Deposition of Asphaltene from Destabilized Dispersions in Heptane

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Deposition of asphaltene from destabilized dispersions in heptane-toluene Sophie Campen, Benjamin Smith, and Janet S. S. Wong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01887 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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Deposition of asphaltene from destabilized dispersions in heptane-toluene Sophie Campen¹, Benjamin Smith² and Janet Wong¹* ¹Department of Mechanical Engineering, Imperial College London, UK SW7 2AZ ²BP Exploration Operating Company Limited, Chertsey road, Sunbury on Thames, Middlesex, UK TW16 7BP *Corresponding author. Telephone: +44 (0)20 7594 8991. E-mail: [email protected]

Abstract Deposition of carbonaceous materials, such as asphaltene, is a major problem in petroleum production. During production, changing environmental conditions destabilize asphaltene, resulting in dispersions that are out of equilibrium, where asphaltene is aggregating or flocculating. Key to developing the most effective strategies for tackling this problem, is a fundamental understanding of asphaltene deposition behaviour. A quartz crystal microbalance with dissipation monitoring (QCM-D) is used to study asphaltene deposition from non-equilibrium dispersions generated by in-line mixing of asphaltene in toluene (a solvent) with n-heptane (a precipitant). The effects of heptane:toluene ratio and destabilization time are investigated. At high heptane:toluene ratio the rate of asphaltene aggregation is faster and large flocs form by the time the flowing liquid reaches the QCM cell. In this case, the rate of deposition decreases with deposition time. At low heptane:toluene ratio the rate of asphaltene aggregation is slower, hence large flocs do not form before the flowing liquid reaches the QCM cell, and deposition of smaller aggregates occurs. Here the deposition rate is constant with time. The deposited mass is greatest before the formation of large flocs and at short destabilization times, where the particle distribution is furthest from equilibrium. Destabilized small particles existing immediately after a destabilization event pose a greater deposition problem than the flocs which subsequently form. This may be a contributing factor in the existence of deposition “hotspots” at certain locations in the production pipeline. Pushing destabilized dispersions to their new equilibrium distributions as quickly as possible may be a preventative strategy to combat deposition. The dissipation-frequency relationship monitored by QCM-D is sensitive to the nature of deposited asphaltene films and may be used as a diagnostic tool.

Keywords deposition; QCM-D; asphaltene; aggregation; flocculation; colloid; crude oil; petroleum

1.

Introduction

Deposition of carbonaceous materials onto engineered and naturally-occurring surfaces throughout all stages of petroleum production significantly reduces the efficiency of operations [1,2]. The economic impact of this is considerable [3]. Deposited carbonaceous materials are often comprised of asphaltene, although deposition of other materials such as waxes also poses a problem [4,5]. The cause of the problem is rooted in the colloidal nature of crude oil and the extreme and changing environmental conditions the crude oil experiences during production [6]. In this paper our focus is on understanding the mechanism of asphaltene deposition encountered near the upper window of the asphaltene precipitation envelope. To this purpose, we have designed an experiment to monitor the mass of deposited material as a function of time for destabilized dispersions such as those encountered in the field.

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Crude oil can be divided into saturate, aromatic, resin and asphaltene (SARA) fractions [7]. Asphaltene is the densest, most polar fraction and is defined by its solubility: heptane-insoluble, toluene-soluble [8]. The heptane-soluble fractions are collectively called maltenes [9,10]. As crude oil travels from the subsea reservoir to the surface, it experiences a pressure drop. This drop in pressure causes a volume expansion of crude oil that is non-equivalent for the different fractions. Light saturates (poor asphaltene solvents) expand at a faster rate and to greater magnitude than heavier fractions (good asphaltene solvents), thereby reducing the solvency of the maltenes for asphaltene [11] which causes the asphaltene molecules to self-associate forming aggregates, clusters, flocs and eventually precipitate. Asphaltene particles can adsorb or deposit onto surfaces resulting in fouling – it has been noted that asphaltene phase separation is a necessary but not an adequate criterion for fouling [12]. The non-equilibrium nature of asphaltene experienced during petroleum production can be replicated by mixing crude oil or model oil (asphaltene in toluene) with heptane (or an alternative precipitant). The heptane:toluene ratio governs the mass fraction of asphaltene that will eventually precipitate. Additionally, the heptane:toluene ratio will affect the kinetics of asphaltene aggregation, displaying a trend of increasing precipitation rate with heptane content [13]. Previously, the crude oil:precipitant ratio above which precipitate was observed was referred to as the “onset” of asphaltene precipitation [10,14]. However, exceptionally slow kinetics of precipitation at very low precipitant concentrations has brought into question the existence of an “onset” [15,16]. Destabilization time (or aging time), the time elapsed since asphaltene mixed with heptane, has an important effect on the particle size distribution of asphaltene at a given time. This is because the particle size distribution does not immediately achieve its new equilibrium distribution when precipitant is added. It takes time for the particle size distribution to evolve. Since the kinetics of asphaltene deposition depend on the particle size distribution, a change in destabilization time may alter the observed deposition behaviour. For deposition studies it is possible to achieve a constant destabilization time by employing in-line mixing of asphaltene in toluene with precipitant. In-line mixing was used to investigate asphaltene deposition in capillary flow [17,18,19,20,21,22,23]. The change in pressure-drop across a length of capillary tubing is measured as a function of time or mass flow. It has been found that the deposit may or may not be uniformly distributed along the length of the capillary. In instances, a thicker deposit was observed at the capillary inlet, compared to the capillary outlet [17,20], perhaps due to local mass-transport limitations [17]. Very large 100 µm flocs were observed in glass capillaries at long run times [20]. Uniform deposition across the capillary length is a necessary requirement for accurate calculation of deposit thickness from the pressuredrop. For destabilized dispersions, fouling is dominated by deposition of sub-micron aggregates rather than larger particles and ‘aged’ asphaltene aggregates do not deposit [17]. By purposefully employing low heptane concentrations to limit the rate of aggregation and prevent formation of particles larger than 500 nm, it was shown that the main factors governing the rate of deposition in a column of packed beads were the concentration of destabilized asphaltenes (that will eventually precipitate) and the fluid flow velocity [24]. In these experiments, the rate of deposition was constant with time and did not vary with destabilization time (distance along the column). Quartz crystal microbalance with dissipation monitoring (QCM-D) has been used to investigate asphaltene deposition from heptane:toluene [25,26,27]. In one study, the total deposited mass increased with increasing volume fraction of heptane up to a point, beyond which, mass decreased [26]. This change in behaviour coincided with the observed “onset” of precipitation and it was argued that large flocs travelled out with the liquid flow owing to their increased inertia [26]. For all tested precipitant concentrations, the rate of deposition decreased with time. Note in these studies, in-line mixing of asphaltene:precipitant was not used and test solutions were aged for 30 min before flowing through the QCM. An additional increase in age would have occurred during QCM experiments and one can expect that the particle size distribution during initial deposition would have been different to


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that at the end of experiments. Quartz crystal resonators have also been used to determine the “onset” of asphaltene precipitation during gas injection into dead oil [28] and the bubble point during depressurisation of live crude oil [29]. QCM-D is well-positioned to provide additional information on the nature of asphaltene deposits forming at the solid-liquid interface. QCM-D is sensitive to the nature of films and elsewhere has been used to determine vesicle-bilayer transitions [30,31,32], the size of depositing nanoparticles [33] and the viscoelastic properties (e.g. shear modulus) of adsorbed polymer gels and brushes [34]. QCM-D thus has the potential to inform on the nature of asphaltene deposits; information that may be difficult to obtain directly, e.g. by optical techniques, owing to the small size of depositing species at short destabilization times. The planar quartz sensors, readily removable from the flow cell, allow easy ex-situ analysis of deposits. Atomic force microscopy (AFM) is used to corroborate the morphology of films deposited under different conditions. Here we use in-line mixing of asphaltene in toluene with heptane:toluene before the QCM-D flow cell to investigate asphaltene deposition from destabilized dispersions. The length of inlet tubing to the QCM-D flow cell is varied to control the destabilization time. Our experiments thus give a snapshot view of what is happening at a single point in time after a destabilization event (mixing with heptane). The observed deposition behaviour will depend on the properties of the dispersion above the sensor surface, which in turn is affected by prior aggregation / flocculation and mass deposition in the tubing before the cell. This paper aims to answer fundamental questions including: what role does asphaltene destabilization play in its ensuing deposition? How does the degree of asphaltene destabilization (governed by heptane volume fraction) and destabilization time affect the deposition behaviour? What is the state of depositing asphaltene? Does asphaltene deposit as individual nanoaggregates / clusters or is it large flocs that are chiefly responsible for deposition? By addressing these questions, it is hoped that fresh insight will be garnered that will help determine the best strategies for preventing thick deposit formation.

2.

Materials and Methods

2.1

Materials and preparation methods

Asphaltene was extracted from a crude oil sample provided by BP. n-Heptane was added to crude oil in a 40:1 mass ratio. This heterogeneous mixture was agitated for 1 h before storing for 2 days in a dark cupboard. The precipitated asphaltene was collected by vacuum filtration on a PTFE filter with 0.45 µm pore size (Merck Millipore). The precipitate was rinsed with an excess of heptane until the filtrate was colourless before drying in an oven at 70 °C for 1 h. The dry extracted asphaltene (dark brown/black solid, yield 4.65 wt%) was dissolved in toluene to give a 1 gL⁻¹ solution; this was ultrasonicated at room temperature for 1 h then aged in a dark cupboard for 2 weeks prior to use. For QCM-D measurements, gold-coated quartz sensors (Q-Sensors) were acquired from Biolin Scientific. The QCM sensors and the flow cell were cleaned using Hellmanex III liquid cleaning concentrate and sodium dodecyl sulfate (≥99.0%, GC, dust-free pellets) respectively, both from Sigma-Aldrich. Cuvettes, used for UV-vis spectroscopy and dynamic light scattering measurements, were also cleaned using Hellmanex III. Before use, Hellmanex III and sodium dodecyl sulfate were dissolved in DI water (resistivity > 18.2 MΩ cm, Milli-Q, Millipore) to give 2 vol % and 5 wt % solutions respectively. The parts were ultrasonicated in the cleaning solutions for 30 min then rinsed thoroughly with DI water, followed by 2-propanol and acetone before drying under a stream of N₂ gas. Before use the QCM sensors were plasma cleaned in a low-pressure cleaner (Diener Electronic) under O₂ for 2 min. The solvents: n-heptane (≥99%, CHROMASOLV), toluene (99.9 %, CHROMASOLV), 2-propanol (99.9%, CHROMASOLV) and acetone (≥99.8%, CHROMASOLV) were acquired from Sigma-Aldrich and used without further purification.


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2.2

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Asphaltene particle size distribution

The particle size distribution of asphaltene was determined indirectly using the method described by Tavakkoli et al. [35]. Aliquots were taken from the 1 gL⁻¹ asphaltene solution in toluene. These were diluted by adding heptane:toluene to give a series of 0.1 gL⁻¹ asphaltene mixtures at various volume fractions of heptane. The mixtures were aged for the allotted time (either 10 min or 24 hours) and then centrifuged in tubes of 3.5 cm length. Speeds of 1400 and 14000 rpm were used to separate particles larger than 1000 and 100 nm diameter respectively as per the Stokes equation, see supplementary information (SI) 3 for details. A sample of the supernatant liquid was transferred to a quartz cuvette and a UV-vis absorption spectrum obtained using a USB4000-UV-vis spectrometer with fibre optic light source from Ocean Optics. The concentration of asphaltene remaining in solution and hence the mass fraction precipitated was determined by comparing the absorption values to those obtained for a series of solutions of known concentration in toluene at the same wavelength, see SI Fig. 4. The particle size distribution of asphaltene in heptane:toluene was determined by dynamic light scattering (DLS) using a Zetasizer Nano ZSP with 633 nm ‘red’ He-Ne laser from Malvern Instruments. Measurements were made using non-invasive back scattering (NIBS) at an angle of 173°. Samples were held in glass cuvettes with PTFE stoppers. The refractive index of heptane:toluene, required for DLS, was measured at the Na D-line using an Abbe refractometer with an external heated oil bath, see SI 4. A refractive index for asphaltene of 1.7 was taken from the literature [10,36,37].

2.3

QCM-D technique and apparatus

A quartz crystal microbalance with dissipation (QCM-D) was used to monitor the adsorption and deposition of asphaltene onto a gold-coated quartz sensor from Q-Sense, Biolin Scientific. This apparatus measures the changes in frequency and dissipation of an oscillating quartz crystal at its fundamental frequency ( ) of 5 MHz and odd overtone numbers () of 3 (15 MHz), 5 (25 MHz), 7 (35 MHz), 9 (45 MHz), 11 (55 MHz) and 13 (65 MHz). Dissipation is a measure of how the oscillation of the quartz crystal decays when the driving voltage is switched off, for further information see SI 2. The adsorbed areal mass can be calculated using the Sauerbrey equation: ∆ = −

2 ∆ (1)

where ∆ is the frequency shift due to adsorption, is the fundamental frequency, is the overtone number, is the specific density of quartz, is the shear wave velocity in quartz, ∆ is the adsorbed mass and is the surface area in contact with the liquid. The Sauerbrey mass is only correct for thin, rigid films that do not dissipate energy. In cases where > 0 and there is spreading of ⁄ for the different overtones, as encountered in our experiments, the mass is calculated using the Voigt viscoelastic model in QTools software from Biolin Scientific. Changes in frequency and dissipation can also be caused by liquid loading and liquid trapping. The former of which was compensated by obtaining frequency and dissipation baselines in the respective solvents used for deposition, see SI 2 for details. For mass modelling, viscosity and density measurements of heptane:toluene and asphaltene solution were made using a Stabinger viscometer, see SI 1. The accuracy of the Voigt mass was independently determined by UV-vis spectroscopy, SI 7. The Voigt fitting was carried out for overtones n=3-13. In addition to mass, the shear viscosity (0.1 – 7 mPa.s) and shear elasticity (0.1 – 0.4 MPa) of asphaltene deposits was obtained, SI 8. QCM-D measurements were made in a liquid flow cell at 20°C. Two pumps were used to flow liquid through the cell: an Ismatec Reglo 12-roller pump with Gore 100CR or Viton pump tubing and a MilliGAT HF pump from Global FIA. All other tubing was of PTFE (Chemfluor, Saint Gobain). O-rings


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and gasket were of chemically resistant Kalrez. Connectors, fittings and ferrules were of PEEK and ETFE (Kinesis).

2.4

QCM-D procedure for monitoring asphaltene deposition

QCM experiments were carried out to investigate the deposition of asphaltene onto a gold surface from destabilized dispersions in heptane-toluene using the experimental set-up shown in Fig. 1. Two reservoirs were used: Reservoir A held asphaltene solution in toluene and Reservoir B held heptane:toluene. This allowed asphaltene and the precipitating/destabilizing agent to be kept separately. Two pumps were used to control the flow from the reservoirs giving 1:9 volume ratio mixing of the liquids. The two liquid streams met at a mixing T-junction before flowing though PTFE tubing of known length to the QCM flow cell. Low-cracking pressure check valves placed immediately before the T-junction prevented liquid backflow. The composition of heptane:toluene in Reservoir B was adjusted to achieve destabilized dispersions after the T-junction with heptane content ranging from 50 to 90 vol %. The experimental procedure involved systematically changing the nature of liquid flowing from the two reservoirs to the QCM cell as described by the series of steps in Table 1. The design of this experiment ensured (i) constant flow rate over the sensor surface, (ii) constant concentration of asphaltene in heptane:toluene and (iii) constant destabilization/incubation time, governed by the length of the PTFE tubing between the T-junction and the flow cell. The residency time of the dispersions above the QCM sensor is 2 s at a flow rate of 20 µLs⁻¹. The effect of destabilization time on the deposition behaviour was investigated by varying the length of tubing from the T-junction to the QCM, illustrated by the dashed line in Fig. 1. Three lengths of tubing were used: 30, 60 and 180 cm, giving destabilization times of 12, 20 and 50 s respectively at a flow rate of 20 µLs⁻¹. These were calculated by considering the internal volumes of the flow cell (100 µL) and tubing (148, 297 and 891 µL).

2.5

Atomic Force Microscopy (AFM) of asphaltene deposits

The clean Au sensor surface and deposited asphaltene films were analysed by tapping mode AFM. Measurements were made in air at ambient temperature (22°C) with a Multimode SPM with Nanoscope V controller or a diCaliber AFM, both from Bruker. Images were acquired using softtapping RTESPA-150 probes with silicon tips and nominal spring constant of 5 Nm⁻¹ and free resonance frequency of 150 kHz (Bruker). The asphaltene samples for AFM analysis were prepared in the QCM-D. After the deposition, the surface was rinsed for a short time with heptane:toluene at the same heptane volume fraction as used for the deposition. This was achieved by filling Reservoir A in Fig. 1 with toluene. Both pumps were then simultaneously stopped, the sensor immediately removed from the flow cell and any remaining solvent allowed to evaporate. This procedure ensured that asphaltene was cleared from the liquid volume above the deposited film. n.b. Stopping the flow without first rinsing with heptane:toluene would have allowed any asphaltene remaining in solution above the sensor the chance to age and subsequently deposit, thereby altering the morphology of the film.

3.

Results

3.1

Asphaltene precipitation and particle size distribution in heptane:toluene

The particle size distribution of asphaltene in heptane:toluene at a concentration of 0.1 gL⁻¹ was determined indirectly by centrifugation-UV-vis spectroscopy, as detailed in section 2.2. At a


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destabilization time of 10 min it is observed that particles of over 1000 nm size exist at heptane fractions of 70, 80 and 90 vol %, (black, Fig. 2(a)). The mass fraction existing as large particles or flocs, increases with increasing heptane vol %. At 70, 80, and 90 vol % heptane there also exist particles of intermediate 100-1000 nm size (white, Fig. 2(a)). The mass fraction of particles possessing this size is independent of heptane:toluene ratio. At 60 vol % heptane there is a very small proportion of particles of 100-1000 nm size whilst at 50 vol % heptane all particles are smaller than 100 nm (grey, Fig. 2(a)). The particle size distribution was also investigated at a longer destabilization time of 24 h, Fig. 2(b). The distribution is altered by aging; the mass fraction existing as large particles of 1000 nm size or greater (black) is increased, whilst the mass fraction of small particles of 100 nm size or less (grey) is decreased slightly. However, the most marked change is in the fraction of particles of intermediate 100-1000 nm size, which is significantly reduced by aging. This reduction, accompanied by an increase in the fraction of particles larger than 1000 nm, suggests that the aggregation of intermediate size particles to form large particles occurs at a higher rate than that of the formation of new intermediate size particles. Similar to the observed distributions at 10 min destabilization time, the mass fraction of asphaltene existing as particles of 1000 nm size or greater increases with increasing vol % heptane, whilst the mass fraction existing as particles of 100-1000 nm size is independent of vol % heptane. At destabilization times of 10 min and 24 h, it is estimated that particles larger than 1000 nm exist at 68 and 57 vol % heptane and over respectively, see SI Fig. 6. Dynamic light scattering (DLS) was used to investigate the particle size distribution for 0.1 gL⁻¹ asphaltene in heptane:toluene, Fig. 3. Measurements were made at destabilization times of 2, 4, 6, 8 and 10 min. In 0, 50 and 60 vol % heptane, the distribution is monomodal and does not change with destabilization time. The average size of particles is 6, 5 and 9 nm in 0, 50 and 60 vol % heptane respectively. DLS does not reveal discrete peaks corresponding to monomer/nanoaggregate and cluster states, as suggested by the Yen-Mullins Model [38], instead there is a continuum of particle size from 3-30 nm, in line with the most current molecular modelling results [39] and theories [40]. It is interesting to note that the particle size distribution in 50 and 60 vol % heptane does not change significantly with destabilization time. However, later we observe deposition from 50 and 60 vol % heptane (section 3.2), suggesting that small particles exhibit a tendency to deposit prior to their aggregation. At low heptane vol%, only a small fraction of the total asphaltenes (those with chemistries most incompatible with being in a non-polar alkane environment) are destabilized, and thus have a tendency to aggregate and, after sufficient time, precipitate. Since the rate of aggregation is reaction-limited in these early stages [41,42], the relatively low concentration of these destabilized asphaltenes results in a slow rate of aggregation and thus means that little change in the particle size distribution is observed at the short destabilization times used in our study. In 70 vol % heptane, the particle size distribution changes with destabilization time, Fig. 3(d). At 2 min, the distribution is monomodal, centred about 500 nm (black solid line). At longer times, a shoulder appears on this peak at c. 1 µm and the peak eventually shifts to a larger size of 2 µm. Additionally, a second separate peak at 5.5 µm is observed at destabilization times of 4 min (grey dotted line) and longer (grey and black dash lines). Similar behaviour is observed in 80 vol % heptane, Fig. 3(e). The initial distribution after 2 min is monomodal and centred about 1.5 µm with a shoulder at 3 µm (black solid line). At longer destabilization times, two separate peaks are observed at 2 and 5.5 µm (grey and black dash lines). In 90 vol % heptane, flocculation appears to be quicker, since the initially measured distribution at 2 min is bimodal corresponding to particles of 2 and 5.5 µm size (black solid line, Fig. 3(f)). Generally, it is observed that the number of large (5.5 µm) particles increases with time. It is noted that for these experiments, sedimentation of large particles may lead to some error in the distribution by volume. Hence, the proportion of large (5.5 µm) particles may be observed to fall with time due to their removal by sedimentation. Previous DLS studies observed similar trends with increasing heptane vol % [43]. At 70, 80 and 90 vol % heptane, contrary to the indirect method with UV-vis spectroscopy, DLS does not detect any particles of size under 100 nm, Fig. 3(d,e,f). It is a known limitation of DLS that, when


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coexisting in liquid, a very small volume fraction of large particles, which are very effective light scatterers, can screen smaller particles that are poorer scatterers [44,45]. Whilst DLS is better suited to provide a temporally resolved size distribution during destabilization, the indirect method gives a more complete and accurate view of asphaltene size distribution. Both methods illustrate that the particle size distribution changes with increasing destabilization time, proving that the equilibration between different states of asphaltene aggregation is slow, continuing for over 10 min.

3.2

Deposition from non-equilibrium dispersions

The frequency and dissipation shifts for =3 during a typical deposition experiment are shown in Fig. 4. Initially, the baseline values were monitored in toluene and heptane:toluene at 50, 60, 70, 80 and 90 vol % heptane (section A, Fig. 4) before returning to pure toluene (0 vol % heptane). The inlet tubing from Reservoir A was then moved from toluene to a new container holding 1 gL⁻¹ asphaltene in toluene; dilution at the T-junction resulted in a final concentration of 0.1 gL⁻¹ asphaltene in pure toluene (section B, Fig. 4). Adsorption of asphaltene onto the gold surface caused small shifts in frequency and dissipation. n.b. Previously, we investigated the adsorption of this same asphaltene onto Au from solution in toluene over a concentration range of 0.001 to 1 gL⁻¹ at 20°C and found that it followed the Langmuir adsorption isotherm giving the mass of complete monolayer as 804 ngcm⁻² [46]. In this study, this pre-existing asphaltene monolayer simplifies our analysis, since throughout the deposition experiments we are always dealing with asphaltene-asphaltene interactions, i.e. multilayer formation. The volume fraction of heptane in the dispersion was then increased to 50 vol % (section C, Fig. 4), resulting in shifts in frequency and dissipation due to both liquid-loading and mass-loading. After a deposition time of 20 min, the inlet tubing from Reservoir B was changed to pure toluene so that once again 0.1 gL⁻¹ asphaltene in 0 vol % heptane flowed through the QCM (section D, Fig. 4). It is observed that frequency and dissipation return to the same values observed prior to the deposition (i.e. frequency and dissipation in sections B and D are similar). Hence, the asphaltene film deposited in 50 vol % heptane can readily be removed by improving the quality of the solvent flowing over the surface. The deposition was repeated at increasing volume fractions of heptane, between which 0.1 gL⁻¹ asphaltene in pure toluene was flowed. At 50 (section D, Fig. 4), 60 and 70 vol % heptane (section E, Fig. 4), the shift in frequency appears to be linear with time, whilst at 80 and 90 vol % heptane (section F, Fig. 4) the shift is non-linear and the rate of change of ∆ decreases with time. After each deposition, flowing asphaltene in 0% heptane leads to complete removal of the deposited film (i.e. both frequency and dissipation return to the level observed in section B). Detailed frequency and dissipation shifts for =3,5,7,9 at different stages of the deposition experiment presented in Fig. 4 are shown in SI Fig. 8. For the baseline measurements (no asphaltene), there is a positive shift in frequency and a negative shift in dissipation due to liquid loading as the vol % heptane is increased, SI Fig. 8(a). This is caused by the lower viscosity and density of heptane (0.3645 mPa⋅s, 0.6837 gcm⁻³) respective to those of toluene (0.5718 mPa⋅s, 0.8660 gcm⁻³). The magnitude of the frequency and dissipation shifts decrease with increasing overtone number. See SI 2 for equations describing liquid loading. The frequency and dissipation shifts during asphaltene deposition from 60, 70 and 80 vol % heptane are shown in SI Fig. 8(b-d). At initial times there is a positive shift in frequency and a negative shift in dissipation. This is due to the liquid-loading effect. After this initial period, shifts due to mass-loading occur in the opposite direction to those of liquid-loading. It is expected that simultaneous massloading and liquid-loading effects occur at initial time. However, decoupling of the effects is complicated and hence analysis of the deposited mass at very short time (and likewise at very long time when changing back to 0 vol % heptane) is not attempted. At 60 and 70 vol % heptane (SI Fig.


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8(b,c)) it is evident that ∆Dn ⁄∆t increases with increasing overtone number (). While at 80 vol % heptane the opposite trend is observed, i.e. ∆Dn ⁄∆t decreases with increasing , SI Fig. 8(d). The mass vs. time plots for asphaltene deposition are given in Fig. 5. At 50, 60 and 70 vol % heptane the rate of asphaltene deposition is constant and increases with heptane content, Fig. 5(a). Note that at time = 0 s, the mass is greater than zero owing to the film already existing on the surface due to asphaltene adsorption from toluene. Above 70 vol % heptane, it is observed that the total deposited mass decreases with increasing heptane fraction, Fig. 5(b). At 80 and 90 vol % heptane the rate of deposition decreases with time. The initial rate of asphaltene deposition is plotted vs. vol % heptane, Fig. 5(c). The initial rate of deposition increases gradually as the vol % heptane is increased from 50 to 60 vol % and more sharply as the heptane content is increased from 60 to 70 vol %. At 80 and 90 vol % heptane the initial rate of deposition shows a very small increase. Note, the initially measured rates may be underestimations of the real values. This is owed to the simultaneous liquid- and mass-loading effects when changing the volume fraction of heptane, which means that the initial rate determination may be offset by c.30 s. This is of little consequence under circumstances where the rate is constant with time (i.e. 70 vol % heptane and under), but may be significant in cases where the rate is known to decrease with time (i.e. 80 vol % heptane and over). After a deposition time of 20 min the rate of asphaltene deposition from 80 and 90 vol % heptane drops significantly and is slower than that from 60 vol % heptane, as indicated by the open circles, Fig. 5(c). The results indicate that there is a deposition “sweet-spot” which, in our experiments, occurs at 70 vol % heptane. This is typified by a steep and linear increase in mass with time, leading to thick multilayer deposition. Understandably, as the volume fraction of heptane increases, the mass fraction of asphaltene that is destabilized increases, which results in a faster rate of deposition. However, beyond a certain point, the deposition behaviour alters, and the rate decreases with time. Considering that at a destabilization time of 10 min, particles larger than 1000 nm occur at heptane:toluene of 68 vol % heptane and over (see section 3.1), we conclude that the change in deposition behaviour is linked to changes in the particle size distribution. Different behaviours are observed before and after the formation of large (> 1000 nm) particles. The results indicate that fouling will be most problematic under conditions representative of this deposition “sweet-spot”. Note the exact position (heptane:toluene ratio) of the deposition “sweet-spot” is specific to the asphaltene sample under investigation and the experimental conditions employed. Use of a higher asphaltene concentration would lead to an increased rate of aggregation [16], and thus alter the particle size distribution within the QCM cell to that consisting of larger particles; this would shift the deposition “sweet-spot” to lower heptane vol %. The nature of depositing asphaltene is further addressed in section 3.4.

3.3

Effect of destabilization time

The effect of destabilization time on asphaltene deposition behaviour was investigated. Experiments were carried out at 70 and 80 vol % heptane, centred about the change in deposition behaviour as shown in Fig. 5. In 70 vol % heptane, the rate of asphaltene deposition decreases with increasing destabilization time. A constant rate of deposition is observed at destabilization times of 12 and 20 s, (solid and open circles respectively, Fig. 6(a)). At a longer destabilization time of 50 s (crosses, Fig. 6(a)), the rate decreases with time. In 80 vol % heptane, the rate of asphaltene deposition is not constant at all destabilization times, Fig. 6(b). The initial rate of deposition decreases with increasing destabilization time. However, after 500 s, the rate of deposition for a 12 s destabilization time (solid circles, Fig. 6(b)) is slower than that for a longer 20 s destabilization time (open circles, Fig. 6(b)). Whilst determination of mass depends on acceptance of several assumptions, the raw frequencydissipation data itself, which is not prone to manipulation, can give insight into the nature of films. A simple way to achieve this is to plot ∆Dn vs. ∆fn ⁄n. This removes the effect of time. In this way two


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films that form at different rates but have similar material properties should possess similar dissipation-frequency relationships. Generally, a large negative shift in frequency indicates a large deposited mass, while a large positive shift in dissipation suggests that the film is “lossy”, i.e. dissipates energy. A film that is soft or viscoelastic will dissipate energy, likewise a film that consists of discrete rigid particles may also dissipate energy through rocking and sliding motions of these particles [47]. The ∆Dn vs. ∆fn ⁄n plots for asphaltene deposition are shown in Fig. 7. Both destabilization time and heptane:toluene ratio affect the dissipation-frequency response and, so it follows, the mechanical properties of the film. Two principal behaviours are observed. The first (type I) is characterized by (i) ∆ that increases with overtone number such that ∆D < ∆D5 < ∆D7 < ∆D9 and (ii) a large spread of ∆ among different overtones that increases with increasing frequency shift. The second (type II) is characterized by (i) ∆ that decreases with overtone number such that ∆D > ∆D5 > ∆D7 > ∆D9 and (ii) a relatively small spread of ∆ among different overtones. Type I behaviour is observed for depositions in 60 vol % heptane at all destabilization times and 70 vol % heptane at 12 and 20 s destabilization times. For these experiments, the rate of deposition was constant, Fig. 6 (a). Type II behaviour is observed for 70 vol % heptane at the longest destabilization time of 50 s and for 80 vol % heptane at all destabilization times, Fig. 7. For these experiments, the rate of deposition decreased with time, Fig. 6 (b). Previously, ∆ that increases with overtone number (i.e. type I behaviour) was observed for sub-monolayer films of weakly-bound nanoparticles with aspect ratio close to 1, which participated in rocking / sliding motions induced by the flow of liquid around particles [48]. Note, in our case we cannot find an asymptotic solution of a complete monolayer with vanishing dissipation (i.e. where ∆Dn /(− ∆fn ⁄n) approaches zero for the different overtones), since we are dealing with multilayer deposition. Finally, although dissipation is clearly non-negligible, the relative dissipation shifts are relatively small (∆Dn /(− ∆fn ⁄n) < 0.2 x 10⁻⁶ Hz⁻¹).

3.4

Morphology of deposited asphaltene films

AFM was used to investigate the morphology of asphaltene films deposited on the Au surface during QCM-D experiments. At a destabilization time of 20 s, the morphology of the deposited asphaltene film is strongly dependent on the heptane:toluene ratio (see Fig. 8(b-d)) and is different to that of the clean Au surface, Fig. 8(a). The films obtained at 60 vol % heptane (Fig. 8(b)) and 70 vol % heptane (Fig. 8(c)) consist of many small particles. These deposited films are reasonably homogeneous, with particles appearing uniformly distributed even at a 50 µm scan size, see for example Fig. 8(g). The increased contrast of Fig. 8(c) respective to Fig. 8(b) suggests greater variance in particle size and/or increased particle size for the deposit from 70 vol % heptane. Using line-profile analysis it is estimated that the diameter of particles is approximately 20-100 nm and 50-300 nm at 60 and 70 vol % heptane respectively. The surface roughness (Ra) of the clean Au surface and the 60 and 70 vol % heptane deposits are 0.843, 1.8 and 5.76 nm respectively at a 10 µm scan size. At 80 vol % heptane (Fig. 8(d)), the morphology of the deposited film is very different to those obtained at lower vol % heptane. Note the increased z-scale. In this case the deposited film is very heterogeneous, consisting of both large flocs and smaller particles. Here, we define clusters to be approximately spherical aggregates with size up to 1000 nm, while flocs are larger than 1000 nm, are comprised of primary particles (clusters) and have looser fractal aggregate structure. Fig. 8(d) shows a floc of 10 µm size, elsewhere other regions contain only smaller particles of 200-1500 nm size, see Fig. 8(h). The globular nature of the floc is visible, containing agglomerated smaller particles (clusters). The morphology of an asphaltene film deposited from 70 vol % heptane at a longer destabilization time of 50 s is shown in Fig. 8(i). At a longer destabilization time, the deposited film contains both large flocs and smaller particles and is very heterogeneous. At a small scan size of 1 µm, the


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particulate nature of the floc is visible, Fig. 8(j). By line profile analysis the diameter of clusters inside the floc is approx. 20 nm, Fig. 8(k). The film is very different to that observed at a shorter destabilization time of 20 s, Fig. 8(c), and appears more like the film deposited from 80 vol % heptane, Fig. 8(d), although the z-dimension is less. In summary, two main asphaltene deposition behaviours are observed. The first, observed at 70 vol % heptane and under, is characterised by a linear increase in mass with time. The second, observed at 80 vol % heptane and over, is characterised by a decreasing deposition rate with time. AFM analysis reveals that the constant deposition rate occurs when there is deposition of clusters and/or small particles of 20-300 nm size, while a decreasing rate with time is observed when there is deposition of both large flocs of c. 10 µm size and smaller clusters/particles. Fig. 9(a) illustrates these two behaviours. We speculate that a similarly fast and constant deposition rate does exist for dispersions in 80 and 90 vol % heptane, however the experimental set-up used here, has not thus far allowed us to probe such short destabilization times, i.e. before the formation of flocs.

4.

Discussion

In this study, we investigate deposition of asphaltene from heptane:toluene onto a gold surface that is already covered by a monolayer film of asphaltene. Use of a T-junction ensured a constant destabilization time, hence the nature of asphaltene flowing over the sensor surface, including its particle size distribution, should not change during each deposition. Deposition onto surfaces (e.g. tubing) before the QCM cell must be considered. The total mass loss throughout the whole flow system, including the inlet tubing, was determined by analysing the concentration of asphaltene in the effluent from the cell, see SI 6. It is observed that the total mass fraction deposited throughout the whole flow system increases with increasing heptane vol %. Interestingly, the QCM-D deposition study does not capture this behaviour, note how the deposited mass from 80 and 90 vol % heptane is lower than that from 70 vol % heptane (Fig. 5 (b)). A reasonable explanation for this observation is that at high heptane vol %, the rate of aggregation of destabilized asphaltene is faster, and large flocs form before reaching the QCM cell. Indeed, Figs. 6 and 7 indicate that flocculation will occur within 20-50 s in 70 vol % heptane and in under 12 s in 80 vol % heptane. Constant rates of deposition are only observed when the depositing species are relatively small, sub-micron aggregates. We speculate that constant rates of deposition do exist in 80 and 90 vol % heptane, but that they occur at much shorter destabilization times in the tubing before the cell. Deposition in the tubing before the cell, would cause the actual concentration of asphaltene above the sensor to be lower than expected. A maximum reduction of 12.5% in the concentration of asphaltene due to mass losses is insufficient to explain the significantly lower deposited mass at 90 vol % heptane respective to that at 70 vol % heptane, Fig. 5 (b). We conclude that lower deposited masses at high heptane vol % are principally due to changes in the depositing species and/or the resultant deposited film. It has been suggested that under turbulent flow large particles, as observed in higher heptane vol % in this study, deposit to a lesser extent than smaller particles owing to their increased inertia which prevents transportation to the wall [26,49,50,51]; this results in a critical maximum particle size for deposition [49]. However, in our experiments we are in the laminar flow regime. n.b. Reynolds number (Re) = 49 in the tubing before the cell and Re = 2.7 above the sensor surface. The reason deposition rate decreases with time beyond the flocculation point is unclear. Large flocs, which form a patchy film, may be more susceptible to shear-removal than smaller adsorbed particles, which form a continuous film. It has been shown that the deposited mass of asphaltene decreases with increasing wall shear stress [51]. A limiting upper mass may be reached in such cases, where the rate of mass gain by particle deposition is equivalent to that of mass loss by shear-removal. Perhaps deposited flocs do not act as sites for subsequent deposition, while smaller particles (clusters) do. Asphaltene clusters are compact, approximately spherical particles [52], while asphaltene flocs, consisting of multiple clusters, have much looser fractal aggregate structure and are highly porous [53]. Depositing clusters which are uniform in size and shape may pack efficiently at the surface forming a uniform viscoelastic film with considerable trapped solvent-component (see SI


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7), and hence large spreading of overtones for dissipation (Fig. 8). Depositing flocs, which have low fractal dimension, cannot pack so efficiently. During multilayer deposition the adsorbed nanoaggregate-depositing floc interaction (at initial time) may thus be stronger than the adsorbed flocdepositing floc interaction (at longer time) due to steric reasons; this could contribute to a decreasing deposition rate with time at high heptane vol %. The asphaltene-asphaltene interaction may change depending on the quality of the solvent. A surface forces apparatus (SFA) study of asphaltene films on mica found that two approaching asphaltene surfaces jumped-in to contact in toluene but not in heptane. The Hamaker constant (representative of the magnitude of the van der Waals interaction) was calculated to be 3.2 × 10⁻²⁰J and 0.11 × 10⁻²⁰ J in toluene and heptane respectively. However, the pull-off (adhesive) force was greater in heptane than in toluene [54]. Very high deposition rates are observed at short destabilization times. Aged asphaltene dispersions in heptane:toluene do not show this deposition behaviour since they do not have the same proliferation of destabilized smaller particles [19,55]. Initially, the asphaltene solution in toluene is fully equilibrated, i.e. the distribution between monomer and aggregated states is such that the free energy is at its minimum. When the heptane:toluene ratio is increased, the solution is destabilized and forced out of equilibrium. Given time, the solution will reach a new equilibrium position. This is achieved by minimising contact between asphaltene and solvent via asphaltene aggregation in the bulk and asphaltene deposition onto surfaces. These two processes are competitive, and it is expected that there may be some analogy between the kinetics of aggregation and deposition, since the thermodynamic driving force for these processes is similar. Previously, it has been assumed that the rate of asphaltene precipitation is proportional to the super-saturation degree of asphaltene, i.e. the difference between the immediate concentration of asphaltene in the liquid and the concentration at equilibrium [55]. At a short destabilization time, the distribution of asphaltene is furthest from equilibrium, hence the deposition rate is highest. As destabilization time increases, the system moves closer to its equilibrium condition. The size of aggregates in the bulk increases and there is a decrease in the deposition rate, even before the formation of flocs, see for example deposition from 70 vol % heptane at 12 and 20 s destabilization times in Fig. 6(a). We have created a hypothetical energy diagram to describe the state of asphaltene during destabilization, Fig. 9(b). In toluene, a good solvent, asphaltene exists as a mixture of nanoaggregates and clusters. Upon addition of heptane precipitant, these asphaltene clusters are destabilized. We believe that it is at this point where the free energy is maximum. The asphaltene clusters are described as having a continuum of different energies. The least stable clusters flocculate, thereby lowering the system free energy. Later flocculation events do occur upon further aging; however, the free energy minimization of these events is lower. n.b. The relative energies of the clusters and flocs are simply shown for illustrative purposes. It has been suggested that the rate of deposition of sub-micron aggregates is mass-transport limited, depending on the diffusivity and thus the size of depositing particles [24,56]. In deposition models the main factors governing deposition rate are the number of destabilized particles, the size of destabilized particles (which governs their diffusivity and transport through the liquid), the flow conditions (hydrodynamics) and other environmental conditions (e.g. temperature) [57,58,59]. Generally, deposition (which involves asphaltene-asphaltene interaction) is not considered to be reaction-limited, even though there is evidence that in the early stages of aggregation and at low heptane vol % the rate of aggregation is reaction-limited [41,42]. In our study, we observe that deposition rate tends to decrease with increasing particle size, see depositions from 70 vol % heptane at increasing destabilization times (Fig. 6 (a)). A decrease in deposition rate with increasing destabilization time from 12 to 20 s may suggest the formation of larger (yet still sub-micron) aggregates with lower diffusivity. However, we postulate that all destabilized particles may not be treated as thermodynamically equivalent (see how destabilized clusters have a spread of different energies as shown in Fig. 9 (b)). That is to say, two particles of the same size may not necessarily


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exhibit the same deposition rate. Indeed the asphaltene fraction is a solubility class containing different molecular chemistries. Sub-fractionation of asphaltenes revealed that the least stable fraction, which precipitates with the smallest volume of precipitant, tends to contain a larger proportion of polar groups [60]. ATR-FTIR studies of asphaltene destabilization by heptane revealed asphaltene molecules with higher oxygen and nitrogen heteroatom content (e.g. sulfoxides, sulfones, ethers, esters, pyrroles and pyridines) were less stable [61]. Hence, when we change the heptane:toluene ratio, we are not only changing the mass fraction of asphaltene that precipitates but also the respective chemical compositions of the solvated and precipitated fractions. For this reason, chemical analysis of the deposited films was attempted. However, our analyses could not conclusively support that there were significant changes in the chemistry of asphaltene deposited at different heptane:toluene ratios, see SI 9. In QCM-D the dissipation-frequency response of the sensor is very sensitive to the nature of deposited films [47]. We show that both heptane:toluene ratio and destabilization time affect the dissipation-frequency response, thereby proving that the mechanical properties of the asphaltene film depend on these two variables. In fact, no two experimental conditions give the exact same response, Fig. 7. Two main behaviours are observed: type I behaviour is characterised by ∆ < ∆& and large spreading in ∆ ⁄∆ for different overtones, while type II behaviour is characterised by ∆ > ∆& and smaller spreading in ∆ ⁄∆ for different overtones. By AFM, we show that type I behaviour is observed when the film is relatively homogenous and consists of smaller particles of 20-300 nm size (i.e. asphaltene clusters or small particles, see Fig. 8(b,c)). On the other hand, type II behaviour is observed when the film is heterogeneous and contains large deposited flocs of c. 10 µm size as well as smaller deposited particles, Fig. 8(d,h,i). In this way the dissipation-frequency relationship from multiple overtones may be used diagnostically to determine the morphology of deposited films, i.e. whether they contain large deposited flocs. Although the responses are specific to our system (asphaltene in heptane:toluene), it is possible that there may be similar changes in the dissipation-frequency relationship with film morphology for other colloidal systems. Dissipation-frequency analysis has previously been used in a more quantitative manner to determine the height of monolayer films of monodisperse colloidal particles of well-defined size, for example, liposomes [62,63]. The thing of note here, is that dissipation-frequency analysis may also reveal important information for multilayer films that are extremely polydisperse in terms of (i) chemistry – asphaltene is extracted from naturally occurring crude oil and contains a variety of different molecular structures; and (ii) morphology – the depositing particles may be found in an array of different sizes owing to the different coexisting states of aggregation. The main finding of this study is that it is not large flocs per se, that pose the biggest threat to flow assurance in upstream oil production. Rather, it is the process of flocculation itself which results in destabilized smaller particles, which give rise to constant deposition rates and thick multilayer formation. Our findings suggest that there will be deposition "hotspots" that occur very shortly after destabilization events. At longer destabilization times, although the flowing crude oil may be very heterogeneous, containing an increased proportion of asphaltene solid, the deposition threat will be lower since the rate of deposition is known to decrease rapidly with time. The question now is: how can this knowledge be applied in practice to tackle the problem of fouling? Depressurization of crude oil during production is unavoidable, hence it is not possible to altogether prevent asphaltene destabilization and its ensuing deposition/aggregation. Firstly, it is recommended that destabilization should be avoided in areas of restricted volume, because in such locations, formation of thick deposits would be particularly undesirable as they could completely block the fluid path. Strategies that increase the rate of aggregation may prove beneficial, since they would allow the equilibrium distribution to be reached more quickly, thereby reducing the time period (and section of pipe) over which destabilized smaller particles exist. If it were possible to induce asphaltene destabilization (effectively driving the deposition behaviour seen at high vol % heptane), a quickly


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growing deposit would form in a relatively short section of pipe. If this deposit can be removed, e.g. by mechanical methods, it may prove more manageable than having a slower growing deposit (with overall lower mass) over a longer section of pipe. Note, in our experiment we have a single destabilization event (mixing with heptane), after which the particle size distribution tends towards its equilibrium condition. However, in reality, changing environmental conditions experienced by crude oil as it travels along the pipe would lead to a shifting equilibrium position and the generation of new destabilized nano-aggregates ripe for deposition. Operational changes, which can be readily implemented, may have significant effect. Shear-induced flocculation and fragmentation of colloidal particles should be considered [64]. There is evidence that asphaltene aggregation [65] and deposition [66] is shear-rate limited, hence control of flow rate may prove effective. Simulations suggest that different asphaltene molecular structures respond differently under an applied shear field [67]. High flow rates and turbulent flow may also help dislodge large deposited flocs [51,68]. Finally, whilst additive packages aimed at dispersing large flocs have use, they may not offer much relief at deposition “hotspots”, if they do not also act to disperse smaller destabilized asphaltene clusters or alter the particle-surface interaction.

5.

Conclusions

A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to investigate the deposition of asphaltene onto a gold surface from heptane:toluene. In-line mixing of asphaltene solution in toluene with heptane precipitant before the QCM-D flow cell created destabilized dispersions where asphaltene was aggregating/flocculating. It is found that the rate of asphaltene deposition strongly depends on both the heptane:toluene ratio and the destabilization time (the time between asphaltene mixing with heptane and the Au sensor surface). A fast and constant rate of asphaltene deposition is observed at low heptane volume fractions and short destabilization times. In this case the depositing species are small (sub-micron) particles. At high heptane volume fractions and long destabilization times, the rate of asphaltene deposition decreases with time and consequently the overall deposited mass is less. In this case the depositing species are both large flocs (>10 micron) and smaller particles. Dissipation vs. frequency plots for asphaltene deposition under different conditions reveal two main behaviours. Type I behaviour, characterised by ∆ < ∆& and large spreading in ∆ ⁄∆ for different overtones, is observed during deposition of smaller particles where the resultant film is relatively laterally homogeneous. Type II behaviour, characterised by ∆ > ∆& and smaller spreading in ∆ ⁄∆ for different overtones, is observed when there is deposition of both smaller particles and large flocs, resulting in a film that is laterally heterogeneous. In our experiments, deposited asphaltene is readily removed by increasing the quality of the solvent in contact with the film, i.e. by flowing asphaltene in toluene (0 vol % heptane). The results suggest that destabilized smaller particles that “want” to aggregate pose the greatest threat to upstream flow assurance since they give rise to the constant deposition rates responsible for thick multilayer formation. Consequently, the management of asphaltene deposition would require pushing destabilized dispersions to their new equilibrium distribution as quickly as possible.

Acknowledgements The authors would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM) which made this research possible.


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Supplementary Material Supplementary information (SI) can be found in one document and contains: SI 1. Viscosity and density of heptane:toluene SI 2. QCM-D theory and loading equations SI 3. Asphaltene particle size distribution by centrifugation-UV-vis spectroscopy SI 4. Refractive index of heptane:toluene SI 5. QCM-D frequency and dissipation shifts during a deposition experiment SI 6. Total mass losses throughout flow system during QCM-D experiments SI 7. Accuracy of Voigt mass SI 8. Mechanical properties of asphaltene deposits from Voigt fitting SI 9. Chemistry of asphaltene deposits

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Energy & Fuels

Figures

Fig. 1. Schematic showing flow-path for QCM-D experiments with in-line mixing of asphaltene solution in toluene with heptane:toluene at a T-junction

Fig. 2. Asphaltene particle size distribution determined indirectly by centrifugation-UV-vis spectroscopy at destabilization times of (a) 10 min and (b) 24 h. Asphaltene (0.1 gL⁻¹) in heptane:toluene.

Fig. 3. Asphaltene particle size distribution by DLS for dispersions in (a) 0, (b) 50, (c) 60, (d) 70, (e) 80 and (f) 90 vol % heptane. Graphs show effect of destabilization time for 0.1 gL⁻¹ asphaltene in heptane:toluene at 20°C.


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Fig. 4. Deposition of asphaltene from heptane:toluene by QCM-D using T-junction. Frequency (top) and dissipation (bottom) for =3, destabilization time = 20 s. Baselines are measured in heptane:toluene, before flowing asphaltene (0.1 gL⁻¹), indicated by arrow. Depositions are carried out at increasing heptane vol %. After each deposition, the volume fraction of heptane is reduced to 0% causing desorption of asphaltene. Letters A-F denote sections described in text.


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Energy & Fuels

Fig. 5. Deposition of asphaltene onto Au from non-equilibrium dispersions in heptane:toluene. Mass vs. time plots for (a) 50 to 70 and (b) 70 to 90 vol % heptane. Asphaltene concentration is 0.1 gL⁻¹, 20 s destabilization time, 20 µLs⁻¹ flow rate, 20°C. Initial rate of deposition vs. heptane volume fraction (c), n.b. open circles indicate measured rate after 20 min.

Fig. 6. Effect of destabilization time on the mass of asphaltene deposited from (a) 70 and (b) 80 vol % heptane. Variation of inlet tube length gives destabilization times of 12, 20 and 50 s. n.b. Inset in (a) highlights decreasing rate with time for 50 s destabilization time.


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Fig. 7. Dissipation (∆Dn ) vs. frequency (∆fn ⁄n) plots for asphaltene on Au from 60, 70 and 80 vol % heptane at destabilization times of 12, 20 and 50 s. Arrows show direction of increasing overtone number (n).

Figure 8. Tapping mode AFM height images of (a) clean gold surface and asphaltene films deposited from (b) 60, (c) 70 and (d) 80 vol % heptane at a destabilization time of 20 s; line profile along dashed line for (e) image c and (f) image d; large scan size images of deposits from (g) 70 and (h) 80 vol %


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Energy & Fuels

heptane; height images (i,j) of film deposited from 70 vol % heptane at a destabilization time of 50 s and (k) line profile along dashed line for image j

Fig. 9. Illustration of asphaltene deposition behaviour from non-equilibrium dispersions before and after the formation of flocs (a), n.b. in-line mixing means nature of asphaltene flowing over the sensor surface is constant with time. Hypothetical potential energy diagram for asphaltene destabilization by heptane (b).


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Page 24 of 24

Tables

Table 1. Steps describing experimental procedure for QCM-D experiments using T-junction Heptane Reservoir A Reservoir B Step Description Section Asphaltene Heptane: No. in Fig. 4 concentration (vol %) Toluene (gL⁻¹) (vol ratio) 1 Baseline A 0 0 Toluene 0:90

Flow rate from A (µLs⁻¹) 2

Flow rate from B (µLs⁻¹) 18

2

Baseline

A

0

50

Toluene

50:40

2

18

3

Baseline

A

0

60

Toluene

60:30

2

18

4

Baseline

A

0

70

Toluene

70:20

2

18

5

Baseline

A

0

80

Toluene

80:10

2

18

6

Baseline

A

0

90

Toluene

90:0

2

18

7

Baseline

A

0

0

0:90

2

18

8

Adsorption B

0.1

0

0:90

2

18

9

Deposition C

0.1

50

50:40

2

18

10

Desorption D

0.1

0

Toluene 1 gL⁻¹ asphaltene in toluene 1 gL⁻¹ asphaltene in toluene 1 gL⁻¹ asphaltene in toluene

0:90

2

18


24

Deposition of Asphaltene from Destabilized Dispersions in Heptane

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