Abstract:
In a method for the solvent extraction of butadiene from a mixture of hydrocarbons having four carbon atoms per molecule, which method inherently produces tars, the extraction process is operated with a tar loading level, relative to the solvent employed, of no more than about 1.6 wt. %.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the solvent extraction of butadiene from a mixture of hydrocarbons having four carbon atoms per molecule (C 4 &#39;s). 
     2. Description of the Prior Art 
     Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce individual olefin products such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In such olefin production plants, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. This mixture, after preheating, is subjected to severe hydrocarbon thermal cracking at elevated temperatures of about 1,450 to 1,550° Fahrenheit (F.) in a pyrolysis furnace (steam cracker or cracker). 
     The cracked product effluent from the pyrolysis furnace contains hot, gaseous hydrocarbons, both saturated and unsaturated, of great variety from 1 to 35 carbon atoms per molecule (C 1  to C 35 ). This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate product streams of high purity, e.g., molecular hydrogen, ethylene, and propylene. After separation of these individual streams, the remaining cracked product contains essentially hydrocarbons with four carbon atoms per molecule (C 4 &#39;s) and heavier. This remainder is fed to a debutanizer wherein a crude C 4  stream is separated as overhead while a C 5  and heavier stream is removed as a bottoms product. 
     The crude C 4  stream has a variety of compounds such as n-butane, isobutane, 1-butene, 2-butenes (cis and trans), isobutylene, butadiene (1,2- and 1.3-), vinyl acetylene, and ethyl acetylene, all of which are known to boil within a narrow range, U.S. Pat. No. 3,436,438. Further, some of these compounds can form an azeotrope. Crude C 4 &#39;s are, therefore, known to be difficult to separate by simple distillation. 
     The crude C 4  fraction, after removal of acetylenes, normally goes to a butadiene extraction unit for separation of butadiene from the fraction. Thereafter, isobutylene can be removed by, for example, reaction with methanol to form methyl-tert-butyl ether (mtbe). Butenes can then be distilled from the mtbe, and 1-butene separated from 2-butenes by simple distillation. 
     The dominating process for separating butadiene from crude C 4 &#39;s is known technically as “fractional extraction,” but is more commonly referred to as “solvent extraction” or “extractive distillation.” However it is termed, this process employs an aprotic polar compound that has a high complexing affinity toward the more polarizable butadiene than other olefins in the crude C 4  stream. Known solvents for this process include acetonitrile, dimethylformamide, furfural, N-methyl-2-pyrrolidone, acetone, dimethylacetamide, and the like. This process and the solvents used therein are well known, U.S. Pat. Nos. 2,993,841 and 4,134,795. It is equally well known that this type of process inherently generates internally tars (tar) that, if not controlled, can affect the quality of the butadiene separated as a product of the process, and even plug equipment, thereby causing an expensive and time consuming shut down and clean out of the plant. Accordingly, there is continuous effort in the industry to which this process pertains to find solvents that reduce tar formation and deposition in equipment. 
     This invention takes a different tack from industry in addressing the control of tar formation and deposition in a butadiene extraction unit, in that it controls tars without changing the known solvents used in such a process. 
     Heretofore, in butadiene extraction plants such as that shown in  FIGS. 1-4  herein below, wherein a primary solvent and a secondary solvent were employed, it was dogma that some tar content suspended in the solvent mixture (primary and secondary) circulating in the system was necessary to keep tar formation and deposition at a minimum in the system as a whole. Accordingly, operators of such extraction plants were required without fail to maintain in the solvent mixture a tar level (load) of not less than 2 weight percent (wt. %) and a total content of tars plus secondary solvent of 5 wt. %, both weight percents being based on the total weight of the solvent mixture plus tar circulating in the system. For example, the unswerving operating specifications for this type of plant known as the Nippon-Zeon design required the tar level to be 2 wt. % minimum and the combined tar and secondary solvent level to be 5 wt. %, i.e., 2 wt. % tar and 3 wt. % secondary solvent, the remainder being 95 wt. % primary solvent. These design criteria were slavishly followed by operators of such plants. 
     SUMMARY OF THE INVENTION 
     It has been found that, at the tar levels heretofore required by the industry as necessary, tar formation control (minimization) was not achieved. It was found that at the above tar and tar/secondary solvent levels deemed necessary by industry, tar formation and deposition were not controlled. In actuality, it has been found that at the levels dictated by industry, the tar present begat more tar instead of controlling tar formation. 
     In accordance with this invention, tar formation and deposition is controlled by deliberately maintaining the level of tars in the butadiene extraction system substantially below that which was considered necessary in the industry to prevent tar formation and deposition, i.e., not greater than about 1.6 wt. % in the solvent (primary alone or primary plus secondary) plus tar system. 
     It was surprisingly found that with the tar loading of this invention, the extraction system as a whole became self-cleaning in that the rate of formation of tars in the system actually decreased, and existing tar deposits were reduced. 
     It was further found that an essentially self-cleaning extraction system, with respect to tars formed therein, is established when the tar level is maintained at no more than about 1.6 wt. % and the total content of tars plus secondary solvent is at the same time maintained at a level of less than 5 wt. %, both wt. % based on the total weight of the solvents plus tar present and circulating in the system. 
     Finally, it was found that such tar and tar/secondary solvent loadings could be achieved in such a system only by alteration of the solvent reclamation system that is normally employed in such plants. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first extractor circuit for a commercial design that employs primary and secondary solvents. 
         FIG. 2  shows a second extractor circuit that is normally combined with the circuit of  FIG. 1 . 
         FIG. 3  shows the solvent loop for an extraction plant that contains the extraction circuits shown in  FIGS. 1 and 2 . 
         FIG. 4  shows a reboiler that is typically employed in the solvent loop of  FIG. 3 . 
         FIG. 5  shows the reboiler of  FIG. 4  modified to operate in accordance with one embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although this invention is discussed, for sake of clarity and brevity, in respect of a Nippon-Zeon type design, it is to be understood that this invention can be employed in other designs so long as the tar minimization advantages of this invention are achieved. 
       FIG. 1  shows the first section of the extraction unit to contain a first reboiled butadiene extractor  1  to which is fed crude C 4  feedstock  2 . Feed  2  is typically first passed into a reboiled vaporizer drum (not shown), and then passed from that drum into extractor  1  at one or more points along the height of that extractor. Lean (essentially C 4  free) solvent  3  is introduced into tower  1  above the point(s) of introduction of feed  2  so that the denser, descending solvent  3  can counter currently contact the feed  2  which is rising inside tower  1 . Solvent  3  extracts butadiene from feed  2  in known manner. Solvent  3  contains tars, but essentially no butadiene. Solvent  3  is the primary solvent present in the system in that it is present in a major amount. A secondary lean solvent  4  can be mixed with primary solvent  3  in a minor amount, e.g., primary solvent equal to or greater than 50 wt. %, and secondary solvent less than 50 wt. %, based on the total weight of the combined solvents. Solvents  3  and  4  contain tars, but essentially no butadiene. 
     Heretofore, the prior art deliberately and rigorously maintained the combination of solvents  3  and  4  at a tar loading of not less than 2 wt. % based on the total weight of the solvent combination plus tar loading. By this invention this tar loading is not to exceed about 1.6 wt. %. The combination of solvents  3  and  4  plus tars was heretofore just as rigorously maintained by the prior art at a secondary solvent  4  plus tars loading of not less than 5 wt. %. By this invention this secondary solvent and tar loading is less than 5 wt. %. All wt. % are based on the total weight of solvents  3  and  4  plus tars. 
     Extractor  1  has at its upper end a conventional reflux circuit that is not shown for sake of clarity but is like that of stripper  10 . Raffinate  5  (C 4  feed  2  essentially minus its butadiene content) is removed from this circuit for further processing elsewhere. Tower  1  has a conventional reboiler loop  6  for heating the tower. 
     Bottoms  7  from tower  1  is a mixture of butadiene and solvent(s) and are passed to first reboiled stripper  10  to separate butadiene from solvent. Primarily butadiene, after separation from the solvent, is recovered as overhead from tower  10  in line  11  and enters a conventional reflux circuit composed of at least one heat exchanger  12  and reflux drum  13 . Liquid reflux is returned to tower  10  by way of line  14 , while a vapor stream rich in butadiene is recovered in line  15  for transport to a compression step shown in  FIG. 2 . 
     Bottoms  16  from tower  10  is primarily lean solvent (primary and secondary) and tars that have been formed in the extraction system, of which at least part is removed for reclamation and reuse in the extraction system as lean solvent. 
       FIG. 2  shows the second section of the overall extraction unit to contain a compression unit  20  that receives butadiene rich vapor stream  15 , and subjects it to at least one compression operation, e.g., a compression step followed by heat exchange and liquid separation followed by a second compression step. The thus compressed material from first stripper  10  is introduced by way of line  21  into second reboiled butadiene extractor  22 . Lean solvent  23 , primary and secondary, is introduced into tower  22  near the top thereof so that, when descending through tower  23  in the same manner described for tower  1  of  FIG. 1 , it counter currently contacts ascending feed  21 . 
     Tower  22  is operated in such a manner, known in the art, that a butadiene rich overhead  24  is recovered and passed through a conventional reflux circuit composed of a heat exchanger  25  and reflux drum  26 . Reflux  27  is split, part being returned by way of line  28  to tower  22 , and part being passed by way of line  29  to first reboiled fractionator  30 . In distillation column  30 , stream  29  is topped and materials lighter than butadiene are removed overhead by line  31  through a conventional reflux circuit (not shown) for ultimate removal from the extraction unit for use elsewhere in the plant, e.g., as fuel. The bottoms of tower  30  are removed by way of line  32  and introduced into reboiled fractionator  33  wherein materials heavier than butadiene are removed as bottoms by way of line  34  for use elsewhere in the plant, e.g., as fuel. The overhead  35  from tower  33  passes through a conventional reflux circuit (not shown) and is then removed as the butadiene product of the olefin plant. 
     Bottoms  40  of tower  22  contains primarily solvent and tars plus slight amounts of butadiene and acetylenes (C 3  and C 4 ), and is passed to a reboiled butadiene recovery column  41 . Column  41  typically does not have a reflux circuit for its overhead  42 . Overhead  42  is returned directly to line  11  of first stripper  10 , and ultimately as a feed material  15  for compression unit  20 . Bottoms  43  of column  41  is primarily solvent plus tars and acetylenes, and is passed to second reboiled stripper  44  wherein light materials are stripped from the solvent(s) and tars and recovered overhead for use elsewhere in the plant, e.g., as fuel. Tower  44  typically has a reflux circuit (not shown). Bottoms  46  of tower  44  contains primarily solvent plus tars originally in the solvents and tars formed during the butadiene extraction process, and are recovered for reuse in the butadiene extraction process including the solvent reclamation unit. 
       FIG. 3  shows the solvent(s) loop for the extraction process of combined  FIGS. 1 and 2 .  FIG. 3  shows first extractor  1  connected by way of line  7  to first stripper  10 , see  FIG. 1 .  FIG. 3  also shows that second extractor  22  is connected (indirectly through column  41 ) by way of line  43  to second stripper  44 , line  40  and column  41  not being shown for sake of clarity. Bottoms  16  of first stripper  10  and bottoms  46  of second stripper  44  are combined into a common stream  50  for reuse as lean solvent in towers  1  and  10  for the extraction of additional butadiene from new feed  2  ( FIG. 1 ). 
     A minor portion of combined solvent stream  50  is taken as a side stream  51  to be processed for the removal of all or substantially all tars there from in solvent reclaiming unit  52 . Solvent essentially devoid of tar is reintroduced into common line  50  by way of line  53 . Tar that has been separated from solvent in unit  52  is removed from the process by way of line  59  for other disposition outside the extraction unit. A mixture of recovered solvent carrying tars and reclaimed solvent containing essentially no tars is passed by way of line  54  through at least one heat exchanger  55 , and then, by way of line  56 , to extractors  1  and  22  for reuse as a lean solvent, stream  56  being split between extractors  1  and  22  by way of lines  57  and  58 , respectively. This split between lines  57  and  58  is not necessarily equal, common practice being the majority (more that 50 wt. % based on the total weight of stream of stream  56 ) going to extractor  1  and the remainder to extractor  10 . An 85/15 split is not uncommon. 
     For sake of clarity, streams  16  and  46 , alone or combined, are characterized herein as “primary” mixtures of solvent/tars, e.g., primary lean solvent streams, while streams  51  and  53  are characterized herein as “secondary” mixtures of solvent/tars. Stream  53  is a secondary lean solvent stream that is essentially tar free, and this stream is used to keep the tar level in the overall solvent loop at the desired tar loading. 
     In the prior art operation of an extraction plant as represented by the combination of  FIGS. 1 and 2 , the quantity (volume or weight) of secondary side stream  51  taken from primary stream  50  is varied so that after the tars free secondary stream  53  is mixed with the primary stream to form stream  54 , the resulting primary stream  54  has essentially the tar content called for by the plant design, e.g., the not less than 2 wt. % tars dictated by the industry prior to this invention. The size of side stream  51  and the amount of tar removed from that side stream in unit  52  before it is added back to the primary solvent stream was used by an operator to meet this 2 wt. % tar goal. The amount of secondary solvent added by way of line  4  ( FIG. 1 ) can be used by an operator to achieve the relative relationship goal of the secondary solvent/tars aforesaid, e.g., secondary solvent  4  plus tars equals 5 wt. %, and also affects the amount of tar free stream  53  that is added to make up stream  54 . Thus, it can be seen that, although the control of the overall tar level in the system is easy to state, it is far from easy to achieve in the actual operation of the overall extraction unit. However, operators of such units are skilled, and capable of controlling the unit to essentially meet target tar loading levels. That is why they could heretofore rigorously maintain a 2 wt. % minimum tar loading, and can hereafter maintain a tar loading of not greater than 1.6 wt. %. 
       FIG. 4  shows solvent reclaimer  52  of  FIG. 3  can be a kettle style reboiler  60 . Such devices are well known in the art. Prior art operation of reboiler  60  was to inject hot (about 325° F.) slightly pressurized (about 20 psia) secondary solvent stream  51  into the bottom of reboiler  60  below the tar body therein. Inside reboiler  60  the prevailing pressure is lower than stream  51 , e.g., about 5 psia, and solvent is vaporized away from the higher molecular weight tars that are more difficult to vaporize than the solvent(s) present in stream  51 . Accordingly, essentially tar free solvent should be recovered overhead in line  54 , thereby leaving a build up of tars  61  in reboiler  60 . Thus, inside reboiler  60  there is present a mixture  61  of tar, primary solvent, and secondary solvent, if any, which has an upper tar level  62  above which is a vapor space  63  in which only solvent vapor should be present for recovery by way of line  54 . When reboiler  60  is operated correctly, the solvent recovered in line  54  should be devoid of tar, i.e., contain essentially no tars. 
     Although for sake of simplicity it is shown in  FIG. 3  that secondary lean solvent  53  is mixed with primary lean solvent  50 , lean solvent  53  is commonly collected in a refined solvent receiver (not shown) and reintroduced into the solvent loop of  FIG. 3  by employing it as pump seal flush for the various solvent pumps employed in the extraction unit. 
     Tars in mixture  61  that have been left behind by the vaporizing solvent(s) collect in the lower portion of reboiler  60 , and when the amount of tars so collected reaches a target level, e.g., reboiler mixture  61  is from about 30 to about 40 wt. % tar based on the total weight of mixture  61 , the collected tars are removed (dumped) from the reboiler and the extraction system by way of line  59 . A good measure of the rate of tar generation in a given extractor system is the number of reboiler  60  dumps that have to be made over a given period of time. For example, pursuant to this invention, given the same feed rate for stream  51  and the same overall extraction operating conditions, a 1 wt. % system tar load, based on the total weight of solvent and tar, can result in a reboiler dump about every 5.5 days, whereas a prior art 2 wt. % system tar load can result in a dump about every 3 days. 
     In the embodiment set forth by  FIGS. 1 ,  2 , and  3 , solvent reclamation unit  52  was operated by the prior art in accordance with design requirements. This meant that the extraction unit and process was rigorously operated in a manner such that the primary solvent streams  54  through  58 , inclusive, carried no less than 2 wt. % tars. This was based on the premise that it took 2 wt. % tars to prevent undesired build up and deposition of tars in the system. When a secondary solvent was employed the prior art operated the extraction process so that not only was the 2 wt. % minimum tar goal maintained, but, also, the total of tars plus secondary solvent in the system was rigorously maintained at no less than 5 wt. %. 
     It was found that even when maintaining the 2 wt. % tar goal, tars still tended to be generated in the system, i.e., tars begetting tars. The same was true when maintaining a loading of 2 wt. % tars and 3 wt. % secondary solvent. 
     Pursuant to this invention it was found that by maintaining the tar level at no more than about 1.6 wt. %, surprisingly and unexpectedly, the system became self-cleaning in that tar deposits that were already present inside the equipment before the 1.6 wt. % or less level was established and maintained started to disappear. The same was true when the tar level of this invention was maintained, and the combined tars and secondary solvent level was also maintained at less than 5 wt. %. All wt. % are based on the total weight of the solvent(s) plus tars. It should be noted here that this invention does not include zero percent tars in the solvent(s) employed in the system. Some finite amount of tars should be present in the solvent, it should just be no more than about 1.6 wt. %, preferably less than 1.6 wt. %, still more preferably from a minimum of about 0.1 wt. % to a maximum of less than 1.6 wt. %, all wt. % based on the total weight of the solvent(s) plus tars. 
     To achieve the lower tar content goal of this invention a larger quantity of secondary stream  51  can be removed for treatment by reboiler  60  in solvent reclamation unit  52  to achieve a consistently larger level of tar removal from the extraction system. In the practice of this invention at least about 0.5 wt. %, desirably, from about 0.5 to about 40 wt. % of primary stream  50  is separated into secondary stream  51 , all wt. % based on the total weight of stream  50 . 
     However, it was found that in actual operation, it was not possible to maintain an overall tar content in the system of less than 1.6 wt. % using the apparatus of  FIG. 4 . The problem was found to reside in the reboiler itself. When hot, slightly pressured stream  51  was injected into tar body  61  of reboiler  60  as shown in  FIG. 4 , it was found that the flashing of solvent that occurred below surface  62  of the tar body coupled with the increased volume of stream  51  to maintain the overall tar level at no more than 1.6 wt. % caused foaming of tars and intrusion of this foam into vapor space  63 . This foaming resulted in undesired, and not heretofore experienced, carryover of tars into the overhead stream  53  which was supposed to be essentially tar free. 
     In accordance with this invention, stream  51  is cooled before injection of same below tar level  62  to a temperature that prevents flashing of solvent while in the tars below level  62 , thereby preventing foaming and carryover of tars into stream  53 . Accordingly, in this embodiment of the invention the temperature of input feed  51  should be no higher than from about 160 to about 180° F. 
     Other solutions will become apparent to those skilled in the art once appraised of this invention, and they are considered to be part of this invention. For example,  FIG. 5  illustrates one such alternative solution.  FIG. 5  shows the same apparatus as  FIG. 4  except that stream  51  has been rerouted by way of line  70  to introduce stream  51  into the vapor space  63  that exists inside reboiler  60  above tar level  62 . In this embodiment, hot, slightly pressured stream  51  need not be cooled, and can be injected into vapor space  63  with no worry of tar carryover into stream  53  because the solvent in stream  51  flashes in vapor space  63  and not inside tar body  61  under surface  62  with no resultant foaming. Of course, a combination of cooler feed to the bottom of reboiler  60  and hotter feed to the vapor space in reboiler  60  can be employed. 
     The solvents employed in this invention can be the same as employed by the prior art discussed hereinabove. Dimethylformamide (DMF) and furfural are particularly effective, although others can be used in this invention. When DMF and furfural are employed together, it is presently preferred that DMF be employed in major amount with furfural the minority remainder, e.g., DMF at least about 90 wt. % and furfural less than about 10 wt. %, both based on the total weight of DMF and furfural combined. When a mixture of two solvents is employed, the secondary solvent can be present in an amount of from about 0.1 wt. % to about 3.0 wt. %, based of the total weight of the mixture. 
     Normal operation of any butadiene extraction unit inherently generates long chains of C 4  compounds no matter how the process is carried out. These long chains are polymers (tars) that have been formed from monomers such as butadiene, vinyl acetylene, furfural, and the like. These polymers can be homopolymers, e.g., polybutadiene, or copolymers, e.g., copolymers of butadiene and vinyl acetylene. These polymers grow from lower molecular weight (lighter and relatively more volatile) to higher molecular weight (heavier and less volatile) the longer they reside in the extraction system. Generally, relative to the C 4 &#39;s present, they are heavier molecular weight materials. These polymers vary from those that resist volatilization under the various temperature and pressure conditions that prevail in the normal operation of a butadiene extraction unit up to those that simply will not volatilize under any of the prevailing conditions of temperature and pressure throughout the extraction unit, the lower molecular weight polymers simply being those that are on their way to becoming higher molecular weight polymers upon continued exposure to the operating conditions prevailing in the extraction unit. Thus, these polymers are difficult, if not impossible, to quantify further, but functional description of these materials is sufficient to inform the art because they are so prevalent in extraction processes. The conventional test procedure for determining the amount of tars that simply will not volatilize and that are present in a fluid such as a lean solvent that is to be used in an extractor is to heat a sample of such fluid at 212° F. and 28 inches of mercury pressure for 2 hours. The non-volatiles that remain constitute the tar fraction of the sample. The tars referred to in describing this invention can include both lighter and heavier molecular weight materials, and thus are not strictly limited to tars as determined by the foregoing test procedure. 
     Based on the practice of this invention a number of advantages arise over the prior art practice. First, there is an increased time of continuous operation between extractor unit shutdowns for cleaning and maintenance, and essentially no such shutdowns based on tar deposition. This saves substantially on maintenance costs and lost production. Reboilers  6  and  17 , and solvent heat exchangers  55  are the first to show signs of plugging due to tar deposition. After that the various towers themselves can be subject to tar deposition and potential plugging. This invention substantially reduces, if not eliminates the need for cleaning tar deposits out of such equipment. When operating under the reduced tar free environment of this invention, particularly as to tar deposits, energy costs are reduced. Finally, less solvent is lost from the reclamation unit. 
     EXAMPLE 1 
     An extraction unit as depicted in  FIGS. 1 through 4  was operated using a crude C 4  feedstock  2  for extractor  1  that contained about 6.8 wt. % n-butane, about 1.3 wt. % isobutane, about 13.5 wt. % 1-butene, about 10.3 wt. % 2-butenes (cis and trans), about 27.9 wt. % isobutylene, about 39.5 wt. % butadiene (1,2 and 1,3), about 0.5 wt. % vinyl acetylene, and about 0.1 wt. % ethyl acetylene, all wt. % being based on the total weight of the feed. Feed  2  was introduced into extractor  1  at a temperature of about 125° F. at about 65 psig, and a flow rate of about 60,000 pounds per hour (pph). 
     The extraction process employed dimethylformamide as the primary solvent and furfural as the secondary solvent. The total weight of the combined solvents, including a tar load of 2 wt. %, contained about 95 wt. % primary solvent and about 3 wt. % secondary solvent. The solvents were introduced into extractor  1  at a temperature of about 104° F. at about 48 psig, and a flow rate of about 415,000 pph. 
     Extractor  1  was operated with a bottom temperature of about 260° F. at 80 psig, and an overhead temperature of about 110° F. at about 50 psig with an external reflux rate of about 45,000 pph. Extractor  1  produced about 35,000 pph of raffinate (C 4 &#39;s essentially free of butadiene) at about 110° F. Butadiene rich solvent was removed as bottoms  7  at a flow rate of about 351,000 pph and passed to first stripper  10 . 
     Stripper  10  had a bottom temperature of about 335° F. at about 6 psig, and a reflux rate in line  14  of about 45 gallons per minute (gpm). About 60,000 pph of butadiene rich gas  15  was removed from reflux drum  13  and passed to compression unit  20 . About 30,000 pph of gas  15  was passed from compression unit  20  to second extractor  22 . About 415,000 pph of combined solvent and tars was removed as bottoms  16  and sent to reboiler  60 . Bottoms  16  was at about 335° F. at 6 psig. 
     Compression unit  20  employed a two stage compression process with heat exchange cooling between the two compression steps. Solvent at about 91° F. and about 90 psig was removed from unit  20  and passed to second extractor  22  at the flow rate of about 30,000 pph. 
     Extractor  22  was operated at a bottom temperature of about 258° F. at about 48 psig, a top temperature of about 110° F. at 50 psig, and a reflux rate of about 30,000 pph. Bottoms  40  of extractor  22  was removed at a rate of about 50,000 pph and sent to butadiene removal column  41 . 
     Column  41  was operated at a bottom temperature of about 260° F. at about 5 psig. Overhead  42 , at about 210° F. and 5 psig, was sent to first stripper  10  at a flow rate of about 3,000 pph. Bottoms  43 , at a flow rate of about 47,000 pph, was sent to second stripper  44 . 
     Second stripper  44  was operated at a bottom temperature of about 325° F. at 3.5 psig using a reflux rate of about 12 gpm. Bottoms  46  was sent to reboiler  60  at the flow rate of about 45,000 pph. Overhead  45  removed from the extraction unit. 
     Overhead  24  of extractor  22  was sent by way of line  29  to first fractionator  30  at the rate of about 27,000 pph. Tower  30  was operated with a bottom temperature of about 116° F. at about 53 psig overhead under total reflux. Bottoms  32  was passed to second fractionator  33  for final separation of butadiene product from the remaining solvent. Tower  33  was operated at a bottom temperature of about 140° F. at 70 psig, with a reflux rate of about 180,000 pph. Final butadiene product was removed by way of overhead stream  35  at the rate of about 26,000 pph. Bottoms  34  was removed from the extraction process. 
     Reboiler  60  was maintained at a temperature of about 190° F. at about minus 9 psig (5 psia). Feed  51  (combined from bottoms  16  of stripper  10  and bottoms  46  of stripper  44 ) was passed into reboiler  60  at a temperature of about 325° F. at about 5 psig (20 psia) at the rate of about 1,700 pph. The tar content of stream  51  was about 2 wt. % based on the weight of stream  51 , the remainder being DMF (about 95 wt. %) and furfural (about 3 wt. %), all wt. % based on the total weight of stream  51 . The secondary solvent (furfural) plus the tar level was about 5 wt. % based on the weight of stream  51 . At this tar level reboiler  60  required emptying over 9 times per month. 
     When operating under the conditions of this Example 1, the longest the extraction unit, as a whole, was operated continuously before shut down was required for tar clean out was 31.7 months. 
     EXAMPLE 2 
     The operation of Example 1 was repeated except that the flow rate of stream  5  to reboiler  60  was increased to about 2,100 pph (a 23.5% increase), and the temperature of stream  51  reduced to about 170° F. at a pressure of minus 10 psig (4.7 psia). 
     After operating under these conditions for an extended time sufficient to stabilize the extraction system as a whole, the tar content of stream  51  had fallen to about 1 wt. % based on the weight of stream  51 , and the secondary solvent plus tar level had fallen to less than 5 wt. % based on the weight of stream  51 . 
     Continued operation of the extraction unit as a whole at this reduced tar loading required emptying of reboiler  60  only 7 times per month in order to maintain the 1% tar loading even though the feed rate of stream  51  had been increased 23.5%. 
     In addition, while operating under this reduced tar level, about 25% less primary solvent was used and the extraction unit experienced a self-cleaning effect in that previous tar deposits were later found to be reduced in volume. The extraction system as a whole experienced less tar fouling in its equipment, particularly the first to foul heat exchangers  6  (e.g., one steam reboiler and up to two solvent reboilers) and  17 . This resulted in significantly reduced extraction unit steam usage and cost. 
     Additional benefits found with the practice of this invention pursuant to this Example 2 were an increase in extractive unit operating capacity, as evidenced by an ability to operate the unit at a higher input rate for C 4  feed  2 , and a decreased energy cost—3,100 btu/pound of butadiene product for Example 2 versus 4,400 btu/pound of butadiene for Example 1. 
     While operating under the conditions of this Example 2, the extraction unit was run continuously for 47 months without need for shut down for polymer (tar) clean out. At 47 months the olefin production plant as a whole was shut down for a regularly scheduled turnaround. During this turnaround, upon internal inspection by experienced personnel, it was found that the upper section of extractor  1  was cleaner of tar than at the end of a prior 32 month run using a 2 wt. % minimum tar loading, hence the self-cleaning advantage. 
     Also, at the 47 month shut down, the lower section of extractor  1  was scheduled to have its trays removed and cleaned, which was previously under prior art operation necessary. However, due to the limited nature of tar deposition found on the trays, they were merely cleaned by hand in place in the tower. This alone saved substantial maintenance time and expense. 
     Based on the substantially reduced polymer deposits actually found in the extraction unit equipment, it was projected that the unit could have run for 60 months without need of a shut down for polymer clean up. A 60 month continuous run time for the unit would mean that the unit could match the run time for the olefin production plant as a whole. 
     This Example 2 demonstrates that this invention not only lowers the rate of tar formation in the extraction process as a whole, but, also, has a self-cleaning effect on equipment that has already experienced polymer deposition.