Adsorption dynamics of p-nitrophenol in structured fixed bed with microfibrous entrapped activated carbon

Chemical Engineering Journal 225 (2013) 481–488

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Adsorption dynamics of p-nitrophenol in structured fixed bed with microfibrous entrapped activated carbon Yan Shao, Huiping Zhang, Ying Yan ⇑ School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China

h i g h l i g h t s  The microfibrous entrapped activated carbon composites are used for PNP adsorption.  The HSDM is used for investigation adsorption dynamics.  Mass transfer has been enhanced in structured fixed bed.  Experimental and two theoretical results confirmed the conclusion.

a r t i c l e

i n f o

Article history: Received 11 February 2013 Received in revised form 26 March 2013 Accepted 27 March 2013 Available online 10 April 2013 Keywords: Adsorption Microfibrous composites Structured fixed bed Modeling

a b s t r a c t Adsorption dynamics of p-nitrophenol (PNP) in the structured fixed bed filled with granular activated carbon in the inlet and microfibrous entrapped activated carbon composites in the outlet at various flow rates and inlet concentrations were studied. A modified version of the homogeneous surface diffusion model (HSDM) involving surface diffusion, film mass transfer and axial dispersion was proposed to simulate the adsorption dynamics of PNP in the structured fixed bed. The breakthrough curve of PNP in the structured fixed bed is steeper than that in the individual activated carbon fixed bed with the same bed height. The length of unused bed (LUB) of the structured bed decreases almost 20% compared with that of the individual bed. The model simulations indicate that intraparticle diffusion resistances, film mass transfer resistances and the resistance due to axial dispersion decrease by adding the microfibrous entrapped activated carbon composites in the outlet of the bed. The experimental and modeled results indicate that the mass transfer and utilization of bed capacity are enhanced by adding the microfibrous entrapped activated carbon composites in the outlet of the structured bed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Activated carbon has been widely applied in the removal of organic pollutants from industrial and municipal wastewater [1–3], in which fixed bed adsorbers filled with activated carbons are commonly used due to easy operation [4]. In view of the high operation cost and low utilization of bed, how to enhance the mass transfer and decrease the length of unused bed (LUB) is still a challenging research topic. Fine adsorbent particles are commonly used to raise the overall adsorption rate. For example, a multi-layered bed with one layer of large adsorbent particles followed by another layer of small adsorbent particles was used to improve the adsorption rate and the utilization of bed capacity by Mathews [5] and Sze and McKay [6]. However, high pressure drop will be induced when the solution flows through the bed with fine particles and low bed voidage [7]. Hence, it is an attractive research topic to develop ⇑ Corresponding author. Tel./fax: +86 20 8711 1975. E-mail address: [email protected] (Y. Yan). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.133

a novel composite which can both enhance mass transfer and reduce the pressure drop. The layered bed filled with different adsorbents in pressure swing adsorption for gas separation has shown its superiority [8]. The applications of microfibres entrapped with smaller adsorbents (such as ZnO, silica and carbon) composing structured fixed beds for gas adsorption were implemented due to their small structural dimensions, uniform structures and adjustable porosity [9], which can effectively reduce the bed pressure drop, enhance the mass transfer and improve adsorption efficiency [9–11]. Chang et al. [12], used microfibrous entrapped supported ZnO sorbent to adsorb hydrogen sulfide exhibiting the enhancement of heat/mass transfer and the improvement of contacting efficiency. Microfibrous entrapped activated carbons composites packed in the bed outlet were also used in the removal of toluene from gas stream by Liu et al. [13]. All the results confirmed that the mass transfer and the utilization of bed can be obviously enhanced by adding the microfibrous entrapped supported sorbent in the bed outlet [11,13]. Although a large variety of experiments have been studied

482

Y. Shao et al. / Chemical Engineering Journal 225 (2013) 481–488

Nomenclature

Alphabet ap external surface area per unit particle volume, m2/m3 C absorbate concentration in liquid phase, mg/L C0i inlet adsorbate concentration in liquid phase in ith layer, mg/L Cb breakthrough concentration, mg/L Ce equilibrium concentration in liquid phase, mg/L Cs adsorbate concentration in liquid phase at adsorbent external surface, mg/L dp particle diameter, m Dab molecular diffusivity, m2/s DL axial dispersion coefficient, m2/s Ds intraparticle surface diffusivity, m2/s Dp average pore diameter, nm kf external film mass transfer coefficient, m/s K K = dq/dC, L/g KF constant in Freundlich isotherm Li length of the ith layer, m MB molecular weight of solvent, g/mol n constant in Freundlich isotherm q amount adsorbed at r and t, mg/g qe concentration in solid phase equilibrated with Ce, mg/g  q average amount adsorbed at t and Z, mg/g r radial coordinate for particle, m rp particle radius, m Re Roynolds number, dimensionless

on the applications of microfibrous composites in gas-phase adsorption and catalytic process, there are scarce publications about the applications of microfibrous activated carbon composites for removal of phenolic compounds from liquid stream, and about adsorption dynamics based on structured fixed beds using mathematical models. Consequently, it is very important to develop an appropriate mathematical model which can exactly predict adsorption dynamics of phenolic compounds in structured fixed bed filled with microfibrous activated carbons in the outlet of fixed bed. There are different particle sizes and flow rates in different layers of the structured fixed bed with specially designed adsorbents. And the boundary conditions of the different layers are variable. Kalluri et al. [10] predicted the breakthrough curves of hexane adsorption in structure fixed bed with microfibrous entrapped sorbents by linear driving force model in terms of the gas-soil mass transfer coefficients. In Yang’s work [11], a modified Amundson model was proposed to predict the breakthrough curves of hydrogen sulfide adsorption in the structured fixed bed filled with microfibrous entrapped zinc oxide sorbents. Do [14] analyzes that, in contrast to the interstitial diffusion and Knudsen diffusion, contribution of surface diffusion increases with larger and heavier adsorbate molecules in adsorbents with higher internal surface area, and thus homogeneous surface diffusion model (HSDM) neglecting the pore diffusion is usually applied to the adsorption in activated carbon. The HSDM combining surface diffusion and external mass transfer has successfully been applied in modeling the fixed bed adsorber [4,6] and layered adsorber with the increase or decrease of packing size [5,6], which is conducive to analyze the mass transfer process in fixed beds. However, the applications of HSDM to simulate the adsorption dynamics of phenolic compounds in structured fixed bed with microfibrous entrapped activated carbon composites have never been reported.

Rf RL Rs SBET Sc Sh t tb t⁄ T v Vtotal Vmeso Vmicro VA Z

film mass transfer resistance, s1 effective axial dispersion, s1 intraparticle diffusion resistance, s1 BET surface area, m2/g Schmidt number, dimensionless Sherwood number, dimensionless time, s breakthrough time, s stoichometric time, s temperature, K interstitial velocity, m/s total pore volume, cm3/g mesopore volume, cm3/g micropore volume, cm3/g volume of solute at its normal boiling temperature, cm3 /mol axial coordinate for bed, m

Greek alphabet eb bed voidage, dimensionless l viscosity of solvent, Pa s u association factor of solvent, dimensionless q liquid density, kg/m3 qp apparent adsorbent density, kg/m3 qs skeletal adsorbent density, kg/m3

The objectives of this paper are (1) to apply the microfibrous entrapped activated carbon composites in structured fixed bed for pnitrophenol (PNP) adsorption, (2) to simulate the adsorption dynamics of PNP in structured fixed bed using modified HSDM, and (3) to analyze the breakthrough curves by the LUB theory and the simulation results.

2. Experimental 2.1. Materials The granular activated carbon (GAC) used in all the experiments was coconut shell activated carbon. The activated carbon was purchased from Shanghai Xing Chang Activated Carbon Co., Ltd. (Shanghai, China). P-nitrophenol (analytical grade) was supplied by Guangzhou Chemical Reagent Factory (Guangzhou, China).

Table 1 Characteristics of materials.

a

Sample parameters

Properties of materials

BET surface area (m2/g) Average pore diameter, Dp (nm) Total pore volume, Vtotal (cm3/g) Micropore volume, Vmicro (cm3/g) Mesopore volume, Vmeso (cm3/g) Apparent particle density, qp (kg/m3) Bed voidage of granular activated carbon MEACCa Bed voidage Mass fraction of fibrous Mass fraction of activated carbon

906 1.93 0.437 0.317 0.120 718.6 0.32 0.68 0.27 0.73

Microfibrous entrapped activated carbon composites.

Y. Shao et al. / Chemical Engineering Journal 225 (2013) 481–488

483

The stainless steel fibers with an average diameter of 6.5 lm and the cellulose were purchased from Huitong Advanced Materials Company (China). Deionized water was used in the composite synthesis process.

breakthrough curves in the form of variations of C/C0 versus time were obtained.

2.2. Preparation of microfibrous entrapped activated carbon composites

3.1. The length of unused bed (LUB)

The activated carbon with an average particle diameter of 1.5 mm were milled and sieved to fine particles of 150–200 lm as adsorbent for preparing the microfibrous composites. The detailed synthesis processes of microfibrous entrapped activated carbon composites have been reported in our previous paper [13]. The microfibrous composites were made up of activated carbon, stainless steel fibers and wood fibers. The wood fibers are removed after the sintering process, and the remaining composition is listed in Table 1.

3. Theory

The LUB is the equivalent length of bed if we were to add up all of the unused adsorption capacity at breakthrough time and equate it to the adsorption capacity of a length of fresh, untouched bed [15]. The LUB can be obtained by analyzing the experimental breakthrough curves according to the following equation

 LUB ¼

2.4. Adsorption isotherm Adsorption isotherms of PNP on the coconut shell activated carbon were carried out with the classical bottled-point method. 50 mL PNP aqueous solution with concentrations of 200– 2200 mg/L and 0.1 g activated carbons were added in 250 mL bottles. Afterwards, the bottles were shaken at 150 rpm at 298 ± 1 K in a thermostatic shaker for 24 h to attain equilibrium. The concentration of PNP was measured by a UV spectrometer (VARIAN carry 50, American) at wavelength of 317 nm. The initial and final concentrations of the PNP were measured to calculate the total amount adsorbed. 2.5. Adsorption dynamics of PNP in different structured fixed beds The structured fixed bed was filled with the granular activated carbon in the inlet and microfibrous entrapped activated carbon composites in outlet of a stainless steel adsorber (2 cm i.d., 15 cm length). The volumetric flow rates were varied from 30 to 60 mL/min. The inlet concentrations (C0) were 150, 300 and 500 mg/L, respectively. A peristaltic pump (Lead fluid company, China) was used to pump PNP solution in upward flow. All of the experiments were carried out at 298 ± 1 K. PNP concentrations at the column outlet were measured by the UV spectrophotometry at wavelength of 317 nm at selected time intervals. The

 tb L t

ð1Þ

where L is bed height, tb is the breakthrough time corresponding to C = Cb (where C/C0 = 0.05), t⁄ is the stoichiometric time, and for unsymmetric breakthrough curves, can be determined from:

2.3. Characterization The characterization of the GAC was obtained by N2 adsorption/ desorption isotherm analysis and mercury intrusion method. The adsorption/desorption isotherm of N2 on the activated carbons at 77 K were measured by ASAP 2010 instrument (Micromeritics instrument corporation, American). The BET specific surface area of the activated carbon (SBET) was calculated by Brunauer–Emmett–Teller (BET) method. The total pore volume (Vtotal) was determined from the amount of nitrogen adsorbed at relative pressure of P/P0 = 0.98, then the average pore diameter (Dp) was found by Dp = 4Vtotal/SBET. The t-plot method was used for the micropore analysis with the P/P0 range from 0.01 to 0.65. The mesopore volume was calculated as the difference between the total pore volume and the micropore volume (Vmeso = Vtotal–Vmicro). The apparent density and skeletal density of activated carbon were determined by AutoPore IV 9500 (Micromeritics instrument corporation, American) through mercury intrusion method. The morphologies of the composites were observed by LEO1530VP field emission scanning electron microscopy (FE-SEM). All of the samples were coated with an ultra-thin film of gold to make them conductive before analysis.

1

t ¼

Z

1

0

  C dt 1 C0

ð2Þ

For a symmetry breakthrough curve, it is equal to the time at which C/C0 = 0.5 [15]. 3.2. Structured fixed bed model development The adsorption in a fixed bed is based on ‘‘two-step mass transport mechanism’’ [4]. The homogeneous surface diffusion model is based on the assumption that the adsorbate penetration rate is determined by the external film mass transfer plus the surface diffusion. The structured fixed bed is made up of two layers of adsorbents each with bed length of Li. The two layers of structured fixed bed have their own eb, rp, v and dynamic parameters. In the layer of microfibrous entrapped activated carbon composites, the effect of the stainless steel fibers on mass transfer rates was found to be insignificant for low Re by other authors [16]. Therefore, the presence of the fibers can be neglected in the mass transfer calculations for that layer, except for the bed voidage calculations [10].The following bed material balance, equilibrium and rate equations apply to the ith layer with the corresponding parameters eb, rp, v, DL and kf. The each layer mass balance in the structured fixed bed is:

v



@C @Z



!      @C ð1  eb Þ @q @2C þ þ qp ¼ DL @t z @t z eb @Z 2 t t

ð3Þ

The corresponding initial and boundary conditions are:

Cjt¼0 ¼ 0 CjZ¼0 ¼ C 0i þ

ð4Þ DL @C v @Z

 @C  ¼0 @Z Z¼Li

ð5Þ

ð6Þ

The last term on the left hand side of Eq. (3) can be described by the rate of adsorbate transfer across the stagnant liquid film. Hence the linear driving force model is used to describe the external mass transfer:

qp

 3kf @q ¼ ðC  C s Þ @t rp

ð7Þ

The adsorption equilibrium at the surface correlated with Freundlich equation:

q ¼ K F C 1=n

ð8Þ

484

Y. Shao et al. / Chemical Engineering Journal 225 (2013) 481–488

The adsorption rate of adsorbate in a spherical particle is governed by the intraparticle surface diffusion and can be represented as:

    @q Ds @ @q ¼ 2 r2 @t @r r @r

700

ð9Þ

500

The initial and boundary conditions for the particle are:

ð10Þ

 @q ¼0 @r r¼0

ð11Þ

 kf @q  ¼ ap ðC  C s Þ @r r¼rp qp

qe / mg

400

qjt¼0 ¼ 0

Ds ap

experimental result Freundlich equation

600

300 200 100

ð12Þ

The boundary conditions of the two layers, which represented by Eq. (5) are different. C0 of the influent to the first layer (granular activated carbon) is constant, and C0 of the influent to the subsequent layer (microfibrous entrapped activated carbon composites) is that of the effluent from the previous layer which is variable. The algorithm of piecewise cubic Hermite interpolating polynomial was used in the process. The partial differential equations containing Eqs. (3)–(17) are coupled and solved with the finite element scheme. A set of ordinary differential equations are resulted from discreting space (Z and r) with the finite element and then integrated in time with Gear’s method. Through testing, even space mesh of 20 divisions and absolute error tolerance 106 for time integration are selected. The solution is programmed with the software package MATLAB. In the modeling, the intraparticle surface diffusivity (Ds) has been obtained from the batch kinetic study of our previous work [17].

0

0

200

400

600

800

1000 1200 1400 1600 1800

Ce / mg Fig. 2. Equilibrium isotherm of PNP on activated carbon at 298 K.

The value of Ds is 4.187  1012 m2/s and its order of magnitude is consistent with that reported by other researchers [18]. The Ds is regarded as not related to activated carbon diameter, flow rate and initial concentration and thus the same to the two layers. The values of kf and DL are determined for each layer with the empirical equations of Coeuret [19] and Wakao [20], respectively. The equations are given by:

Sh ¼ 5:4Re1=3 Sc1=4

ð13Þ

qdp v eb l

ð14Þ

Re ¼

Fig. 1. Photographs of microfibrous entrapped activated carbon composites before (a) and after (b) sintering; SEM images of microfibrous composites before (c) and after (d) sintering.

485

Y. Shao et al. / Chemical Engineering Journal 225 (2013) 481–488

The molecular diffusivity (Dab) of PNP in water is estimated according to Wike-Chang equation [21]. The equation of Coeuret is valid for Reynold’s numbers in the range of 0.04–30.

1.0

0.8

4. Results and discussion 0.6

C/C0

4.1. he microfibrous entrapped activated carbon composites

0.4 15cm GAC

0.2

13cm GAC+ 2cm MEACC 11cm GAC+ 4cm MEACC

0.0 0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

t /min Fig. 3. Breakthrough curves for PNP adsorption in different structured fixed beds at different bed height ratio.

1.0

0.8

C/C0

0.6 30 mL/min (15 cm GAC) 40 mL/min (15 cm GAC) 60 mL/min (15 cm GAC)

0.4

30 mL/min (13cm GAC + 2cm MEACC) 40 mL/min (13cm GAC + 2cm MEACC)

0.2

60 mL/min (13cm GAC + 2cm MEACC)

0.0 0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

t /min Fig. 4. Breakthrough curves for PNP adsorption in fixed beds at different flow rates.

The microfibrous entrapped activated carbon (150–200 lm) composites were fabricated through the wet lay-up papermaking process and sintering process. The photographs and SEM views of the composites before and after sintering are shown in Fig. 1. As seen in Fig. 1b, the microfibrous entrapped activated carbon composites were clipped to a circular cake with diameter of 2 cm, which were filled in the outlet of the structured fixed bed. From Fig. 1c, it is clearly shown that the activated carbon particles were well entrapped into a three dimensional network of stainless steel fibers with cellulose as binder. As seen in Fig. 1d, the cellulose were completely removed through the high temperature sintering process and the junctures of stainless steel fibers were welded together to form a sinter-locked three dimensional network with large voidages. According to Liu’s paper, the pore size distributions of both the original activated carbon and the activated carbon entrapped in the composites were similar [13]. Hence, the test of characterization was only carried out for the original activated carbon in this study and the main characteristics of materials are listed in Table 1. The particle physical properties (apparent density, pore volume, pore size distribution, surface area, etc.) of the activated carbon entrapped in the composites were assumed to be similar to those of the original activated carbon. During the preparing process, the loss of stainless steel fibers was little [13]. Hence, the bed voidage in the composite layer can be calculated based on these assumptions, which also listed in Table 1. 4.2. Adsorption equilibrium The adsorption isotherms of PNP on the activated carbon at 298 K were measured and analyzed by Freundlich Eq. (7) through non-linear regression approach. Freundlich isotherm is one of the commonly used equations [22]. The equilibrium isotherm is shown in Fig. 2, and the corresponding values of KF and n are 32.948 and 2.822 with the units of q and C in mg/g and mg/L, respectively.

Sc ¼

l qDab

ð15Þ

Sh ¼

kf dp Dab

ð16Þ

4.3. Breakthrough curves of PNP in individual and structured fixed beds

ð17Þ

The individual fixed bed was filled with the granular activated carbons in the whole bed (15 cm in height), and the structured fixed bed was filled with the granular activated carbons of height 13 cm or 11 cm in the inlet of bed and the microfibrous entrapped

D L ¼ 2v r p



 20 1 þ ReSc 2

Table 2 Length of unused bed (LUB) for PNP adsorption in fixed beds. Adsorption fixed bed

Flow rate (mL/min)

Inlet concentration (mg/L)

LUB (cm)

DZa (%)

15 cm individual activated carbon fixed bed

30 40 60 30 40 60 40

300 300 300 300 300 300 300

8.52 9.73 12.08 7.08 8.39 10.67 5.34

56 65 81 47 56 71 36

15 cm structured fixed bed (13 cm granular activated carbon + 2 cm microfibrous entrapped activated carbon composites) 15 cm structured fixed bed (11 cm granular activated carbon + 4 cm microfibrous entrapped activated carbon composites) a

Relative percentage of LUB: DZ = 100% LUB/L.

486

Y. Shao et al. / Chemical Engineering Journal 225 (2013) 481–488

Table 3 Values of dynamic parameters for PNP adsorption in fixed bed under different conditions. Bed height (cm) (GAC + MEACCa)

13 + 2 13 + 2 13 + 2 13 + 2 13 + 2 15 + 0 11 + 4

30 40 60 30 30 40 40

Ds  1012 (m2/s)

Inlet concentration (mg/L)

300 300 300 150 500 300 300

kf  105 (m/s)

GAC

MEACC

4.187 4.187 4.187 4.187 4.187 4.187 4.187

4.187 4.187 4.187 4.187 4.187 – 4.187

a

DL  106 (m2/s) a

GAC

MEACC

GAC

MEACCa

2.495 2.744 3.143 2.495 2.495 2.744 2.744

9.559 10.512 12.044 9.559 9.559 – 10.512

3.786 5.016 7.516 3.786 3.786 5.016 5.016

0.260 0.337 0.494 0.260 0.260 – 0.337

Microfibrous entrapped activated carbon composites.

1.0

1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

a

Flow rate (mL/min)

0.4

0.4 15cm GAC

0.2

13cm GAC + 2 cm MEACC

150 mg/L 300 mg/L 500 mg/L

0.2

11cm GAC + 4 cm MEACC

0.0 0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

t /min

0.0 0

200

400

600

800

1000

1200

1400

1600

t /min Fig. 5. Simulated breakthrough curves for PNP adsorption in different structured fixed beds at different bed height ratio.

1.0

0.8

C/C0

0.6

0.4

30 mL/min 40 mL/min 60 mL/min

0.2

0.0 0

200

400

600

800

1000

1200

1400

1600

t /min Fig. 6. Simulated breakthrough curves for PNP adsorption in structured fixed beds at different flow rates.

activated carbon composites of height 2 cm or 4 cm in the outlet of bed. The breakthrough curves of PNP adsorption in two kinds of fixed beds were obtained under the operating conditions of inlet concentration of 300 mg/L and flow rate of 40 mL/min. The experimental breakthrough curves in individual and structured fixed beds are shown in Fig. 3, and the effect of flow rates is shown in Fig. 4.

Fig. 7. Simulated breakthrough curves for PNP adsorption in structured fixed beds at different inlet concentration.

As seen in Fig. 3, the breakthrough time in the structured bed with 2 cm microfibrous composites in the bed outlet is almost the same with that in the individual bed but the breakthrough curves become steeper with adding more microfibrous composites in the bed outlet. However, further increasing the microfibrous composites resulted in decreasing the breakthrough time. It can be also seen in Fig. 4 that all the breakthrough curves in the structured bed at different flow rates become sharper compared with those in the individual fixed bed, further indicating that the mass transfer in the structured bed has been enhanced. Similar results of gas phase adsorption were also obtained in Liu’s paper [13]. The improvement of breakthrough curve is based on the enhancement of mass transfer arising from the smaller carbon particles and the high voidage in the microfibrous entrapped activated carbon composites. Sze and Mckay [6] demonstrated that the adsorption column with stratified decreased particles can effectively enhance the bed utilization. 4.4. LUB values in individual and structured fixed beds The LUB values were calculated by analyzing the experimental breakthrough curves of PNP in both the individual bed and the structured bed according to Eq. (1), and the results calculated at different flow rates are listed in Table 2. As seen in Table 2, it can be found that the LUB values of the structured bed decrease 20% when the microfibrous entrapped activated carbon composites of height 2 cm were packed in the outlet of the bed, comparing with those of the individual bed. Correspondingly, the values of DZ of the structured bed decrease 10% compared with those of

487

Y. Shao et al. / Chemical Engineering Journal 225 (2013) 481–488

Table 4 Values of intraparticle diffusion resistance, film mass transfer resistance and the resistance due to axial dispersion for PNP adsorption in structured fixed bed under different conditions. Bed height (cm) (GAC + MEACCa) 13 + 2 13 + 2 13 + 2 13 + 2 13 + 2 15 + 0 11 + 4 a

Flow rate (mL/min) 30 40 60 30 30 40 40

Inlet concentration (mg/L) 300 300 300 150 500 300 300

Rs (s1)

Rf (s1)

RL (s1)

GAC

MEACCa

GAC

MEACCa

GAC

MEACCa

15.038 15.038 15.038 9.613 20.914 15.038 15.038

0.267 0.267 0.267 0.171 0.372 – 0.267

10.020 9.111 7.954 10.020 10.020 9.111 9.111

0.349 0.317 0.277 0.349 0.349 – 0.317

0.325 0.251 0.161 0.325 0.325 0.251 0.251

0.022 0.016 0.011 0.022 0.022 – 0.016

Microfibrous entrapped activated carbon composites bed.

the individual bed. Therefore, the LUB obviously decreases due to the enhancement of mass transfer by adding the microfibrous entrapped activated carbon composites in the outlet of the structured fixed bed, and thus the utilization of bed will be increased. 4.5. Simulation of adsorption dynamics in the structured fixed bed operation 4.5.1. Effects of bed height ratio, inlet concentration and flow rate The breakthrough curves of PNP adsorption in the structured fixed bed were measured under different flow rates and inlet concentrations. The adsorption dynamics of PNP in the structured fixed bed were simulated by the proposed model in Section 3.2. The values of Ds measured with the batch kinetic study [17] and kf, DL calculated from Eqs. (12)–(16) at different experimental conditions are presented in Table 3. Fig. 5 shows the experimental and predicted breakthrough curves in individual fixed bed and structured fixed bed at the inlet concentration of 300 mg/L and flow rate of 40 mL/min. The experimental and predicted breakthrough curves in structured fixed bed at different flow rates from 30 mL/ min to 60 mL/min and the same inlet concentrations of 300 mg/L are shown in Fig. 6, and those under three inlet concentrations (150, 300 and 500 mg/L) and the same constant flow rate of 30 mL/min are shown in Fig. 7. As seen in Figs. 6 and 7, earlier breakthrough occurs at high flow rate and high inlet concentration. It is because that the higher inlet concentration leads to a quicker saturation of the adsorbent. And the mass transfer zone at higher concentrations will proceed faster than that at lower concentration [23]. The high flow rate will decrease the contact time between the adsorbate and the adsorbent. In Figs. 6 and 7, we also observe that the slope of breakthrough curve slightly increases with the increase in inlet PNP concentrations and flow rates. The high flow rate will reduce the thickness of the mass transfer boundary layer around the adsorbents, leading to reducing external mass transfer resistances [23]. The high concentration gradient will also increase the mass transfer driving force [24]. Hence, it can be concluded that the steepness of the breakthrough curve of PNP in the structured fixed bed was more upright under the conditions of higher flow rate and inlet concentration. Similar phenomenon of the effects of flow rate and inlet concentration on breakthrough was also found by other researchers [25]. From Figs. 5 and 7, the predicted breakthrough curves fit well with the experimental breakthrough curves, indicating that the proposed model is able to describe the adsorption dynamics of PNP in the structured fixed bed. The simulation parameter values are listed in Table 3. As listed in Table 3, the values of Ds are the same for the two layers of bed at different conditions. According to literatures, Ds is only dependent on temperature and surface coverage [26]. Ds is assumed to be independent of particle diameter [6,27], flow rate and not related to initial concentration [25].

After the sintering process, the pore structure of carbon in the microfibrous entrapped activated carbon composites is almost unchanged [13]. The internal structure of activated carbon particles and the adsorbent capacity remains unchanged when the particles are milled to fine ones [5]. Hence, the values of Ds are regarded as the same in the proposed model during the simulation process. Moreover, as seen in Table 3, the parameters of kf and DL are independent on inlet concentration yet increase with the increase in flow rate. 4.5.2. Analysis of mass transfer resistances of PNP in structured fixed bed By analyzing the dynamic parameters and mass transfer resistances for the structured fixed bed, the phenomenon of the enhancement of mass transfer caused by the microfibrous activated carbon composites layer may further be explained. The intraparticle diffusion resistance, film mass transfer resistance and the resistance due to axial dispersion can be expressed as [28].

Rs ¼ r 2p =15K qp Ds ; where K ¼ dq=dC

ð18Þ

Rf ¼ r p =3kf

ð19Þ

RL ¼ DL ð1  eb Þ=v 2 eb

ð20Þ

The corresponding values of intraparticle diffusion resistances, film mass transfer resistances and effective axial dispersions in two layers of the structured fixed bed under various experimental conditions are calculated and listed in Table 4. As seen in Table 4, the intraparticle diffusion resistances in the granular activated carbon layer are almost 50 times larger than that in the microfibrous entrapped activated carbon composite layer. The possible reason is that the particle size of the activated carbon entrapped in microfibrous composite (0.2 mm) is 7.5 times smaller than that of the granular activated carbon (1.5 mm) in the fixed bed. The small particles with a larger external specific surface area and a shorter intraparticle diffusion path length can effectively decrease the intraparticle diffusion resistances and increase the diffusion rate. The small adsorbent particles possess higher adsorption rate, which also agrees with the Fick’s second law of diffusion (Eq. (9)). The enhancement of mass transfer in the structured fixed bed is not only based on the fast adsorption rate due to smaller carbon particles, but also attributed to increasing in external mass transfer and decreasing in axial dispersion. As seen in Table 4, the external mass transfer resistances in the granular activated carbon layer are 30–40 times larger than that in the microfibrous entrapped activated carbon composite layer. The external mass transfer resistances in the microfibrous entrapped activated carbon composite layer decrease due to a smaller particle radius and a lager kf. Moreover, the resistance due to axial dispersion in two layers of the structured fixed bed is compared. Unlike the film mass transfer

488

Y. Shao et al. / Chemical Engineering Journal 225 (2013) 481–488

resistance and intraparticle diffusion resistance, the resistance due to axial dispersion is directly proportional to DL. As seen in Table 4, the resistance due to axial dispersion in the microfibrous activated carbon composite layer is much smaller than that in the granular activated carbon layer. The mass transfer is also enhanced due to the decrease of axial dispersion. Similarly, in Kalluri’s work [10], the flow maldistribution and axial diffusion can also be effectively reduced due to the high voidages and structural uniformity in the microfibrous entrapped sorbents.

[3]

[4] [5]

[6] [7]

5. Conclusion A mathematical model based on HSDM for predicting breakthrough curves of PNP in a structured fixed bed with the granular activated carbon in the inlet and the microfibrous entrapped activated carbon composites in the outlet has been established. It is shown that the agreement between predicted and experimental breakthrough curves is very satisfactory at various flow rates and inlet concentrations. The breakthrough curves of PNP in the structured fixed bed are steeper compared with those in the individual activated carbons fixed bed. The LUB value of the structured bed decreases 20% compared with that of the individual bed. The mass transfer of PNP in the structured bed can be greatly enhanced by adding the microfibrous entrapped activated carbon composites in the outlet of the bed. The outstanding performance of the structured fixed bed with microfibrous entrapped activated carbon composites such as sharp breakthrough curves, high mass transfer rate and high bed utilizations can be explained by simulation results of the modified HSDM. The smaller particles entrapped in the composites can decrease the intraparticle diffusion resistances. The less external film mass transfer resistances and axial dispersion in the composite layer can also enhance the mass transfer. The experimental and modeled results confirm that the mass transfer in the structured fixed bed for PNP adsorption has been enhanced and the bed capacity utilization has been increased.

[8]

[9]

[10]

[11]

[12]

[13] [14] [15] [16]

[17] [18]

[19] [20] [21]

Acknowledgment Financial support from the National Natural Science Foundation of China (No. 21176086) and the Research Fund of Program of Guangdong Provincial Key Laboratory of fuel cell technology are gratefully acknowledged. The authors are very grateful to Dr. J. Guan for the helpful discussion about MATLAB programming.

[22]

[23]

[24]

[25]

References [26] [1] Y.T. Hung, H.H. Lo, L.K. Wang, J.R. Taricska, K.H. Li, Granular activated carbon adsorption, in: L.K. Wang, Y.T. Hung, N.K. Shammas (Eds.), Physiochemical Treatment Processes. Handbook of Environmental Engineering, vol. 3, Humana Press Inc., Totowa, 2005, pp. 573–633. [2] Y.T. Hung, H.H. Lo, L.K. Wang, J.R. Taricska, K.H. Li, Powdered activated carbon adsorption, in: L.K. Wang, Y.T. Hung, N.K. Shammas (Eds.), Advanced

[27] [28]

Physicochemical Treatment Processes. Handbook of Environmental Engineering, vol. 4, Humana Press Inc., Totowa, 2005, pp. 123–153. S. Lin, R. Juang, Adsorption of phenol and its derivatives from water using synthetic resins and low-cost natural adsorbents: a review, J. Environ. Manage. 90 (2009) 1336–1349. J.P. Chen, L. Wang, Characterization of metal adsorption kinetic properties in batch and fixed-bed reactors, Chemosphere 54 (2004) 397–404. A.P. Mathews, Effect of adsorbent particle layering on performance of conventional and tapered fixed-bed adsorbers, J. Environ. Eng. 131 (2005) 1488–1494. M.F.F. Sze, G. McKay, Enhanced mitigation of para-chlorophenol using stratified activated carbon adsorption columns, Water Res. 46 (2012) 700–710. D.O. Cooney, Adsorption Design for Wastewater Treatment, fifth ed., CRC Press LLC, New York, 1999. pp. 178. S. Cavenati, C.A. Grande, A.E. Rodrigues, Separation of CH4/CO2/N2 mixtures by layered pressure swing adsorption for upgrade of natural gas, Chem. Eng. Sci. 61 (2006) 3893–3906. B.K. Chang, Y. Lu, B.J. Tatarchuk, Microfibrous entrapment of small catalyst or sorbent particulates for high contacting-efficiency removal of trace contaminants including CO and H2S from practical reformates for PEM H2– O2 fuel cells, Chem. Eng. J. 115 (2006) 195–202. R.R. Kalluri, D.R. Cahela, B.J. Tatarchuk, Microfibrous entrapped small particle adsorbents for high efficiency heterogeneous contacting, Sep. Purif. Technol. 62 (2008) 304–316. H. Yang, D.R. Cahela, B.J. Tatarchuk, A study of kinetic effects due to using microfibrous entrapped zinc oxide sorbents for hydrogen sulfide removal, Chem. Eng. Sci 63 (2008) 2707–2716. B. Chang, Y. Lu, H. Yang, B.J. Tatarchuk, Facile regeneration vitreous microfibrous entrapped supported ZnO sorbent with high contacting efficiency for bulk H2S removal from reformate streams in fuel cell applications, J. Mater. Eng. Perform. 15 (2006) 439–441. J. Liu, Y. Yan, H. Zhang, Adsorption dynamics of toluene in composite bed with microfibrous entrapped activated carbon, Chem. Eng. J. 173 (2011) 456–462. D.D. Do, Adsorption Analysis: Equilibria and Kinetics, 9th ed., Imperial College Press, London, 1998. D.O. Cooney, Adsorption Design for Wastewater Treatment, fifth ed., CRC Press LLC, New York, 1999. pp. 141. R.R. Kalluri, Microfibrous Entrapped Catalysts and Sorbents: Microstructured Heterogeneous Contacting Systems with Enhanced Efficiency, Ph. D. Dissertation, Auburn University, Auburn, AL, 2008. Y. Shao, H. Zhang, Y. Yan, Characterization of P-nitrophenol adsorption kinetic properties in batch and fixed bed adsorber, J. Wuhan Univ. Technol., in press. J. Fujiki, T. Shinomiya, T. Kawakita, S. Ishibashi, E. Furuya, Experimental determination of fluid-film mass transfer coefficient from adsorption uptake curve, Chem. Eng. J. 173 (2011) 49–54. F. Coeuret, The percolating porous electrode. I – mass transfer in fixed bed, Electrochim. Acta 21 (1976) 185–193. D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley & Sons, New York, 1984. pp. 213. E.J. Wilson, C.J. Geankoplis, Liquid mass transfer at very low Reynolds number in packed beds, Ind. Eng. Chem. Fundam. 5 (1966) 9–14. M.L. Soto, A. Moure, H. Dominguez, J.C. Parajo, Recovery, concentration and purification of phenolic compounds by adsorption: a review, J. Food Eng. 105 (2011) 1–27. X. Zhang, X. Zhao, J. Hu, C. Wei, H. Bi, Adsorption dynamics of trichlorofluoromethane in activated carbon fiber bed, J. Hazard. Mater. 186 (2011) 1816–1822. L.A. Lail, J.A. Bergendahl, R.W. Thompson, Mathematical modeling of chloroform adsorption onto fixed-bed columns of highly siliceous granular zeolites, AICHE J. 31 (2012) 591–596. D.C.K. Ko, J.F. Porter, G. Mckay, Film-pore diffusion model for the fixed-bed sorption of copper and cadmium ions onto bone char, Water Res. 35 (2001) 3876–3886. I. Neretniekst, Adsorption in finite bath and countercurrent flow with systems having a concentration dependant coefficient of diffusion, Chem. Eng. Sci. 31 (1976) 465–471. A.A. Pota, A.P. Mathews, Adsorption dynamics in a stratified convergent tapered bed, Chem. Eng. Sci. 55 (2000) 1399–1409. D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley & Sons, New York, 1984. pp. 243.