Chapter 3 Fluid-Bed Studies of Olefin Production from Methanol R. F. Socha1, C. D. Chang1, R. M. Gould2, S. E. Kane2, and A. A. Avidan2
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1Mobil Research and Development Corporation, Princeton, NJ 08540 2Mobil Research and Development Corporation, Paulsboro, NJ 08066
Studies at Mobil Research have shown that light olefins instead of gasoline can be made from methanol by modifying both the ZSM-5-type MTG (Methanol-to-Gasoline) catalyst and the operating conditions.
Work carried out in micro-scale fluidized-
bed reactors show that converted to a mixture 76 wt% C2-C5 olefins. 9% C1-C5 paraffins, of
methanol can be completely of hydrocarbons containing about The remaining hydrocarbons are which the major component is
isobutane, and 15% C6+, half of which is aromatic.
It is likely that future commercialization of Methanolto-Olefins (MTO) will take place in a fluid-bed reactor for many of the same reasons which encouraged fluid-bed MTG development, including better temperature control and constant product composition. The olefins produced by this process can be readily converted to gasoline, distillate and/or aviation fuels by commercially available technologies such as Mobil's MOGD process. Scale-up of MTO has proceeded through bench-scale and 4 BPD pilot units and has been tested in a 100 BPD reactor.
With newly developed technology, conversion of methanol to hydrocarbons represents the final link in the production of premium transportation fuels from coal or natural gas. The methanol-togasoline (MTG) process has been developed. The more readily scaled fixed-bed version is the heart of the New Zealand Gas-to-gasoline complex, which will produce 14,000 BPD high octane gasoline from 120 million SCFD gas. The fluid-bed version of the process, which is also available for commercial license, has a higher thermal efficiency and possesses substantial yield and octane advantages over the fixed-bed. Successful scale-up was completed in 1984 in a 100 BPD semi -works plant near Cologne, West Germany. The project was funded jointly by the U.S. and German governments and an industrial consortium comprised of Mobil; Union Rheinsche Braunkohlen Kraftstoff, AG; and Uhde, GmbH. 0097-6156/87/0328-0034$06.00/0
© 1987 American Chemical Society In Industrial Chemicals via C1 Processes; Fahey, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
3. SOCHA ET AL.
Olefin Production from Methanol
35
The 100 BPD MTG project was extended recently to demonstrate a related fluid bed process for selective conversion of methanol to light olefins (MTO) . The products of the MTO reaction make an excellent feed to the commercially available Mobil Olefins to Gasoline and Distillate process (MOGD) which selectively converts
olefins to premium transportation fuels (I) . A schematic of the
combined processes is shown in Figure 1. Total liquid fuels production is typically greater than 90 wt% of hydrocarbon in the
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feed.
Distillate/gasoline product ratios from the plant can be
adjusted over a wide range to meet seasonal demands. This paper describes the initial scale-up of the MTO process from a micro-fluid-bed reactor (1-10 grams of catalyst) to a large pilot unit (10-25 kilograms of catalyst). Experimental
The catalysts used in this experimental program were variously modified ZSM-5-type catalysts. The ZSM-5 zeolite has a pore structure which allows the formation of olefins or gasoline-range hydrocarbons without the significant formation of heavier polymethylbenzenes . The feed in most cases was pure methanol, although in one of the micro-fluid-bed runs dimethyl ether was used as the feed. The MTO reaction has been investigated in the temperature
range of 400-500°C and the pressure range of 1-6 atmospheres. Figure 2 is a schematic drawing of the micro-fluid-bed reactor used in the study. It was fabricated from vycor and could hold as much as 10 grams of catalyst in an isothermal zone maintained in a vertically-mounted tube furnace. The catalyst bed was supported by a fritted glass disk. Temperature was monitored by means of thermocouple in an axial thermowell. A glass-wool plug in an expanded disengagement section at the top of the apparatus served as a filter to keep catalyst particles from exiting the reactor. Methanol or dimethyl ether feeds were fed from an ISCO positivedisplacement pump. Liquid products were collected in a roomtemperature trap and gaseous products were analyzed by on-line gas chromatography and volumes measured by a wet- test meter. The catalyst could be regenerated in-situ by switching to a nitrogen/air mixture.
This miniature fluid-bed reactor exhibited isothermicity far superior to that attainable in fixed-bed micro-units.
Because of
the highly exothermic nature of the MTO reaction (11.7 kcal/mole methanol, 49 kJ/mole) , in an adiabatic reactor, there is a temperature profile as depicted in Figure 3. In a micro-scale laboratory fixed-bed reactor, where isothermicity is desired, a hotspot developed (unless a very narrow reactor was used, for example
1/8 inch copper tubing in a sand bath). As the catalyst ages in its
typical "band-aging" fashion (2) , the hot-spot moves along the bed,
as is also shown in Figure 3. In the micro-fluid-bed reactor, axial-mixing of the catalyst provides rapid heat transfer rates throughout the dense bed. As a result, a fairly isothermal zone several centimeters in length can be maintained with a single-zone tube furnace.
The fluid-bed pilot-plant is a 10.2 cm ID reactor, 7.6 m high, and is equipped for external catalyst recirculation and regeneration.
In Industrial Chemicals via C1 Processes; Fahey, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
36
INDUSTRIAL CHEMICALS VIA C, PROCESSES
FUEL
FUEL
GAS
METHANOL
-------------- ».
GAS
._. .JLi ,__, iL MTO
—+>
FRACTIONATION ,, AND
-ï-^
M0GD
»¦ FRACTIONATION
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COMPRESSION
EZZ
I
*" LPG
-j*. GASOLINE
J
T
HYDROGEN ______________ *»
Figure 1.
niCTII . ATC DISTILLATE
HYDROTREATER
U,tbtL
Schematic of combined MTO-MOGD process.
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GLASS WOOL FILTER
THERMOWELL^^^; ^^^
DISENGAGEMENT--^^* ZONE
^^|
V )
O
O
O
O
O
O
,~ n f ,
VAPOR PRODUCT TO GC ANALYSIS AND
WET TEST METER
DROPOUT POT
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II LIQUID PRODUCT
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DIESEL
-.
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^ VYCOR CHIPS
DIMENSIONS:
REACTOR: 28 cm high x 1.0 cm ID DISENGAGEMENT ZONE: 10 cm high x 3.0 cm OD THERMOWELL: 0.3 cm OD
CATALYST BED: 10-20 cm high Figure 2.
Schematic of micro fluid-bed reactor.
In Industrial Chemicals via C1 Processes; Fahey, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
3. SOCHA ET AL.
Olefin Production from Methanol
37
It was earlier used for fluid-bed MTG process development and is more fully described elsewhere (3). In the fluid-bed pilot-plant, uniform temperature profiles were obtained, with as little as 1°C AT over the entire 7.6 meter length. Results and Discussion
The reaction scheme of MTO is similar to that which describes
methanol conversion to gasoline (MTG) (4,5) :
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METHANOL ç==fc DIMETHYL ETHER + WATER METHANOL, DME
? OLEFINS + WATER
Cn OLEFINS + METHANOL -? CR+1 OLEFINS + WATER Cn OLEFINS + Cm OLEFINS a OLEFINS
>
> Cn+m OLEFINS
PARAFFINS + AROMATICS
It is necessary, however, to maximize the intermediate olefin
product at the expense of the aromatic/paraffin product which makes up the gasoline (6) . The olefin yield increases with increasing temperature and decreasing pressure and contact time. Judicious selection of process conditions result in high olefin selectivity and complete methanol conversion. The detailed effect of temperature, pressure, space velocity and catalyst silica/alumina
ratio on conversion and selectivity has been reported earlier (6_) .
The distribution of products from a typical MTO experiment is compared to MTG in Figure 4. Propylene is the most abundant species produced at MTO conditions and greatly exceeds its equilibrium value as seen in the table below for 482 °C. It is apparently the product of autocatalytic reaction (7) between ethylene and methanol (8). Olefins
Equil. (wt%)
Actual (wt%)
C2
8.3
8.1
C3
26.8
59.8
C4
39.0
19.2
C5
25.9
12.9
Since it is highly desirable to operate at complete methanol conversion so as to eliminate recycle of unconverted feed, most experimental results were obtained at conditions which gave complete conversion. The severity of reaction, in terms of position along the reaction pathway from methanol to paraffins and aromatics, was followed not by degree of methanol conversion, but by the ratio of propane to propene products. The higher this ratio, the further one is along the reaction path. Figure 5 shows how the olefin yield varies with propane/propene ratio, and also shows the good
In Industrial Chemicals via C1 Processes; Fahey, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
38
INDUSTRIAL CHEMICALS VIA C, PROCESSES
TRULY ADIABATIC FIXED-BED
1 times t
MICRO FIXED-BED (1-10 g catalyst) Downloaded by PENNSYLVANIA STATE UNIV on July 4, 2012 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch003
H
y \y^\/\
I Max. temp.
S ni/ A/\ \ l=19^2y"c}cm
