US3468793A - Simultaneous solvent extraction of a light and heavy fraction - Google Patents

Description

Heavy Raff/note Sept. 23, 1969 K. o.u|'r1'| 3,468,793

SIMULTANEOUS SOLVENT EXTRACTION OF A LIGHT AND HEAVY FRACTION Filed Aug. 17. 1967 'o N .2 E 9 9 8 1 E rq q Q: m

QB N) 0 N N N N Stripper Extractor Extractor t //v VEN r019 Kennefh D. U/ff/ Heavy Feed Light Raff/'nafe Lighf Fee A TTORNEYS United States Patent Office 3,468,793 Patented Sept. 23, 1969 US. Cl. 208-312 7 Claims ABSTRACT OF THE DISCLOSURE Process for separating and recovering aromatic hydrocarbons, such as benzene, from two different feedstock mixtures via a dual zone solvent extraction technique. The preferred solvent comprises sulfolane and the extraction zones are refluxed individually with specific nonaromatic hydrocarbon streams.

Background of the invention This invention relates to a separation process. It also relates to a solvent extraction process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures utilizing a selective solvent. It particularly relates to a solvent extraction process whereby aromatic hydrocarbons are simultaneously recovered from two different feedstocks utilizing a dual extraction system.

The technique of aromatic hydrocarbon separation by solvent extraction is well known in the prior art. Generally, according to a well known procedure, a mixture of hydrocarbons is introduced into an extraction zone at an intermediate point thereof and contacted therein with a solvent which is selective for aromatic hydrocarbons. The conditions in the extraction zone are sufficient to produce a raflinate phase comprising substantially all of the nonaromatic hydrocarbons in the feedstock and an extract phase comprising the aromatic components of the feedstock dissolved in the selective solvent. The nonaromatic raffinate phase is generally recovered and utilized in gasoline blending. The extract phase is subse quently passed through various separation means for the recovery therefrom of aromatic hydrocarbons in relatively high concentration and high purity with a residue fraction comprising lean solvent which is generally suitable for reuse in the extraction zone. One feature in the prior are process is the substantial freeing of the extract phase of substantially all of the non-aromatic components dissolved by the solvent. This is generally accomplished by introducing a non-aromatic hydrocarbon reflux stream, typically, comprising naphthenes and parafiins into the extraction zone at a point intermediate between the feed introduction point and the extract phase withdrawal point. This reflux phase is more volatile in the presence of the solvent than either of the aromatic or raffinate components of the feedstock and in effect acts as a displacing agent for the non-aromatic hydrocarbons into the raffinate stream.

Normally, in a solvent extraction process of the type described hereinabove, the raflinate stream is recovered as a separate product and aromatic hydrocarbons are recovered as a second product stream. Both of these product streams have well known end uses. Accordingly, petroleum refiners have various hydrocarbon feedstocks possessing different characteristics. It is common practice to pass these feedstocks through the solvent extraction system in blocked-out operation so as to avoid contamination of the various product streams with undesirable components. If the volume of different feedstocks is sufficient, it is also common practice for the prior art to build two separate and distinct solvent extraction systems again in order to avoid cross contamination between the various products. It would be desirable, therefore, to provide a process whereby different hydrocarbon feedstocks could be processed through the solvent extraction system with a minimum of excess equipment and with a technique which would avoid cross contamination of the products.

Summary of the invention Therefore, it is an object of this invention to provide a Separation process.

It is another object of this invention to provide a solvent extraction process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures.

It is still another object of this invention to provide a process for the recovery of aromatic hydrocarbons from two difierent feedstocks via a dual solvent extraction technique in a facile and economic manner.

According to the practice of the present invention there is provided a process for separating aromatic hydrocarbons from a relatively heavy hydrocarbon mixture and from a relatively light hydrocarbon mixture which comprises the steps of: (a) introducing said light mixture into a first solvent extraction zone in contact with a hereinafter specified lean solvent stream and a hereinafter specified light reflux stream under conditions suflicient to produce a light rafiinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein; (b) removing from said first zone said light rafiinate as a first product stream; (0) introducing said heavy mixture into a second solvent extraction zone in contact with hereinafter specified lean solvent stream and hereinafter specified heavy reflux stream under conditions sufficient to produce a heavy raffinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein; (d) removing from said second zone said heavy raflinate as a second product stream; (e) passing the extract stream from said first zone and from said second zone into a first distillation zone maintained under distillation conditions; (f) withdrawing from said first distillation zone an overhead fraction comprising relatively light non-aromatic hydrocarbons, a side-cut fraction comprising relatively heavy non-aromatic hydrocarbons and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein; (g) passing at least a portion of said overhead fraction into said first extraction zone of step (a) as the specified reflux stream therein; (h) passing at least a portion of said side-cut fraction into said second extraction zone of step (c) as the specified reflux stream therein; (i) introducing said first bottoms fraction into a second distillation zone maintained under conditions sufficient to produce an overhead fraction comprising aromatic hydrocarbons and a second bottoms fraction comprising lean solvent; and, (j) returning at least a portion of the lean solvent of step (i) to the first and second extraction zones as specified.

Another embodiment of this invention includes the process hereinabove wherein said lean solvent comprises sulfolane.

A specific embodiment of this invention includes the process hereinabove wherein said overhead fraction of step (i) comprises a combined mixture of aromatic hydrocarbons which were originally present in said' light mixture and originally present in said heavy mixture.

As a broad general class, suitable feedstocks of the satisfactory practice of this invention include fluid hydrocarbon mixtures having a sufiiciently high concentration of aromatic hydrocarbons to economically justify recovery of the aromatics. The present invention is particularly applicable to hydrocarbon feed mixtures which are different in some characteristic. The most applicable characteristic for the practice of this invention include hydrocarbon mixtures which are different in terms of boiling point. Therefore, feedstocks applicable for use in the present invention include a relatively heavy hydrocarbon mixture and a relatively light hydrocarbon mixture. In other words, these different feedstocks include a relatively low boiling aromatic hydrocarbon-com taining feedstock and a relatively high boiling aromatic hydrocarbon-containing feedstock. These difierent feedstocks preferably should contain at least 25% by Weight aromatic hydrocarbons and should have at least one aromatic hydrocarbon type which is common between the two different feedstocks; although, this common expedient is not necessarily a requirement thereof, only a preferred embodiment thereof. The suitable carbon number range of the feedstocks is from about 6 carbon atoms per molecule to about 20 carbon atoms per molecule and, preferably, from about 6 to carbon atoms per molecule. One source of feedstock is debutanized reactor effluent from a catalytic reforming processing unit. Another source is a liquid by-product from a pyrolysis processing unit which has been hydrotreated to saturate diolefins and olefins and to remove other contaminants thereby producing an aromatic hydrocarbon concentrate suitable for solvent extraction techniques. An example of a relatively light hydrocarbon feedstock include a mixture of hexane, cyclohexane, and benzene. An example of a relatively heavy hydrocarbon feedstock include a wide boiling benzene, toluene, xylene stream mixed with the corresponding paraflins and naphthenes which has been obtained from a conventional catalytic reforming unit.

The preferred solvent which may be utilized in the aromatics extraction process of the present invention is a solvent of a sulfolane type. The solvent is Well known and typically possesses a five membered ring containing 1 atom of sulfur and 4 atoms of carbon with 2 oxygen atoms bonded to the sulfur atom of the ring. Those skilled in the aromatics extraction art are Well versed in in the characteristics of this type of solvent and more detail thereof need not be presented at this time.

Other solvents which may be included and may be satisfactorily used in the practice of this invention are the sulfolenes, such as 2-sulfolene and 3-sulfolene. Still other typical solvents which have a high selectivity for separating aromatic hydrocarbons from non-aromatic hydrocarbons and which may be processed within the scope of the present invention are 2-methyl-sulfolane, 2,4-dimethyl-sulfolane, methyl-2-sulfonylether, n-aryl-3-sulfonylamine, 2-sulfonylacetate, diethyleneglycol, various polyethyleneglycols, dipropyleneglycol, various polypropyleneglycols, dimethylsulfoxide, n-methylpyrrolidone, and the like. The specifically preferred solvent for use in the' practice of the invention is sulfolane.

The aromatic selectivity of the solvent can usually be further enhanced by the addition of Water to the solvent. Preferably, the solvents utilized in the practice of this invention contain small quantities of water in order to increase the selectivity of the overall solvent phase for aromatic hydrocarbon over non-aromatic hydrocarbons without reducing substantially the solubility of the solvent phase for aromatic hydrocarbon. The presence of water in the solvent composition further provides a relatively volatile material therein which is distilled from the solvent in the extractor stripper, more fully discussed hereinafter to vaporize the last traces of non-aromatic hydrocarbons from the solvent stream by steam distillation. The composition of the present invention preferably contains from about 0.5% to about by weight water and preferably from about 5% to about 15% by weight depending upon the particular solvent utilized and the process conditions at which the extraction zones and extractive stripper are operated.

The dual extraction zones of the present invention are operated at elevated temperature and at sufiiciently elevated pressure to maintain the feedstock solvent and reflux streams in liquid phase. The operating conditions in each extraction zone may be the same or may be different depending upon the characteristics of the particular feedstock charged to the particular extraction zone. Typically, suitable temperatures when using sulfolane as the solvent are within a range of from about 80 F. to about 400 F., and preferably from about 175 F. to about 300 F. Suitable pressures are generally within the range I from about atmospheric pressure up to about 400 p.s.i.g.

and, preferably, from 50 to 150 p.s.i.g.

It is noted from the description of the present invention thus far that one of the critical problems to which this invention is directed is the displacement of non-aromatic hydrocarbons from the extract phase of the lower end of the extraction by utilizing the technique of a nonaromatic reflux at that point. Therefore, it is a feature of the present invention to use a relatively light reflux material in the extraction zone which is charging the relatively light or low boiling hydrocarbon feedstock. Conversely, it is a feature of this invention to utilize a relatively heavy reflux on the lower end of the extraction zone which charges these relatively heavy or high boiling hydrocarbon feedstocks to the extraction zone. The source of the specific reflux streams utilized herein will be more fully developed hereinbelow. In any event, the volume of reflux material introduced into the lower end of the extraction zone should be at least 10% by volume of the extract phase leaving the extraction zone in question in order toeifectively displace the non-aromatic hydrocarbons from the extract. By operating in this manner, cross-contamination of the respective raffinate stream products is avoided.

The extract stripper is operated at moderate pressures and sufiiciently high reboiler temperatures to drive all of the non-aromatic material and some of the aromatics, water, and solvent out of the bottom portion of the column. Typical stripper pressures are from atmospheric pressure to about p.s.i.g., although, generally the top of the stripper is preferably maintained at from about 1 p.s.i.g. to about 20 p.s.i.g. The reboiler temperature, of course, is dependent upon the composition of the feedstock and the solvent. The column is arranged to take an overhead fraction, at least one side-cut fraction, and a bottoms fraction as separate product streams. Those skilled in the art from a knowledge of the prior art are well versed in the operation of a stripper column and additional details thereof need not be presented here.

The solvent recovery column is operated at low pressures and sufiicien-tly high temperatures to distill the aro matic hydrocarbons overhead as a distillate fraction thereby producing a solvent. bottoms fraction which is generally suitable for reuse in both the solvent extraction zones previously mentioned. Again, the choice of operating conditions depend upon the feedstock characteristics and the composition of the solvent. Preferably, the top of the solvent recovery column is operated at about 100 to 400 mm. Hg absolute. These subatmospheric pressures must be employed in order to maintain a sufiiciently low reboiler temperature to avoid thermal decomposition of the solvent; particularly When the solvent is of the sulfolane type. Preferably, the reboiler temperature should be maintained below about 360 F. when using saturated sulfolane as the solvent.

Thus, from the description of the present invention thus far, the present invention involves a solvent extraction process utilizing a dual extraction zone in a combined stripper-aromatic recovery system whereby solvent is recovered for reuse within the process in both extraction zones and an aromatic hydrocarbon product stream is obtained which contains aromatic hydrocarbons which were originally present in both of the feedstocks.

It is another feature of this invention that the extract streams from the dual extraction zones are passed into a first distillation column which is maintained under conditions sufficient to produce overhead a distillate fraction comprising relatively light non-aromatic hydrocarbons. These conditions are also suflicient to produce as a sidecut fraction a stream comprising relatively heavy nonaromatic hydrocarbons. The residue or bottoms fraction from this distillation zone contains the rich solvent having dissolved therein the desired aromatic hydrocarbons which were originally present in both of these feedstocks.

The overhead distillate fraction from the extractive stripper is returned at least in part to the extIaction zone as reflux therein which originally had charged to it the relatively light feed mixture. Similarly the side-cut fraction is utilized at least in part as reflux on the extraction zone which charged the relatively heavy feed mixture.

The bottoms fraction or residue from the extractive distillation zone is passed in a solvent recovery column which is operated at low pressures, as previously men tioned, in order to separate and recover the aromatic hydrocarbons from the solvent phase and to recover a lean solvent stream suitable for reuse in both of the extraction zones.

Thus, it is seen that the present invention utilizes a dual extraction zone, a common extractive stripper column, a common solvent recovery column which is related to the dual extraction zone in an interdependent manner which effects considerable economies of operation and in capital expense.

The invention may be further understood with reference to the appended drawing which a schematic repre sentation of apparatus for practicing one embodiment of the invention.

Description of the drawing Referring now to the drawing, a relatively heavy hydrocarbon feedstock containing aromatic hydrocarbons, such as a conventional mixed product from a conventional catalytic reforming unit comprising benzene, toluene, and xylene mixed with corresponding naphthenes and paraifins enters the system via line 10. Simultaneously therewith, a relatively light hydrocarbon feedstock, such as one comprising hexane, cyclohexane, and benzene, enters the system via line 11.

Referring now to the relatively heavy feedstock operation: the feed enters extractor column 12 via line at an intermediate point. Aqueous sulfolane as the lean solvent enters the upper portion of extractor 12 via line 17 and a relatively heavy reflux stream, more fully discussed hereinafter, enters extractor 12 at a lower end thereof via line 22. A relatively heavy rafiinate stream is withdrawn from extractor 12 via line 19 and recovered as a separate product. An extract phase comprising solvent having dissolved therein aromatic hydrocarbon and also which is contaminated with small amounts of relatively heavy non-aromatic hydrocarbons, such as naphthenes and paraflins, is withdrawn from extractor 12 via line 18.

Referring now to the extraction step of the relatively light feed: the relatively light hydrocarbon feed enters extractor 13 via line 11. Aqueous sulfolane as the lean solvent enters the extractor at an upper portion thereof via line 16 and a relatively light non-aromatic reflux stream enters extractor 13 at the lower end thereof via line 21, the source of which is more fully developed hereinbelow. A relatively light raffinate stream is withdrawn from extractor 13 via line 14 and recovered as a separate product stream. An extract phase comprising solvent having aromatic hydrocarbons dissolved therein and which is contaminated with minor amounts of relatively light non-aromatic hydrocarbons is withdrawn from extractor 13 via line 15.

As a preferred embodiment of this invention, the relatively heavy extract phase in line 18 is passed into extractive stripper column 20 at an intermediate location therein. Similarly, the relatively light extract phase in line 15 is introduced into extract stripper column 20 at an upper point thereof. Additional solvent may be added to extractor stripper column 20 as needed, from a source not shown, in order to enhance the separation of the nonaromatic hydrocarbons from the solvent containing the aromatic hydrocarbons. Sufficient separation conditions are maintained in extractive stripper 20' to produce a bottoms fraction comprising solvent having dissolved therein the desired aromatic hydrocarbons which are removed from stripper 20 via line 23.

A distillate fraction comprising relatively light nonaromatic hydrocarbons and minor amounts of solvent is removed via line 21 and passed into extractor 13 at the lower end thereof as the relatively light reflux stream, previously mentioned. A side-cut fraction comprising relatively heavy non-aromatic hydrocarbons and minor amounts of solvent is removed via line 22 and passed into the lower portion of extractor 12 as the relatively heavy refiux stream, previously mentioned.

Referring now to the solvent and aromatic hydrocarbon stream in line 23; this material is passed into recovery column 24 which is maintained under conditions sufiicient to separate the aromatic hydrocarbons from the solvent phase. The aromatic hydrocarbons are concentrated as a distillate fraction and removed for recovery via line 25. It is to be noted that the material in line 25 contains, for example, benzene which was originally present in the heavy feed in line 10 and the light feed in line 11 and is removed from the system as a combined product stream. The operation of recovery column 24 includes the use of steam stripping in the column in order to remove the final traces of aromatic hydrocarbons from the solvent. Therefore, the lower end of column 24 contains the desired solvent plus water generally in sufficient amounts such that the material withdrawn from column 24 via line 26 comprises lean solvent generally suitable for reuse in both extraction zones. It is noted that this lean solvent stream in line 26 is split and a portion thereof is passed via line 16 into extractor 13 and another portion passed via line 17 into extractor 12 as previously described.

Thus, it is seen that the practice of the present invention enables the simultaneous handling of two different feedstocks which avoids the contamination of the light raffinate phase with non-aromatic hydrocarbons from the heavy feed system and enables the production of a combined aromatics stream in high concentration and high purity without the necessity of either blocked-out operation or absolute duplicity of equipment.

Preferred embodiment Therefore, from the presentation presented hereinabove, the preferred embodiment of the invention provides a process for separating and recovering a combined aromatic hydrocarbon stream from two different feedstocks which comprises the steps of: (a) introducing a relatively low boiling aromatic hydrocarbon-containing feedstock into a first solvent extraction zone and introducing a hereinafter specified low boiling reflux stream into said first zone, said first solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, sufiicient to separate said low boiling feedstock into a slight rafiinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing light non-aromatic hydrocarbons as contaminants therein; (b) removing from said first zone said light raflinate as a first product stream; (c) introducing a relatively high boiling aromatic hydro- 7 carbon-containing feedstock into a second solvent extraction zone and introducing a hereinafter specified high boiling reflux stream into said second zone, said second solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, sufiicient to separate said high boiling feedstock into a heavy rafiinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing heavy non-aromatic hydrocarbons as contaminants therein; (d) removing from said second zone said heavy rafiinate as a second product stream; (e) passing the extract stream from the first extraction zone into a first distillation zone at the upper portion thereof and passing the extract stream from the second extraction zone into said first column at an intermediate portion thereof, said first column being maintained under conditions suflicient to produce an overhead fraction comprising relatively light non-aromatic hydrocarbons, a sidecut fraction comprising relatively heavy non-aromatic hydrocarbons, and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein; (f) passing at least a portion of said overhead fraction into said first extraction zone of step (a) as the specified reflux stream; (g) passing at least a portion of said sidecut fraction into said second extraction zone of step (c) as the specified reflux stream therein; (h) introducing said first bottoms fraction from step (e) into a second distillation zone maintained under distillation conditions sufiicient to produce a second distillate fraction comprising aromatic hydrocarbons and a second bottoms fraction comprising lean solvent suitable for reuse in said extraction zone; and, (i) recovering said second distillate fraction.

Another preferred embodiment of this invention includes the process hereinabove wherein said solvent comprises sulfolane.

A particularly preferred embodiment of this invention is the process hereinabove wherein said aromatic hydrocarbons in said distillate fraction from the second distillation zone contains benzene.

The invention claimed:

1. Process for separating aromatic hydrocarbons from a relatively heavy hydrocarbon mixture and from a relatively light hydrocarbon mixture which comprises the steps of:

(a) introducing said light mixture into a first solvent extraction zone in contact with a hereinafter specified lean solvent stream and a hereinafter specified light reflux stream under conditions suflicient to produce a light raflinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein;

(b) removing from said first zone said light raflinate as a first product stream;

(c) introducing said heavy mixture into a second solvent extraction zone in contact with hereinafter specified lean solvent stream and hereinafter specified heavy reflux stream under conditions suflicient to produce a heavy raflinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein;

(d) removing from said second zone said heavy raflinate as a second product stream;

(e) passing the extract stream from said first zone and from said second zone into a first distillation zone maintained under distillation conditions;

(f) Withdrawing from said first distillation zone an overhead fraction comprising relatively light nonaromatic hydrocarbons, a side-cut fraction comprising relatively heavy non-aromatic hydrocarbons, and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein;

(g) passing at least a portion of said overhead fraction into said first extraction zone of step (a) as the specified reflux stream therein;

(h) passing .at least a portion of said side-cut fraction into said second extraction zone of step (c) as the specified reflux stream therein;

(i) introducing said first bottoms fraction into a second distillation zone maintained under conditions suflicient to produce an overhead fraction comprising aromatic hydrocarbons and a second bottoms fraction comprising lean solvent; and,

(j) returning at least a portion of the lean solvent of step (i) to the first and second extraction zones as specified.

2. Process according to claim 1 wherein said lean solvent comprises sulfolane.

3. Process according to claim 1 wherein said overhead fraction of step (i) comprises a combined mixture of aromatic hydrocarbons which were originally present in said light mixture and originally present in said heavy mixture. I

4. Process for separating and recovering'a combined aromatic hydrocarbon stream from two different feedstocks which comprises the steps of:

(a) introducing a relatively low boiling aromatic hydrocarbon containing feedstock into a first solvent extraction zone and introducing a hereinafter specified low boiling reflux stream into said first zone, said first solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, suflicient to separate said low boiling feedstock into a light raffinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing light nonaromatic hydrocarbons as contaminants therein;

(b) removing from said first zone said light raflinate as a first product stream;

(0) introducing a relatively high boiling, aromatic hydrocarbon containing feedstock into a second solvent extraction zone and introducing a hereinafter specified high boiling reflux stream into said second zone, said second solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, suflicient to separate said high boiling feedstock into a heavy raffinate streamand an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing heavy non-aromatic hydrocarbons as contaminants therein;

(d) removing from said second zone said heavy raftinate as a second product stream;

(e) passing the extract stream from the first extraction zone into a first distillation zone at the upper portion thereof and passing the extract stream from the second extraction zone into said first column at an intermediate portion thereof, said first column being maintained under conditions suflicient to produce an overhead fraction comprising relatively light nonaromatic hydrocarbons, a side-cut fraction comprising relatively heavy non-aromatic hydrocarbons, and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein;

(f) passing at least a portion of said overhead fraction into said first extraction zone of step (a) as the specified reflux stream;

(g) passing at least a portion of said side-cut fraction into said second extraction zone of step (c) as the specified reflux stream therein;

(h) introducing said first bottoms fraction from step (e) into a second distillation zone maintained under distillation conditions suflicient to produce a second distillate fraction comprising aromatic'hydrocarbons and a second bottoms fraction comprising lean solvent suitable for reuse in said extraction zones; and,

(i) recovering said second distillate fraction.

5. Process according to claim 4 wherein said solvent comprises sulfolane.

6. Process according to claim 5 wherein said aromatic hydrocarbons comprise single ring type aromatic hydrocarbons.

9 10 7. Process according to claim '6 wherein said single 3,262,875 7/1966 (31 m :1 208-321 r g hydrocarbons comprisc bmzcne- 3,361,664 1/1968 Broughton 61 a1. 208-321 References (3M HERBERT LEVINE, Primary Examiner v UNITED STATES PATENTS 5 2,492,787 12/1949 Davis 208-321 2,921,015 1/1960 Shiras 208-412 208-318, 321, 32$;260-674

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