US2983766A - Extraction process - Google Patents

Description

May 9, 1961 LE Rol E. HUTCHINGs ET AL 2,983,766

EXTRACTION PROCESS Filed Dec. 3, 1957 3 Sheets-Sheet 1 INVENTORS LEROI E. HUTCH/NGS BY NATHAN/EL L. REMES WILLIAM A. KREWER ATTORNE;

May 9, 1961 LE Rol E. HuTcHlNGs ET AL 2,983,766

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EXTRACTION PROCESS 5 Sheets-Sheet I5 Filed D60. 3, 1957 INVENTORS LERO/ E. HUTCHINGS NATHANIEL L. REMES WILL/AM A. KRE WER /f ATTORNEY d United States Patent O EXTRACTION PROCESS Le Roi E. Hutchings, Crystal Lake, and William A. Krewer, `Arlington Heights, lll.,` and Nathaniel L. Reines, Miami, Fla., assignors to The Pure Oil Company,` Chicago, lll., a corporation of Ohio Filed Dec. 3, 1957, Ser. No. 700,475

\ 6 Claims. (Cl.`260-674) vThis invention relates toa process for the separation or extraction of aromatic hydrocarbons from hydrocarbon mixtures, including paranic hydrocarbon mixtures, and, more particularly, to an extraction process using dimethyl ammonium dimethyl carbamate with certain critical amounts of water in order to attain high-purity extracts during said process.

`Aromatics such as benzene and alkyl-substituted homologues are present in many hydrocarbon mixtures. Aromatics are present in lubricating oil fractions. Benzene is present in coal atar fractions.` Liquid-phase extraction of'hydrocarbon mixtures of these types with selective solvents, at a temperature substantially below the point at which the solvent is completely rniscible with the mixtures, constitutes one known method Vof separating o'r rening the aromatic content thereof. Although such processes produce an extract which is relatively rich in aromatics and la rainate relatively rich in parainic hydrocarbons, the separation is only par-tial and yields fractions `containing substantial quantities of undesirable constituents; lt is oftenhighly desirable that the purity'of the extract phase be high, e.g., in the production of nitration-grade benzene, purities of 99.5% or higher` are sought. It is well accepted in the art that the presence of an anti-solvent,such as water or other highly polar compounds,` may in 4some instances aid phase separation, improve product purity, etc., but in other instances much better results are obtained by using the selective solvent or solvent mixture alone in substantially-anhyl-` drous condition. No set rule has so far been established d on which one can rely to predict how a given solvent being treated is; completelyV miscible with the solvent.V Here again the use of anti-solvents, including Water,with 'the-t principal solvent has `been employed to overcome:

miscibility to a certain extent." Howevergthose:

processes which employ arsolventl toremove impurities; such as sulfur compoundsor ol'ens' from' a hydrocarbon" Afritioii, either do not recognize 'thislfact or use the Vanti-"` solvent only` to" cause" phase separatiohin" the extract 'fand thereby yrecoverfthe solvent.` In thel extraction procy ges-directed toward separating hydrocarbonsjof an aro- ICC matic nature from associated mixtures, it. is impossible to predict that the water will act satisfactorily as an antisolvent, and if it does so act, what the eii'ect will be on the extract phase or the overall extraction etticiency.

In accordance with this invention, it has been found that the presence of between about 3.0-25.0 volume percent of water at about l50 F. increases the selectivity of dimethyl ammonium dimethyl carbamate without affecting the capacity for aromatics, while at the same time avoiding emulsion diiiiculties and making the process commercially feasible for the preparation of lhigh-purity aromatic products. The invention herein relates at least in part to the discovery that the type of aromatic hydrocarbon in the feed to be treated, the extraction temperature, the reflux ratios and the degree of purity of the end product make it mandatory that the volume percent or proportion of water present in the solvent during the primary extraction be clo'se1y`controlled. The discovery may be briefly stated as the finding that there is a direct relationship between the type of aromatic hydrocarbon sought to be isolated in the extract phase, the temperature of extraction and the contact conditions.

In general, it has been found that to obtain a predominance of aromatic hydrocarbons of maximum purity in the extract phase or highest selectivity, the amount of water required in the dimethyl ammonium dimethyl` carbamate solvent is the least, that is, around 3.0 volume percent, for the xylenes at the lowest extraction temperature of about F; and thegreatest, that is, about 25 volume percent, for benzene at the highest practical extraction temperature of about 140 F. The amount'of water used` may vary from 3.0 volume percent to 25 volume percent as the temperature of the extraction increases, with` the result that the extract will predominate in the aromatic homologues of fewer alkyl substituent groups, or fthe capacity willlbe greater for the lower molecular weight aromatic homologues present in the feed source. Thus, where the feed source contains a predominance of xylenes, the alkyl-substituted homologues of benzene, or` wherethis type `of aromatic` is desiredto be extracted from a mixture of hydrocarbons, theoptimum selectivity andcapacityV therefor is obtained by using about 3.0 to about 5.0 volume percent ofV water in the dimethyl ammonium dimethyl `carbamate at the the optimum selectivity for xylenes at an extraction temperature of about 120 to 140 F. and the optimum Vselectivity for benzene is found at` Waterl concentrations of about 10.0 to 25 volume percent where the extraction temperature varies between" about h11?. to 140" For certaintypesoffeeds, as catalytic reformatecontaining l ,p

benzene, xylenes and-'ethyl benzene, the optimum capac-` Y ity' andselectivity are' obtained byfusing water concen A trationsfrom about 115.0 'to `25:0V Volumepercent'at ex- -In a specific embodimentl ,of the invention, afhigh-l i carbamate with 15.0 to 20.0 volume percent of water at an extraction temperature of about 120 F., using a minimum reflux of about 4.7. Furthermore, if the water content of dimethyl ammonium dimethyl carbamate is increased above 25.0 volume percent, the capacity of the solvent drops to the point where the process is uneconomical and benzene or other aromatics in the solvent begins to force the water out of solution, which results in emulsion diiculties.

Dimethyl `ammonium dimethyl canbamate, hereinafter referred to as DADC tfor simplicity, is a known solvent `for impurities Vin fuel oil and gasoline. DADC is a water-white liquid, boiling at about l40.3 F., having a specifc'gravity of 1.026, an absolute viscosity at 25 C. of 63.3 cps. anda refractive index at 25 C. of about 1.4512. This solvent hask been used to treat a catalytically crackedfuel oil in equal volumes in a single-step extrac tion at ,25 C. with 30 minutes contact, by shaking, allowing the layers to separate, and separately washing with distilled Water to produce al raffinate comprising 88% fand an extract of impurities comprising 10.9% by ing hydrocarbon mixtures, using dimethyl ammonium dimethyl carbamate with certain amounts of water as the solvent.

An object of this invention is to provide Ia process for extracting high-purity xylenes from hydrocarbon mixtures containing same, using dimethyl ammonium dimethyl carbamate with certain amounts of water as the solvent.

Another object of this invention is to provide a process lfor extracting a high-purity benzene-toluene-xylene mixture from a concentrate, obtained by blending selected fractions of catalytic reformate by liquid phase selective solvent extraction using dimethyl ammonium dimethyl carbamate with certainV amounts of water and a minimum solvent-to-feed ratio of 2/5 with a minimum reux ratio to' charge oil -for the extract phase of about 1/4. A linal object of this invention is toprovide a process of selective lliquid-phase solvent extraction to produce high-purity .aromatic hydrocarbons, usingV .from about 15% to 25% by volume of water with the dimethyl am.- monium dimethyl carbamate solvent, where the characteristics of the hydrocarbon mixture require Aoperation of the primary extraction stage at a temperature of about 100 to 120 F.

These and further objects of this invention will be ex- Y plained or beco'me apparent as the description of the in- `In general, those processes which are adaptable to l water dilution to initiate solvent separation vfrom the extract phase are dependent upon a rather steep solubility curve for the solvents in water, and at temperatures of 167 F. or above it is desirable that the solvent be com# pletely miscible with water to elect satisfactory separation and solvent recovery. The use of water in this connection is for the purpose of solvent separation from the extract phase and the high solubility of the solvent in the water mitigates against the presence of water during the extraction process, that is, it'promotes emulsication instead of phase separation in the presence of the ranate. Thus, it'is unexpected to nd that the presence of 3-25 by volume of water in the solvent during the extraction using dimethyl ammonium dimethyl carbamate avoids emulsication and aids phase separation'without undue decrease in capacity, while at the same time increasing the selectivity. Furthermore, it is considered unusual Vtov nd that the solvent extraction efficiency can vention proceeds.v y Y The attached drawings further illustrate the invention wherein: Figure l is a three-phase diagram of the ternary data obtained using dimethyl ammonium dimethyl carbamate,

Figure 4 is athree-phase diagram fo'r the system xylene-n-heptane-DADC, showing the inuence of'water.

Before discussing the drawings in detail, la series of ex- I 3Y perirnents are described to show the development of the be increased to the point where the. purity of the aroingzdimethyl ammonium dimethyl carbamate with-certain amounts of water as the selective solvent.

Another object of the invention is to provide a process for the separation of aromatic hydrocarbons from hydro-V carbon mixtures vby solvent extraction of the yhydrocarbofn mixtures Vwith dimethyl ammonium dimethyl carbamate .containing from about 3.0 to 25.0 volume percent of Water, Vat a temperature from about 80to 140 F.

. A further object of the inventionv is to provide ra process for increasing and maintaining the selectivity of dimethyl j ammonium dimethyl carbam-ate for aromatic'hydr'ocarbons present in admixture with Anon-aromatic hydrocarbons by` employing .particular amounts of water in -relaA Vtionjgtoethe Adesired type of aromatichydrocarbonex.- tracted, the product purity thereof and the process condi- 5 tion'semployfed, particularlythe primary extraction tempe'rature andthe/redux ratiosi- Y ,Anotheryobjectdf the inventionis toY a 'prot-:essl for.i.attracting ihigh-tpuritxtbenzehe. fromsbenzenercoutan;

invention. l

EXAMPLE I In order to demonstrate the efiectof the concentration of water in dimethyl ammonium dimethyl carbamateLa feed composition consisting of 45 volume percent of toluf ene 'and 55 -volume percent of n-heptane was extracted with dimethyl ammonium dimethyl carbamate usingdikfferent concentrations of Water. The volume percent of the toluenerecovered in the extract phases produced,'and the purity o f the toluene therein were determined. A solvent/charge volume ratio of 1/1 and a temperature of 68 F. were used in each run. The results are shown in the following table:

VTABLE I Eect.ojqwater concentration on capacityv and selectivity Y vof DADC for aromatics '68 F.

Solvent Composition Toluene Run No.

- Vol. Per- Vol. Per- `cent;

DADO

that as the concentration of Water is increased, the yield of extract decreases and its purity increases. This means that the water decreases .the capacity and increases the selectivity of the dimethyl ammonium dimethyl carbamate at an extraction temperature of 68 F.

It has been found that the minimumy requirements of water content in the solvent vary when the solvent mixture is used to extract diiferent hydrocrabons. A series of experiments was conducted in which solutions of nheptane mixed with various aromatics were treated with dimethyl ammonium dimethyl oarbamate, and the water content of the solvent was varied to determine the upper and lower limits of water content that can be used. In conducting these experiments, the upper limit of Water concentration was taken las that concentration at which the `capacity of the solvent mixture became unsuitably low. The results are shown in the following Table ll.

TABLE II H2O-Concentrations in DADC to produce substantially pure aromatcs @77 F.

Ratios o1 Components 1 (Parts by Water Content i Volume) in HCl-Mixture Treated of DADC Run No.

Naher talle Lower Limit Toluene Benzene Xylene Upper Limit at 77 F. using DADC (dimethyl ammonium dimethyl carbamate) requires at least the minimum concentrations of water shown.

` This relationship is shown with clarity in the phase diagrams.' In Figure 1, curve 1 outlines the twophase region to show the rainate and extract compositions that are obtained by treating a n-heptane-benzene mixture with pure DADC, containing no water. In this instance, the solvent and benzene `are completely miscible `and point` E on the curve represents the composition of the extract phase which will give the maximum aromatic concentration. Curve 2 outlines the region of two-phases for DADC containing 2.51 wt. percent of water. Some increases in seleotivityandcapacity is noted by the 11p- 'ward shifting of point Ef Curve 3 outlinesthe twophase region for a solvent "comprising DADC with 5.0

Wt. percentof water. All' of the solventl compositions were tested by thoroughly shaking the feedand:` solvent at 80'"` Ffallowing themixture to settlearid separating the extract'` andrafnate phases fori analysis. Figure 2 isa phase diagram for tliesyfstem-'ri-heptane-` benzeneLsolvent at 100 F., where the solvent consists of 88 parts, by weight; ofllQADC and IZfpar-ts of water. Point E again representsthe compositionof thevex-g tract phase which will givethemaximnm aromatic con` centration. However,` by using" the higher water` Vvcncentration, Vsituation isf developedwhichpermits theV recovery of `almost pureiaromaticsfr- If point-E were is obtained. `The tests fonthe" results showrf i'rrFigiiiefZfvV 4 i The waterl-solventmixture in TABLE III- Summary of ternary data for p'xylenep n-heptane-DADC system Ralnnte Extract Vol. Percent t 01.119 151,0,y Yol. Vol. Vol. Vol. Vol. Vol.

Y Percent Percent Percent Percent Percent Percent Xylene DADC Hep Xylene DAD Hep tane tane 13.9 2. 0 84. 1 6.11 90.0 4. 0 28. 1 4. 4 67. 5 9. 81 84. 6 5. 6 39. 4 7.3 53. 3 15. 3 77. 8 6.9 48. 9 11.0 40. 1 20. 1 72. 5 7. 4 57. 4 16. 4 26. 2 31.1 59. 4 9. 5 15. 3 1. 4 83.3 4. 21 92.0 3. 9 29. 3 2. 4 68. 3 8. 0` 88. 2 3.8 41.7 4.0 54. 3 11.1 85.2 3. 7 51.7 6. 8 42. 5 14. 3 82.3 3. 4 60.0 8. 5 31.5 17. 4 79. 4 3. 2 68.0 12.8 19. 2 23. 1 73.9 3. 0 72.3 17. 3 10. 4 31. 3 66.0 2. 7 16.3 1. 3 82. 4 2. 7 95. 6 1. 7 30. 3 2. 3 67. 4 5. 5 92. 5 2.0 42. 6 3. 2 54. 2 8.2 89.9 1.9 53. 1 4. l 42.8 10.7 87. 7 1. 6 69.1 7.0 23.9 13.6 85.4 1.0 77.9 10.7 1154 19. 4 79. 5 1. l 83. fi 16. 4 0 27. 72. l5 0 Figure 4 reilects the results of rliable III as a ternary diagram showing the effect `of water dilution of the DADC. Curve 1 is for 0.0% by volume of water, curve 2 for 2.5% `by volume of water `'and curve 3 for 5.0% by volume of water. 'Dhese results conrm the relationship of the amount of water in the solvent to the extraetion temperatureand the type of aromatic sought.

The, invenion will be better. understood by reference to Figure 3 wherein `is illustrated a process in which the DADC-water solvent is used to extract aromatics from the charge and the solvent is recovered by water extracion ofthe DADC from the aromatics. Subsequent recovery of the DADC is acomplished by distillation. The process is illustrated by the treatment of a catalytic re` formate to lobtain a high-purity, benzene-toluene-xylene mixture. Charge reformate is passed through line 1 into extraction tower 2 which is fitted with the usual plates and/or battles (not shown)` toV increase the degree of contact.Y Tower 2 is operated at a temperature of about to 120 F. and a pressure of about atmospheric to 5 `p.s.i.g. `About` -5 ytol10 parts of solvent, entering at` line 3, are used fori each partofcharge reformate. Extract phase leaves ltower 2 byline 4, passing -to secondary extraction tower-5. .Rainate'from tower 2. is passed by line 61to water-wash tower 7, where it iscontacted with Water from line 8 at a temperature of about 100 to 1207.-` F. This Atreatment results in the separation of a paranie phase, .taken off at Vline 9, and=a water-solvent phase, removedat line 10. .U M f ,TowerwS, whichisconst "cted like `tower 2, receives the extract phase `from line 4, andthis phase iiows countercurrentlyto the water "streamfurnished by line 11. This produces "a so1vent-water mixture which is withdrawn throughrline 12, and mixed with the solvent-water mix-` ture; in line lifrornftower 7,`to forma combined stream which flows through line'` 23 to.solvent `recovery tower 13.- 'Thefoverhead from .tower 5, containing most of the aromatic fphasefpassesthrough line 14 toz'aromatic puriiication-tower.- 15'; `The application of` heat, as by means of s'te'am"coil"16,results in theY removal of the last traces of solvent from theextra'et, .the former passing through line tofcondenserflSand thence, through recycleli'ne `2.0, solventystorageaZS, and line 3,. back totower f2., or throughflirie'19-.back` to tower 15 asrefiuxftherein, the i latter"leavingitowerf atlin'e ,-21... Asportion ofthev g :aromtics `recovered in tower; 5 is recycled back to tower"` une iz joins with that@ i l Vpercent ofarornat'ics. By precise fractionation and blending' to different.v octane l:numbers z it. Was de trmindzih line 10 to form a combined stream which ilows through line 23 to solvent-recovery tower 13, where it is distilled by heating means Y24. The vsolvent passes overhead via line 25 to condenser 26. Part of the condensed solvent is returned to tower 13 as reiiux via line 27; the remainder passes to 'solvent storage 28 via 1in 29. The water passes via line 30 to water storage 31 4from whence it is recycled to,V water-washing towers and 7 viav lines 32,' 8 or 11, or isused to dilute the solvent by passage through line 33 returning to the extraction tower 2. In operating such a system, it has been Afound that the minimum DADC solvent-tochargeoi1 ratio for the extraction of 99.5 percent pure .benzene-toluene-xylene mixture in the primary columnis 2.5, and the actual solvent-to-chargeoil ratio is 6.8. yThe corresponding minimum reflux ratio ofre- 'cycle benzene-toluene-xylene mixture to charge-oil is 1:4 and the actual ratio `is 4:7. The optimum solvent-to-oil ratio is governed by the relationships between the feed composition, water content of the solvent, temperature of extraction, and degree of purity desired, but in general solvent-to-oil ratios from about 1:10 to 10:1 are satisfactory., y L Theprocess lof this invention is applicable to the treatment of any type of hydrocarbon mixture'lfrom which economical amounts 'of aromatics, including 'benzene and homologues thereof, may be recovered. Such mixtures include products obtained from aromatization reactions. Theproducts obtained `from catalytic'reformin'g, hydrocracking,and dehydrocyclization processes may be used as feed for the present process. Any mixtures containingaA paraflinicor naphthenic type hydrocarbon ladmixed with benzene, toluene, or the xylenes as simple or complex multi-component mixtures, may be used'as the start'- ing material. The charge oil should be liquid at ordinary temperatures and not subject to decomposition at the extraction temperatures or reaction with the selective solvents used herein. One suitable charge oil is the 'class of products known in the art as catalytic reformates. These liquid 'products contain a fair concentration of desirable aromatic hydrocarbons. Catalytic 'reformates are obtained by treating naphthas to reforming, dehydrogenation,` |hydrocracltingV and dehydrocyclizationr reactions lat ytemperatures ranging lfrom 850 F. to about 100051?. with pressures up to 500 p.s.i.g. in the presence of a'metal-containing catalyst. As. a more specific'V illustration, .catalytic reformates obtainedl as alresult of the treatment of a virginnaphtha (B. 175.F.400 F., API gravity 50 to 60) with a platinum-alumina catalyst at 875 F. to 975 AF. and pressures rangingfromy 200 to 500 p.s.i.g. maybe used. Reformatesso producedcontain from about 30 to 55 vol. percent'of aromatics and constitute a preferred feed for the'present process. Reformates produced by reforming a 200400F. virgin naphtha at abouti-950'o and r400 catalyst ycomprising about 0.1 n' alumina base are" preferred. These formatesl.iboillbetvveri-f1:00? j to 450?` E., `have `an `gravity 0f` ,40 .tASOQ-andfan aromatic contenti of 45,;55 volume percent-.fThe particularfrefor'rniate,used herein obtainedbysubiectingacharge naphtha have inga boilingrangeof 1017.12.1@418571-7., APIjgravity 49.2;v RON clear ,84.15 RON-e0151 TEL 94:9, @02% sulfur-,and containingabout156.4 vol. percent of paraiins and naphthenes,u1;2 vol. .percent oleiins and 42.4'vol,V

Vduceza'product having. an.API gravity of48.8, LBP.`

` in accordancewith this invention to obtain products hay# 8 i I TABLE IV Aromatics in reformaie feed Estimated Aromatic Vol. Per-V cent Benzene... .0.5 Toluene t 4.4 Mlxed'xylene and ethyl benzene 12. 6 C9 and heavier 28.0

Examples of the compositions of other reformate feed hydrocarbons that may be used are shown in Table IV giving the volume percent of aromatics in each and the research octane level to which the reforming reaction was directed in each instance.

TABLE V Aromatics distribution in various feed reformater (Volume Percent) 85 Re- 90 Re- 95 Re- 95 Re- Aromatic search search Search search Octane Octane Octane Octane Level Level Level Level 2. 96 4. 19 3. 4l 3. 85 9. 64 13. 1 12. 9 (1) 2. 55 2. 60 2. 72 (1) 2. 74 2.85 2.83 (1) A 6. 45 7. 03 73 (1) 3.96 4. 03 8.81 (1) C and heavier aromatics. .p 17. 3 17.2 22. 4 (1) Total 45. 6 51. 0 54. 8

-1 Analyzed for benzene content only.

f In general, these feed stocks contain a mixture in various proportions of i-'butane, n-butane, i-pentane, npent'ane cyclopent'ane, 2,2-dimethylbutane, 2,3-dimethylbutane, l2-methylpentane, 3-methylpentane, n-hexane, methylcyclo'pentane,l 2-2-dirnethylpentane, benzene, 2,4- dimethylpentane, cyclohexane, 2,2,3-trimethylbutane, 3,3- dimethylpentane, 1,1-dimethylcyclopentane, 2,3-dimethylpentane, fZ-Inethyilhexane,Y cis-1,3-dime'thylcyclopentanm trans-1,3-dimethylcyclopentane, 3-methylhexane, transl,2dimethylcyclopentane, 3-ethylpentane, n-heptane,

2,2-4-trimethylpentane, cis-1,2-dimethylpentane, methyl- Y c yclohexane', ethylcyclopentane other C8 hydrocarbons, toluene, ethylbenzle'ne, p-xylene, o-xylene and'm-xylene which represent the vtypes of hydrocarbon mixtures from which they aromatics, as enumerated, may be separated by the present process. In usingreformates as feed hydroca'r'bons, 'onezpurpose is tofseparate'the low-octane compounds so Ithat they )may berecycled or` otherwise upgraded inoctane number, and the high-octane-number productsv rl'ecovzerredv for` gasoliney blending. WAAn ther gtypeof feed mixture comprises various tions and vmixtures offractions of reformates which may bevused inlthisinvention. rFor example; a reformate or other source lof aromatics may be fractionated to form 1-benzene concentrate, a toluene concentrate and a xyleue concentrate; these maybe individually treated or mixed in variousl proportions and Vtreated to solvent extraction ing a. .high concentration 'of4 the desired aromatic. One

TEI93L8J and containing .about 54.0 vol. .percent parainsv arid naphthenes',.1.2-.vol.'.percent: olens and 44.3V vol.-

Vsuch feed material comprises va synthetic mixture of 1.Y i partbenzene concentrate,V 2 parts toluene concentrateand Y l1 part xylene concentrate.y The. purpose of treating such concentratesis. to recover aromatics ofmaximum purity f oruse` asYorganicpintermediates,"solvents, etc... iludescribing theprocess, such'considerationrs 'as rate Voff'tiow through, the systemfhave been'omitted, since they are-,controlled or. limitedl by the type of extraction tower "or fsyste'rn usel,i '.e.,v Aa packedtower,` perforated-plate i iieibel tower." Extraction tower). is operatedk frac# a te' pg tureiijmitspfisof FL @140 from atmslsphererrsafe 19,

or even l0 p.s.i.g., subject to the provisions of water content of the solvent and/or reflux ratios as specied herein. The solvent-to-feed ratio may vary from 1:10 to :1 in tower 2. The purpose of extract recovery tower 5 is to wash a substantial portion of the DADC from the extract phase for which purpose the ratio of water-to-extract phase is adjusted in accordance with known procedures in this art. Water-to-extract ratios of about 0.3:1 to 0.5 :1 or higher may be used in tower 5.

In accordance with sound processing techniques, it is advantageous to operate tower 5 and, for that matter, tower 7 at the same temperature and pressure conditions as tower 2. The purpose of tower 7 is to wash the DADC from the ranate phase using as little water as possible, i.e., water-to-rainate ratios of .01:1 to as high as .05 :1, with .02:1 being optimum. Similarly, the purpose of solvent-recovery tower 13 is to apply enough heat to distill the DADC from the water-wash solutions for which purpose the temperature at the top of the tower will be at least 140 F. and the temperature at the tower bottom will be 212 F. Pressure is used in the system to prevent loss of solvent and maintain the system in the liquid phase. A slight vacuum may be applied to tower 13 to aid the distillation of DADC, which has no tendency to decompose or hydrolyze, and thus adjust its water content. By proper adjustment of the reflux ratio through line 27 and the use of a sulicient number of stages, as is well known in the art, the water content of the distillate DADC may be adjusted to within that range desired for use in tower 2. The water content is determined by following the refractive index of the DADC- water mixture. Final adjustment, or changes therein, is made by addition of water at line 33 or adding fresh solvent to the system. Similarly, in tower 15 the temperature of the partially denuded extract is adjusted so that any remaining DADC and water is removed.

What is claimed is:

1. In the process of separating hydrocarbon mixtures containing aromatic hydrocarbons selected from the group consisting of benzene, toluene, ethyl benzene, and xylenes, and parainic hydrocarbons into -an extract predominating in said aromatic hydrocarbons and a ralinate predominating in said parainic hydrocarbons, the improvement comprising contacting said hydrocarbon mixtures with a solvent consisting of dimethyl ammonium dimethyl carbamate containing about 3.0 to 25 volume percent of water at Ia temperature of about to 150 F., separating the composite mixtures into two components consisting of said extract phase and said raffinate phase and recovering said aromatic hydrocarbons substantially free of other hydrocarbons from said extract phase.

2. The process in accordance with claim 1, in which said water is maintained at -a concentration of about 3.0 to 5.0 volume percent, said temperature is maintained at about to 100 F., and said purified aromatic hydrocarbons are essentially xylenes.

3. 'Ihe process in accordance with claim 1, in which said water is maintained at a concentration of about 6.5 to 10 volume percent, said temperature is maintained at about 77 F., and said purified aromatic hydrocarbon is essentially toluene.

4. The process in accordance with claim 1, in which said water is maintained at a concentration of about 10 to 25 volume percent, said temperature is maintained at about to 140 F., and said puried aromatic hydrocarbon is essentially benzene.

5. The process in accordance with claim 1, in which said hydrocarbon mixture comprises a catalytic reformate having a boiling range of about to 450 F., said water is maintained at a concentration of about l5 to 25 volume percent and said temperature is maintained at about 100 to 120 F.

6. The process in accordance with claim 5, in which said temperature is maintained at about F., said water is maintained at a concentration of about 15 to 20 volume percent and the solvent-to-feed ratio is about 2 to 5, with a minimum reux ratio to feed for the extract phase of about 1 to 4.

References Cited in the tile of this patent UNITED STATES PATENTS 2,183,852 Boyd Dec. 19, 1939 2,433,751 Friedman Dec. 30, 1947 2,594,044 Loder Apr. 22, 1952 2,773,918 Stephens Dec. 11, 1956 2,801,151 Nelson July 30, 1957 2,801,217 Nelson July 30, 1957

Claims (1)

1. IN THE PROCESS OF SEPARATING HYDROCARBON MIXTURES CONTAINING AROMATIC HYDROCARBONS SELECTED FROM THE GROUP CONSISTING OF BENZENE, TOLUENE, ETHYL BENZENE, AND XYLENES, AND PARAFFINIC HYDROCARBONS INTO AN EXTRACT PREDOMINATING IN SAID AROMATIC HYDROCARBONS AND A RAFFINATE PREDOMINATING IN SAID PARAFFINIC HYDROCARBONS, THE IMPROVEMENT COMPRISING CONTACTING SAID HYDROCARBON MIXTURES WITH A SOLVENT CONSISTING OF DIMETHYL AMMONIUM DIMETHYL CARBAMATE CONTAINING ABOUT 3.0 TO 25 VOLUME PERCENT OF WATER AT A TEMPERATURE OF ABOUT 60* TO 150*F., SEPARATING THE COMPOSITE MIXTURES INTO TWO COMPONENTS CONSISTING OF SAID EXTACT PHASE AND SAID RAFFINATE PHASE AND RECOVERING SAID AROMATIC HYDROCARBONS SUBSTANTIALLY FREE OF OTHER HYDROCARBONS FROM SAID EXTACT PHASE.

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