Process for separating mixtures of aromatic C8 hydrocarbons

Abstract: A processing cycle for separating C₈ aromatics (5) (mixtures of ortho-, para-, meta-xylene and ethylbenzene) is disclosed, in which a first subdivision (1) of the mixture is performed into two streams (6,12) respectively constituted of: para-xylene and ethylbenzene the first one (6); meta- and ortho-xylene the second one (12), followed by a separation of the first stream (6) by rectification (2), during which ethylbenzene (7) within specifications is obtained as the overhead fraction, with the second stream (12) being also sent to a separation by rectification (4), with ortho-xylene (14) within specifications being obtained as the bottom fraction: the possible content of C₉ species can be furthermore removed by subsequent distillation.

Description: The present invention relates to a process for separating the constituents of a mixture of aromatic C 8 hydrocarbons, which substantially comprises a preliminary separation of the mixture into two streams, the first one of which contains paraxylene and ethylbenzene, and the other one is constituted by meta- and ortho-xylene, and two subsequent distinct operations of rectification during the course of which ethyl benzene is recovered as the overhead fraction, and ortho-xylene is separated as the bottom fraction. As well-known, the production of aromatic hydrocarbons is greatly interesting for the whole chemical industry. For that purpose, rather simple processes exist, in particular, for the purpose of separating aromatic from non-aromatic species; the resulting mixture of aromatic species is easily separated by distillation into C 6 aromatics (benzene) with a suitable purity for any possible use, and into C 7 aromatics (toluene), also with a high purity level. The situation is completely different as regards C 8 aromatics, the mixtures of which not always can easily be separated, because they are constituted by mixtures of isomers with very close boiling points. The most interesting C 8 mixture from the industrial viewpoint is that mixture which is constituted by ethylbenzene, o-xylene, p-xylene and m-xylene: as such, said mixture is used as solvent or in gasolines, whilst much wider would be the range of possible uses of each individual isomer, if they could be separated with high purity level. So, e.g., a demand exists for high-purity para-xylene for preparing terephthalic acid and dimethyl terephthalate, intermediates for man-made fibre synthesis; ethylbenzene, in its turn, is an interesting intermediate in styrene preparation by dehydrogenation. Usually, the processes used heretofore in order to separate the above said isomers have been mostly based on suitable combinations of superfractionation techniques and of low-temperature crystallization processes: all the above, with high costs and limited yields. According to alternative routes, the para-isomer is separated by adsorption, or by using molecular sieves: the various isomers can then be isomerized and the produced para-xylene can be separated again according to the just mentioned technique. The ortho- and ethylbenzene isomers can also be separated by distillation. However, the just reminded para-isomer separation process by crystallization does not make it possible said isomers to be totally recovered; the presence of the product in the mother liquors sent to the isomerization reduces the isomerization yield per pass, and increases the need for recycles. Furthermore, the presence of ethylbenzene causes even more serious problems, because this product tends to accumulate in the isomerization step, causing still more serious recycle problems; if possible, then, its conversion into dimethyl benzene takes always place with low yields and serious operation problems. From U.S. patent No. 3,698,157, a process is known as well for recovering para-xylene from a mixture of C 8 aromatics by contacting said mixture with a suitably modified zeolite: also the formation is envisaged of two fractions constituted by: meta p boil 139.12°C para p boil 138.37°C ortho p boil 144.41°C ethyl-b p boil 136.19°C Such fractions can then be separated by distillation. The meta/ortho mixture can yield the meta-isomer as the overhead fraction and the ortho-isomer as the bottom fraction, with reflux ratios of the order of 10-20, and with a number of trays comprised within the range of from 150 to 200. The para/ethyl-b mixture can also be separated by distillation, under burdensome conditions (however, obviously, not as burdensome as they would be if the whole stream was submitted to distillation). The present Applicant has found now that the four isomers of the mixture of C 8 aromatics can be separated in a simple and cheap way by adopting a processing cycle which substantially comprises a preliminary separation of said mixture into two streams and subsequent rectification operations. In particular, the present invention concerns a process for the separation of the four isomers of a mixture of C 8 aromatic hydrocarbons comprising the following operations: (a) separating the mixture into two streams respectively containing: para-xylene and ethylbenzene the first one; meta-xylene and ortho-xylene the second one; (b) rectifying the binary para-xylene/ethylbenzene mixture with an overhead fraction consisting of ethylbenzene and a bottom fraction consisting essentially of para-xylene; (c) rectifying the binary meta-xylene/ortho-xylene mixture with a portion consisting of meta-xylene being recovered as an overhead fraction and a mixture consisting essentially of meta-xylene/ortho-xylene being separated as a bottom fraction; (d) purifying the bottom fraction of the (b) step by crystallization in order to obtain para-xylene and mother liquors; said process being characterized by further comprising the following operations: (e) sending the meta-xylene/ortho-xylene bottom fraction of step (c) and the mother liquors of step (d) to isomerization; and (f) distilling the effluent from the isomerization in order to yield a bottom stream consisting essentially of ortho-xylene and an overhead fraction which is sent to crystallization. The composition of a typical mixture of the interesting isomer can be represented as follows: In the first step of the cycle followed in the process according to the present invention, the mixture is sent to a separation zone containing a suitable agent, in which zone para-xylene and ethylbenzene are separated from the other binary mixture consisting of ortho- and meta-xylene. This separation can be carried out by using well known means for those skilled in the art, such as, e.g., those disclosed in the above said U.S. patent No. 3,698,157, or in U.S. patent No. 3,656,278. The ethylbenzene and para-xylene containing stream is then sent to a rectification column operating with a reflux ratio comprised within the range of from 50 to 80, and containing from 300 to 400 trays. Ethylbenzene within specifications and para-xylene are obtained as the overhead and bottom fractions, respectively. The latter may also contain 1 to 10% by weight of other isomers: in such a case, a further purification will be carried out by crystallization, and the mother liquors will be recycled. The parallel ortho-xylene and meta-xylene mixture is sent, in its turn, to a rectification column in which said isomers are separated at respective purity level higher than 99%: ortho-xylene as the bottom fraction, and meta-xylene as the overhead fraction, both of them being obtained with practically quantitative yields relatively to the feedstock. In the ortho-xylene fraction impurities constituted by C 9 species can be still present, which will be simply distilled off. Should the production of meta- and ortho-isomers be not interesting, the mixture can suitably be sent to an isomerization step: in such a case, the operating cycle according to the present invention will produce only ethylbenzene and para-xylene with purity levels within specifications, because also the effluent stream from the isomerization zone will be fed to a separation step on-line with the initial separation step. Also the isomerization, if present, is carried out according to well-known techniques for those skilled in the art. The rectification column for ortho-xylene/meta-xylene mixture operates, according to as provided for by the cycle according to the present invention, with a reflux ratio comprised within the range of from 10 to 20, with a number of trays comprised within the range of from 150 to 200. The cycle according to the present invention will be better understood by referring to the accompanying figure to which reference is made in the following example, which are supplied for the only purpose of better explaining the invention without limiting the scope of protection thereof. EXAMPLE 1 A general example which makes it possible all the isomers to be obtained within specifications and, within certain limits, the production mix to be varied as regards the isomeric xylene species (PX, OX and MX) is based on the scheme reported in Figure 1. Such a scheme makes it possible the process to be operated according to different configurations. For all of said configurations, the production of EB is limited to the amount contained in the feedstock and by the recovery capacity of column 2. The column 6 yields the desired amount of MX according to overhead specifications, with the bottom stream being destined to isomerization 5. The isomerization processes the bottom stream from distillation 6 together with the crystallization mother liquors. A loss of EB due to dealkylation may occur. The column 4 can separate, at its bottom, OX within specifications from C 8 's, or, still as its bottom stream, an OX rich stream can be obtained, with the subsequent crystallization 3 being relieved by increasing PX concentration. Crystallization 3, which may be a multi-step one, makes it possible PX coming from distillation 2 to be purified by removing any residual EB, which is left in the mother liquors, and is sent to isomerization. An exemplifying material balance my be as follows:

Claims: Process for the separation of the four isomers of a mixture of C 8 aromatic hydrocarbons comprising the following operations: (a) separating the mixture into two streams respectively containing: para-xylene and ethylbenzene the first one; meta-xylene and ortho-xylene the second one; (b) rectifying the binary para-xylenelethylbenzene mixture with an overhead fraction consisting of ethylbenzene and a bottom fraction consisting essentially of para-xylene; (c) rectifying the binary meta-xylene/ortho-xylene mixture with a portion consisting of meta-xylene being recovered as an overhead fraction and a mixture consisting essentially of meta-xylene/ortho-xylene being separated as a bottom fraction; (d) purifying the bottom fraction of the (b) step by crystallization in order to obtain para-xylene and mother liquors; said process being characterised by further comprising the following operations (e) sending the meta-xylene/ortho-xylene bottom fraction of step (c) and the mother liquors of step (d) to isomerization; and (f) distilling the effluent from the isomerization in order to yield a bottom stream consisting essentially of ortho-xylene and an overhead fraction which is sent to crystallization. Process according to claim 1, wherein the bottom stream of step (f) is recycled to isomerization.

Inventors: PALUDETTO RENATO, PARET GIANCARLO
Applicants: ENICHEM SPA, ENICHEM S.P.A.
Application year: 1993
Main classification: C07C7
IPC codes: C07C7, C07C15