Residual spray dried particles

Spray drying—Spray drying is applicable to substances that are soluble in a convenient solvent. This method involves atomizing a solution to form a mist. Larger particles are removed by impaction. The remaining flow is mixed with solvent-free air to permit evaporation of solvent from the droplets. Residue particles of the solute are left behind as the solvent evaporates. 

At the conclusion of polymerization, unreacted monomer is recovered by vacuum stripping, then is compressed, condensed, and purified for recycle in the process. A stabilizer, usually sodium carbonate, is then added to the latex at a level of about 0.4%, and the stabilized latex is spray dried. Alternatively some processes involve drum drying following by grinding. In these procedures that involve total drying of the latex, any catalyze residues, emulsifier, buffer, or other additives during the process end up with the product. Particles from emulsion processes are about 1 pm in diameter, about 1/100 of those encountered in suspension polymerization. 

Spray drying is by definition the transformation of feed from a fluid state into a dried particulate form by spraying the feed into a hot drying medium [Masters, 1985]. This is an ideal process where the end-product must comply with precise quality standards regarding particle size distribution, residual moisture content, bulk density, and particle shape. It involves the atomization of a liquid feedstock into a spray of droplets and contacting the droplets with hot air in a drying chamber. The sprays are produced by either rotary (wheel) or nozzle atomizers. Evaporation of solvent of matrix liquid from the droplets and formation of dry particles proceeds under controlled temperature and air flow conditions. 

Most food-processing companies use spray dryers to produce powdered products. Spray drying has the ability to handle heat-sensitive foods with maximum retention of their nutritive content. The flexibility of spray-dryer design enables powders to be produced in the various forms required by consumer and industry. This includes agglomerated and nonagglomerated powders having precise particles size distribution, residual moisture content, and bulk density. As examples, spray drying of milk, tomato juice, tea extracts, and coffee is discussed. 

For the above recipe a reaction temperature of about 50°C is appropriate but if a redox initiating system (see Section 18.5.3.1) is used rapid polymerization may be achieved at about 2(fC, Reaction is allowed to continue, usually for 1—2 hours, until the pressure in the reactor drops and then the resulting latex is stripped of residual monomer and spray dried. The polymer obtained by this type of process is in the form of smooth sperhical particles with diameters in the range lO -lO " cm. 

Serim, T. M., Wanning, S., Lamprecht, A. (n.d.). Determination of residual moisture in spray-freeze-dried particles and optimization of the freeze drying time, San Diego, CA. 

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