Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By contact with solid sorbent
Reexamination Certificate
2002-12-18
2004-12-07
Dang, Thuan Dinh (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By contact with solid sorbent
C585S822000, C585S825000, C585S812000
Reexamination Certificate
active
06828470
ABSTRACT:
The invention relates to a coproduction process for para-xylene and ortho-xylene starting with a batch of hydrocarbons containing them, the process comprising two separation steps.
The production of high-purity para-xylene through separation by adsorption is well known from the prior art. This market is largely developed, its principal outlet is the production of terephthalic acid or its ester, dimethyl terephthalate, used for the production of PET (polyethylene terephthalate) and polyester fibers.
The market for ortho-oxylene is more restricted, its outlet being phthalic anhydride. Ortho-xylene is generally produced by distillation upstream from the para-xylene separation step. But the prior art also knows processes for the production of high purity ortho-xylene by adsorption, for example U.S. Pat. Nos. 4,376,226, 4,529,828 and 4,482,776.
The production of ortho-oxylene by adsorption can be seen as attractive, especially when the desired amount of ortho-oxylene is large and high purity is demanded. It then becomes useful to coproduce para-xylene and ortho-xylene in two adsorption units placed in series and to benefit from the resulting synergies. But the para-xylene must be very pure, typically at least 99.7%, and the meta-xylene must be of a reasonable purity, typically at least 98.5%.
The prior art also describes processes for the coproduction of para- and ortho-xylene, for example EP 0 369 078 B1 uses catalytic isomerization of an aromatic hydrocarbon where the optionally present ethylbenzene is dealkylated, followed by separation of the effluent from the reaction into a light fraction consisting of a benzene-toluene mixture and a heavier tail product rich in p-xylene and o-xylene isomers. This tail product is then treated to separate the p-xylene and the m-xylene from it as individual products. The authors suggest that the ortho-xylene can be separated by distillation and that the para-xylene can be separated by crystallization or by adsorption. But this document does not describe nor does it suggest how it is possible to implement in practice the treatment of the tail product and does not provide any information on the yield and the purity of the desired products. This process further has the drawback of requiring dealkylation of the ethylbenzene, which further reduces the xylene yield.
Patent FR 2 782 714 describes a process for production of para-xylene and meta-xylene starting from a batch having an ethylbenzene content less than 5% by weight that is introduced into a chromatographic column of at least 25 beds, operating as a simulated fluidized bed.
U.S. Pat. No. 3,636,121 describes a process for separation and recovery of ortho-oxylene, para-xylene, and ethylbenzene starting with a stream of C8 aromatics by first separating the para-xylene and the ethylbenzene by selectively retaining them on a crystalline aluminosilicate absorbent in a first separation zone, then by isomerizing at least part of the refined product containing ortho-xylene and meta-xylene, by recovering the ortho-xylene from the effluent of the isomerization zone and by recycling part of the effluent from the isomerization zone into the first separation zone. The separation of para-xylene and ethylbenzene is performed in a second separation zone by adsorption. The proposed solution for obtaining a relatively pure ortho-xylene stream is distillation.
U.S. Pat. No. 3,707,550 describes a process for separating and recovering ortho-xylene, para-xylene, and ethylbenzene from a stream of C8 aromatics, using the same elementary steps as those of U.S. Pat. No. 3,636,121 arranged in a different order. The fresh batch feed is performed upstream from the ortho-xylene separation zone (distillation) and no longer upstream from the first separation zone of U.S. Pat. No. 3,636,121.
U.S. Pat. No. 3,700,744 describes a production process for para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) starting with a stream of C8 aromatics by first performing fractionated distillation to produce an overhead fraction containing ethylbenzene (EB), PX and MX, and free of OX, an intermediate fraction containing a mixture of PX and MX, and a bottom fraction containing relatively pure ortho-xylene. The overhead fraction is then isomerized, then recycled into the fractionation zone. The intermediate fraction containing PX and MX is sent into a zone for separation by adsorption to produce relatively pure para-xylene and meta-xylene. A variant of this process consists in producing para-xylene (PX), ortho-xylene (OX), and meta-xylene (MX) starting with a stream of C8 aromatics by first performing distillation to produce an overhead fraction containing ethylbenzene (EB), PX and MX and OX, a bottom fraction containing a mixture of the three xylenes free of EB. The overhead fraction is then isomerized, then recycled into the fractionation zone. The bottom fraction containing the PX and the MX is sent into a separation zone by adsorption to produce relatively pure para-xylene or meta-xylene as an extract and a refined product containing a mixture of ortho-xylene and of the least retained compound, para-xylene or meta-xylene. The refined product is then distilled to produce relatively pure meta-xylene and ortho-xylene.
U.S. Pat. No. 5,510,562 also describes a separation process for C8 aromatics where the mixture of ortho-xylene, meta-xylene, para-xylene and ethylbenzene is first divided into two streams respectively containing the para-xylene and the ethylbenzene on the one hand, and the meta-xylene and the ortho-xylene on the other hand. The para-xylene is then separated from the ethylbenzene by distillation followed by crystallization and the meta-xylene is separated from the ortho-xylene by distillation.
In all the processes for coproduction of para-xylene and ortho-xylene described in patents EP 0 369 078 B1, U.S. Pat. No. 3,636,121, U.S. Pat. No. 3,707,550, U.S. Pat. No. 3,700,744, U.S. Pat. No. 5,510,562, the ortho-xylene is separated from the other C8 aromatics by distillation. There, the boiling points of these compounds are very close (i.e., respectively 136.1° C. for EB, 138.3° C. for PX, 139.1° C. for MX, and 144.5° C. for OX), which makes separation by distillation very difficult and requires a large column with at least about 150 to 200 plates.
The object of the invention is the coproduction of marketable para-xylene and ortho-xylene starting with a batch of hydrocarbons. A second object of the invention is to obtain para-xylene at a purity of at least 99.7% (with a minimum yield of 98%) and ortho-xylene of a purity equal to at least 98.5%. A third object of the invention is to produce para-xylene and ortho-xylene with two separation units where the size of the second unit is small, the stream entering the second unit being relatively free of para-xylene and ethylbenzene.
More precisely, the invention relates to a process for coproduction of para-xylene and ortho-xylene starting with a batch of hydrocarbons that contains them, the process comprising a first separation step of the batch in a simulated fluidized bed in at least a first chromatographic column (
6
) containing numerous beds with at least one absorbent interconnected in series, said column comprising injection of batch (
1
), withdrawal of a first refined product (
4
), withdrawal of a second refined product (
5
) comprising desorption agent and a mixture containing ortho-xylene and meta-xylene relatively free of ethylbenzene and para-xylene, injection of desorption agent (
2
) and withdrawal of an extract delivering a fraction enriched with para-xylene, the process comprising periodic, simultaneous shifting of the positions where the batch and the desorption agent are injected and of the position where the extract is withdrawn, of a bed in the direction of flow of a main stream circulating in said first column (
6
), the process being characterized in that the second refined product is distilled to eliminate the desorption agent from it, mixture (
12
) containing ortho-xylene and meta-xylene is recovered, a second separation step is performed starting with at
Hotier Gerard
Leflaive Philibert
Methivier Alain
Wolff Luc
Dang Thuan Dinh
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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