SECTION I(b)
(7) I.G. Farben Leuna.
Contrary to I.G.’s work on cobalt catalysts there were not many detailed catalyst studies made on iron catalyst. The effect of variations in operating conditions as studied on a number of Fe catalysts particularly the regular ammonia catalyst used in this plant. The influence of the temperature of reduction was studied on a fused iron ammonia type catalyst. The operating conditions in each case were identical, using watergas as feed.
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Reductions carried out with hydrogen at 1 atm. And 100 V/H/V. |
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Temperature of Reduction |
Length of Reduction |
Yield |
|
850° C |
16 hours |
63 g/m3 ideal gas |
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500° |
10 days |
79 g/m3 ideal gas |
|
450° |
11 days |
102 g/m3 ideal gas |
|
420° |
4 days |
105 g/m3 ideal gas |
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400° |
10 days |
105 g/m3 ideal gas |
At lower reduction temperatures the activity rises, with a special sharp increase near 500° C. The extent of reduction (calculated from the water formation) is apparently only a secondary influence. If the H2 is carried out at elevated pressure, the reduction can be completed even at low temperatures (200 atm. H2, 300° C, 40-60 hrs).
In accordance with all other observers, it was found that increased space velocity and higher temperatures in the synthesis increased the yield of lower boiling products. The reduction temperature, however, influenced the spectrum of the product differently, i.e., low reduction temperature gave low boiling products (all other conditions being equal). It appears that the reduction at lower temperature increases the hydrogenation capacity of the catalyst, which in turn increases the production of low boiling products.
The effect of space velocity was studied using a 15mm. Diameter tubular reactor. Space velocities (VHV) from 100 to 600 were used with 400 V/H/V determined as the practical limit. At higher velocities the catalyst had tendencies to coke up (by CO decomposition) due to local overheating. By taking good care, 500-600 V/H/V were reached for limited intervals. The actual overheating of the catalyst due to the high load is considered the cause of the increase in light products. With higher temperature and higher space velocity, somewhat more branched hydrocarbons were produced but the effect was not very pronounced.
The kinetics of the FT synthesis were formulated by I.G. chemists as follows:
| (a) | CO+2H2→CH2+H2O Formation of methylene By way of carbine formation and hydrogenation of the carbide. | |
| (b) | (1) | n CH2→(CH2 n) polymerization of methylene |
| (2) | CH2+H2→ CH4 Hydrogenation of methylene | |
| (c) | (1) | (CH2)n+H2→Cn H(2n+2) Ending of polymerization due to hydrogenation of olefine. |
| (2) |
CH3(H2)n-3
CH=CH2→CH3)CH2) |
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Ending of polymerization due to shifting of double bond to middle of molecule. |
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It is obvious that the composition of the final product depends on the relative velocities of these reactions and thus on the relative rates of change of these velocities with change in temperature. It should also be noted that iron catalysts have apparently a good activity corresponding to reaction (c)(2).
Another reaction, not shown here but of interest would be the isomerization to branched chain Hydrocarbons. The ability to catalyze this reaction is unfortunately not too good.
It was found that structurally there is little difference between gasoline synthesized over iron and over cobalt. The first gave much more olefins, but upon hydrogenation both gasolines showed the same octane number.
It is still possible to find two “Fe gasolines” with the same olefine content which differ in octane rating due to shifting of the double bond. It may be possible to develop a better control of these different reactions and thus produce a product of substantially only one type of hydrocarbon. Lastly there is still another reaction which is apparently the basis for the Synol process, i.e.; the saturation of the terminal double bond with H2O to from an alcohol. The process is described in Section II.
Carbon deposit is a well known occurrence over Fe catalyst. It is due to the fact the Fe catalyzes the reactions.
I.G. checked this by determining the ratio Fe/Fe3O4 in a new and coked Fe catalyst. The ratio had shifted toward an increase in Fe3O4. In X-ray photographs the carbon did not appear indicating amorphous carbon. The newly formed Fe3O4 is very finely divided compared with the large crystals of Fe3O4 in the fresh catalyst.
CH=CH(CH2)