Ind. Eng. Chem. Res. 2008, 47, 1603-1613
1603
Prediction of Breakthrough Curves for Sorptive Removal of Phenol by Bagasse Fly Ash Packed Bed V. C. Srivastava, B. Prasad, I. M. Mishra, I. D. Mall,* and M. M. Swamy† Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India
Sorptive removal of phenol from synthetic aqueous solutions by bagasse fly ash (BFA) was investigated at 303 K under dynamic conditions in a packed bed. The effects of sorbent bed length (Z ) 40-90 cm), flow rate (Q ) 0.01-0.04 dm3/min), bed diameter (D ) 2-4 cm), and initial concentration (C0 ) 50-500 mg/ dm3) on the sorption characteristics of phenol were investigated at an influent pH of 6.5. More than 99.5% of phenol was removed in the column operated at C0 )100 mg/dm3 of phenol. The column performance improved with increasing Z and decreasing Q. The Bohart-Adams, Thomas, Yoon-Nelson, Clark, and Wolborska models were applied to the experimental data to represent the breakthrough curves and determine the characteristic design parameters of the column. The bed depth service time (BDST) model at 50% breakthrough provided a good fit to the experimental data, and the sorption capacity of the adsorbent was close to the value predicted from a batch study. The sorption performance of the BFA columns could be well described by the Thomas, Yoon-Nelson, and Clark models at effluent-to-influent concentration ratios (C/ C0) higher than 0.08 and lower than 0.99. Application of the Wolborska model to the experimental data for C/C0 pHPZC, whereas the adsorption of anions is favored at pH < pHPZC. Adsorption of phenol was found to decrease with increasing system pH. Up to pH 6.5, the decrease in adsorption was very gradual. However, the sorption dropped dramatically for pH > 6.5. At pH e pHPZC, the presence of oxides of aluminum, calcium, and silicon on BFA leads to the development of charge when adsorbent is in contact with water as follows
M-OH + H+ f M-OH2+ where M represents Al, Ca, and Si. Except for silica, all other oxides have a positive charge for the pH range of interest because the pHPZC values of SiO2, Al2O3, and CaO are 2.2, 8.3, and 11.0, respectively.21 For pH e pHPZC, high electrostatic attraction exists between the positively charged surface of the sorbent and the phenolate ion (C6H5O-). As the system pH increases, the number of positively charged sites decreases, and the number of negatively charged sites increases. A negatively charged surface site on BFA reduces the sorption of C6H5Oas a result of electrostatic repulsion. Moreover, a higher concentration of OH- in the solution leads to competition with C6H5O- for the adsorption sites, resulting in a reduced uptake of phenol. Because the diameter of the phenol molecule is about 6 Å,22 mesoporous adsorbents such as BFA that have 99% mesopores and 1% macropores show better sorption characteristics.17 In the present study, the practical applicability of BFA as a sorbent in a packed column under continuous-flow conditions was investigated for the removal of phenol from aqueous solutions. The effects of such parameters as Z, Q, D, and C0 on the breakthrough curve were investigated. The column sorption models available in the literature were applied to test their validity against the experimental data. 2. Theory The most important criterion in the design of a column adsorber is the prediction of the column breakthrough or the shape of the sorption wave front, which determine the bed length (z) and the operating lifespan and regeneration time of the bed.23 Factors that affect the column breakthrough include both operating variables and characteristics of the sorbate and the sorbent. Several models have been used for the prediction of breakthrough times. Some of these are discussed below. 2.1. Bed Depth Service Time (BDST) Model.24 The BDST model is based on the assumption that the rate of sorption is controlled by the surface reaction between the sorbate and the residual capacity of the sorbent. The Bohart-Adams model24 is used for the description of the initial part of the breakthrough curve, which relates C/C0 to time, t, for a continuous-flow sorber column. This equation is
ln
(
) [ ( ) ]
C0 Z - 1 ) ln exp kN0 - 1 - kC0t C U
(1)
where C0 is the initial concentration of sorbate (mg/dm3), C is the desired concentration of sorbate at time t (mg/dm3), k is the sorption rate constant of the column [dm3/(min mg)], Z is the length of the bed (cm), N0 is the sorptive capacity of the sorbent
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1605
bed (mg/dm3), and U is the linear flow velocity of the feed to the bed (cm/min). Hutchins25 linearized eq 1 to give
t)
(
N0 C0 1 -1 Zln C0U0 kC0 C
)
(
C0 1 -1 ln kC0 C
)
(3)
At 50% breakthrough, C/C0 ) 0.5 and t ) t0.5. Therefore, the last term in eq 2 becomes 0, and the half-time, t0.5, is obtained as
t0.5 )
N0 Z C0U
(4)
Thus, a plot of t0.5, the time at 50% breakthrough, versus Z should be a straight line passing through the origin, and N0 can be calculated from the slope. 2.2. Thomas Model.26 This model26 assumes plug-flow behavior in the bed and uses the Langmuir isotherm for equilibrium and second-order reversible reaction kinetics. It assumes a constant separation factor but is applicable to both favorable and unfavorable sorption conditions. Sorption is usually not limited by chemical reaction kinetics but is often controlled by interphase mass transfer. Therefore, this model is suitable for sorption processes in which external and internal diffusion limitations are absent.27 Application of the model can lead to some error in sorption processes that follow first-order reaction kinetics.1 The Thomas model is given by
1 C ) C0 1 + exp[kT(q0mC - C0Veff)/Q]
(
)
kTq0m C0 -1 ) - kTC0t C Q
(
)
C ) kYNt - t0.5 kYN C0 - C
A)
(6)
(7)
where kYN is the Yoon-Nelson rate constant (min-1). The values of kYN and t0.5 can be obtained from the slope and intercept, respectively, of a linear plot of ln[C/(C0 - C)] versus t.
(
C0n-1
Cbn-1
)
(8)
)
(9)
1/n-1
- 1 ertb
and
β r ) νm(n - 1) U
(10)
where n is the Freundlich constant, Cb is the concentration of sorbate at breakthrough time tb (mg/dm3), and νm is the migration velocity of the concentration front in the bed (cm/ min). νm can be determined from the relationship
UC0 N 0 + C0
νm )
(11)
Equation 8 can be rearranged to the following linear form
ln
[( ) C0 C
n-1
]
- 1 ) -rt + ln A
(12)
For a particular sorption process in a fixed bed with a chosen treatment objective, the values of A and r can be determined by using the above equation, thereby enabling the prediction of the breakthrough curve. 2.5. Wolborska Model.30 Wolborska30 deduced the following relationship for describing the concentration distribution in a bed for the low-concentration range of the breakthrough curve
ln
where Veff/Q ) t. The kinetic coefficient, kT, and the sorption capacity of the bed, q0, can be determined from the intercept and slope, respectively, of a plot of ln[(C0/C) - 1] versus t at a given value of Q. 2.3. Yoon and Nelson Model.28 This is a relatively simple model based on the assumption that the rate of decrease in the probability of sorption for each sorbate molecule is proportional to the probability of sorbate sorption and sorbate breakthrough on the sorbent. The equation for the 50% breakthrough concentration from a fixed bed of sorbent is
ln
with
(5)
where kT is the Thomas rate constant [dm3/(min mg)], q0 is the maximum solid-phase concentration of the solute (mg/g), mC is the mass of sorbent in the column (g), Veff is the throughput volume (dm3), and Q is the volumetric flow rate (dm3/min). The linearized form of the model is given by
ln
(
C 1 ) C0 1 + Ae-rt
(2)
This equation predicts linear BDST plots between t and Z. The sorptive capacity of the system, N0, can be evaluated from the slope of the plot, and the rate constant, k, can be calculated from the intercept, I, where
I)-
2.4. Clark Model.29 Clark29 used the mass-transfer coefficient in combination with the Freundlich isotherm to define a new relation for the breakthrough curve as
βZ C βC0 ) tC0 N0 U
(13)
where β is the kinetic coefficient of external mass transfer (min-1) and the other symbols have their usual meanings. The values of β and N0 can be determined from a plot of ln(C/C0) versus t at a given Z and Q. 2.6. Estimation of Breakthrough Curve. The dynamic behavior of a fixed-bed sorption column can be predicted by using any of the above-described models. Linear regression coefficients (R2) show the adequacy of the fits between the experimental data and the linearized forms of the equations, and an estimation of error between the experimental and predicted values of C/C0 can be calculated from Marquardt’s percent standard deviation (MPSD)31
MPSD ) 100
x
1
n
∑
N - P i)1
[
]
(C/C0)exp - (C/C0)theo (C/C0)exp
2
(14)
i
where N is the number of data points and P is the number of parameters (or the degrees of freedom of the system). Also, the percent deviation between the experimental and theoretical times for breakthrough can be calculated as
) 100
(
)
tb,exp - tb,cal tb,exp
(15)
1606
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008
3. Materials and Methods 3.1. Bagasse Fly Ash (BFA) and Its Characterization. In the present work, phenol was used as a sorbate, and bagasse fly ash (BFA) was used as a sorbent. BFA is a waste collected by the dust collection equipment attached upstream of the flue gas duct of sugarcane-bagasse-fired boilers. It is plentiful and available free of cost. BFA as obtained from U.P. State Sugar Corporation Ltd., Doiwala Unit, Dehradun, India, was used as such without any pretreatment, except for the sieving of very fine particles. The physicochemical characteristics of BFA were determined using standard procedures. Proximate analysis was carried out using the standard procedure.32 Bulk density was determined using a MAC bulk density meter, and particle size analysis was done using standard sieves. X-ray diffraction analyses of BFA were carried out using a Phillips diffraction unit (model PW 1140/90), with copper as the target, nickel as the filter medium, and K radiation maintained at 1.542 Å. The goniometer speed was maintained at 1°/min. Scanning electron microscopic (SEM) analyses of BFA were carried out using a scanning electron microscope (model SEM-501, Phillips, Eindhoven, The Netherlands). The specific surface area and pore diameter of BFA were measured by N2 sorption isotherm using an ASAP 2010 Micromeritics instrument; the BrunauerEmmett-Teller (BET) method was applied, using software from Micromeritics. Nitrogen was used as a cold bath (77.15 K). The Barrett-Joyner-Halenda (BJH) method33 was used to calculate the mesopore distribution. 3.2. Sorbate. Phenol (C6H5OH) of analytical reagent grade supplied by M/S Ranbaxi Laboratories Ltd., Gurgaon, India, was used for the preparation of synthetic sorbate solutions of various initial concentrations (C0). The required quantity of phenol was accurately weighed and dissolved in a small amount of distilled water and subsequently made up to 1 dm3 in a measuring flask. Fresh solution was prepared as required each day and was stored in a brown-colored glass reservoir of 5 dm3 capacity to prevent photo-oxidation. The value of C0 was ascertained before the start of each experimental run. 3.3. Analytical Measurements. The concentration of phenol was determined by measuring the absorbance of the aqueous solution at λmax ) 470 nm using a UV/vis spectrophotometer (model UV 210 A, Shimadzu, Kyoto, Japan). The calibration plot of absorbance versus concentration for phenol showed a linear variation up to a phenol concentration of 40 mg/dm3. Because the value of C0 was kept between 50 and 500 mg/ dm3, the samples were diluted with distilled water, whenever necessary, so that the phenol concentrations of the samples could be determined accurately using the linear portion of the calibration curve. 3.4. Column Studies. Plexiglass columns with inside diameters, D, of 2, 2.54, and 4 cm and a length of 110 cm were used for the column sorption studies. The columns were provided with a feed port at the bottom center of the column. The lower portion of the column was filled with pieces of 0.30.5-cm-diameter glass rods to a height of approximately 10 cm, and this portion was used for the uniform distribution of the solution across the whole cross section of the column and to dampen the fluctuating flow phenomenon induced by the peristaltic pump. This portion was attached to the main column through two flanges with O-ring rubber seals and a sintered metal disc in between. The column was packed with BFA up to different lengths, viz., 40, 60, 75, and 90 cm from the bottom, and was loaded with aqueous solution of phenol using a peristaltic pump (Miclins PP20). The solution flowed upward. Sampling ports were provided at 15, 30, 60, and 75 cm from
Table 1. Particle-Size Analysis of BFA sieve size (µm)
wt %
sieve size (µm)
wt %
>1000 850-1000 710-850 600-710 500-600 355-500 300-355
0.76 0.80 3.58 1.42 0.84 3.84 2.78
150-300 125-150 106-125 75-90 63-75 45-63
