US2541682A - Production of para xylene - Google Patents

Description

4 Sheets-Sheet 1 J- H. ARNOLD PRODUCTION OF' PARA XYLENE Feb. 13, 1951 Filed Aug. 26, 1947 ATTORNEYS Feb. 13, 1951 J, H, ARNOLD 2,541,682

PRODUCTION OF' PARA XYLENE Filed Aug. 265, 1947 4 sheets-sheet 2 |A l 1 FIG. 2 FIG. 3 .a u 7o '7 70 60 s so I ,t Y 9/ E 0. o C sof .so f e 4r \%0F E 40 s /o 20 a0 40 k -70 -ao -90 -loo -l/o PARAFF//vs /N FEED cRrsTALL/zAT/ON TEMP. F

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4 Sheets-Sheet 3 X920 Srv J. H. ARNOLD PRODUCTION 0F PARA XYLENE Feb. 13, 1951 Filed Aug. 26, 1947 /NvEN TOR Jerome//'no/o BY A A 7' TORNE Ks' J. H. ARNOLD PRODUCTION oF PARA xYLENE 4 Sheets-Sheet 4 Filed Aug. 26, 1947 0@ THG FIG. 7

IN VE N TOR er: ATTORNEYS Patented Feb. 13, 1951 OFFICE PRODUCTION OF PARA XYLENE Jerome Howard Arnold, Albany, Calif., assignor to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application August 26, 1947, Serial No. 770,587

, Claims. l

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 nonaromatic petroleum hydrocarbons.

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 is 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 xylene 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 20% by volume. Additionally, 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 identied as parafiinic, which may boil as much as 50 F. below the para xylene and not more than a'bout20 F. above the para isomer. These paraillnic hydrocarbons may be present in amounts of from 25 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 ahe xylenes. Examples of such paraiiinic hydrocarbons are various isomeric octanes and nonanes. The presence of cyclic paraiins, i. e., naphthenes boiling from 50 F. below to 20 F.above para xylene is not precluded.

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

Characteristic boiling ranges of xylene fractions with which this invention dealsvare 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.

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

The recovery of para xylene from such complex mixtures is not simple, since the presence of not only the isomeric xylenes but also paraflns and aromatics isomeric to the xylenes, particularly of ethyl benzene, complicate and obscure the purication problem. Methods for recovering para xylene from its isomers'have been proposed and prior proposals are of two general types, each of which has significant disadvan tages 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 effects of aromatic and non-aromatic hydrocarbons in the xylene fractionand have attempted to solve this problem by removal thereof.

Spannagel Patent No. 1,940,065 allegedly recovers para xylene by crystallization but first purifies the xylene fraction by distilling off any aliphatic hydrocarbons, ethyl benzeneand the like, boiling below para and meta xylene," to avoid the complicating'eiects 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 complieating hydrocarbon impurities, ls utilizedY in the crystallization step. Thus, in prior processes it appears that there has been an attempt to avoid unpredictable, obscuring and complicating effects of ethyl benzene by removal thereof as well as elimination of aliphatic hydrocarbon impurities. These purification 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 paraiilns boiling within the range of from 279 to 285 F. Likewise, the ethyl benzene content of pure xylenes of commerce sometimes called technically pure is less than 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 meta xylenes in the presence of from 5 to 20% or more by volume of ethyl benzenes as well as in the additional presence of from 5 to 20% or more paraiilns 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 ind 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, lpara xylene:

'I'he foregoing list of course is an oversimplication, since it ignores the obscuring elects of the multi-component parafllnic portion of the xylene fraction.

According to the present invention, in brief, para xylene is separated from the above-described xylene-rich hydrocarbon mixtures by chilling to a temperature of from -75 to 120 F., preferably from 80 F. to -l10 F. and more desirably to a temperature below 80 F. and just above that representedl by one of the following v equations:

(l) When theortho xylene content is less than about one-half the percentage of the meta xylene:

where T2V equals minimum temperature in F., X equals per cent paraillns in feed, Y equals per cent ethyl benzene in feed and Z equals per cent ortho xylene in feed.

(2) When the ortho xylene content is greater than one-half the percentage of meta xylene:

Where T2 equals minimum temperature in F., X equals per cent paraflins in feed, Y equals per cent ethyl benzene in feed and Z equals per cent of meta xylene in the feed.

(3) When the ortho xylene content equals onehalf the percentage of meta xylene:

Where T2 equals minimum temperature in F., X equals per cent parafflns in feed and Y equals per cent ethyl benzene in feed.

In practicing the invention in its preferred embodiment, para xylene is produced and recovered from non-aromatic petroleum hydrocarbons. A suitable xylene fraction is obtained by aromatization, preferably by the so-called hydroforming process in which a naphthenic petroleum fraction is aromatized and xylenes are produced. 'Ihis type of process is well-known in the petroleum industry. However, because the chemistry involved and the mixtures obtained are extremely complex, careful coordination of feed stocks and hydroforming conditions is necessary to obtain best results and to yield a preferred xylene fraction for recovery of para xylene in accordance with this invention.

The present invention is particularly adapted to the treatment of an equilibrium xylene mixture from hydroformed non-aromatic petroleum fractions. The term equilibrium xylene mixture" is here utilized to designate a xylene fraction containing ortho, meta and para xylenes in the equilibrium proportions resulting from hydroforming or other suitable aromatization process, that is, in which the relative proportions are about o:m:p::2:6:2. The additional ethyl benzene and parafns hereinbefore described are also present. Although, the invention is particularly adapted to the treatment of this specific type of mixture, it will be understood that the invention is also applicable to other xylene fractions of the compositions hereinbefore described. To avoid prolixity, the remainder 0f this description will be made with reference to xylene fractions derived from hydroforming operations.V

In order to produce para xylene from nonaromatic petroleum hydrocarbons, proper selection of feed stocks and aromatizing conditions is important and essential to the most successful practice of the invention.

FEED STOCKS Naphthenic hydrocarbon mixtures from naphthene-type petroleum crude oils comprise one preferred type of feed stock. vSuch mixtures are normally termed straight run distillates in the petroleum industry, although other aromatizable hydrocarbons or dstillates may be substituted therefor. The hydrocarbons present in this preferred feed stock are believed to consist largely of cyclo-aliphatic hydrocarbons with six carbon atoms in the cyclo-aiphatic ring and with aliphatic side chains attached to the ring. Some five and seven carbon atom cyclo-aliphatic rings may be present. Both the number of side chains and the length of each chain attached to the foregoing rings vary among the many compounds normally present in a petroleum hydrocarbon mixture. In general, these variables are a function of the average molecular weight or, more particularly, the boiling range and distillation -curve .of the petroleum fraction. A naphthenic hydrocarbon mixture consisting essentially of hydrocarbons having from six to twelve carbon atoms in the molecule and preferably composed at 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. to about 320 F. In some instances an even more narrow cut boiling from 230 F. to 275 F. is preferred. Open chain paraillnic hydrocarbon fractions of these boiling ranges are not precluded.

AROMATIZATION As previously set forth, an initial step in the exemplary process comprises aromatization of the particular petroleum feed stock selected. Where a naphthenic hydrocarbon mixture is utilized, the conversion of hydrocarbons to aromatics is believed to occur by dehydrogenation of the six carbon atom rings from cyclo-aliphatic to aromatic while leaving alkyl groups attached to the residual nucleus. For example:

CH-CHI Ortho dimethyl cyclohexane CH,

CII-CH3 HgC Para dimethyl cyclohexane CHqCH;

CHzCHg Ethyl cyclohexane saturated paramns and naphthenes. The over# all complexity of the mixture and the relative proportion of the above-mentioned non-aromatic components depend upon the effectiveness of the particular aromatization process as well as upon the specific hydrocarbon feed stock selected. It is for this reason that a highly naphthenic hydrocarbon feed stock boiling within the ranges previously disclosed are preferred, since the recation products therefrom are better adapted to subsequent processing steps involved in the production of isomeric xylenes. However, it is possible, but less desirable, to obtain operative aromatic fractions from open chain parafiinic lwdrocarbons by known reactions, such as dehydrogenation and cyclization illustrated by the following reactions:

p-Xylene H CH:

\C/ HIC CH3 HIC C/CH: n/a.

Di-isobutyl l (2, Bdimethyl hexane) Processes for effecting such aromatization reactions and catalysts therefor are known in the petroleum art. Likewise, aliphatic olefins are convertible to aromatics by known cyclization and dehydrogenation reactions similar to the foregoing. 'I'hese 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.

The preferred aromatization process knownas 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 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 1000 F., over a vanadium oxide-alumina or molybdenum oxide-alumina 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 Vof from about 30 to about 300 pounds per square inch. The reaction product from such a 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 flow sheet of a typical process and suitable apparatus for practicing the process of this invention.

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

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

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

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

Referring to Fig. 1 of the drawing, a naphthenic hydrocarbon feed is introduced by way of line l to a hydroforming unit and non-aromatic petroleum hydrocarbons such as the naphthenic petroleum distillate boiling within the range of 18o-320 F., as previously described, is converted to a complex aromatic hydrocarbon fraction. Desirably the particular hydroforming operation is that previously described and exemplied as a preferred process. The hydrocarbon eiiiuent iiows by way of line I2 to a fractionating column I3 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 |4 while Cs, C6 and C7 hydrocarbon fractions are removed separately as side streams by way of lines I5, I6 and I1 respectively. C9 and heavier hydrocarbons are discharged as bottoms by way of line I 8. The xylene-rich hydrocarbon mixture from which para xylene is to be recovered is withdrawn from fractionating column I3 by way of lziie I9 and ows through cooler 2| to surge tank The xylene-rich hydrocarbon fraction containing parai'lins and ethyl benzene, as hereinbefore described. flows 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 of ortho to meta xylene in this xylene 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 flows by way of lines 24 and 28 through heat exchanger 21 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.

crystal growing tank 3| 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 filtering 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 iine crystals which can be 'separated from the mother liquor only with great difliculty, if at all. Thus, a cooling rate in the order of 50 F. a minute in a batch process produces such adverse effects, Whereas a cooling rate through the crystallization temperature range in the order of 1 to 10 F. a minute gives a good yield of lterable crystals ofrelatively high purity. More desirably, a cooling rate below about 5 F. a minute through the crystallization temperature range may be utilized.

Crystallizing temperature is maintained in soaking tank 3| 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 31 connected in parallel, as shown, by valve-controlled inlet lines 38, 39 and 4I. Desirably, circulation pump 33 is designed and controlled to force the xylene mixture through the chiller It has been found that recovery of para xylene minutes or more residence time at crystallizingtemperatures. As here shown the xylene stream flows into a suitable heat insulated soaking or tubes at a suflicient velocity and under adequate pressure to cause turbulent ow. The term turbulent flow here is used in the kcommonly accepted hydraulic sense. Such turbulent flow 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 31. 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 3|, the xylene mixture is passed through chiller discharge lines 42, 4'3, 44 and return header 46 to the crystal soaking or growing tank. The chilled xylene mixture is dispersed with the crystal slurry in tank 3| 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. Liquefied ethylene, ethane or methane are examples of suitable refrigerants. As here indicated, temperature controls 35 are provided in the refrigerant discharge line of each of the chillers to regulate the flow of refrigerant therethrough. Desirably, these controls are responsive to the temperature of the xylene mixture in discharge lines 42, 43 and 44 respectively.

Upon completion of the crystal growing operation in tank 3|, the slurry of para xylene crystals in the remaining liquid hydrocarbon mixture is conveyed by suitable means, as indicated by line 41, to a crystal separation and recovery unit. As illustrated herein, crystal separation and purification are eiected by a combination of centrifugal lters and an agitated tank washer. Initially the crystals in slurry from tank 3| are separated in a centrifugal filter 48 at a temperature of from about 75 F. to about 120 F., more desirably 80 F. to 110 F., and preferably from 80 F. to T2 as previously defined, and conveyed as indicated by line 48 to crystal washer 5|. Any Suitable washing fiuid may be utilized, such as isopentane, alcohol or the like, but as here shown a para xylene saturated hydrocarbon mixture is -introduced by way of valve-controlled line 52 with the slurry and the mixture intimately contacted by agitators 53. The resultant slurry flows through outlet line 54 to a second centrifugal filter 56. In order to maintain and control the temperature in washer I, a portion of the washing liquid in stream 54 is by-passed through valvecontrolled line 51, heater 58 and return line 59 to washing tank 5|. Steam or other fluid heating agent is supplied to heater 58 as indicated by inlet and outlet lines 6| and 62.

'I'he crystal slurry from washer 5| is separated in the second stage centrifugal filter 56, and the purified crystals removed and transferred to melting tank 63 as indicated by line 64. The ltrate from this second stage separation is discharged by way oi' line 66. 'I'his filtrate comprises a xylene fraction saturated withv respect to para xylene at ltration temperature. A portion thereof flows by way o f valve-controlled line 52 to be utilized as the washing liquid in tank 5| The remainder of the nitrate from unit 56 passes by way of recycle line 61 through heat exchanger 21, and preferably is blended with the xylene feed stock, before it is introduced into soaking tank 3|.

In some instances it will be found desirable to minimize crystal formation in chillers 34, 36 and 31 by recirculating the filtrate of recycle line 51 through the heat exchanger tubes 38, 39 and 4| together with, or in lieu of, xylenes from crystal forming tank 3|. A by-pass line 61a from recycle line 61 to pump 33 is provided for this purpose.

Purified crystals ofpara xylene in tank 63 are melted and passed to storage 68 by way of line 89. A portion of the melted stock is by-passed through valve-controlled line 1I, heater 12 and return line 13, the heated xylene serving to melt crystals fed to the system. Heat is supplied by hot water or any other suitable fluid introduced through the line 'I4 and discharged through line 15.

'I'he two-stage filtration and crystallization system preferablyis operated with first-stage filter 48 maintained at a lower temperature than second-stage lter 56. A portion of the crystals discharged from washer 5| is allowed Ato melt so that the filtrate from unit 56 is para xylene of the desired purity, thereby furnishing a wash liquid rich in para xylene by way of valve-controlled line 52 for removing entrained less-pure mother liquor from the crystals in washer 5|. Temperature in such a washing operation may be from about +20 F. to +35 F. although lower temperatures may be used, depending upon purity and yields desired.

Mother liquor from first stage filter 48 is discharged by way of outlet conduit 11, and in the embodiment here illustrated passes to fractionating column 18 wherein an ortho xylene fraction is separated by distillation.

In this distillation as relatively high purity ortho xylene fraction (for example, 95% or higher) can be obtained by superfractionation, which is a preferred type of operation for the present invention. The ortho xylene is removed from the l0 distillation as a bottoms fraction by way of discharge line 19. A portion of the ortho xylene desirably is recycled by way of valve-controlled line 23 to feed surge tank 22 in an amount sumcient to adjust the ortho xylene content of the feed as previously disclosed herein. The remainder of the ortho xylene flows to storage by way 'of valve-controlled line 8|. Overhead from superfractionator 18 passes by way of line 82 to'storage 83. This overhead fraction consistsv of a mixture of xylenes, primarily meta xylene with minor amounts of ortho and para xylenes as well as with parafhns and ethyl benzene contained in the original feed stock.

With respect' to the separation of an ortho xylene fraction by distillation and superfractionation, it should be noted that it will be necessary to maintain the non-aromatic hydrocarbon contentl of the xylene fraction supplied to superfractionator 18 below about 15% by weight. When necessary this initial purification may be eiIected in any suitable manner as, for example, by an initial extractive distillation of the xylene, or by liquid phase selective solvent extraction or the like. The superfractionation itself requires a highly eillcient fractionating column. One equivalent to 35 theoretical plates is necessary for practical operation, more de sirably about 45 and preferably about 60 theoretical plates are utilized. Reflux ratios on distillate of, from about 7:1 to about 12:1 have been found satisfactory. Very close temperature regulation is important, and the distillation is so sensitive that control by temperature responsive device has been found to give ineiilcient though operable separation. A preferred method of superfractionation is to operate the fractionating unit continuously at a given constant feed rate while (1) removing overhead distillate and bottoms at a constant ratio corresponding to the feed rate and in a relative proportion such that the desired purity of the ortho xylene may be maintained, and (2) maintaining a constant volume of liquid and still bottoms by controlling the rate of heat inputthereto. Maintenance of the constant volume of bottoms may be effected, for example, by a constant level control which increases the amount of steam admitted to the still heating unit when the level of the still bottoms begins to rise, and decreases steam input when the volume of bottoms begins to drop below the predetermined level. With the benefit of the foregoing instructions, those skilled in the art can eect superfractionation of a mother liquor boiling Within the range of, for example, 275-295 F. and having a non-aromatic hydrocarbon content of less than about 15% by weight.

To further illustrate the invention and guide those skilled in the art in the practice thereof, data showing effective recovery of para xylene in the presence of different amounts oi parains and at different temperatures are presented graphically in Fig. 2. The feeds B and C referred to in Fig. 3 had the following composition:

Figs. 2 and 3 illustrate that the presence of paralllns tends to decrease para xylene recoveryat any given' temperature, but that this decrease in recovery is avoidable by further reducing crystallization temperature 'within the limits and in the manner hereinA disclosed; that is, by lower- 12 TABLE I Eect of paramns on the recovery of para ing the temperature 0.3 F. for each per cent of b renin Feedo parafns present. bi hth fr t ith nc P nt `1'igs.4and5estals ee ecso eprese e meA Perma. of ethyl benzene and show that V it tends to de.. g'lginm" 1kg crease p-xylene recovery at any given temperagiggle 53.5 35.4 ture to a greater extent than do the Paramus 10 Pmmnsmmwmne) fgjg '12 Likewise, the data illustrate that this decrease in recovery is avoidatble by reditisg crxstiarlllicondlfin: 50 l d tion temperature wi hin the i an e rsea r. ee manner herein disclosed, that is, by lowering gargifgmfflghogfcmHs crystallization temperature as previously disclosed (4) Cake if dried 2 minutes- Crystal- Percent Charge Percent Iizing Cooling mme af p-X Pefnt Paramus Tetrllllrrera- Time ,1.31% Cryigta 1s excglg:

"r, Mm. Mm. 60g.B"--..-- 10.2 -75 20 5 80.0 45,9 -75 2s zo 80.5 43.8 -ioo as 1o es um 1oe ao 3o '12 69.5 so g. C"..--- 3a 6 -90 25 2o 74. 7 53.0 we eo 4o 74.3 63.7 no 1o 77 eze about 1.4 F. for each percentage of ethyl benzene In Tablev II are shown data, on the effects of present 1n the feed, 4 l t successively increased percentages of ethyl ben. Figs. 6 and 7 show the effects of ortho xylene to zene obtained by addition of ethylI benzene to the meta xylene ratio on para xylene recovery and original charge stock. y on optimum crystallization temperature. The TABLE n data of these two figures are based on compositions containing para xylene in excess of the xyl- Eect of ethyl benzene on recovery 0f para ene eutectics. Thus, when the ortho-meta xylene xylene ratio is less than one-half, c:rytaltlizoaicnlpteirn Chage stoctmth lbenz I rature should be decrease a ou or ereen e y ene..- .o Egon per cent of ortho xylene present. When 21?- ?nglf"- l the ratio of ortho to meta xylene is greater than 15g; g ggg; mi one-half, the optimum crystallization temperature condliitiri; "t' ".1145 for any given feed containing para xylene in exafg@ c0- uenep yopp est over eed cess of the eutectic proportion should be increased lciliilmcolfgho cm' Hg about 1.3 F. for each per cent of ortho xylene 45 4) Cake alf dried2minutes present in excess of 33, based on the xylenes.

Again, when the ortho to meta xylene ratio is the Per Cent ryscalliz- Cooling 'rime at Pez'cfellg Per Cent optimumv 1:2, then the most desirable crystalliza- Bglg'e 13g l Time gstp y1 Il-Xylenei tion temperature is 84.5 F. decreased by the P crystals scovare correction factors previously disclosed for para- 50 o in and ethyl benzene contents only. 14.0 1'75 35 1o mo 4e s An exemplary process was carried out and data 28.4 -75 3o an 74.3 3813 obtained in a simplied apparatus consisting of jg :gg gg a fritted glass filter surroundedby a cooling bath 14.0 9o lao so 76.8 5&8 to maintain the filtration at specified temperatures. I@ 32 Crystals were ltered from the mother liquor by 38.6 -oo 2o 1o 7z`0 ae's applying vacuum, and the crystal cake of para 22 :13g gg 1g g'lig ggg xylene was air-dried for a measured time interval. 38. 6 los 25 5 ca -2 472 o Crystals were weighed and purity determined by the freezing point method. To regulate the sweato0 A second series of exemplary runs was made ing of the crystal cake and eliminate ice formawith centrifugal separation of para xylene crystion on the lter, the air used for drying was rst tals. The filtration was effected in equipment chilled with an alcohol solid CO2 bath. In these consisting of a perforated basket centrifuge runs the percentage of paraillns was varied by lined with muslin. An agitated chilling vessel adding the unsulfonatable residue (that is, the was provided for cooling the feed stock by inparaiiinic hydrocarbons) of a xylene fraction I ternal refrigeration by direct addition of Dry formed by hydroforming a petroleum hydrocarbon Ice. In addition to the mixing effected by evapofraction as previously described herein. By utilrated CO2, mechanical agitation was utilized to izing this particular mixture of paraillns, repreaid in controlling the temperature of the charge sentative results were obtained without the necesstock and in reducing agglomeration of the sity of identifying the exact composition and prosolid CO2. portions of the different parafilnic components. The para xylene crystal slurry was fed by grav- 'Iables I and II give the results of representative ity into the centrifugal filter, and a pump was .runs made in the foregoing manner, Table I showprovided for recirculation of the cooled mother lng the eirect of the parans:

liquor from the filter back to the agitating ves?.

13 sel. 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 CO: to the agitator and continuous recirculation of the xylene 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 on the mother liquor and allowing the filter cake to rise in temperature suiliciently 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 DI Charge Stock:

Per cont Ethyl benzene i4. Per cent o-Xylene ,-L d l Per cent m-Xylene 49. 3 Per cent Xylene 19. l Per cont arailins 1l. 5

Final Cen- Recovery of Crystalliz- R. P. M. of Purity of ing Temp. ltfgil centrifuge p-Xylene lrlge F. Min. Percent Per cent YV14... liquid from the crystal phase. withdrawing the washed crystal phase as a product, conducting zone during the succeeding cycle and utilizing the remainder as the specified wash liquid to wash the crystal phase produced in the succeeding cycle of operation. 4`

2. A cyclic process for recovering paraxylene from a xylene rich fraction of catalytically reformed naphtha boiling in the range about 270- 300 F., each cycle comprising the steps of`cooling said fraction to a temperature in the range 75 F. to 120 F. for a time sufficient to cause the formation' of a solid crystalphase comprising paraxylene, ltering the cooled mixture to separate the crystalline phase and a mother liquor and withdrawing the mother liquor as a product, washing the crystal phase with a wash liquid having a, paraxylene content substantially greater than that of the mother liquor, filtering the wash liquid from the crystal phase, withdrawing the washed crystal phase as a product, conducting the washing and second mentioned filtering steps at temperatures substantially above the temperature of the cooling zone such that a subtantial portion of the crystal phase is melted during said steps, returning a portion of the filtrate from the second mentioned filtering step to the l Equipment was cork insulated for this and ensuing runs. Low

recovery due to increased speed of centrifuge.

I Some mechanical loss of product from centrifuge.

l Centrifuga modilled to permit measurement of R. P. M. For ggg 'zunptlli pseed of the final centrifuge period was increased to 4 This period was reduced by increasing the air flow through the ntl'lfllge.

It is readily apparent from the foregoing description that various modifications of the process can be made within the spirit 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 drawing but is illustrated only as to major unit operations in the process. Many detailed pumps, valves, condensers, heat exchangers, temperature controls and the like have been omitted, since any suitable form of apparatus incorporating these features can be supplied in obvious manner by those skilled in the art.

I claim:

l. A cyclic process for recovering paraxylene from a hydrocarbon liquid comprising substantial amounts of orthoxylene, metaxylene and paraxylene, each cycle comprising the steps of cooling said liquid to a. temperature in the range 75 F. to 120 F. for a time suflicient to cause the formation of a solid crystal phase comprising paraxylene, filtering the cooled mixture to separate the crystalline phase and a mother liquor and withdrawing the mother liquor as a product, washing the crystal phase with a wash liquid having a paraxylene content substantially greater than that of the mother liquor, filtering the wash cooling zone during the succeeding cycle and utilizing the remainder as the specified wash liquid to wash the crystal phase produced in the succeeding cycle of operation.

3. The method of separating paraxylene from catalytically reformed naphtha which comprises fractionally distilling said naphtha to separate a xylene rich fraction having a boiling range from about 270 F. to about 300 F., passing said fraction into a cooling zone and cooling it to a temperature in the range F. tof-120 F. to separate a solid crystalline phase comprising paraxylene and a mother liquor phase, filtering the cooled fraction without appreciably raising its temperature to separate the crystalline phase from the mother liquor, withdrawing the mother liquor as a product, washing the crystalline phase with a wash liquid having a paraxylene content substantially higher than the mother liquor, fil-` tering the mixture of wash liquid and crystalline phase at a temperature substantially above that at which the crystallization is effected such that a substantial .portion of the crystalline phase is melted, withdrawing the washed crystalline phase as a product, returning a portion of the liquid eilluent from the second mentioned filtration` to the cooling zone and utilizing the remainder as the Wash liquid in washing further quantities of separated crystalline phase.

4. The method as defined in claim 3, characterized by the further steps of fractionally distilling the mother liquor to separate metaxylene as the overhead fraction and a liquid rich in orthoxylene as the kettle product and introducing into the cooling zone together with the xylene rich fraction a portion of said kettle product to raise the ratio of orthoxylene to metaxylene in the resultant mixture.

5. A cyclic process for recovering paraxylene from a hydrocarbon liquid comprising substantial amounts of orthoxylene, metaxylene and paraxylene. each cycle comprising the steps of cooling ing a paraxylene content substantially greater than that of the mother liquor, filtering the wash liquid from the crystal phase, withdrawing'the I0 washed crystal phase as a product, conducting the washing and second mentioned llltering steps at temperatures substantially above the temperature of the cooling zone such that a substantial portion of the crystal phase is melted during said steps, returning a portion of the ltrate Iromthe second mentioned ltering step to the .cooling 16 zoneduring, a. succeeding cycle and utilizing the remainder as the major component of the specied wash liquid to wash the crystal phase produced in the succeeding cycle of operation. y JEROME HOWARD ARIIIOLD.A

` REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number` Name Date 1,940,065 Spannagei Dec. 19, 1933 2,383,174 Weir Aug. 21, 1945 2.398.526 Greenburg Apr. 16, 1946 2,400,883 Keeling et al May 28, 1946 2.435,79?

MCArdle et al. Feb. 10. 191:8

Claims (1)

1. A CYCLIC PROCESS FOR RECOVERING PARAXYLENE FROM A HYDROCARBON LIQUID COMPRISING THE STEPS OF TIAL AMOUNTS OF ORTHOXYLENE, METAXYLENE AND PARAXYLENE, EACH CYCLE COMPRISING THE STEPS OF COOLING SAID LIQUID TO A TEMPERATURE IN THE RANGE -75* F. TO -120* F. FOR A TIME SUFFICIENT TO CAUSE THE FORMATION OF A SOLID CRYSTAL PHASE COMPRISING PARAXYLENE, FILTERING THE COOLED MIXTURE TO SEPARATE THE CRYSTALLINE PHASE AND A MOTHER LIQUOR AND WITHDRAWING THE MOTHER LIQUOR AS A PRODUCT, WASHING THE CRYSTAL PHASE WITH A WASH LIQUID HAVING A PARAXYLENE CONTENT SUBSTANTIALLY GREATER THAN THAT OF THE MOTHER LIQUOR, FILTERING THE WASH LIQUID FROM THE CRYSTAL PHASE, WITHDRAWING THE WASHED CRYSTAL PHASE AS A PRODUCT, CONDUCTING THE WASHING AND SECOND MENTIONED FILTERING STEPS AT TEMPERATURES SUBSTANTIALLY ABOVE THE TEMPERATURE OF THE COOLING ZONE SUCH THAT A SUBSTANTIAL PORTION OF THE CRYSTAL PHASE IS MELTED DURING SAID STEPS, RETURNING A PORTION OF THE FILTRATE FROM THE SECOND MENTIONED FILTERING STEP TO THE COOLING ZONE DURING THE SUCCEEDING CYCLE AND UTILIZING THE REMAINDER AS THE SPECIFIED WASH LIQUID TO WASH THE CRYSTAL PHASE PRODUCED IN THE SUCCEEDING CYCLE OF OPERATION.

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