GB2096637A - Process for obtaining nitrogen- containing aromatic hydrocarbons from aromatic hydrocarbon mixtures - Google Patents

Abstract

Aromatic oil (such as methylnaphthalene oil or anthracene oil distilled from coal tar) is mixed with a porous inorganic carrier (such as aluminium oxide or silica gel) in a weight ratio from 3:1 to 2:5, eluted with a non-polar solvent (such as n- hexane) to separate a fraction of low nitrogen content, and then eluted with an aromatic solvent (such as toluene and/or pyridine) to separate a fraction of high nitrogen content. The oil can be diluted with chloroform prior to the separation. The carrier is regenerated by combustion.

Description

SPECIFICATION Process for obtaining nitrogen-containing aromatic hydrocarbons from aromatic hydrocarbon mixtures The invention relates to a novel process for obtaining nitrogen-containing aromatic hydrocarbons from aromatic hydrocarbon mixtures.

The most important raw materials for obtaining industrial aromatic hydrocarbon mixtures are coaltar oils. Liquid hydrocarbon mixtures from these sources contain, in addition to carbon and hydrogen, considerable amounts of heterocyclic compounds such as for example, compounds of oxygen, sulphur and nitrogen. These heterocyclic compounds are obtained as pure heterocompounds for the chemical industry.

The separation of tar-derived heterocyclic compounds such as quinolin, indole or carbazole from coal tar is an example of the purpose of "obtaining pure nitrogen compounds".

Extraction processes or distillation refining steps are generally used for obtaining these compounds from coal-derived hydrocarbon fractions (see for example H.-G. Franck, G. Collin Heinkohlenteer (Coal Tar), Springer Publishing House, Berlin 1968).

The disadvantage of the above processes lies in the fact that in obtaining industrially pure hydrocarbon mixtures with hetero-atoms combinations of processes having multiple stages are necessary.

The object of the invention is to develop a process by means of which it is possible to obtain from fractions of aromatic hydrocarbons nitrogencontaining compounds having a purity such that they may be passed on directly - or for example after a single step of dissolution and allowing to crystallize -- to further technical treatment.

This object is attained according to the invention in that nitrogen-containing aromatic substance fractions are homogenized with porous inorganic carrier material in a ratio of from 3:1 to 2 :5 in a mixer, are poured into a separation column and with at least two different elution agents are separated into a nitrogen-lean fraction and a nitrogen-rich fraction at pressures of from 1 to 30 bar and at temperatures below the boiling point of the elution agent in question, the elution agents are recovered by distillation and fed back, and the carrier material is at least partially re-used after thermal regeneration.

Without giving preference to a specific theory, it may be assumed that when the hydrocarbons are applied to the carrier material - a product with a high surface activity - a particularly strong association is produced between the carrier material and the nitrogen compounds and permits a variable separation and washing of the hydrocarbons from the carrier.

The stationary phases - customary in conventional column chromatography - on the basis of silica gel, aluminium oxide or their mixtures have proved to be suitable carrier materials.

As expected, differing effects occur, which are affected by pore volume, grain size distribution and degrees of activity. The use of aluminium oxide, acid active, and silica gel having a grain size of from 0.063 to 0.200 mm has proved to be particularly advantageous.

Non-polar hydrocarbons, in particular aliphates such as C5 to Ca0 alkanes or cycloalkanes, may be used, according to the invention, as elution agents in order to separate the nitrogen-depleted fractions. Aromatic compounds, such as for example benzene, toluene, pyridine, quinoline and similar compounds or mixtures thereof, are used according to the invention to obtain the nitrogenenriched fractions.

The performance of the process according to the invention is described with reference to the block diagram illustrated in the Figure. The nitrogen-containing aromatic substance fraction 1 is diluted with a small quantity of a readily volatile auxiliary solvent 11, such as chloroform for example, and is fed to the carrier material present in the mixer 3. After homogenization the auxiliary solvent 11 is evaporated in a vacuum and fed back via condensation 12.

It is likewise possible to apply the aromatic substance fraction 1 directly, where necessary by way of a heat exchanger 15, on to the carrier material 2 without auxiliary solvents 11. In order to avoid evaporation, the boiling point of the aromatic substance fraction 1 should not be exceeded.

The flowable homogeneous mixture is poured into the separation column 4. If the aromatic substance fraction 1 contains constituents which are difficult to elute, it is expedient to use a divided separation column 4 which has up to 80% of its volume filled with an uncharged carrier material 2 as a separating zone. The feeding and the separating zones are separated from one another by a filter layer of coarse material, for example quartz sand. It is unnecessary to regenerate the carrier material of the separating zone after each cycle, in contrast to that of the feeding zone.

The separation column 4 is now charged with the non-polar elution agent 5 which preferably dissolves the non-nitrogen-containing fraction 7 out from the aromatic substance fraction 1. The elution agent 5 is separated from the fraction 7 by a preferably continuous vacuum distillation 9 and fed ihto the citculation.

The nitrogen-containing fraction 8 is then recovered in the same way with the aromatic elution agent 6.

If the atomatic substance fraction 1 contains constituents 1 3 which are difficult to elute, they may be washed out in the reverse direction with an additional solvent 14 and recovered as described as a third fraction.

The elution agent or solvent remaining in the separation column is evaporated under vacuum and after condensation 1 2 is processed for re-use.

Carrier material which has become unusable is fed back for regeneration 10, where it is regenerated in an oxidizing atmosphere at temperatures of from 400 to 7000 C.

An analytical process was introduced for mineral oil residues under the name "Extrographie" ("extrography") (J. Halasz, Erd6I u.

Kohle (Petroleum and Coal), 1979, 32, 571), where carrying out the tests was not suitable for industrial application, but was purely analytically orientated and thus particularly expensive. Further analytically orientated chromatography processes, such as thin layer chromatography for example, which are only suitable for the separation of very small quantities of substances, cannot be carried out on an industrial scale.

Surprisingly, however, it has been found that in a simplified separation process which may be carried out on an industrial scale the selective enrichment and depletion of nitrogen-containing hydrocarbons is possible even from coal-derived products by preparation in one step starting with relatively low concentrations.

For this purpose the process described above may be varied in that only an exact section of the desired oxygen-rich or nitrogen-lean fraction is deliberately eluted, while the remaining material is rapidly removed without reducing the selectivity in the area in question.

One advantage of the process lies in the high degree of enrichment of compounds, which are contained in the starting material in a relatively low concentration, in only one process step.

A further advantage is the high total yield of N-heterocyclic compounds, ensuring a substantial technical isolation of the substances present in the starting material.

A particular advantage of the process lies in the possibility of repeatedly using the inorganic carrier materials. The used carrier soiled with small residues of matter is regenerated by burning off in an air stream at 400 to 7000C. Even after 5 to 7 regeneration cycles, regenerated carrier materials still have a chargeability of 85% and a selectivity ot 75% (ratio of the purities obtained under the same conditions) compared with fresh material.

The process according to the invention is described in greater detail in Examples 1 to 4, without being limited to them.

EXAMPLE 1 A methylnaphthalene oil obtained by distillation from coal tar processing with an indole content of 2.5% was added to the 1.8-fold quantity by weight of silica gel 0.063 to 0.2 mm (Merck), relative to the quantity of charging oil, after dissolution in the 1.3-fold quantity of chloroform.

The chloroform was then evaporated under a pressure of 40 mbar. By elution with the 22-fold quantity by weight of n-hexane an indole-free fraction in a yield of 96.4%, relative to the charging oil, was obtained at room temperature and at a pressure of 3.75 bar. The subsequent elution with the 10-fold quantity of toluene produced an 83.4% indole fraction, in a yield of 2.7% (relative to the quantity of the charge).

A residue of 0.9% remains on the carrier material. The elution agents were completely separated by continuous distillation at a pressure of 40 mbar and were fed back. The indole obtained in this way may, insofar as required for further processing, be processed by simple distillation to high degrees of purity. The soiled silica gel is first liberated from residues of the solvent in a vacuum and then fed from above into a cylindrical, externally heated container at temperatures of 4500C and there the organic phase is burnt off while air is admitted in counterflow. After cooling, the carrier material may be coated again with aromatic hydrocarbon mixtures and used in the separation. The loss of charging capacity per regeneration cycle is less than 4%.

EXAMPLE 2 A methylnaphthalene oil obtained by distillation from coal tar processing with an indole content of 2.5% was heated to 140-1 5O0C and added by mixing to the 1.8-fold quantity by weight of silica gel, grain size 0.063 to 0.2 mm (Merck), relative to the quantity of charging oil. By elution with the 22-fold quantity by weight of n-hexane an indolefree fraction in a yield of 96.0%, relative to the charging oil, was obtained at room temperature and at a pressure of 3.5 bar. The subsequent elution with the 10-fold quantity of toluene produced an 83.1% indole fraction, in a yield of 2.7%.

A residue of 1.3% remains on the carrier material which was regenerated as in Example 1.

The elution agents are recovered as in Example 1 and fed back.

EXAMPLE 3 The anthracene oil occurring in the primary distillation of crude coal tar and having a carbazole content of 2.4% was treated in the same manner, added to the aluminium oxide, neutral active (Riedel-de Haven) and poured into the elution column, as described in Example 1. By elution with the 25-foid quantity by weight of n-hexane a carbazole-free fraction in a yield of 84.0%, relative to the charged anthracene oil, is obtained at room temperature and at a pressure of 5.3 bar. The subsequent treatment with the 12-fold quantity of toluene produces a 26.7% carbazole fraction with a yield of 8.1%. Then a further 5.9% of material is obtained with the 15-fold quantity of pyridine. A residue of 2.0% remains on the carrier material.

The fractions were liberated by vacuum distillation at a pressure of 40 mbar from the solvent which was fed into the circulation.

For the purpose of further carbazole enrichment the carbazole fraction obtained was dissolved in the 6.5-fold quantity by weight of chlorobenzene at from 90 to 1 O00C. The solution was cooled to OOC and the resulting crystal sludge filtered off and dried. 89% carbazole in a yield of 85.5%, relative to the carbazole content of the charged anthracene oil, is obtained.

The main product is thus free of the carbazole, which accompanies boiling in a harmful manner, and may be used for obtaining anthracene. The carbazole obtained is used as a starting product for the preparation of dies, insecticides and special plastics.

The soiled aluminium oxide is first liberated from residues of the solvent in a vacuum and then fed from above into a cylindrical, externally heated container at temperatures of 700 C and there the organic phase is burnt off while air is admitted in counter-flow.

After cooling, the carrier material may be coated again with aromatic hydrocarbon mixtures and used in the separation. The loss of charging capacity is approximately 3%.

EXAMPLE 4 In the production of anthracene from coal tar oils a product occurs which contains approximately 31% anthracene and 25% carbazole. This mixture of substances is dissolved in chloroform as an auxiliary solvent in a ratio of 1:0.9 at approximately 450C and is added to the 0.75-fold quantity of aluminium oxide, acid active (Riedel-de-HaQn). The auxiliary solvent is then removed quantitatively by vacuum distillation. The carrier material coated in this way is poured into the feeding zone of the separation column, the lower half of which is formed as a separating zone and is filled with pure aluminium oxide, neutral active.

In the elution with the 25-fold quantity of n-hexane, relative to the charged substance, a nitrogen-free fraction is obtained in a 64.5% yield.

The subsequent treatment with the 1 2-fold quantity of toluene produces an 82.6% carbazole fraction in a 22.8% yield (relative to the charged substance).

By elution with the 10-fold quantity of pyridine a further fraction is obtained in an 8.0% yield.

4.7% of matter remains on the carrier material. As already described, the fractions are liberated from the solvent by vacuum distillation. Regeneration of the carrier material in the feeding zones is carried out as in Example 3. The carrier material of the separating zone is regenerated only after 5 cycles.

Claims (10)

1. A process for separating nitrogen-containing aromatic hydrocarbons from an aromatic hydrocarbon mixture, wherein the aromatic hydrocarbon mixture after homogenisation with a porous inorganic carrier material in a ratio of from 3 :1 to 2:5 is separated by fractionation using at least 2 different liquid elution agents into a nitrogen-lean fraction and a nitrogen-rich fraction, the elution agents being recovered by distillation for re-use and the carrier material being recovered with thermal regeneration for at least partial reuse.

2. A process according to claim 1, wherein the fractionation is effected at a pressure of from 1 to 30 bar and at a temperature below the boiling point of the elution agent in question.

3. A process according to claim 1 or claim 2, wherein the fractionation is effected in a separation column.

4. A process according to any preceding claim, wherein the aromatic hydrocarbon mixture is heated to a temperature below its boiling point before mixing with the carrier material.

5. A process according to any one of claims 1 to 3, wherein the aromatic hydrocarbon mixture is diluted with a readily-volatile auxiliary solvent before mixing with the carrier material, and the auxiliary solvent is evaporated under vacuum after mixing, and liquefied for re-use.

6. A process according to any preceding claim, wherein the porous inorganic carrier material is aluminium oxide, acid active, or silica gel having a grain size of from 0.063 to 0.2 mm.

7. A process according to any preceding claim, wherein the fractionation is carried out in two zones, a feeding zone and a separating zone, with the separating zone being filled with carrier material and constituting at most 80% of the volume of the two zones.

8. A process according to any preceding claim, wherein the elution agent for the nitrogen-lean fraction is a non-polar hydrocarbon, and the elution agent for the nitrogen-rich fraction is an aromatic compound.

9. A process according to any preceding claim, wherein the used carrier material is regenerated at from 400 to 700 C in an oxidising atmosphere after washing with a further solvent and drying under a vacuum.

10. A process according to any preceding claim, wherein the aromatic hydrocarbon mixture is a fraction from the treatment of coal tar.

1 A process for separating nitrogencontaining aromatic hydrocarbons from an aromatic hydrocarbon mixture, the process being substantially as hereinbefore described with reference to the accompanying drawing.