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Wet Hydrodechlorination of p-Chlorophenol Using Pd Supported on an Activated Carbon Cloth Yu. Shindler, Yu. Matatov-Meytal, and M. Sheintuch* Wolfson Department of Chemical Engineering, TechnionsIsrael Institute of Technology, Haifa, Israel 32000
The use of activated carbon cloths as supports for Pd catalyst has been investigated in a hydrodechlorination process. Using p-chlorophenol (CP) as a model compound, the influence of the catalyst preparation procedure and Pd content have been studied at 30-85 °C, showing good activity and selectivity. Kinetic analysis of these diffusion-independent results showed the reaction to be first-order with respect to each reactant concentration (CP and hydrogen). Analysis of diffusion and reaction shows that diffusion resistance will be significant in granules larger than 100 µm. Several simulations of adsorption, diffusion, and reaction are presented showing the effect of pellet size, of hydrogen pressure, and of adsorbent loading. Introduction Hydrodechlorination (HDC) is a commercially significant process for synthesis of fine chemicals1 as well as for abatement of chlorinated organic pollutants.2 While thermal and/or chemical oxidations are also available for this purpose,3 these approaches are energetically demanding and can generate highly toxic products such as phosgene, dioxins, and chlorine.4 The destruction of chloroorganics at even lower temperatures can be achieved by catalytic oxidation, but the produced chlorine poisons most catalysts or at least reduces their activity (see ref 5 and references therein). Liquid-phase catalytic HDC presents several advantages over the above oxidation methods because the hydrogen chloride formed can be absorbed and the hydrocarbon can be safely burned or adsorbed on a suitable adsorbent. Gas-6,10-12 and liquid-phase7-9 HDC has been reported for various chloroorganics over Pd,6-9,12 Ru,10 and Ni11,12 catalysts supported on powders and pellets of different materials. Out of all tested metals, Pd supported on granular activated carbon (GAC) seems to be the best catalyst for HDC of p-chlorophenol (CP).7,12 The high activity of Pd supported on carbonaceous carriers is due, in part, to the hydrogen spillover onto the carbon surface, a phenomenon widely invoked in hydroprocessing reactions on metal/carbon catalysts.13,14 Even though several researchers have already demonstrated the feasibility of using a Pd/AC catalyst for liquid-phase HDC, these solutions are limited by mass-transfer rates of a GAC or by handling problems of powdered AC. Catalytic carriers in the form of woven fibrous cloths (see, e.g., refs 15 and 16) have recently become available. These have open structures and high geometric surface areas, making them a potential alternative for many processes. Because of mechanical elasticity and geometric flexibility, fibrous cloths may be constructed in the best form to fit the particular use. In this respect, fibrous activated carbon cloths (ACCs) are especially promising as supports for noble-metal catalysts for liquid-phase hydrogenation reaction.18 ACCs have a high apparent surface area, normally in the range of 1500-3000 m2 * To whom correspondence should be addressed. Phone: 97248292920. Fax: 97248230476. E-mail: cermsll@techunix. technion.ac.il.
g-1, and, consequently, a larger adsorption capacity when compared with GAC. They possess inertness in liquid reaction media, a high dispersion of impregnated metal due to a porous network formed mainly by deep pores in a narrow range of sizes (mostly micropores). Thus, ACCs offer low transport resistance while providing fluid permeability and, consequently, higher adsorption and desorption rates and convenience of use. The main objective of this study is to prepare ACCsupported Pd catalysts and to examine their catalytic properties during liquid-phase HDC using CP as a model compound. The adsorption of reactant and product on ACC and Pd/ACC samples was also studied. Experimental Section Materials. Commercial ACC samples (Kynol Europa Gmbh, Germany) woven of fibers of 10 µm in diameter, which were prepared by carbonization and activation of phenolic pitch, were employed as a support. GAC (Calgon Filtrasorb-300, from Chemviron Carbon) was employed as a support in the reference material (see Table 1). Palladium(II) chloride (PdCl2, pure from Fluka) was used as a precursor of the active phase. The stock reactant solutions of CP (better than 99.5% purity, from Aldrich) were prepared with distilled water. Hydrogen (purity of 99.99%, Orgim Israel) was supplied from cylinders. Catalyst Preparation. The as-received ACC contains several inorganic impurities that may interact with the metal and may affect its catalytic activity, sintering resistance, and even catalytic secondary reactions. To remove these impurities, the ACC was washed at room temperature by demineralized water. Several procedures to modify the surface chemistry of ACC were used: In procedure a, the ACC was treated consecutively with 0.6 M aqueous NaOH, demineralized water, 0.5 M aqueous HCl, and distilled water. In procedure b, the ACC was treated with 0.6 N H2O2 for 48 h and then thoroughly washed and dried. Pd was deposited on the ACC by immersing it overnight in a PdCl2 solution. The Pd loading of the catalyst was adjusted by changing the PdCl2 concentration in the impregnation solution. After impregnation the resulting cloth was dried at 100 °C for 5 h and was thoroughly washed to remove chloride ions. Then,
10.1021/ie001019d CCC: $20.00 © 2001 American Chemical Society Published on Web 06/19/2001
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Table 1. Main Characteristics of ACC and Reference GAC Supports Used in This Work sample
commercial name
particle diameter (µm)
density (g L-1)
SSA (m2 g-1)
ACC1 ACC2 GAC
523-15 Kynol 507-15 Kynol Filtrasorb-300
9-10 9-10 300-500
600 350 435
1540 1560 800
several samples were treated after impregnation with nitrogen at 400 °C for 2 h (labeled as c). The Pd/GAC samples were prepared in a similar manner except that the GAC was subjected to Pd in place of ACC. All prepared samples were reduced before reaction with hydrogen for 1 h at 200 °C to obtain an active metal catalyst. Apparatus and Experimental Procedure. Testing of the prepared catalysts was performed using a stirred autoclave reactor of 0.5 L, made of stainless steel and equipped with ports for sampling and addition of liquids at elevated pressures.17 The standard operation procedure used in each experiment was as follows: the reactor was charged with distilled water and the catalyst was set in the cross-shaped basket of the stirrer. Hydrogen was introduced and flowed for 5 min; adding an appropriate amount of concentrated CP solution in order to prepare a 0.4 L of the required initial concentration, we started the reaction. The progress of the HDC reaction was monitored by measuring the liquidphase concentration of CP and of phenol (P). Blank tests intended to ascertain that the reaction did not proceed thermally, in the absence of a catalyst or of hydrogen, were conducted at 85 °C. No signs of reaction were detected after 3 h under these conditions. ACC supports (without the catalytic metal) did not catalyze the reaction. All HDC kinetic experiments were carried out under a constant initial CP concentration (10 mmol L-1), at a maximum rotation speed of 300 rpm and at 30-85 °C. The reactor was kept under pressure to increase the solubility of hydrogen. Because adsorption was found to occur along with the HDC process, independent experiments for batch adsorption equilibrium were performed using the procedure described previously.12 Analyses. The characterization of supports and Pd/ ACC catalysts was carried out by N2 adsorptiondesorption isotherm runs at liquid nitrogen temperature (77 °K) using a Flowsorb 2300 II (Micromeritix); specific surface areas (SSAs) were calculated by employing the Brunauer-Emmett-Teller method. The surface morphology of prepared samples was observed by a scanning electron microscope (SEM, JEOL 5400). The CP and P concentrations were detected by a UV spectrophotometer (ATI Unicam, UV-2) at λmax ) 280 and 270 nm, respectively. A number of reaction mixture samples were also analyzed using a gas chromatograph-mass spectrometer (HP-5890 model). Agreement between the two methods was excellent, with less than 4% difference in the measured values. The solution was analyzed for Pd using an inductively coupled plasma emission spectrometry (ICP-ES, a Perkin-Elmer Optima 3000 DV instrument). Results Results in this section describe the general optimization procedure, the adsorption properties, and the HDC kinetics using one of active catalyst.
Catalyst Activity and Stability. Several steps of optimization of the catalyst preparation have been conducted initially: Tests of several types of ACC, tests of surface modification prior to impregnation with acidic, basic, or oxidation solutions, and tests of thermal treatment after impregnation have been considered. Most of the surface modification procedures led to negative effects on the stability and HDC activity of the corresponding Pd/ACC samples. Interesting results have been obtained by treatment (a) that increased the basicity of the carbon surface and by treatment (b) that introduced oxygen surface groups. Both treatments are known to produce significant changes in the AC surface chemistry,17 and these in turn can have effects on catalyst dispersion and catalytic activity. The Pd/ACC samples have a high apparent surface area, suggesting that impregnation of Pd did not significantly affect the pore-volume distribution and that no pore blockade occurred. The main optimization steps are listed in Table 2, along with the resulting HDC activity. The activities are specified either per unit catalyst weight (as µmol min-1 gcat-1) or per unit metal weight (as mmol min-1 gPd-1). Initial rates were determined from the slope of P production at short time; all experiments were performed at constant catalyst loading. In many cases the reaction was repeated under identical conditions where the rate reproducibility was better than (6%. The optimization resulted in an order of magnitude increase in the activity of the Pd/ACC. Note that the specific surface area (SSAs) of the various Pd/ACC samples did not vary significantly, indicating that the observed differences are due to the surface chemistry of ACC. The highest activity (about 160 mmol of CP min-1 gPd-1) was achieved with 0.5-1.0 wt % of Pd, independent of the ACC type. All Pd/ACC catalysts tested in this work showed 100% selectivity toward HDC of CP to P; no ring hydrogenation products were detected. To study the effect of the support on the catalytic activity, we prepared Pd/GAC samples and compared them with Pd/ACC in the same reactor at identical conditions: Results indicate that Pd/ACC showed a higher HDC specific activity (per g of metal) than Pd/ GAC catalysts (Table 2). The HDC activity varied with the Pd content: the activity (per g of catalyst) monotonically increased with increased palladium content, but the specific activity per g of Pd seems to decline for increased Pd loading (Figure 1). Tests of the stability of the most active Pd/ACC samples (13 and 14) were conducted under reaction conditions during runs of 3 h in length. The remaining solution was analyzed for Pd using ICP-ES, and Pd was not detected. The constant HDC activity and the absence of Pd in the remaining solutions suggest that Pd/ ACC is not destroyed under the reaction conditions and that it stays bonded to the ACC support. Adsorption Properties. All Pd/ACC samples showed strong adsorption even at low solute concentrations (Figure 2a). The adsorption of CP and P on Pd/ACC as well as on the ACC support can be correlated well by the Langmuir isotherm expressing the relation between the adsorbed amount per unit adsorbent mass (qi) and the concentration in solution (i ) CP or P).
Ind. Eng. Chem. Res., Vol. 40, No. 15, 2001 3303 Table 2. Optimization Steps of Catalyst Preparation activitya sample no.
support
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
ACC1 ACC2 ACC1 ACC2 ACC1 ACC1 ACC2 ACC1 ACC2 ACC1 ACC2 ACC2 ACC2 ACC1 ACC1 ACC1 GAC GAC GAC
a
treatment
Pd content (wt %)
a a+c a+c a+c a+c a+c a+c b b b b+c b+c b b a+c b+c b
0.5 0.5 0.5 0.1 0.5 0.1 0.4 0.9 0.5 0.5 0.5 1.0 0.1 1.0 1.9 5.1 0.1 0.5 0.5
SSA
(m2
1460 1460 1410 1500 1450 1490 1480 1410 1440 1460 1470 1410 1500 1430 1340 1150 760 730 660
g-1)
µmol
min-1
gcat-1
460 410 640 130 780 95 480 670 600 500 690 800 155 880 880 1500 50 190 260
mmol min-1 gPd-1 92 82 128 130 156 95 120 75 120 100 138 80 155 88 46 29 50 38 52
CP )10 mmol L-1, 60 °C, hydrogen pressure 2.8 bar.
Figure 2. Batch adsorption isotherm at 60 °C: (a) CP and P adsorption on ACC and on Pd/ACC; (b) linear representation of the data according to eq 2.
Ci 1 1 ) C + qi qmi i Kiqmi
Figure 1. Effect of the Pd content in ACC on the HDC initial activity expressed per g of catalyst (a) or per g of Pd (b). Conditions: 60 °C, hydrogen pressure 2.8 bar, 0.2 g of catalyst loading in the reactor. For comparison, the activity of Pd/GAC is denoted also, by bold symbols.
KiCi qi ) qmi 1 + KiCi The linear form of eq 1
(1)
(2)
is plotted in Figure 2b, and the estimated values of Ki and qmi are tabulated in Table 3. Incorporation of 1 wt % of Pd onto the ACC led to about 6 or 10% loss in the adsorption capacity of ACC for P or CP, respectively. The presence of Pd did not affect the shape of the equilibrium adsorption isotherms: when scaled with respect to the saturation adsorption capacity (qmi), the isotherms for ACC and Pd/ ACC overlap. HDC Kinetics. HDC kinetics was studied in several batches using various charges (g of catalyst) of the active Pd/ACC catalyst (sample 14). Because the disappearance of CP is both due to HDC conversion to P and due to adsorption, we describe below the temporal decline of the CP concentration along with P production and their sum. The difference between the initial concentration (C0) and the sum of reactant and product concentrations (CCP + CP) expresses the effect of adsorption.
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Table 3. Adsorption Equilibrium Data adsorbent
KCP (L mmol-1)
KP (L mmol-1)
qmCP (mmol g-1)
qmP (mmol g-1)
ACC1 1.0 Pd/ACC1 (3-12 mmol L-1, pH 4.9) 1.0 Pd/ACC1 (3-12 mmol L-1, pH 8.3)
4.5 3.5 1.4
1.4 1.2
5.2 4.4 6.0
4.1 3.8
Figure 3. Typical temporal dependencies of CP (a), P (b), and their sum (c) during HDC of CP as a function of weighted time for different catalyst loadings in the reactor. Conditions: catalyst sample 14, 60 °C, hydrogen pressure 2.8 bar. Large deviations of P + CP from their initial values indicate the effect of adsorption.
Figure 4. Temporal dependencies of CP (a), P (b), and their sum (c) during HDC at three different temperatures (left, hydrogen pressure 2.8 bar) and at different hydrogen pressures (right, 60 °C, 0.2 g of catalyst, sample 14).
Plots of concentrations versus a weighted time coordinate (time × mass of catalyst) show a weak dependence of the CP concentration but a strong dependence in the case of P and the sum CCP + CP on the catalyst charge (Figure 3). This effect should be absent in a simple catalytic process and is due to adsorption. The effect of adsorption is strong at short time and with high catalyst charges as is evident by the difference between C0 and the sum CCP + CP. Increasing temperature accelerated the HDC reaction, and it diminished the effect of adsorption (Figure 4ac). Increasing the hydrogen pressure accelerated the HDC reaction, while it had a small effect on adsorption (Figure 4d-f). To distinguish between the effects of adsorption and reaction, the dynamics of CP adsorption was studied with nitrogen in the feed to the reactor instead of hydrogen. At that time (t ) tH), the hydrogen flow was reinstated and P was detected. The formation of P and the disappearance of CP were found to be independent of the CP adsorption time (Figure 5). Bases are frequently used in catalytic dehalogenation reactions to neutralize the liberated acid and to alter the catalyst surface. Increasing the initial pH of the solution to 8.3, by adding ammonium hydroxide (with [NH4OH]/[CP] ) 1.2), did not affect the initial rate, while the adsorption fraction increased. During reaction the pH of the reaction medium monotonically diminished with increased concentration of P and HCl (Figure 6).
Because the disappearance of CP is both due to conversion of CP to P and due to adsorption on ACC, the rate of temporal decline of the CP concentration should account for the two effects:
dqCP V dCCP ) -rCP M dt dt
(3)
Assuming adsorption equilibrium and adsorption isotherms that follow the multicomponent Langmuir expression
KCPCCP ; qCP ) qmCP 1 + KCPCCP + KPCP KPCP (4) qP ) qmP 1 + KCPCCP + KPCP and assuming the absence of mass-transfer resistance, the rate of the HDC process may be expressed as a function of the concentrations of CP and P (at constant hydrogen pressure) as
rCP ) -
qmCPKCP(1 + KPCP) dCCP V dCCP + M dt (1 + K C + K C )2 dt CP
CP
P
P
qmCPKCPKPCP
dCP (5) (1 + KCPCCP + KPCP)2 dt
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Figure 5. (a) Dynamics of adsorption and HDC on Pd/ACC showing temporal profiles of CP (bold symbols) and P (open symbols) in three experiments in which hydrogen was introduced at t ) 0, 120, or 240 min. Dynamics for CP and P with respect to t - tH are presented in b and c, respectively. Conditions: 0.2 g of catalyst, sample 14, 60 °C.
Figure 7. Rate expression calculated using eq 5: at different catalyst loadings in the reactor (a); at different pH’s (b); at different temperatures (c), and at different hydrogen pressures (d).
Figure 8. Linear dependence of the rate constant on hydrogen pressure (catalyst sample 14, 60 °C).
Figure 6. Effect of ammonium hydroxide on temporal dependencies of (a) CP, (b) P, and (c) pH during HDC. Conditions: 0.1 g of catalyst, sample 14, 60 °C.
A linear dependence of the specific rate (rCP), calculated from experimental data and adsorption parameters at pH 4.9 as function of the CP concentration using eq 5, is obtained (Figure 7a): The rate constant is about 0.1 (L min-1 g-1). The ACC density is 600 g L-1 and, thus, the volumetric constant is about 60 min-1. While
pH affects the rate, results indicate that the effect of pH on the HDC rate is small (Figure 7b). A linear dependence of the rate constant was revealed with respect to the hydrogen pressure (Figures 7c and 8). The estimated apparent activation energy calculated from the HDC rates using eq 5 was found to be 24.8 ( 0.4 kJ mol-1 in the temperature range 30-85 °C (Figure 9d). The results obtained indicate that the rate is approximately described by
r ) 4.2 exp(-24800/RT)CPCH2
(6)
Modeling and Analysis. The purpose of this section is to account for the relative importance of adsorption and diffusion in the reactive process considered here and
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V
[
dcA d〈qA〉 ) -M + 〈r〉 dt dt
]
(9)
The concentration of P follows from the overall mass balance
V(cA0 - cA) + M〈qA〉 ) VcB + M〈qB〉
Figure 9. Radial profiles of the CP, P, and hydrogen concentrations during a typical run (initial concentration of 10 mmol L-1, CA and CB are scaled with respect to that initial value, pressure of 2.8 bar, M ) 0.4 g) in pellets of 0.01 cm (a) and 0.1 cm (b) in radius; the five lines are equally intervaled at 0.4 (a) or 2.0 (b) time units.
to predict the resistance of diffusion in GAC. Consider the problem of adsorption of CP and the HDC reaction
A + H2 f B in a catalytic pellet of spherical or Cartesian geometry. If Ci and qi are the fluid- and adsorbed-phase concentrations of both species and of hydrogen (i ) A, B, H) in the pore space and we account for both pore and surface diffusion (with Dp and Ds, respectively), as well as for the reaction at a rate r (qA, qB, CH) [mmol min-1 gcat-1], then the pellet dynamics is described by
∂(CA + FqA)/∂t ) Dp,A∇2CA + Ds,A∇2qA Fr(qA,qB,CH) (7a)
(10)
Using the ACC and under the assumption that diffusion resistance is absent, we found a linear kinetics in CA (Figure 7), r ) k′′CA with k′′ ∼ 0.1 L min-1 gcat-1 at a pressure of 2.8 bar of hydrogen and a temperature of 60 °C. We also assume a linear dependence on CH, as suggested by Figure 8 and thus r ) k′CACH. Apparently, the rate depends also on pH, but the dependence is relatively weak (Figure 7b). Hydrogen adsorption is small, and we assume that the hydrogen balance is under a steady state. Hydrogen solubility in water, under atmospheric pressure at 20 °C, is 1.6 mg L-1 (0.8 mmol L-1), so that apparently hydrogen limits the reaction (at 60 °C, solubility declines to about 1.5 mg L-1). Before simulating the general case that accounts for all of the equations above, let us analyze the dynamics and consider a few simple cases: (1) When adsorption is negligible, the system is described by eq 9, the plots of c(Mt) should be independent of mcat, and the rate can be determined experimentally from
r ) -V
dcA d(Mt)
(2) When diffusion resistance is absent, the system is described by eqs 7 and 9. Note that cA(t) depends on M/V now and the initial conditions are the concentration after instantaneous equilibrium have been achieved. Substituting the isotherms and rearranging eq 9 yield
dcA
[( )
( ) ]
∂qA dcA ∂qA dcB + ∂cA d(Mt) ∂cB d(Mt)
∂(CB + FqB)/∂t ) Dp,B∇2CB + Ds,B∇2qB + Fr(qA,qB,CH) (7b)
r ) -V
∂(CH + FqH)/∂t ) Dp,H∇2CH + Ds,H∇2qH Fr(qCP,qP,CH) (7c)
where the partial derivatives are estimated from the isotherm. A similar equation can be derived from cB(t). This equation can be used to estimate the rate curve if cA(t) and cB(t) are measured. Application of this approach revealed the linear kinetics used here. While we can estimate the pore-diffusion coefficients, when that resistance is significant, the estimation of surface diffusivities is still difficult. The effective pore diffusivity of P in GAC (with porosity, , of 0.66) was estimated by Sheindorf et al.19 to be Dp,B ) 6.7 × 10-5 cm 2 s-1; while the effect of surface diffusion was not negligible, it was incorporated into the pore diffusivity, and we will proceed under the assumption that pore diffusion dominates (and we will drop the subscript p in the diffusivity description). The diffusivity of pbromophenol in that study was quite similar to that of P, and the diffusivity of CP is expected to be similar (i.e., we assume Dp,A ) Dp,B). The pore mass accumulation term (the first term in eq 7) can be ignored most times (except initially where CA ) 0), and the adsorbate accumulation term should be expressed in terms of pore concentrations (CA and CB).
subject to symmetry at the center and
Ci|R ) ci, qi|R ) qi(ci), CH|R ) cHs
(7d)
where cA and cB are the concentrations in the solution and cHs is the hydrogen concentration at saturation and is assumed to be constant. The adsorption isotherms of the two species are assumed to follow the Langmuir expression (eq 4) and hydrogen adsorption is assumed negligible. Now, the flux to the solid phase
V
[
|
dcCP dCCP ) - Dp,CP dt dr
R
+ Ds,CP
|]
dqCP dr
R
× 4πR2N (8)
accounts for the decline in the solute concentration, where N is the number of pellets, each of diameter 2R. Integrating eq 6 with qA . CA and expressing N in terms of catalyst mass (4πR3N/3 ) M/F), we find
d(Mt)
+M
(11)
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At high pressures, where hydrogen is not limiting the rate, the rate is first order in CA. Under steady-state condition the effectiveness factor depends on one parameter, the Thiele modulus, φ2 ) Fk′R2cHs/Dp,A. Thus, at 20 atm (cHs ) 16 mmol L-1) and with cA , cHs, diffusion becomes limiting as φ is of order unity, or R ∼ 0.05 cm. If the situation is reversed and cA . cH, the reaction is first order in cH and zero order in cA. When both concentrations are comparable (as is the situation here at 2.8 bar; cADA ∼ cHDH), then the reaction is practically a second-order one and diffusion effects become even stronger. The diffusivity of hydrogen in water was reported to be 4.5 × 10-5 cm2 s-1, so that the effective diffusivity in porous GAC is DH ∼ 2.0 × 10-5 cm2 s-1. In the general case, after incorporation of the assumptions above, the model is reduced to the following dimensionless forms:
δφ2cA0 δφ2cA0
∂θA(CA,CB) ) ∇ξ2CA - φ2CAXH; CA(1) ) cA ∂τ
∂θB(CA,CB) DB 2 ∇ C + φ2CAXH; CB(1) ) cB ) ∂τ DA ξ B
∂XH DHcHs 2 ) νφ cA0 ∇ X - φ2CAXH; XH(1) ) 1 ∂τ DA ξ H 2
|
∂cA 3 ∂CA )- 2 ∂τ φ ∂ξ
These data will be used in the future to design a continuous process. Conclusions
ξ)1
cA0 - cA + δ〈θA〉cA0 ) cB + δ〈θB〉cA0
(12)
The dimensionless variables are defined below: time is scaled with respect to reaction time, φ is the Thiele modulus, δ is the mass capacity ratio of the two phases, and ν is the pore-space capacity for hydrogen and is a small number.
ξ)
Figure 10. Temporal profiles of the reactor behavior showing the effect (a) of pellet size (CA and CB are scaled with respect to an initial value of 10 mmol L-1; pressure of 2.8 bar, M ) 0.4 g); (b) of hydrogen pressure (pressure in bar, R ) 0.01, other conditions as in part a); (c) of adsorbent loading (M ) 0.4 or 4.0 g, 2.8 bar, R ) 0.01).
k′cHsMt MqmA Fk′cHsR2 r ; τ) ; δ) ; φ2 ) ; R V VcA0 DA cH qA,B McHs ; θA,B ) ; ν) (13) xH ) cHs qmA FVcA0
The simulated radial concentration profiles (Figure 9; concentrations are scaled with respect to the initial value cA0) in spherical pellets of R ) 0.1 or 0.01 cm, in a typical run with conditions that correspond to those employed here (60 °C and 2.8 bar of hydrogen), demonstrate that diffusion resistance is significant even at the smaller pellet, but the resistance declines somewhat with time. Temporal simulations of the concentration in the reactor demonstrate the effects of pellet size (Figure 10a), of hydrogen pressure (Figure 10b), and of reactor loading (Figure 10c): It demonstrates that pellets of R ) 1 mm (0.1 cm, typical of GAC) are too large to be used, while the difference between 0.01 and 0.001 cm (i.e., 10 µm, similar to ACC) pellets may be small enough to use the larger size. Increasing hydrogen pressure will increase the rate, although not linearly because of diffusion resistance. Recall that the dimensionless time already scales cHs and M. Increasing M will increase the rate of the process but again not linearly. The effect of adsorption is denoted in Figure 10c by plotting cA + cB.
Pd supported on ACC can catalyze the HDC of dissolved chloroorganics, showing a high activity and 100% selectivity of CP conversion to P; no ring hydrogenation products were detected under reaction conditions used. Kinetic analysis showed the reaction to first order with respect to each reactant concentration (CP and hydrogen) and the activation energy was determined in the range 30-85 °C. The activity of the most active Pd/ACC catalyst was double that obtained with Pd/GAC, probably because of pore-diffusion resistance. Analysis of diffusion and reaction shows that diffusion resistance will be significant in granules larger than 100 µm. Several simulations of adsorption, diffusion, and reaction are presented, showing the effect of pellet size, of hydrogen pressure, and of adsorbent loading. Acknowledgment This research was supported by the Ministry of the Science, State of Israel. Yu.S. and Yu.M.-M. are partially supported by the Center for Absorption in Science, Ministry of Immigrant Absorption. Simulations were conducted by Dr. Olga Nekhamkina. Symbols Used ACC ) activated carbon cloth GAC ) granulated activated carbon SSA ) specific surface area [m2 g-1] Ci ) concentration of component i [mmol L-1] ci ) concentration of component i in solution [mmol L-1] Dpi, Dsi ) pore and surface diffusivities of component i [cm2 s-1] Ki ) Langmuir adsorption equilibrium constant for component i [L mmol-1] M ) mass of the catalyst [g] N ) number of pellets r ) reaction rate [mmol min-1 gcat-1]
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r′ ) radial coordinate [cm] R ) pellet radius [cm] k′, k′′ ) constants qi, qmi ) amount of adsorbed component and its saturation value [mmol g-1] t ) time [min] xH ) dimensionless dissolved hydrogen concentration V) reactor volume [L] Greek Letters φ ) Thiele modulus δ ) mass capacity ratio of the two phases ν ) pore-space capacity for hydrogen ) porosity τ ) dimensionless time F ) catalyst density [g cm-3] ξ ) dimensionless radial coordinate θ ) dimensionless adsorption capacity Subscripts CP, P, H ) of p-chlorophenol, phenol, and hydrogen A, B ) of compounds A and B 0 ) initial conditions e ) at equilibrium 〈 〉 ) spatial average
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Received for review November 28, 2000 Revised manuscript received April 4, 2001 Accepted April 6, 2001 IE001019D
