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Characterization of membrane fouling distribution in a spiral wound module using high-frequency ultrasound image analysis
Author’s Accepted Manuscript Characterization of membrane fouling distribution in a spiral wound module using High-frequency ultrasound image analysis Kuo-Lun Tung, Hui-Chieh Teoh, Ching-Wei Lee, Yu-Ling Li, Yi-Feng Lin, Ching-Liang Chen, Meng-Shun Huang www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(15)30130-7 http://dx.doi.org/10.1016/j.memsci.2015.08.035 MEMSCI13922
To appear in: Journal of Membrane Science Received date: 3 April 2015 Revised date: 6 August 2015 Accepted date: 15 August 2015 Cite this article as: Kuo-Lun Tung, Hui-Chieh Teoh, Ching-Wei Lee, Yu-Ling Li, Yi-Feng Lin, Ching-Liang Chen and Meng-Shun Huang, Characterization of membrane fouling distribution in a spiral wound module using High-frequency ultrasound image analysis, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Characterization of Membrane Fouling Distribution in a Spiral Wound Module Using High-Frequency Ultrasound Image Analysis Kuo-Lun Tunga, *, Hui-Chieh Teohb, **, Ching-Wei Leea, Yu-Ling Lic, Yi-Feng Lind , Ching-Liang Chend and Meng-Shun Huange a
Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
b
Department of Chemical Engineering, Universiti Tunku Abdul Rahman, 43000 Selangor, Malaysia
c
Department of Products, Taiwan Textile Research Institute, New Taipei City 243, Taiwan
d
R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Taoyuan City 320, Taiwan e
Water Technology Research Division, Material and Chemical Research
Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan
Manuscript revised to Journal of Membrane Science on August 4, 2015
Keywords: Membrane fouling, Spiral wound membrane module, Humic acid, Highfrequency ultrasound, Image analysis * Tel: +886-2-33663027, Fax: +886-2-23623040, E-mail: [email protected] ** Tel: +60-3-90860288, Fax: +60-3-90198868, E-mail: [email protected]
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Abstract The fouling phenomenon in spiral wound membrane modules was investigated using ultrasonic time domain reflectometry (UTDR) and computational fluid dynamic (CFD) simulations. This study utilized a high-frequency 50-MHz ultrasound system to measure the fouling distribution in the spiral wound ultrafiltration and reserve osmosis membrane modules. The results show that for a porous membrane, the voltage decreases as the fouling increases while the opposite is true for a dense membrane. The effects of gravity and module orientation on fouling were also investigated. It is clear from the results that gravity and membrane curvature play a role in the deposition of foulants on the membrane surface. This finding is supported by CFD simulations revealing that the inner membrane experiences higher axial velocities as the fluid flows through the spacers, thus generating a higher wall shear stress in the interior, which can aid in reducing membrane fouling. This study demonstrated that UTDR with a high-frequency transducer can be used to analyze the fouling phenomenon in spiral wound membrane modules.
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1. Introduction In recent years, membrane separation processes have been established as a primary technology for ensuring the purity and efficiency of water treatment. Membrane processes usually operate at room temperature, and thus, they require less energy compared to conventional methods such as distillation. There are four primary configurations for membrane processes: spiral wound, plate and frame, hollow fiber and tubular. For water treatment applications, spiral wound membranes are commonly used because of their high packing density and low cost. In these modules, the membrane layers are wound around a central tube, and the adjacent membrane leaves are kept apart by feed spacers to provide a channel for the feed. One of the key characteristics of the spiral wound membrane is that the feed and permeate channels have curved geometries, where each channel in the module varies in the radial direction from the central tube to the outer layer. The inner layer closer to the central tube has a larger curvature than the outer layer. Although membrane technologies are currently widely used in industry, fouling is a critical factor that degrades the performance of several membrane processes. Membrane fouling is primarily caused by particle deposition on the membrane surface and plugging of the membrane pores. The performance of a membrane separation process is strongly dependent on the degree of fouling deposition and cake layer formation on the surface [1–3]. Fouling will reduce the effective separation area, leading to a reduced flux and ultimately to the need to clean or replace the membrane, which interrupts the operation of the normal process and thereby increases the cost of operation. An early warning system for the determination of fouling problems in membrane processes is crucial to the improvement of membrane operation and the development of a fouling prevention strategy. Several studies have been performed to understand the nature of fouling and its mechanism [3– 10]. Conventional methods for determining the extent of fouling include observing the flux decline and analyzing the morphology using scanning electron microscopy (SEM). In recent decades, new 3
methods have been developed, such as optical methods [11–13] and ultrasound methods [1,2,14– 30], to further predict and accurately determine the thickness of the fouling layer. Ultrasound methods are becoming more popular than conventional methods due to their advantages such as real-time imaging, lower cost and their non-destructive nature. Ultrasound waves are mechanical waves that propagate through a medium; when they encounter an interface between two media of different acoustic impedances, some portion of the energy is reflected. The reflected energy is recorded and can be converted into an image. Mairal et al. [14,15] first proposed a method for using ultrasound (ultrasonic time domain reflectometry, UTDR) to measure the degree of calcium sulfate fouling on a commercial reverse osmosis membrane in real time. Their results showed that this technique is sensitive to the formation of a fouling layer on the membrane surface, demonstrating good agreement with conventional fouling indicators, including flux decline, changes in the permeate concentration, and the magnitude of membrane surface coverage. Li et al. [1,16–20] performed a number of studies employing the UTDR technique for the measurement of membrane fouling. In two of their studies [1,16], the UTDR technique was used to detect the growth of fouling on microfiltration (MF) and ultrafiltration (UF) systems for paper mill effluent. The fouling layer growth corresponded to changes in the amplitude of the ultrasound signal. It was also found that the signal amplitude remains constant when no fouling has occurred. The formation of a second echo representing the feed/fouling interface can be used to quantify the thickness of the fouling layer. Another method, based on a differential signal, was developed to indicate the fouling condition and to provide a warning of advanced fouling during operation. Li et al. also applied the UTDR technique for other purposes such as detecting CaSO4 deposition and growth on a reverse osmosis (RO) membrane surface in cross-flow and dead-end operation [17], monitoring the fouling of bovine serum albumin protein layers at different pH values in flat-sheet [18] and tubular UF membrane modules [19], and investigating the effect of magnetic fields on CaCO3 deposition on a membrane surface during a cross-flow nanofiltration (NF) process [20]. Overall, the UTDR results
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show a good correlation to conventional methods such as flux decline measurements, SEM analysis and weight measurements. Chai et al. [21] combined the UTDR technique with permeate flux, gravimetric and SEM measurements to study the fouling of calcium sulfate dihydrate in an RO spiral wound membrane and demonstrated that UTDR can provide valuable insight for inorganic fouling in a spiral wound membrane module. UTDR has also been used to study fouling in a hollow fiber membrane [22]. In addition to the studies mentioned above, UTDR has been successfully adopted to monitor the fouling of crude oil phospholipids in the UF process [23] and organic fouling on polyvinylidene fluoride (PVDF) flat-sheet membranes [24]. Recently, An et al. used UTDR technique coupled with sound intensity modeling for in situ monitoring of CaSO4 fouling [25-26] and cleaning process [26] in spiral wound membrane module. Their results, using a low-frequency 2.25 MHz transducer, show that the ultrasound is able to penetrate through the multiple layers on the spiral wound module. The results also show that the ultrasonic reflected signal and the corresponding sound intensity declined to a minimum with fouling and then increased subsequently with the formation of fouling layer. When the cleaning process was carried out on the membrane module, the permeate flux and the sound intensity recovered slowly, showing that this technique is capable to evaluate membrane fouling and cleaning process. Kujundzic et al. [27-30] presented a novel extension of UTDR signal processing into a more sensitive ultrasonic frequency-domain reflectometry (UFDR) to monitor biofouling on membrane surface. MF fouling behavior of industrial fermentation broths was better characterized using a combination of UFDR and conventional optical methods, acoustic reflectometry and permeate flux response patterns [29]. The detection of fouling during protein purification was determined by applying rigorous statistical methodology to reflection spectra of ultrasonic signals obtained during membrane fouling [30]. UTDR has been proven to be able to predict and measure the growth of fouling layers. However, most previous studies only utilize lower frequency, usually less than 10-MHz, ultrasound systems, which have poorer resolution. In this study, a high-frequency 50-MHz ultrasound system was used 5
to increase the spatial resolution of the fouling deposition associated with a constant depth (C-mode scanning). The ultrasound technique was adopted to measure the fouling distribution in a spiral wound membrane module and to evaluate the effect of gravity on the extent of fouling.
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2. Materials and methods 2.1. Spiral wound membrane module filtration system Figure 1 presents a schematic representation of the cross-flow filtration system. The feed was driven into the membrane module by a diaphragm pump (Model HF-600, FLUXTEK, Taiwan), and the flow rate was controlled by adjusting the inlet valve A and bypass valve B. The feed flow rate into the module was measured by a flow meter (Model S-111-7, McMillan, USA). The operating pressure of the membrane module was controlled by the retentate valve C and was measured by pressure gauges located at the inlet and outlet of the module. A closed-loop system was employed, in which the permeate and retentate were recycled to the feed tank and recirculated into the system. The mixture in the feed tank was continuously stirred to ensure perfect mixing of the feed solution. Fig. 2 shows a schematic representation of a dead-end filtration system, which is similar to Fig. 1 except that the dead-end filtration system has no retentate stream. In the present study, two types of membranes, UF and RO, were used to investigate the response of the ultrasound voltage signal to the fouling conditions. The characteristics of the two membranes are summarized in Table 1.
2.2. High-frequency ultrasound system Fig. 3 shows a schematic diagram of the high-frequency ultrasound system. The system is composed of a 50-MHz high-frequency transducer (NIH Ultrasonic Transducer Resource Center, USC, LA, USA) attached to a 3D motor stage that can be controlled by a motor controller to scan different sections of a sample. Because ultrasound waves require a medium to propagate, the transducer and the membrane sample were immersed in water. The transducer was driven by a pulser / receiver (Model 5900PR Olympus, USA), where ultrasound waves were generated and received. The signals reflected after the ultrasound passed through the sample were amplified by an amplifier (Model LN1000A, Amplifier Research, USA) and sent through a filter. The filtered signals were digitized by an analog to digital converter (A/D converter) and stored in a computer. 7
Signal processes and image reconstruction were performed on a computer using MATLAB (MathWorks, Inc.). Fig. 4 shows the pulse-echo response of the 50-MHz transducer in the time domain and frequency domain. This high-frequency ultrasound system is able to perform both Bmode and C-mode imaging. B-mode imaging, more commonly known as 2D mode imaging, scans a plane through the sample and produces a 2D image through a series of signal processing procedures using the Hilbert transform and logarithmic compression. The strength of the signals is then converted into a grayscale image, where a higher amplitude corresponds to a higher grayscale level (brighter). In C-mode imaging, the transducer is shifted to sample the required area at a constant depth. The image is formed in a plane normal to that of a B-mode image. In the present study, Cmode images were obtained by moving the ultrasound transducer in an S-path, as indicated in Fig. 5. The transducer was moved 5 mm in the X-direction and then 0.05 mm in the Y-direction. This sequence was repeated for 100 cycles to sweep out an area of 25 mm2.
2.3. Experimental procedures The feed solutions were 1 mg/L and 5 mg/L humic acid solutions (198763, ICN Biomedical, Inc.) with 0.1 ml of polymeric ferric sulfuric solution, PFS (PFS-S, Jongmaw Chemical Co. Ltd.). In this study, high-frequency ultrasound waves were used, which have a high resolution but a shallow penetration depth. Thus, the system was unable to acquire in situ measurements of the fouling conditions in the spiral wound membrane module, and the module had to be dissected for ultrasound analysis. PFS was added to the feed solution to prevent the foulants from detaching from the membrane surface when the module was dissected and analyzed. The PFS-supplemented humic acid solutions were tested in a dead-end filtration unit to determine the effectiveness of PFS in retaining foulants on the membrane surface. The experiments were performed at pressures of 1 bar and 7.85 bar and at a flow rate of 1 L/min. To investigate the effect of gravity on fouling in a spiral wound membrane module, experiments were conducted for two module orientations, horizontal and vertical, as indicated in Fig. 6. The 8
horizontal orientation was further divided into an up-side and a down-side (Fig. 6(a)). The experiments were run for 6 hours at a pressure of 7.85 bar, a flow rate of 1 L/min and a concentration of 1 mg/L humic acid solution with PFS. Deionized water was circulated in the system for 24 hours prior to the start of each experiment to stabilize the membrane surface and to build up a stable flow field. The feed was switched to PFS-supplemented humic acid solution after 24 hours. The feed solution was replaced every 3 hours during the experiments. At the end of each experiment, the spiral wound membrane was dissected, and the different sections of the membrane were labeled as shown in Fig. 7. Sections A, C, and E were located at the feed entrance, with A nearest to the permeate tube and E the furthest away, while sections B, D, and F were located at the retentate outlet, with B nearest to the permeate tube and F the furthest away. The desired sections of the spiral wound membrane were cut and placed in a container filled with water for ultrasound scanning. The fouling distributions of each of these sections, which included both the inner and outer membrane layers on the spacers, were analyzed using high-frequency ultrasound B-mode and C-mode scanning.
2.4. Computational fluid dynamics (CFD) simulation In previous studies, CFD models primarily used a single spacer-filled channel or periodic boundary conditions. These models, although greatly reducing the computational cost, may not accurately represent the actual conditions inside a spiral wound membrane module. In the present work, an entire spiral wound membrane module with 5,802,000 meshes was modeled, as shown in Fig. 8, to elucidate the fluid flow in the module using the commercial CFD software FLUENT®. The simulation was carried out to predict the radial velocity, axial velocity, wall shear stress and pressure drop across the membrane in a spiral wound module. The geometric parameters of the module are tabulated in Table 2, and the operating conditions are reported in Table 3. In order to reduce the computational time and due to the limitation of computational power, the spiral wound membrane module was modeled as a whole without taking into account the characteristics of spacer 9
design such as flow attack angle and filament spacing, but instead utilized a feed and permeate spacer resistance to simulate the spacers’ effect on the fluid flow. The simulation was run under steady-state and isothermal conditions. The fluid was assumed to be a non-compressible Newtonian fluid, and a non-slip wall was assumed. To investigate the effect of curvature on the wall shear stress, the dimensionless radius of curvature Rr is defined as [25]
Rr =
R − Ri hch = 2 o Rave Ro + Ri
(1)
where Rave is the arithmetic mean of the outer membrane radius, Ro, and the inner membrane radius, Ri. An Rr of 0 corresponds to a flat channel. The degree of curvature of a channel increases as the value of Rr increases. Li et al. [25] defined the dimensionless wall shear stress, Es, which is determined by the difference in the wall shear stress between the inner and outer membrane walls:
Es =
τ i −τ o τ ave
(2)
where τave is the arithmetic mean of the outer membrane wall shear stress, τo, and the inner membrane wall shear stress, τi. As Es increases, the variation in shear stress between the inner and outer walls increases. This increase could result in non-uniform performance between the membranes in a module and thus may affect the fouling or concentration polarization conditions.
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3. Results and discussion 3.1. The effectiveness of PFS in retaining foulants on the membrane surface Fig. 9 shows the fouling condition of the PVDF membrane for a PFS-supplemented humic acid feed solution. The results show that the foulants were still attached to the membrane surface after the membrane was removed from its module. The foulants did not detach from the membrane, even when it was immersed in water. This finding shows that PFS is effective in retaining the foulants on the membrane surface.
3.2. Ultrasound analysis of membrane fouling in porous and dense membranes Fig. 10 shows high-frequency ultrasound B-mode images of porous PVDF membranes that were dead-end filtrated with 5 mg/L of PFS-supplemented humic acid solution at different feed volumes. In the images, the topmost interface corresponds to the PVDF membrane surface. As shown in the B-mode images, the fouling became more severe as the volume of the feed increased, and the brightness of the membrane surface declined accordingly. Comparing the image of the clean membrane with those of the fouled membranes, it can be seen that the brightness distribution of the clean membrane tended to be more uniform, whereas for the fouled membranes, the darker values were patchy, indicating uneven fouling deposition on the membrane surface. C-mode images were acquired to further analyze the distribution of the fouling layer, as shown in Fig. 11. The images clearly show that the fouling deposition and density were not evenly distributed on the membrane. The average voltage value (Vavg) decreases as the feed volume increases, showing that more fouling on a porous membrane will cause a lower voltage. The decline in the ultrasound signals for porous membrane ascribed to the presence and growing of fouling layer was also reported in previous 11
literatures [1,18,24]. The signals decreased progressively during the formation of patchy fouling (Fig.13(b)) are owed to the increase in the attenuation of the received signals, the decrease of the difference of acoustic impedance between the fouling layer and porous membrane, and the roughness of fouling layer surface. Fig. 12 shows high-frequency ultrasound C-mode images of dense polyamide composite RO membranes that were dead-end filtrated with 5 mg/L of PFS-supplemented humic acid solution for 30 min and 90 min at 7.85 bar. Lines are observed on the membrane surface in Figs. 12(b) and (c), corresponding to the locations of the spacer supports of the RO spiral wound membrane; these lines were caused by the high operating pressure on the spacer supports. From the images, it can be seen that the fouling became more severe as the filtration time increased, and the voltage increased as more foulants deposited on the dense membrane surface. The increase in the ultrasound signals for fouled dense membrane was reported by Li et al. [17]. The acoustic impedance of the fouling layer increased with the fouling processes, implying that the density of the fouling layer also increased. This finding contrasts the case of the porous membrane, in which more severe fouling led to a lower voltage. As the foulants deposited on the dense membrane, the fouling layer became denser (Fig 13(a)), which increased the ultrasound signals, while in the porous membrane, the pores and gaps between the foulants scatter the ultrasound signals (Fig 13(b)), causing attenuation of the received ultrasound energy.
3.3. The effect of gravity and membrane module orientation on the fouling condition Fig. 12(a) shows a C-mode image for a clean RO spiral wound membrane with an average voltage of approximately 1.38 V. The fouling condition of the RO membranes after the experiments was determined by high-frequency ultrasound C-mode scanning, and in agreement with the findings in Section 3.2, the average voltage increased as more foulants were deposited on the membrane 12
surface. Fig. 14 shows C-mode images for the inner and outer membrane surfaces at the bottom and top of the horizontal spiral wound membrane module at the feed entrance, and Fig. 15 shows Cmode images for the retentate outlet. The figures show traces of blue fouling lines on the C-mode images, which were due to the zig-zag path of the fluid as it flowed past the spacers, causing the foulants to accumulate on the spacers perpendicular to the flow direction. Comparing the fouling conditions of the inner and outer membrane surfaces on the bottom, there was less fouling on the inner membrane at both the feed entrance and the retentate outlet. Li and Tung [32] showed that in a curved spacer-filled channel, the shear stress on the inner surface is greater than that on the outer surface. The inner membrane has a higher curvature than the outer membrane surface, which generates a higher shear stress when fluid flows through the membrane channel, thus increasing the efficiency of the removal of foulants from the inner membrane surface. At the bottom of the horizontal module, the inner membrane is located above the outer membrane, which also reduces the foulants’ tendency to deposit on the inner membrane surface due to the force of gravity. Based on Figs. 14(c), (d) and Figs. 15(c), (d), the difference in the average voltages between the inner and outer membrane surfaces on the top of the module is in the range of 1% to 8%, indicating that the fouling conditions for the two membrane surfaces were of comparable severity. Although the inner membrane surface has a higher shear stress due to its higher curvature, the force of gravity due to its position below the outer membrane on the top of the module promotes greater foulant deposition on its surface. The competition between the shear stress and the gravitational force on the inner surface causes the effect of the membrane curvature to become insignificant. The results in Fig. 14 also show that at the feed entrance, fouling on the inner membrane surface on the top of the module was much more severe than that on the bottom. The opposite is true for the outer membrane, in which the bottom is more fouled. Because the outer membrane is located below the inner membrane on the bottom, the foulants were more likely to deposit on the outer surface at the bottom, primarily due to the force of gravity. For the inner membrane, the higher shear stress combined with the force of gravity prevented the particles from depositing on 13
the inner membrane surface at the bottom. At the top, while the higher shear stress acts to remove foulant deposition on the inner membrane surface, gravity works in the opposite direction and tends to deposit foulant on the surface because on the top, the inner membrane is located below the outer membrane, resulting in much more severe fouling on the inner membrane on the top than on the bottom. These findings show that the force of gravity plays a role in particle deposition on the membrane surface. At the retentate outlet, as shown in Fig. 15, the same fouling condition was observed, where the outer membrane surface on the bottom was more fouled. For the inner membrane, the fouling severities on the top and bottom were comparable, with an average voltage difference of 5% to 8%. To eliminate the effect of gravity on the membrane module, the experiments were repeated with a vertically oriented membrane module. Fig. 16 shows C-mode images for the inner and outer membrane surfaces of the vertical spiral wound membrane module. The results show that the outer membrane is more fouled than the inner membrane, indicating that the inner membrane surface experienced a higher wall shear stress, which aided in removing foulants from the surface. Comparing the membrane sections at the feed entrance and retentate outlet, the results show that the fouling condition at the retentate outlet was more severe compared to that at the feed entrance. This trend was caused by the effect of gravity on the foulants, whereby they were more likely to deposit on the bottom of the vertical module, where the outlet was located.
3.4. Fluid dynamics analysis of the spiral wound membrane module CFD simulations were performed to predict the radial velocity, axial velocity and pressure drop across the membrane in a spiral wound module. Fig. 17 shows the radial velocity of the inner and outer membranes for different membrane resistances, Rm. Odd membrane numbers represent the inner membranes, while even membrane numbers represent the outer membranes. The membrane numbering starts from the center permeate collection tube. Comparing the radial velocities for the different membrane resistances, it was found that when the membrane resistance is lower, the radial 14
velocity is higher. The radial velocities for the UF membrane module (Rm = 2.87 × 1011 m-1) increase as the membrane number decreases (i.e., closer to the permeate collection tube). Because more foulants are deposited on the membrane surface when the radial velocity is higher, the fouling is more severe close to the permeate collection tube in the UF module. In contrast, the radial velocities for the RO membrane module (Rm = 1.12 × 1014 m-1) do not change significantly as the membrane number decreases, showing that the radial velocity does not significantly affect fouling in an RO module. Fig. 17 also shows that the inner membrane radial velocities were higher than the outer membrane radial velocities for the UF module. This outcome is supported by Fig. 18(a). As the membrane number increases (from center to outer), the transmembrane pressure decreases, resulting in a lower radial velocity. However, for the RO module, the radial velocities do not change significantly due to the almost constant transmembrane pressure across the module (Fig. 18(b)). Fig. 19 shows the axial velocity distribution between the inner and outer membranes along a spiral wound module (Z-direction). The y-axis represents the dimensionless distance from the inner membrane, where 0 represents the position at the inner membrane and 1 represents the position at the outer membrane. Fig. 19 shows that the axial velocity decreases significantly from the feed end (Z = 0.051 m) to the retentate end (Z = 0.255 m) for the UF module, while there is no significant change in velocity for the RO module across the same distance. The axial velocity at the center between the inner and outer membranes (dimensionless distance = 0.556) is 1.25 m/s at Z = 0.051 m and 1.17 m/s at Z = 0.255 m for the UF module, whereas it is constant at 1.14 m/s at the same positions for the RO module. For the UF module, the membrane resistance is smaller, which causes its radial velocities to increase; thus, the axial velocities must also increase to maintain the stability of the membrane module. Fig. 20 shows simulation results for the center position between the feed end and the retentate end (Z = 0.1275 m). In the figure, the “Inner” side corresponds to the position next to the center permeate collection tube, and the “Outer” side corresponds to the position next to the external side of the membrane module in the radial direction. The axial velocity for a dimensionless distance = 15
0.556 is 1.22 m/s on the inner side and 1.19 m/s on the outer side of the UF module, whereas it is constant at 1.14 m/s at the same positions in the RO module. In the UF module, as shown in Fig. 17, the radial velocities are higher than in the RO module due to the higher transmembrane pressure observed near the center permeate collection tube (Fig. 18), thus increasing the axial velocities to maintain the stability of the membrane module. For the RO module, the transmembrane pressures were constant from the inner side to the outer side (Fig. 18), and thus, the velocities were constant as well. Fig. 21 shows the effect of membrane curvature on the wall shear stress. In a spiral wound membrane module, the membranes are rolled around a center permeate collection tube, causing the inner and outer membranes to have different curvatures. This geometry, in turn, will cause a difference in the wall shear stress between the inner and outer membranes. The dimensionless wall shear stress, Es, is defined by Eq. 2, where a value of Es larger than 0 indicates that the inner wall shear stress is larger than the outer stress, and vice versa. Fig. 21 shows that for both the UF and RO modules, the inner wall shear stress is always larger than the outer stress, and as the dimensionless radius, Rr, increases, Es also increases. The increase in Es as Rr increases implies that the difference between the inner and outer wall shear stresses becomes greater as the membrane curvature increases. In agreement with the results presented in Fig. 20, the inner membrane experienced higher axial velocities when the fluid flowed through the spacers, which generated a higher wall shear stress at the interior. The comparison between the UF and RO modules in Fig. 21 shows that the dimensionless wall shear stress between the inner and outer membranes in the UF is larger than that in the RO module. Based on the results in Fig. 17, the radial velocity is higher for the UF module (with a smaller membrane resistance), and thus, the axial velocities must also be higher to maintain the stability of the membrane module for constant inlet and outlet pressure boundary conditions.
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4. Conclusions In this study, a high-frequency 50-MHz ultrasound system was used to measure the fouling distribution in UF and RO spiral wound membrane modules. The results show that the voltage decreases as the fouling increases on a porous membrane due to the formation of patchy fouling of humic acid on the membrane surface causing the increase in the attenuation of the received signals, whereas for a dense membrane, the voltage increases as the fouling of humic acid increases due to the increase in the acoustic impedance of the fouling layer, implying the increase in the density of the fouling layer. The study also investigated the effect of gravity and module orientation on the fouling condition. For a horizontal module orientation, there was less fouling on the inner membrane at the bottom due to the higher wall shear stress and the position of the inner membrane above the outer membrane. At the top, the inner membrane is located below the outer membrane; thus, the competition between the shear stress and gravity at the inner membrane causes the effect of membrane curvature to become insignificant. For a vertical module orientation, the fouling condition at the retentate outlet (at the bottom) was more severe than at the feed entrance (at the top). This trend indicates that gravity plays a role in the deposition of foulants on the membrane surface. The results also demonstrated that the outer membrane was always more fouled than the inner membrane, indicating that the inner membrane surface experienced a higher wall shear stress, which aided in removing foulants from the surface. This finding was supported by CFD simulation results demonstrating that the inner membrane experienced higher axial velocities when the fluid flowed through the spacers, which generated a higher wall shear stress in the interior. As the dimensionless radius Rr increases, the dimensionless wall shear stress Es also increases, indicating that the difference between the inner and outer wall shear stresses becomes more significant as the membrane curvature increases.
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5. Acknowledgements The authors would like to thank the Ministry of Science and Technology (MOST), Taiwan, R.O.C. for their financial support (Project numbers 101-2815-C-033 -008-E and 103-2622-E-002003-CC1).
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[10] J.C. Schippers, J. Verdouw, The modified fouling index, a method of determining the fouling characteristics of water, Desalination 32 (1980) 137–148. [11] K.-L. Tung, H.-R. Damodar, R.-A. Damodar, T.-T. Wu, Y.-L. Li, N.-J. Lin, C.-J. Chuang, S.-J. You, K.-J. Hwang, Online monitoring of particle fouling in a submerged membrane filtration system using a photointerrupt sensor array, J. Membr. Sci. 407–408 (2012) 58–70. [12] J. Mendret, C. Guigui, P. Schmitz, C. Cabassud, P. Duru, An optical method for in situ characterization of fouling during filtration, AlChE J. 53 (2007) 2265–2274. [13] D. Hughes, U.K. Tirlapur, R. Field, Z.F. Cui, In situ 3D characterization of membrane by yeast suspensions using two-photon femtosecond near infrared non-linear optical imaging, J. Membr. Sci. 280 (2006) 124–133. [14] A.P Mairal, A.R Greenberg, W.B Krantz, L.J Bond, Real-time measurement of inorganic fouling of RO desalination membranes using ultrasonic time-domain reflectometry, J. Membr. Sci. 159 (1999) 185-196. [15] A.P. Mairal, A.R. Greenberg, W.B. Krantz, Investigation of membrane fouling and cleaning using ultrasonic time-domain reflectometry, Desalination 130 (2000) 45–60. [16] J. Li, R.D. Sanderson, D.K. Hallbauer, V.Y. Hallbauer-Zadorozhnaya, Measurement and modelling of organic fouling deposition in ultrafiltration by ultrasonic transfer signals and reflections, Desalination 146 (2002) 177–185. 19
[17] J. Li, L.J. Koen, D.K. Hallbauer, L. Lorenzen, R.D. Sanderson, Interpretation of calcium sulfate deposition on reverse osmosis membranes using ultrasonic measurements and a simplified model, Desalination 186 (2005) 227–241. [18] J. Li, R.D. Sanderson, G.Y. Chai, D.K. Hallbauer, Development of an ultrasonic technique for in situ investigating the properties of deposited protein during crossflow ultrafiltration, J. Colloid Interface Sci. 284 (2005) 228–238. [19] J.X. Li, R.D. Sanderson, G.Y. Chai, A focused ultrasonic sensor for in situ detection of protein fouling on tubular ultrafiltration membranes, Sens. Actuators B 114 (2006) 182–191. [20] J. Li, J. Liu, T. Yang, C. Xiao, Quantitative study of the effect of electromagnetic field on scale deposition on nanofiltration membranes via UTDR, Water Res. 41 (2007) 4595–4610. [21] G.Y. Chai, A.R. Greenberg, W.B. Krantz, Ultrasound, gravimetric, and SEM studies of inorganic fouling in spiral-wound membrane modules, Desalination 208 (2007) 277–293. [22] X.C. Xu, J.X. Li, H. Li, Y. Cai, Y.H. Cao, B.Q. He, Y.Z. Zhang, Non-invasive monitoring of fouling in hollow fiber membrane via UTDR, J. Membr. Sci. 326 (2009) 103–110. [23] Cheng, L-H, Yang, Y-C, Chen, J, Lin, Y-H, Wang, S-H. A new view of membrane fouling with 3D ultrasonic imaging techniques: Taking the canola oil with phospholipids for example, J. Membr. Sci. 372 (2011) 134-144. [24] Y.-H. Lin, K.-L. Tung, S.-H. Wang, Q. Zhou, K.K. Shung, Distribution and deposition of organic fouling on the microfiltration membrane evaluated by high-frequency ultrasound, J. Membr. Sci. 433 (2013) 100–111. [25] G. An, J. Lin, J. Li, X. Jian, In situ monitoring of membrane fouling in spiral-wound RO modules by UTDR with a sound intensity modeling, Desalination and Water Treatment, 32(13) (2011) 226-233. [26] G. An, J. Lin, J. Li, X. Li, X. Jian, Non-invasive measurement of membrane scaling and cleaning in spiral-wound reverse osmosis modules by ultrasonic time-domain reflectometry with sound intensity calculation, Desalination 283 (2011) 3-9. 20
[27] E. Kujundizic, A.C. Fonseca, E.A. Evans, M. Peterson, A.R. Greenberg, M. Hernandez, Ultrasonic monitoring of early-stage biofilm growth on polymeric surfaces, Journal of Microbiological Methods 68 (2007) 458-467. [28] E. Kujundizic, K. Cobry, A.R. Greenberg, M. Hernandez, Use of ultrasonic sensors for characterization of membrane fouling and cleaning, Journal of Engineerred Fibers and Fabrics, Special Issue (2008) 35-44. [29] E. Kujundizic, A.R. Greenberg, R. Fong, B. Moore, D. Kujundzic M. Hernandez, Biofouling potential of industrial fermentation broth components during microfiltration, J. Membr. Sci. 349 (2010) 44-55. [30] E. Kujundizic, A.R. Greenberg, R. Fong, M. Hernandez, Monitoring protein fouling on polymeric membranes using ultrasonic frequency-domain reflectometry, Membranes 1 (2011) 195-216. [31] Y.L. Li, K.L. Tung “CFD Simulation of Fluid Flow through Spacer-filled Membrane Module: Selecting Suitable Cell Types for Periodic Boundary Conditions,” Desalination 233 (2008) 351-358. [32] Y.L. Li, K.L. Tung, The effect of curvature of a spacer-filled channel on fluid flow in spiralwound membrane modules, J. Membr. Sci. 319 (1/2) (2008) 286–297.
21
List of Tables Table 1
Characteristics of membranes
Table 2
Geometric parameters of spiral wound membrane module used in CFD simulation
Table 3
Operating conditions of spiral wound membrane module used in CFD simulation
List of Figures Figure 1
Schematic representation of cross-flow filtration system.
Figure 2
Schematic representation of dead-end filtration system.
Figure 3
Schematic diagram of the high-frequency ultrasound system.
Figure 4
The pulse-echo response of the 50 MHz transducer.
Figure 5
C-mode scanning with high-frequency ultrasound system.
Figure 6
Spiral wound membrane module orientation, (a) horizontal, (b) vertical.
Figure 7
Labeled sections of spiral wound membrane for high-frequency ultrasound analysis.
Figure 8
Geometry of spiral wound membrane module for CFD simulation. (a) 3D model, (b) cross-sectional area.
Figure 9
Fouling condition of PVDF membrane using PFS-added humic acid feed solution.
Figure 10
High-frequency ultrasound B-mode images of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
Figure 11
High-frequency ultrasound C-mode images and average voltage Vavg of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
Figure 12
High-frequency ultrasound C-mode images and actual images of dense polyamide composited membrane which were dead-end filtrated with 5 mg/L of PFS-added humic acid solution for (a) 0, (b) 30, and (c) 90 min. 22
Figure 13
Scanning electron microscope (SEM) images of fouling membrane, (a) dense polyamide composited membrane, (b) porous PVDF membrane.
Figure 14
High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance for (a) down-side of inner membrane, (b) downside of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
Figure 15
High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the retentate outlet for (a) down-side of inner membrane, (b) downside of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
Figure 16
High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance (section A, C, E) and retentate outlet (section B, D, F) for (a) inner membrane, and (b) outer membrane, after 6 h filtration at vertical orientation.
Figure 17
Radial velocity of inner and outer membranes in a spiral wound module for different membrane resistances, Rm.
Figure 18
Static pressure across the spiral wound membrane module at Z = 0.1275 m for (a) UF module, and (b) RO module.
Figure 19
Axial velocity distributions between the inner and outer membranes along a spiral wound membrane module (Z-direction) for (a) UF module, and (b) RO module.
Figure 20
Axial velocity distributions between the inner and outer membranes from centre permeate collection tube to external module in radial direction at Z = 0.1275 m for (a) UF module, and (b) RO module.
Figure 21
Dimensionless wall shear stress, Es versus dimensionless radius, Rr for different membrane resistances, Rm.
23
↑ Tung .
Fig. 1. Schematic representation of cross-flow filtration system.
24
↑ Tung .
Fig. 2. Schematic representation of dead-end filtration system.
25
↑ Tung .
Fig. 3. Schematic diagram of the high-frequency ultrasound system.
26
↑ Tung .
Fig. 4. The pulse-echo response of the 50 MHz transducer.
27
↑ Tung .
Fig. 5. C-mode scanning with high-frequency ultrasound system.
28
↑ Tung .
(a)
(b) Fig. 6. Spiral wound membrane module orientation, (a) horizontal, (b) vertical. [31]
29
↑ Tung .
Fig. 7. Labeled sections of spiral wound membrane for high-frequency ultrasound analysis.
30
↑ Tung .
(a)
(b) Fig. 8. Geometry of spiral wound membrane module for CFD simulation. (a) 3D model, (b) crosssectional area.
31
↑ Tung .
Fig. 9. Fouling condition of PVDF membrane using PFS-added humic acid feed solution.
32
Depth (mm)
↑ Tung .
5.5
6 2
4
6
8
Distance (mm)
Depth (mm)
(a)
5.5
6 2
4
6
8
Distance (mm)
Depth (mm)
(b)
5.5
6 2
4
6
Distance (mm)
33
8
Depth (mm)
(c)
5.5
6 2
4
6
8
Distance (mm) (d)
Fig. 10. High-frequency ultrasound B-mode images of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
34
↑ Tung .
Vavg = 0.3750 V
Vavg = 0.3659 V
(a)
(b)
Vavg = 0.3004 V
Vavg = 0.2830 V
(c)
(d)
Fig. 11. High-frequency ultrasound C-mode images and average voltage Vavg of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
35
↑ Tung .
(a) Vavg = 1.3796 V
(b) Vavg = 2.0214 V
(c) Vavg = 2.4662 V Fig. 12. High-frequency ultrasound C-mode images and actual images of dense polyamide composited membrane which were dead-end filtrated with 5 mg/L of PFS-added humic acid solution for (a) 0, (b) 30, and (c) 90 min.
36
↑ Tung .
(a)
(b)
Fig. 13. Scanning electron microscope (SEM) images of fouling membrane, (a) dense polyamide composited membrane, (b) porous PVDF membrane.
37
↑ Tung . Section A
Vavg = 1.6324 V
Vavg = 2.0647 V
Vavg = 2.1705 V
Vavg = 2.0301 V
Vavg = 2.0140 V
Vavg = 2.1198 V
Vavg = 2.0912 V
Vavg = 2.3023 V (b)
Vavg = 2.1556 V (c)
Vavg = 2.0293 V (d)
Section C
Vavg = 1.6249 V Section E
Vavg = 1.6960 V (a)
Fig. 14. High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance for (a) down-side of inner membrane, (b) down-side of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
38
↑ Tung .
Section B
Vavg = 1.8997 V
Vavg = 2.1190 V
Vavg = 1.8112 V
Vavg = 1.9198 V
Vavg = 2.0456 V
Vavg = 1.7880 V
Vavg = 1.8264 V
Vavg = 2.0400 V (b)
Vavg = 1.7835 V (c)
Vavg = 1.9292 V (d)
Section D
Vavg = 1.9335 V Section F
Vavg = 1.9328 V (a)
Fig. 15. High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the retentate outlet for (a) down-side of inner membrane, (b) down-side of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
39
↑ Tung .
Section A
Vavg = 1.8347 V
Section B
Vavg = 2.1818 V
Vavg = 1.9913 V Section D
Section C
Vavg = 1.8575 V
Vavg = 2.0822 V
Vavg = 1.9223 V
Vavg = 2.1651 V
Section F
Section E
Vavg = 1.8310 V (a)
Vavg = 2.1825 V
Vavg = 2.1417 V (b)
Vavg = 2.0720 V (a)
Vavg = 2.2266 V (d)
Fig. 16. High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance (section A, C, E) and retentate outlet (section B, D, F) for (a) inner membrane, and (b) outer membrane, after 6 h filtration at vertical orientation.
40
↑ Tung .
Fig. 17. Radial velocity of inner and outer membranes in a spiral wound module for different membrane resistances, Rm.
41
↑ Tung .
Rm = 2.87 × 1011 m-1
Rm = 1.12 × 1014 m-1
(UF module)
(RO module)
(a)
(b)
Fig. 18. Static pressure across the spiral wound membrane module at Z = 0.1275 m for (a) UF module, and (b) RO module.
42
↑ Tung .
Rm = 2.87 × 1011 m-1 (UF module)
Rm = 1.12 × 1014 m-1 (RO module)
(a)
(b)
Fig. 19. Axial velocity distributions between the inner and outer membranes along a spiral wound membrane module (Z-direction) for (a) UF module, and (b) RO module.
43
↑ Tung .
Rm = 2.87 × 1011 m-1 (UF module)
Rm = 1.12 × 1014 m-1 (RO module)
(a)
(b)
Fig. 20. Axial velocity distributions between the inner and outer membranes from centre permeate collection tube to external module in radial direction at Z = 0.1275 m for (a) UF module, and (b) RO module.
44
↑ Tung .
Fig. 21. Dimensionless wall shear stress, Es versus dimensionless radius, Rr for different membrane resistances, Rm.
45
↑ Tung .
Table 1 Characteristics of membranes UF spiral wound membrane module Material
Polyvinylidene fluoride, PVDF
Membrane resistance
2.87 × 1011 m-1
Pore size
0.2, 0.05 µm
Module diameter / length
1.8 in / 10 in
Company / Model
ALPSPRING Co., Ltd / UF-2012
RO spiral wound membrane module Material
Polyamide composited, FILMTEC USA
Membrane resistance
1.12 × 1014 m-1
Module diameter / length
1.8 in / 10 in
Company / Model
ALPSPRING Co., Ltd / TW40-1812-50
46
↑ Tung .
Table 2 Geometric parameters of spiral wound membrane module used in CFD simulation Name
Radius (cm)
Height (cm)
Length (cm)
Feed spacer channel
-
0.048
25.5
Permeate spacer channel
-
0.028
25.5
Membrane
-
0.011
25.5
0.75
-
25.5
-
2
2
Permeate collection tube Hole on permeate collection tubes
47
↑ Tung .
Table 3 Operating conditions of spiral wound membrane module used in CFD simulation Module
UF
RO
Model
Laminar
Laminar
Inlet pressure
1 atm
8 atm
Outlet pressure
0.9 atm
7.9 atm
Permeate pressure
1 atm
1 atm
Membrane resistance
2.87 × 1011 m-1
1.12 × 1014 m-1
Feed spacer resistance
0.048 × 101 m-1
0.048 × 101 m-1
Permeate spacer resistance
2.8 × 101 m-1
2.8 × 103 m-1
48
PII: DOI: Reference:
S0376-7388(15)30130-7 http://dx.doi.org/10.1016/j.memsci.2015.08.035 MEMSCI13922
To appear in: Journal of Membrane Science Received date: 3 April 2015 Revised date: 6 August 2015 Accepted date: 15 August 2015 Cite this article as: Kuo-Lun Tung, Hui-Chieh Teoh, Ching-Wei Lee, Yu-Ling Li, Yi-Feng Lin, Ching-Liang Chen and Meng-Shun Huang, Characterization of membrane fouling distribution in a spiral wound module using High-frequency ultrasound image analysis, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Characterization of Membrane Fouling Distribution in a Spiral Wound Module Using High-Frequency Ultrasound Image Analysis Kuo-Lun Tunga, *, Hui-Chieh Teohb, **, Ching-Wei Leea, Yu-Ling Lic, Yi-Feng Lind , Ching-Liang Chend and Meng-Shun Huange a
Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
b
Department of Chemical Engineering, Universiti Tunku Abdul Rahman, 43000 Selangor, Malaysia
c
Department of Products, Taiwan Textile Research Institute, New Taipei City 243, Taiwan
d
R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Taoyuan City 320, Taiwan e
Water Technology Research Division, Material and Chemical Research
Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan
Manuscript revised to Journal of Membrane Science on August 4, 2015
Keywords: Membrane fouling, Spiral wound membrane module, Humic acid, Highfrequency ultrasound, Image analysis * Tel: +886-2-33663027, Fax: +886-2-23623040, E-mail: [email protected] ** Tel: +60-3-90860288, Fax: +60-3-90198868, E-mail: [email protected]
1
Abstract The fouling phenomenon in spiral wound membrane modules was investigated using ultrasonic time domain reflectometry (UTDR) and computational fluid dynamic (CFD) simulations. This study utilized a high-frequency 50-MHz ultrasound system to measure the fouling distribution in the spiral wound ultrafiltration and reserve osmosis membrane modules. The results show that for a porous membrane, the voltage decreases as the fouling increases while the opposite is true for a dense membrane. The effects of gravity and module orientation on fouling were also investigated. It is clear from the results that gravity and membrane curvature play a role in the deposition of foulants on the membrane surface. This finding is supported by CFD simulations revealing that the inner membrane experiences higher axial velocities as the fluid flows through the spacers, thus generating a higher wall shear stress in the interior, which can aid in reducing membrane fouling. This study demonstrated that UTDR with a high-frequency transducer can be used to analyze the fouling phenomenon in spiral wound membrane modules.
2
1. Introduction In recent years, membrane separation processes have been established as a primary technology for ensuring the purity and efficiency of water treatment. Membrane processes usually operate at room temperature, and thus, they require less energy compared to conventional methods such as distillation. There are four primary configurations for membrane processes: spiral wound, plate and frame, hollow fiber and tubular. For water treatment applications, spiral wound membranes are commonly used because of their high packing density and low cost. In these modules, the membrane layers are wound around a central tube, and the adjacent membrane leaves are kept apart by feed spacers to provide a channel for the feed. One of the key characteristics of the spiral wound membrane is that the feed and permeate channels have curved geometries, where each channel in the module varies in the radial direction from the central tube to the outer layer. The inner layer closer to the central tube has a larger curvature than the outer layer. Although membrane technologies are currently widely used in industry, fouling is a critical factor that degrades the performance of several membrane processes. Membrane fouling is primarily caused by particle deposition on the membrane surface and plugging of the membrane pores. The performance of a membrane separation process is strongly dependent on the degree of fouling deposition and cake layer formation on the surface [1–3]. Fouling will reduce the effective separation area, leading to a reduced flux and ultimately to the need to clean or replace the membrane, which interrupts the operation of the normal process and thereby increases the cost of operation. An early warning system for the determination of fouling problems in membrane processes is crucial to the improvement of membrane operation and the development of a fouling prevention strategy. Several studies have been performed to understand the nature of fouling and its mechanism [3– 10]. Conventional methods for determining the extent of fouling include observing the flux decline and analyzing the morphology using scanning electron microscopy (SEM). In recent decades, new 3
methods have been developed, such as optical methods [11–13] and ultrasound methods [1,2,14– 30], to further predict and accurately determine the thickness of the fouling layer. Ultrasound methods are becoming more popular than conventional methods due to their advantages such as real-time imaging, lower cost and their non-destructive nature. Ultrasound waves are mechanical waves that propagate through a medium; when they encounter an interface between two media of different acoustic impedances, some portion of the energy is reflected. The reflected energy is recorded and can be converted into an image. Mairal et al. [14,15] first proposed a method for using ultrasound (ultrasonic time domain reflectometry, UTDR) to measure the degree of calcium sulfate fouling on a commercial reverse osmosis membrane in real time. Their results showed that this technique is sensitive to the formation of a fouling layer on the membrane surface, demonstrating good agreement with conventional fouling indicators, including flux decline, changes in the permeate concentration, and the magnitude of membrane surface coverage. Li et al. [1,16–20] performed a number of studies employing the UTDR technique for the measurement of membrane fouling. In two of their studies [1,16], the UTDR technique was used to detect the growth of fouling on microfiltration (MF) and ultrafiltration (UF) systems for paper mill effluent. The fouling layer growth corresponded to changes in the amplitude of the ultrasound signal. It was also found that the signal amplitude remains constant when no fouling has occurred. The formation of a second echo representing the feed/fouling interface can be used to quantify the thickness of the fouling layer. Another method, based on a differential signal, was developed to indicate the fouling condition and to provide a warning of advanced fouling during operation. Li et al. also applied the UTDR technique for other purposes such as detecting CaSO4 deposition and growth on a reverse osmosis (RO) membrane surface in cross-flow and dead-end operation [17], monitoring the fouling of bovine serum albumin protein layers at different pH values in flat-sheet [18] and tubular UF membrane modules [19], and investigating the effect of magnetic fields on CaCO3 deposition on a membrane surface during a cross-flow nanofiltration (NF) process [20]. Overall, the UTDR results
4
show a good correlation to conventional methods such as flux decline measurements, SEM analysis and weight measurements. Chai et al. [21] combined the UTDR technique with permeate flux, gravimetric and SEM measurements to study the fouling of calcium sulfate dihydrate in an RO spiral wound membrane and demonstrated that UTDR can provide valuable insight for inorganic fouling in a spiral wound membrane module. UTDR has also been used to study fouling in a hollow fiber membrane [22]. In addition to the studies mentioned above, UTDR has been successfully adopted to monitor the fouling of crude oil phospholipids in the UF process [23] and organic fouling on polyvinylidene fluoride (PVDF) flat-sheet membranes [24]. Recently, An et al. used UTDR technique coupled with sound intensity modeling for in situ monitoring of CaSO4 fouling [25-26] and cleaning process [26] in spiral wound membrane module. Their results, using a low-frequency 2.25 MHz transducer, show that the ultrasound is able to penetrate through the multiple layers on the spiral wound module. The results also show that the ultrasonic reflected signal and the corresponding sound intensity declined to a minimum with fouling and then increased subsequently with the formation of fouling layer. When the cleaning process was carried out on the membrane module, the permeate flux and the sound intensity recovered slowly, showing that this technique is capable to evaluate membrane fouling and cleaning process. Kujundzic et al. [27-30] presented a novel extension of UTDR signal processing into a more sensitive ultrasonic frequency-domain reflectometry (UFDR) to monitor biofouling on membrane surface. MF fouling behavior of industrial fermentation broths was better characterized using a combination of UFDR and conventional optical methods, acoustic reflectometry and permeate flux response patterns [29]. The detection of fouling during protein purification was determined by applying rigorous statistical methodology to reflection spectra of ultrasonic signals obtained during membrane fouling [30]. UTDR has been proven to be able to predict and measure the growth of fouling layers. However, most previous studies only utilize lower frequency, usually less than 10-MHz, ultrasound systems, which have poorer resolution. In this study, a high-frequency 50-MHz ultrasound system was used 5
to increase the spatial resolution of the fouling deposition associated with a constant depth (C-mode scanning). The ultrasound technique was adopted to measure the fouling distribution in a spiral wound membrane module and to evaluate the effect of gravity on the extent of fouling.
6
2. Materials and methods 2.1. Spiral wound membrane module filtration system Figure 1 presents a schematic representation of the cross-flow filtration system. The feed was driven into the membrane module by a diaphragm pump (Model HF-600, FLUXTEK, Taiwan), and the flow rate was controlled by adjusting the inlet valve A and bypass valve B. The feed flow rate into the module was measured by a flow meter (Model S-111-7, McMillan, USA). The operating pressure of the membrane module was controlled by the retentate valve C and was measured by pressure gauges located at the inlet and outlet of the module. A closed-loop system was employed, in which the permeate and retentate were recycled to the feed tank and recirculated into the system. The mixture in the feed tank was continuously stirred to ensure perfect mixing of the feed solution. Fig. 2 shows a schematic representation of a dead-end filtration system, which is similar to Fig. 1 except that the dead-end filtration system has no retentate stream. In the present study, two types of membranes, UF and RO, were used to investigate the response of the ultrasound voltage signal to the fouling conditions. The characteristics of the two membranes are summarized in Table 1.
2.2. High-frequency ultrasound system Fig. 3 shows a schematic diagram of the high-frequency ultrasound system. The system is composed of a 50-MHz high-frequency transducer (NIH Ultrasonic Transducer Resource Center, USC, LA, USA) attached to a 3D motor stage that can be controlled by a motor controller to scan different sections of a sample. Because ultrasound waves require a medium to propagate, the transducer and the membrane sample were immersed in water. The transducer was driven by a pulser / receiver (Model 5900PR Olympus, USA), where ultrasound waves were generated and received. The signals reflected after the ultrasound passed through the sample were amplified by an amplifier (Model LN1000A, Amplifier Research, USA) and sent through a filter. The filtered signals were digitized by an analog to digital converter (A/D converter) and stored in a computer. 7
Signal processes and image reconstruction were performed on a computer using MATLAB (MathWorks, Inc.). Fig. 4 shows the pulse-echo response of the 50-MHz transducer in the time domain and frequency domain. This high-frequency ultrasound system is able to perform both Bmode and C-mode imaging. B-mode imaging, more commonly known as 2D mode imaging, scans a plane through the sample and produces a 2D image through a series of signal processing procedures using the Hilbert transform and logarithmic compression. The strength of the signals is then converted into a grayscale image, where a higher amplitude corresponds to a higher grayscale level (brighter). In C-mode imaging, the transducer is shifted to sample the required area at a constant depth. The image is formed in a plane normal to that of a B-mode image. In the present study, Cmode images were obtained by moving the ultrasound transducer in an S-path, as indicated in Fig. 5. The transducer was moved 5 mm in the X-direction and then 0.05 mm in the Y-direction. This sequence was repeated for 100 cycles to sweep out an area of 25 mm2.
2.3. Experimental procedures The feed solutions were 1 mg/L and 5 mg/L humic acid solutions (198763, ICN Biomedical, Inc.) with 0.1 ml of polymeric ferric sulfuric solution, PFS (PFS-S, Jongmaw Chemical Co. Ltd.). In this study, high-frequency ultrasound waves were used, which have a high resolution but a shallow penetration depth. Thus, the system was unable to acquire in situ measurements of the fouling conditions in the spiral wound membrane module, and the module had to be dissected for ultrasound analysis. PFS was added to the feed solution to prevent the foulants from detaching from the membrane surface when the module was dissected and analyzed. The PFS-supplemented humic acid solutions were tested in a dead-end filtration unit to determine the effectiveness of PFS in retaining foulants on the membrane surface. The experiments were performed at pressures of 1 bar and 7.85 bar and at a flow rate of 1 L/min. To investigate the effect of gravity on fouling in a spiral wound membrane module, experiments were conducted for two module orientations, horizontal and vertical, as indicated in Fig. 6. The 8
horizontal orientation was further divided into an up-side and a down-side (Fig. 6(a)). The experiments were run for 6 hours at a pressure of 7.85 bar, a flow rate of 1 L/min and a concentration of 1 mg/L humic acid solution with PFS. Deionized water was circulated in the system for 24 hours prior to the start of each experiment to stabilize the membrane surface and to build up a stable flow field. The feed was switched to PFS-supplemented humic acid solution after 24 hours. The feed solution was replaced every 3 hours during the experiments. At the end of each experiment, the spiral wound membrane was dissected, and the different sections of the membrane were labeled as shown in Fig. 7. Sections A, C, and E were located at the feed entrance, with A nearest to the permeate tube and E the furthest away, while sections B, D, and F were located at the retentate outlet, with B nearest to the permeate tube and F the furthest away. The desired sections of the spiral wound membrane were cut and placed in a container filled with water for ultrasound scanning. The fouling distributions of each of these sections, which included both the inner and outer membrane layers on the spacers, were analyzed using high-frequency ultrasound B-mode and C-mode scanning.
2.4. Computational fluid dynamics (CFD) simulation In previous studies, CFD models primarily used a single spacer-filled channel or periodic boundary conditions. These models, although greatly reducing the computational cost, may not accurately represent the actual conditions inside a spiral wound membrane module. In the present work, an entire spiral wound membrane module with 5,802,000 meshes was modeled, as shown in Fig. 8, to elucidate the fluid flow in the module using the commercial CFD software FLUENT®. The simulation was carried out to predict the radial velocity, axial velocity, wall shear stress and pressure drop across the membrane in a spiral wound module. The geometric parameters of the module are tabulated in Table 2, and the operating conditions are reported in Table 3. In order to reduce the computational time and due to the limitation of computational power, the spiral wound membrane module was modeled as a whole without taking into account the characteristics of spacer 9
design such as flow attack angle and filament spacing, but instead utilized a feed and permeate spacer resistance to simulate the spacers’ effect on the fluid flow. The simulation was run under steady-state and isothermal conditions. The fluid was assumed to be a non-compressible Newtonian fluid, and a non-slip wall was assumed. To investigate the effect of curvature on the wall shear stress, the dimensionless radius of curvature Rr is defined as [25]
Rr =
R − Ri hch = 2 o Rave Ro + Ri
(1)
where Rave is the arithmetic mean of the outer membrane radius, Ro, and the inner membrane radius, Ri. An Rr of 0 corresponds to a flat channel. The degree of curvature of a channel increases as the value of Rr increases. Li et al. [25] defined the dimensionless wall shear stress, Es, which is determined by the difference in the wall shear stress between the inner and outer membrane walls:
Es =
τ i −τ o τ ave
(2)
where τave is the arithmetic mean of the outer membrane wall shear stress, τo, and the inner membrane wall shear stress, τi. As Es increases, the variation in shear stress between the inner and outer walls increases. This increase could result in non-uniform performance between the membranes in a module and thus may affect the fouling or concentration polarization conditions.
10
3. Results and discussion 3.1. The effectiveness of PFS in retaining foulants on the membrane surface Fig. 9 shows the fouling condition of the PVDF membrane for a PFS-supplemented humic acid feed solution. The results show that the foulants were still attached to the membrane surface after the membrane was removed from its module. The foulants did not detach from the membrane, even when it was immersed in water. This finding shows that PFS is effective in retaining the foulants on the membrane surface.
3.2. Ultrasound analysis of membrane fouling in porous and dense membranes Fig. 10 shows high-frequency ultrasound B-mode images of porous PVDF membranes that were dead-end filtrated with 5 mg/L of PFS-supplemented humic acid solution at different feed volumes. In the images, the topmost interface corresponds to the PVDF membrane surface. As shown in the B-mode images, the fouling became more severe as the volume of the feed increased, and the brightness of the membrane surface declined accordingly. Comparing the image of the clean membrane with those of the fouled membranes, it can be seen that the brightness distribution of the clean membrane tended to be more uniform, whereas for the fouled membranes, the darker values were patchy, indicating uneven fouling deposition on the membrane surface. C-mode images were acquired to further analyze the distribution of the fouling layer, as shown in Fig. 11. The images clearly show that the fouling deposition and density were not evenly distributed on the membrane. The average voltage value (Vavg) decreases as the feed volume increases, showing that more fouling on a porous membrane will cause a lower voltage. The decline in the ultrasound signals for porous membrane ascribed to the presence and growing of fouling layer was also reported in previous 11
literatures [1,18,24]. The signals decreased progressively during the formation of patchy fouling (Fig.13(b)) are owed to the increase in the attenuation of the received signals, the decrease of the difference of acoustic impedance between the fouling layer and porous membrane, and the roughness of fouling layer surface. Fig. 12 shows high-frequency ultrasound C-mode images of dense polyamide composite RO membranes that were dead-end filtrated with 5 mg/L of PFS-supplemented humic acid solution for 30 min and 90 min at 7.85 bar. Lines are observed on the membrane surface in Figs. 12(b) and (c), corresponding to the locations of the spacer supports of the RO spiral wound membrane; these lines were caused by the high operating pressure on the spacer supports. From the images, it can be seen that the fouling became more severe as the filtration time increased, and the voltage increased as more foulants deposited on the dense membrane surface. The increase in the ultrasound signals for fouled dense membrane was reported by Li et al. [17]. The acoustic impedance of the fouling layer increased with the fouling processes, implying that the density of the fouling layer also increased. This finding contrasts the case of the porous membrane, in which more severe fouling led to a lower voltage. As the foulants deposited on the dense membrane, the fouling layer became denser (Fig 13(a)), which increased the ultrasound signals, while in the porous membrane, the pores and gaps between the foulants scatter the ultrasound signals (Fig 13(b)), causing attenuation of the received ultrasound energy.
3.3. The effect of gravity and membrane module orientation on the fouling condition Fig. 12(a) shows a C-mode image for a clean RO spiral wound membrane with an average voltage of approximately 1.38 V. The fouling condition of the RO membranes after the experiments was determined by high-frequency ultrasound C-mode scanning, and in agreement with the findings in Section 3.2, the average voltage increased as more foulants were deposited on the membrane 12
surface. Fig. 14 shows C-mode images for the inner and outer membrane surfaces at the bottom and top of the horizontal spiral wound membrane module at the feed entrance, and Fig. 15 shows Cmode images for the retentate outlet. The figures show traces of blue fouling lines on the C-mode images, which were due to the zig-zag path of the fluid as it flowed past the spacers, causing the foulants to accumulate on the spacers perpendicular to the flow direction. Comparing the fouling conditions of the inner and outer membrane surfaces on the bottom, there was less fouling on the inner membrane at both the feed entrance and the retentate outlet. Li and Tung [32] showed that in a curved spacer-filled channel, the shear stress on the inner surface is greater than that on the outer surface. The inner membrane has a higher curvature than the outer membrane surface, which generates a higher shear stress when fluid flows through the membrane channel, thus increasing the efficiency of the removal of foulants from the inner membrane surface. At the bottom of the horizontal module, the inner membrane is located above the outer membrane, which also reduces the foulants’ tendency to deposit on the inner membrane surface due to the force of gravity. Based on Figs. 14(c), (d) and Figs. 15(c), (d), the difference in the average voltages between the inner and outer membrane surfaces on the top of the module is in the range of 1% to 8%, indicating that the fouling conditions for the two membrane surfaces were of comparable severity. Although the inner membrane surface has a higher shear stress due to its higher curvature, the force of gravity due to its position below the outer membrane on the top of the module promotes greater foulant deposition on its surface. The competition between the shear stress and the gravitational force on the inner surface causes the effect of the membrane curvature to become insignificant. The results in Fig. 14 also show that at the feed entrance, fouling on the inner membrane surface on the top of the module was much more severe than that on the bottom. The opposite is true for the outer membrane, in which the bottom is more fouled. Because the outer membrane is located below the inner membrane on the bottom, the foulants were more likely to deposit on the outer surface at the bottom, primarily due to the force of gravity. For the inner membrane, the higher shear stress combined with the force of gravity prevented the particles from depositing on 13
the inner membrane surface at the bottom. At the top, while the higher shear stress acts to remove foulant deposition on the inner membrane surface, gravity works in the opposite direction and tends to deposit foulant on the surface because on the top, the inner membrane is located below the outer membrane, resulting in much more severe fouling on the inner membrane on the top than on the bottom. These findings show that the force of gravity plays a role in particle deposition on the membrane surface. At the retentate outlet, as shown in Fig. 15, the same fouling condition was observed, where the outer membrane surface on the bottom was more fouled. For the inner membrane, the fouling severities on the top and bottom were comparable, with an average voltage difference of 5% to 8%. To eliminate the effect of gravity on the membrane module, the experiments were repeated with a vertically oriented membrane module. Fig. 16 shows C-mode images for the inner and outer membrane surfaces of the vertical spiral wound membrane module. The results show that the outer membrane is more fouled than the inner membrane, indicating that the inner membrane surface experienced a higher wall shear stress, which aided in removing foulants from the surface. Comparing the membrane sections at the feed entrance and retentate outlet, the results show that the fouling condition at the retentate outlet was more severe compared to that at the feed entrance. This trend was caused by the effect of gravity on the foulants, whereby they were more likely to deposit on the bottom of the vertical module, where the outlet was located.
3.4. Fluid dynamics analysis of the spiral wound membrane module CFD simulations were performed to predict the radial velocity, axial velocity and pressure drop across the membrane in a spiral wound module. Fig. 17 shows the radial velocity of the inner and outer membranes for different membrane resistances, Rm. Odd membrane numbers represent the inner membranes, while even membrane numbers represent the outer membranes. The membrane numbering starts from the center permeate collection tube. Comparing the radial velocities for the different membrane resistances, it was found that when the membrane resistance is lower, the radial 14
velocity is higher. The radial velocities for the UF membrane module (Rm = 2.87 × 1011 m-1) increase as the membrane number decreases (i.e., closer to the permeate collection tube). Because more foulants are deposited on the membrane surface when the radial velocity is higher, the fouling is more severe close to the permeate collection tube in the UF module. In contrast, the radial velocities for the RO membrane module (Rm = 1.12 × 1014 m-1) do not change significantly as the membrane number decreases, showing that the radial velocity does not significantly affect fouling in an RO module. Fig. 17 also shows that the inner membrane radial velocities were higher than the outer membrane radial velocities for the UF module. This outcome is supported by Fig. 18(a). As the membrane number increases (from center to outer), the transmembrane pressure decreases, resulting in a lower radial velocity. However, for the RO module, the radial velocities do not change significantly due to the almost constant transmembrane pressure across the module (Fig. 18(b)). Fig. 19 shows the axial velocity distribution between the inner and outer membranes along a spiral wound module (Z-direction). The y-axis represents the dimensionless distance from the inner membrane, where 0 represents the position at the inner membrane and 1 represents the position at the outer membrane. Fig. 19 shows that the axial velocity decreases significantly from the feed end (Z = 0.051 m) to the retentate end (Z = 0.255 m) for the UF module, while there is no significant change in velocity for the RO module across the same distance. The axial velocity at the center between the inner and outer membranes (dimensionless distance = 0.556) is 1.25 m/s at Z = 0.051 m and 1.17 m/s at Z = 0.255 m for the UF module, whereas it is constant at 1.14 m/s at the same positions for the RO module. For the UF module, the membrane resistance is smaller, which causes its radial velocities to increase; thus, the axial velocities must also increase to maintain the stability of the membrane module. Fig. 20 shows simulation results for the center position between the feed end and the retentate end (Z = 0.1275 m). In the figure, the “Inner” side corresponds to the position next to the center permeate collection tube, and the “Outer” side corresponds to the position next to the external side of the membrane module in the radial direction. The axial velocity for a dimensionless distance = 15
0.556 is 1.22 m/s on the inner side and 1.19 m/s on the outer side of the UF module, whereas it is constant at 1.14 m/s at the same positions in the RO module. In the UF module, as shown in Fig. 17, the radial velocities are higher than in the RO module due to the higher transmembrane pressure observed near the center permeate collection tube (Fig. 18), thus increasing the axial velocities to maintain the stability of the membrane module. For the RO module, the transmembrane pressures were constant from the inner side to the outer side (Fig. 18), and thus, the velocities were constant as well. Fig. 21 shows the effect of membrane curvature on the wall shear stress. In a spiral wound membrane module, the membranes are rolled around a center permeate collection tube, causing the inner and outer membranes to have different curvatures. This geometry, in turn, will cause a difference in the wall shear stress between the inner and outer membranes. The dimensionless wall shear stress, Es, is defined by Eq. 2, where a value of Es larger than 0 indicates that the inner wall shear stress is larger than the outer stress, and vice versa. Fig. 21 shows that for both the UF and RO modules, the inner wall shear stress is always larger than the outer stress, and as the dimensionless radius, Rr, increases, Es also increases. The increase in Es as Rr increases implies that the difference between the inner and outer wall shear stresses becomes greater as the membrane curvature increases. In agreement with the results presented in Fig. 20, the inner membrane experienced higher axial velocities when the fluid flowed through the spacers, which generated a higher wall shear stress at the interior. The comparison between the UF and RO modules in Fig. 21 shows that the dimensionless wall shear stress between the inner and outer membranes in the UF is larger than that in the RO module. Based on the results in Fig. 17, the radial velocity is higher for the UF module (with a smaller membrane resistance), and thus, the axial velocities must also be higher to maintain the stability of the membrane module for constant inlet and outlet pressure boundary conditions.
16
4. Conclusions In this study, a high-frequency 50-MHz ultrasound system was used to measure the fouling distribution in UF and RO spiral wound membrane modules. The results show that the voltage decreases as the fouling increases on a porous membrane due to the formation of patchy fouling of humic acid on the membrane surface causing the increase in the attenuation of the received signals, whereas for a dense membrane, the voltage increases as the fouling of humic acid increases due to the increase in the acoustic impedance of the fouling layer, implying the increase in the density of the fouling layer. The study also investigated the effect of gravity and module orientation on the fouling condition. For a horizontal module orientation, there was less fouling on the inner membrane at the bottom due to the higher wall shear stress and the position of the inner membrane above the outer membrane. At the top, the inner membrane is located below the outer membrane; thus, the competition between the shear stress and gravity at the inner membrane causes the effect of membrane curvature to become insignificant. For a vertical module orientation, the fouling condition at the retentate outlet (at the bottom) was more severe than at the feed entrance (at the top). This trend indicates that gravity plays a role in the deposition of foulants on the membrane surface. The results also demonstrated that the outer membrane was always more fouled than the inner membrane, indicating that the inner membrane surface experienced a higher wall shear stress, which aided in removing foulants from the surface. This finding was supported by CFD simulation results demonstrating that the inner membrane experienced higher axial velocities when the fluid flowed through the spacers, which generated a higher wall shear stress in the interior. As the dimensionless radius Rr increases, the dimensionless wall shear stress Es also increases, indicating that the difference between the inner and outer wall shear stresses becomes more significant as the membrane curvature increases.
17
5. Acknowledgements The authors would like to thank the Ministry of Science and Technology (MOST), Taiwan, R.O.C. for their financial support (Project numbers 101-2815-C-033 -008-E and 103-2622-E-002003-CC1).
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[27] E. Kujundizic, A.C. Fonseca, E.A. Evans, M. Peterson, A.R. Greenberg, M. Hernandez, Ultrasonic monitoring of early-stage biofilm growth on polymeric surfaces, Journal of Microbiological Methods 68 (2007) 458-467. [28] E. Kujundizic, K. Cobry, A.R. Greenberg, M. Hernandez, Use of ultrasonic sensors for characterization of membrane fouling and cleaning, Journal of Engineerred Fibers and Fabrics, Special Issue (2008) 35-44. [29] E. Kujundizic, A.R. Greenberg, R. Fong, B. Moore, D. Kujundzic M. Hernandez, Biofouling potential of industrial fermentation broth components during microfiltration, J. Membr. Sci. 349 (2010) 44-55. [30] E. Kujundizic, A.R. Greenberg, R. Fong, M. Hernandez, Monitoring protein fouling on polymeric membranes using ultrasonic frequency-domain reflectometry, Membranes 1 (2011) 195-216. [31] Y.L. Li, K.L. Tung “CFD Simulation of Fluid Flow through Spacer-filled Membrane Module: Selecting Suitable Cell Types for Periodic Boundary Conditions,” Desalination 233 (2008) 351-358. [32] Y.L. Li, K.L. Tung, The effect of curvature of a spacer-filled channel on fluid flow in spiralwound membrane modules, J. Membr. Sci. 319 (1/2) (2008) 286–297.
21
List of Tables Table 1
Characteristics of membranes
Table 2
Geometric parameters of spiral wound membrane module used in CFD simulation
Table 3
Operating conditions of spiral wound membrane module used in CFD simulation
List of Figures Figure 1
Schematic representation of cross-flow filtration system.
Figure 2
Schematic representation of dead-end filtration system.
Figure 3
Schematic diagram of the high-frequency ultrasound system.
Figure 4
The pulse-echo response of the 50 MHz transducer.
Figure 5
C-mode scanning with high-frequency ultrasound system.
Figure 6
Spiral wound membrane module orientation, (a) horizontal, (b) vertical.
Figure 7
Labeled sections of spiral wound membrane for high-frequency ultrasound analysis.
Figure 8
Geometry of spiral wound membrane module for CFD simulation. (a) 3D model, (b) cross-sectional area.
Figure 9
Fouling condition of PVDF membrane using PFS-added humic acid feed solution.
Figure 10
High-frequency ultrasound B-mode images of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
Figure 11
High-frequency ultrasound C-mode images and average voltage Vavg of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
Figure 12
High-frequency ultrasound C-mode images and actual images of dense polyamide composited membrane which were dead-end filtrated with 5 mg/L of PFS-added humic acid solution for (a) 0, (b) 30, and (c) 90 min. 22
Figure 13
Scanning electron microscope (SEM) images of fouling membrane, (a) dense polyamide composited membrane, (b) porous PVDF membrane.
Figure 14
High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance for (a) down-side of inner membrane, (b) downside of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
Figure 15
High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the retentate outlet for (a) down-side of inner membrane, (b) downside of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
Figure 16
High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance (section A, C, E) and retentate outlet (section B, D, F) for (a) inner membrane, and (b) outer membrane, after 6 h filtration at vertical orientation.
Figure 17
Radial velocity of inner and outer membranes in a spiral wound module for different membrane resistances, Rm.
Figure 18
Static pressure across the spiral wound membrane module at Z = 0.1275 m for (a) UF module, and (b) RO module.
Figure 19
Axial velocity distributions between the inner and outer membranes along a spiral wound membrane module (Z-direction) for (a) UF module, and (b) RO module.
Figure 20
Axial velocity distributions between the inner and outer membranes from centre permeate collection tube to external module in radial direction at Z = 0.1275 m for (a) UF module, and (b) RO module.
Figure 21
Dimensionless wall shear stress, Es versus dimensionless radius, Rr for different membrane resistances, Rm.
23
↑ Tung .
Fig. 1. Schematic representation of cross-flow filtration system.
24
↑ Tung .
Fig. 2. Schematic representation of dead-end filtration system.
25
↑ Tung .
Fig. 3. Schematic diagram of the high-frequency ultrasound system.
26
↑ Tung .
Fig. 4. The pulse-echo response of the 50 MHz transducer.
27
↑ Tung .
Fig. 5. C-mode scanning with high-frequency ultrasound system.
28
↑ Tung .
(a)
(b) Fig. 6. Spiral wound membrane module orientation, (a) horizontal, (b) vertical. [31]
29
↑ Tung .
Fig. 7. Labeled sections of spiral wound membrane for high-frequency ultrasound analysis.
30
↑ Tung .
(a)
(b) Fig. 8. Geometry of spiral wound membrane module for CFD simulation. (a) 3D model, (b) crosssectional area.
31
↑ Tung .
Fig. 9. Fouling condition of PVDF membrane using PFS-added humic acid feed solution.
32
Depth (mm)
↑ Tung .
5.5
6 2
4
6
8
Distance (mm)
Depth (mm)
(a)
5.5
6 2
4
6
8
Distance (mm)
Depth (mm)
(b)
5.5
6 2
4
6
Distance (mm)
33
8
Depth (mm)
(c)
5.5
6 2
4
6
8
Distance (mm) (d)
Fig. 10. High-frequency ultrasound B-mode images of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
34
↑ Tung .
Vavg = 0.3750 V
Vavg = 0.3659 V
(a)
(b)
Vavg = 0.3004 V
Vavg = 0.2830 V
(c)
(d)
Fig. 11. High-frequency ultrasound C-mode images and average voltage Vavg of porous PVDF membrane dead-end filtrated with 5 mg/L of PFS-added humic acid solution; (a) clean membrane, (b) 100 ml, (c) 200 ml, and (d) 400 ml of feed volume.
35
↑ Tung .
(a) Vavg = 1.3796 V
(b) Vavg = 2.0214 V
(c) Vavg = 2.4662 V Fig. 12. High-frequency ultrasound C-mode images and actual images of dense polyamide composited membrane which were dead-end filtrated with 5 mg/L of PFS-added humic acid solution for (a) 0, (b) 30, and (c) 90 min.
36
↑ Tung .
(a)
(b)
Fig. 13. Scanning electron microscope (SEM) images of fouling membrane, (a) dense polyamide composited membrane, (b) porous PVDF membrane.
37
↑ Tung . Section A
Vavg = 1.6324 V
Vavg = 2.0647 V
Vavg = 2.1705 V
Vavg = 2.0301 V
Vavg = 2.0140 V
Vavg = 2.1198 V
Vavg = 2.0912 V
Vavg = 2.3023 V (b)
Vavg = 2.1556 V (c)
Vavg = 2.0293 V (d)
Section C
Vavg = 1.6249 V Section E
Vavg = 1.6960 V (a)
Fig. 14. High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance for (a) down-side of inner membrane, (b) down-side of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
38
↑ Tung .
Section B
Vavg = 1.8997 V
Vavg = 2.1190 V
Vavg = 1.8112 V
Vavg = 1.9198 V
Vavg = 2.0456 V
Vavg = 1.7880 V
Vavg = 1.8264 V
Vavg = 2.0400 V (b)
Vavg = 1.7835 V (c)
Vavg = 1.9292 V (d)
Section D
Vavg = 1.9335 V Section F
Vavg = 1.9328 V (a)
Fig. 15. High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the retentate outlet for (a) down-side of inner membrane, (b) down-side of outer membrane, (c) up-side of inner membrane, and (d) up-side of outer membrane, after 6 h filtration at horizontal orientation.
39
↑ Tung .
Section A
Vavg = 1.8347 V
Section B
Vavg = 2.1818 V
Vavg = 1.9913 V Section D
Section C
Vavg = 1.8575 V
Vavg = 2.0822 V
Vavg = 1.9223 V
Vavg = 2.1651 V
Section F
Section E
Vavg = 1.8310 V (a)
Vavg = 2.1825 V
Vavg = 2.1417 V (b)
Vavg = 2.0720 V (a)
Vavg = 2.2266 V (d)
Fig. 16. High-frequency ultrasound C-mode images and average voltage Vavg of membrane sections located at the feed entrance (section A, C, E) and retentate outlet (section B, D, F) for (a) inner membrane, and (b) outer membrane, after 6 h filtration at vertical orientation.
40
↑ Tung .
Fig. 17. Radial velocity of inner and outer membranes in a spiral wound module for different membrane resistances, Rm.
41
↑ Tung .
Rm = 2.87 × 1011 m-1
Rm = 1.12 × 1014 m-1
(UF module)
(RO module)
(a)
(b)
Fig. 18. Static pressure across the spiral wound membrane module at Z = 0.1275 m for (a) UF module, and (b) RO module.
42
↑ Tung .
Rm = 2.87 × 1011 m-1 (UF module)
Rm = 1.12 × 1014 m-1 (RO module)
(a)
(b)
Fig. 19. Axial velocity distributions between the inner and outer membranes along a spiral wound membrane module (Z-direction) for (a) UF module, and (b) RO module.
43
↑ Tung .
Rm = 2.87 × 1011 m-1 (UF module)
Rm = 1.12 × 1014 m-1 (RO module)
(a)
(b)
Fig. 20. Axial velocity distributions between the inner and outer membranes from centre permeate collection tube to external module in radial direction at Z = 0.1275 m for (a) UF module, and (b) RO module.
44
↑ Tung .
Fig. 21. Dimensionless wall shear stress, Es versus dimensionless radius, Rr for different membrane resistances, Rm.
45
↑ Tung .
Table 1 Characteristics of membranes UF spiral wound membrane module Material
Polyvinylidene fluoride, PVDF
Membrane resistance
2.87 × 1011 m-1
Pore size
0.2, 0.05 µm
Module diameter / length
1.8 in / 10 in
Company / Model
ALPSPRING Co., Ltd / UF-2012
RO spiral wound membrane module Material
Polyamide composited, FILMTEC USA
Membrane resistance
1.12 × 1014 m-1
Module diameter / length
1.8 in / 10 in
Company / Model
ALPSPRING Co., Ltd / TW40-1812-50
46
↑ Tung .
Table 2 Geometric parameters of spiral wound membrane module used in CFD simulation Name
Radius (cm)
Height (cm)
Length (cm)
Feed spacer channel
-
0.048
25.5
Permeate spacer channel
-
0.028
25.5
Membrane
-
0.011
25.5
0.75
-
25.5
-
2
2
Permeate collection tube Hole on permeate collection tubes
47
↑ Tung .
Table 3 Operating conditions of spiral wound membrane module used in CFD simulation Module
UF
RO
Model
Laminar
Laminar
Inlet pressure
1 atm
8 atm
Outlet pressure
0.9 atm
7.9 atm
Permeate pressure
1 atm
1 atm
Membrane resistance
2.87 × 1011 m-1
1.12 × 1014 m-1
Feed spacer resistance
0.048 × 101 m-1
0.048 × 101 m-1
Permeate spacer resistance
2.8 × 101 m-1
2.8 × 103 m-1
48