Para-xylene production process integrating pressure swing...

Details Para-xylene production process integrating pressure swing... Para-xylene production process integrating pressure swing...

2001-07-10

2003-07-29

Grffin, Walter D. (Department: 1764)

Chemistry of hydrocarbon compounds

Purification, separation, or recovery

By contact with solid sorbent

C585S812000, C585S813000, C585S814000, C585S815000, C585S820000, C585S825000, C585S847000

Reexamination Certificate

active

06600083

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for producing para-xylene integrating pressure swing adsorption for separating para-xylene and ethylbenzene from mixed C
8
aromatics with recovery of para-xylene by crystallization. The present invention further relates to a process comprising using a pressure swing adsorption (PSA) process for separating para-xylene and ethylbenzene from mixed C
8
aromatics using a para-selective adsorbent and separating the para-xylene from the para-xylene/ethylbenzene effluent by fractional crystallization to obtain para-xylene product. The para-selective adsorbent is preferably a non-acidic, molecular sieve. The para-selective adsorbent is more preferably a non-acidic, medium pore, molecular sieve. The molecular sieve is preferably of the MFI structure type and the process is preferably operated in the vapor phase at elevated temperatures and pressures wherein the temperature is substantially isothermal. The present invention also relates to a method of pressure swing adsorption which includes a plurality of steps and which provides recovery from a mixture comprising C
8
aromatics of a substantially pure para-xylene or para-xylene and ethylbenzene product stream and a substantially pure meta-xylene and ortho-xylene product stream and separating the para-xylene from the para-xylene/ethylbenzene stream by fractional crystallization to obtain a substantially pure para-xylene product.
A pressure swing adsorption process to separate para-xylene and ethylbenzene from C
8
aromatics which uses a para-selective, non-acidic, medium pore molecular sieve of the MFI structure type and is operated isothermally in the vapor phase at elevated temperatures and pressures is used to produce a para-xylene-rich stream which is then subjected to crystallization to produce a para-xylene product. In the pressure swing adsorption step, a fixed bed of adsorbent is saturated with pX and EB, which are preferentially adsorbed, then the feed to the process is stopped. Lowering the partial pressure desorbs the pX and EB. The process effluent is rich in pX and EB. A stream of non-adsorbed mX and oX may be obtained prior to desorption of pX and EB. The mX/oX stream may be isomerized to produce an equilibrium mixture of xylenes and recycled to the PSA step.
It is known that certain high surface area, porous substances such as silica gel, activated charcoal, and molecular sieves, including zeolites and other molecular sieves, have certain selective adsorption characteristics useful in separating a hydrocarbon mixture into its component parts.
The selective sorption properties of molecular sieves and zeolites have been disclosed in earlier patents and in literature references. Crystalline molecular sieves and zeolites are shape-selective in that they will admit molecules of specific geometry while excluding other molecules.
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. 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 in the equilibrium mixture.
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.
The use of faujasite zeolites, which are large pore type X and Y type zeolites, as adsorbents in liquid phase, chromatographic-type separations is well known.
In the petrochemical production chain, one of the most important streams is the C
6
to C
8
aromatics stream containing benzene, toluene, and xylenes (BTX), which is a source of raw materials for high value downstream products. Of the C
8
aromatics, para-xylene (pX) is the most desirable. However, because the boiling points of ethylbenzene (EB), ortho-xylene (oX), meta-xylene (mX) and para-xylene (collectively referred to as “C
8
aromatics”) are close, they are difficult to separate by fractional distillation. As a consequence, various alternative methods of separating pX from the C
8
aromatics have been developed. Common separation methods are fractional crystallization, which utilizes the difference in freezing points, and liquid phase adsorption (e.g., UOP's Parex process and IFP's Eluxyl process), which uses a faujasite zeolite to chromatographically separate pX from the other C
8
aromatics. The reject stream from the crystallization process or the raffinate from the adsorption process are depleted in pX, and contain relatively high proportions of EB, oX and mX. These streams are typically sent to a catalyst reactor, where the xylenes are isomerized to equilibrium, and at least a portion of the EB is converted to other products, which can be removed from the C
8
aromatics by fractional distillation.
Processes for making pX have typically included combinations of isomerization with fractional crystallization or adsorption separation.
FIG. 1
is a schematic representation of known art combination of an isomerization catalyst reactor and a crystallization unit. Crystallization is a separation process that takes advantage of the fact that pX crystallizes before the other isomers, i.e., pX crystallizes at 13.3° C. (55.9° F.), whereas oX crystallizes at −25.2° C. (13.4° F.) and mX at −47.9° C. (−54.2° F.). In the physical system of the three isomers, there are two binary eutectics of importance, the pX/mX and the pX/oX. As pX is crystallized from the mixture, the remaining mixture (mother liquor) composition approaches one of these eutectic binaries, depending on the starting composition of the mixture. Therefore, in commercial practice, pX is crystallized so that the binary eutectic is only approached but not reached to avoid co-crystallization of the xylene isomers, which would lower the pX purity. Thus, the key disadvantage for crystallization is restricted pX recovery per pass, due to this eutectic limit of the C
8
stream. Typically, the concentration of pX in a mixed C
8
aromatic stream at equilibrium is about 22 wt %. In commercial crystallization operations, the eutectic point of this mixture limits the pX removed per pass to about 65% of that amount.
The problem of the eutectic limit for pX crystallization has been recognized for some time. U.S. Pat. No. 5,329,060 discloses that the eutectic point of the crystallization unit can be overcome by use of a selective adsorption zone that enriches the pX feed to the crystallizer by rejecting most of the mX, oX and EB to the isomerization reactor. Specifically, the disclosure teaches using a faujasite-based, liquid phase adsorption process that can either be selective for pX or selective for mX and oX. The result of this process is a stream enriched in pX, but still containing a substantial portion of mX and oX. Similarly, U.S. Pat. No. 5,922,924 discloses combining at least one liquid phase, simulated moving bed adsorption zone with crystallization to produce high purity pX. Again, pX is enriched, but the stream still c

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