BIOS 1722 - Synthesis at Medium Pressure

Synthesis at Medium Pressure

General

Lurgi and Hoesch discovered that for synthesis at medium pressure the best ratio of H₂/CO in the synthesis gas is 1.5 - 1.6, and for their full-scale plant Hoesch use the following ratios, which they say give the best results: Stage I 1.5 - 1.6, Stage II 1.5, Stage III 1.3 - 1.4. The low ratio in the last stage is chosen to suppress methane formation. Ruhrchemie agree with this, an have adopted a similar scheme for their own medium pressure plant.

Lurgi say that if gas rich in carbon monoxide is used from the very beginning of the catalyst life, it damaged the catalyst due to formation of carbon, but if used after proper running in it is very good. Hoesch go as far as to say that gas with H₂/CO = 1 gives the best results for synthesis on a small scale but cannot be used on the large scale as it causes formation of carbon.

Ruhrchemie drew attention to the fact that the inert content of synthesis gas for medium pressure synthesis will always be less than for atmospheric pressure synthesis gas on account of the ease with which carbon dioxide can be washed out under pressure, and as a result, medium pressure synthesis starts with this advantage over atmospheric pressure synthesis.

In agreement with observations made at the Fuel Research Station, Ruhrchemie also found that if during medium pressure synthesis the gas rate is temporarily reduced and then restored to normal, the activity of the catalyst is permanently ruined. They think that the bad effect is due to the catalyst becoming covered with some products which are not removed by going back to the normal gas rate. It is interesting that the bad effect of low gas rates is not observed during the first few days of synthesis, and during the initial period the high molecular weight products formed a very low gas rates may even be beneficial in protecting the catalyst against the formation of carbon.

These observations have an important bearing on the running of the plant, and if there is a shortage of gas for the Ruhrchemie medium pressure plant some of the reactors are shut down altogether so as to maintain the gas rate for the others.

For a complete shut-down of a reactor the temperature and pressure are maintained at their synthesis levels and the catalyst left in contact with stationary gas. This is for the large plant. For laboratory plant the catalyst is cooled in contact with stationary gas and restarted by restoring the gas flow and then heating up to reaction temperature.

Hoesch-Benzin

Before December, 1940, this plant was arranged for synthesis in two stages, using H₂/CO ratio of 1.8 - 2.0 for Stage I. The total contraction was 69%, the yield of products 130 g./cu.m. (CO + H₂), and the yield/reactor/day was 1.63 t. The catalyst life was about 9 months. Further details for one month's synthesis by this method are given in Table 17.

As a result of experiments carried out at the Hoesch plant in conjunction with Lurgi, Hoesch introduced two independent changes, converting the plant to three-stage working and reducing the ratio of H₂/CO in the synthesis gas. The second of these changes was carried out in spite of Ruhrchemie's advice against it. In this way the methane production was decreased and the specific yield and plant production increased, the latter going up to 2.05 t./reactor/day. Hoesch paid Lurgi a premium on the extra product produced as a result of the change. The big increase much more than balanced the extra expense owing to shorter catalyst life, which was now only 7 - 8 months. The yield of products/kg. cobalt wa 550 - 560 kg., which was much higher than that obtained at the other plants. Thus Essener Steinkohle produced 450 kg. and the other plants 300 - 350 kg. Full details of the three-stage synthesis are shown in Table 18, and a comparison of Tables 17 and 18 shows the salient features of the two methods of working. The change made no difference in the boiling range of the products, but it increased the proportion of olefines, as shown in Table 19.

Hoesch had 40, 20, 8 reactors in Stages I, II, and III respectively but the pipeline connections were not completely adequate, so that even up to 1944 the best method of working could not be used for all the reactors, namely of starting synthesis in Stage III and then switching into Stage II and Finally Stage I. Details are as follows:

Stage III. Freshly reduced catalyst is started here and kept in this stage for 1 month. Synthesis is started at 170°C. and 2,000 cu.m./h. and the temperature is increased to 200°C. in four days. The gas rate is decreased gradually and is only 1,500 cu.m./h. at the end of the month. Hoesch say that it would have been better to have used temperatures of 210 - 215°C., which would not harm the catalyst but which could not be attained with the present reactor fittings. It was proposed to alter these to allow temperatures of 220 - 225°C. to be available if required.

The gas entering this stage had an H₂/CO ratio of 1.3 - 1.4.

Stage II. After switching from Stage III the temperature is reduced to 180°C. and the gas rate set at 1,200 cu.m./h. During the first month in this stage the temperature is increased to 195°C. and the gas rate decreased slowly so as to keep the contraction constant. For the next 4 - 5 months the gas rate is still decreased, down to 900 cu.m./h. at the end of this period. The gas used in this stage has H₂/CO - 1.5.

Stage I The catalyst spends the remaining 1 or 2 months of its life here, starting at 196 and 900 - 1,000 cu.m.h. and finishing at 200°C. and 600 cu.m./h. The gas for this stage has H₂/CO - 1.5 - 1.6.

The details of how the synthesis proceeded in these stages can be seen from the analyses in Table 20.

Ruhrchemie

In the Ruhrchemie medium pressure plant freshly charged reactors are started in Stage III where they are put under gas pressure and heated to 125°C. in stationary gas. A gas rate of 500 cu.m./h. is then set up and the temperature increased by 5°/h. till 160°C. is reached, and then by 1°/h. till the contraction is 45%. The gas rate is then increased to 1,000 cu.m./h. and the temperature raised at 1°/h. till the contraction again reaches 45%. The gas rate is then put up to its normal value of 1,5000 cu.m./h. and the temperature increased till the contraction is 40%. These figures refer to the normal synthesis gas for Stage III and are varied if necessary to correspond to the composition, and in particular to the inert content of this gas. Further details are shown in Figure 5.

For synthesis, the same general principle was used as at atmospheric pressure, namely, to aim at the same output/reactor/day for each stage. Details are given in Table 21.

The temperature were arranged so as to maintain these contractions. A temperature of 203°C. was set as a maximum for Stage III, and when a reactor reached this temperature it was transferred to Stage II and synthesis restarted at 185 - 188°C. It was kept in Stage II till the temperature reached 197°C. and then transferred to Stage I, restarting synthesis at 180°C. The maximum temperature for this stage was set at 203°C, and when this was reached the catalyst was discharged. It usually lasted 6 - 7 months when treated in this way, during which time the gas rate was gradually reduced from 1,000 - 1,200 N.cu.m./h. to 800 N.cu.m./h. Further details are given by the dotted curve in Figure 3.

Hoesch-Benzin say that at the higher gas rates more methane is produced but Ruhrchemie think that this is only due to increased temperatures.

It is considered that there is no effective difference between 5 and 10 atm. pressure as far as results are concerned.

Ruhrchemie followed Hoesch in adding hydrogen to the gas between the stages, but got their plant re-arranged first and actually started synthesis in this way before Hoesch. It may also be mentioned here that Ruhrchemie heated the gas going to the second and third stages to 160°C. in a tubular steam heater, to avoid the separation of spray in the reactors.

The following data for 1934-4 illustrate the working of the Ruhrchemie medium pressure plant:

A volume of 37,000 cu.m./h. of synthesis gas - I enters Stage I, which is run to a contraction of 51.6%. The residual gas is mixed with 4,200cu.m./h. washed converted water-gas to give a volume of 22,000 cu.m./h. synthsis gas - II. Stage II is run to a contraction of 46.5% and the residual gas mixed with 2,000 cu.m./h. washed converted water-gas to give 13,500 cu.m./h. synthesis gas - III. Stage III is run to a contraction of 29.2%. Further details are given in the production data of Table 23, and the gas compositions at the various stages in the plant are given in Table 22.

Schaffgotsch-Benzin

Since there is so little first-hand information about this plant, which is in Eastern Germany, the production data given for it in Table 23 are of interests.