Buoyancy Effects in Dead-End Reverse Osmosis: Visualization by

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Buoyancy Effects in Dead-End Reverse Osmosis: Visualization by Holographic Interferometry Julio Ferna´ ndez-Sempere, Francisco Ruiz-Bevia´ ,* Raquel Salcedo-Dıáz, and Pedro Garcıá-Algado Chemical Engineering Department, UniVersity of Alicante, Apartado 99, E-03080 Alicante, Spain

Real-time holographic interferometry, previously used to measure concentration profiles in the polarized layer during membrane processes, has been applied to visualize the buoyancy effects on dead-end reverse osmosis of salts by rotating the cell 90° and 180° from its original position (0°). Sets of experiments have been carried out, each one with the membrane in a different gravitational orientation, using NaCl and Na2SO4 with a feed concentration ranging from 1 to 7 kg/m3 at a constant pressure of 600 kPa. The interferometric fringe patterns obtained in each membrane position were very different. At the 0° position, the evolution of the polarization layer was observed by means of several interferometric fringes parallel to the membrane surface. At 180° position, the interferometric fringe pattern obtained pointed out the existence of a natural convection or buoyancy flow in the vicinity of the membrane surface which prevented the growth of the polarization layer. Finally, with the membrane vertically placed, an intermediate situation occurred. The combination of the diffusive movement of the solute (horizontal) and the natural convection currents (vertical) caused by the density gradient established in the module produced deformations on the interference fringes. The main consequence of the buoyancy effect is a great enhancement of the membrane performance and, therefore, a higher effectiveness of the reverse osmosis process. 1. Introduction The performance of pressure-driven membrane processes may be significantly improved when unsteady fluid instabilities are superimposed on cross-flow. Winzeler and Belfort1 have comprehensively analyzed various possible mechanisms of unsteady secondary flows resulting from surface roughness, flow pulsation, and centrifugal instabilities with respect to depolarization and defouling of membranes. Another method capable of inducing fluid instability near the membrane surface is the use of natural convection. Previous experimental studies2-4 have observed the effect of buoyancy on reverse osmosis (RO) and ultrafiltration (UF). In an excellent experimental study on the effects of natural convection instability on membrane performance in dead-end and cross-flow UF, Youm et al.5 presented the buoyancy phenomenon in a very clear way. In membrane separations, as a consequence of the variation of solute concentration across the concentration polarization layer, a density variation occurs so that the solution density at the membrane surface is higher than that in the bulk solution. By changing the gravitational orientation of the module, a density inversion is obtained, which may lead to an unstable fluid behavior causing natural convection or buoyancy flow in the vicinity of the membrane surface. In the paper, three different orientations were studied: zero gravity in the flow direction (90°), gravity acting in the same direction as the flow direction (0°), and gravity acting in the opposite direction to the flow (the most gravitationally unstable orientation) (180°). In a more recent paper, Fletcher and Wiley6 studied from a theoretical point of view the effect of buoyancy on the polarization layer in cross-flow reverse osmosis (RO) by simulation with a mathematical model. This was applied to the study of salt water separation in a flat system. Both studies showed that, in cross-flow operations, gravitational effects were only important at low flow rates because * To whom correspondence should be addressed. Tel: +34965903547. Fax: +34-965903826. E-mail: [email protected].

high velocities may overshadow the effect of natural convection. When dextran and bovine serum albumine (BSA) solutions are ultrafiltered,5 permeate flux enhancements by natural convection instability only occurs if the cross-flow velocity is lower than 0.1-0.2 m/s (Re ) 35-90). On the other hand, in the computational fluid dynamics study of buoyancy effects in RO,6 four different inlet velocities were considered (0.002, 0.005, 0.01, and 0.1 m/s, which correspond to Re numbers ranging from 8 to 400). Results of permeate flow showed that the effect of buoyancy was only significant at the lowest inlet velocity (Re ) 8). However, in dead-end operation (UF of dextran and BSA solutions),5 the permeate fluxes at the gravitationally unstable orientation of the cell (180°) are enhanced several times: up to 3.5 times for dextran solutions compared with the results at the stable orientation (0°) and 5.5 times for BSA solution. This flux enhancement implies that natural convection flow induced by the density inversion produces a gravitationally unstable state in the concentration boundary layer which may promote the movement of accumulated solutes away from the membrane surface (depolarizing). During ultrafiltration at 90° (gravitationally semistable), the fluxes were also enhanced, although less than when the 180° position was used. The aim of this work is to visualize, by means of holographic interferometry, buoyancy effects on the polarization layer during RO of salt solutions. According to the results described above, it seems that the best way to study the effects of buoyancy is operating in dead-end conditions. Some researchers, in most cases using optical techniques, have previously reported experimental observations for the existence of natural convection on membrane processes. Hendricks et al.2 used the shadowgraph technique to observe convection patterns in an RO unstirred batch cell, placing the membrane at the top of the cell. A representative shadowgraph was shown in the paper. From the dark membrane at the top of the figure, several bright filaments bounded by darker zones were seen to extend vertically downward for a short distance. Although the average fluid motion in the vicinity of the membrane was upward, the

10.1021/ie061433z CCC: $37.00 © 2007 American Chemical Society Published on Web 02/20/2007

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Figure 1. Schematic diagram of the three positions of the module.

filaments were interpreted as downward-moving jets (or plumes) of increased salinity. This pattern was developed a few minutes after the cell was pressurized, with filaments first appearing at the membrane and then descending slowly. In this same article, the authors also showed profiles of salt concentration obtained with electrical conductivity microprobes in a two-dimensional laminar channel-flow configuration of a desalination system. When the membrane was at the top of the channel, the measurements of concentrations within the diffusion layer were observed to fluctuate. Other authors,7-9 using classic MachZehnder interferometry, studied the effect of gravity on diffusion layers formed around the membranes of graviosmotic systems. The osmotic diffusion flux was significantly higher when the denser liquid was located above and the lighter liquid below the membrane in a horizontally mounted diffusion cell. Water, aqueous ethanol solutions, and aqueous glucose solutions were the liquids used. The authors explained their observations in terms of natural convection instability that reduced boundary layer thickness. In this paper, holographic interferometry, an optical technique by means of which changes in the refractive index can be visualized as interference fringes, is used. During a reverse osmosis experiment, changes in the refractive index distribution, and therefore in the concentration distribution, are visualized as an interference fringe pattern. Holographic interferometry presents some advantages over classic interferometry: it is simpler, only one cell is needed (because the comparison is between two images of the same cell obtained at different times), there is no need for windows of high optical quality on the RO cell, and the inhomogeneities in the windows of the cell do not cause perturbations in the measurements. This technique has been previously used for visualization and measurement of concentration profiles in the polarized layer during ultrafiltration processes.10,11 In these previous papers, only experimental data of concentration profiles were presented and no theoretical model of simulation was used because, during ultrafiltration, some additional processes occur, such as solute adsorption or gel layer formation, which complicate the modeling of ultrafiltration. In a more recent paper,12 the same holographic interferometry technique was used to determine in-situ and real-time concentration profiles during dead-end RO of NaCl and Na2SO4 solutions, where the previously mentioned additional processes are not expected. Concentration profiles determined were compared with those calculated using the mixed convection-diffusion and osmotic pressure theory or Fick’s second law of diffusion, depending on whether the profiles correspond to the develop-

ment or the disappearance of the layer. A reasonable agreement between experimental and calculated results was obtained. In this case, all experiments were performed in a horizontal unstirred batch cell, placing the membrane at the bottom (gravity acting in the same direction as the flow direction (orientation 0°)). In the present paper, to determine the effects of natural convection instability on dead-end reverse osmosis processes, several experiments were carried out changing the gravitational orientation of the module, that is, horizontal, but placing the membrane at the top (gravity acting in the opposite direction to the flow) which is the most gravitationally unstable orientation (180°) and vertical, with zero gravity in the flow direction (90°). The main objectives of this research are (1) to visualize what happens during the process in the vicinity of the membrane, (2) to study the buoyancy effects on the performance of the membrane (permeate flux and salt rejection), and (3) to compare the new results with those obtained in the previous work12 (gravity acting in the same direction as the flow direction (0°)). 2. Experimental Materials, the optical setup (holographic interferometry system), the RO setup, and the experimental methodology were described in a previous paper.12 Two solutes (NaCl and Na2SO4) with different feed concentrations (1-7 kg/m3) have been used. All the experiments have been carried out at a constant pressure of 600 kPa. All the experiments (including those of the previous paper12) were performed using the same RO cell but in three different gravitational orientations obtained by rotating it. In each one, the membrane surface was in a different position: (1) horizontally placed facing upward (0°), in the previous work, (2) horizontally placed facing downward (180°), and (3) vertically placed (90°), in the present work. Figure 1 shows the three positions of the membrane surface. 3. Results and Discussion 3.1. Visualization of the Polarization Layer and the Buoyancy Effects in the Vicinity of the Membrane. Really, as later will be stated, a completely developed polarization layer only exists in the 0° gravitational orientation. In a previous paper,12 a set of experiments were carried out with the RO cell in a horizontal unstirred batch cell, placing the membrane at the bottom (gravity acting in the same direction as the flow direction (orientation 0°)). In the present work, using

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Table 1. Experimental Conditions Cell Orientation 0° experiment

Co (kg/m3)

Jw × 106 (m3/m2 s)

Rm × 10-11 (Pa s/m2)

I II III IV V

1 2 3.5 5 7

NaCl 5.61 5.78 5.71 5.42 5.69

1.08 1.05 1.06 1.12 1.07

VI VII VIII IX X

1 2 3.5 5 7

Na2SO4 6.55 6.61 6.61 6.39 6.47

0.93 0.92 0.92 0.95 0.94

experiment

Co (kg/m3)

Cell Orientation 180° Jw × 106 (m3/m2 s)

Rm × 10-11 (Pa s/m2)

XI XII XII XIV XV

1 2 3.5 5 7

NaCl 5.84 6.31 5.80 5.67 5.73

1.04 0.96 1.05 1.07 1.06

XVI XVII XVIII XIX XX

1 2 3.5 5 7

Na2SO4 6.91 6.91 6.91 6.74 6.62

0.88 0.88 0.88 0.90 0.92

experiment

Co (kg/m3)

Cell Orientation 90° Jw × 106 (m3/m2 s)

Rm × 10-11 (Pa s/m2)

XXI XXII XXIII XXIV XXV

1 2 3.5 5 7

NaCl 4.82 4.84 5.01 4.78 4.98

1.26 1.26 1.21 1.27 1.22

XXVI XXVII XXVIII XXIX XXX

1 2 3.5 5 7

Na2SO4 5.10 5.17 5.14 5.03 5.05

1.19 1.18 1.18 1.21 1.20

the same experimental procedure, two new sets of experiments were carried out, each one with the RO cell in a different gravitational orientation (180° and 90°) (see Figure 1). For each position of the membrane, solutions of different concentrations of NaCl and Na2SO4 were used. The operation conditions of the experiments carried out (including orientation 0°, for comparison) are given in Table 1, where Co is the bulk concentration, Jw is the permeate flux for pure water, and Rm is the membrane hydraulic resistance to the flux with pure water, which is calculated as

Rm )

∆P Jw

(1)

Orientation 0°. With this gravitational orientation of the dead-end cell, the real-time holographic interferometry technique allows the appearance, the evolution with time, and the disappearance of the polarization layer to be visualized as interference fringes. In a previous paper,12 some interferograms corresponding to two experiments with different initial concentrations of Na2SO4 (2 and 5 kg/m3) were shown, as an example, to illustrate the appearance and evolution of the polarized layer. Additionally, in the present work, interferograms corresponding to experiment VIII with 3.5 kg/m3 initial concentration of Na2-

SO4 are shown in Figure 2. A few minutes after the RO process started, some parallel interferometric fringes near the membrane surface appeared. The amount of fringes continued increasing throughout the process, thus indicating that the concentration at the membrane surface (Cm) was increasing as well as the thickness of the boundary layer (δ). During the first few minutes of the experiment, the rate of appearance of new interference fringes was high, but later it decreased. Using the methodology described in previous papers,11,12 concentration profiles were determined and compared with those calculated using the mixed convection-diffusion and osmotic pressure theory. A reasonable agreement between experimental and calculated results was observed. The real-time holographic interferometry technique also allows the disappearance of the polarization layer to be visualized as interference fringes. Once the pressure in the RO process with the 0° gravitational orientation ceased, concentration profiles were measured. A mathematical model, on the basis of a diffusive mechanism, was proposed and agreement between experimental and calculated concentration profiles was obtained. Orientation 180°. This orientation of the cell, gravity acting in the opposite direction to the flow, is the most gravitationally unstable orientation. Natural convection or buoyancy flow in the vicinity of the membrane surface is produced. The realtime holographic interferometry technique allows the gravitational effects on the boundary layer, near the membrane, to be visualized. Video 1 (http://www.ua.es/es/servicios/si/servicios/ videostreaming/iq/Video1.html), showing the first 3 min at the start of experiment XIX (5 kg/m3 initial concentration of Na2SO4), has been prepared. To better perceive the changes occurring, the video reproduces the images at triple the speed of the real process (as a consequence, the video is 1 min long). Additionally, Figure 3 shows interferograms at 52, 54, 55, 57, 112, 115, 116, and 118 s corresponding to the same experiment. All these pictures are taken from video 1. In this case, the fringe patterns are different from those obtained when the module was in the 0° orientation. Instead of a concentration profile continuously increasing similar to that which appeared at 0°, only one or two fringes appeared which were very close and parallel to the membrane. A few minutes after the cell was pressurized, something with a shape like filaments or elongated fluctuating drops and moving downward from the membrane surface was observed. This image was similar to that described by Hendricks et al.2 They interpreted these “filaments” descending from the membrane surface as downward-moving jets (or plumes) of increased salinity. In the video and in Figure 3, some of these filaments or “big fluctuating drops” can be seen. Later, the fringes far from the membrane started to deform and acquired a curved appearance. With time, the fringes became wider, more deformed, and farther away from the membrane. In Figure 4, some interferograms with deformed fringes are shown as an example. At the interferogram corresponding to 20 min after the beginning of the process, a wavy fringe was observed. Later (interferogram at 1 h), the wavy shape of the fringe became more defined. In the interferogram obtained at 2 h, two circular zones had appeared above the previous wavy fringe and at 2 h 30 min, a new wide and deformed fringe near the membrane started to come into sight. A possible explanation of this phenomenon could be the accumulation of solute at the membrane surface as a consequence of the reverse osmosis process. The variation of solute concentration implies a variation of the solution density. In this membrane position, a higher density solution was on top of a lower density solution, and a fluid instability was produced.


Figure 2. Interferograms belonging to experiment VIII at 5, 35, 90, and 120 min.

Figure 3. Interferograms belonging to the beginning of experiment XIX at 52, 54, 55, 57, 112, 115, 116, and 118 s.

Figure 4. Interferograms belonging to experiment XIX-180° at 20, 60, 120, and 150 min.

The material moving downward generated a natural convection (or buoyancy) flow in the vicinity of the membrane. These convection currents produced circular interferometric fringe patterns, differing from the parallel fringe patterns obtained when the membrane was facing upward. Figure 5 shows a diagram of the fluxes possibly involved in the reverse osmosis process under natural convection. When the pressure was removed, the convective solute flux to the membrane surface ceased and the fringe pattern was modified, showing a tendency to form fringes parallel to the

membrane surface. Video 2 (http://www.ua.es/es/servicios/si/ servicios/videostreaming/iq/Video2.html) showing the first 3 min after the pressure ceased in experiment XIX was prepared. To better perceive the changes occurring, the video reproduces the images at triple the speed of the real process (therefore, the video is 1 min long). At the beginning, some perturbations can be observed as a consequence of the pressure disappearing. A few seconds later, because there was an accumulation of solute in the vicinity of the membrane, whereas a more diluted solution was present in the bottom part of the cell, a certain quantity of

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Figure 5. Diagram of the fluxes involved in the reverse osmosis process under natural convection.

Figure 6. Interferograms belonging to experiment XIX at 25, 26, 27, 28, 30, and 32 s after the pressure ceased.

the less dense solution started to move upward, toward the membrane, trying to equalize the densities. This phenomenon can be observed in video 2 and in the interferograms in Figure 6 (taken from video 2 at times 25, 26, 27, 28, 30, and 32 s). In the first interferogram, a bright zone, like a bag, can be seen, bounded by darker zones that rise up from the bottom part of the cell, as an eruption, to extend vertically upward. In the last interferogram, the bright zone has stabilized becoming a kind of channel or chimney connecting the bottom part of the cell with the top, so it may be assumed that the less dense solution flows upward by natural convection. Simultaneously to these processes, some thin fringes, which continue to develop later, begin to form parallel to the membrane surface. The behavior of the fringes was similar to that with the membrane facing upward, but in this case the movement of the solute is not only diffusive but also convective because of the gravitational effect. The fringes became wider and more separated with time than

when the cell was in orientation 0°, as can be seen in Figure 7, interferograms at 5, 15, 30, and 60 min. Similar results were obtained in all the experiments with 180° orientation (two salts, NaCl and Na2SO4, and five concentrations). Orientation 90°. With the membrane vertically placed (90°), an intermediate situation between 0° and 180° was observed. When the process started, a few interference fringes parallel to the membrane surface appeared, which indicate that a concentration profile, similar to that in 0° position, had been developed. The difference between both positions was that, at 0°, new interference fringes were continuously appearing during the process, and at 90° few fringes (three or four, depending on the solute and concentration used) appeared during the first 5-10 min, and then the concentration profile became stable for a long period of time (about 1 h in most of the experiments). Later, the concentration fringe which first appeared (the more distant from the membrane surface) began to change its shape. First, a small rounded fringe appeared at the center of the straight fringe. This new fringe was only a deformation produced on the vertical one. The size of the deformation increased with time and, finally, the originally straight fringe became a completely curved fringe. While this fringe was becoming curved and its distance from the membrane increased, a new vertical fringe close to the membrane surface appeared. So, the concentration profile remained nearly constant during the entire experiment. Some interferograms corresponding to the experiment carried out with Na2SO4 at Co) 5 kg/m3 are shown in Figure 8. In this experiment, for instance, the deformation of the first fringe occurred between 1 h 3 min and 1 h 5 min after the beginning of the process. At 1 h 30 min, the previous curved fringe was very far from the membrane surface and another fringe was becoming deformed. The development of this last deformation was slower than the first one. These deformations could be a consequence of the combination of two effects. Because of the concentration gradient in the vicinity of the membrane surface, a horizontal diffusive movement of the solute occurs. On the other hand, a natural convective flux is produced because of the density gradient established inside the cell. Figure 9 shows a diagram of the fluxes involved in the reverse osmosis process with the membrane vertically placed. Once the pressure was removed, the interference fringes changed their shape and position with respect to the membrane surface. In Figure 10, some oblique fringes can be observed. The initial fringes became wider with time. For instance, at 25 and 35 min, some fringes with their origin near the membrane can be seen. Instead of remaining parallel to the membrane, the fringes started to separate from the surface as a consequence of the diffusion process and abruptly changed their direction. The reason for this change in the fringe pattern could be that, when the pressure was taken off the system, the forced convective flux ceased. However, the diffusive flux of the solute from the membrane surface and the natural convective flux because of the density gradient continued, causing the change in the shape and position of the fringes. Similar results were obtained in all the experiments with 90° orientation (two salts, NaCl and Na2SO4, and five concentrations). 3.2. Membrane Performance. In all the experiments performed, permeate flux (J) and concentration (Cp) were measured. Both of these are important parameters to quantify the membrane performance and, therefore, the process effectiveness. As dead-


Figure 7. Interferograms belonging to experiment XIX at 5, 15, 30, and 60 min after the pressure ceased.

Figure 8. Interferograms belonging to experiment XXIX at 10, 63, 65, and 90 min.

Figure 10. Interferograms belonging to experiment XXIX at 20 s and 5, 25, and 30 min after the pressure ceased.

Figure 9. Diagram of the fluxes involved in RO process with the membrane vertically placed.

Not only the shape of the curves but also the values of the permeate flux were different between the 0° position and the other two. The reduction of the permeate flux related to pure water flux (Jw) has also been calculated. This reduction was much higher at 0° than at 90° and 180°. Data of Jw are presented in Table 1. In Figures 11 and 12, the nondimensional J/Jw curves corresponding to the orientations 180° and 90° are shown for all concentrations. Curves corresponding to orientation 0° were presented in Figure 8 of a previous paper.12 As an example, Figure 13 shows the results of experiments IX, XIX, and XXIX (Co ) 5 kg/m3 Na2SO4) in the three positions (0°, 90°, and 180°) for comparison purposes. As can be appreciated in Figure 13, the evolution of J/Jw at 90° and 180° is very similar, while at 0° this relation is rather different, thus indicating that an important increase in the permeated flux was obtained when positions 90° and 180° were used. This difference between the values of the permeate flux is a consequence of the different behavior of the solution in the vicinity of the membrane, depending on its position. The polarization layer that developed when the membrane was facing upward produced an increase in the osmotic pressure of the solution, which caused a decrease in the driving force of the process (∆P - ∆Π) and, therefore, in the permeate flux. The flow instability induced when the membrane was placed vertically and horizontally facing downward caused a buoyancy flow that tended to destroy the polarization layer. Therefore, the decrease in the driving force was not so high and the flux reduction was less. These results are very similar to those

end reverse osmosis is an unsteady state process, J and Cp are continuously changing. The gravitational orientation of the RO module had a great effect on the evolution of the permeate flux. At the 0° position (Figure 8 in previous paper12), the shape of the permeate flux curves was typical, similar to that observed in other papers about UF and RO in dead-end cells. During the first minutes of the process, a great reduction occurred, and later the flux decreased more slowly. At the other two positions (90° and 180°), the permeate flux underwent a continuous but smooth reduction during the entire process. In these two positions, the shape of the flux curves was nearly linear.

1800


Figure 11. Evolution of the experimental dimensionless flux (J/Jw) at 180°: (a) NaCl and (b) Na2SO4 (/, 1 kg/m3; 4, 2 kg/m3; 0, 3.5 kg/m3; b, 5 kg/m3 × 7 kg/m3).

Figure 12. Evolution of the experimental dimensionless flux (J/Jw) at 90°: (a) NaCl and (b) Na2SO4 (/, 1 kg/m3; 4, 2 kg/m3; 0, 3.5 kg/m3; b, 5 kg/m3 × 7 kg/m3).

As can be observed in Figure 16, membrane rejection at 0° was very different from that at 90° and 180°. In these two last positions, membrane rejection was very similar, with only a small divergence between both curves, mainly at the beginning of the experiments. These results show that the concentration polarization phenomenon has a great effect on membrane rejection, since in both positions where natural convection instability occurred (90° and 180°) and, therefore, the concentration polarization layer was not completely developed, the value of membrane rejection was higher than in the position with no gravitational effects (0°). 4. Conclusions Figure 13. Comparison of the evolution of the dimensionless flux (J/Jw) at 0°-90°-180° (Na2SO4, Co) 5 kg/m3).

presented by Youm et al.,5 who obtained an important enhancement in the permeate flux when the cell was placed in an unstable gravitational orientation. Once the concentration Cp was known, it was possible to calculate the membrane rejection Ro ) 1 - Cp/Co. Figures 14 and 15 show the membrane rejection (Ro), and a behavior similar to that described above can be observed. At 0°, the rejection decreased considerably during the process, while at 90° and 180°, the rejection was nearly constant, decreasing very slowly. For instance, Figure 16 shows the rejection curves corresponding to the same experiments IX, XIX, and XXIX mentioned before, all of them with the same operation conditions (Co ) 5 kg/m3 Na2SO4) but at a different membrane positions (0°, 90°, 180°).

Holographic interferometry is an optical technique, which has proved to be very useful to study mass transfer processes where concentration changes occur. In this work, holographic interferometry has been applied to visualize the buoyancy effects on dead-end reverse osmosis of salts by changing the gravitational orientation of the cell (90° and 180°). A module with special characteristics has been used to carry out three sets of experiments with NaCl and Na2SO4 at concentrations ranging from 1 to 7 kg/m3 at a constant pressure of 600 kPa. Working in dead-end conditions has allowed visualizing the phenomena that occur during the beginning of the filtration process as well as those occurring at longer times. At the same time, it has been possible to visualize the process once the pressure ceased. The fringe patterns obtained were very different depending on the membrane position. As was seen in a previous paper,12


Figure 14. Evolution of the rejection observed at 180°: (a) NaCl and (b) Na2SO4 (/, 1 kg/m3; 4, 2 kg/m3; 0, 3.5 kg/m3; b, 5 kg/m3 × 7 kg/m3).

Figure 15. Evolution of the rejection observed at 90°: (a) NaCl and (b) Na2SO4 (/, 1 kg/m3; 4, 2 kg/m3; 0, 3.5 kg/m3; b, 5 kg/m3 × 7 kg/m3).

Figure 16. Comparison of the evolution of the rejection observed at 0°90°-180° (Na2SO4, Co) 5 kg/m3).

at 0°, interference fringes parallel to the membrane surface appeared continuously during the process because of the development of the concentration polarization layer. At 180°, several minutes after the pressure was applied, some material moving downward from the membrane surface was observed, similar to the jets described by Hendricks et al.2 This phenomenon is caused by the inversion of the density gradient which occurs at this membrane position. Later, some circular fringes appeared which indicates that, in the vicinity of the membrane, circular convection currents existed. At 90°, a semistable situation was observed caused by a combination of the diffusive movement of the solute (horizontal) and the natural convection

currents (vertical) caused by the density gradient established in the module, which produced deformations on the interference fringes. Obviously, the differences observed in the development of the polarization layer in these three positions have an important effect on the membrane performance. Permeate flux as well as membrane rejection are very different, depending on the position of the membrane surface. A great difference was observed not only in the shape of the flux and rejection curves but also in the values obtained. Both flux and rejection were higher in the positions where buoyancy flow was induced (90° and 180°). The results presented in this paper prove the existence of a buoyancy effect in dead-end reverse osmosis of salts, depending on the gravitational orientation of the membrane surface. The consequence of this phenomenon is a great enhancement of the membrane performance and, therefore, a higher effectiveness of the reverse osmosis process. Acknowledgment This research was sponsored by the Plan Nacional de I+D+I BQU2000-0456 (Ministerio de Educacioń y Cultura) and by the Ajuda per a Groups de I+D+I de la Consellerıá d’Empresa, Universitat i Cie`ncia (Generalitat Valenciana). Literature Cited (1) Winzeler, H. B.; Belfort, G. Enhanced performance for pressuredriven membrane processes: the argument for fluid instabilities. J. Membr. Sci. 1993, 80, 35-47.

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(2) Hendricks, T. J.; Macquin, J. F.; Williams, F. A. Observations on buoyant convection in reverse osmosis. Ing. Eng. Chem. Fundam. 1972, 11, 276-279. (3) Derzansk, L.; Gill, W. N. The mechanisms of brine-side mass transfer in a horizontal tubular membrane. AIChE J. 1974, 20, 751-761. (4) Huffman, W. J.; Ward, R. M.; Harshman, R. C. Effects of forced and natural convection during ultrafiltration of protein-saline solutions and whole blood in thin channels. Ind. Eng. Chem. Process Des. DeV. 1975, 14 (2), 166-170. (5) Youm, K. H.; Fane, A. G.; Wiley, D. E. Effects of natural convection instability on membrane performance in dead-end and cross-flow ultrafiltration. J. Membr. Sci. 1996, 116, 229-241. (6) Fletcher, D. F.; Wiley, D. E. A computational fluids dynamics study of buoyancy effects in reverse osmosis. J. Membr. Sci. 2004, 245, 175181. (7) Slezak, K.; Dworecki, K. Gravitational effects on transmembrane flux: the Rayleigh-Taylor convective instability. J. Membr. Sci. 1985, 23, 71-81. (8) Kargol, M.; Dworecki, K. Interferometric studies of diffusive unstirred layers generated in graviosmotic systems. Curr. Top. Biophys. 1993, 18, 99-104.

(9) Dworecki, K.; Wasik, S. The investigation of time-dependent solute transport through horizontally situated membrane: the effect of configuration membrane system. J. Biol. Phys. 1999, 23, 181-194. (10) Fernańdez Torres, M. J.; Ruiz-Bevia´, F.; Fernańdez-Sempere, J.; Lo´pez-Leiva, M. Visualization of the UF Polarizad Layer by Holographic Interferometry. AIChE J. 1998, 44, 1765-1776. (11) Fernańdez-Sempere, J.; Ruiz-Bevia´, F.; Salcedo-Dıáz, R. Measurements by hologrphic interferometry of concentration profiles in dead-end ultrafiltration of polyethylene glycol solutions. J. Membr. Sci. 2004, 229, 187-197. (12) Fernańdez-Sempere, J.; Ruiz-Bevia´, F.; Salcedo-Dıáz, GarcıáAlgado, P. Measurement of Concentration Profiles by Holographic Interferometry and Modelling in Unstirred Batch Reverse Osmosis, Ing. Eng. Chem. Res. 2006, 45, 7219-7231.

ReceiVed for reView November 8, 2006 ReVised manuscript receiVed January 18, 2007 Accepted January 22, 2007 IE061433Z

Buoyancy Effects in Dead-End Reverse Osmosis: Visualization by

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