Note: Descriptions are shown in the official language in which they were submitted.
8~436
F-3957
MULTISTAGE PROCESS FOR CONVERTING
OXYOENATES TO LIQUID HYDROCARBONS AND ETHENE
This invention relates to an integrated system for converting
oxygenates, such as methanol or dimethyl ether (DME), to liquid
hydrocarbons and ethene. In particular it provides a continuous
process for producing hydrocarbon products by converting the
oxygenate feedstock catalytically to an intermediate lower olefinic
stream, separating the ethene and oligomerizing the remalning
olefins to produce distillate and gasoline.
In order to provide an adequate supply of liquid hydrocarbons
for use as synfuels or chemical feedstocks, various processes have
been developed for converting coal and natural gas to gasoline,
distillate and lubricants. A substantial body of technology has
grown to provide oxygenated intermediates, especially methanol.
Large scale plants can convert methanol or similar aliphatic
oxygenates to liquid fuels, especially gasoline. However, the
demand for heavier hydrocarbons has led to the development of
processes for increasing yield of diesel fuel by a multi-stage
technique.
Recent developments in zeolite catalysts and hydrocarbon
conversion processes have created interest in utilizing olefinic
feedstocks for producing C5+ gasoline, diesel fuel, etc. In
addition to the basic work derived from ZSM-5 type zeolite
catalysts, a number of discoveries have contributed to the
development of a new industrial process, known as Mobil Olefins to
Gasoline/Distillate ("MOGD"). This process has significance as a
safe, environmentally acceptable technique for utilizing feedstocks
that contain lower olefins, especially C2-C5 alkenes. This
process may supplant conventional alkylation units. In U.S. Patents
3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose
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conversion of C2-C5 olefins, alone or in admixture with
paraffinic components, into higher hydrocarbons over crystalline
zeolites having controlled acidity. Garwood et al have also
contributed improved processing techniques to the MOGD system, as in
U.S. Patents 4,150,062, 4,211,640 and 4,227,992.
Conversion of lower olefins, especially propene and butenes,
over HZSM-5 is effective at moderately elevated temperatures and
pressures. The conversion products are sought as liquid fuels,
especially the C5+ aliphatic and aromatic hydrocarbons.
Olefinic gasoline is produced in good yield by the MOGD process and
may be recovered as a product or recycled to the oligomerization
reactor system for further conversion to distillate-range products.
Operating details for typical MOGD units are disclosed in U.S.
Patents 4,445,031, 4,456,779 (Owen et al) and 4,433,185 (Tabak).
In addition to their use as shape selective oligomerization
catalysts, the medium pore ZSM-5 type catalysts are useful for
converting methanol and other lower aliphatic alcohols or
corresponding ethers to olefins. Particular interest has been
directed to a catalytic process ("MTO" Methanol to Olefins) for
converting low cost methanol to valuable hydrocarbons rich in ethene
and C3' alkenes. Various processes are described in U.S.
Patents 3,894,107 (Butter et al), 3,928,483 (Chang et al), 4,025,571
(Lago), 4,423,274 (Daviduk et al) and 4,547,616 (Avidan et al). It
is known that the MTO process can be optimized by fluidized bed
catalysis to produce a ma~or fraction of C2-C4 olefins
economically.
It has been found that ethene production can be increased by
supplementing a continuous fluidized bed unit with a smaller
capacity fixed bed unit. In U.S. Patent 4,506,106 (Hsia et al),
there is disclosed a method for increasing the ethene production by
employing a fixed bed reactor operated within a specified range of
conditions. The ethene may be recovered in an interstage sorption
unit as a valuable chemical feedstock, while the remaining olefins
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are oligomerized to predominantly C10-C20 liquid hydrocarbons.
Oligomerization process conditlons tend to convert only a small
portion of ethene as compared to C3+ olefins.
It has been discovered that methanol, dimethyl ether (DME) or
the like may be converted to liquid fuels, particularly distillate,
in a multi-stage continuous process, with integration between the
major process units to provide an increase in ethene production on
demand. The additional ethene is obtained from an
intermittently-operated auxiliary MTO conversion unit. The major
MTO conversion unit is operated in substantially continuous use
under steady state conditions. Process conditions in the auxiliary
unit allow for a significantly higher percentage of ethene in the
C2-C4 olefin fraction from the oxygenate conversion reaction.
When both units are on stream, the ethene production is maximized.
However, a fixed bed oxygenate conversion unit requires maintenance
shutdown for in situ catalyst regeneration offstream. This is
usually a planned periodic interruption. The primary MTO process
hydrocarbon effluent stream, after water and an ethene rich stream
and C5+ liquid hydrocarbons are recovered, can be fed to the
MOGD stage for conversion to heavier hydrocarbons. Part of the
oligomerization stage gasoline rich product may be recycled to the
sorption fractionation unit.
In a preferred embodiment, the invention provides methods for an
integrated continuous technique for converting oxygenated organic
feedstock to liquid hydrocarbons and ethene comprising means for
reacting a major portion of feedstock comprising one or more lower
aliphatic organic compounds in a primary conversion unit comprising
a low pressure fluidized bed reactor containing zeolite oxygenate
conversion catalyst to dehydrate and convert at least a portion of
the feedstock to hydrocarbons containing a major fraction of
C2-C4 olefins having less than 10~ by weight ethene and a minor
fraction of C5+ heavy hydrocarbons; intermittently reactinq a
minor portion of the lower aliphatic oxygenate feedstock in an
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auxiliary conversion unit comprising a lower pressure fixed bed
reactor containing an acidic zeolite catalyst to dehydrate and
convert at least a portion of the feedstock to hydrocarbons
containing a major fraction of C2-C4 olefins having at least 15%
by weight ethene and a minor fraction of C5+ heavy hydrocarbons,
thereby increasing ethene production; cooling and separating the
effluent from the primary conversion unit to provide an aqueous
liquid stream, a heavy hydrocarbon liquid stream, and recovering a
primary light hydrocarbon vapor stream rich in C2-C4 olefins;
cooling and separating the effluent from the auxiliary conversion
unit to provide an aqueous liquid stream, a heavy hydrocarbon liquid
stream, and recovering an auxiliary light hydrocarbon vapor stream
rich in ethene; combining and fractionating the primary and
auxiliary light hydrocarbon vapor streams by compressing and
selectively sorbing C3+ hydrocarbons in a gasoline sorbent stream
to recover an ethene-rich vapor stream and a liquid stream rich in
C3+ sorbate; and contacting the sorbate-rich stream in a reaction
zone with a shape selective medium pore zeolite oligomerization
catalyst at elevated temperature and pressure to convert olefins to
an oligomerization effluent stream comprising olefinic gasoline and
distillate range liquids. The oligomerization effluent stream is
separated to obtain a distillate fraction, a gasoline fraction, and
a lighter hydrocarbon fraction.
Advantageously, the oxygenate conversion and the oligomerization
catalysts comprise acidic ZSM-5 type zeolites and at least a portion
of the gasoline fraction is employed as the sorbent stream.
Other ob~ects and features of the invention will be seen in the
following description and drawings. In the drawings:
FIG. 1 is a process flow sheet showing the ma~or unit operations
and process streams; and
Figure 2 is a preferred embodiment of an integrated olefins
upgrading process.
?
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Numerous oxygenated organic compounds may be contained in the
feedstock material to be converted in the ma~or and auxiliary MT0
units. Since methanol or its ether derivative tDME) are industrial
commodities available from synthesis gas or the like, these
materials are utilized in the description herein as preferred
starting materials. It is understood by those skilled in the art
that MTû-type processes can employ methanol, dimethyl ether and
mixtures thereof, as well as other aliphatic alcohols, ethers,
ketones and/or aldehydes. It is known in the art to partially
convert oxygenates by dehydration, as in the catalytic reaction of
methanol over gamma-alumina to produce DME intermediate. Typically,
an equilibrium mixture (2CH ~H ~~' CH30CH3 ~ H20) is produced
by partial dehydration. This reaction takes place in either
conversion of methanol to lower olefins (MT0) or methanol to
gasoline (MTG).
Catalyst versatility permits the same zeolite to be used in both
the primary conversion stage (MT0) ~nd secondary oligomerization
stage (MOGD). While it is within the inventive concept to employ
substantially different catalysts in these stages, it is
advantageous to employ a standard ZSM-5 having a silica alumina
molar ratio of 70~
The oligomerization catalysts preferred for use herein include
the crystalline aluminosilicate zeolites having a silica to alumina
ratio of at least 12, a constraint index of about 1 to 12 and acid
cracking activity of about 160-200. Representative of the ZSM-5
type zeolites are ZSM-5, ZSM-ll, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and
ZSM-38. ZSM-5 is disclosed and claimed in U.S. Patent No. 3,702,886
and U.S. Patent No. Re. 29,948; ZSM-ll is disclosed and claimed in
U.S. Patent 3,709,979. Also, see U.S. Patent No. 4,076,979. Also,
see U.S. Patent No. 3,832,449 for ZSM-12; U.S. Patent No. 4,076,842
for ZSM-23; U.S. Patent No. 4,016,245 for ZSM-35 and U.S. Patent No.
4,046,839 for ZSM-38.
A suitable catalyst for fixed bed operation is HZSM-5 zeolite
with 35 wt.% alumina binder in the form of cylindrical extrudates
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of about 1-5mm. These medium pore shape selective catalysts are
sometimes known as porotectosilicates or "pentasil" catalysts.
Other catalysts and processes suitable for converting
methanol/DME to lower olefins are disclosed in U.S. Patent 4,393,265
(sonifaz)~ u.s. Patent 4,387,263 (vogt et al.) and European Patent
Application 0081683 (Marosi et al.) published April 17, 1982, and
ZSM-45. In addition to the preferred aluminosilicates, the boro-
silicate, ferrosilicate and "silicalite" materials may be employed.
ZSM-5 type catalysts are particularly advantageous because the SamR
material may be employed for dehydration of methanol to ~ME,
oonversion to lower olefins and oligomerization.
In this description, metric units and parts by weight are
employed unless otherwise stated.
Referring to FIG. 1, a ma~or portion of feedstock (methanol or
DME, for instance) is fed to the primary conversion unit where it is
converted to lower olefins and gasoline hydrocarbon plus water by
dehydration of the oxygenated feedstock. Oyproduct water is
recovered by simple phase separation from the cooled effluent.
Liquid hydrocarbons consisting essentially of C5+ gasoline
range materials is recovered by fractionation. At least a portion
of the vapor phase effluent from the primary conversion unit is
compressed and heated along with oligomerization stage recycle
gasoline sorbent and throughput liquids to oligomerization reaction
temperature, and the combined olefinic stream is reacted at high
pressure and elevated temperature over the shape selective medium
pore zeolite catalyst. Effluent is then separated into light gases,
C5+ gasoline, at least a portion of which can be recycled to
the absorption zone, and distillate range products. The distillate
stream comprises a ma~or fraction of C10-C20 high boiling
aliphatics and may contain a minor amount of aromatics.
A minor portion of the oxygenate feedstock is converted to a
predominantly ethene-rich lower olefins product in a low pressure
fixed bed auxiliary reaction zone to increase ethene production on
demand. About 25 to 90% of the methanol feedstock is converted in
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the auxiliary unit per reactor pass. Diluent water may be cofed
with methanol and/or dimethyl ether in a molar ratio of about 0.1:1
to 5:1, based on methanol equivalents.
The ethene-rich lower olefins from the auxiliary reactor zone
are recovered as a low pressure gas and combined with the primary
MT0 unit olefinic gas stream for recovery of increased ethene and
upgrading C3+ olefins by oligomerization. The C5+
hydrocarbons from the auxiliary MT0 unit is combined with the
C5+ hydrocarbons from the primary unit and compression liquid
from both units. The combined streams are fractionated to separate
a C5+ product gasoline, Cg+ product distillate and a
C4- stream for oligomerization.
In the process for catalytic conversion of olefins to heavier
hydrocarbons by catalytic oligomerization using an acid crystalline
zeolite, such as ZSM-5 type catalyst, process conditions can be
varied to favor the formation of either gasoline or distillate range
products. At moderate temperature and relatively high pressure, the
conversion conditions favor distillate range product having a normal
boiling point of at least 154C (310F). Lower olefinic feedstocks
containing C2-C6 alkenes may be converted selectively; however,
the distillate mode conditions do not convert a ma30r fraction of
ethylene. While propene, butene-l and others may be converted to
the extent of 50 to 99% in the distillate mode, only about 10 to 50%
of the ethene component will be converted.
When the catalytic oligomerization of lower olefins is conducted
at moderate temperature and relatively high pressure, i.e.,
conditions which favor distillate range products, the ethene is
preferably recovered prior to the oligomerization reactor since it
is unreactive as compared to the propenes and other lower olefins.
The integrated system of a primary fluidized bed MT0 conversion
unit and an auxiliary fixed bed MT0 conversion unit is depicted in
Fig. 2. The oxygenate feedstock, which is preferably a mixture of
methanol and dimethylether from an acid catalyzed dehydration
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reaction, is passed via conduit 10 to a valve 11, where a major
portion of the feedstock is conducted through line 20 and is passed
through process heat exchange unit 22 and then furnace 24 to achieve
the temperature for catalytic conversion in fluidized bed reactor 25.
The reactor effluent is sequentially compressed and cooled by
passing through heat exchange unit 40. Cooled effluent enters phase
separator 102 to provide a vapor stream 102V, rich in C2-C4
olefins, a liquid hydrocarbons stream 102L, and by-product water
stream 102W. The liquid (eg-C5 ) stream 102L is combined with
a corresponding liquid HC from succeeding separators and sent to
fractionation unit 60. The vapor stream 102V is polytropically
compressed by multi-stage compressor set 105C, cooled via exchanger
106 and passed to a succeeding separator 104A, at which point the
preceeding phase separation technique is repeated. ~ikewise other
separators 1048 and 104C operate to provide an ethene-rich stream
104V, which is passed to ethylene recovery unit 118. As is
understood by one skilled in the art, ethene can be treated in a
cryogenic plant cold box, de-ethanizer tower, absorption unit or the
like to remove undesirable components prior to recycle 114 and/or
recovery 112. A suitable selective sorption unit is disclosed in
U.S. Patent 4,471,147 (Hsia et al). Preferably, compressed light
hydrocarbons are fractionated to recover a recycle stream containing
at least 90 mole percent ethene. This can be achieved by
selectively absorbing C3+ components in a C5+ liquid
hydrocarbon sorbent stream and then recovering the ethylene in a
cryogenic unit. Advantageously, the MTO effluent is received at
about atmospheric pressure (eg, 100-150 kPa) and compressed in
plural stages to an intermediate pressure of about 1100-3500 kPa
(150-400 psig) and separated in the final vessel 104C at about
ambient temperature (20-60C).
On demand, a minor portion of the lower oxygenate feedstock is
fed through line 30 and is passed through process heat exchange unit
32 and furnace 34 to achieve the temperature for catalytic
conversion in fixed bed reactor 36. The auxiliary conversion unit
is operated in the temperature range of about 260C to 425C, in the
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pressure range of about 170 to 800 kPa, and in the weight hourly
space velocity range of about 0.5 to 1.0 based on zeolite equivalent
catalyst and methanol equivalent in the feedstock.
The reactor effluent is cooled by passing through heat exchange
unit 50. Effluent then undergoes phase separation in separator unit
52 to provide a vapor stream 52V, rich in C2-C4 olefins and
containing at least 15% by weight ethene, a liquid hydrocarbon
stream 52L, and by-product water stream 52W. The vapor stream 52V
is passed to the third stage of the primary MTO unit compressor 106,
and then to phase separator 104C where the amount of ethene which
passes through line 104V and to ethylene unit 118 is substantially
increased. Hydrocarbon stream 52L is combined with stream 102L and
fractionated in unit 60 to provide C5' gasoline 609, Cg+
distillate 60d, and C4 olefins 60p.
The combined processes are an effective means for converting
oxygenated organic compounds, such as methanol, DME, lower aliphatic
ketones, aldehydes, esters, etc., to valuable hydrocarbon products
and ethene. Thermal integration is achieved by employing heat
exchangers between various process streams, towers, absorbers, etc.
In a preferred embodiment, a crude methanol feed is processed in
an integrated MTO-MOGD system with continuous primary fluidized bed
and auxiliary fixed bed MTû reactors. The auxiliary fixed bed
reactor processes about 25-30% of the methanolic feed on demand.
Operation of this system results in a doubling of the net ethylene
recovery, as compared to an integrated MTO-MOGD system with a single
fluidized bed MTO unit. The yields of pressurized gasoline and
blended distillate are similar for both systems. The following
table compares the net product yield for these integrated plants.
Table I gives a comparison of the standard and the improved
MTO-MOGD integrated systems.
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TABLE I
MTO-MOGD- MTO-MOGD-
ALKYLATION UNIT ALKYLATION UNIT
INTEGRATION INTEG MTION WITH
"AUXILIARY" MTO
_EACTOR
Unpressurized Gasoline 8595 7857
(B/SD), barrels per day
Butanes for Pressurizing 348 585
(B/SD)
Blended Distillate (B/SD)8939 8449
Ethylene (thousand metric43.9 85.9
tonnes/yr).
(delivered after purifica-
tion in cryoaenic unit with
98% recovery~
A further advantage may be achieved in the manufacture of
valuable byproducts, particularly durene. The auxiliary MTO unit 36
produces alkyl aromatics, which may be recovered by fractionating
liquid stream 52L, which may contain more than 25 wt% durene,
recoverable from other C8 - C10 aromatics by crystallization.
Various modifications can be made to the system, especially in
the choice of equipment and non-critical processing steps. While
the invention has been described by specific examples, there is no
intent to limit the inventive concept as set forth in the following
claims.
