US3849261A

US3849261A - Method for separating hydrocarbons especially aromatic hydrocarbons and installations therefor

Info

Publication number: US3849261A
Application number: US00229273A
Authority: US
Inventors: G Gau
Original assignee: Agence National de Valorisation de la Recherche ANVAR
Current assignee: Bpifrance Financement SA
Priority date: 1971-02-26
Filing date: 1972-02-25
Publication date: 1974-11-19
Anticipated expiration: 1991-11-19
Also published as: DE2208115A1; BR7201063D0; DE2208115C3; FR2127175A5; LU64839A1; AT326626B; SU460612A3; NL7202516A; CA992489A; ATA154172A; DE2208115B2; SE373122B; RO64224A; ES400659A1; ZA721185B; GB1378951A; IE36118B1; DD99981A5; IE36118L

Abstract

The method and installation are for the enrichment of one of the constituents of an initial mixture containing at least two hydrocarbons, especially with neighboring boiling points, and of which one at least possesses mobile hydrogens. Fractional distillation of this mixture is effected in contact with a phase containing an organo-metallic compound derived from a compound having itself mobile hydrogens and in which the metallic atoms are substituted in reversible manner for these mobile hydrogens. The vapor phase enriched in the hydrocarbon of the initial mixture which has the least affinity for metallic atoms is collected and an unvaporised phase enriched in the hydrocarbon of the initial mixture which has the most affinity for metallic atoms in partially metalled form is also produced. The affinity of the hydrocarbons concerned for the metallic atoms varies sometimes considerably from one hydrocarbon to the other and from one isomer to the other. By conducting operations continuously and in countercurrent in a distillation column relatively few stages suffice to obtain efficient separation of the hydrocarbons. The transmetallation reactions are facilitated by basic catalysts such as tertiary amines and chelating polyamines.

Description

Knitted States Patent [191 Gan [451 Nov. 19, 1974 METHOD FOR SEPARATING HYDROCARBONS ESPECIALLY AROMATIC HYDROCARBONS AND INSTALLATIONS THEREFOR Inventor:

Assignee: Agence Nationale de Valorisation de la Recherche (Anvar), Courbevoie, France Filed: Feb. 25, 1972 Appl. No.: 229,273

Georges Gau, Nancy, France [30] Foreign Application Priority Data References Cited UNITED STATES PATENTS 1 H1960 Krausse et a1. 260/674 A 4/1965 dOstrowick 260/674 WC 5/1966 'Huckins 203/74 2,959,626 3,177,234 3,254,024 3,356,593 3,598,879 3,629,288 12/1971 3,707,577 12/1972 Stenmark ..260/674A FOREIGN PATENTS OR APPLICATIONS 803,558 10/1958 Great Britain 260/674 A Suzuki 260/674 A Kmecak et al. 260/674 A Primary Examiner Wilbur L.- Bascomb, Jr. Attorney, Agent, or Firm-Burgess, Dinklage & Sprung [57] ABSTRACT The method and installation are for the enrichment of one of the constituents of an initial mixture containing at least two hydrocarbons, especially with neighboring boiling points, and of which one at least possesses molie hydrogens, Fractional distillation of this mixture is effected in contact with a phase containing an organometallic compound derived from a compound having itself mobile hydrogens and in which the metallic atoms are substituted in reversible manner for these mobile hydrogens. The vapor phase enriched in the hydrocarbon of the initial mixture which has the least affinity for metallic atoms is collected and an unvaporised phase enriched in the hydrocarbon of the initial mixture which has the most affinity for metallic atoms in partially metalled form is also produced. The affinity of the hydrocarbons concerned for the metallic atoms varies sometimes considerably from one hydrocarbon to the other and from one isomer to the other. By conducting operations continuously and in countercurrent in a distillation column relatively few stages suffice to obtain efficient separation of the hydrocarbons. The transmetallation reactions are facilitated by basic catalysts such as tertiary amines and chelating polyamines.

43 Claims, 3 Drawing Figures PAT Elm my 1 91974 3.849.261 SHEEI 20? 3 PATENTLILGY'! sum.

SHEH 30$ 3 [Illlll llll.

.Illll METHOD FOR SEPARATKNG HYDROCARBONS ESPECIALLY AROMATIC HYDROCARBONS AND INSTALLATIONS THEREFOR The invention relates to a method for separating hydrocarbons especially aromatic hydrocarbons and to an installation therefor. More particularly the invention relates to a method of separating hydrocarbons of which at least one possesses mobile hydrogens, from mixtures which contain them, especially from mixtures of hydrocarbons of which at least one is aromatic, and it relates more especially, since it is in this case that its application seems to offer the most advantage, but not exclusively, to a method of separating those hydrocarbons which have neighboring boiling points such as, for example, isomeric hydrocarbons. To this type of hydrocarbons, of which the separation often assumes very great importance for the chemical industry, belong for example the various xylenes (ortho-, metaand paraxylenes) and ethylbenzene, which are available in large quantities in the form of their mixtures.

Numerous methods have already been suggested to effect the separation of such isomers.

In a first class of methods, it has been proposed to as sociate with the mixture to be separated, a third com pound adapted to form differentiated physical molecular interactions with the various isomers, and to modify their relative volatilities, in order to enable their separation by fractional distillation.

A survey of attempts (azeotropic or extraction or liquid-liquid extraction distillations) carried out in this field, which relates especially to xylenes and ethylbenzene, is presented in the chapter entitled Xylenes and Ethylbenzene in Volume 22, published in 1970, of the second edition of the work ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY of KIRK-OTHMER (lnterscience Publishers, a Division of John Wiley and Sons, Inc., New York).

These methods have however scarcely enabled so far the industrial separation of two aromatic hydrocarbons from one another, the molecular interactions brought into play being weak and consequently of little selectivit The most current industrial method for the separation of two aromatics consists in the carrying out of fractional crystallizations. Thus paraxylene is recovered from paraxylene-methaxylene mixtures, or from ethylbenzene-paraxylene-metaxylene-orthoxylene mixtures. This method is however burdensome: it involves high refrigeration expenses and it is difficult to control effectively the operations of crystallization and of filtration.

There have also been proposed separation techniques bringing into play chemical reactions. For example it has been proposed to have recourse to the chemical reactions of addition of metals or of metallic compounds to certain hydrocarbons. Such methods have in particular been advocated to effect the separation of naphthalenic hydrocarbons, or again of hydrocarbons possessing reactivities and chemical properties which are sharply distinct. The methods essentially contemplate the formation selectively of an addition compound of one of the hydrocarbons of the mixture concemed, the separation of the addition compound and, if necessary, its decomposition in a separate reactor to recover the hydrocarbon in the purified state. Such methods are however inapplicable to the separation of hydrocarbons possessing chemical properties which are often neighboring, which is frequently the case, especially, for isomeric aromatic hydrocarbons.

The best results have been obtained recently with a method combining the processes of liquid-liquid extraction and of chemical reactions. Thus there have been successfully separated effectively, for example. metaxylene from paraxylene by having recourse to liquid-liquid extraction in the presence of a mixture of boron fluoride and hydrofluoric acid. This mixture presents however the considerable drawback of being relatively isomerising and very corrosive.

Consequently it is an object of the invention to obviate to a great extent such difficulties which are inherent in the methods known hitherto and'which have just been discussed, and more particularly to provide a novel method which enables benefit to be derived, in the separation of hydrocarbons of the abovementioned type, especially of hydrocarbons with neighboring volatilities, from known advantages in fractional distillation methods, when the latter are applied to the separation of hydrocarbons of quite different volatilities.

It is another object of the invention to provide an installation for the separation of such closely similar chemical compounds.

The method according to the invention for enriching in one of its constituents an initial mixture containing at least two hydrocarbons, especialy with neighboring boiling points, and of which at least one possesses mobile hydrogens, is characterised in that fractional distillation of this mixture in contact with a phase containing an organo-metallic compound derived from a compound having itself mobile hydrogens and in which the metallic atoms are substituted in reversible manner for these mobile hydrogens is carried out, and in that there is collected, on one hand, the vapor phase enriched in the hydrocarbon of the initial mixture which has the least afiinity for metallic atoms and, on the other hand, an unvaporised phase enriched in the hydrocarbon of the initial mixture which has the most affinity for metallic atoms in partially metallated form.

The method according to the invention takes advantage of the fact that, on one hand, organo-metallic compounds of the type concerned give rise to equilibrium reactions with that or those of the hydrocarbons of the mixture which possess mobile hydrogens, a portion of the latter exchanging reversibly with the metallic atoms of the organo-metallic compound and, on the other hand, the affinity of the hydrocarbons concerned for these metallic atoms varies sometimes considerably, from one hydrocarbon to the other, and from one isomer to the other.

In the following it will often be stated that the hydrocarbon with the greatest affinity for metallic atoms is more acid than the hydrocarbon with less affinity for metallic atoms. Similarly, the substitution of the mobile hydrogens of a hydrocarbon by metallic atoms will be spoken of as resulting in the metallation of the hydrocarbon. Finally, reversible exchange reactions of the metallic atoms between compounds with mobile hydrogens will be called transmetallations.

The placing in contact of a mixture containing for example two hydrocarbons with the organo-metallic compound hence produces, due to the fact of their various acidities, a more extensive metallation of the most acid hydrocarbon and a lesser metallation of the less acid hydrocarbon, whence there is obtained, in the liquid phase of the mixture, relative coneenti'ations of the unmetallated hydrocarbons different from the relative concentrations of these same hydrocarbons in the initial mixture. There results from the nonvolatility of the organo-metallic compound introduced or formed in the abovesaid liquid phase, a concomitant modification of the relative concentrations of the hydrocarbons to be separated in the vapor phase in equilibrium with the liquid phase, to an extent which is a function of the modification of the concentrations of nonmetallated hydrocarbons in the liquid phase after the abovesaid placing in contact. It will be noted in particular that the most extensive metallation of the most acid hydrocarbon, will involve a relative reduction in the concentration of this same hydrocarbon in the liquid phase with respect to the less acid hydrocarbon, consequently a concomitant impoverishment of the vapor phase in equilibrium with the liquid phase of this more acid hydrocarbon and, in other words, an enrichment of the vapor phase in the less acid hydrocarbon.

If these contacting operations are repeated, especially if effected continuously and in countercurrent and in several stages constituted by the plates of a distillation column, a few stages will suffice to obtain a vapor phase free of the more acid or the more metallated hydrocarbon, under conditions consequently sub stantially similar to those which would prevail in a fractional distillation of a mixture of hydrocarbons of sharply distinct volatilities.

It must be noted that the organo-metallic compounds considered are not addition products of metals with hydrocarbons, but substitution compounds of these metals for mobile hydrogens of these hydrocarbons.

The metals which are well suited for these organometallic compounds, due especially to the speed of the exchange reactions that they provide, are lithium, sodium and potassium, the other alkali metals being prohibitive in price, even in the laboratory. These organometallic compounds can be manufactured from any compounds possessing mobile hydrogens. Particularly favorable results are obtained, for example, with aromatic tertiary amines, such as N,N-dimethylaniline, which include in their nuclei hydrogen atoms capable of being substituted by metallic atoms, or with aromatic hydrocarbons, such as isopropylbenzene, isobutylbenzene, trimethyl-l,2,4 benzene, trimethyl-l,2,3 benzene, trimethyl-l,3,5 benzene and other polyalkylbenzenes.

The reaction can be carried out either in a homogeneous liquid phase, or in the mass with a solid organometallic compound: the organo-metallic compounds hence can be, either completely, or partially solubilized in the reaction liquid medium.

In a distillation column, the transmetallation reaction hence occurs on the plates and this, either in the midst of the liquid phase, if the reaction is carried out in a homogeneous phase, or partially or completely in the midst of the solid phase, if the reaction is carried out in a heterogeneous phase. In the latter case, the finely divided solid is kept in suspension in the liquid by the turbulence created by the plates to improve the liquidvapor contact.

The transmetallation reactions are advantageously carried out in the presence of basic catalysts which facilitate, especially accelerate these reactions. These catalysts must be stable toward the organo-metallic compounds. A first group of these catalysts formed by tertiary amines in which the groups attached to the nitrogen atoms are alkyl or cycloalkyl groups.

Nontertiary amines cannot be used, since the hydrogens connected to their nitrogen atoms would always be more mobile than the hydrogens of the hydrocarbons to be separated, so that they would metallate pref erentially to the hydrocarbons.

By way of example of tertiary amines which are well suited, there may be mentioned, for instance, triethylamine, tripropylamine,- tributylamine, N,N-dimethylcyclohexylamine, or bridged amines such as triethylenediamine or quinuclidine.

Advantageously however, especially when it is de sired to operate in homogeneous liquid phase, recourse is had to basic catalysts of a second group which, at the same time, are adapted to form chelates with the metallic atoms of the organo-metallic compounds. These basic catalysts are constituted by tertiary dior polyamines in which the groups attached to the nitrogen atoms are alkyl or cycloalkyl groups, and in which two at least of their nitrogen atoms are sufficiently close to one another to permit the formation of chelates. These tertiary diamines or polyamines enable at the same time the acceleration of the speed of the reactions, the considerable increase in the solubility of the organometallic compounds and the regulation of the progress of these reactions.

Other bases, chelating or not, with electron dipoles,

which can be associated with organo-metallic compounds and which contain oxygen (ethers) or phosphorus (phosphines) cannot be used since they are destroyed by organoalkali compounds of the type concerned.

As examples of chelating diamines, the following products may be mentioned: N,N,N',N'-tetraalkylethylene-diamine; N.N,N',N'-tctraalkylpropylenediamine; N,N,N,N-tetraalkyl-1.2-.

diaminocyclohexane, the alkyl groups being selected from among the methyl, ethyl, propyl or butyl groups.

Similarly, the chelating polyamines corresponding to the general formula:

where x= l. 2, 3, 4 where R and R are methyl, ethyl, propyl, isopropyl, nbutyl, s-butyl, t-butyl or cyclohexyl groups, enable good results to be obtained. For example the following polyamines may be used:

(CH N C H N (CH C H N (CH (CH3)2N 2 4 N 3) C2H4 N 3) a 4 3): which are easily prepared methylation of diethylenetriamine and of triethylenetetramine.

It is not indispensable that all the R groups be identical between themselves or that all the R groups be identical between themselves, but the synthesis of amines with very unsymmetrical formulae is more difficult.

Particularly advantageous results are obtained with N,N,N,N'-tetramethyll ,Z-diamino-cyclohexane whose chelating power is comparable with that of N,N,- N,N-tetramethylenediamine, but whose volatility is less and chemical stability greater. The oraganometallic compounds chelated by this diamine are very soluble in N,N'-dimethylaniline and the metallation, especially in installations which will be described below, can be carried out in a homogeneous phase.

N,N,N ,N '-tetramethyl-1 ,2-diaminocyclohexane can be obtained by methylation of l,2-diaminocyclohexane with formaldehyde and with formic acid according to the method of Eschweiller and Clarke.

Recourse may advantageously also be had to a solvent which enables, either the solution (partial or complete) of the organo-metallic compound to be facilitated, or its carrying in the form 'of a finely divided suspension to be facilitated.

It can be constituted by an excess of the mobile hydrogen compound from which the organo-metallic compound used is derived. It can also be constituted by any medium stable to organo-metallic compounds and to metallation. Ethers are not suitable since they are not stable in the presence of organo-alkali compounds.

I The solvent, for example, could be constituted by aromatic hydrocarbons of weak acidity, such as cumene, tertiobutylbenzene, isobutylbenzene or diisopropylbenzene. One of the preferred solvents is constituted by tertiobutylbenzene, whose chain cannot be metallated. There can also be used, especially in the petroleum industry, gas-oil fractions.

The solvent can also be constituted, especially when greater solubilization of the organo-metallic compound is desired, by a more polar and/or more solubilizing me dium, such as N,N-dimethylaniline, N,N-dimethylcyclohexylamine or any other tertiary amine.

Lastly it will often be advantageous to operate in the presence of a stabilizing organo-metallic compound with a metal of Group II or Group III. Preferred compounds are obviously those show cost price is lowest and more particularly compounds of the formulae:

Mg(R) Zn(R) and Al(R) in which R represents an alkyl or aryl group. It is known that these products enable the solubility of organoalkali compounds to be increased. It has been shown that they also enable the tendency of tertiary amines to decompose with the formation of double bonds, to be reduced. The latter are particularly undesirable to the extent that they would permit the addition of the organo-metallic compounds to the decomposition products obtained. In fact it has been noted that they enable the speed of the hydrogen-alkali metal exchange reactions to be slowed and this all the more as, for example, the ratio Mg/Na is greater. Thus the speed of homogeneous transmetallation by sodium of paraxylene and of metaxylene is too rapid to be measured at 0C. This speed is always rapid, but measurable at 80C if the ratio Mg/Na is equal to 0.25.

The reactive species (carbo-anions) are hence less reactive in the presence of symmetrical organomagnesiums and it is this which explains that the tertiary amines have less tendency to decompose by forming double bonds.

The preparation of the organometallic compounds can be effected in any manner known in itself. There could, for example, be prepared an organo-rnetallic compound of the formula R Na by proceeding as follows:

Firstly phenylsodium is prepared by causing very finely dispersed sodium to react with chlorobenzene introduced drop by drop at 20C. The reaction takes place according to the scheme:

C5H5Cl 'l" 2 Na C H5Na'l' with a yield of chlorobenzene above 99%.

This first organo-metallic compound can be used to transmetallate the compound RH, for example NN- dimethylaniline; the benzene is entirely expelled from the phenylsodium by carrying out the reaction in a flask surmounted by a rectifiying column operating under reflux. To keep the boiling point relatively low, this operation is effected in partial vacuum.

If it is desired to remove the sodium chloride and the excess metallic sodium and to operate subsequently in a homogeneous phase, the compound R Na is solubilized by the use of one of the aforesaid solubilizing basic catalysts and the insolubles are entirely removed by centrifugation.

The stabilizing organo-metallic compounds are either available commercially (example: Trialkylaluminium), or prepared by reactions similar to the following reactions:

C H --Mg-Cl C H Na ClNA Mg (C H Other features and advantages of the invention will emerge from the description which follows, given by way of purely illustrative, but non-limiting, example, and from the drawings in which FIGS. 1 to 3 show in diagrammatic manner various embodiments of installations enabling the application of the method, according to the invention.

The installation of FIG. I lends itself well to the pro- I duction, by a preferred first modification of the method according to the invention, of the separation of two hydrocarbons R I-I and R H, of which at least one possesses mobile hydrogens, from a mixture of the latter, by using an organo-metallic compound R Na derived from a compound with mobile hydrogens R H whose boiling point is above those of the hydrocarbons R,I-I and R H and which has a lesser affinity with respect to sodium than the two hydrocarbons R I-I and R l-l. Procedure can be as follows or in similar manner:

For clarity in the description, it will be considered that in the following RH and R H are, for example, constituted by paraxylene R ii and metaxylene R H respectively, and that R H is constituted, for example, by dimethylaniline, R Na then resulting by the substitution of one of the hydrogen atoms of its nucleus by sodium.

R,H could nonetheless just as well, for example, denote normal heptane (n-heptane) and R H toluene. R l-l could also be constituted by another tertiary aromatic amine or by an aromatic hydrocarbon, such as cumene, isobutylbenzene, etc.

Finally, the organo-sodium compound R Na could also be replaced by a corresponding organo-lithium compound R Li or organo-potassium compound R K or by mixtures of the latter.

The aforesaid separation can then be effected as follows, by having recourse to the installation of FIG. 1, which comprises two distillation columns 2 and 4 of which the plates have been shown diagrammatically by horizontal rows 6 of interrupted lines.

In a first column 2, there is charged respectively: at 8, into the middle of the column, the mixture of hydro carbons R I-I and R H to be separated;

at 10, above the introduction point 8 for the mixture of the abovesaid hydrocarbons, a phase containing the R Na R H R H R Na' It will thus be possible to define the relative acidity of the hydrocarbons by the ratio of the equilibrium constants of the reactions such as (l) and (2). For example R H is more acid than R H, when K is greater than K and K K /K is greater than unity.

Thus metaxylene is more acid than paraxylene, which is itself more acid than N,N-dimethylaniline. The dimethylaniline gives up practically all the sodium from its organo-metallic compound to the two xylenes and preferentially to the metaxylene.

The sodated xylenes, chelated with the diamine, are soluble in N,N-dimethylaniline (which plays here, as can be noticed, the role of R il and of solvent); these chelates are not volatile and the concentrations of the xylenes in the gaseous phase are only a function of the concentrations of unsodated xylenes in the liquid phase. Paraxylene being less combined with the metal than metaxylene, its concentration in the gaseous phase will be higher than would normally be provided by the law of Raoult; this law is only applicable to concentrations of unmetallated xylenes.

The relative overall volatility of the pair para/- metaxylene has thus been modified and the separation of the constituents of the mixture by the method according to the invention hence becomes possible.

The intake point 8 for the mixture of xylenes enables therefore the defining, as in any normal distillation, of the enrichment zone and the depletion zone of the column 2. In addition, the intake point 10 can only be situated at some plates at the head of the column 2, the latter being sufficient to recover the slightly volatile compounds R l-l.

Paraxylene at a very high degree of purity is condensed at the head of the column, in a condenser 12. A portion of the condensate is sent back as reflux by a pipe 14, the other portion being collected through a pipe 16.

The mixture obtained at the base of the column 2 no longer contains paraxylene and the sodium is distributed between the metaxylene and the N,N- dimethylaniline according to the equilibrium (2). This mixture is introduced, through a pipe 18, at a suitable height into a second distillation column 4, a portion of this mixture being, if necessary, taken off, vaporised in a reboiler 19 and reinjected into the base of the column 2.

In the depletion zone of the column 4, the reverse reaction to the reaction (2) is produced, the equilibrium then being displaced toward the terms on the left of the equation, by vaporisation of the metaxylene. The enrichment zone 22 is only used to stop the vapors of amine, of solvent and of R l-l.

There is thus obtained at the head of the column 4, metaxylene of high purity, of which a portion is returned as reflux, after condensation in a condenser 24, by means of a pipe 26. The other portion is collected through a pipe 28.

The mixture obtained at the bottom of the column 'contains therefore the constituents introduced previ ously into the column 2, and can hence in principle be reused indefinitely and recycled to the intake point 10 of the column 2, through a pipe 29.

The reutilization of the mixture organo-alkali compound solvent basic catalyst stabilizing organometallic compound need not however be complete since:

a portion of the basic catalyst (amine) is attacked by the organo-metallic compounds and gives by-products without catalytic or solvating effect;

a portion of the organo-metallic compound is attacked by the traces of water or of sulfurized products present in the mixture of xylenes.

It is therefore indispensable to drain out, through a pipe 30, continuously, a portion of the mixture drawn from the bottom of the column 4, and to introduce continuously through a pipe 32 an equivalent amount of the noncontaminated mixture.

The purged mateial can be treated batchwise by distillation in vacuo, to recover the solvent as well as the undecomposed basic catalyst. After treating with water or with alcohol, the hydrocarbon R H is itself recovered and the sodium present in the purged material can be recovered in the form of soda.

To reduce to the minimum the amount of decomposed amine, three methods are available:

I. To carry out the two distillations in

columns

12 and 22 under high vacuum in order to be able to operate the lowest possible temperature, but this is obviously limited by the loss of charge at the plates. 2. To use the most stable possible amine (basic catalyst), that is to say the most resistant to attack by organo-metallic compounds. A compromise can be found, in each particular case, between the basic power of the amine, its stability, its volatility and its price. These particularly interesting results are obtained by the use of N,N,N,N'-tetramethyl-l-2 diaminocyclohexane; this chelating amine is very solubilizing and enables the obtaining of a great separating power whilst resisting the attack of sodium well. This stability is due to the fact that the cyclohexane nucleus only metallates very slightly and that the formation of a double bond between two neighboring carbon atoms, is hence very slow.

3. To operate in the presence of a stabilizing organometallic compound.

The foregoing description would apply in the same manner, for example, to the separation of n-heptane and toluene. In this particular case, R l-I (which can be also N,N-dimethylaniline or cumene) would have an acidity intermediate between R- H (toluene) and RH (heptane); the acidity of the n-heptane (as that of all the saturated compounds) is very slight and the constant K would be very large as well as the overall relative volatility of the pair n-heptane-toluene, so that the number of distillation plates to be used can be very small.

It is naturally self-evident that the principle of the separation of two hydrocarbons R l-l and R H such as has just been described with respect to FIG. 1, can be extended to separation of a greater number of hydrocarbons.

There could for example be effected the separation of three hydrocarbons such as ethylbenzene, paraxylene and metaxylene in an installation comprising two units of the type shown in FIG. 1. Ethylbenzene is slightly more volatile, but especially less reactive with respect to organo-sodium compounds than paraxylene. There can hence be produced, first of all, in the column 2 of the first unit, the separation of the paraxylene and the ethylbenzene, on one hand, and metaxylene, on the other hand, the paraxylene-ethylbenzene mixture being obtained at the head of column 2 in the vapor phase and the metaxylene being separated in the second column 4 of the first unit, as described above. The separation of the paraxylene and of the ethylbenzene from the abovesaid vapor phase can then be carried out, after condensation of the latter, in the second aforesaid unit, after the introduction of the condensed mixture into the first column 2 of this second unit, at a point of the latter similar to the point 8 described above, the separation then taking place under conditions similar to those which have been described above with respect to the separation of paraxylene and metaxylene.

It will be noted that it is quite possible, in the foregoing double separation, to resort to different organometallic compounds in the first and in the second units respectively. This will be the same as regards the solvents used, etc.

In numerous cases, in the method according to the invention and in order to maintain as long as possible the activity of the organo-alkali compound, it is preferable to work at relatively low temperatures, generally below 100C. The distillations will consequently be carried out under partial vacuum.

It is not however necessary that certain of the columns of FIG. 1 should operate at low temperature; this is especially the case for the upper sections of each column which effect separations of the unmetallated compounds and of the reboilers 19 and 19a associated with the columns 2 and 4.

There is shown in diagrammatic manner in FIG. 2, an installation in which the operations of distillation in the presence of organo-metallic compounds are carried out in columns 2a and 40 under partial vacuum, the operations of separating unmetalled compounds being effected in distinct columns 3a and a respectively, which operate at atmospheric pressure.

In order for example to separate under these conditions paraxylene and metaxylene from mixtures which contain them, procedure is as follows or in analogous manner.

The mixture 8a of paraxylene and metaxylene is introduced at a suitable height into the column 2a whilst the phase containing the compound is introduced this time at the level of the upper plate 40, the liquid reflux being ensured either by a portion of the condensate 41, coming from the condenser 42 of the column 2a, or by the introduction of heavy products from the atmospheric column 3a, by means of a pipe 44, or by both simutaneously.

The condensate 41 of the first column contains paraxylene, compound R l-I liberated by the organometallic compound R Na and possibly solvent. The

more volatile paraxylene is then easily recovered at 45 by normal distillation in the column 3a, the heavy products 44 being returned into the system as indicated above.

This modified installation enables in addition, if desired, the use of the reboiler 46 of the column 3a to produce the vapor necessary for the operation of the column 2a, this vapor being brought to the base of the column 2a by means of the pipe 48. There is thus avoided all the drawbacks associated with the operation of a reboiler operating at the same time under vacuum and in the presence of compounds sensitive to heat and having a tendency to foul up the heat exchange surfaces, which drawbacks increase the losses of metallated products.

The-portion of the installation serving for the regeneration of the compound R Na and for the purification of the metaxylene operate in a manner similar to that which has just been described. The mixture of metaxylene, of the mobile hydrogen compound from which the organo-metallic compound R Na is derived, and of their metallated forms, which is drawn off from the base of the column 2a is introduced at the level of the upper plate 50 of the regeneration column 4a under partial vacuum, by means of a pipe 52. The regenerated organo-metallic compound, withdrawn from the base of the regeneration column 4a, is recycled, by means of a pipe 53, to the intake point 40 of the column 2a, whilst the vapor phase 54, containing a mixture of metaxylene and R H which escapes from the top of the column 40 subjected, after condensation in a condenser 55, to fractional distillation in the column 5a, operating at atmospheric pressure. The purified metaxylene is then collected at 56 from the vapor phase, whilst the compound R H withdrawn from the base of the column 5a is recycled to the level of the upper plate 50 of the column 4a. As in the first portion of the installation described, the reboiler 57 associated with the column 5a supplies the necessary vapor for the operation at the same time of the column 5 and of the column 4a, by means of

pipes

58 and 59.

In the foregoing, there has only been considered the case where the compound R il corresponding to the organo-metallic compound is less volatile and less acid than at least one of the hydrocarbons of the mixture to be separated. There is never any question there of a limiting condition. 0n the contrary, in certain cases, and especially for the separation of very slightly volatile aromatic hydrocarbons, like the tetramethylbenzenes, the diethylbenzenes or the cymenes, recourse can be had to organo-metallic compounds deriving from R H compounds more volatile and more acid than the R,l-I

and R H hydrocarbons. This also remains true besides I for the more volatile hydrocarbons, such as the xylenes, which can be separated by having recourse to benzylsodium, a derivative of toluene, which is both more acid and more volatile than the xylenes.

FIG. 3 shows diagrammatically an installation in which the latter modification of the method according to the invention can be applied.

This installation comprises a column 2b, operating preferably under reduced pressure, in which the mixture of hydrocarbons R I-I and R H introduced into this column at a point 8b, flows in countercurrent with a phase containing the organo-metallic compound introduced at a point 1% of the same column, above the point 8b. The vapor phase obtained at the head of the column 2b, which contains the compound R H and least acid hydrocarbon R H. After condensation in a condenser 60, it is fractionated in its turn in a separate column 3b operating under atmospheric pressure, R H being withdrawn at 62 from the bottom of the column 3b.

The regeneration column 4b supplied, through a pipe 64, with products withdrawn from the bottom of the column 2b, delivers at its head a vapor phase containing a mixture of the compound R il and of the other hydrocarbon R H. This mixture is, after condensation in a condenser 66, fractionated in its turn in a separate column 5b operating at atmospheric pressure, R H being there again withdrawn at 68 from the bottom of this column 5b.

The regeneration in the column 4b will be facilitated by the introduction of an excess of the compound R H at the bottom of the column 4b and by its relatively high acidity. There can also be eliminated the reboiler 70 of the column 4b and there can be introduced in vapor form into the bottom of the column 4b the amount of the compound R H necessary to assure the operation of the column 4b and the entrainment in the vapor phase of the whole of the compound R H con- .tained in the mixture introduced into this column by means of the pipe 64. These amounts of R l-l in vapor form can especially come, by means of a pipe 72, from the head fractions of the columns 3b and 5b and/or from a separate source shown diagrammatically at 74.

If necessary, the two principal columns 2a and 4b can also be supplied at their respective bases with solvent contained in the phase carrying the organo-metallic compound, when this solvent possesses itself a volatility higher than that of the hydrocarbons to be separated and a very weak acidity.

There is thus obtained a method which can be applied in all the most diverse ways and which enables the obtaining of important modifications of the separation coefficients which are normally-established between the hydrocarbons of a mixture in the absence of the organo-metallic compounds of the type concerned, and consequently the achievement of extremely high separating powers for hydrocarbons which can however have volatilities of the same order of magnitude, as emerges especially from the following examples and which are given of course purely by way of illustrative and non-limiting indication.

For clarity in the description, it will firstly be recalled that the separation coefficient will be defined, for a mixture of two hydrocarbons RH and R H, in the presence of their metallation products, by the formula:

Y1 molecular concentration of the compound R H in the vapor phase,

Y2 molecular concentration of the compound R H in the vapor phase,

xl molecular concentration of the compounds R H and R Na in the liquid phase,

x2 molecular concentration of the compounds R H and R Na in the liquid phase.

These two latter concentrations are replaced by aver age molecular concentrations of the solid-liquid suspension when one operates in a solid-liquid heterogeneous reaction.

In the absence of organo-alkali compounds, this factor C becomes the relative volatility of R H with respect to R H.

EXAMPLE 1 0.05 Moles of phenylsodium is solubilized in a mixture of 50 cm3 (0.39 moles) of N,N-dimethylaniline and 20 cm3 (0.153 moles) of N,N,N,N'-tetramethylethylenediamine.

The sodium is immediately distributed by equilibrium of metallation between the benzene and the dimethylaniline; the dimethylaniline being slightly more acid than the benzene and occurring in a higher concentration takes the largest portion of the sodium.

The medium is brought to 50C and at zero time, there is added:

2 cm3 of paraxylene 2 cm3 of metaxylene These two xylenes, more acid than the benzene or the dimethylaniline, metallate preferentially, the metaxylene metallating more than the paraxylene.

The temperature is kept at 50C and, 15 minutes after the addition of the xylenes (at time t 15 min) there is taken off, by vacuum distillation, and condensed a small fraction of which the composition is substantially representative of that of the vapor in equilibrium with a liquid present in the flask at the moment of the withdrawal. Infrared spectrographic analysis of this fraction, after washing with acidified water to eliminate amines, gives the following relative compositions:

Paraxylene: 79% Metaxylene: 21% which corresponds therefore to a separation coefficient between the gaseous and liquid phase of:

C 3.75 namely a separation coefficient very much higher than the normal paraxylene/metaxylene relative volatility at 50C which is:

(ozpm) 50 1.055

EXAMPLE 2 The operation is carried out as in Example 1, except thatthe amounts of xylenes added are doubled, that is to say:

4 cm3 of paraxylene 4 cm3 of metaxylene The relative compositions of the gaseous phase are as follows:

Paraxylene: 62% Metaxylene: 38%

. which corresponds to a separation coefficient:

This example illustrates well that, for a given amount of sodium, the overall percentage of sodated xylene is less when the amount of xylene increases and consequently the separation coefficient is smaller.

EXAMPLE 3 Operation is effected as in Example 1, but the paraxylene is replaced by orthoxylene: there is therefore added to the metallation medium:

2 cm3 of metaxylene 2 cm3 of orthoxylene The relative concentrations of the gaseous phase are as follows:

Metaxylene: 6l% Orthoxylene: 39% which corresponds to a separation coefficient of This value is again higher than that of the liquid vapor equilibrium:

(a mo) 50 1.22 and indicates therefore that, under the conditions used, orthoxylene is more acid than metaxylene.

EXAMPLE 4 The same operational conditions as in Example 1 are used but N,N,N',N'-tetramethylethylenediamine is not added; the organo-metallics are not soluble in the liquid phase and the attack on the solid phenylsodium progresses slowly. At zero time there is added;

3 cm3 of paraxylene 3 cm3 of metaxylene The separation coefficient C develops as a function of time, as emerges from the following Table, in which there is indicated the measured values of the separation coefficient after increasing reaction times.

Reaction Time Separation Coefficient It is possible to greatly icrease the speed of the phenomenon, either by increasing the temperature, or by using nondissolving basic catalysts.

EXAMPLE 5 0.05 Moles of phenylsodium are mixed with 50 cm3 (0.39 moles) of N,N-dimethylaniline and 80 cm3 (0.58 moles) of triethylamine.

The temperature is controlled at 40C and at zero time, there is added:

1 cm3 of paraxylene 3 cm3 of metaxylene As in Example 4, the reaction is heterogeneous, and the separation coefficient between the vapor and liquid phases is very close to unity. The separation coefficient between the vapor and the suspension is a function of time, and it is found at:

Reaction Time Separation Coefficient As can be seen, the reaction speed is more rapid than in the preceding case, which is due to the fact that the triethylamine is a good basic catalyst of the reaction.

EXAMPLE 6 This example shows that there is a reduction in the separation coeffecient when the reaction time is very long. This reduction is observed for the heterogeneous reaction and also for the homogeneous reaction, as is indicated by the following figures:

0.05 Moles of phenylsodium, 50 cm3 (0.39 moles) of N,N-dimethylaniline and cm3 (0.153 moles) of N,N,N',N'-tetramethylethylene diamine, are mixed.

The temperature is controlled at 40c, and at zero time, there is added:

2 cm3 of paraxylene 2 cm3 of metaxylene Distillation is effected in vacuole at 40C at different reaction times. The separation coefficients corresponding to this homogeneous reaction are as follows:

Reaction Time Separation Coefficient r= lOmin r= 30min lhr 000F100 ll ll ll ll ii I] pu-mu-u-Ln This deactivation is more rapid when the temperature of the reaction is higher. The times necessary for the appearance of this deactivation remain however greater than the normal dwell time of the phases on the plates of a distillation column.

EXAMPLE 7 Reaction Time Separation Coefficient Separation Coefticient para/meta para/meta The meta/ortho separation coefiicient shows that orthoxylene is more acid than metaxylene, as has already been shown in Example 3.

EXAMPLE 8 N,N,N',N'-tetramethyl-1,2-diaminocyclohexane is prepared and the test of Example 1 is carried under the same conditions, but replacing the N,N,N',N'-tetramethylethylenediamine by an equivalent amount of N,N,- N',N-tetramethyl-1,Z-diaminocyclohexane. The separation coefficient obtained is slightly higher than that of Example 1 and is equal to 3.92.

The mixture obtained is then heated for 48 hours at C; it is observed that the separation coefficient, within close experimental errors, is again around 3.9,

which indicates a very high stability of the N,N,N',N-

:tramethyl-l ,Z-diaminocyclohexane inthe presence of organoalkyli compounds.

EXAMPLE 9 The method is carried out under the same operational conditions as in Example 1, but in the presence of 0.05 moles of phenylsodium and of 0.0125 moles of Mg (C H obtained by the action of the phenylsodium on phenylmagnesium bromide.

It is observed that, after minutes at 80c, the separation coefficient is:

C 3.70 and that the coefficient remains equal to this value, within close experimental errors, after having kept the mixture for 90 hours at 80C. This example shows the stabilizing role played in the reaction by the magnesium derivative.

It will be obvious to technicians that a strict relationship exists between the compositions of the vapor phases in equilibrium with the liquid phases of the abovesaid hydrocarbon mixtures, in contact with the organometallic compounds used, on one hand, and the equilibrium constants K3 of the transmetallation reactions such as have been defined above, on the other hand. These equilibrium constants can, for example in the preceding examples, be calculated from the measured compositions of the Vapor phases. Reciprocally, the latter can be calculated from the experimental determinations of the equilibrium constants K3.

These constants have been determined, in the following example, for numerous pairs of hydrocarbons RH and R l-l, by determining in their respective liquid phases in contact with an organo-metallic compound R M (M representing an atom of alkali metal), the relative EXAMPLE 10 The abovesaid determinations were effected, by working under the conditions of Example 8, using the corresponding relative proportions of. the hydrocarbons indicated below (transmetallation with the sodium derivative of dimethylaniline in the presence of N,N,N',N'-tetramethyl-l ,Z-diaminocyclohexane). The values of K3 obtained for the abovesaid pairs of hydrocarbons are indicated below:

metaxylene. paraxylene paraxylene, ethylbenzene toluene, paraxylene metaxylene, orthoxylene pseudocumene, paraxylene mesitylene, paraxylene tertiobutylbenzene, paraxylene benzene, paraxylene paraxylene, paracymene mctacymene, paracymene paraxylene, metadiethylbenzene ethylbenzene, isobutylbenzene EXAMPLE 1 1 metaxylene, paraxylene ethylbenzene, paraxylene orthoxylene, metaxylene metacymene, paracymene ll II II ll \musq EXAMPLE 12 0.05 Moles of phenylsodium, 50 cm3 of cumene, 0.05 moles of N,N,N',N'-tetramethyll ,2- diaminocyclohexane are mixed. The temperature is controlled at 40C and at zero time there is added 0.05 moles of paraxylene and 0.05 moles of metaxylene. The reaction is homogeneous. It is distilled under vacuum at t= l min. The separation coefficient is:

It is verified by treatment of the liquid with dimethylsulfate that:

the cumene was practically unmetallated;

the equilibrium constant of the metallation reaction K K /K was equal to 9.5;

the ratio of the concentrations of the unmetallated xylenes in the liquid phase was equal to the ratio of the concentrations of the xylenes in the vapor phase.

EXAMPLE 1 3 0.05 Moles of phenylsodium, 50 ml of t-butylbenzene and 0.05 moles of N,N,N',N'-tetramethyl-1,2- diaminocyclohexane were mixed.

The temperature was controlled at 40C and at time zero there was added 0.10 moles of paraxylene and 0.02 moles of metaxylene. It was distilled under vacuum at time r= 30 seconds and at time t 5 min. The separation coefficient did not vary as a function of time and is equal to:

This value corresponds also to K, 9.5, the constant determined by treatment of the liquid with dimethylsulfate, the t-butylbenzene not being metallated under these conditions.

EXAMPLE 14 0.05 Moles of phenylsodium, 50 ml of t-butylbenzene and 0.05 moles of N,N,N,N'-tetramethyl-1,2- diaminocyclohexane are mixed.

The temperature is controlled at 40C and at time EXAMPLE 15 0.05 Moles of phenylsodium, 50 ml of tbutylbenzene, 0.05 moles of N,N,N,N'-tetramethyl- 1,2-diaminocyclohexane are mixed.

The temperature is controlled at 40C and at time zero there is added 0.02 moles of paraxylene, 0.05 moles of metaxylene and 0.05 moles of pseudocumene. It is distilled under vacuum at time t =l min. The separation coefficient obtained between the para and metaxylene is The equilibrium constant K is equal to 9.5, this corresponding to the equilibrium of the pseudocumene and of the metaxylene, in the presence of the organoalkyli compound, is equal to 2.6. These constants have been determined by treatment of the liquid with dimethylsulfate. I

The very simple method of determining the constants K, which has been illustrated in Examples l0 to 15 enables the rapid determination of the relative acidities of any pair of hydrocarbons of which one at least possesses mobile hydrogens.

From its systematic application to the study of numerous pairs of hydrocarbons the following rules and facts have emerged:

The alkali cation plays a preponderant role, the measured values of K being substantially independent of the R group in the corresponding organo-metallic compound, of the basic catalysts or chelating diamines used. There is not however a single scale of acidity. The latter varies from one alkali cation to the other. Thus, in the case of aromatic hydrocarbons having several substituents on the same nucleus, the relative acidity of the meta compounds with respect to the para compounds is much greater with the organo-potassiums than with the organo-sodiums. Ethylbenzene is less acid than paraxylene in the transmetallation with sodium;

the reverse is true in the transmetallation with potas- I Slum.

However as regards the aromatic hydrocarbons having several substituents on the same nucleus, it is observed that the meta compound is always more acid than the corresponding para compound, and this is still truer with the organo-potassiums than with the organov sodiums.

There is obtained, for each of the pairs concerned, acidity constants intermediate between the values found with the organo-sodium compounds and the organo-potassium compounds respectively, when mixtures of the latter are used.

It will also be noted that it is possible, with a certain degree of accuracy, to calculate the modification of the acidity introduced by an additional substituent by using the linear combination of free energies well-known in organic chemistry. Thus starting from the acidities of the ortho, meta and paraxylenes, it is possible to calculate those of triand tetramethylbenzenes, etc.

There is thus obtained a method which is applicable to the separation in practice of all hydrocarbon mixtures, in which at least one of these hydrocarbons possesses mobile hydrogens. The technician has available possibilities as regards the selection of the reactants and the adjustment of the parameters brought into play in each reaction, that he will always be able in practice to find operational conditions enabling efficient separation of such mixtures, whatever the nature of their constituents. It will be observed in fact that the relative acidity constants are never identical with the various alkali ions, so that it can be affirmed that two compounds which would not be separable by application of one of these alkyli metals, would necessary be so by the application of another.

In other words, it can therefore be confirmed that the probability that two aromatic compounds RH and R H have at the same time the following three characteristics:

0.9 a, 1.l relative volatility R H/R H O.9 K, l.l relative acidity R,H/R H in the presence of Na 0.9 K l.l relative acidity R,H/R ,H in the presence of K hemillitene/pseudocumene/orthoethyltoluene/mesitylene orthoethyltoluene/metaethyltoluene/paraethyltoluene orthocymene/metacymene/paracymene orthodiethylbenzene/ l ,2,3 ,S-tetramethylbenzene/ l ,2,4 S-tetrarnethylbenzene saturated hydrocarbons/a-methylnapththalene/B- methylnaphthalene Separation of dimethylnaphthalenes and more particularly of 2,6-dimethylnaphthalene n-butylbenzene/secbutylbenzene/isobutylbenzene/tertbutylbenzene orthodiisopropylbenzene/metadiisopropylbenzene/paradiisopropylbenzene, etc.

As is self-evident and as emerges already from the foregoing, the invention is in no way limited to those of its types of application and production which have been more especailly contemplated; it encompasses, on the contrary, all modifications.

I claim:

1. Method of enriching one of the constituents of an initial mixture containing at least two hydrocarbons, and of which at least one possesses a replaceable hydrogen atom more readily replaceable by a metal of an organometallic compound than a hydrogen atom of the other hydrocarbon, comprising fractionally distilling said mixture in contact with such an organo-metallic compound having a metal atom which preferentially replaces a replaceable hydrogen atom on one of said hydrocarbons, said metal atom being substituted in a chemically reversible manner for said replaceable hydrogen atoms, effecting replacement of at least one hydrogen atom of said hydrocarbon whose hydrogen atom is more readily replaced by said metal atom with said metal atom, and collecting vapor phase distillate enriched in the hydrocarbon of the initial mixturewhich has the least affinity for said metal and an unvaporized bottoms phase enriched in the hydrocarbon of the initial mixture which has the most affinity for said metal atom in a partially metallated form.

2. Method according to claim 1, wherein the compound with mobile hydrogens from which the abovesaid organosmetallic compound is derived has an affinity for the corresponding metal less than that of at least one of the abovesaid hydrocarbons and is constituted by a liquid whose boiling point is, higher than those of the hydrocarbons to be separated.

3. Method according to claim 1, wherein said unvaporised phase is subjected in its turn to fractional dis tillation to collect in the vapor phase the abovesaid hydrocarbon which has most affinity for metallic atoms of the initial mixture in its unmetallated form.

4. Method according to claim 1, wherein one at least of the constituents of the mixture to be separated is constituted by an aromatic hydrocarbon.

5. Method according to claim 4, wherein the mixture contains at least two hydrocarbons of the group xylene, m-xylene, p-xylene and ethylbenzene.

6. Method according to claim 1,'wherein said organometallic compound-1 alkali compound derived from an aromatic hydrocarbon or from an aromatic 9. Method according to claim 6, wherein said organo' metallic compound is an organo-alkalicompound derived from trimeffiyl-1,2,4' benzene, from trime'thyl- 1,2,3 benzene or from trimethyl-1,3,5 benzene.

10. Method according to claim 1, wherein the organo-metallic compound which is brought into contact with the mixture of hydrocarbons to be separated is in the form of a dispersion a solvent substantially unable to undergo metallation itself and resistant to the action of the organo-metallic compounds.

11. Method according to claim 10, wherein said solvent is constituted by a saturated hydrocarbon or by an aromatic hydrocarbon such as cumene, isobutylbenzene, tertiobutylbenzene or diisopropylbenzene.

12. Method according to claim 10, wherein said solvent is constituted by an aromatic tertiary amine such as N,N-dimethylaniline or N,N-dimethylcyclohexylamine.

13. Method according to claim l, wherein the phase containing the organo-metallic compound which is brought into contact with the mixture of hydrocarbons to be separated is constituted by a solution or a dispersion of this organo-metallic compound in an excess of the compound with mobile hydrogens from which it is derived.

14. Method according to claim 10, wherein said dispersion also contains a basic catalyst constituted by a tertiary amine in which the groups fixed to the nitrogen atom are alkyl or cyclo-alkyl groups.

15. Method according to claim 14, wherein the tertiary amine is a tertiary monoamine such as triethylamine, tripropylamine, tributylamine, N,N-dimethylcyclohexylamine or a bridged amine such as triethylenediamine or quinuclidine.

16. Method according to claim 10, wherein the dispersion contains a chelating agent constituted by a polyamine in which the amine groups are tertiary and the hydrocarbon groups fixed on their nitrogen atoms or connecting the latter are saturated, the nitrogen atoms of at least two of these amines being sufficiently close to one another to permit the formation of chelates with the metallic atoms of the abovesaid organometallic compound.

17. Method according to claim 16, wherein the chelating agent is constituted by a diamine selected from the group consisting of N,N,N,N'-tctraalkylethylenediamine, N,N,N ',N'-tetraalkyll ,2-

diaminocyclohexane, the alkyl groups being selected from the groups methyl, ethyl, propyl and butyl,

18. Method according to claim 17, wherein the chelating agent is constituted by N,N,N,N-tetramethyl- 1 ,2-diaminocyclohexane.

19. Method according to claim 16, wherein the chelating agent is constituted by a polyarnine of the general formula:

in which R is an alkyl or aryl group.

22. Method according to claim 2 to produce the fractionation of said mixture of hydrocarbons, wherein the placing in contact of the initial mixture to be separated with the phase containing said organo-metallic compound is effected in several stages in countercurrent in a distillation column, the organo-metallic compound being introduced into the column, at a sufficient number of stages above the intake point of the mixture to be fractionated for the vapor phase obtained at the level of the intake point of the organo metallic compound to be substantially free of the hydrocarbon which has greatest affinity for metallic atoms and at a number of stages below the top of the column sufficient to enable the practically complete separation of the hydrocarbon with the least affinity for metallic atoms.

23. Method according to claim 22, wherein said unvaporised phase withdrawn at the base of the first abovesaid column, is subjected, in a separate second distillation column, to another fractional distillation to collect the said hydrocarbon which has most affinity for metallic atoms of the initial mixture in the vapor phase and in its unmetallated form.

24. Method according to claim 23, wherein the phase withdrawn from the lower portion of said second column is recycled into said first column, at the intake point for the phase containing said organo-metallic compound.

25. Method according to claim 2 for effecting the fractionation of said mixture of hydrocarbons, wherein the placing in contact of this mixture with the phase containing said organo-metallic compound is effected in several stages and in countercurrent in a first distillation column operating under partial vacuum, the phase containing the abovesaid organo-metallic compound being introduced into the upper portion of this first column and the mixture of hydrocarbons being introduced into this first column at a point situated below this upper portion at a sufficient number of stages for the vapor phase obtained at the head of the column to be practically free of the hydrocarbon of the initial mixture which has the most affinity for metallic atoms, collecting and condensing this vapor phase, proceding to a fractional distillation of at least a portion of the condensate in a second column operating at atmospheric pressure to collect at the head the hydrocarbon which has the least affinity for metallic atoms, and vaporising and recycling a portion at least of the tail products of this second column in the lower portion of the first column.

26. Method according to claim 25, wherein the unvaporised phase withdrawn from the lower portion of the abovesaid first column is introduced'into the upper portion of a third column operating under partial vacuum and placing in contact, in countercurrent in this column, with a phase containing the compound with free mobile hydrogens from which said organo-metallic compound is derived, collecting and condensing the vapor phase obtained at the head of the column, proceding to a fractional distillation of at least a portion of the condensate in a fourth column operating at atmospheric pressure, to collect at the head the hydrocarbon which has most affinity for metallic atoms in the vapor phase and in its unmetallated form, and recycling a portion at least of the tail products from the fourth column into the upper portion of the third column.

27. Method according to claim 26, wherein the unvaporised phase withdrawn from the lower portion of the third column is recycled into the upper portion of the first column.

28. Method according to claim 1, wherein said organo-metallic compound is derived from a mobile hydrogen compound which possesses an affinity for the corresponding metal higher than those of the hydrocarbons to be separated and is constituted by a liquid whose boiling point is lower than those of these hydrocarbons.

29. Method according to claim 28, wherein said placing in contact is effected in several steps in countercurrent in a fractionating column, collecting and condensing the vapor phase obtained at the head of the column, and proceding to a fractional distillation of a portion at least of the condensate to collect said hydrocarbon to be separated.

30. Method according to claim 29, wherein the unvaporised phase withdrawn from the base of said column is subjected to a fractional distillation in a second fractionating column, in the presence of an excess of said mobile hydrogen compound sufficient to entrain in the vapor phase the hydrocarbon of the initial mixture which has most affinity for metallic atoms, collecting the vapor phase, condensing a portion at least of this vapor phase and subjecting it to fractional distillation to separate said last mentioned hydrocarbon.

31. Method according to claim 28, wherein said organo-metallic compound is constituted by benzylsodium.

32. Method according to claim 22, wherein said organo-metallic compound is constituted by an organolithium, organo-sodium or organo-potassium compound.

33. Method according to claim 22, wherein said phase containing the organo-metallic compound is introduced into said first fractionating column in the form of a dispersion or of a solution in a solvent substantially unable itself to undergo metallation and resistant to the action of the abovesaid organo-metallic compounds.

34. Method according to claim 22, wherein said phase containing the organo-metallic compounds contains also a chelating agent constituted by a diamine selected from the group consisting of N,N,N,N-tetraalkyl-ethylenediamine, N,N,N,N'-tetraalkyl-l ,2- diaminocyclohexane, the alkyl groups selected being the methyl, ethyl, propyl or butyl groups.

35. Method according to claim 34, wherein said chelating agent is constituted by N,N,N,N-tetramethyll ,ldiaminocyclohexane.

36. Method according to claim 22, wherein said phase containing the organo-metallic compound contains also a chelating agent constituted by a polyarnine of the general formula:

in which x is equal to l, 2, 3 or 4 and R and R are constituted by the methyl, ethyl, propyl, isopropyl, n-butyl, s-butyl, t-butyl or cyclohexyl groups.

37. Method according to claim 21, wherein said organometallic compound is in the form of a dispersion which contains an additional organo-metallic compound of the formula:

in which R is an alkyl or aryl group.

38. Method according to claim 1, wherein said organo-metallic compound is an organo-alkali compound selected from the group consisting of an organolithium, an organo-sodium and an organo-potassium compound.

39. Method according to claim 1 wherein the organometallic compound which is brought into contact with the mixture of hydrocarbons to be separated-is in the form of a solution in asolvent substantially unable to undergo metallation itself and resistent to the action of the organo-metallic compounds.

40. Method according to claim 21 wherein said organo-metallic compound is in the form of a solution which contained an additional organo-metallic compound of the formula in which R is an alkylor aryl group.

41. Method according to claim 1 wherein said organo-metallic compound is contacted by said mixture of hydrocarbons in the presence of a basic catalyst con stituted by a tertiary amine in which the groups fixed to the nitrogen atom are alkyl or cyclo-alkyl groups.

polyamine in which the amine groups are tertiary and the hydrocarbon groups fixed on the nitrogen atoms or connecting the latter are saturated, the nitrogen atoms of at least two of these amines being sufficiently close to one another to permit the formation of chelates with the metallic atoms of said organo-metallic compound.

Claims

1. METHOD OF ENRICHING ONE OF THE CONSTITUENTS OF AN INITIAL MIXTURE CONTAINING AT LEAST TWO HYDROCARBONS, AND OF WHICH AT LEAST ONE POSSESSES A REPLACEABLE HYDROGEN ATOM MORE READLIY REPLACEABLE BY A METAL OF AN ORGANOMETALLIC COMPOUND THAN A HYDROGEN ATOM OF THE OTHER HYDROCARBON, COMPRISING FRACTIONALLY DISTILLING SAID MIXTURE IN CONTACT WITH SUCH AN ORGANOMETALLIC COMPOUND HAVING A METAL ATOM WHICH PREFERENTIALLY REPLACES A REPLACEABLE HYDROGEN ATOM ON ONE OF SAID HYDROCARBONS, SAID METAL ATOM BEING SUBSTITUTED IN A CHEMICALLY REVERISIBLE MANNER FOR SAID REPLACEMENT HYDROGEN ATOMS, EFFECTING REPLACMENT OF AT LEAST ONE HYDROGEN ATOM OF SAID HYDROCARBON WHOSE HYDROGEN ATOM IS MORE READLY REPLACED BY SAID METAL ATOM WITH SAID METAL ATOM, AND COLLECTING VAPOR PHASE DISTILLATE ENRICHED IN THE HYDROCARBON OF THE INITIAL MIXTURE WHICH HAS THE LEAST AFFINITY FOR SAID METAL AND AN UNVAPORIZED BOTTOMS PHASE ENRICHED IN THE HYDROCARBON OF THE INITIAL MIXTURE WHICH HAS THE MOST AFFINITY FOR SAID METAL ATOM IN A PARTIALLY METALLATED FORM.

2. Method according to claim 1, wherein the compound with mobile hydrogens from which the abovesaid organo-metallic compound is derived has an affinity for the corresponding metal less than that of at least one of the abovesaid hydrocarbons and is constituted by a liquid whose boiling point is higher than those of the hydrocarbons to be separated.

3. Method according to claim 1, wherein said unvaporised phase is subjected in its turn to fractional distillation to collect in the vapor phase the abovesaid hydrocarbon which has most affinity for metallic atoms of the initial mixture in its unmetallated form.

5. Method according to claim 4, wherein the mixture contains at least two hydrocarbons of the group o-xylene, m-xylene, p-xylene and ethylbenzene.

6. Method according to claim 1, wherein said organo-metallic compound are is an organo alkali compounds derived from an aromatic hydrocarbon or from an aromatic tertiary amine, the metal substituting one hydrogen of the aromatic nucleus of the amine.

7. Method according to claim 6, wherein said organo-metallic compounds are organo-alkali compounds derived from N,N-dimethylaniline, the atoms of alkali metals being substituted for hydrogen atoms of its nucleus.

8. Method according to claim 6, wherein said organo-metallic compounds are organo-alkali compounds derived from isopropylbenzene or from isobutylbenzene.

9. Method according to claim 6, wherein said organo-metallic compounds are organo-alkali compounds derived from trimethyl-1,2, 4 benzene, from trimethyl-1,2,3 benzene or from trimethyl-1,3,5 benzene.

13. Method according to claim 1, wherein the phase containing the organo-metallic compound which is brought into contact with the mixture of hydrocarbons to be separated is constituted by a solution or a dispersion of this organo-metallic compound in an excess of the compound with mobile hydrogens from which it is derived.

16. Method according to claim 10, wherein the dispersion contains a chelating agent constituted by a polyamine in which the amine groups are tertiary and the hydrocarbon groups fixed on their nitrogen atoms or connecting the latter are saturated, the nitrogen atoms of at least two of these amines being sufficiently close to one another to permit the formation of chelates with the metallic atoms of the abovesaid organo-metallic compound.

17. Method according to claim 16, wherein the chelating agent is constituted by a diamine selected from the group consisting of N, N,N'',N''-tetraalkyl-ethylenediamine, N,N,N'',N''-tetraalkyl-1,2-diaminocyclohexane, the alkyl groups being selected from the groups methyl, ethyl, propyl and butyl,

18. Method according to claim 17, wherein the chelating agent is constituted by N,N,N'',N''-tetramethyl-1,2-diaminocyclohexane.

19. Method according to claim 16, wherein the chelating agent is constituted by a polyamine of the general formula:

20. Method according to claim 19, wherein the chelating agent is constituted by one of the polyamines of the following formulae: (CH3)2 N-C2H4-N(CH3)-C2H4-N(CH3)2 (CH3)2 N-C2H4-N(CH3)-C2H4-N(CH3)-C2H4-N(CH3)2

21. Method according to claim 6, wherein in addition to said organometallic compound having replaceable metal atom, the hydrocarbon mixture contacts an organometallic compound of the formula Mg(R)2, Zn(R)2 or Al(R)3 in which R is an alkyl or aryl group.

22. Method according to claim 2 to produce the fractionation of said mixture of hydrocarbons, wherein the placing in contact of the initial mixture to be separated with the phase containing said organo-metallic compound is effected in several stages in countercurrent in a distillation column, the organo-metallic compound being introduced into the column, at a sufficient number of stages above the intake point of the mixture to be fractionated for the vapor phase obtained at the level of the intake point of the organo-metallic compound to be substantially free of the hydrocarbon which has greatest affinity for metallic atoms and at a number of stages below the top of the column sufficient to enable the practically complete separation of the hydrocarbon with the least affinity for metallic atoms.

26. Method according to claim 25, wherein the unvaporised phase withdrawn from the lower portion of the abovesaid first column is introduced into the upper portion of a third column operating under partial vacuum and placing in contact, in countercurrent in this column, with a phase containing the compound with free mobile hydrogens from which said organo-metallic compound is derived, collecting and condensing the vapor phase obtained at the head of the column, proceding to a fractional distillation of at least a portion of the condensate in a fourth column operating at atmospheric pressure, to collect at the head the hydrocarbon which has most affinity for metallic atoms in the vapor phase and in its unmetallated form, and recycling a portion at least of the tail products from the fourth column into the upper portion of the third column.

31. Method according to claim 28, wherein said organo-metallic compound is constituted by benzyl-sodium.

32. Method according to claim 22, wherein said organo-metallic compound is constituted by an organo-lithium, organo-sodium or organo-potassium compound.

34. Method according to claim 22, wherein said phase containing the organo-metallic compounds contains also a chelating agent constituted by a diamine selected from the group consisting of N, N,N'',N''-tetraalkyl-ethylenediamine, N,N,N'',N''-tetraalkyl-1,2-diaminocyclohexane, the alkyl groups selected being the methyl, ethyl, propyl or butyl groups.

35. Method according to claim 34, wherein said chelating agent is constituted by N,N,N'',N''-tetramethyl-1,2-diaminocyclohexane.

36. Method according to claim 22, wherein said phase containing the organo-metallic compound contains also a chelating agent constituted by a polyamine of the general formula:

37. Method according to claim 21, wherein said organometallic compound is in the form of a dispersion which contains an additional organo-metallic compound of the formula: Mg(R)2, Zn(R)2 or Al(R)3 in which R is an alkyl or aryl group.

38. Method according to claim 1, wherein said organo-metallic compound is an organo-alkali compound selected from the group consisting of an organo-lithium, an organo-sodium and an organo-potassium compound.

39. Method according to claim 1 wherein the organo-metallic compound which is brought into contact with the mixture of hydrocarbons to be separated is in the form of a solution in a solvent substantially unable to undergo metallation itself and resistent to the action of the organo-metallic compounds.

40. Method according to claim 21 wherein said organo-metallic compound is in the form of a solution which contained an additional organo-metallic compound of the formula Mg(R)2, Zn(R)2 or Al(R)3 in which R is an alkyl or aryl group.

41. Method according to claim 1 wherein said organo-metallic compound is contacted by said mixture of hydrocarbons in the presence of a basic catalyst constituted by a tertiary amine in which the groups fixed to the nitrogen atom are alkyl or cyclo-alkyl groups.

42. Method according to claim 41 wherein the tertiary amine is a tertiary monoamine such as triethylamine, tripropylamine, tributylamine, N,N-dimethylcyclohexylamine or a bridged amine such as triethylenediamine or quinuclidine.

43. Method according to claim 1 wherein in addition to the organo-metallic compound present in the distillation zone there is a chelating agent constituted by a polyamine in which the amine groups are tertiary and the hydrocarbon groups fixed on the nitrogen atoms or connecting the latter are saturated, the nitrogen atoms of at least two of these amines being sufficiently close to one another to permit the formation of chelates with the metallic atoms of said organo-metallic compound.