Ind. Eng. Chem. Res. 2009, 48, 1443–1450
1443
Estimation of Wetting Efficiency in Trickle-Bed Reactors for Nonlinear Kinetics Aswani K. Mogalicherla, Gaurav Sharma, and Deepak Kunzru* Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India
A procedure has been developed for estimating the wetting efficiency in a trickle-bed reactor by using the conversion data obtained for a reaction following nonlinear kinetics. For this purpose, the hydrogenation of R-methylstyrene on 0.5 wt % Pd/Al2O3 catalyst was studied in a batch slurry reactor (1.0-6.0 atm, 313-343 K), as well as in a trickle-bed reactor (313 K, 1.0-2.5 atm). The superficial velocities of gas and liquid in the trickle-bed reactor (TBR) were in the range of 0.25-1.0 cm/s and 0.06-0.24 cm/s, respectively. The intrinsic power-law kinetics, determined from the data obtained in the batch reactor, was used to calculate the effectiveness factors in the TBR. A comparison of these effectiveness factors with those calculated assuming complete external wetting showed that the wetting was not complete. The external wetting efficiencies (f) were determined using the method proposed by Ramachandran and Smith [AIChE J. 1979, 25 (), 538.]. To this end, effectiveness factors for the catalyst completely covered with gas or liquid were estimated for this nonlinear kinetics by a previously published procedure. f varied between 40% and 75% with the liquid superficial velocity. The difference between f values calculated from the conversion data and from the correlation developed using residence time distribution (RTD) data was attributed to the presence of stagnant liquid zones between the catalyst particles. 1. Introduction Trickle-bed reactors (TBRs) are three-phase catalytic reactors, where gas and liquid at low superficial velocities flow cocurrently downward over a stationary catalytic bed. A number of studies have been reported on the hydrodynamics and kinetics in TBRs under different operating conditions.1-6 It is a common observation that, at low gas and liquid superficial velocities, catalyst particles in the reactor are not completely wetted by the liquid. Different techniques, such as residence time distribution (RTD),7-10 tomography,11 pressure drop,12 and colorimetry13 measurements, have been reported for estimating the wetting efficiencies in TBRs. Many correlations are available for estimating the wetting efficiency, but the general applicability of these equations is still doubtful.14 However, it is essential to estimate the partial wetting of catalytic beds in order to predict the performance of trickle-bed reactors. The wetting efficiency can also be determined by using the reaction method. This approach directly gives the chemically active area in the catalytic bed, which is an essential parameter in estimating the reactor performance. Morita and Smith15 studied hydrogenation kinetics of R-methylstyrene (AMS) in a trickle-bed reactor using two catalysts of different activities and estimated the wetting efficiency by solving the mass balance equations. Herskowitz et al.16 investigated the hydrogenation kinetics of AMS at 1 atm and 40 °C in a recycle trickle reactor and developed a procedure to evaluate trickle-bed effectiveness factor in terms of wetting efficiency. In addition, Herskowitz et al.17 used the same reaction to study the effects of gas and liquid superficial velocities on wetting efficiency. In all of the above studies, the kinetics of hydrogenation of AMS was assumed to be first-order. Khadilkar et al.18 compared the performance of trickle-bed and upflow reactors for both gaslimited and liquid-limited conditions, using the hydrogenation of AMS as the test reaction. The intrinsic rate data were obtained in a slurry reactor in the pressure range of 2.04-20.4 atm (30-300 psig) and modeled using Langmuir-Hinshelwood * To whom correspondence should be addressed. E-mail: dkunzru@ iitk.ac.in. Tel: +91-512-2597193. Fax: +91-512-2590104.
kinetics. Additionally, data were obtained in a basket reactor for determining the pellet effectiveness factors. The reactor was modeled using either a heterogeneous plug-flow model together with the experimentally determined apparent kinetics or a pellet scale model with the intrinsic kinetics. Because of the nonlinear kinetics, the mass balance equations were solved numerically, and satisfactory agreement was obtained between the calculated and experimental conversions. The wetting efficiency used in the above models was obtained from correlations. For partially wetted catalyst with complete internal wetting, Ramachandran and Smith19 proposed that the effectiveness factor in a TBR, ηTBR, can be estimated from the effectiveness factors when the catalyst is completely covered with liquid (ηL) and when it is completely surrounded by gas (ηG) as ηTBR ) fηL + (1 - f)ηG 20
(1)
Llano et al. included the effect of a static external wetting surface on the trickle-bed reactor performance and correlated the wetting efficiencies determined from kinetic studies to wetting efficiencies from RTD studies. Different models are available for calculating the wetting efficiencies from kinetic data, but most of them are suitable only for first-order kinetics.21,22 For nonlinear kinetics, it is not possible to obtain an analytical solution for effectiveness factors,23 and to avoid complexity in data analysis, most researchers have avoided the use, of nonlinear kinetics in estimating wetting efficiencies. An additional complexity that can be encountered with LangmuirHinshelwood kinetics is effectiveness multiplicity under certain conditions,24 resulting in a nonunique problem for estimating the wetting efficiency. Some theoretical models have been published for nonlinear kinetics, but the experimental validity of these models has not been ascertained.25,26 Most laboratory kinetic experiments in studies of trickle-bed reactors have been conducted after prewetting of the bed. This reduces the time required to achieve steady state, reduces catalyst deactivation, minimizes side reactions, and improves reproducibility.6,27-29Different prewetting procedures have been reported in the literature. These include Levec mode, Kan-Liquid mode, and super prewetting. In Levec prewetting, the packed catalyst
10.1021/ie801305t CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
1444 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
bed is preflooded and then drained under gas flow, and the liquid flow is increased in steps starting from the lowest operating velocity. In the Kan-Liquid mode of operation, the bed is flooded, the gas and liquid flow rates are set at the desired values, and then the liquid velocity is increased until pulsing flow starts. Subsequently, the liquid velocity is decreased to the desired value. In super prewetting, the flooded bed is subjected to draining by fixing the gas and liquid flow rates to the desired values. van Houwelingen et al.30 reported that, depending on the prewetting procedure, zones with different wetting efficiencies can exist in the trickle-bed reactor, and this can have a strong influence on the reactor performance. Indeed, van Houwelingen et al.31 reported large differences in the wetting efficiencies (>20%) between the Levec and Kan-Liquid prewetting modes of operation. The effects of the prewetting procedure on the performance of a trickle-bed reactor for the hydrogenation of AMS was studied by van der Merwe et al.32 They observed better reactor performance for Kan-Liquid mode than for Levec mode because of channeling effects in the latter mode for gas-limited reactions. The above discussion highlights that, in previous studies, either the external wetting efficiencies in trickle-bed reactors were determined experimentally by assuming first-order kinetics, or the experimental conversions were compared with numerically obtained model predictions by estimating wetting efficiencies from correlations. Because most multiphase reactions of commercial interest follow nonlinear kinetics, the objective of the present study was to develop a procedure for estimating the wetting efficiencies in a TBR using the conversion data obtained for a reaction following nonlinear kinetics. The test reaction was the hydrogenation of AMS. To eliminate the effects of hydrodynamic multiplicity, an identical prewetting procedure was followed for all experimental runs. The calculated efficiencies were compared with an available correlation, and the fraction of surface covered by stagnant liquid was estimated. 2. Experimental Section 2.1. Catalyst Preparation. For all runs, 0.5 wt % Pd/Al2O3 was used as the catalyst. Palladium chloride (Loba Chemie, Mumbai, India) was used as the precursor. For the batch reactor studies, γ-alumina (average particle size, 35 µm; surface area, 155 m2/g) from Grace Chemicals (Columbia, MD) was used as the support. Before impregnation of the metal, the average particle size of the support was reduced to 3 µm in a planetary mono mill (Pulverisette 6, Fritsch GmbH, Idar-Oberstein, Germany). For trickle-bed studies, commercial 1/8-in. γ-alumina pellets (surface area, 170 m2/g) were crushed, and the size fraction between 1.0 and 1.7 mm was used as the support. In both cases, the desired amount of PdCl2 was dissolved in 5 mL of concentrated HCl and added to a slurry of 10 g of alumina in 50 mL of water. The water was removed at 60 °C in a rotary vacuum evaporator, and the catalyst was dried at 110 °C and finally calcined at 450 °C for 4 h. The palladium dispersion on alumina was determined by hydrogen chemisorption on a Micromeritics Pulse Chemisorb 2705 unit. The palladium dispersion was 26.0% for the catalyst used in the batch reactor and 15.5% for the catalyst used in the trickle-bed studies. Preferably, the intrinsic kinetics should have been determined by using crushed trickle-bed catalyst. However, earlier studies have shown that, to eliminate internal diffusional resistance for the hydrogenation of AMS on Pd/Al2O3 catalyst, the average catalyst size should be less than 8 µm.32,34Attempts were made to crush the trickle-bed catalyst to dp Bo
(2)
where L is the length of the catalyst bed; dp is the catalyst particle diameter; Bo is the Bodenstein number; and Co and Cf are the liquid reactant concentrations entering and leaving the active zone, respectively. For laminar flow of liquids (for which the Reynolds number for the liquid, ReL, is
