Importance of the Usage Ratio in Iron-Based Fischer-Tropsch Synthesis with Recycle Nileinindran S. Govender,* ,²,‡ Matthys Janse van Vuuren, ‡ Michael Claeys, ² and Eric van Steen ² Catalysis Research Unit, Department of Chemical Engineering, UniVersity of Cape Town, PriVate Bag X3, Rondebosch 7701, South Africa, and Sasol Technology, Sasolburg 1947, South Africa The present study was undertaken to understand the fundamentals of recycling the tail gas during the Fischer- Tropsch Synthesis with an iron-based catalyst with particular emphasis on the importance of the usage ratio. The usage ratio was ca. 1.4 for the catalyst studied at our reaction conditions (240 °C, 20 bar, 5000 mL/ (g cat ‚h)). The study shows the H 2 /CO ratio in the recycle gas fed to the reactor depends on both the H 2 /CO ratio in the fresh feed and the usage ratio of the catalyst. For an H 2 /CO in the fresh feed higher than the 1.4, the H 2 /CO ratio in the reactor increases with the increasing recycle ratio whereas the converse is true for H 2 /CO ratios lower than 1.4. For negligible change in the Fischer-Tropsch selectivity during external recycle, the H 2 /CO ratio in the fresh feed should match the usage ratio of the catalyst. The presence CO 2 in this recycled gas did not change the overall CO 2 selectivity; hence, CO 2 is regarded as an inert at these conditions. Introduction The Fischer-Tropsch process refers to the synthetic produc- tion of hydrocarbons by catalytically reacting carbon monoxide (CO) and hydrogen (H 2 ). Extensive reviews have already been published in this field. 1-4 The Fischer-Tropsch reaction (eq 1) produces 1 mol of water for every mole of CO converted to carbon in an organic product compound: The water produced in the Fischer-Tropsch reaction can be consumed in the water gas shift reaction (eq 2) yielding CO 2 and H 2 : The usage ratio of hydrogen to carbon monoxide (i.e., the rate at which hydrogen is consumed relative to the rate at which carbon monoxide is being consumed) equals 2 if the water gas shift reaction does not occur. 5 If all the water formed in the Fischer-Tropsch reaction is consumed in the water gas shift reaction, then the overall H 2 /CO usage ratio would be 0.5. Therefore, the exit H 2 /CO ratio in the Fischer-Tropsch synthesis can be lower or higher than the initial H 2 /CO ratio at the inlet of the reactor, depending on whether the initial H 2 /CO ratio is lower or higher than the usage ratio. Espinoza et al. 6 illustrated this change in the exit H 2 /CO ratio of a FT reactor with a cobalt-based catalyst for different conversions when the initial H 2 /CO ratio is below, at, or above the usage ratio of 2.15. Boelee et al. 7 reported similar trends for an iron-based catalyst albeit the extent of the water gas shift reaction is higher for iron-based catalysts. However, such plots for varying recycle ratios have not yet been reported in the open literature to the best of our knowledge. We expect similar trends since with increasing recycle ratio, the overall conversion also increases. Raje et al. 8 argues that the CO conversion per pass of a low- temperature Fischer-Tropsch reactor must be limited, since at high CO conversion the methane selectivity increases. However, a low CO conversion results in a lower rate of hydrocarbon production and typically a higher selectivity for the water gas shift reaction (i.e., a higher CO 2 selectivity). This will thus result in a lower H 2 /CO usage ratio. For the economically viable operation of an iron-based Fischer-Tropsch technology, two options are available: (i) use a diluted feed such as nitrogen-rich synthesis gas, thereby saving on synthesis gas costs 9 or (ii) recycle of the unconverted synthesis gas that leaves the reactor, after condensation of the liquid products (or use a number of reactors in series with intermediate condensation of the products). The most common FT plant schemes recycle the tail gas back to the synthesis gas preparation section with CO 2 removal 5,10,11 or without CO 2 removal. 12,13 Raje and Davis 14 illustrated the benefits of low single-pass synthesis gas conversion with recycle for situations in which the H 2 /CO ratio of the unconverted recycle synthesis gas and the fresh feed are the same. The recycle reactor can process more than double the volume of synthesis gas per weight of iron and produces twice as much hydrocarbons as the single pass reactor. These were mainly light hydrocabons (alkenes), and this increase is expected since the space velocity through the recycle reactor was higher. The removal of CO 2 from the recycle stream was also considered in their study. In our research, we keep the space velocity (residence time in the reactor) constant, and only water is removed from the recycled stream. Recycling cold tail gas back to the reactor may alter the hydrogen to carbon monoxide ratio in the reactor. This is the most important parameter, 2 which controls the selectivity for the precipitated iron catalyst at low temperatures. An increase in the H 2 partial pressure results in termination of the surface species to paraffins while an increase in the CO partial pressure results in a higher probability of chain growth. 15 Thus, one could expect that the higher the H 2 partial pressure and the lower the CO partial pressure or the higher the H 2 /CO ratio, the higher will be the production of CH 4 and lighter hydrocarbons and the lower the production of heavier hydrocarbons such as wax. * Corresponding author. Telephone: +31-40-247-4953. Fax: +31- 40-244-6653. E-mail: g.govender@tue.nl. ² University of Cape Town. ‡ Sasol Technology. CO + 2H 2 f -CH 2 -+ H 2 O (1) CO + H 2 O f CO 2 + H 2 (2) 8629 Ind. Eng. Chem. Res. 2006, 45, 8629-8633 10.1021/ie060415e CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2006