INTRODUCTION
A significant portion of the world crude oil is produced in the form of emulsion.
Also during the lifting, transportation and processing of oil, emulsions and
sludge’s are created. Ninty percent of the oilfield emulsion produced is
the type of water-in-oil (w/o) emulsions (Xia et al.,
2004). Water/oil/solid emulsions are mixtures of ordinarily incompatible
materials. Crude oil is composed of mostly hydrocarbons, both aliphatic and
aromatic, as well as some molecules that naturally occurring surfactants in
crude oil (asphaltenses and resins) have been identified as largely responsible
for the stability of these emulsions. An emulsion may be tight (difficult to
break) or loose (easy to break). Whether an emulsion is tight or loose depends
on a number of factors such as the percentage of oil and water found in the
emulsion, the amount of agitation, the types and amounts of emulsifying agents
present, as well as the properties of oil and water (Ali
and Alqam, 2000).
For economic purpose, pipeline considerations and for efficient refinery operations,
the produced crude oil emulsions must dewatered and necessary to separate the
water completely from the crude oils. The traditional methods of eliminating
these emulsions include high heat and chemical utilizations, which force the
emulsion to separate into water, hydrocarbon and solids. Usually these methods
were expensive, chemical additives are carried into the wastewater streams,
or follow the hydrocarbon into the refining process. The concept of microwave
demulsification was first introduced by Klaila (1983)
and Wolf (1986) in their patent applications. Chan
and Chen (2002) and Fang and Lai (1995), Fang
et al. (1989) reported demulsification of water-in-oil (w/o) emulsions
by microwave radiation. The experimental results showed that the percentage
of water separated from the emulsion by microwave radiation was higher than
80% under certain conditions. This study was conducted to examine a batch microwave
process in demulsification of water-oil- (w/o) emulsions. Also, the study examined
the effect of triton X-100 and Low Sulfur Wax Residue (LSWR) from synthesized
(w/o) emulsions stability and demulsification. Results of the study showed that
emulsion stability is depending on the concentration of the emulsifiers (Triton-X-100
or LSWR). The demulsification or coalescence rate was measured by dividing the
volume of water separated to the total water content. The demulsification efficiency
reaches 90% in a very short time under microwave radiation.
MATERIALS AND METHODS
This research was conducted in 2010 at University Malaysia Pahang’s Laboratories.
In this study, Elba domestic microwave oven model: EMO 808SS, its rated power
output is 900 watts and its operation frequency is 2450 MHz was used in heating
water-in-oil emulsion samples.
|
| Fig. 1: |
Elba microwave oven |
A 900 mL graduated cylindrical glass was used as sample container. The diameter
and height of emulsion sample in the container were 11.5 and 11 cm, respectively.
Three thermocouples type (K-IEC-584-3) were connected to Pico-TC-08 data logging and then connected to microwave oven as shown in Fig. 1. The data logger was connected to PC; with PicoLog R5.08.3 software. The thermocouples were inserted to different locations top, middle and bottom of the emulsion sample to measure local temperatures.
Sample preparation and procedures: The crude oil samples were obtained from Petronas refinery at Malaka city, 50-50% and 20-80% water-in-oil emulsions were prepared. Emulsions were prepared in 900 mL graduated beakers, with ranges by volume of the water and oil phase. The microwave radiation was set to different power settings. The water phase is distilled water. The emulsions were agitated vigorously using a standard three blade propeller at speed of 1800 rpm and temperature 30°C for 8 min. The concentrations of water in samples were 20-50% by volume. The sample placed in a feed tank and used a pump to pull the samples to the Elba domestic microwave oven model: EMO 808SS for radiation. Three thermocouples were inserted in the settling tank of emulsion sample at different locations, top, middle and bottom. The emulsion samples were heated with microwave radiation for 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 sec. Temperature profiles of emulsions inside a cylindrical container during continuous microwave heating at 2450 MHz were recorded by Pico-TC-08 data logging. The surfactants used in this study were the commercially available Triton X-100; this Triton X-100 is a non-ionic water soluble molecule and Low Sulfur Wax Residue (LSWR). The emulsifying agents were used as manufactured without further dilution. In order to prepare water-in-oil emulsions, the agent-in-oil method was followed; that is, in this study, the emulsifying agent (Triton X-100) and LSWR were dissolved in the continuous phase (oil), then water was added gradually to the mixture. The volume of water settled to the bottom was read from the scale on the beaker with different times. The amount of water separation in percent was calculated as separation efficiency (e). From volume of water observed in the beaker as follows:
MICROWAVE RADIATION
A number of studies were carried out on microwave heating (MW) of oil and water
systems. Microwave heating because of its volumetric heating effects, offers
a faster processing rate, also microwave has another unique feature other than
how it interacts with matter, is its penetrating power. Microwave distributes
energy within the bulk of most materials, rather than just on its surface. Any
heat produced at the surface must then be conducted or convicted into the material.
Microwave, because the wave length is relatively long and the method of interaction
so mild, can penetrate deeply into a substance. Penetration energy deposition
by microwave overcomes many surface-limiting characteristics of normal heating.
The purpose of heating water-in-oil (w/o) emulsions with microwave radiation
is to separate water from oil. Therefore, when water-in-oil emulsion is heated
with microwave radiation, two phenomenons will occur; the first one is the increase
of temperature, which causes reduction of viscosity and coalescence. The result
is separation of water without addition of chemicals (Fang
et al., 1988, 1989). According to Stoke’s
law, if oil is the continuous phase, the settling velocity of water droplets
is given by:
where, D is the diameter of the droplets. The viscosity of oil very sensitive to temperature, as temperature increases, viscosity decreases much faster than the density difference (ρw-ρ0) does, the result when viscosity decreases, the droplets size increases. Therefore, microwave heating increases the velocity of water (vw) and accelerates the separation of emulsion. The higher temperature and lower viscosity make the coagulation process easier. The results are larger particle diameter D and rapid separation. Since, microwave heats materials volumetrically, it is possible to calculate the volume rate of microwave heat generation from energy balance equation as:
| Table 1: |
Experimental results of continuous microwave heating (50-50%
w/o emulsions) (Microwave power is: 900 W) |
 |
| Table 2: |
Experimental results of continuous microwave heating (20-80%
w/o emulsions) (Microwave power is: 900W) |
 |
| Table 3: |
Viscosity data for 50-50 % and 20-80 % w/o emulsions |
 |
The above equation assumes that the rate of heat transfer from emulsified water
droplets to the continuous phase (oil) is very rapid; therefore, water and oil
practically have the same temperature. The right hand side of Eq.
3 comprises of three terms, convective heat transfer, radiative heat due
to microwave and conductive heat in the sample respectively. From results of
this study, the effect of radiative term is very small as well as convective
term. Since, the sample container (glass) has low dielectric constant, therefore,
its heat generated assumed to be negligible. For calculation of volume rate
of heat generation in Eq. 3, the density (ρ) and (cp)
of the emulsions calculated from mixing rules as:
The volume rates of microwave heat generation for 50-50% and 20-80% water-in-oil emulsions calculated from temperature measurements and Eq. 3 were summarized and shown in Table 1 and 2, respectively. While viscosity data shown in Table 3.
Dielectric constant and dielectric loss of water used in this work were given
by Wolf (1986):
Von-Hippel (1954) proposed equations for dielectric properties
of various petroleum oils, in this regards, dielectric constant and loss tangent
of crude oil for this study calculated from the equations below.
Table 3 shows viscosity data for 50-50% and 20-80% water-in-oil emulsions, respectively.
RESULTS AND DISCUSSION
The results of coalescence between liquid droplets of 50-50% and 20-80% water-in-oil emulsions were shown in Fig. 2 and 3, respectively.
All experimental results showed that microwave radiation is very effective
in separation of water-in-oil emulsions. Figure 2 and 3
illustrated that, microwave radiation can raise the temperature of emulsion,
reduce the viscosity, increase the velocity and accelerate separation process
as suggested by Eq. (2) and supported by Nour
et al. ( 2006, 2007). It is found that triton
X-100 and the LSWR stabilize water-in-oil emulsions, while in the absence of
triton X-100 and LSWR, emulsions were not stable. The percent of coalesced or
separated water is plotted against the time of sedimentation. It observed that,
the 50-50% w/o was separated faster than 20-80% does, this may attributed due
to high volume fraction for 50-50% (φ = 0.46) compared to 20-80% (φ
= 0.18).
|
| Fig. 2: |
Separation of water from 50-50% water-in-oil emulsion |
|
| Fig. 3: |
Separation of water from 20-80% water-in-oil emulsion |
The water separation in percent was calculated from volume measurements as
described in Eq. 1. It found that the percentage of coalesced
water droplets decreases with the concentration of the triton X-100 reached
up to 0.8% and LSWR up to 2%.
The temperature increasing rates of irradiated samples, dielectric constant, dielectric loss, loss tangents and volume rates of heat generation for 50-50% and 20-80% w/o emulsions were shown in Table 1 and 2, respectively. The rate of temperature increase was calculated from temperature increase divided by radiation time. It is observed that, the rate of temperature increase (dT/dt) is inversely proportional to the increase in temperature ΔT; this was expected result since the dielectric loss of water is small. The rates of temperature increase for 50-50% and 20-80% w/o illustrated in Fig. 4. Equation 3 used to calculate the volume rate of heat generation, from the calculations; it found that the contributions of the heat loss by convective heat transfer and radiative heat loss were very small, while the contribution of heat accumulation in the emulsion is significant.
In application of Eq. 3 for calculation of volume rates of
heat generation, the emulsion density (ρm) and heat capacity
(cpm) were calculated from mixture rules Eq. 4
and 5, respectively. It observed that, the dielectric properties
of emulsions affected by temperature in this regards, Fig. 5
shows dielectric losses for 50-50% and 20-80% w/o emulsions, this supported
by Nour et al. (2010). It is clear from the figure,
dielectric loss for 20-80% w/o less than for 50-50% this may attributed to the
high temperature of 20-80% compared with temperature of 50-50% w/o.
The shear rate, shear stress and viscosity of the emulsion samples were measured with Brookfield (DV-III) Rheometers given in Table 3. The viscosity, μ of an emulsion diminishes when the volume fraction of the dispersed phase φ is reduced.
|
| Fig. 4: |
Rates of temperature increase for 50-50% and 20-80% w/o |
|
| Fig. 5: |
Dielectric vs. temples for 50-50% and 20-80% w/o |
|
| Fig. 6: |
Viscosity vs. temp. For 50-50% and 20-80% w/o |
|
| Fig. 7: |
Viscosity vs. Shear rate. For 50-50% and 20-80% w/o |
Figure 6 shows the viscosity of w/o versus temperature, it’s clear from the Fig. 6; emulsions were very sensitive to temperature. As temperature increases, the viscosity decreases fast.
The viscosities for 50-50% and 20-80% for w/o emulsions are shown in Fig. 7. Increases in the internal phase volume fraction lead to an increase in both the viscosity and the degree of shear thinning.
CONCLUSIONS
The microwave heating process was examined for water, oil and emulsion samples.
Results of this study showed that, microwave radiation is a dielectric heating
technique with the unique characteristics of fast, volumetric and effective
heating is feasible and has the potential to be used an alternative way in the
demulsification of water-in-oil emulsions. From temperature distribution profiles
of irradiated emulsion, it appears water-in-oil emulsion has been heated quickly
and uniformly by microwaves rather than by conventional heating. This new separation
technology does not require chemical addition. Furthermore, microwave radiation
appears to provide faster separation than the conventional heating methods.
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