Low-temperature Fischer-Tropsch

Jager, B., and Espinoza, R. 1995. Advances in low-temperature Fischer-Tropsch synthesis. Catalysis Today 23 17-28. 

In this chapter a two a selectivity model is proposed that is based on the premise that the total product distribution from an Fe-low-temperature Fischer-Tropsch (LIFT) process is a combination of two separate product spectrums that are produced on two different surfaces of the catalyst. A carbide surface is proposed for the production of hydrocarbons (including n- and iso-paraffins and internal olefins), and an oxide surface is proposed for the production of light hydrocarbons (including n-paraffins, 1-olefins, and oxygenates) and the water-gas shift (WGS) reaction. This model was tested against a number of Fe-catalyzed FT runs with full selectivity data available and with catalyst age up to 1,000 h. In all cases the experimental observations could be justified in terms of the model proposed. 

The use of a Fischer-Tropsch (FT) process to produce long-chain hydrocarbons is well known in industry, and achieving the desired selectivity from the FT reaction is crucial for the process to make economic sense. It is, however, well known that a one-alpha model does not describe the product spectrum well. From either a chemicals or fuels perspective, hydrocarbon selectivity in the FT process needs to be thoroughly understood in order to manipulate process conditions and allow the optimization of the required product yield to maximize the plant profitability. There are many unanswered questions regarding the selectivity of the iron-based low-temperature Fischer-Tropsch (Fe-LTFT) synthesis. 

Botes, F.G. 2007. Proposal of a new product characterisation model for the iron-based low-temperature Fischer Tropsch synthesis. Energy Fuels 21 1379. 

Cobalt-based low-temperature Fischer-Tropsch (Co-LTFT)... 

The first commercial Fischer-Tropsch facility was commissioned in 1935, and by the end of the Second World War a total of fourteen plants had been constructed. Of these, nine were in Germany, one in France, three in Japan, and one in China. Both German normal-pressure and medium-pressure processes (Table 18.1) were employed. The cobalt-based low-temperature Fischer-Tropsch (Co-LTFT) syncrude produced in these two processes differed slightly (Table 18.2), with the product from the medium-pressure process being heavier and less olefinic.11 In addition to the hydrocarbon product, the syncrude also contained oxygenates, mostly alcohols and carboxylic acids. 

The synthetic fuels that can be produced by low-temperature Fischer-Tropsch synthesis inherently have a high quality (being sulfur- and aromatics-free) and can therefore be used as quality improvers with conventional components. 

Low-temperature Fischer-Tropsch (LTFT) synthesis runs at temperatures between 200°C and 250°C [23-25]. The chain-growth probability at these conditions is much higher than for the HTFT, and as a consequence, the product distribution extends well into the liquid waxes. LTFT reactors are thus three-phase systems solid catalysts, gaseous reactants, and gaseous and liquid products. Both cobalt and iron... 

Espinoza RL, Steynberg AP, Jager B, Vosloo AC. Low temperature Fischer-Tropsch synthesis from a Sasol perspective. Appl Catal A Gen. 1999 186(1—2) 13—26. 

Catalysts of commercial significance are either iron-based or cobalt-based. Iron-based catalysts are typically not supported, whereas cobalt-based catalysts are usually supported on alumina, silica, or a similar material. The three-phase low-temperature Fischer-Tropsch (LTFT) technology can be operated in either... 

B. Jager, R.C. Kelfkens and A.P. Steynberg, A Slurry Bed Reactor for Low Temperature Fischer-Tropsch , Natural Gas Conversion It, Elsevier Science B.V. (1994)... 

Cobalt-based low temperature Fischer—Tropsch catalysts, appHed at approximately 220 °C and 30 atm, are usually supported on high-surface-area Y-AI2O3 (150—200 m g ) and typically contain 15—30% weight of cobalt. To stabihze them and decrease selectivity to methane, these catalysts may contain small amounts of noble metal promoters (typically 0.05—0.1 wt% of ruthenium, rhodium, platinum, or palladium) or an oxide promoter (e.g., zir-conia, lanthana, cerium oxide, in concentrations of 1—10 wt%) (409). 

Gideon Botes, F. (2007). Water-gas-shift kinetics in the iron-based low-temperature Fischer—Tropsch synthesis. Applied Catalysis A General, 328, 237—242. 

Guettel, R., Turek, T. (2009). Comparison of different reactor types for low temperature Fischer-Tropsch synthesis a simulation study. Chemical Engineering Science, 64, 955—964. Scopus Exact. 

Fig. 1. Muititubular and slurry bed reactors for low temperature Fischer-Tropsch operations (high wax production).

Pig. 3. Low temperature Fischer-Tropsch multitubular catalyst deactivation profiles. Time on stream increases from A to B to C. 

Fig. 6. Relation between the selectivities of the hydrocarhon cuts for the low temperature Fischer-Tropsch process. The selectivities are on a mass % basis.

Fig. 7. Hard wax (boiling point >500°C) selectivity as a function of the H2/CO ratio of the feed gas (low temperature Fischer-Tropsch operation).

Figure 1.17 Slurry bubble column reactor for low-temperature Fischer-Tropsch synthesis.

Figure 2.31 Flow scheme for natural gas- based low-temperature Fischer-Tropsch synthesis [409]. Reproduced with the permission of ACS.

Overall, the fixed-bed reactor choice is easy to operate and scale up. They can be used over a wide temperature range and the liquid/catalyst separation can be performed easily and at low costs, rendering this reactor type suitable for LTFT (low-temperature Fischer—Tropsch). Moreover, in the case of syngas contamination with H2S, the H2S is absorbed by the top catalyst layer and does not affect the rest of the bed, thus no serious loss of activity occurs (Dry, 1996). On the down side, fixed-bed reactors are expensive to construct and the high gas velocities required translate to high gas compression costs for the recycled gas feed. Moreover, it is maintenance- and labor-intensive and has a long downtime due to the costly and time-consuming process of periodical catalyst replacement (Tijmensen et al., 2002). 

Table 18.4 The carbon number distribution of high-temperature Fischer—Tropsch (HTFT) and low-temperature Fischer—Tropsch (LTFT) products, excluding Ci—C2 hydrocarbons...

---