US2878261A - Recovery and separation of naphthalenes by solvent extraction - Google Patents

Description

March 17, 1959 D. B.-BROUGHTON 2,

RECOVERY AND SEPARATION OF NAPHTHALENES BY SOLVENT EXTRACTION Filed Nov. 15, 1956 Bottoms 9 Watqr Make -up Ref/ax- Make-up Solvent I N VE N T 0R: Donald B. Bmughtan Fead Stock.

A r p/my: rs.-

United States Patent RECOVERY AND SEPARATION OF NAPH THALENES BY SOLVENT EXTRACTION Application November 15, 1956,,Serial No. 622,488

14' Claims. (Cl. 260-674) This invention relates to a process for separating and recovering bicyclic aromatic hydrocarbons from mixtures of the same with, other classes of hydrocarbons, such as hydrocarbon mixtures containing said naphthalenes and one or more of the group comprising parafiins, naphthenes, olefins: and aromatics. More specifically, this invention concerns a process for recovering an extract comprising naphthalene and one or more of its homologs from a mixture'of hydrocarbons containing the same and thereafter segregating the extract into specific homologs by means. of a process which involves liquid-liquid phase extraction ofthe hydrocarbon mixture with a solvent for the, naphthalene component selected from the alcohols, the alkylene glycols, and the polyalkyl'ene glycols, recovering the hydrocarbon solute from the resulting rich solvent stream and thereafter fractionally distilling the recovered extract to separate said naphthalene and homologs.

The principal object of this invention, therefore, is to provide a process for segregating substantially pure naphthalene products, each of which consists essentially of an individual" naphthalene homolog from mixtures of the same with other types of hydrocarbons, including other naphthalene homologs. Other objects of the invention concern the preparation of individual naphthalene homologs from petroleum sources in a substantially pure form containing at least 99+% of the individual naphthalenes.

In one of its embodiments this invention comprises contacting a hydrocarbon fraction boiling from about 300 to about 500 F., containing at least two homologous naphthalene hydrocarbons with asolvent for said naphthalene hydrocarbons selected from the group consisting of alcohols, an alkylene glycol, a polyalkylene glycol and ethers of said alcohol and glycols at a temperature sufilcient to extract from said hydrocarbon fraction substantially all of said napthalene hydrocarbons therefrom, providing sutficient pressure to maintain the hydrocarbon mixture and solvent in substantially liquid phase, thereby forming a rich solvent containing the naphthalene components of said hydrocarbon fraction dissolved in said solvent, counter-currently contacting said rich solvent with a reflux comprising a liquid paraffinic hydrocarbon having a boiling point below the boiling point of the naphthalene hydrocarbon, separating said rich solvent from a rafiinate comprising nonaromatic hydrocarbons, stripping a hydrocarbon extract from the rich solvent, separately recovering lean solvent and said hydrocarbon extract, recycling said lean solvent to said first-mentioned contacting step, and separating a naphthalene homolog from said hydrocarbon extract.

A more specific embodiment of this invention concerns a'process for separating naphthalene and methylnaphthalene from ahydrocarbon fraction boiling within the range-of from about 400 'to about 500 P. which comprises subjecting said petroleum fraction to countercurrent contact" with an aqueous solution of diethylene glycolcontaining not more than about-5% by weight of 2,878,261 Patented Mar. 17, 1959 water at a temperature of from about 325 to aboutAOO? F. and at a pressure sufiicient to maintain said fraction and said solution substantially in liquid phase and form thereby a rich solvent stream containing said naphthalene and methylnaphthalene dissolved therein, countercurrently contacting said rich solvent stream with a reflux stream comprising a normally liquid parafiinic hydro: carbon having a boiling point less than naphthalene, thereafter recovering a rich solvent stream from. which the non-aromatic and alkylbenzene components boiling in the feedstock rangehave been displaced by contact of said richysolvent with said parafiinic hydrocarbon, reducing the pressure on said rich solvent stream atsllbstantially isothermal conditions, withdrawing from a, rich solvent, residue a first, vapor fraction comprising; said parafiinic hydrocarbon and recycling, saidgfirst fraction to contact with said rich solvent as at least a. portion of said parafi'lnic reflux hydrocarbon, fractionally distilling a naphthalene hydrocarbon from said residue, withdraw ing a bottoms comprising lean solvent, recycling said lean solvent to the first-mentioned contacting zone; as said diethylene glycol solvent and thereafter fractionally dis tilling the naphthalene-hydrocarbon fractionto thereby segregate naphthalene from its homologs.

Other embodiments of the present invention relating to specific aspects of the present process will be-referred to in greater detail in the following further descriptionof the invention.

It has previously been known that aromatic hydro? carbons, including polycyclic aromatics, may be. ex; tracted from certain boiling range hydrocarbon fractions containing the Same in admixture with parafliniqolefinic and/or naphthenic hydrocarbons by dissolving, the aromatic hydrocarbon in a solvent, such; as an alcohol or glycol, but it has also been realized that the recoveryof the aromatic hydrocarbon from the rich solvent stream formed by contacting the hydrocarbon, fraction with the solvent does not readily release. its aromatic solute by simple distillation methods, except at relatively, high temperatures, at which temperatures the character of the solvent changes rapidly, tending to become highly acidic (presumably because of the adsorptionof oxygen and/or the shift of oxygen atoms in, the molecular coin}: position of the alcohol or glycol solvent from hydroxyl and/or ether oxygen to carboxylgroups) and also,ten d-, ing to becomescontaminated with resinous and tarry. by; products, The acidic contaminants are highly corro-l sive atthe high temperatures utilized in the extraction and/ or stripping stages and the resins and tars precipitate in the equipment, necessitating frequent interruptionsin the process for the removal of the resulting deposits. In effect, therefore, the use of the highly effective alco: hols, alkylene glycols and polyalkylene glycols as solvents for the extraction of, polycyclic, aromatics which are re,-} coverable from the rich solvent stream formed in. the extractor only at relatively high temperatures is madeuna available because of the tendency of the solvent toundere go deterioration into said acidic,resinous, and tarry. by. products at the required high temperatures; the use of, such solvents is therefore impracticable because of the,

constant and recurring problem of cleaning the eqnip creases, the required temperature of operation likeyvise increases and the deterioration of the solvent into acidic;

by-products likewise increases. In accordance with one of the provisions of the process of this invention a, blanket of inert gas is maintained over the surface ofthe, glycol which would normally contact atmospheric, oxy gen, thereby excluding oxygen from, contact with glycol and eliminating the deterioration of the solvent to acidic by-products which cause corrosion of the portions of the equipment coming'into contact with the solvent and the deposition ofresinous and tarry materials in the solvent .reboilers and redistillation equipment. The use of-the inert gas blanket in the present high temperatures for .such solvent extractions is the inthalene and its homologs are of relatively high melting point and would normally tend to crystallize in the extractreceiver as they distill out of the rich solvent. These naphthalenes are also of greater density than water and therefore form a lower, solid phase in the extract receiver below the aqueous layer distilled with thenaph In accordance with the process of this invention a liquid parafiin diluent, in suf-' ficient quantity to completely dissolve the naphthalenes in thalenes from the rich'solvent.

the receiver vessel, is maintained at all times in the receiver vessel to prevent crystallization of the naphthalenes and to reduce their density below that of water .so that the aqueous distillate is presentin the receiver at troduction of a ,light,.- parafiinic hydrocarbon having -a boiling point below, that of the naphthalene extract into the extractor at a critical point and maintaining the-light 'parafiin in the extract receiver vessel as a diluent of the naphthalene extract, .At the required extraction temperatures, that is, at temperatures of 200 F. and higher, the rich solvent stream formed within the extraction zone and containing dissolved naphthalenes increase the solubility of other hydrocarbons (including the normally rafiinate-type hydrocarbons, which in the absence of the naphthalene solute would be substantially insoluble in alcoholic and glycolic solvents) to such an extent that the entire feed stock becomes soluble in the lower portion of the extraction-3 column, thereby rendering the entire process inoperable ass means of separating the desired naphthalene and its homologs from other types of aliphatic and aromatic hydrocarbons which accompany these naphthalenes. in the feed stock. By means of the present process, a. countercurrently flowing reflux stream comprising a light paraflinic hydrocarbon is charged into the lower portion of the extraction column to provide a separate and distinct raffinate phase capable of extracting the non-aromatic constituents of the feed stock and the non-naphthalene aromatics from the rich solvent stream in the lower portion of the extraction zone and thereby maintain a separate liquid hydrocarbon phase into which the rafiinate hydrocarbons dissolved in the rich solvent may, diffuse and which may be removed as a separate liquid phase from the top of the extraction column. In the absence of the light paraflin reflux in the bottom portion of the extraction zone, the maintenance of a separate rafiinate phasein the extraction zone and the recovery of the present naphthalene products becomes inoperable. The boiling point characteristic of the light paraffin reflux enables these hydrocarbons tobe readily distilled from the naphthalene extract recovered in the stripping zone and also enables it to be distilled as a light overhead from the rich solvent in the stripping zone without appreciably vaporizing the naphthalene solute therewith. The preferred light paraflins comprising at least a portion of the reflux charged into the extractor are the parafiins boiling from about 10 to about 150 F. below the boiling point of naphthalene, one of the preferred paraffins for this use being one or a mixture of C to C parafiins, such as a fraction of petroleum. In the process as herein provided, the light vapor overhead from the rich solvent stream distilled in the stripping zone, hereinafter described, is rich in the light paraffins present in the rich solvent by virtue of having displaced heavier feed stock rafiinate components from the rich solvent in the preceding extraction stage. This overhead, which also contains some of the naphthalene and other aromatic components of the rich solvent stream constitutes a major source of the light paraffin reflux stream recycled to the extraction zone.

Another feature of the present process, inherent in the use of a low molecular weight paraffinic reflux which makes feasible the operation of the process is the presence of the light parafiin in the naphthalene product receiver vessel attached to the stripping column. Naphall times as a lower layer, thereby facilitating withdrawal of both the naphthalenes and the aqueous phase from thereceiver vessel. The light paraffin is subsequently readily distilled from the naphthalene extract in the re ceiver forv recovery and recycle of the paraffin reflux in the process and for producing a residue consisting of substantially pure naphthalene product.

Another feature of the present process which makes the operation of a naphthalene extraction process feasible is the use of a solvent selected from a specific class of organic compounds containing a certain limited proportion of, water which adjusts the selectivity and solubility relationships of the solvent to the naphthalene compo :nents as well as to the non-aromatic components of the feed stock in such manner as to make possible. an $60? nom-ically feasiblerange of solvent to feed stock ratios,

N and enables the recovery of a naphtha'lene and methyl? naphthalene product in the form of an extract of substantially purity, while at the same time maintaining the boiling point of the solvent at a. temperature which may be feasibly utilized in a solvent extraction.

and extract recovery process of this'type, including the stripping stage as well as the extraction stage of the process. The water content of the solvent lies within a certain specified range which reduces the boiling point of thesolvent below the decomposition point of the solvent and yet maintains its selectivity and solvency at a high level. h

Suitable feed stocks utilizable in the process of this invention. are hydrocarbon mixtures generally containingany recoverable proportion of naphthalene, methylnaphthalene, or higher naphthalene homologs (herein referred to collectively as naphthalene hydrocarbons) in admixture with other hydrocarbons of aromatic, parafiinic or olefinic structure and of any molecular weight. These mixtures are desirably recovered from petroleum sources or the conversion products of petroleum, the fractions utilized as feed stock boiling at least above about 300 F. and more preferably, from about 400 to about 500 F. The. latter boiling range includes more specifically the desired products of the present process, including naphthalene, having a boiling point of about 424 F., alphamethylnaphthalene having a boiling point of about 473 F., and beta-methylnaphthalene which boils at approximately 466 F., although higher boiling feed stocks con taining the higher alkylated homologs of naphthalene, such as dimethylnaphthalene isomers, ethylnaphthalene and diethylnaphthalene may be utilized in the present process as the source of the desired product. The feed stock may contain one or more naphthalenes and/or homologs, but in the preferred application of the process, the feed stock is prefractionated to separate a fraction containing not more than two species within the range of boiling points for these species. Thus, a fraction boiling from about 400 to about 500 F. contains naphthalene and the alphaand beta-methylnaphthalene isomers and an extract product separated from such a feed stock may be fractionally distilled in the final stages of the process to segregate individual isomers and homologs of substantially 100% purity. Feed stocks which are particularly adapted to the present processare selected from the naturally occurring hydrocarbon mixtures or naphthalene-containing mixtures which are the product or a.-;.fraction of the product of a hydrocarbon conversion process; and which boil. ata constant temperature as an azeotrope normally inseparable by simple or fractional distillation means. Thus, the conversion products of by drogen reforming and many other. petroleum refining processes contain paraflinic, olefinic, aromatic and naphthenic hydrocarbons having a wide range of molecular weights, depending upon the boiling range of such a fraction, which, when distilled yield azeotropic mixtures containing components of the same molecular weight of adjacent homologs. Illustrative of such a separation problem is the segregation of naphthalene and the alphaand beta-methylnaphthalene homologs from a fraction boiling from 400 to about 500 F., saidfraction containing, in addition to said naphthalene homologs, aromatic hydrocarbons containing a total of, from 5, tov 7 carbon atoms in alkyl side, chains attached. to the benzenoid nucleus, as well as dodecane, various dodecane isomers, tridecane, and triand tetradecane isomers, as well as various alkylcyclohexanes, alkylcyclopentanes, etc. Particularly preferred sources of feed stock herein are petroleum or petroleum conversion product fractions in which the non-aromatic components are exclusively of parafiinic and/or naphthenic structure.

The solvents utilized in the present process which are selectively miscible with naphthalene hydrocarbons and therefore capable of selectively extracting said naphthalenes from mixtures containing the same, together withparaflinic, olefinic and naphthenic hydrocarbons are referred to herein as the monoand dihydric alcohols and ethers. These compounds are herein specifically selected for use as solvents because of the stability of these compounds at the high temperatures required for ('JXlIl'ElC-r tion of the present hydrocarbon feed stocks which boil at temperatures above about 300 F. Typical representative alcohols utilizable as solvents for the extraction of naphthalene hydrocarbons from hydrocarbon mixtures are the aliphatic alkanols containing from 1 to about carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec.-butyl alcohol, tert.-butyl alcohol, amyl alcohol, isoamyl alcohol, n-hexanol, cyclohexanol, n-heptanol, n-octanol, lauryl alcohol and the various isomers thereof. The compounds herein referred to as glycols and utilizable as solvents in the present process include the polyhydric alcohols, such as ethylene glycol, propylene glycol, trimethylene glycol, butylene glycol, amylene glycol, glycerol, beta-methylglycerol, etc. and particularly the dihydric glycol ethers, commonly referred to as the polyalkylene glycols, including, for example, diethylene glycol, (HOCH CH OCH CH OH), triethylene glycol, [HO(CH CH O) H], tetraethylene glycol, [HO(CH CH O) H], dipropylene glycol, tripropylene glycol, and mixed ethylene glycol-propylene glycol ethers. Another class of compounds utilizable herein as solvents are the alcohol ethers of the foregoing monoand polyhydric alcohols and polyalkylene glycols, particularly the methanol, ethanol, butanol, etc. ethers of monoand diethylene glycol, known commercially as the carbitols and cellosolve derivatives of these glycols. These materials are selectively miscible with aromatic hydrocarbons, on a comparative basis, that is, to the straight-chain and naphthenic hydrocarbons. The difference in the selectivity of the solvent for aromatic hydrocarbons and aliphatic saturated or naphthenic hyd'rocarbons may be enhanced by incorporating in the solvent composition certain limited amounts of water, for example, up to about 20%, and preferably up to about 10%, by weight of the solvent composition of water when enhancing the selectivity between bicyclic aromatics and non-aromatic hydrocarbons, as in the present process. The solvency may also be varied by compounding the foregoing alcohols, ethers and glycols into a solvent composition, for example, by mixing diethylene. glycol with diprcpylene glycol in. order to enhanee the solvency of the'..composition for the hydrocarbonmixture; without seriouslyfderogating; its selectivityfor naphthalene hydrocarbons, the quantity of hydrocarbons dissolved by the solvent composition increasing as the proportion of glycol containing the longer alkylene groups increases. Particularly preferred solventcompO: sitions, for operation of the presentprocess are those which contain from about 1% to about 30% by weight of dipropylene glycol, from 2% to about.5%v by weight of. water and the balance diethylene glycol orv triethylene glycol. Also preferred for use in the extraction of naph thalene and methylnaphthalene from hydrocarbon fractions containing the same isthe solvent composition con.- sisting of diethylene or triethylene glycol. containing from 0.5% to about 10% by. weightof' water.

The relationship between solubility, volatility and selectivity of various, aromatiehydrocarbons present in a typical feedstock utilizable in the. presentprocess, for example, a fraction boiling from about400 F. to about 500 F. of a hydroreformed gas oil fraction, based on a solvent composition containing 98.2% diethylene glycol and 1.8% water, is illustrated by the data in the follow ing table, these data being based on an extraction tempera: ture of 375 F. and on the basis, of the hydroreformed charge stock containing. about 20% naphthalene and about 26% alphaand beta-methylnaphthalenes by weight, as a typical feed stock, and at the above values as typical of the operating temperature and composition of solvent. It is to be emphasized, however, that such data are merely exemplary of typical conditions, the volatility and solvency relationships for the indicated aromatics being within the same range for other glycol solvents and the relative diiferences in these properties being of the same order of magnitude, regardless-of the identity of the glycol or of the extraction conditions.

TABLE I Extraction of 400-500" F. boiling range petroleum naphtha with diethylene glycol containing 1.8% water at 375 F.

Rel. Soly.

at Extraction Conditions B. P. Hydrocarbon Component F.,'

to be Extracted Normal Rel. Volat. in Absence of Solvent Rel. Volat. in Presence of Solvent Naphthalene a-methyl-naphthalene fl-methyl-naphthalene Monooylclic Aromatics:

fi-Oarbon atoms alkyl in alkyls 7-Oarb0n atoms in alkyls These data indicate the feasibility of a solvent extrac? tion process for separating naphthalene and methylnaphe thalene from a mixture of hydrocarbons containingthem as well as the C11, C and C mono and polyalkyl monocyclic aromatic hydrocarbons and aliphatic parafiins. It will be noted from the above data that of all of the aromatics present in the petroleum naphtha, the naphthalenes are most soluble in the glycol solvent, being, on an average, more than twice as soluble as the alkylbenzenes having boiling pointsinvthe same boiling range as the naphthalenes. This solubility relationship permits substantially complete recovery of the indicated naphthalenes in substantially pure form, free of monocyclic aromatic impurities, particularly in view of the boiling points of the aromatics when dissolved in the rich solvent (i. e., the relatively lesser volatility of the naphthalene hydrocarbons. in the presence of the solvent than the monocyclic aromatics). The efiect of the latter factor is to enable complete vaporization of the monocyclic aromatics present in the rich solvent stream formed in the extraction. column from the rich solvent: by taking overhead from the rich solvent stripper a first vapor fraction relatively richer in said monocyclic aromatics '(these being the lowest boiling aromatics in the presence of the solvent of all the aromatics dissolved in the rich solvent), continuing the rich solvent stripping until the rich solvent is free of monocyclics, and thereafter (but only after freeing the rich solvent of dissolved monocyclics) vaporizing into a separate stripper side-cut fraction the remaining naphthalenes in a high state of purity. Any naphthalenes removed from the rich solvent into the vapor overhead fraction of the stripper are recovered therefrom when the vapor overhead is recycled as a reflux stream to the lower portion of the extraction zone. The naphthalenes thus refluxed into the extraction zone and contacted with the rich solvent stream just prior to its removal from" the extraction into the stripping zone also serve to displace non-aromatic and monocyclic alkyl aromatic raflinate-type feed stock hydrocarbons from the rich solvent by virtue of the selective solubility of said naphthalenes in the solvent, these rafiinate-type components thereby being displaced into the raflinate stream removed from the top of the extraction zone.

The above operating procedures and a fuller description of the principles of operation involved in the present process are further illustrated and described by reference to the accompanying flow diagram.

The process of the accompanying diagram is directed to a combined solvent extraction, rich solvent stripping and extract fractionation process utilizing an aqueous alkylene or polyalkylene glycol solvent in a countercurrent solvent extraction process wherein a reflux stream comprising a relatively volatile paraffin is utilized for the purpose of displacing feed stock raifinate components from the rich solvent stream formed in the extraction vessel. The feed stock, hereinbefore characterized, is charged into the process flow through line 1 in amounts controlled by valve 2, being transferred by means of pump 3 into heat exchanger 4 wherein the hydrocarbon stream is heated to the desired extraction temperature, generally to a temperature of from about 200 to about 450 F., preferably to a temperature of from about 300 to about 400 F., and thereafter charged at a pressure sufficient to maintain the feed stock in substantially liquid phase, generally, not substantially in excess of about 300 pounds per square inch, preferably at a pressure of from about 10 to about 150 pounds per square inch, through line 5 into a countercurrent, liquid-liquid solvent extraction zone designated as extractor 6 in the accompanying diagram, the feed being introduced at a point in the extraction vessel between the raffinate outlet in the upper end of the column and the rich solvent outlet in the lower portion of the column, preferably into the midportion or middle tray of the column. Extraction vessel 6 may be of any suitable type for obtaining countercurrent contact between the relatively more dense lean solvent phase introduced into the upper portion of the column and the relatively less dense liquid hydrocarbon phase introduced into the column below the solvent inlet. The hydrocarbon phase accordingly rises in countercurrent flow relationship through the more dense solvent phase which flows downwardly through the column. The solvent which is an alkylene glycol containing from about 0.5% to about 20% by weight of water, is charged at a temperature of from about 200 to about 500 F., preferably at a temperature of from about 320 to about 400 F. into extractor 6 from line 7 which conveys a recycle lean solvent stream from the extract stripper, hereinafter described, at approximately the stripper reboiler temperature into the extraction zone. In order to maintain the reflux stream, solvent and feed stock in substantially liquid phase in the extraction zone and to provide the advantages of flash distillation in the stripper and isothermal operation of the process the pressure maintained in the extraction zone is substantially in ex cess of the pressure maintained in the stripping zone, here- 7 inafter described, and as previously indicated is preferably within the range of from about 10 to about pounds per square inch, depending upon the extraction temperature. The solvent initially introduced into the process flow and any additional quantities required to replace solvent losses are charged into line 7 through line 8 in amounts controlled by valve 9. As the liquid phase solvent descends through the rising stream of hydrocarbons introduced through line 5, it selectively extracts from the hydrocarbon stream substantially all of the aromatic components present in the feed stock. Thus, as the hydrocarbon phase flows upwardly through the c0lumn it becomes progressively leaner in bicyclic aromatic components, although richer in less preferentially dissolved alkylbenzenes, the ultimate hydrocarbon residue removed from the top of the extraction zone through line 10, herein referred to as raflinate, consisting almost exclusively of non-aromatic and alkylbenzene hydrocarbons present in the initial feed stock and introduced by way of the reflux stream. The raflinate is separately treated, as hereinafter more fully described.

Although the glycol solvent selectively extracts the naphthalene components of the feed stock, including its homologues, the solvent nevertheless dissolves to a certain limited extent the alkylbenzenes and non-aromatic paraffin, olefin and/or naphthene constituents also present in the'feed stock, the non-aromatic content of the solute present in the rich solvent stream being sufficient in amount to contaminate the final product with an impurity (from the standpoint of being a contaminant of the desired naphthalenes) boiling at approximately the same temperature as the desired aromatics. For the purpose of removing the latter, small amount of impurity from the rich solvent stream prior to the stripping stage, a countercurrently flowing reflux stream comprising relatively volatile paraifins (i. e., boiling at a temperature below the initial boiling point of the feed stock) is contacted with the rich solvent stream for the purpose of displacing the feed stock non-aromatic impurities from the rich solvent with parafi'ins which may be subsequently easily separated from the rich solvent and/ or the aromatic extract as a separate and distinct fraction. As heretofore indicated, the reflux stream comprises one or more paraflinic hydrocarbons (such as a light fraction of a paraflinic gasoline) boiling below the initial boiling point of the feed stock (preferably at a temperature from about 50 to about 150 F. below the feed stock initial boiling point). In the process utilizing, for example, a feed stock boiling from about 400 to about 500 F., a suitable reflux paraffin may consist of a mixture of octanes or the octane-decane fraction of a saturated gasoline or a naphthene boiling from 10 to about 150 F. below the initial boiling point of the feed stock such as methylcyclohexane, dimethylcyclohexane, etc. This reflux stream which is made up largely of the light vapor overhead from the rich solvent stripping column, as hereinafter described, is introduced into extraction Zone 6 in the lower portion thereof, preferably contacting the rich solvent just prior to the removal of the latter from the bottom of the column, the reflux hydrocarbon thereafter flowing upwardly in countercurrent relationship to the descending rich solvent in extraction zone 6, the portion in excess of that which replaces the feed stock raflinate solute of the rich solvent joining the raffinate components of the feed stock which leave the extraction column through line 10. In such countercurrent contact between the reflux and rich solvent streams, the raifinate-type alkylbenzenes and non-aromatics boiling at approximately the same temperature as the desired aromatic product are displaced from the rich solvent and their place taken by the reflux hydrocarbon which may subsequently be readily separated from the aromatic extract product by simple fractionation. In the illustration of the process shown in the accompanying drawing the reflux hydrocarbon is charged into extractor 6 through line 1 1 in amounts controlled by valve 12, the

As a consequence of recycling the stripper overhead,

this. stream contains, in addition to the light paraflins, a substantial proportion of the most volatile. aromatics extracted from the feed stock. These aromatics are. re dissolved in the solvent by virtue of the recycle ofv overhead to the lower portion of extraction vessel 6 as a reflux stream and its contact with the solvent stream in said zone 6. That portion of the paraffin components of the reflux, however, which does not enter the rich solvent stream, (i. e., the amount in excess of that required to displace feed stock rafflnate solute of the rich solvent), appears in the raflinate removed. from the top of the extraction vessel through line 10. These excess light paraflins are desirably, recovered from the.

ratfinate stream for recycle into the process and for this purpose, and in accordance with the present process, they are separated and recovered for recycling from the eflluent raflinate stream by any suitable method of separation. Since the paraflins. utilized herein as. reflux have a boiling point substantially below the boiling point of the feed stock, a convenient method of separation comprises fractionally distilling the same from the raflinate stream, although other methods of separation by means known to the art may also be employed. For the purpose of fractionally distilling the raflinate, the latter stream is conveyed by means of pump 16 through line in amounts controlled by valve 17 into line 18, through heater 19 and line 26) into raflinate, still 21, which is generally in the form of a suitably packed or bubble tray column. The rafl'inate which is heated to a temperature above the boiling point of the reflux component in heater 19 flashes upon entering column 21, the flashed vapors being taken overhead through line 22, liquefied in condenser 23 to a. condensate stream removed from condenser 23 through line 24, valve-v 25, and drained into receiver 26. A portion of the liquid condensate comprising the desired light paraflin reflux hydrocarbon is recycled to the upper section of fractionating column 21, that portion being withdrawn from receiver 26 through line 27,. diverted into line 28. in amounts controlled by valve 29 by means of. pump 30 and discharged through line 31 onto an upper plate of column 21 for use as said distillation reflux. The remaining portion of the condensate in receiver 26, com prising said light parafiin overhead of column 21 is discharged in controlled amounts, determined by valve 32,

into light paraffin reflux recycle line 11. The high boiling bottoms or residue of the raflinate from which the recycle reflux has been removed as overhead is. reboiled by removal from the bottom of distillation column 21 through line 33, circulated through reboiler heater 34, and the resulting vaporized portion of the bottoms returned to column 21 through line 35. The, raflinate residue comprising the alkylbenzene and paraffinic, nonextracted portion of the feed stock is removedifrom reboiler 34 through line 36 and valve 37 for discharge from the process flow. The non-extracted residue of the feed stock may be withdrawn from the process flowfor further conversion into aromatic, components by additional reforming or permanently withdrawn from the process, as desired.

The rich solvent. stream. formed in extraction vessel 6 and from which the feed" stock rafl'inate components have, been displaced. by countercurrent contact withthe light paraflin-containing reflux in the lower portion of extraction vessel 6 is withdrawn from column 6 through line 38 in amounts controlled by valve 39 and transferred 7 by means of pump 40 into line41 which conveys the rich solvent into the upper, flashing section of extract stripper column 42. Stripping zone 42 is in the form ofv a distillation column having a flash section in the upper portion and a fractional distillation section in the lower portion, the two sections being divided by a side-to-side pan sealed into the upper portion of the column from the distillation section below it, as illustrated by interior pan 43 in column 42. The pressure maintained in the flashing section above pan 43 is somewhat less than the pressure on the rich solvent stream flowing into stripping vessel 42, such that upon entering column 42 onto pan: 43, the most volatile solute components in the rich solvent, that is, the light paraflin components, immediately flash from the rich solvent by virtue of the sensible; heat content of the rich solvent, thereby providing sub stantially isothermal stripping of the solute from the rich. solvent. Also present in the light vapors, because of their'significant (although substantially lower) vapor pressures at the temperature of the rich solvent are water vapor, substantially all of the monocyclic aromatic components in the extract, a smaller proportion of the naphthalene components extracted by the solvent and a still smaller proportion of the solvent component which,

has the lowest vapor pressure. This relationship in the volatilities of the solute components is shown in the data of Table I, particularly the data relating to the relative volatilities of the various aromatic hydrocarbon feed stock components in the presence of the solvent,

the data indicating that the alkylbenzenes are, on an. average, the most readily vaporized components ofv the; rich solvent, naphthalene, which is the most volatile.

bicyclic aromatic in the presence of the solvent. also constitutes a substantial proportion of the vapor overhead;

from stripper 42, the methylnaphthalene isomers con-.

stituting the least prevalent component. It is evident,

therefore, that all of the monocyclic aromatics will. vaporize from the rich solvent prior to any substantial, vaporization of the naphthalenes or, at least, constitute,

a major aromatic hydrocarbon component of the; over.- head vapor. These are removed from column 42 through light vapor overhead line 44, passing through valve 45 into condenser 46. wherein they are condensed into a.

liquid condensate which are transferred by line 47 into receiver vessel 48 wherein the phase of greatest density, comprising aqueous solvent, settles from an upper hydrocarbon layer in vessel 48. The non-vaporized portion. of the rich solvent stream accumulating on side-to-side. pan 43 is removed from the primary flashing section of column 42 at a reduced pressure. (relative to the extrac tion pressure) through line 49 and valve 50 and discharged at a somewhat lower pressure than the pressure. maintained in the primary flashing section of column 42 onto a lower tray of column 42, the vapors released isothermally from the rich solvent stream at said lower" pressure being removed from the latter secondary flash.

zone through line 51 and valve 52 and discharged into overhead vapor line 44. The secondary flash vapors thus recovered from column 42 comprise the remaining reflux paraflins contained in the rich solvent, additional steam primary flashsectionis preferably a major drop in pres.- I sure (say, one half to two-thirds of the. pressure ditferential between the extraction pressure and the distillation pressure in. column 42), the remaining one half to one 1 I third in the pressure drop taking place in the secondary and subsequent flash sections, if desired.

The secondary flash vapors join the primary flash vapors in line 44, being liquefied in condenser 46 with the primary overhead flash vapors and thereafter accumulate in receiver vessel 48 with the primary vapor overhead. The combined hydrocarbons of the primary and secondary flash zones which separate as an upper layer in receiver vessel 48, and which also consist of light parafiin reflux, as well as the aromatic hydrocarbons vaporized in part from the rich solvent steam are transferred as reflux stream by means of pump 53 from receiver vessel 48 through line 54 and valve 55 into line 13 for recycle into the lower portion of extraction vessel 6 through line 11, as previously described, the light parafiins and aromatics stripped from the rich solvent in vessel 42 being returned to extractor 6 for recovery of the naphthalene hydrocarbons contained in the light vapor overhead and for displacement of feed stock raffinate components in the rich solvent stream, as previously described. Since the aromatics in this stream are redissolved in the rich solvent by recycle as a portion of the reflux stream into extraction zone 6, a gradual build-up of monocyclic aromatics in the recycle into excessively large quantities of total recycle would result unless a portion of this stream were continuously removed from the process flow and not recycled. For this purpose a bleed line 13a containing valve 132), is provided as a draw-off line from recycle line 13 to'remove that portion of the extract representing the alkylbenzene components of the reflux recycle redissolving in the rich solvent.

The lower aqueous phase accumulating in receiver vessel 48 Which contains a small proportion of the monoor polyhydric alcohol slovent component vaporized from the rich solvent stream in the flashing sections of strip ping vessel 42, accumulates in settling leg 56 of receiver 48 as a heavy liquid layer and is continuously drained therefrom through line 57 in amounts controlled by valve 58, into line 59 for reconstituting the aqueous component of the solvent composition prior to recycling the lean solvent to extraction vessel 6 and to provide stripping steam in the reboiling section of column 42. Any additional water required for replacement of water loss and reconstitution of the lean solvent composition to its desired water content may be introduced into the process fiow from storage from line 60 in amounts controlled by valve 61. The water thus added to the solvent is desirably introduced into the reboiling section of column 42 in the lower portion of the column, the water thus producing steam in reboiler 62 which is charged into the lower portion of column 42 as the stripping agent to remove the last traces of aromatic solute from the rich solvent residue in the lower portion of the column.

The rich solvent stream, as it descends through the distillation section of stripper 42, is progressively stripped by means of the aforementioned steam introduced into the reboiling section of the column to substantially completely remove the non-aromatic and alkylbcnzene components of the hydrocarbon solute present in the rich solvent, these hydrocarbons being withdrawn together with the stripping stream and vaporized organic solvent (such as glycol), through line 51, as aforesaid. A higher boiling side-cut fraction comprising the desired naphthalene products of this invention is withdrawn from column 42, together with stripping steam and vaporized solvent, through line 63, liquefied in condenser 64 to form a liquid condensate which is drained by means of line 65 and through valve 66 into side-cut receiver 67. The liquid phase collecting in the receiver separates into a hydrocarbon phase and an aqueous, dilute solvent phase containing a greater proportion of the glycol solvent component than the overhead in receiver 48 because of the substantially higher tcmperature of vapor withdrawal of this side-cut fraction.

The hydrocarbon phase in receiver vessel 67 is made up exclusively of naphthalene and its homologs which normally are solids melting at relatively high temperatures and which have a density greater than water. Except for the special provision herein of a parafiinic diluent of the naphthalene phase present in receiver vessel 67, the hydrocarbons distilled from the rich solvent and collecting in receiver 67 would form a crystalline mass of solid on the bottom of receiver vessel 67. Such a solid phase hydrocarbon would ordinarily be difficult to handle in the presence of the less dense aqueous phase also present in the receiver vessel. In accordance with the present process a volatile paraffin hydrocarbon diluent of low density is continuously maintained within receiver vessel 67 to dissolve and dilute the distilled naphthalenes, maintaining the naphthalenes in solution in the parafiin diluent as'a liquid layer, reducing the density of the by drocarbon phase thereby below the specific gravity of water and maintaining the hydrocarbon phase as an upper liquid layer in the receiver vessel. For this purpose it is preferred that the amount of light paraflin, such as the aforementioned octane fraction, maintained within the receiver vessel be sufiicient to provide at least 0.1 volume of light paraflin per volume of naphthalene distillate and preferably from about 1 to l to about 10 to 1 volumes per volume. The paraflin diluent is supplied to receiver vessel 67 through line 68 by recycle of the same paraflin diluent distilled from a previously recovered upper layer in a subsequent distillation, as hereinafter more fully described. The lower aqueous phase distilled from the rich solvent in column 42 separating as a lower aqueous layer in receiver 67 collects in the dense liquid leg 69 of the receiver vessel and is desirably decanted therefrom by continuous withdrawal through line 70 which connects with line 57 for recycle to the reboiling section of column 42 through line 59, into reboiler 62 to thereby form the stripping steam supplied to column 42. The lower aqueous phase also contains all of the organic solvent (such as glycol) distilled from the rich solvent as the side out fraction; by recycling the lower aqueous phase into column 42, the water thus returned not only provides steam per stripping solute from the rich solvent, but the organic solvent contained therein is returned to the process flow for recycle in the system.

The diluted naphthalene layer accumulating above the aqueous phase in receiver 67 and containing the light from through line and valve 76 into reboiler 74 from which the resulting heated bottoms and light paraflin diluent are returned to the column through line 77 and valve 78 for further vaporization of paraffin diluent therefrom. The vapors of light parafiin are taken overhead from column 73 through line 79, liquefied in condenser 80 and the resulting condensate drained into receiver 81 from which it is transferred through line 82 by means of pump 83 into line 68 and valve 84 for recycle into extract receiver vessel 67. It will thus be noted that the light parafiin diluent which serves to maintain the naphthalene extract collected in receiver vessel 67 in liquid phase is continuously supplied thereto in a closed circuit, being recycled to the extract receiver 67 from the overhead of extract fractionator 73 and returned again to continu ously repeat the cycle to extract fractionator 73 from which it is again vaporized and recycled to extract re ceiver 67.

The bottoms from column 73 comprising the naph thalene extract from which the light parafl'ln diluent has been distilled and comprising a mixture of naphthalene and its homologues initially present in the feed stock is withdrawn from reboiler 74 through line 85 by means of pump 86, transferred via line 87 through valve 88 into naphthalene separation column 89 for fractionation of "themixed naphthalene and homologues into separate components. The heat supplied for distillation and fractionation of the naphthalene mixture is supplied through reboilfer 90, the bottoms from column 89 being withdrawn through line 91, valve 92, heated in said reboiler 90 and the resulting vapors and heated liquid being returned to the column through line 93 and valve 94. The most volatile vapors fractionated in column 89 are removed as an overhead vapor from the column through line 95, condensed to a liquid condensate in cooler 96 and the condensate collected in receiver vessel 97. This condensate which consists exclusively of naphthalene as the naphthalene extract component of greatest volatility is essentially pure naphthalene, free of its higher homologues by virtue of the fractionation in column 89 and control of the overhead vapor temperature. This product may be withdrawn from receiver 97 through line 98 and valve 99 into storage or for other disposition as a substantially pure naphthalene product. Preferably, however, at least a portion of the naphthalene condensate is returned as reflux to naphthalene distillation column 89 by withdrawing from line 98 at least a portion of the condensate through line 100, by means of pump 101 which discharges the naphthalene condensate into reflux return line 102. This reflux is desirably returned in limited amounts, controlled by valve 103, to the upper plate of fractionating column 89 to provide thereby the desired degree of fractionation in the column.

The bottoms from the naphthalene column are transferred by means of pump 104 from reboiler 94 through line 105 and valve 106 into line 107 which conveys the bottoms of column 89 into methylnaphthalene separation column 108. The methylnaphthalene in the bottoms is a mixture of its alpha and beta isomers, which are distilled overhead from column 108 through line 109 into cooler 110 which operates at a temperature sufliciently below the boiling point, but above the melting point of the distillate to liquefy the methylnaphthalene. The resulting liquid condensate is withdrawn from cooler 110 into receiver 111 from which methylnaphthalene may be withdrawn as a product through line 112 and valve 113. At least a portion of the methylnaphthalene condensate in receiver vessel 111 is preferably refluxed to the uppermost. plate in column 108 by withdrawing a portion of the condensate from line 112 and transferring the same by'means of pump 114 through line 115 and valve 116 to said upper plate. tionati'on is introduced into the liquid bottoms of column 108, withdrawn therefrom through line 117, by means ofreboiler 118, the. heated bottoms being recycled back to column 108 through line 119. The net bottoms make, comprising higher boiling homologues of naphthalene, such as dimethylnaphthalene, etc. are removed from the process flow through line 120 and valve 121 for further processing, as desired.

.The solvent residue from which substantially all of the volatile hydrocarbon components have been removed by vaporization and stripping in extract stripper 42 is withdrawn from column 42 as a high boiling residue through line 122 and Valve 123, a major proportion thereof being recycled to the lean solvent inlet of extract temperature, or, in the event that the solvent residue in.

the, lower portion of stripper 42 is at a temperaure below the desired extraction temperature, heat exchanger 126 may be a heater for raising the temperature of the lean solvent to thedesired extraction temperature. In general,

Heat for the above indicated frac however, vaporization of the solute from the rich solvent is effected predominantly by pressure reduction, thereby providing substantially isothermal stripping ofthe rich" solvent stream. The lean solvent leaving the bottom of" the stripper for recycle toextraction column 6 is usually at approximately the desired extraction temperature and may be recycled to column 6 without heating or cooling. Pump also increases the pressure on the lean solvent recycle to the operating pressure maintained in extractor 6, as previously described, thus restoring the pressure dilferential between extraction zone 6 and the primary flash zone of extract stripper 42.

In the preferred operation of column 42; at least a portion of the solvent residue accumulating in the lower.

section of the column is separately reboiled in order to generate steam which supplies the stripping agent for recovery of the aromatic solute'from the rich solvent in column 42. For thispurpose a portion of the lean solvent residue from line 122 is withdrawn therefrom through line 127 in an amount determined by valve 128, mixed with recycle water supplied through line 59 by connection of line 59 with line 127, and reboiled in heater 62. The steam and hot solvent residue thus produced leave reboiler 62 through line 129 for discharge into the stripping. section of column 42, as heretofore indicated. The steam thus charged into the lower section of column 42reconstitutes the solvent to its desired selected composition for use as a lean solvent in extraction column 6, steam distills the aromatic extract from the rich solvent, and reducesthe boiling point of the rich solvent. For the purpose of maintaining the solvent composition at its desired water content, samples of the lean solvent may be withdrawn from line 122 for analysis and adjustment in water content to the preferred selected composition for extraction purposes in column 6.

One of the preferred means of maintaining the glycol solvent component in its unaltered form, that is, as aglycol of the same identity as the component of the solvent composition initially charged into the process and to avoid the development of acidic and resinous or tarry by-products in the composition, comprises main taining a blanket of an inert gas over the glycol vaporized from the rich solvent in stripping column 42 and returned to the process by recycle of the lower aqueous. phases in the various receiver vessels attached to overhead and side cut lines from column 42. By this means, oxygen which unavoidably leaks into the receiving ves:- sels and ordinarily absorbed by the glycol solvent is excluded from the system, thereby eliminating the major cause of glycol deterioration in an extraction process utilizing glycol solvents. A suitable means for maintaining such a blanket of inert gas over the aqueous layers in the. receiver vessels comprises introducing the inert gas,

such as nitrogen, carbon monoxide, etc. into one end of each of the receivers and withdrawing the inert gas from the other end. Thus, in overhead receiver vessel 48, the

inert gas is admitted into the receiver through line 128 and valve. 129, the gas flowing across the vessel, completely covering the liquid contents of the receiver and excluding air therefrom, being withdrawn from the' vessel through line 130 and valve 131. in order to blanket he aqueous solvent in side cut receiver vessel 67 an inert gas inlet line 132 containing valve 133 may be connected to receiver 67 and the gas vented, after passing through the receiver, through line 134 and valve 135.

This invention is further illustrated with respect to several of its embodiments in the following example, the

15 Platforming process to separate a gasoline boiling range cut from the Platformate having an end boiling point of 400 F. The bottoms which represents that portion of the Platformate product boiling above about 400 F. constitutes approximately 3.5% of the total Platformate product and consists almost exclusively of aromatic hydrocarbons, including 20% by weight of naphthalene and 26% by Weight of methylnaphthalene, the remaining aromatic components comprising benzene hydrocarbons of 11, 12 and 13 carbon atoms per molecule, the carbon atoms being present in one or more alltyl side chains attached to the benzene nucleus. The original Platformate from which the bottoms cut boiling above 400 F. was separated as still residue Was prepared in a prior Platforming process by passing a mixture of a straight-run gasoline fraction having an end boiling point of 400 F. and hydrogen (utilizing a hydrogen to hydrocarbon mole ratio of 9:1) at 950 F. and at a pressure of 500 pounds per square inch over a Platforming catalyst comprising a platinum supported on an alumina-halogen composite. The residue of the Platformed product Was separated by fractionally distilling the Platforming effluent, taking overhead the aforementioned gasoline boiling range fraction having an end boiling point of 400 F. from the bottoms residue and the latter utilized as feed stock.

The above bottoms fraction, consisting almost entirely of aromatic components, is a relatively difficult feed stock from which to separate its naphthalene and methylnaphthalene components, because of its highly aromatic character, dissolving substantially in its entirety in the usual solvent compositions selective for aromatic hydrocarbons without producing a separable raflinate in which the undesired components of the fraction are concentrated. In accordance with the present process, separati n of an aromatic concentrate by solvent extraction is effected by charging a low molecular weight paraflinic reflux stream into the extraction zone with the selective solvent and the feed stock, the light paraffins diluting the aromatic-rich feed, reducing its viscosity, its density, and its total miscibility with the solvent and enables the production or a naphthalene product of substantially 100% purity.

The above feed stock is charged at a temperature of 375 F., at a rate of 1000 barrels per day and at a pressure of 100 pounds per square inch on the thirtieth tray of a countercurrent solvent extraction column containing sixty decks. A solvent composition consisting of 98.2% by weight of diethylene glycol and 1.8% by weight of Water at a temperature of 380 F. and at the aforementioned pressure, is charged into the top of the solvent extraction zone at a rate of 6500 barrels per day. A reflux hydrocarbon stream derived in part from an overhead stream of the stripping column utilized to separate the hydrocarbon solute from the rich solvent stream formed in the extraction zone and in part from the light parafiin fraction recovered from the raflinate stream of the extraction zone is charged at a rate of 1700 barrels per day into the bottom of the extraction column. Analysis of this reflux stream indicates that it is made up of approximately 850 barrels per day of octanes, approximately 690 barrels per day of naphthalenes, and the remainder comprises monocyclic aromatic hydrocarbons stripped from the rich solvent stream in the subsequent stripping zone. A raffinate stream comprising the nonextracted components of the combined feed and reflux streams is removed from the top of the extraction column at a rate of 1100 barrels per day, the raffinate retaining only a very small proportion of the naphthalene hydrocarbons present in the original feed stock and substantially all of the monocyclic aromatics contained in the feed. The raflinate was diverted, after being washed with a stream of water to thereby recover the glycol therefrom, into a ralli'nate fractionator wherein the octane components is distilled therefrom and the latter re- 16 cycled to the reflux line leading into the bottom of the extraction zone.

The rich solvent stream formed in the extraction column, at a temperature of 347 F. and at the extractor pressure, that is at pounds per equare inch, is re- ,moved from the bottom of the extraction zone, heated to a temperature of 370 F. and charged into the top of a flash stripping column wherein the pressure is reduced in three stages to atmospheric pressure. In the first stage the pressure is reduced to approximately 50 pounds per square inch, resulting in the flash distillation of a light vapor overhead from the top plate of the stripping column which is separated from the lower plates by a side-to-side pan sealed circumferentially on the inside of the stripping column. The overhead vapor at 370 F. is flashed off at a rate of approximately 675 barrels per day being thereafter blended with the octane overhead from the raffinate fractionator and the combined stream recycled as reflux to the bottom of the extractor. The overhead vapor also contains a portion of the desired naphthalene and methylnaphthalene as well as most of the monocyclic aromatics extracted from the original feed stock by the solvent. In order to prevent a build-up of monocyclic aromatics in the reflux stream, a portion of the overhead from the stripping column, amounting to the net monocyclic aromatic component charged into the system as fresh feed and not removed from the process in the raflinate (about 8 barrels per day) is withdrawn from the stripper overhead stream. A portion of the overhead vapors in the amount of approximately barrels per day comprises the diethylene glycol and water components of the solvent, these being separated from the overhead vapors in a receiver vessel attached to the overhead vapor condenser, the aqueous solvent being separated by decantation from the upper layer reflux hydrocarbons, combined with other aqueous glycol condensates, and charged into the reboiling section of the stripping column.

The rich solvent residue is continuously drained from the primary stage flashing section into a secondary flash section wherein the pressure is reduced from 50 pounds per square inch to 5 pounds per square inch, the resulting secondary flash vapors being taken off at 370 F. at a rate of approximately 1450 liquid barrels per day, of which 750 barrels per day represents hydrocarbons comprising predominantly aromatic components, together with the remaining octanes present in the rich solvent and approximately 700 barrels per day of aqueous glycol. The vapors are condensed by cooling to form a two-layer liquid condensate in the receiver vessel attached to the secondary flash condenser, the glycol layer being drained from the receiver, mixed with the overhead aqueous glycol and charged into the stripper reboiler to provide stripping steam for the column. The hydrocarbon portion of the secondary flash overhead is combined with the hydrocarbon portion of the primary flash overhead and recycled as reflux to the extraction column. The rich solvent residue thereafter flows into the stripping section of the column wherein the pressure on the rich solvent is reduced to atmospheric. A resulting sidecut fraction is recovered from the stripping zone at a temperature of 374 F., the vapors comprising a mixture. of aqueous glycol and the hydrocarbon portion comprising predominantly naphthalene and methylnaphthalenc extract. These vapors are cooled sufliciently to form a liquid condensate which is drained into a receiver vessel. The aqueous glycol liquid layer is drained from the" bottom of the receiver, mixed with the aforementioned aqueous glycol layers, and charged into the reboiling section of the stripping column to provide stripping steam. A light liquid paraffin stream comprising the octane fraction (overhead) recovered from the subsequent extract fractionator and continuously recycled from the extract -fract'ionator in the amount of 222 barrels per day is charged into the extract receiver in order to maintain vessel.

the naphthalene ,e omponents of. the side-cut fraction in liquid phase and to reduce the density of the naphthalene sufliciently to maintain the hydrocarbon distillate of the extract in liquid phase as an upper layer in the receiver The octanes completely dissolve the normally 'solid naphth'alenes in the upperhy'clrocarbon'layer of the receiver contents, the resulting liquid layer being. re-

moved to an extract fractionating column for recovery of the individual components, as hereinafter described.

A reboiler coil is operated as an adjunct to the stripping column, consisting essentially of a gas-fired heater into which the stripped solvent bottoms is charged and heated to a temperature of 380 F. and into which the aqueous solvent phases of the various receiving vessels heretofore referred to are charged in order to generate steam supplied to the lower portion of the stripping column for vaporization of the aromatic solute from the rich solvent residue. A lean solvent stream containing approximatley 1.8% water is continuously removed from the reboiler at a rate of 6500 barrels per day and pumped into-the top of the extraction vessel for use as selective solvent therein.

The bottoms of the extract fractionator from which the octane reflux has been removed for recycle to the extract receiver is removed from the extract fractionator at a temperature of 450 F. and at a rate of 405 barrels per day into a naphthalene fractionating column from which 184 barrels per day of naphthalene of 99.9% purity is taken overhead at a temperature of 424 F. The product crystallizes readily, the crystals melting at 802 C. The bottoms fraction from the naphthalene column, at a temperature of 480 F. is charged into a methylnaphthalene separation column from which an overhead fraction (B. P. 468 F.) in the amount of 215 barrels per day is recovered. This fraction contains both the alphaand beta-methylnaphthalene isomers and contains approximately 1% by weight of monocyclic aro matics. A bottoms fraction is recovered from the methylnaphthalene tower at a temperature of 510 F. in an amount representing approximately 4 barrels per day. This residue contains monocyclic aromatic hydrocarbons having about 13 carbon atoms per molecule, as well as a small proportion of alkyl naphthalenes of higher molecular weight than methylnaphthalenes.

In the above process nitrogen was charged into the overhead receivers and side cut receiver in order to blanket the surface of the aqueous glycol phase with an inert gas and thereby prevent absorption of oxygen by the glycol. For this purpose, nitrogen was bled into the receivers at a rate of 100 ft. per hour, being removed from the opposite ends of each of the receiver vessels. In this manner the glycol was maintained in an unaltered condition for long periods of use, the glycol replacement rate being less than 1.5 barrels per day on the above plant.

Prior to use of the above nitrogen blanket on the aqueous glycol phase, however, the receiver vessels permitted sufiicient leakage of air into the unit to rapidly deteriorate the glycol. It was observed that as operation of the unit continued beyond the first day on stream, the pH of the glycol decreased rapidly, causing rapid corrosion of the steel equipment, formation of tar and loss of glycol from the unit at a rate of more than 60 barrels per day. Glycol deterioration became so rapid that an auxiliary glycol still was required to continuously distill a slip stream of the solvent for removal of tars and degradation products.

I claim as my invention:

1. A process for recovering a naphthalene hydrocarbon from a mixture containing naphthalene and benzene hydrocarbons which comprises subjecting said mixture to countercurrent contact with an aqueous solution of solvent in which said naphthalene hydrocarbon is selectively soluble and selected from the group consisting of an aliphatic alcohol containing up to carbon atoms, an alkylene glycol, a polyalkylene glycol, anda -glycol-ether,-.at a temperature of from about 200 F. to about 450 and at a pressure suflicient. to maintain the mixture in substantially liquidphase, thereby formingsa rich solvent containing said naphthalenehydrocarbon dissolved in'said solvent, countercurrently contacting the resulting .rich solvent with a liquid paraffinic reflux hydrocarbon which boils at a temperature below the boiling point of the lowest boiling naphthalene hydrocarbon in said mixture, sep arating said rich solvent from a raffinate comprising the non-extracted portion of said mixture and at least a portion of said paraflinic reflux hydrocarbon, stripping hydrocarbon extract from said rich solvent, separately recovering hydrocarbon extract from the resulting lean solvent, recycling said lean solvent to the first-mentioned contacting step and separating a naphthalene hydrocarbon from said extract.

2. The process of claim 1 further characterized in that said solvent is an alkylene glycol.

3. The process of claim 2 further characterized in that said alkylene glycol is an ethylene glycol.

4. The process of claim 1 further characterized in that said solvent is diethylene glycol containing from 0.5 to about 10% by weight of water.

5. The process of claim 1 further characterized in that said solvent is triethylene glycol containing from 0.5 to about 10% by weight of water.

6. The process of claim 1 further characterized in that said mixture boils from about 300 to about 500 F.

7. The process of claim 1 further characterized in that said paratfinic reflux hydrocarbon is an octane and said naphthalene hydrocarbon is naphthalene and the alphaand beta-isomers of methylnaphthalene.

8. The process of claim 1 further characterized in that said first-mentioned contacting step is effected at a temperature of from about 300 to about 400 F.

9. The process of claim 8 further characterized in that said extract stripping step is effected by reducing the pressure on said rich solvent.

10. The process of claim 1 further characterized in that the lowest boiling overhead fraction from said extract stripping step is recovered separately from a naphthalenerich fraction stripped from said rich solvent and said vapor overhead fraction, in admixture with said paraffinic reflux hydrocarbon is recycled to said contacting step.

11. The process of claim 10 further characterized in that said naphthalene-rich fraction separately collected in said stripping step is continuously mixed with a light, normally liquid paraflinic diluent in an amount of said diluent suflicient to completely dissolve the naphthalenerich fraction and maintain said fraction in liquid phase.

12. The process of claim 11 further characterized in that the diluent-naphthalene-rich mixture is continuously distilled and the separated light parafiin hydrocarbon continuously recycled to form said diluent-aromatic rich mixture.

13. A process for recovering naphthalene and methylnaphthalene from a hydrocarbon fraction boiling from about 400 to about 500 F. and containing alkylbenzenesv of 11 to 13 carbon atoms which comprises contacting said fraction at a temperature of from about 300 to about 400 F. and at superatmospheric pressure with a solvent selectively miscible with aromatic hydrocarbons comprising diethylene glycol containing from 0.5 to about 10% by weight of water to form thereby a rich solvent and a rafiinate, countercurrently contacting said rich solvent with a light paraffin reflux hydrocarbon to thereby displace from said rich solvent said alkylbenzenes, distilling said rallinate and recovering a recycle fraction of said light paraffin reflux, reducing the pressure on said rich solvent to thereby flash overhead from the rich solvent residue a light hydrocarbon fraction, combining said light hydrocarbon fraction with said light paraffin reflux re- 19 20 v. e'y'cle'," distilling the "rich solvent residue to recover a naph- References (media the file of this patent th alene-niethylnaphthalene extract substantially free of 1 non naphthalene contaminants, and distilling said extract UNITED STATES PATENTS into a naphthalene product and a separately recovered 2,037,677 Connolly et a1. Apr. 14, 1936 methylnaphthalene product. 5 2,400,802 Arnold .L .May 21; 1946 14. The process of claim 13 further characterized in 2,727,854 Brown et al Dec.v 20, 1955 i that said light paraffin reflux is the octane to decane frac- 2,770,663 Grote Nov. 13, 1956 v tion of a gasoline boiling range material.

Claims (1)

1. A PROCESS FOR RECOVERING A NAPHTHALENE HYDROCARBON FROM A MIXTURE CONTAINING NAPHTHALENE AND BENZENE HYDROCARBONS WHICH COMPRISES SUBJECTING SAID MIXTURE TO COUNTERCURRENT CONTACT WITH AN AQUEOUS SOLUTION OF SOLVENT IN WHICH SAID NAPHTHALENE HYDROCARBON IS SELECTIVELY SOLUBLE AND SELECTED FROM THE GROUP CONSISTING OF AN ALIPHATIC ALCOHOL CONTAINING UP TO 10 CARBON ATOMS, AN ALKYLENE GLYCOL, A POLYALKYLENE GLYCOL, AND A GLYCOL ETHER, AT A TEMPERATURE OF FROM ABOUT 200*F. TO ABOUT 450*F. AND AT A PRESSURE SUFFICIENT TO MAINTAIN THE MIXTURE IN SUBSTANTIALLY LIQUID PHASE, THEREBY FORMING A RICH SOLVENT CONTAINING SAID NAPHTHALENE HYDROCARBON DISSOLVED IN SAID SOLVENT, COUNTERCURRENTLY CONTACTING THE RESULTING RICH SOLVENT WITH A LIQUID PARAFFINIC REFLUX HYDROCARBON WHICH BOILS AT A TEMPERATURE BELOW THE BOILING POINT OF THE LOWEST BOILING NAPHTHALENE HYDROCARBON IN SAID MIXTURE, SEPARATING SAID RICH SOLVENT FROM A RAFFINATE COMPRISING THE NON-EXTRACTED PORTION OF SAID MIXTURE AND AT LEAST A PORTION OF SAID PARAFFINIC REFLUX HYDROCARBON, STRIPPING HYDROCARBON EXTRACT FROM SAID RICH SOLVENT, SEPARATELY RECOVERING HYDROCARBON EXTRACT FROM THE RESULTING LEAN SOLVENT, RECYCLING SAID LEAN SOLVENT TO THE FIRST-MENTIONED CONTACTING STEP AND SEPARATING A NAPHTHALENE HYDROCARBON FROM SAID EXTRACT.

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