Physico-chemical properties of heavy crude oil-in-water emulsions stabilized by mixtures of ionic and non-ionic ethoxylated nonylphenol surfactants and medium chain alcohols

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

Contents lists available at ScienceDirect

Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd

Physico-chemical properties of heavy crude oil-in-water emulsions stabilized by mixtures of ionic and non-ionic ethoxylated nonylphenol surfactants and medium chain alcohols ˜ c, Ronaldo Gonc¸alves dos Santos a,∗ , Antonio Carlos Bannwart b , Maria Isabel Briceno d Watson Loh a

School of Chemical Engineering, Universidade Estadual de Campinas, CP 6066, 13083-970 Campinas, SP, Brazil School of Mechanical Engineering, Universidade Estadual de Campinas, CP 6122, 13083-970 Campinas, SP, Brazil c School of Engineering, Universidad de Los Andes, Mérida 5101, Venezuela d Institute of Chemistry, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas, SP, Brazil b

a b s t r a c t This work describes the formulation and evaluation of concentrated, heavy oil-in-water emulsions stabilized by mixtures of ethoxylated surfactants and normal alcohols. The rheology, stability and droplet size of these emulsions were investigated as functions of the emulsification process parameters. The parameters investigated for this study include emulsifier agent composition, presence of additives, pH and salinity of the continuous aqueous phase, emulsification temperature, oil content and emulsion aging. The produced emulsions had viscosities ranging from 30 to 150 mPa s and represent a 30-fold reduction of the crude oil viscosity. Sauter mean diameters of the droplets ranged from 10 to 50 ␮m. The emulsions were produced by mixing the oil with an aqueous solution containing medium normal-chain alcohols and small quantities of a mixture of ethoxylated nonylphenol and ethoxylated amine surfactants. The presence of these alcohols led to a sharp decrease in the droplet size of the emulsion. This size decrease had a direct impact on the emulsions’ stability and apparent viscosity. The rheological parameters of the aged emulsions were also essentially constant over a 42-day period. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Petroleum emulsion; Heavy crude oil; Alcohol; Rheology; Stability; Droplet size

1.

Introduction

The decrease in worldwide reserves of conventional oils and the increase in global fuel demand have driven continuous innovation in the petroleum industry and have spurred the development of new production and transportation technologies for heavy oils. Forecasts point to heavy oils as the world’s main fossil energy resource in the near future (Suslick et al., 2003). This predicted reliance on heavy oils is accompanied by a predicted increase in the market value of these oils. Heavy oil reserves, including bitumen and extra-heavy oils, represent

a significant fraction of the total known reserves (Meyer and Attanasi, 2003). In Brazil, recoverable heavy oils correspond to 3 billion barrels (bb), which is equivalent to 26% of Brazil’s proved reserves. Another 4 bb could be added if adequate technologies were available (Trevisan et al., 2006). Prospective studies claim that the production should be shared in 20% of the total produced volume to maintain a favorable domestic demand/production ratio (Minami et al., 2003). The major barrier to utilization of heavy oil is the high pressure drop that occurs during pipe flow of these oils. These pressure drops are due to the high viscosity of the oil and lead

Abbreviations: EO, ethylene oxide group; EON, number of ethylene oxide group; HLB, hydrophilic–lipophilic balance. Corresponding author. Current address: Centre for Petroleum Studies, Universidade Estadual de Campinas, CP 6122, 13083-970 Campinas, SP, Brazil. Tel.: +55 19 8823 5908; fax: +55 19 3289 4999. E-mail address: [email protected] (R.G. dos Santos). Received 23 February 2010; Received in revised form 5 October 2010; Accepted 29 November 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.11.020 ∗

958

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

Table 1 – Physical–chemical properties of the oil studied. Nomenclature D32 Drmc di n ni K

Sauter mean diameter root mean cubic diameter droplet diameter flow behavior index number of droplet with i-size flow consistency index

Greek letters  apparent viscosity ˙ shear rate  shear tension

Property 3 a

Density at 25 C (kg/m ) API gravity (◦ API)b Surface tension at 25 ◦ C (mN/m)c Interfacial tension (against water) at 25 ◦ C (mN/m)c Water content (wt.%)d Asphaltene content – C5I (wt.%)e Asphaltene content – C7I (wt.%)e Naphthenic acid content (wt.%)e a

b c

d

to increases in the pumping energy required, which makes the oil’s transportation costs unattractive (Bannwart, 2001). Current technologies aim to reduce the oil’s viscosity either by heating or by dilution with lighter oils. However, wide application of these technologies in the oilfield is unsuitable because light oils are becoming increasingly scarce and because diluents such as kerosene and naphtha are very expensive. Alternative technologies that support the oil flow with emulsions of crude oil in water have been investigated for use in heavy oil production. The emulsification of heavy oil can reduce the fluid’s viscosity to significantly lower values, thus making its transportation in pipelines quite feasible (Langevin et al., 2004). Emulsion technologies are designed to disperse the crude oil as droplets within an aqueous phase that contains an emulsifying agent. Use of this configuration is based on experimental evidence that under shear flow, the less viscous fluid of two-phase dispersion systems migrates to the high shear region (i.e., near the wall) and lubricates the flow (Joseph, 1997). Under such conditions, crude oil can be transported with essentially no contact with the pipe wall. The pipeline flow of an oil-in-water (o/w) emulsion requires low pumping energy because pressure drop is controlled by the shear stress at the wall. Crude oil-in-water emulsions can reach a viscosity as low as 50 mPa s, and most pipeline operations of heavy oil are limited to viscosity values up to 400 mPa s (Rimmer et al., 1992), depending on the system dimensions and operational conditions. In addition to improving pipeline operations, emulsion technologies can also improve the recovery factor for both mature and offshore fields (Salager et al., 2001). Emulsions of crude oils are complex systems consisting of sophisticated mixtures of chemical structures. The constituent compounds affect emulsion stability and impact the level of interfacial tension reduction achieved between the phases. Droplets may also present a high density of charges, typically negative. These emulsions exhibit nearly shearthinning rheological behavior (Pal and Rhodes, 1989) resulting from the interplay of several phenomena including surface charge, salinity, disperse fraction volume and dispersed phase viscosity (Langevin et al., 2004; Salager et al., 2001). Several field tests have confirmed the viability of emulsion technology for transporting viscous crude oils. Emulsions of 13 ◦ API heavy oil were tested in the Shanjiasi field (China), and the results showed that a dispersed oil fraction between 0.6 and 0.8 produces an 80% reduction in pressure drop (Zhang et al., 1991). Oil-in-water emulsions have also been continuously pumped and stored for several days with no sign of degradation (Stockwell et al., 1988). Some reports have shown that emulsion technologies can enhance the oil recovery and can

Value ◦

e

940 19 33.1 21.6 0.12 9.0 3.0 0.5

Determined by immersion densitometry in the range 0.9–1.0 kg/m3 . Calculated according to the relation API = (141.5/dr (60/60)) − 131.5. Measured with a Sigma 701 tensiometer using the du Nuoy ring method. Determined by Karl–Fischer titration suing an AF8 titrator, Orion. C5I and C7I refer to asphaltenes precipitated, respectively, with npentane and n-heptane, and they were determined as described elsewhere by Santos et al. (2006).

lead to an increase in the recovery factor of mature fields (Bertero et al., 1994). Despite several demonstrations of the o/w emulsions as a viable technology for transporting viscous oils, proposals to formulate and prepare heavy oil emulsions lack an understanding of the influence of many variables on emulsion properties. For transport technology, the most important properties of heavy oil-in-water emulsions are their stability and their viscosity. Continued examination of emulsification technologies should also enable prediction of the effects that fluid dynamic, preparation and compositional variables have on the emulsions’ stability and viscosity. These predictions are an important component of reaching operational robustness. This paper describes the influence that parameters of the emulsifying process have on the main properties (stability, viscosity and droplet size) of concentrated heavy oil-in-water emulsions stabilized by mixtures of ethoxylated surfactants and normal alcohols. The data shown here describe emulsions (up to 70 wt.% dispersed oil) that, depending on the formulation methodology, exhibit high stability and have rheological behavior obeying a power law. The droplet size of the disperse phase regulates the emulsions’ stability and viscosity. These properties depend on emulsifying temperature, electrolyte concentration and the presence of co-surfactants, and these parameters were used as formulation variables for optimizing the emulsions’ properties in a way that permits transport of viscous oils in conventional pipeline systems. By manipulating the aforementioned parameters, viscosity values as low as 30 mPa s can be reached and could represent a drastically lower pumping pressure drop than that of the original crude oil.

2.

Materials and methods

2.1.

Crude oil

This study used samples of a Brazilian crude oil provided by Petrobras. The oil was free of dissolved gas (i.e., a ‘dead oil’), and its properties are summarized in Table 1.

2.2.

Chemicals

Two types of emulsifying agents were used. The first type was from the ethoxylated nonylphenol family (RENEX) of surfac-

959

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

2.3.

Emulsion preparation

Emulsions were prepared by dispersing crude oil in aqueous solutions. Emulsifying agents (except for the more non-polar alcohols) and other additives were initially dissolved in water; therefore, an additive’s concentration is always expressed on the basis of the water content. Whenever pH control was necessary, the pH of the aqueous phase was measured with a Quimis Q400M1 pH meter and glass electrode prior to emulsification. In the general procedure, both phases were kept separated at a specified temperature and were then mixed with one of the two stirrers. An Ultra-Turrax Ika model T18 was used for emulsifying at high stirring rates (10,000 rpm), and a mechanical stirrer Ika Labortechnik model RW20DZM with the R1342 dispersing element and four 1.27-cm blades was used for emulsifying at low stirring rates (1000 rpm). The emulsion type was confirmed by dilution tests (0.1 mL of emulsion into ca. 50 mL water) and, in some cases, by optical microscopy (as described below). Emulsions with oil content between 50 and 70 wt.% were prepared by three different emulsifying protocols: (1) pre-defined amounts of both the oil and the aqueous phase were manually mixed, left to equilibrate for 60 s and then stirred; (2) an amount corresponding to half of the oil content was mixed with the total amount of the aqueous phase and stirred for 30 s. After an interval of 60 s, the remaining oil was added to the pre-emulsion and stirred again for another 30 s; (3) an amount corresponding to half of the aqueous phase (containing the total amount of surfactant and additives) was mixed with the oil and stirred for 30 s. After an interval of 60 s, the remaining aqueous phase (free of surfactants and additives) was added to the pre-emulsion and stirred for another 30 s. These protocols are referred to as methods 1, 2 and 3, respectively.

2.4.

Determination of emulsion stability

The most common method for measuring relative emulsion stability is the bottle test, which determines stability by quantifying the amount of resolved water as a function of time or after a pre-determined period of time. The centrifugation method is a modified bottle test in which the natural sedimentation force is artificially increased to reduce the experimentation time and to mimic the natural aging of the emulsion. For these studies, emulsion stability was determined by the bottle test, both under rest and under centrifugation at 2000 rpm for 10 min (corresponding to ca. 700 × g) in an Excelsa centrifuge, model 206 BL (Fanem), at temperature near 25 ◦ C. The amount of resolved water was determined, and its percentage was used as a measurement of emulsion stability. The accuracy of prediction was 6 vol.%.

Aqueous phase resolved (Vol.%)

22

80

20 70 18 60

16 14

50

12 40

10

Stability Diameter

8 1

10

Mean Diameter D32 (μm)

tants with different ethoxylated chain lengths ranging from 8 to 100 EO units. The second type of agent was a surfactant with an ethoxylated amine (Ultramina, 5 EO groups). Emulsion breakers were commercial products based on polypropylene glycol, based on a mixture of phenolic resin and polypropylene glycol; and a pure ethoxylated nonylphenol 4EO. All of these surface-active agents were obtained as a kind gift from Oxiteno SA (Brazil) and were used as received. The other chemicals used were the following: sodium chloride (99.5%, Merck), 1-octanol (97%, Merck), 1-butanol (p.a., Nuclear), 1hexanol (>98%, Fluka) and 1-decanol (>95%, Fluka). Deionized and double-distilled water was used throughout the study.

30

100

EON Fig. 1 – Assessment of emulsion stability and average droplet size using formulations containing ethoxylated nonylphenol surfactants with different EO chain lengths.  stability;  mean Sauter diameter D32 . Emulsification conditions: 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min.

2.5.

Evaluation of emulsion rheological behavior

Rheological behavior was investigated with a Haake rotational rheometer, model RheoStress 1, operating at a controlled rate. Cylinder and plate configurations were used with gaps of 4.2 and 1.0 mm, respectively. All of these measurements were performed with temperature controlled within 0.1 ◦ C of the setpoint. An external temperature probe was used to observe the temperature, and a slight temperature increase due to shear effects was occasionally observed. These increases were never greater than 1 ◦ C.

2.6. Determination of particle size and size distribution Droplet sizes were determined with an Olympus optical microscope, model CBA-K, coupled to a CCD camera for image acquisition. Images were analyzed using the Image Tool software package (University of Texas Health Science Center, available at http://ddsdx.uthscsa.edu). Typically, 10 images were collected immediately after emulsion preparation for each sample. Particle size was determined for populations of 200–1500 droplets, and the distribution converged for populations larger than 800 droplets, in agreement with previous reports (Tolosa et al., 2006). Droplet diameters were individually measured and classified according to the class interval. These measurements were then used to produce histograms and statistical curves of frequency and accumulated frequency (using a volumetric basis). Average diameters and the associated standard deviations for each population were determined based on moment analyses, as suggested by Alderliesten (1990).

3.

Results and discussion

3.1.

Optimum HLB for oil emulsification

For determination of the optimum HLB, a series of o/w emulsions was prepared using pure ethoxylated nonylphenol surfactants containing from 8 to 100 EO groups, which corresponds to HLB values ranging from 12.3 to 19.0. The stability of these emulsions changed abruptly with changes in emulsifier HLB (Fig. 1). The semi-logarithmic curve in Fig. 1 exhibits an unambiguous minimum point and reveals that the most stable

960

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

a

3.2.

b

Evaluation of the method of emulsion preparation

Aqueous phase resolved (Vol.%)

emulsions were obtained by using ethoxylated nonylphenol with 12 EO groups (HLB = 14.1). Fig. 1 also shows the corresponding curves representing changes in the Sauter mean diameter, D32 , in relation to the EON for these emulsions. The smallest droplet size was found at an EON equal to 15 (HLB = 15), a result that is similar to the trend seen in the emulsion stability curve. The slight difference in the x-axis position of the minimum point position (corresponding to 3 EO groups) may be explained by the fact that methods for determining emulsifier HLB do not take completely into account for contribution of real interaction between the surfactants and both water and oil phase (Boyd et al., 1972). In addition, the usual distribution of EO groups in commercial ethoxylated surfactants must have an influence on the offset between the minimum points on the stability and droplet size curves.

30

25

20

15

10

5

M

F

M1 M1 M1 M1 M2 M2 M3 M3 M3

A A B B A B A A B

0 0

20

40

60

80

100

120

 ni · d3i D3,2 = i 2 n i i

· di

(1)

where ni represents the number of i-droplets whose diameter is di . In general, a higher stirring rate resulted in smaller emulsion droplets, regardless of the formulation or the method employed for emulsion preparation. Polydispersity of the sample was estimated with the ratio of the Sauter diameter and the root mean cubic diameter (D32 /Drmc ) to estimate the distribution width. This ratio was chosen because Drmc can provide insight into the hydrodynamic interactions occurring through fragmentation and coalescence of two droplets. Additionally, D32 is the most useful parameter for quantitatively describing interfacial phenomena in emulsions. The root mean cubic diameter, Drmc , represents the number of drops per unit volume, and it is calculated as

 (1/3) ni · d3i i  Drmc = n i i

(2)

where ni represents the number of i-droplets whose diameter is di . Polydispersity changes randomly in relation to the method, the formulation and the stirring rate used to prepare the

140

160

Diameter D32 (μm) 160

Apparent Viscosity (mPa.s)

M

The generation of surfactant-stabilized, oil-in-water emulsions was studied using the three different methods of emulsion preparation, as described earlier. Each method was applied to produce emulsions with 70% oil dispersed in the aqueous phase. The emulsions were prepared at 20 ◦ C and were mixed using stirring rates of 1000 and 10,000 rpm. Two different formulations (A and B) for the emulsifying agents were chosen so that the HLB of the surfactant mixture was near the optimum HLB described earlier. Formulation A combined an ethoxylated nonylphenol with 15 EO groups (Renex R-150) and an ethoxylated amine with 5 EO groups (Ultramina TA-50) in a 40:60 ratio (by weight). Formulation B is an 80:20 mixture of an ethoxylated nonylphenol with 100 EO groups (Renex R-1000) and Ultramina TA-50. The utility of each methodology for preparing more stable emulsions was assessed by examining the emulsion’s average droplet size, evaluated using the Sauter diameter, D32 , and by examining the droplet size distribution. The Sauter diameter was calculated as

S

1,000 10,000 1,000 10,000 10,000 10,000 1,000 10,000 10,000

150

M1 M1 M1 M1 M2 M2 M3 M3 M3

Inversion to w/o emulsion

140

80

F

A A B B A B A A B

S

1,000 10,000 1,000 10,000 10,000 10,000 1,000 10,000 10,000

60

40

20 0

20

40

60

80

100

120

140

160

Diameter D32 (μm) Fig. 2 – Relationship between droplet size (measured by the Sauter average diameter) and (a) stability (measured as the amount of resolved) and (b) emulsion apparent viscosity (determined at shear rate value of 500 s−1 ). Legend: M – method of emulsification; F – formulation; S – stirring. emulsions. Despite the lack of strong correlation between D32 /Drmc and the main formulation parameters (i.e., stirring, method and formulation), it can be seen that emulsions with larger droplet sizes (D32 ) present a wider size distribution. The narrowing of the size distribution caused by increasing the stirring speed from 1000 to 10,000 rpm can be a useful tool for evaluating the efficiency of the preparation method. In this respect, method 3 presents advantages over the other methods (Table 1A in Support Information), especially when utilizing formulation A. Droplet size distributions for emulsions prepared with formulation A (Fig. A1 in Supplementary Information) permit direct comparison of the three methods. These data show slightly better performance from method 3. Moreover, the droplet size distribution for these emulsions conforms to a rather symmetric log-normal distribution. The relationships between droplet size, emulsion stability and apparent viscosity are shown in Fig. 2. Fig. 2(a) reveals a direct correlation between emulsion stability (as measured by the amount of resolved water after centrifugation) and droplet size. Emulsions with smaller droplets are more stable, a trend that holds for emulsions prepared using different combinations of the tested parameters. Considering the normal route to emulsion breakdown, which involves increasing droplet

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

961

Aqueous phase resolved (Vol.%)

25

20

15

10

5

0 1

10

100

1000

NEO Fig. 3 – Effect of n-butanol and sodium chloride on emulsion stability.  no additive;  1 wt.% n-butanol;  1 wt.% NaCl. Emulsification conditions: 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min.

diameter via coalescence, emulsions with smaller droplets are farther from the final phase-separation stages. On the other hand, emulsion viscosity increases as droplet size decreases (Fig. 2(b)). This is a general behavior reported for other kinds of emulsions (Salager et al., 2001; Otsubo and Prud’homme, 1994). The trends shown in Fig. 2 are well-defined for emulsions prepared at 10,000 rpm, but there is somewhat greater data scattering for emulsions prepared at 1000 rpm.

3.3.

Effects of additives in the aqueous phase

Additives, including alcohols and electrolytes, are known to affect an emulsion’s properties (Salager et al., 2001). Fig. 3 shows the effect of adding n-butanol and NaCl to emulsions prepared with pure ethoxylated nonylphenol surfactants. In the presence of butanol, the emulsions are more stable, and changing the number of EO-groups in the surfactant molecule from 8 to 100 leads to no more than 5% of separated water. This finding suggests that alcohols are effective as co-surfactants in crude oil-in-water emulsions. The effects of alcohols on the stability of other types of emulsions have already been reported (Shiao et al., 1998; Capek, 2004). In the presence of medium-chain alcohols, better packing of surfaceactive molecules at the interface is achieved and consequently improves the interface’s mechanical stability. This effect is reported for alcohols having alkyl chains of more than four carbon atoms (Graciaa et al., 1993), and it normally increases with increasing alcohol alkyl chain length (Bourrel and Chambu, 1983) because of a decrease in the alcohol’s solubility in the aqueous phase. On the other hand, the addition of sodium chloride has negligible effects on emulsion stability, except that it causes the disappearance of the stability maxima at the previously determined optimum HLB value. Figs. 4 and 5 show the effects that electrolytes and alcohols have on the more complex emulsions generated with 1 wt.% of the Formulation A emulsifying agent. Because of the possibility for using seawater or production water in emulsion preparations, the effect of electrolytes on emulsion properties was evaluated by preparing emulsions in the presence of up to 6 wt.% NaCl (Fig. 4). Droplet size was only slightly affected up to ca. 4.5% NaCl and then displays a 50% increase when NaCl concentration reaches 6%. Accordingly,

Fig. 4 – Effect of NaCl content on emulsion stability, viscosity and average droplet size. Emulsification conditions: 70% oil dispersed, 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min. Formulation A and method 3. emulsion stability remains unaltered at low NaCl content and shows a slight increase between 4.5 and 6% NaCl. Apparent viscosity begins to increase when the NaCl concentration reaches 1.5% and rises to 110 mPa s (an increase of 50%) before it levels off at salt concentrations up to 6% NaCl. The relationship between droplet size and emulsion stability conforms to the general pattern described earlier. This pattern also predicts that an emulsion’s apparent viscosity should decrease with increasing average droplet diameter, but the properties of the emulsions made using 4.5–6.0% NaCl do not seem to follow this prediction. The droplet size distributions (Fig. A2 in Supplementary Information) reveal that these emulsions are rather disperse, and in some cases, a bimodal distribution is observed (e.g., for emulsions prepared with 4.5% NaCl). The behavior shown in Fig. 4 may be due to contributions of populations with different droplet sizes that overcome the predicted tendency, which is based only on the average diameter. This observation also stresses the importance of analyzing the droplet size distribution, especially in cases where more than one population is expected to be present. Such bimodal emulsions are known to display particular rheological behavior when compared to unimodal ones (Salager et al., 2001). Overall, these results suggest that electrolytes have a modest effect on the properties of these emulsions, but they do not have a pronounced effect as expected in the case of emulsions formulated with ionic surfactants. The observed effects may be related to the impact of NaCl on the aqueous solubility of emulsion components, or they may be related to

962

a

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

30

Diameter D32 (μm)

5 4

25

3 20

2 1

15

0 0

2

4

6

8

Aqueous phase resolved (Vol.%)

Stability Diameter D 32

10

N-Butanol (Wt.%)

Diameter D32 (μm)

Stability Diameter D 32

18

15

17 12

16 15

9

14 6 13 12

Aqueous phase resolved (Vol.%)

b

3 0

2

4

6

8

10

N-Octanol (Wt.%) Fig. 5 – Comparison of emulsion properties (average droplet diameter, viscosity and stability) in the presence of different amounts of butanol and octanol. Emulsification conditions: 70% oil dispersed, 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min. Formulation A and method 3. electrostatic interactions with ionic compounds present in the oil. These electrostatic interactions may be especially important for compounds having interfacial activity (e.g., asphaltenes and naphthenic acids) (Santos et al., 2006; Loh et al., 2007). The impact of alcohols on the properties of the more complex emulsions was investigated by employing n-alcohols with different alkyl chain lengths: n-butanol, n-hexanol and noctanol. The goal of these experiments was to assess changes in the emulsion properties and to determine how these effects (if any) varied with hydrophobicity of the alcohol. A more detailed analysis was performed using different concentrations of butanol and octanol, and the results are summarized in Fig. 5. With butanol, a higher amount is needed to produce clear effects, a trend that is probably due to the greater solubility of butanol in water that leads to a smaller fraction of alcohol incorporated into the interface. Moreover, emulsions with butanol contents above 8% display high stability and have no detectable resolved water from the stability tests. Emulsions produced with octanol display significant stability even at lower contents (there is no major change in stability between 2 and 6% octanol), and the addition of more octanol leads to a decrease in stability. This stability decrease at higher octanol content is accompanied by an increase in droplet size, which remains constant between 2 and 6% octanol but undergoes a 25% increase as the octanol content increases to 8%. The mean droplet size for emulsions containing butanol follows the same general trend. Between 2 and 6% butanol, droplet diameters are similar to those observed with octanol, and they undergo a 60% increase at higher butanol contents. Interestingly, at higher butanol contents, an increase in droplet size is

associated with a great increase in emulsion stability, a phenomenon that is contrary to the expected trend. Again, an analysis of the droplet size distributions for emulsions prepared with butanol indicated that the addition of butanol produces emulsions with broad, bimodal size distributions (Fig. A3 in Supplementary Information). Bimodal emulsions are less viscous than their unimodal counterparts, and the separation of droplets in the larger mode is facilitated because of the reduced interactions with the droplets in the smaller mode. It was also found that increased alcohol content corresponded to increased emulsion viscosities for both of the alcohols tested. The emulsion’s viscosity ranges from 100 to 160 mPa s when using butanol, and the values are slightly larger (100–350 mPa s) when using 2–8% octanol. Adding substantial amounts of alcohols at quantities higher than their solubility in water may be responsible for inducing the formation of more ordered surfactant structures in the aqueous phase, which would explain the sharp increase in viscosity (Graciaa et al., 1993; Radford et al., 2004; Tolosa et al., 2006) observed mainly for octanol. The flow curves (i.e., shear tension as a function of shear rate) obtained for emulsions containing different amounts of butanol reveal a deviation from the linear behavior predicted by the Ostwald–de Waele (power law) model (Fig. 6). This deviation becomes more pronounced as the butanol content increases. Additionally, the hysteresis loops (i.e., the area between the up- and down-scanning curves) ascribed to thixotropy also increase as the alcohol concentration increases. This phenomenon is related to the orientation and deformation of emulsion droplets under shear along the flow direction, and it is able to overcome the effects of Brownian motion, which leads to a reduction of the apparent viscosity observed under shear flow when compared to the viscosity determined when the fluid is at rest (Barnes et al., 1989). Both the shear thinning and the thixotropic behavior may also be related to the ability of shear flow to breakdown more ordered structures, induced by the presence of the alcohol, that have formed either at the interface or in the continuous phase. The enlargement of the area between the up-curve and the down-curve is suggestive of increased interactions between the droplets at higher butanol content.

3.4.

Effect of the pH of the aqueous phase

The effects of pH on emulsion properties were evaluated by adding acid (HCl) or base (NaOH) to the aqueous phase prior to emulsion preparation. The changes in droplet average size, emulsion viscosity and emulsion stability were determined for pH values from 4 to 10 (Fig. 7). This range was chosen to encompass the pKa of the ethoxylated amine surfactant, which was determined by potentiometric titration to be 5.6. Therefore, it was expected that this surfactant is positively charged at the lowest pH and is essentially neutral at pH 7 and 10. Fig. 7(a) and (b) shows that the mean droplet diameter decreases as the pH of the aqueous phase increases. This effect is seen mainly between pH 4 and 7. Similar behavior was obtained with respect to the emulsion viscosity, and the stability showed no variation related to pH changes. The changes in the emulsion properties as a function of pH are directly related to the protonation of TA amine groups and other acid/base compounds in the oil. At pH 4, TA-50 is essentially protonated, and in this condition it displays limited surface activity or emulsifying power. In this situation, emulsion properties are

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

963

Fig. 6 – Effect of butanol addition on emulsion rheological curves. Emulsification conditions: 70% oil dispersed, 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min. Formulation A and method 3.

a

50

5

Diameter D32 (μm)

40

4

30

3

20

2

10

2

b

4

6 8 10 pH of Aqueous Phase

1 12

140

50 Viscosity Diameter D32

40

130 120

30 110 20

η (mPa.s)

Diameter D32 (μm)

Aqueous phase resolved (Vol.%)

Stability Diameter D 32

100 [b]

10

90 2

4

6

8

10

12

pH of Aqueous Phase Fig. 7 – Effect of pH of the aqueous phase on emulsion stability, viscosity and droplet size. Emulsification conditions: 70% oil dispersed, 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min. Formulation A and method 3.

determined by the non-ionic surfactant R-150 and by the natural surfactants in the crude oil. After increasing the pH of the aqueous phase, TA-50 loses its positive charge and recovers its surfactant power. Additionally, the naphthenic acids become ionized and decrease the surface tension of the system. These results also indicate that the formulation used is sufficiently robust to produce emulsions that are stable within this moderate range of pH.

3.5.

Effect of the emulsifying temperature

The emulsifying temperature, or emulsion formation temperature, plays an important role in the emulsion formulation and preparation. It has effects on the viscosity of the oil and water phases, and it impacts the solubility of the additives in both phases. The emulsifying temperature also affects the self-assembly and the interfacial adsorption of the emulsifier agent. Emulsifying temperature effects were evaluated by studying o/w emulsions containing 1.5% NaCl and 6% nbutanol. Both the droplet size and the amount of resolved water decreased strongly for emulsions prepared within 10–20 ◦ C, while these properties remain constant if the emulsifying temperature is between 20 ◦ C and 60 ◦ C (Fig. 8). On the other hand, the apparent viscosity of these emulsions decreases linearly for emulsifying temperatures from 10 ◦ C to 60 ◦ C and reaches a value of about 95 mPa s at 60 ◦ C. These trends are due to the dual effects of the emulsifying temperature on the emulsion properties. Increasing the temperature promotes a decrease in the system’s viscosity and interfacial tension and favors the formation of smaller droplets. However, higher temperatures promote destabilization effects derived from the increased Brownian motion and mass transfer across the interface.

964

70

20

10

60

8

50 40

6

30

4

20 2

10 0

16

Diameter (μm)

Stability Diameter D32

Aqueous phase resolved (Vol.%)

Diameter D32 (μm)

a

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

12 8

4

0 0

10

20

30

40

50

60

70

0

80

50

Emulsifying Temperature (ºC)

b

50

130

40 120 30 110 20 100

10

90

0 10

20

30

40

50

60

Emulsifying Temperature (ºC) Fig. 8 – Effect of emulsifying temperature on emulsion stability, viscosity and droplet size. Emulsification conditions: 70% oil dispersed, 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min. Formulation A and method 3. Reduction in apparent viscosity with increasing emulsifying temperature can be attributed to the memory phenomena indicated by Salager et al. (2001). The memory effect permits emulsions to keep properties obtained under unsteady preparation conditions, provided that the changes in these properties are slower than the changes of the system. This way, the properties of the final emulsions (regardless the emulsifying temperature utilized – all of the emulsions were evaluated at 25 ◦ C) show the influence of the preparation conditions as transferred by the memory effect.

3.6.

60

65

70

75

Fig. 9 – Effects of oil content on the mean droplet size of fresh and aged o/w emulsions. Emulsification conditions: 1 wt.% surfactants, T = 20 ◦ C and stirring rate of 10,000 rpm for 1 min. Formulation A and method 3.

η (mPa.s)

Diameter D32 (μm)

60

55

Oil Content (Wt.%)

150 Viscosity Diameter D 32 140

70

t=0 t = 7 days

Effect of the content of dispersed oil

The content of dispersed oil in an emulsion formulated for pipeline transport of crude oils represents the effective productivity of the process because it is related to the oil flow rate. For the study of these effects, emulsions were formulated with 1.5% NaCl and 6% n-butanol. Increasing the oil content promotes an increase in the emulsions’ stability and apparent viscosity (Fig. A4 in Supplementary Information). At the same

temperature, emulsions with 55–70 wt.% of oil exhibit viscosities ranging from 30 to 150 mPa s, which is adequate for use in pumping operations, according to the criteria proposed by Rimmer et al. (1992). The increase in emulsion viscosity when the oil content increases represents a typical behavior reported in the literature (Rimmer et al., 1992; Zaki, 1997), but this increase depends strongly on the droplet size and droplet size distribution. Emulsions formulated for use in pumping processes should also be evaluated for changes in their properties over time. Fig. 9 shows that the mean droplet size increases after the aging time, consistent with the natural instability of dispersed systems. The curves for both the fresh and the aged emulsions have a maximum point achieved at 65% dispersed oil. This behavior is the result of formulation, composition and fluid dynamic effects (Pérez et al., 2002). After the maximum, droplet size tends to decrease as oil content is increased in the direction of the emulsion’s inversion point (Salager et al., 2001; Pérez et al., 2002; Salager, 2000). Flow and viscosity curves for emulsions containing 55%, 60%, 65% and 70% of dispersed oil and curves for the crude oil were measured at 20 ◦ C and in the shear rate range of 10–1000 s−1 (Fig. A5 in Supplementary Information). These data show that the emulsions exhibit shear-thinning behavior that becomes more pronounced at higher oil content. This observation is confirmed by the parameters used to describe the rheological power law behavior (Table 2). Table 2 also shows that decreases in the flow behavior index, n, caused by increasing of the oil content are accompanied by a worsening of the data fit for this model (estimated by the R2 parameter). It is worth noting that emulsions with up to 70% of dispersed oil, at 25 ◦ C and 500 s−1 (a typical shear rate found in oil pumping operations), are still less viscous than the crude oil at 80 ◦ C. Power law behavior is represented by the following expression. n

 = K · () ˙

(3)

Table 2 – Parameters of the Ostwald–de Waele model as evaluate for changes in the oil content in the oil-in-water emulsions. Oil content (wt.%) 50 60 65 70

n 0.89 0.77 0.60 0.57

K (Pa sn )

 ( = 500 s−1 , mPa s)

R2 (10–1000 s−1 )

0.06 0.28 1.66 2.23

31.7 67.9 134.4 151.9

0.9996 0.9990 0.9980 0.9938

965

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

No De-emusifier Ethoxylated Nonylphenol 4 EO Based in Polypropylene glycol Based in phenolic resin / polypropylene glycol

Water Resolved (%)

100 80 60 40 20 0 0

2

4

6

8

10

12

14

16

Time (min) Fig. 11 – Effects of different de-emulsifiers on the amount of water resolved from the emulsion.  no de-emulsifier;  ethoxylated nonyl phenol 4EO;  polypropylene glycol basis;  polypropylene glycol/phenolic resin basis.

Fig. 10 – Effects of aging on the flow behavior index and on the apparent viscosity evaluate by the power law model, as measured at 500 s−1 and 20 ◦ C.

to the initial tendency shown in Fig. 9, and it can be related to several phenomena occurring simultaneously as the emulsion ages. These phenomena are due the changes in emulsion properties, which are responsible for its natural instability. An important finding from the data in Fig. 10 is that, in general, the viscosity increases with increased dispersed oil content and prevails at longer observation times when the memory effects vanish.

3.8. where  is the shear tension,  is the shear rate, K is the flow consistency index (SI units, Pa sn ) and n is the flow behavior index (dimensionless).

3.7.

Aging of o/w emulsions

To evaluate changes in the rheological parameters of emulsions over time, the flow behavior index and the apparent viscosity were measured for emulsions with different oil contents (Fig. 10). The initial “zero-time” values in Fig. 10 represent measurements taken immediately after preparing the emulsion. Then, the emulsion was kept at rest at a controlled temperature until the next rheological evaluation. These properties were measured each week over a 42-day period. Fig. 10 shows that both the behavior index and the apparent viscosity remain almost constant over the 42-day interval for emulsions containing 55% or 70% of dispersed oil, while some changes are observed when the oil content is of 60% or 65%. For 55% and 70% oil emulsions, the absence of large changes in apparent viscosity values with respect to time is attributed to the higher stability of these emulsions. Fig. 9 shows no change in the mean droplet size of these emulsions over 7 days. For the 70% oil emulsion, this phenomenon might be expected because higher oil content improves emulsion stability (Fig. A4 in Supplementary Information). For the 55% emulsion, a lower number of droplet collisions due to the higher dilution of the emulsion may be responsible for the absence of changes in droplet size and consequently may drive the emulsion’s stability over time. During the first 7 days, the properties of the 60% and 65% oil emulsions changed to a greater extent, and then they started to recover their initial values during the measurements taken in the following weeks. In particular, the 65% oil emulsion tends to recover its rheological properties after 42 days. This observation is in agreement

De-emulsification and oil recovery

For being available to be used in the refining process and for marketing, the oil must be de-emulsified and recovered free of water. Three formulations for emulsion breakers were studied by a screening using bottle test, such as described in Section 2.4. These emulsion breakers were commercial products termed: (A) formulation based on polypropylene glycol; (B) formulation based on a mixture of phenolic resin and polypropylene glycol; and (C) pure ethoxylated nonylphenol 4EO. A very stable o/w emulsion was prepared using method 3 at 10,000 rpm and 25 ◦ C to be used in these tests. The emulsion composition was: 70 wt.% oil dispersed, 1 wt.% emulsifier agent (40% TA-50 and 60% R-150), 1.5 wt.% of NaCl and 8 wt.% of n-butanol. Fig. 11 shows the evolution of the amount of water resolved from the emulsion with time after the addition of 40 ppm (based on the oil phase) in tests under rest (no centrifugation) and constant temperature. Curves in Fig. 11 represent the kinetic process of emulsion de-stabilization. Data show that in the absence of de-emulsifier agent the emulsion is kept stable during the test time and longer. On the other hand, the presence of de-emulsifier formulated with basis on either phenolic resin/polypropylene glycol or pure polypropylene glycol promotes a fast emulsion breaking and an effective phase separation in a short period of time. Ethoxylated nonyl phenol accelerates the breaking process, however, it is not effective to resolve more than 20% of aqueous phase from the emulsion. Bottle test plus centrifugation was used to evaluate the action of the de-emulsifiers on more severe situation. An amount of 40 ppm of de-emulsifier was added into each o/w emulsion sample and the mixture was kept at rest at the test temperature (20 ◦ C and 60 ◦ C) during the test time (30 min and 60 min). Then, the mixture was centrifuged at the same conditions described in Section 2.4. After centrifugation, the phase

966

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

Table 3 – Volummetric percentage of residual water present in the crude oil phase after emulsion breaking. Product

Temperature = 20 ◦ C 30 min

A B C D

1.8 4.2 24.0 30

± ± ± ±

0.2 0.2 1.5 2

60 min 1.6 4.2 20.9 30

± ± ± ±

0.2 0.2 1.2 2

Temperature = 60 ◦ C 30 min 0.7 0.9 12.4 17.8

± ± ± ±

0.2 0.2 0.8 1.2

60 min 0.4 0.8 5.4 10.0

± ± ± ±

0.1 0.1 0.2 0.6

was separated and a sample from the oil phase was collected. Finally, the oil sample was evaluated with regard to the water amount by means of a Karl–Fisher titration. Results presented in Table 3 show that the residual amount of water in the oil recovered after the emulsion breaking can reach values lower than 1 wt.%, which means a dry oil for commercial purposes (Triggia et al., 2001). This means that heating and centrifugation, which may be assumed to mimic the aging process, improve significantly the efficiency of de-emulsifier agent to break the emulsion and recover the oil at industrial specification. A final point to mention is that phase inversion phenomenon may occur under changes in the process and formulation parameters, such as it has been demonstrated by Salager (2000), especially for systems with high fraction of dispersed phases, as the ones described here. To ensure that inversion does not occur, it is necessary to carry out a previous screening on the formulation parameters and properties to identify the inversion point and determine how distant from this point the formulation should stay to avoid that small changes from the environment and process conditions lead the system to inversion. Figs. 1 and 3 are well-defined examples for this screening. O/w emulsions formulated and prepared based on the principle described above did not undergo any phase inversion within the range of changes on the parameters investigated in this study.

4.

through a PhD scholarship to R.G.S. and through a research productivity grant to W.L. Support from the Brazilian agency FAPESP is also acknowledged for enabling a visit of M.I.B. to Campinas.

Conclusions

Concentrated oil-in-water emulsions were formulated using low-cost additives and were prepared through a low-energy procedure at laboratory scale. These formulations and procedures were used to reach suitable properties that enable the emulsion to be used for pipeline transportation of heavy oil. The presence of alcohols had pronounced effects on the emulsion properties, mainly the droplet size. Alcohols also improved the emulsifying process and produced emulsions with smaller droplet sizes and rather high stability. Removing the low-boiling-point alcohols, via heating, can change the emulsion stability whenever emulsion breakup is necessary. Aging tests demonstrated the robustness of these emulsions by following their rheological properties over time. The detailed investigation reported here provides a useful device for designing emulsions with specific, required properties for a given application. This report also contributes to understanding of the relationships between the most important properties of the emulsions and the parameters involved in the emulsions’ formulation and preparation.

Acknowledgements The authors gratefully acknowledge support from the Brazilian agency CNPq to this project through the CTPetro program,

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cherd.2010.11.020.

References Alderliesten, M., 1990. Mean particle diameters 1. Evaluation of definition systems. Part. Part. Syst. Charact. 7, 233–241. Bannwart, A.C., 2001. Modeling aspects of oil–water core–annular flows. J. Pet. Sci. Eng. 32, 27–143. Barnes, H.A., Hutton, J.F., Waters, K., 1989. An Introduction to Rheology. Elsevier Science. Bertero, L., Dilullo, A., Lentini, V., Terzi, L., 1994. An innovative way to produce and transport heavy oil through dispersion in water: laboratory study and field test results. In: Proceedings of the 69th Annual Technical Conference and Exhibition , New Orleans, LA, pp. 25–28. Bourrel, M., Chambu, C., 1983. The rules for achieving high solubilization of brine and oil by amphiphilic molecules. Soc. Pet. Eng. J. 23, 327–338. Boyd, J., Parkinson, C.S., Sherman, P.J., 1972. Factors affecting emulsion stability, and HLB concept. Colloid Interface Sci. 41, 359–370. Capek, I, 2004. Degradation of kinetically-stable o/w emulsions. Adv. Colloid Interface Sci. 107, 125–155. Graciaa, A., Lachaise, J., Cucuphat, C., Bourrel, M., Salager, J.L., 1993. Improving solubilization in microemulsions with additives. 2. Long chain alcohols as lipophilic linkers. Langmuir 9, 3371–3374. Joseph, D.D., 1997. Lubricated pipelining. Powder Technol. 94, 211–215. Langevin, D., Poteau, S., Hénaut, I., Argillier, J.F., 2004. Crude oil emulsion properties and their application to heavy oil transportation. Oil Gas Sci. Technol. 59, 511–521. Loh, W., Mohamed, R.S., Santos, R.G., 2007. Crude oil asphaltenes: colloidal aspects. In: Somasundaram, P. (Ed.), Encyclopedia of Colloid and Surface Science. Taylor & Francis, Nova York. Meyer, R.F., Attanasi, E.D., 2003. Heavy Oil and Natural Bitumen: Strategic Petroleum Resources. United States Geological Survey (U.S.G.S.). Minami, K., Almeida, A.S., Diniz, M.A.C., Palagi, C.L., Assayag, M.I., 2003. Campos bas challenges still to overcome. In: Proceeding of the 2003 Offshore Technology Conference , Houston, TX, USA. Otsubo, Y., Prud’homme, R.K., 1994. Effect of drop size distribution on the flow behavior of oil-in-water emulsions. Rheol. Acta 33, 303–306. Pal, R., Rhodes, E.J., 1989. Viscosity/concentration relationships for emulsions. J. Rheol. 33, 1021–1045. Pérez, M., Zambrano, N., Ramirez, M., Tyrode, E., Salager, J.L., 2002. Surfactant–oil–water systems near the affinity inversion. XII. Emulsion drop size versus formulation and composition. J. Dispersion Sci. Technol. 23, 55–63. Radford, S.J., Dickinson, E., Golding, M., 2004. Stability and rheology of emulsions containing sodium caseinate: combined effects of ionic calcium and alcohol. J. Colloid Interface Sci. 274, 673–686. Rimmer, D.P., Gregoli, A.A., Hamshar, J.A., Yildirim, E.I., 1992. Pipeline emulsion transport for heavy oils. In: Schramm, L.L. (Ed.), Emulsions: Fundamentals and Applications in the Petroleum Industry. American Chemical Society, Washington, DC. ˜ M.I., Brancho, C.L., 2001. Heavy hydrocarbon Salager, J.L., Briceno, emulsions. In: Sjöblom, J. (Ed.), Encyclopedic Handbook of Emulsion Technology. Marcel Dekker, New York.

chemical engineering research and design 8 9 ( 2 0 1 1 ) 957–967

Salager, J.L., 2000. Emulsion properties and related know-how to attain them. In: Nielloud, F., Marti-Mestres, G. (Eds.), Pharmaceutical Emulsions and Suspensions. Marcel Dekker, New York. Santos, R.G., Mohamed, R.S., Bannwart, A.C., Loh, W., 2006. Contact angle measurements and wetting behavior of inner surfaces of pipelines exposed to heavy crude oil and water. J. Pet. Sci. Eng. 51, 9–16. Shiao, S.Y., Chhabra, V., Patist, A., Free, M.L., Huibers, P.D.T., Gregory, A., Patel, S., Shah, D.O., 1998. Chain length compatibility effects in mixed surfactant systems for technological application. Adv. Colloid Interface Sci. 74, 1–29. Stockwell, A., Sit, S.P., Hardy, W.A., 1988. Transoil technology for heavy oil transportation. Results of field trials at Wolf Lake. In: Proceedings of the SPE European Petroleum Conference , London, UK, October 16–19. Suslick, S.B., Schiozer, D.J., Nepomuceno, F., Furtado, R., 2003. Forecasting the development of heavy-oil reserves in ultra-deep waters using technological risk models. In: Proceedings of the SPE Hydrocarbon Economics and Evaluation Symposium , Dallas, TX, USA.

967

Tolosa, L.I., Forgiarini, A., Moreno, P., Salager, J.L., 2006. Combined effects of formulation and stirring on emulsion drop size in the vicinity of three-phase behavior of surfactant–oil water systems. Ind. Eng. Chem. Res. 45, 3810–3814. Trevisan, O.V., Lisboa, A.C.L., Franc¸a, F.A., Trindade, W., 2006. Oil production in offshore fields: an overview of the Brazilian technology development program. In: Proceedings of the World Heavy Oil Conference , Beijing, China. Triggia, A.A., Correia, C.A., Filho, C.V., Xavier, J.A.D., Machado, J.C.V., Thomas, J.E., Filho, J.E.S., Paula, J.L., Rossi, N.C.M., Pitombo, N.E.S., Gouveia, P.C.V.M., Carvalho, R.S., Barragan, R.V., 2001. Fundamentos de Engenharia de Petróleo. Interciência, Rio de Janeiro. Zaki, N., 1997. Surfactant stabilized crude oil-in-water emulsion for pipeline transportation of viscous crude oils. Colloids Surf. A 125, 19–25. Zhang, J., Yan, D., Chen, D., Yang, X., Shen, Ch., 1991. Pipelining of heavy crude oil as oil-in-water emulsions. In: Proceeding of the Production Operation Symposium , Oklahoma City, OK, USA, pp. 911–918.