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July 1s, 1946. 2,404,253 V. SCARTH EXTRAC-TÍVE DI S T vILLAT ION WI TH FURFURAL VFiled Dec. 21, 1944 439ans I -lvandund ON NN Il Il In 2 dlLS IMENTCR ' Y vscARTH ATroRNl-:Ysê ì . '7 vPatented July 16, 1946 „ 2,404,253 v UNITED _STATES PATENT oFFIcE 2,404,253 EXTRACTIVE DISTILLATION WITH ' FURFURAL Virgil Scarth, Bartlesville, Okla., .Vassîgnor t to Phillipsrl’etroleum Company, a _corporation Vof Delaware Application December 21, 1944, Serial No. 569,243 s claims. (o1. 19e-13)y 1 2 This invention relates to extractive distillation, more particularly to extractive distillation of hy drocarbon streams containing unsaturated hy drocarbons using furfural as the solvent, and still It is not desired to be limited to any theory as ' to the cause of the furfural becoming corrosive. It is conventional practice to purify the fur- _ fural continuously by “re-running” a side stream of the furfural leaving the stripper and recycled to the absorber. This is carried out in ’accord more particularly to extractive distillation for the recovery of aliphatic monooleñns and/or ali phatic conjugated dioleñns especially normal bu tene and/or butadiene from hydrocarbon streams containing same with furfural as the solvent. It has become customary to recover unsat urated aliphatic hydrocarbons especially butene and/or butadiene from hydrocarbon streams con taining them by extractively distilling the stream ance Vwith U.v S. Patents to K. H. Hachmuth 2,372,668 and 2,350,609, and Buell et al. 2,350,584. 10 large portion of this loss is due to polymerization of the furfural in the re-run unit. At present the furfural is re-run for the purpose of remov ing otherfcorrosive material as Well as the poly in a fractional distillation column or absorber into the top of which there is introduced con 15 mer.; If a feasible method of preventing the tinuously a stream of furfural, usually contain >ing a few per cent of dissolved Water, in amount l suiïicient to dissolve the unsaturated hydrocar bon. However, there is considerable additional loss of furfural during the re-running operation. A The top of the column is refluxed with a accumulation of corrosive material in the furfural in the main system (i. e., in the absorbing and stripping operations) were available, the amount of furfural passed to the re-run unit could be ma portion of condensed overhead vapors , in the 20, terially usual Way and the bottom of the column is pro vided with the conventional reboiler for the pur pose of boiling the rich furfural as it attains the bottomso as to free it from undesired paraffin .and/or olefin. The rich furfural is withdrawn 25 decreased with many advantages princi pal of which is the reduction of polymerization of furfural in the re-running operation. Many other advantageszof such a method will appear hereinafter. w Y - . . ' , The principal object of the present invention `is to provide` an improved method of conducting extractive distillation of unsaturated hydrocar per is provided with reboiler and is refluxed with bons with furfural. Another object is to pro a portion of the condensed overhead. The pur .vide a feasible method of preventing the accu pose of ref‘luxing the absorber and stripper is to 30 mulation'of corrosive material in the furfural in prevent carryover of furfural into the overhead Y an extractive. distillation system wherein unsat urated hydrocarbons especially aliphatic mono vapors. The stripped furfural is then returned from the bottom of the stripper to the absorber. oleflns and dioleñns are separated from hydro . During extractive distillation of unsaturated carbon streams containing same. Another object lwdrocarbons with furfural the furiural becomes 35 is' to provide such an extractive distillation sys contaminated with acids, polymer and other im tem applied to the recovery of normal butene purities which are very deleterious because they and/or butadiene by means of furfural. Another are highly corrosive to the metal equipment. The object is to provide a method which greatly re resulting corrosion'of expensive equipment in duces the amount of furfural which has to bel cluding fractional distillation columns and ac 40 passed to the re-run unit. Another object is to prevent corrosion of the main system (i. e., the cessories such as piping,` reboilers, condensers, accumulators, pumps and so forth presents a extractive distillation system proper as distin serious problem for which various remedies have guished frornthe re-run unit). Another object been proposed none of which is entirely satisfac is to reduce the loss of furfural in the extractive tory, so far as I am aware. The cause of the 45 distillation system and in the re-running opera formation of corrosive impurities in the furfural tion. Another object is to prevent the formation is not known even though the problem has been of‘corrosive impurities in the furfural at the extensively investigated. Factors which may con point where those impurities are most deleterious. tribute to the formation include high tempera Another object is to accomplish the foregoing ob ture, presence of Water and presence of unsat 50 jects in‘a manner which does not cause foaming ’ urated hydrocarbons. It is believed by some in of the furfural. Numerous other objects will vestigators that the furfural selectively absorbs hereinafter appear. \ traces of corrosive material from the hydrocar The accompanying drawing portrays diagram bon feed or of material from the feed which be matically one arrangement of equipment for‘car ;comes corrosive upon reacting with the furfural. 55 rying out the present invention. and fed to a stripping column where the dissolved unsaturated hydrocarbon is removed. The strip ? '3 V casta of the debutanizer overhead with' furfural where- y î‘. I have discovered that the extractive distil- Y _, - n Y 4 by the butadiene andbutene-Z content are dis lation of hydrocarbon streams with furfural as ' the solvent to recover unsaturated hydrocarbons therefrom the furfural may be prevented from solved in the furfural and subsequently recovered therefrom by the stripping operation, and finally fractionally distilling the butadiene-butene-Z becoming corrosive and from corroding the equip mixture to separate pure butadiene from butene 2. This process y»is fully described in the copend ing application ,of K. I-I. Hachmuth, Ser. No. 481, Vment by,` intimately contacting the >furfural in the extractive distillation system with lime. . The term “lime” as used herein denotes calcium hy droxide. I have determined that limestone will'.` -`i 305, filed March 31, 1943, which has now matured 10 into Patent No. 2,386,310, dated Oct. 9, 1945. The ' present invention is applicable to the furfural ex the inné' I tractivedistillation system in step II or in step IV not operate in the process of the present .in vention. ' ' " The contacting of the furfural with is commonly accomplished `by~introducing the lime in finelyfdivided form into -the'furfural‘in >or in both» Or it may be applied in those `bu tadiene plants where normal butylene feed is any suitable manner. The lime may be added 15 catalyticall'y-dehydrogenated-to butadiene and bu tadiene-is> recovered from the eiiluent by extrac n the dry form, as a dispersion or slurry> in Vfur-V fural,> as a dispersion or slurry in water orbyv tive distillation with furfural. other methods. Insu'ch an extractiveV distillation process the i l iurfural enriched in unsaturated hydrocarbon is Probably the most convenient method is to add it in the form of a suspension withdrawn from the bottom >of the absorber and or dispersion in a'portion‘or all of the makeup water whichv is lcommonly added continuously to passed to the stripper where the unsaturated hy drocarbon is stripped therefrom. The stripped . the fur‘fural fed to the absorber or absorbersiin iurfural is then cooled and after addition of makee up water is recycled to the absorber.V Usually a v addition of"make-up water is for the purpose of 25 surge tank is provided inthe recycle line. A side stream of the stripped furfural is continuously replacing water volatilize'd from the furfural in order to impart the desired content of dissolved water to the furfural. It will be understood that fed to a furfural re-run unit wherein it is steam the absorbers and strippers. vThe use of furfural containing dissolved Water as a solvent in the ex distilled, e. g. in the manner of Buell et al. 2,350, ing application of K. H. Ha'chmuth, Ser. No. 438, „ i’rornk polymer and other material relatively non , 584, to give a puriñed material consisting essen tractive distillation of normal butene and/erbu tadiene is disclosed and claimed in the _copend 30 tially of furfural and dissolved water and free 844, filed April 13, 1942. ' volatile Awith steam. The re-run furfural is then LA merge'd with the main recycle stream for passage to furfural storage and ultimatelyto the absorber. iuríural >Wherever the furfural becomes corrosive> 35 'I'he volume of furfural passed to the re-run unit is preferably such as to maintain the concentra due to conditions prevailing in the extractive dis tion of polymer in the furfural in the system at tillation system. The factors which cause the fur below 1% by weight. ' fural to vbecome corrosive are but little under In accordance with the present invention lim stood but it is believed that they include the -high temperatures especially in the reboiling- sections 40 is admixed with the furfural in the extractive distillation system, for example, by suspending. it of' the absorber and stripper, the presence ofwater therein inA any suitable way.> It ispreferredgto andunsaturatèd hydrocarbons especially oleñns limit: the amount of lime to that'required to keep` „ and dioleiins, the presence of air or oxygen. The the acidity at the desired level. 'I'he amount of development of corrosivity in the furfural is ac- ` lco'n'ipani'ed bythe «formation of polymer which 45 lime required is so small that in practice no dif ficulty due to sedimentation of the .lime or its re represents a?lo’ssV ciï furfuralland- causes poor heat The present invention is applied t0 the extrac tive distillation of unsaturated' hydrocarbons `with ' ' action products is encountered.Y The lime >or a transfer ‘and other troubles. The presentv inven portion thereof may dissolve in the furfural; the tion-is especially applicable to the eXtractive-dis presence of dissolved water in the furfural may tillation of butadiene and/or normal Abutene with o fur-tural, this 'being the -principal if notrthe `only 50 enhance any such tendency. commercial application of eXtractive distillation with furfural as a‘means of lrecovering unsat urated. hydrocarbons. However,l vit may also be applied where furi‘ural is used in the recovery by extractive distillation of otherl aliphatic olefins or aliphatic conjugated diole?lns from hydrocar A very convenient method of contacting the furfural with the lime is to employ an excess of a dispersion of lim'efin water,-admix it very in timately with the furfural Yin’any suitable way, 55 for example, by means of >a ycentrifugal mixing` pump, run the resulting churned-up mixture'or emulsion into a settling tank where it is allowed to separate into two layers, an upper layer of> The conversion of normalk butane to butadiene the Water and lime which have. not dissolved in is now commonly effected by a series of stepsin cluding (I) catalytic dehydrogenation of normal 60 the furfural and a, lower layer of furfural which butane to normal butene, (II) recovery of normal , is saturated with water and which may contain dissolved lime. 'I‘hese layers are separately with butene and recycle normal butane by ñrst frac drawn, the water layer being recycled to the mix tional-ly distilling the C4 content ofthe .effluent ing lzone to save its water, lime and dissolved fur >from-step I in a butene-l column> wherein butene 1 is recovered overhead followed by extractive dis 65 fural content and the furfural layerbeing fed'to the absorber. Under conditions where corrosive tillation of the resulting bottoms fraction of~nvness of thefurfural in the system can beikept butane and butene-2 withfurfural to separate down to a satisfactory level Such a treatment as Vthe butene-Z from the normal butane which is re just outlined may be appliedto asidestrearn of cycled to step I, (III) passing the combined bu- ‘ `the recycle furfural, the resulting non-corrosive tene-1 and butene-Z recovered in step II to a sec furfural being intimately admixed with theÍ bal-ond catalytic stage and therein dehydrogenating ance of the recycle furfural and the furfuralfrom Ysame to butadiene, and iinally (IV) recovering bon streams containing the same. ' butadiene from the C4V content of the efliuent of the re-run unit and allowed to stand as in a surge .or storage tank for a. period of'time beforeV being Step III bv a ,Sequence 'Ofsisps .includínsdde propanizing, debutanizing, extractive distillation 75 re-introduced to the absorber. _- " aap/i353 ’ 5 Alternatively 1an excess :of .powdered lime may be intimately'admixed with the'furfural and after a suitable period of time separated therefrom by and suitable means such as'îñltration, centrifug the furfural to. reduce the acid content of the v l. indicated above,> the ,acidcontent ofthe fur fural is not necessarily an absolute index of its corrosiveness. It is',` believed that the concentra-V tion >of corrosive materialin the furfural in creases somewhat in proportion to the acidity. At levels of acidity greater than 0,04 percent itis evident thatl some factor other than acidity furfural in theextractive distillation system be comes into play asa cause of corrosion since be ing or sedimentation. Y -4 _ , It is preferred to introduce enough lime Vinto low 0.06 per cent by weight, measured as acetic tween levels of 0.04 and 0.06 per cent acidity Vse acid, and still more'upreferably below 0.04 per v10 rious corrosion may or` may not -occur and at cent by weight. I have found that below 0.04- per levels` of acidity above-0.06 percent .the corrosion cent of acidity no appreciable corrosion occurs; wliich'occurs is not always serious although it between 0.04 and 0.06 perv cent Íacidity the con >usually is. In any event, whatever the causes of corrosion and’regardless of theory it has been centration of the corrosive substance whatever it isvmay or may not -be suchïthat >serious-corro 15 proven absolutely that by addition of lime to the _sion occurs, apparently depending uponiactors not understood at present; while, at an acidity» above 0.06 per cent corrosion. always occurs >and, is .usually so high that the equipment, especially . furiural in the system in an amount such as to reduce the acidity to, or maintain the acidity at, levels below 0.04 per cent serious corrosion is pre vented and polymerization is inhibited; while re, the fractionating columns (i. e. the absorbers) 20 duction of acidity to, or maintenance atylevels and the strippers, would soon be eaten away be between 0.04 and 0.06 per centiby the addition low the point of safe operation if the plant con tinued to'operate at such levels ofv acidity. Fur thermore, the loss of furfural by polymerization of lime in most cases prevents seriouscorrosion and reduced polymerization of furiural to satis factory levels. ' s ' at levels of acidity above 0.06 per cent becomes 25 In accordance with the present invention the excessive so that furfuralY consumption exceeeds acid content may be _reduced nearly to or to the the available supply vwhich is exceedingly dan Aneutral point or the furfural may be made valka gerous to the synthetic rubber program.- The re line although the latter practice is not ordinarily lationship between level of acidity and poly followed because the tendency of -furiural to pol merization of furfural is at. present not under 30 ymerize is enhanced under y alkaline conditions. stood and I do not wish to be limited to any the- ' It is preferred to add only enough lime to reduce ory with regard thereto. _ the acid content below 0.06 per cent and prefer ably below 0.04 per cent which practically elim fural at a point in the system such that the fur, inates corrosion of carbon steel by the lfurfural, iural is at a temperature of not over 130° F. at _the 35 however, the nearer the neutral point or the point point where the admixture takes place. Usually of zero acidity the’less polymers of furfural will the coldest point- in the furfural system is just be formed. When the acid content of furfural in after the cooler or coolers which are employed the main system is below 0.04 per cent the cor ' to cool the furfural leaving the stripper or strip rosion of carbon steel equipment is negligible and persand so it is preferred to add the lime at 40 the rate of formation oi polymer is substantially this point. The reason for adding the lime at a lower than when the acid content is above 0.06 . It is preferred to introduce the lime to the fur point of `low temperature` is that the possibility of , additional polymerization of furfural upon the introduction of lime to hot furfural is there by eliminated. . ~ The purpose of lime addition is to reduce the acid content of the furfural, reduce the» forma tion of polymer and the corrosiveness of the fur fural. The manner in which vthese several fac tors are inter-related is not understood but it has been deñnitely established that the Vaddition of lime’does prevent the furfural from being cor-v rosive and does` reduce the polymer formation, `whether one causes the other or not. The pres ‘ entv invention may be employed kfrom th'e‘start- of operations tokeep the furfural in the system from ever becoming corrosive or may be applied .per cent. ~ The products formed from the reaction of lime are removed >from the system in there-run unit with the polymers, so no problem due to accu mulation of lime or its reaction products is en countered. In ordinary operation the re-running is carried out at such a rate that the furfural in the entire system is re-run in a short time- -say every two to` three days. If enough lime is added to cause a sediment to form in the main system it may be necessary to provide a trap or filter to remove the sediment from the furfural. My in- vention may be applied by adding lime to aïsmall side stream of furiural andv any sedimentthat „might beformed may be removed by iiltering or some other method before the furfural is re to systems in which the furfural hasbecome Vcor turned to the main stream. However, it is voften .rosive to eliminate lthe corrosivity and keep »the -system thenceforth non-corrosive. Thus the ~ 60 desirable to add only enough lime to reduce’the acid content of the furfural to the extentuindi ,process of the present invention prevents non cated but not enough to result in difficulties due ycorrosive furfural from becoming corrosive and to lime or the reaction products of lime settling also prevents corrosive furfural from exerting its in the system, . deleterious action. The addition of lime inac In accordance with my invention the fresh fur cordance with the present invention involves 65 fural which is added as makeup to the system `more than simple neutralization of corrosive may be treated with lime prior to its introduc acidity since it inhibits polymer yformation as tion into the extractive distillation system. " well. Moreover, it has been found by experimen In accordance with the present invention lime tal work that neither the acid content nor the pH Aof furfuralis necessarily an index of its corro 70 is added in the main system, i. e. in the extractive distillation system proper, as distinguished from siveness. . It will be obvious that acid content and the re-run unit. 'I’he addition of lime to the y VpH are not the same since it is common to have 1a high acid content associated with' a pH rang re-run unit is disclosed and claimed in my copend »ing from 0 to '7 or a lowïacid content anda pI-I ing application Ser. No. 607,212, filed July` 26, l»ranging from‘llto .7.; . _. _ -75 1945. „ . ' s ,_ _ > 8 amount of lime ademend-.the .rate fat :which rur-l 7 ¿it iis-often -desirabie-to-appiy the presentimen tionf-‘to extractive distillation` ?system-s ~ wherein ~furîural »has'rbeoome «excessively corrosive. >In > tion. suchcaseit may be desirable toadd-enough fur-’-y furalin' the mainsystem to `reducethe acidity V0.02 In themanufacture ofibuta'diene by 'twoestage . per-cent each day until it is «below 0.04 perV cent 'dehydrogenation of normal butano. afrecoverv and -to then reduce the amount of <liifneaddeol system, as lshown in the drawing, was employed. Furfural was fed continuously into the ‘absorbers 2 and 4 at the rate of '260,000 gai/hr. entering toa lñgure which will‘hold-theacidityat `thisleveL` Referring to the >accompanying drawing, I have illustrated diagrammatically., .the Y `'application >of , my invention toßarplant in .whichbut‘adiene is io both absorbers. The furfural v'corit‘a'ined-öfïàfo‘iî made .by the twoesta'ge .catalytic dehydregenation Water Áby Wei'ghtrbased on 'the Weight Aof furfn‘ural and water. The stripped ‘furfural >from Yboth of normalbutane. The mixture of normal «butane >and butene-Z >recovered .from .step I'oiîV the .process enters absorber 2 via line I., while the C4 hydro' strippers was cooled to 1Z0-130° F. and the major part' of the cooled product waspassed tothe surge tank 2l. A side'stream of 3500 gah/hr. 'was carbons uderivedirom step III . and >containing butene-Z and. butadiene enter ‘absorber .4 via line Y » withdrawn via line '23 and 'fed >to refrun unit 24. ' 3. Absorbersz and 4 carry out the extractive dis tillation of thefeed with furfural injected into V?l‘he total volume of'fu‘rfural in the entiresystem was 225,000 gallons. ‘Theacidity-ofthe fur*v f'ural in the main system was 0.510 weight per centV >the top via lines 5 and 6, respectively. Absorbers v20 measured as acetic acid.y This vwasexceedingly 2 and 4 areprovided with conventional reboiling corrosive to the equipment and polymer formation means and means for condensing and returning a was excessive. The acidity had> been 'approxi mately 0.10% lfor ten days, during which time portion ofthe overhead as reflux, the latter being indicated diagrammatically by a cooling coil in the polymer content increased> from 0.4 t'o_~1.5 the top of each absorber. . The undissolvednormal 25 per cent by weight. Under these conditions, . namely at 0.10% acidity, [1,5% polymer( concene Y butane leaves absorber 2 via line 1„ while >the »furfural rich in butene~2 is> withdrawn‘asjbottom tration and 3500 gal/hr.y of 'iurfural 're-run, product via line ß` and fed <to ,stripper 9 where j equilibrium 'had been established. ' However., operations were unsatisfactory and it was con the dissolved butene-2 is driven 01T. ¿ Stripper 9 Vis likewise provided‘withy conventional reboiler V."II) sidered imperative to vreduce corrosionV andi poly yandmeans for condensing and returning a portion merization because otherwise the plant would be corroded away in a short time and it would be yof the overhead as reñux. . „ v . The undissolved C4 hydrocarbons leave absorber impossible to obtain ‘sumcient furfural to continue 4 via line I0 While the -furfurall rich in butene-2 and butadiene passes via line II lto stripper I2 operations. Y f' »_ Under the foregoing conditions, 'lime wasadded Whichperformsthe same function asstripper 9, v to the furfural-in the vmainïsystem at a rate vof the butene-2 Vand butadiene mixture leaving via line I3. The stripped furfural streamer-leaving stripper 9 via line I4 and stripper I2 viafline I5, are merged and fed via lline IB to cooler Il. YIt will be understood that a >cooler may be provided 11 lbs/hr.V 'I‘he lime wasI added in the form of a slurry in the makeup water Whichwas employed >to keep the water concentration at approximately 6%. The slurry was admixed with the furfural recycle stream immediately after the cooler.- as for each stripped furfural stream if desired. The cooled furfural ñowing vin line I8 is passed via began to decrease at the rate »of approximately lines I9 and 20 to furfural >surge or storage 2| 0.015% pel' day. When` the acidity reached 0.04%` whence it is recycled via line V22> and lines?) and .G _to thevabsorbers. A side stream ofrîurfuralï is passedrvia line`23 to the re-run unit 2l whereit Yis lbs/hr. Aand the acidity varied ‘from 0.03Y tol 0.030% from then lon 'for an irideñnite-fpe'riod. vshovvn in the drawing. The acidityY immediately the rate of addition of lime was decreased `to 6 continuously pur'iñed, inthe manner new well -The polymer content had'decreased 'to’0.3% and know-n in the art, to removerthe polymer there remained at abouttiiis figure. Vsimultaneo'usly Vfrom.V The re-run furfural -is `passed via line `2'5 15o with the attainment of 0.04% 'acidity,`the'rate 'of into the main recycle stream inline I9. reerunning 'o‘f lfurfu'ral was rreduced 'to 2000 ` In accordance with thepresent invention'lime gaL/hr. whichfsumced to keep 'the polymer con _is continuously fed in the properamounts indi cated above vinto the main recycle stream Vflowing centration from increasing above 0.3%. ' ' As a result of 'the use of lime in the'manner . in line I9. This is accomplished by means of line described inr the foregoing example'the .loss of‘ 26 which, it will be understocd'is `purely .dia furfural'in the 'plant Was vreduced vfrom’ 4000 Yto grammatic. Usually Íboth lime and water are injected into the stream in line I9. As explained Vabove it is'often convenient to inject the make up water at this point. A 'point of injection off lime `close to cooler Il is preferred because the temperature is at substantially kthe 'lowest level at this point and because this gives ajre'latively long line between the point of injection of lime Vand furfural surge 2|. Intimate'mixing of the lime with fur'fural may take place in the long pipe indicated by lines I9_ and ‘20. V_ y f . It will be understood that _the showing in the drawing is purely diagrammatic and that the lime and water may be added "separately or in any desired manner and that »provision> maybe made 'for the removal -of any excess lime'or sediment. It mayor may not be‘necessary' ‘to-provide for' the removal of the reaction ïproduötsïof lime an‘díth'e I impurities in the furfural, depending' uporiltli'e 1500 gaL/day.V ' ' ‘ - The amountV of lime necessary to 'be added'to the rurfurai in accordance with the present 1in vention 'in order to keep the furfural from be coming >corrosive and .the polymer content thereof from increasing is extremely small. ' Under’ordi nary conditionsoi operation; 'it `ranges from 0.000005 to 'O-.OOOOGÄlbs/gal. of kf'urfural> ie‘d into the y»absorber perhour. Thus,A the >total ypounds etlime in the systemat'any onetime isvery small, 'usually less 'thani10'00 lb'sjîfor a system `containing 225,000 ygallons furfural as in thefexample. . ‘From the foregoing, many advantages of the present invention will ‘be’ apparent îto those VVskilled in the The'principaliadvantage'is`Y that corrosion of ’the lequipment'inthe main Y y system> is prevented.' “This allows the ‘use of equipment of ` carbon - steel'which -ismuch' flessrex‘ pensive than corrosion-res'i‘starít’metala‘ïrnn-- _ 2,404,253 other major advantage of the present invention no» problem due to sediment in the main system is presented. 'I'he lime or its reaction products is completely removed from the furfural in the is that it inhibits polymer formation in the main system which of itself gives rise to many advan tages well known to those skilled in the art. The invention accomplishes simultaneous reduction in the level of acid concentration in furfural in the extractive distillation system and reduction in the rate of polymerization of furfural. The invention is simple and can be carried out with out requiring expensive additional equipment or introducing complications into the existing ex tractive distillation system and re-run unit. By using lime, there is no danger of high concen tration of alkaline material at any point which re-running operation. Many other advantages of the process of the present invention will be apparent to those skilled in the art. I claim: _ 1. In the process of >separating unsaturated aliphatic hydrocarbons from a mixed hydrocar bon stream containing such unsaturates by ex tractive distillation wherein said mixed hydro carbon stream is passed upward through an ab sorber tower in countercurrent to a descending stream of solvent composed of furfural contain ing dissolved water; a bottoms product consist ing of furfural rich in unsaturated hydrocarbonV is withdrawn from said absorber tower; said bot tom product is introduced into a stripper 'column and there stripped of its unsaturated hydrocar bon content; a bottoms product consisting of stripped furfural is withdrawn from said strip per column and split into tWo Streams; one of such resulting streams is passed to a re-run unit `and. there distilled to give a. purified material would be harmful because it would cause an un duly high rate of polymerization of furfural; lime is unable to give a highly alkaline condition by reason of its nature and limited solubility in furfural. The invention is extremely economical to apply because of the small amount of lime and because lime is cheap. The invention greatly reduces the amount of furfural which has to be re-run in order to remove polymer. This reduc tion in amount of furfural re-run gives rise to many other advantages, the principal of which is the markedly reduced loss of furfural and the lowered cost of re-running the smaller amount of furfural. It will be understood by those skilled in the art that loss of furfural in the re-run ’ consisting essentially of furfural `and water dis solved therein; the purified furfural is combined with the other stream of stripped furfural and the resulting combined furfural stream is re cycled to said absorber tower as solvent for the system is high because the re-running itself extractive distillation step; that improvement the re-run unit. factors other than acidity by introducing lime causes additional polymerization and because it 30 which consists of simultaneously inhibiting poly merization of furfural corrosion of equipment is necessary to leave aconsiderable amount of due to `acids and corrosion of equipment due to furfural in the polymer in order to get it out of ' The invention is further advantageous in that it accomplishes prevention of formation of im 35 purities at the very point where such impurities are most deleterious, namely in the extractive dis tillation system which embodies very expensive absorber and stripper units. A very marked ad- ' vantage of the present invention is that it causes 40 no foaming of the furfural in the extractive dis tillation system. Foaming of furfural in the ab sorbers and strippers is most objectionable. The behavior of lime introduced in accordance with the present invention is to be compared with that droxide. Undoubtedly such sodium compounds tion of polymers; however, such alkaline-reacting between the place where said stripped furfural stream is split and Ithe place where the recycled stripped furfural is fed into said absorber tower; and controlling the amount of lime so introduced so that the acidity of the furfural in the extrac tive distillation system is maintained between 0.2% and 0.6% by weight, calculated as acetic acid. 2. The process of claim 1 wherein the lime is introduced into the furfural in the form of an aqueous dispersion. 3. The process of claim 1 wherein the introduc of soda ash, sodium bircarbonate or sodium hy would, if added to the furfural in the main sys tem, substantially decrease corrosion and forma into that branch of the stripped furfural stream which bypasses the said re-run unit, at a point 50 alkali metal compounds cause ythe furfural to foam which cannot be tolerated in processes of absorption and stripping. Even traces of sodium hydroxide, carbonate or bicarbonate cause foam ing of furfural so that it would be completely im possible to use such materials in place of lime in accordance with the present invention. Another advantage of the present invention is that the amount of lime required is so small that 60 tion of said lime is accomplished by admixing same with said branch of the stripped furfural stream which bypasses the re-run unit While said furfural is at a temperature of not over 130° F. 4. The process of claim 1 wherein said hydro carbon stream is an aliphatic C4. stream and wherein said unsaturated hydrocarbon is an ali phatic C4 hydrocarbon. ' - 5. The process of claim 1 wherein said unsat urated hydrocarbon comprises butene-Z, 6. The process of claim l wherein said unsat urated hydrocarbon comprises butadiene. . l VIRGIL SCARTI-I.