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SEPARATIONS Liquid-Liquid Extraction in a Rotating-Spray Column: Removal of Cr(VI) by Aliquat 336 D. Bonam, G. Bhattacharyya, A. Bhowal,* and S. Datta Department of Chemical Engineering, JadaVpur UniVersity, Kolkata 700032, India
The effect of centrifugal force on mass-transfer rates in liquid-liquid extraction was investigated experimentally in a rotating-spray column. Extraction with interfacial chemical reaction was studied in this contactor by countercurrent contact of a dilute aqueous solution of hexavalent chromium (pH 2) with an extractantsAliquat 336 (quaternary ammonium salt)sdissolved in kerosene (dispersed phase). The overall volumetric masstransfer coefficient, based on a continuous phase (assuming plug flow of the phases), was measured to be an order higher than that in a conventional spray column. A mathematical model was developed for this contactor, considering axial dispersion in the continuous phase and internal circulation within drops. Simulation results suggested that the variation of kca, relative to the flow rate of the aqueous phase, was not significant at a constant rotor speed and a constant flow rate of the organic phase. Introduction Liquid-liquid extraction has found immense application in chemical process industries and in the cleanup of pollutants. Of the various types of extraction columns, the spray column is the simplest in design and generally is used when the number of theoretical stages required for a given separation is small. The extractor is a vertical tower in which the dispersed phase is sprayed through the continuous phase. The hydrodynamics and, hence, mass-transfer rates in a conventional extraction spray column are dictated by terrestrial gravity. Podbielneik1-4 applied centrifugal force to liquid-liquid extraction. The extractor was a cylindrical drum containing concentric perforated cylinders. The advantages of operating under a centrifugal force field include handling systems with low-density difference, low holdup (hence, rapid attainment of steady state), high throughput, and smaller contactor volume.5 Todd6 studied the residence time distribution in the Podbielniak Model B-10 centrifugal extractor. Jacobsen and Beyer5 and Barson and Beyer7 performed extraction studies with isoamyl alcohol-boric acid-water in a centrifugal extractor that consisted of 18 concentric annuli slotted at 180° intervals. The number of ideal stages varied between 2 and 8. Mass-transfer studies has also been reported8 for this contactor using benzene-acetic acid-water (number of ideal stages ) 5) and methyl isopropyl ketone (MIK)-acetic acid-water (number of ideal stages ) 3.4-12.5). Schilp and Blass9 reported the flooding capacities of perforated cylindrical plates in rotating liquid-liquid systems. Extraction studies in the conventional spray column have been investigated extensively.10-17 However, little information is available in the literature for rotating spray extractors. In this paper, results of an experimental study for extraction of hexavalent chromium from dilute acidic solution by Aliquat 336 (a quaternary ammonium salt commercialized as a mixture of tri-n-alkylammonium chloride) dissolved in kerosene in a rotating spray column is presented. This metal is found in the effluent stream of various industries (leather tanning, metallurgy, * To whom correspondence should be addressed. Tel.: +91 033 24146378. Fax: +91 033 24137121. E-mail:
[email protected].
electroplating, etc.), which causes environmental problems, and liquid-liquid extraction is an effective conventional technique for its removal. A mathematical model was also developed to describe the mass-transfer process in this contactor. Experimental Section Description of the Experimental Setup. Figure 1 shows the schematic of experimental setup. The sketch of internals of rotating extraction column is presented in Figure 2. The spray column is a stainless steel column of 0.15 m in length and 0.025 m in diameter. It is located inside another column that has a diameter of 0.055 m and a length of 0.21 m. The exiting streams of the spray column discharges into this larger column. The latter column was connected to a central shaft through flanges. The central shaft was connected to a motor-by-belt-pulley arrangement. Rotation of this shaft caused rotation of the spray column. A dummy column of the same dimension as the outer column and filled with sand was fitted on the central shaft opposite to the extraction column (see Figure 1), to ensure smooth rotation of the equipment.
Figure 1. Schematic of the experimental setup.
10.1021/ie900012k CCC: $40.75 2009 American Chemical Society Published on Web 07/21/2009
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of operation. The exiting phases were visually observed for the presence of any entrained droplets. The volume of the aqueous phase (VA) in the spray column and outer cylindrical shell under different operating conditions was determined by the method reported by Jacobsen and Beyer.5 The technique involved trapping and displacing the volume of the aqueous phase flowing through the setup (VT) by allowing kerosene to flow into the system through the aqueous phase inlet. The volume VA was obtained by deducting the aqueous phase holdup in the pipelines, rotameter, etc. from VT. An ultraviolet/visible light (UV/vis) spectrophotometer (Perkin-Elmer, Model Lambda 25) was used to analyze the chromium samples. The method is based on the spectrophotometric determination of the magenta chromagen (λmax ) 540 nm), which is formed when 1,5-diphenylcarbazide reacts with hexavalent chromium in a sulfuric acid solution. Mathematical Modeling
Figure 2. Internal details of a rotating-spray column.
Inside the central shaft are two sets of annular pipes rotating along with the shaft, as shown in Figure 2. The inner pipes (denoted as feature “A”) transport streams into the column, and exiting streams flow out through the shorter outer pipes (denoted as feature “B”). The heavier aqueous phase (feed) is introduced near the rotational axis. Within the spray column, the aqueous phase is thrown outward by centrifugal acceleration, thereby displacing the solvent phase inward. The raffinate stream from the periphery of the outer cylinder is channeled radially to the axis. Solvent phase enters through a distributor at the opposite end of the extractor from which the aqueous phase is introduced. The distributor is adjacent to the spray column. To provide unobstructed passage for the aqueous phase to flow out of the spray column, the distributor was constructed of a cylindrical tube with an outer diameter of ∼0.0125 m. There are 12 holes, 0.002 m in diameter, spread over 180° bored on the upper surface (facing the spray column) of the distributor. Along the axis of the distributor tube are 4 perforations, ∼0.005 m apart. The flow rate of the solvent stream exiting the extractor near the axis was controlled by a needle valve. Experimental Procedure. Potassium dichromate was dissolved in distilled water to form Cr(VI) anions (50 mg/L) to constitute the feed solution. The pH of the solution was maintained at pH 2 by adding hydrochloric acid (HCl). The solvent stream comprised of 0.25 vol % Aliquat (Aldrich) and 2 vol % Decanol (Merck), dissolved in kerosene. Kerosene was purchased from the local market and used without further refinement. Decanol was added to prevent the formation of a solid phase of the complex. The distribution coefficient of hexavalent chromium at pH 2 was measured by contacting 100 mL of 50 mg/L Cr(VI) solution with varying volume of solvent in a temperature-controlled shaker. For mass-transfer studies in the spray column, solvent and feed phases stored in respective reservoirs were pumped into the extraction column via rotameters. Opening of the needle valve in exit pipeline of the solvent phase was adjusted to ensure that only the solvent stream exited through this pipeline at the flow rate of the inlet solvent stream. Samples of the exiting aqueous phase were analyzed for the presence of hexavalent chromium during the run. Although runs were continued for 30 min, analysis of the hexavalent chromium in the aqueous phase showed that the column reached steady state after 12 min
Hexavalent chromium is extracted into the solvent phase by reaction with Aliquat 336 at the drop/aqueous phase interface. The resultant soluble complex then diffuses into the solvent phase. Reactions involving extraction of many metal ions with specific reagents are so rapid that the process may be classified as purely diffusion-controlled.18 The material balance over a differential element (dz) in the column using axial dispersion model to describe flow nonideality for the continuous phase is given by
Ec
d2Cc dz
2
+ Vc
dCc - kca(Cc - C*c ) ) 0 dz
(1)
Ec, Vc, and kc are the axial dispersion coefficient, the superficial velocity, and the mass-transfer coefficient of the continuous phase, respectively; a is the interfacial area, and Cc and C *c are the continuous-phase Cr(VI) concentrations in the bulk and at the drop/continuous-phase interface. The boundary conditions are z)0 z)L
dCc )0 dz - Ec
(2)
dCc ) Vc(Ccf - Cc) dz (3)
where L is the length of the spray column and Ccf is the feed concentration of hexavalent chromium in the continuous phase. Dispersed-phase drops formed at the distributor perforations are assumed to be monodispersed and do not interact with each other in the column. The concentration profile of the complex in a drop of radius R at an axial position z in the column is governed by the differential equation Vj
( )
Deff ∂ 2 ∂Cd ∂Cd ) 2 r ∂z ∂r r ∂r
(4)
Vj is the velocity of the drop at any position z, Deff the effective diffusivity of complex in the dispersed droplets, and Cd the complex concentration within the drop at radial distance r. The boundary conditions are z)0
Cd ) 0
(5)
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∂Cd )0 ∂r
r)0 r)R
- Deff
(6)
∂Cd ) kc(C *c - Cc) ∂r
(7)
There seems to be no available design correlation for the rotating-spray column. Therefore, correlations available in the literature for the conventional spray column were used to estimate the model parameters for this contactor by replacing the terrestrial gravity term (g) in the expressions with centrifugal acceleration. The axial dispersion coefficient was estimated through the correlation19 Ec ) 0.35dT4/3
( ) Vdg∆F Fc
1/3
(8)
where dTis the column diameter, ∆F the difference between the continuous-phase density and the dispersed-phase density, Fc the density of the continuous phase, and Vd the superficial velocity of the dispersed phase. The drop diameter from a multiorifice distributor was obtained from the expression proposed by Kumar and Hartland:20
( (
) ( ) (
2 -0.07
∆FdnV d32 ) 1.59 dn γ
d32 ∆FdnV2 ) 1.55 dn γ
-0.02
2
∆Fdn g γ ∆Fdn2g γ
) )
-0.28
(for 0 < We < 2 )
-0.21
(for 2 < We < 8.64 ) (9)
In eq 9, the terms d32, dn, V, and γ refer to the Sauter mean drop diameter, the diameter of the perforations in the distributor, the velocity of the dispersed phase issuing from each perforation, and the interfacial tension, respectively. It was assumed that the liquid drops in the contactor were always at its terminal velocity. The term Vj in eq 4 at any z in the rotating-spray column was estimated from the theoretical relation that was derived by Elgin and Browning21 to determine Vj in a conventional spray column (see eq 10) with Vt being calculated under centrifugal acceleration. Vj ) [VtπdT2 + 4(Fd - Fc)] + {[VtπdT2 - 4(Fd - Fc)]2 - 64FcFd}0.5
dpj 1 ) [kca(Cc,j - C *c,j) - Vcpj] dz Ec Vjj
(
Deff dCd,j,i dCd,j,i dCd,j,i ) 2 rj2 + rj dz dr dr ri
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(13)
)
(14)
where Cc,j is the continuous-phase concentration at the jth grid point of the column and Cd,j,i is the concentration in the drop at the ith grid point within the drop and the jth grid point in the column. The value of Cc at z ) 0 is the concentration of hexavalent chromium in the exiting continuous phase (measured experimentally), and the value of the term (p)z)0 is zero (see eq 2). The value of kc for a given operating condition was obtained by minimizing the difference in (dCc/dz)z)L obtained by numerical simulation of eqs 12-14 (along with other relevant equations) with that computed from eq 3 (i.e., (dCc/dz)z)L ) -(Vc/Ec)(Ccf - Cc)). The volumetric continuous-phase masstransfer coefficient was obtained by multiplying this value of kc by the average interfacial area estimated in the column. Results and Discussion The complexities in the extraction behavior of hexavalent chromium by quaternary ammonium salt has been widely mentioned.22-25 Hexavalent chromium may exist in the aqueous phase in various forms: HCrO4-, CrO4-, Cr2O72-, etc. The distribution of chromate species is dependent on both pH and total Cr(VI) concentration. A predominance diagram was presented by Sengupta et al.,26 with pH and total Cr(VI) concentration as the variables. At low pH, the Cr(VI) species exists mostly as HCrO4- at a total chromium concentration of less than (1.26-1.74) × 10-2 mol/L. The extraction efficiencies of hexavalent chromium with Aliquat 336 decreases as the pH increases,27 because the extractant exhibits reasonable extraction abilities only toward monovalent Cr(VI) species. Therefore, mass-transfer runs were performed at pH 2. Figure 3 shows the variation of distribution coefficient of Cr(VI) species with the aqueous-phase concentration at 30 and 40 °C at pH 2. The distribution coefficient can be represented by 30°C: CD ) 264Ca-0.9
(15)
40°C: CD ) 217Ca-0.905
(16)
2
2πdT
(10)
The terms Fd and Fc are the flow rates of the dispersed phase and the continuous phase, respectively, and Vt is the terminal velocity of the drops. The fractional holdup of the dispersed (φ) is then the volume of the dispersed phase entering the column during the residence time of the drop divided by volume of the contactor, VT, i.e., φ)
Fd VT
∫ 1Vj dz
(11)
The interfacial area and the slip velocity Us (which is defined as Us ) (Vd/φ) + [Vc/(1 - φ)]) was calculated based on the average holdup predicted by eq 11. The concentration profiles in the column and drop was solved numerically using the finite difference technique. At the jth grid point in the column, eqs 1 and 4 can be written as dCc,j ) pj dz
(12)
The dispersed phase within the rotating-spray column could not be distinguished visually. As an alternative, solvent phase holdup (denoted as Vo) was used to determine the dispersed phase. Vo ) VS - VA
(17)
where VS is the inner volume of the outer cylinder, excluding the volume occupied by the wall of the spray column and distributor. The term Vo represents solvent phase holdup inside the spray column plus the coalesced layer of solvent droplets near the exit inside the outer cylinder (see section X in Figure 2). The organic-phase holdup so determined varied mostly in the range of 40-65 cm3. Because the volume of section X is ∼60 cm3, and taking into account the solvent-phase holdup in the spray column, solvent was the dispersed phase for all the experimental conditions. In estimating mass-transfer coefficients, it was assumed that mass transfer in the regions outside the spray column was negligible. An increase in temperature of the exiting streams from ∼30 °C, by ∼10 °C, was also noted during the experi-
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Figure 3. Variation of the distribution coefficient with the aqueous-phase concentration.
Figure 4. Variation of Koca with the superficial velocity of the aqueous phase.
Figure 5. Variation of Koca with the superficial velocity of the organic phase.
mental runs. Therefore, an average value of distribution coefficient was used by linear interpolation between these two temperatures. The overall volumetric mass-transfer coefficient, Koca, obtained from the experimental data, assuming plug flow of the
phases (model 1), is illustrated in Figures 4 and 5. The volumetric mass-transfer coefficient increases with rotational speed, as well as superficial velocity of the aqueous and organic phase flows. Mass-transfer runs for this system were also conducted in a conventional spray column with a diameter of
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Figure 6. Variation of kca with the superficial velocity in the aqueous phase (estimated from the models described by eqs 2 and 3).
Figure 7. Variation of kca with the superficial velocity in the organic phase (estimated from the models described by eqs 2 and 3).
0.025 m using a distributor of the same configuration as that used in the rotating-spray column. Also plotted in Figure 5 are the values of Koca obtained in the conventional-spray column reported by various investigators, as well as that obtained experimentally for this system. Higher values of Koca were obtained when the extraction column was rotated. Values of (HTU)oc varied over a range of 0.06-0.15 m (Vd ) 6 × 10-3-7.8 × 10-3 m/s, Vc ) 6 × 10-3-14.0 × 10-3 m/s). In comparison, the (HTU)oc value that was reported for the conventional spray column for solute transfer from the continuous phase to the dispersed organic phase varied over a range of 0.235-1.0 m for methyl isopropyl ketone (MIK)-propionic acid-water11 (MIK ) (1.7-4) × 10-3 m/s, water ) 3.4 × 10-3 m/s), 0.13-2.5 m for ferric chloride-isopropyl ether-hydrochloric acid solutions12 (ether ) (0.8-5) × 10-3 m/s, water ) 4.7 × 10-3 m/s), 0.2-1.4 m for toluene-acetone-water15 (toluene ) (2-16) × 10-3 m/s, water ) 2.5 × 10-3 m/s), 1.1-2.6 m for this system (water ) (5-6) × 10-3 m/s, kerosene ) (2.5-4.0) × 10-3 m/s). The reason for the decrease in (HTU)oc values in the rotating-spray column could be, among others, more-efficient dispersion of organic drops from the distributor and a higher value of slip velocity caused by rotation of the column. The slip velocity was computed to be ∼4 times higher in the rotating-spray column, compared to the column operating under terrestrial gravity.
Estimating the mass-transfer coefficient using eqs 12-14 requires knowledge of Deff. Salazar et al.24 performed batch studies at 300 rpm (organic phase holdup ) 0.04) for the extraction of hexavcalent chromium by Aliquat 336 (dissolved in kerosene and decanol) and reported a value of Deff R2
) 5.03 × 10-4
(18)
Here, drops have been assumed to behave as rigid spheres. Because the impeller diameter was not reported, drop radius could not be estimated from available correlations for an agitated batch system. Lee and Soong28 studied the Sauter mean drop diameter for the kerosene-water system contaminated by surfactants. At an impeller speed of 350 rpm (six-flat-bladed turbine) and a dispersed phase holdup of 0.2, d32 was reported to be ∼2.5 × 10-4 m in the absence of surfactant. However, even substituting higher mean values of the drop radius in eq 18 (R ) 3 × 10-4 m) for a larger estimate of Deff, the specified change in concentration obtained experimentally could not be simulated using the drop diameter predicted by eq 9. Therefore, the presence of internal circulation was considered within the drops. Various correlations for a modified diffusion coefficient, incorporating internal circulation in the drops, are available in the literature. The correlation used in this study is given below:29
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Deff )
Vtd 2048(1 + κ)
(19)
where κ)
(
Eoc 1 gd2∆F ) 6 6 γ
)
(20)
The values of kca, considering the axial dispersion coefficient and internal circulation (model 2), are reported in Figures 6 and 7. Also given in the figures are the estimates of kca, assuming plug flow behavior of the continuous phase and estimating Deff through eq 19 (model 3). The difference in estimate of kca between the two models is due to neglecting the term Ec in model 3. Figure 6 shows that kca, determined by the assumptions made in model 2, was almost unaffected, relative to variation of the velocity of the aqueous phase at constant rotational speed and the velocity of the organic phase. The reason could be the almost-constant values of interfacial area, holdup, and slip velocity (estimated) with variation in Vc at constant dispersed phase velocity and rotor speed. Also observed was the fact that, under centrifugal force, Vd/φ is significantly greater thanVc. The increase of kca with Vd (see Figure 7) was chiefly due to greater interfacial area. However, with variation in rotor speed from 750 rpm to 1100 rpm, the slip velocity increased by ∼40%, resulting in significant enhancement of the mass-transfer coefficient. Conclusions Liquid-liquid extraction of hexavalent chromium from an acidic solution by Aliquat 336 dissolved in kerosene was performed in a rotating-spray column. The effect of experimental parameters on the mass-transfer coefficient, based on the continuous phase, was estimated through mathematical models developed for the extractor. The volumetric mass-transfer coefficient increased with flow rate of the phases and the centrifugal acceleration. The high values of the volumetric masstransfer coefficient obtained in this equipment, compared to that in the conventional spray column, would lead to a drastic reduction in the size of the liquid-liquid extractor. Considering this potential for process intensification, further studies are warranted to understand hydrodynamics and establish design correlations for this contactor. Acknowledgment Financial assistance provided by All India Council for Technical Education (F. No. 8023/BOR/RPS-19/2006-07) is gratefully acknowledged. Nomenclature Parameters a ) interfacial area in the spray column (m2/m3) Ca ) concentration of Cr(VI) in aqueous phase (mg/L) Cc ) concentration of Cr(VI) in continuous phase in the contactor (mg/L) C *c ) concentration of Cr(VI) in the continuous phase at the drop/ continuous-phase interface (mg/L) Cc,f ) feed concentration of Cr(VI) in the continuous phase (mg/ L) Cd ) complex concentration in the dispersed (solvent) phase (mg/ L) CD ) distribution coefficient
d ) diameter of liquid drop (m) d32 ) Sauter mean diameter (m) Deff ) effective diffusivity of complex in solvent drops (m2/s) dn ) nozzle diameter (m) dT ) diameter of the spray column (m) Ec ) continuous phase axial dispersion coefficient (m2/s) Eoc ) Eo¨tvus number when the droplet has critical diameter Fd ) flow rate of the dispersed phase (m3/s) Fc ) flow rate of the continuous phase (m3/s) g ) terrestrial gravity (m/s2) kc ) continuous-phase mass-transfer coefficient (m/s) koca ) overall volumetric mass-transfer coefficient, based on the continuous phase (1/s) L ) length of the rotating spray column (m) R ) radius of the liquid drop (m) r ) radial distance inside the drop (m) R ) outer radius of the drop (m) Us ) slip velocity (m/s) VA ) aqueous phase holdup in outer cylinder (m3) VT ) inner volume of spray column (m3) Vo ) solvent phase holdup in the extractor (m3) VS ) inner volume of outer cylinder extractor excluding volume occupied by spray column wall and distributor (m3) VT ) experimental collected aqueous phase using the entrapment technique (m3) Vc ) superficial velocity of the continuous phase (m/s) Vd ) superficial velocity of the dispersed phase (m/s) V ) nozzle velocity (m/s) Vt ) terminal velocity of the drops in the column (m/s) Vj ) average velocity of drops (m/s) We ) Weber number z ) distance along the length of the contactor (m) Greek Letters ω ) rotor speed (rpm) φ ) average dispersed phase holdup in the column ∆F ) density difference between the phases (kg/m3) γ ) interfacial tension (N/m) Fc ) density of continuous phase (kg/m3)
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ReceiVed for reView January 5, 2009 ReVised manuscript receiVed June 26, 2009 Accepted June 26, 2009 IE900012K
