US2537459A - Recovery of pure hydrocarbons by extractive distillation - Google Patents

Description

Jan. 9, 1951 .1. GRISWOLD 2,537,459

RECOVERY OF PURE HYDROCARBONS BY EXTRACTIVE DISTILLATION Filed Feb. 5, 1946 4. K ova/1540 /aooucr SOLVE/V7 RECYCLE r550 I n v 2 7 aorrous nooucr FIG. I

3a 40 35 as FIG- Ir. JOHN GR/SWOLD, JNVENTOR.

Patented Jan. 9, 1951 RECOVERY OF PURE HYDROCARBONS BY EXTRACTIVE DISTILLATION John Griswold, Austin, Tex. Application February 5, 1946, Serial No. 645,660

1 Claim.

This invention is directed to a method or process for separating narrow-boiling mixtures of those non-aromatic hydrocarbons which occur in straightrun and in processed petroleum fractions, in synthetic hydrocarbon mixtures such as the products of hydrogenation or of the Fischer- Iropsch synthesis, and in pyrolized or catalytically converted petroleum fractions.

Thus, the various types of hydrocarbons to' which the invention relates are paraffins, naphthenes (cycloparafiins) unsaturated naphthenes, olefins, and diolefins, or, more generally, those non-aromatic hydrocarbons whose boiling points are in the gasoline range, and which have about five to nine carbon atoms in the molecules.

The principal object of the invention is to recover from mixtures, however derived, of such hydrocarbons, each of the several hydrocarbon components in a substantially pure state unmixed with others of such components or with foreign substances.

A further object is the provision of a method for separating from mixtures of the above enumerated types of hydrocarbons the various components thereof by first separating the mixtures into portions or fractions, each containing but a single type of hydrocarbon, and then further treating the portions or fractions individually to separate from each other their constituent hydrocarbons and recover them in substantially pure state. It will be understood, however, that references herein to hydrocarbons, or types of hydrocarbons, "in substantially pure state denote materials whose purities are equivalent to technical grades or better, but at least 90 mol per cent; however, individual hydrocarbons recovered in accordance with the invention are frequently of purity above 98 mol per cent.

It is well known that petroleum distillates boiling within the gasoline range (say, 20 C. to 150 0.) usually are composed of an extremely large number of hydrocarbons of several distinct types. Even a narrow-boiling fraction (say, of 5 C. boiling range) which has been rigorously fractionated from such a distillate contains quite a number of hydrocarbons, of which at least one pair and usually several pairs are present whose closeness of boiling points or whose partial pressure relations are such that the component hydrocarbons cannot be satisfactorily separated and recovered in substantially pure state by conventional distillation processes.

In accordance with this invention however,

the component hydrocarbons may be recovered individually from narrow-boiling mixtures thereof by a succession of operations including conventional fractional distillation and extractive distillation in the presence of a relatively inert foreign liquid vehicle or solvent.

Distillation of a hydrocarbon mixture of the character mentioned, in the presence of a substantial quantity of a higher boiling miscible vehicle or solvent is essential to the invention as hereinafter more fully described, the object of this operation being to separate the close-boiling hydrocarbons constituting the original mixture or feed into a plurality of portions or fractions each containing not more than one of said types of hydrocarbons.

It is known that the various types of nonaromatic hydrocarbons have different activity coefficients or Raoults law deviation in a solvent. In most, but not in all cases, numerical values of activity coefficients of said types of hydrocarbons lie in the same order as the hydrogen/carbon ratio of said types and of the individual hydrocarbons therein. Those hydrocarbons which are high in hydrogen content, e. g., parafiins, have relatively high activity coefiicients and are more volatile from the solvent than are the other types, whereas hydrocarbons which are low in hydrogen content, c. e., aromatics, have relatively low activity coefficients and are less volatile from the solvent than are the other types. Naphthenes, unsaturated naphthenes, olefins and diolefins have activity coefllcients intermediate between those of parafllns and of aromatics. Th activity coefficients of two isomers of the same type and molecular weight, e. g., 2-methylpentane and 3-methylpentane are substantially the same, and when they occur together in substantial proportions in the same mixture their separation from each other thus may offer some difliculties, but their separation, as a type or group, from other constituents is readily eflectuated in accordance with the invention.

However, provided sufliciently large numerical differences between activity coefiicients of said types of hydrocarbons exist; said types in ac-.

cordance with the invention can readily be separated from each other where present in nar row-boiling fraction (say, of boiling range not greater than 15C. or 20 C.). Forexample, the activity coefllcients ('y) of various types of hydrocarbons as defined by the relation,

P= vapor pressure of subject type of hydrocarbons fl= mol fraction of subject type of hydrocarbons in vapor :r=mol fraction of subject type of hydrocarbons in liquid for atmospheric pressure, aniline as the solvent, and 80 mol per cent aniline in the mixture, are approximately:

In general, activity coemcients of all types increase with increasing solvent concentration in the liquid, so that a high proportion of solvent (such as 80 mol per cent) favors hydrocarbon separation.

More specifically, the practice oi this invention comprises (1) initial separation of a narrowboiling mixture into two or more portions in such manner that at least one of said portions contains substantially fewer types of hydrocarbons than the original mixture, or but one type of hydrocarbon, or, one hydrocarbon in substantially pure state; (2) the separation of any of said portions or groups containing more than one type into subgroups each in turn containing fewer hydrocarbon types than the parent portion, or but one substantially pure hydrocarbon; (3) the recovery of substantially pure individual hydrocarbons from each of the groups containing but a single hydrocarbon type.

That part of the invention herein designated extractive distillation may be accomplished with the aid of a unit comprising a fractionating column of any of the well-known types, such as a bubble-tray column. The essentials of a unit appropriate to the performance of the operation are shown diagrammatically in Fig. i of the accompanying drawing with auxiliary apparatus such as heat exchangers, pumps, tanks, valves and control equipment omitted for simplicity.

In Fig. 2 of the drawing a more complete layout of apparatus adapted for performing the invention is shown diagrammatically comprising,

with other apparatus, three interconnected units of the type represented in Fig. 1. The said apparatus, now to be more fully described, is well adapted but not necessarily specifically required for the practice of the invention, and the following descriptions of it and its operations are therefore to be considered merely as illustrative of the nature of the invention and a preferred manner of carrying out the operations it contemplates.

In accordance with the invention I employ, as indicated, a suitable solvent for the hydrocarbon feed. The required properties of the solvent are (1) that the hydrocarbons be soluble therein; (2) that the activity coefiicients of the various types of hydrocarbons be essentially different from one another when dissolved therein, in other words, that selectivity be one of its attributes; (3) that it be reasonably stable and not highly corrosive under conditions of use and recovery; (4) that it be substantially inert toward all hydrocarbons present in the original mixture in appreciable quantity; (5) that it be of sufliciently higher boiling point than any of the hydrocarbons in the mixture to inhibit formation of azeotropes with any of them. and (6) that it preferably be easily separable and puriflable by distillation or other process or by simple combinations of processes. Examples of suitable solvents are aniline,

dichloroethyl ether, furfural, phenol and nitrobenzene, although many other liquid organic compounds or mixtures of organic compounds satisfying the above requirements are known in the art and may be employed and the following description of the practice of the invention is directed to operation with the first mentioned, namely aniline, as a solvent.

The apparatus illustrated in Fig. 1 comprises a column I which may be any suitable type of fractionating reflux distillation column, provided with feed line 2, reboiler 3 or other suitable means for supplying supplemental heat, and an overhead vapor condenser 4 from which line 5 carries part of the overhead condensate back to column I near the top of the column, while a line 6 from the condenser affords an outlet for the remainder of the condensate.

Between the points in the column at which the feed and condensate are admitted through lines 2 and 5 respectively, a line I returns to the column solvent withdrawn therefrom through line 8 during extractive distillation.

Solvent withdrawn through line 8 carries with it the less volatile constituents of the feed, with the exception of such relatively insignificant portions thereof as may escape with the overhead product withdrawn through line 6 and even a part of these are returned for reflux through line 6. The solvent and bottoms hydrocarbons are separated irom each other in a column 5 by ordinary distillation, the column being equipped with heater II) for this purpose, and the overhead, consisting of the bottoms hydrocarbons from the original feed to column I, is withdrawn from column 9 through line H, the stripped solvent 9 being returned to column I through line I. The column I is operated at temperatures between the boiling points of the solvent and the hydrocarbons at the existing pressure of the column. At said temperatures, the vapor portion consists chiefly of hydrocarbons, and the liquid portion consists chiefly of solvent, but with appreciable percentages of hydrocarbons dissolved therein.

The aforesaid operations are conveniently conducted at substantially atmospheric pressure, but may be successfully conducted at subatmospheric pressure or at superatmospheric pressure it preferred, the temperatures depending upon the pressure, the particular solvent employed, the per cent solvent in the liquid reflux, and the composition of the hydrocarbon mixture in the reflux liquid.

For example, at atmospheric pressure with aniline as the solvent and the operation maintained with liquid reflux containing mol percent aniline, the temperature in the solvent column then depends only upon the composition of hydrocarbons dissolved in the solvent. For these,

conditions, the temperatures for compositions consisting chiefly or entirely of single hydrocarbons in the solvent are approximately:

when two or more such hydrocarbons are pres-'- ent the temperature will be between the lowest and the highest given for any of them in the above tabulation.

After a period of use, the solvent becomes contaminated with impurities which may consist of decomposition, oxidation, and/or reaction products, of minor amounts of high-boiling hydrocarbons not removed by the solvent recovery column, of sulfur, nitrogen, oxygen or other bodies present in the ori inal feedstock, and of water from stripping steam where this is used, and for best results the solvent should be occa- 'sionally withdrawn from the system and purified by redistillation and/or other processes.

In the practical application of the above described operation to continuous recovery of substantially pure hydrocarbons from a narrow-boiling straight-run petroleum fraction stock, using aniline as a solvent, apparatus of the nature of that diagrammatically shown in Fig. 2 may be satisfactorily employed and the principal hydrocarbons recovered in purities of 99 mol percent. For simplicity, the columns. distillation units and connecting lines are indicated and all auxiliary apparatus such as reboilers, condensers, heat exchangers, pumps, tanks, valves and control apparatus are omitted.

As an example, I shall now describe the treatment, in accordance with the invention of an original stock having an A. S. T. M. boiling range of 148 F. to 165 F. and ultimate composition as follows:

Vol. percent The narrow-boiling stock is fed to the column of one extractive distillation unit 2i through line 21. This column separates the feed into an overhead containing only parafiins and a bottoms containing all the cyclics except cyclopentane but with a residue of hexane. The cyclopentane tends to accumulate in this column and is eliminated by bleeding out a small portion of the vapor at an appropriate point through line 28. The hydrocarbon bottoms product from the solvent recovery column passes through line 29 to the column of a second extractive distillation unit 22 which eliminates all the hexane along with some methylcyclopentane in its overhead product. This overhead is then recycled to the original feed through line 30 although similar results are attained by having this overhead enter column 2| directly at a point slightly above line 21 as indicated by the broken line in Fig. 2. The bottoms product from extractive distillation unit 22 passes through line 3| to the column of a third extractive distillation unit 23 which separates pure benzene as bottoms product eliminated through line 32, from the naphthenes. The overhead product, containing only the two naphthenes, methylcyclopentane and cyclohexane passes through line 33 to fractionator 26 which separates it into pure methylcyclopentane overhead, removed through line 34, and pure cyclohexane bottoms recovered through line 35.

The paraflln overhead product from column 2| passes to a fractionator 24 through a line 36.

The trace of 3-methylpentane tends to accumulate in the column of fractionator 24 and is eliminated by bleeding out a small portion of the liquid reflux at an appropriate point through line 31. The bottoms product recovered through line 33 is pure hexane.

The overhead from fractionator 24 consists of a mixture of pentane, 2,3-dimethylbutane and 2-methylpentane. It passes through a line 39 into the column of a second fractionator 25, operated to give pure 2-methylpentane'as bottoms product recovered through line 40. The overhead removed from the fractlonator 25 through a line 4| contains the low-boiling minor paraiflns 2,3-dimethylbutane and pentane with some 2-methylpentane, which it is normally not economical to recover, although obviously they can be recovered by further fractionation processes if desired.

As a second illustration, a hexane-hexene mixture from catalytic gasoline whose A. S. T. M. boiling range is 141 F. to 154 F. was separated in a laboratory extractive distillation unit into 55% of an overhead product of substantially pure isomeric hexanes and a 45% bottoms product of substantially pure isomeric hexenes. Said mixture contained no aromatic hydrocarbons and only small amounts of naphthenes and of unsaturated naphthenes.

Elements of the flow diagrams, e. g., the bleeding out of an impurity and the provision for recirculation of a stream so that a column is required to make only one sharp separation have been known and used in the art of distillation for many years, and I do not claim them individually as my invention. Furthermore, all the pure hydrocarbon products described in the illustrations can be obtained from the respective original mixtures with an apparatus which contains only one eflicient column,.

if said column be operated intermittently and batchwise with storage of intermediate products, so that at variou times the column performs essentially the same separations as the six columns indicated in Fig. 2.

It will be recognized from the foregoing that in accordance with my invention it will generally be preferred to remove from the stock in the overhead from the first extractive distillation operation all, or substantially all, the hydrocarbons which are relatively high in hydrogen content and thus have high activity coefllcients in a solvent, and to leave in the last extractive distillation bottoms for recovery therefrom in accordance with any suitable method, of which several are known, the aromatic hydrocarbons which are low in hydrogen content and have relatively low activity coefficients. Intermediate hydrocarbons, such as naphthenes, unsaturated naphthenes, olefins and diolefins,

which may be considered neither high nor low I in hydrogen content or activity coefficients may be recovered initially in the overhead from the first extractive distillation operation, for further refinement as herein described, or may be retained in the first extractive distillation bottoms and removed from the solvent in the overhead from a second or third extractive distillation operation prior to final refinement by ordinary fractional distillation for recovery of the pure substances. The choice in these matters will usually depend upon what other types of hydrocarbons are present in the original stock and the manner in which it is desired to carry out 'the operation, with particular reference to the temperatures, pressures and solvents employed in the recovery. Thus it will be understood that the specific operations herein described by way of example and to illustrate the practice of the invention are susceptible of numerous modifications in detail and in the relationships between various contributing factors, many of which are so interrelated that any change in one may require or produce a corresponding change in others, but these and other changes and modifications will readily occur to those skilled in the art in the light of the foregoing and may be made without departing from the scope of the invention as defined in the appended claims. 7

Having thus described my invention, I claim and desire to protect by Letters Patent the United States:

A method for separating hexane, methyl pentane, methyl cyclopentane cyclohexane and benzene individually from a hydrocarbon mixture containing them which comprises first extractively distilling the mixture in the presence of a substantial concentration of aniline, removing from the first extractive distillation a bottom product and an overhead product, said overhead product containing hexane and 2-methyl pentane, subjecting said overhead product to fractional distillation to separate therefrom hexane as a bottom product and an overhead product containing 2-methyl pentane, subjecting this overhead product to fractional distillation to separate 2-methyl pentane therefrom as a bottom product, subjecting the bottom product of said first extractive distillation to fractional distillation to separate anoverhead product from the aniline, returning the aniline to the first extractive distillation, extractively distilling this last mentioned overhead product in the presence of a substantial concentration of aniline to separate an overhead product con taining hexane and a bottom product containing aniline, methyl cyclopentane, cyclohexane and benzene, returning the hexane-containing overhead product to the first extractive distillation, fractionally distilling said aniline containing bottom product to separate an overhead containing methyl-cyclopentane, cyclohexane and benzene from the aniline, returning said aniline to the second extractive distillation, extractively distilling said methyl-cyclopentaneand-cyclohexane-containing overhead product in the presence of additional aniline in substantial concentration to separate a third overhead product consisting of methyl cyclopentane and cyclohexane from a bottom product of benzene and aniline, and separating the overhead therefrom by fractional distillation into methyl cyclopentane as an overhead product and the cyclohexane as a bottom product, separating the aniline and benzene-containing product by tractional distillation, and returning the aniline to the third extractive distillation.

JOHN GRISWOLD.

\ BEEERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Schubert: The Separation of Hydrocarbons," Ph. D. Thesis. Pennsylvania State College, August 1941. (Copy in Penna. State College Library, pages 99-115.)

Mair et al.: Bureau of Standards Journal of Research, vol. 27, pp. 39-63. (Copy in Sci. Lib.)

Fenske et al.: Industrial and Engineering Chemistry, vol. 39, pp. 1322-1328, Oct. 1947.

(Copy in Sci. Lib.)

Rose et al.: Bureau of Standards Journal of Research, vol., 21, pp. 167-184, Aug. 1938. (Copy in Sci. Lib.)

Brame et al.: Journal of the Institution of Petroleum Technology, vol. 13, pp. 802-811, 1927. (Copy in Sci. Lib.)

Rossini: Refiner and Natural Gasolene Manufacturer, vol. 16. Pp. 545-549, Nov. 1937. (Copy in Sci. Lib.)

Benedict et al.: Transactions of American Institute of Chemical Engineers, vol. 41, pp. 353- 359, June 25, 1945. (Copy in Scientific Library.)

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