|Publication number||US3492222 A|
|Publication date||Jan 27, 1970|
|Filing date||Nov 14, 1967|
|Priority date||Nov 14, 1967|
|Publication number||US 3492222 A, US 3492222A, US-A-3492222, US3492222 A, US3492222A|
|Inventors||Tassell Harry M Van|
|Original Assignee||Universal Oil Prod Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (4), Referenced by (17), Classifications (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
Jam-27, 1970l M. VAN TAssELL 3,492,222
SOLVENT RECOVERY PROCESS Filed Nov. 14. 1967 Relurn Benzene Nef C7* Aromaf/cs uwn/og auazuag Rell/rn Lean Solvent N VEN T0 R Harry M. Van Tasse/l A TTOR/VE'YS United States Patent O 3,492,222 SOLVENT RECOVERY PROCESS Harry M. Van Tassell, Des Plaines, Ill., assiguor to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed Nov. 14, 1967, Ser. No. 682,941 Int. Cl. C10g 21/28, 21/16 U.S. Cl. 208-321 11 Claims ABSTRACT F THE DISCLOSURE Process for recovery of specified solvent from a nonaromatic raffinate which comprises extracting thel solvent therefrom with a iirst aqueous stream to produce a solvent free raffinate and a second aqueous stream containing solvent and non-aromatic hydrocarbons. The second aqueous stream is then contacted with aromatic hydrocarbons in a contacting zone under conditions which produce an aromatic stream containing non-aromatic hydrocarbons and a third aqueous stream containing solvent and having substantial freedom from non-aromatics. The process has particular yapplication in aromatic extraction processing wherein the specified solvent comprises a sulfolane-type solvent, a polyalkylene glycol type solvent, or other typically utilized water soluble solvents.
FIELD OF INVENTION The present invention relates to the solvent extraction of aromatic hydrocarbons from a hydrocarbon charge stream. More particularly, the present invention relates to the recovery of solvents from a non-aromatic raflinate which is produced by the solvent extraction of aromatics from a hydrocarbon charge stream. More specifically, the present invention relates to an improved process for the recovery of solvent from the non-aromatic ratlinate by water Washing the solvent-containing ratlinate and contacting the resulting solvent-containing aqueous stream with aromatic hydrocarbon.
It is well known in the art that the raffinate which leaves the extraction zone of an aromatic hydrocarbon extraction process contains solvent. The solvent which is withdrawn in the raffinate stream must be recovered not only because it may interfere with subsequent rafnate processing or ultimate rainate use, but primarily because continual loss of solvent in the rainate stream is a prohibitive economic expense in the aromatic extraction process. The recovery of the solvent from the raffinate stream may be typically accomplished by distillation of the raffinate or by a secondary solvent extraction process such as the water washing of the raiiinate.
A typical solvent which is utilized in commercial aromatics extraction and which may be recovered in accordance with the practice of this invention is a solvent of the sulfolane type. The solvent possesses a five-membered ring containing one atom of sulfur and four atoms of carbon with two oxygen atoms bonded to the sulfur atom of the ring. Generically, the sulfolane type solvents may be indicated as having the following structural formula:
wherein R1, R2, R3, and R4 are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an alkoxy radical having from one to eight carbon atoms, and an arylalkyl radical having from one to twelve carbon atoms.
3,492,222 Patented Jan. 27, 1970 Other solvents which may be included within this process are the sulfolenes, such as 2-sulfolene or 3-sulfolene which have the following structures:
O O S Cz \CH2 CH=CH Other typical solvents which have a high selectivity for separating aromatics from non-aromatic hydrocarbons and which may be processed within the scope of the present invention are 2-methylsulfolane, 2,4-dimethylsulfolane, methyl- 2 sulfonyl ether, N-aryl-3-sulfonylamine, 2-su1fonyl acetate, diethylene glycol, various polyethylene glycols, dipropylene glycol, various polypropylene glycols, dimethyl-sulfoxide, N-methyl pyrrolidone, etc.
The specifically preferred solvent chemical which is processed within the scope of the present invention is sulfolane having the following structural formula:
Cz \CH2 CH2- CH2 DESCRIPTION OF PRIOR ART The typical solvent composition Within the scope of the present invention comprises a mixture of water and one or more of the solvent chemicals herein noted. A particularly preferred solvent composition of the present invention comprises Water and sulfolane. In extracting aromatic hydrocarbons from the hydrocarbon mixture, it is known that the parains are the least soluble followed in increasing order of solubility by naphthenes, oleiins, diolens, acetylenes, sulfur containing hydrocarbons, nitrogen containing hydrocarbons, oxygen containing hydrocarbons and aromatic hydrocarbons. It is the practice to regulate the solubility of the hydrocarbons within the solvent composition by varying the water content thereof. Thus, by adding more Water to the solvent, the solubility of all components in the hydrocarbon mixture is decreased but the solubility difference between components (selectivity) is increased. 'Ihe net effect is to decrease the number of contacting stages required to achieve a given purity of aromatic extract, or to increase the resulting purity of the aromatic extract when the number of contacting stages is held constant.
The presence of water in the solvent composition provides a further processing benet in that it introduces a relatively volatile material into the fractionation system wherein the aromatic extract is separated from the rich solvent composition. The water of the solvent composition is vaporized at least in part to provide assistance in stripping all traces of non-aromatic hydrocarbons out of the aromatic-rich solvent, and to provide assistance in stripping substantially all of the aromatic extract out of the final lean solvent.
It is, therefore, the practice to provide that the solvent composition contain from about 0.1% to about 20% by wt. of water. When the solvent composition comprises chemical sulfolane, it is preferable that the solvent cornposition contain from about 0.1% to about 1.0% of Water, while a solvent composition comprising a polyalkylene glycol preferably contains from about 6% to 15% of water.
In the aromatic extraction process which is typical of the present invention, the aromatic-containing hydrocarbon feed stock is passed to an extraction zone which may comprise a tower containing suitable packing such as Berl saddles or Raschig rings, or a tower containing suitable trays, or a rotating disc contactor (RDC). The feed stock is contacted therein with a lean solvent composition under conditions sufficient to produce a non-aromatic raflinate and an aromatic rich solvent. The rich solvent composition leaving the extraction zone is passed to a rich-solvent separation zone which typically comprises one or more fractionation columns which are operated in a manner sufcient to remove non-aromatics from the rich solvent and to recover a high purity aromatic extract and the final lean solvent composition. The non-aromatic fraction which is removed is normally returned to the extraction zone to provide a non-aromatic reux. Because the solvent chemicals typically utilized are chemically unstable at elevated temperatures, the aromatic extract is normally removed from the solvent composition with the assistance of steam sripping under operating conditions which minimize thermal decomposition of the solvent chemical. The aromatic extract is separated from the stripping steam condensate and then passed into a fractionation train wherein the extract is separated into its aromatic constituents. The nal lean solvent is withdrawn from the rich solvent separation zone and returned to the extraction zone.
When the aromatic extract comprises a typical benzene, toluene, and xylene composition (a BTX composition), the benzene column of the BTX extract fractionation train will be provided with a side-cut withdrawal means for removal of the benzene product. This side-cut fraction is normally withdrawn as a liquid stream from the fourth or fifth tray below the top of the column in order to assure that it will be substantially free of water and low boiling non-aromaics. The vapor withdrawn overhead from the benzene column will contain the traces of low boiling non-aromatic hydrocarbons as well as the traces of water which are contained in the extract. The water which is contained in the overhead of the benzene column stems from the stripping steam which is normally utilized in stripping the aromatic extract out of the rich solvent in the prior rich solvent separation zone. The hydrocarbon fraction of the benzene column overhead is returned in-part to the benzene column as reflux, and the water which is separated from the condensed benzene overhead vapor is withdrawn from the process. Since the benzene overhead fraction contains low boiling nonaromatic constituents, it is typically the art to withdraw a small slip stream or drag stream from this receiver and pass the stream back to the extraction zone for separation of aromatics and non-aromatics. This drag stream assures that the top of the benzene column will not become loaded with an accumulation of non-aromatic hydrocarbons which may eventually contaminate the benzene side-cut product stream.
The aromatic extraction process which has been broadly summarized hereinabove is clearly set forth by D. B. Broughton et al., in U.S. Patent 3,361,664, wherein the solvent composition comprises sulfolane and water. A typical aromatic extraction process wherein the solvent composition comprises polyalkylene glycol and water is disclosed by I. H. Stephens in U.S. Patent 2,773,918. These patents and other published literature clearly set forth typical processing steps and the operating conditions for the aromatic extraction zone which produces a non-aromatic raflinate and an aromatic rich solvent, and for the subsequent separation zone wherein the rich solvent may be separated to provide a non-aromatic fraction, a high-purity aromatic extract, and a final lean solvent.
As previously noted, the non-aromatic rainate which leaves the extraction zone will contain some solvent. The solvent may be present in the rainate partly as a soluble constituent in low concentration and partly as an entrained dispersion of free solvent phase due to the turbulence within the extraction zone. Because the typical solvent compositions which are utilized in aromatic extraction processing are water soluble, it is the practice to extract the solvent which is contained in. the non-aromatic rainate stream by contacting this raffinate stream with an aqueous stream in a subsequent extraction means. The extraction of the solvent from the ranate with water may be undertaken in any sutiable liquid-liquid contacting means such as in a tower containing suitable packing such as Berl saddles or Raschig rings, or in a tower containing suitable tray devices, or in a. rotating disc contactor (RDC).
The raflinate which then leaves the aqueous extraction zone, or water wash zone, will be substantially free of this solvent composition but the aqueous stream containing the recovered solvent will normally contain some non-aromatic hydrocarbons. The non-aromatic hydrocarbons are contained within the aqueous stream both as a soluble constituent of the aqueous stream and as a free hydrocarbon phase which is dispersed within the solventcontaining aqueous stream. This dispersion of microdroplets of non-aromatic hydrocarbons is entrained within the aqueous phase because of the turbulence which is normally experienced within the rainate water wash zone.
Thi-s solvent-containing aqueous stream is normally sent back to the rich solvent separation zone to provide at least a part of the stripping steam which is utilized in separating the aromatic extract from the rich solvent. As the aqueous stream is generated into stripping steam, the solvent contained therein is thus recovered as a part of the nal lean solvent which remains when the extract has been stripped out of the rich solvent. However, since this aqueous stream contains a substantial portion of nonaromatic hydrocarbons, it is undesirable to utilize this stream for direct generation of stripping steam since the non-aromatic ranate contained therein would be vaporized in the stripping zone and thereby contaminate the resulting high purity aromatic extract. To avoid any contamination of the aromatic extract by non-aromatic constituents contained within the aqueous stream, it is therefore, typically the art to pass this aqueous stream to a prior distillation column or Water still. The water still produces an overhead fraction comprising the nonaromatic raflnate constituents and water, and a bottoms fraction comprising water and solvent having substantial freedom from non-aromatic contaminants. This bottoms fraction is then passed to the extract recovery column in order to provide at least a part of the stripping steam utilized therein, While the non-aromatic overhead fraction may be passed to the extraction zone as a part of the nonaromatic extractor reux.
SUMMARY OF INVENTION It is an object of the present invention to provide an improved extraction process wherein high-purity aromatics are separated from aromatic-containing feed stocks in a more economical and facile manner. It is a particular object of this invention to provide Ia method for the recovery of Water-soluble solvent from non-aromatic rafnate whereby the solvent may be recovered in an aqueous stream which is reused as stripping steam without detrimentally contaminating the high purity aromatic extract with non-aromatics constituents. It is a further particular object of the present invention to provide an improved extraction process for the recovery of high-purity aromatics wherein the solvent-containing aqueous stream from the rainate water wash zone is used as a part of the stripping steam in the extract recovery column by a method which provides for the elimination of the prior art water still.
Other objects and the advantages of the present invention will become apparent in the description which follows hereinbelow and which is made with reference to the accompanying drawing. The attached drawing is a schematic ow diagram of a conventional aromatic extraction process wherein the prior art water still has been eliminated by the practice of the present invention.
It has been determined that the objects of this invention may be achieved by bringing the solvent-containing aqueous stream from the rafnate water Iwash zone into contact with Ian aromatic hydrocarbon stream in a zone of high turbulence. Upon subsequent settling of the resulting mixture, substantially all of the non-aromatic constituents which were present in the solvent-containing aqueous stream will be found in the aromatic hydrocarbon phase. The resulting aqueous phase will contain the solvent recovered from the non-aromatic raffinate sand some aromatic hydrocarbons while having substantial freedom from non-aromatic hydrocarbons.
It is well known in the art that although water is a poor solvent for hydrocarbons, it has a high selectivity for -aromatic hydrocarbons. For example, at a temperature of 15.5 C. (60 F.), 100 grams of water will dissolve 0.168 gram of benzene, but only 0.014 gram of hexane. Similarly, at a temperature of 15.5 C. (60 F.), 100 grams of water will dissolve 0.040 gram of toluene, but only 0.005 gram of heptane (Seidell, A., Solubilities of Organic Compounds, vol. II, 3rd ed., 1941).
By the practice of the present invention, the aromatic hydrocarbon stream is admixed in a contacting zone with the solvent-containing aqueous stream under conditions sufficient to provide that substantially all non-aromatics will preferentially pass out of the aqueous phase and into the hydrocarbon phase Iwhile the solvency of the water for hydrocarbons is utilized by selectively dissolving aromatic hydrocarbons.
Therefore, in accordance with the practice of the present invention a broad embodiment comprises a process for the recovery of water soluble solvent from a tirst raffinate stream comprising non-aromatic hydrocarbons produced in fan aromatic extraction process which comprises; contacting the first raflinate stream with a first aqueous stream in a first contacting zone under conditions suflicient to provide a second rainate stream comprising non-aromatic hydrocarbons having substantial freedom from the solvent, and a second aqueous stream containing solvent and non-aromatic hydrocarbons; contacting the second aqueous stream with a first stream comprising aromatic hydrocarbons in a second contacting zone under conditions suflicient to provide a second aromatic stream containing nonaromatic hydrocarbons, and a third aqueous stream containing solvent and having substantial freedom from non-aromatic hydrocarbons; recovering the second raffinate stream from the first contacting zone and recovering the second aromatic stream from the second contacting zone; and recovering the solvent from the third aqueous stream.
A further embodiment of the present invention may be characterized as the process disclosed in the broad embodiment immediately above, wherein the third aqueous stream is returned to the aromatic extraction process to provide at least a part of the stripping steam required to remove aromatic extract from the rich solvent composition.
A still further embodiment of the present invention may be characterized as the process of the broad embodiment disclosed hereinabove, wherein the first aromatic stream comprises a part of the aromatic extract produced in the aromatic extraction process and the second aromatic stream is returned to the aromatic extraction process.
As has been noted hereinabove, the aromatic extract is normally fractionated into its constituent aromatic hydrocarbons, and in the typical BTX Aromatics Unit a benzene drag stream is removed from the benzene column and sent back to the extraction zone. It is therefore, one preferred embodiment of the present invention to utilize this benzene drag stream as the first aromatic stream of the broad embodiment disclosed above, and to return the benzene drag stream to the extraction zone after it has contacted the solvent-containing raffinate wash water in the second contacting zone and has rendered the wash water substantially free of non-aromatic hydrocarbons.
This preferred embodiment and other embodiments may be more clearly understood by now referring to the accompanying drawing in conjunction with the following disclosure of one specific example wherein the present invention is practiced.
DRAWING AND EXAMPLE In a specific operation illustrating the application of the inventive process, a depentanized catalytic reformate was rerun to remove the high boiling fraction, and then solvent extracted to produce nitration grade benzene, nitration grade toluene, and a mixed C8 aromatics stream. The rerun charge stock was passed to a primary aromatics extraction zone, not shown in the accompanying drawing, at a rate of 8730 b.p.s.d. or 1063.6 lb-mols/hr. wherein the feed was contacted with a sulfolane solvent composition. A nonaromatic raffinate stream was produced at the rate of 4365 b.p.s.d. or 470.8 lbs.mols/hr. An aromaticcontaining rich solvent was produced at a rate sucient to recover 1191.9 lb.mols/hr. of hydrocarbon extract.
Referring now to the attached drawing, the raffinate stream leaves the primary aromatic extraction zone and enters the process of the present invention at the rate of 470.8 mols/hr. via line 1. The raflinate in line 1 passes into a water wash extraction zone 2 which comprises an RDC extraction unit which is typical of the prior art. This raflinate stream comprising 463.8 mols/hr. of hydrocarbon and 7.0 mols/hr. of sulfolane solvent enters the RDC extractor 2 at 60 p.s.i.g. and 100 F. The raflinate is Water washed therein by 621.9 mols/ hr. of water entering the top of RDC extractor 2 via line 3 at 100 F. The raffinate which has now been rendered substantially free of sulfolane solvent leaves RDC extractor 2 via line 4 at the rate of 463.73 mols/hr., at a temperature of F., and at a pressure of 30 p.s.i.g. This raffinate stream is sent to further processing or to raffinate storage, not shown, at the rate of 4320 b.p.s.d.
A rich aqueous phase leaves RDC extractor 2 via line 4S at the rate of 628.97 mols/hr. and at a temperature of 100 F. This stream comprises 621.9 mols/hr. of water, 7.0 mols/hr. of sulfolane solvent, and 0.07 mol/hr. of non-aromatic rainate The non-aromatic hydrocarbon is present in this rich water stream at a concentration which is greater than is normally anticipated based solely upon solubility. This elevated level of non-aromatic hydrocarbon contamination is due to the high turbulence within the RDC extractor 2 which results in the entrainment'of free hydrocarbon phase within the rich water stream of line 45. The disposition of this solvent-rich water stream will be further discussed hereinbelow.
The rich solvent stream from the primary aromatics extraction zone enters the process of the present invention via line 5. This rich solvent stream contains 1191.9 mois/hr. of hydrocarbon extract and comprises a solvent composition containing sulfolane and water. This rich solvent stream passes via line 5 at a temperature of 245 F. and a pressure of 5 p.s.i.g. and enters solvent stripper column 6.
The solvent stripper is operated under conditions suffcient to remove substantially all non-aromatic hydrocarbon contaminants from the solvent-hydrocarbon solution. A stripper overhead vapor leaves solvent stripper 6 via line 7 at the rate of 659.1 mols/hr., at a temperature of 265 F., and at a pressure of 5 p.s.i.g. This vapor stream is cooled in condenser 8 to a temperature of 120 F. and then passed via line 9 into a phase separator 10 which is maintained at substantially atmospheric pressure. The vapor condensate is settled within separator 10 to provide an aqueous phase and a hydrocarbon phase. The hydrocarbon phase comprising non-aromatics is withdrawn via line 11 at the rate of 592.1 mols/hr. and at a temperature of F. This hydrocarbon stream is typically returned to the primary aromatics extraction zone in order to provide a non-aromatic reflux in the extractor vessel. An aqueous phase is separated in separator 10 comprising a portion of the water in the sulfolane-water solvent composition which was vaporized from the solvent composition which entered column 6. This aqueous phase is withdrawn from separator 10 via line 12 at the rate of 67.0 mols/hr. and is processed in a manner which will be set forth hereinbelow.
As the rich solvent liquid passes down solvent stripper column 6, its temperature is elevated by rising hot hydrocarbon and water vapors in a manner sufficient to provide that substantially all non-aromatic hydrocarbon constituents are removed from the liquid solvent phase. A hot solvent liquid is withdrawn from the bottom of stripper column 6 via line 13 at a temperature of 300 F. This solvent stream enters reboiler 14 wherein the temperature is elevated to 350 F. A resulting hot reboiler vapor and liquid mixture leaves reboiler 14 via line 15 and reenters the bottom of stripper column y6 at l0 p.s.i.g.
A resulting stripper bottoms lfraction comprising rich solvent leaves the bottom of the stripper column 6 via line 16 at a temperature of 350 F. This rich solvent stream has been rendered substantially free of non-aromatic hydrocarbon contaminants by the operating conditions within stripper column 6. The stripper bottoms fraction is now passed via line 16 into an extract recovery column 17 at a temperature of 325 F. and a pressure of 300 mm. Hg absolute. This rich solvent feed contains 599.8 mols/hr. of substantially pure aromatic hydrocarbons and comprises a sulfolane solvent composition containing water.
Extract recovery column 17 is operated in a manner sufficient to provide that substantially all aromatic hydrocarbons are separated from the sulfolane solvent composition. An overhead vapor stream leaves column 17 via line 18 at a temperature of 180 F., and at a pressure of 300 mm. Hg absolute. This hot vapor stream enters condenser 19 wherein it is cooled to 100 F. before passing into receiver 21 via line 20. The overhead vapor condensate is separated therein to provide an aromatic hydrocarbon phase and an aqueous phase which results from the stripping steam utilized within column 17. A first portion of the hydrocarbon phase is withdrawn from receiver 21 via line 22 and is returned to extract recovery column 17 as reflux. A second portion of the hydrocarbon phase is withdrawn via line 23 and passed to the subsequent benzene column 32 via line 23 at the rate of 602.8 mols/ hr. The aromatic extract passing via line 23 comprises 599.8 mols/hr. of high-purity aromatic hydrocarbon, and 3.0 mols/hr. of dissolved and entrained water. The aqueous phase comprising stripping steam condensate is settled in receiver 21 and withdrawn therefrom via line 3 at the rate of 621.9 mols/hr. This aqueous stream is then passed to RDC extractor 2 via line 3 as the raffinate water wash stream previously disclosed hereinabove. Because of a continual loss of water in the aromatic extract leaving receiver 21 via line 23, a water make-up stream comprising clean steam condensate is introduced into receiver 21 va line 24 at the rate of 3.0 mols/ hr.
As the aromatic rich solvent passes down extract recovery column 17, it is contacted with stripping steam and hot aromatic vapors in a manner sufficient to strip substantially all hydrocarbons out of the solvent. A resulting lean solvent leaves the bottom of recovery column 17 via line 25 at a temperature of 325 F. A stream comprising stripping steam, hydrocarbon vapor, and sulfolane solvent enters line 25 via line 2-6 from a source to be disclosed hereinbelow, and the resulting steam and solvent stream passes into reboiler 27 wherein it is heated to 335 F. The resulting reboiler vapor and liquid mixture leaves reboiler 27 and enters the bottom of recovery column 17 via line Z8.
A net lean solvent leaves recovery column 17 via line 29 at a temperature of 325 F., and enters a steam generator 30 at .a pressure of 150 p.s.i.g. wherein the stripping steam of line 26 is generated in a manner to be further disclosed hereinbelow. The lean solvent then leaves the steam generator 30 via line 31 and is returned to the primary aromatic extraction zone, not shown.
The high purity aromatic extract in line 23 is passed into benzene column 32 at the rate of 602.8 mols/hr. This feed stream comprises 599.8 mols/hr. of substantially pure aromatic hydrocarbon and 3.0 mols/hr. of dissolved and entrained water, and enters column 32 at a temperature of 225 F. and a pressure of 5 p.s.i.g. Benzene column 32 is operated under conditions sufficient to provide that substantially all the water and low boiling non-aromatic contaminants will Abe removed as an overhead vapor while a substantially pure benzene product which meets nitration grade specifications will be produced as a side-cut fraction. In the instant example, a hot benzene liquid is withdrawn from the fifth deck of benzene column 32 via line 33 at a temperature of 195 F. to provide a net benzene product of 158.6 mols/hr. Upon cooling to F. the net benzene product is passed via line 33 to storage facilities, not shown, at a rate of 960 b.p.s.'d.
The benzene overhead vapor is removed via line 34 at a temperature of F. and a pressure of 5 p.s.i.g. The vapor stream enters condenser 35 wherein it is cooled to 100 F. before passing into receiver 37 via line 36. A rst portion of the benzene overhead fraction is removed from receiver 37 via line 38 and returned to the top tray of column 32 as reflux. An aqueous phase is separated in receiver 37 and removed therefrom via line 39 at the rate of 3.0 Inols/hr. and sent to a disposal system, not shown. In order to provide that non-aromatic hydrocarbon contaminants will not accumulate within receiver 37 or within the top section of benzene column 32 and thereby contaminate the pure benzene withdrawn via line 33, a benzene drag stream is removed from receiver 37 via line 40. This drag stream comprises 8.3 mols/hr. of benzene and 0.008 mol/hr. of non-aromatic hydrocarbons and is processed in a manner which will be set forth hereinbelow.
The heavier aromatic constituents of the aromatic extract are rendered substantially free of benzene in the lower section of benzene column 32. A heavy aromatic stream comprising of C7-I- hydrocarbons is withdrawn from the bottom of benzene column 32 via line 41 and passed into reboiler 42. This reboiler liquid stream is increased in temperature from 270 F. to 280 F. therein, and returned to the bottom of the benzene column 32 via line 43. A net aromatic fraction comprising C7| aromatics is withdrawn from benzene column 32 via line 44 at the rate of 432.9 mols/hr. and at a temperature of 270 F. This C74- fraction is further separated to provide a nitration grade toluene product and a high-purity Cg-laromatic product in a subsequent fractionation train, not shown.
The solvent-containing water stream of line 4S which is removed from the water wash extractor 2 at the rate of 628.97 mols/ hr. is combined with the water stream removed from stripper overhead receiver 10I via line 12 at the rate of 67.0 mols/hr. The resulting aqueous stream continues via line 45 at a temperature of 100 F. and at a rate of 695.97 mols/hr. This total water stream contains 7.0 mols/hr. of sulfolane solvent which was recovered from the raffinate and 0.07 mol/hr. of nonaromatic raffinate which was dissolved and entrained in the raffinate wash water. The total aqueous stream is augmente'd by the benzene drag stream withdrawn from receiver 37 at the rate of 8.308 mols/ hr. The benzene drag stream `containing 0.008 mol/hr. of non-aromatics enters line 45 via line 40, and the total stream of hydrocarbon and water is passed to in-line mixer 46 via line 45 at a rate of 704.278 mols/hr.
The temperature of 100 F. and the degree of turbulence within in-line mixer 46 is sufficient to provide that the non-aromatic hydrocarbon contained in the rafinate wash water is preferentially passed into the hydrocarbon phase and dissolved in the benzene hydrocarbon. Simultaneously, the solvency of the aqueous phase for hydrocarbon is satisfied by selectively dissolving aromatic benzene in the aqueous phase in substitution for the displaced non-aromatic raflinate. The mixture of hydrocarbon and water leaves mixer 46 via line 47 and enters a phase separator or settler 48. A net benzene drag stream leaves separator 48 via line 49 and is returned to the primary aromatics extraction zone, not shown, at the rate of 48.5 b.p.s.d. This returned benzene stream comprising 7.95 mols/hr. of benzene and 0.076 mol/hr. of non-aromatics is returned to the extraction zone for the recovery of the benzene and the rejection of the nonaromatic contaminants into the raffinate stream.
The water phase leaves settler 48 via line 50l at the rate of 696.252 mols/hr. at a temperature of 100 F. This Water stream has now been rendered substantially free of non-aromatic contaminants. The Water stream in line 50 comprises 688.9 mols/hr. of water, 7.0 mols/hr. of sulfolane, 0.35 mol/hr. of benzene, and only 0.002 mol/hr. of non-aromatic contaminants. The amount of hydrocarbon contained in this water stream is higher than anticipated from mere solubility considerations, since the degree of phase separation in settler 48 is imperfect and some free hydrocarbon phase is entrained in the aqueous phase. In addition, the sulfolane solvent which is present in the aqueous phase increases the solubility of the benzene. The water stream passes via line 50 into steam generator 30 wherein the stream is heated from 100 F. to 250 F. by the lean solvent stream, as has 'been previously noted hereinabove. The resulting stripping steam leaves steam generator 30 via line 26 at the rate of 696.252 mols/hr. This stripping stream comprises 650.952 mols/hr. of saturated vapor and 45.3 mols/hr. of saturated liquid. The vapor comprises 650.6 mols/ hr. of steam, 0.35 mol/hr. of benzene vapor, and 0.002 mol/hr. of nonaromatic vapor. The saturated liquid comprises 7.0 mols/ hr. of liquid sulfolane and 38.3 mols/hr. of water. This hot liquid and vapor stream passes from stream generator 30 via line 26 into line 25 to provide the stripping stream required for recovery of the aromatic extract from the nal lean solvent within column 17 as previously disclosed hereinabove.
PREFERRED EMBODIMENTS The effectiveness of the present invention may be noted by comparing the content of the non-aromatic hydrocarbon which was originally contained in the solventcontaining rainate wash water leaving extractor 2, with the iinal non-aromatic in the aqueous stream which was generated into the required stripping steam for the extract recovery column 17. In the foregoing example, the nonaromatic hydrocarbon level was reduced from 0.07 mol/hr. to 0.002 mol/hr. or more comparatively, this reduction of non-aromatic contaminants was from a level of 111.2 p.p.m. to 2.9 p.p.m. Thus, by the practice of the present invention the final aqueous stream of line 50 was rendered substantially free of non-aromatic contaminants, thereby eliminating the need for the prior art water still. Although this reduced non-aromatic hydrocarbon content is passed overhead with the aromatic extract in the recovery column 17, this 2.9 p.p.m. of contaminants in the stripping steam only becomes 3.3 p.p.m. of contaminants in the total extract which is withdrawn via line 23. A1- though this 3.3 p.p.m. of non-aromatic becomes greatly concentrated in the benzene column, it does not appreciably affect the purity of the benzene product withdrawn as the side-cut fraction. The bulk of this non-aromatic contaminant which has been introduced with the aromatic extract feed, passes overhead with the benzene vapor and is ultimately withdrawn as part of the benzene drag stream.
By the elimination of the water still in the process of the present invention both capital expense and operating expense are reduced. The reboiler of the water still is replaced by the steam generator 30. The column of the water still and its other appurtenances are replaced by mixer 46 and separator 48 which are pieces of equipment requiring a lower total capital expense than a fractionating column. It should be noted particularly that mixer 46 need not be an elaborate piece of apparatus but may be any simple mixing device suitable for providing a turbulent contact between the aqueous phase and the aromatic hydrocarbon phase. A typically simple mixing apparatus suitable in this service may comprise One or more mixing orifices.
Although the process of the example uses the benzene drag stream as the aromatic phase passed to the contacting zone, it must be realized that the scope of the present invention is not so limited. Any aromatic stream of high-purity is suitable Within the scope of the present invention and, in fact, an aromatic stream could be derived from some source external to the aromatic extraction processing unit. However, it is preferable that some part of the aromatic extract which is produced in the process should be utilized, such as a slip stream withdrawn from line 23, for example. However, since a benzene drag stream is normally withdrawn in the typical commercial process, this is the most convenient source for the aromatic stream passing to the inventive contacting zone. But it is also within the scope of the present invention to use a toluene stream in the contacting zone or to use a xylene stream for contacting the aqueous phase. It must also be noted that where the aromatic extraction processing unit processes a hydrocarbon charge stock which does not contain benzene, the lowest boiling aromatic constituent of the extract will contain any nonaromatic contaminants contained in the extract. Therefore, it is typical in the art, for example, when fractionating extract comprising only toluene and xylenes to remove the water contaminant and non-aromatic contaminants in the toluene overhead vapor and to remove the pure toluene product as a side-cut fraction in a manner similar to that which has been shown for the benzene column of the example. Thus, in such an aromatic extraction unit it is typically necessary to withdraw a toluene drag stream in order to avoid contamination of the pure toluene sidecut product by an accumulation of non-aromatics in the top of the toluene column. In such an operation, it would be preferable to use the toluene drag stream as the aromatic hydrocarbon passed to the contacting zone.
While any high-purity aromatic mixture or any highpurity aromatic compound is effective within the process of this invention, it is preferable to use the lowest molecular weight aromatic constituent which is available. This is because it is well known that solubility decreases with increasing molecular weight. Thus, in order to most effectively displace the non-aromatic hydrocarbon out of the aqueous phase, the lowest molecular weight constituent of the aromatic extract should preferably be used.
It must be realized that the operating conditions which have been given in the foregoing example are specific to that example and should not be construed as a limitation upon the operation of the present invention. Those skilled in the art may readily ascertain those particular operating conditions which may be required in order to achieve any given separation of non-aromatic raflinate and aromatic extract for any given composition of hydrocarbon charge stock. Broad operating conditions for operation of the primary aromatics extraction zone and for the operation of the rich solvent separation zone may be found in the U.S. patents which have been cited hereinabove or in other well-known publications. Those skilled in the art similarly are able to select the operating conditions which may be required in the benzene column and in the subsequent aromatic fractionating columns utilized in the typical fractionating train for the aromatic extract.
The operating conditions which may be required within the inventive contacting zone comprising mixer 46 and separator 48 are also specific to the example. Any pressure could be imposed upon this contacting zone provided that the level of pressure is sufiicient to keep the two phases in a liquid state under the temperature level which is utilized for contacting the aromatic hydrocarbons with the aqueous phase. The temperature level should be established sufficient to selectively dissolve the aromatic hydrocarbon into the aqueous phase while preferentially dissolving the non-aromatic hydrocarbons into the aromatic phase. It is well known that the separation between aromatic and non-aromatic hydrocarbons will be more selective at a relatively low temperature. Since the nonaromatic raffinate which leaves the aromatic extraction zone is typically at a temperature of from 200 F. to about 300 F., the temperature of the contacting zone should not exceed this level. Indeed, it is preferable that the aromatic phase contact the aqueous at a temperature in the range of from about 60 F. to about 200 F. and, preferably, the temperature selected will be in the neighborhood of from about 80 F. to about 120 F. The turbulence which is experienced in the contacting zone need not be defined with any specific limitation other than that it be sufficient to result in the intimate contacting of the aromatic and aqueous phases.
The amount of aromatic hydrocarbon which should be present in the contacting zone should be as high as is possible frorn the standpoint of equipment size and utility expense. The presence of a massive quantity of aromatic phase in comparison with the non-aromatic contaminants in the aqueous phase will enhance the effectiveness of the present invention. By providing a more massive dilution of the non-aromatics in the aromatic phase, the solubility of the non-aromatics in the aromatic phase is greatly promoted and the transfer of the non-aromatics from the aqueous phase to the aromatic phase becomes more highly preferential. In the example shown above, the ratio of the aromatic hydrocarbon to the non-aromatic hydrocarbon in the contacting zone was 8.30 mols/hr. to 0.0078 mol/ hr. or an effective ratio of about 1063 to one. It is Within the scope of this invention to operate the contacting zone with a ratio of aromatic hydrocarbons to non-aromatic hydrocarbons in the range of from about 100 to one to about 10,000 to one or even higher.
Those skilled in the art can readily ascertain specific operating conditions which may be required within the contacting zone for any specific application of this invention by utilizing the teachings which have been presented hereinabove.
From the foregoing disclosure, one broad embodiment of the present invention may now be summarized as a process for recovering aromatic hydrocarbons from a charge stock containing non-aromatic hydrocarbons which comprises, contacting the charge stock with specified solvent in an extraction zone maintained under conditions sufficient to provide a first raffinate stream comprising solvent-containing non-aromatic hydrocarbons, and a rich solvent stream containing aromatic hydrocarbons; passing the rich solvent stream from the extraction zone into a first separation zone under conditions sufficient to provide a high-purity aromatic stream, a first aqueous stream, and a lean solvent stream substantially free of aromatic hydrocarbons; returning the lean solvent stream to the extraction zone to provide at least a part of the specified solvent; contacting the first raffinate stream with the first aqueous stream in a first contacting zone under conditions sufficient to provide a second rainate stream comprising non-aromatic hydrocarbons having substantial freedom from specified solvent, and a second aqueous stream containing specified solvent and non-aromatic hydrocarbons; contacting the second aqueous stream with a first portion of the high-purity aromatic stream in a second contacting zone under conditions sufficient to provide an impure aromatic stream containing non-aromatic hydrocarbons and a third aqueous stream `containing specified solvent and having substantial freedom from non-aromatic hydrocarbons; passing the impure aromatic stream into the extraction zone under conditions sutiicient to recover aromatics therefrom; passing the third aqueous stream into a heat exchanger means under conditions sufficient to provide a fourth aqueous stream comprising specified solvent and steam; passing the fourth aqueous stream into the first separation zone to provide at least a part of the stripping steam; and recovering the second raffinate stream and recovering a second portion of the high-purity aromatic stream.
The invention claimed is:
1. Process for recovery of water soluble solvent from a first raffinate stream comprising non-aromatic hydrocarbons produced in an aromatic extraction process which comprises:
(a) contacting said first rafiinate stream with a first aqueous stream in a first contacting zone under conditions suiiicient to provide a second raffinate stream comprising non-aromatic hydrocarbons having substantial freedom from solvent, and a second aqueous stream containing solvent and non-aromatic hydrocarbons;
(b) contacting said second aqueous stream with a first aromatic stream comprising high-purity aromatic hydrocarbons in a second contacting zone under conditions sufficient to provide a second aromatic stream containing non-aromatic hydrocarbons, and a third aqueous stream containing solvent and having substantial freedom from non-aromatic hydrocarbons;
(c) recovering said second raffinate stream from first contacting zone, and said second aromatic stream from said second contacting zone; and,
(d) recovering said solvent from said third aqueous stream.
2. Process of claim 1 wherein said third aqueous stream is passed to said aromatic extraction process.
3. Process of claim 1 wherein said first aromatic stream comprises a part of an aromatic extract produced in said aromatic extraction process and said second aromatic stream is lpassed to said aromatic extraction process.
4. Process of claim 1 wherein said solvent comprises a sulfolane-type chemical of the general formula:
Rg-CH CII-R4 Rz-CH-CH-Ra wherein R1, R2, R3, and R4 are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an arylalkyl radical having from one to twelve carbon atoms, and an alkoxy radical having from one to eight carbon atoms.
5. Process of claim 1 wherein said solvent comprises at least one polyalkylene glycol.
6. Process for recovery of aromatic hydrocarbons from a. charge stock containing non-aromatic hydrocarbons which comprises:
(a) contacting said charge stock with specified solvent in an extraction zone maintained under conditions sufficient to provide a first raffinate stream comprising solvent-containing non-aromatic hydrocarbons, and a rich solvent stream containing aromatic hydrocarbons;
(b) passing said rich solvent stream from said extraction zone into a first separation zone;
(c) passing stripping steam into said first separation zone under conditions sufficient to provide a highpurity aromatic stream, a first aqueous stream, and a lean solvent stream substantially free of aromatic hydrocarbons;
(d) returning said lean solvent stream to the extraction zone to provide at least a part of said specified solvent;
(e) contacting said first raffinate stream with said first aqueous stream in a first contacting zone under conditions suflicient to provide a second raffinate stream comprising non-aromatic hydrocarbons having substantial freedom from specified solvent, and a second aqueous stream containing specified solvent and nonaromatic hydrocarbons;
(f) contacting said second aqueous stream with a first portion of said high-purity aromatic stream in a second contacting zone under conditions sufficient to provide an impure aromatic stream containing nonarornatic hydrocarbons and a third aqueous stream containing specified solvent and having substantial freedom from non-aromatic hydrocarbons;
g) passing said impure aromatic stream into said eX- traction zone under conditions sufficient to recover aromatic therefrom; g
(h) passing said third aqueous stream into said first separation zone in a manner sufficient to provide at s least a part of said stripping steam; and,
(i) recovering said second rafiinate stream and recovering a second portion of said high-purity aromatic stream.
7. Process of claim 6 wherein said high-purity aromatic stream is separated at least in part into its aromatic molecular constituents, and said first portion of said highpurity aromatic stream comprises a portion of one of said constituents.
8. Process of claim 7 wherein said first portion of said high-purity aromatic stream comprises a portion of the lowest molecular weight aromatic constituent.
9. Process of claim 6 wherein said specified solvent comprises a sulfolane-type chemical of the general formula:
Ri-CH \CH-Ra Rz- H- H-R:
wherein R1, R2, R3, and R4 are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an arylalkyl radical having from one to twelve carbon atoms, and an alkoxy radical having from one to eight carbon atoms.
10. Process of claim 6 wherein said specified solvent comprises at least one polyalkylene glycol.
11. Process of claim 6 wherein said third aqueous stream is passed into a heat exchanger means under conditions sufficient to provide a fourth aqueous stream comprising specified solvent and steam, and said fourth aqueous stream is passed into said first separation zone in a manner sufi'icient to provide at least a part of said stripping steam.
References Cited UNITED STATES PATENTS 2,936,283 5/1960 Hutchings 208-321 3,179,708 4/1965 Penisten 208-321 3,308,059 3/1967 Deal 208-321 3,338,824 8/1967 Oliver 208-321 HERBERT LEVINE, Primary Examiner U.S. Cl. X.R. 208-325, 333
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US2936283 *||Nov 8, 1957||May 10, 1960||Pure Oil Co||Extraction process wherein the desired material is recovered by azeotropic distillation of the extract|
|US3179708 *||Sep 18, 1961||Apr 20, 1965||Universal Oil Prod Co||Solvent extraction of aromatics from hydrocarbon mixtures|
|US3308059 *||Mar 31, 1965||Mar 7, 1967||Shell Oil Co||Sulfolane solvent recovery from water wash|
|US3338824 *||May 9, 1966||Aug 29, 1967||Shell Oil Co||Water-washing raffinate to recover the sulfolane solvent|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US3864244 *||Nov 23, 1973||Feb 4, 1975||Universal Oil Prod Co||Solvent extraction with internal preparation of stripping steam|
|US3864245 *||Nov 23, 1973||Feb 4, 1975||Universal Oil Prod Co||Solvent extraction with increased polar hydrocarbon purity|
|US4009217 *||May 6, 1975||Feb 22, 1977||Universal Oil Products Company||Process for production and dehydrogenation of ethylbenzene|
|US4009218 *||May 6, 1975||Feb 22, 1977||Universal Oil Products Company||Alkylaromatic hydrocarbon dehydrogenation process|
|US4039602 *||May 6, 1975||Aug 2, 1977||Universal Oil Products Company||Ethylbenzene Dehydrogenation process|
|US4083772 *||Jul 19, 1976||Apr 11, 1978||Uop Inc.||Aromatic hydrocarbon separation process|
|US4333823 *||Sep 8, 1980||Jun 8, 1982||Uop Inc.||Corrosion inhibition in aromatic liquid extraction|
|US5176821 *||Feb 20, 1991||Jan 5, 1993||Uop||Process for the separation of aromatic hydrocarbons with energy redistribution|
|US5225072 *||Aug 3, 1990||Jul 6, 1993||Uop||Processes for the separation of aromatic hydrocarbons from a hydrocarbon mixture|
|US5879540 *||Jul 25, 1997||Mar 9, 1999||Occidental Chemical Corporation||Process for reducing corrosion in a system for separating aromatic hydrocarbons from a mixture with aliphatic hydrocarbons|
|US6576132 *||Feb 12, 2002||Jun 10, 2003||Stone & Webster Process Technology, Inc.||Quench water pretreat process|
|US7288184||Dec 10, 2004||Oct 30, 2007||Exxonmobil Chemical Patents Inc.||Process for mitigating acids in a system for separating aromatic hydrocarbons from a hydrocarbon feedstream|
|US8455709 *||Dec 8, 2009||Jun 4, 2013||Gtc Technology Us, Llc||Heavy hydrocarbon removal systems and methods|
|US20060124509 *||Dec 10, 2004||Jun 15, 2006||Van Nuland Marcus Lambertus H||Process for mitigating acids in a system for separating aromatic hydrocarbons from a hydrocarbon feedstream|
|US20110306816 *||Dec 8, 2009||Dec 15, 2011||Gtc Technology Us, Llc||Heavy hydrocarbon removal systems and methods|
|CN1062007C *||Nov 11, 1997||Feb 14, 2001||中国石油化工总公司||Process for regenerating aromatic hydrocarbon extraction solvent|
|DE2940937A1 *||Oct 9, 1979||Apr 17, 1980||Uop Inc||Verfahren zur fraktionierung von destillierbaren gemischen|
|U.S. Classification||208/321, 208/325, 208/333|
|International Classification||C10G21/00, C10G21/28|