The FT synthesis process can be made generally selective by proper choice of oper­ating conditions and catalysts. Most of the studies over last several decades have been focused in this area. The status of the several FT processes through 1950 is given in Table 4. The yield of C3+ product per cubic meter of synthesis gas given in this table is very important because it directly relates to the purity of the synthesis gas. The cost of the production of purified synthesis gas can be as high as 70% of the total cost of the FT process [16]. Earlier work focused on Fe catalysts and the improvements in these catalysts that would increase their operability and selectivity and decrease the operating costs of FT process [16]. The efforts were made to pre­vent the reaction 2CO = CO2 + C and improve the steady-state life of the catalyst. Significant developmental efforts were also made [17, 18] for a more active and mechanically stable catalyst to further reduce the yields of C1 and C2 [19]. The pro­cess improvement efforts were also focused on developing a catalyst which mini­mized the shift reaction.

It was suggested [8] that further selectivity in FT reactions can be attained by either poisoning acceptable catalysts with sulfur compounds or by selecting sulfides of less frequently used catalysts. Storch et al. [20] reported that the initial effect of small amounts of H2S is the increase in nickel-manganese catalyst activity. This was also confirmed by Herrington and Woodward [21] for cobalt-thoria-kisselguhr

“Kilogram of total product, excluding water, carbon dioxide, methane, ethane, and ethylene per volume of reactor per unit time

bWeight percent of oil plus water-soluble product

‘In converter and its accessories only

dBauxite treated, but no T. F.L. added

‘Very small, less than 1%

(100:18:100) catalysts. In their experiments, H2S is mixed with the synthesis gas in small batches and no H2S was eliminated in the off-gas during the course of sulfur poisoning experiments. The first addition of H2S increased the yield of liquid hydrocarbons at constant temperature. As sulfur addition continued, there was a decrease in the yield of gaseous hydrocarbons. Total hydrocarbon yield increased with sulfur addition to the catalyst until 8 mg. of sulfur was added to each gram of catalyst. This work suggested the advantage of stopping sulfidization at low level (1-4 mg of sulfur/g of catalyst) to obtain the benefits of increased liquid hydrocar­bons yield. The results also suggested that the catalyst might show the same behav­ior if presulfided to the same degree before introducing the synthesis gas. The patent of Storch et al. [22] showed that 69% of CO conversion can be obtained for a molybde­num disulfide catalyst alkalized with 2-3% KOH in a feed of 2H2 + CO at 530°F and

13.6 atm. Products from this synthesis were low boiling with 30% of the product C3+ hydrocarbons and organic oxygenated compounds. Laynes [23] indicated that by allowing the sulfur content of the iron catalyst to build up to an optimum ratio and maintain at that level will minimize CO2 formation during hydrogenation of CO to form hydrocarbons.

Over last several decades, a continuous effort to improve catalyst activity, selec­tivity, and stability has been carried out. Besides looking at different forms of iron catalysts, nickel, cobalt, and ruthenium catalysts have been extensively studied. Both cobalt and ruthenium catalysts with different types of promoters have been extensively examined by Exxon and other oil companies. Their studies indicate that while these catalysts give higher initial activity, they also tend to decay rapidly. Numerous patents on these catalysts have been reported by Exxon and other oil companies. In recent years, MOF (metal organic framework) have been tested to improve the selectivity of FT reactions. This work is being carried out at NETL in Pittsburgh and it is still at the development stage. More work in this area (perhaps using recent developments in nanotechnology) is needed.