Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
NEW MEMBRANE SYSTEMS: INNOVATION and LIMITS Stefan Ioan VOICU, Mihaela Emanuela CRACIUN and Gheorghe NECHIFOR University POLITEHNICA of Bucharest, Faculty of Applied Chemistry and Materials Sciences, POLIZU Str., No. 1-7, Sector 1, Bucharest, 011061
[email protected];
[email protected]
1. POLYSULFONE-POLYANILINE COMPOSITE MEMBRANES Polysulfone based on bisphenol A is a thermoplastic widely used as a membrane material or membrane support for liquid separation processes such as ultrafiltration or reverse osmosis. The constant interest of the membrane scientists for polysulfone is due to its excellent characteristics: good solubility in a wide range of aprotic polar solvents (dimethylformamide, dimethylacetamide, dimethylsulfoxide, halogen derivatives, nitrobenzenem aniline), high thermal resistance (150-170oC), good chemical resistance over the entire pH range, good resistance in oxidative medium (hypochlorite 5-7 %, hydrogen peroxide 3-5 %), high mechanical resistance of the films (fracture, flexure, torsion), moderate reactivity in aromatic electrophilic substitutions reactions (sulfonation, nitration, chloromethylation, acylation, etc.). The good solubility allows all preparation methods known for polysulfone membranes, with special emphasis on phase inversion by immersion precipitation. The chemical resistance allows sterilization (both thermal and chemical), while biocompatibility and the moderate reactivity allows functionalisation by aromatic electrophilic substitution or other reactions. In order to combine the advantages of polysulfone as a membrane material and the requirements for enhanced selectivity, this paper reports the results obtained for the synthesis of polysulfone-polyaniline (PSf−PANI) composite membranes using a
290
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
new technique. This consists of phase inversion by immersion precipitation accompanied by chemical reaction, leading to a composite different from that obtained by electro−polymerization. The polymer (polysulfone-PSf) was supplied by BASF (Ultrason S3010). It was further purified from a chloroformic solution by re-precipitation with methanol, and dried in vacuum at 60oC. N,N’−dimethylformamide (DMF), aniline (Merck), and chloroform (Fluka) were used as solvents for polysulfone. Cyclohexanol, acetone, propanol, methanol, ethanol, ethylic ether, ethyl acetate, and octanol were used as non−solvents for membrane phase inversion. Ammonium peroxodisulfate (Fluka) and hydrochloric acid (HCl) (Merck) were used for aniline polymerization. Preparation of polysulfone solution. The required amount of solvent (aniline) was introduced into an Erlenmeyer glass and small portions of polymer were added under magnetic stirring until the desired concentration was achieved. The air was then removed from solution into a vacuum dessicator for 30 minutes. The membranes formation. Method 1: 5 mL of polymer solution was deposited onto a spectral glass and the blade was fixed at a standard thickness of 250 µm. The polymer film formed was immersed into the coagulation bath (I), containing 200 mL cyclohexanol. Method 2: A polysulfone solution was prepared by combining a DMF polymer solution, 80 %, and an aniline polymer solution, 20 %. 5 mL of this polymer solution was deposited onto a spectral glass and the blade was fixed at a standard thickness of 250 µm. The polymer film formed was immersed into the coagulation bath (II), containing 200 mL methanol aqueous solution (50 %). PSf/aniline membrane functionalisation. The crude membranes (obtained either by method 1 or 2) were transferred from the coagulation bath into a tank (III) containing hydrochloric acid, 1 M and were stored in this solution for 30 minutes. Afterwards, the membrane was introduced into the tank (IV) containing the polymerization initiator (ammonium peroxodisulfate in HCl) for 4 hours. In order to finish the functionalisation (aniline oxidation at polyaniline), the composite membrane was kept into a persulfate acid solution for 24 hours. The obtained membranes were stored in water : methanol mixture (1:1). Membranes characterization. Membrane samples (4.8, 5.2, and 9.0 cm diameter disks) were characterized by Fourrier transformed infrared spectroscopy (FT-IR) using a Bruker Tensor 37 instrument, scanning electron microscopy (SEM) using a FESEM Hitachi S 4500 instrument, and thermo−gravimetric analysis using a
291
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
Shimadzu DTA-TG-51H instrument. Membranes were dried in vacuum at 60oC for 4 hours before characterization. Any membrane was washed and kept in distilled water for 24 hours before running the hydrodynamic tests. Perm-porometry (Porometer®), solvent permeation, and solvent flux determination were perfomed (Sartorius installation). 500 mL of pure solvent (water, methanol, propanol, buthanol) were introduced in the tank, and then a pressure between 2 and 6 atm was applied. The permeate specific flux was then calculated by means of relation (1): Jv=Vp/Sm*τ
cm3/cm2·min
(1)
where Jv = flux, Vp = permeate volume, Sm=membrane surface, τ = time. A relatively small number of papers and researchers reported phase inversion with chemical reaction so far. The regenerated cellulose films were made by spinning cellulose xantogenate (Cell-OCS S-Na+) into a coagulation bath containing sulfuric acid as main reagent and sodium sulfate, or zinc sulfate and surfactant. Mulder et al. tried to promote reactive systems in phase inversion by immersion precipitation using poly benzimidazole / sulfuric acid, in alkaline solution; this technique didn’t develop too much and it was beaten by other phase inversion methods. One of the arguments in favour of this technique is the fact that from a polymer/solvent/non−solvent system, even if there is a chemical reaction between solvent and non−solvent, the membrane is formed from the initial polymer dissolved in solution: polysulfone, polyamide or polybenzimidazole. A previous tentative related to in situ polymer functionalisation by phase inversion using PSf/DMF and POCl3 (Vilsmeyer-Haak reaction) yielding formylated polysulfone membrane did not have the expected impact. This is the reason why results related to polysulfone and polysulfone/polyaniline (PANI) membranes synthesis using a relatively new system, PSf/aniline, are presented in this paper. Aniline is miscible with alcohols, halogen derivatives, ethers, esters, and this fact allows the approach of phase inversion in many ways for the polysulfone/aniline/non solvent system. As an alkaline species (pKa ≈ 9), aniline allows phase inversion with chemical neutralization reaction, as well as different organic reactions such as oxidation and condensation. Polysulfone membranes preparation from polysulfone/solvent/non−solvent systems is well known. One should
292
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
find the adequate non solvent so that starting from a known polymer concentration in solvent to be able to obtain a membrane for micro-, ultra-, nano-filtration or pervaporation. The study has followed two steps: a) the synthesis of polysulfone membranes by classical immersion-precipitation from a polysulfone/aniline/non−solvent system and from a polysulfone/aniline−DMF/non−solvent system, b) the synthesis of polysulfone-polyaniline composite membranes from polysulfone/aniline/non−solvent (oxidant) system and a polysulfone/aniline−DMF/non−solvent (oxidant) system by phase inversion with chemical reaction. The characterization and testing of the membranes for an adequate membrane process are additional goals of the study. For the polysulfone/aniline-dimethylformamide/non−solvent system only three coagulants were tested: iso-propanol in water (50 %), ethanol in water (50 %) and methanol in water (50 %). The membranes obtained from polysulfone/anilinedimethylformamide/propanol system and polysulfone/anilinedimethylformamide/ethanol system have small pores diameter and are not suitable for the polymerization of aniline to polyaniline. The best results were obtained for the polysulfone/anilinedimethylformamide/methanol system. A study related to membrane formation through phase inversion was performed. Additionally, polysulfone solutions in aniline at three concentrations (7 %, 11 % and 15 %) were used. For polysulfone/aniline/non solvent system several coagulants were tried (cyclohexanol, acetone, propanol, ethylether, ethylacetate and octanol, and some mixtures of these solvents). A small number of obtained membranes may be used in a membrane process, most of them presenting surface damages and non-homogeneity. This may be explained only by formation of solvent-non−solvent synergic mixtures which interact with polymer and cause resolubilization, swelling, and local agglomeration of the polymer film. The best results were obtained for polysulfone/aniline/cyclohexanol system. Before using a membrane type for a specific membrane process, this should be characterized by SEM, perm-porometry and solvents permeation (water, methanol, ethanol, i-propanol, buthanol). The data from perm-porometry were correlated with the results obtained from the gravimetric method. The SEM images of the obtained membranes (Fig. 1) give important information related to the material synthesis and characteristics. The membranes obtained from PSf/aniline/cyclohexanol system are more compact than the
293
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
membranes obtained using the classical PSf/DMF/methanol system. The polyaniline obtained in polysulfone membranes from PSf/anilinedimethylformamide/methanol system presents a homogeneous surface, with long (aprox. 1-1.5 µm, fig. 2d) and constant diameter (aprox. 100 nm) polyaniline fibres. This is not observed for the PSf/aniline/cyclohexanol system (fig. 2b). This is explained by the constant structure and the big dimension of pores in membranes obtained from system PSf/aniline-dimethylformamide/methanol.
a
b
c
d
Fig.1. Scanning electron microscopy of obtained membranes: 1 – PSf-PANI from PSf/aniline/cyclhexanone system and 2 – PSf-PANI from PSf/anilineDMF/methanol at a – X5000 and b – X25000
The formation of polysulfone-polyaniline composite membrane is also supported by thermal analysis, where a maximum loss point at 561.49oC for polyaniline is observed (Fig. 2). Further comments related to TGA results will be presented elsewhere.
294
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM TGA %
DTA uV 800.00
100.00 549.92C
80.00
60.00
40.00
Start
450.00CStart
500.00C
End
500.00CEnd
550.00C
W eight Loss
-11.014% W eight Loss
-22.275%
Start
550.00C
End
580.00C
W eight Loss
-18.406%
Start
39.59C
End
600.00C
W eight Loss
-78.493%
561.49C 600.00
400.00
479.07C 200.00
0.00
20.00 100.00
200.00
300.00 Temp [C]
400.00
500.00
600.00
Fig.2. TGA spectra for polysulfone membrane (a) and polysulfonepolyaniline composite membrane (b).
The polyaniline from polysulfone-polyaniline composite membranes was doped with sulfonated β-cyclodextrin. A surface of 10 cm2 of a polysulfone-polyaniline composite membrane is immersed in deionized water with 1g of sulfonated β-cyclodextrin and the solution is stirring for 24 h. After this, the membrane is washed with deionized water for residual sulfonated β-cyclodextrin removal (Fig. 3). CH3
O
C
S
CH3
O
O
O
n CH2OSO3H O OSO3H O 7 OSO3H+ N
H N xN H
O
N 1-x n
CH3
O
C
S
CH3
O
O n
Fig.3. Schematic representation for molecular structure of composite membrane
295
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
The ionic conductivity and capacitance for obtained membrane were determined by Electrochemical Impedance Spectroscopy. The measurements were made using a platinum cell with two electrodes with a specific surface of 0,9503 cm-2 in frequency range 100 kHz-100 mHz at 25oC. The obtained EIS spectra are presented in Fig. 4.
a
b Fig.4. EIS spectra for polysulfone-doped polyaniline composite membrane: Nyquist representation in normal impedance coordinates (a) and Bode representation (b)
At a medium membrane thickness of 124,4 µm, the measured capacitance was C = 2,816 µF/cm2 and the ionic conductivity was σ = 1,76 · 10-4 S/cm-1. These values and properties indicate possible applications for this material like micro-capacitors, support for different sensor devices, electronic and optic use solvents purification, colorants and proteins nanofiltration [1-18].
296
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
In conclusion: PSf-aniline system may lead to membrane formation through phase inversion by immersion precipitation into a coagulation bath which contains cyclohexanol (cyclohexanol gives the best results in comparison with other oxygenated organic solvents such as inferior alcohols, esthers, ethers, ketones), followed by immersion precipitation in cyclohexanol and ammonium persulfate (acid solution) oxidation which leads to polysulfone-polyaniline composite membranes. The polyaniline from obtained membranes can be reversible doped with sulfated β-cyclodextrin in order to increase the ionic conductivity and to improve the electrochemical characteristics for new synthesized material. The measured capacitance was C = 2,816 µF/cm2 and the measured ionic conductivity was σ = 1,76 · 10-4 S/cm-1. 2. FERROFLUID MEMBRANES The bulk liquid membranes consisting of an organic phase, usually with a higher density than water, non- miscible with water, which forms a selective barrier between the aqueous feed phase and the receiving aqueous phase (fig. 5 A) [19-21]. In this section we propose the use of ferrofluids as membrane phase. The following advantages can be mentioned to replace the organic solvent from a liquid membrane with a ferrofluid: small volume and implicitly short distances of diffusion; adjustment of the diffusion distance using a magnetic field and finally enhancement of the permeability through the liquid membrane by means of an external magnetic field. The disadvantages of membranes with ferrofluid are related to the existence of a dispersed system: the decrease in diffusivity through the membrane due to the increase of the medium viscosity without and in the presence of the external magnetic field; and limitations of the membrane system caused by colloidal system instabilities of the ferrofluid based membrane. In terms of transport properties, this system seems similar to the bulk liquid membranes but by decreasing the amount of solvent, the ferrofluid type of membrane exhibit a closer resemblance to immobilised liquid membranes (ILM’s) or supported liquid membranes (SLM’s). In this way a new type of membrane system can be defined as “magnetically immobilised membrane". Using a system of magnets a part of the membrane could be redispersed in the water phase (source or receiving phase) which
297
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
increase the surface accessible for mass transfer. In this way the ferrofluid system is comparable to the emulsion liquid membrane. The liquid membranes based on ferrofluid have a small diffusion length for the chemical species from the feed phase to the receiving phase as has been described in the literature study. However, a short lifetime, in connection to the loss of the liquid membrane from the pores of the spongy support is a disadvantage of this system. In literature many methods are proposed to enhance the stability of the supported liquid membranes (gelation, coating with a nonporous toplayer). Here, a new idea has been presented to solve the problem of membrane stability; a colloidal dispersion of monofield magnetic particles (ferrofluid) is used and this has the advantage that a magnetic field can be applied to keep the ferrofluid within the pores of the membrane. Due to the similar dimensions of the magnetic particles with the pores of the support membrane, the Brownian motion is reduced at minimum. The system becomes very complex by the addition of the magnetic particles and in this way the advantage of a liquid membrane has been diminished. It is necessary to introduce agitation insight the membrane, by using the magnetic field over the magnetic particles. Application of a variable magnetic field in the membrane zone should improve the mass tranfer efficiency. Two main aspects of this type of membrane were investigated: the effect of the frequency of the applied magnetic field and the intensity of the field as well. The experimental device. A new experimental device /25/ has been developed to carry out the experiments. The separation performance, as indicated by selectivity, and transfer rate are dependent on the redox conditions of the phases. The experimental device allows adjusting this redox control. It is obvious that it is extremely complicated to control both phases, which makes it difficult to interpret the results. From a technological point of view (e.g. in relation to some applications as hemodialysis), it is more useful to have a redox controlled system in the acceptor phase. A comparison is presented in figure 5, where "A" represents the device according to ref. [22-24], and "B" the device that is utilised now. Polysulfone was used as support membrane and before each experiment was impregnated with Fe3O4 colloids in dioctyladipate. The membrane area was in all cases 16 cm2 and phenol was used as solute to remove. The two phases had the following characteristics;
298
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
• The receiving phase, a volume of 500 mL of a saturated KCl solution (for an optimal electric conductivity to allow a redox reaction by the applied electric field); • The source phase, a volume of 1000 mL with a solute concentration of 0,1-% phenol in a saturated KCl solution (in order to diminish possible osmotic effects).
A
B Fig 2. Experimental device for a ferrofluid membrane in a magnetic field. A - device according to literature: M-magnet, FS-source phase; FRreceinving phase, LM-ferrofluid membrane; B - device developed in own laboratory: 1.source cell; 2. source phase;receiving cell; 4. receiving phase; 5. mixing ; 6. supported ferrofluid membrane; 7. electrode
A potential difference of 30 V was applied between the electrodes of 200 mm distance which generates a current of 2,3 … 2,6 mA. (in some exceptional cases 3,2 mA.). The phenol transfer rate has generated an increase in current proportionally to its concentration. It is supposed that the frequency, at which the
299
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
magnetic field polarity is changed, induces an additional movement of the Fe3O4 magnetic particles in the support membrane thickness that results in an increase in transfer rate. 2.1. THE INFLUENCE OF THE FREQUENCY OF THE MAGNETIC FIELD
Each experimental series were carried out at different rH values and the results are presented in figure 6 - 8.
Fig.6. The influence of the receiving phase rH value upon the intensity of the electrical power (for 0 Hz)
Fig.7. The influence of the receiving phase rH value upon the intensity of the electrical power (for 50 Hz)
From these results (fig. 6), it can be observed that the transfer rate has a maximum value at rH 34,3 but, the rH window width is larger, and this is possible because of the inherent modifications brought to the experimental device and to the support membrane.
300
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
These experiments show that the maximum position doesn't change but the membrane system selectivity decreases and this is more evident at 100 Hz (fig. 8).
Fig.8. The influence of the receiving phase rH value upon the intensity of the electrical power (for 100 Hz)
It can be observed that the scattering of the experimental results increases once a magnetic field has been applied which induces additional disturbance. Also, the i values obtained in the optimal rH field (rHt = 34,3) are decreasing when the frequency increases (see fig. 9 - dashed line).
Fig.9. The influence of the frequency upon the intensity of the electrical power (at the optimal rH value in the receiving phase)
301
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
An increase in frequency would improve the ferrofluid agitation. Following the experimental results (solid line), a flux increase could be observed at low frequency. In practice, the optimum frequency and the magnetic field intensity depend on the saturation magnetising of the ferrofluid and the solvent viscosity. 2.2. THE INFLUENCE OF THE INTENSITY OF THE MAGNETIC FIELD
The frequency but also, the magnetic field intensity influenced the magnetic particles agitation in the ferrofluid. It was observed from the experiments that the optimum frequency was 50 Hz. Figures 10 and 11 show the experimental data on the intensity of the magnetic field on the flux (the full points represent the i parameter value obtained directly where the empty points are related to the 0,8 mA value which are in agreement with the early observations with respect to rH 42,16.
Fig.10. The influence of the receiving phase rH value upon the intensity of the electrical current (for 2 coils)
From these results it can be seen that the maximum remains the same. In addition, a difference in flux can be observed but the data are too scattered to draw any conclusion. Depending on the field intensity two transport mechanisms can be observed; If a supported liquid membrane is considered in which the pores are filled with ferrofluid, a minimum influence can be observed at a low intensity of the magnetic field. In this case, the transport across the membrane is similar to a semi-permeable membrane with a large hydraulic resistance.
302
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
Fig.11. The influence of the receiving phase rH value upon the intensity of the electrical current (for 6 coils)
Comparing these results with those obtained with the polysulfone membranes, without the liquid membrane presence, and i = 10 mA, the similarity is obvious. However, it should be realised that the Brownian motion of the magnetic particles within the membrane pores induces a small increase in flux. In case of a larger intensity, the transport mechanism is comparable to the liquid membrane. In conclusion: The transfer of some organic substances has been studied using a support membrane with ferrofluid. This membrane system is practically impermeable, in the absence of a specific external force. This force can be a redox gradient between the source phase and the receiving phase (dialysis type) or an electric potential difference between these two phases (electrodialysis type). The ferrofluid membranes allow the transfer of some organic substances, which is influenced by the physical-chemical characteristics of the basic liquid, and by the intensity and frequency of the applied magnetic field to the membrane system. The transfer of these substances (phenol, pesticide) does not occur in the same way;
303
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
it depends on the receiving ferrofluid phase and consequently, it can be adjusted by changing the basic liquid. The selective transfer could be realised by including in the basic liquid some specific carriers.
3. DOUBLE JET LIQUIDMEMBRANE 3. 1. INTRODUCTION
Transport and separation of chemical species through the membrane is, today, a viable alternative for the purification and concentration of substances with various applications in both industrial cleaning processes and environmental analytical chemistry [24,25]. In some cases, transport liquid membrane process is the only possible or effective separation of chemical species from aqueous solutions [26]. The liquid membrane separation process is applied, even if the concentration of the useful component has a small concentration [27,28]. The mechanism of the transfer and transport processes of iodide anion through immobilized membrane is presented in the figure 12. As the model reveals, the iodide anions are transformed by a chemical reaction into iodine molecules in the source phase [29]. Then, the iodine molecules are transferred into the membrane, transported towards receiving phase and afterwards transferred into the receiving phase. Here, the iodine molecules were transformed by a chemical reaction into iodide anions. The source phase and the receiving phase are dispersed in the liquid membrane that represents the continuous phase meant to intensify the transfer processes.
Fig.12. Transfer and transport processes of iodide anion through immobilized membrane.
304
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
This paper presents a hydrodynamic study of the dispersed phases that was generated in the immobilized liquid membrane in order to determine the favorable conditions to implement this separation process to remove the anionic iodide from the aqueous solutions. 3.2. EXPERIMENTAL
The experiments were carried out with the help of a double jet installation presented in figure 13. The installation function as follows: the source phase and the receiving one are introduced in the separation dropping funnels. In the permeator, the chloroform membrane was introduced and the aqueous phases are collected in graded cylinders so that the working flow rate can be evaluated. More information about the experimental installation set-up was presented in a previous paper [29,30]].
Fig.13. Schematic experimental installation.
The hydrodynamic parameters determined were: droplet average diameter, specific interfacial surface and shape factor. The hydrodynamics parameters of the droplets were measured by photographic method [31]. The capture with a video digital camera of the droplet formations and the measurement of these parameters on the photographic slides are realized with specialized processing image program. In order to reduce the experimental errors for each distribution curve, 1000 drops were measured. Experimental results are expressed in the form of histograms for equivalent diameter and
305
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
shape factor of drops. The operating flow rates of the dispersed phases are presented in Table 1. Table 1. Operating flow rates Flow rate I Flow rate II Flow rate III Flow rate IV
source phase receiving phase source phase receiving phase source phase receiving phase source phase receiving phase
1.046 1.042 3.632 3.566 6.475 6.680 8.675 8.718
[ml/min] [ml/min] [ml/min] [mL/min] [mL/min] [mL/min] [mL/min] [mL/min]
3.3. RESULTS AND DISCUSSION
The diameters size distribution and shape factors for different flow rate of source phase are presented in figure 14. 50
flow flow flow flow
% 40
rate rate rate rate
I II III IV
30
20
10
0 1
2
3
4
5
6
d, mm
Fig.14. The influence of source phase flow rate on the droplets’ size distribution.
-
Analyzing these figures the following conclusions will be drawn: At low flow rates (flow rates I and II) the histograms are unimodal, having an aspect of normal distribution and therefore, the representative values for each size are the medium average ones.
306
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
-
For higher flow rates (flow rates III and IV), the distribution curves are bimodal. It has been noticed that in this flow rate interval, the formation of each droplet through the installation nozzle is followed after the separation by the formation of another smaller droplet, known as „pilot droplet”. This explains the distinct bimodal curve distribution on the histograms. In this case, the average diameter equals the medium average for the values of the average diameters, which corresponds to the distribution of the two categories of droplets. This is a consequence of the fact that the number large of droplets is similar to the one of the smaller ones. - The shape of the distribution curve and the variation of the measured values depend on the flow rates values. The experimental results for the hydrodynamic parameters for the receiving phase are shown in Figures 15. 50
%
flow flow flow flow
40
rate rate rate rate
I II III IV
30
20
10
0 0
2
4
6
8
d, mm
Fig.15. The influence of receiving phase flow rate on the droplets’ size distribution.
-
The analysis of the figure presented leads to the following conclusions: Despite the similarities of the formation conditions, the values of the hydrodynamic parameters of both phases are different. This
307
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
-
points out the influence of the physical properties of the dispersed systems on the shape and size of droplets. In this case too, at flow rates lower than 3.5 mL/min, the values of the hydrodynamic parameters are independent from the effluent flow rates. The size distribution in these conditions is normal. At flow rates higher than 3.5 mL/min, the distribution curves are bimodal. The shape of the distribution curve for each group of droplets is normal as well.
The comparative analysis of the results, presented in Figures 14 and 15 leads to the following conclusions: - At low flow rates, for both phases, the shape of the distribution curve is practically the same. The only difference is the interval of the values obtained. - The diameter values depend on the effluent flow rate and on the physical properties of the dispersed phases. - At higher flow rates, the distribution curves are bimodal as a consequence of the „pilot droplet” formation. - An increase of the effluent flow rate determines changes into the distribution of droplet sizes. For the source phase, the distribution curve of the „pilot droplets” remains unchanged, whereas the curve of the large droplets registers smaller values. As for the receiving phase, the distribution curve of large droplets remains unchanged, while the distribution curve of the „pilot droplets” increases its values. This different behaviour causes a decrease of the average diameter in the first case, and an increase of it in the second case. The values of superficial tensions of the dispersed liquids may be the explanation of this phenomenon. The influences of the dispersed flow rate on the mean diameter and on the shape factor of the droplets are presented in Figures 16 and 17. It will also be noticed that at a flow rate of 3.5 mL/min, the mean diameter curve registers a maximum value. The average value of shape factor is characteristic for the ellipsoidal droplets. This conclusion confirms the experimental observations. The increase of the shape factor value is related to the value of the effluent flow rate. This in turn is correlated to a diminished value of the mean diameter of the droplets and demonstrates that at higher flow rates, the droplets’ shape tend to became spheroid. The influence of the flow rate upon the specific area (Fig. 10), shows that from the hydrodynamic point of view, the operating at flow rates higher than 3.5 mL/min is favourable to the separation process.
308
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM 8
1.0
6
0.9
4
0.8 2
source phase receiving phase
souce phase receiving phase
0
0.7 2
4
6
8
10
0
2
Gv, ml/min
4
6
Gv, mL/min
a.
b.
Fig.16. The influence of the dispersed flow rate on the mean diameter (a) and on the shape factor (b) of the droplets.
2400
source phase receiving phase
3
2000
2
0
aspm, m /m
d, mm
φ
1600
1200
800 0
2
4
6
8
10
Gv, mL/min
Fig.17. The influence of the dispersed flow rate on the specific interfacial area of the droplets.
309
8
10
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
The data above prove that for the reproductibility of the experiment, the accurate measure of the effluent flow rate is extremely important. Therefore, the installation was equipped with Mariotte jars throughout the effluent’s circuit to regulate and maintain a constant value. 3.4. CONCLUSION
From the experiment with a new liquid membrane – double-jet liquid membrane system - shown above it can be deduced that: - The efficiency of the membrane separating installation depends on the hydrodynamic conditions of the process carried out and also on the physical properties of the dispersed phases. - In order to avoid accumulation of transfer substance into the membrane, equal specific areas for both dispersed phases are required. These conditions correspond to the flow rate variation ranged between 4 to 6 mL/min. - The value of the dispersed phase flow rate is important for the process and for this reason, the installation is equipped with devices meant to regulate and maintain the pre-established values. Presented double-jet liquid membrane system is available to conducting cleaning and clean processes.
REFERENCES [1] Membrane Technology in the Chemical Industry. Edited by S. P. Nunes and K.-V. Peinemann, Wiley-VCH Verlag GmbH, 2001. [2] C. Saravanan, S. Palaniappan, F. Chandezon, “Synthesis of nanoporous conducting polyaniline using ternary surfactant”, Materials Letter,s 62, p. 882-885, 2008. [3] X. Li, T. Zhuang, G. Wang, Y. Zhao, “Stabilizer-free conducting polyaniline nanofiber aqueous colloids and their stability”, Materials Letters, 62 (2008) pp. 1431-1434, 2008. [4] R.J. Petersen, “Composite reverse osmosis and nanofiltration membranes”, J. Mem. Sci., 83, p. 81, 1993. [5] K.A. Lundy, I. Cabasso, “Analusis and construction of multi-layer composite membranes for the separation of gas mixtures”, Ind. Engng. Chem. Res., 28, p. 742, 1989. [6] Y. Dai, M.D. Guiver, G.P. Robertson, F. Bilodeau, Y.S. Kang, K.J. Lee, J.Y. Jho, J. Won, “Modified polysulfones 5: synthesis and characterization of tetramethyl polysulfones containing trimethylsilyl groups and their gas transport properties”, Polymer, 43, p. 5369, 2002.
310
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
[7] G. Nechifor, S.I. Voicu, A.C. Nechifor, S. Garea, “Nanostructure hybrid membrane polysulfone-carbon nanotubes for hemodyalisis”, Desalination, 243, 2009. [8] S.I. Voicu, A.C. Nechifor, B. Serban, G. Nechifor, M. Miculescu, “Formylated Polysulphone Membranes for Cell Immobilization”, Journal of Optoelectronics and Advanced Materials, 9(11), p. 3423-3426, 2007. [9] M. Mulder, Ph.D. Thesys Pervaporation – separation of ethanol-water and of isomeric xylenes, Twente University, Enschede, Netherlands, p. 81-104, 1986. [1 0] R. Raut e nba ch, R. Albre cht, Membrane Processes, John Wiley, New York, 1989. [11] K. Rodemann, E. Staude, “Synthesis and characterization of affinity membranes made from polysulfone”, J. Mem. Sci., 88, p.271, 1994. [12] B. Serban, M. Bercu, S.I. Voicu, M. Mihaila, G. Nechifor, C. Cobianu, “Calixarene-Doped Polyaniline for Applications in Sensing”, Proceedings of International Semiconductors Conference (CAS), p. 257, 2006. [13] B. Şerban, M. Bercu, Ş.I. Voicu, A.C. Nechifor, C. Cobianu, “Sinteza si caracterziarea unei noi polianiline dopata cu sulfat acid de ciclodextrina”, Revista de Chimie, 57, pg. 978, 2006. [14] A. Higuchi, N. Iwata, M. Tsubaki and T. Nakagawa, “Surface-modified polysulfone hollow fibers”, Journal of Applied Polymer Science, 36, p.1753, 1988. [15] A. Higuchi, S. Mishima and T. Nakagawa, “Separation of proteins by surface modified polysulfone membranes”, J. Mem. Sci., 57, p.175, 1991. [16] Z.A. Hu, L.J. Ren, X.J. Feng, Y.P. Wang, Y.Y. Yang, J. Shi, L.P. Mo and Z.Q. Lei, “Platinum-modified polyaniline/polysulfone composite film electrodes and their electrocatalityc activity for methanol oxidation”, Electrochemistry Communications, 9, p.97, 2007. [17] Y. Yang, X. Shang, C. Kong, H. Zhao and Z. Hu, “Preparation of PANI/PSF conductive composite films and their characteristics”, Frontiers of Chemistry in China, 1, p.484, 2006. [18] Z.-A. Hu, X.-L. Shang, Y.-Y. Yang, C. Kong and H.-Y. Wu, The electrochemical synthesis of polyaniline/polysulfone composite films and electrocatalytic activity for ascorbic acid oxidation, Electrochimica Acta, 51 p.3351, 2006. [19] R. Leysen, Ph. Vermeiren and W. Doyen, in “Future Industrial Prospects of Membrane Processes”, (L. Cecille and J.C. Toussaint eds.), Elsevier Science Publ. Co., London - New York, 1989, p. 268; [20] R.P. Cahn, N.N. Li, Sep. Sci., 1974, 9, 505; [21] S. Schlosser, ‘Advances in membrane phenomena and processes’ Lecture book ESMST Summer School, Gdansk, Poland, 1988, p 163-183. [22] C. V. Uglea, C.V. Zanoaga, J. Membr. Sci., 1989, 47, 285 [23] a. A.M. Nechifor, C.V. Zanoaga, G. Nechifor, unpublished report 19951996; b. A.M. Nechifor, G. Nechifor, M. Piso, G. Tutos, G. Pintilescu, PAT. No. : RO 110 407/5.04.1994 c. A-M. Nechifor, Gh. Nechifor, Ferrofluid Membranes, Roum. J. Colloid Sci. Assoc., 1997, vol. 2, no. 2 ,p. 24-29
311
Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM
[24] G. Nechifor, B. Albu, D. Rata, G.Popescu, Membranes in the separation processes. 7. Organic compounds removal from the aqueous solutions. Rev. Chim., 1996, 47(3), 260-264. [25] A.C. Nechifor, B. Şerban, E. Ruse, G.Nechifor, Calixarene-carriers in + liquid membranes. III. The transport of Pb2 cations through liquid membranes with thiocalix[4]arene, Rev. Chim., 2002, 53(1), 20-26. [26] A. A. Ensafi, H. Eskadari, Efficient and selective extraction of iodide through a liquid membrane, Microchem. Journal, 2001, 69, 45-50. [27] M. Bumbac, B. Serban, C. Luca, G. Nechifor, S.I. Voicu, Studies + + regarding the synergistic effect in the extraction of alkaline metal Na and K cations in the presence of a mixture of p-tert-butylcalix[4]arene –ether crown, Rev. Chim., 2006, 57(9), 927-930. [28] K. Farhadi, M.Shamsipur, Separation study of cadmium as CdI42through a bulk liquid membrane containing ketoconazole and oleic acid, Analytical Science, 2005, 21, 501-505. [29] E.Gheorghe, L.Barbu, G.Nechifor, C.Luca, The Pb2+ cations transport through liquid membrane with macro cycle benzo-18-crown-6, Rev. Chim., 2006, 57(9), 940-944. [30] M.E. Craciun, E. Ruse, G. Nechifor, An efficient iodide separation procedure from aqueous solutions, Rev. Chim., 2006, 57(9), 936-939 [31] M. Mihai, I. Pincovschi, Hydrodynamic of bubbling columns I. Methods for measuring the interfacial specific area, Rev. Chim., 1999, 50, 2, 113-120.
312
