Extraction of 4-nitrophenol from 1-octanol into aqueous solution in a hollow fiber liquid contactor

Extraction of 4-nitrophenol from 1-octanol into aqueous solution in a hollow fiber liquid contactor

Journal of Membrane Science 195 (2001) 193–202 Extraction of 4-nitrophenol from 1-octanol into aqueous solution in a hollow fiber liquid contactor S...

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Journal of Membrane Science 195 (2001) 193–202

Extraction of 4-nitrophenol from 1-octanol into aqueous solution in a hollow fiber liquid contactor S.W. Peretti a,∗ , C.J. Tompkins a,1 , J.L. Goodall a,2 , A.S. Michaels b a

Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695, USA b Alan Sherman Michaels Sc D Inc., 210 Allandale Rd, Chestnut Hill, MA 02467 USA Received 3 October 2000; received in revised form 21 June 2001; accepted 22 June 2001

Abstract p-Nitrophenol (PNP) was extracted from 1-octanol into an aqueous buffered solution using membrane-supported extraction in hollow fiber liquid contactors (HFLCs) containing hydrophobic, microporous polypropylene fibers. PNP is a weak acid and in aqueous solution may dissociate to form nitrophenolate ion (PNP− ), that has negligible solubility in 1-octanol. The ratio of the two species in aqueous solution is governed by pH; therefore, the overall mass transfer coefficient based on solvent phase concentrations was determined as a function of pH. The extraction of PNP is a four-step process, consisting of diffusion across the solvent boundary layer, diffusion through the solvent-filled membrane, reaction (dissociation) at the solvent/aqueous interface, and diffusion across the aqueous boundary layer. The reaction step is assumed much faster than the others; a model of the mass transfer capabilities of the system is presented based on this assumption. The overall mass transfer coefficients were determined experimentally by recirculating the solvent phase through the shell space of a HFLC and the aqueous phase through the fibers. The model closely predicts the experimentally measured trends of the overall mass transfer coefficient. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hollow fiber; Membrane transport; Supported liquid membrane; pH-swing extraction

1. Introduction Industrial processing of chemicals almost invariably leads to streams with multiple components, that must then be subjected to a train of separation/purification operations to isolate the desired product. Components from fermentation broths [1–4], pharmaceutical pro∗ Corresponding author. Tel.: +1-919-515-6397; fax: +1-919-515-3465. E-mail address: [email protected] (S.W. Peretti). 1 Present address: Proligo LLC, 2995 Wilderness Place, Boulder, CO, USA. 2 Present address: Chemical Process R&D, DuPont Pharmaceuticals Company, Chambers Works, PRF (S-1), Deepwater, NJ 08023-0999, USA.

cessing streams [5–8], chemical synthesis [9–12], and metal working fluids [13–15] have all been successfully purified using some form of supported liquid extraction. Extraction is also a promising technology to couple to biodegradation. The successful treatment of synthetic waste streams has been demonstrated for a wide range of pollutants [16]. The large-scale application of bacteria-based systems has met with limited success due to the nature of industrial waste streams, which may contain high salt concentrations, exhibit extreme pH conditions, or consist of a complex mixture of organic compounds [16]. In addition, the concentration of the target pollutant may be too dilute to serve as an effective carbon source for the biomass. As a result, separation of organic pollutants from the

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 5 6 6 - X

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Nomenclature Membrane properties Am surface area of membrane (cm2 ) di internal fiber diameter (␮m) dlm log mean fiber diameter (␮m) do external fiber diameter (␮m) L length of hollow fibers (cm) r membrane pore radius (␮m) T membrane tortuosity δ membrane thickness (␮m) ε membrane porosity Mass transfer coefficients kiw aqueous ion in boundary layer (cm/s) km membrane (cm/s) kpw aqueous PNP in boundary layer (cm/s) solvent boundary layer (cm/s) ks kw aqueous boundary layer (cm/s) overall based on solvent phase (cm/s) Ks Kw overall based on aqueous phase (cm/s) Fluxes J Jiw Jpm Jps Jpw Jw

flux between bulk phases (g/cm2 /s) flux of ion across aqueous boundary layer (g/cm2 /s) flux across membrane (g/cm2 /s) flux across solvent boundary layer (g/cm2 /s) flux of PNP across aqueous boundary layer (g/cm2 /s) total flux across aqueous boundary layer (g/cm2 /s)

Physical constants Dpw aqueous diffusivity of PNP (cm2 /s) solvent diffusivity of PNP (cm2 /s) Dps Ka p-nitrophenol ionization constant Pa apparent partition coefficient Pi inherent partition coefficient of ion inherent partition coefficient of PNP Pp Concentrations Cis ion concentration in solvent (ppm) Ciw ion concentration in water (ppm) Ciwm2 concentration of ion at aqueous side membrane surface (ppm)

Cps Cpw Cpwm2 Cs Csm1 Csm2 Cw Cwo Cso

PNP concentration in solvent (ppm) PNP concentration in water (ppm) concentration of ion at aqueous side membrane surface (ppm) total concentration in solvent (ppm) concentration at solvent side membrane surface (ppm) concentration at aqueous side membrane surface (ppm) total concentration in water (ppm) initial total concentration in water (ppm) initial total concentration in solvent (ppm)

Fluid properties Qs solvent flow rate (ml/min) aqueous flow rate (ml/min) Qw t time (min) v fluid velocity (cm/s) Vs solvent reservoir volume (ml) aqueous reservoir volume (ml) Vw

waste stream using extraction-based processes may facilitate effective biotreatment. To overcome the requirement of waste stream modification, the concept of point-source treatment systems has been proposed [9,16,17]. Close to the point of generation, a process effluent is well-characterized, which facilitates the removal of the target compounds for treatment in a specialized biological system. To realize this approach, we have proposed a novel multi-step bioreactor system in which biodegradation is coupled with a relatively new separation technique, membrane-supported extraction [18]. Using this system, the organic pollutant is extracted from the waste stream and concentrated in an organic solvent phase. Subsequently, the pollutant is extracted into a second aqueous stream and fed directly to a bioreactor. By making the appropriate solvent choice, inhibitory components such as salts or heavy metals are left in the waste stream and do not compromise the biological activity. Weak acids and bases are especially suited for this approach as the partitioning characteristics between the two phases are dictated by the pH of the aqueous phase.

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The primary pollutant in our model waste streams is p-nitrophenol (PNP), an intermediate in large-scale pesticide synthesis, and an EPA listed priority pollutant [19]. In a waste stream from parathion production, the primary components inhibitory to biological survival are the inorganic salts present at high levels. 1-Octanol is used as an extracting solvent; it has been shown to selectively remove PNP from aqueous streams in the presence of KCl [9]. As PNP is a weak acid, the pH of the waste stream may be manipulated to maximize mass transfer to the solvent phase. The pH of the second aqueous (media) stream, however, is constrained by the tolerance of the biological system. Previously [9], we reported the mass transfer analysis for the first step of the coupled process: membrane-supported extraction of PNP from an acidic solution into 1-octanol using a HFLC. The weakly acidic PNP was present in both phases primarily in the un-ionized state, and partitioned between the two phases with a substantially constant, concentration-independent partition coefficient. The overall mass transfer coefficient (Kw ) was calculated based on aqueous phase concentrations using a sum of resistances model. Consistent with classical laminar boundary layer theory, Kw was found to vary with the tube side, aqueous phase flow rate. Over the range studied, it was independent of the shell side, 1-octanol flow rate. The measured membrane mass transfer coefficient was nearly an order of magnitude lower than that predicted by simple Fickian diffusion of a solute through pores. We suggested that extreme shell side flow maldistribution due to fiber clumping was responsible for the disparity between theory and experiment. Analyses of similar hollow fiber systems [14] reported equivalent findings. In this investigation we characterize the reverse transport process: the extraction of PNP from 1-octanol into aqueous solution. At aqueous pH values greater than the solute pKa , the majority of PNP exists in ionized form. The nitrophenolate ion has low solubility in 1-octanol, so that the extraction from the solvent phase into the aqueous phase is strongly favored. The overall process consists of a combination of diffusion and chemical reaction that may be described as following four sequential steps:

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1. Molecular diffusion of PNP from the bulk 1-octanol phase through a stagnant boundary layer at the membrane surface. 2. Permeation (by molecular diffusion) of PNP through the 1-octanol-filled membrane. 3. Reaction of PNP with aqueous alkali at the membrane/water interface to form the phenolate ion (PNP− ) [PNP]m + [OH− ]w → [PNP− ]w + H2 O

(1)

4. Diffusion of the phenolate ion through a stagnant boundary layer of aqueous solution on the membrane surface into the bulk aqueous phase. This extraction process is more difficult to model than a conventional liquid–liquid extraction in which a single transferring solute is present in both liquid phases. The overall mass transfer resistance is the sum of the four independent resistances for the above process steps, and is quantitatively modeled below.

2. Theoretical analysis 2.1. Partition equilibrium of PNP between aqueous phase and 1-octanol The partitioning characteristics of PNP between 1-octanol and aqueous phase have been previously reported [9]. The apparent partition coefficient (Pa ) is well described by the following equation: Pa =

Pp + Pi [a log(pH − pKa )] 1 + a log(pH − pKa )

(2)

The pKa value of PNP is 7.1 [20]. Pp is the inherent partition coefficient of PNP and has a value of 89.2, Pi is the inherent partition coefficient of PNP− and has a value of 0.14. Accordingly, at low pH, P a → P p and at high pH, P a → P i , which has also been substantiated experimentally [9]. At neutral pH, where the ratio of PNP to PNP− in aqueous solution is close to unity, the corresponding ratio in the octanol phase is greater than 600. It is therefore reasonable to assume that PNP is the only species present in the 1-octanol phase.

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Fig. 1. Concentration profiles across the solvent boundary layer, octanol-filled membrane and aqueous boundary layer.

2.2. Mass transport model As described above, the transfer of PNP from solvent phase to aqueous phase consists of four steps, shown in Fig. 1. The driving forces for the first two steps, mass transfer through the octanol side boundary layer and through the membrane, are written in terms of a concentration gradient of PNP solvent boundary layer :

membrane :

Csm2 = Pp Cpwm2

Js = ks (Cs − Csm1 ) = Jps

Jm = km (Csm1 − Csm2 ) = Jpm

as PNP− . Assuming that deprotonation is rapid and that the concentration of buffer in solution is sufficient to maintain a constant pH regardless of the amount of PNP deprotonated, we may assume that the chemical potential or activity of PNP is equal on both sides of the interface. Therefore, octanol and the aqueous phases are in equilibrium with respect to the PNP, such that

(3) (4)

where ks and km are the local mass transfer coefficients in the solvent boundary layer and membrane, respectively and Cs and Csm1 are the PNP concentrations in the bulk solvent phase and the solvent at the solvent-side membrane surface, respectively. Csm2 represents the solute concentration in the solvent at the aqueous side membrane surface, which is also the interface between the solvent and aqueous phases. At the interface between the membrane and aqueous phase, PNP leaving the membrane may follow two pathways. It can migrate through the aqueous phase as PNP or react with a proton acceptor (base) and migrate

(5)

where Cpwm2 is the concentration of PNP ion in the aqueous solution contacting the membrane. Both PNP and PNP− diffuse across the aqueous boundary layer into the bulk aqueous stream. The mass fluxes of the two species are given by Jpw = kpw (Cpwm2 − Cpw )

(6)

Jiw = kiw (Ciwm2 − Ciw )

(7)

where kpw and kiw are the local mass transfer coefficients of PNP and PNP− in the aqueous boundary layer, respectively. The mass transfer coefficients for both species are determined by their diffusivities in solution. The diffusivities and the mass transfer coefficients of both species should be approximately equal

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since the two species are of comparable dimensions. If the system pH is constant, then Cpw Cpwm2 [H+ ] = = = constant Ciwm2 Ciw Ka

(8)

The total flux across the aqueous boundary layer is described by combining Eqs. (6), (7) and (8): Jw = Jpw + Jiw

  Ka = kw (Cpwm2 − Cpw ) 1 + + [H ]

(9)

The concentration difference in Eq. (9) is expressed as aqueous phase PNP concentrations; however, the driving forces for transport through the octanol boundary layer and the membrane phases are defined in terms of solvent phase concentrations. Therefore, solution of the overall mass transfer relationship requires that the driving force in Eq. (9) also be expressed in terms of solvent phase concentrations. This is accomplished by combining the definition of the partition coefficient with Eq. (9):   Ka kw ∗ Jw = 1 + + (Csm2 − Cps ) Pp [H ]  ∗ = kw (Csm2 − Cps )

(10)

∗ Cps

The quantity is the hypothetical solvent concentration of PNP in equilibrium with the bulk aqueous phase. Continuity requires that the solute flux between the bulk phases (J) equal the flux through the solvent phase boundary layer, the membrane, and the aqueous phase boundary layer: J = Jw = Jps = Jpm

(11)

Final solution of the overall local mass transport relationship is obtained by solving Eqs. (3), (4) and (10) with the equality given above, such that 1 ∗ J = ) (Cs − Cps  (1/kw ) + (1/km ) + (1/ks )

(12)

Note that the area corrections for the inner and outer diameters of the fibers have been omitted for clarity. Alternatively, an overall mass transfer coefficient based on solvent phase concentrations may be expressed by 1 1 1 1 =  + + Ks kw km ks

(13)

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Eq. (12) describes the local total flux of PNP from the solvent phase to the aqueous phase. The overall ∗ ), driving force for mass transfer is given by (Cs − Cps and represents the difference between the local concentration of PNP in the bulk octanol phase and the concentration of PNP in a hypothetical octanol phase  in equilibrium with the bulk aqueous phase. Here kw is the only term in Eq. (12) which is pH dependent:   kw Ka  1+ + kw = Pp [H ] kw (1 + a log(pH − pKa )) (14) = Pp Substituting this relationship into Eq. (13) gives an expression for the overall mass transfer coefficient as a function of pH Pp 1 1 1 1 = + + Ks (1 + a log(pH − pKa )) kw km ks

(15)

where kw is the “true” aqueous phase mass transfer coefficient, and is proportional to the ratio of the diffusivity of PNP to the effective boundary layer thick → ness (Dpw /δ w ). If the pH is well below the pKa , kw kw /Pp ; at pH values in excess of the pKa , however,  → ∞. Consequently, a well buffered, basic aquekw ous phase functions as an infinite sink for PNP, regardless of the PNP− concentration. The overall mass transfer coefficient for the extraction process should increase with pH from a lower asymptotic value when pH  pK a to an upper asymptotic value when pH pK a . The two limits are, respectively low pH : high pH :

Pp 1 1 1 → + + Ks kw km ks 1 1 1 → + Ks km ks

(16) (17)

The tube side (aqueous phase) mass transfer coefficient, kw , is a function of the local Sherwood number for well-developed laminar flow in a cylinder [21]:  1/3 di kw di 1/3 Sh = (18) = 1.62 Ret Sc1/3 Dpw L With the appropriate substitutions, this equation becomes     µ 1/3 Dpw 2/3 1/3 kw = 1.62 Ret (19) ρL di

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For diffusion through a porous membrane, km is calculated as previously described [9] km =

Dps ε τδ

(20)

where ε, τ and δ are the membrane porosity, tortuosity and thickness, respectively, and Dps is the diffusivity of PNP in the solvent. Analyses of membrane-based solvent extraction operated in the same manner as utilized in this work have been presented by other authors [3,14], resulting in similar equations. To determine the appropriateness of this model derivation, a series of experiments were conducted in a laboratory scale hollow fiber liquid–liquid contactor. The experiments were designed to specifically investigate the relative contribution of each mass transfer resistance, and the effect of aqueous phase pH on the aqueous phase boundary layer resistance.

3. Materials and methods 3.1. Chemicals Spectrophotometric grade PNP was supplied by Sigma Chemicals (St. Louis, MO) >99% ACS certified 1-octanol was supplied by Aldrich (Milwaukee, WI). 3.2. Hollow fiber liquid contactors Hollow fiber modules, containing Celgard® X-10 240 ␮m polypropylene hollow fibers, were provided by Celanese [22], Separation Products Division (Charlotte, NC). Each unit contained approximately 3600 fibers, with an effective length available for mass transfer of 16 cm [22]. The corresponding mass transfer area per unit was about 4200 cm2 . The relevant characteristics of the fibers are listed in Table 1. The priming volumes of the fibers and the shell side were approximately 50 and 100 ml, respectively. 3.3. Determination of mass transfer coefficients A bench scale system, similar to that described previously [9], was used to measure the mass transfer

Table 1 Celgard® X-10 240 ␮m porous polypropylene hollow fibers [22] Internal diameter (␮m) Wall thickness (␮m) Surface area (m2 /g) Porosity (%) Effective pore size (␮m) Tortuosity (estimated)

240 30 18 30 0.05 2

coefficients for the extraction of PNP from solvent to aqueous phase. The aqueous stream flowed through the fiber lumens and the solvent stream flowed through the shell space; both phases were recirculated to reservoirs. The aqueous phase and solvent phase reservoirs were 4000 and 500 ml, respectively. Aqueous phase flow rate was 490 ml/min resulting in a Ret of 12.1; the solvent phase flow rate was 170 ml/min. The overall mass transfer coefficient was measured for aqueous phase pH varying from 3 to 13. The buffers and buffer concentrations used are listed in Table 2. An over-pressure was applied to the aqueous phase, to ensure emulsion-free operation [9]. Ks values were determined by measuring the concentration of the solute in the aqueous and solvent reservoirs as a function of time. All of the solute was initially in the solvent reservoir. Both streams were recirculated to reservoirs, so the measured concentration asymptotically reached an equilibrium value. The driving force for solute transfer between the two phases during a typical run is shown in Fig. 2; the measured octanol solute concentration has been divided by the partition coefficient. The overall mass transfer coefficient, Kw , for the system is related to the concentration in the aqueous reservoir as a function of time, as previously described [9,23]. In brief, an unsteady state mass balance on the aqueous reservoir is combined with the equation for the concentration changes during

Table 2 Buffer information pH range

Buffer

Concentration (mM)

1.85–3.27 2.77–5.20 5.34–7.94 9.27–10.24

Citrate Sodium formate Sodium phosphate (dibasic) Sodium phosphate (mono basic)

50 50 50 50

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Table 3 Values for evaluation of Eq. (15) Parameter

Valuea

Pp pKa kw (×10−4 cm/s) (km )−1 + (ks )−1 (×10−5 cm/s)

89.2 7.1 1/3 6.88Ret 1.03

a

Fig. 2. Driving force for mass transfer during a typical run. PNP was transferred from aqueous to solvent phase; both phases were recirculated to reservoirs.

a single pass through the unit. After plotting the concentration measurements in dimensionless form, the slope of the resulting line is used to determine Kw . Kw is related to Ks by the relationship K s = K w /P a . 3.4. Determination of solute concentration The maximum absorbance of aqueous PNP was measured at 315 nm, while that for the p-nitrophenolate anion was measured 410 nm. Absorbance assays for the total solute concentration involved adjusting the pH several units above or below the pKa so that one form of the compound predominated. A Shimadzu UV160 UV–VIS recording spectrophotometer with a temperature-controlled cell holder was used for the absorbance assays. Calibration curves were established at pH 11 and pH 4. In organic solution, where the nitrophenolate concentration is negligible, PNP was assayed for at the wavelength of maximum absorbance, 315 nm. All absorbance assays were performed at 28◦ C.

[9].

study of the transfer of PNP from aqueous phase to octanol in an identical hollow fiber contactor yielded an apparent mass transfer coefficient for the membrane and shell (octanol) side boundary layer of about one-tenth the predicted value. We believe the discrepancy is due to extreme shell-side flow maldistribution. Accordingly, we have chosen to use the experimental value (1.03 × 10−5 cm/s) for the combined value of ((1/k s ) + (1/k m )) in our correlation. The value of kw was determined using Eq. (19), corresponding to our previous work [9]. Substituting these constants into Eq. (15) yields 1 1 89.2 = Ks [1 + a log(pH − 7.1)] 6.88 × 10−4 Re1/3 t

1 + 1.03 × 10−5

(21)

The resulting equation and the experimentally determined values of Ks as a function of aqueous phase pH are shown in Fig. 3. The experimental overall mass transfer coefficients increase with pH, as predicted by

4. Results and discussion The theoretical dependence of Ks on pH was determined according to Eq. (15); the required values are shown in Table 3. The value of km , as predicted by Eq. (20), is 1.0 × 10−4 cm/s. However, our previous

Fig. 3. Effect of pH on the overall mass transfer coefficient, Ks (based on solvent phase concentrations).

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Fig. 4. Predicted resistances to mass transfer for the membrane-supported extraction of p-nitrophenol from 1-octanol to aqueous buffer, as expressed in Eq. (15).

Eq. (15). The dependence is small because the magnitude of tube-side mass transfer resistance is small relative to the membrane and solvent boundary layer resistances. A simple way to picture the transport process is to plot each of the three mass transfer resistances and the total resistance as a function of aqueous pH and tube side Reynolds number, as shown in Fig. 4. For comparison purposes, the theoretical membrane resistance (Eq. (20)), has also been plotted. At pH values below the solute pKa , the mass transfer resistance (Rw ) is significantly affected by the tube side aqueous flow rate. In comparison to the membrane and solvent boundary resistances, however, the effect of the tube side aqueous flow rate on the overall mass transport resistance is small. As the aqueous pH increases above the solute pKa , Rw becomes negligible. pH has also been shown to significantly effect reactive extractions involving metals [13–15]. For example, Fontas et al. [13] report that the stripping of rhodium from the loaded organic phase occurs through the reduction of Rh(III) to Rh(I), requiring HSO3 − in the stripping solution. The reaction velocity goes through a maximum at intermediate pH values, leading to maximum transport rates. Kumar et al. [14] reports results similar to those reported earlier for PNP trans-

port [9]; increased rates of transport as feed stream pH values decrease. The mechanism is also similar, with hydrogen ions acting as reactants to stimulate transport of Au(I) from an aqueous phase into an organic extractant (hexane) containing metal chelators. Ho et al. [24] demonstrated that polyglycol liquid membranes could be used to separate p-nitrophenol from other salts and ions present in a waste stream. Cascaval et al. [5] reported on the use of extraction and transport through supported organic liquid membranes to separate penicillin V from phenoxyacetic acid. In both cases, the pH difference between the feed and receiving aqueous phases had a significant effect on the rate of solute transport. The pH of the receiving aqueous phases was sufficiently high that the solute was essentially completely deprotonated upon transfer to that phase. A relevant analysis of interphase transport with reaction has been reported by Basu et al. [25], in which mass transfer coefficients were determined in relationship to an instantaneous chemical reaction occurring on the aqueous side of the membrane. The presence of a reaction enhances the overall mass transfer coefficient by increasing the driving force relative to the unreacted species. A similar effect occurs

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Table 4 Comparison of mass transfer coefficients obtained for supported liquid membranes Solute

Extractant

Receiving fluid

p-Nitrophenol

1-Octanol

Phenol

Kerosene + 0.75N NaOH + nonionic surfactant Kerosene, kerosene + MIBK

Aqueous with 50 mM buffer Extractant

Phenol

for kw in the present system. As the receiving aqueous phase pH increases, deprotonation is favored and Rw becomes negligible, effectively increasing kw and Kw . Unfortunately these effects are presently overshadowed by the large resistance to mass transfer in the membrane and solvent-filled shell side boundary layer. Nanoti et al. [26], and Urtiaga et al. [27,28], have also studied the extraction of phenol from an aqueous stream into an organic-rich extractant phase. The mass transfer coefficients determined by these researchers are similar to those determined in this study, as shown in Table 4. The extremely high values obtained by Urtiaga et al. [27,28] are likely to be due to the use of concentrated caustic (1 M NaOH) as the receiving stream rather than a pH-buffered aqueous phase. This presence of high concentrations of caustic is likely to have diminished the mass transfer resistance of the liquid film on the receiving side of the membrane by allowing the reaction generating sodium phenolate to occur at the interface between the extractant and the receiving fluid. The mass transfer coefficients measured at high pH values were as much as 2.5 times higher than those measured in our earlier study [9]. Under these conditions, the tube side resistance should be negligible, which suggests that variations in fiber packing and fiber spacing may occur during passage of octanol through the shell space. These variations would materially alter the apparent membrane resistance. The scatter of the data at high pH further supports this speculation. The effect of shell side flow maldistribution could be minimized by changing the flow configuration of the unit. Feeding high pH aqueous extractant on the shell side of this module, with rich octanol on the tube side, should result in substantially enhanced mass transfer performance.

1 M NaOH

Mass transfer coefficient (×107 m/s)

Reference

2–3

This work

2–7

[26]

15–20

[27,28]

5. Conclusions Extraction of p-nitrophenol from 1-octanol into buffered aqueous solution in a HFLC follows classical mass transport theory when allowance is made for the deprotonation of p-nitrophenol at the solvent–aqueous interface. The overall mass transfer resistance is pH dependent, exhibiting an asymptotic maximum and minimum values at aqueous pH well below and above the solute pKa , respectively. Anomalously high values for the solvent-filled shell side boundary layer and membrane resistances have been observed, consistent with those found in earlier studies. Flow maldistribution in the shell space due to irregular fiber packing is hypothesized to be responsible for this result. Improvements to the system will be realized by re-designing the unit to minimize or avoid flow maldistribution on the shell side. Placing the solvent on the tube side will further enhance the overall mass transfer coefficient. This analysis will be particularly useful in situations where the resistance in the aqueous phase boundary is of similar magnitude to that in the solvent boundary layer and membrane. In such cases, optimizing the pH of the aqueous stream will also optimize the overall mass transfer coefficient. In addition, situations occur in which a reaction occurs at the liquid–liquid interface, necessitating accounting for two different diffusing species.

Acknowledgements We would like to thank the Monsanto Corporation for funding this project. In addition, we would like to thank Hoechst Celanese for providing the membrane materials used in these experiments free of charge.

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