Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By plural serial diverse separations
Reexamination Certificate
1996-01-25
2002-04-23
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By plural serial diverse separations
C585S819000, C585S477000, C585S478000, C585S481000, C585S321000
Reexamination Certificate
active
06376733
ABSTRACT:
BACKGROUND OF THE INVENTION
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 C
6
to C
8
aromatics stream which is a source of raw materials for high value downstream products. From this stream, benzene, toluene and the C
8
aromatics which are particularly valuable may be obtained. The C
8
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 “C
8
aromatics”) are close, they are difficult to separate by fractional distillation. As a consequence various alternative methods of separating paraxylene from C
8
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 C
8
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 C
8
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 predominantly 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 C
8
naphthene intermediate and/or to C
10
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 C
8
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 C
8
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. No. 4,699,892, U.S. Pat. No. 5,100,596, EP 0481658, EP 0481659, EP 0481660, WO 92/13631, WO 93/00155, WO 94101209, and WO 94/25151. However the prior art does not describe how to use such membranes in actual C
8
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
SUMMARY OF THE INVENTION
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 C
8
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 C
8
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 C
8
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.
DETAILED DESCRIPTION OF THE INVENTION
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 absorbers. 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 C
8
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 ava
Clem Kenneth Ray
Cox Graeme Ian
Ferraro John Michael
Lattner James Richardson
Osman Robert Michael
ExxonMobil Chemical Patents Inc.
Griffin Walter D.
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