DE3930992A1 - METHOD FOR SEPARATING AROMATS - Google Patents

Description

The invention relates to a method for separation and simultaneous presentation of aromatics from aroma hydrocarbons. Starting materials are Mixtures of aromatic and non-aromatic coals Hydrogen, from which the pure aromatics because of the Siedelage and because of the formation of azeotropic mixtures not or only by simple thermal distillation can be separated at an unreasonably high cost. Examples of aromatics to be separated are benzene, Toluene and the xylene isomers, both together and individually, called. Mixtures of this type to be separated fall for example in refineries and petrochemical Attachments.

It is known that the aromatics from the non-aromatics by extractive distillation with a selective solution Means can be separated (DE-AS 15 68 940).

As a selective solvent for this separation task used in technology for example: N-substi tuated morpholines such as N-formyl morpholine (NFM); N-  substituted pyrrolidones, such as N-methyl-pyrrolidone (NMP), letrahydrothiophene dioxide (sulfolane): dimethyl formamide (DMF). In the event that such selective Solvent, for example with water or methanol such mixtures are already miscible been set.

In such an extractive distillation, the non-extracted non-aromatics are drawn off in gaseous form at the top of the extractive distillation column, while a mixture of the selective solvent and the aromatics is taken off as the bottom product. This bottom product is fed to a downstream column (stripper), where the pure aromatic product is removed as the top product and the regenerated selective solvent as the bottom product by distillation. Such a method is explained by way of example with reference to the attached FIG. 1:

Fig. 1 shows the following devices:
An extractive distillation column (I), a raffinate column (II), a stripper (III), two condensers (IV) and (V), a cooler (VI), two bottom circulation heaters (VII) and (VIII) and three heat exchangers (IX ), (X) and (XI).

Fig. 1 shows the following material flows:
The mixture of aromatics and non-aromatics ( 1 ) to be used, the (regenerated) selective solvent to be used ( 2 ), the gaseous raffinate ( 3 ), the liquid raffinate reflux ( 4 ), the raffinate removal ( 5 ) which is carried out with the extracted aromatics loaded selective solvents ( 6 ), the gaseous aromatics ( 7 ), the reflux of the aromatics ( 8 ), the aromatics removal ( 9 ).

The method is briefly described with reference to FIG. 1 as follows:
The feed mixture (1) is at a temperature just below its boiling point under the conditions of Betriebsbedin (I) in (I) is fed. The selective solvent ( 2 ) is supplied at the top of (I). It flows down over the internals of (I), taking up the aromatics. The non-aromatic hydrocarbons come in gaseous form, for example through a chimney floor according to (II), where the raffinate to be removed as the top product is freed of any entrained solvent vapors. (II) can be directly connected to (I) via the chimney tray shown in Fig. 1 or be integrated into the process as a completely separate column. To remove the selective solvent from the raffinate, part is returned as reflux ( 4 ) to the top of (II); the rest is removed as largely aromatic-free raffinate ( 5 ). The liquid bottom product of (I) consists of the selective solvent and the aromatic (s) dissolved in it ( 6 ).

( 6 ) is fed into the middle of (III), where it is separated by distillation into aromatic (s) and selective solvent. The aromatic (s) ( 7 ) is (are) condensed in (V), partly added as reflux ( 8 ) to the top of (III) and partly removed as pure aromatic (s) ( 9 ). The regenerated selective solvent is removed from the bottom of (III), partly pumped through the bottom circulation heater (VIII) and partly as ( 2 ) via the heat exchangers (IX), (X) and optionally (XI) and a cooler (VI) fed again (I). In this way, the heating energy introduced into the process via (VIII) is used to cover the energy requirement of (I) via (IX) and the energy requirement of (II) via (X). In addition, sensible heat can still be given to an external consumer via (XI). Finally, the temperature of ( 2 ) required for feeding into (I) is set in (VI). The bottom circulation heater (VII) is essentially used only for starting.

Procedures of the type described have been around Proven in large-scale plants for years. There are however wishes for further improvement of such ver drive: The top product of the raffinate column (II) is a valuable blending component for the production of Gasoline. As further restrictions in the future with regard to the residual aromatic content (especially the Benzene content) are to be expected in the gasoline it is necessary to determine the remaining aromatic content in the Lower raffinate as much as possible. Furthermore the purity of the aromatics to be processed (remaining content of non-aromatics) an ever higher  Requested further process improvements demand. In addition to the dearomatization of the raffinate and the associated improvement in yield and the Making high purity aromatics is another one Aim to separate the aromatics from the non-aromatics to achieve with as little energy expenditure as possible.

It has been found that the aromatics content in the raffinate by the temperature and the amount of in the solution used in the extractive distillation column be influenced by. In practice there has been one Feed temperature for the selective solvent be that lasts at or just below the working temperature the extractive column is at the feed point. At further lowering the temperature of the selective solution the aromatic content in the Raffinate, but at the same time a higher one dissolves Raffinate content in the solvent, which is only through a higher evaporation rate in the sump circulation of the extractive distillation column can be compensated. Similar Effects are increased by increasing the amount of solvent causes. In both cases there is a significantly higher energy energy consumption and a jamming of the liquid phase in the Column and thus an unstable driving style of the column notice that easily becomes a sudden strong Increase in the non-aromatic content in the sump of the extract tiv distillation column can lead. A decrease others lead the amount of selective solvent on the one hand to an insufficient yield of aromatics and a penetration of remaining aromatics into the Raffinate.  

There has now been a process for the separation of aromatics from aromatic hydrocarbons by Ex tractive distillation using a selective Solvent found, which is characterized in that the selective solvent with a temperature in the extractive distillation column is fed in more than 10 ° C to 40 ° C below that at the feed place in the column prevailing temperature, and that at the same time the amount of solvent by 10 to Reduced 50% of the standard amount of solvent being, lowering the solvent temperature and the reduction in the amount of solvent directly Are correlated to each other.

Since the selective solvent is in many cases due to the given the top of the extractive distillation column the temperature at the Feed point in the column prevails, in general around the condensation temperature of the raffinate.

The solution is preferably lowered mean temperature 15 to 40 ° C and the reduction of Solvent amount 15 to 45% of the standard mentioned quantity, with both values correlating directly with one another.

The standard amount of solvent that the Reduction relates to the respective selectivity this solvent depends. These relationships are known to the person skilled in the art. For the separation of benzene come, for example, when using NMP 5.5-7.5 t / t Benzene, often 5.7-6.5 t NMP / t benzene, in question.  

In the event that the extractive distillation column at is operated at reduced pressure, the maximum Lowering the solvent temperature may be less than at normal pressure.

In a preferred manner of the aforementioned selek tive solvents N-substituted morpholines, N-sub substituted pyrrolidones, tetrahydrothiophene dioxide, DMF or a mixture of several of them used in be particularly preferably N-formyl-morpholine or N-Me thyl pyrrolidone (NMP).

The inventive method is used in a special way on the joint production of benzene, toluene and the Xylene isomers (BTX aromatics), on the common Ge extraction of benzene and toluene and on the extraction of Benzene from corresponding distillation cuts turns; this is invented in a particularly preferred manner process according to the invention on the extraction of benzene a so-called benzene cut applied. Here will in a special way on the use of NMP in the Ben mines turned off.

When obtaining benzene from a benzene cut Using NMP has become an empirical formula proven, in which the amount of solvent L in t / h, the Solvent temperature T at the feed point in ° C and the benzene production E in t / h for a benzene production of 5-50 t / h, preferably 10-30 t / h, are linked as follows:  

L = a × T + b × E-c,

where the factors are values of

a = 1.1-1.2,
b = 3.5-4.5 and
assume c = 20-40.

Extractive distillation and regeneration of the Solvents in the stripper can basically be found under Normal pressure can be carried out. Preferably but they are reduced independently of one another pressure. Choosing the benzene separation one advantageously for the extractive distillation column operating data in the range of 150-400 mbar and 35-65 ° C head temperature.

The lowering of the temperature is advantageous of the selective solvent to be fed in, starting from from a conventional driving style, in small steps lowered to maintain the stable driving style of the column hold. To the same extent, the amount of solvent decreased.

When using the method according to the invention read The following advantages can be achieved:

• 1. The residual aromatic content in the raffinate is clearly ge lowers. For example, dropped in a large-scale Plant for the production of benzene from a distillery tion of pyrolysis gasoline obtained benzene fraction the benzene content of the raffinate decreases from 5 to 2% by volume.  
• 2. The specific energy consumption, expressed by the sum of the energy consumption of the extractive style lation column, the raffinate column and the strip pers and related to aromatics production, reduced was also concerned. In the one described under 1. Case there was a reduced specific vapor consumption by 0.1 t steam / t aromatics.
• 3. The procedure of the extractive distillation column is calmer. It can have any desired non-aromatic content be set in the pure aromatic, d. H. the specifics tion limit of the pure aromatic can be approached very closely will.
• 4. Due to the lower solvent circulation, a Relief of the stripper and thus a better off heating of the solvent reached.

Examples

In the following comparisons, a pre-hydrogenated and desulfurized benz distillation section of a pyrolysis gasoline was used in an extractive distillation apparatus according to FIG. 1 in a large-scale plant. NMP was used as the selective solvent. The desired and achieved specification requirement was a maximum of 300 ppm non-aromatics in the benzene drawn off at the top of the stripper. The operating data listed as case A originate from operational tests before the method according to the invention is implemented; the data listed as case B correspond to the changes according to the inventive method. The work was carried out under two different loads. The numbers given below, which are not exactly the same, correspond to the operational conditions under which the experiments had to be carried out (different benzene content, different head pressure, etc.).

In case B according to the invention, the solvents temperature and the amount of solvent at upright maintaining head pressure and thus maintaining temp temperature in the extractive distillation column. The reflux on the raffinate column could in the can be reduced as required (saving steam). The The benzene content in the raffinate dropped considerably. As well the reflux on the stripper was withdrawn (Steam saving). That regenerated selective solution medium had a lower benzene content. Despite the lower reflux amounts resulted in a constant bene benzene specification and an improved raffinate purity. Overall, the energy supply was around given amount decreased.

Claims (8)

1. A process for the separation of aromatics from aromatics-containing hydrocarbons by extractive distillation and the use of a selective solvent, characterized in that the selective solvent is fed into the extractive distillation column at a temperature which is more than 10 ° C to 40 ° C below that the Einspei solution point in the column prevailing temperature, and that at the same time the amount of solution is reduced by 10 to 50% of the standard amount of solvent, the lowering of the solvent temperature and the reduction in the amount of solvent being directly correlated. 2. The method according to claim 1, characterized in that that the lowering of the solvent temperature 15th up to 40 ° C and the reduction of solvents amount is 15 to 45% and both values directly correlate with each other. 3. The method according to claim 1, characterized in that as a selective solvent N-substituted Morpholines, N-substituted pyrrolidones, tetra hydrothiophene dioxide, dimethylformamide or a Ge can be used in a mixture of several of them.   4. The method according to claim 1, characterized in that as a separation of aromatics the common Ab separation of benzene, toluene and the xylene isomers (BTX aromatics), the common separation from Ben zene and toluene or the separation of benzene suitable distillation cuts made becomes. 5. The method according to claim 4, characterized in that as aromatics separation, the extraction of benzene made from a benzene distillation cut becomes. 6. The method according to claim 5, characterized in that in the separation of benzene with N-methyl pyrrolidone worked as a selective solvent becomes. 7. The method according to claim 6, characterized in that the amount of solvent L in t / h, the solvent temperature T at the entrance to the extractive distillation column in ° C and the benzene production E in t / h for a benzene production of 5-50 t / h , preferably from 10-30 t / h, is linked in the following manner: L = a × T + b × Ec, where the factors have values of a = 1.1-1.2,
b = 3.5-4.5 and
assume c = 20-40. 8. The method according to claim 5, characterized in that the extractive distillation column with a Head pressure from 150 to 400 mbar and a head temperature is operated from 35 to 65 ° C.