US2386523A - Process for separating hydrocarbons - Google Patents

Description

Patented a. 9, 1945 OFFICE rnocnss FOR samaa'rmc nrnaocannons Charles E. Welling, Wilkinsburg, Pa., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application December 13, 1943, Serial N0. 514,157

a claims. ((21. 183-115) This invention relates to the separation of mixtures comprising close-boiling olefin hydrocar bons. More particularly, 'this invention relates to the selective extraction of one or more olefins from agaseous mixture of close-boiling isomeric olefins, utilizing a selective solvent in which branched olefins are more soluble than unbranched or less-branched-olefin s.

Large quantities .of olefinic hydrocarbons are produced in various industrial processes. In general, these olefins when concentrated by known means are found to be mixtures of close-boiling structural isomers; However, it often happens that such a mixture of isomeric olefins is undesirable for special uses, as in chemical syntheses of particular derivatives or products, .andin such cases the known methods of separation of olefinic iii hydrocarbons are either inadequate or impractical from the economic standpoint.

f The difilculties encountered in separating mixtures of close-boiling, isomeric olefins by fracbranched oleilns. The

of structurally isomeric olefins of difierentdegrees of branphing one from another by contacting the olefinic mixture, in the gaseous state,

with a selective solvent, v specifically furfural,

which effects a selective solution of the more to the lessbranched olefins in preference terml'ess-branched olefins as used herein may include fins. After \the contacting, the dissolved olefins may be recovered from the solvent by heating, reduction of pressure, orother suitable, means, and are found to contain a greater proportion of the more-branched olefin or olefins, whereas the undlssolved hydrocarbon vapors are found to contain a lesser proportion of the more-branched In this manner a more or less complete segregation of the olefins in the mixture, in accordance with structure, is brought about. Although furfural and other selective solvents have been proposed for use in separating different types of hydrocarbons, my invention is not contional distillation, for instance, are readily perceived from considerationof the small differences in boiling points that frequently exist. Thus, in

the case of the butenes, the following pair of com- ?ounis of almost identical boiling points is oun ' Boiling Compound pol-list,

Isobutene e -1 531? when the isomeric pentenes are considered, much the same situation isfound, as is shown In general, the number of isomeric olefins increases, and the boiling oint diiferences decrease,

' for higher members of the homologous series,

such as hexenes.

My invention is.a process for the separation sures somewhat cerned with such separations, but with the hitherto unknown separation of one isomeric olefin from another. I

When treating a close-boiling mixture of isomeric olefins according to my invention, the process is preferably carried out by conducting the olefinic mixture, in the gaseous state, into a scrubbing tower countercurrently to the liquid solvent,

so that the gas is depleted in its content of. the

-more-branched oleflns and is withdrawn at-the t p.

and the solution containing the morebranched oleflns is withdrawn at the bottom of the tower. v

As will be clear to one skilled in the art, the temperature and the pressure to be employed i2. practicing my inv 'ntion depend to a considerable extent upon the olefinic'mixture to be treated. In general, it is desirable to operate at temperatures within the range 50-300" F. and at presbelow-the vapor pressure ofthe olefinic mixture at the temperature of operation.

' When treating mixtures of pentenes, I have found a temperature of 150 F. and a pressure-of 30 p. s. i. a. to be suitable conditions for selectively absorbing the branched pentenes in furfural.

Any known arrangement of apparatus to. promote eflicient contacting of gas and liquid may be employed in practicing my'invention. Likewise, a scrubbing tower may be. arranged-with the hydrocarbon feed introducedat some point straight-chain oleintermediate-between the point at which lean solvent is introduced and the point at which rich solvent is withdrawn, and a temperature gradient may be maintained through the tower by reboilingof the rich solvent reaching the bottom-of the tower together with abstraction of heat from the gases rising to the top of the tower. As will be recognized by one skilled in the art, the amount of reboiling of rich solvent and the required efficiency of the scrubbing tower as measured by the number of theoretical gas-liquid contacts that are brought about aredependent upon the ,particular "olefinic mixture to be rich-in pentene-2 and an extract portion which is recovered from the solvent, that is rich in 2- methyl-2-butene. Each portion may then be subjected to thermal cracking to produce diolefins, and the cracking conditions may be so adjusted as to obtain maximum emciency in each case. In this way greater yields are realized from the isomeric pentenes than could be realized by thermal cracking of the original fraction, be-

treated and .also upon the degree of segregation v of the component olefins that is desired; when the latter factors are determined, be readily established, Although the ratio of solvent to olefinic mixture to be separated depends upon thefactors discussed above, I maystate that ordinarily suitable mol ratios of solventto hydrocarbon charge range from around 5:1 to 40: 1.

I have found that I may employ my process for separation of isomeric olefins in mixtures even when hydrocarbons of other types, such as paraffins and dioleflns are also present. I may also state that my process is particularly applicable to the separation of close-boiling mixturesof isomeric oleflnsthat contain from fourto six car-,

bon atoms r molecule. However, when the separatf wide-boiling mixtures of isomeric olefins 'is to be made it sometimes happens that my process is less applicable than conventional fractionation. Thus, although I have found that of twoolefins having about the same-boiling point the more-branched olefin possesses relatively the greater solubility in the solvent phase of a vaporsolvent system and that the difference in solubility is enhanced if the boiling point of the morebranched olefin is higher than that of the less-,

the former may cause of the considerable differences in optimum conditions for cracking these individual hydrocarbons.

In another embodiment of my invention, closeboiling isomeric olefins are separated when a parafiln is also present in the mixture. For example,

a mixture of pentene-2, 2-methyl-butene-2, and n-pentane may be contacted, under suitable conditions, with the solvent. The portion recovered by stripping the enriched furfural is rich in 2- methyl-butene-2. .The overhead from the extraction step containing the pentene-Z and npentane is passed to a second gas-liquid solvent extraction step employing furfural or another a suitable solvent wherein the -pentene-2 is exbranchedolefin, the difference in solubility mayy be reduced or even overcome if the morebranched olefin is considerably lower boiling than the less-branched olefins. '6 apply my. process to mixtures of isomeric 0le-' fins of such a nature that the more-branched olefin or oleflns that are to be preferentially dissolved in the solvent either boil above the lessbranched olefins or in any event do not boil more.

than about 3 to 6 F. below the less-branched olefins. Thus, although my process is advantageoujsly applicable to the separation of isobutene from butene-l, whereby isobutene is selecy dissolved in the solvent,'it is not equally applicable to the separation of a mixture of isobutene and butene-2 I since thismixture can be readily separated by conventional fractionation. In employing furfural as the selective solvent in the process according to my invention, it is sometimes advatageous. toadd small amounts of other solvents in" order to modify the properties 01' the furfural. Thus, for example-I may add a few per cent, usually three to five, of waterto the iurfural. r

In order further to illustrate my invention, I may mention .a practical application whichis indicative of the present; interest in a process capable of eiiecting the separation of isomeric Therefore, I prefer} tracted and the n-pentane removed overhead- The solvent is stripped to recover a portion rich in pentene-2. By meansof my process" a mixture of three. or moreclose-boiling hydrocarbons may be separated into substantially pure fractions.

The selectivity that I have found to be exercised by the solvent of my invention as between moreand less-branched olefins is illustrated and exemplified by the following experimental observations, but the invention is not to be limitedthereby.

Two sets of measurements were made to obtain the solubilities or vapor-liquid equilibria of the low-boiling geometrical isomer of pentene-2 and of 2-methyl-2-but'ene in furfural containing four weight per cent dissolved water. The vaporliquid equilibrium for each olefin' was measured separately; normal pentane was used as a diluent percentage of the same hydrocarbon found in I the solvent phase.

ployed' in calculating the equilibrium constants A similar procedurewas em- -for both 'furfural and water, save that the olefins in concentrated or pure form. However,

this application is not to be taken as limiting in any way the scope of my invention. This application consists in treating, according to my invention, a close-cut fraction comprising pentJne-Z and 2-methyl-2-butene in order toeffect separation of the mixture into a portion that is amounts of these materials in'the g s Phase were estimated from the known vapor pressures of the materials rather than determined by analysis,

because of the difllculties of making accurate analyses for the small amounts involved.

The hydrocarbon mixture containing Z-methyl- 2-butene was composed of 25.5 mol per cent 2- 'methyl-Z-butene and 74.5 mol per cent n-pentane. The compositionoi the mixture containing the low-boiling isomer of pentene-Z was 25.1 mol per cent pentene-2 and'74.9 mol per cent n-pentane.

The compositions of the gas and'liquid phases in thevarious systems and the calculated equilibrium constants are shown in the tabulation below. As'is shown, check runs were made with e ch olefin. v

Equilibrium conditions Gas Liq. System Components of system 2 i s? g Temp Pressure, percent percent v T. p. s. i. a.

2-methyl-2-bntene l7. 3 2. 5 6. 9 A-l n-Pentane 150 30 79. 2 5. 6 l4. 1

Water- 2. 1 l6. 7 0. 127 Furiural. l. 4 75. 2 0. 018

. 2-methyl-2-butene. l8. 8 2. 4 7. 8 A2 n-Pentane 77. 7 5. 7 13. 6

2. 1 l6. 8 0. 127 a1 l. 4 75. l 0. 018

2. 2 l7. 1 0.127 Furiural 1. 4 76. 6, 0. 018

-The average -"K constant for 2-methyl-2- butene in systems A-.1 and A2 is seen to be 7.35, whereas the average K constant for the low-boiling geometrical isomer of pentene-2 in systems B--1 and B-y2 is seen to be 12.05. Thus 2-methyl-2-butene is considerably more soluble in furfural than pentene-2 and the difierence is much more pronounced than the difference in the vapor pressures of the two olefins. The discovery that branched olefins possess a higher natural solubility in furfural than do close-boil-. ing less-branched olefins is the basis for practical application of the process described herein to the separation of olefins inaccordance with the pres'ent'invention. Example. 4

In an example illustrating separation of 2-methylbutene-2 from pentene-2, n-pentane and lighter hydrocarbons, a feed having the composition M01 per cent 2-methyl-2-butene 9 Pentene-2 (mixture of isomers) 17 n-Pentane Lighter than n-pentan'e 34 is subjected to vapor-liquid extraction in an efficientcolumn with about 20 moles furfural (containing 4 weight per centwater) per mole of hydrocarbon feed-at a pressure 01' about 2 atmospheres absolute and a temperature of about 150 F. A reboiler partially strips hydrocarbons wise than as specifically shown herein, and because many modifications and variations of it will be apparent to those skilled in the art, it should not be restricted except as specifically indicated in the following claims.

Iclaim:

l. A process for the resolution of a mixture of close-boiling structurally isomeric aliphatic olefins having from four to six carbon atoms per molecule and being of different degrees of" I branchedness into a fraction of more highly branched olefins and a fraction of less highly branched olefins which comprises extracting said mixture of olefins in the gaseous phase with a solvent composed of liquid iurfural containing 3 to 5 weight per cent of-dissolved waterand thereby efiecting preferential dissolution in the furfural of the more highly branched olefin confrom the furfural reaching the kettle of the column so that the vapor load at the feed tray is approximately twice the, feed. The rich furfural discharged fromthe extraction tower is then stripped in a stripping column at reduced pressure and/or increased temperature; the overhead product is a hydrocarbon stream containing I Mol per cent zemethylbutene-2 53 Pentene-2 27 n-Pentane. v I 12 Lighter than n-pentane 8 I Because the invention may be practiced other- 76 tent of said mixture while causing the less highly branched olefin content of said mixture to remain undissolved, separating the resulting solution from the resulting gaseous phase, and recovering the dissolved more highly branched olefins from said solution by stripping the same.

2. The process of claim 1 wherein said extracting is conducted at a temperature of from 50 to 300 F. and at a pressure below the vapor pressure of said mixture at the temperature 01 extraction.

3. The process of claim 1 in which the more I highly branched olefin content of said mixture --boils at a temperature of not more than 6 degrees F below the less highly branched ole'fln content. I

4. The process of claim 1 wherein the olefin content of said olefin mixture is composed of isobutene and butene-l. I

5. The process of claim 1 wherein the olefin content of said olefin mixture is composed o! 2-methyl-2-butene and pen'tene-2.

6. The process of claim 1 wherein the olefin content of saidolefin mixture is composed of 2-methyl-1-butene and pentene-l.

7. The process or claim 1 whereinthe olefin content of said olefin mixture is composed chiefly of aliphatic pentenes, the more highly branched pentene boils not more than 6 degrees F. below the less highly branched pentene, 1

and wherein the mixture also contains a substantial amount of pentane which appears in the resulting gaseous phase in admixture with the lesslfiihly branched pentene. 1

" 8. The process of claim l'whereinsaidiurtural is employed in 9. mol ratio to said mixture of from 5:1to 40:1.

I CHARLESE. WEIJJNG.