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Separation of oil-in-water emulsion using two coalescers of different geometry
Journal of Hazardous Materials 175 (2010) 1001–1006
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Separation of oil-in-water emulsion using two coalescers of different geometry ˇ cerov ´ Radmila M. Se Sokolovic´ ∗ , Dragan D. Govedarica, Dunja S. Sokolovic´ Faculty of Technology, University of Novi Sad, Cara Lazara 1, 21000 Novi Sad, Serbia
a r t i c l e
i n f o
Article history: Received 28 May 2009 Received in revised form 27 October 2009 Accepted 27 October 2009 Available online 10 November 2009 Keywords: Oily water Bed coalescence Fiber material
a b s t r a c t The objective of this work was to investigate the effect of the coalescer geometry on steady-state bed coalescence. Oil-in-water model emulsions, encompassing 14 oils of different properties, were separated using two commercial bed coalescers, marked “04” and “H”. Experiments were carried out over a wide range of oil properties and fluid velocities. Operation of coalescer “04” was characterized by an extremely low working velocity, determined by its construction. For the same reason, coalescer “H” exhibited better performances. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Separation of emulsions is an important operation in the present technology whether it is chemical processing as liquid extraction, direct contact heat transfer, effluent treatment, or the purification of fuels or chemicals. In all these operations some kind of separation is required. For the efficiency of all these separations the most important data are emulsion quantity, phase ratio, and emulsion stability. If we deal with secondary emulsions, the size of the drops is less than 100 m, and the most common separation method is steady-state fiber-bed coalescence [1]. Coalescers are commonly used for the separation of unstable emulsions, regardless of their quantity and phase ratio. One of their main advantages is the possibility of bed oil self-cleaning. They are compact, easy to install, automatize and maintain. One of their disadvantages is the necessity to replace the bed from time to time, depending on the concentration of solid particles in the influent. Numerous factors that are important for bed coalescence, such as the effect of fluid velocity, bed properties, fiber properties, properties of both liquids, and surface phenomena have been studied in the course of decades-long research [2–7]. In the case of oily waste water, which is in the focus of this paper, we deal with a dilute, relatively unstable emulsion, flowing through the porous bed with the drop size smaller than pore size, involving three simultaneous regimes of oil flow: oil droplets suspended in the aqueous phase, coalesced oil forming a continuous phase and flowing through well-connected channels, and the held-up oil as discrete coalescing globules that act as an inter-
∗ Corresponding author. Tel.: +381 214853677; fax: +381 21450413. ˇ cerov ´ ´ E-mail address: [email protected] (R.M. Se Sokolovic). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.10.109
mediate between the dispersed oil and the continuous oil phase [2,8,9]. Fluid velocity plays an especially important role in oil-in-water emulsion flow as it controls the mechanism and probability of droplets capturing, the distribution of oil phase per each of three regimes of oil flow, and the breakup of the retained oil. Some authors defined very early critical velocity as the velocity at which the effluent concentration of the dispersed phase exceeds a fixed value [3]. Sareen et al. [10] and Hazlett [11] have investigated the effect of bed length on separation efficiency of water-in-oil emulsion. While Sareen et al. concluded that there is an optimal bed length that enables maximal value of critical velocity, Hazlett thinks that the bed length above a minimum, has no influence on coalescence. ˇ cerov ´ Se Sokolovic´ et al. [8,9] have demonstrated that the effect of bed length on steady-state bed coalescence is determined by the operation working range. The most relevant properties of fiber bed in respect of bed coalescence are: morphological and geometrical characteristics, including fiber size, shape, arrangement, physical and chemical properties and heterogeneity [9–11]. The effect of chemical nature of solid surface has been considered very often based only on the classification of bed materials in two groups: low-surface energy and high-surface energy materials. In this way, wettability was ˇ cerov ´ designated as a crucial factor in coalescence phenomena. Se Sokolovic´ et al. [12] investigated materials belonging to low-energy solids and found that they exhibited a marked difference in bed coalescence efficiency. Effects of some dispersed phase properties such as density, viscosity, and interfacial tension have been investigated in [3,13,14], whereas the influence of molecular weight, neutralization number, content of n-alkynes, and some other oil properties were investigated in [15].
R.M. Sˇ e´cerov Sokolovi´c et al. / Journal of Hazardous Materials 175 (2010) 1001–1006
1002
Nomenclature Ci Ce df dp E K0 S
v vk15
influent oil concentration, mg/l effluent oil concentration, mg/l fiber diameter, m mean drop diameter, m separation efficiency, % bed permeability, 10−3 mm2 solid surface, 1/mm fluid velocity, m/h critical velocity for Ce 15 mg/l
Greek letters ε bed porosity viscosity, mPa s
ˇ cerov ´ Only Burganos et al. [16] and Se Sokolovic´ et al. [17] have addressed the problem of fluid-flow orientation. These authors have explicitly pointed out the importance of flow mode in filter operation. Their results showed that the effect of flow mode is determined mainly by the range of velocity and bed heterogeneity. The objective of this work was to compare the separation efficiency of commercial coalescers with two different geometries under the same influent oil concentration, working temperature, and bed permeability over a broad range of fluid velocities and dispersed phase properties. 2. Materials and methods
duced at the bottom of section 4 passes vertical up through the EPS bed, changes its orientation to vertical down at the top of the unit, and then passes through the PU bed. Oil waste is collected at the top of the setup, 5, and discharged discontinuously through the valve 3. The other coalescer, marked “H” (Fig. 2), consisted of a horizontal tube 1 m long, with bed and settling section. The bed is located only at the inlet part of the tube, 2. The settled oil is discharged from the settling section discontinuously through the valve 6. 2.2. Operating conditions Steady-state regime was achieved by preoiling the PU bed, with the pressure drop being constant during all the time of the coalescence experiments. The model oil-in-water emulsion of constant oil concentration of 500 mg/l, constant temperature of 20 ◦ C, and mean drop diameter of about 20 m, measured on an optical microscope, was prepared in two tanks 3 (Fig. 2) by continuous stirring with a stainless steel impeller 4. The emulsion was continuously forced with a membrane dosage pump through the experimental unit. Oil concentration in the mean samples was measured by IR spectroscopy from a carbon tetrachloride extract, after adjusting the pH at 2 with HCl solution, to stabilize the oily water samples. Fluid velocity in all experiments was kept constant. The velocity in unit “04” was 7 m/h, whereas in unit “H” it was possible to obtain higher working velocities with good effluent quality, so that the experiments were carried out at the velocities of 30, 35, 40, 45, 50, 55, and 60 m/h.
2.1. Experimental fiber-bed coalescers
2.3. Properties of bed materials
Coalescers with two different geometries were used in the experiments. The coalescer’s body of the unit marked “04” (Fig. 1) is a vertical pipe-in-pipe system [1]. Pipes are filled in with two different polymer materials: granular expanded polystyrene (EPS, pipe 1) and polyurethane fibers (PU, pipe 2). The waste water intro-
Our previous investigation showed that PU is an advantageous bed material for these purposes, exhibiting high separation efficiency in oily water treatment [18]. Several properties of both filter media (PU and EPS) were measured and the data are given in Table 1. Fiber size, microstructure and surface morphology were characterized by scanning electron microscopy (SEM). In unit “04”, the EPS bed length was 30 cm. The experimental bed of PU was formed using compressible, smooth fibers, wetted well with all investigated oils, to obtain similar bed properties. The PU bed permeability was K0 = 5.39 × 10−9 m2 for both coalescers, and the bed length was 1 m in unit “04”, and 5 cm in unit “H”. 2.4. Properties of dispersed oils Ten samples of oils with different characteristics were investigated in unit “04” and four oils in unit “H”. Some of these samples were crude oils from different domestic fields and the other ones were vacuum oil fractions. Main characteristics of the oil samples are given in Table 2. Experiments were performed in a broad range of oil properties: density (836–974 kg/m3 ), viscosity (7.00–1132 mPa s), mean molecular weight (150–500 kg/kmol), neutralization number (0.10–1.70 mg KOH/l), and interfacial tension (11–34 mN/m). 2.5. Coalescence efficiency Results are analyzed using separation efficiency. The coalescence efficiency was calculated on the basis of oil content in the influent Ci , and effluent Ce , using the expression:
Fig. 1. Schematic diagram of the experimental bed coalescer “04”: (1) EPS filter medium; (2) PU filter medium; (3) valve for oil discharge; (4) pump; and (5) coalescer body.
E=
Ci − Ce × 100 Ci
(1)
R.M. Sˇ e´cerov Sokolovi´c et al. / Journal of Hazardous Materials 175 (2010) 1001–1006
1003
Fig. 2. Schematic diagram of the experimental bed coalescer “H”: (1) coalescer body; (2) filter medium; (3) tanks; (4) stainless steel impellers; (5) pump; and (6) valve for oil discharge. Table 1 Characteristics of bed materials. Property
Material EPS 3
Density (kg/m ) Bulk density (kg/m3 ) Bed porosity (%) Bed permeability (10−3 mm2 ) Solid surface (1/mm) Fiber diameter (m) Granules equivalent diameter (mm) Uniformity coefficient
37 21 45 3.84 2.55 – 1.34 1.74
PU 1200 50 96 5.39 3.36 50 – –
3. Results and discussion 3.1. Introduction In our previous paper [17] we showed that the separation efficiency is highly influenced by flow mode. The effect of flow mode is dominantly determined by the range of fluid velocity. At high fluid velocities, horizontal flow coalescers are more efficient than the vertical ones, whereas at low velocities there is no significant difference. At high fluid velocities, the up-flow operation is the least efficient. Our previous investigations [15] also showed that the nature of dispersed oil had a great effect on the separation efficiency. As already mentioned, the objective of this study was to compare separation efficiency of coalescers with two different geometries in the separation of oils different characteristics. Commercial coalescer “04”, involving both vertical flow modes (up-flow and down-flow), was compared with the coalescer “H” working in horizontal flow mode.
Fig. 3. Dependence of separation efficiency on dispersed oil density for coalescer “04” (empty symbols) and for coalescer “H” (full symbols).
3.2. Effect of oil properties on separation efficiency The effect of oil properties such as density, neutralization number, viscosity and molecular weight was compared for the two coalescers over several working velocities. As can be seen from Fig. 3, the dependence of separation efficiency on oil density is similar for both units. The efficiency of oil removal decreases with the increase in density, reaching a minimum, and then increases with increase in oil density, which is in agreement with the findings of some other authors [10,11].
Table 2 Physical characteristics of oil samples. Sample
Density (kg/m3)
Viscosity (mPa s)
Mean mol. weight (kg/kmol)
Neutral. no. (mg KOH/l)
Interfacial tension (mN/m)
A B C D E F G I II III Ar A1 A4 P1 Interval
912.80 847.15 867.44 898.20 836.04 921.68 866.74 973.58 861.80 844.73 915.50 905.90 918.90 879.00 836–974
56.31 9.22 30.00 180.00 7.79 1132 17.48 23.28 11.64 21.73 43.35 9.18 168.90 10.32 7.00–1132
374 217 251 460 201 500 250 236 271 349 410 150 520 300 150–500
0.98 0.55 0.46 1.61 0.38 1.47 0.50 0.40 0.10 0.23 1.42 1.13 1.71 0.13 0.10–1.70
18.80 20.40 14.80 12.19 17.03 11.56 14.57 13.69 21.32 18.04 18.80 33.80 30.50 32.4 11–34
1004
R.M. Sˇ e´cerov Sokolovi´c et al. / Journal of Hazardous Materials 175 (2010) 1001–1006
Fig. 4. 3D diagram of the dependence of separation efficiency on fluid velocity and oil density for coalescer “H”.
Only at a fluid velocity of 60 m/h, the dependence is different, which will be explained later on. The dependence obtained for unit “H” over all working velocities is even better illustrated in Fig. 4. How can we explain such a trend? In the given bed arrangement, the droplet size is smaller than the pore size, and the droplet capture mechanisms may involve interception, sedimentation, hydrodynamic retardation, London-van der Waals attraction, Brownian diffusion, etc. When the density of droplets decreases, their settling velocity increases. An opposite effect is also operating simultaneously. In the steady-state bed coalescence, droplet attachment is realized on the surface of saturated oil. The droplet capture and coalescence are directly influenced by the amount of saturated oil, which can be correlated to oil density. The amount of saturated oil increases with increasing oil density for all three modes of flow. In the vertical upward flow, lower density oil tends to leave the bed, so that the saturated oil thickness is reduced. Higher density oils show a lower tendency to leave the bed, increasing the amount and thickness of saturated oil. The dependence of the amount of saturated oil on oil density in the horizontal fluid-flow orientation can be explained in the following way. The cross-sectional oil phase distribution is influenced by the oil density. The oil of higher density is well distributed over the coalescer’s cross-section while lower density oil occupies only the upper part of the bed. Consequently, the amount of saturated oil increases with the increase in oil density, contributing thus to a higher coalescence and separation efficiency. Because of all these reasons, the dependence of separation efficiency on oil density shows a minimum. The nature of the dependence and position of the minimum of efficiency for the two investigated units can also be explained in terms of the amount of saturated oil. The horizontal flow mode involves the largest, vertical downward somewhat smaller, and the vertical upward mode the smallest amount of saturated oil. Hence, coalescer “H” has a better performance than coalescer “04”, the minimum of separation efficiency for the latter being 94.6% at an oil density of 921.68 kg/m3 . On the other hand, the minimum of separation efficiency for unit “H” is located at 98.7% for the oil density of 905.90 kg/m3 for all working velocities. The effect of neutralization number, as a measure of the oil polarity, on separation efficiency is very similar to the effect of density, although they are reflecting different oil properties. The efficiency shows a minimum for the two units in a different range of the neutralization number, Fig. 5. For unit “04”, the minimum
Fig. 5. Dependence of the separation efficiency on dispersed oil neutralization number for coalescer “04” (empty symbols) and for coalescer “H” (full symbols).
is located at 1.446 mg KOH/l, while for unit “H” it is located at 1.153 mg KOH/l for all investigated velocities. In the range of low viscosity, the dependence of the efficiency is similar for both units, whereas in the range of high viscosity, the efficiency decreases with increasing oil viscosity for unit “04”, Fig. 6. The effect of oil molecular weight is shown in Figs. 7 and 8. At low working velocities, molecular weight does not influence separation efficiency, whereas at high velocities the efficiency is drastically lower for smaller molecular weights. It seems plausible that the lower oil molecular weight contributes to a decrease of adhesion forces which can hinder coalescence. Interfacial tension had a specific effect on separation efficiency. Since the range of interfacial tension in our investigations was not identical, it was not possible to compare its effect in the two coalescers investigated. However, if we want to compare two coalescers, there is another possibility: to test their sensitivity to the changes of the influent oil concentration under the same working conditions. This test was performed on both coalescers, using oil A and the
Fig. 6. 3D diagram of the dependence of separation efficiency on fluid velocity and oil neutralization number for coalescer “H”.
R.M. Sˇ e´cerov Sokolovi´c et al. / Journal of Hazardous Materials 175 (2010) 1001–1006
1005
Fig. 9. Dependence of the effluent oil concentration on the influent oil concentration for the coalescers “04” and “H”. Fig. 7. Dependence of separation efficiency on dispersed oil viscosity for coalescer “04” (empty symbols), for unit “H” (full symbols).
velocity of 7 m/h, Fig. 9. As can be seen from the figure, the effluent concentration for unit “04” is above 15 mg/l already for the influent concentration of 500 mg/l. Furthermore, the effluent oil concentration considerably increases with increase in the influent concentration, while for unit “H”, the effluent concentration of 15 mg/l can be obtained for the influent concentration up to 10,000 mg/l. 3.3. Effect of unit geometry on working velocity The principle of the work of a steady-state coalescer is that small oil drops get into the bed and coalesce into saturated oil inside pores. At the same time, large globules detach from the saturated oil, exit the bed, and float on the water phase. It looks like small drops are growing bigger while crossing through the bed. In some circumstances, the situation may be completely opposite. Small drops are coming into the bed, but they can be redispersed and leave the bed even smaller. Which process will be dominant, it depends on the fluid velocity. The most important thing for bed coalescence is whether the working velocity is below or above the critical one. When the velocity is below the critical value, coalescence takes place, and the large globules are separated after passing the bed. If
the velocity is higher than the critical one, redispersion takes place, and drop separation is not realized. Critical velocity in our study is defined as the velocity when the effluent oil concentration reaches 15 mg/l. The coalescer geometry does not affect the dependence of the separation efficiency on the nature of dispersed oil. However, the geometry determines dominantly the critical velocity. The question arises as to why coalescer “04” is less efficient than coalescer “H”. Coalescer “04” involves both vertical fluid flow modes (upflow and down-flow), resulting in extremely low working velocity of 7 m/h. This velocity was critical for three of ten investigated oils. For oil A, the obtained effluent concentration was 19.40 mg/l, for oil D 18.30 mg/l, and for oil F 26.90 mg/l. The separation efficiency is much lower in the region above critical velocity because of the system instability. Unit “H” exhibits similar behavior only above the working velocity of 60 m/h. The obtained effluent concentrations were: 16.30 mg/l for oil Ar, 21.12 for oil A1 and 26.97 mg/l for oil P1. The working velocity of 60 m/h for these three oils was above the critical one. On the other hand, the nature of oil A4 caused that even at such high working velocity the effluent concentration was much lower (5.08 mg/l). It is clear that the unit “H” shows two advantages over unit “04”: the working velocity is much higher and the separation efficiency is not sensitive to the changes in the influent concentration. In view of the above, the effect of unit geometry has to be tested for a broad range of fluid velocities, oil properties and oil concentrations.
4. Conclusions
Fig. 8. Dependence of separation efficiency on oil molecular weight for coalescer “04” (empty symbols), for unit “H” (full symbols).
Coalescer geometry does not affect the dependence of separation efficiency on the nature of dispersed oil. However, it dominantly determines the critical velocity, which depends on the existing flow mode involved in the given construction and the nature of dispersed oil. The separation efficiency for both vertical fluid flow modes (up-flow and down-flow) is very low. Coalescer “04” has an extremely low working velocity, which is a consequence of its construction. Coalescer “H” exhibits remarkable performance thanks to the horizontal flow mode. Hence, it can be recommended that the bed coalescence phenomena should be investigated for this type of flow.
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Acknowledgement This work was supported by the Ministry of Science and Technological Development of the Republic of Serbia, Grant number 142045. References ˇ cerov ˇ c, ´ ´ S.M. Sokolovic, ´ S. Sevi ´ Oily water treatment using a [1] R.M. Se Sokolovic, new steady-state fiber-bed coalescer, J. Hazard. Mater. 162 (2009) 410–415. [2] S. Dawar, G.G. Chase, Drag correlation for axial motion of drops on fibers, Sep. Purif. Technol. 60 (2008) 6–13. [3] L.A. Spielman, Separation of finely dispersed liquid–liquid suspensions by flow through fibrous media, Ph.D. Dissertation, University of California, Berkeley (1968). [4] F. Ji, C. Li, X. Dong, Y. Li, D. Wang, Separation of oil from oily wastewater by sorption and coalescence technique using ethanol grafted polyacrylonitrile, J. Hazard. Mater. 164 (2009) 1346–1351. [5] G. Deschamps, H. Caruel, M. Borredon, C. Albasi, J. Riba, C. Bonnin, C. Vignoles, Oil removal from water by sorption on hydrophobic cotton fibers. 2. Study of sorption properties in dynamic mode, Environ. Sci. Technol 37 (2003) 5034–5039. [6] H. Speth, A. Pfenning, M. Chatterjee, H. Franken, Coalescence of secondary dispersions in fiber beds 29 (2002) 113–119. [7] J. Li, Y. Gu, Coalescence of oil-in-water emulsion in fibrous and granular beds, Sep. Purif. Technol. 42 (2005) 1–13.
ˇ cerov ´ ´ S.M. Sokolovic, ´ B.D. Ðokovic, ´ Effect of working con[8] R.M. Se Sokolovic, ditions on bed coalescence of an oil-in-water emulsion using a polyurethane foam bed, Ind. Eng. Chem. Res. 36 (1997) 4949–4953, 11. ˇ cerov ´ ´ T. Vulic, ´ S. Sokolovic, ´ Effect of bed length on steady[9] R.M. Se Sokolovic, state coalescence of oil-in-water emulsion, Sep. Purif. Technol. 56 (2007) 79–84. [10] S.S. Sareen, P.M. Rose, R.C. Gudesen, R.C. Kintner, Coalescence in fibrous beds, AIChE J. 12 (1966) 1045–1050, 6. [11] R.N. Hazlett, Fibrous bed coalescence of water: steps in the coalescence process, Ind. Eng. Chem. Fundam. 8 (1969) 625–632, 4. ˇ cerov ´ ´ S.M. Sokolovic, ´ Effect of the nature of different poly[12] R.M. Se Sokolovic, meric fibers on steady-state bed coalescence of an oil-in-water emulsion, Ind. Eng. Chem. Res. 43 (2004) 6490–6495. [13] J. Golob, V. Grilc, R. Modic, Drop coalescence in liquid–liquid dispersions by flow through glass fibre beds, Chem. Eng. Res. Des. 62 (1984) 48–52. [14] V. Grilc, L. Golob, R. Modic, Drop coalescence in liquid/liquid dispersions by flow through glass fiber beds, Part II, Chem. Eng. Res. Des. 64 (1986) 67–70. ˇ cerov ´ ´ S.M. Sokolovic, ´ Ð.S. Mihajlovic, ´ Influence of oil prop[15] R.M. Se Sokolovic, erties on bed coalescence efficiency, Sep. Sci. Technol. 31 (1996) 2089–2104, 15. [16] V.N. Burganos, C.A. Paraskeva, A.C. Payatakes, Monte Carlo network simulation of horizontal, upflow and downflow depth filtration, AIChE J. 41 (1995) 272–285, 2. ˇ cerov ´ ´ T. Vulic, ´ S. Sokolovic, ´ Effect of fluid flow orientation on [17] R. Se Sokolovic, the coalescence of oil droplets in steady-state bed coalescers, Ind. Eng. Chem. Res. 45 (2006) 3891–3895. ˇ cerov ´ R. Se ´ ´ S. Putnik, Some aspects of application of [18] S. Sokolovic, Sokolovic, waste polymer materials as a coalescence medium for oily waste water treatment, Environ. Technol. Lett. 13 (1992) 987–994, 10.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Separation of oil-in-water emulsion using two coalescers of different geometry ˇ cerov ´ Radmila M. Se Sokolovic´ ∗ , Dragan D. Govedarica, Dunja S. Sokolovic´ Faculty of Technology, University of Novi Sad, Cara Lazara 1, 21000 Novi Sad, Serbia
a r t i c l e
i n f o
Article history: Received 28 May 2009 Received in revised form 27 October 2009 Accepted 27 October 2009 Available online 10 November 2009 Keywords: Oily water Bed coalescence Fiber material
a b s t r a c t The objective of this work was to investigate the effect of the coalescer geometry on steady-state bed coalescence. Oil-in-water model emulsions, encompassing 14 oils of different properties, were separated using two commercial bed coalescers, marked “04” and “H”. Experiments were carried out over a wide range of oil properties and fluid velocities. Operation of coalescer “04” was characterized by an extremely low working velocity, determined by its construction. For the same reason, coalescer “H” exhibited better performances. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Separation of emulsions is an important operation in the present technology whether it is chemical processing as liquid extraction, direct contact heat transfer, effluent treatment, or the purification of fuels or chemicals. In all these operations some kind of separation is required. For the efficiency of all these separations the most important data are emulsion quantity, phase ratio, and emulsion stability. If we deal with secondary emulsions, the size of the drops is less than 100 m, and the most common separation method is steady-state fiber-bed coalescence [1]. Coalescers are commonly used for the separation of unstable emulsions, regardless of their quantity and phase ratio. One of their main advantages is the possibility of bed oil self-cleaning. They are compact, easy to install, automatize and maintain. One of their disadvantages is the necessity to replace the bed from time to time, depending on the concentration of solid particles in the influent. Numerous factors that are important for bed coalescence, such as the effect of fluid velocity, bed properties, fiber properties, properties of both liquids, and surface phenomena have been studied in the course of decades-long research [2–7]. In the case of oily waste water, which is in the focus of this paper, we deal with a dilute, relatively unstable emulsion, flowing through the porous bed with the drop size smaller than pore size, involving three simultaneous regimes of oil flow: oil droplets suspended in the aqueous phase, coalesced oil forming a continuous phase and flowing through well-connected channels, and the held-up oil as discrete coalescing globules that act as an inter-
∗ Corresponding author. Tel.: +381 214853677; fax: +381 21450413. ˇ cerov ´ ´ E-mail address: [email protected] (R.M. Se Sokolovic). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.10.109
mediate between the dispersed oil and the continuous oil phase [2,8,9]. Fluid velocity plays an especially important role in oil-in-water emulsion flow as it controls the mechanism and probability of droplets capturing, the distribution of oil phase per each of three regimes of oil flow, and the breakup of the retained oil. Some authors defined very early critical velocity as the velocity at which the effluent concentration of the dispersed phase exceeds a fixed value [3]. Sareen et al. [10] and Hazlett [11] have investigated the effect of bed length on separation efficiency of water-in-oil emulsion. While Sareen et al. concluded that there is an optimal bed length that enables maximal value of critical velocity, Hazlett thinks that the bed length above a minimum, has no influence on coalescence. ˇ cerov ´ Se Sokolovic´ et al. [8,9] have demonstrated that the effect of bed length on steady-state bed coalescence is determined by the operation working range. The most relevant properties of fiber bed in respect of bed coalescence are: morphological and geometrical characteristics, including fiber size, shape, arrangement, physical and chemical properties and heterogeneity [9–11]. The effect of chemical nature of solid surface has been considered very often based only on the classification of bed materials in two groups: low-surface energy and high-surface energy materials. In this way, wettability was ˇ cerov ´ designated as a crucial factor in coalescence phenomena. Se Sokolovic´ et al. [12] investigated materials belonging to low-energy solids and found that they exhibited a marked difference in bed coalescence efficiency. Effects of some dispersed phase properties such as density, viscosity, and interfacial tension have been investigated in [3,13,14], whereas the influence of molecular weight, neutralization number, content of n-alkynes, and some other oil properties were investigated in [15].
R.M. Sˇ e´cerov Sokolovi´c et al. / Journal of Hazardous Materials 175 (2010) 1001–1006
1002
Nomenclature Ci Ce df dp E K0 S
v vk15
influent oil concentration, mg/l effluent oil concentration, mg/l fiber diameter, m mean drop diameter, m separation efficiency, % bed permeability, 10−3 mm2 solid surface, 1/mm fluid velocity, m/h critical velocity for Ce 15 mg/l
Greek letters ε bed porosity viscosity, mPa s
ˇ cerov ´ Only Burganos et al. [16] and Se Sokolovic´ et al. [17] have addressed the problem of fluid-flow orientation. These authors have explicitly pointed out the importance of flow mode in filter operation. Their results showed that the effect of flow mode is determined mainly by the range of velocity and bed heterogeneity. The objective of this work was to compare the separation efficiency of commercial coalescers with two different geometries under the same influent oil concentration, working temperature, and bed permeability over a broad range of fluid velocities and dispersed phase properties. 2. Materials and methods
duced at the bottom of section 4 passes vertical up through the EPS bed, changes its orientation to vertical down at the top of the unit, and then passes through the PU bed. Oil waste is collected at the top of the setup, 5, and discharged discontinuously through the valve 3. The other coalescer, marked “H” (Fig. 2), consisted of a horizontal tube 1 m long, with bed and settling section. The bed is located only at the inlet part of the tube, 2. The settled oil is discharged from the settling section discontinuously through the valve 6. 2.2. Operating conditions Steady-state regime was achieved by preoiling the PU bed, with the pressure drop being constant during all the time of the coalescence experiments. The model oil-in-water emulsion of constant oil concentration of 500 mg/l, constant temperature of 20 ◦ C, and mean drop diameter of about 20 m, measured on an optical microscope, was prepared in two tanks 3 (Fig. 2) by continuous stirring with a stainless steel impeller 4. The emulsion was continuously forced with a membrane dosage pump through the experimental unit. Oil concentration in the mean samples was measured by IR spectroscopy from a carbon tetrachloride extract, after adjusting the pH at 2 with HCl solution, to stabilize the oily water samples. Fluid velocity in all experiments was kept constant. The velocity in unit “04” was 7 m/h, whereas in unit “H” it was possible to obtain higher working velocities with good effluent quality, so that the experiments were carried out at the velocities of 30, 35, 40, 45, 50, 55, and 60 m/h.
2.1. Experimental fiber-bed coalescers
2.3. Properties of bed materials
Coalescers with two different geometries were used in the experiments. The coalescer’s body of the unit marked “04” (Fig. 1) is a vertical pipe-in-pipe system [1]. Pipes are filled in with two different polymer materials: granular expanded polystyrene (EPS, pipe 1) and polyurethane fibers (PU, pipe 2). The waste water intro-
Our previous investigation showed that PU is an advantageous bed material for these purposes, exhibiting high separation efficiency in oily water treatment [18]. Several properties of both filter media (PU and EPS) were measured and the data are given in Table 1. Fiber size, microstructure and surface morphology were characterized by scanning electron microscopy (SEM). In unit “04”, the EPS bed length was 30 cm. The experimental bed of PU was formed using compressible, smooth fibers, wetted well with all investigated oils, to obtain similar bed properties. The PU bed permeability was K0 = 5.39 × 10−9 m2 for both coalescers, and the bed length was 1 m in unit “04”, and 5 cm in unit “H”. 2.4. Properties of dispersed oils Ten samples of oils with different characteristics were investigated in unit “04” and four oils in unit “H”. Some of these samples were crude oils from different domestic fields and the other ones were vacuum oil fractions. Main characteristics of the oil samples are given in Table 2. Experiments were performed in a broad range of oil properties: density (836–974 kg/m3 ), viscosity (7.00–1132 mPa s), mean molecular weight (150–500 kg/kmol), neutralization number (0.10–1.70 mg KOH/l), and interfacial tension (11–34 mN/m). 2.5. Coalescence efficiency Results are analyzed using separation efficiency. The coalescence efficiency was calculated on the basis of oil content in the influent Ci , and effluent Ce , using the expression:
Fig. 1. Schematic diagram of the experimental bed coalescer “04”: (1) EPS filter medium; (2) PU filter medium; (3) valve for oil discharge; (4) pump; and (5) coalescer body.
E=
Ci − Ce × 100 Ci
(1)
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Fig. 2. Schematic diagram of the experimental bed coalescer “H”: (1) coalescer body; (2) filter medium; (3) tanks; (4) stainless steel impellers; (5) pump; and (6) valve for oil discharge. Table 1 Characteristics of bed materials. Property
Material EPS 3
Density (kg/m ) Bulk density (kg/m3 ) Bed porosity (%) Bed permeability (10−3 mm2 ) Solid surface (1/mm) Fiber diameter (m) Granules equivalent diameter (mm) Uniformity coefficient
37 21 45 3.84 2.55 – 1.34 1.74
PU 1200 50 96 5.39 3.36 50 – –
3. Results and discussion 3.1. Introduction In our previous paper [17] we showed that the separation efficiency is highly influenced by flow mode. The effect of flow mode is dominantly determined by the range of fluid velocity. At high fluid velocities, horizontal flow coalescers are more efficient than the vertical ones, whereas at low velocities there is no significant difference. At high fluid velocities, the up-flow operation is the least efficient. Our previous investigations [15] also showed that the nature of dispersed oil had a great effect on the separation efficiency. As already mentioned, the objective of this study was to compare separation efficiency of coalescers with two different geometries in the separation of oils different characteristics. Commercial coalescer “04”, involving both vertical flow modes (up-flow and down-flow), was compared with the coalescer “H” working in horizontal flow mode.
Fig. 3. Dependence of separation efficiency on dispersed oil density for coalescer “04” (empty symbols) and for coalescer “H” (full symbols).
3.2. Effect of oil properties on separation efficiency The effect of oil properties such as density, neutralization number, viscosity and molecular weight was compared for the two coalescers over several working velocities. As can be seen from Fig. 3, the dependence of separation efficiency on oil density is similar for both units. The efficiency of oil removal decreases with the increase in density, reaching a minimum, and then increases with increase in oil density, which is in agreement with the findings of some other authors [10,11].
Table 2 Physical characteristics of oil samples. Sample
Density (kg/m3)
Viscosity (mPa s)
Mean mol. weight (kg/kmol)
Neutral. no. (mg KOH/l)
Interfacial tension (mN/m)
A B C D E F G I II III Ar A1 A4 P1 Interval
912.80 847.15 867.44 898.20 836.04 921.68 866.74 973.58 861.80 844.73 915.50 905.90 918.90 879.00 836–974
56.31 9.22 30.00 180.00 7.79 1132 17.48 23.28 11.64 21.73 43.35 9.18 168.90 10.32 7.00–1132
374 217 251 460 201 500 250 236 271 349 410 150 520 300 150–500
0.98 0.55 0.46 1.61 0.38 1.47 0.50 0.40 0.10 0.23 1.42 1.13 1.71 0.13 0.10–1.70
18.80 20.40 14.80 12.19 17.03 11.56 14.57 13.69 21.32 18.04 18.80 33.80 30.50 32.4 11–34
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Fig. 4. 3D diagram of the dependence of separation efficiency on fluid velocity and oil density for coalescer “H”.
Only at a fluid velocity of 60 m/h, the dependence is different, which will be explained later on. The dependence obtained for unit “H” over all working velocities is even better illustrated in Fig. 4. How can we explain such a trend? In the given bed arrangement, the droplet size is smaller than the pore size, and the droplet capture mechanisms may involve interception, sedimentation, hydrodynamic retardation, London-van der Waals attraction, Brownian diffusion, etc. When the density of droplets decreases, their settling velocity increases. An opposite effect is also operating simultaneously. In the steady-state bed coalescence, droplet attachment is realized on the surface of saturated oil. The droplet capture and coalescence are directly influenced by the amount of saturated oil, which can be correlated to oil density. The amount of saturated oil increases with increasing oil density for all three modes of flow. In the vertical upward flow, lower density oil tends to leave the bed, so that the saturated oil thickness is reduced. Higher density oils show a lower tendency to leave the bed, increasing the amount and thickness of saturated oil. The dependence of the amount of saturated oil on oil density in the horizontal fluid-flow orientation can be explained in the following way. The cross-sectional oil phase distribution is influenced by the oil density. The oil of higher density is well distributed over the coalescer’s cross-section while lower density oil occupies only the upper part of the bed. Consequently, the amount of saturated oil increases with the increase in oil density, contributing thus to a higher coalescence and separation efficiency. Because of all these reasons, the dependence of separation efficiency on oil density shows a minimum. The nature of the dependence and position of the minimum of efficiency for the two investigated units can also be explained in terms of the amount of saturated oil. The horizontal flow mode involves the largest, vertical downward somewhat smaller, and the vertical upward mode the smallest amount of saturated oil. Hence, coalescer “H” has a better performance than coalescer “04”, the minimum of separation efficiency for the latter being 94.6% at an oil density of 921.68 kg/m3 . On the other hand, the minimum of separation efficiency for unit “H” is located at 98.7% for the oil density of 905.90 kg/m3 for all working velocities. The effect of neutralization number, as a measure of the oil polarity, on separation efficiency is very similar to the effect of density, although they are reflecting different oil properties. The efficiency shows a minimum for the two units in a different range of the neutralization number, Fig. 5. For unit “04”, the minimum
Fig. 5. Dependence of the separation efficiency on dispersed oil neutralization number for coalescer “04” (empty symbols) and for coalescer “H” (full symbols).
is located at 1.446 mg KOH/l, while for unit “H” it is located at 1.153 mg KOH/l for all investigated velocities. In the range of low viscosity, the dependence of the efficiency is similar for both units, whereas in the range of high viscosity, the efficiency decreases with increasing oil viscosity for unit “04”, Fig. 6. The effect of oil molecular weight is shown in Figs. 7 and 8. At low working velocities, molecular weight does not influence separation efficiency, whereas at high velocities the efficiency is drastically lower for smaller molecular weights. It seems plausible that the lower oil molecular weight contributes to a decrease of adhesion forces which can hinder coalescence. Interfacial tension had a specific effect on separation efficiency. Since the range of interfacial tension in our investigations was not identical, it was not possible to compare its effect in the two coalescers investigated. However, if we want to compare two coalescers, there is another possibility: to test their sensitivity to the changes of the influent oil concentration under the same working conditions. This test was performed on both coalescers, using oil A and the
Fig. 6. 3D diagram of the dependence of separation efficiency on fluid velocity and oil neutralization number for coalescer “H”.
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Fig. 9. Dependence of the effluent oil concentration on the influent oil concentration for the coalescers “04” and “H”. Fig. 7. Dependence of separation efficiency on dispersed oil viscosity for coalescer “04” (empty symbols), for unit “H” (full symbols).
velocity of 7 m/h, Fig. 9. As can be seen from the figure, the effluent concentration for unit “04” is above 15 mg/l already for the influent concentration of 500 mg/l. Furthermore, the effluent oil concentration considerably increases with increase in the influent concentration, while for unit “H”, the effluent concentration of 15 mg/l can be obtained for the influent concentration up to 10,000 mg/l. 3.3. Effect of unit geometry on working velocity The principle of the work of a steady-state coalescer is that small oil drops get into the bed and coalesce into saturated oil inside pores. At the same time, large globules detach from the saturated oil, exit the bed, and float on the water phase. It looks like small drops are growing bigger while crossing through the bed. In some circumstances, the situation may be completely opposite. Small drops are coming into the bed, but they can be redispersed and leave the bed even smaller. Which process will be dominant, it depends on the fluid velocity. The most important thing for bed coalescence is whether the working velocity is below or above the critical one. When the velocity is below the critical value, coalescence takes place, and the large globules are separated after passing the bed. If
the velocity is higher than the critical one, redispersion takes place, and drop separation is not realized. Critical velocity in our study is defined as the velocity when the effluent oil concentration reaches 15 mg/l. The coalescer geometry does not affect the dependence of the separation efficiency on the nature of dispersed oil. However, the geometry determines dominantly the critical velocity. The question arises as to why coalescer “04” is less efficient than coalescer “H”. Coalescer “04” involves both vertical fluid flow modes (upflow and down-flow), resulting in extremely low working velocity of 7 m/h. This velocity was critical for three of ten investigated oils. For oil A, the obtained effluent concentration was 19.40 mg/l, for oil D 18.30 mg/l, and for oil F 26.90 mg/l. The separation efficiency is much lower in the region above critical velocity because of the system instability. Unit “H” exhibits similar behavior only above the working velocity of 60 m/h. The obtained effluent concentrations were: 16.30 mg/l for oil Ar, 21.12 for oil A1 and 26.97 mg/l for oil P1. The working velocity of 60 m/h for these three oils was above the critical one. On the other hand, the nature of oil A4 caused that even at such high working velocity the effluent concentration was much lower (5.08 mg/l). It is clear that the unit “H” shows two advantages over unit “04”: the working velocity is much higher and the separation efficiency is not sensitive to the changes in the influent concentration. In view of the above, the effect of unit geometry has to be tested for a broad range of fluid velocities, oil properties and oil concentrations.
4. Conclusions
Fig. 8. Dependence of separation efficiency on oil molecular weight for coalescer “04” (empty symbols), for unit “H” (full symbols).
Coalescer geometry does not affect the dependence of separation efficiency on the nature of dispersed oil. However, it dominantly determines the critical velocity, which depends on the existing flow mode involved in the given construction and the nature of dispersed oil. The separation efficiency for both vertical fluid flow modes (up-flow and down-flow) is very low. Coalescer “04” has an extremely low working velocity, which is a consequence of its construction. Coalescer “H” exhibits remarkable performance thanks to the horizontal flow mode. Hence, it can be recommended that the bed coalescence phenomena should be investigated for this type of flow.
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