Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having growth from a solution comprising a solvent which is...
Reexamination Certificate
2001-12-18
2003-05-20
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having growth from a solution comprising a solvent which is...
C422S245100, C585S412000
Reexamination Certificate
active
06565653
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel energy efficient process for the production of high purity paraxylene (pX) from a feedstock of C
8
aromatics which preferably comprises at least about 60 weight percent (wt %) paraxylene wherein a first portion of high purity paraxylene product is obtained in a first crystallization step at a temperature of from about 10° F. to about 55° F. followed by separation of the washed paraxylene crystals without the need for further reslurry and recrystallization and wherein another portion of the high purity paraxylene product is obtained following a reslurry step that warms crystalline paraxylene obtained from subsequent lower temperature crystallizations to yield a slurry at a temperature of from about 10° F. to about 55° F. without the need for further refrigeration. The paraxylene crystals are separated from the slurry mother liquor to give high purity paraxylene product. In an alternate embodiment of the invention, the feedstock may comprise at least about 55 weight percent paraxylene.
BACKGROUND OF THE INVENTION
The separation of xylene isomers has been of particular interest because of the usefulness of para-xylene in the manufacture of terephthalic acid which is used in the manufacture of polyester fabric. Paraxylene is a chemical intermediate useful for the manufacture of terephthalic acid, the major constituent of polyethylene terephthalate. Paraxylene having a purity of at least about 99.5, more preferably of at least about 99.7 weight percent, is used to manufacture terephthalic acid by the oxidation of paraxylene. Other components of the C
8
aromatic hydrocarbon feedstream from which para-xylene (pX) is generally produced are ortho-xylene (oX), which is used in the manufacture of phthalic anhydride which is used to make phthalate based plasticizers; meta-xylene (mX), which is used in the manufacture of isophthalic acid used in the production of specialty polyester fibers, paints, and resins; and ethylbenzene (EB) which is used in the manufacture of styrene.
A refinery feedstock of aromatic C
8
mixtures containing ethylbenzene and xylenes will typically have the following content:
ethylbenzene
about 0 wt % to about 50 wt %
para-xylene
about 0 wt % to about 25 wt %
ortho-xylene
about 0 wt % to about 35 wt %
meta-xylene
about 20 wt % to about 90 wt %
non-aromatics
about 0 wt % to about 10 wt %
C
9
+
aromatics
about 0 wt % to about 30 wt %
Equilibrium mixtures of C
8
aromatic hydrocarbons generally contain about 22 weight percent para-xylene, about 21 weight percent ortho-xylene, and about 48 weight percent meta-xylene.
Processes to separate xylene isomers include low temperature crystallization, fractional distillation, selective sulfonation with subsequent hydrolysis and selective solvent separation; however, such processes require high operating costs.
Crystallization can be used to separate paraxylene from a C
8
aromatic feedstream comprising paraxylene, meta-xylene, ortho-xylene, and ethylbenzene due to the fact that the components have different melting points. Paraxylene freezes at 13° C., meta-xylene freezes at −48° C., ortho-xylene freezes at −25° C., and ethylbenzene freezes at −95° C.
Crystallization has been used commercially to isolate and purity paraxylene, typically from a mixture of xylenes and ethylbenzene close to chemical equilibrium. Because of the low concentration of paraxylene in these mixed xylene streams, very low temperatures are generally required to effectively recover the paraxylene from a C
8
fraction by crystallization. Furthermore, there is an operational low temperature limit generally taken as the meta-xylene/paraxylene or the ortho-xylene/paraxylene binary eutectic temperature that prevents the complete recovery of all the paraxylene from a C
8
fraction. At or below this limit, either meta-xylene or ortho-xylene will co-crystallize with paraxylene. The use of such low temperatures for crystallization is expensive and requires a substantial use of energy. There is a need for a more energy efficient process for crystallizing and purifying paraxylene from a feed containing paraxylene and other C
8
aromatics.
U.S. Pat. No. 6,111,161 discloses a process for the production of high purity paraxylene from a charge containing C
7
-C
9
aromatic hydrocarbons in which a first fraction is enriched to at least 30% weight with paraxylene and this fraction is purified by at least one high-temperature crystallization in at least one crystallization zone. Said first fraction is crystallized in a crystallization zone at high temperature T1 and advantageously between +10 and −25° C. Crystals in suspension in a mother liquor are recovered, and the crystals are separated from the mother liquor in at least a first separation zone. The crystals obtained are partially melted in at least a zone for partial melting and a suspension of crystals is recovered. The crystals in suspension are separated and washed in at least one separation and washing zone and pure paraxylene crystals and washing liquor are recovered, and pure crystals are optionally completely melted and a liquid stream of melted paraxylene is collected.
U.S. Pat. No. 5,448,005 discloses a process for producing high purity paraxylene from a high weight percent paraxylene feedstock, comprising at least about 70 wt % paraxylene and preferably at least about 80 wt % paraxylene which uses a single temperature crystallization production stage at a temperature in the range of from about 0° F. to about 50° F. and also uses scavenger stages to raise the paraxylene recovery rate. The single temperature production stage crystallizer of the process employs a wash using only paraxylene product.
The present invention has an advantage over other crystallization processes. It reduces the refrigeration requirements compared to designs disclosed in U.S. Pat. Nos. 6,111,161 and 5,448,005. Thus, it requires less energy expenditure and provides a cost savings compared to those designs. It accomplishes this by separating some or most of the final product early in the separation sequence, thereby reducing the amount of material that requires lower temperature refrigeration. It does not recycle cake back to the first crystallizer from the lower temperature stage(s), but rather uses a reslurry drum to sufficiently warm the crystals so that additional para-xylene product can be recovered without the need for more refrigeration. As calculated according to standard engineering practices, the refrigeration compressor horsepower for the invention can be as much as 13% less than that for comparable designs based on the teachings of U.S. Pat. No. 6,111,161.
SUMMARY OF THE INVENTION
The present invention relates to a process for the production of paraxylene from a paraxylene-containing feedstream comprising C
8
aromatic hydrocarbons and having a paraxylene concentration of at least about 60 weight percent, the process comprising:
a) crystallizing said feedstream in a first crystallizer at a temperature of from about 10° F. to about 55° F.;
b) recovering an effluent comprising paraxylene crystals in a mother liquor;
c) separating the paraxylene crystals from the mother liquor in a first separation unit, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting the liquid paraxylene product;
d) transferring at least a portion of filtrate from the first separation unit to a second crystallizer which is operated at a temperature lower than that of the first crystallizer, crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
e) separating the paraxylene crystals from the mother liquor in a second separation unit and sending the paraxylene crystals to a slurry apparatus;
f) transferring at least a portion of filtrate from the second separation unit to a third crystallizer, which is operated at a temperature lower than that of the second crystallizer, crystallizing the filtrate, and recovering an effluent comprising paraxylene
BP Corporation North America Inc.
Hiteshew Felisa
Kanady Mary Jo
Yassen Thomas A.
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