typical hydrocarbon fractions used as feed in diolefin separation processes contain hydrocarbons which are lighter or more volatile than the desired hydrocarbons and some which are heavier or less volatile than the desired hydrocarbons . one or more distillation steps prior to the steps recited in the summary section may be conveniently performed on the crude feed to remove these heavy and light ends of the crude feed stream . this type of operation is shown in u . s . pat . no . 3 , 851 , 010 , col . 3 , lines 21 - 26 and u . s . pat . no . 3 , 317 , 627 , col . 3 , lines 18 - 20 as being accomplished in a single distillation column . heavy ends removal is also illustrated by u . s . pat . nos . 2 , 459 , 403 ; 2 , 426 , 705 ; and 2 , 407 , 997 . the top distillate of the second extractive distillation contains isoprene along with some extraction solvent and water . the water and extraction solvent may be conveniently removed by a water wash or water extraction in which said distillate stream is countercurrently extracted with water . such a process is illustrated by u . s . pat . no . 2 , 426 , 705 , fig1 a and column 4 , lines 52 - 60 , which is hereby incorporated by reference . typically , the water wash system is designed to reduce the solvent concentration in the stream to less than 10 ppm . the washed isoprene - containing stream may yet contain compounds other than isoprene which would adversely effect polymerization . these impurities may be small amounts of hydrocarbons having a different number of carbon atoms than the desired isoprene , and they can also include compounds having the same number of carbon atoms as the desired conjugated diolefin , but whose boiling points are somewhat different from that of the desired diolefin ( e . g . 1 , 3 - pentadienes , cyclopentadiene ). these types of impurities can be removed by standard distillation techniques . such processes are illustrated by the following u . s . pat . nos . : 2 , 426 , 705 , fig1 a and col . 4 , lines 60 - 71 ( which illustrates the removal of heavy impurities ); 3 , 510 , 405 , col . 10 , lines 32 - 56 ( which illustrates the removal of light impurities ); and 3 , 436 , 438 , col . 6 , lines 9 - 15 . this distillation step is called the isoprene refining step . after the removal of heavy acetylenes ( e . g . pentyne - 1 ) in this step , total acetylenes concentration is less than 100 ppm . in the case of isoprene , one of the more important impurities which is removed by isoprene refining is cyclopentadiene which exits in the bottoms of the isoprene refining step . there are other ways to remove cyclopentadiene , as discussed in the background section , but whatever the technique used , it should be reduced to below several ppm concentration in the product isoprene . for purposes of this application , the term &# 34 ; ppm or parts per million with respect to isoprene ( or conjugated diolefin )&# 34 ; means the weight of the measured compound in the stream divided by the weight of isoprene in the stream , which quotient is multiplied by one million . as an aid in understanding the process of this invention , the overall process will be described with reference to the flow diagram . throughout this detailed description , process parameters ( e . g . stream compositions , temperatures , pressures , and equipment description ) will be given . these process parameters are to be considered exemplary only and not limitative as to the scope of this invention . the hydrocarbon feed stream 1 enters the first extractive distillation column 2 at a point in between the points of withdrawal for the distillate and bottoms . stream 1 is normally a liquid , a typical example of which is given in table 1 . table 1______________________________________feed stream 1 typical composition mole weight % component percent or ppm______________________________________nitrogen 0 . 231 932 . 3 ppm1 , 4pd ( 1 , 4 - pentadiene ) 0 . 093 920 . 9 ppm1 - pentane 2 . 918 3 . 033isopropenyl acetylene 0 . 000 5 . 0 ppmpentene - 1 6 . 345 6 . 4102mb1 ( 2 - methyl - 1 - butene ) 9 . 447 9 . 545isoprene 35 . 407 34 . 7461pen4yne ( 1 - pentene - 4 - yne ) 0 . 032 304 . 6 ppmn - pentane 18 . 884 19 . 627pentyne - 1 0 . 094 926 . 1 ppmt - pente2 ( trans - 2 - pentene ) 5 . 889 5 . 950c - pente2 ( cis - 2 - pentene ) 2 . 895 2 . 9252mb2 ( 2 - methyl - 2 - butene ) 3 . 750 3 . 789cpd ( cyclopentadiene ) 3 . 163 3 . 012t - pip ( trans - piperylene ) 6 . 410 6 . 289c - pip ( cis - piperylene ) 2 . 985 2 . 929cyclopentene 1 . 239 1 . 216cyclopentane 0 . 218 0 . 220total 100 . 000 100 . 000total mols / hr . = 937 . 7temp . deg . c . 63pressure , kpa ( kilopascals ) 723______________________________________ the extraction solvent 12 ( preferably acetonitrile ) is supplied from solvent surge tank 7 via pump 10 . said surge tank is supplied with fresh solvent 4 and recycled solvent 5 , and is blanketed with nitrogen to prevent oxygen from entering the system . the first extractive distillation column typically has 72 trays . the feed is on the 36th tray from the top , and the solvent enters on the 10th tray . a typical operating pressure for the column is 20 psig . ( 138 kpa ). the bottom of the column is heated by reboiler 23 and solvent exchanger 20 . reboiler pump 15 circulates a stream 14 from the bottom of the column through the two heat exchangers . solvent exchanger 20 conserves heat by transferring heat from recycle stream 5 to stream 18 ( part of circulating stream 14 ). recycle solvent 5 must be cooled prior to storage in the solvent surge tank 7 , and it is further cooled after solvent exchanger 20 with cooling water in recycle solvent cooler 3 . further savings in steam ( the heating medium for reboiler 23 ) may be realized by transferring heat to another part of circulating stream 14 from other streams with excess heat content ( e . g . condensers in the solvent recovery area of the plant ). the first extractive distillation overhead vapors 26 are condensed in condenser 27 , and condensed stream 28 flows to overhead drum 30 . reflux pump 38 withdraws stream 42 from the reflux drum and said stream is split on the discharge side of said pump into reflux stream 43 and raffinate 44 ( principally monoolefins and paraffins ). any water / solvent mixture which collects as a separate phase in the boot or sump part of reflux drum 30 is withdrawn by water phase pump 39 as stream 40 . the vapors which exist in reflux drum 30 flow through pipe 31 through vent chiller 33 which removes heat from stream 31 by transferring it to a refrigerant such as propane . after being chilled , stream 31 flows into disengaging drum 34 wherein the noncondensables 35 are sent to a flare and the condensed materials 37 are returned to reflux drum 30 . reflux ratio , for purposes of this application expressed as r d , is the ratio of moles per hour reflux to moles per hour distillate ( or raffinate in the case of extractive distillation ). reflux ratio for the first extractive distillation is typically in the range of about 3 to 6 . the isoprene - laden solvent stream ( or extract ) stream 47 is transferred to the second extractive distillation column 50 by bottoms pump 46 . this stream contains essentially all the acetylenes , all the heavy diolefins , 99 % of the isoprene , and some heavy olefins . in addition , it contains essentially all the solvent fed to the first extractive distillation column . at the conditions stated , an extract 47 and a raffinate 44 having the composition shown in table 2 are obtained : table 2______________________________________ stream 44 raffinate stream 47 extract mole mole per - weight % per - weight % compnent cent or ppm cent or ppm______________________________________1 , 4pd 0 . 030 313 . 4 ppm 0 . 008 139 . 1 ppm1 - pentane 4 . 926 5 . 378 0 . 000 0 . 000isoprepenylacetylene 0 . 000 0 . 000 0 . 000 1 . 9 ppmpentene - 1 10 . 708 11 . 362 0 . 000 4 . 8 ppm2mb1 15 . 884 16 . 855 0 . 004 76 . 4 ppmisoprene 0 . 516 0 . 532 3 . 716 6 . 4651pen4yne 0 . 002 24 . 6 ppm 0 . 010 171 . 7 ppmn - pentane 31 . 899 34 . 821 0 . 000 0 . 000pentyne - 1 0 . 009 94 . 8 ppm 0 . 028 491 . 6 ppmt - pente2 9 . 900 10 . 505 0 . 003 55 . 0 ppmc - pente2 4 . 846 5 . 142 0 . 002 50 . 2 ppm2mb2 5 . 454 5 . 787 0 . 055 988 . 5 ppmcpd 0 . 015 159 . 4 ppm 0 . 322 0 . 544t - pip 0 . 089 921 . 6 ppm 0 . 714 1 . 243c - pip 0 . 084 867 . 4 ppm 0 . 399 0 . 695cyclopentene 0 . 007 79 . 6 ppm 0 . 130 0 . 226cyclopentane 0 . 000 3 . 4 ppm 0 . 023 413 . 3 ppmdicyclopentadiene 0 . 000 0 . 000 0 . 146 0 . 493acn 14 . 666 9 . 110 76 . 267 83 . 113water 0 . 964 0 . 262 15 . 173 6 . 982total 100 . 000 100 . 000 100 . 000 100 . 000total moles / hr . 608 . 3 8875 . 2temp . deg . c . 46 . 7 99 . 4pressure , psia 123 . 000 ( 847 . 5 kpa ) 98 . 000 ( 675 . 2 kpa ) ______________________________________ stream 47 enters column 50 , 31 trays down from the top of the tower which typically has 64 trays . column 50 typically operates at about 18 psig ( 124 kpa ) at the top and a bottoms temperature of about 112 ° c . the relative volatility adjustment , which allows the separation between isoprene and acetylene , requires the presence of solvent 85 on the separating trays above the feed as well as below . therefore , extraction solvent must be fed high in the tower in a similar manner to any other conventional extractive distillation step . however , this solvent must be acetylenes free ; otherwise , acetylenes contained in the solvent would contaminate the product . solvent 85 from the bottom of stripper 77 is fed to tray 10 of column 50 . the top 9 trays of column 50 are for solvent disengagement . of the total solvent circulation in this process , approximately 70 % is fed to the first extractive distillation 2 , and approximately 30 % is fed separately to column 50 . the purpose of the second extractive distillation is to reduce acetylenes concentration in the isoprene - containing product to an acceptably low level . the design in this particular instance , is to reduce 1 - pentene - 4 - yne to a level less than 100 ppm with respect to isoprene . however , this operation also reduces the amount of other acetylenes present in the feed to this tower ( isopropenyl acetylene and pentyne - 1 by 70 and 96 %, respectively , based on amount of these in feed stream 47 ). this separation is affected with a reflux ratio ( r d ) generally in the range of about 4 / 1 to 11 / 1 . heat is furnished to the second extractive distillation column by circulating stream 52 from the bottom of said column via bottoms pump 53 through reboiler 54 ( steam heated ) and back into the bottom of the column . the distillate vapors 56 are condensed in condenser 58 , and the condensed stream 59 flows into reflux drum 60 . a stream of the condensed overheads 65 is withdrawn from the reflux drum by overhead pump 64 , and this stream is split into two streams , a reflux stream 66 and product or distillate stream 68 . the product stream contains : nearly all the isoprene fed to the process , most of the heavy diolefins fed to the process ( e . g . piperylenes ), and acetylenes ( reduced to within the desired concentration limits with respect to isoprene ). it has been found that a spray of liquid down onto the tubes of condenser 58 is helpful in inhibiting bacteria growth on the tubes . the spray stream may be a small flow taken from stream 66 and sent to the top of condenser 58 . any of the acetonitrile ( acn )/ water extraction solvent which carries over into the reflux drum 60 and separates as a lower liquid phase in the boot or sump section of said drum is withdrawn as an aqueous phase 62 by water phase pump 61 . the remaining diolefins and almost all of the acetylenes incoming to the process appear in the bottoms 69 of the second extractive distillation with the extraction solvent . typical compositions for streams 68 and 69 are given in table 3 . table 3______________________________________ stream 68 - bottoms stream 69 distillate product mole weight % mole weight % component percent or ppm percent or ppm______________________________________1 , 4pd 0 . 000 0 . 000 0 . 143 0 . 148isopropenylacetylene 0 . 000 1 . 0 ppm 0 . 000 6 . 1 ppmpentene - 1 0 . 000 0 . 000 0 . 004 51 . 8 ppm2mb1 0 . 000 0 . 000 0 . 076 818 . 7 ppmisoprene 0 . 008 152 . 0 ppm 66 . 562 68 . 9941pen4yne 0 . 007 130 . 4 ppm 0 . 003 32 . 5 ppmpentyn - 1 0 . 020 364 . 5 ppm 0 . 018 186 . 7 ppmt - pente 2 0 . 000 0 . 000 0 . 055 589 . 7 ppmc - pente 2 0 . 000 0 . 000 0 . 050 537 . 9 ppm2mb2 0 . 000 0 . 000 0 . 991 1 . 058cpd 0 . 006 116 . 4 ppm 5 . 633 5 . 666t - pip 0 . 046 833 . 4 ppm 11 . 722 12 . 150c - pip 0 . 096 0 . 173 4 . 854 5 . 032cyclopentene 0 . 000 0 . 000 2 . 343 2 . 429cyclopentane 0 . 000 0 . 000 0 . 414 0 . 442dcpd 0 . 156 0 . 547 0 . 000 0 . 000acn 84 . 458 91 . 863 5 . 431 3 . 393water 15 . 203 7 . 257 1 . 701 0 . 466total 100 . 000 100 . 000 100 . 000 100 . 000total moles / hr . flow 11 , 974 . 2 494 . 0temp . deg . c 112 57pressure , kpa 785 841______________________________________ bottoms stream 69 is withdrawn by bottoms pump 71 and splits into three streams on the discharge side of said pump . approximately 67 % of the bottoms flows as stream 5 to solvent exchanger 20 . about 2 percent of the bottoms , as stream 74 , flows to solvent purification , which ( as discussed previously ) may be a distillation which removes impurities such as dicyclopentadiene ( dcpd ) from the solvent as a bottoms stream . the distillate from said distillation is then recycled to the solvent surge tank 7 . both this distillate and the recycle stream 5 are comprised of acn and water ; however , they contain small amounts of impurities , such as 1 , 3 - pentadienes . these contaminants are acceptable for the solvent to be recycled to the first extractive distillation , but not for that to be used in the second extractive distillation . the remainder of the bottoms ( stream 75 ) is fed to the stripper 77 for removal of acetylenes and heavy diolefin . in the stripper , all of the isopropenyl acetylene and pentyne - 1 plus 98 percent of the 1 - pentene - 4 - yne in stream 75 are removed from the solvent by taking them overhead in the distillate . the overheads vapors 87 are condensed in condenser 88 , and the condensed stream 89 flows into overheads drum 90 . stripper reflux pump 98 withdraws a stream 97 comprised of acn , water , heavy diolefins , and acetylenes . this stream is split into two streams , one of which is the reflux 99 to the stripper , and the other ( distillate stream 100 ) is added to the raffinate 44 from the first extractive distillation . a typical composition for stream 100 is given in table 4 . table 4______________________________________distillate stream 100 mole weight % component percent or ppm______________________________________isopropenyl acetylene 0 . 025 265 . 4 ppmisoprene 3 . 887 4 . 2541pen4yne 3 . 375 3 . 584pentyn - 1 9 . 312 10 . 191cpd 3 . 062 3 . 252t - pip 21 . 287 23 . 296c - pip 44 . 262 48 . 439acn 7 . 225 4 . 765water 7 . 562 2 . 189total 100 . 000 100 . 000temp . deg . c . 58pressure , psia ( kpa ) 146 . 0 ( 1006 ) ______________________________________ the combined stream 102 is sent to a process for solvent recovery . such a process may consist of a water wash or water extraction to remove the acn and water as extract . the organic raffinate from said water wash would then contain paraffins , olefins , acetylenes , and other impurities and could be disposed of . the aqueous extract can subsequently be distilled to recover the solvent as distillate which then could be recycled to the solvent surge tank 7 . such a system ( designed , however , for dcpd removal ) is described in u . s . pat . no . 2 , 426 , 705 at fig1 a , col . 4 , line 73 - col . 5 , line 7 and col . 9 , line 53 to col . 10 , line 50 . again , any acn / water phase which may collect in the bottom of the stripper overhead drum is withdrawn as stream 92 by stripper water phase pump 94 . the three water phase streams ( 40 , 62 and 92 ) are combined into stream 95 which is sent to a solvent recovery system ( e . g . a distillation for separation of water from said stream which may thereafter be recycled to solvent surge tank 7 ). stripper 77 is typically a 74 tray tower with the feed on the 12th tray counting from the top , at a pressure of about 15 psig ( 103 kpa ), and about 111 ° c . bottom temperature . under these conditions a nearly acetylene - free solvent stream is obtained as the bottoms 78 . this bottoms stream is withdrawn from the stripper by stripper bottoms pump 80 . part of it , 81 , is circulated through stripper reboiler 79 and returned to the bottom of the stripper in order to furnish heat to the stripper . the extraction solvent for the second extractive distillation 85 flows from the discharge of pump 80 through stripper solvent cooler 84 which lowers the temperature of said stream to approximately that of the tenth tray of column 50 ( e . g . 66 ° c .). a bypass pipe around stripper solvent cooler 84 with a control valve ( controlled by a temperature controller on the discharge side of the solvent cooler ) is a preferred method for adjusting flow through the solvent cooler and thereby controlling the temperature of stream 85 . the difference between the quality of the extraction solvent used for column 2 and that for column 50 is made clearer by table 5 which shows typical compositions for the streams . table 5______________________________________tpyical compositions stream number 12 stream number 85 solvent feed to solvent feed to column 2 column 50 mole weight % mole weight % component percent or ppm percent or ppm______________________________________nitrogen 0 . 000 2 . 0 ppm 0 . 000 0 . 0001 , 4pd 0 . 000 3 . 3 ppm 0 . 000 0 . 0001 - pentane 0 . 030 590 . 3 ppm 0 . 000 0 . 000isopropenylacetylene 0 . 000 1 . 0 ppm 0 . 000 0 . 000pentene - 1 0 . 067 0 . 124 0 . 000 0 . 0002mb1 0 . 099 0 . 185 0 . 000 0 . 000isoprene 0 . 011 205 . 1 ppm 0 . 000 0 . 0001pen4yne 0 . 007 126 . 0 ppm 0 . 000 2 . 3 ppmn - pentane 0 . 199 0 . 382 0 . 000 0 . 000pentyne - 1 0 . 019 325 . 8 ppm 0 . 000 0 . 000t - pente2 0 . 061 0 . 115 0 . 000 0 . 000c - pente2 0 . 030 564 . 5 pm 0 . 000 0 . 0002mb2 0 . 034 635 . 3 ppm 0 . 000 0 . 000cpd 0 . 006 114 . 2 ppm 0 . 000 0 . 000t - pip 0 . 045 814 . 4 ppm 0 . 000 0 . 000c - pip 0 . 092 0 . 168 0 . 000 0 . 000cyclopentene 0 . 000 0 . 8 ppm 0 . 000 0 . 000dcpd 0 . 141 0 . 498 0 . 157 0 . 550acn 82 . 910 90 . 413 84 . 867 92 . 302water 16 . 249 7 . 777 14 . 976 7 . 148total 100 . 000 100 . 000 100 . 000 100 . 000total moles / hr . flow = 8609 3668______________________________________ except where otherwise stated , heat exchangers used to cool process fluids such as condensors in this process may utilize plant cooling water ( typically 32 ° c .) as the cooling medium and heat exchangers used for heating such as reboilers utilize steam as the heating medium ( typically saturated steam at 156 ° c .). other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein . it is intended that the specification , drawing , and detailed description be considered as exemplary only , with the true scope and spirit of the invention being indicated by the following claims .