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Effect of Gas and Liquid Superficial Velocities on the Performance of Monolithic Reactors Aswani Kumar Mogalicherla and Deepak Kunzru* Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India
The effect of gas and liquid velocities on the rate of hydrogenation of R-methyl styrene (AMS) in a monolithic reactor has been investigated at different washcoat loadings. The catalyst used was 0.5 wt % Pd/Al2O3. To cover a wide range of velocities, reaction studies were conducted in multichannel as well as single-channel monolith blocks. The liquid channel velocity was varied from 0.5 cm/s to 22 cm/s, and the gas channel velocity was varied from 1.2 cm/s to 22 cm/s. To study the effect of internal diffusional resistance, the washcoat thickness was varied from 11 µm to 62 µm. Under these operating conditions, both external mass-transfer and internal diffusional resistance affected the rate of reaction. At low liquid velocities (10 cm/s), the rate of reaction decreased with liquid velocity, most probably because of the increase in liquid slug length and film thickness around the gas bubble. The overall effectiveness factor of the monolith catalyst was determined from the measured rate of reaction and the published intrinsic kinetics. Using the approximation of Gottifredi et al. for nonlinear kinetics [Gottifredi et al. Chem. Eng. Sci. 1981, 36, 313-317], the internal effectiveness factor and overall mass-transfer coefficient were determined. The overall mass-transfer coefficients determined from the reaction-rate data have been compared with the values calculated using the available correlations. 1. Introduction Recently, several studies have been reported on the use of monolithic reactors for multiphase reactions. Depending on the gas and liquid flow rates, different flow regimes (such as bubble flow, Taylor flow, churn flow, or film flow) can exist in the monolith channels. During multiphase reactions, the flow pattern in the channels strongly influences the reactor performance.1,2 For multiphase reactions, the two flow patterns of commercial interest are Taylor flow and film flow. At high liquid velocities (>1.7 cm/s) and gas-to-liquid flow-rate ratios in the range of 0.3-2, Taylor flow is the dominant flow pattern.3,4 In Taylor flow, well-separated gas bubbles and liquid slugs move alternately in the channels. A thin liquid film separates the gas bubble from the walls of the channel. Very high mass-transfer coefficients have been reported in the slug flow regime. This is due to recirculation in the liquid slugs and the direct transfer of gaseous reactants to the catalyst wall through the thin liquid film around the gas bubble.5 At very low liquid velocities (on the order of mm/s) and moderate gas velocities, film flow is the dominant flow pattern. In film flow, liquid flows as a film along the channel walls, whereas the gas flows in the central core of the channels. In film flow, the thickness of the liquid film formed along the monolith walls is substantially larger than the film that is formed during slug flow, resulting in lower masstransfer rates.6 In comparison with trickle-bed reactors, limited information is available for monolithic reactors. Hatzlantoniou and Andersson7 studied the liquid-phase hydrogenation of nitrobenzoic acid on palladium-coated monoliths. The liquid superficial velocity (UL) was varied from 1.3 cm/s to 2.0 cm/s and the gas superficial velocity (UG) was varied from 1.8 cm/s to 3.1 cm/s. The reaction rate decreased as UL increased and UG decreased. Under these conditions, slug flow existed in the channels and * To whom correspondence should be addressed. Tel.: +91-5122597193. Fax: +91-512-2590104. E-mail:
[email protected].
the reaction rates were much higher than those for a tricklebed reactor under similar conditions, because of the higher masstransfer rates. However, for a similar range of gas and liquid superficial velocities and relatively slow reactions, such as the hydrodesulfurization of dibenzothiophene and the hydrogenation of acetylene, the measured rate of reaction was not affected by changes in either UG or UL.8-10 Mazzarino and Baldi11 studied the palladium-catalyzed hydrogenation of R-methyl styrene (AMS) on monoliths. The liquid superficial velocity was varied from 0.05 cm/s to 0.34 cm/s and the gas superficial velocity was varied from 0.2 cm/s to 1.2 cm/s. In this range of superficial velocities, the performance of the monolithic reactor was strongly influenced by the gas superficial velocity and was independent of the liquid superficial velocity. Smits et al.12 studied the hydrogenation of mixture of styrene and 1-octene on a palladium-coated monolithic reactor for a wide range of linear velocities (0.05 m/s < UG + UL < 0.5 m/s). They observed an increase in the reaction rate with linear velocities. They attributed the improved performance of the monolithic reactor with linear velocity to the improvement in the gas-liquid distribution. Klinghoffer et al.6 studied the oxidation of acetic acid using Pt/Al2O3-coated monoliths. The liquid superficial velocity was varied over a range of 0.024-0.093 cm/s and UG was kept constant at 2.36 cm/s. The reaction rate increased with liquid velocity. They claimed that the reaction rate increased with liquid velocity, because of the transition from film flow to slug flow. Kreutzer et al.13 studied the hydrogenation of AMS on monoliths coated with Pd/Al2O3 at high linear velocities (>0.2 m/s). In their study, the performance of the monolithic reactor decreased with an increase in linear velocity and cell density of the monolith, whereas at low linear velocities, the performance of the monolithic reactor was independent of the gas and liquid superficial velocities. Liu et al.14 conducted the hydrogenation of a mixture of styrene,1-octene, and toluene in a single-channel monolith in the liquid superficial velocity range
10.1021/ie901442d 2010 American Chemical Society Published on Web 01/15/2010
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of 1-50 cm/s and gas superficial velocities of 1-2000 cm/s. The reaction rate increased as the gas and liquid superficial velocities each increased. Tsoligkas et al.15 studied the hydrogenation of 2-butyne-1-4-diol in a palladium-coated alumina capillary. The liquid velocity was varied over a range of 0.74-14.8 cm/s and the gas superficial velocity was varied over a range of 0.98-3.56 cm/s. They observed a sharp increase in the reaction rate when the flow pattern changed from bubble flow to slug flow. One of the disadvantages of monolithic reactors is that the amount of catalyst loading per unit reactor volume is lower than that in conventional trickle-bed reactors. To increase the catalyst loading, the washcoat thickness must be increased. The thickness and shape of the washcoat can affect the activity, as well as selectivity, of the catalyst.16,17 Hayes et al.18 have reported that, during single-phase flow in monoliths, the thickness and shape of the washcoat can have great influence on the external masstransfer coefficient. However, very few experimental studies have been published on the effect of washcoat thickness on the performance of multiphase monolithic reactors.19,20 Some studies have been reported for estimating the masstransfer coefficient from the conversion data obtained in the slug flow regime. Hatzlantoniou and Anderson7 measured the gas and liquid slug lengths, using a conductivity probe, and modeled nitro-benzoic acid hydrogenation kinetics in monolith reactors. Because of the nonlinear kinetics, the model predictions were in satisfactory agreement with experimental data only at low operating pressures. Kreutzer et al.13 determined the masstransfer coefficient from the hydrogenation kinetics of AMS under a completely mass-transfer-control regime. The model predictions were much higher than the mass-transfer coefficients determined from reaction data. Bercic21 decreased the total hydrogen flux calculated from the model by an empirical factor, to match the experimental data with model predictions. In contrast, Winterbottom et al.22 obtained very good agreement between the experimental results and theoretical predictions during the hydrogenation of 2-butyne-1-4-diol in a monolithic cocurrent downflow contactor. As discussed previously, discrepant results have been reported regarding the effect of gas and liquid superficial velocities on the performance monolithic reactors. The objective of the present work was to investigate the performance of the monolithic reactor at different gas and liquid superficial velocities. Another objective was to study the effect of washcoat loading on the productivity of the monolithic reactor. The hydrogenation of AMS on Pd/Al2O3 was taken as the probe reaction. The overall effectiveness factor of the monolithic reactor and the overall volumetric mass-transfer coefficient were estimated from reaction rate data obtained at various VL and VG values. In earlier studies, either the intrinsic kinetics has been assumed to be firstorder or the internal pore diffusional resistances have been neglected during estimation of the mass-transfer coefficient from reaction rate data. In this work, the effect of internal diffusional resistance and the nonlinearity in intrinsic kinetics were taken into account during the estimation of the overall volumetric mass-transfer coefficient. 2. Experimental Section 2.1. Catalyst Preparation. For all the runs, 0.5 wt % Pd/ Al2O3 was used as the catalyst. Monolithic catalyst was prepared in two steps. First, the monolith channels were washcoated with alumina, and then palladium was deposited on the alumina washcoat. γ-Alumina (with a surface area of 155 m2/g, and an average particle size of 35 µm), obtained from Grace Chemicals,
USA, was used for washcoating the monoliths. The average particle size (d50) of the as-received alumina was reduced to 3 µm in a Planetary monomill (Pulverisette 6, Fritsch GmbH, Germany).The milled γ-alumina powder was added to water that contained dispersible pseudo-boehmite (Disperal P2, Condea) and was milled for 1 h to obtain a uniform slurry. The pH of the slurry was adjusted to 4 by adding HNO3 to obtain a stable alumina suspension. The total solids concentration in the slurry was 35 wt % and the ratio of pseudo-boehmite to γ-alumina was 0.1 (wt/wt). These conditions were optimized to obtain crack-free washcoats. A 5 mm × 5 mm square monolith piece (length: 60 mm; cpsi: 400; channel opening: 1 mm; wall thickness: 185 µm) with 16 channels was vertically immersed into the slurry at a speed of ∼4 cm/min. During this immersion, the slurry rose in the channels by capillary action. The dipping time was 4 min. After this, the monoliths were taken out and the excess slurry removed by blowing them with air. Monoliths were dried at 110 °C and calcined at 500 °C for 5 h. The process was repeated 2, 3, or 5 times to achieve washcoat loadings of 15, 25, and 44 wt %, respectively. To obtain reproducible washcoat loadings, various parameters such as the speed of immersion, dipping time, speed of withdrawal, and air flow rate for blowing the slurry from the monoliths was kept the same for all of the runs. Palladium chloride salt was used to prepare a 0.5 wt % Pd/ Al2O3 monolith catalyst. To determine the incipient volume, γ-alumina washcoated monoliths were soaked in distilled water for 20 min and the excess water was blown out using compressed air. The weight increment was noted. Palladium chloride was then dissolved in concentrated hydrochloric acid and diluted to a concentration, such that water taken in by the monolith from the palladium solution would give the desired palladium loading. The washcoated monoliths were soaked in dilute palladium salt solution for 20 min, and the excess solution blown out using compressed air. The increment in weight was noted. Monoliths were wrapped on the outside with Teflon tape and hot air at 60 °C was blown periodically from both ends of the monolith channels for 1 h. Monoliths were dried at room temperature for 3 h and then dried at 100 °C without a Teflon covering for 3 h. The dried monoliths were calcined at 450 °C for 4 h. In the case of single-channel studies, a monolith block 28 cm long and 1.7 cm in diameter, with 120 channels, was used. The catalyst was deposited in all the channels using the same procedure as that used for the 16-channel monolith. To provide a sufficient amount of catalyst in the channel to obtain measurable AMS conversion, the washcoat loading was kept at 44 wt %. Except for a single channel, the other channels were sealed at the top and bottom using air-set cement (Omegabond 500, Omega, USA). The uniformity and thickness of the coating were examined via scanning electron microscopy (SEM). For this, monoliths were cut at various cross sections, using a diamond wheel. Before cutting, the monoliths were soaked in distilled water for 2 h to minimize the formation of the cracks during cutting. For all other measurements, the monoliths were crushed to a fine powder. The specific surface area of the washcoated monolith catalyst was obtained by the dynamic pulsing technique on a Micromeritics Pulse Chemisorb 2705 unit. Temperatureprogrammed reduction (TPR) studies were performed to determine the reduction characteristics of the catalyst. The palladium dispersion of the catalysts was determined by hydrogen chemisorption at 343 K. Prior to chemisorption, the samples were reduced in situ at 343 K using pure hydrogen for 6 h and then
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Figure 2. SEM images of monoliths: (a) bare monolith and (b) monolith with a washcoat loading of 44 wt %.
Figure 1. Experimental setup for 16-channel studies.
outgassed in helium for 2 h at the same temperature. The uniformity of the palladium dispersion along the monolith length was examined by chemisorption studies on different 1-cm-length monolith pieces cut from the longer monoliths. 2.2. Monolithic Reactor. Because of equipment limitations, two sets of experiments were conducted, to cover a wide range of linear velocities. In the first set, a monolith of rectangular cross section with 16 channels was used, and in the second set, only a single channel of a large monolith block was used. For all the runs, the reaction temperature was maintained at 40 °C. 2.2a. Sixteen-Channel Studies. The experimental setup used for 16-channel studies is shown in Figure 1. The monoliths were placed in a stainless steel tube with an inner diameter of 7.5 mm and a length of 60 cm. A water jacket was arranged along the length of the reactor to maintain a constant temperature inside the reactor. The catalyst-coated monolith pieces (each 6 cm in length) were sandwiched between uncoated monoliths (4 cm length) in the reactor. Depending on the operating pressure (1 or 2.5 atm), the number of monoliths in the active zone were 2 or 1. The washcoated monoliths and bare monoliths were wrapped with Teflon tape and tightly packed in the reactor. The reactor was operated continuously for gas and batchwise for liquid. The liquid was fed by a high-pressure pump (Series III, Lab Alliance, USA) that was provided with a pulse dampener. Hydrogen was fed through Bronkhorst mass-flow controllers. Provisions were made for preheating the liquid and gas feed streams separately. Hot water was circulated through the jacket to maintain the desired temperature in the reactor. Gas and liquid were mixed in a conical distributor and sprayed over the monolith channels. The reactor effluent passed through a condenser and a backpressure regulator to a reservoir. The liquid was recirculated, whereas the off-gases from the reservoir were vented. Toluene was used as the solvent, and, for all of the runs, the initial AMS concentration in the reservoir was 1250 mol/m3. The total volume of the reactants was 960 mL. Before use, traces of water and 4-tert-butylcathecol from AMS were removed via the use of activated alumina. Ten grams (10 g) of activated alumina was added to 90 mL of AMS and continuously rotated for 2 h to remove the impurities. An identical procedure was followed for all of the runs. First, the catalyst was reduced at 65 °C for 8 h under a hydrogen flow of 30 mL/min. The TPR studies showed that the catalyst was easily reducible at room temperature. The reactor was then cooled to 40 °C under flowing hydrogen. After that, the reactor was preflooded with toluene
at 40 °C and 1 atm and maintained there for 3 h to ensure complete internal wetting of the monolith. The toluene in the reactor was then drained out under a flow of hydrogen. Purified AMS was then added to the reservoir. Samples were collected from the reservoir after every 60 min for analysis. The liquid channel velocities (VL) were varied over a range of 0.5-5 cm/s and gas channel velocity (VG) was varied over a range of 1.2-7 cm/s. The monolith reactor was operated at two different pressures (1 and 2.5 atm) and three washcoat loadings (15, 25, and 44 wt %). 2.2b. Single-Channel Studies. The reaction tests were also conducted by directly sending gas and liquid into a single reaction channel of the washcoated monolith. The same experimental setup (Figure 1) was used for single-channel studies. The monolith block with one open channel was housed inside the stainless steel reactor, and a feed delivery needle was cemented in the open channel. In single-channel studies, there is a possibility of liquid leakage into adjacent channels. To check the extent of leakage, cold flow studies were conducted in monoliths that had different washcoat loadings, using toluene and nitrogen. At low loadings (
