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Cleaning strategies in ceramic microfiltration membranes fouled by oil and particulate matter in produced water
Desalination 236 (2009) 160–169
Cleaning strategies in ceramic microfiltration membranes fouled by oil and particulate matter in produced water Sumihar H.D. Silalahi, TorOve Leiknes* NTNU — Norwegian University of Science and Technology, Department of Hydraulic and Environmental Engineering, S.P. Andersensvei 5, N-7491 Trondheim, Norway Tel. þ47 7359 4758; Fax þ47 7359 1298; email: [email protected] Received 30 June 2007; revised accepted 7 October 2007
Abstract Produced water is contaminated water that is extracted together with the oil in oil production operations. Membrane filtration has the potential for a very effective separation of oil from water. The major drawback in applying this technique is the inherent fouling phenomena found in all membrane systems. At a certain point, fouling necessitates an extended cleaning to regain the original permeability of the membrane. This study evaluated the cleaning efficiency of different commercial products that are biodegradable i.e. Ultrasil 115, Ultrasil 73, Surfactron CD 50, Derquimþ, etc on fouled membrane by analogue produced water with membrane nominal pore sizes of 0.1, 0.2 and 0.5 mm. Full restoration of fouled membrane could not be achieved by single cleaning step. The total cleaning efficiency depends on temperature, concentration and TMP of cleaning solution. Keywords: Chemical cleaning; Oil emulsion; Microfiltration; Fouling
1. Introduction During oil and gas production water is produced at the wellhead known as produced water. The composition of produced water varies depending on the type and maturity of the reservoirs, however, mainly it consists of water which contains dissolved organic compounds (including hydrocarbons), heavy metals, dissolved
minerals, suspended oil and oil emulsions, solids (sand and silt) and a variety of production chemicals. Current regulations for installations on the Norwegian Continental Shelf (NCS) have set a limit of 30 mg/L of oil in water when produced water is discharged to the sea [1]. Membrane filtration has the potential for a very effective separation of oil from water. Generally, for feedwaters with 0.1–10% of oil content, microfiltration (MF) or ultrafiltration
*Corresponding author. Presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5–9 November 2007, Sydney, Australia 0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.10.063
S.H.D. Silalahi, T. Leiknes / Desalination 236 (2009) 160–169
(UF) has the capability of producing water with less than 10–100 ppm of oil. The major drawback in applying this technique is the inherent fouling phenomena found in all membrane systems. Membrane fouling by oil emulsions is caused by (a) accumulation of oil droplets at the membrane surface (cake layer formation and/or concentration polarization), (b) fouling from oil drops penetrating pores or adsorption onto the membrane surface and (c) fouling layer compaction due to the permeation drag which causes the membrane matrix to slightly reorganize resulting in lowered volume porosity, increasing membrane resistance and thus lower fluxes [2,3]. During operation, membrane fouling causes a progressive decrease of flux and induces a loss of separation efficiency. At a certain point, fouling necessitates an extended cleaning to regain the original permeability of the membrane. Different cleaning protocols and strategies can be employed depending on the separation process and the type of membrane used. This step in a membrane based treatment of produced water will necessarily produce a waste stream. Regulations on the NCS have a strict code as far as types and quantities of chemicals that are used offshore, which includes those applied for membrane cleaning. Due to the stringent regulations of waste generation and discharge on the NCS, an assessment of cleaning agents is required to evaluate if these chemicals represent a secondary pollutant source to the NCS. To comply with regulations, the use of biodegradable cleaning chemicals is an interesting option. However, the optimal cleaning agent will also need to efficiently clean the membrane and chose of compounds and protocol will be a function of the membrane material, foulant characteristic and cleaning procedure i.e. concentration, temperature, pH, pressure, flow and time [3–6]. The main objective of this paper is to evaluate different types of cleaning agents and cleaning protocols/strategies for efficient flux recovery of -Al2O3 tubular ceramic membranes.
161
Membranes with nominal pore sizes of 0.1, 0.2 and 0.5 mm fouled by produced water have been tested. Different commercial products that are biodegradable were evaluated i.e. Ultrasil 115 and Derquimþ (alkaline solutions), Ultrasil 73 and Surfactron CD 50 (acid solutions). The cleaning efficiency after membrane cleaning is determined by assessing flux recovery compared to the permeability of non-fouled membranes. The efficiency of membrane cleaning depends on several parameters i.e. cleaning sequence, concentrations of cleaning agent and temperature of cleaning solution.
2. Experimental section 2.1. Apparatus A -Al2O3 ceramic membranes provided by ECO-CERAMICS with a nominal pore size of 0.1, 0.2 and 0.5 mm were mounted together in a tubular crossflow module. Each membrane has inside dimensions of 0.8 cm inner diameter, 1.1 cm outer diameter and 34 cm length. The effective membrane area was 85 cm2. This experimental design enables direct comparison of the performance of cleaning for different membrane pore sizes. A peristaltic pump was connected to the permeate stream to give a constant permeate flow. The trans membrane pressure was monitored continuously using a pressure transducer, temperature with temperature transducer and pH with pH meter. A data acquisition unit was connected to all transducers to record the data. An illustration of the membrane filtration device is shown in Fig. 1. 2.2. Feed preparation for fouling substance The oil emulsion feed was made using a crude oil with characteristics as shown in Table 1 and diluted with salt water. The dispersed oil was prepared by mixing oil and surfactant in a Ultraturrax homogenizer at a mixing rate of
Permeate
S.H.D. Silalahi, T. Leiknes / Desalination 236 (2009) 160–169
Retentate
162
Flow Transducer Pump
Pressure transducer
Mixer Rig produced water feed
0.5 µm
T Thermoregulator
Flow Transducer Pump
Pressure transducer
Flow Transducer Pump
Pressure transducer
Valve
0.2 µm
0.1 µm
Pressure transducer
Membrance modules
Pressure transducer
Pump
Computer
Fig. 1. Schematic diagram of experimental setup for crossflow microfiltration. Table 1 Oil characteristics for crude oil at 60 C [7] E-crit (kV/cm)
Density (g/cm3)
Viscosity (cP)
Saturates
Aromatic
Resins
Asph.
IFE
TAN
MW (g/mol)
Wax
0.49
0.85
6.05
51.29
37.46
9.83
1.22
9.72
0.0
267.86
–
2000 rpm for 4 min. The surfactant used was a non-ionic surfactant, SERDOX, at concentration of 200 ppm. The stable emulsion was continuously stirred and thermoregulated in the storage tank (Fig. 1). The pH was kept at 4 and adjusted using HCl 1 N at temperature of 25 +1 C. The oil concentration was 350 ppm and salinity of 3.5%. Solid particles of kaolin were added at a concentration of 50 ppm. Scaling and corrosion inhibitor, concentration 10 ppm, was added to the solution to make the final composition of the produced water analogue. Droplet size distribution was measured by optical sensing and visual analyzes using a Jorin ViPA as shown in Fig. 2. The oil droplet characteristic was
modelled to mimic the produced water effluent from a hydrocylone (<15 mm). 2.3. Simulation membrane fouling The membrane was fouled with the analogue produced water for 2 h. The experiment was carried out with crossflow at 1 m/s and constant flux mode operation. The flux for 0.1, 0.2 and 0.5 mm were 80, 140 and 175 L/m2 h (LMH) respectively. Different fluxes were applied to obtain a comparable TMP growth for each membrane pore size. Both retentate and permeate were circulated back to the feed tank. The permeate transmembrane pressure (TMP) for
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163
100 % 90 % 80 %
Oil
Proportion
70 %
Solid
60 % 50 % 40 % 30 % 20 % 10 % 0% 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28
Particle size (µm)
Particle size (µm) 40 35 30 25 20 15 10 5 0 14
14.2
14.4
14.6
14.8
15
Time (h)
Fig. 2. Droplet size distribution at 25 C.
each membrane pore size during the filtration was monitored to observe the progression of the fouling process to confirm that a reproducible fouling was generated. 2.4. Cleaning procedure The cleaning procedure consisted of flushing with distilled water followed by the chemical sequence. The chemical cleaning was done using two alkaline solutions i.e. Ultrasil 115 (Henkel-Ecolab Ltd.) and Derquimþ (Panreac) and two acid solutions i.e. Ultrasil 73 (HenkelEkolab Ltd.) and SurfactrondCD50 (Champion Tech). Ultrasil 115 is a mixture of KOH solution, Derquimþ solution consists of a mixture of ionic and non-ionic surfactant, Surfactron CD50 is a mixture of non-ionic surfactant and organic acid, and Ultrasil 73 is a mixture of EDTA
solution. The cleaning sequences are shown in Table 2. The assessment of each cleaning agent efficiency was carried out for alkaline as the first phase (1,2) or acid solution as the first phase (3,4). An assessment of alkaline/acid sequence and acid/alkaline sequence was done using cleaning number 2 and 3. The effect of temperature was investigated for acid/alkaline sequence (4–6). Further, the effect of concentration of alkaline (Derquimþ, cleaning number 2,9,10) as the first phase and acid (SurfactronCD50, cleaning number 4,7,8) also as the first step were investigated. In all cleaning tests, the flux was tested to ensure the similar flux. Cleaning solutions were made in 15 L of distilled water. The cleaning solution was operated at TMP of 0.85 + 0.1 bar and crossflow 5.5 m/s. The first 5 L of solution was run through the membrane and discharged while the rest was
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Table 2 Formulated cleaning agent condition Cleaning no.
Phase
Solution
pH
Operating condition (Temp, TMP)
1
1st 2nd
1%v–Ultrasil 115 1%v–Ultrasil 73
12.2 2.65
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
2
1st 2nd
1%v–Derquimþ 1%v–Ultrasil 73
9.8 2.65
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
3
1st 2nd
1%v–Ultrasil 73 1%v–Derquimþ
2.65 9.8
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
4
1st 2nd
1%v–Surfactron CD50 1%v–Derquimþ
2.25 9.8
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
5
1st 2nd
1%v–Surfactron CD50 1%v–Derquimþ
2.25 9.8
60 C; 0.85 + 0.1 bar 60 C; 0.85 + 0.1 bar
6
1st 2nd
1%v–Surfactron CD50 1%v–Derquimþ
2.25 9.8
45 C; 0.85 + 0.1 bar 45 C; 0.85 + 0.1 bar
7
1st 2nd
0.5%v–Surfactron CD50 1%v–Derquimþ
2.85 9.8
45 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
8
1st 2nd
0.25%v–Surfactron CD50 1%v–Derquimþ
3.0 9.8
45 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
9
1st 2nd
0.5%v–Derquimþ 0.5%v–SurfactronCD50
9.25 2.85
80 C; 0.85 + 0.1 bar 45 C; 0.85 + 0.1 bar
10
1st 2nd
2%v–Derquimþ 0.5%v–SurfactronCD50
9.85 2.85
80 C; 0.85 + 0.1 bar 45 C; 0.85 + 0.1 bar
11
1st 2nd
1%v–Surfactron CD50 1%v–Ultrasil 115
2.25 12.2
80 C; 0.25 + 0.1 bar 80 C; 0.25 + 0.1 bar
circulated in the membrane system. The total time for cleaning was 1 h. After chemical cleaning, the membranes were flushed with distilled water. Cleaning efficiency (Jr) was evaluated by comparing the water flux of a cleaned membrane (Jc) with the new membrane (Jw) under similar conditions and expressed as Jr ¼ Jc/Jw. The feed and permeate concentrations were measured using Gas Chromatography, model Agilent 6890 following the standard modification of ISO 9377-2 [8]. The total hydrocarbon (THC) of feed and permeate for each membrane pore size was measured to evaluate the effect of the cleaning agents to the membranes selectivity.
3. Results and discussion 3.1. Fouling behaviour of membranes The profile of the pressure measured during the crossflow microfiltration (CFMF) of the produced water analogue is shown in Fig. 3 for 0.1, 0.2 and 0.5 mm nominal pore size respectively. Fouling was assessed as a function of increasing TMP. Cleaning with distilled water (Fig. 4) gave a moderate flux recovery from 25% to 45% and decreased with increasing membrane pore size. This indicates that the solid particles in the produced water analogue could penetrate the
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165
0.3
1 0.9 0.8
0.2
Flux recovery
TMP (Bar)
0.25
0.15 0.1 µm 0.2 µm
0.1
0.5 µm
0.05 0 0
20
40 60 80 Operation time (minute)
100
120
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Ultrasil115 0.1 µm
Fig. 3. Fouling type for different membrane pore.
Derquim+
0.2 µm
0.5 µm
Alkalines
Membrane pore size
Fig. 5. Effect of different alkaline solutions. 0.5 0.45
1
0.3
0.9
0.25
0.8
0.2 0.15 0.1 0.05 0 0.1 µm
Distilled water 0.2 µm
0.5 µm
Membrane pore size
Fig. 4. Cleaning fouled membrane.
Flux recovery
Flux recovery
0.4 0.35
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Ultrasil 73 0.1 µm
0.2 µm
SurfactronCD50 0.5 µm Acid
Membrane pore size
pores of the membrane with nominal pore size 0.5 mm whereas for 0.1 mm the solid particles mainly create a cake layer on the membrane surface. Further, the foulant on the surface is not strongly bound to the membrane surface and thus the cleaning by high shear rate is helpful. 3.2. Efficiency of cleaning agent Two different commercial alkaline solutions and two different acid solutions were compared. Fig. 5 demonstrates the efficiency of the alkaline solutions and Fig. 6 the acid for different membrane nominal pore sizes. Fig. 5 shows that Derquimþ has a higher flux recovery in comparison with Ultrasil 115 and SurfactronCD50 in
Fig. 6. Effect of different acid solutions.
comparison with Ultrasil 73 for all membrane pore sizes. Although the pH of Ultrasil 115 is higher than Derquimþ, the surfactant mixture in Derquimþ is more efficient since it has a stronger solubilisation effect on the oil on the membrane surface. The pH of Ultrasil 73 is lower than SurfactronCD50, however the cleaning efficiency is higher for SurfactronCD50. This might be due to the difference of the solution properties where SurfactronCD50 contains surfactant and organic acids whereas the Ultrasil 73 is mainly EDTA solution. Therefore the effectiveness of SurfactronCD50 is stronger due to solubilization of oil and inorganic
166
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foulants. Lindau and Jonsson [9] reported the same cleaning efficiency using Ultrasil 75 (pH 1.5) and ultrasil 70 (pH 2) for membranes fouled by oily wastewater. Derquimþ and SurfactronCD50 were therefore used for further assessment of temperature and concentration effects. Both of these chemical types could not totally clean the membrane in a single step. 3.3. Cleaning efficiency of alkaline/acid sequence
3.4. Assessment of temperature effects Results from Fig. 6 show that the SurfactronCD50 resulted in higher flux recovery compared to Ultrasil 73. The effect of temperature on a SurfactronCD50 and Derquimþ solution with respect to flux recovery was therefore investigated. Temperature was found not to give any significant effect for the flux recovery of SurfactronCD50 above 45 C for all membranes pore sizes (Fig. 8). However, the total flux recovery (Fig. 9) for SurfactronCD50/Derquimþ decreased with decreasing temperature, indicating that Derquimþ overall was more efficient at higher temperatures. 3.5. Assessment of concentration effect The effect of concentration for each acid (SurfactronCD50) and alkaline (Derquimþ) on flux recovery was also investigated. SurfactronCD50
1
1
0.9
0.9
0.8
0.8
Flux recovery
Flux recovery
A comparison of efficiencies between different cleaning sequences of alkaline/acid and acid/ alkaline is shown in Fig. 7a and b. Sequence of alkaline/acid with Derquimþ resulted in flux recovery of 0.96, 0.95 and 0.77 for 0.1, 0.2 and 0.5 mm nominal membrane pore size while acid cleaning with Ultrasil 73 increases the flux recovery up to 0.98, 0.97 and 0.84 for 0.1, 0.2 and 0.5 mm. On the other hand, sequence of acid/alkaline with Ultrasil 73 gave flux recovery of 0.87, 0.87, and 0.44 for 0.1, 0.2 and 0.5 mm and increase up to 0.99, 0.9, and 0.60 for 0.1, 0.2 and 0.5 mm using Derquimþ. The efficiency of alkaline/acid sequence appears to have a better effect than an acid/alkaline sequence. Lindau and Jonsson [9] report that alkaline/acid
is better than acid/alkaline for membranes fouled by oily wastewater. Using XPS and FTIR, they observed that the inorganic elements were higher after cleaning with alkaline agent and thus reduce the cleaning efficiency of acid/alkaline sequence.
0.7 0.6 0.5 0.4 0.3
0.7 0.6 0.5 0.4 0.3 0.2
0.2 0.1
Ultrasil73
0 0.1 µm
0.1
Derquim+ 0.2 µm
Membrane pore size
0.5 µm
Sequence
Fig. 7. (a) Derquimþ/Ultrasil 73 and (b) Ultrasil 73/Derquimþ.
Derquim+
0 0.1 µm
Ultrasil73 0.2 µm
Membrane pore size
0.5 µm
Sequence
S.H.D. Silalahi, T. Leiknes / Desalination 236 (2009) 160–169
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
45°C
0.1 µm
60°C
0.2 µm
0.5 µm
Temp.
80°C
Membrane pore size
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.2 µm
0.5 µm
Membrane pore size
5/60 °C
0.1 µm
6/45 °C
3.6. Comparison of all cleaning tests
4/80 °C
Cleaning
1 0.9
Membrane pore size
0.5 µm
Conc.
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.1 µm
0.2 µm
0.5 µm
Membrane pore size
Fig. 10. (a) Effect of SurfactronCD50 concentration and (b) Effect of Derquimþ concentration.
2 %v
0.2 µm
1 %v
0.1 µm
0.5 %v
Flux recovery
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.25 %v
Flux recovery
Fig. 9. Effect of Temperature to the total cleaning.
A comparison of all cleaning combinations tested are summarised in Table 3 and shown in Fig. 11. Efficiency is given as fraction flux recovery. From Fig. 11, at high temperature (80 C, Cleaning 1–4), the total recovery for two steps
1 %v
Flux recovery
Fig. 8. Effect of temperature to the cleaning with SurfactronCD50.
from cleaning number 4,7,8 as the first phase and Derquimþ from cleaning number 2,9,10 as the first phase was compared at different concentrations and shown in Fig. 10a,b. The cleaning agent removes the foulant by (1) displacing foulants from the membrane surface due to their strong surface adsorption, (2) emulsifying oils, and (3) solubilising hydrophobic foulants by incorporating them into surfactant micelles [10]. As shown in Fig. 10a, b, there is an optimum concentration for all membrane pore sizes. The voidage of each membrane pore sizes increase up to the optimum cleaning agent concentration during the contact of the foulant with the cleaning agent. The SurfactronCD50 has 0.5%–v as the optimum concentration while Derquimþ has 1%–v. Increasing above the optimum concentration gave a lower flux recovery which is assumed to be due to adsorption of the excess concentration.
0.5 %v
Flux recovery
1 0.9 0.8
167
Conc.
168
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Table 3 Flux recovery of total cleaning
Table 4 Membrane foulant removals for different cleaning procedures
No. of cleaning
Flux recovery 0.1 mm
0.2 mm
0.5 mm
1 2 3 4 5 6 7 8 9 10 11
0.98 0.98 0.99 0.98 0.93 0.91 0.91 0.82 0.87 0.89 1.03
0.99 0.97 0.90 0.98 0.99 0.91 0.93 0.84 0.77 0.87 1.06
0.85 0.84 0.60 0.90 0.81 0.81 0.69 0.63 0.53 0.71 1.05
SurfactronCD50/ Ultrasil 115 Ultrasil 115/Ultrasil 73 Derquimþ/Ultrasil 73 Ultrasil 73/Derquimþ
Total hydrocarbon of permeate (ppm) 0.1 mm
0.2 mm
0.5 mm
4.27
3.8
4.94
4.89 4.79 4.65
4.83 4.55 4.79
5.01 4.87 4.19
3.7. Membrane retention in total cleaning The application of different cleaning agents did not change the selectivity of the membrane. Table 4 shows that for all cleaning sequences the total hydrocarbon concentrations in the permeate was almost the same. Therefore, all the cleaning agents are compatible with the membrane.
1.2 1
Flux recovery
Cleaning sequence
0.8 0.6
4. Conclusions
0.4
Biodegradable cleaning agents were assessed in this study. All the cleaning agents used could not be employed as a direct single step to fully restore the fouled membranes. At high temperature, the combination of alkaline and acid gave a good cleaning efficiency except for the 0.5 mm membrane pore size. The total cleaning efficiency was affected by temperature, where higher temperatures gave higher flux recoveries. Reducing the cleaning TMP gave significant flux recovery for the membrane with the largest pore size. Further studies on other cleaning agents and strategies are currently being investigated.
0.2 0.1 µm 0.2 µm Membrane 0.5 µm
0 1
2
3
4
pore size
11
Cleaning number
Fig. 11. Total cleaning at 80 C different TMP.
is quite good except for the 0.5 mm pore size. This could possibly be due to the high TMP during cleaning which could compact the foulant into the membrane pores and thus give a lower flux recovery. The sensitivity of TMP during cleaning was tested by combining the low efficient cleaning agent acid/alkaline (surfactronCD50–1%v/ultrasil 115–1%v) option. Reducing the TMP from 0.85 + 0.1 bar to 0.25 + 0.1 bar showed a significant effect and gave higher total flux recovery.
Acknowledgements This investigations were carried out with financial support from Norwegian Research Council (project number: 163505/S30) and
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several industry partners: Shell Technology Norway AS, Statoil, Total E&P Norge AS, Chevron Energy Technology Company, DNV, Champions Technology and Vetco Aibel AS as a part of the ‘‘TOP Water’’ project. The Scaling and Corrosion Inhibitor, SERDOX, and Surfactron CD50 from Champion Technology are gratefully acknowledged.
References [1] OSPAR. http://www.ospar.org/documents/dbase/ decrecs/recommendations/0604e_Rec%20amending% 20Rec%2001-1.doc (2001). [2] T. Bilstad and E. Espedal, Water Sci. Technol., 34 (1996) 239–246.
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[3] J.M. Lee and T. Frankiewicz, SPE Annual Technical Conference and Exhibition, 2005, Paper Number-95735. [4] R. Liikanen, J. Yli-Kuivila and R. Laukkanen, J. Membr. Sci., 195 (2002) 265. [5] M. Bartlett, M.R. Bird and J.A. Howell, J. Membr. Sci., 105 (1995) 147–157. [6] G. Tragardh, Membrane cleaning, Desalination, 71 (1989) 325–335. [7] A. Hannisdal, R. Orr and J. Sjoblom, J. Disp. Sci. Technol., 28 (2007) 361–369. [8] www.ospar.org/documents/dbase/decrecs/agreements/ 05_15e_reference%20method%20oil%20in% 20produced%20water.doc [9] J. Lindau and A-S. Jonsson, J. Membr. Sci., 87 (1994) 71–78. [10] L.J. Zeman and A.L. Zydney, Microfiltration and Ultrafiltration, Marcel Dekker, New York, 1996, pp. 453–461.
Cleaning strategies in ceramic microfiltration membranes fouled by oil and particulate matter in produced water Sumihar H.D. Silalahi, TorOve Leiknes* NTNU — Norwegian University of Science and Technology, Department of Hydraulic and Environmental Engineering, S.P. Andersensvei 5, N-7491 Trondheim, Norway Tel. þ47 7359 4758; Fax þ47 7359 1298; email: [email protected] Received 30 June 2007; revised accepted 7 October 2007
Abstract Produced water is contaminated water that is extracted together with the oil in oil production operations. Membrane filtration has the potential for a very effective separation of oil from water. The major drawback in applying this technique is the inherent fouling phenomena found in all membrane systems. At a certain point, fouling necessitates an extended cleaning to regain the original permeability of the membrane. This study evaluated the cleaning efficiency of different commercial products that are biodegradable i.e. Ultrasil 115, Ultrasil 73, Surfactron CD 50, Derquimþ, etc on fouled membrane by analogue produced water with membrane nominal pore sizes of 0.1, 0.2 and 0.5 mm. Full restoration of fouled membrane could not be achieved by single cleaning step. The total cleaning efficiency depends on temperature, concentration and TMP of cleaning solution. Keywords: Chemical cleaning; Oil emulsion; Microfiltration; Fouling
1. Introduction During oil and gas production water is produced at the wellhead known as produced water. The composition of produced water varies depending on the type and maturity of the reservoirs, however, mainly it consists of water which contains dissolved organic compounds (including hydrocarbons), heavy metals, dissolved
minerals, suspended oil and oil emulsions, solids (sand and silt) and a variety of production chemicals. Current regulations for installations on the Norwegian Continental Shelf (NCS) have set a limit of 30 mg/L of oil in water when produced water is discharged to the sea [1]. Membrane filtration has the potential for a very effective separation of oil from water. Generally, for feedwaters with 0.1–10% of oil content, microfiltration (MF) or ultrafiltration
*Corresponding author. Presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5–9 November 2007, Sydney, Australia 0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.10.063
S.H.D. Silalahi, T. Leiknes / Desalination 236 (2009) 160–169
(UF) has the capability of producing water with less than 10–100 ppm of oil. The major drawback in applying this technique is the inherent fouling phenomena found in all membrane systems. Membrane fouling by oil emulsions is caused by (a) accumulation of oil droplets at the membrane surface (cake layer formation and/or concentration polarization), (b) fouling from oil drops penetrating pores or adsorption onto the membrane surface and (c) fouling layer compaction due to the permeation drag which causes the membrane matrix to slightly reorganize resulting in lowered volume porosity, increasing membrane resistance and thus lower fluxes [2,3]. During operation, membrane fouling causes a progressive decrease of flux and induces a loss of separation efficiency. At a certain point, fouling necessitates an extended cleaning to regain the original permeability of the membrane. Different cleaning protocols and strategies can be employed depending on the separation process and the type of membrane used. This step in a membrane based treatment of produced water will necessarily produce a waste stream. Regulations on the NCS have a strict code as far as types and quantities of chemicals that are used offshore, which includes those applied for membrane cleaning. Due to the stringent regulations of waste generation and discharge on the NCS, an assessment of cleaning agents is required to evaluate if these chemicals represent a secondary pollutant source to the NCS. To comply with regulations, the use of biodegradable cleaning chemicals is an interesting option. However, the optimal cleaning agent will also need to efficiently clean the membrane and chose of compounds and protocol will be a function of the membrane material, foulant characteristic and cleaning procedure i.e. concentration, temperature, pH, pressure, flow and time [3–6]. The main objective of this paper is to evaluate different types of cleaning agents and cleaning protocols/strategies for efficient flux recovery of -Al2O3 tubular ceramic membranes.
161
Membranes with nominal pore sizes of 0.1, 0.2 and 0.5 mm fouled by produced water have been tested. Different commercial products that are biodegradable were evaluated i.e. Ultrasil 115 and Derquimþ (alkaline solutions), Ultrasil 73 and Surfactron CD 50 (acid solutions). The cleaning efficiency after membrane cleaning is determined by assessing flux recovery compared to the permeability of non-fouled membranes. The efficiency of membrane cleaning depends on several parameters i.e. cleaning sequence, concentrations of cleaning agent and temperature of cleaning solution.
2. Experimental section 2.1. Apparatus A -Al2O3 ceramic membranes provided by ECO-CERAMICS with a nominal pore size of 0.1, 0.2 and 0.5 mm were mounted together in a tubular crossflow module. Each membrane has inside dimensions of 0.8 cm inner diameter, 1.1 cm outer diameter and 34 cm length. The effective membrane area was 85 cm2. This experimental design enables direct comparison of the performance of cleaning for different membrane pore sizes. A peristaltic pump was connected to the permeate stream to give a constant permeate flow. The trans membrane pressure was monitored continuously using a pressure transducer, temperature with temperature transducer and pH with pH meter. A data acquisition unit was connected to all transducers to record the data. An illustration of the membrane filtration device is shown in Fig. 1. 2.2. Feed preparation for fouling substance The oil emulsion feed was made using a crude oil with characteristics as shown in Table 1 and diluted with salt water. The dispersed oil was prepared by mixing oil and surfactant in a Ultraturrax homogenizer at a mixing rate of
Permeate
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Retentate
162
Flow Transducer Pump
Pressure transducer
Mixer Rig produced water feed
0.5 µm
T Thermoregulator
Flow Transducer Pump
Pressure transducer
Flow Transducer Pump
Pressure transducer
Valve
0.2 µm
0.1 µm
Pressure transducer
Membrance modules
Pressure transducer
Pump
Computer
Fig. 1. Schematic diagram of experimental setup for crossflow microfiltration. Table 1 Oil characteristics for crude oil at 60 C [7] E-crit (kV/cm)
Density (g/cm3)
Viscosity (cP)
Saturates
Aromatic
Resins
Asph.
IFE
TAN
MW (g/mol)
Wax
0.49
0.85
6.05
51.29
37.46
9.83
1.22
9.72
0.0
267.86
–
2000 rpm for 4 min. The surfactant used was a non-ionic surfactant, SERDOX, at concentration of 200 ppm. The stable emulsion was continuously stirred and thermoregulated in the storage tank (Fig. 1). The pH was kept at 4 and adjusted using HCl 1 N at temperature of 25 +1 C. The oil concentration was 350 ppm and salinity of 3.5%. Solid particles of kaolin were added at a concentration of 50 ppm. Scaling and corrosion inhibitor, concentration 10 ppm, was added to the solution to make the final composition of the produced water analogue. Droplet size distribution was measured by optical sensing and visual analyzes using a Jorin ViPA as shown in Fig. 2. The oil droplet characteristic was
modelled to mimic the produced water effluent from a hydrocylone (<15 mm). 2.3. Simulation membrane fouling The membrane was fouled with the analogue produced water for 2 h. The experiment was carried out with crossflow at 1 m/s and constant flux mode operation. The flux for 0.1, 0.2 and 0.5 mm were 80, 140 and 175 L/m2 h (LMH) respectively. Different fluxes were applied to obtain a comparable TMP growth for each membrane pore size. Both retentate and permeate were circulated back to the feed tank. The permeate transmembrane pressure (TMP) for
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163
100 % 90 % 80 %
Oil
Proportion
70 %
Solid
60 % 50 % 40 % 30 % 20 % 10 % 0% 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28
Particle size (µm)
Particle size (µm) 40 35 30 25 20 15 10 5 0 14
14.2
14.4
14.6
14.8
15
Time (h)
Fig. 2. Droplet size distribution at 25 C.
each membrane pore size during the filtration was monitored to observe the progression of the fouling process to confirm that a reproducible fouling was generated. 2.4. Cleaning procedure The cleaning procedure consisted of flushing with distilled water followed by the chemical sequence. The chemical cleaning was done using two alkaline solutions i.e. Ultrasil 115 (Henkel-Ecolab Ltd.) and Derquimþ (Panreac) and two acid solutions i.e. Ultrasil 73 (HenkelEkolab Ltd.) and SurfactrondCD50 (Champion Tech). Ultrasil 115 is a mixture of KOH solution, Derquimþ solution consists of a mixture of ionic and non-ionic surfactant, Surfactron CD50 is a mixture of non-ionic surfactant and organic acid, and Ultrasil 73 is a mixture of EDTA
solution. The cleaning sequences are shown in Table 2. The assessment of each cleaning agent efficiency was carried out for alkaline as the first phase (1,2) or acid solution as the first phase (3,4). An assessment of alkaline/acid sequence and acid/alkaline sequence was done using cleaning number 2 and 3. The effect of temperature was investigated for acid/alkaline sequence (4–6). Further, the effect of concentration of alkaline (Derquimþ, cleaning number 2,9,10) as the first phase and acid (SurfactronCD50, cleaning number 4,7,8) also as the first step were investigated. In all cleaning tests, the flux was tested to ensure the similar flux. Cleaning solutions were made in 15 L of distilled water. The cleaning solution was operated at TMP of 0.85 + 0.1 bar and crossflow 5.5 m/s. The first 5 L of solution was run through the membrane and discharged while the rest was
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Table 2 Formulated cleaning agent condition Cleaning no.
Phase
Solution
pH
Operating condition (Temp, TMP)
1
1st 2nd
1%v–Ultrasil 115 1%v–Ultrasil 73
12.2 2.65
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
2
1st 2nd
1%v–Derquimþ 1%v–Ultrasil 73
9.8 2.65
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
3
1st 2nd
1%v–Ultrasil 73 1%v–Derquimþ
2.65 9.8
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
4
1st 2nd
1%v–Surfactron CD50 1%v–Derquimþ
2.25 9.8
80 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
5
1st 2nd
1%v–Surfactron CD50 1%v–Derquimþ
2.25 9.8
60 C; 0.85 + 0.1 bar 60 C; 0.85 + 0.1 bar
6
1st 2nd
1%v–Surfactron CD50 1%v–Derquimþ
2.25 9.8
45 C; 0.85 + 0.1 bar 45 C; 0.85 + 0.1 bar
7
1st 2nd
0.5%v–Surfactron CD50 1%v–Derquimþ
2.85 9.8
45 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
8
1st 2nd
0.25%v–Surfactron CD50 1%v–Derquimþ
3.0 9.8
45 C; 0.85 + 0.1 bar 80 C; 0.85 + 0.1 bar
9
1st 2nd
0.5%v–Derquimþ 0.5%v–SurfactronCD50
9.25 2.85
80 C; 0.85 + 0.1 bar 45 C; 0.85 + 0.1 bar
10
1st 2nd
2%v–Derquimþ 0.5%v–SurfactronCD50
9.85 2.85
80 C; 0.85 + 0.1 bar 45 C; 0.85 + 0.1 bar
11
1st 2nd
1%v–Surfactron CD50 1%v–Ultrasil 115
2.25 12.2
80 C; 0.25 + 0.1 bar 80 C; 0.25 + 0.1 bar
circulated in the membrane system. The total time for cleaning was 1 h. After chemical cleaning, the membranes were flushed with distilled water. Cleaning efficiency (Jr) was evaluated by comparing the water flux of a cleaned membrane (Jc) with the new membrane (Jw) under similar conditions and expressed as Jr ¼ Jc/Jw. The feed and permeate concentrations were measured using Gas Chromatography, model Agilent 6890 following the standard modification of ISO 9377-2 [8]. The total hydrocarbon (THC) of feed and permeate for each membrane pore size was measured to evaluate the effect of the cleaning agents to the membranes selectivity.
3. Results and discussion 3.1. Fouling behaviour of membranes The profile of the pressure measured during the crossflow microfiltration (CFMF) of the produced water analogue is shown in Fig. 3 for 0.1, 0.2 and 0.5 mm nominal pore size respectively. Fouling was assessed as a function of increasing TMP. Cleaning with distilled water (Fig. 4) gave a moderate flux recovery from 25% to 45% and decreased with increasing membrane pore size. This indicates that the solid particles in the produced water analogue could penetrate the
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165
0.3
1 0.9 0.8
0.2
Flux recovery
TMP (Bar)
0.25
0.15 0.1 µm 0.2 µm
0.1
0.5 µm
0.05 0 0
20
40 60 80 Operation time (minute)
100
120
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Ultrasil115 0.1 µm
Fig. 3. Fouling type for different membrane pore.
Derquim+
0.2 µm
0.5 µm
Alkalines
Membrane pore size
Fig. 5. Effect of different alkaline solutions. 0.5 0.45
1
0.3
0.9
0.25
0.8
0.2 0.15 0.1 0.05 0 0.1 µm
Distilled water 0.2 µm
0.5 µm
Membrane pore size
Fig. 4. Cleaning fouled membrane.
Flux recovery
Flux recovery
0.4 0.35
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Ultrasil 73 0.1 µm
0.2 µm
SurfactronCD50 0.5 µm Acid
Membrane pore size
pores of the membrane with nominal pore size 0.5 mm whereas for 0.1 mm the solid particles mainly create a cake layer on the membrane surface. Further, the foulant on the surface is not strongly bound to the membrane surface and thus the cleaning by high shear rate is helpful. 3.2. Efficiency of cleaning agent Two different commercial alkaline solutions and two different acid solutions were compared. Fig. 5 demonstrates the efficiency of the alkaline solutions and Fig. 6 the acid for different membrane nominal pore sizes. Fig. 5 shows that Derquimþ has a higher flux recovery in comparison with Ultrasil 115 and SurfactronCD50 in
Fig. 6. Effect of different acid solutions.
comparison with Ultrasil 73 for all membrane pore sizes. Although the pH of Ultrasil 115 is higher than Derquimþ, the surfactant mixture in Derquimþ is more efficient since it has a stronger solubilisation effect on the oil on the membrane surface. The pH of Ultrasil 73 is lower than SurfactronCD50, however the cleaning efficiency is higher for SurfactronCD50. This might be due to the difference of the solution properties where SurfactronCD50 contains surfactant and organic acids whereas the Ultrasil 73 is mainly EDTA solution. Therefore the effectiveness of SurfactronCD50 is stronger due to solubilization of oil and inorganic
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foulants. Lindau and Jonsson [9] reported the same cleaning efficiency using Ultrasil 75 (pH 1.5) and ultrasil 70 (pH 2) for membranes fouled by oily wastewater. Derquimþ and SurfactronCD50 were therefore used for further assessment of temperature and concentration effects. Both of these chemical types could not totally clean the membrane in a single step. 3.3. Cleaning efficiency of alkaline/acid sequence
3.4. Assessment of temperature effects Results from Fig. 6 show that the SurfactronCD50 resulted in higher flux recovery compared to Ultrasil 73. The effect of temperature on a SurfactronCD50 and Derquimþ solution with respect to flux recovery was therefore investigated. Temperature was found not to give any significant effect for the flux recovery of SurfactronCD50 above 45 C for all membranes pore sizes (Fig. 8). However, the total flux recovery (Fig. 9) for SurfactronCD50/Derquimþ decreased with decreasing temperature, indicating that Derquimþ overall was more efficient at higher temperatures. 3.5. Assessment of concentration effect The effect of concentration for each acid (SurfactronCD50) and alkaline (Derquimþ) on flux recovery was also investigated. SurfactronCD50
1
1
0.9
0.9
0.8
0.8
Flux recovery
Flux recovery
A comparison of efficiencies between different cleaning sequences of alkaline/acid and acid/ alkaline is shown in Fig. 7a and b. Sequence of alkaline/acid with Derquimþ resulted in flux recovery of 0.96, 0.95 and 0.77 for 0.1, 0.2 and 0.5 mm nominal membrane pore size while acid cleaning with Ultrasil 73 increases the flux recovery up to 0.98, 0.97 and 0.84 for 0.1, 0.2 and 0.5 mm. On the other hand, sequence of acid/alkaline with Ultrasil 73 gave flux recovery of 0.87, 0.87, and 0.44 for 0.1, 0.2 and 0.5 mm and increase up to 0.99, 0.9, and 0.60 for 0.1, 0.2 and 0.5 mm using Derquimþ. The efficiency of alkaline/acid sequence appears to have a better effect than an acid/alkaline sequence. Lindau and Jonsson [9] report that alkaline/acid
is better than acid/alkaline for membranes fouled by oily wastewater. Using XPS and FTIR, they observed that the inorganic elements were higher after cleaning with alkaline agent and thus reduce the cleaning efficiency of acid/alkaline sequence.
0.7 0.6 0.5 0.4 0.3
0.7 0.6 0.5 0.4 0.3 0.2
0.2 0.1
Ultrasil73
0 0.1 µm
0.1
Derquim+ 0.2 µm
Membrane pore size
0.5 µm
Sequence
Fig. 7. (a) Derquimþ/Ultrasil 73 and (b) Ultrasil 73/Derquimþ.
Derquim+
0 0.1 µm
Ultrasil73 0.2 µm
Membrane pore size
0.5 µm
Sequence
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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
45°C
0.1 µm
60°C
0.2 µm
0.5 µm
Temp.
80°C
Membrane pore size
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.2 µm
0.5 µm
Membrane pore size
5/60 °C
0.1 µm
6/45 °C
3.6. Comparison of all cleaning tests
4/80 °C
Cleaning
1 0.9
Membrane pore size
0.5 µm
Conc.
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.1 µm
0.2 µm
0.5 µm
Membrane pore size
Fig. 10. (a) Effect of SurfactronCD50 concentration and (b) Effect of Derquimþ concentration.
2 %v
0.2 µm
1 %v
0.1 µm
0.5 %v
Flux recovery
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.25 %v
Flux recovery
Fig. 9. Effect of Temperature to the total cleaning.
A comparison of all cleaning combinations tested are summarised in Table 3 and shown in Fig. 11. Efficiency is given as fraction flux recovery. From Fig. 11, at high temperature (80 C, Cleaning 1–4), the total recovery for two steps
1 %v
Flux recovery
Fig. 8. Effect of temperature to the cleaning with SurfactronCD50.
from cleaning number 4,7,8 as the first phase and Derquimþ from cleaning number 2,9,10 as the first phase was compared at different concentrations and shown in Fig. 10a,b. The cleaning agent removes the foulant by (1) displacing foulants from the membrane surface due to their strong surface adsorption, (2) emulsifying oils, and (3) solubilising hydrophobic foulants by incorporating them into surfactant micelles [10]. As shown in Fig. 10a, b, there is an optimum concentration for all membrane pore sizes. The voidage of each membrane pore sizes increase up to the optimum cleaning agent concentration during the contact of the foulant with the cleaning agent. The SurfactronCD50 has 0.5%–v as the optimum concentration while Derquimþ has 1%–v. Increasing above the optimum concentration gave a lower flux recovery which is assumed to be due to adsorption of the excess concentration.
0.5 %v
Flux recovery
1 0.9 0.8
167
Conc.
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Table 3 Flux recovery of total cleaning
Table 4 Membrane foulant removals for different cleaning procedures
No. of cleaning
Flux recovery 0.1 mm
0.2 mm
0.5 mm
1 2 3 4 5 6 7 8 9 10 11
0.98 0.98 0.99 0.98 0.93 0.91 0.91 0.82 0.87 0.89 1.03
0.99 0.97 0.90 0.98 0.99 0.91 0.93 0.84 0.77 0.87 1.06
0.85 0.84 0.60 0.90 0.81 0.81 0.69 0.63 0.53 0.71 1.05
SurfactronCD50/ Ultrasil 115 Ultrasil 115/Ultrasil 73 Derquimþ/Ultrasil 73 Ultrasil 73/Derquimþ
Total hydrocarbon of permeate (ppm) 0.1 mm
0.2 mm
0.5 mm
4.27
3.8
4.94
4.89 4.79 4.65
4.83 4.55 4.79
5.01 4.87 4.19
3.7. Membrane retention in total cleaning The application of different cleaning agents did not change the selectivity of the membrane. Table 4 shows that for all cleaning sequences the total hydrocarbon concentrations in the permeate was almost the same. Therefore, all the cleaning agents are compatible with the membrane.
1.2 1
Flux recovery
Cleaning sequence
0.8 0.6
4. Conclusions
0.4
Biodegradable cleaning agents were assessed in this study. All the cleaning agents used could not be employed as a direct single step to fully restore the fouled membranes. At high temperature, the combination of alkaline and acid gave a good cleaning efficiency except for the 0.5 mm membrane pore size. The total cleaning efficiency was affected by temperature, where higher temperatures gave higher flux recoveries. Reducing the cleaning TMP gave significant flux recovery for the membrane with the largest pore size. Further studies on other cleaning agents and strategies are currently being investigated.
0.2 0.1 µm 0.2 µm Membrane 0.5 µm
0 1
2
3
4
pore size
11
Cleaning number
Fig. 11. Total cleaning at 80 C different TMP.
is quite good except for the 0.5 mm pore size. This could possibly be due to the high TMP during cleaning which could compact the foulant into the membrane pores and thus give a lower flux recovery. The sensitivity of TMP during cleaning was tested by combining the low efficient cleaning agent acid/alkaline (surfactronCD50–1%v/ultrasil 115–1%v) option. Reducing the TMP from 0.85 + 0.1 bar to 0.25 + 0.1 bar showed a significant effect and gave higher total flux recovery.
Acknowledgements This investigations were carried out with financial support from Norwegian Research Council (project number: 163505/S30) and
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several industry partners: Shell Technology Norway AS, Statoil, Total E&P Norge AS, Chevron Energy Technology Company, DNV, Champions Technology and Vetco Aibel AS as a part of the ‘‘TOP Water’’ project. The Scaling and Corrosion Inhibitor, SERDOX, and Surfactron CD50 from Champion Technology are gratefully acknowledged.
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[3] J.M. Lee and T. Frankiewicz, SPE Annual Technical Conference and Exhibition, 2005, Paper Number-95735. [4] R. Liikanen, J. Yli-Kuivila and R. Laukkanen, J. Membr. Sci., 195 (2002) 265. [5] M. Bartlett, M.R. Bird and J.A. Howell, J. Membr. Sci., 105 (1995) 147–157. [6] G. Tragardh, Membrane cleaning, Desalination, 71 (1989) 325–335. [7] A. Hannisdal, R. Orr and J. Sjoblom, J. Disp. Sci. Technol., 28 (2007) 361–369. [8] www.ospar.org/documents/dbase/decrecs/agreements/ 05_15e_reference%20method%20oil%20in% 20produced%20water.doc [9] J. Lindau and A-S. Jonsson, J. Membr. Sci., 87 (1994) 71–78. [10] L.J. Zeman and A.L. Zydney, Microfiltration and Ultrafiltration, Marcel Dekker, New York, 1996, pp. 453–461.