7. Medium Pressure Fischer-Tropsch Units.
(b) Development of 3-stage Operation. The Hoesch MP pilot plant had been designed to operate on the same 2:1 H2:CO gas as the LP units, but it was soon found that the gas formation (CH4) could be suppressed by lowering this ratio. The plant was operated on a feed ratio of 1.94:1 with the following results:
|
Average Values for Five Months: |
||
|
Feed Gas: |
81% CO+H2 |
|
|
Conversion |
90.8% |
|
|
Gasformation |
23.1% |
|
|
Gasol formation (C3 C4) |
6.1% |
|
|
Liquid Product |
126.2 g/m3 Ideal gas |
|
|
Total Product (Incl. C3 C4) |
138.43 |
|
|
Average space velocity |
569 m3/oven |
|
|
Average life of catalyst |
4015 hrs. |
|
|
Coke requirement |
4.99 kg/kg recovered product. |
|
|
Primary Liquid: |
Gasoline-200°C |
48.3% |
|
Kogasin-320°C |
26.9% |
|
|
Hard wax-450°C |
8.9% |
|
|
Olefine content of C3 C4 |
15% |
|
Following one year of operation at this condition, the H2/CO ratio was further decreased to 1.8:1. This resulted in improved liquid yields, higher olefine content and lower gas formation. Unfortunately the average catalyst like decreased from 4015 hrs. to 2306 hrs.
In view of this improvement it was decided to further lower the H2/CO ratio to 1.6:1 Since the ratio of consumption averages nearly 2.15:1, hydrogen had to be added to convert the remaining CO in the second stage. The ratio in the exit gas from the first stage was only 1.04:1. Hoesch had to use converted watergas with a ratio of 3079:1 to correct the ratio as no hydrogen was available.
With the low ratio of H2/CO2 fed to both stages, the total conversion was found to be low. It was therefore decided to add a third stage preceded by another addition of hydrogen (converted watergas) to the exit gas from the second stage. Since this gas is quite high in inert content, the V/H/V and temperature in this final clean-up stage are rather high and correspondingly fewer ovens (10-17% of the total) are required. Results from three-stage operation are as follows
|
Average Values for Five Months. |
||
| Conversion |
95% |
|
| Gas formation |
18.1% |
|
| Gasol formation |
6.7% |
|
| Liquid product |
144.4 g/m3 ideal gas |
|
| Total product (incl. C3+C4) |
156.6 |
|
| Average space velocity |
607 m3/oven |
|
| Average catalyst life |
2745 |
|
| Coke requirement |
4.35 kg/kg recovered product |
|
|
Primary Liquid |
||
| Gasoline |
42.8% |
|
| Kogasin |
25.8% |
|
| Hard Wax 450°C |
14.6% |
|
| Olefine content of C3+C4 |
35 |
|
In order to show the conversion, the gas analysis in the three stages are listed:
|
Feed 1st stage |
Exit 1st stage |
Feed 2nd stage |
Exit 2nd stage |
Feed 3rd stage |
Exit 3rd stage |
Convert gas |
Total Plant |
|
|
CO2 |
8.6 |
20.2 |
16.3 |
30.8 |
27.3 |
37.9 |
5.0 |
7.9 |
|
Cn.Hm. |
- |
1.1 |
0.9 |
1.6 |
1.3 |
2.0 |
- | - |
|
CO |
33.0 |
29.7 |
27.1 |
20.7 |
20.5 |
14.5 |
18.8 |
30.8 |
|
H2 |
53.5 |
30.6 |
40.5 |
15.7 |
22.8 |
6.5 |
70.6 |
56.3 |
|
CH4 |
0.4 |
7.4 |
5.7 |
13.6 |
12.3 |
17.4 |
0.4 |
0.4 |
|
N2 |
4.5 |
11.0 |
9.5 |
17.6 |
15.8 |
21.7 |
5.2 |
4.6 |
|
Ideal |
86.5 |
- |
67.6 |
- |
43.3 |
- |
89.4 |
87.1 |
|
H2/CO |
1.62 |
1.04 |
1.49 |
0.76 |
1.11 |
0.45 |
3.76 |
1.83 |
Note: The low H2/CO ratios; they cannot be used in LP units.
The favorable results of this operation with low H2/CO ratio to increase liquid yield led to the adoption of the three -stage operation in all three MP units.