Expert ForumsMixing

Topic: Re: How do you create an emulsion?

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There is no magic formula for creating an emulsion.  A combination of fluid properties are involved, including surface tension, relative viscosity, density difference, phase ratio, and surfactants or stabilizers.  While mixers, especially high-shear mixers are usually required to form a stable emulsion, the relative amounts of mixing intensity and stabilizing agents must be balanced to make a practical emulsion.  Greater mixing intensity, especially with respect to impeller tip speed, will create smaller dispersed phase droplets, which should help create a more stable emulsion.  In some cases, mixing intensity alone is sufficient to form an emulsion.  In other cases, a stabilizing agent (surface active agent) is necessary to both form small droplets and prevent coalescence. From the very limited process description, the best information available is some general guidelines, which apply to most emulsions.  First, the tank size mentioned is large, especially when the emulsion requirements are uncertain.  Most emulsion recommendations are based on smaller scale test results.  Those tests may provide information about the mixing and chemistry requirements necessary to form a stable emulsion.  Next, this combination of immiscible fluids has a significant density difference.  So Solution 1 would be expected to float on top of Solution 2, and potentially separate quickly.  Some mixing intensity, in terms of flow or bulk motion will be required to circulate these fluids and bring them into contact.  The phase in which the mixer is located usually becomes continuous phase.  The phase ratio between the two solutions is also important.  The relative quantities of the two phases will often influence, which phase is the dispersed phase and which is the continuous phase.  Usually, the phase with the smaller volume will be the dispersed phase.  However, the method by which the phases are brought together and the dispersed drop size can give different results.  The temperatures indicated in the process description means that the viscosity of both phases is at a practical minimum, which should make droplet dispersion and emulsion formation easier. As far as mixing intensity is concerned, a tip speed (velocity at the tip of the impeller) of 1 to 3 m/s is common for liquid-liquid dispersion.  At these tip velocities, strong shear gradients are formed in the fluid, resulting is drop break-up and dispersion.  Nominal drop sizes from 20 to 80 microns are possible with these tip speeds, but the physical properties of the fluids have a strong effect on these results.  High-shear mixers can be of several types from saw-tooth disks, to rotor-stator devices, to even colloid mills and high-pressure homogenizers.  For this application and the vessel size mentioned, a saw-tooth disk impeller would seem to be most practical.  A very large rotor-stator mixer would be required as a stand-alone mixer.  A smaller rotor-stator mixer would have to be combined with high flow impeller to provide mixing and dispersion. The basic answer to all of these questions and uncertainties is that some form of laboratory or pilot test results are needed to begin to predict how to create an emulsion.  If large-scale experience with a similar process were available, those results would be more useful.  Since the question is being asked, the large-scale results must not be available.
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