USRE22379E

USRE22379E - Recovery of pure aromatics

Info

Publication number: USRE22379E
Authority: US
Publication date: 1943-09-28

Description

Sept. 28, 1943. c, 1 DUNN Er 'A Re. 22,379

RECOVERY OF PURE AROMATICS Original Filed July 2'7, 1940 lnvznforszClorzncz L. Du nn Ruben b. MConoughq Reiuued Sept. 28, 1943 RECOVERY OF PURE AROMA'I'I'CS Clarence L. Dunn, Berkeley, and Robert B. McConaughy, El Cerrlto, CaliL, asslgnors to Shell Development Company, San Francisco, Calii'., a corporation of Delaware Criginal No. 2,288,126, dated June 30, 1942, Serial No. 348,046, July 27, 1940. Application for reissue October 19, 1942, Serial No. 2.479%

Claims.

This invention relates to a process for the separation of pure aromatic hydrocarbons such as benzene, toluene, paracymene, naphthalene or mixtures of aromatic hydrocarbons of identical molecular weight, such as xylenes and ethyl benzene, or the isomeric methyl naphthalenes and the like from petroleum hydrocarbon distillates. More particularly, it deals with a process wherein a distillate containing the desired aromatic constituent is fractionated to produce a concentrate containing an increased proportion of the aromatic constituent to be separated and said concentrate is subjected to an extractive distillation or a vapor phase extraction to produce the substantially pure aromatic constituent.

It has long been recognized that petroleum contains a vast number of hydrocarbons and among those minor quantities of aromatic compounds, both the simple relatively low-boiling aromatics such as benzene, toluene, ethyl benzene, xylenes, etc., as well as the relatively highboiling condensed ring compounds such as naphthalenes, phenanthrenes and the like.

These individual hydrocarbons are present in such variety and low concentration each that heretofore it has not been possible to separate them except by expensive, tedious processes in the laboratory, and in many instances it has been impossible to isolate individual compounds in any known manner.

It is known that aromatic compounds, for example, toluene occurring in small quantities in petroleum hydrocarbons cannot usually be concentrated by fractional distillation to produce concentrates containing more than about toluene. Further concentration of toluene by fractional distillation is, however, impossible because of the existence of minimum boiling azeotropic mixtures formed by toluene with some of the other hydrocarbons in the mixture. Therefore, toluene, for instance, of a sufllciently high degree of purity to-meet nitration requirements cannot be produced from petroleum distillates by fractional distillation alone.

It is an object of our invention to recover commercially substantially pure aromatic hydrocarbons or mixtures of aromatic hydrocarbons having the same molecular weight such as benzene, toluene, xylenes and ethyl benzene, paracymene, naphthalene, isomeric methyl naphthalenes and the like from petroleum distillates. Another purpose is to provide a process whereby substantially all of the small amounts of these compounds which are normally present in petroleum distillates can be recovered economically. It is a further object to produce from petroleum fractions aromatic hydrocarbons of high purity, for example. benzene or toluene of sufllciently high purity to meet nitration specifications; and it is another object to produce these compounds in a substantially pure state by employing only physical processes.

We have discovered that it is commercially possible by the method of our invention to produce from petroleum distillates compounds of high purity. Our method comprises producing first a concentrate of the aromatic constituent to be extracted having specific properties and then subjecting this concentrate to a vapor phase extraction or extractive distillation in the presence of a relatively high-boiling selective solvent; which has preferential solvent power for the aromatics to be extracted. In the extraction step, the non-aromatic hydrocarbons are taken overhead while the aromatic is withdrawn from the bottom of the extraction zone with the solvent from which it is readily separated by simple distillation.

The following discussion of the production of toluene of high purity by the method of our invention serves to illustrate it in its essentials.

As already stated, toluene (as well as 'otheraromatic hydrocarbons) forms azeotropes with allphatic and naphthenic hydrocarbons having boiling temperatures close to (both below and above) the boiling temperature of toluene. Because of the distortion of boiling range caused by these azeotropes which boil below the normal boiling temperatures of the toluene and these aliphatic or naphthenic hydrocarbons, it is impossible to exclude non-toluene hydrocarbons from the first concentrate. This fact is illustrated by the following example:

An East Texas gasoline having an A. S. T. M. boiling range from 42 to 133 C. and containing 1.52% toluene was separated by precision distillation into narrow fractions, each of which had a 10 C. true boiling range. A fraction thus produced, having a true boiling range from 95 to 105 C., contained approximately of all the toluene (boiling point 110.6 (2.), and the next higher boiling fraction boiling from 105 to 115 C. contained most or the remainder of the toluene. Thus, the concentrate containing substantially all, i. e., at least about 01 the toluene contained in the gasoline distillate had a true boiling range from to C.

Generally speaking, it has been found that the true boiling range of suitable concentrates ranges from below 101 C. to above 112 C., if substantially only of toluene and solvent. 'niis fat solvent is withdrawn through line ll. whence it passes through distillation colmnn ll equipped with reboiler l and partial condenser 23. In column ll the toluene is distilled, resulting vapors going through line I. to storage not shown. The selective solvent thus freed from toluene emerses from column it through bottom line l1 and through cooler II. The cooled selective solventreturnstothetopoicolmnn ll iorusein another extraction cycle.

For simplicity, the drawing does not show pumps, heat exchangers, valves, by-passes, and other auxiliaries, the proper placement of which will be at once evident to those skilled in the art.

While in the above, our invention has been described in detail with particular reference to the 1,3- di-ethoxr glycerol, lflipmpm s rw -diprop xr lycerol. h l -i proper! sh and 1,3-di-isopropoxy glycerol; the mixed diglycerol ether esters such as l-ethoxy', Z-methnxy glycerol, l-methoxy, 3-propoxy glycerol,

z-isopropoxy glycerol. 7

While the majority of the above-listed solvents canbeusedfor theseparationofthelowerboiling aromatics in the extractive distillaflon step, only the higher boiling members can be em- 'ployedfortheseparationofthehigherboiling separation of toluene from petroleum distillates V which contain toluene naturally associated therewith, it is not limited thereto. Thus, it may be applied as well to the separation of other pure aromatics or mixtures of isomeric aromatic, which may be contained in straight-run, cracked or other petroleum distillates, the boiling range of which embraces the boiling temperatures of these aromatics. Further, while our process is particularly applicable to the treatment of petroleum fractions containing relatively low percentages of aromatics, it has also been successfully applied to the production of pure aromatics from fractions .containing aromatics in relatively high concentrations. concentrate taken for extractive distillation be prepared on the basis of the same general principles as those used in produc ng the toluene'concentrate and that the solvent employed in the extractive distillation have a boiling temperature suiiiciently above the boiling temperatures of the aromatic constituent to be separated so that the two can be readily separated from each other by simple fractional distillation. Also. it is desirable that the solvent should boil sumciently above the boiling temperature of the highest boiling constituent of the concentrate so that there is substantially no loss of solvent to'the residual non-toluene hydrocarbon vapors. The solvents must also be chemically and thermally stable under the conditions of the extraction or extractive distillation.

Examples of solvents which are suitable for the concentration of the various aromatics in the extractive distillation step are phenol, cresylic acids, alkyl phenol mixtures, aniline, alkyl anilines, diphenyl amine, ditolyl amines, carbitols methoxy' glycerol, l-cthoxy glycerol, 2-ethoxy glycerol, l-propoxy glycerol, 2-propoxy glycerol, -isopropoxy glycerol, 2-isopropoxy glycerol; the glycerol di-ethers such as LZ-di-methOXy g cerol, 1,3-di-methoxy glycerol, 1,2-di-ethoxy glycerol,

It is only necessary that the aromatics for the reasons outlined above which serveasaguidefortheselectionofanyspecifle solvent for agivenbase. Itistobeemphasised that the solvents listed above are intended as examplesonlyandonthebasisofourdisclosures otherequivalent solvents will be at once evident to those skilled in the art. a

In separating a given concentrate into its toluene and'non-toluene components, the number of plates in the extractive distillation column required with a given reflux, or conversely, the amount of reflux required with a given number of plates, decreases as the amount of solvent employed relative to. the amount of concentrate treated increases. The amount of solvent necessary increases with the boiling point of the highest boiling non-toluene components present; however, we have found that one to three parts by weight of solvent to one part by weight of concentrate are usually suflicient. Less solvent may be employed with concentrates which contain only low-boiling non-toluene components, and more solvent if the concentrate contains a large amount of toluene, as the solution of toluene and solvent obtained as the bottom product of column II will then not be inconveniently low.

For the successful operation of our process it is necessary that the concentrate which is to be extractively distilled have the following general properties. It should contain at least 5% and preferably not less than about 10% by weight of the aromatic to be extracted. It should have a true initial boiling point at least 10: b 0. lower than the boiling temperature of the pure aromatic or the lowest boiling member of a group of aromatic isomers, and a true final boiling point at least 21% 0. higher than the boiling temperature of said aromatic or the highest boiling member of a group of aromatic isomers. Thus, the boiling point of pure toluene being 110.6 0., the concentrate to be extracted should at least encompass the true boiling range indicated earlier from below 101 C. to above 112' C.

In cases where a group of isomers boil over a considerable'range, it is possible to get a separation between isomers by careful control of the boiling range of the concentrate so as to exclude eiflier lower or higher boiling members of the isomeric group from the concentrate being extractively distilled. It is difiicult to separate isomers in the extractive distillation step since the common selective solvents generally show practically no difference in solvent selectivity for difierent isomers of the type, e. g., ortho, meta or para xylene.

In addition, the concentrate must not contain an amount of components boiling higher than 10: i? C. above the boiling point of the ammatic to be extracted or the highest boiling member of a group of isomers, which amount is greater than the content of this aromatic, nor a content of components boiling higher than 15: 55 C. above the boiling point of said aromatics or highest boiling member ofa group of isom rs.

Table Properties of fraction to be extractively distilled I Content Content ponents Boiling 33; boiling Aromatic to be filing :32 Min.

separated to above g' boiling so stated must not 9 shown be at" from must be s below to nott mter of content above content of oiarometic aromatics of or the total concentrate '0. C. C'. C. Benzene 80 90 95 73-384 Toluene 110. 6 120 125 100-112 o-Xylene 144 agm 133 154 159 121-147 Ethyl benzene 136 p-Cymcne 176 186 191 167-179 Naphthalene 218 2% 233 209-221 The above examples illustrate thegcneral principles-to be applied for the separation of a given aromatic hydrocarbon or a mixture of isomeric aromatic hydrocarbons from hydrocarbon distillates of relatively wide boiling ranges. By following these principles aromatic hydrocarbons not enumerated may be separated from hydrocarbon mixtures without departing from the spirit of the present invention.

The extractive distillation may be carried out adiabatically or isothermally. In the latter case, the stripping and enriching zones are operated at substantially the same temperature. If desired separation may be made under either reduced or elevated pressure. When operating under reduced pressure, it is sometimes possible to employ selective solvents which would not otherwise be applicable because of their tendency to undergo chemical changes at elevated tempcratures.

If the distillate to be treated contains certain sulphur compounds, for example, methyl thiophenc, which commonly occurs in cracked gasolines and which are apt to contaminate the aromatics due to their relatively high solubility in selective solvents for aromatics, these objectionable compounds may be removed prior to treatment by our process, or if preferred, may be removed from the aromatic fraction subsequent to its having been isolated.

Our process for the production of pure aromatics is also applicable to hydrocarbon oils other than petroleum oils which comprise mixtures of aromatics with non-aromatics, i. e., aliphatic, alicyclic, or both hydrocarbons, provided they contain hydrocarbons of the difierent types that boil sufilciently close to each other to form azeotropes during distillation. Examples of such mixtures are certain coal tar distillates, synthetic hydrocarbon mixtures such as may be obtained by cyclization of aliphatics, or dehydrogenation of naphthenes, etc.

Example being larger than the following limitations: (1) 5% of the toluene content, or (2) 70 of the total concentrate. The following table gives examples showing the properties of suitable concentrates for the production of various pure aromatics:

distilled in a precisiondistillation column equivalent to 50 plates to produce a concentrate boiling between 98 C. and 120 C. containing 17.8% toluene. The concentrate was fed, continuously I in the vapor state to a packed column equipped with reboiler having an efficiency equivalent to 44 theoretical plates. The ratio of reflux to take-oil at the still-head was 2 to 1. The temperaturcs at the top and bottom of the column were 102.5 and 158 C., respective1y. Aniline a Mas fed to the upper part. of the column as a.

selective solvent. The aniline feed was 1.3 compared to 1 for the concentrate feed.

The top product contained 98.9% non-toluene hydrocarbons, a little over 1% toluene and substantially no aniline. The bottom product consistcd of 11.6% toluene in aniline with less than 0.2% non-toluene hydrocarbons. Distillation of this bottom product at a. 3 to 1 reflux ratio in a batch still equivalent to eight theoretical plates gave toluene of better than 98% purity, meeting specifications for nitration grade. The bottom aniline" product was suitable for re-use in the extractive distillation column. The toluene recovery was of the total toluene content contained in the original distillate.

We claim as our invention:

1. In a process for separating a. substantially,

aromatics of identical molecular weight from a hydrocarbon mixture containing non-aromatic hydrocarbons and having a relatively wide boiling rangeabove and below the boiling temperatures of said aromatics, the steps of fractionally distilling said mixture to producea concentrate containing not less than about 5% ofv said aromatic fraction, said concentrate Containing an amount of components boiling'higher than about 10 C. above the upper boiling temperature limit of said aromatic fraction, which amount is not greater than the content of said aromatic fraction in said concentrate, and a content of cornponents boiling higher than about 15 C. above said upper boiling temperature limit which content is not greater than the larger of the following; 5% or said content of said aromatic fraction and /2% of the total concentrate, contacting said concentrate in the vapor phase with a l'iqliid solvent selectively dissolving aromatic hydrocarbons to produce a residual vapor and a fat solvent containing absorbed vapors consisting essentially of aromatic hydrocarbons, separating said fat solvent from said residual vapor, and fractionally distilling the separated fat solvent to recover said absorbed aromatics, said solvent boiling at a temperature substantially above the said highest boiling temperature of said pure aroma-tic fraction.

2. The process of claim 1 wherein the hydrocarbon mixture is a petroleum distillate. v

3. In a process for separating a substantially pure aromatic fraction consisting essentially of aromatic hydrocarbons of identical molecular weight having at least 7 carbon atoms from a pctroleum hydrocarbon distillate containing nonaromatic hydrocarbons and having a relatively wide boiling range above and below the boiling temperatures of said aromatics, the steps of fractionally distilling said distillate to produce a concentrate containing not less than about 5% of said aromatic fraction, said concentrate containing an amount of components boiling higher than about 10 C. above the upper boiling temaromatic fraction in said concentrate, and a content or components boiling higher than about 15' C. above said upper boiling temperature limit which is-not greater than the larger of the following: or said content of said aromatic fraction and 36% of the total concentrate, said concentrate having a true initial boiling point at least about C. below the lowest boiling temperature of said pure aromatic fraction and a true final boiling point at least about 2 C. above the highest boiling temperature of said pure aromatic fraction, contacting said concentrate in the vapor phase with a liquid solvent selective for aromatics, to produce a fat solvent containing absorbed aromatics only and a residual hydrocarbon vapor, separating said fat solvent from said vapor, and fractionally distilling the separated fat solvent to recover said absorbed aromatics, said solvent boiling at a temperature substantially above the said highest boiling temperature of said pure aromatic fraction.

4. In a process for separating a substantially pure aromatic fraction consisting essentially of xylenes and ethyl benzene from a petroleum hydrocarbon distillate containing non-aromatic hydrocarbons and having a relatively wide boiling range above and below the boiling temperatures of said xylenes and ethyl benzene, the steps of fractionally distilling said distillate to produce a concentrate containing not less than about 5% of said xylenes and ethyl benzene, said concentrate containing an amount of components boiling higher than about 154 C. not greater than the content of said xylenes and ethyl benzene in said concentrate, and a content oi components boiling higher than about 159 C. not greater than the larger of the iollowing: 5% of said content of said xylenes and ethyl benzene content and /2% 01 the total concentrate, said concentrate having a true initial boiling point not higher than 127 C. and a true final boiling point 01 at least 147 C., contacting said concentrate in the vapor phase with a liquid solvent selectively dissolving aromatic hydrocarbons and having a boiling temperature substantially above 144 C., to produce a residual hydrocarbon vapor and a fat solvent containing absorbed vapors which consist essentially of xylenes and ethyl benzene, separating said fat solvent from said residual vapor, and iractionally distilling the separated fat solvent to recover said absorbed xylenes and ethyl benzene.

5. In a process for separating substantially pure toluene from a petroleum hydrocarbon distillate containing non-aromatic hydrocarbons and having a relatively wide boiling range above and below 110.6 C., the steps oi rractionally distilling said distillate to produce a concentrate containing not less than about 5% toluene, said concentrate containing an amount of components boiling higher than about 120 C. not greater than the content of said toluene fraction in said concentrate, and a content of components boiling higher than about 125 C., being not greater than the larger oi the following: 5% of said toluene content and 70 of the total concentrate, said concentrate having a true initial boiling point not higher than about 100 C., and a true final boiling point at least about 112 C., contacting said concentrate in a vapor phase with a liquid solvent selectively dissolving aromatic hy drocarbons and boiling at a temperature substantially above l10.6 C., to produce a residual bydrocarbon vapor and a fat solvent containin absorbed vapors consisting essentially oi toluene, separating said tat solvent from said vapor and i'ractionally distilling the separatedlat solvent to recover said absorbed toluene. v

6. In a process for separating substantially pure toluene irom a petroleum hydrocarbon distillate containing non-aromatic hydrocarbons and having a relatively wide boiling range above and below 110.6 C., the steps or ifractionally distilling said distillate to produce a concentrate containing not less than about 5% toluene, said concentrate containing an amount of components boiling higher than about 120 C. not greater than the content of said toluene traction in said concentrate, anda content of. components boiling higher than about 125 C., being not greater than the larger 01 the tollowing: 5% of said toluene content and of the total concentrate, contacting said concentrate in a vapor phase with a liquid solvent selectively dissolving aromatic hydrocarbons and boiling at a temperature substantially above 110.6 C. and to produce a fat solvent containing absorbed vapors consisting essentially of toluene, separating said fat solvent from said vapor and fractionally distilling the separated fat solvent to recover said absorbed toluene.

7. In the process according to claim 5, the step of contacting said concentrate in the vapor phase with a mixture of alkyl phenols.

8. In the process according to claim 5, the step of contacting said concentrate in the vapor phase with nitrobenzene.

9. In a process for separating substantially pure toluene from a petroleum hydrocarbon distillate containing non-aromatic hydrocarbons and having a relatively wide boiling range above and below 110.6 C., the steps of fractionally distilling said distillate to produce a concentrate containing more than 2% of toluene, said concentrate containing an amount of components boiling higher than about 120 C. not greater than the content of toluene in said concentrate, and a content of components boiling higher than about 125 C., being not greater than the larger of the following: 5% of said toluene content and of the total concentrate, said concentrate having a true initial boiling point not higher than about C., and a true final boiling point of at least about 112 C., contacting said concentrate in a vapor phase with a liquid solvent se-' lectively dissolving aromatic hydrocarbons and boiling at a temperature substantially above 110.6 C., to produce a residual hydrocarbon vapor and a fat solvent containing absorbed vapors consisting essentially of toluene, separating said fat solvent from said vapor and fractionally distilling the separated fat solvent to recover said absorbed toluene.

10. In a process for separating substantially pure toluene from a straight-run petroleum dis- .tillate having a relatively wide boiling range above and below 110.6 C., the steps or rractionally distilling said distillate to produce a concentrate containing more than 2% of toluene, said concentrate containing an amount of components boiling higher than about C. not greater than the content of toluene in said concentrate, and a content of components boiling higher than about C., being not greater than-the larger of the following: 5% or said toluene content and /3% of the. total concentrate, said concentrate having a true initial boiling point not highpoint 01' at least about 112' (7., contacting said coneentrate in'a vapor phase with a liquid solvent selectively dissolving aromatic hydrocarbons and boiling at a temperature substantially above 110.6 (2., to produce a residual hydrocarbon vapor and a fat solvent containing absorbed vs- ,879 er than about 100' 6.. and a true final boiling pors consisting essentially of toluene. separating said tat solvent from said vapor andiractionally distilling the separated i'st solvent to recover said absorbed toluene.

CLARENCE L. DUNN. ROBERT B. MOCONAUGHY.