US3584068A - Process for c8 aromatic feed fractionation - Google Patents

Abstract

D R A W I N G
MULTIPLE SEPARATION IN A SINGLE FRACTIONATION ZONE OF C8 AROMATICS INTO COMPONENT ETHYLBENZENE, O-XYLENE AND M-AND/OR P-XYLENE UTILIZING THE SAME HEAT FLOW. THE FEEDS TO THE FRACTIONATION ZONE COMPRISE A FIRST FEED RELATIVELY RICH IN ETHYLBENZENE AT A FIRST INTERMEDIATE ZONE LEVEL AND A SECOND FEED RELATIVELY RICH IN O-XYLENE BELOW THE FIRST FEED LEVEL. BOTH FEEDS MAY BE OBTAINED FROM AVAILABLE PROCESS STREAMS OR BE FORMED FROM A SINGLE FEED BY PRELIMINARILY FRACTIONATING THE FEED IN ADVANCE OF THE COMPLEX FRACTIONATION. IN EITHER CASE THE FEED MAY BE DEPLETED OF MXYLENE PRIOR TO FRACTIONATION BY CHEMICALLY COMPLEXING THE M-XYLENE INTO A SEPARABLE FORM E.G. WITH HF-BF3. THE OVERHEAD STREAM, A PREDOMINANTLY O-XYLENE BOTTOMS STREAM AND A SIDESTREAM PREDOMINANTLY OF M-XYLENE AND/OR P-XYLENE. THE SIDESTREAM MAY BE PROCESSED TO SEPARATE P-XYLENE AS A PRODUCT OF THE PROCESS, AND/OR TO CONVERT UNWANTED ISOMERS TO DESIRED ISOMERS, WITH SUCH CONVERTER EFFLUENT BEING RETURNED DIRECTLY TO THE FRACTIONATION ZONE, OR TO THE FEED, FOR FRACTIONATION IN COMMON WITH THE FRESH FEEDS.

Description

PROCESS FOR Ca AROMATIC FEED FRACTIONATION Filed March 23, -19'70 S. B. JACKSON EVAL June 8, y1971 4 Sheets-Sheet 1 I l J A/Evs/ ENDS?- wf i@ N 4m f. j lllllllllli'l M N ,zw/, D U MW wm m y@ m mw E 7\/ lllll 2J aw/ MM im 2. wl? (4 L a 0 ,/L e p y D 0 ,n f. F

s u W rse N 3M Mw I.. W BH E e fr@ E G i zw s m Lm E Ep i w lw p N 4N aww @E j r 1 1mm. X/ 9/ m-aw TQ n am e 7 5 a J ,Zwmf FPJ f /.C L Hipaa. .w @5 p q June 8, 1971 s. B. .JAcKsoN EVAL 3,584,068

PROCESS FOR C8 AROMATIC FEED FRACTIONATION Filed March 23. 1970 4 Sheets-Sheet z T 70m/Eux June 8, 19"/1 s, B JACKSON EIAL 3,584,068

PROCESS FOR C8 AROMATIC FEED FRACTIONATION June 8, 1971 s. B. JACKSON ETAL 3,584,068

PRocEss FOR of', ARoMATxc FEED FRACTIONATION 4 Sheets-Sheet 4 Filed March 23, 1970 IN VEA/Tow. STE vE/v B J2? c/sa/v .R 0G52 Maen# @from/E915.

3,584,068 Patented June 8, 1971 U.S. Cl. 260-668 77 Claims ABSTRACT OF THE DISCLOSURE Multiple separation in a single fractionation zone of C8 aromatics into component ethylbenzene, o-xylene and mand/or p-xylene utilizing the same heat flow. The feeds to the fractionation zone comprise a rst feed relatively rich in ethylbenzene at a first intermediate zone level and a second feed relatively rich in o-xylene below the irst feed level. Both feeds may be obtained from available process streams or be formed from a single feed by preliminarily fractionating the feed in advance of the complex fractionation. In either case the feed may be depleted of mxylene prior to fractionation by chemically complexing the m-xylene into a separable form e.g. with HF-BF3. The fractionation zone provides a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottoms stream and a sidestream predominantly of m-xylene and/or p-xylene. The sidestream may be processed to separate p-xylene as a product of the process, and/or to convert unwanted isomers to desired isomers, with such converter eluent being returned directly to the fractionation zone, or to the feed, 'for fractionation in common with the fresh feeds.

REFERENCE TO RELATED APPLICATION This application is a continuation in part of our copending application Ser. No. 653,177, led July 13, 1967, and now abandoned.

BACKGROUND OF THE INVENTION (l) Field of the invention This invention has to do with the separation from a predominantly CB aromatic hydrocarbon feed, such as is obtainable from catalytic reforming of naphtha, of various commercially desirable component materials in controllable ratios, with low capital cost and with low utilities consumption. Chief among desired materials so obtainable are ethylbenzene, o-xylene and p-xylene. The fourth C8 aromatic isomer, m-xylene, is commercially less important and its output accordingly may be minimized. Other feed components, such as non-aromatic hydrocarbons, toluene, C9 aromatics, etc., also must be separated insofar as is required to meet quality specifications on the various C3 aromatic products. The various components of C8 feeds can 'be heat separated, that is, advantage may be taken of their different boiling points to fractionate the feed, with lower boiling materials e.g. ethylbenzene (B.P. 277 F.) and light ends passing overhead out of a fractionating zone and higher boiling materials e.g. o-xylene (B.P. 291 F.) and heavy ends withdrawn as bottoms from the zone. The m-xylene and pxylene fractions may be withdrawn as overhead or bottoms depending on operation of .the zone. Generally, successive separations are carried out to isolate the different desired products. p-Xylene for example, may be separated from m-xylene in a p-xylene crystallizing plant by freezing the p xylene (F.P. 55 F. and passing the m-xylene (F.P. 54 F.) out of the plant. Because m-xylene has limited commercial usage, it is often isomerized, or converted, to p-xylene and/or oxylene, either preceding or following fractionation operations. This invention is particularly concerned with a complex :fractionation process having low capital costs and low operating expense by virture of the separation of the ethylbenzene and o-xylene components from the m-xylene and p-xylene fractions ywith the same heat ow. Various preliminary and subsequent operations -to the complex fractionation may be combined therewith to give extremely ilexible Ca processing abilities for maximum advantage in use of available feeds to satisfy product requirements.

(2) Prior art Many different schemes have been proposed for the fractionation and conversion of predominantly C5 aromatic hydrocarbon feeds. In general, it has been the practice to rst separate o-xylene and heavy ends together from the remainder of the feed, then in a separate operation, the o-xylene and heavy ends are separated one from another. The remainder of the feed, in another separate operation, is fractionated to separate together ethylbenzene and light ends. The balance of the feed, comprising predominantly m-xylene and p-xylene may be passed to a pxylene crystallization plant and on to a conversion plant to obtain first crystallized p-xylene and, secondly, additional p-xylene formed from m-xylene. o-Xylene and heavy ends in the conversion plant eilluent typically are passed with other xylenes to still another fractionator where they are separated from p-xylene as bottoms, the second o-xylene and heavy ends separation in this operation. The p-xylene rich remainder can be returned directly to the crystallization plant .for processing with additional feed from the ethylbenzene fractionator. In another commercial process the m-xylene component is chemically separated from the feed, and optionally converted to additional p-xylene and o-xylene, in advance of fractionation operations, but the fractionation of remaining components is effected in multiple steps with successive heatings and extensive handling of several streams.

SUMMARY OF THE INVENTION One of the major objectives of the present invention is to achieve a variety of C8 products with fewer separate heating operations, thus to eliminate substantial capital expense and operating costs. This objective is met by separating one from another with a common heat flow in a fractionation zone, the major desired fractions, ethylbenzene, o-xylene and m-/p-xylene. Specifically, this objective, and others to become apparent as the description proceeds are realized by the present process for fractionating a predominantly C8 aromatic hydrocarbon mixture containing ethylbenzene and xylenes into component fractions which includes feeding a first C8 aromatics stream relatively rich in ethylbenzene into a multilevel fractionation zone at a first feed tray located intermediate the top and bottom of the zone, feeding a second C8 aromatics feed stream relatively rich in o-xylene into the zone at a second feed tray located intermediate the top and bottom of the zone and below the first feed tray, and heating both feed streams with a common heat fiow in said zone, and separating a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottom stream and a xylene isomer sidestream at a level of the zone between the first and second feed trays.

The first feed stream, relatively rich in ethylbenzene, typically comprises at least 8 mol percent ethylbenzene; the second feed stream, relatively rich in o-xylene typically comprises at least 8 mol percent o-xylene. The complex fractionation zone may be operated at temperatures between 250 and 500 F., and pressures between 10 and 100 p.s.i.a.

The process in certain embodiments includes processing the xylene isomer sidestream to isolate p-xylene therefrom, andprocesing the resulting p-xylene depleted sidestream in a conversion zone to convert m-xylene isomer present therein to other xylene isomers, and returning the conversion zone effluent to the fractionation zone as the second feed stream relatively rich in o-xylene. The conversion zone may be operated to convert m-xylene to o-xylene or p-xylene or both, with the common heat fiow in the fractionation zone being utilized to enable separation of the o-xylene and p-xylene produced in the conversion zone from one another. The process may further include returning separated o-xylene to the conversion zone for conversion into p-xylene, or conversion of p-xylene to oxylene, as dictated by product requirements. Typically the amount of p-xylene isolated from the sidestream is at least equal to the amount of p-xylene present in the predominantly C8 aromatics mixture.

In certain embodiments, the present process further includes heating the predominantly C8 aromatic mixture in a preliminary fractionation zone, and separating the feed into an overhead portion relatively rich in ethylbenzene and a bottoms portion relatively rich in o-xylene, feeding the overhead portion into a multilevel fractionation zone at a first feed tray located intermediate the top and bottom of the zone, feeding the bottoms portion into the multilevel zone at a second feed tray located intermediate the top and bottom of the zone and below the first feed tray, and heating both feed streams with a common heat flow in the lmultilevel fractionation zone, and separating a predominantly ethylbenzene overhead stream, a predominantly oxylene bottom stream and a xylene isomer sidestream at a level of the multilevel zone between the first and second feed trays. The preliminary fractionation typically is carried out at temperatures in the range of 250 and 600 F. and at pressures between 10 and 150 p.s.i.a.

The first stream, overhead, from the preliminary fractionation is heated incrementally in the multilevel fractionation zone to separate ethylbenzene and light ends from the p-xylene and m-xylene in the first stream.

Heat utilized in the prefractionation is advantageously transferred from the prefractionation zone to the multilevel fractionation zone as may be effected by cycling the preliminary fractionation zone overhead through the fractionation zone above the sidestream draw point or by reboiling hydrocarbons in the multilevel fractionation zone between the feed streams against condensation of the preliminary fractionation zone overhead vapors outside the multilevel fractionation zone. The hydrocarbons for yreboiling may be drawn from the multilevel fractionation zone at or near the level of the sidestream, whereby preliminary fractionation zone heat is utilized in both stripping and rectification operations in the multilevel fractionating zone.

The foregoing embodiment of the present process may further include returning the efiiuent from the conversion zone processing of p-xylene depleted sidestream, fwherein m-xylene has been converted into o-xylene and/or pxylene, to the multilevel fractionation zone below the sidestream level.

m-Xylene may be depleted from the initial feed to the process in advance of fractionation in other embodiments of the process. Thus the present invention contemplates preliminarily depleting the C8 aromatic hydrocarbon mixture of m-xylene, feeding a m-xylene depleted mixture relatively rich in ethylbenzene into the multilevel fractionation zone as a first feed stream at a first feed tray located intermediate the top and bottom of the zone, feeding a second C8 aromatics feed stream relatively rich in o-xylene into the zone at a second feed tray located intermediate the top and bottom of the zone and below the first feed tray and heating both feed streams with a common heat flow in the multilevel fractionation zone, and seperating a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottom stream and a xylene isomer side-stream at a level of the zone between the first and second feed trays. The m-xylene may be depleted from the feed by chemically modifying the m-xylene to a form separable from the feed, as by complexing the mxylene with a. complexing agent such as HF BF3.

The m-xylene initially depleted from the feed mixture may be heated to convert the separated m-xylene to o-xylene and/or p-Xylene, these conversion products then being fed to the multilevel fractionation zone for separation in common with other C8 aromatics in that zone. In other respects, the initial m-xylene depletion embodiment of the invention is similar to the embodiment described above and may include sidestream processing to isolate p-xylene and return of the p-xylene depleted stream to the multilevel fractionation zone or at least a portion thereof may be combined with fresh feed ahead of the m-xylene depletion zone, isolating at least the amount of p-xylene present in the first feed stream from the sidestream, converting separated o-xylene or isolated p-xylene to p-xylene or o-xylene respectively to be fed to the multilevel fractionation zone e.g. at a point above the sidestream level and also preliminarily fractionating the m-Xylene depleted feed mixture and utilizing the preliminary fractionation heat as described above in the multilevel fractionation zone.

In each of the foregoing embodiments the multilevel fractionation zone typically is operated at temperatures between 250 and 500 F. and at pressures between 10 and p.s.i.a., the preliminary fractionation, if any, at temperatures between 250 and 600 F. and pressures between 10 and 150 p.s.i.a. The relatively ethylbenzene rich first feed stream typically may contain from 12 to 25 mol percent ethylbenzene whether derived from an available process stream or from the prefractionation overhead product of a predominantly C8 aromatic feed mixture stream. The relatively o-xylene rich second feed stream may contain from 15 to 30 mol percent o-xylene and generally will contain at least 8 mol percent o-xylene whether derived from available process streams, from prefractionation bottoms, and/ or from converter efiiuent.

Also, in each of the foregoing embodiments, product quality specifications may require that the various products of the process be purified further to remove unwanted contaminants, which purications are effected by conventional methods not comprising an essential part of this invention. Typically, the ethylbenzene is purified by distillation and/ or solvent extraction, the o-Xylene by distillation, and the p-xylene by re-crystallization and/or selective adsorption, generally incorporated within the p-xylene separation unit.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described as to illustrative embodiments in conjunction with the attached drawings in which:

FIG. 1 is a schematic ow sheet of one embodiment of the present process;

FIG. 2 is a schematic flow sheet of an embodiment of the process in which the xylene isomer sidestream is processed to effect p-xylene isomer separation and conversion and return of converter-effluent and/or other isomers to the fractionation zone and depicting ethylbenzene and o-xylene purification operations;

FIG. 3 is a schematic iiow sheet of an embodiment of the present process illustrating particularly the feed mixture prefractionation and intermediate reboiling of multilevel fractionation hydrocarbons against condensation of prefractionator overhead vapor;

FIG. 4 is a schematic flow sheet of an embodiment of the present process similar to FIG. 3, and particularly depicting another means of heat transfer from prefractionator to multilevel fractionator;

FIG. 5 is a schematic flow sheet of an embodiment of the present process generally similar to FIG. 4 and particularly depicting intermediate reboiling of multilevel fractionator hydrocarbons;

FIG. 6 is a schematic ow sheet of an embodiment of the present process, generally similar to FIG. 4, without special provision for heat transfer from prefractionator to multilevel fractionator;

FIG. 7 is a schematic flow sheet of an embodiment of the present process particularly depicting initial depletion of m-xylene from the feed mixture;

FIG. 8 is a schematic fiow sheet of an embodiment of the present process generally similar to FIG. 7 with the addition of preliminary fractionation of the m-xylene depleted feed mixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 11n the ensuing description like numerals will refer to like parts in each ow scheme.

With reference first to FIG. l, a feed relatively rich in ethylbenzene, such as a fresh reformate fraction from catalytically reformed naphtha, is introduced into the system through line 1 as a first feed stream (Feed #1) to a fractionation zone defined by the complex fractionator column 2. The fresh feed may be suitably at a temperature of about 360 F. and may flow directly from a toluene recovery fractionator (not shown). This first feed stream composition may typically be: ethylbenzene 16 mol percent; p-xylene 17 mol percent; m-xylene 42 mol percent; o-xylene 20 mol percent; and other hydrocarbons 5 mol percent. A feed relatively rich in o-xylene such as may be obtained as effluent from a xylene isomerization unit is introduced into the system through line 3 as a second feed stream (Feed #2) to the complex fractionation column 2, suitably at a temperature of about 400 F. The second feed stream composition may typically be ethylbenzene 8 mol percent; p-xylene 21 mol percent; m-xylene 47 mol percent; o-xylene 21 mol percent; and other hydrocarbons 3 mol percent.

The complex fractionation zone in FIG. 1 is shown as a multilevel, plural tray column 2 which may alternatively be two or more interconnected sections. (See FIG. 2.) The first feed stream line 1 enters the fractionator column 2 at a first feed tray 4 located intermediate the top and bottom of the column. The second feed stream line 3 enters the fractionator column 2 at a second feed tray 5 also located intermediate the top and bottom of the column but below the first feed tray 4.

A multiple or complex fractionation is effected 1n column 2. In the upper portion of the column 2, ethylbenzene and light ends are separated from m-xylene and p-xylene, while in the lower portion of the column, oxylene and heavy ends are separated from m-xylene and p-xylene. Importantly, these two different separations are effected with a single or common heat flow in the column. 'Ihat is, heat input to the column 2 by passage of column bottoms through reboiler loop 6 having heat exchanger 7 to raise the column bottoms temperature to about 250 F. to 500 F., or higher, serves to enablerst separation of the o-xylene from m-xylene and p-xylene, and, then, separation of ethylbenzene from m-xylene and p-xylene higher in the column, as the heat travels upward through the column. The overhead of the column 2', comprising predominantly ethylbenzene with light ends enters reux loop 8 having condenser 9 and accumulator 110. A portion of the condensed overhead in reflux loop 8 is returned to the column as reflux, the remainder is passed along line 1v1 to ethylbenzene purification zone 12 where light ends are separated along line 13 and ethylbenzene product is taken out along line 14.

A portion of the column 2 bottoms being continually passed through reboiler loop 6` to maintain desired column temperatures, which comprises mainly o-xylene with heavy ends, is taken off along line 15 out of the fractionation zone to o-xylene purification zone 16 wherein heaxy ends are separated through line 17 and the oxylene product is passed out in line 1'8. With the separation of predominantly ethylbenzene and light ends overhead and predominantly o-xylene and heavy ends below, the remaining substantial components of the two feeds, i.e. m-xylene and p-xylene are separated to further processing along line 19 leading from column 2 at a side draw tray level 20 between the first and second feed trays 4 and 5.

The column 2, as mentioned, may comprise two or more interconnected sections and be coupled to ethylbenzene and o-xylene purification zone 12, and i16. The column may further be connected to xylene isomer separators and/ or converters. With reference to FIG. 2, such a fiow scheme is shown for processing a feed relatively rich in ethylbenzene, such as Feed #1 in the just described embodiment, introduced in the fractionation zone as the first feed stream. The second feed is obtained from a xylenes converter and is relatively rich in o-xylene.

With reference to FIG. 2, the first feed stream enters the system through line 1 at the complex fractionation zone defined by a multisection, plural tray column 2 having three interconnected sections, a lower section 2a, an intermediate section 2b and an upper section 2c. The first feed line 1 enters the intermediate section 2b at a feed-tray 4. Within section 2b an interior temperature is provided which drives the light ends and ethylbenzene components of the feed upward to be carried from the intermediate section along line 21 to the bottom of upper section 2c. In upper section 2c, the overhead from intermediate section 2b is further fractionated so that the predominantly ethylbenzene overhead stream, containing the light ends of the feed is taken off overhead and passed into reflux loop `8 through condenser 9l and accumulator 10 and into line 11 for separation of the ethylbenzene. The bottoms from upper section 2c are returned along line 22 to the intermediate section 2b.

The bottoms in intermediate section 2b, following passage overhead of light ends and ethylbenzene and comprising primarily m-xylene, p-xylene, o-xylene and heavy ends are passed along line 23 from the intermediate section for further fractionation. A portion of these bottoms is taken as a sidestream. along line 24 for processing outside of the fractionation column 2 to isolate the p-xylene in a manner to be described, and the remainder is passed along line 23 to the top of lower section 2a.

Alternatively, the sidestream may be taken from any location in the fractionation column 2 below the entry point of first feed stream line 1 into the column (feed tray 4), with accompanying variation in the o-xylene content of the sidestream within the compositional o-xylene limits of the two feeds to the column, for increased fiexibility of product mix.

In the lower section 2a the bottoms are passed continually through reboiler loop 6 and reboiler 7 to maintain section temperature at desired levels. A portion of the bottoms, including o-xylene and heavy ends, is taken off along line 15 out of the fractionation column 2 for separation of o-xylene. The remainder of the lower section 2a feed, comprising primarily m-xylene and p-xylene is passed overhead along line 25 to the bottom of intermediate section 2b.

It will be apparent that the three sections 2a, b, c are arranged to act as a single column with ethylbenzene and light ends taken out as overhead, o-xylene and heavy ends taken out as bottoms and m-xylene and p-xylene separated as a sidestream.

The predominantly ethylbenzene overhead stream in line 11 is conventionally processed in the ethylbenzene purification zone 12 to strip light ends, as necessary, for recovery of the desired purity ethylbenzene. Or the overhead in line 11 may be processed by extraction or extractive distillation methods, as required when close-boiling non-aromatic contaminants are present. Thus, the overhead stream is passed along line 11 to column 26 wherein light ends are distilled overhead to line 27 and partially returned as reux to column 26y through cooling loop 28, including condenser 29 and accumulator 30 and passed to waste or preferably to return to the naphtha reformer or toluene recovery column (not shown) for recovery of toluene or other values. The column 26 bottoms, predominantly ethylbenzene, is cycled through reboiler loop 31 having heat exchanger 32 and a portion is passed to line 14 for recovery as a product of the process.

The bottoms from fractionation column section 2a in line is conventionally processed in the o-xylene separation zone 16 to separate the o-xylene from the heavy ends. Thus, the bottoms stream is passed along line 15 to column 35 wherein o-xylene is distilled overhead for recovery through line 18, as a product of the process, from retluxing loop 37 having condenser 38 and accumulator 39. The heavy ends from the column 35 are taken off as bottoms through reboiler loop 40 having reboiler 41 to line 17, to Waste or further processing.

The sidestream taken from the intermediate section 2b of the fractionation column 2 is passed along line 24 to the p-xylene isolation zone. The p-xylene is conventionally isolated from its mixture with m-xylene e.g. by cooling in p-xylene separation zone 42 to crystallize the higher melting p-Xylene component, and separating the p-xylene along line 43. Alternatively the p-xylene can be selectively adsorbed or otherwise separated in zone 42. The liquid remaining, high in m-xylene, is passed through line 44 to the converter 4S. There, by well known techniques, not forming a part of this invention, the nonp-xylene isomers may be converted to p-Xylene. The pxylene rich effluent from converter 45, unlike prior processes is not recycled for immediate recovery of p-Xylene, following removal of heavy ends. Rather, the converter effluent is passed along line 3 as the second feed to the fractionation column 2 used to fractionate fresh feed. The line 3 enters the fractionator 2 at feed tray 5 in the lower section 2a and below the sidestream take-out line 23 from intermediate section 2b. Thus, the side dra'w line 23 is between the two feed lines, i.e. between line 1 carrying the first feed stream to the fractionation column 2 and line 3 carrying the second feed stream comprising xylenes converter 45 effluent to the same fractionation column. Light ends produced in the converter 45 may be withdrawn in part through line 46. In the fractionation column 2, the converter 4S efiluent is fractionated much in the way the first feed stream is fractionated, and importantly with the same heat flow. That is, the p-xylene and o-xylene components are commingled with fresh quantities of these materials and are eventually passed out of the fractionation column 2 with corresponding components from the fresh feed.

Importantly, conversion-produced o-xylene is fed along line 3 to the fractionation column 2 and is separated in that zone with fresh feed originated o-xylene for withdrawal from the column 2 along with heavy ends in line 15. Thus, only one separation of o-xylene needs to be made and both of the high utilities demanding steps, i.e. ethylbenzene separation and o-xylene separation, each from m-xylene and p-xylene, are accomplished with the same heat flow.

A particularly advantageous aspect of the FIG. 2

embodiment of the present process is the flexibility of product mix. Referring to FIG. 2 it can be seen that oxylene product separated in column 35 can be returned along line 47 (dotted) to the xylene converter 4S, to be incorporated in the converter feed in line 44 or fed separately to converter 45 if desired, along line 34. Appropriate control of the converter 45 operating conditions, in a known manner, converts the o-xylene at least partially to p-xylene for an increase in p-xylene yield at the expense of o-Xylene. Similarly, the m-xylene content of the sidestream in line 24 by appropriate control of the converter operating conditions, in a -known manner, may be converted to o-Xylene at least partially, for an increase in o-xylene production. Thus, greatly more or greatly less o-xylene can be produced than is provided in the feed stock.

Where a C8 aromatic mixture containing substantial quantities of all four isomers is available, another embodiment of the present process, depicted in FIG. 4 may be employed. As shown in FIG. 4, a feed mixture having typically the composition given above in connection with FIG. l, is introduced into the system along line 48 to the middle of prefractionation column 49 in which column bottoms are heated by continual circulation through reboiler loop 50 having reboiler `51 to temperatures between 250 and 600 F. at pressures in the range of 10 and p.s.i.a., effectively preliminarily fractionating oxylene and some m-xylene and p-xylene in the process feed in line 48. The prefractionator column 49 overhead which is relatively rich in ethylbenzene is passed to line 1 as the first feed to the column 2 at feed tray 4. The prefractionator column 49 bottoms which is relatively rich in o-xylene is passed along line 3 as the second feed stream to the multilevel column 2 to enter the column at feed tray 5 below the first feed tray 4. As in the FIG. 1 embodiment, the first and second feed streams in lines 1 and 3 are subjected to fractionation in column 2, with the ethylbenzene and light ends being separated from m-Xylene and p-xylene in the upper portion of the column and o-xylene and heavy ends being separated from mxylene and p-xylene in the lower portion of the column. As in FIG. 1 and in the other embodiments of the process, the two different separations are effected with a single or common heat flow in the column 2 with heat input by reboiler loop `6 serving to enable first separation of oxylene from m-xylene and p-xylene and then separation of ethylbenzene from m-Xylene and p-xylene higher up in the column as the heat travels upward. The overhead and bottoms of the column 2 are taken off as described above in connection with FIG. 1.

Heat may be transferred from prefractionator column 49 to the complex fractionation column 2 by cycling prefractionator overhead through the fractionator column. Thus, column 49 vapor overhead in line 1 enters column 2 just above the first feed tray 4 and liquid reflux smaller in quantity than the vapor overhead feed exits from column 2 in line 55 from the feed entry level to give a net transfer of overhead product from column 49 to column 2, comprising Feed No. 1 to column 2, and therewith a transfer of heat to column 2 from column 49.

In FIG. 3, the prefractionation in column 49 of a C8 aromatics feed stream from line 48 into an ethylbenzene rich overhead stream in line 1 and an o-Xylene rich bottoms stream in line 3, as first and second feed streams respectively for the complex fractionator 2 is carried out generally as described above in connection with FIG. 4. In this FIG. 3 embodiment, however, intermediate reboiling of fractionator column 2 hydrocarbons is provided. Thus, hydrocarbons in the fractionation column 2 are with drawn from the column at a point between the feed trays 4 and 5 and heat exchanged with the preliminary fractionation column 49 overhead vapors, outside of the multilevel fractionation column 2. To accomplish this form of heat transfer from preliminary fractionation column 49 two intersecting loops 52 and 56 are provided.

Loop 52 is a reflux loop for column 49 and includes combination condenser/reboiler unit 53 and accumulator 54. Loop 56 is an intermediate reboiler loop for column 2 and includes a side draw line 57 passing through the reboiler side of unit 53 back to column 2 above line 57 and below upper feed tray 4. Desirably the side draw line 57 is at or near the level of the m-xylene/p-xylene sidestream line 24 so that the heat from preliminary fractionation column 49 overhead is utilizable in both stripping and rectification operations in the upper part of fractionation column 2. In the FIG. 3 embodiment, the xylene isomer sidestream comprising predominantly m-xylene and p- Xylene is taken off in line 24 and processed to isolate p-xylene therefrom. The sidestream is passed along line 24 to a p-Xylene separation zone 42 which may be a crystallizer as described above in connection with FIG. 2, or other type of separator operating e.g. by selective absorption, complexing, or other technique to isolate pxylene. p-Xylene product is passed out of the separation zone 42 along line 43. The remainder of the xylenes isomer sidestrearn after separation of p-xylene, is high in coutent of m-xylene and is passed along line 44 to converter 4S for production of additional p-xylene .and/or o-xylene in various ratios depending on market requirements. Light ends may be removed at least in part from the converter eluent along line 46. Separated o-Xylene can be passed along line 61 to the converter 45 for isomerization into p-xylene as desired.

In the FIG. 3 embodiment, as in the FIG. 2 embodiment above described, the effluent from converter 45 is recycled to the fractionation column 2 along line 58 to column 2 at a point below the `sidestream take out line 24. The fractionation column 2 thus has a supplementary feed or Feed #3 relatively rich in p-xylene and/ or o-xylene which is simultaneously fractionated with the #1 and `#-2 feeds into component fractions with a common heat ow in the column 2 provided by reboiler loop |6 and supplemented by intermediate reboiler loop 56 as described above. Optionally, a portion or all of the converter 45 eiuent may be returned along line 59 to the prefractionator column 49 for processing there with fresh feed prior to fractionation in the column 2 or the converter 4S effluent may be passed along line 60 to be combined with Feed #2 (prefractionator bottoms) in line 3 downstream of the prefractionator column.

In FIG. 5 the process feed in line 48 having typically the composition given above in the FIG. 1 embodiment description is prefractionated in column 49 to separate the process feed into an o-xylene rich bottoms stream in line 3 and ethylbenzene rich overhead stream in line 1. Heat input to the column 49 is by reboiler loop 50. The column 49 overhead vapor is condensed on the condenser side of combined condenser-reboiler unit 53 in reflux loop 52 against liquid intermediate level hydrocarbons in intermediate reboiler loop 56 vaporizing the liquid hydrocarbons on the reboiler side of the unit 53 for ret-urn t0 the multilevel fractionation column 2. The condensed overhead vapors rich in ethylbenzene are passed in part to column 2 in line 1 as Feed #1 and in part returned to column 49 as reflux through loop 52. Product separation from column 2 in this FIG. 5 embodiment is as in the FIG. 1 embodiment, above described.

In FIG. 6, an embodiment of the present process, particularly useful Where fuel costs are low, is depicted. In this embodiment specific provision for double utilization of prefractionator heat input is not made which may be acceptable with low fuel costs. In FIG. 6 the process feed in line 48 is prefractonated in column 49 as in the FIG. 5 embodiment, just described. Reflux loop 64 including condenser 65 and accumulator 66 returns a portion of the condensed overhead to column 49 and a portion is passed along line 1 to column 2 as the first feed stream. The bottoms in column 49 are circulated through reboiler loop 50 and a portion thereof is passed along line 3 to the multilevel fractionation column 2 as the second feed l0 stream. Operation of the multilevel fractionation column 2 and product recovery therefrom is as described in connection with FIG. l above.

The process in the further em-bodiments shown in FIGS. 7 and 8 includes a preliminary treatment of the process feed to separate m-xylene and/or isomerize m-xylene to o-Xylene and/or p-xylene for the fractionation operations; in reference to FIGS. 7 and 8, a feed of mixed xylenes and ethylbenzene appropriate for fractionation (and conversion) to produce p-xylene as well as ethylbenzene and o-xylene, as above described, is introduced into the system through line l67. The feed passes to a m-xylene separator and isomerizer 68 in which m-xylene is complexed with a complexing agent. The m-xylene complex may be separated from the balance of the feed, then decomposed and the m-xylene all or in part isomerized to oand/ or p-xylene.

Typically, the separator and isomerizer 68 is a known unit which contains HF`-BF3 with which the feed from line 67 is intimately contacted at about 32 to 45 F. which results in essentially all the m-xylene in the feed forming complexes with HFBFB in an HF solution. The m-xylene complex is passed as an HF solution to the isomerizer portion of separator 68 where the complex is heated to a temperature at which it is unstable and decomposes, freeing the m-xylene for recovery as such along line 70, and restoring the HF-BFS complex to its original condition for reuse. Or the m-Xylene-HF-BF`3 complex may be passed as such to the isomerizer portion of separator and isomerizer 68 and at least a portion of the m-xylene component isomerized to oand/or p-xylenes using the HF-BF3 present as a catalyst. The isomerization products are then combined with the m-xylene depleted feed in line 1 to increase the o-xylene and p-xylene content thereof for processing in the fractionation column 2. Optionally, light ends may be separated at least in part in separator 68 and passed out of the system in line 69.

fIn FIG. 7, the m-xylene depleted feed is passed to the multilevel fractionation column 2 as a first feed stream to be heated within the column to simultaneously separate ethylbenzene from oand p-xylenes at an upper level of the Zone and p-xylene from o-xylene on a lower level of the column. The use of a single heat ow in accordance with the teaching of this invention to effect these two separations in a m-xylene depleted feed is a departure from previous practice in processing m-Xylene depleted CB feeds. a

It will be noted that in the FIG. 8 embodiment of the invention a preliminary fractionation of the m-xyleue depleted feed in line 48 is accomplished as described in connection with FIG. 5 above. Thus, the m-xylene depleted feed in line 48 is passed into column 49 and separated into an ethylbenzene rich portion overhead to line 1 and to the fractionation column 2 as the first feed stream and an o-xylene rich portion as bottoms along line 3 to the fractionation column 2 as the second feed stream.

As in previous embodiments, within multilevel column 2, light ends and ethylbenzene components of the column feed are driven upwards to be carried as vapors into the upper reaches of the column where the ethylbenzene is essentially completely separated from p-Xylene by further fractionation so as to pass the ethylbenzene (and light ends) overhead to reflux loop 8 and to pass p-xylene downward. In FIG. 8, the heat flow utilized in the preliminary fractionation of the feed in column 49 derived from heat exchanger 51 is transferred to multilevel fractionation column 2 through the combining condenserreboiler unit 53 in reux loop 52 and intermediate reboiler loop 56 described above, to be combined with the heat flow in multilevel column 2 from reboiler 7 so that the two heat flows are available to facilitate the relatively difficult separation of p-xylene and ethylbenzene, where heat demand is greatest.

Pressures within the columns 49 and 2 in the FIGS. 7 and 8 embodiments as in other embodiments herein are not narrowly critical and typically range between 10 and 150 p.s.i.a., but are not limited to this range where economic considerations suggest other pressure levels.

Product recovery in the present embodiments is accomplished as in FIG. 1 with respect to the ethylbenzene and o-xylene products. Thus overhead from column 2 enters reux loop 8, is condensed in condenser 9 and is partly returned to column 2 as reux and partly passed along line 11 to the ethylbenzene purication zone 12 for separation of light ends along line 13 and separation of the ethylbenzene in line 14. Similarly the o-xylene bottoms stream from column 2 is passed along line 15 to o-xylene 1 2 EXAMPLES For processing a typical Ca-aromatics rich feedstock having a composition of approximately:

lb.-mols/hour Heavy ends (C9-H Apparatus may be arranged according to any of the ow sheets FIGS. l through 8, the choice of flow sheet being dependent upon the desired products of the process, the

purification zone 16 where heavy ends are separated along 15 relative amounts of such products, and desired extent of line 17. The o-xylene product is taken out in line 18. flexibility to change the products in response to changing In FIG. 8 the p-xylene sidestream is taken in line 19 market conditions. The chosen apparatus may include, in just above tray 20 in column 2 and passed to p-Xylene addition to the complex fractionator, one or more of the separator 42. The p-xylene product is passed along line following: a prefractionator; a p-xylene separation unit; 43 out of the system, and the o-xylene rich remainder of 20 a XylefleS SOmeTiZation Unit With o1" Without mXYene the sidestream 19 is passed from separator 42 back io Separati0r1; an ethylbenzene purication unit; and an 0- multilevel fractionation column 2 along line 58 as supxylene Puflealon unltplementary Feed #3 to the column. Optionally, a por- EXAMPLE 1 tion or all of the p-xylene separator eluent in line 58 r may be returned along line 72 (dotted) to the isomerizer 2') A process for fractionation and conversion of the above 68 for conversion of excess xylene isomers from one to fnentioned typical feedstock may be eondueed accord' another, or along line 72 and thence along line 73 (dotted) mg t0 the FIG' 2 OW Sheet to obtaln three Pnmafy Prod' to the prefractionation column 49 to enhance overall ucts as follows: recovery of ethylbenzene product, or along lines 72 and 30 h (f1) Ethyllbenze equYalent to lbout 95 of that 1.n 73 and thence along line 74 (dotted) to be combined in t e ee fstgcj M19 0 mlnlmlm plglty ogtmmf a man; line 3 with prefractionator bottoms as part of Feed #2 tIgTX/len m0 perce 1g t en s an mo percen to column 2. I'f p-xylene product is desired at the expense (2) O Xy'lene equivalent to about 85% of that in .Ehe of o-xylene product, separated o-xylene product 1n lllle feedstock, at 95% minimum purity, containing a maxi- 18 may be returned alongline 75 (dotted) to isomeri'zer 35 mum of 4.0 m01 percent other CSHM material and 1.0 68 for conversion to additional p-xylene product. Simmolpercent heavy ends. ilarly separation of less than all of the p-Xylene in sep- (3) p Xylens at maximum economic yield and 99% arator 42 enables return of p-xylene along line 72 to the minimum purity, consistent with the stated production of isomerizer 68 for conversion, if desired, to additional ethylbenzene and o-xylene, and one further requirement O Xylene product 40 that the by-product C9+ heavy ends contain about 30 mol .In FIG 7, the O Xyleue rich emuent from the p xylene percent xylenes. For this case, the apparatus includes (l) a Separator 42 following Separation of p xylene product multisection complex fractionating column lh avingabout along line 43 is returned to multilevel fractionation col- 1252 theormcal Stags. 370 actual trays dwlded Mito a' umn 2 as ,Feed #2 along line 3' Optionally a portion 45 ower section comprising a column of 70 trays, an intermediate section comprising a column of 150 trays, and an but not au of the P'Xyiene Separator emuent m lm e 3 upper section also comprising a column of 150 trays, havmay be remmef along 1m@ 72 (dott'd) to the lsomenzer ing a reux ratio of 14.0 (fresh feed basis), operable at 68 for Converslon Ofhexss xylene lsomers from .one t0 15.7 p.s.i.a. (accumulator pressure), and capable of a heat aHOthef, 0r along line 72 and thence along llne 73 input of 136.0 MM B.t.u./hour; (2) an ethylbenzene puri- (dotted) to be Combined in line 1 With Feed #1 '[0 CO1- 50 cation column having about 26 theoretical stages, 40 umn 2 to enhance overall recovery of ethylbenzene prodactual trays, a reflux ratio of 1.5, operable at 15.7 p.s.i.a., uct. As in FIG. 8, additional p-xylene product may be and Capable of a heat input of 2-5 MM B-11-/ hour; (3) obtained by returning separated o-xylene product in line an o'xylene Purcation Column having about 35 theo' 18 along line 7S (dotted) to isomerizer 68 for conversion 55 retlcal Stages 55, actual trays a reflux rat10-0f 32 oper' to P Xylene or additional O Xylene p 1 0 duct may be ob able at 15.7 p.s.i.a., and capable of a heat input of 19.4 tained by separation of less than all of the p-xylene in MM B't'l-l/hour; (4) a p-Xylene Separauon umt m Whlch p-xylene is separated by crystallization; and (5) a xylenes Separator Hond retu'fn of .a Pomon of the effluent m isomerization unit in which ni-xylene and excess o-xylene line 3 along line 72 to isomerizer 68 for conversion of the are Converted to p Xy1ene A material balance at various unrecovered p-xylene to o-xylene. 60 points in the process is given in Table 1.

TABLE i Complex fi'actonator (Column 2) o-Xylene purilcation (Unit 16) aan (zarten esta wenn) masas ComponentI pound mols per hour:

C7 3.95 1.09 Etliylbenzene 91. 84 89.03 128. 77 03 03 p-Xyiene 9s. 09 3i 437. 42 i. i3 1. ii o2 m.Xy1em 241. i2 i1 100s. s4 3. 0s 2. 9s i0 O Xylemh 117. 0s 33s. 99 12o. 93 99. 52 21.41 @9+ 32.97 26. 42 51. 27 1.0i 50.26

Total 579. 37 93. 40 1941. 53 17s. 44 104. 65 71. 79

TABLE l-Contiuued Ethylbenzene purification p-Xylene separation Isomerization (Unit 12) nit 42) (Unit 45) Distillate Bottoms p-Xylene Filtrate (to Post light ends light ends ethylbenzene product converter) removal-recycle (Line 27) (Line 14) (Line 43) (Line 44) (Line 3) Comionent, pound mols per hour: 3 76 19 01 1 08 4 77 7 I I 22 12a 55 125l 99 249. 68 187. 74 342. 77 1.67 1007. 17 770. 91 56 338. 43 342. 84 26. 37 44. 72

Total 5. 54 87. 86 252. 19 1689. 34 1632.00

EXAMPLE 2 15 EXAMPLE 3 A process for fractionation of the above mentioned typical feedstock may be conducted according to the FIG. 5 flow sheet to obtain three primary products as follows:

1) Ethylbenzene equivalent to about 95% of that in the feedstock, at 99% minimum purity, containing a maximum of 0.5 mol percent total xylene.

(2) o-Xylene equivalent to about 90% of that in the feed stock, at 95 minimum purity, containing a maximum of 4.0 mol percent other 08H10 material and 1.0 mol percent heavy ends.

(3) A xylene isomer mixture, predominantly p-xylene and m-xylene, at maximum economic yield consistent with the stated production of ethylbenzene and o-xylene. For this case, the apparatus includes (l) a prefractionation column having about 74 theoretical stages, 100 actual trays, a reux ratio of 5.8 (feed basis), operable at `61.1 p.s.i.a. (accumulator pressure), and capable of a heat input of 54.2 MM B.t.u./hr.; (2) a multi-section complex fractionati-ng column having about 298 theoretical stages, 410 actual trays divided into a lower section comprising a column of 75 trays, a first intermediate section also comprising a column of 75 trays, a second intermediate section comprising a column of 130 trays, and an upper section also comprising a column of 130 trays, having a reflux ratio of 11.2 (total feed basis), operable at 14.7 p.s.i.a. (accumulator pressure), and capable of a heat input of 57.2 MM B.t.u./hr. in a bottom reboiler plus 54.2 MM B.t.u./hr. transferred from the prefractionation column in an intermediate reboiler; and (3) an o-xylene purification column, having about `65 theoretical stages, 100 actual trays, a reux ratio of 5.4, operable at 14.7 p.s.i.a., and capable of a heat input of 15.3 MM B.t.u./hr. In this case, an ethylbenzene purification column is not required inasmuch as the overhead product yfrom the complex fractionator meets the ethylbenzene product specifications. Other units for p-xylene separation and xylenes isomerization also are not required to obtain the desired products. A material balance at various points in the process is given in Table 2.

A process for fractionation and conversion of the above mentioned typical feedstock may be conducted according to the FIG. 3 flow sheet to obtain three primary products as follows:

(l) Ethylbenzene equivalent to about 95% of that in the feedstock, at 99% minimum purity, containing a maximum of 0.5 mol percent light ends and 0.5 mol percent total xylene.

(2) o-Xylene at maximum economic yield and 95% minimum purity, containing a maximum of 4.0 mol percent other CgHm material and 1.0 mol percent heavy ends.

y(3) p-Xylene at maximum economic yield and 99% minimum purity, consistent Iwith the stated production of ethylbenzene and o-Xylene. For this case, the apparatus includes (l) a prefractionation column having about 73 theoretical stages, 105 actual trays, a reflux ratio of 5.7 (feed basis), operable at 50.5 p.s.i.a. (accumulator pressure), and capable of a heat input of 54.0 MM B.t.u./ hr.; (2) a multisection complex fractionating column having about '244 theoretical stages, 350 actual trays divided into a lower section comprising a column of trays, a irst intermediate section also comprising a column of 80 trays, a second intermediate section comprising a column of 100 trays, and an upper section comprising a column of trays, having a reflux ratio of 20.4 (total yfresh feed basis), operable at 14.7 p.s.i.a. (accumulator pressure), and capable of a heat input of 152.3 MM B.t.u./hr. in a bottom reboiler plus 54.0 MM B.t.u./hr. transferred from the prefractionation column in an intermediate reboiler; (3) an o-xylene purification column having about 62 theoretical stages, 90 actual trays, a reflux ratio of 5.3, operable at 14.7 p.s.i.a., and capable of a heat input of 27.1 MM B.t.u./hr.; (4) an ethylbenzene purification unit in which ethylbenzene is separated by extractive distillation; (5) a p-xylene separation unit in which p-Xylene is separated by selective adsorption; and (6) a xylene isomerization unit in which m-xylene is converted to additional p-xylene and o-xylene, conversion-generated light ends are-separated and the net isomerate is recycled to the complex fractionating column by being combined (in this case) with the prefractionation column bottoms. A material balance at various points in the process is given 'in Table 3.

TABLE 2 o-Xylene purification Prefractionator Complex fractionator column (column 49) (column 2) (unit 16) Feed Over- Over- Sideo-Xylene Hea stock head Bottoms head stream Bottoms products enilss Component, pound mols per hour: (Line 43% (Line (Line 3) (Line l) (Line 19) (Line 15) (Line 18) (Line 17) 91s4 Qooo "ifi' 87125 "ES'IIIIII 96. 09 64. 27 3l. S2 40 95. 39 30 241. 12 129. 7S 111. 34 03 237. 03 4. 06 117. 08 2. 34 114. 74 9. 56 107. 52 32. 97 32. 97 32. 97

HN @HaHa/HN@ wm .H H .Rw HHH .wv n@ .mH @n .QS Ho .www S .HHN @o .com .mow @H om .www Ho .Ham .Hm .n I H205 -y 5.@ @Hlm HHH .-..lll HHHH` llillllllm 7.5.1---- 55m mH NMS owdH mm.mH mo. @ESN HHNMH ---1.1.1. H HH md woHHH lllll.. .1..-.,ilEHHn .l mo. 1.1-1-1.-- oms @H @om wmmm moH om# mdmn mo. NHNH oHmmH mHHH 1li,.-.1212.-.-..,.I-@HSHHUE low. .--.Illll mH.. NQHHNH no.: Hmdo mn. mm ov. moHm cs w @edm {11:72.21}-.-11-11.22:@HSNAH mm .hm Ho N@ .Hm mw .mm Ho Nm .mm mH w .H co .om .Hm :@HSNEHHHHHHMH om. mH. HH.H mo. QH.. I E.. mw. l hm. IIHD Een En wHoE mHzEoHH EEQQEQO We claim:

1. The process for fractionating predominantly C8 aromatic hydrocarbon mixtures containing ethylbenzene and xylenes into component fractions, that includes feeding a first C8 aromatics stream relatively rich in ethylbenzene into a multilevel fractionation zone at a first feed tray located intermediate the top and bottom of said zone, feeding a second C8 aromatics feed stream relatively rich in o-xylene into said zone at a second feed tray located intermediate the top and bottom of said zone and below said rst feed tray, and heating both feed streams with a common heat ow in the multilevel fractionation zone, and separating a predominantly ethylbenzene overhead stream, a predominantly o-Xylene bottom stream and a xylene isomer sidestream at a level of said zone between said first and second feed trays.

2. Process according to claim 1 in which said fractionation zone is operated at temperatures between 250 and 500 F., and pressures between l0 and 100 p.s.i.a.

3. Process according to claim 1 in which said first feed stream comprises at least 8 mol percent ethylbenzene.

4. Process according to claim 1 in which said second feed stream comprises at least 8 mol percent o-xylene.

5. The process for fractionating predominantly C8 aromatic hydrocarbon mixtures containing ethylbenzene and xylenes into component fractions, that includes feeding a first C8 aromatics stream relatively rich in ethylbenzene into a multilevel fractionation zone at a first feed tray located intermediate the top and bottom of said zone, feeding a second C8 aromatics feed stream relatively rich in o-Xylene into said zone at a second feed tray located intermediate the top and bottom of said zone and below said first feed tray, heating both feed streams with a common heat flow in the multilevel fractionation zone, and separating a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottom stream and a xylene isomer sidestream at a level of said zone between said first and second feed trays, processing said xylene isomer sidestream to isolate p-xylene therefrom, processing the resulting p-xylene depleted sidestream in a conversion zone to convert m-xylene isomer present therein to other Xylene isomers, and returning conversion zone efliuent to the fractionation zone as said second feed stream.

6. Process according to claim 5 in which said fractionation zone is operated at temperatures between 250 and 500 F., and pressures between l0 and 100 p.s.i.a.

7. Process according to claim S in which said first feed stream comprises at least 8 mol percent ethylbenzene.

8. Process according to claim 5 in which said second feed stream comprises at least 8 mol percent o-xylene.

9. Process according to claim 5 including converting m-xylene to o-xylene in said conversion zone.

10. Process according to claim 5 including converting m-xylene to p-xylene in said conversion zone.

11. Process according to claim 5 in which said common heat fiow in the fractionation zone is utilized to separate from each other o-xylene and p-xylene produced in the conversion zone.

12. Process according to claim 5 in which the amount of p-xylene isolated from the sidestream is at least equal to the amount of p-xylene present in the first feed stream.

13. Process according to claim S including converting o-xylene to p-xylene in said conversion zone.

14. Process according to claim 5 including converting p-Xylene to o-xylene in said conversion zone.

15. Process according to claim 13 including returning separated o-xylene to the conversion zone for conversion into p-xylene.

16. The process lfor fractionating a predominantly C8 aromatic hydrocarbon mixture containing ethylbenzene and xylenes into component fractions, that includes heating said mixture in a preliminary fractionation zone, and separating said mixture into an overhead portion relatively rich in ethylbenzene and a bottom portion relatively rich in o-xylene, feeding said overhead portion into a multilevel fractionation zone at a first feed tray located intermediate the top and bottom of said zone, feeding said bottom portion into said multilevel zone at a second feed tray located intermediate the top and bottom of said zone and below said first feed tray, and heating both feed streams with a common heat ow in the multilevel fractionation zone, and separating a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottom stream and a xylene isomer sidestream at a level of said multilevel zone between said first and second feed trays.

17. Process according to claim 16 in which said multilevel fractionation zone is operated at temperatures between 250 and 500 F. and pressures between 10 and 100 p.s.i.a.

18. Process according to claim 16 in which said first `feed stream to the multilevel fractionation zone cornprises at least 8 mol percent ethylbenzene.

19. Process according to claim 16 in which said second feed stream to the multilevel fractionation zone comprises at least 8 mol percent o-xylene.

20. Process according to claim 16 in which said preliminary fractionation zone is operated at temperatures between 250 and 600 F. and pressures between l0 and 150 p.s.i.a.

21. Process according to claim 16 including transferring heat from said preliminary fractionation zone to said fractionation zone.

22. Process according to claim 21 in which said heat transfer is effected by cycling said preliminary fractionation zone overhead through the fractionation zone above the sidestream draw from said zone.

23. Process according to claim 22 including also incrementally heating the first stream in the fractionation zone to separate ethylbenzene and light ends from the pxylene and m-xylene in said first stream.

24. Process according to claim 16 including also reboiling hydrocarbons in the multilevel fractionation zone between the feed streams against condensation of preliminary fractionation zone overhead vapors outside said fractionation zone.

25. Process according to claim 24 including drawing hydrocarbons for reboiling from the fractionation zone at the level of said sidestream, whereby said preliminary fractionation zone heat is utilized in both stripping and rectification operations in the multilevel fractionating zone.

26. The process for fractionating a predominantly C8 aromatic hydrocarbon mixture containing ethylbenzene and xylene into component fractions, that includes heating said mixture in a preliminary fractionation zone, and separating said mixture into an overhead portion relatively rich in ethylbenzene and a bottom portion relatively rich in o-xylene, feeding said overhead portion into a multilevel fractionation zone at a first feed tray located intermediate the top and bottom of said zone, feeding said bottom portion into said zone at a second feed tray located intermediate the top and bottom of said zone and below said first feed tray, heating both feed streams with a common heat flow in the multilevel fractionation zone, and separating a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottom stream and a xylene isomer sidestream at a level of said zone between said first and second feed trays, processing said xylene isomer sidestream to isolate p-xylene therefrom, processing the resulting p-xylene depleted sidestream in a conversion zone to convert m-xylene isomer present therein to other xylene isomers, and returning conversion zone effluent to the multilevel fractionation zone below said sidestream level.

27. Process according to claim 26 in which said multilevel fractionation zone is operated at temperatures between 250 and 500 F. and pressures between l0 and p.s.i.a.

28. Process according to claim 26 in which said overhead portion feed stream to the multilevel fractionation zone comprises at least 8 mol percent ethylbenzene.

29. Process according to claim 26 in which said bottom portion feed stream to the multilevel fractionation zone comprises at least 8 mol percent o-xylene.

30. Process according to claim 26 in which said preliminary fractionation zone is operated at temperatures between 250 and 600 and pressures between l0 and 150 p.s.i.a.

31. Process according to claim 26 including transferring heat from said preliminary fractionation zone to said fractionation zone.

32. Process according to claim 31 in which said heat transfer is effected by cycling said preliminary fractionation zoneoverhead through the fractionation zone above the sidestream draw from said zone.

33. Process according to claim 26 including also reboiling hydrocarbons in the multilevel fractionation zone between the feed streams against condensation of preliminary fractionation zone overhead vapors outside said fractionation zone.

34. Process according to claim 33 including drawing hydrocarbons for reboiling from the fractionation zone at the level of said sidestream, whereby said preliminary fractionation zone heat is utilized in both stripping and rectification operations in the multilevel fractionating zone.

35. Process according to claim 26 including converting m-Xylene to o-xylene in said conversion zone.

36. Process according to claim 26 including converting m-xylene to p-xylene in said conversion zone.

37. Process according to claim 26 in which said common heat flow in the fractionation zone is utilized to separate from each other o-Xylene and p-xylene produced in the conversion zone.

38. Process according to claim 26 in which the amount of p-xylene isolated from the sidestrea-m is at least equal to the amount of p-xylene present in the first feed stream.

39. Process according to claim 26 including converting p-xylene to o-xylene in said conversion zone.

40. Process according to claim 26 including converting o-xylene to p-xylene in the conversion zone.

41. Process according to claim 40 including also returning separated o-Xylene to the conversion zone for conversion into p-xylene.

42. The process for fractionating a predominantly C8 aromatic hydrocarbon mixture containing ethylbenzene and xylenes into component fractions, that includes depleting said mixture of m-xylene, feeding a first feed stream comprising a m-xylene depleted C8 aromatic mixture and relatively rich in ethylbenzene into a multilevel fractionation zone at a first feed tray located intermediate the top and bottom of said zone, feeding a second C8 aromatic feed stream comprising C8 aromatics and relatively rich in o-xylene into said zone at a second feed tray located intermediate the top and bottom of said zone and below said first feed tray, and heating both feed streams with a common heat flow in the multilevel fractionation Zone, and separating a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottom stream and a Xylene isomer sidestream at a level of said zone between said first and second feed trays.

43. yProcess according to claim 42 in which said multilevel fractionation zone is operated at temperatures between 250 and 500 F. and pressures between 10 and 100 p.s.i.a.

44. Process according to claim 42 in which said first feed stream to the multilevel fractionation zone cornprises at least 8 mol percent ethylbenzene.

45. Process according to claim 42 in which said second feed stream to the multilevel fractionation zone cornprises at least 8 mol percent o-xylene.

46. Process according to claim 42 including also chemg ically modifying the m-xylene to a form separable from 20 the first feed stream to facilitate m-xylene depletion therefrom.

47. Process according to claim 46 in which -said m- Xylene is complexed with a complexing agent.

48. Process according to claim 47 in which said m-xylene is complexed with H13-BFS.

49. Process according to claim 42 including also processing said sidestream to isolate p-xylene therefrom and returning the p-xylene depleted stream to said multilevel fractionation zone at a point below said sidestream level, said p-xylene depleted stream comprising all or part of said second feed stream to said fractionation zone.

50. Process according to claim 42 including also processing said sidestream to isolate p-xylene therefrom and returning at least a portion of said p-Xylene depleted stream to the feed ahead of the m-Xylene depleting zone.

51. Process according to claim 42 including also processing said sidestream to isolate p-xylene therefrom and returning a portion of said p-Xylene depleted stream to said multilevel fractionation zone at a point abovel said sidestream level.

52. Process according to claim 42 including also converting m-xylene separated from said mixture in said depleting step to o-xylene and feeding the o-xylene obtained to said multilevel fractionation zone.

53. Process according to claim 42 including also converting m-xylene separated from said mixture in said depleting step to p-xylene and feeding the p-Xylene obtained to said multilevel fractionation zone.

S4. Process according to claim 42 including also converting m-xylene separated from said mixture in said depleting step into o-Xylene and p-xylene and in which said common heat fiow in the multilevel fractionation zone is utilized to separate from each other o-Xylene and p-Xylene so produced.

55. Process according to claim 42 in which the amount of p-Xylene isolated from the sidestream is at least equal to the amount of p-xylene present in said mixture.

56. Process according to claim 42 including also converting separated o-Xylene to p-xylene and feeding the p-xylene so produced to the multilevel fractionation zone.

57. Process according to claim 49 including also converting isolated p-xylene to o-xylene and feeding the o-xylene so produced to the multilevel fractionation zone.

58. The process for fractionating a predominantly C8 aromatic hydrocarbon mixture containing ethylbenzene and xylene into component fractions, that includes chemically separating m-Xylene from the mixture to deplete the feed of m-xylene, heating the m-Xylene depleted mixture in a preliminary fractionation zone, and separating said mixture into an overhead portion relatively rich in ethylbenzene and a bottom portion relatively rich in o-Xylene, feeding said overhead portion into a multilevel fractionation zone as a first feed stream at a first feed tray located intermediate the top and bottom of said zone, feeding said bottom portion into said zone as a second feed stream at a second feed tray located intermediate the top and bottom of said zone and below said first feed tray, heating both feed streams with a common heat flow in the multilevel fractionation zone, and separating a predominantly ethylbenzene overhead stream, a predominantly o-xylene bottom stream and a predominantly pxylene sidestream at a level of said zone between said first and second feed trays.

59. Process according to claim 58 in which said multilevel fractionation zone is operated at temperatures between 250 and 500 F. and pressures between l0 and l0() p.s.1.a.

60. Process according to claim 58 in which said first feed stream to the multilevel fractionation zone comprises at least 8 mol percent ethylbenzene.

61. Process according to claim 58 in which said second feed stream to the multilevel fractionation zone comprises at least 8 mol percent o-xylene.

62. Process according to claim 58 in which said preliminary fractionation zone is operated at temperatures tween 250 and `600" F. and pressures between and 1S() p.s.i.a.

63. Process according to claim 58 in which said m-xylene is complexed with a complexin g agent.

64. Process according to claim 63 in which said m-xylene is complexed with HF-BF3.

65. Process according to claim S8 including also processing said sidestream to isolate p-xylene therefrom and returning at least a portion of the p-xylene depleted stream to said fractionation zone.

66. Process according to claim 58 including also processing said sidestream to isolate p-xylene therefrom and returning at least a portion of the p-xylene depleted stream to said preliminary fractionation zone.

67. Process according to claim 58 including also processing said sidestream to isolate p-xylene therefrom and returning at least a portion of the p-xylene depleted stream to the feed ahead of the m-Xylene depleting zone.

68. Process according to claim 58 including transferring heat from said preliminary fractionation zone to said fractionation zone.

69. Process according to claim 68 in which said heat transfer is effected by cycling said preliminary fractionation zone overhead through the fractionation zone above the sidestream draw from said zone.

70. Process according to claim 58 including also reboiling the hydrocarbon in the fractionation zone between the feed streams against condensation of preliminary fractionation zone overhead vapors outside said fractionation zone. Y

:71. Process according to claim 70 including drawing hydrocarbons for reboiling from the fractionation zone at the level of said sidestreaim, whereby said preliminary 22 fractionation zone heat is utilized in both stripping and rectification operations in the fractionating zone.

72. Process according to claim 58 including also converting m-xylene separated from said mixture to o-xylene and feeding the o-xylene obtained to said multilevel fractionation zone.

73. Process according to claim 58 including also converting m-xylene separated from said mixture to p-xylene and feeding the p-xylene obtained to said multilevel fractionation zone.

74. Process according to claim S8 including also converting m-xylene separated from said mixture into o-xylene and p-xylene and in which said common heat flow in the multilevel fractionation zone is utilized to separate from each other o-xylene and p xylene so produced.

75. Process according to claim in which the amount of p-.xylene isolated from the sidestream is at least equal to the amount of p-xylene present in said mixture.

76. Process according to claim 58 including also converting separated o-xylene to p-xylene and feeding the p-xylene so produced to the multilevel fractionation zone.

77. Process according to claim 65 including also converting isolated p-xylene to o-xylene and feeding the o-xylene so produced to the multilevel fractionation zone.

References Cited UNITED STATES PATENTS CURTIS R. DAVIS, Primary Examiner U.S. Cl. XJR. 260-674A

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