Mark E. Dry

The Fischer–Tropsch process: 1950–2000

Abstract The decision to build a Fischer–Tropsch (FT) plant is still fraught with risk because it has to be based on the perceived future price and availability of petroleum crude oil and on local politics. The most expensive section of an FT complex is the production of purified syngas and so its composition should match the overall usage ratio of the FT reactions, which in turn depends on the product selectivity. The kinetics, reactor requirements, control of selectivity and the life of cobalt and iron catalysts are discussed and compared. Control of the FT conditions coupled with appropriate downstream processes results in high yields of gasoline, excellent quality diesel fuel or high value linear α-olefins. The history of the various FT options and of the improvements in FT reactor technologies over the last 50 years is reviewed. It appears that “new” technologies are re-discovered in cycles of 15–30 years and it often takes the same time for the implementation of new concepts.

Practical and theoretical aspects of the catalytic Fischer-Tropsch process

Abstract The development, current status as well as the future of the Fischer-Tropsch (FT) process is reviewed. In this past decade, several new FT plants and reactors have been built. The first commercial scale slurry (3 phase) reactor and an improved version of the 2-phase fluidized bed reactor were successfully commissioned. The practical chemical, engineering and economic aspects of the FT process and reactors as well as the means of controlling conversion and selectivity are discussed. Despite the uncertainties regarding the chemical mechanism and the rate controlling steps satisfactory kinetic equations have been developed and successfully applied in both the design and control of FT reactors. The various proposed mechanisms are reviewed and a new version is presented. Uncertainties remain, however, and the usefulness of the present mechanistic information is questioned.

Heats of chemisorption on promoted iron surfaces and the role of alkali in Fischer-Tropsch synthesis

The heats of adsorption of CO, CO2 and H2 on unpromoted, on singly and on doubly promoted reduced magnetite were determined calorimetrically. The heats of CO adsorption are similar to those reported on evaporated iron films. The heats of hydrogen adsorption are lower than on iron films but nevertheless similar to those reported on reduced iron oxide samples. Promotion with only MgO does not markedly influence the heats of adsorption of any of the above gases. Promotion with K2O increases the heat of CO adsorption at low coverages while it decreases the initial heats of hydrogen adsorption. The K2O markedly increases the heat of adsorption of CO2 at all coverages. The CO2 appears to chemisorb not only on the surface alkali but also on the metallic iron. The known influence of K2O promotion on both the activity and selectivity of the Fischer-Tropsch synthesis is explained in terms of the observed influence of K2O on the heats of adsorption of CO and of hydrogen on iron catalysts. The postulates are also applied to the case of the ammonia synthesis.

The fischer-tropsch process - commercial aspects

Abstract This review is an update of previous reviews dealing with the Sasol process, the catalysts used, the control of selectivity, the kinetics and the mechanism of the F-T reaction. The emphasis is on the practical aspects. With regard to reactor developments two new types have now been put into commercial production, namely a 45 bar multi-tubular fixed bed reactor for wax production and an alternative type of fluidised bed reactor for light oil production. The latter reactor costs much less to construct and to operate than the older circulating fluidised bed reactors. The slurry phase reactor concept is now entering the demonstration size stage of development. Improved olefin oligomerisation catalysts have resulted in a better quality diesel fuel being made in downstream upgrading. The best way of producing maximum and high quality diesel is to maximise wax production and then to selectivity hydrocrack it. A comparison is made between the two existing Sasol processes and the proposed Shell Middle Distillate process.

Fischer–Tropsch reactions and the environment

Abstract Suitable economic conditions given, the Fischer–Tropsch (FT) process is an alternative route to liquid fuels and chemicals (in particular linear 1-alkenes). Being S and N free and low in aromatics, the fuels are more environment friendly than those produced from crude oil. In particular, the production of environment friendly high quality diesel fuel is an attractive application of the FT process. Relatively large amounts of CO2 are produced in the gasification processes, but whether this will really contribute to global warming is a disputed question. The water effluent from an FT complex is zero.

CO and CO2 hydrogenation study on supported cobalt Fischer–Tropsch synthesis catalysts

Abstract The conversion of CO/H2, CO2/H2 and (CO+CO2)/H2 mixtures using cobalt catalysts under typical Fischer–Tropsch synthesis conditions has been carried out. The results show that in the presence of CO, CO2 hydrogenation is slow. For the cases of only CO or only CO2 hydrogenation, similar catalytic activities were obtained but the selectivities were very different. For CO hydrogenation, normal Fischer–Tropsch synthesis product distributions were observed with an α of about 0.80; in contrast, the CO2 hydrogenation products contained about 70% or more of methane. Thus, CO2 and CO hydrogenation appears to follow different reaction pathways. The catalyst deactivates more rapidly for the conversion of CO than for CO2 even though the H2O/H2 ratio is at least two times larger for the conversion of CO2. Since the catalyst ages more slowly in the presence of the higher H2O/H2 conditions, it is concluded that water alone does not account for the deactivation and that there is a deactivation pathway that involves the assistance of CO.

Fischer-Tropsch synthesis over iron catalysts

Because of the decreased profitability of making synthetic fuels, Sasol intends expanding its production of the higher valued chemicals, in particular waxes and olefins. The advantages and disadvantages of using Fe, Co and Ru catalysts are discussed from the point of view of costs, availability, product selectivity, activity and sensitivity to poisons.The loss of activity and selectivity of iron based catalysts in both fixed and fluidized bed reactors is discussed. The main contributing factors are sulfur poisoning, oxidation and coke fouling. In fixed bed reactors sulfur poisoning and “coke” laydown deactivates the front end of the bed while hydrothermal sintering/oxidation deactivates the back end. In fluidized beds the deposition of large amounts of Boudouard carbon doesnot markedly lower the activity. The smaller catalyst particles end up consisting of small iron carbide entities embedded in a matrix of carbon. The larger catalyst particles consist of cores of inert magnetite surrounded by the carbide/carbon matrix.FT reactor development at Sasol is briefly reviewed.

Rate of the Fischer-Tropsch reaction over iron catalysts

The rate of the Fischer-Tropsch reaction over triply promoted iron catalyst was studied in a differential reactor at 240 °C, in the pressure range 10 to 20 bar with synthesis gases of H2CO ratio varying from 1 to 7. The reaction rate was found to be first order with respect to the hydrogen partial pressure and zero order with respect to the carbon monoxide pressure. These findings are in agreement with the relative adsorption characteristics of H2 and CO on reduced magnetite surfaces. Water is a primary product of the Fischer-Tropsch reaction, while CO2 is a secondary product formed via the water gas shift reaction. With a gas ratio H2CO of 1.9, the activation energy of the Fischer-Tropsch reaction was found to be 16.8 kcal/mole.

Catalytic aspects of industrial Fischer-Tropsch synthesis

Abstract A brief description of the commercial Sasol process is given. Two types of reactors using two different iron-based catalysts are used, the one producing predominantly gasoline and the other waxes. The hydrocarbon product spectrum can be varied over wide ranges ( e.g. CH 4 from ca. 1 to ca. 80%). The probability of chain-growth is influenced by the temperature, the catalyst type and its promoter level and the gas composition. It is not influenced by the type of reactor. By subjecting the products to additional processes, e.g. oligomerisation of olefins or selective hydrocracking of the waxes, the final product slate can be further altered. Thus a 75% overall yield of either gasoline or diesel fuel can be achieved. A major advantage of the Fischer-Tropsch process over other processes which produce fuel from coal is that a very high quality diesel fuel is produced. The exact nature of the chain-growth mechanism on the catalyst surface is still a disputed subject. It is probable that both ‘oxygen-containing’ and ‘oxygen-free’ hydrocarbon fragments participate in the surface reactions. Because of the likelihood that the bonds between the complexes and the catalyst are highly labile and consequently ill-defined, it is no simple task to assign ‘clear-cut’ structures to the intermediate complexes. The proposed reaction scheme accounts for the many different products obtained from CO + H 2 mixtures over various types of catalysts.

Chapter 7 - FT catalysts

Publisher Summary This chapter reviews that only the four group VIII metals, Fe, Co, Ni, and Ru have sufficiently high activities for the hydrogenation of carbon monoxide to warrant possible application in the fischer-tropsch (FT) synthesis. Of the four metals ruthenium is the most active, but its high cost and low availability rules it out for large scale application. Being a powerful hydrogenating catalyst it produces much more methane than Co or Fe catalysts. Nickel forms volatile carbonyls resulting in continuous loss of the metal at temperatures and pressures, at which practical FT plants operate. From the above, it is clear that only cobalt and iron based catalysts can be considered as practical FT catalysts. The chapter highlights that for the production of the high value linear alkenes, iron catalyst, operating at high temperatures in fluidized bed reactors remains the catalyst of choice. The LTFT iron catalyst may also find future applications for the conversion of coal-derived syngas.