This invention relates to a process and a chemical plant for the production of paraxylene. In particular the process and chemical plant utilise zeolite membranes for enhanced paraxylene production.
In the petrochemical production chain one of the most important streams is the C6 to C8 aromatics stream which is a source of raw materials for high value downstream products. From this stream, benzene, toluene and the C8 aromatics which are particularly valuable may be obtained. The C8 aromatics are orthoxylene, metaxylene, paraxylene and ethylbenzene. Paraxylene is often the most desirable of the xylenes; however because the boiling points of ethylbenzene, ortho-, meta- and paraxylene (hereinafter collectively referred to as “C8 aromatics”) are close, they are difficult to separate by fractional distillation. As a consequence various alternative methods of separating paraxylene from C8 aromatics have been developed. The most common of such methods are fractional crystallisation which utilises the difference in freezing points between ethylbenzene, ortho-, meta- and paraxylene, and selective adsorption which commonly utilises zeolite materials to selectively adsorb paraxylene from C8 aromatics streams; the adsorbed paraxylene is recovered after desorbing from the zeolite. When either of these processes are used paraxylene can be recovered in high yields from the C8 aromatics stream. The resulting filtrate from the crystallisation process or the raffinate from the adsorption process are depleted in paraxylene and contain relatively high proportions of ethylbenzene, ortho-, and metaxylene. These streams are typically subjected to further processing downstream of the crystallisation or adsorption process.
Typically one of the additional downstream processes is an isomerisation process which is used to increase the proportion of paraxylene in paraxylene depleted streams from such processes as fractional crystallisation or selective adsorption. The xylenes, which are predominately ortho- and metaxylene, can be contacted with an isomerisation catalyst under appropriate temperature and pressure which results in the conversion of some of the ortho- and metaxylene to paraxylene. It is also usually necessary to convert some of the ethylbenzene to prevent it from building up to high concentrations. A catalyst can be selected to enable conversion of ethylbenzene to benzene, and/or to orthoxylene through a C8 naphthene intermediate and/or to C10 aromatics and benzene via transalkylation. It may be that the catalyst for conversion of ethylbenzene to orthoxylene is also a xylenes isomerisation catalyst in which case the orthoxylene from the ethylbenzene is converted to an equilibrium mixture of xylenes.
Prior art processes for making paraxylene have typically included combinations of isomerization with fractional crystallisation and/or adsorption separation. The problem with this combination is that despite improvements in catalyst performance the isomerisation technology is only able to produce equilibrium or near-equilibrium mixtures of xylenes and may also be relatively inefficient for the conversion of ethylbenzene to benzene or xylenes. The consequence of this is that big recycles of the xylenes stream back through these processes are needed to ensure the conversion of the C8 aromatics stream to paraxylene is maximised with or without the additional recovery if desired of orthoxylene and/or metaxylene. There is a need therefore for improved processes and chemical plants for the production of paraxylene from C8 aromatics streams, which in particular address the problems associated with large recycles and/or low ethylbenzene conversions.
Zeolite membranes have been described in the prior art, for example in U.S. Pat. Nos. 4,699,892, 5,100,596, EP 0481658, EP 0481659, EP 0481660, WO 92/13631, WO 93/00155, WO 94/01209, and WO 9425151. However the prior art does not describe how to use such membranes in actual C8 aromatics processing in the petrochemical cycle nor does the prior art describe how to use such membranes in combination with existing processes to significantly enhance their paraxylene production capability.
The present invention is therefore directed to a chemical plant and process which offers an improvement over the prior art for the production of paraxylene from C8 aromatics streams. The present invention resides in the specific application of a zeolite membrane unit and process in a paraxylene or paraxylene with orthoxylene and/or metaxylene recovery process. This invention utilises zeolite membranes to continuously separate paraxylene and/or ethylbenzene from xylenes, or to isomerise ortho- and metaxylene to paraxylene and/or ethylbenzene to xylenes and simultaneously or subsequently separate paraxylene from the xylenes mixture. The use of a zeolite membrane unit and process in for example a process for paraxylene recovery provides for a significant improvement in paraxylene production when compared to conventional paraxylene recovery processes.
Accordingly the present invention provides a process for recovering paraxylene from a C8 aromatics stream containing paraxylene and at least one other isomer of xylene, ethylbenzene, or mixtures thereof which process comprises:
(a) recovering by means of a paraxylene separation process in a paraxylene recovery unit a portion of said paraxylene from at least a portion of said C8 aromatics stream to produce a first stream having a reduced paraxylene content and containing at least a portion of said other isomers of xylene, said ethylbenzene, or mixtures thereof;
(b) passing at least a portion of said first stream directly or indirectly to a zeolite membrane unit comprising a zeolite membrane and optionally isomerisation catalyst under isomerization conditions, such that the permeate withdrawn through the zeolite membrane and from the zeolite membrane unit is enriched in is paraxylene when compared to the feed to the zeolite membrane unit and
(c) feeding the permeate directly or indirectly back to the paraxylene separation process.
Preferably there is an additional step between (a) and (b) wherein at least a portion of said first stream is subjected to an isomerisation process in an isomerisation unit to produce an isomerate having an enriched paraxylene content compared to that of the first stream; and it is at least a portion of this isomerate stream which is passed to the zeolite membrane unit. Most preferably the permeate withdrawn from the zeolite membrane unit is enriched in paraxylene compared to the equilibrium concentration of paraxylene in a xylenes equilibrium mixture.
The present invention further provides for a paraxylene recovery plant comprising:
(a) paraxylene recovery unit, and
(b) a zeolite membrane unit comprising a zeolite membrane and optionally isomerisation catalyst.
Preferably the paraxylene recovery plant comprises an isomerisation unit in addition to the paraxylene recovery unit and zeolite membrane unit.
The paraxylene recovery unit uses separation technology to produce a paraxylene enriched stream and a paraxylene depleted stream. Such separation technology includes for example the known processes of fractional crystallisation, or selective adsorption using for example molecular sieve adsorbers. The paraxylene recovery unit may therefore be a fractional crystallisation unit which utilises the difference in freezing points between ethylbenzene, ortho-, meta- and paraxylene or it may be a selective adsorption unit which commonly utilises zeolite materials to selectively adsorb paraxylene from C8 aromatics streams; the adsorbed paraxylene is recovered after desorbing from the zeolite. The paraxylene recovery unit may also be a combination of such separation units, or may incorporate other less commonly used techniques such as fractional distillation.
Fractional crystallisation units are well known in the art and are described for example in U.S. Pat. No. 4,120,911. Commercially available processes include the crystallisation isofining process, direct contact CO2 crystallisers, scraped drum crystallisers, and continuous countercurrent crystallisation processes. The crystalliser may operate for example in the manner described in Machell et. al. U.S. Pat. No. 3,662,013. Commercial fractional crystallisation processes typically recover about 60% to 68% of the paraxylene from the feed to the paraxylene recovery unit when this feed is an equilibrium or near equilibrium mixture of xylenes and ethylbenzene. The reason for this is that they are limited by the formation of a eutectic between paraxylene and metaxylene. However the actual recovery depends on the composition of the feed with higher recoveries possible when the paraxylene content of the feed is higher than the xylenes equilibrium content.
Selective adsorption units are also well known in the art and are described for example in U.S. Pat. Nos. 3,706,812, 3,732,325, 4,886,929, and references cited therein, the disclosures of which are hereby incorporated by reference. Commercially available processes include UOP PAREX™, and IFP-Chevron ELUXYL™ processes. Commercial molecular sieve selective adsorption processes may recover higher levels of paraxylene than fractional crystallisation processes; typically they recover over 90% or more typically over 95% of the paraxylene from the feed to the paraxylene recovery unit.
The paraxylene recovery unit produces a paraxylene enriched stream that usually comprises over 99% and may even be as high as 99.9% paraxylene. The exact amount depends on the process used and the design and operating conditions of the specific plant. The balance in this stream being ethylbenzene, ortho-, and metaxylene, toluene, and C9 aromatics, paraffin's, naphthenes and possibly small amounts of other materials. The paraxylene recovery also produces a paraxylene depleted stream containing the balance of ethylbenzene, ortho-, and metaxylene, toluene, C9 aromatics, paraffins, etc. along with any paraxylene fed to the paraxylene recovery unit that is not removed in the paraxylene rich stream. It is this paraxylene depleted stream which is then fed to the isomerisation unit and/or zeolite membrane unit.
The C8 aromatics stream which is used as the feed for the paraxylene separation unit may come from a variety of sources in the petrochemical plant. One possible source is from naphtha reforming. Examples of such processes include Exxon POWERFORMING™, UOP Platforming™, IFP Aromizing™. Another possible source is pyrolysis gasoline from steam cracking processes although this is likely to be a minor source of such streams. A further possible source is the UOP Cyclar process for conversion of C3/C4 hydrocarbon streams to aromatics (see for example U.S. Pat. No. 5,258,563, the disclosure of which is hereby incorporated by reference). A further possible source is from toluene disproportionation and/or C9 aromatics transalkylation. Examples of such processes include UOP TATORAY™, TORAY TAC9™, Mobil Selective Toluene Disproportionation™ (MSTDP), Mobil Toluene Disproportionation™ (MTDP), IFP Xylenes PLUS™ and FINA T2BX™. There are other possible sources of C8 aromatics streams. The source of C8 aromatics stream for the process of the present invention is not critical and may be a single stream or may be a combination of streams from any of the above processes.
The isomerisation unit may be any of the well known units in the art such as those described in U.S. Pat. Nos. 4,236,996, 4,163,028, 4,188,282, 4,224,141, 4,218,573, 4,236,996, 4,899,011, 3,856,872 and Re. 30,157, the disclosures of which are hereby incorporated by reference.
The isomerisation catalyst may be any of the well known catalysts for isomerisation units in the art. There are primarily two types of catalyst system which are used in isomerisation units. The choice of catalyst has an impact on the overall yield and structure of the aromatics complex and also on the plant design and economics. The first type of catalyst is designed to convert ethylbenzene to xylenes and to isomerise the paraxylene depleted feed stock to a near equilibrium xylene composition. This type of catalyst system is generally the choice for aromatics producers whose objective is to maximise para and ortho-xylene production from a fixed quantity of feed stock. A second catalyst system is also designed to isomerise the para xylene depleted feed stock; however rather than converting ethylbenzene to xylenes, this catalyst system dealkylates the ethylbenzene to produce benzene. This catalyst system is often employed when the benzene requirements are high relative to ortho and para xylene production or when feed stock availability is not a limiting factor.
Examples of processes and catalyst systems which include the capability of converting ethylbenzene to benzene are the Mobil MHTI (Mobil High Temperature Isomerisation) process and catalyst (see for example U.S. Pat. Nos. 3,856,871 and 4,638,105, the disclosures of which are hereby incorporated by reference), the Mobil MHAI (Mobil High Activity Isomerisation) process and catalyst, the AMOCO AMSAC process and catalyst and the UOP ISOMAR™ I-100 process and catalyst.
Examples of processes and catalyst systems which include the capability of converting ethylbenzene to xylenes are the IFP/ENGELHARD Octafining and Octafining II processes and catalyst, and the UOP ISOMAR™ I-9 process and catalyst. Other processes include catalysts capable of converting ethylbenzene to C10 aromatics. Other processes do not include ethylbenzene conversion.
Isomerization units typically use a zeolite or mordenite type catalyst. Isomerization catalysts known to promote conversion of ortho and metaxylene to paraxylene include metal promoted molecular sieves such as for example Pt Promoted ZSM-5, Pt promoted Mordenite and metal promoted borosilicates etc. Commercial examples are Mobil MHAI and ISOMAR™ I-9 catalyst.
The isomerization reactor is arranged and effective to isomerise ortho- and metaxylene to paraxylene at these conditions and also advantageously to convert ethylbenzene to benzene and/or xylenes. The term “arranged and effective” is used in this application to denote that conditions in a process unit are as described in this specification to include the temperatures, pressures, space velocities, reaction time, other reactants, and any other process conditions necessary to achieve the desired reaction, conversion or separation that is the normal function of that process unit.
Operating temperatures are typically in the range of 400 to 900° F. and pressures in the range of 25 to 500 PSIG. The weight hourly space velocity (WHSV) based on hydrocarbon feed typically ranges from 0.5 to 20. Most isomerization catalyst systems require a source of hydrogen which can be introduced to the isomerization reactor to promote the isomerization reaction that converts ortho- and metaxylene to paraxylene, to assist in the conversion of ethylbenzene to benzene and or xylenes and assists also in the prevention of coking of the isomerisation catalyst.
In one aspect of the present invention the zeolite membrane unit is used to selectively separate paraxylene and/or ethylbenzene from a stream which comprises ethylbenzene and an equilibrium or near equilibrium mixture of xylenes. In this aspect the zeolite membrane unit may be located downstream of an isomerisation unit and does not have an isomerisation catalyst in combination with the membrane.
In a further aspect of the present invention, a zeolite membrane unit utilises an isomerisation catalyst in combination with the membrane to isomerise ortho- and metaxylene to paraxylene in co-operation with the selective separation function of the membrane and may also include the catalytic conversion of ethylbenzene to benzene or xylenes.
In this aspect of the present invention the zeolite membrane may itself be rendered catalytically active for the isomerisation reaction or an appropriate isomerisation catalyst may be located proximate to the membrane. By proximate to the membrane is meant that the catalyst is arranged and effective to isomerise the ortho- and/or metaxylene and/or ethylbenzene in the material in the zeolite membrane unit but upstream of the zeolite membrane to produce paraxylene. The exact amount of paraxylene which is required to be produced by the isomerisation process in the zeolite membrane unit depends in part on the properties of the zeolite membrane used. If the membrane for example has high flux and/or high selectivity for paraxylene then it may be possible or even desirable for the isomerisation reaction to produce and maintain paraxylene at a none equilibrium concentration compared to its concentration in an equilibrium xylene mixture whilst the membrane selectively removes paraxylene from the upstream material and into the permeate. However the isomerisation catalyst in the zeolite membrane unit should ideally be arranged and effective to produce and maintain paraxylene, upstream of the membrane and inside the zeolite membrane unit, at 50% or more, preferably 80% or more, and most preferably 90% or more of the paraxylene equilibrium concentration whilst the membrane selectively removes paraxylene from the upstream side of the membrane and into the permeate. Depending on membrane properties it may be desirable and preferable to maintain the paraxylene concentration at or near to equilibrium for xylenes isomerisation whilst the membrane selectively removes paraxylene from the retentate into the permeate. Thus the isomerisation catalyst causes the ortho- and metaxylene to convert to paraxylene and the paraxylene selectively permeates through the zeolite membrane to be produced as a permeate stream. Ortho- and metaxylene less readily pass through the zeolite membrane and tend to stay on the upstream side in the retentate stream where they can be further isomerised. The permeate stream from xylenes isomerisation unit may be fractionated to remove materials boiling below and above the boiling point of xylenes e.g. benzene, toluene and C9+ aromatics and then transferred to the paraxylene recovery unit. If the zeolite membrane unit is particularly efficient at isomerisation and separation there may theoretically be no retentate stream as there would be no paraxylene depleted stream to reject. In practice there will however likely be impurities and heavier aromatic compounds such as C9 aromatics which remain in the retentate stream and must be purged from the zeolite membrane unit for further treatment. Thus in the zeolite membrane unit there is a dynamic and coupled process of isomerisation and separation of xylenes. If the catalytic function is also capable of converting ethylbenzene to benzene or xylenes then any ethylbenzene which enters into the retentate stream of the unit is also involved in this dynamic process with the resulting xylenes entering into the xylenes isomerisation reactions or the resulting benzene passing through the membrane into the permeate stream. In this aspect the zeolite membrane unit may be downstream of an isomerisation unit or may be used in place of an isomerisation unit.
In a further aspect the zeolite membrane is used to selectively separate ethylbenzene with a small amount of paraxylene from a paraxylene depleted feedstream as is typically found after a paraxylene separation process. In this aspect the zeolite membrane unit is located between the paraxylene separation unit and an ethylbenzene isomerisation unit. The feed to the isomerisation unit is enriched in ethylbenzene and improves the efficiency of the ethylbenzene isomerisation process in this unit. The output from this isomerisation unit is enriched in paraxylene and passes into a conventional paraxylene isomerisation unit along with the retentate from the zeolite membrane unit. In such a process the paraxylene isomerisation unit is required to convert lower levels of ethylbenzene and therefore may be operated at lower temperatures and may in fact be a liquid phase isomerisation unit which has no ethylbenzene conversion activity. The overall effect of this use of the zeolite membrane is to enhance the conversion of ethylbenzene to useful xylenes and to significantly reduce the xylene losses which usually occur due to the use of high temperature isomerisation units such as ISOMAR™ or MHTI™. A further modification of this aspect of the present invention is to include a catalytic function into the zeolite membrane unit. This catalytic function may be for ethylbenzene conversion and may be located within the membrane itself. This catalytic function may advantageously be located proximate to the membrane on the permeate side of the zeolite membrane. The function of this catalyst is to catalyse the conversion of ethylbenzene to xylenes. The effect of this is to deplete the concentration of ethylbenzene on the permeate side of the membrane and in doing so sets up a concentration gradient across the membrane which increases the quantity of ethylbenzene transferred from the retentate stream into the permeate stream. If the ethylbenzene conversion catalyst in the zeolite membrane unit is particularly efficient there may be no need for the ethylbenzene isomerisation unit which is located downstream of the zeolite membrane unit. In a further embodiment a second zeolite membrane unit for selective paraxylene separation or for selective paraxylene separation and isomerisation, may be located downstream of the paraxylene isomerisation unit. The permeate stream from xylenes isomerisation unit or the second zeolite membrane unit if present may be fractionated to remove materials boiling below and above the boiling point of xylenes e.g. benzene, toluene and C9+ aromatics and then transferred to the paraxylene recovery unit. Optionally, the retentate stream may be combined with the permeate stream and the combined streams fractionated and transferred to the paraxylene recovery unit for recovery of a paraxylene rich stream.
Examples of zeolite membranes which may be used in zeolite membrane units for the present invention are described in the following documents. U.S. Pat. No. 5,110,478, the disclosure of which is hereby incorporated by reference, describes the direct synthesis of zeolite membranes. The membranes produced in accordance with the teachings of U.S. Pat. No. 5,110,478 were discussed in “Synthesis and Characterisation of a Pure Zeolite Membrane,” J. G. Tsikoyiannis and W. Haag, Zeolites (Vol. 12, p. 126., 1992). Such membranes are free standing and are not affixed or attached as layers to any supports. Zeolite membranes have also been grown on supports. See e.g. “High temperature stainless steel supported zeolite (MFI) membranes: Preparation, Module, Construction and Permeation Experiments,” E. R. Geus, H. vanBekkum, J. A. Moulyin, Microporous Materials, Vol. 1, p. 137, 1993; Netherlands Patent Application 91011048; European Patent Application 91309239.1 and U.S. Pat. No. 4,099,692, the disclosures of which are hereby incorporated by reference. Other literature describing supported inorganic crystalline molecular sieve layers includes U.S. Pat. No. 4,699,892; J. C. Jansen et at, Proceedings of 9th International Zeolite Conference 1992 (in which lateral and axial orientations of the crystals with respect to the support surface are described), J. Shi et al, Synthesis of Self-supporting Zeolite Films, 15th Annual Meeting of the British Zeolite Association, 1992, Poster Presentation (in which oriented Gmelinite crystal layers are described); and S. Feng et al, Nature, Apr. 28, 1994, p 834 (which discloses an oriented zeolite X analogue layer), the disclosures of which are hereby incorporated by reference.
Further examples of zeolite membranes which may be used in zeolite membrane units for the present invention are described in the following documents; International Application WO 94/25151, U.S. Ser. No. 267,760 filed Jul. 8, 1994, PCT US95/08512, PCT US95/08514, PCT US95/08513, PCT EP95/02704 and WO94/01209, the disclosures of which are hereby incorporated by reference. In our earlier International Application WO 94/25151 we have described a supported inorganic layer comprising optionally contiguous particles of a crystalline molecular sieve, the mean particle size being within the range of from 20 nm to 1 μm. The support is advantageously porous. When the pores of the support are covered to the extent that they are effectively closed, and the support is continuous, a molecular sieve membrane results; such membranes have the advantage that they may perform catalysis and separation simultaneously if desired. Preferred zeolite membranes are those which are prepared by the Inverted In-Situ-Crystallisation (I-ISC) process, or by using a GEL layer and a Low Alkaline synthesis solution using the Inverted In-Situ-Crystallisation process (GEL-LAI-ISC), or by using a Seeding Layer and a Low-Alkaline-synthesis solution using the Inverted In-Situ Crystallisation (S-LAI-ISC). These processes are described in U.S. Ser. No. 267,760 filed Jul. 8, 1994, PCT US95/08512, PCT US95/08514, PCT US95/08513 and PCT EP95/02704. Zeolite compositions fabricated using the above described LAI-ISC, GEL-LAI-ISC or S-LAI-ISC techniques can have dense zeolite layers in which the zeolite crystals are intergrown such that non-selective permeation pathways in these as-synthesised zeolite layers are virtually non-existent. The zeolite membranes described above may be incorporated into the zeolite membrane unit in the form of a module such as that described in WO94/01209. It is envisaged that the zeolite membrane unit will contain at least one zeolite membrane which may or may not be catalytically active. If the membrane is not catalytically active for the desired process a suitable catalyst may be used in combination with the membrane. This catalyst may be located on the upstream side of the membrane or the downstream side of the membrane depending on the process and the nature and purpose of the catalyst. In one embodiment one or more membranes may be arranged with one or more catalysts to provide alternating membrane and catalyst regions in the zeolite membrane unit. In this arrangement the feedstream to the unit may for example pass through a membrane region with the retentate flowing to a catalyst containing region and then through a second membrane region to a second catalyst region. The exact number of membrane and catalyst regions will depend on the nature of the separations and catalyst processes desired. The separation and catalyst process may be substantially the same for each combination of catalyst and membrane or may be different.
It should be understood that two or more zeolite membrane units with or without isomerisation catalyst in close proximity to the zeolite membrane in each unit may be used in the processes of the present invention. Reference to zeolite membrane unit in this specification should also be taken to include embodiments where two or more zeolite membrane units may be used in sequence to each other with or without any further intervening processes or process units.
The zeolite membrane unit may be installed downstream of an existing xylenes isomerization reactor or installed as a replacement of an isomerization reactor in an existing paraxylene recovery process. The zeolite membrane unit may be added to an existing process solely for separation of paraxylene from xylenes, or for both isomerization and separation. The most preferred option is to have the zeolite membrane unit downstream of a xylenes isomerisation unit and for the zeolite membrane unit to comprise a zeolite membrane and an isomerisation catalyst so that it performs both isomerisation of xylenes, and selective separation of paraxylene; optionally it also catalyses conversion of ethylbenzene to xylenes or benzene. If the zeolite membrane unit catalyses conversion of ethylbenzene to xylenes or benzene then this may allow less conversion of ethylbenzene in the conventional isomerisation unit with less xylene losses due to the lower operating temperature which would be required in the conventional isomerisation unit.
It is preferred that the zeolite membrane unit is incorporated into a conventional xylene recovery loop such as that shown in FIG. 1 and discussed below. The xylene recovery process is referred to as a “loop” because xylenes not converted to paraxylene are recycled to the isomerization unit that is usually a part of the xylene recovery loop again and again until the xylenes are converted to paraxylene and removed from the loop via the paraxylene separation unit. In such a loop orthoxylene may also be a product which is removed from the loop in the xylene splitter if desired. Orthoxylene can sometimes be generated by the isomerisation unit if the feed to that unit has a less than equilibrium orthoxylene concentration.
As indicated above the fresh feed for the xylene recovery loop may come from a variety of sources in the petrochemical cycle. Fresh feed from, for example, a reformer, which is introduced to the xylene recovery loop is usually fractionated before introduction to the paraxylene separation unit to remove materials boiling below the boiling point of xylenes, and may optionally also be fractionated to remove at least part of the material boiling above the boiling point of xylenes. If lower boiling materials are not removed from the fresh feed, it is introduced to a detoluenizer tower (“DETOL”) which removes toluene and lighter materials by distillation. The feed is then introduced to either a xylene rerun tower or splitter. A xylene rerun tower removes C9+ aromatics from the feed. A xylene splitter tower in addition removes at least part of the orthoxylene for subsequent recovery as orthoxylene product in an orthoxylene rerun tower. The fresh feed in a xylenes loop is combined with a recycle stream which comes from the xylene isomerisation unit or in the present invention from the zeolite membrane unit. The overhead stream from the xylene rerun tower or splitter is typically a mixture of compounds which includes 0 to 10 wt % non aromatics, 0 to 5 wt % toluene, 5 to 20 wt % ethylbenzene, 0 to 10 wt % C8 naphthenes, and 70 to 95 wt % xylenes. The exact composition will depend on the fresh feed and the nature of the catalysts used in the isomerisation unit and in the zeolite membrane unit. It should be appreciated that the fresh feed to the xylenes recovery loop could be a combination of two or more feeds such as those discussed above. Thus it could be a combination of a feed from a naphtha reformer with that from a TATORAY™ or MSTDP™ unit.
It should be understood that in the present description when reference is made to a feed to, or material upstream of the membrane, in a zeolite membrane unit being at equilibrium in xylenes this means that it can be a mixture of xylenes which are at the typical respective concentrations for an equilibrium mixture of xylenes as known in the art. In the same context by near equilibrium is meant a composition comprising xylenes in which one or more of the xylenes present are at their none equilibrium concentration with respect to the other xylenes present and includes mixtures where one or more of the xylene isomers are present at a concentration which is greater than their equilibrium concentration. Ideally in such mixtures the paraxylene should be present at 50% or more, preferably 80% or more and most preferably at 90% or more of the paraxylene equilibrium concentration.