US2842484A

US2842484A - Separation of hydrocarbons

Info

Publication number: US2842484A
Application number: US478647A
Authority: US
Inventors: Raymond N Fleck
Original assignee: Union Oil Company of California
Current assignee: Union Oil Company of California
Priority date: 1954-12-30
Filing date: 1954-12-30
Publication date: 1958-07-08
Anticipated expiration: 1975-07-08

Description

July 8, 1958 D FLECK 2,842,484

SEPARATION OF HYDROCARBONS Filed Dec. 50, 1954 [WI/m1. firm/am A! 44241 SEPARATION OF HYDROCAREUNS Raymond N. Fleck, Whittier, Califl, assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application December 30, 1954, Serial No. 47 8,647

19 Claims. (Cl. 20239.5) V

This invention relates to methods for resolving complex hydrocarbon mixtures whereby pure individual naphthenes, pure parafiins, and/or pure aromatic hydrocarbons may be obtained. The process comprises as one of its salient features an extractive distillation step wherein a mixture of close-boiling hydrocarbons, the components of which differ in carbon/hydrogen ratio, is subjected to fractional distillation in the presence of a particular type of mixed solvent which is found to bring about an unusually large change in the relative volatilities of such hydrocarbons. The mixed solvent consists essentially of a lower aliphatic dinitrile as the primary component, plus a secondary component which contains only one nitrogen atom and from 3 to 10 carbon atoms, and is selected from the group consisting of mononitriles, and mono-amines. Preferably the secondary component is aromatic, and should boil lower than the aliphatic dinitrile. A further preference is that both of the solvent components should boil at least about 30 C. higher than any of the individual hydrocarbons in the mixture.

By the methods herein described, it has been found possible to treat low-boiling gasoline fractions whereby such valuable pure hydrocarbons as cyclohexane, methyl cyclopentane, benzene and the like may be recovered at a minimum expenditure for equipment and operating costs. Essentially the same procedures may also be employed to fractionate higher boiling gasoline cuts to obtain for example toluene, methyl cyclohexane, dimethyl cyclopentanes, xylenes and the like. The invention is particularly valuable for separating naphthenes from likeboiling paraffins, a type of separation which is extremely difiicult by extractive distillation with known types of entrainers. The invention is specifically of great value in treating the gasoline cut boiling between about 60 and 80 C.; the resolution of this fraction is normally extremely difiicult because it contains most or all of the benzene, and most of the parafiins and naphthenes boiling normally below about 82 C. form azeotropes with benzene, which renders the separation extremely diflicult.

A primary object of the invention is to provide a novel class of solvents for extractive distillation which will produce an extremely marked shift in the relative volatilities of like-boiling hydrocarbons which diifer in carbon/hydrogen ratio. A more specific object is to provide extractive solvents which will markedly increase the volatility of paraffins relative to naphthenes. Astill further object is to provide solvents which exert such a marked selective depressing effect upon the relative volatilities of the hydrocarbon classes, paraflins, naphthenes and aromatics that mixtures of two or more of those classes of hydrocarbons may be separated class-wise, even though the respective classes may overlap each other as much as 530 C. in boiling point. A specific object is to provide methods for the economical recovery of pure cyclohexane, methyl cyclopentane and benzene from various mineral oil fractions. A. still further object is to provide methods for accomplishing the last named separation without the expensive initial requirement for a two-column heart-cutting operation. Other objects and advantages of the invention will appear from the more detailed description which follows.

It is well known in the art that certain solvents are capable of altering the relative volatilities of two or more hydrocarbons which differ in carbon/hydrogen ratio. As a rule such solvents tend to decrease most markedly the relative volatility of aromatics, then of naphthenes and least markedly that of paraffins, i. e. the normal volatility is increasingly depressed in the order of increasing carbon/hydrogen ratios. The volatility of olefins is usually depressed slightly more than that of like-boiling naphthenes, though in many cases there may be little change in relative volatilities as between naphthenes and olefins. Examples of conventional solvents which are known to function in this manner are phenols, aniline, glycol mono-ethers, chlorinated dialkyl ethers, nitrobenzene, phthalic acid esters, benzyl alcohol, benzonitrile, pyridine, methyl pyridines, quinoline, and many other similar compounds. The measure of elfectiveness of such solvents is ordinarily expressed as the quantity alpha, which is defined as follows for two hydrocarbons, A and B:

Mol ratio A/B, vapor phase Mol ratio A/B, liquid phase when the vapor-and liquid phases are at equilibrium. For example considering the two hydrocarbons, 2,4-dimethyl pentane and cyclohe'xane, which normally exhibit an alpha with respect to each other of around 1.00, most of the above-mentioned solvents are found to shift this alpha value at most to between about 1.1 and 1.3. However, a typical solvent of the present invention, e. g. one composed of 45 volume-percent of B,,8-thiodipropionitrile and 55 volume-percent of benzonitrile is found to shift the alpha value to 1.75, when the liquid phase is a 14 volume-percent solution of the hydrocarbons in thesolvent. The corresponding value for benzonitrile alone is found to be 1.33. The corresponding value for pfi thiodipropionitrile alone is difficult to determine, due to the fact that the hydrocarbons are soluble therein to the extent of less than 2% by volume; hence even if B,/3'-thiodipropionitrile alone did give a high alpha value, it would be impractical because of the inordinately high Alpha solvent/feed ratios which would be required in order to maintain a single liquid phase. The mixed solvents employed herein all exhibit a substantial solvent capacity for the hydrocarbons. In all cases the alpha values are greater than the corresponding values for the secondary component alone, while their solvent capacity for hydrocarbons is greater in all cases than that of the primary component alone. It may also be said that, for mixtures containing about 2-l5% by volume of hydrocrbons and the remainder a solvent, the mixed solvents of this invention will always yield a higher alpha value than would either solvent component alone.

It is clear therefore that the two solvent components exert a synergistic effect in the distillation, causing a much higher shift in relative volatility than would either component alone at practical solvent/feed ratios. The same is found to apply to other pairs of hydrocarbons which differ in carbon/hydrogen ratio. The shift in volatility is in fact much greater between naphthenes and aromatics, or paraifins and aromatics than it is between the chemically very similar types, paraffins and naphthenes. When matic.

' nent and'are hence preferredfrom that standpoint.

separating a mixture of two or more naphthenes from one or more aromatic hydrocarbons, it is found that essentially all of the naphthenes may be distilled overhead even though'the highest boiling naphthene may boil 1020 C. higher than the lowest boiling aromatic. When separating a mixture of two or more paraffins from one or more aromatic hydrocarbonsfit i -possibleto distil all the-paraflins overhead even though the highest boiling parafiin boils at 1030 C. above the lowest boiling aro- When it is desired to separate a mixture of two or more paraflins from a mixture containing at least one naphthene, this-can be accomplished even though the highest boiling paraflin'boils 5-10 C. higher'than the lowest boiling naphthene.

-As-indicated above, the primary component of the mixed solvent-employed herein is a lower aliphatic dinitrile, the lowest boiling of which is malonitrile (B. P. 220 C.). A'preferred subgroup comprises those dinitriles wherein the linking aliphatic chain is interrupted by a-hetero atom of sulfur, oxygen or nitrogen. Suitable examples of such primary components are as follows:

Malononitrile Succinonitrile Glutaronitrile Adiponitrile Pimelonitrile Oxydiacetonitrile Thiodiacetonitrile Iminodiacetonitrile B,B'-Oxydipropionitrile ,8,5'-Thiodipropionitrile B,}8'-Iminodipropionitrile The preferred sub-group may be designated by the formula:

NC--RXR'-CN wherein R and R are lower alkyl groups and X is O, S, or NH. Several of these preferred materials are thermally somewhat unstable, and should preferably not be heated above about 150 C. for. any appreciable length of time, unless precautionary measures are employed such .nitrogen atom and from 3 to 10 carbon atoms inclusive.

These compounds may also contain linking oxy-ether or .thio-ether groups, but other functional groups aregenerallyexcluded. Included within this classification are aromatic nitriles, alkyl nitriles, alkoxy alkyl nitriles, alkoxy aryl nitriles, cycloalkyl nitriles, alkyl mercaptoalkyl nitriles, alkaryl. nitriles, aralkyl nitriles, alkaralkyl nitriles,

primary, secondary and tertiary amines. alkyl amines,

.cycloalkyl amines, N-(alkoxy alkyl) amines, arylamines, 'N,N-'dialkyl aryl amines, N-alkyl arylamines, saturated 'heterocyclic amines, aromatic heterocyclic amines, oxa zines, thiazines, oxazoles, thiazoles, pyrroles, and the like.

' 'Thearomatic mono-nitriles appear to exert the-greatest synergistic effect in conjunction with the primary compo- It is further preferred that the secondary component should boil at least about 30 C. higher than the hydrocarbon component of the feed mixture, in order that the latter may be separated from the solvent Without forming azeotropes,- and it should also boil considerably below (e. g. 50'200 C.)- the boiling point of theprimary component.

This-latter preference reduces the effective boiling point of the solvent, thereby permitting; effectivestripping of 4 feed components therefrom at lower temperatures. Suitable secondary components include for example the following:

Nitriles:

Benzonitrile u-Tolunitrile o-Tolunitrile m-Tolunitrile p-Tolunitrile p-Ethyl benzonitrile p-Methoxy benzonitrile Phenoxy acetonitrile Phenyl mercapto acetonitrile Propionitrile Butyronitrile Valeronitrile Cyclohexyl acetonitrile Ethoxy acetonitrile B-Methoxy propionitrile Isopropoxy acetonitrile fi-Isopropoxy propionitrile Methoxy acetonitrile Isopropyl mercapto acetonitrile fl-(Methyl mercapto) propionitrile Amines:

Triethylamine n-Butyl amine n-Hexyl amine N-methyl, N-(n-butyl) amine Di-isopropyl amine Tri-isopropyl amine N-ethyl, N,N-diisopropyl amine N-ethyl, N-(methoxyethyl) amine N,N-diethyl, N-(methoxyethyl) amine N-isopropyl, N,N-di(methoxyethyl) amine Aniline N,N-dimethyl aniline N-isopropyl aniline N-methyl cyclohexyl amine Piperidine N-methyl piperidine Pyrrolidine N-methyl pyrrolidine N-isopropyl pyrrolidine Pyrrole N-ethyl pyrrole Thiazole Tetrahydro thiazole .Morpholine N-methyl morpholine N-ethyl morpholine N-isopropyl morpholine Pyridine Quinoline Isoqninoline 'y-Picoline B-Picoline a-Picoline cx-COllldil'lC ,S-Collidine -Collidine 2,4-lntidine 2,6-lutidine 3,4-lutidine When the secondary component is relatively high boiling. it is further preferred that its nitrogen atom be bonded exclusively to carbon (thereby excluding primary and secondary amines), inasmuch as such compounds are gen erally more heat stable than those which contain a hydrogen atom bonded to nitrogen.

In addition to the above primary and secondary cenxponents, the solvent may also comprise a tertiary comp-f nent which may be relatively inert, being employed primarily to-lower the effective-boiling point of the solvent,

The ratio of primary to secondary component in the mixed solvent may vary widely, e. g. from about 5 95 to 95% /5% by volume. It is preferred to employ substantial amounts of the secondary component, e. g. -70% by volume, especially where it is the lowest boiling component.

Example I To test the effectiveness of representative examples of the above mixed solvents in comparison to conventional solvents, an Othmer still was employed to establish a onestage vapor-liquid equilibrium condition at atmospheric pressure, for liquid mixtures of the solvent plus a mixture of 2,4-dimethyl pentane and cyclohexane. In all cases the solvent composed about 80-90 volume percent of the liquid phase at equilibrium, the 2,4-dirnethyl pentane about 6-8 volume-percent, and the cyclohexane about 7-9 volume-percent. After equilibrium was established, the hydrocarbons of the liquid phase and the vapor phase were analyzed by either mass spectrometer or by refractive index or both, and the alpha values calculated therefrom. Where refractive index analysis was employed, the samples were acid treated with sulfuric acid to remove solvents and impurities. The results were as follows:

Solvent Alpha, 2, 4- DMP/OH None. 1. 00 1. Pyridlne 1.12-1.15 2. 'y-Picol 1.26 3. Morpholine 1.30 4. Benzonitrile 1.33 5. B, B-Thiodipropionitrile 1 1.06 6. Adipnnitrilp (a) 7. Propionitrile 1. 10 8. 8-Isopropoxy proplonitrile 1. 30 9. Aminoethyl ethanolamine, lmethyl ethan mine, 45% 1.20 10. B, fi-Thiodipropionitrile, 42%; n-Butanol, 58% 1.36 11. B, fl-Thiodipropionitrile, 50%; B-Isopropoxypropionitrile, 50% 1. 46 12. Adiponitrile, 50%; B-Isopropoxypropionitrile, 50% 1. 57 13. B, /3-Thiodipropionitrile, 40%; Morpholine, 60% 1. 74 14. B, fi-Thiodipropionitri1e, 45%; Benzonitrile, 55% 1. 75 15. ,9, fl-Thlodipropionitrile, 42%; -Picoline, 58%.- 1. 59 16. Adiponitrile, 55%; Benzonitrile, 45% 1. 46

1 Two liquid phases 1present; solvent-rich phase contained about 02% of hydrocarbons by v0 ume.

2 All percentages are by volume.

3 Not measurable due to low solubility. See note It will be readily apparent from the above data that solvents 11 through 16 are markedly superior to the individual solvents 1 through 8, and also to the mixed solvents 9 and 10. It is especially desirable herein to employ solvents capable of giving a 2,4-DMP/CH alpha which is substantially in excess of 1.31, preferably above 1.5, because at such higher values the solvent is capable, in one column, of splitting a 0-82 C. cut of gasoline into an overhead which contains substantially all the paraflins, and a bottoms which contains all the aromatics and all the naphthenes, except cyclopentane. This is true notwithstanding the fact that the highest boiling parafiin, 2,4-dimethyl pentane (B. P. 806 C.) boils almost 9 higher than the lowest boiling naphthene, methyl cyclopentane (B. P. 71.9 C.), which is retained in the bottoms. It is thus possible to separate mixed naphthenes from mixed paraflins even though the boiling ranges of the two classes overlap as much as 10 C. or more. Solvents having an alpha of 1.31 or below, or even slightly above, are incapable of effecting this particular separation. in previous processes employing conventional solvents,

difiiculty has been experienced even in separating n-hexane (B. P. 69 C.) from methyl cyclopentane (of. U. S. Patent No. 2,537,459).

The overhead fraction from the above extractive distillation of the light gasoline fraction may then be utilized in gasoline, or further fractionated to recover pure cyclopentane or individual paraffins. The bottoms fraction may be treated to separate the hydrocarbons from the solvent, and the hydrocarbons are then preferably subjected to a second extractive distillation step to separate benzene from the two remaining naphthenes, methyl cyclopentane and cyclohexane. The second extractive distillation may be carried out with the same mixed solvent, or one of the above-mentioned conventional solvents may be utilized inasmuch as this separation is not diflicult. The naphthenic overhead from the second extractive distillation is then fractionated by conventional methods to recover pure cyclohexane and methyl cyclopentane. Pure benzene is recovered from the solvent bottoms.

The above type of separation is more specifically exemplified in the accompany drawing, which is a flowsheet illustrating the recovery of benzene, methyl cyclopentane and cyclohexane from a gasoline fraction. The operational details will be described in connection with the flowsheet, but it should be clearly understood that the invention is, not restricted to such details.

The individual equipment units consist of conventional distillation columns, heat exchangers, storage tanks, surge Vessels, pumps, and the like which will not be described in detail. The feed in storage tank 1 consists of a straight-run gasoline which may be either a light or heavy naphtha. In any case the feed stock is preferably predominantly naphthenic in origin, and hence will be relatively rich in naphthenes and aromatics. The boiling range for example may range between about 50 and 450 F. This feed stock is transferred via line 2 to primary fractionating column 3.

Fractionating column 3 preferably contains 30-65 trays, and may be operated with an overhead reflux ratio of about 1-10. In this column the gasoline is fractionated to recover overhead all of the components normally boiling below about 82 C. The heaw bottoms product is not further utilized herein, and is hence withdrawn through line 4, whence a portion thereof is continuously returned to the column as bottoms reflux via reboiler 4-a, While the net bot-toms is transferred via line 5 to storage tank 6. The overhead product is removed through line 8, condensed and passed into surge vessel 10. Overhead reflux is returned to the column through line 11, while the net overhead is taken off in line 12 and transferred to the primary extractive distillation column 15.

The feed to column 15 may typically comprise, by volume, about 20-40 percent of light ends, 10-30 percent n-hexane, 20-30 percent methyl cyclopentane, 0.2-2 percent of 2,2-dimethyl pentane, 0.2-2 percent of 2,4-dimethylpentane, 3-10 percent benzene, 5-15 percent cyclohexane, and 1-5 percent of heavy material boiling above about C. In column 15 this material is distilled countercurrently to a descending stream of the mixed solvent which is introduced to the column as recycle from line 16. Line 16 preferably enters the column at a point several trays above the feed inlet line 12. The overhead employing solvents which are not stable at high temperatures, resides in the various measures which may be employed for maintaining low temperatures in the bottom of column 15. In the particular type of separation presently exemplified, and with the preferred sOlvents, the solvent/feed ratio, and the overhead reflux ratio, are necessarily high. This in turn entails a low hydrocarbon concentration in the bottom of the column. In order to effectively operate an extractive distillation column, it is necessary to maintain in the bottom of the column a suificiently large boil-up to obtain of" cfive rectification, and to effectively strip the solvent of the paraflinic hydrocarbons which are to be taken overhead. This may be accomplished in several ways. The most obvious method consists in employing high bottoms temperature, but this is feasible only when solvent is not too heat sensitive, or when adequate counter measures, as for example the use of decomposition inhibitors or equilibrium proportions of the decomposition products, are employed. Other measures which may be employed consist in utilizing a low-boiling tertiary component in the solvent, as previously desclibed. The large boil-up of the tertiary component effectively strips the column bottoms of the hydrocarbons which are to be taken overhead. Other methods comprise using inert, low-boiling entrainers, or fixed gases, or employing reduced pressures. These latter modifications however are disadvantageous in that they lower the column capacity considerably.

In the modification herein illustrated, boil-up in the bottom of the column is effected by recycling thereto a portion of the hydrocarbon component which has been stripped from the fat solvent in the succeeding fat-solvent stripping column to be subsequently described. This stripped hydrocarbon component is admitted to column 15 through line 18, the inlet port of which is preferably within 2 or 3 trays of the entry port for bottoms reflux line 19. By suitably adjusting the proportion of hydrocarbon recyclcd through line 13 it is possible to obtain effective separation of paraflinic hydrocarbons in column 15 With temperature ditferentials therein, between stillhead and pot, amounting to only about 3050 C.

The paraflinic overhead from column 15 is removed through line 29, condensed and passed into surge vessel 21. The overhead reflux is returned to the column via line 23, while the net overhead is transferred to gasoline ends storage tank 6 via line 24. The fat solvent bottoms from column 15 is removed via line 26, and a portion thereof is heated in reboiler 27 and returned to the column as bottoms reflux through line 19. The net bottoms from column 15 is passed via line 29 to low pressure solvent stripping column 36, wherein the aromatic and naphthenic hydrocarbons contained therein are removed overhead, while the stripped solvent is recycled to column 15 via line 16. Extensive rectification is not ordinarily required in column 13 and hence this column may comprise only 820 trays for example, and may be operated at low reflux ratios, amounting to about 0.1-1.0. As is well understood in the art, reflux ratios are defined as the ratio of product which is recycled to the column to the net product which is withdrawn.

The stripping of naphthenes and aromatic hydrocarbons from the solvent in column 30 may be accomplished utilizing any of the temperature-mitigating expedients outlined in connection with column 15. In the case illustrated, the column is operated at reduced pressures, for example 1-10 p. s. i. a. While the use of low pressures reduces the column capacity, certain compensating effects are obtained. For example the lower temperature prevailing therein may obviate the cooling of stripped solvent prior to its reentry into column 15. it is essential of course that the solvent which enters column 15 through line 16 be at a lower temperature than prevails at the bottom of column 15. The temperature at the bottom of column 30 may range between about 80 and 190 C. for example. Column 30 embraces a conventional reboiler 31 for reheating the bottoms reflux circulating through line 32.

The overhead from column is removed through line 35, and consists of solvent-free hydrocarbons, mainly naphthenes and aromatics. A portion of this overhead is diverted through line 18 as previously described for recycle to the bottom of column 15. The ratio of recycled hydrocarbons in line 18 to the total overhead of column 30 may range between about 0.1 and 2.0 for example. The portion of overhead which is not recycled is condensed and passed into surge vessel 37. The overhead reflux is cycled back to column 30 through line 38, while the net overhead is withdrawn through line 39.

The overheadproduct from column 30 is then transferred through line 39 to a second extractive distillation column 40, wherein benzene plus solvent is separated as bottoms from an essentially pure naphthenic overhead. This separation may be effected in a manner similar to that described in connection with column 15, and with the same type of solvent. Alternatively, inasmuch as this separation is considerably less diflicult than the separation in column 15, a more conventional type of solvent may be employed, e. g. benzonitrile, aniline, pyridine, phenol or the like. If conventional solvents are employed column 40 will comprise preferably between about 50 and 80 trays, and the reflux ratios may be similar to that in column 15. However if the novel solvents described herein are employed the number of trays may be substantially reduced as well as the reflux ratios. Also, the solvent/feed ratios may be somewhat reduced thereby lowering the bottoms temperature of the column.

The over-all operation of column 40 may be a conventional extractive distillation procedure. The recycled stripped solvent is admitted to the column via line 42, the inlet of which is located several trays above the feed inlet line 39. The naphthenic overhead is taken off through line 43 and condensed into surge tank 44. Overhead reflux is returned via line 45 while the net overhead is passed via line 46 to a fractionating column 47 for separating the naphthenic components from each other. The bottoms from column 40 is withdrawn through line 48, and a part thereof is diverted through reboiler 49 and returned to the column as bottoms reflux through line 50. The net bottoms is transferred via line 52 to solvent stripping column 54.

Column 54 may be operated similarly to previously described column 30 if the novel solvents of this invention are employed therein. If conventional solvents are employed, reduced pressures are ordinarily unnecessary and hence substantially atmospheric pressures may be utilized. In order to completely strip the benzene from the solvent, it is ordinarily necessary to maintain the bottom of column 54 at substantially the boiling point of the solvent. The stripped solvent is removed through line 55, and a portion thereof may be recycled as bottoms reflux through reboiler 56. The net bottoms consisting of stripped solvent is then recycled via line 57, cooler 43, and line 42 to extractive distillation column 40.

The overhead from column 54 is taken off through line 59, condensed and passed into surge vessel 60, from which overhead refl x is recycled via line 61 while the net overhead, consisting essentially of benzene, is passed to storage through line 62.

As previously indicated, fractionating column 47 is operated to separate the naphthenic overhead components from column 40. These components normally comprise essentially, methyl cyclopentane and cyclohexane, together with a small proportion of higher boiling bottoms carried through the process. Column 47 may comprise for example 90 trays, and may utilize overhead reflux ratios between about 3 and 10. The overhead is taken off through line 65, condensed and passed into surge vessel 66, from which the overhead column reflux is recycled via line 67. The net overhead is withdrawn through line 68 and passed to methyl cyclopentane storage facilities. The methyl cyclopentane recovered from this column may be 95-99% pure.

The bottoms from column 47 is withdrawn through line 69, and a portion thereof is recycled via reboiler 70 and line 71 as bottoms reflux to the column. The net bottoms product is cyclohexane of e. g. 65-85% purity, the major part of the remainder being heavier material, and this mixture is passed via line 72 to cyclohexanepurification column 73. This last column is employed in order to obtain as overhead, cyclohexane of 90-99% purity. This may require the use of a 60-90 tray column with appropriate reflux ratios similar to that of column 47. The overhead is removed through line 74, condensed and sent to surge vessel 75, from which overhead reflux is returned to the column via line 76. The final pure cyclohexane is withdrawn through line 77 and sent to storage. The bottoms from column 73 is withdrawn through line 77, and a part thereof recycled as bottoms reflux via reboiler 78. The net bottoms product may be cycled via line 79 to gasoline-ends storage tank 6, along with the light and heavy ends previously recovered in the process. It should be understood however that any of the gasoline ends recovered from column 73 as bottoms, or column 15 as overhead may be further treated for the recovery or" pure paraflins or cyclopentane therefrom.

It should be clearly understood that the recitation hereinof preferred reflux ratios and'plate ratios isintended to serve as a rough guide only. The actual optimum relationships between reflux ratios and plate ratios will depend upon the particular solvents employed, the specific nature of the feed, and the relative economy of unit operations for a given area. These factors are calculated by standard engineering procedures for each particular case.

Example 11 This example illustrates the results obtainable in recovering benzene, cyclohexane, and methyl cyclopentane from a specific feed stock. The feed is a straight-run naphtha having a 93 C. end-point and containing the following components:

V The feed is fractionated (4411 bbl./ day) as described above to recover the 82 C. cut, which is then subjected to extractive distillation in a 45 tray column with a 50% fi,/3-thiodipropionit1ile, 50% benzonitrile solvent. The solvent/feed ratio is about 5/1 by volume. The overhead reflux ratio is about 2/1 and the bottoms reflux is about 3.6/1. About 1457 -bbl. per day of parafiinic overhead is recovered, and the bottoms extract is vacuum stripped at 100 mm. to recover about 1207 bbl. per day of solvent-free hydrocarbons. The extracted hydrocarbons are then subjected to a second extractive distillation with benzonitrile employing a solvent/feed ratio of about 9/ l, and otherwise similar conditions to the first extractive distillation. The naphthenic over-head from the second extractive distillation is then fractionated in a 75 plate column, and the solvent bottoms is stripped of benzene. About 150 bbls. per day of 98% pure benzene is recovered. The fractionation of the naphthenes yields 375 bbls. per day of 76% cyclohexane bottoms, and 661 bbls. per day of 97% methyl cyclopentane overhead. The impure cyclohexane is redistilled to remove heavy ends, giving 266 bbls. per day of 96.5% pure cyclohexane. Thevolume percent recovery of each component, based on the initial feed is,

benzene 87.5%, cyclohexane 78.5% and methyl cyclopentane 92.5%.

The above example should not be construed as limiting in scope. The operating conditions set forth are based on high recoveries and high purities; by aiming at lower recoveries or lower purities, substantial economics in operating conditions may be effected, as for example by employing smalicr columns, increasing the throughput of feed, decreasing the solvent ratios and the like. Substantially similar results are obtained when other solvents within the purview of the invention are employed. Neither should the invention be construed as limited to the particular type of feed stock. Other mineral oil distillates, e. g. cracked gasolines, reformed gasolines, etc. may be similarly treated to recover valuable pure hydrocarbons. It is not intended to limit the invention, except as defined in the following claims.

I claim:

1. A method for resolving a mixture of close-boiling hydrocarbons which differ in carbon/hydrogen ratio which comprises subjecting said mixture to extractive distillation in the presence of a solvent consisting essentially of (1) a primary component which is an aliphatic dinitrile containing 3-7 carbon atoms, and (2) a secondary component selected from the group consisting of mono-nitriles and mono-amines containing one nitrogen atom and 3 to 10 carbon atoms, each of said solvent components having a boiling point at least about 30 C. higher than any of said hydrocarbons, whereby the volatility of those hydrocarbons having a relatively high carbon/hydrogen ratio is decreased relative to the volatility of those hydrocarbons having a lower carbon/hydrogen ratio, recovering overhead from said distillation a hydrocarbon fraction having a relatively low carbon/ hydrogen ratio, and stripping the solvent bottoms from said distillation to recover a hydrocarbon fraction having a relatively high carbon/hydrogen ratio.

2. A method as defined in claim 1 wherein said secondary component has a boiling point substantially lower than said primary component.

3. A method as defined in claim 1 wherein said hydrocarbon mixture boils over a temperature range not exceeding about 20 C., said range being located within the limits of about 40 to C.

4. A method as defined in claim 3 wherein said hydrocarbon mixture comprises a paraffin and a naphthene, said paraflin being recovered overhead, and said naphthene being recovered from said solvent bottoms.

5. A method as defined in claim 3 wherein said hydrocarbon mixture comprises a paraflin and an olefin, said paraffin being recovered overhead, and said olefin being recovered from said solvent bottoms.

6. A method as defined in claim 3 wherein said hydrocarbon mixture comprises a paraffin and an aromatic hydrocarbon, said paraflin being recovered overhead, and said aromatic hydrocarbon being recovered from said solvent bottoms.

7. A method as defined in claim 3 wherein said hydrocarbon mixture comprises a naphthene and an aromatic hydrocarbon, said naphthene being recovered overhead, and said aromatic hydrocarbon being recovered from said solvent bottoms.

8. A method as defined in claim 3 wherein said hydrocarbon mixture comprises an olefin and an aromatic hydrocarbon, said olefln being recovered overhead, and said aromatic hydrocarbon being recovered from said solvent bottoms.

9. A method as defined in claim 3 wherein said hydrocarbon mixture comprises a paraffin, a naphthene and an aromatic hydrocarbon, said paraflin being removed overhead, and said naphthene and aromatic hydrocarbon being recovered from said solvent bottoms.

10. A method as defined in claim 3 wherein said hydrocarbon mixture comprises a paraffin, a naphthene and an aromatic hydrocarbon, said parafifin and naphthene being removed overhead, and saidaromatic hydrocarbon being recovered from said solvent bottoms.

11. A method for resolving a mixture of close-boiling hydrocarbons which difier in carbon/hydrogen ratio which comprises subjecting said mixture to extractive distillation in the presence of a solvent consisting essentially of (1) a primary component which is an aliphatic dinitrile containing 37 carbon atoms and a linking hetero atom selected from the class consisting of oxygen, sulfur and nitrogen, and (2) a secondary component selected from the group consisting of aromatic mononitriles and heterocyclic amines containing one nitrogen atom and 3 to carbon atoms, each of said solvent components having a boiling point atleast about 30 C. higher than any of said hydrocarbons, whereby the volatility of those hydrocarbons having a relatively high carbon/hydrogen ratio is decreased relative to the voltatility of those hydrocarbons having a lower carbon/hydrogen ratio, recovering overhead from said distillation a hydrocarbon fraction having a relatively low carbon/ hydrogen ratio, and stripping the solvent bottoms from said distillation to recover a hydrocarbon fraction having a relatively high carbon/ hydrogen ratio.

12. A process as defined in claim 11 wherein the nitrogen atom of said secondary solvent component is bonded exclusively to carbon.

13. A process as defined in claim 11 wherein said solvent comprises between about 10% and 90% by volume of fi,B'-thiodipropionitrile, and the remainder is substantially benzonitrile.

14. A process as defined in claim 11 wherein said solvent comprises between about 10% and 90% by volume of ,8,,8'-oxydipropionitrile, and the remainder is substantially benzonitrile.

15. A process as defined in claim 11 wherein said solvent comprises between about 10% and 90% by volume of p,fl'-iminodipropionitrile, and the remainder is substantially benzonitrile.

16. A method for recovering substantially pure hydrocarbons boiling between about 70 and 82 C. including benzene, cyclohexane and methyl cyclopentane from a mineral oil fraction containing such hydrocarbons together with higher and lower-boiling hydrocarbons, which comprises (1) fractionating said mineral oil fraction to recover as a first overhead substantially all components boiling below about 82 C., (2) sub'ecting said first overhead to extractive distillation in the presence of a solvent consisting essentially of a primary component which is an aliphatic dinitrile containing 3-7 carbon atoms, and a secondary component selected from the group consisting of mono-nitriles and mono-amines containing one nitrogen atom and 3 to 10 carbon atoms, and distilling therefrom a second overhead which is substantially free of benzene but contains substantially all the paratfins and light ends from said first overhead, (3) removing a solvent-containing bottoms fraction from said extractive distillation and stripping therefrom a benzene-naphthene-rich hydrocarbon fraction, (4) separating benzene from said last-named hydrocarbon fraction #12 leaving a naphthene fraction,and (5) fractionally distilling said naphthene fraction -to-rec0ver therefrom substantially pure cyclohexane and methyl cyclopentane.

17. A method as defined in claim 16 wherein said mineral oil fraction is a straight run gasoline containing n'hexane, methyl cyclopentane, 2,2-dimethyl pentane, 2,4-dimethyl pentane, benzene, and cyclohexane.

18. A method for recovering substantially pure hydrocarbons boiling between about and 82 C. including benzene, cyclohexane and methyl cyclopentane from a mineral oil fraction containing such hydrocarbons together with higher and lower-boiling hydrocarbons, which comprises (1) fractionating said mineral oil fraction to recover as a first overhead substantially all components boiling below about 82 C., (2) subjecting said first overhead to extractive distillation in the presence of a solvent consisting essentially of a primary component which is an aliphatic dinitrile containing 3-7 carbon atoms, and a secondary component selected from the group consisting of mono-nitriles and mono-amines containing one nitrogen atom and 3 to 10 carbon atoms, and distilling therefrom a second overhead which is substantially free of benzene but contains substantially all the parafiins and lightends from said first overhead, (3) removing a solvent-containing bottoms fraction from said extractive distillation and stripping therefrom a benzenenaphthene-rich hydro-carbon fraction, (4) subjecting said last-named hydrocarbon fraction to a second extractive distillation in the presence of a second solvent and distilling therefrom a third overhead which is substantially free of benzene, (5) stripping benzene from the solventbottoms from said second extractive distillation, and (6) fractionating said third overhead to obtain substantially pure cyclohexane and methyl cyclopentane.

19. A method for the treatment of a complex hydrocarbon fraction to separate chemically similar components from other chemically similar components different from said first named chemically similar components contained in said complex hydrocarbon fraction, which comprises subjecting said complex hydrocarbon fraction to extractive distillation in the presence of a solvent, and distilling overhead chemically similar components, and recovering from the solvent bottoms other chemically similar components different from said first named chemically similar components, said solvent consisting essentially of a primary component which is a lower aliphatic dinitrile containing 3-7 carbon atoms, and a secondary component selected from the class consisting of mononitriles and mono-amines containing one nitrogen atom and 3-10 carbon atoms.

References Cited in the file of this patent UNITED STATES PATENTS 2,379,696 Evans July 3, 1945 2,441,827 McKinnis May 18, 1948 2,537,459 Griswald Jan. 9, 1951 2,568,176 Vriens Sept. 18, 1951 2,588,056 Teter Mar. 4, 1952 2,765,356 Skinner Oct. 2, 1956

Claims

1. A METHOD FOR RESOLVING A MIXTURE OF CLOSE-BOILING HYDROCARBONS WHICH DIFFER IN CARBON/HYDROGEN RATIO WHICH COMPRISES SUBJECTING SAID MIXTURE TO EXTRACTIVE DISTILLATION IN THE PRESENCE OF A SOLVENT CONSISTING ESSENTIALLY OF (1) A PRIMARY COMPONENT WHICH IS AN ALIPHATIC DINITRILE CONTAINING 3-7 CARBON ATOMS, AND (2) A SECONDARY COMPONENT SELECTED FROM THE GROUP CONSISTING OF MONO-NITRILES AND MONO-AMINES CONTAINING ONE NITROGEN ATOM AND 3 TO 10 CARBON ATOMS, EACH OF SAID SOLVENT COMPONENTS HAVING A BOILING POINT AT LEAST ABOUT 30*C. HIGHER THAN ANY OF SAID HYDROCARBONS, WHEREBY THE VOLATILITY OF THOSE HYDROCARBONS HAVING A RELATIVELY HIGH CARBON/HYDROGEN RATIO IS DECREASED RELATIVE TO THE VOLATILITY OF THOSE HYDROCARBONS HAVING A LOWER CARBON/HYDROGEN RATIO, RECOVERING OVERHEAD FROM SAID DISTILLATION A HYDROCARBON FRACTION HAVING A RELATIVELY LOW CARBON/ HYDROGEN RATIO, AND STRIPPING THE SOLVENT BOTTOMS FROM SAID DISTILLATION TO RECOVER A HYDROCARBON FRACTION HAVING A RELATIVELY HIGH CARBON/HYDROGEN RATIO.