Effects of Induced Pulsing Flow on Trickle-Bed Reactor Performance

Apr 12, 2003 - In the limiting case of steady flow (R = 1, σ = 0), the column operated in the trickling flow regime. .... were obtained using the cor...
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Ind. Eng. Chem. Res. 2003, 42, 2139-2145

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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 timeaveraged conditions corresponding to the trickling flow regime. For the hydrogenation of phenylacetylene over Pt/γ-Al2O3 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 multiphase 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 improvements in multiphase reactor performance over steadystate operation can result from either cycling or switching 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 SO2 in a trickle-bed reactor. In this system, the gas phase, consisting of dilute SO2 in air, reacted over a bed of activated carbon to form SO3. The aqueous phase was then used to remove the SO3 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 (3045%) in SO2 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 Huare10 investigated the effects of flow switching on the hydrogenation of R-methyl styrene (AMS) in a trickle-bed reactor operating under gaslimited conditions. Unsteady operation allowed the bed to approach runaway conditions in the absence of liquid * To whom correspondence should be addressed. Tel.: 219631-6491. Fax: 219-631-8366. E-mail: [email protected].

Figure 1. Liquid feed strategy for (a) flow cycling, or base/peak modulation, and (b) feed switching, or on/off modulation.

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 conversion of more than 400% over steady-state operation was obtained. Turco and co-workers11 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 Huare10 resulted in ∼50% improvement 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-

10.1021/ie020591x CCC: $25.00 © 2003 American Chemical Society Published on Web 04/12/2003

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liquid mass transfer. Khadilkar et al.12 studied the effects of flow switching and flow cycling on the hydrogenation of AMS under liquid-limited conditions. In both cases, up to 12% enhancement over steady-state operation was reported at low average flows, where periodic flushing of bed maldistributions improved the overall liquid-solid contacting efficiency. Similar findings were reported by Stradiotto et al.13 for the hydrogenation of crotonaldehyde. Transient bed behavior can also be achieved under steady feed conditions by exploiting the inherent hydrodynamics of trickle-bed systems. Depending on factors such as gas and liquid flow rates, physical properties, and nature of the reactor packing, a variety of flow regimes can occur within the packed bed, with trickling and pulsing flows being of primary interest.2 The former, occurring at low feed rates, results in steady flow of both the gas and liquid phases, and is characterized by incomplete wetting and poor contacting efficiency. In the latter, alternating liquid-rich (pulse) and gas-rich (base) regions traverse the column length, resulting in enhanced gas-liquid and liquid-solid transport rates.14,15 Recent studies6,7,16 have demonstrated that, at identical feed conditions, because of the removal of external mass-transport limitations, operation under pulsing flow significantly improves reactor performance as compared to operation under trickling flow. In practice, the advantages of operating under pulsing flow conditions can be offset by the required high gas and liquid throughputs, resulting in reduced contact times. A means of circumventing this drawback was presented recently by Boelhouwer and co-workers,17,18 who showed that cycling the liquid feed can induce pulsing flow at lower time-averaged throughputs. Using this technique, the flow regime within the reactor can be made to cycle between trickling and pulsing flows under the low and high liquid periods, respectively. In light of the recent findings under pulsing flow at steady feed conditions, the induced pulsing mode of operation offers potential for improving new and existing reactor designs. Indeed, this possibility was anticipated and discussed recently by Boelhouwer et al.,19 although no experiments under induced pulsing reaction conditions were reported. In the present work, using the hydrogenation of phenylacetylene to styrene and ethylbenzene as an example, the performance of a trickle-bed reactor under induced pulsing conditions caused by periodic cycling of the liquid feed is compared with the corresponding steady-state trickling flow operation. Initial rates of reaction and selectivity to the desired intermediate styrene are determined experimentally for various cycling conditions and average flows and compared with model predictions. The results from both experiments and simulations are explained in the context of the current literature on the transient operation of tricklebed reactors.

Experimental Section Reaction System. The reaction system employed in this work is the hydrogenation of phenylacetylene (PA), dissolved in n-tetradecane, to styrene (ST) and ethylbenzene (EB) over Pt/γ-Al2O3 catalyst.

This is an important industrial process20,21 that proceeds under relatively mild conditions and presents both conversion and selectivity issues. In addition, the intrinsic kinetics of this system have been developed in the literature,22 allowing for a comparison of experiments with theoretical predictions. Because of these advantages, phenylacetylene hydrogenation is often used as a model system for evaluating reactor performance.7,16,23,24 Apparatus. The reactor consisted of a 2.54 cm i.d. × 40 cm Pyrex tube packed with 20 g of catalyst, with the remaining volume loaded with inert material of identical shape and geometry (Figure 2a). A core-annular inlet section ensured even an distribution of the gas and liquid feeds to the column. The transparent nature of the column allowed for visual confirmation of the flow regime under each set of conditions studied. Details of the apparatus and operating procedure can be found elsewhere.7 Prior to each experiment, liquid was fed at a sufficient rate to completely wet the packing. Cycling of the liquid feed was achieved via a three-way solenoid valve (Burkert PN0330SS) controlled by an Apple Power Macintosh 7100/80 computer equipped with LabView (see Figure 2b). The base and peak flow rates were controlled by separate Gilmont Accucal (GF-4540) rotameters. The initial liquid phase consisted of dilute phenylacetylene in 1 L of n-tetradecane solvent to maintain isothermal conditions, and the gas phase was ultra-high-purity hydrogen. All organic species were supplied by Aldrich Chemical Co., and all gases were from Praxair Specialty Gas. The catalyst employed was 2.5-mm-diameter, 0.5 wt % Pt/γ-Al2O3 (ESCAT-26, Engelhard), with the platinum metal dispersed in an eggshell distribution (with a thickness of 0.23 mm) to minimize intraparticle diffusion limitations. Before use, the catalyst was treated under a 100 mL/min flow of H2 at 400°C for 12 h to ensure complete reduction. Platinum dispersion was measured as 79 ( 4% using the hydrogen chemisorption technique. Samples of the liquid phase were taken at regular time intervals and analyzed using a Hewlett-Packard 5890 gas chromatograph with a 30 mm × 32 mm × 25 µm HP-5 packed capillary column. Peak areas for each species were normalized relative to decane and converted to concentrations via calibration curves generated experimentally, following the internal standard technique. The species concentrations at each sample interval verified a mass balance, typically within (2%. Flow Cycling Strategy. Liquid feed cycling can be described fully by four quantities: the peak and base flow rates (Lp and Lb, respectively) and their corresponding time periods (tp and tb). A combination of these variables yields the following parameters that charac-

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Figure 2. Details of experimental apparatus: (a) packing configuration for all experiments, (b) flow cycling schematic, (1) liquid from reservoir, (2) peak liquid flow meter, (3) base liquid flow meter, (4) three-way solenoid valve, (5) to reactor inlet, (6) computer, (7) I/O board, (8) control voltage, (9) digital relay, (10) 120-V ac current supply.

terize the flow cycling strategy

R ) Lp/Lb

(1)

tc ) tb + tp

(2)

σ ) tp/tc

(3)

L h)

Lbtb + Lptp tc

(4)

where R is the ratio of the peak and base flow rates, tc is the cycle time, σ is the fraction of cycle time spent operating at peak flow, and L h is the time-averaged liquid flow rate. For feed cycling to produce induced pulsing flow, the additional constraint is that the peak liquid flow rate corresponds to the natural pulsing flow regime.17,18 For the constant 3 L/min gas feed employed throughout this study, the steady-state transition to pulsing flow occurred at a liquid flow rate of 223 mL/min, thus defining the minimum peak liquid flow rate required for induced pulsing operation. Results In the present work, all experiments were performed at 90 °C and an initial phenylacetylene concentration of 0.045 M. The reactor performance was measured in terms of the initial rate of phenylacetylene consumption and the maximum styrene yield for various cycling conditions at an average liquid flow of 130 mL/min. In the limiting case of steady flow (R ) 1, σ ) 0), the column operated in the trickling flow regime. The ratio of peak to base liquid flow rates, R, was varied from 1 to 26.3 for a constant cycle split, σ ) 0.333 and time, tc ) 30 s. Under these conditions, the induced pulsing flow regime occurred for all R > 2.7. As shown in Figure 3, if all other parameters are held constant and R is increased, both the initial rate of reaction and the maximum styrene yield decrease steadily from their steady flow values. The effect of the cycle split, σ, was investigated at a constant Lp ) 250 mL/min, sufficient for induced pulsing flow, and at the cycle time tc ) 30 s (Figure 4). As the

Figure 3. Results of feed cycling experiments at L h ) 130 mL/ min, tc ) 30 s, and σ ) 0.333 as a function of R: (a) initial rate of reaction, (b) maximum styrene yield.

fraction of time spent under pulsing flow increased, the base flow rate decreased to maintain a constant average flow. It can be seen that both the initial rate of reaction and the maximum styrene yield decreased with increasing cycle split. Last, the peak flow time was varied at constant R ) 3.57 and σ ) 0.333 (Figure 5). It appears that both the initial phenylacetylene consumption rate and the maximum styrene yield decrease asymptotically to constant values at long peak times. Experiments performed at constant liquid flows of 250 and 70 mL/min resulted in initial rates of 10.12 × 10-2 and 5.67 × 10-2 mol/L‚min, respectively. A weighted average of these values, accounting for a cycle split of 0.333, predicts an initial rate of 7.15 × 10-2 mol/L‚min. This value is within experimental error of that obtained for induced pulsing conditions at a peak time of 10 s, which corresponds to the limit of long cycle times. Additional experiments were performed at L h ) 45 mL/ min to study the benefits of transient operation at low liquid flows. The range of parameters available for study at this flow rate were limited because of the constraint that Lp > 223 mL/min for induced pulsing flow operation. For this reason, only the peak flow time was varied, with σ and R held constant at 0.083 and 10.16, respectively. The results of this study are presented in Figure 6.

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Figure 4. Results of feed cycling experiments at L h ) 130 mL/ min, Lp ) 250 mL/min, and tc ) 30 s as a function of cycle split, σ: (a) initial rate of reaction, (b) maximum styrene yield.

Discussion Over the range of conditions and individual parameters studied, steady trickling flow was found always to be superior to induced pulsing operation. Thus, the benefits of operating under pulsing flow during a fraction of the cycle were outweighed by the disadvantages of operating at reduced flows for the remainder of the cycle. Although initial rates and maximum styrene yields corresponding to shorter cycle times were consistently greater than those corresponding to longer cycle times, transient operation never resulted in a net improvement over steady trickling flow operation (see Figures 3-6). These results need to be examined in the context of prior studies which have demonstrated that cycling or switching the liquid feed enhances the performance of both gas- and liquid-limited systems. Evaluation of the / parameter γ ) De,PAC0PA/De,H2CH yields a value of ∼2.4, 2 indicating that the present system corresponds to mild gas-limiting conditions.25,26 As noted in the Introduction, for the highly exothermic hydrogenation of pure AMS under gas-limited conditions (γ . 1), Castellari and Huare10 and Khadilkar et al.12 demonstrated that switching the liquid feed allowed the much faster gas-solid reaction to occur, owing to volatilization of reactants and the consequent temperature runaway. However, the present system was operated at a sufficiently dilute initial reactant content (0.045 M) to

Figure 5. Results of feed cycling experiments at L h ) 130 mL/ min, R ) 3.57, and σ ) 0.333 as a function of peak time, tp: (a) initial rate of reaction, (b) maximum styrene yield.

maintain isothermality; hence, the reactor performance cannot be enhanced by this route. It has also been shown in the literature that feed cycling in trickle-bed reactors operated under liquidlimited conditions at low average liquid flow rates can result in performance enhancement. Khadilkar et al.12 determined that improvement in conversion over steady flow occurs for AMS hydrogenation at low liquid hourly space velocities (LHSVs), typically 1), the initial rate of reaction decreases for operation under induced pulsing flow, whereas for liquid-limited conditions (γ < 1), performance benefits from induced pulsing (Figure 9a). The reasons for this behavior are believed to be as follows. The weighted-average fractional liquid holdup under induced pulsing (0.347) is less than that under steady flow (0.380), thus reducing the effective reactor volume and causing a decrease in the reaction rate under gas-limited conditions. However, the weightedaverage mass-transfer coefficient for the liquid-phase reactants, K h ls, under induced pulsing flow (5.6 × 10-3 cm/s) is greater than the corresponding steady flow value (4.2 × 10-3 cm/s). This increase in mass-transport rate outweighs the decrease in reactor volume, as

Figure 9. Comparison of model predictions and experimental results as a function of γ: (a) ratio of initial rates of reaction, (b) ratio of maximum styrene yields.

indicated by the improvement in reactor performance obtained under induced pulsing flow for liquid-limiting conditions. An equivalent enhancement does not occur under gas-limited conditions, since the mass transfer coefficient for hydrogen remains constant owing to the assumption that the liquid phase is always saturated with gas.16 During the experiments, the gas and liquid streams are continuously recycled, with fresh H2 fed to compensate for reaction consumption, ensuring equilibrium between the two phases. This, along with low per-pass conversion ( 1), whereas improvements over steady conditions are observed for the case of liquid limitations (γ < 1). Between these two limits, the predicted maximum styrene yield reaches a maximum of 45%, indicating an ideal ratio of hydrogen to phenylacetylene availability within the catalyst, which optimizes production of the intermediate styrene over the formation of ethylbenzene. Unfortunately, the existing apparatus did not permit experiments at pressures higher than atmospheric. However, these enhancements are sufficiently promising to invite future experimental verification to justify the additional complexity associated with induced pulsing operation. Concluding Remarks Previous theoretical and experimental works have demonstrated the superiority of reactor operation under the pulsing flow regime, as compared to trickle flow at identical steady feed conditions.6,7,16 Further, studies by Boelhouwer and co-workers17-19 indicate that, by cycling the liquid feed such that trickling and pulsing flow regimes alternate during the cycle period, an induced pulsing mode of operation can be realized at timeaveraged conditions corresponding to the trickling flow regime. In the present work, the potential benefits of trickle-bed reactor operation under induced pulsing flow conditions are investigated through experimental and modeling efforts. This is the first reported experimental investigation of reactor performance employing liquid feed cycling where two different flow regimes occur during each period of the cycle. Experiments, conducted under mild gas-limiting conditions demonstrate a decrease in reactor performance

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under this mode of operation. On the other hand, model predictions for the case of liquid-limited reaction show improvement of reactor performance, with up to a ∼45% increase in styrene selectivity over the corresponding steady flow results. Judging from these findings and those reported previously in the literature, reaction enhancement owing to induced pulsing flow operation is expected for the case of liquid-limited systems and for low-space-velocity reactors with bed maldistributions, as are often encountered in industrial practice. Acknowledgment We gratefully acknowledge the National Science Foundation (Grant EEC-9700537) and the Arthur J. Schmitt Chair Fund at the University of Notre Dame for support of this research. We thank the Center for Environmental Science and Technology at Notre Dame for use of their facilities. B.A.W. gratefully acknowledges the award of a Schmitt Fellowship. We are also indebted to Dennis Birdsell, Ian Duncanson, and James Smith for their assistance with the experimental work. Notation C ) concentration, mol‚L-1 De,i ) effective diffusivity of species i, m2‚s-1 Kls ) liquid-solid mass transfer coefficient, m‚s-1 L h ) average liquid flow rate, mL‚s-1 L ) liquid flow rate, mL‚s-1 P ) pressure, MPa r ) reaction rate, mol‚L-1‚min-1 R ) ratio of peak to base liquid flow rates t ) time, s T ) temperature, °C X ) maximum styrene yield, % Subscripts 0 ) initial value b ) base c ) cycle H2 ) hydrogen IP ) induced pulsing p ) peak PA ) phenylacetylene SF ) steady flow Superscripts 0 ) initial * ) saturation Greek Letters σ ) cycle split γ ) relative liquid/gas availabilty

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Received for review August 4, 2002 Revised manuscript received March 5, 2003 Accepted March 6, 2003 IE020591X