US2770664A

US2770664A - Aromatic hydrocarbons by solvent extraction with a solvent of diethylene glycol and water

Info

Publication number: US2770664A
Application number: US255688A
Authority: US
Inventors: Lee H Horsley; Victor S Morello; Poffenberger Noland
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1951-11-09
Filing date: 1951-11-09
Publication date: 1956-11-13
Anticipated expiration: 1973-11-13

Description

Nov. 13, 1956 H. HoRsLL-:Y E1' Al. 2,770,554

AROMATIC HYDROCARBONS BY SOLVENT EXTRACTION WITH A SOLVENT 0F' DIETHYLENE GLYCOL AND WATER Filed Nov. 9. 195] 3 Sheets-Sheet 1 INVENTORS. Lee H Hons/ey l//c/o/ 5 /I//O/e//o othwxm,

l WN Ill' mm @N Wm M NN A/o/anc/ Poffene/:gef

ATTORNEYS.

50 60 W/ Per 6220/ erene /n Hydroca/on ayer (Soen/Free 5051's) INVENTORS. Z 5 ee H Hors/ey l//c/Qr' 5. Mare//o /Vo/na of'fene/ger ATTORNEYS.

` Nov. 13; 1956 L. H. HoRsLl-:Y ETAL 2,770,664

l/a/aor Pres.; KAmOSO/ere) AROMATIC HYDROCARBONS BY SOLVENT EXTRACTLON WITH A SOLVENT 0F' DIETHYLENE GLYCOL AND WATER Filed NOV. 9. 1951 3 Sheets-Sheet 5 50 70 so /lo )3o /50 /70 /90 2/0 240 Tempera/Ure C INVENTORS. Lee H Hors/6g Vic/or 6. More//o /Vo/ano/ Pof/'ene/ger ATTORNEYS.

nited States Patent O AROMATIC HYDROCARBONS BY SLVENT EX- TRACTION WITH A. SOLVENT F DIETHYLENE GLYCOL AND WATER Lee H. Horsley, Victor S. Morello, and Noland Pottenberger, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application November 9, 1951, Serial No. 255,688

7 Claims. (Cl. 260-674) This invention relates to an extraction process for recovering individual :aromatic hydrocarbons in high purity from liquid hydrocarbon mixtures, such as those produced by the reforming of gasolines. It particularly concerns the production of benzene, toulene, and the xylenes.

Aromatic hydrocarbons a-re found in quite small proportion in petroleum and in somewhat higher concentration in natural and straight-run gasolines. Richer sources of aromatic hydrocarbons can be produced by the .catalytic reforming or aromatiZat-ion of the naphtha cuts of petroleum and of natural gasolinas. From all these zsources, concentrated fractions of aromatic hydrocarbons can be separated by a number of known processes. The :most common purpose in forming and concentrating such :aromatic fractions is to provide blending stocks for upgrading motor fuels of low antiknock value. ln this use, .no effort is made to lisolate single aromatic hydrocarbons, :and many of the processes used or proposed to date are not suitable for doing so. There have been one or two ,industrial processes for the extraction and recovery of individual aromatics, particularly toluene, xylene, and ihigher hydrocarbons, .from refinery cuts. However, these processes have largely proved complex, expensive, -or otherwise unsuitable, especially when operated for the .manufacture of pure benzene.

ln particular, the known extraction processes have not been capable of isolating individual aromatic hydrocarbons in both high purity and high yield. If a product at least 98 percent pure is to be made directly, not more than perhaps 8O percent of the total available hydrocarbon is recovered. On the other hand, if nearly cornplete recovery is achieved, the product is rarely over 90 percent pure, and must be cleaned up by elaborate treatment, such as washing with sulfuric acid. Indeed as Claussen et al. state in U. S. Patent 2,404,902, No known solvent 'is sufficiently selective to recover by a simple operation all of an aromatic compound from admixture with the yassociated paratfnic and naphthenic materials in a petroleum distillate and at the same time to recover it in a state of sufficiently high purity that it may be subjected directly to nitration as, for instance, in the preparation of explosives, or may be used directly as raw material in other specic chemical processes.

An additional problem in known extraction processes is that, even when loperation is such `as to isolate a single aromatic hydrocarbon in a purity of 9S to 99 percent, the product still does not meet standard U. S. speciiications for a nitration grade, or even an industrial grade, hydrocarbon (A. S. T. M. specification D835 to D850). The product is frequently disoolored and becomes more so `on storage. It may also react peculiarly in certain organic syntheses. There have been hints that roletinic hydrocarbons in the naphtha or other feedstock are in some measure responsible for these difficulties (c.f. Trans. Amer. Inst. Chem. Engrs. 40, 211 (1944)). During solvent extraction, the 'olefins are not cleanly -separated from the aromatic hydrocarbons, but in part accompany the latter even through multi-stage extractions 2,770,664 Patented Nov. 13, 1956 with solvents of high selectivity. ln the final product, the olefins tend to form gums and colored bodies which, even in minute proportions, seriously affect the quality. Further, the presence of the olefins also seem to render the solvent less capable of making a complete separation yof aromatic from naphthenic and parainic hydrocarbons. No adequate way of dealing with the olefin problem has heretofore been available. i

It is therefore the principal object of the present invention to provide a process for the isolation of individualaromatic hydrocarbons in both high yield and high purity from hydrocarbon feedstocks containing them, even when `such stocks contain appreciable proportions of olefins. A related object is to provide a process capable of isolating benzene, as well as toulene .and Zylenes, from refinery fractions in purity sufficient` to `meet all standard specifications entirely by the physical steps of solvent extraction, stripping, Iand fractionation, and without the need for any chemical treatment. A similar object is to provide a process for making virtually pure benzene in good yield from gasoline, natural or straight-run.

These objects are realized, according to the invention, by `an extraction process in which benzene and other aromatic hydrocarbons are separated from mixtures thereof with non-aromatic hydrocarbons by means of a selective solvent consisting essentially .of diethylene glycol, or mixtures thereof with water. The benzenecontaining hydrocarbon mixture is introduced into a counterilow multi-stage extraction system near the middle, while diethylene glycol is introduced into one end of the `system into extractive contact with the hydrocarbon phase therein. Substantially aromatic hydrocarbon-free raffinate is withdrawn from the system at the sameend. Diethylene glycol rich in benzene and other aromatic hydrocarbons is withdrawn from the other end of the system, and Iis stripped to separate the aromatic hydrocarbon portion therefrom. A part of this latter portion is returned to the system at this other end as reflux. The remainder of the aromatic hydrocarbon portion is withdrawn as product, and may be subjected to fractionation to isolate the benzene and other individual aromatic hydrocarbons.

The success of the new process depends mainly upon the exceptional behavior of diethylene glycol as a selective solvent. It was long ago suggested that various glycols, including diethylene glycol, might be used to extract aromatic hydrocarbons from naphthas (British specification 456,958). However, it has not heretofore been `appreciated that, in extracting the particular hydnocarbon benzene, the particular glycol diethylene glycolis almost uniquely advantageous, even as compared to closely-related glycols, both with respect to its physicochemical properties in an extraction system and to the economics of the process. These advantages of diethylene glycol are also observed in the extraction of toluene and to a considerable degree in that yof the xylenes and ethyl benzene.

Unlike many selective solvents, diethylene glycol exhibits phase equilibria with benzene and non-aromatic hydrocarbons in ternary mixtures thereof which impose no physical limit on the maximum purity of benzene obtainable. The solvent is highly selective, with good solvent power for benzene and such low solvent power for nonaromatics that the solvent loss in the raffinate is exceedingly small. Further, this selectivity remains high over the entire range of concentrations existing in a multi-stage counterflow system, even in the enriching stages where the concentration of aromatic hydrocarbon is highest. On the other hand, the solvent power for aromatics is not high enough to reach complete miscibility, so that it is possible to maintain two separate phases at every stage inthe sys- {process inordinary steel equipment. tivelyhigh density of diethylene glycol and its high surface which admit of high recoveries of puriied benzene.

n, tem. Hence, with diethylene glycol, a separation of benzene from other hydrocarbons which is complete for all practical purposes can be achieved in a finite, and in fact va'rcomparatively small, number of extraction stages, at

high-,efficiency even with moderate reflux to product ratios.

yIn addition, the physical properties of diethylene glycol ess steam heating. The low volatility of diethylene glycol makes easy the stripping of aromatic hydrocarbons from the glycolextract phase by simple distillation or steam .distillation, .leaving ythe diethylene glycol as bottoms. The thermal stability of diethylene glycol and its noncorrosiveness `toward metals make it possible to operate the entire Further, the relatension allow ready liquid-liquid separation of the extract i phasefrom the hydrocarbon phase inthe extraction system yand-eliminate the emulsion problems often encountered yin extraction processes.

Another characteristic of diethylene glycol as a selective `solvent lfor isolating benzene and other light aromatic hydrocarbons from naphthas is in its effect on whatever ol'ens-'may 4be present in such stocks. Like other selective solvents, diethylene glycol does not effect complete separation of aromatics from oleiins.

The latter appear partly inthe raffinate and partly in the extract. However, in unprecedented manner, diethylene glycol drastically alters the relative distribution of the individual olens be- Atween raffinate and extract to the point that what olens do remain in the aromatic extract are easily separated from it by simple rectication. This heretofore unknown property of diethylene glycol, further demonstrated in the examples to follow, is to a considerable degree responsible for the success of the invention in producing high purity benzene and other aromatic hydrocarbons meeting all vfour fold by the addition of as little as one percent water,

without serious loss in total benzene solubility. Adding water within the preferred range permits operation at atmospheric pressure and also effects a significant reduction in the viscosity of the solvent, allowing easierhandling, and decreases its boiling point, permitting lower stripping temperatures. There is little if any tendency for the water to separate from the diethylene glycol at any stage of the extraction process, so that the solvent is, for practical purposes, of the single-component type.

It is also important, in the invention, to select or to prepare, as by catalytic treatment, hydrocarbon feedstocks Aromatic mixtures produced by the catalytic reforming of vaporized. gasoline fractions in the vpresence of hydrogen ,underfcyclizing conditions, e.g. over applatinum-containing catalyst, are particularly suitable, as will be further exyplained Such mixtures, even those having wide 'boiling n. ranges, can be separated cleanly into aromatic and non- .aromatic portions according to the invention, without the elaborate predistillations or careful segregation of refinery lstocks-so essential in prior processes.

t of; :materials Fig. 2 is an equilibrium diagram at 25 C. for. the twobottom drain 28a.

phase system benzene: petroleum naphtha (boiling range to 185 E): diethylene glycol solvent, showing the solubility of benzene in the diethylene glycol layer as a function of the benzene concentration in the hydrocarbon layer. Curves are givenyfor ,anhydrous diethylene glycol and for diethylene glycol containing 2 percent and 5 percent by weight of water.

Fig. 3 is another equilibrium diagram for the same system at 25 C., showing the variation of the separation factor ,3 (a measure of the selectivity of the diethylene v.glycol solvent for benzene) asja function of the benzene content of the vhydrocarbon layer; and

Fig. 4 is a graph showing the Vapor pressures (in atmospheres) at different temperatures of diethylene glycol and several diethylene glycol-water mixtures.

ln operating .the system shown in Fig. l, the liquid feed to be processed may, for instance, be a benzene-rich fraction of a catalytically reformed naphtha. The extractant is diethylene glycol, preferably containing a small proportion of water.

The extraction step is carried out ina column 11 designed for multi-stage countercurrent liquid-liquid contact. The hydrocarbon-feed is introduced as a continuous stream through a valved inlet 12 about half way up the column. yThe diethylene glycol solvent, from storage 13, enters the column ll-near the top by way of a valved line 14, pump 15, and indirect water cooler 16. Rafl'lnate, virtually free of benzene, and consisting of naphthenes and parains, `together with part of any oleiins in the feed, leaves the column through a top outlet 1.7. This raflinate l contains virtually no diethylene glycol. The extract, consisting of benzeneV and other aromatic hydrocarbons, if any, and part of any olefins in the feed, together with the diethyleneglycol, is withdrawn from the bottom of the column through aline 18. To insure complete separation of naphthenes and paraflins, and to allow concentration of the aromatic hydrocarbons, a stream of reflux, consisting of essentially solvent-free extract, is returned to the ,zene` and any other aromatic hydrocarbons are vaporizetl from the diethylene glycol solvent. A `small flow of steam may be injected into the still through a line 24 to assist in the vapo-rization, if needed, and to control the water content of the diethylene glycol. The hydrocarbon vapors leave the still through a topcutlet line 25 leading to a condenser 26. The condensate, consisting of crude benzene, Vother aromatic hydrocarbons,.traces of oleiins, and whatever ywater was Vaporized in the still, Hows into a gas separator and decanter 27. A part of the `hydrocarbon distillate is fed `back to the column 22 through a valved line 28; the water layer is removed through a The remainder of the distillate is led through a line 29 to storage 30. From the latter, a valved line 31 and pump 32 return a portiontof the benzene-rich condensate as a continuous-stream to the line 19 leading to the bottom of the extractor `11. The stripped diethylene glycolsolvent, free of benzene, leaves the-still 23 at the bottom through valved line 33, and is forwarded by a pump 34 through the heat exchanger 21 back to storage 13. For atmospheric 'pressure operation of the still 23, the storage tank 30 is openvthrough a valved vent 35, which is open to the separator 27 through a line 36. However, if distillation at reduced pressure, e. g. at 0.5 atmosphere, is desired, the still and auxiliary equipment 'may be exhausted by a purnp37.

Al portion of the benzene-containing distillate from the receiver 30 is continuously led to a finishing system through theline 38. This stream enters the center of a fractionat mg column of a topping still 39, in which a small overhead of light ends boiling lower than benzene and *conarranca taining most of any yoleiins in the extract, is removed through a line 40. Bottoms are withdrawn through a line 41 and pump 42 and sent to the center of the fractionating column `of a finishing still 43. Here benzene is produced as overhead through a product line 44. Hydrocarbons boiling higher than benzene, mainly toluene, leave through a bottom line 45. If desired, the light ends from line 40 and highers from line 45 may be blended and used to 11p-grade gasoline of low antikn'ock value. Alternatively the highers from line 45 may be rectified to isolate toluene, the xylenes, and some ethyl benzene, if present, as purified products.

The apparatus just described is flexible and admits of considerable variation in operating conditions without adverse consequences. In general, however, it is preferred to run the column 11 at about atmospheric pressure and at temperatures up to about 60 C. Under these conditions, a feedstock containing a relatively modest portion of benzene, e. g. 10 to 30 percent by weight, can easily be separated into a finished benzene of high purity.

The foregoing description with respect to Fig. l is illustrative only, it being 'obvious that there are a variety of other apparatus arrangements and operating procedures within the scope of the invention, as will be further explained. For instance, the stripper 23 may be operated without reiiux or fractionation by entering the feed 18 at the top of the column 22. Figs. 2 to 4 present physicochemical data to permit working out the operating details of many such variations.

The diagram of Fig. 2 summarizes the results of a large number of solubility determinations at 25 C. in the twophase system diethylene glycol: benzene: petroleum naphtha (boiling range 150 to 185 F.). The latter component, which was benzene-free, was chosen as typical of the non-aromatic hydrocarbon portion of feedstocks used in the invention. In the diagram, in which all percentages are by weight, the percent of benzene `in the diethylene glycol layer is given as a function of the benzene in the hydrocarbon layer (solvent-'free basis). Curves are given for anhydrous diethylene glycol as the solvent and also for systems in which the diethylene glycol contains 2 percent and percent of water, respectively.

The diagram of Fig. 3 summarizes additional data obtained in the same solubility determinations reported in Fig. 2. In this case, the separation factor is plotted as a function of the percent of benzene in the hydrocarbon layer. This coefficient sometimes termed selectivity factor, was calculated according to the equation ,..rr as YN X B where YB and YN represent the concentration, in weight percent, `of the benzene and petroleum naphtha, respectively, in the glycol-rich phase, and XB and XN yrepresent the corresponding concentrations in the hydrocarbon-rich phase. Curves are given for anhydrous diethylene glycol and for diethylene glycol containing 2 and 5 percent water.` It will be noted that the coeicients are larger when the diethylene glycol contains water, indicating that the watercontaining glycol is more selective a solvent for benzene than the dry glycol.

As will be appreciated from Figs. 2 and 3, both the selectivity and solvent power of the diethylene glycol can be maintained at any desired value within wide limits merely by adjusting the concentration of water in the solvent. While the two graphs show conditions at 25 C., the selectivity and solubility change only slightly with temperature, so the graphs serve as rough guides to operation at any temperature.

Fig. 4 presents vapor pressure data for diethylene glycol and several diethylene glycol-water mixtures. This figure is of assistance in planning the operation 'of the solvent stripper 23, which must be held at a temperature sufiicient to vaporize the extracted hydrocarbons almost completely. To this `end, the temperature in the base of the still should CII approximate the boiling point at the existing pressure of benzene-free solvent, i.` e. diethylene glycol or diethylene glycol-water mixture. As the data shows, the addition of water to the glycol lowers the temperature at which the still 23 need be operated to produce effective stripping into a range easily obtainable with ordinary process steam heating.

The data in Figs. 2 to 4 were determined by careful physico-chemical measurement and are believed largely new to the literature.

As previously mentioned, the process of the invention is applicable generally to the separation of benzene, toluene, and other light aromatic hydrocarbons from admixture with non-aromatic hydrocarbon liquids, both paraiiinic and naphthenic. The process also effects `a partial separation of benzene from olelinic hydrocarbons, and further redistributes the oleiins so that those not extracted are easily removed from the benzene by simple rectification. Hence, the process is capable of operation on feedstocks of wide boiling range, even in the presence of substantial proportions of oleiins. On the other hand, since olens do complicate operation somewhat, it is economically advantageous, in making benzene, not only to select a feed which is rich in that hydrocarbon, containing at least 10 to 30 percent benzene, but which also is relatively free of olelins, eg. containing not over 2 to 3 percent.

Among suitable ultimate sources of benzene for the process of the invention are aromatic-containing naphthas and gasolines produced from petroleum during refining or catalytic conversion processes. Natural gasolines or naphthas recovered from natural gas in lthe Texas Gulf Coast and South Texas areas are particularly desirable. To avoid burdening the extraction step, these source materials are ordinarily subjected Ito a preliminary fractional distillation to effect a concentration of the Ce and Cr hydrocarbons and to exclude high-boiling materials. In general, a comparatively broad still cut, boiling in the range of 60 to 95 C., may be taken, and will be found to contain most of the benzene and parent materials for benzene. When toluene, xylenes, and ethyl benzene are also to be recovered, broader fractions may be employed.

In -many respects, the more desirable feedstocks for extraction of benzene according to the invention are the liquid hydrocarbon products of vapor-phase catalytic reforming and cyclization of Cs and C7 hydrocarbon fractions in the presence of hydrogen. Typical of these reforming processes are hydroforming, i. e. the reforming of naphthas over a molybdenaolumnia catalyst under pressure at 900 to 1000 F. in the presence of excess hydrogen (Trans. Amer. Inst. Chem. Eng. 42, 611 (1946), and platforming, i. e. the reforming of gasolines over a platinum-containing catalyst under pressure in the presence of hydrogen at 800 to 900 F. (Ind. Eng. Chem. 42, 582 (1950) The platforming process makes perhaps the most satisfactory benzene source material in that the product is exceptionally rich in aromatic hydrocarbons and contains little olefinic hydrocarbons. Optimum yields of high-grade benzene appear to be realized when Cra-rich fractions of natural gasoline are platformed, and then extracted according to the invention. Highpurity benzene is produced simply by extraction and dis tillation, without the need for any chemical purification..

As previously explained, in operating on feedstocks con taining a substantial portion of oleiins, a part of the oleiins.

remain in the crude benzene after in it stripped from the diethylene glycol solvent. Such olefins as persist may usually be separated almost completely by fractional distillation of the crude extract during which they tend to\ concentrate in fractions boiling lower than benzene, be-

xylenes.

simple clay treatment similar to 'that used on gasoline.

V.Title diethylene glycol used `as the extractant inthe process of the invention may `be the ordinary commercial product, and may contain small proportions of non-interfering solventswithout serious consequence. As already explained, real advantage is realized by adding to the diethylene glycol a small proportion of water, i. e. 0.5 to -percent by weight. Atfvalues below 0.5 percent, the effect of-water is not pronounced, while above 10 percent the solubility of benzene in the extractant is reduced unn duly and other adverse effects lbegin to appear. For atmospheric pressure operations, loptimum concentration of wateris in the range of 1 to 5 percent, with 1.0 to 2.0 percent being a good working Value. As already mentioned, the ywater tends to remain with the diethylene glycol throughout the process, and nowhere appears as a separate phase, except with the benzene product of the stripper. Here it is trapped and may be returned to the still. However, since water loss may occur during prolonged operation, is may be necessary to test the diethylene glycol at intervals and to `add water as needed to maintain the desired concentration. Such addition may conveniently be madein the form of steam supplied to the stripper, as at line 24 in Fig. l.

In the operation of the countercurrent multistage extraction according to the invention, the temperature should preferably be controlled below 60 C. by cooling the solvent and feedstock, if necessary, to insure maximum efficiency and to minimize hazard due to the presence of' volatile hydrocarbon. The pressure in the extractor should be sufficient to maintain all phases liquid. Atmospheric pressure is ordinarily adequate, and, even under extreme conditions, only low superatmospheric pressures are required. The details of extractor design and operation for any particular set of conditions chosen may be worked out `from lthe data of Figs. 2 to 4 by known engineering principles. In general, however, there should be efficient counter-current extraction, with usually a minimum cf five or more theoretical stages. The solvent: feedstock ratio used is, of course, dependent on the composition of the stock, the number of extraction stages, and the purity of product and the recovery desired. Several volumes of solvent per volume `of feed, eg. at least a 4:1 ratio, are usually used. Likewise, one or more volumes of solvent-free extract are idesirably recycled as reflux to the extraction system per volume subjected to fractionation, such reflux serving to insure virtually cornplete exclusion of non-aromatic hydrocarbons from the extract.

In an alternative mode of operating the process described with respect to Fig. l, the heat-

exchangers

16 and 21 may be eliminated and `the system operated almost isothermally. In this case, the extractor 11 is operated at the same temperature as the stripper reboiler 23. To achieve this equilibrium, Ithe water content of the diethylene glycol must be controlled carefully at the value required to maintain the other conditions chosen, in accordance with the data in Figs. 2 to 4. For instance, if the extractor and stripper are both operated at 150 C., with moderate pressure in the extractor, about 7.5 percent by weight of water should be present in the diethylene glycol. Suchoperation reduces heating and cooling to a minimum, significantly lowering costs.

The following examples will further illustrate the invention.

Example I yThe feedstock was a benzene-rich mixture of aromatic, naphthenic, paraflinic, and oleiinic hydrocarbons, pro duced by passing a vaporized natural gasoline fraction together with hydrogen over 4a platinum-containing catalyst at a temperature of 800 to 900 F. according to the platforming process (Oil and Gas. I. 48 (47), 83 (1950); Petr. Refiner 29 (4), 131 (1950)). It contained about to 30 percent by'weight of aromatic hydrocarbons, primarily benzene. The olefin content corresponded to a lbromine index of 2660. (The bromine index is the bromine measured as milliliters of N/ 100 bromatebromide solution absorbed by 100 milliliters of ole1incontaining material, according tothe procedure described by Johnson et al., Anal. Chem. 19, 869 (1947)).

`For purpose of analysis, a sample of this feedstock was subjected to careful fractional distillation in a three-foot Podbielniak column at 40:1 reflux ratio. The amount of the fraction, weight per cent aromatic hydrocarbon in the fraction (as measured by refractive index), and bromine index of each fraction were determined as follows:

Wt. Per- Aromatie Bromine Boiling Range, C. cent of Content, Index Feed Percent From these Values, it is apparent that the olefns occurred in all boiling ranges and could not b'e separated from the non-olefinic material by fractionation.

The main body of the feedstock was then extracted according `to the invention with diethylene glycol at room temperature in a laboratory countercurrent multistage extraction unit according to the general scheme shown in Fig. 1. The rafnate phase from the top of the extraction column contained only 1.0 percent aromatic hydrocarbon and had a brornine index of 2820. The extract phase from the bottom of the column, after separation from the diethylene glycol solvent, contained 97 percent aromatic hydrocarbons and had a bromine index of 1440. The extraction thus made a clean separation of the aromatic from the non-aromatic portions of the feedstock. However, the iolefins of the feed were distributed in both extract and raffinate.

The aromatic-rich extract was then subjected to a fractional distillation to separate the fraction boiling at temperatures up to C. This fraction, which represented about 80 percent by weight of the total extract, was then subjected to fractional distillation through a three-foot Podbielniak column at 15:1 reflux ratio. The amount of each fraction, aromatic content, bromine index, and freezing point of ythe fractions were determined as Wt. Per- Aromatics, Bromine Freezing Bolling Range, C. cent of Percent Index Point,

Feed

28.5-71 0 3. 7 31 9500 71.0-78.0 4. 4 73 4300 78.0-78 5 30. 7 98 950 3.8 78.5-79. 57. 5 98 275 4. 9 Residue. 3. 7

It will be evident that, in fractional distillation, virtually all the olefinic impurities were removed as forerunnings, leaving final fractions which were at least 98 percent benzene and which met standard specifications for that material.

Example 2 Ja light overhead cut, an intermediate naphtha, andv a heavy naphtha. The intermediate naphtha, representing 31.4 percent by volume of the feedstock, had an A. P. I. gravity of 65, and an A. S. T. M. boiling range of 162 to 207 F. It is consisted mainly of parafiinic and cycloparaiinic hydrocarbons of 6 and 7 carbon atoms per molecule, together with some benzene.

The latter fraction was vaporized `and passed together with an excess of hydrogen over a platinum-containing catalyst at a temperature of 800 to 900 F. and lat a pressure of 500 to 900 p. s, i. g. according to lthe platforming process, and the resulting reformed product was condensed. The liquid condensate had an A. P. I. gravity of 56.0, a Reid vapor pressure of 4.3 pounds, and an A. S. T. M. boiling range of 140 to 284 F., and contained 17.0 percent by weight of benzene.

This condensate was then fractionally distilled to separate a light overhead, a middle cut representing 56.4 percent by volume of the total and containing virtually all the benzene in the condensate, and heavy ends. The middle cut had a specific gravity of 0.765, a boiling range of 154 to 200 F., and contained, by infra-red analysis, 27.6 percent by weight of benzene. It had a bromine index `of 2280.

The middle cut was then forwarded as the feedstock into the extraction and recovery system illustrated in Fig. l. The extractor was of the vertical mixer-settler type and was equivalent to about 10 theoretical extraction stages. The solvent was commercial diethylene glycol to which 1.0 percent by weight of water had been added. Operation was at C. with a solvent to feed ratio of 4:1 and a solvent-free benzene return to the base of the extractor column of 4 volumes per volume forwarded to the finishing stills. The ra'inate from the top of the extractor contained less than 0.1 percent benzene. The solvent-free aromatic hydrocarbon mixture produced as overhead in atmospheric pressure operation of the solvent stripper contained 97 percent byweight of benzene, in a quantity corresponding to 99 percent overall recovery eiciency in the solvent-extraction. 5

For analytical purposes, a sample of this solvent-free overhead was subjected to fractional distillation in a three-foot Podbielniak column at a reux ratio of 15:1. The amount of each fraction, aromatics content, bromine index, and freezing point were determined as follows:

The solvent-free extract having this analysis was processed through the finishing stills, yielding benzene having a freezing point of 5.3 to 5.5 C., a boiling range varying less than 2 C. from beginning to end, and an acid wash color of 3. It met all specifications for industrial grade benzene (A. S. T. M.-specication: D83647). The yield of high-purity benzene thus isolated represented 96.5 percent of the total benzene content of the platformed condensate.

The process of the invention is intended primarily for 30 the production of pure benzene from hydrocarbon mixtures, and is most effective therewith. However, the extraction step with diethylene glycol, or with mixtures thereof with water, may also be used in the separation of toluene and other aromatic hydrocarbons from mixtures thereof with parafnic and cyclo-parainic hydrocarbons, even in the absence of benzene.

This application is a continuation-in-part of our application Serial No. 198,578 tiled December 1, 1950, now abandoned.

, `What is claimed is:

1. A continuous process for isolating benzene in a substantially pure state from the liquid hydrocarbon product of the vapor-phase catalytic reforming and cyclization in the presence of hydrogen of a gasoline fraction containing mainly hydrocarbons of six and seven carbon atoms per molecule, such product containing at least 10 percent by weight of benzene and an appreciable proportion of olefins not exceeding about 3 percent, which comprises introducing a stream of the hydrocarbon product into a counterowmultistage extraction system near the middle, introducing a stream of diethylene glycol containing 0.5 to 10 percent by weight of water into one end of the system into extractive contact with the hydrocarbon phase therein, withdrawing a stream of substantially aromatic hydrocarbon-free raffinate containing part of the oletins from the system at the same end, withdrawing a stream of benzene-containing extract containing the other part of the olens from the other end of the system, subjecting the extract to evaporative conditions to distill a benzene-rich hydrocarbon portion therefrom, returning part of the said separated hydrocarbon portion as a reflux stream to the extraction system at this other end, and subjecting the remainder of the separated hydrocarbon portion to fractional distillation to separate the olens therefrom and to isolate the benzene therein in a substantially pure state.

2. A process accordingv to claim l wherein at least 4 volumes of diethylene glycol-water mixture are introduced into the extraction system per volume of liquid hydrocarobn feed and wherein at least half of the benzene-rich hydrocarbon portion distilled from the extract is returned to the extraction zone as reflux.

3. In a process for isolating light aromatic hydrocarbons of the class consisting of benzene, toluene, ethylbenzene and the xylenes from a mixture thereof with close-boiling parafns and naphthenes, such mixture also containing an appreciable proportion of olens boiling in the same temperature range and not separable by fractional distillation, the steps which comprise introducing a stream of the mixture into a counterow multistage extraction system near the middle, introducing a solvent consisting essentially of diethylene glycol and containing from 0.5 to l0 percent by weight of water into one end of the system into extractive contact with the hydrocarbon phase therein, withdrawing from the system at the same end a stream of substantially aromatic hydrocarbon-free raffinate containing most of the parains, naphthenes, and that part of the oleiins not separable from the aromatic hydrocarbons by fractional distillation, withdrawing a stream of aromatic hydrocarbon-rich extract containing the other part of the oletins from the other end of the system, evaporatively stripping the extract to separate the hydrocarbon portion therefrom, returning part of the separated hydrocarbon portion as a stream to the extraction system at this other end, and subjecting the remainder of the separated hydrocarbon portion to fractional distillation to separate the remaining olens from the aromatic hydrocarbons and to isolate individual aromatic hydrocarbons in the substantially pure state.

4. In a process for isolating benzene and toluene in the substantially pure state from a liquid hydrocarbon mixture produced by catalytic reforming and containing mainly hydrocarbons of six and seven carbon atoms per molecule, such mixture containing 10 to 30 percent by weight of benzene together with paraiiins, naphthenes, and an appreciable proportion of olens boiling in the same temperature range and not separable from the benzene and toluene by fractional distillation, the steps which comprise introducing a stream of the mixture into a counterflow multistage extraction system near the middle, introducing a solvent consisting essentially of diethylene glycol and containing from 0.5 to 10 percent by weight of water into one end of the system into extractive contact with the hydrocarbon phase therein, withdrawing `from the system at the saine end a stream lof raffinate substantially free of benzene and toluene and containing substantially all the parafns, naphthenes, and that part of the olens not separable from the aromatic hydrocarbons by fractional distillation, withdrawing from the other Yend of the system an extract rich in benzene and toluene and containing 'the other part of ytheoleiins, subjecting the extract to vevaporate conditions to distill the `'hydrocarbon portion therefrom, returning part of this "hydrocarbon portion to lthe extraction `system at this other end, and fractionally distilling the remainder of this hydrocarbon portion to separate as individual fractions fore-runnings containing much of the olens, substantially pure lbenzene virtually free of oleiins, and intermediate fraction containing some olefins, and substantially purse toluene virtually free of oleiins 5. In a process for isolating light aromatic hydrocarbons of the class consisting vof benzene, toluene7 ethylbenzene andthe xylenes from a mixture thereof with close-boiling vparaflins and naphthenes, such mixture also containing an appreciable proportion of lolens boiling in the same temperature `range and vnot separable by fractional distillation, the steps lwhich comprise introducing a stream of the mixture into a Vcounterflow multistage extraction system near the middle, introducing a solvent consisting essentially of diethylene glycol and containing a predetermined proportion of water from 0.5 to 10 percent by' weight into one end of the system and passing it therethrough in extractive contact with the khydrocarbon phase therein while maintaining the said concentration of water substantially constant throughout the extraction system, withdrawing from the system at the same end a stream of substantially aromatic-hydrocarbonfree raffinate containing most of the paratlins, naphthenes,

and that part of the oleiins, not separable from the aromatic hydrocarbons by fractional distillation, withdrawing a stream of aromatic-hydrocarbon-rich extract containing the other ypart of the oleiins from the other end of the system, subjecting ythe extract to evaporate conditions to distill the hydrocarbons therefrom in crude form together with at least part of the water in the sol- 'vent while injecting water into the extract vundergoing distillation in a proportion suliicient to maintain the concentration of water therein throughout distillation at Subf stantially the predetermined proportion aforesaid, separating ythe crude hydrocarbon distillate from the `water distilled therewith, recycling part'of theseparated hydrocarbon portion as a stream to the extraction system near the solvent outlet end, and subjecting the remainder of the separated hydrocarbon portion to fractional distillation to separate the remaining olens from the aromatic hydrocarbons and lto isolate individual aromatic hydrocarbons in the substantially pure state.

6. In a process for separating light aromatic hydro- 12 carbons of `the'class vconsisting of benzene, toluene, ethylbenzene and the xylenes Lfrom 'a feed mixture thereof with close-boiling Vnon-aromatic hydrocarbons in a liquidliquid counterflow multistage solvent extraction system 'followed by a solvent stripping zone, wherein the extraction system and the stripper are operated substantially isothermally at a predetermined temperature in the range corresponding to the boiling temperatures at atmospheric pressure of mixtures 4of diethylene glycol with from 0.5 to l0 percent by weight of water, the steps which comprise: introducing the feed mixture into the extraction system near the middle; introducing into one end of the system a selective solvent at the predetermined temperature and consisting essentially of diethylene glycol and containing from 0.5 to 10 percent by weight of water, and passing such solvent into extractive contact with the hydrocarbon phase in the system while maintaining the system under a moderate `pressure suicient to keep all phases liquid; kwithdrawing substantially aromatic hydrocarbon-'free ralfinate from the system at the solvent 'inlet end; `withdrawing aromatic hydrocarbon-rich extract .from the `other end kof `the system, transferring -it withouty substantial temperature change to the solvent stripping zone maintained at about-.atmospheric pressure andtherein boiling it to strip therefrom the hydrocarbon portion and some of the water while injecting -water into the boiling liquid vin a proportion just sufcient to maintain the normal boiling temperature of the stripped solvent :at :the :predetermined temperature aforesaid; withdrawing stripped solvent `from the stripping Zone and recycling yit without substantial cooling to the extraction zone; returning part of the'stripped hydrocarbon portion 'to vthe extraction kzone at `the solvent outlet end; vand fractionally Idistiiling the :remainder of the hydrocarbon portion 'to separate the light aromatic hydrocarbons as product.

7. rA process according to claim 6 wherein the predetermined temperature is about C. and the concentration of water 'in the solvent is about 7.5 percent.

References Cited `in the le of this patent UNITED STATES PATENTS 2,246,297 Duncan et al June 17, 1941 2,302,383 Stratford etal Nov. 17, 1942 2,400,802 Arnold May 21, 1946 2,407,820 Durrum Sept. 17, 1946 2,409,695 Laughlin Oct. 22, 1946 2,449,402 Lipkin et al Sept. y14, 1948 2,478,916 Haensel et al Aug. 16, 1949 2,492,787 Davis Dec. 27, 1949 2,514,997 Floyd July 11, v1950 FOREIGN PATENTS 456,958 Great Britain Nov. 18, 1936

Claims

1. A CONTINUOUS PROCESS FOR ISOLATING BENZENE IN A SUBSTANTIALLY PURE STATE FROM THE LIQUID HYDROCARBON PRODUCT OF THE VAPOR-PHASE CATALYTIC REFORMING AND CYCLIZATION IN THE PRESENCE OF HYDROGEN OF A GASOLINE FRACTION CONTAINING MAINLY HYDROCARBONS OF SIX AND SEVEN CARBON ATOMS PER MOLECULE, SUCH PRODUCT CONTAINING AT LEAST 10 PERCENT BY WEIGHT OF BENZENE AND AN APPRECIABLE PROPORTION OF OLEFINS NOT EXCEEDING ABOUT 3 PERCENT, WHICH COMPRISES INTRODUCING A STREAM OF THE HYDROCARBON PRODUCT INTO A COUNTERFLOW MULTISTAGE EXTRACTION SYSTEM NEAR THE MIDDLE, INTRODUCING A STREAM OF DIETHYLENE GLYCOL CONTAINING 0.5 TO 10 PERCENT BY WEIGHT OF WATER INTO ONE END OF THE SYSTEM INTO EXTRACTIVE CONTACT WITH THE HYDROCARBON PHASE THEREIN, WITHDRAWING A SREAM OF SUBSTANTIALLY