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Supported liquid membranes with organic dispersion for recovery of Cephalexin
Journal of Membrane Science 468 (2014) 90–97
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Supported liquid membranes with organic dispersion for recovery of Cephalexin Zisu Hao a,b, Zihao Wang b, Weidong Zhang b, W.S. Winston Ho a,c,n a
William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, OH 43210-1180, USA Beijing Key Laboratory of Membrane Science and Technology, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China c Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1178, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 13 February 2014 Received in revised form 27 May 2014 Accepted 29 May 2014 Available online 4 June 2014
Supported liquid membranes with organic dispersion (SLM-OD) have been proposed and investigated for simultaneous removal and recovery of Cephalexin from aqueous solutions by using the carrier Aliquat 336 and a commercially available hollow-fiber module (HFM). The organic dispersion, formed by dispersing a small amount of organic membrane solution in the feed solution with a mixer, flowed through the shell side of the HFM. Investigated various parameters including organic-to-feed volume ratio, Aliquat 336 concentration, initial Cephalexin concentration, KCl concentration, shell-side organic dispersion flow rate, and lumen-side strip solution flow rate. The mass transfer mechanism of the SLMOD technique was elucidated, and a mathematical model was developed to calculate the overall mass transfer coefficient. The results showed that the overall mass transfer coefficient increased with an increase of organic-to-feed volume ratio or increase of Aliquat 336 concentration, but reduced with the increase of initial Cephalexin concentration. Furthermore, the results indicated that the lumen and shell side flow rates had little effects on mass transfer performance and that an excess amount of KCl was necessary for the facilitated transport. The fractional mass transfer resistances of SLM-OD were calculated based on the resistance-in-series model, showing that the extraction reaction resistance was greatly reduced. In addition, the SLM-OD was shown to be superior to the supported liquid membranes with strip dispersion (SLM-SD) in terms of the improvement of mass transfer performance and the reduced volume requirement of the organic membrane solution. & 2014 Elsevier B.V. All rights reserved.
Keywords: Supported liquid membrane Organic dispersion Cephalexin Aliquat 336 Mass transfer model
1. Introduction Cephalexin is a first-generation cephalosporin antibiotic, and it has been widely used to treat various infections due to its broad antibacterial activity [1]. The traditional production method is a 10step chemical synthesis process which suffers from significant energy consumption, many side reactions [2] and introducing toxic solvent [3]. To overcome these shortcomings, an enzymatic synthetic method [4] has been developed and proved to be an efficient alternative for the chemical synthesis method [5]. However, the commercial application of the enzymatic method to produce Cephalexin has not been realized because of the low yield and difficulty to separate (purify) Cephalexin from product solution.
n Corresponding author at: William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, OH 43210-1180, USA. Tel.: þ 1 614 292 9970; fax: þ 1 614 292 3769. E-mail address: [email protected] (W.S. Winston Ho).
http://dx.doi.org/10.1016/j.memsci.2014.05.052 0376-7388/& 2014 Elsevier B.V. All rights reserved.
Liquid membrane (LM) is a highly integrated separation technique which combines extraction and stripping processes in one step. Since LM was firstly invented by Li in 1968 [6], various LM techniques have been proposed and extensively investigated, including bulk liquid membrane (BLM) [7], emulsion liquid membrane (ELM) [6], and supported liquid membrane (SLM) [8]. The advantages of LMs include the ease of scale-up, high selectivity, low consumption of extractant and no flooding. Therefore, LM is considered as a promising separation method for wastewater treatment, chemical and biochemical processing, and pharmaceutical and food processes [9]. LM has been proved to be an attractive technique for the separation and recovery of Cephalexin [10,11]. Sahoo et al. [10] reported the facilitated transport of Cephalexin using Aliquat 336 as the extractant in a BLM. Then, Sahoo and Dutta [11] proved that Cephalexin can be successfully separated from an aqueous solution containing 7-amino3-desacetoxicephalosporanic acid (7-ADCA) by means of an ELM system. However, traditional LM techniques have some intrinsic shortcomings, which hinder their commercial application [12].
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
In recent decades, some novel LM techniques have been proposed that offers long-term stability such as hollow fiber contained liquid membrane (HFCLM) [13], hollow fiber renewal liquid membrane (HFRLM) [14], and supported liquid membrane with strip dispersion (SLM-SD) [15]. Among them, SLM-SD has shown effective removal and recovery capabilities and has been applied in wastewater treatment [15–17] as well as separation and purification of valued organic compounds [18,19]. Vilt and Ho firstly used the SLM-SD technique to recover and concentrate Cephalexin from a dilute solution. The results showed that the recovery rate of Cephalexin was around 96–98% with the enrichment ratio ranging from 1.6 to 3.3 [18]. In subsequent work, they developed an enzymatic synthesis process coupled with the SLMSD technique for in-situ selective removal and recovery Cephalexin using a commercially available hollow fiber module (HFM). It was found that the combination of SLM-SD and enzymatic complexation enhanced the maximum yield of Cephalexin from 32% to 42% without an enzyme deactivation problem [4]. Besides the investigation of removal efficiency and enrichment performance, the analysis of mass transfer mechanism of LM is also necessary for the future industrialization. In our previous work, the distribution of mass transfer resistances of SLM-SD for the removal and recovery of Cephalexin was estimated, and the results indicated that the extraction reaction resistance, i.e., the complexation reaction of Cephalexin and Aliquat 336, was dominant [18]. In order to decrease this resistance, a possible approach is to enlarge the mass transfer surface area between the feed and organic phases. As this mass transfer surface area in SLM-SD locates on the inner surface of hollow fibers, this would suggest that this area can be augmented by increasing the number of hollow fibers. But this would enlarge the size or number of hollow-fiber membrane modules and the amount of organic phase, resulting in an increase of the cost. In addition, from the environmental and economic perspectives, the amount of organic phase should be limited. Actually, in the solvent extraction and HFRLM techniques, the mass transfer area can be greatly increased by dispersing the organic solution in the aqueous solution. In a HFRLM technique, the dispersion flowed through the lumen side of HFM, and the organic-to-aqueous volume ratio was normally less than 1:10, which enhanced the mass transfer performance [14]. In this work, the SLM with organic dispersion (SLM-OD) technique was proposed and demonstrated for the removal and recovery of Cephalexin from its dilute aqueous solutions. Organic dispersion, formed by dispersing the organic membrane solution in the feed solution with a mixer, flowed through the shell side of a HFM. A series of important experimental parameters were investigated, including the organic-to-feed volume ratio, Aliquat 336 concentration, initial Cephalexin concentration, KCl concentration, and lumen- and shell-side flow rates. This allowed for identifying the critical parameters. A mathematical model was developed to determine the overall mass transfer coefficient of SLM-OD. Furthermore, according to the resistance-in-series model, the distribution of mass transfer resistances of the SLM-OD system was calculated. This showed a significant reduction of the mass transfer resistance due to the extraction reaction for the SLM-OD vs. the supported liquid membranes with strip dispersion (SLM-SD). In addition, the SLM-OD was shown to be superior to the SLM-SD in terms of the reduced volume requirement of the organic membrane solution.
Strip Solution Out
Dispersed Droplets of Organic Solution
Pf Continuous Feed Phase
Hollow Fiber Pores Filled with Organic
Hollow Fiber Wall Ps Strip Solution In
Fig. 1. A schematic diagram of an enlarged view of the SLM-organic dispersion (SLM-OD) process.
microporous hydrophobic polymer hollow-fibers should be used to obtain a large mass transfer area. In a HFM, the dispersion phase can flow on the shell side while the aqueous strip phase can be on the lumen-side. In order to prevent any sipping of the organic membrane solution, the pressure of the feed phase (Pf) containing the organic dispersion and the aqueous strip phase pressure (Ps) should be approximately the same (about 34.5 kPa, 5 psi). In this work, the organic dispersion was pumped through the shell side of the HFM.
3. Theory 3.1. General transport mechanism In SLM-OD, the organic membrane solution is dispersed in the continuous aqueous feed solution. The multiphase flow behavior is different from that of SLM-SD, suggesting that SLM-OD may have a different transport mechanism, which is described as follows:
Solvent extraction phenomenon in the organic dispersion vessel
2. The SLM-OD technique The SLM-OD technique is illustrated schematically in Fig. 1, in which a small amount of organic membrane solution is dispersed as droplets in the continuous aqueous feed solution by a mixer, while in the SLM-SD technique, the dispersed phase is the aqueous strip solution. Typically, a hollow-fiber module (HFM) containing
91
Due to the direct mixing of the feed and organic phases, a small amount of target species in the feed phase can be quickly extracted into the organic droplets, which are a small fraction in the dispersion. Mass transfer on the shell side of the module On the shell side of HFM, the dispersed organic droplets and the outer surface of the hollow fibers provide a large mass transfer area at the feed/organic interface. There are three types of transport processes. a. At the feed/organic interface located on the outside diameter of each hollow fiber, the target species reacts with the carrier to form the solute–carrier complex and is partitioned into the organic phase, i.e., the supported liquid membrane phase. b. At the feed/organic interface located on the surface of each organic droplet, the target species reacts with the carrier to form the solute–carrier complex in the organic droplet. Due to the small size of each organic droplet in the dispersion, the concentration profile of the solute–carrier complex in the organic droplets is assumed to be uniform. The pumping flow action enhances the fulfillment of this assumption. c. The solute-loaded droplets may collide with the hollow fibers due to the effect of the flow field. In this process, the complex in the droplets may be directly transported to the feed/organic interface located on the outside diameter of hollow fibers. This process may bring about the mass transfer intensification. Mass transfer in the membrane supported in pores Because the microporous hydrophobic polymer hollow-fibers have been wetted by the organic membrane phase, the complex can then diffuse across the membrane supported in the porous wall of each hollow fiber.
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Mass transfer from membrane/strip interface to bulk strip phase At the organic/strip interface, the decomplexation reaction occurs while the target species is partitioned into the aqueous strip phase and the carrier is regenerated. Then, the regenerated carrier diffuses back to the feed side for the next facilitated transport, and the target species can be convectively transported to the bulk strip phase.
(Cod) and strip phase (Cs) [22]. V od
dC od ðtÞ dC s ðtÞ ¼ jNεπdlm L ¼ V s dt dt
where j is the flux of Cephalexin in the pseudo-steady state, and Cod is the Cephalexin concentration in the organic dispersion at time t, which consists of Cephalexin in the feed and organic phases, as shown in Eq. (6), Vod being the volume of organic dispersion. C od ðtÞ ¼
3.2. Cephalexin–Aliquat 336–KCl system Cephalexin is a zwitterionic molecule, and the ionic state of Cephalexin is anionic or cationic when the pH value is higher than 6.88 or below 2.56, respectively. Aliquat 336, a quaternary ammonium chloride, was chosen as the extractant for the ion-exchange extraction used in the process. Detailed information about the Cephalexin–Aliquat 336 system can be found elsewhere [4,18,20]. The extraction and stripping reactions of Cephalexin–Aliquat 336 (QCl)–KCl system are shown in Eqs. (1) and (2), respectively:
ð1Þ
ð2Þ
Cephf þQClo 2QCepho þ Clf
QCepho þCls 2QClo þCephs
Generally, an excess amount of Cl‾ is provided to obtain a high stripping rate. When the rate of the stripping reaction is very fast, its contribution to overall mass transfer resistance can be neglected [4,18,20]. 3.3. Modeling of SLM-OD for the Cephalexin–Aliquat 336 system The SLM-OD technique is one kind of dispersive liquid membrane, which has a different mass transfer mechanism from traditional SLM processes. Thus, for the future industrial application of SLM-OD, it is necessary to develop a mathematical model to evaluate its mass transfer performance. In this study, due to a relatively large volume of the organic dispersion in comparison with the organic liquid membrane phase supported in the pore volume of the hollow fibers, the amount of Cephalexin in the organic dispersion on the shell side is much larger than that in the supported organic liquid membrane. It suggests that the mass transfer from the organic dispersion to go across the membrane can be considered at the pseudo-steady state. To verify this assumption, we can compare the diffusion time across the membrane t D with the process time t P . The value of t D in this work can be obtained by tD ¼
ðδτÞ2 ½ð300 220Þ=2 10 6 2:62 m2 ¼ ¼ 27:64 s Dm m2 s 1 3:913 10 10
ð3Þ
where Dm is the complex diffusion coefficient in the membrane, which was calculated by using the Haydauk and Minhaus equation, and the value was 3.913 10 10 m2 s 1 [21]. t P is approximately 3 h in each experiment, and the ratio of t D to t P is tD 27:64 s ¼ 2:56 10 3 ⪡1 ¼ t P 3600 3 s
ð4Þ
The value of this ratio is far smaller than 1, indicating that the SLM-OD can be considered under the pseudo-steady state. In the supported organic membrane phase, the amount of Cephalexin as in the Cephalexin–Aliquat 336 complex is negligibly small in comparison with that in the organic dispersion. The pseudo-steady state of the SLM-OD allows us to use the steady-state flux across the supported organic liquid membrane to relate to the Cephalexin concentrations in the organic dispersion
ð5Þ
Vo V C o ðtÞ þ f C f ðtÞ V od V od
ð6Þ
where Co is the Cephalexin–Aliquat 336 complex concentration in the dispersed organic phase, which can be calculated by the mass balance shown in Eq. (7). C o ðtÞ ¼
Vf 0 Vs ðC C f ðtÞÞ C s ðtÞ Vo f Vo
ð7Þ
Therefore, j, the flux of Cephalexin in the pseudo-steady state may be expressed by Hs j ¼ K exp C od ðtÞ C s ðtÞ ð8Þ Hf where Kexp is the experimental overall mass transfer coefficient; Hf and Hs are the partition coefficients for the organic dispersion and strip sides, respectively [22,23]. Hf and Hs are defined as Eqs. (9) and (10) Hf ¼
Co C nf
ð9Þ
Hs ¼
Co C ns
ð10Þ
where C nf and C ns are the feed and strip solute concentrations in equilibrium with the organic solute concentration, C o . By substituting Eq. (8) into Eq. (5) and integrating the resultant Eq. (5) [22] with respect to t with the initial conditions C od ð0Þ ¼ C 0od and Cs(0)¼ 0, the Kexp can be obtained as in Eq.(11). " # H f C 0od H s C 0s 1 : ð11Þ K exp ¼ ln H f C od ðtÞ H s C s ðtÞ βt where β is a constant, given by 1 1 Hs β ¼ LN επdlm þ : V od V s H f
ð12Þ
Thus, by linear regression of the experimental data using Eq. (11), Kexp for the SLM-OD system with various experimental conditions can be obtained. Used in Eq. (11), the values of Hf and Hs were 4.41 and 0.39, respectively, which were determined experimentally. 3.4. The fractional mass transfer resistances of the SLM-OD One of the important characteristics of a liquid membrane process is the distribution of the fractional mass transfer resistances. The resistance-in-series model has been extensively used to analyze the fractional mass transfer resistances of various liquid membrane techniques [14,18]. For the SLM-OD, the mass transfer process is also governed by this model. The overall mass transfer resistance of the SLM-OD consists of the following individual mass transfer resistances, including the diffusional resistance of complex in the small organic droplets, extraction reaction resistance, shell side resistance, membrane phase resistance, stripping reaction resistance and lumen side resistance. Because of the small size of the organic droplets and the flow action of the organic dispersion as discussed earlier, the diffusional resistance of Cephalexin–Aliquat 336 complex in the organic droplets can be neglected. Moreover, the stripping
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
93
reaction is instantaneous due to the very high stripping efficiency resulting from the excess stripping reagent used (KCl). Therefore, the overall mass transfer resistance can be expressed as [18] 1 1 1 1 Hs 1 ¼ þ þ þ K exp ksh ker H f km H f kl
ð13Þ
where ker is the mass transfer coefficient for the extraction reaction, kl, the mass transfer coefficient for the lumen side [24], km, the mass transfer coefficient for the membrane phase [15], and ksh, the mass transfer coefficient for the shell side [25]. The values of kl, km and ksh can be estimated by using the following equations: !1=3 2 kl dmi u d ¼ 1:62 b mi ð14Þ Da Da L km ¼
Dm εdlm δτdmo
ð15Þ
ksh de 1Φ Re0:6 Sc0:33 ¼ 5:85 de L Da de ¼
P
P
Hollow Fiber Module P
Strip
Organic
Solution
Dispersion
Fig. 2. Schematic of the SLM-OD set-up with a hollow-fiber module in the recycle mode.
ð16Þ Table 1 Properties of the hollow fiber module used.
2ðR2i nr 2o Þ Ri þnr o
ð17Þ 10
P
2
m /s, was obtained via the Wilke– The value of Da, 4.66 10 Chang equation [1,26]. ker can be calculated by substituting the Kexp obtained from Eq. (11) in Eq. (13), and thus the fractional mass transfer resistances to the overall mass transfer process can be determined. Furthermore, if the extraction reaction resistance (ker) can be neglected, we can use kl, km and ksh to predict the theoretical overall mass transfer coefficient, Ktheo. 1 1 1 Hs 1 ¼ þ þ K theo ksh H f km H f kl
Property
Description
Model no. Material Number of fibers Active surface area Module length Hollow fiber effective length Module diameter Inner diameter of hollow fiber Outer diameter of hollow fiber Tortuosity Porosity
G 542 Polypropylene 7400 0.58 m2 18.1 cm 12.2 cm 4.25 cm 220 μm 300 μm 2.6 40%
ð18Þ
4. Experimental 4.1. Chemicals Cephalexin monohydrate of BP USP grade was purchased from MP Biomedicals, Solon, OH, USA. Sodium carbonate, sodium bicarbonate, citric acid, sodium citrate, and potassium chloride were acquired from Fisher Scientific, Pittsburgh, PA, USA. All of them were ACS reagent grade. Aliquat 336 was kindly donated by Cogins Corporation, Tucson, AZ, USA. Isopar L (isoparrafin hydrocarbon solvent) was purchased from ExxonMobil Chemical Company, Houston, TX, USA. 1-Decanol was bought from Sigma-Aldrich, Milwaukee, WI, USA. All chemicals were used as received without further purification. 4.2. Apparatus and operation of SLM-OD The schematic of the SLM-OD set-up with a hollow-fiber module in the recycle mode is illustrated in Fig. 2. By using digitally controlled peristaltic pumps (Masterflex 7524-40 from Cole-Parmer, Vernon Hills, IL, USA), the organic dispersion and the aqueous strip solution were pumped through the shell and lumen sides of the commercial HFM in the recycling mode, respectively (Table 1 lists the properties of the HFM). The flow rates of the organic dispersion and the strip solution were 200–400 ml/min. Prior to conducting the SLM-OD, the strip solution was pumped first with maintaining a high inlet pressure (34.5 kPa, 5 psi) to prevent the penetration of the organic phase from the shell side to the lumen side of the HFM. Then, a pre-wetting process, recycling a certain amount of organic solution on the shell side of the HFM for at least 10 min, was performed to assure that the
hydrophobic hollow fibers were fully wetted by the organic membrane solution. The aqueous feed phase was then poured into the organic dispersion vessel at the beginning time of the SLM-OD experiment, followed by introducing a pre-determined amount of the organic membrane solution in the same vessel. An overhead mixer (threebladed impeller with 5.0 in. diameter, Model BDC2002, Caframo, Wiarton, Ontario) was used to disperse the organic membrane solution into the continuous feed aqueous solution. All of the experiments were performed at ambient temperature. Each of the data reported represents the average from at least three independent experiments, and the mean relative error is less than 5%. 4.3. Preparation of solutions Feed solutions were prepared by mixing Cephalexin monohydrate with 0.05 M carbonate-bicarbonate buffer solution and adjusted to pH 8.0. The pH values were measured by a digital pH meter (Model Accumet AB15, Fisher Scientific, Pittsburgh, PA). Organic membrane solutions contained 1.25, 2.5 and 5 wt% of Aliquat 336 and 0.625, 1.25 and 2.5 wt% of 1-decanol in Isopar L. Aqueous strip solutions were prepared by dissolving KCl in 500 ml of 0.1 M citric acid–citrate buffer solution; the KCl concentrations investigated were from 0 to 3 M. For Cephalexin concentration analysis, the dispersion samples were centrifuged at 5000 rpm for 2 min, and then the aqueous samples were taken from the bottom portion. Cephalexin concentrations in the feed and strip phases were determined by using a calibrated UVspectrophotometer with the wavelength of 262 nm (Model UV-1700, Shimadzu, Columbia, MD, USA) [18]. A cleaning procedure for the hollow-fiber module was conducted after each experiment using 600 ml ethanol–deionized water and isopropyl alcohol–deionized water in order with the 50/50 volume ratio of alcohol and water as
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the cleaning agent. Afterwards, the module was flushed using the deionized water and then dried for a minimum of 12 h with sweeping dry filtered air.
5
-7
-1
Kexp (10 m s )
4
5. Results and discussion 5.1. Effect of organic-to-feed volume ratio (holdup)
2
Cephalexin Conc. (mM)
8
Cf
Cs Vo/f = 80:500
Cf
Cs Vo/f = 105:500
Cf
Cs Vo/f = 120:500
Cf
Cs Vo/f = 250:500
6 4 2 0
0
50
100 Time (min)
150
200
Fig. 3. Cephalexin concentrations in the feed and strip phases vs. time with varying organic-to-feed volume ratio (Vo/f ml/ml) (C0f ¼ 8 mM; C0QCl ¼ 2.5 wt%; Vs ¼500 ml, CKCl ¼1 M, Qod ¼ 200 ml/min, Qs ¼ 400 ml/min).
1 0.0
0.2 0.4 Organic-to-feed volume ratio (v:v)
0.6
Fig. 4. The values of Kexp vs. organic-to-feed volume ratio (C0f ¼ 8 mM; C0QCl ¼2.5 wt%; Vs ¼500 ml, CKCl ¼ 1 M, Qod ¼ 200 ml/min, Qs ¼ 400 ml/min).
10
Cephalexin Conc. (mM)
Due to the specific multiphase flow behavior, the SLM-OD technique shows different mass transfer characteristics compared with other traditional SLM techniques. In this technique, the organic-to-feed volume ratio (holdup) is an important parameter which determines the special hydrodynamics and thus influences the mass transfer performance. Various organic-to-feed volume ratios, including 80:500, 105:500, 125:500 and 250:500 (ml:ml), were applied to investigate the effect of the volume ratio on the mass transfer process. Fig. 3 shows the Cephalexin concentrations in the feed and strip phases vs. time with varying the organic-to-feed volume ratio. As shown in this figure, an up-hill concentration phenomenon occurred after 80 min, i.e., the Cephalexin concentration in the strip solution was higher than that in the feed solution. It meant that the SLM-OD technique inherits the basic advantage of traditional SLMs – simultaneous extraction and stripping to maximize the driving force across the membrane. On the other hand, in the feed phase, a slight solvent extraction phenomenon was observed in the SLM-OD in the beginning period up to about 60 min, because some Cephalexin was extracted by the dispersed organic membrane solution. By increasing the organicto-feed volume ratio, this phenomenon became more significant, while the stripping rates were slightly reduced. Fig. 4 illustrates the Kexp results obtained as a function of organicto-feed volume ratio. An increase of the organic-to-feed volume ratio slightly reduced the Kexp. This phenomenon is interesting. As we know, increasing the organic-to-feed volume ratio indicated that more organic solution could be dispersed as droplets, by which the mass transfer area could be significantly enlarged. However, the increase of the volume of the organic membrane solution retained more Cephalexin. The driving force of mass transfer on the feed side and in the organic droplets to go across the supported organic liquid membrane was therefore reduced, i.e., not favorable for the mass transfer performance. In addition, in consideration of minimizing the volume of organic membrane solution and maximizing the stripping rate, the feed-to-organic volume ratio at 80:500, i.e., 80 ml of the feed solution with respect to 500 ml of the organic phase, was selected for the subsequent experiments.
10
3
8
0
Cf
Cs
CQCl = 1.25 wt%
Cf
Cs
CQCl = 2.5 wt%
Cf
Cs
CQCl = 5 wt%
0 0
6 4 2 0
0
50
100 Time (min)
150
200
Fig. 5. Cephalexin concentrations in the feed and strip phases vs. time with varying Aliquat 336 concentration (C0f ¼8 mM, Vf ¼ 500 ml; Vo ¼ 80 ml; Vs ¼ 500 ml, CKCl ¼1 M, Qod ¼200 ml/min, Qs ¼ 400 ml/min).
5.2. Effect of Aliquat 336 concentration The effect of Aliquat 336 concentration on the mass transfer performance is shown in Fig. 5. When a lower carrier concentration was used (1.25 wt%), the Cephalexin concentration profile of the feed phase was similar with that for traditional SLM techniques. This indicated that the effect of solvent extraction was not significant, which could be attributed to the low extraction capacity for a small amount of the organic membrane solution in the organic dispersion. When an organic membrane solution with a high Aliquat 336 concentration, i.e., 2.5 wt%, was used, a considerable amount of Cephalexin could be transported to the organic droplets, corresponding to a significant solvent extraction phenomenon. On the other hand, with an increase of Aliquat 336 concentration from 1.25 wt% to 2.5 wt%, the stripping rate was also enhanced. This result could be attributed to the high driving force of Cephalexin–Aliquat 336 complex, i.e., the high concentration of the Cephalexin–Aliquat complex at the feed/organic interface induced by the high extraction capacity, resulting in the high mass transfer across the supported liquid membrane. However, the enhancement of the stripping rate was not obvious when the Aliquat 336 concentration was further increased to 5 wt%. These results indicated that 2.5 wt% of Aliquat 336 concentration was sufficient for the liquid membrane system studied here. Fig. 6 depicts the Kexp values obtained as a function of Aliquat 336 concentration. The Kexp slightly increased as the Aliquat 336 concentration increased from 1.25 wt% to 2.5 wt%, which could be attributed to the enhanced extraction capacity. But with further increasing the Aliquat 336 concentration to 5 wt%, the Kexp was almost the same as
Kexp in this work Ktheo in this work Kexp in ref [18]
3
2
2 4 Aliquat 336 concentration (wt%)
0.8
that for 2.5 wt% of carrier concentration. This result indicated that the extraction capacity provided by 2.5 wt% of Aliquat 336 was sufficient for 8 mM of Cephalexin solution. Also depicted in Fig. 6, the agreement between Kexp and Ktheo was very good for all of the three carrier concentrations tested. This good agreement demonstrated that the extraction reaction resistance was not significant in the SLM-OD technique. In addition, the value of Kexp of the SLM-OD was 3.56 10–7 m/s when the organic-to-feed volume ratio was 80:500, which was higher than that of SLM-SD (2.72 10– 7 m/s) when a large organic-to-feed volume ratio (600:300) was applied [18]. Therefore, the SLM-OD was shown to be superior to the SLM-SD in terms of the improvement of mass transfer performance and the reduced volume requirement of the organic membrane solution. On the other hand, the slight deviation of Kexp and Ktheo could be attributed to the deficiency of the shell side mass transfer correlation (Eq. (16)) for the SLM-OD system under consideration. For the SLM-OD system, the flow pattern in the shell side of the hollow-fiber module is liquid–liquid two-phase dispersed flow. The mass transfer correlation of this flow pattern on the shell side of a hollow-fiber module has not yet been reported in the literature to the best of our knowledge. 5.3. Effect of initial Cephalexin concentration in the feed phase Fig. 7 shows the dimensionless Cephalexin concentration profiles of the feed and strip phases with varying the initial Cephalexin concentrations of 5, 8 and 15 mM. The starting time of the up-hill mass transfer phenomenon increased with an increase of initial Cephalexin concentration. Because the extraction and stripping capacities were fixed, the feed solution with a higher initial Cephalexin concentration required more time for Cephalexin to be removed and recovered. In addition, it could also be found that when 15 mM Cephalexin was used, the dimensionless concentration profile of the feed phase was linearly reduced, indicating that the removal and recovery rates were constant. Fig. 8 illustrates the Kexp values obtained as a function of initial Cephalexin concentration. The Kexp significantly reduced with the increase of initial Cephalexin concentration, and a similar conclusion was also reported in the literature [14,18]. These results suggested that when the initial Cephalexin concentration was high, a suitable Aliquat 336 concentration should be applied to improve the removal efficiency. In this work, 2.5 wt% of Aliquat 336 concentration was selected for the rest of the experiments reported in this work. 5.4. Effect of KCl concentration in the strip phase As an ion-exchange type mass transfer process, a counter-ion, i.e., an anion, should be provide in the aqueous strip phase to facilitate the
0
Cf
Cs
Cf = 5 mM
Cf
Cs
Cf = 8 mM
Cf
Cs
Cf = 15 mM
0 0
0.4 0.2 0.0
6
Fig. 6. The values of Kexp and Ktheo vs. Aliquat 336 concentration (C0f ¼8 mM; Vs ¼ 500 ml, CKCl ¼ 1 M, Qod ¼200 ml/min, Qs ¼400 ml/min).
95
0.6
0
50
100 Time (min)
150
200
Fig. 7. Dimensionless Cephalexin concentrations in the feed and strip phases vs. time with varying initial Cephalexin concentration (Vf ¼ 500 ml; C0QCl ¼2.5 wt%, Vo ¼ 80 ml; Vs ¼ 500 ml, CKCl ¼ 1 M, Qod ¼ 200 ml/min, Qs ¼ 400 ml/min).
7 6 -1
0
1.0
5
-7
1
4
Kexp (10 m s )
4
-7
-1
Kexp or Ktheo(10 m s )
5
Dimensionless Cephalexin Conc.
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
3 2 1 4
6 8 10 12 14 16 Initail Cephalexin concentration (mM)
Fig. 8. The values of Kexp vs. initial Cephalexin concentration (Vf ¼ 500 ml; C0QCl ¼ 2.5 wt%, Vo ¼ 80 ml; Vs ¼500 ml, CKCl ¼1 M, Qod ¼ 200 ml/min, Qs ¼400 ml/min).
transport of the anionic Cephalexin. KCl has been proved to be a great choice [1]. The effect of KCl concentration on the mass transfer performance is shown in Fig. 9. The up-hill mass transfer phenomenon can even be found even with the absence of KCl in the strip phase. This result indicated that the ion-exchange mass transfer process was achieved due to the transfer of citric acid ions used as the buffer in the strip solution. At the same time, however, the transport of citric acid ions impaired the buffer capability and hence resulted in a significant change of pH [18]. On the other hand, an excess amount of KCl (3 M) did not bring about an enhancement of the mass transfer, because the driving force provided by 1 M Cl– was sufficient; this concentration was applied in the rest of the experiments reported in this study. 5.5. Effect of lumen side and shell side flow rates Fig. 10 displays the profiles of Cephalexin concentration in the feed and strip phases by varying the lumen-side and shell-side flow rates. When the shell-side (organic dispersion) flow rate was fixed at 200 ml/min, by increasing the lumen-side flow rate of the stripping solution from 200 ml/min to 400 ml/min, the mass transfer performance was not significantly influenced. A slightly lower mass transfer rate for the lumen-side (stripping solution) flow rate of 400 ml/min could be attributed to the shorter retention time within the first hour, in which a smaller amount of Cephalexin was transferred to the strip phase. The shorter retention time decreased the contact time between the organic dispersion and the liquid membrane. However, because the total contact time (3 h) was kept consistent, the final amount of
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Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
Cephalexin transferred to the strip phase was approximately the same for all of the three cases shown in this figure. On the other hand, with varying the shell-side (organic dispersion) flow rate from 200 ml/min to 400 ml/min while keeping the lumen-side (stripping solution) flow rate constant at 200 ml/min, also displayed in this figure, the mass transfer performance was not significantly influenced either. This also indicated that the mass transfer resistance of the strip side was insignificant. 5.6. The fractional mass transfer resistances to the overall process For the SLM-OD system under consideration with the overall mass transfer coefficients illustrated in Fig. 4, the fractional mass transfer resistances with varying the organic-to-feed volume ratio were determined from Eqs. (14) to (16) for the lumen side, membrane phase and
Cephalexin Conc. (mM)
10 8
Cf
Cs
CKCl = 0 M
Cf
Cs
CKCl = 1 M
Cf
Cs
CKCl = 3 M
6 4 2 0
0
50
100 Time (min)
150
6. Conclusions
200
Fig. 9. Cephalexin concentrations in the feed and strip phases vs. time with varying KCl concentration (C0f ¼ 8 mM, Vf ¼ 500 ml; C0QCl ¼2.5 wt%, Vo ¼80 ml; Vs ¼500 ml, Qod ¼200 ml/min, Qs ¼400 ml/min).
Cephalexin Conc. (mM)
10 8
Cf
Cs
Qod = 200 ml/min Qs = 200 ml/min
Cf
Cs
Qod = 200 ml/min Qs = 400 ml/min
Cf
Cs
Qod = 400 ml/min Qs = 200 ml/min
6 4 2 0
0
50
100 Time (min)
150
shell side, respectively and Eq. (13) for the extraction reaction. The results are illustrated in Table 2. As illustrated in this table, the fractional mass transfer resistance for the shell side was dominant, accounting for more than 76% of the total resistance. The shell-side fractional mass transfer resistance reduced as the organic-to-feed volume ratio increased. The fractional mass transfer resistances for the lumen side and the supported liquid membrane were relatively small and constant, less than 6.25% of the total resistance. Thus, any reduction in the shell-side fractional mass transfer resistance would give an increase on the fractional mass transfer resistance of the extraction reaction. Also illustrated in this table, the fractional mass transfer resistance of the extraction reaction increased from 0.43% to about 18.3% as the organic-to-feed volume ratio increased. These values were much lower than the fractional mass transfer resistance of the extraction reaction reported for SLM-SD, which was larger than 95% [18]. The results indicated that the extraction reaction resistance was greatly reduced due to the organic dispersion in the SLM-OD. These results were consistent with the finding described earlier that the SLM-OD was superior to the SLM-SD in terms of the improvement of mass transfer performance. As illustrated in this table and described earlier, the mass transfer resistance for the shell side was dominant and larger than 76%, and it increased as the organic-to-feed volume ratio reduced. When the organic-to-feed volume ratio was 0.16, i.e., 80:500, the mass transfer resistance for the shell side was even higher than 90%. These results suggested that a proper approach, which can decrease the shell side resistance, should be investigated for the SLM-OD in the future.
200
Fig. 10. Cephalexin concentrations in the feed and strip phases vs. time with varying the flow rates on the shell and lumen sides (C0f ¼8 mM, Vf ¼ 500 ml; C0QCl ¼ 2.5 wt%, Vo ¼ 80 ml; Vs ¼500 ml, CKCl ¼1 M).
Supported liquid membranes with organic dispersion (SLM-OD) have been proposed and applied successfully to extract and recover Cephalexin from aqueous solutions. For the SLM-OD, a mathematical model was developed to determine the experimental and theoretical overall mass transfer coefficients. The effects of various operating parameters on the mass transfer process were studied. The results showed that the optimal operating parameters for the removal and recovery of 8 mM Cephalexin were 2.5 wt% of Aliquat 336 concentration, 1 M of KCl concentration, and 80:500 of organic-to-feed volume ratio. In this condition, the mass transfer coefficient was 3.56 10–7 m/s, which was 30% higher than that of the SLM-SD technique. The flow rates on the lumen and shell sides did not affect the mass transfer process significantly. The calculated overall mass transfer coefficients compared very well with those obtained experimentally, implying that the mass transfer resistance due to extraction reaction was not significant. To verify this conclusion, the fractional mass transfer resistances for the SLM-OD process were determined. The results demonstrated that the extraction reaction resistance was greatly reduced, amounting to less than 18.3% of the total resistance. This value was much lower than the extraction reaction resistance for SLM-SD (larger than 95%). In addition to the mass transfer performance advantage, the SLM-OD was shown to be superior to the SLM-SD in terms of the reduced volume requirement of the organic membrane solution.
Table 2 The fractional mass transfer resistances to the overall process. Organic-to-feed volume ratio
80:500 105:500 125:500 250:500
Kexp (10 7 m/s)
3.56 3.33 3.29 2.92
Fractional mass transfer resistances (%) Δl
Δm
Δsh
Δer
0.43 0.40 0.40 0.35
6.24 5.84 5.77 5.12
92.90 87.05 85.96 76.23
0.43 6.70 7.87 18.29
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
Acknowledgments The authors are grateful to Cognis Corporation for free samples of the carrier Aliquat 336 used in this work. Partial funding to the first author by the Special Fund for Studying Abroad from the Beijing University of Chemical Technology is gratefully acknowledged. The authors would like to thank the National Natural Science Foundation of China for Grant no. CBET 0932511. Part of this material is based upon the work supported by the National Science Foundation under Grant no. CBET 0932511.
Nomenclature A C Ceph– d D H j k K Kexp Ktheo L n P Q Q ro Ri t tD tP ub V Vo/f
surface area, m2 Cephalexin concentration, mol m 3 Cephalexin diameter of the hollow fiber, m diffusion coefficient, m2 s 1 partition coefficient flux of Cephalexin, mol m 2 s 1 individual mass transfer coefficient, m s 1 overall mass transfer coefficient, m s 1 experimental overall mass transfer coefficient, m s1 theoretical overall mass transfer coefficient, m s 1 effective length of the hollow fiber, m number of hollow fibers pressure, Pa flow rate, ml/min (10 6 m3 min 1) Aliquat 336 outer radius of the hollow fiber, m inner radius of the HFM, m time, s diffusion time across the membrane, s process time, s bulk velocity in lumen side, m s 1 volume, m3 organic-to-feed volume ratio
Greek letters Δ δ ε Ф τ
percentage of individual mass transfer resistance membrane thickness, m membrane porosity packing density membrane tortuosity
Superscript 0
initial condition
Subscript a e er
aqueous phase hydraulic (diameter) of the shell side extraction reaction
f l lm m mi mo o od s sh
97
feed phase lumen side log-mean (diameter) membrane phase inner (diameter) of hollow fiber outer (diameter) of hollow fiber organic membrane phase organic dispersion strip phase shell side
References [1] M.E. Vilt, Supported liquid membranes with strip dispersion for recovery of Cephalexin (Ph.D. dissertation), The Ohio State University, 2010. [2] A. Bruggink, Synthesis of β-Lactam Antibiotics, Academic Publishers, The Netherlands, 2001. [3] R. Sheldon, F. van Rantwijk, Biocatalysis for sustainable organic synthesis, Aust. J. Chem. 57 (2004) 281–289. [4] M.E. Vilt, W.S.W. Ho, in situ removal of Cephalexin by supported liquid membrane with strip dispersion, J. Membr. Sci. 367 (2011) 71–77. [5] R. Schoevaart, T. Kieboom, Combined catalytic reactions—nature's way, Chem. Innov. 31 (2011) 33–39. [6] N.N. Li, Separating hydrocarbons with liquid membrane. U.S. Patent 3,410,794, 1968. [7] A.M. Sastre, A. Kumar, J.P. Shukla, R.K. Singh, Improved techniques in liquid membrane separations: an overview, Sep. Purif. Rev. 27 (1998) 213–298. [8] R. Bloch, A. Finkelstein, Metal ion separation by dialysis through solvent membrane, Ind. Eng. Chem. Process Des. Dev. 6 (1967) 231–237. [9] W.S.W. Ho, K.K. Sirkar (Eds.), Chapman & Hall, New York; Kluwer Academic Publishers, Boston, 1992 (reprint edition, 2001). [10] G.C. Sahoo, A.C. Ghosh, N.N. Dutta, Recovery of Cephalexin from dilute solution in a bulk liquid membrane, Process Biochem. 32 (1997) 265–272. [11] G.C. Sahoo, N.N. Dutta, Studies on emulsion liquid membrane extraction of Cephalexin, J. Membr. Sci. 145 (1998) 15–26. [12] A.J.B. Kemperman, D. Bargeman, T. van den Boomgaard, H. Strathmann, Stability of supported liquid membranes state of the art, Sep. Sci. Technol. 31 (1996) 2733–2762. [13] A. Sengupta, R. Basu, K.K. Sirkar, Separation of solutes from aqueous solutions by contained liquid membranes, AIChE J. 34 (1988) 1698–1708. [14] Z.Q. Ren, W.D. Zhang, Y.M. Liu, Y. Dai, C.H. Cui, New liquid membrane technology for simultaneous extraction and stripping of copper(II) from wastewater, Chem. Eng. Sci. 62 (2007) 6090–6101. [15] W.S.W. Ho, T.K. Poddar, New membrane technology for removal and recovery of chromium from waste waters, Environ. Prog. 20 (2001) 44–52. [16] W.S.W. Ho, Removal and recovery of metals and other materials by supported liquid membranes with strip dispersion, Ann. N.Y. Acad. Sci. 984 (2003) 97–122. [17] W.S.W. Ho, B. Wang, Strontium removal by new alkyl phenylphosphonic acids in supported liquid membranes with strip dispersion, Ind. Eng. Chem. Res. 41 (2002) 381–388. [18] M.E. Vilt, W.S.W. Ho, Supported liquid membranes with strip dispersion for the recovery of Cephalexin, J. Membr. Sci. 342 (2009) 80–87. [19] O.Y. Li, X.L. Su, D.S. He, Y.Y. Chen, M. Ma, Q.J. Xie, S.Z. Yao, A study on separation and extraction of four main alkaloids in Macleaya cordata (wild) R. Br. with strip dispersion hybrid liquid membrane, J. Sep. Sci. 33 (2010) 2026–2034. [20] M.E. Vilt, W.S.W. Ho, Selective separation of Cephalexin from multiple component mixtures, Ind. Eng. Chem. Res. 49 (2010) 12022–12030. [21] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, 4th ed., McGraw-Hill Book Company, New York, 1987. [22] E.L. Cussler, Diffusion: Mass Transfer in Fluid Systems, 3rd ed., Cambridge University Press, Cambridge, UK, 2009. [23] I.M. Coelhoso, J.P.S.G. Crespo, M.J.T. Carrondo, Kinetics of liquid membrane extraction in systems with variable distribution coefficient, J. Membr. Sci. 127 (1997) 141–152. [24] M.A. Leveque, Les Lois de la transmission de Chaleur par convection, Ann. Mines 13 (1928) 201–299. [25] R. Prasad, K.K. Sirkar, Dispersion-free solvent extraction with micro-porous hollow fiber modules, AIChE J. 34 (1988) 177–188. [26] C.R. Wilke, P. Chang, Correlation of diffusion coefficients in dilute solutions, AIChE J. 1 (1955) 264–270.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Supported liquid membranes with organic dispersion for recovery of Cephalexin Zisu Hao a,b, Zihao Wang b, Weidong Zhang b, W.S. Winston Ho a,c,n a
William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, OH 43210-1180, USA Beijing Key Laboratory of Membrane Science and Technology, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China c Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1178, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 13 February 2014 Received in revised form 27 May 2014 Accepted 29 May 2014 Available online 4 June 2014
Supported liquid membranes with organic dispersion (SLM-OD) have been proposed and investigated for simultaneous removal and recovery of Cephalexin from aqueous solutions by using the carrier Aliquat 336 and a commercially available hollow-fiber module (HFM). The organic dispersion, formed by dispersing a small amount of organic membrane solution in the feed solution with a mixer, flowed through the shell side of the HFM. Investigated various parameters including organic-to-feed volume ratio, Aliquat 336 concentration, initial Cephalexin concentration, KCl concentration, shell-side organic dispersion flow rate, and lumen-side strip solution flow rate. The mass transfer mechanism of the SLMOD technique was elucidated, and a mathematical model was developed to calculate the overall mass transfer coefficient. The results showed that the overall mass transfer coefficient increased with an increase of organic-to-feed volume ratio or increase of Aliquat 336 concentration, but reduced with the increase of initial Cephalexin concentration. Furthermore, the results indicated that the lumen and shell side flow rates had little effects on mass transfer performance and that an excess amount of KCl was necessary for the facilitated transport. The fractional mass transfer resistances of SLM-OD were calculated based on the resistance-in-series model, showing that the extraction reaction resistance was greatly reduced. In addition, the SLM-OD was shown to be superior to the supported liquid membranes with strip dispersion (SLM-SD) in terms of the improvement of mass transfer performance and the reduced volume requirement of the organic membrane solution. & 2014 Elsevier B.V. All rights reserved.
Keywords: Supported liquid membrane Organic dispersion Cephalexin Aliquat 336 Mass transfer model
1. Introduction Cephalexin is a first-generation cephalosporin antibiotic, and it has been widely used to treat various infections due to its broad antibacterial activity [1]. The traditional production method is a 10step chemical synthesis process which suffers from significant energy consumption, many side reactions [2] and introducing toxic solvent [3]. To overcome these shortcomings, an enzymatic synthetic method [4] has been developed and proved to be an efficient alternative for the chemical synthesis method [5]. However, the commercial application of the enzymatic method to produce Cephalexin has not been realized because of the low yield and difficulty to separate (purify) Cephalexin from product solution.
n Corresponding author at: William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, OH 43210-1180, USA. Tel.: þ 1 614 292 9970; fax: þ 1 614 292 3769. E-mail address: [email protected] (W.S. Winston Ho).
http://dx.doi.org/10.1016/j.memsci.2014.05.052 0376-7388/& 2014 Elsevier B.V. All rights reserved.
Liquid membrane (LM) is a highly integrated separation technique which combines extraction and stripping processes in one step. Since LM was firstly invented by Li in 1968 [6], various LM techniques have been proposed and extensively investigated, including bulk liquid membrane (BLM) [7], emulsion liquid membrane (ELM) [6], and supported liquid membrane (SLM) [8]. The advantages of LMs include the ease of scale-up, high selectivity, low consumption of extractant and no flooding. Therefore, LM is considered as a promising separation method for wastewater treatment, chemical and biochemical processing, and pharmaceutical and food processes [9]. LM has been proved to be an attractive technique for the separation and recovery of Cephalexin [10,11]. Sahoo et al. [10] reported the facilitated transport of Cephalexin using Aliquat 336 as the extractant in a BLM. Then, Sahoo and Dutta [11] proved that Cephalexin can be successfully separated from an aqueous solution containing 7-amino3-desacetoxicephalosporanic acid (7-ADCA) by means of an ELM system. However, traditional LM techniques have some intrinsic shortcomings, which hinder their commercial application [12].
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
In recent decades, some novel LM techniques have been proposed that offers long-term stability such as hollow fiber contained liquid membrane (HFCLM) [13], hollow fiber renewal liquid membrane (HFRLM) [14], and supported liquid membrane with strip dispersion (SLM-SD) [15]. Among them, SLM-SD has shown effective removal and recovery capabilities and has been applied in wastewater treatment [15–17] as well as separation and purification of valued organic compounds [18,19]. Vilt and Ho firstly used the SLM-SD technique to recover and concentrate Cephalexin from a dilute solution. The results showed that the recovery rate of Cephalexin was around 96–98% with the enrichment ratio ranging from 1.6 to 3.3 [18]. In subsequent work, they developed an enzymatic synthesis process coupled with the SLMSD technique for in-situ selective removal and recovery Cephalexin using a commercially available hollow fiber module (HFM). It was found that the combination of SLM-SD and enzymatic complexation enhanced the maximum yield of Cephalexin from 32% to 42% without an enzyme deactivation problem [4]. Besides the investigation of removal efficiency and enrichment performance, the analysis of mass transfer mechanism of LM is also necessary for the future industrialization. In our previous work, the distribution of mass transfer resistances of SLM-SD for the removal and recovery of Cephalexin was estimated, and the results indicated that the extraction reaction resistance, i.e., the complexation reaction of Cephalexin and Aliquat 336, was dominant [18]. In order to decrease this resistance, a possible approach is to enlarge the mass transfer surface area between the feed and organic phases. As this mass transfer surface area in SLM-SD locates on the inner surface of hollow fibers, this would suggest that this area can be augmented by increasing the number of hollow fibers. But this would enlarge the size or number of hollow-fiber membrane modules and the amount of organic phase, resulting in an increase of the cost. In addition, from the environmental and economic perspectives, the amount of organic phase should be limited. Actually, in the solvent extraction and HFRLM techniques, the mass transfer area can be greatly increased by dispersing the organic solution in the aqueous solution. In a HFRLM technique, the dispersion flowed through the lumen side of HFM, and the organic-to-aqueous volume ratio was normally less than 1:10, which enhanced the mass transfer performance [14]. In this work, the SLM with organic dispersion (SLM-OD) technique was proposed and demonstrated for the removal and recovery of Cephalexin from its dilute aqueous solutions. Organic dispersion, formed by dispersing the organic membrane solution in the feed solution with a mixer, flowed through the shell side of a HFM. A series of important experimental parameters were investigated, including the organic-to-feed volume ratio, Aliquat 336 concentration, initial Cephalexin concentration, KCl concentration, and lumen- and shell-side flow rates. This allowed for identifying the critical parameters. A mathematical model was developed to determine the overall mass transfer coefficient of SLM-OD. Furthermore, according to the resistance-in-series model, the distribution of mass transfer resistances of the SLM-OD system was calculated. This showed a significant reduction of the mass transfer resistance due to the extraction reaction for the SLM-OD vs. the supported liquid membranes with strip dispersion (SLM-SD). In addition, the SLM-OD was shown to be superior to the SLM-SD in terms of the reduced volume requirement of the organic membrane solution.
Strip Solution Out
Dispersed Droplets of Organic Solution
Pf Continuous Feed Phase
Hollow Fiber Pores Filled with Organic
Hollow Fiber Wall Ps Strip Solution In
Fig. 1. A schematic diagram of an enlarged view of the SLM-organic dispersion (SLM-OD) process.
microporous hydrophobic polymer hollow-fibers should be used to obtain a large mass transfer area. In a HFM, the dispersion phase can flow on the shell side while the aqueous strip phase can be on the lumen-side. In order to prevent any sipping of the organic membrane solution, the pressure of the feed phase (Pf) containing the organic dispersion and the aqueous strip phase pressure (Ps) should be approximately the same (about 34.5 kPa, 5 psi). In this work, the organic dispersion was pumped through the shell side of the HFM.
3. Theory 3.1. General transport mechanism In SLM-OD, the organic membrane solution is dispersed in the continuous aqueous feed solution. The multiphase flow behavior is different from that of SLM-SD, suggesting that SLM-OD may have a different transport mechanism, which is described as follows:
Solvent extraction phenomenon in the organic dispersion vessel
2. The SLM-OD technique The SLM-OD technique is illustrated schematically in Fig. 1, in which a small amount of organic membrane solution is dispersed as droplets in the continuous aqueous feed solution by a mixer, while in the SLM-SD technique, the dispersed phase is the aqueous strip solution. Typically, a hollow-fiber module (HFM) containing
91
Due to the direct mixing of the feed and organic phases, a small amount of target species in the feed phase can be quickly extracted into the organic droplets, which are a small fraction in the dispersion. Mass transfer on the shell side of the module On the shell side of HFM, the dispersed organic droplets and the outer surface of the hollow fibers provide a large mass transfer area at the feed/organic interface. There are three types of transport processes. a. At the feed/organic interface located on the outside diameter of each hollow fiber, the target species reacts with the carrier to form the solute–carrier complex and is partitioned into the organic phase, i.e., the supported liquid membrane phase. b. At the feed/organic interface located on the surface of each organic droplet, the target species reacts with the carrier to form the solute–carrier complex in the organic droplet. Due to the small size of each organic droplet in the dispersion, the concentration profile of the solute–carrier complex in the organic droplets is assumed to be uniform. The pumping flow action enhances the fulfillment of this assumption. c. The solute-loaded droplets may collide with the hollow fibers due to the effect of the flow field. In this process, the complex in the droplets may be directly transported to the feed/organic interface located on the outside diameter of hollow fibers. This process may bring about the mass transfer intensification. Mass transfer in the membrane supported in pores Because the microporous hydrophobic polymer hollow-fibers have been wetted by the organic membrane phase, the complex can then diffuse across the membrane supported in the porous wall of each hollow fiber.
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Mass transfer from membrane/strip interface to bulk strip phase At the organic/strip interface, the decomplexation reaction occurs while the target species is partitioned into the aqueous strip phase and the carrier is regenerated. Then, the regenerated carrier diffuses back to the feed side for the next facilitated transport, and the target species can be convectively transported to the bulk strip phase.
(Cod) and strip phase (Cs) [22]. V od
dC od ðtÞ dC s ðtÞ ¼ jNεπdlm L ¼ V s dt dt
where j is the flux of Cephalexin in the pseudo-steady state, and Cod is the Cephalexin concentration in the organic dispersion at time t, which consists of Cephalexin in the feed and organic phases, as shown in Eq. (6), Vod being the volume of organic dispersion. C od ðtÞ ¼
3.2. Cephalexin–Aliquat 336–KCl system Cephalexin is a zwitterionic molecule, and the ionic state of Cephalexin is anionic or cationic when the pH value is higher than 6.88 or below 2.56, respectively. Aliquat 336, a quaternary ammonium chloride, was chosen as the extractant for the ion-exchange extraction used in the process. Detailed information about the Cephalexin–Aliquat 336 system can be found elsewhere [4,18,20]. The extraction and stripping reactions of Cephalexin–Aliquat 336 (QCl)–KCl system are shown in Eqs. (1) and (2), respectively:
ð1Þ
ð2Þ
Cephf þQClo 2QCepho þ Clf
QCepho þCls 2QClo þCephs
Generally, an excess amount of Cl‾ is provided to obtain a high stripping rate. When the rate of the stripping reaction is very fast, its contribution to overall mass transfer resistance can be neglected [4,18,20]. 3.3. Modeling of SLM-OD for the Cephalexin–Aliquat 336 system The SLM-OD technique is one kind of dispersive liquid membrane, which has a different mass transfer mechanism from traditional SLM processes. Thus, for the future industrial application of SLM-OD, it is necessary to develop a mathematical model to evaluate its mass transfer performance. In this study, due to a relatively large volume of the organic dispersion in comparison with the organic liquid membrane phase supported in the pore volume of the hollow fibers, the amount of Cephalexin in the organic dispersion on the shell side is much larger than that in the supported organic liquid membrane. It suggests that the mass transfer from the organic dispersion to go across the membrane can be considered at the pseudo-steady state. To verify this assumption, we can compare the diffusion time across the membrane t D with the process time t P . The value of t D in this work can be obtained by tD ¼
ðδτÞ2 ½ð300 220Þ=2 10 6 2:62 m2 ¼ ¼ 27:64 s Dm m2 s 1 3:913 10 10
ð3Þ
where Dm is the complex diffusion coefficient in the membrane, which was calculated by using the Haydauk and Minhaus equation, and the value was 3.913 10 10 m2 s 1 [21]. t P is approximately 3 h in each experiment, and the ratio of t D to t P is tD 27:64 s ¼ 2:56 10 3 ⪡1 ¼ t P 3600 3 s
ð4Þ
The value of this ratio is far smaller than 1, indicating that the SLM-OD can be considered under the pseudo-steady state. In the supported organic membrane phase, the amount of Cephalexin as in the Cephalexin–Aliquat 336 complex is negligibly small in comparison with that in the organic dispersion. The pseudo-steady state of the SLM-OD allows us to use the steady-state flux across the supported organic liquid membrane to relate to the Cephalexin concentrations in the organic dispersion
ð5Þ
Vo V C o ðtÞ þ f C f ðtÞ V od V od
ð6Þ
where Co is the Cephalexin–Aliquat 336 complex concentration in the dispersed organic phase, which can be calculated by the mass balance shown in Eq. (7). C o ðtÞ ¼
Vf 0 Vs ðC C f ðtÞÞ C s ðtÞ Vo f Vo
ð7Þ
Therefore, j, the flux of Cephalexin in the pseudo-steady state may be expressed by Hs j ¼ K exp C od ðtÞ C s ðtÞ ð8Þ Hf where Kexp is the experimental overall mass transfer coefficient; Hf and Hs are the partition coefficients for the organic dispersion and strip sides, respectively [22,23]. Hf and Hs are defined as Eqs. (9) and (10) Hf ¼
Co C nf
ð9Þ
Hs ¼
Co C ns
ð10Þ
where C nf and C ns are the feed and strip solute concentrations in equilibrium with the organic solute concentration, C o . By substituting Eq. (8) into Eq. (5) and integrating the resultant Eq. (5) [22] with respect to t with the initial conditions C od ð0Þ ¼ C 0od and Cs(0)¼ 0, the Kexp can be obtained as in Eq.(11). " # H f C 0od H s C 0s 1 : ð11Þ K exp ¼ ln H f C od ðtÞ H s C s ðtÞ βt where β is a constant, given by 1 1 Hs β ¼ LN επdlm þ : V od V s H f
ð12Þ
Thus, by linear regression of the experimental data using Eq. (11), Kexp for the SLM-OD system with various experimental conditions can be obtained. Used in Eq. (11), the values of Hf and Hs were 4.41 and 0.39, respectively, which were determined experimentally. 3.4. The fractional mass transfer resistances of the SLM-OD One of the important characteristics of a liquid membrane process is the distribution of the fractional mass transfer resistances. The resistance-in-series model has been extensively used to analyze the fractional mass transfer resistances of various liquid membrane techniques [14,18]. For the SLM-OD, the mass transfer process is also governed by this model. The overall mass transfer resistance of the SLM-OD consists of the following individual mass transfer resistances, including the diffusional resistance of complex in the small organic droplets, extraction reaction resistance, shell side resistance, membrane phase resistance, stripping reaction resistance and lumen side resistance. Because of the small size of the organic droplets and the flow action of the organic dispersion as discussed earlier, the diffusional resistance of Cephalexin–Aliquat 336 complex in the organic droplets can be neglected. Moreover, the stripping
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
93
reaction is instantaneous due to the very high stripping efficiency resulting from the excess stripping reagent used (KCl). Therefore, the overall mass transfer resistance can be expressed as [18] 1 1 1 1 Hs 1 ¼ þ þ þ K exp ksh ker H f km H f kl
ð13Þ
where ker is the mass transfer coefficient for the extraction reaction, kl, the mass transfer coefficient for the lumen side [24], km, the mass transfer coefficient for the membrane phase [15], and ksh, the mass transfer coefficient for the shell side [25]. The values of kl, km and ksh can be estimated by using the following equations: !1=3 2 kl dmi u d ¼ 1:62 b mi ð14Þ Da Da L km ¼
Dm εdlm δτdmo
ð15Þ
ksh de 1Φ Re0:6 Sc0:33 ¼ 5:85 de L Da de ¼
P
P
Hollow Fiber Module P
Strip
Organic
Solution
Dispersion
Fig. 2. Schematic of the SLM-OD set-up with a hollow-fiber module in the recycle mode.
ð16Þ Table 1 Properties of the hollow fiber module used.
2ðR2i nr 2o Þ Ri þnr o
ð17Þ 10
P
2
m /s, was obtained via the Wilke– The value of Da, 4.66 10 Chang equation [1,26]. ker can be calculated by substituting the Kexp obtained from Eq. (11) in Eq. (13), and thus the fractional mass transfer resistances to the overall mass transfer process can be determined. Furthermore, if the extraction reaction resistance (ker) can be neglected, we can use kl, km and ksh to predict the theoretical overall mass transfer coefficient, Ktheo. 1 1 1 Hs 1 ¼ þ þ K theo ksh H f km H f kl
Property
Description
Model no. Material Number of fibers Active surface area Module length Hollow fiber effective length Module diameter Inner diameter of hollow fiber Outer diameter of hollow fiber Tortuosity Porosity
G 542 Polypropylene 7400 0.58 m2 18.1 cm 12.2 cm 4.25 cm 220 μm 300 μm 2.6 40%
ð18Þ
4. Experimental 4.1. Chemicals Cephalexin monohydrate of BP USP grade was purchased from MP Biomedicals, Solon, OH, USA. Sodium carbonate, sodium bicarbonate, citric acid, sodium citrate, and potassium chloride were acquired from Fisher Scientific, Pittsburgh, PA, USA. All of them were ACS reagent grade. Aliquat 336 was kindly donated by Cogins Corporation, Tucson, AZ, USA. Isopar L (isoparrafin hydrocarbon solvent) was purchased from ExxonMobil Chemical Company, Houston, TX, USA. 1-Decanol was bought from Sigma-Aldrich, Milwaukee, WI, USA. All chemicals were used as received without further purification. 4.2. Apparatus and operation of SLM-OD The schematic of the SLM-OD set-up with a hollow-fiber module in the recycle mode is illustrated in Fig. 2. By using digitally controlled peristaltic pumps (Masterflex 7524-40 from Cole-Parmer, Vernon Hills, IL, USA), the organic dispersion and the aqueous strip solution were pumped through the shell and lumen sides of the commercial HFM in the recycling mode, respectively (Table 1 lists the properties of the HFM). The flow rates of the organic dispersion and the strip solution were 200–400 ml/min. Prior to conducting the SLM-OD, the strip solution was pumped first with maintaining a high inlet pressure (34.5 kPa, 5 psi) to prevent the penetration of the organic phase from the shell side to the lumen side of the HFM. Then, a pre-wetting process, recycling a certain amount of organic solution on the shell side of the HFM for at least 10 min, was performed to assure that the
hydrophobic hollow fibers were fully wetted by the organic membrane solution. The aqueous feed phase was then poured into the organic dispersion vessel at the beginning time of the SLM-OD experiment, followed by introducing a pre-determined amount of the organic membrane solution in the same vessel. An overhead mixer (threebladed impeller with 5.0 in. diameter, Model BDC2002, Caframo, Wiarton, Ontario) was used to disperse the organic membrane solution into the continuous feed aqueous solution. All of the experiments were performed at ambient temperature. Each of the data reported represents the average from at least three independent experiments, and the mean relative error is less than 5%. 4.3. Preparation of solutions Feed solutions were prepared by mixing Cephalexin monohydrate with 0.05 M carbonate-bicarbonate buffer solution and adjusted to pH 8.0. The pH values were measured by a digital pH meter (Model Accumet AB15, Fisher Scientific, Pittsburgh, PA). Organic membrane solutions contained 1.25, 2.5 and 5 wt% of Aliquat 336 and 0.625, 1.25 and 2.5 wt% of 1-decanol in Isopar L. Aqueous strip solutions were prepared by dissolving KCl in 500 ml of 0.1 M citric acid–citrate buffer solution; the KCl concentrations investigated were from 0 to 3 M. For Cephalexin concentration analysis, the dispersion samples were centrifuged at 5000 rpm for 2 min, and then the aqueous samples were taken from the bottom portion. Cephalexin concentrations in the feed and strip phases were determined by using a calibrated UVspectrophotometer with the wavelength of 262 nm (Model UV-1700, Shimadzu, Columbia, MD, USA) [18]. A cleaning procedure for the hollow-fiber module was conducted after each experiment using 600 ml ethanol–deionized water and isopropyl alcohol–deionized water in order with the 50/50 volume ratio of alcohol and water as
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Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
the cleaning agent. Afterwards, the module was flushed using the deionized water and then dried for a minimum of 12 h with sweeping dry filtered air.
5
-7
-1
Kexp (10 m s )
4
5. Results and discussion 5.1. Effect of organic-to-feed volume ratio (holdup)
2
Cephalexin Conc. (mM)
8
Cf
Cs Vo/f = 80:500
Cf
Cs Vo/f = 105:500
Cf
Cs Vo/f = 120:500
Cf
Cs Vo/f = 250:500
6 4 2 0
0
50
100 Time (min)
150
200
Fig. 3. Cephalexin concentrations in the feed and strip phases vs. time with varying organic-to-feed volume ratio (Vo/f ml/ml) (C0f ¼ 8 mM; C0QCl ¼ 2.5 wt%; Vs ¼500 ml, CKCl ¼1 M, Qod ¼ 200 ml/min, Qs ¼ 400 ml/min).
1 0.0
0.2 0.4 Organic-to-feed volume ratio (v:v)
0.6
Fig. 4. The values of Kexp vs. organic-to-feed volume ratio (C0f ¼ 8 mM; C0QCl ¼2.5 wt%; Vs ¼500 ml, CKCl ¼ 1 M, Qod ¼ 200 ml/min, Qs ¼ 400 ml/min).
10
Cephalexin Conc. (mM)
Due to the specific multiphase flow behavior, the SLM-OD technique shows different mass transfer characteristics compared with other traditional SLM techniques. In this technique, the organic-to-feed volume ratio (holdup) is an important parameter which determines the special hydrodynamics and thus influences the mass transfer performance. Various organic-to-feed volume ratios, including 80:500, 105:500, 125:500 and 250:500 (ml:ml), were applied to investigate the effect of the volume ratio on the mass transfer process. Fig. 3 shows the Cephalexin concentrations in the feed and strip phases vs. time with varying the organic-to-feed volume ratio. As shown in this figure, an up-hill concentration phenomenon occurred after 80 min, i.e., the Cephalexin concentration in the strip solution was higher than that in the feed solution. It meant that the SLM-OD technique inherits the basic advantage of traditional SLMs – simultaneous extraction and stripping to maximize the driving force across the membrane. On the other hand, in the feed phase, a slight solvent extraction phenomenon was observed in the SLM-OD in the beginning period up to about 60 min, because some Cephalexin was extracted by the dispersed organic membrane solution. By increasing the organicto-feed volume ratio, this phenomenon became more significant, while the stripping rates were slightly reduced. Fig. 4 illustrates the Kexp results obtained as a function of organicto-feed volume ratio. An increase of the organic-to-feed volume ratio slightly reduced the Kexp. This phenomenon is interesting. As we know, increasing the organic-to-feed volume ratio indicated that more organic solution could be dispersed as droplets, by which the mass transfer area could be significantly enlarged. However, the increase of the volume of the organic membrane solution retained more Cephalexin. The driving force of mass transfer on the feed side and in the organic droplets to go across the supported organic liquid membrane was therefore reduced, i.e., not favorable for the mass transfer performance. In addition, in consideration of minimizing the volume of organic membrane solution and maximizing the stripping rate, the feed-to-organic volume ratio at 80:500, i.e., 80 ml of the feed solution with respect to 500 ml of the organic phase, was selected for the subsequent experiments.
10
3
8
0
Cf
Cs
CQCl = 1.25 wt%
Cf
Cs
CQCl = 2.5 wt%
Cf
Cs
CQCl = 5 wt%
0 0
6 4 2 0
0
50
100 Time (min)
150
200
Fig. 5. Cephalexin concentrations in the feed and strip phases vs. time with varying Aliquat 336 concentration (C0f ¼8 mM, Vf ¼ 500 ml; Vo ¼ 80 ml; Vs ¼ 500 ml, CKCl ¼1 M, Qod ¼200 ml/min, Qs ¼ 400 ml/min).
5.2. Effect of Aliquat 336 concentration The effect of Aliquat 336 concentration on the mass transfer performance is shown in Fig. 5. When a lower carrier concentration was used (1.25 wt%), the Cephalexin concentration profile of the feed phase was similar with that for traditional SLM techniques. This indicated that the effect of solvent extraction was not significant, which could be attributed to the low extraction capacity for a small amount of the organic membrane solution in the organic dispersion. When an organic membrane solution with a high Aliquat 336 concentration, i.e., 2.5 wt%, was used, a considerable amount of Cephalexin could be transported to the organic droplets, corresponding to a significant solvent extraction phenomenon. On the other hand, with an increase of Aliquat 336 concentration from 1.25 wt% to 2.5 wt%, the stripping rate was also enhanced. This result could be attributed to the high driving force of Cephalexin–Aliquat 336 complex, i.e., the high concentration of the Cephalexin–Aliquat complex at the feed/organic interface induced by the high extraction capacity, resulting in the high mass transfer across the supported liquid membrane. However, the enhancement of the stripping rate was not obvious when the Aliquat 336 concentration was further increased to 5 wt%. These results indicated that 2.5 wt% of Aliquat 336 concentration was sufficient for the liquid membrane system studied here. Fig. 6 depicts the Kexp values obtained as a function of Aliquat 336 concentration. The Kexp slightly increased as the Aliquat 336 concentration increased from 1.25 wt% to 2.5 wt%, which could be attributed to the enhanced extraction capacity. But with further increasing the Aliquat 336 concentration to 5 wt%, the Kexp was almost the same as
Kexp in this work Ktheo in this work Kexp in ref [18]
3
2
2 4 Aliquat 336 concentration (wt%)
0.8
that for 2.5 wt% of carrier concentration. This result indicated that the extraction capacity provided by 2.5 wt% of Aliquat 336 was sufficient for 8 mM of Cephalexin solution. Also depicted in Fig. 6, the agreement between Kexp and Ktheo was very good for all of the three carrier concentrations tested. This good agreement demonstrated that the extraction reaction resistance was not significant in the SLM-OD technique. In addition, the value of Kexp of the SLM-OD was 3.56 10–7 m/s when the organic-to-feed volume ratio was 80:500, which was higher than that of SLM-SD (2.72 10– 7 m/s) when a large organic-to-feed volume ratio (600:300) was applied [18]. Therefore, the SLM-OD was shown to be superior to the SLM-SD in terms of the improvement of mass transfer performance and the reduced volume requirement of the organic membrane solution. On the other hand, the slight deviation of Kexp and Ktheo could be attributed to the deficiency of the shell side mass transfer correlation (Eq. (16)) for the SLM-OD system under consideration. For the SLM-OD system, the flow pattern in the shell side of the hollow-fiber module is liquid–liquid two-phase dispersed flow. The mass transfer correlation of this flow pattern on the shell side of a hollow-fiber module has not yet been reported in the literature to the best of our knowledge. 5.3. Effect of initial Cephalexin concentration in the feed phase Fig. 7 shows the dimensionless Cephalexin concentration profiles of the feed and strip phases with varying the initial Cephalexin concentrations of 5, 8 and 15 mM. The starting time of the up-hill mass transfer phenomenon increased with an increase of initial Cephalexin concentration. Because the extraction and stripping capacities were fixed, the feed solution with a higher initial Cephalexin concentration required more time for Cephalexin to be removed and recovered. In addition, it could also be found that when 15 mM Cephalexin was used, the dimensionless concentration profile of the feed phase was linearly reduced, indicating that the removal and recovery rates were constant. Fig. 8 illustrates the Kexp values obtained as a function of initial Cephalexin concentration. The Kexp significantly reduced with the increase of initial Cephalexin concentration, and a similar conclusion was also reported in the literature [14,18]. These results suggested that when the initial Cephalexin concentration was high, a suitable Aliquat 336 concentration should be applied to improve the removal efficiency. In this work, 2.5 wt% of Aliquat 336 concentration was selected for the rest of the experiments reported in this work. 5.4. Effect of KCl concentration in the strip phase As an ion-exchange type mass transfer process, a counter-ion, i.e., an anion, should be provide in the aqueous strip phase to facilitate the
0
Cf
Cs
Cf = 5 mM
Cf
Cs
Cf = 8 mM
Cf
Cs
Cf = 15 mM
0 0
0.4 0.2 0.0
6
Fig. 6. The values of Kexp and Ktheo vs. Aliquat 336 concentration (C0f ¼8 mM; Vs ¼ 500 ml, CKCl ¼ 1 M, Qod ¼200 ml/min, Qs ¼400 ml/min).
95
0.6
0
50
100 Time (min)
150
200
Fig. 7. Dimensionless Cephalexin concentrations in the feed and strip phases vs. time with varying initial Cephalexin concentration (Vf ¼ 500 ml; C0QCl ¼2.5 wt%, Vo ¼ 80 ml; Vs ¼ 500 ml, CKCl ¼ 1 M, Qod ¼ 200 ml/min, Qs ¼ 400 ml/min).
7 6 -1
0
1.0
5
-7
1
4
Kexp (10 m s )
4
-7
-1
Kexp or Ktheo(10 m s )
5
Dimensionless Cephalexin Conc.
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
3 2 1 4
6 8 10 12 14 16 Initail Cephalexin concentration (mM)
Fig. 8. The values of Kexp vs. initial Cephalexin concentration (Vf ¼ 500 ml; C0QCl ¼ 2.5 wt%, Vo ¼ 80 ml; Vs ¼500 ml, CKCl ¼1 M, Qod ¼ 200 ml/min, Qs ¼400 ml/min).
transport of the anionic Cephalexin. KCl has been proved to be a great choice [1]. The effect of KCl concentration on the mass transfer performance is shown in Fig. 9. The up-hill mass transfer phenomenon can even be found even with the absence of KCl in the strip phase. This result indicated that the ion-exchange mass transfer process was achieved due to the transfer of citric acid ions used as the buffer in the strip solution. At the same time, however, the transport of citric acid ions impaired the buffer capability and hence resulted in a significant change of pH [18]. On the other hand, an excess amount of KCl (3 M) did not bring about an enhancement of the mass transfer, because the driving force provided by 1 M Cl– was sufficient; this concentration was applied in the rest of the experiments reported in this study. 5.5. Effect of lumen side and shell side flow rates Fig. 10 displays the profiles of Cephalexin concentration in the feed and strip phases by varying the lumen-side and shell-side flow rates. When the shell-side (organic dispersion) flow rate was fixed at 200 ml/min, by increasing the lumen-side flow rate of the stripping solution from 200 ml/min to 400 ml/min, the mass transfer performance was not significantly influenced. A slightly lower mass transfer rate for the lumen-side (stripping solution) flow rate of 400 ml/min could be attributed to the shorter retention time within the first hour, in which a smaller amount of Cephalexin was transferred to the strip phase. The shorter retention time decreased the contact time between the organic dispersion and the liquid membrane. However, because the total contact time (3 h) was kept consistent, the final amount of
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Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
Cephalexin transferred to the strip phase was approximately the same for all of the three cases shown in this figure. On the other hand, with varying the shell-side (organic dispersion) flow rate from 200 ml/min to 400 ml/min while keeping the lumen-side (stripping solution) flow rate constant at 200 ml/min, also displayed in this figure, the mass transfer performance was not significantly influenced either. This also indicated that the mass transfer resistance of the strip side was insignificant. 5.6. The fractional mass transfer resistances to the overall process For the SLM-OD system under consideration with the overall mass transfer coefficients illustrated in Fig. 4, the fractional mass transfer resistances with varying the organic-to-feed volume ratio were determined from Eqs. (14) to (16) for the lumen side, membrane phase and
Cephalexin Conc. (mM)
10 8
Cf
Cs
CKCl = 0 M
Cf
Cs
CKCl = 1 M
Cf
Cs
CKCl = 3 M
6 4 2 0
0
50
100 Time (min)
150
6. Conclusions
200
Fig. 9. Cephalexin concentrations in the feed and strip phases vs. time with varying KCl concentration (C0f ¼ 8 mM, Vf ¼ 500 ml; C0QCl ¼2.5 wt%, Vo ¼80 ml; Vs ¼500 ml, Qod ¼200 ml/min, Qs ¼400 ml/min).
Cephalexin Conc. (mM)
10 8
Cf
Cs
Qod = 200 ml/min Qs = 200 ml/min
Cf
Cs
Qod = 200 ml/min Qs = 400 ml/min
Cf
Cs
Qod = 400 ml/min Qs = 200 ml/min
6 4 2 0
0
50
100 Time (min)
150
shell side, respectively and Eq. (13) for the extraction reaction. The results are illustrated in Table 2. As illustrated in this table, the fractional mass transfer resistance for the shell side was dominant, accounting for more than 76% of the total resistance. The shell-side fractional mass transfer resistance reduced as the organic-to-feed volume ratio increased. The fractional mass transfer resistances for the lumen side and the supported liquid membrane were relatively small and constant, less than 6.25% of the total resistance. Thus, any reduction in the shell-side fractional mass transfer resistance would give an increase on the fractional mass transfer resistance of the extraction reaction. Also illustrated in this table, the fractional mass transfer resistance of the extraction reaction increased from 0.43% to about 18.3% as the organic-to-feed volume ratio increased. These values were much lower than the fractional mass transfer resistance of the extraction reaction reported for SLM-SD, which was larger than 95% [18]. The results indicated that the extraction reaction resistance was greatly reduced due to the organic dispersion in the SLM-OD. These results were consistent with the finding described earlier that the SLM-OD was superior to the SLM-SD in terms of the improvement of mass transfer performance. As illustrated in this table and described earlier, the mass transfer resistance for the shell side was dominant and larger than 76%, and it increased as the organic-to-feed volume ratio reduced. When the organic-to-feed volume ratio was 0.16, i.e., 80:500, the mass transfer resistance for the shell side was even higher than 90%. These results suggested that a proper approach, which can decrease the shell side resistance, should be investigated for the SLM-OD in the future.
200
Fig. 10. Cephalexin concentrations in the feed and strip phases vs. time with varying the flow rates on the shell and lumen sides (C0f ¼8 mM, Vf ¼ 500 ml; C0QCl ¼ 2.5 wt%, Vo ¼ 80 ml; Vs ¼500 ml, CKCl ¼1 M).
Supported liquid membranes with organic dispersion (SLM-OD) have been proposed and applied successfully to extract and recover Cephalexin from aqueous solutions. For the SLM-OD, a mathematical model was developed to determine the experimental and theoretical overall mass transfer coefficients. The effects of various operating parameters on the mass transfer process were studied. The results showed that the optimal operating parameters for the removal and recovery of 8 mM Cephalexin were 2.5 wt% of Aliquat 336 concentration, 1 M of KCl concentration, and 80:500 of organic-to-feed volume ratio. In this condition, the mass transfer coefficient was 3.56 10–7 m/s, which was 30% higher than that of the SLM-SD technique. The flow rates on the lumen and shell sides did not affect the mass transfer process significantly. The calculated overall mass transfer coefficients compared very well with those obtained experimentally, implying that the mass transfer resistance due to extraction reaction was not significant. To verify this conclusion, the fractional mass transfer resistances for the SLM-OD process were determined. The results demonstrated that the extraction reaction resistance was greatly reduced, amounting to less than 18.3% of the total resistance. This value was much lower than the extraction reaction resistance for SLM-SD (larger than 95%). In addition to the mass transfer performance advantage, the SLM-OD was shown to be superior to the SLM-SD in terms of the reduced volume requirement of the organic membrane solution.
Table 2 The fractional mass transfer resistances to the overall process. Organic-to-feed volume ratio
80:500 105:500 125:500 250:500
Kexp (10 7 m/s)
3.56 3.33 3.29 2.92
Fractional mass transfer resistances (%) Δl
Δm
Δsh
Δer
0.43 0.40 0.40 0.35
6.24 5.84 5.77 5.12
92.90 87.05 85.96 76.23
0.43 6.70 7.87 18.29
Z. Hao et al. / Journal of Membrane Science 468 (2014) 90–97
Acknowledgments The authors are grateful to Cognis Corporation for free samples of the carrier Aliquat 336 used in this work. Partial funding to the first author by the Special Fund for Studying Abroad from the Beijing University of Chemical Technology is gratefully acknowledged. The authors would like to thank the National Natural Science Foundation of China for Grant no. CBET 0932511. Part of this material is based upon the work supported by the National Science Foundation under Grant no. CBET 0932511.
Nomenclature A C Ceph– d D H j k K Kexp Ktheo L n P Q Q ro Ri t tD tP ub V Vo/f
surface area, m2 Cephalexin concentration, mol m 3 Cephalexin diameter of the hollow fiber, m diffusion coefficient, m2 s 1 partition coefficient flux of Cephalexin, mol m 2 s 1 individual mass transfer coefficient, m s 1 overall mass transfer coefficient, m s 1 experimental overall mass transfer coefficient, m s1 theoretical overall mass transfer coefficient, m s 1 effective length of the hollow fiber, m number of hollow fibers pressure, Pa flow rate, ml/min (10 6 m3 min 1) Aliquat 336 outer radius of the hollow fiber, m inner radius of the HFM, m time, s diffusion time across the membrane, s process time, s bulk velocity in lumen side, m s 1 volume, m3 organic-to-feed volume ratio
Greek letters Δ δ ε Ф τ
percentage of individual mass transfer resistance membrane thickness, m membrane porosity packing density membrane tortuosity
Superscript 0
initial condition
Subscript a e er
aqueous phase hydraulic (diameter) of the shell side extraction reaction
f l lm m mi mo o od s sh
97
feed phase lumen side log-mean (diameter) membrane phase inner (diameter) of hollow fiber outer (diameter) of hollow fiber organic membrane phase organic dispersion strip phase shell side
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