Fixed-Bed Adsorption of Salicylic Acid onto Polymeric Adsorbents and

Fixed-bed adsorption and desorption of salicylic acid onto activated charcoal (Filtrasorb F400) and onto polymeric adsorbents (Sephabeads SP207 and SP...
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Ind. Eng. Chem. Res. 2005, 44, 927-936

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Fixed-Bed Adsorption of Salicylic Acid onto Polymeric Adsorbents and Activated Charcoal Marta Otero,† Miriam Zabkova, Carlos A. Grande, and Alı´rio E. Rodrigues* Laboratory of Separation and Reaction Engineering (LSRE), Department of Chemical Engineering, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal

Fixed-bed adsorption and desorption of salicylic acid onto activated charcoal (Filtrasorb F400) and onto polymeric adsorbents (Sephabeads SP207 and SP206) were studied. A mathematical model using the salicylic acid adsorption equilibrium data and linear driving force rate equations to describe diffusional mass transfer was used for the simulations. With the Sephabeads SP206 adsorbent, we have measured breakthrough curves in a pilot-scale fixed bed. Also, an experiment of parametric pumping was performed to demonstrate the feasibility of water purification from salicylic acid liquid streams. The temperature in the parametric pumping system was cycled between 293 and 333 K. 1. Introduction Salicylic acid is a compound produced from phenol1 and was once used as a painkiller, with it being later replaced by aspirin because it caused severe stomach pain. Salicylic acid has many applications, but the most popular one is the manufacture of acetylsalicylic acid (ASA; aspirin), which is accompanied by the production of phenolic wastewaters. ASA is the salicylate ester of acetic acid1 and, when ingested, becomes in its active form salicylic acid.2,3 More than 10 000 tons of aspirin are consumed each year in the United States alone.4 Traditionally, drugs are rarely viewed as potential environmental pollutants and only recently started to capture the attention of the scientific or popular press;5-9 nevertheless, salicylic acid, which is a xenobiotic organic compound,4 may have several important metabolic effects that contribute to its toxicity.10-17 Also, if drugs are not captured from wastewaters during treatment, they will remain in sewage sludge, which is actually a major pathway by which drugs enter the environment.7 Adsorptive processes are an option for the purification of wastewaters containing pharmaceuticals. An additional interesting aspect of sorption operations, especially when dealing with valuable molecules, is their ability to concentrate solutes. Traditional fixed-bed adsorption systems have two weaknesses: one is the need for a chemical regenerant, which implies an additional waste disposal problem, and the other is the lack of efficiency, because only a fraction of the adsorbent is used in fixed-bed operation. As an alternative, thermal parametric pumping is a cyclic process based upon the fact that the adsorption equilibrium isotherm of solutes changes with temperature. A cyclic change of temperature occurring simultaneously with flow reversal will enable the separation process. In thermal parametric pumping, the temperature change can be imposed through the bed jacket in direct mode or through the temperature change of the * To whom correspondence should be addressed. Tel.: +351 225081671. Fax: +351 225081674. E-mail: [email protected]. † On leave of absence from Chemical Engineering Department, Natural Resources Institute, IRENA-ESTIA, University of Leo´n, Leo´n, Spain.

fluid stream in recuperative mode. If the feed stream (wastewater) passes through a bed of adsorbent in an upward flow at a “hot” temperature and later in a downward flow at a “cold” temperature, this cyclic process will produce a concentrated solution in a top reservoir and a solute-free solution in the bottom one. This will make possible the purification of salicylic acid industrial wastewaters without spending chemicals for regeneration, and therefore avoiding a new pollution problem. The fixed-bed adsorptive performances of two polymeric adsorbents, Sephabeads SP206 and SP207, were compared to that of a well-known standard activated charcoal, Filtrasorb F400,18,19 for the uptake of salicylic acid from an aqueous solution. The influences of the bed height and feed flow rate were considered. Special attention was given to the temperature effect on the adsorption of salicylic acid, with purification technologies such as thermal parametric pumping20-24 being based on a marked differential adsorptive behavior as a function of temperature. In this work, three adsorbents were compared according to their thermal separation potential. With the Sephabeads SP206 pellets, breakthrough curves were measured in a pilot-scale fixed bed at 293 and 333 K, which was subsequently used in a parametric pumping experiment to evaluate the feasibility of this technology for salicylic acid wastewater treatment. 2. Experimental Section 2.1. Chemicals and Adsorbents. Salicylic acid (C7H6O3) was purchased from Sigma-Aldrich (Madrid, Spain). Two different polymeric resins were used in this study: Sephabeads SP206 and SP207 (Mitsubishi Chemical Corp., Tokyo, Japan) were purchased from Resindion (Rome, Italy), and activated carbon Filtrasorb F400 was kindly provided by Chemviron Carbon (Feluy, Belgium). Table 1 shows the physical characteristics data of all of the adsorbents as supplied by the manufacturers. The solutions of salicylic acid were prepared with degassed distilled water. 2.2. Fixed-Bed Adsorption and Desorption of Salicylic Acid. Dynamic column experiments were carried out in a laboratory-scale column with a Watson-

10.1021/ie049227j CCC: $30.25 © 2005 American Chemical Society Published on Web 01/11/2005

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Table 1. Physical Properties of Adsorbents Used for Salicylic Acid Adsorption adsorbent matrix physical form humidity factor (fh) specific surface area (m2 g-1) density (g L-1) particle size (mm)

Filtrasorb F400

Sephabeads SP207

Sephabeads SP206

agglomerated coal-based granular activated carbon black granular carbon 1 1050

aromatic porous resin with hydrophobic substituents brown opaque beads 0.5 627

aromatic porous resin with hydrophobic substituents yellow opaque beads 0.5 556

700 0.7

1180 0.4

1190 0.4

Table 2. Equilibrium and Kinetic Parameters Determined for De/adsorption of Salicylic Acid onto Filtrasorb F400 and Sephabeads SP206 and SP20719 Filtrasorb F400

Sephabeads SP207

Sephabeads SP206

Equilibrium Langmuir Model

Q (mg g-1) K∞L (L mg-1) ∆HL (kJ mol-1)

351.0 1.85 × 10-5 -19.69

qm (mg g-1) K∞N (L mg-1) n ∆HN (kJ mol-1)

432.8 1.51 × 10-5 1.573 -20.29

temp (K) kLDF (min-1)

81.6 2.03 × 10-7 -27.46

45.2 1.27 × 10-8 -37.19

Nitta Model

293 0.007

310 0.018

91.5 8.48 × 10-8 1.711 -30.58 333 0.045

Kinetics 293 310 0.019 0.060

Marlow peristaltic pump used to pump the salicylic acid solution into the column. The temperature of the feed solution was maintained by a thermostatic bath (Edmund Bu¨hler, Bodelshausen, Germany). The salicylic acid concentration at the column outlet was determined by measuring the absorbance at a wavelength of 295 nm by means of a UV-visible spectrophotometer (model 7800; JASCO Corp., Tokyo, Japan). Runs at three different temperatures were carried out for each of the adsorbents by feeding the column with solutions at 293, 310, and 333 K. Different flow rates (10 and 15 mL min-1) were also tested. With respect to the height of the bed, lengths of 15 and 30 cm were used for the polymeric adsorbents and lengths of 10 and 5 cm for the activated carbon. The bed diameter was 1 cm for all runs. The feed concentration was around 100 mg L-1 salicylic acid in all cases. The elution profiles of the salicylic acid saturated beds were also obtained. Distilled water was pumped through the columns at the same flow rate and temperature as those used for the saturation. 2.3. Operation of the Parametric Pumping Pilot Plant. An automated pilot plant20-24 was used to carry out an experiment of water purification with a salicylic acid containing feed. Prior to the parametric pumping experiment, breakthrough curves of a solution with around 100 mg L-1 salicylic acid were produced at 293 and 333 K to check the effects of scale-up (axial dispersion, adsorbent packing, and heat losses). The parametric pumping experiment was performed with the following protocol: after saturation of the bed at 293 K, the salicylic acid solution was pumped through the bed of Sephabeads SP206 in an upward flow (hot half-cycle, at 333 K) followed by a downward-flow pumping of the solution (cold half-cycle, at 293 K). This was repeated for 10 cycles, with each cycle comprising one hot half-cycle and one cold half-cycle; the total duration of each cycle was 185 min, which may be seen in Table 5, together with the other operating variables.

66.2 1.41 × 10-8 1.711 -36.70 333 0.112

293 0.030

310 0.050

333 0.100

Figure 1. Diagram of the thermal parametric pumping operation (T ) thermopar, pp ) peristaltic pumping, tb ) thermostatic bath).

The top reservoir receives the concentrated salicylic acid product when the solution is pumped through the column in an upward flow, and the bottom reservoir receives the purified effluent when the solution is pumped in a downward flow. The system was operated in a recuperative mode; i.e., the temperature change was carried out by the solution itself, which is heated or cooled by means of thermostatic baths. Figure 1 shows a schematic diagram of the unit used for the parametric pumping experiment. 3. Theoretical Section 3.1. Thermal Separation Potential. Modeling of the fixed-bed adsorption of salicylic acid onto Filtrasorb F400 and Sephabeads SP206 and SP207 was made considering the equilibrium and kinetic parameters previously obtained19 and shown in Table 2. Data

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deviation a ) [m(T1) - m(T2)]/2, considering T1 < T2. The capacity parameter is defined as m(T) ) (1 )FfhK(T)/, where K(T) is the slope of the initial linear region of the isotherm, which has been represented either by K(T) ) QKL or by K(T) ) qmKN corresponding to each temperature,  is the bed porosity, F is the density of the adsorbent, and fh is the humidity factor. The largest b values are those corresponding to the system salicylic acid-Sephabeads SP206. As may be seen in Table 3, the higher separation parameter is for salicylic acid-Sephabeads SP206 in the 293-333 K range. 3.2. Mathematical Model. The mathematical model used to simulate the operation of the parametric pumping unit comprises the mass balance in the liquid phase, a global energy balance, the nonlinear adsorption equilibrium isotherm, and a linear driving force (LDF) rate equation to describe the diffusional mass transfer inside adsorbent particles.26,27 The balance of the reservoirs containing the product from the hot and cold cycles, top and bottom reservoirs, is also considered. In the case of the adsorption and desorption runs in a fixed bed, neither the balance in the reservoirs nor the energy balance was included. Mass Balance. The mass balance in a bed volume element is

Dax

∂2C(z,t) ∂z

2

- ui

∂C(z,t) ∂C(z,t) 1 -  ∂〈q(z,t)〉 ) + Ffh ∂z ∂t  ∂t (2)

where Dax is the axial dispersion, ui is the interstitial velocity,  is the bed porosity, z is the axial position, t is the time variable, C is the concentration in the bulk fluid phase, 〈q〉 is the average adsorbed-phase concentration in the particle, F is the density of the adsorbent, and fh is its the humidity factor. The axial dispersion was obtained from the following expression:28 Figure 2. Experimental adsorption equilibrium, qe versus Ce, for the adsorption of salicylic acid onto Filtrasorb F400 (A), Sephabeads SP207 (B), and Sephabeads 206 (C) at different temperatures. Fittings to Langmuir (dotted lines) and Nitta (full lines) models are shown, together with the experimental data points.19 Table 3. Separation Parameter (b) Corresponding to the Different Systems Adsorbent-Salicylic Acid Considered Here temperature range (K) 293-310 Filtrasorb F400 Sephabeads SP207 Sephabeads SP206

310-333

293-333

bL

bN

bL

bL

bN

bL

0.22 0.30 0.40

0.22 0.33 0.39

0.26 0.35 0.46

0.27 0.39 0.46

0.45 0.59 0.72

0.46 0.64 0.72

fittings to the Langmuir model and the Nitta multisite model are shown, together with the experimental points in Figure 2. Taking into account the equilibrium results, the effect of the temperature can be assessed for each of the systems salicylic acid-adsorbent. Table 3 shows for each of the adsorbents considered the corresponding separation parameter (b) regarding the adsorption of salicylic acid. This parameter, introduced by Pigford et al.,25 is indicative of the separation potential:

b)

a 1+m j

(1)

with the average slope m j ) [m(T1) + m(T2)]/2 and the

u0dp/Dax ) 0.2 + 0.011Re0.48

(3)

Re ) uiFsdp/η

(4)

with

The values of the Reynolds number Re and the calculated axial dispersion coefficients Dax are shown in Table 4. Energy Balance. A global energy balance for the column was used:

[FfCpf + FCps(1 - )] FfCpfu0

∂T(z,t) ∂2T(z,t) ( ) Kae ∂t ∂z2

∂T(z,t) ∂〈q(z,t)〉 - hwAw(T - Tamb) + (-∆H) ∂z ∂t (5)

where Kae is the axial thermal conductivity, hw is the heat-transfer coefficient of the wall of the column, Aw is the wall specific area, Ff and F are the densities of the fluid and of the adsorbent, and Cpf and Cps are the heat capacities of the fluid and of the adsorbent, respectively. Considering z ) 0 at the bottom of the column and z ) L at the top, in eq 5, the term (FfCpfu0 [∂T(z,t)/∂z] has the + sign for the cold half-cycle downward and the - sign for the hot half-cycle upward.

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Table 4. Parameters Used for Estimating Dax and the kf and ks Values Used in the Intraparticle Mass-Transfer Equation for the Different Fixed-Bed Systems Considered adsorbent Filtrasorb F400

Sephabeads SP207

Sephabeads SP206

T (K)

L (cm)

flow rate (mL min-1)

Re

Dax (cm2 min-1)

Sc

Sh

Dm (cm2 min-1)

kf (cm min-1)

ks (min-1)

293 310 333 293 310 333 293 310 333 293 310 333 293 310 333 293 310 333 293 310 333 293 310 333 293 310 333

10 10 10 10 10 10 5 5 5 30 30 30 30 30 30 15 15 15 30 30 30 30 30 30 15 15 15

10 10 10 15 15 15 10 10 10 10 10 10 15 15 15 10 10 10 10 10 10 15 15 15 10 10 10

1.45 2.31 3.13 2.18 3.47 4.70 1.45 2.31 3.13 0.83 1.32 1.79 1.24 1.98 2.68 0.83 1.32 1.79 0.83 1.32 1.79 1.24 1.98 2.68 0.83 1.32 1.79

4.18 4.12 4.07 6.19 6.08 5.99 4.18 4.12 4.07 2.42 2.40 2.37 3.60 3.55 3.51 2.42 2.40 2.37 2.42 2.40 2.37 3.60 3.55 3.51 2.42 2.40 2.37

1376.595 816.803 561.481 1376.595 816.803 561.481 1376.595 816.803 561.481 1376.595 816.803 561.481 1376.595 816.803 561.481 1376.595 816.803 561.481 1376.595 816.803 561.481 1376.595 816.803 561.481 1376.595 816.803 561.481

10.044 10.526 10.758 11.851 12.442 12.727 10.044 10.526 10.758 8.080 8.445 8.621 9.447 9.893 10.109 8.080 8.445 8.621 8.080 8.445 8.621 9.447 9.893 10.109 8.080 8.445 8.621

4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4 4.46 × 10-4 4.72 × 10-4 5.07 × 10-4

0.064 0.071 0.078 0.075 0.084 0.092 0.064 0.071 0.078 0.090 0.100 0.109 0.105 0.117 0.128 0.090 0.100 0.109 0.090 0.100 0.109 0.105 0.117 0.128 0.090 0.100 0.109

0.005 0.013 0.031 0.005 0.013 0.032 0.005 0.013 0.031 0.014 0.043 0.075 0.015 0.045 0.078 0.014 0.043 0.075 0.020 0.035 0.072 0.021 0.037 0.075 0.020 0.035 0.072

where θ ) q*/qm is the fractional amount of the species in the adsorbed phase, qm is the saturation capacity, and n is the number of neighboring sites occupied by the adsorbate. KL and KN are the constants corresponding to the Langmuir and Nitta models, respectively, indicated by the subindexes L and N, which in a general form are

Table 5. Characteristic Parameters of the Operated Parametric Pumping System Properties of the Resin F ) 1190 g L-1 fh ) 0.5 rp ) 2 × 10-4 m

p ) 0.61 τ)2

Operating Variables Tc ) 293 K tc ) 85 min Th ) 333 K th ) 100 min Tamb ) 298 K Qc ) 200 mL min-1 VU ) 17 000 mL Qh ) 200 mL min-1 Q(π/ω) ) 20 000 mL QTP ) 62.5 mL min-1 φB ) 0.15 QBP ) 32 mL min-1 φT ) 0.25 CF ) 101 mg L-1 Pe ) 120 L ) 85 cm D ) 9 cm

Model Parameters Bed Characteristics

K ) K∞ exp(-∆H/RT)

Intraparticle Mass Transfer. The mass-transfer rate inside particles can be represented by the LDF model:

∂〈q(z,t)〉/∂t ) ks[q*(z,t) - 〈q(z,t)〉]

Peh ) 80

Adsorption Equilibrium Isotherm. The equilibrium was described by the Langmuir model (eq 6) and the multisite Langmuir model for homogeneous adsorbents derived by Nitta using statistical thermodynamic arguments29 (eq 7):

QKLC 1 + KLC

Langmuir isotherm

(6)

where Q is the maximum adsorptive capacity and KL a parameter that relates to the adsorption energy.

θ ) KNC(1 - θ)n

Nitta isotherm

(9)

where ks is the effective mass-transfer coefficient and q*(C) is the adsorbed-phase concentration in equilibrium with the bulk concentration. The overall effective LDF rate constants for the experiment were estimated from the following correlation:30,31

 ) 0.4 ξη ) 1.205

Transport Parameters Dm(293 K) ) 4.47 × 10-8 m2 min-1 Dm(333 K) ) 5.07 × 10-8 m2 min-1 Dpe(293 K) ) 1.36 × 10-8 m2 min-1 Dpe(333 K) ) 1.55 × 10-8 m2 min-1 hw ) 0.852 kJ m-2 min-1 K-1

q*(z,t) )

(8)

(7)

rp rp2 rp ∂q* ∂q* ∂q* 1 1 ) Ffh Ffh Ffh + ) + ks 3kf ∂C 15Dpe ∂C 3kf ∂C kLDF (10) which considers macropore and film resistances to the mass transfer and where Dpe is the effective pore diffusivity, rp is the radius of the particle of the adsorbent, 15 is the Ω factor that is equal to 15 for spherical particles,32 ks is the global mass-transfer coefficient, kLDF is the LDF kinetic rate constant, kf is the external-film mass-transfer coefficient, F and fh are the density and humidity factor of the adsorbent, and the derivative dq*/dC represents the slope of the equilibrium data. In the second term of the equation, the second summative corresponds to an estimate of 1/kLDF.32 The

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estimated kLDF constants, which appeared in Table 2, had been previously compared with the data experimentally obtained from batch tests19 and were those used for the calculation of ks. The external-film mass-transfer coefficient kf was correlated by the Ranz-Marshall equation:33

( )(

kfdp η Sh ) ) 2.0 + 0.6 Dm F s Dm

1/3

)

u0Fsdp η

1/2

1/2

(11)

where Sh, Sc, and Re stand for the Sherwood, Schmidt, and Reynolds numbers, respectively. The values of the molecular diffusivity of salicylic acid in water, Dm, were estimated according to the Wilke-Chang method, as was described for the batch adsorption tests.19 Re, together with Dm, Sh, and Sc, are shown in Table 4 for all of the systems considered in this work, together with the calculated kf and the resulting values of ks. Balance of the Reservoirs.

(i) Hot half-cycle 〈CBP〉 ) 〈CBP〉n-1

(1 + φT)

(φB + φT) + CF (1 + φT)

(13)

z)L

|

∂C(z,t) ∂z

|

∂C(z,t) ∂z

z)0

) ui[C(0,t) - C0]

)0

z)L

∂C(z,t) )0 ∂z

(22) (23)

∂T(z,t) )0 ∂z

(24)

(ii) Cold half-cycle z)0

∂C(z,t) )0 ∂z

(25) ∂T(z,t) )0 ∂z

(26) (27)

T ) Tc

(28)

The initial conditions were

C(z,0) ) CF

(29)

T(z,0) ) T0

(30)

(14)

where 〈CBP〉 and 〈CTP〉 are the average concentration of salicylic acid in the fluid in the bottom and top reservoirs, respectively, φB and φT are the fraction of the total reservoir displacement volume [Q(π/ω)] that are withdrawn as product from the bottom and top reservoirs, respectively, and n refers to the number of cycles. 3.3. Boundary and Initial Conditions. In the case of the fixed-bed adsorption and desorption, the boundary conditions for the mass balance equation (2) are the Danckwerts boundary conditions:

z ) 0 Dax

(21)

z ) L C(L,t) ) 〈CTP〉n (12)

(ii) Cold half-cycle 〈CBP〉 ) 〈C(0,t)〉n

z ) 0 C(0,t) ) 〈CBP〉n

)

2.0 + 0.6Sc Re

(1 - φB)〈C(L,t)〉n

(i) Hot half-cycle

T ) Th

1/3

〈CTP〉 )

The boundary conditions for the equations included in the parametric pumping model are

(15)

(16)

z)L

The initial conditions for the adsorption step are for a clean bed:

C(z,0) ) 0

(17)

q(z,0) ) 0

(18)

For the simulation of the desorption step, the initial conditions are

C(z,0) ) Ce

(19)

q(z,0) ) q*(Ce)

(20)

The partial differential equations corresponding to the fixed-bed adsorption and desorption were solved using the software package gPROMS (general process modelling system). The orthogonal collocation method on finite elements was used with 50 finite elements and two interior collocation points in each element of the adsorption bed. The parametric pumping model was solved with a package24 previously developed in Visual Basic. 4. Results and Discussion 4.1. Laboratory-Scale Fixed-Bed Adsorption and Desorption of Salicylic Acid. Experimental results corresponding to fixed-bed adsorption of salicylic acid onto the adsorbents here considered may be seen in Figures 3-5, with the desorption being shown in Figure 6. Full lines correspond to simulated results with the model including the mass balance equation, the Nitta equilibrium isotherm, and the intraparticle masstransfer equation considering the global coefficient. No fitting parameters were applied for the prediction of the experimental data. Differences between experimental and predicted capacities are less than 6%, which may be due to experimental errors, either in adsorption equilibrium measurements or in flow-rate measurements in the fixed-bed experiments. The overall effective LDF rate constant was calculated by including the estimated external-film mass transfer and the previously determined pore diffusion resistance.19 Some discrepancies were found between the predicted breakthrough and the experimental runs that may result from overestimation of the external-film mass-transfer coefficient (eq 11). These differences are more pronounced for the polymeric resins than for the activated charcoal.

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Figure 3. Effect of the temperature on the fixed-bed adsorption of salicylic acid onto Filtrasorb F400 (A), Sephabeads SP207 (B), and Sephabeads SP206 (C). The bed length was 10 cm for part A and 30 cm for parts B and C, and the solution was fed at three different temperatures (293, 310, and 333 K) at a flow rates of 10 mL min-1. Symbols are the experimental data, and full lines are the simulated results obtained by the LDF model.

Figure 4. Effect of the flow rate on the fixed-bed adsorption of salicylic acid onto Filtrasorb F400 (A), Sephabeads SP207 (B), and Sephabeads SP206 (C). The bed length was 10 cm for part A and 30 cm for parts B and C, and the solution was feed at 310 K and at two different flow rates (q10 and q15 for 10 and 15 mL min-1, respectively). Symbols are the experimental data, and full lines are the simulated results obtained by the LDF model.

Effect of the Temperature. The effect of temperature on the fixed-bed performance of the adsorbents considered with respect to salicylic acid may be seen in Figure 3, which shows runs at 293, 310, and 333 K. In all cases, the adsorption capacity of the bed decreases with increasing temperature; i.e., the adsorption is favored by lowering the temperature. As for the batch tests,19 this is especially evident for the polymeric resins used here, for which the adsorption capacity of salicylic acid is strongly affected by the temperature. In the case of the activated carbon, the effect of the temperature on the adsorption of salicylic acid is not so marked, even though its adsorptive capacity is much higher that the

ones of Sephabeads SP206 and SP207. From these results, the polymeric resins, especially Sephabeads SP206, would be more appropriate for carrying out a thermal parametric pumping process for the purification of salicylic acid, as was also indicated by the separation parameter b. Effect of the Flow Rate. The effect of the flow rate on the adsorption of salicylic acid may be seen in Figure 4 for Filtrasorb F400, Sephabeads SP207, and Sephabeads SP206 as parts A-C, respectively. The figures show the breakthrough curves corresponding to the pumping of solution at two different flow rates, 10 and

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Figure 5. Effect of the height of the bed on the fixed-bed adsorption of salicylic acid onto Filtrasorb F400 (A), Sephabeads SP207 (B), and Sephabeads SP206 (C). The bed length was 5 and 10 cm (h5 and h10, respectively) for Filtrasorb F400 and 15 and 30 cm (h15 and h30, respectively) for Sephabeads SP206 and SP207. The solution was fed at 333 K at a flow rate of 10 mL min-1. Symbols are the experimental data, and full lines are the simulated results obtained by the LDF model.

15 mL min-1. Lowering the flow rate leads to longer breakthrough times, but as may be seen in Figure 4, for both of the flow rates tested, a constant pattern along the column has been reached. Effect of the Bed Length. Figure 4A shows breakthrough curves corresponding to 10 and 5 cm beds (h10 and h5, respectively) of Filtrasorb F400. Parts B and C of Figure 5 show the effect of the bed length for Sephabeads SP207 and SP206, respectively, for runs carried out in beds of 30 and 15 cm (h30 and h15,

respectively). As may be seen, having more adsorbent, the longer the bed, the longer the column takes to be saturated. Again, a constant pattern along the bed occurs for the different bed lengths tested here, and no entrance effects (more dispersive fronts) have been observed because of the use of a shorter bed. Fixed-Bed Desorption of Salicylic Acid. Desorption was carried out after complete saturation of the columns at the same temperature and flow rate, except that water was pumped instead of a salicylic acid solution. Parts A-C of Figure 6 show saturationdesorption cycles carried out in beds of Filtrasorb F400, Sephabeads SP207, and Sephabeads SP206, respectively. The comparison of the area corresponding to adsorption and desorption is within 5% difference, indicating that the operation is completely reversible. Also, the difference between the experimental stoichiometric time and the calculated stoichiometric time based on adsorption equilibrium data given in Table 5 was within (6%. Because of the nonlinearity of the adsorption equilibrium, the shape of the desorption curves is different from that of the adsorption ones (dispersive front). On the other hand, because of the relative importance of dispersive effects, desorption experimental results slightly deviate from the desorption curve predicted by equilibrium theory. As an example, Figure 7 shows this for the case of Sephabeads SP207 at 293 K. Note that kinetic effects are very noticeable in the case of desorption from Filtrasorb F400 and the lower the temperature, the lower the mass transfer. The saturation-desorption cycles are much longer for the beds of Filtrasorb F400 than for those of Sephabeads SP207 and SP206. 4.2. Pilot-Scale Fixed-Bed Adsorption of Salicylic Acid. With the aid of fixed-bed experiments, it was determined that the adsorbent with the higher separation parameter was the polymeric resin Sephabeads SP206. For this reason, the pilot plant existing in our laboratory was loaded with it for parametric pumping experiments. Before the cyclic experiment, it is desirable to carry out breakthrough experiments to check the fixed-bed behavior in the real system to be used in parametric pumping runs. Fixed-bed adsorption breakthrough curves of salicylic acid were carried out at 293 and 333 K, the lower and upper temperatures where adsorption equilibrium data and kinetics are available. The corresponding experimental and simulated breakthrough curves are shown in Figure 8. Full lines correspond to simulated results with the model including the mass balance equation, the Langmuir equilibrium isotherm, and the intraparticle masstransfer equation considering the global coefficient. As for the laboratory-scale fixed-bed experiments, some discrepancies between the model and the experimental data occur in the initial region of the pilot-scale curves. As explained before, these discrepancies may result from the overestimation of the external-film mass-transfer coefficient. The stoichiometric time at 293 K is nearly twice that at 333 K, indicating the maximum amount of salicylic acid that can be desorbed in the hot half-cycle of the parametric pumping system. 4.3. Parametric Pumping Operation. As mentioned before, the adsorbent Sephabeads SP206 was the one chosen for parametric pumping experiments because of the higher separation parameter b.

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Figure 6. Salicylic acid saturation-desorption cycles in beds of Filtrasorb F400 (A), Sephabeads SP207 (B), and Sephabeads SP206 (C). The bed length was of 5 cm for Filtrasorb F400 and 15 cm for Sephabeads SP207 and SP206. Both the solution (saturation step) and water (desorption step) were fed at a flow rate of 10 mL min-1, and cycles at 293, 310, and 333 K were carried out. Symbols are the experimental data, and full lines are the simulated results obtained by the LDF model.

Figure 7. Comparison of the salicylic acid desorption experimental results with those predicted by the LDF model and by the equilibrium theory. Symbols are the experimental desorption results corresponding to Sephabeads SP207 at 293 K, and lines are the simulated results obtained by the LDF model (full lines) and by the equilibrium theory (dotted lines).

The operating variables used in the parametric pumping experiment (temperatures, flow rates, and step times) are shown in Table 5. The model used to predict the operation of the parametric pumping unit comprises the mass and energy balances, the Langmuir isotherm, and the LDF equations shown. The experimental results corresponding to the operation of the parametric pumping pilot plant are shown in Figure 9, together with the predictions of the LDF model (full lines). The concentrations of salicylic acid in the top and bottom reservoirs are represented as a function of time. It may be seen that the LDF model explains the parametric pumping operation for the system salicylic acid-water-polymeric adsorbent SP206 to a great extent. The concentration of salicylic acid in the purified stream was reduced nearly 100 times when compared with that of the feed stream. Also, a stream with more concentrated salicylic acid was produced that can be employed to recover this drug.

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and of C. Grande from FCT (SFRH/BD/11398/2002) is also gratefully acknowledged. Nomenclature

Figure 8. Breakthrough curves corresponding to the fixed-bed adsorption of salicylic acid onto Sephabeads SP206 at a pilot scale. Symbols are the experimental data, and full lines are the simulated results obtained by the LDF model.

Figure 9. Experimental results (symbols) for the operation of the parametric pumping system salicylic acid/water/Sephabeads SP206, together with the model predictions (full lines). The top and bottom product concentrations are shown as a function of time in recuperative thermal parametric pumping.

5. Conclusions The effects of temperature, bed length, and flow rate were studied on fixed-bed adsorption of salicylic acid using different polymeric adsorbents (Sephabeads SP207 and SP206) and activated carbon (Filtrasorb F400). The simulation of breakthrough curves based on a LDF rate model to describe the diffusional mass transfer seems to fit the experimental results well. The polymeric adsorbents are much more sensitive to temperature when adsorbing salicylic acid than Filtrasorb F400 is. This is especially true for Sephabeads SP206, which shows higher values of the separation parameter b based on the temperature. Thermal parametric pumping in a recuperative mode was carried out in a pilot plant using Sephabeads SP206 for salicylic acid purification. A LDF model explained the top and bottom concentrations of phenol as a function of time. In terms of a purification of salicylic acid, thermal parametric pumping using Sephabeads SP206 as the adsorbent may be an interesting option worth further study. Acknowledgment This research has been supported by a Marie Curie Fellowship of the European Community Fifth Framework Program’s Human Resources and Mobility (HRM) under Contract EVK1-CT-2002-50018. The financial support of M. Zabkova from FCT (SFRH/BD/8007/2002)

a ) deviation in the separation parameter Aw ) wall specific area (4/D) (m-1) ∆H ) enthalpy of adsorption (J mol-1) b ) separation parameter Bi ) nondimensional mass Biot number (kfrp/Dpe) C ) concentration in the bulk fluid phase (g L-1) C0 ) initial liquid-phase concentration of phenol (mg L-1) Ce ) equilibrium liquid-phase concentration of phenol (mg L-1) CF ) feed phenol concentration in the liquid phase (mg L-1) Cpf ) heat capacity of the fluid (kJ kg-1 K-1) Cps ) heat capacity of the solid (kJ kg-1 K-1) 〈Cy〉 ) average concentration of the fluid in a reservoir (mg L-1), y ) BP or TP dp ) diameter of the particle of the adsorbent (cm) D ) bed diameter (mm) Dax ) axial dispersion (m2 min-1) Dpe ) effective pore diffusivity (m2 min-1) Dm ) molecular diffusivity (m2 min-1) fh ) humidity factor (gdry adsorbent gadsorbent-1) hw ) global wall heat-transfer coefficient (kJ m-2 s-1 K-1) Kae ) axial thermal conductivity (kJ m-1 s-1K-1) K ) parameter in the isotherm model (L mg-1) K∞ ) infinite adsorption equilibrium constant for the equilibrium model (L mg-1) kLDF ) linear driving force kinetic rate constant (min-1) L ) bed length (m) m j (T) ) capacity parameter in the separation parameter b m ) average slope in the separation parameter b MB ) molecular weight of solvent B (g mol-1) n ) number of cycles Pe ) Pe´clet number Peh ) thermal Pe´clet number Q ) constant in the Langmuir isotherm model (mg g-1) related to the adsorptive capacity Q(π/ω) ) reservoir displacement volume (mL) Qx ) flow rate of the fluid in the column during a halfcycle (mL min-1), x ) c or h Qy ) flow rate of removing the product from a reservoir (mL min-1), y ) BP or TP 〈q〉 ) average adsorbed-phase concentration per dry mass of adsorbent (mg g-1) q* ) solute adsorbed per dry mass of adsorbent in equilibrium with the solute concentration in solution at a certain time (mg g-1) qe ) solute adsorbed per dry mass of adsorbent at equilibrium (mg g-1) Re ) Reynolds number rp ) radius of the particle of the adsorbent (m) t ) time (min) tx ) half-cycle time (min), x ) c or h T ) absolute temperature (K) Tx ) feeding temperature during a half-cycle (K), x ) c or h Tamb ) ambient temperature (K) U ) flow rate (cm3 min-1) u0 ) superficial velocity (m min-1) ui ) interstitial velocity (m min-1) VU ) volume withdrawn as the top product per cycle (mL) VA ) molar volume of solute a at its normal boiling temperature (cm3 gmol-1) z ) axial coordinate in the bed (cm) Greek Letters  ) bed porosity p ) porosity of the adsorbent

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φ ) dimensionless association factor of solvent B φB and φT ) fractions of the Q(π/ω) that are withdrawn as the bottom and top products, respectively θ ) in the Nitta model, fractional coverage of the solute on the adsorbed phase η ) viscosity of the solution (Pa s) ηB ) viscosity of solvent B (cP) Ω ) LDF factor, which is equal to 3, 8, or 15 for slab, cylindrical, or spherical geometry, respectively F ) density of the adsorbent (g L-1) Ff ) density of the fluid (g L-1) τ ) tortuosity of the adsorbent ξη ) heat capacity parameter ω ) frequency of the temperature change Subindexes L ) Langmuir equilibrium model N ) Nitta equilibrium model c ) cold half-cycle h ) hot half-cycle BP ) bottom product from the cold half-cycle TP ) top product from the hot half-cycle

Literature Cited (1) Hanesian, D.; Perna, A. J. The Sitting, Process analysis and Design of a Manufacturing Facility Using hazardous material in a Residential Community (The manufacture of aspirin); Workbook; New Jersey Institute of Technology: Newark, NJ, 2000; p 65. (2) Lutomski, D. M.; Bottorff, M.; Shanga, K. Pharmacokinetic optimisation of the treatment of embolic disorders. Clin. Pharmacokinet. 1995, 28, 67-92. (3) Insel, P. A. Analgesic-antipyretics and anti-inflamatory agents, drugs employed in the treatment of rheumatoid arthritis and gout. In The Pharmacological Basis of Therapeutics, 8th ed.; Goodman, L. S., Gilman, A., Eds.; Pergamon Press, Inc.: Elmsford, NY, 1990; pp 638-681. (4) Erikson, E.; Auffarth, K.; Eilersen, A.-M.; Henze, M.; Ledin, A. Household chemicals and personal care products as sources of xenobiotic organic compounds. I. Grey wastewater. Water SA 2003, 29, 2. (5) Daughton, C. G.; Ternes, T. A. Pharmaceuticals and Personal Care Products in the Environment: Agents of Subtle Change? Environ. Health Perspect. 1999, 107 (6), 907-938. (6) Halling-Sorenson, B.; Nors Nielsen, S. P.; Lanzky, F.; Ingerslev, F.; Holten Lu¨tzhoft, H. C.; Jergensen, S. E. Occurrence, fate and effects of pharmaceutical substances in the environments a review. Chemosphere 1998, 36 (2), 357-393. (7) Montague, P. Drugs in the water. Rachel’s Environ. Health Weekly 1998, 614. Available in: www.rachel.org. (8) Raloff, J. Drugged waters. Sci. News 1998, 153, 187-189. (9) Velagaleti, R. Drugs Are Accumulating in Sewage Sludge, Surface Waters, and Drinking Water. Drug Inf. J. 1997, 31, 715722. (10) Miyahara, J. T.; Karlen, R. Effect of salicylate on oxidative phosphorylation and respiration of mitochondrial fragments. Biochem J. 1965, 97, 194-198. (11) Hmicek, G.; Skelton, J.; Miller, W. Pulmonary edema and salicylate intoxication. JAMA, J. Am. Med. Assoc. 1974, 230, 866867. (12) Walters, J. S.; Woodying, J. H.; Stelling, C. R. Salicylateinduced pulmonary edema. Radiology 1983, 146, 289-293. (13) Kaplan, E. H.; Kennedy, J.; Davis, J. Effects of salicylate and other benzoates on oxidative enzymes of the tricarboxylic acid cycle in rat tissue homogenates. Arch. Biochem. Biophys. 1954, 51, 47-61.

(14) Schwart, R.; Landry, G. Organic acid excretion in oxidative intoxication. J. Pediatr. (St. Louis) 1965, 66, 658-666. (15) Bartels, P. D.; Lund-Jacobsen, H. Blood lactate and ketone body concentration in salicylate intoxication. Hum. Toxicol. 1986, 5, 363-366. (16) Done, A. K.; Temple, A. R. Treatment of salicylate poisoning. Mod. Treat. 1971, 8, 528-551. (17) Thurston, J. H.; Pollock, P. G.; Warren, S. K. Reduced brain glucose With normal plasma glucose in salicylate poisoning. Clin. Invest. 1970, 49, 2139-2145. (18) Hamadi, N. K.; Chen, X. D.; Farid, M. M.; Lu, M. G. Q. Adsorption kinetics for the removal of chromium(VI) from aqueous solution by adsorbent derived from used tyres and sawdust. Chem. Eng. J. 2001, 84, 95-105. (19) Otero, M.; Grande, C.; Rodrigues, A. Adsorption of salicylic acid onto polymeric adsorbents and activated charcoal. React. Funct. Polym. 2004, 60, 203-213. (20) Ramalho, R.; Costa, C.; Grevillot, G.; Rodrigues, A. Adsorptive parametric pumping for the purification of phenolic effluents. Sep. Technol. 1991, 1, 99-107. (21) Ferreira, L. M.; Rodrigues, A. E. Adsorptive separation by thermal parametric pumping. Part 1: Modeling and simulation. Adsorption 1995, 1, 213-231. (22) Ferreira, L. M.; Rodrigues, A. E. Adsorptive separation by thermal parametric pumping. Part 2: Experimental study of the purification of aqueous phenolic solutions at pilot scale. Adsorption 1995, 1, 233-252. (23) Dı´ez, S.; Leita˜o, A.; Ferreira, L. M.; Rodrigues, A. E. Adsorption of phenylalanine onto polymeric resins: equilibrium, kinetics and operation of a parametric pumping unit. Sep. Purif. Technol. 1998, 13, 25-35. (24) Davesac, R. R.; Pinto, L. T.; Silva, F. A.; Ferreira, L. M.; Rodrigues, A. E. A package for thermal parametric pumping adsorptive processes. Chem. Eng. J. 2000, 76, 115-125. (25) Pigford, R.; Baker, B.; Blun, D. An equilibrium Theory of the Parametric Pumping. Ind. Eng. Chem. Fundam. 1973, 12, 406-412. (26) Glueckauf, E.; Coates, J. E. Theory of chromatography, Part 4. The influence of incomplete equilibrium on the front boundary of chromatogram and on the effectiveness of separation. J. Chem. Soc. 1947, 1315. (27) Shim, W. G.; Lee, J. W.; Rhee, H.; Moon, H. Condensation of Trichloroethylene vapor on mesoporous material. Korean J. Chem. Eng. 2004, 21 (2), 469-476. (28) Guiochon, G.; Shirazi, S. G.; Katti, A. M. Fundamentals of preparative and nonlinear chromatography; Academic Press: Boston, 1994; p 148. (29) Nitta, T.; Shigetomi, T. M.; Kuro-Oka, M.; Katayama, T. J. Chem. Eng. Jpn. 2000, 17, 39-45. (30) Farooq, S.; Ruthven, D. M. Heat effects in adsorption column dynamics: 2. Experimental validation of the one-dimensional model. Ind. Eng. Chem. Res. 1990, 29, 1084. (31) Silva, J. A.; Rodrigues, A. E. Fixed-bed adsorption of n-Pentane/Isopentane mixtures in pellets of 5A zeolite. Ind. Eng. Chem. Res. 1997, 36, 3769. (32) Azevedo, D. C. Separation/Reaction in simulated moving bed, Application to the production of industrial Sugars. Ph.D. Thesis, Laboratory of Separation and Reaction Engineering, School of Engineering, University of Porto, Porto, Portugal, 2001. (33) Yang, R. T. Gas separation by adsorption processes; Butterworth Publishers: Stoneham, MA, 1987; p 106.

Received for review August 24, 2004 Revised manuscript received November 15, 2004 Accepted November 22, 2004 IE049227J