Iron carbon number distribution

The carbon number distribution of Fischer-Tropsch products on both cobalt and iron catalysts can be clearly represented by superposition of two Anderson-Schulz-Flory (ASF) distributions characterized by two chain growth probabilities and the mass or molar fraction of products assigned to one of these distributions.7 10 In particular, this bimodal-type distribution is pronounced for iron catalysts promoted with alkali (e.g., K2C03). Comparing product distributions obtained on alkali-promoted and -unpromoted iron catalysts has shown that the distribution characterized by the lower growth probability a, is not affected by the promoter, while the growth probability a2 and the mass fraction f2 are considerably increased by addition of alkali.9 This is... 

For iron catalysts in general, the incorporation of 1-alkenes is negligible, and that of ethene is much lower than that of cobalt.1617 Therefore, for all published carbon number distributions for iron catalysts, a strict representation by two superimposed ASF distributions is obtained. Examples are given by Schliebs and Gaube,7 Dictor and Bell,8 and Huff and Satterfield.10 Also, the old experiments of the Schwarzheide tests are well represented by this model.7... 

Iron. Fe-Cu-K data are shown in Table 2. Carbon number distribution (Flory plots) and a-olefin/paraffin data are shown in Figures 3 and 4, respectively. As on Ru catalysts, the Flory plot is curved and the a-olefin/paraffin ratio decreases to zero as carbon number increases. The experimental conditions are different for the Ru and Fe systems therefore, we cannot make direct comparisons. Such comparisons will be made in a later publication (14). However, we comment on three important findings. First, C2 and C3 hydrocarbons fall close to the Flory curve (Figure 3) for Fe, in clear contrast with the results on Ru. This suggests that the high rate of ethylene readsorption that leads to low C2 concentrations on Ru (7,8) does not occur on Fe. Secondly, both a-olefins and / -olefins persist at higher carbon numbers than on Ru the Flory plot for Fe shows a more pronounced curvature and the asymptotic value of a is reached at higher carbon number than on Ru. Finally, Ru catalysts produce about 40 wt% C20+ product whereas the Fe... 

Guczi et al. (305) also find antipathetic structure sensitivity for the CO/H2 reaction over Ru/Al203 and Ru/Si02. Alloying with iron has little effect on TOF but increases selectivity toward olefins. In contrast to some previous reports (128, 306), it has been found (307) that the product carbon number distribution is sensitive to particle size. 

In that work, the carbon number distribution of FT products on iron catalyst was studied by use of a modified ASF distribution with two chain-growth probabilities. Based on enol mechanism and two ASF distributions the complete set of elementary reactions is given. Using mechanistic kinetic studies of FTS reaction, the chain-growth probabilities ( i and 2) for two ASF distributions formulated. The calculated two ASF model are carefully fitted with experimental results at low carbon monoxide. Thus, the two ASF model is a useful model for prediction of products distribution on lanthanum-promoted iron catalyst in their experimental conditions at low carbon monoxide conversions. The results for higher carbon monoxide conversions deviate substantially because the FTS reaction highly depends on the hydrogen formed by the WGS as the carbon monoxide conversion increases. 

The fact that constant growth parameters will predict the isomer distribution data reasonably is remarkable. It is not necessary that the kinetic constants governing chain growth are independent of chain length and structure but that certain ratios of these parameters are constant. The fraction of tertiary carbons has been reported to decrease with carbon number beyond Cio (i7). The SCG scheme predicts a maximum and subsequent decrease, but the maxima occur at C12-C14 for products considered in this chapter. For the cobalt product, all schemes predict yields of dimethyl species that are often too large by factors of two to four. The simple schemes with constant growth parameters as described here are unable to predict the isomer distribution sensibly for products from fixed-bed iron (16) and from fixed-bed nickel... 

We have studied the synthesis of fatty acids by the closed Fischer-Tropsch process, using various carbonates as promoters and meteoritic iron as catalyst. The conditions used were D2/CO mole ratio = 1 1, temperature == 400°C, and time = 24-48 hr. Sodium, calcium, magnesium, potassium, and rubidium carbonates were tested as promoters but only potassium carbonate and rubidium carbonate produced fatty acids. These compounds are normal saturated fatty adds ranging from C5 to Cis, showing a unimodal Gaussian distribution without predominance of odd over even carbon-numbered aliphatic chains. The yields in general exceed the yields of aliphatic hydrocarbons obtained under the same conditions. The fatty acids may be derived from aldehydes and alcohols produced under the influence of the promoter and subsequently oxidized to the acids. 

Developments in the Fischer-Tropsch synthesiis at the Bureau of Mines from 19 5 to I960 include a simple mechanism for chain growth and the use of iron nitrides as catalysts. The chain-growth schene can predict the carbon-number and isomer distributions for products from most catalysts with reasonable accuracy using only 2 adjustable parameters. Iron nitrides are active, durable catalysts that produce high yields of alcohols and no wax. During the synthesis, the nitrides are converted to carbonitrides. 

The possible states of electrons are called orbitals. These are indicated by what is known as the principal quantum number and by a letter—s, p, or d. The orbitals are filled one by one as the number of electrons increases. Each orbital can hold a maximum of two electrons, which must have oppositely directed spins. Fig. A shows the distribution of the electrons among the orbitals for each of the elements. For example, the six electrons of carbon (B1) occupy the Is orbital, the 2s orbital, and two 2p orbitals. A filled Is orbital has the same electron configuration as the noble gas helium (He). This region of the electron shell of carbon is therefore abbreviated as He in Fig. A. Below this, the numbers of electrons in each of the other filled orbitals (2s and 2p in the case of carbon) are shown on the right margin. For example, the electron shell of chlorine (B2) consists of that of neon (Ne) and seven additional electrons in 3s and 3p orbitals. In iron (B3), a transition metal of the first series, electrons occupy the 4s orbital even though the 3d orbitals are still partly empty. Many reactions of the transition metals involve empty d orbitals—e.g., redox reactions or the formation of complexes with bases. 

In cast iron the amount of vanadium varies from 0.08 to 0.15 per cent. Its function is that of a scavenger, and its beneficial results are almost wholly indirect. It causes a more even distribution of the carbon, lessens porosity and brittleness, and checks spalling and flaking. The strength of the casting is increased 10-25 per cent by 0.1 per cent of vanadium. It is claimed that the decrease in the number of rejected castings... 

Above about 900 °C all carbon steels form a non-magnetic interstitial solid solution of carbon in y iron, called austenite. The amount of carbon which can be taken up in solid solution is, "however, limited to about 17 per cent by weight, equivalent to 7 5 atomic per cent, and it is therefore clear that the number of carbon atoms is insufficient for them to be arranged in any regular way and that they must be distributed at random in the interstices of the structures. If more carbon is present the excess occurs as graphite, and in this case the system is usually described as cast iron . It is convenient to restrict the term steel to those systems in which the carbon content does not exceed 7-5 atomic per cent. 

Manganese is relatively abundant, constituting about 0.085% of the earth s crust. Among the heavy metals, only iron is more abundant. Although widely distributed, it occurs in a number of substantial deposits, mainly oxides, hydrous oxides or carbonate, and from all these, or the Mn304 obtained by roasting them, the metal can be obtained by reduction with aluminum. 

---