column, the mixture of aromatics and Hichtaromats, withdraws in vapor form from the lower part or a few floors below the inflow of the product to be separated, then to influence the evaporation ratio in vapor form or completely or partially liquefied to an extractive distillation column at one or more points and the pure aromatics together with the selective solvent withdraws from the sump and then separates. 2) Process according to claim 1, characterized in that part of the separated pure aromatics is returned to the bottom of the extractive distillation column. 3) Process according to claims 1 and 2, characterized in that the extra-active distillation column is followed by one or more columns and the aromatics with the lowest boiling point are returned to the bottom of the extractive distillation column. 4) Process according to one or more of the preceding claims 1 to 3, characterized in that the raffinate is withdrawn from the extractive distillation column at the top or at the side. -16- 409827/1052 5) Method according to one or more of the preceding claims 1 to 4, characterized in that the withdrawn raffinate is supplied in vapor and / or liquid form to a raffinate distillation column, ■ the raffinate is removed overhead without solvent and residual solvent is withdrawn from the bottom and fed back to the extractive distillation column . 4098 27/1 052