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Influence of the use of surfactants in the treatment of produced water by ceramic membranes
Journal of Water Process Engineering 32 (2019) 100955
Contents lists available at ScienceDirect
Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Influence of the use of surfactants in the treatment of produced water by ceramic membranes
T
S.E. Weschenfeldera,b, , M.J.C. Fonsecab, B.R.S. Costaa, C.P. Borgesb ⁎
a b
Petrobras Research Center, Rio de Janeiro, Brazil COPPE/Chemical Engineering Program, Federal University of Rio de Janeiro, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Produced water Surfactants Enhanced oil recovery Ceramic membranes Treatment
Surfactant-enhanced oil recovery is a method used to augment residual oil extraction from reservoirsHowever, this kind of procedure can produce stable emulsions of water/oil and oil/water, making the conventional treatment of the produced water more difficult. This study evaluates the performance of ultrafiltration process using ceramic membranes for produced water treatment containing the cationic surfactant Dodecyltrimethylammonium bromide (DTAB). To better understand the process, the effect of DTAB and sodium chloride (NaCl) on interfacial properties of oil/water and oil/water/membrane, such as surface tension, membrane wetting, zeta potential and adhesion map were investigated. It was observed that the surfactant facilitates the oil phase adhesion to the membrane surface, causing a permeate flux reduction. On the other hand, the presence of NaCl in the oily emulsion containing DTAB lead to a permeate flux increase, which indicated that ionic strength modifies the interaction between the oil phase and the membrane surface. Considering the high salinity of the produced water, it is possible to imply that the presence of surfactant can contribute to treatment by ultrafiltration process. Independent of surfactant presence in produced water, the oil and grease (COG) content in the permeate stream was consistently lower than 5 mg/L.
1. Introduction Primary and secondary recovery procedures are capable of recovering only about 35% of the oil contained in the reservoir [1]. In order to extract residual oil in mature reservoirs, enhanced oil recovery (EOR) methods are applied, among them chemical addition methods can be highlighted. The purpose of chemicals is to modify the interaction between the fluid injected and the fluid contained in the reservoir with the rock. Three chemical categories are usually applied - polymers, surfactants and alkaline products - either together or separately [2]. Injection of surfactant solutions has proven to be very efficient in increasing the degree of oil recovery. It its caused by the amphiphilic characteristic of the surfactant, i.e., each molecule has two functional groups, a polar group (hydrophilic) and a nonpolar group (hydrophobic), which are capable of interacting with both water and oil, leading to an improvement in the sweep efficiency and thus the recovery factor. The hydrophobic group is generally comprised of a long hydrocarbon chain (C8 and C18), which may be branched or not, while the hydrophilic group is formed by moieties such as carboxylates, sulfates, sulfonates (anionic), alcohols (nonionic) and quaternary ammonium salts (cationic) [3]. Due to this structure, the surfactant can be
⁎
Corresponding author at: Petrobras Research Center, Rio de Janeiro, Brazil. E-mail address: [email protected] (S.E. Weschenfelder).
https://doi.org/10.1016/j.jwpe.2019.100955 Received 17 July 2019; Accepted 8 September 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
used to emulsify the oil by the strong reduction of interfacial tension (IFT) or to alter the oil wettability in the reservoir [4]. According to Gary and Handwerk [5], in presence of dissolved salt, the surfactants accumulate at the interface oil/water, intensifying formation of an adsorbed film and IFT reduction. Depending on the properties of the crude oil (API gravity, viscosity, sulfur content, salts, metals, among others), the structure of adsorbed surfactant film can vary significantly. The molecular packing, surface viscosity, surface elasticity and surface charge of the adsorbed film are key parameters that determine several phenomena, such as the coalescence of emulsion droplets as well as the migration of oil droplets in porous media [3]. Different surfactants have been investigated with the purpose of altering the wettability of the reservoir rock and, consequently, increasing the oil recovery factor [6]. Among these products, the cationics ones have the advantage of having the same surface charge as carbonate minerals. Standnes and Austad [7] showed that cationic surfactants, such as Dodecyltrimethylammonium Bromide (DTAB), are highly efficient for this purpose, being able to recover approximately 70% of original oil in place. According to the authors, this high rate could be related to the same surface charge presented by the carbonate minerals and the surfactant. Despite the many advantages related to advanced
Journal of Water Process Engineering 32 (2019) 100955
S.E. Weschenfelder, et al.
recovery chemical methods, an important aspect to consider is the impact of chemicals in the produced water treatment cycle. Injection of surfactants can produce stable emulsions of water/oil and oil/water, making the water treatment more difficult and less cost effective due to the need of more specific additives and equipment, in order to keep the topside processing system efficient. According to Argillier et al. [1] and Deng et al. [8], the additional treatment cost is one of the causes that limit application of this technology. Najamudin et al. [9] mentioned that the use of surfactants could increase the water toxicity and affect the ecosystem. Hence, produced water reuse (reinjection) can become indispensable when an EOR is applied for oil recovery. As pointed out by Weschenfelder et al. [10,11], the use of ceramic membranes has shown great potential for produced water treatment specially when the goal is the reinjection in restrictive reservoirs, i.e. low water injectivity, by making the effluent practically free of suspended solids and dispersed oils and greases. In this sense, the objective of this work was to evaluate the effect a surfactant has on the ultrafiltration process using ceramic membranes for produced water treatment. DTAB was chosen for this study due to its potential application for EOR. For a better understanding of the process, the effect of DTAB and salinity on interfacial properties of water/oil/ membrane system, by measuring surface tension, wetting, electrokinetic potential and oil adhesion, was investigated.
2.4. Interfacial properties 2.4.1. CMC determination Surfactant critical micelle concentration (CMC) was determined by measuring the interfacial tension of the solutions with a spinning drop tensiometer (SVT 20 - DataPhysics). Each measurement was performed after oil/solution contact time of twenty minutes. 2.4.2. Contact angle determination The pendant drop method was applied to evaluate the contact angle between the oil droplets and the membrane surface by using OCA 15 Dataphysics goniometer. For the evaluation, one drop of oil (1 μL) was placed in contact with the membrane surface immersed in the solution. For each experimental condition the static drop image was captured. It is noteworthy that for these experiments ZrO2 membranes were supplied by Likuid Nanotek in flat conformation with similar characteristics to tubular membranes used in the permeation tests. 2.4.3. Adhesion mapping The adhesion test was carried out in order to verify the oil/membrane adhesion in presence of different solutions. For the evaluation, 2 g of a flat membrane of ZrO2 was crushed and immersed in different solutions. After a contact time of 5 min, 1 mL of oil was added to the bottom of each graduated cylinder with the assistance of a syringe. After 15 min of contact with the membrane, the volume of supernatant oil and its adhesion to the membrane material were observed.
2. Material and methods 2.1. Membrane
2.4.4. Zeta potential The zeta potential of all emulsions were measured by a Zetasizer Nano ZS90 with the purpose of evaluating the influence of salinity on the mobility and charge of oil particles.
This study was carried out with zirconium oxide supported by TiO2/ Al2O3 multi-channel membranes acquired from Likuid Nanotek, with 0.1 μm pore size, 0.003 m channel diameter, 0.2 m length and 0.0019 m2 surface area.
3. Results and discussion
2.2. Synthetic produced water
3.1. Process efficiency
Crude oil (28.1° API gravity) was emulsified at 50 °C by using Ultra Turrax (Model T-50) at the speed of 11,000 rpm during 5 min. The average concentration of oil and grease (COG) and salt content (CS) were kept at 100 ± 5 mg/L and 100,000 mg/L, respectively, which is usually found in offshore treatment units. For sake of comparison, oily water with no salinity was also investigated. Due to the difficulty of predicting surfactant concentration in produced water, it was decided to evaluate its influence up to the maximum concentration commonly injected (1,000 mg/L). The emulsified particle size was then determined by Malvern Mastersizer 3000E equipment. The procedure and conditions applied in this work were based on the study conducted by Weschenfelder et al. [10,12]. 2.3. Permeation of synthetic produced water
3.1.1. Permeate quality The oil content of all the permeate samples collected after permeation with ceramic membranes was lower than 5 mg / L. Similar results were reported in other studies and indicate that the water can be considered appropriate for reinjection even in restrictive petroleum reservoirs [11,14]. Absorbance analysis results showed that the membrane selected for this study (mean pore size of 0.1 μm) did not retain the surfactant DTAB. This was expected since the molecular size of the surfactant is much smaller than the membrane pore size. This fact can represent a saving of surfactant, because it will reduce the make up needed during reinjection into the reservoir.
The permeation set-up used in the tests is shown in Fig. 1. It consists of a permeation module, capable of accommodating a permeation element (ceramic membrane), a positive displacement recirculation pump, pressure gauges, flow meter, feed tank with mechanical stiring and a tank for permeate mass data collection. The transmembrane pressure (ΔP) of 2.0 bar and across flow velocity of 2.0 m/s (Re ≈ 6000) were selected according to the values proposed by Weschenfelder et al. [10]. As stated by these authors, the permeation time of 60 min can be considered sufficient for the permeate flux stabilization. The feed temperature (50 ± 2 °C) was chosen considering normal operating conditions found in offshore treatment plants of produced water [13]. During permeation experiments COG and surfactant concentration were periodically measured in feed and permeate streams. COG and DTAB concentration were measured with a Spectrophotometer HACH DR 2800 (according to the ASTM D3921 method) and a Spectrophotometer UV–vis HACH DR 6000, respectively.
3.1.2. Permeate flux Fig. 2 shows the normalized permeate flux as a function of DTAB concentration in synthetic produced water with (NaCl concentration of 100,000 mg/L) and without salt added. The permeate flux was normalized by the initial flux (J/J0) to allow comparison of solutions with different viscosities and to minimize experimental fluctuations inherent in the ultrafiltration process. In this figure it is possible to observe different behaviors of normalized permeate flux as a function of salinity. As the concentration of DTAB increases in the emulsion the permeate flux reduces for the NaCl-free emulsion, while the inverse behavior was observed for emulsion with 100,000 mg/L NaCl. At highest DTAB concentration the permeate flux for the emulsion with salt was approximately 6 times higher than the one obtained with the solution with no salt added. The permeate flux augment in the combined presence of DTAB and salinity indicates that the presence of this surfactant may favor the treatment of the produced water by ultrafiltration process. For a better 2
Journal of Water Process Engineering 32 (2019) 100955
S.E. Weschenfelder, et al.
Fig. 1. Photograph and schematic representation of the permeation set-up.
ionic species [15]. It is important to highlight that the concentration of the water injected in the reservoirs is commonly much lower than the CMC (3,500 mg/L) observed in emulsions with no salt added. 3.1.4. Particle size distribution Particle size distribution was determined for the emulsions containing 100 mg/L of oils and greases, 1,000 mg/L of surfactant DTAB, with 0 and 100,000 mg/L NaCl, as portrayed in Fig. 4. Both samples had particle with sizes comparable to the values commonly found in produced water [13]. In this figure, it is possible to observe that the particle size distribution shifts for smaller sizes for emulsion with 100,000 mg/L of NaCl. This fact can be related to the reduction of approximately 15 times in the critical micelle concentration (CMC), as can be observed in Fig. 3. Lower value of CMC increases the stability of oil droplets dispersed in the produced water. Although there is a reduction in particle size, the disperse phase is still significantly larger than the membrane pore size. Furthermore, higher emulsion stability should diminishes membrane fouling by oil deposition, resulting in higher permeate fluxes.
Fig. 2. Normalized permeate flux versus DTAB concentration. (COG = 100 mg/L, T =50 °C, ΔP =2 bar, Re ≈ 6,000)
understanding of these results the oil particle size distribution and interfacial properties related to the process were investigated. 3.1.3. Critical micelle concentration Fig. 3 shows the interfacial tension values for the emulsions prepared with different DTAB concentration with the presence and absence of NaCl. The value of the CMC can be verified at the beginning of the asymptotic region of the interfacial tension, determined by intersection of two curves. As expected, the CMC value is strongly influenced by the solution salinity. In the study, the CMC observed in the solution containing 100 g/L of NaCl was approximately 200 mg/L. Emulsion with 100,000 mg/L NaCl exhibit a high reduction in the CMC (200 mg/L), which favors the oil phase emulsification. This result is related to the surfactant solubility reduction with salinity, intensifying surface segregation phenomenon due to strong interaction between water and
3.1.5. Contact angle The contact angle between oil droplet and the membrane surface was determined by the verification of the interaction between the dispersed phase and the membrane surface in the presence of surfactant and NaCl. Fig. 5 shows the static images of oil droplets in contact with the membrane surface immersed in solutions with different DTAB and NaCl concentrations. When the membrane was immersed in water without surfactant and NaCl, the interaction with the oil phase is low (Fig. 5a), which
Fig. 4. Particle size distribution in synthetic produced water containing DTAB surfactant.
Fig. 3. Interfacial tension as function of DTAB concentration. 3
Journal of Water Process Engineering 32 (2019) 100955
S.E. Weschenfelder, et al.
Fig. 6. Photographs of different vessels with membrane crushed samples used in the adhesion mapping tests. (a) DTAB = 0 mg/L, Cs = 0 mg/L; (b) DTAB = 1,000 mg/L, Cs = 0 mg/L; (c) DTAB = 1,000 mg/L, Cs = 100,000 mg/ L.
1,000 mg/L DTAB and 100,000 mg/L NaCl, the oil trapped with the membrane particles was almost completely displaced. On the other hand, greater oil phase adhesion to the membrane material was observed when only surfactant was present in the solution. This result corroborates with previous analysis of membrane material hydrophilicity and its low affinity for the oil phase. The presence of surfactant facilitates oil adhesion to the membrane, which is reversed in high ionic strength solutions. The permeate flux is a consequence of these effects. 3.1.7. Zeta potential The electrochemical properties of dispersed particles may significantly impact the nature and magnitude of membrane fouling causing a decline in the permeate flux. Fig. 7 shows the zeta potential measured in oil emulsions with different salinities and DTAB concentration. In absence of DTAB, the oil droplets exhibit negative charges as indicated by the zeta potential value. However, the addition of DTAB neutralizes these charges even leading to signal inversion and zeta potential intensity, which can be attributed to surfactant adsorption on the dispersed particles surface. Another aspect observed in Fig. 7 is the salinity influence on the zeta potential. As expected, there is reduction of zeta potential value with ionic strength increase, i.e., the electric double layer undergoes a contraction, reducing charge intensity in the solution. NaCl concentrations above 50,000 mg/L bring the zeta potential value to approximately 0 mV. As commonly known, the ZrO2 membrane isoelectric point is approximately 6. This point indicates the equality between positive and negative charges. Only at pH values lower than the isoelectric point
Fig. 5. Static images of oil droplets in contact with the membrane surface immersed in different solutions. (a) DTAB = 0 mg/L, Cs = 0 mg/L; (b) DTAB = 1,000 mg/L, Cs = 0 mg/L; (c) DTAB = 1,000 mg/L, Cs = 100,000 mg/ L.
characterizes a hydrophilic nature of the membrane. However, increase of DTAB concentration alters the interaction between the membrane and the oil phase (Fig. 5b), which can be attributed to the amphiphilic characteristic of the surfactant, facilitating wetting of the membrane by the oil. This phenomenon has a direct impact on membrane fouling and may justify low values of permeate flux observed in this condition. When the solution contains 1,000 mg/L of DTAB and 100,000 mg/L of NaCl (Fig. 5c) the membrane interaction with the oil is also reduced. It can be attributed to the increase of ionic strength in the solution, which enhances interaction between the DTAB molecules and the oil phase, as evidenced by CMC reduction (Fig. 3) and decrease of oil droplets mean size (Fig. 4). 3.1.6. Adhesion mapping In order to qualitatively evaluate the membrane fouling by the oil phase as a function of the solution composition, adhesion mapping tests were performed (Fig. 6). In these tests, as presented in the experimental methodology, the membrane was crushed and immersed in solutions with different compositions. When the membrane material was immersed in water without surfactant and NaCl, as well as when immersion occurred in solution with
Fig. 7. Zeta potential of emulsions as a function of DTAB and NaCl concentration (pH = 6,5, COG = 100 mg/L). 4
Journal of Water Process Engineering 32 (2019) 100955
S.E. Weschenfelder, et al.
(pH < 6) the membrane has a positive zeta potential [16]. At the produced water pH (commonly around 6–8) a low and negative value of the membrane zeta potential is expected. Hence, the absence of DTAB and NaCl in the emulsion lead to an electro repulsion between the membrane surface and the dispersed oil phase, as evidenced by the oil displacement observed in the adhesion map. On the other hand, the presence of DTAB in the solution prepared without NaCl changes the zeta potential value leading to electro attraction between oil droplets and membrane surface, which can justify the sharp drop in the permeate flux. Moreover, high ionic strength reduces electrostatic interactions by compression of the double electric layer and enhances interaction between the DTAB and the oil phase, as evidenced by CMC reduction. This condition also reduces oil adhesion to the membrane surface, diminishing fouling and favoring water permeation as evidenced by the permeate flux increase.
References [1] J.F. Argillier, I. Henaut, C. Noik, R. Viera, F. Roca Leon, B. Aanesen, Influence of chemical EOR on topside produced Water management, SPE Improved Oil Recovery Symposium, 12-16 April, Tulsa, Oklahoma, USA, 2014 SPE-169067-MS. [2] P. Dwyer, E. Delamaide, Produced Water treatment – preparing for EOR projects, SPE Produced Water Handling and Management Symposium, 20–21 May, Galveston, Texas, USA, 2015 SPE-174537-MS. [3] A.A.A. Olajire, Review of ASP EOR (alkaline surfactant polymer enhanced oil recovery) technology in the petroleum industry: prospects and challenges, Energy 77 (2014) 963–982. [4] J.J. Sheng, Comparison of the effects of wettability alteration and IFT reduction on oil recovery in carbonate reservoirs, Asia-Pac. J. Chem. Eng 8 (2013) 154 e 161. [5] J.H. Gary, G.E. Handwerk, Petroleum refining: technology and economics, 3rd ed., Marcel Dekker, New York, 1994. [6] A. Seethepalli, B. Adibhatla, K.K. Mohanty, Wettability alteration during surfactant flooding of carbonate reservoirs, SPE/DOE Fourteenth Symposium on Improved Oil Recovery, 17-21 Apr, Tulsa, Oklahoma, USA, 2004 SPE 89423. [7] D.C. Standnes, T. Austad, Wettability alteration in carbonates Interaction between cationic surfactant and carboxylates as a key factor in wettability alteration from oil-wet to water-wet conditions, Colloids Surf. A: Physicochem. Eng. Aspects 216 (2003) 243–259. [8] Shubo Deng, Gang Yu, Zhanpeng Jiang, Ruiquan Zhang, Yen Peng Ting, Destabilization of oil droplets in produced water from ASP flooding, Colloids Surf. A Physicochem. Eng. Asp. 252 (2005) 113–119. [9] K.E. Najamudin, N.H. Halim, I.K. Salleh, I.C.C. Hsia, M.Y. Yusof, M.F. Sedaralit, Chemical EOR produced Water management at Malay Basin Field, . Offshore Technology Conference - Asia, 25-28 March, Kuala Lumpur, Malaysia, 2014 OTC24804-MS. [10] S.E. Weschenfelder, C.P. Borges, J.C. Campos, Oilfield produced water treatment by ceramic membranes: mass transfer and process efficiency analysis for model solutions, Sep. Sci. Technol. 50 (2015) 2190–2197. [11] S.E. Weschenfelder, A.M.T. Louvisse, C.P. Borges, E. Meabe, J. Izquierdo, J.C. Campos, Evaluation of ceramic membranes for oilfied produced water treatment aiming reinjection in offshore units, J. Pet. Sci. Eng. 131 (2015) 51–57. [12] S.E. Weschenfelder, M.J.C. Fonseca, C.P. Borges, J.C. Campos, Application of ceramic membranes for water management in offshore oil production platforms: process design and economics, Sep. Purif. Technol. 171 (2016) 214–220. [13] M. Stewart, K. Arnold, Emulsions and oil treating equipment: selection, sizing and troubleshooting, Gulf Professional Publishing, Amsterdam, 2009 304 p. [14] S.E. Weschenfelder, C.P. Borges, J.C. Campos, Oilfied produced water treatment by ceramic membranes: bench and pilot scale evaluation, J. Membr. Sci. 495 (2015) 242–251. [15] A.W. Adamson, Gast. A.P. Physical Chemistry of Surfaces, 6th ed., Wiley, New York, 1997, https://doi.org/10.1126/science.160.3824.179. [16] T. Fengqiu, H. Xiaoxian, Z. Yufeng, G. Jingkun, Effect of dispersants on surface chemical properties of nano-zirconia suspensions, Ceram. Int. 26 (2000) 93–97.
4. Conclusions The ultrafiltration process with ceramic membranes can be considered an effective alternative for produced water treatment even when cationic surfactants are present in the effluent. DTAB was studied as typical surfactant added to injection water, which should be present in the produced water. Interfacial phenomena investigation allowed the broadening of the understanding of the effects promoted by the presence of surfactant in produced water in the separation process. As DTAB concentration increases in the emulsion the oil adhesion to the membrane material is enhaced, resulting in a reduction of the permeate flux. This effect is related to electrical attraction as evidenced by zeta potential. However, produced water has high ionic strength that affects the interaction between emulsified oil droplets and the membrane surface, as well as intensifies adsorption of DTAB in the oil dispersed phase. These phenomena were perceived by low values of zeta potential, reduction of both particle size distribution and membrane wetting by the oily phase. Combination of these effects seems to reduce membrane fouling and facilitate water permeation. The efficiency of the oil phase retention by the membrane was not affected, even at high concentrations of surfactant (1,000 mg/L), being possible to obtain an effluent stream with contents of oils and greases below 5 mg/L.
5
Contents lists available at ScienceDirect
Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Influence of the use of surfactants in the treatment of produced water by ceramic membranes
T
S.E. Weschenfeldera,b, , M.J.C. Fonsecab, B.R.S. Costaa, C.P. Borgesb ⁎
a b
Petrobras Research Center, Rio de Janeiro, Brazil COPPE/Chemical Engineering Program, Federal University of Rio de Janeiro, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Produced water Surfactants Enhanced oil recovery Ceramic membranes Treatment
Surfactant-enhanced oil recovery is a method used to augment residual oil extraction from reservoirsHowever, this kind of procedure can produce stable emulsions of water/oil and oil/water, making the conventional treatment of the produced water more difficult. This study evaluates the performance of ultrafiltration process using ceramic membranes for produced water treatment containing the cationic surfactant Dodecyltrimethylammonium bromide (DTAB). To better understand the process, the effect of DTAB and sodium chloride (NaCl) on interfacial properties of oil/water and oil/water/membrane, such as surface tension, membrane wetting, zeta potential and adhesion map were investigated. It was observed that the surfactant facilitates the oil phase adhesion to the membrane surface, causing a permeate flux reduction. On the other hand, the presence of NaCl in the oily emulsion containing DTAB lead to a permeate flux increase, which indicated that ionic strength modifies the interaction between the oil phase and the membrane surface. Considering the high salinity of the produced water, it is possible to imply that the presence of surfactant can contribute to treatment by ultrafiltration process. Independent of surfactant presence in produced water, the oil and grease (COG) content in the permeate stream was consistently lower than 5 mg/L.
1. Introduction Primary and secondary recovery procedures are capable of recovering only about 35% of the oil contained in the reservoir [1]. In order to extract residual oil in mature reservoirs, enhanced oil recovery (EOR) methods are applied, among them chemical addition methods can be highlighted. The purpose of chemicals is to modify the interaction between the fluid injected and the fluid contained in the reservoir with the rock. Three chemical categories are usually applied - polymers, surfactants and alkaline products - either together or separately [2]. Injection of surfactant solutions has proven to be very efficient in increasing the degree of oil recovery. It its caused by the amphiphilic characteristic of the surfactant, i.e., each molecule has two functional groups, a polar group (hydrophilic) and a nonpolar group (hydrophobic), which are capable of interacting with both water and oil, leading to an improvement in the sweep efficiency and thus the recovery factor. The hydrophobic group is generally comprised of a long hydrocarbon chain (C8 and C18), which may be branched or not, while the hydrophilic group is formed by moieties such as carboxylates, sulfates, sulfonates (anionic), alcohols (nonionic) and quaternary ammonium salts (cationic) [3]. Due to this structure, the surfactant can be
⁎
Corresponding author at: Petrobras Research Center, Rio de Janeiro, Brazil. E-mail address: [email protected] (S.E. Weschenfelder).
https://doi.org/10.1016/j.jwpe.2019.100955 Received 17 July 2019; Accepted 8 September 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
used to emulsify the oil by the strong reduction of interfacial tension (IFT) or to alter the oil wettability in the reservoir [4]. According to Gary and Handwerk [5], in presence of dissolved salt, the surfactants accumulate at the interface oil/water, intensifying formation of an adsorbed film and IFT reduction. Depending on the properties of the crude oil (API gravity, viscosity, sulfur content, salts, metals, among others), the structure of adsorbed surfactant film can vary significantly. The molecular packing, surface viscosity, surface elasticity and surface charge of the adsorbed film are key parameters that determine several phenomena, such as the coalescence of emulsion droplets as well as the migration of oil droplets in porous media [3]. Different surfactants have been investigated with the purpose of altering the wettability of the reservoir rock and, consequently, increasing the oil recovery factor [6]. Among these products, the cationics ones have the advantage of having the same surface charge as carbonate minerals. Standnes and Austad [7] showed that cationic surfactants, such as Dodecyltrimethylammonium Bromide (DTAB), are highly efficient for this purpose, being able to recover approximately 70% of original oil in place. According to the authors, this high rate could be related to the same surface charge presented by the carbonate minerals and the surfactant. Despite the many advantages related to advanced
Journal of Water Process Engineering 32 (2019) 100955
S.E. Weschenfelder, et al.
recovery chemical methods, an important aspect to consider is the impact of chemicals in the produced water treatment cycle. Injection of surfactants can produce stable emulsions of water/oil and oil/water, making the water treatment more difficult and less cost effective due to the need of more specific additives and equipment, in order to keep the topside processing system efficient. According to Argillier et al. [1] and Deng et al. [8], the additional treatment cost is one of the causes that limit application of this technology. Najamudin et al. [9] mentioned that the use of surfactants could increase the water toxicity and affect the ecosystem. Hence, produced water reuse (reinjection) can become indispensable when an EOR is applied for oil recovery. As pointed out by Weschenfelder et al. [10,11], the use of ceramic membranes has shown great potential for produced water treatment specially when the goal is the reinjection in restrictive reservoirs, i.e. low water injectivity, by making the effluent practically free of suspended solids and dispersed oils and greases. In this sense, the objective of this work was to evaluate the effect a surfactant has on the ultrafiltration process using ceramic membranes for produced water treatment. DTAB was chosen for this study due to its potential application for EOR. For a better understanding of the process, the effect of DTAB and salinity on interfacial properties of water/oil/ membrane system, by measuring surface tension, wetting, electrokinetic potential and oil adhesion, was investigated.
2.4. Interfacial properties 2.4.1. CMC determination Surfactant critical micelle concentration (CMC) was determined by measuring the interfacial tension of the solutions with a spinning drop tensiometer (SVT 20 - DataPhysics). Each measurement was performed after oil/solution contact time of twenty minutes. 2.4.2. Contact angle determination The pendant drop method was applied to evaluate the contact angle between the oil droplets and the membrane surface by using OCA 15 Dataphysics goniometer. For the evaluation, one drop of oil (1 μL) was placed in contact with the membrane surface immersed in the solution. For each experimental condition the static drop image was captured. It is noteworthy that for these experiments ZrO2 membranes were supplied by Likuid Nanotek in flat conformation with similar characteristics to tubular membranes used in the permeation tests. 2.4.3. Adhesion mapping The adhesion test was carried out in order to verify the oil/membrane adhesion in presence of different solutions. For the evaluation, 2 g of a flat membrane of ZrO2 was crushed and immersed in different solutions. After a contact time of 5 min, 1 mL of oil was added to the bottom of each graduated cylinder with the assistance of a syringe. After 15 min of contact with the membrane, the volume of supernatant oil and its adhesion to the membrane material were observed.
2. Material and methods 2.1. Membrane
2.4.4. Zeta potential The zeta potential of all emulsions were measured by a Zetasizer Nano ZS90 with the purpose of evaluating the influence of salinity on the mobility and charge of oil particles.
This study was carried out with zirconium oxide supported by TiO2/ Al2O3 multi-channel membranes acquired from Likuid Nanotek, with 0.1 μm pore size, 0.003 m channel diameter, 0.2 m length and 0.0019 m2 surface area.
3. Results and discussion
2.2. Synthetic produced water
3.1. Process efficiency
Crude oil (28.1° API gravity) was emulsified at 50 °C by using Ultra Turrax (Model T-50) at the speed of 11,000 rpm during 5 min. The average concentration of oil and grease (COG) and salt content (CS) were kept at 100 ± 5 mg/L and 100,000 mg/L, respectively, which is usually found in offshore treatment units. For sake of comparison, oily water with no salinity was also investigated. Due to the difficulty of predicting surfactant concentration in produced water, it was decided to evaluate its influence up to the maximum concentration commonly injected (1,000 mg/L). The emulsified particle size was then determined by Malvern Mastersizer 3000E equipment. The procedure and conditions applied in this work were based on the study conducted by Weschenfelder et al. [10,12]. 2.3. Permeation of synthetic produced water
3.1.1. Permeate quality The oil content of all the permeate samples collected after permeation with ceramic membranes was lower than 5 mg / L. Similar results were reported in other studies and indicate that the water can be considered appropriate for reinjection even in restrictive petroleum reservoirs [11,14]. Absorbance analysis results showed that the membrane selected for this study (mean pore size of 0.1 μm) did not retain the surfactant DTAB. This was expected since the molecular size of the surfactant is much smaller than the membrane pore size. This fact can represent a saving of surfactant, because it will reduce the make up needed during reinjection into the reservoir.
The permeation set-up used in the tests is shown in Fig. 1. It consists of a permeation module, capable of accommodating a permeation element (ceramic membrane), a positive displacement recirculation pump, pressure gauges, flow meter, feed tank with mechanical stiring and a tank for permeate mass data collection. The transmembrane pressure (ΔP) of 2.0 bar and across flow velocity of 2.0 m/s (Re ≈ 6000) were selected according to the values proposed by Weschenfelder et al. [10]. As stated by these authors, the permeation time of 60 min can be considered sufficient for the permeate flux stabilization. The feed temperature (50 ± 2 °C) was chosen considering normal operating conditions found in offshore treatment plants of produced water [13]. During permeation experiments COG and surfactant concentration were periodically measured in feed and permeate streams. COG and DTAB concentration were measured with a Spectrophotometer HACH DR 2800 (according to the ASTM D3921 method) and a Spectrophotometer UV–vis HACH DR 6000, respectively.
3.1.2. Permeate flux Fig. 2 shows the normalized permeate flux as a function of DTAB concentration in synthetic produced water with (NaCl concentration of 100,000 mg/L) and without salt added. The permeate flux was normalized by the initial flux (J/J0) to allow comparison of solutions with different viscosities and to minimize experimental fluctuations inherent in the ultrafiltration process. In this figure it is possible to observe different behaviors of normalized permeate flux as a function of salinity. As the concentration of DTAB increases in the emulsion the permeate flux reduces for the NaCl-free emulsion, while the inverse behavior was observed for emulsion with 100,000 mg/L NaCl. At highest DTAB concentration the permeate flux for the emulsion with salt was approximately 6 times higher than the one obtained with the solution with no salt added. The permeate flux augment in the combined presence of DTAB and salinity indicates that the presence of this surfactant may favor the treatment of the produced water by ultrafiltration process. For a better 2
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Fig. 1. Photograph and schematic representation of the permeation set-up.
ionic species [15]. It is important to highlight that the concentration of the water injected in the reservoirs is commonly much lower than the CMC (3,500 mg/L) observed in emulsions with no salt added. 3.1.4. Particle size distribution Particle size distribution was determined for the emulsions containing 100 mg/L of oils and greases, 1,000 mg/L of surfactant DTAB, with 0 and 100,000 mg/L NaCl, as portrayed in Fig. 4. Both samples had particle with sizes comparable to the values commonly found in produced water [13]. In this figure, it is possible to observe that the particle size distribution shifts for smaller sizes for emulsion with 100,000 mg/L of NaCl. This fact can be related to the reduction of approximately 15 times in the critical micelle concentration (CMC), as can be observed in Fig. 3. Lower value of CMC increases the stability of oil droplets dispersed in the produced water. Although there is a reduction in particle size, the disperse phase is still significantly larger than the membrane pore size. Furthermore, higher emulsion stability should diminishes membrane fouling by oil deposition, resulting in higher permeate fluxes.
Fig. 2. Normalized permeate flux versus DTAB concentration. (COG = 100 mg/L, T =50 °C, ΔP =2 bar, Re ≈ 6,000)
understanding of these results the oil particle size distribution and interfacial properties related to the process were investigated. 3.1.3. Critical micelle concentration Fig. 3 shows the interfacial tension values for the emulsions prepared with different DTAB concentration with the presence and absence of NaCl. The value of the CMC can be verified at the beginning of the asymptotic region of the interfacial tension, determined by intersection of two curves. As expected, the CMC value is strongly influenced by the solution salinity. In the study, the CMC observed in the solution containing 100 g/L of NaCl was approximately 200 mg/L. Emulsion with 100,000 mg/L NaCl exhibit a high reduction in the CMC (200 mg/L), which favors the oil phase emulsification. This result is related to the surfactant solubility reduction with salinity, intensifying surface segregation phenomenon due to strong interaction between water and
3.1.5. Contact angle The contact angle between oil droplet and the membrane surface was determined by the verification of the interaction between the dispersed phase and the membrane surface in the presence of surfactant and NaCl. Fig. 5 shows the static images of oil droplets in contact with the membrane surface immersed in solutions with different DTAB and NaCl concentrations. When the membrane was immersed in water without surfactant and NaCl, the interaction with the oil phase is low (Fig. 5a), which
Fig. 4. Particle size distribution in synthetic produced water containing DTAB surfactant.
Fig. 3. Interfacial tension as function of DTAB concentration. 3
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Fig. 6. Photographs of different vessels with membrane crushed samples used in the adhesion mapping tests. (a) DTAB = 0 mg/L, Cs = 0 mg/L; (b) DTAB = 1,000 mg/L, Cs = 0 mg/L; (c) DTAB = 1,000 mg/L, Cs = 100,000 mg/ L.
1,000 mg/L DTAB and 100,000 mg/L NaCl, the oil trapped with the membrane particles was almost completely displaced. On the other hand, greater oil phase adhesion to the membrane material was observed when only surfactant was present in the solution. This result corroborates with previous analysis of membrane material hydrophilicity and its low affinity for the oil phase. The presence of surfactant facilitates oil adhesion to the membrane, which is reversed in high ionic strength solutions. The permeate flux is a consequence of these effects. 3.1.7. Zeta potential The electrochemical properties of dispersed particles may significantly impact the nature and magnitude of membrane fouling causing a decline in the permeate flux. Fig. 7 shows the zeta potential measured in oil emulsions with different salinities and DTAB concentration. In absence of DTAB, the oil droplets exhibit negative charges as indicated by the zeta potential value. However, the addition of DTAB neutralizes these charges even leading to signal inversion and zeta potential intensity, which can be attributed to surfactant adsorption on the dispersed particles surface. Another aspect observed in Fig. 7 is the salinity influence on the zeta potential. As expected, there is reduction of zeta potential value with ionic strength increase, i.e., the electric double layer undergoes a contraction, reducing charge intensity in the solution. NaCl concentrations above 50,000 mg/L bring the zeta potential value to approximately 0 mV. As commonly known, the ZrO2 membrane isoelectric point is approximately 6. This point indicates the equality between positive and negative charges. Only at pH values lower than the isoelectric point
Fig. 5. Static images of oil droplets in contact with the membrane surface immersed in different solutions. (a) DTAB = 0 mg/L, Cs = 0 mg/L; (b) DTAB = 1,000 mg/L, Cs = 0 mg/L; (c) DTAB = 1,000 mg/L, Cs = 100,000 mg/ L.
characterizes a hydrophilic nature of the membrane. However, increase of DTAB concentration alters the interaction between the membrane and the oil phase (Fig. 5b), which can be attributed to the amphiphilic characteristic of the surfactant, facilitating wetting of the membrane by the oil. This phenomenon has a direct impact on membrane fouling and may justify low values of permeate flux observed in this condition. When the solution contains 1,000 mg/L of DTAB and 100,000 mg/L of NaCl (Fig. 5c) the membrane interaction with the oil is also reduced. It can be attributed to the increase of ionic strength in the solution, which enhances interaction between the DTAB molecules and the oil phase, as evidenced by CMC reduction (Fig. 3) and decrease of oil droplets mean size (Fig. 4). 3.1.6. Adhesion mapping In order to qualitatively evaluate the membrane fouling by the oil phase as a function of the solution composition, adhesion mapping tests were performed (Fig. 6). In these tests, as presented in the experimental methodology, the membrane was crushed and immersed in solutions with different compositions. When the membrane material was immersed in water without surfactant and NaCl, as well as when immersion occurred in solution with
Fig. 7. Zeta potential of emulsions as a function of DTAB and NaCl concentration (pH = 6,5, COG = 100 mg/L). 4
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(pH < 6) the membrane has a positive zeta potential [16]. At the produced water pH (commonly around 6–8) a low and negative value of the membrane zeta potential is expected. Hence, the absence of DTAB and NaCl in the emulsion lead to an electro repulsion between the membrane surface and the dispersed oil phase, as evidenced by the oil displacement observed in the adhesion map. On the other hand, the presence of DTAB in the solution prepared without NaCl changes the zeta potential value leading to electro attraction between oil droplets and membrane surface, which can justify the sharp drop in the permeate flux. Moreover, high ionic strength reduces electrostatic interactions by compression of the double electric layer and enhances interaction between the DTAB and the oil phase, as evidenced by CMC reduction. This condition also reduces oil adhesion to the membrane surface, diminishing fouling and favoring water permeation as evidenced by the permeate flux increase.
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4. Conclusions The ultrafiltration process with ceramic membranes can be considered an effective alternative for produced water treatment even when cationic surfactants are present in the effluent. DTAB was studied as typical surfactant added to injection water, which should be present in the produced water. Interfacial phenomena investigation allowed the broadening of the understanding of the effects promoted by the presence of surfactant in produced water in the separation process. As DTAB concentration increases in the emulsion the oil adhesion to the membrane material is enhaced, resulting in a reduction of the permeate flux. This effect is related to electrical attraction as evidenced by zeta potential. However, produced water has high ionic strength that affects the interaction between emulsified oil droplets and the membrane surface, as well as intensifies adsorption of DTAB in the oil dispersed phase. These phenomena were perceived by low values of zeta potential, reduction of both particle size distribution and membrane wetting by the oily phase. Combination of these effects seems to reduce membrane fouling and facilitate water permeation. The efficiency of the oil phase retention by the membrane was not affected, even at high concentrations of surfactant (1,000 mg/L), being possible to obtain an effluent stream with contents of oils and greases below 5 mg/L.
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