Mass-Transfer Resistances in the Catalytic Hydrodechlorination of

Ind. Eng. Chem. Res. 2001, 40, 2011-2016

2011

KINETICS, CATALYSIS, AND REACTION ENGINEERING Mass-Transfer Resistances in the Catalytic Hydrodechlorination of Polychlorobiphenyls. Experimental Results of 2-Chlorobiphenyl Hydrodechlorination in a Slurry Reactor and in a Rotating Basket Reactor Emiddio Schioppa, Fabio Murena,* and Francesco Gioia Department of Chemical Engineering, University of Naples “Federico II”, Piazzale Tecchio 80, 80125 Napoli, Italy

The hydrodechlorination of 2-chlorobiphenyl promoted by a commercial Ni-Mo catalyst has been investigated in a stirred batch reactor (T ) 300 °C and P ) 20 bar) in order to analyze the mass-transfer resistances which might influence the performance of an industrial reactor. Powdered (diameter = 100 µm) and full size particles (cylinders diameter ) 3 mm; length ) 4.5 mm) catalysts have been used. In the second case, a rotating basket reactor has been set up. The results of the investigation indicate that external mass-transfer resistances are not relevant in the majority of the cases. When catalyst pellets are adopted, the diffusion in the catalyst porosity plays a significant role and values of the effectiveness factor in the range 0.05-0.12 have been calculated. 1. Introduction Catalytic hydrodechlorination (HDCl) is a valid process for detoxifying liquid wastes containing aromatic chlorinated compounds.1 It has been demonstrated that Ni-Mo catalysts, usually commercialized for the hydrodesulfurization (HDS) of petroleum stocks, may be used as well for performing the HDCl process of polychlorobiphenyls (PCBs)2-7 and chlorobenzenes8-10 at relatively mild temperatures (250 °C < T < 350 °C) and pressures in the range of 40-100 bar. In the case of HDCl of chlorobenzenes, it has been shown that hydrogen pressure plays a less relevant role in the range of 35-100 bar.11 In strict analogy with the HDS process, several alternatives are possible for the choice of the type of reactor to be used in practice for this three-phase reaction. In the most general case, the design of a threephase reactor is made difficult by the complexity of the fluid mechanics and by the presence of diffusional processes among the three phases. For continuous operation, trickle beds could be a right choice because they are employed commercially for HDS and their designing procedures are well consolidated. For oils with a low PCB contamination level, fixed beds could be used as well. In this case, however, the liquid phase should be saturated with hydrogen before feeding the reactor. A case which is often encountered in the practice is the necessity to detoxify relatively small batches of oils contaminated with PCBs, at different concentration levels. In this circumstance the development of HDCl batch mobile units to accomplish an on-site treatment * To whom correspondence should be addressed. Fax: (+39)081-2391800. E-mail [email protected].

seems very promising. The typical reactor which could be adopted is a slurry reactor in which a powdered catalyst is loaded. As a matter of fact, in our experiments we found that the powdered catalyst is somewhat sticky and tends to agglomerate after the treatment is accomplished. This could give problems in industrial plants during the filtration of the liquid phase to separate the decontaminated oil from the catalyst. In this paper we propose for the first time (for the HDCl process) the use of a rotating basket reactor (RBR) which would avoid any filtration problems. However, in this case, because a catalyst in granular form is adopted, diffusional resistances in the catalyst particles could play a role. At present information about the effect of the masstransfer resistance in the catalyst porosity on the HDCl rate and on the selectivity of HDCl reactions is lacking. The evaluation of the effectiveness factor in the case of the HDCl of PCBs is a hard task. As a matter of fact, HDCl processes of PCBs are a network of chlorine atoms substitution reactions whose complexity depends on the number of chlorine atoms to be substituted.12 In the case of monochlorobiphenyl, the network is made of only one reaction. In the case of decachlorobiphenyl, 840 HDCl reactions are possible. Because HDCl reactions are poorly selective,12 most of the them take place to a significant extent. The evaluation of the effectiveness factor, even in a relatively simple network like A f B f C, is not a simple task13 particularly if the system is not stationary, as is the case of batch reactors. In this paper we have evaluated the kinetic constant of the HDCl reaction of 2-chlorobiphenyl at T ) 300 °C and P ) 20 bar which is a pressure level lower than those generally adopted (P > 40 bar) in HDCl

10.1021/ie000584d CCC: $20.00 © 2001 American Chemical Society Published on Web 04/03/2001

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Table 1. Operating Conditions of the Experimental Runs T P run [°C] [bar] 1 2 3 4 5

300 300 300 300 300

20 20 20 20 20

WHex [g]

Wc [g]

Wcat [g]

reactora

stirrer speed [rps]

119.95 119.94 119.80 119.91 120.37

0.123 0.123 0.126 0.121 0.123

0.125 0.125 0.127 0.123 0.125

SR SR RBR RBR RBR

6.7 10.0 3.3 6.7 10.0

a Catalyst particle dimensions: granules, diameter 100 µm (SR ) slurry reactor); cylinders, diameter 3 mm, length 4.5 mm (RBR ) rotating basket reactor).

processes.4-11 Moreover, we have evaluated both theoretically and experimentally the effectiveness factor for the HDCl reaction (monochlorobiphenyl f biphenyl). This result could be useful to evaluate the role of diffusion in catalyst porosity on the HDCl rate of highly chlorinated PCBs when pelletized catalysts are adopted. 2. Experimental Section A 300 mL stirred batch reactor (SS316) manufactured by Brignole (Italy) was used to perform the experimental runs (diameter of the reactor dr ) 5 cm; diameter of the stirrer da ) 4.6 cm; height of the stirrer Ha ) 3 cm). The reactor is equipped with a magnetic stirrer, two charging lines (one for gases and another for liquids), a sampling line of the liquid phase, and a discharging gas line. A 60 mL stainless steel cylindrical vessel is used as an external charger to load instantaneously the reacting mixture through a globe valve connected to the reactor. Pure hexadecane (99%; Aldrich Co.) was used as the reaction medium. A sulfided form of the Ni-Mo/γAl2O3 catalyst (BASF M8-24PS) was adopted (apparent density = 1 g/cm3; skeletal density = 2 g/cm3). Experimental runs have been carried out at T ) 300 °C and P ) 20 bar. Operating conditions selected for the experimental runs are reported in Table 1. The density of the solution was 0.6 g/cm3 at T ) 300 °C and P ) 20 bar.11 For runs 1 and 2, the catalyst was grounded and sieved and the fraction between 74 and 105 µm was loaded in the liquid phase, thus operating the autoclave as a slurry reactor. In runs 3-5, full size catalyst particles (cylinders: diameter ) 3 mm, length ) 4.5 mm) were loaded into two stainless steel wire grid baskets, which were attached to the stirrer paddles (RBR). Two pellets were loaded into each basket. In this way the weight of catalyst was about constant for all runs (see Table 1). The catalyst and part of the hexadecane (about 95 g) were loaded into the reactor. The headspace of the reactor was purged by nitrogen at ambient pressure. Then the reactor was tightened and heated. When the operating temperature was reached, the external loader was pressurized with hydrogen at 20 bar and the reacting mixture (about 25 g of hexadecane and 0.12 g of 2-chlorobiphenyl) was injected instantaneously into the reactor. This is the time t ) 0 for the run. Analyses of liquid samples were performed with GCECD (HP 6890 micro ECD) equipped with a fused-silica capillary column (HP-5 cross-linked 5% Ph-methyl silicone, length 30 m, internal diameter 0.25 mm, thickness of the film 0.20 µm). Hexachloroethane was used as the internal standard for GC analyses. It was added in a weighed amount to liquid samples withdrawn from the reactor. The operating conditions of the

chromatograph were as follows: inlet pressure 25 psig, inlet temperature 250 °C, carrier gas (helium) flow 5.8 mL/min, split flow 20 cm3/min, oven temperature 200 °C, detector temperature 300 °C, makeup flow (Ar/CH4) 60 cm3/min. On the basis of previous experiments, the addition of a sulfur compound like CS2 to the feed with the aim of keeping the catalyst sulfided was not necessary. As a matter of fact, the rate of the HDCl process was not influenced by the presence of CS2 in the scale of time observed in a run (t < 8 h). Moreover, deactivation of the catalyst was not observed without adding CS2 even for a time as long as about 40 h. Therefore, experimental runs have been carried out without adding any sulfur compound to the feed. 3. Interpretation of Results and Discussion The HDCl process of 2-chlorobiphenyl is a three-phase process: solid phase (catalyst), liquid phase (hexadecane), and gas phase (hydrogen). To evaluate the rate of the overall process, the following steps must be considered: (1) transfer of hydrogen from the gas phase to the liquid phase; (2) transfer of hydrogen and 2chlorobiphenyl from the liquid phase to the external surface of the catalyst particles; (3) transfer of hydrogen and 2-chlorobiphenyl with chemical reaction from the external surface into the porous structure of the catalyst. The analysis of each single step follows. 3.1. Transfer of Hydrogen from the Gas Phase to the Bulk of the Liquid Phase. Hydrogen is dispersed from the headspace of the reactor into the liquid phase. Henry’s constant has been evaluated at the operating conditions (T ) 300 °C; P ) 20 bar) following the procedure reported by Gioia et al.11 and is 440.84 bar. Accordingly, the equilibrium concentration of hydrogen in the liquid phase is c/H ) 1.88 × 10-4 mol/g of solution, while the concentration of 2-chlorobiphenyl loaded in the liquid phase is on the average c°c ) 5.5 × 10-6 mol/g of solution. Thus, if the mass-transfer processes of the two species (hydrogen and 2-chlorobiphenyl) occur with comparable rates, hydrogen is the reactant in excess. The transfer of the hydrogen in the liquid phase takes place from the bubbles which are entrapped in the liquid by the aspirating action of the stirrer. The characteristic stirrer speed (N*) for bubble aspiration has been calculated by the following relationship:14

N*da 4

x

σ1g d F1 r

(

)2

)

H1 - Ha H1

1/2

(1)

In our case, it is N* ) 4.1 rps. Runs 1 and 2 and runs 4 and 5 were carried out at N > N*, while for run 3, it is N < N* (see Table 1). The diameter of the gas bubbles in the liquid phase and the gas-liquid transfer coefficient (liquid side) do not depend on the agitation speed as long as N > 2.5N*. In fact, the gas bubble diameter depends only on the difference of densities of the two phases and on the surface tension of the liquid, as shown in the following equation valid for pure liquids:14

Ind. Eng. Chem. Res., Vol. 40, No. 9, 2001 2013

(

db )

0.41σ1

)

1/2

(2)

g(F1 - Fg)

The gas-liquid transfer coefficient (liquid side) may be calculated by the equation proposed by Van Dierendonck for db < 2 mm:14

kg ) 0.42

( ) µ1g F1

1/3

Sc-1/2Rdb

(3)

where R ) 5 cm-1. In our case, db ) 0.703 mm and kg ) 0.045 cm/s. These values can be safely assumed for runs 2 and 5, where N ) 10 rps = 2.5N*. The same values are also assumed with a certain uncertainty for runs 1 and 4 where N ) 6.7 rps. The fraction of gas holdup in the liquid phase has been calculated by the equation reported by Froment and Bischoff14 in the absence of gas flow:

g ) 0.45

(N - N*)da2 drxgdr

(4)

Thus, the interfacial area ag, defined as the ratio of the surface of the bubbles and liquid volume, is ag ) 6g/db. The values of g, of ag, and of the product kgag are reported in Table 2 for runs 1 and 2 and runs 4 and 5. The same data for run 3 cannot be computed by the above-reported procedure because for this run N < N*. 3.2. Transfer of Hydrogen and 2-Chlorobiphenyl from the Liquid Phase to the External Surface of Catalyst Particles. The transfer of hydrogen and 2-chlorobiphenyl from the liquid phase to the external surface of the catalyst particles depends on the fluid mechanics of the three-phase system. Because we consider two different types of reactors [slurry reactor (SR) for runs 1 and 2 and RBR for runs 3-5], it is necessary to consider them separately. 3.2.1. SR. The minimum stirrer speed that provides a complete suspension of the catalyst has been evaluated through the equation reported by Zwietering.15 This value is, in our case, 2.3 rps and, thus, in runs 1 and 2 complete suspension of the particles of catalyst has been realized because higher stirrer speeds have been adopted (see Table 1). The liquid/particle mass-transfer coefficient (kl) for hydrogen and for 2-chlorobiphenyl has been calculated by the equation reported by Satterfield and Sherwood: 16

(

k1* ) 0.34

)

gµ1(Fs - F1) F12

1/3

Sc-2/3

(5)

where kl* is the mass-transfer coefficient for a small sphere particle falling freely at its terminal velocity. The ratio of the observed mass-transfer coefficient kl to kl* ranges between 1 and 4.16 The mean value 2.5 of the ratio kl/kl* was chosen. The values of the mass-transfer coefficient (kl ) 2.5kl*) for runs 1 and 2 are reported in Table 3. Diffusion coefficients of both hydrogen and 2-chlorobiphenyl in hexadecane at T ) 300 °C and P ) 20 bar have been evaluated using the Wilke-Chang method.17 The values calculated are DH ) 6.99 × 10-4 cm2/s and Dc ) 1.31 × 10-4 cm2/s. 3.2.2. RBR. Continuous rotating basket laboratory reactors have been used for kinetic studies of gas-solid

Table 2. Hold-Up Fraction, Interfacial Area, and Volumetric Gas-Liquid Mass-Transfer Coefficient run

reactor

stirrer speed [rps]

g

ag [cm-1]

kgag [s-1]

1 2 4 5

SR SR RBR RBR

6.7 10.0 6.7 10.0

0.07 0.16 0.07 0.16

5.9 13.7 5.9 13.7

0.60 1.39 0.60 1.39

Table 3. Liquid-Solid Mass-Transfer Coefficients run reactor

stirrer speed [rps]

1

SR

6.7

2

SR

3

RBR

3.3

4

RBR

6.7

5

RBR

10.0

10

reactant

kl [cm s-1]

k las [s-1]

H2 2-chlorobiphenyl H2 2-chlorobiphenyl H2 2-chlorobiphenyl H2 2-chlorobiphenyl H2 2-chlorobiphenyl

0.65 0.21 0.65 0.21 0.13 0.061 0.26 0.14 0.42 0.22

0.24 7.9 × 10-2 0.24 7.9 × 10-2 1.4 × 10-3 6.9 × 10-4 3.0 × 10-3 1.5 × 10-3 4.7 × 10-3 2.5 × 10-3

catalytic processes.18-20 The gas-liquid mass-transfer coefficient is independent of the presence of the basket and can be calculated by eq 3 analogously to the SR. The particle-to-liquid mass-transfer coefficient in a RBR was first determined by Teshima and Ohashi,21 who correlated their experimental data by the equation

( )

Sh ) 2.0 + 0.012

edp4 ν1

3

0.41

Sc0.64

(6)

We used this relationship to evaluate the masstransfer coefficient of hydrogen and 2-chlorobiphenyl from the bulk of the liquid to the external surface of the catalyst pellets in the baskets. The diameter of the cylindrical catalyst particles dp in eq 6 is defined as the diameter of a sphere having the same surface. The power supplied by the stirrer to the liquid per unit of mass of solution (e in eq 6) has been evaluated assuming a power number equal to 6.3 because the Reynolds number is higher than 2 × 104 for all of the runs. The values of the mass-transfer coefficient (kl) for both compounds are slightly smaller than those evaluated for runs 1 and 2 (Table 3). However, in runs carried out with RBR, the interfacial area as is much smaller. In fact, as ) 0.37 cm-1 for the slurry, and as ) 0.011 cm-1 for the cylindrical particles. Therefore, in the RBR the product klas is smaller than that for the SR. The values of the hydrogen mass-transfer coefficients kgag and klas reported in Tables 2 and 3 show that kgag > klas for both the SR and RBR runs. Therefore, hydrogen masstransfer resistance from the bulk of the liquid phase to the external catalyst particles surface is predominant with respect to the mass-transfer resistance from the gas-liquid interface to the liquid bulk. 3.3. Mass Transfer with Chemical Reaction of Hydrogen and 2-Chlorobiphenyl from the External Surface of the Catalyst Particles into the Porous Structure of the Catalyst. The HDCl reaction of 2-chlorobiphenyl can be written as follows:

2-chlorobiphenyl + H2 f biphenyl + HCl

(7)

In the most general case, the kinetic equation of reaction (7) should be represented by a LangmuirHinshelwood expression. Because a low concentration of 2-chlorobiphenyl has been adopted in all of the

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Weisz modulus Φ, based on the observed reaction rate,16 has been used for evaluating η

Table 4. Kinetic Constants Obtained by Fitting Experimental Data with Eq 8

run

reactor

stirrer speed [rps]

1 2 3 4 5

SR SR RBR RBR RBR

6.7 10.0 3.3 6.7 10.0

kwa [g of solution/ (g of catalyst s)] 5.15 × 10-1 5.80 × 10-1 0.73 × 10-2 1.30 × 10-1 1.40 × 10-1

a Kinetic constants of runs 1 and 2 are “intrinsic” kinetic constants, while kinetic constants of runs 3-5 are “observed” kinetic constants.

experimental runs, pseudo-first-order kinetics is a reasonable assumption.4,8-10 In the hypothesis that the process is kinetically controlled, as will be confirmed later, and on the basis of the previous observation that c/H >> c°c, it can be assumed that the hydrogen concentration in the bulk of the liquid phase and in the porous structure of the catalyst is constant during the run and equal to its solubility (i.e., cH ) c/H). Therefore, the equation for the reaction rate is where r is the rate of consumption of

r ) kcc

(8)

2-chlorobiphenyl (moles per unit mass of liquid solution and per unit time), k the pseudo-first-order kinetic constant (s-1) in which the hydrogen concentration is included, and cc the concentration of 2-chlorobiphenyl in the bulk of the liquid phase (moles per unit mass of liquid solution). If diffusional effects of 2-chlorobiphenyl in catalyst particles have to be taken into account, the effectiveness factor η must be introduced and the equation rate becomes

r ) kηcc

(9)

The analyses of experimental results are reported for the two types of reactors adopted. 3.3.1. SR. Equation 8 has been used to fit the concentration data of 2-chlorobiphenyl vs reaction time in the hypothesis that η ) 1. The kinetic constants have been obtained and then expressed per unit weight of catalyst by the relationship

kw ) k(Wsol/Wcat)

(10)

where Wsol and Wcat are the weight of the solution and the weight of catalyst loaded into the reactor, respectively. The values of kw for runs 1 and 2 are reported in Table 4. The kinetic constant of run 1 is similar to that of run 2. This is experimental evidence that the gasliquid mass-transfer resistance is negligible because the stirring speed does not appreciably affect the overall rate. Furthermore, with the values of the mass-transfer coefficients reported in Table 3 and the kinetic constants reported in Table 4, it results that for both runs 1 and 2

kl a s >> 1 k

or

klas >> 1 Wcat kw Wsol

(11)

and because kgag > klas, all of the external resistances may be neglected. To verify the hypothesis η ) 1, the

Φ)

(

)

Lp2 Vsol 1 r De Vcat cc

(12)

where the characteristic length Lp ) R (radius of spherical catalyst particles) for runs 1 and 2. The effective diffusion coefficient of 2-chlorobiphenyl hasbeen evaluated through the equation

De ) Dc(/τ)

(13)

where  ) 0.5 and τ (tortuosity factor) ) 2. In eq 12 we have assumed that the concentration of 2-chlorobiphenyl on the catalytic surface is equal to the concentration of 2-chlorobiphenyl in the bulk liquid. The validity of this assumption is confirmed by eq 11. From eq 12 values of Φ < 0.7 for both runs 1 and 2 have been calculated which correspond to η = 1.16 Consequently, for runs 1 and 2 both the external and internal mass-transfer resistances are negligible; namely, the process is controlled by the reaction kinetics, as hypothesized. The kinetic constants evaluated by fitting the experimental data of runs 1 and 2 with eq 8 and reported in Table 4 are then the intrinsic kinetic constants of HDCl reaction (7). 3.3.2. RBR. Analogously to runs 1 and 2, experimental data of runs 4 and 5 have been fitted by eq 8 and the kinetic constants are reported in Table 4. In this case, as will be shown later, these are “observed” kinetic constants which include the effect of diffusion in the catalyst pellets. The external mass-transfer resistances of 2-chlorobiphenyl can be neglected because condition (11) holds true even for runs 4 and 5. Therefore, the lower values of the kinetic constants of runs 4 and 5 with respect to runs 1 and 2 must be due to diffusion effects within the pellets. The effectiveness factor η has been computed through both the Thiele modulus φ

φ ) Lpxkv/De

(14)

and the Weisz modulus Φ (eq 12) based on the observed reaction rate. The Thiele modulus φ (eq 14) was calculated using as the intrinsic kinetic constant the value obtained from run 2 (see Table 4). The kinetic constant kw has been expressed per unit volume of catalyst (kv) and the characteristic length Lp of the cylindrical pellets as the ratio of the volume to the external surface area of the cylindrical pellets. The results of the calculations give η ) 0.12. In the second case, the effectiveness factor has been evaluated using the Weisz modulus (eq 12), with Lp as in eq 14. In this case η ) 0.05. Thus, the concentration gradient of 2-chlorobiphenyl within the pellet is significant so that the internal diffusion strongly affects the overall rate of the HDCl process. The low value of the observed kinetic constant for run 3 (see Figure 1 and Table 4) carried out at N ) 3.3 rps can be due to an ineffective gas aspiration in the liquid phase. In fact, the value of the stirrer speed adopted in that run is below the characteristic speed N* ) 4.1 rps.

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Figure 1. Experimental data and fitting curves of the ratio cc/c°c as a function of (Wcat/Wsol)t.

In this case it is possible that the transfer of hydrogen into the liquid phase plays a role. 4. Conclusions The extent to which mass-transfer resistances affect the catalytic HDCl of 2-chlorobiphenyl has been investigated at T ) 300 °C and P ) 20 bar. First, small particles (diameter = 100 µm) of catalyst have been used in a SR. In this case both internal and external diffusional resistances are negligible. Thus, the intrinsic kinetic constant has been evaluated. Because large particles are more suitable to be used in an industrial reactor, the extent to which diffusion effects influence the overall HDCl kinetics has been investigated. To avoid the comminution of the full size catalyst particles in the stirred reactor, a RBR was set up. An effectiveness factor which may range between 0.05 and 0.12 has been evaluated, demonstrating that the internal mass-transfer resistance plays a major role. On the contrary, the effect of the external mass transfer was found to be irrelevant in the majority of the cases. Acknowledgment This work was financed by a research grant from CNR (Consiglio Nazionale delle Ricerche), Protezione Civile, Gruppo Nazionale per la difesa dei rischi chimicoindustriali. Nomenclature ag ) gas-liquid interfacial area per unit volume of liquid, cm-1 as ) liquid-solid interfacial area per unit volume of liquid, cm-1 c ) concentration in the bulk of the liquid, mol/g of solution c° ) initial concentration, mol/g of solution c* ) equilibrium concentration, mol/g of solution da ) diameter of the stirrer, cm db ) diameter of the bubbles, cm dp ) diameter of the catalyst particle, cm dr ) diameter of the reactor, cm De ) effective diffusion coefficient of 2-chlorobiphenyl, cm2/s DH ) diffusion coefficient of hydrogen, cm2/s Dc ) diffusion coefficient of 2-chlorobiphenyl, cm2/s

e ) power supplied (by the stirrer) to the liquid per unit of mass, cm2/s3 g ) acceleration due to gravity, 980 cm/s2 Ha ) height of the stirrer, cm Hl ) height of the liquid phase, cm kg ) liquid-side mass-transfer coefficient for hydrogen, cm/s k ) kinetic constant, s-1 kl* ) liquid-particle mass-transfer coefficient (for a sphere settling at its terminal velocity), cm s-1 kl ) liquid-particle mass-transfer coefficient, cm/s kv ) kinetic constant per unit volume of catalyst, s-1 kw ) kinetic constant per unit mass of catalyst, g of solution/(g of catalyst s) Lp ) characteristic length for the Thiele modulus (volume/ surface), cm N ) stirrer speed, rps N* ) characteristic stirrer speed for bubble aspiration, rps P ) pressure, bar r ) rate of the reaction, mol/(g of solution s) t ) reaction time, min Sh ) Sherwood number Sc ) Schmidt number T ) temperature, °C Vcat ) volume of the catalyst, cm3 Vsol ) volume of the solution, cm3 Wcat ) weight of the catalyst, g of catalyst Wc ) weight of o-chlorobiphenyl, g WHex ) weight of hexadecane (WHex = Wsol), g Wsol ) weight of the solution into the reactor, g of solution Greek Symbols R ) see eq 3, cm-1  ) void fraction of the catalyst g ) gas holdup φ ) Thiele modulus (eq 14) Φ ) Weisz modulus (eq 12) η ) effectiveness factor µl ) viscosity of the liquid, g/(cm s) νl ) kinematic viscosity of the liquid, cm2/s Fg ) density of the gas, g/cm3 Fl ) density of the liquid, g/cm3 Fs ) density of the catalyst, g/cm3 σl ) surface tension of the liquid, dyn/cm τ ) tortuosity factor Subscripts a ) stirrer c ) 2-chlorobiphenyl cat ) catalyst g ) gas H ) hydrogen Hex ) hexadecane l ) liquid p ) particle of catalyst s ) solid sol ) solution

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Thesis, Department of Chemical Engineering, University of Naples “Federico II”, Naples, Italy, 1996. (6) Schiraldi, C. Modellazione cinetica del processo di idrodeclorazione catalitica di policlorobifenili. Chemical Engineering Thesis, Department of Chemical Engineering, University of Naples “Federico II”, Naples, Italy, 1996. (7) Murena, F.; Schioppa, E.; Gioia, F. Catalytic hydrodechlorination of a PCB dielectric oil. Environ. Sci. Technol. 2000, 34, 4382-4385. (8) Hagh, B. F.; Allen, D. T. Catalytic hydroprocessing of chlorobenzene and 1,2-dichlorobenzene. AIChE J. 1990, 36, 773778. (9) Hagh, B. F.; Allen, D. T. Catalytic hydroprocessing of chlorinated benzenes. Chem. Eng. Sci. 1990, 45, 2695-2701. (10) Murena, F.; Famiglietti, V.; Gioia, F. Detoxification of chlorinated organic compounds using hydrodechlorination on sulphided Ni-Mo/γ-Al2O3. Environ. Prog. 1993, 12, 231-237. (11) Gioia, F.; Gallagher, E. J.; Famiglietti, V. Effect of hydrogen pressure on detoxification of 1,2,3-trichlorobenzene by catalytic hydrodechlorination with both unsulphided and sulphided NiMo/γ-Al2O3 catalyst. J. Hazard. Mater. 1994, 38, 277-291. (12) Murena, F.; Schioppa, E. Kinetic analysis of catalytic hydrodechlorination process of polychlorinated biphenyls (PCBs). Appl. Catal. B 2000, 27, 257-267. (13) Aris, R. The mathematical theory of diffusion and reaction in permeable catalyst; Clarendon Press: Oxford, U.K., 1975.

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Received for review June 16, 2000 Revised manuscript received January 3, 2001 Accepted January 27, 2001 IE000584D