Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By ring formation from nonring moiety – e.g. – aromatization,...
Reexamination Certificate
1999-10-15
2001-02-13
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By ring formation from nonring moiety, e.g., aromatization,...
C585S319000, C585S324000, C585S330000, C585S639000, C585S640000, C585S408000, C585S407000, C585S418000, C585S469000, C585S648000, C208S069000, C208S070000
Reexamination Certificate
active
06187982
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for converting low value dienes, such as those found in pygas streams from steam cracking, to higher value para-dialkylbenzenes, such as p-xylene, and light olefins such as ethylene and propylene.
BACKGROUND OF THE INVENTION
Steam cracking of ethane to form lower olefins such as ethylene and propylene provides undesirable low value dienes (diolefins) in pygas as well. Diolefins are extremely reactive, unstable, and difficult to process in conventional refinery units. For example, the reactive di-olefins can form gum which plugs conventional hydrotreating beds as well as plugging upstream heat exchangers or heaters. Accordingly, refiners have often had to resort to extreme and expensive measures to process these materials, e.g., high pressure hydrotreating. Catalytic diolefin conversion upstream of conventional hydrotreating is also possible, e.g., UOP, Inc.'s Platfining process, which operates at temperatures low enough to prevent gum formation. U.S. Pat. No. 4,097,367 discloses upgrading of pyrolysis gasoline from steam cracking to make ethylene, by passing naphtha over Pd/Zn/ZSM-5 at 900° to 1200° F. High temperature processing of C5+ fraction converted everything boiling in the BTX range to aromatics, yielding a liquid product with essentially no non-aromatic hydrocarbons boiling above 167° F. U.S. Pat. No. 5,053,579 discloses a process for upgrading unstable olefins, naphthas and dienes, such as coker naphthas, by oligomerizing over shape selective zeolite to gasoline and distillate products. U.S. Pat. No. 4,751,339 discloses a process for upgrading diene-containing olefins to aromatic hydrocarbons by contacting a feedstock of liquid pyrolysis gas byproduct under high severity conditions in a fluidized bed of acidic zeolite catalyst particles, e.g., ZSM-5 in the presence of C3+ alkanes. Aromatics yield is increased by recovering and recycling to the reactor C5+ aliphatic hydrocarbons.
U.S. Pat. No. 4,088,706 describes converting methanol to a mixture of C
2
to C
3
olefins and mononuclear aromatics, particularly p-xylene, by contacting with a shape-selective zeolite having a Constraint Index of 1 to 12 which has been modified by adding oxide of boron or magnesium either alone or in combination or in further combination with oxide of phosphorus. U.S. Pat. Nos. 4,049,573 and 4,088,706 disclose modifying zeolites, such as ZSM-5 with oxides of boron or magnesium, either alone or in combination or in further combination with oxide of phosphorus, increases the yield of p-xylene in the catalytic conversion of methanol to olefins and aromatics. U.S. Pat. No. 4,480,145 discloses increasing ethylene yield in the catalytic conversion of methanol over ZSM-5 by moderating diffusivity of the zeolite by use of large crystal form of the zeolite and by silica “stuffing” of the zeolite pores. U.S. patent application Ser. No. 09/055,478 (Docket No. 10017), filed Apr. 6, 1998, which is a continuation-in-part of U.S patent application Ser. No. 08/725,277, filed Oct. 2, 1996 discloses a catalyst for converting methanol to C
2
to C
4
olefins having a Diffusion Parameter for 2,2-dimethylbutane of 0.1-20 sec
−1
such as phosphorus oxide-containing ZSM-5.
None of the prior art processes describe a way to efficiently convert a mixture of oxygenates and unstable diolefins to high value petrochemicals, e.g., ethylene, propylene, benzene, toluene and xylenes.
It would be desirable to provide a way to efficiently convert unstable by-products of steam cracking to higher value hydrocarbons. It would also be desirable to convert unstable diolefin products to higher value aromatic hydrocarbons such as para-dialkylbenzene products such as p-xylene, as well as to the desired lower olefin products of steam cracking. Inasmuch as steam crackers are built to produce ethylene and propylene, a conversion process for diolefins which provides such low molecular weight monoolefins as necessary by-products would be highly efficient for converting diolefins produced during steam cracking. A steam-cracking facility could thus readily increase its capacity for producing ethylene and propylene by employing a method which would simultaneously upgrade pygas dienes to higher value products such as p-xylene, as well as desired incremental ethylene and propylene.
SUMMARY OF THE INVENTION
The present invention relates to a process for selectively converting a feed comprising C
4
+ dienes and oxygenate to a product comprising increased C
2
+ monoolefins and para-dialkylbenzene levels which comprises
a) contacting said feed under diolefin conversion conditions with a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-100 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa);
b) recovering a diolefins-depleted product stream rich in monoolefins and C
8
+ aromatics, including para-dialkylbenzene; and
c) optionally recovering a C
2
+ monoolefins rich stream from said diolefins depleted product stream to provide a C
8
+ aromatics rich stream.
In another aspect, the present invention relates to a steam cracking process for converting ethane to C
2
+ olefins which comprises
i) contacting an ethane-containing feed with steam under steam cracking conditions to provide a steam cracked product stream comprising C
2
+ olefins and C
4
+ diolefins,
ii) recovering ethylene and propylene from said steam cracked product stream to provide a C
4
+ diolefins rich byproduct stream,
iii) contacting said byproduct stream, or boiling fractions thereof enriched in dienes, in the presence of oxygenate under diolefin conversion conditions with a catalyst in the presence of oxygenate under diolefin conversion conditions with a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-100 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
iv) recovering a diolefins depleted stream rich in C
2
+ mono-olefins and C
8
+ aromatics, including para-xylene, and
v) separating ethylene and propylene from said diolefins depleted stream to provide a C
8
+ aromatics stream rich in para-xylene.
DETAILED DESCRIPTION OF THE INVENTION
Catalysts
The present invention utilizes a catalyst for selectively converting low value pygas, and added oxygenates, preferably C1 to C3 oxygenates, e.g., methanol, dimethyl ether, ethanol, and diethethyl ether, to higher value products such as para-xylene, e.g., p-xylene, and C
2
-C
4
olefins, e.g., ethylene and propylene. The catalyst comprises a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-100 sec
−1
, preferably 0.1-40 sec
−1
and most preferably 3-30 sec
−1
, when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
As used herein, the Diffusion Parameter of a particular porous crystalline material is defined as D/r
2
×10
6
, wherein D is the diffusion coefficient (cm
2
/sec) and r is the crystal radius (cm). The required diffusion parameters can be derived from sorption measurements provided the assumption is made that the plane sheet model describes the diffusion process. Thus for a given sorbate loading Q, the value Q/Q
∞
, where Q
∞
is the equilibrium sorbate loading, is mathematically related to (Dt/r
2
)
½
where t is the time (sec) required to reach the sorbate loading Q. Graphical solutions for the plane sheet model are given by J. Crank in “The Mathematics of Diffusion”, Oxford University Press, Ely House, London, 1967.
The porous crystalline material employed in the process of the invention is preferably a medium-pore size aluminosilicate zeolite. Medium pore zeolites are generally defined as those having a pore size of about 5 to about 7 Angstroms, such that the zeolite freely sorbs molecules such as n-hexane, 3-me
Beck Jeffrey S.
Brown Stephen H.
Weber William A.
Dang Thuan D.
Griffin Walter D.
Mobil Oil Corporation
Roberts Peter W.
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