SECTION I. (b)
(4) Rheinpreussen.
The work done by this organization was somewhat similar to that done by KWI. Here too exact studies of the carbide formation were carried out, and a catalyst based on a cheap raw material for use in liquid phase operation was developed.
The reason why iron catalysts do not lend themselves to operation at low pressure is explained by Dr. Koebel (Rheinpreussen) as follows:
The Synthesis of hydrocarbons over elements of the 8th Group is based in part on the competing reactions of carbide formation and carbide hydrogenation. It seems that in the case of iron at atmospheric pressure the carbide formation is faster than the hydrogenation. At least a minimum partial pressure of hydrogen is apparently required the hydrogenate the carbides as they are formed and thus keep the active points of the catalyst free for further carbide formation. If the H2 partial pressure is below the minimum, the catalyst soon becomes “carbided” and loses its activity. This limiting pressure seems to be H2=0.5 atm. At one atmosphere starting with water gas and at CO conversion of 60%, the PH2 in the tail-gas is below this figure. At 10 atm. Operation, however under equal conditions, the PH2 in the tail-gas is 3 atm.
It follows that the ability to form carbides does not increase with pressure at the same rate as the ability to hydrogenate. This drawback may, however, be overcome by a treatment of the catalyst, consisting in a formation of carbide at normal pressure, before starting operation at elevated pressure. This procedure was recommended by both Rheinprussen and the KWI group who had reached the same conclusion independently from each other.
The carbide is formed during the synthesis, if no special “forming” precedes it, but the formation is slow and may not lead to the same carbide since it is formed under different conditions.
Fe3C is the more desirable catalyst. As the catalyst ages it changes. If the Fe3C content could be kept up, the activity would remain indefinitely. It may be possible that higher carbides such as Fe2C are even more desirable but they are difficult to prepare and are unstable.
There is no better means to determine the carbide in a catalyst than magnetic measurements although hydrogenation could be used to determine the “C in Carbide.”
Rheinpreussen also studied the effect of alkali on the formation of carbide. Pure Fe2O3 was treated with CO to yield carbide, and gave a certain “C in Carbide” value. Upon addition of 1% K2CO3, the “C in C” increased by 30% but upon further addition (i.e., 10% K2CO3), the “C in C” decreased to 25% below the value of pure alkali free Fe2O3. The effect of alkali on catalyst activity is in proportion to these figures.
Copper is frequently added to Fe catalysts. Copper was also found to slightly increase the carbide formation. This is explained by the fact that Cu increases the rate of reduction of Fe2O3 to Fe which must precede the carbide formation. This could be interpreted as a confirmation of a statement by Lurgi that copper does not seem to have a catalytic effect in itself but helps in reproducing a catalyst of constant activity. 1% Cu based on Fe is sufficient for this purpose.
Another catalyst was developed by Rheinpreussen using Luxmasse as raw material and adding 0.15-5.0% K2CO3 and 0=3.0% copper. A catalyst of this type was kept in operation for 32,264 hours (3 ˝ years). At the end of that period the catalyst supposedly gave 85% conversion at a space velocity of 80 V/H/V and 113 g. liquid product/m3 ideal gas, plus 32 gm C3=C4. These figures were taken from the monthly reports of the Rheinpreussen laboratory but the data presented there were rather incoherent and must therefore be used with care.