Journal of Catalysis 214 (2003) 251–260 www.elsevier.com/locate/jcat Wax composition transients during Fischer–Tropsch synthesis Dario Pinna, a Enrico Tronconi, a Luca Lietti, a Roberto Zennaro, b and Pio Forzatti a,∗ a Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta,” Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milan, Italy b Catalysis and ProcessTechnology Research Center, EniTecnologie, Via Maritano, 26, 20097 San Donato Milanese, Italy Received 5 August 2002; revised 30 October 2002; accepted 30 October 2002 Abstract The temporal evolution of the composition of the products collected from a slurry reactor during Fischer–Tropsch synthesis over a Co/Al 2 O 3 catalyst is studied, in order to rationalize the time required for a complete renewal of the liquid phase used to initially dilute the catalyst. The transients are well described by a simple equation with no assumption on the phase repartition of the hydrocarbons in the reactor, which also provides an estimate of the time constant of the system. The mathematical model can be used to predict the time necessary to reach steady state of the composition of the reactor outlet and hence collect reliable data for product selectivity in a slurry reactor. The change in the composition at the outlet of a fixed bed reactor after step changes of a co-fed liquid olefin is also approximately analyzed: it is shown that the transient is much faster for a fixed bed reactor than for a slurry reactor.  2003 Elsevier Science (USA). All rights reserved. Keywords: Fischer–Tropsch synthesis; Slurry reactor; Fixed bed reactor; Unsteady mass balance 1. Introduction The catalytic conversion of synthesis gas into hydrocar- bons (Fischer–Tropsch synthesis, FTS) is recently receiving great attention, as a powerful way of exploiting natural gas wells located in remote areas [1–3]. Syngas, produced from natural gas by partial oxidation or steam reforming, is trans- formed into a complex mixture of gaseous, liquid, and solid hydrocarbons and alcohols that can be further processed to maximize the liquid product yield [3]. In general heavy paraffins (waxes) are the preferred products of FTS, since they can be easily hydrocracked to liquid fractions (gasoline and diesel fuel), whereas light hydrocarbons are undesired products. The renewed interest in FTS has led to many works rel- ative to investigation of the process variables. In particular, various experimental studies were performed to accurately describe activity and product selectivity over different cat- alysts [4]. Both fixed bed and slurry reactors were widely used to test the performance of catalysts, the former system being easier to operate, the latter allowing a better tempera- ture control. Yet the performances of these types of reactor may be different, due to the fluid dynamic and dimensional * Corresponding author. E-mail address: pio.forzatti@polimi.it (P. Forzatti). characteristics of the two systems. In fact it is common prac- tice to initially load a slurry reactor with a liquid, to suspend the fresh catalyst, before the start-up of the unit. For this rea- son, if heavy product selectivity is a variable of interest, the time required to collect reliable data is longer for a slurry re- actor rather than for a fixed-bed reactor. Indeed the presence of the liquid wax in the reactor (either if initially contained in the slurry reactor, or if produced by the reaction and retained in the system) may in principle modify the hydrocarbon dis- tribution in the effluent, by diluting the real liquid products. The replacement of wax during the reaction is a matter of great interest in the operation of both laboratory and indus- trial scale units, being crucial in one case to collect reliable data, in the other to control and predict the specifications of the products. The problem relative to the wax renewal and its con- sequences on reaction selectivities has been approached in different ways in the literature. Some authors explicitly con- sidered the dilution of the produced wax with the initial liquid, by taking into account the time necessary to remove the old wax by the newly synthesized one; some others just considered the gaseous products, whose transient is much faster. Gormley et al. [5], in their study on the effect of the initial wax media on FTS in a slurry reactor, corrected the heavy products distribution by considering the daily disappearance 0021-9517/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9517(02)00151-3