US3179708A

US3179708A - Solvent extraction of aromatics from hydrocarbon mixtures

Info

Publication number: US3179708A
Application number: US139576A
Authority: US
Inventors: Penisten J Robert
Original assignee: Universal Oil Products Co
Current assignee: Universal Oil Products Co
Priority date: 1961-09-18
Filing date: 1961-09-18
Publication date: 1965-04-20
Anticipated expiration: 1982-04-20

Description

April 20, 1965 J. R. PENlsTEN SOLVENT EXTRACTION OF AROMATICS FROM HYDROCARBON MIXTURES Filed Sept. 18, 1961 l SOLVENT EXTRACTN F ARMATL'CS HYDRCARBN MEXTURES J. Robert Penisten, Paiatine, Ill., assiwor to Universal @il Products tCompany, Des Plaines, lll., a corporation of Delaware Filed Sept. 18, 1961, Ser. No. 139,576 8 Claims. (Ci. Zeil-674) This application is a continuaton-in-part of my copending application Serial Number 787,294, filed January 16, 1959, now abandoned.

This invention relates to a solvent extraction process for the recovery of'an aromatic component from a hydrocarbon mixture containing aromatic and non-aromatic hydrocarbons. More particularly this invention relates to a process wherein such hydrocarbon mixture is contacted inan extraction zone with a solvent composition which selectively dissolves the aromatic component of the hydrocarbon charge mixture and which contains water and an organic solvent boiling above the aromatic component, forming thereby a resulting railinate phase comprising one or more aromatic hydrocarbons and an extract phase containing dissolved aromatic component, the extract phase is thereafter separately distilled, yielding `an overhead distillate containing only a portion of the extracted aromatic component and a side cut comprising aromatic hydrocarbon and water are separately recovered from the distillation and the aromatic hydrocarbon recovered from the side cut. t

In the solvent extraction of aromatic hydrocarbons from a hydrocarbon-mixture utilizing a solvent selectively miscible with the aromatic component of the mixture, the solvents utilized for this purpose generally contain a definite Vproportion of water to thereby increase the selectivity of the solvent for the aromatic component of the feed stock and to increase the capacity of the solvent to reject the non-aromatic components in the feed stock, herein referred to as rainate. Thus, by virtue of t such inclusion of water in the solvent composition, the solubilityof aromatic hydrocarbonsin the solvent, although somewhat reduced i-n comparison with a non-aqueous solvent, greatly decreases the solubility of raffinate cornponents in the solvent and also reduces the solubility of solvent in the raffinate effluent. The latter factor, however, cannot be completely eliminated and consequently, the raffinate stream inherently contains more or lessrsolvent, depending upon the proportion of water in the solvent composition. Although the quantity of solvent in the raffinate at any instant is relatively small, the cumulative effect of such small amounts of solvent in a stream removed from the process flow and thus otherwise lost, greatly reduces the efficiency and economy of the solvent extraction process. Accordingly, it is desirable that the solvent dissolved in the ralnate stream be recovered therefrom. Such recovery is usually accomplished by counter-currently washing the ralllnate with water in a separate washing Zone from which an aqueous Wash effluent is recovered containing dissolved therein the solvent recovered from the raffinate.

Although the aqueous solvent composition selectively extracts the aromatic components from the hydrocarbon feed stock, it nevertheless dissolves a small but significant quantity of ramnate components having essentially the same boiling `point as the aromatic extract hydrocarbon. If these are not removed from the extract or rich solvent phase prior to the recovery of the aromatic component therefrom in the subsequent extract distillation or solvent stripping stage, the normally small amount of railinate components dissolved in the rich solvent tend to distill over with the aromatic extract product, and since these hydrocarbons have essentially the same boiling point as the desired aromatic extract, they generally cannot be readily removed from the aromatic product by subsequent distillation. When the aromatic productV is desired in a high state of purity, as, for example, in the production of benzene of nitration grade quality, containing 99+ percent benzene, a slight contamination of the `aromatic product with railinate produces a product which does not meet nitration grade specifications and is not readily subject `to supplementary methods of purification. For this reason, the rich solvent stream (as it is formed in the extraction zone and `containing raffinate- "type solute) is desirably contacted (preferably under countercurrent flow conditions), prior to the recovery of aromatic extract therefrom in the so-called stripping stage, with a light parain-aromatic mixture recovered as overhead distillate from the rich solvent stripping column. During such countercurrent contact of `the-rich solvent with 'the light paraiiin-aromatic mixture, known in the art as reflux, the ratlinate dissolved by the solvent (i.e., of the same boiling point as aromatic extract) is displaced from the rich solvent prior to the removal of the latter from the solvent extraction zone, the raffinate thus being replaced by the light parallin and aromatic components of the reflux. The rich solvent leaving the solvent extraction zone,

. thereby, contains dissolved aromatic extract and `light paraffin present therein by virtue of the contact between the light parafin-aromatic reflux and rich solvent stream in the lower portion of the solvent extraction zone.

Inf the subsequent stripping stage of the process the hydrocarbon solute is distilled from the rich solvent with j the `aid of steam formed by distillation of the aqueous solvent composition and/ or by the injection of water into the lower or so-called reboiling section of the stripping zone which is maintained at a temperature above the boiling point of the solvent; the overhead product fromthe stripping zone will then contain substantially all of the light parains of the reflux as well as a. `portion of the aromatic component of the rich solvent` stream. When the aromatic component of the hydrocarbon feed stock consists of two or more species of different boiling points, the reflux will contain atportion of theirnost volatile aromatic component of the rich solvent` stream. The remaining portion of the aromatic component of the rich solvent is then distilled from the stripping zone as a side cut containing in addition to the volatilized aromatic hydrocarbon(s), vaporized water and organic solvent; at the same time the rich solvent residue obtained from the reboiling section of the stripping zone will contain organic solvent substantially free of hydrocarbon solutes and will generally contain water in an amount depending on the conditions of temperature, pressure and the quantity of water or steam injection employed in the reboiling section. By appropriate selection of these conditions it is possible in the known solvent extraction process and desirable also in the process of the present invention to reconstitute the solvent composition in the stripping zone so that the effluent from the bottom portion of the reboiling section thereof contains the same proportion of water and organic solvent as the desired solvent composition contracted with the hydrocarbon feed stock in the extraction zo-ne. y

A principal object of this invention is to provide a simplified and highly economical process for the recovery of a rafinate-free aqueous phase from the raffinate washing stage of the process to thereby enable the re-use of the recovered aqueous phase in the rich solvent stripping stageas a source of stripping steam and to thereby also a to a minimum the need for water from extraneous sources in the recovery of a substantially pure-solventf ized principally by the feature of utilizing the aromatics dissolved inthe aqueous phase of the stripper side-cut fraction to displace raffinate-hydrocarbons dissolved in the aqueous phase recovered from the raffinate Wash stage of the process to thereby produce recovered aqueous effluents free of contaminating feed stock boiling range raffinate hydrocarbons prior to the distillation of the recovered aqueous phases for the purpose of recovering solvent therefrom.

A more specicernbodirnent of this invention concerns an aromatic hydrocarbon recovery process which comprises contacting a hydrocarbon mixture containing7 aromatic and non-aromatic hydrocarbons with a solvent composition which selectively dissolves the aromatic com- A ponent of said mixture and which contains water and an organic solvent boiling above said aromatic component,

separating a resulting rafiinate phase comprising said nonaromatic hydrocarbon from a rich solvent phase contain- 'l ing dissolved aromatic component, subjecting said rich solvent phase to distillation to thereby form an overhead distillate containing only a portion of the extracted aromatic component and a side-cut fraction containing Water and said aromatic hydrocarbon, separating an aqueous side-cut condensate containing solvent and dissolved aromatic hydrocarbon from a substantially aromatic hydrocarbon extract, washing said raffinate with water and separately recovering an aqueousvvvash effiuent containing solvent Washed from said rainate, thereafter mixing said aqueous wash effluent with at least one of the group consisting of: (l) said overhead distillate, and (2) said aqueous side-cut condensate, recovering substantially raffinate-free aqueous solvent from the mixing step and f supplying said aqueous solvent to said distillation stage.

Most advantageously, the aqueous phase resulting from the washing of the raffinate is separated from the washed rainate Vand then mixed with the aqueous phase recovered from the side-cut stripper and overhead condensates Vof the rich solvent stripping stage, the resultant mixture fis separated into a hydrocarbon phase and an aqueous phase, substantially free from raffinate components, and the resultant mixture of aqueous phases is supplied to the reboiling section of the'rich solvent stripping stage as a f source of stripping steam therein. l

Before the overhead distillate of the rich solvent strip- -ping stageis separated into an aqueous phase and hydrocarbon phase suitable for recirculation to the extraction zone `for reflux purposes, one of the preferred embodiments of the present invention provides for utilizing the aromatic component present in this light distillate to displace the raffinate paraiiin dissolved in the aqueous Wash effluent of the raffinate water-washing step, and this aqueous Wash efiiuent is thereafter charged into the rich solent stripping zone as a source of stripping steam therein. Such additional processing technique will be hereinafter A furtherdescribed.

Typical hydrocarbon mixtures containing aromatic components and utilizable as the hydrocarbon feed Fstock to the present process include hydrocarbon distil- .late fractions (usually boiling within or near the gasoline boiling range) of natural gasoline or straight-run petroleum distillates, and especialiy, hydroreformed naph- Y thas which are rich in aromatic hydrocarbons and are particularly valuable as a source of such mononuclear aromatic hydrocarbons as benzene, toluene and xylene.

Solvent compositions which have a preferential solu- 'rbility for aromatic hydrocarbons generally contain one or greater than the boiling point of water which is included in the solvent composition for enhancing its selectivity,

and in general, must also have a boiling point substantially greater than the end boiling point of the aromatic component to be extracted from the hydrocarbon charge. Preferably the organic polar compound used in the solvent composition boils above the end boiling point of the hydrocarbon mixture supplied as charge to the extraction zone. VIn most instances the solvent composition has a greater density than the hydrocarbon charge and is accordingly introduced into the uppermost portion of the solvent extraction zone, thereafter fiowing Y downwardly, countercurrent to the rising hydrocarbon feed. stock introduced into the extraction zone at its midt point or in the lower portion thereof.

Organic compounds for use as the solvent component of the solvent composition preferably are selected from the group of those oxygen-containing compounds which include the aliphatic and cyclic alcohols, cyclic monomeric sulfones, the glycols and glycol ethers, as well as the glycol esters and glycol ether esters. The monoand polyalkylene glycols in which the alkylene group contains from 2 to 3 carbon atoms, such as ethylene glycol, diethylene glycol, triethylene glycol and tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol, as well as the methyl, ethyl, propyl and butyl ethers of the glycol hydroxyl groups and the acetic acid esters thereof constitute a particularly preferred class of organic solvents useful in admixture with Water as the solvent composition for use in the present process. A particularly preferred cyclic monomeric sulfone is tetrthydrothiophene-l,l-dioxide. Various phenols, such as phenol and resorcinol and their alkyl 'ethers, such as, for example, para-cresol and thymol,

are also effective solvents for aromatic hydrocarbons.

' Aliphatic nitriles and cyano-substituted ethersrand amines 'of appropriate boiling point, such as acetonitrile, the

di-alpha, -di-beta, and cli-gamma, propion-nitriles and the diethers and polyalkylene polyamines constitute another group of useful solvents when combined with Water in certain proportions to provide solvent compositions havinr'the desired selectivity. The proferred polyalkylene glycols and especially, diethylene, triethylene and dipropylene glycols, containing from 2 percent to about 25 j'percent by Weight of water, constitute especially desirable solvent compositions for use in the present process. The improvement in solvent extraction to which the present invention is directed involves separating the aque- 'ous Wash effluent from the raffinate Water-washing step,

recovering the aqueous solvent-containing phases from each of the overhead and side-cut distillate fractions removed from the rich solvent stripping column, mixing said aqueous Wash effluent with the aqueous solvent-containing phase recovered from the stripping column and/or lwith they overhead distillate of the stripper column. In

this manner, the aromatics present in the overhead vapors or dissolved in the aqueous solvent phase of the stripper distillates are utilized for the displacement of the dissolved raffinate from the aqueous Wash effluent of the raffinate Water-washing step. The aromatic extract hydrocarbons -being preferentially soluble in the aqueous phase, these aromatics effectively displace the raiiinate hydrocarbons drssolved in the aqueous Wash effluent of the raffinate water-'washing step. Accordingly when the Wash efliuent is Vsubsequently injected into the stripping column as a source lof steam therein and for the purpose of recovering the solvent in the Wash effluent, the raflinate initially present in the wash effluent is no longer present therein and contarnination of the extract product distilled from the stripping column with azeotrope-forming raffinate hydrocar- 'bofns is avoided.

The present invention and the improvements effected thereby are explained in greater detail in connection with arroyos the accompanying diagram which illustrates a typical process flow in `conformity with this invention. lt is not intended however to limit the scope of the invention to the specic solvent, specific process conditions, and particular apparatus arrangement described with reference to this drawing.

Referring to the accompanying diagram, an aromatic hydrocarbon-containing charge, such as the product of a gasoline reforming process containing benzene, toluene `and xylenes as aromatic components, s'introduced into the process flow through line 1 at a superatmospheric pressure which is sufficient to maintain the charge in substan- .tially` liquid phase at the particular temperature maintained within the extraction zone and which may be in the range of from Yabout to about 300 pounds per square inch, or higher. Depending upon the water content ofthe solvent composition, the temperature of the extraction may be varied to increase or decrease the selectivity and solubility relationships between the solvent and the hydrocarbon feed stock. Utilizing a polyallrylene glycol solvent, for example, containing `from 2 to about percent by weight of water, the process may be operated at temperatures of from about 100o to about `300" C. In order to increase the solubility of aromatics in the solvent, while maintaining the selectivityat a high level, the extraction stage of the process is preferably operated at a temperature in the range of from about 125 drocarbon feed stock at the foregoing temperature and pressure conditions is fed from line 1 at a rate controlled by valve 2 and at a pressure provided by pump 3 into `line 4 containing heater 5 which raises the temperature of the hydrocarbon feed stock to the foregoing extraction temperature. rhe feed stock at the desired temperature `and pressure is charged yinto extraction column 6 at an intermediate point below the solvent inlet (which generally is in the Lipper portion of the column) and above the rich solvent outlet in the lowermost portion of the column. Lean solvent composition from which hydrocarbon solute has been removed in the stripping stage of the process, is charged through line at the desired tempenature and pressure into the upper portion of solvent `extraction zone 6.

Solvent extraction zone d may be of any type suitable for effecting countercurrent contact between two liquid phases at least partially `but not wholly miscible in each other and wherein the relatively more dense solvent may be brought into intimate contact with the relatively less `dense` hydrocarbon phase. Thus, extraction Zone 6 may `be a packed column or may contain a series of horizontal, perforated plates through which the liquid solvent flows in dispersed form and in countercurrent flow relationship to an ascending hydrocarbon stream. The recycle reflux stream consisting in part, preferably predominantly, of a portion (usually the lightest portion) of the aromatic extract component present in the hydrocarbon charge, is `introduced into zone 6 at a point below the feed stock charge line l through line 8. This redux stream usually is made up predominantly of the stream recovered as overhead distillate from the solvent stripping-zone.

As the hydrocarbon feed stock and reflux ow upwardly through column 6 in countercurrent relationship to the descending solvent composition, the solvent selectively extracts the more soluble aromatic hydrocarbon components of the feed stool; and selectively rejects the paraflinic and other non-aromatic components which make up the raffinate. The non-extracted residue, herein referred to as raffinata` comprises predominantly paraliinic hydrocarbons substantially free of the aromatic hydrocarbons to be recovered as extract product of the process; thus f" raihnateis .withdrawn from the top of extraction zone 6 through line` 9, containing pump 10 whichidischarges `the raffinate streamthrough line 11 into raliinate washing zone 12 for recovery of the normally small but significant quantity of solvent dissolved in the effluent raffinate hydrocarbon stream. Since this `stream is customarily employed for blending into gasoline motor fuels, the presence ofthe solvent therein is detrimental toits use and must accordingly be removed therefrom prior to its use as a motor fuel. Moreover, if the solvent dissolved in the raffinate etiluent, even `through the quantity of this dissolved solvent is generally small, werenot recovered therefrom, its continuous removal from the process in the raffinate would represent a substantial `loss on a continuously operating basis, since the rainateistream doesznot reenter the process flow. Accordingly, ratiinate washing zone .12, is incorporated into the process iiow in order to reco er the solvent removed from the extraction `zone in the raffinate.

In order to effect such recovery of solvent from the raffinate on an efficient basis, however, the Vaqueous stream used to Wash the raflinate must be free of solvent and must be conned to the smallest possible quantity suflicient for the purpose, since excess water must be removed from the recovered solvent `solution in order `that it maybe utilized again in the extraction zone. `The most effective aqueous wash stream for this purpose which is recycled within the process iiow in order to maintain the total water inventory constant and in equilibrium is thatrecovered from the overhead or side-out fractions of the stripping column` in the operation according to the present process which produce aqueous condensates containing a high proportion of water suitable for washing the raffinate stream removed from the solvent extraction zone. The source of such aqueous streamsV charged into the top of column 12 through line 51 will be hereinafter more specifically described. ,i Since the aqueous wash stream is of generally greater density than the `hydrocarbon raffinate, countercurrent washing is conveniently effected by introducing the aqueous stream into the top of zone 12, usually a countercurrent contacting column, as the raffinate of lesser density is introduced into the bottom thereof. A resulting washed ratiinate from which the solvent composition has been substantially completely removed is withdrawn from column 12 through line 13 at a rate controlled by valve 14 and thereafter removed to storage or to other processing stages accompanying the present extraction process. An aqueous wash effluent containing recovered solvent is removed from the bottom of washing Zone 12 through 15 by means of pump 16 which discharges the aqueous wash eluent through line 17 containing Valve 1S into an overhead receiver vessel utilized in connection with the solvent stripping stage of the present process, hereinafter referred to in greater detail.

Referring again to the solvent extraction stage of the present process flow, as the rich solvent stream containing dissolved aromatic extract flows downwardly through extraction zone d, it countercurrently contacts the light aromatic-parafhn reflux introduced into the extraction zone through line 3 in the lower portion of the column. The

resulting rich solvent, which now contains dissolved `aroi. matic extract and reflux components and is free from raf.-`

inate hydrocarbons boilingy in the same range as the aro- Vmatic solute is removed from the bottom of extraction zone 6, through line 19 at a rate controlled by valve 2d and transferred by means of pump 2l through line 22 into the upper portion of rich solvent stripping zone 23 for recovery of the hydrocarbon solute therefrom. Stripping zone 23 generally contains a reboiler Zd, in the lower portion of the column which supplies additional heat to therich solvent residue accumulating in the lower portion of column 23 for stripping the last traces of aromatic solute from the rich solvent prior to its removal from the stripping zone.

In the preferred manner of operating the solvent extraction process, extraction Zone 6 is maintained at a pressure suthcient to provide essentially liquid phase conditions in the extractor at the particular extraction temperature, as aforesaid. Thereafter, the resulting rich solvent in liquid phase is transferred into stripping zone Z3 wherein the pressure on the rich solvent is reduced (preferably in several stages) while maintaining the liquid residue substantially at the same temperature by means of reboiler 24. The vapor overhead and side-cut fractions thereby isothermally flashed from the rich solvent are predominantly hydrocarbon, but since the water and organic solvent components of the rich solvent stream also have significant vapor pressures at the temperature of the rich solvent, water and solvent are also present in the vapors, the quantity of the latter components increasing as the boiling point of the respective fraction increases.

The overhead vapor ilashed from the rich solvent contains substantially all of the light paraffin components (e.g., C5, C6 and C7 parains) of the hydrocarbon reflux, which is present in the rich solvent by virtue of having replaced paraiins of greater molecular weight in the extraction zone. The overhead also contains a large proportion of the lightest (lowest molecular weight) aromatics (e.g., benzene) present in the feed stock Which distills overhead with the light paraiiins during the stripping operation and thus make up the hydrocarbon reflux recycled to the bottom of the extraction zone. In any event, the reduction in pressure between the extraction and Stripping zones is sufficient to vaporize completely the parailinic component of the rich solvent stream and if necessary, suflicient heat is introduced into the reboiling portion of the column to supply the heat of vaporization required to completely remove all of the paraffin soluble from the rich solvent into the vapor overhead and to effect such vaporization substantially isothermally. The resulting vapor overhead is removed from the top of the stripping zone through line Z into overhead condenser 26 which liqueiies the vaporized components for withdrawal therefrom through line 27 and valve 28 for discharge into receiver 29. The condensate formed by liquefying the overhead Vapor stream is a mixture of hydrocarbons and aqueous solvent saturated with the light aromatic component of the vapor overhead. in accordance with the present process, the primarily hydrocarbon portion of the condensate may be separated from the aqueous phase in an auxiliary receiver (not shown), if desired, and separately recycled to extraction Zone 6 as recycle reflux while the lower aqueous phase is separately charged into receiver 29 as the source of aromatic for displacement purposes therein. However, in the preferred alternative arrangement, the entire overhead condensate is allowed to enter receiver 29, to thereby provide a source of excess light aromatics therein and a hydrocarbon phase to receive the raiiinate components displaced from the aqueous raflinate Wash effluent.

The lower aqueous layer accumulating in receiver vessel 29 would normally be saturated with the most volatile aromatic component of the feed stock (such as benzene, for'example, when the feed stock is a gasoline boiling range fraction), together with a small amount of solvent vaporized with the hydrocarbon components into the light vapor overhead. ln order to recover not only the dissolved solvent and aromatic components dissolved in the aqueous portion of the condensate, but also to provide steam for strlpping purposes in rich solvent stripping zone '23, the aqueous layer would normally be withdrawn from the receiver vessel and recycled to the reboiling section of stripping zone 23, for the aforementioned purposes. In the present process, however, the aromatic hydrocarbon dissolved in the aqueous phase accomplishes an important auxiliary function comprising the essential inventive concept upon which the present improvement is based. As heretofore indicated, the aqueous wash etiiuent recovered 'from the raffinate washing zone 12 contains feed stock raiiinate (parail'ins) dissolved and/ or entrained within the aqueous washings recovered from the raffinate water washing zone l2. Since the quantity of solvent recovered by the raffinate water wash step necessitates recovery of the solvent therein and such recovery is generally effected by charging the aqueous Water wash effluent into the stripping zone as a source of steam in the latter zone for stripping solute from the rich solvent, the feed stock raffinate paraiiins contained in the aqueous wash effluent would contaminate the aromatic extract product distilled from the stripping zone if not removed from the aqueous etiluent prior to discharge into the reboiling section of rich solvent stripping zone 23. Although the aqueous wash etliuent could be separately distilled to recover the solvent therefrom, such procedure involves the necessity of providing an additional distillation column, the consumption of large quantities of heat for effecting such distillation and other uneconornical procedures which would otherwise increase the cost of recovering the aromatic extract product in a highly puried form.

In accordance with the process of the present invention, however, the raffinate para'ins present in the aqueous wash ei'lluent are displaced therefrom by preferentially soluble aromatics derived from the stripper overhead itself (which is normally recycled to the solvent extraction zone) or from the aqueous layer collected in overhead receiver 29, the aqueous layer containing dissolved light aromatic distilled from the rich solvent in stripper 23. Through such mixing of the two aqueous streams, one containing a dissolved light aromatic from the overhead receiver and the other containing dissolved radinate paratlins from the ratinate washing zone, or by mixing the water wash eifluent with stripper overhead hydrocarbons the raffinate paraiiins present in the aqueous wash effluent are displaced therefrom into the hydrocarbon phase collecting in the overhead receiver, forming thereby a lower aqueous layer in receiver 29 containing only preferentially soluble aromatic hydrocarbons, the raffinate paratlins initially present in the aqueous wash effluent being displaced into the hydrocarbon layer simultaneously accumulating in receiver vessel 29. By virtue of such displacement, the aqueous phase subsequently withdrawn from receiver 29 and recycled to rich solvent stripping zone 23 contains only the desired aromatic product as the dissolved hydrocarbon phase. When subsequently vaporized in zone 23, these aromatic hydrocarbons are recovered as part of the aromatic product of the process, not contaminated by feed stock raffinate hydrocarbons. Aromatic products exceeding nitration grade specifications (i.e., of 99+ percent purity) are thereby made feasible.

As a result of the hydrocarbon exchange which occurs in receiver Z9, the hydrocarbon layer accumulating above the aqueous phase in the receiver is a mixture of light parafrns (if present), the lightest aromatic component of the feed stock (such as benzene), both of which are distilled overhead from the rich solvent in stripping zone 23, and the feed stock raflinate parains displaced from the aqueous Wash etliuent admitted to receiver 29 from the `raffinate wash Zone. The upper hydrocarbon layer is recycled to the bottom of solvent extraction zone 6 to provide the recycle rel-nix heretofore described and for the purposes indicated, being withdrawn from receiver `section of column Z3.

product is taken off as a side-cut vapor. below theflashing section of column Z3, the hydrocarbon vessel 29 through line 30 at a rate `controlled by valve 3l. and transferred by Vmeans of pump 32, into line 8 which conveys the recycle reflux into the lower portion `of solvent extraction zone 6. The volume of light parains and light aromatics in the recycle reflux is generally suflicient to effect countercurrent washing of the rich solvent phase descending into the lowerportion of extraction zone e; but if insufficient to effect the removal of feed stock parafins (raflinate) from the rich solvent before the latter leaves the extraction zone, additional makeup paraffin reflux may be introduced into the process cycle at a rate determined by valve 34, from external source through line 33 connecting with recycle line Si?. In general, the

volume oflight paraffin and/or light aromatic present in the recycle reiluxstream is sutlicient to provide a volumetric ratio of reflux to rich solvent contacted therewith in the lower `portion of extraction Zone 6 of from about 0.1:1 to about 20:1 or more, preferably from 0.5:1 to about 5:1 volumes of reiiux per volume of rich solvent.

The feed stock raiiinate paraflins displaced from the aqueous wash effluent of the raflinate Wash tower and present in the recycle reliux stream do not hinder the rich solvent washing action of the light pararins inthe lower portion of the solvent extraction zone, since these hydrocarbons join the rising hydrocarbon stream in the solvent extraction Zone and are eventually removed from the top of the `extraction zone in admixture with feed stock raiiinate.

The aromatic-containing aqueous stream utilized as one of the alternative sources of light aromatic for displacing ratinate from the aqueous raiiinate Wash eiluent may be supplied to vessel 29 from the aqueous phase recovered as a portion of the vapor overhead or vapor side-cut from the solvent stripping zone, hereafter described. Both of these streams provide a source of light aromatic which saturates the water simultaneously distilled from the solvent composition with the light aromatic and light parpresent in receiver vessel 29 is increased. This supply of aromatic hydrocarbon-saturated aqueous stream is supplied to receiver 29 through line 27, as aforesaid.

As the rich solvent descends through the solventstripping zone, after leaving the iiash vaporization section 1n `the upper portion thereof, it contacts rising vapors of steam and hydrocarbon distilled from the rich solvent in column 23 and by virtue ofsteam generated from the recycled aqueous washings charged into the reboiling Reboilcr 2d is operated at a rate of heat input suihcient to supply the necessary heat of vaporization to complete `the distillation of all of the paratlinsfrom the rich solvent stream in the upper section ofstripping Zone 23, suliicient aromatic solute being distilled with the pararhns to ensure complete removal of the latter parafiins from the rich solvent before aromatic At some point vapor phase in equilibrium with the solvent composition is completely aromatic in composition and at such point the hydrocarbon component of the side-cut vapors withdrawn from column 23 are entirely aromatic extract. Water vapor, and vaporized solvent composition which `also has a linitevapor pressure above the rich solvent .phase comprise the remaining portion of the vapors. The

accordance with this invention, is withdrawn from receiver through line dit and valve .di as the ultimately desired aromatic product of the process, being thereafter dried and fractionated to separate therefrom, substantially pure aromatic components.

Although the solubility of aromatic hydrocarbons in water is relatively small, the aqueous layer accumulating in vessel 39 contains a significant proportion of dissolved aromatic product, generally from about `0.1 to about 2% by weight of the aqueous phase. The aromatic content is directly proportioned to the temperature of this stream. The lower aqueous phase, containing dissolved aromatic product, thus constitutes a desirable source of the aromatic-containing aqueous stream supplied to receiver vessel 29 for the purpose of displacing feed stoel; paraflins from the aqueous washings recovered from the rafinate washing Zone, thereby' supplementing the aqueous, aromatic-containing stream recovered as overhead from the rich solvent stripping zone. When desired, therefore, particularly for the ultimate recovery of `a substantially pure aromatic extract product of `the present process and in order to enhance the displacement raliinate paraflins from the aqueous phase in receiver 29, the lower aqueous layer in receiver vessel 39 is `withdrawn from the bottom of the latter vessel through line d?, at a rate controlled by valve 43 and transferred by means of pump i3 into line da which conveys the aqueous phase into receiver 29. The combinedV aqueous phases supplied to receiver 29 and from which substantially all of the feed stock paraiins have beendisplaced is now in condition for recycling to the lower portion of the stripping zone to be introduced therein as a source of stripping steam and for the recovery of solvent `contained therein. The lowerV aqueous solvent layer in receiver 29 is withdrawn therefrom through line d5 and valve d6 and discharged into line d'7 from which all or only a portion of the aqueous layer is supplied to steam generator 48 which produces `the steam for stripping purposes in i receivers which accumulate the stripper overhead and side cut Vstreams from the rich solvent stripping zone may be and are desirably utilized as the source of the water wash. The amount of aqueous stream removed from line 4;"'7 for the purpose of supplying railinate washing zone 12 with water is controlled by valve 49. and pump 5@ which discharges the amount of water required for raflinate washing purposes into water recycle line Si, feeding into the top of raffinate washing zone l2.

The lean solvent composition fromwhich the hydrocarbon solute has been stripped in the upper portion of zone 23 and which accumulates in the lower portion of the stripping Zone is withdrawn from column 23 at an elevated temperature as a result of reboiling and stripping the last traces of the hydrocarbon solute therefrom, the stream being removed through line 52 at a rate controlled by valve 53 and transferred by means of pump iid into line 5S connecting with heat exchange coil 56 in steam generator d3. Steam is required for stripping purposes in the upper portions of column 23 to reduce the solubility of aromatica in the solvent composition and thereby increase their volatility in equilibrium with the solvent composition. The relatively high temperature of the solvent residue provides an available source of sensible heat from which stripping steam may be generated for injection into the lower portion of stripping zone 23 by heat exchange with the recovered aqueous portion of .the side-cut and overhead vapor condcnsates and the aqueous if wash effluent from zone f2. The steam thus generated at an elevated temperature is discharged from steam generator 46 at a point above the liquid level therein through line 57 which conveys the hot steam into the lower portion of stripping zone 23 at a rate controlled by valve 58. The lean soivent composition from which the reboiling heat has been removed by heat exchange in steam generator 43 is recycled to extraction zone 6 at the desired extraction temperature, depending upon the extent of heat exchange in vessel 48, through line 59 connecting with lean solvent feed supply line 7, thereafter flowing into the top of extraction zone 6 for reuse therein as the lean solvent composition. In the event that additional solvent. composition, sufficient to replace losses in the system is required, such quantity, sufficient to make up for such losses, is supplied from solvent reserve through line 60 containing valve 61 and transferred by means of pump 62 into lean solvent feed line 7.

The residue of solvent composition, after distillation of sufficient water therefrom in steam generator 48 to supply the stripping steam requirements of the process in zone 23 and comprising the solvent recovered from the various aqueous condensate sources of the present process, is removed from the lower portion of steam generator 48 through line 63 at a controlled rate determined by -valve 64 and transferred under pressure by means of pump 65 through line 66 into lean solvent recycle line 59, thereby recovering the solvent present in the raffinate wash effluent and the aqueous layers of the stripper overhead and side-cut fractions for reuse in the extraction stage. The Iamount of water distilled from the aqueous solvent residue in steam generator 43 is also controlled by the quantity of water required to rehydrate the lean `solvent stream withdrawn from the bottom of the stripping zone to the water level required to meet the lean solvent composition for use as extractant in solvent extraction zone 6. Thus, if the water content of the solvent residue in column 23 is sufficient to meet the lean solvent requirements for Aextraction purposes, no additional Water need be added to the solvent recycle stream from line 66 and the water content of the solvent residue in heat exchanger 4S can be reduced to the lean solvent water level.

The process `of the present invention is further illustrated with respect to several of its specific embodiments in the following illustrative examples which, however, are not intended to limit the scope of the invention necessarily in accordance therewith.

In the following examples a series of runs is described which illustrate a selective solvent extraction process operated under various conditions, designed to demonstrate the advantages to be realized by operating the process in accordance with the improvement provided by the present invention. In each case a catalytically reformed gasoline boiling range fraction is utilized as feed stock for the recovery of a mixture of benzene, toluene and xylene, utilizing a diethylene glycol-water mixture as lean solvent composition containing `about 8% by weight of water in a countercurrent, liquid-liquid solvent extraction process effected in an extraction column comprising a series of horizontal perforated plates. The extraction stage of the process results in the formation of a rich solvent composition which is thereafter stripped in the presence of steam to recover the mixture of C6, C7 and C8 aromatic hydrocarbons. The latter extract mixture is thereafter fractionally distilled to separate individual fractions of benzene, toluene and xylene. In Run A the aqueous effluent recovered from washing Ithe raffinate stream with water is recycled to the rich solvent stripping zone without prior treatment to thereby recover the solvent contained in the aqueous wash effluent. In Run B the aqueous effluent is separately distilled to remove the hydrocarbons which are dissolved and/or entrained in the effluent, prior to recycling the distillation residue to the solvent stripping zone. Run C embodies the improvement comprising the present invention wherein the aqueous wash efiiuent from the raffinate washing step (containing raffinate paraf'lins) is mixed in the stripper overhead condensate receiver prior to recycling the resulting combined aqueous layers in the receiver to the reboiling section of the solvent stripping zone and the hydrocarbon layer in the overhead receiver to the bottom of the extraction zone as reiiux therein. In each of the following runs, that is A, B and C, the catalytic reformate is fractionally distilled to separate .a fraction having a boiling range of from about 40 to about 155 C. andthe latter cut is redistilled to remove a fraction boiling from about 40 to about 65 C. which is utilized as light parafiin reflux make-up to the extraction zone .and a feed stock fraction boiling from about 65 to about 155 C. The latter feed stock contains approximately 7.0% by weight of benzene, 25.6% by Weight of toluene and 30.6% by weight of C8 xylenes, and ethylbenzene, the remainder consisting of aliphatic paraiiin hydrocarbons in the C-Cg range. The feed stock is charged at the rate of 1635 bbl./day, at a pressure of pounds per square inch and at a temperature of 290 F. (144 C.) into the mid-section of a countercurrent solvent extraction column of the aforementioned type containing about 60 actual plates, which represent from about 15 to about 20 theoretical plates. As the feed stock is charged at the foregoing conditions into the solvent extraction zone the aqueous diethylene glycol solvent composition, recovered from the reboiling section of the solvent stripping zone, hereinafter described, is charged at a rate of 24,780 bbls./ day and at a temperature of 290 F. and at a pressure of 100 pounds per square inch into the top of the extraction column, thereafter percolating downwardly through the perforations in the horizontal plates against a rising stream of the feed stock charged into the mid-portion of the extraction column. Another stream, herein referred to as a recycle reflux enters the extraction column below the feed stock inlet and more preferably, in the bottom of the extraction column, the reiiux being a mixture of benzene and light parains containing approximately 88% by volume of benzene combined with C5 and C6 aliphatic paraiiins derived from the upper hydrocarbon layer of the stripper overhead condensate, mixed with the volatile portion of the catalytic reformate boiling from about 40 to about 65 C., referred to hereinabove as light paraffin reux make-up. The latter mixture is charged into the Ibottom of the solvent extraction column at a rate of 2102 bbls./day and at the temperature and pressure conditions maintained in the extraction zone.

The non-extracted portion of the hydrocarbon feed stock, herein referred to as rafiinate, is removed from the top of the solvent extraction zone at a rate of 766 bbls./ day and is diverted into the bottom of a countercurrent ow rainate washing zone into the top of which 172 gals/hr. of the recovered lower aqueous phase of the stripper overhead condensate is charged as extractant to remove the small quantity of solvent dissolved in the raffinate stream (approximately 1.7 bbls./day). The recovered, Washed raffinate removed from the top of the raffinate washing column contains less than 0.2% by weight of benzene, approximately 0.5% by Weight of toluene and about 6% by weight of xylene. The recovered aqueous wash effiuent is separately treated in accordance with the procedures herein designated in Runs A, B and C to provide a comparative basis of examination for the different Runs.

In the following Run A the rich solvent stream recovered from the bottom of the solvent extraction column is charged at the temperature and pressure maintained in the extraction column into a solvent stripping zone wherein the hydrocarbon solute in the rich solvent is .allowed to ash distill in the upper section of the solvent stripping column and reboiled in the lower portion of the stripping zone to recover the aromatic hydrocarbon extractl dissolved in the rich solvent. The rich solvent upon i3 entering the stripping zone is flashed by successive reductions of the ambient pressure from 100 pounds per square inch to substantially atmospheric pressure, the resulting overhead vapors being-liquetied in a water-cooled condenser and collected as overhead liquid condensate in a receiver Vessel attached to the condenser. The liquid co'ndensate separates into two phases, an upper layer cornprising a mixture of benzene and light paratiin hydrocarbons containing approximately 86% by volume of benzene and a lower aqueous layer which is predominantly water containing a small amount (about 2 percent by weight) of diethylene glycol. The upper layer is withdrawn `and mixed with the make-up light paraffin redux fraction recovered from the catalytic reformate, and charged at a rate of 1102 bbls./ day into the bottom of the extraction column as recycle reflux. The lower aqueous layer of the overhead condensate, in the amount of 242 bbls./ day, is separately recovered and reserved foruse as stripping steam in the rich solvent stripping zone.

A side-cut fraction is removed from an intermediate plate of the solvent stripping column at atmospheric pressure, liquefied in a water-cooled condenser and collected in a side-cut receiver. A lower aqueous layer is withdrawn from the receiver in ,the amount `of 387 bbls./day, combined with the aqueous phase recovered from the overhead vapor condenser and the aqueous wash eiiiuent of the ratlinate washing zone. The combined aqueous mixture containing approximately 2.3% by weight of diethylene glycol is charged at a rate of 1530 bbls./day into the reboiling section of the solvent stripping zone, the steam thereby generated rising through the stripping column and vaporizing hydrocarbon solute from the rich solvent.

`The aromatic extract (upper layer) accumulating in the sidecut receiver, amounting to 869 bbls./day, is rey served as product. Upon fractionation of the extract in a separate distillation column a benzene overhead boiling from 79.9 C. to 81 C., containing 98.2% benzene is condensed and collected. A toluene cut (98.3% by weight toluene) is separated from the residue in a second distillation column. The residue from the latter distillation, comprising xylenes .and ethylbenzene contains 94.8% by weight of C8 aromatic hydrocarbons. Infra-red analysis of the three aromatic cuts indicates that the irnpurities present in each of the cuts, although relatively small in amount, consist oi' aliphatic paraiins boiling in the same range as the desired aromatic products.

In the following Run B the aqueous wash efuent from the railinate washing zone is separately recovered and distilled in a separate column prior to mixing the distillation residue with the lower aqueous layers recovered from the overhead and side-cut condensate receiver vessels attached to the respective overhead and side-cut `vapor lines of the stripping zone. Upon distillation of the raflinate wash eluent, and overhead vapor consisting predominantly of steam, but also containing feed stock ranate hydrocarbons is recovered from the aqueous residue which contains diethylene glycol in solution. In order to free the aqueous rathnate wash eiiluent completely of paraihnic hydrocarbons, approximately 1/2 of the volume of the re- `covered aqueous wash ellluent must be distilled in the separate distillation column. Redistallation of the aqueous wash eluent requires a separate column containing about 5 theoretical plates and results in the consumption of an additional 8% of the utility requirements (heat and electrical energy) of the process in which the redistillation is omitted. The recovered aromatic products removed from the side-cut receiver of the rich solvent stripping zone, however, are substantially pure, the benzene fraction consisting of 99.99% benzene has a boiling range of from 80.0 to 80.1 C. The toluene fraction `recovered in a separated in a separate column is also of substantially greater purity (99.1% toluene).`

anvar/es 1.4i `In the following RunlC the aqueous wash eiiiuent (approximately 172 gals/hr.) recovered from the raffinate wash zone is allowed to run in the receiver attached to virtue of such mixing the feed stock paraihns dissolved in or entrained in the aqueous raffinate wash elliuent are displaced therefrom into the hydrocarbon layer accumulating in the overhead receiver vessel, and and recycled to the extraction column as recycle rellux. The lower aqueous phase collected in the overhead condensate receiver, after removing a portion (172 gals/hr.) for Y use asthe raiiinate wash water, was mixed with the lower aqueous layer accumulating in the side-cut receiver vessel and the resulting combined -aqueous streams charged at a rate of 1523 bbls./day into the reboiling section of the stripping column. The mixture of Cs-Cg aromatic hydrocarbons recovered as extract product from the stripping.

zone upon fractional distillation yielded benzene of 99.99% purity, toluene of 99.4% purity and a C8 aromatic cut of 98.5% purity. The latter purities were realized without the consumption of additional utilities in either the extraction or stripping sections of the process, compared to the process of Runs A and B, above.

Y I claim as my invention:

l. An Varomatic hydrocarbon recovery process which comprises contacting a hydrocarbon mixture containing aromatic and non-aromatic hydrocarbon components with a solvent composition which selectively dissolves the aromatic component of said mixture and which contains water and an organic solvent boiling above said aromatic component, separating a resulting rainate phase comprising said non-aromatic hydrocarbon from a rich solvent phase comprising solvent containing dissolved aromatic component, distilling said rich solvent phase, recovering 'overhead from the distilling step and aqueous condensate comprising extracted aromatic component of greatest volatility and a side-cut fraction comprising a major proportion of the extracted aromatic hydrocarbon, separating an aromatic-containing aqueous condensate from said side-cut fraction, washing said raffinate with water and separately recovering an aqueous wash effluent containing raffinate component and solvent washed from the said ratlinate, thereafter mixing said aqueous wash ellluent with at least one of said aqueous condensates to displace ralinate component in the wash eiuent with aromatic hydrocarbon contained in the aqueous condensate, recovering a substantially raffinate-free aqueous solvent phase from a hydrocarbon phase comprising rafiinate displaced from said aqueous solvent, and introducing the resulting aqueous solvent into the rich solvent during the distilling step.

2. The process of claim 1 further characterized by recovering an aqueous phase containing dissolved aromatic hydrocarbon from the overhead of said distilling step and utilizing atleast a portion of said aqueous phase as the source of water for washing ralinate.

3. The process of claim 1 further characterized in that said hydrocarbon mixture contains benzene.

4. The process of claim 3 further characterized in that said hydrocarbon mixture contains benzene, toluene and a C8 aromatic hydrocarbon.

5. The process of claim 1 further characterized in that said aqueous wash eiiluent is mixed with both said aqueous condensates.

- "15 is 6. The process of claim 1 further characterized in that References Cited by the Examiner the hydrocarbon mixture formed by mixing said aqueous UNITED STATES PATENTS Wash euent with aqueous condensate is charged into the solvent extraction zone above the point of withdrawing 2534382 12/50 Skelton et al 208-321 v 2,770,663 11/56 Grote 260-674 rich solvent from sald extractlon zone. 5 2 73 9 5 7. The process of claim '1 further characterized in 2; Zg 1:)2/ 6 Stephens 26o-674 that said aqueous wash euent is mixed with the aqueous 78 1 /59 Broughton 208-321 condensate removed overhead from the distiliing Step. 2936283 5/60 Hutchmgs 208-321 2,963,428 12/60 Lindahl A a 260-674 8. The process of claim 1 further characterized in y that said aqueous wash efuent is mixed with the aqueous 10 condensate separate from said side-cut fraction. ALPHONSO D SULLIVAN P'limmy Examme'"

Claims

1. AN AROMATIC HYDROCARBON RECOVERY PROCESS WHICH COMPRISES CONTACTING A HYDROCARBON MIXTURE CONTAINING AROMATIC AND NON-AROMATIC HYDROCARBON COMPONENTS WITH A SOLVENT COMPOSITION WHICH SELECTIVELY DISSOLVES THE AROMATIC COMPONENT OF SAID MIXTURE AND WHIICH CONTAINS WATER AND AN ORGANIC SOLVENT BOILING ABOVE SAID AROMATIC COMPONENT, SEPARATING A RESULTING RAFFINATE PHASE COMPRISING SAID NON-AROMATIC HYDROCARBON FROM A RICH SOLVENT PHASE COMPRISING SOLVENT CONTAINING DISSOLVED AROMATIC COMPONENT, DISTILLING SAID RICH SOLVENT PHASE, RECOVERING OVERHEAD FROM THE DISTILLING STEP AND AQUEOUS CONDENSATE COMPRISING EXTRACTED AROMATIC COMPONENT OF GREATEST VOLATILITY AND A SIDE-CUT FRACTION COMPRISING A MAJOR PROPORTION OF THE EXTRACTED AROMATIC HYDROGEN, SEPARATING AN AROMATIC-CONTAINING AQUEOUS CONDENSATE FROM SAID SIDE-CUT FRACTION, WASHING SAID RAFFINATE WITH WATER AND SEPARATELY RECOVERING AN AQUEOUS WASH EFFLUENT CONTAINING RAFFINATE COMPONENT AND SOLVENT WASHED FROM THE SAID RAFFINATE, THEREAFTER MIXING SAID AQUEOUS WASH EFFLUENT WITH AT LEAST ONE OF SAID AQUEOUS CONDENSATES TO DISPLACE RAFFINATE COMPONENT IN THE WASH EFFLUENT WITH AROMATIC HYDROCARBON CONTAINED IN THE AQUEOUS CONDENSATE, RECOVERING A SUBSTANTIALLY RAFFINATE-FREE AQUEOUS SOLVENT PHASE FROM A HYDROCARBON PHASE COMPRISING RAFFINATE DISPLACED FROM SAID AQUEOUS SOLVENT, AND INTRODUCING THE RESULTING AQUEOUS SOLVENT INTO THE RICH SOLVENT DURING THE DISTILLING STEP.