Disproportionation and Transalkylation of Alkylbenzenes over Zeolite Catalysts

Disproportionation and transalkylation of alkylbenzenes over zeolite catalysts

Tseng-Chang Tsai a ,Shang-Bin Liu b ,Ikai Wang c,*

a

Re?ning and Manufacturing Research Center,Chinese Petroleum Corporation,Chiayi 600,Taiwan b Institute of Atomic and Molecular Sciences,Academia Sinica,PO Box 23-166,Taipei 106,Taiwan c Department of Chemical Engineering,National Tsing-Hua University,Hsinchu 300,Taiwan

Received 13June 1998;received in revised form 3October 1998;accepted 5November 1998

Abstract

Disproportionation and transalkylation are important processes for the interconversion of mono-,di-,and tri-alkylbenzenes.In this review,we discuss the recent advances in process technology with special focus on improvements of para -isomer selectivity and catalyst stability.Extensive patent search and discussion on technology development are presented.The key criteria for process development are identi?ed.The working principles of para -isomer selectivity improvements involve the reduction of diffusivity and the inactivation of external surface.In conjunction with the fundamental research,various practical modi?cation aspects particularly the pre-coking and the silica deposition techniques,are extensively reviewed.The impact of para -isomer selective technology on process economics and product recovery strategy is discussed.Furthermore,perspective trends in related research and development are provided.#1999Elsevier Science B.V .All rights reserved.Keywords:Disproportionation;Transalkylation;Alkylbenzenes;Zeolites;Diffusivity

1.Introduction

Aromatics have a wide variety of applications in the petrochemical and chemical industries.They are an important raw material for many intermediates of commodity petrochemicals and valuable ?ne chemi-cals,such as monomers for polyesters,engineering plastics,intermediates for detergents,pharmaceuti-cals,agricultural-products and explosives [1].Among them,benzene,toluene and xylenes (BTX)are the three basic materials for most intermediates of aro-matic derivatives (Fig.1)[2].

Dialkylbenzenes,a subcategory of aromatics,include xylenes,diethylbenzene (DEB)and dipropyl-benzene (DPB),all of which may be derivable to valuable performance chemicals.For example,xylenes are the key raw materials for polyesters,plasticizers and engineering plastics [3],p -DEB is a high-valued desorbent used in p -xylene adsorptive separation process [4],whereas increasing applica-tions of diisopropylbenzene (DIPB)have been found,ranging from photo-developers,antioxidants to engi-neering plastics [5].Process development in aromatic interconversion is therefore an important research task with great industrial demands.There are many driving forces for the development of a new process.In addition to the

economically Applied Catalysis A:General 181(1999)355±398

*Corresponding author.0926-860X/99/$±see front matter #1999Elsevier Science B.V .All rights reserved.PII:S0926-860X(98)00396-2

relevant variables such as market demand,feedstock availability and cost,and operating cost,legislative aspects such as environmental laws,and new refor-mulated gasoline speci?cations,etc.,also come into play.In response to the worldwide environmental aware-ness,there are active programs to search for clean processes.Solid acid catalysts have long been demon-strated as the keys to the success of the historical efforts.Tanabe et al.[6]comprehensively discussed acid catalyst properties in his well-known review in solid acids and bases.Aromatics alkylation was one typical example of the use of solid acid catalysts in the development of environmentally sound

processes.Fig.1.Derivatives of benzene,toluene and xylene;from refs.[1,2].

356T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398

Zeolites were used to replace the traditional Friedel±Craft catalysts,making the process cleaner,less cor-rosive and more economic competitive.By using Friedel±Craft catalysts,solid and liquid wastes in ethylbenzene (EB)production of 390000tons/year were 500and 800tons/year,respectively.By using ZSM-5catalyst,the wastes were signi?cantly reduced to 35and 264tons/year,respectively [7].

The 1990Clean Air Act (CAA)had re-de?ned the gasoline speci?cations to enforce the so-called ``refor-mulated gasoline''(RFG)act [8].The initial stage of the enforcement applied the Simple Model regulation,by which the maximum benzene content in gasoline is limited to less than 1vol%[9,10].In a later stage,a Complex Model regulation will be applied,by which the maximum content of total aromatics is likely to be limited to as low as 25vol%.As a result,many international projects have been developed to modify re?nery structures in order to meet the challenge.The modi?cations involved switching the application of aromatics from gasoline to petrochemicals,especially to benzene and xylenes.The composition restrictions imposed on the ``reformulated gasoline''therefore not only have signi?cant impact on gasoline composition,but also on the economics of aromatics production processes.

The processes of catalytic reforming and naphtha pyrolysis are the main sources of BTX production.The product yields of those processes are normally controlled by thermodynamics and hence result in a substantial mismatch between the supply and the actual market demands.As shown in Fig.2,the world-

wide production capacity ratio for B:T:X obtained from the two above mentioned processes is 32:36:32[11],and varies with regional locations.This contrasts to their market demands in petrochemical industry (without accounting for the demand in gaso-line pool)of 55:11:34[12±16].In other words,toluene which has the lowest market demand is always in surplus from the production of reformate and pyrolysis gasoline,whereas benzene and xylenes are in strong demand with the average annual growth rates of around 10%[16±18],as shown in Fig.3.As a result of demand and supply,the price for toluene is always lower than the other aromatics.For example,the historical BTX market price in Europe is presented in Fig.4.The conversion of dispensable toluene into the more valuable aromatics therefore has an eco-nomic incentive.A serious discrepancy between pro-duction and market demand was also found for most dialkylbenzene isomers,among which the para -iso-mer apparently has the greatest market demand.In response to market situation and legislation changes,the main areas of new aromatics process innovations were:

1.conversion of surplus

toluene,

bca4c5a103d8ce2f006623d5parison of worldwide BTX distribution patterns of production and market

demand.

Fig.3.Worldwide BTX market growth

curves.

Fig.4.Historical BTX price in Europe market.

T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398357

2.upgrading of heavy aromatics,which are benzene and xylene (B &X)oriented,

3.selective production of para -dialkylbenzene iso-mer against thermodynamic equilibrium,such as p -xylene and p -DEB,and

4.production of dialkylbenzenes with carbon number of alkyl group larger than

3.

Disproportionation and transalkylation are the two major practical processes for the interconversion of aromatics,especially for the production of dialkyl-benzenes.The generic formulas for the two processes are given in reactions (1)and (2),respectively.They are coined as ``alkyl group transfer reactions'',which deal mainly with the alkyl group transfer among different aromatic rings.Such processes are com-monly used in the conversion of toluene into benzene and xylenes.Moreover,disproportionation of EB and isopropylbenzene (IPB)yields diethylbenzene (DEB)and dipropylbenzene (DPB),respectively.

Owing to the recent development in catalytic chem-istry of zeolites,a drastic improvement in aromatic conversion process technology has been found.There has been a growing research interest in both academic and industry.The results obtained from the funda-mental research in turn promote more innovation and development and hence stimulate ?ne-tuning of zeo-lite catalyst from the approach of molecular engineer-ing level.For example,several successful processes have been developed for the production of para -di-alkylbenzenes using zeolitic catalysts.The subject has been reviewed by Weisz [19]and Csicsery [20]on the fundamentals of shape selectivity of catalysis,Kaed-

ing et al.[21]on the Mobil's aromatics processes,Haag et al.[22]on acid catalytic aspects of medium-pore zeolites,and recently,Chen et al.[23]on indus-trial shape selective catalysis,Venuto et al.[24]on microporous catalysis chemistry,Ribeiro et al.[25]on techniques of zeolite modi?cation,Khouw and Davis [26]particularly on metal encapsulation and in the application of electro-and photo-chemical reactions.Improving and ?nding cost effective disproportio-nation and transalkylation catalytic processes are interesting and challenging tasks in industrial research.In recent years,there have been many research attempts in the area of process development.These new processes not only have had a great impact on process economics,such as production cost and supply and demand of aromatics,but also on the optimum process integration between conversion and separation units in a traditional dialkylbenzene production complex.

Several newly developed novel processes produced dialkylbenzenes which are particularly rich in para -isomers compared to their thermodynamic equili-brium compositions,for example,MSTDP SM ,MTPX SM and PX-Plus SM for p -xylene production [27±30]and TSMC's (Taiwan Styrene Monomer)selective PDEB process for p -DEB production [31].In addition,several new emerging heavy aromatics conversion processes with maximum approach to thermodynamic equilibrium xylene yield have also been developed,namely Tatoray SM [32±34]and TransPlus SM [35±38].

The present review is presented from the perspec-tive of process technology of aromatic interconversion along with fundamental research on shape selective catalysis.In particular,the selection of a suitable zeolite for a process and the fundamentals of shape selective catalysis are intensively discussed.For appli-cation point of view,the interplay of market demand and process technology development is reviewed and the development of alkyl group transfer processes from the perspectives of overall economics of produc-tion complex is thoroughly discussed.An extensive patent search and industrial process review have been done along with the evaluation of their possible impacts on the production scheme.Moreover,the existing relevant separation technologies are discussed and a perspective on trend in related research and development is included.

358T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398

2.Zeolite structure

Zeolites are crystalline and porous materials with an open structure that consist of AlO4and SiO4tetra-hedral units linked through oxygen atoms.Owing to their unique pr{o}perties in ion exchange and adsorp-tion capacity and catalytic activity,zeolites have been widely used as adsorbents,molecular sieving agents and catalysts for a variety of different chemical reac-tions.They have also been modi?ed by isomorphous substitution of silicon and aluminium by incorporating other atoms such as titanium,iron,gallium,boron, phosphorous,etc.in the framework.

There are over40known natural zeolites and more than150synthetic zeolites have been reported[39,40].The number of synthetic zeolites with new structure morphologies grows rapidly with time.Based on the size of their pore opening,zeolites can be roughly divided into?ve major categories,namely8-,10-and 12-membered oxygen ring systems,dual pore systems and mesoporous systems[23].Their pore structures can be characterized by crystallography,adsorption measurements and/or through diagnostic reactions. One such diagnostic characterization test is the``con-straint index''test.The concept of constraint index, originally introduced by Frilette et al.[41],was de?ned as the ratio of the cracking rate constant of n-hexane to3-methylpentane.The constraint index of a typical medium-pore zeolite usually ranges from3to 12and those of the large-pore zeolites are in the range

Table1

Structural characteristics of selected zeolites[23,39]

Zeolite Number

of rings Pore

opening(Aê)

Pore/channel

structure

V oid volume

(cc/g)

D Frame a

(g/cc)

CI b

8-membered oxygen ring zeolites

Erionite8 3.6?5.1Intersecting0.35 1.5138 10-membered oxygen ring zeolites

ZSM-510 5.1?5.6Intersecting0.29 1.798.3

5.1?5.5

ZSM-1110 5.3?5.4Intersecting0.29 1.798.7 ZSM-2310 4.5?5.2One-dimensional±±9.1 Dual pore system

Ferrierite(ZSM-35,FU-9)10,8 4.2?5.4One-dimensional0.28 1.76 4.5

3.5?

4.810:8intersecting

MCM-22127.1Capped by six rings±±1±3

10Elliptical Two-dimensional

Mordenite12 6.5?7.0One-dimensional0.28 1.700.5

8 2.6?5.712:8intersecting

Omega(ZSM-4)127.4One-dimensional0.38 1.650.5

8 3.4?5.6One-dimensional

12-membered oxygen ring zeolites

ZSM-1212 5.5?5.9One-dimensional±± 2.3 Beta127.6?6.4Intersecting±±0.6

12 5.5?5.5

Faujasite(X,Y)127.4Intersecting0.48 1.270.4

127.4?6.512:12intersecting

Mesoporous system

VPI-51812.1One-dimensional±±±MCM41-S±16±100One-dimensional±±±

a Framework density.

b Constraint index.

T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398359

1±3.For materials with an open porous structure,such as amorphorous silica alumina,their constraint indices are normally less than1.On the contrary,small-pore zeolites normally have a large constraint index;for example,the index for erionite is ca.38.

A comprehensive bibliography of zeolite structures has been published by the International Zeolite Asso-ciation[39].The structural characteristics of assorted zeolites are summarized in Table1.

Zeolites with10-membered oxygen rings normally possess a high siliceous framework structure.They are of special interest in industrial applications.In fact, they were the?rst family of zeolites that were synthe-sized with organic ammonium salts.With pore open-ings close to the dimensions of many organic molecules,they are particularly useful in shape selec-tive catalysis[23].The10-membered oxygen ring zeolites also possess other important characteristic properties including high activity,high tolerance to coking and high hydrothermal stability.Among the family of10-membered oxygen ring zeolites,the MFI-type(ZSM-5)zeolite(Fig.5(A))is probably the most useful one.ZSM-5zeolite has two types of channel systems of similar size,one with a straight channel of pore opening5.3?5.6Aêand the other with a tortuous channel of pore opening5.1?5.5Aê.Those intersect-ing channels are perpendicular to each other,generat-ing a three-dimensional framework.ZSM-5zeolites with a wide range of SiO2/Al2O3ratio can easily be synthesized.High siliceous ZSM-5zeolites are more hydrophobic[42]and hydrothermally stable[43] compared to many other zeolites.Although the?rst synthetic ZSM-5zeolite was discovered more than two decades ago new interesting applications are still emerging to this day.For example,its recent applica-tions in NO x reduction,especially in the exhaust of lean-burn engine[44],have drawn much attention. Among various zeolite catalysts,ZSM-5zeolite has the greatest number of industrial applications,cover-ing from petrochemical production and re?nery pro-cessing to environmental treatment.

Although the10-membered oxygen ring zeolites were found to possess remarkable shape selectivity, catalysis of large molecules may require a

zeolite

Fig.5.Framework structure of zeolites:(A)ZSM-5(B)Faujasite(C)Beta(D)ZSM-12(E)Mordenite(F)MCM-22;reproduced from refs. [39,45].

360T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398

catalyst with a larger-pore opening.Typical12-mem-bered oxygen ring zeolites,such as faujasite-type zeolites,normally have pore opening greater than ca.5.5Aêand hence are more useful in catalytic applications with large molecules,for example in trimethylbenzene(TMB)conversions.Faujasite(X or Y;Fig.5(B))zeolites can be synthesized using inorganic salts and have been widely used in catalytic cracking since the1960s.The framework structures of zeolite Beta and ZSM-12are shown in Fig.5(C)and Fig.5(D),respectively.

Zeolites with a dual pore system normally possess interconnecting pore channels with two different pore opening sizes.Mordenite is a well-known dual pore zeolite having a12-membered oxygen ring channel with pore opening6.5?7.0Aêwhich is interconnected

to8-membered oxygen ring channel with opening 2.6?5.7Aê(Fig.5(E)).MCM-22,which was found more recently,also possesses a dual pore system. Unlike mordenite,MCM-22consists of10-and12-membered oxygen rings(Fig.5(F))[45]and thus shows prominent potential in future applications.

In the past decade,many research efforts in syn-thetic chemistry have been invested in the discovery of large-pore zeolite with pore diameter greater than12-membered oxygen rings.The recent discovery of mesoporous materials with controllable pore opening (from ca.12to more than100Aê)such as VPI-5[46], MCM-41S[47,48]undoubtedly will shed new light on future catalysis applications.

3.Xylene production process

3.1.Xylene market

Among the three xylene isomers,namely o-(1,2-dimethylbenzene),m-(1,3-dimethylbenzene)and p-xylene(1,4-dimethylbenzene),the last has the great industrial demand.Since p-xylene can be used to produce pure terephthalate and polyester,the annual growth production rate of p-xylene usually coincides with the gross national production rate(GNP)[3]. There is strong demand especially in countries of the Paci?c Rim region.Worldwide growth rates were7% and8.8%in the1990±1995and1995±2000periods, respectively[17].The application for polyester is comprised of73%?ber,14%PET(polyethylene-terephthalate)resin,7%PET?lm and6%miscella-neous[3].Among them,growth rate of PET resin demand was particularly high,up to around17%[17]. It is owing to the high growth rate for p-xylene in the past decade that many major petrochemical compa-nies worldwide have pursued active p-xylene expan-sion projects to meet the strong market demand[49].It is expected that the worldwide p-xylene production will increase by50%in the next decade[3,17,18]. By comparison,o-xylene produces phthalic anhy-dride which is used for plasticizer.The average growth rate of o-xylene was around5±9%per year,which was slightly lower than that of p-xylene[16,50].On the other hand,m-xylene is mostly used in producing isophthalic acid,which is a valuable additive for polyester.Although the demand for m-xylene is low (Fig.6),it shares the same growth rate as p-xylene. While most of the aromatics production processes yield xylene mixtures with a ratio approaching ther-modynamic equilibrium,(24:53:23for p-,m-,and o-isomer,respectively),the market demand for the same xylene isomers is roughly in the ratio of80:2:18[13±16],as shown in Fig.6.Since the amount of p-xylene obtained directly from the reaction mixtures cannot meet market requirement,the surplus m-xylene and o-xylene are further isomerized to p-xylene to balance the market demand.

3.2.Recovery of p-xylene and o-xylene

By convention,the so-called C8aromatics(A8) include four isomers,namely o-,m-and p

-xylene Fig. 6.Spectrum of worldwide production rate and market requirement of xylenes.

T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398361

and ethylbenzene(EB).As shown in Table2[51], these four isomers all have similar physical properties. In comparing its vast demand to the other A8isomers (Fig.6),p-xylene recovery plays the key role in determining the A8separation scheme.

EB has the lowest boiling point but it is only2.18C lower than that of p-xylene.Conventionally,it can be recovered by a superfractionation method developed in the1960s.For example,for a fractionation tower design to have a recovery rate of95%,it required about330theoretical trays and a reˉux ratio up to90. As a result,the requirements for large number of trays and high reˉux ratio inevitably resulted in a substan-tial increase in the recovery cost which is the main reason for the near obsoleteness of the superfractiona-tion method.

The latest technology for EB separation is UOP's Sorbex TM[52]which involves recovery from an A8 mixture.However,benzene ethylation has become the major source of EB due to its lower production cost and the growing demand of the styrene industry.Thus, a state-of-the-art aromatics complex no longer sepa-rates EB from A8mixtures.Instead,EB is converted to other A8isomers by isomerization reaction or to benzene by dealkylation reaction.

A typical aromatic production scheme(Fig.7)nor-mally consists of:

1.aromatics production section which includes reforming process and pyrolysis gasoline,

2.extraction section to separate BTX aromatics from non-aromatics raffinate,

3.toluene conversion section which includes dispro-portionation and transalkylation processes,

4.product recovery section which consists of separa-tion towers of benzene,toluene,xylenes,o-xylene and A9(C9aromatics),

5.p-xylene recovery section by either crystallization or adsorption method[4,53±58]and

6.isomerization section for converting raffinate of the p-xylene recovery section into xylene isomers. Typical commercial specifications of some of the aromatics products are presented in Table3.

3.2.1.o-Xylene recovery

o-Xylene has the highest boiling point and is5.38C higher than that of m-xylene(Table2).o-xylene plus C 9aromatics can?rst be separated out from the other three isomers in the xylene column.A typical xylene column design normally has80±160trays with a reˉux ratio of2±6.Moreover,the system usually maintains a low(30±50%)o-xylene recovery rate to prevent contamination of o-xylene product and to minimize energy consumption of the xylene splitter [59].High purity o-xylene can then be obtained with a purity up to98.5%by separating out A 9in the bottom of o-xylene column.

Table2

Physical properties of dialkylbenzne aromatics[15,51]

Isomer Boiling

point(8C)Melting

point(8C)

d204

Dimethylbenzenes(C8aromatics)

Ethylbenzene136.2à95.00.8670

p-Xylene138.3 13.30.8611

m-Xylene139.1à47.90.8642

o-Xylene144.4à25.20.8802

Methylethylbenzenes(C9aromatics)

p-Ethylmethylbenzene162.0à62.40.8656

m-Ethylmethylbenzene161.3à95.60.8689

o-Ethylmethylbenzene165.2à80.80.8851

Diethylbenzenes(C10aromatics)

p-Diethylbenzene183.8à42.90.8670

m-Diethylbenzene181.1à84.20.8684

o-Diethylbenzene183.5à31.20.8839

Methylpropylbenzene isomers(C10aromatics)

p-Methylisopropylbenzene177.1à67.90.8615

m-Methylisopropylbenzene175.1à63.70.8652

o-Methylisopropylbenzene178.2à71.50.8808

p-Methyl-n-propylbenzene183.3à64.20.8631

m-Methyl-n-propylbenzene181.8à82.20.8659

o-Methyl-n-propylbenzene184.8à60.20.8783

Dipropylbenzenes(C12aromatics)

p-Diisopropylbenzene210.5à17.10.8606

m-Diisopropylbenzene203.2à63.10.8629

Table3

Product specifications of assorted commodity aromatics

Product Specification minimum purity(%)

Benzene99.85

Toluene99.0

o-Xylene98.5

p-Xylene99.5%(typical),99.8%(ultra-pure)

m-Xylene99.5

Cumene99.9

p-Diethylbenzene97±99

362T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398

3.2.2.p-Xylene recovery The mixture of EB,p -,m -xylene,and some residual o -xylene,which is collected from xylene splitter top (Fig.7),is subject for recovering p -xylene.The recov-ery of p -xylene from an A 8mixture can be achieved either by crystallization or adsorption technology.The more conventional crystallization method takes the advantage of the fact that p -xylene has the highest melting point among the A 8isomers (Table 2).Several practical technologies are known,for example,Iso-?ning SM (Esso),Antar SM (HRI,Hydrocarbon Research)and the proprietary processes developed by Krupp Koppers,Maruzen and ARCO (Atlantic Rich?eld).Conventionally,the crystallization process operates at low temperature and utilizes a two-stage crystallizer scheme [60].In the ?rst stage the crystal-lizer,which is maintained in the temperature range from à608C to à708C,yields only a wet cake with a relatively low p -xylene purity.During this stage,m -and p -xylene together form an eutectic mixture which limits the lowest crystallization temperature [61].Refrigeration cost increases substantially with the temperature below à358C,at which ductility of the insulation material becomes an issue [62].The tem-perature of the eutectic mixture controls m -xylene impurity levels and the maximum recovery rate of p -xylene.As shown in Fig.8[63],eutectic tempera-tures can be calculated from the ratio of the concen-tration of m -xylene to the concentration of p -xylene in the feed and the ratio of the concentration of m -xylene and of p -xylene in eutectic mixtures.The eutectic temperature of thermodynamic equilibrium xylene compositions with p -xylene to m -xylene concentration ratio of 0.45is à52.68C [63,64].It increases with higher concentrations of p -xylene,EB and o -xylene in A 8mixture [63].In the second stage the crystallizer is operated in the temperature range from à188C to 48C.This further puri?es the wet cake generated in the ?rst stage.The wet cake from the second stage is then further washed with p -xylene or toluene to obtain p -xylene with 99.5%purity.The two-stage crystallizer scheme described above is operated at a high recycle rate of mother liquid.As shown in Fig.9,for typical thermodynamic equilibrium xylene compositions,

the Fig.7.Typical aromatics production scheme.

T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398363

p -xylene recovery rate obtained by crystallization is ca.65%,as compared to the value of 90±95%obtained by adsorption.The low recovery rate and low crystal-lization temperature of the former resulted in an operating cost 2±6times higher than the latter tech-nique [53].Worldwide unit capacity applying crystal-lization technology is excessively lower than that applying adsorption technology,accounting for 35%and 65%global p -xylene capacity,respectively [30].The adsorption method mostly utilizes modi?ed faujasite as the adsorbent over which p -xylene has the greatest adsorption af?nity among the species in the isomer mixture [53,54].Several industrial tech-

nologies are known,for example,Parex SM (UOP),Aromax SM (Toray)and Eluxyl SM (IFP).The adsorp-tion process is operated in a simulated moving,coun-tercurrent,liquid phase adsorption bed at constant chamber temperature and pressure [4,63,65],with typical ranges of 160±1808C and 8±12atm [63].Separation proceeds through four zone steps,namely adsorption zone,puri?cation zone,desorption zone and buffer zone along the axial positions of adsorption chamber [53,65]in which p -xylene concentration changes with sequential time and chamber positions.The control of the sequential operations is the key characteristics of different technologies.Parex SM Pro-cess applies a patented rotary valve,which is a multi-port valve.A rotary valve comprises connectors of various bed line pipes to deliver liquids of changing concentrations into various zones of the adsorption chamber and product distillation towers and feed lines.The adsorption process requires a stringent speci?ca-tion of A 9content.

Design of Eluxyl SM Process has two versions,a stand-alone version and a so-called hybrid version [54±58].In the stand-alone version,p -xylene was recovered directly from xylene mixtures by the adsorption unit.The hybrid version consists of an adsorption unit and a crystallizer in which p -xylene purity is upgraded ?rst by the adsorption unit from thermodynamic equilibrium composition (with 23%concentration)up to ca.90%,and that product is then puri?ed by crystallization to 99.5% purity.The stand-alone version applies ?ve zones as the adsorp-tion con?guration [54],which has one additional adsorption stage more than Parex SM Process.In con-trast,the hybrid version applies four zone con?gura-tions.It was claimed that the hybrid version has higher productivity,with smaller adsorbent inventory and requires fewer fractionation columns.

After the recovery of p -xylene,the remaining iso-mer mixture,namely A 8aromatic raf?nate,is subject to isomerization which converts the p -xylene lean raf?nate into a thermodynamic equilibrium mixture (Fig.7).The mixture is then recycled back to the separation scheme loop for extinct recovery of o -xylene and p -xylene.The typical ˉow rate of the recycling loop is about three times larger than the fresh A 8mixture.

The key factors which affect the p -xylene recovery rate and purity vary with the recovery technique

used.

Fig.8.Relationship between eutectic temperature and ratio of m -xylene and p -xylene compositions in feedstock and eutectic mixtures;reproduced from ref.

[63].

Fig.9.Schematic comparison of p -xylene recovery rate of different compositions of xylene mixtures by crystallization and adsorption methods;data from refs.[30,53,78].

364T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398

In the case where the crystallization method is applied,the p -xylene recovery rate is found to depend on the overall xylene composition,which dictates the tem-perature limit of the eutectic mixture.In the case of the adsorption method,however,the recovery rate has a strong dependence on the EB content which has a similar adsorption af?nity to p -xylene over the zeolite adsorbent [53,54].

There has been a growing demand from the down-stream PTA (pure terephthalic acid)industries to upgrade the p -xylene purity speci?cation.For exam-ple,high purity p -xylene is indispensable for the production of PET bottle resins and micro?ber polye-ster.As a result,the purity speci?cation of p -xylene has increased from 99.2%in the 1970s to 99.8%in the 1990s [66].Nowadays,ultra-high purity p -xylene occupies about 20%of the worldwide xylene market,as shown in Fig.6.It is believed that the p -xylene recovered by the state-of-art adsorption technology alone is capable of meeting the growing market demand and purity requirements [66].Since the energy consumption cost for p -xylene recovery by the crystallization method is much higher and much more sensitive to product purity than that of the adsorption technique (Fig.9),the conventional crys-tallization method has been gradually replaced by the adsorptive separation process.However,recent advances in p -xylene selective disproportionation pro-cesses seem to favor more on the crystallization method;their impacts are discussed below.3.3.Toluene disproportionation

bca4c5a103d8ce2f006623d5mercial disproportionation processes There are two major techniques to convert surplus toluene into other aromatics.The ?rst is methyl group transfer,are shown earlier in reactions (1)and (2)with the R group representing methyl group.The second (reaction (3))is

hydrodealkylation.

The methyl group transfer technologies,which include disproportionation and transalkylation,con-vert toluene into benzene and xylenes simultaneously whereas the hydrodealkylation scheme mainly pro-

duces benzene.From the data in Fig.10,it is obvious that there is a growing demand for methyl group transfer technologies than for the hydrodealkylation process mainly due to the growing xylene market and partially from the impact of freeing up of benzene from RFG regulation [12].In the US market,the annual growth rates of disproportionation process and hydrodealkylation were 6.9%and 0.7%,respec-tively [18].Compared to the methyl group transfer reactions,hydrodealkylation processes are normally operated at a much higher reaction temperature (ca.6508C)and require higher operation and capital investment cost by about 10%[16].

Disproportionation and transalkylation are both acid catalyzed reactions.In the early days,liquid Friedel±Crafts [67]and HF±BF 3systems [68]were commonly used.Then,the metal oxide catalysts,such as CoO±MoO 3on aluminosilicate/alumina [69]and noble metal or rare earth on alumina were developed and used [70].In modern technology,zeolite catalyst systems,for example zeolite Y ,mordenite,ZSM-5and other large-pore zeolites,are predominant [71].Trans-alkylation processes are normally catalyzed by large-pore zeolites which can also be used for toluene disproportionation.The latter,however,is mostly catalyzed by 10-membered oxygen ring zeolites hav-ing medium-pore size such as ZSM-5zeolite.

The main products of toluene disproportionation are benzene and xylenes.In addition to main reaction,there are some side reactions,including xylene dis-proportionation producing A 9and dealkylation of alkylbenzenes producing light gas.Therefore,char-acteristics of the catalytic processes include

conver-

Fig.10.Worldwide growth curves of distribution of toulene usuage for hydrodealkylation and disproportionation processes and gasoline.

T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398365

sion,product yields and reaction conditions and,more importantly,throughput.

The typical operating conditions for most commer-cial disproportionation processes are depicted in Table4.Among the well-known Mobil disproportio-nation processes,the Toluene Disproportionation Pro-cess version3(TDP-3SM)is known for its high catalyst activity,high stability and low EB yield[72,73],which would reduce the recovery cost of p-xylene in down-stream bca4c5a103d8ce2f006623d5pared to the earlier ZSM-5technol-ogy,the TDP-3SM Process can lower reaction temperature about508C[73].Its space velocity is the highest one among the known processes(Table4). Selectivity is good,having benzene-to-xylene molar ratio of1.1,which is close to the theoretical number of unity.Its?rst cycle length is more than three years. The reactor can be radial or axialˉow design.The TDP-3SM can process toluene feed containing feed up to25%A9.

The T2BX SM Process,which Fina Oil developed in the1980s[74],applied severe operating conditions, such as lower space velocity.Water content is limited to250ppm and hydrogen consumption was around 17.8m3/m3feed.It produced excessively high amounts of A9aromatics with selectivity up to14% (Table4),which is used as a gasoline blending stock. Recently,Fina Oil made improvements on the process [75].

There are three p-xylene selective processes, including the Mobil's Selective Toluene Dispropor-tionation Process(MSTDP SM),(which utilizes ZSM-5 zeolite that is modi?ed by pre-coking treatments [27,76,77])and Mobil's Toluene to para-Xylene (MTPX SM)Process and PX-Plus SM Process,which are known in less detail.According to the patent literature,the ZSM-5zeolite used in the MTPX SM Process involves a silica selectivation treatment [78,79].The PX-Plus SM Process incorporated a pro-

Table4

Summary of commercial toluene disproportionation processes

Process name

TDP-3[73]MSTDP[27]MTPX[79]PX-Plus[30]T2BX[74] Catalyst ZSM-5ZSM-5pre-coked ZSM-5silica modified Not disclosed Not disclosed

Reaction conditions

Reactor type Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Temperature(8C)435455±470$420±390±495 Pressure(kg/cm2)24.5±28.221.1±42.3$21.1±42.3±49.3

H2/HC(mol)1±22±4$2±4±4

WHSV(hà1)62±4±± 1.2±2.3a Conversion(%)45±5030303044

Product Selectivity(%)b

Cà5gas 2.7 6.6 3.7 5.38.1 Benzene42.344.944.746.435.0 Xylenes50.443.548.044.740.8

EB 1.3 2.5 2.0 1.9 2.4

C 9aromatics 3.3 2.5 1.6 1.713.7 Xylene distribution(%)c

p-Xylene25.282.289.890.225.1

m-Xylene52.815.18.28.550.1

o-Xylene22.0 2.7 2.0 1.424.8

B/(X EB)(mol) 1.1 1.3 1.2 1.4 1.1 Cycle length(years)>3>1.5±±>1

a WHSV was estimated from reported LHSV data of1±2.

b Selectivity of PX-Plus was an approximation.

c Xylene distribution of MSTDP was line out data in start-of-run period.

366T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398

prietary selectivation technique,which has not been disclosed to the public.As a result of selectivation, they all produce xylene mixtures with p-xylene con-centration far beyond its thermodynamic equilibrium value.The relevant details of these processes will be discussed in the next section.

3.3.2.Development of selective toluene disproportionation process

The development of a dialkylbenzene process with enrichment of the p-isomer is a challenging techno-logical task.Nonetheless,the pioneering works to improve para selectivity started in the1970s.Yashima et al.[80]observed p-xylene selectivity of45±50%, higher than its thermodynamic equilibrium value,over cation-exchanged zeolite Y.Chen et al.[81,82]found that with a shape selectivity catalyst,p-xylene selec-tivity tended to increase with increasing ZSM-5crys-tal size.Since then,more techniques involving modi?ed zeolite catalysts were applied to increase p-isomer selectivity.

A number of different modi?cation techniques were found to be useful.Kaeding et al.[83±86]found that impregnating phosphorous,silica,calcium or MgO or boron onto ZSM-5could enhance the selectivity of bca4c5a103d8ce2f006623d5rge amine molecules,such as4-methylqui-noline[87],1-methylisoquinoline[88],etc.,can be adsorbed only on the zeolite external surface,and improve para-isomer selectivity.Since the amines desorb at higher temperatures,the method is applic-able only at low reaction temperatures.Germanium was the other modi?cation agent for?ne-tune zeolite pore structures;it introduced metal catalytic function into zeolite acid function[89,90].In terms of external surface coverage,germanium gave less complete coverage than silica[90].Industrial application applied mainly pre-coking and silica deposition tech-niques,with proper selection of crystal size,Si/Al ratio and morphology of the parent zeolite.

The basic principles of para selectivity improve-ment include the reduction of diffusivity and the inactivation of external surface sites.While the p-isomer is the apparent primary product leaving the zeolite pore mouth[91],isomerization proceeds as the secondary reaction on the zeolite external surface. Since sites located on the external surface are more accessible than the sites in zeolite pores[92],inactiva-tion of external sites can inhibit secondary isomeriza-tion and retain the p-xylene selectivity in products coming out from zeolite pores.

Diffusivity in zeolites varies widely with molecular structure con?gurations.For example,it drops sharply in ZSM-5as the number of branch chains increases (Fig.11),such as from n-hexane to3-methylpentane over the range of10à4±10à5cm2/s,and also as the sizes of alkylbenzenes becomes larger,such as p-xylene to o-xylene over the range of10à7±10à10cm2/s[22,93].The diffusion rate of p-xylene is at least1000times faster than that of the other isomers;the increasing diffusion resistance will create more diffusion superiority for p-xylene and conver-sely,more diffusion barrier for o-xylene and m-xylene. Therefore,p-xylene rapidly diffuses out from zeolite pores,inside of which isomerization takes place stea-dily toward equilibrium to provide additional p-xylene isomer for diffusion out.The criteria in coupling of isomerization rate and diffusion rate to enhance p-xylene selectivity is[91]

D p)D m Y o(4.1) K I!D m Y o a r2Y(4.2) K D D T a r2Y(4.3) K I a K D observed1Y(4.4) where K I and K D are the rate constants of isomeriza-tion and disproportionation,respectively,D m,o and D T are the diffusivities of m-xylene or o-xylene and toluene,respectively,and r is the crystal size.The ratio of the intrinsic kinetic rate of isomerization re-action to the disproportionation reaction,K I/K D(intrinsic), is about5000[71].The ratio of the observed kinetic rates,K I/K D(intrinsic),depends on the zeolite pore opening and thus,the constraint index.Moreover, for ZSM-5zeolite,K I/K D decreased from360in small crystals to2in large crystals,in which the criteria of reaction(4.4)were ful?lled and para selectivity was obtained[22,86].

Olson et al.[91]studied the relationship between p-xylene selectivity and the diffusion time of o-xylene over various ZSM-5catalysts,including large crystals and small crystals modi?ed with silica,coke,anti-mony,magnesium,calcium,zinc and boron,as shown in Fig.12.Diffusivity was expressed with a charac-teristic diffusion time,t0.3,required to sorb30%o-xylene at1208C,and para selectivity was achieved

T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398367

only at t 0.3beyond around 50min.Therefore,the diffusivity control mechanism is a useful working principle in assessing p -isomer selectivity.Among those modi?cation methods,large crystal size gener-ally produced better para selectivity than small crys-tals.Furthermore,pre-coking pretreatment can effectively enhanced para -isomer selectivity up to 70±80%(Fig.12).It was believed that coke

selec-Fig.11.Diffusion rate of various aliphatics and aromatics molecules over ZSM-5,with comparison to the pore openings of 8-MR,10-MR and 12-MR zeolites;from ref.

[93].

Fig.12.Relationship between p -xylene selectivity in toluene disproportionation over ZSM-5and their characteristic diffusion time t 0.3(adsorption time to adsorb 30%o -xylene at 1208C);reaction conditions:5508C,41bar,20%conversion;reproduced from ref.[91].368T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398

tively lays down on the external surface,and inacti-vates the external active sites,as shown in Fig.13.Because there is no internal coke deposition,pre-coking treatment can retain adequate acid sites to catalyze reactions.Mathematical models supported well the conclusion,showing that p -xylene concentra-tion is greater than 90%when conversion is low and effectiveness factor,(Kr 2/D )1/2,is greater than 10[94,95].A schematic model (Fig.14)illustrated the working principle,showing the relative diffusivity of xylene isomers and rate constants of isomerization and disproportionation.However,there were disagreements about the key controlling parameters among these above-mentioned parameters.Bhat et al.[96,97]proposed that inactiva-tion of external surface is the prominent control strategy for achieving high para -isomer selectivity.Wang et al.[87]concluded in the studies of EB disproportionation and toluene ethylation that para -isomer selectivity was not solely dependent on diffu-sion barrier and that external surface inactivation was another important factor.They also found that the modi?cation requirement for para -isomer selectivity enhancement varies with reaction types.Niwa et al.[98±101]have conducted extensive studies on the working mechanisms of para -isomer selectivity enhancement of toluene methylation.They prepared various parent zeolite samples with different

Si/Al

Fig.13.Coke selectivation conceptual model;reproduced from ref.

[91].

Fig.14.Conceptual model of selective toluene disproportionation over ZSM-5(modified from refs.[19,22,24,71,84,86,91].T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398369

ratios and crystal sizes,on which silica CVD mod-i?cation was performed.Silica covers the external surface and also reduces the effective pore size by ca.0.1and0.2nm upon the formation of1,2and3 molecular layers of silicon oxide[101].All the sam-ples were characterized by diffusion rate measure-ments,1,3,5-triisopropylbenzene cracking tests and o-xylene isomerization tests.They concluded from the extent of dependency of para-isomer selectivity enhancement on characterization properties that nar-rowing of the pore opening is much more important than the inactivation of external surface.On the other hand,Yashima et al.[102]reported that weaker acidity in metal MFI zeolites catalyzes higher para selectivity

and the working parameter is different for dispropor-tionation of alkylbenzenes and aromatics alkylation [103].Nevertheless,para selectivation depends on various factors,such as morphology of the starting zeolite sample,type of reaction,details of modi?ca-tion techniques,etc.

Haag et al.[76,77]disclosed a coke selectivation technique over ZSM-5zeolite to enhance p-xylene selectivity up to79%during toluene disproportiona-tion.The requirements of treatment time to achieve 40%and60%p-xylene selectivity are shown in Fig.15[76].Effectiveness of the pre-coking treatment increases with increasing coking temperature but raises concerns of catalyst stability.Pretreatment time would be longer than six days with the pre-coking temperature below ca.5208C.The H2/HC ratio during pre-coking treatment is also found to have substantial effect on p-xylene selectivity.As shown in Fig.16, pre-coking time requirements increase dramatically with increasing H2-to-aromatics ratio.Pre-coking was not practical to enhance para-isomer selectivity at H2-to-aromatics molar ratio beyond ca.0.7.According to the patent literature,the typical pre-coking tempera-ture is about55±1008C higher than the normal reac-tion temperature.As a result,the pre-coking treatment incorporated15±25%of coke onto the zeolite[76]and enhanced the p-xylene selectivity to70±80%.

The pre-coking treatment process was industrially applied in the so-called MSTDP SM Process.Its per-formances in the?rst and second cycles were shown in Figs.17±19[27].Its start-up took37h for coke selectivation at reactor inlet temperature,possibly higher than5008C(Fig.17).Reaction temperature was lined out at4558C in the start-of-run period, and gave p-xylene selectivity of82%at30%toluene conversion(Fig.18).The cycle length was over one and half years.At end-of-run period,p-xylene selec-tivity increased with day-on-stream to90%with25% toluene conversion,after which regeneration

was

Fig.15.Effects of selectivation temperature in pre-coking

treatment on the requirement of pre-coking time for achieving

various p-xylene selectivity;reaction conditions:WHSV:6.5±

20hà1,H2/HC 0.5mol/mol,N2/HC 3.5mol/mol,Pressure:

28.2kg/cm2;Data from ref.

[77].

Fig.16.Effects of selectivation H2/HC in pre-coking treatment on

the requirement of pre-coking time for achieving various p-xylene

selectivity;reaction conditions:temperature:566±5938C,WHSV:

13±20hà1,N2/HC 3.5mol/mol,pressure:28.2kg/cm2;data from

ref.

[77].

Fig.17.Plot of reaction temperatures against days-on-stream in

MSTDP commercial unit;feed:toluene,WHSV:2±4hà1,H2/HC:

2±4mol/mol,adapted from ref.[27].

370T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398

required.It was noted that the catalyst used in the MSTDP SM could be completely regenerated.The start-of-cycle temperature of the second cycle was about 4658C,with a p -xylene selectivity of 86%(Fig.19).Comparing to TDP-3SM Process,MSTDP operation is lower in throughput,and higher in reac-tion temperature,resulting in a higher benzene-to-xylene molar ratio of 1.3and higher light gas yield (Table 5).Nevertheless,in a process economics eva-luation,MSTDP SM is more competitive than TDP-3SM ,mainly from improvement in p -xylene selectivity and p -xylene recovery cost [12,30].Although pre-coking technique was successfully used industrially,there were only a few mechanistic studies [85,86,91,104±106].Olson et al.[91]proposed that coke tends to deposit on the external surface of ZSM-5(Fig.13)and passivates the isomerization on external surface.Chen et al.[104]proposed that for fresh ZSM-5catalyst during early time on stream coke is formed preferentially on Brùnsted acid sites that are located in the channels.Fang et al.[105]further demonstrated that the nature and location of coke in toluene disproportionation can be manipulated by a ?ve-stage on-stream treatment with switching differ-ent carrier gases (between nitrogen and hydrogen)under varied temperatures in range of 480±5408C (Fig.20).During the test periods,conversion drops in nitrogen gas and recovers in hydrogen,while p

-Fig.18.First cycle performance of MSTDP commercial unit by plotting p -xylene selectivity against toluene conversion;WHSV:2±4h à1,adapted from ref.

[27].

Fig.19.Second cycle performance of MSTDP commercial unit by plotting p -xylene selectivity against toluene conversion;WHSV:2±4h à1;adapted from ref.[27].Table 5Selectivation of toluene disproportionation over ZSM-5[79]

Treatment

Unmodified DMS modified a PMS modified a Pre-coking Days-on-stream 91963±WHSV (h à1)8674Temperature (8C)404439422446Pressure (kg/cm 2)42.242.242.235.2H 2/aromatics (mol/mol)2222Conversion (%)

30.928.63030.9Yield (%)C à5gas 0.30.7 1.1 1.7Benzene b

12.912.213.414.0Xylenes 17.014.815.014.8C 9aromatics 0.70.90.50.4Benzene/xylene (mol) 1.0 1.1 1.2 1.3p -Xylene selectivity (%)

26.156.986.085.0a

10%modifier silica,H±ZSM-5/silica binder.b By balance of original data.c p -Xylene at thermodynamic equilibrium composition is 23.4%.T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398371

xylene selectivity remains fairly constant in initial stages,Stages I and II,and is enhanced along with catalyst deactivation in Stage III as shown in Fig.20.From measurements on coked catalyst samples from various stages,the coke laydown process involves coke migration and rearrangement.As shown in Fig.20,in Stage I by using nitrogen,coke deposits preferentially inside zeolite pores,with small inho-mogeneous deposition occurring externally.In Stage II,by switching to hydrogen,internal coke was stripped out to external surface,resulting in reduction of total coke content.In Stage III by using nitrogen,coke deposited heavily inside zeolite pores,and on the external surface and pore openings,leading to severe deactivation in catalyst activity,with a gradual improvement in p -xylene selectivity.In Stage IV by switching to hydrogen,similarly to Stage II,internal coke was stripped out toward the external surface,resulting in a large coverage of the external surface.Catalyst activity restored and p -xylene selectivity stayed high.As evidenced by 129Xe NMR measure-ments,with coke content of 19%,there was no pore opening change,whereas void space was 94%of the original level.In Stage V ,reaction temperature was reduced to extend normal operation cycle,coke lay-down rate was slow,and p -xylene selectivity remained high.Their work indicated clearly that coverage of external surface sites contributes to p -xylene selectiv-ity enhancement in toluene disproportionation at med-ium para -isomer selectivity,ca.50%.Surface modi?cation by silica deposition is the other widely applied industrial practice to improve para selectivity over ZSM-5.It is believed that the catalyst applied industrially in the MTPX SM Process was silica modi?ed.Such modi?cation can be achieved either by in situ silica deposition or ex situ impregnation.The effectiveness of silica deposition depends on the silica sources,deposition methods,and also the nature of the zeolite raw material.Impregna-tion is conducted either using organic solutions (par-ticularly in hexane,decane and dodecane)of organic silicones onto parent ZSM-5either synthesized with [79,107±111]or without organic templates [112],or with water emulsion of silicones [79,113±115].Multi-ple-impregnations [116±118]followed by steaming [119,120]are also applied as pretreatment procedures.Interestingly,steaming alone gives no improvement in para -isomer selectivity;in contrast,steaming treat-ment at mild conditions,ca.2h at 3408C,following silica impregnation can enhance catalyst activity and also reduce D /r 2ratio to enhance p -xylene selectivity [116,120].Signi?cant improvement in p -xylene selec-tivity is obtained with various modi?cations achieving D /r 2ratio lower than ca.10à7±10à8s à1in comparison to the original samples at 10à5s à1[111,112,116,120].Chang et al.[79]used different silica compounds such as phenylmethylsilicone (Dow-710TM ,PMS),dimethylphenylmethylpolysiloxane (Dow-550TM ),or dimethylsilicone (DMS)as impregnation treatment agents to improve p -xylene selectivity.As shown

in Fig.20.Conversion and p -xylene selectivity in toluene disproportionation over ZSM-5with five stage coke selectivation treatment;reaction conditions:WHSV:6.5h à1,carrier gas/toluene:4mol/mol,pressure:28.2kg/cm 2;from ref.[105].

372T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398

Table 5,during toluene disproportionation,the resul-tant p -xylene selectivity is higher while using PMS as the impregnation source instead of DMS.Moreover,at similar conversion levels,selectivation by silicon impregnation from PMS is found to retain higher catalyst activity than the pre-coking modi?cation,as shown in Fig.21.In general,the silica modi?cation method suffers less from lower activity than the pre-coking selectivation technique.The modi?cation by pre-coking method produces higher benzene and gas yield and a higher benzene-to-xylene ratio,which may be due to the higher reaction temperature in this context.Moreover,the silicon modi?cation by PMS tends to provide a stable activity.Overall,the two modi?cation methods both yield a similar line-out p -xylene selectivity of ca.86%,as shown in Fig.22,however,silica deposition produces lower A 9than pre-coking (Table 4).Sensitivity studies showed that process economics of toluene disproportionation pro-cess greatly improves with reducing benzene-to-xylene ratio.So far,the MTPX SM Process has the lowest benzene-to-xylene ratio (Table 4)and best economics [12,121].

In situ silica modi?cation techniques included sur-face silylation by using silane [107]or chemical vapor deposition method by using organosilicon compounds [122,123],the so-called CVD method.Silica deposi-tion pretreatment can be either a separate procedure or co-feeding with reactants.In the latter case,silicone

reagent was co-fed with toluene to modify the catalyst and conduct toluene disproportionation simulta-neously.The silicone reagent was discontinued when desired toluene conversion and p -xylene selectivity were reached.The technique can be combined with ex situ silica impregnation to ?ne tune zeolite structural features and has been termed ``trim selectivation''[118].

Chang et al.[123]found that HMDS (hexamethyl-disiloxane)selectivation sustains a better activity retention,at ca.25%toluene conversion and ca.88%p -xylene selectivity,than the other siloxane modi?cation agents,such as TMDS (1,1,2,2-tetra-methyldisiloxane)or PMDS (pentamethyldisiloxane),and TEOS (tetra-ethyl-orthosilicate).TEOS selectiva-tion resulted in a more rapid catalyst deactivation.However,the CVD technique using orthosilicate com-pounds,with the structure of SiR n OR 4àn ,was suc-cessfully applied in EB disproportionation,as discussed below.Niwa et al.[124]pioneered the technique in the modi?cation of the pore structure of mordenite.

Encouraged by the success of MSTDP SM Process and MTPX SM Process,there have been many active research attempts to ?ne tune selectivation techniques to further improve para -isomer selectivity,while also reducing side products and approaching benzene-to-xylene molar ratio of unity.James et al.[30]of UOP (Union Oil Products)reported recently a new toluene disproportionation process,PX-Plus SM ,with a high p -xylene selectivity of ca.90%at toluene

conversion

Fig.21.Plot of reaction temperature against days-on-stream by using silica selectivation (cycles 1and 2)and pre-coking selectivation to achieve comparable toluene conversion and p -xylene selectivity various modification treatments;data from refs.

[27,79].

Fig.22.Plot of p -xylene selectivity of silica deposition selectiva-tion method (cycles 1and 2)against days-on-stream in comparison to pre-coking selectivation method;reaction conditions:tempera-ture:4228C,WHSV:7h à1,H 2/HC:2mol/mol;data from refs.[27,79]

T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398373

level of30%.It applies an on-line selectivation pre-treatment in the start-of-cycle,consisting of three stages,i.e.,oil-in at typical operating conditions,a selectivation stage for para-isomer selectivity improvement,and then lined out normal operation at enhanced para-isomer selectivity.However,the relevant details of their selectivation techniques were not disclosed.The benzene-to-xylene molar ratio of PX-PLUS SM is1.37,which is close to that observed in MSTDP SM(1.3)(Table4).

3.4.Effects of para-isomer selectivity on production scheme

As mentioned earlier in Section3.2,for p-xylene recovery,the adsorptive separation method is more favorable than the crystallization method,mainly because the former not only has higher recovery rate but also yields high purity p-xylene.Recent advances in para-selectivity enhancement,as discussed above, have created a signi?cant impact on the production scheme of high purity p-xylene.The concentration of p-xylene is found to reach up to80±90%with MSTDP SM[27],MTPX SM[78,79]and PX-Plus SM [30].Crystallization processes for various composi-tions of A8aromatics,obtained from different xylene processes with and without thermodynamic equili-brium,are listed in Table6.As long as the p-xylene concentration in A8mixtures increases,then tempera-ture of the eutectic mixture decreases,whereas isomer contamination in the p-xylene product decreases cor-respondingly(Fig.8).There is no requirement for the p-xylene enriched mixtures in crystallization to cool down to the eutectic temperature,such as for A8 mixtures with thermodynamic equilibrium composi-tion,high recovery rate and high quality(99.5%)of p-xylene is achieved in a single stage[78].Typical operating temperatures for the crystallization method are in the range ofà20±48C,with a high recovery rate up to ca.90%,and hence the production cost decreases substantially(Table6,Fig.9).

Advances in the disproportionation process thus revived crystallization technology.Owing to the high recovery rate of p-xylene,there is a much less fraction of raf?nate stream as compared to the conventional thermodynamic equilibrium mixture.The raf?nate stream can be directly charged into an existing adsorp-tion recovery process or combined into conventional xylene mixtures.In addition,a new production scheme producing p-xylene only can be designed as a stand alone p-xylene complex comprising p-xylene selec-tive disproportionation processes(such as MTPX SM) and utilizing the crystallization method for the recov-ery unit in the absence of a xylene isomerization unit. Such a new production scheme(Fig.23)is very simple,and should have much lower capital invest-ment cost and minimal,more economic size compared to the conventional p-xylene complex(Fig.7). According to Mobil Oil,the MTPX SM Process,the latest commercial process,requires an even lower capital investment and operating costs by10±15% than the MSTDP SM Process[121].It should further reduce the capital and operating costs compare to the other conventional p-xylene processes.

3.5.Development of transalkylation process

The transalkylation process deals with methyl group transfer between toluene and A9molecules, which are readily available from catalytic reforming and naphtha cracking.Both the transalkylation pro-cess and toluene disproportionation process can con-vert toluene into xylene and benzene.They are used in most aromatic complexes to increase the p-xylene production from catalytic reforming.As shown in Fig.24,p-xylene production was evaluated with va-rious types of BTX complex.They include reforming unit alone(Reforming),reforming plus toluene dis-proportionation(Ref TDP)to use up surplus toluene,

Table6

Crystallization processes for various xylene compositions A8component Mix-xylene production process

Thermodynamic equilibrium process a Para selective process b

m-Xylene(%)483±14

o-Xylene(%)211±3

Ethylbenzene(%)105±8

Crystallization process

Number of stages21

Temperature(8C)$à70$à20±4

Recovery rate(%)6580±90

a Produced from A7disproportionation and A9transalkylation.

b Produced from MSTDP SM,MTPX SM and PX-Plus SM.

374T.-C.Tsai et al./Applied Catalysis A:General181(1999)355±398

and reforming unit plus transalkylation,either typical transalkylation processes (Ref Tans)or particularly TransPlus SM Process (Ref TransPlus),by conversion of surplus toluene and additional A 9.In Fig.24,the BTX production rate of the reforming complex is the base case.By comparison with the base case,the production rates of benzene and xylenes in Ref TDP complex increase by 88%and 48%respectively;in the transalkylation process scheme,they increase by 88%and 119%for Ref TransPlus scheme,and by 177%and 96%for Ref Trans scheme,respectively [3,33,37].Total feed and BTX production rates remain the same for the former case and increase by around 35%for the transalkylation case.Transalkylation thus boosts more xylene production than disproportiona-tion.The transalkylation reaction is thermodynamically controlled,where the equilibrium aromatic composi-tions are mainly dictated by the methyl group per benzene ring (M /R ratio)of the system,as shown in Fig.25.For disproportionation of pure toluene feed,the M /R ratio was unity.In practical operation,M /R ratio increases with increasing percentage of heavy aromatics,especially A 9and A 10,in the feed compo-sition.By increasing the A 9blending percentage in toluene feed,xylene yield is enhanced at the expense of benzene yield [125].Therefore,transalkylation process is more selective for xylene production than toluene disproportionation process.Maximum ther-modynamic equilibrium xylene yield was achieved at M /R ratio of 2.Transalkylation process is thus more attractive to re?neries having naphtha-cracking units which produce excessive A 9products or marketplaces having higher demand for xylene than benzene.Unlike the toluene disproportionation process,whose performance is commonly characterized

by Fig.23.Simple p -xylene production scheme using selective toluene disproportionation processes (STDP)and crystallization recovery

technology.

bca4c5a103d8ce2f006623d5parison of BTX production rate among various process integration schemes by using reforming process as base;data from refs.[3,33,37].

T.-C.Tsai et al./Applied Catalysis A:General 181(1999)355±398375

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