US3436436A - Method for separation of conjugated diolefin by back wash in extractive distillation - Google Patents

Description

Aprll 1, 1969 SUSUMU TAKAO ET AL 3,436,436

METHOD FOR SEPARATION OF CONJUGATED DIOLEF'IN BY BACK WASH INYEXTRACTIVE DISTILLATION Filed Sept. 20. 1966 STEAM Q2 /2 5 g: 57'EAM STEAM *3 STEAM INVENTORS S US UM U TARA 0 HIROSHI HOKA RI .4 rronue'ys United States Patent U.S. Cl. 260681.5 8 Claims ABSTRACT OF THE DISCLOSURE Method for the separation of conjugated diolefin from a C or C -hydrocarbon mixture by fractionation of the hydrocarbon mixture to a more readily soluble hydrocarbon fraction and a less readily soluble hydrocarbon fraction. The hydrocarbon mixture is introduced to the intermediate portion of an extractive distillation zone wherein a polar solvent is flowed downwardly. In this manner, a less readily soluble hydrocarbon fraction is recovered as the top distillate of the zone while a more readily soluble hydrocarbon fraction together with the solvent is recovered as the bottom liquid of the zone. The bottom liquid is then transferred to a recovery zone which is operated at a lower pressure than the extractive distillation zone. An overhead vapor stream from the recovery zone is returned into the extractive distillation zone while the bottom liquid of the recovery zone is transferred to a stripping zone. The more readily soluble hydrocarbon fraction is recovered as the top distillate while the solvent is withdrawn as the bottom liquid.

Summary of the disclosure This invention relates to a method for the separation of conjugated diolefins from a hydrocarbon mixture. More particularly, this invention is concerned with a process for fractionating a hydrocarbon mixture comprising conjugated diolefin with or without higher acetylene by extractive distillation thereby to obtain a more readily soluble hydrocarbon fraction and a less readily soluble hydrocarbon fraction, by using an extractive distillation column, a stripping column and a recovery column or flash tank (sometimes these are generically referred to as a recovery tower) placed between the former two and being operated at a lower pressure than that of the extractive distillation column, which process comprises passing the bottom liquid of said extractive distillation column into said recover tower, returning the recovered gas from the recovery tower to the extractive distillation column by means of a compressor and introducing the bottom liquid of the recovery tower into the stripping column to recover the extracted materials (conjugated diolefin and/ or higher acetylene), whereby the extractive distillation is carried out without raising the bottom temperature of the extractive distillation column so that the diolefin and higher acetylene may be prevented from polymerization.

The terms a more readily soluble hydrocarbon fraction and a less readily soluble hydrocarbon fraction used herein do not mean the absolute solubility of individual hydrocarbon components of the fraction into a solvent used, but means the relative solubility of the individual components into the solvent with reference to the solubility of an objective component to be recovered. Whether the objective component can be recovered as a more readily soluble hydrocarbon fraction or as a less readily soluble hydrocarbon fraction will be dependent on the chemical composition of a hydrocarbon mixture as 3,436,436 Patented Apr. 1, 1969 feedstock and on the class of hydrocarbon components to be separated from said mixture. For instance, 1,3-butadiene as an objective component in Example 1 was recovered together with higher acetylene as a more readily soluble hydrocarbon fraction, while extractive distillation of a mixture consisting of 1,3-butadiene and higher acetylene (Example 2) gave 1,3-butadiene as a less readily soluble hydrocarbon fraction and higher acetylenes as a more readily soluble hydrocarbon fraction.

The term conjugated diolefin used herein means unsaturated hydrocarbons having a conjugated double bond, e.g. 1,3-butadiene, isoprene, 1,3-pentadiene, cyclopentadiene, etc. The term, higher acetylene used herein means acetylenically unsaturated hydrocarbons having a carbon-to-carbon triple bond, e.g. methylacetylene, ethylacetylene, dimethylacetylene, vinylacetylene, pentynes etc., and allennically unsaturated hydrocarbons having a cumulated double bond, e.g. 1,2-butadiene, ethyl allenes, etc.

In the manufacture of ethylene and/or propylene by thermal cracking of a petroleum fraction, e.g. LPG, naphtha, etc., a hydrocarbon fraction containing conjugated diolefin is obtained as byproduct, from which are recovered a c -hydrocarbon mixture (C -fraction) comprising l,3-buta-diene and a C -hydrocarbon mixture (C fraction) comprising isoprene 1,3-pentadiene and cycl0- pentadiene. The C -fraction usually contains butanes, nbutene, isobutene, 1,3-butadiene, vinyl acetylene, ethyl acetylene, 1,2-butadiene, etc. and the C -fraction contains pentanes, n-pentenes, iso-amylene, cyclopentene, isoprene, transor cis-l,3pentadiene, cyclopentadiene, higher ace tylenes, etc. Further, the recovery of a C -fraction containing 1,3-butadiene is known in the catalystic dehydrogenation of n-butane and/ or n-butene, and that of a C fraction containing isoprene is known in the dehydrogenation of isopentane and/or isoamylene. It is also known that a thermally cracked oil obtained b cracking of a petroleum fraction contains conjugated diolefin. Usually these fractions contain the small amount of higher acetylenes.

Extractive distillation is known as an effective method for separating desirable conjugated diolefin from the above-mentioned, conjugated diolefin-containing hydrocarbon mixture. For instance, the individual components of the C -hydrocarbon mixture usually have boiling points falling within a very narrow range, and moreover, they sometimes form an azeotrope. Because of these facts, an ordinary distillation method is not effective for separation of pure 1,3-butadiene from the said mixture.

Extractive distillation usually is carried out in an apparatus comprising an extractive distillation column and a stripping column. In the extractive distillation column, a starting feedstock is introduced at the intermediate portion of that column, while a polar solvent is flowed downwardly from the top of the column. By the action of a reboiler attached to the bottom of said column, a part of the top distillate is refluxed to the extractive distillation column, thereby separating the feedstock to a more readily soluble hydrocarbon fraction and a less readily soluble hydrocarbon fraction. The bottom liquid taken out from the column bottom (said liquid comprising the extracted, more readily soluble hydrocarbon fraction and the polar solvent) is heated in the stripping column, from which the more readily soluble hydrocarbon fraction and the polar solvent are recovered respectively as the column top vapor and the column bottom liquid. The solvent is then cooled and returned back to the extractive distillation column for cyclic use. The concentration of the polar solvent used in the extractive distillation column generally is 40 to by mole. Suitable polar solvents include, for example, acetonitrile, furfural, N-rnethyl pyrrolidone, butyrolactone, dimethylformamide, acetone and their combination with water.

In the extractive distillation wherein one component or a mixture of a definite composition is to be extracted with a solvent, the volume of the extracted solution generally depends upon the kind and volume of the solvent used, and the operating pressure and temperature employed. In the extractive distillation operation, the kind of the solvent and the volume ratio of the solvent to the feedstock are suitably selected to conform to its optimum operation, and the pressure corresponds to the temperature at which the top distillate is condensed by a coolant. Usually water is used as a coolant. Accordingly, the bottom temperature of the extractive distillation column must be kept at a certain level so that the volume of the substance to be extracted may be equal to the volume of the more readily soluble hydrocarbon fraction of the feedstock.

Where a hydrocarbon mixture containing conjugated diolefin together with or without higher acetylene is subjected to the above-mentioned extractive distillation, a

solvent containing conjugated diolefin or higher acetylene is inavoidably exposed to relatively high temperatures, e.g., 120 to 230 C. or higher. As the result, polymerization of the conjugated diolefin and/or higher acetylene takes place until the separated gummy polymer causes choking or plugging of columns, heat exchangers, pipings, etc. or deposition of the polymer on the inner wall of the apparatus. It is known that the polymerization of the conjugated diolefin and/or higher acetylene in the solvent occurs even at 120 C. Thus, it is practically very difficult to carry out the continuous operation over a long period of time. In order to avoid such disadvantages as above, addition of a polymerization inhibitor and/or a chain transfer agent to the solvent has been proposed to have decreased tendency for the polymerization of the conjugated diolefin and/r higher acetylene. However, this is still unsatisfactory because a certain high temperature, e.g., more than 160 C., is required for extractive distillation.

It is accordingly one object of the present invention to provide a method for the extractive distillation of conjugated diolefin, which method is free from the inconveniences caused by the polymer formation. Another object of the present invention is to provide an improved extractive distillation method for conjugated diolefin, by which method desirable conjugated diolefin can be obtained in high purity and at good efficiency, even by using a relatively low temperature for the bottom of the extractive distillation column. Still another object of the present invention is to provide an efficient method for obtaining highly pure 1,3-butadiene from a feedstock containing both 1,3-butadiene and higher acetylene by removal of higher acetylene. Other objects, features, capabilities and advantages as comprehended by the invention will be apparent from the description and claims which follow.

The gist of the present invention resides in a method for the recovery of conjugated diolefin from a hydrocarbon feedstock containing conjugated diolefin together with or without higher acetylene by extractive distillation through an extractive distillation column and a stripping column between which a recovery column or flash tank (these are generically referred to as a recovery tower) is further provided, which method comprises passing the bottom liquid of the extractive distillation column to the recovery tower, returning the vapor recovered from the recovery tower back to the extractive distillation column and introducing the bottom liquid of the recovery tower into the stripping column to recover the extracted material and solvent.

In the present invention, wherein it is possible to uptake a part of the extracted hydrocarbon fraction as vapor from the top of the recovery tower, the bottom temperature of the extractive distillation column can be maintained at a relatively low level in comparison with an ordinary extractive distillation. Thus, the extracted material (conjugated diolefin and/or higher acetylene) is not exposed to a high temperature during the operation of extractive distillation, and according to the present invention, it is possible to carry out the continuous distillation over a long period of time, with preventing the formation of gummy polymer.

In view of the economical reasons, a refrigeration system sometimes is omitted for the extractive distillation column, in which consequently high pressure prevails. In such case, too, the present invention is effective and provides remarkable technical advantages. In the prior art where the operating pressure is high, it is generally required to adopt a high temperature for the bottom of the extractive distillation column. Under such circumstance, there may be developed several troubles including choking of the reboiler with polymerized conjugated diolefin or higher acetylene, decomposition of the solvent, corrosion of the apparatus, etc. These troubles can be settled by employing the present invention which allows us to maintain the bottom temperature at a low temperature level. Particularly effective is the present invention when a high boiling solvent with high solubility, such as N-methyl pyrrolidone, dimethyl formamide, furfural, butyrolactone, etc., is used. The present invention is further applicable to the separation of higher acetylene safely by using a recovery column operating at a relatively low pressure, because higher acetylene can not be separated by itself in the extractive distillation column due to the hazardous problem. Higher acetylene such as vinylacetylene, ethyl acetylene, etc. is extremely explosive when it is highly concentrated at such high temperature and high pressure under which the extractive distillation column is operated.

Generally when a hydrocarbon mixture containing paraffinic hydrocarbons, monoolefinic hydrocarbons, conjugated diolefin and higher acetylene is subjected to extractive distillation, paraffinic hydrocarbons and monoolefinic hydrocarbons (which constitute a less readily soluble hydrocarbon fraction) are recovered as the top distillate of the extractive distillation column, while conjugated diolefin and higher acetylene (which constitute a more readily soluble hydrocarbon fraction) recovered as the bottom liquid of said solumn. The extracted hydrocarbon fraction contained in said bottom liquid is further subjected to extractive distillation whereby highly pure conjugated diolefin is recovered as the top distillate while the higher acetylene is recovered as the bottom liquid. Thus the aforementioned two-stage extractive distillation process can be carried out to recover the conjugated diolefin. Strictly speaking, a hydrocarbon mixture containing a desirable conjugated diolefin, more soluble hydrocarbons than said conjugated diolefin and less soluble hydrocarbons than said conjugated diolefin is subjected to the first-stage extractive distillation whereby a mixture of said conjugated diolefin and the more soluble hydrocarbons is obtained as the more readily soluble hydrocarbon fraction while the less soluble hydrocarbons are retained as the less readily soluble hydrocarbon fraction, and then the more readily soluble hydrocarbon fraction of the above first-stage is subjected to the second-stage extractive distillation whereby said conjugated diolefin is recovered as the less readily soluble hydrocarbon fraction while the more soluble hydrocarbons are retained as the more readily soluble hydrocarbon fraction.

For example, the extractive distillation of a C -fraction firstly yields 1,3-butadiene, ethylacetylene and 1,2 butadiene (more readily soluble hydrocarbon fraction) as the bottom liquid, which, when subjected to further extractive distillation, produces l,3-butadiene (now a less readily soluble hydrocarbon fraction) as top distillate while ethylacetylene, 1,2-butadiene or other higher acetylene are yielded as the bottom liquid. Extractive distillation of a C -fraction is somewhat more complicated than that of the C -fraction because of the existence of other additional conjugated diolefins than the desired conjugated diolefin. In the case of the C -fraction, too, however, the desired conjugated diolefin is obtained by the two-stage extractive distillation. For example, if isoprene is desired, the extractive distillation of a C -fraction firstly yields isoprene, cyclopentadiene, 1,3-pentadiene, propylacetylene, cyclopentene and other higher acetylene (more readily soluble hydrocarbon fraction) as the bottom liquid, which, when subjected to further extractive distillation yields isoprene (now a less readily soluble hydrocarbon fraction) as top distillate while cyclopentadiene, 1,3- pentadiene, propylacetylene, cyclopentene and other higher acetylene are contained in the bottom liquid. Thus, a method of the present invention is noted to be successfully applicable to the two-stage extractive distillation as mentioned above.

No special apparatus is required for the recovery column or flash tank used in the invention. Which of the recovery column or flash tank is to be used will be determined depending on the necessity of the further separation of the extract of the extractive distillation column. A recovery column or flash tank should be operated under a pressure lower than that of the extractive distillation column. The bottom temperature of the recovery column or flash tank should be kept at a temperature equal to or lower than that of the extractive distillation column. Further the pressure and bottom temperature of the recovery column or flash tank should be optionally determined depending on the content of the more readily soluble hydrocarbon fraction of the feedstock fed to the extractive distillation column and the type and amount of the solvent used. Generally, since the separation is completed in the extractive distillation column, a flash tank of the usual hollow cylindrical type is satisfactorily used instead of a recovery column as the intended performance is simple liquidvapor separation. On the other hand, if the separation is not completed in the extractive distillation column, for instance, in cases where it is impossible to keep higher acetylene at a high concentration in the extractive distillation column due to the danger of explosion under high pressure and high temperature, a multi-plate column or packed tower can be conveniently used for further separation under lower pressure than the extractive distillation column. A flash tank should preferably be equipped at its bottom with a reboiler in order to facilitate the control of dissolution of the extract into the solvent.

In carrying out the invention, the extracting solvent containing a polymerization inhibitor and/or a chain transfer agent should preferably be used. Any polymerization inhibitor which inhibits polymerization of conjugated diolefins, acetylenic and allenic hydrocarbons, or any chain transfer agent which inhibits formation of high polymers may be used. Furfural, benzaldehyde or aromatic nitro compound is particularly etfective for the extractive distillation of conjugated diolefin, and they can prevent polymerization to a considerable degree even in the presence of iron rust which catalyzes the polymerization. The combinations of the above polymerization inhibitor with sodium nitrate, methylene blue, sulfur, phenols, aromatic amines, etc. which are known as stabilizers for unsaturated compounds, are further effective. These additives may be used in the amount of about 0.0-1 to based on the solvent.

Now reference will be made in conjunction with the attached drawing.

In FIG. 1 which illustrates the ordinary extractive distillation operations schematically, a hydrocarbon mixture as feedstock is fed through a pipe 1 to an extractive distillation column 7 at its middle portion. The said column consists of the upper part which part which is an absorbing section and the lower part which is a stripping section. A cold solvent is introduced through a pipe 2 into the column at its top. Extractive distillation is conducted by heating of a reboiler with steam, said reboiler being provided to the bottom of the extractive distillation column. A less readily soluble hydrocarbon fraction is distilled out from the column top and cooled by a condenser 9. A part of the condensate is refluxed to the column through a pipe 3. The remaining part is taken away through a pipe 4. From the bottom of the extractive distillation column 7, a bottom liquid -(i.e. a solvent solution containing the more readily soluble hydrocarbons) is taken out and then it is passed to the top of a stripping column '8 through a pipe 5. In the stripping column, stripping is made at a temperature approximating to the bubble point of the solvent under the operating pressure. The extract is stripped through a pipe 6. Regenerated hot solvent is withdrawn from the bottom of the stripping column "8 and then passed through a pump 11 and a cooler 10 to the extractive distillation column for reuse. When a C fraction is subjected to extractive distillation thereby to obtain 1,3- butadiene, the output from the pipe 6 usually is about 9 8%-pure 1,3-butadiene, entrained with a little amount of ethyl acetylene or the like higher acetylene.

In FIG. 2 which illustrates the flow diagram of one embodiment of the present invention, a hydrocarbon mixture containing conjugated diolefin and/ or higher acetylenes is fed through a pipe 1 into an extractive distillation column 7. A more readily soluble hydrocarbon fraction, separated from a less readily soluble hydrocarbon fraction in the extractive distillation column, is passed through a pipe 12 to a recovery tower or flash tank 13. The recovery tower is operated at a temperature equal to or below the bottom temperature of the extractive distillation column and at a pressure lower than the operating pressure of the extractive distillation column, so as to realize substantially complete dissolution of all the more readily soluble hydrocarbon fraction contained in the hydrocarbon feedstock. The hydrocarbon fraction which entrains due to excessive dissolution because of the low bottom temperature of the extractive distillation column is returned through a pipe 14 and a compressor 15 back to any portion (for example, the bottom portion as shown in FIG. 2) of the extractive distillation column 7. If the separation is not complete in the extractive distillation column, a multiplate column or packed tower is conveniently used for further separation. In such case, refining operation can be supplemented in a recovery tower 13 by the provision. of a cooler 17 prior to entering into the tower 13. However, if extractive distillation is complete in the extractive distillation column 7, the requirement for the performance of a recovery tower 13 is simply to return the partly entrained excess hydrocarbon fraction back to the extractive distillation column. In this case, the recovery tower may be a hollow cylindrical flask tank preferably equipped with a reboiler, and a cooler 17 is omissible.

The more readily soluble hydrocarbo-n-containing solution is passed from the recovery tower 13 through a pipe 16 to a stripping column 8, in which the dissolved more readily soluble hydrocarbons are stripped at the bubble point of the solvent under the operating pressure. The regenerated solvent is recycled through a pump 11, a cooler 10 and a pipe 2 to the top of the extractive distillation column.

The bottom temperature of an extractive distillation column is set so as to effect substantially complete dissolution of the contained more readily soluble hydrocarbon at the operating pressure. If the operating temperature is too high, the entrainment of the less readily soluble hydrocarbons into the solution is advantageously avoidable, and however, the discharge of the more readily soluble hydrocarbons from the column top increases. On the contrary, if the operating temperature of the extractive distillation column is too low, a part of the less readily soluble hydrocarbon is taken away along with the solution from the bottom, thereby to cause an adverse influence on separation of the more readily soluble hydrocarbon fraction and the less readily soluble hydrocarbon fraction.

In accordance with the present invention, the bottom temperature of the extractive distillation column is kept below a certain critical temperature. The hydrocarbons excessively dissolved in the used solvent is passed to the recovery column or flash tank, from which the recovered gas is returned back to the extractive distillation column. Thus, the deposition of the polymer or other troubles can be avoided. Accordingly, desired conjugated diolefin can be separated at a high purity and without a loss. Further, if the operating pressure of the extractive distillation column is relatively high, it is possible to keep the bottom temperature of the extractive distillation column at a low temperature thereby to have eflicient separation of the more readily soluble hydrocarbon fraction and the less readily soluble hydrocarbon fraction.

1,3-butadiene, isoprene, cyclopentadiene and 1,3-pentadiene which are obtainable by application of the present invention are valuable substances for use in the manufacture of syntheti rubbers, pharmaceuticals and the like.

The following examples describe certain ways in which the principle of the invention has been applied, but are not to be construed as limiting its scope.

Example 1 Composition Vol. percent Classification C hydroearbon 0. 1 Gig-hydrocarbon-.. 2. 3 iutane 4. 6

Less readily soluble n-Butane 10.0 v and i Butenes 46. 2 hydrocarbon fraction. Trans-Z-butene. 6. 5 Cis-2-butene..- 4. 1 1,3-butadiene. 26. }More readily soluble Higher acetylene- 0. 2 hydrocarbon fraction.

The feed rate of the hydrocarbon feedstock in the extractive distillation column is 15 Nm. /hr. The solvent amount used is 250 l./hr. and the reflux amount is 45 kg./hr. Under the operating conditions specified below, the following results are obtained.

Run 1 Run 2 Operating pressures (kg/cm. G):

Extractive distillation column... 4 5 Flash tank 3 3 Stripping column 0. 2 0. 2 Botton temperatures 0.):

Extractive distillation column. 150 150 FlMh tank 150 150 Stripping column 163 163 Vapor amount returned back irom flash tank (limi /hr.) 0.87 1. 28 Purity of more readily soluble hydrocarbons (1,3-butediene and higher acetylene) (percent). 98. 5 98. 5

In each run, highly pure 1,3-butadiene can be obtained even at a low bottom temperature of the extractive distillation column. After the operation over 2,000 hours, no polymer deposition is observed in the bottom of the extractive distillation column.

For comparison, extractive distillation is conducted according to the prior art process not using a flash tank between the extractive distillation column and the stripping column. The type and feed rate of the hydrocarbon feedstock as well as the solvent amount and the reflux amount are same as before. The results are in the following table:

set forth Operating pressures (kg/em. G):

Extractive distillation column 4 4 5 5 Stripping column 0. 2 0. 2 0. 2 0. 2 Bottom temperature 0.):

Extractive distillation column 150 210 Stripping column 163 163 163 163 Purity of more readily soluble hydrocarbons (1,3-butadiene and higher acetylene) (percent) 85. 2 98. 5 70. 5 08. 5

Example 2 By using an extractive distillation apparatus (FIG. 2) which comprises an extractive distillation column having 36 plates, a stripping column having 20 plates, and a recovery column having 30 plates, higher acetylene is removed as an extract (more readily soluble hydrocarbon) from a hydrocarbon mixture (obtained in Example 1) mainly comprising 1,3-butadiene and higher acetylene, thereby to recover highly pure 1,3-butadiene from the top of the extractive distillation column. The recovered gas from the top of the recovery column is returned to the feed plate of the extractive distillation column. The solvent used is the same as in Example 1. The hydrocarbon mixture as feedstock has the following composition:

Composition V01. percent Classification Mono-olefins 0. 6 Less readily soluble hydro- 1,3-butadiene 98. 1 carbon 1,2-butadiene- 0. 4 Ethyl acetylene 0. 3 More readily soluble hydro- Monovinyl acetylene 0. 6 carbon.

The feed rate of the hydrocarbon feedstock is 4.4 Nmfi/hr. The solvent amount is 26 l./ hr. and the reflux Temperature at the outlet of the heat exchanger (17 of FIG. 2) (C.) Vapor amount obtained at the top of the extractive distillation column (NmF/hr.) 4.3 Vapor amount returned back from the recovery tower (NmF/hr.) 0.56 Exhaust amount of the stripping column (Nm. /hr.) 0.10

Analysis (vol. percent) 'Iop dis- Bottom Top distillate of extract of tillate of the extracthe extraethe strip- Classification tive distlve disping tillation tillation column column column Mono-olefins 0.6 0.1 Less readily 1,3-butadiene 99.4 92.0 42. 5 hydm carbon. 1,2-butadiene.. 3. 5 17. 6 p um acetlylenefl 2.0 13.2 gg ffi onovin 1 2.4 acetyleirwl 26 7 hydrocarbon.

1 Less than 10 p.p.m.

As indicated above, higher acetylenes can be concentrated up to 57.5% by volume by extractive distillation at the bottom temperature of 150 C. unless a pressure of kg./cm. g.

It is assumed that in an ordinary extractive distillation process using an extractive distillation column and a stripping column, if the concentration of higher acetylenes up to 57.5% is required, the bottom temperature of the extractive distillation column should be about 230 C. Under such operating temperature, a long running operation is not possible because of the polymerization of 1,3-butadiene and/or higher acetylene and decomposition of the solvent used.

In this example, because of the danger of explosion of the higher acetylene at a high concentration under high pressure and high temperature, the separation of 1,3- butadiene and higher acetylene was incompletely carried out in the extractive distillation column, and the further separation was done in the recovery tower under a lower pressure than in the extractive distillation column. If the higher acetylene had no tendency of explosion, it is assumed that the separation could be completely carried out by the combination of an extractive distillation column and a flash tank as in Example 1. The ratio of 1,3- butadiene to the higher acetylene in the stripped gas from the stripping column was relatively high (42.5% by volume), but the complete separation in the recovery column is uneconomical because the absolute volume of 1,3- butadiene is very small.

What we claim is:

1. A method for the separation of conjugated diolefin from a C or C -hydrocarbon mixture by fractionation of the hydrocarbon mixture to a more readily soluble hydrocarbon fraction and a less readily soluble hydrocarbon fraction, which comprises introducing said hydrocarbon mixture to the intermediate portion of an extractive distillation zone wherein a polar solvent is flowed downwardly, thereby to recover a less readily soluble hydrocarbon fraction as the top distillate of said zone and a more readily soluble hydrocarbon fraction together with the solvent as the bottom liquid of said zone; transferring said bottom liquid to a recovery zone being operated at a lower pressure than the extractive distillation zone; returning an overhead vapor stream from said recovery zone into the extractive distillation zone; transferring the bottom liquid of said recovery zone to a stripping zone; and recovering the more readily soluble hydrocarbon fraction as the top distillate while the solvent is withdrawn as the bottom liquid.

2. A method as claimed in claim 1, wherein the starting hydrocarbon mixture is a mixture comprising a conjugated diolefin and less soluble hydrocarbons than said conjugated diolefin with or without more .soluble hydrocarbons than said conjugated diolefin, and said conjugated diolefin with or without said more soluble hydrocarbons are recovered as the more readily soluble hydrocarbon fraction.

3. A method as claimed in claim 1, wherein the starting hydrocarbon mixture is a mixture consisting of a conjugated diolefin and more soluble hydrocarbons than said conjugated diolefin, and said conjugated diolefin is recovered as the less readily soluble hydrocarbon fraction.

4. A method as claimed in claim 1, wherein the starting hydrocarbon mixture is a C or C -fraction obtained by thermal cracking of a petroleum fraction, a C -fraction obtained by catalytic dehydrogenation of n-butane and/ or n-butene, or a C -fraction obtained by dehydrogenation of isopentane and/0r isoamylene.

5. A method as claimed in claim 1, wherein the polar solvent is selected from dimethyl formamide, acetonitrile, furfural, N-methylpyrrolidone and butyrolactone.

6. A method as claimed in claim 5, wherein the polar solvent contains a polymerization inhibitor and/or a chain transfer agent.

7. A method for the separation of a conjugated diolefin from a C or C -hydrocarbon mixture containing said conjugated diolefin, more soluble hydrocarbons than said conjugated diolefin and less soluble hydrocarbons than said conjugated diolefin, which comprises the firststage wherein the said hydrocarbon mixture is treated by the method of claim 1 thereby to recover a mixture of said conjugated diolefin and the more soluble hydrocarbons as the more readily soluble hydrocarbon fraction while the less soluble hydrocarbons are retained as the less readily soluble hydrocarbon fraction; and the second stage wherein the more readily soluble hydrocarbon fraction of the first stage is treated again by the method of claim 1 thereby to recover said conjugated diolefin as the less readily soluble fraction while the more soluble hydrocarbons are retained as the more readily soluble hydrocarbon fraction.

8. A method for the separation of 1,3-butadiene from a C -hydrocarbon mixture containing parafiinic and olefinic hydrocarbons and higher acetylene as well as 1,3- butadiene, which comprises the first-stage wherein the said hydrocarbon mixture is treated by the method of claim 1 thereby to recover 1,3-butadiene and higher acetylene as the more readily soluble hydrocarbon fraction while other components are retained as the less readily soluble hydrocarbon fraction; and the second-stage wherein the more readily soluble fraction of the first stage is treated again by the method of claim 1 thereby to recover 1,3-butadiene as the less readily soluble fraction while higher acetylene is retained as the more readily soluble hydrocarbon fraction.

References Cited UNITED STATES PATENTS 2,993,841 7/1961 Sarno 202-395 3,000,794 9/1961 Tschopp 202-395 3,114,783 12/1963 Butler et al 260-674 3,146,190 8/1964 Papadopoulos et al. 208-313 3,230,157 1/1966 Hill et a1 208-53 3,242,227 3/1966 Kroeper et al 260-6815 3,265,591 8/1966 Halliwell 203-33 3,293,316 12/1966 Clay 260-6815 3,309,412 3/1967 Sakuragi et al 260-6665 3,317,627 5/1967 King et al 260-6815 3,338,823 8/1967 Voetter 208-313 2,388,041 10/1945 Craig 203-9 2,905,637 9/1959 Scofield et a1 208-311 3,080,437 3/1963 Scofield et a1 260-6815 DELBERTZ E. GANTZ, Primary Examiner. G. E. SCHMITKONS, Assistant Examiner.

US. Cl. X.R. 203-60

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