CHEMICAL ENGINEERING TRANSACTIONS VOL. 32, 2013 A publication of The Italian Association of Chemical Engineering Online at: www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2013, AIDIC Servizi S.r.l., ISBN 978-88-95608-23-5; ISSN 1974-9791 Modeling of Fischer-Tropsch Product Distribution over Fe-based Catalyst Branislav Todic a,b , Tomasz Olewski a , Nikola Nikacevic b , Dragomir B. Bukur* ,a,c a Chemical Engineering Program, Texas A&M University at Qatar, PO Box 23874, Doha, Qatar b Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade, Serbia c Artie McFerrin Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, TX 77843, USA dragomir.bukur@qatar.tamu.edu The kinetic models of Fischer-Tropsch synthesis (FTS) product distribution can be classified into two major groups: hydrocarbon selectivity models and detailed Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic models. In this study the two approaches to FTS product distribution modeling are presented and compared using the experimental data obtained in a stirred tank slurry reactor with promoted iron catalyst over a wide range of process conditions. Positive deviations from the classical Anderson-Schulz-Flory distribution and an exponential decrease in olefin-to-paraffin ratio with carbon number are predicted by the inclusion of solubility-enhanced 1-olefin readsorption and/or chain length dependent 1-olefin desorption concepts. In general the agreement between the model predictions and experimental data was very good, and modeling approaches are discussed in terms of fit quality, physical meaningfulness and practical utility. 1. Introduction The main products of the FTS reaction are n-paraffins and 1- and 2-olefins. These products are obtained from a mixture of CO and H 2 over a heterogeneous catalyst (Bhatelia et al., 2011). FTS product distribution was initially described with the so called Anderson-Schulz-Flory (ASF) distribution characterized by the chain growth probability factor (α), independent of the number of carbon atoms in the product molecule. Later a change in growth probability with carbon number was observed over the Fe-based catalysts and today it is well known that deviations from the ASF distribution over all FTS catalyst types (Fe, Co, Ru etc.) include: high yield of methane, low yield of ethene and increasing chain growth probabilities with increasing carbon number (van der Laan and Beenackers, 1999a). Another important feature of the experimental product distribution is the exponential decrease in olefin-to-paraffin ratio (OPR) with carbon number (for C 3+ hydrocarbons). In order to account for the behavior of the FTS product distribution many different models have been proposed in the literature. They can be classified into two major groups: hydrocarbon selectivity models and detailed Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic models. The selectivity models are based on FTS reaction networks (simplified reaction mechanisms) and most can be grouped either as the double-alpha models or the olefin readsorption models. Latter are more frequently used because they calculate selectivity for various product species (paraffin and olefin), while the double-alpha models predict only the total hydrocarbon formation (lumped paraffin and olefin). Recently Botes (2007) proposed a third type of selectivity model for the Fe catalyst, based on the new hypothesis of chain length dependent olefin desorption, where the activation energy for olefin desorption increases with carbon number due to the effect of weak interactions between desorbing chain and the catalyst surface. Similar to the readsorption models, this model was also capable of predicting the deviations from ASF and the change of OPR with carbon number. Contrary to these, the detailed LHHW kinetic models consider the entire FTS mechanism: adsorption of reactants, formation of monomer, chain initiation, propagation and termination. It is important to note that 793