8. Kreislauf Operation.
(See also reference I(a)/10 and I(a)/11 at end of this Section).
While the specific output had thus considerably been improved, there was still the low olefine content of the products which had to be overcome. This was finally done by the use of gas recirculation.
It had been found that at higher pressure the cobalt catalyst reduced the CO almost exclusively under formation of H2O. Thus it is possible to feed a mixture to the reactor, with very high CO:H2 ratio, without at the same time losing any CO to CO2. The increase in the ratio does however raise the olefine content of the product. The tailgas of such an oven is obviously very high in CO and low in H2 and thus affords a means to further increase the CO/H2 ratio of the feed to this oven by recycling the gas, preferably after removing all products. This is important to prohibit possible hydrogenation.
The operation with Kreislauf is thus simply an extension of the three-stage operation described above. In practice, it was found that any increase in the ratio recycle-gas: feed-gas over 3:1 does not give sufficient further increase in olefins to warrant the expense. The olefine content of the gasoline out (200°C EP) is around 70%, the Kogasin cut (200°-325°C) is around 45%.
The temperature required to give adequate conversion is somewhat higher than in ordinary cobalt operation but des not exceed 225° C. This can be reached with the existing steam-cooled reactors.
The Kreislauf was considered of great importance and the entire MP section of the RCH plant was to be used as first stage with Kreislauf. The LP Section would be operated with the exit gas from the MP plus the required hydrogen addition to give the 2:1 ratio, which is imperative in LP cobalt plants. In this arrangement 80% of the total plant production would have come from the Kreislauf.
Pilot Plant Data on Kreislauf Operation. At Hoesch a single reactor had been operated at length with gas recirculation and data from two identical ovens, one with, the other without Kreislauf, are given below:
|
Kreislauf |
Once Through |
|
|
Space velocity (basis 10m3 cat.) |
1060 m3/hr. |
1015 m3/hr. |
|
Ideal gas velocity |
861 m3/hr. |
827 m3/hr. |
|
Catalyst space velocity |
1.35 m3/hr/kg cobalt |
1.23 m3/hr/kg cobalt |
|
Contraction |
69% |
57% |
|
CO Conversion |
85% |
69.5% |
|
CO+H2 Conversion |
89.8%` |
74.3% |
|
Temperature |
195° C |
189Q C |
|
Average daily production (liquid) |
2715 kg. |
1835 kg. |
|
Yield |
131 gm/m3 ideal gas |
98 g/m3 ideal gas |
The test oven was operated 3650 hours. During the first 2764 hours (almost four months) the oven temperature remained unchanged at 195° C, the pressure at 7 atm. The space velocity was 1200 m3/hr. for 1 month and 1000 m3/hr. for the rest of the period. Recycle to fresh feed ratio was 2:1 in the beginning and later raised 3:1, but the difference in the result was not large within that range. The oven contained 2660 kg. catalyst with 785 kg. cobalt. The catalyst was the standard cob alt catalyst used throughout the plant.
The advantages derived from this operation are as follows:
(1) The oven may be brought on stream without lose of time usually required for this operation. In about 5 hours the oven is in full production. Synthesis gas is allowed to enter slowly and the recycle blower is started up while at the same time the pressure is allowed to build up. This is done at around 100° C. Next the temperatures is raised and fresh gas is added as the conversion starts in order to keep up the pressure. Finally the exit valve is opened and the unit is on stream.
(2) The yield per cubic meter synthesis gas is increased. Thus in the first stage alone, the same yield may be reached as formerly in two stages.
(3) Higher space velocity. Therefore fewer ovens are required for a given output. The increase is in the ratio of 3:2 if compared with two-stage once-through operation.
(4) Lower catalyst cost. This is apparent from (3).
(5) The unit is less susceptible to operating disturbances of variation in feed gas composition.
(6) Higher olefine concentration in the products.
In general the application of recycle lowers the boiling point of the product towards lighter materials, but this may be corrected by raising the CO:H2 ratio as described above.