Published: March 10, 2011 r2011 American Chemical Society 4359 dx.doi.org/10.1021/ie102095c | Ind. Eng. Chem. Res. 2011, 50, 4359–4365 ARTICLE pubs.acs.org/IECR A New Way to Look at FischerÀTropsch Synthesis Using Flushing Experiments Xiaojun Lu, Xiaowei Zhu, Diane Hildebrandt,* Xinying Liu, and David Glasser Center of Material and Process Synthesis, University of the Witwatersrand, Johannesburg, South Africa ABSTRACT: When FischerÀTropsch synthesis (FTS) reaction experiments were conducted in a gasÀsolid system with a TiO 2 - supported cobalt catalyst in a continuous stirred tank reactor (CSTR), we observed significant changes in the reaction rate and product selectivity at an early stage of time on stream (TOS) when all the reaction conditions were kept constant. 1 We designed flushing experiments with an inert gas that started when the FTS reaction had reached steady state. After the completion of flushing, the FTS reaction was resumed with syngas. We then compared the results of the FTS reaction rate and product selectivity both before and after flushing. The flushing experimental results suggested that the marked variations we observed were caused (either wholly or mainly) by liquid products deposited in the catalyst rather than by the change in the properties of the catalyst surface. The concentrations and the relative amount of the reactant in the flushed out stream were examined and the implications of the high H 2 / CO ratio for the reaction kinetics and product selectivity are discussed. On the basis of the dynamic concentrations of C 1 ÀC 8 in the flushed outgas, we propose that reaction among the products takes place under moderate FTS reaction conditions. 1. INTRODUCTION Our previous paper 1 showed that when low-temperature FTS was conducted on a TiO 2 -supported cobalt catalyst (10% Co/ 90% TiO 2 ) in a CSTR, rapid and substantial changes occurred in the FTS reaction rate and product selectivity at a certain time on stream (TOS). These changes can be clearly seen in Figures 1 and 2. In these examples, changes were observed to start at around 25 h of TOS. The time at which these changes occurred varied with the reaction temperature. 1 Two pseudosteady states (from around 8À25 h and after 85 h in Figures 1 and 2) were observed, and the secondary steady state was maintained without any further change for long TOS. The possible reasons we suggested for these large and sudden changes were: alterations in catalyst surface properties in a syngas environment or the deposit of liquid phase products in the catalyst. The latter of these could affect the mass transfer of reactants and products, and consequently alter the reaction rate and product selectivity. These transient phenomena are believed by some to be partly the result of the accumulation of the liquid in the catalyst. 2,3 Anderson et al. 4 first reported that intraparticle diffusion restric- tions on the rate of reactant arrival to hydrocarbon synthesis sites controlled the CO conversion rate of Fe-based catalysts. Post et al. 5 report a simplified transport-reaction model that describes only H 2 transport limitations, although CO is the more probable diffusion-limited reactant in Fe and Co catalysts; they address only rate effectiveness factors for the primary CO hydrogenation reaction and do not discuss transport effects on synthesis selectivity or on secondary reactions. Iglesia et al. 6 report a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) transport processes; these processes control the rate of arrival of CO and H 2 and the rate of removal of reactive products within catalyst pellets and reactors. The transport limitation enhanced the secondary reaction of the R-olefins. However there was no experimental evidence to prove the effect is from the liquid products in the catalyst directly, or to explain the extent to which the performance of the FTS could be affected. On the other hand, a supported Co FT catalyst is believed by others to reconstruct in a syngas atmosphere, and alter the surface properties of the catalyst, which in turn will affect its performance, such as reaction rate and product selectivity. Schulz 7,8 and his co- workers reported that the change in product selectivity and increase of activity during reaction were caused by the “catalyst construction”. CO chemisorbs strongly on cobalt (as well as on Ni and Ru) and it has been pointed out by Pichler 9 that FT synthesis performs under conditions not so far from those which allow (thermodynamically) carbonyl formation from these metals. Thus the reaction of CO with the metal surface can be assumed to induce surface restructuring. 7 Images of a cobalt metal surface which had been used for FT synthesis were obtained by Wilson and de Groot 10 through scanning tunneling electron microscopy. It is deduced from those pictures that segrega- tion produces an ordered surface structure under a syngas atmosphere. As a precise explanation for these phenomena could not be given on the basis of the experiments and the subsequent analysis we performed, we concluded further experiments need to be designed and carried out. 1 A group of flushing experiments with inert gas (argon) at various temperatures, plus FTS runs with syngas after flushing, were designed and conducted. The reac- tants and hydrocarbons from the reactor system during and after flushing were analyzed. The results are discussed below. 2. EXPERIMENTAL SECTION 2.1. Standard FTS Experiments. The FT experiments were carried out in a 100 mL continuous stirred tank reactor (CSTR) Received: October 15, 2010 Accepted: February 28, 2011 Revised: February 21, 2011