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IV. COBALT CATALYSTS
C. Catalyst Reworking.
Spent catalyst from all Fischer-Tropsch plants in West Germany was returned to Sterkrade for reworking. A flow diagram for this reworking plant was furnished by Dr. Bloechel (12) and is reproduced as Figure 3. The operation of this plant was not discussed in detail. The equipment was almost entirely destroyed by bombing.
D. Synthesis Procedure
1. General Considerations
Regarding synthesis operating conditions in general, Martin (9) stated that each chamber contains one metric ton of cobalt and produces 1.5 metric ton of primary product per day on the average, including chambers in all stages. The operating rate given will produce a yield of 150 g. primary product per m3 of ideal (pure) synthesis gas. Some plants use a slower operating rate than above and thereby attain yields as high as 165 g. per m3. For the 150 g. per m3 yield noted above synthesis gas flow is initially 1000 m3 per metric ton of cobalt per hour in the first stage chambers, but as the catalyst becomes older, this rate is lowered until it is 800 m3 per ton per hour just before regeneration of the catalyst. The general policy of chamber rotation is to put fresh catalyst in the last stage in contact with older gas wile older catalyst is put into the first stage in contact with fresh synthesis gas.
Martin explained that each plant had been given a free choice as to the use of atmospheric or medium pressure. The later has the advantage of giving three times as much hard paraffin as does the normal-pressure, and hard paraffin can be used for the manufacture of higher alcohols, such as C25, which can be made by no other process. Also the medium-pressure operation gives a somewhat longer catalyst life (about 2 months longer). The normal-pressure process gives a high yield of gasoline, but under peace conditions and a free world economy Martin thought that such gasoline could not compete with that made from natural petroleum, so this is no advantage. Summing up, Martin said that the medium-pressure process should be chosen for any post-war project.
In all of the commercial Fischer-Tropsch plants inspected by the oil team, the construction of units and the conditions and results of operation were essentially the same as already known in this country. Therefore, only brief comments on the operation of these units will be made here, although much more detailed information is available in one form or another.
2. Hoesch Benzin.
Hoesch Benzin at Dortmund (1) had 68 middle pressure reactors of conventional design, and of these normally 64 were in service as follow:
|40 chambers in Stage 1|
|16-20 chambers in Stage 2|
|4-8 chambers in Stage 3|
All chambers were operated at 180-205°C. under 10 atmospheres pressure. The rate of synthesis gas feed to the first stage was 1000m3 per hour per cent gas volume contraction was 50-60% in Stage 1, 35-40 % in Stage 2, and 30% in Stage 3.
Normal production was about 50,000 tons of primary liquid product per year.
Ruhrbenzin at Sterkrade-Holten (2) operated about 48 atmospheric pressure reactors and 72 middle pressure reactors, all of conventional design. The normal pressure ovens are operated in two stages on charge gas containing the usual 2 volumes of H2 and 1 volume of CO. No further details of this operation are given since it is well known.
The arrangement of the Sterkrade middle pressure reactors is shown by Figure 4. In this operation the H2:CO ratio of the inlet gas to the three stages is adjusted to 1.4, 1.6, and 1.8 respectively by introducing the requisite amount of converted water gas before each stage. For Stage 1 of the medium-pressure operation the inlet gas contraction 50%. Similar conditions obtain for Stage 2. In Stage 3, where all newely-charged ovens are started, higher rates (2000m3/oven/hour) and lower temperatures (165-185°C.) can be employed. the overall contraction is about 75% and the yield, 150 gm. C3 and higher hydrocarbons /m3 ideal gas.
The advantage of the scheme of commencing with a gas relatively deficient in hydrogen and then increasing the proportion of hydrogen in later stages lies in combining the beneficial effects of low hydrogen proportion on olefine content and methane production with the efficient utilisation of the carbon monoxide achieved with normal synthesis gas (H2:CO=2:1).
The products produced by this method of operation are as follows:
|C3 + C4||= 10% (Olefines 40%) (13% of C4H8 is iso)|
|C5-170°C||= 25% (Olefines 24%) O. N. (Motor) ca. 45.|
|170 - 280°C||= 30% (Olefines 9%)|
|280 - 340°C||= 20% Soft Wax|
|Residue||= 15 Hard Wax M. P. ca. 90°C.|
(This includes 3% of wax extracted from the catalyst at the end of its life).
Using normal synthesis gas throughout, 14% of the total hydrocarbons produced is methane, but with the above scheme the proportion is reduced to 10%.
The annual production of primary liquid products at Sterkrade was normally about 70,000 tons.
Rheinpresussen at Moers (4) operated % conventional Ruhrchemie atmospheric pressure reactors of which 60 were normally in the first stage and 30 were in the second stage. The temperature was stated to be 195 to 200°C.
From a plant record book a rather detailed picture of the performance of the synthesis ovens during the last half of 1941 and of the first half of 1942 was obtained. During this period the synthesis gas charge rate averaged abut 630 cu.m. per hour per oven. The charge rate was in general quite steady, but on some days averaged as low as 520 M3 per hour, and on others as high as 740 M3 per hour. The product yield figures, for some unknown reason, showed much greater fluctuations from day to day with no apparent correlation with charge rate. The total liquid product, including "Gasol" (C3 and C4 hydrocarbons) averaged 150-155 grams per cubic meter, with a daily minimum of 135 gm/M3 and a maximum of 175 gms/M3. The yield of products heavier than the "Gasol" averaged about 135 gm/M3 with a minimum of 112 gm/M3 and a maximum of 155 gm/M3.
The production in 1944, until stopped by bombing in July was as follows:
|Kogasin for mixed diesel fuel||1,765||5.6|
|Light diesel fuel||3,219||10.2|
|Gatsch (Crude paraffin Wax)||2,415||7.6|
Total production for preceding years had ranged from 52,000 tons to 71,000.
Klocknerwerke A. G. at Castrop-Rauxel (5) operated 63 atmospheric pressure ovens of conventional Ruhrchemie design. Of this Number, 53-55 are on stream at one time with approximately 33 in the first stage and the remainder in the second stage.
Stage 1 receives the input of 900,000 M3 of synthesis gas per day; the gas contraction is about 50 per cent, and the temperature is regulated to maintain this contraction. The out-going gases from Stage 1 pass through both direct condensers and active carbon absorbers before entering Stage 2. The overall gas contraction is 75 per cent.
The average working yield of C3 + higher hydrocarbons varies between 135 and 150 gm./Nm3 ideal gas. The composition is as follows:
|Gasoline - 160°C.||:50 per cent|
|Diesel oil 160-230°C.||:20 per cent|
|Heavy oil 230-320°C.||:20 per cent|
|Gatsch, over 320°C.||:10 per cent|
|Gasol||(65 per cent C3 (25 % olefines)||:15% of total products.|
|(35 per cent C4 (75% olefines)|
|Methane: 14-15 per cent of total products.|
|Normal production was about 40,000 tons of primary products per year.|
E. Catalyst Conditioning and Regeneration.
At Ruhrbenzin, Sterkrade (2) each charge of fresh catalyst is started in Stage 3 and run for 10-14 days. It is then put on Stage 2 where it may remain for the rest of the catalyst's life or it may be transferred after a period to Stage 1. Alternatively it may go direct from Stage 3 to Stage 1. The precise history depends on the demands of Stages 1 and 2 for new ovens, but the charge of catalyst always remains on stream for approximately the same total time i.e. 9 to 11 months, depending on smoothness of operation, the lack of interruptions, etc.
According to Alberts (2) Ruhrchemie at Sterkrade never used regeneration with solvent or hydrogen in medium pressure synthesis and only in the first stage of atmospheric-pressure synthesis. In the second stage it is unnecessary as little or no wax is formed. Originally hydrogen was used, for the first time after 4 weeks' operation then at 14-day intervals. Then, to save hydrogen, solvent treatment was substituted, but this was not so effective as hydrogen treatment. finally solvent treatment followed by hydrogen treatment was used and this was the most successful method of all. The time intervals used are the same as those given above.
The high-temperature re-reduction treatment was a Roelen invention. It has never been tried on the full-scale but Alberts is convinced it will work and was preparing to try it at Castrop Rauxel just before the bombing started. The process consists in treating the oven contents at 400°C. for 3 hours in pure hydrogen at 2000 m3/hour. The object of the treatment is the removal of persistent carbonaceous deposits.
At Klocknerwerke A. G., Castrop Rauxel, (5) a freshly-charged oven is placed on Stage 2 at about 185°C. It remains there for 30-35 days during which time the temperature may have reached 192°C. will have been reached. It is then treated with solvent and subsequently at 14-day intervals. It remains on this stage for 120 days before the catalyst is discharged and returned to Ruhrchemie for regeneration. The life of an oven filling is thus 150-160 days.
During the last working year (1944), immediately following the 5th or 6th washing and when the catalyst had reached the stage when it would normally have been discharged, the catalyst was treated with pure H2 at 200°C. for 8 hours at 1500m3/oven/hour. The subsequent synthesis temperature was found to be only 176-177°C. to obtain the normal activity. However, after only 20 days synthesis the plant was put out of action by bombing and the test could not be completed. The plant personnel were convinced that by using this combination of solvent and hydrogen treatment the ultimate life of the catalyst would be well over 200 days. The procedure was first tried by von Lopmann of Kamen-Dortmund.
The solvent extraction process consists in allowing 28 m3 of F. T. benzin to trickle down over the catalyst during a period of 7 hours.
The gasoline used for catalyst extraction is distilled separately and he recovered hard wax, (m.p.90°, 1 1/2 metric tons per oven filling or ca. 3 per cent of total products) sold as such.
At Rheinpreussen, Moers (4) he normal life of a charge of a catalyst was about four months. Three or four times during this period the catalyst was dewaxed by washing for 12 to 15 hours at 160°C. with synthetic gasoline having a boiling range of about 140 to 180°C. hydrogenation was originally used for this purpose, the hydrogen being made by the iron-steam reaction. It was stated that gasoline washing had been used for the last two years because it was much more "effective," but it seems probable that the reason for the change was the desire to recover the high melting point wax instead of largely destroying it by hydro-cracking on the catalyst. The gasoline was distilled from the extract and the residual wax, known as "Hart Paraffin" was cast in pans to make cakes for shipment.
At Hoesch-Benzin (1) it was stated that fresh catalyst is put into operation in any stage of the plant which is contrary to the general practice of starting a fresh catalyst in the last stage. Here regeneration was accomplished by periodic washing with middle oil and after 1500-2000 hours' service, the catalyst was returned to Ruhrchemie for working.
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