US2406695A - Separation of hydrocarbons by distillation - Google Patents

Description

' G. R. LAKE SEPAATION OF HYDROCARBONS BY DISTILLATIQN Aug. 27, 1946.

Filed April 25, 1941 Patented Aug. 27, 1946 SEPARATION OF HYDROCARBONS BY DISTILLATION George R. Lake, Long Beach, Calif., assigner to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application April 25, 1941, Serial No. 390,264

4 Claims.

This invention relates to the preparation of pure hydrocarbons from petroleum, these pure hydrocarbons being contained in a fraction of petroleum hydrocarbons whose components have small differences in boiling points, which renders them inseparable by ordinary fractional distillation.

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/or extraction with selective solvents. Y

Another object of the invention is to prepare from a given fraction of petroleum, such as gasoline, kerosene, or a narrow boiling range hydro- .carbon fraction prepared from such materials,

these fractions consisting of a mixture of parafnic, iso-paraiiinic, naphthenic, olenic and aromatic hydrocarbons, a fraction that is essentially paraiiinic or iso-paranic or naphthenic or olenic or aromatic.

A particular object of my invention is to separate aromatic hydrocarbons from non-aromatic hydrocarbons having the same boiling point or range, particularly from gasoline fractions containing relatively small quantities of the aromatic hydrocarbons desired to be recovered from the gasoline. 1

A particular object of my invention is to separate a component or class of components from a hydrocarbon mixture, the components of which Yhave substantially the same boiling point or boiling point range, by fractional distillation in the presence of an added substance, hereinafter referred to as an azeotrope former which is adapted to form an azeotrope or lower boiling mixture with one of the components or class of componentsand also effecting the distillation in the presence of a second added substance, hereinafter referred to as a vapor pressure depressant which is adapted to lower the vapor pressure of the other component or class of components. In other words, it is an object of my invention to increase the difference between the vapor pressures of the components desired to be separated by fractional distillation.

It is known to separate specific hydrocarbons or hydrocarbon fractions from a mixture of diftion wherein a substance is added to the mixture to be distilled which will form an azeotrope boiling at a lower temperature with one or more of the components so as to permit separation from the other component. For this purpose, many azeotrope formers, some of which will be referred to hereafter, have been used. While the process of azeotropic distillation to separate different hydrocarbons of substantially the same boiling point has its advantages over other methodsfor effecting the separation of these components, such as extraction with selective solvents, the process has the disadvantage of frequently requiring large and efficient fractionating columns due to the fact that in many cases the difference in boiling point between the mixture of azeotrope former and the component associated therewith on one hand and the component remaining undistilled which may or may not be associated with azeotrope former on ther other hand, is small. For example, in the case of separating toluene from paran and/or other non-aromatic hydrocarbons having substantially the same boiling point as the toluene employing methanol as the azeotrope former, the azeotrope of methanol and parain and/ or other non-aromatic hydrocarbons is distilled at a temperature ofabout 143 F.while the azeotrope of methanol and toluene is distilled at a temperature of about 148 YF. Hence, while the small difference in the distillation temperatures ofthe two azeotropes permits the separation of the toluene from the non-aromatic hydrocarbons, whereas without the azeotrope former such separation would be practically impossible by fractional distillation, yet this small difference in distillation temperatures requires eilicient fractionation equipment in order to effect the desired separation. Also, in some cases, it is impossible to separate one class of components from another class of components from a relatively wide boiling range hydrocarbon fraction due to the overlap in distillation temperatures of the azeotrope consisting of the azeotrope former and the heavier components of one class of components and the lighter components of the other class of components. My invention relates to an improvement in the foregoing process whereby I effect an increase in the difference between the aforesaid distillation temperatures, thereby facilitating the separation of dilerent components or different classes of components.:V

I have discovered that in the aforesaid process of azeotropic distillation, the presence 0f a certain substance, i. e. a vapor pressure depressant which has an aflinity for the higher boiling component or higher boiling azeotrope ordinarily remaining as still bottoms during the aforesaid azeotropic distillation has the effect of depressing the vapor pressure or retarding the distillation of the component remaining undistilled. At the same time, the distillation temperature of the `azeotrope formed with one of the components which is first distilled from the mix-ture is not substantially aifected. For example, in the af orementioned case of separating toluene from paraffin and/or other non-aromatic hydrocarbons employing an azeotrope former, the presence of a vapor pressure depressant during the distillation does not appreciably affect the distillation temperature at which the azeotrope consisting of azeotrope former and the parailin and/or other non-aromatic hydrocarbons is distilled; however, the vapor pressure depressant materially retards the distillation of the higher boiling azeotrope of the azeotrope former and toluene so that it Will require a somewhat higher still temperature, depending of course, upon the particular `substance employed as the vapor pressure depressant in order to distill it, than whenk the distillation is eiected in the absence of the vapor pressure depressant. This is particularly true in the case where the azeotrope former only forms an azeotrope with the non-aromatic hydrocarbons and does not form an azeotrope with the undistilled aromatic hydrocarbons. In this manner, the difference in distillation ternperatures of the paranin or non-aromatic hydrocarbons associated with azeotrope former and the toluene, whether associated with azeotrope former or not, is increased considerably so that it is possible to effect a sharper and more enicient separation of the toluene fromv the paraliin and/or other non-aromatic hydrocarbons. It is possible that the presence of the vapor pressure depressant will require a higher still temperature to remove the azeotrope of paraffin and/or other' non-aromatic hydrocarbons than without its presence, yet the still temperature required to distill the azeotrope of toluene and azeotrope former is increased to a greater extent or proportion than without its presence so that the net' difference inv distillation temperatures of the foregoing azeotropes is greater than when the distillation is effected in the absence of the vapor pressure depressant. Y

While the invention is adapted for the separation of hydrocarbons of characteristics different lfrom each other, I have found that this process is particularly useful for producing toluene having a very high degree of purity from gasoline fractions produced from straight run or synthetic gasolines such as lthose produced by cracking, polymerizing orv reforming. The production of substantially pure toluene is highly important particularly when it is to be used in the manufacture of explosives by nitrating the toluene since small amounts of impurities seriously impair the ni-tration process.

As azeotrope formers which I have found usefull are aliphatic ketones such as methyl ethyl ketone, acetone and cyclic aliphatic ketones such as cyclohexanone.

Of'the above mentioned azeotrope formers, I have found' methyl ethyl ketone and acetone, particularly eicient azeotrope formers for separating a hydrocarbon fraction having a boiling range between G-300 li'. into hydrocarbon components of different chemical characteristics and are particularly useful for separating'parafn and/or naphthene hydrocarbons having 7 to 10 carbon atoms from aromatic hydrocarbons having 9 or less carbon atoms, as for example,

when separating heptanes, octanes, nonanes, decanes and/or naphthene hydrocarbons having similar carbon atoms from toluene, xylenes and ethyl benzene. Some of the azeotrope formers mentioned herein .are more eflicient when used in thepresence of water while others must be used in substantially anhydrous form in order to obtain the best results. For exampley methyl ethyl ketone may contain from 0 to 25% by volume of water to be operative.

As vapor pressure depressants which I have found are phenolic compounds such as cresylic acid, phenol, Xylenol and resorcinol.

In general, when choosing azeotrope formers and vapor pressure depressants for eiecting the distillation, it is preferable to employ an azeotrope former having substantially the same boiling point as the stock, and preferably boiling not more than about E'. below or about 40 F. above the boiling point of the stock. When choosingV a vapor pressure depressant, it is preferable to choose one boiling substantially above the boiling point of the stock, preferably boiling greater than about 100 F; above that of the stock. Moreover, when choosing azeotrope formers and vapor pressure depressants for effecting the distillation it is, of course, necessary toselect pairs which arenot chemically reactive with each other because if they did react the reaction products would not necessarily be either azeotrope formers or vapor pressure depressants.

In carrying out they process, the distillation of the mixture of hydrocarbon fraction, azeotrope former and vapor pressure depressant is continued until all of one of the components has been distilled from the mixture, This component which` is Vaporized is condensed together with the azeotrope former. In order to separate the azeotrope former from the oil condensate, it is merely necessary to cool and/or mix the-condensate mixture with water which dissolves in the azeotrope former and allows the hydrocarbons to separate from the azeotropeformer. By allowing this mixture to settle, two distinct layers are formed, an upper layer consisting of the hydrocarbon and a lower layer of diluted azeotrope former. When the azeotrope former isA used in the presence of water in the distillation operation, thecondensate will ordinarily stratify into the two above-mentioned layers. However, it may be desirable to add additional water to the condensate' in order to insure complete separation of the azeotrope former from the hydrocarbon. The azeotrope former may berecovered from the water by simple distillation. If desired, the azeotrope former may be returned to the still withI additional feed stock, it being apparent that when the azeotrope former is used in the presence 0f water and a separate layer forms when the overhead is condensed that the separated layer of Water and azeotrope former may be returned directly to the still. When all of the component has* been distilled, the still temperature is raised and the other component is distilled from the vapor pressure depressant whether alone or associated with azeotrope former.

When separating one class of hydrocarbons from another class of hydrocarbons in an oil fraction having a Wide boiling range, it is preferable to first fractionate the wideA boiling range stock into a plurality of fractions, each having a narrow boiling range. Each of' these fractions may then be separately subjected to azeotropic distillation in the presence of a vapor pressure depressant in order toeffect the separation into the 'may be fractionally distilled to produce ten fractions, each having a'` boiling range difference of about twenty degrees. Each fraction may then be subjected to azeotropic distillation in the presence of vapor pressure depressantl to separate paraffin hydrocarbons from aromatic hydrocarbons. The various paraiiin hydrocarbons may then be blended to produce a hydrocarbon fraction having the same boiling range as the original stock andcomposed of substantially paraffin hydrocarbons.

This may be done with the'various aromatic hydrocarbons. I have found that by carrying out the process in this manner that a sharper separation of components may be obtained from stocks having wide boiling ranges than if these stocks were subjected directly to the distillation without prior separation into narrow .boiling range fractions.

The foregoing procedure of separating a wide boiling range fraction into narrow outs and subjecting each fraction to separate azeotropic distillation in the presence of a vapor pressure depressant and then blending the components of like characteristics is particularly useful in the production of high viscosity index lubricating oils. In order to produce high quality lubricating oils from naphthene base crude oils, it is generally vnecessary to subject the lubricating oil fractions to extraction with solvents capable of effecting a separation of relatively non-parainic from relatively parainic oil fractions. However, due to the wide boiling range of the lubricating oil fractions, it has been impossible to accomplish this by azeotropic distillation of the entire lubricating oil fraction. However, by carrying out the jazeotropic distillation in the presence of a vapor pressure depressant, the lubricating oil stock may be separated into fewer fractions, each having a wide boiling range and thereby reducing the number of azeotropic distillations necessary to effect the desired separation. s While the foregoing separation of a wide boiling range oil fraction into narrow cuts before azeotropic distillation of each cut in the presence portion of the boiling range of the other class of components in the hydrocarbon mixture. For

:examplalet us consider a hydrocarbon fraction boiling between 200 and 500 F. and composed of paramn and aromatic hydrocarbons. By adding an azeotrope former only to this mixture, an azeotrope is formed with the paraiiin hydrocarbons distilling between 100 and 400 F. Thus, the hydrocarbon material distilling from 100 F, to 200 F. will be substantially paraffnic and the hydrocarbon fraction distilling between 200 and 100 F. will be a mixture of parafnic and aromatic hydrocarbons, while the fraction remaining in the still which distills above 400 F.

vwill be substantially aromatic.

By adding a vapor pressure depressant to the above mixture, this will tend to shift the distillation range of the aromatic hydrocarbons so that instead of the aromatic fraction having a distillation range of 200 to 500 F., it will nowhave a higher distillation range of, for example, SOO-600 F. so that the fraction now distilling between 100-300 F. will be substantially paraflinic, that distilling between 300400 F. will be a mixture of parafnic and aromatic fractions while the fraction remaining in the still which distills above 400 F. in the presence of the vapor pressure depressant will be substantially aromatic. Thus, by effecting the distillation and separately collecting the above fractions and removing azeotrope former and vapor pressure depressant, we will now have the three oil fractions named above, i. e. one paraliinic boiling between 20D-400 F., one aromatic boiling between 3D0-500 F. and one a mixture of the two. Since the aromatic hydrocarbons boiling between 3D0-500 F. and the paraflinic hydrocarbons boiling'between 20G-400r F. have been separately recovered, the mixed fraction will be composed of aromatic hydrocarbons boiling in the lower boiling range of the stock, i. e. 20G-300 F. and parafn hydrocarbons boiling in the higher range 0f the stock, i. e. 40o-500 F. After removing the azeotrope former by mixing with water and stratifying, the aromatic hydrocarbons may be readily separated from the paraflinic hydrocarbons by fractional distillation. Since there is a Wide diierenoe in boiling points between these aromatic and parainic hydrocarbons, separation is readily accomplished. By blending all' of the aromatic hydrocarbons thus separated, we will have an aromatic hydrocarbon fraction having the same boiling range as the original stock. The same can be done with the various paraflinic fractions. This same procedure can be used to treat lubricating oil stocks in order to recover the more paraflinic oil fractions which give high quality lubricants.

The foregoing process is also applicable to the separation of wax from oils containing the same. This is possible due to the fact that the wax components 'are pure paraffin hydrocarbons while the oil fractions are not pure paran hydrocarbons. Thus, the wax components will form an aZeotrope with azeotrope formers having a lower boiling point than the fluid oil components in the lubricating oil stock. The boiling point of the latter is depressed by the presence of the vapor pressure depressant. When a lubricating stock is Vcomposed of wax, relatively paraflinic and relaf tively more paraflinic fractions which may be separately collected. This wax separating procedure may be carried out on either narrow boiling fractions as described above or on a wide boiling fraction which may be separated intoits component classes as also described above.

Other objects, featuresand advantages of my invention will be apparent to those skilled in the art from the followingdescription of the invention as taken from the drawing which represents a diagrammatic arrangement of apparatus for carrying out my invention.

In the drawing, the hydrocarbon feed to be resolved into components of similar characteristics, as for example, a hydrocarbon fraction produced from catalytically treated or reformed gasoline having a boiling range of about 220 to 235 F.

and; consisting of substantially 45%I toluene,- 6% olens and the remainder parafiins andv naphthenes, is taken from tank l via line il controlledv by valve I2 and pumped by pump i4 into line l5. Azeotrope former, such as methyl ethyl ketone, containing about 10% water, is taken from tank i6 via line I1 controlled by valve I8 and is pumped by pump l0 through line 20 into line l5. Vapor pressure depressant, such as cresylic acid, is taken from tank 2l via line 22 controlled by valve 23 and is pumped by pump 24 through line 25, Valve 20 and line 2l into line l5. This mixture in line l consisting of the hydrocarbonv feed, azeotrope former and vapor pressure depressant in the ratio of approximately one part by volume of hydrocarbons,l one part of methyl ethyl ketone and water and ve parts of cresylic acid, is then passed into fractionating column 23 Where the mixture is subjected to fractionation, heat being supplied by closed steam coil 2S. If desired, the vapor pressure depressant may be introduced directly into the fractionating column via line 2l controlled by valve EES and throughV either valve 25a or 2Gb depending upon whether it is desired to introduce it into the top or bottom of the column. In some cases, it may be desired to introduce the vapor pressure depressant into theA top of the column so as to act as reflux therein. In the fractionating column, the distillation is controlled so'as to distilloverhead an azeotrope consisting or the paraffin, olefin and naphthene hydrocarbons and the methyl ethyl ketone and water. In the example herein given, this is accomplished at an overhead temperature of approximately 160 F. If desired, the azeotropic distillation may be carried out either at atmospheric orl superatmospheric pressure or under a vacuum. The above overhead mixture is removed from the fractionating column via line 30, condensed in condenser 3l and passed via line 32 into the bottorn of Washer 33 provided with packing material, such as broken tile 33a, for effecting intimate countercurrent contact with` water which is introduced into the washer from tank 34 Via line 35 controlled by valve- 35 and pumped by pump 3l through line 38 into `the washer. The contact of the condensate of methyl ethyl ketone, water and hydrocarbons introduced into the Washer with the Water causes the condensate to separate into two phases, i. e. an upper phase consisting of the paraffin, olefin and naphthene hydrocarbons and a lower phase consisting of methyl Vethyl ketone and water. The upper phase is withdrawn via line 39 and is passed through cooler 40 and line 4l to storage tank 42. The washing operation' is preferably carried out at an elevated temperature of approximately 300 F. under superatmospheric pressure.

The lower phase is withdrawn via line 43- controlled by Valve 44 and is pumped by pump 45 through line 40 into fractionating column 41 where the methyl ethyl ketone containing the desired amount of water, i. e. about is re- Y moved as an azeotrope aided by heat in steam coil 48, as an overhead vapor via line 49, condensed in condenser 50 and returned via line 5l to= storage tank l5. The undistilled Water substantially free from methyl ethyl ketone is removed Via line 49 controlled by valve 50 and is pumped by pump 5l through line 52 to storage tank 34.

The bottoms in the fractionating column 28 consisting of aromatic hydrocarbons and vapor pressure depressant, i. e. cresylic acid, are removed via line 53 controlled by valve 54 and pumped by pump 55 through line 5Sv into frac vtionating column 51 in which the aromatic hydrocarbon, i. e. toluene, is distilled by steam heat in coil 58 and is removed via line 59, condensed in 60 and passed through line 5l into storage tank 62. The bottoms in the fractionating column consisting of cresylic acid are withdrawn via line $3 controlled by valve 64 and pumped by pump 55 through line 60 into storage tank 2l. In the foregoing, whilenot disclosed, a portion of the condensate obtained by condensing the overhead from each fractionating column such as 23, 41 and 51 may be recycled tothe top of the column to act as reflux and control the fractionation.

The foregoing description is not to be taken as limiting my invention but4 only as illustrative as many variations may be made by those skilled in the art without departing from the scope of the following claims.

l'. claim:

1. A process for the treatment of a narrow boiling range complex hydrocarbon fraction to separate at least one component from other components contained therein which ordinarily distill from the hydrocarbon fraction in the same temperature range as said component distills therefrom. which comprises fractionally distilling said complexy hydrocarbon fraction in the presence of a suiilcient amount of an aliphatic ketone having a boiling point within F. below and 40 F. above the average boiling point of said complex hydrocarbon fraction to vaporize at least one component of said complex hydrocarbon fraction together with said. aliphatic ketone and in the presence of a phenolic compound having a boiling point of at least 100 F. above the average boiling point of saidr complex hydrocarbon fraction adapted to remain in the residue together with at least one component of said complex hydrocarbon fraction, thereby leaving at least one component of said complex hydrocarbon fraction in the residue together with said phenolic compound substantially completely separated from at least one component of said complex hydrocarbon fraction. Y

2. A process for the treatment of a narrow boiling range complex hydrocarbon fraction containing relatively aromatic and relatively nonaromatic hydrocarbons to separate the relatively aromatic hydrocarbons from the relatively nonaromatic hydrocarbons contained therein which ordinarily distill from said complex hydrocarbon fraction in the same temperature range as the relatively aromatic hydrocarbons distill therefrom which comprises distilling said complex hydrocarbon fraction in the presence of a suiiicient amount .of an aliphatic ketone having a boiling point within 100o 1i'. below and 40 El. above the average boiling point of said complex hydrocarbon fraction to vaporize the relatively non-aromatic hydrocarbons together with said aliphatic ketone and in the presence of a suicient amount of a phenolic compound having a boiling point of at least 100 F. above the average boiling point of said complex hydrocarbon fraction adapted to remain in the residue together with said relatively aromatic hydrocarbons, thereby leavingk said relatively aromatic hydrocarbons in the residue substantially completely separated from the hydrocarbons other than said relatively aromatic hydrocarbons.

3. A process for separating toluene from parafiin and other non-aromatic hydrocarbons having similar boiling points which comprises addingv an 9 aliphatic ketone adapted to form an azeotrope with the parafn and other non-aromatic hydrocarbons, said aliphatic ketone having av boiling point Within 100 F. vbelow and 40 F. above the boiling pointof said hydrocarbons and adding also a phenolic compound adapted to depress the vapor pressure of the toluenesaid phenolic compound having a boiling point of at least 100 F. above the boiling point of said hydrocarbons and fractionally distilling the mixture to vaporize an azeotrope of said aliphatic ketone and non-aromatic hydrocarbons from said toluene and phenolic compound.

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