Study on Demulsification of Water-in-Oil Emulsions Via Microwave Heating Technology

MATERIALS AND METHODS

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. 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 Pico Log 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, two types of crude oil were collected namely, heavy oil and light crude oil. 50-50 and 20-80% water-in-oil emulsions were prepared using the same volumes of oil and water. Emulsions were prepared in 900 mL graduated beakers, with ranges by volume of the water and oil phase. The microwave radiation was set to its highest power setting. The water phase is tab water. The emulsions were agitated vigorously using a standard three blade propeller at speed of 1600 rpm and temperature 28°C for 7 min. The concentrations of water in samples were 20-50% by volume. The container of emulsion sample was placed in the center of Elba domestic microwave oven model: EMO 808SS. Three thermocouples were inserted in the 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 batch microwave heating at 2450 MHz were recorded by Pico-TC-08 data logging. The surfactant used in this study was the commercially available Triton X-100; this Triton X-100 is a non-ionic water soluble molecule. The emulsifying agent was 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) was 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:

(1)

The prepared emulsion was used to check for w/o or o/w emulsions. All emulsions investigated were water-in-oil (w/o) emulsion (oil-continuous).

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. The separation of emulsified water from crude oil has several stages, due to gravity settling, water droplet/droplet flocculation takes place as water droplets approach each other (Young et al., 1996). The purpose of heating water-in-oil emulsions with microwave radiation is to separate water from oil. When water-in-oil emulsion is heated with microwave radiation, two phenomena 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:

(2)

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, (ρ_w-ρ₀) does, the result when viscosity decreases, the droplets size increases. Therefore, microwave heating increases the velocity of water (V_w) and accelerates the separation of emulsion. The second phenomenon is coagulation. The higher temperature and lower viscosity make the coagulation process easier. The results are larger particle diameter D and rapid separation.

Microwave power generation: Using microwaves as a source of heat in the processing (heating, melting, drying and thawing) of materials is one of the advantageous because it results in faster, more uniform heating than conventional heating does. This study, focus on generation of microwaves in the oven, temperature distribution, microwave power absorption as well as separation of emulsified water from crude oil. The variables affecting microwave power absorption by an element are dielectric constant and dielectric loss, location and microwave power incident at the load. For a sample in cylinder container, the local microwave power flux calculated as:

(3)

Where, m is mass (g) of the sample, A is sample’s container area. The microwave power absorption density at any location within the sample is one of the interesting terms, in this regards, the electric field attenuates (decay) exponentially in x and y directions within the sample due to dissipation as heat and can be expressed as Swami (1982).

(4)

Where, P_z is microwave power transmitted. The attenuation factor can be calculated from the electromagnetic field theory given by Von Hippel (1954) as:

(5)

The above equation will be used for calculation of the volume rate of heat generation by microwave radiation as:

(6)

If the dielectric properties are assumed to be independent of temperature at Frequency 2450 MHz, the wavelength λ_m and penetration depth D_p within a sample for a radiation of the above frequency (2450 MHz) are related to dielectric constant ε’r and dielectric loss ε”r as follows:

(7)

and

(8)

Since microwave heats materials volumetrically, it is possible to calculate the volume rate of microwave heat generation from energy balance equation as:

(9)

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 (Fang and Lai, 1995). The right hand side of Eq. 9 comprises of three terms, convective heat transfer, radiative heat due to microwave and conductive heat in the sample, respectively.

Table 1:	Experimental results of microwave heating (water and crude oil)

Table 2:	Experimental results of microwave heating for emulsions

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. 9, the density (ρ) and (C_p) of the emulsions calculated from mixing rules as:

(10)

(11)

The volume rate of microwave heat generation of the water and crude oil calculated from temperature measurements and Eq. 9 are shown in Table 1 while heat generation values for emulsion samples shown in Table 2.

RESULTS AND DISCUSSION

In this study, a batch microwave heating technology was successfully applied and examined on water, oil and emulsion samples. For temperature distributions, a three temperature readings were placed at the top, middle and bottom of the cylindrical sample container. Figure 2 and 3 depict temperature distribution of 50-50 and 20-80% water-in-oil emulsions respectively, for microwave irradiation time varies from 20, 40, 60, 80, 100, 120, 140, 160 and 180 sec. Figure 2 and 3 show the motion and distribution of temperature of water-in-oil emulsions in horizontal direction were slow and emulsions were heated uniformly through the microwaves. This was expected result since the sample container was covered with aluminum foil at the top and bottom and microwaves enter only on the side surface of samples. Also these results shown the temperature rise at a given location was linear, this may attribute due to the small dielectric loss of oil. The same findings were reported by Fang et al. (1995).

Fig. 2:	Temperature distributions of 50-50% w/o emulsions

Fig. 3:	Temperature distributions of 20-80% w/o emulsions

Fig. 4:	Separation of water from 50-50% w/o emulsions

Fig. 5:	Separation of water from 20-80% w/o emulsions

Fig. 6:	Rates of temperature increase for water and oil

The temperature increase rates of irradiated samples and their volume rates of heat generation were shown in Table 1 and 2 respectively. These samples comprise water, crude oil, 50-50 and 20-80% water-in-oil emulsions. The temperatures of emulsions were obtained from the average values of three location temperature readings.

Table 3:	Volume rates of heat generation of water and crude oil by microwave radiation

Table 4:	Volume rates of heat generation of emulsions by microwave radiation

The rate of temperature increase was calculated from temperature increase divided by radiation time. The average rates of temperature increase of 50-50 and 20-80% water-in-oil emulsions are 0.351 and 0.437 C/sec, respectively. It observed that, the rates of temperature were decreases at temperature increases; this was the expected results since the dielectric loss of water is small.

The energy balance Eq. 9 was used to calculate the volume rates of heat generation; this Equation included three terms, convective heat transfer, irradiative heat transfer due to microwave and conduction heat transfer respectively.

Fig. 7:	Rates of temperature increase for 50-50 and 20-80% w/o

Fig. 8:	Rate of temperature increase vs. radiation time for water, oil and emulsions

From calculations of this study, the contributions of convective and irradiation terms are very small. Since the sample container is a glass cylinder (transparent to microwave and has very low dielectric constant), its heat loss assumed to be zero. The volume rates of microwave heat generation of water and crude oil calculated from the temperature measurements and Eq. 9 are shown in Table 1, while for 50-50 and 20-80% water-in-oil emulsions are shown in Table 2. In application of Eq. 9 for determination of volume rates of heat generation, the emulsion density (ρ_m) and heat capacity (C_p,m) calculated from Eq. 10 and 11, respectively. The calculated volume rates of heat generation of water and oil from Eq. 3 through Eq. 11 were shown in Table 3. While for emulsions were shown in Table 4. It observed that, for water and emulsions, the experimental results greater than the calculated, while for oil the reverse, the calculated results greater than the experimental, this attributed due to shortage of oil properties and used literature values. Since the purpose of heating water-in-oil emulsions with microwave is to separate water from oil, therefore, the separation efficiency of 50-50 and 20-80% water-in-oil emulsions calculated by using Eq. 1 were shown in Fig. 4 and 5, respectively. The same trend was reported by Fang et al. (1988), Fang et al. (1989) and Chan et al. (2002).

All experimental results showed that microwave radiation is very effective in separation of water-in-oil emulsions. Figure 4 and 5 shows that, microwave radiation can raise the temperature of emulsion, reduce the viscosity and accelerate separation process as suggested by Eq. 2. The rates of temperature increase were decrease at higher temperatures, Fig. 6 shows the phenomenon for water and oil, while Fig. 7 shown the same phenomenon for 50-50 and 20-80% water-in-oil emulsions, respectively.

The wavelength (λ_m) and penetration depth (D_p) were found 1.39 and 3.427 cm respectively. Figure 8 shows the heating rate of temperature increase for water, oil and emulsions versus the radiation time.

CONCLUSIONS

The batch 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.