Ind. Eng. Chem. Res. 2005, 44, 9575-9580
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Radial-Flow Reactor Packed with a Catalytic Cloth: Nitrate Reduction in Hydrogen-Saturated Water Uri I. Matatov-Meytal† Department Chemical Engineering, TechnionsIsrael Institute of Technology, Haifa, Israel 32000
A novel flow reactor packed with a fibrous catalytic cloth is proposed to carry out a gas-liquid reaction. Radial flow and separation of the transport of the gaseous reactant and the reaction are the main features of the new design. An activated carbon cloth supported 2 wt % Pd-0.6 wt % Cu bimetallic catalyst installed in the reactor was applied for the water-phase nitrate hydrogenation, showing selectivity up to 84% toward nitrogen at a nitrate conversion of up to 93%. The reactor performance has been evaluated using the diagnostic criteria associated with varying the hydrogen pressure, liquid flow rate, and catalyst loading in the reactor; it was described by a simplistic model, assuming one-dimensional, plug-flow, and steady-state operation. The significance of mass-transfer effects on the reactor performance has been analyzed. The results obtained indicate that the reactor investigated has a high potential for further development and scale-up. 1. Introduction Microfibrous catalysts in the form of woven cloths1 are emerging as an attractive alternative for application to gas-liquid (G-L) reactions. These reactions have been used in fine chemical production, where cloth-type catalysts can combine the benefits of traditional slurry and trickle-bed technology, as well as in water hydrotreatment,2 where cloth-type catalysts are preferable to monoliths, for example, when rapid fluctuations in the flow regime occur, as is often encountered in environmental applications. The fiber catalyst with a diameter on the order of several microns that corresponds to those of traditional suspension catalysts helps to overcome problems related with the use catalyst particles of small diameter required to avoid internal mass-transfer limitations. Important advantages of catalytic cloths also include the unique ability to combine an open macrostructure with mechanical elasticity and flexibility. However, although some advances of catalytic cloths are claimed, they have not been widely used for multiphase reactor design. The present study is aimed at the development of the novel flow reactor with a catalytic cloth for G-L reactions. The hydrogenation of nitrate in water over the most active bimetallic Pd-Cu catalyst was chosen as a model reaction. This process is a relatively new research object, first discovered by Vorlop and coworkers,3 and numerous studies of catalytic hydrogenation of dissolved nitrate have been reported in batch reactors. Not wishing to overburden the paper with references, the author refers the reader to reviews4,5 where reported works are summarized. The hydrogenation of nitrate in water can be described as6,7
Few investigations were published on the nitrate reduction in continuous-flow reactors.8-11 Sell et al.8 carried out the catalytic nitrate reduction with hydrogen †
E-mail:
[email protected].
predissolved in the liquid phase prior to the hydrogenation reactor. With an expanded-bed reactor, rates of nitrate reduction of up to 2.5 g/kg of supported Pd-Cu catalyst per hour were achieved, while the nitrate disappearance rate measured in the “liquid-full” fixedbed reactor was lower by about 5 times, which is attributed to mass-transfer limitations. Lecloux9 studied the liquid-phase nitrate reduction in batch and continuous-flow reactors. In a fixed-bed laboratory reactor, 90% of the nitrate ions (50 mg/L) at a flow rate of about 50 L/h are converted on Pd-Cu alloys, supported on alumina gels into nitrite ions with a conversion rate of about 8 mg of nitrate/g of catalyst per hour and the remaining 10% into ammonium ions. The catalyst with core-impregnated particles larger than 100 µm works with important external and internal diffusion limitations; the effectiveness factor is about 0.85 for both external and internal diffusion. Pintar and Batista also found that the nitrate disappearance rate over a PdCu/alumina powder in a bubble-column flow reactor10 is mainly determined by the mass transfer of hydrogen from the gas phase into the bulk liquid phase, and only about 10% of the Pd-Cu/alumina pellets present in the reactor was exploited. To overcome at least part of the above-mentioned problems, Renken and co-workers12-14 proposed a flow reactor with catalytic fibrous cloths stacked in the bubble column. This reactor was evaluated for nitrite hydrogenation using a Pd/GFC catalyst12 and, more recently, for 2-butyne-1,4-diol hydrogenation over a Pd/ activated carbon cloth (ACC) catalyst.13 The measured rates for the second reaction, for example, compare favorably with the reported results in the trickle-bed reactor. Stacked fibrous cloths were shown to have a low pressure drop during gas and liquid passage through the catalytic bed and a narrow residence time distribution.14 The main features of the design of a continuous reactor presented in this work are a radial-flow configuration and separation of the transport of the gaseous reactant and the reaction. The use of a catalytic cloth permits the various flow arrangements in the reactor, and radial flow allows a good flow distribution and low pressure drop. It should be especially suitable for
10.1021/ie050260v CCC: $30.25 © 2005 American Chemical Society Published on Web 06/30/2005
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Figure 1. Scanning electron micrograph view of ACC and ACF as received.
Figure 2. (left) Schematic diagram of the experimental setup: (1) feed tank; (2) hydrogen cylinder; (3) liquid pump; (4) damping and heat-exchanger vessel; (5) liquid flowmeter; (6) gas mass-flow controller; (7) back-pressure regulator; (8) autoclave; (9) inert fixed bed; (10) reactor; (11) pressure reducer; (12) G-L separator with vent; (13) liquid sample port. (right) Reactor and catalytic cloth.
processes requiring high pressures. A 2 wt % Pd-0.6 wt % Cu supported on an ACC was used as the catalyst for the water-phase nitrate hydrogenation. The reactor performance has been evaluated by varying the hydrogen pressure, liquid flow rate, and catalyst loading in the reactor; it was described by a simplistic model, assuming one-dimensional, plug-flow, and steady-state operation. The importance of mass-transfer effects has been analyzed. 2. Experimental Section 2.1. Materials. A commercial ash-free ACC (523-15 Kynol), employed in this work as a support (Figure 1), was woven from 4-6-cm-long threads with a diameter of about 0.02 cm; threads consist of a bundle of elementary fibers (ACF-15) of 9-10-µm diameter. The basic characteristics of ACF-15 were the following: cloth density, 0.35 g/cm3; Brunauer-Emmett-Teller specific surface area (SSA), 1540 m2/g. The micropore volume and cloth porosity were 0.605 mL/g and 0.264, respectively. A nitrate solution was prepared from NaNO3 (reagent grade, Fluka) in distilled water; hydrogen gas was supplied from a cylinder (Orgim, Israel). 2.2. Catalyst. A 2 wt % Pd-0.6 wt % Cu/ACC catalyst used in this work was prepared by the controlled reduction of a copper precursor on Pd/ACC.15 Palladium(II) chloride (PdCl2, pure, Fluka) was used as a precursor for the metallic palladium catalyst. The loading of metals in the catalyst was determined by inductively coupled plasma emission spectrometry (Perkin-Elmer Optima 3000 DV instrument). The main characteristics of the catalyst used are the following: average metal
particle size, 5.7 nm; SSA of supported metal particles, 1.2 m2/g (high-resolution scanning electron microscopy; LEO 982 Zeiss-Leica instrument); palladium dispersion and SSA of 4.1% and 0.4 m2/g, respectively (CO chemisorption; ASAP 2010 Chemi Micromeritics). The point of zero charge of the catalytic cloth used, i.e., the pH value above which the total surface of the carbon is negatively charged, determined by an equilibrium pH drift method, was found to be 8.3. 2.3. Experimental Setup. The nitrate hydrogenation runs were carried out in a laboratory-scale test unit constructed according to Figure 2. Important elements in the design are an autoclave in which the liquid is saturated with hydrogen and the reactor. The reactor was designed as a tubular cartridge with a volume of 28 cm3 (total length of 14 cm and outer diameter of 1.7 cm) and can house up to 10 layers of the catalytic cloth. Rectangular pieces of the catalytic cloth were cut to reactor dimensions and spirally wound around a central cylindrical core with a diameter of 0.6 cm. Hydrogen was predissolved in the liquid phase in an autoclave (280 cm3) at an elevated operating pressure prior to entering into the catalytic reactor. The high volume ratio of the autoclave to the reactor provides the reaction in the catalytic bed carried out with dissolved gas. To ensure this, blank tests intended to ascertain the absence of gas bubbles after the autoclave were conducted with saturated liquid and a glass tube at the location of the reactor. No gas bubbles were observed even at the highest liquid flow rates. The reaction was carried out at 25 °C, maintained by a heat exchanger
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Figure 3. Typical time on stream of a reactant/products distribution profile: catalyst loading of 9.5 g of 1.82 mmol/L nitrate in a feed volumetric liquid flow rate of 0.25 cm3/s at a pressure of 6 bar.
assuming that nitrite and ammonium ions are the only dissolved side products formed. The relative error is estimated to be on the order of 4%.
Table 1. Experimental Conditions Used in This Work value parameter C°NO3-, mmol/L F, cm3/s hydrogen pressure, bar W, g no. of layers Vcat, cm3 y, cm
1.8 0.08-0.83 2-10 2.2 2 6.3 0.2
3. Results and Discussion 9.5 9 27.2 0.9
installed on the liquid feed line. Other experimental conditions of the hydrogenation runs are listed in Table 1. 2.4. Analyses. The concentrations of the nitrate and nitrite ions were monitored by ion chromatography (761 Compact IC, Methrom instrument with 813 Compact Methrom Autosampler) using conductivity detection. Separation and elution of the anions were carried out on an anion analytical column METROSEP A SUPP 4 (4 × 250 mm); the mobile phase was a carbonate/ bicarbonate effluent and a sulfuric acid regenerant. Before measurement, the (Millipore HAWP) liquid samples filtered through a 0.2-µm membrane filter were diluted to achieve a nitrate (nitrite) concentration lower than 10 mg/L. The concentration of the ammonium ions was measured spectrometrically with a Nessler reagent at λmax ) 500 nm. 2.5. Calculations. The primary result of each hydrogenation run is the reactant/products distribution curve in the reactor outlet as a function of time; from that, the nitrate conversion (for any reaction time) can be directly determined:
X)
C°NO3 - CNO3 C°NO3
(2)
The yield of nitrite and ammonium ions was determined as
Yi )
Ci C°NO3
(3)
where i ) NO2- or NH4+. The total yield of gaseous products (i.e., N2 and, possibly, N2O and NO) was calculated as follows:
TNR ) [C°NO3 - (CNO3 + CNO2 + CNH4)]/C°NO3
(4)
A typical reactant/products distribution profile is presented in Figure 3. The main reaction products in the outlet liquid phase were found to be nitrite and ammonium ions. It can be seen that the reactor system reached a steady state within about 1 h and the outlet concentrations remained nearly constant for more than 10 h. The reaction is accompanied by an increase of the outlet pH values of approximately 0.2 units per 0.2 mmol/L nitrate converted and achieved up to 7.4, leading to a weak decrease in the ability of the catalyst to convert nitrate. The inhibition effect of forming hydroxyl ions on the nitrate removal activity of the PdCu catalyst was recognized early.4 The main reason for the Pd-Cu/ACC catalyst deactivation is, probably, the decrease of the negative charge of the catalyst surface, a phenomenon that was already confirmed for nitrite reduction over the Pd/ACC catalyst.16 To reduce this negative impact of the pH gradient, the catalyst was periodically washed in the reactor with a dilute hydrochloric acid solution and the activity of the Pd-Cu/ACC catalyst was easily replenished. The chemical and mechanical stability of the catalyst used is very good over the whole period even when the catalyst is washed with hydrochloric acid. The active metal contents remain very stable (metal loss of 0.04% and 0.1% for Pd and Cu, respectively). This result is consistent with that of Yoshinaga et al.,11 who have demonstrated that active carbon was superior in conversion, selectivity, and stability over other supports (silica, alumina, and zirconia) studied in nitrate reduction on supported PdCu alloys at elevated temperature (60 °C). After 6 h of initial startup and, therefore, under steady-state conditions, the 2 wt % Pd-0.6 wt % Cu/ ACC catalyst used can reduce the nitrate level from 1.82 to about 0.2 mmol/L, while the nitrate conversion and total nitrogen removal (TNR) reached ca. 93% and 84%, respectively. The reactor performance compares favorably with the performance of a conventional fixed-bed reactor: productivity of the reactor studied was found to be 14.3 g of nitrate/kg of catalyst per hour, which is 5 times higher than that reported for a powdered catalyst in an expanded-bed reactor with hydrogen
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Figure 4. Effect of the hydrogen pressure on nitrate conversion: catalyst loading of 9.5 g; volumetric liquid flow rate of 0.25 cm3/s.
predissolved in the liquid phase.8 The resulting concentrations of nitrites and ammonium were about 4.6 and 0.9 ppm, respectively, which are both, however, higher the maximum admissible contaminant concentration (0.1 and 0.5 ppm nitrite and ammonium, respectively). To evaluate the reactor performance, the operation conditions were varied and the influence of the hydrogen pressure and liquid flow rate for the two catalyst loadings was studied with respect to the extent of TNR and reaction selectivity. All experimental data were taken under steady-state conditions. As shown in Figure 4, the increasing the pressure (here almost the total pressure) has a positive influence on the nitrate conversion and there was a significant increase in the conversion of nitrate when the hydrogen pressure was increased to about 6 bar; above that, the nitrate conversion was essentially insensitive to any increase in the hydrogen pressure. Therefore, above 6 bar for 1.82 mmol/L nitrate in the feed, reactor operation is removed from the gas-adsorption-controlled region. Within this region, the nitrate conversion and yields of the reaction products are considerably influenced by the variation of the volumetric liquid flow rate and catalyst loading (Figures 5-7). Figure 5 demonstrated that, with an increase in W/FF (residence time), nitrite appeared at short residence times and the yield of nitrite decreased rapidly through a maximum value as the residence time increased further. Ammonium ions (Figure 5) are formed in parallel with nitrogen (TNR; Figure 6) at all residence times. Therefore, the experimental results indicate that nitrite is intermediate in consecutive reactions of nitrate hydrogenation and nitrite produced nitrogen and ammonium ions concurrently over PdCu/ACC, supporting reported reaction pathways:4,6,10
Figure 5. Yield of nitrite and ammonium ions as a function of W/FF at a pressure of 6 bar.
Figure 6. Calculated yield of gaseous products as a function of W/FF. Conditions are as in Figure 5.
respect to nitrate, the model material balance equation can be written as
UL
dC ) -kappC dy
(5)
Integrating this equation along the radial coordinate between the reactor inlet (C°NO3 at y ) 0) and outlet (CNO3) gives
ln
C°NO3 C°NO3
)-
kapp y ) -kappτ UL
(6)
Here, τ ) W/FF(1 - ) is the space time the nitrate spends in the catalytic bed, and the rate expression then becomes Kinetic analysis of the reaction rate on catalytic cloth beds follows the same chemical engineering principles as those of packed-bed reactors. Assuming the one-phase plug flow through the catalyst bed, with hydrogen being predissolved in the liquid, steady-state operation, and the reaction to obey pseudo-first-order kinetics with
ln
(1 -1 X) ) -k
W FF(1 - )
app
(7)
By using eq 7, the predicted nitrate conversion vs W/FF(1 - ) is shown in Figure 7 by a solid line. It can be seen that good agreement between the measured and
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9579 Table 2. Mass Transfer in the Liquid Phase and in the Catalyst Bed (25 °C) parameter Sherwood numbera ShNO3L ) kLSdf/DNO3L specific interfacial area, external area of the fiber per unit volume (m-1) aS ) 4/df diffusion coefficient of nitrate in water (m2/s)c
DNO3L )
7.4 × 10-12(ζML)0.5T
value 2b 4.0 × 108
1.9 × 10-9
0.6
VcNO3 υL
diffusion coefficient of hydrogen in water (m2/s)22 mass-transfer coefficient of liquid-solid aSkLS ) 8DNO3L/df2
4.5 × 10-9 1.2
a Estimated from empirical correlation20 Sh ) 2.0 + CRenScm. Assuming that the relative velocity between fine fibers and fluids is quite low and the Reynolds number (Re) approaches zero.21 c Estimated using the Wilke-Chang equation, which is an empirical modification of the Stokes-Einstein relation.22 b
Figure 7. Nitrate conversion as a function of W/FF(1 - ) (symbols, experiment; line, model).
calculated parameters is achieved, which indicates that the assumptions and model developed are reliable. The average value of the apparent nitrate removal rate constant kapp was found to be equal to 0.02 s-1. To assess the importance of interplay of the reaction and mass-transfer resistance by that the overall rate of chemical transformation is governed in the fibrous catalytic cloth in the reactor system studied, the overall mass-transfer factors were estimated. The following mass-transfer and reaction steps proceed simultaneously: (a) H2 dissolution in the liquid feed: H2 diffusion through gas and liquid films at the G-L interface; (b) dissolved nitrate and H2 diffusion through the bulk liquid at the liquid-solid (L-S) interface; (c) reaction and diffusion at the external/internal surface of catalyst; (d) the reverse transport of products into the bulk solution. (a) G-L Mass Transfer. The equilibrium concentration of the dissolved hydrogen in a liquid feed after the autoclave depends exclusively on the hydrogen pressure and physical properties of the nitrate solution in water and can be determined by the gas solubility as / CH ) PH2RH2 2
(8)
According to the reaction (1) stoichiometry, the concentration of dissolved hydrogen at the reactor inlet should / / ) 2.5CNO . Taking into account that some be C°H2 ) CH 2 3 of the hydrogen can be spent on byreactions (about 10%) and that the hydrogen solubility constant for water RH2 ) 0.89 mmol of H2/L‚bar17 (that required for C°H3 ) 1.82 mmol/L at 25 °C), the pressure of hydrogen should be PH2 ) 2.5C°H3/0.9RH2 ) (2.5 × 1.82)/(0.9 × 0.89) ) 5.6 bar. In principle, Henry’s constant (RH2) in pure water should be corrected to correspond to the conditions of the actual nitrate solution. In this paper, the effect of electrolyte on the hydrogen solubility was ignored because of the secondary importance for low nitrate concentrations.18 Therefore, in order for the reactor to operate in the “reaction-limited”, with respect to hydrogen, region, the hydrogen mass-transfer limitations can be first minimized by adjusting the hydrogen pressure to the nitrate concentration.
(b) L-S Mass Transfer. The importance of the external L-S mass transfer of the nitrate ion as a limiting reactant9 can be evaluated primarily by calculation. In the absence of an interphase concentration gradient in an isothermal system, the Carberry number with respect to dissolved nitrate for a first-order reaction should be Ca ) rapp/rmax < 0.1,19 where rmax can be deducted from eq 9, which represents the mass transfer at the L-S interface, with CNO3S as the concentration at the external surface of the catalytic fiber:
r ) aSkLS(CNO3L - CNO3S)
(9)
When the reaction is fast (CNO3S ∼ 0), the following expression for rmax is obtained:
rmax ) aSkLSCNO3L
(10)
kapp rapp ) rmax aSkLS
(11)
Therefore,
Ca )
Taking the experimental value kapp and the calculated value aSkLS (Table 2), Ca ) 0.02/1.2 ) 0.017, which is significantly less than 0.1. This result suggests that L-S interface mass transport of nitrate can be considered to be of little importance. This analysis provides a rational starting point for further reactor development. 4. Conclusions A study was conducted to evaluate the potential of a novel radial flow reactor with a microfibrous cloth-type catalyst for the water-phase nitrate hydrogenation. Application of this radial flow with separate dissolution of gaseous hydrogen in the liquid feed to nitrate hydrogenation demonstrates a stable performance with ACC-supported 2 wt % Pd-0.6 wt % Cu catalyst over the conditions studied. Under steady-state conditions, nitrate conversion in the reactor reached nearly 93% while the calculated TNR achieved 84%. However, the resulting concentrations of nitrates and ammonium were found to be higher than the maximum admissible contaminant concentration. The reactor performance primarily has been evaluated experimentally using the diagnostic criteria as-
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sociated with varying hydrogen pressure, liquid flow rate, and catalyst loading in the reactor. The reactor behavior was described by a simplistic model, assuming one-dimensional, plug-flow, and steady-state operation. The significance of mass-transfer effects on the reactor design has been analyzed. In order for the reactor to operate in the “reaction-limited”, with respect to hydrogen, region, the hydrogen mass-transfer limitations can be first minimized by adjusting the hydrogen pressure to the nitrate concentration. Mass transport of dissolved nitrate ion through the L-S interface was evaluated to be of little importance. The productivity of the reactor studied was higher than those reported for fixed-bed reactors. The results obtained indicate that the reactor investigated has a high potential for further development and scale-up. Acknowledgment The author gratefully acknowledges the partial financial support of the Centre for Adsorption in Science of the Ministry of Immigrant Absorption and the Committee for Planning and Budgeting of the Council for Higher Education (under the framework of the KAMEA Program). The author also thanks Prof. Moshe Sheintuch for his support during the work implementation. Symbols Used aS ) specific interfacial area, external area of the fiber per unit volume (m-1) C ) concentration (mmol/L) df ) fiber diameter (m) F ) volumetric liquid flow rate (cm3/s) kLS ) liquid-solid mass-transfer coefficient for a liquid reactant (m/s) ML ) molecular weight (for water ) 18 g/mol) P ) pressure (bar) T ) absolute temperature (K) W ) catalyst weight (g) VR ) volume of the reactor (cm3) VCNO3 ) critical molar volume of a dissolved nitrate ion (104.5 cm3/mol) UL ) superficial velocity of the liquid phase in the reactor (m/s) X ) conversion y ) thickness of the catalytic bed in a flow coordinate (cm) Greek Letters R ) solubility constant (mol/L‚bar) ) bed porosity F ) bulk density of the catalytic bed (g/cm3) σ ) association factor (2.26 for water) υL ) viscosity (0.891 cP for water) Subscripts L ) related to liquid S ) related to solid H2 ) related to hydrogen NO3 ) related to nitrate
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(2) Matatov-Meytal, Yu.; Sheintuch, M. Hydrotreatment processes for catalytic abatement of water pollutants. Catal. Today 2002, 75, 63. (3) Horold, S.; Tacke, T.; Vorlop, K.-D. Catalytic removal of nitrate and nitrite from drinking water: Screening for hydrogenation catalysts and influence of reaction conditions on activity and selectivity. Environ. Technol. 1993, 14, 931. (4) Prusse, U.; Hahnlein, M.; Daum, J.; Vorlop, K.-D. Catal. Today 2000, 55, 79-90. (5) Centi, G.; Perathoner, S. Remediation of water contamination using catalytic technologies. Appl. Catal. B 2003, 41, 15. (6) Warna, J.; Turinen, I.; Salmi, T.; Maunula, T. Kinetics of nitrate reduction in monolith reactor. Chem. Eng. Sci. 1994, 49, 5763. (7) Pintar, A.; Batista, J.; Levec, J.; Kajiuchi, Kinetics of the catalytic liquid-phase hydrogenation of aqueous nitrate solutions. Appl. Catal. B 1996, 11, 81. (8) Sell, M.; Bishott, M.; Bonse, D. Katalytische Nitratreduction in Trinkenwasser. Egebnisse und Erfahrungen aus technischen Pilotversuchen. Vom Wasser 1993, 72, 129. (9) Lecloux, A. J. Chemical, biological and physical constrains in catalytic reduction processes for purification of drinking water. Catal. Today 1999, 53, 23. (10) Pintar, A.; Batista, J. Catalytic hydrogenation of aqueous nitrate solutions in fixed-bed reactors. Catal. Today 1999, 53, 35. (11) Yoshinaga, Y.; Akita, T.; Mikami, I.; Okura, T. Hydrogenation of nitrate in water to nitrogen over Pd-Cu supported on active carbon. J. Catal. 2002, 207, 37. (12) Holler, V.; Yuranov, I.; Kiwi-Minsker, L.; Renken, A. Reduction of nitrite-ions in water over Pd-supported on structured fibrous materials. Appl. Catal. B 2001, 32, 143. (13) Kiwi-Minsker, L.; Jannet, E.; Renken, A. Loop reactor staged with structured fibrous catalytic layers for liquid-phase hydrogenations. Chem. Eng. Sci. 2004, 59, 4919. (14) Holler, V.; Wedricht, D.; Kiwi-Minsker, L.; Renken, A. Fibrous structured catalytic beds for three-phase reaction engineering: Hydrodynamic study in staged bubble column. Catal. Today 2000, 60, 51. (15) Matatov-Meytal, U.; Sheintuch, M. Activated carbon cloth supported Pd-Cu bimetallic catalyst. Synthesis, characterization and optimization for the continuous water denitrification. Proceedings of the International Symposium on Carbon for Catalysis, CarboCatal, Lausanne, Switzerland, July 18-20, 2004. (16) Matatov-Meytal, Yu.; Shindler, Yu.; Sheintuch, M. Cloth catalyst in water denitrification. III. pH inhibition of nitrite hydrogenation over Pd/ACC. Appl. Catal. B 2003, 45, 127. (17) Stephen, H.; Szephen, T. Solubilities of Inorganic and Organic Compounds; Pergamon Press: Oxford, U.K., 1963; Vol. 1, Part 1. (18) Cianetto, A., Silveston, P., Eds. Multiphase Chemical Reactors; Hemisphere Publishing Corp.: Bristol, PA, 1986; p 92. (19) Butt, J. B. Reaction kinetics and reactor design; Marcel Dekker Inc.: New York, 2000. (20) Sherwood, T. K.; Pigford, R. L.; Wilke, C. R. Mass Transfer; McGraw-Hill: New York, 1975. (21) Deen, W. M. Analysis of transport phenomena; Oxford University Press: New York, 1998. (22) Cussler, E. L. Diffusion: mass transfer in fluid systems; Cambridge University Press: Cambridge, U.K., 1997.
Received for review February 28, 2005 Revised manuscript received May 17, 2005 Accepted June 2, 2005 IE050260V
