Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By contact with solid sorbent
Reexamination Certificate
2000-03-31
2001-08-28
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By contact with solid sorbent
C585S828000, C585S831000
Reexamination Certificate
active
06281406
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the separation of xylene isomers and ethylbenzene comprising the use of an adsorption process for paraxylene recovery and purification, to produce high purity paraxylene.
BACKGROUND OF THE INVENTION
Paraxylene is a valuable petrochemical precursor for the production of terephthalate polymers such as polyethylene terephthalate. Polyethylene terephthalate is the polymer in several large scale end uses including polyester fibers and PET bottle resin. The large market for paraxylene for fiber and PET resin creates a need for large scale production and purification facilities for paraxylene. Paraxylene (PX) is typically isolated in refineries from the reformate produced by catalytic reformers although it can come other sources such as pygas. Catalytic reformers produce xylene molecules by the dehydrocyclization of straight and branched chain paraffins, the dehydrogenation of naphthenes, and to a small extent the dealkylation of polyalkylaromatics. The C
8
aromatic portion of the reformate contains more than just paraxylene. The other aromatic isomers present in C
8
boiling range reformate are orthoxylene (OX), metaxylene (MX), and ethylbenzene (EB). These isomers are difficult or impossible to separate from the paraxylene by distillation because they have close boiling points.
Two methods that have been developed to isolate the paraxylene (PX) from the other C
8
isomers are crystallization and adsorption. Continuous simulated moving bed adsorption is currently used in most grass roots PX separation plants. Examples of such PX adsorption processes include Parex (UOP) and Eluxyl (IFP). These processes utilize an X or Y zeolite adsorbent in combination with either toluene or para-diethylbenzene as a desorbent. Selection of a desorbent is an important consideration and has significant impact on the design and economics of the process. In general the desorbent should be adsorbed about as well as the feed components on the selected adsorbent. If the desorbent is adsorbed too tightly to the adsorbent, an excessive fraction of the total capacity of the adsorbent is occupied by the desorbent. This effectively reduces the portion of the adsorbent that is available to adsorb and separate the C
8
aromatic isomers, and increases the amount of adsorbent that must be used. If the desorbent is not adsorbed tightly enough large amounts of desorbent must be used causing a high desorbent to feed ratio. A high ratio of desorbent to feed results in the need for larger extract and raffinate distillation columns. The extract column separates the PX product from the desorbent. In a PX process the raffinate column separates the desorbent from the other C
8
aromatic isomers. These distillation columns are a major part of the expense of a PX separation process. They are both a large portion of the capital cost of the process and cause a large portion of the operating cost of the PX separation plant. Thus it is generally best to minimize the amount of desorbent used in the process. Another important parameter of the desorbent is its boiling point. Since the desorbent is separated from the raffinate and the extract by distillation it is desirable to select a desorbent with a boiling point that is substantially different from the extract and raffinate boiling ranges. The closer the desorbent boiling point to the PX extract or C
8
aromatic raffinate boiling range, the larger the column required and the higher the utility costs to operate the column. Also it is important to select a desorbent that does not form an azeotrope with any of the feed components in order to have a clean separation.
As discussed above adsorbents used for commercial PX separation from mixed xylenes typically comprise X or Y zeolites. X or Y zeolites are typically used because they have a relatively large adsorption capacity, are commercially available on a large scale, and can be formulated to adsorb PX relatively stronger than all three other C
8
aromatic isomers, EB, OX, and MX as exhibited by the selectivity. To a great extent the properties desirable for a good adsorbent are a compromise and require the careful balancing of the properties of the adsorbent or the adsorbent/desorbent pair. For example the PX selectivity of X or Y zeolites relative to the other C
8
aromatic isomers is only satisfactory.
Prior Patents that discuss the preparation and use of some of the preferred adsorbents useful in the present invention include U.S. Pat. No. 4,826,667 (SSZ-25), U.S. Pat. No. 4,439,409, and U.S. Pat. No. 4,954,325. The ′667 patent to Zones et al. discusses preparation of SSZ-25 using an adamantane based template. The ′667 patent also discloses on column 13 line 6 that “SSZ-25 can also be used as an adsorbent. . . .” However, the ′667 does not provide any elaboration or disclosure as to the use of SSZ-25 as an adsorbent.
An adsorbent having improved PX selectivity relative to the other C
8
aromatic isomers can have substantial economic advantages over prior art processes The process of the present invention provides just such advantages.
SUMMARY OF THE INVENTION
The present invention relates to an adsorptive process for the separation and purification of paraxylene from a hydrocarbon feed comprising a mixture of C
8
aromatic hydrocarbons comprising:
a) contacting the feed, at adsorption conditions with an adsorbent bed comprising an adsorbent selected from the group consisting of SSZ-25, MCM-22, PSH-3, ERB- 1, and ITQ- 1 thereby adsorbing paraxylene from the feed;
b) contacting the adsorbent bed at desorption conditions with a desorbent material comprising benzene;
c) withdrawing a stream comprising desorbent and less selectively adsorbed components of the feed from the adsorbent bed;
d) withdrawing from the adsorbent bed a stream comprising desorbent and said paraxylene;
e) separating the stream comprising desorbent and less selectively adsorbed components into a second stream comprising desorbent and a stream comprising less selectively adsorbed components;
f) separating the stream comprising desorbent and said paraxylene into a third stream comprising desorbent and a stream comprising paraxylene.
An alternative embodiment of the present invention employs a two stage process for the production of paraxylene from a feed comprising C
8
aromatic isomers, comprising:
an adsorption step, using an adsorbent selected from the group consisting of SSZ-25, MCM-22, PSH-3, ERB-1, and ITQ-1 to form a stream concentrated in paraxylene from the feed comprising C
8
aromatic isomers; and
a crystallization step to purify the stream concentrated in paraxylene to obtain high purity paraxylene product suitable for use in the production of terephthalic acid or dimethylterephthalate.
Another embodiment of the present invention comprises a process for the production of high purity paraxylene from a feed comprising paraxylene and at least one other C
8
aromatic hydrocarbon, comprising:
a) adsorption of the paraxylene from the feed on an adsorbent selected from the group consisting of SSZ-25, MCM-22, PSH-3, ERB-1, and ITQ-1 at adsorption conditions;
b) desorption of the paraxylene from the adsorbent using a desorbent to form a stream comprising desorbent and paraxylene; and
c) separation of the stream comprising desorbent and paraxylene to form a stream comprising paraxylene and a stream comprising desorbent
Among other factors the present invention is based on the finding that an adsorbent comprising SSZ-25 zeolite or its isostructural equivalents MCM-22, PSH-3, ERB- 1, and ITQ- 1 is particularly useful in an adsorption process for the separation and purification of paraxylene from a feed comprising xylenes and ethylbenzene. In a particularly preferred embodiment the zeolite adsorbent has at least a portion of exchangeable cationic sites exchanged with Cesium. In another particularly preferred embodiment of the present invention, the desorbent used in the present invention comprises benzene. Use of benzene desorbent results in a much greater boiling point difference between xylenes
Chevron Corporation
Griffin Walter D.
Nguyen Tam M.
Stewart Charles W.
Tuck David M.
LandOfFree
Adsorption process for paraxylene purifacation using Cs... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Adsorption process for paraxylene purifacation using Cs..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Adsorption process for paraxylene purifacation using Cs... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2467259