The SEM process has been tested in various application fields. This chapter will give an overview over the conducted research.
4.1. Preparation of Hybrid Nanoparticles
Hybrid nanoparticles are of scientific and industrial interest as they can be used for several applications—e.g., paints of high color intensity [
60,
61,
62], electronic devices [
63,
64], and medical applications [
65,
66,
67]. Hybrid nanoparticles can be prepared by miniemulsion polymerization in a two-stage process [
68,
69,
70]: first, a nanoparticle-in-monomer suspension is emulsified in a continuous phase and then the polymerization of the filled submicron-sized monomer droplets is conducted [
55]. Hecht et al. consider high-pressure homogenizers to be the most suitable device to produce small sizes for particle filled monomer droplets at a high throughput [
55].
Figure 6 displays some nanostructured particles produced in dynamic high-pressure processes via miniemulsions [
11].
In the past, ultrasonic systems have been used to produce particle loaded droplets [
71,
72,
73]. Since nanoparticles can cause abrasion in homogenization valves [
12], Hecht et al. used the SEM process for the miniemulsion polymerization especially with regard to a high degree of nanoparticle filling (up to 60 wt %) [
55]. As disruption unit, a simple orifice (
Figure 5a) was used. To avoid abrasion of the orifice, operational mode 1 (see
Figure 4) was chosen: pure continuous phase was pumped through the orifice, while the premix emulsion containing the nanoparticle-in-monomer-droplets was inserted as mixing stream. Droplets of the desired size range (<500 nm) were produced, which demonstrates that the particle-loaded droplets did not require an elongation prior to their break-up [
55]. Winkelmann et al. [
44] used a similar SEM process to produce zinc oxide nanoparticles by miniemulsion precipitation. Experimental investigations and computational fluid dynamics (CFD) simulations showed that different factors influence the mixing quality: the homogenization pressure, the disruption unit geometry, and the distance between the outlet of the disruption unit and the inlet of the second feed stream. Mixing quality was shown to be responsible for the size of precipitated nanoparticles especially when the process was run in the single-emulsion-mode. Here, precursor 1 of the particles to be precipitated is dissolved in the miniemulsion droplets, while precursor 2 is mixed into the continuous phase. Its transport into the miniemulsion droplet starts precipitation. On top of that, SEM homogenizers can also be efficiently used to disperse nanoparticles in a liquid [
13].
4.2. Particle Stabilized Emulsions
Particle stabilized emulsions (PSE), also called Pickering emulsions, use small particles to stabilize the interface of emulsions [
74,
75]. They have regained interest in scientific literature [
76] as the availability of suitable particles has increased [
58]. Since the energy supplied during emulsification determines the droplet break-up for emulsions including PSE [
77], HPH of PSE could be of great interest [
58], although the particles may cause abrasion as already discussed.
By using an SEM process in which the stabilizing particles are added in the mixing stream, this problem can be reduced. Köhler et al. investigated the influence of process parameters, composition and operational mode on the homogenization results of (o/w)-emulsions prepared in the SEM process which were stabilized by Stober silica particles [
58]. The investigated operational modes were numbers 1, 3, and 6 (see
Figure 4). In all operational modes, the particles were added in the mixing stream. First experiments in the operational mode 3 revealed that small particles in the range of 12 nm were needed to achieve fast stabilization kinetics. Larger particles in the range of 200 nm resulted in significantly larger droplet sizes. At lower homogenizing pressures (100–500 bar), the obtained droplet sizes in all operational modes were comparable to emulsions stabilized by a conventional homogenizer. At higher pressures (800–1000 bar) however, droplet sizes could not be further reduced since the stabilization kinetics of the particles was apparently not fast enough and droplets recoalesced.
4.3. Dairy Homogenization
For decades, drinking milk and dairy products have been homogenized either in full-stream or partial-stream processes [
51]. Homogenization reduces the milk fat globule diameter from around 4 µm to 0.6–0.7 µm [
78]. The lower droplet size is crucial to prevent e.g., creaming of the fat droplets within the shelf life of milk. In the partial-stream process, the milk is first separated into cream and skim milk, before both are mixed again to yield a fat content of maximum 17 vol % [
10,
51] and homogenized. In full-stream homogenization, the fat content is adjusted to the final fat content (e.g., 3.5 vol %) upstream the homogenization step and the whole volume is homogenized.
The partial-stream homogenization process allows a fat content up to 17 vol % upstream of the homogenization step, which results in saved energy due to the reduced over-all processed volume. In a downstream standardization step, the fat content is then adjusted to the final fat content.
Raising the fat content during milk homogenization above 17 vol % is the key to save energy during homogenization [
79]. Nevertheless, in conventional partial stream milk homogenization the fat content cannot exceed 17 vol % due to coalescence and aggregation of the fat droplets [
4] after the homogenization step.
Köhler et al. [
4] used SEM homogenization in order to enable the homogenization of cream with up to 42 vol % fat content which corresponds to the concentration at which cream exits the separation process in conventional dairy processing lines. In this case, the operational mode 3 and therefore a SpEM process was used. The homogenized cream was diluted with skim milk, also coming from the separation process, in the micromixer unit instantly after droplet breakup. Using this setup, fat globule aggregation could be prevented while still allowing for the homogenization of increased fat contents. Since the product volume to be pressurized was reduced, energy and investment costs could be cut. Furthermore, it was possible to simplify the process line because two mixing units could be eliminated. From the application point of view, the SpEM process allows an increase of the throughput of a dairy process line by a factor of up to 8 without investment in new high-pressure pumps [
10,
79].
In order to further improve the SpEM process of milk homogenization, Köhler et al. [
4] investigated the influence of the distance between the exit of the orifice and the inlet of the mixing stream (distance d in
Figure 3) on the milk fat globule size and mixing quality. Both experiments and CFD simulations of the process indicated that there is an optimal distance for the injection of the skim milk as mixing stream. Short distances improve the mixing quality while long distances enable an undisturbed disruption process. As compromise, the mixing stream was inserted after a distance d = 5 mm in order to obtain the smallest droplets possible.
In subsequent experiments, Köhler et al. [
57] found that the required product quality can be maintained by homogenizing cream with 32–42 vol %. Compared to conventional dairy HPH, the aggregation rate was rather low, even for those high fat contents. However, it must be stated that Köhler et al. found no further reduction of droplet sizes by increasing the homogenization pressure over 200 bar, which indicates that coalescence could not be fully prevented. On top of that, it was shown that using the SEM process provides the opportunity to increase the cream temperature, as the heat-sensitive milk proteins are mainly included in the skim milk [
53]. In this way, homogenization results can be further improved as the droplet viscosity decreases with increasing temperature which simplifies droplet deformation and break-up [
10]. According to Köhler et al. in [
56], both the length of the mixing zone and the geometrical shape of the flow channel had no significant influence on the homogenization results.
The experiments on the homogenization of milk in the SEM process described so far had been conducted using orifices as disruption units. However, in industrial dairy processes, flat valves are used instead [
51]. Schlender et al. developed a modified flat valve allowing for SEM processing of e.g., drinking and chocolate milk [
79]. The working principle of the SEM flat valve set-up is shown in
Figure 7. The flat valve was successfully integrated in an industrial homogenization process with 1000 L/h throughput in operational mode 3. At 29 vol % fat content and 220 bar homogenization pressure, the droplet size distribution was comparable to commonly homogenized milk. Based on energy consumption data from this production line, 70%–80% energy could be saved in plants of similar dimensions. Furthermore, the authors also managed to further decrease fat globule droplet size for ESL (extended shelf life) dairy products, and to produce a cocoa drink that fits the standards of quality by adding a cacao mixture in the mixing stream.
4.4. Melt Emulsification
Nowadays, small particles of wax are commonly produced by wet milling [
16]. Using a melt emulsification process instead of wet milling can lead to shorter process times, energy savings, and new product characteristics [
16]. The new product characteristics result from the spherical shape of the particles obtained by the melt emulsification process [
18]. The process consists of three main steps: melting the disperse phase, emulsification, and producing a suspension by cooling [
80]. One of the main challenges is finding a suitable emulsifier that can stabilize the emulsion at higher and lower temperatures as well as the suspension [
18].
Köhler et al. showed that it is possible to produce monomodal particle size distributions in the order of 1 µm [
7] using a SEM melt emulsification process in operational mode 3. Different waxes were homogenized at pressures between 100 and 1000 bar. Cold water of 20°C was used as a side stream to realize an instant cooling of the droplets. Due to the instant cooling, it was possible to stabilize the droplets and maintain their spherical shape without any emulsifier added to the formulation [
81].
4.6. Economical Interest
The SEM technology opens up opportunities in several operational fields like process costs, new products, and process design for all industrial sectors. The following economical aspects focus mainly on the dairy and beverage industry. Beside the already mentioned (
Section 4.3) energy optimization with 30%–80% savings of mechanical energy cf. conventional processes, advantages in maintenance are expected too. Due to the effective dispersion after the homogenization gap, wear intensive additives like cocoa, sugar, or others can be added to the product with the mixing stream. By that, abrasion of the high-pressure unit can be minimized. Depending on its application, SEM homogenization provides potential energy and maintenance cost benefits of up to 70 vol %.
When the existing machinery is replaced, the equipment size could be reduced. This results in optimized investment and maintenance needs. Existing older and energy inefficient equipment can be upgraded to an economically profitable production level.
In case of process design, the usage of SEM processes results in new levels of freedom in product development, process configuration, and plant design. Besides a flexible product flow management for a cost optimized process design, rapid product changeover by in-line adjustment of mixing ratios between main and mixing stream are the focus of industrial interest. A further potential economical benefit can be achieved by developing new products with enhanced properties. As examples, the gentle treatment of shear- or temperature sensitive ingredients and additives while adding them to the mixing stream can be named. In other industrial sectors, production improvements of emulsions based products are often limited by batch processes. By using an SEM based continuous process instead, the production performance might be increased significantly. Especially in case of melt emulsification not only the production performance and process variability can be increased. The possible ultra-high cooling rate between the main and mixing stream also allows developing new product properties and can reduce the addition of environmentally questionable additives to a minimum. Here, a solvent-free production process can be reached [
82].