US3720605A - Process for the extraction of aromatic hydrocarbons by keto and aldo-morpholines - Google Patents

Abstract

A multi-stage countercurrent process for the solvent extraction of aromatic hydrocarbons from a mixed feed stock is disclosed wherein the feed is contacted with a keto- or aldo-morpholine and extracted in a first stage, extract from the first stage is purified in a second stage by countercurrent contact with an overhead fraction from a third stage wherein the second stage extract is distilled, and rectification is effected in a fourth stage wherein water is refluxed and a part of the water, obtained as reflux free of solvent, is vaporized and recycled to the bottom of this rectification stage to recover the sensible heat of lean solvent.

Description

United States Patent Paret et al.

M IMarch 13, 1973 1 PROCESS FOR THE EXTRACTION 0F 3,551,327 12/1970 Kelly et al. ..208/321 AROMATICHYDROCARBONS BY 3,558,480 1/1971 Broughton ....208/32l m0 ALDO-MORPHOHNES 2 522 233 24:22? 2:;::::;s-.--- "@2232:

[75] Inventors: Giancarlp BagtLMilg-no; Ermanno Cinelli, San Donato Milanese, both Primary Examiner fle beft Levine of Italy Attorney-Ralph M. Watson Assignee: Snam ProgettiS.p.A., Milan, Italy 7 BSTR T 221 Filed: July 17,1970 [5 1 A AC A multi-stage countercurrent process for the solvent [21] Appl' 55908 extraction of aromatic hydrocarbons from a mixed feed stock is disclosed wherein the feed is contacted [30] Foreign Application Priority Data with a ketoor aldo-morpholine and extracted in a first stage, extract from the first stage is purified in a July 18, 1969 Italy ..l98l2 A/69 Second stage by countetcuttent Contact with an ovet head fraction from a third stage wherein the second 5 CL "208/321, 208/313 208/317 stage extract is distilled, and rectification is effected in 208/318 208/326 a fourth stage wherein water is refluxed and a part of [51] Int. Cl. ..C10g 21/28 the water, obtained as reflux free of solvent, is

[ Fleld 0t Search 326 vaporized and recycled to the bottom of this rectification stage to recover the sensible heat of lean solvent.

[56] References Cited UNITED STATES PATENTS 7 Claims, 5 Drawing Figures 3,487,013 12/1969 Van Tassel] ..208/32l PATH-HFUHAR 1 31913 SHEET 3 OF 3 PROCESS FOR THE EXTRACTION F AROMATHC HYDROCARBONS BY KETO AND ALDO- MORPHOLINES The present invention relates to a process for the extraction of aromatic hydrocarbons from an hydrocarbon mixture.

More particularly the process according to the present invention relates to an improved process for the extraction of aromatic hydrocarbons by using a well defined class of solvents selected from keto and aldomorpholines.

Several processes for extracting aromatic hydrocarbons from petroleum and petro-chemical mixtures are well known.

Said processes make use of solvents which selectively dissolve aromatic hydrocarbons.

Generally the processes of extraction by solvent of aromatic hydrocarbons comprises four main states (see FIG. 1):

l. The first stage consists in contacting the feed mix ture with the solvent, generally in a countercurrent contact system. The two streams obtained are a raffinate with a very low contentof aromatic hydrocarbons and an extract containing practically all the aromatic hydrocarbons present in the feed mixture;

2. The second stage, again a liquid-liquid extraction, consists in contacting in countercurrent relation the extract of the first stage with the overhead products of the third stage.

This contact is realized with the purpose of removing the heavier non-aromatic hydrocarbons present in the extract of the first stage and coming from the feed mixture.

The two streamscoming from the second stage are a light one, sent to the first stage, and a bottom product sent to the third stage.

Particularly this bottom product is constituted by solvent, aromatic'hydrocarbons and saturated hydrocarbons too.

Said saturated hydrocarbons are not the same present in the extract of the first stage either as composition or generally as amount.

For this reason generally a solvent is selected that presents a good selectivity between aromatic and nonaromatic hydrocarbons and at the same time a good selectivity for lower boiling homologous hydrocarbons having different numbers of carbon atoms.

3. The third stage consists in an extractive distillation for the extract coming from the second stage.

Said distillation gives an overhead product consisting of the saturated hydrocarbons, present in the extract of the second stage, and part of the lighter aromatic hydrocarbons. i

This overhead stream is refluxed to the second stage, while the bottom product of the extractive distillation consists of solvent and of aromatics, the latter already at the desired purity.

4. The fourth stage consists in a distillation of the bottom product coming from the third stage. In this way aromatics are obtained as overhead and solvent-as bottom product.

The first and second stages may be carried out in a single apparatus. The third and fourth stages are on the contrary preferably carried out in different apparatuses.

Also when the aromatic hydrocarbons in the overhead of the third stage are already at the desired purity, the use of two different apparatuses for the third and fourth stages allows an easier reaching of the desired purity and the use of heat at lower thermal level.

In the connection of these four stages above mentioned several variations are possible that allow, according to different conditions, with the same aromatics recovery, important consumption savings and improvements in the purity of the extracted aromatics.

It is known for instance to introduce into the reflux to the second stage light saturated hydrocarbons with the result of removing the heavy saturated hydrocarbons from the extract sent to the second stage. Since these light saturated hydrocarbons have boiling points different from the ones of the aromatics to be recovered, their separation becomes easier in the subsequent stages.

It is also known to introduce water into the reflux to the second stage. This water dissolves into the solvent and it increases the solvent selectivity improving the removal from the extract of the hydrocarbons having lower polarity, i.e. increasing the purity of the aromatics contained in said extract.

it is an object of the present invention to provide an improved cycle of extraction which allows one to obtain aromatic hydrocarbons at high recovery and purity degree with reduced heat consumption.

According to the present invention the overhead from the third stage, to be sent to the second stage, is rectified in absence of solvent (see FIG. 2). In this way from the bottom of the rectifying stage the heavier products containing saturated hydrocarbons having high boiling points and non-aromatic hydrocarbons having a very high polarity as some hydrocarbons in the class of naphthenes, biand polycyclic naphthenes, olephines, cyclo-olephines and derived compounds, are obtained. As overhead products saturated and light aromatic hydrocarbons are obtained and sent to the second stage, while the bottom may be drained or sent to suitable points of the extraction cycle, so as to avoid its accumulation into the cycle. Said stream, i.e. the bottom, may be introduced into the first stage between the feed inlet and the solvent inlet. Said separation is possible also, in the easiest cases, by condensation at two different thermal levels, refluxing to the second stage the condensate coming from the lower thermal level.

The overhead of the third and fourth stages contains large amount of water. The water contained in the overhead of the third stage is sent, as it is, to the second stage, according to known techniques.

It is a feature of the process according to the present invention to make use of the water separated from the overhead of the fourth stage. This water does not contain any solvent.

According to the process of the present invention said water is divided in four variable streams depending on the operative conditions and it is utilized as follows (see FIG. 3). One stream is refluxed to the top of the stripper, i.e. fourth stage, with the advantage of washing the rising vapors and of avoiding the presence of solvent in the overhead. A second stream is recycled to the second stage, while a third stream is utilized to wash the rafflnate and then is mixed with the preceding stream.

The remaining water is vaporized by indirect heat exchange with the lean solvent coming from the bottom of the fourth stage and is introduced at the bottom of the stripper, i.e. the fourth stage.

Said operation, carried out in presence of water does not contain any solvent, allows one to lower the thermal level and therefore to improve the heat recovery yield. According to the described process, the aldehydic and ketonic derivatives of morpholine and particularly the N-formyl morpholine with water content up to 30 percent by weight are particularly efficient, though any other solvent presenting a good selectivity and transport capacity for the aromatics may be utilized in the described process. The temperature in the extraction operations of the first and second stages ranges from C and 90C while in the distillation operations of the third and fourth stages is ranges from 50C and 200 C.

The behavior of N-formyl morpholine mixed with water at the temperature above mentioned, with respect to its thermal stability and corrosiveness of carbon steel has been surprisingly good, when particular expedients are adopted.

Said expedients consist mainly in an accurate removing of the oxygen and in the use of suitable inhibitors.

When the feed has a very high content in aromatics, in the overhead of the third stage it is preferred, according to the process of the present invention, to obtain aromatics at the desired value of purity and to recycle them as reflux to the second stage.

Also in this case it is a feature of the present invention to effect the third and fourth stages in two different apparatuses. In the other cases the achievement of said conditions should be too onerous therefore the reflux consists of aromatic and saturated hydrocarbons.

in said conditions there is the possibility of accumulation of the non-aromatics presenting as anomalous polarity and already mentioned, and the above mentioned rectification operation is necessary when this condition comes true.

It is worthy of notice that said operation proved itself to be advantageous also when there is not an accumulation of not desired products; the above separation allows one, in any case, to obtain from the third stage an extract containing aromatics of higher purity.

According to the fundamental cycle of solvent extraction, when a light saturated fraction is added to the overhead of the third stage, to be recycled to the second stage, exploiting in that way the selectivity between light and heavy saturated hydrocarbons of the solvent, we found very satisfactory the following way of operation. As the light saturated fraction has the tendency to pass into the raffinate of the second stage to the first stage a recovery is necessary so as to maintain in the cycle a constant amount of said light saturated hydrocarbons without large make up. The make up practice can be effected either on the raffinate of the first stage or on the raffinate of the second stage.

We have noticed that by effecting it on the second stage raffinate, instead of the first stage one, several operative advantages are gotten, like a reduction of the solvent amount necessary for the first operation and therefore of the column dimension and of the heat consumption of the whole cycle.

As illustrative but not limitative example we will describe the process of the present invention referring to FIG. 4.

Through pipe 1 the feed mixture is sent to the lower part of the extractor 2 where a multi-stage contact in countercurrent relation is effected, the solvent is introduced at the top of the extractor through line 3.

The raffinate leaves the extractor from the top through line 4 and is fed to the bottom of the washing column 5 where, to the top, water is fed through 6. From the top the raffinate leaves the column 5 through 7 while from the bottom, through line 8, the washing water containing traces of solvent before present in the raffinate leaves the same column.

From the bottom of column 2, through line 9, the aromatic enriched solvent goes to the top of columns 10, where the purification agent is introduced at the bottom of said column through 1 I.

Said stream, in the present case, consists of water, light aliphatic hydrocarbons and the lightest aromatics.

From the top of the extractor 10, through 12 the raffinate leaves said purification extractor l0. Said stream is fed to the rectification column 13, through 12, where the light aliphatic hydrocarbons are separated and withdrawn as overhead through 14, while the heavier fraction is obtained as bottom product and recycled through 15 to the bottom of the extractor 2. It is evident that the light aliphatics, except the amount sent to the extractive distillation column, effect their function in a closed loop between the columns 10 and 13, while the amount sent to the extractive distillation works in a wider closed loop.

From the extractor 10, as bottom product, an extract having a high purity is obtained and through line 16 is fed to the extractive distillation column 17. In said column, according to the above mentioned techniques, the extractive distillation of the obtained extract is effected.

The selective function of the solvent allows one to obtain as overhead product, through line 18, all the non-aromatic hydrocarbon compounds together with a part of the present lighter aromatics.

Said stream is condensed and sent to the column 19 where a rectification is effected and an overhead is separated, withdrawn through 20 and reused in closed loop.

From the bottom of column 19, through line 21, an high boiling fraction is withdrawn containing also the non-aromatic hydrocarbons presenting anomalous polarity, said fraction is recycled to the extractor 2 at a point above the feed inlet. In this way an accumulation of the anomalous polarity hydrocarbons is avoided. if said compounds are not present said stream may be added to the feed mixture. From the bottom of column 17 the solvent, containing the specification purity aromatics, is obtained.

Said stream, through line 22, is fed to the column 23 where the separation of the hydrocarbons from the solvent is carried out.

The lean solvent is withdrawn from the bottom through line 24, sent to the exchanger 25 where most of its sensible heat content is recovered, and then the solvent is reused in the extraction. To the top of the column 23 water is refluxed through 26 so as to obtain an overhead product without solvent.

Through line 27 an overhead is obtained, condensed and decanted in the decanter 28 wherein the upper The lower water consisting layer is withdrawn through line 30.

Part of this water is refluxed to the column 23 through 26, while another part is sent as washing agent to the column 5 through line 6.

The remaining water is divided and one part is fed, through 31, to the exchanger 25 and sent vaporized to the bottom of column 23. The remainder is mixed through line 32, with the streams of lines 20, 14, 8 and enters, as purification agent, the extractor through 11.

For the purpose of better illustrating the process of the present invention a non-limitative example is given:

EXAMPLE With reference to the simplified scheme on FIG. 5, as feed mixture a gasoline of the following composition has been utilized:

Benzene 7o b.w. Toluene X: b.w. Xylene 25 b.w. Saturated hydrocarbons b.w.

Said feed was sent to the 24th tray of the liquid-liquid extraction column 1, provided with 60 perforated trays, at a flow rate of 5 kg/hr. .To the top of column 1 as solvent for formyl morpholine at 5% b.w. of water was fed, at a flow rate of 15kg/hr; to the bottom of the column 1 the lighter fraction of the column 2 overhead was fed, at a flow rate of 1.8 kg/hr with 0.7 kg of water; the heavier fraction of the overhead of column 2 condensed at temperature higher than 120C. at a flow rate of 0.2 kg/hr was mixed with the feed.

The column operated at temperature ranging from and C.

The extract flowing out from the extractor 1 was heated to 80C and fed to the head tray of the column 2 provided with 27 trays, operating without reflux; the

, bottom temperature of column 2 was kept at 120C and the overhead condensed at two different temperatures in the condensers 3 and 4; in the condenser 3 0.2 kg of hydrocarbons are condensed, in the condenser 4 1.8 kg.

The stream condensed in the condenser 3, operating at higher temperature, was mixed with the feed, the stream condensed in 4 was sent to the bottom of the extractor l. The enriched solvent flowing out from the bottom of the column 2 was sent to the next distillation column 5, provided with 22 trays, where at the top pure aromatics with 2.2 kg/hr of water and at the bottom the solvent containing 5% b.w. of water were obtained; to the bottom of column 5, operating at 152C, steam was fed at a rate of 1.5 kg/hr.

The solvent flowing out from the bottom of column 5 contained only traces of aromatics and was cooled and refluxed to the top of the extractive column 1.

In said conditions from the next distillation of the aromatic mixture, xylenes with 300 ppm of non-aromatics were obtained.

Operating at the same conditions, but without the separation effected by the condenser 3, the non-aromatics content was of 800 ppm.

What we claim is:

l. A process for the extraction of aromatic hydrocarbons corn rising a first stage of solvent extraction wherein t e feed mixture 15 contacted with solvent selected from the group consisting of anhydrous and aqueous ketonic and aldehydic derivatives of morpholine and with a low boiling fraction of hydrocarbons coming from a subsequent stage in a multi-stage system in countercurrent, a second stage of purification of the extract of the first stage by solvent extraction wherein said first stage extract is contacted in countercurrent with a hydrocarbon fraction coming at least in part from a subsequent third stage of extractive distillation, a third stage of extractive distillation of the extract, coming from the second stage, which gives an overhead to be utilized in the second stage as purification agent and finally a fourth stage of rectification of the bottom of the third stage so as to separate the aromatics from said solvent which is then recycled to the first stage characterized in the combination of at least two of the following steps consisting in:

carrying out the final rectification with water reflux; separating the raffinate of the second stage into a fraction of light aliphatic hydrocarbons and a fraction of heavier aliphatic hydrocarbons and recycling said heavier aliphatic hydrocarbons to the first stage; separating the overhead of the third stage into a low boiling fraction and a high boiling fraction;

utilizing the low boiling fraction of the overhead of the third stage of extractive distillation as reflux to the second stage of purification;

recycling the high boiling fraction of the overhead of the third stage of extractive distillation to the first stage, at a point intermediate the introduction of feed and solvent to said first stage;

vaporizing a part of the water, free of solvent, ob-

tained as overhead of the fourth stage of final rectification and recycling said vapor to-the bottom of the same rectification stage, so as to recover the sensible heat of the lean solvent.

2. A process according to claim 1 characterized in that the solvent is anhydrous or aqueous formylmorpholine.

3. A process according to claim 1 characterized in that the separation of the low boiling fraction of the overhead from the third stage of extractive distillation is carried out by rectification in absence of solvent.

4. A process according to claim 1 characterized in that the separation of the low boiling fraction of the overhead coming from the third stage of extractive distillation is carried out by condensation in two or more stages.

5. A process according to claim 1 characterized in that a low boiling aliphatic fraction is added to the reflux fraction of the second stage of extract purification.

6. A process according to claim 1 characterized in that an aqueous stream is added to the reflux fraction of the second stage of purification.

7. A process according to claim 1 characterized in that the reflux fraction of the second stage of purification of the extract is constituted by aromatic hydrocarbons already at final specification purity and it is produced in a stripper different from that one which gives the final product.

Claims (6)

1. A process for the extraction of aromatic hydrocarbons comprising a first stage of solvent extraction wherein the feed mixture is contacted with solvent selected from the group consisting of anhydrous and aqueous ketonic and aldehydic derivatives of morpholine and with a low boiling fraction of hydrocarbons coming from a subsequent stage in a multi-stage system in countercurrent, a second stage of purification of the extract of the first stage by solvent extraction wherein said first stage extract is contacted in countercurrent with a hydrocarbon fraction coming at least in part from a subsequent third stage of extractive distillation, a third stage of extractive distillation of the extract, coming from the second stage, which gives an overhead to be utilized in the second stage as purification agent and finally a fourth stage of rectification of the bottom of the third stage so as to separate the aromatics from said solvent which is then recycled to the first stage characterized in the combination of at least two of the following steps consisting in: carrying out the final rectification with water reflux; separating the raffinate of the second stage into a fraction of light aliphatic hydrocarbons and a fraction of heavier aliphatic hydrocarbons and recycling said heavier aliphatic hydrocarbons to the first stage; separating the overhead of the third stage into a low boiling fraction and a high boiling fraction; utilizing the low boiling fraction of the overhead of the third stage of extractive distillation as reflux to the second stage of purification; recycling the high boiling fraction of the overhead of the third stage of extractive distillation to the first stage, at a point intermediate the introduction of feed and solvent to said first stage; vaporizing a part of the water, free of solvent, obtained as overhead of the fourth stage of final rectification and recycling said vapor to the bottom of the same rectification stage, so as to recover the sensible heat of the lean solvent.

2. A process according to claim 1 characterized in that the solvent is anhydrous or aqueous formyl-morpholine.

3. A process according to claim 1 characterized in that the separation of the low boiling fraction of the overhead from the third stage of extractive distillation is carried out by rectification in absence of solvent.

4. A process according to claim 1 characterized in that the separation of the low boiling fraction of the overhead coming from the third stage of extractive distillation is carried out by condensation in two or more stages.

5. A process according to claim 1 characterized in that a low boiling aliphaTic fraction is added to the reflux fraction of the second stage of extract purification.

6. A process according to claim 1 characterized in that an aqueous stream is added to the reflux fraction of the second stage of purification.

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