Effects of Induced Pulsing Flow on Trickle-Bed Reactor Performance B. A. Wilhite, X. Huang, M. J. McCready, and A. Varma* Department of Chemical Engineering and Center for Molecularly Engineered Materials, University of Notre Dame, Notre Dame, Indiana 46556 The benefits of trickle-bed reactor operation under the induced pulsing flow regime are investigated using experiments and modeling. Under these conditions, by cycling the liquid feed, trickling and pulsing flow regimes can be made to alternate during the cycle period under time- averaged conditions corresponding to the trickling flow regime. For the hydrogenation of phenylacetylene over Pt/γ-Al 2 O 3 catalyst, experimental results obtained in a laboratory-scale reactor operating under mild gas-limiting conditions indicate better performance for steady flow, as opposed to induced pulsing flow. The model predictions compare well with the experimental data. Further, simulations of a trickle-bed reactor over a wide range of initial reactant concentrations and pressures predict up to 45% improvement in styrene selectivity for induced pulsing flow under liquid-limited conditions. The findings suggest that enhancements in reactor performance due to induced pulsing can be expected for liquid-limited systems, which generally operate at low liquid flow rates, as are commonly encountered in industrial practice. Introduction Trickle-bed reactors, in which gas and liquid reactants are fed in cocurrent downflow over a packed bed of catalyst, are commonly utilized for conducting mul- tiphase reactions in the chemical, petrochemical, and pharmaceutical industries. 1,2 Numerous studies are available in the literature investigating important bed characteristics, including liquid distribution, contacting efficiency, partial wetting, and local hydrodynamic regime, that significantly influence reaction behavior. 3-7 These factors can then be incorporated into the design and operation of such systems to improve reactor productivity. Recent studies have shown that significant improve- ments in multiphase reactor performance over steady- state operation can result from either cycling or switch- ing the liquid feed, as illustrated in Figure 1. The first experimental demonstration of the latter technique was presented by Huare et al. 8 for the catalytic oxidation of SO 2 in a trickle-bed reactor. In this system, the gas phase, consisting of dilute SO 2 in air, reacted over a bed of activated carbon to form SO 3 . The aqueous phase was then used to remove the SO 3 from the catalyst surface, allowing the gas-solid reaction to proceed. Thus, by switching the liquid feed, rather than operating under steady flow conditions, a significant improvement (30- 45%) in SO 2 conversion was obtained. Further studies aimed at determining the optimal split and cycle times attempted to balance the tradeoff between the flushing (nonzero liquid flow) and reaction (zero liquid flow) periods. 9 Castellari and Huare 10 investigated the effects of flow switching on the hydrogenation of R-methyl styrene (AMS) in a trickle-bed reactor operating under gas- limited conditions. Unsteady operation allowed the bed to approach runaway conditions in the absence of liquid flow, as indicated by a temperature rise of over 30 °C, with periodic quenching of the bed during the remainder of the cycle. In this manner, an improvement in conver- sion of more than 400% over steady-state operation was obtained. Turco and co-workers 11 studied the effects of liquid feed cycling on the same reaction under similar gas-limited conditions. Isothermal reactor operation at 40 °C and average flows nearly double those employed by Castellari and Huare 10 resulted in ∼50% improve- ment in conversion over the corresponding steady-state performance. In this case, the benefits from flow cycling were not a consequence of controlled runaway conditions occurring as a result of transient operation; instead, cycling of the liquid feed caused a transition from the low interaction regime (trickling) to the high interaction regime (foaming), which significantly improved gas- * To whom correspondence should be addressed. Tel.: 219- 631-6491. Fax: 219-631-8366. E-mail: avarma@nd.edu. Figure 1. Liquid feed strategy for (a) flow cycling, or base/peak modulation, and (b) feed switching, or on/off modulation. 2139 Ind. Eng. Chem. Res. 2003, 42, 2139-2145 10.1021/ie020591x CCC: $25.00 © 2003 American Chemical Society Published on Web 04/12/2003