CA2001645C - Adsorptive separation of para-xylene using a tetralin desorbent - Google Patents

Abstract

Para-xylene is separated from a feed mixture of C8 isomers of xylene and C9 aromatics by contacting the para-xylene-containing feed mixture with an X or Y zeolite adsorbent having Group IA or IIA cations, e.g., barium and/or potassium, at exchangeable cationic sites. The para-xylene components are selectively adsorbed onto the adsorbent. The non-adsorbed feed is then removed from the adsorbent and the para-xylene recovered from the resulting rich adsorbent by desorption with a desorbent comprising tetralin or alkyl or dialkyl derivatives of tetralin or mixtures thereof. The majority of the C9 aromatics and the C8 xylene isomers are typically recovered in the raffinate andcan be separated from the desorbent by fractionation of the raffinate and the recovered desorbent recycled to the process.

Description

200~64~;

~ADSORPTIVE SEPARATION OF PARA-XYLENE
- ,~ USING A TETRALIN DESORBENr s FIELD OF THE INVENTION

The field of art to which the claimed invention pertains is ad~o"vti~e hy,l~ ocarbon separation. More spe~ if;cally, the invention relates to a processfor separating para-xylene from a feed mixture co" ".risir,~ at least one other 10 xylene isG",er and Cg aro",~tic hyJroc~rl,ons, which process employs a zeolitic a"~orbent and a tetralin ,lesorl,enl. It is particularly adva, l~ageo~ ~s in a process in which the feed contains large amounts of Cg aro" Idtic hyd~ oca, L,o"s, whichare known to cause problems in prior acJsor~ti~/e ssparation ,~- ocesses of thistype.

BACKGROUND OF THE INVENTION

In numerous processes Jssc,iLeJ in the patent literature, for example U.S. F'atents 3,~,0~ to Neuzil, 3 663 638 to Neuzil, 3 665,046 to 2 0 deRosset, 3,668 ?66 to Chen et al., 3,686,342 to Neuzil et al., 3,700,744 toBerger et al., 3,734,974 to Neuzil, 3,894,109 to Rosl,~ck, 3,997,620 to Neuzil and 4,014,949 to Hedge, particular zeolitic aJsorL,ents are used to separats thepara isomer of dialkyl substit~ed monocyclic aro",~tics from the othe MSGI ~er:j, particularly para-xylene from other xylene iso" ,ers. Many of the above ~.atents25 teach the use of ben~ene, toluene, or p-diethylben~ene as the dssorbe, L
P-diethyll,en~el,e (p-DEB) has bsco,ne a colr""erc;al sta"Jard for this separation. However p-DEB is a "heavy~ desGrl,ent (higher boiling than p-xylene) which ca~ ~ses problems in a process for aJsorpti~e separatiol, of p-xylene when the feed mixtures also contain Cg arol "~ics bec~use the boiling 3 o point of p-DEB is too close to the boiling point of Cg arol "atics in the feed and thus separ~tion by simple ht.ctiol ,ation is difficult. Rec~use the Cg arol"alics if allowed in the feed to the ~IsGr~ti~/e separ~ion are difficult to se~arate from p-DEB by simple f~a~1ionation, the Cg aro,natics would gradually build up in thedesGr6en~, which must be recycled for eco,-ol,lic reasGns. In the prior art ~r 2 ~a~l645 ~
process for recovering p-xylene from feed mixtures containing isomers using para-diethylbenzene as the desorbent, therefore, it has been necessary to reduce Cg aromatics in the feed to below about 0.1 wt.%
prior to the adsorptive separation of p-xylenes. This is usually done by 5 distillation in a so-called xylene splitter column. Of course, substantial costs associated with this practice, such as capital costs of the xylene splitter and utilities necess~ry to achieve substantially complete removal of the Cg aromatics, could be reduced greatly or eliminated if it were not necessary to first remove Cg aromatics. Thus, while U.S. Patent 3,686,342, supra, mentions tetralin as a possible heavy desorbent for the para-xylene separation process, that reference clearly states that p-DEB is the best desorbent for the separation and, further, does not address the problem that the preferred desorbents may have in separating feeds containing Cg aromatics. Therefore, a higher boiling point material, that 15 meets the selectivity requirements for desorbents and can be separated from Cg aromatics, has been long sought and is still desirable.
It is also known that crystalline aluminosilicates or zeolites are used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance.
20 Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to 25 convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a JJ :vs '~ 3 2001 64S i countercurrent simulated moving bed system, such as described in Broughton U.S. Patent 2,985,589. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Patents 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Patent 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of galions per hour.
The invention may also be practiced in a cocurrent, pulsed batch process, like that described in U.S. Patent 4,159,284 or in a cocurrent, pulsed continuous process, like that in Gerhold Patents 4,402,832 and 4,478,721.
Also, in some cases illustrated herein, it is necessary to remove three effluent streams from the adsorptive separation step in order to obtain a desired product intermediate in adsorption strength from an extract and a raffinate stream. This intermediate stream can be termed a second raffinate stream, as in U.S. Patent 4,313,015 or a second extract stream, as in U.S. Patent 3,723,302. This case pertains when a contaminating component in the feed, such as p-ethyltoluene, is more strongly adsorbed than the desired product, p-xylene. lt is not always necessary to remove p-ethyltoluene from p-xylene, e.g., where terephthalic acid is the final product of the oxidation of p-xylene, since oxidation of p-ethyltoluene results in the same product. However, if it is desired to keep the concentration of the contaminating component in the product as low as possible, a first extract is taken off, high in concentration of the desired component and lower in the contaminating product followed by a second extract, withdrawn at a point between the desorbent inlet and the first extract point, containing a high concentration of the contaminant and a lower concentration of the desired product. It is not necessary, however, -, JJ :vs ~, .
~,-?

'~ 3a ~001645 to use a second desorbent, if the desorbent is able to first desorb the lightly held product and then desorb the remaining more strongly held contaminants, as disclosed in the aforementioned 3,723,302 patent. If the contaminating component in high concentrations and purity is desired, this 5 can be achieved by withdrawing a second extract in the cocurrent pulsed batch process mentioned above.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, see for example, Zinnen et al. U.S. Patent 4,642,397.
A process employing a zeolite adsorbent for the separation of p-xylene from its isomers using a desorbent which is a substantial improvement in a process for separating xylene isomers where the feed mixture also contains Cg aromatic impurities has now been discovered.

JJ :vs ~,~, , Z00164s ~' 4 SUMMARY OF THE INVENTION

In brief summary, the invention iS a ~ h lalCIyl ~,uhic ,ul oc~ss for s6~.af~ti--5a p-xylene from a feed mixture cGr"~,ri~iny p-xylene, Cg afomatic s hyJroca-6O--s and one or more additional xylene iSGIllel- (including ethyllJe"~ens) cG",~.risiny contactiny said feed mixture with sn X- or Y-type zeolite exchanged with Group IA or IIA metal ions at excl ,angeable cationic sites to effect the selective adsGI ptiGI ~ of said p-xylene and produce a r~finate co" ,~,ris;. ~y Cg aro,ndtic hy~roca, 6~"s and the non-paraxylene isG" ,er~, 10 induding ethyl~e"~e"e. P-xylene is ll ,ered~ler recovered by co. Ita~1ing theresulting p-xylene rich a~l~G,6enl with a ~lesGr6ent cG",~.risin~a 1,2,3,4 tetrahydrona~ tl ,alene (tetralin) or an alkyl, or dialkyl derivative of tetralin and mixtures ll,ereof. These desG,l,ents are all higher boiling (tetralin-b.p. 206~C) than the Cg a~ or"a~ics, making it possible to separate the Cg afo")atics from the 15 .les~r6ent by simple h ~ctionation so that the ~JesGr6e,~t can be reused in the process without building up Cg ar~l "a~ics in the recycled d~SGI ~nt.

BRIEF DESCRIPTION OF THE DRAWINGS

2 0 Figure 1 is a chfomatoy, aphic represen~a~ion of the s~,ai ~io., of p-xylene from a mixture of xylene isG",er~ and Cg afolna~ics with a K-e,(cl ,anged Y zeolite and a desor6erlt cGm~.ri~i-)y a 30/70 mixture of tetralin and n-i,eptane.
Figure 2 is similar to Figure 1 except that the adsor6ent is BaX.
Figure 3 is similar to Figure 1 except that the desGr6~1lt diluent is n-octane and shows the chr~" ,atogfaphic separ~tion of para-ethyl toluene and para-xylene from other C8 and Cg ar~" ,atics.
Figure 4 is similar to Figure 2 except that the desGrL,el)t diluent is n-octane and shows the separation of p-xylene from p-ethyltoluene and other 30 iSGm6rS of xylene.
In the figures, n~""erals are used to indicate the CG~ Gl 161 1~ Iines in the graph of relative concen~f~ti~ns vs. ~ etentio" volume (ml.) as follows: thetracer 1; mesitylene 2; m-xylene 3; n-propyll,en~ene 4; o-xylene 5; 1,2,3-l,i,netllyl~6l,~ene 6; ethylLen~el)e 7; 1,2,4-trimethyl~el,~ne 8; cumene 9;
35 p-xylene 10 and p-ethyltoluene 11.

DETAILED DESCRIPTION OF THE INVENTION

AcJsGrL,e"t~ to be used in the ~.rocess of this invention CG m~.rise specific crystalline aluminosilicate zeolites or molecular sieves" ~a, - .ely X and Y
5 zeolites. The zeolites have known cage structures in which the alumina and silica t~al,Gvd~a are inti,,,c.tely cGnnev~d in an open three-di",ensional network to form cage-like structures with v~. ,d~vl~ like pores. The tetral ,ed~ a are cross-linked by the sl ,a, ing of oxygen atoms with sl,~ces bat~ccn the tetld hGvdra occupied by water molecules prior to partial or total dehyd~ dtion of this zeolite.
lO The dehy,l~ ~ti~n of the zeolite results in crystals interlaced with cells having molecular diloensions and thus, the crystalline aluminosilicates are often referred to as ~molecular sieves~ when the se,~af~t,on which they effect is ~Jeper.dent essenlially upon dH~vrei ,ces l,~t~ e., the sizes of the feed molecules as, for inslance, when smaller nGr",al pa,d~" molecules are sepafaled from 15 larger isGpar~fFin molecules by using a particular molecular sieve. In the prooess of this invention, howe~r, the term Rmolecular sieves~, although widely used, is not strictly suitable since the separdtion of spec~c ar~",atic is~",er;j is appare, Itly dcvper,dGv, It on dHrere"ces in elvvbocl ,efrlical atb~actiGn of the dNferent isGmer~ and the ~JsGrL.enl rather than on pure physical size 2 o dfflerences in the isomer molecules.
In hyd~ ~ed form, the crystalline aluminosilicates encGmpass type X
zeolites which are represented by Formula 1 below in terms of moles of o~ides:

Formula 1 (o.9+ o.2) M 2/n o:Al2 o 3:(2-5~ o-5)sio 2 y H 2 o where "M is a cation having a valence of not more than 3 which balances the elec~,o~alence of the alumina tetrahedra and is generally refe"~.l to as an 3 0 excl ,ar,yeable cationic site, "n" represents the valence of the cation, and ~y", which represen~s the moles of water, is a value up to about 9 depending upon the identity of ~M and the deg~ee of hyd~ a~iGn of the crystal. As noted from Formula 1, the SiO2/A1203 mole ratio is 2.5t0.5. The cation ~M may be monovalent, divalent ortrivalent ca~iGIls or mixtures ll,ereof.

.

..
The type Y structured zeolite, in the hyJ~ate.l or partially hyJIated form, can be similarly represen~ed in terms of moles of oxides as in Formula 2 below:

Formula 2 (o~o.2)M2/no:Al2o3:wsio2 yH2o where ~M", ~n" and "y~ are the same as above and W is a value y-ealer than o about 3 up to about 6. The SiO2/AI2O3 mole ratio for type Y structured zeolites can thus be from about 3 to about 6. For both zeolites, the cation ~M" may be one or more of a variety of cations but, as the zeolites are initially prepareJ, the cation ~M" is also usually preJo" ~inately sodium. The type Y zeolite containing~,re.~o"linately sodium c~tions at the exehangeable cationic sites is, U ,er~f~re, 15 r~f~rr~J to as a sodium-excl ,ang~-~ type-Y, or NaY, zeolite. Depending upon the purity of the reaclantà used to make the zeolite, other cations ",enlion~d above may be present, ho~owr, as impurities.
The zeolites useful in the invention are typical as ~esc, i~J above.
I loJIe\,cr, the oxchanye of the cation of the as-man~actured zeolite by ions 2 0 from Group IA or IIA, e.g., barium or I ~PssiLIm or mixtures lhereof, is necessa~ to achieve the sepa- ~tion.
Typically, a-JSGI 6e, It~ used in sq.ardti~/e processes contain the crystalline material disperaeJ in an ar, IGr~hous, inorganic matrix or binder, having channels and cavities therein which enable liquid ~ccess to the 25 crystalline material. Silica, alumina, clay or mixtures II,ereof are typical of such inorganic matrix mdterials. The binder aids in forming or agglomer~tir,y the crystalline particles which ~tl ,e~;se would co,l-prise a fine poJ:Jer. The ~Jsor6ent may thus be in the form of pa, licles such as extnJr~tes, ayyl eg~es, tablets, macrospl ,eres or granules having a JesireJ particle range, preferal,ly30 from about 16 to about 60 mesh (Slanda"~ U.S. Mesh) (250 to 1190 microns).
Feed mixtures which can be utilized in the ,. rocess of this invention will COI "~,riae para-xylene, at least one other C8 aro,n~tic iso" .er, and may also contain one or more Cg arolna~ics as impurities. Mixtures containing sl~l,al~n~ial quan~i~ies of para-xylene and other C8 aroll-atic isol"era and Cg 35 aro-llalics generally are pro~uced by reforming and isomeri~aiion processes, processes which are well known to the refining and petrochemical arts. Many 200~645 of the Cg afo"-atics have boiling points in the range of 160-170~C and cannot be easily removed by distillation from the sla"da, d JesG, ban~, p-diethyll,e"~e,~e. In the current process, Iherefore, the Cg's are usually removed from the feed by distillation before the aJ~Gr~ti~/e se~.a, dtiGI, step and 5 co, ~se~luent CGI Ita~;t with the "or",al desvrbe, n. A desu, benl has now been discovered which can be easily separated from the Cg aro" IdtiCS by h actiGna~i~n after the aJ~Gr~ti~n step and does not thus require the large column and quantity of utilities to ~,ret. eat the feed, resulting in sub~'~nlial cost sav;ngs.
I tef~r" ling ~,rocesses can provide feed mixtures for the prwess of this invention. In reforming processes, a, lapl nl ,a feed is oontacleJ with a platinum-halogen-containing catalyst at severities selected to produce an effluent containing C8 afo")~tic is~" ,ers. Generally, the refo" ,-ate is then hacti~nateJ to cG,-centrate the C8 aro natic is~",er:j in a C8 ha~tion which will lS contain the C8 aro",dtic isG",ers as well as C8 nGnaro",atic and Cg arorr,dtics.
Feed mixtures for the process of this invention may also be obtained from is~")erkati~n and transalkylation proo~sses Xylene mixtures which are deficient in one or more isGI~ers can be iso-neri~ad, at isom~ ation conditions,to produce an emuent containing C8 a,~"-atic iSGIllelS, e.g., t",ri~;l-eJ in 2 o p-xylene, as well as C8 nonaro,na~ics and Cg ar~matics. The Cg ar~r"alic CGi Itent of isomerkeJ xylene iso" ,el :j can be as much as 1-2 wt.% Jepe"ding on isG",eri~tiGn conditions. Ukewise, transalkylation of mixtures of C7 and Cg ar~" ,atics produces xylene isG" ,ers. In all of these catalytic routes, the xylene splitter column must be employed to remove Cg a~o-"atics from C8 a~l-ldtics 25 before conventiG"al aclsor~ti-/e xylene sepaf~tion ",etl,Gds can be employed.Thus the feed mixtures to the process of this invention can contain qual Itities of C9 arGmatics and may also contain qua, Itities of straight or branel ,e.l chain par~ffins, cyclopar~ffins, or olefinic ",ate,ial. It is prefer~ble to have thesequa, Itities at a minimum amount in order to prevent conlamination of products 3 0 *om this ~,rocess by Il ,~e, ials which are not selectively aJSGrbed or sepa~led by the adsor~llt. r, efer~bly, the above-",el Itioned CG- Itaminants should be less than about 20% of the volume of the feed mixture passe~ ~ into the pfocess.To separa~e the para-xylene *om a feed mixture containin~ para-xylene, at least one other C8 a, om~ic and C9 ar~r,Idtics, the mixture is 35 conlacted with the aJSGrLent at a-JsGrpliGn conditions and the para-xylene (and para-ethyltoluene, if prese, It) is more selectively adsGrl,e.l and retained by the ~ 2 0 0 1 6 4 ~ ~
~J~r~nt while the other co" ,po"e, ~ are relatively unadsGl bed and are rermoved from the i, l~er:,~tial void sp~ces L et~ec., the pa, ~ eS of adsGr~nt and from the surFace of the aJ~O, l~nt. The aJ~G, benl containing the more SeleCt;VBIY aJSGIbe.l para-xylene is r~errbJ to 8S a ~fich~ aJsG,~nt-rich in the5 rnore selsctively aJsorbed para-xylene. The para-xylene is then recovered fromths rich aJsGrL enl by conta.,tin~ the rich aJSGI be, n with the instant tetralin or tetralin derivative ~Jesorl~, n l"alerial at desor,~tion conditions.
In this process, which employszeolitic ~J~Grbe, ns and which is generally operateJ continuously at subs~ tially co, Istanl pressures and o te, ~ ef~tlJres to ensure liquid phase, the desGr~"t ",dte, ial relied upon must beJudiciously selecteJ to satisfy several criteria. ~irst, the ~eEG,~ent ",dte,ial should dis,~lace an extract c~",ponent from the ~Is~, L~l n with reasonable mass flow rates without itseH bein~ so str~" Igly hdS0l L eJ as to unduly prevent the extract component from d;sp'~c;n~ the Jeso, Lent n ~at~rial in a following 15 ~Isorption cycle Seco, Idll, the JesGrbent ",~tefial must be c~,npatible withthe particular adsorl.ent and the particular feed mixture More s~c~lly, they must not reduce or destroy the critical selectivity of the ~dsGIL~nt for an ~Atla~t c~mpGI ,enl with ,es~ect to the r~ ale compGnenl or react chemically with the feed c~",p~nents Desorbsllt l"dterials should additionally be easily sepu~le 20 from the feed mixture that is ~J~sse J into the ~,rocess. Both the r~t;ndte co",~ onenis and the extract component~ are typically removed from the ~JsGrL,ent in aJInixlure with ,lesorl,enl r"aterial and without a "lethG J of s~par~tin~ at least a po, tion of ~Jesorbenl l"a~erial, the purity of the extract product and the ,~tfinate product would not be very high nor would the 25 ,lesGrbent "laterial be available for reuse in the ~.rocess It is, tl,er~Dre,c~, Ite",plated that the ,JesG,bent "~alerial used in this ~,,ocess will have a s~ l~)stantially different average boiling point than that of the feed mixture or any of its co,npo"~nts, i e, more than about 5~C d~rence, to allow se~,af~tion of at least a po, tion of the desorLenl ",aterial from feed COmPGI ,e~ in the extract 3 0 and r~Fi,)ate sl, ea" ,s by simple ~ actional distillation, U lereL,~ permitting reuse of ~s~, ~nl 1l ,aterial in the process Finally, JesGr~nt " ,atefials should be readily available and r~asonable in cost I lo~over, a suitable JesGrL ent or desG, ~*nts for a particular separation with spec;~c aJsorL.ent are not always predictable In the 35 p.~er,eJ is~tl,er",al, isol)a,ic, liquid-phase operation of the process of this invention, when the feed ",aterial to the separdtio,- process contains more than 20~)~645 .' g , about 0.1 wt.% Cg aro" ,atics, it has been found that a dasGrL~I ,~ " ,~terial "",rising tetralin or alkyl or dialkyl derivaUves of tetralin or mixtures ll ,ereof will ~bs-,rl, the extract from the aJ:3GI ~I l~ and can be se~arateJ from the Cg ar~",atics containing ~fFina~e by distillation.
s Suitable alkyl-substituted derivatives of tetralin include methyl tetralin, ethyl tetralin, propyl tetralin, is~prop~ltetralin, etc. Suitable dialkyl-substituted derivatives of tetralin include methyl-ethyl tetralin, dimethyl tetralin, diethyltetralin, etc. Mixtures of tetralin wmi one or more of these derivatives as well as mixtures of these derivatives also may be used with good results. In lO addition to tetralin, methyl tetralin and 2,~dimethyl tetralin are pre~r,~J as des~rL~nt~ in this ~,rocess. All po~ition is~,ners and mixtures are inler,JeJ tobe induded when any tetralin derivative is ~efer,eJ to herein.
A.Jsor~ion cGnJitions will include a temper~ure range of from about 20 to about 250~C with about 60 to about 200~C being more preferred and a 15 pressure just sufficient to maintain liquid phase, which may be from about atl "os,c I ,e, ic to 600 psig. DesG, ptiGn conditions will indude the same range of t~ "~.er~tures and pressure as used for &Jsorp~ion conditions.
A dynamic testing app~us is employed to test various &J~GrLenls and JesGr~nt material with a particular feed mixture to measure the aJ Gl L~nt 20 ~ar~l1erislics of aJ~or~Jti-/e cap~c~ and e,~cl,ange rate. The a~,par~tus cons;sls of a helical &J~orl,e, n cl ,an)ber of a~,pro,~i",ately 70 cc volume havin~
inlet and outlet ,uGI lions at oppo~ite ends of the cha"l6er. The cha"ll,er is contained within a teri ~r~ture control means and, in a~ ;o~)~ pressure conlrol equipment is used to oper~te the c;ha" Iber at a co"slanl pre.J~termined 25 pressure. Qu~ntitdti~e and qualitative equipment, such as reh~acto",eter~, pola(i" ,etera, cl " oma~oyl ~1 ,s, etc., can be ~tlacl ,eJ to the outlet line of the cha"lber and used to analyze, "on-st,e6"" the emuent sl,ea", lo~v;ng the ~JSGr6ellt ~I,al,ll~er.
A pulse test, p61 For,neJ using this appar~t~; and the folbwing 3 0 ~3eneral ~.roceJure is used to determine data, e.g., selectivities, for various aJSOI bel ~ systems. The aJsorL ent is filled to equilibrium with a particular .JesGr~enl by ~assi"~ the ~lesorbe"l material through the a~lsor~nl c~,a"lLer.
At a convenient time, a pulse of feed containing known concentrdtions of a tracer and of a particular extract cor"ponel ,t or of a r~Finate componen~, or 3 5 both all diluted in desor~ent " ,dterial is injected for a duration of several minutes. Desorbent flow is resumed, and the tracer and the exlr~ct and ;~OOi645 r~ffinate co"-~one"ts are eluted as in 8 liquid-solid cl,,omat~fa,uhic ope(a~ion.
The emuent can be analyzed by on-stream cl ,ro",~tog~phic equipment and traces of the env~lopes of cor,esponding co,r,~nent peaks developed.
Aller"dti~ely, emuent samples can be collected periodically and later analyzed 5 sepaf~tely by gas ~:I,rul,,d~uy,~l,y.
From i, If ~r,n~tion derived from the test, adsor6enl ~, Forrns"ce can be rated in terms of void volume, fetentio,., volume for an extract or a r~fFi"a~e co" ~po~ ~el)t, and the rate of ~bsor~Jtion of an extract co" ,pone, It from theadsor6ent and selectivity. Void volume is the non seltcli~/e volume of the o adsor6ent, which is e~ressed by the amount of .bso, l~, 1~ pumped during the interval from initial flow to the center of the peak envelope of the tracer. The net retel ~tio" volume (net) of an extract or a f~fFi,)ate c~r"~neut may be ~;hAra~;t~ d by the di,t~nce ~tv~eon the center of the peak envelope (gross retentio" volume) of the extract or r~fi,)ale compone, It and the center of the peak envelope (void volume) of the tracer Co",~l ,e, It or some other known reference point. It is e~resse.J in terms of the volume in cubic ce, ~imeter~ of.lesorbel It material pumped during this time interval r~pres6l Ited by the distance betv~con the peak envelopes. The rate of oxd ,&n~e or ~.leSGI ~tion rate of an extract cGm~cl Isnl with the desGrlJent ,),aterial can generally be 20 c:har~..~ri~c.l by the width of the peak env- llopes at half inlens~. The n&r,o~/er the peak width, the faster the deso, ption rate. The desGr~ tio" rate can also be c;l)aracteri~ed by the d;sta"ce L,et~ee.) the center of the tracer peak envelopeand the ~isaf)rea~ance of an extract col"pol~entwhich has just been .lesGrbed.
This ~i~lance is again the volume of desGr6ent ll,dte,ial pumped during this time 25 interval. Selectivity, ~, is determined by the ratio of the net rete"tion volumes of the more sbul ,yly adsGrbed co,npone, l~ to each of the other CGIll~Gl lent~.
The following non-limiting examples are presenlecl to illustrate the process of the present invention and are not inte"ded to unduly rest, i~ the scope of the claims ~tla~;hecl hereto.
EXAMP~E I

In this e~eri",ellt, a pulse test, using the a~"~af~tus as desc,iLed above, was pel ~ur",ed to evaluate the ability of the present invention to 35 sepaf~te para-xylene ~nD.p. 138~C) from the other xylene isG",er~ and ethylben~e"e (b.p's. from 136-145~C) and from Cg arol"dtics. The adsorbent ll used was a Y f~uj~s~e exchar"Je-l with ~otassiurn, dried to yield a loss upon i~nition (LOI) of 1.26% at 900~C combined with 15 wt.% of an amor~l ,ous clay bindér.
For each pulse test, the column was maintained at a ten",eral.Jre of 5 ~50~C and at a pressure of 165 psig (1240 kPa) so as to maintain liquid-phase operalio"s. Gas cl ,ro" ,~tG53rsphic analysis equipment was dttached to the column emuent stfear" in order to determine the cGmpo~ition of the emuent n,aterial at given time intervals. The feed r~ixture employed for each test was 5 cc of a mixture containing 0.45 cc each of the xylene i~l"er~ and ethylbel ~Lel ,e lO and each of the following Cg arol"atics: cumene, n-propylL~"~e ,e, p-ethyltoluene, mesrtylene, 1,2,4-~im~tl,ylL,e"Le.)e and 1,2,3-bi",etl,~lben ane.
Normal nGnane (0.45 cc) was used as a tracer and 4.95 cc ~eso-Le,tt ",aterial was added to the feed. The desGILsnt n,alerial cG"-prised 30 vol.% tetralin withthe remainder being n~7 par~n. The oper~tions taking place for the test 15 were as follows: The dssorl,enl mctterial was run continuously at a rate of about 1.44 cc per minute. At some convenient time interval, the deso- bont was stopl ~d and the feed mixture was run for a 3.47 minute interval. The ~E3G- Lenlst- e~ " was then resumed and continued to pass into the ~- l a- ~ column until all of the feed arol"atics had been eluted from the column as determined 20 by chro",dtog-aphic analysis of the emuent ",dterial Ic~v;n~a the ~Jso-~iol, column.
The results of the tests shown in Table 1 and the c hl Ol l l~tOyl aphic lldc;rly of Figure 1 illustrate the invention. The table lists the gross ratenlion volume (GRV) and net retention volume (NRV) for each ccl",~"en~ of the feed 25 and the selectivity, ~, for each cor"ponen~ with respe.1 to the refare"ce, p-xylene.

Z0016~5 ' 12 TABi E 1 Gross Net Fbure 1Retention ll~t~,r)tbn Se~ectiv ty Bdlin~
on6lQ nefe.ence No.Vdume(ml) Vdume(mi) ~B Poht (~) n N~ )ane 1 47.7~ ~0.0 (Tracer) Ethyii~,.~ene 7 59.8 12.1 1.43 136 10 p-Xyene 10 65.0 17.3 1.00 ~ef.) 138 Cumene 9 63.3 15.7 1.11 153 ~Xyene 5 55.6 7.9 2.18 144 n-Propybenzene 4 53.9 6.3 2.77 159 ~Ethytduene 11 75.2 27.5 0.63 162 15 Mesityene 2 53.6 5.9 2.92 163 1,2,4-T,i~,~tl,~ >ene 8 62.9 15.3 1.13 168 1,2,3-Trimethybenzene 6 60.3 12.6 1.37 175 m-Xyiene 3 53.0 5.3 3.26 139 20 E)CAMPLE ll i xample I was rq,e~e~ I except that the zeolite was a barium-excl ,~"ged X-type zeolite dried to yield an LOI of 4.7% at 900~C. The results are shown in Figure 2 and the following Table 2.

- 2001645 ' .

TABi E 2 Gross Net Fi~ure 2 Retention n~erhion S~iectivity Co,.,~ne.~l Reference No. Vdume(ml) Vdume(ml) n ~J~ )e 1 42.1 0.0 (Tracer) Ethybenzene 7 53.5 11.3 1.86 i~Xyiene 10 63.2 21.1 1.00 (Ref ) Cumene 9 52.6 10.5 2.01 ~Xyene 5 50.1 8.0 2.64 n~ropybenzene 4 50.9 8.8 2.40 i~Ethytduene 11 65.4 23.3 0.90 Mesityene 2 45.4 3.2 6.50 1,2,4-Trimethybenzene 8 53.4 11.2 1.88 1,2,3-Trimethybenzene 6 50.9 8.8 2.40 m-Xyene 3 49.2 7.1 2.97 20 =)(AMPI E 111 Another pulse test was run under the same conditions and with the same 1 l Idler;als as Example 1, except that the desoriJent was diluted with n-octane inste~d of n-l~eptane and the tracer was n-C6 instead of n-Cg The feed 25 was 2 cc of a solution containing 0 5 9 each of the following Illdteri~ls n-C6;
ethyl~,e"~ene, p-xylene, m-xylene, o-xylene and p-ethyltoluene. The ad~orL e"~
was dried to yield an LOI of 0 64% The results are shown in Figure 3 and the following Table 3.

2~0~

T~RI F 3 Gross Net Peak Width Figure 3 net~ lion Retention at HaH Selectlvlty Co~"~one,~Reference No.Vdume(ml) Vdume(ml) Hei~ht(rnl) n~6 1 43.8 0 8.3 Tracer Ethyl~,u~ne 7 56.9 13.1 12.3 1.50 ~Xyiene 10 63.5 19.7 11 (F~ef.) rn-Xyiene 3 49.1 5.3 11.2 3.72 0-Xyiene 5 51.8 8 12.6 2.46 p-Ethy1tduene 11 78 34.2 16.9 0.58 EXAMPLE IV

The pulse test of Example 111 was ~e~ e~l except that the bJSo- benl 2 0 was a barium oxel ,ange~i X-type zeolite dried to yield an LOI of 2% at 900~C.
The resuits are shown in Figure 4 and the following Table 4.

2 5 Gross Net Peak WWth Fi~ure 4Retention Retention At Half Selecthhty G,""~one,~ReferenceNo.Volume(ml) Vdume(ml) Hei~ht(ml) ,~
n ll~,~ne 1 39.4 o 9.9 Tracer Ethylben~ene 7 49.1 9.7 18.5 2.84 3 0 ~Xyene 10 66.9 27.5 18.8 Refer.
~Xyene 3 52.1 12.7 22.1 2.17 ~Xylene 5 49.4 10 19 2.75 ~Ethyltduene 11 58.4 19 26.4 1.45 - :~0~645 EXAMPLE V

Example ll was repea~ed except that the desorbent was a 30/70 wt.% mixture of methyl tetralin and I ,e,~lta"e, the LOI of the adsGrbent, BaX, was 5 4.02% at 500~C and the column flow rate was 1.23 cc/min. The methyl tetralin was a 35/65 wt.% mixture of the two possible iso" ,er~, 5-methyl tetralin and 6-methyl tetralin. The results are shown in the following Table 5, indicati,ly improved selectivity for p-xylene over ethy~e"~ene and slightly better selectivity over p-ethyl toluene.

Gross Net Retention Retention Selectivity Cor"~onent Volume (ml)Volume (ml) n-Cg 39 7 ~ ~ (Tracer) EthylL,e"~ene 52.5 12.8 2.20 p-Xylene 67.9 28.2 1.00 (Ref.) m-Xylene 47.3 7.6 3.71 2 0 o-Xylene 46.8 7.1 3.97 Cumene 51.1 11.3 2.50 n-Propyl~er,~ene 47.7 7-9 3-57 p-Ethyltoluene 64.6 24.9 1.13 Mesitylene 43.2 3.5 8.06 1 24-Trimethyll,enzene 48.9 9.1 3.10 1 23-Trimethylbenzene 46.1 6.3 4.48 m-Xylene 49.2 7.1 3.97

Claims (7)

1. A process for separating p-xylene from a mixture comprising p-xylene, at least 0.1 wt. % C9 aromatic hydrocarbons and at least one other isomer of xylene which process comprises contacting said mixture with an adsorbent comprising a crystalline aluminosilicate zeolite containing a Group IA or IIA metal ion at exchangeable cationic sites at adsorption conditions to effect the selective adsorption of said p-xylene by said adsorbent and to produce a raffinate stream comprising the less strongly adsorbed C9 hydrocarbons and said other xylene isomers and contacting said adsorbent with a desorbent comprising 1,2,3,4-tetrahydronaphtalene at desorption conditions to effect the removal of p-xylene from said adsorbent as a first extract stream.

2. The process of Claim 1 wherein said adsorbent is selected from the group consisting of type X and type Y zeolites.

3. The process of Claim 1 wherein a second extract stream is recovered in the desorption step after the first extract stream comprising more strongly held C9 aromatic hydrocarbons.

4. The process of Claim 1 wherein said crystalline aluminosilicate zeolite is exchanged with potassium or barium, or a mixture thereof, at exchangeable sites.

5. The process of Claim 1 wherein said raffinate stream is fractionated to separately recover C9 aromatic hydrocarbons, said desorbent material substantially free of C9 aromatic hydrocarbons and said other xylene isomers and the resulting recovered desorbent material is recycled to the desorption step of said process.

6. The process of Claim 1, 2, 3, 4, or 5 wherein the process is practiced in a cocurrent, pulsed system.

7. The process of Claim 1, 2, 3, 4, or 5 wherein the process is practiced in a countercurrent simulated moving bed system.