PATENT
Conversion
Of Oxygenates To Propylene With Selective Hydrogen Treatment Of Heavy Olefin
Recycle Stream
European Patent EP1794103
Inventors:
Kalnes, Tom Uop Llc25 East Algonquin Road N. (P.O. Box 5017, Des Plaines, Il
60017-5017, US)
Wei, Daniel Uop Llc25 East Algonquin Road H. (P.O. Box 5017, Des Plaines, IL
60017-5017, US)
GLOVER, Bryan K.UOP LLC (25 East Algonquin RoadP.O. Box 5017, Des Plaines, IL
60017-5017, US)
Application Number:
EP20050790056
Publication Date:
01/25/2012
Assignee:
UOP LLC
BACKGROUND OF THE INVENTION
The
present invention relates to an improved catalytic oxygenate to propylene (OTP)
process that uses a combination of dual-function catalyst technology and
selective hydrotreating of one or more heavy olefin recycle streams to hold
average cycle catalytic activity and propylene selectivity nearer to the start
of cycle levels during the entire on-stream cycle time, thereby sharply
improving the average propylene yield achieved by the catalyst over its life
cycle relative to that achievable with the same or similar dual-function catalyst
in a heavy olefin recycle system that does not employ the selective
hydrotreating step.
The present invention recognizes that propylene selectivity
achievable over dual-function OTP catalyst systems known in the art operating
in heavy olefin recycle mode is very sensitive to deactivation by coke
deposition and by hydrothermal dealumination (accelerated by exposure to high
temperatures in the presence of steam which temperatures are needed both in the
OTP reaction step and in the OTP catalyst regeneration step to compensate for
the activity loss caused by this excessive coke deposition) and recognition
that coke precursors such as dienes, acetylenic hydrocarbons and other highly
unsaturated hydrocarbons are concentrated in this heavy olefin recycle stream.
Accordingly the propylene selectivity in an OTP catalytic process operating
with a dual-function catalyst and in heavy olefin recycle mode can be
continuously maintained at or near start-of-run levels if selective
hydrogenation treatment technology of the heavy olefin recycle stream is used
to minimize and control detrimental coke deposition on the dual-function OTP
catalyst system.
A major portion of the worldwide petrochemical industry is concerned with the
production of light olefin materials and their subsequent use in the production
of numerous important chemical products via polymerization, oligomerization,
alkylation and the like well-known chemical reactions. Light olefins include
ethylene, propylene and mixtures thereof. The art has long sought a source
other than petroleum for the massive quantities of raw materials that are
needed to supply the demand for these light olefin materials. Oxygenates are
particularly attractive alternate source because they can be produced from such
widely available materials as coal and natural gas. The art of making methanol
and other oxygenates from these types of raw materials is well established. The
prior art has revealed essentially two major techniques that are discussed for
conversion of methanol to light olefins (MTO). The first of these MTO processes
is represented in
US-A-4,387,263.
US-A-4,587,373 discloses the need to operate at a substantial pressure to make
the commercial equipment of reasonable size and the diversion of a portion of
the methanol feed to the DME absorption zone to downsize the scrubbing zone.
To control the amounts of undesired C4+ hydrocarbon products produced by ZSM-5
type of catalyst systems later prior art uses a non-zeolitic molecular sieve
catalytic material.
US-A-5,095,163;
US-A-5,126,308 and
US-A-5,191,141 disclose a metal aluminophosphate (ELAPO) and more specifically
a silicoaluminophosphate molecular sieve (SAPO), with a strong preference for
SAPO-34.
The classical oxygenate to olefin (OTO) technology produces a mixture of light
olefins primarily ethylene and propylene along with various higher boiling
olefins. Although the classical OTO process technology possesses the capability
of shifting the major olefin product recovered therefrom from ethylene to propylene
by various adjustments of conditions maintained in the reaction zone, the art
has long sought an OTP technology that would provide better yields of propylene
relative to the classical OTO technology.
Publication No.
US2003/0139635A1 describes a process for selectively converting methanol to
propylene (MTP) and/or converting DME to propylene utilizing three reactors
containing fixed beds of a pentasil-type (i.e. ZSM-5 type) oxygenate conversion
catalysts in a parallel flow arrangement with respect to the oxygenate feed and
a serial flow arrangement with respect to the effluents of the first reactor
and the second reactor. Rothaemel et al. describe this MTP process in
"Demonstrating the New Methanol to Propylene (MTP) Process"
(presented to the ERTC Petrochemical Conference in March of 2003 at Paris,
France) as having an expected on-stream portion of the process cycle of 500 to
700 hours before in situ regeneration becomes necessary. The conventional
procedure compensates for activity decay in a catalytic operation by increasing
the average reactor temperature to hold conversion in the targeted range of
greater than 94% of the oxygenate charge.
The problem addressed by the present invention is then to modify this OTP
process of the prior art which uses heavy olefin recycle to enhance its average
propylene selectivity over its on-stream cycle time and thereby diminish the
requirement for recycle of olefin products other than propylene to compensate
for lower propylene selectivity. It is recognized that propylene selectivity is
a function not only of reaction conditions but also of average coke level
deposited on the OTP conversion catalyst during the on-stream portion of the
process cycle and that a major source of coke precursors is the heavy olefin recycle
stream since it contains detrimental amounts of highly unsaturated
hydrocarbons. Thus, average propylene selectivity in an OTP process operated
with heavy olefin recycle can be significantly enhanced if the amount of
detrimental carbonaceous deposits laid down on the dual-function catalyst
during the on-stream portion of the process cycle is controlled by selective
hydrogen treatment of at least a portion of the heavy olefin recycle stream to
convert highly unsaturated hydrocarbons, such as dienes and acetylenic
hydrocarbons, contained therein to the corresponding olefin compounds, thereby
eliminating coke precursors. This catalytic hydrotreatment step enables
oxygenate conversion and propylene selectivity to be maintained over the
process cycle near or at essentially start-of-cycle levels. Quite surprisingly
it was found that if selective hydrogen treatment of the heavy olefin recycle
stream is used, then the average propylene selectivity over the process cycle
will be improved by 1.5% to 5.5% or more, relative to the prior art due to the
high level of coke deposition on the catalyst during the latter portion of the
on-stream cycle.
SUMMARY OF THE INVENTION
The primary objective of the present invention is to provide a solution to the
problem of propylene selectivity loss during the on-stream cycle of the prior
art fixed bed OTP process when operated with at least one heavy olefin recycle
stream. A secondary objective is to improve the economics of the OTP process by
controlling coke deposition on the dual-function catalyst to maintain oxygenate
conversion and propylene selectivity at higher levels. Another object of the
present invention is to avoid severe deactivation of the dual-function OTP
catalyst utilized in this OTP process to minimize the severity of the necessary
regeneration step thereby minimizing hydrothermal damage and prolonging
catalyst life. A more general objective combines selective hydrogen treatment
of at least a portion of heavy olefin recycle stream in such an OTP process
with moving bed technology that minimizes attrition losses.
In one embodiment the invention is an OTP process for the selective conversion
of an oxygenate feed to a propylene utilizing dual-function catalyst technology
and selective
hydrogen treatment of a heavy olefin
recycle stream to maintain catalyst performance near or at essentially
start-of-cycle levels, thereby enhancing the average cycle yield of propylene
and minimizing oxygenate breakthrough into the product stream. In the first
step the process contacts an oxygenate feed and a diluent in an amount
corresponding to 0.1:1 to 5:1 moles of diluent per mole of oxygenate with a
dual-function catalyst containing a zeolitic molecular sieve, an ELAPO
molecular sieve, or a mixture thereof, capable of converting at least a portion
of the oxygenate to propylene and interconverting C2 and C4+ olefins to C3
olefins. This OTP conversion step is performed in an OTP reaction zone
containing at least one fixed bed or moving bed reactor which is operated at
oxygenate conversion conditions effective to selectively convert oxygenate to
propylene and to convert any ethylene or heavy olefins recycled thereto to
propylene. An effluent stream is withdrawn from the OTP reaction zone and
contains major amounts of a C3 olefin product and a water by-product, lesser
amounts of a C2 olefin, C4+ olefins, C1 to C4+ saturated hydrocarbons and minor
amounts of unreacted oxygenate, by-product oxygenates, dienes, acetylenic
hydrocarbons and aromatic hydrocarbons. The effluent stream passes to a
separation zone for cooling and separation into a vaporous fraction rich in C3
olefins, a water fraction containing unreacted oxygenate and by-product
oxygenates and a liquid hydrocarbon fraction containing heavier olefins,
heavier saturated hydrocarbons and minor amounts of dienes, acetylenic
hydrocarbons and aromatic hydrocarbons. At least a portion of the water
fraction from the separation step is recycled to the oxygenate conversion step
to provide at least a portion of the diluent used therein. The vaporous
fraction recovered in this separation step is further separated in a second
separating zone into a C2 olefin-rich fraction, a C3 olefin-rich product
fraction and a C4+ olefin-rich fraction containing highly unsaturated
hydrocarbons such as dienes and acetylenic hydrocarbons. The C3 olefin-rich
product fraction is recovered as a principal product stream. At least a portion
of the C4+ olefin-rich fraction is charged to a selective hydrogen treating
step. The selective hydrogen treating step selectively converts highly
unsaturated compounds contained in this C4+ olefin-rich stream into the
corresponding olefin, thereby eliminating coke precursors from the OTP
conversion step. This catalytic hydrotreating step contacts at least a portion
of this C4+ olefin-rich stream and hydrogen with a metal-containing
hydrogenation catalyst at selective hydrogenation conditions effective to
convert highly unsaturated hydrocarbons contained therein to the corresponding
olefin and to produce a selectively hydrotreated C4+ olefin-rich fraction that
is recycled to the OTP conversion step to interconvert these heavier olefinic
materials into additional quantities of the desired propylene product.
A second embodiment involves a process for the selective conversion of an oxygenate
feed to propylene as described in the first embodiment wherein the
dual-function catalyst contains a zeolitic molecular sieve having a structure
corresponding to ZSM-5 or an ELAPO molecular sieve having a structure
corresponding to SAPO-34 or a mixture of these materials.
Another embodiment comprises a process for selective conversion of an oxygenate
feed to propylene as described above in the first embodiment wherein the OTP
reaction zone contains at least 3 moving bed reactors which are connected in a
serial flow or parallel flow configuration with respect to oxygenate feed and
in a serial flow configuration with respect to the stream of catalyst particles
that passes therethrough.
A highly preferred embodiment of the present invention comprises a process for
the selective conversion of an oxygenate feed to propylene as described above
in the first embodiment wherein the oxygenate feed contains methanol or
dimethylether or a mixture thereof. In this embodiment the instant process is
referred to herein as a methanol to propylene embodiment (MTP).
A high propylene yield embodiment of the instant process involves the process
for selective conversion of an oxygenate feed to propylene as described in any
of the previous embodiments wherein the liquid hydrocarbon fraction recovered
in the first separation step is further separated into a second C4+ olefin-rich
fraction and a naphtha product fraction and at least a portion of the resulting
C4+ olefin-rich fraction is charged to the selective hydrogen treatment step
and thereafter the resulting hydrogen-treated product is recycled to the OTP
conversion step to interconvert these heavier olefins into propylene.
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