Analysis of Competition between Multicomponent BTX Compounds for

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Analysis of Competition between Multicomponent BTX Compounds for the Active Site of Adsorption in a Fixed-Bed Column Adriana Dervanoski da Luz,*,† Selene Maria de Arruda Guelli Ulson de Souza,† Cleuzir da Luz,† Josiane Maria Muneron de Mello,‡ and Antônio Augusto Ulson de Souza† †

Chemical Engineering Department, Federal University of Santa Catarina, Laboratory of Numerical Simulation of Chemical Systems, Campus Universitário, 88040-900 Florianópolis, SC, Brazil ‡ Environmental Sciences Postgraduate Program, Chapecó Region Community University. PO Box 1141, 89809-000, Chapecó, SC, Brazil ABSTRACT: An experimental and numerical study of the individual and competitive adsorption of BTX compounds (benzene, toluene, and o-xylene) in aqueous solution was carried out in a fixed-bed column filled with activated carbon. The equations of transport were discretized using the finite volumes method and an algorithm was implemented in the Fortran programming language. The results obtained in all cases showed that o-xylene is the contaminant which is most competitive for the active site of the adsorbent, and over time it is able to desorb the compounds which have a lower affinity and adsorb to the free active sites. The result is that the local concentration of the weakly adsorbed component in the fluid phase is higher and surpasses the dimensionless concentration. This finding appeared to be related to the Biot number. As the Biot number increased the rate of competitive adsorption decreased, and the form of the breakthrough curves is flat with a lower breakpoint. This is due to the low intraparticle resistance and also the reduced contact time required to reach saturation. prediction of different operational conditions.21 The study of these effects is of great importance for obtaining the appropriate operating conditions in an adsorption column for multicomponent removal in a continuous process. Studies to determine the characteristics of the adsorption process in relation to the removal of VOCs in multicomponent solutions showed very good performance when activated carbon was used as the adsorbent.2,22,23 According to Wibowo et al.,13 Leitão and Rodrigues24 and Chatzopoulos et al.,25 adsorption with activated carbon is a proven and reliable technology for removing small amounts of soluble organic compounds from water and wastewater, which can thus be reused several times in the same process. The aim of this study was to investigate the singlecomponent and multicomponent adsorption of BTX compounds using thermally activated carbon (obtained from coconut shell) as the adsorbent. The parameters of the thermodynamic equilibrium were obtained from Luz et al.2 and the isotherm model which best represented the equilibrium data was the Langmuir model (mono- and multicomponent). Multicomponent adsorption tests were carried out in a fixedbed column in order to verify the competition between contaminants for the active site of the adsorbent. The results for the multicomponent adsorption were compared with those obtained for the individual compounds. In the case of the bicomponent mixtures, all combinations were tested and the concentration of each contaminant was 50 mg/L. The same concentration was used for each contaminant in the

1. INTRODUCTION The removal of volatile organic compounds (VOCs), such as benzene, toluene, and xylenes, collectively known as BTX, from effluents produced by the petrochemical industry is of considerable interest because of the harmful effects of these contaminants.1−4 BTX compounds are powerful central nervous system depressants, with chronic toxicity and mutagenic potential even at low concentrations. Benzene is among the most toxic compounds due to its carcinogenic activity, causing leukemia and tumors in multiple organs. Acute exposure by inhalation or ingestion can even cause death in humans.5−7 Environmental legislation, monitoring instruments, and economic implications have been key instruments of environmental policy related to the release of these effluents. Thus, studies have been directed to the treatment of polluted streams (integrated approach) and the final effluent treatment (end-ofpipe approach), for which the adsorption method has been used quite effectively.8−11 Investigations on the removal of most hydrocarbon groups, particularly VOCs, by adsorption have focused on the individual components.10−16 However, in industrial effluents there is a mixture of toxic compounds to be removed. The experimental measurement of the multicomponent adsorption equilibrium is complex, especially when the number of components exceeds two.17−20 An optimized design for multicomponent adsorption must take into account the interactions between the mixture of compounds and various factors associated with the physical and chemical nature of the adsorbent and the adsorbate. Thus, the design process must proceed through experimentation with the subsequent development of rigorous mathematical models that can be used to validate the numerical methodology and © 2013 American Chemical Society

Received: Revised: Accepted: Published: 16911

July 29, 2013 October 25, 2013 November 15, 2013 November 15, 2013 dx.doi.org/10.1021/ie402452h | Ind. Eng. Chem. Res. 2013, 52, 16911−16921

Industrial & Engineering Chemistry Research

Article

was determined in a continuous process using a fixed-bed column packed with activated carbon. The feed solutions composed of the mono- and multicomponent BTX mixtures were prepared in glass vials with a capacity of 4 L. To keep the solution homogeneous a magnetic stirrer was used. This methodology allowed a reproducible concentration to be obtained, which generally remained within 2−3% of its average value during the adsorption experiments, measured at the entry point of the column before the activated carbon bed. A Gilson peristaltic pump was employed to transfer the feed solution to the column with upward flow. The glass column was 7.0 cm in length and 1.2 cm in diameter. Before the activated carbon bed there was a sampling point to allow the withdrawal of liquid samples using glass syringes to monitor the concentration of the contaminant at the entrance of the bed. A porous plate was placed after the entry point of the column in order to suspend the activated carbon bed, as illustrated in Figure 1. The activated carbon was packed in the column between the porous plate and a zone of glass balls that was used to support the activated carbon in the bed.

tricomponent mixtures. In addition, a study on the competitive adsorption was conducted varying the concentrations of the contaminants. This paper also presents a mathematical model that describes the mono- and multicomponent adsorption process. To perform simulations to study the adsorption in a fixed bed, the finite volume method was applied to the discretization of the transport equations, using the weight upstream differencing scheme (WUDS) and central differencing scheme (CDS), and the algorithm was implemented in the Fortran programming language.

2. MATERIALS AND METHODS 2.1. Materials. The adsorbent used was carbon produced from coconut shell and thermally activated with steam and carbon dioxide in the temperature range of 800 to 1000 °C. The solvents used were distilled water (to prepare the BTX solutions), benzene (HPLC grade; Fluka), toluene (UV/ HPLC, spectroscopic grade; VETEC) and o-xylene (98%; HPLC grade; Sigma-Aldrich). Milli-Q water and methanol (HPLC grade; Carlo Erba Series Gold) were used in the high performance liquid chromatography (HPLC). 2.2. Methods. 2.2.1. Experimental Procedure. Prior to the laboratory tests, samples underwent treatment consisting of adjusting the particle size of the activated carbon (18/20 mesh) to around 0.85 mm followed by washing for a period of 10 days with distilled water and drying at 110 °C for 3 h. The concentrations of the BTX compounds were determined on an HPLC (GC) system connected to a UV−vis detector (model 437 CG-B) using a Nucleosil C18 reverse phase column (250 mm) with an internal diameter of 4.6 mm. The mobile phase used in the HPLC was composed of MilliQ water and methanol (80:20) at a flow rate of 0.8 mL/min. The BTX compounds were identified at a wavelength of 254 nm. All experiments were performed in triplicate, and the mean error adopted was less than 5%. 2.2.1.1. Characterization of Adsorbent. The characterization of the adsorbent was carried out by determining the following parameters: particle size and hardness, and moisture, volatile compounds, fixed carbon, and ash contents. We also performed experiments to determine the surface functional groups of the activated carbon. BET (Brunauer, Emmett, and Teller) and BJH (Barrett, Joyner, and Halenda) tests were carried out to determine the surface area, pore volume, and pore size distribution of the adsorbent used.26,27 Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDAX) were performed to examine the physical structure and chemical composition of the sample. 2.2.1.2. Adsorption Isotherms Obtained in the Batch Reactor. The thermodynamic equilibrium between the adsorbent and the adsorbate was investigated to determine the maximum adsorption capacity of the adsorbent for the BTX compounds as single components. The solutions were prepared with distilled water to obtain the individual BTX compounds in the following concentrations: 30, 50, 70, 90, 110, 130, and 150 mg/L respecting the solubility limits of the contaminants (B, 1790 mg/L; T, 530 mg/L; and o-xylene, 175 mg/L). The initial adsorption pH was approximately 6.4. The mass of adsorbent used for all concentrations was 1.0 g. All experiments were conducted in batch, using 250 mL Erlenmeyer flasks with PTFE covers, at a temperature of 23 ± 1 °C and at 120 rpm. The equilibration time was 15 h for each experiment. 2.2.1.3. Kinetics of Adsorption in Fixed-Bed Column. The adsorption kinetics of the mono- and multicomponent mixtures

Figure 1. Representative scheme used to study the adsorption of BTX in fixed-bed column with activated carbon.

Before starting the adsorption experiments a certain amount of activated carbon (3.90 g) was placed in flasks with distilled water under stirring in a weakly agitated bath to remove air from the pores of the adsorbent. The activated carbon was packed into the glass column together with some water in order to reduce the porosity of the bed. Subsequently, distilled water was pumped through the system for around 10 min to measure the volumetric flow rate with the aid of a beaker and a stopwatch. The solution flow for the experiments was directed upward at the desired flow rate. To minimize axial dispersion effects, the relationship between the length of the bed and the particle diameter (L/dp) in all experiments was greater than 20 and, in most cases, exceeded 50. The feed concentration taken as a reference in the kinetics experiments was 50 mg/L, and this was the concentration chosen for the multicomponent analysis, considering that the sum of the concentrations of three contaminants cannot surpass the o-xylene solubility limit of 175 mg/L. The concentrations of BTX at the inlet and outlet of the column were monitored over time by collecting liquid samples which were immediately analyzed by HPLC. 2.2.2. Mathematical Modeling and Numerical Methods. The simulation method used is based on the mathematical 16912

dx.doi.org/10.1021/ie402452h | Ind. Eng. Chem. Res. 2013, 52, 16911−16921


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Table 1. Adsorption Isotherm Models for the BTX Compounds isotherm model

monocomponent

Langmuir (Sulaymon and Ahmed21)

qei =

1 + bi LCie

qei = kFCei1/ nF

Freundlich (Ruthven28)

∂t

=

Deff ∂ ⎛ 2 ∂qi ⎞ ⎜r ⎟ r 2 ∂r ⎝ ∂r ⎠

∂r

(1a)

∂qi ∂r

= k f (Ci − Cie) r=R

(1c)

where ρs is the apparent density of the solid, kf is the external mass transfer coefficient, and Cie is the solute concentration in the liquid phase at the solid−liquid interface. The initial condition used in the model for the solid phase established that in ∀ r and ∀ z along the bed at time zero the concentration in the solid phase is equal to zero for adsorption. The boundary conditions used in the model are symmetry and equality of flows. In the absence of axial dispersion of the solute in the bed, the mass balance in the fluid phase and the initial and boundary conditions are expressed by eqs 2, 2a, and 2b, respectively. ∂Ci v ∂Ci 3 (1 − εL) =− s − k f (Ci − Cie) ∂t εL ∂z R εL

(2)

IC: t = 0, 0 ≤ z ≤ L , Ci = 0

Cei

Bii

+ aijCej

q =

Bij ej

ajCej Bj + Bjj Cej Bjj + ajiCi Bji

(4b)

(2b)

3. RESULT AND DISCUSSION 3.1. Characterization of Activated Carbon. The physical and chemical analysis showed that the activated carbon used in the adsorption of BTX compounds had low moisture (0.03% dry basis) and ash (1.4% dry basis) contents and a high amount of fixed carbon (94.99% dry basis). For the determination of the surface functional groups the Boehm titration method was used. The results obtained showed that the activated carbon had higher amounts of basic functional groups (8.19 × 10−4 mEq/100 g) compared to acid functional groups (mEq/100 2.86593 × 10−4), indicating that the carbon had a basic character. Also, lactones (4.35 × 10−5 mEq/100g) and phenolic compounds (2.43 × 10−4 mEq/ 100g) were present. The structural analysis of the carbon involved determining the surface area, extent of microporosity, and pore size distribution, as seen in Figure 2. Figure 2a shows the isotherm for the adsorption and desorption of nitrogen at 77 K, and Figure 2b illustrates the distribution of the pore size according to the BJH method for activated carbon. The BET isotherm, which is represented by the curve in Figure 2a, obtained by means of N2 adsorption, can be classified as type I (typical of solids with high microporosity). Measurements of the relative pressure and volume of N2 gas adsorbed are commonly used in mathematical models to

(1b)

BC2: t > 0, 0 ≤ z ≤ L , Deff ρs

aiCe1

(3b)

Bi + Bii

where vs is the liquid superficial velocity in the bed, εL is the bed porosity, and R is the radius of the adsorbent particle. The boundary condition of the variable time at the entrance of the bed in eq 2b was mainly used to give more flexibility to the model, so that it can be applied to experiments under conditions of variable input concentration. The concentrations of solute in the liquid and solid phases can be related through an adsorption equilibrium isotherm at the solid−liquid interface. The isotherm models used in this study were the Langmuir and Freundlich (mono- and multicomponent) models, shown in eqs 3 and 4 in Table 1. The finite volume method29 is used to discretize the conservation equations. These were selected to ensure the conservation of the magnitudes involved both at the elementary and global levels. Numerous authors have used this method for the discretization of this phenomenon.4,30−32 In this study we used the explicit formulation, and the one-dimensional structured mesh for storing discrete points. In the computational mesh, the arrangement used was colocated variables, where all variables are stored in the central control volumes. To assess the variables (and their derivatives) of the faces the control volume interpolation functions were used: the weight upstream differencing scheme (wuds) through the column and central differencing scheme (CDS) through the particle.

=0 r=0

n

1 + ∑i = 1 biCei

t > 0, z = 0, Ci = C in(t )

BC1: ∂qi

qmax ib iC ei

BC:

with the following initial and boundary conditions: IC:

t > 0, 0 ≤ z ≤ L ,

qei =

(4a)

(1)

t = 0, 0 ≤ r ≤ R , 0 ≤ z ≤ L , qi = 0

qei =

(3a)

model detailed by Varma and Chatzopoulos,16 which describes toluene removal by adsorption in a fixed-bed column, using activated carbon as the adsorbent. This is a pore diffusion model, which considers the mass transfer resistance inside and outside the adsorbent particle. The mathematical modeling of this adsorption process involves the conservation equations of chemical species for liquid and solid phases, as well as the initial and boundary conditions. The equations describe the change in the concentration of the solute inside the column and particles as a function of time and position. The concentration of the solute in the liquid phase, Ci, varies with the axial position, z, and time, t, while the concentration in the solid phase, qi, is a function of the radial position, r, within the particle. Assuming an isothermal process due to the high heat capacity of water, spherical adsorbent particles, and fast adsorption kinetics, the mass balance of the solute in the solid phase is given by eq 1: ∂qi

multicomponent

qmax ibiLCie

(2a) 16913



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adsorbent. Through elemental analysis (Figure 4) it was confirmed that carbon was the element found in the highest percentage, the second most abundant element being oxygen (around 9% by weight). Small amounts of Mg, Al, Si, K, and Fe were also present. From these results it can be seen that the activated carbon had a large surface area and a large pore volume, which characterizes the activated carbon obtained from coconut shell as an excellent adsorbent for the adsorption of BTX. 3.2. Equilibrium Study. 3.2.1. Adsorption Isotherms for BTX Compounds. All experimental results for the adsorption equilibrium of the BTX compounds were fitted by the leastsquares method using the software STATISTICA 7.0, following the Langmuir and Freundlich adsorption isotherm models for single-component compounds. The multicomponent isotherm models were used in the computational algorithm using the parameters for the monocomponent simulation of the concentration profiles in the mixture. The experimental results and the model parameters of the adsorption isotherms can be found in Luz et al.2 3.3. Experimental and Numerical Results Obtained in Fixed-Bed Column. 3.3.1. Single Component Adsorption Kinetics in Fixed-bed Column. The breakthrough curves were obtained in a packed fixed-bed column. The average diameter of the adsorbent obtained by previous sieving was 0.85 mm and the average particle mass was 4.62 × 10−4 g. The bulk density was 1.44 g/cm3, the real density was 0.49 g/cm3 bed, and the average mass of the adsorbent used in the bed was 3.90 g. The porosity of the bed was found to be 0.41. Prior to beginning the experiments it was necessary to determine the best operating conditions for the adsorption of BTX. On the basis of a computer code, it was possible to determine the optimum dimensions for the fixed-bed column used in the adsorption experiments, which were found to be a length of 7.0 cm and internal diameter of 1.2 cm, which allows for a reasonable residence time for the contaminants in the bed, minimizing the axial dispersion. Table 2 shows the conditions and parameters required for the determination of the breakthrough curves for the BTX compounds. The validation of the method and the numerical parameters for thermodynamic equilibrium can be obtained from Luz et al.2 The isotherm used to describe the equilibrium data obtained experimentally was the Langmuir isotherm, which presented the best average value for the correlation coefficients. In Table 2, Dm is the molecular diffusivity of BTX in water,16 Ds is the effective diffusivity obtained from fitting the experimental breakthrough curves, and kf is the coefficient of mass transfer in the liquid film obtained through the correlation of Wilson and Geankoplis.33 Figure 5a−c shows the breakthrough curves of the experimental and numerical monocomponent tests for BTX. The experiments were performed in duplicate and the error bars shown in the figures represent the standard deviation of the experimental data. In Figure 5a−c it can be observed that for the same operating conditions the adsorbent is saturated with toluene in a time of around 23 h, whereas for the benzene and o-xylene the saturations times are 40 and 35 h, respectively. The results for the breakthrough curves showed good agreement between the experimental and numerical data for the single component adsorption of BTX.

Figure 2. (a) N2 adsorption and desorption isotherms at 77 K for activated carbon; (b) pore size distribution of activated carbon determined by the BJH method.

calculate the coverage of the monolayer of N2 adsorbed on the adsorbent surface. The BET model was applied using a N2 adsorption relative pressures of 0.05 to 0.35, where the coverage of the monolayer of N2 molecules is assumed to be complete, thus obtaining the surface area of the activated carbon. Figure 2b shows the pore size distribution for the activated carbon, investigated within the range of 18 to 400 Ǻ , which indicated micro- and mesopores (diameter around 21 Ǻ ). According to Ruthven,28 adsorption occurs intensely in the micropores, but the mesopores and macropores are very important for the movement of the adsorbate into the adsorbent particle. The results for textural characterization of the adsorbent indicated a surface area of 724 m2/g. For the extent of microporosity the values were pore volume = 0.39 cm3/g; average pore diameter = 21.35 Ǻ ; micropore volume = 0.31 cm3/g; micropore area = 614 m2/g, and the pore size was distributed between the minimum value of 18 Ǻ and maximum value of 400 Ǻ . Figure 3 shows the results for the scanning electron microscopy (SEM) for the adsorbent used, where the magnifications are 30, 125, 500, and 1000 times the size of the particle. Figure 4 shows the spectrum for the activated carbon samples obtained by EDAX. It can be seen in Figure 3 that there is a large number of pores, as shown by the structural characterization of the 16914



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Figure 3. Micrographs of coconut shell activated carbon.

Table 2. Conditions and Parameters Necessary for the Determination of the Breakthrough Curves and Concentration Profiles for the BTX

Figure 4. Spectrum for the sample of coconut shell activated carbon.

3.3.2. Kinetics of Multicomponent Adsorption in Fixed-Bed Column. 3.3.2.1. Bicomponent Adsorption. Figure 6 shows the experimental and numerical results for the BTX breakthrough curves for all binary combinations of the contaminants. The isotherm used to describe the equilibrium data obtained experimentally was the multicomponent Langmuir isotherm, using the parameters of the single-component isotherms. On the basis of the Figures 6a−c, competition between the studied compounds for the active sites of the adsorbent can be verified.

parameters

benzene

toluene

o-xylene

Cin (mg/L) εL (dimensionless) Ds (cm2/s) Dm (cm2/s) ρs (g/L) dp (cm) Dc (cm) kf (cm/s) Q (mL/min) T (°C) L (cm) b (L/g) qmax (mg/g)

50 0.41 5.12 × 10−9 9.8 × 10−6 1850 0.085 1.20 6.3107 × 10−3 40 23 7.0 0.0490 124.77

50 0.41 4.03 × 10−8 8.6 × 10−6 1850 0.085 1.20 6.3107 × 10−3 40 23 7.0 0.0497 150.42

50 0.41 1.11 × 10−8 8.4 × 10−6 1850 0.085 1.20 6.3107 × 10−3 40 23 7.0 0.0405 165.07

As can be observed in Figure 6a, for the conditions studied, benzene saturated the column in 2.5 h and toluene took approximately 11 h to reach saturation. This can be explained by a higher affinity of toluene for the stationary phase, compared to benzene. Thus, the competition favored toluene in the presence of benzene. In Figure 6b, which compares the adsorption of toluene and o-xylene, it can be verified that toluene saturated the column in 16915



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Figure 6. Experimental and numerical breakthrough curves for BTX: (a) benzene and toluene (b) o-xylene and toluene, and (c) benzene and o-xylene; Cin = 50 mg/L for each contaminant in the mixture, L = 7.0 cm, and Q = 40 mL/min.

Sulaymon and Ahmed,21 who studied the competitive adsorption of furfural and phenolic in a fixed bed of activated carbon. The result is that the weakly adsorbed components in the fluid phase inside the fixed bed of adsorbent are present in local concentrations exceeding the dimensionless concentration (1). Another explanation is that the concentration profile of the breakthrough curve for the fixed bed of adsorbent is related to the initial concentration of the solute and the Biot number (Bi M ). The Biot number is a dimensionless number representing the ratio between the resistances to mass transfer by diffusion and internal resistance to external mass transfer by convection, as shown in eq 5.

Figure 5. Experimental and numerical breakthrough curves for (a) benzene, (b) toluene, and (c) o-xylene with the standard error.

5.0 h and after 11 h the column was 85% saturated with oxylene. This is explained by the fact that o-xylene has a greater affinity for the adsorbent than toluene. Therefore, the competition favored o-xylene in the presence of toluene. Figure 6c shows the data for the adsorption of benzene and o-xylene. Benzene saturated the column in 7.0 h, and after 11 h the column was 60% saturated with o-xylene. Once again oxylene has a higher affinity for the stationary phase, this time compared to benzene. Therefore, in relation to the adsorption at the active site the competition would favor o-xylene in the presence of benzene and toluene. In the case of multicomponent systems, in the initial stage there is a large amount of active sites on the adsorbent and the components are weakly or strongly adsorbed to the active sites. As the process proceeds, the components which are weakly adsorbed move with the fluid until the end of the column. These results are in agreement with results reported by

BiM =

k f Lc Deff

(5)

Since the initial solute concentration in the fluid phase slightly increased, the breakthrough curve becomes flat and its break point is rapidly reached. This can be explained by the fact that the driving force for mass transfer increased with increasing 16916



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Table 3. Effective Diffusivity and Biot Number for Bicomponent Mixtures compound a mixture (compound a − compound b) B−T T−X B−X

Deff (cm2/s)

compound b Deff (cm2/s)

BiM

−11

1.92 × 10 4.75 × 107 3.94 × 108

2.30 × 10 9.30 × 10−10 1.12 × 10−10

9

−7

2.70 × 10 7.90 × 10−8 7.90 × 10−6

BiM 1.64 × 105 5.59 × 105 5.59 × 103

Figure 7. Experimental results for the breakthrough curves for BTX for the combinations (a) Cin = (30B, 30T, 50X) mg/L); (b) Cin = (30B, 50T, 30X) mg/L; (c) Cin = (50B, 30T, 30X) mg/L; and (d) experimental and numerical Cin = (50B, 50T, 50X) mg/L; L = 7 cm and Q = 40 mL/min.

these compounds have lower intraparticle diffusivity compared to o-xylene. According to Sulaymon and Ahamed,21 this is due to lower intraparticle resistance and also a shorter contact time required to reach saturation. For the bicomponent breakthrough curves the numerical results showed a slight deviation compared with the experimental data, mainly for the compounds with lower affinity for the solid phase, that is, benzene and toluene (compared with o-xylene). This deviation can also be explained by the short bed and high flow rate, which means that the compounds adsorbed at the top of the bed are easily desorbed. 3.3.2.2. Tricomponent Adsorption. Figure 7 shows the experimental data (a−c) and the experimental and numerical results (d) for the breakthrough curves for some of the BTX ternary combinations. In the numerical study, the isotherm used to describe the equilibrium data was the competitive Langmuir isotherm using the parameters of single-component isotherms. On analyzing Figure 7a, which shows the results obtained for the tricomponent mixture with initial concentrations of 30 mg/ L of benzene, 30 mg/L of toluene, and 50 mg/L of o-xylene, it

solute concentration, resulting in a reduction in the competitive adsorption.21 As the Biot number for each solute increases, the competitive adsorption rate will decrease and the shape of the breakthrough curves will be flat with a lower break point. This is due to the low intraparticle resistance and also to a shorter contact time being required to achieve saturation. With the increasing height of the bed, the adsorption rate will increase and the competitive displacement of weak components will be increased.21 Table 3 shows the effective diffusivity and the Biot number for each solute in the bicomponent mixture, and the effective diffusivity was measured by fitting the experimental breakthrough curves for the column. Table 3 shows that the lowest effective diffusivity was found for benzene, in both the toluene−benzene mixture and the benzene−xylene mixture. For the toluene−xylene mixture, toluene showed a lower effective diffusivity. The lower value found for the effective diffusivity explains the lower interaction between these contaminants and the active sites in the micropores of the adsorbent. The highest Biot number can be observed for benzene in the benzene/toluene mixture, and 16917



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can be observed that when the concentration of o-xylene (50 mg/L) is higher compared with that of benzene (30 mg/L) and toluene (30 mg/L), o-xylene is the contaminant which is adsorbed to the greatest extent by the adsorbent. It was verified that after 11 h of adsorption the column was not saturated with this contaminant (70% saturation), whereas for benzene the column saturated in 5 h and for toluene in 5.5 h. As discussed above, it is known that at the beginning of the adsorption process there is a large quantity of active sites available and over time the contaminant which is most favored will occupy these sites. Since o-xylene is the most competitive contaminant with the greatest capacity for adsorption it will be adsorbed in greater quantity. After 6 h of adsorption of the three contaminants a competitive effect between benzene and toluene can be observed, which begin to compete for the available active sites of the adsorbent. Since o-xylene is the most competitive of these contaminants it displaces the other compounds. After 6 h of adsorption, toluene, which had been adsorbed in a greater quantity than benzene after the saturation of both compounds, begins to be desorbed and benzene begins to adsorb to the available active sites. The same behavior can be verified in Figure 7b, but this time the tricomponent mixture had initial concentrations of 30, 50, and 30 mg/L of benzene, toluene, and o-xylene, respectively. It can be observed that even though the concentration of toluene (50 mg/L) is higher compared with benzene (30 mg/L) and oxylene (30 mg/L), o-xylene is still the contaminant with the greatest amount of adsorption onto the adsorbent surface. Toluene has a greater concentration but even so o-xylene is more competitive. It was verified that after 11 h of adsorption the column was not saturated with o-xylene (75% saturation), whereas for benzene the column saturated in 4 h and for toluene in 5.5 h, the same times as those observed for the lower concentration (30 mg/L). In this figure, the competition described above can once again be observed between benzene and toluene Figure 7c shows the same behavior, but in this case the initial concentrations in the tricomponent mixture are 50, 30, 30 mg/ L of benzene, toluene, and o-xylene, respectively. On analyzing the figure it can be observed that even though the benzene concentration (50 mg/L) is higher compared with that of toluene (30 mg/L) and o-xylene (30 mg/L), o-xylene is once again the contaminant which shows the greatest amount of adsorption by the adsorbent. The effect of the competition between benzene and toluene can be verified, benzene being the contaminant with the highest concentration and the lowest affinity toward the active sites of the adsorbent. Benzene begins to leave the column in the shortest time (around 3.5 h) while toluene saturates the column after 6 h and o-xylene is found with 60% saturation after 11 h. This highlights a significant problem since benzene represents a serious risk to the environment. According to Manahan,6 benzene is a strong depressor of the central nervous system, presenting chronic toxicity and mutagenic potential, even at low concentrations, this being the most toxic of the BTX compounds. Thus, ideally, this contaminant should be adsorbed in greater quantity by the adsorbent. On analyzing Figure 7d, which shows the numerical and experimental results for the adsorption of the tricomponent mixture, at an initial concentration of 50 mg/L for each contaminant, the same competitive behavior found for the previous concentrations can be observed from the experimental breakthrough curves, a greater amount of o-xylene being

adsorbed on the adsorbent surface compared with the other two compounds. It can be verified that after 11 h of adsorption the column was saturated with o-xylene, while benzene saturated the column after 2.5 h and toluene after 4 h. The effective diffusivity values were 9.30 × 10−11 cm2/s, 9.70 × 10−9 cm2/s, and 9.50 × 10−7 cm2/s for benzene, toluene and oxylene, respectively. For the multicomponent breakthrough curves the numerical results showed greater deviation than the experimental data, mainly for the compounds which present lower affinity for the solid phase, that is, benzene and toluene (compared with oxylene). This deviation can also be explained by the short bed and high flow rate, which mean that the compounds adsorbed at the top of the bed are desorbed with greater facility. According to Sulaymon and Ahamed,21 a probable explanation for this deviation in the dimensionless concentration on the breakthrough curve, mainly for compounds which are weakly adsorbed, is related to the Biot number. In this case the following Biot numbers were obtained for the BTX mixture [BiMB = 4.7499 × 108; BiMT = 4.5541 × 106; BiMX = 4.6499 × 104]. Also, according to Sulaymon and Ahamed,21 the rate of competitive adsorption will decrease as a function of the increase in the average Biot number and the breakthrough curves will be flat with a lower break point. This is due to the low intraparticle resistance and also the reduction in the contact time required to reach saturation. A higher Biot number can be observed for benzene and toluene in the BTX mixture, these contaminants having lower intraparticle diffusivity compared with o-xylene. 3.3.3. Competitive Study. A series of experiments with ternary combinations were performed to analyze the competitiveness of the active site of adsorption. In the case of benzene, in Figure 8, all concentrations relate to 50 mg/L

Figure 8. Breakthrough curves for adsorption of pure benzene and benzene in the tricomponent mixture with toluene and o-xylene, evaluating the influence of different concentrations of toluene and xylene.

benzene [pure, (50B 30T 30X), and (50B 50T 50X)] and the influence of toluene and o-xylene on the adsorption of benzene was evaluated. In the case of toluene, in Figure 9, all concentrations relate to 50 mg/L toluene [pure, (50T 30B 30X), and (50T 50B 50X)] and the influence of benzene and oxylene on the adsorption of toluene was evaluated. In the case of o-xylene, in Figure 10, all concentrations relate to 50 mg/L of o-xylene [pure, (50X 30B 30T), and (50T 50B 50X)] and the influence of benzene and toluene on the adsorption of oxylene was evaluated. 16918



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column saturated in around 4 h and in the mixture with (50B, 50T, 50X) mg/L the column saturated in around 2.5 h. 3.3.3.2. Effect of o-Xylene and Benzene on the Adsorption of Toluene. Figure 9 shows the breakthrough curves for pure toluene at an initial concentration of 50 mg/L and the simultaneous presence of toluene with benzene and xylene in different concentrations; that is, Cin = (30B, 50T, 30X) and Cin = (50B, 50T, 50X), applying a flow rate of 40 mL/min. In the tricomponent mixture only the concentration of toluene was investigated in order to compare the adsorption of pure toluene and toluene in the presence of benzene and xylene. On analyzing Figure 9 it can be seen that the behavior was same as that observed for benzene; that is, the adsorption of toluene was inhibited by the presence of different concentrations of benzene and o-xylene, this effect increasing with an increase in the contaminant concentrations. In the case of pure toluene the column was not saturated within the time shown, while for the mixture with (30B, 50T, 30X) mg/L the column was saturated with toluene after around 6 h and for the mixture with (50B, 50T, 50X) mg/L the column was saturated after around 4 h. Also, in the case of toluene, the presence of other contaminants resulted in the more rapid saturation of the column, once again due to the occupation of the adsorbent sites by the other contaminants. 3.3.3.3. Effect of Benzene and Toluene on the Adsorption of o-Xylene. Figure 10 shows the breakthrough curves for pure o-xylene at an initial concentration of 50 mg/L and o-xylene in the simultaneous presence of benzene and toluene in different concentrations; that is, (mg/L) Cin = (30B, 30T, 30X) and Cin = (50B, 50T, 50X), with a flow rate of 40 mL/min. In the tricomponent mixture only the concentration of o-xylene was analyzed in order to compare the adsorption of pure o-xylene with that of the same compound in the presence of benzene and toluene. On analyzing Figure 10 it can be observed that the behavior is the same as that shown by the results reported in Figures 8 and 9, when comparing the monocomponent adsorption with the multicomponent adsorption; that is, in the former case a longer time is required for the saturation of the column. In relation to the format of the curve, it can be noted that the behavior differs from the results obtained for benzene and toluene. The breakthrough curves for o-xylene in the mixture follow a linear tendency. Regarding the competition, it can be observed that the greater the concentration of the other two contaminants is, the greater the negative influence on the oxylene adsorption will be. For o-xylene in the mixture with (30B, 30T, 50X) mg/L, that is, in the presence of relatively low concentrations of the contaminants benzene and toluene, the column is not saturated within 11 h of adsorption. For higher benzene and toluene concentrations (50B, 50T, 50X) mg/L, the column saturates in around 11 h, as expected. On comparing the results in Figures 8, 9, and 10, which show the results for the breakthrough curves for the mono- and multicomponent BTX systems, it can be noted that o-xylene is the contaminant which is the most competitive for the active site of adsorption. As mentioned previously, this is associated with the high affinity of this contaminant for the adsorbent and its polarity, since activated carbon is a material with an apolar surface. The effective diffusivity for each solute in the tricomponent mixture with (50B, 50T, 50X) mg/L was evaluated in the column experiments, and the behavior observed was similar to that of the bicomponent mixtures, where benzene showed

Figure 9. Breakthrough curves for pure toluene and toluene in the tricomponent mixture with benzene and o-xylene, evaluating the influence of different concentrations of benzene and o-xylene.

Figure 10. Breakthrough curves for the adsorption of pure o-xylene and o-xylene in the tricomponent mixture with benzene and toluene, evaluating the influence of the different concentrations of benzene and toluene.

3.3.3.1. Effect of Toluene and Xylene on the Adsorption of Benzene. Figure 8 shows the breakthrough curves for pure benzene at an initial concentration of 50 mg/L and the simultaneous presence of benzene, toluene, and xylene for different concentrations, that is, Cin = (50B, 30T, 30X) and Cin = (50B, 50T, 50X), applying a flow rate of 40 mL/min. In the tricomponent mixture only the concentration of benzene was investigated, in order to compare the behavior of pure benzene with that of benzene in the presence of toluene and o-xylene. In Figure 8 it can be observed that the presence of other contaminants resulted in more rapid saturation of the adsorbent compared with the monocomponent adsorption seen on the breakthrough curves, which is due to the occupation of the active sites of the adsorbent by the contaminants. For competitive adsorption, the time required for the adsorption to occur differs from that for the same monocomponents, this being shorter for the mixture than for the individual compounds. The establishment of the adsorption equilibrium for the multicomponent compounds of lower molar mass occurs more rapidly and requires only minutes or tenths of a minute depending on their concentration in the solution.34 From Figure 8 it can be noted that the benzene adsorption was inhibited in the presence of different concentrations of toluene and o-xylene, this effect being slightly greater as the contaminant concentrations increase. For pure benzene the column saturated in around 15 h, while for benzene in the mixture with (50B, 30T, 30X) mg/L the 16919



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bi = adsorption equilibrium constant, (L3 M−1) qmax = maximum adsorption capacity (from Langmuir Eq.), (M M−1) kF = Freundlich pre-exponential constant (dimensionless) nF = exponent of the Freundlich isotherm (dimensionless) aij, bij = parameter of the Freundlich bicomponente isotherm (dimensionless) Dm = molecular diffusivity, (L2 t−1) Deff = effective diffusivity, (L2 t−1) k = parameter defined in eq 2, (dimensionless) R = particle radius, (L) dp = particle diameter, (L) Dc = internal diameter of bed, (L) ρs = density of solid, (M L−3) BiM = Biot number, (dimensionless) kf = external mass transfer coefficient (L/t) εL = bed porosity, (dimensionless) vs = interstitial velocity, (L t−1) Q = fluid flow rate, (L3 t−1) L = bed length (L) T = temperature, (T) t = time, (t) z = axial position in bed, (L) r = radial position inside the adsorbent particle, (L)

lower effective diffusivity compared with toluene and o-xylene. As suggested by Al-Duri and McKay,35 there is mutual interaction between the components in the presence of a second species in the adsorption system. For all of the mixtures there was a change in the adsorption rate and quantity adsorbed compared with the monocomponent compounds. The kinetics results presented show that the interactions between the BTX compounds in the mixture are complex since they involve several factors associated with the physical and chemical nature of the material and the solute. According to Kouyoumdjiev,18 these effects are very complex.

4. CONCLUSIONS An experimental and numerical study on the adsorption of BTX compounds in aqueous solution at 23 ± 1 °C was carried out. Thermally activated carbon originating from coconut shell was used as the adsorbent. The results obtained in adsorption tests on the multicomponent mixture in a fixed-bed column were compared with those for the pure compounds. In the case of the multicomponent mixture o-xylene was the contaminant which had the greatest affinity for the active sites of the adsorbent and over time it was able to force the other compounds, which had a lower affinity for these active sites, to desorb and then adsorb to the active sites which had become free. As a result, the local concentration of the weakly adsorbed component in the fluid phase is higher, surpassing the dimensionless concentration. One probable explanation for this deviation, mainly for weakly adsorbed compounds, is related to the Biot number. As the Biot number increases for each solute, the rate of competitive adsorption will decrease, and the curve will be flat with a lower break point. This is due to the low intraparticle resistance and also the reduction in the contact time required to reach saturation. A higher Biot number was observed for benzene and toluene in the BTX mixture, these contaminants showing a lower intraparticle diffusivity compared with o-xylene. The kinetics results showed that the interactions between the BTX compounds in the mixture are complex, since they are related to several factors associated with the physical and chemical nature of the material and the solute. For all mixtures the adsorption rate and the quantity adsorbed differed from the values observed for the monocomponents. The results for the breakthrough curves obtained through the simulation showed good agreement with the experimental data allowing other situations to be simulated.

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Subscripts

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e = equilibrium value in = value at bed inlet (z = 0) i = i component (for example: BTX)

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +55 (49) 3311 9314. Fax: +55 (49) 3311 9300. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to ANP/MECPETRO for financial support and UFSC/LABSIN-LABMASSA for the infrastructure provided for this study.

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NOMENCLATURE q = concentration of solute in solid phase, (M M−1) C = concentration in fluid phase, (M L−3) V = volume of solution, (L3) M = mass of adsorbent, (M) 16920



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Analysis of Competition between Multicomponent BTX Compounds for

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