Regeneration of Fixed-Bed Adsorbers Saturated with Single and

Ind. Eng. Chem. Res. 2002, 41, 6165-6174

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Regeneration of Fixed-Bed Adsorbers Saturated with Single and Binary Mixtures of Phenol and m-Cresol Agostinho Garcia, Jose´ Silva, Licinio Ferreira, Anabela Leita˜ o, and Alı´rio Rodrigues* Laboratory of Separation and Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

The regeneration with sodium hydroxide of fixed-bed adsorbers saturated with single and binary mixtures of phenol and m-cresol has been investigated by performing a set of desorption experiments. The process simulation is accomplished by the development of a mathematical model considering the effects of axial dispersion in the liquid phase, film diffusion, intraparticle mass transfer, and reaction between sodium hydroxide and phenol/m-cresol species. The particle equations were discretized by orthogonal collocation on finite elements, and the resulting ordinary differential equations together with the mass balance equations were numerically solved with the PDECOL package. The effects of the flow rate of the regenerant and bed initial conditions on single and binary experiments are addressed. Experimental data show that regeneration with sodium hydroxide is efficient and faster than conventional desorption with a solvent. Introduction Adsorption processes are gaining interest as methods of purifying industrial effluents. Most industries discharge effluents containing several components. Pollution prevention must rely on efficient material recycling and reuse approaches. Prior to recycling and material recovery from process or effluent waste streams, it is often necessary to separate and concentrate the useful or toxic materials. Their removal from wastewater and process streams and contaminated water supplies can best be achieved by adsorption methods. As an example, the regeneration with sodium hydroxide (desorption with chemical reaction) compared with regeneration with a solvent (without reaction) leads to much lower volumes of regenerating polluted solution. In single systems this solution will be highly concentrated (say in phenate), and it can be transformed in a phenol concentrated solution (by an exchange resin in the H+ form) which can be recycled to a process (as in the phenol/formaldehyde resin production). Increasing concern for public health and environmental quality has led to the establishment of limits on the acceptable environmental levels of specific pollutants. Polynuclear aromatic hydrocarbons (PAHs) and phenolic compounds are two classes of compounds widely prevalent in the environment and classified by EPA as priority pollutants. Phenolic compounds, entering the aquatic environment through direct discharge from coke ovens in steel plants, refineries, pulp and paper industries, etc., impart objectionable taste and odor to drinking water at concentrations as low as 0.005 mg/L. Consequently, there has been a growing interest in developing processes of removing these compounds from water. Adsorption is often the preferred separation process because it can be used for removal of a variety of organics from aqueous systems.1-4 Adsorption of phenolic compounds onto microporous activated carbon is of great interest because of the high * To whom correspondence should be addressed. Tel: 351 225081671. Fax: 351 225081674. E-mail: [email protected].

adsorption capacity of this adsorbent.3 However, the regeneration of spent carbon is not easy because of considerable irreversible adsorption. Because the regeneration efficiency is a critical factor for the economy of the overall adsorption process, polymeric adsorbents are used as an alternative to activated carbon for the removal of organic substances from aqueous solutions.4-7 The regeneration of these adsorbents can be accomplished by leaching with alkaline solutions.4,5 Polymeric resins are available in a range of different functionalities and thus can be used for selective removal from multicomponent systems. Unfortunately, the feasibility for specific pollutant removal in multicomponent situations cannot be predicted from the existing body of knowledge. Information is also lacking on the dynamics of multisolute adsorption onto commercial polymeric resins and regeneration in packed columns. Although there are now several methods for predicting and correlating multisolute adsorption from aqueous systems, to date, the various prediction and correlation methods have not been rigorously tested for polymeric resins with well-defined multicomponent mixtures containing more than two components, in part because of the lack of experimental data.1,8-10 Polymeric resins can be easily regenerated in situ, and they have been found to have sorption capacities which approach or exceed that of activated carbon in some cases. Adsorption of phenolic species onto a nonfunctional macroreticular polymeric sorbent was carried out in a finite batch adsorber, multicomponent adsorption equilibrium data were experimentally measured, and the single species isotherms were assumed to be of the Langmuir type.2 The adsorption equilibria of the mixtures of phenol and p-nitrophenol from an aqueous solution on activated carbon have been studied, namely, the effect of pH on the adsorbent selectivity.11 Equilibria of dissolved organic compounds on activated carbon and polymeric resins were studied by Seidel and Gelbin,12 Van Lier,13 Kouyoumdjiev,14 and Ramalho,15 respectively; binary and ternary adsorption equilibria were predicted with the IAS theory.16 Multicomponent adsorption equilibria on zeolites at atmospheric pressure

10.1021/ie010888f CCC: $22.00 © 2002 American Chemical Society Published on Web 10/17/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 Table 2. Experimental Conditions for Regeneration Experimentsa

Figure 1. Experimental setup used for regenerating experiments.

run

flow rate, U (mL/min)

phenol, C1i (mg/L)

m-cresol, C2i (mg/L)

1 2 3 4 5 6 7 8 9 10

43 117 50 58 17 50 50 67 50 47

110 96 60 158 200 0 181 118 0 108

110 93 90 49 150 204 0 0 100 102

a In all experiments the sodium hydroxide concentration is C 5f ) 1 M, except for run 3, where C5f ) 1.1 M, and run 7, where C5f ) 1.2 M.

and 230 °C were studied by Paludetto et al.17 and predicted via a nonideal thermodynamic adsorption model based only on binary interaction parameters. Modeling of equilibrium and nonequilibrium multisolute adsorption processes was addressed by Mansour.18 Basic concepts needed for the interpretation and modeling of fixed-bed adsorption have been given by Rodrigues.19 The objective of this work is the study of the regeneration of adsorption beds saturated with phenol and m-cresol using sodium hydroxide as a regeneration agent. A mathematical model for the adsorption system is developed, and regenerative experiments were performed under various experimental conditions showing the effects of flow rate, feed concentration, and bed initial conditions. Finally the reliability of the model has been verified in its capability of reproducing the experimental runs.

of the column in response to a step input of sodium hydroxide concentrations were collected in a sample collector and analyzed by gas chromatography with a Varian Aerograph 1400 equipment, with a flame ionization detector and a 1829 mm × 2 mm glass column packed with 0.1% SP-1000 in Carbopack C. The column temperature was set at 225 °C, and the carrier gas was nitrogen at 20 mL/min. After a saturation-regeneration experiment, there is a washing step to remove sodium hydroxide from the column. Experimental Procedure. Prior to every experiment, the column is washed with pure water contained in the reservoir PW. After that, the column is saturated using the solution (single or binary mixture of phenol and m-cresol) contained in the reservoir SS. Once the adsorption column AC is saturated, the switching valve SV1 is rotated, allowing the pumping of a regenerating solution RS. Time zero in the experiments is controlled by switching valve SV2 that allows the cleaning of pipes between the solution reservoirs and the column entry using the discharge pipe. The effluent of the column is collected in a sample collector system SC with the aid of switching valve SV3 that at a predetermined period of time is activated. Once the samples are collected, they are analyzed in the gas chromatograph system GC.

Experimental Section

Adsorption Equilibrium

A schematic diagram of the experimental setup is shown in Figure 1. The apparatus consists of three major sections: (i) feed section; (ii) packed column; (iii) analytical section. (i) The feed section consists of three different solutions: the saturating solution (a single or binary mixture of phenol and m-cresol), the regenerating solution (sodium hydroxide), and pure water for washing. A constant flow rate in the column is set up by a peristaltic pump. All of the experiments were carried out at 20 °C, by using flow rates up to 265 mL/min, feed concentrations of sodium hydroxide (Cf5) of around 1 M, and bed initial conditions saturated with phenol (C1i) and/or m-cresol (C2i) as depicted in Table 2. (ii) The regeneration experiments were conducted in a fixed-bed adsorber of 39 cm length and 2.18 cm diameter. The adsorbent is a polystyrene matrix crosslinked with divinylbenzene [Duolite ES-861 (Rohm and Haas) macroporous polymeric adsorbent] with characteristics summarized in Table 1. (iii) The total concentrations of phenol + phenate and m-cresol + m-cresate in the liquid phase at the outlet

Single solute adsorption equilibrium isotherms for phenol and m-cresol onto the polymeric adsorbent Duolite ES-861 were obtained by Ramalho15 and were represented by Langmuir isotherms. In the present work, the binary adsorption equilibrium is predicted by the extended Langmuir equation:

Table 1. Properties of Duolite ES-861 Resin wet density, Fh (gwet resin/Lresin) apparent density, Fa (gdry resin/Lresin) real density, Fr (gdry resin/Lpolymer) BET surface area (m2/g) particle porosity, p particle diameter, dp (mm)

1020 285.6 1040 500 0.72 0.47

qi )

qmaxbiCi 1+

∑i biCi

(1)

where qi is the adsorbed concentration of component i in equilibrium with the liquid concentration Ci and qmax is the maximum adsorbed concentration. The adsorption equilibrium isotherm parameters qmax ) 141 mg/gads, b1 ) 0.0017 L/mg, and b2 ) 0.0065 L/mg were estimated from the single-component equilibrium data. Kinetics The regeneration of polymeric adsorbents saturated with phenol and m-cresol can be achieved by using

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sodium hydroxide. The reactions taking place are the following:

Mass Balance inside the Adsorbent Particle.

p

(i) For phenol C6H5OH + OH- / C6H5O- + H2O

)

(2)

(ii) For m-cresol CH3C6H4OH + OH- / CH3C6H4O-+ H2O

(

∂qi ∂Cpi ∂Cpi 1 ∂ + (1 - p) ) pDpi 2 R2 + pRi ∂t ∂t ∂R R ∂R (7)

(3)

The kinetics of the reactions were approximated by a first-order law on both reactants resulting in the overall rate laws

r1 ) k11C1C5 - k12C3

(4)

r2 ) k21C2C5 - k22C4

(5)

where r1 and r2 represent the velocity of phenol and m-cresol disappearance, respectively; k11, k12, k22, and k33 are reaction constants; and C1, C2, C3, C4, and C5 are concentrations of phenol, m-cresol, phenate, mcresate, and sodium hydroxide, respectively. For the system under consideration, the equilibrium constant for the phenol reaction is keq1 ) k11/k12 ) 1.02 × 104 L/mol at 20 °C, and for the m-cresol reaction, it is keq2 ) k21/k22 ) 8.13 × 103 L/mol at 20 °C (Ramalho15). Mathematical Model for Fixed-Bed Regeneration The mathematical model for the fixed-bed regeneration is based on the following assumptions: (a) intraparticle mass transfer takes place by diffusion in the liquid phase filling the pores; (b) the effective diffusivities are taken to be constant; (c) the adsorption equilibrium is instantaneously achieved at the pore/wall interface; (d) the adsorbent particles are spherical; (e) the hydrodynamics of the fluid flow is described by the dispersed plug-flow model; (f) phenate, m-cresate, and hydroxide can diffuse inside particles. The following mass balance equations, with respective initial and boundary conditions, may be written for the species i (i ) 1, 2, ..., 5) where the subscripts are as follows: 1, phenol; 2, m-cresol; 3, phenate; 4, m-cresate; 5, sodium hydroxide. Mass Balance in the Bulk Fluid Phase.

∂Cbi ∂2Cbi ∂Cbi 3 1 - b -u kfi(Cbi ) Dax 2 ∂t ∂z R b ∂z p Cpi|R)Rp) + Ri (6) where Cbi is the bulk concentration in the bed fluid for species i, Cpi is the concentration in the porous adsorbent particle for species i, Dax is the bed axial dispersion, u is the bed interstitial velocity, kfi is the film masstransfer coefficient, z is the axial coordinate in the bed, Rp is the particle radius, b is the bed porosity, t is the time, and Ri is the rate law for species i: for phenol (i ) 1), Ri ) -r1; for cresol (i ) 2), Ri ) -r2; for phenate (i ) 3), Ri ) r1; for cresate (i ) 4), Ri ) r2; for sodium hydroxide (i ) 5), Ri ) -(r1 + r2).

where Dpi is the pore diffusivity, R is the radial coordinate of the particle, p is the porosity of the adsorbent, and qi is the adsorbed phase concentration for species i (qi ) 0 for i ) 3-5). Boundary and Initial Conditions. (i) Boundary Conditions in the Bed.

z ) 0; Cbi ) 0, Cb5 ) C5f

(8)

where the subscript i refers to species (i ) 1, 2, ..., 4) and C5f is the concentration of the regenerant sodium hydroxide at the bed inlet (feed condition).

∂Cbi )0 ∂z

z ) L;

(9)

where the subscript i refers to species (i ) 1, 2, ..., 5). (ii) Initial Conditions in the Bed.

t ) 0; Cb1 ) C1,ini, Cb2 ) C2,ini, Cb3 ) Cb4 ) Cb5 ) 0 (10) where C1,ini and C2,ini are the initial fluid concentrations in the bed saturated with phenol and m-cresol. (iii) Boundary Conditions in the Particle.

R ) Rp; pDpi

|

∂Cpi ∂R

R)Rp

) kfi(Cbi - Cpi|R)Rp)

(11)

where the subscript i refers to species (i ) 1, 2, ..., 5).

∂Cpi )0 ∂R

R ) 0;

(12)

where the subscript i refer to species (i ) 1, 2, ..., 5). (iv) Initial Conditions in the Particle.

t ) 0; Cp1 ) C1,ini, Cp2 ) C2,ini, Cp3 ) Cp4 ) Cp5 ) 0 (13) Numerical Solution First, the bed fluid coordinate z and particle radial coordinate R were written in dimensionless form by introducing the set of reduced variables: z* ) z/L and R* ) R/Rp for axial and radial coordinates, respectively. Then, model equations were solved by using the following numerical technique: the mass balances for adsorbent particles, eq 7, were discretized along the particle radial coordinate, R*, by using orthogonal collocation on finite elements with cubic Hermite polynomials as basis functions (Finlayson20). The interval 0 e R* e 1 was divided into NE subintervals, with two collocation points within each subinterval. The solutions Cpi, with i ) 1, 2, ..., 5, in the kth subinterval of R* were approximated by 4

Cpi(gj,R*,t) )

m ai+2k-2 (R*,t) Hji ∑ i)1

(14)

where gj, with j ) 1 and 2, are the collocation points within each subinterval of u*. After the discretization process, we obtained a system of 5 × 2NE ) 10NE

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Table 3. Model Parameters for Experimental Runsa run Dax (cm2/min) kf1 (cm/min) kf2 (cm/min) kf3 (cm/min) kf4 (cm/min) kf5 (cm/min)

1

2

3

4

5

6

7

8

9

10

8.9 0.61 0.59 0.58 0.55 0.40

24.4 0.95 0.91 0.90 0.86 0.62

10.5 0.65 0.63 0.62 0.59 0.42

12.0 0.69 0.66 0.65 0.67 0.45

3.5 0.38 0.37 0.39 0.37 0.25

10.4 0.65 0.63 0.62 0.59 0.43

10.4 0.65 0.63 0.62 0.59 0.43

13.9 0.74 0.71 0.70 0.67 0.48

10.5 0.65 0.63 0.62 0.59 0.42

9.7 0.63 0.61 0.60 0.57 0.41

a In all experiments, D -4 cm2/min, D -4 cm2/min, D -4 cm2/min, D -4 cm2/min, and p1 ) 3.8 × 10 p2 ) 3.6 × 10 p3 ) 3.6 × 10 p4 ) 3.4 × 10 Dp5 ) 3.5 × 10-4 cm2/min.

Figure 2. Regeneration with sodium hydroxide of a fixed-bed saturated with phenol (a) and m-cresol (b). Part i represents phenol or m-cresol desorption; part ii represents phenate + phenol (4) or m-cresate + m-cresol (]) desorption; part iii represents sodium hydroxide desorption. Points are experimental data (Table 2) and lines model predictions with parameters shown in Table 3.


ordinary differential equations (ODEs) in the basis function coefficients am dependent on the bed axial coordinate, z*, and time, t. Finally, the system of 10NE ODEs resulting from the discretization step of the particle equations and the mass balance equations in the bulk fluid phase, eq 6, was numerically integrated with the PDECOL package.21 Results and Discussion A set of regeneration experiments were performed in order to study the effect of operating variables such as the effect of the flow rate of sodium hydroxide regenerant and bed initial saturation of phenol and m-cresol. Table 2 shows the number of experiments performed and the respective experimental conditions. All experiments were conducted at a very high concentration of sodium hydroxide (1 N) because, as shown by Costa and Rodrigues,4,5 high values of the pH favor the desorption of phenol and m-cresol. At the same time, the reliability of the mathematical model is verified in its capability to simulate the experimental runs. Model parameters were obtained from both experiments and correlations. The bed axial dispersion coeffiecient, Dax, was determined from tracer experiments.22 The pore diffusivity Dpm was obtained by the expression Dm/Tp, where Dm is the molecular diffusivity and Tp ()1.4) is the tortuosity factor for the adsorbent found in a previous work.22 The film masstransfer coefficients, kfi, were calculated by using the correlation jD ) 7.32Re-0.567, where jD ) (kf/u0)(µ/FDm)2/3. The molecular diffusivities were estimated by the Wilke-Chang equation. Table 3 shows the values of the model parameters for the experimental runs. Single-Component Experiments. Parts a and b of Figure 2 show the regeneration with sodium hydroxide of a bed previously saturated with phenol and m-cresol, respectively. The fluid concentration used to saturate the bed was 118 mg/L for phenol and 100 mg/L for m-cresol. The concentration of regenerant sodium hydroxide is 1 N. Regeneration of the fixed bed is very fast for both components (15 min for phenol and 25 min for m-cresol), as can be seen from the desorption curves of phenate + phenol and m-cresate + m-cresol; however, it takes more time to regenerate the bed saturated with m-cresol because of the higher adsorption capacity of the adsorbent for this species. It should be noted that for the same flow rate it takes around 200 min to saturate the bed with phenol and 400 min to m-cresol as reported in a previous work.23 Lines in Figure 2a,b are model predictions. The model was fitted to the experimental data in Figure 2 using the kinetic constants k11 and k21 as adjustable parameters. The values obtained are k11 ) 55.8 L/mol‚min and k21 ) 82.6 L/mol‚min. These values were kept constant in all single and binary simulations performed in this work. The fast regeneration of the bed seen in Figure 2a,b can be clearly explained by the influence of the sodium hydroxide concentration on the adsorption equilibrium of phenol onto the polymeric adsorbent. For one adsorbed species (phenol), the adsorption equilibrium can be represented by

q1 )

qmaxb1C1 1 + b1C1

(15)

Figure 3. Effect of the sodium hydroxide concentration (pH) in the adsorption equilibrium of phenol.

The equilibrium constant for the phenol reaction with sodium hydroxide (eq 2) is

keq1 )

C3 C1C5

(16)

Considering CT ) C1 + C3, where CT is the total concentration of phenol plus phenate, and combining it with eqs 15 and 16, we obtain

q1 )

qmaxb1CT 1 + keq1C5 + b1CT

(17)

This equation explains the influence of the sodium hydroxide concentration on the adsorption equilibrium which is shown in Figure 3. It clearly shows that the presence of sodium hydroxide in the fluid phase at concentrations higher than 1 × 10-7 M (pH ) 7) decreases significantly the amount of phenol adsorbed. This interpretation can be extended to the adsorption of m-cresol and binary systems. It should be noted that sodium hydroxide is not adsorbed in this polymeric adsorbent. In fact, this species breaks at a time given by total porosity × column volume/flow rate. The same happens with phenate and m-cresate species. Moreover, a simple model based on the analogy with the penetration theory allows us to predict the maximum peak concentration and the time at which it occurs. As an example for run 8 (Figure 2a) the time at which the maximum peak concentration should appear (based in a model without axial dispersion and instantaneous reaction) is total porosity × column volume/flow rate ) 1.82 min. The penetration theory allows us to calculate a penetration distance δ for OH- ions in the polymeric particle during a contact time tc ) dp/u, where dp is the particle diameter and u is the interstitial velocity; in fact, the penetration theory says that the mass-transfer coefficient is k ∝ xD/tc and so δ ) xDtc, where D is the diffusivity of sodium hydroxide. For run 8, δ ) 4.8 × 10-3 cm, which is around 1/10 of the particle diameter. The sodium hydroxide will react with the phenol adsorbed in the layer of the particle between the surface and 2dp/5, i.e., a volume V1 ) (61/125)Vp, and the amount of phenol adsorbed in that layer for the whole adsorbent bed is n1 ) q1 (mg/mL) × Vadsorbent (mL) × (V1/Vp) ) 23.5 mg/mL × 84.4 mL × (61/125) ) 968 mg.

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Figure 4. Effect of the bed initial concentration of phenol in the regeneration of the fixed bed. In part a, the initial concentration of phenol is 181 mg/L, and in part b, it is 118 mg/L. Part i represents phenol desorption; part ii represents phenate + phenol (4) desorption; part iii represents sodium hydroxide desorption. Points are experimental data (Table 2) and lines model predictions with parameters shown in Table 3.

The fluid concentration at the column outlet at 1.82 min (maximum peak concentration) would be c1 ) n1/(total porosity × column volume) ) 968 mg/122.2 mL ) 7920 mg/L. On the basis of this simple model, the regeneration would be complete at a time equal to Rp/δ (total porosity × column volume/flow rate), which for run 8 is 9.1 min (well compared with experimental data shown in Figure 2a). In general, the regeneration time from this simple model would be proportional to 1/xflowrate. Effect of the Bed Initial Condition on Regeneration Curves. Parts a and b of Figure 4 show the effect

of the bed initial concentration of phenol in regeneration curves. In Figure 4a the initial saturation concentration of phenol in the bed is 181 mg/L, and in Figure 4b, it is 118 mg/L. Figure 4 shows that the initial saturation concentration does not affect the time needed to regenerate the fixed bed. This can be explained because the reaction of phenol and sodium hydroxide is very fast. Once sodium hydroxide breaks the column (practically at the space time in both experiments), all of the phenol in the bed is converted to phenate and consequently the phenol disappears from the outlet of


Figure 5. Effect of the bed initial concentration of m-cresol in the regeneration of the fixed bed. In part a, the initial concentration is 100 mg/L, and in part b, it is 204 mg/L. Part i represents phenol desorption; part ii represents m-cresate + m-cresol (]); part iii represents sodium hydroxide. Points are experimental data (Table 2) and lines model predictions with parameters shown in Table 3.

the column, as can be seen in Figure 4 (i). After this period of time, the regeneration will proceed only by a diffusive effect of phenate in the pores of the adsorbent and a diffusive and convective effect in the fixed bed, which is similar for the concentrations under investigation. Parts a and b of Figure 5 show the effect of the bed initial concentration of m-cresol in regeneration curves. The behavior is similar to that found for phenol (Figure 4). In these experiments regeneration takes more time

(approximately 20 min compared to 10 min for phenol). This can be explained because the bed adsorption capacity practically doubles for m-cresol. Again, Figure 5 shows that the initial saturation concentration does not affect the time needed to regenerate the fixed bed, and the explanation for this behavior is the same as that pointed out for phenol. A reasonable agreement is observed between experimental and calculated concentration curves, validating this model for single-component experiments.

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Figure 6. Effect of the flow rate of sodium hydroxide on the regeneration of a fixed bed saturated with binary mixtures of phenol and m-cresol. In part a, the flow rate of sodium hydroxide is 117 mL/min, in part b, it is 43 mL/min, and in part c, it is 17 mL/min. Part i represents phenol and m-cresol desorption; part ii represents phenate + phenol (4) and m-cresate + m-cresol (]); part iii represents sodium hydroxide. Points are experimental data (Table 2) and lines model predictions with parameters shown in Table 3.

Binary Regeneration Experiments. Binary regeneration experiments were performed because this is the most important case from the industrial point of view. The fixed bed was previously saturated with binary mixtures of phenol and m-cresol and afterward regenerated with sodium hydroxide. The effects of regenerant flow rate and bed initial concentrations of phenol and m-cresol are studied. At the same time, the mathematical model developed in this work with parameters independently determined is verified in its capability of predicting the binary experimental runs. Effect of the Flow Rate of Sodium Hydroxide on the Regeneration of the Fixed Bed. Figure 6 shows the effect of the flow rate of sodium hydroxide on the regeneration of the fixed bed. The bed is saturated with similar concentrations of phenol and m-cresol and regenerated with different flow rates of sodium hydroxide. In Figure 6a, the flow rate of a sodium hydroxide solution is 117 mL/min, in Figure 6b, it is 43 mL/min, and in Figure 6c, it is 17 mL/min. Figure 6 shows that the flow rate of sodium hydroxide has only the effect of diluting the concentration of phenate and m-cresate at

the outlet of the bed. The time needed to clean the fixed bed is practically the same for the three runs, approximately 20 min. This can be explained because sodium hydroxide is fed at a very high concentration and in excess for the three runs; therefore, when sodium hydroxide breaks the column at a time equal to the space time, practically all of the phenol and m-cresol are converted to phenate and m-cresate in the column. The regeneration will proceed by the diffusive effect of phenate and m-cresate in the porous adsorbent that does not depend on the flow rate. These experiments show that it is not necessary to use a very high flow rate of sodium hydroxide to regenerate the fixed bed if sodium hydroxide is fed at a very high concentration as in the present case. Figure 6 also shows that phenate is cleaned from the fixed bed in a shorter time than the time needed to clean the bed from m-cresate. This is so because the fixed bed is saturated with similar concentrations of phenol and m-cresol; because m-cresol has an higher affinity to the adsorbent the amount of mass in the bed is also higher, consequently the time needed to clean m-cresate species increases. A reasonable


Figure 7. Effect of the bed initial conditions on the regeneration of a fixed bed saturated with binary mixtures of phenol and m-cresol. In part a, the bed saturation concentration of phenol is 60 mg/L and that of m-cresol 90 mg/L; in part b, the phenol concentration is 158 mg/L and that of m-cresol 49 mg/L; in part c, phenol is 108 mg/L and m-cresol 102 mg/L. Part i represents phenol and m-cresol desorption; part ii represents phenate + phenol (4) and m-cresate + m-cresol (]); part iii represents sodium hydroxide. Points are experimental data (Table 2) and lines model predictions with parameters shown in Table 3.

agreement is observed between experimental and calculated concentration curves in the experiments, validating the model to predict binary regeneration of the fixed bed. Effect of the Bed Initial Conditions on the Regeneration of the Fixed Bed. Figure 7 shows the effect of the bed initial conditions on the regeneration of the fixed bed. The bed is saturated with different concentrations of phenol and m-cresol and regenerated with sodium hydroxide at the same flow rate and concentration. In Figure 7a, the fluid concentrations in the bed after saturation are 60 mg/L for phenol and 90 mg/L for m-cresol; in Figure 7b, the phenol concentration is 158 mg/L and that for m-cresol is 49 mg/L, and in Figure 7c, the phenol concentration is 108 mg/L and that for m-cresol is 102 mg/L. Figure 7 illustrates that the desorption curves of phenate and m-cresate become closer as the bed initial condition of the phenol concentration in the bed increases and that of m-cresol decreases. This is because the amounts of both components in the bed change because of the adsorption

equilibrium. Again the model is in close agreement with the experiments, being a valuable tool for the study and operation of fixed-bed adsorbers regenerated with bases. These experimental data on reactive regeneration seem to support the use of sodium hydroxide as a regenerant agent as recommended by Fiorentino24 and Fox.25 Conclusions In this work the regeneration with sodium hydroxide of fixed-bed adsorbers packed with a polymeric adsorbent and saturated with phenol and m-cresol mixtures was studied. Both single-component and binary regeneration experiments were performed in order to study the effects of the flow rate of regenerant and bed initial conditions on desorption curves. It is shown that when the concentration of sodium hydroxide is high, the flow rate of the regenerant does not affect the time needed to regenerate the fixed bed. Also the regeneration with sodium hydroxide is very fast, decreasing significantly

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the cycle time for this process, compared with the conventional regeneration with a solvent. A mathematical model is developed considering dispersed plug flow for the bulk liquid, external masstransfer resistance, intraparticle mass transfer by pore diffusion, instantaneous equilibrium of adsorption at the pore-wall interface, and a kinetic law for the reaction between sodium hydroxide and phenol/m-cresol. The particle equations were discretized by orthogonal collocation on finite elements and the resulting ODEs together with the mass balance equations were numerically solved by the method of lines with the PDECOL package. Model results are in close agreement with experimental data, showing that the simulation is an effective tool to the study and operation of fixed-bed adsorbers regenerated with an alkaline solution. Nomenclature bi ) Langmuir equilibrium constant, L/mg Cbi ) concentrations of species (i ) 1-5) in the bulk liquid phase, mg/L Cpi ) concentrations of species (i ) 1-5) in the particle, mg/L CT ) total concentration of phenol plus phenate, mg/L dc ) column diameter, cm dp ) adsorbent particle diameter, cm Dax ) axial dispersion coefficient, cm2/s Dpi ) solute effective diffusivity of species (i ) 1-5) in the particle pores, cm2/s kfi ) film mass-transfer coefficient, cm/s k11 ) rate constant for direct phenol reaction, L/mol‚min k21 ) rate constant for direct m-cresol reaction, L/mol‚min Keq,1 ) equilibrium constant for the phenol reaction, L/mol Keq,2 ) equilibrium constant for the m-cresol reaction, L/mol L ) column length, cm qi ) adsorbed solute concentration in equilibrium with the liquid concentration, mg/Lads qmax ) maximum capacity of the adsorbent, mg/Lads r ) rate law Ri ) rate law for species i R ) particle radial coordinate, cm Rp ) particle radius, cm t ) time, s u ) interstitial fluid velocity, cm/s u0 ) superficial velocity, cm/s U ) flow rate, mL/min z ) axial column coordinate, cm Greek Words δ ) penetration distance, cm b ) external porosity p ) particle porosity Tp ) tortuosity factor Fa ) apparent density, g/cm3 Fh ) wet density of the adsorbent, g/cm3 Subscripts 1 ) phenol 2 ) m-cresol 3 ) phenate 4 ) m-cresate 5 ) sodium hydroxide ini ) initial f ) feed conditions i ) species

Literature Cited (1) Gusler, G. M.; Browne, T. E.; Yuram C. Sorption of organics from aqueous solution onto polymeric resins. Ind. Eng. Chem. Res. 1993, 32, 2727. (2) Moon, H.; Kook, S. K.; Park, H. C. Adsorption of Phenols onto polymeric Sorbent. Korean J. Chem. Eng. 1991, 8 (3), 168. (3) Calleja, G.; Serna J.; Rodrı´guez, J. Kinetics of Adsorption of Phenolic Compounds from Wastewater onto Activated Carbon. Carbon 1993, 31, 691. (4) Costa, C.; Rodrigues, A. E. Design of Cyclic Fixed Bed Adsorption Process. II. Phenol Adsorption on Polymeric Adsorbents. AIChE J. 1985, 31, 1645. (5) Costa, C.; Rodrigues, A. E. Design of Cyclic Fixed Bed Adsorption Process. I. Regeneration and Cyclic Operations. AIChE J. 1985, 31, 1655. (6) Garcia, A.; King, C. The use of basic polymer sorbent for the recovery of acetic acid from dilute aqueous solution. Ind. Eng. Chem. Res. 1989, 28, 204. (7) Rixey, W.; King, C. Fixed bed multisolute adsorption characteristic of non-wet adsorbents. AIChE J. 1989, 35, 69. (8) Browne, T. E.; Cohen, Y. Aqueous Phase Adsorption of Trichloroethylene and Chloroform by Polymeric Resins and Activated Carbon. Ind. Eng. Chem. Res. 1990, 29, 1338. (9) Browne, T. E.; Cohen, Y. Polymer-Grafted Silica: A Screening System for Polymeric Adsorption Resin Development. Ind. Eng. Chem. Res. 1993, 32, 716. (10) Gusler, G. M.; Cohen, Y. Swelling Equilibrium of Higly Cross-linked Polymeric Resins. Ind. Eng. Chem. Res. 1994, 33, 2345. (11) Siedel, A.; Reschke, G.; Friedrich, S.; Gelbin, D. Equilibrium adsorption of two component organic solutes from aqueos solutions on activated carbon. Adsorpt. Sci. Technol. 1986, 3, 189. (12) Seidel, A.; Gelbin, D. On applying the ideal adsorbed solution theory to multicomponent adsorption equilibria of dissolved organic components on activated carbon. Chem. Eng. Sci. 1988, 43 (1), 79. (13) Van Lier, W. C. Mass transfer to activated carbon in aqueous solutions. Ph.D. Thesis, T. U. Delft, Delft, The Netherlands, 1989. (14) Kouyoumdjiev, M. S. Kinetics of adsorption from liquid phase on activated carbon. Ph.D. Thesis, T. U. Eindhoven, Eindhoven, The Netherlands, 1992. (15) Ramalho, E. Adsorç aõ multicomponente de fenois. Ph.D. Thesis, University of Porto, Porto, Portugal, 1993. (16) Myers, A.; Prausnitz, J. Thermodynamics of mixed gas adsorption. AIChE J. 1965, 11, 121. (17) Paludetto, R.; Gamba, G.; Sorti, G.; Carra, S. Multicomponent adsorption equilibria of highly non-ideal mixtures: the case of choroaromatic mixtures on Zeolite. Chem. Eng. Sci. 1987, 11, 2713. (18) Mansour, A. R. Comparison of equilibrium and nonequilibrium models in the simulation of multicomponent sorption processes. Sep. Sci. Technol. 1987, 22 (4), 1219. (19) Rodrigues, A. E. Modeling of percolation processes. In Percolation Processes: Theory and Applications; Rodrigues, A. E., Tondeur, D., Eds.; Sijthoff & Noordhoff: Alphen aan den Rijn, The Netherlands, 1981; p 31. (20) Finlayson, B. Nonlinear Analysis in Chemical Engineering; MacGraw-Hill: New York, 1980. (21) Madsen, N.; Sincovec, R. PDECOL: General Collocation Software for Partial Differential Equations. ACM Trans. Math. Software 1979, 5, 326. (22) Ferreira, L. Dinaˆmica de Processos de Sorç aõ. Ph.D. Thesis, University of Porto, Porto, Portugal, 1994. (23) Garcia, A.; Ferreira, L.; Leitaõ, A.; Rodrigues, A. E. Binary Adsorption of Phenol and m-cresol Mixtures onto Polymeric Adsorbent. Adsorption 1999, 5, 359. (24) Fiorentino, E. Adsorption Process. U.S. Patent 4,067,854, Jan 1978. (25) Fox, C. R. Plant uses prove phenol recovery with resins. Hydrocarbon Process. 1978, 57, 269.

Resubmitted for review April 15, 2002 Revised manuscript received June 28, 2002 Accepted September 9, 2002 IE010888F

Regeneration of Fixed-Bed Adsorbers Saturated with Single and

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