US2844527A - Separation of c-9-alkaryl hydrocarbons by azeotropic distillation - Google Patents

Description

ly 22, 1958 A. c. MCKINNIS 2,844,527

SEPARATION O C-9-ALKARYL HYDROCARBONS BY AZEOTROPIC DISTILLATION Filed Nov. 16, 195s A XML aim MM .afiinscor naphthenes.

tiri'cle, and similar materials.

United rate Patim SEPARATION OF C.9-ALKARYL HYDROCARBONS BY AZEOTROPIC DISTILLATION This .invention relates to methods for separating aromatic hydrocarbons boiling above about 150 C. from :other hydrocarbons boiling in the same range, e. g. par- Broadly stated the method-consists in azeotropically distilling the mixture to be separated :with nitromethane as theazeotrope former. It is found that nitromethane is capable of forming low-boiling azeotropes with'paraffins and'naphthenes boiling between about 150 and 220 C.,' and that the aromatic hydro- .carbons boiling in that range-do not form azeotrop'eswith nitromethane. Olefinic hydrocarbons in this boiling range may exhibit slight azeotropy with :nitronrethane, but generally such azeotropes are higher boiling than the parafiin or naphthene azeotropes and may henceibe separted therefrom.

The. invention is -particularly concerned with the alkaryl hydrocarbons.boiling-apprordmately in the 160-170 C. .range, which includes most of vthejfollowing C-9 alkaryl Certain of these materials have become'increasingly important as raw materials for the synthesis of various chemica'ls'such as terephthalic acid, t-oluic acid, phthalic anhy- A particularly rich. source material for such hydrocarbons consists of the gasoline fractions obtained from modern reforming ofhydroforming operations wherein cracked or straight-run gasoline is subjected to catalytic hydroforming to'increase the arorn'aticity and improve the octane rating-and general stability ofthe gasoline. The naphthene components in the raw gasolineare largelydehydrogenated to form the corresponding aromatic hydrocarbons. Substantial isomer- .iza'tion .and cyclization also may take place. Olefins which may have been present in the original gasoline are largely hydrogenated .during reforming to form paraffins and naphthenes.

When such reformed gasolines are subjected to distillation a narrow heart-cut boiling between about 160 and 170 C. may be obtained which will ordinarily be'more .than.50% aromatics, the remainder'being mostly parafiins and naphthenes. The separation of the aromatics from the nonaromatics in the 160170 0. fraction presents parafi'lns overhead with nitromethane.

2,844,527. Patented July 22, 1958 carbons in preference to aromatic'hydrocarbons, but the commonazeotrope formers are found to be subject to several disadvantages when theyare employed for azeotroping high-boiling parafli'ns. For example the aliphatic acids, alcohols, Cellosolves, nitriles, pyridine and acetylacetone are found either to'form azeotropes with pseudocuznene, or the paraifin azeotrope taken overhead contains only a small percentof hydrocarbons. The present invention is based upon my discovery that nitromethane is an exceptionally efiective'azeotroping agent for separating high'boiling paraflins, i. e. those boiling at 150 C. or above, from other hydrocarbons'inthe same boiling range.

It is therefore an objectof this invention to provide economical methods for separating C-9 alkaryl hydrocarbons, or other aromatics boiling inthe same or higher ranges from like-boilingparaflin and naphthene hydrocarbons. Afurther object of the invention is to provide economicalmeansior separating any desired group of high-boiling aromatic hydrocarbons from reformed gasoline fractions. A specific object is to provide efiective means for removing the C-9:alkaryl hydrocarbons from such reformates in substantially 100% purity. Other objects of the invention will be apparent from the more detailed description which follows.

It is known in the art (Geckler et al., U. S. Patent No. 2,316,126) that toluene (B. P. 110.6" C.) may be separated from like-boiling parafiins by-azeotroping the The azeotropic overhead contains about nitromethane and 55% paraffin hydrocarbons. Toluene also azeotropes, but at a higher temperature. The behaviorof nitromethane as an azeotropingagent for paraifins has alsobeen explored in connection with paraffins boiling up-to 128 C. .The n-octane azeotrope with nitromethane contains about 53% nitrornethane (Lecat, Tables Azeotropiques, 2nd edition, 1949, page 311). Theoretically therefore, nitromethane would be expected to be uneconomical as an azeotrope formerforhighboiling parafiins, since it would benecessaryto volatilize largequantities of nitrornethane per volume of hydrocarbon. It has been unexpectedly discovered however that When a '1'60170 C. heart-cut .of catalytically reformed naphtha is azeotroped with nitromethane, the azeotrope formed is much richer in hydrocarbons than would'be expected.

considerable difficulties. Solvent extraction is inherently It is wellknown in azeotropy that thereis a definite relationship between the absolute difference in boiling points of'the'azeotrope components, [A], and the difference in boiling points between the azeotrope and the lower boiling component thereof, '8. Lecat [Compt. rend. 183,880 (1926)] has in fact set up a mathematical relationship between these factors as follows:

A=(boiling point of component A) (boiling point of component B),

|A|=diiference in boiling point of A and B (absolute 'value of A),

5=difiference in boiling point of azeotrope and the lower boiling component,

a, b, c=constants for a-given series ofrelated azeotropesisuch as methanol-hydrocarbons.

In the case-of nitromethane'azeotropes with paratfinsthe relationship of IA] with 6 is set forth in Figure 'L curve B. It will be noted that n-hep'ta'ne, n-octane, n-hex'ane and cyclopentane all lie on essentially the same continuous curve. As 8nfor the curve approaches zero, the likelihood of any appreciable azeotropeformationbecomes less and less. At 6:0 no azeotrope would theoretically be formedbecause such azeutrope would boil at the same clay, bauxite, etc.

temperature as the low boiling component. As the curve B shows, from the known data in regard to parafiins up to n-octane, it would be expected that when {A[ reaches about 60", little or no azeotrope formation would be obtained. This would cast serious doubt on the feasibility of using nitromethane for azeotroping parafiins boiling above 160 C. It has been found however that the |A|/5 relationship in nitromethane-paraffin azeotropes is also anomalous in the higher boiling range. This may be seen by the inflection in curve B at about ]A|=40. Thus, at the point where ]A{=73, it is found that 6 is about 2' thus permitting effective azeotroping in that range. Such effective azeotroping continues outwardly until IA] reaches about 120. This means that nitromethane may be effectively employed for azeotroping parafiins and naphthenes boiling up to about 220 C.

The advantageous relationship existing between nitromethane and higher parafiins does not accrue to the other nitroparafiins. The higher nitroparaffins are substantially more soluble in hydrocarbons and are hence less effective than nitromethane. Nitromethane moreover does not form azeotropes with aromatic hydrocarbons boiling ing point 9799 C.) is taken overhead and condensed.

Upon condensation two liquid phases are formed, a lower solvent-rich phase and a supernatant paraflin-rich phase.

The phases are then separated and separately topped in distillation columns to strip out any dissolved nitromethane or paraflin as azeotrope, and the resulting azeotropes are condensed and returned to the liquid-liquid phase separator to be further resolved. The stripped nitromethane is then returned to the primary azeotroping column, and the stripped parafiins or naphthenes are recovered as bottoms from the paraflin stripping column. The -9 aromatics which are removed as bottoms from the azeotroping column are recovered in approximately 99% purity and 99% yield.

The operation of this process in connection with a specific gasoline fraction may be more readily understood by referring to the drawing which is a fiowsheet of one particular modification of the process. A stream of cracked or straight run gasoline, or a blend thereof, is brought in as feed through line 1, preheated in heat ex changer 2, in heat-exchange relationship with hot product gases in line 3, and transferred via line 4 to a heater 5. 'Recycle hydrogen is admitted to the gasoline stream through line 6 and the vaporous mixture of gasoline and hydrogen is heated in heater to the desired reforming temperature'e. g. 900l200 F. The preheated mixture is then passed into catalytic reactor 7 wherein the hydrocarbons are subjected to reforming as described above. The conditions for reforming are conventional in the art and hence will not be described in detail. The catalyst in reactor 7 may comprise any of the known hydroforming catalysts e. g. platinum on an alumina or silica carrier,

cobalt molybdate on a suitable carrier such as alumina, Any of the oxides or sulfides of the transitional metals may be employed as catalyst, e. g. nickel, cobalt, tungsten, chromium, molybdenum, vanadium, iron, etc. The pressure in the reactor may vary between about 0 and 1000 p. s. i. g. The space velocity may be between about 0.2 and 10 volumes of liquid feed per volume of catalyst per hour. The recycle hydrogen may vary between about 1000 and 10,000 S. C. F. per barrel of feed.

The gaseous products from reactor 7 are withdrawn tylene.

through line 3, condensed in heat exchange relationship with the feed as previously described, and the liquid products are then transferred via line 9 to a gas-liquid separator 10. The noncondensable gases comprising hydrogen, methane, ethane, propane and small amounts of hydrogen sulfide, are withdrawn through line 11. Inasmuch as there is ordinarily a net make of hydrogen in the reactor, a part of the gas stream may be bled ofi through line 12 and utilized as fuel or for other purposes. The hydrogen for recycle is diverted through line 6 and returned to be admixed with fresh feed as previously described. In order to prevent the buildup of inert gases in the system, it may be desirable to bleed 011 a stream of recycle gas from line 6 and purify it by conventional means not shownin order to enrich the hydrogen gas which is actually recycled through heater 5.'

The reformed gasoline in separator 10 is withdrawn through line 14 and transferred to a first fractionating column 15. In this column the low boiling fraction boiling below e. g. is taken off as overhead through line 16, and the high boiling bottoms is transferred via line 17 to a second fractionating column 18. Column 18 is operated so as to take overhead through line 19 a cut boiling between about 160 and 170 C., and preferably between about and C. An even closer boiling cut boiling at 167-l68 may be taken if desired. This overhead ordinarily comprises at least about 50% aromatics. If the feed material was high in naphthenes the percent of aromatics in the overhead in line 19 may range up to 80-90%. Substantially all of the remaining hydrocarbons in the overhead are paraffins and naphthenes with substantially no olefins. The bottoms from distillation column 18 is withdrawn through line 20 and comprises the higher boiling gasoline hydrocarbons which may be employed as blending stock or for any other desired purpose.

The aromatic stream in line 19 is next transferred to azeotropic distillation column 22. Recycle nitromethane is introduced through line 23. The azeotropic overhead from column 22 boils at about 96-99 C. and is taken off through line 24, condensed in condenser 25 and transferred via line 26 to liquid-liquid phase separator 27. The lower phase formed in separator 27 consists of nitro methane together with a small amount of dissolved by drocarbons. This lower phase is taken off through line 28 and topped in column 30 to remove the hydrocarbons as azeotrope with part of the nitromethane. The azeotrope is removed overhead through line 31, condensed and recycled via line 32 to separator 27. The stripped nitromethane is removed as bottoms from column 30 through line 23, and recycled to azeotroping column 22 as previously described. The supernatant phase in separator 27 consists of paraflins and small amounts of naphthenes, together with a small proportion of dissolved nitromethane. This mixture is taken off through line 33 and transferred to column 34 wherein the dissolved nitromethane is removed as azeotrope through line 35, condensed and recycled to separator 27. The bottoms from column 34 is taken off through line 36 and consists almost entirely of C8, C-9, and C-10 parafiins which may be utilized for any desired purpose.

The bottoms from azeotroping column 22 is taken off through line 38, and consists of substantially pure C-9 aromatic hydrocarbons, mainly pseudocumene and mesi- This material may be utilized directly as raw material for various chemical syntheses involving for example nitration, oxidation, or the like.

The above described azeotroping system may obviously be modified in many of its details without departing from the essential scope of the invention. By adjusting the cut points in distillation columns 15 and 13 substantially any desired high boiling hydrocarbon fraction may be azeotroped in column 22 to obtain the corresponding higher boiling aromatics. The following example may serve to illustrate more specifically the type of separations which may be obtained, but should not be interpreted as limiting the scope of the invention.

A sample of the reformate is then fractionated in a 20 plate column at a /1 reflux ratio. A heart-cut is taken oflf boiling between 167.1 and 168.2 C., which consists of 81% aromatic hydrocarbons and 19% paraffins plus naphthenes. The aromatic fraction is comprised of 51% pseudocumene and 11% mesitylene.

36.8 ml. of the above heart-cut is mixed with 31 ml. of nitromethane and the mixture is then subjected to azeotropic distillation, taking overhead the following cuts which are condensed to form two liquid phases:

29.1 ml. of hydrocarbons are recovered in the bottoms, which are found to be 99+ percent aromatics. This represents a recovery of about 98-99% of the aromatic hydrocarbons initially present.

This example demonstrates the remarkable efliciency with which high boiling aromatics may be recovered from a predominantly aromatic heart-cut of reformate. In fact, no other known azeotrope former is capable of giving as efiicient a separation.

The foregoing disclosure is not to be considered as limiting the scope of the invention since many variations may be made by those skilled in the art without departing from the scope or spirit of the following claims.

I claim:

1. A process for recovering an aromatic hydrocarbon fraction consisting mainly of pseudocumene and mesitylene from catalytically reformed gasoline which comprises fractionating said gasoline to obtain a predominantly aromatic heart-cut boiling between about and C., subjecting said heart-cut to azeotropic distillation with nitromethane, removing overhead an azeotrope of nitromethane and non-aromatic hydrocarbons, and recovering as bottoms a paraffin-free, naphthene-free aromatic hydrocarbon fraction which is predominantly pseudocumene and mesitylene.

2. A process for recovering C-9 alkaryl hydrocarbons from catalytically reformed gasoline which comprises fractionating said gasoline to obtain a predominantly aromatic fraction boiling between about 160 and 170 C., subjecting said fraction to azeotropic distillation with nitromethane, removing overhead an azeotrope of nitromethane and non-aromatic hydrocarbons, and recovering as bottoms a paraffin-free, naphthene-free aromatic hydrocarbon fraction.

3. A process as defined in claim 2 including the steps of condensing said azeotropic overhead to form a nitromethane-rich liquid phase and a hydrocarbon-rich liquid phase, separating said liquid phases, separately distilling said liquid phases to strip nitromethane azeotropically from said hydrocarbon phase and to strip hydrocarbons from said nitromethane phase, and recovering substantially pure non-aromatic hydrocarbons and nitromethane as bottoms from said respective distillations.

References Cited in the file of this patent UNITED STATES PATENTS 2,162,963 McKittrick June 20, 1939 2,316,126 Geekler et al. Apr. 6, 1943 2,385,610 Clark Sept. 25, 1945 2,415,192 Rittenhouse Feb. 4, 1947 2,432,771 Lake Dec. 16, 1947 2,477,715 Berg et al. Aug. 2, 1949 2,481,734 Engel Sept. 13, 1949 2,506,858 Davidson May 9, 1950 FOREIGN PATENTS 456,616 Canada May 10, 1949 UNT IED STATES PATENT- OFFICE Certificate of Correction Patent n 2,844,527

" Art 0. McKinnis It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below. 1

Column 1 line 49, for reforming of readreforming or; column 2, line 46, beginning with It is well known in azeotropy, strike out the entire subject matter to and including boiling up to about 220 C. in column 3, line 16; column 5, line 3, after scope of the invention. insert the following EXAMPLE I follows:

temperature 950 F. pressure 100 p. s. i. g. space velocity 1.0.

H SCF/Bb1feed 5000.

column 5, lines 15 to 25, the third and fourth columns of the table should appear as shown below instead of as in the patent Volume (m1.)of

CHzNO! y carbon Attest: KARL H. AXLINE, ROBERT C. WATSON, Attesfi/ng Ofieer. Uomwvissiomr of Patents.

July 22, 1958

Claims (1)

1. 2. A PROCESS FOR RECOVERING C-9 ALKARYL HYDROCARBONS FROM CATALYTICALLY REFORMED GASOLINE WHICH COMPRISES FRACTIONATING SAID GASOLINE TO OBTAIN A PREDOMINANTLY AROMATIC FRACTION BOILING BETWEEN ABOU 160* AND 170* C., SUBJECTING SAID FRACTION TO AZEOTROPIC DISTILLATION WITH NITROMETHANE, REMOVING OVERHEAD AN AZEOTROPE OF NITROMTHANE AND NON-AROMATIC HYDROCARBONS, AND RECOVERING AS BOTTOMS A PARAFFIN-FREE, NAPHTHENE-FREE AROMATIC HYDROCARBON FRACTION.

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