1. Introduction
One of the main issues of the oil production industry is the separation of
water from crude oil. The crude oil extracted from the wellbore is naturally in
an emulsion form.[1,2] The formation of water-in-oil emulsion happens once the heterogonous mixture flows in the piping valves
and porous rocks and endures turbulence at high temperatures or high pressure.[3,4] Emulsions are extremely stable due to the presence of interface-active
species such as surfactants, asphaltenes, resins, naphthenic acids, and fine
solids.[5–7] The formation of highly stabilized emulsion with saline water causes
numerous problems during oil processing and transportation, including an
increase in viscosity related to the presence of tiny water droplets in crude
oil, obstruction during transit, the high cost of pumping and transportation as
well as pipelines corrosion and catalysts poisoning throughout the refining
process.[8–10] The emulsion must be split into two phases or demulsified to maintain a
smooth refining process in the oil production industry. Therefore, elaborating
the efficient and qualitative method of reducing the stability of water-in-oil
emulsions is the primary concern among oilfield researchers.
Several demulsification techniques were developed in recent years for the
separation of crude oil emulsions such as chemical, biological,[11–13] membrane,[14–16] electrical[17–19] and microwave
irradiation.[20–22] Chemical treatment is the most widely used demulsification method, which involves the addition
of demulsifying chemicals to improve emulsion separation.[1,23,24] Demulsifiers should have a higher interfacial
activity than natural emulsifiers and less mechanical strength of the
interfacial film that facilitates the coalescence of water droplets. The
demulsification mechanism involves replacing surface active natural stabilizers
with demulsifier molecules from the water-oil interface.[25,26] In recent years, plentiful research studies have been dedicated to using
various types of chemical additives to estimate the influence of demulsifiers’
composition, structure, concentration and molecular weight on demulsifier
effectiveness.[3,27–30] Chemical demulsifiers
are well-known for their effective separation of oil from water as well as for
reducing the operational and disposal costs. However, they are associated with
environmental pollution as a result of the discharge of demulsified water
(removed from the crude) into the environment and contamination of the
separated product.[31] In addition, it is necessary to select or optimize the demulsifying agent
regarding the composition of crude oil emulsion which can be varied depending
on the oil field and depth of production prior to separation.[25]
The coal fly ash (CFA) could be considered as the auspicious alternative
to chemical additives. Recently, several studies have reported the CFA as a
promising demulsifying agent for stable W/O emulsions due to the high content
of silica, iron and alumina oxides.[32] The main advantage of CFA compared to chemical demulsifiers is that it
can be easily removed from the water phase after breaking the emulsion and
recycled, thus, reducing the operational costs.[33] Furthermore, CFA is the
waste product of thermoelectric power stations,[34] making it relatively
cheap and available, and its implementation in the petroleum industry may be
beneficial both for the economy and ecology since CFA can result in issues with
the skin, eyes, respiratory, cardiovascular, and neurological and even cause
cancer.[35] Fly ash is usually stored in open ash dumps and contributes to the content
of PM2.5 particles (inhalable particles with diameters of 2.5 µm and
smaller) in the air.[36] Also, the use of ash is
a waste-free process and may reduce the burden on the environment of large
cities in Kazakhstan. Fly ash's demulsifying capabilities have recently been
studied in Kazakhstan,[37] but more research is
still needed to determine how it affects oil emulsions with different
compositions and stability.
In this work, highly stable water-in-crude oil emulsions were treated
under different temperatures in the presence of CFA. The correlation between particle size distribution
of coal fly ash and demulsify/cation efficiency was revealed. In addition, the
combination effect of CFA with a surface-active agent Tween 20 was investigated
and the separation mechanism of CFA was described.
2. Experimental section
2.1. Materials
Crude oil samples are taken from North-West Qonys oilfield in Kyzylorda
region, Kazakhstan. The selected properties of crude oil were determined and
are presented in Table 1.
Table 1. Crude oil of
North-West Qonys oilfield characteristics.
|
Сharacteristic
|
Value
|
Determination method
|
|
Asphaltenes
|
0.19 wt%
|
GOST 11858-66, ASTMD 6560
|
|
Waxes
|
0.69 wt%
|
GOST 11858-66, ASTMD 6560
|
|
Paraffines
|
11.5 wt%
|
GOST 11851-85
|
|
Dynamic viscosity
|
52.8 mPas (at 30 °C)
|
Brookfield
digital rheometer DV-III+
|
|
Density
|
843 kg/m3
|
GOST 33364-2015
|
|
API Gravity
|
36.35°API
|
ASTM D-287
|
|
Water content
|
6% (vol.)
|
ASTM D-4006
|
Tween 20 was used as a surface-active agent. Tween 20 – polyoxyethylene
(20) sorbitan monolaurate (Fig. 1). The hydrophilic
properties of Tweens are provided by ethylene oxide groups - (CH2CH2O)
- and a carboxylic acid polyester, and the lipophilic properties are provided
by polysorbitan.
Fig. 1 Tween 20
structural formula.
2.2. Methods
The CFA samples were taken from thermal power station № 2 in Almaty
city, Kazakhstan. CFA was dried at 105 °C for 2 hours and sieved using
vibratory sieve shaker Analysette 3 with the smallest mesh size of 63 μm
and labelled as СFA-1. The sieved fraction of CFA was
then milled in a planetary ball mill (SQM-0.4 L) at 400 rpm for 10 minutes.
The grinded CFA sample was labelled as CFA-2.
Emulsions were mixed with 1 wt% of coal fly ash powders and homogenized
for 5 minutes. This amount of CFA is found as an optimal weight for
demulsification.[38] Afterwards, emulsions with ash were thermostated at various temperatures
for different periods of time and then were centrifuged for the ash particles
sedimentation. The laboratory centrifuge СМ-6М at 1500 rpm
during 5 min was used for centrifugation of W/O emulsion.
The demulsification efficiency (DE) was studied using the bottle test
method at different temperatures.[39]
The crude oil model emulsion was transferred into 50 ml graduated glass test
tubes and placed into a thermostat. The aqueous phase separation was visually
monitored at regular time intervals. DE
was calculated according to the equation taking into account the initial volume
of water in crude oil:
× 
X-ray diffraction patterns of samples were obtained on Dron-4
diffractometer in digital form using copper radiation. Sample recording modes
are as follows: X-ray tube voltage 35 kV, tube current 20 mA, goniometer
movement step 0.05° 2θ, and intensity measurement time at a point - 1.5
sec. During the shooting, the sample rotated in its own plane at a speed of 60
rpm. The phase analysis was carried out using the PCPDFWIN and EVA programs
with the PDF-2 diffraction database.
Scanning electron microscopy (SEM) images were derived via Quanta 3D 200i
system and microphotographs were obtained via optical microscope Leica DM 6000
M at Kazakh National university’s National nanotechnology laboratory of open
type.
The Fourier Transform
Infrared Spectra (FT-IR) of crude oil samples were measured on PerkinElmer
Spectrum 65 and zeta potential measurements were carried out using Anton Paar
zeta potential analyzer.
The particle size analysis was determined via the laser scattering
particle size distribution analyser Partica LA-960. The dynamic viscosity
measurements were analysed via Brookfield
digital rheometer model DV-III+. The water content was
defined through Dean-Stark distillation method (ASTMD-4006).
3. Results and discussion
3.1 Determination the physicochemical
characteristics of coal fly ash particles
In the first stage of this work, the characteristics of CFA particles were
studied. According to the XRD analysis results (Fig. 2, Table 2), CFA is mainly represented by crystals of mullite and quartz with
insignificant amounts of aluminium and iron (III) oxides. This result
correlates with other research papers dedicated to fly ash.[40–42]
Conforming to the ASTM C-618 standard, CFA belongs to the F type. Measured zeta
potential was equal to -25mV and it can be explained by the presence of
aluminum oxide carrying the negative charge.
Fig. 2 XRD analysis of CFA.
Table 2. Mineral composition of CFA.
|
Mineral
|
Weight composition (%)
|
|
Mullite
|
67.3
|
|
Quartz
|
25.8
|
|
Al2O3
|
3.7
|
|
Fe3O4
|
3.2
|
According to the SEM image (Fig. 3) CFA-1 is represented mainly
by spherical and polydispersed particles with sizes from less than 1 µm to 15
µm.
Fig. 3 Scanning electron microscopy image of CFA-1.
From Fig. 4 it is evident that the ash grinding leads to a shift of the peak maxima in
the particle size distribution curve from 11.5 to 4.4 µm. It favors increasing
in the specific surface area of CFA powder and more uniform distribution of ash
particles in crude oil emulsion. During grinding, the particles become finer
and more monodisperse, which correlates well with SEM image of CFA-2 (Fig. 5). Finally, the ash particle's shape is altered from a spherical to a more
irregular form.
Fig. 4 Particle size distribution
curve of CFA.
Fig. 5 Scanning electron
microscopy image of CFA-2.
3.2 Demulsification performance of the coal fly
ash particles
W/O emulsions of various concentrations from 10% to 50% (vol.) were stable
at room temperature, and the phase separation was not observed. Natural water-in-oil emulsions are usually produced during oil
extraction and transportation. Water content of the emulsions varies between
30-50% (vol.) according to Zolfaghari et al. and 60-80% (vol.) according to
Fingas and Fieldhouse.[6,43] This is why water concentration from 10% to 50% (vol.) were chosen. In
addition, the watering degree of w/o emulsions of Kazakhstan oilfields varies
closed to these concentrations.[44–46] W/o emulsions treatment with CFA at room temperature did not yield in any
phase separation. This is why thermal treatment was decided to apply. Optimal
temperature of 60 °C was chosen due to the highest DE for 50% (vol.) w/o
emulsion which is the closest to the natural emulsions by water content.
Moreover, due to the high stability of crude oil emulsions, temperatures below
50-60 °C are not applied and the application of higher temperatures is not
justified due to evaporation of light oil fractions. The emulsions were thermostated for 90 minutes and then they were
centrifuged. The effect of the thermal treatment was investigated on w/o
emulsions, and it was clear that temperatures from 50 °C to 60 °C are optimal
and yield the highest DE. Nevertheless, thermal treatment is not enough for the
dewatering of the w/o emulsion, because the DE did not exceed 20% at the
optimal temperature.
To study the demulsification in the presence of CFA particles, w/o
emulsions were treated with the fly ash powder under different temperatures and
concentrations of emulsions, and the results are presented in Fig. 6. Fig. 6 shows that DE rises with higher water content in the emulsions and reaches
nearly 80% for 40% (vol.) w/o emulsion, whereas for 20% (vol.) and 10% (vol.)
does not exceed 60%. It is noteworthy that demulsification of 10% and 20% w/o
emulsions at different time intervals happened suddenly. It can be explained by
the fact that phase boundary was not clear first 20-50 minutes, but values
after 50 minutes were visible and reached the plateau.
Fig. 6 DE
of CFA-1 at 60 °C.
Particle size decrease positively affects DE as shown in Fig. 7. For example, CFA-2 applied at the same conditions to 20% (vol.) w/o
emulsion increases the DE nearly by 10% compared with CFA-1. Nevertheless, DE
was improved nearly by 1% for 40% (vol.) w/o using CFA-2.
Fig. 7 DE of CFA-2 at 60 °C.
Effect of temperature in combination with CFA-2 addition was measured onto
w/o emulsions of various concentrations. Table 3 displays that
increasing the temperature higher than 50-60°C leads to lowering of DE. Some
results deviate from the general trend due to the absence of clear phase
boundary observed during the experiments.
Table. 3 DE (%) of CFA-2 (1 wt%) for w/o emulsions at 90th
minute.
|
Water content
Temperature
|
10%
|
20%
|
40%
|
50%
|
|
50 °C
|
64.9
|
80.6
|
91.7
|
75.5
|
|
60 °C
|
-
|
64.5
|
82.6
|
86.8
|
|
70 °C
|
52
|
56.5
|
62.2
|
71.7
|
Microphotographs (Fig. 8) illustrate that 50%
(vol.) w/o emulsion with water droplet diameter from 1.3 to 22 µm was broken
after 1-hour treatment with CFA-2 at 60 °C. Complete breaking of the crude oil
emulsion can be justified by the absence of water droplets on the
microphotograph image. The demulsification via CFA-2 yields higher DE, but
causes the transition of fine CFA-2 particles suspended in the oil phase.
However, it is known that fine ash particles (up to 75 µm) mixed with fluids in
the gaps of the pumping unit's bearings and bushings cause increased vibrations
and subsequent jamming.[47]
Therefore, the centrifugal treatment of w/o emulsions was applied after
thermostating stage.
Fig. 8 Microphotographs of a) -
50% w/o emulsion; b) - 50% w/o emulsion after demulsification by CFA-2 at 60
°C.
Surface active agents are widely used to split the w/o emulsions into two
phases. For instance, the group of researchers applied Tweens onto w/o emulsions
at various concentrations, and the effective concentrations varied from 700 ppm
to 800 ppm.[48] As a result of their natural fatty acid and sugar alcohol origins, Tweens
are acknowledged as green emulsifiers, detergents, and dispersion agents for food,
cosmetic, and personal care applications.[49] The demulsifying effect of Tweens is based on the fact that they contain
a sufficiently large number of ethylene oxide groups, for Tween 20 it equals to
20. Polymeric non-ionic surfactants are used in practice mainly to destabilize
the water-in-oil emulsion. Tween 20 has the highest HLB value among Tweens.[48] Polymeric demulsifiers
with high HLB values can be adsorbed at the water-oil interface and destroy the
adsorption layer of emulsifiers. That is, they have a developed hydrophilic
part that can penetrate into the interfacial layer around the water globule.
In the current investigation, Tween 20 was applied onto w/o emulsions at
concentrations of 800 ppm and 1600 ppm. At concentrations of 800 ppm and room
temperature, Tween 20 was not effective and did not show two separate phases.
Heating up to 60 °C and increasing the concentration of Tween 20 to 1600 ppm
allowed to reach DE of 3.8% only. Fig. 9 demonstrates that the
Tween 20 has a significant impact on the DE in combination with CFA-2. The
combined effect of Tween 20 and CFA-2 was studied. DE reached 100% at 1600 ppm
of Tween 20 and 1% wt of CFA-2 for 20% (vol.) w/o emulsion, while for 40% and 50%
(vol.) w/o emulsions DE was around 90%.
Fig. 9 Cumulative effect of
Tween 20 (1600 ppm) and CFA-2.
3.3 The
demulsification mechanism of CFA particles
The combination of Tween 20 and CFA-2 led to the highest DE even with the
lowest water content emulsions at 60°C. The consistent explanation of this
could be the effects of adsorption of natural stabilizers on the surface of
CFA-2 (Figs. 10a, b) and the subsequent replacement of the residual stabilizers by Tween 20 (Fig. 10c). Electrostatic interaction between the negative
charge of CFA particles and positively charged nitrogen heteroatoms from
asphaltene molecules could be a reason for the adsorptive properties of CFA. Thus,
the interfacial film became less stable and water droplets are coalesced (Fig. 10 c,d). The proposed separation mechanism is illustrated in the Fig. 10.
To justify the concept of adsorptive action of CFA-2, the FT-IR spectra
were measured. Fig. 11 displays the FT-IR spectrum of crude oil sample, pristine CFA-2 and CFA-2
used to treat 40% w/o emulsion. CFA-2 sample after using
it for dewatering the 40% w/o emulsion was collected from the bottom of a test
tube and did not contain the water as it was oily already. As shown in Fig. 11c, the crude oil sample produce the strongest bands that correlates with
aliphatic C-H stretching and could be related to methyl, methylene and methine groups
attached to the heteroatoms such as O and N and aromatic rings (largest bands
at about 2850 and 2925 cm-1) in asphaltene and resins aggregates.
Same patterns of bands observed were found and described by Asemani et al.[50] These bands were detected in CFA-2 sample after using it for oil emulsion
dewatering (Fig. 11b), while they were not spotted in pure CFA-2 sample (Fig. 11a).
The cost-effectiveness of the proposed method involves various factors
including assessment of cost and availability of fly ash and Tween 20. Fly ash
is a waste product on thermal power plants and its price is relatively low.
Prices of fly ash can range from $14 to $26 per ton, depending on location and
particle size distribution. Almaty Thermal Power Plant #2 can be considered as
a potential supplier. Tween 20 is a surfactant that is more expensive than ash.
The price of Tween 20 depends on the purity and quantity purchased. Usually,
Tween 20 costs around $8 to $108 per kilogram (kg). Consumption of Tween 20 and
fly ash, when converted to 1 ton of the crude oil emulsion, is (the water
content of crude oil emulsion is assumed to be very high and density of the
emulsion is close to 1g/l) 1.56 kg (1600 ppm) and 10 kg (1% wt.) respectively.
Thus, fly ash and Tween 20 prices per 1 ton of emulsion may vary from $12.64 to
$167 in total. Also, the final cost of the proposed method depends on labour,
equipment and energy costs. Finally, potential scale-up implies the
multiplication of costs by several times. Even though, the price of the oil
separated and ecological benefits overweight the expenses.
Fig. 10 The suggested
demulsification mechanism of CFA action. a) representation
of stable w/o emulsion with CFA introduced; b) natural stabilizers adsorption
on CFA; c) Formation of water droplets stabilized by Tween 20 molecules and
their approaching; d) water droplets coalescence.
Fig. 11 FT-IR
spectroscopy of a) CFA-2; b) CFA-2 after treating 40% w/o emulsion, c) crude
oil sample.
To benchmark the
effectiveness of fly ash and Tween 20 combination several studies were
compared. Ramalho et al. investigated the effect of commercial poly(ethylene
oxide-b-propylene oxide) demulsifier on the separation efficiency of
water-in-crude oil emulsions and the maximum water separation index was 57.2%
with 50 ppm demulsifier concentration.[51]
Similarly, Adilbekova et
al. tested the effect of commercial demulsifiers represented as block
copolymers in which the central polypropylene glycol group is flanked by two
polyethylene glycol groups and yielded a separation efficiency of 96% for 30%
(vol.) water-in-crude oil emulsion.[52]
Hajivand and Vaziri have
evaluated the plethora of commercial demulsifiers present on the market and
justified that separation efficiency did not exceed 60% for various oil- and
water-soluble demulsifiers applied to water-in-oil emulsions at 70°C, pH 5.5
and demulsifier concentration 10-5 volume fraction.[53] Obviously, the degree
of separation depends on the oil type and other conditions like temperature,
treatment time and demulsifier concentrations.
4. Conclusions
The study is dedicated to investigating coal fly ash dewatering ability of
water-in-oil emulsions. Fly ash samples were collected from the local power
plant in Almaty city, Kazakhstan and characterized using various techniques.
Fly ash was represented mainly by silica and aluminium oxides with an average
particle diameter of 11.5µm. The utilization of a planetary ball mill for
dispersion caused a change in the peak maximum of the curve representing the
distribution of particle sizes, moving it from 11.5 to 4.4 µm.
Stable water-in-oil emulsions were prepared and treated with dispersed
CFA-2 at different temperatures and concentrations. The role of centrifugation
was determined as an additional treatment for producing the clear phase
boundary and increase of demulsification degree. The effect of thermal
treatment with and without CFA-2 was explored. The optimal conditions for CFA-2
(1 wt%) were 60 °C and 1-hour treatment with the DE of about 80%. The cumulative effect of
CFA-2 and Tween 20 at 60 °C was observed with the DE higher than 90% in less
than 1 hour. It has been noticed also that with the raise in the water content
in the emulsions, the DE increases at any applied conditions.
The rationale for
selecting grinded CFA in demulsification is based on several factors such as
chemical composition, high surface area, adsorption capacity, availability,
cost-effectiveness and environmental benefits. According to the American
Society for Testing and Materials CFA is classified into C type and F type
based on its chemical composition.[54] The main difference
between C and F types is the amount of calcium oxide (more than 20% wt. in
class C). Therefore, utilizing C class CFA should consider the interaction of
calcium oxide with the water and oil phase. In particular, fatty acids present
in minor amounts and playing a surfactant role in w/o emulsions may precipitate
when reacted with calcium oxide resulting in destabilization of emulsions.
Additionally, water present in emulsions can react with calcium oxide to
produce calcium hydroxide which is slightly soluble in water and increase the
temperature of the emulsion. In contrast, insoluble particles produced may lead
to the formation of Pickering emulsions. Finally, C class CFA may have
potential as a demulsifier as well as F type and requires additional attention.
Crude oil differs by
type and chemical composition from one well to another well. Furthermore, the
presence of impurities in crude oil is not an obstacle because using fly ash in
combination with Tween 20 considers the flexibility of the method due to the
demulsification mechanism proposed.
Tween 20 is a surfactant
that is commonly stated as inexpensive, non-toxic, environmentally friendly and
water-soluble. Nevertheless, some studies show its disruptive properties onto
some plant membranes at concentrations from 0.001% (v/v) to 0.01% (v/v).[55,56]
A possible mechanism of CFA-2 and Tween 20 combination action was
proposed. The combination of Tween 20 and dispersed CFA-2 results in the
adsorption of natural stabilizers on the CFA surface and the subsequent
substitution of the natural stabilizers by Tween 20.
Acknowledgements
The research for this article was funded by a PhD
program grant for the educational program "Chemical
Engineering", S. Faizullayev and authors are grateful to the Ministry
of Science and Higher Education
of the Republic of Kazakhstan for their support.
Conflict of Interest
There is no conflict of interest.
Supporting Information
Not applicable.
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