US2415006A - Butadiene separation - Google Patents

Description

Jan. 28, 1947.

K. H. HAACHMUTH BUTADINE SEPARATION Filed. Aug. 10, 1942 HOLVNOILDVtId lN'vl-:NToR KARL- H. HACHMUTH ATTO Patented .,Jan. 28,

BUTAIENE SEPARATION Karl H. Hachmuth, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application August 10, 1942, Serial No. 454,312

This invention relates to the recovery of butadiene-1,3 from a complex petroleum mixture comprising bo'th normally gaseous and normallyliquid hydrocarbons of both saturated and unsaturated linkages. More specifically, it relates to a process for recovering substantially pure butadiene- 1,3 from a petroleum fraction consisting principally ofpropane, propylene and lighter hydrocarbons, isobutane, isobutylene, butene-l, butadiene-1,3, n-butane, butene-Z (both high and low boiling), and a heavy fraction of and more carbon atoms containing a large percentage of diolens. The invention further relates to the recovery of butadiene-1,3 from a complex petroleum fraction containing, in addition to such hydrocarbons as described above, small quantities of.

both methylacetylene and vinylacetylene.

One source of butadiene-1,3 is from various hydrocarbon cracking and reforming processes in which the butadiene may be recovered as a byproduct from the'reaction effluents. The economical recovery of substantially pure butadiene from the other reaction products presents a difficult separation problem, for a number of reasons among which are the following:

1. Butadiene is usually present in small concentrations in the reaction effluents, thereby requiring the handling of large volumes of material.

2. Many four carbon atom hydrocarbons present with the butadiene in the reaction mixture have boiling points very close to that of butadiene. For this reason, and also because of the fact that the concentration of butadiene in the reaction mixture is usually very small, extremelyl careful fractionation must be practiced to obtain the maximum recovery of high purity-butadiene.

3. An azeotrope is formed between butadiene and n-butane. Some means other than conventional fractionation must therefore be employed to remove the butane from the butadiene if the butadiene recovery and purity are` to be kept at amaximum.

4. The C5 and heavier fraction usually contains a large percentage of diolens, for example, cyclopentadiene, isoprene, and piperylene, which undergo non-catalytic polymerization at or above room temperature. It is advantageous, and usu ally essential, for reasons disclosed below to remove these materials early in the separation process.

5. Small amounts of acetylenes, principally methylacetylene and vinylacetylene, may be pres-l ent in the C4 fraction. Since any appreciablev quantity of these compounds is inJurious to the 4Clalms. (Cl. 202-39) quality of synthetic rubber manufactured from the butadiene. the separation steps used should include a provision for their removal.

Therefore, adwith'the above considerationsin mind, the principal object of this invention is to provide an economical and practical arrangement of separation steps for the recovery of butadiene-1,3- from a complex hydrocarbon mixture such as previously described. Numerous other objects will more fully appear hereinafter.

The accompanying drawing portrays diagrammatically one arrangement of equipment which has been found useful in carrying out thepresent invention in one embodiment thereof.

My invention will first be described in connection with the preferred embodiment wherein a butadiene-containing stream which also contains Ca hydrocarbons including methylacetylene,

'C4 hydrocarbons boiling close to butadiene 1nd including butene-Z, normal butane,fvinylacetylene, possibly isobutane although it may be absent, generally butene-l, generallyisobutylene, usually C5 hydrocarbons, and frequently heavier hydrocarbons than C5.

The butadiene-containing feed is first subjected to a depropanizing step. This step has for its particular purpose, in addition to removal of propane and propylene, the removal of the methylacetylene which would otherwise appear as a contaminant in the butadiene. Consequently, this-step is necessay whether or not the volume 0f Cs hvdrocarbons in the initial feed stream is large or small. If the volume of Ca hydrocarbons in the initial feed stream is large, this step, of course, has the further advantage in that it reduces the load on subsequentI equipment.

The extent of the removal of methylacetylene is dependent upon the extent or degree of depropanization; that is, the more complete the depropanization of the feed the more complete the removal of the methylacetylene. For example, the following tabulation gives an approximate indication of the relation between the extent of depropanization and the extent of removal of methylacetylene by a fractionator having a moderate number of trays: g

Per cent propane Per cent methyl-` remov acetylene removal It can be seen from the above tabulation that essentially complete depropanization of the feed is highly desirable to reduce the methylacetylene content to a low value. Also, it can be seen that with essentially complete propane removal, roughly 20 to 30 per cent of the methylacetylene originally present will remain in the feed. However, the amount of this component in the orisinal feed stream to the depropanizing step is usually less than one per cent. Consequently, when '10 to 80 per cent of this one per cent of methylacetylene is removed in the depropanizing step only a small fraction remains. This small amount that remains in the bottom emuent from the depropanizing step is later removed in the absorption step.

The bottom fraction from the depropanizing step Iis subjected to a further fractionation which is essentially for the purpose of removing the Cs and heavier hydrocarbons and a large portion of the butenes-2. Not all of the butenes-2 can be economically removed by fractionation because .of the closeness of their boiling points (cls and trans) to those of n-butane, butadiene, butane-1, and isobutylene, Consequently, the fractionation is carried out to remove as much of the butane-2 as possible without loss of butadiene in order to reduce the load in the subsequent absorption step elimination of normal butane along with butene-l with furfural. The final removal o1' butene-2 is effected by a fractionation step after the furfural absorption andstripplng steps.

As pointed out, the removal of as much of the butenes-2 as possible before the absorption step is desirable from the standpoint of reducing the load on the absorption equipment. However, since removal of all of the butenes-2 cannot be effected economically in the second fractionation step, and since a third fractionation step has to be employed after the absorbing stripping step to remove the residual butenes-2, etc., it is frequently desirable to balance the load of butenes-2 removal between these two fractionation steps rather than operating the second fractionation step before the absorption-stripping steps to remove as much of the butenes-2 as possible. Also. the second fractionation step removes someof the vinylacetylene- 'Ihe residual vinylacetylene is removed in the final distillation step after the furfural absorption and stripping steps along with butene-Z. A

The overhead from the second fractionation step is subjected to a furfural absorption treatment which selectively absorbs butadiene and vinylacetylene to the essential exclusion 'of the other hydrocarbons, except butenes-Z, in the stream. The butenes-Z are distributed between the overhead and bottom product. The primary purpose of this step is to separate the isobutylene Y and butene-l from the butadiene. However, this step also results in an additional advantage in that removal of normal butane which forms an azeotrope with butadiene is simultaneously effected. The reason for employing this step to remove Y butene-l and isobutylene is that it is difllcult to and isobutylene which would otherwise have to be removed by some other means.

It is also to be noted that normal butane cannot be removed from butadiene by simple fractional distillation because of the azeotrope formed by these two compounds. Usually, however, normal butane represents only a comparatively small percentage of a butadiene-containing mixed hydrocarbon stream, and since the azeotrope is the minimum boiling type and, therefore, boils at a lower temperature than butadiene, it would appear that by taking slightly more butadiene overhead than required to form the azeotrope with the normal butane present, the normal butane could be eliminated from the butadiene retained in the bottom product. However, in practice the separation cannot be effected in this manner which probably means that the other components in the hydrocarbon mixture tend to partially break the azeotrope. In the previously discussed 200 plate fractionator with' a reflux ratio oi 150 to 1. the indications were that if about 25 per cent of the butadiene-1,3 was taken overhead with the butene-I and isobutylene, approximately only 50 per cent of the normal butane was taken overhead. When using a furfural absorption system in accordance with my invention, the normal butane can be removed overhead along with the butene-l and isobutylenepractically 10.0 per cent with less than one per cent loss of butadiene. The combined amount of-butene-l and isobutylene retained in the furfural along with butadiene is usually less than one per cent.

. As pointed out above, the overhead products from the second fractionation step are, in accordance with my invention, subjected to a furfural absorption treatment wherein the butadiene, vinylacetylene, and most of the butene-2 are selectively absorbed While butane-1, isobutylene, butanes, and a small portion of the butene-Z are not. However, to effect this separation it is essential that a certain amount of reboil heat be applied to the furfural absorber. Furfural is selective to varying degrees towards all of the different hydrocarbons. Those towards which it is less selective are easily stripped from it while those towards which it is most selective are more diflicultly removed. ,The reboil heat which is applied at the bottom of, the furfural absorber functions to strip out the less selectively absorbed compounds consisting, in the usual case, primarily of butene-l', isobutylene, and normal butane. The butadiene, vinylacetylene, and most of the butene-Z are retained in the furfural. Of course, to remove normal butane, isobutylene, and butene-l essentially completely, it is necessary to strip out some butadiene in the bottom of the absorber. However, the butadiene and undesirable components after being released pass upwardly through the furfural vtowards the top where the furfural is cooler than at the reboiler, and the butadiene is again selectively absorbed while the undesirable lessv selectively absorbe components, after the upper layers of furfural once become essentially saturated with them, are discharged overhead. y

The rich furfural from the absorption step is "separated from the absorbed hydrocarbons in a stripping step. The resulting lean furfural is returned to the absorption' zone. The overhead products from the stripping zone are subjected to a fractional distillation to remove butene2 and vinylacetylene from the butadiene to give essentially pure butadiene. The pure butadiene is recovered as an overhead product. Part of the purpose of this final fractionation step has already been pointed out in the discussion of the second fractionation step. It also serves to remove any furfural carried over from the stripping step and any polymer of butadiene or furfural that might be present.

Butadiene' of very high purity can be obtained by my process. In plant operations. butadiene of better than 99.5 per cent purity has been obtained, namely 99.8, with about 98.5 to 99 per cent being about the average purity obtained consistently. In comparison with this, the data previously presented for the recovery of butadiene from butylenes, butanes, etc., by fractionation alone indicates that butadiene of high purity can be Obtained provided normal butane is not present or its concentration in the feed stock is very W. but even so, considerable butadiene necessarily has to be wasted to effect essentially complete removal of the butylenes. However, normal butane, as wellas isobutylene and butene-l, is usually present in butadiene-containing feed streams and when present inquantities equal to or less than that required to form the azeotrope with butadiene, it tends to become the limiting factor in regard to the purity of the butadiene that can be recovered by fractionation alone, bec'ause in fractionation it goes overhead with the butadiene. In concentrations higher than that required to form the azeotrope, the purity of the butadiene recovered will be approximately equal to its concentration in the azeotrope, that is, about 80 per cent.

In accordance with my invention, a predominantly C4 stream comprising butadiene and butene2, with or-without other hydrocarbons, may be subjected to selective solvent extraction with asolvent for the butadiene under conditions such that only a portion of the butene2 is dissolved. The rich solvent is then stripped in the usual :vay driving off the dissolved butadiene and butene2.

, Thereupon, and this is a most important feature of this embodiment of my invention, the recovered mixture is fractionally distilled to separate yessentially pure butadiene, 'i. e. -at least 98% pure, on the one hand from the butene2 on the other hand.

In most cases the butadiene stream will contain normal butane. which has heretofore involved difficulties of separation from butadiene. Such a feed stream may be treated in 'accordance with my invention to readily recover the butadiene in a high state of purity, at least 98%, by first extracting with a. solvent selective for solvent extraction step is primarily or exclusively a C4 stream. In addition to the speciilc C4 hydrocarbons referred to above, namely, butadiene,

'toseparate from the other components present by using conventional methods, such as fractional distillation, because of the closeness of its boiling point to that of some of the other components of the mixture, because it possibly forms azeotropes with other components of the mixture, and possibly rbecause of its tendency to decompose or polymerize.

When proceeding in accordance with my 4invention, however, any vinylacetylene present inthe C4 stream being treated is dissolved, along with the butadiene, in the extraction solvent. During the fractionation of the mixture recovered by stripping the rich solvent the butadiene is readily separated overhead while the vinylacetylene is removed in the bottoms along with butene2 if present.

The butadiene-containing C4 stream may advantageously be prepared from any available stream containing hydrocarbons lighter and/or heavier than C4, in accordance with a more speciilc aspect of my invention. For example, such a stream, which may have been produced by cracking, dehydrogenation, etc., may be flrst fractionated in a depropanizing step to separate C3, methylacetylene and lighter hydrocarbons; whereupon the bottoms may be fractionated in a debutanizing operation to recover overhead all the C4 constituents originally present except a part of the butene2., This C4 overhead product may then, either with or without interposition of still another fractionation to separate any isobutane present, be subjected to solvent extraction and the succeeding steps described above forI the recovery of the pure butadiene.

Removal of any isobutane in the manner just alluded to further lowers the volume load on the solvent extraction step.

Frequently the original stream, except for a stream obtained by dehydrogenating C4 hydrocarbons, vbefore separation of the C4 stream by depropanizing and debutanizing, will preponderantly comprise Cs and lighter. Often these will amountl to or more of the original stream. However, for a stream obtained by dehydrogena-` tion of C4 hydrocarbons the depropanizing step is still necessary for the removal of methylacetylene.

The original stream may contain appreciable amounts of methylacetylene. This is essentially completely removed in the first` fractionation step. Also the Cs content of the stream may comprise appreciable amounts of Cs conjugated di butadiene whereby all the butadiene and only 4a portion of the butene2 in the feed are dissolved butnone or only a trace of the normal butane is dissolved. Upon stripping and fractionating as described in the preceding paragraph, pure butadiene is recovered as before.

pentadiene. These go out as bottoms in the debutanizing step,.and likewise are prevented from interfering with the operation of the process.

Isobutane, it present, may or may not have been removed from the C4 stream subjected to the solvent extraction. This may have been ef- 10 fected in the depropanizing step by appropriate adjustment of the cut, or in an `intermediate fractionation step as referred to above. Isobutane is the lowest boiling of the C4 hydrocarbons apt to be encountered and therefore readily re'- It will be seen that the stream treated by the Il moved. For reference purposes the boiling points 1 of the C: and C4 hydrocarbon apt to be present in the original stream are tabulated as follows:

As the selective solvent, furfural is by far the most highly preferred. However, other solvents having similar selectivity for butadiene may be used in place of furfurai, though less desirable. Examples are: nitrobenzene, dichloridiethyl ether, dimethyl formamide, methyl levulinate, glycol diacetate, diethyl acetamide, pyridine, phenol, phenol plus water, benzyl alcohol, etc.

The furfural used may be anhydrous but preferably contains up to 5% of water, usually 4 to 5%, in order to increase its selectivity for butadiene, lower its boiling point, 'and bring about other advantages. Use of furfural containing significant amounts of water is disclosed and claimed' in my copending applications, Serial No. 430,307, filed February l0, 1942 and Serial No.

438.844,-1'lled April 13, 1942. Since the water is lost from the `system by going overhead with the hydrocarbons evolved in the stripping column, it is convenient to add water lto the system from time to time or continuously. For example, this water may be introduced to the stripping column, say at the top thereof, or to the lean furfural stream fed into the top of the extraction column. It may be desirable to maintain the furfural lin the system and especially in the absorber completely or substantially saturated with water at the temperatures involved.

The furfural or the like breaks any azeotrope of normal butane and butadiene or prevents the formation of such azeotrope by dissolving the butadiene. In fact the invention is applicable to an azeotrope of butadiene and normal butane. For example, such an azeotrope, which contains approximately 80% butadiene and 20% normal butane, and formed in any suitable manner, may be solventl extracted as described above preferably with furfural to recover pure butadiene.

The final fractionation to separate butadiene of at least 98.5% purity from butane-2 may ad-- vantageously be conducted in a columnof preferably not less than 100 trays. The ilnal fractionation separates ,out in the bottoms product any iurfural or other selective solvent carried in the overhead from the stripping column.

Referring to the drawing, from a source of supply, not shown, a petroleum fraction consisting principally of propane, propylene and lighter hydrocarbons, isobutanerisobutylene, butene-l, butadiene-1,3, normal` butane, butene-2 (both high and low boiling), and a C5 and heavier fraction containing a large percentage of dioleilns is charged to fractionatcr 2 through line I. The mixture may or may not contain detect- -ablequantities of all of the above hydrocarbons and also may or .may not contain detectable quantities of acetylenes. Substantially all the propane, propylene and lighter hydrocarbons. which will ordinarily represent the greatest p0rtion of the feed to fractionator 2, as well as most of any methyl-acetylene that may be present in the feed, are removed from the top of fractionator 2 through line 3. Substantially all of the C4 and heavier hydrocarbons, along with any vinylacetylene that may be present in the feed, are removed from the bottom of the fractionator through line 4 and charged to fractionator 5.

In a butadiene by-product recovery plant wherein its feed consists essentially of the reaction products from a petroleum cracking and/or reforming operation, large quantities of hydrocarbons boiling below C4's may ordinarily be expected to be present in the charge stock to the recovery plant. Therefore, early depropanization has the advantage of lowering the volume of material to be handled throughout the rest of the separation steps thereby reducing equipment size and investment cost. 'Ihis step also has the further advantage of removing the major portion of any methylacetylene that may be present in the reaction mixture and which, if not removed, will be absorbed by the lean furfural (which is employed subsequently as a selective solvent) in absorber 8; upon stripping of the rich furfural in stripper il, the methyl acetylene will pass overhead through line I2 to fractionator Il and will be removed overhead from this fractionator through linev l5 thus appearing as an impurity in the nished butadiene product which is also withdrawn through line I5.

Fractionator 5 is operated in such a manner as to remove substantially all of the Css and heavier components and part of the butene-2 from the bottom ofthe fractionator through line 1. The overhead product, consisting of butanes, butadiene-1,3, isobutylene, butene-l and butene- 2 along with any vinylacetylene is removed via7 line 6 and charged to absorber 8. Several advantages are gained by the fractionation step conducted in fractionator -5. Among these advantages are;

1. Certain of the dioleflnic compounds that may be present in the Cs andv heavier fraction, and especially conjugated diolefins, such as isoprene, piperylene and cyclopentadiene are quite valuable.. Such diolefinic hydrocarbons tend to polymerize at or. above room temperatures, the polymerization rate increasing rapidly with temperature. By removing the Css early in the process the time of processing them is reduced and they are not subjected to the high temperatures of the absorbing and stripping operations, thus minimizing any loss of these valuable hydrocarbons through polymerization reactions.

2. Over a period of time some of the olefinic compounds, particularly the diolens, present in the C5 and heavier fraction polymerize to gummy materials. Gum is not only undesirable because it deposits on heating elements causing a reduction in heat transfer or because it may deposit in lines, valves, etc., thereby reducing the ow,

but also because it will be absorbed in the furfural thus making its regeneration more difficult. Thus -by early removal of the Cs and heavier, the extent of gum formation is markedly reduced.

3. Removal of the heavier material early in the process lowers subsequent stripping and fractionation temperatures thereby reducing the loss of butadiene by polymerization.

4. A further advantage obtained by the use of fractionatorl 5 is that the load on the absorbing y bottom temperature of 175 F. Substantially all of the Cas and lighter hydrocarbons, consisting of about 'I0 parts of propane and propylene, 30

` parts of ethane and ethylene and a trace of ing coil 8A is employed in the bottom ofabsorber I 8 to give a cleaner separation therein, inaccordance with principles now well-known to the art. yRich furiural containing substantially all of the bute/diene charged to the absorber plus some butene-2 and any vinylacetylene that may be present is withdrawn from the bottom of the absorber through line III and charged to stripper II equipped with heating coil IIA. The product withdrawn from the top of absorber 8 contains A substantially all of the isobutylene, butane-1 and butanes in the feed to the absorber plus some of the butene-2 as well as any propane that may have been retained in the product withdrawn from the bottom ci fractionator 2. y

In column II the rich furfural is wstripped of butadiene, butene-Z and vinylacetylene, if pres ent. These hydrocarbons pass overhead through line I2 to fractionator I4. The lean furfural is withdrawn from the bottom of the stripper through line I3 and is recycled`to absorber 8. Since n-butanerv and butadiene form an azeotrope, pure butadiene can not be obtained by l conventional fractionation. By my use of solvent extraction to separate the butadiene from the n-butane at this stage in the process, these difficulties are completely overcome in a simple and economical manner.

In the Ui'inal fractionation, butene-2 and vinylacetylene, if any, are charged to fractionator I4 through line I2; substantially pure butadiene is removed overhead through line I5 and the butene-2 and any vinylacetylene, plus any small amounts or furfural and polymer of butadiene or furfural that may have carriedfover from the stripper column I'Iare removed from the bottom of fractionator I4 through line I6. Butadiene may be separated from Vadmixture with normal butane in azeotropic or any other proportions and with or' without other hydrocarbons which may be close-boiling or not, by selective solvent extraction with a solvent which' is selective for butadiene but which does not appreciably dissolve normal butane.` Upon'strippingy of the butadiene-rich solvent there is obtained butadiene free from normal .butaneand the formation of an azeotrope of the butadiene with the normal butane, which occurs when efthe butadiene,

methane and ',methylacetylene were removed overhead through line 3. rSubstantially all of the C4 and heavier hydrocarbons, consistingof about 32 parts butadiene, 7, parts butene-l, 3

parts isobutylene, 12 parts butene-2 (both high and low boiling), 2 parts butane, and 44 parts of C5 and higher along with a small quantity of vinylacetylene were withdrawn from the bottom of fractionator 2 through line 4 and charged to fractionator 5.- 4 I From f ractionator 5, which was operated at rI0 pounds per square inch absolute, reflux ratio of 25: 1, top temperature of 103 F. and bottom temperature of 220 F., all of the .Cs and heavier material and about one-half ofthe butene-2 was removed from the kettle. Y The overhead product, consisting i of about 3 parts propane, 63 parts butadiene, 13 parts butene-L 5 parts isobutylene,

y13 parts butene-2, and 3- parts of butane along with a trace of vinylacetylene was charged to absorber 8.

v Lean furfural from stripper II was introduced into absorber 8 through line I3. Rich furfural. containing principally about 4 parts of butadiene and 0.3 part of butene-2 (both high and low boiling) along with a trace of C5+ and vinylacetylerie Y was withdrawn from the bottom of absorber 8 forts are made to separate by fractional distillation a mixture comprising these two ingredients, is effectively prevented.

Example As an example of the operation of my invention, an unsaturated petroleum fraction, from:

a source not shown(, containing about per cent through line I0 and charged to stripper II.4 The product withdrawn from thetop of absorber 8 through line 9 contained about 45 parts of butene-l, 10 parts of propane, 17 parts isobutylene,

'17 -parts of butene-2 (mostly low boiling) and 11 parts butane.- The absorber was operated at a pressure of 65 pounds per square inch absolute, reux ratio of 16.611, top temperature of 120 F. and bottom temperature of 240 F.

The-rich furfural was introduced into stripper II through line II). The stripper was operated at apressure of v65 poundsper square inch absolute, reflux ratio of 1:1, top temperature of 103 F. and bottom temperature of 300F. Lean furfural containing 99.9 per cent furfural and 0.1 per cent C4s was withdrawn from the bottom of the stripper through linel I3 and recycled to absorber 8.. The stripper overhead product, consisting of 93 parts butadiene and 7 parts of butene-2 along with traces of isobutylene, butane-1, 05+, yvinylacetylene and furfural was charged to fractionator I 4 through line I2.

In the nnal fractionator I4, operated at 65 pounds per square inch absolute, reux ratio or 14:1, top and bottom temperatures of 103vand F., respectively.'a finished butadiene product of about 98.5 per cent purity was withdrawn overhead throughline I5. The bottoms product withdrawn from line' I6 consisted of about 97 parts butene-Z, 1 part butadiene, and 2 parts of Cs and heavier, furfural and vinylacetylene.

From the foregoing description a great man? advantages of. my invention will be apparent to those skilled in the art. Many of these have been referred to in detail above, Many of the quanti- 'tative advantages will not be discussedin great 11 lowed by fractionation in the manner described in detail makes possible the clean separation of extremely pure butadiene. The invention enables the complete separation of vinylacetylene from the butadiene which should be free from vinylacetylene to be -acceptable as a starting material in synthetic rubber production. 'I'he invention also makes possible starting with a crude broad cracking, refinery; or dehydrogenation stream and enables ready elimination of the 'C3 and lighter including objectionable methylacetylene and of the C and heavier including objectionable C5 dioleiins together with a substantial portion of Ythe butene-2 which is found in the bottoms from lthe second fractonator 5. Initial removal of all these materials lightens the load on the solvent extraction unit and prevents interference with subsequent steps in. the process. Also this removal contributes materially to the high purity of the product obtained. Moreover the invention enables one to readily and economically obtain maximum yields-of butadiene even though the concentration in the C4 stream is low and though the original concentration in the ins itial composite feed is extremely low. lA great many more advantages will be readily apparent to those skilled in the art.

By the terms fractionation and ufractionating" as used herein and in the claims. I mean fractional distillation. As used herein butadiene means butadienel. The terms butene-2" and butenes-2" mean both the low and high boiling isomers.

1. A process forthe recovery of butadiene from a mixture of hydrocarbons comprising butadiene and other closely-boiling C4 hydrocarbons including butene-l and butene-Z which comprises subjecting said mixture to a ist fractional distillation keffecting removal of a portion of the butene-2, subjecting the remainder of said mixture to selective solvent extraction with a solvent which is selective for butadiene under conditions such that a fraction consisting essentially of butadiene and a further portion of the butene-2 is absorbed without substantial absorption of the butene-l and the remainder of the butene-2, stripping said fraction of butadiene and butene-2 from the solvent, and subjecting said fraction to a second fractional distillation eiecting separation of the butadiene from the butene-Z.

2. A process as dened in claim 1 wherein said solvent is f urfural.

3. A process for the recovery of butadiene from amixture of hydrocarbons comprising butadiene and other closely-boiling C4 hydrocarbons including normal butane, butene-l, and vinylacetylene, which comprises subjecting said mixture to a rst fractional distillation effecting removal of a portion of the butene-2; subjecting the remainder of said mixture to select ive solvent extraction with a solvent which is selective for. butadiene under conditions such that a fraction consisting essen`- tially of butadiene, vinylacetylene, and a further portion of the butene-Z is adsorbed without substantial adsorption of the butene-l and the remainder of the butene-Z; stripping said fraction of butadiene, vinylacetylene, and butene-2 from the solvent; and subjecting said fraction to a second fractional distillation eiecting separation of the butadiene from the vinylacetylene and butene-2.

4. A process as deiined in claim 3 solvent is furfural.

wherein said KARL H. HACHMUTH.

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