US3422163A - Recovery of aromatics from multiple hydrocarbon streams - Google Patents

Description

Jan. 14, 1969 G. F. AssELlN 3,422,163

RECOVERY OF AROMATICS FROM MULTIPLE HYDROCARBON STREAMS Filed OCT.. 2l, 1965 United States Patent O 3,422,163 RECOVERY F AROMATlCS FROM MULTIPLE HYDROCARBON STREAMS George F. Asselin, Mount Prospect, lll., assigner to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed Oct. 21, 1965, Ser. No. 499,972

U.S. Cl. 260-674 11 Claims Int. Cl. C07c 7/10; C10g 21/00 ABSTRACT 0F THE DISCLOSURE Solvent extraction process to which plural feed hydrocarbon streams are charged. A first feed, comprising aromatics and paraflins, is introduced to a lower intermediate point in the height of a countercurrent solvent extraction column. A second feed, comprising aromatics and naphthenes, is introduced to a middle intermediate point of the extraction column. An aromatics-enriched solvent extract phase, withdrawn from the bottom of the extraction column, is subjected to extractive distillation to strip substantially all of the remaining non-aromatics therefrom.

This invention relates to a process for the separation of hydrocarbons from at least two hydrocarbon mixtures with the aid of a selective solvent for the purpose of separating and recovering the aromatic hydrocarbon components of the mixtures. The process of the present invention serves to segregate the particular species of aromatic hydrocarbons, such as benzene, toluene and/or C8 aromatics from other classes of hydrocarbons that are normally contained in petroleum distillates, and utilizes a solvent which may be indefinitely recycled in the system, yields the desired hydrocarbon product in a state of high purity and separates the same substantially in its entirety from the feed stocks charged to the process.

The present invention is concerned with an improvement in the type of separation process wherein at least two mixtures of various classes of hydrocarbons, the nonaromatic components of which in one mixture are more readily stripped out of a solvent-aromatic mixture in an extractive stripper than the non-aromatic components of the other mixture,` are introduced into an extraction zone at an intermediate pointthereof and are countercurrently contacted therein with a solvent selective for aromatic hydrocarbons, a lean solvent is introduced into an upper point in the extraction zone, a rainate phase comprising substantially all of the nonaromatic hydrocarbons in the feed stocks is removed from the upper end portion of the extraction zone, an extract comprising the aromatic components of the feed stocks dissolved in the solvent phase is removed from the lower end portion of the extraction zone and the aromatic solute is subsequently recovered by extractive stripping and fractionation of the extract phase.

In accordance with the present invention, said one mixture containing non-aromatic components having a tendency to be more readily stripped out of the extract phase in an extractive stripper than the non-aromatic components in the other mixture is introduced into the extraction zone at a point closer to the withdrawal of extract from the zone than the other mixture in order to improve the aromatics purity and/ or yield as described hereinafter.

In one of its embodiments, this invention relates to a process for the separation and recovery of aromatic hydrocarbons from at least two hydrocarbon containing fluids having a solvent extraction step to form an extract phase and a raffinate phase and an extractive distillation step to remove non-aromatic hydrocarbons from the 3,422,163 Patented Jan. 14, 1969 extract phase in which a substantial portion of the nonaromatic components of the first fluid have an increased volatility in the extractive distillation step when compared to a substantial portion of the non-aromatic components of the other fluid which comprises: introducing a lean solvent into an upper intermediate point in the height of an extraction zone; introducing the other fluid into a middle intermediate point in the height of an extraction zone; introducing the first fluid into .a lower intermediate point in the height of an extraction zone; countercurrently contacting the solvent with the fluids in the extraction Zone; removing a hydrocarbon reaflinate phase consisting essentially of non-aromatics yfrom an upper point of said zone; removing a solvent extract phase enriched in aromatics content from a lower point of said Zone; introducing the extract phase into an extractive distillation column and stripping essentially all of the non-aromatic hydrocarbons from the extract phase; and separating the aromatics from the solvent.

Solvent extraction processes for the recovery of aromatic hydrocarbons from hydrocarbon mixtures have been known for many years. This is usually accomplished by methods such as extractive distillation or alternately by countercurrently contacting the feed with a solvent selective for aromatics in the liquid phase within an extractor. The present process relates to both of these methods. In general, the solvents described hereinafter exhibit an increasing solubility selectivity for hydrocarbons as the hydrocarbons degree of unsaturation increases. Thus, the selectivity of a solvent is -greater for aromatics than naphthenes, greater for naphthenes than parai'lins, etc. Likewise in general a solvent exhibits an increasing solubility selectivity for a given class of hydrocarbons as the number of carbon atoms per molecule decreases. In the past it has been a common practice to introduce the feed or feeds into an intermediate point in an extraction zone. In cases wherein the solvent is denser than the feed, the solvent is introduced into an upper point in the extraction zone and the feed and solvent being immiscible, countercurrently contact each other therein. The more unsaturated feed cornponents tend to dissolve in the solvent phase to a greater extent than the more saturated feed components. However, a completely clean separation of aromatics cannot be attained since a small amount of the more saturated feed components will also dissolve into the solvent phase. Therefore, the extract phase is sent to an extractive stripper in order to vaporize the more saturated feed components and leave a solvent phase containing only aromatic hydrocarbons. The stripper is able to remove the non-aromatic components of the extract phase because the presence of the solvent in the stripper has less effect on the boiling point of the non-aromatic components dissolved in the solvent than it does on the aromatic compounds also dissolved in the solvent. In general, the solvent tends to retain in solution the more soluble aromatic component even at temperatures considerably above the normal boiling point of the aromatics alone. This effect can be further emphasized by introducing additional lean solvent into the stripper along with the extract phase. However, the amount of suppression of boiling point of the aromatics is limited and when processing charge stocks of wide boiling ranges it becomes impossible to vaporize the heavy non-aromatics without also vaporizing the light aromatics. lf these heavier non-aromatics are not removed from the extract phase in the stripper they will ultimately reduce the purity of and contaminate the heavier aromatics. Attempts have been made in the prior art to overcome this problem by methods such as the use of a light paraffin backwash introduced into a point in the extraction zone between the feed introduction and extract withdrawal to displace the heavier and more diffi- ICC cult to separate non-aromatic components from the extract phase as shown in U.S. Patent No. 3,037,062. In many cases the use of a light backwash material is not necessary since the feed is of intermediate boiling range and although satisfactory aromatic purity and recoveries can be made, nevertheless, the concentration of non-aromatics in the aromatics is higher than desired. In these cases it may not be economically justified to buy the extra equipment. Likewise the feed may be of such a narrow boiling range that a backwash stream is not very effective. For example, when using a feed stock composed of a reformate and a hydrotreated pyrol-ysis oil, both of which are of about the same narrow boiling range, the present invention is employed to produce just as -good results as could be obtained by employing a backwash stream. It is also possible to combine the present invention along with the backwash step to further improve the process for the production of aromatics.

It is an object of this invention to overcome the foregoing contaminant problem in solvent extraction processes in a simple, inexpensive manner.

It is another object of this invention to use the more saturated non-aromatic components of one feed to displace the more unsaturated non-aromatic components of a second feed from the extract phase.

It is still another object of this invention to use the lower molecular weight non-aromatic components of one feed to displace the higher molecular weight non-aromatit` components of a second feed from the extract phase.

It is a further object of this invention to increase the yield and/ or purity of recovered aromatics in a solvent extraction process.

These and other objects will become more apparent especially in the light of the following detailed description.

Referring to the accompanying drawing, feed `#l is introduced into ow conduit where it picks up a reux stream from a source described hereinafter and flows into a lower intermediate point in extraction zone 3. Preferably, feed `.iii-'1 is introduced into extractor 3 from about 1A; to about 1/12 of the way along its height from the bottorn. Feed #2 is introduced into flow conduit 4 where it ows into a middle intermediate point in extractor 3. Preferably, feed #2 is introduced into extractor 3 from about 1A; to about 1/10 of the way along its height from the bottom. The distance between the lower intermediate point and the middle intermediate point is preferably at least 4% of the total height of extractor 3. Lean solvent from a source described hereinafter is introduced into an upper intermediate point through flow conduit 6 whereupon the solvent ows downflow through extractor 3 and countercurrently contacts the feeds which flow upow through extractor 3. The extractor contains contacting means such as bafe plates, sieve decks, rotary discs, packing, etc., to eiciently intermix the immiscible solvent and feed phases. The extractor is maintained at an elevated temperature and pressure sufficient to maintain the solvent and the feeds in the liquid phase. Since the solvent has a selectivity for the aromatics and the solvent is immiscible with the feeds, aromatics will preferentially dissolve into the solvent phase. Thus, as the solvent passes downow through the upper portion of the extractor, the aromatics concentration therein gradually increases while as the feed hydrocarbon phase passes uptow through the extractor due to its lower density (than the solvent phase) the aromatics concentration therein gradually decreases. When the hydrocarbon phase passes the upper intermediate point, it contains essentially no aromatics. This material is called the raffinate phase and is removed from extractor 3 through an upper point where it passes into flow conduit 7. The raflinate may be fractionated and/ or water-washed to remove small quantities of dissolved solvent.

The extract phase comprising solvent, substantially-all the aromatics in the Ifeeds, and some non-aromatics is removed from a lower point in extractor 3 where it passes into ow conduit 8. The non-aromatics present in the extract phase are derived from the non-aromatics present in the material flowing in flow conduit 5, which is the recycle stream in flow conduit 15 and feed #1. Feed #1 contains non-aromatics which are more readily vaporized in the extractive stripper described hereinafter than the non-aromatics present in feed .#2. For example, a feed #l having the non-aromatic portion rich in paraftins and a feed #2 having the non-aromatics portion rich in naphthenes are employed as feeds and are introduced into extractor 3 as described hereinabove. Paraflins are less selectively absorbed by the solvent phase :but since their concentration is higher in the hydrocarbon phase between ow conduits 4 and 5 in the height of the extractor, there is suflicient driving force to allow the paraflins to displace the naphthenes from the solvent phase to the hydrocarbon phase. Therefore, the extract phase leaving extractor 3 through ow conduit 8 contains predominantly parafns as the non-aromatic portion. These parafiins are easier to strip out of the extract in extractive stripper 11, described hereinafter, and will result in higher aromatic purities in the finished aromatic product. As another example feed #1 is lighter in molecular weight and lower in boiling point than feed '#2, In this case, light nonaromatic hydrocarbons are doubly effective in displacing the 4heavier non-aromatics since they are more selectively absorbed by the solvent phase and are present in high concentrations in the hydrocarbon phase between flow conduits 4 and 5 (relative to the heavy non-aromatics). Therefore, the extract phase leaving extractor 3 through flow conduit 8 contains predominantly light non-aromatics as the Anon-aromatic portion. This latter example can be economically and efficiently carried out even with a single feed stock by introducing said feed into a splitter column, removing a light feed stock portion overhead and introducing it into the lower inter-mediate point in the extraction zone while removing a heavy feed stock portion from the bottom and introducing it into the middle intermediate point. The important point in all these cases is that the feed whose non-aromatic components are more volatile in an extractive stripper is introduced into a point closer to the extract withdrawal point than another feed whose non-aromatic components are less volatile in the extractive stripper.

The extract phase leaves extractor 3 through ow conduit `f5 where it mixes `with additional lean solvent owing in ow conduit 9, if desired, and the total mixture flows through flow conduit 10' and into extractive stripper 11. The stripper is operated at elevated temperatures and intermediate pressures (relative to the extractor pressure) in order to remove substantially all of the nonaromatics, some of the water and aromatics and a small amount of the solvent overhead. This overhead stream is removed from stripper 11 through flow conduit 12 where it passes through condenser 13 and into overhead receiver 14. The overhead stream is condensed and separated into two phases, one a water-solvent phase and the other a hydrocarbon phase. This hydrocarbon phase is withdrawn from receiver 14 through flow conduit 15 Where it is recycled to ow conduit 5 and eventually into extractor 3. Recycling this stream assures complete recovery of aromatics while also providing an addition source of volatile non-aromatics to displace the less volatile nonaromatics from feed #2. The water-solvent phase settles in boot 16 where it is withdrawn therefrom through ow conduit 17 and sent to solvent recovery means not shown.

A bottoms stream consisting essentially of aromatic hydrocarbons and solve-nt is withdrawn from the bottom of stripepr 11 and through fiow conduit 18 where a portion thereof ows through flow conduit 19, reboiler heater 20 and return to stripper 1t1. The remaining bottoms portion ows through ow conduit 21 and into solvent recovery column 22.

Column 22 is preferably operated at low pressures (relative to the extractor pressure) and elevated temperatures to separate the solvent from the aromatics. The aromatics andsome of the water are removed from column 22 as a vapor and pass through ow conduit 23, condenser 24 and into receiver 25. The overhead is separated into a hydrocarbon phase comprising aromatics and a Water phase. The aromatics are withdrawn from receiver 25 through ow conduit 26 where a portion thereof ret-urns to column 22 through flow conduit 27 as reflux -while the' remaining portion of aromatics is withdrawn through flow conduit 28 as net product. Generally, the net product is sent to a series of fractionators to recover the individual aromatics as substantially pure components such as benzene, toluene, ortho-xylene, ethylbenzene, etc. A bottom stream consisting essentially of lean solvent is withdrawn from column 22 through flow conduit 31 where a portion thereof flows through flow conduit 32, reboiler heater 33 and returns to column 22. The remaining bottoms portion iiows through flow conduit 34 where a fraction thereof flows into flow conduit 6 and into the upper-intermediate point of extractor 3 while the other fraction ows into ow conduit 9 where it mixes with extract phase before entering stripper 11. A small amount of decomposition of solvent and other sludge forming reactions may occur in the process and it is desirable to remove a small slipstream of lean solvent from flow conduit 34 through ow conduit 35 and regenerate the solvent in equipment not shown in order to prevent the build up of sludge. The regeneration preferably is done by rerunning the solvent and thereafter the regenerated solvent is returned to flow conduit 34 by means of `flow conduit 36. In addition suicient water is added to the lean solvent in equipment not shown to adjust the Water content of the lean solvent to the desired level. This is conviently done by introducing the water into flow conduit 36.

Suitable feed stocks for the process of this invention comprises fluid hydrocarbon mixtures containing at least 6 carbon atoms per molecule. Feed #l is preferably selected in relation to feed #2 in order to attain the desired non-aromatic displacement referred to hereinbefore. A preferable combination is for feed #1 to be debutanized or 1depentanized reformate (which is lean in napthenes) and for feed #2 to be a debutanized or depentanized hydrotreated pyrolysis liquid (which is rich in naphthenes). Another preferable combination is for feed #1 to be a lighter fluid hydrocarbon mixture such as in the C5 to C7 range and for feed #2 to be a heavier fluid hydrocarbon mixture such as in the C7 to C10 range. It is readily apparent that many other suitable pairings of feed stocks may be used to attain the eicient stripping referred to hereinbefore and it is intended that these pairings be included within the scope of this invention.

Likewise, many solvents may advantageously be ernployed in the process of this invention providing they possess a solubility selectivity for aromatics over more saturated hydrocarbons. Preferable solvents include the glycols, sulfolanes, sulfoxides, pyrrolidones, etc. Especially preferable solvents include diethylene glycol, various polyethylene glycols, dipropylene glycol, various polypropylene glycols, dimethyl sulfoxide, N-methyl pyrrolidone, saturated sulfolane (C4H8SO2), Sulfolene (CaHsSOz) etc. It is also within the scope of this invention to add a polar diluent to the solvent such as water to further improve the solvents selectivity for aromatic hydrocarbons. Preferably, the present solvents contain a small amount of water dissolved therein (from about 0.5% to about 20%) to increase the selectivity of the overall solvent phase for aromatic hydrocarbons over non-aromatic hydrocarbons without reducing substantially the solubility of the solvent phase for aromatic hydrocarbons over non-aromatic hydrocarbons. The presence of water in the solvent composition furthermore provides a relatively volatile material therein which is distilled from the extract phase in the extractive stripper 'which aids in vaporizing the last traces of non-aromatic hydrocarbon from the extract phase by steam stripping. Depending upon the particular solvent employed, water is added thereto in concentrations of from about 0.1% to about 20% by weight.

The extractor is operated at elevated temperatures and at a sufficiently elevated pressure to maintain the feed stocks, recycle stream and solvent in the liquidphase. Suitable temperatures are within the range of from F. to about 400 F. and preferably from about 150l F. to about 300 IF. Suitable pressures are withinthe range of from about atmospheric pressure up to about `400 p.s.i.g. and preferably from about 50 p.s.i.g. to about 150 p.s.i.g. Suitable contacting means in the extractor vessel to efficiently contact the solvent phase with the hydrocarbon phase include sieve decks, baffles, rotary discs, packing, etc.

The extractive stripper is operated at moderate pressures and suiciently high reboiler temperatures to vaporize all the non-aromatic hydrocarbons, and some of the aromatics, water and solvent. .The stripper is preferably operated at lower pressures than the extractor, preferably from a slight vacuum up to about p.s.i.g. The reboiler temperature is dependent upon the composition of the feed and the solvent.

The solvent recovery column is preferably operated at lower pressures than the stripper and sufficiently high temperatures to vaporize the aromatic hydrocarbons without thermally decomposing the solvent. In many cases, moderate to high vacuum must be employed in order to maintain a sufficiently low reboiler temperature to avoid thermal decomposition while permitting vaporization of the aromatics. Pressures may be from about 100 mm. mercury absolute to moderate superatmospheric pressures in the order of 20 p.s.i.g.

The apparatus employed in the process of this invention may be any conventional convenient type known to those skilled in the art. For simplicity the drawing does not show all the pumps, tanks, heat exchangers, valves, bypasses, vents, preheaters, coolers, control valves, means for actuating control valves and other auxiliaries that may be necessary for the proper operation of the process but the inclusion of which will be evident to those skilled in the art.

The following example is presented to further illustrate the process of this invention but it is not intended to limit the invention to the materials used or the operaing conditions disclosed therein.

A rst feed consisting of a reformate in the C6-C8 carbon number range is introduced into flow conduit 5 (see the drawing) at a rate of 100 moles/hr. whereupon it flows into the lower intermediate point (Ma of the height from the bottom) in extractor 3. The reformate contains approximately 50 volume percent aromatics, 46 volume percent paraiiins and 4 volume percent napthenes. A second feed consisting of a hydrotreated pyrolysis gasoline in the C6-C8 carbon number range is introduced into flow conduit 4 at a rate of 1000 moles/hr. whereupon it flows into the middle intermediate point in extractor 3 (1A of the height of the extractor). The pyrolysis gasoline 'contains approximately 65 volume percent aromatics, 30 volume percent naphthenes and 5 volume percent paraflins. Saturated sulfolane (C4H8SO2) is employed as the organic solvent for this example. Lean solvent consisting of sulfolane containing about 5% (mole) water is introduced into the upper intermediate point in extractor 3 through flow conduit 6 at a rate of 600 moles/hr. A recycle stream owing through ow conduit 15 is introduced into extractor 3 at a rate of 140 moles/hr. The extractor is maintained at a pressure of 100 p.s.i.g. and a temperature of 200 F. A raflinate product is withdrawn from the upper point in extractor 3 through ilow conduit 7 at a rate of 86 moles/hr. An extract stream is withdrawn from the lower point in extractor 3 through ow conduit 8 at a rate of 854 moles/hr. where it contacts an additional 200 moles/hr. of lean solvent flowing through flow conduit 9. The total mixture is introduced into ow conduit 10 where it flows into stripper 11.

Stripper 11 is operated at a top pressure of 13 p.s.i.g., a top temperature of about 225 F. and a bottom temperature of about 350 F. These conditions allow essentially complete vaporization of the non-aromatics in stripper 11. The overhead vapors from stripper 11 are condensed and settle into receiver 14. The hydrocarbon phase separates therein and is recycled through flow conduit 15 at a rate of about 140 moles/hr.

The stripper bottoms consisting essentially of aromatic hydrocarbons, organic solvent and a small amount of water is introduced into solvent recovery column 22. Column 22 is operated at a top pressure of about 200 mm. mercury absolute, a top temperature of about 150 F. and a bottom temperature of about 350 F. The aromatics are removed overhead in column 22 and the organic solvent is removed out the bottom of column 22. Sufficient water is added to this organic solvent to bring the water concentration back to about 5%. The aromatics overhead is fractionated into the individual compounds, its purity is analyzed and a material balance is calculated. The purity of the benzene and toluene is at least nitration grade and the purity of the C8 aromatics is above 99.5% The recovery of benzene is about 99%, the recovery oi toluene is above 98% and the recovery of C8 aromatics is above 95%. It is expected that had the feeds been introduced into the same point in the extractor the purity of the aromatics at the same operating conditions would have been lower.

I claim as my invention:

1. A process for the separation and recovery of aromatic hydrocarbons from at least two hydrocarbon feeds including a rst feed comprising aromatics and a nonaromatic portion rich in parains and a second feed comprising aromatics and a non-aromatic portion rich in naphthenes, which process comprises:

introducing a lean solvent into an upper intermediate point in the height of an extraction zone;

introducing said second feed into a middle intermediate point in the height of said extraction zone; introducing said first feed into a lower intermediate point in the height of said extraction zone; countercurrently contacting the solvent with said feeds in the extraction zone; removing a hydrocarbon raffinate phase consisting essentially of non-aromatics from an upper point of said extraction zone;

removing a solvent extract phase enriched in aromatics content from a lower point of said extraction zone; introducing the extract phase into an extractive distillation column and stripping essentially all of the nonaromatic hydrocarbons from the extract phase; and separating the aromatics from the solvent.

2. The process of claim 1 further characterized in that the solvent comprises sulfolane.

3. The process of claim 2 further characterized in that the lean sulfolane solvent contains up to by volume of water.

4. The process of claim 1 further characterized in that the solvent comprises sulfolene.

5. The process of claim 1 further characterized in that the solvent is selected from the group consisting of the polyethylene glycols and the polypropylene glycols.

6. The process of claim 1 further characterized in that the solvent comprises dimethyl sulfoxide.

7. The process of claim 1 further characterized in that the solvent comprises N-methyl pyrrolidone. Y

8. A solvent extraction process for the separation and recovery of aromatic hydrocarbons from reformate feed and from a hydrorened pyrolysis feed, said reformate feed comprising aromatics and a non-aromatic portion rich in parains and said pyrolysis feed comprising aromatics and a non-aromatic portion rich in naphthenes, which process comprises:

introducing a lean solvent into an upper-intermediate point in the height of an extraction zone;

introducing the pyrolysis feed into a 4middle-intermediate point in the height of said extraction zone; introducing the reformate feed into a lower-intermediate point in the height of said extraction zone; countercurrently contacting the solvent with the feeds in the extraction zone; removing a hydrocarbon raffinate phase consisting essentially of non-aromatics from an upper point in the height of said extraction zone;

removing a solvent extract phase enriched in aromatics from a lower point in the height of said extraction zone;

introducing the extract phase into an extractive distillation column and stripping essentially all of the non-aromatic hydrocarbons from the extract phase; and separating the aromatics from the solvent.

9. The process of claim 8 further characterized in that the reformate and the pyrolysis feeds are within the C6 to C10 carbon number range.

10. The process of claim 1 further characterized in that the overhead hydrocarbon material from the extractive distillation column is recycled to said lower-intermediate point.

11. The process of claim 1 further characterized in that additional lean solvent is mixed with the solvent extract phase before the extract phase is introduced into the extractive distillation column.

References Cited UNITED STATES PATENTS 2,365,898 12/1944 Morris et al. 260-674 2,407,820 9/1946 Durrum 260-674 2,770,663 11/1956 Grote 260-574 2,886,610 5/1959 Georgian 260-674 3,037,062 5/1962 Gerhold 260-674 3,200,165 8/ 1965 Eisenlohr et al 260-674 DELBERT E. GANTZ, Primary Examiner.

C. E. SPRESSER, JR., Assistant Exdmner.

U.S. C1. X.R. 208-321, 325

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