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Dewatering of petroleum crude oil emulsions using modified Schiff base polymeric surfactants
Journal of Petroleum Science and Engineering 122 (2014) 719–728
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Dewatering of petroleum crude oil emulsions using modified Schiff base polymeric surfactants Ayman M. Atta a,b,n, Hamad A. Allohedan a, Gamal A. El-Mahdy a a b
Surfactants Research Chair, Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia Petroleum Application Department, Egyptian Petroleum Research Institute, Nasr City 11727, Cairo, Egypt
art ic l e i nf o
a b s t r a c t
Article history: Received 27 August 2012 Accepted 3 September 2014 Available online 5 October 2014
The new discoveries of increasingly heavy oil represent a major challenge for the oil industry because of its undesirable properties, such as its tendency to form stable emulsions and high viscosity and acidity. Dependent upon the production scheme, very stable water-in-oil emulsions can be formed from these oils, for which the complete separation of oil and water phases using conventional processes requires excessive heating, chemical addition, and high residence time. In the present work, the improvement of existing technologies and the development of new additives to decrease the temperature of the treatment processes of these emulsions are very important to ensure the processing of these oils in view of productivity gains. In this respect, the rate of demulsification of different types of crude oil emulsions using series of Schiff base polyethoxylates as demulsifiers was evaluated. The chemical structure of the prepared surfactants was determined by FTIR and 1H NMR analyses. Characterization of the surface activity of the prepared surfactants is performed to investigate the relation between the structure of surfactants and their performances. The interfacial properties of the prepared demulsifiers at the oil–water interfaces were investigated by means of the interfacial tension relaxation method. The results showed that the slow relaxation processes at low concentration because of stronger adsorption ability of the prepared demulsifiers. This was attributed mainly to rearrangement in the conformation of the molecules appeared with increasing demulsifier concentration. The demulsification efficiency of the prepared demulsifiers for the synthetic crude oil emulsions was determined at different temperatures using bottle test. The demulsification rate increased with surfactant concentration up to the onset of surfactant aggregation in the oil, water phase and a third, surfactant-rich phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: Schiff base demulsifier interface interfacial tensions crude oil emulsions emulsion stability
1. Introduction Most of the oil fields around the globe are producing oil that is often accompanied by significant amounts of water. Many types of emulsions are created in the same produced fluids during production. These emulsions may be oil-in-water (O/W), water-in-oil (W/O), or complex emulsions such as oil-in-water-in-oil (O/W/O) or water-in-oil in-water (W/O/W), depending on the water, the oil, energy in the flow, and oil-to-water ratios (Dodd, 1960; Mohammed et al., 1993; Acevedo et al., 1993; McLean et al., 1998; Ese et al., 1998). Petroleum dewatering has been a challenge in conventional crude oil industry for several decades (McLean et al., 1998). Most of the studies on stability mechanisms of water incrude oil emulsions can be divided into two categories: interfacial tension and interfacial rheology studies, (Dodd, 1960; Mohammed
n
Corresponding author.
http://dx.doi.org/10.1016/j.petrol.2014.09.017 0920-4105/& 2014 Elsevier B.V. All rights reserved.
et al., 1993; Acevedo et al., 1993; Ese et al., 1998). Comparative studies on emulsion stability using modeling of components and solvents (McLean and Kilpatrick, 1997,) indicated that the properties of interfacial material are responsible for emulsions stability (Hunter, 1986; McLean and Kilpatrick, 1997; Layrisse et al., 1984; Acevedo et al., 1992). The majority of the studies showed that water-in-crude oil emulsions are stabilized by a rigid film or “skin” at the oil–water interface (Dodd, 1960; Taylor, 1992; Acevedo et al., 1993). A typical dehydration plant operation usually comprises the following six major steps: separation by gravity settling, chemical injection, and heating, addition of fresh (less salty) water, mixing, and electrical coalescing. The treatment involves allowing time for water drops to settle out and be drained off. Settling time and draining are accomplished in wash tanks, separators, and desalting vessels. Settling and draining can be speeded up using one or more of the following actions: injecting chemicals (demulsifier), applying heat, adding diluents (freshwater), and applying electricity (AlOtaibi, 2004). Therefore, it is expected that the most important
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parameters affecting performance of the dehydration are settling time, mixing time, chemical dosage, crude temperature, and wash water flow rate ratio. In heavy oil production, the efficiency of extracting clean dry oil from the produced fluids depends on the process conditions as well as the choice of chemical additives used to destabilize the emulsions. The effectiveness of the measurements used to destabilize these emulsions depends on the properties of the surfactants such as the hydrophile–lyophile balance (HLB) as well as on properties of the oil and water phase. The complication arises from the treatment and production of the fluids was attributed to the presence of solids in the fluid (Angle, 2004). Demulsification of crude oil emulsions has been attracted great attention and many published work was recently reviewed, (Angle, 2004; Sjoblom et al., 2001) and gained an importance discussion to specific cases (Bailes and Kuipa, 2001; Xia et al., 2004; Stark and Asomaning, 2005; Zhang et al., 2005). The majority of the studies focused on simple model systems, and few studies focused on complex field emulsions, which are often more difficult to obtain. The demulsifier combinations often used for emulsion treatments become ineffective as more complex emulsions are produced. Commercial demulsifiers are polymeric surfactants, such as copolymers of polyoxyethylene (PE) and polyoxypropylene (PO) or alkylphenol–formaldehyde resins, dodecylbenzenesulphonic acid, or blends of different surface-active compounds (Kokal, 2005). In the previous works (Atta et al., 2008a, 2008b) new surfactant blends were prepared by etherification of Schiff base surfactants. There are two challenges faced us in this work to separate water from the crude emulsions which are emulsion stability and demulsification at temperature below 60 1C. In our quest for new polymeric demulsifiers, we have focused on Schiff bases, condensed organic derivatives containing azomethine groups C ¼N–. In this work we have synthesized a new class of soluble such Schiff bases and evaluated them as demulsifiers for crude petroleum at temperatures not exceeding 60 1C. The correlation between the surface activity of the prepared Schiff base surfactants and their performance as demulsifiers is another goal of the present work. The proposed work aims also to examine the interfacial behaviors to determine and improve the poor performance of the demulsifiers.
2.2. Preparation of Schiff base surfactants 2.2.1. Synthesis of Schiff base monomers The reactions were completed in three-necked flask (0.5 L capacity) equipped with a condenser, magnetic stirrer, thermometer, dropping funnel and nitrogen atmosphere inlet. The flask was charged with ODA or DDA (0.25 mol) and 200 ml absolute ethanol. HBA (0.25 mol) were dissolved in 200 ml absolute ethanol and the solution was added into flask within 1 h with stirring in the presence of nitrogen gas. The temperature of reaction was gradually increased up to reflux for 3 h. The reaction was cooled to filtrate the solution. The products of condensation of HBA with ODA and DDA were designated as ODS and DDS, respectively. 2.2.2. Ethoxylation of Schiff-base monomers A high pressure stainless steel autoclave (Parr model 4530, USA) of 1 L capacity, 400 psi maximum pressure and 180 1C maximum temperature was used for ethoxylation reaction. The autoclave is equipped with a magnetic drive stirrer, an electric heating mantle with a thermocouple inserted in the reactor body, a cooling coil, a pressure gauge and a drain valve. The prepared ODS, DDS and their polymers were charged into the reaction vessel with Na metal as a catalyst (0.3 wt%), the reaction mixture was heated to 180 1C with continuous stirring while passing a stream of nitrogen gas through the system for 10 min to flush out air. The nitrogen stream was then replaced by ethylene oxide. The ethylene oxide was introduced through the inlet gas valve until the desired amount of ethylene oxide was reacted. Generally, as a result of the introduction of ethylene oxide, the pressure was substantially increased as indicated by the pressure gauge, until it reached a maximum value. The pressure drop indicates ethylene oxide consumption. The reaction completion was established when the pressure reached its minimum value. At this stage, heating was stopped and the content was cooled gradually to ambient temperature by means of the cooling coil connected to the reactor carrying cold water. After cooling, the product obtained was discharged, weighed and neutralized with HCl. The ethoxylated products of ODS and DDS at different ethoxylation times can be designated as EODS, EDDS-1 and EDDS-2, respectively. 2.3. Measurements
2. Experimental 2.1. Materials Octadecyl amine (ODA), dodecyl amine (DDA), and p-hydroxybenzaldehyde (HBA) were obtained from Aldrich Chemical Co., and used without purification. All solvents were purchased from Aldrich Chemical Co. (Germany). Table 1 shows details of the Land Balayium crude oil (produced from Petrobel Co., Egypt) and its origin. On the other hand, the formation water was obtained from production wells (produced from Petrobel Co., Egypt). Table 1 Physicochemical properties of the tested land belayim crude oil. Test
Method
Results
API gravity Specific gravity 60/60 (1F) Wax content (wt%) Asphaltene content (wt%) Pour point (1C)
Calculated IP 160/87 UOP 46/64 IP 143/84 IP 15/67(86)
25 0.896 5 13 18
The resin and asphaltene contents of crude oil were determined according to standard procedure ASTM D2000 methods (ASTM-2007D, 2000). The relative solubility number (RSN) values of the surfactants were measured by the newly developed method using ethylene glycol dimethyl ether and toluene as titration solvents. In this method, 1 g of surfactant was dissolved in 30 mL of solvent consisting of toluene and ethylene glycol dimethyl ether and the resultant solution was titrated with deionized water until the solution became persistently turbid. The volume of water in mL was recorded as the RSN number. The solubility tests were performed in different solvent media for EODS, EDDS-1 and EDDS-2, at a concentration of 40% (w/v), at room temperature, with xylene, ethanol, and xylene/ethanol (75:25). The nitrogen content was measured with a Tecator Kjeltech auto analyzer. A Tecator 1007 digester was used for the initial digestion of the samples. Infrared spectra were determined with a Perkin-Elmer model 1720 FTIR (KBR). While, 1H NMR spectra of the prepared Schiff base monomers and polymers were recorded on a 400 MHz Bruker Avance DRX-400 spectrometer.
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Microscope images of petroleum crude oil emulsions sealed in 200-μm deep Helma cells were observed and photographed using a Nikon E600 microscope and a Nikon AE850 digital camera. The surface and interfacial tensions between formation water and crude oil were measured at 25 1C by means of the pendent drop technique using a drop shape analyzer (model DSA-100, Kruss, Germany). In this method the shape of a pendent drop was fitted to the theoretical drop profile according to the Laplace equation, using surface tension as one of the adjustable parameters. The error limits of these measurements were of the order of 0.1 mN/m or less. The DSA-100 analysis required accurate density measurements, which were obtained as functions of temperature and surfactants concentration with an AP Paar DMA45 MC 1296 densitometer. Pendant drops were formed on the tip of a needle with an outside diameter of 1.84 mm.
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OH
OH + R-NH 2 R=C12H25 =C18H37
CHO OH
CH=N-R O
O-[CH2-CH2-O]n-H
Na CH=N-R
CH=N-R
2.4. Preparation of water crude-oil emulsions
Scheme 1. Preparation of Schiff polymeric surfactants.
Water-in-oil emulsions collected in the field were free of demulsifier. Shortly after collection, samples were drained of any free water. Free water is defined as water that separates rapidly and is not emulsified. All emulsions were prepared with a total volume of 100 mL. The ratio between crude petroleum and the aqueous phase (formation water) was in the range 10–50 vol%. The emulsions were prepared by mixing using a Silverstone homogenizer. In a 500 ml beaker, the crude petroleum was stirred at 35 1C (3500 rpm for 1 h) while sea water was added gradually until the two phases become homogenous. The ratios crude oil: water were 90:10, 80:20, 70:30 and 50:50. 2.5. Demulsification of the prepared emulsions The bottle test is used to estimate the capability of the prepared demulsifiers in breaking of water in oil emulsions. Demulsification was studied at 45–60 1C using gravitational settling with graduated cone-shaped centrifuge tube. The prepared Schiff base surfactant solutions were injected into the emulsion using a micropipette. After the contents in the tube had been shaken in an oscillating shaker for 1 min, the tube was placed in a water bath at 45–60 1C to allow the emulsion to separate. The phase separation was recorded as a function of time. The interface between the emulsion and separated water phase can be easily observed during the settling process. The demulsification rate (DDE%) can be calculated from the following equation; DDE% ¼Vo 100/V1, where Vo and V1 are volume of emulsified and separated water, respectively.
3. Results and discussion Schiff base compounds containing an imino group (–C ¼N–) are employed for several petroleum applications as corrosion inhibitors and demulsifiers (Atta et al., 2008a, 2008b; Migahed et al., 2011). The purity of the prepared Schiff base monomer is very important when used as starting materials to produce watersoluble Schiff base polymeric surfactants. The nonionic Schiff base surfactants were prepared from the ethoxylation of Schiff base monomer as illustrated in experimental section and Scheme 1. The reaction between p-hydoxybenzaldehyde and octadecyl amine or dodecyl amine produced 4-hydroxybenzaldehyde-N-octadecylimine (ODS) or 4-hydroxybenzaldehyde-N-dodecylimine (DDS), respectively. The physicochemical characteristics of the prepared Schiff base monomers were listed in Table 2. Nitrogen content indicates good agreements between the experimental and theoretical value. This reveals that the method of synthesis and purification was performed successfully. On the other hand, the Schiff base
Table 2 Physicochemical characteristics of Schiff base polymeric surfactants. Compounds
ODS DDS
Nitrogen content
Melting temperature
Yield
Theoretical (wt%)
Determined (1C)
(%)
3.86 4.86
3.82 4.81
112 128
90 85
monomers were ethoxylated to produce EODS, EDDS-1, and EDDS2 as illustrated in both Scheme 1 and experimental section in the presence of sodium as catalyst. 3.1. Chemical structure of ehoxylated Schiff base surfactants The chemical structure of Schiff base monomers and their ethoxylated derivatives were determined from FTIR and 1H NMR analyses. The FTIR spectra of DDS, and EDDS-1 were selected as representative samples and listed in Fig. 1a–b. 1H NMR spectra of EDDS-1, EDDS-2 and EODS were represented in Fig. 2a–d. The lack of O–H stretching band in the 3400–3500 cm 1 (Fig. 1) in the spectra of the ethoxylated derivatives confirms complete ethoxylation of ODS and DDS. The appearance of strong peaks in the spectra of both Schiff base and ethoxylated derivatives in the region 1580–1670 cm 1 (Fig. 1a–b) indicates the presence of C ¼N stretching bands (Atta et al., 2008a, 2008b). The disappearance of strong bands of aldehyde groups at 2750 and 1700 cm 1 which assigned for CH and C ¼ O stretching of aldehyde groups indicates the formation Schiff base monomer with high purity grade. The 1H NMR spectra of ethoxylated Schiff base, Fig. 2, show a complicated aromatic proton multiplet (7.1–7.6 ppm) and an imino proton singlet at 6.8 ppm downfield from the usual aromatic proton region. The triplet peak, observed in spectra of EDDS-1, EDDS-2 and EODS at 4.2 ppm can be assigned to CH2 groups attached directly to azomethine group. While, multiple peaks at 2.1 ppm (Fig. 2) can be attributed to (CH2)n of alkyl group. The ethoxylation of Schiff base monomers can be proved by appearance of new peaks of oxyethylene units at 3.6 ppm and –OH proton of PEO 2.5 ppm. The 1H NMR spectroscopic analysis was used previously for determining the propylene oxide/ethylene oxide ratio for the PPO–PEO block copolymers (Huang et al., 2002). In the present investigation the 1H NMR spectroscopic analysis is used to determine the degree of ethoxylation based on calculation integration ratio between the number of oxyethylene
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Fig. 1. FTIR spectra of (a) ODS, (b) DDS and (c) EDDS-1.
unit from the oxyethylene units (4 protons) in the hydrophilic moiety and the phenyl rings (4 protons) in the hydrophobic moiety. In this respect, the data for number of oxyethylene units was determined and listed in Table 2. On the other hand, the degree of ethoxylation can be calculated from the yield % of the purified ethoxylated Schiff base monomers. The data was used to calculate the theoretical molecular weights of ethoxylated Schiff base monomers. The calculated ethoxylation number (EO)n from 1 H NMR data is inconsistent with the data that determined from weight measurements.
3.2. Solubility of the prepared surfactants It was found that toluene and xylene/ethanol (75:25) solvents are the normal dispersants among the other solvents and used for formulations containing demulsifying additives (Razi et al., 2011). In this respect, solubility tests of the EODS, EDDS-1, and EDDS-2 at room temperature were carried out in different solvent media. The solubility tests of the EODS, EDDS-1, and EDDS-2 in water, toluene, and xylene/ethanol (75:25) were performed and the results indicated that all prepared surfactants were soluble in water, and
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xylene/ethanol (75:25). The difference in solubility was attributed to the hydrophil–lipophil balance (HLB) of the surfactants. In this respect, the HLB values were calculated by using the general formula for nonionic surfactants (Griffin, 1954). The equation applied as follows: HLB ¼ ½ðMH 20Þ=ðMH þ MLÞ
ð1Þ
where MH and ML are molecular weights of both hydrophile and hydrophobe moieties, respectively. HLB values of the prepared nonionic surfactants were calculated and listed in Table 3. The solubility of nonionic surfactants in polar solvents increases with increasing their HLB values. It is noticed that the HLB values of the prepared surfactants ranged from 11.2 to 13.9 which indicates the good solubility of these surfactants in polar solvent. Relative solubility numbers (RSNs) can be used to represent the hydrophobicity of the surfactant (Xu et al., 2005) due to the experimental difficulty for HLB determination. The data of RSN (vol%)
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were measured and listed in Table 3. The RSN values of the three series of Schiff base nonionic surfactants used in this work indicated that, at the fixed linear alkyl chain, the RSN value of the surfactant increases with an increment in EO number as determined for EDDS-1 and EDDS-2. This can be attributed to an increasing EO number enhances the hydrophilicity of the surfactant, while an increasing of alkyl chain length enhances its hydrophobicity. It is well known that, the activity of the chemical additives affected by the solubility of these additives at different temperatures. The cloud point is the temperature below which there is a single phase of a molecular solution. Above the cloud point, a nonionic surfactant loses its solubility in water, causing the formation of two phases and giving the mixture a cloudy aspect (Hergeth et al., 1991). To investigate the effect of the temperature on the solubility of the EODS, and EDDS was measured in aqueous medium (up to 85 1C), the cloud points were measured at concentrations of 100, 1000, and 10,000 ppm. The data of cloud temperatures were measured and listed in Table 3. It can be expected that the solubility will be hampered with increasing the concentration, i.e., as the surfactant concentration increased, the cloud point temperatures decreased. It was also noted that EDDS surfactants were more soluble in water than EODS. This means that the alkyl substituents are the main factor for solubility over polyoxyethylene units as shown in previous studies (Mansur et al., 2004, 1997). 3.3. IFT of Schiff base surfactants at water oil interface There are natural surface-active fractions (such as asphaltenes, resin, naphthenic acid, and porphyrin materials) that can be adsorbed onto the interface and form a firm film at the oil–water interface, resulting in high stability of crude oil emulsions. The measurements of the W–O interfacial tension were performed using the pendant drop method. This method allows us to follow the variation of the W–O interfacial tension values as a function of time. The interfacial tension values can be calculated after the processes of diffusion, adsorption, reorganization at the interface, desorption, and transfer of the molecular mass to the other phase have attained the equilibrium. It was previously reported that, the faster the minimum interfacial tension values the quicker the diffusion of the molecules to the interface. This in turn leads to the formation of thinner and less elastic films that rupture more readily (Yen, 1974). The demulsification of produced crude oil emulsions is an important problem in oil field industry. Chemical demulsification using demulsifiers is in common use in oil field chemistry. The demulsification ability of a demulsifier is mainly controlled by two factors: one is the hydrophilic–hydrophobic ability; the other is the ability to destroy the interfacial film. The structure of the demulsifier can influence both of the above two factors. To observe the influence of the structure of the ethoxylated Schiff base the EODS, EDDS-1, and EDDS-2 polymeric surfactants on the interfacial tension measurements, they were dissolved in a mixture of xylene/ethanol (75:25) or water using
Fig. 2. 1H NMR spectra of (a) EDDS-1, (b)EODS, and (c) EDDS-2.
Table 3 Degree of ethoxylation and cloud temperature of the prepared Schiff polymeric surfactants. Surfactants
Number of ethylene oxide unit
Theoretical molecular weight
HLB
RSN
(g/mol) EODS EDDS-1 EDDS-2
18 15 10
1155 948 728
13.7 13.9 12.01
14 18 12
Cloud temperature (1C) at different surfactant concentrations 100 (ppm)
1000
10,000
80–82 485 77–79
77–78 485 73–75
75–77 485 70–71
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ruptured occurred after 10 min of analysis. This behavior is related to the higher ability of molecule to adsorb at the interface which favors the process of diffusion in the oil. The water forms the interface with the oil when water used as solvent for the prepared surfactants, then adding this solution to the water. It can be concluded that there was a great reduction in interfacial tension after 25 min when the aqueous surfactant solution was added to the water phase. This result reflects the hydrophobicity of these surfactants which tend to migrate quickly to the interface and minimizes its contact with the water. The xylene/ethanol (75:25) mixture provided the fastest reduction of the interfacial tension, reaching a value of 2.5 mN/m after 8 min among the other solvent media in which surfactants dispersed in the oil phase. The tension decreased gradually followed by a slight decline after long time measurement (25 min). The W–O interfacial tension values recorded during the last stage of test can be attributed to the strong influence of the solvent medium on the surfactant ability to reduce interfacial tension. The effectiveness of these three media can be arranged in descending order: xylene/ethanol (75:25) 4toluene 4water. It can be confirmed that the higher the hydrophilic solvent, the easier the dispersion of the surfactant in the crude oil emulsions. 3.4. Evaluation of the performance of the Schiff base surfactants as demulsifier Fig. 3. Microscope graph of (a) W/O and O/W emulsions.
Table 4 IFT data between different Schiff base polymeric surfactant solutions and crude oil interface at 25 1C. Surfactants
Concentrations
IFT (mN/m) Xylene/ethanol (75:25)
Water
(ppm) EODS
0 50 100 250
18.1 14.1 7.2 4.2
28.1 26 22 14
EDDS-1
50 100 250
18.0 18.0 12.2
26.0 22.2 16.5
EDDS-2
50 100 250
9.9 5.8 3.2
18.1 14.3 12.1
different concentrations. The interfacial tension (IFT) between surfactant solution and crude oil interface was measured using a Kruss drop shape analyzer (DSA100) at 25 1C. Fig. 3 shows the dynamic interfacial tension of crude oil/EDDS-2 of different concentrations interface. The results of IFT were measured and listed in Table 4. The interfacial tension of crude oil/formation water was 28.5 70.9 mN/m. Upon addition of different concentrations of the Schiff base surfactants, reduction of the interfacial tension was obtained down to about 3.5 mN/m at 250 ppm of EDDS-1 (Fig. 3) with the least time for equilibrium. The variation of IFT data with time indicated that all of the Schiff base surfactants reduced the W–O interfacial tension. The interfacial tension values initially decline with time due to gradual migration of surfactant molecules to the interface and adsorbed as monolayers according to the Gibbs isotherm. Data indicated that EDDS-2 and EODS were most effective in reducing the interfacial tension and the droplet
Crude oil emulsions commonly exist in the form of water in-oil emulsions. The stability of the emulsion was mainly determined by the mechanical strength of the film and the rheological properties of liquid–liquid films (Angle, 2001). It is well known that asphaltene responsible for stabilization of petroleum crude oil because it acts as stabilizer for different types of crude oil emulsions (Mohammed et al., 1993; Acevedo et al., 1993; Ese et al., 1998). Wax and mineral particles have also been shown to stabilize water-in-crude oil emulsions (Menon and Wasan, 1986). Asphaltene has HLB value 11.1 and assists to form stable crude oil emulsions (Kloet et al., 2002). In this respect, Land Balayium was owing to its high asphaltene content (15 wt%) as listed in Table 1. The crude oil was blend with different volume ratios of sea water ranging from 10% to 50% as illustrated in the experimental section. The produced emulsions show high emulsion stability even at high temperature (65 1C). Water does not separate up to 14 days. The microscope images of water-in-oil (W/O) emulsion and the oil inwater (O/W) emulsion were represented in Fig. 4. The average oil drop size of the O/W is larger than water droplet of the W/O for the typical heavy oil water emulsions. The data indicated that the
Fig. 4. Dynamic IFT oil versus drop age time for crude oil emulsions and different concentrations of EDDS-2 and EODS in xylene/ethanol (75/25).
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Fig. 5. DDE of PDDS-2 measured at different concentrations for petroleum crude oil emulsions having different oil/water compositions (A) 90/10, (B) 80/20 and (C) 70/30 at 60 1C.
W/O emulsion inversed to form O/W emulsion when the water volume percentage reached 30% as illustrated in Fig. 4. The dropping test of emulsion in toluene gave an evidence for the same results. The demulsification ability of a demulsifier is mainly controlled by two factors: one is the hydrophilic–lipophilic ability. The other is the ability to destroy the interfacial film. The structure of a demulsifier can influence both of the above two factors. The
removal of emulsified water and the solids associated with the diluted froth by both chemical and physical means is practiced in commercial operations on a trial-and-error type basis. Surface active demulsifier molecules will orient at the interface and displace the natural asphaltenes, eliminating the barrier to coalescence and promoting destabilization of the emulsion. The commercial surfactants are typically high molecular weight polymers
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Table 5 DDE data of the prepared surfactants at different concentrations and separation temperature 60 1C. Surfactants
Conc.
DDE% O/W 90/10
O/W 80/20
O/W 70/30
O/W 50/50
DDE
Time (min)
DDE
Time (min)
DDE
DDE
Time (min)
(ppm) EODS
50 100 250
100 100 100
300 90 60
100 100 100
270 180 270
53 60 100
360 360 330
100 100 100
360 90 60
EDDS-1
50 100 250
80 100 100
360 60 45
100 100 100
300 180 330
67 53 100
360 360 330
100 100 100
240 60 90
EDDS-2
50 100 250
80 100 100
360 60 45
100 100 100
240 90 210
60 100 100
360 45 105
100 100 100
45 45 45
Table 6 DDE data of the prepared surfactants at concentration 250 ppm and separation temperature 45 1C. Surfactants
Conc.
DDE% O/W 90/10
O/W 80/20
O/W 70/30
O/W 50/50
DDE
Time (min)
DDE
Time (min)
DDE
DDE
Time (min)
100 100 100
420 300 180
100 100 100
330 330 240
33 40 80
88 88 100
480 480 45
(ppm) EODS EDDS-1 EDDS-2
250 250 250
480 480 480
intended to flocculate the emulsion droplets. It is thought that the droplets then coalesce due to an increase in the collision frequency and under the influence of treatment processes such as centrifugation. In the present work an attempt is made to use new demulsifiers based on Schiff base polymeric surfactants in terms of the HLB system. In this study, the aim is directed to prepare low molar mass affective surfactants to add as demulsifier of stable water-in-crude oil emulsions. The demulsification using the pure Schiff base polymeric nonionic surfactants will be investigated by using different HLB surfactants. In the previous section, based on solubility of the prepared surfactants and IFT data, xylene/ethanol (75:25) selected as the solvent to dissolute the prepared surfactants with respect to its ability to break up water-in-oil emulsions. This enhanced solubilization is reflected in a better dispersion in crude oil–water emulsion and, consequently, in a better demulsification performance. Different concentrations of Schiff base surfactants ranged from 50 to 250 ppm. Bottle tests performed in the field showed that the emulsion breaker showed that the oil was dry, less than 3% BS&W. The relation between demulsification efficiency (DDE) and time of water separation of EDDS-2 was selected as representative sample and plotted in Fig. 5 at different crude oil water emulsion compositions and different temperatures 60 and 45 1C. The data for separation time and DDE was determined at different temperature 45 and 60 1C, and listed in Tables 5 and 6. It was noticed that, the emulsions color changed (becoming clearer), at the bottom of the tube used in the assays. The experiment result indicated that the solvent medium and alcohol can be acting as co-additives in the demulsification process and as a co-surfactant, respectively. It was found that the demulsification performance of a demulsifier is mainly dependent on the interaction between dissolute surfactants and water droplet
through diffusion and adsorption, which permits faster transport of demulsifier molecules to the water droplet interface. Moreover, dissolute demulsifiers give a better separation of the phases (water and crude oil phases) than an undiluted demulsifier does. The steady state observed in Fig. 5 was attributed to constant rate of dehydration for some time then increases again and eventually attaining the final percent of dehydration for each surfactant. Crude oil emulsions are stabilized by surfactants, viz., asphaltenes and resins and do not develop high surface pressures. Hence, steric stabilization of water-in-crude-oil emulsions is the most plausible mechanism of stabilization of such emulsions. Demulsifier molecules are adsorbed at the interface when they having the tendency to lower interfacial tension more than the natural surfactants and the film become unstable in the direction of coalescence of water droplets. Emulsions formed in the petroleum industry are predominantly water-in-oil or regular emulsions, in which the oil is the continuous or external phase and the dispersed water droplets, form the dispersed or internal phase. The water droplets become bigger with time after demulsifier is added. The role of the demulsifier is to change the interfacial properties and to destabilize the surfactant-stabilized emulsion film in the demulsification process. Small and uniform drops flocculate and some large drops begin to form during the initial stage at 30 min. Two or more large drops continue to form a single larger drop due to occurrence of coalescence during the middle stage. The droplet size eventually grows fast with a reduction in droplet number with the addition of demulsifier during the last stage. It can be concluded that coalescence of water droplets destroyed emulsions. One can quietly distinct three terms related to stability of crude oil emulsion. The first term is flocculation, which refers to the mutual attachment of individual emulsion drops to form flocs or loose assemblies. The reversible process occurs in many cases during flocculation can be avoided by imposing (forcing) less energy than required in the original emulsification process. The second term is coalescence, which describes the joining of two or more drops to form a single drop of greater volume, with small interfacial area. Although coalescence will result in significant microscopic changes in the condition of the dispersed phase, it may not immediately result in a macroscopically apparent alteration of the system. The third term is breaking of an emulsion. It explains the occurrence of the gross separation of the two phases. The identity of individual drops is lost owing to the change in the physical and chemical properties of the emulsion and followed by loss of stability in the emulsion. It has been established that the kinetics of chemical demulsification is complicated by the interaction of three main effects. These are the displacement of the asphaltenic film from the oil water interface by the demulsifier; flocculation and coalescence of water drops (Kim and Wasan, 1996; Carmen and Carbognani, 2001). Careful inspection of data indicated that in all cases, an increase in the concentration of the additive improved its performance in destabilizing the emulsion. EDDS-2 was the most efficient additive in breaking down the emulsion (Fig. 5). At approximate concentrations (100 and 250 ppm) 90% of the water had separated out after 15 min. Complete phase separation was achieved at a concentration of 50 ppm after 45 min. The phase separation was much less efficient for the other surfactants EDDS-1 and EDOS. In general, the efficiency of the gravitational separation for the samples tested increased in the following order: EDDS-24 EODS4 EDDS-1. Although EDDS-2 was the most efficient in reducing the interfacial tension and EDDS-1 was the least efficient (Table 5), there was no direct relationship between the efficiencies of the additives and this property. The results showed that there is some relation between the gravitational separation and the solubility preference of the additive in the oil phase (Table 3). The best performance displayed by EDDS-2 may be attributed to the highest bulk phase
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concentration. The performance of the poorly soluble polymer additives in the aqueous phase is better than that are soluble in both phases. However, there was a relationship among the efficiency of the gravitational separation, the molecular architecture, and the area occupied at the interface. The gravitational separation efficiency increased as the area occupied per molecule at the W–O interface declined (Table 4). The smallest areas were attained by the molecules with adjacent architecture. The better efficiency of EDDS-2 in relation to EDDS-1 can be related to the tighter packing of the molecules of the former. The high demulsification power of surfactants based on EDDS-2 and EODS reflects the hydration of long hydrophilic groups into bulk phase which prevents the crude oil to penetrate the exterior region of micelle to arrive to their interior core and solubilize there (Rosen, 1978). In the same time, the long hydrophilic chain reduces the hydrophobicity of the surfactant molecule; hence, the solubilization efficiency is reduced (Cowell et al., 2000). However, in terms of understanding mechanisms this kind of representation can be misleading. 3.5. Effect of the Schiff base surfactant concentrations on DEE In particular, a given demulsifier concentration will be above the critical value required for aggregation for some surfactants but below for others. In general nonionic surfactants added at concentrations below that required for aggregation to a nonpolar oil/ water mixture distribute as monomers between the two phases. For equilibrated oil/water mixtures containing the present homologous series of nonionic surfactants, the lower EO monomer species partition predominantly to the oil phase, whereas the higher EO members are predominantly located in the aqueous phase (Crook et al., 1965). Above a certain critical total concentration, all further surfactant added is involved in the formation of aggregates in either the oil, the water, or a third, surfactant-rich phase depending on the HLB of the system. The interfacial tension in such systems decreases with increasing surfactant monomer concentration but remains virtually constant above the “critical aggregation concentration” (cac). The cac values for three Schiff base surfactants in crude oil/water systems were determined using surface tensiometry by vigorously shaken known quantities of surfactant with crude oil and water and the mixtures were equilibrated at 40 1C for 10 days. The volume ratio of water to oil was kept constant at the value used in the emulsions (10 vol%). In order to avoid making tension measurements in the presence of crude oil, the surface tensions of the separated aqueous phases were measured. The cac values were determined as the break points in plots of the surface tension versus the logarithm of the
727
total surfactant concentration as shown in Fig. 6. The cac values applied only to the water volume fraction used (10%) and refer to the total systems. In order to compare demulsification rates, we have chosen to take the initial slope of the percentage resolution versus time curves (Fig. 5) as a measure of the demulsification effectiveness. The variation of the initial resolution rate with demulsifier concentration for EDDS-2, with n ¼10, exhibit maxima which coincide closely with the measured cac values. The cac values for EDDS-2, EODS and EDDS-1 are, 180, 120 and 80 ppm, respectively. From the given results it was noticed that the dehydration rates reached 100% for all surfactants indicating the high efficiency of them for treating crude oil emulsions. It is clear that the surfactants have optimum HLB values ranged from 12.0 to 13.7 and have the highest the demulsification efficiency from the present work data dealing with a water-in-oil emulsion. It can be concluded that the lowest hydration rates of the surfactants was reported for 70:30 W–O emulsions. It was previously reported that, the emulsions have composition of oil (70) and water (30) can be converted from W/O emulsion to O/W due to increased salinity of brine water. From these results, it is clear that the aqueous phase of the initial emulsion is below optimum salinity and, hence, is an O/W microemulsion. Accordingly, the separation of water from cannot be easily occurred from micro-emulsion. Surfactant having HLB between 1 and 8 promotes the formation of oil-in-water emulsions while surfactant having HLB between 8 and 12 may promote either type of demulsifiers. The data presented in our work indicated that the surfactants have HLB (12.0–13.7) show good demulsification efficiencies, while surfactants having HLB above 13.8 show low demulsification efficiencies. This behavior reflects the dependence of the demulsification power on the presence of surfactant molecules in the soluble form rather than micelles (Cowell et al., 2000). This can be attributed to the solubilization of crude oil into the hydrophobic interior core of micelle (Hirasaki et al., 2011). The data listed in Tables 5 and 6 indicated that the DDE was affected by the temperature of separation. The dehydration rate of the surfactants evaluated at 45 1C for different crude oil emulsion compositions using 250 ppm of surfactants solutions. The data indicated that the DDE was reduced in 70:30 (oil:water) emulsions and the time of water separations increased in the other emulsions determined at the same temperature 60 1C. These behaviors can be referred to solubility of the prepared surfactants at different temperatures. The good solubility of surfactants in oil phase can be referred to the presence of aromatic moieties in the structure of surfactant. Visual observation of the aqueous phase during the end of the bottle tests indicated that when water was used as solvent medium, a large amount of oil was trapped on the walls of the glass tube, indicating that this medium does not favor diffusion of the additive molecules in the oil phase. This behavior was also observed when toluene was used as the solvent medium.
4. Conclusions
Fig. 6. Surface tension versus log [surfactant] in the total system for Schiff base polymeric surfactants.
We have found that certain combinations of low molar mass Schiff base surfactants are easily used to demulsify water-in-crude oil emulsions. Schiff base surfactants and solvents can be added in small amounts to cause significant reduced water contents of crude oil emulsions. This is in general agreement with the use of high HLB surfactants to destabilize water-in-oil emulsions in, for example, oil emulsion treatment. The existence of an optimum surfactant concentration for such beneficial additives correlates with a minimum in interfacial tension and is consistent with conventional oilfield demulsifier experience. The results obtained by the interfacial tension relaxation method showed that the IFT values reduced near the critical micelle concentration of the prepared surfactants. With
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an increase in the Schiff base concentration, the IFT gradually decreased and the contribution of them also changed disciplinarily. Efficiency of gravitational separation of synthetic emulsion W–O of the Schiff base surfactants show the best results for water separation when added to the emulsion at a concentration of 100 and 250 ppm and solubilized in xylene/ethanol (75:25). The efficiency of the water gravitational separation for the samples tested increased in the following order: EDDS-24EODS4EDDS-1. The data of IFT measurement indicated that the EDDS-2 is the most efficient and EDDS-1, was the least efficient of the prepared surfactants.
Acknowledgments The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group no. RGP-VPP-235. References Acevedo, S., Escobar, G., Gutierrez, L., Rivas, H., 1992. Isolation and characterization of natural surfactants from extra heavy crude oils, asphaltenes and maltenes. Interpretation of their interfacial tension-pH behaviour in terms of ion pair formation. Fuel 71, 619–623. Acevedo, S., Escobar, G., Gutierrez, L.B., Rivas, H., Gutierrez, X., 1993. Interfacial rheological studies of extra-heavy crude oils and asphaltenes: role of the dispersion effect of resins in the adsorption of asphaltenes at the interface of water-in-crude oil emulsions. `Colloids Surf. A 71, 65–71. Al-Otaibi, M., 2004. Modelling and Optimizing of Crude Oil Desalting Process (Ph.D. thesis). Loughborough University, Leicestershire, England. Angle, C.W., 2001. Chemical demulsification of stable crude oil and bitumen emulsions in petroleum recovery review. In: Sjoblom, J. (Ed.), Encyclopedic Handbook of Emulsion Technology, 1st ed. Marcel Dekker, New York, pp. 541–594. Angle, C.W., 2004. Effects of sand fraction on toluene-diluted heavy oil in water emulsions in turbulent flow. Can. J. Chem. Eng. 82, 722–734. ASTM-2007D, 2000. Fractionation of Crudes by SARA Analysis. ASTM, West Conshohocken, PA. Atta, A.M., Abdel-Rahman, A.H., Elsaeed, S.M., Abou-Elfatouh, S., Hamad, N.A., 2008a. Demulsification of crude oil emulsions using some new water-soluble Schiff base surfactant blends. J. Dispers. Sci. Technol. 29, 1484–1495. Atta, A.M., Abdel-Rahman, A.H., Hamad, N.A., 2008b. Synthesis and evaluation of some Schiff base surfactants for treating crude oil emulsions. J. Dispers. Sci. Technol. 29, 1222–1232. Bailes, P.J., Kuipa, P.K., 2001. The effect of air sparging on the electrical resolution of water-in-oil emulsions. Chem. Eng. Sci. 56, 6279–6284. Carmen, C.M., Carbognani, L., 2001. Asphaltene paraffin structural interactions. Effect on crude oil stability. Energy Fuels 15, 1021–1027. Cowell, M.A., Kibbey, T.C.G., Zimmerman, J.B., Hayes, K.F., 2000. Partitioning of ethoxylated nonionic surfactants in water/NAPL systems: effects of surfactant and NAPL properties. Environ. Sci. Technol. 34, 1583–1588. Crook, E.H., Fordyce, D.B., Trebbi, G.F., 1965. Molecular weight distribution of nonionic surfactants: II. Partition coefficients of normal distribution and homogeneous p, t-octylphenoxyethoxy-ethanols (OPE's). J. Colloid Sci. 20, 191–204. Dodd, C.G., 1960. The rheological properties of films at crude petroleum–water interfaces. J. Phys. Chem. 64, 544–550. Ese, M.-H., Yang, X., Sjoblom, J., 1998. Film forming properties of asphaltenes and resins. A comparative Langmuir–Blodgett study of crude oils from North Sea, European continent and Venezuela. Colloid Polym. Sci. 276, 800–809. Griffin, W.C., 1954. Calculation of HLB values of nonionic surfactants. J. Soc. Cosmet. Chem. 5, 249–256.
Hergeth, W.D., Alig, I., Lange, J., Lochmann, J.R., Scherzer, T., Wartewig, S., 1991. On the molecular mechanism of clouding in aqueous solution of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblock copolymers. Makromol. Chem. Macromol. Symp. 52, 289–296. Hirasaki, G.J., Miller, C.A., Raney, O.G., Poindexter, M.K., Nguyen, D.T., Hera, J., 2011. Separation of produced emulsions from surfactant enhanced oil recovery processes. Energy Fuels 25, 555–561. Huang, K., Lee, B.P., Ingram, D.R., Messersmith, P.B., 2002. Synthesis and characterization of self-assembling block copolymers containing bioadhesive end groups. Biomacromolecules 3, 397–406. Hunter, R.J., 1986. Foundations of Colloid Science. Clarendon Press, Oxford, U.K., pp. 55–76. Kim, Y.H., Wasan, D.T., 1996. Effect of demulsifier partitioning on the destabilization of water-in-oil emulsions. Ind. Eng. Chem. Res. 35, 1141–1149. Kloet, J.V., Schramm, L.L., Shelfantook, B., 2002. Application of the hydrophile– lipophile balance concept to the classification of demulsifiers and bituminous froth and its components. Fuel Process. Technol. 75, 9–26. Kokal, S., 2005. Crude oil emulsions: a state-of-the-art. Review. SPE Prod. Facil. 20, 5–13. Layrisse, I., Rivas, H., Acevedo, S., 1984. Isolation and characterization of natural surfactants present in extra heavy crude oils. J. Dispers. Sci. Technol. 5, 1–18. Mansur, C.R.E., Barboza, S.P., Gonzalez, G., Lucas, E.F., 2004. PLURONIC TETRONIC polyols: study of their properties and performance in the destabilization of emulsions formed in the petroleum industry. J. Colloid Interface Sci. 271, 232–240. Mansur, C.R.E., Oliveira, C.M.F., Gonzalez, G., Lucas, E.F., 1997. Phase behavior of aqueous systems containing block copolymers of poly(ethylene oxide) and poly (propylene oxide). J. Appl. Polym. Sci. 66, 1767–1772. McLean, J.D., Kilpatrick, P.K., 1997. Effects of asphaltene solvency on stability of water-in-crude-oil emulsions effects of asphaltene solvency on stability of water-in-crude-oil emulsions. J. Colloid Interface Sci. 189, 242–253. McLean, J.D., Kilpatrick, P.K., 1997. Effects of asphaltene aggregation in model heptane–toluene mixtures on stability of water-in-oil emulsions. J. Colloid Interface Sci. 196, 23–34. McLean, J.D., Spiecher, P.M., Sullivan, A.P., Kilpatrick, P.K., 1998. The role of petroleum asphaltenes in the stabilization of water-in-oil emulsions. In: Mullins, E.Y., Sheu, E.Y. (Eds.), Struct. Dyn. Asph.. Plenum Press, New York, pp. 53–66. Menon, V.B., Wasan, D.T., 1986. Particle–fluid interactions with application to solidstabilized emulsions part I. The effect of asphaltene adsorption. Colloids Surf. 19, 89–105. Migahed, M.A., Farag, A.A., Elsaed, S.M., Kamal, R., Mostfa, M., Abd El-Bary, H., 2011. Synthesis of a new family of Schiff base nonionic surfactants and evaluation of their corrosion inhibition effect on X-65 type tubing steel in deep oil wells formation water. Mater. Chem. Phys. 125, 125–135. Mohammed, R.A., Bailey, A.I., Luckham, P.F., Taylor, S.E., 1993. Dewatering of crude oil emulsions 1. Rheological behaviour of the crude oil–water interface. Colloids Surf. A 80, 223–235. Razi, M., Rahimpour, M.R., Jahanmiri, A., Azad, F., 2011. Effect of a different formulation of demulsifiers on the efficiency of chemical demulsification of heavy crude oil. J. Chem. Eng. Data 56, 2936–2945. Rosen, M.J., 1978. Surfactants and Interfacial Phenomena. John Wiley and Sons, New York, pp. 53–68. Sjoblom, J., Johnsen, E.E., Westvik, A., Ese, M., Djuve, J., Auflem, I.H., Kallevik, H., 2001. Demulsifiers in the oil industry. In: Sjoblom, J. (Ed.), Encyclopedic Handbook of Emulsion Technology, 1st ed. Marcel Dekker, New York, pp. 595–619. Stark, J.L., Asomaning, S., 2005. Synergies between asphaltene stabilizers and demulsifying agents giving improved demulsification of asphaltene-stabilized emulsions. Energy Fuels 19, 1342–1345. Taylor, S., 1992. Resolving crude-oil emulsions. Chem. Ind. 20, 770–773. Xia, L., Lu, S., Cao, G., 2004. Stability and demulsification of emulsions stabilized by asphaltenes or resins. J. Colloid Interface Sci. 271, 504–506. Xu, Y., Wu, J., Dabros, T., Hamza, H., 2005. Optimizing the polyethylene oxide and polypropylene oxide contents in diethylenetriamine-based surfactants for destabilization of a water-in-oil emulsion. Energy Fuels 19, 916–921. Yen, T.F., 1974. Structure of petroleum asphaltene and its significance. Energy Sour. 1, 447–463. Zhang, Z., Xu, G., Wang, F., Dong, S., Chen, Y., 2005. Demulsification by amphiphilic dendrimer copolymers. J. Colloid Interface Sci. 282, 1–4.
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
Dewatering of petroleum crude oil emulsions using modified Schiff base polymeric surfactants Ayman M. Atta a,b,n, Hamad A. Allohedan a, Gamal A. El-Mahdy a a b
Surfactants Research Chair, Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia Petroleum Application Department, Egyptian Petroleum Research Institute, Nasr City 11727, Cairo, Egypt
art ic l e i nf o
a b s t r a c t
Article history: Received 27 August 2012 Accepted 3 September 2014 Available online 5 October 2014
The new discoveries of increasingly heavy oil represent a major challenge for the oil industry because of its undesirable properties, such as its tendency to form stable emulsions and high viscosity and acidity. Dependent upon the production scheme, very stable water-in-oil emulsions can be formed from these oils, for which the complete separation of oil and water phases using conventional processes requires excessive heating, chemical addition, and high residence time. In the present work, the improvement of existing technologies and the development of new additives to decrease the temperature of the treatment processes of these emulsions are very important to ensure the processing of these oils in view of productivity gains. In this respect, the rate of demulsification of different types of crude oil emulsions using series of Schiff base polyethoxylates as demulsifiers was evaluated. The chemical structure of the prepared surfactants was determined by FTIR and 1H NMR analyses. Characterization of the surface activity of the prepared surfactants is performed to investigate the relation between the structure of surfactants and their performances. The interfacial properties of the prepared demulsifiers at the oil–water interfaces were investigated by means of the interfacial tension relaxation method. The results showed that the slow relaxation processes at low concentration because of stronger adsorption ability of the prepared demulsifiers. This was attributed mainly to rearrangement in the conformation of the molecules appeared with increasing demulsifier concentration. The demulsification efficiency of the prepared demulsifiers for the synthetic crude oil emulsions was determined at different temperatures using bottle test. The demulsification rate increased with surfactant concentration up to the onset of surfactant aggregation in the oil, water phase and a third, surfactant-rich phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: Schiff base demulsifier interface interfacial tensions crude oil emulsions emulsion stability
1. Introduction Most of the oil fields around the globe are producing oil that is often accompanied by significant amounts of water. Many types of emulsions are created in the same produced fluids during production. These emulsions may be oil-in-water (O/W), water-in-oil (W/O), or complex emulsions such as oil-in-water-in-oil (O/W/O) or water-in-oil in-water (W/O/W), depending on the water, the oil, energy in the flow, and oil-to-water ratios (Dodd, 1960; Mohammed et al., 1993; Acevedo et al., 1993; McLean et al., 1998; Ese et al., 1998). Petroleum dewatering has been a challenge in conventional crude oil industry for several decades (McLean et al., 1998). Most of the studies on stability mechanisms of water incrude oil emulsions can be divided into two categories: interfacial tension and interfacial rheology studies, (Dodd, 1960; Mohammed
n
Corresponding author.
http://dx.doi.org/10.1016/j.petrol.2014.09.017 0920-4105/& 2014 Elsevier B.V. All rights reserved.
et al., 1993; Acevedo et al., 1993; Ese et al., 1998). Comparative studies on emulsion stability using modeling of components and solvents (McLean and Kilpatrick, 1997,) indicated that the properties of interfacial material are responsible for emulsions stability (Hunter, 1986; McLean and Kilpatrick, 1997; Layrisse et al., 1984; Acevedo et al., 1992). The majority of the studies showed that water-in-crude oil emulsions are stabilized by a rigid film or “skin” at the oil–water interface (Dodd, 1960; Taylor, 1992; Acevedo et al., 1993). A typical dehydration plant operation usually comprises the following six major steps: separation by gravity settling, chemical injection, and heating, addition of fresh (less salty) water, mixing, and electrical coalescing. The treatment involves allowing time for water drops to settle out and be drained off. Settling time and draining are accomplished in wash tanks, separators, and desalting vessels. Settling and draining can be speeded up using one or more of the following actions: injecting chemicals (demulsifier), applying heat, adding diluents (freshwater), and applying electricity (AlOtaibi, 2004). Therefore, it is expected that the most important
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parameters affecting performance of the dehydration are settling time, mixing time, chemical dosage, crude temperature, and wash water flow rate ratio. In heavy oil production, the efficiency of extracting clean dry oil from the produced fluids depends on the process conditions as well as the choice of chemical additives used to destabilize the emulsions. The effectiveness of the measurements used to destabilize these emulsions depends on the properties of the surfactants such as the hydrophile–lyophile balance (HLB) as well as on properties of the oil and water phase. The complication arises from the treatment and production of the fluids was attributed to the presence of solids in the fluid (Angle, 2004). Demulsification of crude oil emulsions has been attracted great attention and many published work was recently reviewed, (Angle, 2004; Sjoblom et al., 2001) and gained an importance discussion to specific cases (Bailes and Kuipa, 2001; Xia et al., 2004; Stark and Asomaning, 2005; Zhang et al., 2005). The majority of the studies focused on simple model systems, and few studies focused on complex field emulsions, which are often more difficult to obtain. The demulsifier combinations often used for emulsion treatments become ineffective as more complex emulsions are produced. Commercial demulsifiers are polymeric surfactants, such as copolymers of polyoxyethylene (PE) and polyoxypropylene (PO) or alkylphenol–formaldehyde resins, dodecylbenzenesulphonic acid, or blends of different surface-active compounds (Kokal, 2005). In the previous works (Atta et al., 2008a, 2008b) new surfactant blends were prepared by etherification of Schiff base surfactants. There are two challenges faced us in this work to separate water from the crude emulsions which are emulsion stability and demulsification at temperature below 60 1C. In our quest for new polymeric demulsifiers, we have focused on Schiff bases, condensed organic derivatives containing azomethine groups C ¼N–. In this work we have synthesized a new class of soluble such Schiff bases and evaluated them as demulsifiers for crude petroleum at temperatures not exceeding 60 1C. The correlation between the surface activity of the prepared Schiff base surfactants and their performance as demulsifiers is another goal of the present work. The proposed work aims also to examine the interfacial behaviors to determine and improve the poor performance of the demulsifiers.
2.2. Preparation of Schiff base surfactants 2.2.1. Synthesis of Schiff base monomers The reactions were completed in three-necked flask (0.5 L capacity) equipped with a condenser, magnetic stirrer, thermometer, dropping funnel and nitrogen atmosphere inlet. The flask was charged with ODA or DDA (0.25 mol) and 200 ml absolute ethanol. HBA (0.25 mol) were dissolved in 200 ml absolute ethanol and the solution was added into flask within 1 h with stirring in the presence of nitrogen gas. The temperature of reaction was gradually increased up to reflux for 3 h. The reaction was cooled to filtrate the solution. The products of condensation of HBA with ODA and DDA were designated as ODS and DDS, respectively. 2.2.2. Ethoxylation of Schiff-base monomers A high pressure stainless steel autoclave (Parr model 4530, USA) of 1 L capacity, 400 psi maximum pressure and 180 1C maximum temperature was used for ethoxylation reaction. The autoclave is equipped with a magnetic drive stirrer, an electric heating mantle with a thermocouple inserted in the reactor body, a cooling coil, a pressure gauge and a drain valve. The prepared ODS, DDS and their polymers were charged into the reaction vessel with Na metal as a catalyst (0.3 wt%), the reaction mixture was heated to 180 1C with continuous stirring while passing a stream of nitrogen gas through the system for 10 min to flush out air. The nitrogen stream was then replaced by ethylene oxide. The ethylene oxide was introduced through the inlet gas valve until the desired amount of ethylene oxide was reacted. Generally, as a result of the introduction of ethylene oxide, the pressure was substantially increased as indicated by the pressure gauge, until it reached a maximum value. The pressure drop indicates ethylene oxide consumption. The reaction completion was established when the pressure reached its minimum value. At this stage, heating was stopped and the content was cooled gradually to ambient temperature by means of the cooling coil connected to the reactor carrying cold water. After cooling, the product obtained was discharged, weighed and neutralized with HCl. The ethoxylated products of ODS and DDS at different ethoxylation times can be designated as EODS, EDDS-1 and EDDS-2, respectively. 2.3. Measurements
2. Experimental 2.1. Materials Octadecyl amine (ODA), dodecyl amine (DDA), and p-hydroxybenzaldehyde (HBA) were obtained from Aldrich Chemical Co., and used without purification. All solvents were purchased from Aldrich Chemical Co. (Germany). Table 1 shows details of the Land Balayium crude oil (produced from Petrobel Co., Egypt) and its origin. On the other hand, the formation water was obtained from production wells (produced from Petrobel Co., Egypt). Table 1 Physicochemical properties of the tested land belayim crude oil. Test
Method
Results
API gravity Specific gravity 60/60 (1F) Wax content (wt%) Asphaltene content (wt%) Pour point (1C)
Calculated IP 160/87 UOP 46/64 IP 143/84 IP 15/67(86)
25 0.896 5 13 18
The resin and asphaltene contents of crude oil were determined according to standard procedure ASTM D2000 methods (ASTM-2007D, 2000). The relative solubility number (RSN) values of the surfactants were measured by the newly developed method using ethylene glycol dimethyl ether and toluene as titration solvents. In this method, 1 g of surfactant was dissolved in 30 mL of solvent consisting of toluene and ethylene glycol dimethyl ether and the resultant solution was titrated with deionized water until the solution became persistently turbid. The volume of water in mL was recorded as the RSN number. The solubility tests were performed in different solvent media for EODS, EDDS-1 and EDDS-2, at a concentration of 40% (w/v), at room temperature, with xylene, ethanol, and xylene/ethanol (75:25). The nitrogen content was measured with a Tecator Kjeltech auto analyzer. A Tecator 1007 digester was used for the initial digestion of the samples. Infrared spectra were determined with a Perkin-Elmer model 1720 FTIR (KBR). While, 1H NMR spectra of the prepared Schiff base monomers and polymers were recorded on a 400 MHz Bruker Avance DRX-400 spectrometer.
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Microscope images of petroleum crude oil emulsions sealed in 200-μm deep Helma cells were observed and photographed using a Nikon E600 microscope and a Nikon AE850 digital camera. The surface and interfacial tensions between formation water and crude oil were measured at 25 1C by means of the pendent drop technique using a drop shape analyzer (model DSA-100, Kruss, Germany). In this method the shape of a pendent drop was fitted to the theoretical drop profile according to the Laplace equation, using surface tension as one of the adjustable parameters. The error limits of these measurements were of the order of 0.1 mN/m or less. The DSA-100 analysis required accurate density measurements, which were obtained as functions of temperature and surfactants concentration with an AP Paar DMA45 MC 1296 densitometer. Pendant drops were formed on the tip of a needle with an outside diameter of 1.84 mm.
721
OH
OH + R-NH 2 R=C12H25 =C18H37
CHO OH
CH=N-R O
O-[CH2-CH2-O]n-H
Na CH=N-R
CH=N-R
2.4. Preparation of water crude-oil emulsions
Scheme 1. Preparation of Schiff polymeric surfactants.
Water-in-oil emulsions collected in the field were free of demulsifier. Shortly after collection, samples were drained of any free water. Free water is defined as water that separates rapidly and is not emulsified. All emulsions were prepared with a total volume of 100 mL. The ratio between crude petroleum and the aqueous phase (formation water) was in the range 10–50 vol%. The emulsions were prepared by mixing using a Silverstone homogenizer. In a 500 ml beaker, the crude petroleum was stirred at 35 1C (3500 rpm for 1 h) while sea water was added gradually until the two phases become homogenous. The ratios crude oil: water were 90:10, 80:20, 70:30 and 50:50. 2.5. Demulsification of the prepared emulsions The bottle test is used to estimate the capability of the prepared demulsifiers in breaking of water in oil emulsions. Demulsification was studied at 45–60 1C using gravitational settling with graduated cone-shaped centrifuge tube. The prepared Schiff base surfactant solutions were injected into the emulsion using a micropipette. After the contents in the tube had been shaken in an oscillating shaker for 1 min, the tube was placed in a water bath at 45–60 1C to allow the emulsion to separate. The phase separation was recorded as a function of time. The interface between the emulsion and separated water phase can be easily observed during the settling process. The demulsification rate (DDE%) can be calculated from the following equation; DDE% ¼Vo 100/V1, where Vo and V1 are volume of emulsified and separated water, respectively.
3. Results and discussion Schiff base compounds containing an imino group (–C ¼N–) are employed for several petroleum applications as corrosion inhibitors and demulsifiers (Atta et al., 2008a, 2008b; Migahed et al., 2011). The purity of the prepared Schiff base monomer is very important when used as starting materials to produce watersoluble Schiff base polymeric surfactants. The nonionic Schiff base surfactants were prepared from the ethoxylation of Schiff base monomer as illustrated in experimental section and Scheme 1. The reaction between p-hydoxybenzaldehyde and octadecyl amine or dodecyl amine produced 4-hydroxybenzaldehyde-N-octadecylimine (ODS) or 4-hydroxybenzaldehyde-N-dodecylimine (DDS), respectively. The physicochemical characteristics of the prepared Schiff base monomers were listed in Table 2. Nitrogen content indicates good agreements between the experimental and theoretical value. This reveals that the method of synthesis and purification was performed successfully. On the other hand, the Schiff base
Table 2 Physicochemical characteristics of Schiff base polymeric surfactants. Compounds
ODS DDS
Nitrogen content
Melting temperature
Yield
Theoretical (wt%)
Determined (1C)
(%)
3.86 4.86
3.82 4.81
112 128
90 85
monomers were ethoxylated to produce EODS, EDDS-1, and EDDS2 as illustrated in both Scheme 1 and experimental section in the presence of sodium as catalyst. 3.1. Chemical structure of ehoxylated Schiff base surfactants The chemical structure of Schiff base monomers and their ethoxylated derivatives were determined from FTIR and 1H NMR analyses. The FTIR spectra of DDS, and EDDS-1 were selected as representative samples and listed in Fig. 1a–b. 1H NMR spectra of EDDS-1, EDDS-2 and EODS were represented in Fig. 2a–d. The lack of O–H stretching band in the 3400–3500 cm 1 (Fig. 1) in the spectra of the ethoxylated derivatives confirms complete ethoxylation of ODS and DDS. The appearance of strong peaks in the spectra of both Schiff base and ethoxylated derivatives in the region 1580–1670 cm 1 (Fig. 1a–b) indicates the presence of C ¼N stretching bands (Atta et al., 2008a, 2008b). The disappearance of strong bands of aldehyde groups at 2750 and 1700 cm 1 which assigned for CH and C ¼ O stretching of aldehyde groups indicates the formation Schiff base monomer with high purity grade. The 1H NMR spectra of ethoxylated Schiff base, Fig. 2, show a complicated aromatic proton multiplet (7.1–7.6 ppm) and an imino proton singlet at 6.8 ppm downfield from the usual aromatic proton region. The triplet peak, observed in spectra of EDDS-1, EDDS-2 and EODS at 4.2 ppm can be assigned to CH2 groups attached directly to azomethine group. While, multiple peaks at 2.1 ppm (Fig. 2) can be attributed to (CH2)n of alkyl group. The ethoxylation of Schiff base monomers can be proved by appearance of new peaks of oxyethylene units at 3.6 ppm and –OH proton of PEO 2.5 ppm. The 1H NMR spectroscopic analysis was used previously for determining the propylene oxide/ethylene oxide ratio for the PPO–PEO block copolymers (Huang et al., 2002). In the present investigation the 1H NMR spectroscopic analysis is used to determine the degree of ethoxylation based on calculation integration ratio between the number of oxyethylene
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Fig. 1. FTIR spectra of (a) ODS, (b) DDS and (c) EDDS-1.
unit from the oxyethylene units (4 protons) in the hydrophilic moiety and the phenyl rings (4 protons) in the hydrophobic moiety. In this respect, the data for number of oxyethylene units was determined and listed in Table 2. On the other hand, the degree of ethoxylation can be calculated from the yield % of the purified ethoxylated Schiff base monomers. The data was used to calculate the theoretical molecular weights of ethoxylated Schiff base monomers. The calculated ethoxylation number (EO)n from 1 H NMR data is inconsistent with the data that determined from weight measurements.
3.2. Solubility of the prepared surfactants It was found that toluene and xylene/ethanol (75:25) solvents are the normal dispersants among the other solvents and used for formulations containing demulsifying additives (Razi et al., 2011). In this respect, solubility tests of the EODS, EDDS-1, and EDDS-2 at room temperature were carried out in different solvent media. The solubility tests of the EODS, EDDS-1, and EDDS-2 in water, toluene, and xylene/ethanol (75:25) were performed and the results indicated that all prepared surfactants were soluble in water, and
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xylene/ethanol (75:25). The difference in solubility was attributed to the hydrophil–lipophil balance (HLB) of the surfactants. In this respect, the HLB values were calculated by using the general formula for nonionic surfactants (Griffin, 1954). The equation applied as follows: HLB ¼ ½ðMH 20Þ=ðMH þ MLÞ
ð1Þ
where MH and ML are molecular weights of both hydrophile and hydrophobe moieties, respectively. HLB values of the prepared nonionic surfactants were calculated and listed in Table 3. The solubility of nonionic surfactants in polar solvents increases with increasing their HLB values. It is noticed that the HLB values of the prepared surfactants ranged from 11.2 to 13.9 which indicates the good solubility of these surfactants in polar solvent. Relative solubility numbers (RSNs) can be used to represent the hydrophobicity of the surfactant (Xu et al., 2005) due to the experimental difficulty for HLB determination. The data of RSN (vol%)
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were measured and listed in Table 3. The RSN values of the three series of Schiff base nonionic surfactants used in this work indicated that, at the fixed linear alkyl chain, the RSN value of the surfactant increases with an increment in EO number as determined for EDDS-1 and EDDS-2. This can be attributed to an increasing EO number enhances the hydrophilicity of the surfactant, while an increasing of alkyl chain length enhances its hydrophobicity. It is well known that, the activity of the chemical additives affected by the solubility of these additives at different temperatures. The cloud point is the temperature below which there is a single phase of a molecular solution. Above the cloud point, a nonionic surfactant loses its solubility in water, causing the formation of two phases and giving the mixture a cloudy aspect (Hergeth et al., 1991). To investigate the effect of the temperature on the solubility of the EODS, and EDDS was measured in aqueous medium (up to 85 1C), the cloud points were measured at concentrations of 100, 1000, and 10,000 ppm. The data of cloud temperatures were measured and listed in Table 3. It can be expected that the solubility will be hampered with increasing the concentration, i.e., as the surfactant concentration increased, the cloud point temperatures decreased. It was also noted that EDDS surfactants were more soluble in water than EODS. This means that the alkyl substituents are the main factor for solubility over polyoxyethylene units as shown in previous studies (Mansur et al., 2004, 1997). 3.3. IFT of Schiff base surfactants at water oil interface There are natural surface-active fractions (such as asphaltenes, resin, naphthenic acid, and porphyrin materials) that can be adsorbed onto the interface and form a firm film at the oil–water interface, resulting in high stability of crude oil emulsions. The measurements of the W–O interfacial tension were performed using the pendant drop method. This method allows us to follow the variation of the W–O interfacial tension values as a function of time. The interfacial tension values can be calculated after the processes of diffusion, adsorption, reorganization at the interface, desorption, and transfer of the molecular mass to the other phase have attained the equilibrium. It was previously reported that, the faster the minimum interfacial tension values the quicker the diffusion of the molecules to the interface. This in turn leads to the formation of thinner and less elastic films that rupture more readily (Yen, 1974). The demulsification of produced crude oil emulsions is an important problem in oil field industry. Chemical demulsification using demulsifiers is in common use in oil field chemistry. The demulsification ability of a demulsifier is mainly controlled by two factors: one is the hydrophilic–hydrophobic ability; the other is the ability to destroy the interfacial film. The structure of the demulsifier can influence both of the above two factors. To observe the influence of the structure of the ethoxylated Schiff base the EODS, EDDS-1, and EDDS-2 polymeric surfactants on the interfacial tension measurements, they were dissolved in a mixture of xylene/ethanol (75:25) or water using
Fig. 2. 1H NMR spectra of (a) EDDS-1, (b)EODS, and (c) EDDS-2.
Table 3 Degree of ethoxylation and cloud temperature of the prepared Schiff polymeric surfactants. Surfactants
Number of ethylene oxide unit
Theoretical molecular weight
HLB
RSN
(g/mol) EODS EDDS-1 EDDS-2
18 15 10
1155 948 728
13.7 13.9 12.01
14 18 12
Cloud temperature (1C) at different surfactant concentrations 100 (ppm)
1000
10,000
80–82 485 77–79
77–78 485 73–75
75–77 485 70–71
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ruptured occurred after 10 min of analysis. This behavior is related to the higher ability of molecule to adsorb at the interface which favors the process of diffusion in the oil. The water forms the interface with the oil when water used as solvent for the prepared surfactants, then adding this solution to the water. It can be concluded that there was a great reduction in interfacial tension after 25 min when the aqueous surfactant solution was added to the water phase. This result reflects the hydrophobicity of these surfactants which tend to migrate quickly to the interface and minimizes its contact with the water. The xylene/ethanol (75:25) mixture provided the fastest reduction of the interfacial tension, reaching a value of 2.5 mN/m after 8 min among the other solvent media in which surfactants dispersed in the oil phase. The tension decreased gradually followed by a slight decline after long time measurement (25 min). The W–O interfacial tension values recorded during the last stage of test can be attributed to the strong influence of the solvent medium on the surfactant ability to reduce interfacial tension. The effectiveness of these three media can be arranged in descending order: xylene/ethanol (75:25) 4toluene 4water. It can be confirmed that the higher the hydrophilic solvent, the easier the dispersion of the surfactant in the crude oil emulsions. 3.4. Evaluation of the performance of the Schiff base surfactants as demulsifier Fig. 3. Microscope graph of (a) W/O and O/W emulsions.
Table 4 IFT data between different Schiff base polymeric surfactant solutions and crude oil interface at 25 1C. Surfactants
Concentrations
IFT (mN/m) Xylene/ethanol (75:25)
Water
(ppm) EODS
0 50 100 250
18.1 14.1 7.2 4.2
28.1 26 22 14
EDDS-1
50 100 250
18.0 18.0 12.2
26.0 22.2 16.5
EDDS-2
50 100 250
9.9 5.8 3.2
18.1 14.3 12.1
different concentrations. The interfacial tension (IFT) between surfactant solution and crude oil interface was measured using a Kruss drop shape analyzer (DSA100) at 25 1C. Fig. 3 shows the dynamic interfacial tension of crude oil/EDDS-2 of different concentrations interface. The results of IFT were measured and listed in Table 4. The interfacial tension of crude oil/formation water was 28.5 70.9 mN/m. Upon addition of different concentrations of the Schiff base surfactants, reduction of the interfacial tension was obtained down to about 3.5 mN/m at 250 ppm of EDDS-1 (Fig. 3) with the least time for equilibrium. The variation of IFT data with time indicated that all of the Schiff base surfactants reduced the W–O interfacial tension. The interfacial tension values initially decline with time due to gradual migration of surfactant molecules to the interface and adsorbed as monolayers according to the Gibbs isotherm. Data indicated that EDDS-2 and EODS were most effective in reducing the interfacial tension and the droplet
Crude oil emulsions commonly exist in the form of water in-oil emulsions. The stability of the emulsion was mainly determined by the mechanical strength of the film and the rheological properties of liquid–liquid films (Angle, 2001). It is well known that asphaltene responsible for stabilization of petroleum crude oil because it acts as stabilizer for different types of crude oil emulsions (Mohammed et al., 1993; Acevedo et al., 1993; Ese et al., 1998). Wax and mineral particles have also been shown to stabilize water-in-crude oil emulsions (Menon and Wasan, 1986). Asphaltene has HLB value 11.1 and assists to form stable crude oil emulsions (Kloet et al., 2002). In this respect, Land Balayium was owing to its high asphaltene content (15 wt%) as listed in Table 1. The crude oil was blend with different volume ratios of sea water ranging from 10% to 50% as illustrated in the experimental section. The produced emulsions show high emulsion stability even at high temperature (65 1C). Water does not separate up to 14 days. The microscope images of water-in-oil (W/O) emulsion and the oil inwater (O/W) emulsion were represented in Fig. 4. The average oil drop size of the O/W is larger than water droplet of the W/O for the typical heavy oil water emulsions. The data indicated that the
Fig. 4. Dynamic IFT oil versus drop age time for crude oil emulsions and different concentrations of EDDS-2 and EODS in xylene/ethanol (75/25).
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Fig. 5. DDE of PDDS-2 measured at different concentrations for petroleum crude oil emulsions having different oil/water compositions (A) 90/10, (B) 80/20 and (C) 70/30 at 60 1C.
W/O emulsion inversed to form O/W emulsion when the water volume percentage reached 30% as illustrated in Fig. 4. The dropping test of emulsion in toluene gave an evidence for the same results. The demulsification ability of a demulsifier is mainly controlled by two factors: one is the hydrophilic–lipophilic ability. The other is the ability to destroy the interfacial film. The structure of a demulsifier can influence both of the above two factors. The
removal of emulsified water and the solids associated with the diluted froth by both chemical and physical means is practiced in commercial operations on a trial-and-error type basis. Surface active demulsifier molecules will orient at the interface and displace the natural asphaltenes, eliminating the barrier to coalescence and promoting destabilization of the emulsion. The commercial surfactants are typically high molecular weight polymers
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Table 5 DDE data of the prepared surfactants at different concentrations and separation temperature 60 1C. Surfactants
Conc.
DDE% O/W 90/10
O/W 80/20
O/W 70/30
O/W 50/50
DDE
Time (min)
DDE
Time (min)
DDE
DDE
Time (min)
(ppm) EODS
50 100 250
100 100 100
300 90 60
100 100 100
270 180 270
53 60 100
360 360 330
100 100 100
360 90 60
EDDS-1
50 100 250
80 100 100
360 60 45
100 100 100
300 180 330
67 53 100
360 360 330
100 100 100
240 60 90
EDDS-2
50 100 250
80 100 100
360 60 45
100 100 100
240 90 210
60 100 100
360 45 105
100 100 100
45 45 45
Table 6 DDE data of the prepared surfactants at concentration 250 ppm and separation temperature 45 1C. Surfactants
Conc.
DDE% O/W 90/10
O/W 80/20
O/W 70/30
O/W 50/50
DDE
Time (min)
DDE
Time (min)
DDE
DDE
Time (min)
100 100 100
420 300 180
100 100 100
330 330 240
33 40 80
88 88 100
480 480 45
(ppm) EODS EDDS-1 EDDS-2
250 250 250
480 480 480
intended to flocculate the emulsion droplets. It is thought that the droplets then coalesce due to an increase in the collision frequency and under the influence of treatment processes such as centrifugation. In the present work an attempt is made to use new demulsifiers based on Schiff base polymeric surfactants in terms of the HLB system. In this study, the aim is directed to prepare low molar mass affective surfactants to add as demulsifier of stable water-in-crude oil emulsions. The demulsification using the pure Schiff base polymeric nonionic surfactants will be investigated by using different HLB surfactants. In the previous section, based on solubility of the prepared surfactants and IFT data, xylene/ethanol (75:25) selected as the solvent to dissolute the prepared surfactants with respect to its ability to break up water-in-oil emulsions. This enhanced solubilization is reflected in a better dispersion in crude oil–water emulsion and, consequently, in a better demulsification performance. Different concentrations of Schiff base surfactants ranged from 50 to 250 ppm. Bottle tests performed in the field showed that the emulsion breaker showed that the oil was dry, less than 3% BS&W. The relation between demulsification efficiency (DDE) and time of water separation of EDDS-2 was selected as representative sample and plotted in Fig. 5 at different crude oil water emulsion compositions and different temperatures 60 and 45 1C. The data for separation time and DDE was determined at different temperature 45 and 60 1C, and listed in Tables 5 and 6. It was noticed that, the emulsions color changed (becoming clearer), at the bottom of the tube used in the assays. The experiment result indicated that the solvent medium and alcohol can be acting as co-additives in the demulsification process and as a co-surfactant, respectively. It was found that the demulsification performance of a demulsifier is mainly dependent on the interaction between dissolute surfactants and water droplet
through diffusion and adsorption, which permits faster transport of demulsifier molecules to the water droplet interface. Moreover, dissolute demulsifiers give a better separation of the phases (water and crude oil phases) than an undiluted demulsifier does. The steady state observed in Fig. 5 was attributed to constant rate of dehydration for some time then increases again and eventually attaining the final percent of dehydration for each surfactant. Crude oil emulsions are stabilized by surfactants, viz., asphaltenes and resins and do not develop high surface pressures. Hence, steric stabilization of water-in-crude-oil emulsions is the most plausible mechanism of stabilization of such emulsions. Demulsifier molecules are adsorbed at the interface when they having the tendency to lower interfacial tension more than the natural surfactants and the film become unstable in the direction of coalescence of water droplets. Emulsions formed in the petroleum industry are predominantly water-in-oil or regular emulsions, in which the oil is the continuous or external phase and the dispersed water droplets, form the dispersed or internal phase. The water droplets become bigger with time after demulsifier is added. The role of the demulsifier is to change the interfacial properties and to destabilize the surfactant-stabilized emulsion film in the demulsification process. Small and uniform drops flocculate and some large drops begin to form during the initial stage at 30 min. Two or more large drops continue to form a single larger drop due to occurrence of coalescence during the middle stage. The droplet size eventually grows fast with a reduction in droplet number with the addition of demulsifier during the last stage. It can be concluded that coalescence of water droplets destroyed emulsions. One can quietly distinct three terms related to stability of crude oil emulsion. The first term is flocculation, which refers to the mutual attachment of individual emulsion drops to form flocs or loose assemblies. The reversible process occurs in many cases during flocculation can be avoided by imposing (forcing) less energy than required in the original emulsification process. The second term is coalescence, which describes the joining of two or more drops to form a single drop of greater volume, with small interfacial area. Although coalescence will result in significant microscopic changes in the condition of the dispersed phase, it may not immediately result in a macroscopically apparent alteration of the system. The third term is breaking of an emulsion. It explains the occurrence of the gross separation of the two phases. The identity of individual drops is lost owing to the change in the physical and chemical properties of the emulsion and followed by loss of stability in the emulsion. It has been established that the kinetics of chemical demulsification is complicated by the interaction of three main effects. These are the displacement of the asphaltenic film from the oil water interface by the demulsifier; flocculation and coalescence of water drops (Kim and Wasan, 1996; Carmen and Carbognani, 2001). Careful inspection of data indicated that in all cases, an increase in the concentration of the additive improved its performance in destabilizing the emulsion. EDDS-2 was the most efficient additive in breaking down the emulsion (Fig. 5). At approximate concentrations (100 and 250 ppm) 90% of the water had separated out after 15 min. Complete phase separation was achieved at a concentration of 50 ppm after 45 min. The phase separation was much less efficient for the other surfactants EDDS-1 and EDOS. In general, the efficiency of the gravitational separation for the samples tested increased in the following order: EDDS-24 EODS4 EDDS-1. Although EDDS-2 was the most efficient in reducing the interfacial tension and EDDS-1 was the least efficient (Table 5), there was no direct relationship between the efficiencies of the additives and this property. The results showed that there is some relation between the gravitational separation and the solubility preference of the additive in the oil phase (Table 3). The best performance displayed by EDDS-2 may be attributed to the highest bulk phase
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concentration. The performance of the poorly soluble polymer additives in the aqueous phase is better than that are soluble in both phases. However, there was a relationship among the efficiency of the gravitational separation, the molecular architecture, and the area occupied at the interface. The gravitational separation efficiency increased as the area occupied per molecule at the W–O interface declined (Table 4). The smallest areas were attained by the molecules with adjacent architecture. The better efficiency of EDDS-2 in relation to EDDS-1 can be related to the tighter packing of the molecules of the former. The high demulsification power of surfactants based on EDDS-2 and EODS reflects the hydration of long hydrophilic groups into bulk phase which prevents the crude oil to penetrate the exterior region of micelle to arrive to their interior core and solubilize there (Rosen, 1978). In the same time, the long hydrophilic chain reduces the hydrophobicity of the surfactant molecule; hence, the solubilization efficiency is reduced (Cowell et al., 2000). However, in terms of understanding mechanisms this kind of representation can be misleading. 3.5. Effect of the Schiff base surfactant concentrations on DEE In particular, a given demulsifier concentration will be above the critical value required for aggregation for some surfactants but below for others. In general nonionic surfactants added at concentrations below that required for aggregation to a nonpolar oil/ water mixture distribute as monomers between the two phases. For equilibrated oil/water mixtures containing the present homologous series of nonionic surfactants, the lower EO monomer species partition predominantly to the oil phase, whereas the higher EO members are predominantly located in the aqueous phase (Crook et al., 1965). Above a certain critical total concentration, all further surfactant added is involved in the formation of aggregates in either the oil, the water, or a third, surfactant-rich phase depending on the HLB of the system. The interfacial tension in such systems decreases with increasing surfactant monomer concentration but remains virtually constant above the “critical aggregation concentration” (cac). The cac values for three Schiff base surfactants in crude oil/water systems were determined using surface tensiometry by vigorously shaken known quantities of surfactant with crude oil and water and the mixtures were equilibrated at 40 1C for 10 days. The volume ratio of water to oil was kept constant at the value used in the emulsions (10 vol%). In order to avoid making tension measurements in the presence of crude oil, the surface tensions of the separated aqueous phases were measured. The cac values were determined as the break points in plots of the surface tension versus the logarithm of the
727
total surfactant concentration as shown in Fig. 6. The cac values applied only to the water volume fraction used (10%) and refer to the total systems. In order to compare demulsification rates, we have chosen to take the initial slope of the percentage resolution versus time curves (Fig. 5) as a measure of the demulsification effectiveness. The variation of the initial resolution rate with demulsifier concentration for EDDS-2, with n ¼10, exhibit maxima which coincide closely with the measured cac values. The cac values for EDDS-2, EODS and EDDS-1 are, 180, 120 and 80 ppm, respectively. From the given results it was noticed that the dehydration rates reached 100% for all surfactants indicating the high efficiency of them for treating crude oil emulsions. It is clear that the surfactants have optimum HLB values ranged from 12.0 to 13.7 and have the highest the demulsification efficiency from the present work data dealing with a water-in-oil emulsion. It can be concluded that the lowest hydration rates of the surfactants was reported for 70:30 W–O emulsions. It was previously reported that, the emulsions have composition of oil (70) and water (30) can be converted from W/O emulsion to O/W due to increased salinity of brine water. From these results, it is clear that the aqueous phase of the initial emulsion is below optimum salinity and, hence, is an O/W microemulsion. Accordingly, the separation of water from cannot be easily occurred from micro-emulsion. Surfactant having HLB between 1 and 8 promotes the formation of oil-in-water emulsions while surfactant having HLB between 8 and 12 may promote either type of demulsifiers. The data presented in our work indicated that the surfactants have HLB (12.0–13.7) show good demulsification efficiencies, while surfactants having HLB above 13.8 show low demulsification efficiencies. This behavior reflects the dependence of the demulsification power on the presence of surfactant molecules in the soluble form rather than micelles (Cowell et al., 2000). This can be attributed to the solubilization of crude oil into the hydrophobic interior core of micelle (Hirasaki et al., 2011). The data listed in Tables 5 and 6 indicated that the DDE was affected by the temperature of separation. The dehydration rate of the surfactants evaluated at 45 1C for different crude oil emulsion compositions using 250 ppm of surfactants solutions. The data indicated that the DDE was reduced in 70:30 (oil:water) emulsions and the time of water separations increased in the other emulsions determined at the same temperature 60 1C. These behaviors can be referred to solubility of the prepared surfactants at different temperatures. The good solubility of surfactants in oil phase can be referred to the presence of aromatic moieties in the structure of surfactant. Visual observation of the aqueous phase during the end of the bottle tests indicated that when water was used as solvent medium, a large amount of oil was trapped on the walls of the glass tube, indicating that this medium does not favor diffusion of the additive molecules in the oil phase. This behavior was also observed when toluene was used as the solvent medium.
4. Conclusions
Fig. 6. Surface tension versus log [surfactant] in the total system for Schiff base polymeric surfactants.
We have found that certain combinations of low molar mass Schiff base surfactants are easily used to demulsify water-in-crude oil emulsions. Schiff base surfactants and solvents can be added in small amounts to cause significant reduced water contents of crude oil emulsions. This is in general agreement with the use of high HLB surfactants to destabilize water-in-oil emulsions in, for example, oil emulsion treatment. The existence of an optimum surfactant concentration for such beneficial additives correlates with a minimum in interfacial tension and is consistent with conventional oilfield demulsifier experience. The results obtained by the interfacial tension relaxation method showed that the IFT values reduced near the critical micelle concentration of the prepared surfactants. With
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an increase in the Schiff base concentration, the IFT gradually decreased and the contribution of them also changed disciplinarily. Efficiency of gravitational separation of synthetic emulsion W–O of the Schiff base surfactants show the best results for water separation when added to the emulsion at a concentration of 100 and 250 ppm and solubilized in xylene/ethanol (75:25). The efficiency of the water gravitational separation for the samples tested increased in the following order: EDDS-24EODS4EDDS-1. The data of IFT measurement indicated that the EDDS-2 is the most efficient and EDDS-1, was the least efficient of the prepared surfactants.
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