GB1588646A

GB1588646A - Process for increasing the yield of aromatic compounds in extractive distillation

Info

Publication number: GB1588646A
Application number: GB4417977A
Authority: GB
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1976-10-27
Filing date: 1977-10-24
Publication date: 1981-04-29
Also published as: ES463576A1; NL7710105A; JPS5356622A; IT1087179B; BE860158A; DE2648638A1; PT67190A; NL183936C; PT67190B; FR2371401A1; NL183936B; FR2371401B1; TR19830A

Description

(54) A PROCESS FOR INCREASING THE YIELD OF AROMATIC COMPOUNDS IN EXTRACTWE DISTILLATION (71) We, METALLGESELLSCHAFT AKTIENGESELLSCHAFT, a body corporate organised under the laws of the German Federal Republic, of Reuterweg 14, 6000 Frankfurton-Main, German Federal Republic, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for increasing the yield of aromatic compounds and of simultaneously decreasing the solvent content of the refined product in the recovery of pure aromatic compounds by extractive distillation.

It has been proposed to improve the yield of extracted product and to obtain an end product of higher purity in extractive distillation by conducting a selective solvent for the substances to be recovered in one stream or in a plurality of partial streams opposite to the mixed feed vapours which rise in a distillation column, the partial stream of solvent fed to an upper plate being kept at a lower temperature than the partial stream or streams of solvent fed to a lower plate, and distillate recycled to the top of the column being replaced at least in part by cold solvent.

It is also been proposed to simplify the recovery of solvent from the refined product obtained by extractive distillation by distilling the refined product and carrying out this re covery at a lower heat cost. In this proposal, the overhead product of the extractive distilla tion is fed as vapour to a distillation column on an intermediate level, solvent, free condensate is recovered as a condensed overhead product of the distillation in said column and is partly refluxed to the distillation column, and solvent which is free from refined product is withdrawn from the heated sump of the distillation column and is recycled to the extractive distil lation column.

In those previously proposed processes, the vapours rising in the extractive distillation col umn must be condensed at such a rate on the solvent-receiving plate that the rate of flow of non-condensed vapours exactly equals the rate at which the refined product is to be withdrawn. This condensation is effected by feeding the solvent at a temperature below the temperature on the solvent-receiving plate. As a result of the inevitable fluctuations in a commercial plant, the rate at which the refined product is withdrawn varies too. If these fluctuations have the result that refined product is withdrawn at a rate which is smaller than the rate of nonaromatic compounds contained in the feed, the surplus non-aromatic compounds can leave the extractive distillation column only together with the pure aromatic compounds. In this case, the content of non-aromatic impurities in the pure aromatic compounds is larger than the upper limit usually specified even though the solvent circulation rate, heat supply rate and number of plates would be sufficient for compliance with the usual purity requirements, e.g., 500 ppm. non-aromatic compounds in pure benzene.

In order to avoid such an occurrence, aromatic compounds must be left in the refined product in such an amount that even in case of fluctuations of the rate of the overhead vapours the rate at which refined product is withdrawn is always at least as high as the rate of nonaromatic compounds in the feed. This practice results in an undesired loss of aromatic compounds in the refined product. Whereas this loss can be avoided if the surplus overhead vapours are refluxed to the top of the column, this results in another disadvantage because the concentration of the non-aromatic compounds having different boiling points is changed on the plates on which the solvent is removed from the refined product. As a result, the non-aromatic compounds having the highest boiling points remain in the solvent which is removed from the refined product and recycled to the extractive distillation column. Because these nonaromatic compounds having the highest boiling points can be removed from the pure aromatic compound only with the greatest difficulty, such reflux which improves the yield will result in a lower purity.

Another disadvantage of the previously pro posed processes is that all hydrocarbons should be removed from the solvent which is removed it a side stream column from the overhead vapours withdrawn from the extractive distillation column. To this end, the solvent must be heated to its boiling point and it is not possible to utilize the heat content of the hot solvent coming from the solvent-stripping column.

Even a hydrocarbon content of 15% may lower the boiling point of the sump product of the side stream column by 50 C. and a hydrocarbon content of 50% results in a temperature decrease of 100"C.

Furthermore, it has surprisingly been found that when the sump product of the side stream column contains more than 50% solvent, the aromatic compound, rather than the nonaromatic compounds having the highest boiling points, is enriched most strongly in comparison to the composition of the refined product. As a result, the yield of the aromatic compound can be slightly improved but difficulty separable non-aromatic compounds are recycled at a very low rate.

It is an object of the invention to avoid the above-mentioned disadvantages and to recycle overhead vapours in excess of the desired rate of refined product to the extractive distillation column without the higher boiling non-aromatic compounds being enriched above their concentration in the refined product. Furthermore, it is desirable that the heat content of the hot solvent coming from the stripping column should be utilized for the removal of the solvent from the refined product.

According to the present invention there is provided a process for increasing the yield of aromatic compounds and of simultaneously decreasing the solvent content of the refined product in the recovery of pure aromatic compounds by extractive distillation, wherein a) a feed consisting of mixed aromatic and non-aromatic compounds to be separated is fed to approximately the middle of an extractive distillation column, b) the solvent is fed to the extractive distillation column at a location between the top of the column and the feed-receiving plate and liquid product consisting of non-aromatic compounds, traces of aromatic compounds and traces of solvent is laterally withdrawn above the solvent-receiving plate, c) the entire liquid product which has been laterally withdrawn is fed to the top of a side stream column, which has 2 to 5 plates and is heated by reboiler, d) the sump product contains at least 50% by weight solvent, is fed to the extractive distillation column between the solvent-receiving plate and the feed-receiving plate; e) the overhead product of the side stream column is fed in the form of vapour to the extractive distillation column at a plate above the plate at which liquid product is laterally withdrawn, f) some of the raffinate which becomes available at the top of the extractive distillation column is withdrawn at a rate which is equal to the sum of the rate of non-aromatic compounds contained in the feed and 0.1 to 5% by weight of benzene in the feed, while the remainder of the raffinate is fed to the extractive distillation column, part of said remainder being fed as reflux at such a rate that all solvent is scrubbed from the vapours rising above the solventreceiving plate, and the rest of said remainder being fed to the extractive distillation column between the solvent-receiving plate and the feed-receiving plate, and g) sump product consisting of solvent and aromatic compounds is withdrawn from the extractive distillation column and is separated in a stripping column into aromatic compounds and solvent.

In one embodiment of the present process, theside stream column is integrated into the extractive distillation column and includes a reboiler, which is disposed in the extractive distillation column above the solvent-receiving plate and the vapours rising from the solventreceiving plate by-pass the integrated side stream column and are fed to the distillation column at a level which is 2 to 5 plates above the reboiler. The liquid product draining from the upper most plates of the extractive distillation column is permitted to flow over the reboiler, and the resulting vapours are fed to the upper plates of the extractive distillation column, while the surplus liquid, or sump product of the integrated side stream column consisting of at least 50% by weight solvent is fed to the extractive distillation column below the solvent-receiving plate. The sump product of the side stream column preferably consists of 80 to 95% by weight solvent.

The reboiler of the side stream column is conveniently heated with hot solvent from the sump of the stripping column.

If the extractive distillation column has 30 to 60 actual plates, then the feed-receiving plate is conveniently disposed approximately at the middle of the column and there are 10 to 30 plates above the feed-receiving plate. In this case, the solvent receiving plate is 5 to 15 plates below the top of the column and 5 to 15 plates above the feed-receiving plate.

The present process may be conducted in a particularly economical manner so that the yield of aromatic compounds to be recovered is improved and the solvent content of the refined product is decreased at the same time so that the purity of the non-aromatic compounds is improved. The aromatic compounds obtained are of a high purity.

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawings, which illustrate diagrammatically and by way of example two embodiments thereof, and in which: Fig. 1 is a plant diagram of a first embodi ment of an extractive distillation plant, and Fig. 2 is a plant diagram of a second embodiment of an extractive distillation plant.

Referring now to Fig. 1 of the drawings, there is shown an extractive distillation plant in which a feed consisting of mixed aromatic and non-aromatic compounds is fed through a conduit 2 to a receiving plate 4, which is disposed approximately at the middle of an extractive distillation column 1 having 30 to 60 actual plates. Heat is supplied to the column 1 by a reboiler 3 and there are 10 to 30 plates above the receiving plate 4. Solvent is fed through a conduit 6 to a solvent-receiving plate 5, which is disposed 5 to 15 plates above the receiving plate 4 and 5 to 15 plates below the top of the column and reflux is fed through a conduit 7 to the uppermost plate. Solvent-free hydrocarbon vapours flow through a conduit 8 from the uppermost plate into a condenser 9 and are condensed therein, the condensate flowing through a conduit 10 into a reflux container 11. Some of the condensate flows through a conduit 13 to a pump and is fed by the latter through the conduit 7 as reflux to the top of the extractive distillation column 1, while some of the condensate is withdrawn through a conduit 14 as refined product. The surplus condensate is fed through a conduit 12 to the extractive distillation column 1 between the feedreceiving plate 4 and the solvent-receiving plate 5.

Instead of the normal weit for the overflow to the next lower plate, the next plate above the solvent-receiving plate 5 is provided with a drain tray, from which all the liquid product is withdrawn through a lateral outlet 15. This liquid product is fed to the top of a side stream column 16, which has 2 to 5 plates. The lower portion of the column is heated in a heat exchanger-reboiler 17 by means of hot solvent, which has been stripped in a column 8 from the aromatic compounds and is fed at a temperature of 170 to 1800C through a conduit 19 into the heat exchanger-reboiler 17. The solvent is subsequently fed through a conduit 20, a cooler 21 and the conduit 6 to the extractive distillation column 1. The vapours rising in the side stream column 16 are fed through a conduit 22 to the extractive distillation column 1 above the solvent-receiving plate 5. The sump product of the side stream column 16 contains at least 50% by weight solvent, and is fed through a conduit 23 to the extractive distillation column 1 between the feed-receiving plate 4 and the solvent-receiving plate 5.

A mixture of solvent and pure aromatic compounds is withdrawn from the sump of the extractive distillation column 1 and is fed through a conduit 25 to the solvent-stripping column 18, in which pure aromatic hydrocarbons are recoverd as overhead product and leave the plant through a conduit 24. The hydrocarbon-free solvent is withdrawn from the solvent-stripping column 18 through a conduit 19 and returned to the extractive distillation column 1 through the heat exchangereboiler 17, the conduit 20, the cooler 21, and the conduit 6.

The plant shown in Fig. 2 differs from that shown in Fig. 1 in that the side stream column 16 is integrated into the main column 1, and the heat exchanger-reboiler 17 is fitted in the column above the solvent-receiving plate 5. A plate 26 having no passage is disposed between the solvent-receiving plate 5 and the reboiler 17 and liquid product on the plate 26 flows from a drain tray through a conduit 23 to a plate which is disposed between the solventreceiving plate 5 and the feed-receiving plate 4.

The vapours rising from the solvent-receiving plate 5 flow through a vapour conduit 28 into the column 1 at a level above a plate 27 which is disposed 2 to 5 plates above the reboiler 17.

There are 5 to 15 plates between the plate 27 and the top of the column.

The invention will now be further illustrated by an example of the operation of an extractive distillation plant constructed in accordance with Fig. 1.

A mixture consisting of 77.3% by weight benzene and 22.7% non-aromatic hydrocarbons having substantially the same boiling characteristics as the benzene are fed at a temperature of 600 C. and at a rate of 1300 kg.lh. on the 20th plate of a distillation column, which has 50 actual plates and is operated under a top pressure of 1 kg./cm.2 aboslute pressure. A selective solvent consisting of N-methylpyrrolidone, hereinafter referred to as NMP, is fed on the 35th plate at a temperature of 85"C. and at a rate of 4000 kg./h.. The reboiler is heated with steam at 10 kg./cm.2 above atmospheric pressure fed at a rate of 500 kg./h. Mainly nonaromatic hydrocarbons containing 1.7% by weight of benzene are withdrawn from the top of the column at a rate of 650 kg./h. and are condensed, the condensate being collected in a reflux container. Condensate at a rate of 300 kg./h. is pumped by a pump from the reflux container to the uppermost plate of the distillation column. Additional condensate at a rate of 300 kg./h. is withdrawn from the plant as refined product, and condensate at a rate of 50 kg./h. is fed to the 30th plate of the distillation column.

Vapours which still contain 7 kg./h. NMP rise from the 35th plate to the 36th plate at a rate of 360 kg./h. From the 36th plate, the entire liquid product flows at a rate of 310 kg./ h. via a drain tray onto the uppermost plate of a side stream column having 3 actual plates.

The reboiler of the side stream column is heated with hot solvent, which is fed at 1750C.

from the stripping column and leaves the reboiler at 155 C., a temperature of 135"C. prevailing in the sump of the side stream column.

Vapours at a rate of 300 kg./h. leave the side stream column at its top and are fed to the 36th plate of the main column. Mixed liquids which contain 70% NMP are withdrawn at a rate of 10 kg./h. from the sump of the side stream column and are fed to the 25th plate of the main column. A mixture of 80% by weight NMP and 20% benzene is withdrawn from the sump of the main column at a rate of 5000 kg./ h. and at a temperature of 145"C. and is fed to the 10th plate of the stripping column.

The stripping column has 30 actual plates and is operated at a top pressure of 0.3 kg./ cm.2 absolute pressure and a reflux ratio of 0.5:1. Pure benzene at a rate of 1000 kg./h.

is withdrawn from the top of the column and in dependence upon the composition of the non-aromatic compounds contained in the feed contains 50 to 500 ppm. non-aromatic compounds, mainly methyl cyclohexane. NMP at a temperature of 175"C. and at a rate of 4000 kg./h. is withdrawn from the sump of the stripping column and by delivering part of its heat content in the reboiler of the side stream column is cooled to 1550 C. and in an aftercooler is cooled to 850C before returning to the main column.

WHAT WE CLAIM IS: 1. A process for increasing the yield of aromatic compounds and of simultaneously decreasing the solvent content of the refined product in the recovery of pure aromatic compounds by extractive distillation, wherein a) a feed consisting of mixed aromatic and non-aromatic compounds to be separated is fed to approximately the middle of an extractive distillation column, b) the solvent is fed to the extractive distillation column at a location between the top of column and the feed-receiving plate and liquid product consisting of non-aromatic compounds, traces of aromatic compounds and traces of solvent is laterally withdrawn above the solventreceiving plate, c) the entire liquid product which has been laterally withdrawn is fed to the top of a side stream column, which has 2 to 5 plates and is heated by a reboiler, d) the sump product of the side stream column, which sump product contains at least 50% by weight solvent, is fed to the extractive distillation column between the solvent-receiving plate and the feed-receiving plate; e) the overhead product of the side stream column is fed in the form of vapour to the extractive distillation column at a plate above the plate at which liquid product is laterally withdrawn, f) some of the raffinate which becomes available at the top of the extractive distillation column is withdrawn at a rate which is equal to the sum of the rate of non-aromatic compounds contained in the feed and 0.1 to 5% by weight of benzene in the feed, while the remainder of the raffinate is fed to the extractive distillation column, part of said remainder being fed as reflux at such a rate that all solvent is scrubbed from the vapours rising above the solventreceiving plate, and the rest of said remainder being fed to the extractive distillation column between the solvent-receiving plate and the feed-receiving plate, and g) sump product consisting of solvent and aromatic compounds is withdrawn from the extractive distillation column and is separated in a stripping column into aromatic compounds and solvent.

2. A process as claimed in claim 1, wherein the side stream column is integrated into the extractive distillation column and includes a reboiler, which is disposed in the extractive distillation column above the solvent-receiving plate and the vapours rising from the solvent-receiving plate by-pass the integrated side stream column and are fed to the distillation column at a level which is 2 to 5 plates above the reboiler, and wherein the liquid product draining from the uppermost plates of the extractive distillation column is permitted to flow over the reboiler, and the resulting vapours are fed to the upper plates of the extractive distillation column, while the surplus liquid or sump product of the integrated side stream column consisting of at least 50% by weight solvent is fed to the extractive distillation column below the solventreceiving plate.

3. A process as claimed in claim 1 or 2, wherein the reboiler of the side stream is heated with hot solvent from the sump of the stripping column.

4. A process as claimed in any preceding claim, wherein the sump product of the side stream column consists of 80 to 95% by weight solvent.

5. A process as claimed in any preceding claim, wherein the extractive distillation column has 30 to 60 actual plates, the feedreceiving plate is disposed approximately at the middle of the column and there are 10 to 30 plates above the feed-recieving plate, and wherein the solvent receiving plate is 5 to 1 5 plates below the top of the column and 5 to 15 plates above the feed-receiving plate.

6. An extractive distillation process substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. which contain 70% NMP are withdrawn at a rate of 10 kg./h. from the sump of the side stream column and are fed to the 25th plate of the main column. A mixture of 80% by weight NMP and 20% benzene is withdrawn from the sump of the main column at a rate of 5000 kg./ h. and at a temperature of 145"C. and is fed to the 10th plate of the stripping column. The stripping column has 30 actual plates and is operated at a top pressure of 0.3 kg./ cm.2 absolute pressure and a reflux ratio of 0.5:1. Pure benzene at a rate of 1000 kg./h. is withdrawn from the top of the column and in dependence upon the composition of the non-aromatic compounds contained in the feed contains 50 to 500 ppm. non-aromatic compounds, mainly methyl cyclohexane. NMP at a temperature of 175"C. and at a rate of 4000 kg./h. is withdrawn from the sump of the stripping column and by delivering part of its heat content in the reboiler of the side stream column is cooled to 1550 C. and in an aftercooler is cooled to 850C before returning to the main column. WHAT WE CLAIM IS:

1. A process for increasing the yield of aromatic compounds and of simultaneously decreasing the solvent content of the refined product in the recovery of pure aromatic compounds by extractive distillation, wherein a) a feed consisting of mixed aromatic and non-aromatic compounds to be separated is fed to approximately the middle of an extractive distillation column, b) the solvent is fed to the extractive distillation column at a location between the top of column and the feed-receiving plate and liquid product consisting of non-aromatic compounds, traces of aromatic compounds and traces of solvent is laterally withdrawn above the solventreceiving plate, c) the entire liquid product which has been laterally withdrawn is fed to the top of a side stream column, which has 2 to 5 plates and is heated by a reboiler, d) the sump product of the side stream column, which sump product contains at least 50% by weight solvent, is fed to the extractive distillation column between the solvent-receiving plate and the feed-receiving plate; e) the overhead product of the side stream column is fed in the form of vapour to the extractive distillation column at a plate above the plate at which liquid product is laterally withdrawn, f) some of the raffinate which becomes available at the top of the extractive distillation column is withdrawn at a rate which is equal to the sum of the rate of non-aromatic compounds contained in the feed and 0.1 to 5% by weight of benzene in the feed, while the remainder of the raffinate is fed to the extractive distillation column, part of said remainder being fed as reflux at such a rate that all solvent is scrubbed from the vapours rising above the solventreceiving plate, and the rest of said remainder being fed to the extractive distillation column between the solvent-receiving plate and the feed-receiving plate, and g) sump product consisting of solvent and aromatic compounds is withdrawn from the extractive distillation column and is separated in a stripping column into aromatic compounds and solvent.