Selective Extraction of Lactic Acid from Aqueous Media through a

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Selective Extraction of Lactic Acid from Aqueous Media through a Hydrophobic H‑Beta Zeolite/PVDF Mixed Matrix Membrane Contactor M. Madhumala,† D. Satyasri,† T. Sankarshana,‡ and S. Sridhar*,† †

Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India University College of Technology, Osmania University, Hyderabad 500 007, India

‡

ABSTRACT: 2-Hydroxypropionic acid, commonly known as lactic acid (LA), is a valuable chemical widely used for the manufacture of green solvents such as ethyl lactate and biodegradable polymers such as poly(lactic acid) (PLA). LA is manufactured by fermentation molasses and whey. Isolation of LA from aqueous broths by conventional methods is energy intensive. Reactive extraction through membranes using specific reagents could prove to be a cost-effective alternative for LA recovery. This study focuses on reactive separation of LA using a novel indigenously developed hydrophobic H-beta zeolite/ polyvinylidene fluoride (PVDF) mixed matrix membrane. Experiments were conducted using a stirred cell assembly consisting of two bell shaped glass pipe reducers containing aqueous LA separated by the membrane from an organic solution of tri-noctylamine (TOA) carrier in alcoholic medium. Effects of experimental parameters such as the concentration of TOA in organic phase and zeolite loading on the rate of acid extraction were evaluated by increasing the TOA concentration from 206 to 620 mol/m3 and the extent of zeolite loading from 1 to 25% (w/w) of (PVDF) polymer. SEM analysis was carried out to oversee zeolite distribution on the PVDF surface, whereas TGA was used to determine the maximum operating temperature. XRD study was done to investigate the influence of zeolite loading on intersegmental spacing in the polymer, while FT-IR helped in the identification of interactions between the inorganic filler and organic polymer. A mass transfer correlation was deduced by taking into account all possible reactions involved in formation of the complexes. An optimum extraction of nearly 34% was obtained using 25% zeolite loading, 206 mol/m3 TOA in 1-octanol, and 100 mol/m3 acid concentration, at a stirring rate of 400 rpm over a processing time of 1 h. Continuous separation of LA by a membrane contactor could help improve the fermentation yield of the acid by preventing the inhibition of lactate dehydrogenase enzyme, which is affected by the product itself. Such reactive extractions by membrane contactors could be successfully scaled up using a hollow fiber modular configuration.

1. INTRODUCTION Lactic acid (LA) is a beneficial chemical, produced either by synthetic organic chemistry or through microbial action on sugars, and can be processed further to produce alcohols, acrylics, and esters of industrial importance.1,2 The stipulation for production of lactic acid has grown significantly, due to its wide application in many areas which include the chemical, food, leather, pharmaceutical, and cosmetic industries and drug delivery.3,4 Currently, LA is one of the predominant feedstocks for the synthesis of poly(lactic acid) (PLA), which has properties similar to petrochemicals such as polyolefins, with the added advantage of being biodegradable.5,6 PLA has found wide application in controlled drug delivery systems, surgical sutures, orthopedic implants, and other biomedical applications. Another important function of LA is in the synthesis of green solvents such as lactate esters which could substitute for the traditional but polluting and corrosive liquids obtained by processing petroleum feedstocks.7 Several conventional methods such as adsorption onto a porous solid surface, evaporation, distillation, solvent extraction, and precipitation have been used for separation of LA from fermentation broths in the recent past.8 Separation processes applied thus far have several disadvantages which include the following: (1) production of a large quantity of sludge as solid waste during downstream processing, (2) high energy consumption due to phase change, (3) requirement of © 2014 American Chemical Society

huge capital investment, (4) complexity of operation, and (5) low separation efficiency. Thus, alternate separation techniques are under development for downstream processing of LA to reduce the running cost and environmental pollution. In this context membrane processes are of great significance owing to enhanced economy, process safety, and eco-friendly nature over conventional methods.9 Amine based reactive separation of LA and other solutes through a liquid−liquid membrane contactor is considered a potential alternative.10 This route offers easy integration with fermentation units or reactors owing to advantages of large contact area per unit volume for rapid mass transfer, with the absence of any loading constraints, flooding, back mixing, or solvent holdup. Membrane contactors employ a microporous hydrophobic membrane which forms a barrier between the aqueous carboxylic acid and the organic medium that contains an amine extractant to promote mass transfer.11,12 The amine interacts with the acid to form a complex which becomes solubilized in the inert organic solvent, also known as diluent.13 The performance of the membrane contactor mainly depends on the choice of extractant and properties of the membrane Received: Revised: Accepted: Published: 17770

June 26, 2014 October 12, 2014 October 20, 2014 October 20, 2014 dx.doi.org/10.1021/ie502566b | Ind. Eng. Chem. Res. 2014, 53, 17770−17781

Industrial & Engineering Chemistry Research

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2. THEORY Membrane contactors are devices that use a microporous hydrophobic membrane as an interface between two phases, aqueous and organic, for selective removal of solute without dispersion of one phase within another (Figure 1). The

material. Tertiary amines are the preferred extractants, while alcohols are the best diluents.9 Due to their higher viscosity and corrosive properties, tertiary amines are mixed with alcohols to maintain the viscosity of the nonaqueous medium within a desirable range. In general, hydrophobic membranes such as polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF), and polyethylene (PE) are more commonly used, in order to prevent wetting of the porous barrier or physical mixing of organic and aqueous solutions.14 There are several reports in the literature on reactive extraction of LA using microporous flat sheet and hollow fiber membrane contactor systems.4,10,15−21 Some of the extractants employed are quaternary ammonium salts, tertiary and quaternary amines, and organophosphates. Researchers have reported that tertiary amines dissolved in polar or proton diluents such as alcohols are suitable to achieve higher extraction rates of carboxylic acids. Apart from this, researchers have also carried out kinetics to study the stoichiometry of acid−amine reactions occurring during the process. Incorporation of organic/inorganic fillers into a polymer matrix has drawn great attention in membrane technology, due to several advantages which include improvement in the permeability of the selective component, in the thermal stability, in the mechanical strength, and in the surface area per unit volume of a polymeric membrane.22 Among the various types of fillers available, zeolites, which are crystalline aluminosilicates, are considered to be among the most widely used microporous molecular sieves due to their unique pore diameters, intrinsic channel structure, high thermal resistance, durability, and eco-friendly nature.23,24 Due to its large pore size, high silica content arising from a high Si/Al ratio, and three-dimensional framework, beta zeolite has found application in various fields; as a catalyst in the petrochemical industry, in separation of C5−C8 alkane isomers, as ion exchange media for softening and purification of water, and as a filler in membranes for separation of gaseous mixtures and organic liquid mixtures. In addition to this, the high Si/Al ratio of beta zeolite makes it hydrophobic and thermally stable even at higher temperatures.25,26 When beta zeolite undergoes calcination at ≥500 °C, it converts into a stable form of zeolite. To the best of our knowledge, there is no work carried out with mixed matrix membranes (MMMs) for reactive extraction of carboxylic acids by the membrane contactor technique, which constitutes the focus of this research work. Preparation and utilization of a new hydrophobic barrier made of organic polymer incorporated with inorganic filler for isolation of LA is the major objective. The intention to dope PVDF with H-beta zeolite is to improve its hydrophobic nature by increasing the contact angle, enhance the thermal stability, and enhance the rate of LA transport. The indigenously synthesized MMMs were tested to understand the kinetics of reactive separation of LA from aqueous media. These membranes were examined by scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), X-ray diffractometry (XRD), and Fourier transform infrared spectroscopy (FT-IR) to assess the porosity and filler distribution, temperature resistance, polymer crystallinity, and structure−property relationship, respectively. The effect of varying experimental parameters on the acid extraction rate was also evaluated to optimize operating conditions. A mass transfer correlation was established by taking into account all feasible reactions involved in the formation of acid−amine complexes.

Figure 1. Membrane contactor system separating two phases by membrane.

concentration gradient across the membrane transfers the solute from one phase to another phase. The separation performance of the system can be determined from the distribution coefficient (KD) of the solute in two immiscible phases. During reactive separation, the membrane is not preferentially wetted by the aqueous feed solution at the upstream side, but the organic solution circulated at the downstream side readily wets the membrane. To prevent the permeation of aqueous solution into organic solution though the membrane, the applied pressure (Δp) should be less than a critical value known as the breakthrough pressure given by the Young−Laplace equation.27

Δp =

2γ cos θ r

(1)

Transport mechanisms involved during the separation of a solute (acid) from one phase to another through a microporous hydrophobic film is described well in the literature.28,29 Initially the carboxylic acid diffuses from the bulk portion in the aqueous phase to the surface of the membrane. Subsequently the acid reacts with an extractant present in the tortuous pores of the microporous membrane to form a chemical complex at the interface between the membrane and the organic solution. At steady state, the rate of solute transport (N) from bulk of the water based solution to the organic medium is expressed as (2)

N = K ΔC

The overall mass transfer resistances in aqueous and organic phases are determined from the following equations:28,29 1 1 1 1 = + + Ka ka m1k m ko (3a) m 1 1 1 = 1 + + Ko ka km ko

(3b)

30

As described elsewhere, the overall mass transfer coefficients (MTCs) in water (Ka) and kerosene phases (Ko) can be determined from the rate of iodine solute transport from water 17771

dx.doi.org/10.1021/ie502566b | Ind. Eng. Chem. Res. 2014, 53, 17770−17781


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to the kerosene phase and acetic acid solute in the opposite direction, i.e., from kerosene to the water phase, respectively. As the distribution coefficient of acetic acid in the water/kerosene system is very small, the m1/ka term in eq 3b tends to zero. On the other hand, iodine exhibits a large diffusion coefficient and therefore the resistance offered by the organic phase can be neglected. By substituting 1/ko = 0 in eq 3a and m1 = 0 in eq 3b, the following equations are obtained: 1 1 1 = + Ka ka m1k m

(4a)

1 1 1 = + Ko km ko

(4b)

Table 1. Physical and Chemical Properties of H-Beta Zeolite pore diam (nm) 0.76 × 0.64 and 0.55 × 0.55

km =

(5a)

Dj ϵ δmτ

(5b)

where γ in eq 5a represents the kinematic viscosity of the solvent, whereas the tortuosity factor (τ) in eq 5b can be calculated using the correlation suggested by Macki-Meares.32 The diffusivity of species in different phases is determined by Wilke−Chang and Minhas−Hayduk correlations.33 DAB =

DAB =

■

(7.4 × 10−8)(φBMB)0.5 T μB ϑA 0.6

(6a)

(1.55 × 10−8)T1.29(PB 0.5/PA 0.42) μB 0.92 ϑB 0.23

SiO2/Al2O3 mole ratio

nominal cation form

Na2O (wt %)

salt form

710

38:1

ammonium

0.05

hydrogen

temperature for 6 h to obtain a homogeneous polymer solution. The obtained bubble-free solution was cast on a macroporous nonwoven backing made of polyester that had been tautly stuck on a clean glass plate using cellophane tape at the edges. The gap between the glass plate and casting knife was fixed using a doctor’s blade to adjust the thickness of the casting. Immediately after casting of the solution, the nonwoven backing with the plate was submerged for 10 min in a nonsolvent bath of ice-cold water to obtain a porous membrane by phase inversion. MMMs of different compositions were prepared by increasing the zeolite content from 1 to 25 wt % of the PVDF polymer weight in the casting solutions. The membranes having zeolite loading in the range 1−7.5% of the polymer weight were designated as 15PVDF-Z1, 15PVDF-Z2.5, 15PVDF-Z5, and 15PVDF-Z7.5, whereas those containing zeolite in the range 10−25% of the polymer weight were stipulated as 12PVDF-Z10, 12PVDF-Z15, 12PVDF-Z20, and 12PVDF-Z25, respectively. The numbers “15” and “12” indicate the percent weight per volume (% w/v) concentrations of PVDF in the DMF, solutions whereas the values given after “Z” pertain to zeolite concentrations with respect to the polymer weight. Excess zeolite contents could cause brittleness in the membranes and agglomerations. Concentrations above 25% zeolite rendered the membranes brittle and unfit for application and hence were considered optimum since agglomeration had just begun as revealed by SEM study. 3.3. Experimental Procedure. A membrane stirred cell assembly (Figure 2) was employed to perform the kinetic study by extracting LA from aqueous solution. The cell consists of two chambers separating a porous hydrophobic MMM with an active layer thickness of 20 μm and an effective area of 20 cm2. An organic solution of 1-octanol with TOA (89 mL) was poured into the bottom portion of the cell while ensuring that

In the literature, researchers have used the following relationships to calculate the individual MTCs of various solutes in different phases.28−31 ka , ko ∝ Dj 2/3γ −1/6

surf. area (m2/g)

(6b)

EXPERIMENTAL SECTION 3.1. Materials and Methods. PVDF was procured from Akanksha Enterprises (Pune, India), while zeolite beta ammonium was purchased from Alfa Aesar, (Heysham, U.K.). LA (88% pure), acetic acid, N,N-dimethylformamide (DMF), NaOH, potassium hydrogen phthalate, phenolphthalein indicator, potassium iodide, H2SO4, and sodium thiosulfate anhydrous were purchased locally from sd Fine Chem. (Hyderabad, India). Extra pure soluble starch was obtained from Merck Specialties Private Ltd. (Mumbai, India). Tri-noctylamine (TOA, 95%) was obtained from Sigma-Aldrich, and 1-octanol (99%) was obtained from Hychem Sainor Chemicals Pvt. Ltd. Synthetic aqueous solutions of LA of varying compositions were prepared in deionized water generated by a laboratory reverse osmosis system. LA concentrations in aqueous and organic phases were determined by the titration method and material balance, respectively. The NH4 form of beta zeolite was procured and subjected to calcination at 450 °C for 24 h to obtain the catalytically active proton form of the zeolite, which is H-beta. The physical and chemical properties of H-beta zeolite are provided in Table 1. 3.2. Synthesis of Microporous Hydrophobic Zeolite/ PVDF Mixed Matrix Membrane. A preweighed amount of H-beta zeolite was introduced into DMF with stirring, and the resultant solution was subjected to ultrasonication for 2 h at 30 °C for uniform dispersion of zeolite particles. After that, PVDF polymer was added to the solution with stirring at ambient

Figure 2. (a) Schematic representation of liquid−liquid membrane contactor system. (b) Laboratory experimental setup of membrane contactor system. 17772



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Figure 3. FT-IR spectra of (a) pristine zeolite H-beta, (b) pristine 12PVDF, and (c) 12PVDF-Z25 membranes.

water and kerosene phases and acetic acid transport in the opposite direction using the indigenously developed membrane contactor system (Figure 2). Due to differences in the concentration gradient and affinity toward the respective phases, the solute (iodine/acetic acid) permeates from a region of higher concentration to the lower one. From correlations concerning rates of mass transfer, the following relationships are obtained:30

the hydrophobic barrier contacted the 1-octanol solution in the downstream side without the presence of any air gap or air bubbles. The same quantity of aqueous LA was taken in the upper feed chamber, and both halves of the cell were clamped together tightly to assemble a leakproof arrangement. The solutions present in the lower and upper chambers were stirred at constant rates of 400 revolutions/min (rpm) each using a magnetic bead in the bottom half of the cell and a digital overhead stirrer (IKA RW 20 digital) with an impeller in the top half of the cell with the revolutions induced in opposite directions. As time progresses, LA from aqueous solution is expected to diffuse though the membrane into the organic solution. Separation of LA is facilitated by the presence of TOA. The membrane is highly hydrophobic, and sorption related losses of LA would be a bare minimum as the PVDF surface cannot be wetted by water or LA. Periodic collection of miniature samples from the aqueous phase was carried out to determine the amount of acid that would have been transferred to the organic phase due to the concentration gradient and the phenomenon of phase transfer catalysis. A material balance of LA before and after the experiment was performed to rule out any losses into the membrane or surroundings. All the experiments were performed at 303 K and atmospheric pressure for a period of 1 h each. Original contents of LA in water and TOA in 1-octanol were varied over the ranges of 173−370 and 206−620 mol/m3, respectively. 3.4. Equilibrium Experiments. Similar quantities (50 cm 3 ) of both aqueous solution containing a known concentration of LA and organic diluent containing the desired amount of tertiary amine were brought into contact at ambient temperature. The mixture was subjected to vigorous shaking for 24 h followed by settling in a separating flask for 1 h. LA contents in aqueous and organic phases were estimated through titration of the aqueous sample against caustic solution using phenolphthalein indicator followed by mass balance to determine the LA concentration in the organic phase. The initial concentration of LA in water was varied from 91 to 650 mol/m3 with the TOA concentration kept constant at 206 mol/ m3. 3.5. Diffusional Studies. Diffusion experiments were performed to study the rate of iodine transport across the

⎛ [AA] ⎞ ⎛ S ⎞ o ⎟ = ⎜ ⎟Kot ln⎜ ⎝ [AA]t ⎠ ⎝ Vo ⎠

(7a)

and ⎛ [I] ⎞ ⎛ S ⎞ ln⎜ a ⎟ = ⎜ ⎟K at ⎝ [I]t ⎠ ⎝ Va ⎠

(7b)

The initial concentrations of acetic acid in the organic phase and iodine in the aqueous phase were taken as 400 and 1.2 mol/m3, respectively. Acetic acid and iodine present in aqueous solutions were estimated by titration of the sample against the known normality of aqueous NaOH and hypo solutions, respectively. 3.6. Membrane Characterization. 3.6.1. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR study of indigenously synthesized membranes was done by the KBr pellet method to look for inter- and intramolecular connections within the composite membranes, which were subjected to scanning in the wavenumber range 400−4000 cm−1 using a Nicolet Model No. 740 FT-IR instrument (PerkinElmer, Boston, MA). 3.6.2. X-Ray Diffraction Studies (XRD). A powder X-ray diffractometer, Siemens Model D 5000 (Siemens, Washington, DC), was used to assess the crystallinity and intersegmental dspacing of the indigenously synthesized membranes. X-rays of 1.5406 Å wavelength were produced by a Cu Kα source. The diffraction angle was changed from 2 to 60° to assess transformation in the physical structure and the gap between adjacent segments in the pristine and modified polymers. 3.6.3. Thermal Gravimetric Analysis (TGA). Decomposition of the membranes was observed over a 25−700 °C range of temperature with the rate of heating maintained at 10 °C/min 17773



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Figure 4. X-ray diffractograms of (a) pristine H-beta zeolite, (b) pristine 12PVDF, and (c) 12PVDF-Z25 membranes.

Figure 5. TGA curves of (a) pristine 12PVDF and (b) 12PVDF-Z25 membranes.

4. RESULTS AND DISCUSSION

using a Seiko 220TG/DTA instrument (Seiko, Tokyo, Japan). During the trial the sample was continuously flushed with a 200 mL/min flow of N2 gas. 3.6.4. Scanning Electron Microscopy (SEM). An SEM instrument, Model JEOL JSM 5410 (JEOL, Tokyo, Japan), was utilized to capture magnified images of the surface and thickness of the indigenous MMMs.

4.1. Membrane Characterization. 4.1.1. FT-IR. Figure 3 represents FT-IR spectra of (a) plain H-beta zeolite, (b) pristine PVDF, and (c) 12PVDF-Z25. H-beta zeolite (Figure 3a) exhibited an absorption band in the range 1050−1150 cm−1 which corresponds to the asymmetric stretching vibrations of the bonds inside SiO4 and AlO4 tetrahedrons. The symmetric 17774



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Figure 6. SEM pictures of (a) 12PVDF surface, (b) cross-sectional view of 12PVDF, (c) 15PVDF-Z7.5 surface, (d) 15PVDF-Z7.5 cross section, (e) 12PVDF-Z25 surface, and (f) 12PVDF-Z25 cross section.

H-beta zeolite represented by Figure 4a revealed the crystalline nature of zeolite particles with two distinct sharp peaks at 7 and 22.5° of the 2θ scale, whereas pure PVDF exhibited a predominantly amorphous nature (Figure 4b). With incorporation of zeolite particles into the PVDF matrix, the spectra of the composite membrane revealed a semicrystalline nature with four distinct peaks at 8, 20.5, 23, and 31.8° of the 2θ scale (Figure 4c) pertaining to both the inorganic zeolite and PVDF organic polymer structures. Peaks at 8, 23, and 31.8° of the 2θ scale belong to the zeolite, whereas 20.5° represents PVDF polymer. When compared to pristine PVDF, a shrinkage in the d-spacing value was observed in the XRD spectra of the MMM which meant a reduction in intersegmental spacing that was caused by compatibility of H-beta zeolite with PVDF polymer. This is owing to H-bonding between the electronegative fluorine of PVDF and hydrogen of the zeolite which may cause a shift in the peaks of the MMM. However, though the PVDF pore structure may tighten, the number of overall porous sites available for diffusion of LA would increase due to the zeolite’s porous nature which induces a higher permeation rate. 4.1.3. TGA Studies. The thermal stability of the membrane was assessed by observing the change in mass of the polymer film under a condition of rising temperature. TGA curves of (a) pristine PVDF and (b) 12PVDF-Z25 membranes are shown in Figure 5. The membranes appear to have undergone a distinct two-step thermal decomposition process. The first weight loss for pristine PVDF was observed at 450 °C with a subsequent

stretching region and sharp medium bands between 450 and 700 cm−1 are either due to SiO4 and AlO4 bending or the motion of the external linkage of the AlO 4 and SiO 4 tetrahedrons. Corresponding to the wavenumber of 3340 cm−1, a wide band is seen as a result of −OH groups’ stretching in an asymmetric manner, whereas the band at 1570 cm−1 arises from vibrations pertaining to the bending of H−OH bands.34,35 The peaks at 3026 and 2984 cm−1 are a consequence of symmetrical and asymmetrical vibrations arising from stretching of CH3 band (Figure 3b). The peak corresponding to 1441 cm−1 is visible due to in-plane bending or scissoring of the CH2 group. The spectral peaks obtained at 1179 cm−1 are caused by the CF stretching vibration mode of PVDF.36 Figure 3c illustrates spectra of the 12PVDF-Z25 membrane, wherein the broad peak at 3448 cm−1 pertains to the combined intensity of both H-beta zeolite and PVDF peaks. Peaks at 3026 and 2984 cm−1 related to the CH3 band are also observed. However, the spectra obtained at 950−1250 cm−1 might be due to hydrogen bonding between the zeolite and CF group. Thus, H-beta zeolite indulges in both physical and chemical interactions with PVDF polymer. Physical interactions could occur through dispersive and van der Waals forces and chemical interaction from hydrogen bonding between the electronegative fluorine atoms of PVDF and hydrogen atoms of H-beta zeolite. 4.1.2. XRD. The outcome of X-ray analysis of pristine hydrophobic H-beta zeolite, unmodified PVDF, and 12PVDFZ25 hydrophobic barriers is displayed in Figure 4. Hydrophobic 17775



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Figure 7. Reaction mechanism of formation of acid−amine complex during LA separation.

weight loss at 500 °C, whereas 12PVDF-Z25 membrane exhibited a rapid drop in weight at 470 °C followed by a second weight reduction at 485 °C. From the curves it can also be observed that the weight of the 12PVDF-Z25 polymer film reduced from 6.1 to 2.3 mg at 800 °C whereas that of pristine PVDF reduced from 6.5 to 1.4 mg, indicating decomposition of the PVDF structure. However, it could be concluded that the 12PVDF-Z25 membrane exhibited greater thermal stability than pristine PVDF membrane over the range of temperature studied. 4.1.4. SEM. Figure 6 reveals magnified images of pristine PVDF and zeolite loaded PVDF membranes as viewed from the top face and edge of the film. Figure 6a illustrates that pristine PVDF membrane is porous in nature with the pores distributed unevenly throughout the surface. With the incorporation of Hbeta into PVDF, the pore diameter of the membrane appears to become enhanced (Figure 6c). At higher zeolite loadings, agglomeration of fillers was observed over the membrane surface (Figure 6e). The cross-sectional morphologies of the membranes (Figure 6b,d,f) revealed two main layers, including the active upper membrane layer which penetrated into the bottom layer made of nonwoven polyester backing. Zeolite particles occupy approximately 5−25% of the polymer surface as seen from Figure 6c,e when compared to pristine PVDF membrane in Figure 6a. The zeolite particle size is much smaller than the membrane thickness of 20 μm, and they are mostly distributed on the surface of the membrane rather than within the matrix as seen from SEM cross sections in Figure 6d,f which do not reveal the presence of any zeolite particles. 4.2. Equilibrium and Kinetic Study. 4.2.1. Determination of Equilibrium Constant. The reactive extraction of LA using tertiary amine gives a reaction complex in the organic phase. In this section, the objective is to determine the dominant LA:TOA ratio in the LA−TOA complexes, and to obtain the

corresponding equilibrium constant. The stoichiometry of the acid−amine reaction was investigated by changing the LA content in the water phase from 91 to 650 mol/m3. The complex thus formed is supposed to be stable owing to Hbonding with 1-octanol.37 Figure 7 shows the reaction mechanism taking place during the formation of the acid− amine complex. Initially, one acid molecule reacts with one TOA molecule in the organic phase to constitute a LA−TOA complex as illustrated in Scheme 1. This complex then reacts with one molecule of acid and forms two molecules of acid− amine complex (Scheme 2). Similarly, the reaction of n acid molecules with amine moieties is given by the following equation (Figure 7, Scheme 3): n(LA)aq + (TOA)org ↔ (TOA)[(LA)]n ,org

(8)

The equilibrium constant of complex formation (KEn) for the above-described reaction can be written as KEn =

(TOA)[(LA)]n ,org [LA]aq n [TOA]org

(9)

The degree of loading of LA in the organic phase determines the loading ratio (z) which is given in terms of content of LA in 1-octanol medium to that of TOA in the same phase.38 z=

[LA]org [TOA]org

(10)

The z value mainly depends upon the initial concentration of LA in aqueous solution and the degree of acid extractability but does not rely on the tertiary amine concentration. In his study, Connors39 stated that if chemical interactions between acid and amine are stronger than physical interactions in the system, 17776



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(m3/kmol)2 slope value that indicates the formation of a 2:1 complex. 4.2.2. Influence of Zeolite Loading on Extent of LA Separation. Huang et al.40 used beta zeolite embedded in a poly(vinyl alcohol) (PVA) polymer matrix for separation of water from ethanol by pervaporation. Upon addition of zeolite into the PVA matrix, both the separation factor and flux were found to improve. Shen and Lua41 incorporated inorganic fillers in a PVDF matrix for separation of gaseous mixtures of He, CO2, O2, and N2. Higher gas permeabilities were obtained when the filler loading in PVDF membrane was enhanced. In our study, the performance of pristine PVDF and MMMs based on zeolite−PVDF composite were investigated to verify the rate of LA extraction from aqueous solutions. LA can move more easily through the pores of the zeolites as can be seen from Figure 9, which reveals that the extraction ratio

there is a possibility of formation of complexes with different stoichiometries. The stoichiometry of the acid−amine reaction, i.e., 1:1 or 2:1 or 3:1, depends on the z value. By combining eqs 9 and 10, we obtain the following equation relating the z value, the acid content in the water phase, and the constant for complex formation at equilibrium. z = KEn[LA]E,aq n n−z

(11)

where n denotes the number of reacting acid molecules with one amine molecule. As stated by Wasewar et al.,38 if z values are observed to be 0.5, a 2:1 chemical complex results from a plot between z/(2 − z) and [LA]E,aq2, which again exhibits a straight line passing though the origin with slope KE2. In our study, the z values were observed to be in the range 0.34−1.61 at a constant amine concentration of 206 mol/m3 (Figure 8a). To prove the existence of 2:1 acid− amine complex, a graph of the calculated z/(2 − z) values against [LA]E,aq2 was drawn as seen in Figure 8b, which again shows a similar line passing through the origin with a 38.77

Figure 9. Effect of zeolite loading on extent of LA separation.

continuously increased with increasing zeolite content. The formation of the larger LA−TOA complex would hinder the upward diffusion of TOA into the aqueous chamber against gravity. Figure 9 reveals that an increase in zeolite loading from 1 to 25% in the polymer matrix facilitates the percent extraction of LA into the organic layer to enhance significantly from 13.7 to 34.36%. With increase of the zeolite concentration in the polymer matrix, the degree of hydrophobicity as well as the pore size of the membrane increases.22 As a result, the interfacial contact area between the two phases and the permeability of LA from aqueous to organic further improves. When compared to pristine PVDF membrane, maximum extractions of 18.67 and 34.36% were obtained for 15PVDFZ7.5 and 12PVDF-Z25 membranes, respectively. The original contents of LA and TOA were taken as 100 and 206 mol/m3, respectively. 4.2.3. Estimation of Mass Transfer Coefficients. The kinetic and diffusional studies were carried out using the 12PVDF-Z25 membrane to evaluate the overall MTCs of organic and aqueous phases as this membrane exhibited the maximum rate of extraction with excellent film forming property and mechanical stability. Figure 10 shows the plots of ln([I]a/[I]t) vs (S/Va)t and ln([AA]o/[AA]t) vs (S/Vo)t. With an increment in time of the experiment, a gradual enhancement in the extent of permeation of the solutes, acetic acid and iodine, from one phase to the other phase was observed. The slopes of the plots

Figure 8. (a) Loading of organic layer with lactic acid. (b) Determination of equilibrium constant for 2:1 LA−TOA complex formation at an extractant concentration of 206 mol/m3. 17777



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4.2.4. Determination of Forward Extraction Rates and Interfacial Concentrations of Solutes. Microporous hydrophobic membranes having high porosity and pore size are usually wetted by organic solvents and hydrocarbons.20 Therefore, the concentration of species at the membrane/ organic interface is same as that of the bulk organic phase.43 Concentration profiles of LA and TOA across a hydrophobic flat sheet membrane are shown in Figure 11. The chemical

Figure 10. Estimation of overall coefficients of mass transfer.

gave overall MTCs of 3.94 × 10−5 and 4 × 10−6 m/s for aqueous (Ka) and organic (Ko) phases, respectively. Using eqs 4a, 4b, and 5b, the individual MTCs of acetic and iodine in aqueous, organic, and membrane phases was evaluated. By substituting the obtained values of ka and ko in eq 5a, the proportionality constants were determined. From known values of proportionality constants, the MTCs of various solutes such as LA, TOA, and acid−amine complex in all three phases were calculated from eqs 5a and 5b as displayed in Table 2. Kinematic viscosities of water, 1-octanol, and kerosene at 30 °C were taken as 0.8 × 10−6, 7.36 × 10−6, and 1.52 × 10−6 m2/s, respectively.30

Figure 11. Concentration profiles of LA and TOA around a flat sheet membrane.

reaction between LA and tertiary amine usually takes place at the membrane/organic interface, due to poor solubility of extractant and diluent in aqueous medium. The rate of mass transport of species i from bulk of the aqueous phase to bulk of the organic phase through microporous hydrophobic membrane is generally described by Fick’s law of diffusion. At steady state, diffusion of LA from bulk of aqueous feed toward the membrane and that of the extractant through the organic phase and hydrophobic barrier are related by the following expression:10

Table 2. Mass Transfer and Diffusion Coefficients at 303 K system (solute/medium) LA/water LA/1-octanol LA/membrane/(1-octanol) TOA/1-octanol TOA/membrane/(1-octanol) 2:1 acid−amine/1-octanol 2:1 acid−amine/membrane/ (1-octanol)

DAB (m2/s) 1.22 × 10−9 3.35 × 10−11 4.15 × 10−10 1.09 × 10−10

mass transfer coeff (m/s) 3.41 2.08 6.28 1.11 7.78 4.58 2.06

× × × × × × ×

10−5 10−6 10−7 10−5 10−6 10−6 10−6

Jf = R f − R b = kLA([LA]b − [LA]i ) = K TOA([TOA]b − [TOA]i )

where K TOA

Diffusivities of solutes were determined using eqs 6a and 6b and are provided in Table 2. The molar volumes of LA, 1octanol, TOA, and acid−amine complex at their normal boiling points have been reported earlier.42 Parachor values of organic phase (kerosene), LA, 1-octanol, TOA, and LA−amine chemical complex were calculated by structural group contribution methods and are provided in Table 3.42 The parachor of LA in 1-octanol is doubled since it exists as a dimer, whereas for the TOA/1-octanol system the value of molar volume and parachor values of 1-octanol are multiplied by 8μB, with viscosity in centipoise.33

ϑo (cm3/mol)

P (cm3·g1/4·s−1/2)

1-octanol LA TOA 1:1 acid−amine 2:1 acid−amine

190.82 98 555.9 653.9 733.5

365.5 183.6 1024 1713 2000

⎡⎛ 1 ⎞ ⎛ 1 ⎞⎤−1 = ⎢⎜ ⎟ + ⎜ ⎟⎥ ⎢⎣⎝ k m ⎠ ⎝ ko ⎠⎥⎦

(13)

From eq 13, the value of overall MTC of TOA is obtained as 4.58 × 10−6 m/s. A chemical reaction occurring during the formation of a complex is generally reversible. To avoid problems arising from reversibility, initial rates were determined over a period of 1 h. At the early stage of forward extraction, both the immiscible mediums contain free molecules of acid and amine, respectively. Therefore, the reverse reaction of the complex is neglected (Rb = 0). The rate of forward reaction is calculated using initial rate method:10

Jf =

Table 3. Molar Volumes and Parachor Values for Different Solutes at 303 K solute

(12)

⎛ Vo ⎞⎛ d[LA] ⎞ ⎜ ⎟⎜ ⎟ ⎝ S ⎠⎝ d t ⎠

t=0

(14)

From the concentration vs time profiles of LA in the water phase at different amine concentrations, the preliminary reaction rates could be estimated. Using eq 14, the specific rates of extraction of LA were determined accordingly. By substituting the known values of specific extraction rates, bulk concentrations of LA and TOA, and MTC values in eq 12, the interfacial concentrations were evaluated. The results are exhibited in Table 4. 17778



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R f = k f [LA]i 2 [TOA]i 0.3

Table 4. Interfacial Concentrations of Lactic Acid and Tri-noctylamine at 303 K CTOA (mol/m3)

CLA (mol/m3)

Jf × 10−4 (mol/m2 s)

CLA,i (mol/m3)

CTOA,i (mol/m3)

206 410 620 206 410 620 206 410 620 206 410 620

173 173 173 205 205 205 286 286 286 370 370 370

1.55 2.22 2.67 2.35 3.29 4.16 5.11 6.67 8.01 7.99 11.66 14.02

168.77 166.81 165.51 199.53 197.24 194.98 271.32 266.75 262.83 345.87 335.16 328.24

172.00 361.43 561.72 154.70 338.18 529.19 94.29 264.29 445.15 31.15 155.49 314.01

(15)

From Figure 13 the rate constant value, kf, was found to be 2 × 10−9 m4.9/mol1.3 s at 303 K.

4.2.5. Determination of Order of Reaction with Respect to Interfacial Concentrations of Species. Figure 12 shows the log−log plots of interfacial concentrations of LA and TOA on the rate of forward reaction. The reaction order was found to be 2 with respect to LA and 0.3 with respect to TOA, using regression analysis. Thus, the intrinsic rate of formation of complex (Rf) is given by eq 15:

Figure 13. Determination of intrinsic reaction rate constant.

The conventional method for separation of acids from aqueous streams can be replaced with a hybrid membrane contactor−stripping process to increase extraction rates with reduction in energy consumption and yield of carboxylic acids. Figure 14 shows the hypothetical scheme of LA extraction

Figure 14. Schematic for scale-up of membrane contactor based hybrid process for lactic acid extraction.

using a hydrophobic membrane module followed by stripping wherein the complex that is formed can be broken by the addition of aqueous sodium hydroxide solution with subsequent acidification to obtain pure LA. 4.3. Comparison of Experimental Results with Literature. Table 5 depicts a comparison of results obtained during the present study with data of LA extraction reported in the literature. When compared to previous research attempts, a more rapid and greater extent of LA extraction was achieved with the indigenously developed zeolite−PVDF mixed matrix membranes, which establishes its potential for scale-up and commercial application.

5. CONCLUSIONS Application of microporous hydrophobic H-beta zeolite/ polyvinylidene fluoride mixed matrix membrane for recovering lactic acid from aqueous streams was investigated. Membrane

Figure 12. Effect of (a) LA concentration in water phase and (b) TOA content in organic medium on forward reaction rate at 303 K. 17779



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Table 5. Comparison of Contactor Performance with Indigenous Membrane against Data Reported in the Literaturea no. 1

a

membrane type

2

H-beta zeolite/PVDF microporous mixed matrix membrane PVDF hydrophobic membrane

3 4

PEEK-WC 14% membrane microporous PVDF membrane

5

Celgard X-30 240 polypropylene hollow fiber

concentration

extractant/diluent

LA, 100 mol/m3; TOA, 206 mol/ m3 LA, 100 mol/m3; TOA, 400 mol/ m3 LA, 10 g/L; TOA, 5% LA, 10 mol/m3; TOA, 300 mol/ m3 LA, 10 mol/m3; TOA, 300 mol/ m3

TOA/1-octanol

extent of extraction in 1 h (%) 34.3

ref present study

∼11

10

TOA/n-heptane TOA/xylene

35 ∼7

18 20

TOA/xylene

30

44

TOA/xylene

PVDF, polyvinylidene fluoride; LA, lactic acid; TOA, tri-n-octylamine; PEEK, polyether ether ketone.

τ = tortuosity factor of membrane φ = factor of association M = molecular weight ϑ = molar volume of species (cm3/mol) P = parachor (cm3·g1/4·s−1/2) S = cross-sectional area of membrane (m2) V = solution volume (m3) t = time (s) [AA]o = initial concentration of acetic acid in organic layer (mol/m3) [AA]t = acetic acid concentration in kerosene at time t (mol/ m3) [I]a = original iodine content in aqueous phase (mol/m3) [I]t = iodine concentration in aqueous phase at time t (mol/ m3) KEn = equilibrium complexation constant z = loading ratio Jf = forward extraction rate (mol/m2 s) Rf = intrinsic rate of formation of complex (mol/m2 s) Rb = intrinsic rate of dissociation of complex (mol/m2 s) K[TOA] = overall MTC of TOA (m/s) [LA]b = LA concentration in bulk of aqueous solution (mol/ m3) [LA]i = LA concentration at the interface of aqueous medium−membrane (mol/m3) [TOA]b = amine concentration in bulk of organic medium (mol/m3) [TOA]i = amine concentration at the interface of organic medium and membrane (mol/m3) kf = rate constant

characterization revealed an enhancement in the porosity and thermal stability of the membrane after incorporation of zeolite particles. A 25 wt % zeolite filled PVDF mixed matrix membrane prepared from 12% w/v DMF solution exhibited an optimum extraction rate when compared to pristine PVDF membrane. It was observed that an increase in acid concentration in the aqueous phase enhanced the forward extraction rate. The interfacial concentrations of species adjacent to the membrane in aqueous and organic chambers are influenced by mass transfer coefficients, the bulk concentration of species, and forward extraction rates. Overall mass transfer rates improve with zeolite addition due to kinetics of complex formation and diffusion. However, the improvement in extraction yield originates from the tendency to attain thermodynamic equilibrium. The overall order of reaction pertaining to interfacial concentrations of lactic acid and tri-noctylamine was found to be 2.3 by regression analysis. From the study, it can be concluded that continuous separation of carboxylic acids from fermentation reactors can be done using a membrane contactor system to enhance the fermentation yield which is generally inhibited by the product itself, such as LA, which deactivates the lactate dehydrogenase enzyme of Lactobacillus bulgaricus microorganism used in its production.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +9140-27193408. Fax: +9140-27193626. Notes

The authors declare no competing financial interest.

■

Subscripts

ACKNOWLEDGMENTS The authors express their gratitude to CSIR, New Delhi, for supporting this research work under the MATES XII FYP Program.

■

NOMENCLATURE γ = interfacial tension between aqueous and organic phases θ = angle of contact between air, membrane, and water r = radius of membrane pore k = MTC of an individual component (m/s) K = overall coefficient of mass transfer (m/s) ΔC = concentration gradient (mol/m3) m1 = distribution coefficient D = diffusion coefficient of species (m2/s) γ = viscosity (kinematic) (m2/s) μ = dynamic viscosity (cP) ϵ = porosity of membrane δm = membrane thickness (m)

■

a, aq = aqueous or water phase o, org = organic or nonaqueous phase m = membrane i = interface b = bulk A = solute B = solvent E = equilibrium

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Selective Extraction of Lactic Acid from Aqueous Media through a

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