US2695323A - Production of para xylene - Google Patents

Production of para xylene Download PDF

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US2695323A
US2695323A US206278A US20627851A US2695323A US 2695323 A US2695323 A US 2695323A US 206278 A US206278 A US 206278A US 20627851 A US20627851 A US 20627851A US 2695323 A US2695323 A US 2695323A
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xylene
para
ortho
xylenes
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Arnold Jerome Howard
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D9/00Crystallisation
    • B01D9/0004Crystallisation cooling by heat exchange
    • B01D9/0013Crystallisation cooling by heat exchange by indirect heat exchange
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D9/00Crystallisation
    • B01D9/0004Crystallisation cooling by heat exchange
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D9/00Crystallisation
    • B01D9/0036Crystallisation on to a bed of product crystals; Seeding
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D9/00Crystallisation
    • B01D9/004Fractional crystallisation; Fractionating or rectifying columns
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D9/00Crystallisation
    • B01D9/0059General arrangements of crystallisation plant, e.g. flow sheets
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C7/00Purification; Separation; Use of additives
    • C07C7/14Purification; Separation; Use of additives by crystallisation; Purification or separation of the crystals
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10STECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S203/00Distillation: processes, separatory
    • Y10S203/19Sidestream

Description

Nov. 23, 1954 J. H. ARNOLD PRODUCTION` 0F PARA XYLENE 4 Sheets-Sheet l Original Filed April '26. 1947 Nov. 23, 1954 J. H. ARNOLD PRODUCTION OF PARAn XYLENE original Filed April 26. 1947 /o THEORETICAL PARA-XYLENE RECOVERY /o THEORETICAL PARA-XYLENE RECOVERY FIG. 8O 2 IO 2O 30 40 /0 PARAFF|NS IN FEED FIG. 4

2o 3ov ETHYL BENZENE IN FEE o /o THEORETICAL PARA-XYLENE RECOVERY /o THEORETICAL PARA-XYLENE RECOVERY 4 Sheets-Sheet 2 FIG. 5

CRYSTALLIZATION TEM P.

0 '-90 -IOO -110 Fl G. 5

CRYSTALLIZATION TEMP.

-ao -Qo -ioo -no I NVE NTOR ATToNz-:Ys n

Nov. 23, 1954 J. H. ARNOLD PRODUCTION OF PARA XYLENE 4 Shees-Sheet. 5

Original Filed April 26. 1947 INVENTOR JEROME ARNOLD Nov. 23, 1954 J. H. ARNoLn PRODUCTION OF'PARA XYLENE:

Original Filed April 26. 1947 4^ Sheets-Sheet 4 l IOOVo INVENTOR JEROME H. ARNOLD ATTO EY United States Patent PRODUCTION OF PARA XYLENE Jerome Howard Arnold, Albany, Calif., assignor to Cali- `fomia Research Corporation, San Francisco, Calif., a

corporation of Delaware Original application August 26, 1947, Serial No. 770,587. Divided and this application January 16, 1951, Serial No. 206,278

3 claims. (cl. 2604-674) This invention relates to the recovery of para xylene from a xylene rich fraction consisting essentially of a complex mixture of xylenes with aromatic and non-aromatic hydrocarbons boiling in the same range as the para xylene. More particularly, the invention involves the production of para xylene from a mixture of non-aromatic petroleum hydrocarbons.

This application is a division of application Serial No. 770,587, filed August 26, 1947, now United States Patent No. 2,541,682.

A complex hydrocarbon fraction with which the present invention is concerned primarily and from which para xylene may be recovered, typically contains only a minor proportion of para xylene. The proportion of para xylene in such mixtures seldom isy more than 30% by volume and usually is less than about 21% by volume of the hydrocarbon fraction but should be more than about of the xylenes present in the mixture. The major portion of the mixture comprises aromatic hydrocarbons boiling Within 11 F. of the para xylene and including from at least about 5% up to as much as 20% or more of ethyl benzene based on the entire hydrocarbon fraction. The ethyl benzene content may be from 50 to 100% by volume of the para xylene content. Of these aromatic hydrocarbons at least about 50% by volume of the xylenes in the fraction is meta xylene, with minor amounts of ortho xylene not exceeding about by volume. AdditionalIy,-the xylene fraction usually contains at least about 5% and up to 20% or more (based on the entire hydrocarbon fraction) of unsulfonatable hydrocarbons of unknown constitution, generally identified as parainic, which may boil as much as 50 F. below the para xylene and not more than about 20 F. above the para isomer. These paraitnc hydrocarbons may be present in amounts of from to 100% by volume based on the para xylene content and include acyclic saturated hydrocarbons which either boil within the range or form constant boiling mixtures with the xylenes. Examples of such paralnic hydrocarbons are various isomeric octanes and nonanes. The presence of cyclic parafiins, i. e., naphthenes boiling from 50 F. below to 20 F. above para xylene is not precluded. l

An analysis of a xylene fraction typifying the above discussed composition is: Hydrocarbon:

Per cent by volume Ortho xylene 1 Meta xylene 48 Para xylene 18 Ethyl benzene 9 Parains and/ or naphthenes 13 Characteristic boiling ranges of xylene fractions with which this invention deals are from 230 F. to about 300 F., more desirably boiling within the range of from 270 to about 300 F. and preferably within the range of from 270 to about 290 F.

Y The foregoing specific example has the following boiling range characteristics in an ASTM-D-86 distillation:

Initial Temperature, F. 80 276 278 279 End point 314 The recovery of para xylene from such complex mixtures is not simple, since the presence of not only the isomeric xylenes but also parains and aromatics isomeric to the xylenes, particularly of ethyl benzene, complicate and obscure the purification problem. Methods for recovering para xylene from its isomers have been proposed and prior which has'signitcant disadvantages and high cost factors. One type of proposal has involved extensive chemical alteration of one or more of the hydrocarbon components in the xylene system to afford elimination and separation of the components. Such methods involve relatively expensive chemical conversions with attendant loss and normally require reconversion of the resulting chemical derivatives back to the desired hydrocarbon with additional loss at this stage as well as an overall useless consumption of chemical treating agent. Alternatively, physical methods heretofore proposed have recognized the complicating and obscure ettects of aromatic and nonaromatic hydrocarbons in the xylene fraction and have attempted to solve this problem by removal thereof.

Spannagel Patent No. 1,940,065 allegedly recovers para xylene by crystallization but iirst puriiies the xylene fraction by distilling oii any aliphatic hydrocarbons, ethyl benzene and the like, boiling below para and meta xylene, to avoid the complicating effects of these impurities. In this patent ortho xylene also is removed and an intermediate meta, para xylene cut boiling from 13G-140 C., and evidently free of complicating hydrocarbon impurities, is utilized in the crystallization step. Thus, in prior tion of aliphatic hydrocarbon impurities. These puritcation treatments require extensive, elaborate and costly equipment particularly in the elimination of ethyl benzene by distillation.

Reference also has been made to the use of technically pure xylene of commerce for the separation of ortho, meta and para xylenes. As distinguished from crude xylenes the pure xylenes of commerce contain no more than 3% and usually less than 1% of paratiins boiling within the range of from 279 to 285 F. Likewise, ythe ethyl benzene content of pure xylenes of commerce sometimes called technically pure is less than 15%.

Contrary to the apparent beliefs of those skilled in the art, it has been-discovered that para xylene can be recovered in relatively good purity by crystallization from ortho and metal xylenes in the presence of from 5 to 20% or more by volume of ethyl benzenes as well asin the additional presence of from 5 to 20% or more parafiins boiling within' the range of from 230 to 300 F.

The unpredictability of this discovery can be better appreciated when it is noted that these hydrocarbons not only alter the crystallization temperature of para xylene by solvent action but that para xylene forms binary, ternary and quaternary crystals with various of the components and that the various components likewise form such complex crystals and eutectic mixtures with each other. The complexity and unpredictability of the system is illustrated by the following list of crystal types in the four-component system-ethyl benzene, ortho, meta, para xylene:

Para, ortho, xylene binary Para, meta, xylene binary Ortho, meta, xylene binary Ortho, meta, para xylene ternary Para xylene, ethyl benzene binary Para xylene, ortho xylene, ethyl benzene ternary Para xylene, metal xylene, ethyl benzene ternary Ortho xylene, meta xylene, ethyl benzene ternary Ortho xylene, ethyl benzene binary Meta xylene, ethyl benzene binary Para xylene, ortho xylene, meta xylene, ethyl benzene quaternary proposals are of two general types, each of t@ The foregoing list of course is an oversimplitcation,

since'i't-igno'res" the obscurin'g' effects of 'themmultiwomf Iwhere Tzequals minimumy temperature .in .F.jX-eguls iper cent parainsvin feed, Y equalsper cent ethyl benzene in `feed and Zequals per cent ortho xylene in feed.

-. 2) VWhen the. orthoxylene content .is greater than onevhalf thel percentage of .meta xylene:

(3) When the ortho xylene'content-equals one-half the percentage of meta xylene:

where T2 equals minimum temperature in F., .'Xequals iper cent parains -in feed v.and Y lequals percent ethyl benzene in feed.

VIn practicing the invention in its preferred embodiment,.para xylene is produced :and recovered frornnon- `aromatic petroleum hydrocarbons. A 'suitable `xylene :fraction is obtained by 'faromatizatiom vpreferably byzthe .so-called hydroforming .process .in Vwhich a '.naphthenic vpetroleum fraction :is iaromatzed and v-x-ylenes are produced. lThis type "of'process iswellfknownxin'the .petro- `leum industry. However, because 4the 'ichemstry vinvolved and the mixtures obtained'are extremelyicomplex, careful coordination of feed tstocks and hydroforming vconditions is necessary to obtain best-results'andzto yield a preferred xylene fraction yfor irecovery Yof para xylene yin accordance with thisinvention.

The present invention is particularly adapted ito ithe treatment of anequilibrium'xylene mixture; from fhydroformed non-aromatic petroleum fractions. '.'The term .equilibriumxylene` mixture is there' utilized tof designate axylene: fraction containingortho, 'metatand para-xylenes vinthe equilibrium proportions resultingfromhydroform- -ingor other suitable aromatization process, athat lis, ,in -which the relative proportions .are Vabout ozmzp: :2: 6:2. -The additional ethyl 4benzene an'd paratlnshereinbefore. described are lalso present. iAlthough, the invention is Aparticularly adapted to thetreatment of .this specific type zof mixture, it will be understoodfthat theinventionyis .also applicable to other xylenefractionsof the compositions ,hereinbefore described. To avoid prolixity, the remainder .of this description will be. made with reference;to ,-xylene fractions derived .from .hydroforrning oper-ations.

In order tovproduce para vxylene from .non-aromatic Apetroleum hydrocarbons, proper. selection not .feedstocks -and aromatizing conditions -is :important .and Aessential to the most successful practicevof -theinvention.

VFEED STOCKS Naphthenic hydrocarbon :mixtures from 'naphthenetype petroleum crude oils comprise one :preferred type of feed stock. ASuch mixtures are normally termed straight Atrun "distillates .1in .the Ypetroleum zindustry, :although other aromatizable hydrocarbons :or .distillates may be substituted therefor. Thephydrocarbons.present in this preferred feed stock arebelievedftoconsist.largely of cyclo-aliphatic hydrocarbons with A.six Y carbon .atoms in the cyclo-aliphatic ringv and with aliphaticside chains attached to the ring. Some 'five andzseven carbon atom cyclo-aliphatic rings may be present. lBoththe number of side chains and the length of each chain yattached to the foregoingrings .vary Aamong the many .compounds-normally present in a petroleum.hydrocarbonmixture. ,fln general, these variablesare afuncton of .the average molecular Weightor, more,particularly,.the'.boiling range and distillation curve of Vthe petroleum'fraction. ...A naphthenic hydrocarbon mixture consisting essentially of hy- ,.dehydrogenation.

drocarbons having from six to twelve carbon atoms in the molecule and'preferablycomposedat'least'predominantly of hydrocarbons containing from seven to eight carbon atoms at present is regarded as a more desirable feed stock. The fraction selected desirably should boil within the range of from about 180 F. to about 420 F. and preferably .from about ,180 .F.toabout 320 F. In some instances van even more narrow cut 'boiling from Vr23.0 .F.to 275 F. .is preferred. .Open chain .'parafnic .hydrocarbon fractions ,of .these 'boiling ranges `arefnot precluded.

AROMATIZATION fAs previously set forth, an initialtstep iin Vthe :exemplary process comprises aromatization of the particular petroleum feed stock selected. Where a naphthenic hydrocarbon mixture is utilized, :.therconversionfof hydrocarbons to aromatics is believed to occur by dehydrogenation of Jhesixcarbon atomzrings'from cycloaliphaticlto 4.aromatic awhile leaving alkyl :groups :attached to 'the residual nucleus. .Forexamplez H20 CH:

1GHz .im

*Meta dimethyl cyclohexane .Para dimethylcyelohexane CHCH:

l H :C HzQH:

' rnc on.

Ethyl cyclohexanc Eisomer-ization of any C1 alicyclic rings'present and dehydrogenation'to aromatic compounds also is believed to occur. LikewiseyCs alicyclic'rings containing side chains are jconvertedvtoy aromaticsby isomerization and These "various reactions represent an Aover-'simplication `of-the aromatization reactions which Amayactually occur, since de-alkylation and shortening lof side chainsas .byf cracking undoubtedly take-place. In any eventfthe -aromatization reaction'product comprises a highlyy complex` mixture ofvaromatics and `also contains non-aromatic hydrocarbons, such assaturated or unsatu-1 fratedyparatiins andxnaphthenes. The overall .complexity ofnthe.mixturefan'dthe relative proportion of theabove- .mentioned non-,aromatic components depend upon the` .-eifectiveness'of-the particular aromatization process as `Wellas upony the `specifichydrocarbon feed stock selected.

ltisfor this reason that a` highly naphthenic hydrocarbon jfeed stock boiling within the ranges previously disclosed .arepreferred, since the reaction products'therefrom are better adapte'dto subsequent processing steps involved in the production of isomeric xylenes. However, it-is pos- .s1ble,but' less desirable, to obtain operative aromatic fractionsfrom open chain partinichydrocarbonsby known reactions, such as dehydrogenation and cyclization illusi trated by the following reactions:

Processes for effecting such aromatization reactions and catalysts therefor are known in the petroleum art. Likewise, aliphatic olens are convertible to aromatics by known cyclization and dehydrogenation reactions similar to the foregoing. These various known processes may be utilized within the broader aspects of this invention and are embraced within the term aromatization as used in the present specification.

4The preferred aromatization process known as hydroforming is characterized by aromatization in the presence of controlled amounts of hydrogen and a vanadium oxide or molybdenum oxide catalyst. As an example of the preferred process, a hydrocarbon feed, such as a naphthenic petroleum distillate, boiling within the range of 180 F. to 320 F., and obtained, for instance, by fractional distillation of a crude petroleum (from Kettleman Hills Oil Field in California) is passed at from about 900 F. to about 1200 F., desirably about l000 F., over a vanadium oxide-alumina o r molybdenum oxidealumina catalyst. Space rate desirably is from 0.1 to about 2.0 volumes of liquid hydrocarbon feed per volume of catalyst per hour, and it is preferred to maintain a partial pressure of hydrogen in the reaction zone of from about 30 to about 300 pounds per square inch. The reaction product from such va hydroforming operation will contain not only the desired xylenes and additional aromatic hydrocarbon but also aliphatic hydrocarbons boiling over a wide range, including C4 and like materials. Initially, therefore, it is necessary to recover a xylene fraction from this reaction mixture.

In the drawing, Fig. 1 is a schematic ow sheet of a typical process and suitable apparatus for practicing the process of this invention.

Figs. 2 and 3 illustrate graphically the effect of parafiins upon crystallization temperature and recovery of para xylene.

Figs. 4 and 5 illustrate the effect of ethyl benzene on crystallization temperature and recovery of para xylene; and

Fig. 6 reveals the influence of relative proportions of the xylene isomers on crystallization recovery of para X lene.

yFig. 7 shows the effect of ortho xylene concentration on optimum crystallization temperatures and at diierent ratios of ortho to meta xylenes.

Referring to Fig. l of the drawing, a naphthenic hydrocarbon feed is introduced by way of line 10 to a hydroforming unit 11 and non-aromatic petroleum hydrocarbons such as the naphthenic petroleum distillate boiling within the range of ISO-320 F., as previously described, is converted to a complex aromatic hydrocarbon fraction. Desirably the particular hydroforming operation is that previously described and exemplified as a preferred process. The hydrocarbon efuent ows by way of line 12 to a fractionating column 13 where separation is effected. As here shown, the fractionation is effected in a single column although a multiplicity of fractionating units may be utilized. C4 and lighter hydrocarbons are taken as overhead through line 14 while C5, Cs and C7 hydrocarbon fractions are removed separately as side streams by way of lines 15, 16 and 17 respectively. C9 and heavier hydrocarbons are discharged as bottoms by way of line 18.

The xylene-rich hydrocarbon mixture from which para xylene is to be recovered is withdrawn from fractionating column 13 by way of line 19 and flows through cooler 21 to surge tank 22.

The xylene-rich hydrocarbon fraction containing parains and ethyl benzene, as hereinbefore described, ows from surge tank 22 to and through the para xylene recovery system. Although not essential to operability of the process, it will be found highly desirable in various instances to adjust the ratio ofV ortho to meta xylene in thisixylene feed stock in order to enhance recovery of the para isomer. The ortho xylene content of a fraction prepared by hydroforming is less than the preferred ratio, and as here shown ortho xylene is added to the hydrocarbon mixture in surge tank 22 by line 23, and the ortho to meta xylene ratio is thereby adjusted to approximately 1:2. The blended hydrocarbon mixture so formed then iiows by way of lines 24 and 26 through heat exchanger 27 where the temperature of the mixture is initially lowered, most desirably by indirect heat exchange with mother liquor from the crystallization operation. This mother liquor flows through inlet and outlet conduits 28 and 29, but for purposes of simplicity connections with the mother liquor lines are not shown.

It has been found that recovery of para xylene can be enhanced and superior results obtained by avoiding shock cooling of the entire xylene stream or conversely by maintaining the stock at crystallizing temperature for a substantial length of time to allow growth of crystals and insure dissipation of any adverse effects of localized shock cooling. Best results have been obtained with at least ten minutes and more desirably with twenty minutes or more residence time at crystallizing temperatures. As here shown the xylene stream flows into a suitable heat insulated soaking or crystal growing tank 31 where it is reduced to crystallizing temperature by mixing with previously cooled xylene stock. The xylene stock is retained at crystallizing temperature until the desired crystal form is obtained, that is, until shock crystals are largely removed by remelting and recrystallization or by equilibrium exchange with larger crystals which will be retained and recovered satisfactorily in subsequent iiltering operations. Generally, a residence time of about twenty minutes is preferred.

Extremely rapid cooling of the incoming xylene stream adversely effects para xylene recovery, tends to lower the purity -of product, and produces undesirably fine crystals which can be separated from the mother liquor only with great difficulty, if at all. Thus, a cooling rate in the order of 50 F. a minute in a batch process produces such adverse eifects, whereas a cooling rate through the crystallization temperature range in the order of 1 to 10 F. a minute gives a good yield of iilterable' crystals of relatively high purity. More desirably, a cooling rate below about 5 F. a minute through the crystallization temperature range may be utilized.

VCrystallizing temperature is maintained in soaking tank 31 by circulation of a xylene side stream through chillers by way of line 32. Thus, circulation pump 33 forces the xylene through temperature-controlled chillers 34,36 and 37 connected in parallel, as shown, by valve-controlled inlet lines 38, 39 and 41. Desirably, circulation pump 33 is designed and controlled to force the xylene mixture through the chiller tubes at a suicient velocity and under adequate pressure to cause turbulent flow. The term turbulent ow here is used in the commonly accepted hydraulic sense. Such turbulent ow is adapted to prevent or minimize localized shock cooling of the xylenes at the surface of the heat exchange tubes in coolers 34, 36 and 37. Additionally, crystal growth and adherence on the walls of such heat exchange tubes is reduced to a minimum by the use of high velocities, especially those exceeding the minimum for turbulent flow. For example, supercooling may be effected in the heat exchange tubes and the supercooled liquid returned to the crystallization tank before crystal formation is completed. After reduction to a crystallization temperature at least as low as that to be maintained in soaking tank 31, the xylene mixture is passed through chiller discharge lines 42, 43, 44 and return header 46 to the crystal soaking or growing tank. The chilled xylene mixture is dispersed with the crystal slurry in tank 31 and an equilibrium temperature condition is reached therewith.

Any suitable refrigerant is supplied to the chillers by way of inlet header 25 and outlet 30,V Liqueled ethylene,l

nagegaan-'s ethaine iorflrnetliane A are examples iof:-siiitable" refgerants. a As-hereFindicated-gtemperaturefeontrolsf35 areprov-ided= in`-thelrefrigerant-xdischarge linefoffeach of -thfl`chillrs to f regulate the ow of refrigerant therethrough; Desirablyf! these. controls are responsive`4 tothe-temperature -vofilthe xylel'ne mixture f inf'dischdrgel lines S42; 1'43 and 'A14-renacer i tive yf Upon completionv of lthe crystal growingoperatio'ninvv tank` 31;- the slurry/of para'xylenecrystalsffinthe-remain' ing liquid-hydrocarbon mixture is conveyedby sutablemeansrasindicatedl-by A line -47, '-to fa crystal sepa-rationJ andi/recovery unit!" Asl'illustrated herein, crystal 'sepa-- ration e and f purification 4are effected by a combinationfof' centrifugal filters :and an agitated tank iwasher.` n Initially-- thejcrystalsin slurryvvfrommtanle'-are'separatedin a f centrifugal -iilter4 *4S-l at a temperature'. of from. -about- -75l R-etdaboutff-IZGQ tlifmore-des'irably "-80 F.- t6-f-1lO-F;}fandepreferablyJfronr b8(}"`` F.'to T2 asl previously define`d,=and conveyed as indicated byb lin'el 49 "f tocrystal `VwasherfSI.. 1' Anyjsuitable :washing f fluid may' begutilizethmsuch *asrsopentaney-alcohol Unor the-1v like; but I as.,he're'-`shownI a pararxylene saturated hydrocarbonv mixf ture ris introducedby-wayofvalvecontrolled line-52` with theLslurry and the mixture intimately-contacted" by 'agi-*- tatorsgSlsl The resultant` slurry 'o'ws through-'outlet line 54to a second centrifugal filter 5611 ln'order'to'maintain s and control uthe temperature in washer`51; a portion of the washing liquid infstream541is 4bypassed through valvecontrolledrline 57,'heater S8`and return line 59 to *washingV tankjSl. SteamV or other fluidheatinglagent is supplied to heater 58 as indicatedby inlet and outlet-lines`61 and '62. The' 'crystal-slurry from=washer;51' is `separated in the second'fstage jcentrifugal filter -56,"and the purified crystals"1removed "and transferredto meltingtank 63 as indi?- cated by line ,64. The-filtrate'from'thissecond stage separation is discharged -byway of 4line 66.l This'ltrate com. prises a xylenefraction.saturated Witli'ijrespect repara' xylene `atiiltrationV temperature. Aportion thereof flows by wayof valve-controlled line 52 to bentilized 'asjtheg washingliquid in tankjSl'. The .remainder `of the filtrate from .unit 56passes by way of recycleline 67"through' .l heatexchanger'zand preferablyA is blended with`the xylene feed stockfbeforeit is introducedinto..soaking, tankL In 4s or'ne instances it willbeY found'desirable to minimize. crystalfoimation in chillrs 34', 36 andf37. by recirculab.l ing the" filtrate of-.recycle11in'e.,.67 `through thehe'atex.` changer tubes 38,139 'and-41 togetherwith, orinlie'u of, xylenes from crystalforrning l.tank A31.-. A byfpasslinef 67d from recycleline 67. topump'SS is lprovidedfor .this

purpose.

Puried crystals of paraxylene ,intank63fare melted. and` passed to storage; 68'bylway of;,1ine,69. A portion.v ofthe .melted stock is by-passed ythroughvalve-controlled line 71, heater 72 and return Vline 73,.the,heated Xylene servingtormclt. crystals fed to the` system., Heat is supplied Vby. hot water.v or. any other .suitable fluid. introduced s through lineY 74 anddischarged throughzline..v

The v two-stage., filtration., and .crystallization system preferably Vis Operated with first-stagefilter 4Slmaintained at'a lower temperature than s econd-stageghlter `Sti-A portion of .the` crystals discharged fromawasher 51 is allowed. to melt. so that the filtrate from`unit56y is para Xylene of thedesired purity, therebyfurnishing awash liquid rich in para xylene 52' for-removing entrained less-pure-mother-liquor from the crystalsiinwasherV 51.l Temperature'in sucha wash, ing-Yoperationmay befrorn about .rl-20" -F. to-|35` F.-

althoughl lower temperatures mayAV lbe. used, dependingupon purityand yields desired.

Mother` liquor fromAirst Vstageiilter .48 is ,discharged by, Wayof outletrconduit V77, andinthe-embodiment here illustrated passes to fractionatingt-column'l where- Y in an ortho-Xylenefraction is separated by distillation.

In this distillation a relatively high purity ortho-Xylene A fraction (for example, .95% :or higher) can be obtained; by superfractionation, which is a preferred type of .operaf tion for the presentinvention. The ortho Xylene is .re.v moved from the. distillation as a bottoms fraction by' way ofl discharge liney 79'. A .portion of thefortho Xylene desirably. is recycled by way of valve-controlled line A23 to-feedv surge tank 22 in an amount suflicient'to adjustIV thertho- .Xylene'content-ofithe feedv aspreviously dis closediherein."v The remainder of'the ortho xylene fio'wsto storage-byway-of7 va1ve-controlled \li1ie-81'a Overhead:-

8 frmfsuper'frctio'nator i#78?fpa'sses'flbyfwayf of/l'iliei-Slto storage 83. This overheadffractiomconsistsiofa mixture# of xylenes, primarilynneta xylene with minor amounts of ortho and para Xylenes as Well as with paraffin'sand ethyl 5 Y benzene contained" in the original feedstocki :by way-.of Yvalve-controlled line .'65

supplied With-*respect to the? separation of an ortho xylene fraction by rdis'tillatiomand superfrctionation, itfshould be noted that it willbe necessary to maintain the nonaromatic hydrocarbon content of the xylene fraction to superfractionator 78 below about 15% by weight. When necessary this initial purification may be effected in any suitable manner as, -for example, by an initial extractive distillation of the xylene,'or by liquid phase selective solventextraction or the like. The super- -fractionation itself acquires;V a highly t, efficient fractionating column-..One equivalent to 35 theoretical plates is necessary for practical operation, more desirably about 45 and preferably about60"theoretical'plates'are utilized. Reflux ratios on distillate of from about 7:1 to about 12:1 have been found satisfactory. VeryN close temperature regulation is important, and th distillation is so sensitive that control -by'. temperature; responsive device has been found to give inefficient 'though "operable separation. A preferred method of superfractionation is .to operate"therfractionating unit continr'r'ouslyatf'a given constant feed rate'iwhile l removing 4overhead distillate and bo'ttorns'zata constant*ratio'corresponding to the feed: rate'fand ina relative"proportion'such that the-desired"L purity ofkv theV ortho xylene `may be' maintained; 'and (2)' maintaininga constantvolurneofrliq'uid and'still bottoms by'controllir'igthe rateof heat inputthereto." Maintenr anceY ofthe constant volumeV of bottoms maybe effected;J fo'r'example, by a constant level 'controlwhich increases th'eiamount ofA steam"'admitted to "theV stillheating unit' whenthejlevel of the'.stilljbottomsbegins 'to-rise, and

decreasessteaminput when the' volume of bottoms begins to idrop (below thepredet'erinined. lethal.v WirthA the benefit ofltheioregoingjinstructions, those 'skilled in the Yart .i caniefec't{superfrctionation/of'a mother liquor boiling!V l withinjthelrarigc' Jof.. for fexample 275-295 and hav-.

ingfa ,non-aromatic `hydr`ocarboni-content ,of less'A than` about. 15% by; weightl L Toifuther illustratej.thelinvention andguide'. those'v skilledini. theart lin.A thepractice. thereof, fdata.' showing effective vrecovery. of .,para-Xylene .in the.. presence .of difyl.

ferent amounts of'parafhns and atjdifeifent temperatures-^ are. presentedgraphically .in .Fig"."2. Th'el feedsl B `and .C 1I referred ,to in IFig..3 hadrthe following composition:

tend.to.deerease para. Xylene. recovery at any .given tem..` peratur'e, but'that'this decrease in recovery is avoidable'. byv further.v reducingV crystallization .temperature Within. the limits and-..in.the` manner :herein `disclosed;:that. is, by:v lowering the temperature 0.3`F. lfor each per cent.of.- paraffins present.

Figs5fz4 and -5 establishfthe eects:of=the.presence.of.. ethyl benzene and show thatit tends to decreasep-xylenerecovery at any given temperature to .a greater. extent than-.ado the -paraflins Likewse,. the: datai illustrate that this decrease in recovery .is avoidable by reducing crystallizationrtemperaturewithin the/limits and in the' man- -i nenherein disclosedfthat isfbyloweringcrystallization tempera-ture aspreviouslyoisclosedvabout A14 FA for: eaehape'rcentage;of ethyl; benzene present in the feed-.1-

Figs.;'6 and# showltheletfects of ortho'x-yleneto meta Xylenefratioon para xylene-recovery and on optimurn Crystallizationetemperature. The data of these twofgures f arefbased on :compositions 'containing ypara .Xylenein -ex.. cessgof the xylenecutectics.'.` Thus, when theortho-meta- Xyleneuratio is less Vthan-fonefhalf; crystallization1.tem..' peraturei-should be :decreasedabout 0.65 F.'for each., percent 'of"orthoaxyleneipresentt` Whenfthestratio4 of" ortho,;to meta .xylene is :greater than .onef'half,l thel opti# mumscrystallization :temperature v-forl' any given -feed con-1f;

taining para xylene in excess of the eutectic proportion should be increased about 1.3" F. for each per cent of ortho xylene present in excess of 33, based on the xylenes. Again, when the ortho to meta xylene ratio is the optimum 1:2, then the most desirable crystallization temperature is 84.5 F. decreased by the correction factors previous- 1y disclosed for paraffin and ethyl benzene contents only.

An exemplary process was carried out and data obtained in a simplified apparatus consisting of a fritted glass filter surrounded by a cooling bath to maintain the filtration at specified temperatures. Crystals were filtered from the mother liquor by applying vacuum, and the crystal cake of para xylene was air-dried for a measured time interval. Crystals were weighed and purity determined by the freezing point method. To regulate the sweating of the crystal cake and eliminate ice formation on the filter, the air used for drying was first chilled with an alcohol solid CO2 bath. In these runs the percentage of paraffins was varied by adding the unsulfonatable residue (that is, the parafiinic hydrocarbons) of a xylene fraction formed by hydroforming -a petroleum hydrocarbon fraction as previously described herein. By utilizing this particular mixture of parains, representative results were obtained without the necessity of identifying the exact composition and proportions of the different paraflnic components. Tables I and IIgive the results of representative runs made in the foregoing manner, Table I showing lthe Aeffect of parans:

TABLEI Eect of parans on the recovery of para xylene Feed B Feed C Percent ethyl benzene ll. 8 8. 4 Percent o-xylene. 4. 5 4. 3 Percent m-xylene. 53. 5 35. 4 Percent p-xyleue. 20. 13. 3 Percent parans (Kattwinkle) 10.2 38. 6

Conditions:

(1) Charge: 50 g. feed (2) Vacuum: Flowmeter with #30 orifice, 20 cm. Hg (3) Drying air cooled in alcohol C02 bath (4) Cake air dried 2 minutes Crystal- Time at Percent Percent Cooling Percent Charge paraftelgw tim ggg; ll'lxie recovery ns ture, F. minutes minutes tals p`xylene Z5 20 25 82 y 5 28 0 50g- "B 1' 2 10o 35 1o es 66.2 -106 30 30 72 69. 5 `9D 25 20 74. 7 53. 0 50 g. C... 38.6 -106 60 40 74.3 63. 7 -110 25 10 77 67. 6

In Table II are shown data on the effects of successively increased percentages of ethyl benzene obtained by addition of ethyl benzene to the original charge stock.

TABLE II Effect of ethyl benzene on recovery of para xylene (1) hChlrge: 50 cc. toluene plant topping still overea (2) IYlacuum: Flowmeter with #30 orifice, 20 cm.

g (3) Drying air cooled in alcohol CO2 bath (4) Cake air dried 2 minutes Crystalliz- Percent eeyrflfb ing tem- Cooling ig p-xylene lg zene peaure' time temp. mtgll7s` recovered A second series of exemplary runs was made with centrifugal separation of para Xylene crystals. The filtration was effected in equipment consisting of a per-' forated basket centrifuge lined with muslin. An agitated chilling vessel was provided for cooling the feedstock by internal refrigeration by direct addition of Dry Ice; In addition to the mixing effected by evaporated CO2, mechanical agitation was utilized to aid in controlling the" temperature of the charge stock and in reducing agglomeration-of the solid CO2. v

The para xylene crystal slurry was fed by gravity into' the centrifugal filter, and a pump was provided for r`ecir= culation of the cooled mother liquor from the filter 'back to the agitating vessel. The centrifugal pump, agitator and pipe lines were suitably insulated to maintain low temperatures. Means for measuring temperatures in the agitator and of the inlet and outlet of the centrifuge were provided. In operation, the whole system was gradually cooled to the desired crystallization temperature by addition of solid CO2 to the agitator and continuous recirculation of the xvlene mother liquor through the agitator and centrifugal filter. Purification of the crystals in situ Was effected in two stages; first, extraction of impurities by circulation of the mother liquor through the filter cake for a substantial period after crystallization temperature is reached; and secondly, by drawing ofi the mother liquor and allowing the filter ture sufficiently to sweat out hydrocarbon impurities while continuing operation of the centrifuge to remove liquefied impurities so released. Data from these runs are given in Table III:

TABLE III Charge stock: Percent ethyl benzene 14.0 o-xylene 6. 1 m-xylene 49.3 p-xylene 19.1 parafiins 1 1.5

Final Recover Crystallizing centrifuge R. P. M. of purify of of p-xylelire temp., F. period, centrifuge p'c' charged, minutes p percent 93 9. 7 87. 2 30. 7 85. 5 48. 0 83. 5 58. 4 88. 2 56. 8 86. 3 58. 7 87. 0 l 30. 5 92. 0 42. 2 95. 7 57. E 88 58 96 b 50. 6 50 98 58 50 1, 450 90 (b) i 68 450 95 It is readily apparent from the foregoing description that various modifications of the process can be made within thejgspirit of the present invention and the scope of the appended claims. For the sake of simplicity and clarity, apparatus has not been shown in detail in the cake to rise in tempera- Nnie Dit@ :L "s: ins-3,17# wief Aug;- Zl, 1945 2,398,526 gfebiiig: Api.` 16,. 1.946 2,400,883 Kli eti iii.' May 28, 1946 2,435,792 MCArdke et al Feb. 10, 1948 2,541,682 Arnold Feb. 13, 1951

Claims (1)

1. IN A PROCESS OF RECOVERING PARA XYLENE FROM MIXTURES OF ORTHO, META AND PARA XYLENES BY CHILLING A XYLENE FRACTION TO A TEMPERATURE OF FROM ABOUT -75* F. TO -120* F., THE STEPS OF FEEDING SAID XYLENE FRACTION TO A CRYSTAL FORMING ZONE COMPRISING A RELATIVELY LARGE BODY OF LIQUID XYLENES CONTAINING PARA XYLENE CRYSTALS SUSPENDED THEREIN, CONTINUOUSLY CIRCULATING A PORTION OF THE XYLENE LIQUID FROM SAID CRYSTAL FORMING ZONE THROUGH A CHILLER AND THERE COOLING IT BY INDIRECT HEAT EXCHANGE TO A TEMPERATURE BELOW THAT OF THE CRYSTAL-FORMING ZONE, RETURNING CHILLED XYLENES TO SAID CRYSTALLIZING ZONE WHEREBY CRYSTAL FORMING TEMPERATURES ARE MAINTAINED, AND RE-
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