Premix disperse phase fraction

In premix membrane emulsification, emulsion premixes are pressed through the membrane pores. A phase inversion while passing the membrane is induced if the membrane surface is wetted by the droplet phase of the premix. Thus emulsions of high disperse phase fractions, even above 80%, can be produced at a throughput of up to several tons per square meter of membrane surface per hour. A narrow droplet size distribution requires a low number (2-3) of recirculations through the membrane. The mean droplet size is about twice the pore size [51]. 

Influence of Disperse Phase Fraction With increasing dispersed phase fraction, droplet collisions and thus droplet coalescence frequencies increase. Therefore, usually, the lower the disperse phase concentrations, the smaller are the mean emulsion droplet sizes found. An example is given in Figure 13.13. In this example, an emulsion premix with 40% of dispersed phase and a Sauter diameter of... 

From the present point of view, premix membrane emulsification is the technique of choice for producing emulsions with high dispersed phase fractions at reasonable flux (see Figure 13.14) [32]. 

SEM systems offer the possibility to use the disperse phase fraction in the emulsification and mixing stream as an additional process parameter. In general, the disperse phase fraction can be in the range of 0-100% in both streams (see Figure 5.3). When both streams have the same phase content of 0% or 100%, no emulsion will be produced. If the three concentrations of tp = 0% (pure continuous phase),

disperse phase), and 0%operational modes are possible and described below for o/w emulsions. 

We now consider a 40% silicone oil premixed emulsion dispersed in an aqueous phase. In Fig. 9 the evolution of mean diameter is plotted as a function of the applied shear rate. The dispersed phase volume fraction is kept constant at 75%, while the emulsifier concentration in the continuous medium is varied from 15 wt % to 45 wt %. The error bars show the distribution width deduced from the measured uniformity. At a given shear rate, smaller droplets with lower uniformity are produced (see Fig. 9) when surfactant concentration increases. For example at 45% of Ifralan 205 the uniformity never exceeds 15% whatever the applied shear rate, whereas it is of the order of 25% for 15% of Ifralan 205. Some microscope pictures of the emulsions obtained are given in Fig. 10. To understand the evolution, we may argue that the continuous phase viscosity increases... 

Premix membrane emulsification is a powerful technique, especially where the system does not induce internal fouling of the membrane used, and where high volume fractions of the dispersed phase are required. This probably limits the applications mostly to (fine) chemicals production, but one may expect a few applications in the food industry as well. 

Inverted W/O emulsion. The premixed emulsion was composed of aqueous droplets dispersed in an oil phase containing a lipophilic surfactant. The droplet mass fraction (/>, was set to 80%. This crude emulsion was sheared into the Couette-type cell [159] at constant shear rate V = 10000 s and a... 

Let us first consider an inverted W/O emulsion made of 10% of 0.1 M NaCl large droplets dispersed in sorbitan monooleate (Span 80), a liquid surfactant which also acts as the dispersing continuous phase. At this low droplet volume fraction, the rheological properties of the premixed emulsion is essentially determined by the continuous medium. The rheological behavior of the oil phase can be described as follows it exhibits a Newtonian behavior with a viscosity of 1 Pa s up to 1000 s 1 and a pronounced shear thinning behavior above this threshold value. Between 1000 s 1 and 3000 s1, although the stress is approximately unchanged, the viscosity ratio is increased by a factor of 4. 

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