FR2743068A1 - Purified para:xylene production, for synthesis of terephthalic acid - Google Patents

Abstract

The preparation of very high purity paraxylene from an aromatic mixture containing paraxylene and at least one of its isomers comprises: (a) admitting the hydrocarbon mixture (1) containing paraxylene to a primary adsorption zone (2) containing an adsorbent which is regenerated by displacement with a solvent S, producing paraxylene of less than 99% purity mixed with desorbent S and a mixture of xylenes depleted in paraxylene and desorbent; (b) optionally distilling to separate the paraxylene from (a) from desorbent by distillation, the desorbent being partially recycled; (c) the purification of paraxylene from (b) in a secondary adsorption zone (7) containing an adsorbent which is regenerated by S, producing 99%+ paraxylene mixed with S and a xylene mixture depleted in paraxylene and S; (d) distilling to separate the pure paraxylene from (c) from S, S being partially re-cycled; and (e) distilling to separate the mixtures of paraxylene-depleted xylenes from (a) and (c) from the desorbent, which is recycled.

Description

The invention relates to a process for preparing very high purity paraxylene from a mixture of isomers of aromatic hydrocarbons having at least 8 carbon atoms and more particularly from a mixture of aromatic isomers with eight carbon atoms . It finds its application in the preparation of paraxylene which serves as an intermediary in the manufacture of terephthalic acid for the synthesis of synthetic fibers.

Several types of process are known to date for producing paraxylene - the crystallization at very low temperature then at high temperature of a mixture containing 18 to 24% of paraxylene makes it possible to produce paraxylene of very high purity at the price of a very high energy expenditure, especially at very low temperatures. In addition, the yield observed is limited (around 65%), so that recovery is not total. Paraxylene is therefore recycled with a mother liquor resulting from a separation step, in the isomerization unit which processes the mother liquor.

 - adsorption, in particular in a simulated counter-current or cocurrent moving bed (US 2985589 and US 4498991, US 4402832 incorporated as references) makes it possible to obtain paraxylene of very good purity but the device for implementation is costly in investment and its control is very difficult to reach the highest degrees of purity.

- adsorption coupling and at least one crystallization at high temperature (+ 10 to -250C for example), US 5284992 finally ensures, thanks to the adsorption which makes it possible to achieve with a yield greater than 98% a purity of the paraxylene product of 75 to 98% and, thanks to the subsequent crystallization step, a purification of 99.7% paraxylene for example under energetically favorable conditions.

However, this technique has the drawback of requiring both the equipment necessary for a simulated counter-current moving bed adsorption operation and that which is required for a crystallization operation, for example a refrigerating machine.

An object of the invention is therefore to remedy the drawbacks of the prior art.

Another object is to produce paraxylene from a hydrocarbon feed containing it by two-stage adsorption under more advantageous conditions, the first adsorption stage making it possible to achieve very high productivity and a purity of 40 at 99% and the second to treat only the fraction enriched in paraxylene and therefore to obtain excellent purification of the paraxylene produced.

More specifically, the invention relates to a process for preparing very high purity paraxylene from a mixture of aromatic hydrocarbons containing paraxylene and at least one of its isomers, the process being characterized in that it comprises at least the following steps (a) a step during which the mixture of hydrocarbons comprising paraxylene is sent to a first adsorption zone containing an adsorbent, the adsorbent being regenerated by displacement using a desorbent S (or solvent), this first step producing paraxylene at less than 99% and preferably at less than 95% purity, as a mixture with the desorbent S and a mixture of xylenes depleted in paraxylene and of desorbent (b) possibly a distillation stage during which the paraxylene originating from stage (a) is separated at least in part from the desorbent by distillation, the desorbent S being recycled at least by tie (c) a step of purifying the paraxylene originating from step (b) in a second adsorption zone containing an adsorbent, the adsorbent being regenerated by displacement using the desorbent S, this second step producing paraxylene at more than 99% purity mixed with the desorbent S and a mixture of xylenes depleted in paraxylene and desorbent (d) a distillation step during which the pure paraxylene from step (c) is separated at least partly of the desorbent by distillation, the desorbent S being recycled at least in part and the pure paraxylene being recovered, and (e) a distillation stage during which the mixtures of xylenes depleted in paraxylene originating from stages (a) and ( c) are separated at least in part from the desorbent by distillation, the desorbent S being recycled at least in part.

In the case of the use in the first adsorption step of a zeolite strongly adsorbing the desorbent, the concentration of desorbent of the extract can be such that it is not necessary to distill the paraxylenesorbent mixture before l 'introduce into the second adsorption zone.

According to a first characteristic of the process, the solvent originating at least in part from the distillation step (b) and at least in part from the distillation step (d) can be recycled to the purification step (c). In this case, the purest solvent is therefore used for the paraxylene purification step, which makes it possible to achieve better purity. To this end, it is preferable to carry out an additional stripping step of solvent originating from the distillation step (b) used for the purification step (c) so that it is as pure as possible, c that is to say devoid of meta- and orthoxylene in particular.

According to a second characteristic, the solvent originating at least partly from stage (b) of distillation and at least partly from stage (e) of distillation of the mixture of xylenes depleted in paraxylene can be recycled to stage ( a) adsorption using the first adsorption zone. It is understood that the solvent delivered by the distillation unit of the fraction enriched in paraxylene with less than 99% purity which comes from the first adsorption zone can contribute to desorbing both the first and the second zone d 'adsorption.

According to a first variant of the process, the adsorbent of the first adsorption zone and that of the second adsorption zone are a selective adsorbent for paraxylene. It may preferably be, for example, a zeolite Y exchanged with barium or (and) with potassium, at least in part, as described in US Pat. No. 3,894,109 incorporated by reference. Zeolites exchanged with cations alone or in combination can be used, such as for example those described in US Patents 3626020,
US 3878127, 3878129, US 3894109, US 3943183 and US 3960774 incorporated as references. By combining with the adsorbent a solvent containing toluene as a desorbent, excellent results have been obtained (US Pat. No. 3,558,732).

According to a second variant of the process, the adsorbent of the first adsorption zone and that of the second adsorption zone are an X zeolite exchanged at least in part, with barium for example as described in the above patents. Very good results have been obtained when this zeolite was combined with a solvent containing paradiethylbenzene (US Pat. No. 3,686,342, for example incorporated as reference).

According to a third variant of the process making it possible to adapt the adsorbent / solvent couple to the purity and to the targeted yield, the adsorbent of the first adsorption zone can be a selective adsorbent for paraxylene (para-selective) while the adsorbent of the second adsorption zone can be a selective adsorbent for the mixture of xylenes depleted in paraxylene (meta selective). In this case, the paraxylene isomers (metaxylene, possibly orthoxylene, and ethylbenzene) are collected as an extract in the second adsorption zone and the paraxylene as a raffinate. The patents US 4940830 and US 5382747 incorporated as references describe a meta-selective adsorption allowing in particular to better adsorb ethylbenzene on the adsorbent compared to paraxylene.

For example, the adsorbent of the second adsorption zone which is selective for the isomers of paraxylene and of ethylbenzene in particular may comprise a zeolite Y exchanged for sodium or exchanged for sodium and lithium (at least 5% by mole sodium ions which can be exchanged with lithium) or a zeolite Y at least partially exchanged with at least one element chosen from the group formed by the metals of groups IB and VIII, preferably copper and nickel.

The adsorbents can also contain an amorphous binder.

Desorbents, in general, must be compatible with the adsorbent and the filler and will be chosen, for example, from the list of patents above to obtain the best selectivity and the best possible productivity for a given adsorbent. They can be used alone or in admixture with diluents such as saturated hydrocarbons. They can have either a boiling point lower than that of the filler, like toluene, or a boiling point higher than that of the filler like insane or diethylbenzenes and preferably paradiethylbenzene.

At least one of the adsorption steps can be carried out in a simulated moving bed.

Preferably, the first and second adsorption steps are simulated mobile beds that can operate either against the current or co-current, like those described in the patents cited as references.

The first and second adsorption stage can in this case consist of a simulated moving bed of adsorbent with at least three zones, each zone having a different function comprising a plurality of stages whose flow inputs and outputs are controlled by a plurality of all-or-nothing valves connected to a control and command center. A single multi-position valve can also be used.

The operating conditions are generally the following for the two adsorption stages:
number of floors (beds): 6 to 24, preferably 16 to 24
solvent flow ratio on charge 1 to 2.5
Temperature 20 to 220 "C, preferably 100 to 200" C
Generally the number of stages in the second adsorption step is greater than that of the first adsorption step.

It has been found that the solvent and the charge of the mixture of hydrocarbons can be circulated in the first adsorption zone in a ratio of solvent flow rate to charge flow rate (mixture of hydrocarbons) lower than the ratio of solvent flow rates. on charge (paraxylene) flowing in the second adsorption zone.

The invention will be better understood from the attached figure schematically illustrating an embodiment of the process in which a desorbent with a higher boiling point than that of paraxylene is used.

A charge comprising aromatic hydrocarbons with 8 carbon atoms, previously freed from orthoxylene and hydrocarbons having at least 9 carbon atoms is introduced by a line 1 into a first adsorption unit 2 to 4 zones operating against the current simulated. This simulated moving bed contains an adsorbent which is an X zeolite exchanged with barium operating in the presence of a desorbent, paradiethylbenzene. A fraction depleted in paraxylene and enriched in isomers of paraxylene (metaxylene and ethylbenzene and optionally orthoxylene) called raffinate and containing desorbent is collected by a line 12 and sent to a distillation unit 15 through a line 14.

A second fraction desorbed by the desorbent is collected as an extract by a line 3. It contains desorbent, paraxylene at less than 95% purity and other isomers as impurities. This fraction is distilled in a distillation unit 4 which produces as residue in line 6, desorbent which is recycled at least in part by a line 6a to line 18 and to the first adsorption zone 2. The unit of distillation 4 also produces a distillate containing paraxylene through a line 5 which introduces it into a second adsorption unit 7 operating in a simulated moving bed against the current with 4 zones. The adsorbent and the desorbent introduced by a line 1 1 are the same as those of the first zone.

A first fraction containing a mixture of desorbent and xylenes depleted in paraxylene is withdrawn by a line 13 as a raffinate. This first fraction is introduced as a mixture with line 12 into the same distillate distillation unit 15. A distillate containing the mixtures of xylenes (metaxylene, possibly orthoxylene and ethylbenzene) is collected by a line 16 and sent to a xylene isomerization unit not shown in the figure, the collected isomerate being able to be at least partly recycled after separation light hydrocarbons, in the first adsorption unit 2.

The distillation residue 15 is drawn off through a line 17 and recycled at least in part with at least part of the residue from the first distillation (line 6a), in the first adsorption unit 2 thanks to the line 18.

A second fraction, called an extract, is desorbed from the adsorption zone 7 by paradiethylbenzene and drawn off by a line 8. It contains desorbent and paraxylene at more than 99.5% purity and is subjected to distillation in a second unit. distillation 9. A distillate containing paraxylene with more than 99.5% purity is withdrawn by a line 10 and possibly redistilled (not shown in the figure) to remove the traces of toluene remaining contained in the initial charge.

Finally, a residue containing desorbent containing substantially only paraxylene is withdrawn and recycled at least in part by line 11 in the second adsorption unit 7.

Another addition of desorbent can be provided at least in part by the residue of first distillation 4 by means of a line 6b connected to line 6 and connected to line 11. The quality of this addition of desorbent can be improved by stripping d '' at least part of the residue from the first distillation, which contributes to desorbing the extract of line 8 only with paradiethylbenzene of excellent quality since it contains only a tiny part of impurities. The purity of the paraxylene obtained by line 10 is therefore further improved.

As has been said, the figure illustrates the case where a less volatile desorbent is used than paraxylene or other C8 aromatic hydrocarbons.

In the case where toluene is used, which is more volatile than the C8 aromatic isomers, the figure illustrating the process according to the invention would remain the same, except that, in the various distillation units, the desorbent would be withdrawn as that distillate and the various C8 hydrocarbon fractions would be drawn off as a residue.

Example
It is sought to produce paraxylene with 99.7% purity in a counter-current simulated moving bed adsorption process comprising 4 zones in each adsorption unit, from a charge of the following composition: Paraxylene: 22% , Metaxylene: 88%. The charge flow rate to be treated is 100 liters / min or 87 kg / min.

To carry out this operation, one can work in two adsorption steps according to the invention. The solvent used during the two stages is toluene.

In a first adsorption zone comprising 16 beds, a selective sieve for paraxylene will be used, for example a KBaY sieve. In a second adsorption zone comprising 24 beds, a selective sieve for the other isomers will be used, for example a NaY sieve.

The first step will make it possible to produce 85% purity paraxylene as an extract mixed with toluene. The yield from this operation will be 96%. Given a sieve productivity of 0.07 g of paraxylene / (cm3 h), it will be necessary to use 15.6 m3 of sieve to carry out this operation. The amount of solvent required is 135 l / min.

The second stage makes it possible to process the extract of the previous stage distilled to increase its purity. The feed rate to be treated is this time 21.6 kg / min. The sieve used being selective for metaxylene, it will make it possible to produce a mixture of these two constituents at 80% purity as an extract. The yield of this operation is 98%, which makes it possible to recover, as raffinate, paraxylene with 99.7% purity as a mixture in toluene. Given a productivity of this stage of 0.04 g of metaxylene / (cm3 h), the quantity of sieve necessary for this operation is 4.8 m3. The amount of solvent used is 38 l / min. The 99.7% purity paraxylene is distilled to recover the toluene which is recycled to the first adsorption zone.

The two stages therefore make it possible to produce pure paraxylene with an overall paraxylene yield of 92%. The total quantity of sieves involved is 20.4 m3, which represents a gain of 20% compared to a separation carried out in a single step.

Claims (16)

1. Process for the preparation of very high purity paraxylene from a  mixture of aromatic hydrocarbons containing paraxylene and  at least one of its isomers, the process being characterized in that it  includes at least the following steps  (a) a step during which the mixture of hydrocarbons (1)  comprising paraxylene is sent to a first zone (2)  adsorption containing an adsorbent, the adsorbent being regenerated by  displacement using a desorbent S, this first step producing  paraxylene with less than 99% purity mixed with  desorbent S and a mixture of xylenes depleted in paraxylene and desorbent  (b) optionally a distillation step (4) during which the  paraxylene from step (a) is separated at least in part from  desorbent by distillation, the desorbent S being recycled at least in part  (c) a step for purifying the paraxylene originating from step (b)  in a second adsorption zone (7) containing an adsorbent,  The adsorbent being regenerated by displacement using the desorbent S,  this second stage producing paraxylene with more than 99% of  purity mixed with desorbent S and a mixture of xylenes  depleted in paraxylene and desorbent;  (d) a distillation step (9) during which the pure paraxylene  from step (c) is separated at least in part from the desorbent by  distillation, the desorbent S being recycled at least in part and the  pure paraxylene being recovered; and  (e) a distillation step (15) during which the mixtures of  xylenes depleted in paraxylene from steps (a) and (c) are  separated at least in part from the desorbent by distillation, the desorbent S  being at least partially recycled. 2. The method of claim 1, wherein the desorbent from  at least in part from step (b) and at least in part from step (d) is  recycled to the purification step (c). 3. Method according to claim 1 or 2, wherein the desorbent  originating at least in part from step (e) and at least in part from  step (b) is recycled to step (a). 4. The method of claim 1 or 2, wherein the desorbent used  for the purification step (c) coming from step (b) undergoes at the end  from step (b) a complementary stripping step. 5. Method according to one of claims 1 to 4 wherein one circulates  the desorbent and the mixture of hydrocarbons in the first zone  adsorption in a ratio of desorbent flows on mixture  of hydrocarbons lower than the solvent flow rate ratio on paraxylene of  step (b) flowing in the second adsorption zone. 6. Method according to one of claims 1 to 5 wherein the adsorbent  of the first adsorption zone and the adsorbent of the second zone  adsorption are a selective adsorbent for paraxylene. 7. The method of claim 6 wherein the adsorbent of the  first adsorption zone and second zone adsorbent  adsorption are a Y zeolite exchanged with barium and potassium,  at least in part. 8. The method of claim 6 wherein the adsorbent of  first adsorption zone and the second adsorption zone are  an X zeolite exchanged with barium, at least in part. 9. Method according to one of claims 1 to 5 wherein the adsorbent  of the first adsorption zone is a selective adsorbent for the  paraxylene and the adsorbent of the second adsorption zone is a  selective adsorbent for said isomers of paraxylene. 10. The method of claim 9 wherein the adsorbent of  second adsorption zone, selective for said isomers of  paraxylene and for ethylbenzene includes an exchanged Y zeolite  with sodium or a zeolite Y at least partly exchanged with at  minus one element chosen from the group formed by the metals of  groups IB and VIII, preferably copper and nickel. 11. The method of claim 9 or 10, wherein the desorbent of the  second adsorption zone contains paradiethylbenzene. 12. Method according to one of claims 1 to 7 wherein the desorbent  contains toluene. 13. Method according to one of claims 1 to 6 and 8 wherein the  desorbent contains paradiethylbenzene. 14. Method according to one of claims 1 to 13 wherein the first  and the second adsorption zone are simulated moving beds. 15. Method according to one of claims 1 to 14 wherein the beds  mobiles operate against simulated counter current. 16. Method according to one of claims 14 to 15, wherein the  first and second adsorption zones include at least three  zones and a plurality of on-off valves controlling the inlet and the  exit from the floors of each zone.