Extraction of dye from aqueous solution in rotating packed bed

Extraction of dye from aqueous solution in rotating packed bed

Journal of Hazardous Materials 304 (2016) 337–342 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.els...

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Journal of Hazardous Materials 304 (2016) 337–342

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Extraction of dye from aqueous solution in rotating packed bed Jayant B. Modak, Avijit Bhowal ∗ , Siddhartha Datta Department of Chemical Engineering, Jadavpur University, Kolkata 700032, India

h i g h l i g h t s • • • •

Liquid–liquid extraction in rotating packed bed for removal of methyl red. Determination of mass transfer coefficient that is not yet available in literature. Maximum efficiency and volumetric mass transfer coefficient was ∼0.98 and 0.2 1/s. Volumetric mass transfer coefficient in RPB much higher than conventional ones.

a r t i c l e

i n f o

Article history: Received 25 May 2015 Received in revised form 14 October 2015 Accepted 26 October 2015 Available online 11 November 2015 Keywords: Liquid–liquid extraction Co-current Rotating packed bed Overall volumetric mass transfer coefficient Stage efficiency

a b s t r a c t The influence of centrifugal acceleration on mass transfer rates in liquid–liquid extraction was investigated experimentally in rotating packed bed (RPB) contactor. The extraction of methyl red using xylene was studied in the equipment. The effect of rotational speed (300–900 rpm), flow rate of the aqueous (4.17–20.8 × 10−6 m3 /s), and organic phase (0.83–2.5 × 10−6 m3 /s) on the mass transfer performance was examined. The maximum stage efficiency attained was ∼0.98 at aqueous to organic flow rate ratio of 10. The results suggest that contactor volume required to carry out a given separation can be reduced by an order of magnitude with RPB in comparison to conventional extractors. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Dyes are widely used in various industries such as textile, leather tanning, paper, plastics, food, cosmetic, printing etc. for the coloration of their related products [1]. Most of these dyes and their metabolites are toxic and potentially carcinogenic in nature [2]. Further, the presence of these colored compounds in wastewater is aesthetically displeasing. They also affect photosynthesis by inhibiting penetration of sunlight into water bodies [3]. Liquid–liquid extraction has been extensively investigated for removal and recovery of textile dyes from wastewater [4–9]. This technique describes a method of separating the solute present in a solution by adding another immiscible liquid in which the solute is transferred preferentially. Liquid–liquid extraction has traditionally been accomplished in contactors such as mixer settlers [10–12], spray [13–15], packed [15–17] and mechanically agitated columns [18–20]. Considerable effort has been directed to improve upon the design of these

∗ Corresponding author. Fax: +91 33 24146414. E-mail address: [email protected] (A. Bhowal). http://dx.doi.org/10.1016/j.jhazmat.2015.10.062 0304-3894/© 2015 Elsevier B.V. All rights reserved.

conventional contactors for enhancing the mass transfer efficiency. Hollow fiber contactor wherein the phases were contacted across micro-porous hollow fibers was studied for extraction of succinic acid [21], pesticide [22], ibuprofen [23] among others. Several contactors have been devised to replace terrestrial gravity governing the flow hydrodynamics in these conventional contactors by centrifugal acceleration. The advantages of operating under centrifugal force field include handling systems with low density difference, rapid attainment of steady state due to low holdup and higher throughput [24]. In impinging stream contactor [25], the phases flow outward between a rotating and a stationary circular disk. Annular centrifugal extractors were developed based on the principle of Taylor–Couette flow. Mixing of the streams occur between two coaxial rotating cylinders [26–28]. In rotating spray column [29], the drops were dispersed under centrifugal acceleration into the continuous phase. The range of the volumetric mass transfer coefficient in the above stated contactors is listed in Table 1. Increase of the magnitude of the volumetric mass transfer coefficient would decrease the contactor volume required for a given separation, and consequently also the capital and operating cost. In recent years, rotating packed bed (RPB) has emerged as a promising alternative for intensifying

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Notations A C EOc h KL a (KL a)m m Q r V

cross section area of contactor (m2 ) concentration (mol/m3 ) stage efficiency axial height of packed bed (m) overall volumetric mass transfer coefficient (1/s) overall volumetric mass transfer coefficient for perfect mixing condition (1/s) distribution ratio flow rate (m3 /s) radial distance (m) velocity (m/s) volume (m3 )

Subscripts o organic phase a aqueous phase Superscripts i inlet e exit Greek symbols rotational speed (rpm) ω

mass transfer in a wide variety of applications. In this equipment, the phases are contacted between two co-axially rotating cylindrical disks filled with packing materials. The liquid is accelerated under the action of centrifugal force and splits into fine droplets, threads, and thin films in the rotating packing. This results in significant intensification of micro-mixing and mass transfer. Though RPB has been extensively investigated for gas–liquid systems [30–35], only a few studies have reported on mass transfer between two liquid phases. These include polymerization [36], bromination of butyl rubber [37] and biodiesel production [38]. The results of these studies indicated significant improvement in performance. The above study suggests the possibility of reducing the required contactor volume for carrying out mass transfer between the solvent and feed stream. Previous studies [4–9] on decolorization of dye solution by liquid–liquid extraction have mainly focused on determining the optimum experimental conditions required for maximizing extraction, and were carried out in batch mode of operation. The advantages of RPB suggest the possibility for reducing the extractor volume for continuous decolorization of dye solution in this contactor. However, there appears to be no data available in the literature to evaluate the performance of this contactor for liquid–liquid extraction process. In this study, solvent extraction of methyl red from textile wastewater using xylene was investigated in RPB. Muthuraman and Teng [9] had performed batch studies for this system. The results indicated that this solvent is effective for

extraction of methyl red from its aqueous solution. In this work, the influence of process parameters such as rotor speed, aqueous phase and organic phase flow rate on stage efficiency and overall volumetric mass transfer coefficient has been presented.

2. Experimental 2.1. Materials Methyl red, xylene and hydrochloric acid were obtained from Merck. Aqueous solution of methyl red was prepared by dissolving the solute in double distilled water. The pH of the solution was adjusted to the desired value using hydrochloric acid (0.1 N) with the help of pH meter (EUTECH pH Tutor). A schematic diagram of the experimental setup used for continuous mass transfer experiments is shown in Fig. 1. The rotor used was a pair of stainless steel circular disks of 160 mm diameter fastened coaxially. The axial distance between the two disks was 20 mm. The space between the disks was packed with a stack of 10 stainless steel wire mesh (10 mesh size). The two disks were connected through a shaft to an AC motor and rotated around the horizontal axis. The rotating packed bed was housed inside a stationary cylindrical casing of diameter 180 mm and axial length 50 mm. Dye solution and xylene were stored in separate reservoirs. The two phases were mixed online prior to entering the RPB and sprayed onto the inner periphery of the packing from a stationary distributor. The diameter of the distributor was 34 mm and consisted of 24 openings of 1 mm diameter. The dispersion flowed towards the outer periphery of the packing under the action of the centrifugal force. It exited the equipment through an opening in the bottom of the casing wall. The concentration of dye in the raffinate phase after phase separation was analyzed at 523 nm wavelength using UV–vis spectrophotometer (PerkinElmer Lambda 25). The reported data was based on the average of three experimental runs. The maximum deviation of the percentage removal was less than 2.5% (standard deviation = ∼1.6). 2.2. Mathematical modeling The mass balance of the solute over a differential volume in the rotor assuming plug flow of the phases is given by Qa dCa = −KL a (Ca − Ca∗ ) dV

(1)

where the differential volume dV is dV = 2rdrh

(2)

The term Qa is the aqueous phase flow rate, Ca and Ca∗ is the bulk concentration of dye in the aqueous phase, and that in equilibrium with the organic phase respectively at a radius r from the rotational

Table 1 Comparison of overall volumetric mass transfer coefficient with other contactors. Ref.

Extractor

Chemical system

Qo(m3 /s)x106 ; Qa(m3 /s)x106

KL a(1/s)

[12]

Mixer–settler

Water–Acetone–Toluene

0.88; 0.78

0.0015–0.005

[15] [15] [20] [23] [25] [28] [29] Present study

Spray column Packed bed column Kühni column Hollow fiber Impinging jet Annular centrifuge Rotating spray column Rotating packed bed

Water–Acetone–Toluene Water–Acetone–Toluene Water–Acetone–Toluene Water–Ibuprofen–Octanol Water–Succinic acid–Butanol Water–Benzyl alcohol–White mineral oil Water–Cr (VI)-Aliquat 336 diluted in kerosene Water–Methyl red–Xylene

16–130; 20–130 16–130; 8–65 3.9–8.9; 3.9–8.9 2.7–7.1; 3.6–7.9 1.83–5.0; 1.83–5.0 0.33; 0.33 2.3–3.9; 2.5–6.8 0.83–2.1; 4.16–20.83

0.0005–0.008 0.0005–0.0055 0.005–0.0125 0.0045–0.042 0.077–0.25 0.002–0.0127 0.06–0.12 0.015–0.205 (Eq. (10))

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339

Fig. 1. Schematic of experimental setup.

500

axis, h is the thickness of the packed bed, and KL a is the overall volumetric mass transfer coefficient. Eq. (1) can be re-written as



Co dCa = −KL a Ca − Qa m dr

450



(3)

400

where Co is the concentration of dye in the organic phase, and m is defined as the ratio of the concentration of the dye in the organic to aqueous phase at equilibrium. Substituting the expression of Co obtained from the mass balance between the inner periphery of the packed bed, (ri ) and r i.e.

350

Qa Cai + Qo Coi = Qa Ca + Qo Co

(4)

into Eq. (3) yields the following relationship, dCa = −KL a Qa dr



1 Qa 1+ m Qo



Ca (r = ri ) = Cai

(5) (6)

where Cai and Coi are the inlet concentration of the dye in the aqueous and organic phase respectively. The value of Coi is zero as fresh xylene was used in this study. The distribution ratio, m was experimentally determined to be related to the equilibrium conin centration  the aqueous phase at pH 2.65 by the following relation R2 = 0.99 m = 521.1 − 1602Ca

300 250

200 150 100



 1  Ca − Qa Cai + Qo Coi 2rh. mQo

Distribuon rao

2rh

(7)

The volumetric mass transfer coefficient was obtained by integrating Eq. (5) between the inlet (ri ) and outlet (ro ) radius of the packed bed. 3. Results and discussions The equilibrium distribution of methyl red between xylene and water at different pH (initial solute concentration = 0.439 mol/m3 ) is shown in Fig. 2. It is seen that m increased from 124 at pH 1.2

50

0 0

1

2

3

4

5

6

7

pH Fig. 2. Equilibrium distribution of methyl red at different pH.

and attained a maximum value of 454 at pH 2.65. The distribution ratio decreased thereafter with further increase of pH. Similar trend in the variation of the percentage equilibrium extraction has also been reported by Muthuraman and Teng [9] for this system. The nature of the m-pH curve can be related to the existence of three different forms of methyl red in aqueous solution [39] namely, m − H2 MR+ (diprotic), HMR (monoprotic), and MR− (basic). The predominance of each fraction depends on pH of the solution. The fraction of the former species increases at pH below 3, whereas the basic form is dominant at higher pH. The maximum value of m obtained at pH 2.65 is due to the dominance of the organic soluble nonionic form of methyl red around this pH.

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1.00

1.00

Qo = 2.08 x 10-6 m3/s

Qa = 4.167x10-6 m3/s, Qo = 1.25x10-6 m3/s Qa = 1.25x10-5 m3/s, Qo = 2.08x10-6 m3/s

0.90

0.85 0.80

0.70

200

400

600 Rotaonal speed (rpm)

800

0.94

0.90

1000

0.00

Fig. 3. Effect of rotational speed on stage efficiency.

0.25 Overall volumetric mass transfer coefficient (1/s)

Qa = 4.17 x 10-6 m3/s , Qo = 1.25 x 10-6 m3/s Qa = 12.5 x 10-6 m3/s , Qo = 2.08 x 10-6 m3/s

0.10

5.00

10.00 15.00 20.00 Aqueous phase flow rate x 106 (m3/s)

25.00

Fig. 5. Effect of aqueous phase feed flow rate on stage efficiency at 900 rpm.

0.12

0.08

0.06

0.04

0.02

rpm = 700, Qo = 0.83 x 10-6 m3/s rpm = 900, Qo = 0.83 x 10-6 m3/s rpm = 700, Qo = 2.08 x 10-6 m3/s

0.20

rpm = 900, Qo = 2.08 x 10-6 m3/s

0.15

0.10

0.05

0.00

0.00 200

400

600 Rotaonal speed (rpm)

800

1000

Fig. 4. Effect of rotational speed on overall volumetric mass transfer coefficient.

Continuous extraction of methyl red in RPB was carried out with initial dye concentration 0.186 mol/m3 and pH 2.65. There was no significant change in pH between the inlet and outlet aqueous stream. The effect of rotational speed and feed flow rate on the stage efficiency (EOc ) based on concentration of the solute in the raffinate, and KL a is depicted in Figs. 3–7 . The stage efficiency was defined as EOc =

0.96

0.92

0.75

Overall volumetric mass transfer coefficient (1/s)

Qo = 0.83 x 10-6 m3/s

0.98

Stage efficiency

Stage efficiency

0.95

Cai − Cae Cai − Cae∗

(8)

where Cae∗ is the concentration of the solute in the raffinate in equilibrium with the exiting organic phase. It is seen that the stage efficiency (Fig. 3) increased from approximately 0.72 to 0.93 as the rotational speed was varied from 300 rpm to 900 rpm at feed and xylene flow rates of 4.17 × 10−6 m3 /s and 1.25 × 10−6 m3 /s respectively. The variation of KL a with rotational speed is presented in Fig. 4. The magnitude of the volumetric mass transfer coefficient was 0.05 1/s and 0.098 at 300 rpm and 900 rpm respectively for these flow rates. The droplets become finer and the film thinner with increase of rotational speed due to the strong shear under the action of centrifugal force. Therefore, the mass transfer resistance reduces and the interfacial area increases as the rotational speed was varied from 300 rpm to 900 rpm. Consequently, higher values of volumetric mass transfer coefficient and stage efficiency were obtained.

0.00

5.00

10.00 15.00 Aqueous phase flowrate x 106 (m3/s)

20.00

25.00

Fig. 6. Effect of aqueous phase flow rate on overall volumetric mass transfer coefficient.

Fig. 5 illustrates the effect of aqueous phase flow rate on stage efficiency (ω = 900 rpm) at constant organic phase flow rate of 8.33 × 10−7 m3 /s and 2.08 × 10−6 m3 /s respectively. The stage efficiency is noted to increase with both aqueous and organic phase flow rate. The effect of aqueous feed and organic phase flow rate on KL a is shown in Figs. 6 and 7 respectively. The volumetric mass transfer coefficient increases with flow rate of both dye solution and xylene. For example, KL a varies from 0.057 1/s at Qo = 8.33 × 10−7 m3 /s to 0.060 at Qo = 2.08 × 10−6 m3 /s at constant aqueous phase flow rate and rotational speed of 8.33 × 10−6 m3 /s and 900 rpm respectively. It was observed during preliminary investigations that liquid is dispersed from increasing number of the distributor openings as the flow rate was increased. Thus, the liquid spreads over a wider area on the packing surface at higher flow rates. Yan et al. [40] reported that the number of liquid trajectories over a given radial distance in the RPB increases with liquid flow rate at constant rotational speed. These phenomena contribute to increase the packing surface area wetted by the liquid. The interfacial area for mass transfer between the two liquid phases consequently increases. This could be the possible reason for the increase of stage efficiency and volumetric mass transfer with flow rate noted in Figs. 5–7. The experimental study suggested that the overall volumetric mass transfer coefficient varies with rotational speed, aqueous and organic phase flow rate. A correlation for KL a was developed

J.B. Modak et al. / Journal of Hazardous Materials 304 (2016) 337–342

30

rpm = 700, Qa = 8.33 x 10-6 m3/s rpm = 900, Qa = 8.33 x 10-6 m3/s rpm = 700, Qa = 16.67x 10-6 m3/s rpm = 900, Qa = 16.67 x 10-6 m3/s

0.16 0.14 0.12 0.10 0.08 0.06 0.04

Qa = 12.5 x 10-6 m3/s , Qo = 2.08 x 10-6 m3/s

20 15 10 5

0.02

0

0.00

0.50

0.00

1.00 1.50 2.00 Organic phase flow rate x 106 (m3/s)

2.50

250

3.00

Fig. 7. Effect of organic phase flow rate on overall volumetric mass transfer coefficient.

0.25

500 750 Rotaonal speed (rpm)

1000

Fig. 9. Variation of overall volumetric mass transfer coefficient for perfect mixing conditions with rotational speed.

The results are illustrated in Fig. 9 for some of the experimental conditions.

0.2

Comparison with other contactors

+15% KLa theorecal (1/s)

Qa = 4.17 x 10-6 m3/s , Qo = 1.25 x 10-6 m3/s

25 KLa for perfect mixing (1/s)

Overall volumetric mass transfer coefficient (1/s)

0.18

341

0.15 -15% 0.1

0.05

0 0

0.05

0.1 0.15 KLa experimental (1/s)

0.2

0.25

Fig. 8. Comparison between experimental and theoretical values of overall volumetric mass transfer coefficient.

assuming it can be related to these variables through the following equation. y

KL a = pωx Va Voz

(9)

where, ω is the rotational speed (rad/s), Va and Vo are the velocity (m/s) of the aqueous and organic phase at the average radius  of the packed bed 12 (ri + ro ) . The constants of Eq. (6) p, x, y and z were obtained from the experimental data through non-linear regression. The final form of the correlation is given by KL a = 5.7588ω0.5991 Va1.1319 Vo0.07213

(10)

Fig. 8 shows parity plot of the experimental values ofKL a and that calculated using Eq. (10). The deviations are within ±15%. Eq. (5) was derived assuming plug flow of the phases. Guo et al. [41] reported that the extent of liquid back mixing in their study was equivalent to two CSTR in series. In this study, another estimate of overall volumetric mass transfer coefficient (KL a)m was obtained assuming perfect mixing condition in the RPB. The following expression was derived from mass balance equations for making the calculation

(11)

The comparison of KL a (plug flow) obtained in RPB with that of other contactors is given in Table 1. It is seen that the overall volumetric mass transfer coefficient obtained in RPB is nearly two orders of magnitude higher in comparison to traditional extractors like mixer settler, spray, packed and mechanically agitated columns. The coefficient is higher than achieved in hollow fiber and annular centrifuge, and comparable with rotating spray column and impinging stream contactor. The volume of the countercurrent contactors can be estimated by integrating the expression given below derived assuming plug flow of the phases Qa

dCa = −KL a dr



1−

1 Qa m Qo



Ca −



 1  Qa Cae − Qo Coi Adz mQo

(12)

The percentage extraction obtained in the RPB studied (volume 3.8 × 10−4 m3 ) at feed and organic phase flow rate of 2.08 × 10−5 m3 /s and 2.08 × 10−6 m3 /s was 95.76. The volume of spray and Kühni column calculated using Eq. (12) to achieve this percentage extraction based on the maximum value of KL a reported in the table is 8.63 × 10−3 m3 and 5.35 × 10−3 m3 respectively. The volume of the mixer required for the same separation estimated using Eq. (11) is 0.183 m3 . The volume requirement in RPB is therefore ∼20 times lower compared to the countercurrent extractors and ∼500 times lower compared to mixer for the same separation. The volume of annular centrifugal extractor will be approximately 10 times higher than the volume of RPB. In other words, the contactor volume could be drastically decreased by carrying out the solvent extraction process under centrifugal acceleration in RPB. 4. Conclusions In this investigation, continuous removal of methyl red from aqueous dye solution by liquid–liquid extraction was studied in a rotating packed bed contactor. The higher values of overall volumetric mass transfer coefficient obtained in this contactor in comparison to conventional contactors indicate the suitability for enhancing separation efficiency for extraction of dye. In absence of any available literature on the overall volumetric mass transfer coefficient in this contactor, this work would be useful in providing an estimate of the parameter for similar mass transfer processes.

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