BE616755A - - Google Patents

Description

       

   <Desc / Clms Page number 1>
 



  Separation of aromatic hydrocarbons.



   The present invention relates to the separation of aromatic hydrocarbons, in particular the separation of ethylbenzene from a refining fraction which comprises ethylbenzene in admixture with isomeric xylenes and certain non-aromatic hydrocarbons boiling at high temperature. roughly in the same interval.



   According to US Pat. No. 2,959,626, ethylbenzene can be efficiently separated using a super-distillation column from other C8 aromatics, namely isomeric xylenes, under critical distillation conditions. , in a column of at least 150 trays and with a reflux ratio greater than 40: 1.

   However, according to this patent it is necessary to eliminate

 <Desc / Clms Page number 2>

 First, substantially all non-aromatic hydrocarbons boiling in the critical range of about 130-140 C. It was believed, indeed, that these non-aromatic impurities boiling in the critical range would prevent a clean separation of the products. - mixed aromatics, sowed under super-distillation conditions, and furthermore any non-aromatic compounds - lament entrained by the separated ethylbenzene would impair the efficiency.
 EMI2.1
 The subsequent catalytic dehydrogenation of ethylbenzene to styrene results in the destruction of the catalyst or difficult separation of the styrene.

   It has now been discovered that despite the presence of appreciable amounts of non-aromatic compounds in the system, ethylbenzene can be effectively separated and, if this is subsequently dehydrogenated, styrene of a purity greater than. 99.0 and usually 99.7%.



   According to the invention, ethylbenzene is separated from a mixture of aromatic compounds containing an appreciable amount of ethylbenzene, the ortho-meta- and paraxylene and a large non-critical amount, which is preferably less. to about 5%, of non-aromatic compounds by fractional distillation of the mixture of ethylbenzene and non-aromatic compounds, at a reflux ratio greater than 40: 1, in one or more distillation columns having at least 150 trays. During the distillation, the non-aromatic compounds accompany the ethylbenzene, which is however substantially completely separated from the other C8 aromatics.

   Surprisingly, non-aromatic compounds predominate with ethylbenzene, although they have higher boiling points than ethylbenzene and xylenes. Only a minor amount, eg about 20%, of the non-aromatics is withdrawn from the bottom of the column. This is due to the formation of an azeotope between ethylbenzene and the non-aromatic compounds. After the ethylbenzene-rich distillate has been separated, it can be treated to remove: non-aromatic compounds, for example by further distillation or by solvent extraction.

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   The separation of ethylbenzene by the process of the invention is much easier than by the process described in US Pat. No. 2,959,626, because it is not necessary to extract the fraction first. whole crude aroma from the feed to remove trace amounts of non-aromatic compounds boiling in the critical boiling range of the C8 compounds before starting super-distillation. Another advantage of the process is that the mixture of ethylbenzene and non-aromatics overhead and substantially free from other aromatics has a smaller volume than the initial feed, so that it is less expensive to separate the non-aromatic compounds.



   It has further been found that non-aromatic impurities overhead with ethylbenzene during distillation do not adversely affect the subsequent production of substantially pure styrene by dehydrogenation of ethylbenzene. Although non-aromatic impurities are partially cracked. in the dehydrogenation reactor, the formation of coke on the commonly used dehydrogenation catalyst is not a problem. Cyclic operation of the reactor to ensure coke removal is not necessary as in many processes using the same general type of catalyst for the dehydrogenation of non-aromatics.

   The styrene is advantageously separated from the residual compounds (which usually include non-aromatic compounds, benzene, toluene and ethylbenzene) by distillation under reduced pressure and at a temperature at which the styrene does not polymerize. When the styrene is separated from the dehydrogenation reaction mixture, the non-aromatic impurities remain in the ethylbenzene unchanged. A substantial amount of the non-aromatic compounds boil around the boiling range of styrene, but no more than the aromatics other than styrene, are drawn off at the bottom of the fractionator.

   Therefore, it is possible

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 to produce substantially pure styrene from the dehydrogenation reaction mixture despite the presence of substantial amounts of non-aromatic compounds in the feed to the device:, -! dehydrogenation. :
 EMI4.1
 According to a preferred process, 'e, thYlbenzene' is separated from the residual compounds, for example by fractional distillation and usually recycled to the dehydrogenation reaction. The ethyl-- -benzene separated in this way is usually contaminated with non-aromatic compounds and recycling tends to lead to enrichment.
 EMI4.2
 : '# -' *>: # - '3p.ï. ., v'.ï \, of the-system in non-aromatic compounds.

   Various methods allow ': \, to separate from the "system the excess of non-aromatic compounds. This ### #V #';, -, '1; I!" "' '.; ..,. Non-aromatic compounds can be obtained by various methods, as described below "" '.,. My usual carrier is Dnt.iur., le,'. i #Au:. '.; ".- w,,, - ..;

     . The overhead ethylbenzene recycle stream separated from the crude mixture from the styrene-forming reaction. - i ','. * #, '#', l,. : .r. -''- '': "j.:."I. --- * ## '"*' ,,: ,, '."' "# '' u Note that if the non-aromatic compounds **, - ..?, Note the y ,,; :; if "iT, o, a. u 7 '""' (1 (1 '.i ", -, -..." "-1;"'., - 'Í ,,, .i'. can accumulate to form approximately 5 at $ 20: 'of the current of ..;. t :: "1,: ## v' - '. # total, # s #: - #: t;:; g3ri {f ;; ttit :: total recycling of ethylbenzene, which is a relative concentration]. '-'. "# '...;"' J ...., .. "".) ", '1,.;" "' Compared with the initial super distillation mixture fed to the styrene reactor, this build-up of non-aromatics in the recycle stream will ultimately become easily controlled.

   Economically, it is possible to simply discard a sufficient portion of the recycle concentrate to avoid the build-up of non-aromatic compounds in the ethylbenzene recycle stream. However, there are other more economical methods which provide sufficient separation.
 EMI4.3
 health of aromatic compounds to avoid this accumulation, of - 1 .......-. 1 '::': '.. "" ".. ... ¯; r .. - 1Î \" non-aromatic compounds in the current of recyç14g ': Aé.thYlb, -.



  .¯..¯ - .. ¯ ¯z,:; '-x' .. - "-. ¯ ¯; - .. ¯¯, ... ¯.: -¯., ... c.¯: .. ¯ -.....--- .. , -. 3 ,, -.- "...- ¯ - ,,:, t.; = Â.-x - .. '.., r: ¯: -. T: = ,.:.



  ,;> :; t- &. ";:. In an ordinary distillation of a 'f; ct' :: r; ' niàÚluè ": - '1.' ''" t. !! " * ... ,, .- x ..... ..;: "" ....,.>: 4ar '..; Nr'. ' 2..a.r \: J '' '' 't -, -.-.'.-; j ..v ^.' - ''. "; - * -., r '#' M. #, r ..- T.-.-w.M.



  ., ', 7ên C8., The content. in non-aromaticslpeù1b¯être! ;; abalssée à z envii ^ on,; la, .à; 2: 'cians le procïüit f, .xai.: Par.çosgûentïliën' q, ué '/ t &;:! t, r! .2 \ en .... tg \ '' ''; .f> f'- 'tl <Ó \ .. "...: f! t: f1j ± ...' 1; J ;. z ^ ¯" "" .. \ l ":; U²'J: ', 4 ... 1T'1 t {. "T, wtJt7lfetiori' 'of the device' of: supe., .." Fi ??,, 'ssé': 'tale-: r1t:' yd :: Y ' , i'x "'' 's ^ 5> .." N' ..., ";,:", \ 1 '. ,,: j'fr!:!' .-i-: ",, '. (;;,.": 1;! ,, f ::'! "., 1 <: ''. ',.:.,. ;;;;.:

 <Desc / Clms Page number 5>

 
 EMI5.1
 up to 5% non-aromatic compounds without critically affecting the system, it is often advantageous to operate a distillation;
 EMI5.2
 preliminary C aromatic feed to reduce the content of non-aromatics to about 1 to 2% prior to comaen;
 EMI5.3
 cer super-distillation.

   Thus, in one embodiment of the invention, the ethylbenzene recycle stream from the dehydrogenation reactor can be passed through a prefraction device.
 EMI5.4
 neraent to separate excess non-aromatic compounds, ethylben- ###: ';' # <# / #. "# -" # '## j;' .; ifv-iv.
 EMI5.5
 zene being again :, 'distilled' mixed with. : the 'fraction in Cg
 EMI5.6
 : ..:. <... \ r: ": -J." H., ..:,., ",,.: t" ..;,: ...:. '.;,,: J. ; '. 1' l¯. #; * r. vaAr bzz - '"- - #"' # * 'i.V. iBi, ".. ', -.;, total in the sUPer'; istil1atioxi.

   We; re! J1arquera "that by this process ..... 4 r1" '. ? '.' (;;: "('
 EMI5.7
 ethylbenzene is completely recycled to the prefraction device.
 EMI5.8
 '# "# Ï--' .- .... -"; ... "'7f,. /.,;,'" ''11. (\ T' "" t ..-- ./. . \ .e '". operation and then passes through the super-fractionation device L,, ^,, d. .. ,, zr,. i, r.,' 9, F" 4. '.' x C. ', - before reaching the reactor e:', és $? Gn.

   This process is sufficient
 EMI5.9
 fit to avoid the continuous accumulation of; non-aromatic compounds
 EMI5.10
 :.:. {1: \ "': Jt:': J r.;, 1; I: <ï /#|;%f.;Ht''ir - '' -r", ² aans rs .: ., "',' '' ..I ... ... '., In the system. \ -'L /' èfJf'X 'ï & i: -," "' '
 EMI5.11
 A small amount of benzene and '; toluene is formed in
 EMI5.12
 s'. ,: ..,? "c ^ Fr, T. *. '. * - ^' r 't," zx:' r, ::: '' '\ ,,; '- "'": \ '' 1f: t.1I "'i.' '': '(:' ':;). ::,, .I, r.
 EMI5.13
 the reactor and. these products are usually: distilled before entering
 EMI5.14
 , ",. ',' i" - 'Il, .I \ ..' I !; ; Ww, y. '; .....' \ t "'.'", '. t
 EMI5.15
 undertake the purification of, -styrene '.

   The benzene-toluene fraction
 EMI5.16
 '"..':" '> f' '\ f'},, "7; J w ir '<: ii. ^" Îxy .. usually must be "separated by: vacuum distillation to avoid to subject the styrene on mother; 61rsable to tepera-. #. \ .. '"" i -;.?.:!,' ..1 (f "" '!' high tures favoring its polymerization. ; Deletion of 't! V,.,' R (- / ". '-:"'..;:.! "Preliminary distillation of 'KbarizJënè; ^ and toluene and its replacement - ...... ..;., ': 1'- ::' - 1 "., R.wrJ: L '!' ..... t;, -; j. T '" "1 J"' "''" ..-: -'-'''.- ..-. '; "' r .. '
 EMI5.17
 ment by the prior purification of the styrene by a distillation
 EMI5.18
 "11 'j' '' '- .."; s;,:! .,: iâcz ;;. aß:. r, - under vacuum constitute a king that is imïànt with respect to .. 1 .... /..-##"'-">.'.;.# '? (t .4 (\' r 1. '1 /: 1, known methods.

   The ethylbenzene passing overhead can then be treated under the ', press: tonlatmÓsphé: iqii'1; -'; q {ii "''; t''pÓssiblë 'after the -. ¯ep.arati9dlJ .. sty ;: ene, d.! Ístabe., Cet'rê9!), duie ..: gu .... 1; 8;, ioeen tj .. "'¯. .sr.rT ^.-.-. t-rtre-9-r.,. ". '" * "" --- t- is advantageous independent of the eo, iear'râiôn7Jdës' "non-aromatic compounds in the a6ékToe6êd.êpëat''Têàai.Te, the; t dimension, of the drspbitr: \ "; 'f1' ':' ': d *: uene.



  '- i'} -. 4-- ",,,,," I ... ';. p..1 ^ ;, - ..' 1 ',. ""': '-': '. .!.; '! f: ... i'I..r.: ..: t .... 7 "" X?' f: # '- # "' '#", $' In -t ': ç. w.tx,; s'Ft-t'Ë, .., M, '' a '"Nial3 ..f." 3 ¯ or de; 6ti: .concm ra- & recycling r; ..' (.. }. '/ r, / ""; - .. (1 ";' ...., ;; i ''. J $ i:."; '-;' * tr 1 ', ..-}. . ,,, ... '; (' -: M "; - '1; .1' \ .., ..>:> o;." 'Ft; rJ r. ,,, J,. \, - (...... I ... of ethylbenzene passing Em "têé; la :: t; r8: ctio ,,., E'.r, total cycle pas- '".:;, I, .. ¯.,; S; 3> ::? Y '<: -; w. S, rx:;. "." -; ...

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 The head is taken to a distillation apparatus for benzene and toluene. The non-aromatic compounds are overhead with the benzene and toluene as an azeotrope, leaving recycle bottoms of ethylbenzene from which the non-aromatic compounds have been completely or almost completely separated.

   The accumulation of non-aromatic compounds in the ethylbenzene bottoms recycled to the dehydrogenation reactor is thus controlled and this accumulation in the system is therefore also controlled.



   In a third process, after separation of the benzene and
 EMI6.1
 toluene, a small part of the ethylbenzene and recycled non-aromatic compounds is taken up in bypass current and passed through a small extraction unit where the non-aromatic compounds
 EMI6.2
 ,. , '1: \\ f., "- l" "<t / ..' X., '... are separated, but usually only in sufficient quantity to avoid the accumulation of non-aromatic compounds in the ali- total mention of the dehydrogenation reactor.



   In a fourth variant of the process, a second distillation apparatus placed downstream of the column for separating the benzene and toluene can receive a small part of the recycle stream, taken as a bypass, distilling therefrom a sufficient quantity of the non-aromatic compounds. and returning it more concentrated in ethylbenzene to the ethylbenzene recycle stream. The amount of non-aromatics separated off is controlled so as to be just sufficient to avoid the build-up of non-aromatics in the feed to the reactor. This variation is particularly applicable to a system in which benzene and toluene are removed before separation of the recycle stream of ethylbenzene from the dehydrogenation reaction mixture.
 EMI6.3
 



  The invention is -illù.5tréë-par-lësaës'sins-ahriëxéff: in the- z "...! O '.



  ,, <.. "" II. ' / ;, ', which:
Fig.l is a flow chart of a process comprising the
 EMI6.4
 ; # '# ## #'; '> *'> *? * '% * îf'j! &' "'#; #' 4.:**,. ,, fy.'Vf. pre-fractionation and t; ;. 01, ² '; I ..' \ t .. \) ,,;, ..;. '.... pf. I!;: "'" Trans- ', 1' \ ': ,, , xx. LK; TE - formation to styrene, the "separation" of Btyrene, thus regulating ethylbenzene and recycling it to the dehy reactor.

 <Desc / Clms Page number 7>

 drogenation.



   Fig. 2 is a flow chart illustrating the removal of a sufficient amount of non-aromatics with the benzene toluene distillate to prevent build-up of non-aromatics in the ethylbenzens recycle stream.



     Fig. 3 is a similar operating table in which a bypass stream taken from the recycle of ethylbenzene is solvent extracted to separate a sufficient quantity of non-aromatic compounds and to avoid the accumulation of these.



   Fig. 4 is a process chart similar to that of Fig. 3, but the non-aromatic compounds are separated from the by-pass stream by distillation.



   Fig. 5 is a flow chart illustrating the recycling of ethylbenzene to the pre-fractionator to separate excess non-aromatic compounds by redistillation in the distillation column.



   Fig. 6 is a detail showing the use of an additional column to distill from the distilled mixture of ethylbenzene and non-aromatic compounds a concentrate rich in non-aromatic compounds which is discharged from the system so as to avoid substantial accumulation. of non-aromatics in the ethylbenzene recycle stream.



   Fig. 7 shows a similar operating table in which an additional column is connected in series with the super-distillation column to separate non-aromatic compounds from
 EMI7.1
 the fraction of ttēg vi tez¯a.ns :. their excessive accumulation. In the drawings:. Fig. 1 illustrates a method for treating a feed
 EMI7.2
 mixed aromatic from a source 10 and. which contains a \ id ",,},, \ l substantial amount, for example 2 to 10%, of non-aromatic compounds boiling in the boiling range of the compounds

 <Desc / Clms Page number 8>

 aromatics in C8. The feed typically contains toluene,
 EMI8.1
 ortho-, meta-, and parax3lene non-aromatic hydrocarbons and ethylbenzene.

   The power goes through a pipe
14 in a pre-fractionation column 12, where conditions are set to separate at the top through line 16 a fraction having a substantial content of non-aromatics, leaving bottoms which are withdrawn through line 18 and contain ha bitude 1 to 3% of non-aromatic compounds. Prefraction food. The still passes to a still column 20. The still column 20 actually includes several columns totaling more than
150 trays and is described in more detail in Fig. 5 of U.S. Patent No. 2,959,626.

   Column 20 separates the C8 aromatics so that the overhead taken off through line 22 is a mixture of non-aromatics and ethylbenzene, the bottoms drawn off through line 24 being substantially pure xylene. led to storage. For example, if the initial feed to column 20 contains 20% ethylbenzene, 78.8% xylenes and 1.2% non-aromatics, the overhead portion passing through line 22 contains about 95, 3% ethylbenzene, about 4.5% non-aromatic compounds passing as an azeotrope with ethylbenzene and only traces, for example about 0.2% meta- and paraxylenes.

   The mixture of ethylbenzene and non-aromatic compounds then passes through the pipes
22 and 56 in a conventional dehydrogenation reactor 26 containing a bed of alkaline-promoted iron oxide catalyst known to those skilled in the art, for example Shell Catalyst 105 and those described in US Patents Nos. 2,414. 585 and 2,460,811. In the reactor, the mixture of hydrocarbons is passed over the catalyst with water vapor at elevated temperature, at approximately 500 - 800 C, with a water vapor: hydrocarbon ratio of approximately 15: to
 EMI8.2
 ï, - ¯ 30: ¯l.

   Consequently a mixture of styrene..d "thylbenzene, of other

 <Desc / Clms Page number 9>

 Very aromatics and residual non-aromatics which have not been cracked in the catalytic reactor enter a settling drum 28 through line 27.



   The crude reaction mixture, after sedimentation of the water vapor condensed in drum 28, passes through line 30 to a recycle column 32 where. the unchanged ethylbenzene and the non-aromatic compounds are distilled off as an overhead with a small amount of benzene and toluene forced into the reactor 26 and recycled through line 34, as described below. The bottoms of the recycle column 32 which include styrene and a substantial amount of ethylbenzene pass through line 37 to still 36. Ethylbenzene drawn from still 36 as a fraction of head through line 38 is returned to recycling column 32.

   The partially distilled styrene withdrawn in the form of bottoms from the still 36 through line 40 reaches the styrene scrub column 42 where pure styrene is withdrawn overhead through line 44 while tar bottoms and polystyrene are withdrawn through line 46. Columns 32, 36 and 42 operate at a pressure below atmospheric pressure and at a temperature below the polymerization temperature of the styrene to minimize polymerization losses. The freshly formed polymers leave the system as bottoms through line 46 of the styrene scrub column 42.



  The top fraction from recycling column 32 passes through line 34 to a still 48 upstream of the styrene separation. The still 48, which may be called the benzene-toluene column, can be operated at pressures equal to or greater than that of the atmosphere and the corresponding distillation temperatures desired. The toluene and benzene more volatile than the recycle fraction go overhead and are withdrawn through line 50, while the bottoms containing ethylbenzene are withdrawn through line 52 and returned to

 <Desc / Clms Page number 10>

 largely sent to the dehydrogenation reactor 26 through lines 54 and 56 and valve 80.



   If all the tails of column 48 go to the reactor
26, non-aromatic compounds tend to accumulate in this system; for example, if the feed entering the column
20 through line 18 contains 1% non-aromatic compounds and approximately 20% ethylbenzene, the mixture of non-aromatic compounds and ethylbenzene withdrawn through line 22 may contain 95% ethylbenzene and 5% non-aromatic compounds before its entry into the dehydrogenation reactor 26.

     Since the per-pass conversion rate of ethylbenzene over the dehydrogenation catalyst is significantly greater than that of non-aromatics, the ratio of non-aromatics to ethylbenzene in the recycle stream to the Line 34 is greater than the ratio in the raw feed of line 22. As a result, non-aromatics are continually accumulating in the system. Since the non-aromatics are more concentrated in this recycle line, the build-up of the non-aromatics in the system can be avoided by withdrawing the recycle material.

   To this end, the bottoms withdrawn from the benzene-toluene column 48 through line 52 are divided and a portion is removed from the system at 53. In this way, an excessive build-up of non-aromatic compounds in the system is avoided. .



   Fig.2 shows the part of fig.l starting at line 22 but not including valve 80 and line 53.



  It illustrates one way in which the build-up of non-aromatics in the system can be avoided by adjusting the conditions in column 48 so that excess non-aromatics are removed in the overhead. This is convenient because non-aromatic compounds, although they have higher boiling points, tend to form an azeotrope.

 <Desc / Clms Page number 11>

 
 EMI11.1
 with the? compounds drO.rt? 2.7 .'- 3C-'S.

   The CO: 30sés non aror.:atC!':E:S, ", -", t to some extent form an azeotrope with the more volatile aromatic compounds, benzene and toluene, airsi than with ethylbenzene. Therefore, instead of withdrawing part of the ethylbenzene recycle stream through line 53 of Fig. 1, this variant of the process based on the formation of an azeotrope can be used.

   From an economic point of view, it is possible to simply withdraw a heavier fraction from the still 48, which removes a part of the non-aromatics with the benzene and the toluene as well as a little. ethylbenzene to control the buildup of non-aromatic coubosates in the system; in this way, the loss of ethylbenzene from the system is less than that caused by the process illustrated in fig.l.



   However, it is also possible and often desirable to have recourse to a variant illustrated in FIG. 3. In this process, the mixture of ethylbenzene and non-aromatic compounds leaving the benzene-toluene column 48 as a bottoms fraction through line 52 can be processed in extractor 60.



  If all of the bottoms from column 48 passing through line 52 are to be removed, they are all diverted through line 58 to a solvent extraction column 60 in which aromatic and non-aromatic compounds are separated. The non-aromatic compounds are withdrawn from line 62 and ethylbenzene through line 64, hence it passes to line 54. However, it is not necessary to separate all the non-aromatic compounds in the extractor. 60, but only an amount sufficient to prevent their accumulation in the system.

   The extraction process can alternatively be performed using a control valve 59 which restricts the flow in line 54 and diverts a sufficient amount through line 58 into extraction unit 60. The rest of the tails of column 48 with toluene and benzene passes through

 <Desc / Clms Page number 12>

 line 54- and mixes with pure ethylbenzene from line 64 which does not contain non-aromatic constituents or in any case a little less than the feed supplied by line 22 to the dehydrogenation reactor 26 in passing through line 56,
 EMI12.1
 in! ',,' "- The extraction unit 60 can also be supplied by the head fraction in line 22.

   The mixture of non-aromatic compounds and ethylbenzene can be taken up entirely without
 EMI12.2
 .r-, y :. 'partly from line 22 through line 22a and fed to the extraction unit 60 through line 58. The quantity of the mixture supplied ,,,,,
 EMI12.3
 bzz,:; '1 "'.: ;; '. ::;' '' !: o ';:,:".' Extraction unit 60 is regulated by valve 63.

   In this way. wÊçostS * before "passing through the dehydrogenation reactor 26, the '.: - #"' L, ::. l, ';;, Ó.>, "....., non-aromatic ss are extracted complete,.; m1'i; .9; .en, part ::. y, v x.,' 1 .valve . is' open- or '. # ,;' G'xv; P; j, "f ,, k '". P, ", ¯4 .a' '! -'. tf: 1 .." {"t, '\, ::, 1" #. # -.' ".- # .. '." "'", "... <" "4%;)) 3-t,; x. ,, ¯ =. ., 'The quantity passing through line 58 is minimum.: #' "''! M- when very efficient solvent extraction is carried out in
 EMI12.4
 W #, '# ,, ¯a y,; ry1 # ## /; ', extraction funity 60, so that substantially all of the compo-fCf:; 4KV'- - ## "# <- ##'.; # .-. '. r #. -, v,; 4' : * Non-aromatic substances are separated from ethyl7.benzene.'U, advantage, de - ", t, ', #' ##: - ... , '. ; ' , .. A, fy '' r'râ # '"pr ..' s, fQ; ß 1" .. 1l., <1, ''.:

   'of a very efficient extraction is .qu'dl; ne i'-faut .-.' bzz,, f.ioh, with a 'very' extraction - * # '# *' "- '#' # ': #: V #'> ter a relatively small -volume from pipe 52.



  1rVlJt, I "'l"' "r,"; ;,., Y .: r ", I.c 7. Consequently the extractor 60 can be relatively small and
 EMI12.5
 i! .îèt: -r certainly very small compared to the extraction unit. previously used in the process described in US Pat. No. 2,959,626. Alternatively, a larger part or representing
 EMI12.6
 Also, all of the top fractions passing through line 52. "; xt :, will be diverted through line 58 by actuating,;. valve 59.



  ,. "'5!: -.".' 0-. thus, can use! me unitf, .. thus yielding, one can use a larger unit 60 func4 '¯ # àmsnt "little" efficiently, since only one has to be separated, "' j't1" '"-'". ' . 'tf #' v. # - ## !: '# / .. # - # -' # '- r *. lower relative oportipn.of non-aromatic compounds ... The 'i'i-iyjWiWiY'ïiSiMSi! .saor-rrr-rr: # * # # r # # # ¯ #: # r # # # * .'- '.' ':.-'--' '. "'-"' "# ''! ' 'in ti it is obviously- to separate'ér: 1ine'; qÍi8rlti té .uf-; 3 - <.? t '' 1; ", j. ; # #; -, ',).: "i'-, i'ITjfnte'desîccimposés non aromatic, by extraction-for é vit ers. #vf? -: ,, w4, h ,, .u $ y'.a ',> ftY tt. Lt. - \\ -' \ -.'... / - to 4''k1f ': ^' ',,.:. "aJ9," if "' Myj.



  Ion at 'd:' 1 ':;, /' d "1t IS .. 4 ,,%, f1; ,,;:;, 'J' \ ': cuMUtipn. At pipe 56. * v & V ? "* â- * 4k '' -.j

 <Desc / Clms Page number 13>

 
In a fourth variant, illustrated by FIG. 4, at least part of the mixture of non-aromatic compounds and ethylbenzene leaving the bottom fraction of the benzene-toluene column 48 is distilled in a column 66 before reaching the line 22. , which removes a sufficient amount of the non-aromatic compounds as an overhead to prevent their accumulation in the system.

   The quantity of the mixture passing through column 66 is regulated by a valve. 59..The fractions, of
 EMI13.1
 \ i; , r - ###>. # "" '.,. l "\.' \" 'f' .; 1 '1f: .t'I. "' I":} ;. the "> '# J tail of column 66 are discharged through line. 6 $ and return to line 54' downstream of ' valve 59, serving that the recycling continues through line 54 to reach the burnt feed.
 EMI13.2
 .> 1 - '...,: "F; Y" ..' "'": te of' the} line 22. The distillation column 66 can operate ", \,. ':' \ '!;" 1t ,,; l "t" "'r; .. II. ,, -; \ 1 ->', - '- - # # #!'; '' 'I \ -'. J economically without regard to efficiency.

   The fundamental requirement of this process. distillation is the removal of a quantity
 EMI13.3
 j ': i. * # * # ## - <; ' # ":?; '' # * '' ī #, '-' - .. '##' - sufficient side of the non-aromatic compounds to reduce their content # -;> ##? - -K -r'fC, .. ',, U-,;

   S, '. ,;. 0IIj centration in the pipe 54 so that by combination with. ,; ,. "'' \;:> 1,." '; ± 1 ......' ... 1'0 .if: 1 "..- ', - #; -;.", 4i.l; ¯- '' j #les, compounds' of line 22 oh obtain a concentration con- s- *,;,; t'5-'f * '"ç' ->: # -" ##; .. # "',' * '*.;' '#,, - .. ,, stante on non-aromatic compounds during continuous operation of the debydrogenation system.:
In a fifth variant illustrated by FIG. 5, the mixture of ethylbenzene and non-aromatic compounds: withdrawn from the,
 EMI13.4
 ... t1. ,, r- .., J;

   '*' ¯ "d,,,, w yt, 'vi" r *, Y' ^, column 48 by there. Pipe .52 can be divided using, valves 7 "" ", ';' i (: '> ;;). \ o;' 1 \ l; "", 1 .. "oI - .. 1- <? r '' 59 and 61, one part being recycled through line 54 to line 22 and entering dehydrogenation reactor 26 through line 56, the other part passing through line 70 to line. admitted...
 EMI13.5
 



  #> iïu, '<,, Ây.i & i-rr>% Mx "-' '#' '#' '- # - =. ##. t- * - # \ i sion 14: raw power to be recycled by the dtspo3- '' '' ..''- '' '' 'rc';, :: d1S? tif'a, éf "'raotiol2; De'gett ag4. ,, .F,: .-, - <- Ti? , ..; "t>: 7,: -", - ,; 'i ":: r; -ri", :: rC-f.t' '' '""' ¯ k "..., x 'y' '',. ',' ......?; ** "<'.' non-aromatic; is: significantly reduced in the device ds pr4- '"'; '' '(-'''.-. - 1.- r" "" - ..' '10 ..? "-.... 4. t <"t:; t1èîit'i2: (-: = oss' leave 'this unit in the form': r '' * # of ** frafttlon¯ldê !, # fceteîipar '.' j.aconduite lo ïL * aliaaa1 viojïjpené" ccaût. i | g '.' d, & s: being 'ôgê'nation' ,, 26; by: J.à '!' ccindúi.i # ê f56 a # ie'-te'v f;; t "" ": 1 '.: .- ii' 4:" sr '"ü ^ -', t, rv F-.xç" '<.:%' i4 ",.:! F7 jt; <t:, {: .:. "l '.,. J,' '"' hour "in" non-aromatic compounds somewhat greater than that of O '... d -.,. (\. \ J',. \ 'l '', l (}. ('\. r -: .....

   (t ...... I..J ../: 'f -'1.' '' '..,' ', - ..: r the feed in, pipe 22 due to the content relati- j

 <Desc / Clms Page number 14>

 higher rate of non-aromatics in the recycle stream passing through line 54. The ratio of flow rates through lines 54 and 70 can be adjusted such that the selected higher content of non-aromatics in the system passing through. line 56 remains substantially constant and stops increasing.



   In a sixth embodiment of the invention, illustrated by FIG. 6, the feed mixture is passed to the still column 20 and is separated into a top fraction, drawn off through line 22, and into a bottom fraction, drawn off through line 22.
 EMI14.1
 L) .... j. 4 aT, r 4,. line 24. The tail fraction mainly comprises iso. mothers of xylene. The overhead in line 22 comprises significant amounts of ethylbenzene and non-aromatic compounds. Via line 22, the top fraction is fed into an ordinary distillation column 72 which is adjusted so as to separate a large part of the non-aromatic compounds as well as part of the ethylbenzene, as the top fraction withdrawn by. the pipe 74.

   Part of the overhead is returned to the column as reflux. The bottom fraction from column 72 is taken back through line 23 and sent to line 56 where it combines with the recycle stream from column 48 (not shown) sent directly to dehydrogenation device 26. Valve 80 serves to control the flow rate in line 54 so that part of the recycle stream from column 48 can be sent through line 78 to column 72. If valve 80 is fully closed, the total recycled material is. supplied to line 22 for distillation, then via line 23 to the dehydrogenation device
26.

   The amount recycled through line 54 is mainly based
 EMI14.2
 on the percentage "<ïe '" coNposed non-aromatics in line 23 and on the amount of non-aromatic compounds which is harmful to the
 EMI14.3
  x :. ,. i: k "1 .:>, - '"' ''.,; ", '' '' ecOû m1qite operation of the dehydrogenation device 26.Noe% - z ::;, 'u."'! -. 1; I "" "'I .... t'" 1 '":::"' \ 'f) q! Jrt r iù, t; ..;. "" T.,. -. - =,:,. J .. '1t.t.: O. ,, 1u-W fT ". ; "" '\:' 4, "#" '1' .., + 'l' '! (i \ .- t! t. mothering, the bottom fraction of column 72 contains up to, about 5% non-aromatics and is sent by the con-
 EMI14.4
 about 5 of non-aromatic compounds and it is sent by the con-. <r '1 .. f'.: .. / t4 ':

   ] .. f .. 'fr t,.)?:, .., pick 23 to the dehydrogenation reactor 26 with the recycling pro-

 <Desc / Clms Page number 15>

 coming from line 54 and passing through line 56. In this embodiment, the pre-fractionation device 12 is not essential to the operation. Crude C8 aromatics containing up to about 5% non-aromatics can be fed initially through line 18 to super-distillation column 20 to separate ethylbenzene from the other aro components. matics in Cg. The mixture of ethylbenzene and non-aromatic compounds. It is only fractionated afterwards to reduce the content of non-aromatic compounds immediately before the dehydrogenation.

   The build-up of non-aromatic compounds in the system can therefore be prevented by any of these methods described above.



   In a seventh embodiment, illustrated by FIG.



   7, the still 20 comprises not only three distillation columns 101, 102 and 103 (as previously indicated these will be shown in detail in Fig. 5 of US Pat. No. 2,959,626), but also an additional column
108 facilitating the regulation of the accumulation of non-aromatic compounds. ques. In fig. 7, the raw feed entering through line 18 first passes into column 102, the bottom fraction of which is returned as reflux through line 19 to previous column 103.



   The bottom fraction from column 103 consists essentially of pure xylenes and is withdrawn through line 21. The top fraction from column 103 is sent to column 102 through line. te 106 and the top fraction of column 102 goes to column
101 through line 105.

   The top fraction of column 101, consisting of relatively refined ethylbenzene and certain. The quantity of non-aromatic compounds is sent through the pipe
107 to the final column 108, a reflux stream consisting of relatively pure ethylbenzene is returned to the preceding column 101 through line 109. The top fraction of column 108, which is withdrawn through line 110, is a mixture of most non-compounds. aromatics and a little ethylbenzene; part of the top fraction is condensed in condenser 114 and

 <Desc / Clms Page number 16>

 returned to column 108 through line 111, the remainder being withdrawn through line 112.

   The relatively refined ethylbenzene produced has a very low content of non-aromatic compounds and is withdrawn through line 113 to. an intermediate level. The feed to line 113 is combined with the recycle stream from column 48 in line 54 and passes to dehydrogenation reactor 26. The content of non-arostatic compounds present in the reactor can also be controlled. the recycle stream from line 54 so as to avoid a build-up which interferes with the dehydrogenation reaction in reactor 26.

   Therefore, the previously described methods of controlling the accumulation of non-aromatic compounds in the system can be easily applied to the flow of.
 EMI16.1
  # ¯īt¯¯ ¯ ¯ Y. ".n. Recycling of line 54 when using additional column 108.

   If desired, additional column 108 can also separate some of the non-aromatic compounds from the recycle stream from line 54. The following examples further illustrate the invention.
 EMI16.2
 4 EXAMPLE I.- - ¯ '"
Following the procedure illustrated in Fig. 1, a crude feed mixture comprising, by weight, 3.0% non-aromatic compounds, 1.5% toluene, 20.0% ethylbenzene. , $ 10.5. p-xylene and 9.0%. de.2-xylene is introduced into a column of! pre-fractionation. The pre-fractionation column consists of a distillation column comprising 60 trays and operating at a reflux ratio of 15: 1.

   The top fraction continuously withdrawn comprises 55.6% of non-aromatic compounds, 41.6% of toluè-; ne and 2.8% ethylbenzene. Analysis of the tail-fraction of the
 EMI16.3
 Prefractional column indicates that it contains 580%.-xy = lene, 9.4% o-xylene; 10.9 ± -xylene, $ 20.7. ethylbenzene and 1.0% non-aromatic compounds. '
 EMI16.4
 'j> \.' 4.:. The tail fraction of the pre-fractionation column is -..... t.j -t. T ###. ! 1 .- :: "1L: ac? E .... ¯ .. L ri .nY'vPtr? ..... r., ...... r -rH \ <" ,; l, ; "''> 'r- <J ::' 11 ''. 1 '' '....." 1 "introduced into a three-column distillati on total-.; du1 in,' -" - distillation , three columns totaling 390 trays.

   Thefraction, in C8 sent to the

 <Desc / Clms Page number 17>

 Three column distillation is first heated in a heat exchanger (not shown in Fig. 1) where. the temperature is high enough to volatilize some of the C8 aromatic compounds. After leaving the heat exchanger, the feed enters the first column of three columns each with 130 trays, and is distilled at a reflux ratio of 75: 1.

   The overhead vapors from the first column are introduced to the bottom of the intermediate column and the overhead vapors from the intermediate column pass to the bottom of the third column - At the same time, the liquid collected as a bottom fraction in the intermediate column and in the third column is recycled to the previous column near the top. The total effect of the three columns corresponds to the effect of a single column comprising 300 plates and operating with a reflux ratio of 75: 1. The top fraction of the third column, consisting of 95.3% ethylbenzene, 4.5% non-aromatics and 0.2% xylenes is sent to a dehydrogenation reactor. The xylenic bottom fraction from the first column is sent to the storage vessels.



  The tail fraction has the following approximate composition:
 EMI17.1
 85, 1 ± of Z- and ± -xylenes, 3 of ethylbenzene, 11.5 ± of-xylene and 0.3% of non-aromatic compounds.



   The overhead fraction consisting of concentrated ethylbenzene from the third column is mixed with recycle ethylbenzene and introduced into a catalytic dehydrogenation reactor. The reactor operates at a temperature of about 600 C and a pressure of 0.7 kg / cm2 and with a water vapor: hydrocarbon ratio of 17: 1. The liquid part of the dehydrogenation product formed consists of 0.6% benzene, 2.5% toluene, $ 6.6. of non-aromatic compounds, 56.8% unchanged ethylbenzene and 32.8% styrene. The gases produced leave the system through a vent (not shown.

   The mixture is introduced into a crude styrene distillation apparatus in which a recycle head fraction rich in ethylbenzene is obtained. The tail fraction

 <Desc / Clms Page number 18>

 consists substantially of styrene and a minor amount of decomposition products. The crude styrene distillation apparatus operates at a pressure below atmospheric pressure and at a temperature which minimizes polymerization of the styrene. The ethylbenzene recycle stream passing overhead
 EMI18.1
 includes 84s1 ethylbenzene, 99 non-aromatic compounds, 0.9% benzene, 3.8% toluene and 1.0% xylene.

   The overhead recycle stream is passed to a benzene toluene column where the benzene and toluene are coarsely separated to give a product which then comprises 10% non-aromatics 86% ethylbenzene, 3 % toluene and 1% xylenes. The dehydrogenation reactor gives about 40% per pass conversion of ethylbenzene while the conversion of non-aromatics on the catalyst is only about 15%. In this continuous process, all of the unconverted ethylbenzene is returned to the reactor as a recycle. Since the conversion of the non-aromatic compounds is only on the order of 1/3 of the conversion of ethylbenzene, this total recycle has the effect of increasing the ratio of non-aromatics to ethylbenzene in the system.

   This increase can be adjusted according to the process shown schematically with regard to FIG. 1 by withdrawing part of the recycle stream from the reactor through line 53. By this means the system is brought into a steady state and the concentration of the non-aromatic compounds in the recycle stream fed to the reactor can be regulated. at 10%. in weight.



   As this example shows, the content of non-aromatic compounds in the recycle stream, which is 6% by weight, increases continuously to finally reach 10%. When the content of non-aromatic compounds in the recycle stream reaches 10%, 14% of this recycle stream is continuously withdrawn.



  Since virtually all non-aromatic compounds pass with the recycle stream rather than with the styrene, the amount withdrawn from the recycle stream depends primarily on

 <Desc / Clms Page number 19>

 the concentration of non-aromatic compounds tolerated by the dehydrogenation reactor. When the permitted concentration of ncn aromatics is reduced to 8% by weight of the recycle stream supplied to the dehydrogenation reactor, the amount withdrawn from the recycle stream is. 32%. Therefore, the accumulation of non-aromatics in the system varies inversely with the fraction of recycle stream withdrawn from the system.



  However, the production of sytene from super-distilled ethylbenzene decreases with the increase in the fraction withdrawn from the recycle stream.



    EXAMPLE II. - The process of Example I is repeated, but with the benzene and toluene column operating so as to take a heavier fraction of the recycle stream while sufficiently removing non-aromatic compounds as well as a little ethylbenzene to reduce to an acceptable level the content of non-aromatics in the recycle stream which mixes with the product leaving the super-distillation column before passing to the dehydrogenation reactor. The benzene-toluene column is run to form an overhead of about 14% toluene, 9.5%. benzene, 12% non-aromatic compounds and 64.5% ethylbenzene.

   The bottom fraction of the benzene column ..et. to toluene is recycled to the dehydrogenation reactor and mainly comprises ethylbenzene containing 10% non-aromatics and traces of benzene and toluene. The losses of ethylbenzene are reduced to about half that which occurs by the procedure of Example I, while the accumulation of the non-aromatic compounds always remains under control.

 <Desc / Clms Page number 20>

 



   EXAMPLE III The procedure of Example I is repeated again, except that, instead of rejecting the part of the recycle stream which is withdrawn, it is treated to separate the non-aromatic compounds therefrom or an appreciable part of these compounds. This operating mode represented by FIG. 3.

   For example, 7% of the recycle stream has passed through a solvent extraction unit 60, where substantially all of the non-aromatic compounds are removed by any known process; This unit -operates, for example at about 140 C and under a pressure of 9.0 kg / cm2 using as a solvent diethyleneglycol with 6% water:
 EMI20.1
 process between "in Ié3 * extraction system by a. *. #,", #>. "> /; -, - '' -" 'if ",' ## **. '# ta heat preheating it to almost 140 C and is sent from there to an intermediate level of the extractor.

   Hot solvent at 140 ° C and 9.0 kg / cm2 enters the upper part of the extractor in a ratio of about 10 parts of solvent to 1 part of recycle stream. At the same time, a reflux of hydrocarbons from the top fraction of the solvent dephlegmator is brought to the bottom of the extractor. The paraffinic recycle current represents about 15% of the solvent. -
The non-aromatic compounds contained in the recycle stream supplied to the extraction apparatus pass through the extractor teat and are thus removed from the system. The hot solvent extraction solution rich in aromatic compounds is drawn off at the bottom of. the extractor and cooled to 110 C in a heat exchanger.

   The solvent is then sent to a solvent dephlegator. The solvent dephlegmator causes by lowering
 EMI20.2
 '},' "{# in pressures at less than about 130 kg / cm 3, the vaporization instantaneous, without the volatile non-aromatic hydrocarbons which. f '' '' * -. '' µ- v * '<' '.' 'non-volatilized solvent.' '' ,,; ci compounds aromâita çlwr.ôrid, 'de.ssection of' vaporization of 0 ..¯. .. ':' jMSf: .-. t ^, '; w '; The instantaneous vaporization is directed by a valve controlled by a float towards a point near the top of the solvent dephlegmation section.

 <Desc / Clms Page number 21>

 



   As the non-aromatics dissolved in the solvent are the most volatile, they are efficiently removed at the top of the column along with some of the more volatile aromatics. This material is condensed in a heat exchanger and sent as reflux to the bottom of the extractor.

   Almost 100% pure ethylbenzene stream is taken from the bypass dephlegmator with water vapor. This mixture of vapors is cooled in a heat exchanger to condense the ethylbenzene which is collected in an accumulator from which it is withdrawn and sent to the recycle stream feeding the dehydrogenation reactor. '
 EMI21.1
 BPLP iv.- ##; ## $ ± '#' # ''
An effect similar to that obtained in Example III is achieved by replacing as shown in fig. 4, the extraction unit by a distillation column 66.

   The separate distillation column serves to distill off a quantity of the non-aromatic compounds rather than to remove them as previously described.



   The feed enters the column at an intermediate point. The column works with a reflux ratio of 10: 1 and gives an overhead fraction comprising 15% non-aromatic compounds and 85% ethylbenzene with traces of residues.



  The relatively purer ethylbenzene fraction is withdrawn from the column as a bottom fraction / 50% of the fraction going to the recycle stream and the remainder being recycled to the distillation column. The column operates with low efficiency and returns 9.5% of non-aromatic compounds to the recycling stream.
 EMI21.2
 #clago Eiâiityiûe3it.ôis $ .s'aâ coursât dcceyclss is mixed with the top fraction of the super-distillation column as in Example I, and treated in the dehydrogenation reactor.

   The
 EMI21.3
 combination of the current 'is; ëâélâgéy', 8 'with' jlt¯hyl'benzèné 'super-distilled containing..5t'pt: deoaapôsés'r: one aromatics gives in ,,. f in 3e- {co3Hpte toi product <xmataire to taTïm non-aromatic compounds which feeds the dehydrogenation reactor.

 <Desc / Clms Page number 22>

 



     EXAMPLE V.-
In a variant of the procedure of Example III, the overhead fraction in line 22 containing 4.5% of non-aromatic compounds from the super-distillation column 20 is fed through line 22a to the unit of. extraction 60, as shown in FIG. 3. The enriched ethylbenzene stream containing 1.8% non-aromatics is then passed as feed through line 54 to dehydrogenation reactor 26.

   The. dehydrogenation product is treated as described in Example I, except that no fraction is subtracted from the recycle stream of the system. A sufficient amount of the non-aromatics can be separated by treating only 50% of the overhead fraction. the super-distillation column in the extraction unit to regulate the build-up in the system.



  EXAMPLE VI.-
It is somewhat more economical to use the pre-fractionation device 12 to fractionate part of the recycle stream passing through line 54, as illustrated in FIG. 5. To this end, 15% of the recycle stream from the line 54 is passed, using the valves 59 and 61, through the line 70 entering the pre-fractionation device 12, the remaining fraction joining the line 22. and combining therewith to form the current supplied to the dehydrogenation reactor via line 56.

   The tail fraction of the pre-fractionator contains approximately the same amount of flavor compounds as in the case of FIG. 1, but it is significantly
 EMI22.1
 richer "in etP.IDer2cié: awcmposz xôn: sortrcWeI '" Gta de la I ,,! "' '"'> '<IF "...-' ...



  '. ; "." ..;. . -.rI! ' C: super-distillation column and happy passing .. dehydration reactor - #. # *, ..



  . Fl) "" '' generation remains unchanged. And the system 'works the same way 1 $. /. "...... .....'" .. t; "\. (. '1. ... ",: #; ç.-t-; - (o! -" \. ,, ... '.... as-described in the example., rec.et-¯: ex2 ;! that the pre-fractionation device is used, which eliminates the need for the enrichment column 66. This process results in a slightly higher load on the super-distillation column.

 <Desc / Clms Page number 23>

 



  EXAMPLE VII.-
In another process, the pre-fractionation device 12 can be omitted, as shown in FIG. 6. According to this example, a distillation apparatus 72 comprising about 60 trays is inserted in the circuit of the pipe 22. The top fraction of non-aromatic compounds and ethylbenzene is introduced in the middle of the column 72 which operates with a reflux ratio of 10: 1 The overhead fraction in line 22 comprises, due to the omission of the slow pre-fractionation device 12, about 12.9% by weight of non-aromatic compounds and 0.7 ; toluene, the remainder being ethylbenzene. The bottoms fraction withdrawn through line 23 contains 3.0% non-flavored compounds, the remainder being ethylbenzene.

   The bottoms fraction is then sent to dehydrogenation reactor 26. Part of the recycle stream in line 54 can be withdrawn through line 78 and sent to inlet line 22 of still apparatus 72. in this way all the functions of the pre-fractionator are performed by the still apparatus 72 which, by removing excess non-aromatic compounds, renders the pre-fractionator unnecessary. Additionally, a small portion of the recycle stream is distilled to prevent excessive build-up of non-aromatic compounds in the system.

   For example, by operating valve 80, 15% of the recycle stream from line 54 is diverted to still 72; the remainder, which has the composition indicated in Example I, joins line 23 and the combined feed is
 EMI23.1
 - a'.tezra by 56.;! 1.t ... "", êç¯i!, 11:!. ' dehydrogenation 26.



  EXAMPLE VIII.-
It is possible to operate the distillation column shown in US Pat. No. 2,959,626, Fig. 5, with an additional branch 108 homologous to the distillation apparatus 72 to effect a separation of non-aromatics.

   The advantage is that non-aromatic compounds can

 <Desc / Clms Page number 24>

 can be separated more efficiently and ethylbenzene supplied. in line 22 can be separated or bypass at one point. '*; device intermediary. Therefore, the current to the dehydrogenation reactor is about the same as in the previous examples. Accumulation of excess non-aromatic compounds is limited by operating according to either of the preceding examples.



  As this patent indicates, considerable savings are possible in the processing of a crude mixture of C8 aromatic compounds to produce a substantially pure system, obtained in each case with as high a degree of purity as in the known processes. , namely 99.7% and more. Prior expensive pre-extraction processes considered necessary to remove all non-aromatic compounds, as the earlier patent indicates, can now be omitted and it is easy to process a fraction having a relatively high compound content. non-aromatic.

   Preferably, the treatment comprises a pre-fractionation the object of which is simple slow to reduce the content of non-aromatic compounds to a reasonable value of up to 5%, the distillate then being super-distilled according to the present invention. It appears that these non-aromatic compounds do not prevent the fine super-distillation of ethylbenzene from the xylene isomers in the C 8 fraction.

   Non-aromatic compounds boiling in the critical range of C8 compounds generally consist of nonanes whose boiling point is higher than that of ethylbenzene, but surprisingly form azeotropic mixtures with the ethylbenzene with which they go overhead. during the super-distillation. In addition, these non-aromatic compounds are not
 EMI24.1
 -: {1 ;; ::: ',: / h.' '"' ¯n from the dehydrogenation of ethyl-,:; - #: bzene 'but" tend .. to - :. 3'. To accumulate in the reaction system .....



    No coke formation is observed on the catalyst by cracking of these non-aromatic compounds. The presence of compounds

 <Desc / Clms Page number 25>

 non-aromatics in the styrene-ethylbenzene separation system does not adversely affect the purity of the styrene.


    

Claims (1)

In addition, known methods can be used for separating non-aromatic compounds from aromatic hydrocarbons, such as the use of molecular sieves, filtration, extractive distillation, and the like. SUMMARY - - - - - - The invention comprises: 1.- A process for the separation of ethylbenzene from a mixture of ethylbenzene with xylenes also containing non-aromatic hydrocarbons, boiling in approximately the same temperature range, for example from a C8 aromatic refining fraction, characterized in that ethylbenzene and non-aromatic hydrocarbons are separated from the mixture by fractional distillation with a reflux ratio greater than 40: 1 in a column of distillation or in a series of distillation columns comprising at least 150 trays. This process may further exhibit one or more of; The following features: (a) The mixture of ethylbenzene and xylenes contains up to 5% by volume of non-aromatic hydrocarbons. (b) The non-aromatic hydrocarbon content of the mixture is reduced during a pre-fractionation stage. (c) The distillate is treated to separate a substantial proportion of non-aromatic hydrocarbons from ethylbenzene, for example by subjecting it to further fractional distillation or solvent extraction. 2. - Ethylbenzene obtained by a process specified under 1, 3. - A process for preparing styrene, in which EMI25.1 ethylbenzene is separated from a mixture of ethylene-xylene and xylenes also containing non-aromatic hydrocarbons boiling in approximately the same temperature range by distillation. <Desc / Clms Page number 26> fractionation of ethylbenzene and non-aromatic hydrocarbons from the mixture, with a reflux ratio greater than 40: 1, in a distillation column or in a series of distillation columns comprising at least 150 trays, the distillate is dehydrogenated to convert part of the ethylbenzene to styrene, and the styrene is separated from the residual compounds. This process may further exhibit one or more of the following features: (a) The mixture of ethylbenzene and xylenes contains up to 5% non-aromatic hydrocarbons. (b) The non-aromatic hydrocarbon content of the mixture is reduced to less than 5% during a pre-fractionation stage. (c) The mixture of xylenes, ethylbenzene and non-aromatic hydrocarbons is a C8 aromatic refining fraction. (d) The residual compounds from the dehydrogenation reaction are separated by distillation under reduced pressure and at a temperature at which styrene does not polymerize. (e) Residual compounds are processed to separate EMI26.1 ethylbenzene and at least a portion of the separated tenzene is recycled to the dehydrogenation reaction. (f) The residual composites are subjected to fractional distillation under conditions such that all of the ethylbenzene-rich fraction can be recycled to the dehydrogenation reaction without causing the accumulation of non-aromatic hydrocarbons in the system. (g) Residual compounds are subjected to fractional distillation and a sufficient amount of the ethylbenzene-rich fraction is subtracted prior to recycling to avoid accumulation of non-aromatic hydrocarbons in the system, the subtracted fraction preferably being recycled to the mixture which feeds the distillation column comprising at least 150 plates <Desc / Clms Page number 27> faults. (h) After processing, the? residual compounds to separate ethylbenzene therefrom, a sufficient quantity of non-aromatic hydrocarbons is separated from this ethylbenzene by extraction EMI27.1 nap solvent tjouï "avoid a cc" '"i3-2tion'? Ir" 'c! rocsrb "r'5s not srciis # Dar solv? nt --o-lar avoid e ¯.c¯.¯¯ on. ,, Ro, .¯. ticks in the system and preferably at least part of the distillate from the distillation column having at least 150 trays is subjected to solvent extraction with separated ethylbenzene. 4 -.- The styrene prepared by a process specified under 3 and the polymers of this styrene.