Accepted Manuscript

Title: Studying Demulsification Mechanisms ofWater-in-Crude Oil Emulsions using a Modified Thin LiquidFilm Technique

Authors: Fan Yang, Plamen Tchoukov, Peiqi Qiao, Xinrui Ma,Erica Pensini, Tadeusz Dabros, Jan Czarnecki, Zhenghe Xu

PII: S0927-7757(17)31153-6DOI: https://doi.org/10.1016/j.colsurfa.2017.12.056Reference: COLSUA 22180

To appear in: Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: 2-11-2017Revised date: 19-12-2017Accepted date: 21-12-2017

Please cite this article as: Yang F, Tchoukov P, Qiao P, Ma X, PensiniE, Dabros T, Czarnecki J, Xu Z, Studying Demulsification Mechanisms ofWater-in-Crude Oil Emulsions using a Modified Thin Liquid Film Technique,Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010),https://doi.org/10.1016/j.colsurfa.2017.12.056

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https://doi.org/10.1016/j.colsurfa.2017.12.056https://doi.org/10.1016/j.colsurfa.2017.12.056

1

Studying Demulsification Mechanisms of Water-in-Crude Oil Emulsions using a Modified Thin Liquid Film Technique

Fan Yang1, Plamen Tchoukov1, Peiqi Qiao1, Xinrui Ma1, Erica Pensini1, Tadeusz Dabros2, Jan Czarnecki1, Zhenghe Xu1

1 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada 2 CanmetENERGY, Natural Resources Canada, 1 Oil Patch Drive, Devon, Alberta, Canada

Graphical Abstract

Abstract

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The thin liquid film (TLF) technique equipped with Scheludko-Exerowa cell has been

demonstrated to be a valuable tool for studying stability of dispersed systems of soft matter such

as emulsions and foams. Many industrial applications involve the addition of various chemical

aids to obtain desired properties or resolve technical problems. One of the examples is the use of

chemical demulsifiers to remove emulsified water from petroleum emulsions. To study the role of

chemicals in this process, a TLF technique with a dosing system is required to add demulsifiers

into already formed thin liquid films stabilized by interfacial layers formed from indigenous

surface active molecules. Herein, we present a modified design of the Scheludko-Exerowa cell

with a dosing mechanism which allows for addition of desired chemicals to the intervening organic

liquid film already formed. The stability of the water-in-oil emulsions and the mechanisms of their

chemical destabilization were studied by determining the thinning process of intervening thin oil

films before and after dosing a controlled amount of demulsifiers into the film. Dosing of a

biodegradable polymer demulsifier (EC300) into the intervening oil film converted a stable film

formed by asphaltene-in-heptol solutions into an unstable film with lifetimes of less than 20

seconds. The observed change in the film drainage rate and stability is related to the disruption of

asphaltene network formed at the oil-water interface by EC300 demulsifiers. The experiments with

the modified TLF cell also revealed the overdosing effect of EC300 demulsifier, forming more

stable films at high EC300 concentrations. A comparison of the experimental results between a

dosed system and a premixed system confirmed the value of the modification in the current TLF

technique to study the stabilization/destabilization processes of water-in-oil emulsions that mimic

more closely the real petroleum emulsions. The modified TLF technique allows for a better

understanding of the underlying demulsification mechanisms.

Keywords: Water-in-oil emulsions, Demulsification, Thin Liquid Film, Bitumen, Asphaltenes

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1. Introduction

During the production of crude oil from various sources, stable water-in-oil (W/O) emulsions are

inevitably formed [1]. These emulsions are highly undesirable as they create problems during

downstream transportation and upgrading of crude oil products. The most efficient approach to

remove the emulsified water is by chemical demulsifiers which are designed to promote

flocculation and/or enhance coalescence of otherwise stable water droplets [2]. Demulsifiers

modify the physicochemical properties of the oil-water interfaces to avoid or break interfacial

conditions that favor stable emulsions. In the case of water-in-crude oil emulsions, the conditions

that promote emulsion stabilization are often described as rigid and mechanically robust

interfaces which set physical barrier to hinder droplet coalescence [3]. Demulsification are often

achieved by softening the interfaces, shown by lower interfacial elastic moduli [4-6]. The softening

is often attributed to the ability of demulsifiers to replace the adsorbed asphaltenes which form

interlinking aggregates at the interface, converting an elastic (rigid) interface to a viscous (soft)

interface. In general, due to their high interfacial activity, demulsifiers can themselves stabilize

emulsions if overdosed [7-9]. Depending on their physicochemical properties, demulsifiers can

stabilize emulsions by mechanisms similar to those of surfactants (lowered interfacial tension),

polymers (steric repulsion) or solids (Pickering emulsion by demulsifier aggregates) [5-6, 10-11].

In the commercial demulsification practice, polymeric surfactants are commonly used as

demulsifiers due to their ability to penetrate and weaken the oil-water interface, while extending

their lyophilic segments into the oil phase to induce flocculation. Biodegradable ethylcellulose

(EC) macromolecules have been shown to exhibit a superior performance in removing emulsified

water from the diluted bitumen system [12]. The efficient demulsification by EC polymers has

been attributed to their high interfacial activity and strong ability to break up stable interfacial

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layers [5]. Langmuir trough compression isotherm further revealed that the EC addition to the

diluted bitumen-water interface drastically increased the compressibility of the interface [5]. A

number of studies [13-16] revealed an important correlation between the viscoelastic properties of

the interfacial layers and stability of the emulsions. While giving valuable insights, these studies

interpreted the destabilization mechanisms exclusively based on the properties of the oil-water

interface.

Attempts to understand the emulsion stability based only on their interfacial properties were not

always successful. In addition to being influenced by the properties of the oil-water interfaces,

coalescence and flocculation of emulsified water droplets are largely controlled by the surface

forces in the intervening oil film. When two approaching water droplets are within a small distance,

a flat thin liquid film forms between the two droplets. Emulsions remain stable when the repulsive

disjoining pressure is sufficiently strong to counter-balance the capillary pressure difference across

the curved liquid-liquid interfaces. Film drainage and stability can be conveniently studied using

Thin Liquid Film (TLF) techniques [17-23] and effectively modeled by Stefan-Reynolds-Young-

Laplace governing equations.

In the past, the film forming properties of asphaltenes (stabilization mechanisms) have been

studied and compared with those of its parent bitumen and deasphalted bitumen using the TLF

technique [22]. In a further attempt to understand the role of different bitumen/asphaltene

components responsible for stabilization of W/O petroleum emulsions, the asphaltenes were sub-

fractionated into interfacially active and remaining asphaltenes [24-26]. Distinctive differences

were observed in film drainage kinetics and film lifetime among the films formed using different

bitumen components in a Scheludko-Exerowa cell [17,18] under well controlled conditions

(temperature and pressure). For example, thin oil films formed from the whole asphaltenes and

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interfacially active sub-fraction of asphaltenes in toluene solutions showed a severe aging effect,

with the film becoming thicker and more heterogeneous with time. The aggregation in the film

and self-organization at the oil-water interface of interfacially active asphaltenes led to a gradual

formation of film networks to such an extent that the liquid in the film behaved as a non-Newtonian

fluid shown by the presence of a yield stress, thus stabilizing water-in-crude oil emulsions [22, 27,

28].

Although the existing TLF technique is extremely useful in studying stabilization mechanisms of

W/O petroleum emulsions, its application to studying destabilization mechanisms and hence

developing novel chemical demulsifiers poses challenges. In dewatering operations, the

demulsifiers and other chemical aids are usually added to the system after the formation of a stable

emulsion, where stabilizing layers are already well developed at the water-oil interface. In a

petroleum system, the added demulsifiers have to disturb and displace interfacial asphaltene layers

instead of competing for adsorption at the oil-water interface and/or preventing the formation of

interfacial asphaltene layers by solubilizing interfacially active asphaltenes. Therefore studies on

systems of premixed demulsifiers in crude oil do not represent the situation encountered in real

industrial processes of demulsifying petroleum emulsions. To establish conditions that are similar

to the real industrial demulsification process, the TLF technique needs to be modified by

introducing a dosing system which allows changing the composition of the studied film in situ and

monitoring the changes in the film properties in real time.

In this study, we propose a modified thin liquid film cell, which enables us to dose demulsifiers

into a thin liquid film after the formation of the stable thin film and /or interfacial layers at the oil-

water interface as encountered in real industrial petroleum demulsification processes. This

modified TLF cell allows for a direct assessment of the effect of the demulsifier addition on the

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thin film drainage dynamics, morphology and stability of the film and interfacial layers. The results

obtained in this study clearly demonstrate that the crude oil - demulsifier premixed system is not

representative of the real demulsification process, which is significantly affected by the time at

which the chemicals are added to the system. The results confirm clearly that the utilization of a

modified TLF cell is essential to achieve a better understanding of the demulsification process.

2. Materials and Experimental Methods

2.1. Materials

Demulsifier ethylcellulose (EC300) was purchased from Sigma-Aldrich and used as received. The

molecular weight of EC300 was determined from its intrinsic viscosity to be 182 kDa [12].

Athabasca coker feed bitumen was provided by Syncrude Canada Ltd. Asphaltenes were extracted

from the bitumen using n-pentane (>98%, purchased from EMD Chemical Inc.), following the

procedures described elsewhere [24]. Toluene (Optima grade) and heptane (HPLC grade) both

purchased from Fisher Scientific Canada were used as received. Heptane-toluene mixtures (termed

as heptol) were used as solvent for preparation of bitumen and asphaltene solutions. For example,

a mixture of 5 parts heptane and 5 parts of toluene by volume would be referred to as (5:5 heptol),

with 0:10 heptol being toluene only. Milli Q water with a resistivity of 18.2 Mcm at 25 C was

used as aqueous phase in the experiments.

2.2. Shear Rheology Measurement

The shear elastic (G) and viscous (G) moduli of the asphaltene-solution/water interface were

measured using an AR-G2 rheometer with a Double Wall Ring geometry of 35 mm radius (TA

instruments, USA). Contaminants on the ring were removed by soaking the ring in toluene and

acetone followed by burning any residues using a butane flame gun before each experiment. After

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placing 19.2 ml of water into the sample holder in a Delrin trough, the cleaned double wall ring

was carefully positioned onto the surface of the water sub-phase, followed by the addition of 15

ml of asphaltene-in-(5:5 heptol) solution as the top phase. Solvent evaporation was minimized by

covering the trough using a Teflon cap. The temperature of the system during the experiment was

controlled to be at 23.0 oC using a Peltier plate. The viscoelasticity of the interfacial film was

measured by applying a small oscillation of interfacial strain to the interface using the double wall

ring. The angular frequency and strain during the time sweep in this study were set at 0.5 Hz and

0.8%, respectively. Further details regarding this technique and experimental procedures can be

found elsewhere [29].

The interfacial layer was allowed to build for half an hour and the development of elastic moduli

G' and viscous moduli G'' was tracked in time. EC300-in-heptol solution was then carefully dosed

into the top oil phase to obtain a demulsifier concentration of 23 ppm. The change in the G' and

G'' moduli of the interface after emulsifier addition was measured at a 30-s interval for at least

another 5,000 s.

2.3. Modified Thin Liquid Film Cell for Demulsifier Addition to Pre-aged Films

As discussed in the introduction, understanding the demulsification mechanisms requires an

experimental method to observe the effect of demulsifier addition to the properties of already

formed and aged thin emulsion films. Thin emulsion films can be conveniently obtained and

studied using Scheludko-Exerowa cell [17, 18]. The film is formed in a hole drilled in a porous

glass holder by withdrawing the liquid through a capillary. Briefly, to generate a W/O emulsion

film, the porous glass plate with a hole drilled through its thickness is first soaked in the oil phase

to saturate the pores and the hole with oil and then immersed in the water-filled bottom part of the

measuring cell to form two fresh oil-water interfaces. By withdrawing the oil held in the hole back

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into the capillary tube, the two oil-water interfaces can be brought closer and closer to each other

until a thin plane parallel film, with its oil core sandwiched by the two oil-water interfaces and

surrounding aqueous phases, is formed. Such a thin liquid film can be used to represent the thin

liquid film formed between two water droplets in oil when they are in close proximity (< 100 nm).

However, the existing design of the Scheludko-Exerowa cell does not allow for changing the

composition and concentration of thin liquid films once the cell is loaded. Moreover, studying

systems that already contain demulsifier is not representative of the real demulsification process

in crude oil processing where interfacial films are formed prior to introducing the demulsifiers into

the system. To mimic the real demulsification process, we proposed a new cell design that allows

dosing demulsifiers or other chemical additives of interest into the emulsion films when they are

already formed and aged. A schematic drawing of the modified Scheludko-Exerowa cell is shown

in Figure 1. In this design, an outlet is open to the side wall of the bottom part of the measuring

glass cell. A rubber membrane is installed to the end of the outlet to make it gas tight. A micro-

syringe containing demulsifier solutions is used to punctuate the membrane and add the solutions

directly into the porous glass plate.

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Figure 1. Schematics of the modified Scheludko-Exerowa cell for studying the effect of

demulsifier addition on morphology and stability of thin emulsion films with aged interfacial

layers.

In our experiments, the cell was first loaded with asphaltene-in-solvent solution and placed in

water. A thick film (~2 mm) was initially formed in the hole and oil-water interfaces were aged

for a desired period of time. Then a thin oil film was formed in the center of the hole as the oil

solution was withdrawn slowly through the capillary using a gas tight syringe pump. Once the thin

film was formed, the pressure in the measurement cell was adjusted by a micro-syringe to obtain

the desired film radius of ~100 m. The film remained undisturbed and was allowed to drain under

the capillary pressure due to the curvature of the interfaces in the neighboring meniscus. The

morphology and lifetime of the thin liquid film were determined in real time. The thickness of the

film was then increased once again to ~2 mm by pushing oil from the capillary reservoir into the

glass plate, followed by dosing of demulsifier solution into the film through the deposition of small

droplets of demulsifier solution onto the porous plate through the micro-syringe. Due to the

hydrophobic nature of the treated glass plate, the oil droplets were very quickly absorbed into the

continuous oil phase. To ensure more uniform distribution of the demulsifiers, each dose of

demulsifier was separated into a few ~2 l droplets and deposited at different locations on the glass

plate. After deposition of demulsifier solutions, more oil from the glass capillary was pumped into

the plate and then withdrawn back into the capillary for at least five times to mix the demulsifiers

homogenously throughout the entire oil phase inside the film and oil reservoir in the capillary tube.

Such a dosing method represents the process of demulsifier addition as encountered in real

industrial practices of breaking water-in-oil petroleum emulsions where demulsifiers are added AC

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into the continuous oil phase. The final concentration of demulsifiers in the thin liquid film (Cd)

was calculated using following equation:

= ( )/( + ) (1)

where Cd0 is the concentration of demulsifier in the mother solution that was dosed in, Vd0 is the

volume of demulsifier solution added and Vr is the total volume of the oil, which includes the

volume of oil in the glass porous plate and also in the capillary tube. The volumes of oil pumped

in and out were kept relatively small so that the disturbance of the film interfaces was minimized.

After demulsifier dosing, the interfaces were aged for a desired period of time before thin liquid

films were generated again prior to measuring the film properties.

The modified cell was placed in the thin film instrument which was already described in our

previous work [21]. The films were observed in the reflected light using an Axio Observer inverted

microscope (Carl Zeiss, Germany). A high-resolution and high-sensitivity DFC500 digital camera

(Leica, Germany) was used to take the film images. A custom built LabVIEW program was used

to control the experiment and perform data acquisition. More details on the TLF method and

experimental setup can be found elsewhere [17-21]. During the experiments, the cell was kept at

a constant temperature of 23 0.1 C. The porous glass plate was made hydrophobic by soaking

in a 20% dichlorodimethylsilane (> 99.5%, Fluka) in cyclohexene (reagent grade, Fisher

Scientific) solution for 24 hours.

3. Results and Discussion

3.1. Destabilization of thin films formed in asphaltene-in-heptol by EC300 demulsifiers

As shown in our previous work [22], asphaltenes in heavy oil are responsible for the formation of

relatively thick and stable emulsion films. Here, the modified cell described above was used to

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study the effect of EC300 addition on stability of these stable films. After 30 min of aging, thin

liquid films obtained from 2 g/L asphaltene-in-(5:5 heptol) solution appeared to be thick and

inhomogeneous, as shown in Figure 2A. The inhomogeneity in the film thickness as indicated by

the observed Newton rings reflects the formation of asphaltene aggregates in the solution. As

shown in Figure 2A, little drainage was observed in the film, leaving the background of the film

with thickness greater than 100 nm. The films formed as such were stable and did not rupture

within 20 min. After ageing the film for 30 min, 23 ppm of EC300 was dosed into the film

following the procedure described in section 2.3. The addition of 23 ppm EC300 to the film

transformed the stable films to unstable films with a film lifetime shorter than 20 s (at 20 s, the

film already ruptured as shown by the last image in Figure 2B). The morphology of the film

changed dramatically from thick, inhomogeneous and rigid to thin, homogeneous and fluid, as

indicated by the black thin film with spherical dimples in Figure 2B. Dark spots (areas in the film

with thickness less than 15 nm) were formed in the film and propagated quickly as soon as the film

was generated. Within a few seconds, these thinner black spots occupied almost the entire film

area except for some brighter spots and colored lenses, corresponding to dimples formed by the

trapped film liquid. One thing worth noticing here is that without demulsifier addition, the

interfacial layer formed by asphaltenes is robust and features thicker domains. The relative location

of these thicker domains is preserved when the films drain or during the injection or withdraw of

the liquid into/from the film. Similar observations were reported in our previous work on aqueous

films sandwiched between bitumen diluted in poor solvents [21, 27]. Similar phenomena were

observed for polymer-stabilized foam films [30]. The drastic change observed in the features of

films when the demulsifier was added is a result of disrupting the interfacial layers by demulsifier

molecules that prevent the formation of such rigid asphaltene domains at the interface.

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Figure 2. Evolution of thin liquid films formed from 2 g/L asphaltene-in-(5:5 heptol) solutions:

(A) without demulsifier; (B) with 23 ppm of EC300 dosed; (C) with 43 ppm of EC300 dosed.

By further increasing the demulsifier dosage to 43 ppm, the thin liquid film became more stable

and had lifetime greater than 2 min, indicating an overdose of EC300 at 43 ppm for films formed

from 2 g/L asphaltene-in-(5:5 heptol) solutions. However, the film contained features similar to

those of films formed in the presence of 23 ppm EC300: A black film occupied the entire film area

within seconds after the film formation, showing a large dimple and a few smaller lenses of trapped

liquid in the film. Higher stability of films formed with 43 ppm EC300 compared to those with 23

ppm EC300 addition is most likely due to the higher density of demulsifier molecules at the oil-

water interface and their intrinsic emulsion stabilizing property. Increasing the demulsifier dosage

to 63 ppm led to a further increase in film lifetime, indicating even higher film stability, although

the features of the film remained the same, i.e., black (thinner) and more uniform film background

in contrast to brighter (thicker) and more heterogeneous films formed from asphaltene solutions

without EC300 addition.

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As shown in Figure 3A, the thin liquid film formed from 10 g/L asphaltene-in-(5:5 heptol) solution

was much thicker and more inhomogeneous in thickness than the film formed from 2 g/L

asphaltene solutions. The features of the film remained the same after 15 min, indicating a

negligible drainage of the thin liquid film. The formation of aggregates with variable size was

evident from the uneven thickness of the film as shown by rich color variations in the film images.

The domains in the film appeared irregular in shape, suggesting a rigid interface with a negligible

liquid flow in the film. The film was stable with a film lifetime longer than 15 min which is an

observation consistent with the stable emulsions under similar conditions [31]. The addition of 23

ppm of EC300 into such stable films caused cracking of the rigid interfacial layer, resulting in

two types of domains: a thinner domain with dimples inside and thicker region with solid-like

features. Such an observation is in agreement with findings from previous studies, in which AFM

imaging on interfacial materials deposited using Langmuir-Blodgett method revealed a change in

the morphology of interfacial materials formed at the water-bitumen solution interface from a

relatively smooth and homogeneous interface to a heterogeneous interface with micro-domains

upon addition of EC demulsifiers [32]. With the help of the new cell design, this phenomenon was

observed in situ for the first time. One important observation worth noticing is that the thicker and

solid-like domains in the film moved on the interface without changing their shape or features

when the film was expanded or contracted in thickness by pumping or withdrawing the liquid in

the film, making it apparent that they were not inside the liquid film but at the oil-water interface.

These observations suggest that the demulsifiers significantly altered the oil-water interface,

disrupted the already formed interfacial asphaltene networks and formed a new interface that the

solid-like asphaltene domains cannot attached to easily. The rapid drainage of the liquid in the film

around the dimple indicates the rheological properties of the liquid inside the thinner film region

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is of a Newtonian nature as discussed in our previous work [22, 27]. Despite the extremely stable

film formed by high concentration of asphaltene-in-(5:5 heptol) solutions, the stability of the film

is now governed by the nature of the thin areas in the film, leading to a film life time of ~8 s.

Figure 3. Evolution of thin liquid films formed by 10 g/L asphaltene-in-(5:5 heptol) solutions: (A)

without demulsifier, (B) with 23 ppm of EC300 dosed into the film, and (C) with 60 ppm of EC300

dosed into the film.

By further increasing the dosage of EC300 to 60 ppm, the thin liquid films were found to be stable

again with the film becoming significantly more heterogeneous in thickness and morphology than

the film formed in the presence of 23 ppm EC300. A slow drainage of the film is apparent from

images in Figure 3C. A close examination of the thinner film background revealed that the

morphology of these thinner areas is thicker than the films formed by the demulsifier alone, as

shown by the inset of Figure 4. The overall appearance of the film although highly inhomogeneous

is different from films formed by asphaltenes without EC300 (Figure 3A). The latter has minimal

drainage and thickness change during the experiment. The heterogeneous morphology of the film

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in the presence of 60 ppm EC300 is likely caused by the debris of asphaltene aggregates formed

following the rupture of the original interface by the demulsifier.

Figure 4 shows the lifetime of the thin liquid films after adding different amounts of EC300

demulsifiers. Film lifetime is defined as the time span from the moment of film formation until

film rupture (coalescence event). To minimize the influence of the film size on the life time, the

radius of the films was kept consistent at 100 10 m for all the experiments. The hydrodynamic

inhomogeneity in the film thickness, which were reported for large films [33] are insignificant for

films of radius ~100 m [34]. For rupturing films, at least 8 measurements were made for each

concentration to produce the error bars in Figure 4. For the case of stable films, at least 4

measurements were made for each concentration.

Effective demulsification was found at very low EC300 concentration (~ 5ppm), which is similar

to the reported effective demulsifier concentration in bottle tests under similar conditions [35].

The working dosage of EC300 was found to be lower than 40 ppm, regardless of the concentration

of asphaltenes in (5:5 heptol) solutions investigated. Any higher demulsifier concentration would

cause overdose, suggesting that demulsifier EC300 itself can stabilize the emulsion. To confirm

such effect of stabilization by the demulsifier, 200 ppm of EC300 in (5:5 heptol) solution was used

instead of asphaltene solution in the TLF experiment, and a stable (up to 20 min) homogenous film

with dimple was observed (see the inset image of Figure 4).

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Figure 4. Thin liquid film life time of 2 g/L and 10 g/L asphaltene-in-(5:5 heptol) solutions dosed

with EC300 at different concentrations. The inset shows image of thin liquid film stabilized by

200 ppm EC300 in (5:5 heptol) without asphaltenes.

3.2. Comparison of the dosed and premixed EC300 / asphaltene-in-(5:5 heptol) system

In order to compare the demulsification behavior in the premixed system with that in the dosed

system, thin liquid films were generated from premixed solutions containing 10g/L asphaltene-in-

(5:5 heptol) with various amounts of EC300 demulsifiers pre-blended in the bulk oil phase. To

understand the intrinsic stabilization capability of EC300, thin liquid films with EC300

demulsifiers as the only stabilizing agent was also studied at a concentration of 200 ppm in (5:5

heptol). In all experiments, the oil-water interfaces were aged for 30 minutes before the thin liquid

films were formed.

Figure 5A showed that the thin liquid film that was stabilized by 200 ppm EC300 demulsifiers

alone drains into a thin film with thickness of less than 15 nm (black film) in the barrier ring region.

During the film drainage, a big dimple was formed initially and drained slowly into the meniscus.

After about 1 minute, the size of the dimple stopped changing, probably due to the slow drainage

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caused by the thin barrier ring. The stability of such a film can therefore be considered to be

governed by the stability of the thin area in the film (the barrier ring). Such film drainage behaviors

clearly show that EC300 demulsifiers have intrinsic film stabilization capability when used in

higher dosages.

With 20 ppm of EC300 premixed in the 10g/L asphaltene in (5:5 heptol) solution, the lifetime of

the stable 10g/L asphaltene in (5:5 heptol) films was shortened to a few seconds. Due to the very

short lifetimes and film rupturing during the very early stage of its drainage, it is difficult to draw

conclusions on possible changes in the properties of interfacial layers caused by the demulsifier

addition. However, the smooth and near circular Newton rings observed in Figure 5B suggest that

the presence of demulsifier led to interfacial layers that are not as rigid and thick with less

aggregates than those observed in Figure 3A that eventually contributed to film stabilization.

Increasing the demulsifier concentration further to 60 ppm led to higher film stability, allowing

sufficient time to see the formation of a barrier ring in the film. The barrier ring formed in these

films appeared thicker and more heterogeneous when compared with the barrier rings formed by

EC300 alone and much thinner than the film formed by asphaltene solutions without demulsifiers.

White dots observed in the film suggest the presence of aggregates which are likely composites of

asphaltene and demulsifier molecules.

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Figure 5. Evolution of thin liquid films formed by (A) 200 ppm EC300 demulsifier alone in (5:5

heptol) solution, (B) 20 ppm of EC300 premixed with 10g/L asphaltene in (5:5 heptol) solution,

(C) 60 ppm of EC300 premixed with 10g/L asphaltene in (5:5 heptol) solution, and (D) 200 ppm

of EC300 premixed with 10g/L asphaltene in (5:5 heptol) solution.

When the premixed demulsifier concentration was increased to 200 ppm, more obvious overdosing

effect was observed in which the films were stable for up to 5 minutes. The film morphology in

the barrier ring area is very similar to that observed with 60 ppm of demulsifier concentration but

thick colorful lenses and dimples indicate the formation of larger aggregates. ACCE

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Figure 6. Film lifetimes of 10g/L asphaltene-in-(5:5 heptol) emulsion films as a function of EC300

concentration added by premixing or dosing method.

Figure 6 compares the lifetime of thin liquid films formed in a premixed system and dosed system

at various EC300 concentrations. The dosed system and premixed system both showed overdosing

effect but such an effect happened at distinctively different demulsifier concentrations. For the

dosed system, apparent overdosing was observed at a demulsifier concentration of 50 ppm,

increasing the stability of thin liquid film abruptly to have a lifetime of more than 20 minutes. The

films formed in a premixed system on the other hand were generally less stable comparing with

films in the dosed systems that contain the same amount of EC300 demulsifiers. Obvious

overdosing was only observed in the premixed system when the demulsifier concentration was

increased to 200 ppm. Such drastically different thin film behaviors suggest that demulsifiers have

different destabilization mechanisms in these two systems.

Comparing the film morphologies at the optimal demulsifier concentrations for the dosed system

(Figure 3B) and premixed system (Figure 5B-C), the formation of thin area (

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in the latter case. Such thin area of the similar thickness and morphology to the thin liquid films

stabilized only by EC300 is the evidence that the original asphaltene interface was disrupted and

the interface was replaced and dominated by the demulsifiers. The EC300 demulsifiers can

penetrate into the asphaltene interfacial layer due to their higher surface activity through entry

points provided by a loosely packed interfacial asphaltene structure built on associations between

a variety of asphaltene structures and interactive sites. When demulsifiers penetrate into the

asphaltene interfacial layer, it is speculated that EC300 demulsifiers can weaken the bonding

between the asphaltenes and also between the asphaltenes and the interface, eventually causing the

original interface to break into smaller pieces that can either rearrange to loosely attach to the

interface and form thick domains or detach from the interface to move back into the bulk oil.

Similar phenomenon was reported before. [36].

The films formed in the premixed system on the other hand does not show formation of very thick

and rigid films as in the case of asphaltene in (5:5 heptol) system nor thin and fluid films as in the

case of pure EC300 in (5:5 heptol) system. When demulsifiers were added to water-in-oil

petroleum emulsions, the demulsifiers have to penetrate through the cross-linked asphaltene

interfacial films and force the asphaltenes into patches but not fully displace them. In contrast,

premixing demulsifiers into the bulk oil system prior to emulsion formation allows the demulsifier

molecules to compete for the interface and dominate the adsorption properties of the system. In

this case most likely a mixed (asphaltenes/EC300) adsorption layer is formed where the presence

of EC300 limits the ability of asphaltenes to form thick and rigid interfacial layers. In such a case,

it is speculated that asphaltenes and asphaltenes-EC300 aggregates occupy the interfaces, where

the stable films of asphaltenes or EC300 are not fully developed. AC

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Thus, the stability of such films showed negative synergy and were less stable compared with the

pure asphaltene systems. Such negative synergy is likely caused by two factors: (1) the effective

concentration of EC300 demulsifiers (demulsifiers that are immediately available to the interface)

is much lower due to consumption of EC300 by adsorption onto asphaltenes dissolved in the bulk,

and (2) the asphaltene aggregates have a modified interfacial activity due to surface modification

by demulsifiers and weaker ability to interlink with each other to form a 3D structured asphaltene

interphase due to inactivation of certain sites on the asphaltene molecules/aggregates.

3.3. Viscoelastic properties of the oil-water interface

Measurement of interfacial shear rheology could provide valuable information not only on the

accumulation of the materials at the interface, but also on the viscoelastic properties of the

interface. Throughout the first 30 min, we followed the evolution (aging) of asphaltene-in-(5:5

heptol) solution-water interface by measuring the interfacial elastic (G') and viscous (G'') moduli.

The results in Figure 7 show an increase in both moduli with aging time and a much higher value

of G than G''. This finding is in good agreement with previous observations of asphaltenes forming

a solid-like interfacial layer dominated by G' [37]. It is worth noting that a transition time at

which the interface transitioned from viscous (G' < G'') to elastic (G' > G'') was observed at lower

asphaltene concentrations (0.4 g/L in 5:5 heptol). The coalescence rate between water droplets was

found to decrease drastically when such transition of viscoelasticity occurred [38]. At the 30-min

mark, demulsifiers were dosed into the top oil phase at a bulk concentration of 23 ppm in organic

phase with an immediate and steady decrease in both G' and G'' values being observed. Over the

next 2,500 s, the viscous and elastic moduli decreased to the values close to zero, indicating a

complete disruption of the original rigid interface to make the interface completely fluid-like as

observed for the oil-water interface formed with the demulsifiers alone.

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Figure 7. Evolution of interfacial elastic and viscous moduli: oil phase is 2 g/L asphaltene-in-(5:5

heptol) solution with 23 ppm of EC300 added at 1800 s mark. For comparison, shown as the inset

is the evolution of interfacial elastic and viscous moduli of the oil phase being a premixed solution

of 2 g/L asphaltenes and 23 ppm of EC300 in (5:5 heptol).

To understand the interaction of EC300 and asphaltene molecules and their competitive

adsorption, a second set of measurements was conducted using asphaltene solutions at the same

concentration but with 23 ppm of EC300 demulsifiers premixed into the solution. The results are

shown as the inset in Figure 7. In contrast to the interface formed by solutions of asphaltene, the

interface formed in asphaltene solutions premixed with 23 ppm of EC300 demulsifiers was found

to be fluid-like with negligible viscoelastic moduli. The results confirm that in the presence of

highly interfacially active demulsifier molecules, the asphaltenes cannot effectively compete for

the interface, preventing the formation of solid-like interfaces. The contrast results obtained for

the systems of different methods to introduce EC300 underline the importance of modifying the

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existing thin film cells to study the demulsification processes which are more relevant to the real

process conditions.

The results in this section, together with the results from the comparison of the dosed and premixed

system, clearly illustrate that the destabilization of W/O emulsions by demulsifiers has its own

dynamics and strongly depends on the order and time of demulsifier addition. It is evident that

studies of the premixed system cannot be representative of the real industrial application of

demulsifiers to break petroleum emulsions. In this respect, the modification of the Scheludko-

Exerowa cell as described in this work provides new opportunities to study the efficiency and

mechanisms of demulsification and to determine optimal dosages and methods of demulsifier

application.

4. Conclusions

Recent thin liquid film work demonstrated that the asphaltenes of crude oil are responsible for the

formation of thick and stable films by the formation of rigid interfaces with ageing. The main

approach of destabilization of petroleum emulsions is based on the addition of demulsifiers, which

displace and soften the adsorbed asphaltene layers at water/oil interfaces [39-42]. The design of

the classical Scheludko-Exerowa cell commonly used in TLF studies does not allow adding

demulsifiers to already-formed and developed stable emulsion films, which is essential to mimic

industrial applications and gain a deeper understanding of the demulsification mechanism. In this

work, we proposed and built a new cell design that allowed us to study the process of destabilizing

asphaltene-in-heptol emulsions by EC300 at different concentrations. Such a modified technique

avoided drawbacks of studying thin liquid film properties of oil/demulsifier mixture and can

closely mimic the real demulsification process, in which demulsifiers act on existing and well-

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structured oil/water interfacial layers. At lower asphaltene concentration (2 g/L), the asphaltene

network formation was found to be destructed by the addition of EC300, and a dramatic transition

to a much thinner black film was observed. At higher asphaltene concentration of 10 g/L, EC300

demulsifiers had a similar effect of reducing the film thickness by penetrating the much thicker

film interfaces, peeling them off from the interface and/or breaking them into small debris.

Addition of demulsifiers at concentrations within the optimal range led to very unstable and short

living films (film lifetimes of several seconds). For both asphaltene concentrations, the addition of

EC300 at concentrations above 40 ppm caused overdosing of demulsifiers as that the film lifetime

increased. The results for premixed system showed negative synergy between the EC300

demulsifiers and asphaltenes in stabilizing emulsion films. Such negative synergy is believed to

be related to the limitation of the growth of asphaltene aggregates in the bulk and at the interface

when EC300 demulsifiers were premixed with asphaltenes in the bulk oil phase. In all cases, it

was found that addition of EC300 softens the film interfaces and reduces the film thickness. The

time sweep measurements of the shear viscoelastic moduli of 2 g/L asphaltene-in-(5:5 heptol) film

showed a build-up of strong elastic dominated interfacial layer (G'>G''), which was reversed and

gradually softened to fully fluid-like water/oil interface (G'~0 N/m) after addition of EC300. For

the premixed asphaltene/EC300 system, negligible shear viscoelastic moduli were observed.

While similar changes in the interfacial rheological properties of asphaltene layers were reported

before, they are directly related to the observed reduction in film thickness and sharp decrease in

film lifetimes as key parameters affecting the emulsion stability for first time in this work. It is

worth noticing that studies on single water/oil interfaces are not able to detect and predict the

overdosing effect, which was clearly demonstrated using thin liquid film technique. In this work,

the modified thin film cell was proven to be a useful tool to study demulsification mechanism

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under conditions that mimic industrial processes. The modified cell could be a successful

instrument to study a much broader spectrum of colloidal and interfacial phenomena, for which

the order of chemical addition and dynamics of the process are essential.

Acknowledgement

The authors acknowledge the financial support from Natural Sciences and Engineering Research

Council of Canada (NSERC), and NSERC Chair in Oil Sands Engineering. The authors also

acknowledge the provision of oil sand samples by Syncrude Canada Ltd.

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