INTRODUCTION
Demulsification of crude oil forms an integral part of crude oil production
and chemical demulsification is an important step in demulsification sequence.
In chemical demulsification, chemical known as demulsifier is added to the water-in-crude
oil emulsion. These demulsifiers are surface active agents (surfactants) and
develop high surface pressure at crude oil-water interface (Graham
et al., 1980; Bhardwaj et al. 1994).
It results in replacement of rigid film of natural crude oil surfactants by
a film which is conducive to coalescence of water droplets. Only qualitative
information is available about the rates of coalescence of water droplets in
such emulsions, i.e. the rate is fast or slow. Chemical demulsification is the
most widely applied method of treating water-in-oil and oil-in-water emulsions
and involves the use of chemical additives to accelerate the emulsion breaking
process. The formulation of an emulsion demulsifier for a specific petroleum
emulsion is a complicated undertaking. In petroleum system, asphaltenes and
resinous substances comprise a major portion of the interfacially active components
of the oil (Sjoblom et al., 1992; Johansen
et al., 1989; Urdahl et al., 1992;
Siffert et al., 1984). Asphaltenes and resinous
are large polyaromatic and polycyclic condensed ring compounds containing heteroatoms
(Urdahl et al., 1992; Moschopedis
et al., 1976). Chemically, asphaltenes and resins are represent
the pentane or hexane insoluble portion of the oil (Anderson
and Bird, 1990; Moschopedis et al., 1976;
Ferworn et al., 1993). The objective of this research
is to qualitatively study the coalescence and separation of water droplets in
emulsions.
MATERIALS AND METHODS
Crude oil samples were obtained from Petronas Refinery at Melaka. A detailed
procedure for the water-in-crude oil (w/o) emulsions preparation and their procedures
including the formation of w/o emulsion, their characteristics and methods of
preparation are thoroughly described in a previous study by Abdurahmman
at al. (2006). Here the work merely describes the main experimental
steps. Three crude oils were used: crude A from Iran oilfield, crude B from
Kuwait oilfield and crude C from Malaysia.
| Table 2: |
Physical properties of crude oils used in the experiments |
 |
| Table 3: |
Composition of w/o emulsion formulations and their corresponding
stabilities |
 |
Their respective compositions and fractions were shown in Table
1 and 2, respectively. For preparation of water-in-crude
oil emulsions, the agent in oil method was implemented, that is; the Emulsifying
agent dissolved in the continuous phase (oil) and water added gradually to the
mixture (oil + emulsifying agent).emulsions were agitated vigorously using a
standard three blade propeller at room temperature (25-30 °C). The volume
of water settled to the bottom was read from the scale on the beaker with different
times. The prepared emulsions were used to check for w/o or o/w emulsions. All
emulsions investigated were type of water-in-oil emulsions (oil continuous).
Coalescence of water droplets was studied at two different demulsifier concentrations,
50 and 100 mg L-1 on the basis of emulsion volume. Surfactants used
in this study are the commercially available Triton-X-100, Sodium Dedocyl-Sulphate
(SDDS), sorbitan monooleate (Span 83) and Low Sulphur Wax Residue (LSWR).
The concentrations of water in samples were 10-90% by volume. The composition
of w/o emulsion formulation and their corresponding stabilities are given
in Table 3 which shows the surfactants used for the
present study.
RESULTS AND DISCUSSION
The first part of this research deals with the formation, production and stabilization
of w/o emulsions, while in the second part discusses the emulsion breaking (demulsification)
of w/o. Table 3 shows emulsions made of LSWR and Triton-X-100
with 50, 55, 60 and 70% (v/v) internal phase. For the 50 and 55% emulsion, a
higher solids concentration was found (6 mg mL-1 oil versus 2 mg
mL-1) allowed easier emulsification and slowed the settling process.
The 60 and 70% emulsions were appeared fairly stable with little settling.
|
| Fig. 1: |
Change of emulsion stability for crude oil A emulsions (50-50%
w/o) as function of processing time and emulsifier applied |
|
| Fig. 2: |
Change of emulsion stability for crude oil C emulsions (50-50%
w/o) as function of processing time and emulsifier applied |
The LSWR and Triton-X-100 stabilized emulsions exhibited only slight coalescence
over two days (Fig. 1). Some globule formation was observed
and settling occurred. In contrast, the Span 83 emulsions were different from
LSWR and Triton-X-100 emulsions, even at similar dispersed phase volume fractions.
The difference between surfactant concentrations for the 50 and 55% (v/v) emulsions
made by Span 83 appeared very significant on emulsions stability. High Span
83 concentrations increased emulsion stability; therefore, for high concentration
of Span 83, the viscosity of w/o emulsion increased considerably and the emulsion
droplets lost their shape. The emulsion stability for crude oil A and C were
examined as function of processing time and emulsifier applied. Stability was
evaluated via the ratio of the total water separated. As depicted in Fig.
1 and 2, in most cases, stability of emulsion increases
with processing time. It is worth noticing that all surfactants permit a very
long time for separation of the water phase (emulsion more stable). However,
the maximum amount of water separated from Crude oil A was 50% and crude oil
C was 60% (Fig. 1, 2), respectively. From
these observations, the classification in terms of decreasing stability efficiency
is therefore, the following; SDDS > Triton-X-100 > LSWR > Span 83, respectively.
Effects of resin/asphaltene ratios on emulsion stability:
The presence of asphaltene and resin in crude oil can stabilize the emulsion.
Asphaltene and resin act as an emulsifying agent, which reduce the interfacial
tension and to induce repulsive forces between the droplets.
|
| Fig. 3: |
Effect of adding resins (varying R/A) from different crude
oils on emulsion stability |
Therefore, the resin/asphaltene ratio (R/A) is an important parameter to predict
the emulsion stability. Resins increase the solubility of asphaltene in the
crude and minimize the asphaltene interaction with water droplets. The resin/asphaltene
ratio (R/A) may be expected to provide valuable information on tight emulsion
formation. In this regards, the R/A ratio for oils A, B and C were found as
6, 12 and 8, respectively. Our experiment results showed that crude oil B fractions
which has higher R/A ratio, 13 was separated easily than that of lower R/A ratio
(crude A and C). It may concluded that, when R/A ratio decreased, the emulsions
are become tighter and harder to break. High resin concentration keeps more
of the asphaltene dissolved in the oil phase, based on this fact, crude oil
A found more stable than crude B and C.
Effects of added resins and resin type: The next logical step
in the investigation was to determine the effect of increasing the amount
of added resins (i.e., increasing the R/A ratio) at constant asphaltene
concentration and to investigate the effect of adding resins isolated
from different crude types. Figure 3 shows the effect
of adding resins from all three crude types on the stability of emulsions
produced from model oil containing 0.3 w% crude A asphaltenes. The first
order effect was that R/A ratios ≥3 did good diminish the emulsifying
propensity of these model oils to a significant extent. The emulsion were
completely resolved when R/A ratio = 4 with crude B. When R/A = 6, all
emulsions were resolved. It should also be noted that the stability observed
for emulsions at an R/A = 3 the amount of water separated from the three
crude oils A, B and C as; 41, 75 and 55%, respectively. The maximum amount
of water decanted when R/A ratio = 6, which 92, 83 and 96%, respectively.
Effects of asphaltene concentration: The concentration of asphaltenes
in the oil should also have a substantial effect on the emulsion stability.
The results in Fig. 4 where the concentration of asphaltenes
was increased both at a constant resin concentration of 1.3 w% and at
a constant R/A ratio of 1, do indeed prove this to be true. In comparing
the two curves in Fig. 4, the constant resin concentration
curve shows more water resolution (100%) than the constant R/A ratio (68%)
curve at asphaltene concentration of 0.5%.
Demulsification is the breaking of a crude oil emulsion into oil and water
phases. From point view, the oil producer is interested in two aspects of demulsification;
the rate or speed at which this separation takes place and the amount of water
left in the crude oil after separation. Coalescence of water droplets was studied
at two different demulsidier concentrations, 50 and 100 mg L-1 on
the basis of emulsion volume. Emulsion was stirred at 750 rpm.
|
| Fig. 4: |
Comparison of varying versus fixed R/A ratio on emulsion stability |
|
| Fig. 5: |
Variation in droplet diameter with time for 50 mg L-1
demulsifier |
|
| Fig. 6: |
Variation in droplet diameter with time for 100 mg L-1
demulsifier |
Water droplet size was measured till 150 min and the first measurement was
taken after 5 min. These experiments were carried out at 25 and 50°C. The
results of these experiments are shown in Fig. 5 and 6,
respectively. It was observed that the droplet size grows very rapid during
the early droplet time (first 7 min) and then followed by very slow increase
in size. It observed also increase in demulsifier will accelerate the coalescence
of droplet faster.
|
| Fig. 7: |
Variation in droplet diameter with time for short duration |
Since the initial coalescence was very rapid, therefore, another set of experiments
was conducted to observe growth in droplet size within shorter time frame. In
this experiment changes in droplet size were observed from 1 to 10 min and temperature
during the experiment was 25°C. It was observed that from these results,
the first minute is most important in droplet coalescence, as shown in Fig.
7.
Due to limitations inherent in the experimental procedures, it was not
possible to run experiments at shorter periods with good reliability of
time scale. Coalescence of water droplets leads to decrease in surface
area of dispersed phase.
CONCLUSIONS
Water-in-crude oil emulsions have great importance in the oil industry.
Results from this study tend to support the proposed mechanism in which
emulsion stability is governed primarily by the state of solubility of
asphaltenes in the crude oil. Coalescence of water droplets in water-in-crude
oil emulsions, in the presence of a demulsifier is characterized by very
short initial coalescence time (few sec). High resin-asphaltene ratio
R/A decreases emulsion stability.
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