Water agglomeration

Milk powder particles <100 pm in size are often difficult to wet and tend to become lumpy when dispersed in water (Schubert, 1987). Hence, milk powder is agglomerated during manufacture to make the powder instantly wettable in warm (>45°C) water. Agglomeration is the process of forming porous clusters of powder particles to increase the volume of occluded air, thereby increasing its dispersibility and making the powder... 

Imaging instruments at synchrotron X-ray sources provide the opportunity to study water distributions within small areas (around a few millimeters) at high spatial resolution up to <1 pm. The high photon flux of synchrotron sources (typically between 10 and 10 photons mm s ) fadUtates monochromatic measurements with short exposure times (typically a few seconds or milliseconds) and offers excellent conditions for precise quantitative analyses of even the smallest water agglomerations. 

Figure 18.12 Radiographic series of water evolution for different current densities for the cell at 60°C and 100% relative humidity inlet gas feed. Liquid water agglomerations appear bright. Reproduced from [23] by permission of ECS-The Electrochemical Society.

GDL hydrophobicity can be observed as CO2, water, and current density are similar in botb partitions of the cell. However, when the cathode GDL is split instead, the current density on the side with the hydrophihc GDL (Figure 18.17c) is strongly reduced. The reason is obvious in the radiographic image flooding on the left side due to excessive Hquid water agglomeration. On the hydrophobic side, the water is still balanced and the current density is about 60% higher than that on the hydrophihc side. 

A perspective view of the interior of the visualized fuel-ceU section is depicted in Figure 18.23. The fiber structure of the GDL is clearly visible, and also hquid water agglomerations in the flow field channels at both the anode and cathode sides. At the back side of the channel, water agglomerates to form a film. Most droplets gather at the bottom of the chaimel, reflecting the gravitational influence. 

Water-agglomerated superabsorbent fine particles dissociate upon contact or swelling with an aqueous solution. This results in a concentration of swollen free fine particles that will contribute to an increased gel blocking. 

FIGURE 3.24 Sketch with slide positions of a cathodic gas diffusion layer. Section A is located in the flow field structure, whereas sections B-D represent sUces in the gas diffusion layer substrate (left). Water agglomerations were visuaUzed in each section [89]. 

Prepare a saturated solution of sodium sulphide, preferably from the fused technical sodium polysulphide, and saturate it with sulphur the sulphur content should approximate to that of sodium tetrasulphide. To 50 ml. of the saturated sodium tetrasulphide solution contained in a 500 ml. round-bottomed flask provided with a reflux condenser, add 12 -5 ml. of ethylene dichloride, followed by 1 g. of magnesium oxide to act as catalyst. Heat the mixture until the ethylene dichloride commences to reflux and remove the flame. An exothermic reaction sets in and small particles of Thiokol are formed at the interface between the tetrasulphide solution and the ethylene chloride these float to the surface, agglomerate, and then sink to the bottom of the flask. Decant the hquid, and wash the sohd several times with water. Remove the Thiokol with forceps or tongs and test its rubber-like properties (stretching, etc.). 

The thin dispersion rapidly thickens into a gelled matrix and coagulates into a water-repellent agglomeration that doats on the aqueous medium as the mechanical agitation is continued. The agglomeration is dried gentiy shearing must be avoided. 

Purely aqueous polymerization systems give copolymers that are not wetted by the reaction medium. The products agglomerate and plug valves, nozzles, and tubing, and adhere to stirrer blades, thermocouples, or reactor walls. These problems do not occur in organic media or mixtures of these with water. 

When the recycle soot in the feedstock is too viscous to be pumped at temperatures below 93°C, the water—carbon slurry is first contacted with naphtha carbon—naphtha agglomerates are removed from the water slurry and mixed with additional naphtha. The resultant carbon—naphtha mixture is combined with the hot gasification feedstock which may be as viscous as deasphalter pitch. The feedstock carbon—naphtha mixture is heated and flashed, and then fed to a naphtha stripper where naphtha is recovered for recycle to the carbon—water separation step. The carbon remains dispersed in the hot feedstock leaving the bottom of the naphtha stripper column and is recycled to the gasification reactor. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

---