Refrigeration – Separator for solidified constituent of liquid mixture
Reexamination Certificate
1999-12-17
2002-01-08
Capossela, Ronald (Department: 3744)
Refrigeration
Separator for solidified constituent of liquid mixture
C062S533000
Reexamination Certificate
active
06336334
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for crystallizing a component from a liquid mixture that contains it, and its use particularly for separating paraxylene from a hydrocarbon mixture that contains aromatic isomers with 8 carbon atoms.
The invention also applies very particularly to the purification to 99.9%, for example, of paraxylene for the preparation of terephthalic acid, an intermediate petrochemical for the synthesis of textile fibers.
BACKGROUND OF THE INVENTION
Among the previous processes that are suitable for the purification of paraxylene, crystallization is the one that has been used most, even though it is limited by a low recovery level due to the existence of a eutectic whose paraxylene concentration in the mixture reaches about 10 to 13%.
With the development of techniques for separation by adsorption in a molecular sieve, it is possible to achieve an excellent paraxylene yield, greater than, for example, 98%, independently of the eutectic composition limitation.
Usually, a recovery in excess of 98% of paraxylene is obtained with the processes of a simulated countercurrent fluid bed (U.S. Pat. No. 2,985,589) when the purity of the product is close to 95% by weight A higher purity, exceeding 99%, however, can be reached at the expense of the yield.
Since an adsorption process makes it possible to carry out the separation of paraxylene with a high yield at the expense of purity while a crystallization process makes it possible to obtain a more pure product to the detriment of the yield, the applicant has proposed a hybrid process that combines adsorption in a molecular sieve of aromatic C
8
isomers, followed by crystallization of the paraxylene-enriched fraction (U.S. Pat. No. 5,401,476, and EP-B.531191 that are incorporated as references).
This process thus combines the advantages of a high yield and a very high purity of the wanted product compared to the processes of adsorption or of crystallization used separately.
Furthermore, the technology of crystallization is very old and has hardly been updated, considering the industrial breakthrough of the adsorption process in the simulated fluid bed.
The technological background is illustrated by the following documents: EP-A 611 589, EP-A 455 243, DE-A 2 222 755 and DE-C 972 036.
Crystallization for separating the paraxylene from a mixture of xylenes is generally carried out at very low temperatures, located in the range of those that can be attained by refrigeration with ethane or with ethylene. The costs of refrigeration and the consumption of energy are high, particularly because it is necessary to produce a cascade of cold cycles, with intermediate refrigeration with propane or with propene.
Certainly, this consumption is reduced when the operation is carried out in a higher range of crystallization temperatures, +10 to −30° C., for example, as is the case when the crystallization feedstock is enriched to more than 50%, for example, preferably with more than 70% paraxylene, by an enrichment process by adsorption of xylenes or by paraselective dismutation of toluene, for example.
Moreover, the process of continuous crystallization by indirect exchange generally requires that the exchangers be scraped, which is an operation that is delicate and energy-intensive, regardless of the selected cooling level.
One of the objects of the invention is to remedy the drawbacks that are mentioned above.
Another object is to propose a crystallization technique by direct exchange of kilogram calories with a feedstock which is simpler and easier to use.
Another object is the use of a process of crystallization and a device that correspond, at any desired level of cooling temperature and at very much the same operating expense, knowing, of course, that the power of the gas compressors will depend on the desired cooling temperature.
SUMMARY OF THE INVENTION
It has therefore been observed that it was possible to crystallize a product in a liquid mixture that contains it, constituting the crystallization feedstock, by direct exchange of cold with a cold cryogenic gas that is obtained by approximately isentropic expansion of this gas, under very favorable and very economical conditions.
In particular, the scope of the present invention is not limited to any particular geometry of a crystallizer since the present invention provides the broad concept of any type of crystallizer combined with a turbo-expander which can cool the fluid obtained from the crystallizer for purposes of recycling it to the crystallizer.
Thus, there is provided a device for crystallization of a component from a liquid mixture comprising a chamber, a first conduit connected to said chamber for supplying said mixture, a second conduit connected to and in combination with said chamber for introducing cold gas, a third conduit for withdrawing warmed gas from said crystallizer, a turbo-expander for cooling gas withdrawn from said chamber, and a fourth conduit leading from the turbo-expander to said chamber.
Described even more specifically is a device for crystallization of a component from a liquid mixture that contains it and that comprises an elongated chamber that has in its upper part means for supplying said mixture, collecting means, and in its lower part, crystals of the component in suspension in a mother liquor, connected to means for separation and for purification of the crystals that are obtained, means for supplying a cold gaseous fluid that is introduced at at least one point in the lower part of the chamber and means for drawing off the so-called hot gaseous fluid that is placed at the upper part of the chamber, resulting from the direct countercurrent heat exchange of the cold gaseous fluid with the mixture, whereby said device also comprises means for suitable shaping of a descending flow of the mixture such that the cold gaseous fluid that circulates upward exchanges directly from the cold with the mixture that is shaped, whereby the means for drawing off the hot gaseous fluid are connected to at least one fluid recompression means (
24
b
), whereby the recompression means has an output that is connected to at least one heat exchanger, whereby the heat exchanger has an output that is connected to a turbo-expander (
30
), and whereby the turbo-expander has an output that is connected to cold gaseous fluid supply means (
31
).
According to an embodiment of the device, the means for suitable shaping of the descending flow of the mixture comprise at least one sprayer that is suitable for forming droplets of the mixture measuring between 50 and 500 micrometers and preferably according to the distribution of sizes of 150 to 200 micrometers for at least 80% of the droplets.
According to another embodiment of the device, the chamber can contain a large number of filaments that are suspended, non-contiguous, and approximately parallel to its longitudinal axis, supported by attachment means that are placed in the vicinity of, for example, the upper end of the chamber and that occupy approximately the entire section of the chamber.
The sprayer that is mentioned above shapes the droplets of the mixture that constitutes the feedstock, which can be deposited on the filaments in the form of a thin film which is subjected to crystallization by direct contact with the cold gas that rises into the crystallization chamber.
These vertical filaments can be hollow, tubes, for example, to lighten the device and/or to make possible the circulation of a hot fluid that reheats that wall of the filament to promote the presence of a liquid film on the latter and the downtake of the crystals into the lower part of the crystallization chamber.
This hot fluid can be a portion of the gaseous fluid that is recovered from the crystallization chamber and recompressed.
According to a variant, the flowing of the liquid mixture onto the filaments can be carried out differently.
Actually, the means for suitable shaping of the flow comprise a chamber for communication with said means for supplying the mixture that comprise a large number of flow orific
Mikitenko Paul
Minkkinen Ari
Capossela Ronald
Institut Francais de Petrole
Millen White Zelano & Branigan P.C.
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