BE513051A - - Google Patents

Description

       

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  PROCESS FOR THE SEPARATION OF MIXTURES OF ORGANIC COMPOUNDS.



   The present invention relates to a process for the separation of a mixture of organic compounds with the aid of a selective solvent in order to separate and recover a specific organic compound or a mixture of compounds having the characteristic of having relatively greater polarity or greater solubility in the selective solvent than the other compounds present in the mixture.

   In one of its more specific applications, the process according to the present invention serves to effect the separation of a particular species of aromatic hydrocarbon, such as benzene and / or toluene, from other classes of hydrocarbons. such as those contained in a petroleum distillate, and the process makes use of a solvent which can be recirculated indefinitely, allowing the desired hydrocarbon product to be obtained in a very pure state and carrying out the process. almost complete separation of the raw material introduced into the process.



   A mixture of organic compounds is separable by the present process when it contains several compounds, or classes of compounds, of different solubility in a selective solvent which may be a single component liquid of an essentially pure primary solvent having a. higher boiling point than the extract to be produced by the process, or which may consist of two components, notably a normally liquid mixture of the primary solvent and a secondary solvent having a lower boiling point than that of the primary solvent and being substantially insoluble in the refined product to be produced by the present process but soluble in the primary solvent.

   The compound, or class of compounds, for which the selective solvent has the greatest solubility will be mentioned below under the designation "class A constituent" of the mixture which is to be separated; the remaining part of the mixture will be mentioned below under the name "constituent class B",

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 that is to say the constituent for which the selective solvent has a lower solubility than for the constituent class A.



   The present invention relates to an improvement of the type of separation process in which a mixture of organic compounds is introduced into an extraction zone at an intermediate point thereof and is brought into contact therein countercurrently with the selective solvent. , a refined product phase containing substantially all of the class B component is removed from that of the ends of the extraction zone where the solvent is introduced into said zone, an "extract" phase comprising the class A component dissolved in it. the selective solvent is removed at the other end of the extraction zone and the extracted class A component is then recovered by fractional distillation of the "extracted" base.



   According to the present invention, the extracted phase is substantially freed, before its removal from the extraction zone, of the residual class B component of the mixture fed into the extraction zone by introducing a class B compound which is more volatile than the component. class B of the mixture in the extraction zone at a point closer to the point where the "extracted" phase is removed than to the point of admission of said mixture, and by displacing the residual class B constituent by the compound class B more volatile within the extraction zone in the refined product phase, and separating the more volatile class B compound contained in the resulting extracted base by fractional distillation thereof after it has been removed of the extraction zone.

   Generally, the volatile class B compound thus employed according to the present invention has a lower solubility and a greater volatility than the class A component which is soluble in a preferential manner in the selective solvent.



   According to a preferred embodiment of the present process, the extracted phase in which the residual class B component has been appreciably displaced by the more volatile class B compound, is introduced from the extraction zone in the upper part of the chamber. a fractional distillation zone, substantially all of the volatile class B compound is evaporated from the extracted phase in the distillation zone and is removed as a vapor stream from the upper part of this zone and returned to the extraction zone at a point near the point where the extracted phase is removed from the zone,

   the class A component substantially freed from both the class B component and the more volatile class B component and the selective solvent is removed as a side stream from the distillation zone and the residue of the distillation comprising the solvent is removed from the lower part of the distillation area and returned to the extraction area to be used again.



   When the extraction is carried out with a selective solvent consisting of a two-component mixture of primary and secondary solvents of the type mentioned above, and the secondary solvent has the property of distilling at least in part with the compound class B volatile but is practically immiscible therewith, a more specific and particularly advantageous embodiment of the present invention comprises the condensation of the vapor stream leaving the upper part of the fractional distillation zone and its separation in a first liquid distillate containing the volatile class B compound and in a second liquid distillate consisting essentially of the secondary solvent,

   the return of the first liquid distillate to the extraction zone at a point located close to the point where the phase extracted from this zone is removed, the return of the distillation residue which contains the primary solvent at a higher concentration that the concentration of the primary solvent in the selective solvent used in the extraction zone, from the lower part of the fractional distillation zone to the extraction zone,

   and returning the primary solvent entrained in the refined product phase obtained in the extraction zone to the extraction process by first contacting a stream of said second countercurrent distillate with the refined product phase in the circuit thereof beyond the point of introduction of the distillation residue into the extraction zone and then mixing this stream of second distillate with the distillation residue within the

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 extraction zone to thereby form the selective two-component solvent.



  In addition, when the secondary solvent has the property of distilling at least in part with the class A component and is substantially immiscible therewith, it is desirable and advantageous that a side stream comprising the class A component extracted. and a part of the secondary solvent introduced into the fractional distillation zone are removed from this zone and subsequently separated into a liquid distillate consisting essentially of constituting class A or product extracted from the process and a second liquid distillate consisting essentially of the secondary solvent, a stream of this second distillate then being introduced to the lower part of the distillation zone to remove the residual Class A component from the distillation residue therein.



   In its application to the separation of mixtures of hydrocarbons, the invention is particularly well suited for separating and recovering aromatic hydrocarbons from a mixture of hydrocarbons containing aromatic and non-aromatic hydrocarbons. A preferred embodiment of such an application of the invention comprises extracting the mixture of hydrocarbons in a countercurrent liquid phase extraction zone with a selective solvent having particularly high selective solubility. for the aromatic component and which consists of an aqueous solution of oxy-diethylene glycol containing from 5 to 15% by weight of water (secondary solvent), the component glycol diethylene (primary solvent)

   of this selective solvent being introduced at the upper part of the extraction zone to form a solvent-extract phase flowing in a downward direction, the mixture of hydrocarbons being introduced in an intermediate part of the extraction zone. tion at a temperature of 100 to 1500C. at about and at supra-atmospheric pressure sufficient to maintain a substantially liquid phase in the extraction zone and to form a refined product phase comprising the non-aromatic hydrocarbon component (class B component), and a phase of liquid extract comprising essentially solvent and the aromatic hydrocarbon component (class A component), the introduction of a non-aromatic hydrocarbon fraction more volatile than the non-aromatic component of the mixture of hydrocarbons,

   to the lower part of the extraction column in an amount sufficient to displace the non-aromatic hydrocarbons which form azeotropes with the aromatic hydrocarbon component of the mixture, removing the refined product phase from the upper part of the mixture. the extraction column and the extract phase from the lower part of the column, the introduction of the extract phase at the temperature mentioned above and at atmospheric pressure in the upper part of a zone of fractional distillation, the separation of a vapor fraction comprising the volatile non-aromatic hydrocarbons dissolved in the extract phase, the introduction of the resulting vapor fraction at the lower part of the zone of extraction to mix with the solvent-extract stream flowing in a downward direction,

   the separation of a higher boiling fraction in the fractional distillation zone and consisting essentially of water and said aromatic hydrocarbon component, the separate recovery of the water and the aromatic hydrocarbon component and the introduction water in the extraction column in contact with the refined product phase effluent above the point at which the diethylene glycol is introduced into this column to thereby form a selective solvent stream flowing in a downward direction. countercurrent contact with the phase of the refined product.



   The process according to the present invention is characterized more particularly by its economy and by the great flexibility of operation to give either a product of high purity at a relatively low continuous working rate or a higher production rate of a separate product of lower purity. The particular economy of operation achieved by the preferred separation method of the present invention is the result of the work of both the extraction column and the extractive distillation as well as the solvent removal column. 'an isothermal way.

   Except for the small amount of heat required to vaporize the residual volatile materials.

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 By dissolving in the extract phase during removal and recovery of the product, the process can be carried out in a substantially adiabatic manner, which is a particularly important factor for the commercial application of the process.

   The great industrial demand for high purity raw materials in the synthesis of products, such as, for example, "nitration" toluene with a purity of 98 to 99 +% or nearly pure benzene has been satisfied by the company. The present separation process which provides a special recirculation technique whereby the process is made more selective for the recovery of preferred hydrocarbon species as a product.



   Mixtures of organic compounds suitable as raw materials for the present separation process contain at least one component which is relatively more polar than other components of the mixture. Thus, phenol and / or thiophene containing the polar groups -OH and -SH respectively can be separated from hydrocarbons such as benzene. Benzene is relatively more polar than a paraffinic or naphthenic hydrocarbon and can be extracted. itself from its mixtures with these hydrocarbons. Similarly, the mercaptans and alkyl sulphides which contain the relatively polar sulfhydryl (-SH) can be separated from the hydrocarbon moieties.

   Compounds containing the non-carboxylic, mono-nitro, mono-amino, mono-sulfo, mono-hydroxyl, and other electronegative groups can be recovered from hydrocarbons or from symmetrical bi-substituted compounds in which there is no There is no electromeric displacement due to an equilibrium of electric forces within the molecule. The presence of unsaturated bonds between carbon atoms or between carbon and oxygen or other elements results in polarization, unless the electromeric displacement is balanced by a tendency to displacement in the opposite direction.

   Thus, mono-olefins, and more particularly cyclo-olefins, can be separated from paraffins; aromatic hydrocarbons are separable from olefins; and polycyclic aromatic hydrocarbons are separable from mononuclear aromatic hydrocarbons. In each case specified in this specification, the component which is separable from the remaining components in the feed mixture is referred to as a compound of relatively greater miscibility with the selective solvent or having greater solubility in the selected solvent. elective, while the unextracted residue comprises elements of less polarity and therefore less solubility in the selective solvent than the extracted compound.

   In the case of mixtures of hydrocarbons comprising constituents of different structural classes, the solubility of the constituents in the selective solvents present decreases in the following general order: aromatic, cyclo-olefinic, naphthenic, chain olefinic compounds. - side chain olefins containing less than 8 carbon atoms per molecule, aliphatic olefins, including side chain olefins containing more than 7 carbon atoms per molecule, and aliphatic paraffinic hydrocarbons. Thus, any element or individual class at the front part of the series can generally be separated from any element or class which follows in the series, using the selective solvent and the extraction method according to the invention.

   In any individual class of hydrocarbons, the solubility in the solvent generally decreases with an increase in the molecular weight of the compound, except that polycyclic aromatics are generally more soluble in the solvent than a hydrocarbon of the series. benzene, the solubility of the substituted mono- and polyalkyl aromatics being less than that of the unsubstituted analogs. The raw material in the present separation process may also consist of a mixture of one or more classes of hydrocarbon structures, the component to be separated having preferential or selective miscibility in the solvent over the other components of the solvent. mixed.

   A general characteristic of the present selective solvents is that they are generally more miscible with so-called "unsaturated" hydrocarbons, including both aromatic and olefinic unsaturated types, although in the case of olefinic hydrocarbons the relatively high molecular weight straight chains of such type containing more than about 7 carbon atoms per molecule are in a ca-

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 characteristic more paraffinic in their property concerning their solubility in the selective solvent present than olefinic.

   Lower molecular weight olefinic hydrocarbons containing less than about 8 carbon atoms per molecule, side chains, and cycoolefins on the other hand are typically unsaturated hydrocarbons and are selectively extracted by present solvent of aliphatic hydrocarbons contained in the raw material.



   Typical mixtures of hydrocarbons which may be used as a raw material in the present process include catalytically cracked naphtha distillate fractions, specific boiling point fractions of natural or "straight run" petroleum distillate. , and more particularly certain reformed or hydroformed naphtha which are relatively richer in aromatic hydrocarbons and are particularly suitable as a source of extracted benzene and toluene.

   It should be emphasized that whatever process is applicable to the simultaneous extraction of more than one type or class of hydrocarbon from raw materials containing more than one species of hydrocarbons which are extracted from it. In a relatively easier way, for example aromatic olefins and cycloolefins, the process is particularly suited to the individual extraction of a single type of hydrocarbon, such as the aromatic component of a hydrocarbon. mixture of hydrocarbons, adjusting the operating conditions to be most advantageous for the extraction of the particular class of hydrocarbon desired.



   One of the most important and useful applications of the present method of separation, providing the means of treating a mixture of constituents which are difficult to separate into its individual constituents by ordinary methods of separation, for example by fractional distillation. , is the separation of an azeotropic mixture of hydrocarbons such as a C6 fraction of a petroleum distillate containing isomers of benzene, hexane and heptane or a toluene-heptane-octane mixture. These azeotropes boil at temperatures between very large limits and contain varying proportions of the aromatic hydrocarbon component.

   By means of the present separation method, an aromatic concentrate representing substantially all of the aromatic constituents of the initial mixture of hydrocarbons and of a purity exceeding 95% can be recovered as the primary product of the present separation process, although one can. separating a product of such purity by other means such as fractional distillation but only at high cost or by complicated separation processes.

   Although a small amount of refined product hydrocarbons in admixture with the desired extracted hydrocarbon to be recovered from the raw material generally accompanies the extract, its proportion in the final product can be controlled to maintain it at a low level. low value, and when the process is carried out according to a preferred embodiment, the proportion of contaminant in the product recovered can be made practically negligible and thus obtain a product with a purity very close to 100%. of a specific hydrocarbon, for example an aromatic hydrocarbon.



   The selective solvent used in the separation process according to the invention may consist of a liquid having a single component of an essentially pure primary solvent or else of a normally liquid mixture of two components, one of the components, referred to as the solvent. primary, having selective miscibility with an organic compound of the raw material mixture which is soluble in a preferential manner in the solvent, and the other component, called secondary solvent, having a lower boiling point to that of the primary solvent and being substantially immiscible with the refined product phase produced in the extraction process but miscible in all proportions with the primary solvent.

   Although the presence of the secondary solvent in the selective mixture used in the present process has a tendency to reduce the dissolving capacity of the selective solvent for the extracted compounds which it is desired to recover from the raw material and therefore has a tendency to reduce since the volume of raw material can be extracted into a column of given dimensions, the secondary solvent has the effect of increasing the selectivity of the solvent mixture by reducing the solu-

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 The reliability of the refined product compounds in the extract phase during extraction, more particularly when it is desired that the end product be a single individual compound as free of refined product contamination as possible.

   By keeping the ratio of solvent to raw material fed into the extraction phase of the process at a high level according to a preferred mode of operation, a relatively high yield can be obtained and the desired objective of recovering a product which is highly concentrated in one component and virtually free of other contaminants.



   The presence of the secondary solvent with a lower boiling point than that of the primary solvent in the selective solvent mixture allows the extractive distillation or solvent removal phase to be carried out at a lower temperature in a corresponding manner. in its absence and makes it possible to recover all, or practically all of the compounds which are dissolved in the extract phase by an evaporation operation. Thus, in the extraction of an aromatic hydrocarbon solvent formed from a single component, an extract phase is formed containing the dissolved aromatic hydrocarbon.

   During the subsequent extractive distillation, although a fairly large proportion of the aromatic hydrocarbon can normally be evaporated from the extract phase and removed from the distillation column as a side distillate fraction, the solvent residue which accumulates in the re-bubbling section of the still column generally contains a small but definite amount of retained aromatic hydrocarbon.

   The incorporation of the lower boiling point secondary solvent into the solvent mixture which is preferred to be employed as a selective solvent in the process according to the invention increases the vapor pressure of the aromatic component in the aromatic phase. extracts and allows the operation of the extractive distillation column or solvent removal device at a lower temperature corresponding to the boiling point of the secondary solvent in order to effect complete evaporation and removal of the solvent. aromatic component out of the solvent residue, thereby avoiding the excessive temperatures normally required to evaporate the dissolved aromatic component from the extract residue in the re-boiling section of the solvent removal device.



   A particularly desirable arrangement of the solvent removal unit of the present invention is to remove the secondary solvent as a bypass from the solvent removal unit and to recirculate it continuously as a stream. to the re-boiling section where it will reduce the partial pressure of the aromatic hydrocarbon dissolved in the extract phase, thereby continuously and completely removing the aromatic hydrocarbon from the extract phase to a temperature below the decomposition temperature of the primary solvent.



   Compounds suitable for use as a primary constituent of the selective solvent are selected from the general group characterized generally as oxygen-containing organic compounds selectively miscible with compounds having the greatest polarity of the mixture of matter. first and with hydrocarbons at the front of the series described above. Particularly advantageous primary solvents are aliphatic and cyclic alcohols, glycols and glycol ethers (also referred to as polyalkylene glycols) as well as glycol esters and glycol ether esters.

   The alkylene glycols and polyoxy-polyalkylene glycols which are a particularly effective class of selective solvents include bi-, tri-, and tetra-oxy-ethylene glycols, more particularly oxy-diethylene glycol, mono-glycols. , di-, and tri-oxy-propylene and the glycols mono-, di-, and tri-oxy-butylene;

   certain glycol ethers, such as the cellosolve series of compounds (defined structurally as the alkyl ethers of ethylene glycol), including methyl-, ethyl-, propyl, and butylcellosolve; carbitols (defined as structural formula as being the alkyl ethers of diethylene glycol), such as methyl-, ethyl-propyl-, and butyl-carbitols; glycol and polyoxyalkylene glycol esters of low molecular weight organic acids such as acetates and propionates; aliphatic alcohols, such as propanol, isopropanol, n-butanol, tert-butanol, etc. ; certain cyclic alcohols, such

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 as cyclopentanol, cyclohexanol, cycloheptanol, etc .;

   and other organic oxygen-containing compounds such as phenol, resorcinol, pyrocatechol, etc .; various alkylated phenols, such as ortho-, meta-, and para- cresols, thynol, etc .; esters of organic acids, and more particularly fatty acid esters of aliphatic alcohols and more particularly esters of organic acids of relatively low molecular weight, such as acetates, propionates, butyrates and valarates, and other solvents of the general class described above and which are generally known in the art.

   Preferred polyoxyalkylene glycols have the following empirical formulas:
HO (CnH2nO) xH, where n has a value of 2 to 5 inclusive and x has a value of 2 to 15 when n is equal to 2, from 2 to 13 when n is equal to 3, from 2 to 12 when n is equal to 4 and from 2 to 10 when n is equal to 5. The primary constituent of the present selective solvent mixture is a compound which necessarily when used in the present solvent extraction process must have a boiling point. relatively high, exceeding at least the boiling point of the extracted compound, and -) reference above approximately 150 ° C..



   The secondary solvent which can optionally be used in admixture with the primary solvent to reduce the miscibility of the latter with the refined compounds present in the initial raw material thereby increasing the selectivity of the solvent mixture for the extracted component recovered in first in the present process, is a compound which is preferably substantially immiscible with, or insoluble in the constituents of the raw material.

   Its insolubility in the raw material and its solubility in the primary solvent allow the secondary solvent to be put back into the circuit at the top of the extraction column and to be mixed there with the phase of the refined product effluent to remove the normally small amount of primary solvent which tends to dissolve in the refined product as the latter flows countercurrently through the solvent in the liquid phase extraction column.

   In general, secondary solvents have this common characteristic of having a sufficient number of polar groups per molecule and a sufficiently high ratio of polar groups to methylene or methylidene groups per molecule to render the compound only partially miscible or preferably substantially non-miscible. - miscible with the refined part of the raw material.

   Normally the liquid compounds having the above properties and preferably having a boiling point substantially lower than the boiling point of the primary solvent are such substances as water, furan, furfuraldehyde. furfuryl alcohol, cyanides and low molecular weight alkylated nitriles, such as ethyl isocyanide, perfluoro hydrocarbons, such as perfluorohydrocarbons, such as perfluoropentane, perhalo- substituted mixed hydrocarbons, acids, alcohols, and the like, such as 1,2-bi-chloro-1,2-tetrafluoroethane, perfluoroacetic acid and the like. One of the preferred secondary solvents used in admixture with the primary solvents is water, present in the mixed selective solvents in amounts sufficient to provide mixtures containing from 5 to about 75% water.

   In admixture with the oxy-diethylene glycol, the proportion of water is from 5 to 35% approximately and preferably from 5 to 15% approximately by weight of the mixture. For the oxypolypropylene glycols, the proportion of water in the mixed selective solvent is from 8 to 50% approximately, and preferably from 10 to 40% approximately by weight of the mixture. As the chain length of the alkylene group in the oxy-polyalkylene glycols increases, their selectivity decreases and the hydrocarbons as well as other organic compounds become more soluble in the solvent.

   It thus becomes essential to increase the proportion of secondary solvent, such as water, in the solvent mixture (up to about 75% by weight of the mixture) in order to increase the selectivity of the solvent as the primary solvent becomes less. selective.



   Further embodiments of the invention and a correlation of the principles on which the present separation process is based are shown in the accompanying schematic drawing which shows the typical circuit.

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 according to a preferred embodiment of the invention for the separation of a mixture of class A and class B organic compounds, this circuit being for reasons of simplicity described with reference to an oil fraction of relatively high concentration of aromatic hydrocarbons , such as the product of a naphtha hydroforming process.

   The diagram is described on the basis of the recovery of a product comprising a benzene concentrate containing at least 95% benzene, and for this purpose a particularly suitable solvent is a mixture of water and oxygenated glycol. diethylene preferably containing from about 5 to 15% by weight of water. When the desired product in the process is a specific aromatic hydrocarbon to be recovered in substantially pure form, an appropriate fraction of the initial petroleum distillate which boils between limits such as hydrocarbons is selected. - aromatic bides with boiling points higher and lower than that of the desired product, are eliminated.

   Thus for benzene, the petroleum distillate, normally liquid, is a fraction with a boiling point below 110 ° C., that is to say a temperature a little lower than the boiling point of toluene. , and preferably at the upper limit of the boiling temperature of the azeotropes of benzene in the fraction, which in the case of a hydroformed petroleum fraction containing paraffin homologues in admixture with benzene, is about 79 to 80. It is very advantageous to use a debutanized raw material, and preferably a derivatization of the gasoline fraction having a boiling point of about 40 ° -80 ° C..



  When the object of the process is to remove all of the aromatic constituents from a given liquid fraction of hydrocarbons, such as a hydroformed gasoline distillate, or to remove the sulfur-containing constituents from a fraction of p- trole for example, we can introduce the whole fraction in the process if desired.



   Referring to the attached circuit diagram, a petroleum distillate fraction containing benzene with a boiling point of about 40-80 ° C. is introduced into a fractionation column 1 through line 2, and the Raw material is roughly fractionated in this column into a vapor which leaves the top of column 1 as a light fraction consisting mainly of C5 hydrocarbons, such as the isomers of pentane and cyclopentane. The vapor is removed from the column through line 3, is condensed, and is discharged from the process for storage or for other use.

   An intermediate fraction consisting of azeotropes of pentane with side chain hexanes as well as the corresponding C5 and C6 olefins, if any, and naphthene is removed from column 1 through line 4 which may be connected to a heater and bypass fractionator, not shown, in order to separate a more selected C6 paraffin and naphthene fraction, if desired. The intermediate bypass removed from column 1 through line 4 is preferably directed into the subsequent stages of the process as described below, but it can also be returned to the column as reflux or removed from the circuit in part. or entirely.

   A "bottoms" fraction consisting of benzene, benzene azeotropes, and higher boiling paraffinic, naphthenic, and olefinic hydrocarbons, if any, is removed from fractionation column 1 through line 5 and is introduced as a raw material (containing benzene) in the solvent extraction process according to the invention. The raw material sent into the circuit through line 5 passes through a heat exchanger 6 containing a heating coil of sufficient capacity to increase the temperature of the raw material to a level of 100 to 150 ° C. approximately, preferably from 125 to 140 C approximately.

   The fraction is maintained in the liquid phase at the above temperature by compression at a supra-atmospheric pressure, the fraction being pumped from the heater 6 through line 7, by means of the pump 8 and being compressed to a pressure. preferably about 5 to 25 atmospheres, but higher pressures can also be employed without affecting the operation of the process.



   The liquid raw material at the pressure indicated above is transferred through line 9 to the liquid solvent extraction column

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 10 and preferably is introduced therein at an intermediate point between the upper part and the lower part of the column, at a point between the extract and effluent outlet ports of refined products respectively in order to provide the contact between the countercurrent circuit of the volatile paraffin and the extract, as described below.

   The mixture of benzene and other hydrocarbons introduced as a raw material into the solvent extraction column 10 is contacted in countercurrently in the column with an aqueous solution of oxy-diethylene glycol containing about 5 to 15%. by weight of water, the glycol being introduced into the column through line 11 at approximately the top of the column.

   The extraction column 10 is a suitable arrangement of liquid-liquid extraction installation of the usual type intended to bring into intimate contact and to mix two liquids which are at least partially immiscible with each other, by a flow at counter-current in the column and the latter can be constituted by a filling tower containing a filling with a large contact surface, for example Berl saddles or the like, or it can be an extraction column with bubbling plates of well-known construction and manufacture.



   The raw material hydrocarbons have at their point of entry into the extraction column 10 a density much less than that of the aqueous solution of oxy-diethylene glycol admitted to the top of the column. 10 and therefore has a tendency to percolate upwardly through the higher specific gravity solvent-extract stream flowing in a downward direction when the two substantially immiscible liquids are introduced at the specified points of column 10.

   Because of the selective solubility of the benzene component relative to the other hydrocarbons of the raw material in the aqueous solution of glycol diethylene, benzene has a tendency to dissolve in the solvent phase, leaving a phase consisting substantially of water. a hydrocarbon, with a relatively low benzene content and consisting mainly of paraffins present in the raw material.

   Although the benzene component is preferably soluble in the solvent and the latter contains enough water to selectively reject the paraffinic, naphthenic and olefinic components (if any) present in the raw material, a low amount of the latter hydrocarbons also tends to dissolve in the selective solvent, and the resulting extract comprising predominantly solvent, benzene, and a small amount of paraffinic, naphthenic, or olefinic hydrocarbons. Depending on the initial composition of the raw material, descends by gravity in column 10 against a stream of refined type hydrocarbon towards the lower part of column 10.

   Because of the selective solubility of the benzene component in the selective solvent, the benzene remaining in the upward circulating hydrocarbon phase has a tendency to pass into the solvent and displace the dissolved paraffinic and other refined hydrocarbons therein. in the solvent. The refined product therefore becomes progressively richer in aliphatic paraffins and olefins and becomes relatively depleted of benzene and other hydrocarbons which tend to dissolve in the extractant as the refined product s Rises in the column in countercurrent contact with the aqueous glycol extract.



   The refined hydrocarbon product comprising hydrocarbons substantially insoluble in the solvent also contains a smaller amount of dissolved solvent, which although small in the absolute sense, represents an appreciable loss in large-scale operations if it is continually. removed from the circuit and if it is not recovered from the refined product effluent.

   In order to effect this recovery, the water required in the system to hydrate the primary solvent to its selective composition, in an amount sufficient to give a selective solvent containing from 5 to 35% by weight of water, and preferably of 5 to 15% by weight of water is introduced into the extraction column at a point above the entry point of the dehydrated solvent, such as the refined product phase rising in the , column is washed with water just before removal of the refined product from the system and beyond the point at which the refined product last comes into contact

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 with the solvent introduced into the system.

   The water thus introduced into the circuit rapidly dissolves the diethylene glycol present in the refined hydrocarbons and thus recovers the primary solvent which will be used in the extraction treatment.



   The water introduced for this purpose in the circuit is fed into the column 10 through the line 12 containing a valve 13, unites with the dehydrated solvent glycol diethylene introduced through the line 11 and mixes with it to form the selective solvent according to the process. Preferred means of introducing water into column 10 consist of a spray head such as 14 which distributes the stream of water in the form of finely divided droplets in the refined product to which the water mixes in an intimate way. The water-washed refined product from which the primary solvent has been removed substantially completely is removed from the extraction column 10 through line 15 and is removed from the circuit to be sent to storage, or to storage. other uses.



   The primary solvent (oxy-diethylene glycol) enters extraction column 10 in substantially anhydrous form as the residue of high boiling "botoms" from the solvent removal device, which will be removed. described in detail below, and dissolves a sufficient quantity of the water introduced to the upper part of the extraction column to increase its water content to the critical content essential for efficient selective extraction of the extraction column. benzene component of the raw material from about 5 to 35% by weight, and preferably from about 5 to 15% by weight, of the selective solvent. The water content of the solvent is maintained at this level by controlling the rate of recirculation of the water introduced into the upper part of the column, the amount being practically constant in a given separation system.



   The rate of flow of the selective solvent in the extraction column relative to the rate of feed of the raw material depends on a number of other factors existing in the system and on the desired purity of the product. final. Thus, a raw material containing a relatively low concentration of the component to be recovered, such as benzene, will allow a lower solvent loading rate, although this rate can be maintained at a high value even when the raw material has. a low concentration of the desired component in order to recover substantially all of this component from the raw material or to recover it in a very high state of purity.

   The loading rate of the raw material can also be maintained at a lower value when the selectivity of the solvent is increased by the incorporation of a greater proportion of secondary solvent, as in the case of water in a solvent. aqueous solution of oxy-diethylene glycol. In general, the ratio of the selective solvent to the raw material fed to the solvent extraction column can vary from about 0.5 to 1 up to about 10 to 1 proportions by volume depending on the variable factors mentioned above.



   At the bottom of the extraction column, below the point where the raw material enters the column, the solvent or phase extracted becomes progressively richer in benzene as it advances in a downward direction and progressively more poor in paraffinic and naphthenic constituents of low volatility which are dissolved in small quantities by the solvent of the hydrocarbons supplied. Although these constituents of paraffinic, naphthenic and olefinic hydrocarbons are dissolved with benzene in the solvent or in the phase extracted in small quantities relative to the total volume of extract,

   their amount when based on the volume of benzene in the extract is sufficient to contaminate the recovered benzene product and form a less desirable and less economically valuable product. In addition, since these contaminants have boiling points between limits corresponding to the limits of the boiling points of benzene azeotropes during the solvent removal operation, they are normally inseparable from the product by simple fractional distillation. .



  According to one of the characteristic features of the present working method, a more volatile paraffinic fraction is brought into contact against the current with

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 the extract which descends present in the lower part of the extraction column 10 and is introduced there preferably at a point very close to the exit lumen of the extract, generally at the lowest part of the extract. column.

   The volatile paraffinic fraction can be derived from the initial petroleum product subjected to fractionation in column 1, as a lower boiling point distillate fraction than that of the hydrocarbon constituents capable of forming azeotropes with benzene. , for example the intermediate fraction removed from column 1 by line 4. Alternatively, the paraffinic fraction can be constituted by volatile paraffinic hydrocarbons circulating continuously in the system by the fact that they are dissolved in the phase extracted in the lower part of the extraction column 10, are distilled off from the phase extracted in the solvent removal column 21, and are switched back on in the extraction column 10, as described below.

   Another alternative operation involves combining the light paraffin recirculation fraction from the solvent removal column 21 with the light paraffin laterally extracted from column 1, the combined light paraffin hydrocarbons being fed to the column 1. bottoms of extraction column 10. In all of these alternatives, the volatile paraffin fraction displaces from the phase extracted in extraction column 10 the heavier or less volatile paraffins, naphthenes and olefins which form azeotropes.

   This embodiment of the present invention constitutes an appreciable improvement over known methods of circulation; contact of the countercurrently flowing extract with the light paraffinic hydrocarbon fraction establishes a desirable liquid phase transfer of the light paraffins in the extract as well as a simultaneous displacement of the less volatile paraffins, formative of azeotropes from the extract in the refined product phase.

   Thus, the heavy paraffin hydrocarbons which inevitably dissolve in the solvent in a limited but appreciable amount during the solvent extraction operation and which normally reappear as a contaminant (generally in amounts of about 5 to 15% by weight). ) of the benzene product as a result of their inherent tendency to form azeotropes, are moved from the extracted phase in the extraction zone to the refined product phase; light paraffins are easily separable by distillation from the benzene product in the solvent removal phase, since their boiling points are sufficiently below the boiling point of benzene so that they do not form azeotropes with the benzene constituent of the extract.

   The final benzene product recovered from the process may therefore, by the very simple expedient given above, be substantially pure benzene and may contain up to 99 +% of the desired aromatic depending on the rate of recirculation of the stream. volatile paraffinic in the extraction column 10. The non-aromatic, less volatile constituents, such as azeotropic-forming paraffins, displaced from the extracted phase are removed from the circuit by inclusion in the refined product effluent which is permanently removed from the circuit through -channel 15.



   It is obvious that the greater the stream of volatile paraffin at the bottom of column 10, and the greater the proportion of said paraffins relative to the less volatile paraffinic hydrocarbons, the greater the purity of the benzene product finally recovered by the process. .



  Thus, for example, when the extracted phase contains hydrocarbons in the proportion of 70 mole percent of benzene and 30 mole percent of paraffinic hydrocarbons of low volatility at the point of entry of the hydrocarbons forming raw material into the column of extraction and when it is desired to obtain a 90% pure benzene product, 14 theoretical plates will be required below the entry point of the raw material when the volume of the more volatile paraffinic hydrocarbons introduced at the bottom of the extraction column and brought into contact with the extract is equal to the volume of hydrocarbons in the extracted phase.

   On the other hand, the number of theoretical plates required to produce a final benzene product containing 98% benzene is reduced to 4 when two volume proportions of more volatile paraffinic hydrocarbons are used per one volume of. hydrocarbons dissolved in the extracted phase. The number of theoretical plates required is further reduced

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 Ge and the purity of the recovered benzene product is further increased if the volume of volatile parffin is further increased in countercurrent contact with the phase extracted in the part of the extraction column which is below the point of. introduction of the hydrocarbons forming the raw material.



   Referring again to the attached schematic diagram of the circuit, the more volatile paraffinic fraction recovered as a side bypass of fractionation column 1 and having a boiling point below about 80 ° C. when the process is used for recovering benzene as the final product, may contain various isomers of hexane, such as 2,2-dimethyl-butane, 2,3-dimethyl-butane, 2-methylpentane, 3-methyl-pentane, naphthenes such as cyclopentane, methylcyclobutane, and can additionally contain various olefinic isomers with boiling points of 40 to 65 C. approximately.

   The by-pass fraction of more volatile paraffinic products removed through line 4 passes into heater 16 and compressor 17 to increase the temperature and pressure of this paraffinic fraction, final up to the extraction conditions existing in the extraction column 10, that is in general at temperatures of approximately 90 to 250 C. (and preferably of approximately 100 to 150 C.) and at supra-atmospheric pressures of approximately 5 to 35 atmospheres and is then transferred through line 18 to the lower part of extraction column 10 where the volatile paraffins are brought into countercurrent contact with the descending extract before it is removed from column 10 through line 19.

   The extracted phase consisting in the first place of aqueous oxy-diethylene solvent which contains the benzene in the dissolved state and a small amount, relative to the amount of benzene contained in the extracted phase, of volatile paraffinic hydrocarbons which are present because they have displaced the less volatile paraffins, azeotropic-forming paraffins and other non-aromatic hydrocarbons from the raw material, is transferred through line 19 containing valve 20 to the top of the removal column. solvent 21.

   This column is preferably constructed to operate at substantially the same temperature as the solvent extraction column 10, but at a somewhat lower pressure sufficient to evaporate the constituents of the extract which are more volatile than the solvent. glycol diethylene, and preferably at a pressure substantially equal to atmospheric pressure. The light vapors separated by flash-distillation from the liquid extract introduced at the upper part of the stripping column 21 are removed through the vapor line 22. The vapors essentially comprise the volatile paraffinic hydrocarbons introduced into the vapor line. the extraction column 10 via line 4 and dissolved in the extract phase as described above.

   In the preferred method of operating column 21, the solvent is recirculated through line 23 and valve 24 at the top of column 21 above the point at which the extracted phase is introduced into the column, but even in the absence of such recirculation of the solvent, the presence of the solvent in the phase withdrawn from line 19 decreases the volatility of the dissolved aromatic component to that of the light paraffinic component which is also present in the liquid. extracts and thus minimizes the proportion of aromatic hydrocarbon having a tendency to evaporate in the vapor fraction subjected to flash-distillation of the solvent and entering line 22.



   The vapor stream removed from column 21 through line 22 may be partially cooled in heat exchanger 25 in order to condense the water vapor present in this stream. The partially cooled stream is introduced through line 26 into side bypass fractionator 27 where the paraffinic hydrocarbon fraction vaporizes from a liquid condensate consisting primarily of water. Steam is removed from fractionator 27 through line 28. Aqueous condensate is removed from fractionator 27 through line 29 which unites with line 30.

   In an alternate arrangement, the vapor flash distilled off from column 21 through line 22 can condense entirely in heat exchanger 25 and the resulting condensate can flow into a vessel.

 <Desc / Clms Page number 13>

 sedimentation not shown, in which the liquefied hydrocarbon layer containing the volatile paraffin is separated by decantation from the aqueous condensate and withdrawn from the sedimentator via line 28.

   The vapor removed from fractionator 27 comprises paraffinic hydrocarbon more volatile than benzene in the extract and is preferably returned to extraction column 10 through line 28, containing a compressor 31 which liquefies. the volatile paraffinic fraction and increases the pressure of the hydrocarbons up to the working pressure in column 10. The fraction leaving the compressor 31 via line 32 in line 4 is mixed with the paraffin bypass volatile separated by fractionation from the initial reformed raw material, as said above.

   The mixed volatile paraffins thus introduced into the heat exchanger 16 and compressed by means of the pump 17 are introduced to the lower part of the column 10 under the desired operating conditions mentioned above. The volume of paraffin fraction introduced into column 10 is preferably as large as possible to obtain the greatest possible quantitative displacement in column 10 of the less volatile paraffins dissolved in the extract and thus to produce a final benzene product. of greater purity. In some cases, however, it is preferable to obtain a larger feed of raw material and a higher production rate of the recovered product on a volumetric basis when, for example, greater tolerance is allowed for paraffinic contaminants in the product. benzene.

   The loading of the liquid volume in column 10 can be reduced in such cases by reducing the volume of volatile paraffin introduced to displace the less volatile, azeotropic-forming paraffins from the extract. During operation of the process, the supply of the volatile paraffinic sidestream from column 1 through line 4 can be eliminated and the light paraffin circuit recovered from column 21 can be used as the sole source of volatile paraffins for the process. moving in column 10.

   Alternatively, the volatile paraffin circuit from column 21 to column 10 can be eliminated and the only source of these paraffins introduced to the lower part of column 10 can be the side fraction recovered from fractionation column 1 by the line 4, as described above.



   Introduction of the extract recovered from column 10 into column 21 at superatmospheric pressure and at temperature considerably above the boiling points of the volatile components present in the. extracted phase, allows the column 21 to work in an isothermal and practically adiabatic manner when the separation between the boiling points of the primary and secondary solvents is sufficient to obtain a "flash-distillation" of the most volatile components of the extract without appreciable evaporation of the component of the high boiling solvent for aromatic hydrocarbons referred to herein as "primary solvent".

   The reduction in the pressure of the extracted phase as it passes through column 21 through line 19 is preferably sufficient to achieve only the desired evaporation of the light components and to allow their separation roughly into a volatile paraffinic vapor which may contain. small amounts of water, in an intermediate derivation with a slightly higher boiling point comprising benzene and water,

   and a high boiling point distillation residue substantially free of volatile component and comprising the dehydrated primary solvent of oxy-diethylene glycol. The intermediate fraction recovered from column 21 is removed as a liquid bypass through line 33 which includes heat exchanger 34 in which the intermediate liquid fraction is converted to vapor and is returned to the side fractionator 35 for separation. substantially pure benzene vapor removed from fractionator 35 through line 36,

   of a liquid water condensate removed from the fractionator 35 through line 37. The fraction of benzene vapor removed from the fractionator 35 through line 36 passes into heat exchanger 38 and is there condensed; the liquid condensate is transferred through line 39 into vessel 40 for storage or for other uses. The benzene thus recovered consists of substantially pure benzene freed from paraffin impurities and other hydrocarbon impurities when column 10 is operated with a sufficiently high volatile paraffin feed rate.

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   Since water and benzene are practically immiscible in the liquid phase, the intermediate liquid fraction removed from column 21 through line 33 can be discharged directly into a liquid container, in which the resulting phase which is mostly liquid. water, separates from the phase consisting mainly of benzene; phases are removed separately as lower and upper phases respectively, the lower water phase being saturated with benzene and the upper phase being wet benzene.



  The latter product can then be dried if desired, for example by fractionation to separate a water-benzene azeotrope from a dry benzene product.



  The separated aqueous phase (s) can then be directed to line 37 as explained above.



   When column 21 is operated in the preferred mode of operation and under the preferred temperature and pressure conditions mentioned above, the extract residue which accumulates at the bottom of column 21 is generally virtually freed from more volatile components, such as water and light paraffins, present in the extract phase removed from column 10.

   However, because of the tendency of the solvent to retain the aromatic hydrocarbons which are dissolved therein more particularly as the oxy-diethylene glycol solvent dehydrates in the lower part of the column and the solubility of the aromatic hydrocarbons in the column. the solvent tends to increase as the extract loses water by evaporation, it becomes more and more difficult to remove the last traces of benzene from the solvent. In order to recover the dissolved benzene from the extract residue, the latter is heated, for example, by means of a heating coil 41 housed at the bottom of column 21.

   Coil 41 may contain a high temperature circulating heating fluid heated in heat exchanger 42 to temperatures above the boiling point of the least volatile component of the extract which boils below the glycol solvent. oxy-diethylene. However, in the case of some solvents such as the present oxy-diethylene glycol, the last trace of residual dissolved benzene is removed by simple fractionation of the extract residue in column 21 only when heated to extremely high temperatures. high above the thermal stability temperature for this solvent.

   In order to reduce the temperature required in the heater to evaporate the last traces of hydrocarbon extracted (dissolved benzene) from the solvent, a volatile liquid or a vaporized liquid is introduced into the heater of column 21 where the added vapor exerts its own partial pressure to reduce the vapor pressure of the residual benzene component present in the "bottoms" of the extract. A preferred vapor for this purpose for this operation is water vapor, preferably vaporized water recovered as a liquid residue from fractionators 27 and 35.

   The water distillate removed from the fractionator 27 through line 29 which joins line 30 may be re-energized partially or entirely by transfer through line 30, valve 43 and line 44 which joins. the line 37, the latter in turn joining the line 46 which leads to the heater of the column 21.

   Water condensate from fractionator 27 can be combined with water condensate from fractionator 35 and the two streams can be combined by joining line 44 to line 37 and adjusting the valves. 43 and 45 in an appropriate manner; the combined aqueous streams can be pumped by means of a pump 47 connecting the pipe 46 with the exchanger 48, can be vaporized there and the resulting vapor, preferably at a temperature of 100 to 150 ° C. can be vaporized therein. sent through line 49 to the heater in column 21.

   The resulting water vapor contacted with the extract residue in the heater of column 21 reduces the vapor pressure of the dissolved benzene residue present in the extract and accompanies the benzene-water intermediate fraction removed. vee of column 21 through line 33 and separated in the fractionation apparatus 35.

   Alternatively the volatile paraffinic vapor removed from the fractionator 27 through line 28 can be used as extract cleaning vapor by directing the paraffinic vapors from line 28 into line 46 containing valve 50 and after being heated. at a temperature of approximately 100 to 150 C. and compressed

 <Desc / Clms Page number 15>

 in the pump 47 up to the working pressure of the column 21 are introduced at the bottom of this column by its heater in a manner similar to the introduction of said water vapor into the heater.



   The extract residue comprising the dehydrated primary solvent of oxy-dietbylene glycol from which the volatile benzene component has been substantially removed is removed from column 21 through line 51 containing valve 52 and is re-connected to pressure prevailing in the extraction column 10 by means of the pump 53 towards the upper part of the column 10 where the line 51 joins the line 11 for the solvent supply. A determined part of the re-circuited oxy-diethylene glycol solvent can be directed from line 51 into line 23, through valve 24 and then fed to the top of column 21 in order to supply the extracting solvent into said line. column in greater quantity than that supplied by the phase extracted from pipe 19.



   The solvent recirculated in column 10 is hydrated again to the critical concentration of water required for the selective solvent extraction of aromatics from a mixed raw material of aromatic hydrocarbons according to the present process. means the water introduced through line 12 at the top of column 10 and that amount of water required to increase the water content of the selective solvent to the appropriate critical level within the limits of. About 5 to 35% concentration is achieved by continuously returning at least the required part of the liquid condensate of water removed from the solvent cleaning column to the fractionator 27 and 35 through the line into the circuit. 30 containing the valve 54 and the pump 55 in the supply line 12,

   the pump 55 increasing the pressure of the water put back into the circuit to the level at which the column 10 is working. In the event that water is lost from the system, the loss can be compensated by introducing water from the storage in the system through line 12. Although recirculation of water is not necessarily essential for the operation of the process, continuous recirculation of a constant quantity of water is particularly advantageous when all the The system is in equilibrium and provides a suitable means for the elimination of variations in the quality of the selective solvent.



   Although the operation of the process as described above with reference to the accompanying scheme uses an aqueous solution of diethylene glycol as the selective solvent, it is evident that a practical, albeit less selective, mode of separation and less efficient and producing a less concentrated benzene product, can be obtained by using a solvent consisting of only one primary constituent, such as oxy-diethylene glycol freed from the secondary constituent, such as the water mentioned. above.

   For such an operation, the water supply pipe 12, the bypass fractionation devices 2, 7 and 35, the water recirculation pipes 37 must be omitted from the attached diagram. and 30 and the benzene cleaner comprising lines 46, pump 47, heater 48 and line 49, although the latter arrangement of apparatus may be maintained to re-circuit the volatile paraffin in the column heater. 21, as shown in the AC circuit.

   Removing the water from the selective solvent, however, would cause column 21 to operate at a higher temperature and / or lower pressure or to recirculate an extract residue from the lower portion of the column. column 21 via line 51 containing a greater proportion of benzene.



   The invention will be illustrated when at various. embodiments in the accompanying examples, which, however, should not be construed as limiting the scope of the invention.



  EXAMPLE I.



   A selective solvent extraction system is provided comprising the preferred arrangement of apparatus described in connection with the accompanying schematic drawing for recovering benzene from a hydroformed straight run gasoline fraction. The hydroformate is subjected to fractional distillation to

 <Desc / Clms Page number 16>

 separate a fraction having boiling points from about 40 to about 81 C., and the last distillate is further fractionated into a light paraffinic fraction with boiling point from 40 to about 65 C., and a raw material fraction of Feed at a boiling point from about 65 to about 81 ° C. the latter fraction containing isomeric C6 and C7 paraffins and about 36% benzene.

   This fraction of raw material feed is introduced in the liquid phase at a pressure of about 13.3 atmospheres and at a temperature of 130 ° C. at the midpoint of a selective solvent extraction column at the rate. of 113 liters per hour o The column consists of a vertical tube filled with Berl salts, through which the fraction of hydrocarbon in liquid phase introduced into the column flows at the maximum rate indicated above.

   The primary solvent consisting of oxy-diethylene glycol is introduced at the rate of 682 liters per hour into the column at a point below the water inlet lumen at the top of the column; is fed into the column at a rate of 121 liters per hour through a sprinkler head and removes the oxy-diethylene glycol solvent which tends to dissolve in the refined, predominantly paraffinic product phase which is removed from the upper part of the column.

   The water flows downwardly through the filling and dissolves in the primary solvent to form an aqueous solution of oxy-diethylene glycol containing about 15% water o The paraffinic fraction volatile at point d 'boiling between 40 and about 65 ° C., separated from the reformed initial product is introduced into the fractionation column at a temperature of 130 ° C. and at a pressure of about 13.3 atmospheres at the bottom of the extraction column in an adjacent point of the "extracted" phase exhaust lumen formed in the column and is percolated upwardly through the fill against the downwardly flowing "extracted" phase stream.

   The "extract" phase which leaves the extraction column consists essentially of a selective solvent containing dissolved benzene and a small amount (not exceeding about 10% by weight of the benzene contained in the extract phase) of the volatile fraction d. paraffinic hydrocarbon; it is removed from the bottom of the solvent extraction column at a temperature of 130 G. and a pressure of 13.3 atmospheres and is transferred to a bubbling plate fractionation column operating at atmospheric pressure, the "extract" phase being fed into the column at a point approximately two trays from the top of the column.

   A re-boiling section is contained at the bottom of the fractionating column which maintains the high boiling liquid "bottoms" accumulating at the bottom of the column at a temperature of about 130 to. 140 C. A fraction of light vapors is immediately distilled by "flash-distillation" of this fractionation column and passes through a heat exchanger which cools the vapor to about 80 C .;

   the resulting cooled vapor passes to a bypass fractionation column containing an upper condenser operating at about 70 ° C., which allows the light wax fraction to pass through the condenser, but liquefies the constituents to. higher boiling point than the light paraffin, including water which is removed in liquid form at the bottom of the bypass fractionation column.



   An intermediate boiling point fraction removed from the solvent removal column in liquid form is vaporized in a small heater and the resulting vapor passes to a bypass fractionation column which is recovered as vapor. a benzene-water azeotrope with a boiling point of about 69-70 C. and containing about 91% benzene, which is condensed in liquid form to a fraction which separates into a layer of water containing a small amount of dissolved benzene and in a layer of benzene containing about 99% benzene. The total benzene recovery represents about 93% of the benzene present in the initial raw material.

   The aqueous stream recovered from the bypass column separates into two streams, one of which, at a rate of 121 liters per hour, is recirculated to the water inlet port of the solvent extraction column. and the other of which, at a supra-atmospheric pressure of 0.7 atmospheres and at a temperature of 140 C. is re-introduced into the re-boiling section of the

 <Desc / Clms Page number 17>

 Solvent removal column at a rate of 37.9 liters per hour to remove benzene dissolved in the oxy-ethylene glycol solvent which normally accumulates at the lower section of the column.

   The dehydrated oxy-diethylene glycol solvent is removed from the bottom of the solvent removal column at a rate of 682 liters per hour and is re-introduced into the circuit to the solvent inlet port of the stripping column. solvent where it combines with the water re-introduced into the circuit to form the selective solvent.



   In an operation similar to the process described above for the recovery of benzene from a hydroformed straight run gasoline fraction, except that the volatile paraffinic fraction which boils at 40 to 65 ° C. is not introduced into the gasoline. At the bottom of the solvent extraction column, the process has a yield of benzene product containing about 85% by weight of benzene. The vapor exiting the solvent removal column is a benzene-hexane-heptane azeotrope which is recirculated at the bottom of the solvent extraction column but which is appreciably less effective in displacing paraffinic hydrocarbons from the mixture. 'feed of the "extract" phase than the operation described above.

   The intermediate fraction recovered from the solvent removal column and consisting first of benzene is also contaminated with paraffins which form constant boiling point mixtures during the distillation of the primary benzene product.



  EXAMPLE II.



   An operation similar to that of Example I is carried out to recover toluene from a raw material which is constituted by the fraction which boils from about 103 to about 110 ° C., separated from the fraction of hydroformed "straight rune" gasoline indicated. in Example I. A volatile paraffin fraction which boils between about 81 and about 100 C. also separates from the hydroformed raw material and is used as the volatile paraffin fraction introduced into the solvent extraction column for the displacement of Less volatile paraffins which lead to the formation of a-zeotropic, toluene extract in the extraction column.



   The toluene product recovered from the water-glycol oxy-ethylene extract consists essentially of pure toluene and the product recovered represents about 90% of the toluene contained in the initial straight run gasoline reform product.



  EXAMPLE III
The solvent extraction plant used in Example I can be used for the specific purpose of removing sulfur-containing compounds consisting of mercaptans, thiols, sulfides and bi-sulfides from a mixture of hydrocarbons. oils, such as a gasoline fraction which may contain aromatic hydrocarbons in addition to hydrocarbon constituents of lower polarity.

   The sulfur-containing compounds, which are the components of the highest polarity of the mixture, can be recovered in a concentration practically free of hydrocarbons, including the aromatics, although these are also relatively polar by nature. relative to the paraffinic, naphthenic and olefinic constituents present in the gasoline fraction o In order to effect the indicated selective separation of the sulfur-containing constituents from the mixture of hydrocarbons, a solvent consisting of an aqueous solution of oxy-diethylene glycol is used. containing 30% by weight of water as the selective solvent, which because of its greater water content,

   exerts greater selectivity for constituents of higher polarity and rejects constituents of intermediate and lower polarity, such as aromatic hydrocarbons, requiring a solvent containing a greater proportion of the first solvent constituent to effect a selective extraction of other types of hydrocarbons.



   A fraction of the boiling point gasoline product between

 <Desc / Clms Page number 18>

 about 80 and about 225 C., separated from a raw material consisting of high sulfur petroleum and containing 0.95% sulfur, 35% paraffinic hydrocarbons, 15% naphthenic hydrocarbons, 36% d The olefinic hydrocarbons and 14% aromatic hydrocarbons are introduced at a flow rate of 682 liters per hour into the solvent extraction column used in Example I above, at about the midpoint of the column and at a temperature of 110 ° C. and a superatmospheric pressure of 10.2 atmospheres.

   This gasoline fraction is contacted in the column with a downwardly flowing solvent consisting of the aqueous solution of oxy-diethylene glycol mentioned above. The oxy-diethylene glycol (anhydrous) is introduced at a rate of 113 liters per hour into the primary solvent inlet port about 10 centimeters below the water inlet port, while water is introduced. at a rate of 37.9 liters per hour through a spray head in the upper section of the column where it countercurrently contacts the refined product stream which is to be removed from the top of the column.



   The "extract" phase passing to the bottom of the column is composed primarily of solvent, but also contains a large proportion of the sulfur compounds of the starting gasoline, with a small proportion of aromatic compounds of which. the boiling points are above those of the azeotropes of benzene, i.e. above about 80 C.

   In order to liberate the "extracted" phase from aromatic constituents with a boiling point above about 80 ° C., which normally would tend to form aseotropes with the sulfur compounds contained in the gasoline fraction and which have also boiling points greater than about 80 C., a light fraction (boiling point between about 65 and 80 C.) of the initial gasoline product, this light fraction consisting of azeotropes of benzene and containing about 36% by weight of benzene, being introduced at a rate of about 208 liters per hour at the bottom of the solvent extraction column and brought into countercurrent contact with the extract present at the bottom of the column.

   The benzene contained in this latter fraction tends to selectively displace higher molecular weight aromatic hydrocarbons from the extract, which normally (if not displaced with molecular aromatics) form azeotropes with the aromatics. sulfur compounds with approximately the same boiling points as the higher molecular weight aromatics in the gasoline fraction.



   The resulting extract passes through a solvent removal column operating at atmospheric pressure where the benzene and other light hydrocarbons dissolved in the extract, as well as part of the water contained in the solvent, are distilled by "flash-distillation" of the extracted phase and are then condensed and separated into separate fractions of hydrocarbons and water; phase of separated hydrocarbons is re-introduced into the circuit at the bottom of the solvent extraction column. An intermediate fraction comprising sulfur compounds and water is removed from the column, distilled in a bypass column and the resulting sulfur compounds and water are separated.

   This water is recirculated at the rate of 56.5 liters per hour in the heater of the solvent removal column to effect steam distillation of the sulfur compounds in the extract. Dehydrated solvent removed at the bottom of the solvent removal column is returned to the circuit to the primary solvent inlet port near the head of the solvent removal column.



   The refined hydrocarbon stream removed from the head of the solvent extraction column is substantially free of sulfur compounds and meets the requirements of the specifications for "desulfurized" gasoline. This refined product representing the relatively high boiling gasoline fraction processed in the desulfurization process described above can be combined with the lower boiling end products of the initial gasoline fraction for forming a gasoline product having the desired ratio of low boiling point components to meet gasoline vapor pressure specifications.


    

Claims (1)

CLAIMS AND SUMMARY. 1. A process for the separation of a mixture of organic compounds with the aid of a selective solvent having a plus. high solubility for one of the classes of compounds contained in the mixture and which will be designated below under the name of "class A constituent", than for another class of compounds contained in the mixture and which will be designated below under the name of "class B constituent", in which the mixture is introduced into the extraction zone at an intermediate point thereof, and is brought into contact against the current with said selective solvent, a phase of refined product con - holding practically all of the class B constituent is removed at the end of the extraction zone where said solvent is introduced into said zone, an extract phase comprising the class A component dissolved in said selective solvent is removed at the other end of said extraction zone and the extracted class A component is then separated for fractional distillation from the "phase". extract ", said process being characterized in that the extracted phase is substantially freed, before its removal from said extraction zone, of the residual class B component of said mixture by introducing a class B compound more volatile than the class B component of said mixture into said extraction zone at a point closer to the point where said extracted phase is removed than to the point of introduction of said mixture, said residual class B component being separated by said more volatile component in the phase extraction zone the refined product therein, and the more volatile compound contained in the resulting extract phase is separated by fractional distillation from this extract when it is removed from the extraction zone. 2. Method according to 1, characterized in that the "extracted" phase from which the residual class B constituent has been substantially separated by the more volatile class B compound is introduced into the extraction zone at the upper part of a fractional distillation zone, substantially all of the class B volatile compound is evaporated from the phase extracted in the distillation zone and is removed as a vapor stream at the top thereof and returned to the zone of 'extraction at a point near the point where the extracted phase is removed from this zone, the class A component being substantially free from both the class B component and the more volatile class B component, as well as the selective solvent being removed as a bypass stream from the distillation zone, and the still residue comprising the solvent being removed to the lower part of the distillation zone and being returned to the extraction zone. 3. Method according to 2, characterized in that the extraction is carried out with a selective solvent which is a mixture of a primary solvent and a secondary solvent, said primary solvent being soluble in said secondary solvent and having a lower volatility than that of said secondary solvent and of the class A component, and said secondary solvent distilling at least in part with the volatile class B component but being substantially immiscible therewith, the vapor stream exiting the upper part of the distillation zone. fraction being condensed and separated into a first liquid distillate containing the volatile component class B and a second liquid distillate which consists essentially of said secondary solvent, said first liquid distillate being returned to the extraction zone at a point at proximity to the point where the "extracted" phase is removed from this zone, the distillation residue having a higher concentration of primary solvent than that of the selective solvent employed in the extraction zone, being returned from the lower part of the chamber. fractional distillation zone to the extraction zone, and the primary solvent entrained in the product phase refined in the extraction zone being returned to the extraction process by first contacting a stream of said second distillate against the current of the product phase refined in the circuit d 'flow thereof past the point of introduction of said distillation residue into the extraction zone and then mixing said second distillate stream- <Desc / Clms Page number 20> tion with said distillation residue within said extraction zone. 4. Method according to 2, characterized in that the extraction is carried out with a selective solvent which is a mixture of a primary solvent and a secondary solvent, said primary solvent being soluble in said secondary solvent and having a lower volatility than that of said secondary solvent and of the class A component, and said secondary solvent distilling at least in part with said class A component but being substantially immiscible therewith, a bypass stream comprising said class A component and a portion of the secondary solvent introduced into the fractional distillation zone by the feed of the phase extracted therein being removed from said distillation zone and then being separated into a distillation - liquid lat consisting essentially of constituent class A and of a second liquid distillate consisting essentially of secondary solvent, and the residual class A constituent being removed from the distillation residue by introducing a stream of the second distillate separated into the lower part of the the distillation zone. 5. Method according to 3, characterized in that the extraction is carried out in an extraction zone elongated in the vertical direction, the "refined product" phase is removed at the upper part of the extraction zone and the phase "extracted" at the lower part of this zone, a stream of the second distillate is introduced into the extraction zone at a point near the point where the refined product is removed from this zone, and the solvent stream removed from the lower part of the fractional distillation zone is introduced into the extraction zone at a point situated vertically between the points of introduction of said stream of second distillate and of the mixture to be separated. 6. - Process according to 3 or 4, characterized in that the second distillate separated from the vapor stream removed from the upper part of the fractional distillation zone is mixed with the second distillate separated from said side stream bypassing the fractional distillation zone and part of the mixed stream of second distillate is directed to the extraction zone while the remaining part is directed to the lower part of the fractional distillation zone. 7. Method according to 1, characterized in that the selective solvent consists essentially, while it is used in the extraction zone, of a mixture of primary solvent and secondary solvent, said primary solvent being a compound. organic containing combined oxygen, of a higher boiling point than that of the treated feed mixture, and having preferential solubility for the class A component of said feed mixture, and said secondary solvent being soluble in said primary solvent but substantially insoluble in the class A component and in the class B component of said feed mixture and having a boiling point lower than said primary solvent. 8. Method according to 7, characterized in that the primary solvent is an oxycycloalkylene glycol. 9. Process according to 8, characterized in that the primary solvent is an oxy-diethylene glycol, the secondary solvent is water, and the latter is used in the extraction treatment in an amount from 5 to 35% by weight. selective solvent. 10. Method according to 3, characterized in that the selective solvent in the extraction zone contains from 5 to 15% by weight of water as secondary solvent dissolved in oxy-diethylene glycol as primary solvent, and the selective solvent is used. as an extractant for the aromatic hydrocarbons from a mixture of hydrocarbons having a lower boiling point than the oxy-diethylene glycol and containing the aromatic hydrocarbons as a class A constituent, while the other hydrocarbons of said mixture of hydrocarbons have a lower solubility than that of the class A component in oxy-diethylene glycol. <Desc / Clms Page number 21> 11. Method according to the, characterized in that the extraction treatment and the displacement of the residual class A component of the phase are carried out in the extraction zone at a temperature between 1000 and 150 C. and at supra-atmospheric pressure. between 5 and 20 atmospheres, and the volatile class B compound used to displace said residual class B component is distilled from the extracted phase in the fractional distillation zone at a pressure appreciably lower than that maintained in the extraction zone . 12. Method according to 11, characterized in that the mixture of hydrocarbons to be separated is a fraction of naphtha containing benzene hydrocarbons as a class A component, and the class B volatile compound used to displace the class B residual component. of the extracted phase is a paraffinic hydrocarbon fraction which boils below the benzene hydrocarbons in the fractional distillation zone and does not form azeotropes with said benzene hydrocarbons. 13. A process for the separation of organic compounds, substantially as described with reference to the accompanying drawings. in appendix 1 drawing.