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V. IRON CATALYSTS
A. Sintered Iron Catalyst - Hot Gas Recycle Process
Since about 1935 the I. G. Farbenindustrie has been experimenting with iron catalysts for the synthesis of hydrocarbons and oxygenated organic compounds from hydrogen and carbon monoxide. A report (14) dated April 18, 1939 concerns the use of a sintered iron catalyst in a hot gas recycle process. An iron powder obtained by the decomposition of iron carbonyl was mixed with 1 per cent of its weight of sodium borate in aqueous solution so as to provide an extrudable paste. The paste was extruded into pieces about 1.5 cms. long. These were then sintered in hydrogen for about four hours at 850°C. The true density of the catalyst was about 7 and the apparent or bulk density about 2.5.
Experiments were done using rapid recycle of the gases through a heat-exchanger and back to the reaction vessel. The gas is passed over the catalyst so rapidly that the temperature does not rise beyond the optimum upper limit. The heat evolved in the synthesis (about 4kgm. cals. per kgm. of product) is absorbed in the external heat exchanger using a 10°C. drop in temperature of the recycle gas. The conversion is about 1 per cent per pass. About 1 per cent of fresh synthesis gas is added per cycle, and an equivalent amount of gas from the reactor is removed and passed through a condensation system and thence to a second stage which is a smaller recycle system (about 1/4 the size of the first stage). In order to minimize power consumption for gas recycling, it is desirable to keep the pressure drop through the catalyst bed not greater than one meter of water. This may be done by limiting the depth of the bed to about 1 meter. The space velocity (vol. of gas per hour) is given in the patent literature as between 5000 and 35000.
In the synthesis products the oxygen appears as water and as carbon dioxide in about equal proportions. It is therefore desirable to operate with a hydrogen to carbon monoxide ratio of about 0.9. Sulfur must be reduced to about 5 milligrams per cubic meter. The operating pressure is 20 atmospheres and the temperature 320° to 350°C. In between the two stages the gases are cooled to 0°C. and carbon dioxide is scrubbed out. The off-gas from the second stage passes in turn to a water-cooler, an ammonia cooler, and to a scrubbing tower (to remove ethylene).
A pilot plant using the above-outlined procedure was operated for several months using 400 liters of catalyst. The yield per liter of catalyst per day was 0.8 to 1.0 kilograms. The conversion in the first stage was 78 percent; in the second stage an additional 13 1/2 per cent was obtained. The off-gas from the second stage contained virtually no carbon monoxide and consisted largely of hydrocarbons and carbon dioxide. The catalyst failed after 2 1/2 months of operation. However, this is not the true catalyst life, for at the end of 2 1/2 months a power failure resulted in overheating of the catalyst with consequent carbon deposition. Upon oxidation with air and subsequent resintering in hydrogen the original activity was restored.
The products contained hydrocarbons of the entire methane series up to high melting paraffins, and a smaller proportion of oxygenated compounds, mainly alcohols. It is characteristic of the process that gasoline is the main product and only little middle oil is obtained. A high degree of unsaturation is characteristic of all of the gasoline and oil fractions. The amount of paraffin wax obtained is about 8 per cent at 300°C. and about 1 per cent at 320°-350°C. Removal of oxygenated compounds from the gasoline is readily accomplished by passage of the vapors over bauxite. A considerable increase in octane number results from the bauxite treatment; after additional treatment with commercial pretreated bleaching earth in the usual way and addition of the naphthol, the stabilized gasoline may be stored, and it fills all motor gasoline specifications.
Grams per N m3 of Synthesis Gas
Percent of Refined Product
Research Octane or Cetane NOS..
|Unrefined Product||Refined Product|
Used for poly-gasoline production Octane No. 97 (Research)
|a/ 85-90% olefines, 60-65% isobutane
b/ 75-85% propylene
c/ Composition in percent: 5 acetaldehyde, 10 acetone, 55 ethanol, 20 propanol, 10 butanol and higher.
The data in Table were virtually identical with a similar table for the hot gas recycle process given in a document (15) dated July 1, 1941. This document mentions some difficulty due to turbulence of the gas at the point of entry to the converter; and also states that precipitated iron catalysts are too friable for use in the hot gas recycle process.
B. I. G. Experiments with Precipitated and Fused
(Synthetic Ammonia-type) Catalysts
Table 2 contains a summary (15) of I. G. Farbenindustrie's tests on various types of iron catalysts and different modes of operation so as to produce varying amounts of alcohols and paraffin wax, in addition to gasoline and Diesel oil. The first column probably refers to the hot gas recycle process described in (a) above. The row labelled "space velocity" refers to the fresh gas feed and hence no data are given on the total gas velocity. The second column refers to the "Synol" process catalyst which may be a synthetic ammonia type catalyst.
Columns 3, 4, and 5 refer to a liquid phase operation in which iron powder prepared from iron carbonyl is mixed with oil and the gas contacted with this liquid suspension. This process yields more Diesel oil than the hot gas recycle process. The cetene number was 60-70. Another advantage of this liquid phase process is the very low methane yield. The gasoline obtained by operation at 310°C. had a research octane number of 90. The converter has a ceramic porous plate for distribution of the gas, and an internal bayonet-type heat exchanger.
Columns 6 and 7 refer to an oil recycle process in which a cooling oil was passed concurrent with the synthesis gas over a synthetic ammonia type catalyst. The cooling was effected by recycling the heated oil through an external heat exchanger. The conditions of Column 6 yield a gasoline of 63-68 octane no. (research?), a diesel oil of 78 cetene number, a wax of 95°C. melting point. The alcohol produced contained 25% methanol, 50% ethanol, and 25% of higher alcohols, acetaldehyde, acetone, etc.
With increasing pressure (experiments were made at 25-100, 150, and 180 atms) the oxygen content of the product increased; at 180atms. large amounts of low boiling and only minor amounts of high boiling alcohols were obtained. To obtain chiefly alcohols, it is necessary to operate at small conversion. Using 1:1, CO:H2, 180 atms. 280°-290°C., with 28-30% conversion, alcohols in per cent of the total liquid yield were as follows: 8.5 CH3OH, 21 C2H5OH, 10 C3H7OH, 6.5 C4 TO C11 alcohols, 2.5 C12 TO C20 alcohols. Hydrocarbons constituted 33.5 per cent of the liquid yield: -26.5 gasoline, 3.5 middle oil, 3.5 boiling over 300°C. Fatty acids comprised 18 percent of the liquid yield, 11 per cent was water soluble, 5% C5 to C11 and 2% C12 to C2O. Alcohols disappeared from the product at high conversions.
In a document (16) dated July 28, 1941 it is stated that using precipitated catalysts at higher pressures (above 50 atmos.) a good yield of alcohol is obtained without noticeable carbonyl formation. The following boiling range of the liquid product is cited:
|Boiling Point||Per cent of Liquid Product||Per Cent of Alcohol in Fraction|
The total liquid yield was 70 grams /N m3 per pass at 200 atms, and space velocity of 500.
With sintered iron catalysts and approximately the same working conditions, chiefly lower alcohols were obtained.
C. Lurgi Laboratory Research on Gas Recycle Processes Using Iron Catalysts (13)
Lurgi has been particularly interested in recycle operation of the Fischer-Tropsch process using a conventional externally cooled intermediate pressure reactor and a relatively low recycle ratio. The development work was done in a pilot unit installed at Hoesch Benzin at Dortmund. This unit consisted one tube of the standard intermediate pressure reactor, which has been described in Section IV F of this report/
Dr. Herbert (research director of Lurgi) stated that the best iron catalyst developed by his laboratory has the following composition:
100 Fe, 25 Cu, 9 Al2O3, 2 K2O, 30 SiO2.
The catalyst is prepared by dissolving the copper and aluminum nitrates in a 10% solution of ferric nitrate in such quantities as will give the specified ratio of metals. The solution is heated to boiling and a 10% solution of sodium carbonate is added rapidly at about 70°C. in quantity required to precipitate the metals as hydroxides. The kieselguhr is then added rapidly, stirred for about 1 minute, and the mixture washed to a pH of 8.0 after which it is washed with a potassium carbonate solution to incorporate the specified quantity of K2O. The product is dried in a centrifuge sufficiently to permit its extrusion and is further dried on a conveyor belt by a blast of hot air to facilitate cutting into desired lengths. Final drying is done at 100°C.
This catalyst is preferably reduced in the synthesis unit with hydrogen at 250-350°C. for 1 to 4 hours. The rate of hydrogen flow is one cubic meter per kilogram of catalyst per hour. Reduction with synthesis gas is possible. Synthesis is started at about 180°C. and the temperature is raised from 220°C. to 230°C. Such a catalyst has not been run to exhaustion by Lurgi, but it is believed that its life would be about one year. Other conditions of the operation and yields are as follows:
- Space Velocities (vols. gas/vol. catalyst/hr.):
- 1st Stage, Fresh Gas 100, Recycle Gas 250 Wax, oil, and gasoline removed before recycling.
- 2nd Stage (Size not given) off-gas 48 vols.100 vols. of fresh gas to Stage 1.
- 20 atmospheres
CO2 CO H2 CH4 N2 CnHm Fresh gas 5.8 37.6 48.1 0.1 8.4 0.0 Total Feed gas 22.9 26.9 31.8 2.0 15.0 1.4 Tail gas 28.2 22.6 27.0 2.7 17.6 1.9
Yield in Grams per N m3 from 2 Stage Operation: Hard Wax 66 Gasoline 29b/ Paraffin 22 Alcohols 9 Oil 29 a/ Gasol (C3+C4) c/ 15 C1+C2 22 a/ 45%olefins b/ 60% olefins c/ 60% olefins
Total yield of liquid products + gasol = 170 gms/N m3
An iron catalyst which yields a higher proportion of gasoline in the product is made by impregnating Luxmasse with a solution of copper ammonium nitrate so as to obtain 3% by weight of metallic copper in the finished catalyst. The resulting catalyst is not highly active and requires a high operating temperature, but at this temperature it gives a relatively low boiling product with less catalyst deterioration due to carbon deposition than would occur with a more active catalyst. The operating temperature is 275°C., and pressure 20 atms. The spaced velocity of fresh gas is 100, of recycle gas is 300; and the tail gas volume is 50.5 per cent of the fresh gas volume. Wax, oil, and gasoline are removed before recycling. The gas composition and yield and product distribution follow:
|Total Feed gas||48.0||16.7||11.3||5.3||16.5||2.2|
|Yield in Grams per NM3 (single stage operation)|
|Oil||20a/||Gasol (C3+C4) c/||32|
a/ olefin content 60%
b/ olefin content 75%
c/ olefin content 70%
Total yield of liquid products + gasol 135 gms/N m3