US2555650A - Azeotropic distillation of hydrocarbons - Google Patents

Description

Patented June 5, 1951 AZEOTROPIC DISTILLATION OF HYDROCARBONS George R. Lake, Long Beach, Calif., assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California No Drawing. Application January 12, 1946, Serial No. 6't1,009

1 Claim.

This invention relates to the preparation of pure hydrocarbons from petroleum, and particularly the separation of naphthenic hydrocarbons from paraffinio hydrocarbons. These hydrocar: bons when contained in a hydrocarbon mixture of relatively narrow boiling range are virtually impossible to separate by ordinary fractional dis tillation and the present invention contemplates their separation by means of azeotropic distillation in the manner as hereinafter set forth. This invention is a continuation-in-part of my ccpending application Serial No. 412,814, now abandoned. a

An object of the present invention is to further the progress in preparing pure compounds from a heterogeneous petroleum mixture, using in this particular case a method which involves fewer steps than a chemical method and which yields a purer product than that produced by careful fractional distillation and/r extraction with selective solvents.

Another object of the invention is to prepare from a given fraction of petroleum, such as gasoline, kerosene, or a narrow boiling range hydrocarbon fraction prepared from such materials, these fractions consisting of a mixture of parafiinic, isoparaffinic, naphthenic, olefinic and aromatic hydrocarbons, a fraction that is essentially parafiinic or isoparaffinic or naphthenic in character.

A. particular object of my invention is to separate naphthenic hydrocarbons from parafiinic hydrocarbons by distilling the complex hydrocarbon fraction in the presence of a saturated heterxocyclic organic compound having four or more atoms, particularly one in which at least one of the atoms in the ring is oxygen, sulfur or nitroin such manner that the partial vapor pressure orfugacity of at least one component in the fraction is changed sufficiently to permit its separation by controlled fractional distillation. This type of fractional distillation will be referred to hereinafter as azeotropic distillation and the substance or substances which are added to the fraction which effect the aforementioned chang will be referred to as azeotrope formers.

According to my invention, the separation of a specific hydrocarbonor hydrocarbon fraction from a mixture of hydrocarbons is accomplished by azeotropic distillation whereina saturated heterocyclic organic compound having four or more atoms is added to the petroleum fraction and the mixture is subjected to controlled .fractional distillation. The addition of the azeotrope former to the petroleum fraction results in forming a more volatile azeotrope with certain of the hydrocarbon components which may thenbe distilled from the remaining hydrocarbon components. Thus when it is desired to segregate naphthene hydrocarbons from paraiiin hydrocarbons, the fractional distillation of this mixture to which an azeotrope, former has been added results in the formation of an azeotrope consisting of the paraffin hydrocarbons and the azeotrope former which is more volatile than-the naphthene hydrocarbons either in themselves or in azeotropic mixture with the azeotrope former employed. Thefractional distillation of the mixture results in distilling overhead the paraffin hydrocarbons in admixture with the azeotrope former leaving the naphthene hydrocarbons as undistilled bottoms, While it is preferred to per form the azeotropic distillation on a fraction containing predominantly naphthenic and paraffinic hydrocarbons and thus effect the distillation in such a manner that one of the components-in the fraction, 1. e., the naphthenic hydrocarbons remain in the substantially pure form as Idist'illation bottoms, the process of the present invention may be employed on a hydrocarbon fraction containing as well as paraflinic and naphthenic hydrocarbons, aromatic hydrocarbons and olefinic hydrocarbons. If such be the nature of the hydrocarbon fraction, a separation between the naphthenic and the paraffinic hydrocarbons, according to the present invention, may still be performed in which casethe naphthenic residue from the distillation will be contaminated with aromatic and olefinic hydrocarbons. The naph thenic hydrocarbons, however, may be separated therefrom by continuing the azeotropic distillation \vtih the azeotrope, formers herein employed or by subjecting theinaphthenic rich residue to a second azeotropic distillation with the present azeotrope former or with other suitable azeotrope formers.

In such cases where the hydrocarbon fraction contains more than two components of different chemical characteristics, as for examp1e,aro-'- matics, naphthenes and parafiins, and it is de 3. sired to separate one or more of these components from the other component or components, the separation may be accomplished by stage azeotropic distillation to remove first one component and then another component. For example, an azeotrope former, such as dioxolane, may be added to a mixture of aromatics, naphthenes'and paraffins having a boiling range of approximately 200 F. to 240 F. and the mixture then distilled to remove as overhead fractions, first an azeotrope of the paraffins with dioxolane and then an azeotrope of the naphthenes with more dioxolane leaving the aromatics as undistilled. bottoms which may or may not contain dioxolane. The point at which one component, the paraifins, for example, is substantially completely distilled from the remaining components may be observed by a rise in distillation temperature necessary to effect further distillation of the material in the still. Thus, in the above example, the distillation is initially carried out at an overhead temperature of approximately 162 F. at which temperature the paralfin hydrocarbons together with azeotrope former distill from the remaining hydrocarbon components. --When substantially .all'of -the parafiin components have been evaporated from the mixture, it will be necessary to raise the distillation temperature so that the overhead temperature will be increased to, for example, approximately 164 F. in order to effect further removal of hydrocarbon components. This increase in temperature indicates that substantially all of the parafiins were previously distilled from the mixture and that the next hydrocarbon components, for example the naphthenes,

'former C there is an azeotrope formed between A and C and also between B and C, which azeotropic mixtures may have no greater boiling point spread than existed between the original components A and B. However, it has been shown many times over that in the majority Of cases azeotropes formed with components A and B by the azeotrope former C are more readily separated by distillation than are components A and B in the absence of az'eotrope former despite the same or even smaller difierences in'boiling points existing between the azeotropes. I have found this to be true in many instances of azeotropic distillation of naphthenes and parafiins with the a'zeotrope formers of the present invention. It should be emphasized therefore that I do not wish to limit my invention to the usage of azeo trope formers which form an azeotrope with only one component in the hydrocarbon mixture, nor to the usage of azeotrope formers which form azeotropes with more than one component in the hydrocarbon mixture which azeotropes boil no further apart than the components themselves.

While the invention is adapted to, the separation of hydrocarbons of characteristics different from each other I have. found that this process particularly useful for producing naphthenic hydrocarbons such as methylcyclohexane, dimethylcyclohexane, dimethylcyclopentane, and

the like, having a high degree of purity from gasoline fractions produced from straight run or synthetic gasoline, such as those produced by cracking, polymerization or reforming. The production of substantially pure naphthenic hydro? carbons is becoming increasingly important as a result of the expanding usage of naphthenic hydrocarbons as intermediates in the synthesis of organic chemicals suchas adipic acid, succinic acid, and the like.

Besides dioxolane mentioned above, other saturated heterocyclic organic compounds may be employed asazeotrope formers. These include the six membered compounds in which at least one of the atoms in the ring is oxygen, nitrogen .or sulfur, such as oxane, trioxane, piperidine,

thiane, dimethyl dioxane, morpholine, thioxane,

piperazine, dithiane, thioformaldehyde, etc., the derivatives of such six membered compounds, such as dibutanol, n-ethyl piperidine, n-methyl mcrphoiine. n-morpholine ethanol, n-phenyl morpholine, pentamethylene sulfone, etc., the five membered compounds in which at least one of the atoms in the ring is oxygen, nitrogen or sulfur, such as oxolane, pyrrolidine, thiolane, dioxolane, methyl dioxolane, etc., and the derivatives of such five member-ed compounds such as n-ethyl pyrrolidine, tetra-methylene sulfide,-tetrae hydrofurfuryl alcohol, etc., the four membered compounds, such as trimethylene sulfide, tri methylene oxide, trimethylenimine, etc., and the derivatives of such four membered compounds, such as trimethylene sulfone, n-ethyl trimethyh' enimine, etc. 1

Of the above-mentioned azeotrope formers. .I have found the heterocyclic oxygen containing compounds and particularly dioxane, dioxolane, and trioxane are very eiiicient azeotrope formers for separating a naphthene-parafiin hydrocarbon fraction having a narrow boiling range preferably not more than 50 F. into a naphthenic and parafiinic rich fraction. The azeotrope former should be chosen with respect to the boiling point of the hydrocarbon fraction, the higher the boiling point of this fraction the higher will be the boiling point of the azeotrope former capable of eiiecting the azeotropic distillation. The boilingpoint of the azeotrope former should be not more than about 35 C. above 0r35 C. below the average boiling point of the hydrocarbon fraction to be separated. In other words the hydrocarbon fraction to be separated should have an average boiling point not more than 35 C. above or more than 35 C. below the boiling point of the specific azeotrope former to be'employed for its A separation. Thus, hydrocarbon fractions to be separated with dioxolane, for example, as the azeotrope former should have an average boiling point between 39 C. and 109 C., dioxolane having a boiling point of 74 C. In this regard, I have found that in most cases it is preferable to employ an azeotrope former boiling below the hydrocarbon mixture for it is found that in this manner a greater azeotropic spread is obtained. However, this is not always true for example dioxane boiling at 214 F. is an effective azeotrope former for separating n-heptane boiling at 208 F. from methylcyclohexane boiling at 21.3.5 F. Further, in this respect I have, found, that the greatest azeotropic spread is obtained when employing an azeotrope ,former boiling. in the lower limits of this lower temperaturerange; However, there must also be considered in em. ploying such an azeotrope former the fact that the greater boiling point .difierential between the mll llllilullwww azeotrope former and the hydrocarbon fraction, the greater will be the amount of azeotrope former necessary to accomplish a given separation. It is further necessary to balance these two opposing factors in determining which azeotrope former is to be employed, and in this regard consideration of the composition of the hydrocarbon fraction, the availability of the azeotrope formers and the like must be taken into consideration. Thus, in a fraction comprised predominantly of naphthenic hydrocarbons containing only a minor amount of paraffinic hydrocarbons as impurities it may be economically practical to employ an azeotrope former boiling some 25 to 35 C. below the average boiling point of the hydrocarbon fraction; whereas, in a fraction comprised of approximately equal proportions of naphthenic and parafiinic hydrocarbons it may be deemed advisable to employ an azeotrope former boiling in the range of C. to C. below the average boil ing point of the hydrocarbon fraction. These generalizations concerning the selection of the azeotrope former to be employed are intended only to be helpful in the utilization of my invention but should not be considered as limiting its application in any way whatsoever inasmuch as I may use as azeotrope formers compounds of the above identified type boiling not more than about C. below the hydrocarbon fraction, and not more than about 35 C. above the hydrocarbon fraction.

The type of distillation to be used depends somewhat on the quantity of the aforementioned azeotrope former employed. I may take any portion of azeotrope former to the petroleum fraction that I desire, depending upon the efficiency of the operation or the purity of the product desired, and the technique to be used in the distillation. The proportion of the azeotrope former may be readily adjusted to an ideal point, again depending on whether I desire the portion high in naphthenic hydrocarbon content to remain as bottoms in the distillation in practically pure state, i. e., free from paraffinic hydrocarbon, or whether I wish to distill a portion of the parafiinic hydrocarbons leaving a portion thereof as bottoms together with the naphthenic hydrocarbon. Also the distillation temperature and amount of azeotrope former may be adjusted to effect the distillation of all of the paraflinic hydrocarbons together with a portion of the naphthenic hydrocarbons. In other words, the desired efficiency of separation of the naphthenic and parafiinic hydrocarbons is dependent upon the proportion of azeotrope former selected, the efficiency of the tillation column and similar factors.

rther, I have found that in conducting the distillation in a batchwise manner it is preferable to introduce the azeotrope former in increments during the course of the distillation for in this manner there is avoided the loss of any uncombined azeotrope former in the distillate, which normally occurs if the entire quantity thereof is initially employed. By permitting the attainment of an equilibrium in the composition of the residue in a continuous distillation the azeotrope former may be added incremently in such a distillation whereby the same beneficial effect is obtained as in the batchwise operation.

In order to separate the azeotrope former from the azeotropic distillate, it is merely necessary to extract the condensate mixture with a solvent adapted to extract or dissolve the azeotrope former and substantially none of the hydrocarbons. By allowing this mixture to settle, two dis- .6 tinct layers are formed, an upper layer consisting of the hydrocarbon and a lower layer of azeotrope former dissolved in the solvent. Solvents useful for the purpose include th nitroparaifins; such as nitromethane, nitroethane, nitropropane, propylene glycol and diethylene glycol, and even the saturated heterocyclic organic compounds ,having different boiling points than the hydro carbons to be separated from the azeotropic distillate. In some cases, the separation of the'azeotrope former from the hydrocarbons may be accomplished by cooling the azeotropic distillate sufficiently, as for example, below 70 F. in order to reject-the hydrocarbons from the azeotrope former. Most of the azeotrope formers disclosed herein are water soluble and these are preferably extracted from the azeotropic distillate with water at anappropriate temperature to effect the desired result. The azeotrope former may be recovered from the non-aqueous solvent or water by simple distillation, the overhead being either the azeotrope former or the solvent depending upon the relative boiling points of these two materials.

Other objects, features and advantages of my invention. will become apparent to those skilled in the art from the following examples thereof. However, it will be observed that these examples are not to be taken as limiting my invention since the process is applicable to separate other complex hydrocarbon fractions employing the other azeotrope formers disclosed herein for effecting the desired separation.

EXAMPLE I A blend of equal parts of methylcyclohexane and n-heptane was prepared and a portion of this blend was subjected to simple fractional distillation while another portion was subjected to an azeotropic distillation in the presence of dioxolane as the azeotrope former. Both of these distillations were made on a 64 plate still at an internal reflux ratio of 64 to 1. The results of the simple distillation are summarized briefly in Table 1 below:

The other portion of the methylcyclohexanen-heptane fraction was distilled in the presence of two parts dioxolane, the results of the distillation being shown in Table 2 below:

TABLE 2 Distillation of 50-50 mixture of n-heptane and methylcyclohemane in the presence of two volumes dioxolane Volume M. O. H. N -heptane Fraction Per Cent Vol. Per Vol. Per 3 of Charge. Cent Cent Initial 36.0 4. 4 95. 6 162 Intermediate 18. 5 32. 6 67. 4 l62164 Bottoms 45. 5 96. 0 4. 0

! Calculated on a dioxolane free basis.

It is immediately apparent that heptane and methylcyclohexane cannot be efficiently separated by'simple-fractional distillation in a column of this character; whereas, a high. degree of separation is obtainable by an azeotropic distillationin the presence of dioxolane.

EXAMPLE II had been distilled together with dioxane, leaving about 30% of the stockv in the still. After separating the diozane from these fractions thus produced, bywashing with water, the hydrocarbons. recovered in the first distillation had a gravity of 72 A. P. I. indicating about 90% by volume of paraflinhydrocarbons, the; hydrocarbons recovered from the second distillation had a gravity of 63 A. P. I. indicating about equalvolumes of parafiin and naphthene hydrocarbons and the distillation bottoms had a gravity of'52A. P. I. indicating about 95 by volume of naphthene hydrocarbons.

The. foregoing description and eXamDle-sotmy invention are not intended to be taken; as limiting my invention but only as illustrative thereof since many variations may be madelby those skilled in the art without departingfrom the spirit or scope of the following claim:

I claim;

A process for the separation of heptane and methcyclohexane which comprises azeotropically distilling said methylcyclohexane and heptane mixture in the presence of a suflicient amount of dioxolane to vaporize the heptane contained in said mixture together with said dioxolane thereby leaving said methylcyclohexane in the residue.

GEORGE R. LAKE.

REFERENCES CITED The following references are of recordinth file of this patent:

UNITED STATES PATENTS Number Name Date 2,313,537 Greenburg Mar, 9,1943 2,352,534 Greenburg June 27, 1944 2,367,701 Tooke J an.23, 1945 2,368,050 Tooke Jam-23, 1945 2,368,597 Morris et a1. Jan.' 30, 1945 2,397,839 Clark Apr. 2, '1946 OTHER, REFERENCES Mair et a1 Bureau of Standards Journal of Re search, vol. 27, pages. 45, 44, 46, and 49 through 54.