V. IRON CATALYST
D. Kaiser-Wilhelm Institute Work on Iron Catalysts
The following information was obtained upon interrogation of Dr. H. Pichler (3):
In the case of the iron catalysts, their main application is in the middle pressure field since, although they can be used at normal pressure, the life of the catalyst at normal pressure is short. The presence of copper, in amounts ranging from 0.5 to 3.0, is definitely desirable. Iron catalysts containing no copper will work also, but reproducible catalysts are difficult to obtain. The presence of alkali is not essential for activity, but it is necessary for the preparation of a catalyst that can be reproduced. The amount of alkali added is reflected in the product distribution. Thus, 1/8 to 1/4 per cent alkali will give more C3-C4 hydrocarbons; with higher alkali contents, the concentration of higher hydrocarbons in the product is increased until at 1% alkali a maximum concentration of higher compounds is attained. However, the catalyst life when 12% alkali is present is definitely shortened. Still higher alkali contents result in the formation of oxygen containing compounds. A suitable catalyst has the following composition:
1/2 to 1% alkali, 2-3% copper, rest iron.
It has been found that the effect of catalyst pretreatment is much greater than that of adding other materials to the catalyst. The pretreatment has little effect upon the product distribution, but primarily upon activity and life. The catalyst pretreatment must be carried out at low pressure, either normal or subatmospheric, the pretreating gas varying in composition from CO alone to water-gas or even synthesis gas. When pretreating with CO alone a typical example would be to use 1/10 atms. absolute pressure at 320°C. and passing 4 liters of CO (NTP) per hour over 10 gms. of Fe for 25 hours. A rate of 25 1. of gas can be employed in conjunction with a total reduction time of 4 hours. With water gas a temperature of 240-250°C. is employed during the reduction. The more hydrogen that is present in the reduction gas, the less is the tendency to build up carbon during the reduction and the synthesis that follows. The reduction with CO alone does result in the formation of specially active catalysts; however, the highly active catalyst ten to give carbon deposition. In the preparation of the iron catalyst, it was found that the use of pure ferrous or pure ferric compounds is not desirable, while a mixture of the two produces good results. The ratio of ferrous to ferric compounds in the mixture is not important, and wide limits can be tolerated. Ferrous chloride in dilute solution (50 g.Fe per liter of solution) and ferric nitrate are precipitated very rapidly at the boiling point. The precipitate is quickly filtered, washed free of alkali, then mixed with water and potassium carbonate is added in the required amount. Actually, all sorts of potassium salts can be used to provide the alkali. The catalyst is dried in air, since the oxidation that does take place does not matter, and reduced.
The magnetic-chemical investigation of the iron catalyst was undertaken by Pichler in connecton with the mechanism of the reaction and changes in the catalyst during its lifetime. The work was started in 1944 and is being continued at the present time. The earlier theories on the carbides of iron were that higher carbides are formed during the synthesis and are necessary for the reaction. By higher carbides are meant Fe2C and possibly even more carbon-rich iron compounds. The present a study showed that, although higher carbides can be formed at 220°C, these decompose at higher temperature (such as 300-400°C.), leaving Fe3C as the only stable component. It was found that the most active catalysts were those containing the most Fe3C. When the catalyst is pretreated at vacuum, that is at about 1/10 atms absolute pressure, more Fe3C is obtained than when the pretreatment is carried out at atmospheric pressure. During the synthesis, the amount of Fe3C decreases as the catalyst loses activity, while the concentration of Fe3O4 increases. An equilibrium, or possibly a pseudo-equilibrium mixture, of Fe3C and Fe3O4, appears to exist in the catalyst. Thus, when a catalyst containing this apparent equilibrium mixture is used in the synthesis, there is no rapid drop in activity, while a higher concentration of Fe3C does produce a decrease in activity.
In previous work, attempts were made to prepare Fe2C. This was by passing hydrogen over iron to get reduction and then passing CO over it. The amount of CO2 formed was measured. Then H2 was again passed over the catalyst at higher temperatures and the amount of methane formed was determined. From the data it was concluded that Fe2O has been formed under these conditions. However, later magnetic measurements showed that Fe3C is also present. This indicates that, along with Fe2C,. and Fe3C a third carbide must be present, and this compound must be still richer in carbon than Fe2C.
At higher temperatures, Fe3C becomes sintered, and care has to be taken to maintain the temperature low enough to prevent it. The formation of Fe3C does not take place if you charge the fresh untreated catalyst with water gas under pressure at 220°C. Some Fe3C can be obtained at 280°C. under such conditions, but the catalyst produced is not good.
3. Synthesis with Iron Catalysts
The preferred operating pressure is 20 atms., since a higher conversion is attained and the paraffins produced have a higher molecular weight. At 10 atms essentially the same catalyst life is attained, but below 10 atms, the life of the catalyst decreases. Above 20 atms the tendency is to produce oxygen containing compounds, and at 50 atms, the catalyst life is decreased. In actual operation, taking apparatus costs into consideration, an operating pressure of 10 atms has been favored.
An equimolar mixture of CO and H2 is considered to be a suitable synthesis gas composition. The higher the CO concentration in the synthesis, the higher is the required conversion temperature. A higher conversion is obtained with a gas mixture containing 2H2:3CO (cannot use this mixture at 220°C.), but there is danger of carbon formation. With water gas, a temperature of 200°C. can be used, and when a gas mixture containing 4H2:1CO is employed (no practical significance), the synthesis can be started at 180°C. and continued at 200°C., whereby the products obtained are very similar to those formed using the cobalt catalyst (solid paraffin). The life of the catalyst increases as the concentration of hydrogen in the synthesis gas is increased.
In the beginning of the run, the product from the first few hours is carbon dioxide, which is followed by the formation of hydrocarbons along with both CO2 and H2O. Fifty per cent of the oxygen from the carbon monoxide goes to CO2 and 50 per cent to H2O. In the actual operation with water gas at 20 atms, and starting at 215-220°C., a normal cubic meter of gas produces, exclusive of methane, 130 g. of product. After three months of operation, during which time the temperature is gradually increased to 220-225°C., the yield is 10% lower. a catalyst life of one year with a yield of 120 g. was obtained in one case employing a CO enriched water gas and increasing the temperature to 235°C. In another experiment, using a gas mixture consisting of 3 CO:2H2, and employing an inclined reaction tube, the catalyst was in operation for two years. In the last case, a vertical reaction tube could not be employed because the catalyst volume increases with increased carbon deposition which would result in plugging.
A typical product from the synthesis using water gas has the following composition:
20% C3-C4 hydrocarbons containing 50% olefins
40% gasoline, with an olefin content of 50%
20% of fraction boiling at at 200-300°C.
20% of solid paraffin having a melting point close to room temperature
In the beginning of the run, more solid paraffin is produced. The wax formation is favored by the following factors:
Increasing alkali
Increasing pressure
Decreasing temperature
Wile the olefin content of the product increases with the following changes:
Decreasing hydrogen in synthesis gas
Decreasing pressure
Increasing temperature
Small effect of changing alkali concentration
In an interrogation of Dr. F. martin (9) the following information was obtained:
In another phase of the interrogation, Martin was asked about iron catalysts. Martin reviewed the statement of Dr. Franz Fischer that a temperature of 240°C. was necessary for successful operation with the iron catalyst. Such a temperature required a water pressure in the cooling system of the catalyst chamber of 50 atmospheres. Since such a pressure presented many construction problems, it was most desirable to obtain and iron catalyst that would operate at a lower temperature. By a special process developed by Ruhrchemie, an iron catalyst was produced that could operate at 215-225°C. (average 220°C.) and the pressure then was such that the concentric double tube catalyst chambers of the medium-pressure then was such that the concentric double tube catalyst chambers of the medium-pressure process could be used.
To make this modified iron catalyst, the iron nitrate was precipitated by potassium carbonate rather than sodium carbonate. This involved added expense, but the potassium nitrate in the filtrate could be removed and sold for a partial credit against the cost. The precipitation is carried out rapidly from a hot solution, using for for every 100 parts of iron (Fe) 30 parts kieselguhr and about 3-4 parts copper (Cu). After precipitation, the precipitate is washed on the filter until samples taken from the filter show a pH of 8.0. If over-washed, the pH may be brought back to 8.0 by addition of a weak alkali solution.
The reduction of this material is easier than that of the cobalt catalyst and is carried out at 325-350°C. for a somewhat longer time than the cobalt reduction. As in the case of cobalt, the iron catalyst must be only partially reduced, about 60 to 70%, since the presence of some iron oxide seems to be essential to the efficient working of the catalyst.
Martin believes that the use of potassium carbonate causes a metamorphism of the iron oxides and hydroxides into a form that makes the catalyst more effective at lower temperature. Ruhrchemie had operated a pilot plant with the iron catalyst, using 100 m3/hr. synthesis gas for about a year. Experience with this pilot plant had furnished the data for the Italian product which had been set up for use of iron catalyst.
Martin was asked about the sintered iron catalyst of I. G. He said that I. G. had used this sintered iron catalyst exclusively for the manufacture of C9 alcohols for plasticizers, etc., and that this had no relation to the Fischer-Tropsch process as ordinarily conducted. As an aside at this point, Martin said that the Fischer-Tropsch process had been offered to I. G. in 1930 for about 500,000 marks, but that they had refused it.
In connection with the iron catalyst Martin was also interrogated regarding recirculation type of operation, since Ruhrchemie had also carried out such experiments in addition to those with cobalt catalysts. He said that the use of water gas (approximately equal content of hydrogen and carbon monoxide) as the synthesis gas with iron catalyst had given trouble due to deposition of carbon on the catalyst particles, thereby destroying its activity. by recirculation of two parts of recycle gas taken after the first stage to one part of fresh water gas through the first stage only, this deposition of carbon was prevented. Fresh gas feed was held at normal rate, so that this type of operation meant a gas rate of three times normal through the first stage chambers.
The oxygen products from the operation with water gas were both carbon dioxide and water instead of substantially all water as from the hydrogen-richer normal synthesis gas. The life of the catalyst was changed by this recirculation system, being still 5 to 6 months. Also the yield was still 150 g. per m3 of synthesis gas.
In connection with the operation of the catalyst chambers, Martin again commented on other advantages and disadvantages of the iron catalysts. When the iron catalyst is operated at 220° (special iron catalyst as prepared by Ruchchemie) the yield of paraffins is high, being 40 to 50% of the primary product. One important disadvantage of the operation with a high ratio of carbon monoxide to hydrogen, using the iron catalyst, is that the main oxidation product is carbon dioxide which must be scrubbed out of the gas between stages, whereas when water is the main oxidation product, it may be removed simply by cooling and condensation. Martin stated that oil obtained by use of the iron catalyst had been considered good stock for production of lubricating oil without any preliminary cracking as was required for the product from normal Fischer-Tropsch operation. However, before the iron catalyst oil can be so used, it is necessary to remove all oxygen compounds first. In the first laboratory tests, metallic sodium was used for this purpose. Later a catalytic process was developed that was considered to be more commercially feasible than use of sodium. The catalyst was clay that had been treated with sulphuric acid and then heated. By passing the vapor of the lubricating oil stock over this catalyst at a temperature which never exceeded 350°, the oxygen compounds were destroyed with out any undesirable shifting of double bonds that might have affected the usefulness of this oil for lubricating oil manufacture.