Development of a continuous emulsification process for a highly viscous dispersed phase using microstructured devices

2.1 Flow properties

The viscosity dependence on shear rate was examined first. The temperature was set to 25°C and 95°C, respectively, and the shear rate range was set to cover the range of 0–1000 s^-1. The moment of torque of the rheometer was limited to 100 mNm. Due to the high viscosity of Oil at 25°C, the required shear stress for obtaining high shear rates could not be reached. Thus, only the shear rate range of 0–30 s^-1was achieved, however, the full studied range was reached for the measurement at 95°C. The purchased batch-fabricated emulsion and the emulsifier were still studied at 25°C.

The results are shown in Figure 1A. The viscosity of Oil at 95°C remained constant for the whole shear rate range of 0–1000 s^-1, the deviation is <1%, and only a small viscosity drop at low shear rates close to 0 (Figure 1B) can be seen, which shows Newtonian flow behavior under the measurement conditions. The emulsifier exhibits similar behavior at 25°C. There are evident viscosity changes at low shear rates in the curves of the emulsions. To substantiate these results, tests on different emulsion samples were carried out and two of them are presented in Figure 1A and B. It should be noted that the flow curves of the two measurements overlapped exactly. As the dispersed phase droplets may be deformed at high shear rates, the experiments were limited to a shear rate of 320 s^-1.

Figure 1

(A) Flow curves of Oil, emulsifier and emulsions (orange legends for shear stress and black legends for viscosity). (B) Flow curves magnification of Figure 1A (orange legends for shear stress and black legends for viscosity).

It was found that at low shear rates, there were no evident viscosity changes for Oil and the emulsifier, but indeed a small viscosity decrease of the emulsion was observed, which indicates that the emulsion may show Bingham flow behavior. In this study, the curves shear stress versus shear rate of the emulsion do not follow Bingham’s linear function at low shear rates. De Larrard et al. [6] developed a modified Bingham model, which can fit well with our experimental data. The yield stress and the equivalent plastic viscosity can be determined by [6]:

where a and b are constants, τ is shear stress, is shear rate, and is the maximal shear rate of the studied range. The fitted parameters are presented in Table 1. It is found the emulsion has a yield stress of 0.66 Pa and an equivalent plastic viscosity of 0.070 Pa·s. The slight viscosity changes observed at low shear rates were probably caused by the deformation of the droplets. The viscosity of the emulsions became almost constant as the shear rate surpassed 30 s^-1. In conclusion, the emulsion shows Newtonian behavior with a low yield stress (Bingham behavior); Oil and the emulsifier are Newtonian fluids without yield stresses.

Table 1

Determination of the yield stresses and the plastic viscosities.

Fluids	a	b	(s-1)	(Pa)	(Pa·s)	R2
Emulsion at 25°C-1	0.04884	1.065	321.3	0.63	0.070	1
Emulsion at 25°C-2	0.04354	1.087	320.0	0.69	0.070	1
Average	–	–	–	0.66	0.070	–

2.2 Temperature dependence of viscosity

The viscosity of Oil is about 123 Pa·s at 20°C at a shear rate of 10 s^-1. Besides its influence on the flow regimes in the emulsification process, the high viscosity will also lead to high pressure losses in microstructured devices. Thus, it is necessary to find a way to reduce the viscosity efficiently. Two options were tested, increasing the temperature of Oil before mixing it with water, or mixing Oil with the emulsifier first. In the latter case, the emulsifier is expected to reduce the viscosity of Oil.

The viscosities of Oil and Oil-emulsifier mixture were studied over the temperature range of 17–98°C at a shear rate of 10 s^-1 as shown in Figure 2 where the logarithm scale was applied for the Y-coordinate due to the big viscosity difference between the studied fluids. The viscosity of Oil decreases dramatically with the temperature increase. For example, the viscosity at 95°C is <0.3% of that at 17°C. The addition of the emulsifier also has an evident effect on viscosity reduction, especially at low temperatures. The viscosity of Oil-emulsifier mixture at 25°C is 55% lower than that of Oil.

Figure 2

Viscosities of Oil, Oil-emulsifier mixture and emulsion as a function of temperature (at 10 s^-1).

The gap between the C60/1° cone and the plate is 52 µm in height at room temperature and gradually becomes smaller during temperature increase. The studied materials will expand due to temperature increase, so parts of the materials can be pushed out of the gap. Thus, the measured shear stress will be overestimated. As a result, the measured viscosity is higher than the real values. In this case, the viscosity was measured in temperature steps of 20°C. That is the reason for the discontinuity shown in the plots. The slight viscosity difference between two adjacent measurements at the same temperature was also due to the volume changes mentioned above. The viscosities of the emulsions were also studied in the temperature range of 25–45°C and they are closer to the viscosities of the continuous phase than that of the dispersed phase.

3.1 Analysis of emulsions

In this section, a few emulsification experiments were performed in a batch using different mixing orders of the components. The correspondent droplet size distributions of the emulsions were analyzed and compared with the purchased emulsion, which will contribute to the process design. The stability of the emulsions over time was studied, too.

The mass ratio of water to Oil was set to be about 1. The emulsifier content in the emulsions is about 2% in mass. The relatively small quantity of the emulsifier compared to the continuous phase or the dispersed phase indicates the difficulty of its mixing with water or Oil using a µMixer in a continuous process with the precise composition. It would be easier to dissolve the emulsifier in water or Oil in a batch container first, and then mix with Oil or water to create the emulsions in a µMixer. In both cases, only a single µMixer will be needed for the process.

Droplet size distribution is one of the most important parameters for the emulsion quality evaluation. It is related to the emulsions’ stability, viscosity, etc. The droplet size distribution was measured by static light scattering using a Beckman Coulter LS 230.

A commercially available product was used as reference (Process Ref). Different mixing arrangements referring to the components were tested to obtain emulsions: first dissolving the emulsifier in water, then mixing with Oil (Process 1); first dissolving the emulsifier in Oil, then mixing with water (Process 2). To test the solubility of Oil in water, the water was mixed with Oil directly. Then the emulsifier was added, and emulsions were generated after stirring (Process 3). Furthermore, to test the stability of the purchased emulsion, one sample was analysed after it was stored for about three weeks at room temperature [Process Ref (aged emulsion)].

For all the studied processes, Oil was preheated to 80°C before mixing with other components. For example, in Process 2, due to the large viscosity difference between Oil and the emulsifier, Oil was preheated to 80°C so that Oil had a similar viscosity to the emulsifier to facilitate the dissolution. In this study, only batch emulsifications were carried out under magnetic agitation. The results are presented in Figure 3 within reasonable uncertainties (<4%).

Figure 3

Characteristic droplet sizes of emulsions.

The volume weighted particle sizes d₁₀, d₅₀, d₉₀ and the Sauter diameter d_(3,2) are often used to characterize droplet size distributions of emulsions. The results of the emulsions produced in this work are presented in Figure 3. Practically, d_(3,2) is related to the energy density input to the system during emulsification [7] and can be used to compare different emulsification processes. The span is used to indicate the width of the droplet size distributions. It is defined by equation (3):

Emulsions produced by Process 1 with 50% Oil content had been tested before. In these investigations it was found that Process 1 could generate a similar droplet size distribution as it was seen in the purchased emulsion. Here the experiments were repeated, but with slowly increasing the Oil content in the emulsion (about 60%) to test the possibility of high-content dispersed phase by this emulsification process. The results show that the emulsion produced exhibits lowest d_(3,2), d₁₀, and d₅₀, which may be a result of the high energy input into the system due to longer stirring time. The relatively high d₉₀ was achieved due to the high content of dispersed phase. Processes 2 and 3 show similar droplet size distributions to Process Ref.

The span of the studied processes are given in Table 2. All the processes except Process 1 show similar span compared to Process Ref, which means the width of the droplet size distribution fits the requirements well. Process 1 shows a very high span due to the high content of dispersed phase in the emulsion.

Finally, the stability of emulsions was studied. It is found that d₁₀, d₅₀, d₉₀, and d_(3,2) all increased after three weeks of storage at room temperature (aged emulsion), which shows the stability of the emulsions cannot be guaranteed over a long time period at room temperature. This was due to the sedimentation effect and the coalescence of droplets. To reduce these effects, the product needs to be stored at lower temperatures (e.g., 5°C) or has to be used directly after production.

3.2 Design of the process

In previous work [8, 9], the possibility of using µMixers for emulsification was shown. Here, a special µMixer will be designed according to the ratio of the dispersed phase to the continuous phase, the correspondent pressure losses, etc. The results given in Section 3.1 showed that all the three batch processes (mixing Oil-emulsifier mixture with water, mixing water-emulsifier mixture with Oil, or mixing Oil-water mixture with emulsifier) could produce emulsions and there is no major difference in terms of the emulsion qualities between those three processes. In the last case, the residence time needs to be long enough to cover the interfaces. In a continuous emulsification process, the residence time in µMixers is usually very short and the emulsification may not be ensured. Thus, only the two other possibilities will be discussed as follows. A preliminary flow diagram is developed and shown in Figure 4.

Figure 4

Preliminary designed flow diagram for continuous emulsification.

In Figure 4, the emulsification takes place in the µMixer. From the discussion on viscosity given above, Oil should be heated before the mixing device. A microstructured heat exchanger (µHX) is preferred for its high efficiency in heat transfer. Pump 1 will be a special pump designed for handling highly viscous fluids. Depending on the pressure drop after the µHX, an additional pump (Pump 3) may be needed for the emulsification process. So far, a specially designed µHX for highly viscous fluids has not been described in literature. The development of an efficient µHX will be one of the major tasks in the development process.

The premixing of the emulsifier in water or in Oil is discussible. Dissolving the emulsifier in water is much easier, this premixing does not require much energy input and can be carried out in water containers with low or no agitation, i.e., there is no process step for obtaining the pre-mixture. If the emulsifier is dissolved in Oil, the pressure drop in the µHX can be reduced due to the effect of viscosity reduction by adding the emulsifier. In this case, most likely no additional pump (Pump 3) is needed. The disadvantage of this procedure is the difficulty of preparing the Oil-emulsifier mixture due to the high viscosity of Oil.

After emulsification, the temperature of the emulsion will be about 45°C. Therefore, at a later stage of this project, this portion of heat is planned to be recuperated? for preheating Oil by using the process shown in Figure 5. The blockage due to viscosity increase can only take place in the heating fluid passage (the emulsion, with the temperature decrease) and not in the heated fluid passage (Oil, with the temperature increase). The temperature of the emulsion will be lowered down from 45°C. Figure 2 shows that its viscosity is 0.106 Pa·s at 45°C and 0.128 Pa·s at 25°C. This moderate viscosity increase reduces the possibility of temperature change induced blockage in the heating fluid passage.

Figure 5

Flow diagram for continuous emulsification process using microstructured devices and energy recuperation.

In the suggested process, the thermal energy consumption is optimized. However, the more microstructured devices are incorporated, the more pressure losses are introduced. Thus, one or two additional pumps will be needed. The energy dissipation due to flow through microstructured devices may become important.