Effect of Surfactant on Stability and Size Distribution of Gas

Apr 14, 2014 - In specific, oil-in-water (O/W) emulsions are used in industrial operations involving two immiscible fluids and emulsifier agents that ...
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Effect of Surfactant on Stability and Size Distribution of Gas Condensate Droplets in Water Hossein Esmaeili, Feridun Esmaeilzadeh,* and Dariush Mowla Environmental Research Center in Petroleum and Petrochemical Industries, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Fars 71345-1789, Iran S Supporting Information *

ABSTRACT: Emulsions are mixtures of two immiscible liquids in which one liquid is dispersed in the other as fine droplets. In this work, different surfactants, including nonionic, anionic, and cationic surfactants, are used to form gas condensates in a water emulsion along with four kinds of surfactant: Span 80, Tween 80, CTAB, SDS, and the mixture of Span 80 and Tween 80. In order to evaluate the stability of these emulsions, the droplet size distribution is measured by an optical microscope. Having the lowest Sauter mean diameter of droplets, emulsions with Tween 80 and the combined Tween 80 and Span 80 surfactants had the most stability. The effect of gas condensate concentration in water was assessed, and the results revealed that solutions containing 0.022 mass fraction of gas condensate are more stable than those containing 0.044. The addition of nonionic surfactants had a better potential to improve emulsion stability, in comparison with cationic and anionic surfactants. Moreover, to investigate the stability of the emulsion, some stable emulsions were then demulsified with an EO/PO block copolymer of dodecan-1-ol. Results signified adding demulsifier caused gas condensate droplets to grow (about 200% growths), which resulted in a good coalescence.

1. INTRODUCTION Emulsions are mixtures of two or more immiscible liquids where one liquid is present in the other in the form of droplets. Emulsions are divided into water-in-oil and oil-in-water emulsions depending on continuous and dispersed phases.1,2 Emulsions are fundamental in many applications (food processing, petroleum, and detergents industries) and industrial products (foods, pharmaceuticals, or cosmetics). In specific, oilin-water (O/W) emulsions are used in industrial operations involving two immiscible fluids and emulsifier agents that allow emulsion formation and stabilization.3,4 The role of emulsifiers in emulsion formation and stabilization has been extensively described in the literature of food colloids.5 If these two phases are mixed with surfactant (emulsifier) using an agitator, an emulsion mixture is prepared.6−8 Chemical structure of surfactant often consists of a relatively long molecule with a hydrophobic end and a hydrophilic end. The hydrophilic group is polar and is attracted by water, whereas the hydrophobic group is nonpolar, is mainly a long-chain hydrocarbon, and is absorbed by oil.9−12 Surfactants are categorized in three main categories of anionic, cationic and nonionic. Selection of surfactant to form an emulsion has a special formulation that depends on the balance of hydrophilic−lipophilic groups. Hydrophilic− lipophilic balance (HLB) is a parameter that has a specific value for each surfactant and a special surfactant that has a given HLB must be used to develop an emulsion solution.4,13,14 In some cases lipophilic surfactant and in other cases hydrophilic surfactant are used to develop an emulsion solution, and © 2014 American Chemical Society

sometimes combinations of them are used for which the HLB is between those materials.15,16 Oil droplets in water are stabilized by a monolayer of surfactant or cosurfactant molecules enveloped around these droplets.17 One of the methods of evaluating the stability of emulsions is via distribution of droplets size of dispersed phase in continuous phase. Microscopy techniques have been used in analysis of colloidal suspensions, particles, and aggregated sediments to examine the characteristics of particles.18 A number of droplet size distribution methods have been used to characterize coalescence of droplets in emulsions.19,20 One common way to characterize the droplets size distribution of the emulsions is the use of parameters that indicate the dispersion, such as median drop size, Sauter mean diameter (Smd), and diameter of the largest stable drop size.3 Stability of emulsions containing larger drops is lower at equivalent other conditions.21 Usually, effluents wastewaters of refineries are contaminated by small fractions of gas condensates emulsion that must be removed. Optimal wastewater emulsion breaking requires understanding the conditions that promote a stable emulsion then use the best means to enhance demulsification. Petroleum refinery oily wastewater emulsion breaking is usually aided by the addition of demulsifier.22 Demulsification process consist of addition of little amounts of chemical compounds to emulsion.23−29 Received: November 15, 2013 Accepted: March 26, 2014 Published: April 14, 2014 1461

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Table 1. Characterization of Used Surfactants name

IUPAC name

formulation

HLB

type

purity/mass fraction

Span 80 Tween 80 CTAB SDS

[2-(3,4-dihydroxyoxolan-2-yl)-2-hydroxyethyl] octadec-9-enoate polyoxyethylene sorbitan monooleate hexadecyl-trimethyl-ammonium bromide sodium dodecyl sulfate

C24H44O6 C64H124O26 (C16H33)N(CH3)3Br CH3(CH2)11OSO3Na

4.3 15 10 40

non-ionic non-ionic cationic anionic

>0.99 >0.99 >0.99 >0.99

1000 rpm. The emulsion container was 150 mL with 100 mL working volume. After the emulsion formation, solution was immediately analyzed by an optical microscope. To evaluate the stability of developed emulsions, the droplets size distribution for three droplets of different parts of emulsion were measured by the optical microscope. All the data were reported as mean. 2.3. Bottle Test Method. An EO/PO block copolymer of dodecan-1-ol was used to demulsify emulsion solutions. The emulsions formed, according to the above-mentioned procedure, were transferred to graduated glass bottles (100 mL). The 0.015 mL of demulsifier was added to the 50 mL emulsion immediately after emulsion preparation using the micro sampler, and then the bottles were stopped and shaken for 1 min to thoroughly mix the demulsifier and emulsion. The bottles were then placed in the oven at a constant temperature of 60 °C for 2 h.32 Finally, the samples are instantly analyzed by an optical microscope, and three droplets from different parts of emulsion are taken and their droplet size distribution are measured, then their average is obtained. 2.4. Droplets Size Distribution. The mean droplets size and droplets size distribution were determined from the captured images of optical microscope. The diameters of the recorded oil drops were afterward measured (one by one) by image processing of Matlab Software. The microstructure of emulsions was evaluated using an optical microscope equipped with a camera. To do so, a few droplets of homogenized sample were put on sampling glass and the microstructure pictures of samples were observed at magnification ratios of 10, 40, and 100, given the fact that the homogeneity of samples should have been verified. Finally, an average diameter was calculated for each emulsion sample. Values of the Smd, d3,2, which is inversely proportional to the specific surface area of droplets, were obtained as follows:2,3,33

Coalescence of oil droplets in emulsions occurs when droplets come into contact with each other. As a result, the size distribution of oil droplets changes over time. Droplets grow in size until they become too large.2,30,31 The objectives of this study were to elucidate the droplets size distribution of gas condensates in water after adding some surfactants and its effect on emulsion stability. The effect of gas condensate concentrations in water on the droplets size distribution and emulsion stability and, finally, the effect of demulsifier on the growth and coalescence of gas condensate droplets in water emulsion were also evaluated.

2. EXPERIMENTAL METHODS 2.1. Materials. All general chemicals (surfactants) used were purchased from Sigma-Aldrich Chemical Co. Table 1 shows the formulation of surfactants with their HLB values. Distilled water was used for the preparation of all solutions. Sodium dodecyl sulfate (SDS), Polyoxyethylene sorbitan monooleate (Tween 80), [2-(3,4-dihydroxyoxolan-2-yl)-2hydroxyethyl]octadec-9-enoate (Span 80) and hexadecyltrimethylammonium bromide (CTAB) surfactants were used to develop the emulsion of gas condensate donated by the South Pars Gas Company in distilled water. The main components of gas condensate were C5 to C14 and the other components were less. Table 2 shows characterization of the used gas Table 2. Characterization of Gas Condensate density/kg·m−3 at 15 °C Mwa/ kg·kmol−1 Tb/K mole fraction of alkanes mole fraction of cycloalkanes mole fraction of aromatics mole fraction of alkanes + cycloalkanes (C15+) mole fraction of aromatics (C15+) a

741 113 325 to 500 0.636 0.248 0.114 0.001 0.001

d3,2 =

b

Molecular weight. Boiling point range.

∑ nidi3 ∑ nidi2

(1)

where ni is the number of droplets with a diameter di. condensate. An optical microscope (Nickon, E400, Japan) equipped with a display was used to observe the droplets of hydrocarbon compound. The demulsifier was then used to demulsify the emulsion, which was an oxirane/epoxypropane (EO/PO) block copolymer of dodecan-1-ol (C12H25(OCH2CH2)0.42(OCH2 CH2 CH2)1.3OH) and purchased from Isfahan Copolymer Company (Iran). The molecular weight and HLB of the demulsifier are 279 g/mol and 6.7 g/mol, respectively. 2.2. Emulsions Preparation. In this work, the effects of four types of surfactants (SDS, CTAB, Span 80, and Tween 80) with different HLBs on the formation of gas condensate emulsion in water were evaluated. The mixtures containing 2·10−4 mass fraction of surfactant in distilled water and gas condensate are prepared in such a way that the resulting mixtures are containing 0.022 and 0.044 mass fraction of gas condensate. The mixture was then thoroughly blended for 2 h at ambient temperature by a stirrer magnet with stirrer speed of

3. RESULTS AND DISCUSSION 3.1. Surfactant Effect. To evaluate the effect of surfactant on emulsion stability, the effects of four types of surfactant (SDS, Span 80, Tween 80, and CTAB) were studied. Figures 1 and 2 show the images of gas condensates droplets in water using the optical microscope for prepared emulsions with different surfactants, including 0.022 and 0.044 mass fraction of gas condensate in water, respectively. Figures 3 and 4 also show the number of droplets (NOD) versus droplets size (DS) of these emulsions. Table 3 represents the type of surfactants which are used to synthesize emulsion, the Sauter mean diameter of 0.022 (Smd1) and 0.044 (Smd2) mass fraction of gas condensate droplets in water and the strength of emulsion stability. As can be seen in Table 3, the emulsions formed with SDS, CTAB, and Tween 80 in solution containing 0.022 mass fraction of gas condensate had Sauter mean diameters of 12 μm, 12 μm, 1462

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Figure 1. Optical microscope images of 0.044 mass fraction of gas condensate in water emulsion formation with 0.02 g of different surfactants in 100 mL of solution at ambient temperature with a mixing time of 2 h, including (a) SDS, (b) Span 80, (c) Tween 80, (d) CTAB, and (e) Tween 80 + Span 80.

Figure 2. Optical microscope images of 0.022 mass fraction of gas condensate in water emulsion formation with 0.02 g of different surfactants in 100 mL of solution at ambient temperature with a mixing time of 2 h, including (a) SDS, (b) Span 80, (c) Tween 80, (d) CTAB, and (e) Tween 80 + Span 80. 1463

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Figure 3. Distribution of droplet sizes of 0.044 mass fraction of gas condensate in water emulsion formation with 0.02 g of different surfactants in 100 mL of solution at ambient temperature with a mixing time of 2 h, including (a) SDS, (b) Span 80, (c) Tween 80, (d) CTAB, and (e) Tween 80 + Span 80.

Figure 4. Distribution of droplet sizes of 0.022 mass fraction of gas condensate in water emulsion formation with 0.02 g of different surfactants in 100 mL of solution at ambient temperature with a mixing time of 2 h, including (a) SDS, (b) Span 80, (c) Tween 80, (d) CTAB, and (e) Tween 80 + Span 80.

and 8 μm, respectively. However, the emulsion, formed with the same materials in solution containing 0.044 mass fraction of gas condensate, had bigger Sauter mean diameters. It was found that

solutions containing 0.022 mass fraction of gas condensate in water had higher stability than those containing 0.044. The emulsion, formed with Tween 80, was slightly more stable than 1464

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Table 3. Sauter Mean Diameter of Droplets in the Formed Emulsions Containing 0.022 and 0.044 Mass Fraction of Gas Condensate in Water with 0.02 g of Different Surfactants in 100 mL of Solution surfactant

Sca/kg·m−3

106·Smd1b/m

stability

106·Smd2c/m

stability

Tween 80 Span 80 + Tween 80 CTAB Span 80 SDS

0.2 0.2 0.2 0.2 0.2

8 5 12 17 12

stable approximately stable stable stable stable

18 6.5 22 72 23

approximately stable stable approximately stable unstable approximately stable

a

Surfactant concentration. bSauter mean diameter for 0.022 mass fraction of gas condensate in water. cSauter mean diameter for 0.044 mass fraction of gas condensate in water.

Figure 5. Optical microscopic images of gas condensate droplets in water after adding 0.015 mL of an EO/PO block copolymer of dodecan-1-ol as a demulsifier to the 50 mL emulsions including (a) 0.044 mass fraction of gas condensate in water containing Tween 80, (b) 0.022 mass fraction of gas condensate in water containing Tween 80, (c) 0.044 mass fraction of gas condensate in water containing Tween 80 + Span 80, and (d) 0.022 mass fraction of gas condensate in water containing CTAB.

the emulsions, formed with SDS and CTAB. Additionally, the solution, containing 0.022 mass fraction of gas condensate in water with Tween 80, had droplets with very small sizes with the Sauter mean diameter of 8 μm, which is a small one that indicates a very stable emulsion. Figure 1b shows microscopic image of emulsion formation with Span 80 in solution containing 0.044 mass fraction of gas condensate. As can be seen, the formed emulsion is as water/ gas condensate/water (w/o/w). This means that water droplets are in gas condensate droplets and vice versa. The sizes of droplets are big and the Sauter mean diameter is about 72 μm. With respect to the size of droplets, one can say that this emulsion was not stable. Consequently, Span 80 is not a suitable material for the formation of a stable emulsion of this kind of solution (0.044 mass fraction of gas condensate in water). The HLB of this surfactant is 4.3, which provides the meaning that for a preparation of gas condensate in water, surfactants with low HLB are not suitable. However, the Sauter mean diameter for formed emulsion with this surfactant in solution containing 0.022 mass fraction of gas condensate was 17 μm, indicating approximately stable emulsion.

Finally, an emulsion of 0.01 mL of Span 80 with 0.01 mL of Tween 80 in 100 mL of solution was formed. Because droplets within the emulsion were small, a microscopic image with a magnification of 100 was applied to determine the distribution of droplet sizes. Figure (1e) shows an image of this emulsion. As can be seen in Figure (3e), all drops were formed with a size approximately in the range of 4 to 6 μm and the nanoscale droplets were also observed within the emulsion, indicating the high stability of this emulsion. The Smd for this emulsion was 6.5 μm, which was very small. To evaluate the stability of this emulsion, an amount of it was evaluated one month later, and the average diameter of its droplets did not change during this time, confirming the high stability of this emulsion. The effect of this surfactant on the stability of the emulsion of water solution, containing 0.022 mass fraction of gas condensate, was also studied as shown in Figure 3b. It shows that the droplets have very small size and the droplets with diameter larger than 10 μm is rarely seen. In addition, the majority of droplets have sizes between 0.5 and 2 μm. The Smd of this emulsion, as is shown in Table 3, is 5 μm, which is a very small. Therefore, this 1465

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Figure 6. Distribution of droplets size after adding 0.015 of an EO/PO block copolymer of dodecan-1-ol as a demulsifier to the 50 mL of emulsion. The initial emulsions included (a) 0.044 mass fraction of gas condensate in water containing Tween 80, (b) 0.022 mass fraction of gas condensate in water containing Tween 80, (c) 0.044 mass fraction of gas condensate in water containing Tween 80 + Span 80, and (d) 0.022 mass fraction of gas condensate in water containing CTAB.

from 18 μm to 53 μm. These increases in droplet volume justify the effectiveness of demulsifier on reduction of emulsion stability. Similarly, Figures 5 and 6 show the increase in droplet volume of other samples after adding demulsifier. According to Table 4, although the sample containing Tween 80 + Span 80 has the maximum increasing of droplets volume, approximately a 200% increase in droplet volume in all samples is seen. It can be said that the demulsifier resulted in the coalescence of gas condensate droplets in water. Hence, it can be used as a proper demulsifier for these types of emulsions (gas condensate in water).

combination of surfactants of Tween 80 and Span 80 was quite proper for the formation of a stable emulsion. Additionally, the results showed that, because the droplets of hydrocarbon compounds dispersed in emulsion had smaller sizes, 0.022 mass fraction of gas condensate emulsion in water, in comparison with 0.044, has much more stability. Among the surfactants used in this research, Tween 80 and the combination of Tween 80 and Span 80 had more stability. Moreover, nonionic surfactants formed more stable emulsions than cationic and anionic surfactants and stable emulsions, consisted from surfactants, had an HLB of approximately 10. 3.2. Demulsification of Produced Stable Emulsions. The stability of the studied emulsions (the growth and coalescence of droplets) was evaluated by adding a demulsifier. The demulsifier was an EO/PO block copolymer of dodecan1-ol. Figures 5 and 6 show the microscopic images and droplets size distribution of all samples after adding the demulsifier. Table 4 shows the Sauter mean diameter that are observed before (Smdi) and after (Smdf) adding the demulsifier.

4. CONCLUSIONS Four surfactants including SDS, Span 80, CTAB, and Tween 80 and also the combination of Span 80 and Tween 80 with different HLBs were used for the preparation of 0.022 and 0.044 mass fraction of gas condensate in water emulsion. The results showed Tween 80, CTAB, SDS, and the combination of Tween 80 and Span 80 provided high stable emulsions containing 0.022 mass fraction of gas condensate. The Smd of droplets was, thus, about 10 μm. Among these four samples, the combination of Span 80 and Tween 80 prepared an emulsion with a Smd of about 6 μm. In other words, the blend of a low HLB and a high HLB surfactant gave better emulsion stability. Additionally, in solutions containing 0.044 mass fraction of gas condensate in water, the combination of Span 80 and Tween 80 formed a stable emulsion. However, emulsion formed with Span 80 was relatively unstable. It was found that solutions containing 0.022 mass fraction of gas condensate in water had higher stability than those containing 0.044. Eventually, the effect of demulsifier on some stable emulsions was evaluated and the coalescence of droplets in emulsions was assessed. The 0.015 mL of an EO/PO block copolymer of dodecan-1-ol as a demulsifier was added at 60 °C to all stable emulsions (50 mL of solution). According to optical microscope analysis, after the addition of demulsifier, the coalescence of droplets caused an increase (about 200% growth) in the droplets’ diameter in emulsions.

Table 4. Effect of Demulsifier (an EO/PO Block Copolymer of Dodecan-1-ol) on the Growth of Droplets in the Emulsion after 2 h in 60°Ca surfactant

Dcb/kg·m−3

wc

106·Smdid/ m

106·Smdfe/m

Tween 80 Tween 80 Tween 80 + Span 80 CTAB

0.3 0.3 0.3 0.3

0.044 0.022 0.044 0.022

18 8 7 12

53 22 30 27

a

The 0.015 mL of demulsifier was added to 50 mL of emulsion. Demulsifier concentration. cMass fraction of gas condensate in the water. dSauter mean diameter before adding demulsifier. eSauter mean diameter after adding demulsifier. b

As indicated in Figures 5a and 6a, by addition of the demulsifier, the droplets’ Sauter mean diameter of Tween 80 emulsion (0.044 mass fraction of gas condensate) increased 1466

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*F. Esmaeilzadeh. Tel.: +989177135623. Fax:+98 711 635 9875. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Shiraz University for supporting this research. REFERENCES

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