US2372941A - Process for concentrating diolefins in mixtures containing same - Google Patents

Description

Patented Apr. 3, 1945 PROCESS FOR CONCENTRATING DIOLEFINS IN MIXTURES CONTAINING SAME Theodore W. Evans, Oakland, Rupert 0. Morris,

Berkeley, and Edward C. Shokal, Oakland, Calif assignors to Shell Development Company, San Francisco,

Delaware No Drawing.

rlgin 1941, Serial No. 409,292.

Calla, a corporation of al application September 2,

Divided and this application April 21, 1942, Serial No. 439,914

8 Claims.

'This invention relates to increasing the concentration of diolefins in fluid mixtures containing diolefins and mono-olefins, and particularly to the concentration of isoprene and/or piperylene in fluid mixtures containing these acyclic diolefins and mono-olefins of such closely adjacent boiling points that practical concentration by ordinary fractional distillation of the mixtures is very difficult, if not impossible.

When petroleum products are subjected to cracking or pyrolysis, the resulting products, besides the relatively high boiling hydrocarbons,- also contain relatively large quantities of the relatively low boiling unsaturated hydrocarbons which consist of'or contain various mono-olefins and diolefins. For example, when straight run gasoline fractions, e. g., a fraction having a specific gravity D42--0.804, an initial boiling temperature of about 162 C., a maximum boiling temp'erature of 220 0., and an octane number of 40.5 (as determined by the C. F. R. Motor Method), are subjected to pyrolysis at temperatures of between about 700 C. and about 800 C., in the presence or absence of reaction-promoting catalysts, the eflluent fractions frequently contain as much as 50% or more of hy ocarbons boiling below about 50 C. A large ceritage of this relatively low boiling fraction consists of monoolefins and diolefins containin from two to five carbon atoms per molecule. Such hydrocarbon mixtures may be subjected to fractional distillation to obtain fractions each of which consists of, or at least predominates in, hydrocarbons having the same number of carbon atoms per mole; cule. For instance, by means of an ordinary fractional distillation, it is possible to recover separately a fraction which consists of, or predominates in, hydrocarbons having five carbon atoms per molecule, this fraction containing the various five-carbon atom mono-olefins and diolefins, such bon fraction boiling between 28 C. and 35 C., v

as pentene-l, pentene-Z, 2-methyl-butene-1, 2-

methyl-butene-2, 3-methyl-butene-1, isoprene, piperylene, cyclopentadiene, etc. It is known that members of each group of hydrocarbons (i. e.,

following table gives the boiling temperatures of most of the olefins (i. e.-monoand diolefins) having five carbon atoms per molecule:

This proximity of the boiling temperature of the individual five-carbon monoand diolefins renders it practically impossible to separate or even concentrate any one or more of them in a hy-. drocarbon fraction consisting of several or all of these and other five-carbon atom olefins. For

instance, as will be seen from an example described hereinbelow, when an olefinic hydrocarand containing approximately 34.8% ofdiolefins,

is subjected even to a careful fractional distilla-- tion in the absence of anyauxiliary agent, the

maximum concentration of the diolefins in any one cut is about 55%-56%, this fraction consti tuting only approximately 7% of the charge subjected to distillation.

It is, however,'frequently desirable to effect a more or less complete separation of the hydrocarbons in such olefinic fractions into at least two parts, one of which consists of, or greatly predominates in, the diolefins while the other predominates in the mono-olefins. For instance;

both isoprene and piperylene (whether individually or in combination with each other) are suitable as substitutes for butadiene in various polymerization and co-polymerization processes in which the presence'of the five-carbon atom monoolefins, at least in relatively large percentages, is undesirable. Other instances of the desirability or necessity of separating the mono-olefins from the diolefins for the, best utilization of one part or the other, as well as the concentration of the C5 diolefins in mixtures thereof with the corresponding mono-olefins, are familiar to those skilled in the field of hydrocarbon reactions.

Although such separation or concentration of diolefins in olefinic fractions having narrow boillng ranges have been attempted, none of the methods employed heretofore, including the various chemical processes, has been found to be simple or to be economically practical.

It is, therefore, the main object of the invention to provide a simple and economical process for the concentration or separation of isoprene and/or piperylene from hydrocarbon mixtures containing either or both of these acyclic diolefins and mono-olefins having boiling temperatures which are close to those of the mentioned diolefins. Another object is to provide an economical process whereby hydrocarbon fractions consisting of substantial quantities of monoolefins and of diolefins having five carbon atoms per molecule may be treated to obtain one or more fractions in which the diolefin or diolefins are present in relatively greater concentrations.

According to the present invention, the above and other objects are attained by subjecting the mixtures containing isoprene and/or piperylene and one or more of the five-carbon atom monoolefins, to fractional distillation in the presence of a certain compound which forms minimumboiling azeotropic mixtures with 'the hydrocarbons subjected to treatment, these azeotropic mixtures containing not less than mol per cent nor more than 90 mol per cent of said azeotrope-forming compound. More specifically stated, the present invention comprises a process in which a relatively narrow boiling hydrocarbon fraction consisting of, or predominating in, monoand diolefins, is subjected to fractional distillation in the presence of acetone.

The quantity of acetone to be added to the hydrocarbon fraction to be distilled may vary within relatively wide limits, depending, in part, on the composition of the hydrocarbon mixture to be fractionated and the distillation equipment employed. Acetone, when added to five-carbon atom olefinic fractions, forms minimum-boiling mixtures with each of the monoand dioleflns constituting the mixture. In other words the acetone causes a lowering of the effective distillation temperatures of the various constituents of such mixture. The drop in the distillation temperature, however, is not uniform for the diiferent olefins, the acetone lowering the'distillation temperatures of the five-carbon atom mono-olefins to a greater extent than the the distillation temperatures of the diolefins, e. g., isoprene and piperylene. Therefore, the azeotropes of the mono-oleflns boil before the azeotropes of the diolefins, thus permitting an economic, efficient and ready method for the separation or concentration of isoprene and/or piperylene present in oleflnic hydrocarbon fractions, and particularly those boiling within such a narrow boiling range as to render practical concentration by ordinary or simple fractional distillation very difllcult and uneconomical, if not substantially impossible.

In general, it is preferable to employ the acetone in a quantity substantially sufficient to form azeotropes with each and every olefin present in the mixture treated. However, greater or lesser amounts may also be used. If the acetone is added to a mixture of five-carbon hydrocarbons in an amount only suflicient to form the azeotropes of the mono-oleflns present, the fractional distillation of this mixture will permit the removal of the azeotropes containing the mono-oleflns, while the dioleflns may then be left behind as a residue in the desired concentrated condition. However, because of the fact that the-selective or preferential solvent action .olefins have thus been removed in the overhead fraction or fractions. Also, the use of excess amounts of the acetone, besides having no deleterious effect upon the separation of the specified mono-oleflns from the diolefins, is advantageous in that it permits the utilization of smaller fractionation towers, i. e., towers having comparatively fewer plates.

The distillation according to the present invention may be effected at' atmospheric pressure, although higher or lower pressures may also be used. It must noted, however, that the use of pressures other than those substantially in the neighborhood of atmospheric may change the ratio of the compounds in the azeotropic mixture. Also, the distillation temperatures for the azeotropic mixtures containing the different olefins will change depending on the pressure maintained in the distillation unit.

The present process of concentration of the acyclic diolefins, i. e., isoprene and piperylene, may be applied to any oleflnic hydrocarbon mixture containing either or both of these diolefins.

However, it is generally preferable to fractionate preliminarily the hydrocarbon mixture, and then to apply the present process of azeotropic distillation to the relatively narrow boiling olefinic fraction or fractions thus produced. For instance, the acyclic diolefins may be separated from or concentrated in a fraction predominating in or consisting of the five-carbon atom olefins, this fraction boiling between about 10 C. and about 50 C. to 55 C. However, when such a fraction is distilled in the presence of acetone, the fraction predominating in the diolefins will contain both isoprene and piperylene. Also, probably because of the difference in the boiling temperatures of v the azeotropes containing the acetone and the isoprene or piperylene, the concentration or separation of mixtures thereof from olefinic fractions containing both is not attained as readily as when the fractions contain only one of these acyclic diolefins. Therefore, for the purpose of eflecting the concentration of these diolefins in the most economic and practical manner, it is generally preferable first to subject the live-- carbon atom olefinic fraction to an ordinary fractionation to obtain two relatively narrow cuts, one

of which contains the isoprene and mono-oleflns having boiling temperatures in the close vicinity of the boiling temperatures in this diolefin, and.

the other containing piperylene and mono-oleflns boiling close thereto. These narrow boiling fractions may then be separately treated according to the present process to obtain cuts consisting of or greatly predominating in the respective diolefins, the degree of concentration of these acyclic diolefins depending, in part, on the efliciency of the fractionating tower employed. The presence of cyclopentadiene in the fraction subjected to azeotropic distillation according to the present process is notdetrimental to the eflicient concentration of the acyclic diolefln or dioleflns present therein. However, since this cyclic diolefln is readily polymerizable even at the distillation temperatures employed, it is generally preferable to .pretreat the fractions to separate the cyclopentadiene present therein. Such separation may, for example, be efiected by heating the fraction 'to selectively polymerize this cyclic diolefin, separating the polymer thus formed, and, if desired, depolymerizing the polymer to recover the monomeric cyclopentadiene.

The following detailed examples are given for the purpose of illustrating the present process and the modes of executing the invention. These examples are indicative of the advantages derived from efiecting the separation or concentration of the five-carbon atom acyclic dioleflns accord-- ing to the present process. It is to be understood, however, that the invention is not to be considered as limited to the specific embodiments, modes or conditions of operation disclosed.

Example I Approximately 215 grams of an oleflnic fraction boiling between about 28 C. and 35 C., and containing approximately 34.8% (mainly isoprene), were mixed with acetone in such proportions that the mixture contained about 46.8% by weight of the olefins to be treated.

This mixture was then distilled in a column containing 15 to 18 theoretical plates, the upper end of which was in communication with a condenser. The results of this treatment are given in the following table:

The third and fourth cuts of this distillation, representing 19.8% of the total charge, contained 54.7% of the total diolefins in the charge, the

pentadiene concentration in this combined fraction being about 62.3%.

Example II For purposes of comparison, approximately 210 grams of the above-described olefin fraction boiling between 28 C. and 35 C. and containing approximately 34.8% diolefins, were distilled in the same apparatus as that employed in the preceding example. In this case, however, no acetone was added. The'results of this distillation are as follows:

W1... emes e g Per cent Boiling Cut number of cut, in the in grams charge range a C overhead fraction A comparison of the above data shows the superiority of effecting the concentration of C5 diolefins by subjecting them to distillation in the presence of acetone. a

The separation of the acetone from the various fractions obtained may be effected, for example, by means of cooling, addition of water, or by a of diolefins which predominates in piperyle'ne.

treatment of the azeotropic mixtures in any other convenient manner known to those skilled in the We claim as our invention:

1. A process for separating a mixture of isoprene and mono-olefins having boiling temperatures close to that of isoprene which comprises adding acetone to said mixture in a quantity at least suflicient to form minimum-boiling azeotropic mixtures with 'all of the hydrocarbons, subjecting the mixture thus formed to fractional distillation to distill off an azeotropic fraction the hydrocarbon content-of which predominates in the mono-olefins, continuing the fractional distillation to distill ofi a fraction comprising an azeotropic mixture the hydrocarbon content of which predominates in isoprene, and separating the isoprene from said last-mentioned fraction.

2. A process of concentrating piperylene in a hydrocarbon mixture predominating in five-carbon atom diolefins and mono-olefinshavin'g boil-y ing temperatures close to that of piperylene which comprises adding acetone to said mixture in a quantity at least suflicient to form minimum-boiling azeotropic mixtures with all of the hydrocarbons, and subjecting the mixture thus formed to fractional distillation to obtain at least two overhead azeotropic'fractions, the first of which predominates in a minimum-boiling azeotropic mixture of the added acetone and the mono-olefins, while the second of said fractions comprises an azeotropic mixture the hydrocarbon content of 3. A process of concentrating isoprene in a hydrocarbon mixture predominating in-flve-carbon atom diolefins and mono-olefins having boiling 40 temperatures close to that of isoprene which com prises adding acetone to said mixture in a quantity at least suiiicient to 'form minimum-boilingazeotropic mixtures with all of the hydrocarbons, and subjecting the mixture thus formed to fractional distillation to obtain at least two overhead azeotropic fractions, the first of which predomimates in a minimum-boiling azeotropic mixture of the added acetone and, the mono-olefins, while the second of said fraction comprises an azeotropic mixture the hydrocarbon content of which predominates in isoprene.

4. A process for separating a mixture of acyclic diolefins and mono-olefins having five carbon atoms per molecule into a fraction predominating in the mono-olefins and'a fraction predominating in the diolefins, which comprises adding acetone to said mixture in a quantity at least suflicient to form minimum-boiling azeotropic mixtures with all of the hydrocarbons, subjecting the mixture thus formed to fractional distillation to distill off an initial fraction comprising anazeotropic mixture the hydrocarbon content of which predominates in the mono-olefins, continuing the frac-- tional distillation to distill ofi a fraction compris- I ing an azeotropic mixturethe hydrocarbon content of which predominates in the diolefins, and

separating the hydrocarbons from each of said azeotropic mixtures.

5. A process for separating a hydrocarbon mixwhich comprises adding acetone to said mixture in a quantity at least suflicient to form minimumlMs application is a division of our copen'ding boiling azeotropic mixtures with all of the monoolefins in the mixture, and subjecting the mixture thus formed to fractional distillation to distill off an overhead fraction comprising an azeotropic mixture predominating in acetone and the monooleflns.

6. A process of concentrating acyclic diolefins in a mixture consisting of five-carbon atom hydrocarbons containing diolefin and mono-olefins which comprises separating cyclic diolefins from said mixture, adding acetone to the remaining mixture in a quantity at least sufficient to form quantity at least sufllcient to form minimumboiling azeotropic mixtures with all of the monoolefins and the diolefins in the mixture, and subjecting the mixture thus formed to fractional distillation to obtain at least two overhead tractionsthe first of which predominates in a minimum-boiling azeotropic mixture of the added ace tone and the mono-olefins, while the second of said fractions predominates in a minimum-boiling azeotropic mixture of the acetone and the dioleflns.

8. A process of concentrating dioleflns in a hydrocarbon mixture predominating in five-carbon atom diolefins and mono-oleflns which comprises adding acetone to said mixtur in a quantity at least suflicient to form minimum-boiling azeotropic mixtures with all of the hydrocarbons except the diolefins in the mixture, and subjecting .the mixture thus formed to fractional distillation to distill of! as an overhead fraction a minimumboiling azeotropic mixture the hydrocarbon content of which predominates in mono-oleflns.

THEODORE W. EVANS. RUPERT C. MORRIS. EDWARD C. SHOKAL.