- Email: [email protected]
Capillary hollow fiber nanofiltration membranes
Separation and Purification Technology 22-23 (2001) 499– 506 www.elsevier.com/locate/seppur
Capillary hollow fiber nanofiltration membranes M. Frank a, G. Bargeman b, A. Zwijnenburg c, M. Wessling a,* b
a Akzo Nobel, Chemicals Research, PO Box 9300, 6800 SB Arnhem, The Netherlands NIZO Food Research (Netherlands Institute for Dairy Research), PO Box 20, 6710 BA Ede, The Netherlands c Stork Friesland BV, PO Box 13, 8400 AA Gorredijk, The Netherlands
Abstract Spiral-wound modules generally have high packing densities, low costs, but require extensive feedwater pretreatment and have a high fouling potential. Tubular membrane modules have beneficial fouling properties, can be backflushed, but have low packing densities and are expensive. Both module types exist for the nanofiltration process. However, the membrane module type combining the superior properties of both types, namely capillary hollow fiber membrane modules, have not been developed yet. This paper presents the flux and retention performance data of new developed nanofiltration capillary hollow fiber membrane modules. The new modules show retention performances comparable with those of best-performing spiral-wound modules. They can typically be applied for water softening and decoloring. Continuous experiments on surface water and nanofiltration whey permeate demonstrate that the fouling behavior of the new capillary modules is indeed better than the spiral-wound modules due to its well-defined feed channel geometry. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nanofiltration; Capillary hollow fiber module; Spiral-wound module; Water softening; Water decoloring; Fouling
1. Introduction Nanofiltration is a pressure driven process that forms the transition between ultrafiltration and reverse osmosis with respect to its flux and separation performance. Ultrafiltration membranes reject macromolecules but pass dissolved ions whereas reverse osmosis rejects macromolecules as well as all ionic species. Nanofiltration membranes, however, pass mono-valent to a certain extend, but they can reject multi-valent ions and low molecular weight molecules such as glucose * Corresponding author. Fax: +31-26-3665109. E-mail address: [email protected] (M. Wessling).
with retention coefficients close to 100%. Nanofiltration is therefore a membrane process suitable for separation and treatment processes such as water softening, color removal and chemical oxygen demand (COD) reduction [1,2]. Nanofiltration membranes can also be used to separate or adjust the ratio of mono and bi-valent ions [3]. Membrane module forms applied in nanofiltration processes are either spiral-wound modules or tubular modules. The characteristics of both technologies are described in Table 1. Spiral-wound modules show benefits with respect to packing density and cost per module. Tubular modules show superior performance over spirals with re-
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spect to the extend of pretreatment and the option to backflush. Aim of the presented work is to bridge the gap between the two technologies by developing a capillary hollow fiber membrane having in inner diameter significantly smaller than the tubular membrane modules in the order of 1.5 mm and lower. The following paper will demonstrate that such a development combines the benefits of the low fouling potential of the tubular membrane modules with the high packing density of hollow fiber membrane modules.
2. Membrane and module preparation
2.1. Ultrafiltration support membranes and modules Polyethersulfone based ultrafiltration membranes (UltraPES) used as a support membrane were developed by Membrana GmbH, Wuppertal/Germany. Internal diameter of the UltraPES capillaries is 1.5 mm, however, the coating procedure is not limited to this membrane. The clean water flux (CWF) of the 1.5 mm capillaries is 400 –600 l/m2 h bar. The number of capillaries is 50 in a 1¦-module and 512 in a 3¦-module resulting in an effective membrane area of 0.2 and 2.2 m2, respectively. The capillary membranes are potted in modules with a length of 1 m. Before coating the support module with the polyamide toplayer, integrity of the UF modules is measured by performing a pressure hold test at 0.5 bar. A critical time required to hold the gas pressure Table 1 Comparison between the technologies of spiral-wound, tubular and hollow fiber membrane modules (adapted from Ref. [3]) Criteria
Spiral-wound
Tubular
Capillary
Packing density Cost of module Quality of pretreatment required Backflushing
++ +++b −
−a − +++
++ +++ ++
−
+
+++
a b
Clear disadvantage. Clear advantage.
indicates whether the membrane module contains defects.
2.2. Coating procedure Interfacial polymerization of a polyacylhalides and multifunctional amines on support membranes result in layers having reverse osmosis or nanofiltration separation properties. For the membrane modules developed here, the polyamide toplayer is formed on the lumen (inner) side of the capillaries. The UltraPES support membranes are first wetted with an aqueous solution containing tetramethyl-tetrakis-aminomethylmethane (hereafter referred to as TTAM). Information about this proprietary aminocompound is given in [4]. Suitable aqueous solutions of TTAM may vary from 0.1 to 2.0 wt.% and may also include acid scavengers to promote the interfacial reaction and surfactants to promote proper wetting. After draining, the amine-loaded supports are contacted during a short time with an organic solution containing polyacylhalides. The polyacylhalide reactants are present in a concentration from 0.1 to 1 wt.%. After the reaction with the organic phase the modules are heatcured. Varying the concentration of the polyfunctional amine in the aqueous phase and the polyacylhalides in the organic phase can change the characteristics of the polyamide toplayer. Moreover, changes in coating parameters such as drainage time and velocity influence the ultimate performance of the capillary nanofiltration module. The capillary modules can be backwashed for effective cleaning. Trans-membrane pressure (TMP) range during backwash 20–100 kPa with a backflush time of 30 s. Effectiveness of backwashing can be improved by the use of forward flush on the human side. Tests with 10.000 backwash shocks at a TMP of 150 kPa performed with a 1¦-module showed no drop in rejection. pH stability of these capillary nanofiltration membranes is limited by the polyamide toplayer. pH range for regular use 4–10 (35°C); pH range during cleaning 3–11 (B 35°C).
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Table 2 Separation and flux performance of capillary hollow fiber and spiral-wound nanofiltration membranesa Performance criteria
Capillary hollow fiber
Spiral wound NF-45
Spiral wound NF-70
Clean water flux (l/m2 h bar)
22/22
4
11
Retention NaCl CaCl2 Na2SO4 MgSO4 Glucose
40/40 92/96 70/75 98/99 80/80
45 90 \99 \99 96
88
a The two values for the capillary hollow fiber module indicate two different modules. The first value gives the performance of the module used during surface water treatment: the second describes the module used for the NF whey permeate.
2.3. Testing procedure To characterize the performance of composite nanofiltration membranes, modules are tested on different salts in RO-water at a trans-membrane pressure (TMP) of 6 bar. The concentration of salts like NaCl, MgSO4, CaCl2 and Na2SO4 is 0.1 wt%. Due to concentration polarisation, flux and rejection values depend on the flow velocity. The feed flow velocity ranges between 0.5 and 2 m/s.
with the Dow Chemicals NF-45: sodium chloride and calcium chloride retention have about the same value. Retentions of the NF-45 for sodium and magnesium sulfate as well as glucose are higher than the capillary hollow fiber. The pressure-normalized flux of the capillary membrane is significantly higher by a factor 5. The NF-70 in general shows much higher retentions and very high flux considering the high retentions. The newly developed capillary nanofiltration module will be primarily applied for water softening and decoloring processes.
3. Membrane module performance
3.2. Surface water treatment 3.1. Synthetic feed mixtures The clean water flux of a typical 1.5 mm NF membrane module developed for water softening is 25 l/m2 h bar with a MgSO4 rejection of 97% and a NaCl rejection of 40% at Reynolds numbers of 2500 and a trans-membrane pressure of 6 bar. Due to the high flux modules may be operated at low pressure which reduces operating costs significantly. A summary of performance data is given in Table 2 comparing the newly developed capillary nanofiltration membrane with two frequently used nanofiltration spiral-wound modules from Dow Chemicals (NF-45 and NF-70). The NF-45 is a spiral-wound module typically used to soften and decolorize water. The NF-70 has the additional function to desalt as well, hence retaining mono-valent ions. Table 2 indicates that the capillary hollow fiber membranes compare with respect to rejections
After characterizing the basic performance data, 3¦ capillary hollow fiber membrane modules containing 2.2 m2 membrane were tested on surface water. A few cubic meters of surface were pumped from the river Rhine at Arnhem into containers and shipped to the process labs of Akzo Nobel. Here, the volume was split into two batches to be used in two total-recycle filtration experiments: one with the capillary hollow fiber module, the other for a spiral wound. Before entering the membrane system, the surface water had to pass a metal screen filter having a pore diameter of 5 mm. No further pretreatment was applied to the water. The typical composition of the surface water is given in Table 3. Table 4 compares typical retentions of the capillary hollow membrane and the two spiral-wound modules NF-45 and NF-70 for the particular compounds of the river Rhine water.
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Table 3 Typical composition of the surface water obtained from the river Rhine Component
Concentration
Conductivity (mS/cm) DOC (ppm) Na (ppm) K (ppm) Ca (ppm) Mg (ppm) Cl (ppm) SO4 (ppm)
670 15.6 49 4.4 65 10.3 63 60
In long-term experiments, we followed the flux as a function of time. This experiment was aimed to determine the fouling tendency for the two different module types. Due to its superior retention performance, the capillary module was compared with the NF-70. To guarantee that the amount of fouling matter deposited is equal for both membrane modules, the modules were operated at equal flux resulting in a feed pressure of 5 bar for the capillary module and 16 bar for the spiral-wound module. Fig. 1 shows a clear flux decline for both membrane modules during the initial period of the experiment. However, clearly the flux decline is more pronounced for the spiralwound module. After 6 h of operation, the capillary module shows a flux of 50 l/m2 h bar whereas the spiral-wound modules only retains a flux of 20 m2 h bar. Visually, one could observe that the feed water fed to the spiral-wound mod-
Fig. 1. Flux decline as a function of time for the capillary hollow fiber membrane in comparison with the spiral-wound module NF-70 of Dow Chemicals.
ule lost turbidity indicating deposition of matter inside the module. The turbidity of feed water fed to the capillary module remained constant throughout the experiment indicating less matter deposition inside the module. These qualitative observations are in accordance with the measured flux declines. One may argue that the hydrodynamic conditions at the feed side of the modules may have been advantageous for the hollow fiber module. Table 5 summarizes the parameters at which both modules were operated. The mass transfer coefficients and the concentration polarization modulus for the rejection of MgSO4 are estimated from experimental rejection data procedures described in literature [5,6]. The data indicate higher mass transfer coefficients for the spiral-wound module
Table 4 Comparison of the retention performance between the capillary hollow fiber module and the two Dow Chemicals spiral-wound modules NF-45 and NF-70 Retained component
Conductivity (mS/cm) DOC (ppm) Na (ppm) K (ppm) Ca (ppm) Mg (ppm) Cl (ppm) SO4 (ppm)
Retention (%) capillary module
69 73 26 29 95 97 67 95
Retention (%) spiral-wound NF-45
NF-70
77 79 34 37 92 \96 67 \98
94 86 88 89 98 97 92 99
M. Frank et al. / Separation/Purification Technology 22-23 (2001) 499–506
and higher susceptibility for concentration polarization for the hollow fiber module. Nonetheless, the hollow fiber performs better with respect to fouling tendency. Recent research [7] on fouling of nanofiltration membranes indicates that fouling sets in when a module operates outside the initial linear relationship between flux and feed pressure. For the experiments presented here, both modules are operated in the linear part of the curve flux versus transmembrane pressure as one can conclude from the product of clean water flux and transmembrane pressure applied during the surface water testing which equals the initial water flux obtained on the surface water. After the significant flux decline, both modules were cleaned with Ultrasil 10, an alkaline cleaning Table 5 Hydrodynamic conditions in the capillary hollow fiber and spiral-wound module and estimated mass transport coefficients Capillary module Trans-membrane 5.1 ( 90.1) pressure (bar) Initial clean water flux 20 (l/m2 h bar) Feed flow to module 4.8 (m3/h) Average linear velocity 1.7 (m/s) Mass transfer coefficient 2.5 (m/s) [4] Concentration 2.4 polarization modulus (–) [6]
NF-70 spiral-wound 15.9 ( 9 0.1) 7 1 0.1 4.7 1.6
Table 6 Feed composition of a permeate resulting from desalting and concentration of cheese whey using nanofiltration COD (mg O2/l) N (ppm) NO3 (ppm) PO4 (ppm) Cl (ppm) Ca (ppm) K (ppm) Na (ppm)
480 179 32 34 1050 25 810 223
503
agent supplied by Henkel. The capillary hollow fiber modules was cleaned at an Ultrasil 10 concentration of 0. 1 wt.% The concentration for the Dow module was 0.5 wt.%. After the cleaning, the flux recovered to 88% of its initial value for the capillary module and to 78% for the spiralwound module. There is no hard evidence for the better result of the cleaning procedure to be related to the geometry of the module instead of being related to the surface chemistry and properties of the polyamide coating layer. However, one may speculate that the well defined inside of the hollow fiber allows better cleaning than the feed channel geometry of a spiral-wound module. Also after the cleaning procedure, the flux drops for both modules: again, more distinct for the spiral-wound. The extend flux decline is smaller in the last part of the experiment because the experiments are run in a total recycle mode: there is only a finite amount of organic matter that can be deposited on the membrane and after deposition of all the matter the membrane flux will not decline any further.
3.3. Whey permeate filtration The capillary membrane module listed in Table 2 was tested for filtration of NF whey permeate as well. The composition of the feedstock, which was obtained by desalting and concentrating cheese whey with nanofiltration, is listed in Table 6. The aim of this test program was to obtain information on the retention characteristics, membrane fouling and the filtration capacity of the developed membrane in relation to a commercially top ranked spiral-wound membrane with similar single solute retention characteristics. DOW Chemicals’ NF-45 was selected as spiral wound module. Single solute characteristics for both modules are listed in Table 2. The capillary module containing 2.2 m2 membrane surface was tested in continuous singlestage operation as shown in Fig. 3. The unit was operated at a concentration factor cf of 5, a trans-membrane pressure (TMP) of 5 bar and a cross-flow rate of 4.8 m3/h at the inlet of the module. This resulted in a pressure-drop over the module of 0.3 bar. The selected operating condi-
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Fig. 2. Experimental set-up for filtration of NF whey permeate.
tions were optimal for the capillary module within the experimental constraints for the unit. After the cleaning procedure, which resulted in complete restoration of the dean water flux, a second experiment was done at cf =10 with otherwise unchanged operating conditions. NF-45 was also operated in continuous mode using feedstock from the same batch. Two NF-45 modules consisting of 5.6 m2 of membrane surface each were operated in series using a concentrate recycle over the individual modules (a two-stage operation mode as shown in Fig. 2). The cross-flow rate at the inlet of each of the modules was 4.8 m3/h, resulting in a pressure-drop over each of the modules of 0.7 bar. Also, for this situation optimal operating conditions were selected for the spiral wound modules to avoid concentration polarization as much as possible, while remaining
within the pressure-drop constraints of the modules. The average TMP was 26 bar at an overall concentration factor of 10. The higher operating pressure for NF-45 was selected on the basis of the ratio of the clean water fluxes for NF-45 and the developed capillary module and the wish for similar product fluxes during the tests. All experiments were performed at 10°C to avoid microbial growth in the membrane system, the concentrate and permeate. Operation with the developed capillary module resulted in a product flux of around 53 l/m2 h at cf = 5 as shown in Fig. 3. An increase of the concentration factor to cf = 10 reduced the product flux by less than 10%. The obtained product flux is in line with the observed clean water flux taken into account differences in the permeate viscosity and osmotic pressure influences. Also for NF-45 the product flux was marginally affected by the concentration factor. For operation at an overall concentration factor of 10 this resulted in a two fold concentration of the feedstock in the first stage followed by an additional five times concentration of the first stage concentrate in the second stage. Fluxes for both stages are represented in Fig. 3. As anticipated, the product flux for NF-45 at cf = 10 was similar to that found for the capillary module, despite the five times higher operating pressure used for NF45. Consequently, the required capital investment and the expected energy costs for filtration of NF whey permeate with the capillary module will be clearly lower than for operation with NF-45.
Fig. 3. Product fluxes as a function of time for the filtration of NF whey permeate with NF-45 and the capillary membrane module.
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Table 7 Comparison of the COD retentions for the capillary nanofiltration membrane and the spiral-wound module NF-45 from Dow Chemicals for filtration of NF whey permeate Membrane module
Stage
Overall concentration factor (–)
COD retention per stage (%)
Capillary membrane module Capillary membrane module NF-45 NF-45
1 1 1 2
5 10 2 10
82 88 74 91
Although the reduction of the flux as function of time was limited to less than 10% in 6 h of operation, the flux reduction with time for the second stage of NF-45 was clearly higher than for the capillary membrane module as shown in Fig. 3. On the basis of the total permeate flux from the unit, the fouling rate for NF-45 was more than a factor 2 higher than for the capillary module at an overall concentration factor of 10. The retention for COD for the capillary module was slightly lower than for NF-45 as shown in Table 7. At an overall concentration factor of 10 the COD of the total permeate obtained for NF45 was consequently lower than that for the capillary modules with 245 vs. 298 mg O2/l. Although the COD retentions for both membranes are high, the reduction of COD with respect to the feedstock is less than 50% for both cases due to the high concentration factor of 10. Mono-valent cations and chlorine retentions were below 40% for both membrane modules. Consequently, at cf = 10 concentrations of these components in the permeates were only marginally lower than in the feedstock for both membrane modules. For the use of the permeates in the food and dairy industry as alternative for drinking water or treated groundwater lower COD contents and mono-valent ion concentrations are required. Further development of the capillary membranes will therefore be aimed at production of tighter NF membranes. The currently developed capillary membranes are amongst others believed to be useful for the filtration of waste water treatment plant effluent at limited concentration factors to produce water that can be re-used in the dairy and food industry.
4. Summary The results presented in this paper demonstrate that it is technically feasible to combine the benefits of spiral-wound and tubular modules in capillary nanofiltration membrane modules. The newly developed capillary modules were compared with state-of-the-art spiral-wound modules with respect to their flux, retention and fouling performance. Retention performance of the capillary NF module compare well with spiral-wound modules normally used for water softening and decoloring. The flux performance however is superior over classical spirals such as the NF-45 of Dow Chemicals. To attain the desalting capacity (high retention for mono-valent ions) and maintaining a high flux as observed with the NF-70 will be the challenge for future developments in capillary hollow fiber nanofiltration membranes.
Acknowledgements We particularly appreciate the support of the Dutch R&D program Ecology, Economy and Technology, EET. The advice of R. Ligtermoet (FCDF), G. Hols (Nutricia Cuyk) and H. Kroes (Heineken) was greatly appreciated. M. DohmenSpeelmans, J. Klok, J. Leenders, J. Houwing and N. Vaandrager of NIZO food research are thanked for their contribution in this research project. The Dutch dairy industry is kindly thanked for their financial support. B. Tuin, C. Dirix and J.B. Westerink, J. Sluys of Akzo Nobel. H. Futselaar, R. Kolkman, J.W. de Rijk of Stork Friesland are thanked for the contribution to this research project. Membrana GmbH, Wuppertal
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Germany, particularly U. Seidel and F. Wechs, developed the support membrane and are thanked for their contributions.
References [1] J. Cadotte, R. Forester, M. Kim, R. Petersen, T. Stocker, Nanofiltration membranes broaden the use of membrane separation technology, Desalination 70 (1988) 77–88. [2] L.P. Raman, M. Cheryan, N. Rajagopalan, Consider nanofiltration for membrane separations, Chem. Eng. Prog. 90 (1994) 68 – 74.
.
[3] R.G.J. Radier, C.W. van Oers, A. Steenbergen, M. Wessling, Desalting a process cooling water using nanofiltration, Sep. Purif. Technol. (submitted for publication). [4] S.B. McCray, Bend research, US Patent 4 876 009, October, 1989. [5] F. Evangelista, An improved analytical method for the design of spiral-wound modules, Chem. Eng. J. 38 (1988) 33 – 40. [6] R.D. Noble, S.A. Stern, Membrane Separations, Technology Principles and Applications, Elsevier, Amsterdam, 1995. [7] M. Marittari, L. Puro, J. Nuortila-Jokinen, M. Nystrom, Fouling effects of polysaccharides and humic acid in nanofiltration, J. Membr. Sci. 165 (2000) 1 – 17?1?2?3?4?5?6?7.
Capillary hollow fiber nanofiltration membranes M. Frank a, G. Bargeman b, A. Zwijnenburg c, M. Wessling a,* b
a Akzo Nobel, Chemicals Research, PO Box 9300, 6800 SB Arnhem, The Netherlands NIZO Food Research (Netherlands Institute for Dairy Research), PO Box 20, 6710 BA Ede, The Netherlands c Stork Friesland BV, PO Box 13, 8400 AA Gorredijk, The Netherlands
Abstract Spiral-wound modules generally have high packing densities, low costs, but require extensive feedwater pretreatment and have a high fouling potential. Tubular membrane modules have beneficial fouling properties, can be backflushed, but have low packing densities and are expensive. Both module types exist for the nanofiltration process. However, the membrane module type combining the superior properties of both types, namely capillary hollow fiber membrane modules, have not been developed yet. This paper presents the flux and retention performance data of new developed nanofiltration capillary hollow fiber membrane modules. The new modules show retention performances comparable with those of best-performing spiral-wound modules. They can typically be applied for water softening and decoloring. Continuous experiments on surface water and nanofiltration whey permeate demonstrate that the fouling behavior of the new capillary modules is indeed better than the spiral-wound modules due to its well-defined feed channel geometry. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nanofiltration; Capillary hollow fiber module; Spiral-wound module; Water softening; Water decoloring; Fouling
1. Introduction Nanofiltration is a pressure driven process that forms the transition between ultrafiltration and reverse osmosis with respect to its flux and separation performance. Ultrafiltration membranes reject macromolecules but pass dissolved ions whereas reverse osmosis rejects macromolecules as well as all ionic species. Nanofiltration membranes, however, pass mono-valent to a certain extend, but they can reject multi-valent ions and low molecular weight molecules such as glucose * Corresponding author. Fax: +31-26-3665109. E-mail address: [email protected] (M. Wessling).
with retention coefficients close to 100%. Nanofiltration is therefore a membrane process suitable for separation and treatment processes such as water softening, color removal and chemical oxygen demand (COD) reduction [1,2]. Nanofiltration membranes can also be used to separate or adjust the ratio of mono and bi-valent ions [3]. Membrane module forms applied in nanofiltration processes are either spiral-wound modules or tubular modules. The characteristics of both technologies are described in Table 1. Spiral-wound modules show benefits with respect to packing density and cost per module. Tubular modules show superior performance over spirals with re-
1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 0 ) 0 0 1 7 1 - 4
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500
spect to the extend of pretreatment and the option to backflush. Aim of the presented work is to bridge the gap between the two technologies by developing a capillary hollow fiber membrane having in inner diameter significantly smaller than the tubular membrane modules in the order of 1.5 mm and lower. The following paper will demonstrate that such a development combines the benefits of the low fouling potential of the tubular membrane modules with the high packing density of hollow fiber membrane modules.
2. Membrane and module preparation
2.1. Ultrafiltration support membranes and modules Polyethersulfone based ultrafiltration membranes (UltraPES) used as a support membrane were developed by Membrana GmbH, Wuppertal/Germany. Internal diameter of the UltraPES capillaries is 1.5 mm, however, the coating procedure is not limited to this membrane. The clean water flux (CWF) of the 1.5 mm capillaries is 400 –600 l/m2 h bar. The number of capillaries is 50 in a 1¦-module and 512 in a 3¦-module resulting in an effective membrane area of 0.2 and 2.2 m2, respectively. The capillary membranes are potted in modules with a length of 1 m. Before coating the support module with the polyamide toplayer, integrity of the UF modules is measured by performing a pressure hold test at 0.5 bar. A critical time required to hold the gas pressure Table 1 Comparison between the technologies of spiral-wound, tubular and hollow fiber membrane modules (adapted from Ref. [3]) Criteria
Spiral-wound
Tubular
Capillary
Packing density Cost of module Quality of pretreatment required Backflushing
++ +++b −
−a − +++
++ +++ ++
−
+
+++
a b
Clear disadvantage. Clear advantage.
indicates whether the membrane module contains defects.
2.2. Coating procedure Interfacial polymerization of a polyacylhalides and multifunctional amines on support membranes result in layers having reverse osmosis or nanofiltration separation properties. For the membrane modules developed here, the polyamide toplayer is formed on the lumen (inner) side of the capillaries. The UltraPES support membranes are first wetted with an aqueous solution containing tetramethyl-tetrakis-aminomethylmethane (hereafter referred to as TTAM). Information about this proprietary aminocompound is given in [4]. Suitable aqueous solutions of TTAM may vary from 0.1 to 2.0 wt.% and may also include acid scavengers to promote the interfacial reaction and surfactants to promote proper wetting. After draining, the amine-loaded supports are contacted during a short time with an organic solution containing polyacylhalides. The polyacylhalide reactants are present in a concentration from 0.1 to 1 wt.%. After the reaction with the organic phase the modules are heatcured. Varying the concentration of the polyfunctional amine in the aqueous phase and the polyacylhalides in the organic phase can change the characteristics of the polyamide toplayer. Moreover, changes in coating parameters such as drainage time and velocity influence the ultimate performance of the capillary nanofiltration module. The capillary modules can be backwashed for effective cleaning. Trans-membrane pressure (TMP) range during backwash 20–100 kPa with a backflush time of 30 s. Effectiveness of backwashing can be improved by the use of forward flush on the human side. Tests with 10.000 backwash shocks at a TMP of 150 kPa performed with a 1¦-module showed no drop in rejection. pH stability of these capillary nanofiltration membranes is limited by the polyamide toplayer. pH range for regular use 4–10 (35°C); pH range during cleaning 3–11 (B 35°C).
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Table 2 Separation and flux performance of capillary hollow fiber and spiral-wound nanofiltration membranesa Performance criteria
Capillary hollow fiber
Spiral wound NF-45
Spiral wound NF-70
Clean water flux (l/m2 h bar)
22/22
4
11
Retention NaCl CaCl2 Na2SO4 MgSO4 Glucose
40/40 92/96 70/75 98/99 80/80
45 90 \99 \99 96
88
a The two values for the capillary hollow fiber module indicate two different modules. The first value gives the performance of the module used during surface water treatment: the second describes the module used for the NF whey permeate.
2.3. Testing procedure To characterize the performance of composite nanofiltration membranes, modules are tested on different salts in RO-water at a trans-membrane pressure (TMP) of 6 bar. The concentration of salts like NaCl, MgSO4, CaCl2 and Na2SO4 is 0.1 wt%. Due to concentration polarisation, flux and rejection values depend on the flow velocity. The feed flow velocity ranges between 0.5 and 2 m/s.
with the Dow Chemicals NF-45: sodium chloride and calcium chloride retention have about the same value. Retentions of the NF-45 for sodium and magnesium sulfate as well as glucose are higher than the capillary hollow fiber. The pressure-normalized flux of the capillary membrane is significantly higher by a factor 5. The NF-70 in general shows much higher retentions and very high flux considering the high retentions. The newly developed capillary nanofiltration module will be primarily applied for water softening and decoloring processes.
3. Membrane module performance
3.2. Surface water treatment 3.1. Synthetic feed mixtures The clean water flux of a typical 1.5 mm NF membrane module developed for water softening is 25 l/m2 h bar with a MgSO4 rejection of 97% and a NaCl rejection of 40% at Reynolds numbers of 2500 and a trans-membrane pressure of 6 bar. Due to the high flux modules may be operated at low pressure which reduces operating costs significantly. A summary of performance data is given in Table 2 comparing the newly developed capillary nanofiltration membrane with two frequently used nanofiltration spiral-wound modules from Dow Chemicals (NF-45 and NF-70). The NF-45 is a spiral-wound module typically used to soften and decolorize water. The NF-70 has the additional function to desalt as well, hence retaining mono-valent ions. Table 2 indicates that the capillary hollow fiber membranes compare with respect to rejections
After characterizing the basic performance data, 3¦ capillary hollow fiber membrane modules containing 2.2 m2 membrane were tested on surface water. A few cubic meters of surface were pumped from the river Rhine at Arnhem into containers and shipped to the process labs of Akzo Nobel. Here, the volume was split into two batches to be used in two total-recycle filtration experiments: one with the capillary hollow fiber module, the other for a spiral wound. Before entering the membrane system, the surface water had to pass a metal screen filter having a pore diameter of 5 mm. No further pretreatment was applied to the water. The typical composition of the surface water is given in Table 3. Table 4 compares typical retentions of the capillary hollow membrane and the two spiral-wound modules NF-45 and NF-70 for the particular compounds of the river Rhine water.
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Table 3 Typical composition of the surface water obtained from the river Rhine Component
Concentration
Conductivity (mS/cm) DOC (ppm) Na (ppm) K (ppm) Ca (ppm) Mg (ppm) Cl (ppm) SO4 (ppm)
670 15.6 49 4.4 65 10.3 63 60
In long-term experiments, we followed the flux as a function of time. This experiment was aimed to determine the fouling tendency for the two different module types. Due to its superior retention performance, the capillary module was compared with the NF-70. To guarantee that the amount of fouling matter deposited is equal for both membrane modules, the modules were operated at equal flux resulting in a feed pressure of 5 bar for the capillary module and 16 bar for the spiral-wound module. Fig. 1 shows a clear flux decline for both membrane modules during the initial period of the experiment. However, clearly the flux decline is more pronounced for the spiralwound module. After 6 h of operation, the capillary module shows a flux of 50 l/m2 h bar whereas the spiral-wound modules only retains a flux of 20 m2 h bar. Visually, one could observe that the feed water fed to the spiral-wound mod-
Fig. 1. Flux decline as a function of time for the capillary hollow fiber membrane in comparison with the spiral-wound module NF-70 of Dow Chemicals.
ule lost turbidity indicating deposition of matter inside the module. The turbidity of feed water fed to the capillary module remained constant throughout the experiment indicating less matter deposition inside the module. These qualitative observations are in accordance with the measured flux declines. One may argue that the hydrodynamic conditions at the feed side of the modules may have been advantageous for the hollow fiber module. Table 5 summarizes the parameters at which both modules were operated. The mass transfer coefficients and the concentration polarization modulus for the rejection of MgSO4 are estimated from experimental rejection data procedures described in literature [5,6]. The data indicate higher mass transfer coefficients for the spiral-wound module
Table 4 Comparison of the retention performance between the capillary hollow fiber module and the two Dow Chemicals spiral-wound modules NF-45 and NF-70 Retained component
Conductivity (mS/cm) DOC (ppm) Na (ppm) K (ppm) Ca (ppm) Mg (ppm) Cl (ppm) SO4 (ppm)
Retention (%) capillary module
69 73 26 29 95 97 67 95
Retention (%) spiral-wound NF-45
NF-70
77 79 34 37 92 \96 67 \98
94 86 88 89 98 97 92 99
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and higher susceptibility for concentration polarization for the hollow fiber module. Nonetheless, the hollow fiber performs better with respect to fouling tendency. Recent research [7] on fouling of nanofiltration membranes indicates that fouling sets in when a module operates outside the initial linear relationship between flux and feed pressure. For the experiments presented here, both modules are operated in the linear part of the curve flux versus transmembrane pressure as one can conclude from the product of clean water flux and transmembrane pressure applied during the surface water testing which equals the initial water flux obtained on the surface water. After the significant flux decline, both modules were cleaned with Ultrasil 10, an alkaline cleaning Table 5 Hydrodynamic conditions in the capillary hollow fiber and spiral-wound module and estimated mass transport coefficients Capillary module Trans-membrane 5.1 ( 90.1) pressure (bar) Initial clean water flux 20 (l/m2 h bar) Feed flow to module 4.8 (m3/h) Average linear velocity 1.7 (m/s) Mass transfer coefficient 2.5 (m/s) [4] Concentration 2.4 polarization modulus (–) [6]
NF-70 spiral-wound 15.9 ( 9 0.1) 7 1 0.1 4.7 1.6
Table 6 Feed composition of a permeate resulting from desalting and concentration of cheese whey using nanofiltration COD (mg O2/l) N (ppm) NO3 (ppm) PO4 (ppm) Cl (ppm) Ca (ppm) K (ppm) Na (ppm)
480 179 32 34 1050 25 810 223
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agent supplied by Henkel. The capillary hollow fiber modules was cleaned at an Ultrasil 10 concentration of 0. 1 wt.% The concentration for the Dow module was 0.5 wt.%. After the cleaning, the flux recovered to 88% of its initial value for the capillary module and to 78% for the spiralwound module. There is no hard evidence for the better result of the cleaning procedure to be related to the geometry of the module instead of being related to the surface chemistry and properties of the polyamide coating layer. However, one may speculate that the well defined inside of the hollow fiber allows better cleaning than the feed channel geometry of a spiral-wound module. Also after the cleaning procedure, the flux drops for both modules: again, more distinct for the spiral-wound. The extend flux decline is smaller in the last part of the experiment because the experiments are run in a total recycle mode: there is only a finite amount of organic matter that can be deposited on the membrane and after deposition of all the matter the membrane flux will not decline any further.
3.3. Whey permeate filtration The capillary membrane module listed in Table 2 was tested for filtration of NF whey permeate as well. The composition of the feedstock, which was obtained by desalting and concentrating cheese whey with nanofiltration, is listed in Table 6. The aim of this test program was to obtain information on the retention characteristics, membrane fouling and the filtration capacity of the developed membrane in relation to a commercially top ranked spiral-wound membrane with similar single solute retention characteristics. DOW Chemicals’ NF-45 was selected as spiral wound module. Single solute characteristics for both modules are listed in Table 2. The capillary module containing 2.2 m2 membrane surface was tested in continuous singlestage operation as shown in Fig. 3. The unit was operated at a concentration factor cf of 5, a trans-membrane pressure (TMP) of 5 bar and a cross-flow rate of 4.8 m3/h at the inlet of the module. This resulted in a pressure-drop over the module of 0.3 bar. The selected operating condi-
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Fig. 2. Experimental set-up for filtration of NF whey permeate.
tions were optimal for the capillary module within the experimental constraints for the unit. After the cleaning procedure, which resulted in complete restoration of the dean water flux, a second experiment was done at cf =10 with otherwise unchanged operating conditions. NF-45 was also operated in continuous mode using feedstock from the same batch. Two NF-45 modules consisting of 5.6 m2 of membrane surface each were operated in series using a concentrate recycle over the individual modules (a two-stage operation mode as shown in Fig. 2). The cross-flow rate at the inlet of each of the modules was 4.8 m3/h, resulting in a pressure-drop over each of the modules of 0.7 bar. Also, for this situation optimal operating conditions were selected for the spiral wound modules to avoid concentration polarization as much as possible, while remaining
within the pressure-drop constraints of the modules. The average TMP was 26 bar at an overall concentration factor of 10. The higher operating pressure for NF-45 was selected on the basis of the ratio of the clean water fluxes for NF-45 and the developed capillary module and the wish for similar product fluxes during the tests. All experiments were performed at 10°C to avoid microbial growth in the membrane system, the concentrate and permeate. Operation with the developed capillary module resulted in a product flux of around 53 l/m2 h at cf = 5 as shown in Fig. 3. An increase of the concentration factor to cf = 10 reduced the product flux by less than 10%. The obtained product flux is in line with the observed clean water flux taken into account differences in the permeate viscosity and osmotic pressure influences. Also for NF-45 the product flux was marginally affected by the concentration factor. For operation at an overall concentration factor of 10 this resulted in a two fold concentration of the feedstock in the first stage followed by an additional five times concentration of the first stage concentrate in the second stage. Fluxes for both stages are represented in Fig. 3. As anticipated, the product flux for NF-45 at cf = 10 was similar to that found for the capillary module, despite the five times higher operating pressure used for NF45. Consequently, the required capital investment and the expected energy costs for filtration of NF whey permeate with the capillary module will be clearly lower than for operation with NF-45.
Fig. 3. Product fluxes as a function of time for the filtration of NF whey permeate with NF-45 and the capillary membrane module.
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Table 7 Comparison of the COD retentions for the capillary nanofiltration membrane and the spiral-wound module NF-45 from Dow Chemicals for filtration of NF whey permeate Membrane module
Stage
Overall concentration factor (–)
COD retention per stage (%)
Capillary membrane module Capillary membrane module NF-45 NF-45
1 1 1 2
5 10 2 10
82 88 74 91
Although the reduction of the flux as function of time was limited to less than 10% in 6 h of operation, the flux reduction with time for the second stage of NF-45 was clearly higher than for the capillary membrane module as shown in Fig. 3. On the basis of the total permeate flux from the unit, the fouling rate for NF-45 was more than a factor 2 higher than for the capillary module at an overall concentration factor of 10. The retention for COD for the capillary module was slightly lower than for NF-45 as shown in Table 7. At an overall concentration factor of 10 the COD of the total permeate obtained for NF45 was consequently lower than that for the capillary modules with 245 vs. 298 mg O2/l. Although the COD retentions for both membranes are high, the reduction of COD with respect to the feedstock is less than 50% for both cases due to the high concentration factor of 10. Mono-valent cations and chlorine retentions were below 40% for both membrane modules. Consequently, at cf = 10 concentrations of these components in the permeates were only marginally lower than in the feedstock for both membrane modules. For the use of the permeates in the food and dairy industry as alternative for drinking water or treated groundwater lower COD contents and mono-valent ion concentrations are required. Further development of the capillary membranes will therefore be aimed at production of tighter NF membranes. The currently developed capillary membranes are amongst others believed to be useful for the filtration of waste water treatment plant effluent at limited concentration factors to produce water that can be re-used in the dairy and food industry.
4. Summary The results presented in this paper demonstrate that it is technically feasible to combine the benefits of spiral-wound and tubular modules in capillary nanofiltration membrane modules. The newly developed capillary modules were compared with state-of-the-art spiral-wound modules with respect to their flux, retention and fouling performance. Retention performance of the capillary NF module compare well with spiral-wound modules normally used for water softening and decoloring. The flux performance however is superior over classical spirals such as the NF-45 of Dow Chemicals. To attain the desalting capacity (high retention for mono-valent ions) and maintaining a high flux as observed with the NF-70 will be the challenge for future developments in capillary hollow fiber nanofiltration membranes.
Acknowledgements We particularly appreciate the support of the Dutch R&D program Ecology, Economy and Technology, EET. The advice of R. Ligtermoet (FCDF), G. Hols (Nutricia Cuyk) and H. Kroes (Heineken) was greatly appreciated. M. DohmenSpeelmans, J. Klok, J. Leenders, J. Houwing and N. Vaandrager of NIZO food research are thanked for their contribution in this research project. The Dutch dairy industry is kindly thanked for their financial support. B. Tuin, C. Dirix and J.B. Westerink, J. Sluys of Akzo Nobel. H. Futselaar, R. Kolkman, J.W. de Rijk of Stork Friesland are thanked for the contribution to this research project. Membrana GmbH, Wuppertal
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Germany, particularly U. Seidel and F. Wechs, developed the support membrane and are thanked for their contributions.
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[3] R.G.J. Radier, C.W. van Oers, A. Steenbergen, M. Wessling, Desalting a process cooling water using nanofiltration, Sep. Purif. Technol. (submitted for publication). [4] S.B. McCray, Bend research, US Patent 4 876 009, October, 1989. [5] F. Evangelista, An improved analytical method for the design of spiral-wound modules, Chem. Eng. J. 38 (1988) 33 – 40. [6] R.D. Noble, S.A. Stern, Membrane Separations, Technology Principles and Applications, Elsevier, Amsterdam, 1995. [7] M. Marittari, L. Puro, J. Nuortila-Jokinen, M. Nystrom, Fouling effects of polysaccharides and humic acid in nanofiltration, J. Membr. Sci. 165 (2000) 1 – 17?1?2?3?4?5?6?7.