Hollow beads

One of the most widely used spray-drying techniques for coffee extract involves spraying percolated coffee extract at 40°F into the top of a baffled tower of hot air at 480°F initially. The cooler wet air is removed toward the bottom of the tower and finally a powder of the coffee extract solids leaves the tower at about 90°F. These solids are in the form of hollow beads and contain 3% moisture. 

In the supported systems the catalyst can be coated on the walls of the reactor, supported on a solid substrate or deposited around the case of the light source. Many are the supported materials used in literature, such as glass beads, and tubes [69], silica-based materials [70], hollow beads, membranes [71], optical fibers, zeolites, activated carbon, organic fibers [72], and so on. 

All raw materials are procured either in liquid or powder form. While ensuring a certain quality, the selection is mostly based on price. The physical characteristics of the components is not important since they will be dispersed or dissolved and mixed with other liquids to form a solution or slurry. This slurry is pumped by low- or medium-pressure pumps to the spray dryer (Fig. 6.3-2). An air vessel (pressure accumulator) is used to even-out pressure peaks. The spray towers can work in con- or counter-current fashion [B.97]. Concurrent contact yields light powders (mostly made-up of hollow particles) with a bulk density of about 100-150 g/L and a moisture content of 3-10%. Also, the hollow beads tend to break-up and form dust. Counter-current drying (Fig. 6.3-2) produces powder with a bulk density of 300-500 g/L and a moisture content of 7-15% (commonly 10%). Most plants use this method because the higher bulk density is almost always desired. 

Microscopically small hollow beads of resin or even glass (e.g., microballoons) are embedded in a resin matrix. 

The purpose of the second extraction is to confirm that complete extraction was achieved by the first. The second flask should contain no solids, or at most a few milligrams. The hollow bead structure of spray-dried powders can make it difficult to obtain a complete extraction by the Soxhlet method, and the following procedure is then better. 

In Brenn et al. (2001) a proposal is presented for distinguishing between the formation of hollow beads and compact particles obtained from crystallizing substances by means of the parameter... 

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