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Ind. Eng. Chem. Res. 2008, 47, 4230–4235
SEPARATIONS Continuous Biosorption in Rotating Packed-Bed Contactor Aivi Das, Avijit Bhowal,* and Siddhartha Datta Department of Chemical Engineering, JadaVpur UniVersity, Kolkata 700 032, India
Biosorption process has been studied in fixed-bed contactors where terrestrial gravity dictates attainable mass transfer rates. This work explored the mass transfer characteristics of continuous biosorption under centrifugal acceleration (many times earth’s gravity) in a rotating packed-bed (RPB) contactor. The system studied was copper(II) ion adsorption on the packing of the contactor constituted by scales of Catla catla fish. Experiments were performed by continuously recycling a fixed volume of solution through the bed. The uptake of copper by the biosorbent increases with centrifugal force and liquid flow rate, but compaction of the bed negates the advantages of using high values of these. A low-packing-density bed demonstrated comparable efficiency in adsorbing copper as a bed nearly twice its packing density. Mass transfer rate per unit volume of packed bed was enhanced in the RPB by about 2.4 times compared to the fixed bed. A simple mass transfer model predicted the continuous biosorption experimental data in RPB fairly well. Introduction Biosorption is the passive uptake of heavy metals from aqueous solutions by chemical sites present naturally and functionally in dead biomass (biosorbents). It has emerged as one of the most promising processes for the removal of toxic metals from industrial waste streams and natural waters. The advantage of using biosorption lies in using biomass raw materials that are abundant in nature or are wastes in industrial operations. Compared with conventional methods for removing toxic metals from industrial effluents (such as precipitation with lime, ion exchange, membranes, solvent extraction, etc.), the biosorption process offers the advantages of low operating cost, minimization of the volume of chemical and/or biological sludge to be disposed of, and high efficiency in detoxifying very dilute effluents. The fixed bed has been extensively studied for continuous biosorption of metal ions. The operating conditions of some of the investigations are presented in Table 1. It is seen from the table that the bed height used for these experiments varied between 0.1 and 0.4 m and the maximum liquid flow rate was about 0.5 × 10-6 m3/s. In a fixed-bed contactor, the liquid flows under the influence of terrestrial gravity, which dictates allowable liquid throughputs and attainable mass transfer rates. A promising alternative for intensifying mass transfer in a fixed bed is subjecting the liquid flowing through the packing to a high gravity field. By rotating the packed bed, centrifugal forces hundreds of times the terrestrial gravity can be achieved. Ramshaw and Mallinson14 first developed a rotating packed bed (RPB) for enhancing mass transfer efficiency in distillation and absorption process. The advantages of carrying out a mass transfer process in a rotating packed bed over a conventional fixed bed include higher throughput, formation of thinner liquid film over the packing resulting in decreased external mass transfer resistance, use of higher surface packing per unit volume of packed bed, better distribution of liquid over the packing, lower static holdup, etc.15 As a result, the volume of the contactor can be reduced as compared to that in a conventional * To whom correspondence is to be addressed. E-mail: avijit_bh@ yahoo.co.in. Tel.: 91(033)24146378. Fax: 91(033) 24146414.
packed bed, with lower capital and perhaps also operating costs. Situations where the rotating packed bed could be advantageously used compared to the fixed bed include mobile pollution-abatement units, plant sites where space is at a premium. Some results have been published on the influence of centrifugal force on solid-liquid mass transfer. Munjal et al.15 studied mass transfer of a naphthalene-water system in a rotating packed bed at rotational speeds of 950, 1150, and 1575 rpm and a liquid flow rate of 2.3 × 10-6 m3/s. On the basis of the experimental data, they concluded that the liquid-solid volumetric mass transfer coefficient, Ksa, increases with rotational speed. Also, Ksa values in a rotating packed bed were 4-6 times higher at a centrifugal acceleration of 50-150 times the gravity force than in gravity flow. Lin and Liu16 studied dye adsorption by activated carbon in a rotating packed bed at varying rotational speeds and dye concentrations by continuously recycling a fixed volume of solution through the bed at a fixed recirculation flow rate of 0.4 × 10-6 m3/s. They also reported increased mass transfer with centrifugal force. The results for these solid-liquid systems suggest that mass transfer rate is also likely to be enhanced for continuous biosorptionsa process also involving mass transfer between liquid to solid phase in a high gravity field. Scrutiny of the literature revealed no previous studies into application of RPB in this field. The objective of the present work is to evaluate mass transfer performance of a rotating packed-bed contactor for continuous removal of species by biosorption. The system studied in this work involves uptake of copper ion from an aqueous solution by the biomass-scales of Catla catla fish. Experimental Section Raw scales of Catla catla fish were collected from a local fish market. The fish scales were pretreated thermally following the procedure outlined in a previous work.17 The collected scales were rinsed with distilled water and then boiled with deionized water for 30 min. The wet scales were dried overnight in a laboratory oven at around 80 °C. The dried scales were stored
10.1021/ie070679g CCC: $40.75 2008 American Chemical Society Published on Web 05/21/2008
Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 4231 Table 1. Literature on Metal Uptake by Biosorption in Fixed Bed metal removed
biomass
Cu Cu Cu Pb, Cd, Ni, Zn Cu Cu Cu Ni Cu Cu, Co, Ni Pb, Cd, Hg Cu, Pb Cu, Pb, Ni, Zn,
S. fluitans. Cladophora sp. Ca-alginate, agarose Mucor rouxii. Sargassum sp. Padina sp. S. filipendula. Crab shell Turbinaria ornate. UlVa reticulate M. aeruginosa. chaff seaweed, sunflower waste, maize, etc.
bed height (cm)
flowrate (mL/min)
reference
20, 40 7.8 12.3 29 30.6 8.4 41 15-25 15-25 15-25 10 30 20
7.5 0.36-6 0.8-3.3 2.22-2.66 6 1.5 15-20 5-20 5-20 5-20 0.75 3.6-8.3 27
1 2 3 4 5 6 7 8 9 10 11 12 13
in a refrigerator. Copper solutions were prepared by dissolving accurately weighed samples of CuSO4·5H2O (Merck) in deionized water. The equilibrium uptake of Cu(II) ion by the biomass was determined by contacting 100 mL of copper sulfate solution of different concentrations with 1 g of fish scales in flasks kept in a temperature-controlled shaker set at 30 °C for 24 h. The initial pH of the copper sulfate solution was around 5.6. There was no significant change in pH after contact with the biosorbent. The adsorbed amount was determined from the difference in concentration of copper ion present in the initial and residual copper sulfate solutions. These uptake experiments were carried out in a sterile environment using sterilized deionized-distilled water. The concentration of Cu(II) ions in the aqueous solution was determined by a spectrophotometer (Perkin-Elmer-Lambda 25). The samples were prepared following the methodology adopted by Aksu et al.2 The absorbance of the colored complex of Cu(II) ion with sodium diethyl dithiocarbamate was read at 460 nm. Figure 1 shows a sketch of the experimental setup used for continuous biosorption experiments in a rotating packed-bed contactor. The contactor consists of a rotor housed in a stationary casing. The rotor was a pair of disks of 0.2 m diameter made of a stainless steel sheet. The disk spacing was kept at 0.02 m by fastening the disks coaxially using spacers. Liquid was fed to the contactor through a distributor also rotating with the rotor.
The distributor was a hollow cylinder of outer diameter 0.0254 m provided with 48 holes of 0.5 mm diameter. The holes are arranged in vertical groups of eight, and the groups were spaced 45 ° apart. A known weight of fish scale was used to fill up the intervening space between the distributor and a stainless steel wire mesh encircling the packing between the two disks. The radial distance between the distributor and the wire mesh (i.e., bed depth) is 0.08 m. The contactor rotates about a horizontal axis. A known volume (5 × 10-3 m3) of copper sulfate solution was stored in a reservoir. The liquid entered the inner periphery of the packed bed and flew outward through the packing because of centrifugal force. The liquid flow rate was monitored by a rotameter. After exiting the packing, the liquid impacted the casing wall, was collected on the bottom of the casing, and flowed by gravity out of the unit back to the reservoir. The change in concentration of the solution in the reservoir was monitored by withdrawing samples at intervals. Fixed-bed studies were carried out in a glass column of length 0.4 m and diameter 0.05 m. The biosorbent bed was supported on a stainless steel wire mesh in the column. Copper sulfate solution present in a reservoir was introduced at the top of the bed through a distributor provided with 20 holes of 1 mm diameter. The solution from the bottom of the column exited into the reservoir and was continuously recirculated through the fixed bed. Mathematical Modeling. A simplified mathematical model is used to describe the mass transfer in the biosorption process in the RPB. The uptake rate of adsorbate, dqt/dt, is given by dqt ) Kp(q/ - qt) dt
(1)
assuming a linear driving force for the sorption process and combined film and intraparticle mass transfer resistance. In the above equation, Kp represents the overall particle-liquid mass transfer coefficient, q*, and qt represents the amount of copper adsorbed in equilibrium with the bulk concentration of copper and average copper concentration in the biosorbent at time t, respectively. The holdup of the liquid phase in the RPB is assumed to be negligible. The copper concentration in the aqueous solution does not change significantly during the residence time (∼0.2 s at 1200 rpm for wire mesh packing18) in the RPB, and q* is estimated based on the concentration of solution entering the packed bed, i.e., the reservoir concentration. The change in concentration of copper in the reservoir assuming a constant value of qt throughout the entire the bed depth is given by d[Cu]res dqt ) -FbedVbed (2) dt dt The terms Vres, [Cu]res, Fbed, and Vbed refer to the volume of solution in the reservoir, the concentration of copper in the reservoir, the density of the packed bed, and the volume of the bed, respectively. The value of qt and [Cu]res in eqs 1 and 2 at t ) 0 are 0 mmol/g and 0.79 mmol/L, respectively. Vres
Results and Discussion The adsorption isotherm of copper ion by scales of Catla catla fish is presented in Figure 2. The Langmuir adsorption isotherm
Figure 1. Schematic of experimental setup.
q/ bC ) qmax 1 + bC
(3)
4232 Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008
Figure 2. Adsorption isotherm of copper2+ on fish scale. Figure 5. Variation of qt with bed compaction.
Figure 3. Variation of qt with rotational speed.
Figure 6. Variation of qt with packing density.
qt )
Figure 4. Variation of qt with liquid flow rate.
was used to correlate the isotherm data. In the above equation, the term q* (mmol/g) represents the value of solid-phase concentration of copper in equilibrium with aqueous-phase concentration of copper denoted by C (mmol/L). The values of the parameters of the Langmuir model, qmax and b, determined by regression of the data are 1.1 mmol/g and 1.838 L/mmol, respectively. The model fitted the equilibrium data fairly accurately. In continuous biosorption experiments, the concentration of copper ion in the reservoir gradually decreased due to adsorption by the fish scale. The effect of various operating parameters on uptake by the biosorbent is presented in Figures 3-6 by plotting qt, the amount of copper(II) ion removed from the solution by unit weight of biosorbent at various experimental times, t. The value of qt was obtained from the experimental data using the mass balance equation given below
o Vres([Cu]res - [Cu]res) FbedVbed
(4)
In writing the above equation, the liquid holdup in the RPB, piping, casing, etc. was considered to be negligible. All the experiments were repeated twice, and the average value is reported here. The deviations between the repeated data were within 3.5%. The initial concentration of copper ion in the reservoir was 0.79 mmol/L. The uptake rate by the biosorbent given by the slope of the qt-versus-t curve is seen to gradually decrease as the contact time progresses. At the beginning of the experiment, mass transfer rate is high as solute is being transferred from a solution at its highest concentration (feed) to a fresh adsorbent. As time progresses, the uptake rate decreases as equilibrium is approached. The influence of centrifugal force on uptake by the biosorbent was studied at a recirculation flow rate of 33.34 × 10-6 m3/s using a packing density of 235 kg/m3. The data are plotted in Figure 3. The bed was operated between 500-1500 rpm, which provided centrifugal acceleration between 50-110g based on arithmetic mean radius of the bed. The higher values of qt at 1000 rpm compared to 500 rpm result from the increase of volumetric mass transfer coefficient (and, hence, mass transfer rate) with rotational speed.15 However, the qt values decreased with further increase of rotor speed to 1500 rpm. When the bed was observed after the conclusion of the run under this operating condition, it was found that the inner periphery of the packed bed was pushed outward from the distributor by over 0.01 m. So though volumetric mass transfer coefficient increases with rotational speed, the decreased flow path resulted in lowering the uptake at 1500 rpm.
Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 4233 Table 2. Equilibrium Values of qt for the Operating Conditions of RPB Fbed
o [Cu]res
t)∞ [Cu]res
qtt)∞
FbedVbedqtt)∞
1 - FbedVbedqtt)∞⁄ 0 Vres[Cu]res
235 235 124
0.79 0.39 0.79
0.0157 0.008 0.030
0.031 0.015 0.058
3.875 1.912 3.8
0.019 0.039 0.019
The effect of recirculation liquid flow rate on qt values at constant rotational speed and packing density of 1000 rpm and 235 kg/m3, respectively, is illustrated in Figure 4. As the flow rate was increased from 16.67 × 10-6 to 33.34 × 10-6 m3/s, the uptake improved significantly. High liquid flow rate appears to cause more liquid spreading over the packing surface and is reflected in improved mass transfer performance. With further increase of flow rate to 50.0 × 10-6 m3/s, the uptake decreased significantly again, owing to the compaction of the bed. This phenomenon occurs as a result of drag force exerted by the liquid on the movable packing particles and gains significance at high rotational speed and liquid flow rate. The advantages of reducing bed compaction are demonstrated in Figure 5 by comparing qt values at constant rotor speed and recirculation liquid flow rate and packing densities of 1500 m3/ s, 16.67 × 10-6 m3/s, and 235 kg/m3, respectively, for two bed packings. The difference between the types of packing is that, in one of them, glass beads (∼15 cm3) were mixed with fish scales to restrict the movement of the scales. The data show that mass transfer efficiency is improved in the presence of glass beads as the bed compaction was reduced to about 0.0025 m from nearly 0.01 m in the absence of glass beads. Experimental data of biosorption studies performed in two beds with packing densities of 235 and 125 kg/m3, respectively, at fixed recirculation liquid flow rate and rotational speed of 16.67 × 10-6 m3/s and 1000, respectively, are shown in Figure 6. To minimize compaction (observed in preliminary runs) of the packed bed with the lower packing density bed, fish scales were mixed with glass beads (∼15 cm3) to constitute the bed. The figure shows that the fractional decrease in copper concentration in the reservoir at various contact times obtained with both packing densities nearly overlaps. In other words, the amount of biosorbent in the lower packing density bed can remove copper equally efficiently as the biosorbent present at nearly twice this amount in the higher packing density bed. This fact can be explained based on the data given in Table 2. This table shows the values of qt; [Cu]res; the total amount adsorbed by the biosorbent, FbedVbedqt; and the fractional decrease of copper concentration at equilibrium (last column) estimated using the Langmuir adsorption model for the operating conditions used for this study. It is seen that, when equilibrium is achieved, though the values of qt differ significantly for the different packing density beds, the total uptake by the biosorbent in the bed and the fractional removal of solute is nearly the same. Experimental data indicate that these values are approached at nearly the same rate. Figure 7a and 7b compare biosorption results for RPB and a traditional fixed-bed (diameter ) 0.05 m) contactor in terms of copper uptake per unit weight of biosorbent and per unit volume of contactor. Both the contactors were packed with 0.125 kg of fish scales and operated at a recirculation liquid flow rate of 16.67 × 10-6 m3/s. The other operating parameters for the experiments are given in Table 3. The values of qt are slightly higher in the RPB operated at 1000 rpm compared to the fixed bed of bed height 0.19 cm (Figure 7a). However, when the residence time of the liquid in the fixed bed was increased by raising the bed height to 0.29 cm (Figure 7b), qt values for the
Figure 7. (a) Comparison between fixed-bed and rotating packed-bed contactor and (b) comparison between fixed-bed and rotating-bed contactor. Table 3. Experimental Condition for Data Presented in Parts a and b of Figure 7 results in
contactor
bed depth (m)
Figure 7a
RPB fixed bed RPB fixed bed
0.08 0.19 0.08 0.29
Figure 7b
rotational speed (rpm) 1000 1000, 1500
[Cu]ro(mmol/L) 0.39 0.39 0.79 0.79
fixed bed were higher compared to that obtained in the RPB operated at 1000 rpm. In other words, the concentration of copper is depleted more rapidly in the fixed bed compared to the RPB. When the rotational speed of the RPB was increased to 1500 rpm, the qt values for RPB (glass bead mixed with biosorbent) and fixed bed were nearly the same. However, the performance of the RPB is much improved over that of the fixed bed when comparison between the two contactors is made in terms of mass transfer per unit volume of packed bed. For example, in Figure 7b, this parameter is 2.2-2.4 times higher throughout the experiment in the RPB operated at 1500 rpm compared to that in the fixed bed. In other words, the use of centrifugal force reduces the volume of the packed bed required to bring about a given change in solute concentration within a given time. Uptake studies17 of metal ions (Cu, Pb, Co, and Ni) with fish scales of Mojarra tilapia demonstrated that, though both the two main fractionssthe organic or protein fraction and the inorganic fraction mainly composed of hydroxyapatitescontribute to the adsorption phenomena, ion-exchange reaction was the main mechanism for copper uptake by fish scale with calcium ion being exchanged for metallic ion. So the ion-exchange concept based on ion-exchange equilibrium constants was applied for
4234 Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008
concentration of copper, qext. The value of K that minimized F was 5.1 and was taken as the value of reaction equilibrium constant. Prediction of time-variant profiles of adsorbed-phase concentration in the RPB contactor was achieved by simultaneously solving eqs 1 and 3 along with eq 8 given by d[Ca]res d[Cu]res )dt dt
Figure 8. Comparison between experimental data and model predictions.
for estimating calcium ion concentration in the reservoir. The term q* in eq 1 corresponding to copper and calcium concentration in the liquid phase was determined using eq 6. Figure 8 shows the comparison between experimentally obtained qt values and those predicted using the model equations. The legend in the figure lists the operating condition in the following order: rotor speed in rpm, liquid flow in m3/s, and packing density in kg/m3. The term Kp was obtained by fitting the model equations to the experimental data and is given in the figure. The mass transfer coefficient Kp varies with rotor speed, recirculation flow rate, and packing density. A correlation for Kp was developed assuming it can be related to the variables through the following equation Kp ) C(ω2Ravg)xQyFzbed
Figure 9. Prediction of continuous biosorption in RPB using ion-exchange and Langmuir model.
modeling the biosorption process in RPB. The generalized ionexchange reaction as given by Figueira et al.19 is n+ m+ n+ nMm+ 1 + mM2 T nM1 + mM2
(5)
In the above equation, the species M1 (of valence m+) in the liquid phase is exchanged for the bound species M2 (of valence n+) present in the solid biosorbent, resulting in the species M1 being bounded to the biosorbent (M1) and species M2 being released to the solution. For this case, Cu2+ and Ca2+ represent M1m+ and M2n+, respectively. The reaction equilibrium constant, K, of this copper-calcium exchange, assuming that bound calcium ion in the fish scale is in large excess, is represented by K)
n
∑
(6)
[Cu2+(l)]
i)1
(
qexp - qtheo qexp
(9)
where ω, RaVg, Q represent rotor speed, average packed bed radius of the RPB, and flowrate, respectively. The values of the constants, C, x, y, and z, in the above expression were determined from the experimental and modeling data through nonlinear regression. The final form of the correlation is given below Kp ) 6.46 x 10-4(ω2Ravg)0.392Q0.322Fbed-1.035
(10)
The accuracy in predicting experimental data is tested in Figure 9 for an operating condition not used in developing the above correlation. The comparison shows that the model predicts the data fairly well. Further, the model prediction of the same operating condition using Langmuir model given in 3 to estimate q* is shown in the figure. It is seen that the Langmuir model does not fit this data (or the other operating conditions given in Figure 8) accurately. The reason appears to be the failure of the single-component Langmuir model to correctly estimate the equilibrium amount of copper adsorbed, q*, for the entire range of calcium concentration in the liquid phase. Conclusions
[q*][Ca2+(l)]
The terms [q*], [Ca2+(l)], and [Cu2+(l)] represent the equilibrium adsorbed-phase concentration of copper and that of calcium ion and copper ion in solution, respectively. To estimate K from the equilibrium adsorption data, the calcium ion concentration in the liquid phase at these data points was determined based on the fact that an equivalent amount of calcium ion is exchanged to the copper ion absorbed. At an assumed value of K, the estimated amount of copper adsorbed, qtheo was calculated using eq 6. The objective function, F, given by F)
(8)
)
2
(7)
was evaluated to compare the fitting of qtheo obtained for this assumed K to the experimentally measured adsorbed-phase
In this work, continuous biosorption operation that has so far been studied in traditional fixed-bed contactors has been performed in a rotating packed bed. The influence of various operational parameters on solute uptake by biosorbent in the contactor has been presented. The removal of copper from an aqueous solution has been done by scales of Catla catla fish. Analysis of the experimental data shows that the mass transfer rate in RPB is a function of rotational speed, liquid flow rate, and density of packing. Compaction of the biosorbent bed at high liquid flow rates and rotational speed needs to be avoided to achieve the full benefits of centrifugal acceleration on mass transfer. Moreover, comparison of mass transfer data with terrestrial gravity-driven liquid flow in conventional fixed-bed biosorption contactors indicated that centrifugal force plays an important role in reducing the contactor volume. The linear driving force model coupled with the ion-exchange model for adsorption equilibrium provided good representation of the experimentally obtained biosorption data in the RPB.
Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 4235
Nomenclature [Cu]res ) initial copper concentration in the reservoir, mmol/L [Cu]res ) copper concentration at any time in the reservoir, mmol/L [Ca]res ) calcium concentration at any time in the reservoir, mmol/L K ) equilibrium constant for ion-exchange model, mmol/g Kp ) overall particle-liquid-phase mass transfer constant, 1/s Q ) liquid flow rate, m3/s q* ) equilibrium copper adsorbed in the biosorbent, mmol/g of biosorbent qt ) average biosorbent-phase copper concentration at time t, mmol/g Ravg ) arithmetic average bed radius, m t ) time, s Vres ) volume of solution in the reservoir, m3 Vbed ) volume of packed bed, m3 o
Greek Symbols Fbed ) packing density, kg/m3 ω ) rotor speed, rad/s
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ReceiVed for reView May 14, 2007 ReVised manuscript receiVed February 14, 2008 Accepted February 25, 2008 IE070679G
