Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By contact with solid sorbent
Reexamination Certificate
2001-04-26
2004-02-24
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By contact with solid sorbent
C585S820000, C585S827000, C585S828000
Reexamination Certificate
active
06696616
ABSTRACT:
The invention relates to a process for co-production of paraxylene and metaxylene from a feedstock that contains aromatic hydrocarbons with 8 carbon atoms.
The production of paraxylene with high purity by separation by adsorption in a simulated moving bed is well known from the prior art. This market is extensively developed: its outlets are the production of terephthalic acid, phthalic anhydride and polyethylene terephthalate resins. In contrast, the metaxylene market is still restricted, whereby its outlet is isophthalic acid. It was recently perceived that the addition of small amounts of polyethylene isophthalate to polyethylene terephthalate improved the properties of the latter. It therefore becomes advantageous to co-produce paraxylene and metaxylene in the same aromatic compound production complex provided that the market requirements are satisfied: the amount of paraxylene that is produced should be much larger than that of metaxylene: typically 5 to 40 times larger, the paraxylene should be very pure, typically at least 99.6%, and the metaxylene should have reasonable purity, typically at least 99.0%.
The prior art knows metaxylene production processes, for example U.S. Pat. No. 4,326,092 where the adsorbent is a Y zeolite of an Si/Al molar ratio on the order of 4.5 to 5 exchanged with sodium and where the separation is carried out by the technique of adsorption in a simulated moving bed in liquid phase. In U.S. Pat. No. 5,382,747, the same separation is carried out on a Y zeolite that is exchanged with lithium and sodium, in a restricted range of temperature and degree of hydration by using the toluene as a desorbent. To co-produce the paraxylene in the great majority and metaxylene, the drawback of these processes is to require two separate units of very different sizes, without a possibility of finding any synergy in the co-production of the two isomers.
The prior art also describes processes of co-production of paraxylene and metaxylene; for example, U.S. Pat. No. 4,368,347 uses a vapor phase process with intermediate fraction recycling: in addition to the complication that is linked to recycling of intermediate fractions, this document does not suggest how it is possible to use in a practical way such a process that operates at a pressure of between 1 and 2 bar and at a temperature of 150 to 200° C. with a feedstock whose bubble point is 145° C. and with fixed beds that have pressure drops of at least 0.1 bar and probably more to operate economically. Patent FR 2 651 148 uses two different solvents to separate the C8-aromatic fraction into three effluents, which greatly limits its scope since the distillations that result from the simulated moving bed separation unit are multiplied. Patent WO 93/22022 describes various cases of separations of feedstocks of three components into three effluents, however the technology that is used that involves very high pressures, pressure regulation and flow rate regulation at the same time in each of the three or four zones of the process and beds that are each separated in a column is justified economically only for products of high added value.
U.S. Pat. No. 4,306,107 describes a simulated moving bed process in liquid phase where the metaxylene is sampled in the form of extract; the paraxylene, orthoxylene and a fraction of ethylbenzene are sampled as an intermediate raffinate; and finally the ethylbenzene is sampled as a raffinate. This process naturally does not allow a majority of paraxylene and an accompanying stream of metaxylene to be co-produced.
The document of the prior art that comes closest to the invention is U.S. Pat. No. 4,313,015; this document describes the separation of a simulated moving bed, in liquid phase, on zeolite X that is exchanged with barium, whereby the desorbent is diethylbenzene. The extract consists of paraxylene that is too impure (99.44%) to be marketed at current standards (current standard=99.6 mini) and with a yield of 97.5%; the intermediate raffinate consists of ethylbenzene, metaxylene and orthoxylene and a little paraxylene; finally the raffinate consists primarily of a mixture of orthoxylene and metaxylene, whereby the metaxylene can be separated by distillation. The text specifies that the intermediate raffinate is sampled approximately in the middle of the zone between the introduction point of the feedstock and the sampling point of the raffinate. The feedstock that is dealt with in the example is not completely representative of a feedstock that is found in a refinery: the latter always contain at least traces and sometimes up to 5% of paraffins and naphthenes with eight and nine carbon atoms, which distill in the same temperature interval as xylenes. This document does not specify how the paraffins and naphthenes are separated between the raffinate and the intermediate raffinate, the total number of beds and the number of beds per zone used, the order of magnitude of the flow rates in each of the zones of the process, and more particularly the one in zone 1, therefore the necessary solvent level, and the switching time of the beds (circulation speed of the solid).
One of the joint inventors of this application with two other inventors filed a patent application now publication FR 2782714 in which a process for continuous co-production of paraxylene and metaxylene from a hydrocarbon feedstock in at least one chromatographic column that contains at least twenty-five beds that are distributed in five zones is described. At least five beds should be located in zone 3B that is between the point for continuous draw-off of an intermediate raffinate that contains metaxylene, orthoxylene, ethylbenzene, solvent and paraxylene, and the point for continuous draw-off of a raffinate that contains metaxylene and orthoxylene and solvent. In addition to the large number of beds necessary for the implementation of the process (
30
, for example), the hydrocarbon feedstock should have an ethylbenzene content that is less than 5%, which is restricting. Furthermore, despite these constraints, although the purity of the metaxylene that is obtained after distillation under economical conditions of said raffinate is very advantageous (for example 99.6%), the yield of metaxylene relative to the metaxylene that is contained in the feedstock of the adsorption unit is about 10%, which is low.
The object of the invention is therefore the co-production of paraxylene with a purity of at least 99.6% and with a minimum yield of 98%, and metaxylene with a purity after distillation that is at least equal to 99% and with a yield that is greater than 10%, according to the definition above, and preferably greater than 15%, for example between 15 and 20%.
Another object is to produce in particular metaxylene from a feedstock that is not limited in terms of ethylbenzene.
More specifically, the invention relates to a process for co-production of metaxylene and paraxylene from a hydrocarbon feedstock that comprises them, whereby the process comprises a separation stage of said mixture in a simulated moving bed in at least one chromatographic column that contains a number of beds of an adsorbent that are interconnected in a loop, whereby said column comprises an injection of a feedstock, a draw-off of a first raffinate, a draw-off of a second raffinate that comprises metaxylene, an injection of desorbent and a draw-off of an extract that delivers paraxylene, whereby the process comprises the simultaneous periodic offsetting of injection positions and the draw-off position of the extract from a bed in the direction of flow of the main flux that circulates in the column, whereby the process is characterized in that a first raffinate that comprises desorbent, orthoxylene, metaxylene and ethylbenzene is drawn off continuously or intermittently, in that a second raffinate R
2
that comprises orthoxylene and metaxylene is drawn off intermittently, whereby the process is also characterized in that the second raffinate is distilled in such a way as to recover, separately, orthoxylene and metaxylene with at least 99% of p
Hotier Gerard
Pavone Didier
Griffin Walter D.
Institut Français du Pétrole
Millen White Zelano & Branigan P.C.
Nguyen Tam M.
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