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Energy-saving potential of cross-flow ultrafiltration with inserted static mixer: Application to an oil-in-water emulsion
Separation and Purification Technology 57 (2007) 134–139
Energy-saving potential of cross-flow ultrafiltration with inserted static mixer: Application to an oil-in-water emulsion夽 Darko M. Krsti´c a,∗ , Wilhelm H¨oflinger b,1 , Andr´as K. Koris c,2 , Gyula N. Vatai c,2 a
Faculty of Technology, University of Novi Sad, Bulevar Cara Lazara 1, Novi Sad 21000, Serbia and Montenegro b Mechanical Process Engineering and Air Pollution Control Techniques, Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, A-1060 Vienna, Austria c Department of Food Engineering, Corvinus University of Budapest, M´ enesi St. 44, H-1118 Budapest, Hungary Received 11 January 2007; received in revised form 26 March 2007; accepted 26 March 2007
Abstract Ultrafiltration has been accepted as a highly attractive separation process to treat oily wastewaters. The use of ultrafiltration is particularly interesting for the treatment of stable oil-in-water emulsions, such as cutting oil emulsions, since the value of the recovered solid oil waste is high. However, the efficiency of ultrafiltration is limited by membrane fouling and concentration polarisation, resulting in a permeate flux decrease, and therefore, increasing energy consumption of the process. The aim of this study was to investigate the potential for energy saving in cross-flow ultrafiltration of a stable oil-in-water emulsion by using a static mixer as a turbulence promoter. Experimental investigations were performed on a zirconia membrane with a nominal pore size of 20 nm using the KenicsTM static mixer as a turbulence promoter. The reduction of the specific energy consumption of over 40% accompanied by the flux increase of up to 600% has been observed during ultrafiltration of fresh cutting oil. Furthermore, the results showed the way how operation parameters should be selected in order to achieve optimal process performance of the ultrafiltration with inserted static mixer. © 2007 Elsevier B.V. All rights reserved. Keywords: Cross-flow ultrafiltration; Energy saving; Turbulence promotion; Static mixer; Oil-in-water emulsion
1. Introduction Cutting oil fluids are extensively used in metal-working industry for cooling, lubrication, surface cleaning and corrosion prevention. Depending on specific application, the cutting oil fluid can consist of up to 97% water, the rest being a complex mixture of free and emulsified oils, surfactants, antifoaming agents, bactericides, rust inhibitors and other additives [1]. The temperature of the cutting oil fluid is usually between 30 and
夽 Presented at the 5th European Meeting on Chemical Industry and Environment (EMhIE), Vienna, Austria, 3–5 May 2006. ∗ Corresponding author. Present address: Novel Technologies (Malta) Ltd., 2 Zinja, Triq il-Kappara, Marsascala ZBR12, Malta. Tel.: +356 21 63 3737; fax: +356 21 63 3737. E-mail addresses: [email protected] (D.M. Krsti´c), [email protected] (W. H¨oflinger), [email protected] (G.N. Vatai). 1 Tel.: +43 1 58801 15910; fax: +43 1 58801 16699. 2 Tel.: +36 1 37 26 234.
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.03.023
90 ◦ C [1] because of the heat it removes from the metal surfaces. Therefore, cutting oil fluids need to be replaced periodically because of the effects of thermal degradation, contamination by particles and biological contamination. Used cutting oil fluids produce a large amount of oil-in-water emulsion wastes which are considered hazardous industrial wastes and require further treatment before its disposal. Cutting oil wastes, together with oily wastewaters, represent two of the main pollutants discharged to the water environment. They cannot be discharged to the sewer because of a high oil content and high residual organic pollution. Hence, they have to be treated in order to obtain an oily phase as concentrated in oil as possible in order to reuse the oil and an aqueous phase in accordance with the regulation levels for industrial wastewater. The conventional methods for treatment of cutting oil emulsions can be classified as chemical, mechanical and thermal [2]. The chemical methods involve chemical pretreatment of emulsified oil to destabilize the emulsion followed by gravity separation. The mechanical and thermal methods are primary based on the phenomenon of gravitational and thermal emulsion
D.M. Krsti´c et al. / Separation and Purification Technology 57 (2007) 134–139
breaking. However, the conventional methods for treatment of oil emulsions have several disadvantages, such as a low efficiency, operational difficulties and high operation costs. Used cutting oil emulsions may contain inorganic, organic and biological contaminants. Generally, cutting oil wastes have a high COD level (more than 300 g L−1 ), with the total oil content of 1–10%, and conventional treatment is usually not effective enough to meet new regulations levels for industrial wastewater (2 g O2 L−1 and 15 mg oil L−1 ) [3,4]. To address this problem membrane processes, such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis, have been increasingly investigated for treating oil emulsions. For more than 25 years, ultrafiltration has been considered as an attractive method for the separation of stable oil-in-water emulsions. The use of ultrafiltration for treatment of cutting oil emulsions has been reported in the literature [5–10]. This process has proved to be more effective then conventional methods, because it produces a water phase of higher quality and oil phase which can be recycled. In recent years, ultrafiltration employing ceramic membranes is being increasingly used to treat oil emulsions. Ceramic membranes have gained popularity due to their better mechanical, thermal and chemical stability over polymeric membranes, despite that the available pore size range is still limited. However, the efficiency of ceramic ultrafiltration membranes is reduced by membrane fouling. Oil-in-water emulsions induce three kinds of fouling mechanisms: oil drop deposit, concentration polarisation and adsorption of dissolved organic compounds [3]. Membrane fouling results in substantial decline in initial membrane hydraulic permeability leading to a permeate flux decline over time, and therefore increased energy consumption is necessary in order to maintain a required membrane productivity. Fouling and permeate flux behaviour of ceramic membranes during ultrafiltration of oil emulsions have been the focus of several studies [9–12]. Since a sufficiently high value of the permeate flux can assure lower investment costs and the operation at lower energy consumption, the research work has been mostly focused on membrane fouling reduction and flux enhancement. Koris et al. [11] have reported flux values of about 90 L m−2 h−1 during ultrafiltration of 5% (w/w) stable oil-in-water emulsion with a 20 nm zirconia membrane. Benito et al. [12] have reported flux values as high as 250 L m−2 h−1 when ultrafiltering oilin-water emulsion with a 50 nm zirconia membrane. However, filtration and demulsification followed by centrifugation were used as pretreatment steps and more than 95% of the oil was removed from the feed before the ultrafiltration step. Viadero Jr. et al. [9] have shown that high-shear rotary ultrafiltration allows concentration of oil beyond the typical operating limitations of conventional ultrafiltration modules. Limiting flux values of up to 550 L m−2 h−1 have been reported in the experiments with permeate recirculation to the feed tank. Among the different ways to improve the permeate flux in ultrafiltration of oil-in-water emulsions, another interesting technique is the use of static turbulence promoters. Although they reduce hold-up in the feed channel, static turbulence promoters increase wall shear rates and may produce secondary flows and instabilities, and the significant flux enhancements compared to
135
a conventional cross-flow process have been reported [10,11]. Krsti´c et al. [10] have shown that the use of the KenicsTM static mixer as a turbulence promoter during ultrafiltration of a stable oil-in-water emulsion with a 20 nm zirconia membrane can provide flux values of almost 300 L m−2 h−1 ; about five times higher compared to the fluxes obtained during operation without using the static mixer. However, the increase in pressure drop along the membrane module due to presence of the static mixer resulted in the increase in energy consumption. In the experiment involving the feed concentration with a 5 nm titania membrane, an increase in the specific energy consumption was around 90% at a volumetric concentration factor of 6, despite of flux improvement of more than 300% obtained at this concentration factor by using the static mixer. The results obtained during ultrafiltration with an inserted static mixer suggest that the principles of process intensification should be applied in order to improve membrane productivity with substantial reduction in energy consumption, and therefore operating costs. The aim of this study was to investigate the potential for energy saving in cross-flow ultrafiltration of a stable oil-in-water emulsion by using a static mixer as a turbulence promoter. 2. Materials and methods The experiments were carried out in cross-flow mode using a conventional ultrafiltration set-up with a laboratory tubular single-channel membrane module (Fig. 1). The membrane used was a ZrO2 membrane (Exekia, Pall, USA) with a nominal pore size of 20 nm, length of 250 mm and diameter of 6.8 mm. The KenicsTM static mixer (FMX8124-AC, Omega, USA), consisting of 38 mixing elements with a diameter of 6.35 mm, was placed in the membrane (Fig. 2). Detailed characteristics of the static mixer can be found elsewhere [13]. A stable oil/water emulsion was prepared from a non-used water-soluble cutting oil (Unisol, Mol, Hungary) in batches of 8 L. The oil concentration in the emulsion was 5% (w/w). All experiments were carried out at 50 ◦ C. The density and dynamic viscosity of used emulsion were ρ = 992.3 kg m−3 and μ = 1.278 × 10−3 Pa s, respectively. The feed was pumped from a tank to the membrane module and then recirculated. The volume flow rate (Q) and transmembrane pressure (TMP) were controlled by means of regulation valves. The permeate flux
Fig. 1. Schematic diagram of the experimental set-up: (1) feed tank; (2) circulation pump; (3) pressure gauge; (4) membrane module; (5) liquid flow meter.
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the velocities during operation without the use of the static mixer. Therefore, in order to compare these two modes of operation at similar cross-flow velocity, the experiment without using the static mixer had to be run at higher flow rate than the one during SM mode of operation. Volumetric concentration factor (VCF) during concentration of the emulsion was determined as the ratio of feed volume at the beginning of operation (Vfeed,i ) to retentate volume in time t (Vret,t ): VCF = Fig. 2. The KenicsTM static mixer.
(Jp ) was calculated from the time needed to collect 10 mL of permeate. The permeate side was opened to the atmosphere, so TMP could be taken as the average of the gauge readings. The liquid flow rate was varied from 50 to 150 L h−1 . Comparison of the process performances during operation without the use of the static mixer (NSM mode of operation) and when the static mixer was used (SM mode of operation) was carried out in the conditions of total recirculation of the feed and with volumetric concentration of the emulsion. The aim of experiments with total recirculation was to determine the optimal operation conditions (Q and TMP) for both modes of operation. On the other hand, the aim of experiments with volumetric concentration was to demonstrate practical benefits of using a static mixer in ultrafiltration of an oil-in-water emulsion. As ultrafiltration systems are usually analysed via liquid velocities far more often than via liquid flow rates, the comparison of NSM and SM modes during volumetric concentration of the feed was carried out at the same cross-flow velocity. The liquid velocity is calculated as a superficial velocity: U=
Q S
(1)
where S represents the effective cross-section area of the membrane tube. However, when the static mixer was used, the effective crosssection area was smaller for the value of a sectional area of the static mixer than that of the empty tube. Different effective crosssectional areas resulted in about 20% higher velocities at the same flow rates in the case of using the static mixer compared to
Vfeed,i Vret,t
(2)
The membrane was cleaned according to the recommendation of the manufacturer prior to each experiment and the pure water flux of the cleaned membrane was measured. The cleaning procedure was repeated until the original water flux was restored. Beside permeate flux, one of the most important parameter from an economical point of view is the specific energy consumption (E) defined as the power dissipated per unit volume of permeate. The hydraulic dissipated power is directly related to the pressure drop along the membrane module (P) and the specific energy consumption can be calculated as [13]: E=
QP Jp A
(3)
where Jp is the permeate flux and A is the membrane surface area. Energy-saving potential of the configuration with the static mixer was checked through the reduction of the specific energy consumption, determined as a relative reduction of E by using the static mixer compared to the value of E obtained without the static mixer: ER =
ENSM − ESM × 100 ENSM
(4)
3. Results and discussion 3.1. Optimization of the operating conditions The influence of transmembrane pressure and feed flow rate on the process performance with the inserted static mixer (SM mode of operation) was investigated during recirculation of the
Fig. 3. The variations of the permeate flux with TMP during NSM and SM modes of operations. Feed flow rate: (A) 100 L h−1 ; (B) 150 L h−1 .
D.M. Krsti´c et al. / Separation and Purification Technology 57 (2007) 134–139
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Fig. 4. Specific energy consumption vs. TMP during NSM and SM modes of operations. Feed flow rate: (A) 100 L h−1 ; (B) 150 L h−1 .
permeate to the feed tank. Furthermore, in order to compare results, for a given flow rate, the experiments were carried out without using the static mixer (NSM mode of operation). The pseudo-steady state permeate flux values obtained at different TMPs for the feed flow rates of 100 and 150 L h−1 are shown in Fig. 3. Fig. 3 shows that the use of the static mixer provided a significant flux enhancement compared to the operation without the static mixer. Positive effect of the static mixer on permeate flux during ultrafiltration of an oil-in-water emulsion has already been reported in the literature [10,11]. The flow field generated by the static mixer induces hydraulic turbulence and increases the wall shear stress in the vicinity of the membrane which minimizes the effect of concentration polarisation as well as decreasing the potential for membrane fouling. However, the main limitation of the configuration with the static mixer is the increase of pressure drop along the membrane module (P). According to Eq. (3), an increase of pressure drop along the membrane at a given flow rate through the module leads to an increase in dissipated power and energy consumption. Therefore, the specific energy consumption (E) during the operation with the static mixer should be checked as it is directly proportional to the operating costs. Fig. 4 shows the variation of E with TMP at flow rates of 100 and 150 L h−1 during both NSM and SM modes of operation. Despite the significant flux enhancement, the energy consumption was even higher at a flow rate of 150 L h−1 during the SM mode of operation due to a high-pressure drop along the membrane compared to the one without using the static mixer. However, the results from Fig. 4A show that certain energy saving can be obtained at a flow rate of 100 L h−1 by using the static mixer. The values of the reduction of the specific energy consumption (ER ) and flux enhancement achieved during the
operation in NSM and SM modes at the same feed flow rate are shown in Table 1. From Table 1 it is clear that the static mixer is more efficient at higher TMPs; significant flux enhancement, and therefore lower specific energy consumption were obtained when the static mixer was used at higher TMPs. The reduction of the specific energy consumption of over 40% accompanied by the flux increase of up to 600% has been observed during the operation at a TMP of 300 kPa and flow rate of 100 L h−1 . The results presented in Figs. 3 and 4, as well as in Table 1, suggest that the pressure drop along the membrane is too high during the operation in SM mode at a flow rate of 150 L h−1 , and that the use of the static mixer is unlikely favourable from the energetic point of view at this flow rate. On the other hand, the flow field generated by the static mixer enables a high degree of turbulence near the membrane surface at low cross-flow velocities. In order to examine which value of cross-flow velocity can provide considerable energy saving during the operation with the inserted static mixer, the main process parameters were determined at several feed flow rates. Fig. 5 shows the variations of the permeate flux and specific energy consumption with TMP at the feed flow rates in the range from 50 to 150 L h−1 , which correspond to the cross-flow velocities in the range from 0.46 to 1.4 m s−1 . From Fig. 5 it can be noticed that the limiting flux was achieved at a TMP as low as 200 kPa during the operation at a flow rate of 50 L h−1 . Although low energy consumption was observed at this flow rate, the vortices generated by the static mixer cannot efficiently scour the membrane surface, especially at higher transmembrane pressures. On the other hand, a high pressure drop along the membrane at a flow rate of 150 L h−1 lead to increased energy consumption, despite high permeate fluxes. The permeate flux of over 300 L m−2 h−1 , with the spe-
Table 1 Comparison of NSM and SM modes of operation at different TMP and flow rates TMP (kPa)
70 150 220 300
ER (%)
Flux enhancement (%)
Q = 100 L h−1
Q = 150 L h−1
Q = 100 L h−1
Q = 150 L h−1
−37 10 35 42
−139 −86 −35 0
206 334 521 607
109 193 285 430
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Fig. 5. The variations of permeate flux (A) and specific energy consumption (B) with TMP during the operation with the static mixer at different feed flow rates.
cific energy consumption lower than 2 kWh m−3 , obtained at a flow rate of 100 L h−1 are very promising results. The specific energy consumption of 3.2 kWh m−3 has been reported during conventional ultrafiltration of oil-in-water emulsion [3]. Therefore, it seems that a flow rate of 100 L h−1 , which corresponds to a cross-flow velocity of 0.92 m s−1 , could be the optimal feed flow rate for improved process performance with inserted static mixer. The improved performance of cross-flow microfiltration of skimmed milk by using the same static mixer inserted in the membrane with the same geometric characteristics has been reported at cross-flow velocities around 1 m s−1 [10]. These results suggest that the geometric characteristics of used static mixer and membrane determine the efficiency of the configuration with static mixer more than membrane pore size, in the cases when surface fouling is predominant.
velocity (1.2 m s−1 ). In addition, TMP of 160 kPa proved to be too low to obtain any significant values of permeate flux during NSM mode of operation, so TMP of 200 kPa was used throughout this experiment. It should be emphasized that the permeate fluxes not higher than 75 L m−2 h−1 were obtained during the operation without the static mixer at cross-flow velocities as high as 2 m s−1 [10]. On the other hand, despite operation at lower TMP the static mixer provided the initial flux of over 300 L m−2 h−1 with the flux of around 130 L m−2 h−1 at a VCF of 5 during the operation at a cross-flow velocity of only 0.92 m s−1 , resulting in lower specific energy consumption (Fig. 6B); the flux enhancement of 314% with the reduction
3.2. Volumetric concentration of oil/water emulsion The experiments with the recirculation of the permeate provided the guidelines of how ultrafiltration involving the use of static mixer should be carried out in order to improve the permeate flux and concurrently reduce energy consumption. Furthermore, the previous results obtained during the use of the same configuration [10] suggest that the static mixer is even more effective at higher oil concentrations. Therefore, volumetric concentration of the feed using the ultrafiltration configuration with the static mixer was carried out at a flow rate of 100 L h−1 . For comparison, the experiment without using the static mixer was carried out at a flow rate of 150 L h−1 to compensate the “loss” of effective cross-section area and increased turbulence due to presence of the static mixer. An increase of pressure drop along a membrane and consequently increase of TMP is usually indicated as the main disadvantage of a configuration with an inserted static mixer. To demonstrate opposite, volumetric concentration of the emulsion in SM mode of operation was carried out at TMP as low as 160 kPa. The variations of the permeate flux and specific energy consumption with VCF are shown in Fig. 6. The permeate flux decreased due to fouling and increasing of the viscosity as the feed is concentrated. Low permeate flux during NSM mode of operation can be explained by insufficient turbulence near the membrane surface at the given cross-flow
Fig. 6. Permeate flux (A) and specific energy consumption (B) as a function of VCF for NSM and SM modes of operation. NSM: Q = 150 L h−1 ; TMP = 200 kPa; SM: Q = 100 L h−1 ; TMP = 160 kPa.
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of the specific energy consumption of 44% was achieved at a VCF of 2. The experiments involving volumetric concentration of the feed indicated that the appropriate use of static mixer could result in significant energy saving and improved ultrafiltration of oil/water emulsion. 4. Conclusions The present study clearly shows the energy-saving potential of cross-flow ultrafiltration with the KenicsTM static mixer inserted in the membrane tube. The results obtained with a stable oil-in-water emulsion as a feed demonstrated that the use of the static mixer at operating cross-flow velocities as low as 1 m s−1 is crucial in order to achieve the improved process performance. Considerable energy saving and higher fluxes were achieved compared to those obtained with a conventional crossflow ultrafiltration without using the static mixer. Acknowledgements This research was supported by the Ministry for Science and Environment Protection, Republic of Serbia (project no. 142045), CEEPUS Foundation (network no. H-0158-04/05) and the Federal Ministry for Education, Science and Culture, Republic of Austria (Ernst Mach Grant). References [1] S.H. Lin, W.J. Lan, Waste oil/water emulsion treatment by membrane processes, J. Hazard. Mater. 59 (1998) 189–199.
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[2] M. Cheryan, N. Rajagopalan, Membrane processing of oily streams. Wastewater treatment and waste reduction, J. Membr. Sci. 151 (1998) 13–28. [3] G. Ducom, H. Matamoros, C. Cabassud, Air sparging for flux enhancement in nanofiltration membranes: application to O/W stabilised and non-stabilised emulsions, J. Membr. Sci. 204 (2002) 221–236. [4] A.Y. Tremblay, H. Peng, D.E. Veinot, Design and performance of an inorganic MF/polymeric UF hybrid system for the treatment of a difficult waste stream containing both colloidal and micron size particles, Desalination 149 (2002) 151–152. [5] M. Belkacem, D. Hadjiev, Y. Aurelle, A model for calculating the steady state flux of organic ultrafiltration membranes for the case of cutting oil emulsions, Chem. Eng. J. 56 (1995) 27–32. [6] X.G. Hu, E. Bekassy-Molnar, Gy. Vatai, Study of ultrafiltration behaviour of emulsified metalworking fluids, Desalination 149 (2002) 191–197. [7] D.A. Masciola, R.C. Viadero Jr., B.E. Reed, Tubular ultrafiltration flux prediction for oil-in-water emulsions: analysis of series resistances, J. Membr. Sci. 184 (2001) 197–208. [8] X. Hu, E. B´ek´assy-Moln´ar, A. Koris, Study on modelling transmembrane pressure and gel resistance in ultrafiltration of oily emulsion, Desalination 163 (2004) 355–360. [9] R.C. Viadero Jr., D.A. Masciola, B.E. Reed, R.L. Vaughan Jr., Two-phase limiting flux in high-shear rotary ultrafiltration of oil-in-water emulsions, J. Membr. Sci. 175 (2000) 85–96. [10] D.M. Krsti´c, A.K. Koris, M.N. Teki´c, Do static turbulence promoters have potential in cross-flow membrane filtration applications, Desalination 191 (2006) 371–375. [11] A. Koris, D. Krstic, X. Hu, Gy. Vatai, Ultrafiltration of oil in water emulsion: flux enhancement with static mixer, in: Proceedings of the Membrane Science and Technology Conference of Visegrad Group—PERMEA 2005, Polanica Zdr´oj, Poland, September 18–22, 2005. [12] J.M. Benito, G. R´ıos, E. Ortea, E. Fern´andez, A. Cambiella, C. Pazos, J. Coca, Design and construction of a modular pilot plant for the treatment of oil-containing wastewaters, Desalination 147 (2002) 5–10. [13] D.M. Krsti´c, M.N. Teki´c, M.D. Cari´c, S.D. Milanovi´c, Static turbulence promoter in cross-flow microfiltration of skim milk, Desalination 163 (2004) 297–309.
Energy-saving potential of cross-flow ultrafiltration with inserted static mixer: Application to an oil-in-water emulsion夽 Darko M. Krsti´c a,∗ , Wilhelm H¨oflinger b,1 , Andr´as K. Koris c,2 , Gyula N. Vatai c,2 a
Faculty of Technology, University of Novi Sad, Bulevar Cara Lazara 1, Novi Sad 21000, Serbia and Montenegro b Mechanical Process Engineering and Air Pollution Control Techniques, Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, A-1060 Vienna, Austria c Department of Food Engineering, Corvinus University of Budapest, M´ enesi St. 44, H-1118 Budapest, Hungary Received 11 January 2007; received in revised form 26 March 2007; accepted 26 March 2007
Abstract Ultrafiltration has been accepted as a highly attractive separation process to treat oily wastewaters. The use of ultrafiltration is particularly interesting for the treatment of stable oil-in-water emulsions, such as cutting oil emulsions, since the value of the recovered solid oil waste is high. However, the efficiency of ultrafiltration is limited by membrane fouling and concentration polarisation, resulting in a permeate flux decrease, and therefore, increasing energy consumption of the process. The aim of this study was to investigate the potential for energy saving in cross-flow ultrafiltration of a stable oil-in-water emulsion by using a static mixer as a turbulence promoter. Experimental investigations were performed on a zirconia membrane with a nominal pore size of 20 nm using the KenicsTM static mixer as a turbulence promoter. The reduction of the specific energy consumption of over 40% accompanied by the flux increase of up to 600% has been observed during ultrafiltration of fresh cutting oil. Furthermore, the results showed the way how operation parameters should be selected in order to achieve optimal process performance of the ultrafiltration with inserted static mixer. © 2007 Elsevier B.V. All rights reserved. Keywords: Cross-flow ultrafiltration; Energy saving; Turbulence promotion; Static mixer; Oil-in-water emulsion
1. Introduction Cutting oil fluids are extensively used in metal-working industry for cooling, lubrication, surface cleaning and corrosion prevention. Depending on specific application, the cutting oil fluid can consist of up to 97% water, the rest being a complex mixture of free and emulsified oils, surfactants, antifoaming agents, bactericides, rust inhibitors and other additives [1]. The temperature of the cutting oil fluid is usually between 30 and
夽 Presented at the 5th European Meeting on Chemical Industry and Environment (EMhIE), Vienna, Austria, 3–5 May 2006. ∗ Corresponding author. Present address: Novel Technologies (Malta) Ltd., 2 Zinja, Triq il-Kappara, Marsascala ZBR12, Malta. Tel.: +356 21 63 3737; fax: +356 21 63 3737. E-mail addresses: [email protected] (D.M. Krsti´c), [email protected] (W. H¨oflinger), [email protected] (G.N. Vatai). 1 Tel.: +43 1 58801 15910; fax: +43 1 58801 16699. 2 Tel.: +36 1 37 26 234.
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.03.023
90 ◦ C [1] because of the heat it removes from the metal surfaces. Therefore, cutting oil fluids need to be replaced periodically because of the effects of thermal degradation, contamination by particles and biological contamination. Used cutting oil fluids produce a large amount of oil-in-water emulsion wastes which are considered hazardous industrial wastes and require further treatment before its disposal. Cutting oil wastes, together with oily wastewaters, represent two of the main pollutants discharged to the water environment. They cannot be discharged to the sewer because of a high oil content and high residual organic pollution. Hence, they have to be treated in order to obtain an oily phase as concentrated in oil as possible in order to reuse the oil and an aqueous phase in accordance with the regulation levels for industrial wastewater. The conventional methods for treatment of cutting oil emulsions can be classified as chemical, mechanical and thermal [2]. The chemical methods involve chemical pretreatment of emulsified oil to destabilize the emulsion followed by gravity separation. The mechanical and thermal methods are primary based on the phenomenon of gravitational and thermal emulsion
D.M. Krsti´c et al. / Separation and Purification Technology 57 (2007) 134–139
breaking. However, the conventional methods for treatment of oil emulsions have several disadvantages, such as a low efficiency, operational difficulties and high operation costs. Used cutting oil emulsions may contain inorganic, organic and biological contaminants. Generally, cutting oil wastes have a high COD level (more than 300 g L−1 ), with the total oil content of 1–10%, and conventional treatment is usually not effective enough to meet new regulations levels for industrial wastewater (2 g O2 L−1 and 15 mg oil L−1 ) [3,4]. To address this problem membrane processes, such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis, have been increasingly investigated for treating oil emulsions. For more than 25 years, ultrafiltration has been considered as an attractive method for the separation of stable oil-in-water emulsions. The use of ultrafiltration for treatment of cutting oil emulsions has been reported in the literature [5–10]. This process has proved to be more effective then conventional methods, because it produces a water phase of higher quality and oil phase which can be recycled. In recent years, ultrafiltration employing ceramic membranes is being increasingly used to treat oil emulsions. Ceramic membranes have gained popularity due to their better mechanical, thermal and chemical stability over polymeric membranes, despite that the available pore size range is still limited. However, the efficiency of ceramic ultrafiltration membranes is reduced by membrane fouling. Oil-in-water emulsions induce three kinds of fouling mechanisms: oil drop deposit, concentration polarisation and adsorption of dissolved organic compounds [3]. Membrane fouling results in substantial decline in initial membrane hydraulic permeability leading to a permeate flux decline over time, and therefore increased energy consumption is necessary in order to maintain a required membrane productivity. Fouling and permeate flux behaviour of ceramic membranes during ultrafiltration of oil emulsions have been the focus of several studies [9–12]. Since a sufficiently high value of the permeate flux can assure lower investment costs and the operation at lower energy consumption, the research work has been mostly focused on membrane fouling reduction and flux enhancement. Koris et al. [11] have reported flux values of about 90 L m−2 h−1 during ultrafiltration of 5% (w/w) stable oil-in-water emulsion with a 20 nm zirconia membrane. Benito et al. [12] have reported flux values as high as 250 L m−2 h−1 when ultrafiltering oilin-water emulsion with a 50 nm zirconia membrane. However, filtration and demulsification followed by centrifugation were used as pretreatment steps and more than 95% of the oil was removed from the feed before the ultrafiltration step. Viadero Jr. et al. [9] have shown that high-shear rotary ultrafiltration allows concentration of oil beyond the typical operating limitations of conventional ultrafiltration modules. Limiting flux values of up to 550 L m−2 h−1 have been reported in the experiments with permeate recirculation to the feed tank. Among the different ways to improve the permeate flux in ultrafiltration of oil-in-water emulsions, another interesting technique is the use of static turbulence promoters. Although they reduce hold-up in the feed channel, static turbulence promoters increase wall shear rates and may produce secondary flows and instabilities, and the significant flux enhancements compared to
135
a conventional cross-flow process have been reported [10,11]. Krsti´c et al. [10] have shown that the use of the KenicsTM static mixer as a turbulence promoter during ultrafiltration of a stable oil-in-water emulsion with a 20 nm zirconia membrane can provide flux values of almost 300 L m−2 h−1 ; about five times higher compared to the fluxes obtained during operation without using the static mixer. However, the increase in pressure drop along the membrane module due to presence of the static mixer resulted in the increase in energy consumption. In the experiment involving the feed concentration with a 5 nm titania membrane, an increase in the specific energy consumption was around 90% at a volumetric concentration factor of 6, despite of flux improvement of more than 300% obtained at this concentration factor by using the static mixer. The results obtained during ultrafiltration with an inserted static mixer suggest that the principles of process intensification should be applied in order to improve membrane productivity with substantial reduction in energy consumption, and therefore operating costs. The aim of this study was to investigate the potential for energy saving in cross-flow ultrafiltration of a stable oil-in-water emulsion by using a static mixer as a turbulence promoter. 2. Materials and methods The experiments were carried out in cross-flow mode using a conventional ultrafiltration set-up with a laboratory tubular single-channel membrane module (Fig. 1). The membrane used was a ZrO2 membrane (Exekia, Pall, USA) with a nominal pore size of 20 nm, length of 250 mm and diameter of 6.8 mm. The KenicsTM static mixer (FMX8124-AC, Omega, USA), consisting of 38 mixing elements with a diameter of 6.35 mm, was placed in the membrane (Fig. 2). Detailed characteristics of the static mixer can be found elsewhere [13]. A stable oil/water emulsion was prepared from a non-used water-soluble cutting oil (Unisol, Mol, Hungary) in batches of 8 L. The oil concentration in the emulsion was 5% (w/w). All experiments were carried out at 50 ◦ C. The density and dynamic viscosity of used emulsion were ρ = 992.3 kg m−3 and μ = 1.278 × 10−3 Pa s, respectively. The feed was pumped from a tank to the membrane module and then recirculated. The volume flow rate (Q) and transmembrane pressure (TMP) were controlled by means of regulation valves. The permeate flux
Fig. 1. Schematic diagram of the experimental set-up: (1) feed tank; (2) circulation pump; (3) pressure gauge; (4) membrane module; (5) liquid flow meter.
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the velocities during operation without the use of the static mixer. Therefore, in order to compare these two modes of operation at similar cross-flow velocity, the experiment without using the static mixer had to be run at higher flow rate than the one during SM mode of operation. Volumetric concentration factor (VCF) during concentration of the emulsion was determined as the ratio of feed volume at the beginning of operation (Vfeed,i ) to retentate volume in time t (Vret,t ): VCF = Fig. 2. The KenicsTM static mixer.
(Jp ) was calculated from the time needed to collect 10 mL of permeate. The permeate side was opened to the atmosphere, so TMP could be taken as the average of the gauge readings. The liquid flow rate was varied from 50 to 150 L h−1 . Comparison of the process performances during operation without the use of the static mixer (NSM mode of operation) and when the static mixer was used (SM mode of operation) was carried out in the conditions of total recirculation of the feed and with volumetric concentration of the emulsion. The aim of experiments with total recirculation was to determine the optimal operation conditions (Q and TMP) for both modes of operation. On the other hand, the aim of experiments with volumetric concentration was to demonstrate practical benefits of using a static mixer in ultrafiltration of an oil-in-water emulsion. As ultrafiltration systems are usually analysed via liquid velocities far more often than via liquid flow rates, the comparison of NSM and SM modes during volumetric concentration of the feed was carried out at the same cross-flow velocity. The liquid velocity is calculated as a superficial velocity: U=
Q S
(1)
where S represents the effective cross-section area of the membrane tube. However, when the static mixer was used, the effective crosssection area was smaller for the value of a sectional area of the static mixer than that of the empty tube. Different effective crosssectional areas resulted in about 20% higher velocities at the same flow rates in the case of using the static mixer compared to
Vfeed,i Vret,t
(2)
The membrane was cleaned according to the recommendation of the manufacturer prior to each experiment and the pure water flux of the cleaned membrane was measured. The cleaning procedure was repeated until the original water flux was restored. Beside permeate flux, one of the most important parameter from an economical point of view is the specific energy consumption (E) defined as the power dissipated per unit volume of permeate. The hydraulic dissipated power is directly related to the pressure drop along the membrane module (P) and the specific energy consumption can be calculated as [13]: E=
QP Jp A
(3)
where Jp is the permeate flux and A is the membrane surface area. Energy-saving potential of the configuration with the static mixer was checked through the reduction of the specific energy consumption, determined as a relative reduction of E by using the static mixer compared to the value of E obtained without the static mixer: ER =
ENSM − ESM × 100 ENSM
(4)
3. Results and discussion 3.1. Optimization of the operating conditions The influence of transmembrane pressure and feed flow rate on the process performance with the inserted static mixer (SM mode of operation) was investigated during recirculation of the
Fig. 3. The variations of the permeate flux with TMP during NSM and SM modes of operations. Feed flow rate: (A) 100 L h−1 ; (B) 150 L h−1 .
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Fig. 4. Specific energy consumption vs. TMP during NSM and SM modes of operations. Feed flow rate: (A) 100 L h−1 ; (B) 150 L h−1 .
permeate to the feed tank. Furthermore, in order to compare results, for a given flow rate, the experiments were carried out without using the static mixer (NSM mode of operation). The pseudo-steady state permeate flux values obtained at different TMPs for the feed flow rates of 100 and 150 L h−1 are shown in Fig. 3. Fig. 3 shows that the use of the static mixer provided a significant flux enhancement compared to the operation without the static mixer. Positive effect of the static mixer on permeate flux during ultrafiltration of an oil-in-water emulsion has already been reported in the literature [10,11]. The flow field generated by the static mixer induces hydraulic turbulence and increases the wall shear stress in the vicinity of the membrane which minimizes the effect of concentration polarisation as well as decreasing the potential for membrane fouling. However, the main limitation of the configuration with the static mixer is the increase of pressure drop along the membrane module (P). According to Eq. (3), an increase of pressure drop along the membrane at a given flow rate through the module leads to an increase in dissipated power and energy consumption. Therefore, the specific energy consumption (E) during the operation with the static mixer should be checked as it is directly proportional to the operating costs. Fig. 4 shows the variation of E with TMP at flow rates of 100 and 150 L h−1 during both NSM and SM modes of operation. Despite the significant flux enhancement, the energy consumption was even higher at a flow rate of 150 L h−1 during the SM mode of operation due to a high-pressure drop along the membrane compared to the one without using the static mixer. However, the results from Fig. 4A show that certain energy saving can be obtained at a flow rate of 100 L h−1 by using the static mixer. The values of the reduction of the specific energy consumption (ER ) and flux enhancement achieved during the
operation in NSM and SM modes at the same feed flow rate are shown in Table 1. From Table 1 it is clear that the static mixer is more efficient at higher TMPs; significant flux enhancement, and therefore lower specific energy consumption were obtained when the static mixer was used at higher TMPs. The reduction of the specific energy consumption of over 40% accompanied by the flux increase of up to 600% has been observed during the operation at a TMP of 300 kPa and flow rate of 100 L h−1 . The results presented in Figs. 3 and 4, as well as in Table 1, suggest that the pressure drop along the membrane is too high during the operation in SM mode at a flow rate of 150 L h−1 , and that the use of the static mixer is unlikely favourable from the energetic point of view at this flow rate. On the other hand, the flow field generated by the static mixer enables a high degree of turbulence near the membrane surface at low cross-flow velocities. In order to examine which value of cross-flow velocity can provide considerable energy saving during the operation with the inserted static mixer, the main process parameters were determined at several feed flow rates. Fig. 5 shows the variations of the permeate flux and specific energy consumption with TMP at the feed flow rates in the range from 50 to 150 L h−1 , which correspond to the cross-flow velocities in the range from 0.46 to 1.4 m s−1 . From Fig. 5 it can be noticed that the limiting flux was achieved at a TMP as low as 200 kPa during the operation at a flow rate of 50 L h−1 . Although low energy consumption was observed at this flow rate, the vortices generated by the static mixer cannot efficiently scour the membrane surface, especially at higher transmembrane pressures. On the other hand, a high pressure drop along the membrane at a flow rate of 150 L h−1 lead to increased energy consumption, despite high permeate fluxes. The permeate flux of over 300 L m−2 h−1 , with the spe-
Table 1 Comparison of NSM and SM modes of operation at different TMP and flow rates TMP (kPa)
70 150 220 300
ER (%)
Flux enhancement (%)
Q = 100 L h−1
Q = 150 L h−1
Q = 100 L h−1
Q = 150 L h−1
−37 10 35 42
−139 −86 −35 0
206 334 521 607
109 193 285 430
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Fig. 5. The variations of permeate flux (A) and specific energy consumption (B) with TMP during the operation with the static mixer at different feed flow rates.
cific energy consumption lower than 2 kWh m−3 , obtained at a flow rate of 100 L h−1 are very promising results. The specific energy consumption of 3.2 kWh m−3 has been reported during conventional ultrafiltration of oil-in-water emulsion [3]. Therefore, it seems that a flow rate of 100 L h−1 , which corresponds to a cross-flow velocity of 0.92 m s−1 , could be the optimal feed flow rate for improved process performance with inserted static mixer. The improved performance of cross-flow microfiltration of skimmed milk by using the same static mixer inserted in the membrane with the same geometric characteristics has been reported at cross-flow velocities around 1 m s−1 [10]. These results suggest that the geometric characteristics of used static mixer and membrane determine the efficiency of the configuration with static mixer more than membrane pore size, in the cases when surface fouling is predominant.
velocity (1.2 m s−1 ). In addition, TMP of 160 kPa proved to be too low to obtain any significant values of permeate flux during NSM mode of operation, so TMP of 200 kPa was used throughout this experiment. It should be emphasized that the permeate fluxes not higher than 75 L m−2 h−1 were obtained during the operation without the static mixer at cross-flow velocities as high as 2 m s−1 [10]. On the other hand, despite operation at lower TMP the static mixer provided the initial flux of over 300 L m−2 h−1 with the flux of around 130 L m−2 h−1 at a VCF of 5 during the operation at a cross-flow velocity of only 0.92 m s−1 , resulting in lower specific energy consumption (Fig. 6B); the flux enhancement of 314% with the reduction
3.2. Volumetric concentration of oil/water emulsion The experiments with the recirculation of the permeate provided the guidelines of how ultrafiltration involving the use of static mixer should be carried out in order to improve the permeate flux and concurrently reduce energy consumption. Furthermore, the previous results obtained during the use of the same configuration [10] suggest that the static mixer is even more effective at higher oil concentrations. Therefore, volumetric concentration of the feed using the ultrafiltration configuration with the static mixer was carried out at a flow rate of 100 L h−1 . For comparison, the experiment without using the static mixer was carried out at a flow rate of 150 L h−1 to compensate the “loss” of effective cross-section area and increased turbulence due to presence of the static mixer. An increase of pressure drop along a membrane and consequently increase of TMP is usually indicated as the main disadvantage of a configuration with an inserted static mixer. To demonstrate opposite, volumetric concentration of the emulsion in SM mode of operation was carried out at TMP as low as 160 kPa. The variations of the permeate flux and specific energy consumption with VCF are shown in Fig. 6. The permeate flux decreased due to fouling and increasing of the viscosity as the feed is concentrated. Low permeate flux during NSM mode of operation can be explained by insufficient turbulence near the membrane surface at the given cross-flow
Fig. 6. Permeate flux (A) and specific energy consumption (B) as a function of VCF for NSM and SM modes of operation. NSM: Q = 150 L h−1 ; TMP = 200 kPa; SM: Q = 100 L h−1 ; TMP = 160 kPa.
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of the specific energy consumption of 44% was achieved at a VCF of 2. The experiments involving volumetric concentration of the feed indicated that the appropriate use of static mixer could result in significant energy saving and improved ultrafiltration of oil/water emulsion. 4. Conclusions The present study clearly shows the energy-saving potential of cross-flow ultrafiltration with the KenicsTM static mixer inserted in the membrane tube. The results obtained with a stable oil-in-water emulsion as a feed demonstrated that the use of the static mixer at operating cross-flow velocities as low as 1 m s−1 is crucial in order to achieve the improved process performance. Considerable energy saving and higher fluxes were achieved compared to those obtained with a conventional crossflow ultrafiltration without using the static mixer. Acknowledgements This research was supported by the Ministry for Science and Environment Protection, Republic of Serbia (project no. 142045), CEEPUS Foundation (network no. H-0158-04/05) and the Federal Ministry for Education, Science and Culture, Republic of Austria (Ernst Mach Grant). References [1] S.H. Lin, W.J. Lan, Waste oil/water emulsion treatment by membrane processes, J. Hazard. Mater. 59 (1998) 189–199.
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