DE2263344C2 - Process for the production of pure aromatics from hydrocarbon mixtures by extractive distillation - Google Patents

Description

The invention relates to a process for the production of pure aromatics from hydrocarbon mixtures by extractive distillation with a selective solvent by separating the hydrocarbon mixture in a stabilizing column into high-boiling hydrocarbons that come from the bottom of the stabilizing column in an overhead product, which is recycled as condensate to the top of the stabilizing column and the non-condensable components as exhaust gas in a mixture of aromatics and non-aromatics and Introduction of the mixture into an extractive distillation column, the aromatics together with the selective solvent drawn off from the bottom of the extractive distillation column and in nachgescha'lteten Columns in pure aromatics and selective p \; Solvents are separated and one that is selective

Hf; Solvent on the head and optionally part of the

; ■ £ pure aromatics in the lower part of the extractive de-

; S initiates stillative column, as well as separation of the steam

U ' shaped non-aromatics as raffinate overhead.

Γ In such procedures, careful work

: '' ■ [ wise required to assign continuous operation

'ensure and a high degree of purity of the

\ - to achieve aromatics to be separated off.

It is known to use pure aromatics from mixtures with non-aromatics to meet these requirements to be obtained by extractive distillation by introducing the mixture to be separated into the middle of the column and brought into contact with the selective solvents, dividing them into two or more Partial flows divided and with such a temperature <> 5 is introduced that the rising vapors are essentially condensed and the rest split up and at a lower temperature than that prevailing on the relevant feed tray of the column Temperature is introduced (DE-OS 15 43 104).

It is also known to use N-methylpyrrolidone as the selective solvent in extractive distillation and in this way to separate pure xylene from a hydrocarbon cut (DE-OS 18 08 758).

It has also been proposed to use a certain ratio in the extractive distillation of benzene to be observed between solvent circulation and supplied thermal energy in order to optimize the Procedure (BE-PS 7 67 693).

When separating aromatic hydrocarbons, the aim is to achieve the greatest possible purity to use the lowest possible expenditure in terms of resources and investment costs. The known procedures however do not achieve this goal. Usually a higher solvent circulation is required, which is a requires high energy.

In the known processes, the solvent flows up in the extractive distillation column Counteracting vapors, whereby the solvent dissolves the aromatics and the vapor pressure of the non-aromatics is fully preserved. Since the solvent also dissolves traces of non-aromatics in addition to aromatics, this is im Solvent-aromatic mixture at the bottom of the column only then free from non-aromatics if the Dampfmevige coming from the bottom 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 five trays below the top of the extractive distillation column given up. In the overhead vapors, however, proportions of the solvent are appropriate the equilibrium and as a result of the entrainment effect. Accordingly, in order to reduce the solvent losses to reduce, work with high reflux rates. This will get non-aromatics instead of the solvent to be dispensed, as a result of which the selective value of the solvents falls.

The invention is based on the object of avoiding these and other disadvantages of the prior art and to provide a method that optimizes the system with the most economical solvent circulation the extractive distillation allowed and the selectivity of the solvent is not impaired.

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

According to the invention, this object is achieved in that some trays are removed from the stabilizing column a mixture of aromatics below the inflow of the product to be separated and above the sump and non-aromatics withdraws in vapor form and vapor and liquefies in several places in the Introduces extractive distillation column.

By this measure of the task of the mixture of aromatics and non-aromatics in partly vaporous, partly in liquid form, the evaporation ratio in the upper and lower part of the extractive distillation column to be influenced.

The invention is shown schematically and simplified in the drawing and will be described in more detail below described.

The mixture to be processed is fed via line 1,

a fully hydrogenated Si non-stabilized coke oven benzene (., pyrolysis gasoline, consisting of aromatics and Non-aromatics fed into the stabilizing column 2. The top product is via line 3, condenser 5, The reflux tank 6, the pump suction line 7, the reflux pump 8 and the reflux line 9 of the stabilizing column 2 abandoned again. The gaseous top product (off gas) is via line 4 containing hydrogen sulphide Gases withdrawn.

The stabilizing column 2 is heated with the reboiler 49. The stabilizing column 2 is from the bottom A residue is drawn off via line 10. This residue contains high boilers, some of which are in the Hydrogenation were formed and which in the downstream extractive distillation of the solvent cannot be separated off by distillation since they boil in the same boiling range as the solvent or z. T. are heavier boiling. Removal of these high boilers from the solvent could result in · μγ a downstream water heat takes place. '

Below the inflow to the stabilizing column 2, the aromatic-non-aromatic mixture becomes vaporous withdrawn via line 11, which ensures that the high-boiling residue is no longer in the Vapors are present and, since these vapors rise from the bottom of the stabilizing column, are also released of gaseous components from hydrogenation.

Line 11 branches into lines 12 and 13. Line 12 leads into a condenser 14 in which a part the vapors can be precipitated. This condensate flows into a container 15 and is over Line 16, pump 17, line 18, cooler 19 and line 20 as a liquid feed to the extractive distillation column 21 abandoned.

Through line 13 aromatic-non-aromatic vapors are above the liquid feed 20 in the Extraktiydestillationskolonne 21 fed.

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

Due to the partially vaporous and liquid task, one is able to influence the evaporation ratio in the upper and lower columns of the extractive distillation column 21. If the entire product is applied in liquid form via line 20, then the evaporation ratio for the lower column is a maximum of ^4:> mum. If the feed mixture is only applied in vapor form via line 13, the evaporation ratio is a maximum for the upper column and a minimum for the lower column. Since the purity of the aromatics obtained with by the Verdampfungsver- ^{3 {)} ratio, which is the rising in the column amount of steam to the won amount of aromatics, is dependent on the du ^ ch lines 20 and 13, respectively discontinued product streams can be by varying the Influence the evaporation ratio through optimal operation of the extractive distillation.

The top of the extractive distillation column 21 has a selective solvent applied to it. Above Line 22, the non-aromatics are drawn off in vapor form, with traces of the solvent being entrained will. These vapors are processed further in a raffinate distillation column 23. Above Line 24, condenser 25, reflux vessel 26, reflux pump 27, reflux line 28 and raffinate line 29 the non-aromatics z. T. abandoned as reflux to the column 23, partly as the end product won. This raffinate is solvent-free.

In the bottom of column 23, which is heated with reboiler 48, solvent is transferred via line 30 withdrawn, the operation of the column 23 can be adjusted so that this solvent is still contains a portion of raffinate, the raffinate portion can be set as high as desired. Line 30 opens into line 45, via which the cooled selective solvent of the extractive distillation column 21 is supplied. Instead of letting the line 30 open into line 45, line 30 can also be separated be passed to the top tray of the column 21.

The extractive distillation column 21 is heated with reboiler 46 and heat exchanger reboiler 42. The bottom product, which consists of solvent and aromatics, is fed via line 31 to the solvent stripper 32 supplied

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 line 39, the aromatics are obtained or partly given to the stripper 32 as reflux. As the bottom product of the solvent stripper 32, the Solvent, which may contain a residual load of aromatics, via line 41, heat exchanger reboiler 42, line 43, cooler 44 and line 45 withdrawn.

In many cases it is convenient to first pass the hot solvent from line 43 through the heating system 48 in order to heat the column 23 partially or completely and then via cooler 44 and feed line 45 to column 21. This makes better use of the heat from the solvent

It is expedient to have one from line 38 via line 40 into the bottom of the extractive distillation column 21 To give partial flow of the pure aromatics obtained in order to influence the purity of the aromatics obtained can. It is possible, if several aromatics are to be obtained at the same time, from which then downstream aromatic distillation the light boiling Attributed to aromatics.

This measure increases the evaporation ratio of the lower column, which is lower Concentration of aromatics in the feed mixture may be required.

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 the higher concentration of the lower-boiling aromatics, which increases the heat exchange in the Heating system 42 is enlarged.

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

The proposed method enables the simultaneous separation of several aromatics, is simple to be carried out and very economical.

By giving up the mixture to be separated in partly vaporous and partly liquid State it is possible to adjust the evaporation ratio in the upper and lower column of the external active distillation column to influence. This can be achieved by simply varying the product streams introduced ensure optimal operation.

Since the invention greatly reduces the amount of solvent in circulation, only small amounts of non-aromatics in the bottom of the extractive distillation column. The ascending Vapor quantities can be increased by introducing a partial flow of pure aromatics obtained.

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 can be run, whereby the amount of solvent contained in the raffinate can be kept as small as desired. There If no non-aromatics come into contact with the solvent, the solvent retains its full selectivity on the feed floor.

Since up to now the aromatic-non-aromatic mixture to be extracted that is obtained in the bottom product of the stabilizing column Containing higher-boiling constituents tending to polymerization reactions and these with in the ι j Extraction go, the circulating solvent was always contaminated and had to be washed or through Purified distillation.

This disadvantage is eliminated by the invention. in the The high-boiling ends are drawn off at the bottom of the stabilizing column. The aromatic fraction becomes vaporous deducted. As a result of this measure, the aromatic fraction contains no or only small amounts of low-boiling polymer formers. These can be obtained very easily, either together with the raffinate, or by means of a small distillation device that is easy to operate and installed in parallel in the solvent cycle removed.

Embodiment

30th

Non-aromatics 3 718 kg / h benzene 14th 044 kg / h toluene 20 kg / h Heavy boilers 60 kg / h

This example describes the extraction of pure benzene from a pyrolysis gasoline with the help of extractive distillation. A Pyrolysebenzinsc! _; nitt, which is fully hydrogenated but not yet stabilized, is introduced into column 2. The composition of pyrolysis gasoline is as follows:

40

The upper part of column 2 works as a stabilizing column; its operating pressure is 5.5 ata. _{Light product containing H 2} S is withdrawn from column 2 at a temperature of 85 ° C. via line 3. The condensable is condensed in condenser 5 and collected in container 6. Via line 7 with pump 8 and via line 9, 1871 kg / h is returned to the top of column 2 as reflux. From the degassing line 4, which is attached to the container 6. H ₂ S-containing gas / vapor mixture is discharged as off gas (exhaust gas) at a temperature of 35 ° C. and a pressure of 4.8 ata at a pressure of 5.7 ata and a temperature of 195 ° C. 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 fed Benzene fraction consisting of benzene and non-aromatics is withdrawn from column 2. 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 14. destroyed h and 12000 kg / h of condensate in vessel 15 is collected, this is then performed at a temperature of 8O ⁰ C, via line 16 to pump 17 via line 18 and cooled in the cooler 19 to 50 ° C, wherein in the cooler 19180000 kcal / h are to be discharged The 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, 5423 kg / h of the side draw of column 2 are in vapor form at a temperature of 155 ° C given up on the 25th tray of the column 21.

The column 21 receives its heat of distillation via heating system 46 and heating system 42. In heating system 46 Heating steam can be supplied for the required heat of distillation. In the present case, the required amount of heating steam 0 kg / h. In the heating system 42 is with the hot solvent, which over Line 41 is withdrawn from the bottom of the stripper 32, an amount of heat of 2,280,000 kcal is supplied. The solvent N-methylpyrrolidone (NMP) enters the liquid at a temperature of 180 ° C Heating system 42 on and off at 110 ° C.

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

The solvent is cooled in cooler 44 from 110 ° C to 60 C ^c. In the course of this, 1,300,000 kcal / h are deducted 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 return for the column 21 cold solvent stream is used and no further floors between the solvent feed and the vaporous overhead takeoff are present, is with that flowing off as a distillate of the column 21 Overhead vapor 2% N-methylpyrrolidone entrained. The amount of vapor flowing through line 22 is 3950 kg / h. In order to free the raffinate from the solvent, column 23 is provided. the Column 23 receives the required heat of distillation from heating system 48. The bottom temperature of this Column is about 98 ° C, the pressure in the bottom 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, the Column 21 abandoned.

The required amount of heat to be supplied to the column 23 in the heating system 48 is 158,800 kcal / h. In the example, the supplied heat is intended as steam; it is easily possible, by appropriately dimensioning the heating system, ^{to send the 110 °} C. hot solvent through the heating register 48 in heat exchange before entering the cooler 44, with the amount of heat to be drawn off in the cooler 44 correspondingly humiliated.

Via the top of column 23 is conducted at a temperature of 6O ⁰ C and a pressure of 0.61 ata via line 24 a Kopfdämpfmenge of 5700 kg / h performed in the condenser 25, in which 600 000. destroyed kcal / h. The liquefied vapors are passed at a temperature of 50 ⁰ C in container 26 and to pump 27 via line 28 at a rate of 1900 kg / h as reflux to the column to the uppermost tray 23, while kg via line 29 3800 / h raffinate is obtained as a finished product.

The column 21 works via the column 23 under a slight negative pressure, which in a vacuum system, which is not described in detail here, is generated.

Via line 31, the bottom product from column 21, consisting of pure benzene and N-Me-

thylpyrrolidone, passed into column 32. As already described, the bottom product of column 32 is 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. At a pressure of 0.5 ata and a temperature of 58 ° C., pure benzene in vapor form in an amount of 20,500 kg / h is passed into the condenser 34 at the top of the column 32. In the condenser 34 an amount of heat of 2,180,000 kcal is withdrawn. The effluent from the condenser 34 in container 35 has a quantity of distillate temperature of 50 ⁰ C. With pump 36 is fed via line 37 to the return line with 6877 kg / h of the column 32 abandoned. 13,623 kg / h of pure 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 substream.

This partial flow can be, for example, 4000 kg / h.

A pure benzene of the following purity is obtained:

Methylcyclohexane

benzene

toluene

i-heptane +

i-octane

0.0014 wt%

99.9892 wt%

0.0024 wt%

0.0070 wt%

The above embodiment for the production of pure benzene can be applied analogously to the Expand the production of multiple aromatics. For this purpose, the top of the column 32 via line 39 is the Mixture of aromatics, which can consist of two or three aromatics, e.g. B. benzene and toluene or toluene and xylene or benzene, toluene and xylene, withdrawn and fed to a downstream system in which the Aromatics are separated, the lightest-boiling aromatic in the circle expediently via line 40 to be led. The downstream system can, for. B. be a distillation plant.

1 sheet of drawings

Claims (1)

Claim: Process for the production of pure aromatics from hydrocarbon mixtures by extractive distillation with a selective solvent 5 by separating the hydrocarbon mixture in a stabilizing column in high-boiling hydrocarbons, which are from the bottom of the Stabilizing column in an overhead product, which is recycled as condensate to the top of the Stabilizing column and the non-condensable constituents as exhaust gas in a mixture ;; Aromatics and non-aromatics and introduction of the mixture into an extractive distillation column, the Aromatics together with the selective solvent from the bottom of the extractive distillation column withdrawn and in downstream columns in pure aromatics and selective solvent be separated, and one the selective solvent at the top and optionally part of the pure Introducing aromatics into the lower part of the extractive distillation column, as well as separating off the vaporous ones Non-aromatics as raffinate overhead, characterized in that some floors below the stabilizing column Inflow of the product to be separated and a mixture of aromatics and above the bottom Non-aromatics withdraws in vapor form and in vapor form and liquefies in several places in the Introduces extractive distillation column.