US 3862254 A
Valuable aromatic hydrocarbons are produced from a hydrocarbon feedstock particularly rich in C8+ aromatics. The feedstock is initially fractionated into (a) a first stream consisting mainly of benzene, toluene and non-aromatics boiling in the same temperature range along with a minor proportion of C8 aromatics, (b) a second stream consisting mainly of C8 aromatics, and (c) a third stream consisting predominantly of C9 and C10 aromatics. Stream (a) is subjected to solvent extraction to obtain a non-aromatic raffinate and an aromatic extract, which latter is distilled to obtain as products benzene, toluene and a C8 aromatics fraction. Stream (b) may be utilized as such or extractively distilled to remove any nonaromatic hydrocarbon impurities which may be present. Stream (c) may, if desired, be separated into a C9 fraction and a C10 fraction with the former being hydrodealkylated to yield additional quantities of benzene, toluene and C8 aromatics.
Description (OCR text may contain errors)
Unite States Patent Eisenlohr et al.
[451 Jan. 21, 1975 PRODUCTION OF AROMATIC HYDROCARBONS Primary Examiner-Delbert E. Gantz Assistant ExaminerC. E. Spresser  Inventors: Karl Heinz E senlohr, Buchschlag, Attorney, Agent or Firm HarOld A. Hermann;
Germany Wmfned Grosshans Richard A Dannells Jr Barr Mo erman Marlton, N.J.; Toshiyuki Nakajima, y y Tok o, .la an 7 A A l (1 t (1 Ch l I  ABSTRACT s c., 3] sslgnees n: m s an -emlca n Valuable aromatic hydrocarbons are produced from a Philadelphia, Pa., Nippon hydrocarbon feedstock particularly rich in C aro- Petrochemicals Company Ltd., Th f d k U f d Tokyo, Japan; Metangesenschafl matics. e ee stoc 1s inltia y ractionate into (a) Akfiengeseneschafl Frankfurt/M a first stream consisting mainly of benzene, toluene German and non-aromatics boiling in the same temperature y range along with a minor proportion of C aromatics,  Filed: Oct. 16, 1970 (b) a second stream consisting mainly of C aromatics, and (c) a third stream consisting predominantly of C  Appl' 81566 and C aromatics. Stream (a) is subjected to solvent extraction to obtain a non-aromatic raffinate and an Cl 260/674 2 R aromatic extract, which latter is distilled to obtain as  Int. Cl. C07c 7/10 produ t benzene, toluene and a C aromatics frac-  Field Of Search 260/672 R, 674 SE tion, Stream (b) may be utilized as such or extractively distilled to remove any nonaromatic hydrocar-  References Cited bon impurities which may be present. Stream (c) may, UNITED STATES PATENTS if desired, be separated into a C fraction and a C fraction with the former being hydrodealkylated to 2,799,629 7/1957 Clough et al. 260/674 2,840,620 6/1958 Gerhold et a1 260/674 yield additional quantities of benzene, toluene and C 3,304,340 2/l967 Noll v 260/672 aromatics- 3,366,568 1/1968 Eisenlohr et al... 260/674 3,433,849 3/1969 Eisenlohr et al 260/674 4 Clams 1 D'awmg SOLVENT EXTRACTlON B-FRACTIONATION T- FRACTIONATION SPLITTER EXTRACTIVE DlSTILLATlON I Ca-AROMATICS DISTILLATION Ca-AROMATICS W FRACTIONATION HVDRODEALKYLATION c AROMATICS DISTILLATION PRODUCTION OF AROMATIC HYDROCARBONS BACKGROUND OF THE INVENTION a. Field of the Invention The invention relates to the production of aromatic 5 hydrocarbons from hydrocarbon feedstocks rich in aromatics. More particularly, it relates to fractionation of such mixtures prior to solvent extraction thereof.
b. Prior Art Heretofore, a number of solvent extraction processes have been proposed and put into practice for the recovery of aromatic hydrocarbons such as benzene, toluene and C aromatics from hydrocarbon mixtures containing such aromatic components. A typical example of such solvent extraction is the process in which an aromatics-containing feedstock is separated into raffinate and extract; the latter, being a stream essentially of pure aromatics, is then fractionated into benzene, toluene and C aromatics, respectively, while higher boiling aromatic hydrocarbon fractions (C may be, if desired, subjected to dealkylation to form benzene, toluene and C aromatics and recycled for the fractionation and purification.
Processes of the type just referred to as in conventional practice have the disadvantage that C aromatics, C and C aromatic hydrocarbons, in particular, are difficult to recover in commercially satisfactory yields without the use of extravagant quantities of solvents and thus without resort to increased operation and utility costs. Moreover, with these prior-art processes it has been quite difficult to obtain the desired C C and C aromatics at any higher yield than 95 percent, 70 percent and 50 percent, respectively, depending upon the feedstock composition even with the use of large quantities of solvent.
SUMMARY OF THE INVENTION The present invention owes its merits to the discovery, after exhaustive research of the relations between given classes of aromatic hydrocarbon compounds and solvents for the selective extraction thereof, that there may be obtained superior results from isolating C aromatics, C and C aromatics by fractionation from the starting material prior to charge to the extraction stage thereby making it possible to provide high yields of these aromatics fractions as well as substantial increases in the yields of benzene, toluene, and expecially C aromatics with a minimum of solvent and extractrecycle requirement.
Briefly summarized, the present invention comprises a process wherein a feedstock consisting of aromaticscontaining hydrocarbon mixtures is subjected to fractional distillation in a splitter without first charge to a liquid-liquid extraction stage and separated into a first fraction consisting of the major portions of benzene, toluene and non-aromatics boiling at the same range and a minor proportion of C aromatics, into a second fraction consisting of mainly C aromatics, and into a third fraction consisting predominantly of C and C aromatics and wherein the said first fraction is further subjected to liquid-liquid extraction, the extract thereby obtained being fractionated and purified into final benzene, toluene and C aromatics products. The said second fraction may be utilized per se as a material for suitable chemical processing or as end product C aromatics. Alternatively, it may be, if desired, extractively distilled to provide high-purity C aromatics. The
raffinate containing a minor proportion of C aromatics and non-aromatics is taken overhead from the extractive distillation column and may be recycled to the liquid-liquid extraction stage. The said third fraction may be dealkylated together or after it is separated into C and C aromatics fractions, which C aromatics may be converted suitably into benzene, toluene and C aromatics. The thus obtained dealkylates may be joined with the aromatics extract stream from the liquid-liquid extraction stage and charged to the benzene, toluene and C aromatics fractional distillation stages hereinafter described.
The main fractional distillation employed at the beginning of the process according to the invention for splitting the starting feedstock into three different fractions may be carried out in a variety of modes of operation depending upon the particular feedstock composition used. A typical embodiment involves subjecting the feedstock to fractional distillation in a splitter of known type where the major proportions of benzene, toluene, non-aromatics and a minor proportion of C aromatics are taken as a first fraction or stream from the top of the splitter for introduction as a charge to liquid-liquid extraction stage to recover benzene, toluene and C aromatics, and subjecting the bottoms product of the splitter consisting of the major proportions of C aromatics and C and C aromatics to distillation thereby obtaining C aromatics as a second stream from the overhead of the distillation column, said second stream consisting mainly of C aromatics and nonaromatics of the same boiling range, and C and C aromatics as a third stream from the bottom of the column.
Another embodiment consists in separating the feedstock by distillation into an overhead product consisting of benzene, toluene and C aromatics and a bottoms product as a third stream consisting predominantly of C and C aromatics, and in further separating said overhead product by distillation into a first stream (overhead product) consisting mainly of benzene, toluene and non-aromatics of the same boiling range and trace amounts of C aromatics and a second stream (bottoms product) predominantly of C aromatics.
Another embodiment comprises separating the feedstock by distillation under selected conditions to obtain the first, second and third streams, respectively, from the top, middle and bottom of a single distillation column.
Any combination of the foregoing embodiments relative to the main fractionation of a feedstock may be chosen in accordance with a specific feed composition, and each fractional distillate may be adjusted in composition and flow rate in a manner desired to effectively accomplish the subsequent unit processing.
Accordingly, it is an object of the present invention to eliminate the above-noted difficulties of the prior-art processes.
Another object of the invention is to provide a new, useful process for the separation and purification of aromatic hydrocarbons which can be carried out with desirably reduced solvent ratios related to total starting material and in greater yields than anticipated by the conventional processes.
Still another object of the invention is to provide a new, useful process capable of recovering increased yields particularly of C C and C aromatic hydrocarbons in addition to benzene and toluene.
These and other objects and features of the invention will be apparent to those skilled in the art from the following description and claims taken in connection with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING The drawing is a diagrammatic illustration of an embodiment of the invention and includes modifications thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the invention will now be discussed in further detail with reference to the accompanying drawing which is utilized to particularly explain that mode of operation which employs the combination of a splitter and a C aromatics distillation column. The other processing techniques referred to hereinabove may, of course, be employed with similar results.
The aromatics-rich feedstock suitable for the invention may be hydrocarbon mixtures available typically from catalytic reformates, hydrogenated pyrolysis gasoline, crude benzole and mixtures thereof. It is to be noted, however, that the invention proves its outstanding advantage over any prior-art processes in that a starting feedstock containing over percent of C aromatics can be processed with success.
The reference numeral 1 designates a feed line for charging the starting material to a splitter 2, which is operated under selected temperature conditions to produce trace amounts of C aromatics in the overhead with the major portions of C aromatics in the bottom. Such temperature conditions will be shown in the Example which appears hereinafter. From the top of this splitter column are taken as overhead via line 3 benzene, toluene and non-aromatics of the same boiling range, as well as a minor proportion of C aromatics. The major proportions of C C and C aromatics and non-aromatics are withdrawn via line 16 from the bottom of the splitter 2.
Now, the overhead stream from the splitter 2 is charged, via line 3, to a solvent extraction unit 4 where it is treated in well-known manner and separated into benzene, toluene, C aromatics and nonaromatics. More specifically, this unit 4 consists of a multi-stage counter-current extractor, an extract-recycle column, a solvent stripper and auxiliary equipment. Preferably, the stream fed via line 3 is charged to the middle section of the extractor and a solvent enters the top thereof so that the solvent descends in counter-current contact with the ascending aromatics-rich hydrocarbon.
In the extraction unit 4, the non-aromatic fraction is withdrawn as a raffinate via line 5 and the aromatics extract is charged via line 6 to an aromatics fractionation system. Since the feed to be charged to the extraction unit contains, in addition to benzene and toluene, a minor part of C aromatics but no C or C aromatics, the selectivity of the solvent is greatly enhanced.
The solvents for use in liquid-liquid extraction according to the invention include N-methylpyrrolidone, N-hydroxyethylpyrrolidone, butyrolactone, ethylene or diethylene glycol, dimethylformamide, sulfolane, dimethylsulfoxide and the like or mixtures thereof. These solvents may be used solely or in mixtures thereof, or may be diluted with water or organic solvents with a low aromatics capacity to improve the selectivity of those mixtures and preferably have a higher boiling point than the aromatics to be recovered.
As already stated, the overhead product of the splitter 2 is substantially free of C C and other higher boiling materials including only trace amounts of C aromatics, which results in reducing the amount of solvent required for the extraction of benzene, toluene and C aromatics. This, together with reduced ratios of solvent to feed, is another important aspect of the invention, as will be noted specifically in the example hereinafter given.
The aromatics distillates may be treated with clay, in a known manner, to remove the olefins and impurities which may be present. Thereafter they are forwarded through line 6 to the aromatics fractionation system. At the same time, a product stream from the dealkylation unit may be supplied to this system via line 26.
The aromatics fractionation system consists of a benzene distillation column 7, a toluene distillation column 10 and a C aromatics distillation column 13. The aro matics extract is charged through line 6 to the benzene distillation column 7 wherein pure benzene is separated and taken as liquid a few trays below the top tray. The bottoms stream consisting of toluene and minor parts of C aromatics is introduced, via line 9, to the toluene distillation column 10. Pure toluene is drawn off as liquid a few trays below the top tray of column 10, via line 11. The bottoms, consisting of pure C aromatics, are withdrawn via line 12. Unlike the conventional processes, the charge to the toluene column 10 is very low in C aromatics content with the result that toluene is easily and effectively separated from C aromatics.
The bottoms product from the toluene distillation column 10 consists essentially of pure C aromatics and hence may be utilized, per se, as a chemical processing material or as an end-product. However, it may be, if desired, further processed in a first C aromatics distillation column 13, from the top of which column, via line 14, there may be obtained pure C aromatics. The bottoms of this column 13, withdrawn from line 15, contains small amounts of higher boiling impurities produced from the dealkylation unit. The C aromatics in line 12 may be alternatively sent via line 31 to a second C aromatics distillation column 17 later described. It will be appreciated that ultimate purity of the desired aromatic hydrocarbons may be improved by recycling some slip streams from the benzene and toluene column tops to the solvent extraction unit 4. Yields of the various aromatics recovered therein are illustrated in relation to starting feed and compared with those obtainable by prior-art in the following example.
The bottoms product of the splitter 2 is withdrawn, via line 16, and charged to a second C aromatics distillation column 17 where C aromatics are separated and taken overhead via line 18. C and C aromatics are withdrawn as bottoms via line 22. Preferred temperature conditions for this column are about -150C. The C aromatics taken from the top of the column 17 may be of a quality suitable for immediate use as a chemical processing material or as an end-product. However, where utmost purity is desired, the C aromatics may be clay-treated and forwarded to an extractive distillation unit 19 to remove objectionable nonaromatic hydrocarbon impurities.
More specifically, the C aromatics fraction is directed substantially to the lower part of the extractive distillation unit or column 19, while recycled solvent is fed to the top thereof. The column 19 is operated under selected conditions to allow a mixture containing the non-aromatics in the C aromatics fraction and small amounts of C aromatics to pass in vapor phase out through the overhead of the column. The bottoms from this column consist essentially of C aromatics and solvent and are sent to a C aromatics stripper (not shown) where pure C aromatics and solvent are withdrawn from the top and bottom, respectively; the solvent being further recycled to the extractive distillation unit 19.
The extractive distillation may be conducted preferably with a polar solvent boiling above the C aromatics and especially one forming no azeotropes with the hydrocarbons contained in the C aromatics. Suitable solvents for this purpose are N-methylpyrrolidone, sulfolane and polyethylene glycol. These solvents may be advantageously used for both extractive distillation and liquid-liquid extraction. Such solvents as may form azeotropes with the hydrocarbons in the C aromatics stream may also be used if there is a suitable solvent recovery unit provided. This extractive distillation gives as another overhead product a raffinate consisting of the non-aromatics contained in the C aromatics which may be withdrawn, via line 21, and charged together with the stream in line 3 to the extraction unit 4 for recovery of the small amounts of C aromatics contained therein. The C aromatics is thus purified to less than 500 ppm. of non-aromatics content and is withdrawn via line 20.
Where the liquid-liquid extraction and the extractivedistillation are combined with a hydrodealkylation process, the C aromatics stream withdrawn from the bottom of the toluene distillation column 10 may be charged via lines 31 and 16 to the second C aromatics distillation column 17 to separate high boiling materials formed during the hydrodealkylation later described. Alternatively, these bottoms may be treated in the first C aromatics column 13. The C and C aromatics fraction which has been withdrawn from the bottom of the second C aromatics column and which contains small amounts of non-aromatics, may be, if desired, charged via line 22 directly to the hydrodealkylation unit 25 for conversion to benzene, toluene and C aromatics, and thereafter passed from line 26 to line 6 and thence into the aromatics fractionation system. For better results, the C aromatics fraction may be separated from the C aromatics prior to such hydrodealkylation. In which instance, the bottoms stream from the second C aromatics column 17 is forwarded via line 22 to a C aromatics distillation column 23 therein to separate and remove C aromatics and to take C aromatics overhead for introduction through line 24 to the unit 25. This hydrodealkylation process is well known and conducted in the presence of hydrogen and catalyst or thermally in the absence of catalyst.
Typically, the catalytic hydrodealkylation is performed preferably at about 500C800C, under -76 Kglcm g. and in the presence of hydrogen and known catalysts such as chrome-alumina, nickel, cobalt oxide and molybdenum oxide. These operating conditions are such that small amounts of C and C paraffins undergo decomposition and turn into low molecular weight gases, while C and C aromatics become progressively demethylated and converted to benzene, toluene and C aromatics. The thermal dealkylation takes place under similar or somewhat higher temperature and pressure conditions and in the presence of hydrogen, but without catalysts.
The following example is illustrative of operation of the process in accordance with the invention, with the flow rates given in metric tons/hr. (t/hr).
The splitter 2 contained 50 plates and was operated at a top temperature of C and a bottom temperature of C and under a pressure of 0.6 kglcm G. 50 percent by weight of N-methylpyrrolidone and 50 percent by weight of monoethylene glycol were blended and used as a solvent in the extraction unit 4, which comprised a 24-tray multi-stage extractor and which was operated at about 60C.
The feed to splitter 2, which entered through line 1, was a catalytic reformate having the following composition and flow rate.
The flow of solvent and solvent to feed ratio required in the extraction unit 4 according to the invention were compared with those by prior-art.
TABLE III Extract Feed Solvent Recycle Solvent] Charge Rate Rate Feed (t/hr) (t/hr) (t/hr) Ratio Invention 56.0 35 3.5 Prior-Art 93.0 392 78 4.2
It is thus obvious that the process of the invention can be carried out with far less solvent and less extract recycle than required by the prior-art process. The liquidliquid extraction according to the invention was conducted at a solvent to feedstock (line 1) ratio of 2.1 :1 and an extract recycle to feed ratio of 0.4:1.
Line stream compositions provided from the second C aromatics distillation column 17 were:
- TABLE IV noted that the process increases the yields of valuable aromatic hydrocarbons (e.g., C C and C compared iii Line 22 to the prior art at a saving in solvent used as reflected, Content Flow Content Flow for example, by reduced solvent to feed ratios in excomponem (mm mm) traction unit 4. Further, the same solvent was used for (38 aromatics 99, 59 both extraction and extractive distillation, further ene 9 3 105 hancing the economic viability of the instant process.
aromatlcs 3.7 0.4 nomammatics Q8 H 02 While the invention has been described and illus- Total 100.0 26.1 100.0 10.9 m trated as to specific embodiments thereof, it will be understood that various changes and modifications may be made, which would be obvious to those skilled in the In the distillation column 17, C aromatics were sepa-- without dfaparting from the Spirit or Scope of the rated from C and C aromatics. Line stream 18, conappends? Clalmstaining the majority of C aromatics, was charged to the 5 We Clalml extractive distillation unit 19, which was equipped with In a Process for the Separation and recovery of 60 trays and which was operated at top temperature of benzene, toluene and s aromatic hydrocarbons from 00 bottom temperature f 1 top pressure f an aromatic hydrocarbon-rich charge stock selected 0.3 atmospheric absolute and bottom pressure of 0.6 from the g p consisting of catalytic reform-ates, yatmospheric absolute, d i h presence f N. 20 drogenated pyrolysis gasoline, crude benzole and mixh l lid Solvent, h l t to f d ratio tures thereof, which includes the steps of fractionating being 3.111 and the flow of solvent being 80 t/hr. and solvent extraction, wherein the improvement, The non-aromatics obtained as raffinate from the exw ereby enhanced yields particularly of C aromatic tractive distillation were withdrawn via line 21 and hydrocarbons are obtained comprisess charged together with the stream in line 3 at a feed rate 25 a. fractionating said aromatic hydrocarbon-rich of 0.4 t/hr. of C aromatics and of 0.2 t/hr. of noncharge stock to separately obtain aromatics to the extraction stage. The C aromatics i. a first fraction containing substantially all of the thus purified were withdrawn via line 20. benzene, toluene and non aromatic hydrocar- Table V below is provided to show the yield of each bons boiling in the same range and a minor of the aromatics per feed in each strategic point of the amount of C aromatic hydrocarbons, process according to the invention in comparison with ii. a second fraction containing substantially all of one prior-art. the C aromatic hydrocarbons and non-aromatic TABLE V invention Prior-Art Line 6 Line 20 Line 22 Line 6+20+22 Aromatics Extracts Component (A) (B) (A) (B) (A) (B) (A) (B) (A) (B) Benzene 100 3 .5 100 3.8 99.5 3.6 Toluene 99.5 23.6 99.5 23.6 98.0 23.2 c, aromatics 95.0 2.9 921.5 25 s 99.6 224.4 95.0 27.0 cu om t 100 10.5 100 10.5 40.0 4.2 C, aromatics 100 0.4 100 (1.4 0 0 Total Recovery 86.5 58.0 Flow of solvent": 275( t/hr) 392(t/hr) Solvent/feed ratio: 2.96 42 Note: (A) and (B) ahovc denolu. re pcctituly. yield 1)i du\ig|1atcd aromatics by weight purccnl and rate of ruL'nVcr) thcruol' in tons pur hour *Tltc llo ol MiiVL'lll according to the inwntion includes lhc amount of solve-n1 supphcd lo thu extractive distillation unit.
The C aromatics fraction was processed in the hydrodealkylation unit 25, which was operated at an average reaction temperature of 560C and a pressure of 60 kglcm G (865 psig), With a hydrogen to feed ratio of 11:1 and in the presence of chrome-alumina catalyst, thus hydrodealkylated and withdrawn via line 26 at the feed rate of 9.0 t/hr. and combined with the stream in line 6 to go into the aromatics fractionation system and thereby to add to the total yields of benzene, toluene and C aromatics.
All aromatics shown in Table V as present in line 6 were fractionated in their respective fractionation columns and recovered as pure benzene, toluene and C aromatics respectively, each substantially in 100 percent yield. The benzene was noted to have a crystallization point of 552C. The toluene and C aromatics were 99.5 percent and 99.0 percent, respectively, by gas-chromatography.
From the above exemplary embodiments it should be hydrocarbons boiling in the same range, and
iii. a third fraction containing substantially all of the C and C aromatic hydrocarbons and other hydrocarbons boiling above the second fraction;
b. subjecting the first fraction to liquid-liquid solvent extraction and recovering an aromatic extract;
c. hydrodealkylating the third fraction to yield a product stream comprising benzene, toluene and C aromatic hydrocarbons;
d. combining said product stream of step (c) with said aromatic extract of step (b);
e. fractionating the combined stream of step (d) to separate and recover substantially pure benzene, toluene, and C aromatic hydrocarbons; and
f. extractively distilling the second fraction of step (a) to separate and recover substantially pure C aromatic hydrocarbons;
whereby C aromatic hydrocarbons are recovered in an amount in excess of the amount of C aromatic hydrotillation column operated at a top temperature of about C. and a bottom temperature of about C. under 0.3 atmospheric absolute at the top and 0.6 atmospheric absolute at the bottom.
4. The process as in claim 1 wherein said third fraction is hydrodealkylated in step (c) at temperatures in the range of 500C. 800C, under pressures in the range of 35-76 kglcm G, and in the presence of hydrogen and chrome-alumina catalyst.