Normal pressure synthesis

In the classical normal pressure synthesis (16), higher hydrocarbons are produced by net reactions similar to those observed in the eady 1900s, but at temperatures below the level at which methane is formed ... 

During World War II, nine commercial plants were operated in Germany, five using the normal pressure synthesis, two the medium pressure process, and two having converters of both types. The largest plants had capacities of ca 400 mr / d (2500 bbl/d) of Hquid products. Cobalt catalysts were used exclusively. 

Medium Pressure Synthesis. Pressures of 500—2000 kPa (5—20 atm) were typical for the medium pressure Fischer-Tropsch process. Cobalt catalysts similar to those used for the normal pressure synthesis were typically used at temperatures ranging from 170 to 200°C ia tubular "heat exchanger" type reactors. 

The introduction of iron-zinc catalysts led to the low pressure nthesis of liquid and solid hydrocarbons from CO/Hj in 1925 [19. 20. However, it was found that these catalysts were deactivated rapidly and thus further investigations concentrated on nickel and cobalt catalysts. They led to the introduction of a standardized cobalt-based catalyst for llic normal-pressure synthesis of mainly saturated hydrocarbons at temperatures below 200 C. In 1936, the first four commercial plants went on stream. Until 1945 the Fischer-Tropscit synthesis was carried out in nine plants in Germany, one plant in France, four plants in Japan and one plant in Manchuria. The total capacity amounted to approximately one million tons of hydrocarbons per year in 1943. The catalysts used consisted of Co (1(X) parts), ThO (5 parts). MgO (8 parts), and kieselgur (200 parts) and were prepared by precipitation of the nitrates. These catalysts were used in fixed-bed reactors at normal or medium pressures (< 10 bar) and produced mainly saturated straightproduct obtained consisted of 46% gasoline. 23% diesel oil, 3% lubricating oil and 28% waxes (3.15). 

Fischer-Tropsch synthesis. (Synthol process Oxo synthesis). Synthesis of hydrocarbons, aliphatic alcohols, aldehydes, and ketones by the catalytic hydrogenation of carbon monoxide using enriched synthesis gas from passage of steam over heated coke. The ratio of products varies with conditions. The high-pressure Synthol process gives mainly oxygenated products and addition of olefins in the presence of cobalt catalyst (Oxo synthesis) produces aldehydes. Normal-pressure synthesis leads mainly to petroleum-like hydrocarbons. 

Fig. 4. Molar activity of synthesis products as a function of C number in the Co normal pressure synthesis after addition of [l- C]-n-hexadecene-l to the synthesis gas (relative molar activity of -C,4 in product 291,000) (O) n-paraffin ( ) monomethyl-paraffin ( ) n- and isoparaffins. From (20).

Tropsch synthesis. Four important publications precede the discovery of the normal-pressure synthesis ... 

C. The periodic treatment of the cobalt catalyst, necessary in the case of the normal-pressure synthesis, was not necessary at the conditions of medium-pressure synthesis in spite of the higher molecular weight of the reaction products. The life of the catalyst (without regeneration) was increased many times. 

Medium-pressure synthesis with iron catalysts. Up to January, 1935, the maximum yields of C5+ hydrocarbons obtained with iron catalysts at atmospheric pressure were 30-40 g./m.3 synthesis gas. The decline of catalyst activity amounted to 20% within 8 days (19). Fischer and Meyer (20) improved the yields of the normal-pressure synthesis with iron catalysts (in 1934-1936) to 50-60 g./m.3 synthesis gas and the lifetime of the catalyst from 8 days to about 30 days. These results were obtained with iron-copper precipitation catalysts (1 atm., 230-240°C.). The decline of catalyst activity was closely connected with changes of the composition of the reaction products. The color of the synthetic products changed from white to yellow and formation of fatty acids and organic iron salts was detected. Increased carbon monoxide content of the synthesis gas and increased alkali content of the catalyst accelerated this phenomenon. 

The table shows the nature and quantity of the products formed in the presence of various alkalized precipitation catalysts during operation at 235°C. and 15 atm. with a synthesis gas of three parts of carbon monoxide and two parts of hydrogen. During the first month, the yields in grams per normal cubic meter of ideal gas fluctuated between 140 and 160 g. It was apparent that the addition of alkali was not necessary for the preparation of an active catalyst (in contrast to the normal-pressure synthesis). The yields obtained over a long period of time with a catalyst prepared by precipitation with ammonia and containing no... 

The contract between Studien und Verwertungsgesellschaft and Ruhrchemie required that all further improvements in the field of normal-pressure synthesis, invented at the Kaiser Wilhelm Institut, had to be made known to Ruhrchemie. A separate agreement later extended these conditions to the medium-pressure synthesis. 

It is possible to increase the olefin content of the synthetic hydrocarbons by increasing the carbon monoxide content of the synthesis gas. This step cannot be taken in the case of the normal-pressure synthesis, because of its deleterious influence on the activity of the cobalt catalyst. High carbon monoxide content of the synthesis gas, however, can be used with advantage for the medium-pressure synthesis. 

Fischer and Tropsch used cheap catalysts (alkalized iron turnings) and high temperature and pressures in their initial development work. The products of the reaction were oxygenated compounds (Synthol). Later they used catalysts of high activity, low temperature, and low pressure. As a result of these experiments the opinion was established that the optimum conditions for production of hydrocarbons are atmospheric pressure and lowest possible temperatures (normal-pressure-synthesis). 

Thermodynamic questions often were discussed in connection with the reaction mechanism of the synthesis (102). Montgomery and Weinberger (103) report that the distribution of paraffins obtained at normal-pressure synthesis on cobalt is similar to that predicted by thermodynamic equilibrium calculations. 

Fischer and Koch (104) carried out experiments on the formation of carbon dioxide and water as by-products of the normal-pressure synthesis. They came to the following conclusions ... 

Figure 29 Molar activity of the hydrogenated synthesis product as a function of the carbon number In the cobalt normal pressure synthesis with the addition of ethene [ C] to the synthesis gas (O, n-paraffins , monomethyl-paraffins) (redrawn from Reference 74).

Figure 31 Radioactivity distribution In the synthesis products. Conversion with 0.1 vol% 1-hexa-decene-[1- C] In the synthesis gas, cobalt normal pressure synthesis (redrawn from Reference 74).

Results were also obtained for the conversion of syngas containing C-labeled eth-ene or propene using a precipitated promoted iron catalyst. In addition, a fused iron catalyst was employed in a run with labeled ethene at 20 atm pressure. They found that the cracking reaction of ethene was of secondary importance with the iron catalyst, unlike the case with cobalt. The distribution of the synthesis products from C-ethene showed that about 50 percent of the transformation was to the C3 product the transformation to higher hydrocarbons decreased much quicker than for the cobalt normal pressure synthesis (Figure 33). With the addition of C-ethene the iso-paraffins had a lower activity than the normal paraffins this is consistent with the data for cobalt (Figure 34). 

Schulz et al. observed that during the cobalt normal-pressure synthesis, 2-propanol and acetone tracer compounds were interconverted by fast hydrogenation/dehydrogen-ation reactions thus, both acetone and 2-propanol initiated chain growth. With either acetone or 2-propanol, propane was found to be one of the major products. 

Total yield per cubic meter of synthesis gas normal-pressure synthesis, 148 g medium-pressure synthesis, 145 g of liquid products and 10 g gasol. 

---