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Optimum cleaning-in-place conditions for stainless steel microfiltration membrane fouled by terephthalic acid solids
Journal of Membrane Science 209 (2002) 233–240
Optimum cleaning-in-place conditions for stainless steel microfiltration membrane fouled by terephthalic acid solids Young-Beom Kim a , Kisay Lee a,∗ , Jung-Hoon Chung b a
Department of Environmental Engineering and Biotechnology, Myongji University, Yongin, Kyongki 449-728, South Korea b Dongjoo Engineering Inc., Seoul 150-756, South Korea Received 21 December 2001; received in revised form 16 July 2002; accepted 16 July 2002
Abstract Terephthalic acid (TPA) is a raw material of polyester fiber and polyethylene terephthalate. When TPA is produced by catalytic air oxidation of p-xylene in the presence of acetic acid solvent, most of produced TPA exists in the form of crystalline suspended solids. A microfiltration process may be used to recover TPA, but the microfilters are subjected to fouling and therefore cleaning-in-place (CIP) regimes need to be developed. In this research, the effects of variations to CIP conditions were investigated on the flux recovery accomplished in a TiO2 -sintered stainless steel microfiltration membrane (0.1 m pore size) fouled with TPA. The extent of flux recovery was estimated as the ratio of the stabilized flux obtained during CIP to the water flux value achieved under corresponding operational conditions. Based upon batch solubility tests, sodium hydroxide (NaOH) was chosen as the major cleaning agent for the present experiment. The extent of flux recovery increased with increasing NaOH concentration over the range of 3–4% (w/v) NaOH, but decreased at NaOH concentrations above 4%. The flux recovery was favored at high cross-flow velocities, high temperatures and low transmembrane pressures. A high temperature run of cleaning did not produce any adverse effects up to 70 ◦ C. The addition of surfactants (SDS and Tween 80) to the caustic cleaning agent led to a significant reduction in cleaning efficiency. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cleaning-in-place (CIP); Microfiltration; Stainless steel membrane; Terephthalic acid (TPA)
1. Introduction Purified terephthalic acid (PTA), an essential raw material in the manufacture of polyester fiber and polyethylene terephthalate (PET), can be manufactured by catalytic air oxidation of p-xylene in the presence of acetic acid solvent. Most of the terephthalic acid (TPA) produced in this reaction exists in the form of crystalline suspended solids (SS) due to its low solubility. The reacted mixture is conven∗ Corresponding author. Tel.: +82-31-330-6689; fax: +82-31-336-6336. E-mail address: [email protected] (K. Lee).
tionally filtered with a mechanical separation device such as a drum or centrifugal disc filter. The TPA recovered as a solid then undergoes purification and drying, and the supernatant mother liquor is sent to a wastewater treatment facility. Since the efficiencies in solid–liquid separation of conventional mechanical separation devices are not satisfactory, a significant quantity of synthesized TPA is still contained in the generated mother liquor, which often imposes a serious burden on wastewater treatment [1]. Recently we are investigating a microfiltration process as an alternative method of TPA recovery in order to enhance the TPA recovery yield in the filtration step by reducing the TPA concentration in the
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discarded mother liquor. A stainless-steel microfiltration membrane was selected for this purpose. The aim of the present study is to optimize the cleaning-in-place (CIP) conditions for use after membrane fouling has been established. The cleaning agent and its concentration were selected by a series of solubility tests. The effects of variations in CIP conditions on flux recovery were then investigated after the membrane was fouled by SS which contained TPA. The parameters investigated were the cleaning agent concentration, transmembrane pressure (TMP), cross-flow velocity (CFV), and temperature. A further investigation was carried out on the effect of adding surfactant to the cleaning solution on the flux recovery.
2. Experimental 2.1. Membrane A SCEPTER® stainless steel tubular membrane (Graver Technologies Inc., DW, USA) with 0.1 m pore size and 6.5 mm i.d. was used. This membrane was made with a porous 316L stainless steel tube surface-coated with a sintered TiO2 layer [2]. Two modules of different filtration area were used: 0.07 and 0.0117 m2 . The bigger unit was used for the filtration of large quantities of commercial TPA product solution to see the flux decline tendency. The smaller unit was used for the tests to find optimum CIP conditions by intentionally inducing rapid fouling.
hydrocarbon chain. The CMC of Tween 80 is approximately 1 mM at 25 ◦ C. 2.3. Cleaning-in-place The membrane module with 0.07 m2 surface area was used for the filtration of commercial TPA product solution obtained from a local PTA manufacturing plant to examine the general tendency of filtration flux decline. The cross-flow filtration was run at 5 bar TMP, 4 m/s CFV, and 40 ◦ C. To investigate the optimization of CIP conditions, fast fouling of the membrane was intentionally induced using the 0.0117 m2 module and a model feed solution containing 3% (w/v) TPA in distilled water. The permeate stream was not recycled to the feed tank. After its flux had declined to 50% of the water flux for the clean membrane, the CIP conditions were varied to investigate their effects on the extent of flux recovery. This percentage extent was estimated by the ratio of the stabilized flux that was obtained during CIP to the water flux value under the corresponding operation conditions. Between each run of CIP tests, the initial water flux was retained by strong cleaning with 4% (w/v) NaOH solution at 70 ◦ C and a high CFV. The membrane material is chemically stable TiO2 -sintered 316L stainless steel and connections were welded, no noticeable change was observed in membrane integrity over the period of present study, despite the strong cleaning condition. This strong cleaning was necessary because under certain mild CIP conditions, the permeate flux during CIP was sometimes not recovered to the clean membrane level.
2.2. Cleaning agents Sodium hydroxide was used as the basic cleaning agent and was compared with a formulated cleaning agent, Ultrasil 10 (Henkel). Ultrasil 10 is a caustic based reagent containing several surfactants and is known to be effective in the cleaning of membranes fouled with organic compounds. The effect of two surfactant additives to the sodium hydroxide solution, sodium dodecylsulfate (SDS) and Tween 80, were therefore examined. SDS is an anionic surfactant with a critical micelle concentration (CMC) of approximately 8 mM at 25 ◦ C. Tween 80 is a nonionic surfactant which has a sorbitan ring and 20 polyoxyethylene (POE) units as a hydrophilic moiety and a monooleate
3. Results and discussion 3.1. Flux decline during filtration Fig. 1 shows a typical flux decline occurring during the filtration of TPA product solution of the composition given in Table 1. The TPA content in the feed solution was 0.55% (w/v), most of it in the form of white crystalline SS. Benzoic acid and p-toluic acid were the major side products formed during the TPA-producing reaction, and they themselves may be converted to TPA if returned to the reactor. The permeate flux declined within 1050–1100 l/(m2 h) (LMH) down to
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Fig. 1. Flux decline during the filtration of TPA-product solution. Membrane area: 0.07 m2 ; TMP: 5 bar; CFV: 4 m/s, and temperature: 40 ◦ C.
600 LMH over the 5 h of filtration due to membrane fouling caused by the accumulation of SS cake. This fouling cake was sampled and analyzed after the test and more than 90% of the dried cake mass was confirmed to be TPA. The insoluble fraction of feed TPA was almost completely removed, thus leaving only soluble fractions of TPA, p-toluic acid, and benzoic acid in the resulting permeate stream. Chemical oxygen demand (COD) was also reduced from 14,200 to 2800 ppm (Table 1). The pH value of feed, retentate, permeate was unchanged in the range of 3.4–3.5. 3.2. Selection of cleaning agent A batch solubility test can assist in the selection of a more effective cleaning agent because the solubilization of foulants from the membrane surface is one of the actions in which cleaning agents are involved. The variation in TPA solubility in solutions containing sodium hydroxide (NaOH), sodium carbonate (Na2 CO3 ), or EDTA and pure water was examined. The solubility of TPA increased linearly with increasing temperature from 20 to 80 ◦ C, but the extents of increase were small in solutions containing 2% (w/v) chemical cleaning agent: 33–37 g/l in NaOH, 16–18 g/l in Na2 CO3 , and 4–6 g/l in EDTA. TPA solubility in water was 1.5 g/l at 30 ◦ C and
steadily increased to 9 g/l at 80 ◦ C. The solutions of NaOH and Na2 CO3 displayed much enhanced and temperature-independent TPA solubility. Both NaOH and Na2 CO3 are alkaline cleaning agents, often used for the cleaning of organic foulants [3]. Since NaOH showed better TPA solubility than Na2 CO3 over the tested temperature range, it was used as the major cleaning agent in this study and the effect of NaOH concentration on TPA solubility was examined (Fig. 2). Of immediate note was the optimal NaOH concentration of about 7% (w/v) which produces maximum TPA solubility. At NaOH concentrations below this level, the TPA solubility increased steadily, but decreased thereafter, giving rise to a prominent peak in TPA solubility. The reason is not Table 1 Comparison of feed, retentate and permeate after microfiltration of TPA product solution
SS (%, w/v) TPA (%, w/v) p-Toluic acid (%, w/v) Benzoic acid (%, w/v) Acetic acid (%, v/v) COD (ppm) a
Feed
Retentate
Permeate
0.60 0.55a 0.1 0.08 0.1 14200
11.9 10.1a 1.9 0.09 0.1 –
≈0 0.03 0.005 0.08 0.1 2800
Sum of TPA amounts in soluble and solid fractions.
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Fig. 2. Change of TPA solubility with NaOH concentration variation. (䊉) 30 ◦ C; (䊊) 40 ◦ C; (䉲) 50 ◦ C; () 60 ◦ C; (䊏) 70 ◦ C; (䊐) 80 ◦ C.
clear for the decreased TPA solubility at NaOH concentration higher than 7%. It seems that the solvating power of NaOH solution for TPA is reduced at highly viscous state of such high NaOH concentration range.
3.3. Effect of NaOH concentration on cleaning Fig. 3 displays the time course of flux recovery during the CIP, depending on the NaOH concentration
Fig. 3. Influence of NaOH concentration on flux recovery in the cleaning of fouled membrane. Membrane area: 0.0117 m2 ; TMP: 1 bar; CFV: 0.5 m/s, and temperature: 25 ◦ C. NaOH concentration: (䊉) 1%; (䊊) 2%; (䉲) 3%; () 4%; (䊏) 5%; (䊐) 6%; (䉬) 7%.
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in the cleaning solution. The operating conditions of this set of CIP tests were 2 bar TMP, 2 m/s CFV, and 30 ◦ C. Over the range of 2–4% NaOH concentration, the recovered flux value quickly increased to a maximum within 15 min before decaying to a stabilized flux value. The observation of a maximum flux occurring at such an early stage of the cleaning process has been reported in the caustic cleaning of a stainless steel MF membrane which was fouled with whey protein concentrate [4]. Meanwhile, the flux increase was steady at NaOH concentrations higher than 5% with no decay. The flux was recovered to a higher value as NaOH concentration increased over the range of 1–3% NaOH, with the maximum flux recovery level being achieved at 3–4% NaOH. The use of 3% NaOH resulted in the highest flux recovery, this being 92% of the initial water flux. This NaOH concentration is a lower one than the optimal 7% concentration, which had been estimated from the batch solubility test (Fig. 2). It implies that there exist other criteria for selecting cleaning agent in actual membrane system. The rheological aspects and interactions of cleaning agent with filtered cake or membrane material can influence the cleaning efficiency and optimum concentration of cleaning agent. In Fig. 2, the extent of
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flux recovery was reduced at NaOH concentrations above 5%. The existence of an optimal cleaning agent concentration has been observed in other membrane systems [4–6]. In the present study, the retentate became highly viscous at high NaOH concentrations, an unfavorable outcome for effective circulation and for TPA cake cleaning from the membrane surface [7]. 3.4. Influence of TMP and CFV on cleaning Fig. 4 shows the influence of variations in TMP and CVF on the flux recovery during the cleaning process. Operational temperature was fixed at 30 ◦ C and 2% NaOH was used as the cleaning solution. The flux decreased with increasing TMP over the pressure range of 1–4 bar under constant CFV. Meanwhile, the flux recovery was enhanced with increasing CFV over the velocity range of 0.5–3 m/s under constant TMP. It should be noted that the achieved flux recovery was greater than 100% (i.e. higher than the clean membrane water flux) under certain conditions of low TMP and high CFV. Since such phenomenon was also observed in clean membrane filtering NaOH solution, especially at high CFV values, it is speculated that NaOH solution could be permeable more easily than
Fig. 4. Influence of TMP and CFV on flux recovery. Temperature: 30 ◦ C; cleaning solution: 2% NaOH; CFV: (䊊) 0.5 m/s; () 1.0 m/s; (䊐) 2.0 m/s; (䉫) 3.0 m/s.
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water in the case of the stainless steel membrane which was used in this study. A high TMP is known to lead to more compact cake formation or in-pore fouling, which accelerates the flux decline and reduces the cleaning efficiency [8–10]. This tendency was aggravated when CFV was lowered, as indicated by the flux recovery of less than 70% at 0.5 m/s CFV and 3 bar TMP. The increase in recovered flux at higher CFV values is due to the higher shear force which carries the particles away from the fouled cake as well as reduces the concentration polarization [11,12]. At low TMP levels such as 1 bar, the change of cleaning efficiency with variations in CFV value was quite small over the CFV range of 0.5–2 m/s. 3.5. Influence of temperature Increasing temperature produced a pronounced enhancement of flux recovery during the cleaning process. Fig. 5 shows the flux recovery at different temperatures under conditions of 3 bar TMP and 2 m/s CFV. The extent of permeate flux recovery was found to increase with temperature, for example the flux at 60 ◦ C was more than double that at 25 ◦ C. No adverse effects associated with a high temperature cleaning
process were noted over the range of temperature and the membrane system used in the present experiments. However, an optimal temperature does exist for maintaining a high flux in some other membrane filtration systems [4,10]. The reason for the flux enhancement observed at high temperature in this study was not clear. A high temperature improvement of the solubilization power of the cleaning agent was not considered to be the cause of the increased flux because, as noted in Fig. 2, the TPA solubility in NaOH solution was not severely temperature-dependent. 3.6. Effect of surfactants The effect of surfactant additions to the cleaning solution on flux recovery in the TPA-fouled membrane was compared with that of simple caustic cleaner. The cleaning conditions were 3 bar TMP and 2 m/s CFV at 30 ◦ C. Ultrasil 10 is a formulated cleaning agent, which is known to be a caustic-based reagent with the addition of surfactants, and is often used in the cleaning of organic-fouled membranes [13]. However, Fig. 6 clearly indicates that the cleaning efficiency of Ultrasil 10 was significantly lower than that of NaOH. The recovered flux with 2% (w/v) Ultrasil solution was less than 40% of that achieved with 2% NaOH
Fig. 5. Influence of temperature on flux recovery. TMP: 3 bar; CFV: 2 m/s, and 2% NaOH solution. (䊉) 25 ◦ C; (䉲) 50 ◦ C; (䊏) 60 ◦ C; (䉬) 70 ◦ C.
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Fig. 6. Influence of surfactant in cleaning solution on flux recovery. TMP: 3 bar; CFV: 2 m/s, and temperature: 30 ◦ C. (䊊) 2% NaOH; () 2% Ultrasil 10; (䊐) 2% NaOH + 1% SDS; (䉫) 2% NaOH + 1% Tween 80; () water.
solution. The flux level obtained with Ultrasil 10 was even less than that with pure water only. The addition of either Tween 80 or SDS also merely resulted in a significant reduction in recovered flux level. Although surfactants are valuable components of cleaning solutions in many cases, a loss of filtration flux due to adsorption, precipitation, or foaming has been reported [11]. The reason for surfactants having such poor cleaning effect is not readily apparent in the present specific case of TPA-fouled sintered stainless steel membrane. It seems that such surfactants do not contribute to TPA solubilization from the fouled membrane surface and that the surfactant molecules themselves act as an additional foulant and thus aggravate surface or in-pore fouling. In the case of the anionic surfactant SDS, another possible cause could be the formation of insoluble precipitates if SDS encounters some metal ions which leaked from the catalyst used in the TPA-producing reaction. Acknowledgements This study was financially supported by G-7 projects of the Ministry of Environment, Korea. Y.B.
Kim is also thankful to the BK21 scholarship of the Ministry of Education, Korea.
References [1] R. Kleerebezem, G. Lettinga, High-rate anaerobic treatment of purified terephthalic acid wastewater, Water Sci. Technol. 42 (2000) 250–268. [2] Microfiltration with sintered stainless steel SCEPTER crossflow membranes, Graver Technologies, http://www.eurodia. com/html/ulb.html. [3] L.J. Zeman, A.L. Zydney, Microfiltration and Ultrafiltration, Marcel Dekker, New York, USA, 1996 (Chapter 10). [4] M. Barlett, M.R. Bird, J.A. Howell, An experimental study for the development of a qualitative membrane cleaning model, J. Membr. Sci. 105 (1995) 147–157. [5] K.J. Kim, P. Sun, V. Chen, D.E. Wiley, A.G. Fane, The cleaning of ultrafiltration membranes fouled by protein, J. Membr. Sci. 80 (1993) 241–249. [6] S.S. Madaeni, T. Mohamamdi, M.K. Moghadam, Chemical cleaning of reverse osmosis membranes, Desalination 134 (2001) 77–82. [7] S.K.S. Razavi, J.L. Harris, F. Sherkat, Fouling and cleaning of membranes in the ultrafiltration of the aqueous extract of soy flour, J. Membr. Sci. 114 (1996) 92–104. [8] M.L. Cabero, F.A. Riera, R. Alvarez, Rinsing of ultrafiltration ceramic membranes fouled with whey proteins: effects
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on cleaning procedures, J. Membr. Sci. 154 (1999) 239– 250. [9] Q. Gan, J.A. Howell, R.W. Field, R. England, M.R. Bird, M.T. McKechinie, Synergetic cleaning procedure for a ceramic membrane fouled by beer microfiltration, J. Membr. Sci. 155 (1999) 277–289. [10] P. Wang, N. Xu, J. Shi, A pilot study of the treatment of waste rolling emusion using zirconia microfiltration membranes, J. Membr. Sci. 173 (2000) 159–166.
[11] M. Cheryan, Ultrafiltration and Microfiltration Handbook, Technomic, PA, USA, 1998 (Chapter 6). [12] H.K. Vyas, R.J. Bennett, A.D. Marshall, Influence of operating conditions on membrane fouling in crossflow microfiltration of particulate suspensions, Int. Diary J. 10 (2000) 477–487. [13] M. Nystrom, H. Zhu, Characterization of cleaning results using combined flux and streaming potential methods, J. Membr. Sci. 131 (1997) 195–205.
Optimum cleaning-in-place conditions for stainless steel microfiltration membrane fouled by terephthalic acid solids Young-Beom Kim a , Kisay Lee a,∗ , Jung-Hoon Chung b a
Department of Environmental Engineering and Biotechnology, Myongji University, Yongin, Kyongki 449-728, South Korea b Dongjoo Engineering Inc., Seoul 150-756, South Korea Received 21 December 2001; received in revised form 16 July 2002; accepted 16 July 2002
Abstract Terephthalic acid (TPA) is a raw material of polyester fiber and polyethylene terephthalate. When TPA is produced by catalytic air oxidation of p-xylene in the presence of acetic acid solvent, most of produced TPA exists in the form of crystalline suspended solids. A microfiltration process may be used to recover TPA, but the microfilters are subjected to fouling and therefore cleaning-in-place (CIP) regimes need to be developed. In this research, the effects of variations to CIP conditions were investigated on the flux recovery accomplished in a TiO2 -sintered stainless steel microfiltration membrane (0.1 m pore size) fouled with TPA. The extent of flux recovery was estimated as the ratio of the stabilized flux obtained during CIP to the water flux value achieved under corresponding operational conditions. Based upon batch solubility tests, sodium hydroxide (NaOH) was chosen as the major cleaning agent for the present experiment. The extent of flux recovery increased with increasing NaOH concentration over the range of 3–4% (w/v) NaOH, but decreased at NaOH concentrations above 4%. The flux recovery was favored at high cross-flow velocities, high temperatures and low transmembrane pressures. A high temperature run of cleaning did not produce any adverse effects up to 70 ◦ C. The addition of surfactants (SDS and Tween 80) to the caustic cleaning agent led to a significant reduction in cleaning efficiency. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cleaning-in-place (CIP); Microfiltration; Stainless steel membrane; Terephthalic acid (TPA)
1. Introduction Purified terephthalic acid (PTA), an essential raw material in the manufacture of polyester fiber and polyethylene terephthalate (PET), can be manufactured by catalytic air oxidation of p-xylene in the presence of acetic acid solvent. Most of the terephthalic acid (TPA) produced in this reaction exists in the form of crystalline suspended solids (SS) due to its low solubility. The reacted mixture is conven∗ Corresponding author. Tel.: +82-31-330-6689; fax: +82-31-336-6336. E-mail address: [email protected] (K. Lee).
tionally filtered with a mechanical separation device such as a drum or centrifugal disc filter. The TPA recovered as a solid then undergoes purification and drying, and the supernatant mother liquor is sent to a wastewater treatment facility. Since the efficiencies in solid–liquid separation of conventional mechanical separation devices are not satisfactory, a significant quantity of synthesized TPA is still contained in the generated mother liquor, which often imposes a serious burden on wastewater treatment [1]. Recently we are investigating a microfiltration process as an alternative method of TPA recovery in order to enhance the TPA recovery yield in the filtration step by reducing the TPA concentration in the
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discarded mother liquor. A stainless-steel microfiltration membrane was selected for this purpose. The aim of the present study is to optimize the cleaning-in-place (CIP) conditions for use after membrane fouling has been established. The cleaning agent and its concentration were selected by a series of solubility tests. The effects of variations in CIP conditions on flux recovery were then investigated after the membrane was fouled by SS which contained TPA. The parameters investigated were the cleaning agent concentration, transmembrane pressure (TMP), cross-flow velocity (CFV), and temperature. A further investigation was carried out on the effect of adding surfactant to the cleaning solution on the flux recovery.
2. Experimental 2.1. Membrane A SCEPTER® stainless steel tubular membrane (Graver Technologies Inc., DW, USA) with 0.1 m pore size and 6.5 mm i.d. was used. This membrane was made with a porous 316L stainless steel tube surface-coated with a sintered TiO2 layer [2]. Two modules of different filtration area were used: 0.07 and 0.0117 m2 . The bigger unit was used for the filtration of large quantities of commercial TPA product solution to see the flux decline tendency. The smaller unit was used for the tests to find optimum CIP conditions by intentionally inducing rapid fouling.
hydrocarbon chain. The CMC of Tween 80 is approximately 1 mM at 25 ◦ C. 2.3. Cleaning-in-place The membrane module with 0.07 m2 surface area was used for the filtration of commercial TPA product solution obtained from a local PTA manufacturing plant to examine the general tendency of filtration flux decline. The cross-flow filtration was run at 5 bar TMP, 4 m/s CFV, and 40 ◦ C. To investigate the optimization of CIP conditions, fast fouling of the membrane was intentionally induced using the 0.0117 m2 module and a model feed solution containing 3% (w/v) TPA in distilled water. The permeate stream was not recycled to the feed tank. After its flux had declined to 50% of the water flux for the clean membrane, the CIP conditions were varied to investigate their effects on the extent of flux recovery. This percentage extent was estimated by the ratio of the stabilized flux that was obtained during CIP to the water flux value under the corresponding operation conditions. Between each run of CIP tests, the initial water flux was retained by strong cleaning with 4% (w/v) NaOH solution at 70 ◦ C and a high CFV. The membrane material is chemically stable TiO2 -sintered 316L stainless steel and connections were welded, no noticeable change was observed in membrane integrity over the period of present study, despite the strong cleaning condition. This strong cleaning was necessary because under certain mild CIP conditions, the permeate flux during CIP was sometimes not recovered to the clean membrane level.
2.2. Cleaning agents Sodium hydroxide was used as the basic cleaning agent and was compared with a formulated cleaning agent, Ultrasil 10 (Henkel). Ultrasil 10 is a caustic based reagent containing several surfactants and is known to be effective in the cleaning of membranes fouled with organic compounds. The effect of two surfactant additives to the sodium hydroxide solution, sodium dodecylsulfate (SDS) and Tween 80, were therefore examined. SDS is an anionic surfactant with a critical micelle concentration (CMC) of approximately 8 mM at 25 ◦ C. Tween 80 is a nonionic surfactant which has a sorbitan ring and 20 polyoxyethylene (POE) units as a hydrophilic moiety and a monooleate
3. Results and discussion 3.1. Flux decline during filtration Fig. 1 shows a typical flux decline occurring during the filtration of TPA product solution of the composition given in Table 1. The TPA content in the feed solution was 0.55% (w/v), most of it in the form of white crystalline SS. Benzoic acid and p-toluic acid were the major side products formed during the TPA-producing reaction, and they themselves may be converted to TPA if returned to the reactor. The permeate flux declined within 1050–1100 l/(m2 h) (LMH) down to
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Fig. 1. Flux decline during the filtration of TPA-product solution. Membrane area: 0.07 m2 ; TMP: 5 bar; CFV: 4 m/s, and temperature: 40 ◦ C.
600 LMH over the 5 h of filtration due to membrane fouling caused by the accumulation of SS cake. This fouling cake was sampled and analyzed after the test and more than 90% of the dried cake mass was confirmed to be TPA. The insoluble fraction of feed TPA was almost completely removed, thus leaving only soluble fractions of TPA, p-toluic acid, and benzoic acid in the resulting permeate stream. Chemical oxygen demand (COD) was also reduced from 14,200 to 2800 ppm (Table 1). The pH value of feed, retentate, permeate was unchanged in the range of 3.4–3.5. 3.2. Selection of cleaning agent A batch solubility test can assist in the selection of a more effective cleaning agent because the solubilization of foulants from the membrane surface is one of the actions in which cleaning agents are involved. The variation in TPA solubility in solutions containing sodium hydroxide (NaOH), sodium carbonate (Na2 CO3 ), or EDTA and pure water was examined. The solubility of TPA increased linearly with increasing temperature from 20 to 80 ◦ C, but the extents of increase were small in solutions containing 2% (w/v) chemical cleaning agent: 33–37 g/l in NaOH, 16–18 g/l in Na2 CO3 , and 4–6 g/l in EDTA. TPA solubility in water was 1.5 g/l at 30 ◦ C and
steadily increased to 9 g/l at 80 ◦ C. The solutions of NaOH and Na2 CO3 displayed much enhanced and temperature-independent TPA solubility. Both NaOH and Na2 CO3 are alkaline cleaning agents, often used for the cleaning of organic foulants [3]. Since NaOH showed better TPA solubility than Na2 CO3 over the tested temperature range, it was used as the major cleaning agent in this study and the effect of NaOH concentration on TPA solubility was examined (Fig. 2). Of immediate note was the optimal NaOH concentration of about 7% (w/v) which produces maximum TPA solubility. At NaOH concentrations below this level, the TPA solubility increased steadily, but decreased thereafter, giving rise to a prominent peak in TPA solubility. The reason is not Table 1 Comparison of feed, retentate and permeate after microfiltration of TPA product solution
SS (%, w/v) TPA (%, w/v) p-Toluic acid (%, w/v) Benzoic acid (%, w/v) Acetic acid (%, v/v) COD (ppm) a
Feed
Retentate
Permeate
0.60 0.55a 0.1 0.08 0.1 14200
11.9 10.1a 1.9 0.09 0.1 –
≈0 0.03 0.005 0.08 0.1 2800
Sum of TPA amounts in soluble and solid fractions.
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Fig. 2. Change of TPA solubility with NaOH concentration variation. (䊉) 30 ◦ C; (䊊) 40 ◦ C; (䉲) 50 ◦ C; () 60 ◦ C; (䊏) 70 ◦ C; (䊐) 80 ◦ C.
clear for the decreased TPA solubility at NaOH concentration higher than 7%. It seems that the solvating power of NaOH solution for TPA is reduced at highly viscous state of such high NaOH concentration range.
3.3. Effect of NaOH concentration on cleaning Fig. 3 displays the time course of flux recovery during the CIP, depending on the NaOH concentration
Fig. 3. Influence of NaOH concentration on flux recovery in the cleaning of fouled membrane. Membrane area: 0.0117 m2 ; TMP: 1 bar; CFV: 0.5 m/s, and temperature: 25 ◦ C. NaOH concentration: (䊉) 1%; (䊊) 2%; (䉲) 3%; () 4%; (䊏) 5%; (䊐) 6%; (䉬) 7%.
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in the cleaning solution. The operating conditions of this set of CIP tests were 2 bar TMP, 2 m/s CFV, and 30 ◦ C. Over the range of 2–4% NaOH concentration, the recovered flux value quickly increased to a maximum within 15 min before decaying to a stabilized flux value. The observation of a maximum flux occurring at such an early stage of the cleaning process has been reported in the caustic cleaning of a stainless steel MF membrane which was fouled with whey protein concentrate [4]. Meanwhile, the flux increase was steady at NaOH concentrations higher than 5% with no decay. The flux was recovered to a higher value as NaOH concentration increased over the range of 1–3% NaOH, with the maximum flux recovery level being achieved at 3–4% NaOH. The use of 3% NaOH resulted in the highest flux recovery, this being 92% of the initial water flux. This NaOH concentration is a lower one than the optimal 7% concentration, which had been estimated from the batch solubility test (Fig. 2). It implies that there exist other criteria for selecting cleaning agent in actual membrane system. The rheological aspects and interactions of cleaning agent with filtered cake or membrane material can influence the cleaning efficiency and optimum concentration of cleaning agent. In Fig. 2, the extent of
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flux recovery was reduced at NaOH concentrations above 5%. The existence of an optimal cleaning agent concentration has been observed in other membrane systems [4–6]. In the present study, the retentate became highly viscous at high NaOH concentrations, an unfavorable outcome for effective circulation and for TPA cake cleaning from the membrane surface [7]. 3.4. Influence of TMP and CFV on cleaning Fig. 4 shows the influence of variations in TMP and CVF on the flux recovery during the cleaning process. Operational temperature was fixed at 30 ◦ C and 2% NaOH was used as the cleaning solution. The flux decreased with increasing TMP over the pressure range of 1–4 bar under constant CFV. Meanwhile, the flux recovery was enhanced with increasing CFV over the velocity range of 0.5–3 m/s under constant TMP. It should be noted that the achieved flux recovery was greater than 100% (i.e. higher than the clean membrane water flux) under certain conditions of low TMP and high CFV. Since such phenomenon was also observed in clean membrane filtering NaOH solution, especially at high CFV values, it is speculated that NaOH solution could be permeable more easily than
Fig. 4. Influence of TMP and CFV on flux recovery. Temperature: 30 ◦ C; cleaning solution: 2% NaOH; CFV: (䊊) 0.5 m/s; () 1.0 m/s; (䊐) 2.0 m/s; (䉫) 3.0 m/s.
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water in the case of the stainless steel membrane which was used in this study. A high TMP is known to lead to more compact cake formation or in-pore fouling, which accelerates the flux decline and reduces the cleaning efficiency [8–10]. This tendency was aggravated when CFV was lowered, as indicated by the flux recovery of less than 70% at 0.5 m/s CFV and 3 bar TMP. The increase in recovered flux at higher CFV values is due to the higher shear force which carries the particles away from the fouled cake as well as reduces the concentration polarization [11,12]. At low TMP levels such as 1 bar, the change of cleaning efficiency with variations in CFV value was quite small over the CFV range of 0.5–2 m/s. 3.5. Influence of temperature Increasing temperature produced a pronounced enhancement of flux recovery during the cleaning process. Fig. 5 shows the flux recovery at different temperatures under conditions of 3 bar TMP and 2 m/s CFV. The extent of permeate flux recovery was found to increase with temperature, for example the flux at 60 ◦ C was more than double that at 25 ◦ C. No adverse effects associated with a high temperature cleaning
process were noted over the range of temperature and the membrane system used in the present experiments. However, an optimal temperature does exist for maintaining a high flux in some other membrane filtration systems [4,10]. The reason for the flux enhancement observed at high temperature in this study was not clear. A high temperature improvement of the solubilization power of the cleaning agent was not considered to be the cause of the increased flux because, as noted in Fig. 2, the TPA solubility in NaOH solution was not severely temperature-dependent. 3.6. Effect of surfactants The effect of surfactant additions to the cleaning solution on flux recovery in the TPA-fouled membrane was compared with that of simple caustic cleaner. The cleaning conditions were 3 bar TMP and 2 m/s CFV at 30 ◦ C. Ultrasil 10 is a formulated cleaning agent, which is known to be a caustic-based reagent with the addition of surfactants, and is often used in the cleaning of organic-fouled membranes [13]. However, Fig. 6 clearly indicates that the cleaning efficiency of Ultrasil 10 was significantly lower than that of NaOH. The recovered flux with 2% (w/v) Ultrasil solution was less than 40% of that achieved with 2% NaOH
Fig. 5. Influence of temperature on flux recovery. TMP: 3 bar; CFV: 2 m/s, and 2% NaOH solution. (䊉) 25 ◦ C; (䉲) 50 ◦ C; (䊏) 60 ◦ C; (䉬) 70 ◦ C.
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Fig. 6. Influence of surfactant in cleaning solution on flux recovery. TMP: 3 bar; CFV: 2 m/s, and temperature: 30 ◦ C. (䊊) 2% NaOH; () 2% Ultrasil 10; (䊐) 2% NaOH + 1% SDS; (䉫) 2% NaOH + 1% Tween 80; () water.
solution. The flux level obtained with Ultrasil 10 was even less than that with pure water only. The addition of either Tween 80 or SDS also merely resulted in a significant reduction in recovered flux level. Although surfactants are valuable components of cleaning solutions in many cases, a loss of filtration flux due to adsorption, precipitation, or foaming has been reported [11]. The reason for surfactants having such poor cleaning effect is not readily apparent in the present specific case of TPA-fouled sintered stainless steel membrane. It seems that such surfactants do not contribute to TPA solubilization from the fouled membrane surface and that the surfactant molecules themselves act as an additional foulant and thus aggravate surface or in-pore fouling. In the case of the anionic surfactant SDS, another possible cause could be the formation of insoluble precipitates if SDS encounters some metal ions which leaked from the catalyst used in the TPA-producing reaction. Acknowledgements This study was financially supported by G-7 projects of the Ministry of Environment, Korea. Y.B.
Kim is also thankful to the BK21 scholarship of the Ministry of Education, Korea.
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