US2834822A

US2834822A - Toluene

Info

Publication number: US2834822A
Authority: US
Publication date: 1958-05-13
Anticipated expiration: 1975-05-13

Description

Filed Aug. Vsa, 1954 KTTORNEYS R# S. WORTHINGTON ET AL DISTILLATION PROCESS FOR RECOVERING AROMATICS m3 mvaHN-IZNBQ Nl slNvNlwvlNoo May 13, 1958 2,834,822 Patented May 13, 1958 rl`his invention relates to an etta'ctive distillation process for recovering Vsubstantially pure aromatic compounds from a hydrocarbon distillate fraction containing the desired aromatic compound along with other 'aromatic and nonaromatic constituents. In more particular aspect, the

. process is one whereby benzene is recovered from a benzene concentrate obtained from a catalytic reformate, which concentrate contains 'the benzene along with a minor percentage of toluene and a large proportion of nona'roma'tics associated with the aromatic components by `reason of boiling point relationships.

The established specifications for substantially lpure (nitration grade) aromatic compounds 'suchas benzene, toluene and Xylene are such as to permit the presence of only trace amounts of other aromatic compounds than the desired prodct (as well as of nonaromatic contami nants) if the product is to meet `the required distillation range specifications. This is borne out by the fact that the presence of as little as 0.1% of the next higher aromatic, in the case of either benzene ort'oluene, is suilicient to bring the boiling range to the limit 'of the speciiication which calls for a distillation range of l C., initial to dry. Since the distillate fractions initially obtained following various petroleum refining and other operations which are productive of aromatic Yproducts normally contain substantial amounts of more than one aromatic compound (along with various nona'romatic material), the practice which is now followed by a large segment ofthe industry is to redistill such fraction under rigorous conditions whereby a fraction is obtained which contains the desired aromatic compound but is free of other aromatic contaminants. Thus, in the case of the benzene-containing fraction recovered from a reforming operation, which fraction contains a substantial amount of toluene -along with associated nonaromatic components, vthe-practice is to subject the material initially recovered to one or more preliminary fractional distillation steps designed to provide a benzene'heart cut which is substantially'free (less than 0.1% by volume in terms of the benzene content) of any toluene, and to thereafter subject this concentrate to an extractive distillation treatment whereby the contaminating nonaromatic components are removed overhead. The bottoms, which contain the benzene along with the eXtractive solvent, are then distilled so as to recover substantially pure benzene.

The foregoing method has the disadvantage that a heart cut which is free of contaminating aromatic components can only be obtained by a practice of the most exacting techniques involving the use of high redux ratios and distillationl columns containing as many as 65 to 80 or more trays. The necessity for obtaining a distillate of this character, which contains but a single aromatic compound, greatly adds to the cost of the recovery procedure, and it would be desirable if a method embodying all of 4the advantages of the previously practiced recovery processes were available which, while yielding aromatics of the desired purity, did not require' a complete separation of aromatic contaminants from the feed to the extractive distillation zone and thus could be practiced with less A elaborate and more economical preliminary distillation means. p

Accordingly, it is a primary object of this invention to provide a method for recovering pure benzene, toluene or C8 aromatics from a reformate or other distillate wherein the fraction supplied to the extractive distillation column is one which, in addition to the desired aromatic compound (or compounds, in the case of C8 aromatics) and associated nonaromatic compounds, also contains an appreciable amount of one or more higher boiling. aromatic compounds. In this connection, the term pure, or substantially pure, is employed to designate aromatics which are at least 99% pure, by volume, and which contain not more than 0.1% by volumeof aromatic contaminants. A more particular object is to supply a method of this character for producing pure benzene from a reformate containing appreciable amounts of both benzene and toluene, in which method the severity of the distillation procedures employed in any prefractionation step may, if desired, be reduced (as by lowering the number of trays 'in the column and/ or by .reducing the redux ratio, for example) since the benzene concentrate supplied to the extractive distillation column is one which contains an appreciable amount of toluene. A still further object is to provide a method whereby the pure benzene or other desired aromatic compound is recovered directly as an overhead from .a vsolventstripping column to Awhich the bottoms from the extractive distillation column are supplied.

The present invention is 'based upon the .discovery that the foregoing objects are achieved, with pure benzene, toluene or C8 aromatics (including Xylencs and ethylbenzene) being'readily produced, by a rproceure whereby the hydrocarbon feed stock, after being subjectedto a catalytic Vconversion processproductive of a reaction mixture containing va desired aromatic compound, or conipounds, along'with one or more higher boiling aromatic compounds and various 'nonaromatic components, is fractionally distilled to produce a concentrate, or heart cut, which includes substantially all ofthe desired aromatic compound'together with a total of from about 0.2 to 5% by volume, and preferably from 0.5 to 2.5%, of at least one aromatic compound boiling above the desired product, said percentages being in terms of the content of the latter product. With benzene, thev higher boilingaromatic contaminant will normally be toluene, whereas when toluene is the desired product, the contaminant may be Xylenes and ethylbenzene. When one or more of the latter compounds are the desired product, the contaminant will usually bea C9 aromatic such as cuniene. .Following the preparation of the concentrate, the latter is introduced into a `distillation column to which a higher boiling extractive solvent (e. g., phenol) is also supplied, the conditions within this extractive distillation .column being suchthat the lower boiling nonaromatic components of the feed concentrate are taken overhead, while the aromatic components of the feed, along with any remaining higher boiling nonaromatic components, are withdrawn as bottoms. Said bottoms arethen passed to a solvent stripping column wherein it has been found that the desired aromatic component can readily betalten overhead in the desired Ydegree of purity. The nbottomsfrom the solvent stripping column arethen `recycled back to the extractive distillation column,with'at least aportion of this recycle stream preferably being diverted to a second stripping column (operated at somewhat higher temperatures than the solvent stripping column) kwhere thehi'gher boiling aromatic contaminant (normally toluene-in the case where benzene is theproduct lfaltenoverhcadinthe solvent 'stripping column) is Jtaken'overhead along with any higher boiling nonaromatic component remaining in the recycle stream. if desired, this second stripping column may be operated either continuously or intermittently at still higher temperatures so that the column acts as a clean-up still for the removal of accumulated light and heavy impurities in the solvent. The extractive solvent is recovered as bottoms from this stripping column and is returned to the extractive distillation zone along with the balance of the recycle stream,

While the process has 'been described above in its broader aspects, the invention is especially applicable to the preparation of individual pure aromatic compounds, such as benzene, from catalytic reformate fractions, particularly Platformates derived lby the catalytic reforming of naphtha fractions over a platinum-alumina reforming catalyst. In this process, as it applies to the production of benzene, a petroleum naphtha fraction consisting essentially of a mixture of naphthenic, paraftinic and aromatic hydrocarbons having from six to seven carbon.

volume of toluene, -based on volume of benzene in the n concentrate. IFollowing the preparation of the concentrate, the latter is extractively distilled in the presence of a suitable solvent (e. g., phenol) under such conditions that the relatively lower boiling nonaromatic components are separated as an overhead product, While all of the 'benzene and toluene, along with the higher boiling nonaromatic portion of the concentrate, is withdrawn as bottoms. Thereafter, the bottoms are passed to a distillation zone maintained at an elevated temperature adapted to distill benzene of the desired purity overhead Without vaporizing the other components of lthe feed, while the bottoms, which includes the selective solvent, the toluene and a minor percentage of lhigher boiling nonaromatics, is recycled to the extractive distillation column for 'ad-mixture with the incoming benzene concentrate. Preferably, a portion of said recycle stream, as indicated above, is passed to a distillation zone maintained at a suitable temperature to separate the toluene and said nonaromatic portion overhead, while the extractive solvent is recycled back to the extractive distillation zone.

While phenol has been given above as an example of an extractive solvent, and is, in fact, the preferred solvent for use in this invention when recovering benzene or toluene, a wide variety of other solvents can be employed. In general, the solvent selected should have a boiling point which is Well above the end point of the stock being treated and which can thus be readily separated from the Idissolved components. Thus, suitable solvents which can be employed in a practice of this invention are: phenol, aniline, cresylic acids, alkyl phenols, various polyglycols including di-, triand tetra-ethylene glycol, carbitols, diethanolamine, nitrobenzene, quinoline, furfural alcohol; 'furfu'rah monoglycerol ethers, resorcinol, xylidine, acetone and acetone-Water mixtures.

The mode of practice of the invention with reference to several embodiments and the various features and advantages thereof will be apparent from the following detailed description with reference to the appended drawing wherein the iigure represents a flow diagram of a process for recovering pure benzene from a petroleum naphtha feed stock. In the ligure, a feed mixture comprising a petroleum naphtha fraction consisting primarily of a mixture of naphthenic, parainic and aromatic hydrocarbons having from six to seven carbon atoms in the 'molecule -is passed through line 10 in` the vapor phase into contact with a reforming catalyst in reforming zone 11 under elevated conditions Aof temperature and pressure suitable for effecting the desired conversion of the C6 napthenes to benzene and the C, naphthenes to toluene, and to largely isomerize the straight chain paraiinic hydrocarbons present to branched chain compounds. From the reforming zone the resulting reformate passes through line 12 into a stabilizer column 13 provided with a suitable reboiler 14, from which column the lighter fractions comprising predominantly C3-C4 hydrocarbons are taken overhead through line 15 and condenser 16, with a portion of said lighter hydrocarbons being sent to product recovery through line 17, while the balance is returned as reux to the column through line 18.

The bottoms from stabilizer column 13 are withdrawn through 'line 19 and passed into a topping still 20 provided with a reboiler 21. lFrom this still a lighter fraction comprising essentially C and C8 hydrocarbons boiling below hexane is taken overhead through line 22 and condenser 23, with a portion of the condensate being sent to product recovery through line 24, while the balance is returned to the column as reflux through line 25.

The bottoms from the topping still are withdrawn through line 26 and charged into a mid-portion of a rerun column 30 provided with a reboiler 31. From this column a benzene heart cut 'fraction is taken overhead through line 32 and condenser 33, which fraction contains substantially all of the benzene supplied to the column, as well as toluene in an amount equal to from about 0.2 to 5% by volume, based on the benzene content of the fraction, along with a substantial proportion of nonaromatic components. Of the resulting condensate, a portion is returned to the column as reflux through line 34, while the balance of the stream is passed through line 35 into a mid-point of an extractive distillation column 36 having a reboiler 3S. The balance of the toluene introduced into rerun column 30 through line 26 is removed from said column through line 37, along with nonaromatic hydrocarbons boiling above benzene, and is sent to a suitable toluene recovery system (not shown).

ln a representative run, the benzene concentrate in line 35 contains 28.7% benzene, 1.3% toluene and 70% nonaromatics, and -is supplied to the extractiva distillation column 36 at a rate of 100 volumes per day, with phenol being introduced to the column through line at a rate of 500 volumes per day. The temperature in column 36 is so regulated that the Ihydrocarbons boiling below benzene are taken overhead through line 39 and condenser 40 at a rate of 380 volumes per day, with 310 volumes per day being returned to the column as reflux through line 42, while a rainate stream amounting to volumes per day is sent to a suitable recovery system (not shown) through line 41, said raffinate (at a still head temperature of 178 F. and under the described reflux conditions) consisting of 96.4% nonaromatics and lbut 3.6% benzene.

The bottoms from the column 36, which contain the desired benzene along with associated higher boiling aromatic (toluene) and nonaromatic contaminants, as well as the solvent, are supplied to a mid-point of a solvent stripper column 44 provided with a reboiler 45. From the solvent stripper column 44 (when operated, for example, at a still head temperature of 183 F. and at a reux ratio of about 6 to l), a benzene stream consisting of 99.2% benzene and 0.8% uonarornatics is taken overhead through line 46 and condenser 47 at a rate of 186 volumes per day of which 26 volumes is discharged through line 4S, while the balance is returned to the column as retlux through line 49. The stream in line 43 may, if desired, bc passed to a treating zone where the benzene is subjected to the conventional nishing treatments required for the production of a product of nitration grade.

The bottoms from solvent stripper 4d, which contain the extractive solvent, along with a minor percentage of toluene and of various contaminants, are withdrawn through line 50 and recycled back to the extractive distillation column 36. A slip stream from said recycle stream is withdrawn through line 51 and is introduced into toluene stripping column 52, provided with reboiler 53, at a rate of approximately 50 volumes per day, the relative amount of said slip stream, if any, being regulated by the setting of valves 54 and 55. From the column 52, operated at an overhead temperature of 235 F. and at a reflux ratio of approximately 16:1, an overhead stream containing toluene and other heavier contaminants present in the benzene heart cut is taken through line 56 and condenser 57 at a rate of 16 volumes per day. A toluene reject stream made up of 59.3% toluene, 6.3% benzene and 34.4% of nonaromatics is withdrawn through line 58, while the balance of the overhead from column 52 is returned to the column as reflux through line 59. The

bottoms from column 52, made up substantially of the extractive solvent, are withdrawn through line 60 for return to column 36 through line 50.

In the foregoing description, the various percentages given are on a volume basis. Further, in the flow diagram various of the required pumps, heat exchangers, valves and other items of flow control equipment have been omitted in the interests of simplicity and clarity of expression, the placement of such auxiliary equipment being evident to those skilled in the art.

While the character of this invention has been described in detail and numerous illustrative examples given, this has been done by way of illustration only and with thc intention that no limitation should be imposed upon the invention thereby. It will be apparent to those skilled in the art that numerous modications and variations may be effected in the practice of this invention which is of the scope of the claims appended hereto.

We claim:

l. In a process for recovering substantially pure benzene from a catalytic reformate containing benzene, toluene and nonaromatic components, the steps comprising fractionally distilling said reformate to produce a concentrate containing substantially all the benzene content of said reformate, along with nonaromatic components and from about 0.2 to 5% toluene, based on the volume of benzene present in said concentrate; passing said concentrate to an extractive distillation zone, and there contacting the concentrate with a selective solvent at elevated temperatures whereby the relatively low boiling non-aromatic components are separated as an overhead product, while withdrawing a bottoms product containing substantially all the benzene and toluene content of the concentrate, along with solvent; passing said bottoms product through a first stripping zone maintained at a temperature adapted to distill the benzene content thereof overhead as a pure benzene stream, while withdrawing a bottoms product containing solvent, toluene and any higher boiling nonaromatic components; passing at least a portion of said latter bottoms product to a second stripping zone maintained at a temperature sufficient to distill selected from the group consisting of benzene, toluene and a mixture of C8 aromatic hydrocarbons from a catalytic conversion reaction mixture containing the desired product, along with nonaromatic hydrocarbons and at least one higher boiling aromatic compound, the steps f' comprising fractionally distilling said reaction mixture to produce a concentrate containing substantially all of the content of the desired aromatic product in said mixture, along with nonaromatic components and from 0.2 to 5%, in terms of the volume of the desired aromatic product, of at least one higher boiling aromatic component; passing said concentrate to an extractive distillation zone, and there contacting the concentrate with a selective solvent at elevated temperatures whereby any relatively low boiling nonaromatic components are separated as an overhead product, while withdrawing a bottoms product containing substantially all the aromatic content of the concentrate, along with solvent and any higher boiling nonaromatic components; passing said bottoms product to a first stripping zone maintained at a temperature adapted to distill overhead the desired pure aromatic compound selected from the group consisting of benzene, toluene and a mixture of C8 aromatic hydrocarbons, while withdrawing a bottoms product containing solvent, a higher boiling aromatic compound than that taken overhead and any higher boiling nonaromatic components present in the concentrate; passing at least a portion of said latter bottoms product to a second stripping zone maintained at a temperature suflicient to distill overhead said higher boiling aromatic and nonaromatic components in the feed to said zone, while withdrawing a solvent-containing bottoms product; and recycling to the extractive distillation Zone said last-mentioned bottoms product, along with any bottoms from the rst stripping zone not passed to the second stripping zone.

4. The process of claim 3 wherein the solvent employed in the extractive distillation zone is phenol.

References Cited in the le` of this patent UNITED STATES PATENTS 2,325,379 Durrum July 27, 1943 2,366,570 Souders et al. Jan. 2, 1945 2,727,077 Fragen et al. Dec. 13, 1955 OTHER REFERENCES Haensel: Gas-Oil Journal, August 9, 1951, pages 80, 82, 84, 88 and 101.

Claims

1. IN A PROCESS FOR RECOVERING SUBSTANTIALLY PURE BENZENE FROM A CATALYTIC REFORMATE CONTAINING BENZENE, TOLUENE AND NONAROMATIC COMPONENTS, THE STEPS COMPRISING FRACTIONALLY DISTILLING SAID REFORMATE TO PRODUCE A CONCENTRATE CONTAINING SUBSTANTIALLY ALL THE BENZENE CONTENT OF SAID REFORMATE, ALONG WITH NONAROMATIC COMPONENTS AND FROM ABOUT 0.2 TO 5% TOLUENE, BASED ON THE VOLUME OF BENZENE PRESENT IN SAID CONCENTRATE; PASSING SAID CONCENTRATE TO AN EXTRACTIVE DISTILLATION ZONE, AND THERE CONTACTING THE CONCENTRATE WITH A SELECTIVE SOLVENT AT ELEVATED TEMPERATURES WHEREBY THE RELATIVELY LOW BOILING NON-AROMATIC COMPONENTS ARE SEPARATED AS AN OVERHEAD PRODUCT, WHILE WITHDRAWING A BOTTOMS PRODUCT CONTAINING SUBSTANTIALLY ALL THE BENZENE AND TOLUENE CONTENT OF THE CONCENTRATE, ALONG WITH SOLVENT; PASSING SAID BOTTOMS PRODUCT THROUGH A FIRST STRIPPING ZONE MAINTAINED AT A TEMPERATURE ADAPTED TO DISTILL THE BENZENE CONTENT THEREOF OVERHEAD AS A PURE BENZENE STREAM, WHILE WITHDRAWING A BOTTOMS PRODUCT CONTAINING SOLVENT, TOLUENE AND ANY HIGHER BOILING NONAROMATIC COMPONENTS; PASSING AT LEAST A PORTION OF SAID LATTER BOTTOMS PRODUCT TO A SECOND STRIPPING ZONE MAINTAINED AT A TEMPERATURE SUFFICIENT TO DISTILL TOLUENE AND ANY NONAROMATIC COMPONENTS OVERHEAD WHILE WITHDRAWING A SOLVENT-CONTAINING BOTTOMS PRODUCT; AND RECYCLING TO THE EXTRACTIVE DISTILLATION ZONE SAID LAST MENTIONED BOTTOMS PRODUCT, ALONG WITH ANY BOTTOMS FROM THE FIRST STRIPPING ZONE NOT PASSED TO THE SECOND STRIPPING ZONE.

2. THE PROCESS OF CLAIM 1 WHEREIN THE SELECTIVE SOLVENT EMPLOYED IN THE EXTRACTIVE DISTILLATION ZONE IS PHENOL.