GB2168075A - Process for the recovery of naphthalene from residual oils of pyrolysis - Google Patents

Abstract

A naphthalene fraction, which is crystallized, is recovered by distillation from the residual oil of pyrolysis of petroleum fractions to form olefins: After the separation of the expressed oil, which may be used as a resin oil, the crystallizate is washed with an aromatic oil originating from carbon, e.g. an expressed oil (cresol oil) from the crystallization of naphthalene oil from coal tar: This is followed by extraction, e.g. with methanol. By the additional washing stage with the aromatic oil, in particular lower boiling contaminates, such as indene and methylindenes, are reduced.

Description

SPECIFICATION Improved process for the recovery of naphthalene from residual oils of pyrolysis The subject of the invention is an improved process for the recovery of naphthalene from residual oils of pyrolysis of mineral oil fractions to form olefins and other unsaturated hydrocarbons by distillation, crystallization and extraction, it being necessary to enrich the compounds, indene and methylindenes, contained in the naphthalene fraction of the residual oil in a fraction.

It is known to recover a naphthalene with a purity of approximately 98% from residual oils of pyrolysis in a similar manner as from coal tar (Ullmans Enzyklopadie der technischen Chemie [Ullmann's Encyclopaedia of Technical Chemistry], 4th Edition, Volume 17, pages 77-79). In this connexion a naphthalene fraction, which boils approximately between 210 and 220"C and which in addition to naphthalene essentially contains indane, indene, methylindanes, methylindenes, and methylnaphthalenes as boiling attendants, is recovered by distillation from the residual oil of pyrolysis. The distillation of residual oils of pyrolysis for the recovery of the naphthalene fraction is described for example in the German Patent 18 15 568.

The reconditioning of naphthalene oil from coal tar is performed in stirring apparatus for example, in which the naphthalene oil is crystallized by direct or indirect cooling. The crystallizate is separated by centrifuging or dripping from the expressed oil (moth liquor).

After a two-stage extraction with methanol a naphthalene is recovered in a purity which is adequate for a number of areas of industrial application.

This process has proved successful for naphthalene fractions originating from coal tar. When transferred to naphthalene recovery from residual oils of pyrolysis, however, it has been found that even after extraction with methanol has been carried out three times an excessive content in particular of indene and methylindenes remained in the naphthalene.

The object is therefore to improve the process for the recovery of naphthalene from residual oils of pyrolysis by separation by distillation of a naphthalene fraction, crystallization of the said fraction in order to separate the naphthalene crystallizate from the expressed oil, it being possible for the expressed oil to be returned at least in part to the crystallization stage, and extraction of the crystallizate with methanol in one or more stages, in such a way that on the one hand a naphthalene of greater purity is recovered and on the other hand an expressed oil which is rich in indene and methylindenes is recovered.

This object is attained in that before the extraction with methanol the crystallizate is additionally washed with an aromatic oil originating from carbon and which boils to 90% by weight within a temperature range of from 170 to 250"C under normal pressure and which has an aromatic acid content of from 5 to 35% by weight. The washing is carried out in a temperature range of from 20 to 50 C, preferably at between 25 and 35"C.

A fraction from the reconditioning of mixtures of aromatic substances, such as occur in the pyrolysis, hydration or extraction of lignite or coal, can be used as the aromatic oil originating from carbon. It is referred to below as carbo-oil. This carbo-oil is normally liquid in the temperature range indicated. It can, however, also contain additives of up to 70% of a naphthalene fraction which is at least partially crystallized, as explained in greater detail in Example 3. What is important, however, is that in any case the non-crystallized oil of the residual oil fraction of pyrolysis, the so-called expressed oil, should be substantially separated from the crystallizate before washing.

The invention is described in greater detail with reference to the following examples, without being restricted to them.

Example 1 The course of the process is outlined in Figure 1. without the recovery of the naphthalene fraction by distillation. A naphthalene fraction with a solidification point of 59"C and a naphthalene content of 70.2% by weight is recovered from a residual oil of benzine pyrolysis to form ethylene by fractionated distillation. It contains as the main accompanying substances in percentages by weight: aromatic substances boiling below 177"C 2.0 indane 1.2 methylindanes 2.5 indene 5.4 methylindenes 6.4 methylnaphthalenes 11.1 75 parts by weight of the liquid naphthalene fraction (1) are mixed with 25 parts by weight of expressed oil (2) from the preceding crystallization in an agitator vessel.The mixture has the following composition in percentages by weight: aromatic substances boiling below 177"C 2.3 indane 1.6 methylindanes 4.3 durene 0.2 indene 7.0 tetralin 0.4 methylindenes 8.5 naphthalene 60.2 methylnaphthalenes 14.7 The naphthalene content in the mixture should not fall below 45% by weight and should preferably amount to between 55 and 65% by weight. The temperature of the mixture is reduced to 25 C by indirect cooling. In this connexion a crystal mass is produced, from which 50 parts by weight of expressed oil are separated in a filter (4) by means of reduced pressure.The remaining crystallizate is then washed at a temperature of 30"C with 36 parts by weight of a filtered naphthalene oil (5) with a boiling range of from 175 to 235 C, such as occurs during the crystallization of naphthalene oil from coal tar.

The filtered naphthalene oil used as carbo-oil has the following composition in percentages by weight: aromatic substances boiling below 177"C 3.8 indane 5.7 indene 1.9 methylindenes 7.9 naphthalene 30.8 methylnaphthalenes 18.8 It contains 20.5% phenols and 3% nitrogen bases and it can be used again at least in part, before being discharged out of the process for reconditioning.

The washing stage is followed by a two-stage extraction with methanol at 40"C. For the first extraction, 35 parts by weight of a methanol (6) already adulterated with expressed oil from the second extraction stage of the previous process with a methanol content (anhydrous) of 89% by weight are used. For the second extraction stage, 35 parts by weight of a pure methanol (7) with a water content of 12% by weight from the reconditioning (8) of methanol by distillation from the first extraction stage are used.

The distillation residue (9) is added for the further reconditioning of the naphthalene fraction of the pyrolysis oil. In this way a naphthalene (10) with a solidification point of 79"C is recovered in a yield of 78% by weight relative to the naphthalene content of the naphthalene fraction used. The gas chromatographic analyses of the naphthalene and the expressed oil are reproduced in Table I in percentages by weight.

TABLE 1 expressed oil naphthalene aromatic substances boiling below 177"C 2.6 indane 2.6 methylindanes 7.6 durene 2.3 1.2 indene 10.6 tetralin 0.6 methylindenes 13.0 naphthalene 24.0 98.1 methylnaphthalenes K 22.1 0.6 Example 2 As in Example 1 a crystallizate is produced by separating the expressed oil from a mixture of 75 parts by weight of a naphthalene fraction and 25 parts by weight of expressed oil. In this case the same raw materials are used as in Example 1.

For washing, 35 parts by weight of a fraction - boiling between 180 and 250"C --from the reconditioning of oils by distillation from the hydration of coals with a phenol content of 23.3% by weight is used.

Since this carbo-oil has only a very low naphthalene content, it dissolves the naphthalene very intensely, so that after the two-stage extraction with methanol, as described in Example 1, a naphthalene with a purity of 98.0% is recovered only in a yield of 59.8% by weight. The naphthalene contains 1.4% by weight of impurities boiling at a lower temperature and 0.5% by weight of methyl naphthalenes. After saturation of the carbo-oil fraction with naphthaiene the yield can be increased to approximately 76% by weight while the naphthalene quality remains approximately the same.

Example 3 The diagram of this process variant is illustrated in Figure 2. 70 parts by weight of a naphthalene oil from coal tar (11) with a solidification point of 59C are mixed with 30 parts by weight of a filtered naphthalene oil (5) as in Example 1 and are cooled in an agitator vessel (12) to 25"C. The mixture boils in a temperature range of from 175 to 240"C and contains 8.5% by weight of phenols. At the same times 75% by weight of a naphthalene fraction (1) from oil of pyrolysis with a solidification point of 60"C is cooled to 25"C in a mixture with 25 parts by weight of expressed oil (2) in the agitator vessel.The analyses of the naphthalene oil mixtures are reproduced in Table 2 in percentages by weight. 45 parts by weight of expressed oil are separated from the naphthalene crystal mass of oil of pyrolysis in the filtering centrifuge (13) and are partly fed back to the subsequent process and are partly further processed as resin oil.

With respect to the naphthalene crystallizate recovered in this way the crystal mass originating from coal tar is drained into the centrifuge. In this connexion the crystallizate from the oil of pyrolysis is washed out from the naphthalene oil which originates from coal tar and which is discharged for further reconditioning.

The subsequent two-stage extraction with methanol is carried out as described in Example 1. In this connexion a mixture of naphthalene originating from coal tar and oil of pyrolysis is recovered in a yield of 78.9% by weight, relative to a naphthalene content of the naphthalene fraction used, with a solidification point of 79.1"C. The analyses of the expressed oil and the naphthalene are reproduced in Table 3 in percentages by weight.

TABLE 2 naphthalene naphthalene oil mixture oil mixture from coal tar from oil of p yrolysis aromatic substances boiling below 177"C 1.5 2.2 indane 3.0 1.1 methylindanes 1.1 3.5 durene 0.1 0.1 indene 0.7 5.7 tetralin 0.2 0.4 methylindenes 2.4 7.7 naphthalene 68.6 64.9 thionaphthene 1.0 methylnaphthalenes 10.8 14.0 others 10.6 0.3 TABLE 3 expressed oil naphthalene aromatic substances boiling below 177"C 2.8 indane 2.8 methylindanes 7.0 durene 0.6 0.7 indene 10.1 tetralin 0.7 methylindenes 13.8 naphthalene 25.7 98.5 thionaphthene - 0.4 methylnaphthalenes 22.3 0.4 Example 4 {Comparison) As described in Example 1, 75 parts by weight of a naphthalene fraction of oil of pyrolysis (1) and 25 parts by weight of expressed oil (2) are mixed and are cooled to 25"C. After 43 parts by weight of expressed oil have been filtered out of the crystal mass the crystallizate is extracted three times in each case with 35 parts by weight of pure methanol with a water content of 12% by weight at 400C. T'.-e naphthalene recovered in this way in a yield of 69% by weight and having melting point of 77.8"C contains 1.5% by weight of indene and 0.8% by weight of methylindenes in addition to other accompanying substances. The analyses of the charge mixture and the naphthalene are reproduced in Table 4 in percentages by weight.

TABLE 4 charge mixture naphthalene aromatic substances boiling below 177"C 1.1 indane 0.8 methylindanes 3.3 durene 0.2 2.4 indene 6.5 tetralin 0.5 methylindenes 12.4 naphthalene 66.5 96.8 methylnaphthalene 8.5 0.8 As a comparison of the naphthalene analyses shows, the content of impurities boiling at a lower temperature such as indene and methylindenes is considerably reduced in particular by washing with filtered naphthalene oil.

This surprising effect presumably results from the phenols present in the filtered naphthalene oil.

In order to examine this unexpected effect more closely, the following test was carried out: Example 5 {Comparison) Of 100 parts by weight of a crystal mass with the following composition in percentages by weight: aromatic substances boiling below 177"C 3.4 indane 2.4 methylindanes 6.5 durene 0.3 indene 6.5 tetralin 0.5 methylindenes 8.2 naphthalene 51.4 methylnaphthaienes 13.5 63 parts by weight of expressed oil are separated in a filter as in Test 1. The remaining crystallizate is washed with 36 parts by weight of the dephenolated filtered naphthalene oil of the same boiling range as in Example 1 and is then washed in three stages with pure methanol in each case.The naphthalene recovered in this way contains 2.5% by weight of impurities boiling at a lower temperature and 1.9% by weight of methylnaphthalenes in addition to 95.3% by weight of naphthalene. When Example 1 is repeated the crystallizate is washed in two stages with pure methanol. The purity of the naphthalene corresponds to the data in Table 1. On account of the different charge products the results are not directly comparable. If the analyses of the methanol solutions after the extraction stages are compared, however, it is evident that the washing with phenol-containing oils improves the selectivity of the methanol as a solvent for the boiling attendants of naphthalene. The naphthalene recovered in Example 1 contains 1.2% by weight of lower boiling constituents, the main component of which is indene.The methanol after the first washing contains 8.1% by weight and that after the second washing contains 1.1% by weight The naphthalene recovered in Example 5 contains 1.1% by weight of indene, the methanol after the first washing contains 2.7% by weight, that after the second washing contains 1.8% by weight and that after the third washing contains 0.6% by weight of indene. The naphthalene in Example 1 is adulterated with 0.6% by weight of 2-methylnaphthalene, the methanol after the first washing is adulterated with 5.2% by weight and that after the second washing is adulterated with 1.1% by weight. The naphthalene in Example 5 contains 1.5% by weight of methylnaphthalene, the methanol after the first washing contains 5.6% by weight, that after the second washing contains 3.8% by weight and that after the third washing contains 2.0% by weight. The naphthalene absorption in the first and second methanol washings in Example 1 corresponds to approximately 14.8 and 8.0% by weight respectively of that of the second and third methanol washings in Example 5 with 14.1 and 8.1% by weight.

The proposed process can be carried out particularly advantageously in plants in which naphthalene oils originating from both carbon and petroleum are reconditioned. The naphthalene in the washing methanol can be recovered up to approximately 80% and from the expressed oil up to approxi,-..3tely 65%. In Example 1 approximately 10% losses of naphthalene occur in the process as a whole during the complete return feed of the filtered naphthalene oil (5) and the naphthalene dissolved in the methanol and reconditioning the resin oil fraction. In the case of the process according to Example 3 the losses are similar to those of Example 1.

On account of the separate filtration of the expressed oil in a quantity of from 88 to 90% by weight relative to the theoretical quantity, a high-valance resin oii is additionally recovered, which can be added without further reconditioning of the resin oil fraction from the reconditioning by distillation of the residual oil of pyrolysis. That means that lower demands may be made upon the separation by distillation of resin oil and naphthalene oil. A common reconditioning of naphthalene fractions from coal tar and residual oil of pyrolysis, on the other hand, would render the recovery of a resin oil fraction considerably more difficult on account of the slight indene and hydrindene content in the naphthalene oil originating from carbon.

Claims (10)

1. A process for the recovery of naphthalene from residual oils of pyrolysis by the steps of distilling the residual oil to produce a naphthalene - containing fraction; crystallizing the said fraction, separate the crystallizate from the expressed oil, (which expressed oil may be returned at least in part to the crystallization stage); and extracting of the crystallizate in one or more stages; in which, prior to the extraction step, the crystallizate is additionally washed with an aromatic oil originating from carbon.

2. A process according to claim 1 in which the aromatic oil is an expressed oil (cresol oil) originating from coal tar from naphthalene crystallization.

3. A process according to Claim 1, in which the aromatic oil is a coal tar oil.

4. A process according to Claim 1, in which the aromatic oil is an oil fraction from carbon hydrogenation.

5. A process according to Claim 1, in which the aromatic oil is fed back at least in part into the process.

6. A process according to Claim 3, in which coal tar oil mixed with naphthalene crystallizate originating from coal tar is used for washing.

7. A process according to any one of Claim 1 to 6 in which the washing is carried out at a temperature of from 20 to 50"C, preferably from 25 to 350C.

8. A process according to any one of Claims 1 to 7 in which the starting product for crystallization has a naphthalene content of from 50 to 65% by weight.

9. A process according to Claim 1 in which the expressed oil of the pyrolysis oil fraction is subsequently used as a resin oil.

10. A process according to Claim 1 substantially as hereinbefore described with reference to the Examples.